Polystyrene (PS) is a widely used thermoplastic known for its rigidity, dimensional stability, and water-clear optical clarity in its general purpose form, and for toughness and excellent thermoformability in its rubber-modified high-impact form. Discovered in 1839 and commercialized at scale in the 1930s, polystyrene remains one of the highest-volume commodity thermoplastics in the world, valued for its low cost, ease of processing, and versatility across packaging, appliances, electronics, and medical devices.
As a polystyrene resin supplier, Formerra provides access to GPPS and HIPS grades from AmSty, INEOS Styrolution, and Trinseo, covering general purpose transparent grades, rubber-toughened impact grades, and specialty formulations for food contact, medical, and structural applications.
PS is produced by free-radical polymerization of styrene monomer, linking styrene units into long, amorphous chains without the crystalline structure found in polyolefins. The result is a rigid, glassy material with a glass transition temperature (Tg) of 90-100 degrees C. HIPS grades are produced by dissolving polybutadiene rubber in styrene monomer before polymerization, creating a two-phase microstructure where dispersed rubber particles absorb impact energy and arrest crack propagation through the matrix.
PS granules and pellets are available in GPPS grades covering melt flow indexes from 1 to over 25 g/10 min for injection molding and extrusion, and HIPS grades offering a range of impact levels, surface finishes, and sheet thermoforming performance. Food contact grades comply with FDA 21 CFR 177.1640, and specialty grades include UV-stabilized outdoor formulations and flame-retardant grades for electronics.
Optical clarity is the defining property of general purpose polystyrene. GPPS grades transmit 85-90% of visible light with a refractive index of 1.59, among the highest of standard thermoplastics. The amorphous structure eliminates light-scattering crystalline domains, producing water-clear transparency suitable for display packaging, medical labware, lighting diffusers, and consumer covers requiring unobstructed visibility at commodity pricing.
Impact resistance is the defining advantage of high impact polystyrene over GPPS. Dispersed polybutadiene rubber particles act as energy absorbers during impact, arresting crack growth and delivering elongation at break of 5-50% compared to 1-3% for unmodified PS. HIPS grades tolerate rough handling in packaging, consumer products, and appliance components without the brittle fracture failure characteristic of GPPS. The rubber content, typically 5-12% by weight, trades optical transparency and some stiffness for dramatically improved toughness.
Processing ease makes both GPPS and HIPS attractive for high-volume production. Both grades process on standard injection molding, extrusion, and thermoforming equipment at melt temperatures of 200-260 degrees C, with short cycle times and reliable mold filling in thin sections. HIPS is particularly well suited to thermoforming of large sheets for refrigerator liners, food trays, and display panels. PS granules generate minimal volatile byproducts and do not require pre-drying under most storage conditions.
HIPS grades produce fine, reproducible surface finishes suited to thermoforming, where sheets are heated and formed over molds to create large, complex shapes. HIPS is the benchmark thermoforming material for refrigerator door liners and interior components due to its combination of formability, toughness, surface quality, and food contact compliance at competitive cost.
GPPS 35-55 MPa;
HIPS 20-35 MPa
GPPS 1-3%;
HIPS 5-50%
GPPS 3.0-3.5 GPa;
HIPS 1.8-2.5 GPa
GPPS 15-25 J/m;
HIPS 50-200 J/m
GPPS R70-R90; HIPS R50-R80
GPPS 3.0-3.5 GPa; HIPS 1.8-2.5 GPa
GPPS 90-100 degrees C;
HIPS 80-95 degrees C
GPPS 70-90 degrees C at 0.46 MPa;
HIPS 70-85 degrees C
-20 to 70-80 degrees C continuous use
200-260 degrees C
GPPS 85-103 degrees C;
HIPS 75-95 degrees C
60-80 x 10^-6 /degrees C
Less than 0.1% in 24 h at 23 degrees C. PS absorbs negligible moisture due to its non-polar aromatic hydrocarbon structure. Pre-drying is not required before processing under normal storage conditions. Parts maintain dimensional stability in humid service environments throughout their service life without measurable moisture-driven property changes.
Poor without stabilizers. PS yellows rapidly under UV exposure due to oxidation of the aromatic backbone. Unprotected PS loses color and surface integrity within weeks of continuous outdoor sun exposure. UV-stabilized grades improve resistance for moderate outdoor applications but do not match the inherent UV stability of ASA, acrylic, or UV-stabilized polypropylene for long-term exterior use. Indoor applications are unaffected by UV.
Excellent. PS shows no measurable degradation from water immersion at ambient or elevated temperatures. The non-polar aromatic backbone resists hydrolysis entirely. Medical labware, food packaging, and water-contact applications perform reliably without property loss from moisture throughout the product service life.
Moderate to high. PS is susceptible to environmental stress cracking (ESC) when exposed to fats, oils, and organic solvents, particularly under mechanical stress. Contact with cleaning solvents, mold release agents, and lipid-containing foods causes surface crazing or cracking. HIPS shows lower ESC susceptibility than GPPS due to rubber phase energy absorption. Designing to minimize stress concentrations and avoiding solvent contact reduce ESC risk.
