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Abstract

Silicone rubber’s special features such as “organosiloxanes polymer” has been originated from its unique molecular structure that they carry both inorganic and organic properties unlike other organic rubbers. In other words, due to the Si–O bond of silicone rubber and its inorganic properties, silicone rubber was superior to ordinary organic rubbers in terms of heat resistance, chemical stability, electrical insulating, abrasion resistance, weatherability and ozone resistance. With these unique characteristics, silicone rubber has been widely used to replace petrochemical products in various industries like aerospace, munitions industry, automobile, construction, electric and electronics, medical and food processing industry. Recently, these scopes of silicone applications have been expanding at a great speed by the demand of industries that want more reliable elastomer. This paper reviews on synthesis, general properties, applications and nanocomposites of silicone rubber.
1 23
National Academy Science Letters
ISSN 0250-541X
Natl. Acad. Sci. Lett.
DOI 10.1007/s40009-013-0150-2
A Review on Silicone Rubber
Subhas C.Shit & Pathik Shah
1 23
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REVIEW
A Review on Silicone Rubber
Subhas C. Shit Pathik Shah
Received: 4 January 2013 / Revised: 19 February 2013 / Accepted: 1 April 2013
ÓThe National Academy of Sciences, India 2013
Abstract Silicone rubber’s special features such as ‘‘or-
ganosiloxanes polymer’’ has been originated from its
unique molecular structure that they carry both inorganic
and organic properties unlike other organic rubbers. In
other words, due to the Si–O bond of silicone rubber and its
inorganic properties, silicone rubber was superior to
ordinary organic rubbers in terms of heat resistance,
chemical stability, electrical insulating, abrasion resistance,
weatherability and ozone resistance. With these unique
characteristics, silicone rubber has been widely used to
replace petrochemical products in various industries like
aerospace, munitions industry, automobile, construction,
electric and electronics, medical and food processing
industry. Recently, these scopes of silicone applications
have been expanding at a great speed by the demand of
industries that want more reliable elastomer. This paper
reviews on synthesis, general properties, applications and
nanocomposites of silicone rubber.
Keywords Polydialkylsiloxanes Elastomer
Room temperature vulcanization Liquid silicone rubbers
Nanocomposite
Introduction
Silicone rubber is an elastomer (rubber-like material)
composed of silicone—itself a polymer—containing sil-
icon together with carbon, hydrogen, and oxygen. The
name silicone was given in 1901 by Kipping to describe
new compounds of the generic formula R
2
SiO. These
were rapidly identified as being polymeric and actu-
ally corresponding to polydialkylsiloxanes, with the
formulation:
Where R represents methyl or phenyl or vinyl or tri-
fluoropropyl. In 1871, Ladenburg observed that, in the
presence of a diluted acid, diethyldiethoxysilane
{(C
2
H
5
)
2
Si (OC
2
H
5
)
2
} gave an oil that decomposed only
at a ‘‘very high temperature.’ Kipping laid the founda-
tion of organosilicon chemistry with, among other
things, the preparation of various silanes by means of
Grignard reactions and the hydrolysis of chlorosilanes to
yield ‘‘large molecules’’. The most common silicones are
the polydimethylsiloxanes trimethylsilyloxy terminated
with following structure. Where Me represents methyl
group. These were linear polymers and liquids, even for
large values of n. The main chain unit—(SiMe
2
O)—was
often shortened by the letter D because, as the silicon
atom was connected with two oxygen atoms, this unit
was capable of expanding within the polymer in two
directions. In a similar way, M, T and Q units can be
defined corresponding to following structures:
S. C. Shit (&)P. Shah
HLC, Central Institute of Plastics Engineering & Technology,
Plot No. 630, Phase IV, Vatva GIDC, Ahmedabad, Gujarat, India
e-mail: cipetahmd@gmail.com
P. Shah
e-mail: pathikas@yahoo.co.in
123
Natl. Acad. Sci. Lett.
DOI 10.1007/s40009-013-0150-2
Author's personal copy
The simultaneous presence of ‘‘organic’’ groups
attached to an ‘‘inorganic’’ backbone gives silicones a
combination of unique properties. The unique properties of
polysilanes stem from the extensive delocalization of r-
electrons along the silicon backbone and depend on the
nature of the substituent attached to the polymer backbone,
the polymer conformation, and the polymer molecular
weight. The Si–O bond energy was significantly greater
than that of a C–C bond. This has far-reaching effects on
the stability and resistance of silicones to a variety of
influences. For example, silicones have remarkable thermal
and thermo oxidative resistance. Silicones were also far
less readily attacked by electromagnetic and particle radi-
ation (UV, alpha, beta and gamma rays) than organic
plastics. This allows their use in fields as different as
aerospace (low and high temperature performance), elec-
tronics (electrical insulation), health care (excellent bio-
compatibility) or in the building industries (resistance to
weathering).
The studies reported in this literature deal with the
properties of silicone rubber. The objective of this article
was to give the curious reader a scientific overview about
the ways silicones were prepared, their key properties and
how these properties allow silicones to perform in many
different applications. It also discovers contemporary
technologies of silicone rubber indenture with nano
composites.
Synthesis of Silicone Rubbers
Silicone rubbers were obtained in a three-step synthesis:
(1) Chlorosilane synthesis.
(2) Chlorosilane hydrolysis.
(3) Polymerization and polycondensation.
Chlorosilane Synthesis
Silicones were obtained commercially from chlorosilanes,
prepared following the direct process of Rochow [1]. The
reaction giving chlorosilanes takes place in a fluidized bed
of silicon metal powder in which a stream of methyl
chloride flows, usually at temperatures of 250–350 °C and
at pressures of 1–5 bars using copper metal. A mixture of
different silanes were obtained containing mainly the
dimethyldichlorosilane, Me
2
SiCl
2
XSi þYCH3Cl !Me2SiCl2þMeSiCl3þMe3SiCl
þMeHSiCl2
The reaction was exothermic and has a yield of 85–90 %.
