High Impact Polystyrene HIPS: Properties and Applications

High Impact Polystyrene (HIPS) - Complete Overview

What is High Impact Polystyrene?

High Impact Polystyrene (HIPS) is an amorphous thermoplastic material that is a graft copolymer of styrene with polybutadiene or other synthetic rubbers, as well as a mixture of styrene copolymers. It is a modified form of conventional polystyrene that, through the addition of rubber, acquires significantly higher impact toughness.

HIPS is an opaque thermoplastic consisting of a two-phase system: a continuous polystyrene phase with dispersed rubber particles. This structure provides a unique combination of the stiffness inherent in polystyrene and the impact toughness characteristic of rubber. The material is widely used across many industries due to its ease of processing, good gloss, and well-balanced mechanical properties.

How is HIPS Produced?

High Impact Polystyrene is produced by copolymerization of styrene with an elastomer - typically polybutadiene rubber. The rubber content in the material ranges from 5 to 12%, and in some grades up to 15%.

Currently, the graft polymerization method is predominantly used for HIPS production, where the elastomer (polybutadiene or butadiene-styrene rubber) is dissolved in styrene, followed by polymerization. The elastomer is distributed throughout the polystyrene mass as tiny particles, forming a multiphase structure. A certain amount of graft copolymer is formed, which improves compatibility between the phases.

Previously, mechanical blending was used but produced lower quality and less stable results. Modern technologies allow the production of material with various properties by varying the type and content of elastomer, particle size, polystyrene molecular weight, and the addition of various additives.

Various additives may be incorporated into HIPS: flame retardants to increase fire resistance (allowing the glow wire test temperature to be increased from 650 to 960 °C); light stabilizers to improve UV resistance; anti-blocking agents to improve processability.

Key Physical and Mechanical Properties

High Impact Polystyrene possesses a number of unique properties that determine its wide application.

Density is 1.03-1.06 g/cm³. Typical values are 1.04-1.05 g/cm³.

Glass transition temperature is 93-105 °C. Melting (softening) temperature is 95-105 °C. Vicat softening temperature is about 89-99 °C. Heat deflection temperature (1.82 MPa) is 77-96 °C.

Tensile strength (yield) is 14.5-41.1 MPa, according to other sources - 20-38 MPa or 15-25 MPa. Flexural strength is 35-62 MPa.

Impact toughness is the key advantage of HIPS. Impact strength (Izod notched) reaches 15 kJ/m², according to other sources - 9.5 kJ/m². The impact toughness of HIPS is several times higher than that of General Purpose Polystyrene (GPPS).

Flexural modulus is 1103-2483 MPa, elastic modulus is not less than 1800 MPa.

Elongation at break is 20-65%.

Rockwell hardness is R 50-110.

Water absorption is low, less than 0.1%, according to other sources - 0.05-0.7%.

Mold shrinkage is low, ranging from 0.2 to 0.7%.

Electrical properties - volume resistivity is 10¹⁵ Ω·cm, which is somewhat lower than GPPS but comparable to SAN and ABS plastics.

Operating temperature range is from -30 °C to +70 °C. Some sources indicate a range down to -40 °C.

Chemical resistance - like General Purpose Polystyrene, HIPS is resistant to hydrolysis, dilute acids, alkalis, and alcohols. However, the material is not resistant to UV radiation and has limited chemical resistance.

Processing Features

HIPS is easily processed by all major thermoplastic processing methods.

Main processing methods:

• Injection molding - the main method for producing housings and precision parts. Recommended processing temperature is 180-230 °C.

• Extrusion - production of sheets, profiles, and films. The material has good melt strength and a wide processing window.

• Thermoforming - various products are formed from extruded sheets, including food packaging.

• Co-extrusion - co-extrusion with General Purpose Polystyrene.

Due to high dimensional stability and low shrinkage, HIPS is compatible with anti-blocking additives, avoiding additional mold lubrication costs. Low shrinkage allows the use of molds designed for General Purpose Polystyrene.

Pre-drying of the material at 80 °C for 2-3 hours is recommended before processing. Recommended mold temperature is 30-60 °C.

