Rubber Trim

Chapter 1: What is Rubber Trim?

Rubber trim, which can be extruded or molded, serves to shield edges and surfaces from impact damage. Commonly used on panels, windows, doors, removable covers, and hatches, it also acts as a sealant for the edges of mating components, functioning similarly to rubber gaskets or O-rings. This highly flexible material, made from dense rubber, offers excellent durability and resistance to ozone, fitting securely and snugly.

Sometimes, rubber trim comes with a pre-applied adhesive, such as hot melt or butyl adhesive, to enhance its grip and prevent slippage. Its popularity is attributed to its strong resistance to abrasion from frequent impacts and scrapes.

Rubber trims are made with different profiles and are often times customized for a specific application. The profiles are engineered and milled onto a die, then installed onto an extrusion machine where the rubber product is created. The most common is a U-channel used for edge protection. The edge of the part or substrate is inserted into the channel's open side and is locked in place by fasteners or adhesives. Other designs have protrusions or tongues along the inside of the channel to lock the rubber trim in place without the need for a permanent fastening.

In applications where a dependable seal is crucial, a solid or hollow cord or profile is incorporated into the channel. This setup, known as edge trim with an integrated seal, is manufactured using a co-extrusion technique, where two distinct types of rubber, each with unique properties, are extruded simultaneously. Upon compression, the cord adapts and conforms to the mating surfaces, ensuring a tight seal, while the edge trim maintains the overall structural integrity. Rubber trims are favored for their versatility in achieving various hardness levels and their robust resistance to chemicals, moisture, and high temperatures.

Chapter 2: What are the types of rubber used in rubber trim production?

Rubbers belong to a group of polymers known as elastomers, characterized by their exceptional elasticity, which is achieved through the cross-linking of long polymer chains into amorphous structures. The relatively weak intermolecular forces between these polymer chains enable them to rearrange when subjected to stress. This flexibility allows elastomeric materials to adapt seamlessly to the contours of adjacent surfaces.

Elastomers can be vulnerable to degradation depending on the curing process or vulcanization used. Exposure to water, ultraviolet light, oils, and specific solvents can lead to deterioration. High temperatures may cause expansion and oxidation or aging, while low temperatures can render the materials brittle. Common elastomers employed in rubber trim production include nitrile (NBR), ethylene propylene diene monomer (EPDM), neoprene, silicone, and natural rubber.

• Natural Rubber (NR): Natural rubber comes from latex harvested from the bark of plants and trees, mainly Hevea trees. It contains the polymer chain polyisoprene. Natural rubber is preferred because of its excellent heat buildup and fatigue resistance when compared with other types of rubbers. The properties of natural rubber are high strength and the ability to be stressed and stretched without breaking.

• Polybutadiene Rubber (BR):Polybutadiene rubber is produced from the polymerization of butadiene monomers containing four carbon atoms and six hydrogen atoms (C4H6). The three types of polybutadiene rubber are high Cis polybutadiene, lithium-based polybutadiene, and high trans polybutadiene, which have different isomers of butadiene.

  The types of polybutadiene rubbers are made using a solution process that includes transition metals like neodymium, nickel, or copper complexes or an alkyl metal like butyllithium as catalysts. Generally, butadiene rubber has good cracking, abrasion, and rolling resistance but is prone to ozone degradation.

• Styrene-Butadiene Rubber (SBR): Butyl rubber, or polyisobutylene, is a copolymer of isobutylene and isoprene, which consists of around 3% of the copolymer and provides the required unsaturation for vulcanization. The low unsaturation of IIR enables it to repel most chemicals (gas and liquids) and is highly resistant to aging when vulcanized properly. Butyl rubber has good shock absorption and low gas and moisture permeability.

• Butyl Rubber (IIR): This is a copolymer of isobutylene and isoprene; hence, the abbreviation IIR. Isoprene only consists of around 3% of the copolymer which gives the required unsaturation for vulcanization. The low unsaturation of IIR enables it to repel most chemicals (gas and liquids) and is highly resistant to aging when vulcanized properly.

