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Introduction:
As an important engineering material, rubber’s unique viscoelastic properties pose numerous technical challenges during processing that distinguish it from other materials. Rubber technology systematically investigates the underlying principles and process-control methods throughout the entire production chain, from raw materials to finished products. The manufacture of rubber goods typically involves key steps such as raw‑rubber plastication, mixing, calendering, extrusion, and vulcanization; the precise control of each step directly influences the quality of the final product.

 

Chapter 1: Raw Rubber:
I. Overview:
Raw rubber is the fundamental material for manufacturing rubber products and can be divided into two major categories: natural rubber and synthetic rubber. At present, natural rubber accounts for approximately 30% to 40% of global rubber consumption, while styrene–butadiene rubber makes up about 40% to 50% of synthetic rubber production. Among general-purpose synthetic rubbers, the highest‑volume producers are, in order, styrene–butadiene rubber, polybutadiene rubber, ethylene–propylene rubber, butyl rubber, chloroprene rubber, and nitrile rubber.

II. Natural Rubber:
1. Source and Classification:
Natural rubber is produced by coagulating and processing the latex harvested from the Hevea brasiliensis tree. Depending on the manufacturing process and physical form, it is classified into smoked sheet rubber, granulated rubber (standard rubber), crepe rubber, and other types. Among these, smoked sheet rubber is graded into six levels, ranging from Extra‑Grade to Grade V, based on its visual quality; standard rubber, on the other hand, is categorized according to physicochemical parameters such as impurity content, plasticity retention rate, initial plasticity, nitrogen content, volatile matter content, ash content, and color index, with designations like SCR5, SCR10, and SCR20.

The plasticity retention index (PRI) is an important parameter for evaluating the resistance of raw rubber to thermal–oxidative aging. It is defined as the ratio of the Wallerstein plasticity before and after heating at 140°C for 30 minutes: PRI = P/P₀ × 100%. A higher PRI value indicates better resistance to thermal–oxidative aging.

2. Influence of Non-Rubber Components:

In addition to rubber hydrocarbons, natural rubber contains non‑rubber constituents such as proteins, acetone‑extractables, ash, and moisture; these components significantly influence its processing and service performance.

Protein: It has anti‑aging effects, and the amino acids released upon hydrolysis can promote sulfidation; however, its strong hygroscopicity reduces the insulating properties of the product and makes it prone to mold.

Acetone extract: includes higher fatty acids, sterols, phospholipids, and others. Higher fatty acids act as softening agents and vulcanization activators; sterols provide anti‑aging properties; and the decomposition of phospholipids releases choline, which promotes vulcanization.

Ash content: Primarily composed of metallic elements such as Ca, Mg, K, Na, Cu, and Mn. Among these, K, Na, Ca, and Mg affect electrical performance, while redox‑active metals like Cu and Mn accelerate aging; their concentrations must be kept below 3 ppm.

3. Structure and Properties:
Molecular structure: Natural rubber contains more than 97% cis‑1,4‑polyisoprene, with about 2% in the 3,4‑configuration, and exhibits 100% head‑to‑tail linkages. In contrast, eucommia gum consists of trans‑1,4‑linkages; although its chemical composition is identical, its properties differ markedly.

Crystallinity: Natural rubber is amorphous at room temperature; it begins to crystallize below 10°C, with the fastest crystallization occurring at −25°C. During stretching, oriented crystallization develops, and this self-reinforcing behavior imparts natural rubber with relatively high mechanical strength even in the absence of reinforcing agents.

Physical and Mechanical Properties:

Glass transition temperature Tg = −73°C; exhibits good elasticity even at −50°C.

Green strength of raw rubber: 1.4–2.5 MPa

Tensile strength of virgin rubber vulcanizate: 17–28 MPa

Tensile strength of carbon-black-reinforced vulcanizates: 25–35 MPa

Tensile strength: 98 kN/m

Volume resistivity: 10¹⁴–10¹⁵ Ω·cm

Flexural fatigue resistance: more than 200,000 cycles

Chemical properties: The double bonds in the natural rubber molecular chain confer the typical reactivity of unsaturated rubbers: it can undergo sulfur‑curing crosslinking; react with chlorine to form chlorinated natural rubber; react with hydrochloric acid to yield a white powder (used as an adhesive); and react with peracetic acid to produce epoxidized natural rubber (ENR‑50, which exhibits gas impermeability comparable to butyl rubber and oil resistance similar to nitrile rubber with a moderate acrylonitrile content).