The various silanes were separated by distillation. As the
boiling points were closed together, long distillation columns
were always seen at silicone factories. The
dimethyldichlorosilane, which was separated, becomes the
monomer for the preparation by hydrolysis of
polydimethylsiloxanes. Redistribution reactions can be used
to convert the other silanes and increase the commercial yield
of the production equipment. Ethyl- and phenylchlorosilanes
[2] can also be obtained through similar reactions to the direct
process described above. Phenylchlorosilanes can also be
prepared through a Grignard reaction [3]:
MeSiCl3þC6H5MgBr !Me C6H5
ðÞSiCl2þMgClBr:
Other chlorosilanes were prepared from an existing
silane, e.g., methylvinyldichlorosilane, was obtained by the
addition of methyldichlorosilane on acetylene using a Pt
complex as catalyst [4]. It was also possible to replace the
chlorine groups by alcoholysis:
SiCl þROH !SiOR þHCl
In this way, various silanes with different functionalities
can be prepared, e.g., alkoxy and vinyl. These allow
coupling reactions to take place between inorganic surfaces
and polymers in composite manufacturing [4].
Chlorosilanes Hydrolysis
Polydimethylsiloxanes were obtained by the hydrolysis of the
dimethyldichlorosilane in the presence of an excess of water
[5]. This heterogeneous and exothermic reaction yields a
disilanol, Me
2
Si(OH)
2
, which readily condenses, with HCl
acting as a catalyst, to give a mixture oflinear [2]orcyclic[3]
oligomers by inter- or intramolecular condensation.
S. C. Shit, P. Shah
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where n=20–50, m=3, 4, 5 (mainly 4). This mixture
separates from the aqueous acid phase, with the ratio
between the two oligomers depending on the hydrolysis
conditions (concentrations, pH, and solvents). These olig-
omers were water-washed, neutralized and dried. The HCl
was recycled and reacted with methanol to give the
methylchloride used in the direct process described above.
Polymerization and Polycondensation
Linear Condensation
This reaction was catalyzed by many acids or bases. A
distribution of chain length was obtained and longer chains
were favored when working under vacuum and/or at ele-
vated temperatures to reduce the residual water concen-
tration [6]. Acid catalysts were more efficient when the
organosilanol carries electron-donating groups, base cata-
lysts when it carries electron-withdrawing groups.
Some catalysts can induce redistribution by attacking
the polymer chain with the formation of cyclic. This was
important when condensing a mixture of linear oligomers
such as dimethyl- and methylphenylpolysiloxanes. A
sequential polymer will be obtained in the absence of
redistribution, while a random polymer will result if a
catalyst capable of opening the main chain was used.
Cyclic Polymerization
Opening and polymerizing cyclic, (R
2
SiO)m, to form long
linear chains was catalyzed by many acid or base com-
pounds and give at equilibrium a mixture of cyclic oligo-
mers plus a distribution of polymers [7]. The proportion of
cyclic will depend on the substituent along the chain, the
temperature and the presence of a solvent. Polymer chain
length will depend on the presence of substances capable of
giving chain ends. For example, in the polymerization of
(Me
2
SiO)
4
with KOH, the average length of the polymer
chains will depend on the KOH concentration:
xMe2SiOðÞ
4þKOH !Me2SiOðÞ
yþKO Me2SiOðÞ
zH
A stable and –OH terminated polymer, HO (Me
2
SiO)
z
H,
can be isolated after neutralizing and stripping the above
mixture, under vacuum, of the remaining cyclic. In fact, a
distribution of chains with different lengths was achieved. The
reaction can also be made in the presence of Me
3
SiOSiMe
3
,
which will act as a chain end blocker according to:
Me2SiOK þMe3SiOSiMe3!Me2SiOSiMe3
þMe3SiOK
The Me
3
SiOK formed will attack another chain to
reduce the average molecular weight of the linear polymer
formed. The copolymerization of (Me
2
SiO)
4
in the
presence of Me
3
SiOSiMe
3
with Me
4
NOH as a catalyst
displays a surprising viscosity change over time. First, a
peak or viscosity maximum was observed at the beginning
of the reaction. With such a base catalyst, the presence of
two oxygen atoms on each silicon in the cyclic makes them
more susceptible to a nucleophilic attack by the catalyst
than the silicon of the end blocker, which was substituted
by one oxygen atom. The cyclic were polymerized first in
very long, viscous chains that were subsequently reduced
in length by the addition of terminal groups provided by the
end blocker, which was slower to react. This reaction can
be described as follows:
Me3SiOSiMe3þxMe2SiOðÞ
4!Me3SiO Me2SiOðÞ
nSiMe3
Most catalysts used to prepare silicones can also
catalyze the depolymerization (attack along the chain),
particularly in the presence of water traces at elevated
temperatures:
Me2SiOðÞ
nþH2O!Me2SiOðÞ
yHþHO Me2SiOðÞ
z
It was therefore essential to remove all remaining traces
of the catalyst to benefit as much as possible from the
silicone’s thermal stability. Labile catalysts have been
developed. These decompose or were volatilized above the
optimum polymerization temperature and so can be
eliminated by a brief overheating; in this way, catalyst
neutralization or filtration can be avoided. The cyclic
trimer, (Me
2
SiO)
3
, was characterized by an internal ring
tension and can be polymerized without re-equilibration of
the resulting polymers. With this cyclic, polymers with
narrow molecular weight distribution can be prepared, but
also polymers carrying only one terminal reactive function
(living polymerization). Starting from a mixture of
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different ‘‘tense’’ cyclic also allows the preparation of
block or sequential polymers [8].
Cross Linking by Addition
The shrinkage problem can be eliminated when using an
addition reaction to achieve cross-linking. Here, cross-
linking was achieved using vinyl end blocked polymers
and reacting them with SiH groups carried by functional
oligomers [9]. A few polymers can be bonded to these
functional oligomers. The addition occurs mainly on the
terminal carbon and was catalyzed by Pt or Rh metal
complexes, preferably organometallic compounds to
enhance their compatibility. There was no by-product with
this reaction. Molded pieces made with a product using this
cure mechanism were very accurate (no shrinkage). How-
ever, handling these two-part products (polymer and Pt
catalyst in one component, SiH oligomer in the other)
requires some precautions. The Pt in the complex was
easily bonded to electron-donating substances such as
amine or organosulphur compounds to form stable com-
plexes with these poisons, rendering the catalyst inactive
(inhibition).
