Deterioration of rubber articles in various exposures and their protection.
As such, they are subject to several forms of deterioration: ozone induced cracking, thermal embrittlement, the effects of direct solar radiation and possible mechanical damage. These affect the performance of the tires when they are put into service presumably by exacerbating the reduction of the adhesion between the plies as the tires are run. Much of the deterioration may be estimated from laboratory tests.
Exposure of small items such as grommets, o-rings, etc., to temperatures which may reach 70 [degrees] C (158 [degrees] F), if protected by a variety of films, showed that only PE/aluminum laminates gave substantial additional protection to all elastomers except CR and EPDM which are already very age resistant. Many parts, such as shock absorbers and oil retainers, are often combined with steel inserts which are preferably protected by a vapor phase corrosion inhibitor (VCI) in order to avoid the use of greases. We have found that the presence of VCI has no influence whatsoever on the aging of the rubber component
Figure 1 compares the behavior of some elastomer compounds on aging in plain PE, Kraft paper envelopes either alone or with talc to prevent the development of stickiness and those made out of laminated PE/Al. In view of the fact that we did not find any substantial difference between the protection offered by the former different envelopes and completely unprotected specimens, we have lumped these together and recorded on the graphs the arithmetic average of the observed extent of deterioration. We have purposely discarded the curves for EPDM and CR which are of no particular interest in order to avoid cluttering the graphs. After 150 days of aging, both elastomers retained over 80% of their original elongation at break in all envelopes.
In the graphs of figure 1, the lower case letters refer to the average values as above, while the capitals are for those in the PE/Al envelopes. The improvement in elongation retention is immediately obvious. The curves have been fitted to the experimental points by a hyperbolic regression as shown in table 1. The curve for the aging of SBR in aluminized polyethylene is unexpected and is possibly due to a fault or nick in the test piece which reduced the average of three tests after 60 days aging.
Table 1 A general algebraic expression for a hyperbola is y = (ax +b)/(sx+p) In our particular case, we are only interested in one quadrant of the hyperbola passing through the origin, in which case the equation may be simplified as y = 100 - at/(b+t) For such a small number of points as we have here, the constants may very simply be determined by trial, although the development of a least squares algorithm should not be insuperable.
In previous work (ref. 1), we assumed an Arrhenius aging process to conclude that the average annual temperature under cover in our storage depots is just over 28 [degrees] C, and we have checked that direct sunlight raises the temperature of the stored tires to some 45 [degrees].
Based on the criterion of an elongation at break of 150%, we estimated that the open ak storage life of a properly compounded CR article is less than 15 or 16 years. NR and SBR being the principal components of tire rubber would be expected to show a much shorter storage life than this.
Nevertheless, it is important to consider the effect of the resulting handicap that a shorter period of storage, say for example seven years, would have on the expected running life after storage. The graph of figure 2, which is based on experimental evidence on small articles (but not for tires), was developed over many years. It gives average residual lives of general purpose rubbers after various periods of storage, always rejecting rubber when its elongation drops to 150%. This criterion may seem to be somewhat harsh, but in view of the statement of the first paragraph of this article, it is reasonable. Thus, for example, the IDF will use an article which had been stored for four years in PE/Al for another five years of service, but for only another four years if stored in other envelopes.
Tire and track studies
In these, we had a series of 12 tires, both bias and radial and new or used studied by a reputable U.S. laboratory (ref. 4). The size of this sample is rather small for far reaching conclusions to be made, but the results obtained were certainly indicative. It was found that the age of the tire had a fairly pronounced influence on interply adhesion and particularly on the average dynamometer endurance, and it was observed that bias fires were considerably worse in this respect than radial fires in this respect. These conclusions are obvious from figure 3.
The tracks for the armored personnel carriers (APC) also suffer from exposure cracks, but compared to the track thickness, they are probably insignificant, principally because the tensile stress on the tracks is borne by the reinforcing steel cables.
However, abrasion loss may prove to be seriously influenced by open air aging. The graph in figure 4 shows that unless properly compounded, this may actually be a problem as our field experience has demonstrated.
Much of the work in this paper has been previously reported on in other publications (refs. 1-3), but we feel that it is important to summarize the information in one journal.
"Understanding the IRHD and Shore methods used in rubber hardness testing" is based on a paper given at the September, 1999 Rubber Division meeting.
"Factory testing and control of raw natural rubber and mixing batches using the rubber process analyzer" is based on a paper given at the September, 1999 Rubber Division meeting.
(1.) E. Tuval, A. Thurm and Z. Rigbi, "The shelf life of rubber compounds," Polym. Der. Stability, 58, 291 (1997).
(2.) E. Tuval, A. Thurm and Z. Rigbi, "The aging of tires and similar objects in open air Storage," J. Polym. Engng. 9, 209 (1999).
(3.) A. Tuval, H. Ma'ayan, A. Thurm and Z. Rigbi, "Packaging for rubber parts," Kau. Gummi Kunstst. 5, 266 (2000).
(4.) Smithers Scientific Services, Akron, OH, "Military tire aging study for Israeli Ministry of Defense, 1995."
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|Date:||Jan 1, 2001|
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