Silicone Chemistry Overview
`
`ALISON – Ex. 1014
`Alison v. Aspen
`IPR2017-00201
`
`

`
`Silicone Chemistry Overview
`
`1. Introduction
`
`By analogy with ketones, the name silicone was given in 1901 by Kipping to describe new
`compounds of the generic formula R2SiO. These were rapidly identified as being
`polymeric and actually corresponding to polydialkylsiloxanes, with the formulation:
`
`R
`(Si O )n
`R
`
`The name silicone was adopted by the industry and most of the time refers to polymers
`where R = Me (polydimethylsiloxane). The methyls along the chain can be substituted
`by many other groups, e.g., phenyl, vinyl or trifluoropropyl. The simultaneous presence
`of “organic” groups attached to an “inorganic” backbone gives silicones a combination of
`unique properties and allows their use in fields as different as aerospace (low and high
`temperature flexibility), electronics (high electrical resistance), medical (excellent bio-
`compatibility) and construction (resistance to weathering).
`
`2. Historic
`
`The principal steps in the development of silicone chemistry are1,2:
`
`• The discovery of silicon by Berzelius in 1824 from the reduction of potassium
`fluorosilicate with potassium:
`
`4 K + K2SiF6
`
`Si + 6 KF
`
`Reacting silicon with chlorine gave a volatile compound later identified as
`tetrachlorosilane, SiCl4:
`
`Si + 2 Cl2
`
`SiCl4
`
`• The next step was made by Friedel and Craft with the synthesis of the first silicon
`organic compound in 1863, tetraethylsilane:
`
`2 Zn(C2H5)2 + SiCl4
`
`Si(C2H5)4 + 2 ZnCl2
`
`• In 1871, Ladenburg observed that, in the presence of a diluted acid,
`diethyldiethoxysilane, (C2H5)2Si(OC2H5)2, gave an oil that decomposed only at a “very
`high temperature.”
`
`• Kipping laid the foundation 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 polymeric nature of the silicones was
`confirmed by the work of Stock.
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`
`• In the 1940s, silicones became commercial materials after Hyde of Dow Corning
`demonstrated the thermal stability and high electrical resistance of silicone resins, and
`Rochow of General Electric found a direct method of preparing silicones from Si and
`MeCl.
`
`3. Nomenclature
`
`The most common silicones are the polydimethylsiloxanes trimethylsilyloxy terminated,
`with the following structure:
`
`or
`
`Me3SiO(SiMe2O)nSiMe3
`
`(n = 0, 1, ...)
`
`Me
`
`O
`
`Me
`Me
`Me
`Si Me
`(Si O)n
`Si
`Me
`Me
`Me
`These are linear polymers and liquids, even for large values of n. The main chain unit,
`—(SiMe2O)—, is often shortened by the letter D because, as the silicon atom is connected
`with two oxygen atoms, this unit is capable of expanding within the polymer in two
`directions. In a similar way, M, T and Q units can be defined corresponding to3:
`
`O
`
`Me
`Si O
`Me
`
`D
`Me2SiO2/2
`
`O S
`
`i O
`O
`
`O
`
`Q
`SiO4/2
`
`Me
`
`Me
`Si
`Me
`
`O
`
`M
`Me3SiO1/2
`
`O
`
`Me
`Si O
`O
`
`T
`MeSiO3/2
`
`The above polymer can also be described as MDnM. It is possible to simplify the
`description of various structures like (Me3SiO)4Si or tetrakis(trimethylsilyloxy)silane,
`which becomes M4Q (superscripts are sometimes used to indicate groups other than
`methyl).
`
`4. From Sand to
`Silicones
`
`Silicones are obtained in a three-step synthesis:
`- chlorosilane synthesis
`- chlorosilane hydrolysis
`- polymerization and polycondensation
`
`4.1 Chlorosilane
`Synthesis
`
`Today, silicones are obtained commercially (± 500,000 t/y) from chlorosilanes prepared
`following the direct process of Rochow2 and using Si metal obtained from the reduction of
`sand at high temperature:
`
`SiO2 + 2 C →
`
`Si + 2 CO
`
`and methylchloride obtained by condensation of methanol with hydrochloric acid:
`
`CH3OH + HCl →
`
`cat
`
`CH3Cl + H2O
`
`The reaction giving chlorosilanes takes place in a fluidized bed of silicon metal powder in
`which a stream of methylchloride flows, usually at temperatures of 250 to 350 °C and at
`
`Silicone Chemistry Overview
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`3
`
`

`
`pressures of 1 to 5 bars. A mixture of different silanes is obtained containing mainly the
`
`dimethyldichlorosilane, Me2SiCl 32 :
`
`x Si + y CH3Cl →
`
`cat
`
`Me2SiCl2 [1]
`MeSiCl3
`Me3SiCl
`MeHSiCl2
`other silanes
`
`Yield (weight %)
`> 50
`10-30
`< 10
`< 5
`5
`
`Bp (°C)
`70.0
`66.4
`57.9
`41.0
`-
`
`The reaction is exothermic and has a yield of 85 to 90%. A copper-based catalyst is used.
