INTRODUCTION
The Achilles heel of rubber and plastics is that, compared with metals, inorganic materials and natural materials such as leather, they are easily degraded and short lived. Figure 1 compares the rate of degradation of natural organic materials with rubber and plastic materials. Although rubber and plastics are clearly well placed in their initial properties, natural materials have the better properties once a long time has passed. An even starker difference emerges in comparison with metals and inorganic materials. Stored under the correct conditions, metals and other inorganic materials will, like Japanese swords, maintain their properties semipermanently despite the passage of several hundred years. Rubber and plastics are created by unnaturally enforced polymerisation using high performance catalysts at artifi cially imposed temperatures and pressures; inevitably, therefore, they are unstable compared with materials nature has wrought over a very long time. As their quality has improved, they have acquired the status of everyday necessities and are now central to much industrial activity. However, they are actually less robust over the long term than is generally supposed: exposed to the natural environment, they slowly lose their intrinsic properties through physical and chemical action until they are no longer serviceable and disintegrate or decompose. Even if it falls short of disintegration, degradation induces all kinds of fault as properties decline. This is because diverse factors progressively degrade the length and state of association of the individual molecular chains wherein the strength of polymer materials resides.
INTRODUCTION
The Environmental Degradation Research Group of the Japan Society of Rubber Industry has made a follow-up survey of the changes with time over a 15 year period when dumbbells of natural rubber and seven different synthetic rubbers made from standard recipes were kept in an unloaded state in instrument shelters [1] . The Group reported that around half the rubbers exhibited a decline in strength of around 50%. It is clear from the report's fi ndings that rubber in particular is prey to marked changes with time. Nippon Gomu Kyokaishi (Journal of the Society of Rubber Industry, Japan), No. 11, 2006, pp. 529-536 Degradation of vulcanised rubber products -Problems and solutions (Degradation by residual chlorine in tap water, metals and ozone due to deterioration in the global environment) Furthermore, during use, rubber and plastics are greatly affected by hot water, metal contamination, oils, and more recently also by atmospheric nitrogen oxides (NO x ), sulphur oxides (SO x ) and ozone (O 3 ) due to deterioration in the environment; although some imperfections inherent in the material are not evident as problems in initial use, the inherent imperfections are prominently exposed once the moulded article as a whole becomes embrittled with advancing degradation. A great deal of effort has been invested in developing or selecting better age-resistors to prevent degradation and enhance the stability and durability of such polymer materials. Nevertheless, it must be accepted that the materials remain "easily degraded and short-lived".
INTRODUCTION
This review looks at the problems associated with the degrad ation of rubber and ways of combating the problems, dealing in turn with the water degradation (degradation by residual chlorine in mains water), degradation by copper ions dissolved in environmental water, and ozone degradation associated with global environmental deterioration, a topic that has recently been receiving particular attention.
WATER DEGRADATION

Analysis of the Water Degradation of EPDM Packings Due to Residual Chlorine in Mains Water
The residual chlorine concentration in mains water has tended to increase in recent years as the environment has deteriorated. The trend is said to be especially marked around big conurbations in an industrial belt. Figure 2 shows the residual chlorine concentration of mains water in a number of Japanese cities. The "black speck" phenomenon the rubber industry has been muttering about refers to the so-called softening degradation whereby carbon compounded in the rubber and some of the rubber itself adhere to the fi ngers when the surface of EPDM mouldings that have been in constant contact with mains water are vigorously rubbed with the fi ngers. While no fracture or leakage is involved, tiny fragments of degraded EPDM are shed into the tap water a little at a time from equipment fi tted with hose or packings thus exposed. Despite their small size, the black fragments are highly visible as foreign matter when the water is run into a cup for drinking. This degradation by residual chlorine is of two kinds: the softening degradation just referred to, and hardening degradation whereby cracks, etc, develop until eventually the water leaks. The overall picture presented by a problem item is that it is free from cracks or fl aws and has no loss of rubbery elasticity to the touch; on the contrary, it has a soft feel compared with the unused item. Examination of the surface by SEM (scanning electron microscopy) revealed the situation shown in Figure 3 . Thus, along with severe irregularities in profi le, some of the surface appears to be on the verge of detachment or exfoliation, and alongside severe degradation, it is clear signifi cant damage has been done by physical factors of external origin such as water fl ow. Measurements of surface hardness with a microhardness meter, which generally demonstrate hardening (crosslinking degradation) as a degrad ation characteristic of EPDM, reveal the opposite in items recovered from service, i.e. substantial softening (depolymer isation), and in areas exposed to water fl ow in particular, the results point to a softening phenomenon, as shown in Table 1 . 
