Rubber Compounding Basics
Industries require different types of rubber for the end application. The strength and finish requirement for each rubber varies from industry to industry.
For instance, if you use rubber in the form of belts, you need it to be flexible. On the other hand, if you need to use it in chemical industries, you need it to be highly chemical resistant.
For different end applications, you chemically modify the structure and composition of rubber by adding. This refers to rubber compounding.
It helps you achieve the final rubber product’s desired physical, chemical, and mechanical properties.
Step-by-Step Rubber Compounding Process
Multiple steps lead to effective rubber compounding. Here is the list of steps for your reference:
1. Base Material Formation
The base material is an underlying or primary material (polymer). This material is the base polymer for adding additives to enhance various properties. You can use the following materials as the base material in rubber compounding.
Natural rubber refers to a rubber that you acquire naturally. You can get it from latex. It is a milky liquid that you can find within the cells of plants that produces rubber.
You can use natural rubber to enhance its properties like abrasion resistance, tensile strength, and tear resistance. It ages quickly due to acid, heat, alkali, and ozone impact. This compound offers a lower compression set, and you can subject it to lower temperatures.
This base polymer has characteristics similar to rubber, but it belongs to a synthetic class. With this material, you can enjoy high resistance to weathering, oils, and solvents.
You cannot subject it to alkalis and acids. Its performance is poor, especially at low temperatures. However, you can compound it for applications of flame resistance.
You can define Nitrile as a synthetic copolymer. It results from the combination of butadiene and acrylonitrile. Nitrile offers better resistance to solvents as well as oils.
You can get a better compression set, high abrasion resistance, and slight resistance to acids and alkalis. Nitrile offers you bad weathering resistance. Apart from this, you can also subject Nitrile to low temperatures without compromising performance.
Hydrogenated Nitrile (HNBR)
As the name suggests, you can get hydrogenated Nitrile due to the hydrogenation of conventional Nitrile. This polymer type does not contain a double carbon bond as in Nitrile.
You can experience higher levels of ozone, oil, and heat resistance. Offers poor resistance to alkalis as well as acids. An option exists to use this base polymer for lower-temperature applications.
You can define styrene butadiene as a rubber class that combines polymer butadiene and styrene. Styrene butadiene also refers to a low-cost alternative to natural rubber.
The properties of this base polymer include high resistance to water and abrasion but poor resistance to weather. You cannot use it with solvents, alkalis, acids, and oils.
Butyl, you can refer to the synthetic rubber resulting from polymerizing isoprene and isobutylene.
Butyl offers unmatchable resistance to various gases, weathering, and ozone. Apart from this, you can also use it with concentrated alkalis and acids. Butyl is not suitable when it comes to applications of petroleum products.
Another synthetic rubber EPDM combines propylene, ethylene, and diene monomers.
EPDM is also famous for its remarkable properties, like resistance to ozone, water, weathering, gases, heat, and steam. An option exists that you can expose it to concentrated alkalis and acids but not solvents and oils. s
The term FKM refers to synthetic rubber that results in because of highly fluorinated hydrocarbons. Regarding the characteristics of FKM, it is resistant to oils, solvents, chemicals, and oxidation. The FKM or FKM rubber is often regarded as a high-performance material.
2. Filling Operation
The fillers refer to the additives you can use in rubber compounding to boost particular characteristics of rubber composites. Fillers also contribute to lowering the cost of rubber composites. Get insights into various types of stuffing in the following section:
As the name reflects, the reinforcing filler plays a role in reinforcing different characteristics of the specific rubber material. The following are the key reinforcing fillers:
Carbon black is a fine powder material containing carbon as a key element. It results from the partial combustion of coal tar/ coal/ petroleum products/ vegetable matter.
Carbon black increases the reinforcement of tires, paints, plastics, and inks. It can impart better electrical conductivity and a high strengthening effect. An option exists to adjust the viscosity and resultant compound strength with carbon black filler.
The precipitated silica refers to the white powder that forms due to the precipitation of silicate salts.
You can use precipitated silica as a filler in the rubber compound to accelerate tear resistance. An option exists to prevent cuts or cracking in the end material. Precipitated silica has high desirability among power transmission belts and conveyor belts.
Another name for fumed silica is pyrogenic silica. It results in formation because of the flaming of amorphous silica. The flaming then contributes to the shape of a chain-like three-dimensional structure, referred to as fumed silica.
