Silicone - SpeedyLook encyclopedia

Uses

While varying the length of the chains - Si-O, the fixed groups and the connections between chains, silicones provide a large variety of materials. Their consistency goes from the Liquide (Implant mammaire) to the plastic hard, while passing by the Gel and the Gomme. Silicones are present a little everywhere in the daily newspaper, in the form of Mastic S, Colle S, joint S, additives antifoaming for powders detergent, cosmetic, medical device, insulating sleeves of electric cables, greases high efficiency, etc

History

While seeking to combine the properties of made up of carbon (plastics) with those of composed of silicon (glasses), a researcher of Corning (the USA), J.F. Hyde invented silicones in 1938. The company Dow Corning was founded in 1943 to use this invention which led to a great number of resins, varnish, elastomers and of another human use etc

Classification

Chemical nomenclature

The nomenclature of polymers in general is complex and of a difficult use. That of silicones was simplified by the use of the letters M, D, T and Q to represent monomeric units mono, di, sorting, and tétrafonctionnelles.

Nomenclature of silicones

physical shape

oil or gum (various viscosities)
resins

elastomers (formed starting from materials above)

vulcanizable cold (EVF)
vulcanizable hot (EVC)

Properties

Silicones are characterized by two fundamental properties:

“strong” Si_O connection
* great chemical inertia
* good behavior with UV (ultraviolet)
* high breaking down temperature

flexibility of the polymeric chain
* weak Viscosity
* Temperature of vitreous transition low
* weak dependence from the mechanical properties (viscosity,…) with the temperature.

Manufacture

The raw material is the pure Silicium, obtained starting from the quartz by electrometallurgy. One makes react this silicon in chemical engines with Chlorure of methyl (reaction of Rochow), to obtain Méthylchlorosilane S, of which most important is the Diméthyldichlorosilane (DMDCS) having for chemical formula:

(CH3) ₂Cl₂Si.

The DMDCS is then hydrolized to eliminate the Chlore, then a polycondensation (polymerization with water elimination) led to the famous chain - If-O-iF-O-If-O. It is necessary to adjust the length of the chain, the connections, and then to graft the functions necessary to the concerned use.

Industry

A monograph

History of silicones

The history of silicones is due to the conjugation of university research and industrial.

Raw material

The primary product of the chemistry of silicones is the quartz, i.e. the silicon silica or oxide, SiO2. The name " silicium" comes from Latin flint, silicis : stone.

Silicon exists in nature only in combined form mainly dioxide of silicon and silicates. They account for 25,8% in weight of the Earth's crust what makes of it the second chemical element by its abundance (behind oxygen) and the primary product most important of mineral materials. The presence of silicon was even proven in the moon rocks and the meteorites.

In the most moved back historical times, the man employed construction materials and matters containing of silicon, for example the ceramic sands, clay and materials. One of the oldest uses of silica was the manufacture of the Verre.

But as silicon does not meet in nature in a native state, it was obtained only relatively late.

Academic research

It is Jöns Jacob Berzelius which, the first, isolated silicon in 1824 by treating fluosilicate from potassium (K2SiF6) with a metal excess of Potassium. By continuing its research, it made heat silicon in a current of Chlore, which had like effect a vigorous Combustion. The product thus obtained being the silicon tetrachloride, one of the current primary products in the manufacture of silicones.

During all the 19th century, chemists were interested in chemistry of " … substances in which, silicon played the same part dominating as that plays carbon in the compounds organiques" (F. Wöhler). One counts among them names as famous as Persoz, Ebelman, Wöhler, Friedel, Crafts… All this work had however led to no industrial realization.

First half of the 20th century saw increasing the number of researchers in the field of the organic derivatives of silicon. Stock, Moissan, Spalling hammers, Renning… could prepare silicon compounds while passing via silicide of magnesium (SiMg2) and an acid. But it is F.S. Kipping which carried out the academic projection most important, showing that there did not exist any proof that the chemistry of silicon was really similar to that of carbon.

