The Basics of Adhesion

1. The Basics of Adhesion • Generating strong interphase adhesion, requires: • 1. Wetting the surfaces to generate intimate contact between • surfaces • High Contact Angle Zero Contact Angle • Poor wetting of surface Complete Spreading • 2. Solidification to a cohesively strong solid that provides • sufficient adhesive strength with the matrix such that applied • loads are withstood. J.S. Parent

2. Adhesion in Polymer Systems • An advanced treatment of adhesion in polymeric systems requires an analysis of three phenomena: • 1. Characterizing the surface properties/interactions of dissimilar • materials: • origin of interfacial forces • surface energy of liquids and solids • 2. Interaction of polymer surfaces with liquids: • wetting, spreading • work of adhesion • 3. Interaction of solid polymers with other solids: • contact adhesion of viscoelastic materials J.S. Parent

3. Conceptual Models for Adhesive Bonding • The actual mechanism of adhesive attachment is not thoroughly understood - no single theory explains the phenomenon universally. • Adsorption theory: • Adhesion results from the adsorption of adhesive molecules onto the substrate, the resulting secondary attractive forces giving rise to the observed bond strength. • Adhesive must therefore make intimate, molecular contact with the adherends. • Mechanical theory: • Adhesion occurs by the filling of micro-cavities within the substrate by the adhesive formulation. Bond strength is derived from cured/dried polymer properties. • Adhesive strength favours large bond areas, and surface roughness. J.S. Parent

4. Interfacial and Bulk Material Forces J.S. Parent

5. Forces Acting at a Liquid/Vapour Interface • Molecules near an air-liquid interface are subjected to an unequal distribution of the forces that act to maintain a condensed phase. • The result is a net inward • force experienced by • molecules at the surface, • and a contraction of the • fluid into its lowest energy • state. • The surface energy (commonly called the surface tension of a liquid) characterizes this effect. • The surface energy of a substance is the work required to increase the area of a surface (usually in air) by unit amount. • Interfacial surface energy (interfacial tensions) represent the imbalance of forces at the surface of two liquids. J.S. Parent

6. Surface Energy of Liquid/Vapour Interfaces • The energy associated with a phase changing its surface area is described in a thermodynamic sense by the concept of surface energy. In terms of the Helmholtz free energy (F(T,V) as opposed to G(T,P)): • A droplet, for instance, can lower its free energy by reducing its surface area (dA<0) to the point where internal pressure offsets the contribution of area. • Capillary Rise Measurement: • Surface energies of liquids in their saturated vapour • are readily measured with a capillary rise tube, • shown to the right: • where r is the capillary radius, h the height difference, • g the gravitational constant and  the fluid densities. J.S. Parent

7. dG ? Liq. B Liq. B Liq. A Liq. A Surface Energy of Solid-Liquid Interfaces • Given that our system exists under static conditions, we can use a thermodynamic approach to examine surface phenomena. • What changes in Gibbs energy result from increased contact area between liquids A and B? • As contact between A and B changes, the area of exposure of A to the vapour changes (dAA), as does that of B to the vapour (dAB) and A to B (dAAB). The Gibbs energy responds as follows: • (1) • or, • (2) • where gi represents the surface/interfacial free energy (J/m2). J.S. Parent

8. Spreading Coefficient - Complete Wetting • When will a liquid completely wet a surface? Predictions can be made on the basis of the spreading coefficient. • Realizing that if liquid B increases in area it is at the expense of A and the benefit of AB, • then equation 2 is simplified by substitution: • The spreading coefficient SA/B, defined by: • represents how the Gibbs energy of the system will respond to a change in the surface area of liquid A. • For surface wetting to occur, SA/B must be positive • Under this condition, wetting lowers G and is spontaneous. J.S. Parent

9. Work of Adhesion • Failure of an adhesive joint creates two new surfaces. The energy expended per unit area should be the sum of the two surface energies, gLV and gSV. However, intermolecular forces were present at the joint prior to failure. • The work of adhesion per • unit area, WA, is given by • the Dupré equation: • Difficulties arise in trying to use this equation to represent liquid/solid and solid/solid adhesion: • Absolute values of gSV and gSL cannot be measured • Strain induced at the interface upon solidification is not described • This treatment describes only reversible adhesive failure J.S. Parent

