Kumho Petrochemical SAN 350 is a styrene-acrylonitrile copolymer resin identified in the producer’s SAN series as a transparent, rigid injection-moulding material. The polymer is an amorphous single-phase copolymer of styrene and acrylonitrile; the absence of a butadiene rubber phase differentiates it from ABS grades and preserves optical clarity while reducing notched impact energy. The polar nitrile group in the acrylonitrile comonomer raises tensile yield stress, heat deflection temperature, and resistance to aliphatic oils compared with general-purpose polystyrene. The 350 model designation is used by the producer to mark a grade with a melt-flow position suited to thin-wall and moderate-flow technical parts. It is normally supplied as cylindrical pellets, and like other amorphous SAN resins it requires predrying before processing to avoid hydrolytic degradation and splay.

Which property clusters control the selection of SAN 350 over lower-acrylonitrile SAN copolymers?

Commercial SAN resins are typically specified by acrylonitrile content in the range 24 wt% to 34 wt%. Lower AN grades in the 24–26 wt% interval show reduced solvent resistance, lower heat deflection, and lower melt viscosity. Higher AN grades in the 30–34 wt% interval show improved oil resistance and tensile strength but require tighter thermal control because the nitrile group increases melt viscosity and shear heating. SAN 350 is positioned in the higher end of the clarity-oriented SAN range; public database values for this specific grade are limited, and the producer’s certificate of analysis should be used for exact specifications. Representative physical-property ranges for commercial SAN copolymers in the 30–32 wt% acrylonitrile class are density 1.07–1.09 g/cm³, tensile yield stress 60–75 MPa, flexural modulus 3.0–3.6 GPa, elongation at break 2–4 %, and Vicat softening temperature 104–110 °C under ISO 306 method A50. The melt volume-flow rate of lot specimens is batch-specific and is reported under ISO 1133-1:2022 at 220 °C/10 kg.

Before compounding on a production line, pellets are dried in a closed-loop desiccant dryer at 80 °C for 2–4 h. In humid production environments with ambient relative humidity above 60 %, drying must be extended and the dew point of the drying air maintained at or below −20 °C. Residual moisture in the melt above approximately 0.03 wt% produces splay, silver streaks, and reduced tensile elongation in thick sections. Moisture content can be verified by loss-in-weight analysis under ISO 15512 or by Karl Fischer titration. On single-screw extruders, a general-purpose three-zone screw with L/D 20–25 and compression ratio 2.0–2.5 is adequate for plastication; when throughput exceeds 100 kg/h, a barrier screw with a Maddock shear mixer and separate vent is commonly applied. Barrel settings for injection moulding are typically profiled from 190 °C at the feed throat to 230–250 °C at the nozzle. Melt temperature at the nozzle should not exceed 250 °C; residence time above 250 °C should be kept below 5–10 min to avoid yellowing. The shot size should be 30–80 % of barrel capacity to limit hold-up time. In injection moulding, hold pressure is commonly set between 50–70 MPa hydraulic, with backpressure of 3–7 bar and decompression of 2–5 mm before screw rotation. The gate must freeze after the hold stage; for a 2 mm wall thickness, gate land length of 0.8–1.5 mm is a common starting point. Mould cooling is normally set between 50 °C and 80 °C; low tool temperature creates high residual stress at the gate and can reduce solvent stress-cracking resistance.

Thermal degradation window and gate-freeze limits in SAN 350

The thermal stability of styrene-acrylonitrile copolymers is limited by the formation of chromophoric nitrile degradation products at elevated temperature. Differential scanning calorimetry of commercial SAN typically shows a glass transition between 100 °C and 112 °C; no melting endotherm is present. Degradation onset is not a single number but a function of time-temperature history. At 220 °C, a residence-time limit of 15 min is often used; at 250 °C, the limit drops to 5 min; at 270 °C, visible yellowing can occur within 1–2 min in unsaturated screw zones. Therefore, shear heating at high backpressure and high screw speed must be measured directly with a needle thermocouple in the melt stream. A screw speed increase from 50 rpm to 150 rpm can raise melt temperature by 10–20 °C depending on screw design. For transparent parts, the hold-pressure profile determines gate freeze and part mass. If the gate freezes before the holding phase is complete, the part will show sink marks and elevated moulded-in stress. If the gate remains open too long, overpacking raises part mass and can increase birefringence in polarized-light inspection. Gate freeze time can be estimated by observing part mass as a function of hold time; when mass stops increasing, hold time is longer than necessary. For a 2 mm part, gate freeze often occurs between 2 s and 6 s. The exact value depends on gate geometry, melt temperature, and tool temperature.

