SiliconeRubber-中英文.doc

Silicone Rubber 1 BASIC STRUCTURE 基本结构基本结构 Technically known as polyorganosiloxanes silicones are polymeric compounds in which silicon atoms join together with oxygen as chains or networks The remaining valences of silicon link with organic groups mainly methyl groups 技术上我们称之为聚有机硅氧烷类 硅酮是硅原子与氧连接或网接而成的聚合物 硅链上 剩余的原子价上连接有机基团 主要是甲基 The diagram shows a typical silicone molecule 图片是一个典型的硅酮分子 It is essentially an organically modified quartz i e two oxygen atoms attached to every silicon atom have been replaced by methyl groups This changes the structure from the three dimensional backbone of quartz to linear polymer molecules Silicones therefore are composed of the elements Si O C and H Silicone molecules are usually tangled up in each other However the methyl groups are free to rotate about the Si O Si chain 它其实就是一个有机的改良的石英 也就是说 每个硅原子上附加两个氧原子而且被甲基 重组 这是石英从线性聚合分子向三维主链的结构转化 因此硅酮由元素硅 氧 碳 氢 组成 硅酮分子经常是相互之间缠绕 但是 甲基基团是可以围绕硅氧链进行自由旋转的 In this way silicones unite the durability of quartz and the many qualities of modern plastics Other groups can be attached to the silicon oxygen backbone instead of methyl groups The linear silicone polymers can be crosslinked to each other to different extents i e covalently via groups of atoms This explains the various different properties of the corresponding silicones 其他基团可以附加在硅氧主链上来替代甲基 这些线性硅聚合物可以不同程度的相互进行 交联 也就是说 借助原子基团的共价键 这说明相应的硅酮会有不同的特性 2 A SHORT HISTORY OF SILICONES PRODUCTION Silicones are industrially produced compounds derived from the element silicon Although silicon is the second most common element in the earth s crust its high affinity for oxygen means that it is only found in compounds with oxygen namely as silicates and silicon dioxide that make up minerals and sand Elemental silicon was discovered at a relatively late stage because of the very high silicon oxygen bond energies The principal steps in the development of silicone chemistry were The discovery of silicon by Berzelius in 1824 from the reduction of silicon tetrafluoride with potassium Kipping generally considered the father of silicone chemistry laid the foundation of the industry with among other things the preparation of various silanes by means of Grignard reactions and the hydrolysis of chlorosilanes to yield large molecules 3 SILICON The element silicon 元素硅元素硅 Derived from the Latin silex meaning flint silicon is the commonest building block of our planet after oxygen It makes up 26 of the planet s crust and has also been found in meteorites and lunar rocks It is always found in chemically and thermically stable mineral combinations but never in its pure form Silicon is the key to all silicone chemistry as its atomic structure dictates the properties of silicones 26 of the Earth s crust is made of Silicon Chemistry Silicon and oxygen have a strong chemical affinity and therefore silicon only occurs naturally in the particularly stable form of Si O compounds such as calcium magnesium and iron silicates as well as SiO2 as sand and quartz The tetravelant structure is common to all compounds in which silicon is surrounded by oxygen atoms Today elemental silicon is obtained through the electro thermic reduction of SiO2 with carbon at 1 400 degrees Celsius Elemental silicon is a dark grey metallic shiny hard and brittle octaedric material It has a melting point of 1 423 C and a boiling point of 2 630 C Like carbon silicon has a crystalline structure similar to that of diamonds As a representative of the fourth main group of the Periodic Table silicon like carbon displays both metallic and non metallic qualities There are nevertheless some important differences between the two The chemical compounds of Si and C are predominantly tetravalent however both are capable of forming higher or lower coordination numbers The difference between Si and C lies in the significantly lower electro negativity of Si compared to C 1 8 as opposed to 2 5 and in the larger atomic diameter 1 15 as opposed to 0 77 Silicon forms very stable single bonds with the electro negative oxygen while carbon may also form double bonds Double bonds in silicon chemistry are generally limited to a small number of unstable silane compounds SILANES Silanes are the homologues of saturated carbon hydrogen compounds that is of alkanes They all share the general formula SinH2n 2 Unsubstituted silanes made of silicon and hydrogen only are very unstable and can only be produced in the absence of oxygen Much more important are the methylsilanes where