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Rubber material

🕙 2023-11-06 👆

1、 What is rubber material: Rubber is the only material with high elasticity

1. Definition of rubber: American Society for Materials Measurement (ASTM D1566): Rubber is a material that can quickly recover its deformation under large deformations;

Rubber can be modified, and the modified rubber cannot dissolve in organic solvents, but can swell. Essentially, it refers to the vulcanization (i.e. crosslinking) of rubber.

The vulcanized rubber material has high elasticity, characterized by a sample that can be stretched twice in 1 minute at a temperature of 18-29 ℃. When the external force is removed, the sample can shrink to at least 1.5 times its original length within 1 minute.

2. Rubber and Elastomer

II. Development History

Natural rubber, synthetic rubber, and thermoplastic elastomers.

The application of rubber materials: gradually becoming the fourth largest strategic resource after petroleum, iron ore, and non-ferrous metals!

Automobile tires; Production line conveyor belt; Engine transmission; High speed railway vibration reduction; Seals and other seals for manned spaceflight and space stations; Rubber packers ensure safe production of oil and gas deep production wells; Rubber parts for fighter jets and tanks; Submarine sound-absorbing tiles; Earthquake resistance and disaster reduction; Thermal conductive elastomer, heat dissipation for microelectronic equipment; Reliability and Safety of High tech Equipment

Challenges and Possible Solutions for Sustainable Development of the Three Rubber Industry

1. Resource issue: The planting area of natural rubber trees is limited within the Tropic of Cancer, and the yield is limited! Expanding the planting area of natural rubber through biological and genetic engineering

Expansion: 1000 tons of rubber require planting 3 million trees, 5500 management and planting personnel, and an investment of 1300 US dollars per ton.

2. Energy conservation issues: Internal consumption wastes driving energy, wasting 10% of car fuel; Green tires

Low rolling resistance, fuel saving 2-5%; The wear resistance and wet slip resistance remain unchanged. High performance rubber nanocomposites are the key to developing fuel-efficient and safe tires, and tire rubber materials are multi-level and multi-scale nanocomposites; The fuel saving performance, wet slip resistance, and wear resistance of tires are closely related to the structure and properties of rubber nanocomposites

2. Recycling issue: The annual consumption of rubber is over 7 million tons; Chemical cross-linked rubber is difficult to recycle and reuse; Black pollution ", encroaching on useful land, polluting the environment, and easily causing fires.

Recycling and reuse of rubber materials: prototype utilization; Thermal cracking fuel; Rubber powder; Illegal oil refining; Regenerated rubber (the main method), 71.3%, taking into account the issues of recycling and insufficient resources

Regenerated rubber: Rubber regeneration refers to the process of using physical, chemical, or biological methods to open a three-dimensional cross-linking network and form a processable quasi linear regenerated rubber. From traditional rubber regeneration process to green, efficient, and continuous dynamic desulfurization regeneration of waste rubber technology and its application

Thermoplastic elastomers: low energy consumption, high efficiency, and convenient recycling during forming and processing

4、 Fundamentals of Rubber (1)

1. Classification:

Classification by source and purpose:

Natural rubber; Comprehensive performance

synthetic rubber

²  Universal synthetic rubber (alternative): Synthetic rubber that can partially or completely replace natural rubber, with varying performance and usage effects. However, due to its unrestricted geographical conditions, it has developed rapidly and its production and consumption have exceeded that of natural rubber

Styrene butadiene rubber (SBR, the largest production synthetic rubber, tire),

Polybutadiene rubber (BR, with the best elasticity and compatibility),

Isoprene rubber (IR, synthetic natural rubber, with a cis-1,4 structural content (92% -97%) that is not as high as natural rubber (>98%)),

Large variety of special rubber: butyl rubber (IIR, best airtight; modified, brominated butyl rubber),

Nitrile rubber (NBR, currently the most widely used special synthetic rubber, has higher oil resistance than chloroprene and is second only to polysulfide, silicon, and acrylate),