Fumed silica is desirable to make different base polymers thick and prevent free flow. It imparts paste-like characteristics. Besides that, it also adds up the durability and strength properties of a specific rubber composite.
2. Non-Reinforcing Filler
The non-reinforcing filler uses recycle rubber to replace raw materials within rubber composites. It decreases production costs and offers great help in resource recovery. Consider the key reinforcing fillers given below:
Mica refers to the mineral group with the same chemical and physical traits. In general, they refer to platy or sheet silicate minerals.
Mica is highly desirable as a non-reinforcing filler. It contributes to the properties like incredible hardness, tear resistance, high surface finish, and high weather resistance.
Talc refers to the mineral that occurs naturally. Four elements contribute to the formation of talc hydrogen, oxygen, magnesium, as well as silicone.
Calcium carbonate refers to a compound that occurs naturally. It forms as a result when calcium ions react with carbonate ions in the presence of hard water.
Adding calcium carbonate to the rubber compounds will lead you to experience effective stiffness and resistance to abrasion. An option exists to use calcium carbonate as an extender detacktifying agent.
You can define a plasticizer as a substance you can add to a rubber compound to improve flexibility and plasticity. You can also do this to decrease brittleness. Keep reading to get familiar with plasticizers used in rubber compounds.
Silica is a compound that is colorless and non-reactive. It is a key constituting element of various rocks as well as sandstones.
As a filler, you cannot ignore the importance of silica. It adds resistance to wear and tear, offers high-end modulus, and greater fatigue resistance. Silica also plays its part in increasing traction in tire applications.
Clay is also a mineral having a fine size of the particle. It features a gray-white to tan color. You can use it in rubber compounds to promote binding, extending, or anti-caking characteristics. An option exists to use clay in specific materials or compounds to boost tensile strength.
Another name for glycerol is glycerin. You can refer to it as an odorless, colorless, and highly viscous liquid that is non-toxic.
You can add this to various and multiple rubber compounds to improve the plasticity. Doing so also improves other characteristics like viscosity and dispersion.
3. Protection Additives Addition
The protection additives lead to the effective protection of the specific rubber compound throughout its life. It prevents failure and leads to effective longevity. This effectively contributes to or prevents material loss from the rubber compound’s surface. Get information highlighted on the following protection additives.
Anti-degradant also refers to the deterioration inhibitor. It is an essential ingredient within the rubber compound. Anti-degradant plays an effective role in preventing the aging of the products. Antidegradants are of two types, including antioxidants and antiozonants.
Antiozonants refer to the waxes and chemicals that you apply or bleed directly on the rubber surface. You can do this to ensure protection from ozone. For instance, hydrocarbons and di-alkyl, alkyl aryl, and di-aryl p-phenylenediamine are different antiozonants. The cracks appear on the rubber surface because ozone is perpendicular to the rubber particle’s direction.
Antioxidants refer to chemicals you can use or apply on rubber surfaces directly. This will help protect the rubber from the oxygen radicals in the atmosphere. For instance, the popular anti-oxidants are phenolic antioxidants based on amine, 2-dihydroquinoline, and 2,2,4-trimethyl-1 antioxidants. The cracks appear on the rubber material’s surface because oxygen is random in shape and direction.
2. Addition of Curing Agents
You can define a curing agent as a chemical or additive that plays its part in cross-linking long-chain polymers. The curing agent prevents the free movement of the polymers. It also plays a crucial role in stretching specific material under stress and ensure returning to original shape upon stress release. Find insights on different curing agents highlighted below:
1. Vulcanizing Agents
You can define vulcanizing agents as chemical crosslinks. You add up these cross-links within the rubber matrix. This will lead you to enjoy a three-dimensional network of polymer links.
Vulcanizing agents prevent permanent flow, specifically under heat/ deformation. It also protects a specific rubber material, especially solvents, from complete dissolution. The widely used curing agents are peroxides, sulfurs, and metal oxides.
Activators refer to organic as well as inorganic chemicals. You can use an activator to boost the overall time of the vulcanization process. It does this by speeding up the vulcanization process rate.
Apart from this, you can also benefit from the activator, which leads you to experience better vulcanization efficiency and end properties. One popular activator in the vulcanization process is the ZnO (Zinc Oxide). It is specifically suitable for sulfur vulcanization.
