Table of Contents
Overview
Natural rubber is a natural, highly elastic material made by coagulating, sheeting, drying, and processing plant latex. There are over 800 species of plants containing rubber latex, including trees, shrubs, vines, and herbs. Among them, the Hevea brasiliensis (three-leaf rubber tree) and the wild silver rubber guayule are known for their high-quality rubber and economic value. These plants mainly grow in hot and humid regions. Rubber latex can be processed using different methods: smoke drying produces smoked sheets; the use of drying agents and air drying produces air-dried sheets; and the use of patterned rollers to remove moisture produces crepe rubber. Further processing can yield granular rubber. The main components of natural rubber are rubber hydrocarbons (92%–94%), proteins (2.5%–5.5%), and small amounts of inorganic salts, fatty acids, and ash. The rubber hydrocarbon is composed of isoprene, with cis-1,4 structures accounting for over 98% of its composition, and a molecular weight ranging from 800,000 to 1,500,000.
Structure and Chemical Formula
Every fourth bond in the molecule is a double bond, and every fourth carbon atom has a methyl group, resulting in a highly regular molecular structure. Natural rubber has no definite melting point; it softens when heated, becomes molten at 130–140°C, begins to decompose at 200°C, and decomposes rapidly at 270°C. The molecular main chain consists of single and double bonds with weak steric effects, resulting in low rotational energy barriers for the molecular chains. Its glass transition temperature is -69 to -74°C. When cooled to -70°C, it becomes brittle, but its elasticity fully recovers upon heating.
Properties of Raw Rubber
When unstretched, the intermolecular forces in natural rubber are small, and the molecular chains are easily deformed and flow. Under high stress, its stereoregular molecules orient and crystallize, making it a crystalline rubber with excellent mechanical strength. The tensile strength of unfilled vulcanizates can reach 17–25 MPa, which can be increased to 25–35 MPa with carbon black reinforcement. At 90°C, the strength decreases by about 35%. The maximum elongation can reach 100%, but it decreases as the hardness of the vulcanizate increases. Natural rubber has excellent resilience, with a rebound elasticity of 50%–85% in the temperature range of 0–100°C. Due to the regularity of its molecular and branch chains, it has good gas barrier properties, with a permeability coefficient of 2.969×10⁻¹² cm²/(s·Pa). At 15°C and 1 atm (0.101825 MPa), the air and CO₂ permeability rates for natural rubber are 2.5 cm³/(cm²·24h) and 28 cm³/(cm²·24h), respectively. This is 10 times higher than butyl rubber, slightly worse than styrene-butadiene (SBR) and ethylene-propylene rubber (EPR), but better than cis-polybutadiene rubber (BR). It is not suitable for sealing high-vacuum devices.
The molecular chains of natural rubber contain many unsaturated double bonds, which are highly chemically active. They can participate in addition, substitution, and cyclization reactions to produce various modified natural rubber products, such as cyclized natural rubber, epoxidized rubber, and chlorinated rubber. The double bonds on the molecular chains are also active sites for vulcanization. When natural rubber is mixed with sulfur, sulfur-containing accelerators, or peroxides, the double bonds open under certain temperature and pressure to form cross-links between molecular chains, creating a network structure. Oxygen in the air can also cause oxidation reactions at the double bonds. This reaction is an autocatalytic chain reaction that can break molecular chains or cause excessive cross-linking, leading to stickiness and cracking of the rubber. The gradual degradation of the physical and mechanical properties of vulcanizates under light, heat, and high-energy radiation is called aging. Natural rubber without antioxidants will crack after 34–72 hours of exposure to sunlight. If exposed to a certain concentration of ozone, cracks can appear within minutes. Thus, the biggest drawback of natural rubber is its poor aging resistance. Adding antioxidants to the compound significantly improves aging resistance, allowing long-term use below 80°C. Natural rubber products can be stored for up to 2 years in a dark warehouse.
