long-term durability of rigid PVC plastics?

Greetings. I am seeking information on the long-term durability of rigid PVC plastics. My question involves the dehydrochlorination degradation process,
and whether long-term degradation is inevitable or not in rigid PVC.
My understanding had been that dehydrochlorination was primarily a concern in plastisized PVC plastics, and that well formulated and produced rigid PVC plastics were stabilized and internally protected against this problem. It had been our belief that properly made rigid PVC plastics would not degrade "on their own", and that if they were not damaged by UV, heat, or physical stress, they would not significantly degrade even over very long time frames, and perhaps even indefinitely.
Is the dehydrochlorination degradation process inevitable in all PVC plastics, or can properly made rigid PVC plastics be considered stable?
I am wondering what research has been done on the (long term) stability or non-stability of rigid, non-plastisized PVC plastics. Can you possibly tell me more about that, and/or help direct me to find that information? If you are familiar with it - or have performed it yourself! - might you be able to summarize it for me as well?
Any help that I can receive in answering these questions would be tremendously helpful.
Thanks so much!
--
From: Paul Solomon

59 O'Neil Street - Kingston, NY 12401
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Let me start by saying that your question sounds slightly suspect as if you are asking more for political reasons than for scientific or developmental ones. I say this given that rigid PVC has a well know history in long term durable outdoor applications. Rigid PVC has long been used in applications such as vinyl siding and vinyl window profiles with warranties ranging from anywhere from 10-20 years. Many window manufacturers have products in which wooded frames are cladded with vinyl in order to length the lifespan of the window. Many products have lasted well beyond their warranty life. PVC irrigation and reticulation pipe is designed for lifetimes of 50-100 years minimum. How long do you want? How stable is stable? The blow molded HDPE slide that I bought my daughter a couple of years ago is starting to show the effects of the hot Texas sun. Does that make HDPE unstable? Of course not.
Now with that spiel out of the way, let me address the specifics of your question.
Dehydrochlorination is a process that is a function of time and temperature. During processing, properly stabilized rigid AND flexible PVC can be safely processed at temperaures up to ~210 C. However, you question dealt with long term degradation. At processing temperatures, dehyrdochlorination can be quite rapid and is an area of great concern. At ambient temperatures, dehydrochlorination is very slow process and is readily quenched by the more mobile stabilizers employed. As such PVC is considered stable at ambient conditions.
UV exposure is damaging to just about every organic material. Just look what it does to one's skin over a lifetime (and skin is being constantly replenished). Depending on the heat stabilization system used, PVC typically requires about 5-10 parts of a rutile titantium dioxide (TiO2) to provide adequate UV protection. It does not typically require the help of hindered amines or other light stabilizers that are commonly used in other thermoplastics.
Larry Effler
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Larry,
Thanks so much for responding to my posting.
First, as to your concerns about my motives for posting: I have designed a product made of [rigid expanded] PVC plastic. My field is artist materials (art supplies), so the questions that arise about "long term" durability are concerned with longer periods of time than standard industrial or consumer products; which is to say, what is likely to happen to a material over the course of centuries, or even longer! The product is a panel - a painting surface to replace wood panels as a substrate for panel paintings. Since the base plastic materials were designed for different primary purposes, no testing has been done to assess how they might hold up that far out. I am trying to collect information from which educated deductions can be made about this kind of "extreme long term" durability.
In the field of art conservation and the conservation of plastic materials in museum collections, most of the study that has been done regarding PVC plastics centers around the degradation that is effecting plastisized PVC materials in museums. The material that people in the field have read regarding PVC plastics is about that degradation, and this seems to give them an uneasy-ness about PVC in general (and what I consider an unfair bias against PVC). I would like to be able to keep them from generalizing those experiences with plastisized PVC and extending them "by association" to all PVC.
My understanding, as I said, had been that well formulated rigid PVC plastics could be considered "stable" in the sense that minimal or no change or degradation would be taking place within the material [under the conditions that you would expect an artwork to encounter; indoor conditions]. I am searching for information, be it testing research or simple logic, that would back up my understanding.
I am trying to combat a fear that degradation processes such as dehydrochlorination are inherent to all PVC, and that therefore any PVC plastic is unsuitable for use as an art supply product.
You wrote: "At ambient temperatures, dehydrochlorination is [a] very slow process and is readily quenched by the more mobile stabilizers employed. As such PVC is considered stable at ambient conditions."
That was my belief as well. When you say "is considered stable", do you know if that is indeed a generally held view among polymer scientists?
One colleague (whose opinion is rather critical to my product's success at this point) is concerned, I believe, that even a very slow dehydrochlorination process may eventually feed on itself, "snowballing" and thus eventually causing significant degradation. My understanding is that the material is formulated to contain HCL "scavenger" stabilizers that would head off that type of run-away degradation, or at least keep the degradation process from becoming anything but an extremely slow and ultimately insignificant process. Can you comment on this?
Any ideas where I might find any concrete testing, published research, etc, that would bear on this? Could you suggest any other approaches that I might take to substantiate my beliefs in the stability of the material?
Thanks, - Paul Solomon

