SILRES^® K

1. Glass Fibre Laminates

Please note: manufacturing prepregs and laminates is a complex process that needs a thorough control of various process parameters, such as temperature, catalyst quantity, nature of catalyst, solvent type, machine specific conditions, etc. Therefore, the following indications are inteded as a guide only, and we recommend running preliminary tests to optimize the conditions of the particular process.

Since fully cured SILRES® K provides water repellency, excellent electrical insulation properties, outstanding heat resistance and long term stability against weathering, moisture and UV light, this silicone resin is often used as a binder for laminates made of fibrous fillers, cloths, woven or non-woven reinforcing materials. The following process illustrates a typical production sequence for the manufacture of glass cloth laminates.

SILRES® K is diluted with toluene, or any other suitable organic solvent, to form a resin solution with a solids content of about 10-20 % by weight. The curing catalyst is added right prior processing. For the purpose of storage-stable prepregs WACKER^® Catalyst K83 proved most useful; it is added in a quantity of 2 to 5 wt.%, based on the solids content of the silicone resin solution prepared.

The glass cloth is then impregnated by the catalyzed silicone resin solution either by spraying, or by running the fabric through an impregnation bath. The solvent subsequently is evaporated in a drying oven at moderate temperature (90 to 110 °C). Please note: the drying temperature must not be too high in order to avoid premature curing of the B-stage resin and of the prepreg web, respectively. Also a very long drying cycle can impair the storage stability of the prepregs made.

Depending on the target shape of the laminate product the prepreg web either is rolled-up in layers on a stainless steel mandrel, or it is cut up into smaller pieces which in turn are sandwiched to a layered stack. By applying heat (e.g. 200 °C) and high pressure (for the layered stack assembly: >10 bar) the B-stage resin converts into the C-stage to form a durable and flexurally very rigid composite with very high heat resistance.

Please note: inserting a PTFE coated glass fabric between the pressing plates and the layered prepreg stack can help to prevent the composite from sticking to the plates. Besides it has proven useful to first cool the composite down to 80 °C (or lower) before the laminating press is opened, as this minimizes the delamination risk during the decompression phase.

2. Mica Laminates

Please note: manufacturing prepregs and laminates is a complex process that needs a thorough control of various process parameters, such as temperature, catalyst quantity, nature of catalyst, solvent type, machine spedific conditions, etc. Therefore, the following indications are inteded as a guide only, and we recommend running preliminary tests to optimize the conditions for the particular process.

The process of making mica laminates is similar to the production of glass fibre laminates. Important: the quality of the final product strongly depends on the type of mica, its particle size distribution and the uniformity of the mica paper used.

Similar to the process described for glass fibre laminates, the binder-free mica paper is first impregntated by a catalyzed, diluted solution of SILRES® K (solids content of about 10-20 % by weight, which will result in a silicone resin content of approx. 8 to 15 wt.% in the final composite). After a short residence time the solvent is evaporated in a drying oven at moderate temperature (90 to 110 °C) to yield a physically dry, prepreg web that can be rolled-up in layers for storage.

For producing mica laminates, the prepreg web is cut up into smaller pieces, sandwiched to a layered stack of desired thickness and pressed for approximately 1 hour at 200 °C under high pressure (>10 bar).

Please note: inserting a PTFE coated glass fabric between the pressing plates and the layered prepreg stack can help to prevent the composite from sticking to the plates. Besides it has proven useful to first cool the composite down to 80 °C (or lower) before the laminating press is opened, as this minimizes the delamination risk during the decompression phase. Pressureless post-curing at 250 °C can further improve the lmainate's properties in terms of hardness and cohesive strength.

3. Binder for Resistor/Capacitor Coatings

As SILRES® K can be highly filled with mineral fillers, it often is used as binder in coatings for resistors and capacitors. The respective silicone resin based blends are highly fire-retardant, and they safely protect the electrical parts from moisture or any other environmental impact. Besides the thermal management is improved, which extends the lifecycle of the particular electrical element - even when continuously run at high temperature levels.

For the production of resistor and capacitor coatings SILRES® K is diluted with toluene, xylene or any other suitable organic solvent, to form a resin solution with a solids content of about 25 to 35 % by weight. Then the finely ground fillers and pigments, which in total account for up to 75 % of the final coating formulation, are added and thoroughly mixed in a kneader. Finally, a curing catalyst (e.g. WACKER® Catalyst F 100%, WACKER® Catalyst K 83 or GENIOSIL® GF 91) is admixed in a quantity of some 1.5 to 3 wt.%, based on the amount of solid silicone resin used. If necessary, more solvent can be added to adjust the consistency of the coating slurry to the particular needs.

For coating the respective electrical part is dipped into the slurry; alternatively, resistors can be coated by rolling them over the surface of the slurry. The solvent subsequently is evaporated at moderate temperature, e.g. in a drying oven. Finally the green body is cured at high temperature (150 °C, 90 min.) to give a hard, durable coating of 0.5 to 1 mm thickness. Note: to increase the coating thickness multiple dip-coating is possible.