High temperatures can accelerate the aging rate of rubber. For every 10°C increase in temperature, the aging rate of natural rubber approximately doubles. Therefore, the service life of natural rubber at 100°C is less than several tens of hours, and the most suitable working temperature should not exceed 80°C.
Natural rubber molecules contain no polar groups, making it a non-polar rubber that easily crystallizes. The volume and mass changes of natural rubber in various chemical media are shown in Table 1-1.
From the solvent resistance tests of natural rubber, it can be seen that this type of rubber cannot be used in non-polar solvents and is not suitable for manufacturing rubber products that come into contact with petroleum-based or bisphenol-based oils.
Natural rubber has good resistance to alkalis but poor resistance to acids. It maintains good physical and mechanical properties even in contact with soda ash and ammonia water at concentrations up to 90%, ensuring a long service life. When exposed to working media such as nitric acid, hydrochloric acid, or sulfuric acid, the acid concentration should not exceed 30%. Particularly, due to the strong oxidizing nature of nitric acid, its concentration should not exceed 10%. Natural rubber has excellent water resistance, but its temperature resistance is poor, making it unsuitable for use in boiling water or superheated water.
Volume and Mass Change in Chemical Media
| Solvent | Max Swell Volume ΔV/% | Mass Increase Δm/% (Room Temp, 100h) |
|---|---|---|
| Chloroform | 651 | 24 |
| Carbon Tetrachloride | 659 | 8.9 |
| Carbon Disulfide | 583 | 18 |
| Tetrachloronaphthalene | 564 | 17.9 |
| Decalin | 510 | 19.6 |
| Turpentine | 483 | 69 |
| Benzene | 498 | 9 |
| Toluene | 504 | – |
| Xylene | 501 | 7.6 |
| Petroleum Ether | 234 | – |
| Gasoline | 389 | 8.4 |
| Paraffin Oil | 303 | 1 |
| Cyclohexane | 458 | – |
| Cyclohexanone | 158 | – |
| Cyclohexanol | 50 | 8.7 |
| Cyclohexyl Acetate | 307 | – |
| Anisole | 323 | 3.4 |
| Nitrobenzene | 145 | 3.3 |
| Amyl Acetate | 237 | 13 |
| Ethanol | 243 | 3 |
| Acetone | 3 | – |
Physical and Mechanical Properties of Vulcanizates
Natural rubber compounds are made by mixing No. 1 smoked sheet with vulcanizing agents, fillers, and various additives. After vulcanization, they exhibit high strength and good elasticity. They are suitable for manufacturing seals, instrument damping pads, and parts for oxygen system equipment that operate in water, air, ethanol-glycerol antifreeze, and low-concentration (below 20%) acid-base solutions. The operating temperature range is -50 to 80°C, with short-term (24 hours) use in air up to 100°C. Natural rubber products have poor weather aging resistance and are not suitable for use in petroleum-based oils.
Processing Technology of Compounds
Natural rubber compounds have low Mooney viscosity, good flow during molding, and excellent mill bagging during mixing and re-milling. Therefore, they can be processed by molding, calendering, and extrusion to produce various rubber products. Molded products such as seals, films, and damping pads of various shapes and sizes are formed and vulcanized simultaneously using molds and hot presses (steam or electric heating). Smooth and patterned rubber sheets and rubber-coated fabrics are made into sheets or blanks by calendering and then vulcanized in steam-heated autoclaves or electric pressure vessels. Tubes of various diameters, profiles of different cross-sectional shapes, and rubber strips or ropes are formed by extrusion and then vulcanized in autoclaves or hot pressure vessels.
