Vintage plastics

P&P I dit a lot of natural rubber processing as a student and cutting lumps of natural rubber and then mixed with sulphur and other ingredients on a two roll mill then cured sheets for tear , resiliency and tensile testing  – so your vulcanite test sounds good. 

Polypropylene has a methyl side group that can be distributed either side of the chain – so when it is synthesized it can be produced as random, alternating or all on one side – which affects its crystallization and physical properties.  The tech terms are  atactic, syndiotactic and isotactic. Large use in ropes as it is very aliphatic and doesn’t get affected by water.

Biaxially Orientated Polypropylene = crisp packet, with micron deep aluminium by vapour deposition. Blame NASA, its the cladding on the Apollo Lunar landers. Also emergency ‘blankets’ for hypothermia. Spongecake and crumpet can be in old school cellophane. 

Oooh, an intelligent, serious and informative discussion concerning a world problem, not to be ignored.  My contribution is not meant to be flippant or thread sidetracking, but should be slightly considered on a similar theme.  What is the name or construction material that potato crisp packets are made from?  It would appear to be a unique product for one item except maybe for sponge cakes as well.

In finishing with the Hart style spadegrip, the hard, black, rippled coating material is described as Vulcanite. In the US, I understand it is more commonly called Ebonite. This is an ‘ancient plastic’, basically a form of vulcanised rubber evolved in the 19th century. In order to test the material, a small piece was subject to the flame test, giving off a sulphurous smell, so surely a rubber compound mixed with sulphur and ‘vulcanised’ with heat. When you look closely at the grip material, you can see how it was originally applied as small pieces, perhaps only an inch wide, blended into each other. Where there is a sudden change in direction, where the spade grip thickens at 3 and 9 o’clock, the Vulcanite looks almost welded, as if it was a thermoplastic. This makes me think that unvulcanised, soft, warm material was worked on to the grip, then the whole grip was placed in an oven, perhaps in an enveloping jig, to allow the material to vulcanise and become hard. In looking at the ‘stripple pattern’ on the Vulcanite, across two grips, you can see a repeat of the texturing. So the theory is that pre- textured Vulcanite material was used, rather than some part of the heating process causing the texture, like ripple paint. Vulcanite was used in the 19th century in many applications : dentures, pipe stems, musical reeds, black ‘mourning jewellery’, soda bottle stoppers and artificial limbs. It would have been a well understood material in 1930. The application of a hard, durable coating to a metal spade grip would be logical. The material has lasted well, particularly because the stainless substrate has lasted well. It can’t be ‘unvulcanised’, only scraped off. Vulcanite was often used on early camera cases but modern substitutes or leatherettes used on cameras are more likely to be PVC. If I am right about the original process, where squares of pre textured, unvulcanised material in a ‘goldilocks’ warmed, partially vulcanised state were hand applied, followed by stoving, then recovering an old spade grip  requires patient work and process investment. I can imagine BSA did this in the 30’s, with benches of gossiping ladies bent over their sulphurous work, truly witches’ work. 

Ahh, Farlam, I can feel a co-authored, very limited edition book, lavishly illustrated, called “Early Plastics in Aviation”. In thinking through plastics, which are now everywhere, the early use of bakelite for ‘durable’ consumer goods like radios, light switches etc was limited by the Depression. It were the aircraft of WW2 that were the first mass application of these novel materials as ‘disposable goods’. The scale of the chemical industry, funded by War budgets, then allowed the 1950’s excursion into the disposable plastic society we are reacting to, with untutored and unkind dismay, today. Our little book will have two purposes. Firstly to document the claim of WW2 aircraft as the Genesis of the Plastic Age and secondly to guide restorers, archaeologists and sundry riff raff on how to preserve or recreate items. I do have an old bakelite press, and would like to make some distributor caps for old engines, but there is much to learn. Traditional bakelite tooling is too expensive for short run items, but 3D printing of tooling might allow it. In any case, first we must learn about Plastic. The historicotechnic currents are fascinating, at least to me. 

P&P there are many types of degradation  – thermal, oxidation, hydrolysis etc. It depends on what is exposed to. Here is a paper on it.

https://www.sciencedirect.com/science/article/abs/pii/S0141391099001834

As you can see Oxidation is of the residues left over from the synthesis. Thermal is by a different route.

The sensitivity depends on the energy to rearrange the molecule into a good leaving group e.g. hydrogen chloride in PVC.

Natural rubber is very unstable to oils as it is a similar chemical structure and like dissolves like – but despite this it is used in auto engine mounts as it is under compression and only the edges are potentially affected.

