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stampsrart

An opposing view on the subject of plastic

The following informative response was given by Roger Rhoads - RRRhoads@aol.com - Chariman of the PRESERVATION AND CARE
OF PHILATELIC MATERIALS at www.stamps.org. Mr Rhoads was a plastics engineer for 40+ years in all phases of the industry with two degrees in chemical engineering.

Please allow me to discuss the degradation process of plastics. It seems that many folks assume that they degrade and give off gases much like municipal garbage, grass clippings, etc. The process is different in that the long molecular chains simply break down from the effect of UV light, environmental temperature and old age (i.e. oxidation). They do not outgas and give off methane, carbon dioxide, etc. The polymer chains simply break into smaller lengths to the point where the material no longer has acceptable properties (e.g. brittleness, loss of clarity, crazing, etc.).

An example is polyethylene used as a film and fabricated into somewhat hazy sleeves for cover protection. (Please be aware that this material is not related to polyethylene terephthlate, aka PET, even though the names are a bit similar.) PE is composed of very long lengths of ethylene molecules. It can be accurately thought of as a very high molecular weight version of paraffin wax. When it degrades (oxidizes, etc.) the long chains become shorter and shorter and finally approaches what appears to be wax.

Polymers do not “rot” in the normally accepted definition of the word. Therefore is not surprising that Dr. Souder found no decomposition gases. By the way, should you wish to know more about his testing procedure, he has an email address at the end of the article. In the interest of full disclosure, prior to his work being started, I was an advisor to him, but had no direct knowledge of his results or his interpretation of the data. I did not see his work until it was published.

As for the question as to how long we can expect plastic materials to adequately protect our philatelic materials, there seems to be little published on the subject. In my many years in the industry, though I have searched for such, I have found only one authoritative publication on the subject. I guess this should not be a great surprise in that plastics as we know them in the hobby have been widely used less than 40 years.

Attached is a two page technical publication on the thermal life of polypropylene put out in March 1985 by Himont. That company was formed in about that year as a joint venture owned by Hercules Chemical Co. and Montedison of Italy for the purpose of manufacturing polypropylene resin that had been made by them since 1967. Since that time its name has changed first to Montell and now to Basell and is the world’s largest manufacture of this plastic. It does not make any products other than the raw materials from which plastic products are made. I retired from that company after 16 years. I have never worked for any company that has produced films used in the philatelic hobby nor have I helped sell PP resin to known hobby suppliers. Therefore I have no monetary interest for expressing my opinions.

The information presented in this brochure is based on laboratory tests carried out over a period of years but certainly not nearly to the extent as extrapolated in the graph. The graph is an Arrhenius plot of the data, a commonly accepted way of extrapolating data from a chemical reaction. Though the tests were not carried out on films (generally considered to be <10 mils) but on molded 40 and 125 mil plaques, it can be considered useful. Note, assuming a continuous exposure of the PP to 140°F, the expected life of the 40 mil plaque would be in excess of 1,000,000 hours or 114 years. By the way, the brochure is apparently not copyrighted, and it has been the policy of the company not to copyright such brochures.

In my opinion, PET, PS, rigid PVC and PE should be considered to have similar lives if protected from extremes of heat and UV light, both of which can drastically influence chain degradation. Moisture has no effect on these materials.

Dr. Souder used alternating temperatures of 75 and 95°F in his oven and relative humidities of 70 and 85%. Humidity was not considered in the Himont testing as PP (as well as that vast majority of other plastics) is not affected by moisture. It is hydrophobic (repels water) and will not absorb such nor be affected by it. Therefore it is consistent with this graph that his tests should have shown no decomposition affect within the time span of the testing.

There are four different plastics used in the hobby: PET, rigid PVC, PP, PS (polystyrene) and PE, or more precisely linear low density PE (there are two other similar varieties that are possible). Plasticized PVC is not, to my knowledge, used in our hobby due to the fugitive nature of the large amount of plasticizers that can readily transfer to stamps. Note its use as album pages for photos and slides are rapidly being discontinued in favor of PP due to this problem.

Each was chosen based on its ease of being readily processed into films, its clarity and cost. While PET is a wise choice due to its clarity and relative stiffness, it suffers from being very difficult to heat seal into sleeves, etc. and its relatively high cost. Rigid PVC is not easily manufactured in thin films but is quite clear and the film can be readily fabricated through heat sealing. PS film suffers as it will shrink readily at temperature nearing 140°F (think Crystal Mounts of an earlier generation) and over time can shrink stamps if they are fully enclosed. PE is cheap but relatively cloudy and quite limp. Therefore we generally see it only as cover protectors. PP can be quite clear and is intermediate in stiffness.

Both PE and PP are generally formulated with slip agents and antiblock compounds in very low concentrations (< 3,000 ppm or 0.030%). Theses additives are generally not found in PET, rigid PVC or PS films. The former is to allow the films to have low coefficients of friction and to be relatively slippery, and thus allow the films to be readily processed on forming equipment. The later is composed of very fine (3 micron) inorganic materials to provide micro roughness to the films and prevent film layers from blocking or sticking when they are unwound from rolls.

These additives must be considered when the films are used in contact with philatelic collectables. The slip agents are oily materials that exude to the surfaces of the films. Thus we could expect them to exude and transfer to the collectibles as well. Such transfer to the inks on stamps would, in theory, make them soft and the inks might transfer back to the films. Dr. Souder’s work showed the transfer of ink to have been found with rigid PVC, PET (Mylar) and PS, but not with PP. His footnote b states, “The changed stamps were often the same in each plastic, e.g., the same 63 stamps has either gum glazing or disturbance in all four plastics listed here and it is only the distributions of their severity ratings (1, 2, and 3) that differ.” This would indicate that it was the nature of the inks/gums that caused the issues and not the chemical composition of the films.

I hope that I have educated you a bit on the subject of plastics and philatelic collectibles.

Roger Rhoads

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