`The reaction mechanism is not completely understood. Chemisorption phenomenons on
`active sites seem preferred to the radical-based mechanism originally proposed. The
`various silanes are separated by distillation. As the boiling points are close together, long
`distillation columns are always seen at silicone factories. The dimethyldichlorosilane [1]
`which is separated becomes the monomer for the preparation by hydrolysis of
`polydimethylsiloxanes (see further). Redistribution reactions can be used to convert the
`other silanes and increase the commercial yield of the production equipment 4.
`
`Ethyl- and phenylchlorosilanes can also be obtained through similar reactions to the
`direct process described above. Phenylchlorosilanes can also be prepared through a
`Grignard reaction 3:
`
`MeSiCl3 + C6H5MgBr → Me(C6H5)SiCl2 + MgClBr
`
`Other chlorosilanes are prepared from an existing silane, e.g., the methylvinyl-
`dichlorosilane is obtained by the addition of methyldichlorosilane on acetylene using a Pt
`complex as catalyst 4:
`
`Pt
`MeHSiCl2 + HC CH → MeViSiCl2
`
`It is also possible to replace the chlorine groups by alcoholysis:
`
`SiCl + ROH →
`
` SiOR + HCl
`
`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 5.
`
`4.2 Chlorosilane
`Hydrolysis
`
`Polydimethylsiloxanes are obtained by the hydrolysis of the dimethyldichlorosilane in the
`presence of an excess of water according to3:
`
`+H2O
`x Me2SiCl2
`
`-HCl
`
`y HO(Me2SiO)nH + z (Me2SiO)m
`
`[1]
`
`linears
`[2]
`
`cyclics
`[3]
`
`with n = 20 - 50 and m = 3, 4, 5, ...(mainly 4).
`
`This heterogeneous and exothermic reaction yields a disilanol, Me2Si(OH)2, which readily
`condenses, with HCl acting as a catalyst, to give a mixture of linear [2] or cyclic [3]
`oligomers by inter- or intramolecular condensation. This mixture separates from the
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`
`4.3
`Polymerization
`and
`Polycondensation
`
`aqueous acid phase, with the ratio between the two oligomers depending on the hydrolysis
`conditions (concentrations, pH, solvents). These oligomers are water-washed, neutralized
`and dried. The HCl is recycled and reacted with methanol to give the methylchloride used
`in the direct process described above.
`
`The linear [2] and cyclic [3] oligomers obtained by hydrolysis of the dimethyldi-
`chlorosilane have too short a chain for most applications. They must be condensed
`(linears) or polymerized (cyclics) to give macromolecules of sufficient length6.
`
`4.3.1 Cyclic polymerization
`
`Opening and polymerizing cyclics, (R2SiO)m, to form long linear chains is catalyzed by
`many acid or base compounds 4 and gives at equilibrium a mixture of cyclic oligomers plus
`a distribution of polymers. The proportion of cyclics will depend on the substituents
`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 (Me2SiO)4 with KOH, the average length of the polymer chains will
`depend on the KOH concentration:
`
`x (Me2SiO)4 + KOH →
`
`(Me2SiO)y + KO(Me2SiO)zH
`
`[3]
`
`A stable and –OH terminated polymer, HO(Me2SiO)zH, can be isolated after neutralizing
`and stripping the above mixture, under vacuum, of the remaining cyclics. In fact, a
`distribution of chains with different lengths is achieved.
`
`The reaction can also be made in the presence of Me3SiOSiMe3, which will act as a chain
`endblocker according to:
`
`Me2SiOK + Me3SiOSiMe3
`
`Me2SiOSiMe3 + Me3SiOK
`
`where
`
`is the main chain.
