Synthetic Rubber Production: From Monomers to Vulcanization

Rubber processing employs multiple technologies: internal mixers combine raw rubber with additives at high temperatures; extruders shape compounds into continuous profiles; calenders produce thin sheets using heated rollers; and molds create complex shapes under heat and pressure. Vulcanization introduces crosslinking as a final step because once crosslinked, materials cannot be remolded. Thermoplastic elastomers bypass vulcanization entirely, enabling direct processing. Major applications include tires (75% of production), seals, gaskets, hoses, conveyor belts, footwear, automotive components, and medical devices. The global rubber industry is dominated by China, followed by the US and Western Europe. Quality control relies on standardized testing for mechanical properties ensuring consistent performance.

Calendaring transforms rubber into sheets and strips by rolling it to precise thickness using heated rollers. These sheets are cut into narrow strands for weaving into elastic products like suspenders and garters, or applied to textiles to create watertight flexible diaphragms. Extrusion represents a major manufacturing method where rubber is worked into a soft mass on warm-up mills, then fed continuously through extruding machines using augur-like screws. Steel dies shape the rubber into complex cross-sections with tight tolerances, producing items such as windshield weather seals, food freezer gaskets, and automotive shims. Both processes require subsequent vulcanization in autoclaves or heaters with steam atmosphere to develop the necessary physical properties.

Three primary processing methods convert silicone from unvulcanized (deformable) to vulcanized (elastic rubber) state: Extrusion uses screws to force material through dies, requiring HCR with clay-like consistency that maintains shape during curing; Molding fills cavities with uncured silicone and applies heat/pressure, accommodating various viscosities including dip coating techniques; Injection molding processes liquid silicone rubbers for complex geometries. All methods require vulcanization to achieve elastic properties. Processing method selection drives material choices and vice versa, making this relationship critical for successful device development.

Polymer processing is divided into three main categories: molding processes (injection molding, blow molding, rotational molding, extrusion, calendaring for thermoplastics; compression molding for thermosets), forming processes (vacuum forming, thermoforming, line bending for thermoplastics), and joining processes (screws, snap fixings, captive nuts, adhesives, thermal welding, ultrasonic welding for thermoplastics; limited to screws and adhesives for thermosets due to their irreversible curing).

Extrusion forces plasticized material through dies to create profiles, with two main variants: solidification (rapid cooling preserves shape) and deformation (material is softened before reshaping). The extruder includes a hopper, screw conveyor, and heated barrel where temperatures increase toward the tooling. Profiles pass through formers maintaining shape during cooling. Blow molding extrusion creates films by inflating tubes with compressed air, forming bubbles that are cooled and wound onto reels for later processing into bags and packaging. For hollow containers, a parison is extruded, placed in a mold, and inflated to conform to the mold's internal shape, with the neck formed by blow gun-mold interaction. Calendering produces flooring sheets using heated cylinders that roll material into precise thickness, sometimes with engraved cylinders for surface designs. These processes demonstrate how polymer processing adapts to different product requirements, from flexible films to rigid containers and decorative surfaces.

Toyota Synthesis has developed a breakthrough rubber recycling technology that solves the industry's long-standing challenge of recycling synthetic rubber, which is difficult to recycle once vulcanized. The key innovation is a steam-based odor removal system that captures sulfur compounds released during the desulfurization process, reducing odor levels from 853 to 263 (approximately one-third) and enabling recycled rubber to be blended at 20% concentration instead of the previous 5% limit. This technology, now used in Toyota's RAV4 SUV, addresses the environmental challenge of Japan's 170,000 tons of annual waste rubber and paves the way for expanded recycling including end-of-life vehicle rubber.

