MODIFICATION OF TUNG OIL FOR BIO-BASED COATING
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Narin Thanamongkollit
August, 2008 MODIFICATION OF TUNG OIL FOR BIO-BASED COATING
Narin Thanamongkollit
Thesis
Approved: Accepted:
______Advisor Dean of the College Dr. Mark D. Soucek Dr. Ronald F. Levant
______Faculty Reader Dean of the Graduate School Dr. Wiley J. Youngs Dr. George R. Newkome
______Department Chair Date Dr. Kim C. Calvo
ii ABSTRACT
Tung oil was used as a diene for modification with acrylate dienophiles via a
Diels-Alder reaction. In this thesis, the research was divided into two related parts. In the
first part, UV-curable resins were prepared from tung oil and tung oil alkyd for a high
solids coating application. In the second part, tung oil alkyd was modified with three
different acrylate monomers, possessing either alkoxysilane, triallyl ether, or fluorinated
groups. The structures of the modified tung oil and alkyds were characterized by 1H
NMR, 13C NMR, MALDI-TOF mass spectroscopy, and gel permeation chromatography
(GPC).
Two UV-curable tung oil-based resins in the first part were synthesized via the
Diels-Alder cycloaddition. A UV-curable Tung Oil (UVTO) was prepared from bodied tung oil and trimethylolpropane trimethacrylate (TMPTMA) by a two-step reaction.
Bodied tung oil was primarily prepared by treatment at high temperature, and then reacted with TMPTMA on the α-eleosterate of tung oil triglyceride via the Diels-Alder
reaction. An inhibitor, phenothiazine, was added to avoid homopolymerziaton of
TMPTMA. UV-curable Tung Oil Alkyd (UVTA) was prepared by the monoglyceride
process, and then reacted with TMPTMA via the Diels-Alder reaction. The UVTO and
UVTA were formulated with a free radical reactive diluent, tripropylene glycol diacrylate
(TPGDA) and photoinitiator Irgacure 2100. Photo Differential Scanning Calorimeter
iii
(Photo-DSC) was used to investigate the curing kinetics of the UVTO and the UVTA.
The data showed that the UVTA formula was cured faster than to the UVTO formula.
In the second part, the α-eleosterate pendent fatty acid of tung oil alkyd was
functionalized via a Diels-Alder reaction with three different acrylate groups, 2,2,2-
trifluoroethyl methacrylate, 3-methacryloxypropyl trimethoxysilane, and triallyl ether
acrylate. Drying time and viscoelastic properties of the alkyd-modified film were investigated. Dynamic Mechanical Thermal Analysis (DMTA) was employed to evaluate the viscoelastic properties of the alkyd-modified films. The viscoelastic and drying time result shows that the alkyd modified with siloxane and triallyl ether group shows a faster drying time, higher crosslink density, and higher glass transition temperature compared to the unmodified alkyd, whereas the fluorinated alkyd showed surface active properties, but lacks in drying and crosslink density.
iv
TABLE OF CONTENTS
Page
LIST OF TABLES ...... viii
LIST OF FIGURES ...... ix
CHAPTER
I. INTRODUCTION ...... 1
II. LITERATURE REVIEW ...... 4
2.1 Drying Oils[1] ...... 4
2.2 Tung Oil[19] ...... 7
2.3 Heat Treatment of Drying Oil[1] ...... 9
2.4 Oxidative Curing and Drying Mechanisms[1] ...... 10
2.5 Alkyd Resin ...... 12
2.6 Diels-Alder reaction ...... 15
2.7 Reactive Diluents ...... 17
2.8 UV Cure Coating[31] ...... 18
2.9 Sol-gel Chemistry ...... 20
2.10 Fluorinated Polymers ...... 22
III. EXPERIMENT ...... 24
v
3.1 Materials ...... 24
3.2 Synthesis of UV-Curable Tung Oil (UVTO) ...... 25
3.3 Synthesis of UV-Curable Tung Oil Alkyd (UVTA) ...... 26
3.4 Synthesis of fluorine functionalized tung-alkyd (FTO-Alkyd) resin ...... 28
3.5 Synthesis of Siloxane functionalized tung-Alkyd (SFTO-Alkyd) resin ...... 30
3.6 Synthesis of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin ...... 31
3.7 Formulation and Curing Kinetic of UV-Curable materials (Chapter IV) ...... 33
3.8 Formulation and Film Preparation of modified alkyd (Chapter V) ...... 34
3.9 Instrument and Characterization ...... 34
IV. SYNTHESIS OF UV-CURABLE TUNG OIL BASED RESIN ...... 36
4.1 Introduction ...... 36
4.2 Characterization of UV-Curable Resins ...... 38
4.3 Evaluation of Curing Kinetic ...... 51
4.4 Discussion ...... 52
4.5 Conclusion ...... 54
V. SYNTHESIS AND PROPERTIES OF ACRYLATE FUNCTIONALIZED ALKYD...... 55
5.1 Introduction ...... 55
5.2 Characterization result of modified alkyd ...... 56
5.3 Drying time study ...... 67
vi
5.4 Viscoelastic Properties ...... 68
5.5 Contact Angle ...... 70
5.6 Discussion ...... 72
5.7 Conclusion ...... 74
VI. CONCLUSIONS ...... 75
REFERENCES ...... 76
vii
LIST OF TABLES
Table Page
1 Typical Fatty acid compositions of vegetable oils ...... 6
2 The assigned mass spectra of bodied tung oil ...... 25
3 The assigned mass spectra of acrylated tung oil ...... 25
4 1H-NMR Chemical Shift of acrylated tung oil ...... 26
5 13C-NMR Chemical Shift of acrylated tung oil ...... 26
6 1H-NMR Chemical Shift of acrylated tung oil alkyd ...... 28
7 13C-NMR Chemical Shift of acrylated tung oil alkyd ...... 28
8 1H-NMR Chemical Shift of fluorinated tung oil alkyd ...... 29
9 13C-NMR Chemical Shift of fluorinated tung oil alkyd ...... 29
10 1H-NMR Chemical Shift of siloxane tung oil alkyd ...... 31
11 13C-NMR Chemical Shift of siloxane tung oil alkyd ...... 31
12 1H-NMR Chemical Shift of triallyl ether tung oil alkyd ...... 33
13 13C-NMR Chemical Shift of triallyl ether tung oil alkyd ...... 33
14 Drying time at 25 oC ...... 68
15 Viscoelastic properties of the alkyd-modified cure films ...... 70
16 Contact angle measurement ...... 71
viii
LIST OF FIGURES
Figure Page
1 General triglyceride structure ...... 5
2 Autoxidative Mechanism of Drying Oil ...... 10
3 Catalyzed decomposition of hydroperoxides ...... 11
4 Monoglyceride process of alkyd preparation ...... 13
5 Fatty acid process of alkyd preparation ...... 14
6 General Diels-Alder Reaction ...... 15
7 The rotation of s-trans to s-cis conformation ...... 16
8 The formation of free radical in photoinitiator ...... 18
9 Photopolymerization path of free radical system ...... 19
10 Depiction of the sol-gel reactions ...... 20
11 Acid catalyzed Hydrolysis mechanism of alkoxysilane ...... 21
12 Base catalyzed hydrolysis mechanism of alkoxysilane ...... 21
13 Phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide) ...... 24
14 a.) Reaction of Resole[38] b.) Reaction of Novolac [39] ...... 37
15 Reaction scheme of UV-Curable Tung Oil (UVTO) ...... 40
16 Mass spectra of bodied tung oil ...... 41
17 Mass spectra of acrylated tung oil ...... 41
18 1H-NMR Spectra of (a) Bodied tung oil (b) Acrylted tung oil ...... 42
ix
19 13C-NMR Spectra of (a) Bodied tung oil (b) Acrylated tung oil ...... 43
20 Tung oil modified phenolic resin ...... 44
21 Reaction scheme of UV-Curable Tung-Based Alkyd (UVTA) ...... 45
22 Synthesis path of a monoglyceride process of acrylate functionalized tung oil ...... 47
23 Synthesis path of acrylate functionalized tung oil monoglyceride ...... 47
24 1H-NMR Spectra of (a) Tung oil alkyd (b) Acrylated tung oil alkyd ...... 49
25 13C-NMR Spectra of (a) Tung oil alkyd b) Acrylated tung oil alkyd ...... 50
26 Thermogram of a.) UV-Curable Tung Oil (UVTO) formula and b.) UV-Curable Tung oil Alkyd (UVTA) formula ...... 51
27 Autoxidation of allyl ether ...... 58
28 A crosslinking network of triallyl ether pendent group ...... 58
29 1H NMR of Tung Oil Alkyd ...... 59
30 13C NMR of Tung Oil Alkyd ...... 59
31 Reaction of Fluorine functionalized tung-alkyd (FTO-Alkyd) resin ...... 60
32 1H NMR of Fluorine functionalized tung-alkyd (FTO-Alkyd) resin ...... 61
33 13C NMR of Fluorine functionalized tung-alkyd (FTO-Alkyd) resin ...... 61
34 Reaction of siloxane functionalized tung-alkyd (SFTO-Alkyd) resin ...... 63
35 1H NMR of siloxane functionalized tung-alkyd (SFTO-Alkyd) resin ...... 63
36 13C NMR of siloxane functionalized tung-alkyd (SFTO-Alkyd) resin ...... 64
37 Reaction of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin ...... 65
38 1H NMR of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin ...... 66
x
39 13C NMR of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin ...... 66
40 Modulus (E’) as a function of temperature for the modified alkyd system ...... 69
41 Tan δ as a function of temperature for the modified alkyd system ...... 69
42 Contact angle measurement of FTO-Alkyd film ...... 71
43 Oligomerization of drying oil via a Diels-Alder reaction ...... 72
xi
CHAPTER I
INTRODUCTION
Seed oil, such as tung oil and linseed oil, has been used as a varnish coating for
centuries. A number of methods including bodying process, blown oil process, and alkyd
synthesis, have been employed to treat the natural seed oil for a coating purpose.[1]
Various types of additive are also incorporated during the oil treating step in order to improve the coating properties. Treating the seed oil with phenolic resin at certain temperature, for instance, will enhance the durability, water resistance, and chemical resistance of the coating.[2]
In the coating application, the most significant factor of the varnish-based coating is its relatively high viscosity. Therefore, the introduction of organic solvent in the varnish formula is required to adjust the viscosity application. However, the volatility of the organic solvent has been an environmental as well as toxic issue for decades.[3]
Several researches have also been reported concerning the possibility of lung cancer and respiratory system problem due to the inhalation of volatile organic compounds (VOCs) into the human organism.[4-7] From these problems, the regulation of the VOCs emission for oil-based coating was found to control the amount of organic solvent used in the formula and also created a pressure on the coating industry to look for alternative methods in order to enforce the new VOCs regulation.[8]
1
A high solid coating is one of the possible solutions to solve the VOCs restriction.
