Contents
Section 1: Silicone Rubber – Basic Principles
General characteristics of silicones and why WACKER silicones are so successful. Chemical fundamentals,
the most important silicone rubber grades, their components and curing mechanisms
Section 2: Silicone Rubber – Material and Processing Advantages
Why and when silicone rubber is the material of choice. Material and processing advantages
Section 3: WACKER Silicone Rubber Grades
Which applications and properties are typical? Overview of the most important applications and properties
Section 4: Material Preparation
Temperature, viscosity, additives and what to consider in advance. Storing and compounding silicone rubber
Section 5: Processing of Material
How to process silicone rubber and where to be careful Extrusion, compression molding, injection molding
and coating of textiles
Section 6: Secondary Finishing
How to finish cured silicone rubber. Post-curing, bonding technology, coating and printing
Section 7: Service
How we support you along the entire process chain. Advice on product choice, technical support, customized ready-to-use compounds, WACKER ACADEMY, e-business
Section 8: Rubber Solutions
Facts and figures about silicone. SECTION 1: SILICONE RUBBER – BASIC PRINCIPLES
Silicones are made from quartz sand, a raw material available in practically unlimited quantities.
WACKER manufactures silicone monomers in a closedloop integrated production system based on the
Müller-Rochow process, creating the precursors for around 3,000 silicone products.
The highly integrated energy and rawmaterial production system is exclusive to WACKER. The by-products of the chemical processes are either used immediately or returned elsewhere to the production loop, so that virtually no waste is generated. Waste heat is also recovered and utilized, increasing the system's overall energy efficiency.
Uncured silicone rubber contains polymers of different chain lengths. It always comprises a principal siliconoxygen chain (the siloxane backbone) and an organic moiety bound to the silicon. A silicon atom has four valence electrons, which is why silicone rubber is often abbreviated with a Q for “quaternary group”. The properties of silicone rubber vary greatly depending on the organic groups and the chemical structure. The organic groups may be methyl, vinyl, phenyl or other groups. Depending on which organic groups are present, silicone polymers in common use are classified as follows: MQ
MQ, or polydimethylsiloxane (PDMS), denotes a polymer in which two methyl groups are bound to the
siloxane backbone.
VMQ
VMQ stands for a polydimethylsiloxane in which a small number of methyl groups have been replaced by
vinyl groups.
PVMQ
PVMQ stands for an VMQ in which a small number of methyl groups have been replaced by phenyl groups.
FVMQ
FVMQ stands for an VMQ in which a mall number of methyl groups have been replaced by
trifluoropropyl substituents
1.2 MAJOR CLASSES
Solid Silicone Rubber
Solid silicone rubber contains polymers
with a high molecular weight and relatively
long polymer chains. It is referred as HTV
(= high-temperature-vulcanizing).
WACKER offers two product
ranges based on solid silicone rubber:
a range of peroxide-curing grades
marketed as ELASTOSIL® R (R = Rubber) and a range of addition-curing (platinum-catalyzed) grades
marketed as ELASTOSIL® R plus. The standard delivery form is as bars measuring 90 x 100 mm
(crosssection), which are wrapped in PE film and shipped in cartons. For special requirements, we also
offer our readyto- use blends in the form of strips, round-cords, pellets and profile strips
Liquid Silicone Rubber
Liquid silicone rubber contains polymers of lower molecular weight and hence shorter chains. It has better
flow properties. Liquid silicone rubber is always addition-curing (platinum-catalyzed),
and is marketed by WACKER as ELASTOSIL® LR (LR = Liquid Rubber). The A1 and B2
components are supplied in 20-kg or 200-kg containers:
• 20-kg pails with a PE inliner (ø inside 280 mm)
• 200-kg drums with a PE inliner (ø inside 571.5 mm)
1.3 COMPONENTS
Crosslinkers
A crosslinker is required to convert the raw rubber into a mechanically stable cured product. Use is made
of peroxides or platinum catalyst systems (see section 4.3, pages 36 and 37).
Fillers
Fillers are needed to reinforce the elastic silicone network. The nature, composition and quantity of the fillers
have a crucial influence on the properties of the raw and cured rubber.
• Reinforcing fillers:
Pyrogenic silica with very high BET surface areas (more than 100 m2/g) is the most frequently used
reinforcing filler. WACKER HDK® has proven to be especially effective here, although precipitated
silica and carbon black can also be used.
• Non-reinforcing fillers:
These have merely a fill-up function. Quartz, for example, increases the cured rubber's resistance to various
media.
Additives
Compared with other elastomers, silicone rubber requires few additives because the essential properties are
determined by the siloxane polymer used. Thus, a finished polymer compound may consist only of polymer
and filler. Particularly notable is the fact that silicone rubber is free of curing accelerators or retarders,
organic plasticizers and organic antioxidants. Additives include stabilizers, masticating aids and colorants.