Softening Degradation

Measurement of Chlorine Penetration by EPMA
Since items recovered from service were found to have adsorbed moisture during degradation, it was possible that chlorine was penetrating the rubber during service. The chlorine distri bution over the cross-section in the recovered material from the surface downward was examined by line analysis with an EPMA (electron probe micro analyzer). As shown in Figure 4 , there is a high concentration of chlorine in the surface of recovered material: going from the surface towards the interior, the concentration remains high to a depth of ca. 300 µm and then falls abruptly, reaching a constant value at a depth of ca. 400µm. Compared with accelerated degradation, the diffusion-penetration of chlorine clearly extends to some depth in recovered material, suggesting slow degradation over a long time. Thus, as water is adsorbed, chlorine compounds are also adsorbed by the carbon black (CB) with the result that penetration-diffusion of chlorine compounds extends to a far greater depth, and degradation is more vigorous, in items recovered from service than in items exposed to accelerated degradation in residual chlorine of high concentration.
Measurement of Crosslink Density
The chlorine compounds in items recovered from service penetrate and adsorb more deeply than in accelerated degradation. The possibility was therefore inferred that proportionately more polymer chains are damaged; the change in crosslink density due to degradation was therefore determined by the swelling method. It was found that when the unused material was subjected to accelerated degradation in high concentration residual chlorine, typical hardening degradation occurred with increase in crosslink density; in the recovered material, on the other hand, the crosslink density decreased and softening degradation occurred, with decrease in the molecular weight of the polymer. This bears out the above fi ndings of hardness measurements on the surface with a microhardness meter, in which the recovered item showed a tendency to much reduced hardness compared with the unused item.
Measurement of Crosslink Density
The fi ndings thus point to a very different reaction mechanism in the degradation of recovered material, where degradation occurs over a long time in a relatively mild environment, and accelerated degradation in high concentration residual chlorine. A further analysis of the degradation reaction mechanism was therefore made as follows.
FT-IR and Solid 13 C-NMR Analysis of Degradation Mechanism
The results of FT-IR observations on EPDM degraded by residual chlorine are shown in Figure 5 . Thus, the material recovered from service gave a spectrum with C=O absorption at 1720 cm -1 and a well-defi ned absorption band at 1640 cm -1 , indicative of a C=C bond in the backbone chain, neither of which are seen in the unused material. At the same time, relative to the unused material, the results of 13 C-NMR (nuclear magnetic resonance) spectrometry showed an increase in peak intensity corresponding to a main chain methyl group (Figure 6 ) and suggested the possibility that main chain scission had occurred in the recovered material. Figure 7 shows the mechanism of degrad ation of EPDM packing recovered from service as deduced from the results of the above FT-IR and 13 C-NMR analysis. Stage 1 consists in adsorption of hypochlorous acid little by little from the mains water onto the CB compounded in the EPDM. In stage 2, chlorination would occur in material subjected to accelerated degradation, whereas in the recovered material >C=O groups form by oxidation. Proceeding to stage 3, degrad ation produces CH 3 and >C=C by disproportionation of free radical pairs formed by Norrish type I scission of the main chain. Thus, degradation was shown to be accompanied by a decrease in crosslink density and decline in molecular weight due to scission of the main chain, leading to partial breakdown or disintegration of the packing and giving rise to the black speck phenomenon.
Hardening Degradation of Packings Due to High Concen tration Residual Chlorine [3]
When the container contents in a soft drinks factory are being switched from, say, coffee to Japanese tea, the containers are fl ushed with water containing chlorine at high concentration to eliminate odour. However, the packings used in the water line undergo hardening degradation, cracks then form, and the line leaks. As noted above, a rise in hardness and increase in crosslink density are seen fi rst. A peak indicative of the C-Cl (2p) bond is detected in XPS (X-ray photoelectron spectrometry), while the results of 13 C-NMR show that methyl groups in the side chain of the diene component are chlorinated. The degradation mechanism was shown to comprise crosslinking degradation by the reaction scheme shown in Figure 8 . In short, it was established that (1) the carbon black compounded in EPDM adsorbs chlorine, (2) the chlorine attacks the methyl groups in the side chain of the EPDM diene component, and (3) owing to chlorination of the methyl groups, interaction with other EPDM occurs, the crosslink density rises, the packing hardens, cracks form and water leaks. 1) Crosslink density: Setting this on the high side suppresses increase in the rates of change of mass and volume, and is effective in preventing penetration of chlorine from the water into the rubber. A highish crosslink density in the range that preserves functionality as a rubber product is effective.