The idea of using reactive diluents instead of the conventional solvent has been carried
out and extensively studied to prepare the coating with high solid content.
[9-11] Despite its function as a regular solvent, the reactive diluent also reacts with the
resin and becomes a part of the film once the curing process is complete. The technology
of UV-curing system is another solution to prepare the high solid coating. A solvent-free
system can be achieved with the UV-curing process, and this leads the industry into the
new era of environmental-friendly coatings.[12]
Nevertheless, most coating materials are mainly derived from petroleum product.
The skyrocketing price and the high demand of crude oil in recent years has affected the
raw material supply throughout the industry. A considerable factor affecting the crude oil
price fluctuation is the prediction that the crude oil supply will soon be depleted.
Therefore, many researchers have been working to solve this crisis. The coating derived
from bio-based materials is the alternative approach to reduce the use of petroleum
products in coating application. For instance, the reactive diluents and UV-curable
materials such as epoxy soybean oil acrylate and epoxynorbornane linseed oil were developed for the bio-based coating.[13,14]
In this thesis, the objective of the research was to develop a new class of bio- based material for coating application. The work was divided into two research areas.
The first study (Chapter IV) was focused on the modification of tung oil and its alkyd
form for the UV-curable coating. A multi-functional acrylate monomer, which was the
photo-reactive molecule, was incorporated onto the structure of tung oil and the tung oil
2
alkyd. The UV-curing kinetic of both products were also investigated for the comparison purpose.
In the second study (Chapter V), the idea of modification method was related to the technique of preparing the UV-curable materials in the Chapter IV. The tung oil alkyd
was prepared and modified with three types of acrylate monomers, 3-methacryloxypropyl
trimethoxysilane, 2,2,2-trifluoroethyl ethacrylate, and triallyl ether acrylate. The
properties of the new alkyd-modified resins depended on the pendent group of acrylate,
which introduced on the alkyd molecule. Nuclear magnetic resonance was used to
characterize the modified oil and alkyd structure.
3
CHAPTER II
LITERATURE REVIEW
2.1 Drying Oils[1]
Drying oils, in general, are the liquid form of oils found in animal, vegetable, and
mineral materials, which can react with atmospheric oxygen to form a solid film.
Although drying oils can be obtained from several sources, the majority is derived from
the seeds of vegetables by roasting and pressing processes. Among the advantages of a
solid film formation, drying oils have been used as a coating varnish for wood finishing
and as a paint binder since prehistoric times. The coating with drying oils will achieve
superior appearance, chemical resistance, and outstanding physical properties. However,
properties of the coating are mainly dependent on the type of drying oils. Therefore, an
understanding of drying oil chemistry is necessary in order to develop the coating
properties to satisfy the particular application.
The major component of natural drying oils is the mixture of triglycerides, which
are triesters of glycerol and different fatty acids distributed among the molecules. The
non-glyceride components such as coloring matters, phosphatides, sterols, and
tocopherol, are also contained as impurities with varying amounts in the drying oils.[15]
The properties of drying oils differ depending on the fatty acid composition in the
triglyceride molecule.
4
Figure 1: General triglyceride structure
Common fatty acids found in the drying oils are steric acid, palmitic acid, oleic
acid, linoleic acid, linolenic acid, pinolenic acid, ricinoleic acid, and α-eleostearic acid.
Fatty acid of drying oil glyceride can be either a saturated or unsaturated variety
depending on the type of oil. Typical fatty acid contents of some vegetable oils are given
in Table 1. A quality control specification of drying oil can be defined by using their
iodine value, which is, grams of iodine required to saturate the double bonds of 100 g of
an oil. If the iodine value of the oil is greater than 140, it can be defined as a drying oil. A
semi-drying oil has the iodine value between 125 and 140, whereas the iodine value less
than 125 indicates a non-drying oil.[16]
Drying oils are also classified into two groups: nonconjugated drying oil and
conjugated drying oil. The nonconjugated drying oil can be indicated by the equation
given below
Drying index = (% linoleic acid) + 2(% linolenic acid)
5
Table 1: Typical Fatty acid compositions of vegetable oils
Fatty Acid Oil Saturated Oleic Linoleic Linolenic Other Linseed 10 22 16 52 Safflower 11 13 75 1 Soybean 15 25 51 9 Sunflower, MN 13 26 61 trace Sunflower, TX 11 51 38 trace Tung 5 8 4 80a - Tall oil fatty acidb 846413 2 Tall oil fatty acidc 2.530451 14 Castor 3 7 3 87 Coconut 91 7 2
a: α-eleostearic acid b: North American origin c: European origin
According to the equation, if the drying index of nonconjugated oil is greater than
70, it can be defined as a drying oil. For instance, the drying index of linseed oil is 120,
so it is a drying oil, while soybean oil is semi-drying oil since its drying index is 69.
Conjugated drying oil, such as tung oil, contains conjugated double bonds on the
triglyceride molecule, which makes the coating derived from this type of oil dry more
rapidly than the nonconjugated system. It was also found that the ether crosslink during
the film curing process of the conjugated system is relatively stable and gives the film
superiority in water and alkali resistance. Although the coating derived from tung oil
shows rapid dry time and excellent properties, it is somewhat expensive due to the high
demand in the market.
6
Therefore, many studies make an effort to synthesize conjugated oils to
compensate the use of tung oil. One approach is to isomerize nonconjugated oil by
heating at high temperature in the presence of a catalyst such as alkali hydroxide.[17]
One study reported that partially conjugated oils could be achieved when tall oil fatty acid was treated at high temperatures in an alkali hydroxide solution. A preparation of conjugated oils from castor oil has also been reported.[19] Ricinoleic acid, or (12-
hydroxy-9-cis-octadecenoic) acid, is a major fatty acid component (~87%) of castor oil
triglyceride. Based on ricinoleic acid structure, the study showed that the dehydration of
the hydroxyl group would achieve the conjugated system on the triglyceride molecule.
The dehydrated castor oil also has the ability to dry rapidly at room temperature similar
to the coating varnish derived from tung oil.
2.2 Tung Oil[19]
Tung oil, also known as “Chinese wood oil”, was obtained by expressing the seed
(oil content in the range of 50-60 wt%) of the nut of the tung tree, which was originally
planted mostly in Asian countries, including China and some parts of Japan. In the early
period of its discovery, tung oil was used broadly in wood, paper, and fabrics, for water-
proofing purposes. In China, Aleurites Fordii, from the central and western parts, and
Aleurites Montana, from the southwestern part, are the two main species commonly used in coating varnish. The difference between the two trees is that the oil from the Montana species is somewhat slower in gelation time compared to the oil from the Fordii species.
In commerce, it is possible to mix the oils of both species for quality control before shipping since both oils have similar end-use properties.
7
For general properties of tung oil, the total fatty acid composition is ~80% α- eleostearic acid (triple conjugated diene), 15% oleic acid or other unsaturated acids, and
5% saturated acids. With a high level of α-eleostearic acid content in tung oil, it is considered as a conjugated drying oil, and its viscosity is relatively higher than most other drying oils. The color of the oil may vary from a clear pale yellow to yellowish brown depending on the species and planting area of the tung tree. Its characteristic odor is easily detected, even in a drying film of varnish.
In the conjugated system of α-eleostearate, the cis and trans conformation exist in the mixture form, which can be divided into two varieties: the alpha form or the cis, trans, trans and the beta form or the trans, trans, trans. In general, the beta form of the oil is more stable than the alpha form, which tends to convert slowly, for years, to the beta form without a catalyst. Ultraviolet light and heat treatment in the presence of a catalyst such as iodine, potassium iodide, sulfur, and strong acid, can possibly accelerate the conversion of the alpha to beta form.
The heat treatment of tung oils is also a fundamental process in order to prepare the oil for coating application. A number of the treatment methods such as blown oil, heated body oil, or alkyd preparation, can be use to improve the properties of the coating.
When tung oil is heated at high temperatures, it tends to undergo gelation more rapidly than the other types of oils. This can be explained that the high content of triple conjugated systems in tung oil, which plays an important role in the self-polymerization of the tung oil triglyceride when exposed to high heat. Therefore, time and temperature need to be taken into consideration for the heat treating process of tung oil in order to avoid an unwanted gelation.
8
2.3 Heat Treatment of Drying Oil[1]
Two heat treatment processes, which are regularly used to treat most drying oils,
are “Blown Oils” and “Heat-Bodied Oil”. Heat-Bodied Oil is the process for heating
drying oils up to 300 oC under an inert atmosphere such as nitrogen and carbon dioxide,
whereas the process of making blown oils can be achieved by passing air or oxygen gas
through the oils during heating at the temperature of 140 to 150 oC. The reaction of
blown oil process is somewhat similar to the autoxidative mechanism in the film drying
process since oxygen, which is passed into the reaction, is reacted with the oil and
forming the peroxide or ether linkage between triglyceride molecules.
Both processes can be applied to treat either conjugated or nonconjugated drying
oil. Some dimerization and oligomerization of the drying oil also occurs during heating,
and this causes the viscosity of the system to increase. During the treatment process, the
formation of glyceride dimer via Diels-Alder reaction is possible in conjugated drying
oil, or even in nonconjugated oil, which can transform to the conjugated system by the
thermal rearrangement process. Some natural rosin and additives such as phenolic resin
are also added in the oil treating step to improve some end-use properties such as
durability, chemical and thermal resistance, and solubility.
At very high viscosity and long period of processing time, the risk of gelation can
possibly take place since the generation of free radical on the triglyceride molecules will
lead the crosslinking reaction of the polymerized oil. Therefore, it is necessary for the
processing time and temperature of both methods to be controlled by checking the
viscosity of the process until the design viscosity is achieved.
9
2.4 Oxidative Curing and Drying Mechanisms[1]
A transformation of liquid into a solid film is the most important feature for all
types of coating. For most oil-based coating, the film curing process takes place via autoxidation. In early studies, an understanding of the autoxidative mechanism was fairly limited due to its complicated and limited instrument ability. Recently, new techniques and updated instrumental technology of instrument has made it possible to understand the entire the mechanism. The autoxidative mechanism scheme is given in Figure 2.
Figure 2: Autoxidative Mechanism of Drying Oil
10
In the initiation step, naturally present hydroperoxides decompose to generate free radicals (a). In this step, the formation of free radicals can be accelerated by increasing
temperature or incorporating with a drier, which is an inorganic compound and acts as a
catalyst for autoxidative system. Cobalt, manganese, and calcium salts are a group of
catalysts, which are widely used as a drier package in the coating formulation. A function
of the drier in accelerating autoxidation is shown in Figure 3.