Natural rubber is typically compounded with a sulfur‑vulcanization system; common accelerators include thiazoles, sulfenamides, and thiurams, while zinc oxide and stearic acid serve as activators. Carbon black is the primary reinforcing filler, though precipitated silica, calcium carbonate, and clay can also be used. Among protective systems, phenylene diamine antioxidants deliver the best performance. Plasticizing agents commonly comprise pine tar, trichlorobenzene oil, rosin, coumarone, and paraffin wax.

III. General-Purpose Synthetic Rubbers:
1. Styrene-butadiene rubber (SBR):
Styrene–butadiene rubber is currently the highest‑volume synthetic rubber, produced by the copolymerization of butadiene and styrene, with butadiene accounting for more than two‑thirds and styrene less than one‑third. Based on the polymerization method, it is classified into emulsion‑polymerized styrene–butadiene rubber and solution‑polymerized styrene–butadiene rubber.

2. Polybutadiene Rubber (BR):
The glass transition temperature of polybutadiene rubber depends on its vinyl content: the Tg of the cis‑1,4 structure is −105 °C, while that of the 1,2 structure is −15 °C. A higher cis‑content results in improved reinforcement. Solution‑polymerized BR exhibits a narrow molecular weight distribution, limited branching and gel formation, but poorer processability; emulsion‑polymerized BR, by contrast, has a broader molecular weight distribution and better processability.

3. Ethylene Propylene Diene Monomer (EPDM):
Ethylene–propylene rubber is classified into ethylene–propylene diene monomer (EPM) and ethylene–propylene–diene monomer (EPDM). EPM is a fully saturated rubber with exceptionally high chemical and thermal stability; it is nonpolar and exhibits excellent electrical insulation properties. EPDM is a terpolymer of ethylene, propylene, and a non-conjugated diene, typically containing 30% to 40% (mol) propylene. It offers superior resistance to oxidation, ozone, and corrosive environments and is widely used in roofing waterproofing membranes.

4. Butyl Rubber (IIR):
Butyl rubber is a low‑temperature copolymer of isobutylene and a small amount of isoprene (1%–5%). Due to the absence of polar or reactive functional groups in its structure, it exhibits poor self‑adhesion and inter‑adhesion. Halogenated butyl rubber can accelerate vulcanization, enhance compatibility with unsaturated rubbers, and improve both self‑adhesion and inter‑adhesion; it is primarily used in inner tubes, airtight layers for tubeless tires, medical stoppers, and similar applications.

5. Nitrile rubber (NBR):
The typical acrylonitrile content in nitrile rubber is 34%. As the acrylonitrile (ACN) content increases, the polarity rises and oil resistance improves, but low-temperature performance deteriorates. Hydrogenated nitrile rubber (HNBR) exhibits even superior oil resistance and heat resistance. Nitrile rubber is difficult to plasticize and requires low‑temperature, thin‑pass mixing; it generates significant heat during compounding and can be vulcanized at elevated temperatures without exhibiting reversion.

6. Chloroprene rubber (CR):
Chloroprene rubber was the first synthetic rubber to be industrialized in China. The electron‑withdrawing nature of the chlorine atoms reduces the reactivity of the double bonds, precluding sulfur vulcanization; instead, metal oxides such as ZnO and MgO are used for vulcanization. Chloroprene rubber exhibits self‑reinforcement, boasts high mechanical properties, and demonstrates good oil resistance—though slightly inferior to NBR. Its resistance to aging and ozone is superior to that of NR, SBR, BR, and NBR, while its flame retardancy is excellent, with an oxygen index of 38–41. However, its low‑temperature performance is relatively poor, with a minimum service temperature of −30°C.

IV. Specialty Rubbers and Thermoplastic Elastomers:
Fluororubber (FPM): Exhibits the best high-temperature resistance among rubbers, with outstanding oil and chemical resistance; it is resistant to aqua regia and possesses self-extinguishing properties. Vulcanization requires peroxides, organic amines, or their derivatives.

Silicone rubber (Q): Exhibits the best low-temperature resistance among all elastomers, with outstanding resistance to ozone and thermal‑oxygen aging, excellent electrical insulation properties, and favorable biomedical performance. Its drawbacks include relatively low tensile strength and tear strength. The most effective reinforcing agent is fumed silica.

Polyurethane rubber (U): Its wear resistance is nine times that of natural rubber, and it exhibits excellent oil resistance. It comes in three types: cast, compounded, and thermoplastic. However, its water resistance and high-temperature performance are relatively poor.

Thermoplastic elastomers exhibit plastic flow at elevated temperatures, allowing them to be processed and molded like plastics without the need for vulcanization, while maintaining rubber-like elasticity at room temperature. Typical examples include styrene‑block copolymers (SBS), polyolefin‑based, and polyester‑based thermoplastic elastomers.

