As they age, O-rings and other rubber products can undergo changes in physical properties. They may even become unusable due to excessive hardening, softening, cracking, crazing, or other surface degradations. These changes may be the result of a single factor or a combination of factors, such as the action of oxygen, ozone, light, heat, humidity, oils, water, or other solvents. Detrimental effects can be minimized, however, by proper storage conditions. (Our warehouse can be seen in Figure 71.)

TEMPERATURE
The optimum storage temperature is between 40° F and 80° F. High temperatures accelerate the deterioration of rubber products. Heat sources should be arranged so that the temperature of stored items never exceeds 120° F. Low temperature effects are neither as damaging nor as permanent, but rubber articles will stiffen. Care should be taken to avoid distorting them at temperatures below 30° F.

HUMIDITY
Expressed as a percentage, relative humidity is the ratio of the amount of water vapor present in the air to the greatest amount that could be present at a given temperature. Ideally, the relative humidity in the storage area should be below 75%. Very moist or very dry conditions must be avoided. Where ventilation is necessary, keep it to a minimum. Condensation cannot be allowed to occur. Some materials, such as polyester-based polyurethanes, are hygroscopic (they absorb moisture from the air). This moisture attacks the polymer’s chemical backbone, resulting in chain scission (division of the polymer chain into smaller, weaker segments). Over time, the material becomes soft and cheesy. In humid environments, this can occur in a matter of weeks unless precautions are taken.

LIGHT
O-rings and other rubber products should always be protected from light, especially natural sunlight. Strong artificial light with a high ultraviolet (UV) content is also dangerous. Regardless of the source, UV rays can cause chain scission. Use of polyethylene bags stored inside large cardboard containers is recommended. Alternatively, polyethylene-lined craft bags also offer good protection.

OXYGEN AND OZONE
Oxygen (O2) and ozone (O3) are very damaging to rubber products. Whenever possible, O-rings and other molded articles should be stored in hermetic (airtight) containers to protect them from circulating air. Oxygen (especially in combination with heat) causes rubber articles to form additional cross-links, leading to unwanted hardening of the seal. As with water and UV light, ozone is capable of causing chain scission. Rubber products should be kept away from ozone generators such as electric motors, mercury vapor lamps, and high voltage electrical equipment.

DEFORMATION
Rubber products should be stored in a relaxed condition, free from tension, compression, or other deformation which can lead to cracking or permanent shape change. Large O-rings and seals should not be stored on pegs.

SHELF LIFE
In normal warehouse conditions, the shelf life of even relatively age-sensitive elastomers is considerable. This is largely due to advances in compounding. Table 35 lists some of the generally recommended limitations of many compounds. Taken from Military Handbook 695, this table is quite conservative.

STORAGE AND CODING
All O-rings inventoried and shipped by R.L. Hudson & Company are stored in either zip lock or heat-sealed plastic bags. These bags feature bar-coded labels indicating both the contents’ cure date and batch number.