Hardening Degradation of Packings Due to High Concen tration Residual Chlorine [3]
2) Carbon black: Adequate reinforcement can be imparted by choosing an FT carbon of low water absorption but lower reinforcing capacity, or using it in combination with highly reinforcing HAF black.
Hardening Degradation of Packings Due to High Concen tration Residual Chlorine [3]
3) Silica: Acidic grades confer some chlorine resistance but alkaline grade silica inhibits chlorination and oxidation of the polymer and fi ller by neutralising the hypochlorous acid.
Hardening Degradation of Packings Due to High Concen tration Residual Chlorine [3]
4) Age resistor: Choose an age resistor that does not leach into the water fl ow, e.g. one with a large molecular weight.
Hardening Degradation of Packings Due to High Concen tration Residual Chlorine [3]
Although any one of (1)- (4) will impart resistance to chlorinated water, combinations of the measures should make it possible to design a compound for packings of good chlorinated water resistance in which a synergistic effect or extended service life can be expected.
DEGRADATION BY METALS, ESPECIALLY COPPER [5]
Susceptibility to metals denotes degradation of the polymer through the catalytic effect (redox reaction) of metal ions in promoting auto-oxidation of the polymer. Reaction is especially sensitive to copper: degradation is suffi ciently promoted to warrant description as "copper attack", having a major impact on the polymer. It can even occur with the non-polar polymer polyethylene (PE) and the champion among resistant synthetic rubbersfl uorinated rubber.
DEGRADATION BY METALS, ESPECIALLY COPPER [5]
The order of effect of different metal elements on rubbers and plastics is as follows:
DEGRADATION BY METALS, ESPECIALLY COPPER [5]
The metals of greatest effect, such as cobalt (Co) and manganese (Mn), usually have little opportunity to make contact with rubber and plastics in an everyday setting. However, the metals copper (Cu) and iron (Fe) are used in large amounts, and copper especially causes noticeable damage. There is often direct contact with these metals after forming: the sheathing of copper wire with polyvinyl chloride (PVC) or polyethylene (PE) is a good example. As we shall see later, recent demands for energy conservation have created many more opportunities for contact with copper because of its high thermal conductivity.
DEGRADATION BY METALS, ESPECIALLY COPPER [5]
Likewise with rubber, when an NR rubber ring is hooked on a brass-plated catch, the material in contact with the plating becomes tacky and unserviceable in less than a week. The effect is due to molecular fragmentation, allowing the rubber to be sheared by the slightest force. When the rubber ring is hooked on an iron nail, on the other hand, nothing untoward happens. The brass used for plating is an alloy of copper and zinc (Zn), and although the copper concentration is lower than for pure copper, the NR contains double bonds in the polymer chain and is therefore susceptible to attack. We need to be aware of copper attack when designing materials. The damage by metals increases in severity the higher the temperature and humidity, high temperature additionally promoting thermo-oxidative degradation. The principle is as follows: the metal ions catalytically decompose hydroperoxide to free radicals by redox reaction, thus promoting chain reaction.
DEGRADATION BY METALS, ESPECIALLY COPPER [5]
In the initial stages the metal ions react directly with the polymer to generate free radicals. There has recently been an increase in the number of buildings fi tted with installations that have an energy recycling function, aimed at conservation of the global environment; in long term use, metal ions elute from the copper heat exchangers, pipes, etc, and the metal ion concentration of the mains water rises. In particular, ionisation is encouraged by residual chlorine in the water and the higher temperature. This imposes harsh conditions on rubber O-rings and has an intimate bearing on the increase in water leakages. Furthermore, the metal combines synergist ically with the previously considered degradation by residual chlorine in the water to cause even more severe degradation of the rubber. This is because copper ions penetrate the interior of the mouldings via voids and crazing in the partially chlorine-degraded and disintegrating rubber surface. It should be possible to increase endurance substantially by adding copper inhibitors to control metal attack (copper attack) and by raising the cure density above normal, as advocated to combat residual chlorine.
