{For full references see literature.}
POLYMER-METAL and polymer-polymer ADHESION, MOULD RELEASE AND POLYMER STABILITY.
Much of the research interest is centred around understanding a fundamental aspect of adhesion: the relationship between the physical and chemical structure of the interface and the macroscopic properties of the bonded joint. Studies have been made of the effects of substrate surface topography on the adhesion and durability of structural adhesive bonds with metals including aluminium, steel and copper. At the other end of the spectrum of adhesion values, work has been done on mould adhesion in elastomers. Measured adhesion is affected by mechanical properties of the polymer and by residues therein. Adhesion between incompatible polymers is also of interest and current work is seeking to understand the interaction between interfacial morphology, chain interpenetration and adhesion energy. Projects have also been conducted on the influence of residues on the vulcanisation and crosslink structure of nitrile rubber mouldings.
Most of the projects mentioned involve problems of obvious industrial relevance. It has therefore been appropriate to work on the values and philosophies which lie behind the present pressure for increasing collaboration between industry and universities. Analyses have been made drawing on the ideas of Alasdair MacIntyre and Isaiah Berlin on value incommensurability and applying them to university-industry interaction.
Some further details of these and other projects follow.
SILICONE ADHESION
The adhesion of silicones to a range of substrates is being investigated. Both 'filled' and unfilled silicones based on polymers such as poly(dimethylsiloxane) are involved. It is intended to rationalise the different levels of adhesion observed, in terms of the properties of the polymers and of the interfaces.
STRUCTURAL ADHESIVE BONDING OF ALUMINIUM.
Pretreatment: topography, surface chemistry and bond durability.
Work has involved joints between a clad aluminium alloy with different pretreatments, bonded with epoxy resin. The datum for comparison of the pretreatments is the Boeing phosphoric acid anodisation. Other treatments include sulphuric acid anodising in combination with a phosphoric acid dip and a sulphuric acid/ferric sulphate etch, as well as application of silane coupling agents.
The topographical structure of the surface layers formed have been examined using scanning and transmission electron microscopy in conjunction with ultramicrotome sectioning. The chemical composition of pretreated surfaces has been characterised by X-ray photoelectron spectroscopy and depth-profiling using argon ion etch performed.
Adhesive bonds have been prepared and their durability assessed using wedge test specimens. The data collected has been used to calculate crack growth and strain energy release rate as functions of time for the bonds produced. Surfaces of failed specimens have been examined to establish the locus of failure.
RUBBER TO METAL BONDING.
Pretreatment: adhesion mechanisms, reproducibility
The purpose of this project was to examine some of the fundamentals of the industrially important vulcanisation bonding process. Adhesion of a natural rubber compound was measured using a peel test: various peel angles were investigated. A steadier peel trace and smoother fracture surface with failure closer to the substrate were associated with 45° peel angle, suggesting the suitability of this peel angle as a 'standard' in this type of work. Adhesion to differently phosphated steel surfaces was compared with that to acid-etched steel. In all cases good adhesion was found with failure mostly cohesive within the rubber. This indicated that the rough phosphated surface was not a prerequisite to effective bonding in the circumstances. Work with a model bonding agent found evidence that interaction between chlorinated rubber and natural rubber at vulcanisation temperatures led to homogenisation and improved adhesion.
This work was supported by the E.P.S.R.C and Avon Clevite Ltd. For a summary of the final report click here
MOULD RELEASE IN RUBBER PROCESSING.
Residues, the mesophase and consistency.
When compounds based on nitrile rubber (NBR) are vulcanised in a mould a complex interfacial layer is formed at the surface of the mould. The nature of this layer depends strongly on the grade of base rubber involved. The components of the layer include vulcanisation and processing aids as well as residues of the NBR manufacturing process. When the moulding is removed parting occurs within the inter-layer: its constituents exert a dominant effect on the adhesion.
