1. Basic Concept of Rubber and Elastic Materials

Rubber and elastic materials are polymeric materials that undergo large deformation under relatively small forces and recover close to their original shape when the load is removed.

This behavior is generally referred to as elasticity or rubber elasticity.

The mechanism of elasticity in rubber differs from that in metals.

In metals, elastic deformation occurs through small changes in atomic bond spacing. In rubber, large deformation occurs as long and flexible polymer chains uncoil, stretch, and reorient.

For a rubber material to function effectively as an elastomer, it generally requires:

• Long and flexible polymer chains

• A glass-transition temperature below the service temperature

• A limited crosslinked structure between molecular chains

• A structure without excessive crystallization

• A tendency for stretched chains to return to a disordered state

Rubber materials are mainly used for:

• Sealing

• Vibration isolation

• Shock absorption

• Noise reduction

• Friction control

• Flexible connection

• Pressure transmission

• Prevention of oil, water, and gas leakage

• Electrical insulation

• Surface protection

Representative mechanical components include:

• O-rings

• Oil seals

• Gaskets

• Packings

• Hoses

• Belts

• Rollers

• Vibration isolators

• Diaphragms

• Flexible couplings

• Wiper seals

• Boots

• Cable jackets

2. Composition of Rubber Materials

Rubber products are not manufactured from the base polymer alone.

A typical rubber compound contains:

1. Base rubber

2. Reinforcing filler

3. Non-reinforcing filler

4. Curing agent

5. Vulcanization accelerator

6. Vulcanization activator

7. Plasticizer or processing oil

8. Antioxidant

9. Antiozonant

10. Adhesion promoter or coupling agent

11. Pigment

12. Flame retardant

13. Blowing agent

14. Functional additives

2.1 Base Rubber

The base rubber determines the fundamental oil resistance, heat resistance, weather resistance, low-temperature flexibility, and elasticity of the product.

Representative base rubbers:

• NR

• SBR

• BR

• NBR

• HNBR

• EPDM

• CR

• IIR

• VMQ

• FKM

• FFKM

• ACM

• ECO

• AU and EU

• TPE and TPU

2.2 Meaning of PHR

Rubber formulations commonly use the unit PHR in addition to ordinary mass percentage.

PHR means:

Parts per Hundred Rubber

It expresses the amount of each additive relative to 100 parts by weight of the base rubber.

For example:

• NBR: 100 PHR

• Carbon black: 50 PHR

• Plasticizer: 10 PHR

• Zinc oxide: 5 PHR

• Sulfur: 1.5 PHR

The total formulation is 166.5 parts by weight.

When converted into mass percentage of the finished compound, the base rubber content is approximately 60%.

Therefore, “50 PHR carbon black” does not mean that 50% of the finished product is carbon black.

2.3 Typical Composition of Rubber Compounds

Actual formulations vary greatly depending on hardness, color, food-contact requirements, flame resistance, oil resistance, and manufacturing process.

3. Mechanism of Rubber Elasticity

3.1 Molecular Chain Structure

Rubber molecular chains exist in a randomly coiled configuration at room temperature.

When rubber is stretched, the chains extend and align in the loading direction.

When the load is removed, the molecular chains tend to return to a disordered state.

This produces large elastic recovery.

3.2 Crosslinked Structure

Crosslinking chemically connects separate polymer chains.

Without crosslinks, polymer chains can slide past one another over time, resulting in permanent deformation.

With an appropriate degree of crosslinking:

• Elastic recovery improves.

• Tensile strength increases.

• Heat resistance increases.

• Solvent resistance improves.

• Compression set decreases.

• Creep decreases.

• Tackiness decreases.

If crosslink density is too high:

• Hardness increases.

• Elastic deformation decreases.

• Elongation decreases.

• Shock absorption decreases.

• Brittleness increases.

• Fatigue cracking may increase.

3.3 Glass-Transition Temperature

For rubber to behave as a flexible elastomer, its service temperature must be sufficiently above its glass-transition temperature.

Below the glass-transition temperature:

• The rubber becomes hard.

• Elasticity decreases.

• Impact brittleness increases.

• Seal contact force may decrease.

• Cracking risk increases.

Therefore, low-temperature seal design must consider not only the minimum temperature but also actual recovery under compression.

4. Curing Methods

4.1 Sulfur Vulcanization

Sulfur vulcanization forms sulfur crosslinks between polymer chains.

Commonly used for:

• NR

• SBR

• BR

• NBR

• Some EPDM grades

• CR-related compounds

Advantages:

• Excellent elasticity

• High tensile strength

• Good fatigue resistance

• Economic processing

• Well-established manufacturing technology

Limitations:

• Sulfur crosslinks may degrade at high temperature.