10. Work of Adhesion • Polymers capable of strong dipole association and/or hydrogen bonding are favoured in adhesive formulations due to strong associations with high-energy surfaces. • Bonds derived from these polymers are often susceptible to interfacial failure by the action of water. J.S. Parent

11. Solid/Liquid Interfaces - Contact Angle • When a liquid placed on a solid generates SA/B < 0, it remains as a droplet with a defined angle of contact, . • Spreading increases contact • between liquid and solid, • altering the Gibbs energy of the • system. • Spreading creates dA of liquid-solid area, dA cos  of liquid-vapour area and -dA of solid-vapour area: • At equilibrium, dG=0: • Young Equation J.S. Parent

12. Work of Adhesion • The indeterminable parameters in our original expression for the work of adhesion, • can be replaced by the contact angle using Young’s equation, thereby transforming WA into: • This simple treatment of adhesion (Young-Dupre equation) relates bond strength to determinable quantities, the liquid-vapour surface tension and the contact angle the liquid makes with the solid. • Note however: • this relationship states that WA can only vary by a factor of two from the surface energy of the liquid. • Solidification of the liquid can develop stress concentrations. • Adhesive failure is irreversible, depending as much on rheology and fracture mechanics as interfacial thermodynamics J.S. Parent

13. Critical Surface Energy • In the Young equation only gLV and cos are measurable. However, gSV and gSL have are useful parameters for predicting adhesion. • Pioneering work by Zisman revealed a linear relationship between contact angle and gLV for a series of liquids: • where gc is defined as the critical surface energy for the solid and b is a constant. • The critical surface tension for the solid is defined as the intercept of the horizontal line cos=1 with the extrapolated straight line of cos against gLV. • A hypothetical test liquid of this gLV would spread on the solid, while one with a greater gLV would wet the solid with a measurable contact angle. J.S. Parent

14. Critical Surface Energy • Shown to the right is a • Zisman plot for determining • the critical surface tension • of poly(tetrafluoroethylene). • Test liquids are n-alkanes • of different surface tensions. • The choice of test liquids • affects the results, and • differentvalues of gc can • be obtained depending upon whether polar, non-polar or hydrogen-bonding fluids are used. J.S. Parent

15. Critical Surface Tension • To assure spreading and wetting of an adhesive formulation on a substrate, the fluid adhesive should have a surface tension no higher than the critical surface tension of the solid adherend. • A solid can induce liquids of lower, but not higher surface tension, to wet it (if no surface chemical reaction occurs). • gc (20°C): mN m-1 • Poly(tetrafluoroethylene) 18 • Silicone, polydimethyl 24 • Poly(ethylene) 31 • cis-Poly(isoprene) 31 • Poly(styrene) 33 • Poly(vinyl alcohol) 37 • Poly(methyl methacrylate) 39 • Poly(vinyl chloride) 40 • Poly(acrylonitrile) 44 • Amine-cured epoxide 44 • Poly(ethylene terephthalate) 45 • Cellulose 45 • Poly(hexamethylene adipamide) (nylon 6,6) 46 • Aluminum ~500 • Copper ~1000 J.S. Parent

16. Critical Surface Energy - Coatings J.S. Parent

17. Graft Copolymers: ABS Resins • TEM of an emulsion-prepared ABS. Typically emulsion rubber domains contain little occluded copolymer of styrene and acrylonitrile. TEM of a suspension-prepared ABS. Typically rubber domains in suspension-derived polymer contain substantial amounts of occluded copolymer of styrene and acrylonitrile. J.S. Parent

18. Graft-Modified Polyolefins • Melt grafting of maleic anhydride and vinylalkoxysilanes to polyolefins generates reactive materials that can improve phase adhesion between high-energy and low-energy materials • Nylon and polyethylene • Silica and polyethylene J.S. Parent