some or all hydrogen atoms in the silanes are formally replaced by the methyl groups If the hydrogen atoms are partially replaced by chlorine atoms and other hydrogen atoms by methyl groups the important class of methylchlorosilanes results often abbreviated as chlorosilanes Methylchlorosilanes form the basis for all silicone chemistry Methylsilanes Methylsilanes are the raw materials for manufacturing silicones They are produced by direct reaction between silicon and methyl chloride in the Mueller Rochow synthesis They are extremely mobile colourless liquids that are soluble in organic solvents and in some cases in anhydrous alcohol Silanes have low molecular weights and are thus highly volatile Industrial silicone production has its commercial basis in the direct synthesis of methylchlorosilanes from silicon and methylchloride via a process called the Mueller Rochow synthesis This technique developed independently in 1940 41 by professors R Mueller in Germany and E G Rochow in the United States takes place in the presence of a copper catalyst at approximately 280 C Finely ground and well mixed Si and Cu are brought together in a fluid bed reactor with methyl chloride in gaseous form This produces a silane mixture from which are derived the most important organochlorosilanes Decisive for the commercial production process is to gain dimethyldichlorosilane CH3 2SiCl2 the most important organochlorosilane Optimal production depends on the selectivity and activity of the copper catalyst as well as on the homogeneity of the Si CH3Cl mixture and an even temperature distribution in the reactor avoiding hot spots Contamination of the copper catalyst for example by lead has a drastic negative effect while antimony additives on the other hand can promote the yield Modern fluid bed reactors have a capacity of approximately 40 000 tonnes of raw silanes per year and more Today European manufacturers no longer consider as commercially viable plants producing less than 60 000 tonnes per year During the process the solid elements of the reaction mixture such as for example unconverted elemental silicon and the copper catalyst are separated from the gases The solids are fed back into the reaction process and the gases after condensation separated into liquid crude mixture and gaseous methylchloride The latter is then also fed back Due to the sometimes very small differences between the boiling points of silanes eg methyltrichlorosilane at 66 C dimethyldichlorosilane at 70 C distillation units have to fractionate them in several stages to obtain the individual silanes The distillation columns therefore have many plates and thus high separation efficiency Even small amounts of contaminants e g CH3SiCl3 in CH3 2SiCl2 in the parts per million ppm range interfere with the further processing of the organochlorosilanes to silicones Organochlorosilanes are very sensitive to hydrolysis that is they react readily with water and vigorously give off hydrochloric acid For safety reasons in case of leaks distillation columns are usually not cooled with water but with air Special Silane Syntheses Manufacturers use other methods to produce chlorosilanes to laboratory standard or to manufacture special silanes such as those containing phenyl groups The following syntheses still play an important role in addition to the Mueller Rochow direct synthesis of silanes Addition reaction hydrosilylation Nucleophilic substitution Grignard reaction The purpose of all these syntheses is to incorporate organic functional groups Typical Reactions of the Chlorosilanes The hallmark of chlorosilane chemistry is their pronounced tendency to polycondensation Organochlorosilanes react violently with water releasing hydrochloric acid Care is needed since typically nearly 250 l HCl are released per kg of dimethyldichlorosilane In silicone production the hydrochloric acid formed is returned to the process and reacted with methanol to produce the feedstock material methylchloride Silanols The reaction of the monomer organochlorosilanes with water hydrolosis or methanol methanolysis produces silanols which using HCl catalysis lead directly to further reacted oligomers or polymer siloxanes sil represents silicon ox stands for oxygen and ane describes the saturated nature of the bond The description silicone for the whole class of polysiloxanes is an imitation of the oxygen carbon bonds of carbon chemistry which are known as ketones The latter however because of their particular