• Compression set may increase.

• Possible metal staining or contamination

• Residual substances require review for food and medical applications.

4.2 Peroxide Curing

Organic peroxides form carbon-carbon crosslinks.

Commonly used for:

• EPDM

• Silicone rubber

• HNBR

• Some NBR grades

• FKM

• Polyethylene-based elastomers

Advantages:

• High heat resistance

• Good compression-set performance

• Potential for low-extractable formulations

• Often advantageous in steam and hot-water service

• Reduced risk of metal corrosion or staining

Limitations:

• Tear and fatigue properties may be lower than sulfur-cured compounds.

• Oxygen can interfere with curing.

• Odor and by-products must be controlled.

• Some plasticizers and additives are incompatible.

4.3 Metal-Oxide Curing

This system is mainly used for CR and some halogen-containing rubbers.

Representative curing agents:

• Zinc oxide

• Magnesium oxide

Metal oxides contribute to crosslinking and also neutralize acidic by-products.

4.4 Platinum-Catalyzed Addition Curing

This curing system is commonly used for silicone rubber.

Advantages:

• Low by-product generation

• Low odor

• Suitable for food and medical products

• High-purity formulations possible

• Good dimensional stability

Considerations:

• Sulfur, amines, tin compounds, and certain contaminants may poison the catalyst.

• Strict control of material and mold contamination is required.

5. Natural Rubber — NR

5.1 Basic Composition

Natural rubber is obtained from rubber-tree latex and consists mainly of cis-1,4-polyisoprene.

The theoretical elemental mass ratio of the repeating unit is approximately:

• Carbon: 88.2%

• Hydrogen: 11.8%

5.2 Representative Formulation

5.3 Properties

Advantages:

• Very high elasticity

• Excellent tensile strength

• High tear strength

• Excellent dynamic fatigue resistance

• Good wear resistance

• Low heat buildup

• Good adhesion

Limitations:

• Poor oil resistance

• Poor fuel resistance

• Poor ozone and ultraviolet resistance

• Limited heat resistance

• Cracking under long-term oxidation

• Possible allergy from natural proteins

5.4 Main Applications

• Vibration mounts

• Tires

• Conveyor belts

• Rollers

• Couplings

• High-elasticity pads

• Shock absorbers

• Rubber springs

6. Styrene-Butadiene Rubber — SBR

6.1 Basic Composition

SBR is a copolymer of styrene and butadiene.

Representative monomer ratio:

A widely used general-purpose SBR grade contains approximately 23.5% styrene.

6.2 Role of Each Component

Butadiene

• Provides elasticity

• Provides low-temperature flexibility

• Improves dynamic fatigue behavior

Styrene

• Increases strength

• Increases hardness

• Improves wear resistance

• Improves processability

• Excessive styrene may reduce low-temperature flexibility

6.3 Representative Formulation

6.4 Properties

• Good wear resistance

• Good processability

• Uniform quality

• Lower tensile and tear strength than NR

• Poor oil and fuel resistance

• Limited ozone resistance

• Relatively low cost

6.5 Main Applications

• Tires

• General belts

• Mats

• Shoe soles

• General-purpose gaskets

• Vibration components

• Rubber sheets

7. Butadiene Rubber — BR

7.1 Basic Composition

BR is produced by polymerizing butadiene.

High-cis BR grades contain a very high proportion of cis-1,4 structure.

7.2 Properties

• Very low glass-transition temperature

• Excellent low-temperature elasticity

• Low heat buildup

• High rebound resilience

• Good wear resistance

• Poor standalone processability

• Limited tear resistance

• Commonly blended with NR or SBR

7.3 Main Applications

• Tire tread and sidewall compounds

• High-rebound rollers

• Vibration components

• Wear-resistant rubber

• Golf balls

• Conveyor belts

8. Nitrile Rubber — NBR

8.1 Basic Composition

NBR is a copolymer of acrylonitrile and butadiene.

