Introduction Thermosets refer to the set of materials formed by polymers joined by chemical bonds acquiring a highly cross-linked final structure. The highly cross-linked structure of thermoset materials is directly responsible for the high mechanical and physical resistance (stresses or loads, temperature…) of these materials compared to thermoplastics and elastomers. On the …
Silicone is an odorless and colorless synthetic rubber widely used in various sectors thanks to its special properties. It is a polymer of high molecular weight and three-dimensional crosslinking; they are formed by a main chain of silicon and oxygen atoms, which unlike quartz (SiO2), has an organic group R every four atoms. All silicone resins are made up of 30 to 80 trifunctional silicon units and their molecular weights range between 2000 and 5000 g / mol, very small compared to those of organic resins.
The chemical nature of silicone gives it many advantages over organic elastomers and sealants. The main chemical characteristic of silicone is the presence of a high number of Si-O bonds with a bond energy that is much greater than that of C-O and C-C in organic polymers. From a chemical point of view, silicone resins belong to an intermediate group between inorganic and pure organic substances. Silicone is inert and stable at high temperatures, which makes it useful in a wide variety of industrial applications, such as lubricants, adhesives, waterproofing, and in medical applications, such as cardiac valve prostheses and breast implants. Silicones can be classified according to the length of their molecules, influencing their use:
- Soft drinks: less than 10 basic units.
- Oils: between 10 and 100 basic units.
- Resins: between 100 and 500 basic units.
- Rubber: between 500 and 2000 basic units.
Chemical structure of polydimethylsiloxane (PDMS).
Silicone resins are obtained from polycondensation of silanes; from the reaction, viscous or solid siloxanes (liquid or solid resins), soluble in organic solvents, are generated. From the ecological point of view, it is appropriate to mention that they are formulated with low volatile solvent content (VOC). The three-dimensional crosslinking that forms the silicone resins gives it excellent water repellency due to its hydrophobic nature, suitable permeability to water vapor and carbon dioxide, high resistance to weathering and biological attacks; These binders also contribute to the aesthetic appearance and facilitate the cleaning of the surface. Being resins, they are polymers of relatively low molecular mass that have a three-dimensional structure. Silicone resin has thermal resistance, is a good insulator, and is water repellent. Its main uses are:
- Insulating varnishes
- Industrial paints
- Encapsulation and impregnation agents
In the case of silicone resins are generally used for the manufacture of glass fiber laminates, formulations for molding in space ships are used for their heat resistance as in the case of airplanes.
Chemical structure of silicone resin.
Silicone resin paints have become one of the most efficient facade cladding systems, they are also widely used for coating electronic equipment and many others. Silicone resin paints can be described as a traditional latex paint in which a part of the binder is replaced by silicone resin; it allows to develop formulations that use a lower content of binders (high PVC values) due to the known ability of silicones to reinforce and fix inorganic materials (pigment and auxiliary elements) and organic materials (binders).
Protecting electronic circuits is vital to ensure optimum performance and performance in your electronic applications. Currently, electronic devices are present in all industries. Highlights automotive, aerospace, industrial electronics, medical sector and telecommunications among others.
Smartphones, LEDs, cars are everyday objects that contain the most advanced technology. In each of these components, the molding resins that are not visible but are essential for its operation. Molding resins, for example, cover electrical components, such as plates, or fill other components. Therefore, the precise operation of the dosing and mixing systems for encapsulation in the electrical and electronic industry is of special importance. For the encapsulation, mainly molding resins, such as silicone, polyurethane and epoxy resin in the liquid state, are processed. In solid state, the molding resins protect the components from moisture, dust and damage, guaranteeing a long shelf life.
Circuit coating with silicone
In-depth knowledge of the application is essential to select the electronic encapsulants in order to offer the best performance and protection. These systems can be of very different natures. From epoxy based products, to silicones and polyurethanes.
As for electrical components, there are possible dangers due to improper wiring. The insulating material of the cable must be suitable for voltage and resistant to environmental conditions. The insulating material prevents the wires carrying current from coming into contact with people and animals, which protects them from electric shock. Buried cables must also have a cover of insulating material that is resistant to fire, moisture, fungus and corrosion. On the other hand, the danger due to exposed electrical components is one more reason to protect them properly.
Why, where and how to use resins to perform an electrical engine or potting for electronics?
The silicone adhesive products for bonding, fixing-sealing, coating and filling electrical and electronic parts show, unlike conventional RTV rubbers silicone-free alcohol, excellent surface and deep healing properties, being resistant to corrosion. Therefore, electronic components are sometimes wrapped, in a process called encapsulation, in silicone to increase stability against mechanical and electrical impacts, radiation and vibrations. Silicones are used when durability and high performance are required in difficult conditions, such as in space (satellite technology).
