LSR News You Can Use

Liquid Silicone Rubber (LSR) is often utilized for seals, valves and diaphragms due to its high elongation (between 400 and 700% at room temperature) and tensile strength over a wide temperature range. The tensile properties of thermoset rubbers and thermoplastic elastomers need to be measured/tested in order to verify that the quality control standards are met as well as to determine whether or not the material is fit for its purpose. The test should be performed according to ASTM D412 which describes two test methods: A and B.

The hydrophobic methyl side groups of Liquid Silicone Rubber (LSR) account for its low surface energy and water repellence. An average value for silicones is 24 dynes/cm or 0.024 N/m. The surface hydrophobicity of a solid surface is determined by its free surface energy. ASTMD2578 (or ISO 8296) is the most employed technique to study loss and recovery of hydrophobicity of silicones, which is calculated by measuring the contact angle. The method is very time efficient and inexpensive instruments can be utilized. It is often defined on the basis of the static contact angle between the surface and a water droplet in which a surface can be considered hydrophilic if the contact angle is <90˚, or hydrophobic if the angle is >90˚. The fundamental equation for the measurement of solid surface tension by contact angle is described by Young's Equation in the image shown below:

Like all silicones, Liquid Silicone Rubber (LSR) is characterized by a flexible, fully saturated backbone of alternating silicon and oxygen atoms. The siloxane bonds have a partial ionic nature and give the material its high strength. The backbone is shielded by closely packed hydrophobic methyl side groups which have low intermolecular forces between them. This allows the surface energy or surface tension to be low while also being hydrophobic. This characteristic enables LSR to be water repellent while allowing it to adhere only to materials with higher surface energy such as polyamide 66 or polycarbonates.

While today's popular non-stick bakeware products manufactured out of Liquid Silicone Rubber (LSR) are flexible to the touch, they maintain a certain degree of resistance to deformation. While ear buds or in-ear headphones, on the other hand, require a close and comfortable fit with a flexibility that enables it to match the shape of the inner ear for a more natural feel.

The elemental structure of Liquid Silicone Rubber (LSR) enables the material to have a very low compression set; normally in the range of 20 - 40% (22 h @ 350˚C). A low compression set means LSR retains its elastic properties even during prolonged application of compressive stresses (resist creep) such as in sealing or vibration dampening part applications. This resistance to creep allows LSR to maintain its high sealing force during a product's lifetime.

Baby bottle nipples are manufactured out of Liquid Silicone Rubber (LSR). What makes LSR the material of choice for applications with such a high physical demand, is its high tear strength, which is normally in the range of 160 - 250 lb/in. Even emerging baby teeth cannot tear the nipples or bite off small pieces, eliminating the risk of swallowing particles or chunks of rubber. The tear strength of thermoset rubbers, thermoplastic elastomers and silicones can be measured according to American Society for Testing and Materials (ASTM) D624. It is defined as the strength required to initiate a tear in a material. This test provides a measure of resistance to tearing. Tear strength may also be used to illustrate the anisotropy of a material.

Professor Tim Osswald, Director of the Technical Advisory Board of SIMTEC Silicone Parts, presented breakthrough research on the viscoelastic behavior of Liquid Silicone Rubber (LSR) at Silicone Elastomers US, the first Silicone Elastomers’ event held in North America.

Professor Tim A. Osswald, Director of SIMTEC Silicone Parts' Technical Advisory Board will be one of the prestigious speakers presenting at this year's Silicone Elastomers US 2011 event. Held at the Hyatt Rosemont Hotel in Rosemont, IL, Professor Osswald will be discussing the viscoelastic behavior of Liquid Silicone Rubber (LSR) at 11:45 CST, Wednesday, December 7th.

The density or its reciprocal, the specific volume, is important for the shrinkage during processing and is greatly affected by temperature and pressure. The specific volume is often plotted as a function of pressure and temperature, in what is known as a pvT diagram (see image on the left below for an example of a pvT diagram). The measurement is defined in ISO 17744. With increasing temperatures, Liquid Silicone Rubber (LSR) expands. When pressure is applied, the overall specific volume shows a lower value, which means the density is higher. In addition, the expansion is less pronounced under pressure, which is shown by the decreasing slope of the curves with increasing pressure.

Semi-crystalline thermoplastic polymers show more order than amorphous thermoplastics. The molecules align in a structured crystalline form. The size of the crystals or spherulites is much larger than the wavelength of visible light, making semi-crystalline materials translucent rather than transparent. Crystalline regions are small with molecular chains comprised of both crystalline and amorphous regions. The degree of crystallinity in a typical thermoplastic will vary from grade to grade, as for example in polyethylene, where the degree of crystallinity depends on the branching and the cooling rate.