characteristics form double bonds instead of single bonds Mono di tri or tetrafunctional siloxane units with Si O bonds arise from polycondensation according to the number of chlorine atoms of the basic silane molecule The diverse halogenated silanes serve as building blocks for the synthesis of the various product types of silicones such as fluids or resins Dimethyldichlorosilane enables the formation of long Si O chains At first the hydrolysis or methanolysis of dimethyldichlorosilane gives a mixture of short chained difunctional and therefore linear siloxanes with OH and groups as well as cyclic siloxanes having normally between three and six chain units The linear siloxanes show a helix structure with the methyl groups being freely able to rotate All silicone fluids emulsions and rubbers are based on dimethyldichlorosilane This is therefore the decisive base product If the trichloro compounds are used a cross linking between the linear chains is produced as a result of the three reactive sites of silicon A three dimensional network is the consequence This process is crucial in the formation of silicone resins Monochlorosilanes on the other hand because of their single reactive site can only be used for the terminating of the chain growth by polycondensation They react as a sort of capping agent for the growing silicone chain POLYDIMETHYLSILOXANES PDMS Polydimethylsiloxane is the basic and most commonly available silicone By adjusting Si O chain lengths the functionality of the side groups and the crosslinking between molecular chains silicones can be synthesized into an almost infinite variety of materials Polydimethylsiloxanes are obtained by the hydrolysis of the dimethyldichlorosilane in the presence of an excess of water according to Condensation and Polymerisation However the linear and cyclic oligomers obtained by hydrolysis of the dimethyldichlorosilane have too short a chain for most applications They need to be condensed linears or polymerised cyclics to give macromolecules of sufficient length ORGANOMODIFIED SILOXANES By adding groups to a polysiloxane it is possible to create a silicon based organomodified molecule If the organic group is able for further chemical reactions e g polymerisation the organomodified siloxane is also called organoreactive siloxane The chemical combination of the organic groups attached to the polysiloxane backbone leads to products with outstanding chemical and physical properties This organomodified silicon based molecule can be made to any size Any number of reactive groups can be designed into it In many cases the organic groups are separated from the silicon by a three carbon chain This chain separates the organic reactivity from the effects of the silicon Organic groups can be attached to polysiloxanes in a number of ways Pendant to the polysiloxane backbone at both ends of the polysiloxane chain and at one end of the polysiloxane chain In cases where the organic groups are still chemically reactive organoreactive siloxanes additional processing steps can be carried out in order to generate the final product CHEMICAL REACTIONS ON THE FINISHED SILICONE CROSSLINKING AND MODIFICATION OF SILICONES Whereas hydrolysing the appropriate silanes yields finished silicone fluids and silicone resins the components of a silicone rubber still have to be crosslinked with each other vulcanised or cured There are three different types of crosslinking reactions Peroxide initiated curing where polymer contains vinyl groups Platinum catalysed addition curing where polymer contains vinyl groups and crosslinking agent contains Si H groups Tin catalysed condensation curing between dihydroxypolydimethylsiloxanes and silicic acid esters Aside from the necessary reagents and reaction conditions addition curing and condensation curing also require a suitable catalyst A platinum catalyst is needed for addition and a tin catalyst for condensation curing systems In contrast peroxide initiated curing does not require a catalyst Peroxide curing HTV rubber To carry out peroxide curing it is first necessary to generate free radicals This can be done either with heat or with radiation Different organic peroxides may serve as free radical generators for initiating this type of curing Addition Curing RTV 2 rubber As already mentioned addition curing functions by attaching Si H groups to double bonds Salts or complexes of platinum palladium or rhodium may serve as catalyst If platinum olefin complexes are used curing