The superior mechanical properties of K•Prene® urethane are not greatly affected by many environmental influences. In comparison with other elastomers, K•Prene® shows outstanding performance over a wide temperature range. K•Prene® exhibits excellent resistance to most common lubricants and aliphatic solvents, and has outstanding water resistance. This section covers the effects upon K•Prene® of normally encountered environmental influences.
K•Prene® remains flexible at very low temperatures and possesses outstanding resistance to thermal shock. Standard grades remain flexible at temperatures below -80°F., although stiffening does occur as the temperature is reduced below 0°F. Special compositions can be formulated which retain a degree of flexibility at temperatures as low as -125°F.
K•Prene® can be used for service applications at temperatures up to 200-250°F., with only a minor decrease in physical properties as tested at ambient temperatures (75°F.). Use of K•Prene® above 250°F., even if only intermittent, is not recommended and each contemplated use at 200°F., or above should be evaluated individually beforehand.
Resistance to Aging
Properly compounded products of K•Prene® are not affected by the passage of time under static ambient temperature conditions. This statement can be substantiated by data on specimens stored for periods of six years or more. Prolonged use in normal service is not a problem, nor is there any indication of shelf aging.
However, the introduction of certain other environmental influences combined with the passage of time can accelerate deterioration. For instance, chemical agents or dynamic forces acting upon a K•Prene® product over a period of time can intensify the aging process. Normally in multiple attacks such as these the effects strictly due to aging may be so masked as to become unrecognizable.
Weatherability and Color Stability
Prolonged exposure to ultraviolet light will gradually degrade physical properties of K•Prene®. For applications in which long term UV exposure is required, special formulations are available.
Products made from K•Prene® are very stable under high vacuum conditions. They show extremely low weight losses in standard outgassing tests. This property, combined with excellent low temperature performance, suggests logical applications in the aerospace field.
All K•Prene® grades are either extremely slow burning or self-extinguishing as tested in accordance with ASTM 0635. It is possible to obtain special compounds containing flame retardants which are completely non burning under test conditions.
Mold, Mildew, Fungus Resistance
Standard compounds of K•Prene® do not support fungus and are generally resistant to fungus attack and mildew growth. Samples buried for one year in moist inoculated soil showed no evidence whatever of attack, and the vulcanizate properties were essentially unchanged.
Oxygen and Ozone Resistance
K•Prene® resists oxidation and is immune to the powerfully destructive attack of ozone. Oxygen and ozone in atmospheric concentrations have no observable effect on K•Prene®. In one test specific, K•Prene® grades were exposed to three parts per million ozone under 20% static strain conditions for 500 hours with no signs of surface cracking or crazing. It is this property in combination with insulating electrical properties that makes K•Prene® ideal for use around electrical equipment where ozone concentrations are highest. Likewise, stress-strain properties of K•Prene® are little changed after oxygen exposure for four weeks at 158°F., under 300 PSI oxygen pressure.