Representative copolymer composition:

Classification by ACN content:

• Low nitrile: Approximately 18–25%

• Medium nitrile: Approximately 26–35%

• High nitrile: Approximately 36–50%

8.2 Relationship Between Composition and Properties

Increasing Acrylonitrile Content

• Increases oil resistance

• Increases fuel resistance

• Reduces gas permeability

• May increase tensile strength and hardness

• Reduces low-temperature flexibility

• Reduces elasticity

• May increase compression set

Increasing Butadiene Content

• Improves low-temperature flexibility

• Improves elasticity

• Increases oil swelling

• Reduces gas-barrier performance

8.3 Representative Formulation

8.4 Properties

Advantages:

• Excellent mineral-oil resistance

• Excellent grease resistance

• Good fuel resistance

• Good wear resistance

• Relatively high mechanical strength

• Low gas permeability

Limitations:

• Poor ozone and ultraviolet resistance

• Limited high-temperature air aging

• Limited extreme low-temperature flexibility

• Unsuitable for ketones, esters, and some polar solvents

• Generally unsuitable for steam service

8.5 Main Applications

• Oil seals

• O-rings

• Hydraulic seals

• Pneumatic seals

• Fuel hoses

• Oil hoses

• Diaphragms

• Rubber rollers

• Gaskets

9. Hydrogenated Nitrile Rubber — HNBR

9.1 Basic Composition

HNBR is produced by hydrogenating the unsaturated bonds in the butadiene portion of NBR.

Main composition:

• Acrylonitrile

• Hydrogenated butadiene-based chain

• Small amount of residual unsaturation

The hydrogenation level is generally controlled to a very high degree.

9.2 Effects of Hydrogenation

Reducing double bonds in the butadiene chain improves:

• Heat resistance

• Oxidation resistance

• Ozone resistance

• Mechanical strength

• Wear resistance

• Compression-set performance

• High-temperature oil resistance

HNBR may be less favorable than NBR in low-temperature flexibility and cost.

9.3 Properties

• High heat resistance

• High tensile strength

• Excellent oil and fuel resistance

• Excellent ozone resistance

• Good wear and fatigue performance

• Compatible with peroxide curing

• More expensive than NBR

9.4 Main Applications

• Automotive air-conditioning seals

• High-temperature hydraulic seals

• Engine seals

• High-pressure O-rings

• Timing belts

• Oilfield-equipment seals

• High-durability diaphragms

10. EPDM

10.1 Basic Composition

EPDM is a terpolymer of ethylene, propylene, and a small amount of non-conjugated diene.

Representative composition:

Representative diene monomers:

• ENB

• DCPD

• VNB

10.2 Relationship Between Composition and Properties

Increasing Ethylene Content

• May increase crystallinity

• Increases tensile strength

• Improves extrusion shape retention

• May reduce low-temperature elasticity

• Changes mixing behavior

Increasing Propylene Content

• Increases flexibility

• May improve low-temperature properties

• Reduces crystallinity

• May reduce strength and extrusion stability

Increasing Diene Content

• Increases cure rate

• Increases crosslinking capability

• Improves mechanical strength

• Excessive content may reduce aging resistance

10.3 Representative Formulation

EPDM can accept large quantities of oil and filler, allowing a wide range of hardness and cost adjustment.

10.4 Properties

Advantages:

• Excellent weather resistance

• Excellent ozone resistance

• Excellent hot-water resistance

• Suitable for some steam environments

• Relatively good resistance to acids and alkalis

• Good electrical insulation

• Wide temperature capability

Limitations:

• Poor mineral-oil resistance

• Poor fuel resistance

• Unsuitable for hydrocarbon solvents

• Metal adhesion may require special treatment

10.5 Main Applications

• Water-service O-rings

• Steam gaskets

• Hygienic seals

• Outdoor packings

• Automotive weatherstrips

• Radiator hoses

• Roofing membranes

• Electrical cable jackets

11. Chloroprene Rubber — CR

11.1 Basic Composition

CR is produced by polymerizing chloroprene and is also widely known by the trade name Neoprene.

11.2 Effect of Chlorine

Chlorine contributes to:

• Improved flame resistance

• Improved weather resistance

• Improved ozone resistance

• Moderate improvement in oil resistance

• Increased polarity

• Increased density

11.3 Representative Formulation

11.4 Properties

• Good weather and ozone resistance

• Good flame resistance

• Better oil resistance than NR and SBR

• Good mechanical strength

• Good adhesion

• Limited low-temperature flexibility

• Lower oil resistance than NBR

• Electrical properties may be affected by humidity

11.5 Main Applications

• General industrial hoses

• Belts

• Vibration mounts

• Wire jackets

• Adhesives

• Diaphragms

• Flame-resistant gaskets

• Outdoor rubber components

12. Butyl Rubber — IIR

12.1 Basic Composition

IIR is a copolymer of isobutylene and a small amount of isoprene.

Representative composition:

The small amount of isoprene provides unsaturation for vulcanization.

12.2 Properties

• Very low gas permeability

• Excellent air retention

• Excellent ozone and weather resistance

• Good vibration damping

• Relatively good resistance to acids and alkalis

• Low rebound resilience

• Poor resistance to oil and hydrocarbon fuels

• Difficult adhesion to metals and other rubbers

12.3 Halobutyl Rubber

Halobutyl rubber is produced by introducing chlorine or bromine into IIR.

