Basic Silicone Chemistry

What are Silicones? are synthetic polymers with a linear, repeating silicon-oxygen backbone, the same bond that is found in quartz, glass and sand. Silicone polymers do not have carbon as part of the backbone structure Have a high melting and boiling point Depending on the number of repeat units in the polymer chain and the degree of cross-linking, six classes of commercially important products can be produced Some disadvantages of Silicone are increased costs and lower mechanical property values when compared to carbon based materials

Forms & Compositions Fluids, Emulsions, Compounds, Lubricants, Resins and rubbers Varies from liquid to gel, or rubber to hard plastic odorless and colorless, water resistant, chemical resistant, oxidation resistant, stable at high temperature, and have weak forces of attraction, low surface tension, low freezing points and do not conduct electricity seem to be impervious to the effects of aging, weather, sunlight, moisture, heat, cold, and some chemical assaults

Silicone Family Tree Si Elastomers Fluids & Emulsions Silicone Resins Dimethyl Compounds Silanes Organo-Silicones Silicone Polyethers Volatile Methyl Siloxanes Amino Silicones Si

Flexibility of Siloxane Chemistry Non-volatile Antifoam Slippery Water Insoluble Excellent Depth of Gloss Incompatible in Organics Durable Volatile Profoam Sticky Water Soluble Shiny Compatible Transient

HISTORY The first silicone elastomers were developed in thesearch for better insulating materials for electric motors andgenerators. Resin-impregnated glass fibers were the state-of-the-art materials at the time. The glass was very heat resistant, but the phenolic resins would not withstand higher temperatures that were being encountered in new smaller electric motors.Chemists at Corning Glass and General Electric were investigating heat-resistant materials for use as resinous binders when they synthesized the first silicone polymers, demonstrated that they work well and found a route to produce polydimethylsiloxanen commercially. .

PROPERTIES Silicone rubber offers good resistance to extreme temperatures,being able to operate normally from -55°C to +300°C.At the extreme temperatures, the tensile strength, elongation, tear strength and compression set can be far superior to conventional rubbers although still low relative to other materials.Organic rubber has a carbon to carbon backbone which can leave them susceptible to ozone, UV, heat and other ageing factors that silicone rubber can withstand well. This makes it one of the elastomers of choice in many extreme environments.Compared to other organic rubbers, however, silicone rubber has a very low tensile strength. For this reason, care is needed in designing products to withstand even low imposed loads. Silicone rubber is a highly inert material and does not react with most chemicals. Due to its inertness,it is used in many medical applications and in medical implants. However, typical medical products have failed because of poor design.

STRUCTURE silicone rubber chain Polysiloxane differ from other polymers in that their backbones consist of Si-O-Si units unlike many other polymers that contain carbon backbones. One interesting characteristic is an extremely low glass transition temperatureof about - 127˚C.  Polysiloxane is very flexible due to large bond angles and bond lengths when compared to those found in more basic polymers such as polyethylene. e.g. A C-C backbone unit has a bond length of 1.54 Å and a bond angle  of 112˚, whereas the siloxane backbone unit Si-O has a bond length of 1.63 Å and a bond angle of 130˚.  

Silicone rubber chain

The siloxane backbone differs greatly from the basic polyethylene backbone, yielding a much more flexible polymer. Because the bond lengths are longer, they can move further and change conformation easily, making for a flexible advantage of polysiloxanes is in their stability. Siliconis in the same group (IV) on the periodic table as carbon, but the properties of these elements are quite different. Silicon has the same oxidation state as carbon, but has the ability to use 3D orbitals for bonding by expanding its valence shell. Si-Si bonds have far less energy than C-C bonds and so are more stable, though in practice Si-Si-bonds are very hard to create.

Repeat unit of Silicone Rubber

Silicone Nomenclature SILICA O Si O O X SILANES X Si X X R SILOXANES O Si O R

Silicone Nomenclature Shorthand Precursor Silanol Siloxane Structure Short hand Me Me Cl-Si-Cl HO-Si-OH Linear Structures D unit Me-Si-Cl Me-Si-OH End-cap group M unit Cl-Si-Cl HO-Si-OH Branched Structures T unit Cl OH Cl-Si-Cl HO-Si-OH Silica Core Q unit Me Me Me Me Me Me Me Me Me-Si-O-Si-O-Si-O-Si-O-Si-Me = Me-Si-(O-Si)3-O-Si-Me = MD3M

SILICONES APPLICATIONS Dow Corning’s products and specialty materials are used by customers in virtually every major industry. Aerospace Automotive Chemicals/ Petrochemicals Construction Consumer Products Electrical/Electronics Food Processing Industrial Maintenance Production Medical Products Paints & Coatings Personal, Household & Automotive Care Pharmaceuticals Plastics Pressure-Sensitive Adhesives Textiles & Leather

Synthesis of Silicones The most common method for preparing silicones involves reacting a chlorosilane with water. This produces a hydroxyl intermediate, which condenses to form a polymer-type structure. The basic reaction sequence is represented as:

Raw Materials Initial material is quartz SiO4/2 26% of the Earth’s crust Reduce to Si metal with carbon at 2500F Methanol is converted to MeCl with recycled HCl

