Polyester electrical properties

The polyester alkyd resins are lower in cost than the DAP resins but are weaker mechanically, have a lower resistance to cracking round inserts and do not maintain their electrical properties so well under severe humid conditions. Fast-curing grades are available which will cure in as little as 20 seconds. 

High strength Stiffness at elevated temperatures Mineral reinforced Most economical Low warpagc Polyester (glass reinforced) High stiffness Lowest creep Excellent electrical properties properties... 

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

Polyesters exhibit excellent electrical properties. They resist breakdown when exposed to continual electrical loads and have a high electrical resistance. For this reason, they are often used in electrical housings, as insulating films in electrical components, and as wire insulation where high temperatures are likely to be encountered. 

Polyesters exhibit excellent high temperature strength and electrical properties making them a good choice for many demanding applications. They also are physiologically inert allowing them to be used in food contact applications. The two common polyesters, polyethylene terephthalate and polybutylene terephthalate, are both used in injection molded products. Polyethylene terephthalate is often used in both extrusion and blow molded processes also. 

Poly(butylene terephthalate) (PBT) is a semicrystalline, thermoplastic polyester which is completely analogous to PET except that it has a longer, more flexible butylene chain linkage which imparts a rapid crystallization rate, thus making PBT well suited to injection moulding processes. This polyester is used widely for electrical and electronic components due to its high temperature resistance and good electrical properties (Chapter 8). 

The characteristics of the three most common thermoset resin systems used in pultrusion are compiled in Table 11.2 [3]. It is noteworthy that unreinforced polyesters and vinylesters shrink 7-9% upon crosslinking, whereas epoxies shrink much less and tend to adhere to the die. These epoxy characteristics translate into processing difficulties, reduced processing speed, and inferior component surface finish. It is normal practice to use resin additives to improve processability, mechanical properties, electrical properties, shrinkage, environmental resistance, temperature tolerance, fire tolerance, color, cost, and volatile evaporation. It is normally the resin, or rather its reactivity, that determines the pulling speed. Typical pulling speeds for polyesters tend to be on the order of 10-20 mm/s, whereas speeds may exceed lOOmm/s under certain circumstances. Apart from the resins characterized in Table 11.2, several other thermosets, such as phenolics, acrylics, and polyurethanes, have been tried, as have several thermoplastics (as will be discussed in Sec. 11.2.6). 

POLYARYLATES. These are clear, amorphous thermoplastics that combine clarity, high heat deflection temperatures, high impact strength, good surface hardness, and good electrical properties with inherent ultraviolet stability and flame retardance. No additives or stabilizers are required to provide these properties. Polyarylates are aromatic polyesters that are manufactured from various ratios of iso- and terephthalic acids with bisphenol A.1 The resultant products are free-flowing pellets which can be processed by a variety of thermoplastic techniques in transparent and... 

Polyimide (PI) caps all other polymers in its temperature range of use (-200 to 260 °C in air short-time even up to 500 °C). Because of its high price, it is used in special cases only, such as space vehicles, nuclear reactors and some electronic parts. Newer developments, related to polyimide, are the polyether imides (e.g. Ultem ), polyester imides and polyamide imides (e.g. Torlon ), all with very good mechanical, thermal and electrical properties and self-extinguishing. 

Epoxy casting resins are frequently applied for encapsulation of electro-technical parts such as coils and transformers. The electrical properties of epoxies are, in general, better than those of polyesters. 

Other anhydrides such as dodecyl succinic anhydride (DDS A) or adducts of DDS A with polyglycols, can also be used for formulating heat cured epoxy adhesives. These have excellent electrical properties and good thermal shock resistance. Anhydride cured epoxies are also useful for bonding plastics, notably polyester such as Mylar.8... 

Alkyds. Alkyd resins consist of a combination of unsaturated polyester resins, a monomer, and fillers. Alkyd compounds generally contain glass fiber filler, but they may also include clay, calcium carbonate, alumina, and other fillers. Alkyds have good heat, chemical, and water resistance, and they have good arc resistance and electrical properties. Alkyds are easy to mold and economical to use. Postmolding shrinkage is small. Then-greatest limitation is extremes of temperature (above 175°C) and humidity. 

These PEI plastics show good physical and electrical properties at high temperatures and a 20,000 h service life at 230C (446F). Their thermal-oxidative stabilities are described as being less than the best polyimides, but better than the aromatic polyesters. 

Saran (Dow polyvinylidene dichloride) is a tough, chemically resistant plastic available in a variety of forms that are useful in the laboratory. Saran pipe or tubing can easily be welded to itself or sealed to glass and is useful for handling corrosive solutions. Thin Saran film, available commercially as a packaging material, is useful for windows, support films, etc. Mylar (du Pont polyethylene terephthalate) film and other polyester films are also useful for these purposes. Mylar is chemically inert and has excellent electrical properties for electrical insulation and for use as a dielectric medium in capacitors. Much thinner than these are films that can be made in the laboratory by allowing a dilute ethylene dichloride solution of Formvar (polyvinyl acetal) to spread on a water surface and dry. 

---