Leveling agents in floor polishes. Embrittling/anti-resoil agents in rug shampoos. Pigment dispersants in ink, paint and plastics. Coatings for paper and plastics. For years, low-molecular-weight styrene/maleic anhydride (SMA) resins satisfied an increased demand for materials that offer low solution viscosity, high functionality, light color, and water solubility. From carpets and floor care to paper and coatings to leather retanning and textiles, SMA resins have a history of improving performance.

Though they are typically low-level additives, SMA resins are used as matting agents to reduce gloss for epoxy powder coating applications. They provide properties such as heat resistance and leveling for traditional coatings or overprint varnish (OPV) systems. However, SMA resins normally are associated with a distinct odor. A result of the manufacturing process, the odor often eliminates the resins as viable alternatives to competitive additives for a range of applications — until now.

Recent developments in the manufacturing process of SMA resins will enable more coatings to be formulated with SMA resins. In fact, SMA LO resins, with reduced odor and VOC content, can be considered for applications that manufacturers once deemed inappropriate for SMA resins.

'Family' Value

SMA resins are a family of styrene/maleic anhydride copolymers and their half-esters. Available in flake, powder or aqueous form, the resins function as dispersants/emulsifiers (polymeric surfactants), crosslinking agents and chemical intermediates. As dispersants, they provide high aqueous alkaline solubility, high surface activity and low molecular weight — low viscosity. As crosslinking agents, the resins provide a high level of functionality and reactivity. SMA resins also function as chemical intermediates, or building blocks, for specialty polymers using their anhydride and aromatic reactivity.

Base resins and esters with varying styrene/maleic anhydride ratios provide features including varying hydrophobic/hydrophilic balance and a range of thermal properties. The glass-transition temperatures (Tg) can range from 60ºC to 155ºC.

Base Resins vs. Ester Resins

SMA base resins, including SMA 1000, SMA 2000, SMA 3000 and EF30/EF40, are unmodified styrene/maleic anhydride polymers with SMA ratios ranging from 1:1 to 4:1. Characteristics of SMA base resins include high melting ranges, high thermal stability, high melt viscosities, low solution viscosities over a range of pH values, and high solubility in a variety of organic solvents. Both the aqueous solution viscosity and solubility in aromatic solvents increase when the styrene content increases. SMA ester resins retain the desirable characteristics of base resins. SMA 1440, SMA 2625, SMA 3840 and SMA 17352 resins have specific physical and chemical properties that are a function of the starting base resin, the esterifying alcohols, and the degree of esterification.

SMA LO Makes the 'Grade'

SMA LO resins provide the same quality, reliability and consistency as existing SMA resin grades without acetophenone, the odor-causing component normally associated with these resins. In the past, SMA resins were manufactured from styrene and maleic anhydride monomers using a polymerization initiator that produced acetophenone as a thermal byproduct. Though much of the acetophenone was removed during product isolation, traditional SMA resins typically contain 0.2–0.4 wt% residual acetophenone.

A new manufacturing process removes acetophenone by using a new free-radical initiator and light-end removal upgrades. (Light-end removal equipment upgrades refer to the removal of materials that are light-end; have low boiling points. The volatile materials are boiled out.) In response, volatile non-odorous thermal byproducts are formed and are then removed during the resin isolation process. These byproducts cannot be detected in the final resin product (<5 ppm detection limit). The acetophenone odor is not removed; rather, it is never created during the manufacturing process.

The lack of initiator byproducts also reduces the total VOC content of SMA LO base resins to half that of traditional SMA resins. Typical VOC content for these lower odor grades has decreased from 0.5 wt% to 0.25 wt%, and consequently the percent non-volatile material specification has increased from 98 wt% to 99 wt%.

Same Polymer Structure

While this new manufacturing process eliminates acetophenone odor and reduces the VOC content of SMA resins, it does not change the structure of the styrene-maleic anhydride polymer. In fact, NMR and ESI-MS analysis confirm that the SMA LO polymer block structure and end-groups are the same as polymers produced with the standard initiator. Properties such as acid number, solution viscosity, molecular weight and Tg also remain the same. Additionally, there is no change in CAS numbering or TSCA registration when transitioning to the SMA LO product line.

As noted previously, the sole change in product specification is an increase in non-volatile materials from 98 wt% to 99 wt%, reflecting the lower VOC content. Cost of SMA LO compared to existing SMA resins remains the same.

The elimination of acetophenone odor and the reduction of VOC content enable SMA LO resins to serve as drop-in replacements for existing SMA resin grades for a range of applications, including OPV for printed packaging for consumer products; ink and coatings for paper plates, dishes and food containers; and carpet shampoos and spot cleaners. SMA LO-series resins are also suitable for powder coatings and coatings systems applications.

Table 1 / Typical Physical Properties of SMA Resins Base and Partial Esters Resins

Powder Coatings Applications

SMA and SMA LO resins exhibit properties that allow for dry blending, extruding and grinding, necessary processes in the production of powder coatings. While SMA resins are traditionally used in epoxy formulations, recent developments have led to using these materials in carboxyl-terminated polyester — epoxy hybrid systems as well. Low gloss values (60º gloss below 20%) are achieved by way of formulation of SMA and SMA LO resins in hybrid powder paint.

Table 2 / Commercially Available Aqueous Solutions of SMA Resins
As additives in powder coatings, SMA resins are typically used at levels of 1–10% and contribute to a number of unique properties, including pigment dispersion, surface characteristics, adhesion promoting, curing, and chemical resistance.

Most importantly, SMA resins contribute to the specified lower levels of gloss that powder coating manufacturers demand for improving aesthetic quality. The SMA resin line offers a very low gloss coating that produces a look not easily achieved with competitive additives. Currently, the epoxy hybrids market often demands a more “moderate gloss.” When properly formulated, SMA resins can provide a range of surface gloss properties from low to moderate.

Coatings Systems Applications

SMA resins offer unique properties for water or solventborne coatings and OPV systems. They modify surface properties such as heat resistance and gloss without significantly decreasing water resistance. A post-addition of 5–10% SMA will solve a variety of coatings issues.

SMA resins such as SMA 1440 and SMA 17352 are outstanding as additives for improving adhesion and wetting on low-energy substrates such as polyethylene and polypropylene. In general, SMA resins, including SMA LO, are vital when formulating for polymeric substrates. Tables 1 and 2 provide some properties of SMA resins.

Replace and Reduce

While the history of SMA resins is remarkable, the future of these materials seems evident. There’s no doubt that SMA resins have satisfied increased demand for materials that have low solution viscosity, high functionality, light color, and water solubility. However, SMA LO resins, with reduced odor and VOC content, can now be considered by manufacturers for more applications. SMA LO resins can serve as drop-in replacements for existing SMA resin grades for a range of applications, including consumer products. What’s more, SMA LO resins are a direct result of an effort to improve the quality, consistency and reliability of SMA resin manufacturing operations.

For more information on SMA resins, contact ATOFINA Chemicals Inc., 2000 Market St., Philadelphia, PA 19103-3222; Dave Barbieri, e-mail dbarbieri@ato.com, call 215/419.7848 ; Charles C. Mateer, e-mail cmateer@ato.com, call 215/419.7476 or fax 215/419.5902; visit www.Atofina Chemicals.com.