1. Introduction

Uncured silicone rubber contains polymers of different chain lengths. It always comprises a principal silicon-oxygen chain (the siloxane backbone) and an organic moiety bound to the silicon. A silicon atom has four valence electrons, that is the reason why the silicone rubber is often abbreviated with a Q for “quaternary group”. The properties of silicone rubber are greatly depending on the organic groups and on the chemical structure. The organic groups may be methyl, vinyl, phenyl or other organic groups [1].

Depending on which organic groups are present, silicone polymers in common use are classified as follows:

– MQ, or polydimethylsiloxane (PDMS), denotes a polymer in which two methyl groups are bound to the siloxane backbone.
– VMQ stands for a polydimethylsiloxane in which a small number of methyl groups have been replaced by vinyl groups.
– PVMQ stands for an VMQ in which a small number of methyl groups have been replaced by phenyl groups.
– FVMQ stands for an VMQ in which a small number of methyl groups have been replaced by trifluoropropyl substituents.

Aside from the polymer structure, the viscosity frequently serves as a classification feature. A distinction is made between liquid and solid silicone rubber components as follows:

1. Contains platinum catalyst.
2. Contains crosslinker.

Rubber solid silicone rubber contains polymers with a high molecular weight and relatively long polymer chains. It is referred to as HTV (i.e. high-temperature-vulcanizing).

2. Liquid silicone rubber ( LSR )

Liquid silicone rubber contains polymers of lower molecular weight and hence shorter chains. The uncured silicone has better flow properties in respect to the solid cured polymers ( an example is ELASTOSIL® LR , where LR means Liquid Rubber). Synthetic silicas have been known for many years in the silicone industry. Both fumed and precipitated silicas are used to control the properties of silicone rubber. The fumed silicas, in the form of different hydrophilic and hydrophobic (i.e. AEROSIL® products ) are applied in numerous silicone systems [2].

As effective thixotropic additives, they control the viscosity and stability of uncured silicone sealing compounds. Their efficiency as reinforcing components also positively affects the mechanical properties of the different silicone products. The use of special AEROSIL® silicas grades even makes it possible to manufacture products with excellent optical properties (transparency). Manufactured by flame hydrolysis of chlorosilanes, and with a silicon dioxide content of more than 99.8%, AEROSIL® is one of the purest silicas commercially available. These inorganic substances, which are completely amorphous, fulfill all consumer requirements regarding reliable, physically safe raw materials. Another example is a metallic oxide (i.e. AEROXIDE® TiO2 P 25 from Evonik Co.) which can be used in silicone rubber to improve the heat stability.

2.1 Liquid Silicone Rubber for injection systems (LSR/LIMS)

The addition cross-linking liquid silicone rubber (LSR) was developed at the end of the 1970’s by Dow Corning. Liquid silicone rubbers have become increasingly important over the past few years, as a result of their good material and processing properties. Due to their fluid consistency and short crosslinking times, a high rate of productivity can be achieved with high-speed, automatic, environmentally-friendly processing methods using liquid injection molding systems (LIMS). The changeover in production from compression molding to automatic processing is an economic factor requiring serious consideration. The result would be a tangible reduction in the piece price.

Here are some of the most significant fields of application:

• Construction of automobiles and other vehicles
• Electronics and electrical engineering
• Human and food sector
• Medical technology
• Sanitary and household goods.

Consequently, the fields of application for LSR products can be found in almost all branches of industry. For example, some important products are keypads, seals and gaskets, insulators in high voltage technology and baby bottle nipples. Due to the fact that the material can be sterilized and displays good biocompatibility properties, it is also used in numerous medical applications. The spectrum of building components ranges at present from small and precision parts weighing less than 0.05 g per piece, and produced in large quantities, to high-volume components of up to 80 kg in weight, manufactured in small or medium-sized series. The diversity of suitable applications is currently undergoing a new revolution. Thanks to the two-component or dual color injection molding technology now available for use with liquid silicone rubber, it is also possible to bond plastics with silicone rubber in one working step.

