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Silicone use and Applications PDF Print E-mail
Written by Nigel Smith, (I.Eng, L Inst MC)   
Saturday, 24 July 2004
The article by Anders Johnsson the Engineer from Sweden, on "Silicone - Carbon Brush Poison !" featured in the July 2001 Edition of Kabinews was excellent in it's content and the problems that it highlighted. Having shown the article to a number of people, it created quite a lot of interest, especially among my well qualified and experienced engineering friends and colleagues. They come from a variety of disciplines both electronic, electrical and mechanical, and the notes below contain a summary of information that we are all agree on. In the real world, of the local garage, motor factor, DIY store or builders merchant, there can be found on the shelves, many items known generally as "Silicone Sealants". In this guise they are often sold to be used for engine / machine joint sealants, bath sealants, window sealants both for windscreen rubber repair and for where bonding is required to brickwork etc. Sealants and 'Liquid Gasket' Materials -Whilst some silicone rubber sealant materials are superb for engine casing seals and similar applications, they give off acidic gasses as they cure. Although implicated in brush and commutator problems, they may also etch a metallic material that it is applied to. This can sometimes be advantageous, but it can also etch and corrode plated components. I believe that the blue silicone gasket referred to in the July 2001 issue of Kabinews, which has been implicated in the Messerschmitt Dynastart Brush problems, may well be of the acidic variety. Silicone gasket materials are physically difficult to remove from engine casing joints once cured, that's why they seal so well! I personally have found the acidic based silicones very difficult to remove; The neutral curing ones less so, and indeed this makes it easier to separate a casing again without damage if required. In my work on industrial machine tools, in the absence of my preferred non silicone jointing compounds such as "Hylomar" and "Golden or Red Hermetite", I will use the neutral curing variety on flanges where appropriate. However, as precaution, I would not use it in the presence of brush gear. Remember, if gaskets are omitted and a sealant of whatever type is being used in it's place, the mating faces will have changed in dimensions. In cases such as close tolerance gearboxes where shimming for end floats have to be made, (as in the Sachs 200), this must be allowed for.

Acidic Cure Sealant When curing, this material gives off a distinct smell of Acetic acid. It is often used for engine and bath sealants etc, but which due to the acid given off will etch metals and destroy plated parts, often leading to rust or corrosion etc. This type usually has a high modulus of elasticity. This type must not be used on building materials such as brick or limestone etc, as it will be likely to chemically eat the material it is supposed to be bonding and etch any metals etc, that the fumes can get to! This type of sealant usually warns the user on the container that acid fumes are released whilst curing; Take Note!

Neutral cure sealants. This is a neutral curing silicone, which in my experience, is less tough that the above acidic type. It does have a low modulus of elasticity, but still with good flexibility. It does not release of acid fumes whilst curing, and does not generally directly destroy anything it is in near contact with. I have also found it is easier to get off if required! It will yield good results in terms of sealing, but with little or no risk of metallic etching. Due to the neutral cure, this silicone family is often used in the electronics industry for potting, or for the protection of high voltage circuits to good effect This makes this type of silicone suitable for bonding to brick and stone work, sensitive metal surfaces such as the perspex dome frame.. I cannot comment on whether this type of silicone will also have adverse effects on brushes in Dynastart units, but if in doubt I would restrict it's use for emergency clutch case sealing for example, which will be well away from the Dynastart.

Aerosols & Water repellents by definition contain small particles of semi atomised fluid in a carrier gas. Two examples being anti-damp spray for ignition systems and another in the form of the various silicone based polishes and interior trim cleaning / polishing products. Small particles can get into very small gaps, and once there if silicone, can "Creep". Aerosols can also be drawn into air intakes such as the cooling fan on a Sachs engine, and hence onto / into the dynostart brush / flywheel contacts area. "Damp Start" solutions often contain silicone used as a water dispersant, and if this mist is drawn int a Dynostart unit, this may well lead contamination of the Dynostart leading to a reduction in brush life and damage as described in the recent "Carbon Brush Poison " Article in Kabinews.

