Adhesion and Cohesion, Liquid Penetration

Adhesion and cohesion, Liquid penetration

CHEM-E2135 - Converting of Web-Based Products Spring 2019 Jouni Paltakari, Eero Hiltunen

1 Why glue is gluing?

• Adhesion: – ”how well glue is attached to the substate surface” – Work needed to separate glue layer from the substrate surface • Cohesion: – ”Internal strenght of glue” – Work needed to split the glue layer

2 Adhesion • ”State in which dissimilar bodies are held together by intimate interfacial contact so that mechanical force can be transferred across the interface”. • Force/work required to separate the bodies.

3 Work of adhesion

e.g. glue

Substrate B

Dupré equation

where Is surfacepintaenergia energy

Work of adhesion, Wa, is needed to separate dissimilar bodies along their interface, to form two separate material-air interfaces.

4 Work of cohesion

e.g. glue two glue ”piece” ”pieces”

where Is surface energy

Work of cohesion, Wc, is needed to form two air-body interfaces from one body.

5 Cohesion and Adhesion: case liquid state • Molecules in the liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. • For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute surface tension.

6 H2O is a polar molecule

H2O is a polar molecule, with an electrical dipole moment The oxygen atom has a higher electronegativity than hydrogen atoms

By User Qwerter at Czech wikipedia: Qwerter. Transferred from cs.wikipedia to Commons by sevela.p. Translated to english by by Michal Maňas (User:snek01). Vectorized by Magasjukur2 - File:3D model hydrogen bonds in water.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/ind 7 ex.php?curid=14929959 Cohesion and Adhesion: case liquid state • When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces which leads to an upward turning meniscus at the walls of the vessel and contribute to capillary action.

• The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds. 8 Adhesion-Cohesion-Surface tension • Cohesion causes water to form droplets • Surface tension causes them to be nearly spherical • Adhesion keeps the droplets in place

9 Adhesion is a critical factor in several papermaking and converting processes

• Fiber-fiber bonding • Retention of size and fillers to paper structure

• Side gluing of sacks and boxes • Combining liner and fluting by gluing in corrugated board manufacturing • Combining plastic film and paper in extrusion process • Attachment of coating material to the substrate in dispersion-, pigment-, or hot-melt coating • Interaction between label and release paper and sticking of label to final surface • Gluability of wall paper to the wall surface material 10 Theories of adhesion

Mechanical interlocking ”Specific theory”

Thermodynamic Electrostatic Chemical Weak boundary Diffusion Polarisation adsorption theory adhesion theory

Induced Hydrogen Dipole dipole bonding

11 Theories of adhesion

Understanding of wetting, adhesion and surface phenomena has increased a lot in recent years

BUT

There is still no single theory explaining adhesion completely!

12 Mechanical interlocking

• Main mechanism in adhesion is the penetration of one component into irregularities in the other surface. • Phenomenon with porous substrates; e.g. wood, paper, textiles.

Example: Penetration of size/glue into the paper structure and adhesion when glue is hardening.

13 Peel strength vs. PE-LD coating weight on three papers with different surface smoothness (calendering treatment).

Filled symbols indicate paper matrix failure. 14 Diffusion theory

• Compatible materials with equal solubility parameters can form a transition zone where interdiffusion of macromolecules or molecule segments can occur. • Close molecular contact by wetting is necessary for interdiffusion. • Adhesion due to diffusion only occurs with identical or compatible polymers. Examples are adhesion between different layers in coextrusion or autohesion (layers of the same polymer) • Segmental interdiffusion and an interfacial layer of 10-1000 Å forms between two incompatible polymers. • Other important cases where diffusion occurs are the heat sealing of thermoplastics and the film formation of latexes Not present, when adhesion to metal or glass surface

Probably not a main mechanism for fibre surfaces Possible fibre examples: * Diffusion of size with small molecules to fiber cell wall in paper * Strenght between board layers (diffusion of hemicellulose)

15 Wetting • The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces • Wetting is important in the bonding or adherence of two materials • As the tendency of a drop to spread out over a flat, solid surface increases, the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability • A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is very favorable • For water, a wettable surface may also be termed hydrophilic and a nonwettable surface hydrophobic

Water beads on a fabric that has been made nonwetting (hydrophobic) by chemical treatment16 Wikipedia – Creative Commons Diffusion theory – example: adhesives on PET

