ELASTOSIL® AUX G 3242 and G 3243 are two solvent-based WACKER primers designed for surface preparation prior to the vulcanization of solid silicone elastomers from the ELASTOSIL® R group, i.e. classic peroxide-cured HCR/HTV compounds. Both products belong to the same chemical family – they are solutions of siloxanes and reactive silanes in a mixture of Isopar E isoparaffin and xylene – and after the solvent has evaporated both form a rigid resinous film on the substrate that provides strong bonding of the silicone to metals, ceramics, glass and many polymeric materials. Despite this common foundation they differ in sufficiently important parameters that the choice between them has a real impact on adhesion quality, process efficiency and result repeatability.
The most visible difference lies in the solids content. AUX G 3242 contains only about seven percent of film-forming substance, while AUX G 3243 offers almost three times more – twenty-three percent. This difference translates directly into the thickness and character of the resulting layer. With the same amount of applied liquid, the product with the lower solids content leaves a thinner, more delicate coating, whereas the higher-concentration version allows a somewhat thicker and denser resin film to be obtained with fewer passes. In practice both formulations are designed so that the final film thickness falls within the range of one to ten micrometres, corresponding to a surface loading of five to fifty grams per square metre. The difference in concentration means, however, that G 3242 more often requires a slightly larger quantity of material or gentle dilution in order to achieve uniform coverage, while G 3243 already in the as-supplied form produces a satisfactory film with lower consumption.
The second key parameter is viscosity. AUX G 3242 is characterised by an exceptionally low kinematic viscosity – only five square millimetres per second at twenty-five degrees – which makes it almost as fluid as water. Such rheology favours spray and dip applications and any situation where a very thin, uniform layer without runs or streaks is required. AUX G 3243 at the same temperature reaches a viscosity of five hundred and fifty square millimetres per second and is therefore noticeably thicker. As a result it holds better on vertical surfaces, runs less when applied by brush or roller and allows more precise local thickness control. The difference in viscosity also affects the dilution approach. Both primers can be diluted with white spirit or Isopar E in ratios of up to one to ten; in the case of G 3242 dilution serves mainly to lower the already low viscosity further or to increase coverage, whereas with G 3243 dilution is often necessary to adapt the product for spraying or to obtain an even thinner layer.
The density of both products is similar – 0.8 g/cm³ for G 3242 and 0.85 g/cm³ for G 3243 – which in practice means that differences when converting consumption to surface area are marginal. Appearance is likewise not decisive, although it is worth noting. G 3242 is colourless to pale yellow, G 3243 is distinctly yellow. If visual control of the applied layer is important, the manufacturer allows the addition of a red colour concentrate that can be ordered separately.
The application process for both primers is almost identical and rests on a few critical principles. The substrate must first be carefully degreased, preferably successively with white spirit and then with ethanol or acetone. The primer must be thoroughly stirred or shaken before use because it is sensitive to humidity; prolonged contact with air causes it to become turbid and lose part of its adhesive properties. The layer is applied as thinly as possible by one of the classic methods – brush, dipping, spraying or roller – and is then left for the solvent to evaporate at room temperature and moderate atmospheric humidity for at least two hours. During this time a coherent silicone-resin film is formed. The silicone should be applied as soon as possible afterwards, at the latest within two days of priming. In some cases additional heating of the primed part to between 100 and 150 °C for up to one hour improves the final bond strength. It is worth remembering that the initial adhesion is often sufficient for handling the part, while the full adhesive strength is reached only after several days.
The choice between G 3242 and G 3243 therefore comes down mainly to the nature of the process and the geometry of the part. When a very thin, uniform layer is required, when spraying is used, or when the substrate is complex and local material build-up is easy, the low-viscosity G 3242 with its seven-percent solids content remains the better choice. When, on the other hand, a somewhat thicker, more robust film is desired, when large flat surfaces are being painted by brush, or when the number of passes and solvent consumption need to be reduced, G 3243 becomes the more advantageous option. Both products are intended for the same groups of substrates and the same types of peroxide-cured silicones, so the final decision should always be confirmed by trials on actual parts, taking into account the specific degree of surface preparation, atmospheric conditions and the time and temperature of vulcanisation.
Storage issues should also be emphasised. Both primers are sensitive to humidity, so after opening the container must be tightly closed and the product thoroughly mixed before each use. They are available in small bottles of approximately 0.75 to 0.8 kg, in 20 kg pails and in 150 kg drums, allowing the packaging form to be matched to the scale of production. The “best use before” date shown on the label is not an absolute limit – after this date the product may still be usable provided it retains the required properties, which must be verified as part of quality control.
In summary, ELASTOSIL® AUX G 3242 and AUX G 3243 are not competing products in the sense that one is superior to the other, but complementary tools with different rheology and concentration. Understanding the influence of solids content and viscosity on the character of the resulting film makes it possible to match the primer consciously to the specific process and thereby achieve repeatable, high adhesion of the silicone to the substrate.
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