• CIIR: Chlorinated butyl rubber

• BIIR: Brominated butyl rubber

Effects of halogenation:

• Faster curing

• Improved adhesion

• Better co-vulcanization with other rubbers

• Retained heat resistance and gas impermeability

12.4 Main Applications

• Tire inner liners

• Vacuum seals

• Pharmaceutical closures

• Gas-barrier membranes

• Vibration-damping pads

• Diaphragms

• Airtight hoses

13. Silicone Rubber — VMQ

13.1 Basic Composition

Silicone rubber has a silicon-oxygen backbone rather than a carbon-carbon backbone.

VMQ refers to vinyl methyl silicone rubber.

13.2 Typical Compound Composition

High-hardness products may contain increased amounts of silica and other fillers.

13.3 Role of Silica

Silicone polymer alone has relatively low mechanical strength.

Fine silica increases:

• Tensile strength

• Tear strength

• Hardness

• Compression performance

• Viscosity

• Processing resistance

Excessive interaction between silica and polymer can produce structuring during storage, so structure-control agents are often used.

13.4 Properties

Advantages:

• Very wide service-temperature range

• Excellent low-temperature flexibility

• Excellent weather and ozone resistance

• Good electrical insulation

• Food- and medical-grade compounds available

• Wide color selection

• Potential for low-toxicity formulations

• Soft touch and good compressibility

Limitations:

• Tear strength may be limited.

• Wear resistance is relatively low.

• Gas permeability is high.

• Resistance to mineral oils and fuels is limited.

• Susceptible to tearing at sharp edges

• Higher cost than general-purpose rubber

13.5 Main Applications

• Food-machinery gaskets

• Hygienic hoses

• Oven seals

• Medical tubing

• Electrical insulation components

• High-temperature packings

• Keypads

• Diaphragms

• Vacuum components

14. Fluoroelastomer — FKM

14.1 Basic Composition

FKM is a family of high-performance synthetic rubbers containing fluorine.

Representative monomer combinations:

• Vinylidene fluoride, VDF

• Hexafluoropropylene, HFP

• Tetrafluoroethylene, TFE

• Perfluoromethyl vinyl ether, PMVE

FKM generally contains approximately 65–71 wt.% fluorine.

As fluorine content increases, resistance to fuels and chemicals generally improves.

14.2 Composition by Type

Type 1

• VDF + HFP

• General-purpose fluoroelastomer

• Good oil and fuel resistance

Type 2

• VDF + HFP + TFE

• Higher fluorine content

• Improved chemical and fuel resistance

Type 3

• VDF + TFE + PMVE

• Balanced low-temperature flexibility and chemical resistance

Types 4 and 5

• Special copolymer compositions

• Adjusted for high heat, chemical resistance, or low-temperature performance

14.3 Typical Compound Composition

14.4 Properties

Advantages:

• High heat resistance

• Excellent mineral-oil resistance

• Excellent fuel resistance

• Resistance to many solvents

• Low gas permeability

• Excellent ozone and ultraviolet resistance

• Good flame resistance

• Good high-temperature compression-set performance

Limitations:

• Limited low-temperature flexibility

• Hot water and steam require grade-specific review.

• Limited resistance to amines, ketones, and some esters

• High cost

• Post-curing may be required

14.5 Main Applications

• High-temperature O-rings

• Fuel-system seals

• Chemical-equipment gaskets

• Vacuum seals

• High-temperature oil seals

• Aircraft fuel components

• Semiconductor-equipment seals

15. Perfluoroelastomer — FFKM

15.1 Basic Composition

FFKM is an elastomer in which most of the polymer backbone is fully fluorinated.

Main components:

• TFE-based monomers

• Perfluorovinyl ethers

• Cure-site monomers

Its very high fluorine content provides chemical stability similar to PTFE while maintaining elastomeric behavior.

15.2 Properties

• Extremely high chemical resistance

• Very high heat resistance

• Low-outgassing grades available

• Suitable for vacuum and semiconductor processes

• Excellent high-temperature compression-set performance

• Some grades resist nearly all process fluids

Limitations:

• Extremely high cost

• Low-temperature properties depend on grade.

• Mechanical and tear strength vary by grade.

• Exact selection based on fluid and temperature is essential.

15.3 Main Applications

• Semiconductor-process seals

• High-purity chemical equipment

• O-rings for strong acids and alkalis

• Aerospace seals

• High-temperature reactors

• Pharmaceutical-production equipment

16. Acrylic Rubber — ACM

16.1 Basic Composition

ACM is based primarily on acrylic ester monomers.

