HTV (High Temperature Vulcanizing) Silicone Rubber are also known as Heat Cured Silicone Rubber. They are made from reactive silicone gums, in other words straight chains, with very high molecular weight macromolecules containing various reactive groups. Fumed or precipitated silicas with high specific surface area to improve mechanical properties, heat stabilizers and various other additives such as plasticizers or anti-structuring agents. Cross-linking occurs using organic peroxides in polycondensation systems or platinum catalysts in polyaddition systems. Compounds cure at the high temperature (100-200°C) after mixing with catalyst. Cross-linking occurs using organic peroxides in polycondensation systems or platinum catalysts in polyaddition systems. HTV silicone rubbers come in two different physical states: liquid and solid.
Types of HTV Silicone Rubber
The method of dividing HTV is generally according to the application and performance, such as general-purpose type, low-compression permanent deformation type, low-shrinkage type, flame-retardant type, solvent-resistant type, high-temperature type, etc.
Based on the different cure and processing methods, HTV silicone rubber can be classified as solid silicone (HCR) and liquid silicone (LSR).
HCR - High Consistency Rubber
High Consistency Rubber (Millable Type Silicone Rubber ) is composed mainly of Polyorgarnosilioxan (Silicone Polymer) and Silica with various additives to grant diversified characteristics. It is added with a reinforcing agent and various additives for mixing. The additives include fillers, vulcanizing agents (generally raw rubber without peroxide), structure control agent and modifier, etc.,and then the rubber compound is vulcanized in a mold at a pressure of about 5 MPa, and vulcanized at a high temperature of 120 to 130 ° C about 10 to 30 minutes. And after demolding ,product need second vulcanization for 24 hours in an oven at 200 to 250 ° C.
LSR - Liquid Silicone Rubber
LSR is Liquid Type and High Temperature Vulcanization Silicone Rubber. LSR differs from Millable Type Silicone Rubber and RTV (Room Temperature Vulcanization) by its degree of viscosity and curing temperature. LSR (Liquid Silicone Rubber) is perfect rubber material for automated injection molding due to its excellent liquidity. Also, LSR (Liquid Silicone Rubber) is ideal for complex molds, demanding design and tolerance because it can easily fill the most complex part of a mold.
HCR vs LSR
The performance characteristics of HCR and LSR are relatively similar; however, viscosity is a key differentiator between LSR and HCR, and has a significant impact on the equipment and processes used to manufacture each of these elastomers.
HCR silicone is produced as gummy, high viscosity solid rubber that as partially vulcanzied. Process ways includes: compression molding, transfer molding, and extrusion of rubber tubing.
LSR silicone on the other hand are supplied in drum or pails in liquid rubber form, due to their low viscosity, injection molding is the perfect way to process LSR.
Compared to other silicone products such as liquid silicone rubber (LSR), HCRs are generally stronger materials with more robust physical properties.
LSR has a lower viscosity than HCR. LSRs are typically injection molded and generally have a longer pot life when compared to HCR.
LSR is easier to manufacture and better intended for use on complex geometries. HCR can reach better overall properties. LSR, meanwhile, offers a larger variety of material choices.
HCR and LSR Silicone Rubber offer many of the same performance advantages. The best material for your project will ultimately depend on how many parts you need to produce. HCRs are less expensive than LSRs both for raw material and tool mold cost. Generally, HCR Silicone materials are better suited to smaller production runs, while LSR Silicone is ideal for high-volume manufacturing or tight tolerances because it keeps unit costs down.
HTV Silicone Advantages
Durable: Silicone rubber is a material of choice in industry when retention of initial shape and mechanical strength are desired under heavy thermal stress or sub-zero temperatures. Organic rubber has a carbon-to-carbon backbone which can leave it susceptible to ozone, UV, heat and other ageing factors that silicone rubber can withstand well. This makes silicone rubber one of the elastomers of choice in many extreme environments.
Chemical resistance: HTV silicone is highly inert and does not react with most chemicals. Due to its inertness, it is used in many medical applications including medical implants. It is biocompatible, hypoallergenic and non-allergenic, which makes it suitable for baby care products, and food contact in general.
Temperature resistance: HTV silicone offers good resistance to extreme temperatures, being able to operate normally from −100 to 300 °C.
Mechanical properties: properties such as elongation, creep, cyclic flexing, tear strength, compression set, dielectric strength (at high voltage), thermal conductivity, fire resistance and in some cases tensile strength can be—at extreme temperatures—far superior to organic rubbers in general, although a few of these properties are still lower than for some specialty materials.
Electrical properties: HTV silicone has excellent insulating properties.
Water repellency & Easy colouring
HTV silicone has been widely used to replace petrochemical products in all industries including aerospace, munitions industry, automobile, fine chemicals, construction, electric and electronics, food processing, mechanical engineering, medical and pharmaceutical, cosmetics, home appliances, paper film, solar batteries, and semi conductor. Recently, the scope of silicone application has been expanding at a greater speed.
Heat-curing silicone rubber outstanding properties are used in many different industrial applications: sealing and piping in fluid circuitry (joints for building construction and aeronautical applications, automotive gaskets, boots and hoses), electrical protection (office equipment safety cables and insulators for electrotechnical applications, spark plug boots, cables and connectors for automotive application), food, pharmaceutical and biomedical applications (tubes, profiled sections).
HCR and LSR silicone rubbers can be processed in a variety of ways, each type of molding has differences in processing methods, and has its own advantages and disadvantages.
Extrusion is a continuous manufacturing process in which silicone rubber is forced through a die and then vulcanized. The die gives the extrudate its shape. The necessary pressure is produced via a conveying screw, in which the material is homogenized, compressed and vented.
Extruded products include: Tubing, Profiles, Cables, Flat tapes & Round cords.
Compression and Transfer Molding
Compression molding processes are used for a wide range of applications. Solid silicone rubber is cured in molds by heat and pressure. Compression and transfer molding are the preferred methods for solid silicone rubber.
Injection molding is the most popular and efficient process for mass-producing products that must meet strict demands for consistently high quality. It is characterized by high dimensional accuracy, produces high-quality parts without the need for secondary finishing, and allows much shorter production cycles than other processes. The disadvantages are higher mold and machine costs, though these are offset by the high productivity.
Injection molding is ideal for small to medium-sized parts and large production runs, such as Nipples for baby bottles and Gasket rings.
Knife coating (on textiles)
Silicones can be used for coating nearly any type of fabric. The art of formulation requires a great deal of experience and creativity, and a wealth of knowledge about relevant chemical and processing relationships.
HTV Silicone Storage
HCR Silicone Storage
- Solid silicone rubber can be stored up to 12 months depending on the grade.
- Products stored longer than the above may still be usable…properties should be checked.
- Long term storage of HTV silicone may cause a rise in Mooney” viscosity…this is reversible.
- Rubber stored to long may be restored by replasticating on a roll mill.
- Recommended storage conditions.
- Store in closed containers in cool rooms at 0 to 30°C
- No direct sunlight.
- Keep separate from organic rubber and crosslinking chemicals.
LSR Silicone Storage
If properly stored liquid silicone rubber has a shelf life of up to 12 months.
- Even if the product is stored for longer than recommended.
- storage you can usually still use it…properties should be checked.
- As with HTV silicones viscosity may increase during storage…this can be reversed.
- Material can still be processed despite higher viscosity.
- Recommended storage conditions.
- Store in closed containers in a cool room at 0 to 30°C.
- No exposure to direct sunlight.
- Higher average temperatures may shorten shelf life.