long-term durability of rigid PVC plastics? 1

[hommealone]

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.

I am trying to licence a product to a large mfgr. It is made of expanded (rigid) PVC plastic, with exceptional built-in UV protection. It is intended for indoor use and exposure only, but the utmost durability is important. In our field, long-term durability and stability of materials is required (i.e., centuries). The product was developed to replace existing wooden products with something more durable.

This manufacturer currently produces other types of products made of acrylics, and has expressed concern about the dehydrochlorination degradation process in PVC, and is afraid that this process might make our product unsuitable in terms of long term durability.

Our 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.

We would like to find out whether our beliefs are true or not, and if they are, we would like to be able to convince this mfgr. of that as well.

Since the manufactured plastic which we use to make our product is not otherwise used for purposes requiring such long time-frames for durability, its mfgr. has not performed testing to verify its longevity. And I am pretty sure that they will not reveal any details about how this particular product is formulated vis-a-vis stabilizers, additives, etc.

Would you consider all PVC plastics - both non-rigid and plastisized - to be unstable? Do you know if this dehydrochlorination degradation process is 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?

What are the parameters that promote and/or mitigate/protect against degradation in rigid PVC?

What are the time frames that you would expect to be associated with any eventual degradation process in rigid PVCs? Would the degradation process likely be a slow and steady progression, or would it eventually speed up after an initial period of slower or less apparent degradation?

If the process is inevitable in all PVC, does it also inevitably result in the release of acidic by-products (eg HCL)?

Any help that I can receive in answering these questions would be tremendously and vitally helpful. Should it be easiest for you, please feel free to contact me by phone at 845 - 338 - 8603. If you prefer to email me directly, you can use this address: paul [at] hudsonhighland [dot] com.

Thanks so much!
- Paul

 

[RichGeoffroy]

Paul:

Dehydrochlorination, the evolution of HCl from PVC, is primarily the result of thermal degradation of PVC. Essentially heat and shear during processing causes hydrogen and chlorine from adjacent carbons to be liberated as HCl, leaving behind a carbon-carbon double bond. While the reaction predominately occurs during processing, it can also occur during post processing exposure to elevated temperature. Once initiated it will continue to proliferate or “unzip” through the molecule, although at much slower rates at room temperature, to produce an alternating structure of:

–(CH₂-CH=CH-CH₂-CH=CH-CH₂)–.​

This structure produces the reddish brown color evident in thermally decomposed PVC.

Sunlight, which initiates photolytic degradation, has little or no effect on the regular repeating head-to-tail configuration of PVC because the light absorption of this material lies in the far UV region of the spectrum. Photochemical reactions, however, can take place:
• on chromophores, such as carbonyl or hydroperoxy groups, formed through heat-degradation during thermoprocessing;
• on structural irregularities formed during polymerization -- such as chain branching or head-to-head configuration; and
• on impurities -- such as residual traces of initiator, surfactant, or metal ions. Of particular importance here is the selection of thermal stabilizer.

Therefore, purity of the polymer, and careful selection of the additives in the compound is essential to long-term photodegradation resistance of rigid PVC.

The photodegradation resistance of PVC can be enhanced through the use of UV stabilizers. However, there is relatively little usage of UV stabilizers in PVC due to the following factors:

• ultraviolet screening is often gained through the use of pigments such as TiO₂;
• epoxidized plasticizers and phosphites are widely used and contribute to light stability;
• antioxidants often are added to impede degradation during high-temperature processing -- they also effectively inhibit reactions that may have been UV initiated.

Other aspects which can significantly affect the UV resistance of PVC formulations include the oxygen in air, temperature, time, humidity, the presence of fungi, and angle and direction of UV exposure.

In a pure sense, one has to admit that dehydrochlorination of PVC is inevitable --- because it simply will occur to some extent. However, the key question is, “Will it occur to any significant extent under the proposed use conditions?”. It is unlikely that a carefully selected PVC compound, properly processed into a part which is “intended for indoor use and exposure only” will experience any significant degree of dehydrochlorination during long-term use.




Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com