Figure 3: Catalyzed decomposition of hydroperoxides
In the propagation step, the diallylic methylene hydrogen is readily abstracted by
the high reactivity free radical (c), which is produced in the initiation step. At this point,
the free electron at the diallylic position will delocalize to form π bond with the carbon in
α position, and, at the same time, the terminal carbon will react with oxygen to
predominantly give a conjugated peroxy free radical (d). This peroxy free radical can
abstract hydrogen from neighboring diallylic methylene groups to form another
hydroperoxide as in (a) and generate a free radical on diallylic methylene position as in
(c); hence, a chain reaction is established, resulting in autoxidation.
In the termination step, the radical-radical combination will end the autoxidative
process. The formation of a carbon-carbon bond, ether, and peroxide bonds by the
combination are the cross-linking section between the diallylic methylene positions of
each molecule (e-g).
11
2.5 Alkyd Resin
The technology of alkyd has been studied since 1847 when Berzelius accidentally
heated glyceryl with tartaric acid resulting in the first polymeric ester. Smith, in 1901,
also synthesized glyceryl phthalate, which is the general alkyd structure using today.[20]
However, those alkyd studies were no practical use until 1927, when Kienle discovered a new class of alkyds.[21,22] He reported that the alkyd resin, which was prepared from
phthalic anhydride, glycerol, and unsaturated fatty acid, such as linoleic, linolenic, and
oleostearic acid, showed more flexibility and toughness than the alkyd derived from
polyhydric alcohol and polybasic acid alone. The first alkyd resin sold commercially was
introduced by the General Electric Company under the name “Glypat”.[20] Since then,
alkyd resin has been extensively used in several applications, such as plasticizer,
adhesives, printing inks, molding, etc., but mostly used as a binder in coating field.
The term “alkyd” refers to an ester-based polymer, which consists of pendent
groups of fatty acid ester on an alkyd backbone. Alkyd resin can be prepared from the
reaction of polybasic acid, polyalcohol, and fatty acid. There are two methods for alkyd
preparation. The first is the “Monoglyceride process”, and the second is the “Fatty acid
process”.[1] The difference between the two methods is the use of starting material and
the processing step. In the monoglyceride process, seed oil is used and reacted with
polyalcohol, commonly used glyceride, to generate a monoglyceride product via
tranesterification in the first step. Then polybasic acid, such as phthalic anhydride, is
added to the monoglyceride product to form alkyd resin via esterification. Figure 4
depicts the schematic of alkyd preparation via a monoglyceride process.
12
Triglyceride
O O O O
O O
O OH O R OH O O Catalyst + OH O R O O R OH O R OH
Polyol Monoglyceride
O O O O * O O O O O n* O O OH O O O O
O R +
OH
Monoglyceride Polyacid Alkyd Resin
Figure 4: Monoglyceride process of alkyd preparation
13
For the fatty acid process, free fatty acid is used instead of seed oil in the reaction.
Also, all starting materials are reacted to form an alkyd in just one step. The fatty acid process has the advantage of more process control, while the monoglyceride has the advantage of price. The schematic of the fatty acid process is shown in Figure 5.
Figure 5: Fatty acid process of alkyd preparation
14
2.6 Diels-Alder reaction
The cycloaddition of the alkene and conjugate diene to form a six-member ring,
known as the Diels-Alder reaction, has been a fundamental method in organic synthesis
since Otto Diels and Kurt Alder introduced their discovery in 1928.[23] The recognition
of this reaction became a prevalent synthesis method due to the ability of a hexene ring
formation in either a simple or complex molecule with no complicated procedure. From
being useful and highly potential in the organic synthesis of the Diels-Alder
cycloaddition, this well-known reaction was awarded the Noble Prize in 1950.
In the Diels-Alder reaction, the formation of the six-member ring takes place via a
1,4-addition of an electrophile and a nucleophile of an alkene, or dienophile, to a
conjugate diene. The two double bonds of conjugate diene and one double bond of
dienophile convert to one new double bond and two new single bonds on the cyclic ring.
A substituted group on the dienophile and the conjugate molecule also plays an important
role in the regioselectivity for the new cyclic system.[24] The general Diels-Alder
reaction is depicted in Figure 6.
Figure 6: General Diels-Alder Reaction
However, the structure of conjugate diene is required to be in the s-cis configuration
to allow the Diels-Alder reaction to work. With this configuration, the activation energy
of the reaction is suitable for the π-electron to delocalize between the highest occupied
15
orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the
dienophile to form the cyclic ring. In some conditions such as in solution or under
thermal process, the conversion of the s-trans to s-cis conformation by the rotation of
carbon-carbon single bond between the conjugate system is possible, and the equilibrium
of the mixture s-trans and s-cis conformation is achieved.[24] The kinetic rate of the
Diels-Alder reaction is considered as well. The study of Yates and Eaton mentioned that the use of acid catalysts such as TiCl4, AlCl3 and ZrCl4, under certain thermal conditions
could accelerate the formation of a six-member ring via the Diels-Alder reaction.[25]
Figure 7: The rotation of s-trans to s-cis conformation
In the coating field, several studies took the advantages of the Diels-Alder
reaction to modify the drying oil with the dienophile molecule. Most studies used an
acrylate monomer. Tung oil is also considered as a potential diene system for the Diels-
Alder reaction since it contains ~80% of α-eleosterate on the molecule. A number of
studies have reported the capability of the cyclic formation by the Diels-Alder reaction of
tung oil and various dienophile molecules. Brunner and Tucker reported that the cyclic
product, by refluxing the mixture of tung oil and styrene in xylene solution, could be
formed via the Diels-Alder cycloaddition.[26] A copolymer based on tung oil was also
studied by Trumbo and Mote.[27] They employed the Diels-Alder reaction to prepare a tung oil copolymer by reacting the tung oil with diacrylate monomers. Time and
16
temperature of the reaction were varied to observe the conversion of the copolymers.
According to their results, ~80% conversion of copolymer between tung oil and 1,6-
hexanediol diacrylate can be achieved by heating the reaction up to 150 oC for two hours
without using a catalyst.
2.7 Reactive Diluents
In the coating application, key to controlling the viscosity of the conventional
formulation is the amount of organic solvents incorporated in the formula. Solvents such
as toluene, xylene, mineral spirit, etc., however, have been considered as a health issue in
these recent years since all volatile solvents are environmental contaminants after being
used. Therefore, the use of reactive diluents to replace the conventional solvent is
becoming a fascinating idea in the high-solid technology of the coating industry.
Reactive diluents, according to its name, can function as a solvent in the coating
formula to adjust the viscosity application and can also be converted to an integral part of
the film during the cure process. The key characteristic of such materials are high
reactivity, low viscosity, low volatility, low toxicity, and a solubility parameter.
However, the concept of reactive diluents is not that new of an innovation. This
concept has been used in coating area for a long time. In the oil-based varnish, linseed oil
is considered as the traditional reactive diluent since it is used to adjust the viscosity of
the varnish system and also reacts with the resin via autoxidation.[28] Although linseed
oil is still in use, it is not as important as a reactive diluent as before. The reactivity is
relatively low for air-drying of the varnish system. In addition, the viscosity of the linseed
oil is too high in the modern use. Allyl ethers are also used as reactive diluents. One
17
study reported that the cure process of allyl ether can take place via metal-catalyzed
oxidation with subsequent copolymerization with fatty acid derivatives, hence bringing about drying of alkyd.[29] Muizebelt et al. studied the reactivity of reactive diluents including allyl ether and allyl ester groups in a high-solids alkyd system by using NMR and mass spectroscopy to investigate the crosslink mechanisms.[30] With their results, it was found that allyl ether appears to react faster than the allyl esters, which show a much lower reactivity.
2.8 UV Cure Coating[31]
UV curing is the process of film transformation from a reactive liquid into a solid by radiation, rather than heat, of the light in the ultraviolet-energy region. The curing process can be divided into two classes based on the polymerization reaction: free radical and cation-initiated chain-growth polymerization. A photoinitiator, which is a light- sensitive molecule, is also presented in the system to initiate the photopolymerization.
The schematic of free radical formation of photoinitiator is given in Figure 8.
Figure 8: The formation of free radical in photoinitiator
In the free radical system, acrylate and methacrylate are the most common
functional groups introduced onto the resin structure to make it into a UV-curable
material. The chain propagation and termination as well as cross-linking of the
18
acrylate/methacrylate system undergo a radical mechanism. Figure 9 shows the
mechanism path of free radical photopolymerization.
Figure 9: Photopolymerization path of free radical system
19
2.9 Sol-gel Chemistry
The process of an inorganic network gel formation through a colloidal suspension
is called sol-gel. Metal alkoxides are generally used as the precursors to synthesize sol colloids due to their reactivity with water. A hydrolysis of alkoxysilane and the
condensation with water according to sol-gel chemistry is depicted in Figure 10. The pH,
catalyst, concentration of alkoxysilane, and temperature of the reaction results properties
of inorganic network. Therefore, it is possible to obtain a wide variety of materials by
controlling these variables.
Figure 10: Depiction of the sol-gel reactions
20
A hydrolysis reaction under acidic conditions occurs in two steps via the SN2 type mechanism as depicted as in Figure 11.[56] First, an alkoxide group is protonated rapidly.
A silanol group forms after the alcohol is released from the transition state. Base catalyzed hydrolysis reactions of alkoxysilanes are much slower compared to acid catalyzed reactions. The hydroxyl anion is attached to the silicon atom of the alkoxysilane via the SN2 mechanism as shown in Figure 12. The base alkoxide group
inhibits the hydroxyl anion and decrease the rate of reaction. Once the first hydrolysis
reaction takes place, subsequent hydrolysis of the remaining alkoxides are easier.
Figure 11: Acid catalyzed Hydrolysis mechanism of alkoxysilane
- H R R H2O + Si OR O O Si O HOSi + H H H
Figure 12: Base catalyzed hydrolysis mechanism of alkoxysilane
Schmidt et al. introduced “Ormosils” which could be prepared by a sol-gel process. In their work, the addition of the inorganic component is the continuous phase
21
and organic polymers for gas permeability, flexibility and toughness.[67-70] Wikes et
al.[71] introduced the second kind of iorganic-organic hybrid materials having covalent
bonds between the two phases. In these materials, the organic part was the continuous
matrix. Low molecular weight organic polymers were modified with trialkoxysilanes to
give covalent bonds between the polymer and inorganic network.[72]
Organofunctional silanes have an alkoxysilane group and organic functional group and have been used as coupling agents for organic binders [1, 73, 74] Soucek and co-workers reported that alkoxysilanes play the role of the compatibilizer and the coupling agent for polyurethane/TEOS based inorganic/organic hybrids.[75, 76] They also reported that the alkoxysilane groups functioned as a nucleation site for silicon-oxo- cluster growth. Therefore, it provides a template for uniform dispersion of the silicon- oxo-nanophase.
