LG Chem SAN 90HR is a high-acrylonitrile styrene-acrylonitrile copolymer supplied in pellet form for injection molding of transparent and translucent parts that require elevated heat resistance and chemical resistance. In the manufacturer’s published technical data, the material is specified with a nominal specific gravity of 1.08 g/cm³ determined under ISO 1183-1, and a melt mass-flow rate of 4.0 g/10 min at 220 °C with a 10 kg load under ISO 1133-1:2022. The lower melt flow rate relative to high-flow SAN grades indicates a higher average molecular weight and more restricted chain mobility associated with the acrylonitrile comonomer fraction. The same structural feature raises the heat deflection temperature and improves barrier characteristics against aliphatic hydrocarbons and detergent solutions. Optical data published for the grade include a total luminous transmittance of 90 % on a 3 mm specimen under ISO 13468-1, and haze in the range of 0.5–1.0 % under ASTM D1003.

Compared with general-purpose polystyrene, 90HR offers higher tensile strength, better resistance to oils and detergents, and a higher heat deflection temperature. The increased acrylonitrile content also produces a slightly elevated melt viscosity and a more yellow cast in thick sections relative to crystal-grade polystyrene. The material is not rubber-modified, so it retains transparency but has low impact strength. It is typically supplied in natural transparent pellets, and color concentrates based on SAN carrier resin are preferred for uniform dispersion.

How Does 90HR Differ from High-Flow SAN and ABS in Mechanical and Optical Performance?

The primary rheological difference between LG SAN 90HR and high-flow SAN grades is the melt flow index. The lower flow index of 90HR corresponds to a higher molecular weight that increases load-bearing and heat resistance but also increases injection pressure and reduces cavity-fill ease in long, thin sections. In tensile testing, the material exhibits a yield stress of 75.5 MPa under ISO 527-2/1A/50 and a flexural modulus of 3,530 MPa under ISO 178. The heat deflection temperature is reported at 88.9 °C under ISO 75-2/A at 1.80 MPa, while Vicat softening is 108 °C under ISO 306/B50. General-purpose SAN grades typically exhibit heat deflection temperatures in the 82–85 °C range under the same test condition. Compared with ABS, the defining difference is the absence of a polybutadiene rubber phase. This absence gives 90HR superior optical transparency and a lower notched Izod impact strength, published at 19 J/m for a 3.18 mm specimen under ISO 180/A, while many ABS grades fall between 100 J/m and 300 J/m. The lack of rubber-phase oxidation also means that 90HR can be selected where contact with detergents or aliphatic oils is expected, provided the molded stress state remains below the environmental stress cracking threshold. In high-flow SAN grades, the same comonomer chemistry may be present, but the lower molecular weight reduces both melt viscosity and resistance to creep under elevated temperature.

The tabulated values are representative of injection-molded specimens and should not be used as design allowables without component-specific testing. The heat distortion behavior of 90HR is particularly relevant in electrical and electronic housings where internal heat from printed circuit boards or lamp ballasts can create localized skin temperatures. A part molded in 90HR at 2–3 mm wall thickness retains its shape at temperatures that would relax stresses in lower-heat transparent styrenics. The low notched Izod impact strength means that snap-fit features must be designed with generous radii and should avoid sharp corners, because the material cannot redistribute localized stress through rubber-toughening mechanisms. For applications requiring repeated drop impact, ABS or polycarbonate/ABS is usually substituted despite the accompanying loss of transparency.

Pellet conditioning is required before injection molding because styrene-acrylonitrile copolymers are hygroscopic enough to produce surface splay in transparent sections when surface moisture is present. A desiccant dryer set at 80 °C for 2–4 h is recommended, with a dew point at or below −20 °C and residual moisture held below 0.05 wt%. When relative humidity exceeds 60 %, drying time should be extended to 4 h or the hopper should be supplied with a dry-air purge. Melt temperatures between 200 °C and 250 °C are typical for standard screws with 20:1–25:1 L/D. Barrel profiles are usually set flat or slightly declining toward the nozzle to limit viscous shear heating. Mold temperatures from 30 °C to 70 °C are acceptable, with the upper half of that range used for low-residual-stress transparent parts. Back pressure of 5–15 bar and screw recovery speeds below 100 rpm help maintain melt homogeneity without generating yellowing from long residence. At melt temperatures above 240 °C, total residence time should not exceed 10 min, and purging with general-purpose polystyrene or acrylic is standard when changing from incompatible polymers. In production-scale tools, gate design should anticipate the lower melt flow of 90HR; sharp gates and undersized runners tend to produce gate blush, internal stress, and optical haze visible under polarized-light inspection.