`
`The Me3SiOK formed will attack another chain to reduce the average molecular weight of
`the linear polymer formed.
`
`The copolymerization of (Me2SiO)4 in the presence of Me3SiOSiMe3 with Me4NOH as a
`catalyst displays a surprising viscosity change over time6. First, a peak or viscosity maximum
`is observed at the beginning of the reaction. With such a base catalyst, the presence of two
`oxygen atoms on each silicon in the cyclics makes them more susceptible to a nucleophilic
`attack by the catalyst than the silicon of the endblocker, which is substituted by one
`oxygen atom. The cyclics are polymerized first in very long, viscous chains that are
`subsequently reduced in length by the addition of terminal groups provided by the
`endblocker, which is slower to react. This reaction can be described as follows:
`
`Me3SiOSiMe3 + x (Me2SiO)4
`
`cat
`→ Me3SiO(Me2SiO)nSiMe3
`
`The ratio between D and M units will define the average molecular weight of the
`polymer formed.
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`Catalyst removal (or neutralization) is always an important step in silicone preparation.
`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:
`
`(Me2SiO)n
`
`cat
`+ H2O→
`
`(Me2SiO)yH + HO(Me2SiO)z
`
`It is 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 are volatilized above the optimum polymerization temperature and so can
`be eliminated by a brief overheating; in this way, catalyst neutralization or filtration can be
`avoided6.
`
`The cyclic trimer, (Me2SiO)3, is 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
`different “tense” cyclics also allows the preparation of block or sequential polymers6.
`
`4.3.2 Linear condensation
`
`This reaction is catalyzed by many acids or bases4, 6:
`
`Me
`OSi
`Me
`
`[2]
`
`HO
`
`Me
`SiO
`Me
`
`[2]
`
`Me
`OSi O
`Me
`
`Me
`SiO
`Me
`
`to give long chains by intermolecular condensation of terminals SiOH. A distribution of
`chain length is obtained and longer chains are favored when working under vacuum
`and/or at elevated temperatures to reduce the residual water concentration. Acid catalysts
`are more efficient when the organosilanol carries electron-donating groups, base catalysts
`when it carries electron-withdrawing groups. Some catalysts can induce a redistribution by
`attacking the polymer chain with the formation of cyclics. This is important when
`condensing a mixture of linear oligomers such as dimethyl- and methylphenyl-
`polysiloxanes. 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 is used.
`
`4.3.3 Other polymers
`
`Apart from the above polymers, reactive polymers can also be prepared. This can be
`achieved when re-equilibrating oligomers or existing polymers:
`
`Me3SiOSiMe3 + x (Me2SiO)4 + Me3SiO(MeHSiO)ySiMe3
`
`cat
`
`cyclics + Me3SiO(Me2SiO)z(MeHSiO)wSiMe3
`
`[4]
`
`to obtain a polydimethyl-methylhydrogenosiloxane, MDzDH
`further functionalized using an addition reaction:
`
`wM. This polymer can be
`
`Silicone Chemistry Overview
`
`6
`
`→
`

`
`Me3SiO(Me2SiO)z(MeHSiO)wSiMe3 + H2C
`[4]
`
`Pt
`
`CHR →
`
` Me3SiO(Me2SiO)z(MeSiO)wSiMe3
`CH2CH2R
`
`(R= alkyl, polyglycol,...)
`
`The above polymers are all linear apart from the cyclics, but these are also made up of
`difunctional units, D. Apart from these, branched polymers or resins can be prepared if,
`during hydrolysis, a certain number of T or Q units are included, which will allow an
`expansion of the material, not in two but in three or four directions. This can be
`described if considering the hydrolysis of the methyltrichlorosilane in the presence of
`trimethylchlorosilane, which leads to:
`
`O
`
`Me
`SiO
`OH
`
`Me
`Si
`
`O O
`
`SiO
`
`SiMe3
`
`+H2O
`x Me3SiCl + y MeSiCl3 →
` z Me3Si
`-HCl
`
`O
`
`Me
`
`or (Me3SiO1/2)x (MeSiO3/2)y or MxTy. The formation, by hydrolysis, of three silanols on
`the MeSiCl3, yields a three-dimensional structure or resin, rather than a linear polymer.