Stabilizers
Stabilizers are available for special applications in order to optimize properties such as heat and media
resistance. Colorants Silicone rubber is generally transparent and can be colored as desired: from
transparent through translucent to opaque. WACKER will supply you with suitable pigment pastes, both
for liquid silicone rubber and solid silicone rubber. These pigment pastes are tailored specifically to the
rubber grade in question and are easily blended into the compound while on the roll mill or via metering
equipment during injection molding. It should be remembered that some additives are themselves
inherently colored. You will find additional information on fillers, additives, stabilizers and colorants in
section 4.3, as from page 31.
CURING
PEROXIDE CURING
To become an elastomeric material, raw silicone rubber has always to be cured. This can be done either
by peroxide or addition curing. Peroxide curing involves the use of organic peroxides. At elevated
temperatures, they decompose to form highly reactive radicals which chemically crosslink the polymer
chains. The result is a highly elastic, three-dimensional network. WACKER offers peroxide crosslinkers in
paste or powder form.
Advantages
Peroxide curing is a time-tested and technically mature process. Peroxidecuring silicone rubber from
WACKER has been in use for more than 50 years and is refined continuously in close cooperation with
our customers. The same applies to the production processes, which ensure consistently high quality.
The rubber compounds are characterized by low sensitivity to catalyst poisons and have a particularly long
shelf life.
Processing Information
Some peroxide crosslinkers (Elastosil® AUX Crosslinker C1 and Elastosil® AUX Crosslinker C6)
are inhibited by atmospheric oxygen. Any surface coming into contact with air during curing will cure
incompletely and remain sticky. You will find additional information on the various crosslinkers in section
4.3, page 36.
1.4 CURING
PLATINUM-CATALYZE ADDITION CURING (NEXT TIME )
Liquid silicone rubber contains polymers of lower molecular weight and hence shorter chains. It has better
flow properties. Liquid silicone rubber is always addition-curing (platinum-catalyzed),
and is marketed by WACKER as ELASTOSIL® LR (LR = Liquid Rubber). The A1 and B2
components are supplied in 20-kg or 200-kg containers:
• 20-kg pails with a PE inliner (ø inside 280 mm)
• 200-kg drums with a PE inliner (ø inside 571.5 mm)
1.3 COMPONENTS
Crosslinkers
A crosslinker is required to convert the raw rubber into a mechanically stable cured product. Use is made
of peroxides or platinum catalyst systems (see section 4.3, pages 36 and 37).
Fillers
Fillers are needed to reinforce the elastic silicone network. The nature, composition and quantity of the fillers
have a crucial influence on the properties of the raw and cured rubber.
• Reinforcing fillers:
Pyrogenic silica with very high BET surface areas (more than 100 m2/g) is the most frequently used
reinforcing filler. WACKER HDK® has proven to be especially effective here, although precipitated
silica and carbon black can also be used.
• Non-reinforcing fillers:
These have merely a fill-up function. Quartz, for example, increases the cured rubber's resistance to various
media.
Compared with other elastomers, silicone rubber requires few additives because the essential properties are
determined by the siloxane polymer used. Thus, a finished polymer compound may consist only of polymer
and filler. Particularly notable is the fact that silicone rubber is free of curing accelerators or retarders,
organic plasticizers and organic antioxidants. Additives include stabilizers, masticating aids and colorants.
Stabilizers
Stabilizers are available for special applications in order to optimize properties such as heat and media
resistance. Colorants Silicone rubber is generally transparent and can be colored as desired: from
transparent through translucent to opaque. WACKER will supply you with suitable pigment pastes, both
for liquid silicone rubber and solid silicone rubber. These pigment pastes are tailored specifically to the
rubber grade in question and are easily blended into the compound while on the roll mill or via metering
equipment during injection molding. It should be remembered that some additives are themselves
inherently colored. You will find additional information on fillers, additives, stabilizers and colorants in
section 4.3, as from page 31.
CURING
PEROXIDE CURING
To become an elastomeric material, raw silicone rubber has always to be cured. This can be done either
by peroxide or addition curing. Peroxide curing involves the use of organic peroxides. At elevated
temperatures, they decompose to form highly reactive radicals which chemically crosslink the polymer
chains. The result is a highly elastic, three-dimensional network. WACKER offers peroxide crosslinkers in
paste or powder form.
Advantages
Peroxide curing is a time-tested and technically mature process. Peroxidecuring silicone rubber from
WACKER has been in use for more than 50 years and is refined continuously in close cooperation with
our customers. The same applies to the production processes, which ensure consistently high quality.
The rubber compounds are characterized by low sensitivity to catalyst poisons and have a particularly long
shelf life.
Processing Information
Some peroxide crosslinkers (Elastosil® AUX Crosslinker C1 and Elastosil® AUX Crosslinker C6)
are inhibited by atmospheric oxygen. Any surface coming into contact with air during curing will cure
incompletely and remain sticky. You will find additional information on the various crosslinkers in section
4.3, page 36.
1.4 CURING
PLATINUM-CATALYZE ADDITION CURING (NEXT TIME )
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