Cross Linking by Condensation
This method was used in sealants such as the ones available
in do-it-yourself shops. These products were ready-to-use
and require no mixing. Cross-linking starts when the
product was squeezed from the cartridge and comes into
contact with moisture. They were formulated from a
reactive polymer prepared from a hydroxy endblocked
polydimethylsiloxane [6] and a large excess of methyltri-
acetoxysilane [10]:
HO Me2SiOðÞ
xHþMeSi OAcðÞ
3
!AcOðÞ
2MeSiO Me2SiOðÞ
xOSiMe OAcðÞ
2þ2 AcOH
As a large excess of silane was used, the probability of
two different chains reacting with the same silane molecule
was remote and all the chains were endblocked with two
OAc (acetoxy group) functions. The resulting product was
still liquid and can be stored in sealed cartridges. Upon
opening and contact with the moisture of the air, the
acetoxy groups are hydrolyzed to give silanols that allow
further condensation to occur:
In this way, two chains have been linked, and the reaction
will proceed further from the remaining acetoxy groups. An
organometallic tin catalyst was normally used [11]. This cross-
linking requires that moisture diffuses within the product and
the cure will proceed from the outside surface toward the inside.
These sealants were called one-part room temperature vulca-
nization (RTV) sealants, but they actually require moisture as a
second component [12]. Acetic acid was released as a by-
product of the reaction and corrosion problems were possible on
substrates such as concrete, with the formation of a water-sol-
uble salt at the interface. To overcome this, other systems have
been developed, including one-part sealants releasing less
corrosive or noncorrosive byproducts, e.g., oxime using the
oximosilane [6] or alcohol using the alkoxysilane [13]instead
of the above acetoxysilane. Condensation cure was also used in
two-part systems where cross-linking starts upon mixing the
two components, e.g., a hydroxy endblocked polymer [14]and
an alkoxysilane such as tetra n-propoxysilan [15].
S. C. Shit, P. Shah
123
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Here, no atmospheric moisture was needed. Usually an
organo tin salt was used as catalyst; however, to do so
limits the stability of the resulting elastomer at high tem-
peratures. Alcohol was released as a by-product of the cure,
leading to a slight shrinkage upon cure. This precludes the
fabrication of very precise objects (0.5–1 % linear
shrinkage).
Curing Additives
With the exception of RTV and liquid curing systems, silicone
rubbers were usually cured using peroxides such as benzoyl
peroxide [16], 2,4-dichlorobenzoyl peroxide [17], t-butyl
perbenzoate [18] and dicumyl peroxide [19]. Alkyl hydroper-
oxides and dialkyl peroxides have also been used successfully
with vinyl containing silicones. Hydrosilylation or hydrosila-
tion was an alternative curing method for vinyl containing
silicones and utilises hydrosilane materials and platinum con-
taining compounds for catalysts. It was a two part process
requiring mixing of two separate components, with the
resulting material having a limited shelf life. Curing does not
produce volatiles and heat cured conventional silicones with
high tear strengths can be cured in this way. Reinforcing fillers
were added to improve the otherwise poor tensile strength of
silicones. Silica, in the form of silica fume with particle sizes in
the range 10–40 nm was the most preferred filler, although
carbon black has been used. Fillers do interact with the vul-
canisate, forming pseudo-vulcanization [20]. This can occur
either during mixing (creep hardening) or in storage (bin age-
ing). Although milling can break down these structures, it was
also common to add structure control additives or ant-structure
additives to combat these reactions. Examples of these mate-
rials were siloxane-based materials such as diphenylsilane and
pinacoxydimethylsilane. Silicones have better fire resistant
properties compared to natural rubbers. This property can be
improved by the addition flame retardant additives such as
platinum compounds, carbon black, aluminium trihydrate, zinc
or ceric compounds [21]. It should be noted that carbon black
addition also increases electrical conductivity [22]. Ferric oxide
mayalsobeaddedtoimproveheatstability[23], titanium
dioxide and other organometallic compounds as pigments [24].
Manufacture
Silicones can be mixed/compounded using mixers of mills.
However, due to the low viscosity close-fitting scrapers and
cheek plates need to be used to ensure complete mixing.
Forming can be carried out by conventional techniques
such as injection moulding, extrusion and compression
moulding. Care must be taken to take into account rela-
tively large curing shrinkages and to avoid entrapped air.
Curing was generally rapid for most grades and followed
by a post cure treatment in an air oven at 200–250 °C, for a
period of 4–24 h. This process serves to improve properties
and remove residual peroxide products.
Liquid Silicone Rubbers
These were essentially two-part systems, supplied deaer-
ated ready for use often in premetered equipment. Low
injection pressures and low pressure forming techniques
were sufficient. They cure after mixing the two separate
portions, by processes such as hydrosilylation. Curing was
often complete in as little as a few seconds at temperatures
of about 200 °C and post-curing was not usually required.
The low capital investment required for production mean
that liquid silicone rubbers (LSRs) can compete with
conventional silicones and organic rubbers. Physical
properties were comparable to general purpose grades and
high strength peroxide cured elastomers. Furthermore, they
exhibit self-extinguishing properties, with carbon black
additions enabling them to satisfy UL-94 tests [25].
Room Temperature Vulcanizing (RTV) Rubbers
These were available in one-part (RTV-1) and two-part
(RTV-2) systems [26,27]. Single part systems consist of
polydialkylsiloxane with terminal hydroxyl groups, which
are reacted with organosilicon cross-linking agents. This
operation was carried out in a moisture-free environment and
results in the formation of a tetra functional structure. Curing
takes place when materials were exposed to moisture.
Atmospheric moisture was sufficient to trigger the reaction,
and thickness should be limited if only one side was exposed
to the moisture source. Curing was also relatively slow,
reliant on moisture ingress into the polymer. Two pack
systems can be divided into two categories; condensation
cross-linked materials and addition cross-linked polymers.
Condensation systems involve the reaction of silanol-ter-
minated polydimethylsiloxanes with organosilicon cross-
linking agents such as Si(RO)
4
. Storage life depends on the
catalyst employed and ambient conditions. Addition-cured
materials must be processed under clean conditions as curing
can be affected by contaminants such as solvents and cata-
lysts used in condensation RTVs. These materials were
suited to use with polyurethane casting materials.
General Properties of Silicone Rubber
High Binding Energy, Intermolecular Force and Coil
Formation
Silicone rubber has siloxane bond (Si–O) of molecular
structure as the main chains. While carbon bond, C–C,
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carries 84.9 kcal/mol, siloxane bond carries 106.0 kcal/mol.
