Natural Rubber: In-depth Analysis and Industrial Applications

Natural Rubber: In-depth Analysis and Industrial Applications

Natural Rubber (NR), as an indispensable industrial raw material, plays a pivotal role in various fields such as tire manufacturing, hose production, sealant making, and medical supplies. Its unique physicochemical properties make it a material that cannot be fully replaced by synthetic rubber in many aspects.

I. Origin and Production Areas of Natural Rubber

1.Origin of Natural Rubber

Natural rubber primarily originates from the milky white latex of the rubber tree (Hevea brasiliensis). This tropical tree is native to the Amazon River basin in South America and was introduced to Southeast Asia in the late 19th century. Today, over 90% of the world’s natural rubber is produced in Southeast Asia, with Thailand, Indonesia, Vietnam, Malaysia, and India being the major producers.

2.Latex Harvesting

Latex harvesting is mainly carried out through the tapping process. A spiral cut is made on the bark of the rubber tree, and the latex flows out through the ducts and is collected in containers. Typically, a single rubber tree can produce about 30-50 milliliters of latex per day, and the harvesting must be done in the morning to avoid high temperatures that can cause the latex to coagulate.

II. Production Process of Natural Rubber

The production process of natural rubber encompasses several steps including latex harvesting, coagulation, pressing, drying, and grading:

1.Latex Processing

Freshly harvested latex is filtered to remove impurities and then mixed with 0.5%-1% ammonia to prevent microbial spoilage.

2.Coagulation Process

Acetic acid or formic acid (with a pH level controlled between 4.5-4.8) is added to coagulate the rubber particles in the latex into blocks. The coagulation time directly affects the molecular weight and properties of the raw rubber.

3.Pressing and Drying

Coagulated rubber blocks are pressed into thin sheets with a thickness of about 3-5 millimeters using a pressing machine and then dried at a temperature of 50-60°C until the moisture content is below 1%.

4.Grading and Packaging

Natural rubber is classified into grades such as RSS (Ribbed Smoked Sheet) and ADS (Air-Dried Sheet) based on color, impurity content, and other indicators, and is finally packaged in block or sheet form.

III. Types of Natural Raw Rubber

Based on processing methods and applications, natural raw rubber can be divided into the following types:

Type	Production Process	Characteristics	Applications
RSS	Smoke-dried, contains smoke components	Dark yellow in color, low in impurities, good aging resistance	High-end tires, industrial products
ADS	Air-dried or hot-air dried	Light yellow in color, higher impurity content, low cost	Shoe soles, hoses
TSR	Mechanized granulation and drying	Standardized impurity content, stable processing performance	General rubber products
CV	Addition of hydroxylamine substances to inhibit crystallization	Stable Mooney viscosity, no need for plasticizing	Precision molded products
LPNR	Multiple centrifugations to remove proteins	High biocompatibility, low allergy risk	Medical gloves, catheters

IV. Physical Properties of Natural Rubber

1.Hardness (Hardness)

Test Method: Shore A Hardness Tester (ASTM D2240).
Typical Value: Unvulcanized rubber has a hardness of about 30-40 Shore A, which can reach 50-80 Shore A after vulcanization.
Influencing Factors: Filler content (such as carbon black) and degree of vulcanization.

2.Mooney Viscosity

Definition: An indicator of the processing fluidity of raw rubber (ASTM D1646).
Typical Value: The Mooney viscosity (ML(1+4) 100°C) of natural rubber is typically between 60-90.
Significance: Raw rubber with high Mooney viscosity requires extended plasticizing time to reduce viscosity.

3.Density (Density)

Range: 0.91-0.93 g/cm³, lower than most synthetic rubbers (e.g., styrene-butadiene rubber has a density of 1.05 g/cm³).

4.Aging Resistance

Weakness: Natural rubber is susceptible to attack by oxygen, ozone, and ultraviolet light due to its double bonds.
Improvement Methods: Adding antioxidants (such as 6PPD, TMQ) or blending with antioxidant polymers.

5.Elasticity (Elasticity)

Rebound Rate: Vulcanized rubber can achieve a rebound rate of 70-85% (ASTM D2632).
Application Advantage: Suitable for high-dynamic fatigue scenarios, such as tire treads.

6.Low-Temperature Flexibility

Glass Transition Temperature (Tg): Approximately -70°C, superior to most synthetic rubbers.
Brittle Temperature: Remains flexible below -50°C.

7.Stress Relaxation

Mechanism: Stress decreases over time due to molecular chain sliding.
Data: Under constant strain, the stress retention rate is about 60-70% after 24 hours.

8.Compression Set

Test Standard: ASTM D395.
Typical Value: Vulcanized rubber exhibits a compression set of 20-30% under conditions of 70°C × 22 hours.

9.Other Key Properties

• Tensile Strength: Unfilled vulcanized rubber can reach 25-35 MPa (can reach 30-40 MPa after reinforcement with carbon black).
• Tear Strength: 45-55 kN/m (higher than styrene-butadiene rubber).
• Abrasion Resistance: The abrasion loss (DIN 53516) after reinforcement with carbon black is less than 80 mm³.

V. Chemical Properties of Natural Rubber

Solvent Resistance

• Non-polar Solvents: Easily soluble in hydrocarbons such as benzene and gasoline, with a swelling rate of up to 350-550%.
• Polar Solvents: Good resistance to polar solvents such as alcohols and acetone, with a swelling rate of less than 15%.
  Improvement Methods: Enhancing oil resistance through chlorination or epoxidation modification

VI. Key Considerations for Mixing and Vulcanization Processes

1.Mixing Process Key Points

• Temperature Control: The internal mixer temperature should not exceed 120°C to prevent scorching (premature vulcanization).
• Feeding Order: Raw rubber → solid additives (such as zinc oxide, stearic acid) → carbon black → liquid softeners → vulcanizing agent.
• Dispersion: Ensure uniform dispersion of fillers to avoid agglomeration (detectable via scanning electron microscopy).

2.Vulcanization Process Control

• Vulcanization System: Commonly used sulfur-accelerator system (e.g., a ratio of CBS/DM to sulfur ranging from 1.5:1 to 2.5:1).
• Temperature and Time: Plate vulcanization typically uses a temperature of 145-155°C for 10-25 minutes.
• Pressure Requirements: Mold vulcanization requires maintaining a pressure of 10-15 MPa to eliminate bubbles.

3.Common Issues and Countermeasures

• Scorching: Due to excessively high mixing temperature or premature decomposition of the vulcanizing agent, adding an appropriate amount of scorch retarder CTP is necessary.
• Under-vulcanization: Extend the vulcanization time or increase the mold temperature to ensure complete vulcanization.
• Bubble Defects: Increase pressure or enhance the Mooney viscosity of the rubber compound to reduce bubbles.

II. Contact Us for Custom Rubber Products

If you have further customization needs for natural rubber and its products, please contact DX. As a leading enterprise with over 30 years of experience in rubber product design and production, we are committed to providing you with high-quality rubber product solutions. From raw material selection to production process optimization, as well as finished product testing and quality control, we adhere to strict standards and possess a professional team. We look forward to collaborating with you to create a bright future!