Plastics ExplainedOctober 7th, 2016
Plastics are divided into two broad groups – Thermopolymers and Thermosets. Thermopolymers can be heated, softened and formed, again and again. They include PE, PVC, CPVC, PP, ABS, PVDF and numerous others. On the other hand, Thermoset plastics react chemically when heated, polymerising and also forming cross links. Thermosets can only be formed once. Thermosets include polyester resin (fibreglass), vinylesters, epoxys as well as others.
Here is a range of the plastics that we work with.
PE – Polyethylene also Polythene and Alkathene
Polyethylene, accounts for more than half of all the plastic manufactured. It is tougher, less rigid, lighter and less expensive than PVC. It has broad chemical resistance and is resistant to most acids, alkalines and many solvents. It has excellent wear resistance, toughness, weld-ability and formability and can be used at temperatures between -50C and 70C. It is used extensively for hazardous substance storage tanks, water tanks including storm water detention tanks, piping, bonded concrete liners and geo-membranes, as well as numerous other applications. Because of its composition, it is made almost entirely of hydrogen and carbon. Polyethylene is a food grade material, however, additives used in polyethylene may not be.
Polyethylene is available in a wide range of molecular weights. Its density, rigidity, strength and wear resistance, all increase as molecular weight increases. The lower molecular weight grades are generally used for rotational molding, the high density material (HDPE) for tanks, piping and fabrication and the high (HMWPE) and ultra high molecular weight (UHMWPE) grades for wear surfaces. Recent catalyst developments have allowed more precise control of the polymerisation process and significant improvements in material properties particularly long term performance. With the exception of UHMW grades, PE is the most easily welded plastic.
PP – Polypropylene
Polypropylene as its name suggests, is very similar to polyethylene. The significant practical differences are its 50% higher stiffness and higher temperature rating. It can be used at temperatures between 0C and 100C. Like PE, it is intrinsically a food grade material and has very broad chemical resistance. Its numerous applications include – tanks of all sorts, food processing equipment, chemical processing equipment, scrubbers, degassing towers, pipe work, and ducting. Fabric backed sheets are available for dual laminate construction and studded, or ribbed sheet for bonded concrete liners.
PVC – Polyvinyl Chloride PVC
Polyvinyl Chloride PVC has the best general chemical resistance of all the common thermoplastics as well has having good formability and weldability. It is generally suitable for temperatures between 10C and 60C. Industrial applications include acid tanks, plating tanks, pipe work and ducting, fans and stacks. It can be glued as well as welded. Because PVC is a blend of up to 15 components care must be taken to ensure the quality of the PVC does not affect chemical resistance. In addition to plain PVC fabrications, using a proprietary method, we are able to bond PVC to fibreglass for dual laminate construction.
CPVC – Cross-linked Polyvinyl Chloride
Cross linked PVC is tougher than PVC and can be used at higher temperatures, up to 100C and still has PVCs broad chemical resistance. Applications include tanks, pipe work and equipment where PVCs chemical resistance is required at higher temperatures.
ABS – Acrylonitrile Butadiene
Acrylonitrile Butadiene Styrene ABS piping has traditionally been used for pump stations, low temperature piping and compressed air and is suitable for temperatures between -40C and 60C. It has broad chemical resistance, but is very susceptible to strong acids. It is tougher than PVC and less expensive than CPVC. It can be glued or welded. Its main use continues to be for piping particularly HVAC plant, pumping stations and chilled water piping.
PVDF – Polyvinylidene Fluoride
PVDF is a Fluoropolymer, one of a family of plastics that includes, ECTFE, CTFE, PTFE (Teflon), MFA, and PFA. Fluoropolymers consist of long carbon chains with some, or all of the Hydrogen atoms replaced with Fluorine atoms. The very strong bonds between the Carbon and Fluorine atoms give Fluoropolymers their extraordinary chemical and temperature resistance. PVDF is resistant to many highly corrosive chemicals at temperatures of up to 150C. It can be used at temperatures as low as -40C and it also has good weldability and formability. This performance comes at a considerable cost. PVDF is many times the price of the more common Thermo-Plastics, however it is used for an increasing range of applications, including hot concentrated mixed acids and ultra-pure water systems. As with Polyethylene and Polypropylene, fabric backed sheet is used for dual laminate construction. The other Fluoro-Plastics have even higher chemical resistance and working temperatures and are used in extremely corrosive and/or hot applications.
GRP – Glass Reinforced Plastic AKA fibreglass
GRP, commonly known as Fibreglass, is a composite material consisting of glass fibres bound together with a thermoset resin. The fibres provide strength and the plastic resin serves to bind the glass fibres. However, glass fibres are attacked by many corrosive materials, so in corrosion resistant applications (when used without a Thermo-Plastic liner) it is the surface layer of resin alone which provides chemical resistance. Resins are available that are resistant to many chemicals at temperatures up to 180 deg C. Applications include; large tanks, pressure vessels, filter vessels and pipe work.
Dual Laminate is the term used to describe tanks and other equipment made with a chemically resistant Thermoplastic liner bonded to a GRP support layer. Commonly used lining plastics include PVC, PP, PE and PVDF. PVC is chemically bonded to the GRP, for the others, a fabric backed sheet is used to give a mechanical bond.