It shows that siloxane bond has greater capacity and sta-
bility. As a result, silicone rubber has better heat resistance,
electric conductivity and chemical stability than any other
ordinary organic rubbers [28]. Siloxane bond’s energetic
stability was secured due to sharp difference between Si and
O in terms of electro-negativity making Si–O to be closest to
ionic bond. Silicone molecules were helical and intermo-
lecular force was low, resulting in high elasticity, high
compressibility and excellent resistance to cold temperatures
[29]. Furthermore, the methyl groups located on the outside
the coil structure can rotate freely. This characteristic gives
silicone its distinctive interfacial properties including water
repellency and good releasability.
Heat and Cold Resistance
Heat resistance of silicone rubber was the one of its most
excellent properties and provides the basis for its creation.
Silicone rubber was far better than organic rubbers in terms
of heat resistance. At 150 °C, almost no alterations of
properties take place that it may be used semi permanently.
Furthermore, silicone rubber withstands use for over
10,000 consecutive hours even at 200 °C and, if used for a
shorter term, it may also be used at 300 °C as well.
Boasting this excellent heat resistance, silicone rubbers
were widely used to manufacture rubber components and
parts used in high-temperature places. Silicone rubber also
has excellent resistance to cold temperatures. The
Embrittlement point of typical organic rubbers was
between -20 to -30 °C, compared to -60 to -70 °C for
silicone rubbers. Even at temperature at which organic
rubber turn brittle, silicone rubber remains elastic. Silicone
rubber hardens when heated in air with decreasing elon-
gation as it deteriorates. But in sealed conditions it softens
as it deteriorates and its operating life at high temperatures
was shorter in sealed conditions than in air. This softening
results from the degradation of the silixone polymer.
Adjusting the silicone rubber formula using a different
curing agent, and/or post curing can help prevent softening
in hot, sealed conditions [30].
Weatherability, Moisture and Stream Resistance
Silicone rubbers have exceptional weatherability. Ozone
created by corona discharge rapidly deteriorates most
organic rubbers but has almost no effect on silicone rubber.
In addition, silicone rubber can be exposed to wind, rain
and UV rays for long periods with virtually no change in its
physical properties [31]. Silicone rubber absorbs only 1 %
of moisture even after experiencing long exposure to water
without being affected in mechanical strength or electric
properties [6]. Typically under ordinary pressure contact
with stream causes almost no deterioration of silicone
rubber. With pressurized stream, however the effects
increases as stream pressure increase. High pressure stream
at temperatures over 150 °C causes breakdown of the sil-
ixone polymer and decline in the properties of the rubber.
This effect can be ameliorated by adjusting the silicone
rubber formula, selecting a proper curing agent and/or post
curing. There were numerous products available with
improved resistance to stream and hot water.
Tear Strength, Tensile Strength and Flex Fatigue
Resistance
The tear strength of silicone was generally around 9.8 kN/m.
There were high strength types available with tear strength
between 29.4 and 49.0 kN/m, achieved through polymer
modification and judicious selection of fillers and cross
linkers. These products were ideal for molding large items,
reverse tapered forms, and complexly, shaped items when
tear strength was required [32]. The strength of silicone
rubber against dynamic stress was no greater than that of
organic rubbers. But Shin-Etsu has overcome this short-
coming, developing silicone rubbers with flex fatigue
resistance that was 8–20 times higher than conventional
product [33].
Compression Set
The compression set of silicone rubber was consistent over
a wide temperature range from -60 to ?250 °C. Although
the compression set of typical organic rubber was relatively
low around room temperature, it increases significantly as
temperature rise [34]. Silicone rubbers of proper curing
agent are particularly recommended when using silicone
rubber to make molded items for which low compression
set was desired.
Thermal Conductivity and Flame Retardancy
The thermal conductivity of silicone rubber was about
0.2 W/mXK, a value higher than that of common organic
rubbers. Some silicone rubbers contain a high proportion of
special inorganic fillers to improve thermal conductivity
and these were used to make products including thermal
interface sheets and heating rollers [35]. If silicone rubber
was brought close to a flame, it will not ignite easily; but
once ignited it will continue burning. It was possible to
impart flame retardancy and/or self-extinguishing proper-
ties by adding a small amount of flame retardant [36].
Flame retardant silicone rubbers presently in use would
scarcely produce toxic gas during combustion since they do
not contain organic halogen compounds discovered in
organic polymers. They are used in consumer electronics
S. C. Shit, P. Shah
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and business equipment; in closed spaces such as aircraft,
subways, and building interiors. These silicone rubbers
contribute to making all these environments safer.
Electrical Conductivity
Conductive silicone rubber was a compound comprising
conductive materials such as carbon black, silver and
copper [37]. A range of products were available, with
resistance varying from 0.01 to 10 Xm. Their other
properties were basically the same as general purpose sil-
icone rubbers. Conductive silicone rubber was also being
used for keyboard interfaces, antistatic parts, and shield
materials for high voltage cables. Silicone rubber has high
insulation resistance of 1–100 TXm, and its insulating
properties were stable over a wide range of temperatures
and across a wide frequency spectrum [38]. There was
almost no decline in performance even when immersed in
water, making silicone rubber an ideal insulating material.
It has particularly good resistance to corona discharge and
arcing at high voltages. Silicone rubber was thus used
extensively as an insulator in high voltage applications.
Resistance to Chemicals and Oil
Silicone rubber has outstanding resistance to oil at high
temperatures. Among common organic rubbers, nitrile
rubber and chloroprene rubber have somewhat higher oil
resistance at temperatures below 100 °C, but at higher
temperatures silicone rubber was superior [39]. Silicone
rubber also has excellent resistance to solvents and other
chemicals. It was essentially unaffected by polar organic
compounds (aniline, alcohol, etc.) or dilute acids or bases,
with the increase in volume due to swelling in the range of
only 10–15 %. Silicone rubber does swell in non-polar
organic compounds like benzene, toluene and gasoline; but
unlike most organic rubbers, it does not decompose or
dissolve, and will return to its former state when the sol-
vent was removed. Silicone rubber was, however,
adversely affected by strong acids and bases, so it should
not be used where it will come in contact with such
chemicals. Typically, the effects of solvents on silicone
were evidenced by the swelling, softening and reduced
strength of the rubber; the extent of these effects depends
on the type of solvent involved [40].