Rubber-mould adhesion has been studied by measuring the mould sticking index with a TMS rheometer and investigating the surfaces formed by X-ray photoelectron spectroscopy. Residues which exert a significant effect include organo-sulphur emulsifiers, fatty acids and coagulant cations - all are common constituents of the 'mesophase'.
THERMO-OXIDATIVE AGEING OF RUBBER.
Consistency, compound and base polymer.
Network changes occurring during thermo-oxidative ageing of sulphur-cured N.B.R. vulcanizates have been characterised by treatment with thiol-amine and methyl iodide chemical probes followed by crosslink density measurement. Further information was obtained from stress relaxation studies and X.P.S. analysis.
Commercial polymers, of identical specification, were formulated with two sulphur based gumstock compounds differing in the addition of diphenyl amine antioxidant. The results differed in detail according to which commercial base polymer was used.
Diphenyl amine accelerates vulcanization and increases the proportion of mono-sulphide crosslinks through a reduction in the sulphur:zinc ratio in the sulphur-accelerator-zinc complex.
During thermo-oxidative degradation there is initially a conversion of di-sulphide and poly-sulphide crosslinks to mono-sulphides with little change in overall crosslink density. The process is accelerated by diphenyl amine. With further ageing there is a rise in overall crosslink density. Mono-sulphide crosslinks are cleaved and the new crosslinks formed are predominantly C-C. At extended ageing times some di-sulphide crosslinks are formed through interaction of pendant thiols and other oxidation structures.
Oxidation is accompanied by an increase in oxygen content, associated with ketone and carboxyl groups and a reduction in nitrogen. Water is a major product of oxidation.
POLYVINYL ACETATE COMPOSITES AND ADHESION
Collaborative work with Professor Vera Kovacevic's group at the University of Zagreb.
Much work has been done on structure-property relationships in polyvinyl acetate composites. The work is concerned with the properties of polymer films produced from polyvinyl acetate emulsions into which calcite fillers have been incorporated. Of particular interest is the influence of loading level, surface area and shape of filler particles and especially the effect of surface treatment of the filler by different levels of stearic acid. Thus there are now available a range of emulsions from which filled polymer films can be produced with a range of properties. Procedures for producing PVAc-metal bonds and for measuring the adhesion obtained have been established, and it is hoped to developed in the future.
ADHESION OF polypropylene to ethylene-octene copolymers
Copolymers of ethylene and octene can act as "thermoplastic elastomers" with the crystallinity's providing "physical crosslinks". In some applications they are melt bonded to polypropylene. It is of interest to consider the effect of density (crystallinity) and melt flow index of the copolymers on the adhesion obtained. Adhesion is strongly dependent upon the moulding temperature. The magnitude of the peel energy depends on the mechanical properties of the particular copolymer and on the morphological changes which occur in the bonding process.
DEGRADATION OF P.V.C. -Ideas for a future project
Environmental benefit: recycling and reduction in heavy metal additions
As a polymer, P.V.C. has poor thermal and photostability, readily undergoing an unzipping reaction in which hydrogen chloride is given off leaving a conjugated sequence of single and double carbon-carbon bonds in the residual polymer. This feature imparts colour to the material, yellow, brown and eventually black, as the length of the conjugated sequence increases. Although the main features of P.V.C. degradation have been known for many years, the mechanism is still a matter for debate. Thus P.V.C. would not be able to function in most of its service applications except for the inclusion of stabilisers which are commonly compounds of toxic metals such as lead and cadmium.
Studies on low molecular weight 'model compounds' suggest that the instability of unstabilised P.V.C. may be connected with structural irregularities in the polymers. This raises the question of the extent to which the stability could be manipulated by (i) removal of sites of initiation and (ii) inclusion of features which might limit the extent of the hydrogen chloride unzipping reaction.
Considerable environmental benefits could benefit from work in this area, including greater use of recycling and reduction of the use of toxic heavy metals.
FURTHER DETAILS FROM
David Packham,
Materials Research Centre, University of Bath, Claverton Down, Bath, BA2 7AY.
Telephone: 01225-386570
Fax: 01225-386098
D.E.Packham@bath.ac.uk
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