will take place at room temperature Platinum complexes containing nitrogen are used for effecting addition curing at elevated temperatures e g Pt complexes with pyridine benzonitrile or benzotriazole Condensation curing RTV 2 rubber Typical catalysts for condensation curing are dibutyltin dilaurate and dibutyltin octoate They catalyse the reaction between dihydroxypolydimethylsiloxanes and silicic acid esters Water has a strong accelerating effect on the rate of reaction The rate of reaction also depends on the crosslinking agent its functionality concentration and chemical structure and on the type of catalyst Unlike organic latexes and rubbers no sulfur is used for curing silicone rubbers Condensation curing RTV 1 rubber RTV 1 silicone rubbers are one component products that are free flowing or paste like in consistency They react with atmospheric humidity to form flexible rubbers RTV 1 Room Temperature Vulcanizing 1 component By virtue of their outstanding properties these silicone rubbers are ideal for many sealing bonding and coating applications During the manufacturing process terminal OH groups of the polysiloxane react with the crosslinking agent generating curable products The reaction itself takes place on exposure to atmospheric moisture and is accompanied by the liberation of hydrolosis products This reaction which is also referred to as vulcanization starts with the formation of a skin at the surface of the rubber and continues gradually towards the inside Differences between condensation and addition curing RTV 2 rubbers Condensation curingAddition curing Blending ratio of silicone rubber and catalyst variable within limits Blending ratio of the two components is fixed Crosslinker agent and catalyst are both contained in the catalyst Cross linking agent H siloxane in rubber component 1 catalyst platinum complex in rubber component 2 Curing impaired only by lack of water Curing impaired by various substances sulfur compounds etc Curing rate largely independent of temperature Curing rate heavily dependent on temperature Chemical shrinkage due to release of alcohol Practically no shrinkage Release products alcohol may cause reversion from 80 C and above No reversion possible Long pot life and hence long curing times Where pot life is long curing can be accelerated by exposure to elevated temperatures FLUIDS Silicone fluids are linear polydimethylsiloxanes whose chains contain between 2 and well over 1 000 silicon atoms and are obtained through condensation of dimethyldichlorosilane The condensation to higher molecular units as well as the ring opening of the cyclic by products is done with acidic or alkaline catalysts Adding monofunctional trimethylchlorosilane generally ends the growth of the molecular chain The resulting trimethylsilyl terminated silicone fluids are henceforth chemically inert Modified silicone fluids eg alkyl vinyl or aminofunctional polysiloxanes can be produced via the hydrolysis of organofunctional silanes The implementation of ethylene oxide CH2 CH2O or propylene oxide units generates copolymer systems that are distinguished by their specific hydrophilic character or their properties as surfactants Properties Silicone fluids are mostly clear inert hydrophobic colourless and odourless liquids Depending on the chain length they have a molecular weight from 176 to 10 000 and a viscosity of in the extreme case only 0 65 to 1 000 000 mPA s They resist temperatures between 60 C and 300 C and have extremely low volatility excellent shear stability a low surface tension and high water repellency Their electrical characteristics are also insensitive to a wide range of temperatures Physical properties of silicone fluid polydimethylsiloxane of viscosity 350 mPa s Molecular weight number average Ca 10 000 Flash point 300 C Solidifying point 50 C Thermostability 200 C in air Ignition pointAround 500 C Thermal conductivity at 50 C 0 15 W K m Dielectric strength14 kV mm Resistivity6 X 1015 cm Surface tension21mN m Typical applications Polydimethylsiloxanes make ideal hydraulic fluids damping liquids diffusion pump oils thermostable lubricants dielectrics defoamers and release agents Special types serve as impregnating agents for textiles and leather They are also added in tiny amounts to surface coatings Other major areas of application include the cosmetics pharmaceuticals and healthcare industries ELASTOMERS Silicone elastomers or rubbers are made from linear polymers that bear hydroxyl vinyl or o