Polymer Data File:

Polymethyl Methacrylate
- PMMA (Acrylic)

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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

PMMA is one of the earliest polymers and is well known around the world by a variety of trade names Lucite, Oroglas, Perspex and Plexiglas which vary with the country you are in. The original PMMA was seen as a replacement for glass in a variety of applications and is currently used extensively in glazing applications.The material is one of the hardest polymers, rigid, glass-clear with glossy finish and good weather resistance.

One original application as automotive windshields was not possible due to the low scratch resistance of the PMMA relative to glass. Despite this PMMA was used extensively as aircraft windows for many aircraft.

My own first involvement with PMMA was in the now famous 'frozen chicken' testing: this testing was carried out to investigate the effects of 'bird strike' on aircraft windshields (hitting a bird at 600 mph is dangerous to both bird and aircraft). The tests involved defrosted 'frozen chickens', a linear accelerator and large sheets of Perspex. You can imagine the rest. We moved on to bullets, small arms and finally slow indentation - It was worth a Ph. D. so don't laugh too much. We also learnt how to make 'flat lenses' by varying the density of PMMA and overall what a wonderful material it is.

Typical Applications

Optics: Dust covers for hi-fi equipment, sunglasses, watch glasses, lenses, magnifying glasses

Vehicles: Rear lights, indicators, tachometer covers, warning triangles

Electrical engineering: Lamp covers, switch parts, dials, control buttons

Office equipment: Writing and drawing instruments, pens

Medicine: Packaging for tablets, pills, capsules, suppositories, urine containers, sterilisable equipment.

Others: Leaflet dispensers, shatter-resistant glazing, shower cubicles, transparent pipelines, illuminated signs, toys.

Physical and Mechanical Properties

General physical properties

PMMA is a glassy polymer with an amorphous structure. It has a density of 1.19 g/cm3 and has a very low water absorption . The refractive index ranges from 1.49 to 1.51 depending on the type.

Mechanical properties

Parts made of PMMA have high mechanical strength and good dimensional stability. Other properties include a high Young's modulus and good hardness with low elongation at break. PMMA does not shatter on rupture. PMMA is one of the hardest thermoplastics and is also highly scratch resistant.

Property

Approximate Value

Tensile strength

55 - 80 MN/m2

Tensile Modulus

2-3 GN/m2

Elongation at Break

<10 %

Flexural Strength

100 - 150 MN/m2

Notched Impact Strength

< 3 kJ/m2

Specific Heat

1.25 - 1.7 kJ/kg/oC

Glass Transition Temperature

100 oC

Heat Deflection Temperature

<100 oC

Coefficient of Thermal Expansion

5 - 10 x 10-5 / oC

Long Term Service Temperature

<100 oC

Specific Gravity

1.0 to 1.2

Mould Shrinkage

0.001 - 0.005 m/m

Water Absorption

0.1 - 0.5 % (50% rh)

Transparency

Transparent

Thermal, Electrical and Optical Properties

Thermal properties

The thermal stability of standard PMMA is only 65oC. Heat-stabilised types can withstand temperatures of up to 100oC. PMMA can withstand temperatures as low as -70oC. Resistance to temperature changes is very good.

Fire behaviour

PMMA ignites very quickly. It burns with a blue glow, even outside the flame, and crackles with white spurts.

Electrical properties

PMMA has good insulating properties, a high dielectric strength and high tracking resistance. The relatively high surface resistance, however, encourages electrostatic charges on the surface of moulded parts; this can be largely overcome by the use of antistatic agents.

Optical properties

PMMA is naturally transparent and colourless. The transmission for visible light is 92%. The refractive index is 1.492 for PMMA. There are types that transmit UV rays, and types that absorb it almost completely, as a result of which sensitive dyes on painted surfaces behind are protected from fading.

Natural colour

PMMA is crystal clear and has a high surface gloss. It can be produced in all colours, transparent and muted.

Chemical Resistance Properties

General

PMMA is resistant to aliphatic hydrocarbons, cycloaliphatic compounds, fats and oils, and also to dilute acids at temperatures of up to 60oC.
Chlorinated aliphatic hydrocarbons, ketones, alcohols, ethers, esters, aromatics, petrol, spirit, nitrocellulose varnishes and certain plasticisers cause PMMA to swell or produce stress cracks.