Indeed after eight years of work, it could show that an asymmetrical silicon atom is unable to deviate the polarized Lumière. In the same way it concluded that silicon does not present double connections with carbon nor with the Oxygène, (assertion recently blamed by Chojnowski).

Kipping discovered that the Hard grindstone reaction (see further) constituted a very effective means of fixing of the organic groups to silicon and that in certain cases one even managed to break the connection - If-O-If.

Up to 1940, little work refers to made up of high molecular weight: the viscous oil formation had been announced, but these products had been rather obtained by accident that voluntarily and, so had never been studied systematically. For the discharge of these researchers it should be said that the chemistry of polymers was still with its stammerings, because 1930 should have been waited until to see definite the principles of polymerization. Nothing at the time could encourage the researcher to suppose that these undesirable products had on the contrary a great practical importance.

Industrial research

Work of Staudinger on the macromolecules upset accustomed of a traditional chemistry which rested on insulation by distillation or recrystallization of the pure compounds and opened the way at the era of the plastics, which gradually start to become products commercial.

Some of these products were transparent and the manufacturers of glass immediately envisaged competition. Thus Corning Glass Works immediately was interested in high polymers containing at the same time organic and inorganic components, perceived like hybrid polymers halfway between glass and the plastic.

The first application of these products was the substitution of the resinous binders in the manufacture of ribbons of glass fibers used in the electric insulation, but to exploit to the maximum qualities of these ribbons, one recognized the need for conferring a good sensitivity to heat to them. The properties of these new products interested the General Electric Co much, at the point to create a new research laboratory under the direction of Dr. Rochow. At the same time Corning Works Knell and Dow Chemical were intended to finance the creation of Dow Corning Corporation, whose scientific department was entrusted to Dr. McGregor.

The Second world war doped research in this type of products to the unknown properties hitherto, intended in priority for the armed forces. The fluids silicone, very stable and isoviscous, still manufactured in small quantities, were used for the garnishing of the very sensitive cushioning devices of apparatuses, used by the military aviation. Later, the circuits of lighting of the engines of planes were isolated by greases from silicone, and appeared the first antifoaming ones.

A close cooperation between the army and industry will facilitate these rapids progress.

Finally in 1945 Dow Corning and General Electric announced simultaneously the development of a silicone rubber preserving its properties at temperatures to which the rubber S organics are not usable.

With the end of the war the needs for the army decreased abruptly, but the diversity of the possible applications of these products allowed important outlets in civil industry. For example like, product release agents of polishing, lubricants, manufacture of varnish, fabric proofing… This development involved a reduction in the prices, and thus the opening of new fields of application still requiring a larger production and cycle is repeated.

The industrial property

Because of war, the development of the industry of silicones in Europe was very delayed. However Rhône-Poulenc in France and Bayer in Germany had begun research what enabled them to start to manufacture as of the end of the war.

However the management of the Patent S of manufacture of silicones being particularly complex, these firms and other industrialists in several countries of Europe and Japan decided to go purchasers of licenses of manufactoring processes of General Electric and Dow Corning in order to profit from the American experiment.

Table n° 1: Transfer of industrial licenses in Europe

At the end of 2006, the production activity of silicones of General Electric and shares that this one had in collaboration (" Joint-Venture") with Bayer and Toshiba were gathered and yielded to a financial holding, Apollo. The new unit took the name of Momentive Performance Materials. At the beginning of 2007, the silicones activity of Rhodia was sold in Bluestar Silicones, a Chinese company.

In the Eastern European countries, research on the organic derivatives of silicone was placed under the direction of official organizations, but they had evil to fill the delay accumulated during all first half of the 20th century. Although the Russian researchers had very early had the prescience of the industrial interest which these compounds could present, the lack of Magnésium in Russia prohibited manufacture according to the method of Hard grindstone.

