Article No : a24_057
Silicones
HANS-HEINRICH MORETTO, Bayer AG, Leverkusen, Federal Republic of Germany
MANFRED SCHULZE, Bayer AG, Leverkusen, Federal Republic of Germany
GEBHARD WAGNER, Bayer AG, Leverkusen, Federal Republic of Germany
1.
2.
2.1.
2.1.1.
2.1.2.
2.1.3.
2.1.4.
2.1.5.
2.1.6.
2.2.
2.3.
3.
3.1.
3.2.
3.3.
3.4.
3.5.
3.6.
4.
4.1.
4.2.
4.3.
4.4.
4.4.1.
4.4.2.
4.4.3.
4.4.4.
4.4.5.
4.5.
Introduction. . . . . . . . . . . . . . . . . . . . . . . .
Linear and Cyclic Polyorganosiloxanes . . .
Production . . . . . . . . . . . . . . . . . . . . . . . . .
Hydrolysis. . . . . . . . . . . . . . . . . . . . . . . . . .
Methanolysis . . . . . . . . . . . . . . . . . . . . . . . .
Cyclization . . . . . . . . . . . . . . . . . . . . . . . . .
Polymerization . . . . . . . . . . . . . . . . . . . . . .
Polycondensation. . . . . . . . . . . . . . . . . . . . .
Industrial Production of Linear Polysiloxanes
Polydimethylsiloxanes . . . . . . . . . . . . . . . .
Siloxane-Based Copolymers . . . . . . . . . . . .
Silicone Fluids . . . . . . . . . . . . . . . . . . . . . .
Methylsilicone Fluids . . . . . . . . . . . . . . . . .
Methylphenylsilicone Fluids. . . . . . . . . . . .
Other Types of Silicone Fluids. . . . . . . . . .
Properties . . . . . . . . . . . . . . . . . . . . . . . . .
Formulation . . . . . . . . . . . . . . . . . . . . . . . .
Applications . . . . . . . . . . . . . . . . . . . . . . . .
Silicone Rubbers and Elastomers. . . . . . . .
General Properties. . . . . . . . . . . . . . . . . . .
Rubber Compounds. . . . . . . . . . . . . . . . . .
Rheology . . . . . . . . . . . . . . . . . . . . . . . . . .
Curing Systems . . . . . . . . . . . . . . . . . . . . .
Radical Curing with Peroxides . . . . . . . . . . .
Hydrosilylation Curing . . . . . . . . . . . . . . . .
Condensation Curing . . . . . . . . . . . . . . . . . .
Radiation Curing . . . . . . . . . . . . . . . . . . . . .
Oxidative Coupling . . . . . . . . . . . . . . . . . . .
Peroxide-Cured High-Temperature
Vulcanizing Silicone Rubbers. . . . . . . . . . . . .
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4.7.1.
4.7.2.
4.8.
4.8.1.
4.8.2.
4.9.
4.10.
5.
5.1.
5.2.
5.3.
5.4.
5.5.
6.
6.1.
6.2.
6.3.
6.4.
7.
8.
9.
10.
Liquid Silicone Rubbers . . . . . . . . . . . . .
Room Temperature Curing Silicone
Rubbers . . . . . . . . . . . . . . . . . . . . . . . . . .
Two-Component RTV Systems . . . . . . . . .
One-Component RTV Systems. . . . . . . . . .
Paper and Textile Coatings . . . . . . . . . . .
Paper Coating . . . . . . . . . . . . . . . . . . . . . .
Textile Coating . . . . . . . . . . . . . . . . . . . . .
Properties of Silicone Elastomers. . . . . . .
Applications . . . . . . . . . . . . . . . . . . . . . . .
Silicone Resins . . . . . . . . . . . . . . . . . . . . .
Structure and General Properties . . . . . .
Production . . . . . . . . . . . . . . . . . . . . . . . .
Curing . . . . . . . . . . . . . . . . . . . . . . . . . . .
Properties . . . . . . . . . . . . . . . . . . . . . . . .
Applications . . . . . . . . . . . . . . . . . . . . . . .
Block and Graft Copolymers . . . . . . . . . .
Polysiloxane – Polyether Copolymers . . .
Other Block Copolymers . . . . . . . . . . . . .
Graft Copolymers . . . . . . . . . . . . . . . . . .
Applications of Block and Graft
Copolymers . . . . . . . . . . . . . . . . . . . . . . .
Analysis . . . . . . . . . . . . . . . . . . . . . . . . . .
Toxicology . . . . . . . . . . . . . . . . . . . . . . . .
Environmental Aspects . . . . . . . . . . . . . .
Economic Aspects . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
Nomenclature and Structure. The term
silicones is used for compounds in which silicon
atoms are linked via oxygen atoms, each silicon
atom bearing one or several organic groups. In
industrially important silicones, these groups are
usually methyl or phenyl. The silicones are
known as polyorganosiloxanes according to
IUPAC rules.
The structure of the industrially important
‘‘methylsilicones’’ can involve the units listed
in Table 1.
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/14356007.a24_057
4.6.
4.7.
Polymer structures can be described by using
the letters M, D, T, and Q to designate the
monomer units. Linear silicone fluids are composed mainly of D units. The base polymers for
silicone elastomers or silicone rubbers consist of
D units that bear cross-linkable functional groups.
The main structural feature of the highly branched
silicone resins are T units, often combined with D
units to make the resins more flexible. Silicone
resins can also contain Q and M units.
General Properties. Silicones have many
outstanding properties, which are described in
676
Silicones
Table 1. Origin, functionality, and fields of application of silicone
structural units
Vol. 32
osilanes. Other chlorosilanes that contain silicon-bound H, C6H5, CH2¼CH, or CF3CH2-CH2
groups, either exclusively or in combination with
methyl groups, are produced in smaller quantities. Chlorosilanes and siloxanes containing other organic ligands, such as C2H5 or HOCH2, are
not discussed in detail here because of their
minor importance.
In the following sections, the processes of
hydrolysis, methanolysis, polymerization, and
polycondensation are described in more detail;
the production of oligomeric polydimethylsiloxanes from dimethyldichlorosilane is used as an
example throughout.
2.1.1. Hydrolysis
The complete hydrolysis of dimethyldichlorosilane leads to an oligomer mixture consisting of
cyclic dimethylsiloxanes and hydroxyl-terminated dimethylsiloxanes:
nðCH3 Þ2 SiCl2 þn H2 O!½ðCH3 Þ2 SiOn þ2n HCl n
¼ 3; 4; 5; etc:
detail for the individual product groups. In general,
methylsilicones exhibit greater stability to high
temperature, UV radiation, and weathering than
organic polymers; marked surface-active behavior
(low surface tension, high spreading power); good
dielectric properties, as well as low temperature
dependence of their physical properties.
and simultaneously:
mðCH3 Þ2 SiCl2 þðmþ1ÞH2 O!HO½ðCH3 Þ2 SiOm
Hþ2m HCl m ¼ 4!100
Hydrolysis with a deficiency of water gives
linear dimethylsiloxanes with terminal SiCl
groups:
ðnþ2ÞðCH3 Þ2 SiCl2 þðnþ1ÞH2 O!ClðCH3 Þ2 SiO
2. Linear and Cyclic
Polyorganosiloxanes
2.1. Production
Linear and cyclic polyorganosiloxanes are generally produced by reacting organodichlorosilanes with water. The mixture of oligomeric
siloxanes arising from hydrolysis can be converted either entirely to cyclic siloxanes (e.g.,
octamethylcyclotetrasiloxane) or directly polymerized to linear polysiloxanes. A special case of
the production of cyclic or linear oligomeric
dimethylsiloxanes that has gained in importance
is the methanolysis of dimethyldichlorosilane.
The most important chlorosilanes used industrially (> 90 % of the total) are the methylchlor-
½ðCH3 Þ2 SiOn SiðCH3 Þ2 Clþ2ðnþ1ÞHCl
Complete hydrolysis with excess water is
carried out continuously in the liquid phase with
ca. 25 % hydrochloric acid or in the gas phase at
ca. 100 C. Liquid-phase hydrolysis gives cyclic
and linear oligomeric dimethylsiloxanes in the
approximate ratio 1: 1 to 1: 2, depending on
reaction conditions, together with ca. 30 % hydrochloric acid. The hydrochloric acid can be
reacted with methanol to produce chloromethane, which is used for producing methylchlorosilanes in the Rochow synthesis (! Silicon Compounds, Organic) [1–4] (chlorine
recycling).
The ratio of cyclic to linear dimethylsiloxanes
and the chain length of the linear oligomers can
Vol. 32
be varied over a relatively wide range by means
of hydrolysis conditions. For example, rapid
removal of hydrochloric acid from the reaction
mixture by neutralization leads almost exclusively to short- chain siloxanediols. Cyclic siloxanes represent up to two-thirds of the reaction
product if prolonged contact with HCl occurs.
The tetramer octamethylcyclotetrasiloxane is the
predominant cyclic siloxane.
Control of the hydrolysis reaction to give
predominantly cyclic or linear oligomeric dimethylsiloxanes is important because highmolecular mass polydimethylsiloxanes are produced both by equilibrating polymerization and
by polycondensation. The preferred starting materials for equilibrating polymerization are cyclic
siloxanes, while polycondensation is only possible with hydroxyl-terminated oligomers.
The hydrolysis process can also be used to
produce organosiloxanes modified with functional groups. For example, the reaction of a
mixture of dimethyldichlorosilane, methylhydrogendichlorosilane, and trimethylchlorosilane with excess water affords a trimethylsilyl
end-stopped linear polymethylsiloxane with a
random distribution of dimethylsiloxy and
methylhydrogensiloxy units. The ratio of the
two difunctional methylsiloxane units in the
mixture can be varied at will. The average chain
length of the siloxanes formed decreases with
increasing trimethylchlorosilane content in the
silane mixture.
nðCH3 Þ2 SiCl2 þm CH3 ðHÞSiCl2 þ2ðCH3 Þ3 SiCl
þðnþmþ1ÞH2 O!ðCH3 Þ3 SiO½ðCH3 Þ2 SiOn ½CH3 ðHÞSiOm
SiðCH3 Þ3 þ2ðnþmþ1ÞHCl
The hydrolysis product mixture is influenced
to a certain degree by the addition of organic
solvents. This method is especially useful for
obtaining siloxanes that contain silanol groups
and silicone resins (see Section 5.2).
Figure 1 shows a schematic of the hydrolysis
process.
2.1.2. Methanolysis
The methanolysis process for the production of
siloxanes from dimethyldichlorosilane allows
direct recovery of the chlorine from methylchlorosilanes as chloromethane. The silane reacts with
Silicones
677
Figure 1. Continuous hydrolysis of dichlorodimethylsilane
in a circulation apparatus a) Cooler; b) Exhaust; c) Phase
separation; d) Settling vessel; e) Water separator; f ) Neutralization, g) Pump
methanol to give oligomeric dimethylsiloxanes
and chloromethane. Two variants of this process
exist, one of which leads to linear hydroxyl endstopped oligomeric siloxanes, whereas the other
affords mainly cyclic siloxanes. In the former,
siloxanes are removed from the bottom of the
reaction column, and the lower-boiling cyclic
compounds in the overhead are returned to the
process; in the latter, volatile cyclic oligomers
are removed continuously from the reaction mixture by distillation.
In both cases the methanolysis plants are
operated in association with methylchlorosilane
production. Figures 2 and 3 show how methanolysis or hydrolysis of dimethyldichlorosilane
is integrated with the Rochow synthesis and the
plants for further processing.
Methanolysis occurs according to the following overall equations:
nðCH3 Þ2 SiCl2 þ2n CH3 OH!HO½ðCH3 Þ2 SiOn H
þ2n CH3 Clþðn1ÞH2 O
nðCH3 Þ2 SiCl2 þ2n CH3 OH!½ðCH3 Þ2 SiOn þ2n CH3 Clþn H2 O
Dimethyl ether formed as a byproduct is
purged from the process or allowed to react with
hydrochloric acid to form chloromethane, depending on the process variant.
If the synthesis and methanolysis of dimethyldichlorosilane are regarded as a single process
unit, then overall, dimethylsiloxane is formally
produced from silicon and methanol, with water
678
Silicones
Vol. 32
Figure 2. Integration of the hydrolysis or methanolysis of dimethyldichlorosilane via intermediate linear dimethylsiloxanes in
silicone production
as the only byproduct. Recycling of chlorine as
chloromethane avoids HCl waste, making silicone production cleaner and more efficient.
Rochow synthesis:
n Siþ2n CH3 Cl!nðCH3 Þ2 SiCl2
Methanolysis:
nðCH3 Þ2 SiCl2 þ2n CH3 OH!½ðCH3 Þ2 SiOn
þ2n CH3 Clþn H2 O
Overall:
n Siþ2n CH3 OH!½ðCH3 Þ2 SiOn þn H2 O
products, which are either marketed as such or
used for the production of polydimethylsiloxanes. Cyclization is performed by heating the
hydrolysis or methanolysis mixture with potassium hydroxide. Process aids are used to prevent
polymerization of the siloxanes to high-viscosity
liquids. Potassium hydroxide catalyzes an equilibration reaction in which the Si – O – Si bonds
are cleaved and reformed. During the reaction,
the lower-boiling octamethylcyclotetrasiloxane
and decamethylcyclopentasiloxane are distilled
continuously from the reaction mixture. The
siloxanes are constantly reformed to maintain
the equilibrium until the siloxane mixture is
completely converted to the desired cyclic
siloxanes.