SHORT WAVELENGTH ULTRAVIOLET RADIATION AND OZONOLYSIS [6]

Mechanism of Ozone Degradation
Diene rubber products with double bonds in the polymer chain, such as NR, SBR and NBR, are highly vulnerable to attack by ozone. The recent deterioration in global environment is producing a gradual rise in the ozone concentration of the atmosphere (normally 0-4 pphm). Ozonolysis is a classic form of degradation and it has long been supposed that the sundry problems of ozonolysis had been solved. However, deterior ation in the global environment has given rise to a serious resurgence of the problem for rubber materials. First we need to consider how ozone and the short wavelengths of ultraviolet radiation are linked.
Mechanism of Formation of Ozone
The O 3 that occurs in nature is evolved as air ascends. The UV wavelength close to the earth's surface is at its shortest about 290 nm and O 3 does not form. This is because formation of O 3 requires the high energy carried by wavelengths of less than 240 nm; only then does O 2 dissociate to monatomic oxygen, which binds immediately with O 2 present in the vicinity to form O 3 . This can be proven by switching on a low pressure mercury lamp emitting wavelengths below 240 nm, whereupon the odour of ozone immediately pervades the surrounding air. Sunlight carries a substantial dose of low wavelength radiation once the height above ground level exceeds 90 km: its action in dissociating O 2 to O is intense, and as there is little O 2 available, oxygen exists only as O. At 70 km above the ground, however, just the right amount of O 2 is present for O to produce O 3 , and so a thick ozone layer is formed at 50-10 km, protecting the earth from harmful ultraviolet radiation. The ozone we normally encounter has been transported close to the earth's surface from the ozone layer in minute traces by meteorological factors. Since ozone is very highly oxidising, it had been presumed that this ozone, which oxidises dust and other pollutants, is immediately consumed in polluted areas of intense urban activity; thus, while the ozone level was high along the coast and in the mountains where the air is clean, almost no ozone could be found in cities. More recently, because a gas mixture of oxygen plus the NO x and hydrocarbons discharged from motor vehicles is converted to ozone by short wavelengths (300 nm or less) even in the weak ultraviolet reaching the ground, the converse is happening and ozone is being evolved at high concentration in conurbations where NO x is abundant. As remarked above, therefore, ozone damage is on the increase again. Figure 9 shows the ozone generation mechanism for exhaust gas from urban vehicles. The variation in ozone concentration close to an urban intersection is illustrated in Figure 10 . Figure 11 shows the action of ozone on double bonds. Ozone attacks the double bond to produce an ozonide, which is unstable and cleaved at the O-O linkage, with formation of a zwitterion. Aldehyde or ketone is also formed. The intermediate products are stabilised by four routes [8] . As to the mechanism of ozone cracking initiation, on the other hand, opinions diverge. Generally speaking, a vulcanised rubber surface contains numerous minute fl aws and irregularities in profi le which are enlarged by elongation of the rubber. Ozone also has a great effect when residual stresses from forming and processing persist within the rubber. When the rubber is under elongation, the amount of ozone penetrating and rate of permeationdiffusion are large compared with the unstretched rubber; the ozone can then attack the double bonds in the defects directly, facilitating crack initiation. If the breaks in the molecular chain then recombine, crosslinking occurs and the polymer stabilises. Hence, if rubber is left in a high concentration ozone atmosphere in an unstretched state with the molecular chains unoriented, the surface hardens as a result of crosslinking; the material then feels solid to the touch, but since ozonolysis is a surface reaction and hardening is limited to the extreme outer surface, countless cracks develop the instant the slightest of stresses is applied.
The Action of Ozone on Diene Rubbers and the Crack Initiation Mechanism
Thus, the outer surface and interior will present completely different rubbery states. Inspection of appearance to identify microcrack initiation is a technique much used for routine appraisal of ozonolysis, but ozonolysis is in fact very diffi cult to appraise by observation since it coexists with fatigue (in which microcracks develop as in ozonolysis) and ordinary oxidative degradation. 
Appraisal of Ozonolysis by Examination of Appearance
From time to time, rubber products that have developed microcracks are brought to the author's laboratory at the Chemicals Evaluation and Research Institute for appraisal of this kind. Figure 12 shows a typical scanning electron micrograph of the surface in ozone- degraded NBR. Countless microcracks have formed in the surface. Characteristically, examination of the crosssection reveals the presence of countless cracks, just as at the surface. The worrying aspect of ozone cracks is that each grows a little at a time by a process of stress concentration from the exterior, sooner or later acting as the starting point for bulk fracture.
