Polymer Data File:

Polypropylene
- PP

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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

In the words of Alan Griffiths 'Polypropylene is the new mild steel' and PP has a huge market penetration in a wide range of markets and is used in a huge variety of applications. 

The good mechanical characteristics and the low cost make it a first material choice for many applications and PP has grown in some cases by displacing other polymers.

PP has a very low surface energy and this makes it difficult to bond, join or coat due to the difficulty of 'wetting' the surface. 

PP is a crystalline polymer and the crystal formation after processing can lead to significant dimensional changes for some time after ejection from the mould.

The ability to orient the polymer chains in PP gives rise to the integral hinge effect used to such good effect by product designers.

PP was made possible by the development of stereo-specific catalysts by Zeigler and Natta (Plastics Timeline) that allowed precise positioning of the molecules along the polymer chain. PP was the first of the stereoregular plastics and the resulting crystallinity produces a stiff polymer chain and exceptional properties. 

Homopolymers: The choice of catalyst allows the production of isotactic, syndiotactic or atactic varieties of homopolymer PP and all have varying properties. It is therefore difficult to characterise PP by one set of statements regarding the properties.

Random Copolymers: These are a type of where the basic structure of the polymer chain has been modified by the incorporation of a different polymer molecule. Ethylene is the most commonly used molecule and this is inserted randomly along the polymer backbone at about 1-7% concentration. The random copolymers give increased clarity and impact strength.

Further details on random co-polymer PP.

Further details on controlled rheology grades of PP.

Typical Applications

Vehicles and mechanical engineering: Battery cases, air duct casings, parcel shelves, fan wheels, dashboards, gas pedals, sunroofs, cooling water containers, air filters, heater housings, headlamp housings.

Electrical engineering: Housings for electrical components, electric kettles, cable drums, plugs, lawnmower housings, egg poachers, mixers.

Medicine: Sterilisable hospital ware, disposable syringes, infusion bottles, tablet tubes, test tubes, metering units.

Packaging: Storage and transport containers, bottle crates, buckets, margarine tubs, desserts, bottle closures, packaging film, bottles.

Others: Adhesive strips, transparent wrappings, index cards, loose-leaf folders, plant tubs, garden furniture, chair shells, ink cartridges, suitcases, vacuum flasks, cases with hinges.

Articles using integral moulded hinges are almost always PP.

Physical and Mechanical Properties

General

PP is a semi-rigid, translucent polymer with good toughness and weather resistance properties. It has low water absorption and is easily moulded as low cost. Polypropylene is a largely non-polar, partially crystalline thermoplastic with a crystallinity of 60 to 70%. PP has a density of 0.90 to 0.91 g/cm3 which is amongst the lowest densities for all plastics. PP does not absorb moisture.

The glass transition temperature of PP homopolymers is about 0oC. The material is viscous above this temperature. The glass transition temperature of the elastomer component of block copolymers containing elastomers is 50oC. This gives a higher impact strength at 0 to 40oC. The creep strength of PP falls very sharply at high temperatures. The strength in one direction can be increased many times over by stretching. This is due to orientation of the macromolecules under a tensile stress.

The homopolymer is translucent, and the transparency can be improved still further by stretching. Block copolymers give dull to opaque moulded parts but this is improved on contact with liquids and the level of containers filled with liquid can be seen from the outside.

Property

Approximate Value

Tensile strength

30 - 55 MN/m2

Tensile Modulus

1 - 2 GN/m2

Elongation at Break

>500 %

Flexural Strength

<50 MN/m2

Notched Impact Strength

>20 kJ/m2

Specific Heat

1.70 - 2.15 kJ/kg/ oC

Glass Transition Temperature

-10 oC

Heat Deflection Temperature

<100 oC

Coefficient of Thermal Expansion

10 - 15 x 10-5 / oC

Long Term Service Temperature

80 oC

Specific Gravity

0.91 -  0.93

Mould Shrinkage

0.01 - 0.025 m/m

Water Absorption

<0.1 % (50% rh)

Transparency

Opaque

Thermal, Electrical and Optical Properties

Thermal properties

The melting point of homopolymers is 155 to 160oC and that of block copolymers is 160 to 165oC. If the PP parts are not subjected to mechanical stresses, the service temperature limit is 110oC. Over short periods without any external load, a temperature of 140oC is permissible.  

Fire behaviour

PP can be ignited by flames and burns with a weak flame with burning drips.

Electrical properties

PP has a very good electrical insulation capacity and parts easily become electrostatically charged and attract dust.

Optical properties

Moulded parts produced from natural homopolymer are translucent. The transparency can be improved still further by stretching below the crystallisation temperature. The refractive index is 1.50.

Chemical Resistance Properties

General

PP has very good chemical resistance. It is stable to aqueous solutions of salts, acids and alkalis. Polypropylene is resistant to alcohol, solvents up to 60oC and detergent solutions. Aromatic and halogenated hydrocarbons and, at high temperatures, fats, oils and waxes, cause PP to swell.

