After long service, filter at left has become filled with dirt but, otherwise, is in normal condition. Filter at right developed a brittle outer sleeve, as evidenced by its discoloration. Acids in the fluid degraded the nylon sleeve and formed a brittle material constituent that can break off and contaminate the system.Although the end caps of the element are nylon, their mass is much greater than the protective sleeving material's. Therefore, the end caps have a longer life before degradation occurs.
Nylon is a hydrophilic material, meaning it can absorb water — in this particular case, up to about 8% of its weight. Over its service life, the element shown lost its lubricity and eventually hardened, became brittle, and broke apart.
Nylon is a polyamide, which is a compound consisting of repeating structural units that impart flexibility. Acids in water can break down the polymer chain, resulting in the degradation of the nylon. The acids introduced by oxidation and hydrolysis were contained in the water absorbed by the nylon. This particular element was removed from a lubrication circuit in a rotary screw compressor. Compressors are especially vulnerable to oxidation, due to their large intake of ambient air. Additionally, high temperatures are generated in the compression process, which accelerate the chemical reaction.
Fluids themselves have varying degrees of chemical aggressiveness. Issues of compatibility are commonly found in long-life synthetic fluids with a polyglycol or silicone base. These fluids are used primarily in machines that operate at temperatures around 160° F. The fluids remain chemically stable at these temperatures and still achieve long service life. However, at these temperatures, these fluids threaten the integrity of plastic compound materials, such as the nylon component of filter elements.
One solution
Working with synthetic material suppliers, Parker Hannifin developed a proprietary material that is hydrophobic in nature and has been tested in aggressive synthetic fluids. This material absorbs water less than 1% of its weight, as compared to the 8% absorption factor of nylon. An anti-oxidant additive resists the effects of acids formed through oxidation.