  The structure of butyl rubber is similar to polyethylene and polypropylene, with every other carbon atom being substituted with two methyl groups instead of one. It is the only elastomer that is resistant to gasses and is biocompatible with resistance to acidic or alkaline chemicals, ozone, heat, and weather and includes good aging properties. Butyl rubber does not perform well in the presence of petroleum-based fluids, hydrocarbons, or flames.

  

  
  
• Halogenated Butyl Rubber (CIIR, BIIR): This is a rubber compound produced from the modification of IIR. Halogenation is done by inserting allylic chlorine (CIIR) and bromine (BIIR) into the double bonds of the isoprene monomer creating new cross-linking chemistry. Like the IIR, halogenated IIR has superior air impermeability and good resistance to moisture, chemicals, and ozone.

• Acrylonitrile Butadiene (Nitrile) Rubber (NBR): This rubber is a copolymer of acrylonitrile and butadiene. They are polymerized in an emulsion similar to SBR polymerization systems. NBR is widely used due to its resistance to oils and petroleum-based solvents. However, they have low tensile strength and poor low-temperature performance. Reinforcing fillers are added to solve these problems.

  

  
  
• Chloroprene (Neoprene) Rubber (CR): Neoprene is a polymer of chloroprene produced from emulsion polymerization. The presence of chlorine in the polymer chain improves resistance to oxidation, ozone, and oil. However, they are more expensive than natural rubber and have poor low-temperature performance.

• Polyisoprene Rubber (IR): Isoprene rubbers are general-purpose rubbers created from the polymerization of isoprene monomers. The polymer chain is similar to that of natural rubber. Because IR is synthesized in a controlled environment, it is chemically purer than natural rubber with similar or more superior characteristics.

  

  
  
• Ethylene Propylene Rubber (EPM, EPDM): These rubbers are products of the copolymerization of ethylene and propylene. Initially, only ethylene and propylene are copolymerized, which results in a rubber compound that can only be cured by peroxide. The addition of diene enables the polymer to be curable with sulfur. The main desirable characteristics of EPM/EPDM are good weathering resistance, good insulating and dielectric properties, excellent mechanical properties both at high and low temperatures, and chemical resistance.

• Silicone Rubber: These polymers, instead of having a carbon backbone, have a silicon-oxygen chain with groups of methyl, vinyl, and phenyl. Silicone rubbers have good oxygen, ozone, heat, light, and moisture resistance. However, they are more expensive and have poorer mechanical properties than organic rubber.

  

  
  
• Polyurethane Rubber: Polyurethane elastomer is prepared by the reaction of a polyether or polyester glycol with diisocyanates and curatives. They can be formulated to have different degrees of hardness with good tensile and abrasion resistance.
• Thermoplastic Elastomers (TPE): Thermoplastic elastomers are polymer materials with the characteristics of thermoset vulcanized rubber and thermoplastic. The three basic characteristics of TPE are its ability to be stretched and return to its original shape, its ability to be processed at high temperatures, and the absence of creep after being installed. A unique quality of TPE, unlike vulcanized rubber, is its capability to be processed, shaped, formed, and recycled for reprocessing, making it environmentally friendly.

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Chapter 3: What is the manufacturing process for extruded rubber?

The manufacturing of extruded rubber is a very complicated process. The processes can be divided into two: the production of the raw rubber and the downstream processes that mainly involve compounding, forming (extrusion), and curing. The production of raw rubber converts gaseous or liquid petroleum-based compounds into polymers. This process is usually done in petrochemical plants. The rest is done in the rubber manufacturing plant, where the raw rubber and other materials are combined and formed to create the final product.

Production of Raw Rubber

Rubbers are created through the polymerization of organic compounds, which form long chains and are cross-linked to achieve their elastomeric properties. This polymerization can occur via either step-growth polymerization or chain-growth polymerization.