Figure 2. Peel strength and solubility parameter difference for various adhesives on poly (ethylene terephthalate).17 Electrostatic theory • Idea of an electrical double layer forming when different materials come into contact. • Adhesive and substrate act as a capacitor, and adhesion is an attractive force across the double layer. • Has a role in polymer-metal adhesion bonds

No big meaning for fibre surfaces 18 Thermodynamic adsorption (or Wetting theory) • Wetting is a precondition for adhesion in this instance

• Interdiffusion is not necessary to achieve good adhesion

• Close contact and molecular and physical interactions between materials are essential to adsorption theory

Has a big meaning for fibre surface 19 a) Covalent b) Hydrogen bond c) Van der Waals nm 1Å = 0.1 nm

Figure. Potential energy curves for different types of interaction/interatomic forces. 20 Contact angle (q), Surface energy (g), Work of adhesion (Wa)

Vapour

gLV gSV = gSL + gLVcosq Liquid q gSV g – g Solid g SV SL SL cosq = gLV

Wa = gLV (1+ cosq)

21 DEFINITIONS:

WETTING: Wetting refers to the intermolecular interactions when forming an interface (liquid-solid). Wetting occurs if change in free energy <0. A system always tries to reach a lower energy state. The amount of wetting depends on the energies (or surface tensions) of the interfaces involved such that the total energy is minimized.

The criteria for wetting, formation of a contact angle; work of adhesion is greater than zero .

DGw = Gafter –Gbefore = gSL-(gS+gL) = -Wa<0 => Wa>0.

Contact angle with water q >0 (0< q <90°).

Precondition for wetting: Work of adhesion > work of cohesion, if liquid is wetting the surface but not spreading over it

22 DEFINITIONS:

SPREADING: Spreading refers to the replacement of the solid surface by a solid with liquid interface and a liquid surface after the contact.

DGS = Gafter –Gbefore = (gSL+gL) -gS <0

gSLis often small compared to gL à spreading occurs if liquid surface energy is lower than that of a solid.

Spreading occurs when the contact angle is zero (q =0) and also indicates the movement of liquid regardless of the contact angle.

23 Hysteresis and metastable state

• Hysteresis is the difference between advancing and receding angles. • If the size of a liquid drop on a solid increases or decreases, the same contact angle does not occur. • Common reasons for hysteresis are – roughness – Morphological and chemical heterogeneity of the surface, – interactions between the solid and liquid – variation of areas with low and high free energy on a surface.

24 Figure 5. A drop of liquid in two metastable configurations.

25 Chemical adhesion • In addition to van der Waals forces, stronger metallic bonds exist: – ionic – covalent • Work of adhesion can be 25-40 times larger – metallic bonds

26 Bond energy Bond length Bond (kJ/mole) 8* (Å) 2

chemical covalent 63-710 1-2 ionic 590-1050 metallic 113-347

van der Waals 3.6 dispersion 0.08-42 dipole-dipole 4-21 dipole-ind. dipole max. 2

hydrogen 2.7 involv. fluorine max. 42 excluding fluorine 10-26 * Bond energies originally from Pauling and Good according to Kinloch 8.

Table. Various bonds, their energies, and lengths. 27 Examples • Chemical bonding: Interfaces between a polymer and metall. • Covalent and ionic bonding: formation of an ester from an anhydride in a tie resin with the hydroxyl group in ethylene vinyl alcohol during coextrusion. • Example of covalent bonding is between oxygen plasma treated paper and untreated polyethylene due to the hydroperoxides formed on the paper surface.

28 Weak boundary layer theory • Adhesion based on molecular interaction forces: – Dispersion forces – Hydrogen bonds – Induction forces

Theory states that a failure within a WBL near the interface causes poor adhesion.

WBL theory describes differences in bonding

Has a big meaning for paper products 29 Weak boundary layer theory • The surface of paper can have a WBL due to fewer or less bonded fibers. • Practical examples of the WBL theory: – linting – adhesive bonding – paper coating – In pigment coating, polyvinylacetate as a binder forms WBL

30 Testing of adhesion and cohesion in practise

31 ADHESION-COHESION TESTING PEEL TEST

Figure 8. Peel test configurations. 32 Value Criteria of evaluation

0 layers do not adhere 1 layers peel off each other 2 layers peel off each other, some fibers are removed 3 fiber tear < 50% of surface area 4 fiber tear > 50% of surface area 5 total fiber tear

Table. Criteria of evaluation in hand test.

