Polymer Data File:

Polystyrene
- PS

Previous

Up

Next

Introduction

Advantages and Limitations

Typical Applications

Processing

Physical and Mechanical Properties

Finishing

Thermal, Electrical and Optical Properties

Health and Safety

Chemical Resistance Properties

Other Information


Introduction

Polystyrene is one of the styrenic family (two of the others are ABS - acrylonitrile butadiene styrene and SAN - styrene acrylonitrile) and all of the family tend to be relatively brittle with poor outdoor performance. Basic PS is brittle, rigid, transparent, easy to process (shrinkage is low), is low cost and free from odour and taste. High Impact grades (PS-HI) are a rubber modified grade of PS where elastomers are introduced into the base polymer to improve the impact resistance and deformation before fracture.

Sometimes PS is referred to as crystal PS, this refers to the clarity of the finished product and does not imply that there the molecular structure is crystalline (as with most of the polyolefins such as PP etc.). In fact the lack of a crystalline structure is responsible for many of the good points of PS such as the clarity of the product, the low energy input required for processing (no crystal to melt) and the ease of processing with low shrinkage.

General purpose PS is available in various grades such as easy flow, intermediate flow and high heat resins. PS is easily recycled and there is a great deal of experience of recycling PS.

The PS-HI (HIPS in the old terminology) grades incorporate a rubber component and by using different elastomers into the chain, products with a wide range of properties can be produced.

A particular application of PS is in the production of EPS or expanded polystyrene which is a foamed plastic with is extensively used in packaging applications. This product has a completely different set of properties and is not covered by this data sheet.

Typical Applications

Packaging and disposables industry: Transparent yoghurt pots, vending and portion cups, jewellery boxes, food dishes, stacking and sorting crates, toys.

Electrical engineering: Tape recorder and film spools, distributor boxes, housings for kitchen appliances, relay parts, coil shells, slide frames, speaker units.

Household goods: Food storage containers, safety razors, party cutlery, flower tubs, freezer boxes, refrigerator liners and boxes, clothes hangers.

Physical and Mechanical Properties

General

Polystyrene is an amorphous thermoplastic with a density of 1.05 g/cm3. Moisture absorption is extremely low, namely 0.05%.

Mechanical properties

Polystyrene is a hard, stiff and dimensionally stable but relatively inextensible material with a high tensile strength and low elongation at break. The mechanical strength is affected to a very large degree by the processing conditions. The highest values can be obtained with free flowing materials at a low processing temperature.

Property

Approximate Value

PS

PS-HI (High Impact)

Tensile strength

55 - 80 MN/m2

30 - 55 MN/m2

Tensile Modulus

3-4 GN/m2

2 - 3 GN/m2

Elongation at Break

<10 %

10 - 50 %

Flexural Strength

50 - 100 MN/m2

50 - 100 MN/m2

Notched Impact Strength

<3 kJ/m2

3 - 15 kJ/m2

Specific Heat

1.25 - 1.70 kJ/kg/oC

1.25 - 1.70 kJ/kg/oC

Glass Transition Temperature

100 oC

100 oC

Heat Deflection Temperature

<100 oC

<100 oC

Coefficient of Thermal Expansion

5 - 10 x 10-5 / oC

5 - 15 x 10-5 / oC

Long Term Service Temperature

70 - 85 oC

60 - 80 oC

Specific Gravity

1.0 - 1.2

1.0 - 1.2

Mould Shrinkage

0.001 - 0.005 m/m

0.001 - 0.01 m/m

Water Absorption

0.1 - 0.5 % (50% rh)

0.1 - 0.5 % (50% rh)

Transparency

Transparent

Opaque

Thermal, Electrical and Optical Properties

Thermal properties

The glass transition zone is between 80 to 120oC and in this region the strength and hardness fall dramatically. Polystyrene can be used down to temperatures just below this range (70oC). PS can also be used at low temperatures down to -70oC.

Fire behaviour

PP can be ignited by a flame and continues to burn with a glowing, very sooty flame even after the source of ignition has been removed. It has the UL classification of 94 HB. Fire retardant PS is rated 94 V-2. When burning molten drips of burning polymer are created.

