ABS is a flammable material and is classified as HB according to UL94. ABS burns quickly when it catches fire and emits a lot of toxic gas and black smoke, which is not conducive to practical applications. With the progress of science and technology and the improvement of quality of life, people have become more and more safety conscious, and strict fire-retardant requirements have been put forward for plastic materials used in automobiles, buildings, household appliances, office supplies, etc. at home and abroad, and corresponding technical standards and specifications have been formulated.
Flame retardants are added to ABS by common methods, including inorganic flame retardants and organic flame retardants (e.g., halogen compounds, phosphorus-based flame retardants). Flame-retardant types have efficient flame retardancy but may have other properties that are not good (e.g., aging, high cost). Chemically modified ABS requires a specific production process and a more complex process. Currently, flame retardant modification of ABS materials is based on the addition of highly efficient halogen-containing flame retardants.
Heat-resistant ABS resins generally have a heat deflection temperature of 90 to 105°C and have good heat resistance, toughness, and fluidity. ABS heat resistance can be improved by reducing rubber content and increasing SAN molecular weight and acrylic content, but the method of developing heat-resistant grade ABS by adding heat-resistant monomer or heat-resistant additives is of more interest.
MS, MI, etc. are copolymerized with styrene and acrylic as the third monomer to increase the rigidity of the base resin and improve its Tg . The copolymer of MA and AN has better heat resistance than SAN with the same AN content, and its Vicat softening temperature is 123°C and 103°C, respectively. The chemical resistance and physical properties of both are similar and the production methods can be common. The color of MS-based material is more yellow than that of styrene-based material. In actual production, MS only partially replaces styrene to obtain an SMSAN copolymer. The substitution ratio depends on the heat resistance requirement.
The styrene-maleic anhydride copolymer (SMA) or styrene-maleimide copolymer (SMI), which has better heat resistance, is added to the ABS resin as a component of the blend. However, SMA is unstable at higher temperatures. It releases carbon dioxide and must be released to remain stable enough to be machinable below 260°C, otherwise, the part will have radiolucent patterns.
SMA can be stabilized by adding 1% of hindered phenol antioxidant and thioester co-activator, and SMI can be used in applications that require higher heat resistance than SMA and MS can provide. The production process of copolymerization of styrene and MI is the same as that of SAN, and SMA can be prepared by reaction with ammonia or amine and amidation.
The electrical resistance of SAN should be AN polar nature and slightly lower than PS but still has sufficient insulation. the surface electrostatic charge of ABS in processing, transportation, and use triggers adsorption of dust, demolding difficulties, electrostatic discharge, and other problems. Therefore, ABS products need to have some electrical conductivity to prevent internal discharge and even to shield the outside world from electromagnetic interference with sophisticated electronic components. The addition of antistatic agents can prevent the generation or dissipation of electrostatic charges and is the main method for preparing antistatic grade ABS.
Antistatic agents can be added before or during the processing of ABS. The types of antistatic agents, depending on how they are added, are as follows.
(1) External antistatic agents, in the form of water or alcohol solutions coated on the surface of ABS products, are most often used as quaternary ammonium salts. Quaternary ammonium salts can reduce the surface resistance of the parts, but they are easily removed during the use and cleaning of the parts, so they are widely used for short-term purposes such as preventing dust accumulation on display parts.
(2) Internal antistatic agents, added during ABS processing, add a small amount (0.1% to 3%), and ABS a certain degree of compatibility. Internal antistatic agents are divided into migratory antistatic agents and permanent antistatic agents. Migratory antistatic agents are ionic or non-ionic surfactants with limited compatibility with ABS in the hydrophobic part, and the hydrophilic part can adsorb water to increase surface conductivity after migrating to the surface within ABS.
After the surface antistatic agent is removed, the internal antistatic agent can continue to migrate to the surface until it is fully depleted, thus using it for a longer period of time than the external antistatic agent. Permanent antistatic agents do not migrate, including high conductivity fillers (such as carbon black, metal-coated carbon fiber, stainless steel wire), hydrophilic polymer materials (such as polyoxyethylene copolymer) and conductive polymer materials (such as polypropylene clear, polythiophene). Among them, hydrophilic polymers are low and mainly used in office automation equipment. Conductive fillers are not suitable for applications where color and transparency are indicated, as well as in electronics.