Step-growth polymerization, also known as polyaddition polymerization, involves a reaction mechanism where monomers with at least two reactive sites or functional groups link together to create oligomers. These oligomers further connect to form longer polymer chains. The process is managed by introducing monomers with a single functional group, which reacts with the oligomer's active sites. This method, known for its straightforwardness, is among the earliest used for polymerization. Common elastomers made using step-growth polymerization include polysiloxane (silicone) and polyurethane.

Chain-growth polymerization, also referred to as chain polymerization, involves the sequential addition of monomers to a growing polymer chain. This process is initiated by reactive species or intermediates, which start the reaction by breaking a bond in a monomer, creating an active site for further monomer addition. These initiators can include free radicals, cations, anions, or organometallic catalysts.

During chain propagation, the active site on the growing chain facilitates the addition of new monomers, which in turn opens one of their bonds, creating more active sites. The polymer chain continues to grow until it encounters another chain or a terminating agent, leading to chain termination.

To control the molecular weight of the polymer, a process known as chain transfer is used. This method helps regulate the average length of the polymer chains. Common rubber compounds produced through chain-growth polymerization include butadiene, isoprene, and chloroprene. In addition to homopolymers (polymers made from a single type of monomer), this process also allows for the creation of copolymers, such as styrene-butadiene (SBR), nitrile butadiene (NBR), and ethylene propylene diene monomer (EPDM).

Compounding Chemicals and Rubber

Compounding is the process of mixing specific chemicals with raw rubber to enhance its physical and chemical properties, reduce costs, and improve its processability and vulcanization. This formulation process aims to tailor the rubber's characteristics to suit its intended use. In addition to the base rubber (whether natural or synthetic), common ingredients used in compounding include:

• Filler Systems: Filler systems include carbon blacks, clays, silica, and calcium carbonate, which are added to reinforce the elastomer structure to meet the required tensile strength and impact and abrasive resistance.
  

  Carbon black is the most widely used reinforcing filler, which is produced from the incomplete combustion of heavy petroleum-based compounds, such as oil and tar. There are different types of carbon blacks, including super abrasion furnace (SAF), high abrasion furnace (HAF), fast extruding furnace (FEF), and semi-reinforcing furnace (SRF). Carbon black reinforces the elastomer by entangling the elastomer molecules on the porous surface of the filler. The deposition of carbon black particles into the elastomer matrix creates a composite with improved mechanical properties.

  

  
  

  Non-carbon-based fillers, such as silica, clay, and calcium carbonate, are added to improve tear strength, reduce heat buildup, and create better compound adhesion for multicomponent products. These fillers do not bind well with elastomers in comparison with carbon blacks, resulting in a lower level of reinforcement, a problem that is solved by adding a silane coupling agent.

• Stabilizer Systems: Stabilizer systems include antioxidants and antiozonants, which are added to prevent the degradation of rubber. Polymers, especially elastomers, tend to degrade due to the presence of carbon-to-carbon double bonds that can easily be broken and bonded onto by oxygen in the presence of ultraviolet light (photochemical reaction). The result is a broken polymer chain and a decrease in cross-linking density of the elastomer.

  A broken polymer chain softens the elastomer, which decreases its abrasion resistance, while the decrease in cross-linking reduces the elastic property of the rubber and makes it stiff and prone to fatigue failure. Adding stabilizers helps limit the rate of oxidation by scavenging free radicals that can break the bonds of the polymer chain and prevents compounds, such as peroxides and hydroperoxides, from producing more free radicals by decomposing them.

  Common antioxidants are phosphites, phenols, and hydroquinones. For the antiozonant, PPDs or paraphenylene diamines are the only classes used in significant quantities due to their potency and ability to further improve the fatigue and heat resistance of the rubber.