2.10 Fluorinated Polymers
Fluorinated polymers are known as high performance materials due to their excellent properties, such as chemical resistance, high thermal stability, low refractive index and friction coefficient, good hydrophobicity and lipophobicity, and etc.[57] The insolubility in any solvent and high fusion of fluorinated polymer such as polytetrafluoroethylene (PTFE) contributed this type of polymer to the greatest exterior durability and heat resistance.[77] A sinter effect of fluorinated polymer pariticle is possibly occurred at the service temperature above 425 oC after application. A low
surface free energy is typically property of fluorinated compound, which contribute good
hydrophobicity and lipophobicity. This attributes to a “nonstick” cooking surface.
22
Fluorinated copolymers with molecule contained crosslinking site will improve the
durability of the materials. Fluorinated copolymers with hydroxyl-functional monomer
such as inylidene fluoride (VDF) can be crosslinked with a polyisocanate. This type of
polymer give coatings with superior wet adhesion and corrosion as compared with
poly(vinylidene fluoride) PVDF homopolymer.[78]
23
CHAPTER III
EXPERIMENT
3.1 Materials
Trimethylolpropane trimethacrylate (TMPTMA) and tripropylene glycol
diacrylate (TPGDA) were received from Sartomer. Tung oil, 3-methacryloxypropyl
trimethoxysilane, phenothiazine, pentaerythritol allyl ether (PETAE), acrylic acid (AA),
p-toluenesulfonic acid, 2,2,2-trifluoroethyl methacrylate, phenothiazine, xylene isomer,
phthalic anhydride (99%), Lead (IV) oxide (97%), and glycerol (99%) were purchased
from Aldrich. cobalt Hydro-Cure II, Zirconium Hydro-Cure, and Calcium Hydro-Cure
were obtained from OMG Group. Phenolic resin was purchased from Georgia-Pacific
Resins Incorporation. Photo initiator Irgacure 2100 (Phenylbis(2,4,6-trimethylbenzoyl)-
phosphine oxide), structure shows in Figure 13, was obtained from Ciba Company. All
chemicals were used as received without further purification.
Figure 13: Phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide)
24
3.2 Synthesis of UV-Curable Tung Oil (UVTO)
Tung oil (100 g) and phenolic resin (7 %wt of tung oil) were charged into a 500
mL three-necked round bottom flask under nitrogen atmosphere and equipped with mechanical stirrer at moderate speed. The reaction was performed at 200 oC for 40
minutes, and then was cooled to 150 oC in order to prepare for the next step reaction.
Trimethylolpropane trimethacrylate (46 g, 0.136 mol) and phenothiazine (0.05 %wt of
TMPTMA) were then charged into the reaction mixture. The reaction temperature was held for addition 1½ hr, and then cooled to room temperature. The UVTO product was characterized the chemical structure by 1H NMR, 13C NMR, and MALDI-TOF MS.
Table 2 and 3 also the assigned chemical shift of 1H NMR and 13C NMR of acrylated
tung oil, respectively. The assigned mass spectra of bodied tung oil and acrylated tung oil
are also given in Table 4 and 5, respectively.
Table 2: The assigned mass spectra of bodied tung oil
m/z Component 927-1233 Phenolic resin 895 Tung Oil triglyceride 1768 Dimer of tung oil triglyceride 2640 Trimer of tung oil triglyceride * See Figure 16
Table 3: The assigned mass spectra of acrylated tung oil
m/z Component 1233 Mono-addition of TMPTMA on tung oil triglyceride 1572 Di-addition of TMPTMA on tung oil triglyceride Di-addition of TMPTMA on the dimer of tung oil 2444 triglyceride * See Figure 17
25
Table 4: 1H-NMR Chemical Shift of Table 5: 13C-NMR Chemical Shift of acrylated tung oil acrylated tung oil
Chemical shift (δ) Position of Proton Chemical shift (δ) Position of Carbon ppm ppm 0.89 1,29 7.64 29 1.06 24 14.10 1 1.30 2,12-15 18.46 32 1.60 3 , 16 , 28 23.59 2,24 1.94 32 24.97 16 2.00 - 2.18 4 ,7,10,11 3-5, 8, 11-15, 27.95 - 34.32 2.31 17 17,22 4.12 - 4.29 21 , 22 41.28 27 4.16 26 62.24 19,21 5.27 20 64.42 26 5.5 - 5.6 8,9 69.00 20 5.58/6.10 33 126.20 33 5.95 5,6 128.26 6,8 * See Figure 18b 128.87 10 130.68 9 136.02 31 167.10 30 173.40 18 176.77 25 * See Figure 19b
3.3 Synthesis of UV-Curable Tung Oil Alkyd (UVTA)
All modified alkyds were modified based on monoglyceride process and
controlled oil length at 70. Oil length of the alkyd system was calculated by the equation,
which gives in eq. (1).[4]
Tung oil (100 g), glycerol (19 g, 0.206 mol), and lead (IV) oxide (0.04 %wt of
tung oil), were charged into a 500 mL four-necked round bottom flask under nitrogen
26
atmosphere and equipped with mechanical stirrer. The reaction was heated to 300 oC for
8 min. Temperature was then decreased to 170 oC, and a 20 mL Dean-Stark trap was
connected to the flask. Phenothiazine (0.1 %wt of tung oil), phthalic anhydride (37.81 g,
0.255 mol), and xylene (80 mL) were charged into the system. After addition of the
reagents, the reaction temperature dropped to 140 oC and was heated to 170 oC for ~1 hr
or until acid number of the system reached ~30. The acid number of the alkyd product
was determined according to ASTM D 1639-90 by titration with 0.1N KOH in xylene
solution. The system was cooled to 150 oC. Then, the Dean-Stark trap was removed and
replaced with condenser. Trimethylolpropane trimethacrylate (TMPTMA) (35 g, 0.104
mol) was then charged into the reaction, and temperature was maintained for another 1½
hr. The resultant product was then cooled to room temperature, and then equipped with
vacuum distillation to remove the xylene. Acid number of the alkyd product was checked
again and had the value was ~30. The structure of UVTA product was characterized by
1H NMR, 13C NMR. Table 6 and 7 show the assigned chemical shift of 1H NMR and
13C NMR of acrylated tung oil alkyd, respectively.
27
Table 6: 1H-NMR Chemical Shift of Table 7: 13C-NMR Chemical Shift of acrylated tung oil alkyd acrylated tung oil alkyd
Chemical shift (δ) Position of Proton Chemical shift (δ) Position of Carbon ppm ppm 0.88-0.92 1,11,37 7.64 37 1.08 32 14.27 1 1.26 2,3,12-15 18.43 41 1.57-1.62 16,36 41.34 35 1.91 41 62.26 19,21 2.31 17 64.48 34 4.17 34 68.11 20 5.34 20 126.17 40 5.57/6.09 40 136.06 39 7.55 25,26 167.14 38 7.75 24,27 169-172 33 * See Figure 25b 174.05 18 * See Figure 26b
3.4 Synthesis of fluorine functionalized tung-alkyd (FTO-Alkyd) resin
Tung oil (100 g), glycerol (19 g, 0.206 mol), and lead (IV) oxide (0.04 %wt of tung oil), were charged into a 500 mL four-neck round bottom flask under nitrogen atmosphere equipped with mechanical stirrer and reflux condenser. Temperature of the reaction was held at 300 oC for 8 min, then temperature was decreased to 170 oC. A 20
mL Dean-Stark trap was then connected to the flask and phenothiazine (0.1 %wt of tung oil), phthalic anhydride (37.81 g, 0.255 mol), and xylene (80 mL) were charged into the system. The reaction was controlled at 170 oC until the acid number of the system was
~20. The acid number of the alkyd product was determined according to ASTM D-1639-
90.
The temperature was cooled to 150 oC, and then Dean-Stark trap was replaced by
a condenser. The 2,2,2-trifluoroethyl methacrylate (10 g, 0.06 mol) was charged into the reaction and refluxed (150 oC) for 2 h, then the reaction was cooled to room temperature.
28
The acid number of the modified alkyd was checked again and was ~20. About 15 mL of
xylene, which used as solvent in the reaction, contained in the product. The xylene
content was determined by calculating the weight of the product before and after
volatilizing the xylene out using a rotovap (6 h). The alkyd-modified product was stored
in an amber bottle under nitrogen environment to prevent autoxidation of the product.
The structure of UVTA product was characterized by 1H NMR, 13C NMR. Table 8 and 9
show the assigned chemical shift of 1H NMR and 13C NMR of FTO-Alkyd, respectively.
Table 8. 1H-NMR Chemical Shift of Table 9: 13C-NMR Chemical Shift of fluorinated tung oil alkyd fluorinated tung oil alkyd
Chemical shift (δ) Position of Proton Chemical shift (δ) Position of Carbon ppm ppm 14.28 1 0.88-0.92 1 22.83 2 1.08 32 25.00 16 1.26 2,3,12-15 27.38-32.06 3,4,11-15,17 1.57-1.62 16 62.24 19,21 2.05 4,11 68.07 20 2.31 17 124.07 35 4.10-4.51 19,21 126.16-137.87 5-10,23-28 4.57 34 168.30-166.45 22,29 5.34 20 169-172 33 5.47-4.70 5-10 174.05 18 * See Figure 33 5.98-6.09 6-9 7.55 25,26 7.75 24,27 * See Figure 32
29
3.5 Synthesis of Siloxane functionalized tung-Alkyd (SFTO-Alkyd) resin
Tung oil (100 g), glycerol (19 g, 0.206 mol), and lead (IV) oxide (0.04 %wt of
tung oil), were charged into a 500 mL four-neck round bottom flask under nitrogen purge
equipped with mechanical agitator and reflux condenser. Temperature of the reaction was
held at 300 oC for 8 min, then temperature was decreased to 150 oC. A 20 mL Dean-Stark
trap was then connected to the flask and phenothiazine (0.1 %wt of tung oil), phthalic anhydride (37.81 g, 0.255 mol), and 3-methacryloxypropyl trimethoxysilane (28 g, 0.11
mol) were charged into the system. The mixture was controlled at 170 oC until the acid
number of the system was ~30. If the acid number goes below this point, the reaction is
in risk of gelation. The acid number of the alkyd product was determined according to
ASTM D 1639-90 by titration with 0.1N KOH methanol solution. Once the reaction was
cooled to 80 oC, 15 mL of xylene was added into the alkyd-modified product to reduce
the viscosity of the system. The resultant product was further cooled to atmospheric
temperature and stored in an amber bottom under nitrogen environment to prevent
autoxidation and moisture cure of the product. The structure of UVTA product was
characterized by 1H NMR, 13C NMR. Table 11 and 12 show the assigned chemical shift
of 1H NMR and 13C NMR of TAETO-Alkyd, respectively.
