A STUDY OF THE EFFECTS OF FUNCTIONALITY ON CERTAIN ASPECTS
OF CROSSLINKABLE LATEX SYSTEMS
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Erika P. Pedraza
May, 2006 A STUDY OF THE EFFECTS OF FUNCTIONALITY ON CERTAIN ASPECTS
OF CROSSLINKABLE LATEX SYSTEMS
Erika P. Pedraza
Thesis
Approved: Accepted:
______Advisor Department Chair Dr. Mark D. Soucek Dr. Sadhan C. Jana
______Committee Member Dean of the College Dr. Sadhan C. Jana Dr. Frank N. Kelley
______Committee Member Dean of the Graduate School Dr. Thein Kyu Dr. George R. Newkome
______Date
ii ABSTRACT
Thermoset or “crosslinkable” latexes provide the cohesive strength and
solvent resistance lacking in traditional thermoplastic latexes but required for
industrial applications. Crosslinking ability is achieved by incorporation of functional groups and subsequent ionic or covalent bond formation with a crosslinking resin. However, the type and level of functional groups, as well as the addition and localization within the latex usually influences processes of synthesis, film formation and mechanical properties. The present work focused on assessing the effect of functionality on certain aspects of the preparation, film formation and properties development of acrylic crosslinkable latexes.
Colloidal stability, particle size and distribution, film properties and
morphology of core-shell latexes were studied as functions of increasing content
of functional monomers. Stability of the system during synthesis limited the addition of functional groups. A bimodal particle size distribution was observed for high concentrations of functional monomers. Increase in carboxyl and hydroxyl functionalities improved tensile strength and modulus even for un- crosslinked films. The incorporation of functionality along with crosslinking ability
iii into acrylic bimodal latex blends was also investigated. Bimodal latexes with varying functionality location were synthesized and characterized. A melamine formaldehyde resin was used to crosslink the films. The properties of the component latexes affected tensile strength and the structure of the films.
Packing of the composite systems was dependent on small to large particle content ratio and was affected by the presence of the resin used for crosslinking.
Finally, acidic functionalized acrylic latexes were synthesized to study the
influence of acid-base interactions on film forming properties. Two acids of
varying strength were introduced through the copolymerization of MAA or 2-
sulfoethyl methacrylate (SEM). Amines with varying boiling point and base
strength were used for neutralization of the acid groups. Drying parameters, pH
and amine evolution were monitored during drying of neutralized and un-
neutralized latex samples during drying. Surface morphology was monitored
during later stages of coalescence. At ambient temperature, amine volatility was
the controlling factor on de-blocking of weak acid groups. For the stronger acid,
volatility was no longer significant, as the process of de-blocking was governed
by the acid-base equilibrium.
iv DEDICATION
I dedicate this work to the loves of my life:
My husband Camilo, my parents Dario & Ines and my sister Andrea.
v ACKNOWLEDGEMENTS
I would like to thank my advisor Dr. Mark Soucek. I also thank Dr. Sadhan
Jana and Dr. Thein Kyu for being part of my thesis evaluation committee.
I express my gratitude to Dr. R. Byron Pipes, Purdue University, for his constant counseling and support throughout my graduate studies.
Thanks are also to my research colleagues, especially Dr. Dave Dworak,
Ahmet Nebioglu and Dr. Zengang Zong, as well as Dr. Rong Bai from Polymer
Engineering, for their technical help and discussions.
To a great group of friends in Akron; my special thanks to Neissa, Carlos,
Manuela, Jairo, Liliana, Isabella, Felipe, Camila, Carla, Dave, Mayela, Elif, Luis,
Sabrina, Faisal and Antonio for all the good moments and their sincere
friendship.
To my father Dario, my mother Ines and my sister Andrea, for their
unconditional love and support always throughout my studies.
To my husband, my love, my soul mate and my colleague, Camilo. You
not only gave me the courage to take this journey, but patiently shared with me
every step of the way with your love, your encouragement and your expert
technical advice.
vi TABLE OF CONTENTS Page
LIST OF TABLES ...... xi
LIST OF FIGURES ...... xii
CHAPTER
I. INTRODUCTION ...... 1
II. BACKGROUND...... 6
2.1 Emulsion Polymerization...... 6
2.1.1 Components ...... 6
2.1.2 Emulsion Polymerization Process ...... 8
2.1.3 Quantitative Treatment ...... 12
2.2 Particle Morphology Development in Emulsion Polymerization Processes ...... 15
2.3 Film Formation from Latex Dispersions ...... 18
2.3.1 Process of Film Formation...... 20
2.3.2 Drying of Latex Dispersions...... 20
2.3.3 Particles Packing and Deformation...... 23
2.3.4 Coalescence of Particles and Interdiffusion of Polymer through Latex Particles Boundaries...... 24
2.4 Thermosetting Latexes ...... 30
vii 2.4.1 Crosslinkers...... 31
2.5 The role of functionality in thermoplastic and thermosetting latexes...... 37
III. ACRYLIC LATEXES WITH INCREASING CONCENTRATION OF FUNCTIONAL MONOMERS ...... 41
3.1 Introduction ...... 41
3.2 Experimental...... 44
3.2.1 Materials...... 45
3.2.2 Preparation of Latexes ...... 45
3.2.3 Latex Characterization...... 47
3.2.4 Film Formation and Characterization...... 48
3.3 Results...... 51
3.3.1 Latex Properties ...... 53
3.3.2 Film Properties ...... 56
3.4 Discussion ...... 64
3.5 Conclusions ...... 71
IV. FUNCTIONALITY AND PARTICLE SIZE VARIATIONS IN LATEXES FOR THERMOSETTING DISPERSIONS: ENHANCED CROSSLINKED STRUCTURES ...... 72
4.1 Introduction ...... 72
4.2 Experimental...... 76
4.2.1 Materials...... 76
4.2.2 Preparation of Latexes ...... 76
4.2.3 Latex Characterization...... 78
4.2.4 Preparation of Blends and Film Characterization ...... 79
4.3 Results...... 81 viii
4.3.1 Surface structure of uncrosslinked films: Effect of OH groups in packing of bimodal latex dispersions...... 82
4.3.2 Effect of blend ratio on mechanical and thermo-mechanical properties of crosslinked films from bimodal hydroxyl-functional latex blends ...... 84
4.3.3 Effect of hydroxyl functionality placement on mechanical and thermo-mechanical properties of crosslinked films from bimodal latex blends ...... 88
4.4 Discussion ...... 93
4.5 Conclusions ...... 101
V. INFLUENCE OF ACID-BASE INTERACTIONS ON FILM FORMING PROPERTIES OF ACID FUNCTIONALIZED ACRYLIC LATEXES...... 102
5.1 Introduction ...... 102
5.2 Experimental...... 105
5.2.1 Materials...... 105
5.2.2 Preparation of Latexes ...... 106
5.2.3 Latex Characterization...... 107
5.2.4 Drying and Coalescence of Latex Films ...... 108
5.2.5 pH Development and Amine Evolution ...... 110
5.3 Results...... 111
5.3.1 Drying behavior ...... 113
5.3.2 pH development ...... 119
5.3.3 Amine evolution ...... 121
5.3.4 Further film aging...... 122
5.4 Discussion ...... 126
5.5 Conclusions ...... 132 ix VI. CONCLUSIONS...... 134
6.1 Future Work ...... 137
BIBLIOGRAPHY...... 139
x LIST OF TABLES
Table Page
3.1 Core-shell compositions for latexes designed with Tg = 10 °C …… 49
3.2 Core-shell compositions for latexes designed with Tg = -5 °C …..... 50
3.3 Molecular weight and particle size for latex series T( -5)…………….. 55
3.4 Coating testing for crosslinked latex films from series T(10)………. 56
3.5 Dynamic properties and crosslink density for crosslinked films from series T(-5)……………………..………………………………………… 63
3.6 Reactivity ratios for pairs of HEMA, MMA and BA...…………………… 65
4.1 Small and large particle size latex compositions for bimodal blends ………………………………………………………………………………….. 77
4.2 Physical properties of latexes used in the preparation of bimodal latex blends………………………………………………………………………….. 80
4.3 Dynamic properties and crosslink density values of films from bimodal crosslinkable latex blends in series FSL………………….. 87
5.1 Feeding stage components in semi-continuous emulsion polymerizations of acid functionalized acrylic latexes...... 108
5.2 Characterization of acid functionalized acrylic latexes…………….. 109
5.3 Physical properties of amines used for neutralization……………... 114
xi LIST OF FIGURES
Figure Page
2.1 Schematic representation of an emulsion polymerization system … 10
2.2 Morphological structures of polymeric particles in dispersion …..…. 17
2.3 General characteristics of latexes during film formation ………..…. 19
2.4 Structure and crosslinking reaction between a melamine formaldehyde resin and a hydroxyl functional polymer ….………… 34
2.5 Crosslinking reaction of hydroxy functional acrylic latex with cycloaliphatic epoxide ………………………………………………….. 37
2.6 Crosslinking reaction of carboxyl functional acrylic latex with cycloaliphatic epoxide ………………………………………………… 38
3.1 Crosslinking reactions of carboxyl and hydroxyl groups with cycloaliphatic diepoxide in a latex system …..………………………. 44
3.2 STEM image of core-shell latex containing 9 wt% HEMA and 6 wt% MAA …………………………………...……….…………………... 54
3.3 Tensile properties of films from latex series T(-5)…………………… 57
3.4 Tensile properties of crosslinked films from latex series T(-5)……... 58
3.5 Storage modulus of films from latex series T(-5)………………….…. 60
3.6 Tan δ of films from latex series T(-5)………………………….…...... 60
3.7 Storage modulus of films from latex series T(-5)………..…………… 61
3.8 Tan δ of films from latex series T(-5)...……………………………….. 61
3.9 Storage modulus of crosslinked films from latex series T(-5)…..….. 62
xii
3.10 Tan δ of crosslinked films from latex series T(-5).………….………… 62
3.11 Tapping mode AFM images (height mode) of films with increasing functionality ………………………………….…………………………… 63
3.12 Representation of particle nucleation dependence on oligomeric radical solubility……………...... 67
4.1 Tapping mode AFM images of surface morphology of blends in series FSL cast without crosslinker ………………….………………... 83
4.2 Tensile strength for crosslinked films prepared from bimodal crosslinkable latex blends in series FSL…………………………….... 85
4.3 Storage modulus of crosslinked films obtained from bimodal crosslinkable latex blends in series FSL …..…………………………. 86
4.4 Tan δ of crosslinked films from blends in series FSL ……………….. 88
4.5 Tensile strength for films containing increasing fractions of crosslinkable, hydroxyl functional small particles in series FS…...… 89
4.6 Storage modulus (E’) for films containing increasing fractions of crosslinkable, hydroxyl functional small particles in series FS…...… 90
4.7 Tan δ for films containing increasing fractions of crosslinkable, hydroxyl functional small particles in series FS ….……………….…. 90
4.8 Tensile strength of films containing increasing fractions of unfunctionalized small particles in series FL ………………………… 91
4.9 Storage modulus (E’) of films containing increasing fractions of unfunctionalized small particles in series FL …………………………. 92
4.10 Tan δ of films containing increasing fractions of unfunctionalized small particles in series FL …………………………………………….. 92
4.11 AFM images of surface morphology of films from blends in series FSL with crosslinker …………………………………………………….. 96
4.12 Crosslinked structures of bimodal latex blends with varying functionality location……………………………………………………... 100
xiii
5.1 Cumulative weight loss and evaporation rate of latex LCA…………. 115
5.2 Drying parameters for latexes containing carboxylic acid and/or sulfonic acid groups …………………………………………………….. 116
5.3 Rate of drying of acid functionalized latexes neutralized with three tertiary amines …………………………………………………………… 117
5.4 Time needed to lose 99 wt% of water from acid functionalized latexes with three amines ………………………………………………. 118
5.5 Development of pH in drying films……………………………………... 120
5.6 Evolution of amines from latex films…………………………………… 122
5.7 AFM height images of LCA latexes aging during a period of 21 days…………………………………………………………………...... 123
5.8 RMS roughness of AFM height images during aging at 30 °C..……. 124
5.9 RMS roughness of AFM height images during annealing at 60 °C… 125
xiv CHAPTER I
INTRODUCTION
Latexes are aqueous dispersions of solid polymeric particles obtained through an emulsion polymerization technique. Due to its environmental friendly attributes, as well as the special features of the synthesis and its structure- property interrelated characteristics, emulsion polymerization has been the subject of extensive research during the past sixty years. Polymers obtained by emulsion polymerization have the advantage of reaching high molecular weights without developing the increased viscosity characteristic of other polymers (for instance, those synthesized by solution polymerization). In this way, a wide variety of homopolymers and copolymers obtained by emulsion polymerization have proven extremely versatile for applications in fields such as coatings, adhesives, synthetic rubbers, floor polishes and additives amongst others.
The performance of traditional thermoplastic acrylic and methacrylic latexes varies according to the final intended application. These latexes are usually designed for film formation at ambient temperatures with low emission of volatile organic compounds (VOC) for applications such as coatings and adhesives. The development of cohesive strength within the solid polymer is of 1 crucial importance and must be achieved whenever film integrity and performance are required for a specific application. In emulsion polymers, such cohesive strength is achieved through interdiffusion of polymer chains from neighboring particles and it can be enhanced by chemical crosslinking. Latexes that can be crosslinked are also referred to as thermosetting latexes.
Present technology on thermosetting latexes has shown the successful crosslinking of latexes with different crosslinking agents, either external or copolymerized within the polymer backbone. However, the incorporation of crosslinking technology to common thermoplastic latexes involves additional variables that will affect the processes of latex production and property development on films. Besides the types of chemistry involved in the crosslinking reactions, one of the most important variables is the incorporation of functional monomers and crosslinking agents to the dispersions. The type of functional groups and crosslinkers, as well as the addition and localization would be expected to influence processes of synthesis, film formation and mechanical behavior of latex films.
With respect to the addition of functional monomers, it is anticipated that the presence of increased functionality during synthesis would influence physical properties of the particles and the structure of the film. On the other hand, the overall mechanical properties and solvent resistance of latex films may not be enhanced if the polymer is not sufficiently crosslinked. Thus, functionality and
2 crosslinker levels must be enough to provide solvent resistance and cohesive
strength, without disrupting other desired properties mandated by the
requirements of specific applications such as flexibility or impact resistance.
The distribution of functional groups within a crosslinked latex film can be
considered as a controlling factor for structure-property relationships if it is
believed that the enhancement of strength through crosslinking is achieved
primarily when the crosslinker is located in the proximities of functional groups.
Ideally, systems with specially controlled functionality location would allow specific crosslinked structures with similar or even improved mechanical behavior compared to homogeneous crosslinked systems. Such control of functionality location could be synergistically combined with packing optimization through the use of different particle sizes within a single latex dispersion (e.g. bimodal latex dispersions). However, as a result of the incorporation of functional groups within the polymer backbone and the addition of crosslinking agents, packing and distribution of particles during film formation could present dissimilar features compared to traditional monomodal thermoplastic latexes.
In some thermosetting latex dispersions, crosslinking reactions are
benefited by the presence of strong or weak acid groups. For instance,
crosslinking reactions of hydroxyl and/or carboxyl groups in acrylic latex
dispersions with melamine-formaldehyde based resins or cycloaliphatic epoxide resins are usually enhanced by the addition of strong acid catalysts such as
3 sulfonic acids. The presence of such strong acids along with weak acids (as
reactive groups) on the surface of latex particles gives an acidic character to the
dispersion. In this way, weak bases are typically used for pH control, further
stabilization of particles and additionally as blocking agents for reactive and
catalyst acid sites. In this way, acid-base interactions between catalysts,
blocking agents and reacting groups are expected to affect film forming
processes and crosslinking reactions.
In spite of many studies on the addition of hydroxyl, carboxylic acid and other functional groups to latex systems, the combined effects on the preparation and properties of latexes designed for crosslinking and on films obtained therein have been left to study. The present study is aimed at assessing and analyzing the effect of the presence of different types of functionality on crucial aspects of the preparation, film formation and development of mechanical properties in films obtained from crosslinkable acrylic latexes.
The document is divided into six chapters. Following the present
introduction (chapter I), chapter II covers the review on basic concepts in
emulsion polymerization, film formation, film morphology and recent advances in
thermosetting latexes. The study of increasing levels of functional monomers
and the effects on the stability and film properties of crosslinkable latexes of the
core-shell type are discussed in chapter III. The evaluation of combined bimodal
particle size and functionality placement variations in crosslinkable latexes is
4 presented in chapter IV. Chapter V deals with the effect of acid and basic groups on processes of film formation. Finally, chapter VI summarizes the conclusions, findings and contributions of the present work.
5 CHAPTER II
BACKGROUND
2.1 Emulsion Polymerization
Emulsion polymerization involves the unique process of polymer synthesis
by free radical chain polymerization through the stabilization of the reacting
components into an emulsion. It has been used for production of synthetic latexes since early requirements for production of rubber during World War II.
Understanding of the emulsion polymerization process was acquired and developed further since the 1940’s, but major technological advances have occurred in the last 30 years. With the implementation of recent environmental regulations, especially on the field of coatings and adhesives, the emulsion polymerization process has gained increased interest for applications that were otherwise fulfilled by solvent-borne products. [1]
2.1.1 Components
The principal components in an emulsion polymerization system are
monomers, dispersing medium (water or sometimes solvent), emulsifying agent
and initiator. The most common monomers used in this type of polymerizations
6 have been classified into families. [1] Some monomers may belong to more than
one family; however the following is a general classification:
• Butadiene-styrene-acrylonitrile
• Vinyl chloride-vinyl acetate
• Vinylidene chloride
• Acrylates and methacrylates
• Water-soluble monomers: acrylic and methacrylic acids, acrylamide
• Other functional monomers
The dispersing medium is usually water, in which the other materials are dispersed and stabilized by means of an emulsifying agent, also known as
surfactant. Emulsifier is added to the system for several purposes, including the
stabilization of monomer droplets in emulsion, the formation of monomer swollen
micelles that would serve as nucleation sites, and stabilization of growing
particles. Surfactants are usually categorized into ionic (cationic and anionic),
non-ionic and amphoteric (zwitterionic). Anionic and non-ionic surfactants are the most widely used due to compatibility issues. Mixtures of surfactants are
sometimes preferred to enhance colloidal stability in commercial processes.
Initiators are the source of free radicals needed for emulsion
polymerization. Depending on the phase in which the free radicals are
generated, initiators are either water or oil soluble. A good initiator should have
7 an efficient free radical generation at reasonable temperatures and it should be stable at room temperature. The most common initiation technique used is the thermal homolytic dissociation of a compound possessing bonds with dissociation energies between 100 and 170 kJ/mole. Water-soluble thermal initiators include sodium, potassium and ammonium persulfates, which decompose in the range of 50-90ºC. Oil soluble thermal initiators are typically peroxides and azo compounds. Initiation can also occur by single electron transfer to or from an ion, most commonly known as a redox reaction between oxidizing and reductant agents to produce free radicals. Persulfate-bisulfite and persulfate-hydrosulfite are two common redox pairs used in emulsion polymerization. [1]
There are other components added during or after synthesis of emulsion polymers for different purposes. Buffers are added to regulate pH; chain transfer agents are used to control molecular weight; rheological modifiers, plasticizers, biocides, antioxidants and uv-absorbers are usually added post-synthesis to modify the latex properties for a specific application or to protect it against degradation or decomposition. [1]
2.1.2 Emulsion Polymerization Process
The location of the components in distinct phases during emulsion polymerization is one of the most important characteristics of this process. When 8 surfactant is added to water, micelles are formed if the concentration exceeds the
critical micelle concentration (CMC) for a given surfactant. When water-insoluble monomers are used, a very small portion of the monomer is dissolved in water,
another small portion enters the micelles, but the largest amount is located in
monomer droplets stabilized by surfactant molecules adsorbed on the surface of
the droplet. Monomer droplets are comparatively larger than micelles; however
the micelles total surface area is larger than that of the droplets. [2]
Water-soluble initiator molecules are located in the aqueous phase, where
free radicals are produced. Since initiation does not occur until a free radical
encounters monomer molecules and form growing chains, it was traditionally
considered that initiation would take place in micelles upon primary radical
entrance. This mechanism has been called micellar nucleation. However, the
likelihood of a charged free radical entering a micelle bearing a similar charge is
very small. For this reason it is probable that initiation occurs in the aqueous
phase, where a few monomer units are added to the free radical until the
molecule acquires surfactant-like attributes and enters the monomer-swollen
micelles. Although the concept of oligomeric surfactant entering the micelles
changes the traditional view of initiation, it is still considered to contribute to
micellar nucleation. Another mechanism for particle nucleation has been
discussed, in which oligomers formed in the aqueous phase precipitate from the
water and become stabilized by surfactant, creating new particles. This type of
particle formation is called homogeneous nucleation. [3] The extent to which one 9 type of nucleation or the other is more likely in a specific polymerization depends strongly on water solubility of the monomer and on surfactant concentration.