`The average molecular weight depends upon the number of M units that come from the
`trimethylchlorosilane, which limits the growth of the resin molecule. Most of these resins
`are prepared in a solvent and usually contain some residual hydroxyl groups. These could
`subsequently be used to cross-link the resin and form a continuous network or varnish.
`
`5. Silicone
`Cross-linking
`
`The silicone polymers are easily transformed into a three-dimensional network and an
`elastomer via a cross-linking reaction that allows the formation of chemical bonds between
`adjacent chains. This is achieved according to one of the following reactions.
`
`5.1 Cross-linking
`with Radicals
`
`Efficient cross-linking with radicals is achieved only when some vinyl groups are present
`on the polymer chains. The following mechanism has been proposed for the cross-linking
`made by radicals generated from an organic peroxide4:
`
`R• + CH2 CH Si
`
`R CH2 CH•
`
`Si
`
`RCH2 CH•
`
`Si
`
`+ CH3
`
`Si
`
`RCH2 CH2
`
`Si
`
`+
`
`Si CH2•
`
`SiCH2• + CH2 CH Si
`
`Si CH2 CH2 CH•
`
`Si
`
`Si CH2 CH2 CH•
`
`Si
`
`Si CH3
`+
`Si CH2 CH2 CH2
`
`Si
`
`+
`
`Si CH2•
`
`2
`
`Si CH2• →
`
`Si CH2 CH2
`
`Si
`
`where
`
`represents 2 methyl groups and the rest of the polymer chain.
`
`This reaction is used for high-consistency silicone rubbers (HCRs) like the ones used in
`extrusion or injection molding and which are cross-linked at elevated temperatures. The
`peroxide is added before use. During cure, some precautions are needed to avoid the
`formation of voids by the peroxide’s volatile residues. Postcure may also be necessary to
`remove these volatiles, which can act as depolymerization catalysts at high temperatures.
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`5.2 Cross-linking
`by Condensation
`
`This method is used in sealants such as the ones available in do-it-yourself shops. These
`products are ready-to-use and require no mixing. Cross-linking starts when the product is
`squeezed from the cartridge and comes into contact with moisture. They are formulated
`from a reactive polymer prepared from a hydroxy endblocked polydimethylsiloxane and a
`large excess of methyltriacetoxysilane:
`
`HO (Me2SiO)x H + exc. MeSi(OAc)3
`-2 AcOH
`
`(AcO)2MeSiO(Me2SiO)xOSiMe(OAc)2
`
`[5]
`
`As a large excess of silane is used, the probability of two different chains reacting with the
`same silane molecule is remote and all the chains are endblocked with 2 OAc functions.
`The resulting product is 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:
`
`O
`
`O
`
`Me
`Si
`OAc
`
`[5]
`
`Me
`Si
`OAc
`
`[6]
`
`H2O
`
`O
`
`Me
`OH
`Si
`OAc
`
`[6]
`
`O
`
`Me
`Si
`O
`OAc
`
`Me
`Si
`O
`OAc
`
`–AcOH
`
`Me
`Si
`O
`OAc
`
`[5]
`
`In this way, two chains have been linked, and the reaction will proceed further from
`the remaining acetoxy groups. An organometallic tin catalyst is normally used. This
`cross-linking requires that moisture diffuses within the product and the cure will proceed
`from the outside surface toward the inside. These sealants are called one-part RTV (room
`temperature vulcanization) sealants, but they actually require moisture as a second
`component. Acetic acid is released as a by-product of the reaction and corrosion problems
`are possible on substrates such as concrete, with the formation of a water-soluble salt at the
`interface (and loss of adhesion at the first rain!). To overcome this, other systems have
`been developed, including one-part sealants releasing less corrosive or noncorrosive by-
`products, e.g., oxime using the oximosilane RSi(ON CR’2)3 or alcohol using the
`alkoxysilane RSi(OR’)3 instead of the above acetoxysilane.
`
`Condensation cure is also used in two-part systems where cross-linking starts upon mixing
`the two components, e.g., a hydroxy endblocked polymer and an alkoxysilane such as tetra
`n-propoxysilane6:
`
`4
`
`cat
`
` →
`– 4 nPrOH
`
`Me2SiO
`
`OSiMe2
`Si
`OSiMe2
`OSiMe2
`
`Silicone Chemistry Overview
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`→
`→
`

`
`5.3 Cross-linking
`by Addition
`
`Here, no atmospheric moisture is needed. Usually an organotin salt is used as catalyst;
`however, to do so limits the stability of the resulting elastomer at high temperatures.