Before producing molded products, the compound is first re-milled on a two-roll mill. The roll gap is locked and the material is thinly passed five times to wrap around the roll, expel air bubbles, and then sheeted off. Blanks are cut according to the shape and thickness of the product to be molded, with a mass allowance of 5% to 10%. The cut blanks should be placed in a clean, covered container and used within 48 hours. The molding pressure depends on the hardness of the compound. For compounds with a Shore A hardness below 50, the unit mold pressure is approximately 1.5 to 3 MPa. For compounds with a hardness above 60, a pressure of 4 to 7 MPa can be selected. The vulcanization temperature and time must be strictly controlled to prevent over-vulcanization caused by prolonged exposure to high temperature and pressure. Natural rubber typically employs a vulcanization system composed of sulfur and accelerators. The sulfur-sulfur crosslinks formed during vulcanization can break at high temperatures, a phenomenon known as reversion, leading to a reduction in physical and mechanical properties. This over-vulcanization becomes particularly evident when the time exceeds 40 minutes at temperatures above 150°C. Therefore, for thick and complex-shaped natural rubber products, the vulcanization temperature is often chosen within the range of 130°C to 140°C. When molding products, if the maximum cross-sectional thickness exceeds 6 mm, the vulcanization time can be extended by 10% to 20% based on the standard test piece vulcanization time. When molding natural rubber and metal bonded components, the vulcanization conditions should be appropriately adjusted according to the curing speed of the adhesive used. For complex-shaped bonded parts, it is advisable to remove the component from the mold after the mold temperature has dropped to 70°C to avoid damage to the bonded interface during demolding at high temperatures.
When producing extruded products, attention should be paid to the temperatures of various parts of the extruder. The re-milled compound is cut into strips and fed into the feed port. The barrel temperature of the extruder is controlled at 50–60°C, the head temperature at 80–85°C, and the die temperature at 90–95°C. Natural rubber compounds have high thermoplasticity, low shrinkage, and a tendency to stick to hot rolls. The calendering process is relatively easy to master. During calendering, the temperature differences between the rolls should be controlled to ensure smooth operation. When using a three-roll calender, the top roll temperature is controlled at 100–110°C, the middle roll at 85–90°C, and the bottom roll at 60–70°C.
The molding shrinkage rate of the compound is related to the hardness of the compound and the structural dimensions of the part. In mold design, a shrinkage rate of 1.2%–2.0% is generally selected. For producing precision-sized seals, the mold dimensions must be repeatedly adjusted based on actual measurements of the part dimensions to ensure accuracy.
Rubber compounds should be stored in a warehouse at 0–28°C with a relative humidity not exceeding 80%. They should be protected from direct sunlight, kept at least 1 m away from heat sources, and at least 0.3 m above the ground. During storage, contact with oils, acids, alkalis, and other substances harmful to the compound should be prevented. Processing of the compound should be completed within the storage period. Beyond the storage period, the physical and mechanical properties and processing performance of the compound will deteriorate. Packaged natural rubber products can be stored for 2–3 years under light-proof conditions.
Applications
Due to its high strength, good elasticity, simple processing, and low cost, natural rubber is widely used in various fields of the national economy, especially in daily products. Currently, natural rubber accounts for 35%–40% of the total global rubber consumption, ranking first among all types of rubber. The biggest drawback of natural rubber is its poor aging resistance. It ages and cracks when exposed to sunlight, and it cannot come into contact with oils or strong acids. The long-term use temperature should not exceed 80°C, although it can be used short-term at 100°C. Therefore, in weapons and equipment, except for tires, synthetic rubber with better heat and aging resistance is used to replace natural rubber. Natural rubber compounds are mainly used to manufacture tires and conveyor belts, sealing products, damping products, and acid- and alkali-resistant hoses. By combining compounds with synthetic fibers or metal wires, various load-bearing tires and transmission belts can be produced. Seals, gaskets, and films of various shapes and sizes can be molded, and sealing products such as rubber strips and profiles can be extruded. Using adhesives to bond natural rubber to metals allows the molding of various instrument damping pads, elastic bearings for helicopters, and bridge vibration isolation supports. Additionally, natural rubber can be extruded into hoses of various inner diameters for sealing, damping, and transporting air, water, dilute acids and alkalis, and as sleeves for wires.