D. M. Brewis

Adhesion to polyethylene and polypropylene

Abstract Adhesion to polyethylene and polypropylene is a complex subject requiring understanding of (a) the poor adhesive characteristics of these polymers; (b) the superior performance following certain pretreatments and (c) the nature of the changes brought about by these pretreatments and the mechanisms involved. This review discusses work on these topics and examines the impact of recent data resulting from the application of surface analytical techniques. The roles of ‘weak boundary layers’, surface energy and wettability and specific interactions are discussed in some detail.

Review of surface pretreatments for titanium alloys

Many treatments to enhance the adhesion to titanium have been studied, especially with respect to their ability to improve the durability of bonded joints. This review examines the effectiveness of a diverse range of treatments in many durability studies and using a variety of tests. The review also identifies 23 techniques that have been used to examine the surfaces of pretreated titanium alloy. These range from complex and costly ultra-high vacuum based surface analytical techniques, such as X-ray photoelectron spectroscopy, to those which are more simple and commonly available, such as contact angles. Overall, the most effective methods are sodium hydroxide and chromic acid anodizing. The recently developed plasma spray treatment is showing much promise.

The durability of some epoxide adhesive-bonded joints on exposure to moist warm air

Abstract Single lap joints bonded with an epoxide adhesive have been exposed to warm moist air for periods up to one year, and then subjected to strength measurement. Surface treatment of the aluminum alloy adherends had an effect upon the ability of joints to resist exposure, the order of surface treatment efficiency in this respect being chromic acid anodize > chromic acid etch > sandblast > degrease. A linear relationship has been observed between joint strength and the water content of joints, leading to the conclusion that water enters a joint by diffusion through the adhesive, rather than by passage along the interface.

X-ray photoelectron spectroscopy studies of polymer surfaces: Part 3 Flame treatment of polyethylene

X-ray photoelectron spectroscopy showed that a normal flame treatment caused a high level of oxidation in low-density polyethylene. 0.02% of the antioxidant 2,6-ditertbuty-p-cresol did not reduce the degree of oxidation or the level of adhesion in contrast to the extrusion of low-density polyethylene. It is estimated that the depth of oxidation is between 40 and 90 Å which is much less than for a moderate chromic acid treatment or with extrusion. There were no significant changes in the XP-spectra or adhesion levels of flame treated samples after 12 months.

X-ray photoelectron spectroscopy studies of polymer surfaces

The effect of melting polyethylene on aluminium has been re-examined using X-ray photoelectron spectroscopy. Even at 150° C, low and high density polyethylenes show degrees of oxidation similar to that observed with conventional pretreatments; large increases in adhesion are also observed. The results are discussed in relation to transcrystalline regions and weak boundary layers.

The effect of humidity on the durability of aluminium-epoxide joints

Abstract Single lap joints of sandblasted aluminium alloy have been prepared with anepoxide adhesive based on dgeba and 1, 3-diaminobenzene. They have been exposed to air at 50°C and to a range of humidities for up to 10080 h, and then tested. After 10080 h ageing the results give some evidence for a critical relative humidity at 65%, such that joints weaken to a greater extent if this value is exceeded. Measurement of mass uptake of films of the adhesive suspended in air at various humidities has provided the water absorption isotherm and diffusion coefficients. The isotherm is gently curved and of BET type IV; there are no sharp changes that might account for a critical relative humidity. The presence of salt hydrates at the interface is a feasible explanation for this.

Pretreatment of poly(vinyl fluoride) and poly(vinylidene fluoride) with potassium hydroxide

Abstract Treatment of poly(vinyl fluoride) (PVF) and poly(vinylidene fluoride) (PVdF) with hot aqueous solutions of potassium hydroxide resulted in large increases in adhesion. With PVF, the addition of small quantities of tetrabutylammonium bromide to an aqueous solution or the replacement of an aqueous with an alcoholic solution resulted in much more rapid treatment under a given set of conditions. Effective treatments of just 30 and 10s were developed for PVF and PVdF, respectively. Substantial changes in surface chemistry were observed using XPS for most of the pretreatment conditions used; the changes were mainly in the form of defluorination plus introduction of oxygen. The changes were most marked for PVdF treated with solutions containing a small amount of TBAB. Elimination reactions are likely to be of particular importance with PVdF, a view supported by the existence of CC and CC bonds. Substantial chemical modification of PVF did not necessarily result in large increases in adhesion. This is attributed to the chemical modification of a weak boundary layer without its removal.

Influence of surface macroroughness on the durability of epoxide-aluminium joints

The influence of varying the adherend surface macroroughness on the durability of epoxide-aluminium joints has been studied. It is concluded that variations in surface macroroughness as observed by scanning electron microscopy and measured by profilometry have a small but significant effect on durability. Good durability was obtained with degreased-only aluminium despite a relatively high level of magnesium in the outermost surface.

Pretreatments of Fluoropolymers

Abstract In the present study the mechanisms and effectiveness of various pretreatments for fluoropolymers were studied. The pretreatments were “Tetra-Etch,” various plasmas, flame and potassium hydroxide. “Tetra-Etch” was found to be much more reactive than potassium hydroxide (KOH) towards fluoropolymers. The plasma treatment of PTFE showed that it was possible to get substantial increases in adhesion with little or no chemical change to the polymer. However, to obtain large increases in adhesion it may be necessary to modify PTFE chemically as with “Tetra-Etch.” Consideration of the bonding of these fluoropolymers shows that sharp interfaces between these substrates and adhesives do not exist.

Further studies into the effectiveness of CO2-laser treatment of metals for adhesive bonding

Abstract The effect of CO2-laser treatment on the wettability of mild steel is presented. In addition, data are presented on the initial joint strengths and durability of joints formed between a single-part epoxide and both mild steel and aluminium. A large increase in stressed durability performance was observed with the laser-treated aluminium compared with degreased-only controls. The laser treatment was shown to efficiently remove the organic contamination from the metallic substrates. Auger analysis showed that the laser interacts more with the mild steel than the aluminium adherends, to produce a relatively thick surface oxide. The changes to the mild steel surface introduced by the CO2-laser treatment facilitate an durability trials was greater with the laser-treated adherends than with degreased-only controls.