Homogeneous nucleation is favored by monomers with high water solubility and by low surfactant concentrations. Studies on highly water-insoluble monomers like styrene have shown mainly micellar nucleation, while for partially water- soluble monomers such as methyl methacrylate or vinyl acetate homogeneous nucleation is more predominant. [4- 7]
Micelle Monomer-contai ni ng Micelle I I I
Fr eesurfa ctant Polymer Particle
I I I I I
Monomer and surfactant supplied by monomer droplets and micelles, respectively. I I I I
I Monomer Monomer Droplet I Initiator Freesurfa ctant
Figure 2.1 Schematic representation of an emulsion polymerization system. Interaction between micelles, monomer-containing micelles and growing polymer particles.
The original description of emulsion polymerization by Harkins [8] was the basis for the quantitative treatment of Smith and Ewart [9], and other
10 researchers. [10-12] Harkins considered the emulsion polymerization process to
be divided in three intervals, based on the very important interaction between
monomer droplets, inactive micelles (no polymerization taking place) and
polymer particles (monomer-swollen micelles with growing chains). These particles are shown in Figure 2.1. Interval I is characterized by initiation and
nucleation. During this stage polymer particles grow with time, keeping a volume
fraction of monomer enough to swell the polymer. Such monomer is available
from solution in water, which is saturated due to continuous dissolution of
monomer from monomer droplets. Micelles provide the surfactant needed for
stabilization of growing particles and for formation of new ones, as shown in
Figure 2.1. Interval I of the polymerization involves not only the nucleation
process but also the depletion of micelles used up for particle formation. Interval
I ends at a different conversion depending on the monomer being polymerized;
for instance, conversion decreases with increasing water solubility of the
monomer and with reduction in surfactant concentration. The number of particles
increases during Interval I but remains constant after that. During Interval II monomer droplets supply monomer and surfactant to the growing polymer particles through the aqueous solution. The monomer concentration in polymer particles is very high during intervals I and II (saturation), however it decreases at the beginning of interval III when monomer droplets have been consumed.
Polymerization rate increases during interval I and remains constant through interval II. At the beginning of interval III polymerization occurs at a slower rate as monomer concentration in the particles decreases and monomer droplets are
11 no longer present to serve as monomer reservoirs. Final conversions are very
close to 100% and particles are spherical in shape, ranging between 50 to 400 nm in diameter.
2.1.3 Quantitative Treatment
One of the most important characteristics in emulsion polymerization is the
possibility of simultaneously increasing polymerization rate and molecular weight
of the polymer. Such condition is possible due to the increased number of
polymerization sites (i.e. polymer particles) and the high monomer concentration
within the particles. The increase in size of one radical inside a particle can be
described by the general free radical expression for the propagation step:
= pp [Mkr ]p (2.1)
where kp is the propagation rate coefficient and [M]p is the equilibrium swelling
concentration of monomer in the particle. Since polymerization is occurring in all
particles that have captured one radical, then the expression changes to:
MNkn ][ pp Rp = (2.2) N A
12 where N is the total number of particles and n is the average number of radicals
per particle. Haward showed that when a polymerizing system is subdivided
several times, the growing radicals become isolated into individual particles, [13]
and in this way, when another free radical enters such particles termination
occurs almost immediately. The value of n in equation (2) is a very important
parameter as it determines the rate and molar mass profiles during
polymerization. Three limiting cases have been generally discussed. It is
important to consider case 2 initially, as it is most commonly applicable to most
emulsion polymerizations. In case 2, the rate of radical entry to the particles is
very high and the desorption rate is low. The average number of radicals per
particle is n = 0.5, since at any given moment each particle will contain either one
or zero growing chains. Once it has entered the particle, each radical is trapped and propagates until another radical enters and instantaneously terminates the growing chain. [14,15] This case is favored by negligible desorption rates and
when the particle size is too small to accommodate more than one radical. It is
typical of monomers like styrene, with low transfer to monomer constants. Case
1 corresponds to values of n << 0.5. This case is common in monomers with high
water solubility, which have higher transfer constants and produce more active
radicals such as vinyl acetate. It applies when radical desorption is high with
respect to entry rate and when termination rates in water are not negligible. In
case 3, the value n is much higher than unity. This case is favored when
particles are relatively large, but more importantly when termination constants
are low and termination in water and desorption of radicals are not important.
13
According to the Smith-Ewart theory, [9] the main nucleation sites are
surfactant micelles. The number of particles that can be stabilized is dependent
on the total surface area of surfactant in the system, expressed as asS, where as is the interfacial surface area covered by a surfactant molecule and S is the total concentration of surfactant in the system. However, the number of particles also depends on the rate of radical generation Ri as well. Smith and Ewart described
the number of particles by equation 3:
4.0 ⎛ Ri ⎞ 6.0 = kN ⎜ ⎟ ()sSa (2.3) ⎝ μ ⎠
where μ is the rate of particle growth and k is the proportionality constant.
Nomura [16], Hansen and Ugelstad [17] considered that chain-transfer to
monomer and radical adsorption/desorption during interval I were important for
certain monomers different than styrene. In such cases, equation (3) was
modified to obtain:
(1− z ) ⎛ Ri ⎞ z = kN ⎜ ⎟ ()sSa (2.4) ⎝ μ ⎠
where 0.6 < z < 1.0 depending on the chain transfer constant to monomer and
water solubility of the monomer.
14 2.2 Particle Morphology Development in Emulsion Polymerization Processes
During the last few years, research in emulsion polymerization and
emulsion polymers has been directed towards controlling the morphology of latex
particles to obtain tailored physical and chemical properties. Heterogeneous
latex particles are usually obtained by sequential polymerization (two or more stages) using different monomers or monomer mixtures for each stage. Such structures are known as “core-shell” particles, meaning that the first stage polymer is located at the centre (core) and the second stage polymer remains in the outer layer (shell). It is also possible to obtain other types of structures depending on the polymerization conditions.
Controlling particle morphology of core-shell structures is influenced by
two main factors: thermodynamic principles affect the morphology that the
particle will have in equilibrium, and kinetic factors will determine if such
equilibrium morphology is achieved or not. Torza and Mason [18] studied the
thermodynamic morphology in a system of three immiscible liquids. Hobbs [19]
investigated a three-component polymer blend system. In both cases, under
thermodynamic control, the equilibrium morphology is determined by the
minimum interfacial free energy of the system Gs, which can be defined in
equation (5) as function of the interfacial areas Aij and interfacial tensions γij between phases i and j.
s = ∑γ AG ijij (2.5) 15
Sundberg et al. [20] analyzed systems of oil droplets encapsulated by several polymers. In equilibrium, the interfacial tension of each phase was the main factor affecting the type of particles obtained: core-shell, hemispheres or individual particles. Chen et al. [21] described the free energy change for five
different morphologies between water and two polymers. Figure 2.2 shows such
structures. The general expression for these systems, shown in equation (6),
includes values for interfacial tensions between both polymers γij and between
polymer 1 (seed) and water γ1w, as well as the interfacial areas:
1 ∑ −=Δ γγ wijij AAG 01 (2.6)
The expressions for each case are complex and involve volume ratios and
geometric parameters in each term. The results of such mathematical modeling
resulted in the prediction of equilibrium morphology if interfacial tensions between polymer phases and between each polymer and water are known, as well as the composition of each phase. A core-shell is favored when the interfacial tension between the seed and water is higher than the interfacial tension between the second stage polymer and water. An inverted core-shell is obtained for the contrary case. Other structures like hemispheres will develop depending on the interfacial tensions and the area between phases, which also changes with phase separation.
16
Figure 2.2 Morphological structures of polymeric particles in dispersion.
Kinetic factors affecting latex particle morphologies have been
mathematically modeled and studied in order to determine which polymerization
variables influence such morphologies. [22,23] Different types of configurations
were considered, including internal or external occluded morphologies, and
structures with surface domains, which are usually observed in latexes. Relative stabilities were determined by calculating the free energy change for all
17 morphologies as function of conversion during the polymerization process. The
studies show that only core-shell, inverted core-shell and hemispheres are
thermodynamically stable. Other morphologies such as raspberry and occluded
particles are non-equilibrium structures obtained by processes in which
polymerization conditions allowed for faster reaction kinetics and slower phase
separation. Among the numerous variables in emulsion polymerization
processes, the major contribution to non-equilibrium morphologies are attributed
to the fraction of second stage monomer, polymerization rate, Tg of the polymers
and level of intraparticle crosslinking. [22,24,25] Variations in initiator type and
chain transfer agents influenced particle morphology of heterogeneous latexes,
although such effects were only observed under specific conditions. [26]
2.3 Film Formation from Latex Dispersions
All the features that differentiate emulsion polymers from polymers
obtained by bulk, solution or suspension polymerizations are the same
characteristics that make film formation in latexes quite a complex process. For instance, similarly to polymers in the melt state “welding” between polymer fronts
(particles boundaries in the latex) is required for the interface to heal. Water
must evaporate from the emulsion in order to form a solid film. Subsequently, it
is worthy to understand the influence of latexes unique characteristics on film
formation and on the properties of the films obtained.
18
Figure 2.3 General characteristics of latexes during film formation.
19
2.3.1 Process of Film Formation
The process by which latexes turn into solid continuous films has been
typically realized as a series of events occurring during three main stages, as
shown in Figure 2.3a). During the first stage, water evaporates from the film
while particles come into close contact. The second stage is characterized by
deformation of particles into a closed packed structure. In the third stage,
individual entities are no longer defined due to sintering of particles into a solid, continuous film strengthened by interdiffusion of chain segments between neighboring particles. However, the actual film forming process is by no means as simple as described above but involves a number of complexities, as will be reviewed.
2.3.2 Drying of Latex Dispersions
Drying of latexes has been studied as function of the variation of water
evaporation rate from films. Typically, water evaporates at a constant rate at the
beginning of drying, and then it slows down gradually until water has totally
evaporated from the film. Vanderhoff [27] proposed a classic model for latex
drying, which occurs in three stages as follows: during stage I particles move
around with characteristic Brownian motion; water evaporation rate is high and
constant as shown in Figure 2.3b). In stage 2, particles deform, come in contact
and coalesce. Upon reduction in the total surface area, water evaporation rate
20 decreases. During stage 3, water loss from the film is very slow, occurring by
diffusion through capillaries or through the polymer layer. In spite of its simplicity,
this model does not account for some aspects of the process. For instance it
finds no differences between the water evaporation rate from dispersions and
from typical water surfaces. At the end of the drying process, the assumption
that water diffuses through a continuous polymer film may be applicable only under certain conditions, for example, when polymer and water molecules do not interact or when water molecules do not find membranes or pathways that facilitate movement through it.
Croll [28] proposed a more comprehensive model, as a result of
simultaneously studying water evaporation from latexes and from pigment
suspensions. From his findings, he proposed a different view of the drying
process. During the first stage, water evaporates from a flocculated surface at
the air interface. The evaporation boundary retreats through the thickness of the
film leaving behind a porous layer that grows thicker with time and through which
water vapor escapes as if it was a percolated network of voids. Underneath the
evaporation boundary, there is a transition layer of packed particles with some
concentration of liquid water followed by a layer of the original liquid latex
dispersion. Evaporation rate remains constant during this first stage,
independent of solid content of the film. This is explained by percolation
concepts, which imply that the percolation threshold should fall below a critical
value before water loss is limited by diffusion. The second stage of Croll’s model
21 is reached when the constant supply of water changes for some reason. For
instance, when the transition layer encounters the substrate, water can no longer
be supplied in the same steady fashion to the evaporating front. In this model,
film formation depends more on the coalescence of particles than on kinetics of
drying since the former is a much slower process. A different situation should
take place if coalescence occurred faster than water evaporation.
It can be now realized that the drying process should not be referred to as
a separate, independent stage during film forming. Particle deformation,
packing, and even coalescence may occur simultaneously with it depending on
the type of system. Additional studies have been performed to elucidate more
detailed relationships between drying and other phenomena. Joanicot and
Chevalier [29] studied packing and interparticle spacing related to drying of
latexes by small angle neutron scattering. They found that latexes did not show
a significant change in interparticle spacing when reaching the volume fraction
for closest packing of spheres. Such observation evidenced deformation of
particles during drying. The data indicated that the latex film dried from the
periphery inwards. The center part of the film remained wet and thick while there
were no changes in the intensity of scattering during most of the drying time.
Above a volume fraction of 0.75 a reduction in intensity indicated that the ordered
structure had been lost. The authors proposed a coalescence front moving
towards the center, evidenced by collapse of the intensity peak when the front
passed the neutron beam.
22
After the proposed propagating drying front from Joanicot and Chevalier,
[29] and based on drying experiments of soft and hard latex blends, another
model for drying latexes was developed by Winnik and Feng. [30] The qualitative
model describes a dry region formed at the edge of the film, followed by a
transition region or drying front that moves inward. The inner wet dispersion is
located at the center and is the last portion to dry. Quantitative calculations and
modeling for drying and deformation of particles in a system with a propagating
front were reported some time later. [31,32]
2.3.3 Particles Packing and Deformation
Packing and deformation of particles during film formation has been
studied in various types of latexes as function of film morphology prior to
coalescence. Packing depends on ambient humidity, which determines drying
rate. For instance, at very high humidity it has been shown that surfactant-free latexes tend to form face-centered-cubic packing structures with extensive long- range order. Less ordered structures were observed for latexes dried very rapidly, at humidity close to zero. Another important factor in particle packing is the presence of hydrophilic species at the surface. For instance, latexes
synthesized in the presence of surfactant and salt tend to show more random
packing and disordered regions. [33,34]
23 Different methods for deformation of latex particles have been proposed.
Two proposed methods are based on the deformation of viscous spheres as a
result of the balance between the work done by surface tension against viscous
dissipation inside the particle. One of the methods, by Vanderhoff et al. [35] is based on wet coalescence driven by polymer-water interfacial tension; the other by Dillon et al. [36] considers dry coalescence driven by polymer-air interfacial tension. Brown [37] proposed that elastic deformation of particles occurs by capillary pressure due to water-air surface tension. Sheetz’s method [38]
combines Brown’s deformation (only for the surface layer of particles) with
compressive forces caused by flow of evaporating water through this layer.
2.3.4 Coalescence of Particles and Interdiffusion of Polymer through Latex
Particles Boundaries
The last stage in the film formation process is defined by the transformation of closely packed discrete particles into a continuous solid film.
The term “coalescence” has been widely employed for the process of fusion between latex particles, similar to the process of fusion between two liquid droplets into a continuous phase. Upon coalescence of the particles, mechanical properties of the film are built up gradually due to diffusion of polymer segments across particles boundaries. Polymer interdiffusion from both sides of an interface is a very important process, not only for latex particles coalescence but also in polymer-polymer welding, crack healing or injection and compression
24 molded parts. For this reason, new techniques and theories have been
developed in order to describe it and understand it. A general view of the theory
and some experimental treatments are summarized.
Chain interdiffusion across an interface develops with time and/or
temperature and promotes randomization of the initially ordered symmetric
interface. Such changes in chains conformations produce an entropic driving
force for further diffusion of chains. Fick’s laws can explain diffusion of short
polymer chains; however, molecular weights are typically very high in situations
where mechanical strength is required and consequently, diffusion coefficients
deviate from such behavior. Diffusion of long polymeric chains has been
explained by reptation theory, [39] which considers that motion of chains occurs
in one dimension only as if it was moving through a tube. Chain ends should
move first in order to allow an entangled molecule to diffuse through the
boundary for a considerable distance. [40,41]
The diffusion process and the concentration profiles are then quite
different than those from systems described by Fickian diffusion. The
concentration profile for reptating chains has a discontinuity during the time prior
to the reptation time Tr for the polymer, which is defined as the time elapsed until the surface chains have diffused a distance equivalent to a radius of gyration Rg.
After such period of time, the diffusion is still reptative, but the centre-of-mass
diffusion of the polymer is Fickian. [41]
25
Experimental evidence of polymer interdiffusion has been characterized
by techniques that require labeling of one of the polymers to obtain contrast.
Deuterated samples were mixed with unlabelled ones, and the radius of gyration
was obtained by analysis of the scattering data during film formation.
Interdiffusion was interpreted by means of the increase of Rg with time of the deuterated sample in the mixture. Hahn et al. [42] analyzed samples of PBMA
from deuterated BMA and unlabelled PBMA, and found that averaged diffusion
coefficients Deff decreased when particles where larger; they also found that
coefficients were strongly dependent on temperature. Another technique that
has been used is direct non-radioactive energy transfer (DET) using fluorescent
dyes. [43,44] A donor dye (phenanthrene, Phe) was used to label one of the latexes and an acceptor dye (anthracene, An) was used for the other. When interdiffusion proceeded, DET occurred between dyes. Annealed samples were evaluated at different periods of time for fluorescent decay profiles of the phenantrene emission. Energy transfer quenches Phe emission, evidenced by faster decays and increased quantum efficiency of energy transfer, which was related to the extent of mixing of both labeled samples. Averaged diffusion
coefficients were obtained from comparison of the fluorescence decay curves,
and very similar values to those calculated by Hahn were found. [42]
The above techniques have been used to explore the effects of structural
features of the latexes on diffusion rate by means of diffusion coefficients. A very
26 important structural feature of polymer chains is their nature of motion by reptation due to high molar mass restrictions. However, reptation effects have not been observed during diffusion of polymer chains through particles interface in latex systems since all studies have employed Fickian models very successfully. One possible explanation for this is the fact that broad molar mass distributions broadened the concentration profile at the interface and this caused insensitivity of the measurement to both diffusion mechanisms. Another reason for this was insensitivity of SANS experiments to early-time diffusion, when reptation effects should be most pronounced. For this reason it is possible that
DET experiments on polymers of narrow molar mass distribution would be more sensitive than SANS to reptation effects in the early stages of diffusion.
Effects on polymer interdiffusion have been observed from hydrophilic components present in the outer layers of the latex particles. Outer layers may vary from very thin membranes made of excess surfactant or hydrophilic homopolymers, to very thick shells made of hydrophilic homopolymers or copolymers of hydrophilic monomers such as acrylic or methacrylic acid. The first case (thin membranes) was studied by Chevalier et al on latexes of poly(styrene-co-butyl acrylate) characterized by SANS. [29b] For both types of membranes (surfactant and homopolymers), deformation of particles began with flattening of the surfaces coming into contact. At the end, the hydrophilic material was confined to the faces of the polyhedra formed, resembling a foam structure.
27
Latexes with thin surfactant membranes on the surfaces underwent
coalescence (interdiffusion of polymer chains from the core) due to
destabilization of the membranes and break up of the “foam” structure. As
evidence of coalescence, they observed an irreversible collapse of the scattering
intensity peak and failure to rehydration of dry films with water vapor to
regenerate any hydrophilic structure. On the other hand, latexes covered with a
very thin poly(acrylic acid) layer did not coalesce. Rehydration was possible
even after complete drying, and scattering peaks showed very similar patterns to
those obtained during drying. They concluded that the structure of membranes
was preserved, which allowed for water diffusion back into the film.