Chemical approach

Definition

A silicone can be defined as a compound which contains the elements silicon and oxygen and of the organic groups, silicon being present in sufficient proportion to affect the properties of the sensitive product of way. The expression " silicone" was initially applied to any compound in which there is presence of silicon, each atom of silicon being surrounded of two atoms of oxygen and two atoms of carbon. As the study of these products developed, this expression took a more general direction little by little.

Nomenclature

In parallel, a nomenclature was created in order to facilitate the definition of silicones. The nomenclature of polymers in general is complex and of a difficult use, that of silicones was simplified by the use of the letters M, D, T and Q to represent monomeric units mono, di, sorting, and tétrafonctionnelles.

Diagram n° 1: Nomenclature of silicones

Table N° 2 gives some indications for the use of this nomenclature.

Table n° 2: Nomenclature of silicones

Organic substituents: M: aliphatic Me: aromatic.

Structure

The chains siloxanes are very flexible and rotation around the axis Si-O is very easy, especially with substituents of small size. Rotation is also possible around the axis If - C for methyl silicones.

The characteristics of the connections appear in table N° 3.

Table n° 3: Characteristics of the connections

From this a great freedom of movement results from the polymeric chains, with important intermolecular distances and weak interactions.

Sand with polymer

The first operation, in the preparation of a silicone, is the manufacture of silicon. Oxide SiO2 the silicon is tiny room to the silicon state by heating to the electric furnace, in the presence of carbon

Silicon is appeared as a solid, gray body black, having a metal reflection. If it is inert at the ordinary temperature, it combines easily with oxygen with the high temperatures.

Diagram n° 2: Chemical conversions of silica

It is thus necessary to transform silicon into reactive and volatile substance to obtain compounds which one will still make react to produce silicones commercial.

Figure 2 represents the preparation of silicones schematically.

The potential reactional intermediaries can have various reactive functions, but only used industrially are methylchlorosilanes.

Methylchlorosilanes

Methylchlorosilanes are the starting matter of the méthylsilicones. Industrially, they are manufactured by reaction of methyl chloride with silicon.

The operation is carried out in a system with fluxed bed. Metal silicon is crushed and mixed with copper in fine particles. The methyl chloride in the liquid state, is vaporized under pressure and is diffused through the bed at important speeds. The percentages of conversion are significant for short residence times, with 5 - 10% of metal copper. The catalytic effect of copper in this reaction is due to the oxidizing action of methyl chloride on copper, compound with the reduction of silicon with respect to copper salts.

The required product is the diméthyldichlorosilane, and it is obtained with outputs higher than 50%. It is separated from the other products of the reaction by distillation. The selectivity of the reaction is determined by the relationship between the méthyltrichlorosilane and the diméthyldichlorosilane (T/D): it must be about 0,1 - 0,2.

The production of each compound which cannot be controlled, one proceeds to interconversions in order to answer the request.

With balance the mixture contains approximately 70% of the required product.

The connection silicon-chlorine is stable with heat, but reacts highly with ammonia, water, alcohol, the organic acids and the reagents containing of the hydroxyls groups: they can be hydrolized by the moisture of the air during the reaction of formation with release of HCl and formation of a polymeric gel. The danger is then triple because, in addition to the toxic and corrosive effects, one attends the conjugation of an overpressure due to the stopping of the pipes by freezing and to an increase in the temperature because of the exothermy of the hydrolysis.

All, in the industrial process (hermetic steel engines, bore…) is made so that this does not occur.

Chlorine is easily replaced by various organic groupings; this high reactivity, at the same time as the facility with which methylchlorosilanes can be obtained, are the reasons of their great interest in the synthesis of silicones.

Polycondensation by hydrolysis

In a current way, the silicone oligomers and polymers are formed by reaction of a organohalogénosilane and water. The reaction of assessment is the following one:

The first stage is the hydrolysis of the chlorosilane in silanols, which condense quickly out of siloxanes.

Polydiméthylsiloxanes (PDMS) constitute by far the greatest volume of homopolymers produces today.