The resistance to weathering of PMMA is very good.

Resistance to stress cracking

Stress cracks can be formed by cleaning agents and disinfectants. Certain organic solvents and metal salts can also cause stress cracking.

We do not have a current detailed Chemical Resistance Chart for PMMA. We are currently putting this together from published information. Click below for what we have at the moment:

PMMA Chemical Resistance Chart

  Advantages and Limitations

 

Advantages

Limitations

1. Material is very hard.

1. Brittle under impact conditions and failure is by shattering.

2. Material is clear and can be coloured in any colour from opaque to translucent.

2. Difficult to mould thin walled products because of poor flow properties.

3. Good weathering resistance.

3. Poor hot-melt strength limits processing methods.

4. Good optical properties.

 4. Flow properties make processing slow compared to other materials.

5. High gloss.

5. Does not have significant elastic deformation before failure i.e. goes straight to brittle fracture.

6. Scratch resistant (but not as good as glass because it does scratch - this is why car windscreens are not made from PMMA).

Processing

Injection moulding

Pre-drying is not necessary if a vented cylinder is used but if a normal cylinder is used then PMMA must be processed dry and it is advisable to pre-dry the granules for up to 8 hours at 70 to 100oC, depending on the type. Surface defects and blisters will form if damp granules are processed. 

Processing parameters: The mould temperature should be between 40 and 80oC, depending on the type. The material temperature should be between 200 and 250oC. As the temperature rises, molecular orientation and internal stresses decrease, but the risk of sink spots increases.

As a general rule, high injection pressures are needed because of poor flow properties and it may be necessary to inject slowly to get the correct flow. This is particularly important for optical moldings where visible weld lines will form if the correct parameters are not used. For thick mouldings a high follow-up pressure is needed for a long time (2 to 3 minutes).

Large gates are needed because of the poor flow but hot runners are possible. 

Reprocessing is possible if the material has been thoroughly dried and has an opaque colour. Optically good mouldings are not generally possible with regrind even if the regrind is glass clear.

Shrinkage is relatively low: 0.4 to 0.8% depending on the grade used. Avoid wall thicknesses of less than 1 mm.

Extrusion

Extrusion temperatures are between 180 and 250oC. Metering type screws with a compression  ratio of 3:1 are generally recommended. Relatively high molecular weight formulations are used for the extrusion of sheets and profiles. A degassing screw with an L/D ratio of 20 to 30 is best.

Processing Method

Applicable

Injection Moulding

Yes.

Extrusion

Yes

Extrusion Blow Moulding

Yes. 

It is possible for some grades (impact modified) to be injection or blow moulded but the poor hot-melt strength makes extrusion blow moulding difficult and injection blow moulding is generally used to produce a parison.

Rotational Moulding

No.

Poor heat fusion characteristics of powders prevent rotational moulding.

Thermoforming

Yes

Casting

Yes

Bending and joining

Yes

  Finishing

Machining

In order to avoid sticking, the cutting speeds should not be too high. Tools must be sharp, and a cutting coolant should be used. When drilling, the angle of point should be 60 to 90o. Thin sheets can be broken along a scribed line. High machining speeds can be used if there is sufficient cooling.

Surface treatment

Care should be taken to ensure that the solvents in the lacquer do not attack the PMMA too severely. Petrol or toluene-based lacquers are suitable. Warm storage is recommended before lacquering. Metallising does not cause any problems provided there are no residues of fat or release agent on the surface.

Welding

PMMA can be welded by all the plastics welding processes such as hot-blade, hot-gas, ultrasonic or spin welding.

Bonding

Solvents can cause stress cracks if internal stresses are present in the moulded part, so it is advisable to temper the part at 60 to 90oC before bonding. Methylene chloride, polymerisation, epoxy resin, contact and impact adhesives are suitable. Adhesion with solvent-based and polymerisation adhesives is better than with contact adhesives.

Health and Safety

Health and Safety

PMMA is odourless and tasteless and physiologically safe.

Other Information

Identification

  • Material is easily flammable and burns with a bright flame after ignition source has been removed.

  • Flame crackles and produces sooty fruity/sweetish smell.

Last edited: 11/03/10

Tangram Technology Ltd. 2000

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