`Alcohol is 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 to 1 % linear shrinkage).
`
`The above shrinkage problem can be eliminated when using an addition reaction to
`achieve cross-linking. Here, cross-linking is achieved using vinyl endblocked polymers and
`reacting them with SiH groups carried by functional oligomers such as those described
`above [4]. A few polymers can be bonded to this functional oligomer [4], as follows 4:
`
`OMe2Si CH CH2 + H Si
`
`cat
`
`OMe2Si CH2 CH2
`
`Si
`
`where
`
`represents the remaining valences of the Si in [4].
`
`[4]
`
`The addition occurs mainly on the terminal carbon and is catalyzed by Pt or Rh metal
`complexes, preferably organometallic compounds to enhance their compatibility. The
`following mechanism has been proposed (oxidative addition of the
`SiH on the Pt, H
`transfer on the double bond, and reductive elimination of the product):
`
`Si CH
`
`CH2
`
`Si CH2 CH2 Pt Si
`
`Si CH2 CH2 Si
`
`–Pt
`
`•••
`
`Pt
`Si H
`
`[8]
`
`where to simplify, other Pt ligands and other Si substituents are omitted.
`
`There is no by-product with this reaction. Molded pieces made with a product using this
`cure mechanism are very accurate (no shrinkage). However, 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 is easily bonded to electron-donating substances
`such as amine or organosulphur compounds to form stable complexes with these poisons,
`rendering the catalyst inactive (inhibition).
`
`6. Properties of
`Silicones
`
`Silicon, just under carbon in the periodic table, led to a belief in the existence of analogue
`compounds where silicon would replace carbon. Most of these analogue compounds do
`not exist or behave very differently. There are few similarities between Si–X bonds in
`silicones and C–X bonds 3, 4:
`
`Element (X)
`
`Si
`C
`H
`O
`
`Bond length (Å)
`Si X C X
`2.34
`1.88
`1.88
`1.54
`1.47
`1.07
`1.63
`1.42
`
`Ionic character (%)
`Si X C X
`-
`12
`12
`-
`2
`4
`50
`22
`
`Between any given element and Si, bond lengths are longer than for C with this element.
`The lower Si electronegativity (1.8) vs. C (2.5) leads to a very polarized Si–O bond, highly
`ionic and with a large bond energy, 452 kJ/mole (108 kcal/mol). The Si–C bond has a
`bond energy of ±318 kJ/mole (76 kcal/mol), slightly lower than a C–C bond, while the
`Si–Si bond is weak, 193 kJ/mole (46.4 kcal/mole). These values partially explain the
`
`Silicone Chemistry Overview
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`
`stability of silicones. The Si–O bond is highly resistant to homolytic scission. On the other
`hand, heterolytic scissions are easy, as demonstrated by the re-equilibration reactions
`occurring during polymerizations catalyzed by acids or bases (see above). Si atoms do not
`form stable double or triple bonds of the type sp2 or sp with other elements, yet the
`proximity of the d orbitals allows dπ-pπ retro-coordination. Because of this retro-
`coordination, trialkylsilanols are more acidic than the corresponding alcohols. Another
`example of the difference between analogues is the tetravalent diphenyldisilanol,
`(C6H5)2Si(OH)2, which is stable while its carbon equivalent, a gem-diol, will dehydrate.
`The Si–H bond is weakly polarized, but here in the direction of a hydride, and is more
`reactive than the C–H bond. Overall, there are few similarities between a silicone polymer
`and a hydrocarbon polymer.
`
`Silicones display the unusual combination of an inorganic chain similar to silicates and
`often associated with high surface energy but with side methyl groups that are, on the
`contrary, very organic and often associated with low surface energy 7. The Si–O bonds
`are strongly polarized and without protection should lead to strong intermolecular
`interactions 4. Yet the methyl groups, only weakly interacting with each other, shield the
`main chain. This is made easier by the high flexibility of the siloxane chain. Rotation
`barriers are low and the siloxane chain can adopt many configurations. Rotation energy
`around a CH2–CH2 bond in polyethylene is 13.8 kJ/mol but only 3.3 kJ/mol around a
`Me2Si–O bond, corresponding to a nearly free rotation. The siloxane chain adopts a
`configuration that can be idealized by saying that the chain exposes a maximum number
`of methyl groups to the outside, while in hydrocarbon polymers, the relative backbone
`rigidity does not allow a “selective” exposure of the most organic or hydrophobic methyl
`groups. Chain-to-chain interactions are low and the distance between adjacent chains is
`also higher in silicones. Despite a very polar chain, silicones can be compared to paraffin,
`with a low critical surface tension of wetting 7.