The molar mass is controlled by the addition of the finishing monomers of chain which can be reactive or not. The nonreactive monomer more used is the triméthylchlorosilane. The purpose of the final reactive groupings are to be able to reticulate the chains by condensation subsequently (amine groups, alkoxy, hydroxy, acetate, oxime, silanol…) or radicalairement (vinyl groups…).

General properties of silicones

The polydiméthylsiloxane with terminations triméthylsiloxy is the silicone manufactured. It is used as a fluid, pasty product or in the form of réticulé elastomer.

Because of the weak intermolecular forces, the polymers always have points of boiling and temperatures of vitreous transition very low and, under normal conditions, they do not crystallize. The freedom of rotation around the bond siloxane confers on the chains siloxane a great flexibility, and in comparison with other polymers, weak changes of the physical properties with the molar mass and the temperature.

The PDMS of low molar mass are Newtoniens fluids (viscosity does not vary with the rate of shearing), but become non-Newtonian when the molar mass increases. An interesting characteristic rheological is the practically negligible effect of the temperature on viscosity what was worth to them to be used preferentially with the hydrocarbon fluids.

The great aptitude for compression was of continuation development by the use of silicones like shock absorber products (12 to 15% of compression compared with 8% for other mineral oils with 200MPa).

They also have excellent dielectric properties, a great strength to the temperature but have a more important permeability to gases than other polymers.

But the principal silicone characteristic is their weak energy of surface. This is the base of their antifoaming application like agents, lubricants, agent unmoulding or anti-adhesive. They present a single dispersive component of the energy of surface of about 18 - 22 mJ/m2.

The whole of these properties makes silicones in general, a family of polymers completely different from organic polymers.

Two industrial methods of obtaining the basic commodities

As one saw in the preceding chapter, the preparation of polymeric silicones comprises, initially, the preparation of the organohalogénosilanes, then the hydrolysis of suitable mixture of these bodies and finally the condensation of polymers to finish the construction of the required molecules.

A choice between the methods of preparation intervenes only for the first stage, the second and the third being generally reached in only one manner, whatever the mode of obtaining the intermediaries. The problem thus returns, from the synthetic point of view, the preparation of the methylated chlorosilanes, since they are most important and the only intermediaries requested for the industrially elaborate products.

Method of Hard grindstone

The most direct solution would consist in adapting the traditional methods of synthesis to the laboratory on an industrial scale, i.e. the method of Hard grindstone. To prepare the diméthylsilicone starting from methyl chloride one would have successively:

While going back to the rough raw materials, it is necessary to also count obtaining sand, coke, chlorine, methane (or methanol) and the magnesium, whose choice of preparation is usually determined by local conditions and does not intervene in the general considerations relating to the method.

All the Hard grindstone process can be summarized in only one équation*:

This reveals that the weight of the auxiliary bodies as chlorine or magnesium is 4,5 times that of the méthylsilicone. If one wants to recover them, one would have to consume a considerable electrical energy.

The method of Hard grindstone is a process of substitution leading to a mixture of products. The theoretical yield of 70% in principal product is tiny room to 50% under industrial conditions of distillation.

On the preceding bases, the complete manufacture of a polymeric silicone by this method can be schematized by the diagram of figure 3.

The Hard grindstone process has the advantage of being able to be applied to the preparation of others organochlorosilanes. At the same time, the well-known objections which concentrate on the handling of the magnesian, unstable reagents and too credits remain on an industrial scale.

From an economic standpoint, the principal disadvantages are:

multiplicity of the operations;
obligation to use, as source of silicon, the tetrachloride of alkyl silicon or silicate, whose silicon contents are lower than 15%.

These objections not constituting insurmountable obstacles with the industrial exploitation of the method, but they handicap it considerably.

Diagram n° 3: Method of Hard grindstone

Direct method or of Rochow

Under this title one knows another process agreeing with the manufacture of the dialcoyldichlorosilanes. This method is the fruit of work undertaken to prepare compounds organosilicés while being freed from the traditional methods of substitution using the Hard grindstone reagents. In theory, this process rests on the action of halogenous carbides on free silicon to obtain mixtures of alcoyl halosilanes of general formula RaSiXb where + B has = 4. As we indicated in the preceding chapter, copper is employed like catalyst.

