SILICONES
INTRODUCTION
The name “ Silicone “ was given by Kipping in 1901 to the new compounds of the brut formula R2SiO that rapidly identified as polymeric and corresponding to polydialkylsiloxanes and the most common one are polydimethylsiloxanes ( PDMS). In PDMS, flexible backbone, low intermolecular forces, low surface tension, high permeability and low viscosity, t good chemical and thermal stability and association of some other properties has led has led to the successful industrial application of silicones.
Silicones in Coatings
silicone polymers as additives :
Polydimethylsiloxane (PDMS) fluids of low-to-medium viscosity readily dissolve in solvent-borne paints, reducing the surface tension of the liquid and enabling it to wet substrates. This reduces the appearance of film defects known commonly as “fisheyes” and “pinholes.” The silicone also reduces surface tension gradients across the coating film as it dries so a smooth surface is obtained rather than the undesirable “orange peel” effect.
Silicone polymers can be modified by grafting polyether groups to give silicone-polyether copolymers. These behave as surfactants in aqueous media as they have both hydrophobic and hydrophilic components. Such surface active materials can perform many functions in inks, paints and coatings. The main uses of silicone surfactants are to provide defoaming, deaerating, improved substrate wetting, and enhanced slip properties.
silicone resins as the main component, or binder: . silicone resins can be major components of the coating. Here they are used as binders or co-binders, imparting important benefits such as durability throughout the life of the coating. Silicone resins offer resistance to weathering in paints for exterior surfaces and provide water repellence to masonry surfaces and reduces coating “chalking “ .Silicone resins have greater resistance to high temperatures than organic resins and are used in paints for ovens, chimneys, car exhausts and barbecues. In these examples, the resilience of the silicone materials allows reduced frequency of maintenance painting and consequently reduced volumes of paint used over the lifetime of the coated item. The success of silicone-alkyds in naval applications led to the evaluation of silicone organic copolymers in coil coatings for residential and commercial aluminum sidings.
Blends of silicone and organic resins are suitable for applications up to about 400 °C but For temperatures above 400 °C, silicone resins are used only as binders.
Seawater fouling on the boat and ships solid surfaces increase drag on the hull and increases fuel consumption , in this case the most effective anti-fouling release system are modified silicone polymers with a combination of methyl and phenyl substituents .
In food industries PDMS act as surface active ingredient in antifoams since foams encounters from production to end use and uncontrolled foaming media can cause sever loss of production capacity , including inefficient mixing or pumping, downtime from clogged lines, overflows, spillage hazards and product waste. Chemical additives for reducing foaming problems are called antifoam agents or defoamers . for food industries PDMS are the best choice because of their low surface tension, water insolubility, thermal stability and chemical inertness , they do not penetrate though biological membranes, and orally not metabolized. On the other hand straight- chain PDMS are suitable as food grade lubricating oil to follow FDA regulations . Immiscibility with many organic fluids, low temperature dependence of their physical characteristics, physiological inertness and high temperature stability are some of the key properties of silicon oils that makes them a better option .
In pulp and paper industries , organo-siloxane materials as digester additives to improve the impregnation of active alkali in the wood chips and improve the cooking , As antifoams to help de-airing or drainage in the pulp washing and papermaking processes , as additives that silicones contribute in the finishing process of paper and tissues , as de-inking aids in the recycling of papers and silicone materials are used as surface treatment for tissue softening .
In textile industries , during fiber processing , Low viscosity polydimethylsiloxane (PDMS) is generally used in combination with solid particles and because of its properties such as heat stability and good lubrication, silicones can provide a reduction of the dynamic coefficient of friction, reducing the risk of fiber melting and breakage, needle overheating , during production. During the manufacturing of artificial fibers, PDMS can also be used as a lubricant to avoid adhesion of the thermoplastic fiber material to the spinneret, Silicones can also be used to achieve low coefficients of friction between the fibers themselves and once produced, fibers can be treated with silicones to impart initial softness to the textiles made from these fibers.
About household cleaning applications of silicone additives, their main use is foam control in consumer washing machines and fabric softening and they can provide some extra benefits fabric dewatering, antiwrinkle characteristics, ease of ironing or improved water absorbency, reducing fabric mechanical losses over time or to improve perfume release.
In construction industries, that represents the largest market segment for silicones , mostly they are used as sealant and adhesives since silicone sealants are durable , have good adhesion to different substrates, outstanding UV stability , very good elastomeric properties excellent resistance to the combined effect of the key weathering factors as well as their resistance to harsh environmental condition .
We can notice the silicones applications in some other fields such as release coating for pressure sensitive adhesives , as two part mold-making elastomers , as protective layer in electronic parts and high voltage electrical parts such as silicone cable end terminators , in skin , hair and beauty care products and also in medical devices and pharmaceuticals .
Fumed Silica
Fumed Silica , or silicon dioxide is a synthetic amorphous pyrogenic(fumed) silica, and in two group of hydrophilic and hydrophobic , is applied as a thickening and thixotropic agent in many organic systems, e.g. in Unsaturated Polyesters, Coatings, Paint, Printing inks, Adhesive, Sealant and others. It is used as a reinforcing filler in elastomers, mainly RTV silicone and HTV silicone. It acts as a free flow additive in the production of technical powders.
Fuctions
- Reinforcement
- Ease of Compounding
- Elongation Resistance
- Thixotropy
- Dispersability
- Rheology control
- HTV silicone rubber
- Thickening
- Anti-settling
- Free-flow
- Heat and sound insulation
- Anti-sag
- Thermal Stability
Applications
- Paints & coating & printing inks
- Unsaturated polyester resins
- HTV silicone rubber
- Liquid silicone rubber
- RTV silicone sealant
- Adhesive, e.g. instant glue and hot-melt adhesives
- Toners for photocopying machines
- Thermal insulation material
- Vacuum insulation panel
- Electrical insulator
- Storage battery
- Cable compounds
- Agricultural chemical
- Mechanical polishing
- Pharmaceuticals
- Cosmetic
- Food
OH Polymer
Hydroxy-terminated polydimethylsiloxane ( OH Polymer )that are series of silanol functional fluids with various viscosities and silanol contents Widely used for in variety of materials . They are specially useful in the treatment of fillers and as Anti-Structuring additives in high consistency silicone rubber and silicone Room Temperature Vulcanizing (RTV) formulation .
Silicone Oils
The most common silicone oils are linear polysiloxane compounds, that orient in spiral chains of silicone and oxygen, with each silicone atom also having two carbon-based substituents, which are most often methyl groups. The silicone oils can possess a degree of polymerization, in other words the average number of repeating siloxane groups in a polymer chain, ranging from several units to several thousand units. The viscosity of silicone oils can vary greatly 0.65cs to 2,000,000cs and the viscosity remains constant across a wide range of temperatures.
Silicone oils are used as hydraulic or damping fluids, dielectric fluids, electronics-grade fluids, heating or cooling fluids, diffusion pump fluids, thermostatic fluids, paint additives and homecare product additives, lubricants and release agents.
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