85-90% (clear GPPS grades)
less than 1% (excellent optical clarity)
1.59
85-95 GU (injection molded GPPS)
17-20 kV/mm
2.4-2.7 at 1 MHz (very low)
0.0001-0.0003 at 1 MHz (excellent)
10^15-10^17 ohm-cm
10^14-10^16 ohm
GPPS 1.04-1.065 g/cm3;
HIPS 1.03-1.06 g/cm3
1-25+ g/10 min (grade dependent)
0.3-0.7% (low and isotropic)
UL94 HB standard;
V-0 and V-2 available with FR additives
GPPS water-clear; HIPS opaque (natural, white, or pigmented)
Water, dilute inorganic acids, dilute inorganic bases, salts, lower alcohols (short contact)
Some aliphatic oils and greases (short contact, room temperature)
Aliphatic hydrocarbons, higher alcohols, fats and fatty foods, some cleaning agents at elevated temperature
Aromatic hydrocarbons (benzene, toluene, xylene), ketones, esters, chlorinated solvents, most organic solvents
Surface finish capability
PS accepts a wide range of mold surface finishes from high gloss to matte and textured surfaces. GPPS reproduces fine detail with excellent optical gloss in clear packaging and display parts. HIPS replicates mold texture accurately, including wood-grain and leather-grain surfaces for consumer products. Both grades eject cleanly from standard steel tooling with minimal draft angle requirements. Surface quality is sensitive to melt temperature and injection speed; optimized parameters deliver consistent, high-gloss surfaces.
Sink, warpage, and visible defect tendency
Low shrinkage (0.3-0.7%) and the amorphous structure minimize sink marks and warpage compared to semicrystalline polymers. PS parts show predictable, isotropic shrinkage that simplifies mold compensation. Weld lines are visible on clear GPPS parts and require gate placement to relocate them to non-critical areas. Jetting and hesitation marks appear on thin-walled GPPS parts at incorrect gate locations. Overall, PS is well suited to tight-tolerance, thin-wall injection molding with straightforward tooling design.
Colorability
HIPS accepts a full range of opaque and translucent colors with excellent consistency and vibrant depth. GPPS colors in transparent and translucent shades with good optical clarity. Both grades disperse masterbatch colorants readily in the melt. Metallic and special-effect pigments achieve strong visual impact in HIPS consumer products. Color stability under indoor light exposure is good for typical product service lifetimes; UV exposure causes yellowing in unstabilized grades.
Color stability
PS yellows under UV exposure due to oxidation of the aromatic ring structure. Indoor color stability is excellent across the service life of most consumer and packaging applications. UV-stabilized grades maintain color for moderate outdoor exposure in temperate climates. Long-term outdoor applications require more UV-resistant polymers or UV-stabilized grades with confirmed accelerated weathering data. High-temperature processing above 260 degrees C causes thermal yellowing regardless of UV stabilization.
Transparency and clarity
GPPS delivers the best optical clarity among commodity thermoplastics with light transmittance of 85-90% and haze below 1%. Clarity competes favorably with acrylic and polycarbonate at significantly lower cost where UV resistance and high impact are not required. HIPS is opaque in all standard grades due to light scattering from the rubber phase and is not suitable for any clarity application. Clear GPPS grades maintain optical performance across the service life of indoor packaging and display applications.
Abrasion and chemical mar resistance
PS surfaces scratch readily with fingernails, keys, and sharp objects. GPPS surface hardness (Rockwell R70-R90) provides modest scratch resistance. HIPS surfaces are softer and more susceptible to marring from abrasive contact. Chemical mar from solvent exposure causes permanent surface damage. For high-cosmetic applications, protective UV-curable coatings or harder polymer alternatives are preferred. Consumer products with managed handling requirements tolerate PS surface hardness without functional failure.
Marking methods
Laser engraving produces clean, high-contrast marks on both GPPS and HIPS. Hot stamping delivers metallic and decorative finishes on HIPS consumer products. In-mold labeling produces permanent-bond graphics during injection molding or thermoforming. Pad printing and screen printing adhere reliably to PS without surface treatment. Inkjet printing works on HIPS without primer for date coding and variable data. PS accepts thermal printing for labeling and traceability in packaging applications.
Coating, painting, and plating suitability
PS accepts solvent-free water-based coatings and paints without surface treatment. Solvent-based paints and adhesives attack PS and require formulation verification before use. UV-curable hard coats improve scratch resistance on GPPS optical covers and display panels. Vacuum metallization produces bright metallic finishes on GPPS and HIPS for decorative cosmetics and consumer electronics applications. Chrome electroplating is generally not practical on standard PS grades. Adhesion is good with water-based primer systems on untreated surfaces.
