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Mattblack

Polystyrene packaging and tyres

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Just posting this as a heads up... my finished models have been packed away for about 10 months, all packed in boxes with polystyrene 'chips' type packing. last night I got them out to put back on display and found that the chips had stuck to the tyres on the rat rod I built from the old Airfix Chrysler Imperial kit (ex MPC). I managed to get the marks off with careful use of spray furniture polish, it was the only model affected. So if you have any older models packed away it may be worth checking them.

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I guess I have been lucky in this regard as my only problems occurred many years ago. It maybe in an instance such as yours that it maybe best to put the model it's self in a plastic Zip Lock (@ trade mark) bag and then in polystyrene chips. I think the problem begins with the tires made of similar base stock as the polystyrene.   

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Vinyl tires are the enemy of polystyrene packing peanuts, chips, and kit parts. Generally the softer the tire the more likely they are to adversely effect styrene.

Further discussion here... http://www.modelcarsmag.com/forums/topic/122957-why-havent-the-model-companies-addressed-the-problem-with-tire-melt/

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Vinyl tires are the enemy of polystyrene packing peanuts, chips, and kit parts. Generally the softer the tire the more likely they are to adversely effect styrene.

Further discussion here... http://www.modelcarsmag.com/forums/topic/122957-why-havent-the-model-companies-addressed-the-problem-with-tire-melt/

John's got the basic answer here:   "Vinyl Tire Disease" didn't really crop up until the very early 60's, when model companies (at the time, AMT, Revell & JoHan) began using a then-much-softer variety of PVC, for molding model car kit tires, in order to get more realism and finer detail than had been possible with the harder PVC formula used on promotional model cas and the earliest of 1/25 scale model car kits having PVC tires molded in black (the black pigment was never the problem).  I think that coincided with the introduction of Saran Wrp (and other generic brands of clear, clinging food wrap).

Unfortunately, plastic model kit mfr's are very much at the mercy of the plastics industry--being able to access only such materials as have been developed and manufactured.  As such, certainly where softer plastic compounds are concerned, it appears that there are not many (at all) that can be used to mold an acceptable soft model kit tire, it having a truly dark black color, the ability to at least stretch a little bit over a model car kit rim, and not require a lot of prep or painting work. And even PVC has never been "uniform" as used for model car kit tires--while those of us with more white than color in our hair certainly remember the dreaded "Revell Tire Disease" of the 60's into at least the middle 70's, for the most part even that wasn't completely a uniform problem--not even all Revell car kits suffered that malady, even "back in the day".  And yet, today, I have many model car kits that were produced in the 1960's through the mid-late 70's, in which the tires were never placed in polybags (such as Zip-Lock or similar) that have NEVER marked a body shell or window glass--leading me to be convinced that Revell notwithstanding, this was never a universal, nor a situation guaranteed to occur.

Other materials have been used to make model car kit tires--for example, AMT Corporation used up a fairly large quantity of the soft-rubber Firestone Nascar Tires they developed for their neat (but commercially unsuccessful "AMT Authentic Model Turnpike" 1/25 scale motorized racing sets of the early 1960's.  Those tires did not eat up the rims on which they were mounted, but in fairly quick order, the rubber compound (Neoprene perhaps?) tended to dry out, shrink, and split apart.   Tamiya used rubber from the very beginning on their model car kits--had many Tamiya model car kits produced and built in the late-1960's well into the 1970's suffered the same drying out and cracking of those tires  (I have a Taimiya 1/12 scale Lotus 49 F1 model that was built in 1971, whose hollow tires first collapsed under the weight of the model, then dried out, crumbled.

A sure-fire solution to this problem (certainly with newly molded model car kits) is to place the tires in a polyethylene bag ASAP upon opening the kit--the bags do not even have to be sealable--just large enough to hold the PVC tires with the bag completely wrapped around those tires.  Upon building the model, IF this tire melt is a worry--two preventative options:  Before painting the wheels, wipe a thin coat of epoxy glue around the mating surface of the rim--just enough to be a barrier between PVC and the styrene wheel.  Or, a strip of Bare Metal Foil can be used to do exactly the very same thing--both epoxies (in my experience) and any metal foil will serve as a barrier to any PVC Monomer that might seep out of a vinyl tire.  I've done both, whenever I've had any concern about tire melt, pretty much "end of the problem".

Art

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Neoprene rubber is actually type of synthetic rubber which is very resistant to degradation. Quoting https://en.wikipedia.org/wiki/Neoprene :

Neoprene resists degradation more than natural or synthetic rubber. This relative inertness makes it well suited for demanding applications such as gaskets, hoses, and corrosion-resistant coatings.[1] It can be used as a base for adhesives, noise isolation in power transformer installations, and as padding in external metal cases to protect the contents while allowing a snug fit. It resists burning better than exclusively hydrocarbon based rubbers,[8] resulting in its appearance in weather stripping for fire doors and in combat related attire such as gloves and face masks. Because of its tolerance of extreme conditions, neoprene is used to line landfills. Neoprene's burn point is around 260°C (500°F).[9]

I suspect that those rubber tires popular with Japanese kit manufacturers are some sort of natural rubber (similar to what is used on 1:1 tires which also crack with age).

As far as the vinyl tires melting styrene goes, it is not PVC (momomer) that is softening the polystyrene. Pure PVC is naturally rigid (like PVC sewer and water pipes). It is made soft by addition of plasticizers. In some instances (poor compatibility or inferior material quality perhaps?) the plasticizers leaches out of the PVC tire and ends up softening the polystyrene parts of the model.

Here is some info on plasticizers from https://en.wikipedia.org/wiki/Polyvinyl_chloride

Phthalate plasticizers

Most vinyl products contain plasticizers which dramatically improve their performance characteristic. The most common plasticizers are derivatives of phthalic acid. The materials are selected on their compatibility with the polymer, low volatility levels, and cost. These materials are usually oily colourless substances that mix well with the PVC particles.

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