Ethylene propylene rubber (EPM, EPDM, heat resistance, aging resistance, and water resistance are the best in synthesis, with the lowest density, used for window sealing strips, radiator hoses, etc.),

Neoprene rubber (CR, weather resistant, flame resistant, oil resistant, chemical corrosion resistant, etc., with the highest density, flame retardant, oil resistant, adhesive, and the best universality);

Special synthetic rubber, small variety special rubber: Synthetic rubber with special properties and special applications that can adapt to harsh conditions, such as high and low temperature resistance

Silicone rubber (SiR), acrylic rubber (ACM, heat-resistant to oxygen, ozone, and oil, second only to fluorine, poor hydrolysis and cold resistance, automotive seals), fluororubber (FKM), chloroether rubber (CO, ECO)..

Classification by chemical structure:

Carbon chain rubber: Unsaturated non-polar (natural rubber (NR), styrene butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR)); Unsaturated polarity (nitrile rubber (NBR), chloroprene rubber (CR)); Saturated non polarity (ethylene propylene rubber (EPM), butyl rubber (IIR)); Saturated polarity (hydrogenated nitrile rubber (HNBR))

Heterogeneous chain rubber: Silicone rubber (SiR); Polyurethane rubber (PU); Chloroether rubber (CO, ECO)

By crosslinking method: traditional rubber chemically crosslinked; Thermoplastic Elastomer - Third Generation Rubber

According to the raw material form: block solid rubber; Powder rubber; Liquid rubber

According to consumption ratio: NR has the highest usage, accounting for 40% of the total rubber usage; The second is SBR, which accounts for 40% -50% of the amount of synthetic rubber used; Special rubber accounts for approximately 1%.

With the increasing requirements for the performance of rubber products, the amount of special rubber used will become increasingly high.

Elastic Theory of Rubber Materials

Conditions for A to have rubber elasticity:

Flexible long chain - causes its curled molecules to undergo conformational changes through chain segment movement under external force, and then unwinds, restoring to a curled state (entropy elasticity) after removing external force.

Moderate cross-linking - viscous flow that can prevent the displacement of the center of mass between molecular chains, allowing it to fully display high elasticity.

Keywords: Long chain is flexible enough for cross-linking

Structural influencing factors of rubber elasticity: flexibility of molecular chains

B high elasticity - the unique mechanical properties exhibited by polymers (above Tg) in a high elastic state, also known as rubber elasticity.

Rubber, plastic, and biopolymers can exhibit a certain degree of high elasticity between Tg and Tf.

The Tg of rubber is much lower than room temperature, and it does not crystallize or crystallizes very little at room temperature.

C High elasticity characteristics:

Low elastic modulus; Large deformation variable; The elastic modulus increases with increasing temperature; High elastic deformation has a time-dependent mechanical relaxation characteristic; The deformation process has a significant thermal effect (thermoelastic effect), stretching releasing heat, and retracting absorbing heat.

Quantitative characterization of rubber high elasticity using impact rebound rate. The higher the impact rebound rate, the better the rubber elasticity.

3. Processing and Preparation Technology of Rubber

A rubber compounding - refers to the determination of the types and amounts of rubber and various compounding agents to meet the performance, processing technology, and cost requirements of rubber products.

Rubber compounding systems typically include five major systems:

(1) Raw rubber system - parent and matrix materials

(2) Vulcanization system - causes cross-linking of rubber

(3) Filling reinforcement system - improving mechanical properties and reducing costs

(4) Softening and plasticizing system - improving processing performance, enhancing product softness and cold resistance

(5) Protective system - improving the aging resistance of rubber

B. Terminology:

Raw rubber: Rubber that has not been added with a compounding agent and has not yet been cross-linked. Generally composed of linear macromolecules or linear macromolecules with branched chains, they can be soluble in organic solvents.

Blended rubber: Rubber that has been processed and mixed evenly with a blending agent, and has not been cross-linked. Commonly used compounding agents include vulcanizing agents, accelerators, activators, reinforcing fillers, anti-aging agents, etc.

