Rubber Trim

Chapter 1: What is Rubber Trim?

Rubber trim is an extruded, sometimes molded, elastomer used to protect the edges or surfaces of objects from sudden impacts. It can be found on panels, windows, doors, removable covers, and hatches. Additionally, rubber trims provide sealing for the edges of mating parts, acting like rubber gaskets or O-rings. Rubber trim is an exceptionally flexible protective material made of dense rubber that has excellent wear and ozone resistance and fits snug and tight.

In some instances, rubber trim is manufactured with a pre-applied hot melt adhesive or butyl adhesive to further enhance its tight fit and avoid slippage. Rubber trim is popular due to its resistance to abrasion caused by frequent bumps 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.

For applications requiring a reliable seal, a solid or hollow cord or profile is integrated into the channel. The combined channel (edge trim) and the cord (seal) is produced through a co-extrusion process where two different types of rubber are extruded with different sets of properties. When compressed, the cord deforms and flows along the mating surfaces while the edge trim provides the structural stability of the seal. Rubber trims are preferred due to the broad range of hardness that can be obtained, along with their inherent resistance to chemicals, moisture, and heat.

Chapter 2: Types of Rubber in Rubber Trim Production

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. In elastomers, the intermolecular forces between the polymer chains are relatively weak, allowing them to be reconfigured upon application of stress. This property allows elastomeric materials to easily conform to the profile of the adjacent surfaces.

Depending on the type of curing reaction or vulcanization, elastomers can degrade under the presence of water, ultraviolet light, oils, and certain solvents. High temperatures can easily expand and oxidize or age elastomers, while low temperatures can make them brittle. Typical elastomers used for the manufacturing of rubber trims are 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.

Chapter 3: Extruded Rubber Manufacturing Process

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 made by polymerizing organic compounds into long chains and cross-linking them to give them an elastomeric structure. Polymerization is achieved by either step-growth polymerization or chain-growth polymerization.

Step-growth polymerization, orpolyaddition polymerization, is a reaction mechanism that involves a monomer with at least two active sites or functional groups that can bind with other monomers, forming an oligomer. The oligomers join to form longer chains until the polymer is formed. Polymerization is controlled by adding monomers with only one functional group, which consumes an active site of the oligomer. This process was one of the earliest methods of polymerization because of its simplicity. Popular elastomers produced through step-growth polymerization are polysiloxane (silicone) and polyurethane.

Chain-growth polymerization or chain polymerization involves the continuous addition of monomers one at a time to a growing chain which is initiated by some reactive species or intermediates. These reactive species initiate the reaction by opening a bond of a monomer which becomes an active site for binding with other monomers. They can be free radicals, cations, anions, or organometallic catalysts.

The polymer chain grows through chain propagation, where the newly added monomers to the chain open one of its bonds which becomes an active site. The chain propagation ends when one chain binds with another chain or with a terminating agent. This is known as chain termination.

Another type of reaction, known as chain transfer, is used to control the average molecular weight of the polymer. Common organic compounds used for rubber production synthesized through chain polymerization are butadiene, isoprene, and chloroprene. Aside from creating homopolymers (a polymer composed of only one monomer), copolymers can also be synthesized. Popular copolymers are styrene-butadiene (SBR), nitrile butadiene (NBR), and ethylene propylene diene monomer (EPDM).

Compounding Chemicals and Rubber

Compounding is the formulation process where certain chemicals are added to the raw rubber to modify its final mechanical and chemical properties, to lower its cost, and aid its processability and vulcanization. Compounding optimizes the properties of the rubber to meet the requirements of its intended application. Aside from the raw rubber (natural or synthetic), typical compounding ingredients are:

• 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 integrated with compounding, wherein the polymer chain is subjected to high shearing forces that mechanically break down the molecule to create smaller chains. This makes the polymer receptive to the compounding ingredients. Not all polymers require mastication, especially for those with uniform chemistry and high stability. The addition of chemical plasticizers or peptizers improves the mastication process by serving as oxidation catalysts. They prevent the recombination of broken polymer chains by removing the free radicals formed during mixing.

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 means the addition of the compounding ingredients into the masticated polymer except for the vulcanizing system. Masterbatching can induce premature vulcanization due to the heat generated by the mixing action. Masterbatching homogenizes the rubber blend to accomplish the maximum carbon black dispersion and the final viscosity.

Final mixing involves the addition of the vulcanizing system. Final mixing is usually done on separate mixers that operate at slower speeds. The required distribution of the final mix must be achieved before premature vulcanization occurs. After mixing, the final blend is cooled and formed into slabs or bales, ready for the subsequent forming operations.

Rubber Extrusion Process

In most manufacturing systems, the product of the mastication and mixing processes goes directly into an extrusion machine with a single-screw or twin screws that further mixes the rubber compound, compresses it, and forces it to flow against a die.

The die has a cross-section of an engineered part that is milled according to the dimensions of the final product with added compensation for any change in dimension after die swelling and vulcanization. Steam jackets, water jackets, or electric heaters are traced along the barrels (screw enclosures) that control the temperature in the extruder.

Other configurations of extrusion machines combine the compounding, masticating, and mixing processes. For these types of machines, the extruder uses screw elements with different profiles. One set of screw elements is for mastication (kneading) while another is for mixing, with the last one being for compressing. The extrudate then goes through a heater or oven to initiate the vulcanization process.

Rubber Curing Process

As mentioned earlier, curing or vulcanization is the process of creating cross-links between elastomer chains. This makes the rubber more stable, enabling it to resist the effects of heat, cold, and solvents. Curing was the general term used for the cross-linking process, while vulcanization was originally the term used for curing rubber with sulfur. The earliest curing agent used to cross-link natural rubber or latex was sulfur. Today, other curing processes do not use sulfur as the main curing system. These curing processes use agents such as peroxides, and rarely, phenols and metal oxides.

Vulcanization occurs when the rubber compound containing the curing additives is heated, which breaks the bonds of the sulfur molecules (S8) or sulfur donors, freeing up sites for binding. Cross-links are created when elemental sulfur or sulfur donors bind with the functional groups or on the unsaturated bonds of two elastomer chains. Initially, vulcanization is done with elemental sulfur, which requires temperatures of around 284 °F (140 °C) for five hours. The result is rubber with good physical properties that degrades easily through aging. The introduction of activators, particularly zinc oxide, decreases the time to about three hours. Activators coupled with accelerators further reduce the curing time to several minutes. Activators and accelerators reduce the curing time of the rubber and improve its aging properties.

Peroxide curing agents work almost the same as sulfur curing agents. Heating splits the peroxide into free radicals that remove a hydrogen atom from the polymer chain, freeing up a bond for cross-linking. Generally, peroxide curing is better than sulfur curing in terms of temperature resistance, aging properties, solvent resistance, compression set, and odor. It is a process that does not require activators and accelerators that can potentially form carcinogens known as nitrosamines.

Secondary Processes

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. For rubber trims, aside from cutting, typical secondary processes are splicing, slip-coating, sealant injection, and adhesive taping.

• 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 done to determine the final properties of the product and to ensure the uniformity and consistency of the product. Large manufacturing plants with compounding processes usually have an internal testing laboratory. Tests conducted on the final rubber compound may include:

Note that not all tests must be performed. The required tests depend on the intended customer and the application of the product. After quality inspection, the products are rolled, bundled, or packed preparing them 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.