2.1.3. Cyclization
2.1.4. Polymerization
Pure cyclic siloxanes are produced by cyclization. Octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane are major industrial
Linear polyorganosiloxanes can be prepared
from cyclic organosiloxanes by equilibrating
Vol. 32
Silicones
679
Figure 3. Integration of the hydrolysis or methanolysis of dimethyldichlorosilane via intermediate cyclic dimethylsiloxanes in
silicone production
ring-opening polymerization, which is promoted by both anionic and cationic catalysts [5],
[6]. Octamethylcyclotetrasiloxane is the preferred starting material. Other siloxanes are
used for chain end-stopping or for producing
copolymers.
Anionic Polymerization with alkali-metal
hydroxide catalysts is of industrial importance.
The catalytic activity decreases in the series
Cs > Rb > K > Na > Li [7]. KOH is the most
common catalyst. Rapid polymerization is observed above 140 C on adding as little as a few
ppm KOH, e.g., as a suspension in octamethylcyclotetrasiloxane. The polymerization mechanism
involves initial formation of potassium siloxanolate, which then catalyzes chain growth and equilibration via cleavage of Si – O – Si bonds.
The polymerization reaction leads to an equilibrium mixture of linear polysiloxanes and ca.
15 – 18 wt % cyclic siloxanes. The average
chain length of the linear polysiloxanes is controlled by the ratio of end groups to D units in the
polymerization mixture. Various substances
(called regulators) added during polymerization
680
Silicones
determine the end groups. A Poisson distribution
of molecular mass is obtained.
For example, hydroxyl-terminated polydimethylsiloxanes are formed in the presence of
water. The average molecular mass depends on
the amount of water added. If trimethylsiloxycontaining siloxanes (e.g., MD2M) are added to
the polymerization mixture then trimethylsilylterminated polymethylsiloxanes are formed.
Cation- complexing additives such as poly(ethylene glycol), crown ethers, methyl ethyl
ketone, dimethylformamide, and dimethyl sulfoxide accelerate the reaction [5], [8] by promoting the dissociation of inactive silanolates:
After the molecular mass distribution has
reached equilibrium, the catalyst must be deactivated. Numerous deactivation methods have
been described. KOH is generally neutralized
with phosphoric acid or chlorosilanes. Careful
neutralization is essential for the stability of the
polymer, since both alkaline and acid residues
lead to degradation of the siloxane chain by
depolymerization. After neutralization, the volatile low molecular mass constituents (mostly
cyclic compounds) are removed by distillation
and recycled to the polymerization process.
Vol. 32
produced, since purification of the siloxanediols
by distillation is not possible.
Polycondensation is carried out as a batch or
continuous process in the presence of acid catalysts, preferably polychlorophosphazenes
(PNCl2)x [9–11]. The water produced in the
reaction must be removed. When the desired
chain length is reached, the catalyst is deactivated with ammonia or an amine [12], [13].
Polycondensation of siloxanediols proceeds
very rapidly at elevated temperature under vacuum
(Fig. 4, curve a). In the presence of a short- chain,
R(CH3)2SiO-terminated dimethylsiloxane (R ¼
CH3, CH¼CH2, H) that regulates the chain length
via equilibration, a combination of polycondensation and slower equilibrating polymerization occurs, and the system therefore passes through a
marked viscosity maximum (Fig. 4, curve b).
With a siloxanol as chain-length regulator, the
final viscosity is reached more quickly (Fig. 4,
curve c) because only condensation steps occur.
R(CH3)2SiO-terminated polydimethylsiloxanes can be obtained by polycondensation and
equilibration of the siloxanediols with R(CH3)2
SiO-terminated dimethylsiloxanes or by polycondensation of the siloxanediols followed by silylation of the silanol end groups with hexaorganodisilazanes, R(CH3)2SiNHSi(CH3)2R, for example.
Cationic Polymerization of cyclosiloxanes is
carried out with strong protic or Lewis acids.
Industrially important catalysts are perfluoroalkanesulfonic acids or sulfuric acid [5], [6]. The reaction mechanism has not yet been fully elucidated.
Another method of cationic polymerization
employs acidic solids such as ion-exchange resins and acid-activated silicates as catalysts.
Cationic polymerization also leads to an equilibrium mixture of linear polysiloxanes.
2.1.5. Polycondensation
Linear oligomeric dimethylsiloxanes from the
hydrolysis or methanolysis process can be polymerized by polycondensation. This process requires the use of high-purity dimethyldichlorosilane if strictly linear siloxane polymers are to be
Figure 4. Polycondensation – equilibration of siloxane diols
a) HO[(CH3)2SiO]nH; b) HO[(CH3)2SiO]nH þR(CH3)2
SiO[Si(CH)2O]xSi(CH3)2R; c) HO[(CH3)2SiO]nH þ R
(CH3)2SiO[Si(CH3)2O]xH R ¼ CH3, CH ¼ CH2
Vol. 32
Silicones
681
2.1.6. Industrial Production of Linear
Polysiloxanes
Depending on the type of polymerization
reaction, the following process steps:
For small-volume products, the polymerization
of oligomeric siloxanes is carried out in stirred
vessels; batches up to 15 t are readily controllable. Larger quantities are produced in continuous
plants (Fig. 5).
1.
2.
3.
4.
5.
Purifying and drying the starting materials
Catalyst and regulator metering
Establishing equilibrium and condensation
Neutralization
Oligomer removal and distillate recycling
Figure 5. Industrial processes for production of silicone polymers A) Fixed-bed catalysis; B) Stirred-vessel cascade;
C) Continuous process in a screw mixer a) D4 cyclosiloxane vessel; b) Dryer; c) Static premixer; d) Metering pump; e) Screw
mixer; f ) Process viscometer; g) Degassing extruder; h) Condenser; i) Vacuum pump; j) Vessel for regulator; k) Vessel for
vinylmethylsiloxane; l) Vessel for potassium siloxanolate; m) Vessel for phosphoric acid; n) Control of metering pumps;
o) Recycle to c; p) Finished polymer; q) Falling-film evaporator; r) Vessel; s) Ion-exchange column
682
Silicones
Vol. 32
Table 2. Polymerization processes
Starting material
D4
D4
HO[(CH3)2SiO]nH
Catalyst
Amount of catalyst, ppm
Polymerization time, min
Reaction temperature, C
Volatile oligomers in the product mixture, %
Regulator
Neutralization
KOH
5 – 20
10 – 90
140 – 180
13
MDxM
H3PO4
H2SO4
100 – 1000
15 – 30
20 – 160
13
MDxM
ZnO
Na2CO3
(PNCl2)x
5 – 200
10 – 20
40 – 160
2
[R(CH3)2Si]2NH
amines
pose different technical requirements, which are
summarized in Table 2.
When the variants are considered, five process
designs for performing the polymerization have
essentially prevailed:
1.
2.
3.
4.
Single-stage polymerization vessel
Stirred-tank cascade
Screw extruder reactor
Cell reactor (tubular reactor with spiral stirrers that generate approximately plug flow)
5. Solid (catalyst) reactor
obtain special properties. With a few exceptions,
these copolymers have the general structure
RðCH3 ÞSiO½ðCH3 Þ2 SiOx ½ðR1 ÞðR2 ÞSiOy SiðCH3 Þ2 R
The substituents R, R1, and R2 can be identical or
different. The most important D units are:
2.2. Polydimethylsiloxanes
Linear polydimethylsiloxanes are the most important industrial polysiloxanes. The polymers
are classified according to their viscosity (i.e.,
average chain length) and the nature of the end
groups. The most important polymer types characterized by their end groups are listed in Table 3.
The endgroups determine the use. For example, trimethylsilyl-terminated polydimethylsiloxanes are typical silicone fluids. Hydroxy- and
vinyl-terminated polymers find major application in silicone rubbers.
2.3. Siloxane-Based Copolymers
Siloxane copolymers containing other siloxy groups
in addition to dimethylsiloxy groups are used to
Table 3. Types of polymer
End group
Structure
Methyl
OH
Vinyl
(CH3)3SiO[(CH3)2SiO]xSi(CH3)3
HO(CH3)2SiO[(CH3)2SiO]xSi(CH3)2 OH
CH2¼CH(CH3)2SiO[(CH3)2SiO]x
Si(CH3)2CH¼CH2
H(CH3)2SiO[(CH3)2SiO]xSi(CH3)2H
H
The copolymers are usually produced by copolymerization of the appropriately substituted cyclosiloxanes and D4 under equilibrating conditions.
In this way, the various D groups are distributed
randomly along the polymer chain. Exceptions are
block copolymers that are produced by anionic
polymerization under nonequilibrating conditions.
Polymers with trifluoro groups are also prepared by the kinetically controlled polymerization of [CH3(CF3CH2CH2)SiO]3, since 96 wt %
cyclics are formed at equilibrium [14].
For example, polydimethyl – polydiphenyl
block copolymers are produced by stepwise polymerization of D3 and (DPh2)3 with Li- containing bases in tetrahydrofuran. The resulting block
copolymers have a twophase morphology and
elastomeric properties.
3. Silicone Fluids
The structure of linear silicone fluids can generally be described by the composition MDxM
(x ¼ 2 – 4000).
Vol. 32
Silicones
683
Table 4. Important types of silicone fluid
Silicone fluids are distinguished from common organic fluids by a number of unique
properties:
most important silicone fluids are listed in
Table 4.
1. Good thermal stability (150 – 250 C)
2. Good
low-temperature
performance
(< 70 C)
3. Strong hydrophobicity
4. Excellent release properties
5. Antifriction and lubricating properties
6. Pronounced surface activity
7. Good dielectric properties
8. Very good damping behavior
9. Good radiation resistance
10. High solubility of gases
11. Physiological inertness
12. Low temperature dependence of physical
properties
3.1. Methylsilicone Fluids
These properties can be modified over a wide
range by varying the organic substituents. The
The most important silicone fluids are the
methylsilicone fluids (polydimethylsiloxanes,
PDMS). Silicone fluids exhibit a chain-length
distribution. The average chain length largely
determines the viscosity. Low molecular mass
volatile constituents are removed during production, which increases the flash point of the final
product. Fluids with viscosities ranging between
1 and 106 mPa s are commercially available.
The most important physical properties of polydimethylsiloxanes are listed in Table 5, which
shows that the physical properties of silicone fluids
depend on the molecular mass only up to a certain
degree of polymerization. With increasing molecular mass, they reach a limiting value.
684
Silicones
Vol. 32
Table 5. Physical properties of polydimethylsiloxanes*
Average molecular mass
Property
Viscosity (25 C), mPa s
Viscosity – temperature coefficient
Density (25 C), g/cm3
Flash point (DIN 51 376), C
Pour point, C
Refractive index, n25
D
Thermal conductivity (150 C), W m1 K1
Specific heat (20 C), J g1 K1
Thermal expansion coefficient (0 – 150 C),
cm3 cm3 K1
Surface tension, mN/m
Dielectric constant (25 C, 50 Hz)
Dielectric strength, kV/mm
Resistivity (25 C), W cm
Loss factor tan d (25 C, 50 Hz)
*
600
1800
5800
26 000
62 000
160 000
3
0.55
0.90
62
100
1.390
0.10
1.50
10
0.57
0.94
170
90
1.398
0.14
1.50
100
0.60
0.97
300
50
1.402
0.16
1.50
1000
0.61
0.97
320
50
1.403
0.17
1.50
12 500
0.61
0.97
> 320
50
1.404
0.17
1.50
500 000
0.61
0.97
> 320
40
1.404
0.17
1.50
11.4104
19.3
2.5
13
21014
5105
10.3104
19.9
2.6
14
21014
5105
9.9104
20.9
2.6
14
21014
5105
9.9104
21.2
2.7
14
21014
5105
9.9104
21.4
2.8
15
21014
5105
9.9104
21.5
2.8
15
21014
5105
Some of these are approximate values that can vary depending on the producer.
3.2. Methylphenylsilicone Fluids
3.3. Other Types of Silicone Fluids
Methylphenylsilicone fluids exhibit higher thermal stability, better low-temperature properties,
more powerful solvent action for organic substances, and better lubricating properties than
PDMS. Their radiation resistance is also particularly high. The physical properties of methylphenylsilicone fluids of high and low phenyl
content are listed in Table 6.
Methylhydrogensilicone fluids have silicon –hydrogen groups that react with protic compounds
with the formation of hydrogen. Inorganic and
organic surfaces having reactive groups, OH or
NH2 groups (such as glass), for example, can be
modified. As a result these surfaces acquire
completely new properties (see Section 3.6).
Fluorosilicone fluids have gained importance
due to their outstanding low-temperature lubricating properties, low solubility in mineral oils,
and the unique properties of fluorosilicone-treated surfaces.
Methylalkylsilicone fluids containing C2 –
C14 alkyl groups have better lubricating properties than pure PDMS. Like the methylphenylsiloxanes, they have better compatibility with
organic substances (e.g., solvents or organic
paint systems).
