Liquid silicones belong to the group of silicone rubbers. They are also known by the abbreviation LSR, which stands for Liquid Silicone Rubber.

They consist of a main chain in which the silicon atoms are linked via oxygen atoms. The side chains contain additional, so-called functional groups that can be used to crosslink the linear main chains. Compared to solid silicone rubbers, the main chains of liquid silicones are shorter and more mobile. This is the reason for their low viscosity.

Due to its low viscosity, liquid silicone has good flow properties, which plays an important role in processing. The products made from it are characterized by excellent mechanical, thermal and chemical properties.

Physical properties of LSR

Liquid silicones and the products made from them exhibit very good aging resistance.

Thus they are affected by

• solar radiation and UV light
• humidity
• rain
• ozone
• and other weathering influences, such as cold weather
  are virtually unaffected.
  
  

In addition, liquid silicones exhibit very good electrical insulating properties and high gas permeability.

Mechanical properties of LSR

High elasticity and flexibility is one of the most outstanding material properties of liquid silicone-based products. They show high elongation at break, which means that they can be subjected to high mechanical loads by pulling or stretching.

They also show improved notch strength compared to other plastics, such as PVC. Thus, even if a crack or notch is present in the material, products will not tear beyond this point. Hardness varies, depending on the additives added, from rather soft, with a so-called Shore value around 20, to hard, with Shore values around 70.

Thermal properties of LSR

Liquid silicones exhibit high temperature resistance over a wide temperature range. For example, liquid silicone products are stable at temperatures from -55°C to +210°C. Physical or mechanical properties, such as their flexibility, do not change over this entire temperature spectrum.

Chemical properties of LSR

Liquid silicone products are chemically inert, meaning they are not attacked by many chemicals, such as acids, alkalis or alcohols.

However, some organic solvents, such as toluene or benzene, and fuels, such as gasoline, can cause liquid silicone products to swell and change their properties. Resistance lists can be consulted for evaluation (see sources).

Other properties of LSR

Another important property is physiological safety. For this reason, many products used in medicine, dental technology or food technology are made of liquid silicones.

As with all silicones, liquid silicones are manufactured from silicon dust, which is reacted with methyl chloride to form a mixture of chloromethylsilanes. This mixture of the components

• tetrachlorosilane
• trichloromethylsilane
• dichlorodimethylsilane
• trimethylchlorosilane
• tetramethylsilane

is separated into its various components by fractional distillation. Dimethyldichlorosilane serves as the starting product for the production of liquid silicones. These are first reacted to form an intermediate product, the silanols, which react further to form oligomers, i.e. short-chain linear silicones, as well as cyclic (ring-shaped) silicones.

In the next step, linear silicones, which have two linkage sites, react with each other. In addition, monofunctional silicones, which have only one linkage site, are added to the mixture. If these are added to a polymer chain, the chain can no longer be extended and thus ends. The chain length can thus be determined by the ratio of these two components. Liquid silicones have shorter polymer chains with a lower molecular weight, while solid silicone rubber contains longer polymer chains.

Liquid silicones are available as liquid two-component systems. One component contains a platinum catalyst and crosslinkable polydimethylsiloxane components. The second component contains the crosslinker. In addition, fillers such as silica may be present to stabilize the end product. Color pigments may also be added, depending on the desired end product. Only when both components are added together does the chemical reaction start.

This process is known as vulcanization. The advantage of this processing method is that the vulcanization process is very fast, allowing short production cycles. Secondly, this crosslinking reaction, which is a so-called addition crosslinking, does not produce any other by-products.

LSR injection molding

Liquid silicones are processed by injection molding. Both components are mixed in a mixing block in a ratio of 1:1 and then fed under pressure into a so-called LSR cylinder. It is important that this mixture is cooled to prevent premature crosslinking. From there, it is injected into the mold, which is heated to 170°C to 200°C. At these temperatures, the liquid silicone crosslinks within a few seconds. Molded parts injection-molded in this way can usually be used directly.

LSR annealing

Annealing is a thermal post-treatment. Silicone products produced by injection molding are dried for several hours at temperatures of around 200°C. The products are then ready for use.

For products that are to be used for food applications or in contact with infants and young children, annealing is required by law. Annealing removes volatile components that can be a byproduct of silicone rubber production. Newer liquid silicone compounds are sometimes already designed in such a way that the time-consuming and expensive step of annealing can be omitted.

Due to the broad resistance of liquid silicone to chemicals, temperature influences or mechanical stress, this material is used in many areas. Furthermore, the material properties can be further optimized by the use of fillers, such as fumed silica or quartz, additives and stabilizers. Finally, silicone rubbers made from liquid silicone can be colored as desired with specially adapted color pigments.

LSR in medicine

Many products in modern medical technology are based on silicone rubbers and especially on liquid silicones. For example, medical products such as

• insulin pumps
• pre-filled syringes
• tubes
• and inhalation devices
  
  

are made from liquid silicone. LSR is also used as a starting material for heart catheters and respiratory masks. Due to the physiological harmlessness of silicone, it is used in a wide range of applications.

LSR in the laboratory

Products made of LSR are inert to many chemicals, i.e. they are not attacked. For example, hoses for transporting chemicals or seals in laboratory equipment that come into contact with chemicals are often based on liquid silicone.

Mold making with LSR

Because of its good processability, liquid silicone can be used to manufacture molded parts with a wide variety of geometries. Even complicated shapes can be realized with it.

LSR in electrical engineering

One of the most outstanding properties of silicones is its good electrical insulating ability, which is why it is used in the electrical industry for the production of small as well as larger components. These include:

• Electrical plugs
• Connectors
• Safety mats
• and seals for various electrical components.
  
  

LSR in the automotive industry

In the automotive industry, many components that are required to have good electrical insulating properties or to withstand high temperatures and various weathering conditions are made of liquid silicone. These include:

• Housing and connector seals
• Protective jackets for spark plug connectors
• Headlight seals
• and rain sensors.
  
  

LSR in the food industry

Many products that come into contact with food are made from liquid silicones because of their physiological safety. These include, for example, elastic baking pans, which can be found in many households. In food technology, it is hoses or seals for whose manufacture liquid silicones are used.

LSR in other areas

Finally, liquid silicones serve as the starting material for a wide range of other products. Contact lenses, for example, are made of special, transparent liquid silicones, and pacifiers and teats for drinking bottles are also made from them. Particularly when it comes to plastic products that have to be physiologically and toxically safe, people like to use products made of silicone.