30
Table 10: 1H-NMR Chemical Shift of Table 11: 13C-NMR Chemical Shift of siloxane tung oil alkyd siloxane tung oil alkyd
Chemical shift (δ) Position of Proton Chemical shift (δ) Position of Carbon ppm ppm 0.72 36 6.05-10.13 36 1.08 32 14.28 1,35 0.88-0.92 1 22.83 2,32 1.26 2,3,12-15 25.00 16 1.57-1.62 16 27.38-32.06 3,4,11-15,17 1.74 35 50.93 37 2.05 4,11 62.24 19,21 2.31 17 68.07 20 3.64 37 126.16-137.87 5-10,23-28 4.10-4.51 19,21 168.30-166.45 22,29 5.34 20 169-172 33 5.47-4.70 5-10 174.05 18 5.98-6.09 6-9 * See Figure 36
7.55 25,26 7.75 24,27 * See Figure 35
3.6 Synthesis of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin
In the synthesis of TAETO-Alkyd, triallyl ether acrylate was prepared following
previously reported procedures by Wutthichareonwong by esterification of
pentaerythritol allyl ether (100 g, 0.390 mol) and acrylic acid (56.16 g, 0.780 mol) in the
presence of p-toluenesulfonic acid, which used as an acid catalyst. Phenothiazine (1 g,
0.005 mol) was used as an inhibitor to prevent homopolymerization of acrylic acid. The reaction was refluxed (45 oC) in dichloromethane (200 mL) for 3 h. The ester product
was isolated by removing the dichloromethane via extraction and dried with MgSO4 anhydrous.[16]
For TAETO-Alkyd synthesis, Tung oil (100 g), glycerol (19 g, 0.206 mol), and lead (IV) oxide (0.04 %wt of tung oil), were charged into a 500 mL four-neck round
31
bottom flask under nitrogen atmosphere equipped with mechanical stirrer and condenser.
Temperature of the reaction was held at 300 oC for 8 min, then temperature was
decreased to 170 oC. A 20 mL Dean-Stark trap was then connected to the flask and
phenothiazine (0.1 %wt of tung oil), phthalic anhydride (37.81 g, 0.255 mol), and xylene
(80 mL) were charged into the system. The controlled at 170 oC until the acid number of
the system was ~20. The acid number of the alkyd product was determined according to
ASTM D 1639-90 titration.
The temperature of the system was cooled to 150 oC, and then Dean-Stark trap
was replaced by condenser. The triallyl ether acrylate (30 g, 0.098 mol) was charged into
the reaction and controlled the temperature at 150 oC for 2 hr. Then, the reaction was
stopped and the resultant product was cooled to room temperature. Acid number of the
modified alkyd was checked again and was ~20. About 15 mL of xylene, which used as
solvent in the reaction, contained in the product. The xylene content was determined by
calculating the weight of the product before and after volatilizing all xylene out. The
alkyd-modified product was stored in an amber bottle under nitrogen environment to
prevent autoxidation of the product. The UVTA product was characterized the structure
by 1H NMR, 13C NMR. Table 13 and 14 show the assigned chemical shift of 1H NMR
and 13C NMR of TAETO-Alkyd, respectively.
32
Table 12: 1H-NMR Chemical Shift of Table 13: 13C-NMR Chemical Shift of triallyl ether tung oil alkyd triallyl ether tung oil alkyd
Chemical shift (δ) Position of Proton Chemical shift (δ) Position of Carbon ppm ppm 0.88-0.92 1 14.28 1 1.26 2,3,12-15 22.83 2 1.57-1.62 16 25.00 16 2.05 4,11 27.38-32.06 3,4,11-15,17 2.31 17 44.73 34 3.43 35 69.38 35 3.74 36 71.07 36 3.92 33 118.43 38 4.10-4.51 19,21 126.16-137.87 5-10,23-28 5.23 38 168.30-166.45 22,29 5.83 37 169-172 33 7.55 25,26 174.05 18 7.75 24,27 * See Figure 39
* See Figure 38
3.7 Formulation and Curing Kinetic of UV-Curable materials (Chapter IV)
UV-Curable Tung Oil and UV-Curable Tung Oil Alkyd (85 wt% of each product)
was formulated with tripropylene glycol diacrylate (TPGDA) and Irgacure 2100 10 wt%
and 5 wt%, respectively. Both formulations were prepared in 20 mL vials and were
mixed on a roller mill until the mixture was mixed homogeneously (~2 hr) and free of
bubble. Once the mixture mixed well, the vials were wrapped with aluminum foil in order
to keep away from any source of light to prevent premature cure. Both UV-Curable
formulations were investigated the curing kinetic by using Photo-DSC.
33
3.8 Formulation and Film Preparation of modified alkyd (Chapter V)
Each modified tung-alkyd resin was formulated with 2 wt% drier package and
diluted with xylene (15 mL) in 20 mL vial. The formulations were allowed to mix on
roller mil for about an hour until the solution was mix homogeneous and free of bubble.
Films of each formulation were casted by a draw down bar on glass panels (8 mils wet film) for DMTA investigating and aluminum panels (3 mils film) for drying time testing and contact angle measurement ( for FTO‐Alkyd and unmodified alkyd system). Wet
films on glass panels were cured in the temperature-controlled oven at 120 oC for 6 hours, and another 24 hour at 40 oC. The films on glass panels were kept another 1 week before
investigating with DMTA to ensure a complete cure was achieved.
3.9 Instrument and Characterization
Mercury-300 MHz spectrometer (Varian) was used to record 1H and 13C NMR spectra at room temperature with using CDCl3 + TMS (0.01%) as reference solvent.
Matrix Assisted Laser Desorption Ionization Mass Spectroscopy (Bruker MALDI-TOF
REFLEX III) was used to determine the exact molecular weight of the products, which were bodied tung oil in the first step reaction and UV-Curable Tung Oil in the second step reaction. To prepare solution for MALDI-TOF, UV-Curable Tung Oil or bodied tung oil (10 mg/ml of THF, Dithranol (20 mg/ml), and NaTFA (10 mg/ml) were mixed in the ratio of 2:10:1, respectively. TA Instrument DSC-Q1000 (Photo DSC), which equipped with a Novacure N2001-A1 as UV-light source, was used to investigate heat flow of curing product. The intensity of the UV-light source was 250 mW/cm2. Perkin-Elmer
Rheometric Scientific dynamic mechanical thermal analyzer (DMTA) was utilized to
34
investigate the viscoelastic properties of the films. Performing temperature of DMTA was
set in the range of -20 oC and 80 oC at the frequency of 1 Hz with the heating rate 3
oC/min. The gap distance of the specimens was set at 1 mm. Drying time of the films was monitored according to ASTM standard (D 5895-03).
35
CHAPTER IV
SYNTHESIS OF UV-CURABLE TUNG OIL BASED RESIN
4.1 Introduction
Tung oil varnish and its alkyd-modified resin have been used extensively in
coating applications for long time since its superior coating properties such as fast-drying
time, water resistance, and high hardness were discovered. In varnish manufacturing,
several types of additive are also introduced into the varnish system in order to achieve
designed-coating properties. For instance, the use of phenolic resin in varnish will
increase the durability, wetting, and leveling of the coating film. [32-37] Two types of
phenolic resin, Novolac and Resole, can react with tung oil by forming a chroman ring in
the temperature range between 200 oC and 300 oC.[38-40] The reaction of each type of
phenolic resin with drying oil is shown in Figure 14. In Novolac reaction, base catalysts, such as hexamethylenetetramine, were necessary to accelerate the rate of reaction; while
Resole-type resin can react directly without using catalyst.
In conventional varnish-based coating, solvents, such as mineral spirit, MEK,
toluene, etc., are incorporated in the formula to reduce the application viscosity.[1]
However, volatile organic compounds (VOCs) in an oil-based coating formulation have been an environment and toxic concerns for worldwide.[3,8] Therefore, many researchers have been working to diminish the use of organic solvent in the coating formulation.
36
From this point, many new classes of reactive diluents have been studying vastly to
resolve the problem of volatile solvent in the formulation [43-45]. The advantage of
reactive diluents is used to replace conventional organic solvents to reduce the viscosity
of the coating formula and become a part of the resin once the wet coating transform into
solid film.
Figure 14: a.) Reaction of Resole[38] b.) Reaction of Novolac [39]
The UV curing for coating applications has been as attractive option to decreasing
VOCs.[31] With the increasing of environmental regulations in recent years, many natural products were studied and modified for UV-Curable materials.[42,46,47] Soucek, et al., demonstrated the synthesis of the photopolymerizable of linseed-modified resin under UV light.[42] In their study, norbornylized linseed oil was first prepared by reacting linseed oil with cyclopentadiene via the Diels-Alder reaction under pressure
~200 psi and temperature at 240 oC in the present of 2,6-di-tert-butyl-4-methylphenol
(BHT) as a free radical retarder, and then followed by epoxidation of norbornylized
linseed oil with hydrogen peroxide and quaternary ammonium tetrakis(diperoxotungsto)
37
phosphate(3-) as the epoxidation catalyst. The norbornyl epoxidized linseed oil product
shows the capability of photopolymerization under UV light when initiate with cationic
photoinitiator.
The objective of this chapter was to develop UV-curable resin derived from
bodied tung oil and tung oil alkyd. The bodied oil and the alkyd were prepared by
bodying tung oil with phenolic resin and the monoglyceride process, respectively, and
then trimethylolpropane trimethacrylate (TMPTMA) ester was functionalized onto the
bodied oil and the alkyd molecule via the Diels-Alder cycloadition. UV-Curable Tung
Oil (UVTO) and Tung Oil Alkyd (UVTA) were characterized by 1H NMR, 13C NMR,
MALDI-TOF mass, and Gel Permeation Chromatography (GPC). Photo-curing kinetics
of the UVTO and the UVTA were investigated by Photo-DSC.
4.2 Characterization of UV-Curable Resins
Several natural materials such as natural rubber,[47] linseed oil,[13] and castor
oil,[59] have been studied and modified by different methods for UV-curing purpose.
Acrylate monomers, in the present of photoinitiator, are also known to undergo
photopolymerization when expose to UV light via free radical mechanism.[12] From this
concept, the use of acrylate molecule is a main idea to modify tung oil and tung oil alkyd
for UV-curable resin.
In this chapter, a new class of UV-Curable resin was synthesized by introducing a
trifunctional methacrylate, which provides additional functionality for photo-initiated
crosslinking, on the bodied tung oil and tung oil alkyd. Trimethylolpropane
trimethacrylate (TMPTMA) was a target monomer on account of its previously report
38
ability to UV-cure.[79-81] The Diels-Alder reaction was employed since the α-eleosterate of tung oil is a diene and the methacrylates were good dienophile. On account of the trifunctionality, gelation was a concern in both the synthesis of the modified tung oil and alkyd. Therefore, time and temperature for each step reaction was close monitored and needed to be adjusted in order to prevent the unwanted gelation.