Main components:

• Ethyl acrylate

• Butyl acrylate

• Cure-site monomer

Ester side groups provide resistance to oil and heat.

16.2 Properties

• Excellent high-temperature oil resistance

• Good oxidation and ozone resistance

• Suitable for automatic-transmission fluids

• Limited low-temperature flexibility

• Limited water and steam resistance

• Lower fuel resistance than FKM

• Requires caution in hydrolytic environments

16.3 Main Applications

• Automotive oil seals

• Transmission seals

• High-temperature oil hoses

• Engine gaskets

• Lubricant-service packings

17. Epichlorohydrin Rubber — ECO

17.1 Basic Composition

ECO is an elastomer containing epichlorohydrin-based monomers.

Representative types:

• CO: Epichlorohydrin homopolymer

• ECO: Copolymer of epichlorohydrin and ethylene oxide

• GECO: Terpolymer containing a cure-site monomer

17.2 Properties

• Low fuel permeability

• Good oil and fuel resistance

• Excellent ozone resistance

• Relatively good low-temperature performance

• Good heat resistance

• Adjustable electrical conductivity or antistatic properties

• Possible limitations in water and steam

• Acidic by-products require control

17.3 Main Applications

• Fuel hoses

• Automotive diaphragms

• Gaskets

• Fuel-vapor barrier components

• Antistatic rollers

18. Polyurethane Elastomers — AU and EU

18.1 Basic Composition

Polyurethane elastomers are composed of polyols, isocyanates, and chain extenders.

Main types:

• AU: Polyester-based polyurethane rubber

• EU: Polyether-based polyurethane rubber

18.2 Polyester-Based vs. Polyether-Based

Polyester-Based

• High mechanical strength

• Excellent wear resistance

• Good oil resistance

• May be susceptible to hydrolysis

Polyether-Based

• Excellent water and moisture resistance

• Good low-temperature flexibility

• Good microbial resistance

• May have lower oil and solvent resistance than polyester grades

18.3 Properties

• Very high wear resistance

• High tensile strength

• High tear strength

• Wide hardness range

• High load-carrying capacity

• Good elasticity and shock absorption

• Requires control of frictional and internal heat buildup

• Limited continuous high-temperature capability

• Hydrolysis and microbial degradation must be reviewed

18.4 Main Applications

• Industrial rollers

• Wheels

• Scrapers

• Hydraulic seals

• Shock absorbers

• Coupling elements

• Belts

• Wear liners

19. Thermoplastic Elastomers — TPE

19.1 Basic Concept

TPE combines rubber-like elasticity with thermoplastic melt processability.

Unlike conventional vulcanized rubber, chemical vulcanization can be omitted or minimized.

Advantages:

• Injection moldable

• Extrudable

• Reprocessable and recyclable

• Short production cycles

• Suitable for two-shot molding with metals or plastics

• Wide color capability

Limitations:

• High-temperature creep

• Compression set

• Limited long-term heat resistance

• Limited oil and chemical resistance

• Elastic recovery may be lower than permanently crosslinked rubber

19.2 TPE-S

TPE-S is a styrenic thermoplastic elastomer.

Representative types:

• SBS

• SEBS

• SIS

Structure:

• Hard styrene blocks

• Flexible butadiene or ethylene-butylene blocks

Typical compound composition:

Properties:

• Soft touch

• Excellent moldability

• Transparent grades available

• Good low-temperature flexibility

• Limited high-temperature durability

• Limited oil resistance

• SEBS is generally more suitable than SBS for outdoor service

Main applications:

• Handles

• Grips

• Seals

• Cushions

• Wire jackets

• Consumer-product overmolding

19.3 TPO

TPO is a blend of PP and EPDM or another olefin elastomer.

Typical composition:

Properties:

• Low density

• Excellent weather resistance

• Good impact strength

• Relatively high stiffness

• Injection and extrusion processable

• Lower compression recovery than vulcanized rubber

• Limited oil resistance

Main applications:

• Automotive bumpers

• Covers

• Waterproof sheets

• Exterior components

• Flexible ducts

19.4 TPV

TPV is a dynamically vulcanized material in which cured rubber particles are dispersed in a thermoplastic matrix.

Typical structure:

• Continuous phase: PP

• Dispersed phase: Vulcanized EPDM

Typical composition:

Properties:

• Better elastic recovery than conventional TPO

• Excellent weather and ozone resistance

• Injection and extrusion processable

• Recyclable

• Compression set may be higher than EPDM vulcanizates

• Limited resistance to mineral oil and fuel

Main applications:

• Weatherstrips

• Flexible hoses

• Seals

• Grommets

• Automotive ducts

• Electrical-connector seals

19.5 TPU

TPU is a thermoplastic polyurethane elastomer.