Electrical properties

Good electrical resistance values that are virtually independent of the moisture content. PS shows a marked tendency to form electrostatic charges - the old style PS rulers can be used to demonstrate static electricity with small pieces of paper, the classic demonstration.  

Optical properties

The transparent types of standard polystyrene have a light transmission of about 90 in the visible range. Absorption increases considerably in the UV range. The refractive index is 1.59.

Natural Colour

Transparent with a high surface gloss and can be produced in all colours, transparent or muted.

Chemical Resistance Properties

PS has good chemical resistance. It is resistant to alkalis, dilute mineral acids, alkalis, alcohols, water and aqueous salt solutions.

PS is attacked by aromatic and chlorinated hydrocarbons, ethers, ketones, esters and strong oxidising agents.

Resistance to weathering

PS is not recommended for long-term use in the open air as it is degraded by UV rays. This is reflected in yellowing and loss of surface gloss and by a decrease in mechanical strength. Darker formulations perform better than pale or transparent types.

Resistance to stress cracking

Polystyrene is very susceptible to stress cracking. Parts with internal stresses can form stress cracks even in media to which PS is usually resistant. It is therefore advisable to produce injection moulded parts with as few internal stresses as possible.  

Click below for a chemical resistance chart for PS.

PS Chemical Resistance Chart

  Advantages and Limitations

Advantages

Limitations

1. Low cost.

1. Negligible mechanical properties above 70oC

2. Easy to process.

2. Brittle at room temperature.

3. Low shrinkage.

3. Degrades rapidly in outdoor use due to UV.

4. Transparent and wide range of colours available.

 

5. Suitable for food use.

 

6. PS-HI products can be tailored to application.

 

7. Good chemical resistance properties.

 

Processing

Injection moulding

A 2 to 3 hour pre-drying operation at 80oC is recommended for antistatic formulations or when high transparency and high surface gloss is required. Predrying is not necessary for standard grades of PS but may be necessary for some grades of PS-HI.

Low injection pressures are possible due to the easy flow properties and a relatively short and low (30-60%) follow-up pressure is required. The injection speed should be as high as possible, especially for thin walled mouldings and high screw speeds are possible. Processing parameters: PS should be processed at between 180 and 280oC. The material may be thermally degraded at high temperatures. The temperature of the mould chosen can be between 10 and 50oC. Shrinkage is between 0.4 and 0.7% depending on the grade used..

Extrusion

Styrene types with a low melt index and high Vicat softening point are suitable for extrusion. The materials are not hygroscopic and do not require drying.

For non-foamed applications a two-stage vented extruder is generally used with extrusion temperatures of between 180 and 220oC (in the final zone).

Processing Method

 

Applicable

PS

PS-HI (High Impact)

Injection Moulding

Yes.

Yes.

Extrusion

Yes

Yes

Injection Blow Moulding

Yes

Yes

Rotational Moulding

Yes

Yes

Thermoforming

Yes

Yes

Casting

No

No

Bending and joining

Yes

Yes

  Finishing

Machining

The low thermal conductivity and the relatively low softening temperatures of the material, the cutting surfaces must either be fanned with air or cooled with water. If this is not done the PS will melt due to heat generated during machining. The impact resistant types are less likely to splinter.

Surface treatment

Printing, lacquering and hot stamping are all possible. PS can be given a mirror finish with a metallic gloss by high-vacuum metallising with a clear lacquer applied for protection.  

Welding

Ultrasonic welding is normally used. High strengths can be achieved with impulse welding. HF welding is not suitable.

Bonding

Only parts of the same type can be bonded together. Solvent adhesives such as toluene or methylene chloride are normally used. Impact adhesives are used for bonding with different materials.

Health and Safety

Health and Safety

PS is physiologically safe.

Other Information

Identification

  • Easily ignited by a flame and burns with a glowing, very sooty flame even after the source of ignition has been removed. Burning drips fall from the sample. Sweet smelling odour.

  • Standard PS has a very distinctive ringing sound when a sample is tapped on a hard surface. The sound is less distinct for the PS-HI variety but experience can identify the PS family simply from the sound on a surface.

Last edited: 11/03/10

Tangram Technology Ltd. 2000

Our standard disclaimer regarding Internet data applies.