Reclaimed rubber is produced by subjecting waste rubber products to processes such as crushing and desulfurization. During the regeneration process, the combined effects of plasticizers (softeners and activators), oxygen, heat, and mechanical shear cause the molecular chains and crosslinking points of vulcanized rubber to break. Commonly used activators include aromatic thiols and their zinc salts, as well as aromatic disulfides.

 

Chapter 2: The Vulcanization System of Rubber:
I. Definition and History of Vulcanization:
Vulcanization is the process by which linear polymers, under physical or chemical action, form a three-dimensional network structure; it is a critical step in transforming plasticized rubber compounds into highly elastic elastomers. The vulcanization process can be divided into three stages:

Induction stage: The vulcanizing agent, activator, and accelerator react to form active intermediate compounds, which in turn initiate the generation of crosslinkable free radicals or ions on the rubber molecular chains.

Crosslinking reaction stage: Crosslinkable free radicals or ions initiate chain reactions with the rubber molecular chains, forming crosslink bonds.

Network formation stage: rearrangement and shortening of crosslinking bonds, as well as modification and cleavage of the main chain.

II. Vulcanization Parameters:
T10: The time required for the torque to rise to 10% of the maximum torque.

Induction period (scorch time): The time required from the moment the rubber compound is placed in the mold until slight vulcanization begins, reflecting processing safety.

T90 (Process Curing Time): The time required for the torque to rise to 90% of its maximum value.

Sulfurization reversal: the phenomenon in which crosslinking structures degrade and material properties deteriorate during the over‑sulfurization stage.

An ideal vulcanization curve should exhibit: a sufficiently long induction period to ensure processing safety; a rapid vulcanization rate to enhance production efficiency; and a long, flat plateau phase to ensure uniform vulcanization throughout the product.

III. Sulfur Vulcanization System:
Conventional sulfur vulcanization system (CV): With a relatively high sulfur dosage, the vulcanizate exhibits excellent initial fatigue resistance and favorable static and dynamic properties at room temperature, but its resistance to thermal–oxidative aging is poor.

Effective Vulcanization System (EV): Comprises more than 90% monosulfide and disulfide linkages, exhibits excellent resistance to thermal–oxidative aging, but has poor initial dynamic performance; it is suitable for high‑temperature static applications.

Semi‑effective vulcanization system (SEV): Offers both excellent dynamic performance and moderate resistance to thermal‑oxidative aging, making it suitable for tire sidewalls.

Equilibrium vulcanization system (EC): By employing additives such as Si69, the crosslink density is maintained in a dynamic equilibrium, resulting in high strength, excellent resistance to moisture, superior heat‑oxygen aging resistance, and outstanding resistance to reversion during vulcanization.

IV. Non-Sulfur Vulcanization Systems:
Peroxide vulcanization: Suitable for saturated rubbers (such as EPM) and heterochain rubbers (such as silicone rubber), with DCP commonly used.

Metal oxide sulfidation: Primarily used for CR, CIIR, CSM, etc.; the combined use of ZnO and MgO yields the best results.

Radiation vulcanization: crosslinking of polymers using high-energy radiation.

V. Relationship Between the Structure and Properties of Vulcanizates:
The order of the influence of crosslinking bond types on performance is:

Tensile strength: polysulfide bonds > disulfide bonds > thiol bonds > carbon–carbon bonds

Dynamic performance: polysulfide bonds > disulfide bonds > thiol bonds

Thermal stability: C–C bond > mono‑sulfide bond > disulfide bond > polysulfide bond

 

Chapter 3: Reinforcement and Filling Systems for Rubber:
I. Basic Concepts:
Reinforcement: The addition of reinforcing agents significantly enhances the wear resistance, tear strength, tensile strength, modulus, and swelling resistance of vulcanizates. Common reinforcing agents include carbon black, precipitated silica, and certain ultrafine inorganic fillers.

Filling: Fillers are added to increase volume, reduce costs, and enhance processing performance. Common fillers include clay, calcium carbonate, rubber powder, wood flour, and others.

II. Reinforcing Effect of Carbon Black:
Carbon black is produced by the incomplete combustion or pyrolysis of hydrocarbon materials, and its microcrystals exhibit a quasi‑graphitic structure. Particle size (specific surface area), structural complexity, and surface activity are the three fundamental properties of carbon black, collectively known as the “three reinforcing factors.”

Particle size: The smaller the particle size, the higher the tear strength, tensile stress at a given elongation, and hardness, while elasticity and elongation at break decrease.

Structural: The higher the DBP oil absorption value, the greater the structural integrity, with a significant impact on set stress and hardness.

