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  • Different variants of PET as substrates for paintingApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2017-09-05 18:12:26 ... Most recent comment 2017-09-11 15:15:23
    Acrylic Oil Paint Rigid Supports Industrial and Non-Traditional Products
    Question

    I have read that Polyethylene terephthalate is a good support for painting with acrylics, and decided to try painting on the sheet version of it. However, there are different variants of plastic sold under the general name "PET". I can buy:

    • APET (amorphous PET)
    • PETG (Polyethylene terephthalate glycol-modified)​
    • other variants with additions that claim to enhance UV-resistance, etc.

    My question is: are all of these equally fine as painting substrates? From what I've read, the glycol-modified version doesn't become hazy or brittle when heat-treated, but I don't know what type of heat would be required to be relevant for painting. Also, I'm not sure whether any of them would be more likely to leach anything (glycol?), or be more susceptible to propylene glycol present in paints.

    As a side question: is sheet PET compatible with oil paints?

Answers and Comments
  • ApproveRejectUn-ApproveUser Comment

    ​Interesting coincindence, I've just started using PETG recently.

    I've done some reading on it and I believe PETG is a form of Polyester which is used as a substrate for painting. However PETG in clear form can be quickly degraded by UV light which is an issue.

    I paint with opaque iron oxide and cadmium pigments and have applied Gamblin varnish which has some UV protection once dry. I'm hoping that as long as displayed outside of direct sunlight (as all paintings should be) then it will be an acceptable protection (although admittedly not as good as other surfaces).

    I like the transparent nature which allows me to have an underdrawing and not worry about coverage issues.


    2017-09-06 12:48:23
  • EditDeleteModerator Answer

    ​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...

    Kristin deGhetaldi
    2017-09-06 14:24:38
  • EditDeleteModerator Answer

    ​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

    2017-09-06 18:18:30
  • EditDeleteModerator Answer

    ​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

    Sarah Sands, Senior Technical Specialist, Golden Artist Colors
    2017-09-06 20:22:24
  • ApproveRejectUn-ApproveUser Comment

    ​Thank you all! Your comments are very helpful and interesting.

    I've painted a picture in oils on sanded PETG and adhesion seemed fine. I also tested clear Gesso on PETG and it scrapes off easily if the surface is not sanded. If sanded well though it doesn't come off unless scratched off.

    2017-09-07 03:40:26
  • ApproveRejectUn-ApproveUser Comment

    Thanks, everyone.

    Sarah, could you ask Dr. Raghavan about the APET vs PETG vs other types of PET when it comes to painting? I would like to know which are recommended and which ones are to be avoided.

    Also, I don't want to use transparent ones, there are opaque sheets that you can buy which (I think) are pigmented during production. Not sure if this makes a difference.​

    2017-09-07 19:14:03
  • EditDeleteModerator Answer

    ​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

    Hope that helps further.

    Sarah Sands
    Senior Technical Specialist
    Golden Artist Colors



    Sarah Sands, Senior Technical Specialist, Golden Artist Colors
    2017-09-07 20:07:48
  • ApproveRejectUn-ApproveUser Comment

    ​Excellent! Looks like transparant PETG would work ok then. My only concern is how much it can degrade with UV light, but this would be behind a layer of opaque paint pigment. :)

    2017-09-08 01:53:52
  • ApproveRejectUn-ApproveUser Comment

    Thank you, Sarah, that cleared up a lot. There's still two more things I'm not sure about:

    1. 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? (By "average" I mean what you would expect in places like an inhabited house, gallery or museum.)
    2. 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?

    2017-09-09 19:46:41
  • EditDeleteModerator Answer

    ​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!

    Sarah Sands, Senior Technical Specialist, Golden Artist Colors
    2017-09-10 15:57:48
  • ApproveRejectUn-ApproveUser Comment

    Thanks a lot, that cleared everything up!​

    2017-09-11 14:02:25
  • EditDeleteModerator Answer

    ​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.

     

    Sarah Sands, Senior Technical Specialist, Golden Artist Colors
    2017-09-11 15:15:23
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