Poly(butyl acrylate) latexes containing thicker shells with small fractions of
acid groups (methacrylic acid) were used by Winnik et al to study interdiffusion
during film formation. [45,46] They established that interdiffusion occurred during
annealing of the samples at 100 °C, however it was retarded when compared to samples without carboxylic groups. They also saw that retardation was more pronounced when content of carboxylic groups was increased. The difference in
results between Chevalier and Winnik was explained as a consequence of the different types of particles studied in each case. Chevalier used particles with
very thin membranes made of poly(acrylic acid), which have a significantly higher
Tg than the core polymer. At temperatures below the Tg of the membrane,
interdiffusion was restricted. Upon annealing of the films above Tg of the
28 membrane (above 100 °C), the “foam” structure breaks up to allow polymer from the core (lower Tg) to diffuse. Particles used by Winnik et al have a very thick layer of a copolymer containing MAA, which has higher Tg than the core, but lower than annealing temperatures employed in the experiments. For this reason they observed interdiffusion at early stages of annealing. In the end, results from both groups were consistent with the view of interdiffusion of polymer molecules
(coalescence of particles) occurring upon annealing at temperatures above the
Tg of the boundary polymer.
The effect of neutralization of acid groups on polymer interdiffusion has been studied by different methods, including DET experiments and force modulation atomic force microscopy. From DET experiments, Winnik et al found
[46] that neutralization of acid groups slowed down polymer interdiffusion.
Ammonia-neutralized latexes showed a slight retardation on interdiffusion compared to un-neutralized samples. Sodium hydroxide and barium hydroxide had very pronounced effects, since both slowed down interdiffusion. Barium hydroxide had a more severe influence. The authors proposed that upon neutralization, an ionomer membrane is formed within the film, which is immiscible with the rest of the polymer and probably has a higher Tg than in the case of un-neutralized acid groups. This ionomer phase must interfere in the process of interdiffusion due to its physical characteristics. The fact that neutralization with ammonia showed lower levels of retardation that in the case of the other bases was attributed to the ability of ammonia to easily evaporate. [46]
29
Besides topographic imaging of latexes, atomic force microscopy (AFM)
has been used to study local friction in surfaces by means of lateral force
microscopy (LFM) and variations in surface elasticity with force modulation and
phase imaging. Such capabilities have been recently used to qualitatively
monitor film formation and polymer interdiffusion in latexes by observation of surface roughness and measurement of local elasticity. Hellgren et al [47] used
AFM techniques to investigate the influence of neutralizing agents in the film
formation of carboxylated styrene-butyl acrylate latexes. Force modulation and
topographic images were correlated by comparison of surfactant migration
between films neutralized either with ammonia or NaOH. It was proposed that a
cell wall was formed during film formation of latexes neutralized with NaOH,
acting as a barrier against interdiffusion.
2.4 Thermosetting Latexes
The field of applications for commonly known latexes has grown
considerably in recent years due to the environmental-friendly features of this
type of dispersions. However, cohesive strength and solvent resistance of films
obtained from this type of latexes are relatively poor for certain industrial
applications. In order to achieve higher standards of performance, thermosetting
latexes have been developed with improved mechanical properties and solvent
resistance. Many types of thermosetting latexes have been reported. Bufkin and
30 Grawe’s review [48a] covered an extensive amount of information on a number
of technologies developed until the mid –1970’s. Daniels and Klein [48b] and
Taylor and Winnik [48c] covered more recent developments.
One method for obtaining thermosetting latexes has been based on the
copolymerization of special monomers (self-condensable or autoxidative) along
with traditional monomers. For instance, polymerization of styrene, butyl acrylate
and m-isopropenyl benzyl isocyanate resulted in crosslinkable latexes even when
methacrylic acid was not added to the formulation. [49] Another example was the incorporation of acetoacetate ethyl methacrylate to the formulation of typical acrylic latexes. Crosslinking was obtained by addition of polyamines; however pot life of the mixed systems was very short due to rapid reaction of the acetate groups with the amine. [50] Reactive allyl moieties were used to obtain latexes
that crosslinked upon drying at ambient temperature in the presence of air by an
autoxidation process. [51] Nabburs et al. synthesized acrylic latexes by pre-
emulsifying an alkyd with monomers in water with the aid of surfactants.
Hardness was superior for the hybrid systems than for the alkyd emulsion and
the acrylic dispersion alone. [52]
2.4.1 Crosslinkers
Another method for preparation of crosslinkable latexes is the
incorporation of functional monomers that are reactive towards an external
crosslinker. For instance, acrylic and methacrylic monomers bearing carboxylic
31 acids, hydroxyl or amine groups are usually copolymerized into acrylic latexes.
Crosslinkers may be added either during or after latex synthesis, although
latexes can also be prepared around these components. The following are the most important materials used as crosslinking agents for carboxylic and or hydroxyl groups:
Etherified melamine-formaldehyde (MF) resins have been introduced to
emulsion dispersions of polymers bearing hydroxyl moieties. Figure 2.4 shows
the corresponding crosslinking reaction. Two procedures were evaluated to
obtain MF crosslinkable latexes: the resin was added after latex synthesis or
latexes were synthesized in the presence of the resin. [53,54] Either system was
crosslinked at different temperatures and to several extents, with or without
presence of strong acid catalysis. The usage of these types of resins has been
very important commercially; however some fractions of volatile organic
compounds evolving during film formation and crosslinking has been considered
an undesirable characteristic.
Carbodiimides have been studied as crosslinking agents for carboxylic functional polymers. [55] Multifunctional carbodiimides were prepared, emulsified and mixed with styrene-acrylic waterborne polymers containing acrylic acid.
Films cast from such mixtures were successfully crosslinked as evidenced by swelling experiments in tetrahydrofuran and by double rubs with methyl ethyl
32 ketone. Hydrolysis of the carbodiimide seemed to be a disadvantage when
introduced in waterborne systems.
Aziridines were used as crosslinking agents for latexes containing
carboxylic groups to form aminoester crosslinks due to the high reactivity of the
aziridine ring towards such acids. [56] Preparation of tri-functional aziridines from
ethyleneimide or propyleneimide were achieved by reaction with a multi-
functional acrylate such as trimethylolpropane triacrylate (TMPTA) or
pentaerythritol triacrylate (PETA). Such multifunctional aziridines were added
neat or as solutions to the emulsion dispersion. Stability of the reactive system
after mixing (pot life) was between 48 to 72 hours due to slow hydrolysis in the
aqueous media. Films from these systems showed less elongation and higher
modulus with respect to films without crosslinker. Solvent swelling tests
demonstrated higher crosslink densities of films crosslinked with aziridines as
compared with other crosslinking systems like MF resins and carbodiimides.
However, aziridines have been considered to be skin irritants and sensitizers,
and there has been some controversy about mutagenicity.
Reactive latexes have also been prepared with epoxy silanes added as post-additives. Latexes with a carboxylic or amine functionality were mixed with emulsified epoxy-silanes such as β-(3,4-epoxycyclohexyl)ethyltriethoxysilane and
γ-Glycidoxypropylmethyl diethoxysilane among others. Pencil hardness values
33 and MEK double rubs for these films were higher in comparison to control emulsions without crosslinker. [57]
Figure 2.4 Structure and crosslinking reaction between a melamine formaldehyde resin and a hydroxyl functional polymer
Cycloaliphatic epoxides were recently studied for crosslinking of latexes
containing hydroxyl or carboxyl functionalities. [58,59] Cycloaliphatic diepoxide
34 (external crosslinker) was added as an emulsion after latex polymerization.
Figures 2.5 and 2.6 show the corresponding crosslinking reactions. Results
revealed that crosslinking effectively took place between epoxide and functional
groups as observed from film properties. Water absorption decreased and gel content, pencil hardness and pull-off adhesion increased in general upon addition of cycloaliphatic epoxide. The amount of crosslinker was studied in both latexes.
Hydroxyl functional latexes showed a more pronounced enhancement of the film properties than carboxylic ones when crosslinker content was increased. [59]
The type of catalyst was evaluated for effects on crosslinking and final properties of the coatings. [60] Sulfonic and phosphonic acids were used as catalysts. Copolymerizable acids were 2-sulfoethyl methacrylate, SEM and phosphoxyethyl methacrylate, PEM and free acids for comparison were p-
toluene sulfonic acid, pTSA and phenylphosphonic acid, PPA. Hydroxyl
functionalized latexes revealed enhanced film properties when catalysts were
used and even better when latexes were neutralized. On the contrary, latexes
bearing carboxylic groups without catalysts showed better properties than latexes
containing catalyst. Neutralized carboxyl functional latexes without catalyst
showed the best crosslinking overall. According to this study as pH was low, a
larger fraction of oxirane rings in the diepoxide were hydrolyzed than under basic
conditions. High pH conditions improved stability of the epoxide against
hydrolysis, allowing for larger extents of crosslinking. [60]
35 Soon after, core-shell latex particles containing hydroxyl-functional cores and carboxylic-functional shells were successfully crosslinked with the cycloaliphatic diepoxide. [61,62] The effect of addition mode of epoxide was
investigated in terms of morphology and properties of the films. Epoxide was
introduced during or after latex synthesis, either as emulsions or solutions.
Colloidal stability of the system was highly affected when all the epoxide was
added during synthesis, for which half the amount was added during
polymerization and the other half was added afterwards (either as emulsion or
solution). When cycloaliphatic diepoxide was added as a solution, an increase in
hardness and tensile modulus was observed, possibly due to a more uniform
adsorption of the epoxide on the particles surface. Addition of emulsified
diepoxide led to films with lower hardness and tensile modulus. It was postulated
that addition of the epoxide as an emulsion restricted the interaction between reactive groups in the latex and the epoxide emulsion droplets. [63]
In a separate study, the effect of location of hydroxyl functionality was
investigated. Phase inversion of the core-shell structure did not occur when
hydroxyl functionality was introduced during the first stage of polymerization,
indicating concentration of the hydroxyl group within the core. When the hydroxyl
functionality was located in the core, it promoted hardness and tensile modulus,
and when it was placed in the shell, a higher crosslink density was observed.
Distribution of the functionality between the two stages did not generally improve
film properties. [64]
36
CH3 2 MMA BA H2C C MMA C O
OCH2CH2OH + O
O O O
H+ (Acid Catalysis)
H3C
MMA BA H2C C MMA C O
HO OCH2CH2O
O O
HO OCH2CH2O O C BA MMA C C BA MMA H2 CH3
Figure 2.5 Crosslinking reaction of hydroxy functional acrylic latex with cycloaliphatic epoxide.
2.5 The role of functionality in thermoplastic and thermosetting latexes
In latex polymers, final application requirements are satisfied in great proportion by properties of the bulk polymer. In the widely used “family” of monomers comprising acrylic and methacrylic esters, two major groups are commonly known. The first group corresponds to monomers such as n-Butyl
Methacrylate (nBMA), Ethyl Acrylate (EA) or 2-Ethylhexyl Acrylate (2-EHA) that impart flexibility, internal plasticization and adhesion. The second group includes monomers such as Methyl Methacrylate or Ethyl Methacrylate which provide 37 characteristics like hardness and strength. On the other hand, requirements related to surface or colloidal properties are most often governed by functional monomers. These monomers may be copolymerized in small amounts (1 – 3 wt
%) for purposes of surface modification, crosslinking ability and/or colloidal stability. Functional monomers are classified according to reactive groups present in the molecule, as follows: carboxylic acids, hydroxyl groups, chloride moieties, amines, isocyanates, derivatives of acrylamide, epoxy and sulfonate groups. [1]
CH3 2 MMA BA H2C C MMA C O
OH + O
O O O
H+ (Acid Catalysis)
H3C
MMA BA H2C C MMA C O HO O
O O
HO O CO BA MMA C C BA MMA H2 CH3
Figure 2.6 Crosslinking reaction of carboxyl functional acrylic latex with cycloaliphatic epoxide.
In many commercial formulations of thermoplastic and thermosetting latexes, the most common functional groups introduced are carboxylic acids and
38 hydroxyl groups. Carboxylic acid groups are usually incorporated in the latex via
copolymerization of Acrylic or Methacrylic Acid, or less commonly, Itaconic
and/or Fumaric Acids. Carboxyl groups usually improve mechanical and shear
stability of latexes, film hardness and adhesion to substrates. Crosslinking is
possible ionically or covalently, upon addition of external crosslinkers such as
carbodiimides, aziridines or some epoxides, as discussed previously. [55-59]
Additionally, carboxylic acids are often added for rheology modification or to
improve colloidal stability of the particles in the dispersion. On the other hand,
hydroxyl groups may be incorporated through copolymerization of 2-Hydroxyethyl
Methacrylate (HEMA) or Hydroxypropyl Methacrylate (HPMA). These monomers
have been successfully crosslinked with melamine-formaldehyde resins as
reviewed earlier. [53,54] Carboxyl and hydroxyl functional monomers are
typically water soluble and as a result these monomers tend to partition between
the aqueous and the dispersed phases during polymerization. [65] The presence of higher amounts of monomers in the aqueous phase may affect the path of polymerization and consequently it may influence the final latex physical properties. [66, 67]
With regards to thermosetting latexes, the phenomena involved in film
formation and crosslinking development in latex films can be affected by the
presence of functional monomers during synthesis and application of these
coating systems. On one side, functional monomers such as MAA and HEMA
are expected to affect synthesis pathways as well as latex stability and
39 properties. [65-67] Additionally, the degree of crosslinking and optimum
mechanical properties development of the films are expected to be highly
dependent on functionality content. Furthermore, in acid catalyzed crosslinkable
dispersions using resins such as melamine-formaldehyde or cycloaliphatic
epoxides, the processes of film formation and crosslinking are affected by the
presence of functional groups such as weak and strong acids (commonly the
reacting functionality and catalysts) and also the blocking agents (typically weak
bases). These particular features that had not been yet addressed by studies
performed on thermoplastic latexes are the subjects of research in the present study and will be developed in the following chapters.
40 CHAPTER III
ACRYLIC LATEXES WITH INCREASING CONCENTRATION OF FUNCTIONAL
MONOMERS
3.1 Introduction
Environmental regulations have been continuously restricting the amount of volatile organic compounds (VOCs) in coatings formulations, and as a consequence waterborne systems have developed, and will continue to do so, as the logical choice for many applications in the coatings industry. [68] Latex based and water-reducible coatings are two major classes of waterborne coatings replacing solvent-borne applications. Latex dispersions form films at ambient temperatures by coalescence of relatively soft particles containing solid polymer. Cohesive strength development occurs during coalescence when interdiffusion of polymer chains takes place, but also by crosslinking of the polymer prior to or during film formation. [69] Generally, crosslinking is introduced
into thermoplastic latex dispersions for improvement of mechanical properties and solvent resistance. Several types of thermosetting latexes have been reported in the literature in which functional monomers have been added during
synthesis and then chemically crosslinked. [48]
41 Previous studies have shown that dispersions of latex particles containing
hydroxyl-functional core and carboxylic-functional shell were successfully
crosslinked with a cycloaliphatic diepoxide. [59-62] A schematic representation is
shown in Figure 3.1. The crosslinking reactions for the system were influenced
by several factors including the amount and addition mode of diepoxide, location
of functional monomer, type and amount of catalyst, among others. Soucek et al.
[63] investigated epoxide addition in terms of morphology and film properties obtained from the latex system. When cycloaliphatic diepoxide was added in the
form of solution, increase in hardness and tensile modulus was observed
possibly due to more uniform adsorption of the diepoxide on the particles
surface, whereas addition of emulsified diepoxide showed lower hardness and tensile modulus. This was postulated to be a result of decreased interaction of emulsified epoxide with the latex particles. [63]
In a separate study, the effect of location of hydroxyl functionality was
investigated. [64] Phase inversion of the core-shell structure did not occur when
hydroxyl functionality was introduced during the first stage of polymerization,
indicating concentration of hydroxyl groups within the core. When hydroxyl functionality was located in the core, it promoted hardness and tensile modulus, and when it was present in the shell of the particles, higher crosslink density was observed. Distribution of functionality between both stages did not generally improve film properties. [64] The effect of type and concentration of catalyst was
studied previously for latexes with both hydroxyl and carboxyl functional groups
42 as sole functionality (monofunctional systems). Strong acid catalysis was
required for crosslinking of hydroxyl functional latexes with cycloaliphatic
diepoxide, whereas it did not necessarily improve the coating properties of
latexes containing carboxyl groups. [60] It was postulated that the conjugate
base of the strong acid catalyst did not undergo re-protonation under the curing
conditions.
Purposeful design of these thermosetting latexes relies on the evaluation
of composition variables such as addition mode of the crosslinker, type of
catalyst and functionality concentration. It is desirable to have 10 – 20 wt% of
total monomer available as functional groups to participate in crosslinking
reactions; however other properties could also be affected by increase in
functionality content. Polymerization pathways, particles structure and ultimately,
film properties might be influenced by such change in composition. In many of
previously studied formulations in the literature, functionality varies between 2
and 8 wt. % based on total monomer weight. Although such films show improved
solvent resistance and enhanced mechanical properties, there is no specific information on the extent of crosslinking. [48-58]
In the present study, optimization of the total amount of functional
monomers was investigated, as well as consequent effects on latex and film
properties. Two series of latexes were prepared with different glass transition
temperatures (Tg = 10 °C and Tg = -5 °C) and with increasing content of carboxyl
43 and hydroxyl functional groups. The effects of the reacting system on colloidal stability and on particle size distribution were evaluated with varying functionality.
Tensile, thermo-mechanical properties and film morphology were also evaluated.
O CH O 3 OOO HO CC OH HO C O C C C H3C OHCH2CH2 O O H3C O C C HO CH3 OOO O H C CC 3 C C OCH2CH2OH O HO H3C C C O CH O H3C 3 HO CC OCH2CH2OH C HO O C C C H3C O O H3C CH 3 O OOO CC OH
CH3 CH H 2 H 2 3 MMA BA C C BA MMA C C MMA HO C O O C + O OH O O O O
CH CH 3 H 2 3 MMA BA C C MMA BA C C MMA H 2 C C O O O O O OH HO
O O O O
HO HO O O O O O C C MMA BA C C MMA BA C C MMA H 2 CH H 2 CH3 3
Figure 3.1 Crosslinking reactions of carboxyl and hydroxyl groups with cycloaliphatic diepoxide in a latex system.
44 3.2 Experimental
3.2.1 Materials
Butyl Acrylate (BA), Methyl Methacrylate (MMA), Hydroxyethyl
Methacrylate (HEMA), Methacrylic Acid (MAA), Benzyl Methacrylate (BzMA),
Ammonium Persulfate (APS), Sodium Bicarbonate and isopropanol were
technical grade chemicals purchased from Aldrich Chemical and used as received. Dow Chemical supplied the cycloaliphatic diepoxide (UVR-6105) and
surfactants Triton-200 (sodium alkylaryl polyether sulfonate) and Tergitol XJ
(polyalkylene glycol monobutyl ether). Sulfoethyl methacrylate (SEM) was supplied by Hampshire / Dow Chemical. Deionized water was used in the preparation of the latexes.
3.2.2 Preparation of Latexes
Latex dispersions were prepared by seeded semi-continuous emulsion
polymerization with a monomer emulsion feed. All seeds used in the latexes
synthesis were obtained from single seed latex prepared batch-wise.
Seed: A solution of NaHCO3 (1.5 g) and Triton-200 (0.5 g) in water (150
g) was charged to a 500 mL flask equipped with a condenser, stirrer and a
nitrogen atmosphere. After reaching 75 °C, a pre-emulsion of BA (41.68 g, 0.326
mol) and MMA (38.32 g, 0.3832 mol) with NaHCO3 (0.1 g) and Triton-200 (3.6 g)
was charged in the flask followed by addition of a 2 wt. % aqueous solution of
ammonium persulfate (102.2 g). Polymerization continued for 30 min at 75 °C.
45 The same experimental apparatus for seed preparation was used for the core
and shell stages.
Core: The seed (20 wt% of the weight of the pre-emulsion) was charged in the reactor and heated to 75 °C. A pre-emulsion of BA, MMA and HEMA along with an initiator solution, were fed continuously for 180 min. Tables 3.1 and 3.2 list the components and amounts for each latex formulation. Contents were heated and stirred for additional 180 min to ensure monomer consumption. For
STEM, BzMA was used instead of MMA in those latexes prepared for the differential staining.