Gas Permeability and Radiation Resistance
Compared to organic rubber or plastic films, thin films of
silicone rubber have better gas and vapor permeability, and
they have selectivity. One application for silicone rubber
being investigated was as a gas and water separation
membrane in oxygen enrichment systems [41]. Compares
to other organic rubbers, ordinary (dimethyl) silicone
rubber has no special performance in terms of anti radia-
tion. Methylphenyl silicone rubber, which features phenyl
groups added to the polymer molecules, resists radiation
and is used in the manufacturing of the cables and con-
nectors used in nuclear power plants [42].
Physiological Inert
Living tissues were affected by contact with silicone rubber
to a lesser degree than by exposure to other organic poly-
mers. Silicone rubber was physiologically inert, and was
thus used for baby bottle nipples and stoppers in medical
applications [43,44]. In addition, it was pleasant to the
touch with a high-grade feel, making it ideal for leisure
items such as swimming caps and goggles.
Applications
Many industries have embraced silicone rubber and its
technology and continue to discover new applications to
meet their market demands. Some applications are dis-
cussed below.
Automotive Industry
Silicone rubber was used in virtually every aspect of the
automotive industry. Excellent electrical insulation proper-
ties, heat and chemical resistance, weather ability, adhesive
properties and tear strength were but a few of the key prop-
erties that make silicone rubber particularly useful for auto-
motive manufacturers and component suppliers. Examples of
applications: sealants, gaskets, connectors, spark plugs tires,
radiators,heat exchangers, water-pump gaskets, cylinder head
gaskets, engine covers, valve covers, oil pumps or pans.
Aviation and Aerospace
Extreme temperature resistance, stability with extreme
environmental and chemical stress and durability were
some of the many reasons silicone rubber benefits aviation
and aerospace. Silicone rubber sealants fasten interior and
exterior doors, windows and paneling. Fluid resistance
makes rubber ideal for fuel control diaphragms, hydraulic
lines and cable clamp blocks. Silicone rubber keypads were
even used in computers on earth and in space.
Bakeware and Cookware
Bakeware was sturdier, convenient, easy to use and long-
lasting when made with silicone rubber. The flexible, non-
stick surface was easy to clean and do not impart any flavor
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or odor. Materials can go from the freezer to the oven,
microwave or dish-washer without affecting the quality of
the product or the food. Products enhanced with silicone
rubber include: baking mats, baking molds, cake pans,
flexible bakeware, garlic peelers, ice cube molds, pan,
gadgets holders, spatulas, and utensils.
Cable Accessories and Electronics Industry
The cable sector uses silicone rubber for cable termina-
tions, connectors, insulators and surge arrestors used
indoors and outdoors. High voltage liquid silicone rubber
is ideal for energy transmission and distribution. Silicone
rubber was essential to the electronics sector, where highly
specialized applications make extreme demands on sealing
materials. Silicone rubber was used to insulate, seal and
protect circuits, engine gaskets, control unit gaskets,
electronic encapsulation and special elements for decou-
pling noise. Silicones were important in industrial manu-
facturing where engines place extreme demands on the
heater hoses and coolant hoses. Due to their stability with
temperature fluctuations and burst strength properties, sil-
icone rubber hoses remain resistant to kinks and vacuum
collapse.
Medical Devices and Veterinary
Health care professionals can use silicone rubber products
with confidence. Silicone rubber retains its performance
qualities and does not contain plasticizers, animal-derived
ingredients or natural rubber latex that could generate
unwelcome by-products that could affect people or
animals.
Molding
Silicones were ideal for making all kinds of sturdy, yet
flexible molds for industrial manufacturers, artists or
craftspeople. Silicone rubber compounds were easy to
process, require no expensive equipment and have adjust-
able working times and cure rates. Flexibility and out-
standing release properties mean silicone rubbers separate
easily from a model and can be used over and over again.
Silicone rubber prototypes were used for design and
working models, wax models or small production runs.
Silicone molds were helpful in all types of reproduction
material including wax, plaster, concrete, casting resin and
low-melting alloy. Silicone rubber was used in reproduc-
tion and copies of: archeological findings, architectural
fabrication, furniture parts, hobbyist applications, museum
pieces, reconstituted stone items.
Semi-Conductors and Toy Manufacturing
Silicone rubber-encapsulated semi-conductors were used in
industrial motors, locomotives and power supplies requiring
high voltage switches; high frequency communications such
as cell phone base stations, satellites and radar; and high
temperature applications for aircraft engines, oil drilling and
automotive electronics. Toys and play equipment made with
silicone rubber were durable, tear-resistant, chew-resistant,
weather-resistant, water-resistant, pleasing to touch, easy to
color and can be sterilized at high temperatures.
Nanocomposites of Silicone Rubber
During the last 10 years, preparation and characterization
of organic–inorganic nanocomposites have attracted a great
deal of attention in material science. Nanocomposites offer
the potential for the diversification of applications for
polymers due to their excellent properties, such as high
temperature resistance, dimensional stability, improved
barrier property, flame retardancy and enhanced thermo-
mechanical property. To improve the surface hydropho-
bicity, electrical conductivity, relative permittivity and
thermal conductivity of polymeric materials, nanofillers
were added. Special attention was put on the property
modification of silicone rubber brought about by filler
incorporation for outdoor insulation applications [45]. The
selective deposition of hydrophilic nanofilms on silicone
rubber substrates to create micro patterns has important
applications in biomedical and chemical sensors, tissue
engineering, drug screening, and optical devices. Nano-
composite membrane based on polydimethylsiloxane
(PDMS) and nanoscale SiO
2
particles were prepared by a
convenient and mild sol–gel copolymerization of tetraeth-
oxysilane as well as cross-linking reaction [46]. The results
showed that the nanocomposite membranes exhibited good
membrane-forming ability, superior mechanical properties,
and high solvent resistance as well as excellent oxygen-
enriching properties for air purification.