Process Chemistry of Methyl Train Me2SiCl2 MeHSiCl2 Me3SiCl H2O Me2 Hydro SiH fluid EBB Chlorosilane Mix Waste & Recovery Si MeCl Copper Catalysts

Silicone Classifications by Physical Form (1) Fluids (hydraulic, release agents, cosmetics, heat transfer media, polishes, lubricants, damping, dry cleaning) Polymer chains of difunctional units (D) terminated with monofunctional (M) units OR cyclics (Dx) (2) Gums (high temperature heat transfer fluids, lubricants, greases, cosmetic and health care additives) Same structure as PDMS fluids, but much higher molecular weight (viscosities >1,000,000 cSt). (3) Resins (varnishes, protective coatings, release coatings, molding compounds, electronic insulation) Rigid solids based on trifunctional (T) and tetrafunctional (Q) units. Surface modification with (M) units (4) Elastomers (Heat cured and RTVs: tubing and hoses, medical implants, sealants, adhesives, surgical aids, electrical insulation, fuel resistant rubber parts, rollers, etc) Soft solids based on crosslinked SiH Fluids

involving the reaction of elemental silicone with an alkyl halide. This is the favoured route although other raw materials such as alkoxysilanes can be used. Chlorosilanes and other silicone precursors are synthesised using the “Direct Process”, involving the reaction of elemental silicone with an alkyl halide. thus, preparation of silicone elastomers requires the formation of high molecular weight (generally greater than500000g/mol). To produce these types of materials requires di-functional precursors, which form linear polymer structures. Mono and tri-functional precursors form terminal structures and branched structures respectively. Si + RX →RnSiX4-n (where n = 0-4)

Other components – curing additives With the exception of RTV and liquid curing systems,silicone rubbers are usually cured using peroxides suchas benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, t-butyl perbenzoate and dicumyl peroxide. Alkyl hydroperoxides and dialkyl peroxides have also been used successfully with vinyl containing silicones. Hydrosilylation or hydrosilation is an alternative curing method for vinyl containing silicones and utilises hydrosilane materials and platinum containing compounds for catalysts. It is a 2-part process requiring mixing of 2 separatecomponents, with the resulting material having alimited shelf life.

Fillers Reinforcing fillers are added to improve the otherwise poor tensile strength of silicones. Silica, in the form of silica fume with particle sizes in the range 10-40nm is the most preferred filler,although carbon black has been used. Fillers do interact with the vulcanisate, forming apseudo-vulcanisation. This can occur either during mixing (creep hardening) or in storage (bin ageing). Examples of these materials are siloxane-based materials such as diphenylsilane and pinacoxydimethylsilane.

Other Additives Silicones have better fire resistant properties compared to natural rubbers. This property can be improved by addition flame retardant additives such as platinum compounds,carbon black, aluminium trihydrate, zinc or ceric compounds. It should be noted that carbon black addition also increase electrical conductivity. Ferric oxide may also be added to improve heat stability, titanium dioxide and other organometallic compounds as pigments.

Volatile Polydimethylsiloxane Fluids Cyclomethicone CH3 Si - O n = 3 Trimer n = 4 Tetramer n = 5 Pentamer n = 6 Hexamer PENTAMER (D5) Si CH3 O n

Volatile Polydimethylsiloxane (PDMS) Fluids R = CH3 INCI: Dimethicone R = OH INCI: Dimethiconol CH3 CH3 CH3 R - Si - O - Si - O - Si - R CH3 CH3 m CH3 When m = 0, R= CH3 called Hexamethyldisiloxane or 200 Fluid,0.65 cS (.65,1, 1.5 and 2.0 cS are volatile)

Properties of Siloxanes Despite the Fact that Silicon and Carbon are both Group IV elements their chemistry is very different Unique flexibility of Si-O bond Si-O-Si C-C-C C-O-C units bond length 1.63 1.54 1.42 angstroms bond angle 130 112 111 degree bond energy 106 83 86 Kcal/mol bond barrier 0.2 3.6 2.7 Kcal/mol

Siloxane Polymers vs Carbon Polymers Barrier to Rotation ( kcal/mole ) Polyethylene 3.3 Polytetrafluoroethylene 4.7 Polydimethylsiloxane < 0.2 Key Point: Siloxane (Si-O-Si) polymers are stronger than carbon polymers, yet the polymer chains are more open and flexible

