Silicone Compounding

The Definition of Silicone Compounding

Silicone compounding refers to the preparation process in which various additives are added to the basic polymer to form the desired product properties through physical mixing. The mixed materials usually have specific properties, which are achieved by adjusting the mixing formula according to your usage requirements. Silicone compounding manufacturers can produce silicone mixtures with different properties according to your requirements. The products of silicone mixture can be applied in various fields, including aerospace, electronics, construction and more.

Purposefulness and Significance

Overall, the purpose of silicone mixing is to improve the mechanical properties or other special functions of silicone materials, which is beneficial for the subsequent production of silicone. For instance, adding reinforcing materials to silicone materials can significantly improve their strength, wear resistance and tear resistance.Adding internal release agents in the material can make the compound easier to extruder and mold and improve the processing performance of the compound. Adding conductive fillers, flame retardants, foaming agents and more agents inside the material can make silicone to achieve specific functions such as conductivity, flame retardancy and foaming.

Primary Ingredients

Base Ingredients

The base polymer serves as the cornerstone of the entire silicone compound formula. Basic polymers can provide fundamental elastomer properties and chemical structures. Choose different types of base polymers according to your different usage requirements. The following are three basic polymers:

Methyl Vinyl Silicone Rubber(VMQ) : This is one of the most commonly used base polymers. It can improve the vulcanization efficiency and the physical and mechanical properties of the vulcanized silicone.Especially in the compression of permanent deformation, it can be greatly improved. It can be applied to the vast majority of high-temperature vulcanized silicone products, with a medium cost and excellent cost performance.

Methyl Silicone Rubber(MQ): This base polymer vulcanization activity is low, the strength and temperature range of vulcanized silicone is not as good as the first one. It is usually used in fields that do not require extremely high strength, such as fabric gluing. But its cost is very low. It is currently rarely used in silicone compounding.

Phenyl Silicone Rubber(PVMQ): It is usually classified into low-phenyl silicone and high-phenyl silicone based on the different phenyl content. Low-phenyl silicone has extremely strong low-temperature resistance and improved radiation resistance. High phenyl silicone has better flame retardancy.Therefore, it is often used in aerospace or extremely cold environment seals, coatings. Correspondingly, its cost is more expensive.

Reinforcing Filler

The tensile strength of virgin silicone is extremely low. The mechanical strength, wear resistance, tear resistance and other physical properties of silicone can be greatly strengthened by adding reinforcing fillers.

Fumed silica: It is a mandatory filler for almost all high performance solid HTV silicone rubber. Its reinforcing effect is positively correlated with its high specific surface area.The better the reinforcement effect, the more difficult the mixing. Its cost is high, and the price rises with the increase of purity and other factors.

Precipitated silica: Its specific surface area is relatively low, and its reinforcing effect is inferior to that of the former. It is usually used in situations where mechanical properties are not highly demanded. Or it can also be used as a semi-reinforcing filler to reduce costs. Its cost is also far lower than the former.

Formulating Aids

The addition of additives is typically used to adjust the processing performance of materials, achieve vulcanization, endow materials with special functions, and improve the specific properties of the final product.

Structured Control Agents

The use of precipitated silica will cause the compound to harden and decrease plasticity after parking, which makes the processing difficult.And adding a structure control agent can inhibit compound harden and improve the processability and the storage stability of the compound.

Vulcanizing Agent

Under the influence of heat or a catalyst, the vulcanizing agent can trigger chemical cross-linking between silicone molecules, thereby endowing the silicone with elasticity. The main vulcanizing agents are organic peroxides and platinum catalysts. The former is usually used for high-temperature vulcanized silicone rubber. The latter is used for addition reaction curing, such as some solid silicone and liquid silicone rubber(LSR).

Mold Release Agent

Mold release agents can reduce adhesion and form a layer of isolation film between the compound and the mold. In this way, the products can be easily demolded, improving production efficiency and the service life of the molds.

Color Paste

Color pastes mainly provide colors for silicone products based on your customized product requirements. It is mainly to satisfy the aesthetic or identification requirements of the products. The cost of color pastes varies depending on their color and temperature resistance grade.

Besides the above-mentioned additives, there are many other types of additives. For instance, thermal conductive fillers for aluminium oxide and aluminium nitride, conductive fillers for conductive carbon black and carbon nanotubes, flame retardants for aluminium hydroxide and platinum compounds.

Primary Processing Equipment

Internal Mixer

The internal mixer is the preferred and standard equipment for large-scale production of solid high-temperature vulcanized silicone. It is applicable to the entire mixing process and is particularly suitable for handling high-viscosity solid silicone and high-filling formulations.The most important is that it can be dispersed efficiently in shear demanding packing, this is key to get high performance silicone. The internal mixer features high efficiency and high output, making it highly suitable for continuous batch production. The adoption of automatic control in the internal mixer can guarantee the consistency of products in batches.