UV-Curable Tung Oil (UVTO)
In the preparation of UVTO, bodied tung oil was prepared in the first step by treating tung oil with Novolac-typed phenolic resin at high temperature. Then, trimethylolpropane trimethacrylate (TMPTMA) ester was functionalized on the bodied tung oil via a Diels-Alder reaction. The synthesis path of the UVTO was presented in
Figure 15. In the earlier study, the synthesis of UVTO was directly funtionalized with
TMPTMA on tung oil molecule without bodying tung oil with phenolic resin. The major problem of synthesized the UVTO by this method was the low cure speed and the high brittleness of the film. In addtion, the film of UVTO needed expose to the UV-light for minutes for complete cure. However, the viscosity of the UVTO using this method was relatively low, which is a good part for some coating application. To solve these problems, the idea of bodying tung oil with phenolic resin before functionalized with
TMPMTA was inspired and carried out in order to achieve higer cure speed and better film properties.
39
Figure 15: Reaction scheme of UV-Curable Tung Oil (UVTO)
In the first step reaction, phenolic resin was expected to react with tung oil to form a tung oil oligomer, but a mass spectra of bodied tung oil in Figure 16 showed that at mass 895 m/z, 1768 m/z, and 2640 were tung oil monomer, dimer, and trimer, respectively, where as mass spectra in the mass range from 927 m/z to 1223 m/z were the mass pattern of phenolic resin, which mean that there was no reaction occurred between tung oil and phenolic resin. Although, phenolic resin does not react with tung oil as expected, it was anticipated to increase the chemical resistance and durability of the varnish formulation. From mass spectra of acrylated tung oil in Figure 17, there are a couple of spectra shown that trimethylolpropane trimethacrylate, from calculation, reacts with α-eleosterate of tung oil triglyceride. Mass spectra at 1233.60 m/z and 1572.78 m/z are the mass of mono- and di-addition of trimethylolpropane trimethacrylate, respectively. Mass spectra also indicate that at 2444 m/z is di-addition of trimethylolpropane trimethacrylate on the tung oil dimer.
40
Figure 16: Mass spectra of Bodied Tung Oil
Figure 17: Mass spectra of UV-Curable Tung Oil (UVTO)
41
Figure 18: 1H-NMR Spectra of (a) bodied tung oil (b) acrylated tung oil
42
Figure 19: 13C-NMR Spectra of (a) bodied tung oil (b) acrylated tung oil
43
Figure 18 and Figure 19 show 1H-NMR and 13C-NMR spectra of bodied tung oil
and acrylated tung oil, respectively. As explained in mass spectra and to confirm the
result, the 1H-NMR and 13C-NMR spectra of the bodied tung oil ( see Figure 18a and
19a) show no evidence of the reaction between tung oil and phenolic resin. If the reaction
resin took place and the ether ring was formed, the chemical shift of proton at position A,
as shown in Figure 20, should appear between δ 3.5-4.0 ppm in 1H NMR, and around δ
85 ppm of carbon at position A in 13C-NMR.
Figure 20: Tung oil modified phenolic resin
For 1H-NMR spectra of acrylated tung oil in Figure 18b, the new resonance appeared at δ 5.5-5.6 ppm, which indicated the protons of double bond on the cyclohexene ring. The resonance at δ 1.06 ppm, which is the proton of methyl group shifting from δ 1.94 ppm, also confirms the six-member ring formation via the Diels-
Alder reaction. The 13C NMR of acrylated tung oil in Figure 19b also appeared the new
resonances at δ 126 and 128 ppm which indicated the new double bond on the
cyclohexene ring. At δ 177 ppm showed a new carbonyl resonance which confirms the
ring formation due to the Diels-Alder reaction.
44
Figure 21: Reaction scheme of UV-Curable Tung-Based Alkyd (UVTA)
UV-Curable Tung-Based Alkyd (UVTA)
The UVTA was also prepared in order to investigate the feasibility of the UV-
curable Alkyds. It is anticipated that since alkyd is an ester-based polymer deriving from
the drying oil, this is expected that the cure speed of the tung-alkyd modification for UV
curing would be faster than the UVTO system. The UVTA was obtained by two step
reaction. First, the tung alkyd was prepared followed the monoglyceride process (see
Figure 4) in the present of lead catalyst, and then the alkyd modification for UV-Curable
resin was achieved by functionalized with TMPTMA via the Diels-Alder reaction. The
synthesis pathway of tung oil alkyd and acrylated tung oil alkyd are shown in Figure 21
and 22, respectively.
The other method, which used to prepare the UVTA, was also studied in the
beginning of the work. First, the TMPTMA was functionalized on the tung oil molecule
as the same procedure of the UVTO preparation and then followed by the monoglyceride
process. In a second approach, the monoglyceride was functionalized then the alkyd was
45
formed. Unfortunately, the trans-esterification of the modified tung oil (see Figure 23)
and the alkyd formation of acrylated monoglyceride (see Figure 24) resulted in gelation.
Several pathways were also attempted to prepare the acrylated alkyd. A number
of different catalysts including lithium hydroxide monohydrate, calcium hydroxide, and
sodium methoxide were used in the monoglyceride step. Once the alkyd was formed by
reacting with phthalic anhydride and modified the alkyd for UV-curable resin by
functionalized with TMPTMA via the Diels-Alder reaction, it was found that the cure
speed of the UVTA film was slow, even with increased the amount of photoinitiator in
the formulation. This was attributed to a low conversion of monoglyceride and low
activity of the catalyst in the monoglyceride step. The use of very long time (~1 hr) and
high temperature (~240 oC) during the monoglyceride preparation resulted in the
destruction of reactive diene in the α-eleosterate, which is necessary for the Diels-Alder cycloaddition. However, the use of lead catalyst proved to be an effective choice in the
preparation of tung oil monoglyceride. This method was first introduced by
Goldblatt.[48] Although, the toxicity of lead compounds was an environment and
carcinogen issue in the recent years, however this was the most efficient method to obtain the acrylated alkyd. A more exhaustive study to replace the lead as a catalyst will be the focus of future work. Another advantage of preparing tung oil alkyd at high temperature
(~300 oC) is that the coating will perform gasproof property, which prevents the flare
effect of coating surface. In coating applications, the flare effect is possible when the
coating varnish deriving from tung oil contacts with a gas, such as carbon monoxide,
carbon dioxide, nitrogen oxides, etc.
46
Figure 22: Synthesis path of a monoglyceride process of acrylate functionalized tung oil
Figure 23: Synthesis path of acrylate functionalized tung oil monoglyceride
The alkyds have proved difficult to characterize due to the complex mixtured products. Therefore, NMR was the only method to identify the structure of the modified alkyd due to the difficulty of interpreting the mass spectra of the alkyd resin. The NMR
47
spectra of UV-Curable Tung Oil (UVTO) was used to compare and identify new
resonances of acrylated tung oil alkyd. The 1H and 13C NMR of non-modified tung oil
alkyd were also used for comparison as shown in Figure 25 and 26, which are 1H-NMR and 13C-NMR spectra of the tung oil alkyd and acrylated tung oil alkyd, respectively. The
broad resonances at δ 7.72 and 7.54 ppm in 1H NMR of tung oil alkyd (see Figure 25a)
show the protons in benzene ring which shifted from δ 7.9 and 7.8 ppm, once the
formation of alkyd took place. Some of the resonances would be assigned in a group
because of the overlapping of the resonance. In 13C NMR (see Figure 26a), the broad
resonances at δ 166-168 ppm confirm the carbonyl group, which are on the alkyd
backbone, whereas the broad resonances at δ 172-174 ppm indicate the carbonyl of α- eleosterate pendent group on the alkyd structure.
For the acrylated tung oil alkyd, a new resonance in 1H NMR (see Figure 25b) at
δ 1.08 ppm confirmed the formation of cyclic ring formed during the Diels-Alder
reaction. This resonance is according to the proton of methyl group, which shifted from δ
1.94 ppm. In 13C NMR of the UVTA (see Figure 26b), the Diels-Alder reaction between
methacrylate monomer and conjugated double bond of the eleosterate was confirmed
with the appearance of a new resonance at δ 169-172 ppm, which corresponded to the
carbonyl group attaching to a new cyclohexene ring. Gel permeation chromatography
(GPC) was also used to determine the molecular weight of FTO-Alkyd. The result
showed that average molecular weight and weight average molecular weight are 2336
and 72530, respectively, and has the polydispersity index (PDI) of 31.05.
48
Figure 24: 1H-NMR Spectra of (a) Tung oil alkyd (b) Acrylated tung oil alkyd
49
Figure 25: 13C-NMR Spectra of (a) Tung oil alkyd (b) Acrylated tung oil alkyd
50
4.3 Evaluation of Curing Kinetic
Photo-DSC was used to investigate the curing kinetics of the UV-curable Tung
Oil (UVTO) and UV-curable Tung Oil Alkyd (UVTA). A free radical photoinitiator
Irgacure 2100 was used in the formulation to initiate photopolymerization. A major
component of Irgacure 2100 is Phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide)
(BAPO), which is unimolecular-typed photoinitiators. BAPO is an effective
photoinitiator due to the absorption ability of near UV-visible radiation, which makes
possible curing of relatively thick films of white pigment coatings. A relatively small
amount of reactive diluent, TPGDA, was also added to reduce viscosity of the system and
increase the efficiency of the UVTO or UVTA.
Figure 26: Thermogram of a.) UV-Curable Tung Oil (UVTO) formula and b.) UV-
Curable Tung oil Alkyd (UVTA) formula
The thermogram of UVTO and UVTA in Figure 27 shows the heat flow as a
function of time of UVTO and UVTO Alkyd formula. The method of determining
51
photopolymerization of acrylate system is according to a study of Tryson and Shultz.[49]
The exothermic reaction showed the significant changed at the initial state of exposure time, which indicated the polymerization of UVTO and UVTA took place, and then it
gradually decrease due to terminating of the reaction. The thermogram also shows that
the cure speed of the UVTA formula is faster than to the UVTO formula
4.4 Discussion
Although there have been UV-curable derivatives of drying oils, this study is the
first UV-curable tung oil alkyd in the literature. Since cationic curing of alkyd would
comprise the polyester backbone, a free radical route was chosen. It is anticipated that the
lead catalyst, surely a detriment to commercial acceptance of proposed synthesis route.
However, it will be replaced with a more environmentally friendly catalyst as this work
proceeds. Nevertheless, UV-curable as a concept will have great potential for widespread
usage in the field of coatings. With respect to UV-curable drying oils, the tung oil based
drying oil has proven to have relatively fast drying speed and good overall coating
properties especially for wood, and most importantly recoatability for multiple coating
for repair. Both the UV-curable drying oil and alkyd represent a considerable step
forward in the replacement of solvents for the compliance of both North America and
European environmental regulations.