Typical composition:

• Polyol-based soft segments

• Isocyanate-based hard segments

• Chain extenders

• Stabilizers

• Lubricants

• Pigments

Properties:

• Very high wear resistance

• High tensile strength

• Excellent tear strength

• Transparent grades possible

• Injection and extrusion processable

• Relatively good resistance to oils and greases

• Compression set may be higher than cast polyurethane

• Hydrolysis requires attention in hot and humid environments

Main applications:

• Belts

• Hoses

• Cable jackets

• Wheels

• Rollers

• Flexible couplings

• Protective films

• Pneumatic tubing

20. Roles of Fillers and Additives

20.1 Carbon Black

Carbon black is the most widely used reinforcing filler in rubber.

Its main constituent is carbon.

Effects:

• Increases tensile strength

• Increases wear resistance

• Increases tear strength

• Increases hardness

• Provides ultraviolet protection

• Adjusts electrical conductivity

• Controls cost

Smaller particle size and higher surface area generally increase reinforcement but may also increase viscosity and heat buildup.

20.2 Silica

The principal component of silica is silicon dioxide.

Effects:

• Reinforces rubber

• Improves tensile and tear strength

• Enables transparent or colored compounds

• Maintains electrical insulation

• Adjusts heat buildup and friction behavior

Because silica is polar, silane coupling agents may be used to improve bonding with nonpolar rubbers.

20.3 Calcium Carbonate

Effects:

• Reduces cost

• Adjusts hardness

• Improves dimensional stability

• Improves extrusion shape retention

• Modifies surface properties

Excessive loading may reduce tensile strength and elasticity.

20.4 Clay and Talc

Effects:

• Increase hardness

• Improve extrusion and dimensional stability

• May reduce gas permeability

• Reduce cost

• Support flame resistance

20.5 Process Oils and Plasticizers

Main types:

• Paraffinic oils

• Naphthenic oils

• Aromatic oils

• Ester plasticizers

• High-molecular-weight plasticizers

Effects:

• Reduce hardness

• Increase flexibility

• Improve low-temperature properties

• Improve mixing and extrusion

• Improve filler dispersion

• Reduce cost

Considerations:

• Oil extraction

• Volume swelling

• Volatilization and hardening

• Food-contact compliance

• Compatibility with sealing fluid

• Changes in low- and high-temperature properties

20.6 Antioxidants

Antioxidants suppress molecular-chain degradation caused by heat and oxygen.

Effects:

• Delay hardening and cracking

• Maintain tensile strength

• Extend service life

• Improve heat-aging resistance

20.7 Antiozonants

Antiozonants reduce attack of unsaturated bonds by ozone.

Countermeasures include:

• Chemical antiozonants

• Wax protective films

• Selection of saturated rubbers such as EPDM, silicone, and FKM

• Reduced tensile strain

• Ultraviolet shielding

20.8 Flame Retardants

Flame retardants delay combustion or improve self-extinguishing behavior.

Main types:

• Halogen-based

• Phosphorus-based

• Metal hydroxides

• Intumescent systems

Flame retardants may affect:

• Tensile strength

• Elongation

• Hardness

• Density

• Electrical properties

• Compression set

• Smoke and combustion gases

21. Main Mechanical Properties of Rubber

21.1 Hardness

Rubber hardness is commonly measured using Shore A.

Approximate application ranges:

As hardness increases:

• Deformation resistance increases.

• Compression force increases.

• Extrusion resistance improves.

• Assembly force increases.

• Low-pressure sealing may become more difficult.

• Shock absorption may decrease.

21.2 Tensile Strength

Rubber tensile strength is affected by:

• Base rubber

• Crosslink density

• Carbon black and silica

• Plasticizer

• Test temperature

• Strain rate

• Aging condition

• Fiber orientation

NR, HNBR, and polyurethane can achieve high tensile strength.

Silicone and FFKM provide excellent environmental resistance, but mechanical strength may be relatively lower depending on formulation.

21.3 Elongation

Elongation indicates how far rubber can stretch before breaking.

Rubber materials often have elongation values of several hundred percent.

High elongation does not necessarily mean high durability.

Actual seal design must also consider:

• Repeated deformation

• Notches

• Surface defects

• Compression set

• Temperature

• Chemical swelling

21.4 Tear Strength

Tear strength indicates resistance to crack propagation after a cut or flaw has formed.