Table 6. Physical properties of methylphenylsilicone fluids
Baysilone fluid Baysilone fluid
PN 200*
PH 1000*
Ratio of phenyl to methyl groups
Viscosity (25 C), mm2/s
Viscosity temperature coefficient
Density (25 C), g/cm3
Flash point, C
Pour point, C
Refractive index, n25
D
Thermal conductivity
1
1
(50 C), W m K
Specific heat, J g1 K1
Thermal expansion
coefficient (25 – 180 C), K1
Surface tension, mN/m
Dielectric constant (25 C, 50 Hz)
Dielectric strength, kV/mm
Loss factor tan d (25 C, 50 Hz)
*
Trademarks of Bayer.
4.5
200
0.74
1.03
ca. 300
ca. 65
1.471
0.13
1.5
1000
0.89
1.08
ca. 300
ca. 30
1.512
0.13
1.56
8.7104
1.60
7.8104
23
2.9
15
1104
25
2.8
15
3104
3.4. Properties
Viscosity and Molecular Mass. The relation between the viscosity and the molecular
mass of PDMS can be expressed approximately
by the equation
M¼
464ðh25 Þ0:825
2þ0:0905ðh25 Þ0:555
Vol. 32
Silicones
Other equations that are applicable to specific
molecular mass ranges are given in [15], [16].
Determination of the molecular mass of linear
and cyclic polymers from the intrinsic viscosity
[h] (in mL/g) by means of the formula [h] ¼
K M a is described in [17].
Temperature Dependence of Viscosity.
An important property of silicone fluids is the
small dependence of the viscosity on temperature, compared to other fluids. The viscosity of
PDMS as a function of temperature is shown in
Figure 6, which includes mineral oils for
comparison.
The relatively low viscosity of silicone fluids
at low temperature and the low pour point are
important in many applications. The viscosity –
temperature coefficient (VTC)
VTC ¼ 1
n99 C
n38 C
which is frequently reported in the technical
literature, is ca. 0.6 for silicone fluids.
As also shown in Figure 6, increasing the
phenyl content (phenylsilicone fluids PH 1000)
increases the VTC, and the viscosity – temperature behavior approaches that of mineral oils.
685
Mixture Viscosity. Formulas are available
for determining the viscosity of a mixture of two
silicone fluids [18]. Silicone fluid manufacturers
provide this information in mixture diagrams
(Fig. 7).
The required mixing ratio can be determined
with the aid of the mixture diagram. For example,
if a fluid of viscosity 6000 mPa s is to be
blended at 25 C from a silicone fluid with
viscosity of 1000 mPa s and a silicone fluid
with a viscosity of 12 500 mPa s, the value
1000 mPa s is marked on the left-hand side
and the value of 12 500 on the right-hand side,
and the two values are joined by a straight line.
The intersection of this straight line with a line
drawn parallel to the x-axis through the value of
6000 mPa s indicates the mixing ratio on the xaxis, in this case 70 % silicone fluid of viscosity
12 500 mPa s and 30 % silicone fluid of viscosity 1000 mPa s.
Newtonian Behavior and Pseudoplasticity. Silicone fluids of low or medium viscosity
exhibit Newtonian behavior up to high shear
rates (see Fig. 8). However, high-viscosity silicone fluids have lower Newtonian plateaus and
show pseudoplastic behavior at lower shear rates.
Figure 6. Temperature dependence of the viscosity of silicone fluids compared with mineral oils
686
Silicones
Vol. 32
listed in Tables 5 and 6. Both values are nearly
independent of temperature.
The thermal stability of dimethylsilicone
fluids is very high. Little change is observed in
the physical properties even after prolonged
exposure to temperatures of 150 – 200 C in air.
Methylphenylsilicone fluids are stable at temperatures as much as 50 C higher. In inert
atmosphere or under vacuum the thermal stability is even higher.
At higher temperature, atmospheric oxygen
leads to cleavage of the organic groups and
consequently to gelling by cross-linking. By contrast, prolonged heating in a closed system in the
absence of oxygen causes chain scission and a
decrease in viscosity. These processes are very
dependent on the nature of the contacting surfaces
and the presence of catalyst traces or impurities.
Figure 7. Viscosity adjustment by blending
If the viscosity is decreased by raising the temperature, the pseudoplastic behavior begins at a
higher shear rate.
Compressibility. The very high compressibility of polydimethylsiloxanes is apparent
in the compressibility coefficient of 100
1011 m2/N. Methylphenylsilicone fluids have
lower values of ca. 601011 m2/N, whereas mineral oils exhibit coefficients of ca. 501011 m2/N.
Thermal Properties. Thermal conductivities and specific heats of various silicon fluids are
Figure 8. Flow behavior of silicone fluids at 25 C
Solubility. The PDMS and methylphenylsilicone fluids are highly soluble in aliphatic,
aromatic, and chlorinated hydrocarbons, as well
as in most ethers, esters, and higher alcohols.
They are insoluble in water, methanol, and ethylene glycol. The solubility in organic solvents
decreases with increasing viscosity of the polymer. PDMS with viscosities > 20 mPa s exhibit only limited solubility in acetone, ethanol,
butanol, and isopropanol. PDMS of different
viscosities can be mixed, but PDMS and methylphenylsilicone fluids are immiscible.
The solubility of gases in polydimethylsiloxanes is considerable. Thus, 1 g of silicone fluid
dissolves 0.19 mL of air, 0.17 mL of N2, or 1 mL
of CO2 at room temperature.
Interfacial Properties. Silicone polymers
exhibit low surface tension: ca. 20 mN/m for
polydimethylsiloxanes, and ca. 25 mN/m for
methylphenylsiloxanes (see Tables 5 and 6).
Since most solids have surface tensions
> 20 mN/m, silicone fluids readily form surface
films. Exceptions are polyolefins and polytetrafluoroethylene, whose surface tensions are also
low. The interfacial tension of polydimethylsiloxane against air and water is listed in Table 7.
A water drop on siliconized glass has a contact
angle of 80 – 110 , which decreases on prolonged contact due to changes in the state of
orientation of the silicone film [19]. Four different states are observed. The spreading of silicone
fluids on water leads to ordered systems [20].
Vol. 32
Silicones
Table 7. Interfacial tension s of polydimethylsiloxane (140 mm2/s)
against air and water at different temperatures [18]
s, mN/m
Temperature, C
Against air
Against H2O
10
20
30
40
50
90
22.9
22.2
21.5
20.9
20.2
17.5
28.5
29.5
30.5
32.5
33.0
3.5. Formulation
Some properties of polyorganosiloxanes can be
particularly advantageous in certain formulations. Silicone fluid-in-water emulsions are preferred forms for applying thin films or for metering very small amounts. Silicone fluid emulsions
are used, for example, as release agents, for
impregnating surfaces, for water repellency
treatment, in antifriction applications, and as
antifoam agents. Emulsions are generally sold
as 10 – 35 % concentrates, which are diluted to
usually less than 1 %, sometimes only a few parts
per million, before use. Silicone microemulsions,
some of which are transparent, are aqueous systems with silicone particle sizes of 10 – 80 nm
[21]. For certain applications, silicone fluids are
formulated with silica or other consistencyincreasing additives such as metal soaps, polytetrafluoroethylene (PTFE), boron nitride, and
ureas to make pastes (see Section 3.6).
3.6. Applications
The characteristic properties of silicone fluids
are exploited in numerous applications [22].
Thus, because of their high thermal stability
and good low-temperature performance, they
are used as heat-transfer media in heating circuits in the chemical, petrochemical, pharmaceutical, and food industries and in solar power
plants, and as refrigerants in cryostats, freeze
dryers, and climate simulation plants. Their low
surface tension leads to use as release agents in
the processing of plastics and rubber articles.
Silicone fluids, especially functional copolymers with Si – H or Si – OH groups, spread on
surfaces to form oriented films with their hy-
687
drophobic organic groups aligned opposite to
the phase boundary of the substrate. These
functional copolymer fluids have many uses as
additives in water-repellent polishes, as waterproofing agents for textiles, and as protective
coatings for building materials. Their high water vapor permeability is a particular advantage,
permitting good ventilation of water vapor
while largely preventing the entry of liquid
water.
Because of their surface activity, polydimethylsiloxanes are used as antifoams in aqueous systems, in petroleum processing, and in
laundry detergents. Special formulations with
highly dispersed silica are effective antifoams
even in the part-per-million range. In human and
veterinary medicine, siloxane antifoams are
used as antiflatulent agents. The antifriction
properties of polydimethylsiloxanes make them
useful as lubricants (e.g., for films, yarn, medical articles, wine corks, and fillers). Special
silicone fluids that dissolve spermicides of the
Nonoxinol-9 type can be used as prophylactic
coatings for condoms. Silicone fluids are increasingly important as dielectric coolants for
transformers and rectifiers because of their
flame resistance [23], resistance to ageing, material compatibility, and physiological inertness. These properties are also important for
use as power transmission fluids in viscous and
fan couplings [24] and as hydraulic and damping fluids in shock absorbers, railway buffers,
and vibration insulation systems, at both high
and low temperature. The more advantageous
lubricating properties of siloxanes compared to
pure organic products in the temperature range
from less than 20 C to > 150 C have led to
their use as lubricants. The greases or pastes
obtained by incorporating consistency improvers exhibit a particularly good lubricating behavior when the base fluids are modified by
phenyl, long- chain alkyl, or fluoro groups. They
are used chiefly for lubricating electric motors,
motor bearings of furnace blowers and ventilators, pump bearings for liquid gases, and machine bearings for low-temperature operation.
Flexure-stable silicone fluid pastes are used as
embedding compounds for glass-fiber cables in
communication engineering. Since silicone
fluids have a good dissolving power for organic
vapors, they can be used as absorbents for
organic vapors of low water solubility in off-gas
688
Silicones
purification. Modified silicone fluids are used as
paint additives to influence the leveling or to
obtain special finishes (e.g., hammer effect).
Their tolerance by the skin and their physiological
inertness [25] make silicone fluids suitable as
additives for ointments and cosmetic
preparations.
4. Silicone Rubbers and Elastomers
4.1. General Properties
Silicone polymers exhibit low glass transition
and equilibrium melting temperatures, weak intermolecular interactions, and high chain mobility. These properties make them highly suitable
for use in rubbers.
Silicone polymers can incorporate a variety of
functional groups as potential cross-linking
points. The position (exclusively chain ends, or
along the polymer chain) and the content of these
functional units can be readily varied. Together
with the wide range of polymers and their compound viscosities these different cross-linking
systems lead to many applications.
The development of silicone elastomers is
reviewed in [26–28]. Specialized rubbers with
unique properties such as low-temperature flexibility down to 70 C according to DIN 53548
(phenyl, ethyl) [29], oleophobicity and solvent
resistance (fluoroalkyl, cyanoalkyl), or higher
surface tension (polyether) are obtained by replacing the methyl groups with other organic
groups.
Curing (vulcanization) converts unvulcanized
silicone compounds into silicone rubbers/elastomers. Unlike other rubber polymers, unfilled
silicone rubbers achieve only low mechanical
strengths when cured. Adequate strengths are
only obtained by incorporating reinforcing fillers. High surface area silicas are used almost
exclusively for this purpose.
The stress – strain curves of filler-free siloxane networks at small deformations can be
well described by the semi-empirical Mooney – Rivlin equation [30], [31]. The ultimate
strength of silicon elastomers is determined by
the attainable average chain length between
cross-links [32]. The Einstein – Guth – Gold
equation is also used to describe filled silicone
elastomers [33].
Vol. 32
4.2. Rubber Compounds
Reinforcing Fillers. Whereas carbon black
is most commonly used to reinforce standard
organic elastomers, the best reinforcing fillers
for silicones are finely-divided silicas. Both pyrogenic and precipitated silicas with BET surface
areas of 150 – 400 m2/g are used. These silicas
have average primary particles of only 7 –
30 nm, which, however, are present in silicone
compounds as larger aggregates [34]. Transparent silicone elastomers can be produced with
silica fillers because of the small primary particle
size.
The tensile strength of silicone elastomers
reinforced with high surface area silicas can be
as much as 50 times that of unfilled systems,
reaching values of up to 12 MPa. Common organic elastomers are significantly stronger and
can attain tensile strengths of 20 – 25 MPa.
The silica filler is normally added to the
silicone polymer during formulation. However,
precipitated silicas can also be generated in situ
in solvents [35] or even directly in the rubber
polymer from orthosilicate esters [36], [37]. Silica fillers cause a large increase in the viscosity of
the formulated rubber [38]. Fillers also influence
the low-temperature crystallization rate of the
elastomer [26].
Reinforced silicone elastomers usually contain 5 – 38 wt % silica filler. An increase in the
viscosity is observed above 2 wt % filler [34]. A
higher filler content, like a higher cross-link
density, leads to an increase in both the modulus
and the hardness of cured rubbers. Increasing the
filler content or filler surface area also raises the
tensile strength, while at the same time causing a
deterioration of the relaxation behavior of the
polymer network. The tensile strength reaches a
maximum at filler concentrations of 25 –
30 wt % [34]. The relaxation behavior is influenced by the inflexible filler – filler network,
which hinders reversible deformation of the
polymer. The stress – strain curves of filled elastomers show a large reduction in stress and
hardness after the first deformation (Mullin effect: breakdown of the solid – solid interactions)
[39]. A significant increase in the storage modulus is observed above 20 wt % filler. At this filler
concentration, known as the percolation point,
the silica aggregates meet and interpenetrate the
entire polymer network [40].