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As more an artist than a polymer scientist, I'd be more concerned of how the surface feels to be painted on. What is the surface texture like? How will the various types of paint adhere to the surface now and in the future. Will the plastic react with the paints causing discoloration or other issues? Certainly things to consider more than just the substrate. John
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John, Because of the way that we are able to "microabrade" the surface, paint handling and adhesion are both extaordinary. No artist paints or solvents react chemically with the material. Really the only big question left is is long-term stability of the PVC. That's why we need more expert opinions on that matter. Any idea of where I can find those?

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I can't guide you to anymore information other than to try to seek out someone or some company with data on the material. Searching Google with PVC longevity seemed to indicate that it is pretty stable in its more common uses. I would imagine any accelerated tests would evaluate the material in environments that it is commonly used (siding, piping ect.) to see if it will last the 20 or 30 year span typical of these products.
Even if it does last centuries, you have some effort in convincing artists, art collectors and art conservationists that the material is stable. John
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John,
You are right that ultimatley convincing artists is the most important part of this. Artists buy the materials and make paintings on it, so everything starts with them. The influence of collectors and conservators is less direct, though I value the opinions of conservators highly and they have good insights into durability. Artists are very familiar with the many problems that affect wood and hardboard panel paintings, and many are finding a plastic panel to be an appealing and sensible alternative. Plus, they just simply work better from a paint handling/brush-work perspective!
The most important point in convincing them, and ultimately everyone who might have an interest in the material, is having good science to back up the material, and being able to make the best and most solid case for the product.
Thanks for your input; I appreciate it!

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Paul,
Now that I know more about your application, I can give you a better answer. And I apologize if I sounded slightly paranoid in my previous answer. As you can see with your brief experience with PVC, many people have an axe to grind against it.
PVC should be stable over a the time frame you're talking about from a structural standpoint. So, I have few qualms about the PVC water pipe that buired along roadsides last 100+ years. However, I might have some concerns with and hydrochloric acid (HCl) that is developed near the surface reacting with the paint on the surface. This is what you need to worry about. Will HCl migrate to the surface and alter the artwork? Perhaps? Probably? I can't give you a definitive answer. Lot's of materials contain acids, either naturally or from a result of processing (e.g. paper). As I am sure you realize most woods contain a number of tannic acids, this is particularly true of softwoods such as pine. While not as aggressive as HCl, these tannins can cause changes in murals painted on them over the centuries. So, your concern is valid.
So, how to avoid this in your case? Over the centuries, PVC will slowly degrade and form HCl. The tin or calcium-zinc stabilizers are there primarily for processing and insert themselves into the PVC chain to prevent the "unzipping" of the chlorine atoms off the polymer chain. In addition to these process stabilizers, a good PVC formulation will also contain acid traps to neutralize any HCl that's developed. Typical traps include inorganic materials such as calcium carbonate, magnesium oxide, or magnesium hydroxide. Their main advantage is that they are cheap. A treated, fine particle size calcium carbonate will typically cost around $0.12/lb ($0.26/kg). Their disadvantage is that being inorganic, they're not mobile and as such one relies on the HCl migrating to them and not the other way around. So, for a more proactive defence against HCl, some will use organic acid traps, such as epoxy resins. You can check the Handbook of Polymer Degradation, S. Halim Hamid, editor, for more details into PVC degradation mechanisms and stabilization techniques. You might also want to consult the Encyclopedia of PVC, Leonard Nass, editor as well.
As far as testing, what might work is to accelerate the aging mechanism by heating sections of your formulation in an oven heat to 60-70 C. At progressively longer time intervals, pull out sections and check the surface with IR or some other analytical tool to check for HCl that might have migrated to the surface. For example, mechanical creep tests on PVC pipe are performed out to 10,000 hours (416 days) in order to make predictions about 50 year lifespans.
Is the painted surface itself PVC or are you working with a laminate structure? For example a thin wood panel fixed to a rigid PVC backing? If so, HCl evolution may not be as important. If you want to avoid the how chlorine issue entirely, you might want to try a talc filled polypropylene or TPO. You'll have to oxide the surface in some manner to make pain adhere. Perhaps corona or plasma etching will do the job.
Good luck and I hope I helped.
Larry Effler
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