Two-roll Mill

The two-roll mill is mainly composed of two parallel and horizontally placed rollers at different speeds. It is mainly used for supplementary mixing or for small-batch production. It can also be used to add color masterbatches or vulcanizing agents to compounds. The cost of this kind of equipment is much lower than an internal mixer. Compared with the internal mixer, its operation is more flexible, allowing direct observation of the state of the compound, which is convenient for timely sampling and adjustment. However, it needs highly skilled and safety-conscious workers to operate it manually. Moreover, its production efficiency is low and it is difficult to achieve large-scale production.

Planetary Mixer

It has one or more stirring paddles to achieve true dead zone mixing. Usually, the mixer is also equipped with a vacuum system, which can deaerate during the mixing process. It is mainly applied in the mixing of high-viscosity liquid silicone, sealant.This mixing device has a very good mixing uniformity. The vacuum system endows it with an outstanding deaeration system.

Compounding Process

Raw Material Preparation and Pretreatment

Before mixing, it is necessary to determine the types of raw materials and the precise proportion required based on the final performance of your customized product. When designing raw material formulas, it is also necessary to consider the interactions among various raw materials. Before mixing, it is necessary to check whether the raw materials meet the specifications and are in good processing condition.

Besides the inspection of raw materials, it is also necessary to inspect and clean the equipment. It is essential to make sure that there are no residual impurities or moisture inside the equipment. Guarantee that the equipment can operate normally.

Main Compounding Process

Raw Material Plasticizing and Preheating

The weighed raw material is put into the internal mixer and idled for a short time. The duration should be around one minute. After plasticizing, the raw material becomes softer and its viscosity decreases, which makes it more convenient to add fillers and additives in the subsequent process and prepares for better dispersion of fillers.

Add Fillers and Additives

At this stage, fillers and additives need to be evenly mixed into the base polymer. First, add structured control agents and silica to the base polymer, or add structured control agents. It can make preparation for inhibiting structuring. During the process of adding additives, it is necessary to control the temperature of the equipment well to prevent the compound from undergoing chemical reactions too early.

Mixing

After adding fillers, the rotational speed of the equipment needs to be increased. Under the action of increasing shear force, the nano-scale dispersion of the filler is realized. This is the important step to get high-performance silicone. To achieve good dispersion, there must be sufficient temperature and shearing time. However, it is also necessary to avoid excessively high temperatures and prolonged durations. This will cause the compound to vulcanize prematurely or its performance to decline.

Discharge and Cooling

After the compound is mixed, the compound needs to be unloaded from the internal mixer. Unload the compound onto the material cart or conveyor belt. Since the compound has been mixed and has a high temperature, it is necessary to divide the compound into small pieces to prevent heat accumulation and coking. Large pieces of compound need to be cooled and reduced in temperature through a cooling bath.

Supplementary Processing

Refining

The thin sheet of compound is covered on the roller of the two-roll mill, and the knife is used to cut and turn. This can better help the compound dissipate heat and also remove the gas carried in the compound. Furthermore, through operations such as folding in the two-roll mill, can eliminate any possible uneven compound.

The vulcanizing agent and color paste can only be added after the compound temperature has cooled to a safe level. If the vulcanizing agent is added too early, it will cause the compound to vulcanize in the internal mixer. This will cause the compound to be scrapped and may lead to equipment damage.

Cutting Compound

After the refining, the roller distance needs to be adjusted to press the compound into a smooth film of uniform thickness. Then the pressed film is wound up or cut into sheets. The compound is made into a unified specification of film, which is more convenient for storage, transportation and subsequent processing.

Post-processing

Park

Place the cut compound in a dry and clean environment for 24 hours. During storage, it is necessary to avoid direct sunlight and prevent the compound from contact with dust and chemical products. Placing the compound allows the fillers and additives to interact further, reaching a state of chemical equilibrium. Moreover, the processing performance of the compound will also be improved after it has been parked.

Filter

If the product you customize has high requirements for the purity of the compound, then the compound also needs to pass through a filter to remove possible tiny impurities and gel particles.

Quality Control System

Silicone compounding is a complex material preparation process.Therefore, to make sure your customized product has stable performance and conforms to the specifications,it’s must to establish a comprehensive quality testing system.

Raw Material Inspection

The main purpose of raw material inspection is to eliminate quality problems from the source. Qualified raw materials are the foundation of high-quality products. Raw material testing mainly examines the viscosity of base polymer and vinyl volatilization and so on.The surface area, pH value and impurity content of silica should also be detected. The content of active ingredients, purity, moisture and impurities of additives are also key factors in whether high-quality customized products can be produced. If you have color customization requirements, it’s also necessary to test the color difference and heavy metal content of the color paste. All the tests are conducted to make sure the consistency of product production.

