The number of individual engineering polymer products has exploded in the past years due to specialized requirements on the polymer being addressed by a variety of additives. Additives and the function they provide include:
Flame Retardants: Most connector housings are required to meet flammability requirements, burning rates and combustion products, defined by various product safety organizations. Some polymers are intrinsically flame retardant, but the majority require additives to provide that function (most of them are halogen).
Reinforcing Agents: Reinforcing agents are intended to enhance the mechanical properties of polymers. As noted previously, glass fibers are the most common reinforcing agents. The enhancement is dependent on the percentage of fiber, the length of the fiber, the bonding between the polymer and the fiber and the processing.
Stabilizing Agents: Stabilizing agents are intended to reduce the sensitivity of a polymer to moisture, oxidation, Ultra-violet radiation etc.
Colorants/Plasticizers: Colorants, of course, are used to add color to the polymer. Plasticizers are additives intended to improve polymer processibility.
Given the variety of products available in these families the discussion will be qualitative. Refer to the supplier literature for details on specific products in each family.
Polyamides: Polyamides are generally known as “nylon”. There are several varieties of nylon use in connectors depending on the component mers used for polymerization. The generic designations are 66 and 46 with 66 being the oldest and most commonly used. Nylons are semicrystalline polymers and have a number of desirable characteristics and one unfortunate negative.
66 nylon once dominated the connector market as a housing material due to its combination of strength, toughness, abrasion resistance and chemical stability. The negative characteristic of nylons is high moisture absorption, with 66 nylon being the most susceptible. High moisture absorption results in growth in dimensions of the part, and subsequent shrinkage if the part later dries out. This compromises the dimensional stability and limits the size of housings that can be made with 66 nylon.
Moisture absorption also compromises the mechanical and electrical properties of nylons. For 66 nylon the flexural modulus and flexural strength both decrease by more than 50 percent from the Dry as Molded to samples conditioned at 100 percent Relative Humidity. The volume resistance also degrades from 1015 to 109 W/cm, a significant decline, but still a very high resistance. Other nylons are less susceptible to this problem. Nylons are still widely used in automotive, commercial, white goods and industrial markets.
Polyesters: Polyesters, too, are semicrystalline polymers with a good balance of electrical and chemical properties and good mechanical properties with glass reinforcement. There are three polyesters used in connectors, PET (PolyEthylene Terephthalate), PBT (PolyButylene Terephthalate) and PCT (PolyCyclohexylenedimethylene Terephthalate). The different mers used for each polyester affect the polymer chain stiffness which affect the mechanical properties directly, but also affect the crystallization of the polymers which influences processing parameters.
Apart from Surface Mount Technology (SMT) soldering applications where the temperature capability of PBT is not adequate, it is the most widely used of the polyesters.
PBT has excellent chemical resistance, very good electrical properties and moderate mechanical properties. It is also sensitive to moisture during processing.
PET is similar in properties to PBT but has better mechanical properties and temperature capabilities and could be used in some SMT applications (before lead free issue). It is also sensitive to moisture during processing.
PCT has better short term temperature capabilities than PBT and PET and was suitable for SMT applications before lead free but not so much used any more.
PolyPhenyleneSulfide (PPS): PPS is a crystalline polymer and has high temperature capabilities for SMT and continuous use applications. The crystallinity of PPS, and, therefore, the properties, are process sensitive. Most PPS polymers are glass reinforced for both mechanical properties and to reduce brittleness and flash for enhanced molding performance. PPS is inherently flame retardant and has outstanding chemical resistance. The properties of PPS are stable over a large temperature range.
Liquid Crystal Polymers (LCP): LCP, as noted previously, is crystalline even in the molten state due to its rod like structure. This impacts several processing and performance characteristics of LCP. The high crystallinity of LCP results in a good balance of flexural modulus and strength over a range of temperatures. The high temperature stability of LCP makes it suitable for SMT and high continuous use applications. However the intrinsic crystallinity of LCP during mold flow can produce highly anisotropic properties due to the rod alignment in the flow direction. The anisotropy in crystallinity creates a tendency towards part warpage and results in weak “weld lines”, the interface where two fronts of injected polymer meet in the mold. This tendency is countered by the use of glass reinforcement and other fillers to reduce the anisotropy. LCP is inherently flame retardant and has outstanding chemical resistance. LCP has outstanding mold flow allowing the molding of thin sections and excellent dimensional stability an important characteristic for molding of miniaturized connector housings.