A variety of fillers can be used in siloxane rubber to
change its properties, but most of them are nonreinforcing
and decrease the tensile properties. Blending siloxane
rubber with carbon black gives the benefit of light weight,
compared with blending ferrite in a rubber matrix [47]. The
material was composite of carbon black and silicone rubber
filled nano-SiO
2
as the main raw material, with a view to
obtain sensitive material of flexible tactile sensor with both
flexibility and tactile function [48]. A kind of new material
suitable for flexible tactile sensor and its preparation
method will be introduced. The tactile sensor applied this
new material and method can be used in many kinds of
S. C. Shit, P. Shah
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occasions which contacting pressure measurement such as
humanoid robot sensitive skin, medicine and so on. The
dispersion of the nanometer-sized multiwalled carbon
nanotubes (MWCNTs) in a silicone matrix leads to a
marked improvement in the properties of the silicone based
composite. Silicone rubber/MWCNTs nanocomposite was
successfully prepared by functionalizing MWCNTs with
silane compound. This allowed a homogeneous dispersion
of functionalized MWCNTs in the silicone matrix. The
silicone rubber/functionalized MWCNTs (1 wt%) com-
posites showed that the tensile strength and modulus of the
composites improved dramatically by about 50 and 28 %,
respectively, compared with silicone rubber [49].
To take advantage of the merits for both hydroxyapatite
(HA) and silicone rubber (SR) in medical applications, a
novel method to produce a new nano-composite of these
two widely used biomaterials. In this work, n-HA (nano-
hydroxyapatite) slurry, which was modified by silane
coupling agent, was mixed with silicone rubber and the
nano-composite could therefore be achieved. It is found
that the dispersion states of the n-HA particles in SR are
homogeneous and the mechanical properties of the com-
posite can be improved significantly. The composites with
micrometer HA (l-HA) and SR were taken as the contrast
for comparison. It finds that the tensile strength of the
n-HA/SR composite can be enhanced greatly in compari-
son with that of the l-HA/SR composite [50]. Antibacterial
polymer was grafted onto nano-sized silica surface and the
surface properties of silicone rubber filled with the silica
were investigated. It was found that silicone rubber filled
with the poly (St–SO
3-
P
?
Bu
3
R)-grafted silica shows
extremely strong antibacterial activity. The silicone rubber
surface has an ability to inhibit the reproduction of a
Staphylococcus aureus and an Escherichia coli [51].
Silicone based elastomers have been mixed with single-
wall carbon nanotubes or larger carbon nanofibrils. Tensile
tests show a dramatic enhancement of the initial modulus
of the resulting specimens as a function of filler load,
accompanied by a reduction of the ultimate properties. It
shows that the unique properties of the carbon nanoparti-
cles were important and effective in the reinforcement. The
modulus enhancement of the composites initially increases
as a function of applied strain, and then at around 10–20 %
strain the enhancement effect was lost in all of the samples
[52]. Siloxane modified montmorillonite clay was prepared
from sodium montmorillonite and quaternary ammonium-
containing siloxane surfactant. The siloxane modified
montmorillonite clay and commercial organo-modified
montmorillonite and bentonite clays were incorporated into
a rapid-cure liquid silicone rubber (LSR) matrix. Up to a
20 % reduction in water vapor permeability was achieved
as well as a 24 % improvement in tear strength and a 40 %
improvement in the compression set [53].
A novel electrically conductive nanocomposite was suc-
cessfully fabricated by dispersing homogeneously conduc-
tive graphite nanosheets (GN) in an insulating silicone
rubber (SR) matrix. GN was prepared by powdering
expanded graphite with sonication in aqueous alcoholic
solution. The particular geometry of GN 30–80 nm in
thickness with high aspect ratio contributes to the advantage
of forming the conducting network, so that the percolation
threshold of SR/GN nanocomposite was about 0.009, much
lower than that of composites with conventional graphite.
The SR/GN nanocomposite presents a remarkable piezore-
sistive behavior under much low pressure, related to the low
elastic modulus of the composite [54].
A superhydrophobic fabric coating made of a cross-
linked polydimethylsiloxane elastomer, containing well-
dispersed hydrophobic silica nanoparticles and fluorinated
alkyl silane, shows remarkable durability against repeated
machine washes, severe abrasion, strong acid or base,
boiling water or beverages and excellent stain resistance
[55]. A conductive silicone rubber (SR) composite, filled
with both carbon nanotubes (CNTs) and carbon black (CB)
was prepared by a simple ball milling method. Because of
the good dispersion and synergistic effects of CNT and CB,
the SR composite (SR with 2.5 phr CB and 1.0 phr CNT
hybrid fillers) shows improvement in mechanical properties
such as tensile strength and strain to failure. As well, due to
the assembly of conductive pathways generated by the
CNT and CB, the nanocomposite becomes highly con-
ductive at a comparatively low concentration, with high
sensitivity for tensile and compressive stress. Long-term
measurement of properties shows that the SR composite
maintains the excellent electrical properties under different
strain histories. These outstanding properties show that the
SR composite has potential applications in tensile and
pressure sensors [56].
An ultrathin (300 nm) homogeneous silicalite–
poly(dimethylsiloxane) (PDMS) nanocomposite membrane
was fabricated on capillary support by a ‘‘packing–filling’
method. Firstly, silicalite-1 nano-crystals were deposited
onto a porous alumina capillary support using dip-coating
technique (packing); secondly, the interspaces among the
nano-crystals were filled with PDMS phase (filling). The
membrane possesses very high flux (5.0–11.2 kg m
-2
h
-1
)
and good separation factor (25.0–41.6) for the pervapora-
tive recovery of iso-butanol from aqueous solution
(0.2–3 wt%) at 80 °C. Such properties offer great potential
towards applications in fermentation–pervaporation cou-
pled processes [57].
Bionanotechnology deals with nanoscopic interactions
between nanostructured materials and biological systems.
Organomodified-nanoclay with negatively charged silicate
layers was incorporated into biomedical grade silicone
rubber. Nanoparticle loading has been tailored to enhance
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cell behavior. Addition of nanoparticles led to improved
mechanical properties of substrate with enhanced strength
and stiffness while no toxic effects was observed. Results
indicated improved viability and proliferation of cells by
addition of nanofillers. The improved mechanical proper-
ties of the matrix result in proper cell response through
adjustment and arrangement of cytoskeletal fibers. Results
can be applied in tissue engineering when enhanced sub-
strates were required for improvement of cell behavior for
in vivo applications [58].