Surface activity: Surface functional groups (such as carboxyl, hydroxyl, and quinone groups) influence the bonding affinity with rubber.

Bound rubber (carbon black gel): The portion of rubber in carbon black‑filled compounds that is insoluble in a good solvent serves as direct evidence of the interaction between carbon black and rubber. The amount of bound rubber increases with increasing carbon black specific surface area and with rising mixing temperature.

III. Reinforcement with Precipitated Silica:
Precipitated silica has silanol groups (Si–OH) on its surface, making it hydrophilic and difficult to disperse in nonpolar rubbers. The use of a silane coupling agent—such as bis(3-triethoxysilylpropyl) tetrasulfide (TESPT)—can render it hydrophobic, thereby significantly enhancing its reinforcing performance.

Precipitated silica offers advantages in enhancing tire wet‑skid resistance and reducing rolling resistance, and its use in high‑performance tires has become increasingly widespread in recent years.

 

Chapter 4: Aging and Protection of Rubber:
I. Aging Phenomena and Mechanisms:
Rubber aging is an irreversible chemical process that entails changes in appearance, structure, and properties. It typically manifests as softening and tackiness (in natural rubber under thermal oxidation), hardening and embrittlement (in cis‑1,4‑polybutadiene and nitrile rubber under thermal oxidation), and cracking (due to ozone aging or photo‑oxidative aging), among other phenomena.

Thermal-oxidative aging mechanism: a free-radical chain reaction. Oxygen reacts with rubber molecules to form peroxy radicals, which initiate an autocatalytic oxidation process.

Ozone aging mechanism: Ozone attacks the double bonds in rubber molecular chains, causing chain scission and resulting in cracking.

II. Protective Measures:
Physical protection: incorporation of paraffin to form a protective film; rubber–plastic blending; surface electroplating or coating with paints.

Chemical protection: Add chemical antioxidants. Classified according to their mechanism of action:

Chain‑terminating antioxidants: convert peroxyl radicals into stable species.

Peroxide-decomposing antioxidants: decompose hydroperoxides into non-radical compounds.

Heavy metal ion passivator: complexes harmful metal ions.

Phenylene diamine antioxidants (such as 4010, 4020, and 4010NA) exhibit excellent overall performance and demonstrate remarkable effectiveness in resisting flexural cracking and ozone aging.

 

Chapter 5: Plasticizing Systems for Rubber:
I. Basic Concepts:
Plasticizers (softeners) are low-molecular-weight compounds that reduce intermolecular forces between rubber chains and improve processing performance.

Softener: a nonpolar substance, often derived from natural sources, used in nonpolar rubbers.

Plasticizers: polar substances, mostly synthetically produced, used in polar rubbers and plastics.

II. Classification and Characteristics:
Petroleum-based plasticizers include paraffinic oils, naphthenic oils, and aromatic oils. Aromatic oils offer the most effective viscosity reduction, while naphthenic oils exhibit excellent overall performance, making them the most ideal rubber plasticizers.

Vegetable and animal oil-based plasticizers: Rosin can enhance plasticity and adhesion but exhibits a delayed vulcanization effect; stearic acid is the primary vulcanization activator.

Coal-tar-based softener: Coumarone‑indene resin is suitable for NR and a variety of synthetic rubbers.

Ester plasticizers—such as phthalates, phosphates, and aliphatic dialkyl esters—are primarily used in polar rubbers.

III. Selection Principles:
Adhere to the principles of “like dissolves like” and “similar solubility parameters.” Aromatic‑based plasticizers are primarily used in SBR, BR, NR, and CR; paraffinic‑based plasticizers are mainly employed in EPDM and IIR; while naphthenic‑based plasticizers enjoy the broadest range of applications. For products with stringent health requirements, environmentally friendly plasticizers should be selected.

 

Chapter 6: Plasticizing and Mixing Processes:
I. Plasticization:
Mastication is a processing step in which raw rubber is transformed from a highly elastic state into a soft, plastic state through mechanical stress, heat, oxygen, or chemical agents. Its objectives include reducing elasticity, enhancing plasticity, lowering viscosity, improving flowability, and increasing the solubility of the rubber compound as well as its molding adhesion.

Plasticizing mechanism: Low-temperature plasticization primarily involves mechanical forces that cleave polymer chains, whereas high-temperature plasticization is dominated by oxygen‑initiated chain scission.

Plasticizing characteristics of different rubbers:

Natural Rubber: Low-Temperature Thin-Blade Calendering

Styrene-butadiene rubber: High-temperature plastication, with strict control of temperature and time.

Polybutadiene rubber: Generally does not require mastication.