LSR is made up of a two-component system, whose components A and B generally require mixing in a 1:1 ratio. They consist of polysiloxane polymers or copolymers, containing active or inactive fillers and additives. Prior to vulcanization, LSR is a mixture of silicone polymers containing vinyl groups and a cross-linking component with Si-H groups. Their consistency and cross-linking behavior allow excellent processing. Their viscosity is very low compared to that of solid silicone rubber . The result of this chemical reaction is a highly elastic, three-dimensional network with excellent mechanical, electrical and optical properties. Unlike with peroxide curing, no by-products are released during the vulcanization of LSR compounds. Good storage stability is achieved by the use of inhibitors and by the right choice of silica.

2.2. LSR Processing:

The much faster vulcanization process, compared to peroxide curing, is a deciding factor for the processing. Short cycles mean lower piece prices. Generally, the injection molding of LSR is fully automatic. The components A and B are metered during production at room temperature, together with a pigment paste or other additives, if required, from a dosing apparatus into a static mixer and subsequently injected into a hot mold. During the dwell time in the hot mold, the mixture cures to a solid, elastic component, which may be extracted by means of an automatic handling system, or manually, after the mold is opened. The cycle times obtainable with liquid silicone rubber vary between a few seconds and several minutes. In order to release volatiles, and to bring about a more complete vulcanization, the molded parts can be further post cured at about 200°C. In most cases, a curing time of four hours is sufficient for this operation.

Liquid silicone rubber (LSR) is a very low viscosity silicone that can be injection molded or cast into silicone rubber sheets. “Liquid” is explicit in the name to differentiate LSRs from high consistency “ gum” based silicone products. LSRs are addition cure / platinum cure systems in contrast with most “gum” based silicones that are peroxide cured.

In its raw uncured form, LSR consists of two parts liquid that are mixed together (i.e. mixing of Part A and Part B starts the curing process) and heated to accelerate the curing process. Raw gum based silicones look and feel similar to clay. LSRs are generally injection molded or cast while gum based silicones are typically compression molded. Note that injection molding of liquid silicone rubber is the inverse of molding thermoplastics : a room temperature liquid silicone compound is injected into and cured in a hot mold vs. the plastic pellets melted into hot liquid and forced into a cooled mold.[3]

The performance properties and the fast cure cycles make liquid silicone rubber a good choice for small molded components. Unlike most thermoplastic elastomers, LSR remains flexible and elastic down to – 58°F and retains its properties up to 392°F. Liquid silicone rubber is a good candidate for gaskets, seals and cushioning pads in portable communications, data acquisition instruments and ruggedized electronic devices where outdoor conditions are demanding.

LSR is available in a hardness range of 10 to 80 durometer Shore A. A 10 Shore durometer LSR is very soft and can feel softer than some extra firm silicone foam and silicone sponge products. A 80 Shore durometer LSR is as firm as many thermoplastic elastomer products. Most applications for LSR are specified in the 20 to 60 durometer Shore A range.

2.3. LSR Recycling

The LSR recycling has been tried from many standpoints of view.

Processes have been proposed in which LSR was decomposed into monomers or oligomers, which can be recycled, by using orthocarboxylic ester compounds, compounds containing active hydrogen group and/or catalysts.

However, these processes produced an unwanted by-product through the hydrolysis of the orthocarboxylic ester. Therefore, the above processes are not practical and resulted in an obstruction to the implementation on an industrial scale.

It is therefore necessary to provide a process in which the silicone monomers and/or silicone oligomers are produced without developing any byproducts.

Instead of using the traditional method of landfills and /or incineration, Eco USA uses environmental friendly and innovative alternatives for recycling unused material into raw material substitute and reusing it effectively. They attack the disposition of process material and recycle the material into a raw material substitute, recovering all materials generated utilizing energy recovery.

References:
[ 1 ] www.wacker.com
[ 2 ] www.aerosil.com
[ 3 ] www.stockwell.com