A novel exfoliated silicone rubber/clay nanocomposite
has been synthesized by hydroxyl-terminated poly-
dimethylsiloxane and organoclay. HTAB and TPAC were
used as swelling agents to treat Na-montmorillonite for
forming organoclay. The results proved that the nanometer-
scale silicate layers of TPAC-mont were completely exfo-
liated in silicone rubber matrix in the cases of 1–10 %
TPAC-mont content. The nanocomposites exhibit markedly
improved mechanical properties and thermal stability when
compared with the pure polymer or conventional aerosilica-
filled silicone rubber. 200–300 % increase in the tensile
strength and a 100 % increase in the elongation at break
were found for TPAC-mont/silicone rubber as compared to
that of pure silicone rubber [59]. Liquid silicone rubber
nanocomposite was prepared from the compounding of
VPMPS, HPDMS, catalyst, and alumina trihydrate modified
with 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane. The
mechanical property and electrical property for insulation
materials were measured by Jung indicating the high tensile
strength and the good short-circuit property [60].
Silicone rubber formulations resist temperatures and
chemicals, and the fast curing properties maximize productivity
in industrial packaging and assembly. Professionals can use a
ready-to-process compound to make an end-product or they can
combine the compound with others for a particular product with
complicated technical requirements. Silicone rubbers were
essential in sophisticated, high-tech applications and have an
increasing number of applications for new markets. Used with
the right technologies, silicone rubber provided innovative
manufacturing solutions, particularly when it comes to reor-
ganization automated production. It was the preferred choice in
many high-end processing and manufacturing applications that
require quality, cost-effective production. Because it comes in a
variety of formulations for curing, molding and manufacturing,
it helps businesses people streamline their processing while
ensuring consistent quality production.
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... This material has several key properties that are beneficial to coral aquaculture. PC-LSR is chemically inert, which means it will not leach chemicals into the water nor react with other chemicals (Moretto et al., 2000;Shit & Shah, 2013;Selim et al., 2017). It is also UV-resistant (Moretto et al., 2000;Shit & Shah, 2013;Selim et al., 2017) and will not breakdown when exposed to UV radiation, which is vital for coral culture because corals need sunlight for their algal symbionts to photosynthesize. ...
... PC-LSR is chemically inert, which means it will not leach chemicals into the water nor react with other chemicals (Moretto et al., 2000;Shit & Shah, 2013;Selim et al., 2017). It is also UV-resistant (Moretto et al., 2000;Shit & Shah, 2013;Selim et al., 2017) and will not breakdown when exposed to UV radiation, which is vital for coral culture because corals need sunlight for their algal symbionts to photosynthesize. Most materials, especially plastics, eventually degrade due to prolonged or high UV exposure (Yousif & Haddad, 2013). ...
... Most materials, especially plastics, eventually degrade due to prolonged or high UV exposure (Yousif & Haddad, 2013). PC-LSR has a wide thermal tolerance ranging from -60 to 300 ºC (Moretto et al., 2000;Shit & Shah, 2013;Selim et al., 2017), making it ideal for aquaculture settings. Corals are cultured well within the temperature range that this material can withstand, and as a result, there is no concern about temperature compromising the material. ...
Thesis
Full-text available
Biofouling is a major concern in coral aquaculture and hinders the upscaling of coral reef restoration efforts as it leads to increased juvenile coral mortality through smothering. This study proposed using platinum-cured liquid silicone rubber (PC-LSR), also known as “food-grade” or “food-safe” silicone, as an antifouling substrate for coral larvae to settle onto and for corals to grow on. To determine if algae grow on the silicone, silicone squares and ceramic tiles were kept in four different tanks (with different algal communities) for one month. Scanning electron microscope (SEM) analysis was used to determine if algae can attach to silicone. The suitability of silicone squares (PC-LSR) as settlement substrate was tested using larvae from five stony coral species (Montastraea. cavernosa, Pseudodiploria. strigosa, Pseudodiploria. clivosa, Colpophyllia natans, and Orbicella faveolata); ceramic tiles were tested as a control. To assess if corals can grow on the silicone, a silicone ‘skirt’ was placed around two corals. In all tanks except one, algal growth was observed on silicone squares; algal growth was observed on all ceramic tiles. There was no algal attachment to silicone squares in tanks R, H, and 3, and minimal attachment in tank 4. Larval settlement of M. cavernosa, P. strigosa, and O. faveolata was not significantly different between substrates, but P. clivosa and C. natans larvae settled significantly more on ceramic tiles. New tissue growth was observed after a 3-month period on both silicone ‘skirts.’ One M. cavernosa colony grew eight new polyps with a total tissue growth area of 0.792 cm2, but experienced a disturbance, losing three polyps (0.353 cm2). A second M. cavernosa colony grew at a slower rate, producing two new polyps with a total tissue growth of 0.245 cm2. This study suggests PC-LSR can prevent or minimize algal attachment, resulting in lower algal growth in tanks, especially turf algae. While PC-LSR can be used as a larval settlement substrate for some coral species, the survival and growth of newly settled corals on this substrate seems vulnerable due to adhesion issues. Consequently, PC-LSR appears to be an effective way to prevent algal overgrowth around corals, especially when deployed as a skirt around corals that are already attached to a substrate, and thus should be the subject of further study.
... Compared with traditional organic rubber, silicone rubber has much better heat resistance. Silicone rubber has been demonstrated to be able to work continuously for 10,000 h at 200 • C without significant degradation, [50,51]. For cold resistance, typical organic rubber has an embrittlement point of − 20 • C to − 30 • C, while silicone rubber has a much lower embrittlement point of − 60 • C to − 70 • C. Silicone rubber can still retain its elastic property when the temperature is not suitable for organic rubber [52]. ...
... The properties of the above-mentioned sealing materials for PEMFCs have been sorted out in Table 3. Silicone rubber has both outstanding heat resistance and chemical corrosion resistance, which can meet the need of PEMFCs, but the compression set of silicone rubber is high [50]. Fluororubber has excellent sealing properties but is not conducive to being used as a sealing gasket for PEMFCs and poor for the disassembly of the cell [85]. ...