When enhanced chemical resistance to oils and detergents is specified, the acrylonitrile content in SAN 350 becomes the controlling variable

Substitution of general-purpose polystyrene by SAN 350 in transparent parts is usually driven by environmental stress-cracking resistance and practical contact with household oils or dilute cleaning agents. The nitrile group increases the cohesive energy density of the copolymer relative to polystyrene, reducing the uptake of nonpolar aliphatic solvents. In laboratory evaluation, SAN copolymers exposed to white mineral oil at 23 °C show mass uptake below 0.5 wt% after 24 h, whereas general-purpose polystyrene can swell and craze under the same external stress. However, SAN is not resistant to ketones, esters, chlorinated solvents, or aromatic hydrocarbons. At flexural strain above 0.5 %, contact with ethanol or lanolin-based hand lotions can initiate grazing in areas with high moulded-in stress. The susceptibility is assessed using constant-strain environmental stress-cracking tests patterned after ISO 22088, with the fluid and strain level set by the end-use specification. Annealing may be used to reduce moulded-in stress. A thermal annealing cycle at 70–80 °C for 1–2 h can relax part of the orientation stress in amorphous SAN without significant dimensional change. The actual stress relaxation rate depends on part thickness and tool temperature history.

Transparent injection-moulded parts commonly produced from SAN 350 include cosmetic jars, refrigerator control-panel lenses, office equipment covers, and light-diffusing components where high clarity and moderate heat resistance are specified. In cosmetic packaging, the resin is selected over general-purpose polystyrene because it resists cracking from oil-based formulations and provides a hard shatter-resistant surface; it is selected over transparent ABS when optical transmission and colour purity are more important than impact strength. Moulding of thin-walled closures with wall stock below 1.5 mm requires high injection speed to fill before the melt front freezes, and adequate venting to avoid burn marks. Field failures in such parts are most frequently associated with weld-line splitting around core pins, gate blush, and stress cracking at insert interfaces. A press-fit insert should use a minimum boss radius of 0.5 mm and interference not exceeding 0.2 mm for moderate-toughness SAN. For snap-fit or load-bearing designs, Charpy notched impact energy should be measured under ISO 179-1/1eA; if the value is below 2 kJ/m² at 23 °C, a higher-impact transparent material may be required.

What standards should a receiving-inspection protocol apply to current lots?

Receiving inspection for SAN 350 should compare supplier certificates against methods recognised by the producer’s technical data sheet. The following table identifies the minimum verification methods and common test conditions for a transparent SAN moulding compound. The acceptance limit for each lot is the producer’s specification entered on the certificate of analysis; this document should not be used as an incoming source-control limit.

Regulatory verification generally requires documentation rather than new testing. Under REACH, European Union importers confirm that the substance and any intentionally added components above threshold are registered or exempt under Regulation (EC) No 1907/2006. Under RoHS, homogeneous material testing for lead, mercury, cadmium, hexavalent chromium, and brominated flame retardants is conducted according to the IEC 62321 series. For food-contact uses, the grade must be evaluated under the applicable national framework, such as US FDA 21 CFR or the European Union Regulation (EU) No 10/2011, for the specific food type and use temperature. Published data for this specific configuration is limited; end users should obtain a current regulatory certificate from the supplier for each lot series.

Against transparent ABS, SAN 350 sacrifices low-temperature impact strength and ductility because it contains no dispersed butadiene rubber particles; the result is lower haze and better optical transmission in a 2 mm plaque under ASTM D1003. Against polycarbonate, SAN 350 has lower heat resistance and lower toughness but processes at lower melt temperatures and generally resists aliphatic oils without surface attack. Against PMMA, SAN 350 exhibits better resistance to notch-sensitivity in moulded parts but lower scratch hardness; PMMA typically has higher surface hardness under pencil-scratch tests but is more sensitive to isopropanol and other polar solvents. These material substitutions are supported only when the part-level requirements and test criteria are fixed, because each alternative shifts the failure mode in a predictable but geometry-dependent way.