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Silicones from Dow Corning
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Siloxane Assets and Attributes

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How To Use Siloxane Masterbatches

The effectiveness of silicone masterbatches as process aids or property modifiers for thermoplastics depends on several factors, including:

  • The way in which they are incorporated into the polymer or compound
  • The choice of other compounding materials
  • The equipment used

The level of masterbatch loading required will depend on these variables and on whether it is being used as a process aid or as a final property modifier.

How to Use Siloxane Masterbatches:

Method of Incorporation
Effects of Other Materials
Masterbatches As Processing Aids
Filled vs. Unfilled Compounds
Conducting A Trial
Start With Temperature

Method of Incorporation

The way in which the masterbatch is incorporated into the polymer in injection molding applications will depend on the mixing efficiency of the molding machine screw. When the mixing efficiency is good, the siloxane masterbatch may be mixed with all of the other components by batch, tumble or dry blending, and added via the main hopper. It can also be added from a subsidiary feeder, if necessary, with one or more other masterbatches.

If the mixing efficiency is poor then pre-compounding will be necessary. In this case, the masterbatch can be conveniently added with other solid components.

Since the mixing efficiency of feeding extruders used in film and profile manufacture is usually much better than that of injection molding screws, pre-compounding may not be necessary. As in the case of some base polymers, however, it is advisable to dry the masterbatch before use. This is recommended for MB50-010/010A and MB40/50-011.

Effects of Other Materials

The melting points and melt flow indices (MFI) of the base polymer and masterbatch carrier are also important. In general, masterbatches disperse better in base polymers of equal or lower MFI. If the base polymer has a higher MFI than the masterbatch carrier, it will probably be necessary to pre-compound to avoid splash marks from poorly dispersed masterbatch pellets.

Allied to this is the use of masterbatches in dissimilar base polymers or compounds. In this case, the melting point of the masterbatch should be lower than that of the base polymer to avoid imperfect melting with consequent poor dispersion.

Masterbatches As Processing Aids

When using siloxane masterbatches as processing aids in the injection molding of unfilled polymers, the best results are obtained in molds with difficult flow paths such as, for example, thin sections, convoluted flow paths or lots of reinforcing ribs or struts.

Siloxane masterbatches should not be used with long, deep, parallel moldings because they are non-migrating and may not provide release as effectively as other methods such as sprays. However this problem can sometimes be overcome by the use of higher loadings of masterbatch to 1%, for example, versus the more normal 0.4%.

Extrusion of unfilled polymers requires similar loadings of siloxane to reduce film-to-metal friction during processing.

Filled vs. Unfilled Compounds

When using siloxane masterbatches to improve the processing of filled compounds, the siloxane loading will depend on the filler type and loading. Typically, 1-3% siloxane is required.

In this case the polymer MFI is secondary to the melt rheology of the compound, so the processing of long glass fiber reinforced compounds based on polymers of MFI 35 and above, for example, can be improved. The siloxane masterbatches can also be used at higher loadings of 2-5% for the modification of surface properties such as scratch resistance, friction, wear and abrasion resistance or appearance. Again, the results obtained will depend on the character of the base polymer, and on the bearing surface in tests and use.

In friction tests, the results will also depend on the shear conditions in use. The siloxane used as the basis of the masterbatches is of ultra high viscosity and requires higher levels of spreading energy than, for example, silicone fluids. Thus the static coefficient of friction of a system containing a silicone masterbatch may be comparatively poor, but the dynamic coefficient will improve with increasing shear, whereas that of a silicone fluid based system will degrade.

Scratch resistance of filled compounds is becoming an important consideration, especially for automotive moldings such as trim, where demolding is an important secondary feature.

Conducting A Trial

Before conducting a trial, specific objectives should be established with the customer. Attempting to accomplish too much with any one trial should be avoided. When used as a process aid in polyolefin injection molding, the masterbatches have been seen to affect specific segments of the molding cycle and not others.

Plasticizing, for example, may require less energy but the time required may not be affected at all. Mold filling time, cooling time, melt temperature and operating pressure will all be reduced to varying degrees depending on materials, equipment and conditions.

First of all, set a baseline by operating at standard conditions. Introduce the siloxane material by draining the main or secondary hoppers (depending on the mixing method used) and refilling them with the test lot.

Start With Temperature

After a short period running at standard conditions, modify the selected segments of the molding cycle. The cooling period is a good initial option. Continue reducing it until just the point at which poor moldings are obtained, then work back up to give a suitable margin. Without changing the temperature profile, it should be possible, with suitable molds and materials, to reduce cycle time by up to 25%.

Changing the melt temperature is a longer and potentially more difficult task but can be rewarding. Reductions in melt temperature of 20-50°C are possible, with consequent energy savings or reductions in cooling time.

The optimum balance between isothermal cycle time reduction and reduced melt temperature will be different for each material processed and for each mold, and may require several trials to achieve.

In extrusion, a similar regimen should be employed. Here, the benefits of isothermal operation are reduced motor torque (and thus power demand) and, in some cases, improved throughput. Lower operating temperatures are also an option, especially in the case of heat sensitive polymers.

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< back to Siloxane Assets and Attributes Home Page  
How to use Siloxane Masterbatches

  1. What are Siloxane Additives?

  2. What Are Siloxane Masterbatches?

  3. How To Use Siloxane Masterbatches

  4. What Are Siloxane Powders?

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