Important applications include:

• Diaphragms

• Belts

• Thin seal lips

• Boots

• Roller coverings

• Repeated-flexing components

NR and polyurethane generally have high tear resistance, while silicone requires caution at sharp defects.

21.5 Rebound Resilience

Rebound resilience indicates how much impact energy is recovered.

High-rebound materials:

• NR

• BR

• Some polyurethane grades

Low-rebound materials:

• IIR

• Some highly filled compounds

• Damping-oriented formulations

In vibration isolation, loss factor and damping are as important as elasticity.

21.6 Compression Set

Compression set is the percentage of permanent thickness loss after a rubber specimen is compressed for a specified time and then released.

Lower compression set is generally better for maintaining long-term sealing force.

Influencing factors:

• Temperature

• Compression ratio

• Time

• Curing system

• Crosslink density

• Plasticizer

• Fluid swelling

• Rubber thickness

• Post-curing

For O-rings, gaskets, and packings, compression set may be more important than tensile strength.

21.7 Creep and Stress Relaxation

When rubber is held at constant deformation, reaction force decreases over time.

This is called stress relaxation.

Possible sealing problems include:

• Reduced contact pressure

• Leakage

• Reduced clamping force

• Changes in vibration-isolation performance

21.8 Fatigue and Crack Growth

Microcracks may grow under repeated bending and tension.

Influencing factors:

• Rubber type

• Filler dispersion

• Ozone exposure

• Surface defects

• Mean strain

• Frequency

• Self-heating

• Temperature

• Fluid exposure

22. Thermal Properties

22.1 Service Temperature

Rubber service temperature must consider:

• Low-temperature flexibility

• Glass-transition temperature

• High-temperature oxidation

• Crosslink degradation

• Compression set

• Combined fluid and temperature effects

• Short-term and continuous temperature

General relative tendencies:

Actual temperature capability depends on formulation and service fluid, so supplier data must be used.

22.2 Heat Aging

At elevated temperature, rubber may:

• Harden

• Soften

• Crack

• Lose plasticizer

• Undergo crosslink scission

• Undergo additional crosslinking

• Lose strength

• Show increased compression set

NBR and NR tend to harden during high-temperature oxidation, while some polyurethane grades may soften or degrade under combined heat and moisture.

22.3 Self-Heating

Repeated deformation generates heat through molecular hysteresis.

Self-heating is important in:

• Tires

• High-speed rollers

• Belts

• Couplings

• Vibration mounts

• Reciprocating diaphragms

Accumulated heat may cause:

• Reduced strength

• Accelerated aging

• Crosslink degradation

• Bond-line damage

• Reduced fatigue life

23. Compatibility with Fluids and Chemicals

23.1 Swelling

Rubber may absorb fluid and increase in volume.

Effects of swelling:

• Reduced hardness

• Reduced strength

• Dimensional growth

• Change in seal compression

• Increased friction

• Interference in assemblies

• Extrusion damage

Some fluids may extract plasticizers and cause shrinkage and hardening.

23.2 Polarity and Compatibility

Materials of similar polarity tend to dissolve or swell each other more readily.

Examples:

• Nonpolar NR and EPDM tend to swell in hydrocarbon oils.

• Polar NBR resists mineral oils but may be vulnerable to some polar solvents.

• Highly fluorinated FKM and FFKM are stable in many fluids.

However, actual compatibility depends on temperature, additives, and mixed fluids, so polarity alone is insufficient for final selection.

23.3 General Compatibility Trends

24. Seal-Design Considerations

24.1 Compression Ratio

Rubber seals generate contact pressure through controlled compression.

Insufficient compression may cause:

• Leakage

• Sensitivity to assembly tolerance

• Failure at low pressure

Excessive compression may cause:

• Increased friction

• Assembly damage

• Increased compression set

• Reduced seal life

• Extrusion from the groove

24.2 Stretch Ratio

Excessive stretching of an O-ring during installation reduces its cross-sectional area and changes contact pressure.

Items to review:

• Installation stretch

• Cross-section reduction

• Temperature change

• Rubber swelling

• Twisting during assembly

24.3 Extrusion Gap

At high pressure, rubber may be forced into metal gaps and damaged.

Countermeasures:

• Increase hardness

• Reduce clearance

• Use backup rings

• Reduce pressure gradients

• Use higher-strength materials

• Consider temperature and swelling

24.4 Dynamic-Seal Friction

Dynamic seals require review of:

• Breakaway friction

• Running friction

• Lubrication

• Surface roughness

• Eccentricity

• Speed

• Heat generation

• Stick-slip

• Wear particles

• Mating-surface coating

25. Vibration Isolation and Damping Design

Rubber vibration isolators act as both springs and dampers.