Sometimes, they are selected above the polyurethane or epoxy encapsulation when a wide range of operating temperatures (-65 to 315 ° C) is required. Silicones also have the advantage of presenting low exothermic heat during curing, low toxicity, good electrical properties and high purity. There are products on the market for coating of curable printed circuits at room temperature (RTV: Room Temperature Vulcanization) which have the advantage that there is no damage to the electronic components is possible due to the effect of temperature.
Silicones are also used in the thermal paste composition in contact with transistors or integrated circuits with heat sinks to improve heat transfer and prevent overheating of electronic components.
Humidity is the main enemy of electrical and electronic equipment since, apart from causing short circuits, it can also lead to premature deterioration of the components due to corrosion. There is a solution for this, which is why resins are so important.
It may also be necessary to protect electrical or electronic components that may come into contact with chemicals, including acids, alkalis, solvents and an amout of other substances that pose a threat to delicate circuits and components. Chemical resistance is usually an area for epoxy resins, although some rigid polyurethane, as well as some specially formulated silicones, thet will provide a suitable degree of protection.
Another essential point of how to use resins is the importance of protecting the components from mechanical damage and vibration. Here, a more flexible cured product are likely to provide the best protection, as it provides cushioning that will help not transmit nor stress the majority of components and devices that are inside the package. Silicone resins offer excellent flexibility and will also tolerate high operating temperatures.
The thermic shock can also have a devastating effect on electronic components, shortening their shelf life at best, and destroying them completely at worst. The adverse effects of thermal stress can be improved by using two-component epoxy resins with thermic behavior similar to that of components and substrates, which are better for those applications that undergo severe temperature cycles or are prone to thermic shock. Some of them also offer the additional benefit of being flame retardants. As previously mentioned, silicones have the advantage of presenting low exothermic heat during curing, as well as low toxicity, good electrical properties and high purity.
Finally, it is worth mentioning that, apart from providing all the aforementioned protections, electrical encapsulation or potting with resin also serves to hide what is under it. This could provide an effective sheet against counterfeiters or those who wish to copy a circuit path, helping to protect intellectual property.
Silicone resin paints (or siloxane) are coating systems that provide decisive advantages:
- Great durability.
- Dirt repellency.
- Water repellency.
- Weather resistance.
- Great vapor permeability.
- Light resistance and stable colors.
- Without solvents (water based).
- Mineral finish.
- Easy workability (manual or mechanical).
- High covering power.
- Absence of tensions.
- High abrasion resistance.
- Heat resistance.
Polysiloxanes are characterized by having a low glass transition temperature, excellent thermal and oxidative stability, low surface energy, high gas permeability and great stability against UV radiation.
The function of an encapsulation or insulator of this type is twofold: on one hand, you have to insulate the components of a circuit and, on the other, protect them from the environment and from mechanical fractures that may compromise the operation and characteristics of the assembly, at the same time it needs to be flexible enough to absorb contractions due to thermic expansion differentials, in order to transmit it to other parts of the circuit and cause failures.
All types of resin can be cleaned in different ways. Depending on the product to be treated, water, isopropyl alcohol, a mild detergent, among others, can be used. Special care must be taken in regard to cleaning solvents, since there are multiple products which purpose is precisely to remove silicone debris from the surface and we are not interested in this case. What we are looking for, is that the silicone based resin resin equipment/ device remains intact and continues to provide its coating functions. So we will take into account what solvent to use when performing maintenance and cleaning of the equipment coated with silicone resins in order to avoid damage of the resin or the coated equipment.
For example, there are non-flammable hydrocarbon-based cleaning solutions that contain a catalytic polymer depolymerization catalyst. The solvent quickly digests and dissolves the cured silicone, therefore, special care must be taken and this type of products should not be used for cleaning equipment coated with silicone resins.
A suitable tool such as water-based surface cleaning wipes, serve perfectly for the effective removal of oils, greases, fuels, hydraulic fluids and other general surface contaminants that we could use without compromising the integrity of the silicon based resin equipment surface.
Depending on the equipment coated with silicone resin, an example of a cleaning product might be the Electrolube ULS Ultrasolve that replaces CFC solvents such as 113 trichloroethane due to its rapid cleaning and rapid evaporation rate. Ultrasolve is 100% ozone friendly. A full range of cleaning solvents is available at Electrolube, including fast and slow drying solvents and water based products. Its properties include excellent degreasing properties, it also eliminates PCB flow residues, It can be used to remove conformal acrylic coatings and, above all, it is harmless to most plastics, rubbers and elastomers with a rapid evaporation rate.
Electrolube – Chemical products for the electronics industry
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