Some miscellaneous comments on adhesion

 

D. E. Packham,

Materials Research Centre,

University of Bath.

 

For a collection of short, self-contained articles, which together give comprehensive account of the scientific, engineering and industrial aspects of adhesion, see:

 

D.E. Packham (ed.), Handbook of Adhesion, 2nd edition, Wiley, 2005.

available on line:

http://www3.interscience.wiley.com/cgi-bin/bookhome/111084161

 

However, some miscellaneous comments on adhesion are provided below

 

Bonds at the interface

Organo-silane adhesion promoters are widely used to enhance the adhesion and bond durability with glass and metals. Typical structure (a) and postulated mode of action (b) for a silane adhesion promoter. There is much evidence that their mode of action is much more complex than this, e.g. see discussion in

D.E. Packham, Some contributions of surface analysis to the development of adhesion theories, J. Adhesion  84,240–255(2008).

 

The next figure shows various interactions postulated between PMMA and surfaces of differing acidic and basic nature. It is based on work of

S.R. Leadley, and J.F. Watts, J. Adhesion 60(1-4) 175-196 (1997). 

E.G.      (Si)                              (Al)                  (Ni)

H-bond (Si)                              ionic (Al)*        ion/dipole (Ni)

* indicating hydrolysis of the ester and adsorption through the carboxylate anion

 

Rough surfaces

With rough surfaces there may be serious problems of the adhesive's being abl to enter proes and crevices on the substrate surface. This figure shows equilibrium penetration of a liquid into cylindrical and "ink bottle" pores, after de Bruyne.

 

A rough surface will have a higher surface energy than a corresponding smooth one. This is shown in the schematic diagram below. In this simplified two dimensional representation, the bulk atom (B) has 6 nearest neighbours, the atom on a plane surface (S) has 4, and that on an asperity on a rough surface (A) only 2.

 

 

 

 

 

Some macroscopically rough surfaces are known to enhance adhesion.  Here undercut TiW islands enhance adhesion of copper to silica. Based on A.M.T. van der Putten, J. Electrochem. Soc. 140, 2376(1993). (i) TiW islands deposited (ii) Pd activator adsorbed and HF etching; (iii) electroless Cu deposited; (iv) Cu electrodeposited.

 

Some very rough surfaces can result from pretreatment prior to adhesive bonding. Here are some microfibrous surfaces (l to r): dendrites of zinc electrodeposited onto a zinc surface; black CuO layer produced on copper; PTFE irradiated by argon ions. The last based on S.K. Koh, S.C. Park, S.R. Kim, W.K. Choi, H.J. Jung, and K.D. Pae, J. Appl. Polym. Sci., 64,1913(1997), courtesy of the authors.

 

 

 

 

 

The increased adhesion to microfibrous surfaces is often associated with increased plastic deformation of the polymer during failure of the adhesive bond, e.g. substrate surface after peeling LDPE from (a) polished copper and (b) copper with a microfibrous oxide surface:

 

 

The concentration of stress at the "fibre" tips can be demonstrated using a photoelastic model [J.R.G. Evans, Ph.D. thesis, University of Bath 1977]:

 

 

 

 

 

 

Polymer-polymer bonding

Bonds between immiscible polymers (A & B) are usually very weak, but some interdiffusion can occur if the interaction parameter is not too high, cf. R.P. Wool: Polymer interfaces: structure and strength, Hanser, Munich (1995) & in Adhesion Science and Engineering, Vol. II, Surfaces, Chemistry and Applications, Assoc. Ed. Chaudhury, Manoj K. and Pocius, A.V. Elsevier, p. 351-402, 2002

 

The figure shows how the laminating temperature affects the adhesion of four different ethylene-octene copolymers to polypropylene (ethylene-octene copolymers and polypropylene are formally incompatible):

L. Godail and D.E. Packham, J. Adhesion Sci. Technol. 15(11), 1305(2001).

 

 

The interface between immiscible polymers (A & B) may be strengthened by introduction of a diblock copolymer AB. Here the A-block length is below entanglement length, so the overall toughening is modest.

 

However, with a long chain length diblock copolymer, much higher toughness is obtainable:

 

Results like these have been published by Creton and colleagues, among others:

C. Creton, H.R. Brown, and V.R. Deline, Macromolecules 27, 1774(1994).

C. Creton, H.R. Brown, and K.R. Shull, Macromolecules 27, 3174(1994). 

 

 

 

Random copolymers – usually cheaper than diblocks – can also be used if the incompatibility between the homopolymers is not too large. The molecule of a random copolymer will form coils wandering many times across interface, forming many "stiches". For large incompatibility the copolymer will simply form a collapsed globules at interface, giving no enhancement of adhesion. Incompatibility increases in the order (a), (b), (c):