  

  
  

• Vulcanizing or Curing Agents: Curing or vulcanizing agents are sulfur, peroxides, accelerators, activators, and retarders. The process of vulcanizing is where elastomers react with a system of compounds that inserts cross-links between the elastomer chains. It is often associated with sulfur systems, which are the most common method.

  Sulfur curing systems consist of three main components: the vulcanizing agent (sulfur), the accelerators, and the activators. Accelerators act as catalysts that promote the vulcanization process by increasing the rate of the cross-linking process resulting in faster curing time and lower reaction temperatures. Accelerators increase the degree of cross-linking, resulting in a rubber matrix with better mechanical properties. Each type of accelerator works through different reaction mechanisms. Common accelerators are benzothiazoles, thiocarbamates, and amines.

  Activators enable the efficient use of sulfur to increase the density of cross-links. The most common activator is zinc oxide combined with stearic acid. Zinc, when compared to other metal oxides, is most suited to forming sulfurating intermediates.

  

  
  

  Adding the vulcanizing agents, accelerators, and activators to the elastomer prepares it for curing, which can occur prematurely during processing, making it harder to form. Premature vulcanization is termed as scorching. Retarders or inhibitors are added to prevent scorching without affecting the rate of vulcanization. The most popular rubber retarder is cyclohexylthiophthalimide (CTP), which is simply termed PVI or pre-vulcanization inhibitor.

• Special Ingredients: Colorants, process oils, resins, and fibers are special ingredients used to give specific properties to the rubber. Colorants and pigments are supported by various compounds such as silica and titanium dioxide. Rubber is usually black due to the addition of carbon black.

  Process oils are added to improve the processability of rubber. They can be paraffinic, naphthenic, and aromatic. Process oils also act as an extender to lower the cost of the rubber bulk. The type of process must be compatible with the elastomer to prevent it from migrating out of the compound.

  Resins and fibers are added to improve the mechanical properties of the rubber. Hydrocarbon resins increase the viscosity of the compound when heated to achieve better flow, while fibers increase tensile strength and chipping resistance.

  

  
  

Mastication, Masterbatching, and Final Mixing

Mastication is a key step in the compounding process where the polymer chains are exposed to intense shear forces, which break the molecules into shorter chains. This treatment enhances the polymer’s ability to integrate with compounding ingredients. However, not all polymers need mastication, particularly those with stable and consistent chemistry. The use of chemical plasticizers or peptizers can aid in mastication by acting as oxidation inhibitors. These additives help prevent the reformation of broken polymer chains by neutralizing free radicals produced during the mixing process.

Mastication is usually done in open roll mills or internal mixers. Open mills consist of two rollers heated for temperature control. The rubber compound is sheared as it passes through the fixed clearance between the rollers. Internal mixers are high shear mixers that can either be a tangential rotor type or an intermeshing rotor type. The tangential rotor design shears the rubber between the clearance of the rotor and the walls or stator parts of the mixer. The intermeshing rotor design performs shearing by impinging the rubber between two rotors.

Masterbatching involves integrating most of the compounding ingredients into the masticated polymer, excluding the vulcanizing agents. This process helps prevent premature vulcanization, which could occur due to the heat generated during mixing. Masterbatching ensures an even dispersion of carbon black and achieves the desired viscosity of the rubber blend.

Final mixing is the stage where the vulcanizing agents are introduced. This step is typically performed using separate mixers that operate at lower speeds to control the mixing process. It is crucial to ensure thorough distribution of the final mix to avoid early vulcanization. Once mixing is complete, the blend is cooled and formed into slabs or bales, preparing it for the next stages of production.

Rubber Extrusion Process

In many manufacturing setups, the output from the mastication and mixing stages is fed directly into an extrusion machine equipped with either a single-screw or twin-screw system. This machine further blends the rubber compound, applies compression, and forces it through a die.

The die is crafted to the specific cross-section of the intended final product, accounting for any dimensional changes due to die swelling and vulcanization. Temperature control within the extruder is managed by steam jackets, water jackets, or electric heaters along the barrel to maintain optimal conditions.

