Polymer Data File:

Polyamide
- PA (Nylon)

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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

Nylon is used as a generic name for the polyamide group of polymers but was originally the name given by Du Pont (in the USA) to the fibre discovered by Wallace Hume Carothers in 1934 (See Plastics Timeline under 1934 and 1937). 

The polyamide products can be formed by two general methods and this gives two distinct classes of polyamides.

  1. Condensation polymerisation:
    The polymers formed by condensation reactions (See Polymer Chemistry) of a dibasic acid and a diamine are the PA 6/6 types of PA. Other variants (which are created by using different dibasic acids) are PA 6/10 and PA 6/12.
  2. Addition polymerisation.
    The polymers formed by addition reactions (See Polymer Chemistry) of ring compounds. Typical variants are PA 6, PA 11, PA 12 and PA 46

It is theoretically possible to produce many different types of PA polymers and all tend to be labelled carelessly under the heading of "Nylon". Given the multitude of variants and range of physical and chemical properties this is unwise. In this data file we will try to be specific with regard to the variant being referred to but sometimes the source information is not always clear. The largest volume sales are of the PA 6 and PA 6/6 variants. The other materials tend to be specialist polymers at specialist prices. In most cases the information refers to these materials.

As a general family the PA group is rigid, translucent with good fatigue, creep and wear resistance. Chemical resistance varies with the variant but is generally good.

Typical Applications

Mechanical: Gear wheels, bearings, rollers, cam-wheels, screws, control rollers.

Consumer goods: Bicycle wheels, hair brushes and combs, per tool housings, Strimmer cable.

Automotive: Fans, fuel system pipe work, oil filters, floats, bearings, tachometer drive pinions, speedometer and windshield wiper gears, jets for windscreen washer systems, radiator header tanks.

Electrical: Coil formers, card guides, terminal blocks, distribution boxes, vacuum cleaners, hand lamps, abrasion-resistant cable ducts.

Miscellaneous: Zips, fibres, nuts and bolts, pressure tubing, pump housings, laboratory equipment, furniture fittings, medical instruments.

Physical and Mechanical Properties

All the properties of PA depend on the moisture content of the plastic and the crystallinity of PA variant being used. PA can be tough when the moisture content is right (2 - 3% in air at 20oC) but dry products tend to become brittle. The equilibrium moisture content depends on the time, temperature and wall thickness of the part. Dimensional and physical property changes take place during moisture absorption and injection moulded parts are often annealed or conditioned after processing to optimise the physical properties quickly.

PA has good temperature stability, abrasion resistance and fatigue resistance.

The crystalline nature of PA also affects the properties significantly and highly crystalline types can be stiff and hard. The use of glass fibre and other reinforcements can improve the mechanical properties (modulus and strength).

The density, equilibrium moisture content and shrinkage varies with the PA variant used.

Property

Approximate Value

PA 6

PA6/6

PA 11

PA 12

Tensile strength

55 - 80 MN/m

55 - 80 MN/m2

30 - 55 MN/m2

30 - 55 MN/m2

Tensile Modulus

1 - 2 GN/m2

2 - 3 GN/m2

1 - 2 GN/m2

<1 GN/m2

Elongation at Break

100 - 500 %

100 - 500 %

100 - 500 %

100 - 500 %

Flexural Strength

50 - 100 MN/m2

50 - 100 MN/m2

50 - 100 MN/m2

50 - 100 MN/m2

Notched Impact Strength

3 - 10 kJ/m2

3 - 10 kJ/m2

3 - 10 kJ/m2

10 - 20 kJ/m2

Specific Heat

1.25 - 1.70 kJ/kg/oC

1.25 - 1.70 kJ/kg/oC

2.15 - 2.60 kJ/kg/oC

1.70 - 2.15 kJ/kg/oC

Glass Transition Temperature

50oC

66oC

approx. 50oC

approx 50oC

Heat Deflection Temperature

<100oC

100 - 150oC

<100oC

<100oC

Coefficient of Thermal Expansion

10 - 15 x 10-5 / oC

10 - 15 x 10-5 / oC

10 - 15 x 10-5 / oC

10 - 15 x 10-5 / oC

Long Term Service Temperature

<100 oC

<100 oC

<100 oC

<100 oC

Specific Gravity

1.0 - 1.2

1.0 - 1.2

1.0 - 1.2

1.0 - 1.2

Mould Shrinkage

0.01 - 0.025 m/m

0.01 - 0.025 m/m

0.01 - 0.025 m/m

0.005 - 0.02 m/m

Water Absorption

>2 % (50% rh)

>2 % (50% rh)

>2 % (50% rh)

1.0 - 2.0 % (50% rh)

Transparency

Opaque

Opaque

Opaque

Opaque

Thermal, Electrical and Optical Properties

Thermal properties

PA has good high temperature performance and depending on the variant can be used between 80 and 120oC, some stabilised variants can be used for short periods at up to 200oC. 

PA can be used down to about -40oC, at this temperature the copolymers will still fail in a ductile manner but the homopolymers will generally fail in a brittle manner.

PA has a low  linear coefficient of thermal expansion and is dimensionally stable nearly up to the melt region when unloaded. Reinforcement with glass fibre gives excellent dimensional stability with changes in temperature.

Fire behaviour

Most PA variants are flammable and start to decompose at above 300oC. Above 450 to 500oC the burning becomes self sustaining and the PA will continue to burn when the source of ignition is removed. When burning, the material bubbles and drips and threads can be pulled from the molten material. The flame is blue and has the odour of burning horn. Some variants are self-extinguishing.

Electrical properties

The electrical properties of all PA variants are strongly dependent on moisture content. The polar nature of PA means that the electrical properties are also dependent on the frequency. Specific details relevant to the variants and grade to be used should be obtained before using PA in electrical applications.

