Polymers are widely used in industry and in our daily life because of their diverse functionality, light weight, low cost and excellent chemical stability. However, on some applications such as heat exchangers and electronic packaging, the low thermal conductivity of polymers is one of the major technological barriers.
Long-term, reliable protection of sensitive electronic components is essential to many electronic applications today. Increasingly small systems and rising circuit densities have resulted in hotter operating temperatures, and driven demand for high-performance solutions for heat dissipation.
Siloxanes are chemical compounds with a backbone of alternating silicon (Si) and oxygen (O) atoms, each silicon atom bearing one or several organic groups, as be seen in the image 1, are very flexible due to large bond angles and bond lengths compared to those found in more basic polymers. Siloxanes are building blocks for silicone products or make part of other products, such as cosmetics or paint. In colloquial language the term silicones is often used synonymously with siloxanes.
Image 1: Representation of the chemical structure of Silicones (1)
Since in one side have an inorganic nature, which gives it stability against heat and cold, as well as its marked chemical resistance. And, on the other side, the plastic properties of silicones, their oily, plastic or resinous constitution and the action of repelling water are due, above all, to their organic part [1] Silicones have amazing array of properties, as it can be observed in the list below:Silicones are inert synthetic compounds that come in a variety of forms: oils and gums that can be formulated in fluids (emulsions, resins, greases, compounds) or elastomers. Thanks to its versatility, silicones are used in a wide range of applications. Typically, silicones are presents in every technologic device for protecting micro-electronics with potting and encapsulation, as well as in every car in the world for sealing and bonding functionalities.
On the other hand, in the image 2, it can be observed that some properties are inherent to siloxane linkage and other characteristics attributable to molecular structure (spatial arrangement)
Image 2: Silicone compounds properties depending on the chemical characteristics (2)
Finally, in list below, it can be observed the performance characteristics silicone versus others resins
Table 1: Properties of different types of resins
The properties of the siloxanes and the silicone products depend on the length of the Si-O backbone, the chemical groups attached to the backbone and the presence of cross-links between the backbones, so the silicone rubber is possible to tailor specific properties depending on the final application. Silicone products are grouped into silicone fluids, elastomers and resins. Silicone fluids are used for a wide range of applications, silicone elastomers are mainly used for sealants and rubbers, and resins are mainly used for paints. The most common siloxanes are polydimethylsiloxane (PDMS) with different modifications.
On the other hand, depending on the stoichiometric quantity (ratio [crosslinker]/[silicone]), molecular weight of siloxane, crosslinker type and crosslinker funtionality it is possible to obtain [2]:
In the following tables will show the typical functional groups that are attached to the backbone of silicones. Special groups can be attached for specific final applications, like, amine, epoxy, carbinol, methacrylate and acrylate …etc.
Table 2: Organic groups vs properties
As it can be saw in the Table 2, depending on the attached chemical groups, the silicone rubbers can be classified as:
Table 3: Silicone rubbers classifications
On the other hand, as is mentioned above, depending on the spatial disposition of the backbone, it possible to achieved different types of features. Most typical spatial arrangement are listed below:
In its uncured state, silicone rubber is a highly-adhesive gel or liquid. In order to convert to a solid, it must be cured, vulcanized or catalyzed. Silicone elastomers are the most important group of silicones serving in the industry. Owing to the wide range of potential applications, they differ with respect to viscosity and curing systems. There are three major groups of these elastomers:
Table 4: Different types of silicone depending on the type curing
Various technologies exist to cross-link polymers such as silicone elastomers with three having achieved broad commercial application. Silicone rubber may be cured by a platinum-catalyzed cure system, a condensation cure system, a peroxide cure system. For the platinum-catalyzed cure system, the curing process can be accelerated by adding heat or pressure.
Image 4: Types of RTV silicone rubber (2)
In a platinum-based silicone cure system, also called an addition system (because the key reaction-building polymer is an addition reaction), a hydride– and a vinyl-functional siloxane polymer react in the presence of a platinum complex catalyst (Pt-catalyst), creating an ethyl bridge between the two[3]. The reaction has no byproducts. Such silicone rubbers cure quickly, though the rate of or even ability to cure is easily inhibited in the presence of elemental tin, sulfur, and many amine compounds[3]
Image 5: Additon cure system reaction (2)
There exists a wide range of Pt catalyst and they differ in their reaction speeds and in which the concentrations are formulated. It must be borne in mind that in order to develop a robust elastomer, relatively quick initial crosslinking must be take place, simply due to the fact that if the elastomer sets too slowly, then the polymer chains are allowed to disentangle and release minute stressed, thus resulting in a softer elastomer.