Vulcanized rubber: Rubber obtained by cross-linking a linear macromolecule into a three-dimensional network structure of mixed rubber under a certain temperature, pressure, and time. Generally insoluble in solvents.

Forming process:

Plasticization: In the process of rubber processing, the raw rubber is first transformed from a strong elastic state to a soft and easy to process plastic state through mechanical, thermal, oxygen, and the addition of chemical reagents.

Reducing the length of rubber molecular chains (reducing molecular weight) is the most effective method to reduce intermolecular forces and increase the plastic deformation of rubber molecules.

Mixing: In order to meet the performance requirements of various rubber products, improve processing performance, save raw rubber, and reduce costs, it is necessary to add various additives to the raw rubber. The process of adding various additives to the plastic raw rubber on the rubber mixer to make the mixed rubber is called mixing.

To ensure the performance of semi-finished products and products, it is necessary to control the quality of the mixed rubber.

Sulfurization: The process of forming a network of linear polymers through cross-linking.

The determination of vulcanization process conditions includes three factors: vulcanization temperature, pressure, and time - vulcanization.

Basic Knowledge of Five Rubbers (2)

1. Natural rubber

A. Manufacturing of natural rubber

Plants, rubber picking, solidification, dehydration, drying, smoking, tablet pressing, and other processes.

B. Composition of natural rubber: rubber hydrocarbons: 92-95%, non rubber components: 7% (protein: 2-3%, acetone extract: 1.5-4.5%, small amount of ash: 0.2-0.5%, moisture: 0.3-1.0%)

Protein: has an anti-aging effect; Decomposing and releasing amino acids to promote rubber vulcanization; Make rubber easy to absorb moisture and mold; Water absorption reduces the insulation of the product; Allergic (soluble protein mass fraction exceeding 110 × 10-6).

Acetone extract: advanced fatty acids: softeners, vulcanization activators; Sterols: Antioxidants; A small amount of carotene: a physical antioxidant

Ash: Inorganic salt substances, mainly Ca, Mg, K, Na, Cu, Mn, etc. Affects electrical and aging performance.

Moisture: High content can cause bubbles in the product.

C. Classification of natural rubber: source and classification.

The content of non rubber components is an important indicator for NR grading. (Appearance quality, physical and chemical indicators)

D. The structure of natural rubber:

1) Primary structure: cis polyisoprene

More than 97% of the cis 1,4-structure, about 2% of the 3,4-structure, and 100% of the head to tail connection.

Eucommia ulmoides gum has a trans 1,4 structure, which has the same chemical composition as NR but different properties.

2) Secondary structure: Molecular weight and molecular weight distribution. The degree of polymerization of NR is very high, ranging from 10000 to 10000, with a molecular weight range of 30-30 million and a high molecular weight, averaging 300000. The molecular weight distribution of NR is wide and exhibits a bimodal distribution pattern. The low molecular weight portion is beneficial for the processing of NR. The high molecular weight portion gives NR high strength and performance.

Gel in NR: 10%~70% gel

3) Crystalline properties: The cis 1,4-structure of NR can crystallize at low temperatures or under tension, belonging to self reinforcing rubber.

Crystallization begins below 10 ℃, with -25 ℃ being the fastest and slower at room temperature. Coexistence of crystalline and amorphous structures.

Self reinforcing property: Under the condition of not adding reinforcing agents, rubber can crystallize at low temperatures or during stretching, and the grains are distributed in amorphous rubber to act as physical cross-linking points, thereby improving its own strength.

E. Properties of natural rubber

Chemical properties: Having the chemical properties of alkenes: (Fast vulcanization rate, easy aging, and easy chemical modification)

It has double bonds and can react with free radicals, oxygen, peroxides, ultraviolet light, and free radical inhibitors;

The presence of methyl groups increases the electron cloud density of double bonds, α- The increased activity of H makes NR more reactive.