Shell: Following the core preparation, continuous addition of a new pre- emulsion (BA, MMA and MAA) along with fresh initiator solution proceeded over a period of 240 min. During the same time a SEM aqueous solution (5 wt%) was fed as well (this monomer was added in order to incorporate sulfonic acid groups for catalysis of crosslinking reactions after film casting). The emulsion was allowed to react for another 240 min after addition was complete. The latexes were then filtrated through a woven polymer micro-filtration mesh (20 μm mesh
opening) followed by coagulum and solid contents measurements.
Nomenclature is as follows: latex with formulated Tg of -5 °C and 7.4 wt%
functional monomers (4.4 wt% HEMA and 3 wt% MAA) based on total content of
monomer is designated as T(-5) F7 (see Tables 3.1 and 3.2 for other
compositions). It should be noted that actual latexes glass transition
temperatures are expected to differ from the values formulated, which were used
for nomenclature purposes.
46
3.2.3 Latex Characterization
Latexes were cleaned using an ion-exchange resin (AG-501-X8) to
remove water-soluble ionic materials. Twice the weight of resin (based on latex
solids) was added to dilute latexes (10 % solids); the mixture was stirred at
ambient temperature for 2 h, and then vacuum filtered. The process was
repeated twice. Particle morphology was examined by STEM using a FEI Tecnai
12 Microscope. Particle sizes and particle size distributions were measured by
dynamic light scattering using a PSS NICOMP (Santa Barbara, CA) equipped with a He-Ne laser operating at 652 nm and a triple detector. Measurements
were carried out at 23 °C and at a fixed angle of 90 °C on very diluted emulsions
(< 0.1 vol%). Molecular weight was determined by gel permeation chromatography (GPC) using high-resolution Waters columns with THF at 1
mL/min. A triple detector was used comprising a Viscotek viscometer, a Waters
differential refractometer and a Wyatt Dawn EOS light scattering detector at 90°.
Particle morphology was evaluated for latexes containing benzyl methacrylate.
This monomer was introduced during the first stage of the polymerization to
differentiate between the core and shell via RuO4 staining. For STEM sample preparation, a drop of clean and very dilute latex (0.5 wt. %) was placed onto a carbon coated copper grid and set to dry for 2 h at ambient temperature.
Samples on grids were exposed to RuO4 vapors for 15 min and dried under
ambient conditions for 24 h prior to imaging.
47
3.2.4 Film Formation and Characterization
Cycloaliphatic diepoxide was dissolved in isopropanol (50 wt%). Latexes
were neutralized with ammonia at ambient temperature to pH = 7. Stoichiometric
amounts of the diepoxide solution were added, and mixtures were stirred for 15
min (see Table 3.1). Then, latexes were cast on glass and aluminum substrates
using a draw down bar #8 (8 mils for wet thickness). The final thickness of the
samples was between 40 and 60 μm. Dry films were obtained either after 48 h of
drying at ambient temperature or after drying 2 h at ambient temperature and heating at 170 °C for 1 h. Films cast on glass were removed with a razor blade and cut (80 x 16 mm or 25 x 5 mm) for tensile testing on an Instron Universal
Electromechanical Tester 5567 and dynamic mechanical thermal analysis using a Rheometrics Scientific analyzer. Tensile testing was performed with a load cell of 100 N and a crosshead speed of 20 mm/min. More than 10 specimens were tested and those with closest values were selected to obtain average values
(selection criteria according to ASTM standard D2370-98). Dynamic mechanical thermal analyses were carried-out with a constant temperature ramp (3 °C/min), a fixed oscillating frequency (1 Hz) and a controlled strain (0.5 %). Films cast on aluminum were used for coatings testing such as pencil hardness (D 3363-00), impact resistance (ASTM G 14), flexibility (conical mandrel testing ASTM D 522) and MEK double rubs (ASTM D 4752-98). Latexes were spin-coated onto pre-
48 treated silica wafers and examined for dry film morphology in a Multimode
Scanning Probe Microscope (Digital Instruments) set to tapping mode.
Table 3.1 Core-shell compositions for latexes designed with Tg = 10 °C. Increasing percentages of functional monomers based on total monomer content.
T(10) F7 a T(10) F15 a T(10) F17 a (7.5 wt%) b (15 wt%) b (17.5 wt%) b Core Shell Core Shell Core Shell
Component Weight (g) Weight (g) Weight (g) Weight (g) Weight (g) Weight (g)
BA 35.75 37.89 35.32 39.66 35.18 40.23
MMA 37.10 37.38 30.28 30.74 27.98 28.62
HEMA 7.14 14.4 16.9
(0.055 eq.) - (0.11 eq) - (0.13 eq) -
MAA 4.73 9.6 11.2
- (0.055 eq.) - (0.11 eq) - (0.13eq)
SEMe - 0.57 - 0.57 - 0.57
NaHCO3 0.1 0.1 0.1 0.1 0.1 0.1
Triton-200 3.6 3.6 3.6 3.6 3.6 3.6
Tergitol-XJ 0.4 0.4 0.4 0.4 0.4 0.4
Water 80.0 80.0 80.0 80.0 80.0 80.0
Diepoxide c 13.86 27.72 32.76 iso-Propanol 13.86 27.72 32.76
Coagulum in grams d 0.05 1.09 1.91
Solid content
(%) 38.5 37.3 36.7
a T stands for formulated Tg in degrees centigrade (°C); F stands for functionality; the last number shows total functionality percentage based on total monomer content b Total functional monomers weight percentage based on total monomer content c Crosslinker was added after latex synthesis d Coagulum formed during reaction e SEM stands for Sulfoethyl methacrylate
49
Table 3.2 Core-shell compositions for latexes designed with Tg = -5 °C. Increasing percentages of functional monomers based on total monomer content.
T(-5) F7 a T(-5) F15 a T(-5) F21 a (7.5 wt%) b (15 wt%) (21 wt%) Component Core Shell Core Shell Core Shell
Weight (g) Weight (g) Weight (g) Weight (g) Weight (g) Weight (g)
BA 44.14 46.29 43.71 48.06 40.48 46.53
MMA 28.71 28.98 21.89 22.34 19.52 20.09
HEMA 7.14 14.4 20.0
(0.055 eq.) - (0.11 eq) - (0.154 eq) -
MAA 4.73 9.6 11.2
- (0.055 eq.) - (0.11 eq) - (0.154 eq)
SEM - 0.57 - 0.57 - 0.57
NaHCO3 0.1 0.1 0.1 0.1 0.1 0.1
Triton-200 3.6 3.6 3.6 3.6 3.6 3.6
Tergitol-XJ 0.4 0.4 0.4 0.4 0.4 0.4
Water 80.0 80.0 80.0 80.0 80.0 80.0
Diepoxide c 13.86 27.72 38.81
iso-Propanol 13.86 27.72 38.81
Coagulum in
grams d 0.21 1.63 3.72
Solid content
(%) 39.3 37.4 36.8
a T stands for formulated Tg in degrees centigrade (°C); F stands for functionality; the last number shows total functionality percentage based on total monomer content b Total functional monomers weight percentage based on total monomer content c Crosslinker was added after latex synthesis d Coagulum formed during reaction e SEM stands for Sulfoethyl methacrylate
50 3.3 Results
The primary objective of this study was to investigate the effect of
functional monomers concentration and concomitant equimolar addition of
crosslinker on stability and film properties of crosslinkable latexes. Reactions of
attached carboxyl and hydroxyl functionalities with cycloaliphatic diepoxides are
influenced by the concentration of reacting groups, by mobility of the polymer
segments and by acid catalysis during crosslinking. Increasing functional
monomer concentration is not only expected to affect crosslinking reactions, but
it can also introduce changes during synthesis and to latex properties. [48]
Moreover, it is anticipated that increasing hydroxyl functionality in the core and
carboxyl functionality in the shell would bring dissimilar chemical and physical
properties. The relationship between monomer composition (including functional
monomers) and glass transition temperature was taken into account in designing the latexes. Consequently, two series of latexes were prepared with different Tg and increasing equimolar content of HEMA and MAA varying from 7.4 to 21 wt%.
Glass transition temperatures of -5 °C and 10 °C were calculated by the Fox equation. [70]
The double functionality approach was preferred due to superior film
properties obtained through crosslinking, compared to mono-functional systems.
[61] The choice of placement of functional groups was based on two reasons.
First, previous studies in this group showed that HEMA-containing copolymer
located in the core of particles did not cause core-shell inversion of the latex
51 particles under the polymerization conditions used. [64] Second, when carboxylic
acid groups were added during the core stage and hydroxyl groups during the
shell stage, latexes were unstable. [62] As a result, the preferred morphology in
terms of latex stability and films performance consisted of hydroxyl groups
located in the core and carboxylic acid groups placed in the shell.
The addition of diepoxide to the latex was performed in the form of a
solution, due to the very low solubility of the epoxide in water. As investigated
previously, [63] addition of the crosslinker as a solution resulted in overall best
films properties. This result was attributed to increased adsorption on particles
surface. AFM images supported such consideration. Additionally, in a previous
study, [64] successful crosslinking was obtained for latexes containing only hydroxyl functionality incorporated during the core stage. The extent of
crosslinking was lower than in the case of hydroxyl groups located in the shell,
but crosslinking was achieved. Additionally, films properties were enhanced
compared to uncrosslinked systems. Such results suggested that a small portion
of epoxide migrates within the polymer during film formation. It is then expected
that a portion of diepoxide, located in the proximity of particles in the emulsion, are driven towards the surface of particles by its low water solubility. It is also expected that a small portion of epoxide might continue to move inside the particle. Consequently, addition of epoxide as a solution in isopropanol was considered highly beneficial in terms of film property development.
52 Total maxima of 17.5 wt% and 21 wt% functional monomers were successfully incorporated in the formulation of series T(10) and T(-5) respectively. The upper functionality limit in these latexes was determined by the maximum HEMA concentration introduced during the first stage of polymerization without causing instability of the emulsion. For series T(10), adding more than
10 wt% HEMA in the co-monomer feed led to disruption of colloidal stability during the first stage of polymerization. However, for series T(-5), it was possible to add up to 12.5 wt% HEMA without emulsion break-up. Coagulum measured after polymerization increased upon addition of larger functionality amounts in the co-monomer feed, which indicated some degree of instability in the system during reaction.
3.3.1 Latex Properties
During the synthesis process or afterward it is possible that the desired core-shell structure of the particles might have not been obtained, since particle morphology is highly affected not only by thermodynamic parameters but also by kinetic factors. [20-26,71] In this particular case, it was anticipated that increments in functional monomer concentration might have interfered with the equilibrium morphology of the system. For this reason, the morphology of the latexes was evaluated by STEM. During synthesis of some of the latexes, a fraction of non-functional monomer mixture was replaced by benzyl methacrylate in the core polymerization stage. Benzyl methacrylate (BzMA) was introduced to
53 achieve facile staining with RuO4 and was preferred over styrene because it is less chemically dissimilar to the acrylic system. An image of latex particles with
Tg= -5 °C and 15 wt. % functional monomers is shown in Figure 3.2. The first stage polymer seems to be located at the centre of the particle as evidenced by dark domains. This suggests that the core-shell structure was not disrupted by increased fractions of HEMA and MAA during polymerization.
Figure 3.2 STEM image of core-shell latex with 9 wt% HEMA and 6 wt% MAA
Together with changes in particle structure, other latex properties might have been altered upon increasing functionality content. As general latex characterization, molecular weight and particle size were evaluated. For molecular weight measurements, dry films were immersed in THF for several
54 days; however complete dissolution was not achieved. A few minutes under
sonication dissolved the remaining gel. Same sonication time was allowed for all
samples; however the sample with 0 wt. % functional monomers retained a small
amount of gel that plugged the filter prior to injection. For such reason these
values are considered as relative in this case. Molecular weight and particle size
of latexes from series T(-5) are presented in Table 3.3. Weight-average
molecular weight values showed a decreasing trend as HEMA and MAA concentrations increased. Latexes with up to 15 wt. % functionality showed similar particle size (around 350 nm). In the case of maximum functionality
content (21 wt. %) two populations of particles with different sizes were observed.
Table 3.3 Molecular weight and particle size for latex series T( -5).
Functionality Molecular Weight Particle Size
Name (wt % total (g/mol) (nm)
monomer) Mw Peak 1 / Std. Dev. Peak 2 / Std. Dev.
T(-5) F0 0 1328,000 364.0 / 57.9 -
T(-5) F7 7.4 836,000 348.0 / 55.0 -
T(-5) F15 15.0 562,000 345.6 / 12.1 -
T(-5) F21 21.0 440,000 215.8 (51.7 %) a 704.0 (48.3 %)
a Standard deviation not available. Proportion of particles for each size reported as percentage.
55 3.3.2 Film Properties
Films were evaluated for typical coatings tests, tensile properties and
dynamic mechanical properties. Table 3.4 presents coatings properties for films
obtained from mixtures of latexes series T(10) and cycloaliphatic epoxide solution (films were cast and dried under ambient conditions for 2 h followed by heating at 170 °C for 1 h). As seen from the table, typical coating tests such as
pencil hardness, impact resistance, conical mandrel flexibility and MEK double
rubs exhibited only small variations for these films. For instance, all films showed conical mandrel flexibilities superior to 32 % and MEK double rubs above 200.
For this reason, tensile testing and dynamic mechanical analysis were preferred
for evaluation and contrast of film properties.
Table 3.4 Coating testing for crosslinked latex films from series T(10). Total functionality varied from 7.4 to 17.5 wt%.
Test T(10) F7 T(10) F15 T(10) F17
Pencil Hardness 2H 4H 3H
Conical Mandrel (% elongation) > 32 > 32 > 32
Impact Resistance (Direct /
Inverse) > 40 / > 40 30 / > 40 > 40 / > 40
MEK double rubs >200 >200 >200
Tensile testing is a very useful tool for evaluation of mechanical properties in free films. Tensile properties of latexes from series T(-5) (Tg = -5°C) are
56 summarized in Figures 3.3 and 3.4. Figure 3.3 clearly shows that tensile
strength and tensile modulus values uniformly increased upon introduction of increasing levels of HEMA and MAA for films cast from latexes without
crosslinker. Conversely, elongation-at-break was severely reduced as
functionality content was increasing in the latex formulation. When crosslinker
was introduced, tensile strength and modulus were improved significantly as
expected, with respect to films without crosslinker especially at very high
functionality content. Elongation-at-break reduction was not as severe for crosslinked films as compared to films without crosslinker, for which elongation decreased from approximately 500 to 100 % or lower.
Figure 3.3 Tensile properties of films from latex series T(-5).
57
Figure 3.4 Tensile properties of crosslinked films from latex series T(-5)
Dynamic mechanical thermal analysis was performed on films cast from latex series T(-5) (Tg = -5°C). The storage modulus E´ at the rubbery equilibrium region above Tg is related to the crosslink density by the classical expression
[72,73]
E´ ν = (3.1) e 3RT where νe is the crosslink density expressed as moles of elastically effective network chains per cubic meter of sample. E’ is the storage modulus at the equilibrium plateau, R is the gas constant (8.3145 Pa*m3/mol*K) and T is the
58 temperature in Kelvin. Figures 3.5 and 3.6 show the temperature dependence of
storage modulus E’ and loss tangent (tan δ) for films without crosslinker dried at
ambient temperature. It was observed that as concentration of functional
monomers increases in the films, there was progressive broadening in the transition region detected on both, storage modulus E’ and tan δ peak signals. In the same way, the maximum value for tan δ decreased and the temperature at
that point shifted toward higher temperatures as functionality content was
increased. Figures 3.7 and 3.8 show storage modulus E’ and tan δ responses for
the same type of films (without crosslinker) dried at ambient temperature for 2 h
and heated at 170 °C for 1 h. Heating did not seem to cause much effect on the
broadening of the transition region observed for films just dried at room
temperature. However, the storage modulus signal reaches a plateau above the
glass transition temperature for latex T(-5)F21, which holds the maximum
functional monomer content possible for this series.
Storage modulus and loss tangent plots for films cast from mixtures of
latexes T(-5) with cycloaliphatic epoxide solution are shown in Figures 3.9 and
3.10. As similarly observed for uncrosslinked films, the relatively sharp drop in
storage modulus characteristic of the glass transition region was broader as
functional monomers concentration was incremented. It was also observed that
upon increments in the functionality content the one tan δ peak originally
detected developed into two broad and indistinct peaks. Table 3.5 presents a summary of the characteristic dynamic properties obtained for these films. Glass 59 transition temperatures, storage moduli at the rubbery plateau and crosslink densities were observed to increase as HEMA and MAA contents were augmented.
Figure 3.5 Storage modulus of films from latex series T(-5). Films were dried at 25 °C for 48 h (no crosslinker added).
Figure 3.6 Tan δ of films from latex seriesT(-5). Films were dried at 25 °C for 48 h (no crosslinker added). 60
Figure 3.7 Storage modulus of films from latex series T(-5). Films were dried at 25 °C for 2 h and heated at 170 °C for 1 h (no crosslinker added).
Figure 3.8 Tan δ of films from latex series T(-5). Films were dried at 25 °C for 2 h and heated at 170 °C for 1 h (no crosslinker added).
61
Figure 3.9 Storage modulus of crosslinked films from latex series T(-5).
Figure 3.10 Tan δ of crosslinked films from latex series T(-5).
62 Table 3.5 Dynamic properties and crosslink density for crosslinked films from series T(-5).
3 Tg (°C) Max. Tan δ Peak Width (°C) E´ (Mpa) νe (mol/m ) T(-5) F7 59.2 1.16455 44.99 1.4 + 106 146.8 T(-5) F15 69.7 0.44994 122.32 2.15 + 106 211.7 T(-5) F21 93.7 0.30816 128.19 8.94+ 106 859.8
Figure 3.11 Tapping mode AFM images (height mode) of films with increasing functionality. Latexes with 0, 7.4 and 21 wt% functional monomers were dried at 25 °C for 48 hours (a, b and c respectively). Same films were heated at 170 °C for 1 hour (d, e and f).
63
Morphology of films obtained through AFM imaging is shown in Figure
3.11. Latexes T(-5) with 0 and 7.4 wt% functional monomers (Fig. 10a and 10b
respectively) showed similar average particle size. However Figure 3.11c
evidenced two different populations of particles: one of small particles and one of
larger particles. Such observation appeared in agreement with particle size
measurements. Images from films dried at 25 °C during 2 h and heated for 1 h at
170 °C showed reduced presence of discrete particles, although incomplete
coalescence was observed.
3.4 Discussion
Several reasons motivated the study of functionality content in thermosetting acrylic core-shell latexes. Initially, optimization of the upper limit functionality concentration was basic to achieve maximum strengthening in these types of films. Additionally, it was anticipated that the polymerization process and latex characteristics could also be affected by incorporation of increasing content of functional monomers. For instance, the pathway of polymerization is often dependent on the type of monomers involved, affecting molecular weight and particle size. Consequently, it was necessary to evaluate the effect of increasing co-monomer functionality content on latex stability and on films properties.
64 The larger fraction of HEMA (and total HEMA and MAA content)
successfully incorporated in series T(-5) compared to series T(10) and its effect
on latexes particle populations can be explained by monomer solubility and
copolymerization effects. It should be considered that as the fraction of HEMA in
the monomer mixture was increased, the concentration of this monomer in the
aqueous phase was higher due to its increased water solubility compared to
MMA or BA (solubilities in water: 81, 1.6 and 0.2 g/100 mL respectively).
[3,74,75] Additionally, taking into account reactivities for the three monomers in
Table 3.6, [76] it could be expected that HEMA molecules were more prone to
react with other HEMA molecules than with MMA and BA molecules present in
the system.