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The 3rd edition of The Science and Technology of Rubber provides a broad survey of elastomers with special emphasis on materials with a rubber-like elasticity. As in the 2nd edition, the emphasis remains on a unified treatment of the material; exploring topics from the chemical aspects such as elastomer synthesis and curing, through recent theoretical developments and characterization of equilibrium and dynamic properties, to the final applications of rubber, including tire engineering and manufacturing. Many advances have been made in polymer and elastomers research over the past ten years since the 2nd edition was published. Updated material stresses the continuous relationship between the ongoing research in synthesis, physics, structure and mechanics of rubber technology and industrial applications. Special attention is paid to recent advances in rubber-like elasticity theory and new processing techniques for elastomers. This new edition is comprised of 20% new material, including a new chapter on environmental issues and tire recycling.
Book
The 3rd edition of The Science and Technology of Rubber provides a broad survey of elastomers with special emphasis on materials with a rubber-like elasticity. As in the 2nd edition, the emphasis remains on a unified treatment of the material; exploring topics from the chemical aspects such as elastomer synthesis and curing, through recent theoretical developments and characterization of equilibrium and dynamic properties, to the final applications of rubber, including tire engineering and manufacturing. Many advances have been made in polymer and elastomers research over the past ten years since the 2nd edition was published. Updated material stresses the continuous relationship between the ongoing research in synthesis, physics, structure and mechanics of rubber technology and industrial applications. Special attention is paid to recent advances in rubber-like elasticity theory and new processing techniques for elastomers. This new edition is comprised of 20% new material, including a new chapter on environmental issues and tire recycling. Explores new applications of rubber within the tire industry, from new filler materials to "green tires" (a tire that has yet to undergo curing and vulcanization). 30% of the material has been revised from the previous edition with the addition of 20% new material, including a chapter on the environment. A mixture of theory, experiments, and practical procedures will offer value to students, practitioners, and research & development departments in industry.
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This chapter reviews the general structures and properties of silicone polymers. It describes the cross linking chemistry and the properties of the cross linked networks. It then discusses the promotion of adhesive and cohesive strength. The buildup of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. The surface of the substrate, the silicone/substrate interface, and the bulk properties of silicones play significant and influential roles that affect practical adhesion and the performance of the silicone. The design of silicone adhesives, sealants, coatings, encapsulants, or any products where adhesion property is needed requires the development chemist to have a thorough understanding of both silicone chemistry and adhesion phenomena. The chapter presents a detailed review of the theories that are proposed for the mechanisms of adhesion. The various theories underlying each mechanism is briefly outlined and qualitatively illustrated in the chapter with specific examples.
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This paper discusses recent topics in the biomedical applications of rubbers and elastomers. It focuses on the current problems obtaining materials for implants from manufacturers due to concern about potential liability by raw materials manufacturers. The many problems and opportunities for development of polyurethane and silicone implants are discussed along with a review of a few other rubbers and elastomers used for implants. Some of the many other rubbers and elastomers available in medical grades and used in non-implant biomedical applications are presented.
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This is a review of silicone rubber and challenging it is since the silicones, a relatively new group of completely synthetic materials, are growing very rapidly even when viewed against today's rapidly advancing technology. The silicones bring together something of the properties of inorganic materials with their silicon-oxygen-silicon polymeric backbones combined with a sheath of organic groups, and the properties which result have won for the silicones an important position in our modern array of new materials. By silicones we mean structures which contain silicon-oxygen polymeric backbones with organic groups attached to the silicon. The name arose when Professor F. S. Kipping and thers in the early 1900's began to work with these materials which they thought at first to be analogous to the organic ketones. For this reason they called them “silicoketones” or “silicones” for short. These materials are not at all, however, related to the organic compounds from which they derive their name for they do not contain any silicon double bonds. Instead the polymers are linked together in giant networks of linear and crosslinked polymers by single bonded silicon-oxygen-silicon bonds. Although the silicones have only recently become important they had a relatively early beginning. It was approximately one hundred years ago, in the same year that Kekule proposed the cyclic structure for benzene, that Friedel and Crafts made the first material which could be classified as a silicone. In the intervening years Professor F. S. Kipping published a monumental series of papers on the chemistry of the silicone intermediates up through the year 1937. However, Professor Kipping was mostly interested in the chemistry of the small molecules and placed very little emphasis on the high molecular weight polymers which were to become so important later on. There was a stirring of interest in the industrial potentials for silicones in the 1930's. Dr. Winton Patnode initiated work at General Electric on silicon compounds containing organic groups as did Dr. J. Franklin Hyde at Corning Glass. Meanwhile, in Russia, B. N. Dolgov and K. A. Andrianov were also working in the field of organo-silicon chemistry. In the early 1940's these efforts finally lead to the blossoming of the commercially important silicones. The Direct Process was discovered by Dr. E. G. Rochow and this important process coupled with other basic rubber technology formed the basis for commercial silicone rubber. Since that time the technology has advanced very rapidly and although prior to 1940 there were virtually no patents on silicones and mostly Kipping's papers from the academic point, there are now approximately 10,000 publications throughout the world including U.S. and foreign patents. Activity has expanded at such a rate that the majority of this literature has been generated in the last few years. The scope of the silicone industry is indicated to some extent by the products as shown in Figure 1. Each of the types of products shown here include many individual products so that there are many hundreds of actual products manufactured and sold. The total business at this time in the United States represents approximately $75 million and silicone rubbers account for a substantial fraction of this total.
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Methyl vinyl silicone rubber/zinc dimethacrylate (VMQ/ZDMA) composites were prepared through in situ poly- merization of ZDMA monomers during the peroxide curing. The polymerization conversion of ZDMA and morphology of the VMQ/ZDMA composites were studied. The results showed that most of the ZDMA monomers participated in the in situ polymerization during the cross-linking of the VMQ matrix and uniform nanophases were formed in the composites. The 'dissolving-diffusion' model was used to explain the micro-nano transformation of ZDMA. According to the model, a uni- form nano-dispersed structure could be obtained through the in situ reaction even though the initial dispersion of ZDMA in the blends was poor. In addition, tensile tests of VMQ/ZDMA composites showed that ZDMA had a significant reinforce- ment on the mechanical properties of VMQ, and the best mechanical properties were obtained when the amounts of perox- ide and ZDMA were 5 and 40 phr, respectively. The gross crosslink density and ionic crosslink density increased as the amount of ZDMA increased, but the covalent crosslink density decreased slightly. These results indicated that the ionic crosslink structure had a significant effect on the mechanical properties of VMQ/ZDMA composites.