Vol. 32
Inert Fillers. Lower surface area, ‘‘inert’’
fillers are used for cost reduction and for adjusting certain properties. Common inert fillers are
quartz powders, diatomaceaous earth, siliceous
and other chalks, talcs, micas, calcium or zirconium silicates, and alumina trihydrate. Inert fillers can be used to modify the stress – strain
behavior. Substitution of silica by inert fillers
leads to a lower hardness and a decrease in the
modulus. Treatment of inert fillers with special
silane coupling agents can partially compensate
for this effect [41].
Inert fillers increase the thermal conductivity
of the elastomers [42], [43]. Electrically conducting elastomers can be produced by addition of
special furnace blacks or other conductive materials (carbon fibers, metal powders) [44], [45].
Process Aids. Pyrogenic silicas used in reinforcing silicone elastomers have a pronounced
thickening action due to hydrogen bonds between the filler aggregates. Three-dimensionally
cross-linked structures, formed in the nonpolar
silicone polymer matrix, hinder flow even at low
concentration (> 2 wt %). At high filler concentration (ca. 25 wt %), such as are necessary for
optimal reinforcement, the viscosity is so high
that the compounds can no longer be remilled.
Process aids are used in silicone rubber
manufacturing to reduce the viscosity. These
process aids react to form a hydrophobic filler
surface and thereby restrict filler – filler interactions. Silylating agents such as silylamines and
silylacetamides [46] are particularly effective.
Hexaalkyldisilazanes are preferred.
Silicones
689
cess aids when a higher pseudoplasticity is required. This applies to solid rubbers, for which
stability under load is necessary for processing
(e.g., extrusion).
As an alternative to treatment of fillers during
rubber manufacture, pretreated fillers can also be
used.
Stabilizers. Specific performance properties of silicone elastomers, such as resistance to
hot air, chemicals, or fire, can be improved by
using additives. Generally, very small quantities
(0.001 – 10 wt %) are required. Metal oxides;
salts of iron, titanium, zirconium, cesium, nickel,
copper, cobalt, or manganese; or carbon blacks
are suitable for inhibiting the cross-linking
by hydroxyl radicals produced in hot air at
> 220 C [48], [49].
The action of water, acids, or bases can lead to
depolymerization of the siloxane polymer network [50]. Stabilizers against these agents are
alkaline-earth silicates, certain amphoteric hydroxides, and specific organic polymers [51],
[52].
Silicone elastomers can be protected from
continued burning following ignition by addition
of conventional flame retardants such as haloaromatics with Sb2O3 [53] or Al(OH)3 with zinc
borate [54], [55].A method of flame retardation
specific to silicones is the use of platinum (10 –
60 ppm), TiO2 [56], Fe2O3, or carbon black, and
certain nitrogen compounds [57], which under
the action of fire result in ceramization of the
surface and reduced depolymerization [58], [59].
4.3. Rheology
Silylation decreases the proportion of surfacebound rubber (gel content or bound rubber) and
prevents hardening and viscosity increase on
storage (crepe hardening). However, hydrophobic fillers are less strongly reinforcing [47].
Siloxanediols and alkoxysilanes are used as pro-
Both the viscosity and its dependence on shear
rate are important in the processing of silicone
rubber compounds. The requirements of various
rubber processing methods can be readily met
because of the large range of silicone polymer
viscosities and the different methods of hydrophobic filler treatment [60].
Maximum strength is achieved with highviscosity polymers and small primary particle
size silica fillers. In the case of pyrogenic silicas,
the best fillers are those with the highest surface
area [34].
Deviations from Newtonian flow behavior
can be adjusted for the various silicone rubbers
690
Silicones
by means of the filler surface area, filler dispersion, and the filler surface treatment. Thus rubbers with weak (two- component RTV) or strong
(HTV) shear thinning are available, as well as
pumpable liquid silicone rubbers (LSR) and rubbers with thixotropic behavior (one- component
RTV).
4.4. Curing Systems
4.4.1. Radical Curing with Peroxides
Silicone rubbers are cross-linked at high temperatures with peroxides but not with sulfur. Both
peroxides common in the rubber industry and
specific to silicone processing are used. Crosslinkable silicone polymers must contain unsaturated groups in sufficient amounts. Preferred
polymers contain 0.03 – 2 wt % methylvinylsiloxy groups. The concentration of methylvinylsiloxy groups determines the cross-link density
and thus important elastomeric properties such as
elongation. The vinyl group distribution in the
polymer can be determined by means of 29Si
NMR spectroscopy [61].
Four groups of peroxides are commonly used
industrially: dialkyl, peroxyketal, diaroyl, and
alkyl aroyl. The peroxides used preferably are:
Dicumyl peroxide
tert-Butyl peroxybenzoate
tert-Butyl cumyl peroxide
2,5-Dimethylbis(2,5-tert-butylperoxy)hexane
Bis(2,4-dichlorobenzoyl) peroxide
Bis(4-methylbenzoyl) peroxide.
Dialkyl peroxides cross-link mainly via the
vinyl groups [62], [63]. The more reactive diaroyl peroxides are less specific and give the
higher cure rates required for pressureless curing.
The structure of the polymer cross-links has
been elucidated by 29Si NMR spectroscopy and
by depolymerization to low-molecular weight
units in several cases [62]. Nonspecific peroxides
cross-link via both methyl and vinyl groups,
forming mostly propylene and butylene bridges,
but also a few ethylene bridges, between polymer
chains [63], [64]. Diaroyl peroxides can react
with the vinyl silyl groups to form ester and ether
groups [65]. The major side products of peroxide
decomposition are acids.
Vol. 32
The cross-link yield with low-vinyl content
polymers can be increased by using co- crosslinkers such as triallyl isocyanurates or acrylates
[66].
4.4.2. Hydrosilylation Curing
The highly selective hydrosilylation curing process is becoming increasingly important for the
vulcanization of silicone rubbers. The cross-linking reaction is based on the addition of Si – H
groups to C¼C double bonds (see ! Silicon
Compounds, Organic). Usually, long- chain
polydimethylsiloxanes containing two or more
vinyl groups are reacted with short- chain
methylhydrogensiloxanes (cross-linkers) in the
presence of metal catalysts according to the
equation:
Preferred catalysts are platinum compounds.
Concentrations as low as a few ppm platinum
lead to adequate curing rates.
Hydrosilylation curing, unlike cross-linking
by peroxides, produces no decomposition products. The reaction rate can be varied over a wide
range by means of the catalyst concentration and
by using inhibitors (e.g., 2-methyl-3-butyn-2-ol)
[67]. Inhibitors also extend the processing time
(pot life) at room temperature. The catalysts are
poisoned by various substances such as compounds
of heavy metals (e.g., tin), sulfur compounds (thiols,
sulfides, etc.), and nitrogen compounds (amines,
isocyanates, etc.).
The molar ratio of the reactants is especially
important for optimum curing. A 1.5- to 2-fold
molar excess of Si – H groups is generally used
to achieve the optimum cross-link density (i.e.,
by complete reaction of the vinyl groups). It is
also possible to produce very soft or gel-like
products by incomplete cross-linking.
4.4.3. Condensation Curing
Condensation- curing compounds cure at room
temperature and are known as RTV (room temperature vulcanizing) compounds. The elastomeric network is formed by reaction of hydroxy
Vol. 32
Silicones
Table 8. Typical silane cross-linkers
Silane
Cleavage product
Si(OC2H5)4, [SiO(OC2H5)2]n, Si(OC3H7)4,
CH3Si(OCH3)3, CH3Si(OC2H5)3
alcohols
CH3Si(OCOCH3)3, C2H5Si(OCOCH3)3
acetic acid
CH3Si[NH(sC4H9)]3, CH3Si(NHC6H11)3
amines
CH3Si[ON¼C(CH3)C2H5]3,
Si[ON¼C(CH3)C2H5]4,
CH2¼CHSi[ON¼C(CH3)C2H5]3
butanonoxime
CH3SiOC2H5[N(CH3)COC6H5]2
N-methylbenzamide,
ethanol
functional polysiloxanes and tri- or tetrafunctional silanes containing hydrolyzable Si – O or
Si – N bonds (Table 8). The hydrolyzable
groups react with SiOH groups of the polymer
or with water to form cross-links.
Most silane cross-linking agents react spontaneously with SiOH groups or with water. Metal
catalysts (Sn or Ti) are generally added to these
systems to give complete curing and improve the
properties. Alkoxysilanes are an example of
cross-linkers that do not cure in the absence of
appropriate catalysts.
Condensation curing is used in one- and twocomponent products, which differ in composition and cure rate.
One- component RTV compounds contain
excess cross-linking agent. During compounding
the hydroxyl functional polysiloxanes and the
cross-linker react to produce polymers with reactive end groups, for example:
691
sulting SiOH groups react with unhydrolyzed
groups to form an elastomer network. The following simplified reaction scheme applies:
One- component RTV compounds are distinguished according to the cure byproducts as
acidic, basic or, neutral cure systems (Table 8).
Acidic systems contain methyl- or ethyltriacetoxysilane as curing agent and produce acetic
acid as byproduct [68]. Basic systems produce
amines [69]. Several neutral systems are commercially available. Alkoxy systems use mainly
methyltrimethoxysilane [70], [71]. Oxime systems contain methyl(tributanone oximo)silane
and give butanone oxime as byproduct [72].
Benzamide systems produce N-methylbenzamide as a typical hydrolysis byproduct. The latter
remains as a finely divided solid in the cured
elastomer [73].
Tetrafunctional alkoxysilanes are generally
used as cross-linkers in two- component RTV
compounds. They must be used in combination
with tin catalysts, whose specific reaction mechanism ensures cross-linking of the OH-functional
polysiloxanes, even in the presence of excess
cross-linker [74], [75]. The presence of water is
also necessary for curing. Unlike one- component RTV compounds, the process does not
involve an intermediate endstopped polysiloxane. The compounds cure uniformly, in contrast
to one- component RTV systems, which cure
from the surface inward.
4.4.4. Radiation Curing
In this way, cross-linking of the polymers can
be repressed. One- component RTV products are
sold in containers sealed against humidity. Curing starts when the compounds are exposed to
atmospheric moisture during application. As the
remaining free cross-linking agent and the reactive polymer endgroups are hydrolyzed, the re-
Radiation-cured products have been unimportant
hitherto. This kind of cross-linking is applied
principally in processes for which high curing
temperatures are not possible [76].
Curing by Ultraviolet Light. Ultraviolet
curing has been described for coating temperature-sensitive substrates (paper, films) [77].
692
Silicones
Cross-Linking by g Rays and Electron
Beams. Cross-linking under the influence of
g rays or electron beams proceeds via the formation of free radicals [78].
Polydimethylsiloxane rubbers cured with 1 –
5 Mrad of g rays have similar mechanical properties to peroxide- cured rubbers.
4.4.5. Oxidative Coupling
Polysiloxanes containing mercaptoalkyl groups
react rapidly at room temperature in the presence
of metal catalysts to form elastomers crosslinked by disulfide bridges [79]. These rubbers
lack some of the advantages typical of silicones,
such as odorlessness, physiological inertness,
and weather resistance.
4.5. Peroxide-Cured High-Temperature
Vulcanizing Silicone Rubbers
High-temperature vulcanizing (HTV) silicon
rubbers are designed for the processing methods
and equipment of the rubber industry.
The viscosity of HTV compounds is in the
range of 20 – 100 Mooney units at 25 C (concentric-disc viscometer DIN 53 523), typical of
other rubbers but only at temperatures of 80 –
120 C.
The base polymers are high-viscosity, vinylcontaining polysiloxanes with only methyl
(VMQ), or a mixture of methyl and phenyl
(PVMQ) or trifluoropropyl (FVMQ) groups. The
vinyl substituents can be present as end groups and
along the polymer chain. The vinyl group concentration is commonly between 0.03 and 2.0 mol %.
Production. HTV silicone rubbers can be
produced batchwise or continuously from the
siloxane polymers, silica fillers, and process aids
in conventional rubber mixing equipment.
Suitable mixing equipment includes e.g.,
twinscrew compounders (2 – 6000 L), internal
mixers (Banbury, 1 – 300 L), roll mills (1 –
100 L), twinscrew extruders or Buss co-kneaders
(2 – 500 kg/h). The individual machines have
different mixing times, but comparable space –
time yields [80], [81].
Since hydrophobic silica fillers are not usually
employed for mixing, dispersion at temperatures
Vol. 32
up to ca. 160 C is often necessary. Unlike the
production of temperature-sensitive, high-viscosity organic rubbers, heat – not cooling – must be
applied during silicone rubber production.
The rubbers are usually manufactured as pigment- and cross-linker-free compounds. The addition of cross-linking agents (e.g., peroxides),
pigments, stabilizers, and optional processing
aids is usually carried out by the user on a smaller
scale in easily cleaned roll mills. Ready-to-use
compounds are marketed on a limited scale by
silicone producers.
Processing. HTV silicone rubbers are pasty,
translucent or colored materials. In the uncured
state at 25 C they have a lower green strength
than other rubbers. To improve the processibility
on roll mills (roll workability) and for better
demolding, release agents [82], [83], or viscosifying additives such as PTFE powder [84] are
sometimes used.