Short shots in multi-cavity tools are usually corrected by increasing melt temperature in steps of 5 °C within the 200–250 °C window or by reducing the cavity count. Splay in humid production environments is commonly traced to incomplete drying or condensation on pellets transferred from cold storage into a warm molding cell. Gate blush and surface streaks are shear-related defects and are minimized by lowering injection velocity and enlarging the gate diameter to at least 0.8–1.0 mm for transparent parts. Weld lines in transparent articles can be made less visible by increasing mold temperature and positioning the weld line away from the primary viewing face. These process adjustments are standard on open-loop hydraulic and electric injection molding machines with closed-loop cushion control, where shot-size consistency is a critical measurable because small viscosity shifts alter optical quality.

Regrind from sprues and runners can be reincorporated at levels up to 20 wt% in transparent parts that are not sold as premium optical-grade components. Lower addition levels are recommended when haze below 1.0 % is required because repeated heat history increases yellowness index and reduces total luminous transmittance. Granulate from rejected parts showing visible contamination, burning, or high dust load should be excluded. Drying of regrind and virgin pellets should be performed together in the same desiccant dryer to prevent moisture inhomogeneity. On machines with grooved feed throats, feed-throat temperature should be maintained below 40 °C to prevent clumping when high concentrations of irregular regrind are used.

Mold shrinkage of 90HR is typically in the 0.4–0.6 % range under ISO 294-4, depending on gate geometry, wall thickness, and holding pressure. This value is lower than many semicrystalline resins but must be anticipated in flat transparent panels because nonuniform shrinkage produces warpage and optical distortion. Holding pressure is set to pack the cavity until the gate freezes; insufficient holding time produces sink marks at bosses and undercuts. In multi-cavity molds, balancing the runner system to less than 5 % fill-time variation reduces part-to-part shrinkage scatter and lowers residual stress levels. Post-molding annealing at 70 °C for 1–2 h can reduce residual stress in parts that will contact aggressive media, but annealing may increase shrinkage and should be validated on prototype tooling before production release.

When Thin-Wall Transparent Housings Require Low Residual Stress and High Dimensional Stability

In transparent and translucent enclosures for office automation devices, display frames, appliance trim, and certain lighting components, 90HR is specified where internal heat accumulation can soften lower-heat styrenics. Components with wall thickness below 1.5 mm solidify quickly and require higher effective injection pressure because of the reduced melt flow. On injection molding machines with clamping force between 80 t and 250 t, polished cavity surfaces are used because surface gloss and light transmission are sensitive to mold roughness. Residual birefringence is minimized by maintaining uniform filling speed, avoiding abrupt gate expansion, and using mold temperatures near 60 °C. Hot-runner systems can process 90HR, but runner channels and gate diameters should be sized for medium-viscosity styrenics; gates below 0.5 mm may cause shear heating and visible streaking. In automated optical inspection, parts are checked for haze, flow marks, and particulate contamination at light-box stations. Secondary operations such as ultrasonic welding are possible when joint design follows styrenic welding recommendations and the weld horn is profiled to avoid excessive surface marking on the transparent surface.

In service environments with occasional contact with isopropyl alcohol or ethanol-based cleaning agents, 90HR shows better resistance than polycarbonate, which is prone to solvent crazing under stress. However, the SAN grade is not intended for continuous immersion in these solvents or for parts that must withstand repeated cleaning with quaternary ammonium disinfectants at elevated temperature. Testing under ASTM D543 at the intended strain level is required because molded-in stress from sharp corners or metal inserts can reduce the apparent chemical resistance. External mold lubricants containing silicone should be avoided on transparent grades because surface contamination interferes with adhesion of printing inks and coatings.

Solvent Contact Limits and Stress-Cracking Boundaries in Service

Sustained contact with aromatic hydrocarbons, chlorinated solvents, esters, ketones, or concentrated oxidizing acids is not recommended for 90HR, because these agents can cause stress cracking or surface crazing in molded parts with high frozen-in stress. The grade exhibits improved resistance to many aliphatic hydrocarbons, aqueous detergent solutions, diluted acids, and alkali compared with general-purpose styrenics. Environmental stress cracking resistance is geometry- and load-dependent; published data for specific chemical exposure durations under load is limited, and compatibility testing should follow ISO 22088-1 or ASTM D543. For external parts, unpigmented SAN yellows under prolonged outdoor exposure, and UV-stabilized or opaque pigmented versions are preferred for sunlight-facing components. Compliance for electrical and electronic applications is generally aligned with REACH and RoHS 2011/65/EU, subject to final formulation and colorant additives. Where food-contact status is required, the applicable acrylonitrile copolymer citation is FDA 21 CFR 181.32, but compliance is formulation-specific and must be confirmed with the compounder or conversion operation. The maximum continuous use temperature is usually reported in the 70–80 °C range for unstressed transparent parts, with short-term peaks not exceeding 90 °C. At service temperatures above these levels, load-bearing performance must be verified by component testing rather than extrapolated from short-term heat deflection values.