Joining methods
Ultrasonic welding produces strong, clean joints in both GPPS and HIPS with short cycle times suited to high-volume assembly. Vibration and spin welding suit larger parts. Solvent bonding with compatible solvents such as cyclohexane produces strong lap joints but requires process control to avoid stress crazing. Cyanoacrylate and two-part epoxy adhesives bond PS for structural joints. Heat staking and hot plate welding join to compatible substrates. Mechanical fastening works reliably in HIPS; GPPS snap fits risk brittle fracture under deflection and require conservative design.
GPPS (general purpose polystyrene) and HIPS (high impact polystyrene) share the same styrene backbone but differ in mechanical performance and optical properties. GPPS is transparent, rigid, and stiff with a flexural modulus of 3.0-3.5 GPa, but brittle with only 1-3% elongation at break and notched impact of 15-25 J/m. GPPS is the material of choice for applications requiring optical clarity, such as clear food packaging, medical labware, and display covers.
HIPS adds 5-12% polybutadiene rubber as a dispersed phase, producing elongation at break of 5-50% and notched impact of 50-200 J/m, at the cost of optical transparency. HIPS is opaque in all standard grades and suited for applications where toughness, thermoformability, and processability outweigh the need for clarity. Choose GPPS for transparency and dimensional accuracy in low-impact applications. Choose HIPS for toughness, thermoforming, and structural packaging applications where optical clarity is not required.
No. PS does not require pre-drying before processing under standard storage and handling conditions. Moisture absorption is below 0.1% in 24 hours at 23 degrees C, far below the threshold that causes processing defects. PS granules do not need desiccant storage, sealed containers, or drying equipment for routine production. This is a significant workflow advantage over hygroscopic resins such as nylon (which requires 4-8 hours at 80 degrees C), PET, and TPU.
If material has been exposed to water, condensation, or very humid storage (above 90% RH for extended periods), brief drying at 70-80 degrees C for 1-2 hours removes surface moisture and restores normal processing. Under standard plant conditions, PS runs immediately from the bag without any pre-treatment.
PS is attacked by a broad range of organic solvents and should be evaluated carefully for any chemical contact application. Avoid aromatic hydrocarbons including benzene, toluene, and xylene; ketones including acetone and MEK; esters including ethyl acetate; and all chlorinated solvents including methylene chloride and TCE. These chemicals dissolve or severely swell PS rapidly. Fats, fatty foods, and cooking oils cause surface stress cracking, particularly under mechanical stress.
Safe for PS include water, dilute inorganic acids (below 10% concentration), dilute inorganic bases (below 10% concentration), alcohols at short contact times and ambient temperature, and most aqueous cleaning solutions formulated without solvents. Cleaning products in contact with PS parts must be verified as solvent-free before use. When in doubt, immerse a PS coupon in the candidate chemical for 24-72 hours and check for weight change, dimensional change, and crazing before approving contact in production.
PS yellows by two distinct mechanisms. UV yellowing occurs when aromatic rings in the PS backbone absorb UV radiation and oxidize, forming chromophoric groups that absorb visible light. This is most pronounced for outdoor applications and begins within days of UV exposure in unstabilized grades. Thermal yellowing occurs at processing temperatures above 260-270 degrees C when the PS backbone begins to degrade, releasing styrene monomer and forming colored degradation products.
Preventing UV yellowing requires UV stabilizer packages containing UV absorbers and hindered amine light stabilizers (HALS). Specify UV-stabilized GPPS or HIPS grades for applications exposed to sunlight or UV sources, and validate with accelerated weathering testing against your service life target. Preventing thermal yellowing requires strict melt temperature control during processing, short residence times in the barrel, and purging the machine during extended shutdown. Process PS below 260 degrees C whenever possible and use correctly sized machines to minimize residence time.
Multiple GPPS and HIPS grades from AmSty and INEOS Styrolution comply with US and international food contact regulations. In the US, polystyrene for food contact must comply with FDA 21 CFR 177.1640, which permits PS in contact with all food types under specified conditions of use. Compliant grades are available from all three Formerra suppliers for food packaging, dairy containers, disposable cutlery, and food service items.
For food contact applications in the EU, PS must comply with EU 10/2011 (plastics for food contact materials). Both regulations restrict specific additives and migration limits. When specifying PS for food contact, request the supplier declaration of compliance for the specific grade and confirm that all additives in the formulation appear on the positive list for the target regulation. Formerra technical representatives provide regulatory support and documentation to assist with compliance verification for your specific application and market.
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Polystyrene (PS) Overview. SpecialChem / Omnexus. 2024.
Polystyrene GPPS and HIPS Product Portfolio. AmSty (Americas Styrenics). 2024.
Styrolution PS General Purpose and High Impact Polystyrene. INEOS Styrolution. 2024.
Polystyrene GPPS and HIPS Technical Product Guide. Trinseo. 2024.
Polystyrene Properties and Applications. MatWeb Material Property Database. 2024.
