PP - Controlled rheology grades



Controlled rheology (CR) grades of polypropylene extend and improve the characteristics of PP. In order to maximise the benefits obtainable with these products a knowledge of the process and basic properties of the materials is very useful. This review gives some background information to allow understanding of these factors.

Basic concepts

Controlled rheology PP is produced by degrading normal PP to give a product with a high melt flow index (MFI), lower molecular weight (MW), narrower molecular weight distribution (MWD) and hence easier and more consistent flow.

This is a broad statement and may confuse more than explain. The basic concepts of molecular weight, molecular weight distribution and MFI are explained below.

Molecular weight (MW)

A polymer (poly = many) is made up of many repeated units of a monomer (mono = one). For polypropylene the repeat unit is the propylene monomer. This can be illustrated as below:

The length of the polymer chain is specified by the number of repeat units in the chain. This is called the 'degree of polymerisation' (DP). The molecular weight of the polymer is defined by:

MW = Degree of polymerisation x molecular weight of repeat unit.

All processes used in polymer production lead to chains of varying lengths and hence varying molecular weights. The distribution of molecular weights for a typical polymer is shown below.

This shows that it is difficult to describe the molecular weight by use of a single number and any number quoted will always be an average of some type. Two averages are generally used. These are:

Number-average molecular weight (Mn). This is generally measured by methods such as vapour pressure lowering or other colligative property measurements. Essentially the techniques involve counting the number of molecules in a known mass of material. Mn normally lies near the peak of the weight distribution curve i.e. the most probable molecular weight.

Weight-average molecular weight (Mw). This is generally measured by light scattering or other methods and tends to emphasise the contribution of heavier or longer molecules. The value for Mw is equal to or higher than that of Mn.

The two values are shown in Figure 1 and the ratio Mw/Mn is used as a measure of the breadth of the molecular weight distribution. Values of Mw/Mn, range from 1.5 to 50. In general a narrow distribution gives more regular and repeatable production characteristics.

For commercial polypropylenes the molecular weights are approximately:

Mn = 38,000 - 60,000

Mw = 220,000 - 700,000

Mw/Mn = 5.6 - 11.9

Melt Flow Index (MFI)

In injection moulding the heated and softened material is forced into a mould and the way in which the material flows is very important. The most commonly used test or indicator for flow characteristic is the Melt Flow Index (MFI) or Melt Flow Rate (MFR).

The test involves heating a polymer to a specified temperature in a piston and then loading the piston with a specified weight. The piston extrudes the polymer through a hole and the amount of material extruded in a given time (usually 10 minutes) is the MFI value. The test suffers from some disadvantages in that the shear rate is very low and varies throughout the test. Processing at much higher shear rates can lead to a great reduction in apparent viscosity. It is thus possible for two materials to have identical MFI values but process differently. MFI is to be regarded as an indicator test only.

When dealing with MFI results it is essential that the temperature and piston load used are noted. Common conditions used are:

230oC/2.16kg - Used for granules and general purpose materials.

190oC/10kg - Used for powders where the increased load is required to avoid air bubbles and give good packing.

The results of each condition are not equivalent and for the same material in different forms e.g. granule copolymer and powder copolymer the results can vary by up to a factor of 10.
Conditions of test must be noted otherwise the results are useless.

MFI thus gives an indication of the viscosity of a polymer melt. A high MFI indicates that the polymer will be very liquid and flow very easily at a given temperature. A lower MFI material will not flow as easily and will require more effort to fill a given mould.

The relationship between MFI and MW is such that low MW materials give high MFI values and vice versa.

For injection moulding it is usual to choose the highest MFI material that will provide sufficient strength in order to obtain maximum output.

Production of High MFI PP

High MFI PP can be produced by two basic methods. These are:

  1. Polymerisation control - This involves changing the polymerisation conditions to control the molecular weight of the final product. This method may lead to a high value of Mw/Mn, i.e. a broad MW distribution. Production by this method should be cheaper as it is a one-step method but the volume of the batch may affect this. MFI values achievable by this method range from 4 to 1500, the upper value being used for high rate fibre production to give very soft fibres for non-woven fabric production.

  2. Post-Treatment - This involves taking PP from the reactor and subjecting it to a variety of treatments to break the main chain and reduce the MW. This is a degradation treatment and can be achieved by:

  • Thermal mechanical treatment

  • Gamma radiation

  • Oxidation

  • Addition of organic peroxides

Peroxide addition is most widely used and these are added prior to extrusion and pelletising. When a peroxide and a PP polymer mixture is heated the peroxide will produce free radicals that react with the PP molecules. The peroxide attacks randomly but statistically the longest molecules are most susceptible to attack. This results in a narrow MW distribution and increased MFI.

The anti-oxidant system used in CR grades must be very carefully chosen. Anti-oxidants are added to PP in order to retard the degradation of the polymer under the effects of heat and sunlight. For CR grades the system must first allow controlled degradation during production and later protect the polymer from further degradation due to heat and sunlight. Pit the same time the system must be suitable for food contact and satisfy the relevant legislation.

Properties of CR grades

Homopolymer PP

The influence of MW (and MFI) on the bulk properties of PP is often the reverse of that found with other polymers. The main reason for this is that crystal formation is more difficult at high MW and this affects the properties via degree of crystallisation rather than simply through MW.

In common with most polymers a decrease in MW (increase in MFI) gives a decrease in melt viscosity i.e. easier flow. The effect of MFI on impact strength is such that impact strength is at a minimum for an MFI of approximately 11 and increases if MFI is increased or decreased. Other property changes are summarised below:

Increase in MFI (e.g. 11 => 25) gives:

In addition CR grades show considerably fewer gels than conventional polymers.

Copolymer PP

The conventional method of producing a block copolymer i.e. adding 4-15% ethylene to the polypropylene gives much improved impact strength and considerably lowers the brittle point such that it is well below normal service temperatures i.e. -15 to -20oC. This greatly improves the service performance of copolymer PP relative to homopolymer PP.

With CR grades of copolymer the MW is not only reduced by chain cleavage but the ethylene containing block of the copolymer also experiences chain growth. This can lead to a very small decrease in impact strength but the improved flow properties more than counteract this.

Processing of CR grades

Due to the decreased melt viscosity of CR grades the application of these materials is mainly in the following areas:

  1. Thin walled packaging.

  2. Straight sided and thin walled boxes.

  3. Products requiring a long flow path.

CR grades show several processing advantages over conventional grades. The most significant of these are:

  1. The decreased viscosity means less injection pressure is required for mould filling.

  2. Lower melt temperatures are required to achieve the same viscosity i.e. approximately 30oC lower mass temperature.

  3. Cycle times can generally be reduced by 10-15%.

  4. Lower temperatures mean that warpage on demoulding is reduced.

  5. Pigment/colour mixing may be improved due to decreased viscosity and better flow at a given temperature.

  6. The narrower MWD should show minimal differences parallel to and across the flow direction.

  7. The decreased injection pressure may allow a machine with a lower clamping force to be used.

Last edited: 29/12/05

© Tangram Technology Ltd. 2002

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