Molding. Peroxide- containing rubbers are
prepared for molding as sheets, strips, or granules. Compression-molded articles are produced
in steel molds filled by means of a piston or in
automatic transfer molding presses and casting
machines [60]. Tubes and insulated wires are
produced continuously by extrusion. Coatings or
coverings are produced on calenders.
Vulcanization. Vulcanization of HTV
moldings is carried out in hot compression molds,
in steam chambers under pressure, or in unpressurized hot air ovens. Diaroyl peroxides are used
for pressureless vulcanization because of their
low decomposition temperature (60 – 90 C) and
high cure rate (5 – 20 min at 100 C).
Curing with diaryl peroxides occurs in a few
minutes at 160 – 180 C. The cure kinetics can
be determined with conventional curemeters,
which measure an increase in torque as an index
of the degree of cross-linking. The time required
for a 50 % increase in torque roughly corresponds
to the half-life of the peroxide [85]. After curing,
volatile siloxanes (0.5 – 1.5 %) and residual peroxide decomposition products must be removed.
This is normally accomplished by heat treatment
(post- curing) for 2 – 6 h at 200 C.
High-viscosity silicone rubbers can also be vulcanized by hydrosilylation curing. A precondition
is a sufficient concentration of cross-linkable vinyl
Vol. 32
groups and their appropriate distribution in the
polymer. These systems produce high-strength,
transparent elastomers free of discoloration. Both
compression molding and unpressurized curing can
be employed [86], [87].
Cross-linking by condensation reactions is not
common for these rubbers due to the long cure
times.
Other Forms of Application. Dispersions
of HTV rubbers in organic solvents are used to
produce coatings such as coated glass fibers and
braided cables.
Silicone foam rubber can be produced by
adding blowing agents or by hydrosilylation
curing in the presence of SiOH groups or water.
The latter method involves the evolution of
hydrogen (see ! Foamed Plastics, Section 4.7.).
4.6. Liquid Silicone Rubbers
Liquid silicone rubber (LSR) is a new class of
rubber that has grown rapidly in importance
since the early 1980s. This rubber was developed
specifically for processing on injection molding
equipment. LSR compounds exhibit a number of
favorable processing properties such as low viscosity and high curing rate and represent an economical alternative to conventional rubber processing due to the lower cost of the finished article.
Liquid silicone rubbers are medium-viscosity
materials that can be pumped from the storage
vessel to the injection molding machine. Hydrosilylation curing is the exclusive mechanism of
vulcanization for this class of rubbers (see Section 4.4.2). Peroxide- cured liquid silicone rubbers are of no commercial importance.
Formulation. Liquid silicone rubber is
based on vinyl- containing polydimethylsiloxanes with viscosities about 1000 times lower
than for HTV rubber stock. However, the chain
length, which is decisive for the development of
the elastomer network is only six times lower. To
compensate for the relatively short chain length
and still maintain an adequate network density,
the majority of the polymer must have vinyl end
groups. Acceptable reinforcing fillers are pyrogenic or precipitated silicas made hydrophobic
by treatment with silylating agents. Filler treatment reduces the mutual interaction of the filler
Silicones
693
agglomerates to produce compounds that are free
flowing despite the high filler content.
Liquid silicone rubbers are formulated as
two- component systems to ensure a long shelf
life. Only after the two components (A and B) are
mixed can the rubber be cured. The A component
usually contains the platinum catalyst in addition
to the base polymer, while the B component
contains the cross-linker (polymethylhydrogensiloxane) and an optional inhibitor in addition to
the base polymer.
Liquid silicone rubbers with other base polymers [e.g., methylphenylsiloxane (PVMQ) or
methyltrifluoropropylsiloxane (FVMQ)] [88],
[89] or other fillers (e.g., carbon black for electrically conductive types) are also available [45].
Processing. Liquid silicone rubber is supplied ready for processing. The two components
are conveyed with a metering pump to a mixing
head in which an optional dye paste can be
separately charged. The material then flows
through a static mixer into the automatic injection molding machine (Fig. 9). In a predetermined cycle, the material is injected into the hot
mold, held under pressure to cure, then automatically demolded. The molds are mainly of the so-
Figure 9. LSR processing
694
Silicones
called cold-runner type, in which the sprue channels are cooled so that no loss of material occurs.
Liquid silicone rubber can be inexpensively
processed like thermoplastic elastomers, but has
the advantage of being chemically cross-linked.
Thus, use temperatures of well above 100 C are
possible.
The cure temperature of LSR is generally
between 170 and 230 C. Cycle times of 15 –
60 s can be achieved, depending upon the wall
thickness of the moldings. Many moldings are
used as obtained without further treatment. With
correct mold design, flash removal is unnecessary. For certain applications, e.g., in the foods
industry, post- curing the articles (usually 4 h at
200 C) is required to remove residual volatiles.
Post- curing also removes excess SiH groups
by hydrolysis and partial condensation.
Post- curing results in new disiloxane bridges
and an increase in the network density. After
post- curing, further reactions of the Si-H groups
are no longer possible. As a consequence, the
compression set, an important property for many
applications, is reduced to a desirably low level.
Liquid silicone rubber is suitable for producing coatings due to the relatively low viscosity.
Special grades with low filler content and, therefore, lower viscosity are available for this application. Unlike many other elastic coating materials, LSR formulations contain no solvent.
4.7. Room Temperature Curing
Silicone Rubbers
Silicone RTV (room temperature vulcanizing)
systems are classified according to their cure
mechanism (hydrosilylation or condensation
cure). One- or two- component systems have
been developed for different applications. Twocomponent products, which must be mixed before application, are available with either curing
mechanism. One-component RTV compounds
are based almost exclusively on a condensation
mechanism. These systems cure upon exposure
to atmospheric moisture.
Typical RTV silicone products contain polymers with a polydimethylsiloxane backbone
Vol. 32
[90]. One- component systems, composed of organic polymers with silane end groups [91], are
not discussed here.
Water-based silicone sealants are unique onecomponent systems that consist of an aqueous
emulsion of a silicone polymer, fillers, an organotin condensation catalyst, and an anionic surfactant. On application, the water evaporates to
give a material that is cross-linked mainly by the
filler [92], [93].
4.7.1. Two-Component RTV Systems
Two- component RTV systems are used as pourable potting or casting compounds.
Condensation-Cured Systems.
Composition. The base component contains a
hydroxyl-end-capped polydimethylsiloxane (M ca.
104 – 105), which may also contain nonreinforcing
fillers, unreactive silicone fluid and pigments, as
well as small amounts of water and other additives.
The second component (cross-linker), which
is sensitive to hydrolysis, contains the crosslinking agent (generally an alkoxysilane) and a
catalyst (often the condensation product of an
alkoxysilane with a dialkyltin compound).
Processing. The two components are generally used in ratios between 100 : 1 and 10 : 1
(base component to cross-linker). Automatic
mixing and dispensing equipment requires a ratio
of 10: 1 for reliable operation.
Curing begins shortly after the two components are mixed and is usually complete within
several hours to one day at room temperature.
Alcohol is generally eliminated during curing
and must completely evaporate from the cured
elastomer to prevent reversion. The rate of crosslinking and evaporation of the alcohol can be
accelerated by raising the temperature.
Hydrosilylation-Cured Systems.
Composition. Hydrosilylation- cured systems consist of two components which are often
mixed in the ratio 1: 1. One component contains
a polydimethylsiloxane with vinyl end groups,
optional fillers, pigments, and the platinum catalyst. The second component contains the cross-
Vol. 32
linking agent, which is either a polymethylhydrogensiloxane or a copolymer thereof with a
polydimethylsiloxane. This component can also
contain fillers and a vinyl- containing siloxane.
Processing. Curing begins immediately after mixing the two components. The reaction can
be retarded, and hence the pot life increased, by
adding low molecular mass cross-linkable components or inhibitors.
4.7.2. One-Component RTV Systems
Uses. One- component RTV silicone compounds are mainly used as sealants in building
construction owing to their good stability to
weathering [94], [95]. The pastes are formulated
to be nonsagging and easily pumpable, so that
they can be readily applied. The pastes are packaged in plastic cartridges or, increasingly, tubular
plastic bags, which prevent the ingress of atmospheric moisture prior to use.
Numerous industrial applications also exist in
which one- component RTV compounds are used
as sealants, adhesives, and coatings. The properties of the product are adjusted to suit the application, ranging from easily pourable to highly
viscous or rigid compounds, and from low-modulus rubbers to adhesives with high tear strength.
Examples include automobile sealants [96], textile coatings, production of insulating glass, contruction adhesives (e.g., for cladding panels),
sealing of furnaces, and applications in electrical
and electronic engineering [89].
Composition. The base polymers are polydimethylsiloxanes with terminal hydroxyl groups
and a viscosity of 1 – 500 Pa s, preferably 10 –
150 Pa s. For building construction, where soft
vulcanizates are mostly required, nonfunctional
polydimethylsiloxane fluids are added as plasticizers. The rigidity is adjusted by adding 7 –
15 wt % pyrogenic silica. Silica of low surface
area is preferred (130 – 150 m2/g). Pigmented
compounds often contain inert fillers in addition
to silica to lower the cost; ground natural chalk,
treated with stearic acid, is used preferentially.
The silane cross-linking agents are used in
quantities of 3 – 6 wt %. The amount of crosslinking agent must exceed the quantity required to
react with the hydroxyl groups of the raw materi-
Silicones
695
als. Neutral systems generally contain an organofunctional silane as coupling agent (e.g., aminopropyltrialkoxysilanes). Dialkyltin carboxylates or chelate complexes of Ti(IV), preferably
with acetylacetone or esters of acetylacetic acid,
are used as catalysts. Tin compounds are generally employed in amounts of < 0.5 wt %, titanium compounds are used in higher quantities [97].
Building sealants contain inorganic pigments
as additional auxiliaries for coloring and biocides
to prevent the growth of mold in damp rooms.
Processing. The material is applied by direct
injection into the joint from the cartridge or tube
by using manual or compressed-air pistols. After a
few minutes, an elastic skin forms on the surface
under the action of atmospheric moisture. Curing
proceeds from the surface to the inside and, with
usual joint dimensions, is complete after at most a
few days. The rate of curing depends not only on
the ambient temperature and humidity, but also on
the type of sealant. Silicone sealants generally
exhibit high flexibility and good adhesion to a
wide range of substrates. Use of a primer may be
necessary for porous substrates and in the case of
insufficient adhesion.
Whereas the classical acetate systems dominate in the sanitary sector, neutral systems are
being used in increasing amounts, for example,
as window sealants. Low-odor, noncorrosive
systems can be expected to further increase in
importance.
General guidelines for processing of silicones
for industrial fabrication cannot be given because
of the wide range of applications.
4.8. Paper and Textile Coatings
Silicone elastomers are used in a variety of
coating applications. The most important substrates are paper and textiles. The choice of
polymer and curing system, the formulation and
the coating method depend on the substrate and
the desired properties.
4.8.1. Paper Coating
Paper and films are often coated with very thin
layers of silicone to make them repellent to
adhesive substances.
696
Silicones
Paper coating plants operate at high speed (up to
500 m/min), and thus require silicone coating
systems that cure rapidly while maintaining a
sufficient pot life. Silicone coating materials are
two- or multicomponent systems that cure at substrate temperatures of 100 – 130 C in 5 – 15 s.
The coating mixture must exhibit good wetability, both for the rolls and for the substrate
surface, to give pore-free films. Completely cured
coatings contain no migrating constituents that
can impair the adhesive properties of a subsequently applied adhesive layer. The adhesive is
applied directly (on-line process) or in a later
manufacturing step. The adhesive forces between
the adhesive layer and the silicone coating must
remain constant over fairly long periods of storage. This is especially important when controlledrelease additives are used to increase adhesion
between the silicone and adhesive layers.
At present, the following silicone coating
systems are used:
Solvent-Containing Systems.
Condensation Systems (I) consist of hydroxyl-end- capped polydimethylsiloxanes and polymethylhydrogensiloxanes as cross-linkers. The
cross-linking reaction is catalyzed by organotin
compounds:
The system is used in dilute form (3 – 5 % in
organic solvents). In emulsion form it is diluted
to about 10 % with water before use.
Condensation Systems (II) contain reactive
silanes instead of polymethylhydrogensiloxane.
Their curing mechanism is the same as for the
moisture- cured RTV compounds (see Section 4.4.3). Systems I and II have been virtually
displaced from the market by hydrosilylationcured systems because of disadvantages such as
sensitivity to variations in atmospheric humidity
and slow cure even at high curing temperature
(20 – 40 s at 120 – 180 C).
Hydrosilylation- Cured Systems consist of
higher viscosity polydimethylsiloxanes with reactive vinyl groups and polymethylhydrogensiloxanes as cross-linkers. They cure in the presence of platinum complexes (30 – 120 ppm Pt)
Vol. 32
in 15 – 20 s at 120 – 140 C (for curing mechanism see Section 4.4.2). They are applied as
solutions in organic solvents or as emulsions.
With sufficient dilution it is possible to prepare
very thin films. The solvent is removed by evaporation during cross-linking and is recovered or
burned.
Solvent-Free Systems.