Neoprene: Generally does not require mastication.

Nitrile rubber: low-temperature calendering, staged plastication.

Butyl rubber: In the internal mixer, at temperatures above 120°C, add the peroxide curing agent DCP.

II. Mixing:
Mixing is the process by which various compounding agents are uniformly incorporated into rubber on a rubber mixer to produce compounded rubber, and it constitutes the core operation in rubber processing.

Principle of ingredient addition sequence: Add compounding agents used in small amounts and difficult to disperse first; add those used in large amounts and easy to disperse later; sulfur and ultra‑accelerators are added last. For NR compounds, the typical sequence is: masticated rubber → solid softener → accelerators, activators, antioxidants, and anti‑scorch agents → reinforcing fillers → liquid softener → sulfur and ultra‑accelerators.

Mixing methods: conventional single-stage mixing, staged rubber‑feeding single-stage mixing, two‑stage mixing, and reverse mixing.

Rubber compound resting: Typically, the compound should be allowed to rest for at least 4 to 8 hours. This process serves to relax the rubber matrix, reduce shrinkage, facilitate further dispersion of compounding agents, and promote additional interaction between the rubber and carbon black, thereby generating more bound rubber.

 

Chapter 7: Calendering and Extrusion Processes:
I. Calendering:
Calendering is the process of converting compounded rubber into a sheet or, in combination with reinforcing materials, into a semi‑finished tape. The calendering effect refers to the phenomenon whereby the physical and mechanical properties of the rubber sheet differ in the machine direction and the transverse direction after calendering, resulting from the oriented alignment of rubber molecules and filler particles.

Calendering process type:

Tableting: Pressing into tablets of a specified thickness and width.

Lamination: Two thin films are bonded together to form a single layer.

Rolling: Formed into a specific cross-sectional shape.

Textile adhesive application: includes adhesive lamination and adhesive wiping.

II. Extrusion:
Extrusion is a manufacturing process in which rubber compounds are continuously formed into semi-finished products of various shapes by passing them through an extruder.

Die swell: the phenomenon whereby the cross-sectional dimensions of a polymer melt increase after extrusion through the die, primarily dependent on the shear rate. Reducing the screw speed or increasing the die lip length can mitigate the degree of swell.

Melt fracture: Occurs when shear stress exceeds a critical threshold, resulting in surface defects. Measures to increase extrusion output include using a meshing, finely divided feed screw, a longer barrel, and installing a gear pump at the die head, among others.

 

Chapter 8: Vulcanization Process:
I. The three essential elements of vulcanization:
Vulcanization temperature, vulcanization time, and vulcanization pressure are the three key factors that determine vulcanization quality; they are known as the three essential elements of vulcanization.

Vulcanization temperature: For every 10°C increase in temperature, the vulcanization time is approximately halved. The optimal temperature for NR is below 143°C, while for SBR and NBR it is below 180°C. Excessively high temperatures can cause chain scission, leading to a deterioration in performance.

Vulcanization time must be precisely maintained within the optimum vulcanization range. If the time is too short, under‑vulcanization occurs; if it is too long, over‑vulcanization or reversion may result.

Curing pressure: prevents bubble formation, promotes the flow of the rubber compound to fully fill the mold cavity, and enhances adhesion between the rubber compound and the reinforcing materials. Generally, thicker products and multi-layer structures require higher pressures.

II. Vulcanization Method:
By medium: direct vulcanization (hot water or steam), indirect vulcanization (hot air), and mixed-gas vulcanization.

Classification by production method: flat vulcanization, individual vulcanization, injection‑pressure vulcanization, and continuous vulcanization (including salt‑bath, fluidized‑bed, microwave, and high‑frequency processes).

III. Determination of Vulcanization Conditions:
During the vulcanization of thick products, the temperature difference between the inner and outer layers is significant, necessitating either low‑temperature, long‑duration vulcanization or a stepwise heating approach. The equivalent vulcanization time is used to control the vulcanization outcome at different temperatures.

 

Conclusion:
Rubber technology encompasses a comprehensive technical framework, spanning from raw material selection to finished-product manufacturing. Mastering the properties of various raw rubbers, the mechanisms of action of compounding agents, and the key process‑control parameters at each stage is essential for ensuring product quality and enhancing production efficiency. Any adjustment to process parameters can have multifaceted effects on the final product’s performance; therefore, such changes must be validated through systematic experimentation before being implemented in production. As environmental regulations become more stringent and demand for high‑performance rubber products continues to grow, emerging environmentally friendly compounding agents, advanced vulcanization systems, and nano‑reinforcement technologies will further drive technological advancement in the rubber industry.