Table 3.6 Reactivity ratios for pairs of HEMA, MMA and BA (from reference 75)
M1 M2 r1 r2
BA MMA 0.13 0.92
BA HEMA 0.09 4.75
MMA HEMA 0.192 0.81
It is commonly known that the initiation process during most emulsion polymerizations usually occurs in the aqueous phase with formation of hydrophilic oligomeric radicals. Upon growth, these oligomeric radicals would not enter monomer-swollen micelles until reaching a critical entry chain length for which a hydrophilic-hydrophobic balance would be achieved. These oligomeric 65 radicals might or might not become soluble; if not soluble, such oligomers
precipitate out of solution and are stabilized by adsorption of surfactant, forming
new particles (homogeneous nucleation). [2] Such scenario is represented in
Figure 3.12. In these systems, as BA / MMA ratio was larger (1.13 in series
T(10) to 1.87 in series T(-5) on average), formation of oligomeric radicals was
influenced by the presence of the less water-soluble BA, possibly by changing
the solubility of oligomeric species at the critical chain length at which such oligomers would either precipitate out of solution or enter the organic phase.
Poehlein [77] studied the emulsion polymerization of styrene in the
presence of water-soluble co-monomers and found that for styrene-methacrylic
acid systems, the entry size and composition of oligomeric species changed as the ratio between monomers varied. As more MAA was used, a higher
MAA/Styrene ratio was observed in the oligomers. Moreover, the molecular weight of the species increased. [77] Correspondingly, if increasing concentration
of HEMA molecules in the aqueous phase was determinant in the stability of the
system during polymerization, it would be most reasonable to think that less-
soluble oligomers led to formation of new particles. This hypothesis would
explain the two particle sizes observed for the latex containing the maximum
amount of functional monomers (21 wt. %). In fact, generation of particles
during polymerization of monomers with high water solubility such as vinyl
acetate (solubility 2.5 g / 100 mL) has been shown to occur via precipitation of
aqueous oligomeric radicals. [7] Similarly, previous studies on polymerization of
66 methyl methacrylate in aqueous solution have shown particle formation during reaction. [5]
Figure 3.12 Representation of particle nucleation dependence on oligomeric radical solubility. (a) Water solubility of oligomeric radicals determine the stability of the system, (b)Homogeneous nucleation may occur for many of the HEMA- rich oligomeric radicals
67 In the case of the latex with two particle populations that was observed in
this study (at 12.5 wt. %), the presence of water-soluble HEMA molecules in the
aqueous phase and its prominent reactivity probably boosted homogeneous
nucleation, which would not be very significant compared to micellar nucleation in
the polymerization of water-insoluble monomers. When concentration of water-
soluble HEMA was increased to this level, more HEMA-rich oligomeric radicals
could have led to secondary nucleation of particles. Above 12.5 wt. % HEMA
content, too many oligomeric radicals were probably generated, and stability of
the colloidal system was compromised.
Stress-strain analyses as well as dynamic mechanical analysis were
performed on films obtained from latexes with increasing functionality and
crosslinker content. Upon addition of crosslinker, films showed pronounced
enhancement in tensile strength and tensile modulus as HEMA and MAA
concentrations were increased. This improvement is evidence of cohesive strength development within films achieved through crosslinking. Flexibility of the films was reduced, as measured by elongation at the moment of breakage. Such reduction in elongation was expected due to higher concentration of joint sites between polymer chains, which restrict movement upon stretching. More interestingly, tensile strength and tensile modulus for films without crosslinker improved uniformly upon increasing content of HEMA and MAA. Such behavior might be the result of increasing interaction forces between polar groups in polymer chains (i.e. hydrogen bonding among carboxyl and hydroxyl groups).
68
The dynamic mechanical analysis of films with increasing functionality
concentrations showed two major trends. One corresponded to broadening of
the transition region and the other represented the shift of the maximum tan δ value toward lower numbers and higher temperatures. Hidalgo and co-workers found a similar broadening for polystyrene/poly(butyl acrylate-methacrylic acid) core-shell latexes with increasing content of MAA (MAA fractions were smaller than in this study). [78] It was considered as a result of composition drift during
copolymerization due to difference in reactivity ratios that produced chains with wide composition distribution and wide range of transition temperatures. This may also be the case for latexes T(-5). Through the transition region, the storage modulus values were larger as functional monomer content was increased. The role of hydrogen bonding discussed previously as being the reason for increased strength in uncrosslinked films, may as well explain the increase in storage modulus and the rubbery plateau observed for the sample containing 21 wt. % functionality. Additionally to the broadening of the transition region, it was observed that a secondary tan δ peak developed upon increments in functionality
fraction. Broad transitions have been often related to samples that undergo
partial phase separation during crosslinking. Furthermore, the presence of two
indistinct and broad peaks in the tan δ signal, as observed in this case, may
indicate phase separation (of the core-shell structure) with substantial mixing at
the boundaries between domains. [72]
69 Images from films dried at 25 °C during 2 h and heated for 1 h at 170 °C, showed incomplete coalescence since discrete particles were still visible with some degree of deformation. Polymer interdiffusion in systems with particles containing hydrophilic shells such as this type of latexes has been previously studied. It was shown that interdiffusion was retarded at temperatures below the
Tg of the polymer located in the shell. [45] It is proposed that inter-particle
crosslinking may be the major factor favoring cohesive strength within these
films. Such crosslinking position accounted for large increases in tensile
strength, tensile modulus and thermo-mechanical properties even tough
interdiffusion across particle boundaries was not complete when crosslinking
reactions occurred.
The implications of increasing functionality, in particular combinations of
hydroxyl and carboxyl groups on latex dispersions are complex. Before the
consummate intention of crosslinking, controlled preparation and stability of the
system may be compromised. Secondary nucleation, if controlled, would have
certain advantages with respect to enhanced coalescence and separation of
functional groups on a mesoscale; thus, this is an area of further study at this
point. Optimum functionality content of a latex system that was stable
corresponded to 9 wt. % hydroxyl functional monomer in the core, and 6 wt. %
carboxyl functional monomer in the shell, for a total functionality of 15 wt. % based on total monomer.
70 3.5 Conclusions
Latexes with high functionality content (HEMA and MAA monomers) were synthesized by a two-stage emulsion polymerization. The maximum possible
content of functional monomers incorporated was determined by colloidal stability
of the system. When large fractions of functional monomers were copolymerized, presumed secondary nucleation occurred during the first stage of
polymerization. Pronounced increment in films strength observed for
uncrosslinked films was explained as a consequence of increased interactions
between functional groups within the films.
71 CHAPTER IV
FUNCTIONALITY AND PARTICLE SIZE VARIATIONS IN LATEXES FOR
THERMOSETTING DISPERSIONS: ENHANCED CROSSLINKED
STRUCTURES
4.1 Introduction
Blends of latexes have been actively studied due to the inherent
advantages brought by adjusting overall properties of the final latex from the
individual properties of its components. Studies have focused on blends
obtained from latexes differing in hardness or particle size, although combinations of these two characteristics have also been subject of investigation.
[30] Studies of blends with varying hardness, meaning blends of a soft, low glass
transition temperature (Tg) latex and a hard latex (Tg above ambient temperature)
have shown that it is possible to reduce volatile organic compound levels by
minimizing the amount of coalescing aids added to latex coatings. [30] Film formation, mechanical properties and polymer diffusion of hard/soft blends have also been investigated. [80-82] During film formation it was observed that the
presence of hard particles promoted clustering of these particles creating voids
not likely to disappear during aging. [80] Tensile properties had also been
72 evaluated for un-crosslinked soft-hard latex blends. The elongation-at-break of
hard/soft blends of similar particle sizes decreased upon addition of increasing
amounts of hard particles. The increase in tensile strength at low volume
fractions of hard particles was attributed to energy dissipation and deflection of growing cracks. At higher fractions of hard particles tensile strength decreased,
possibly due to clustering effects. [81]
Bimodal particle size latexes have also been the subject of extensive
work. Peters et al. [83] found that blending latexes of different particle sizes
(about 50 and 350 nm) resulted in beneficial characteristics regarding film
formation and rheology. For their blends, a minimum value of the minimum film
formation temperature was observed at the blend weight ratio of 80/20
large/small particles. At this same blend ratio, a minimum in water absorption
and a maximum in tensile strength were observed. Such findings were explained
as a result of packing maximization at this blend ratio. [83] Later, Eckersley and
Helmer [84] found that controlling the large/small particle size ratio and the
hard/soft particle concentration proved beneficial to produce films with desired film formation and increased block resistance.
More recently, the concept of phase continuity previously realized by Kusy
was employed by other researchers to study the enhancement of packing in
bimodal particle size latexes. [84,85] In a previous study by Tzitzinou et al., the
critical volume fraction, VC, was defined as the minimum volume fraction of small
73 particles required to form a continuous phase. This volume fraction is a function
of particle size ratio. Thus VC decreased as the large/small particle size ratio
increased. In their study, the ideal concentration of small particles required for
obtaining improved packing was investigated in terms of interstitial void concentration and roughness of the films. [85,86] It was observed that void
concentration and roughness of films decreased dramatically upon incorporation
of small particles to the large particle dispersion up to about 16.5 wt. %. After
that point the decrease in void content and film roughness was less pronounced.
In spite of the several investigations on conventional thermoplastic latex blends, the incorporation of functional monomers to these systems and the subsequent addition of crosslinking components have not been subject of much investigation. As is commonly known, crosslinking technologies for functional latexes have been studied since the late 1950’s. [48] Generally, incorporating
crosslinking reactivity into thermoplastic latexes improves mechanical
performance and solvent resistance. Many crosslinking chemistries have been
investigated for thermosetting latexes bearing functional groups such as
hydroxyl, carboxyl and amine groups. [48-58] However, such studies have only
focused on conventional monomodal latexes, and not on bimodal latex
dispersions.
The incorporation of any crosslinking chemistry into a bimodal latex
dispersion implies the introduction of reactive groups through the
74 copolymerization of functional monomers, and the addition of a crosslinker. The
mere presence of functionality on the particle surface imparts dissimilar physical
properties with respect to conventional thermoplastic latexes. Furthermore, it
would be expected that the combination of a bimodal distribution of particle size
and the crosslinking ability would create a synergistic effect in terms of
mechanical properties of the crosslinked films. However, the integration of
reactive groups and crosslinking resins would introduce changes in packing
characteristics with respect to thermoplastic latexes. In this context, one of the
objectives in this study was that of investigating the influence of bimodal particle
size on the structure of crosslinked films obtained from thermosetting latex
systems. For this purpose, it was important to study the effect of increasing
small particle content and functionality location (and consequently crosslinker
location) on the mechanical behavior of films.
In this work, three different series of latexes were prepared with varying
particle sizes and functionality locations. Latexes were prepared by the copolymerization of butyl acrylate (BA) and methyl methacrylate (MMA) and
when appropriate, hydroxyethyl methacrylate (HEMA) was added. Functionality
was incorporated into the small or the large particle latexes or both. A
methylated melamine-formaldehyde resin was used with the functional latexes and blends of small and large particle size latexes were prepared in different proportions. The effects of blend ratio and functionality placement were
75 determined through observations of tensile strength, thermo-mechanical properties and surface morphology of the films.
4.2 Experimental
4.2.1 Materials
Butyl Acrylate (BA), Methyl Methacrylate (MMA), Hydroxyethyl
Methacrylate (HEMA), Ammonium Persulfate (APS) and Sodium Bicarbonate
were technical grade chemicals purchased from Aldrich Chemical and used as received. The surfactant used was a sodium alkylaryl polyether sulfonate (Triton
X-200) kindly supplied by Farma International. Deionized water was used in the preparation of the latexes. A sample of a methylated melamine (Luwipal 066F) was received from BASF and used as the crosslinking resin in this study. The resin is reported as Hexamethoxymethyl melamine (HMMM) [87] and MALDI-
TOF mass spectrometry confirmed the presence of mostly the monomeric
species.
4.2.2 Preparation of Latexes
Latexes designed with large particle size were synthesized by seeded
semi-continuous emulsion polymerization with monomer emulsion feed. All
seeds used for preparing the large particle size latexes were obtained from single seed latex prepared batch-wise.
76 Seed: NaHCO3 (1.5 g) and Triton-200 (0.5 g) as solution in water (150 g)
were added to a 500 mL flask equipped with a condenser, stirrer and a nitrogen
atmosphere. This solution was heated to 75 °C; then, a pre-emulsion of
monomers was charged to the flask. The pre-emulsion contained BA (31.69 g,
0.247 mol) and MMA (25.18 g, 0.251 mol) with NaHCO3 (0.07 g) and Triton-200
(3.45 g). A 2 wt. % aqueous solution of ammonium persulfate (61.4 g) was then charged to the flask. Polymerization was allowed to progress for 60 min at 75
°C. The same experimental apparatus for seed preparation was used for the
semi-continuous stage.
Table 4.1 Small and large particle size latex compositions for bimodal blends.
Latex Large 1 Large 2 Large 3 Component (g) (g) (g) Pre-emulsion
BA 88.32 89.15 88.72 MMA 57.28 70.85 64.08 HEMA 14.40 - 7.20 NaHCO3 0.20 0.20 0.20 Triton-200 7.20 7.20 7.20 Water 160.00 160.00 160.00
Seed (18.66 % 42.88 42.88 42.88 solid content)
Small 1 Small 2 Small 3 (g) (g) (g) Pre-emulsion
BA 31.69 31.38 31.53 MMA 25.18 20.37 22.78 HEMA - 5.12 2.56 NaHCO3 - - - Triton-200 12.04 12.04 12.04 Water 81.24 81.24 81.24
77 Semi-continuous stage: The seed (5 wt% of the weight of monomer) was
charged in the reactor and heated to 75 °C. A pre-emulsion of BA, MMA and
HEMA (if required) along with an initiator solution, were fed continuously for 240
min. Table 4.1 presents the components and amounts for small and large
particle size latex formulations. Contents were heated and stirred for additional
240 min after the feed was complete.
Latexes designed with small particle size were prepared via a batch-wise
emulsion polymerization process similar to the preparation of the seed for the
large particle size latexes. The solution of NaHCO3 (1.0 g) and Triton-200 (10.08
g) in water (150 g) was charged to the 500 mL flask equipped with the same
features as for seed preparation. After reaching 75 °C, a pre-emulsion of BA,
MMA and HEMA (when required) with Triton-200 (3.45 g) was charged in the
flask followed by the addition of a 2 wt.% aqueous solution of ammonium
persulfate (60.75 g). Polymerization continued for 120 min at 75 °C.
4.2.3 Latex Characterization
Latexes were cleaned using a dialysis membrane to remove excess
amount of surfactant and other water-soluble ionic materials. A regenerated
cellulose dialysis membrane (MWCO 12000-14000) was cleaned to remove
soluble residual materials and was rinsed thoroughly with distilled water. A
weighted amount of latex was placed inside the membrane and into a container
78 with distilled water. Water was replaced every 12 h until the conductivity of the
external water was approximately 0.02 μS.
Particle size and particle size distributions were measured by photon correlation spectroscopy (PCS) using a BI-200SM goniometer and laser light scattering system from Brookhaven Instruments, equipped with a BI-90 digital photon correlator and a 652 nm He-Ne laser. Measurements were carried out at
25 °C and at a fixed angle of 90 °C on very diluted emulsions (< 0.1 vol%) .
Glass transition temperatures were obtained by Differential Scanning Calorimetry
(DSC) analysis performed on a Q1000 DSC from TA instruments using a heating
ramp of 20 °C/min.
4.2.4 Preparation of Blends and Film Characterization
Each one of the latexes prepared for this study would vary in particle size
and on hydroxyl functionality content according to any of three blend series
designed. The three series and the nomenclature are explained as follows.
Series FSL: Blends with Functionality placed in Small and Large particles.
Series FS: Blends with Functionality placed in Small particles.
Series FL: Blends with Functionality placed in Large particles.
Physical properties for each series of latexes are summarized in Table
4.2. The ratio of large to small particles was chosen to be close to RL/RS= 6. At
79 this ratio the minimum volume fraction of small particles required to form a
continuous phase around the large particles is about VC = 0.18. These latexes
allowed for studying systems that are comparatively similar in particle size and
particle size ratio to unfunctionalized bimodal latexes studied previously.
For preparation of crosslinkable systems HMMM was dissolved in methyl
alcohol, added to the hydroxyl functionalized latexes and stirred for 1 h prior to
blend preparation. The molar ratio of HMMM/HEMA = 3 was kept constant for all functional latexes. Latex blends were prepared by mixing large and small
particle size dispersions in different weight ratios. Blends were stirred
continuously for 3 h and cast onto glass plates. Films were dried for 12 h at 25
°C and 45 % humidity; then films were introduced in a convection oven at 150 °C
for 30 min. [88]
Table 4.2 Physical properties of latexes used in the preparation of bimodal latex blends.
Particle Functionality Particle size Polydispersity Latex d) Tg ( °C) Size Ratio (wt %) (nm) e) RL/RS Series Small 4.5 53.5 0.108 10.0 a) 6.5 FSL Large 4.5 346.5 0.084 12.4 Series Small 9.0 55.4 0.106 11.6 b) 6.2 FS Large 0.0 341.0 0.056 8.1 Series Small 0.0 53.6 0.089 13.3 c) 6.6 FL Large 9.0 353.9 0.043 11.4
a) Functionality in Small and Large particles b) Functionality in Small particles c) Functionality in Large particles d) Monodispersity is considered when polydispersity indexes are below 0.025. e) RL stands for Large particles radius and RS stands for Small particles radius. 80 Films were removed with a razor blade and cut (80 x 16 mm or 25 x 5
mm) for tensile testing on an Instron Universal Electromechanical Tester 5567
and for dynamic mechanical thermal analysis using a Rheometrics Scientific
analyzer. Tensile testing was performed with a load cell of 100 N and a
crosshead speed of 12 mm/min. A minimum of 10 specimens were tested
choosing those with closest values to obtain average values (according to the
selection criteria in ASTM standard D2370-98). Dynamic mechanical thermal
analyses were carried-out with a constant temperature ramp (3 °C / min), a fixed oscillating frequency (1 Hz) and a controlled strain (0.5 %). Latexes were cast onto pre-treated silica wafers or on glass slides, and examined for dry film morphology in a Multimode Scanning Probe Microscope (Digital Instruments,
Nanoscope III) set to tapping mode. Samples were scanned after 12 h of drying in order to observe morphology features prior to crosslinking.
4.3 Results
The purpose of this investigation was that of studying the effect of
blending small and large particle size latexes on packing structure and film
properties of crosslinkable latex blends. Initially it would be expected that the
combination of small and large particles sizes would improve packing
characteristics in films. However, the presence of functional groups and
crosslinking resins could interfere with expected packing arrangements observed in uncrosslinked systems. Moreover, along with particle size differences,
81 variations in functionality location (and consequently crosslinker location) could also play an important role in the development of mechanical properties of films
obtained from the blends. The choice of functionality placement either in small or large particles, or in both, would introduce specific structural characteristics to the crosslinked films. Thus, in order to study both aspects, i.e. particle size differences and specific functionality placement, three series of latexes were designed (see Table 4.2). Blends in series FSL were prepared from small and large particle size, hydroxyl functional latexes. In series FS blends were obtained by mixing a small particle size, hydroxyl functional latex with unfunctionalized, large particle size latex. The third series, blends FL were mixtures of small, unfunctionalized particle size latex and large particle, hydroxyl- functional latex. These, series of latexes allowed the study of the effect of blending hydroxyl functional small and large latex particles and functionality placement in either of the component latexes.
4.3.1 Surface structure of uncrosslinked films: Effect of OH groups in packing of
bimodal latex dispersions.
During drying and packing of conventional latex films into close-packed
structures, certain features of the latex may affect the packing arrangement of
particles. Such is the case of polar groups present on the surface of the
particles, such as excess surfactants or salts. [69]. It is possible that the
presence of hydroxyl groups on one or both latex components in the blends
82 might influence particle packing features with respect to unfunctionalized latexes
studied in the literature. Therefore, previous to exploring parameters such as particle size and functionality location in crosslinkable bimodal latex dispersions, it was important to observe any special features occurring in packing due to the presence of functionality, with respect to unfunctionalized latexes. For this, surface morphology was evaluated through tapping mode AFM imaging. Images of uncrosslinked films cast from bimodal hydroxyl-functional latexes (series FSL) without crosslinker are shown in Figure 4.1 as function of small particle content.