By adopting same conventions on the raw materials as in the discussion of the method Hard grindstone, the stages are:

These phases can be summarized in a single equation *:

It is obvious that this process is simpler than the method of Hard grindstone, from the point of view of the raw materials and the chemical operations. One does not consume chlorine, because the HCl released by the hydrolysis of the diméthyldichlorosilane is consumed, in its totality, during its reaction with methanol. This stage of the study we see already two favor compared to the preceding method:

not of consumption of electrical energy for reconversion of HCl,
not of cycle of magnesium.

The percentage of copper is about 10% in weight, and it completely is found when silicon is consumed. Its recovery is in possible theory, but being given its trifling value, it is necessary to stick to simplicity of the process.

Figure 4 shows the transformations of the raw materials leading to the méthylsilicone starting from methanol.

Diagram n° 4: Direct method

Others alkylchlorosilanes can also be prepared by this méthode*.

Selectivities

The method of Hard grindstone is a process of substitution leading to a mixture of five products: SiCl4, RSiCl 3, R 2SiCl 2, R 3SiCl and R4Si, the proportion of each one in the mixture being function of the molar report/ratio of the magnesian reagent to CCL4.

The required product is the diméthyldichlorosilane. SiCl4 and RSiCl3 can be separate and recycled for a subsequent alcoxylation to increase the output, the others made up must be separate mixture and eliminated, unless holding them for special uses.

The direct method is less flexible. As in the preceding case, it provides a mixture of methylchlorosilanes, but under adapted conditions, the required product is the constituent main thing. However, in the direct synthesis it is not possible to recycle and, consequently, any transformation requires additional operations. Possibly, the minority products can be recycled for the method of Hard grindstone.

With regard to the manufacture of alcoysilanestrichlorés, it is enough to write only one equation to realize that the direct method is not appropriate for the manufacture on a large scale.

For a molecule of trichlorosilane produced, one consumes three methyl chloride molecules. In practice, it is formed in minor amount in the direct reaction and the proportion increases when the temperature rises.

This product being usable in quantity reduced in polymeric siloxanes to transverse connections, that which is produced by the direct process can be absorbed entirely.

Practical industrial

Industrially, the direct method only is currently used. Diagram n° 5: Industrial method of manufacture of silicones This method allows, starting from a limited number of chemical components and operations, to arrive at an infinity of components at the various applications. The diagram n° 5 watch obtaining these principal products.

The operations necessary for the transformation of the intermediaries into polymers (distillation, hydrolysis, condensation .......) are traditional and absolutely identical whatever the methods followed to manufacture them.

The first problem arising is that of distillation. The points of boiling of the tri and di-chlorinated intermediaries being 66°C and 70°C, their separation by correction is difficult. Not only the capacity dephlegmator of the column must be high, but it is necessary to take account of the chemical properties of the bodies to distill: all handling must be carried out safe from moisture, because potentially releasable HCl can corrode the metals usually employed in chemical engineering.

The operation of hydrolysis comprises the handling of HCl, aqueous or gas: the use of an equipment able to resist its corrosive action is a requirement.

The secondary treatment of polymers is simpler, from the point of view of industrial chemistry: equipment used is common to all elastomers.

Silicone elastomers

Big families of silicones

The approach that we carried out until now enabled us to see the immense possibilities of making of materials which the chemistry of siloxanes can offer to us. Indeed, and according to the nature of the reactional groups that we introduce into the chain, their quantities…, we can obtain products inert, reactive, that can be reticulated…

Generally one agrees to gather the whole of these products in three big families:

fluids;
resins;
elastomers;

according to their physical statuses (viscosity, percentage of reticulation, mechanical properties).