FYI Carbon Fibre is made from acrylic fibre by a thermal process that drives off the cyano groups and hydrogen in th polymer to leave only carbon.

Cellulose Acetate and Cellulose Nitrate were some of the first semi synthetic polymers that were developed from 1865. The frames of my glasses are still made from cellulose acetate as are cigarette filters.

Bazv, many bakelite components in the engine bay are also loaded with asbestos fibres. Getting to know the hazards around dealing with vintage plastics would also be good to understand. 

Farlam, polyhexamethyleneadipamide ! I wonder how the Welsh would spell it ! Thank you for your detailed and informative reply. I am no expert on plastics, just curious. When you start to look at 30’s onwards aircraft, they appear as an increasing part of the mass, with no readily available information on restoration of these materials. Please continue to contribute your thoughts on the subject, I would like to learn more. On polychloropene/neoprene/Duprene, how does this material typically degrade? What is it reverting to? Is there anything that can be done chemically to slow down this process? 

I love the smell of paxolin in the morning 🙂

Very distinctive smell whilst being drilled or filed – probably very toxic though  🙁

I use the term ‘paxolin’ loosely as there were quite a few similar materials such as Tufnol etc

P&P I think that you need to think about these not as generic “plastics” with tradenames but as the individual polymer chains and how they react.

Nylon is a tradename for a family of polyamides- the most frequent being Nylon 6,6 or polyhexamethyleneadipamide. It is a thermoplastic with no cross linking. It is highly aliphatic from the 6 long carbon chains between each of the CO NH  linkages.

Bakelite is a phenyl formaldehyde resin is a thermoset ( heavily cross linked ) and not a thermoplastic.

Neoprene is a generic term for polychloroprene. It is neither a thermoset or a thermoplastic it is an elastomer with limited crosslinking between the polymer chains meaning that the chains are returned to the initial position when force is applied and removed It also has lower crystallisation than the resin and thermoplastics.  

All three materials mentioned behave very differently in use and in stability to light, water and oxygen.

Wind turbine blades are composites of glass or carbon fibre impregnated with a resin.

Well those parts are not going to degrade any time soon, a bit like Wind Turbine blades

Maybe Max Bygraves threw them into the dump …..

‘You’re a pink toothbrush, I’m a blue toothbrush …..’

Oh, bother, the second one is green rather than blue!

Here is a British BSA grip,  associated these days with the classic Hart biplanes of the 30’s. These grips were a standard Air Ministry issue that I have seen fitted, in historic photographs, to everything from Bristol Bulldogs to Vickers Wellesleys. They all seem to be date stamped 1935, and perhaps there was a supply contract issued to BSA around that time. I have seen production serials on these grips from 981 to 3063, so perhaps thousands were made. Their core substance is a very high quality stainless steel, containing traces of exotic alloys like tungsten, so they have lasted far longer than 1920’s cast aluminium grips made from early silicon -aluminium alloys, like all unreliable vintage British motorcycle castings, or later Wartime magnesium castings. What is interesting about this late serial BSA grip is that the triggers for the Constantinescu-Colley firing mechanism are covered in a foam rubber, which is called Duprene in 1935. Today we know it better as Neoprene, ‘wetsuit material’. Duprene was an invention of DuPont, and from their 1934 technical manual, introducing Duprene to the world, they state that supplies are very limited. To have Duprene from an experimental DuPont plant in the USA on a 1935 BSA grip in the UK is quite remarkable. Why put wetsuit material on a trigger ? I have only ever seen these BSA trigger ‘socks’  in a photograph of a Vickers Wellesley cockpit. I can only think that higher altitudes and freezing conditions may have made the stainless grips too cold for numb, gloved hands to accurately place a thumb on an individual trigger, so they needed to make it easier. In any case, it is remarkable that the Duprene, albeit oxidised and ‘crusty’, has survived. The grip has been kept out of sunlight for perhaps 85 years, on some darkened mantlepiece. It is probably the oldest example of neoprene in the world, which might be of interest to plastiqarians, a select bunch of people who become more attractive the more beer we drink. Most people would be more anxious to discuss the material which is coating the grip, which we can do on another day. There is nothing I can do with the Duprene, as even DuPont acknowledges in 1934 that it will oxidise, in time, in the sun. All surfers know that wetsuits soon fall apart in sunlight. The best thing is to leave it alone and out of the light, waiting for the day a Vickers Wellesley returns to flight.