Resistance to weathering

PP has adequate stability to radiation within the region of visible light. UV light causes embrittlement and surface damage. This oxidation process is accelerated by high temperatures. Weathering resistance can be improved by the addition of carbon black but this reduces the resistance to thermal ageing.

Resistance to stress cracking

The resistance to stress cracking is good.

For a full Chemical Resistance Chart, click below:

PP Chemical Resistance Chart

  Advantages and Limitations

Advantages

Limitations

1. Chemical resistance similar to that of polyethylenes (ie. resistant to most inorganic acids, alkalis, and salts) but without susceptibility to environmental stress cracking when in contact with alcohols, esters, detergents, or polar hydrocarbons. 

1. More expensive than polyethylene.

2. Very good fatigue resistance; possesses integral-hinge property.

2. Embrittles below -17oC.

3. Excellent dielectric properties.

3. Upper service temperature 90-120oC, depending on grade and anti-oxidant content (upper limit applies to filled grades, lower limit to flame-retardant grades).

4. More rigid than polyethylenes and because of high melting point retains mechanical properties at elevated temperatures.

4. Heat-ageing stability is adversely affected by contact with metals.

5. Mechanical and electrical properties unaffected by submersion in water.

5. Attacked by highly oxidising acids, such as fuming sulphuric, liquid and gaseous halogens; swell rapidly in chlorinated solvents and aromatics.

6. Low density.

6. 2 per cent carbon black needed to prevent degradation by UV light.

7. Non-toxic grades available for use with foodstuffs; can be steam sterilized.

7. Colour purity adversely affected by reinforcing materials.

8. Filled grades available with improved stiffness, these are comparable to unfilled nylon but of lower price.

8. Not readily thermoformed because of low melt strength and hence requires close control of processing parameters.

9. Can be joined by hot-gas, hot-tool, induction, or friction welding.

9. Joined more easily to other materials, for example wood and aluminium, than to itself. 

10. Available as pipe, sheet, rod and injection mouldings.

10. Post moulding dimensional changes due to crystallinity effects.

Processing

Injection moulding

PP has a wide processing range. The material temperatures are between 180 and 300oC. The mould temperature can be between 20 and 60oC and the higher the mould temperatures then the better the surface gloss. A high injection pressure 1200 - 1800 bar is generally needed and to avoid sink marks a hold time of approximately 50% of the injection time is needed. A high injection speed is also recommended.

Up to 100% regrind can be used.

Extrusion

The extrusion temperature is between 240 and 270oC and it is best to use a short compression screw with an L/D ratio of 20 to 25.

Blow moulding

Hollow bodies with a volume up to 5 litres and above can be blow moulded. The material temperature should be between 190 and 220oC and the mould temperature should be between 20 and 40oC.

Processing Method

Applicable

Injection Moulding

Yes.

The crystalline nature of PP means that post moulded articles are not always dimensionally stable until full crystallisation has taken place. This can take up to 24 hours and involve significant movement in the article. 

Extrusion

Yes

PP extrusion needs a low flow rate formula to give adequate melt strength.

PP is also used to produce monolayer and co-extruded blown film.

Extrusion Blow Moulding

Yes

Rotational Moulding

No

Thermoforming

Yes

Casting

Yes

Bending and joining

Yes

Difficult to join unless pre-treated.

  Finishing

Machining

High cutting speeds and a slow rate of travel should be chosen in order to achieve clean surfaces. Knurling, grinding, filing and polishing are less suitable.

Surface treatment

PUR or acrylic-based one or two component paints should be used for painting. If a small-scale printing operation is required, hot stamping is used. A new process is printing under hot conditions, where the ink is applied to the untreated surface and then fixed at high temperature.  For the best results in high vacuum metallising, the surfaces should be pre-treated electronically. Good results can be obtained using primers and a base coat.

Welding

Hot gas, heated tool, friction and vibration welding are suitable for welding PP. Ultrasonic welding can be carried out only in the near-field. HF welding is not possible because of the low dissipation factor.

Bonding

As the high resistance to solvents prevents partial dissolution of the surface of the moulded part, only impact adhesives can be used. In order to improve adhesion, the surface must be pre-treated, e.g. by a flame treatment, application of primers, by corona discharge and by immersion in a bath of chromosulphuric acid. A new adhesive system recently released by 3M claims to be able to satisfactorily bond PP.

Health and Safety

Health and Safety

PP is odourless and tasteless and is physiologically safe, apart from those grades with certain stabiliser types e.g. copper and UV radiation. PP will, however, pick up aromas from materials and can transmit these to the contents of packaging.

Other Information

Identification

  • Material is flammable. It drips and continues to burn once alight.

  • The flame is bright with a blue core and there is an odour of burning paraffin similar to that of a candle which has just been snuffed out.

  • The smell of PP when it is being processed is easily recognisable.

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Last edited: 11/03/10

Tangram Technology Ltd. 2000

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