Process Inspection and Control

Real-time monitoring should also be carried out during the production process. Monitoring the stability of the product can promptly detect whether there are deviations in the production process parameters and make timely corrections.The process parameter monitoring of equipment is very important. The mixing time, rotational speed and temperature all determine the success or failure of the compound production. Additionally, it is also necessary to test the sheeting or slicing of the compound. Take a small amount of the compound for small-scale vulcanization and determine whether the vulcanization characteristics and appearance meet the production requirements.

Final Product Inspection

Ultimately, the manufacturer will also test the final product to make sure that the performance of your customized product meets the design standards and usage requirements. The testing of physical and mechanical properties is indispensable.The hardness of the product is the most basic and critical index. Secondly, the tensile strength and tear resistance of the product are also key indicators of the product’s usage requirements.

Besides physical and mechanical properties, the chemical and functional properties of the product are equally important. The heat resistance, flame retardancy and thermal conductivity of a product directly affect its performance in application scenarios, and these are also important indicators of the product qualification rate. Whether there is a color difference in the product and whether there are impurities inside both need to be inspected in detail.

Overall, a comprehensive quality inspection process can guarantee stable product performance and safe and reliable use. Testing can promptly identify and handle issues, prevent batch scrapping of products, and reduce quality costs.

Silicone Compound Categories

With different processing formulas and the adoption of various marriage and processing techniques, silicone products that meet your usage requirements are ultimately produced. The following are several common silicone compounds.

High-Temperature Vulcanized Silicone(HTV) 

High-temperature vulcanized silicone is a common type of silicone. It is usually in solid form and can be roughly classified into general-purpose HTV, high-strength HTV and high-temperature resistant HTV. This type of silicone has good strength, tear resistance and temperature resistance. It can be made into some industrial sealing parts such as silicone gaskets and O-rings, or some kitchenware and highly elastic silicone miscellaneous parts.

Room-Temperature Vulcanized Silicone(RTV) 

Room-temperature vulcanized silicone can be vulcanized at room temperature. It is usually in paste form and is divided into single-component and two-component types. It is often used as a sealant for sealing doors and windows, bonding and sealing water tank groups. It can also provide insulation, moisture-proof and shock-proof protection for electronic components.

Liquid Silicone Rubber(LSR) 

Liquid silicone rubber usually takes the form of a viscous liquid. This type of silicone usually has good fluidity and higher purity. It is usually used to produce some complex-structured products, such as some multi-short structural components. Or high-precision components such as baby pacifiers and food-grade sealing rings. Pure and high-quality liquid silicone rubber is also applied in the field of medical devices, such as medical masks and implantable catheters.

Specialty Silicone 

Most special silicone is applied in the industrial field to meet special industrial demands. For example, conductive silicone is used to seal electronic products or shield against electromagnetic interference. Flame-retardant silicone can be used in wires and cables. Medical-grade silicone has high purity and excellent biocompatibility, making it safe to use. Foamed silicone can be used for sealing, buffering and heat insulation.

Application Areas

Construction Industry

Silicone applied in the construction field mainly serves the functions of sealing, bonding and weather-resistant protection. Weather-resistant sealant has a good elastic recovery rate, is resistant to aging and is not prone to cracking. It is often used to seal building doors and windows to prevent rainwater and dust from seeping in. Fireproof sealant can effectively seal the holes through which pipes and cables pass through walls, and can prevent the spread of flames and toxic smoke.

Industrial Sector

In the industrial field, silicone is usually made into sealing rings and gaskets. For instance, some industrial valve gaskets, transmission ring seals can keep the integrity of the seal in extreme temperatures and chemical substances. Electromagnetic sealing silicone  is often used for the sealing and shielding of aerospace electronic compartments. And conductive silicone is often used for the conductive buttons of medical equipment keyboards. Additionally, there are thermal conductive gaskets, thermal conductive pastes, thermal conductive insulating sheets. These thermal conductive silicone products can fill the air gap between the heat-generating element and the heat sink, and conduct heat efficiently.

Healthcare Industry

Silicone products used in the medical field have high biocompatibility, high purity, and are soft and non-irritating to the skin. It is colorless and odorless, and has stable chemical properties. Common infusion tubes, various types of catheters, and sealing gaskets are all made of silicone.

Consumer Goods Industry

Silicone can also be made into kitchenware, personal care tools and some wearable devices. Food-grade silicone needs to have food and certification, high transparency and a wide temperature resistance range. Soft and skin-friendly silicone can also be made into some care and beauty tools. It can provide a comfortable skin feel and is less likely to cause allergies. Common silicone products also include the straps of smartwatches and silicone cases for headphones. Silicone is skin-friendly and sweat-proof, and can provide users with a comfortable wearing experience.

Final Thought

As a professional silicone manufacturer, Legensilico possesses mature production techniques and complete production equipment. If you want to customize silicone products, Legensilico is definitely a reliable choice.We provide excellent customized service for you,contact us now.