|Styrene maleic anhydride (SMA) is known as a polymer that enhances heat deflection of polymers like PVC that are amorphous. SMA can also be used as compatibilizer in amorphous thermoplastics and copolymers to improve thermal performance, surface polarity and miscibility with other resins in blends and alloys.
SMA, long established as a high performance thermoplastic in its own right, is now finding increasing use as a functional additive and compatibilizer in other amorphous and to some extent crystalline thermoplastics. SMA additive increases value of ABS, PMMA by boosting thermal performance, paintability and compatibility in multi-material systems.
Major SMA producer Polyscope is leading the way in expanding the use of this polymer as a property enhancer. The company�s Xiran SMA additives can boost performance and value of various virgin and recycled resins, particularly ABS and polymethyl methacrylate (PMMA). They can improve thermal stability and widen the processing window, and make parts easier to bond, paint, print on, or plate at cost significantly below that of several other additives and proprietary polymer modification techniques. They also act as excellent compatibilizers between resins that are normally immiscible, such as PA/ABS. SMA�s special properties derive from the combination of polar maleic anhydride (which also contributes stiffness, thermal stability, and chemical reactivity) and non-polar styrene (which also provides processing ease). This block copolymer is miscible in a wide range of plastics, which enables it to be used as a compatibilizer in multi-polymer systems.
Xiran SMA�s high glass-transition temperature (Tg) range of 145-175°C makes it a very cost-effective means of boosting high temperature performance in PMMA and ABS resins. Dimensional stability is also improved. The addition of between 1.5 and 3.3% SMA typically raises Vicat softening point by around 1°C. Compounders and recyclers can take lower value, standard- or scrap-grade ABS and transform it into value-added high-heat ABS using SMA resin, which is less costly and far more accessible than competitive technologies like N-phenyl maleimide (NPI) and alpha-methyl styrene acrylonitrile (AMSAN). In ABS, SMA can yield Vicat B values of 85-120°C. Since SMA is fully miscible in PMMA and is also clear, it can improve acrylic�s thermal performance as well as resistance to stress cracking and exposure to chemicals like detergents � all while maintaining optical properties. For every 1°C increase in thermal performance desired in the base compound, as little as 1.8-2.3% SMA needs to be added.
These additives also increase the polarity of the base resin, making parts easier to paint, plate, print on, and bond to a wide variety of other materials. Processers can often skip the primer step in painting operations, saving both time and cost to produce finished parts. The additives also increase the compatibility of PMMA and ABS base resins with glass and natural fibres, rubber particles, thermoplastic elastomers and polymers such as polyamides with which they are normally immiscible. Grades of Xiran® SMA are available in granular form for either injection molding or extrusion. A liquid form factor is also available for fiber-coating treatments. Typical letdown rates range from 20-40 wt-% SMA, with a maximum of 60% used depending on the application requirements. Compounding can be done on most conventional equipment with good temperature control. SMA also is a very-effective purging compound due to its inherent polarity, so compounding or molding with it helps ensure clean equipment. Typical automotive applications include instrument-panel (IP) carriers, air ducts, and other high-precision interior parts. Common non-automotive uses for SMA additives in ABS or PMMA resins include food packaging, chrome-plated sanitary parts, and components for building & construction, electrical and electronics, appliances/white goods, and lighting applications. Styrene maleic anhydride (SMA) compounds have long been used as moulding resins in the automotive, packaging, and building and construction industries, and maleic anhydride itself is a well-known coupling agent between glass or natural fibres and resin matrices. A lesser known but growing use for neat (unreinforced, non-impact modified) SMA grades is as an additive/compatibiliser in amorphous thermoplastics and copolymers to improve thermal performance, surface polarity, and miscibility with other resins in blends and alloys. Another useful property that can be improved in ABS and PMMA resins through the addition of SMA additive is a change in the base resin's polarity. By increasing polarity or surface reactivity/adhesion, moulded parts are easier to paint, plate, print on, or bond with adhesives, skins, foams, and other decorative or functional treatments. In fact, use of SMA additive can often allow processers to skip the primer step in painting operations, saving both time and cost to produce finished parts. This feature is especially attractive to processors supplying parts for painted, metalised, in-mould decorated, or slush-moulded automotive-interior applications, or for other industries with similar needs for high aesthetic and greater thermal performance on parts requiring secondary-finishing operations. Yet another area of functional improvement in PMMA and ABS base resins when SMA additives are used is compatibilisation with various reinforcements and other polymers in multi-material systems. The broad miscibility and adjustable polarity of SMA helps reduce phase separation and improves bonding between the base resin and glass or natural fibres, rubber particles, thermoplastic polyurethanes (TPUs), thermoplastic elastomers (TPEs) and vulcanizates (TPVs) like styrene-ethylene/butylene-styrene (SEBS), and polymers that normally are immiscible with PMMA and ABS, such as nylon (polyamide (PA). Compounding can be done on most conventional equipment with good temperature control. The best mixing will be achieved with a twin-screw extruder (TSE), although a single-screw extruder (SSE) may also be used where lower per centages of SMA are being compounded into the base resin. For typical extrusion-compounding conditions, feed-zone temperature should be around 120°C (248°F) and the die should be around 220°C (428°F), with an optimum melt temperature between 240-260°C (464-500°F).