33 Interaction of liquids with porous materials (paper, board)

• Essential event in, e.g. – printing – surface sizing – coating – impregnation Driving force • Dynamic event: Rate = Resisting force • Lucas-Washburn equation & Hagen-Poiseuille eq.

34 Control of water penetration

Base paper properties and liquid interaction – Lucas-Washburn equation: capillary penetration; surface chemistry – Hagen-Poiseuille equation: pressure penetration; driving pressure, pore diameter Capillary penetration

• Modified Lucas-Washburn equation

t×g×r×cosq V = p×r2× – k×Dz 2h

V Amount of liquid penetrated into the pore r Radius of the pore g Surface tension of liquid q Contact angle of liquid h Viscosity of liquid t Time k×Dz Effect of swelling Assumptions behind Lucas-Washburn equation a) Cylindrical pores assumed! b) Driving force, pressure difference, is constant all the time c) Flow resistance is caused only by the laminar flow resistance

These assumptions are often not fully valid in paper/board applications e.g. fibre swelling by water absorption changes pore dimensions

37 Capillary penetration, background for Lucas Washburn • ”Filter cake equation” (Xian et al; 2000, 2004)

t ×r ×g ×cosq V = e × t×g×r×cosq æ e 2 ×f ×r 2 ö V = p×r2× 2×hç1+ s ÷ 2h ç ÷ è 8× K ×f f ×(1-fs ) ø

NOTE: V Amount of liquid penetrated into the pore When permeability r Radius of the pore Kà ∞ g Surface tension of liquid this equation reduces q Contact angle of liquid to Lucas-Washburn equation h Viscosity of liquid t Time K Permeability (Darcy coeff.) e void fraction (eqv. to surface porosity) Fs solids volume fraction in original fluid Ff solids volume fraction in filter cake Restrictions of Lucas-Washburn equation

In reality limitations and restriction come from: • Inter-fiber-pores are not cylindrical capillaries • Tortuous flow paths • Liquid fills pores only partially (film flow vs. ”piston flow”) • L-W does not take into account liquid inertia

à L-W equation does not adequately describe water transport in paper during short contact times Pressure penetration

• Hagen-Poiseuille equation

p×r4×p×t V = 8×L×h

V Amount of liquid penetrated into the pore r Radius of the pore p External pressure t Time L Length of the pore h Viscosity of liquid

• Cylindrical pores assumed! Water absorption of coating base paper • Normal pressure ) 2 /m 3

1/2 Absorbed water amount (cm Time (s )

1) LWC base paper containing refiner pulp, 2) SC-paper 3) LWC base paper containing groundwood pulp 4) Hydrophobe sized woodfree paper. Measurements accomplished with P. Salminen’s water absorption equipment.

41 Water absorption of coating base paper • 0.5 bar pressure ) 2 /m 3 Absorbed water amount (cm Time (s1/2)

42 1(3) Water sorption applications

Sizing Water penetration into paper is controlled with hydrofobic sizing. Sizing is carried out either by stock sizing or surface sizing (size press) or by combination of these.

Surface sizing Surface sizing aims at increasing surface strength and tensile strength of paper. Sorption properties are changed simultaneously.

43 2(3)

Coating Paper coating improves printability and appearance of paper. Pore size and pore volume are smaller in coat layer compared to base paper and board. Printing ink liquid phase penetration is much slower into coated paper than into uncoated paper. This results in lower ink demand and better printing result and print gloss.

Printing It is essential in printing that printing ink can wet paper surface easily; adhesion forces between paper and ink has to overcome the cohesive forces in printing ink. In gravure printing the adhesion forces between paper and the raster cup have to transfer the printing ink from the cup to paper surface.

44 Dimension stability 3(3) Paper dimensions changes relate to water sorption of paper and fiber swelling.

Greaseproof papers Greaseproof papers are used as wrappings for butter and other food products. The greaseproof character comes from extensive beating of the paper fibers and heavy calendering. Sulfuric acid treatment is also used.

Absorbent papers Typical absorbent papers are tissue papers, filter papers and papers for impregnation process. High adsorption capacity is achieved with low beating and controlling paper making process not to transfer wood fiber resins to hydrofobic form. Paper creping is used to increase void volume and adsorption capacity.