In previous study, Thames and his coworker[60] introduced the synthesis pathway
and formulation for UV-curable based tung oil. In their work, maleic anhydride was
introduced onto the tung oil molecule via a Diels-Alder reaction, and then esterified with
ethylene glycol to form tung oil-modified polyol. The addition of cycloaliphatic epoxide,
52
on irradiation, was a key of cationic curing in the tung oil-modified polyol via
etherification. However, the modification of tung oil for UV curing in this chapter was
based on free radical polymerization of acrylate group, which introduced on the tung oil
molecule.
In the acrylate-functionalized step of this study, a Diels-Alder reaction was a key
to functionalize acrylate molecule on tung oil and the tung oil alkyd backbone. The high
content of α-eleosterate (~80 wt%) of tung oil triglyceride is considered as a major source
of conjugated diene; while acrylate molecules is considered as a good dienophile since
the carbonyl group on acrylate, which served as an electron withdrawing group, plays an
important role to lower the activated energy of the reaction. The study of Trumbo and
Mote [27] also reported the capability of the Diels-Alder cycloaddition between tung oil
and diacrylate monomers, 1,6-hexanediol and 1,4-butanediol diacrylate, at elevate
temperature without using catalyst. However, the resultant product in their study was
copolymers of tung oil and diacrylate.
In comparison, our study used a trifunctional acrylate instead of diacrylate
monomer and the reaction conditions were carefully controlled providing an excess of
acrylic functionality. The flexibility and lack of steric hindrance of the previously report
diacrylate system were the primary factors in both the telechelic acrylic end groups
reacting with the diene. In contrast, after the first Diels-Alder reaction of the triacrylate
with the α-eleosterate, the subsequent Diels-Alder reactions were steric hindered. This
left the remaining two acrylic groups available for UV-polymerization.
53
4.5 Conclusion
UV-curable tung oil (UVTO) and UV-curable tung-based alkyd (UVTA) were
prepared by reacting trimethylolpropane trimethacrylate (TMPTMA) onto α-eleosterate
of a tung oil and tung oil alkyd molecule via a Diels-Alder reaction. The reactions were
conducts at elevate temperature and atmospheric pressure. The structure of UVTO and
UVTA was confirmed by 1H NMR 13C NMR. MALDI-TOF spectroscopy was also
confirmed the molecular weight of the UVTO. The curing kinetics of the UVTO and
UVTA was observed by using Photo-DSC. The change of heat flow as a function of time
from DSC thermogram proved the capability of the UV curing of both modified tung oil,
UVTO and UVTA, when exposed to UV-light. The thermogram also indicated that the
formula of UVTA performed faster curing speed than to the formula of UVTO.
54
CHAPTER V
SYNTHESIS AND PROPERTIES OF ACRYLATE FUNCTIONALIZED ALKYD
5.1 Introduction
Any type of seed oils, such as linseed oil, soya oil, safflower oil, tall oil,
sunflower oil, etc., can be used as starting material for the alkyd preparation.[2] Tung oil
is one of alternative sources giving the alkyd resin performs superior properties; these are
fast drying time, water resistance, and high hardness.[20] However, the process of synthesizing tung oil alkyd is relatively difficult of account of the conjugated native of the α-eleostearate in comparison with other seed oil based alkyds. In 1961, Goldblatt and his co-worker introduced the synthesis pathway for tung oil alkyd. In the monoglyceride step, tung oil and glycerol were heated to 300 oC for about 8 min in the present of
litharge, which used as a catalyst, and followed by the addition of a polybasic
acid.[48,50] They also mentioned that at this extreme cooking temperature the alkyd resin
would achieve “gasproof”, or “gas-checking”, property, which prevented the coating
away from webbing effect when the surface of coating presence of corrosive
atmospheres such as CO, CO2, sulfur dioxide fumes, nitrogen oxide, and the like.[2]
The competitive cost of the alkyd resin lead many attempts to modify the resin in
order to improve its original properties and expand its usage in coating applications.[51-
54] A Diels-Alder reaction can be employed to modify the alkyd, which contained
conjugated double bonds on the fatty acid pendent group, such as α-eleosterate of tung
55
oil triglyceride, with dienophile molecule.[26,27,41] Brunner and Tucker reported that
the resultant product in their reaction was form via the Diels-Alder reaction by refluxing
the mixture of tung oil and styrene in xylene solution.[26] In previous work, Soucek and coworker prepared three reactive diluents by functionalizing tung oil with three difference functional group of acrylate monomers including alkylsiloxane, triallyl ether,
and fluorine, via the Diels-Alder reaction.[15] They reported that the viscosity of the linseed oil-based alkyd systems was decreased when the reactive diluents was incorporated into the alkyd systems. Also, each reactive diluent performed the properties
of the coating, such as antigraffiti, fast drying time, and moisture cure, related to the
pendent groups of the acrylates.
The objective of this chapter is to synthesize a new series of tung oil alkyd with
three different acrylate molecules, 3-methacryloxypropyl trimethoxysilane, 2,2,2-
trifluoroethyl ethacrylate, and triallyl ether acrylate, via the Diels-Alder reaction. The
structures of the alkyd-modified product were characterized by 1H NMR, 13C NMR, and
gel permeation chromatography (GPC). Viscoelastic properties of dry films were
investigated by using Dynamic Mechanical Thermal Analysis (DMTA). Drying time of
each alkyd-modified system was also monitored. Surface tension of fluorinated system was determined by measuring the contact angle of coating film.
5.2 Characterization result of modified alkyd
The main idea of this chapter was created a new series of tung-based alkyds. Tung
oil alkyd was prepared via a monoglyceride process. Exceed glycerol were reacted with tung oil in the present of litharge catalyst to form tung monoglyceride, and then phthalic
56
anhydride was added for the formation of tung oil alkyd (see Figure 4). A Diels-Alder reaction was employed to modify the tung oil alkyd by incorporating with three difference acrylate monomers. Each acrylate monomer would play a different role on the coating property when it was functionalized on the alkyd molecule. Alkoxysilane group is also known to undergo hydrolysis and condensation reactions with water according to sol-gel chemistry (see Figure 10).[56] Once the siloxane group was grafted on the alkyd backbone, a dual-cure system of autoxidative (see Figure 2) and moisture cure was possible. Fluorine-modified group would have improved a good chemical stability and low surface energy.[57] Triallyl ether group has been shown to assists the autoxidative curing system by increasing the functionality of the alkyd. Figure 27 and 28 shows the autoxidative mechanism of allyl ether group and the crosslink network of triallyl ether pendent group of alkyd system.
Figure 29 and 30 show 1H NMR and 13C NMR spectra of tung oil alkyd and also shows the assigned structure of the spectra. In 1H NMR, the board resonances at δ 7.72
and 7.54 ppm show The protons on aromatic ring of phthalic anhydride part, which
disappeared from δ 7.9 and 7.8 ppm, once the formation of alkyd took place. Some of the
resonances would be assigned in a group because of the overlapping of the resonance. In
13C NMR, the broad resonances at δ 166-168 ppm confirmed the carbonyl group, which are on the alkyd backbone, whereas the broad resonances at δ 172-174 ppm indicate the
carbonyl of α-eleosterate pendent group on the alkyd structure. As described before,
some resonances would be assigned in a group because of the overlapping of resonance.
57
Figure 27: Autoxidation of allyl ether
Figure 28: A crosslinking network of triallyl ether pendent group
58
Figure 29: 1H NMR of Tung Oil Alkyd
Figure 30: 13C NMR of Tung Oil Alkyd
59
Characterization of fluorinated functionalized tung-alkyd (FTO-Alkyd) resin
Fluorinated tung oil alkyd was prepared via a Diels-Alder reaction by the six-
member ring formation between tung oil alkyd and 2,2,2-trifluoroethyl methacrylate. The
tung oil alkyd was prepared by a monoglyceride process in the present of litharge catalyst
before modification with fluorinated methacrylate monomer. The terminal fluorine
pendent groups contribute the methacrylate molecule as a good dienophile due to the high
electronegativity of fluorine atom. The synthesis diagram of FTO-Alkyd is outlined in
Figure 31. FTO-Alkyd was characterized with 1H and 13C NMR to identify a structure of the product. The assigned 1H- and 13C-NMR spectra of FTO-Alkyd is also shown in
Figure 32 and Figure 33, respectively.
Figure 31: Reaction of Fluorine functionalized tung-alkyd (FTO-Alkyd) resin
60
Figure 32: 1H NMR of Fluorine functionalized tung-alkyd (FTO-Alkyd) resin
Figure 33: 13C NMR of Fluorine functionalized tung-alkyd (FTO-Alkyd) resin
61
The propensity of the α-eleosterate to oligomerize results in the reactions that
complete with the Diels-Alder cyclization. As a consequence, the 1H- and 13C-NMR spectra of the modified alkyd were relatively complex. Therefore, the resonance were identified by comparison with the previously report acrylate tung oils [16] and the tung oil based alkyd (starting reactant material). The 1H-NMR spectra shows the appearance
of a resonance at δ 1.13 ppm and a subsequent disappearance of a resonance at δ 1.94
ppm. The disappearance of the δ 1.94 ppm resonance was attributed to the reaction of the
methacrylate group to form the six-membered cyclohexyl ring. 13C-NMR spectra of the
FTO-Alkyd also showed a Diels-Alder reaction occurred between fluorine methacrylate
monomer and conjugate double bond. The resonance of the carbonyl group attached to
cyclohexene ring was observed at δ 169-172 ppm. Gel permeation chromatography
(GPC) was also used to determine the molecular weight of FTO-Alkyd. The result
showed that average molecular weight and weight average molecular weight are 3531
and 35720, respectively, and has the polydispersity index (PDI) of 10.12.
Characterization of siloxane functionalized tung-alkyd (SFTO-Alkyd) resin
SFTO-Alkyd resin was prepared via Diels-Alder reaction by the six-member ring
formation of tung-alkyd resin and 3-methacryloxypropyl trimethoxysilane. The siloxane
side group was known to undergo moisture cure via the sol-gel chemistry. The synthesis
diagram of SFTO-Alkyd is outlined in Figure 34. SFTO-Alkyd was characterized with 1H
and 13C NMR to identify a structure of the product. The assigned 1H- and 13C-NMR
spectra of SFTO-Alkyd is also shown in Figure 35 and Figure 36, respectively.