The fluids are linear systems of PDMS, in which, the number of silicon atoms in the chain can be higher than 1000. Compared with mineral oils, they have a constant viscosity in a broad beach of temperatures. The fluids are characterized by a helicoid structure and a capacity of high spreading out which is accompanied by the possibility of developing special properties like the hydrophobia or the anti-foamer effect. In the same way, the non-polar and nonassociable groupings methyl being, the chains slip the ones on the others to be spread out in extremely thin layers. Their inert character can be modified by introducing reactive groupings.

The resins are chains siloxane extending from the intermediate products to the resins of raised molecular weight and extremely variable structure. But all the resins have a common point: their high degree of reticulation. The intermediate products open multiple possibilities of association to organic resins to form copolymers. The reticulation proceeds with high temperature over one prolonged enough duration during which the resin passes by a thermoplastic phase.

The elastomers are products which present good elastic properties by a weak vulcanization of the different components of the formulation.

Silicone elastomers

Certain reactions in which a silanol group and another siloxane containing a group If - X*, are concerned, were especially studied for the formulation of silicone elastomers. The macromolecules thus formed have specific properties, but the preparation of polymeric materials having improved mechanical properties (elasticity, in particular), requires the modification of linear silicones mixed beforehand with loads of reinforcement.

Rubbery elasticity corresponds to an easy deformation of the macromolecules due to a great freedom of rotation around the bonds Si-O. When a stretched rubber turns over in its initial state, each macromolecule finds its most probable form corresponding to the highest entropy; the stretched state, more organized, corresponds to a lower entropy.

This return supposes that during drawing, there no was slip of the macromolecular chains the ones compared to the others. In this case, there is a permanent deformation; to avoid it the chains are reticulated: it is the operation of vulcanization.

The operation of vulcanization thus consists in creating a certain number of covalent bonds between the elastomer chains in order to form a three-dimensional network and to thus prevent the Fluage, which would inevitably occur at the time of a constraint.

The silicone elastomers are generally classified according to the form of vulcanization employed.

Vulcanizable elastomers hot

The vulcanizable silicone elastomers hot are composed by linear chains PDMS of the type:

A certain percentage of methyl groups can be substituted to confer on elastomer better properties. Thus vinyl groups improve vulcanization and the permanent deformation after compression, by the groups phenyl or ethyl can increase flexibility at low temperature, of the trifluoropropyle groups are supposed to increase resistance to solvents…

Vulcanization, of ridicalizing type, is carried out in a few minutes with higher temperatures with 110°C, using one or more organic peroxides (peroxides of benzoyl and dicumyle for the simplest cases) in small proportion (1 to 2%). The mechanism of vulcanization comprises the formation of bridges ethylene by creation of free radicals on the methyl group.

When the polymer contains small percentages of groups vinyl (<1%), the radical peroxidizes attack the double connection to give a radical which leads to vulcanization. In all the cases, it is often necessary to prolong vulcanization by a post-cooking of a few hours to high temperatures (150° - 200°C).

The typical composition of a vulcanizable silicone elastomer hot is given in table n° 4.

Table n° 4: Standard formulation of a vulcanizable silicone elastomer hot

Component Left in weight PDMS 100 Charge reinforcing (silica) 30 Charge not reinforcing 70 Various additives (of which peroxides) 10 Pigments 1

In a general way, this type of products are presented in the form of one only component, but in order to prevent a possible evolution of the mixture, it is possible to obtain systems in which the peroxide is added at the time of the use. Commercially one finds also dispersions in a solvent like xylene to decrease the viscosity of the product.

Vulcanizable elastomers cold (EVF)

These silicone elastomers can be presented in two different forms. The vulcanization of the products monocomposants is started by the moisture of the air. That of the products bicomposants is carried out after mixture of the two components, one of them containing catalyst; two components being conditioned separately.

