Liquid rubber resins

Liquid rubber resins are excellent for realizing soft to gel-like compounds despite their high hydrophobicity.

Such systems are extremely advantageous for the encapsulation of sensitive electronic components. The mechanical load on such components is low when subjected to thermal shock.

Potting compounds that are mainly based on so-called polybutadiene polyols (Fig. 1) are referred to as liquid rubber casting resins. Due to their very non-polar character, they are ideal for producing casting resins with high hydrophobicity, which also often have soft to gel-like properties.

This has an extremely positive effect on factors such as water absorption and diffusion permeability to water and moisture, meaning that a high cross-linking density is not essential. Low gas permeability is another advantageous property of liquid rubber systems.

Such systems are also extremely advantageous for potting sensitive electronic components. The mechanical load on such components is low when subjected to thermal shock.

By using suitable additives, the resistance to UV radiation and the limited temperature stability can be significantly increased. By adding thinners to the formulation, the comparatively high viscosities can be reduced to a very practicable level.

Liquid rubber cast resins can be divided into two classes:

Polyurethane liquid rubber cast resins

These are systems that are cured using isocyanates. They have good hydrolysis stability, as there are no unstable groups in the underlying polyol and water is virtually not absorbed due to the low polarity of the casting resin. Glass transition points here are usually in the range of approx. -50°C.

Polyester – liquid rubber resins

These are built up by the reaction of a polybutadiene polyol (Fig. 1) and a maleicised polybutadiene. The maleic anhydride groups react with the hydroxyl groups to form an ester bond. The exothermicity of the reaction is very low, so that no component stresses occur. Nevertheless, casting resins of this type can be adjusted to very short processing temperatures using suitable catalysers. Many of the properties mentioned above apply to these pure polybutadiene polyesters in an even more pronounced form, as the hardener component here is also a non-polar polybutadiene. The glass transition temperatures of these systems can be as low as -80°C.

In summary, liquid rubber resins are characterized by special properties that are difficult to achieve with conventional PUR casting resins based on polyether/polyester polyols.                 The advantage of these systems is that they do not require the use of an isocyanate. This means that they are not subject to labelling under current German law. These cast resins are characterized by special properties that are difficult to achieve with conventional casting resins based on polyether/polyester polyols.