62
Figure 34: Reaction of siloxane functionalized tung-alkyd (SFTO-Alkyd) resin
Figure 35: 1H NMR of siloxane functionalized tung-alkyd (SFTO-Alkyd) resin
63
Figure 36: 13C NMR of siloxane functionalized tung-alkyd (SFTO-Alkyd) resin
As expected, The 1H NMR show the new resonance at δ 1.13 ppm, which is
corresponded to the protons of methyl group, which indicated the formation of
cyclohexene ring due to the Diels-Alder reaction. The 13C-NMR spectra also showed a new resonance of carbonyl group attached to the cyclohexene ring. This also confirmed the six-member ring formation between α-eleostearate of tung oil alkyd and siloxane methacrylate. Gel permeation chromatography (GPC) was also used to determine the molecular weight of SFTO-Alkyd. The result showed that average molecular weight and weight average molecular weight are 2252 and 21660, respectively, and has the polydispersity index (PDI) of 9.62.
64
Characterization of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin
TAETO-Alkyd resin was prepared via Diels-Alder reaction by the six-member
ring formation of tung-alkyd resin and triallyl ether acrylate. The tung oil alkyd was
prepared via a monoglyceride process in the presence of litharge catalyst, and then
reacted with triallyl ether acrylate for modification. The synthesis diagram of TAETO-
Alkyd is shown in Figure 37. TAETO-Alkyd was characterized by 1H and 13C NMR to
identify a structure of the product. The assigned 1H- and 13C-NMR spectra of FTO-Alkyd
is also shown in Figure 38 and Figure 39, respectively.
Figure 37: Reaction of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin
65
Figure 38: 1H NMR of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin
Figure 39: 13C NMR of triallyl ether functionalized tung-alkyd (TAETO-Alkyd) resin
66
As discussed in the 1H NMR of FTO-Alkyd and SFTO-Alkyd, a new resonance of
proton on the methyl group around δ 1.10 ppm confirmed the evidence of cyclohexyl formation. However, the methyl group, which used to confirm the formation of cyclohexyl ring, could not use to identify the Diels-Alder reaction between triallyl ether
acrylate and α-eleosterate of tung oil alkyd. Therefore, the 1H NMR of TAETO-Alkyd showed only the assigned resonance of triallyl ether group. In 13C NMR of TAETO-
Alkyd, the resonances at δ 131-132 ppm are corresponded to the carbonyl attached to
cyclohexene ring, which confirmed the formation of cyclohexyl ring via the Diels-Alder
reaction. Gel permeation chromatography (GPC) was also used to determine the
molecular weight of FTO-Alkyd. The result showed that average molecular weight and weight average molecular weight are 2073 and 32750, respectively, and has the polydispersity index (PDI) of 15.80.
5.3 Drying time study
The process of film drying took place via an autoxidative cure when the oxygen in the environment was consumed for the cross-linking reaction. The same drier package was used for all the alkyds. The drying of the modified tung oil alkyds were compared to the unmodified alkyd system. The drying time comparison of each alkyd-modified system is give in Table 14. It should be noted the drying ability of the FTO-Alkyd would be diminished by the consumption of the conjugated double bonds, which are important in autoxidative cure, with non reactive fluorinated groups. The SFTO-Alkyd would have a dual cure system with both the moisture (see Figure 10) and autoxidative process (see
67
Figure 2). For TAETO-Alkyd, the triallyl ether pendent group increased the functionality
of the modified alkyd (see Figure 28), which improved the speed of autoxidative cure.
Table 14: Drying time at 25 oC
Sample Drying Time (hr)
Unmodified Alkyd 17
FTO-Alkyd 20
SFTO-Alkyd 14
TAETO-Alkyd 15
5.4 Viscoelastic Properties
The storage modulus and tan δ of each alkyd-modified system was measured via
DMTA and are shown in Figure 40 and 41, respectively. The glass transition temperature
(Tg) was derived from the maximum α-transition (see Figure 41) shown in the tan δ data.
The value of storage modulus (E’) can be used to determine a crosslink density of a viscoelastic material by the equation which give in eq (2).[58]
(2)
where Ve is the crosslink density, R is the gas constant (8.314 Nm/g mol*K), T is
o the absolute temperature ( K), E’min is the minimum storage modulus in the rubbery
2 plateau (N/m ). The value of minimum modulus (E’min), Crosslink density (Ve), and glass
transition temperature (Tg) of each system also shows in table 15.
68
Figure 40: Modulus (E’) as a function of temperature for the modified alkyd system
Figure 41: Tan δ as a function of temperature for the modified alkyd system
69
Table 15: Viscoelastic properties of the alkyd-modified cure films
Tg E’m Ve
oC N/m2 (mol/m3)
Unmodified Alkyd 48 2.54E+06 265.12
FTO-Alkyd 47 2.02E+06 237.62
SFTO-Alkyd 50 4.27E+06 445.87
TAETO-Alkyd 55 3.19E+06 328.43
As expected, the SFTO-Alkyd and TAETO-Alkyd showed a higher crosslink
density and glass transition temperature compared to an unmodified alkyd. The increase
of glass transition temperature and crosslink density of SFTO-Alkyd and TAETO-Alkyd
can be attributed crosslinking site, which were incorporated onto the α-eleosterate of the
modified alkyd. However, the result showed that SFTO-Alkyd has higher crosslink
density compared to TAETO-Alkyd. The higher of mole ratio of siloxane methacrylate
resulted in increasing crosslinking functionality. The lower crosslink density of FTO-
Alkyd compared to the unmodified alkyd is due to the functionalizing fluorinated group
on the alkyd backbone cause the decreasing of crosslinking site.
5.5 Contact Angle
The surface properties of FTO-Alkyd film was also investigated by measuring
contact angle of a water droplet compared to the unmodified alkyd system. An example
of water contact of FTO-Alkyd coating is shown in Figure 42. The contact angles were
taken 8-10 readings per sample. The result contact angle of FTO-Alkyd and unmodified
70
are shown in Table 16. As expected, the contact angle of FTO‐Alkyd system is higher compared to the unmodified alkyd. Fluorine pendent group of the modified alkyd performed surface activity. With respect to the result, it has been proposed that the fluorine‐modified alkyd improved the surface properties of the coating.
Figure 42: Contact angle measurement of FTO-Alkyd film
Table 16: Contact angle measurement
Sample Contact Angle
Unmodified Alkyd 84o
FTO‐Alkyd 93o
71
5.6 Discussion
In this chapter, gelation was a concern for all of the modified alkyd. The condition
of each acrylate-modified reaction needed to be controlled carefully. Although the
reactions were conducted under inert atmosphere, gelation still occurred due to the
oligomerization of the α-eleosterate pendent group via a Diels-Alder reaction. A
schematic of a Diels-Alder reaction between unsaturated drying oil shows in Figure 43.
Gelation also found during the modification of SFTO-Alkyd. An alkoxysilane group are
also known to undergo moisture cure (see Figure 10) through the sol-gel process.[56]
Free acid and water in the alkyd system possibly will accelerate the sol-gel process.
Therefore, the addition of MEA in the alkyd preparation step was used to solve the
problem, and this let the Diels-Alder reaction and alkyd formation (esterification) taking
place in the same step. A high pressure reaction could be an alternative pathway to avoid
gelation in the preparation of acrylic modified tung oil alkyd. At high pressure, the
reaction could be performed at lower temperature. There were several studied reported
that Diels-Alder reaction can be induced accelerated by pressure.[61-63]
+
Conjugated Diene Drying oil of Drying oil
Figure 43: Oligomerization of drying oil via a Diels-Alder reaction
72
For the film properties of modified alkyds, the alkoxysilane pendent group of
SFTO-Alkyd was used to form hybrid inorganic-organic coating by the sol-gel reaction.
The process involved in situ polycondensation of metal or silicon alkoxide with an
organic polymer matrix.[64-67] The autoxidative cure also involved in the film formation
of SFTO-Alkyd. Therefore, the possibility of dual-cure system was achieved.
Autoxidative and moisture cure of SFTO-Alkyd is depicted in Figure 44.
Figure 44: Dual-cure system of alkoxysilane functionalized tung oil alkyd
With the concept as mentioned, SFTO-Alkyd film could perform high crosslink
density and lower drying time when compared to the unmodified alkyd system. The increasing of allyl ether group of TAETO-Alkyd also played the same role in crosslink density and drying time of SFTO-Alkyd system. The allyl ether pendent group also undergo autoxidative cure under catalytic system as the same that found in conjugated diene and diallylic system of drying oil. For FTO-Alkyd, the lower crosslink density and
73
longer drying time was as expected compared to the unmodified alkyd, and the other two
modified alkyd, which mention above. The fluorinated pendent group does not involve in
autoxidative cure. The incorporation of fluorine acrylate on the conjugated diene would
reduce the functionality in oxidative curing system. However, the fluorinated brings
surface activity and can be used as a additives into other autoxidative or moisture cured alkyds for better mechanical properties. It is anticipated that the fluorinated group could be used as antigraffiti coatings.
5.7 Conclusion
Three different acrylate monomers, 2,2,2-trifluoroethyl methacrylate, 3- methacryloxypropyl trimethoxysilane, and triallyl ether acrylate, were functionalized on tung-alkyd backbone via a Diels-Alder reaction. The NMR spectra confirm the capability of each acrylate monomer to functionalize on the tung-based alkyd. Each alkyd-modified film demonstrated the difference in a drying time and viscoelastic properties. For the siloxane- and triallyl-modified alkyd, the increase of crosslinking site causes a faster drying time, higher glass transition temperature, and higher crosslink density, compared to the unmodified alkyd. Whereas, the fluorinated-modified alkyd showed a slower drying time, lower crosslink density, and lower glass transition temperature, due to the decreasing of crosslinking site.
74
CHAPTER VI
CONCLUSIONS
In this thesis, two focus areas related on the modification of tung oil for bio-based
coating were studied. First, two UV-curable materials based on tung oil, UV-curable
Tung Oil (UVTO) and UV-curable Tung Alkyd (UVTA), were synthesized. Bodied tung
oil and tung-based alkyd were prepared, and then modified with a trimethylolpropane
trimethacrylate (TMPTMA) monomer via the Diels-Alder reaction. The reactions of
acrylate-modified oil were conducted at elevated temperatures and found to be
convenient at atmospheric pressure. Evaluation of acrylate modified tung oil as UV-
curable materials showed that the modified tung oil and tung oil alkyd had the capability
of photopolymerization when exposed to the UV-light. It was also found that the reaction
rate of UVTA is faster than UVTO.
Second, tung oil alkyd was prepared via the same method used in chapter IV, and
then functionalized with three different acrylate monomers via the Diels-Alder reaction.
All alkyd-modified reactions were conducted at elevated temperatures and atmospheric
pressure. The results showed that fluorine-modified alkyd increase the surface activity,
but lacks in crosslink density and drying due to the decrease of crosslinking functionality.
The increase of crosslink density and the decrease of drying time of siloxane- and triallyl
ether-modified alkyd are attributed to the increase of the crosslinking functionality
compared to the unmodified alkyd.
75
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