Optical properties

Natural PA is milky-opaque but can be easily coloured in a wide variety of colours. PA has a refractive index which varies with the variant and for most types it is in the region of 1.52 to 1.53.

Chemical Resistance Properties

PA has good chemical resistance to aliphatic and aromatic hydrocarbons, benzene, oils, fats, certain alcohols, esters, ketones, ether, organic and inorganic bases up to medium concentration and to chlorinated hydrocarbons. Some chemicals e.g. chloroform and methylene chloride, produce significant swelling and others are e.g. alcohols, cause similar swelling to water. PA is not resistant to mineral acids, strong caustic solutions, solutions of oxidising agents, formic acid, phenols, cresols and glycols. 

Weathering resistance

PA has good resistance to ageing and weathering and this can be improved with suitable additives (carbon black improves UV and general weathering performance). Reinforced PA suffers from more surface attack at the glass fibre interfaces and significant visible damage can occur in a short time, this does not generally significantly affect the mechanical properties. 

Stress cracking resistance

Low tendency to stress cracking.

PA 6 Chemical Resistance Chart

PA 6/6 Chemical Resistance Chart

PA 11 Chemical Resistance Chart

PA 12 Chemical Resistance Chart

Advantages and Limitations

Advantages

Limitations

1. Good combination of mechanical properties (fatigue and creep strength, stiffness, toughness and resilience), which are only slightly inferior to those of polyacetals.

1. All PA variants absorb or give up moisture to achieve equilibrium with ambient conditions; moisture acts as a plasticiser, decreasing tensile and creep strengths and increasing impact strength and the dimensions of the component.

2. Good abrasion resistance and self-lubricating properties give widespread use in gears and bearings.

2. Electrical properties are greatly influenced by moisture content.

3. Suitable for prolonged service temperatures of 80-100oCand up to 140oC with heat-stabilized grades.

3. Thermal expansion varies with temperature and moisture content.

4. Can be used in contact with most foodstuffs at room temperature; sterilized by steam or infra-red radiation.

4. All PA variants are attacked by strong mineral acids and acetic acid, and are dissolved by phenols.

5. Resistant to fuels, oils, fats, and most technical solvents, such as aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons, esters, ketones, and alcohols.

5. Unstabilised PA is are attacked by UV, which causes embrittlement in a comparatively short time.

6. Good alkali resistance.

6. PA has sharply defined melting points, and high shrinkage values occur on moulding of thick sections.

7. Wide range of fillers and additives can improve specific properties and reduce limitations of unmodified materials, e.g. affects of moisture on dimensions and properties can be greatly reduced by use of glass-fibre filler.

7. The crystalline nature of PA gives longer cycle times in moulding.

8. Almost all plastics processing methods can be used.

 

Processing

PA is hygroscopic and materials should be stored in sealed and moisture resistant containers. Hoppers should also be sealed to prevent moisture uptake. Moisture contents of greater than 0.25% can give problems during processing. Pre-drying at 80oC for 16 hours is recommended if the material has been left open to the air.

PA has a broad processing range and the easy processability means that almost all of the conventional polymer processing methods can be used with PA.

Injection moulding

Mould temperature is usually between 60 and 90oC but this can be increased to 100oC or even 120oC where high dimensional accuracy is required. This gives a higher level of crystallinity and better mechanical properties.

Processing parameters differ with the variants and the PA 6 types generally require cooler processing temperatures (220 - 230oC) than the PA 6/6 types (270 - 280oC).

Back pressures should be set low (20 - 50 bar) and very precisely with vented cylinders to prevent material oozing from the vent port.

Injection speed should be relatively high and a high screw speed can be used.

After moulding the crystallinity of PA may require post treatment. For mouldings to be used over 60oC then annealing is needed and steam conditioning is best. Unreinforced PA will change dimensionally by about 0.9% by volume for each % of moisture absorbed. PA parts will shrink after the moulding operation and then swell as moisture absorption takes place. The control of tolerances is therefore critically dependent on the moisture and the processing parameters. The absolute amount of shrinkage that will take place is dependent on the processing parameters, the variant used, the filler content, the type of filler and the part geometry.

Extrusion

PA requires a properly designed screw to cope with the sharp melting point and low melt viscosity. Extruders must be able to keep the temperature profile constant to control the melt viscosity. High melt viscosity types of PA are preferred for extrusion.

Regrind (provided it is dry) can be used up to 30% without adverse effects.

Processing Method

Applicable

Injection Moulding

Yes.

Extrusion

Yes

Extrusion Blow Moulding

Yes

Injection Blow Moulding

Yes

Rotational Moulding

Yes

Thermoforming

Yes

Casting

Yes - PA 12

Bending and joining

Yes

Finishing

Machining

PA can be easily machined and the use of coolants allows high cutting speeds whilst still producing a good surface finish. Tools should be sharp and have good clearance and cutting angle.

Surface treatment

PA can be printed or hot-stamped without pre-treatment. PA can be electroplated, metallised or painted.

Welding

All processes suitable for thermoplastics can be used e.g. ultrasonic, friction, hotplate, high frequency and heat impulse welding. The hygroscopic nature of PA can affect welding performance. When PA is reinforced with glass fibre this can also make welding difficult.

Bonding

PA can be bonded with phenol or resorcinol based adhesives, isocyanates or reactive adhesives.

Health and Safety

Health and Safety

PA does not constitute a health hazard but in food contact applications care must be taken in the choice of additives used.

Other Information

Identification

  • PA is flammable and continues to burn when the source of ignition is removed.
  • The material bubbles and drips and threads can be pulled from the molten material.
  • The flame is blue and has the odour of burning horn.

Last edited: 20/05/15

© Tangram Technology Ltd. 2001

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