Some extrusion systems integrate the processes of compounding, masticating, and mixing into a single unit. These machines feature screw elements with varied profiles to perform different functions: one set for mastication (kneading), another for mixing, and a third for compression. The extrudate is then passed through a heater or oven to commence the vulcanization process.

Rubber Curing Process

As previously discussed, the process of curing, also known as vulcanization, involves forming cross-links between elastomer chains. This process enhances the stability of the rubber, allowing it to withstand various environmental factors like heat, cold, and solvents. Historically, "curing" was the general term for this cross-linking process, while "vulcanization" specifically referred to curing rubber with sulfur. Sulfur was the first curing agent used to cross-link natural rubber or latex. Nowadays, other curing methods are employed that do not rely on sulfur as the primary agent. These modern methods use alternatives such as peroxides, and occasionally phenols and metal oxides.

Vulcanization is achieved by heating the rubber compound, which contains curing agents, to activate the sulfur molecules (S8) or sulfur donors. This heating process breaks the sulfur bonds, creating sites for cross-linking. Cross-links are formed when elemental sulfur or sulfur donors interact with the functional groups or unsaturated bonds on elastomer chains. Traditional vulcanization with elemental sulfur involves heating at approximately 284 °F (140 °C) for about five hours. This method produces rubber with good physical properties but tends to age quickly. The addition of activators, such as zinc oxide, shortens the curing time to around three hours. When combined with accelerators, this process further reduces the curing duration to just a few minutes, enhancing the rubber's aging resistance and overall properties.

Peroxide curing agents operate similarly to sulfur-based curing agents. Heating decomposes the peroxide into free radicals, which extract hydrogen atoms from the polymer chain, creating sites for cross-linking. Generally, peroxide curing offers superior temperature resistance, aging characteristics, solvent resistance, and reduces odor compared to sulfur curing. Additionally, peroxide curing does not require the use of activators and accelerators that can potentially produce carcinogenic nitrosamines.

Secondary Processes

Secondary processes are supplementary operations performed on products to tailor them for specific uses. These processes might include cutting, drilling, notching, splicing, coating, and other modifications. For rubber trims, common secondary processes involve splicing, applying slip coatings, injecting sealants, and using adhesive tapes, in addition to cutting.

• Splicing creates a continuous loop of extruded rubber for sealing without any weak points.
• Slip coating improves the abrasion resistance of edge trims by creating a surface with low friction.
• Seal injection is for rubber trims with pre-applied sealant to eliminate the additional sealing process during installation.
• Adhesive taping is used to simplify the installation process by creating pre-applied pressure or temperature-activated adhesives for attaching the rubber trim onto the substrate.

Quality Control and Testing

Testing and quality control are essential for assessing the final properties of the product and ensuring its uniformity and consistency. Major manufacturing facilities with compounding operations typically maintain an in-house testing laboratory. The final rubber compound is subjected to various tests, which may include:

It is important to note that not all tests are mandatory; the specific tests required depend on the customer’s needs and the product’s intended application. Once quality inspections are completed, the products are typically rolled, bundled, or packaged in preparation for distribution.

Conclusion

• Rubber trim is an extruded, sometimes molded, elastomer used that is used to protect the edges or surfaces of objects from sudden impacts.
• Aside from edge and surface protection, rubber trims also provide sealing on mating parts, acting the same way as rubber gaskets and O-rings.
• Rubbers are a class of polymers (elastomers) that have a highly elastic nature created by the cross-linking of long polymer chains into amorphous structures.
• The extruded rubber production processes can be divided into two: the production of the raw rubber and the downstream processes, which mainly involve compounding, forming (extrusion), and curing.
• Secondary processes are additional processes done on the product to suit a particular application. This can include cutting, drilling, notching, splicing, coating, and so on.
• Testing and quality control are done to determine the final properties of the product and to ensure the uniformity and consistency of the product.

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