Electrically conductive elastomer HF seals are mainly used in electronics for two reasons:

• to protect a device from adverse environmental influences and
• to prevent electromagnetic interference radiation from affecting the device or being emitted by it.

These composite materials contain finely dispersed silver particles or silver-plated metal particles embedded in silicone or fluorosilicone rubber. As with conventional rubber gaskets, metal-reinforced rubber or knitted wire RF gaskets, certain precautions should be taken when handling them.

Transport and storage

•  The packaging of all CHO-SEAL gaskets contains the following information: “Caution – these parts are packed in sulphur-free material. Please do not repack these parts or store them in or near materials containing sulphur or sulphur compounds (e.g. cardboard, rubber, etc.)”. Sulfur reacts with silver to non-conductive silver fluid.
•  It is not necessary to store CHO-SEAL seals protected from light. Unlike conventional rubber made of neoprene or butyl, silicone rubber does not crack or break even after twenty years in sunlight – even under stress.
•  Normal temperature fluctuations do not damage these materials. At 120°C, the service life is 20 years. At 200° C the life is reduced to 5 years, except for materials such as CHO-SEAL 1215 and 1217, which are filled with silver-plated copper particles and should not be used at temperatures higher than 125° C, as the metal powder would oxidize.
• Moisture does not harm these materials. Even after seven days of underwater storage, no measurable changes in properties occur. However, storage in steam under overpressure is not recommended: after seven days the conductivity and hardness have deteriorated. Salt spray does not affect materials filled with pure silver or silver-plated aluminium particles, but materials filled with silver-plated copper particles (CHO-SEAL 1215 and 1217) lose their conductivity and form a layer of green corrosion products.
•  Silicone materials are very resistant to ozone. Even at 300 ppm ozone and 74°C they meet ASTM D1149 p.10- whose normal test value for common rubbers is 0.25 ppm and 32°C.
•  Silicone materials are very resistant to ionising radiation. The normal radiation doses are 110 mrad at 23°C and 30 mrad at 200°C.
• Silicone rubber is resistant to fungal attack according to MIL-F-8261 or MIL-E-5272A.
•  Silicone rubber burns to non-conductive silicon dioxide ash when held in a flame. Silicone composite materials such as CHO-SEAL 1215, 1224 and 1285 do not release toxic combustion products when burned. However, fluorosilicone materials such as CHO-SEAL 1217, 1221 and 1287 may release toxic fumes containing fluorine when burned or heated above 288°C. In these cases, appropriate precautions should be taken.

Cleaning

•  Since the electrical contact to a metal surface is decisive for an optimum shielding effect, the contact surfaces should be clean and free of electrically insulating materials. Silicone rubbers are sensitive to certain organic substances that can be absorbed and cause the rubber to swell. This swelling affects electrical conductivity by separating the contacting metal particles. This damage is usually reversible and can be corrected.
•  Metal surfaces should be clean and free of cutting oil, grease, dirt or hydraulic oil. The usual cleaning methods can be used, but any traces of cleaning agents should be removed before installing the CHO-SEAL seals.
• Silicone composite seals can be cleaned with soapy water or 20% ethanol and soap. To remove soap residues, we recommend rinsing with water or 20% ethanol.
• Solvents that are particularly harmful to silicone rubber are, for example carbon tetrachloride, isooctane, yxlol, tuluene, JP-4, hydraulic oil and silicone oil. If a seal should accidentally come into contact with one of these liquids (e.g. by splashing), the seal should be dried thoroughly (e.g. in a circulating air oven at low temperature) and checked with regard to electrical conductivity.
• Fluorosilicone seals such as CHO-SEAL 1217, 1221 and 1287 are resistant to such liquids. However, we recommend the same cleaning method as described for silicone materials.

Mechanical treatment

• Seals should always be used under pressure, never stretched. This applies generally to normal gaskets as well as to conductive composite gaskets. Expansion damages conductive composite materials by loosening the cross-linking of the metal particles and reducing conductivity. CHO-SEAL gaskets are designed so that low expansion (less than 10%) will not cause permanent damage as long as the expansion lasts only a few seconds and the material can recover for a few minutes.
• Seals can be damaged by over-compression. Silicone rubber behaves like a liquid and is therefore practically incompressible. If a seal is deformed, the volume of the material remains constant, so that the dimension perpendicular to the force exerted increases. Since the rubber molecules are cross-linked, over-compression can cause the chemical bonds in this network to break up, resulting in permanent deformation of the seal structure. Depending on the cross-sectional shape of the gasket, different degrees of deformation are permissible, ranging from about 15% for a solid rectangular profile to 70% for a hollow round profile.
• In the case of rubber seals, permanent compression deformation occurs. If they are deformed for a period of time due to load, especially at higher temperatures, the gasket will not regain its original height after relief. Silicone composite gaskets such as CHO-SEAL 1215, 1224 and 1285 recover at least 70% of their deformed height according to the ASTM-D395 test. Fluorosilicone composite materials such as CHO-SEAL 1217, 1221 and 1287 recover at least 40% of the deformed height. We recommend that these materials be stored flat and not at elevated temperatures.
• During installation, flat gaskets should be carefully aligned on the mounting surface. Composite elastomers may stretch under their own weight and therefore may not fit properly. The gasket should be kept even during installation. The usual screw tightening methods should be used when screwing the mating part together: first tighten all the screws all round just finger tight, then tighten as instructed, working from the centre of the side towards the corners, alternating with the sides.
• Gaskets for groove installation should be handled as carefully as flat gaskets. In addition, when inserting the gasket into the groove, you should not press the gasket with your finger, but rather guide it in with a light “slap”. Rubbing can stretch the gasket, making it no longer fit.
• We generally recommend wearing light protective gloves when handling these elastomers. Although the greases and salts of the skin as such do not damage the seal, residues from the skin under the influence of extreme environmental conditions can cause local changes in conductivity which can adversely affect the shielding effect.

Damaged seals

• If mechanical damage to a gasket is suspected, it should be inspected in the usual way with regard to dimensions and visually for surface irregularities and/or defects.
• If an electrical defect is suspected, the conductivity should be measured on site using the resistance tester and a milliohmmeter, as described in Technical Bulletin No. 69. Using the specific resistance limit value according to the material specification, the cross-sectional area of the gasket and the electrode distance of the tester (2.54 cm), the conductivity of the gasket can be quickly checked using the formula given in the Technical Bulletin.

Author Charles H. Kuist, Chomerics Europe