Silicone oils, polishes , once present on a surface, in a switch or Dynastart for example, cannot be cleaned by normal solvents. To remove silicone effectively, a specialised silicone digester must be used. In my industry where silicone mould release is used in the production of base belts, further chemical bonding will not be reliable unless such action is taken. Typically this will involve immersion in, (preferably agitated), digester fluid for some for some 24 hours or so to remove all traces of silicone. In the vehicle metal finishing areas, I believe that special silicone paint wipes are available for the removal of silicone polishes etc, prior to paint application. Greases and compounds - These are often used for plug type cocks, spark plugs, and ball bearings which must operate at extreme temperatures and speeds, but with low loadings.

Varnishes These are extensively used in electrical insulation where temperatures are high. Glass cloth impregnated with such resins will withstand prolonged exposure to temperatures up to 260 Deg C, whilst maintaining their insulating and encapsulation characteristics. Due to the age of an "original" Siba or Bosch Dynastart, it is most unlikely that a silicone based varnish has been used in the construction and insulation of the various coils involved. These products have only come into common use over the past 20 to 25 years, although I cannot confirm this; Prior to this "natural" varnishes were used, with a different chemical base, which are not as durable in the longer term. In the event of discovering a silicone infested Dynastart, or armature involving brushes, if possible, restrict the application of the silicone digester primarily to the brush / copper contact areas. This is a precaution just in case a silicone varnish has been used as a primary insulator on the wires of the coils, or as a secondary protective layer used for encapsulation purposes. Remember, the digester may inadvertently remove the insulation for you!

Note, silicone varnishes, unlike oils, polishes, greases etc, are designed for the job of encapsulating armature windings etc, and once cured by specific heat treatment, are very stable and long lived, and will not contaminate the brush gear to which they are attached !

Silicone polishes, fluids and aerosols have been shown when inadvertently applied to electrical and electronic equipment, to effectively make switch contacts inoperative; This effect being due to silicone's excellent insulating properties even when present in a very fine film. For a switch, this is not very desirable, even if the contacts will be protected from corrosion and damp! The main culprit in the home environment is silicone based aerosol polishes which are very good at producing this effect and making equipment inoperative, as well as attracting dust due to the any static electricity that is held on it! Silicone Polishes are often used on vehicle paint finishes etc, where they give a good shine and disperse water magnificently.

Silicone fluids and greases, on smooth polished surfaces creep and climb over surfaces better than on rough ones. This problem has been demonstrated in the instrumentation systems used for pressure calibration of high stability pressure transducers in a laboratory, where silicone fluid has been used as a seal media.

Brake fluid is not an issue for most Schmitt owners where cable operation is standard. However, some Tg500 owners use silicone brake fluids in their vehicle's hydraulic systems, to avoid the problems associated with standard DOT 3 and above brake fluids. (DOT = United States Department of Transportation - an approval authority). Standard DOT brake fluid is hygroscopic, meaning that it absorbs moisture from the atmosphere. I have been told this absorption can be typically up to 1% per year, to a max of 4 - 5% by volume. This introduces corrosion to the internals of a hydraulic system, lowering of the brake fluid boiling point. This can cause disc brake systems, (and sometimes drum brakes), to vapour lock due to boiling of the water. This normally occurs after repeated heavy use. For some vehicles, (such as 2 CV's I believe), seals made of different materials must be fitted with to ensure leaks do not develop. For Tg owners, silicone brake fluid offers many advantages - particularly as many Tg's have extended periods out of use. There is some doubt about the use of silicone brake fluid in competition applications because it has a slight compressibility whereas DOT 3 and higher rated fluids have zero compressibility. This may result in a slightly longer brake pedal travel. However the advantages of eliminating the corrosion risk are significant especially with the cast metal reservoir fitted to Tg. At least if silicone brake fluid is accidentally spilled on the paintwork of the car it will not strip it off !!

Silicone Rubbers There are several types. Generally they are noted for their resiliency from -45 to 270 Deg C, but with a much lower strength than many synthetic rubbers. They are often used for shaft seals, oven gaskets, refrigerator gaskets, vacuum gaskets.

To sum up, Silicone based products are great, but take care how one uses them !
Last Updated ( Saturday, 24 July 2004 )
 
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