Figure 4.1 Tapping mode AFM images of surface morphology of blends in series FSL cast without crosslinker.
83 In Figure 4.1a, corresponding to the large particle size monomodal parent latex, particles tend to form close domains separated from each other by empty spaces in the film. On the other hand, Figure 4.1e shows a much close packed structure from the small monomodal parent dispersion. The image in Figure 4.1b corresponds to the blend prepared with 20 vol. % of small particles. It shows large particles separated from each other and completely surrounded by small particles. The distance between large particles is in average 96 nm, which is a little less than the diameter of two small particles. At 80 vol. % of small particles
the film surface of this blend seems very similar to the image of the monomodal
large particle parent latex.
4.3.2 Effect of blend ratio on mechanical and thermo-mechanical properties of
crosslinked films from bimodal hydroxyl-functional latex blends
Studying the effects of blending crosslinkable small and large latex
particles on film properties was pursued through observation of mechanical and
thermo-mechanical properties of crosslinked films. Tensile testing allowed
evaluating the overall strength of the films while dynamic mechanical analysis
provided information on the extent of crosslinking and film structure. Blends in
series FSL were prepared from two hydroxyl functional latexes, one designed
with small particles and the other with large particles. A solution of the melamine
resin used as crosslinking agent was added to each one of the latexes and after
prolonged mixing with the resin the two latexes were blended. The blend ratio
84 between small and large particles was varied in order to establish a relation between blend fractions and mechanical behavior. Tensile testing and dynamic mechanical thermal analysis were performed on films that had been dried and heated to induce crosslinking.
Tensile strengths of crosslinked films obtained from Series FSL are shown in Figure 4.2. The behavior of tensile strength values was fitted to a sigmoidal line just as a guide. Tensile strength values increased for films containing over
35 vol. % of small particles. Above 80 vol. % the increasing tendency leveled off.
It was also observed that higher tensile strength was obtained for the small particle parent latex compared to the large particle parent latex.
10
8
6
4
Tensile Strength (MPa) Tensile 2
0 0 20406080100 vol % small particles
Figure 4.2 Tensile strength for crosslinked films prepared from bimodal crosslinkable latex blends in series FSL. Both types of particles (small and large) bear hydroxyl functionality.
85 1e+10
1e+9
1e+8
1e+7 FSL-S0 log E' (MPa) log E' 1e+6 FSL-S20 FSL-S50 FSL-S80 1e+5 FSL-S100
1e+4 -150 -100 -50 0 50 100 150 200 250 Temperature ( °C)
Figure 4.3 Storage modulus of crosslinked films obtained from bimodal crosslinkable latex blends in series FSL. Both types of particles (small and large) bear hydroxyl functionality.
The storage modulus variation with temperature for blends of series FSL is shown in Figure 4.3. Two transitions were detected for films with 20 and 50 vol. % small particles, contrary to the rest of films for which only one sharp transition was observed. On the other hand, all storage moduli signals reached the rubbery equilibrium region characteristic of crosslinked systems. The storage modulus value at the rubbery equilibrium region above Tg has been frequently related to the crosslink density of films by the equation
E ' υ = (4.1) e 3RT
86 where νe is the crosslink density expressed as moles of elastically effective network chains per cubic meter of sample. E’ is the storage modulus at the
equilibrium plateau, R is the gas constant and T is temperature. Table 4.3 shows
calculated crosslink densities of films in series FSL. Similar values obtained for
FSL-S0 and FSL-S100 indicated similar extents of crosslinking for these two
films. Crosslink densities for FSL blends were lower than crosslink densities of
the parent latexes; however, values increased when the small particle content
was increased.
Table 4.3 Dynamic properties and crosslink density values of films from bimodal crosslinkable latex blends in series FSL. Both types of particles (small and large) bear hydroxyl functionality.
E' νe Tan δ Tg Latex (MPa) (mol/m3) (max) ( °C)
FSL-S0 2.65E+06 251.3 1.03 13.18 FSL-S20 1.05E+06 99.7 0.96 17.76 FSL-S50 1.23E+06 116.4 0.97 25.59 FSL-S80 1.88E+06 177.9 0.99 49.71 FSL-S100 2.83E+06 268.3 1.18 45.94
Tan δ transitions for latex blends in series FSL are plotted in Figure 4.4.
Narrow transitions were observed for latexes FSL-S0, FSL-S80 and FSL-S100
and much broader peaks were observed in the case of blends FSL-S20 and FSL-
S50. The broad transitions were a consequence of the two transitions observed
in the storage moduli plots in Figure 4.3. Values of Tg and Tan δ maxima were
calculated and are summarized in Table 4.3 as well. The Tan δ maxima were
87 higher for the parent latexes FSL-S0 and FSL-S100 than for blends. The Tg increased uniformly upon addition of small particles to the blends.
1.4 FSL-S0 1.2 FSL-S20 1.0 FSL-S50 FSL-S80 0.8 FSL-S100 δ 0.6 Tan 0.4
0.2
0.0
-150 -100 -50 0 50 100 150 200 250 Temperature ( °C)
Figure 4.4 Tan δ of crosslinked films from blends in series FSL. Both types of particles (small and large) bear hydroxyl functionality.
4.3.3 Effect of hydroxyl functionality placement on mechanical and thermo-
mechanical properties of crosslinked films from bimodal latex blends
Additionally to studying the mechanical performance of films obtained from crosslinkable small and large particle latex blends, it was important to examine the effect of functionality location on the mechanical behavior of crosslinked films and the development of film structures. Two series of latexes were designed in which hydroxyl functionality and crosslinking resin would be present in either the small or the large particles. Blends in series FS were prepared by combining a small particle, hydroxyl functional latex and a large particle, unfunctionalized latex at different blend ratios. The hydroxyl functionality was placed in the small 88 particles and consequently the crosslinker was added to this latex prior to
preparation of the blend. A complementary set of blends corresponded to series
FL, in which functionality and crosslinker were placed in the large particles only.
10
8
6
4
Tensile Strength (MPa) Strength Tensile 2
0 020406080100 vol. % small particles containing crosslinker Figure 4.5 Tensile strength for films containing increasing fractions of crosslinkable, hydroxyl functional small particles in series FS.
The variation of tensile strength with blend ratio for series FS is shown in
Figure 4.5. The trend in tensile strength was again fitted to a sigmoidal curve as a guide. Tensile strength values were small at low fractions of small particles; however, values increased dramatically when the small particle content increased over 20 vol. %. The storage moduli and Tan δ signals for this series
are shown in Figures 4.6 and 4.7, respectively. The equilibrium plateau above Tg was reached only in the case of blends containing small particle contents of 50 vol % and higher. It was also observed that samples containing 20, 50 and 80 vol % small particles showed secondary transitions, contrary to parent latexes
FS-S0 and FS-S100. Tan δ profiles showed that increasing the concentration of 89 crosslinkable small particles in the blends caused values of Tan δ maximum to decrease and transition regions to broaden, revealing secondary transitions for blends.
1e+10
1e+9
1e+8
1e+7 FS-S0 FS-S20
Log E' (MPa) Log 1e+6 FS-S50 FS-S80 1e+5 FS-S100
1e+4 -150 -100 -50 0 50 100 150 200 Temperature (°C)
Figure 4.6 Storage modulus (E’) for films containing increasing fractions of crosslinkable, hydroxyl functional small particles in series FS.
2.5
2.0 FS-S0 FS-S20 1.5 FS-S50 FS-S80 δ 1.0 FS-S100 Tan
0.5
0.0
-150 -100 -50 0 50 100 150 200 Temperature ( °C)
Figure 4.7 Tan δ for films containing increasing fractions of crosslinkable, hydroxyl functional small particles in series FS.
90 14
12
10
8
Tensile Strength (MPa) 6
4 0 20406080100 vol. % small particles without crosslinker
Figure 4.8 Tensile strength of films containing increasing fractions of unfunctionalized small particles in series FL
The third set of blends, series FL, was designed to contain hydroxyl
groups and crosslinker in the large particles, with increasing fractions of
uncrosslinkable small particles. Figure 4.8 shows the behavior of tensile strength
for films obtained from these series. The values showed an initial steep
decreasing trend starting from the large particle parent latex. For contents of
small particles above 20 vol. % the decreasing tendency was more moderate.
The storage modulus and loss tangent Tan δ signals for films in series FL are
illustrated in Figures 4.9 and 4.10, respectively. The storage moduli variation
with temperature showed that crosslinked structures were obtained for films
containing fractions of uncrosslinkable small particles up to 50 vol. %. Contrary
to series FS, secondary transitions were not observed from the storage moduli signals of any of the films. Upon increasing the content of small particles, loss
91 tangent signals showed increasing values of Tan δ maximum but broader transitions regions.
1e+10
1e+9
1e+8
1e+7 FL-S0 FL-S20
Log E' (MPa) 1e+6 FL-S50 FL-S80 1e+5 FL-S100
1e+4 -150 -100 -50 0 50 100 150 200 250 . Temperature ( °C) Figure 4.9 Storage modulus (E’) of films containing increasing fractions of unfunctionalized small particles in series FL.
1.6 1.4 FL-S0 FL-S20 1.2 FL-S50 1.0 FL-S80 FL-S100 δ 0.8
Tan 0.6 0.4 0.2 0.0
-150 -100 -50 0 50 100 150 200 250 Temperature ( °C)
Figure 4.10 Tan δ of films containing increasing fractions of unfunctionalized small particles in series FL.
92 4.4 Discussion
In this study, three series of latex blends with varying properties such as
particle size and functionality location were designed with the purpose of
studying the effect of such variables on packing structures and mechanical
strengthening of bimodal crosslinkable latex dispersions. Initially, it was
important to observe any changes in packing introduced by the presence of
hydroxyl groups on both types of particles. Also, adding a crosslinking resin into
functional bimodal latex systems was expected to enhance mechanical properties, although it was suspected that the extent of strengthening would be dependent on the ratio of small to large particles. Additionally, it was anticipated that pre-determining the location of the functional groups into the films would allow for strengthening at lower functionality and crosslinking contents through the formation of “reinforced” crosslinked structures. In this way, the design of three latex blend systems was devised to explore first, the effect of blend ratio on fully functional bimodal latex blends and second, the effect of functionality placement on partially functionalized bimodal latex blends.
Upon the observation of the surface morphology of blends prepared from
functional small and large particle latexes at several small particle contents,
(Figure 4.1) the packing of particles was analyzed and compared to observations
previously reported for unfunctionalized latexes. Many of the features observed
for hydroxyl functionalized films here were in good agreement with findings by
Tzitzinou et al. on their unfunctionalized soft latex blends studies. For their
93 systems, they found that at volume fractions close to VC (VC = 0.18) large
particles were not in close contact with each other, but surrounded by small
particles. Such an arrangement would be consistent with the idea of phase
continuity occurring close to the VC of small particles. In the particular case of
functional latex blends, at a content of 20 vol. % of small particles, complete
“disconnection” of large particles was observed in Figure 4.1b, and the small
particles seemed to form a continuous network or phase surrounding the large
particles. According to the observations made from the functionalized blends in
series FSL without crosslinker, it seemed that the level of hydroxyl functional
groups present on the particles did not cause major packing changes with
respect to unfunctionalized latexes.
Considering the aims of bimodal soft latex dispersions regarding improved packing, it was conceived that the combination of this advantageous feature along with crosslinking ability would result in enhancement of mechanical properties through the increase in the extent of crosslinking. Such improvement would be a consequence of closer contact between particles and consequently increased interaction between reactive groups. However, the presence of external crosslinking agents would be expected to interfere during particle packing.
Regarding differences in particle sizes, tensile strength values obtained
from FSL-S0 and FSL-S100 indicated that strengthening of films was favored for
94 the small particle parent latex. However, crosslink density values showed that the reason for this increase was not an improvement in crosslinking of FSL-S100 as the crosslink density values for both parent latexes (FSL-S0 and FSL-S100) were very close to each other. These results indicated that neither of the films
was crosslinked to higher extent than the other, instead, higher strengthening in
FSL-S100 was a result of enhanced particle coalescence brought about by closer
packing between small particles compared to large particles.
With respect to the overall trend in tensile strength values, increments were observed only when the content of small particles was over 35 vol. %. This increase in tensile strength revealed that the packing characteristics of these blends were different from those of bimodal thermoplastic latex dispersions. At
the particle size ratio of these latexes, RL/RS= 6, the critical volume fraction at
which small particles would form a continuous phase or network around the large
particles would be close to VC = 0.18. Just as tensile strength showed a
maximum value in Peters’ work on thermoplastic bimodal latexes close to VC for their systems, [83] it was expected that the latexes in this study would display
higher strength at blend ratios close to VC (0.18 or 0.2), when packing would be
optimized. However, the fact that tensile strength increment occurred at a higher
blend ratio indicated that packing behavior of these blends was affected, possibly by the presence of the crosslinker on the surface of the particles.
95
Figure 4.11 AFM images of surface morphology of films from blends in series FSL with crosslinker. Height images are to the left and phase mode images are to the right. Films were cast and let to dry for 2 h before imaging.
As means of detecting differences in film packing between blends containing crosslinker and blends without crosslinker, AFM images of films cast from blends with crosslinker were acquired after 2 h of drying. The images of
96 films cast from blends FSL mixed with crosslinking resin are shown in Figure
4.11. The presence of the crosslinking resin on the surface of the particles
deteriorated the resolution of particles boundaries. For this reason phase images
(right) were much more illustrating than height images (left). Although not
differentiated in the height image, a portion of the crosslinking resin was present
in the interstices between particles, as observed in Figure 4.11a, for parent latex
FSL-S0. At 20 vol. % of small particles, the distribution of large particles showed
some small close-contact clusters not completely separated by small particles, as
was the case for blends obtained without crosslinker (see Figure 4.1). Upon
addition of 35 vol. % small particles, large particles appeared more scattered,
although close contact was still predominant between some. The image of the
blend with 50 vol. % showed large particles scattered apart from each other,
surrounded by small particles. A larger content of small particles than 20 vol. %
was necessary in order to obtain morphology similar to that observed in the
functional bimodal blend without crosslinker.
The characteristic features describing temperature-dependent mechanical
behavior of crosslinked films were suitable for revealing information about the
crosslinked microstructure of films from series FSL. Usually narrow loss tangent
peaks are common of homogenous materials, while partially phase separated
systems usually present very broad transitions caused by partially overlapping
peaks. [72] Films obtained from parent latexes FSL-S0 and FSL-S100 seemed to be more homogeneous and uniformly distributed structures than films obtained
97 from blends FSL-S20, FSL-S50 and FSL-S80, which appeared moderately heterogeneous, suspected from the broadness of the transition regions.
The evaluation of mechanical and thermo-mechanical properties of films obtained from latex blends with varying functionality location allowed obtaining crosslinked films, although lower functionality and crosslinker contents were used than otherwise required for monomodal systems. The tensile strength improvement after blending little over 20 wt.% small particles into FS blends was explained by the connectivity of small particles achieved when the fraction of small particles was large enough to form a continuous phase around the large particles (at VC for this series). This small particle fraction somewhat over 20 vol.
% was lower compared to blends in series FSL for which improvement in tensile strength was observed in blends containing over 35 vol. %. The basic difference between blends in series FSL and FS was functionality placement. Both latexes in FSL blends contained hydroxyl groups and crosslinker, while blends FS contained hydroxyl groups and crosslinker that had been added only to the small particle latex. The fact that the fraction of small particles necessary to form a continuous phase around large particles for series FSL was higher than for series
FS was attributed to the presence of crosslinker resin on both latexes, instead of only in the small particles as in FS blends. It is possible that a portion of crosslinker could be adsorbed on the surface latex particles. In this way, the presence of crosslinker on both types of particles in series FSL caused a more pronounced effect on packing arrangement than in series FS.
98 The study of series FL was particularly interesting because it allowed
observing the effect of adding small “uncrosslinkable” particles to a system of
crosslinkable large particles. In fact, upon addition of up to 50 vol. % of
uncrosslinkable small particles, the decline observed in tensile strength was
more pronounced than observed at larger fractions. This could be interpreted as
if by adding up to 50 vol. % uncrosslinkable small particles had not yet disrupted
what would be the fully crosslinked network of pure crosslinkable large particle
latex film. Additionally, storage moduli profiles showed that at 50 vol. % a crosslinked structure was still present. This indicated that packing of particles in
this series was also affected by the presence of the crosslinking resin.
Comparison of transition regions of crosslinked blends from series FSL
and FS revealed that these structures shared common elements. When the
crosslinker was added to both small and large particles in series FSL, transitions
were broad and showed certain degree of phase separation. Addition of the
crosslinker only to small particles in series FS resulted in even broader transition
regions evidencing more heterogeneous structures. The presence of two
overlapping peaks or very broad transitions in both systems evidenced that a
partially phase separated morphology was promoted by the difference in extents
of crosslinking of the different particle size latex components. In this way the
structure of the films could be regarded as comprised of regions with varying
crosslink density, depending whether functionality was added to large particles or
not. Schematic representations for both cases are shown in Figure 4.12.
99 Transition regions in series FSL were not as broad as in series FS, possibly because both latex components held similar contents of functional groups and crosslinker. Consequently, sections of the polymer with slightly different extents of crosslinking were formed. On the contrary, series FS showed a larger degree of phase separation because of the intrinsic differences between both types of particles. A network of highly crosslinked polymer (from small particles) surrounding uncrosslinked domains (large particles) resulted from such design of bimodal dispersions.
Figure 4.12 Crosslinked structures of bimodal latex blends with varying functionality location.
100 4.5 Conclusions
The mechanical and thermo-mechanical characteristics of crosslinked
films prepared from bimodal crosslinkable latex blends evidenced different
behavior of these films compared to un-crosslinked latexes studied in the
literature. Films prepared from blends of small and large crosslinkable particles
showed tensile strength values that increased only upon addition of more than 35 vol. %. It is possible that the presence of crosslinking resin affected packing of particles into the films. If that was the case, the minimum fraction of small particles required to form a continuous phase around the large particles would be much higher than for regular uncrosslinked latexes. Consequently, enhancement of mechanical properties was observed at higher fractions of small particles than the expected VC for these types of systems. Placement of hydroxyl functionality
(and crosslinker) in either one of the component latexes in blends also evidenced
interference of the crosslinker during packing, although it was not as pronounced
as in the case when the crosslinker was added to both components. Enhanced
crosslinked structures were obtained when functionality was placed in the small
particles since crosslinked networks were obtained by introducing relatively low levels of crosslinkable particles.
101 CHAPTER V
INFLUENCE OF ACID-BASE INTERACTIONS ON FILM FORMING
PROPERTIES OF ACID FUNCTIONALIZED ACRYLIC LATEXES
5.1 Introduction
The use of latex-based coatings has extended to application areas
requiring high levels of performance which are not often achieved by common
latex polymers. Introduction of crosslinking technologies has promoted the
growth of latex dispersions into more versatile systems that could offer
comparable or improved performance over that of traditional solvent borne
coatings. [89] Performance of these crosslinkable latexes is evaluated with
regards to film properties developed through film formation and crosslinking
processes. Polymer characteristics such as molecular weight, polymer
composition and glass transition temperature have been found to affect film-
forming properties of acrylic latexes. [90-92] The presence of reactive
functionality and other additives in the system has been shown to affect film formation and polymer interdiffusion substantially, modifying the end properties of
the resulting films. [45,46]
102 Many crosslinking chemistries have been reported in the literature, some
of which have been commercialized for industrial applications. [48-58] Among
commercially important systems, hydroxyl functional latexes have been
successfully crosslinked with melamine and other formaldehyde-based resins.
[48,54] The use of strong acid catalysts (sulfonic acids) allowed for lower curing
temperatures or faster curing at shorter heating periods. Melamine-
formaldehyde resins can also react with carboxylic acid functional polymers in
the case of water-reducible acrylic resins. [89] Another crosslinking system
benefited from the use of acid catalysis corresponds to hydroxyl and carboxylic
acid-containing latexes for crosslinking with cycloaliphatic diepoxides. [60] This
type of latex particles bearing carboxylic acid groups and strong acid catalysts
are typically neutralized to control pH and to prevent premature crosslinking viay blocking of acid groups.