The vulcanizable elastomers bicomposants cold, are presented in two parts, called has and B. In this type of EVF, the crosslinking agent must be added to basic polymer just at the time of its use. Generally it is about a silicate alkyl tétrafonctionnel in the presence of a organostanneux catalyst. The mechanism of vulcanization is the following: The reaction is complete at the end of one day, but as it is not very sensitive to the influence of the temperature, it is useless to work beyond 40 - 50°C. On the other hand, it can be accelerated by addition of a platinum salt; the substitution of a certain number of methyl groups by vinyl groups accelerates this reaction.

Contrary to the first case, this reaction of addition is very sensitive to the temperature. For this reason one qualifies them elastomers " with catch accélérable hot ". The time necessary to obtain a completely vulcanized film is of approximately a day to room temperature, but passes to 1 a.m. when the temperature reached 150°C.

Various loads and additives can be added to obtain the wished properties, but of the preliminary tests of compatibility must be carried out in order to envisage the poisoning of catalyst. This one is generally built-in the part has EVF with the PDMS containing of the vinyl functions and the polysiloxane being used as agent of vulcanization is conditioned in the part B. According to the awaited properties, the proportions of the two parts can enormously vary; it is thus extremely difficult to provide a standard formulation of the EVF bicomposants.

This category of silicone elastomers are characterized to have a null adherence on practically all surfaces, from where them employment for the manufacture of flexible moulds. Nevertheless, one can obtain a certain adhesion by use of primary educations of fixing.

The vulcanizable elastomers monocomposants cold are réticulés by condensation. As its name indicates it, they are presented in only one component and of this fact are ready with employment.

The totality of vulcanizable the elastomers compositions to room temperature contain the following components: a hydroxylated polysiloxane a-w; a reticulating agent of the RSiX3 type or If X4, in which group X can be hydrolized; a reinforcing load, generally silica; an accelerator, for example a metal salt; various additives (dyes, fungicides…).

The " catalyseur" reticulation is external, since it is about the moisture of the air. The reticulation thus starts, as soon as the product is in contact with the air and it is propagated outside towards the interior; it is however relatively slow because the speed of diffusion of the steam through the mass is low.

The chemical principle of reticulation is same whatever the hydrolysable group:

The chemical nature of X can be very varied. The first and most used monocomposantes formulations present, like groups hydrolysable a system acétoxysilane, but alcoxy groups, amino, amido…, can also be used.

The reaction between a polysiloxane with terminations silanol and an agent reticulating méthyltriacétoxysilane was already protected in 1957 by the German patent N° 1.121.329 from Rhône-Poulenc. The composition thus described remained stable during a storage of several months in a confined atmosphere but reticulated in wet atmosphere in a few hours. Condensation can be catalyzed by metal salts and more particularly of tin and titanium or mixtures of both.

Industrially, the triacétoxy group is brought in the following way:

VI: CH2 = CH - ac: CH3 - CO -

Vulcanization is carried out by hydrolysis of the acétoxyloxysilanes under the action of the humidity of the atmosphere: the formed silanol condenses with another acétoxyloxysilane group. To accelerate vulcanization substantially, one introduces products which release from water by reaction with the acetic acid.

Table n° 5 provides us the standard formulation of a monocomposant EVF.

Table n° 5: Standard composition of a monocomposant EVF

Component Left in weight Base PDMS 100 Loads renforçantes 20 Additives and pigments 15 Vulcanizer Méthyltriacétoxysilane 5 Accelerator 0,1

Properties and applications of silicone elastomers

As one could note it, only one small portion of the polysiloxanes manufactured is employed as an elastomer. However, being given the enormous possibilities of chemical and physical modification varying their properties, their scopes of application are very varied.

Properties

The silicone vulcanisats are proof of a stability with respect to the diluted acids and of alkalis, just as with respect to polar solvents. They have a remarkable stability with the bad weather and ageing. As example one can quote the estimate which appears in table n° 6D.

Table n° 6: Estimate of the lifespan of a silicone elastomer

Also they have pronounced thermal stability. Temperatures being able to reach 180°C practically do not deteriorate their elasticity; but it is necessary to take care not to subject rubber silicone to a constraint of heat before vulcanization is not completely finished. They remain elastic until approximately -50°C, but at lower temperature, they lose most of their flexibility.