Siloxane Physical Properties Very low glass transition temperature (Tg = -120 °C) high molecular weights but not a solid Ability to spread out on a wide variety of substrates silky, smooth, non-tacky, aesthetic enhancing flowability and film forming Lowest surface shear viscosity and low surface tension lubricating, antifoaming, waterproofing, release properties High gas permeability Excellent dielectric properties Very good thermo-oxidative stability good chemical inertness and temperature resistance The flexibility of the backbone provides one very unique distinguishing point compared to organics: very low glass transition temperature that allows high molecular weights (as high as 500,000) to still be fluid. At the same time, the chemistry kit will allows for water-like volatiles, to rubber-like, tacky or solid resins. Further, since the molecule can easily rotate, the surface tension becomes low from the methyl groups and allows for easy spread for silky, smooth, non-tacky aesthetic non-greasy or oily feel. Compared with conventional organic polymers, many silicones show superior thermal properties. Some silicone elastomers, for example, remain flexible at - 100C and retain their properties for long periods at 200 C. Most of the common silicone polymers are quite hydrophobic but are permeable to water vapor and to other gases. Their resistance to unltraviolet and other radiation means that they weather well. They are good electrical insulators. The low surface tensioin of polydimethylsiloxanes (~20 dynes/cm) permit them to spread easily over irregular surfaces. They are often used as additives to promote easier application and to give additional water repellency to the surface. Some silicones are excellent release agents; even the most aggressive organic adhesives do not adhere to them. Most silicones are not particularily solvent resistent; however, elastomers made from siloxnes substituted with fluoro groups show good resistance to hydrocarbons ans such.

Anionic Ring Opening Equilibrations D4 Ring : Chain Equilibrium 10-15% : 85 – 90% PDI = 2.0

Anionic Ring Opening Equilibrations End Blockers

Anionic Ring Opening Equilibrations End Blockers Maximum in viscosity involves incorporation of end blocker (which is less reactive than cyclic) Viscosity time

Living Anionic Ring Opening Polymerization sec-Butyl – Li + D3 Living anionic polymerization

Industrial classification Industrial Classifications: There are three main industrial classifications of silicone rubbers: • High Temperature Vulcanising (HTV)  – Sometimes called heatcurable, these are usually in a semi-solid gum form in theuncured state. They require rubber-type processing to producefinished items. Room Temperature Vulcanising (RTV) Usually come as aflowable liquid and are used for sealants, mould making,encapsulation and potting. These materials are not generallyused as conventional rubbers. Liquid Silicone Rubbers (LSR) Sometimes called heat curableliquid materials, these materials are processed on speciallydesigned injection moulding and extrusion productionequipment.

Liquid Silicone Rubbers These are essentially two-part systems, supplied deaerated ready fo ruse often in premetered equipment. Low injection pressures and low pressure forming techniques are sufficient. They cure after mixing the two separate portions, by processes such as hydrosilylation. Curing is often complete in as little as a few seconds at temperatures of about 200°C and post-curing is notusually required. The low capital investment required for production mean that LSR scan compete with conventional silicones and organic rubbers. Physical properties are comparable to general purpose grades and high strength peroxide cured elastomers. Furthermore, they exhibit self-extinguishing properties, with carbonblack additions .

Room Temperature Vulcanising (RTV) Rubbers These are available in one (RTV-1) and two-part (RTV-2) systems. Single part systems consist of polydialkylsiloxane with terminal hydroxyl groups, which are reacted with organosilicon cross-linking agents. This operation is carried out in a moisture-free environment and results in the formation of a tetrafunctional structure.Curing takes place when materials are exposed to moisture. Atmospheric moisture is sufficient to trigger the reaction, and thickness should be limited if only one side is exposed to the moisture source. Curing is 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-terminated 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 catalysts used in condensation RTVs. These materials are suited to use with polyurethane casting materials.

Phosphazenes are a class of chemical compounds in which a phosphorus atom is covalently linked to a nitrogen atom by a double bond and to three other atoms or radicals by single bonds. While other substitutions produce relatively persistent compounds, in organic synthesis the term largely refers to species with three amino substituents bound to phosphorus. The compounds are unusually stable examples of the phosphorane class of molecules and have a remarkable proton affinity. As such, they are one of the eminent examples of neutral, organic superbases. Two examples are hexachlorocyclotriphosphazene and bis(triphenylphosphine)iminium chloride. Phosphazenes are also known as iminophosphoranes and phosphine imides.

Phosphazene bases are strong non-metallic non-ionic and low-nucleophilic bases. They are stronger bases than regular amine or amidine bases such as Hünig's base or DBU. Protonation takes place at a doubly bonded nitrogen atom. Related to phosphazene bases are the proazaphosphatrane bases, which have a saturated P(NR)3 structure and protonate at phosphorus. Though the simplest phosphazene superbase, P1-Me, was first synthesized in 1975, chemists assumed that the compounds were highly unstable, like their alkyl-substituted derivatives. The species was regarded at that time as little more than an academic curiosity.

By now phosphazene bases are established reagents in organic synthesis By now phosphazene bases are established reagents in organic synthesis. Perhaps the best known phosphazene bases are BEMP with an acetonitrile pKa of the conjugate acid of 27.6 and the phosphorimidic triamide t-Bu-P4 (pKBH+ = 42.7) also known as Schwesinger base after one of its inventors.[2] In one application t-Bu-P4 is employed in a nucleophilic addition converting the pivalaldehyde to the alcohol:[3

The active nucleophile is believed to be a highly reactive phosphazenium species with full negative charge on the arene sp2 carbon. Besides organic synthesis, phosphazene bases are used as basic titrants in non-aqueous acid-base titration. Their advantages for this are: they are very strong bases in many solvents and their conjugate acids are inert and non-HBD cations.