Process Aids

The use of polymer process aids has increased as polymer processors look for higher quality and better output of their finished products.

Wells Plastics supplies an extensive range of polymer process aid masterbatches to the extrusion and moulding market places.

The Wells product range is highly efficient and only requires low dosage levels to offer large benefits such as melt fracture removal, die lip build up reduction, die swell reduction, reductions of gels and surface defects and lower energy and temperature requirements.

Specific formulations have been developed for polyolefins such as metallocene, styrenics and polyamides.

Application Examples
Film, sheet, profile extrusion, injection moulding and others.

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Polymer Process Aids

  • Enhance processability and reduce production costs
  • Grades for commodity & engineering polymers
  • Suitable for film, fibre, extrusion & injection moulding applications
  • Combination products for the complete solution

Introduction

The term “process aid” covers several types of materials that are used to improve the processability and handling of high molecular weight polymers.

These products are used to:

  • Eliminate melt fracture and improve surface finish.
  • Reduce die-drool.
  • Improve throughput & minimise energy consumption.
  • Reduce the incidence of gel formation.
  • Improve processability of recycled or reground polymer.

They find use in a wide variety of applications from blown film to fibre, injection moulding, profile extrusion, pipe/tubing and cabling processes. Process aids can be used with LLDPE, LDPE, HDPE, mLLDPE, PP, polystyrene, polyamides, acrylics and others. They can be particularly effective when used with metallocene graes of polymer.

Mode of Action

Process aids are formulated to form a microscopic dispersed phase within the polymer carrier. During processing the applied shear-field causes the process aid to phase separate from the carrier polymer and migrate to the die wall gradually forming a continuous layer. Once the coating is complete, the backpressure of the extruder decreases to a minimum as the differential between the surface energies of the polymer and the coating allow for reduced friction during extrusion. Other additives in the system, such as pigments and anti-blocks, as well as process conditions affect the coating and removal rates

A number of factors affect the coating equilibrium including concentration, dispersion, throughput, viscosity, shear gradient and temperature. To achieve the coating in a reasonable time the process aid masterbatch is typically added at 1% until the beneficial effects are seen. After this the addition rate can be reduced to 0.5% down to even 0.25% as long as the effect continues to be evident. A small addition must be maintained as the coating is dynamic, with the polymer melt both abrading the coating and depositing further process aid.

Advantages

Melt Fracture Reduction

Process aids enable polymer to be extruded at higher shear rates without the melt fracture (surface roughness/shark skin effect) that may occur with untreated polymer melts. The addition of process aids results in good product produced at higher output rates.

Reduced Die-drool & Gel Formation

During processing, deposits of low molecular weight polymer, additives and degradation products can build up on the die and extruder surfaces. Such deposits can create die lines and when/if they are released, defects on the extrudate. A process aid is able to reduce die build up because as the coating layer develops any degraded material clinging to the die is displaced and also stagnation in the extrusion process is minimised reducing the production of thermal degradation products.

The low energy surface provided by the coating reduces internal build up and subsequent degradation leading to crosslinked and oxidised gels. Reducing unmelted/unmixed gels is believed to be related to the prevention of solid bed break-up and the process aid may prevent premature melting in the feed section thus helping to maintain a coherent solid bed.

Process Efficiency Improvements

The use of a process aid reduces resistance between the polymer melt and the die, resulting in die pressure and motor torque to fall. If process temperature and outputs are kept constant process aids can reduce motor energy consumption, while if process temperature and extrusion pressures are kept constant higher outputs can be achieved.

As a result of process aids reducing die build up the extruder does not need to be stopped as frequently during production to clean such deposits, thus limiting down time. Industrial trials have also found reductions in the time for colour or resin changes to be completed.

Recycling Plastics

Process aids can to help reduce extruder pressure when processing blends of polymers such as LLDPE with LDPE and LDPE with HDPE. While this does not solve incompatibility issues it can increase processing flexibility/options.

Applications

Blown & Cast Film

Using a process aid can enable LLDPE to be run on narrow die gaps resulting in improved impact strength without effecting other physical properties as caused by blending with LDPE. By reducing the stress and elongation of the polymer chains in the flow direction, a process aid can also improve impact property balance, machine vs. cross section. In cast film, the use of a process aid has been shown to improve control over thickness.

Cable & Tubing Extrusion

Here the process aid can lower apparent viscosity of the polymer, reduce die build up or produce a smoother, glossier surface.

Blow Moulding

Process aids are used to reduce melt fracture, increase surface gloss and enable lower extrusion temperatures to be used resulting in higher melt strength.

Fibre Extrusion

Due to process aids reducing the apparent viscosity of the polymer melt, fibre producers are able to process higher molecular weight materials at similar conditions for lower molecular weight resins, thus providing the physical property benefits like higher strength, less fibre breakage and less die swell. Also the coating produced at the polymer/metal interface helps extruders stay clean and so can extend runs between screen pack changes, fibre breakage or machine time.

Wells Plastics Ltd

Wells has many years of experience in developing the correct formulation for precise requirements and we are able to either offer a product from our large range of standard grades or produce a carefully formulated “special” to meet even the most demanding application.

This information is correct to the best of our knowledge, but we would recommend that users make their own assessment to confirm that the material meets their requirements. We accept no liability for any damage, loss or injury resulting from the use of this information. Freedom from patent rights must not be assumed.

Wells Plastics Ltd

Emerald Way,
Stone Business Park, Stone,
Staffordshire, ST15 0SR, UK

Tel: +44 (0) 1785 817421
Fax: +44 (0) 1785 816357