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Alternate Materials Thermoplastics There are numerous commercially available thermoplastics. In the context of most industrial corrosion resistant applications, the more common competitive encounters with vinyl ester or polyester composites involve the use of thermoplastics which are glass reinforced. Apart from specialized and costly so-called engineered plastics, most of these reinforced thermoplastics are polyolefins, such as isotactic polypropylene or polyethylene. These polymers tend to be high in molecular weight and display good resistance to solvents and many other chemical environments. A major disadvantage to thermoplastics involves restrictions to the size of equipment. Thermoplastics normally require extrusion, injection molding, blow molding, or other methods either impractical or prohibitively costly for some of the sizes commonly involved with lay-up or filament wound composites. However, fairly large diameter extruded plastic pipe (usually not reinforced) is commonly used. Often plasticizers are necessary, which in some cases can detract from chemical or thermal resistance, and furthermore may introduce extraction concerns in the final application. Glass and other fibrous reinforcement can be difficult to wet -out or bind with thermoplastics. Special coupling agents are normally required. Longer fibers improve physical properties, but extrusion and molding operating degrade longer fibers. Thus, glass reinforced thermoplastics are limited to fairly short fibers and cannot be employed with many of the directional or multi -compositional reinforcements common to the composites industry. Although reinforcement greatly improves heat distortion and thermal expansion properties, thermoplastic resins differ quite distinctly from thermosetting resins (such as crosslinked vinyl esters or polyesters). Thermoplastics display distinct glass transition temperatures and can melt or distort at elevated temperatures, so quite often they cannot be considered in high temperature applications. Another problem with thermoplastics relate to water absorption or permeation, which plagues even expensive and highly corrosion resistant plastics such as fluoro -polymers. Due to water permeation, cracks or other damages with thermoplastics are difficult, if not impossible, to repair. Cracking of thermoplastics is common due to loss of ductility especially at low temperatures, and secondary bonding or painting can be a big problem. Some relatively large thermoplastic tanks are mass produced by roto -molding techniques. These can be made from thermoplastic powders by thermal rotational casting methods, to avoid sophisticated high pressure injection equipment. Most often, the polymer is a crosslinkable polyethylene. High temperature peroxide initiators are used to crosslink through vinyl unsaturation incorporated into the polymer. Most often, these tanks are used in municipal applications (such as for storage of hypochlorite) or for agricultural uses and liquid transport. Common problems involve cracking and difficulties in repair. A variety of hybrids or combined technologies have evolved. Sheet stocks of specially reinforced thermoplastics can be bonded to FRP surfaces during manufacturing, to make so-called dual laminates. Various thermoplastic coatings are also quite common. At times, thermoplastic piping may be filament wound with a thermosetting composite to improve structural strength. Other Thermosetting Polymers Epoxy The composites described in this guide are focused on resins based on vinyl esters and polyesters. Although vinyl esters employ epoxies in their formulation, the epoxy (glycidal) functionality is extended and chemically modified for vinyl curing, and should not be confused with direct use of epoxy resins. Both Bisphenol-A as well as novolac epoxies may be used directly in fiber reinforced composites. They are cured on a two -component basis with aromatic or aliphatic amines, diamines, or polyamides. Most epoxy composite applications involve high glass content filament wound pipe used largely in oil recovery applications. Generally speaking, viscosities are higher, and glass wet -out and compatibility is always a concern. At times solvents or reactive diluents are used to reduce viscosity. Toughness is good, but thermal properties are inferior to those of premium vinyl esters and polyesters. A medium viscosity general purpose aliphatic amine cured epoxy heat distortion temperature can be typically only 155- 160° F. Alkali and solvent resistance are generally good, but acid resistance can sometimes present limitations and is highly dependent on the curing system. Curing and hardness development can be another limitation, which may require heat activation and post -curing.