With room temperature, they have electric properties comparable with those of other insulating materials, but they have the advantage of preserving them in a broad beach of temperatures. While burning, they give a nonconducting structure from where them employment in electricals appliance.

Certain intrinsic properties of silicone elastomers can be modified or improved by addition of specific additives. One of the properties most commonly wished is the adhesiveness on supports of different nature. Thus one could raise the additives which appear in table n°7 being used to improve this adhesiveness.

Table n° 7: Additives for formulations of silicone elastomers

Additive Support If (GOLD) X (OCOR') X Aluminum Salts of Zirconium Aluminum, Steel Epoxysilanes Glass, Metals Méthyléthylsilicate Metals, PVC Other silicone Metals resins

Silicones present, on average, a permeability to gases with room temperature 10 times higher than that of the natural rubber, but approaches some towards 100-150°C. According to the nature of gas, replacements of certain methyl groups can be considered. Table n° 8: Permeability of silicone elastomers

Type of silicone Permeability to 25°C in mmol/(m.s.GPa) CO2 O2 N2 Methyl 1,1 200 93 Aromatic 250 42 16 Fluorinated - 38 16

Beside multiple advantages, the EVF present a major disadvantage because of their mode of vulcanization, which in certain cases can become a serious handicap for their use. Indeed the products given off during the hydrolysis can produce the corrosion of certain metal substrates.

Silicone elastomers different from organic elastomers. The most important difference is the degree with which the mechanical properties depend on the reinforcement conferred by the incorporation of certain loads, silica pyro in particular: stress the rupture in traction can be multiplied by 50. The selection of the loads (natural, percentage, granulometry) are then extremely important.

Applications

Because of their exceptional ageing and their single mechanical properties, the silicone elastomers did not cease increasing their market shares in a multitude of applications in the sectors industrialist, of the building and general public. In the past, they were regarded as exotic specialities, whose cost justified their application only in cases where very high efficiencies were required.

The widening of their use, in the 15 last years, made competitive these products in vast sectors, particularly when their price is balanced, relatively high, by their higher properties. The request in the industrial sector for products with high efficiencies did not cease increasing.

Applications of silicone elastomers

Car, Building, Rubber, Chemistry: joints in situ'; other joints; radiators; visco-clutches; connectors; headlights; air filters; shock absorbers; bumper; hydrofugation; impregnation of surfaces; binders of painting; stone conservation; cements of jointing; stuck external glazing; shaped joints; splice plates; release agents; plastic additives; moulded articles; agrochemicals; food industry; oil industry; detergents; agents of glossing; industry of the tire… Mechanical engineering Cosmetics Electronic Electric household appliances Machines and installationsAppareils of mesureFiltrageSalles blanchesBatteries Care capillairesSoins of the peauDéodorantsMaquillageSoins bucaux Radiators tubulairesFers with repasserCuisinièresPots in verreFours microwaves AutomobileGrand publicSemiconducteursTech. photovoltaic Insulation Metallurgy Paper Painting and varnish Glass fiber/stratifiéMicaMoteurs and générateursElectroaimantsTransformateursEclairage Run précisionEnduction of tôlesLaitiersAdjuvants of welding Papers antiadhérantsPapiers intercalairesFilms Adjuv. peinturesPeintures résistantesRevêtementsAnticorrosionAlkydesEncres of printing works Pharmacy Medicine Figures Textile Reproduction and cuirH Subst. pharmaceutiquesPréparations diversesProthèsesTubes diversSoufflets of respirationMoulage impressed AntiadhérantsLubrifiantsAgents of adherence Oils for cylindresCylindres/rubansToner AdoucissantsHydrofugationAntimoussesAgents of accrochageAgents of oiling Transmissions Transport (others) Insulating S composites Coating of Additional insulators of cables Aeronautics Astronautics Naval construction Railroad Staff (flexible moulds: to see with staff)

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