Physical and mechanical properties:

1) High elasticity: easy internal rotation of C-C bonds adjacent to double bonds. The number of side methyl groups on the molecular chain is small. NR has low non polar intermolecular forces. So:

NR has good elasticity, with an elastic modulus of 2-4MPa, approximately 1/30000 of that of steel, and an elongation of 300 times that of steel. The rebound rate of NR can reach over 50-85% in the range of 0-100 ℃. When heated to 130 ℃, it can still maintain its normal performance. When it is below -70 ℃, it loses its elasticity and becomes a brittle material. Tg~-72oC

2) Excellent mechanical properties --- self reinforcing, containing gel, high molecular weight

NR is a crystalline rubber that can be stretched and crystallized, and has a high molecular weight.

The strength of pure rubber after vulcanization can reach 25MPa; After being reinforced with carbon black, it can reach 35MPa.

The tear strength of NR is also very high, up to 98kN/m.

Comparison of mechanical strength of various rubbers: NR>CR>IIR>NBR>SBR>BR self reinforcing>non self reinforcing

Aging resistance: Aging resistance is a fatal weakness of natural rubber.

Natural rubber is prone to a chain reaction of automatic catalytic oxidation with oxygen in the air, resulting in molecular chain breakage or excessive cross-linking, causing rubber to stick and crack, leading to a decrease in physical and mechanical properties, which is called aging.

Light, heat, bending deformation, and metal can all promote rubber aging;

Rubber without anti-aging agents will crack after being exposed to strong sunlight for 4-7 days;

Contact with a certain concentration of ozone can even cause cracks within seconds;

However, by adding antioxidants, its aging performance can be improved.

Blending and processing of natural rubber

Plasticizing: Except for uniform and constant viscosity NR, all need to be plasticized. Easy to shape, but easy to over refine.

Mixing: Good wettability of the compounding agent, easy to mix.

Rolling and extrusion: Good rolling and extrusion performance, low shrinkage rate of adhesive, good self-adhesion and mutual adhesion.

Vulcanization: Fast vulcanization speed, easy to control parameters, and suitable vulcanization temperature of 143 ℃. High temperature vulcanization is easy to return to its original state.

Application of G natural rubber

Good comprehensive performance, can be used to make various rubber products. (Except for products with special performance requirements, such as oil resistance and heat resistance)

Widely used in: tires, rubber hoses, tape, and various industrial rubber products.

Good processing performance, can be used together with other rubbers

It is the most widely used type of rubber, with natural rubber accounting for 35% of the total rubber consumption.

2. Styrene butadiene rubber (SBR)

Definition A: Copolymer of butadiene and styrene. It is the largest type of synthetic rubber used; It accounts for approximately 55% of synthetic rubber and 34% of total rubber usage. The English name is styrene-butadiene rubber, abbreviated as SBR.

B. Synthesis:

Lotion method (E-SBR): high temperature emulsion polymerization: 50 ℃ - peroxide initiation, high gel content; Low temperature emulsion polymerization: 5 ℃ - redox initiation, less gel, good processability. Mechanism of free radical polymerization

Solution method (S-SBR): It was put into industrial production in 1960 and has been widely used in green tires.

Compared with E-SBR, it has a 20% to 30% lower rolling resistance, a 3% higher wet slip resistance, and a 10% higher wear resistance. Anionic polymerization mechanism

C. Classification: according to the preparation method, lotion polymerized styrene butadiene rubber, solution polymerized styrene butadiene rubber

D. Brand: Styrene content (23.5%, 40%, 50% -70%), whether Mooney viscosity is oil filled, filled, etc

E. Structure and Performance of SBR

(1) The Connotation of SBR Molecular Structure

Macrostructural parameters: monomer ratio, average molecular weight, molecular weight distribution, linearity of molecular structure, gel content.

Microstructure parameters: cis 1,4 in the butadiene chain segment; The proportion of structures 1, 4, and 1, 2; The distribution of styrene and butadiene units (block copolymerization/random copolymerization).

2) Monomer ratio

With the increase of styrene content, the glass transition temperature increases, the modulus (constant elongation strength) increases, the elasticity decreases, and the processing performance improves.

