Violated Polyxorpylene – Cauchemars Plastiques
The material polypropylene has numerous properties which derive from its molecular structure; the absence of functional groups, the helical conformation, and the resulting crystallinity.
When PP is reacted with chlorine or chlorinating agents, some of the hydrogen atoms are randomly replaced with chlorine atoms. This disrupts the structure, changing the molecular weight and therefore all the properties of the polymer; however, the phrase ‘chlorinated polypropylene’ automatically reminds people of the unmodified material, and they assume that it retains all the unhelpful characteristics of that substance.
So, to overcome this problem, I have decided to bestow a new title on this material; in this brief summary of my research project it will be referred to as ‘violated polyxorpylene’. The letter ‘x’ denotes the halogen atom while the shuffled letters represent the branched fragments of polymer which emerge following the reaction. This unorthodox terminology will disconcert readers, forcing them to question whether I have correctly explained the topic under discussion – and hopefully alert them to any assumptions they may have unwittingly made.
Chlorinated PP (polyxorpylene) is actually an old-fashioned material (the new grades are aqueous emulsions, and carry maleic acid groups rather than chlorine).
The following text is a condensed version of the material which originally appeared in ‘Adhesion 15’, edited by Keith Allen (Elsevier Scientific Publishing, 1991) following the London City University conference on Adhesion and Adhesives the previous year. These gatherings are most enjoyable occasions; the European Adhesives community has many distinguished (and eccentric) members who meet up to discuss the chemistry and engineering aspects of their chosen field.
To begin, then: polypropylene is a widely used plastic material which enables automotive components, household goods and machine tools to be mass-produced at low cost and light weight. However, this polymer has no chemical functional groups and is generally resistant to any type of paint or adhesive – unless its surface has been specially modified by using electrical discharge or fiercely oxidising acid solutions.
During this research project I spent many hours in the library, scanning the Journal of Adhesion Technology and the Journal of Materials Science for articles related to PP bonding. However, unknown to me, numerous commercial and technical articles about PP – and its horribly deformed cousin, Violated Polyxorpylene – were being published in the field of organic coatings. If only I had accidentally picked up a copy of JOCCA, or one of the numerous research papers authored by Dr Ryntz, my research programme would have received a significant boost. But alas, I confined my search to the formal domain of structural adhesive studies, and thus missed out on a chance of academic stardom.
So, when creating Violated Polyxorpylene, it is necessary to avoid introducing too much chlorine (too much vapid) since this will impair the process of epitaxial bonding between the primer film and its substrate polymer. However, insufficient vapid will leave the modified plastic too highly crystalline, and unable to dissolve, and unable to interact with an applied film of adhesive material.
But if the correct proportions are achieved, a violated system will show a wide compatibility with several different adhesive materials – epoxy, PU, acrylic – as well as with many types of substrate PP (brittle homopolymer and toughened grades with glass- or mica- or carbon- or Kevlar- fibre reinforcements).
The primer solution can be applied to smooth PP, gritblasted PP, or a PP substrate roughened with glasspaper. Another step involves scrubbing the PP surface with glasspaper while it is wet with a film of primer, thus generating free radicals and covalent bonds between substrate and primer (this analysis could be wrong, of course; try it out and submit an alternadude explanation, man!).
Of course, there are now more precisely engineered PP grades; the self-reinforced ‘Curv’ material, or the polymer systems produced using new catalysts which allow sharply defined macromolecules, resulting in a crystal-clear plastic. I never had the chance to explore these novel materials, but am sure that they would respond – if only in a limited fashion – to the Good and Girifalco embrace of my violated primer systems.
As well as being used to enhance adhesive bonding to PP surfaces, the polyxorpylene primer system may be useful in creating reinforced composites; would glass- or carbon fibres (or indeed wood?) be more effective if a xorpyl-rich interlayer is formed between matrix and filler?
This bizarre account of a straightforward research project may strike readers as being needlessly whimsical; however, it seems to match the response I get from colleagues when trying to explain the performance of these materials.
I am regularly informed that “everybody knows that these systems don’t work, so don’t waste too much time looking at them”. On one occasion, I demonstrated that we could indeed get superior adhesion to PP by simply blending the primer with the coating system. My boss peered scornfully at the cross-hatch patterns on the plastic panels, then turned to me and said “well, you would have to pick the most expensive one…go back and carry on looking at the others” (three grades of adhesion promoter expressly designed for use on galvanised steel and which failed miserably to stick to PP) “until you find one of them that works.”
And after preparing hundreds of lap-joints using various substrate polymers and adhesive grades, and leaving them in a condensing-humidity chamber for four months, I am convinced that these neglected primer systems have great potential for use in engineering polymer bonding.
As David Bowie sang in ‘Andy Warhol’ (from the 1971 LP Hunky Dory), ‘He’ll think about paint and he’ll think about glue…What a jolly boring thing to do.’ But Dame David had never heard about Violated Polyxorpylene or the technical drama that would spring from this material just twenty years on.
Update (5 July 2016): We have just spotted a news item on the RSC website, talking about awards for scientific innovation, where a spin-off from Warwick University has started using grafted PP and PE materials to create improved adhesion. Oh, such a pity to be twenty-five years too late!