Silicon rubber

Its about manufacturing of silicon rubber and its applications.

1. SILICON RUBBER Presented by : Muhammad Waleed & Shakir jamal

2. Structure of Silicon Rubber

3. How silicon is produced? Silicon does not exist on its own in nature. Most of it is bound with oxygen in materials like sand and quartz. The silicon oxygen bond in quartz is so stable it can only be broken by white heat. First step ( reduction ): The first step in the production of the silicone involves the reduction of silica (SiO 2 ) to silicon (Si) in an electric arc furnace at 1900°C in the presence of charcoal.  SiO 2 + 2C 1900 o C Si + 2CO The silicon obtained is solid hard material, so ground to 100 to 350 nm

4. Second step ( formation of chlorosilanes ) In the second step we form chlorosilanes from silicon and the process is known as “ Rochow process”. Methyl chloride is reacted with solid silicon metal in the presence of copper as catalyst at 300 o C. The reaction of silicon with methyl chloride has a high activation energy, but the reaction itself is highly exothermic and if uncontrolled, it leads to impurities. 2 CH 3 -Cl + Si 300 o C (CH 3 ) 2 SiCl 2 80 % copper Other compounds: CH 3 -Si-Cl 3 12 % B.P = 65.7 o C (CH 3 ) 3 -Si-Cl 4 % B.P = 57.3 o C CH 3 HSiCl 2 3 % B.P = 40.7 o C Si-Cl 4 B.P = 56.7 o C

5. The various components of the reaction are isolated by fractionation. Cu is used around 10% of the weight of the silicon metal. Third Step ( synthesis of oligomeric siloxane ) This is done either by “Hydrolysis “ or by “ Methanolysis ” 1) Hydrolysis: Chlorosilanes react with excess water in order to convert them into a mixture of linear and cyclic compounds. (CH 3 ) 2 SiCl 2 + 2H 2 O OH-(CH 3 ) 2 Si-OH + 2HCl OH-(CH 3 ) 2 Si-OH + OH-(CH 3 ) 2 Si-OH HO-Si(CH 3 ) 2 -[O-Si(CH 3 ) 2 ] n -OH + nH 2 O Polymerization continues until a chain terminator is added like trimethyl silanol . ( CH 3 ) 3 SiOH + HO-Si(CH 3 ) 2 -[O-Si(CH 3 ) 2 ] n -OH + HO-Si(CH 3 ) 3 (CH 3 ) 3 Si-O-Si(CH 3 ) 2 -[O-Si(CH 3 ) 2 ] n -O-Si(CH 3 ) 3

6.  Complete hydrolysis with excess water is carried out continuously in the liquid phase with 25% HCl or in the gas phase at 100ºC. The ratio of cyclic to linear dimethyl siloxanes and the chain length of the linear oligomers can be varied over a wide range by means of hydrolysis conditions.  For example rapid removal of HCl from the reaction mixture by neutralization leads almost exclusively to short chain siloxane diols . Cyclic siloxanes represent up to two thirds of the reaction product if prolonged contact with HCl occurs . 2) Methanolysis : The methanolysis process for the production of siloxanes from dimethyldichlorosilane allows direct recovery of the chlorine from methylchlorosilanes as chloromethane. Chlorosilanes react with methanol to produce linear and cyclic compound.

7. m(CH 3 ) 2 SiCl 2 + 2nCH 3 OH HO[(CH 3 ) 2 SiO] n H + 2nCH 3 Cl +(n-1)H 2 O n(CH 3 ) 2 SiCl 2 + 2nCH 3 OH [(CH 3 ) 2 SiO] n + 2nCH 3 Cl + nH 2 O Curing : Silicone elastomers are cured (or “vulcanized”) to turn from thick liquids or semi-solid pastes into solids. The two most common curing processes are peroxide initiated curing and platinum catalyzed curing. 1)Peroxide curing: Initially, all silicone elastomers were cured with peroxide-based chemicals. This curing system is also called free radical cure

8. system or a high-temperature vulcanizing system because the peroxides generate free radicals that decompose at high temperatures and initiate the cross-linking reaction.

9. 2 ) Platinum catalyzed curing: Platinum-based curing systems are sometimes called addition cure systems, because there is a silicone hydride (Si – H) polymer that adds across the double bond of a vinyl functional siloxane polymer in the presence of a platinum-based catalyst to create a cross-linking reaction . Curing agent Oligomer chain pt

10. General properties of silicon rubber: 1) High binding energy: The siloxane bonds (-Si-O-Si-) that form the backbone of silicone rubber highly stable. At 433 kJ/mol, their binding energy is higher than that of carbon bonds (C-C), at 355 kJ/mol. Thus compared to common organic polymers , silicon rubber have higher: Heat resistance Flame retardancy Chemical stability 2) Low intermolecular force with spiral structure: With its coil shaped spiral structure and low intermolecular force, silicone ( dimethylpolysiloxane ) is highly elastic and compressible. Furthermore as methyl groups are located in the outside of coil structure, they are free to rotate on its own.

11. As a result silicone rubber has outstanding, water repellency and contact resistance 3) Heat and Cold resistance: Silicon rubber withstands high and low temprature far better than organic rubbers, Because it has broad operating-temperature range from - 50 °C to +250 °C,(specialty grades : -110 °C to +300 °C). 4) Flexibility: In general silicon rubber is flexible at low temperatures due to its low glass transition temperature ( T g ). However, they also tend to stiffen up at higher temperatures. 5) Gas permeability: Compare to organic rubber or plastic films, thin films of silicon rubber have better gas and vapor permeability.

12. 6) Physiologically inert: Living tissues are affected by contact with silicone rubber to a lesser degree than by exposure to other organic polymers. Silicone rubber is physiologically inert, and is thus used for baby bottle nipples and stoppers in medical applications. 7) High purity: High Purity Compared to other elastomers , silicone rubber is exceptionally pure and is therefore also suitable for use in the food and medical sectors. 8) Transparency and coloring properties: Most organic rubbers are black due to their carbon content. In contrast, it is possible to make highly transparent silicone rubber by using fine silica. Its high transparency makes silicon rubber easy to color with pigments.

13. Uses of silicon rubber: 1) Electronics: There is no end to the number of silicone rubber applications in the field of electronics today. It is often used to seal the inner circuits and processors of most electronic gadgets, protecting them from heat, corrosion, moisture and other conditions that can cause wear and tear. 2) Gaskets and seals:   Gaskets and seals are used in between mechanical parts and to prevent leaks Rubber is generally a good gasket and seal material because of its compressibility, flexibility, and its resistance to many liquids. Silicone is particularly an effective material due to its ability to withstand a wide range of temperatures and its resilience to chemicals.

14. 3) Medical devices: The low toxicity of the rubber and its chemical inertness allow the rubber to be used in medical applications. The elastomer is easy to mold, very flexible and is available in translucent forms for monitoring purposes. Tubing for dialysis equipment, catheters and baby bottle nipples are all made of silicone. 4) Construction and Restoration: Due to its adhesive properties, it can be used to bind together building material. Such as concrete , plastic ,glass etc. This enhance the strength and durability of a structure and protect it against moisture pollution, heat and other element that cause wear and tear. 5) Office automation Equipment: Silicon rubber also used in office automation equipment such as keypads of calculator & keyboard of computer, photocopier rollers and printer rollers.

15. THANK YOU  6) Leisure: Due to high transparency, and physiological inertness silicon rubber is used in swimming goggles , snorkels, Mouthpiece and goggles band.