The benzene ring also has the function of dispersing stress and improving wear resistance.

The heat resistance and oxygen aging performance of u are improved, while the cold resistance is reduced.

Based on various properties, the styrene content of ordinary SBR is generally about 23.5%.

(3) Molecular weight and molecular weight distribution

The average molecular weight of SBR ranges from 100000 to 130000.

Wide molecular weight distribution: low strength, multiple chain ends, high internal friction, and good processing performance.

Narrow molecular weight distribution: high strength, few chain ends, low internal friction, and poor processing performance.

(4) Molecular chain branching degree: High branching degree is beneficial for processing performance, but not for mechanical strength; At the same time, it causes multiple chain ends and high internal friction.

(5) Gel (partially crosslinked rubber) content: high gel content will cause mechanical properties to decline. The gel is in cross-linked state, and the compounding agent cannot be mixed.

(6) Structure of polybutadiene

An increase in vinyl content will lead to an increase in Tg, a decrease in elasticity, an increase in processing performance, an increase in friction coefficient between rubber and the ground, and an improvement in wet slip resistance of rubber; But the wear resistance will slightly decrease.

The molecular chains with cis 1,4 structure have high resilience; An increase in its content will lead to an increase in elasticity, a decrease in processing performance, and strength.

The molecular chains of trans 1,4 have high symmetry and tight intermolecular arrangement; Its content increases, elasticity decreases, modulus and processing performance improve.

(7) Distribution of styrene and butadiene units

The block sequence structure is more capable of exhibiting the performance characteristics of this microstructure than the random structure.

The S-SBR obtained by solution polymerization is an anionic polymerization mechanism, which involves controlling the catalyst and reaction conditions to achieve a partial block sequence structure, i.e. a random block structure.

(8) Comparison of structural properties between emulsion polymerized SBR and solution polymerized SBR

Low temperature ESBR has better comprehensive performance than high temperature ESBR.

Compared with low-temperature ESBR, SSBR has high elasticity, low internal friction, and low rolling resistance; Improved wear resistance, unchanged or slightly improved wet slip resistance.

But the prominent drawback of S-SBR is its poor processing performance.

Note: In addition to the structure of the rubber itself, the strength of the interaction between the reinforcing filler and the rubber, as well as the uniformity of its dispersion in the rubber, also play a decisive role in the final dynamic loss characteristics of the material.

Performance of E SBR (compared to NR)

There are a large number of molecular side groups (styrene, vinyl) with relatively rigid molecular chains. The Tg of SBR with a styrene content of 23.5% is about -45oC, which is much lower than the Tg of NR -72oC - with poor elasticity and cold resistance.

Non crystalline rubber does not have self reinforcing properties, and the strength of pure vulcanized rubber is only 2-3MPa. Reinforced fillers must be used for reinforcement. The strength after reinforcement can reach the level of pure NR vulcanizate.

The tear resistance is also lower than NR.

High internal friction, dynamic heat generation and rolling resistance higher than NR.

The bending fatigue resistance is lower than NR.

The double bond content and reaction activity on the main chain are lower than those of natural rubber - with better heat resistance, oxygen aging resistance, ozone resistance, and wear resistance (high temperature, long-term) than NR. The upper limit temperature of SBR is 10-20 ℃ higher than NR. However, the vulcanization rate is slower than that of NR (due to the lower double bond concentration of SBR and the volume steric hindrance effect of the benzene ring).

The solvent resistance and electrical insulation properties of SBR are similar to NR, as they are both non-polar diene rubber.

The processing performance is slightly worse than NR, especially S-SBR with poor roll wrapping and poor self-adhesive mutual adhesion.

Application of F SBR

SBR is the most commonly used rubber with the highest consumption and is widely used in general occasions, except for special situations requiring oil resistance, heat resistance, and resistance to special media.

Mainly used in the tire industry, 54.4%; Rubber overshoe industry: 16.5%; Rubber hose and tape: 11.2%; Power tire: 9.3%