Accelerator refers to the compound or a chemical/ additive that plays its role in enhancing vulcanization speed. Apart from this, it also plays its part in efficiently processing vulcanization and maintaining a lower temperature.
An accelerator is also crucial for decreasing the sulfur amount you need to cross-link the specific rubber material. It helps you enjoy more longevity with the end rubber composite. The popular accelerators are thiuram, dithiocarbamate, thiourea, isopropyl xanthate, etc.
4. Scorch Retarders
The scorch retarder refers to a material you can add to a rubber compound. It helps retardation in the development of scorch, especially during milling and during calendaring temperature.
The scorch retarder has zero effect on the curing rate and temperature. It slows down the process of aging of specific rubber and does not influence the material’s color. The popular scorch retarders are mineral oil, pine tar, and rosin oil.
Criteria of Compounding
A number of things like the below need your attention or consideration while working with the rubber compounds. Neglecting them can lead you to a non-effective end product.
1. Avoid Coagulation
Coagulation is a phenomenon in which particles combine to represent a mass or group. You can also refer to it as a clot or curdle.
To process rubber effectively, you need to avoid coagulation. Several things contribute to this phenomenon, such as dilution, the processing temperature etc. It is essential to avoid this phenomenon. You can use an alkali-like ammonia solution to prevent the formation of coagulants.
2. Match Particle Size
The same particle size does matter when processing the rubber. It leads to better homogeneity and adds up the life of the rubber material. On the other hand, using different particle sizes can lead you to experience difficulty in processing and high abrasion resistance. The recommended particle size should be in the range of 0.8 up to 6.5 µm for different materials.
3. Prevention of Hardwater Usage
You cannot use hard water while processing rubber compounding. This will eventually lead you to face destabilization. The hard water makes the reaction process extremely slow. It also decreases the adsorption rate throughout.
4. Slow Stirring
While processing rubber compounds to get better results, employ slow stirring. Increasing the stirring speed will lead you to experience several by-products during the reaction. This will prevent you from achieving proper rubber compounding.
5. No Contact Between Stirrer/Container
It is the paradigm to facilitate no contact between the container and the stirrer. Doing so will lead to excessive friction between the stirrer and the container. In the end, this will result in coagulation which you must avoid. So, ensure a specific distance between the stirrer and the container.
Selection Criteria for Raw Rubber
The base polymer or rubber is the key ingredient in rubber compounding. It greatly influences the end properties that you are going to get. So, consider the below aspects and choose them wisely.
The cost possesses great importance when selecting raw rubber. You cannot make a big profit if the price is high. Choose a material that facilitates a balance between profit and expenses.
2. Mixing Ease
Consider the mixing process for the particular raw material you select if it needs additional or specialty mixing equipment. Avoid choosing the material which requires other mixing equipment and procedures. Go for the alternate materials.
3. Oil Resistance
The level of resistance a particular material offers to oil matters. It contributes to the valuable end features. You can subject such materials to oily applications without worrying about affecting the surface finish or appearance.
4. Service Temperature
Another aspect that needs your attention is the service temperature it can withstand. Some materials cannot cater to a higher temperature. This is because it affects their chemical characteristics and distorts their physical appearance.
Flammability determines the capability or characteristic of a specific material to catch fire. It also refers to the level of intensity with which a particular material burns. Consider this while choosing the raw rubber material. This will aid in completing the rubber processing with greater ease.
6. Fatigue Requirements
The fatigue requirements of a specific rubber material represent how much force it can endure before breakage. You need to select a material that can offer you higher fatigue resistance.
7. Chemical Resistivity
Pay attention to the chemical resistivity traits of a specific material. It represents how good or bad a particular material can resist the action of chemicals. Suppose you can use it with strong alkalis or acids. In case yes, you can enjoy many applications with such material.
8. Abrasion Resistance
You can define abrasion as the chipping or removal of particles from the material’s surface. Consider the raw rubber material, which offers low abrasion resistance. With such material, you can experience a comparatively longer life.
9. Elongation Requirements
Elongation means to what extent you can stretch your material without breaking it. Such materials are widely desirable for applications that require the subsequent amount of stretch.
10. Strength Requirement
In the end, the thing which matters the most is the overall strength of the raw rubber material. The higher power means a higher level of suitability for rough and tough applications. Don’t ignore this aspect while making your choice.
Process of Compounding Rubber – Source: Science Direct
Rubber Technology – Source: Wikipedia
Rubber Extrusion – Source: IQS Directory