One factor affecting film formation and crosslinking reactions in acid-
catalyzed crosslinkable latexes is the presence of weak acid groups (reacting
functionality), blocking agents (typically weak bases) and strong acids (catalysts),
and interactions among these groups. In fact, the presence of carboxylic acid
groups alone introduces varying characteristics on film formation. Drying studies
of latexes containing carboxyl groups showed that drying rate decreased as the
content of carboxyl group increased. The effects of neutralization on drying
parameters were dependent on the extent of carboxyl groups present on the
particles, on the extent of neutralization and on the type of base used. [93]
103 Polymer interdiffusion across particles boundaries was studied as a function of
carboxylic acid content. It was found that the presence of carboxyl groups on the
particle surface retarded polymer interdiffusion. [45] Upon neutralization of
carboxylic acid groups, polymer interdiffusion was further reduced. The effect
was most pronounced for latexes neutralized with sodium hydroxide than for
latexes neutralized with ammonia. [46]
Variables such as acid strength and chemical and physical properties of
the base used for neutralization are particularly important for latexes designed for
crosslinking via acid-catalyzed reactions. It is expected then, that film-forming
properties of acid-containing latexes will not only be affected by such groups, but
also by the type of neutralizing agent used. These variables also influence other
phenomena such as drying and coalescence, amine evolution from drying films, and acid-base interactions within the films. The presence of different acid groups and amines in acid-containing latexes would be expected to influence drying parameters. Observations made from drying experiments would provide information on the development of acid-base interactions.
Another phenomenon involved in the process of film formation of acidic
latex dispersions is amine evolution. This phenomenon could be treated as
solvent evaporation from a film and has been reviewed, previously. [94] Two
factors were identified: vapor pressure of the solvent at the surface and diffusion
through the film. However, salt formation occurs upon addition of amines to the
104 acid-containing system. The evaporation of amines requires the dissociation of
salts formed. Therefore, weak base-weak acid and weak base-strong acid
equilibria would be expected to be a controlling factor in amine loss.
Underlying the study of acid-base blocking and de-blocking processes in
acid-catalyzed crosslinkable latexes, there is the need to understand the
influence of polar groups present on the latex particles on film-forming properties.
The purpose of this work was to evaluate and analyze the effects caused by
different attached acids and different free bases on the film forming properties of
acidic acrylic latexes. Two different acids (sulfonic and/or carboxylic acid) were
incorporated into the latex through copolymerization of functional monomers
along with butyl acrylate and methyl methacrylate. Three amines with varying
physical properties (boiling point and base strength) were used for neutralization.
The study included the determination of drying parameters and the observation
of pH development and amine evolution during the drying stages of
uncrosslinked acrylic acidic latexes. In addition, surface morphology of the films
was monitored during a longer period of maturation.
5.2 Experimental
5.2.1 Materials
Butyl Acrylate (BA), Methyl Methacrylate (MMA), Methacrylic Acid (MAA),
Ammonium Persulfate (APS), Sodium Bicarbonate, N,N-dimethyl-methylamine
105 (Trimethylamine, TMA), N,N-dimethyl-1-butanamine (DMBA), Hydrogen Chloride
(HCl), Sodium Hydroxide (NaOH) and Methanol (HPLC grade) were purchased from Aldrich Chemical and used as received. Hunstman Corporation supplied N- methyl-morpholine (NMM). Dow Chemical supplied the surfactant Triton-X200
(sodium alkylaryl polyether sulfonate). Sulfoethyl methacrylate (SEM) was supplied by Hampshire / Dow Chemical. Deionized water was used in the preparation of the latexes.
5.2.2 Preparation of Latexes
Latexes were synthesized by a seeded semi-continuous emulsion
polymerization process.
Seed polymer: All latex seeds were obtained from single seed latex
prepared batch-wise. Initially, a 500 mL flask equipped with a condenser, stirrer and nitrogen blanket was charged with a solution of NaHCO3 (1.5 g) and Triton-
200 (0.5 g) in water (150 g). The flask was heated to 75 °C and a pre-emulsion
of BA (41.68 g, 0.326 mol) and MMA (38.32 g, 0.3832 mol) with NaHCO3 (0.1 g) and Triton-200 (3.6 g) was charged, followed immediately by addition of a 2 wt.
% aqueous solution of ammonium persulfate (25.55 g). The polymerization was allowed to continue for 60 min at 75 °C.
Second stage polymer: The seed (10 wt% of the weight of the pre- emulsion) was charged in the reactor and heated to 75 °C. The latex containing carboxylic acid groups was prepared by continuously feeding a pre-emulsion of
106 BA, MMA and MAA for 360 min. The latex containing sulphonic acid groups was prepared by feeding a pre-emulsion of BA and MAA along with a 5 wt % solution
of SEM in water during 360 min. Preparation of the latex containing carboxylic acid and sulphonic acid groups was performed by simultaneously feeding a pre- emulsion containing BA, MMA and MAA, as well as a 5 wt % SEM aqueous solution. An initiator solution was fed simultaneously with monomer pre- emulsions and solutions. Table 5.1 lists the components and amounts for each latex formulation. After the feed process was completed, contents were allowed to continue for additional 180 min. The latexes were then allowed to cool down and filtered through a woven polymer micro-filtration mesh (20 μm mesh
opening).
5.2.3 Latex Characterization
Latexes were cleaned with an ion-exchange resin (AG-501-X8) in order to
remove water-soluble ionic materials. Twice the weight of resin (based on latex
solids) was added to dilute latexes (10 % solids); the mixture was stirred at
ambient temperature for 2 h, and then vacuum filtered. The process was
repeated twice. Particle size was measured by photon correlation spectroscopy
(PCS) using a BI-200SM goniometer and laser light scattering system from
Brookhaven Instruments equipped with a BI-90 digital photon correlator and a
652 nm He-Ne laser. Measurements were carried out at 25 °C, at a fixed angle
of 90 °C on very diluted emulsions (< 0.1 vol %). Glass transition temperatures
107 were obtained by Differential Scanning Calorimetry analysis performed on a
Q1000 DSC from TA instruments using a heating ramp of 20 °C/min.
Acid groups on the particles surface were quantified by potentiometric
titration of clean latex samples with 0.0048 M aqueous solution of NaOH (titration
of surface groups). [45] Excess base was added to reach high pH and the latexes were stirred for 1h. After the stabilization period latexes were back titrated with 0.1 N HCl (standard solution) in order to detect acid groups present in the subsurface layer. Titratable charges are reported in Table 5.2 along with other characterization results.
Table 5.1 Feeding stage components in semi-continuous emulsion polymerizations of acid functionalized acrylic latexes.
Component LCA a LSA a LCSA a
BA (g) 42.98 41.12 43.6
MMA (g) 33.42 38.08 32.0
MAA (g) 3.6 - 3.6
SEM (g) - 0.8 0.8
NaHCO3 (g) 0.1 0.1 0.1
Triton-200 (g) 1.0 0.5 0.5
Water (g) 80 80 80
a LCA stands of carboxylic acid groups only, LSA for sulfonic acid groups only and LCSA for carboxylic acid and sulfonic acid groups
5.2.4 Drying and Coalescence of Latex Films
Samples of each one of the latexes were mixed with known amounts of
amines equivalent to acid groups in the polymers measured through titrations. 108 Weight loss from neutralized and un-neutralized latex films were recorded during
the drying stage in a TA Instruments Thermo-Gravimetric Analyzer Q500 under
constant conditions of air flow and temperature. Measurements were performed
with air flow rate set to 45 ml/min (air velocity = 32 cm/min approximately) and
samples were identical in weight to within 3%.
Table 5.2 Characterization of acid functionalized acrylic latexes.
LCA a LSA a LCSA a
Acid groups (wt %) COOH (4.5) SO3H (1.0) COOH and SO3H (4.5 and 1.0) Mean particle size 290 298 291 (nm) Polydispersity index 0.062 0.040 0.053 b Charge density (10-5 eq/g) Strong acid groups 0.51 3.03 3.78 Weak acid groups 3.59 - 3.92 Tg DSC ( °C) 17 14 16 a Abbreviations: LCA stands for Latex containing Carboxylic Acid groups, LSA stands for Latex containing Sulfonic Acid groups and LCSA represents the Latex bearing Carboxylic and Sulfonic Acid Groups b Monodispersity would be reported in the case of polydispersity indexes below 0.025 groups.
Changes on the surface structure of films during later stages in the
coalescence process were monitored with the aid of a Multimode Scanning
Probe Microscope (Digital Instruments) set to tapping mode. Latexes were coated onto pre-treated silica wafers and allowed to dry at 25 °C and 40 % relative humidity. The samples were subsequently scanned at later times during maturation. Images of 5 x 5 µm regions were statistically analyzed with
Nanoscope III software to obtain the RMS (root mean square) average of peaks and valleys of the image, defined as: 109 N 2 1 2 RMS ∑[−= ZZ avgi ] (5.1) N 1
where N is the number of Z values, Zi is the height of point i and Zavg is the
average height.
5.2.5 pH Development and Amine Evolution
Measurements of pH during drying were performed using a flat probe
micro-electrode connected to a Corning pH-meter. In order to achieve a similar
air velocity as used during weight loss experiments, a drying chamber with
adjusted air velocity was used. The latex was cast onto a glass slide (wet
thickness = 8 mils) and placed inside the chamber. The air flow was controlled by means of a flow meter and the electrode was carefully placed on the film through an opening on the upper plate of the chamber. The pH measurement was started immediately by manually recording the readings during drying
(approximately 2 h).
Loss of amines during drying was studied by analyzing the evolved gases
from the Thermo-Gravimetric Analyzer (Q 500 TGA TA Instruments), using a gas
chromatograph/mass spectrometer system (Saturn 2200 Varian Inc.). Gases
evolved from the furnace were collected via a 16 gauge cannula into a test tube
capped with a septa seal. [95,96] When latexes neutralized with NMM or DMBA
were evaluated, gases were passed through the test tube containing HPLC
grade methanol placed in a 5 °C trap and out to a mineral oil bubbler. When 110 latexes neutralized with TMA were evaluated, the test tube was placed in a bath
of liquid nitrogen. Gases were collected continuously for a pre-determined period
of time, then the cannula was carefully removed from the test tube and the sample was manually injected into the GC-MS equipped with a 30 m long and
0.32 mm i.d CP-Volamine column (Varian Inc.). The GC was programmed to
start at 40 °C with a heating ramp of 5 °C/min to 230 °C and held constant for 5
min. The injection port temperature was set to 250 °C and Helium was used as
the carrier gas.
5.3 Results
This work was intended to investigate and analyze the effects caused by
acid and base groups on the film forming properties of acidic acrylic latexes.
Consequently, latexes were designed to contain relatively high concentrations of carboxylic acid groups and/or sulfonic acid groups on the surface of the particles in order to observe the causal effect of each group, as well as combined contributions. The placement of either one or both acid groups on different latexes and the amounts used in this study, appeared to be an appropriate approach to investigate very essential parameters influencing film formation of acid-catalyzed reactive latex systems. Such latex dispersions usually contain carboxylic acid groups as the reactive moiety (among other such as hydroxyl groups) and strong acids as catalysts. Amines are typically introduced in these systems for stabilization, pH adjusting and blocking of acid groups to prevent
111 premature crosslinking reactions. It was anticipated that the presence of these
polar groups on the surface of the particles would interfere with drying, packing
and coalescence of particles during film formation.
The choice of amines for this study relied on the desire to consider amine
properties such as volatility and base strength, while avoiding reactivity of amines
towards the polymer. Reactivity of primary amines and amino-alcohols towards
ester groups in the polymer was found to be a possible influencing factor in
amine loss from reactive water-reducible polymers. [97] Consequently, tertiary
amines such as Trimethylamine (TMA), N-methyl-morpholine (NMM) and N,N-
dimethyl-1-butanamine (DMBA) were chosen to contrast differences in boiling
point and base strength. Table 5.3 presents physical properties of the amines
mentioned.
To study the influence of acid/base systems on the film formation of latex
dispersions, the effect of other parameters had to be eliminated. Therefore, latex
particles were cleaned to remove excess surfactant and other ionic materials
present in the aqueous phase after synthesis. The latex particle sizes, particle
size distributions and glass transition temperatures were comparatively similar
among the group as observed in Table 5.2. For instance all latexes particle sizes were in the vicinity of 290 nm and polydispersity indices showed similar distributions of particle size. Upon cleaning, latexes were titrated to obtain charge densities on the particles surface and subsurface. Forward titration of
112 LCA allowed measuring acid groups immediately available on the surface of the
particles (small amount of strong acids from initiator fragments and surfactant).
The back titration allowed detecting a larger number of groups buried underneath
the surface which became available upon addition of the base (weak acids). This
was in agreement with similar carboxylated latexes studied in the past. [45] For
LSA the number of acid groups detected in the forward titration was very close to
that detected in the back titration, which indicated that all titratable groups in these particles were immediately available on the surface of the particle. For
LCSA, the charge density measured during the forward titration was much larger than in LCA, indicating a higher amount of strong acid groups present on the surface of LSA particles compared to LCA particles. In this case similarly to
LCA, the number of acid groups measured during the backward titration was larger than during the forward titration since buried acid groups became available upon addition of the base.
5.3.1 Drying behavior
Film formation has been studied in latexes prepared from acrylic and methacrylic ester monomers and in latexes containing polar materials on the
surface of the particle. [45,93] It was found that drying parameters and
interdiffusion of polymer chains in latexes was influenced by the presence of
carboxylic acid groups. The characteristics of the polar material (acid/base)
located at the surface of latex particles prepared for this study, are expected to
113 influence drying parameters as well. Such differences in drying parameters will
lead to important information on the acid-base system, which is of great
significance in the developing of acid-cure crosslinking reactions in this type of
latexes.
Table 5.3 Physical properties of amines used for neutralization. N-Methyl morpholine N,N-dimethylbutyl amine Trimethylamine
N Structural N N Formula O
M.W. (g/mol) 101.15 a 101.19 c 59.11 d
B.P. ( °C) 112.2 a 95 c 2.87 d
Solubility in water (g/L) Soluble 35.2 c Soluble
b b b pKb 6.62 3.81 4.2
a From reference 98 b From reference 99 c From reference 75 d From reference 100
Weight loss from latex films was recorded for up to 24 h in order to
observe the water loss to a very large extent (> 99 wt %). Figure 5.1 shows a
typical cumulative weight loss curve corresponding to a latex film drying at 30 °C.
The drying behavior was characterized by two parameters: 1) rate of drying and
2) total drying time. The rate of drying was calculated from the slope of the weight loss curve during the first stage of drying when the rate was constant.
The second parameter used to compare the drying behavior of these latexes was the total time needed to lose 99 wt % of the water from films. Figure 5.2 shows the dependence on temperature of drying rates and total times needed to lose 99
114 wt. % of water for films cast from latexes containing carboxylic acid and/or sulfonic acid groups. At low temperature (30 °C) there was little difference on the drying rate between latexes containing distinct acid groups on the surface of the particles. However, as drying temperature was higher, the drying rate of the latex containing sulfonic acid groups (LSA) seemed higher than for the other latexes. Comparing the total time needed to lose a determined amount of water a trend was observed. It indicated that loss of 99 wt % of water occurred later for latexes containing carboxylic acid groups than for latexes containing sulfonic acid groups alone or even both types of acids.
50 0.5
0.4 40
0.3 30 0.2 20 0.1
10 rateEvaporation (mg/min) 0.0 Cumulative weigth loss (wt loss (wt %) weigth Cumulative
0 -0.1 0 20 40 60 80 100 120 140 160 180 Time (min)
Figure 5.1 Cumulative weight loss and evaporation rate of latex LCA.
Samples of each of the latexes were neutralized by adding equivalent amounts of amine to acid groups measured by titration (corresponding to groups
located on the surface and subsurface of the particles). Drying rates and times required to lose 99 wt % of water were also measured for neutralized latexes. 115 Figure 5.3 shows drying rates of LSA, LCA and LCSA (Figures 5.3a, 5.3b and
5.3c respectively) neutralized with TMA, DMBA or NMM. Neutralization with
different amines caused differences in drying rates of LCA samples. The TMA
neutralized sample showed higher drying rates than DMBA or NMM neutralized samples. Variations in drying rate between samples neutralized with different
amines were not as pronounced in the case of LSA latex and were completely
absent in LCSA neutralized samples. In addition to drying rates, the time
elapsed during the loss of 99 wt % of water from the films was also recorded for
neutralized latexes.
a) 350
300
(g/min) 250 -5 200
150
100 LCA LSA 50 LCSA Rate of Drying * 10 * Drying of Rate 0 20 30 40 50 60 70 80 90
Temperature (°C) b) 400
LCA 300 LSA LCSA 200
100
Time to lose 99 wt % water (min) % Time to water 99 wt lose 0 20 30 40 50 60 70 80 90 Temperature (°C)
Figure 5.2 Drying parameters for latexes containing carboxylic acid and/or sulfonic acid groups. a) Rate of drying and b) time needed to loose 99 wt% of water
116 400 a) LCA + TMA 350 LCA + DMBA LCA + NMM
(g/min) 300
-5 250 200 150 100 50 Rate of Drying * 10 0 20 30 40 50 60 70 80 90
b) 350 Temperature (°C) 300
(g/min) 250 -5 200
150
100 LSA + TMA LSA + DMBA 50 LSA + NMM Rate of Drying * 10 * Drying of Rate 0 20 30 40 50 60 70 80 90 c) 300 Temperature (°C)
250 (g/min)
-5 200
150
100 LCSA + TMA 50 LCSA + DMBA LCSA + NMM Rate of Drying * 10 * Drying of Rate 0 20 30 40 50 60 70 80 90 Temperature (°C)
Figure 5.3 Rate of drying of acid functionalized latexes neutralized with three tertiary amines. Latexes are a) LCA b) LSA and c) LCSA.
117 a) 200 LCA + TMA LCA + DMBA 150 LCA + NMM
100
50 Time (min) water to lose 99 wt% 0 20 30 40 50 60 70 80 90
b) Temperature (°C) 160
140 LSA + TMA 120 LSA + DMBA LSA + NMM 100
80
60
40
20 Time (min) water to lose 99 wt% 0 20 30 40 50 60 70 80 90
c) Temperature (°C)
500
LCSA + TMA 400 LCSA + DMBA LCSA + NMM 300
200
100 Time (min) water loose wt% to 99 0 20 30 40 50 60 70 80 90 Temperature (°C) Figure 5.4 Time needed to lose 99 wt% of water from acid functionalized latexes with three amines. Latexes are a) LCA b) LSA and c) LCSA.
118 Figure 5.4 shows the times for all neutralized latexes. Different trends were observed depending on the type of acids present on the particles. Drying times of neutralized LCA samples were quite influenced by the type of amine used. The NMM neutralized LCA sample had the longest drying time, while TMA showed the shortest of the three. In the case of latexes LSA and LCSA, samples neutralized with different amines did not show major changes on drying times. It was observed, however that the times required to lose 99 wt % of water for neutralized LCSA samples were in general much longer than for neutralized LSA and LCA samples.
5.3.2 pH development
Samples of un-neutralized latexes were examined for pH changes during the initial drying stages up to the point were stable pH readings were no longer possible. In that moment, the water content in the films was so low that continuity of the aqueous phase was compromised and the reading was no longer steady. Figure 5.5a shows pH readings obtained from un-neutralized
LCA, LSA and LCSA. Although the changes in pH were subtle, it was observed that a larger decrease in pH was obtained for the latex containing mostly carboxylic acid groups compared to the other latexes. The pH behavior of latexes containing both carboxylic and sulfonic acid groups (LCSA) and only sulfonic acid groups (LSA) were similar to each other.