The platinum catalyst form complexes with certain other compounds, and the catalytic action is inhibited. They are also sensitive to substances (organic compounds that contain elements which include nitrogen, phosphorous and sulfur, ionic compounds of heavy metals such as tin, mercury and arsenic and finally, organic compounds that contain unsaturated groups) that can cause cure inhibition or poisoning the Pt-catalyst. There exists a wide range of Pt catalyst and they differ in their reaction speeds and in which the concentrations are formulated. It must be borne in mind that in order to develop a robust elastomer, relatively quick initial crosslinking must be take place, simply due to the fact that if the elastomer sets too slowly, then the polymer chains are allowed to disentangle and release minute stressed, thus resulting in a softer elastomer.
Condensation curing systems can be one-part or two-part systems. In one-part system, a cross-linker exposed to ambient humidity (i.e., water) experiences a hydrolysis step and is left with a hydroxyl or silanol group. The silanol condenses further with another hydrolysable group on the polymer or cross-linker and continues until the system is fully cured. Such a system will cure on its own at room temperature and (unlike the platinum-based addition cure system) is not easily inhibited by contact with other chemicals, though the process may be affected by contact with some plastics or metals and may not take place at all if placed in contact with already-cured silicone compounds.
The crosslinkers used in condensation cure systems are typically alkoxy, acetoxy, ester, enoxy or oxime silanes. In many cases an additional condensation catalyst is added to fully cure the RTV system and achieve a tack-free surface. Depending on the type of detached molecule, it is possible to classify silicone systems as acidic, neutral or alkaline.
Image 6 : Condensation cure system reaction (2)
Peroxide curing is widely used for curing silicone rubber. The curing process leaves behind byproducts, which can be an issue in food contact and medical applications. However, these products are usually treated in a postcure oven which greatly reduces the peroxide breakdown product content.
Image 7 : Peroxide cure system reaction (3)
Even when cross-linked, the silicone polymer network is mechanically weak relative to many other elastomer systems. Pure silicone rubber shows little tracking and erosion resistance. In order to extend service life and improve service effect, some properties of silicone rubber need to be improved. The fillers are added to the polymer to improve specific properties and also to reduce cost.
The extent of nanocomposite property improvement depends on filler concentration, morphology, such as particle size and structure, the degree of dispersion and orientation in the matrix, and also the degree of adhesion with the polymer chains. To improve particle dispersion, several techniques other than mixing are available.
There are two main types of fillers: reinforcing and extending. The reinforcing type can be used to improve tensile strength, modulus, and tear strength and abrasion resistance. The extending filler is a semi-reinforcing or no- reinforcing material. It may be used to impact some desirable property and also extend the formulation [6].
Challenging and versatile demands placed on components in the power sector require a portfolio of different materials and technologies for silicone elastomers. Polymeric structure and functionality, volume and nature of filler as well as cross-linking density and applied curing method all help give silicone elastomers outstanding properties. But deep understanding of all possible interactions between ingredients is essential to achieve stability during storage, reliable processability and product performance that is best suited to the needs of the application.
As a summary, to tailor the silicon rubber according to the customer needs different steps to follow:
(2) Karol Pietrak, Tomasz S. Wi´sniewski, A review of models for effective thermal conductivity of composite materials, Journal of Power Technologies 95 (1) (2015) 14–24
(3) Gel-est Brochure: Reactive siloxane
(4) Brochure: RTV Silicone rubbers for electrical and electronic applications. (Shin-Etsu)
(5) Brochure: Battery Thermal Management in Hybrid & Electric Vehicles: Efficient Insulation or Conduction Silicone (www.silicones.elkem.com)
(6) G.Momen and M.Farzaneh, Survey of Micro/Nano filler use to improve silicone rubber for outdoor insulators, Rev Adv Sci. 27(2011) 1-13
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