Heat-Cured Systems are also based on the
principle of hydrosilylation curing and differ from
the corresponding solvent- containing systems by
having shorter- chain vinyl- containing polymethylsiloxanes with a viscosity of 200 – 500 mPa s. In
contrast to solvent- containing systems, the thickness of the solvent-free coating does not change on
curing. Therefore, stringent requirements must be
met by both the application equipment and the
substrates to obtain pore-free films with reproducible coating thicknesses of ca. 1 mm (ca. 1 g/m2)
silicone. Solvent-free systems cure in 5 – 15 s at
substrate temperatures of 100 – 120 C. About
50 % of all substrates are coated with solvent-free
systems, and the proportion is increasing.
Photochemically Cured Systems are not well
established in paper coating, in spite of the
advantage provided by minimal thermal treatment of the substrates. This method is, however,
used for special coating applications [98]. The
cross-linking mechanisms are varied, but most
systems use UV-absorbing sensitizers to increase
the quantum yield.
Radiation-Cured Systems use high-energy
radiation (e.g., electron beams) and do not require sensitizers, but they do require a nitrogen
atmosphere to avoid formation of interfering
oxygen radicals.
Silicone- coated papers and films are assessed
by the following criteria:
Curing of the coating
Abrasion resistance (no rub-off)
Adhesion to the substrate
Porosity of the coating
Magnitude of the release force values
Constancy of the release force values
Residual adhesive forces
Coating strength
Film smoothness
Vol. 32
Fields of application for the abhesive coated
papers are [100]:
Self-adhesive labels (74 %)
Adhesive tapes (10 %)
Hygienic articles (6 %)
Construction, packaging, decoration, etc. (10 %)
4.8.2. Textile Coating
Textiles can be rendered water-repellent with
special silicone fluids that simultaneously provide the fabric with a soft feel. Elastomer systems
that are similar in composition to those described
for paper coating or for two- component and LSR
silicone rubbers are used for waterproof textile
coatings that are permeable to water vapor.
4.9. Properties of Silicone Elastomers
The typical properties of silicone polymers are also
exhibited by cured silica-reinforced products.
They account for the many applications of silicone
elastomers. Different combinations of properties
can be obtained by choice of polymer, cross-linking system, and compound composition.
Temperature Limits for Silicone Elastomers. Silicone elastomers have the widest operating temperature range of commercially important rubbers. VMQ rubbers exhibit better
relaxation properties than other synthetic rubbers, i.e., the compression set has a smaller
temperature dependence in the range 40 to
170 C (Fig. 10) [99].
Figure 10. Compression set versus temperature for some
elastomers
Silicones
697
Mechanical properties such as tensile strength
and tear strength also show only a small temperature dependence between 40 and 150 C. The
tensile strength of VMQ is strongly enhanced at
40 C.
This general behavior must be distinguished
from heat stability or changes in the room temperature properties that occur at elevated temperature. The heat stability of elastomers is evaluated by measuring the retained properties after
high-temperature ageing (see hot-air resistance).
A phenyl content of 5 – 12 mol % extends the
low-temperature flexibility (measured as torsional stiffness, DIN 53 548) from 45 to 70 C
and the brittleness point from 55 to 105 C
(ASTM D 380) [88].
Phenyl groups improve the resistance to g rays
by a factor of 2 – 5.
Hot-Air and Oxidation Resistance. The
high-temperature and oxidation resistance of
pure cross-linked polydimethylsiloxane polymers is adequate only up to 200 C. Above this
temperature, oxidation reactions at the alkylene
cross-links and methyl groups cause embrittlement. Oxidation can be inhibited by using hot-air
stabilizers (see Section 4.2). Further improvement is achieved by limiting the content of acidic
Si – OH groups in the silica filler.
Since even low contents of acidic and alkaline
impurities lead to depolymerization (reversion)
of the network, the polymers must be thoroughly
neutralized. Stabilizers that neutralize acidic decomposition products from cross-linking agents,
for example, are added optionally. Elastomers
stabilized in this way withstand exposure to hot
air at 300 C for up to 21 d. After an induction
period, embrittlement increases logarithmically
with time.
Steam, Chemicals and Oil. Despite the
sensitivity of the Si – O bond to acids, bases,
and to water at elevated temperature, the stability
of silicone elastomers can be significantly improved by increasing the cross-linking density
and by using additives [52], [101].
Patents describe the use of combinations of
CaO and silanes containing methacrylic groups
to improve the oil resistance [102]. Additives
such as mica or diatomaceous earth have a similar effect [103], [104]. The swelling rate can be
appreciably reduced by using fairly large
698
Silicones
amounts of vinyl resins [105]. Stabilized VMQ
rubbers withstand immersion in ASTM 2 oil for
up to 2000 h and in ASTM 3 oil for up to 500 h.
Swelling rate depends on the solubility parameters c of the siloxane and the contact medium.
Similar c values result in higher swelling rate
[106]. Additional degradation by acids or bases
occurs in technical applications. Silicone elastomers with enhanced oil resistance usually have a
high filler content and thus a reduced siloxane
and stabilizer content.
A large decrease in the swelling rate can be
achieved by changing the solubility parameter of
the polymer (e.g., by using fluoroalkyl-modified
siloxanes) [89].
Another method to improve the oil resistance
is to blend silicone elastomers with solvent-resistant rubbers such as acrylonitrile or fluorinated
rubbers [107–110]. These blends can exhibit
improved low-temperature and swelling behavior. Interest in blends has increased with the
introduction of new peroxide- curable fluorinated rubbers [107], [111].
Reversion – Compression Set. Reversion
and increased compression set are often induced
by the action of water vapor and acid or base. The
resulting breakdown of the polymer network can
be counteracted by polymer neutralization and by
using the maximum possible cross-link density.
Compression set can also be reduced by using
neutralizing additives or by the addition of
methylhydrogensiloxanes [52], [101], [112].
High-Strength Formulations. The roomtemperature tensile strength of silicone elastomers of 6 – 12 MPa is lower than that of other
common elastomers (10 – 30 MPa). The tear
strength is also only moderate (10 – 30 N/mm,
compared to 20 – 80 N/mm), according to
ASTM 624 D die B. Much effort has been targeted at improving the tear strength of silicone
elastomers [113–119]. The tear propagation
strength can be increased to 40 – 55 N/mm by
optimizing the polymer structure.
Adhesion. Silicone elastomers are often
used in composite structures with other materials. Here, adhesion of the elastomer to the other
material is particularly important.
Owing to their cross-linking mechanism, condensation cross-linking systems show good ad-
Vol. 32
hesion to surfaces with hydroxyl groups. The
hydroxyl groups on the surface react with the
SiOH or SiOR groups of the polymer. Good
adhesion is observed to glass and to metals with
oxide or hydrated surfaces, such as aluminum
and iron. Condensation systems with and without
solvent are also used as coupling agents or adhesives for other silicone elastomers.
For good adhesion peroxide- and hydrosilylation- cured rubbers frequently require primers
that are applied as coupling agents to the surface
of the material. With the proper choice of primer,
good adhesion can even be obtained on many
plastic surfaces [120–123].
Electrical Properties. Siloxanes and their
mixtures with pyrogenic silica have a very good
electrical insulating capacity. The electrical
properties are temperature dependent and are
strongly affected by exposure to water. Accordingly, the resistivity of 1016 W cm under dry
conditions at 20 C decreases to 1012 W cm at
160 C and 50 % R.H. [32].
The use of precipitated instead of pyrogenic
silica leads to a decrease in resistivity and, because of the higher water content of the filler, to
an increase in dielectric constant. The addition of
carbon blacks [124], sometimes in combination
with conducting powders or fibers [125], [126],
can lower the resistivity to 3 W cm (ISO 1853).
Thermal Properties. The thermal conductivity of silicone elastomers can be increased by
large amounts of inert fillers such as quartz, AlN,
Al2O3, Si3N4, or MgO [41], [43], [127], [128].
Burning Properties. The heat evolution
from siloxanes on burning is lower and accordingly more favorable than that of pure organic
elastomers [129], [130]. A residual electrical
insulating capacity is maintained, even after burning, as a result of the remaining silica framework.
Flame retardants can effectively suppress the
further combustion of silicones after ignition. In
this way, transparent or colored elastomers can
be produced that pass flammability tests such as
UL 94 V1 or V0 [59]. The combustion gases
formed are less toxic than those from halogen- or
nitrogen-containing elastomers [129–132].
Gas Permeability. Silicone elastomers dissolve many gases well and are highly permeable.
Vol. 32
Silicones
699
The high gas permeability [133] is somewhat
selective and depends on the substituents
[134–137]. Silicone elastomers are therefore
suitable for the production of gas-separation
membranes [138] and for oxygen-permeable
contact lenses or dressings for wounds
[139–141].
Examples of uses of silicone elastomers are
listed below:
Transparency. The extremely small primary particles of well-dispersed high surface area
silica fillers in silicone elastomers cause only
minor light scattering. With a UV transmission
(DIN 5033) of up to 90 %, they are almost
transparent [46] and are therefore used as contact
and other lenses [140]. This high transparency is
achieved by special hydrophobic treatment of the
filler with strong silylating agents [142]. A further enhancement of the transparency is possible
by incorporating phenyl groups into the polymer,
which shifts the refractive index n20
D from 1.39 to
1.43 – 1.47 [143–145]. Recently, more rigid silicone – acrylate copolymers have been preferred
for contact lenses [146].
Electronic
anode caps
coatings
encapsulation
fiber-optic coatings
Physiological Behavior. Because silicone
elastomers exhibit low surface tension and are
abhesive, interactions with surfaces of living
tissues, cells, or blood platelets are very weak
[146–148]. Therefore, filler-free elastomers, in
particular, are suitable for implant applications.
Numerous surgical and medical applications
exist. Another advantage of silicone elastomers
is that they can be sterilized in steam above
100 C.
4.10. Applications
The typical properties of silicone elastomers lead
to their application in areas involving:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Electrical insulation or conduction
High and/or low temperatures
Weather and UV exposure
Oil and hot-air contact
Dynamic stress
Flame resistance, arcing resistance
Gas permeability
Contact with foods
Contact with living tissue
Abhesive surfaces
Transparent articles of high optical quality
Electrical
wire insulation
cable sheaths
power cable endcaps
Household
pot seals
coffeemaker tubes and seals
baby bottle nipples
oven gaskets
anti-stick papers
Automobile
gaskets
cable guides and connectors
headlight seals
shaft seals
O-rings
cooling and turbo hoses
oil pan gaskets
spark plug boots
air bags
ignition cables
fuel line diaphragmas and seals
exhaust pipe hangers
axle boots
Airplane
headlight seals
window seals
interior joints and floors
vibrational damping
Office machines
keyboard pads
copier rolls
Medicine and dentistry
tooth impression compounds
implants
pumps
heart valve seals
catheters
700
Silicones
membranes
lenses
controlled drug release
Paper and Textiles
coatings
conveyor belts
General
casting forms
membranes
impression molding compounds
anti-stick bladders
divers’ masks
protective masks
5. Silicone Resins
5.1. Structure and General Properties
Alongside fluids and elastomers, silicone resins
are the third important class of materials produced by hydrolysis or alcoholysis of organochlorosilanes [149], [150].
Whereas silicone fluids and elastomers are
based on linear polymers, silicone resins are
highly branched, containing significant quantities
of trifunctional (T) or tetrafunctional (Q) units.
By combination of T and Q units with difunctional (D) and monofunctional (M) units, a large
number of different resin structures can be
formed, in which the proportion of the various
silicone units can vary over a wide range.
The properties of silicone resins are strongly
influenced by the organic groups bonded to silicon. The most frequently used monomers are
methyltrichlorosilane, phenyltrichlorosilane, dimethyldichlorosilane, and diphenyldichlorosilane. Pure T resins are relatively brittle and
exhibit little thermoplasticity. The hardness can
be decreased by adding D and M units. An
increased dimethylsiloxane content, in particular, leads to products of increased elasticity and
adhesion, but also increased tendency to yellow
at high temperature. With increasing temperature, the hardness of the resins decreases. This
effect is stronger when diphenyldichlorosilane is
used. The introduction of phenyl groups has a
plasticizing effect. Simultaneously, the resistance of the polyorganosiloxanes to heat and
weathering is increased, and their compatibility
Vol. 32
with organic polymers, their pigmentability, and
their yellowing resistance are improved. Products with increased alkali resistance required
for masonry protection are obtained by using
special silicone resins.
In addition to pure silicone resins, combination resins also play an important role [151],
[152].
Combination resins are produced by cocondensation of silicone resins having hydroxyl or
alkoxy groups with functionalized organic resins
such as polyester, alkyd, acrylic, or epoxy resins.
Compatibility of the resins is improved by using
methylphenylsilicones.
5.2. Production
Silicone resins and their precursors are generally
produced from mixtures of the corresponding
organochlorosilanes by hydrolysis or alcoholysis. From a given mixture, completely different
cross-linked structures with markedly different
properties can be obtained, depending on process
conditions. The simplest case – addition of the
silane mixture to the hydrolysis medium – is
known as direct hydrolysis. Since the chlorosilanes are immiscible with water and the individual silanes hydrolyze at different rates, this can
lead to nonuniform products and, in the worst
case, gelation.
In reverse hydrolysis, in which the hydrolyzing medium (usually an alcohol – water mixture) is added to the silane mixture, a more
uniform cohydrolysis is achieved and gelling is
largely avoided. However, low-boiling starting
materials may be lost, together with escaping
hydrogen chloride.
