MITRA Forum Question Details

​We are not so sure about glycol migration...but it seems that most testing of PET in general has shown it to be a fairly stable material. As for mediums that involve heat...well encaustic and distemper might not be great mediums for this type of support. Oils and acrylics are likely alright. But again more research is needed to confirm this. No matter what you end up going with please make a note on the back which type it is (if it is not obviously labeled already). Often when paint begins to delaminate and/or flake conservators often need to use heat to set down unstable paint so it would be nice to give folks a heads up later on down the line should something unfortunate occur to your painting. Hopefully others with more knowledge about PET studies will weigh in...

​PET seems to be about as good as a plastic can be, but it is hard to get things to stick to it. If the paint clings well, it should be suitable, for indoors, but as sculpture conservators can attest, keeping paint  stable in direct sunlight and outdoor weather is a very tall order. Recent research that has shown that organic components in oil paint react with pigments like lead carbonate to form motile inclusions, the migration of which may be driven by climate cycling. The low permeability of the PET (PETG) sheet will slow this but outdoor display is likely to accelerate it. It is also worth remembering that the pH of rain is rather low and that when water gets into paint films, as it will, and then freezes, it has an expansive power that is sufficient to exfoliate granite.

Hugh Phibbs

​Well, first of all, just in the interest of disambiguation, PET (Polyethylene terephthalate) is the more technical name for what is commonly called polyester. And one form of it, BoPET (Biaxially-oriented polyethylene terephthalate), is better known simply as Mylar. So yes, it is a very suitable substrate for acrylic paints and is one of the plastics covered in an article Acrylics on Plastics we published in our Just Paint Newsletter:

http://www.justpaint.org/acrylics-on-plastics/

I can check with the author of that article, Dr. Vaikunt Raghavan, who is one of our chief chemists, about the variations you mention to see what he says. Also, keep in mind for acrylics (or really any paints) you will want to make sure the surface is thoroughly cleaned (we suggest alcohol) and adhesion is best when the surface is lightly abraded. Finally, I do not have as much experience with oils on PET but would assume that with light abrasion adhesion should likewise be good. If concerned, you could always look to a high quality commercial bonding primer made for non-porous and hard surfaces as an initial coat, but then you would lose the transparency of the support that you apparently like. 

In terms of studies or conservation articles, nearly everything we know of would be focused on the material as being safe or stable in terms of storage or secondary support - such as polyester canvas - but do not see anything on it as an actual primary painting support itself. That said, this one entry can be helpful to realize that not all treatments or surface coatings might be safe:

http://cool.conservation-us.org/byorg/abbey/an/an13/an13-5/an13-507.html

Because of this, when using polyester films ourselves, we take care to use those sold by conservation supply stores, like Talas or Conservation Support Systems, that carry the uncoated, untreated varieties.

Sarah Sands
Senior Technical Specialist
Golden Artist Colors

​Here is more information from our chemist,  Vaikunt Raghavan:

Listed below are the salient points of different variants:

 PET

Semi-crystalline resin.  Higher the crystallinity, higher the opacity.   Depending on crystallinity and processing, products made could be flexible (PSA tapes, magnetic tapes, food packaging) to semi-rigid (bottles) to rigid . 

APET

Amorphous PET.  Amorphous PET is very transparent due to the rapid cooling below Tg during processing as opposed to slow cooling of crystalline PET.

PETG

This is also clear and amorphous.  The backbone is built with CHDM, a much larger molecule in place of conventional ethylene glycol, thus reducing crystallinity and lowering Tm.  Used in 3D printable thermoplastics as lower Tm helps reduce thermal degradation, reduce chain scissions and enhance optical clarity.  This is a moot point for painting applications as one would not be processing the plastic at or near Tm.

ENHANCED UV RESISTANCE

Some plastics are inherently more UV stable than others.  Polyimides are very stable.  Fluoropolymers such as Teflon and PVDF are stable.  PET is fair.  Polycarbonate, Acetal and ABS are poor.  With UV absorbers and UV blockers added one can improve the performance and such improvement is most appreciated when PET is used in thin film solar cells considering the temperature, optical clarity and dimensional stability requirements of these cells.  Your guess is as good as mine on whether they are useful in painting substrates.

GENERAL COMMENTS

 PROPYLENE GLYCOL

Thermoplastics are susceptible to specific solvent attack, but propylene glycol is not harsh enough to do any harm to PET.

ADHESION

With respect to adhesion, there are two distinct schools of thought.  One emphasizes surface energy and the other amorphisation. 

The surface energy school believes that flame, plasma, corona, abrading treatment etc. change the surface energy promoting adhesion in difficult to bond substrates as proved by water contact angle measurements or dyne pens.  Bad wetting leads to poor adhesion and so good wetting must lead to good adhesion (?).

The amorphisation school believes that the surface energy effects are too small to be of any relevance to bonding, but flame, plasma and corona treatments make the semi crystalline polymers like PET more amorphous and coatings are able to intermingle and entangle with amorphous surface that have much less order than crystalline surface.

No matter what school you believe in,  choosing amorphous PET (APET) or PETG over crystalline PET along with a bit of surface abrading and cleaning should put you in a winning spot.

Thanks,

Dr. Vaikunt Raghavan

​Hi -

Happy to help further. See my answers to your questions below

Q1: Is there a risk of ethylene glycol being released from the substrate and into the paint layers due to breakdown? Or is PET stable enough to undergo practically no degradation in average indoor conditions? 

Under normal conditions there should be no risks of ethylene glycol leaching or outgassing from the material. Polyester is extremely stable - among the most stable of plastics - and the ethylene glycol on which it is based is not in there as a free substance but rather it is bound up into the chemical structure of the molecule. While highly technical the following will give you some appreciation of the process:

http://pslc.ws/macrog/pet.htm

But also TOTALLY cool to learn that Nat Wyeth, Andrew Wyeth's brother, was the person who discovered how to form PET into a bottle.

Ah.....art and science.....

Also check out this page as well:

http://pslc.ws/macrog/petsyn.htm

Q2: Are there any risks connected to using PET that has been colored? Or can I assume that the pigments are bound tightly enough to the polymer matrix to not influence the paint layers?The pigments would be tightly bound in and will not influence the paint layers. You can rest easy....

Okay? Now.....go paint!

​One last qualification to add on the issues of colored PET:

If it is amorphous PET or PETG with mold-in-color, in other words colored in the process of being made, it is fine.

Though amorphous PET is usually clear, crystalline PET (which is not recommended) is a mix of translucent and opaque regions – not uniformly white - depending on the formed spherulites.   But it can then be colored to create a uniform appearance. 

So, in the end, make sure you know what type of PET it is - crystaline (PET),  amorphous (APET), or PETG.