`
`The ease with which silicones adopt many configurations is confirmed by monolayer
`absorption studies on water. Two structures have been proposed, an open one in which
`the Si–O–Si bonds are oriented toward the aqueous phase, and a more compact one in
`which the chain adopts a helicoidal structure. The important point is the low energy
`difference between these two structures, again demonstrating the flexibility of the siloxane
`chain6.
`
`The surface activity of silicones is displayed in many circumstances7:
`
`• The polydimethysiloxanes have a low surface tension (20.4 mN/m) and are capable of
`wetting most surfaces. With the methyl groups pointing to the outside, this gives very
`hydrophobic films and a surface with good release properties, particularly if the film is
`cured after application. Silicone surface tension is also in the most promising range for
`bio-compatible elastomers (20 to 30 mN/m).
`
`• Silicones have a critical surface tension of wetting (24 mN/m) higher than their own
`surface tension; this means that silicones are capable of wetting themselves, which
`promotes good film formation and good surface covering.
`
`• Silicone organic copolymers can be prepared with surfactant properties, with the
`silicone as the hydrophobic part, e.g., in silicone glycol copolymers.
`
`Silicone Chemistry Overview
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`10
`
`

`
`The low intermolecular interactions in silicones have other consequences 7:
`
`• Glass transition temperatures are very low, e.g., 146 K for a polydimethylsiloxane
`compared to 200 K for polyisobutylene, the analogue hydrocarbon.
`
`• The presence of a high free volume compared to hydrocarbons explains the high
`solubility and high diffusion coefficient of gas into silicones. Silicones have a high
`permeability to oxygen, nitrogen or water vapor, even if in this case liquid water is not
`capable of wetting a silicone surface. As expected, silicone compressibility is also high.
`
`• In silicone, the activation energy to the viscous movement is very low, and the viscosity of
`silicone is less dependent on temperature than are the viscosities of hydrocarbon
`polymers. Moreover, chain entanglements are involved at higher temperature and limit
`the viscosity reduction.4
`
`The presence of groups other than methyl along the chain leads to a reduction of the
`polymer’s thermal stability, but with this substitution, some of the above properties can be
`modified:
`
`• A small percentage of phenyl groups along the chain alters it sufficiently to affect
`crystallization and allow the polymer to remain flexible at very low temperatures. The
`phenyl groups also increase the refractive index.
`
`• Trifluoropropyl groups along the chain change the solubility parameter of the polymer
`from 7.5 to 9.5 cal1/2·cm-3/2. These copolymers are used to prepare elastomers with little
`swelling in alkane or aromatic solvents.
`
`Note: this article (here revised) was originally published in Chimie Nouvelle, vol. 8 (30), 847 (1990)
`by A. Colas from Dow Corning Corporation.
`
`7. References
`
`1 FEARON, F.W.G., High performance polymers, edit. R. Seymour and G. Kirsenbaum,
`Elsevier, 1986.
`2 ROCHOW, E.G., Silicon and silicones, Springler-Verlag, 1987.
`3 HARDMAN, B., Silicones, Encycl. Polym. Sci. Eng., vol. 15, 204, 1989.
`4 STARK, F.O., et coll., Silicones, Comprehensive Organometallic Chemistry, vol. 2, 305,
`Pergamon Press, 1982.
`5 PLUEDDEMAN, E.P., Silane coupling agents, Plenum Press, NY, 1982.
`6 NOLL, W., Chemistry and technology of silicones, Academic Press, 1968.
`7 OWEN, M.J., Chemtech, 11, 288, 1981.
`
`Silicone Chemistry Overview
`
`11
`
`

`
`The information and data contained herein are based on
`information we believe reliable. You should thoroughly test
`any application, and independently conclude satisfactory
`performance before use. Suggestions of uses should not be
`taken as inducements to infringe any particular patent.
`
`©1997 Dow Corning Corporation.
`All rights reserved.
`Printed in USA
`
`AMPM0105/397
`
`Form No. 51-960A-97
`
`Dow Corning Corporation
`Midland, Michigan 48686-0994