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SMA as additive for amorphous thermoplastics

21st February 2013

Styrene maleic anhydride (SMA) boosting thermal performance, paintability and compatibility in multi-material systems. Patrick Muezers reports.

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.

However, 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. XIRAN SMA additives can boost performance and value of recycled or virgin resins such as acrylonitrile butadiene styrene (ABS), and polymethyl methacrylate (PMMA), and extending thermal stability, improving the processing window, and making parts moulded from these resins easier to bond, paint, print on, or plate at a modest cost vs. other additives and proprietary polymer-modification techniques for these same resin families.

Expanding the use of SMA as a performance enhancer for thermoplastics is Polyscope Polymers BV. Sold under the tradename Xiran, the company claims to have the broadest offering of SMA resin additives and moulding compounds in terms of both range of molecular weights and percent maleic anhydride - two variables that can be manipulated to change final molded-part properties. SMA has unique functional properties due to the combination of polar maleic anhydride (which also contributes stiffness, thermal stability, and chemical reactivity) and non-polar styrene (which also provides processing ease). Manipulation of the ratio of styrene to maleic anhydride allows polarity to be adjusted. This copolymer is miscible in a wide range of plastics, which enables it to be used as a compatibilizer in multi-polymer systems.

Thanks to XIRAN SMA's high glass-transition temperature (Tg) range of 145-175°C (293-347°F), it is a cost-effective way to boost thermal performance in PMMA and ABS resins. Properties like heat-deflection temperature (HDT), continuous-use temperature (CUT), Vicat softening, and heat aging can all be improved. Incidentally, the base resin's processing window also is broadened, and its dimensional stability and ability to hold tight tolerances after moulding are increased coincident with improvements in its thermal performance.

Depending on the starting ABS copolymer and the grade of XIRAN SMA used, generally for every 1 per cent of SMA added to the compound, the thermal performance of the base resin is shifted upward by 0.7°C (1.2°F). This means that 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 heat-boosting technologies like N-phenyl maleimide (NPI) and alpha-methyl styrene acrylonitrile (AMSAN). SMA additives allow customisable improvements in the thermal performance of ABS, with Vicat B ranges of 85-120°C (185-248°F) or higher typically seen.

Since SMA is fully miscible in PMMA and is also clear, it offers the ability to improve acrylic's thermal performance as well as resistance to stress cracking and exposure to chemicals like detergents - all while maintaining its water-clear transparency and optical properties, with minimal impact on the refractive index of moulded acrylic parts. These are attractive features for PMMA used in a variety of industries. Depending on the grade of acrylic XIRAN SMA used, generally for every 1 per cent of SMA added to the compound, the thermal performance of the base resin is shifted upward by 0.4 to 0.6°C (0.8 to 1.0°F). Or, said another way, for every 1°C increase in thermal performance desired in the base compound, between 1.8 and 2.3 per cent SMA should be added to the compound (1.0 to 1.3 per cent to increase 1°F).

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).

Several different grades of SMA additive are commercially available in the XIRAN product line to provide broad flexibility in adjusting base resin properties for ABS and PMMA. Grades are available for either injection moulding or extrusion and are offered granulate form. A liquid form factor is also available for fibre-coating treatments. These can be let down at the rate a processor requires to achieve specific molded-part properties. Typical letdown per centages range from 20-40 wt-per cent SMA, with a maximum of 60 per cent used depending on the application requirements.

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).

Other additives (eg, release agents, antistatic agents, fillers and reinforcements, UV stabilisers, pigments, etc) may either be dry-blended at the start or introduced via side or top feeders during compounding. Ironically, SMA also is a very effective purging compound due to its inherent polarity, so compounding or moulding with it helps ensure clean equipment. In Japan, SMA is used as the preferred purging compound rather than polyethylene (PE) as is more common in Europe and North America.

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Patrick Muezers is managing director, Polyscope Polymers BV, Geleen, The Netherlands.

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