Front Page| Johanson DMS, Inc. | Product Descriptions | Stock Heaters
Johanson DMS, Inc.
|Product Description page contents|
(Room Temperature Vulcanized)
Silicone rubber heaters may be adhered by applying a one part room temperature curing adhesive to the underside of the part and mating it to the surface to be heated.
We recommend the following adhesives:
The adhesive is supplied in collapsible aluminum squeeze tubes, caulking cartridges, and in bulk containers. It is of a paste consistency and should be rapidly spread to a thickness of 15 to 50 mils.
Curing time is dependent on temperature, relative humidity, joint configuration, degree of confinement, sealant thickness and substrate porosity. Warm temperatures (not over 100 degrees F) and humid conditions will shorten the cure.
The heater should be firmly fixed in position and allowed to cure from 24 to 72 hours before use. Energizing the heater before a complete cure is accomplished may cause a failure of the bond line, as deep section cures require more time.
Adhesives and primers have a limited shelf life, and the containers should be marked with the last use date. Observation of shelf life is very important in obtaining proper bonds. Primer containers should not be left uncovered any longer than necessary, as the effectiveness is diminished after exposure to atmospheric moisture.
Adequate bond strength may be attained without the use of primer, but the use of it is strongly recommended.
Service Life The service life of silicone rubber is defined by the following chart. The service life was determined when the elongation value dropped from an original value of 300% to 50%. Please bear in mind that test conditions cannot be compared to service conditions directly, but they serve as a guide to determining life expectancy.
|Silicone rubber is a good selection for use in flexible heaters, as it has many unique
properties. Silicone belongs to a family of synthetic polymers which are partly organic
and partly inorganic. They have a quartz-like polymer structure, being made up of
alternating silicon and oxygen atoms rather than the carbon-to-carbon backbone which is a
characteristic of the organic polymers.
Processing The heat-cured, or vulcanized vinyl/methyl silicone rubber is processed as gum stock, since it is a high viscosity, high molecular weight fluid polymer. Coloring agents are added to change the color from an off-white to a variety of others, the most popular being red. Red iron oxide is employed to impart the color, adding to the high temperature stability of the rubber. Resistance to fuel oil and certain greases may be improved by blending in fluorosilicone, but at a higher cost.
Mechanical A typical silicone rubber would exhibit a hardness of 60 Durometer (Shore A-2), a tensile strength of 950 psi, an elongation of 300% and a compression set of 33% after 22 hours at 350 F.
Temperature Limits Heaters made with silicone rubber can be expected to perform from -100 to +450 F, and the maximum short term service temperature can be extended as high as 500 F with the addition of certain heat stabilizing agents. Please consult the factory for details on additives.
Fluid Resistance Silicone rubber exhibits excellent resistance to moisture, sunlight and ozone. It has good resistance to bases such as ammonium and sodium hydroxide, and excellent resistance to salts such as sodium carbonate. Acid resistance depends on the acid, but generally acetic and low concentrations of hydrochloric and nitric have little or no effect. The use of silicone rubber in phosphoric or sulfuric environments is not recommended. Motor oils, transmission fluids, and mineral oil degrade the rubber, while brake fluid and gasoline have less of an effect. Silicone exhibits good resistance to acetone, ethyl alcohol and xylene, and fair to poor resistance to benzene, carbon tetrachloride and toluene.
Moisture Resistance The moisture resistance of silicone rubber heaters may be enhanced by selecting the proper configuration. Generally. fiberglass cloth is included in heater thickness selections as it dramatically improves tensile strength and tear resistance. Moisture can travel through the cross-sectional fibers of the glass, however, allowing deep penetration and reduction of electrical insulation resistance. Plies of fiberglass can be cut to less than full size, thereby interrupting the moisture path by allowing fiberglass-free silicone rubber to seal the edges.
Another potential leakage path for water is the location where the lead wires are installed. Teflon covered lead wire does not bond well, even if etched, to silicone rubber. Silicone rubber lead wire should be selected, as it bonds integrally to the rubber composition of the heater element. Silicone rubber lead wire is available with a fiberglass overbraid to impart abrasion resistance, but the glass overbraid should be trimmed short where it enters the heater connection area, again removing a moisture leak path.
Front Page | Johanson DMS, Inc | Product Descriptions
| Stock Heaters
Useful Formulas | Ordering Information | Therm. Cond. of Materials