Applications of HIPS

Thanks to its unique combination of impact toughness, stiffness, and ease of processing, High Impact Polystyrene has found the widest application in various industries.

Electronics and household appliances - one of the key applications. HIPS is used for housings of televisions and monitors, printers and other office equipment, radio housings, as well as refrigerator parts and other household appliances.

Automotive industry - interior parts, interior components, upholstery, decorative elements.

Packaging industry - food packaging, containers, trays, beverage cups, dairy packaging. HIPS is widely used in the food industry, especially for yogurt cups, due to its durability and safety.

Refrigeration industry - refrigerator inner liners, parts operating at low temperatures.

Consumer goods - toys, kitchenware, storage containers, stationery.

Construction - plumbing, drainage pipes, decorative elements.

Medicine - disposable medical products, packaging.

Electrical engineering - switch keys, sockets, and other electrical products (especially in flame-retardant grades).

Special applications - HIPS is used in 3D printing for prototypes and as a support material.

Comparison of HIPS and GPPS

High Impact Polystyrene (HIPS) and General Purpose Polystyrene (GPPS) are two related but significantly different materials.

Main difference - the presence of rubber modification in HIPS, which provides significantly higher impact toughness. While GPPS is transparent and brittle, HIPS is opaque and impact-resistant.

Mechanical properties - HIPS has higher impact toughness (several times higher than GPPS), but is inferior to GPPS in tensile strength (by 30% or more) and flexural strength (by 20% or more).

Optical properties - GPPS is transparent (light transmission up to 90%), HIPS is opaque.

Electrical properties - HIPS has somewhat lower values than GPPS.

Processing - both materials are easily processed, have low shrinkage and high dimensional stability. Both materials comply with FDA requirements for food contact and have UL 94HB flammability rating.

Cost - HIPS is somewhat more expensive than GPPS but cheaper than ABS plastic, making it a cost-effective alternative for many applications.

Major HIPS Producing Countries

The global High Impact Polystyrene market is characterized by high production concentration. Major producers are based in the USA, China, Western Europe, Saudi Arabia, South Korea, and Taiwan.

Major manufacturers:

• INEOS Styrolution (UK/Germany) - one of the world leaders

• Trinseo (USA)

• Total (France/USA)

• Americas Styrenics (AmSty) (USA) - produces GPPS and HIPS

• Chimei Corporation (Taiwan)

• Chevron Phillips Chemical (USA)

• LG Chem (South Korea)

• Toyo Styrene (Japan)

• PS Japan (Japan)

• Formosa Chemicals & Fibre Corporation (FCFC) (Taiwan)

• Sinopec (China)

• CNPC (China)

• TPSC Asia (Singapore) - capacity 100,000 tons per year

• Resirene (Mexico)

• Unigel (Brazil)

Production regions:

• North America: USA (AmSty, Trinseo, Chevron Phillips Chemical), Mexico (Resirene)

• Europe: UK (INEOS), France (Total), Italy (Versalis)

• Asia-Pacific: China (Sinopec, CNPC), Taiwan (Chimei, FCFC), Japan (PS Japan, Toyo Styrene), South Korea (LG Chem), Singapore (TPSC Asia), India

• Middle East: Iran (Tabriz Petrochemical)

• Latin America: Brazil (Unigel)

In Russia, High Impact Polystyrene is produced by SIBUR (grades UPS 825ES, UPS 825). Imported grades from Iran (Tabriz Petrochemical), Italy (Versalis), and other countries are also available on the market.

Conclusion

High Impact Polystyrene is one of the most important engineering thermoplastics, occupying a significant place in the global polymer market. Thanks to its unique combination of impact toughness, stiffness, ease of processing, and affordable price, it is widely used in the production of housings for household appliances, automotive parts, refrigeration equipment, packaging, and consumer goods. Although HIPS is inferior to GPPS in transparency and tensile strength, and has limited UV resistance, its high impact resistance and cold resistance make it an indispensable material for many industries. Understanding the specifics of production, properties, and global trends helps market participants make informed purchasing decisions for this raw material.