Vulcanization creates permanent cross-links that define rubber's shape and properties, making recycling extremely difficult. Only 10% of rubber applications (thermoplastic rubbers) can be recycled, while 90% (particularly tires) cannot. During WWII, devulcanization was practiced when natural rubber was scarce, but became obsolete when supplies returned. EPDM succeeded in devulcanization because its strong carbon-carbon main chains survive while weaker sulfur bonds break. Passenger car tires using SBR/BR face greater challenges because their butadiene components spontaneously rebond after breaking. The recycling hierarchy prioritizes approaches from least to most sustainable: landfill/incineration, pyrolysis, mechanical recycling, chemical recycling, and finally devulcanization at the top. Tires contain 12-20 different components with varying compounds, making separation nearly impossible. The ideal solution would separate tires back into original building blocks before processing, but this remains technically challenging.

This section addresses the environmental challenges and solutions related to synthetic rubber waste. Tires represent one of the largest waste products from synthetic rubber and are difficult to dispose of due to their non-biodegradable nature. Improper disposal leads to land pollution, creates breeding grounds for mosquitoes, and emits toxic chemicals that pollute the environment. Recycling methods include converting tires into rubber powder, which can be used to make rubber tiles for playgrounds, rubber mats, and stadium tracks. The section also covers natural polymers (protein, starch, cotton, cellulose, isoprene) and the vulcanization process where sulfur atoms form cross-links between rubber polymer chains at carbon-carbon double bonds, improving elasticity and strength.

New Rubber Technologies has developed a patented devulcanization process that reverses the vulcanization reaction in rubber waste, transforming granulated rubber crumb into chemically reactive polymer capable of serving as a partial or complete substitute for virgin rubber in both rubber compounds and thermoplastic elastomer applications, thereby addressing the environmental challenge of disposing of vulcanized rubber waste that cannot be easily recycled through conventional means.

Rubber recycling involves mechanical processing (fringing, granulating, milling) followed by chemical methods like devulcanization (breaking sulfur bonds to restore malleability) and pyrolysis (thermal decomposition in oxygen-free environments yielding oil, gas, and char), while sustainable alternatives like guayule and Russian dandelion offer renewable rubber sources with smaller carbon footprints, representing key innovations in addressing rubber waste challenges.

Vulcanized rubber has increased tensile strength, reduced water absorption, and improved resistance to oxidation. It can be stretched to twice its original length.

Rubber composites are characterized using various techniques: mechanical properties (tensile strength, modulus, elongation at break) measured using Universal Testing Machine (UTM); tear resistance testing; thermal properties analyzed using TGA (Thermogravimetric Analysis) for thermal stability; dynamic mechanical analysis (DMA) for viscoelastic properties; morphology examined using SEM for particle dispersion; spectroscopy (FTIR, Raman) for molecular structure analysis. MDR (Moving Die Rheometer) analyzes curing characteristics, providing parameters like T90 (time to 90% maximum torque), minimum torque (compound viscosity), maximum torque (cross-link density), and optimum cure time.

Vulcanized rubber has improved properties: (1) Higher elasticity and tensile strength, (2) Better resistance to oxidation, (3) Reduced water absorption, (4) Improved electrical insulation properties, and (5) Better resistance to temperature changes. These properties make vulcanized rubber suitable for various applications including tires, electrical wire coatings, and industrial products. The relationship between sulfur content and rubber hardness allows manufacturers to control properties for specific applications.

Mechanical testing evaluates material performance through systematic bend and tensile strength tests. The bend test applies increasing force until failure, measuring both force requirements and deformation capacity. TGM7 demonstrated exceptional flexibility, bending many times before breaking, while XVN50 showed greater resistance to initial bending. Tensile strength testing measures breaking force, with XVN50 achieving fifth-place performance among tested resins. Results reveal fundamental trade-offs: softer materials bend more easily but require less force to initiate failure, while rigid materials offer higher tensile strength at the expense of flexibility. Post-curing effects cause gradual strength development over the first month.

Elastomers are classified into synthetic and natural types, with material formulation involving selection of raw materials and recipe development. Testing methods include: (1) Rheometers to determine rubber maturity and processing time, (2) Carbon dispersion analysis to ensure uniform filler distribution, (3) Mechanical property testing (tensile, compression, hardness), (4) Dynamic mechanical analysis (DMA) to measure material properties under movement, and (5) Temperature testing to measure heat generation under stress. These tests ensure materials meet specifications before manufacturing.