Both processes can be improved by adding
solvents. Solvents immiscible with water, such as
toluene, xylene, or cyclohexane, lower the silane
concentration and the concentration of reactive
hydrolysis condensate at the interface, and repress the influence of hydrochloric acid, which
promotes condensation. The process can be further improved by using solvents miscible with
both water and silane (two-phase solvents), such
as acetone. A short- chain alcohol such as butanol
is often used to lower the reactivity of the system
by forming alkoxysilanes. A drawback of the
two-phase solvent is the relatively high concentration of organic substances in the wastewater.
Vol. 32
Silicone resins are commonly bodied following hydrolysis by heating in the presence of mild
condensation catalysts. This step is used to control the viscosity and cure properties of the resin.
In this reaction the functional groups condense
according to
Si OHþHO Si
!
Si O Si
þH2 O
or
Silicones
701
degradation occurs and is accompanied by curing, for example,
This curing mechanism is exploited in the use
of silicone resins as binders in the paint industry,
for example, in high-temperature anticorrosion
coatings, release coatings, or decorative coatings
for kitchenware [156].
Si OHþRO Si
!
Si O Si
þROH
The condensation process is frequently accompanied by viscosity monitoring and terminated by cooling, neutralization, or dilution when
the desired properties are attained.
The nature and concentration of the remaining
reactive groups and the molecular mass distribution determine many of the further processing
and product properties.
Combination resins are formed by reaction of
siloxane precursors with hydroxy-functional organic esters (e.g., polyesters), alkyd resins, acrylates, or epoxy resins
Si OHþHO C
!
Si O C
þH2 O
and
Si OHþRO C
!
Si O C
þROH
The hydrolysis of chlorosilanes with subsequent condensation is irreversible. Alcoholysis
products can be improved by an optional acid- or
base- catalyzed equilibration stage. Although silicone resins can be produced continuously [145],
[153–155], many products with relatively low
demand are produced in batch processes.
5.4. Properties
The characteristic properties of silicone resins are
their high resistance to temperature and weathering. The weather resistance includes UV and
oxidative stability, as well as stability to aqueous
acids, fats, and oils. The behavior toward fats and
oils also enables the use of silicone resins as
release coatings. Silicone resins are water repellent and, like all silicone polymers, permeable to
gases. They also show good wetting properties on
many inorganic and organic substrates.
After curing, silicone resins exhibit good adhesion to most surfaces due to chemical or physical
bonding to the surface. They are electrically nonconductive, and their electrical and mechanical
properties show little dependence on temperature.
The susceptibility to oxidation, especially of the
pure methylsilicone resins, is low. Decorative and
gloss-retaining effects, as well as elasticity and
pigment compatibility, increase with increasing
phenyl content, but the hardness decreases simultaneously. By varying the organic substituents of the
resin, desirable resin properties can be adjusted
selectively. Provided certain preconditions are met,
silicone resins are physiologically inert [157], [158].
5.3. Curing
5.5. Applications
Silicone resins can be cured by the same reaction
mechanisms as silicone elastomers. The most
common method of curing silicone resins is
metal- catalyzed condensation, preferably with
complexes of tin or titanium. Resins containing
vinyl groups can be cross-linked by peroxides.
The simultaneous presence of silicon-bonded
hydrogen and of vinyl groups enables curing by
noble-metal catalyzed hydrosilylation (see Section 4.2). Radiation-induced curing reactions are
also possible. Above 150 C in air, oxidative
Silicone resins have a wide range of applications,
the most important being paint binders and masonry protection. In the electrical industry, silicone resins are used as binders for compression
molding compounds and laminates (glass fabric,
mica), as well as for impregnating resins and
insulating varnishes. Electronic chips and other
components are protected from moisture, dust,
and chemicals by silicone-resin-based coatings.
Unlike conventional coating materials such as
702
Silicones
phenolic or epoxy resins, silicones exhibit both
water repellency and good heat resistance. For
high-temperature applications [155], methylsilicone resins are generally used. Siliconeresin-bonded mica laminates are widely used as
supporting elements for resistance heaters. In
anticorrosion paints, the resins are pigmented
with aluminum, zinc dust, or micaceous iron
oxide. In decorative coatings, where paint properties such as pigmentability, elasticity, adhesion,
and gloss are the overriding concern, methylphenylsilicone resins are generally used. The required temperature resistance of between 250 C
(household appliances) and 400 C (ovens) can
be met. Silicone resins are also used as release
coatings (e.g., for baking pans). Silicone resins
can be used to protect buildings against graffiti.
Heat-resistant stoving paints, such as those used
for the decorative coating of household appliances
or for coating metal cladding panels by the coilcoating process [159], frequently consist of silicone – polyester combination resins. The silicone
resin gives increased resistance to weathering and
chalking, especially in the coil- coating field.
Special silicone resins are used as paint additives
to improve specific paint properties [160]. Silicone
resins are used in masonry protection. Porous inorganic building materials are rendered water repellent by impregnation with silicone resin. This also
increases the resistance to chemicals while maintaining gas and water vapor permeability.
The effectiveness of silicone resins in the
protection of building materials depends on the
depth of penetration into the substrate. To meet
the demand for solvent-free products, silicone
emulsions and microemulsions are available
[161], [162]. Microemulsions contain up to
20 wt % solids and have particle sizes of 10 –
80 nm; they have good penetrating power for
most building materials. Silicone resin microemulsions are diluted with water before use.
Other applications of silicone resins are as
scratchproof coatings for glass and plastics and in
polishes. QM resins are commonly used for coatings and in elastomer reinforcement.
6. Block and Graft Copolymers
Polysiloxanes (A) and organic polymers (B) can
be chemically linked to give block or graft copolymers. Block copolymers with linear AB,
Vol. 32
ABA, or (AB)n structures can be prepared selectively. Grafting reactions lead to statistically
distributed branches on the base polymer.
6.1. Polysiloxane – Polyether
Copolymers
The most important siloxane- containing copolymers are the polysiloxane – polyether copolymers [163–170]. Both linear and branched block
copolymer structures are possible. Branched copolymers consisting of a polysiloxane core and
linear polyether side chains are of particular
importance. The polyether segments can be polyoxyethylene, polyoxypropylene, or poly(oxyethylene – oxypropylene) units (see ! Polyoxyalkylenes). The copolymers can have SiOC
as well as SiC linkages between the polysiloxane and polyether segments. Copolymers with
Si – O – C linkages are produced by reaction of
siloxanes having reactive end groups with polyether alcohols, for example,
Si XþHOðCH2 CH2 OÞn R!
Si O
ðCH2 CH2 OÞn RþHX
X ¼ Cl, OR, OAc, NR2, H
The Si – C linked copolymers are generally
produced by platinum- catalyzed hydrosilylation
of polyethers having unsaturated end groups with
SiH- containing siloxanes
SiHþCH2 ¼ CHCH2 OðCH2 CH2 OÞn R!
SiCH2 CH2 CH2 OðCH2 CH2 OÞn R
R¼ H, alkyl
Polysiloxane – polyether copolymers have surfactant properties. The surface tension of aqueous
solutions of these copolymers can be as low as
20 mN/m (see Table 9) [169], which is 5 –
8 mN/m lower than comparable organic surfactants. The chain length and structure (linear or
branched) of the siloxane component have a
strong influence on surface tension, whereas the
chain length of the polyether segment has only a
minor effect.
Unlike most organic surfactants, polysiloxane – polyether copolymers can also lower the
surface tension of organic liquids [169], [170].
Polysiloxane – polyether copolymers are
used in large quantities as foam stabilizers in the
production of polyurethane foams [164–170].
Vol. 32
Table 9. Surface tension of aqueous solutions of polysiloxane –
polyether copolymers
In this application, the role of the copolymer includes not only lowering the surface energy to
promote nucleation, buildup, and stabilization of
the foam cells, but also emulsification of the polymer and blowing agent. Branched copolymers with
poly(oxyethylene – oxypropylene) segments have
a particularly good combination of these properties.
The foam-stabilizing action of these copolymers
can also be applied to phenolic resin foams [171].
6.2. Other Block Copolymers
Siloxane- containing block copolymers are
known with a variety of organic building blocks,
such as polycarbonate [172–175], polyurea
[176], polyimide [177], polyester [178], polyurethane [179], [180], polyarylether [181], [182],
polylactam [183], polystyrene [184–191], polybutadiene [192], and carbodiimide [193].
Suitable synthetic routes involve terminally
reactive polysiloxanes, which either are present
during polymerization of the organic monomer
[172]:
Silicones
703
or react with stoichiometric amounts of functional organic polymer blocks [178]:
Block copolymers can be prepared from some
organic monomers by means of living anionic
polymerization (e.g., styrene, a-methylstyrene,
and butadiene). In this process, the organic monomer is polymerized anionically; then the stillactive ends of the polymer are used to polymerize
hexamethylcyclotrisiloxane under nonequilibrating conditions [163], [184–189]. By varying this
process, AB, ABA, or (AB)n block copolymers
can be produced selectively. Polysiloxanes with
bis(silylpinacolate) groups initiate radical polymerization of certain organic monomers to give
block copolymers [190], [191].
704
Silicones
Silarylenesiloxane – siloxane block copolymers with different silylarylene groups and various organic substituents on the siloxane segments
are known:
Vol. 32
can then behave as reinforced elastomers. If
hydroxyl-end- capped polysiloxanes are grafted
with thermoplastics, copolymers are obtained
that can be formulated with condensation
cross-linking agents to give one- or two- component RTV systems. These elastomers can have
good mechanical properties and have various
applications as embedding materials and coating
compounds [207], [208].
6.4. Applications of Block and Graft
Copolymers
The silarylene segments readily form crystalline phases, which give the copolymers elastomeric properties. Copolymers have been synthesized with good mechanical properties, stability
to oxidation at elevated temperature, and good
chemical resistance [194–197].
The polysiloxane and polyorgano segments of
block copolymers are usually incompatible.
Most block copolymers therefore have a detectable two-phase morphology. For example, in
dynamic – mechanical studies on polysiloxane – polystyrene (AB)n copolymers, two glass
transition temperatures were detected, one at
110 C for the polysiloxane phase and a second
at þ90 C for the polystyrene phase [189].
Block copolymers with thermoplastic segments can behave as thermoplastic elastomers
(TPE). The thermoplastic phase causes physical
cross-linking of the elastic polysiloxane segments [198], [199]. The morphology and mechanical properties of the TPEs depend critically
on the content of thermoplastic and the chain
length of the siloxane segments. The mechanical
properties obtained are in some cases superior to
those of conventional filled silicone elastomers.
6.3. Graft Copolymers
The radical copolymerization of polysiloxanes
with various vinyl monomers (e.g., styrene, acrylates, or vinyl acetate) leads to multiphase graft
copolymers [200–206]. Polysiloxanes with reactive functional groups on the siloxane chain, such
as mercaptopropyl and vinyl groups, can also be
grafted onto organic polymers with reactive
groups.
Graft copolymers can stabilize dispersions of
organic thermoplastics in polysiloxanes, which
Polysiloxane – polyether copolymers are important surfactants that are used not only as foam
stabilizers but also as leveling agents in paints,
release aids, textile auxiliaries, and additives for
cosmetics and polishing agents. Siloxane- containing copolymers have various applications as
modifiers for thermoplastics. They improve the
impact resistance [209] and surface properties
[210], and act as flame retardants [211]. The
stability of polysiloxanes toward oxidation at
elevated temperature and the good low-temperature flexibility are advantageous in these applications. Because of the high oxygen permeability
of polysiloxane- containing copolymers, they
can be used for making contact lenses. The high
gas permeability of polysiloxanes leads to applications as gas-separation membranes. Their use
in numerous biomedical applications such as
dialysis membranes is described in [141].
7. Analysis
The wide range of chemical and physical methods available for analysis of silicones is outlined
below. A comprehensive updated description of
these chemical analyses, spectroscopic, chromatographic, X-ray, and microscopic methods,
together with ca. 1400 references to the original
literature, can be found in a standard work on
silicone analysis [212].
Silicon Content. In many cases, determination of the silicon content is sufficient to characterize the silicone content of the substance to be
analyzed. Gravimetric, spectrophotometric, and
atomic spectroscopic methods are available. To
an increasing extent, however, species-specific
Vol. 32
analysis is required. This is performed by gas
chromatography or by combining separation
methods with spectroscopic determination.
Determination of Total Silicon Content by
Gravimetric and Spectrophotometric Methods. The standard gravimetric method for determining total silicon involves fusing the sample
with Na2O2 in a nickel bomb. The fusion cake is
treated with water, and silicon is separated as SiO2
by repeated fuming with hydrochloric acid. The
final determination is performed by differential
weighing before and after removal of the silicon
as SiF4 by fuming with HF – H2SO4 [213].
Another established gravimetric method is to
convert the silicon in the fusion solution to
molybdosilicate, to precipitate the latter with
organic nitrogen compounds (e.g., pyridine
hydrochloride), and to heat the precipitate to
give the stoichiometrically defined mixed oxide
SiO2 12MoO3. Because of the favorable weight
increase, this method is particularly suitable for
determining low silicon contents [214].
Very low silicon contents are determined
spectrophotometrically after conversion to molybdosilicate. Both the yellow coloration at
390 nm and the blue coloration at 815 nm obtained after reduction with ascorbic acid or 1amino-2-naphthol-4-sulfonic acid are suitable
for this purpose [215].