FT-IR Microscopy (Fourier Transform Infrared Spectral Analysis)
The difference between fatigue degradation and ozonolysis is that ozonolysis is characterised by the appearance of prominent absorption by hydroxyl groups (OH) at 3400 cm -1 in assessment by surface refl ection. The appearance of this absorption is a characteristic result of ozonolysis in diene rubber and affords an extremely effective way of assessing degradation. Although it occurs after only a short time in the presence of ozone, the reaction responsible is a typical surface reaction, i.e. it does not extend into the interior of the rubber layer. It can therefore be effi ciently detected by surface refl ection mode FT-IR microscopy.
FT-IR Microscopy (Fourier Transform Infrared Spectral Analysis)
Since a large amount of carbon black is invariably added as reinforcer to diene rubbers, the IR beam is absorbed in surface refl ection analysis by conventional FT-IR, making it virtually impossible to obtain a good information signal. In refl ection mode microscopy, however, the beam from the light source is stopped down by a beam condenser to a diameter of several tens of nm and therefore supplies intense light energy per unit area; this means that a refl ection spectrum carrying a whole range of information is easily obtained. In fatigue degradation, peroxy radicals are generated in the polymer before cracks form by fatigue fracture; stress then promotes fragmentation of the polymer oxyradicals formed by auto-oxidation, facilitating formation of terminal groups. Consequently, the concentration of the terminal groups shown in Table 2 increases. In physical terms, fatigue in cured rubber entails scission of vulcanization bonds and a decrease in crosslink density; it is therefore desirable to look for depression of the glass transition point by DSC (differential scanning calorimetry).
FT-IR Microscopy (Fourier Transform Infrared Spectral Analysis)
In ozonolysis, on the other hand, we have seen that degradation is a surface reaction, and measurement of glass transition point therefore requires a sample of at least 5 mg; as it is impossible to detect a change, no depression in glass transition point is observed.
Measures to Combat Degradation
Anti-ozone measures have already been implemented in products such as belts. HNBR is used for this purpose, since its polymer is resistant to ozone. Satisfactory resistance is thereby obtained to ozone at the concentration levels that have prevailed over the past decade or so. However, because of environmental deterioration, the ozone concentration in the environment has now increased to the stage where present means of combating ozone are simply overwhelmed.
Measures to Combat Degradation
Specifi cally, this means "highly hydrogenated HNBR" of stronger ozone resistance must be chosen as the polymer. Figure 14 shows the relation between the fractional hydrogen ation (iodine number) of HNBR and the time to ozone cracking. Ozone resistance increases the more highly hydrogenated the HNBR. This approach naturally entails the design of compounds resistant to ozone. Under static conditions, for instance, compounds are easily designed by combining paraffi n waxes of different molecular weight with microcrystalline wax which, while slow to bloom, provides a protective fi lm permitting highly stable and enduring control of ozonolysis.
Measures to Combat Degradation
The above approach is not so effective for "dynamic rubber". Under dynamic conditions, "compound design combining amine-based age resistors" is effective for preventing ozonolysis. For instance, with the more ozone-sensitive NR and NBR, the p-phenylenediamine age resistors IPPD and 6PPD may be combined. This combination is effective for NBR. NBR itself is a polar rubber, bloom is minimal, and hence while not as effective as with NR and SBR, the action lasts for a long time. Ozonolysis is often seen in items making contact with air, for example at "sites much exposed to the outside air" in the case of auto motive components, or pneumatic control components such as diaphragms and packings. The aforementioned solution of compound design is hence to be recommended and should afford a longer service life.
CONCLUSIONS
Recent summers have been exceptionally warm, setting new records. This change in global environment is in turn producing all kinds of "change" in our immediate surroundings. In the local rice fi elds in Tochigi Prefecture where the writer lives, the hull comprising the outermost shell of the grain has been growing appreciably longer and thicker than hitherto to protect the kernel from the exceptional heat [9] . Like rice, the organic materials rubber and plastics will have to adapt if they are to be protected from changes in the global environment. Likewise with mankind, the incidence of cataract in the elderly is increasing in response to shorter wavelength radiation. This has meant that we ourselves are now having to use sunglasses.
CONCLUSIONS
Many different highly functional polymers are now marketed. However, to hold down the cost of responding to environmental challenge, we should not forget the orthodox approach of modifying existing, inexpensive polymers.

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