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.
“Detrimental effects can be minimized by proper storage conditions.”
Figure
71
Table 35