119 a) b) 5 6
LCA 4 LSA 5 LCSA 3 4 pH pH 2 LCA neutralized with TMA 3 1 LCA neutralized with DMBA LCA neutralized with NMM
0 2 0 20 40 60 80 100 120 0 20 40 60 80 100 120 c) Time (min) d) Time (min)
6 6
5 5
4 4 pH pH
LSA neutralized with TMA LCSA neutralized with TMA 3 LSA neutralized with DMBA 3 LCSA neutralized with DMBA LSA neutralized with NMM LCSA neutralized with NMM
2 2 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Time (min) Time (min)
Figure 5.5 Development of pH in drying films. a) neat acidic latexes LSA and LCA, b) neutralized samples of latex containing carboxylic groups and c) neutralized samples of latex containing sulfonic acid groups.
Neutralized samples of the latexes were also monitored for pH changes during drying. The pH development obtained for neutralized LCA, LSA and
LCSA samples is shown in Figure 5.5b, 5.5c and 5.5d, respectively. The initial pH was acidic as it would be expected at the equivalent point of this type of acid/base reactions in aqueous media. [45] Changes in pH were similarly small
for samples neutralized with NMM and DMBA. The variation of pH was more
pronounced for the TMA neutralized sample. For LSA, no significant differences 120 were observed between pH readings of samples neutralized with TMA, DMBA or
NMM. All readings seemed to be almost constant throughout the measurement.
The behavior of neutralized LCSA samples was very similar to that of neutralized
LSA samples. Varying properties of the amines did not seem to affect the
progress of pH during the initial drying stages of latexes containing sulfonic acids
or combined sulfonic and carboxylic acid groups.
5.3.3 Amine evolution
As water evaporates from latex films during drying, it is reasonable that
some portion of the amine is also lost, depending on base volatility and drying temperature. In order to monitor the amount of amine lost during drying, and the
effect of volatility and base strength on this process, amine loss was measured
from drying films. Figure 5.6 presents amine losses for LCA and LSA latexes at
30 and 60 °C. Similarly as observed in the weight loss curve in Figure 5.1, at 30
°C most of the amine loss occurred between the initial 80 to 100 min of drying.
For both types of acid groups the portion of amines retained in the films after 2 h
was quite large, above 85 wt. %. In the case of latexes containing carboxylic
acid groups, TMA seemed to evolve in larger quantities than DMBA and NMM,
about 10 wt. % for TMA compared to 2 wt. % for DMBA and 4 wt. % for NMM.
When the drying temperature was increased to 60 °C, only a slight increase in
amine losses was observed. Neutralized samples of latex LSA showed lower
amine losses compared to neutralized LCA samples. A larger percentage of
121 TMA was lost in larger proportion than DMBA and NMM (about 7 wt. %),
however this amount was still lower than in the case of neutralized carboxyl
functional latex samples, LCA.
a) b)
100 100
80 80
60 60 TMA TMA DMBA DMBA
wt % amine % retained wt 40 amine % retained wt 40 NMM NMM
20 20 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140
Time (min) Time (min)
c) d)
100 100
80 80
60 60 TMA TMA DMBA DMBA wt% amineretained wt%
40 amine % retained wt 40 NMM NMM
20 20 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Time (min) Time (min)
Figure 5.6 Evolution of amines from latex films. a) neutralized LCA samples at 30 °C, b) neutralized LCA samples at 60 °C, c) neutralized LSA samples at 30 °C and d) neutralized LSA samples at 60 °C
5.3.4 Further film aging
After the first stages of film formation, when most of the water and other volatiles are lost, subsequent processes of drying, deformation and particle
122 coalescence lead to a close-packed, solid film. One technique that has been
recently used for qualitatively as well as quantitative studies of coalescence
during maturation of latex films is atomic force microscopy (AFM). [Error!
Reference source not found.-104] In order to evaluate the effect of both acid
and base groups on later stages of the film forming process, AFM was used to
follow film morphology as function of time. Representative images obtained from
a LCA samples are shown in Figure 5.7. The effect of particle coalescence
occurring in the films as time elapsed was clearly observed in the images by the
loss of contrast corresponding to a decrease in surface roughness. Furthermore, statistical analysis of the roughness allowed for comparison of latexes surfaces during the maturation process. Figure 5.8 summarizes the variation in roughness
(RMS) as a function of time for all systems. The decrease in RMS established that coalescence occurred in the films to a certain extent, although not completely even after several weeks. The analysis also showed that coalescence occurred to a higher extent for LSA than for LCA films as observed from the total decrease in roughness.
Figure 5.7 AFM height images of LCA latexes aging during a period of 21 days.
123
a) 20
15
10 RMS (nm) LCA 5 LSA
0 0 5 10 15 20 25 b) 30 Time (days)
25
20
15
RMS (nm) 10 LCA LCA + TMA 5 LCA + DMBA LCA + NMM 0 0 5 10 15 20 25 c) 18 Time (days) 16 14 12 10 8 RMS (nm) 6 LSA 4 LSA + TMA 2 LSA + DMBA LSA + NMM 0 0 5 10 15 20 25
Time (days)
Figure 5.8 RMS roughness of AFM height images during aging at 30 °C. a) neat LCA and LSA, b) neutralized samples of LCA and c) neutralized samples of LSA. 124 a) 18 16 LCA LSA 14 12 10 8
RMS (nm) 6 4 2 0 0 20406080100120140160
Time (h) b) 16 14 12 10 8 LCA
RMS (nm) 6 LCA + TMA 4 LCA + DMBA 2 LCA + NMM 0 0 20 40 60 80 100 120 140 160
Time (h) c) 16 LSA 14 LSA + TMA 12 LSA + DMBA 10 LSA + NMM 8
RMS (nm)RMS 6 4 2 0 0 20 40 60 80 100 120 140 160
Time (h) Figure 5.9 RMS roughness of AFM height images during annealing at 60 °C. a) neat LCA and LSA, b) neutralized samples of LCA and c) neutralized samples of LSA 125
Neutralized samples of latexes LCA and LSA were also monitored during
aging. The initial roughness of the LCA sample neutralized with DMBA was
lower than samples neutralized with the other amines. Figure 5.8b shows the
decrease in surface roughness for LCA samples neutralized with TMA, DMBA or
NMM. The film roughness decreased more pronouncedly during the first two
days of maturation for DMBA neutralized LCA latex than for samples neutralized
with any of the other amines. Similar trends were observed for neutralized
samples of latex LSA. DMBA neutralized samples had lower initial surface
roughness than other amines, and the decline was more evident during the initial
days. As expected, increasing drying temperature (60 °C) promoted surface
flattening during the initial stages of film formation as evidenced from the initial
surface roughness values. Similarly to samples aged at 30 °C, DMBA
neutralized samples showed lower roughness values than other samples
neutralized with TMA and NMM.
5.4 Discussion
The principal objective of this work was to understand the dependence of film forming properties on acid-base interactions occurring in acrylic latexes.
Initially, it was important to observe the effect of two different acid groups during
drying and on later stages of film formation. Moreover, the addition of
neutralizing agents usually used in these systems was anticipated to introduce
126 variations in comparison with un-neutralized latexes. [101] It was suspected that
the presence of acid and base groups on the aqueous phase and on the surface of the particles would influence drying parameters and further coalescence of particles. [46,93]
Upon examination of drying parameters observed during the first stages of
drying from un-neutralized samples, the time for losing water was shorter from
the latex containing sulfonic acid groups than from latexes containing carboxyl
groups or both types of acids. This may be explained by hydrogen bonding
effects. In the presence of water, carboxylic acid groups are only partially
ionized. [105] Undissociated carboxylic acid molecules involve in hydrogen bonding with water molecules more strongly than in the case of sulfonic acid
groups which being completely ionized in water undergo hydrogen bond
formation through the hydration of the anions formed. [106] The stronger
hydrogen bonding between water and undissociated carboxylic acid groups
decreases the activity of water. Consequently, the partial vapor pressure of
water is lowered, the drying rate is decreased and the time for evaporation of
most of the water is longer than in the case when sulfonic acid groups are present.
The drying behavior of neutralized samples was marked by two features of
the latexes related to acid strength and base volatility. First, the higher acid
127 strength of sulfonic acid groups compared to carboxylic acids1 appeared to be
more controlling during drying, than properties of the amines such as volatility or
base strength. This was evidenced by the comparatively unchanged drying rates
and times observed for LSA and LCSA samples neutralized with the three
amines. It is most likely that upon dissociation of the strong acids in contact with
water, the ion pairs formed with amines were more strongly bonded compared to
ion pairs formed by carboxylic acid groups. The second feature was observed
only for LCA samples. Drying parameters showed that volatility of TMA seemed
to favor drying of neutralized LCA samples compared to the other amines.
The development of pH within un-neutralized drying films showed that the
pH decline was more pronounced for films cast from LCA, than from LSA or
LCSA samples, as shown in Figure 5.5a. It is possible that such differences in
pH behavior would be a consequence of plasticization effects of water in
hydrophilic polymers. Winnik et al. [109] found that water has much larger effect
on the diffusion of hydrophilic copolymer chains of MAA and butyl methacrylate
(BMA) than on the diffusion of hydrophobic PBMA molecules, possibly due to
plasticization effects during annealing. In this case, latex LCA contained a large amount of carboxylic acid groups that imparted certain hydrophilic character to the polymer. Water surrounding the polymer located on the outer layers of the
particles would be expected to cause plasticization of these molecules. During
drying and deformation of particles, the presence of water may increase diffusion
1 For aromatic sulfonic acids pKa value is about -6.5, from reference [107]. For carboxylic acids, pKa ~ 4 to 5 from reference [108] 128 across boundaries of the particles promoting the uncovering of carboxylic acid
groups buried underneath the outer layers of the particle. The release of these
groups towards the aqueous media would be evidenced by reduced pH values
observed. The lower initial pH and the decline of pH during drying of LSA and
LCSA films related to the fact that the majority of sulfonic acid groups were
located already on the surface of the particles, as found from acid titration. This explained the sensitivity of pH to additional uncovered carboxylic groups rearranging towards the surface, and the slight pH change when strong sulfonic acid groups were present.
The pH behavior of neutralized samples of carboxyl functional latex was
dependent on the type of amine used for neutralization. It was expected that the
pH behavior would be related to the strength of the base as well as to volatility
and solubility. In these samples, the higher volatility of TMA promoted a larger
pH reduction during drying than was the case for DMBA and NMM. Upon
release of some initial free TMA, the acid-base equilibrium would shift towards
the dissociated ion pair, releasing a portion of protonated carboxylic acid groups,
which decreased the pH in the dispersion
- + RCOOH + N(CH3)3 RCOO + H N(CH3)3 (5.1)
Conversely, lower volatility of DMBA and NMM did not contribute to major release of carboxylic acid groups. However, presence of DMBA seemed to 129 cause a slightly more rapid pH decrease than for NMM neutralized samples.
Although these two amines have relatively close boiling points, it appears that during drying the limited water solubility of DMBA had provoked rearrangement of molecules towards the inside of the particles. On the other hand, pH profiles obtained from latexes containing sulfonic acid groups or both carboxyl and sulfonic acid groups, exhibited very little variation. This could be a result of the high strength of sulfonic acid groups compared to carboxylic acid groups.
Although the presence of TMA caused a slightly more decreased pH, volatility of this amine did not overcome the inherently strength effect of the sulfonic acid on the acid-base equilibrium (see equation 5.2). The conjugate base of the sulfonic acid is so very weak compared to un-dissociated amines present in the aqueous media that re-protonation of the conjugate base to obtain the acid would be very unlikely.
RSO - + H +N(CH ) RSO3H + N(CH3)3 3 3 3 (5.2)
The amine evolution from neutralized carboxyl functional latexes was favored by volatility of the amine and only secondarily by consideration of base strength. The more volatile TMA was lost in higher proportion than NMM and
DMBA. Even though NMM is a relatively weaker base than DMBA, the acid-base equilibria between carboxylic acid groups and any of these two amines appeared to have remained towards the conjugated species since no difference in evolution was observed. Thus, volatility of the amine controlled amine evolution 130 from carboxylic functional latex samples. On the other hand, almost similar interactions between sulfonic acid groups and each of the neutralizing amines could be suggested from the amine evolution results. For all neutralized samples, amine losses were less significant than in the case of carboxyl functional latex samples, and very similar between each other. Lower volatility of
NMM and DMBA was still evidenced by the slightly reduced amine losses. The strong character of the sulfonic acid seemed to have overcome any small difference in base strength among the group of amines. The equilibria of all three bases were shifted towards the salt formed due to the unlikely re- protonation of the conjugate base.
Although during aging film flattening evidenced coalescence in both latexes bearing different acid groups, complete extent of coalescence was not achieved even after several days of maturation at 30 °C or several hours at 60
°C. This was in agreement with aging processes in which coalescence was retarded for latexes containing carboxylic acid groups. [45] The lower roughness values for samples bearing sulfonic acid groups could be related to the fact that the outer layer of particles in LSA latex must contain a thinner layer of hydrophilic groups than LCA latex. These layers act as “barriers” against interdiffusion of polymer chains. [110] Possibly, chain motion across particles boundaries in LCA latex would have been more restricted than in LSA particles affecting the extent of coalescence.
131 Upon addition of neutralizing amines to acidic latexes, surface roughness measurements showed very small differences between samples neutralized with
TMA than with the less volatile, weaker base NMM. On the contrary, overall roughness values were much lower for samples neutralized with DMBA. This would indicate that higher level of coalescence occurred for DMBA neutralized samples, which would not be expected with respect to volatility or even base strength. Although DMBA has the higher boiling point and the lowest pKb of the amines, it has limited water solubility. This could be an important factor after drying, when residual water and other hydrophilic materials are confined to hydrophilic channels. It is commonly known that chain interdiffusion across interfaces develops with time and/or temperature and promotes randomization of the initially ordered symmetric interfaces. Changes in polymer chains conformations produce an entropic driving force for further diffusion of chains. It is possible that migration of DMBA molecules away from hydrophilic domains would have driven rearrangement of polymer chains, possibly promoting interdiffusion across particle boundaries. This would not have been the case with water soluble amines TMA and NMM.
5.5 Conclusions
Addition of specific acid and/or base functional groups to acid catalyzed acrylic latex dispersions is not only expected to facilitate crosslinking or to result in better stabilization of the colloidal systems; the presence of these species was
132 observed to affect film formation due to acid-base interactions occurring with the aqueous media. During early stages of drying, the effect of base volatility was evident; however, at later stages of maturation when volatile losses are no longer significant, base volatility ceased to be a controlling factor. At ambient temperatures, acid strength and amine volatility were the determinant factors in de-blocking processes of acid sites. For a weak acid, de-blocking is favored and accelerated by volatility of the base. However, in the case of a strong acid, base volatility is no longer a factor as the acid-base equilibrium would be favored towards the ion pair and not towards the free species.
133 CHAPTER VI
CONCLUSIONS
In thermosetting latex systems the development of film properties such as mechanical strength and solvent resistance occurs during coalescence of particles, not only through interdiffusion of polymer chains, but also by crosslinking of the polymer prior, during and after film formation. Many aspects of latex design and raw materials’ properties play important roles during synthesis and application of crosslinkable latex dispersions. Two of the most important parameters that can be varied for many purposes are the type and amount of functionality entered in the system. The following is a summary of the findings and conclusions originated from studying several aspects of the introduction of functionality into crosslinkable acrylic latexes.
In order to achieve enhanced strength and resistance of latex films, crosslinking should be maximized through the incorporation of an increased concentration of functional groups. However, the presence of highly polar groups such as hydroxyl and carboxylic acid moieties was expected to not only influence polymerization pathways but also latex and film properties. In this fashion, core- shell latexes with increasing functionality content were prepared, characterized 134 and used for preparation of films. The maximum possible content of functional monomers incorporated was determined by colloidal stability of the system during polymerization. At this maximum content two particle populations were obtained. It was proposed that the increased content of hydrophilic HEMA in the aqueous phase boosted secondary particle formation through homogeneous nucleation. As expected, mechanical strength and extent of crosslinking in films were favored by the presence of increased functionality. Film strengthening was also observed even when crosslinker was not added to the latexes, which was explained as a result of strong intermolecular attractions within the polymer, i.e. hydrogen bonding. It was suggested that the presence of different particle sizes
(product of the secondary nucleation) enhanced packing within films and created propitious conditions for increased interactions between reactive groups.
As a result of the findings previously mentioned, bimodal latexes were further studied in light of the advantages that particles of different sizes bring on packing of traditional latexes. Such improvement in packing has been proven to occur at a volume fraction of small particles such that a continuous phase of these particles is formed around the large ones. This fraction is known as the critical volume fraction. In spite of many previous documented investigations in the literature on bimodal thermoplastic latex dispersions, the incorporation of functionality and the subsequent addition of crosslinking resins to this type of systems had not been the subject of much study. Latex dispersions with varying particle sizes and functionality placement were prepared. In fully functionalized
135 small/large latex blends the presence of hydroxyl groups did not affect particle packing with respect to unfunctionalized latexes. Upon incorporation of the crosslinking resin, mechanical and thermo-mechanical properties of films revealed that enhancement of properties was observed at a higher fraction than observed for un-crosslinkable latexes. It was suggested that the formation of the continuous phase or network of small particles had been affected due to the presence of the resin. Partially functionalized bimodal latex dispersions allowed evaluating the effect of adding the crosslinking resin to either type of particle.
When functionality was placed in the small particles it was observed that strengthening of the films occurred at a lower fraction than in the case of fully functionalized latexes, but it was still higher than the fraction observed for un- functionalized latexes. These films were considered as “reinforced” structures conformed by un-crosslinked domains surrounded by a crosslinked network.
An important factor influencing film formation and crosslinking in acid- catalyzed crosslinkable latexes is the incorporation of weak acids (usually as one of the reacting functionalities), strong acids (as catalysts) and blocking agents
(typically weak bases), as well as the interactions occurring among these groups.
Variables such as acid strength, as well as chemical and physical properties of the base used for neutralization of acid groups are particularly important for crosslinking via acid-catalyzed reactions. These variables are believed to be related to phenomena occurring during film formation such as drying and coalescence of particles. In order to study these variables and the effect that
136 acid-base interactions might have on film formation, latexes with either weak or strong acids, or both, were prepared. These latexes were studied during drying and later stages of film maturation, in the acid form, as well as neutralized by one of three amines. These amines were chosen with variations in physical properties such as base strength, boiling point and solubility. During drying it was observed that the presence of carboxylic acids decreased drying rates at temperatures higher that ambient. The presence of sulfonic acid groups however, caused an increase in drying rates with respect to carboxylic acids. This was attributed to differences in the strength of the hydrogen bonding present between protonated and dissociated species of each acid in aqueous solution. During drying, the effect of base volatility was more pronounced than at later stages of film aging.
After that stage, loss of water and volatiles is rather insignificant and base volatility is no longer a controlling factor. Acid strength and amine volatility were found to be determinant factors in the de-blocking of acid groups at ambient temperature. For the weak acid, de-blocking was favored when a volatile base was used; however, in the case of the strong acid, base volatility did not cause the same effect. The acid-base equilibrium of the system would likely be favored towards the ion pair instead of the free species in every case.
6.1 Future Work
The aim of bimodal latex blends in producing films with optimized packing characteristics proved beneficial for crosslinkable systems by allowing
137 strategically placement of functional groups in the films. Like this, it would seem that controlled secondary nucleation during synthesis would be a promising area of study with the purpose of achieving systems with specific distributions of particle size and functionality obtained in situ. This would be preferable instead of separate preparation of dispersions with varying characteristics and subsequent mixing to obtain bimodal partially functionalized blends. In a future work the controlled preparation of such systems could be attained by first attempting to theoretically model secondary nucleation during the synthesis process and then corroborating experimentally the range of conditions that would lead to such generation of new particles.
The investigation of bimodal latex dispersions showed that some degree of phase separation occurred between small and large particle populations, possibly due to different polymer composition and the presence of the crosslinking resin. Accordingly, if bimodal particle size dispersions could be prepared with different characteristics, self-stratification of layers with gradient properties could be induced by external driving forces. For instance, dispersions bearing not only functional groups but other types of active species such as magnetic or metallic particles or just partially miscible copolymers could be manipulated to form layered films through the choice of substrate. Future work exploring these possibilities could lead to many interesting functionally gradient structures.
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