Determination of Total Silicon Content by
Atomic Spectroscopic Methods. Organosilicon
compounds can be determined in organic solvents by atomic absorption spectrometry in a
nitrous oxide – acetylene flame or by atomic
emission spectrometry in an argon plasma. The
most suitable solvents are methyl isobutyl ketone,
toluene, and xylene. These methods are readily
applicable only to nonvolatile organosilicon compounds. Volatile compounds give variable signals, which causes problems in calibration.
Determination of Functional Groups.
Si – Cl. The Si – Cl groups are hydrolyzed
to Cl, which is titrated argentometrically. Potentiometric end-point detection is recommended, with a chloride-selective or silver electrode as indicator electrode [216]. With low chloride concentrations, the titration should be performed in acetic acid – water mixtures (ca. 4: 1).
Silicones
705
Si – H. The Si – H bond is reacted with
excess oxidizing agent (e.g., HgCl2 [217], bromine [218], or bromosuccinimide [219] ), and the
excess oxidizing agent is determined iodometrically. The solvents used are di-sec-butyl ether,
carbon tetrachloride, acetic acid, or mixtures
thereof. The Si – H bond can also be determined
gas volumetrically after decomposition by alkali.
n-Butanol is suitable as solvent and sodium butoxide as decomposition reagent.
Si – OH is determined gas volumetrically,
the method being based on the formation of
methane in the Zerewitinoff reaction with
methylmagnesium chloride or on the formation
of hydrogen in the reaction with lithium aluminum hydride [216].
Si – vinyl is determined gas chromatographically as ethylene after alkaline cleavage of the
Si – C bond [220]. Iodometric titration of the
Si – vinyl bond is possible after reaction with
excess bromine solution in a 1: 1 acetic acid –
carbon tetrachloride mixture. In the case of polymers, HgCl2 may be required as catalyst. The
method is not applicable in the presence of
compounds containing other unsaturated groups
or SiH groups [221].
Infrared Spectroscopy. The effectiveness
of IR spectroscopy in qualitative and quantitative
analysis has been increased markedly by the use
of computer-interfaced FT-IR spectrometers. It
is a rapid and easily used method for qualitative
characterization, even of mixtures. The very
specific IR bands of silicones allow their detection in a wide variety of preparations. Infrared
spectroscopy thus complements other chemical
and physical methods. Aside from the usual
absorption techniques, ATR (attenuated total
reflection) and FMIR (frustrated multiple internal reflection) are important for the investigation
of cross-linked, insoluble silicones (rubbers,
coatings).
Quantitative analysis can also be conveniently
performed by using the specific IR bands of
silicones. The intense Si – CH3 band at
1258 cm1 is suitable for determination of the
silicone content in the principal and trace ranges.
The Si – OH content is determined from the
intensity of the free SiOH band at 3685 cm1 in
dilute CCl4 solutions by differential spectroscopy
706
Silicones
Vol. 32
Table 10. Important IR bands of silicones
Group
Wave number, cm1, and intensity* of band
SiOH
3685
3200 – 3500
(m)
(s)
SiH
SiCH3
SiOSi
2100
1250
860
1120
1590
1130
(s)
(s)
(s)
(s)
(m)
(s)
SiCH¼CH2
1400
SiC6H5
*
– 2300
– 1280
– 750
– 1000
free silanol groups
associated silanol
groups
broad, possibly
several maxima
(s)
s ¼ strong; m ¼ medium.
Table 12. Typical 29Si NMR shifts in silicone polymers (relative to
SiMe4, solvent CDCl3)
Si (CH3)3
Si (CH3)2(H)
Si (CH3)2(CH¼CH2)
Si (CH3)2(OH)
Si (CH3)2(OCH3)
¼Si (CH3)2
¼Si (CH3)(H)
¼Si (C6H5)2
¼Si (CH3)(OCH3)
Si (CH3)
Si (C6H5)
¼Si¼
*
against an SiOH-free siloxane [222]. Another
method uses the Si – OD bands after deuteration
[223].
Table 10 lists some particularly prominent IR
bands of silicones. Further details can be found in
[212].
Nuclear Magnetic Resonance Spectroscopy. Of the three possible methods, 1H, 13C,
and 29Si NMR spectroscopy, 1H and 29Si are
preferred for the analysis of silicone polymers.
The recording of a 1H NMR spectrum requires
3 – 10 min, and that of a 29Si NMR spectrum 3 –
10 h. The 13C spectrum usually yields no information beyond that of 1H NMR, while requiring a
longer acquisition time.
1
H NMR. The typical chemical shifts of substituents bonded to silicon permit both qualitative
identification and quantitative determination
of the structural groups in silicone polymers.
Table 11 lists typical shift values.
Use of 29Si NMR spectroscopy provides detailed information on the silicone backbone (see
also Table 12). It enables:
Table 11. Typical 1H NMR chemical shifts of silicone polymers
(relative to SiMe4, solvent CDCl3)
Group
Chemical shift, ppm
SiCH3
SiCH2
SiOCH3
SiH
SiCH¼CH2
SiC6H5
0.0
0.5
3.5
4.7
5.8 – 6.2
7.3 (intensity 3); 7.6 (intensity 2)
Chemical
shift*, ppm
Group
(M)
(D)
(T)
(Q)
þ 8
5
5
10
12
20
35
45
56
65
78
100
Typical values when all other substituents of Si are attached via
OSi.
1. Quantification of the proportions of M, D, T,
and Q units in the polymer
2. Qualitative and quantitative analysis of neighboring groups
3. Determination of the proportion of functional
groups and end groups in the polymer
4. Conclusions about asymmetry, tacticity, ring
conformations, etc.
8. Toxicology
The toxicology of silicones, especially polydimethylsiloxanes, has been thoroughly studied
because they are used in medicine and medical
technology, as well as in cosmetics.
The inertness of silicones toward warmblooded animals has been demonstrated in a
number of tests [224]. A single large oral dose
of PDMS has only a laxative effect in test animals. The LD50 in rats is > 50 g/kg. After single
dermal or inhalative administration, no effects on
animals were observed. Skin contact with PDMS
does not cause irritation. After contact with the
eyes, temporary irritation of the conjunctiva and
possible lachrymation can occur.
Studies of chronic oral administration of PDMS
to various animals did not reveal harmful effects.
Oral intake of PDMS by humans (e.g., in the
treatment of flatulence) causes no undesirable side
effects. Regular dermal application in humans
(e.g., as a skin cream) is also tolerated without
problems, and sensitization has not been observed.
The inhalation of PDMS- containing aerosol for
Vol. 32
90 d by experimental animals had no effect. However, injection into the trachea led to pathological
changes in the lungs. In animals, subcutaneously
injected PDMS remains at the place of injection in
subcutaneous cysts. Acute and chronic studies
show that polymeric siloxanes cannot penetrate
cells, and that cleavage of the Si – C or Si – O
bonds does not occur in the body.
Various tests have given no indication of
mutagenicity.
Studies on the carcinogenic effect of PDMS
after oral administration in rats and mice showed
no positive findings.
Repeated subcutaneous injection of PDMS in
animals led to increased tumor formation at the
site of injection. The tumors were not attributed
to the chemical activity of PDMS but to the
physical properties of the substance.
Implantation of weakly cross-linked silicone
gels under the skin of human beings is currently a
subject of controversy. Cases are reported in
which health problems have arisen.
A panel of experts of the FAO and WHO
considers the daily intake by human beings of
1.5 mg of PDMS per kilogram of body weight in
the form of food additives to be unobjectionable.
The molecular mass of the PDMS used must
exceed 200.
In Germany, silicones are permitted in the
production of consumer articles for food contact,
provided Recommendation XV of the BGA is
followed. Similar EC regulations are currently
being prepared on the use of silicones in the
production of consumer articles that come in
contact with foods.
In the case of organofunctional polysiloxanes,
toxic effects cannot be excluded, because of the
reactivity of the organofunctional groups. Polysiloxanes containing trifluoropropyl groups form
toxic decomposition products when heated above
280 C [225].
9. Environmental Aspects
Introduction into the Enviroment, Behavior in the Environment. Owing to their diverse
applications, silicones, especially silicone fluids,
enter the environment from diffuse sources (e.g.,
in wastewater).
Polydimethylsiloxanes are nonbiodegradable
[226]. However, PDMS in wastewater is largely
Silicones
707
eliminated in treatment plants by absorption on
sewage sludge [227]. Sewage sludge is burned or
applied, either as compost or directly, to areas
under agricultural use. Traces of siloxanes have
been detected in soil [228]. As a result of dumping sewage sludge at sea, siloxanes enter marine
sediments [229].
Siloxanes in natural bodies of water are adsorbed on suspended particles and deposited in
the sediment [230]. Traces of siloxanes have
been found in sediment [228], [229], [231].
Abiotic degradation paths of siloxanes have
been demonstrated in soil and aqueous media.
Siloxanes adsorbed on soil are degraded by the
catalytic effect of certain clay minerals [232].
Siloxanes dissolved in water (e.g., siloxanols)
can be degraded to silicates by indirect photochemical reaction [233], [234].
Oligomeric siloxanes can enter the atmosphere due to their volatility. In the atmosphere
these compounds are degraded with a half-life of
a few days by photochemically induced oxidation [235–237].
After use, solid silicones (e.g., silicone rubber,
resins) are disposed of as waste or incinerated.
On complete combustion, SiO2, CO2, and H2O
are formed. Solid SiO2 is removed from the flue
gas by dedusting. It is not known whether silicones are degraded (biotic or abiotic) in landfill
sites.
Behavior toward Organisms in the Environment. The solubility of PDMS in water is
very low. Polydimethylsiloxanes dissolved in
water have no harmful effects on aquatic organisms (plankton, crustacea, mussels, fish) [238].
Silicone fluid administered with food exhibits no
effect on fish [239]. The symbiosis of microorganisms (bacteria, algae, protozoa) is also not
disturbed by siloxanes dissolved in water [230].
Polydimethylsiloxanes in the form of an
emulsion enter surface water at higher concentrations than the solubility of pure PDMS. In
tests, emulsion concentrations up to 10 000 ppm
PDMS in water exhibited toxic effects on fish,
but these are attributed to the emulsifier [226],
[240], [241].
Sediment enriched in PDMS (up to 1000 mg/
kg) has no effect on organisms living in the
sediment [242].
Polymeric siloxanes of high molecular mass
do not accumulate in aquatic organisms, even
708
Silicones
when they are fed PDMS- containing nutrients
[241], [243].
Oligomeric siloxanes of molecular mass up to
ca. 1000 are absorbed by tissue and accumulated.
After transfer of the fish to uncontaminated
water, absorbed siloxanes were rapidly eliminated from the tissue [243–246].
Summary. Liquid and volatile siloxanes enter the environment as a result of use. Soil
(sewage sludge) and sediment are sinks, and
siloxanes are found in trace amounts in these
environmental compartments. Siloxanes have no
marked harmful effects on organisms in the
environment. Therefore they are assumed not to
represent an environmental hazard.
10. Economic Aspects
Silicones have been produced industrially since
the early 1940s. With the industrial-scale realization of methylchlorosilane synthesis (Rochow
direct synthesis), the basis was created for a rapid
increase in sales worldwide, beginning in the
United States. Given the considerable innovation
potential of this product group, this trend is likely
to continue.
One reason for this development is the diverse
molecular structure of siloxanes, and the intimate
interrelationship of structure and properties not
obtainable for any other class of polymers. Thousands of industrial products have been developed, ranging from fluids to rubbers and resins.
Silicones as such or in various formulations, are
used in most industries and spheres of life.
Worldwide silicone production can be estimated from the production of organochlorosilane
precursors. From 1 kg of dimethyldichlorosilane, about 0.5 kg of dimethylsiloxane is obtained. Thus, an organochlorosilane production
of ca. 800 000 t/a in 1991 corresponds to a
siloxane output of ca. 400 000 t/a. Actual sales
are higher due to formulation of silicones with
fillers, solvents, and auxiliary chemicals.
The sales of > 600 000 t/a of silicone products in 1992 represented a business volume of
ca. $ 5.5109. The United States is the largest
market (ca. 40 %), followed by Europe and
Japan.
All economically important regions of the
world have integrated silane – silicone produc-
Vol. 32
tion facilities. The individual producers are as
follows:
United States:
Germany:
France:
United Kingdom:
Japan:
Dow Corning
General Electric
Union Carbide
Wacker (no silane production)
Bayer
Goldschmidt (no silane production)
H€uls
Wacker
Rhône-Poulenc
Dow Corning
Shin Etsu
Toray Silicones
Toshiba Silicones
Brazil, Mexico, Italy, Spain, China, the CIS,
and India also have production facilities for
silicones and in some cases organochlorosilanes,
which however are far less important than those
in the United States, Europe, and Japan.
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Further Reading
M. Butts et al.: Silicones, ‘‘Kirk Othmer Encyclopedia of
Chemical Technology’’, 5th edition, John Wiley & Sons,
Hoboken,
NJ,
online
DOI:
10.1002/
0471238961.1909120918090308.a01.pub2.
M. Forrest: Food Contact Materials, Smithers, Shawbury
2009.
A. J. O’Lenick: Silicones for Personal Care, 2nd ed., Allured
Pub., Carol Stream, IL 2008.