Design factors:

• Static load

• Dynamic stiffness

• Natural frequency

• Excitation frequency

• Loss factor

• Temperature

• Compression and shear direction

• Long-term creep

• Oil and ozone exposure

The dynamic stiffness of rubber can differ from its static stiffness and varies with frequency and temperature.

NR is widely used in vibration mounts because of its high elasticity and fatigue resistance. EPDM or CR may be selected for outdoor or elevated-temperature environments.

26. Rubber Materials in Food Machinery

26.1 Commonly Used Materials

• EPDM

• Silicone rubber

• FKM

• NBR

• HNBR

• PTFE-composite seals

• TPU

• Food-contact natural rubber

Food compliance cannot be determined from the generic rubber name alone.

The following ingredients must also be reviewed:

• Carbon black

• Pigments

• Plasticizers

• Antioxidants

• Accelerators

• Mold-release agents

• Fillers

• Cure by-products

• Adhesives

26.2 EPDM

Suitable environments:

• Water

• Hot water

• Steam

• Acid and alkaline cleaning

• Outdoor exposure

Considerations:

• Edible oils and lubricants

• Fatty foods

• Hydrocarbon cleaners

26.3 Silicone Rubber

Suitable environments:

• High temperature

• Low temperature

• Hygienic hoses

• Food contact

• Ovens and heating equipment

• Transparent seals

Considerations:

• Tear

• Wear

• High pressure

• Oil

• Gas permeability

• Sharp grooves

Platinum-cured silicone is advantageous where low extractables and hygiene are important.

26.4 NBR and HNBR

Suitable environments:

• Edible oils

• Greases

• Lubricants

• Some fatty foods

Considerations:

• High-temperature steam

• Ozone

• Strong oxidizing cleaners

• Actual food-contact certification

26.5 FKM

Suitable environments:

• High-temperature oils

• Fats

• Fuels

• Some aggressive cleaners

• Elevated temperature

Considerations:

• Dedicated grades are required for steam and hot-water service.

• Low-temperature performance

• Amine-based cleaners

• Cost

• Food-contact compound verification

26.6 Metal-Detectable Rubber

Food machinery may use metal-detectable or X-ray-detectable rubber to identify fragments from damaged seals.

Typical composition:

• EPDM, silicone, NBR, or another base rubber

• Metallic or high-density inorganic detectable additive

• Blue pigment

• Reinforcement and stabilizers

Considerations:

• Detectability depends on fragment size.

• Additives may affect tear strength and compression set.

• Detection must be validated with the actual equipment.

• Food-contact compliance must be verified.

27. Comparison of Representative Materials

28. Material Selection by Component

28.1 Oil Seals

Recommended materials:

• NBR

• HNBR

• FKM

• ACM

Selection criteria:

• General mineral oil: NBR

• High temperature and high pressure: HNBR

• High-temperature oil and fuel: FKM

• Automotive high-temperature lubricant: ACM

28.2 Water and Steam Gaskets

Recommended materials:

• EPDM

• Peroxide-cured EPDM

• Silicone

• Dedicated FFKM grades

Selection criteria:

• General water and hot water: EPDM

• High-temperature steam: Dedicated EPDM

• Hygiene and wide temperature range: Silicone

• Extreme chemicals and steam: Dedicated FFKM

28.3 Vibration Mounts

Recommended materials:

• NR

• CR

• EPDM

• IIR

Selection criteria:

• High elasticity and fatigue resistance: NR

• Outdoor exposure and flame resistance: CR

• Weather and heat resistance: EPDM

• High damping and gas impermeability: IIR

28.4 High-Wear Rollers

Recommended materials:

• Polyurethane

• HNBR

• NR

• NBR

Selection criteria:

• Maximum wear resistance: PU

• Oil and heat resistance: HNBR

• High elasticity and impact resistance: NR

• Oil environment: NBR

28.5 O-Rings for Chemical Equipment

Recommended materials:

• FKM

• FFKM

• EPDM

• PTFE-composite seals

Selection must be based on the exact fluid and temperature. A general claim of “good chemical resistance” is not sufficient.

29. Drawing and Specification Designation

It is insufficient to specify only “rubber,” “NBR,” or “silicone.”

The following should be specified together.

29.1 Base Material

Examples:

• NBR

• HNBR

• EPDM

• VMQ

• FKM

• TPU

29.2 Hardness

Examples:

• NBR 70 Shore A

• EPDM 60 ±5 Shore A

• Silicone 50 Shore A

The test method and allowable tolerance should also be specified.

29.3 Color

Examples:

• Black

• White

• Blue

• Translucent

Color may affect pigment selection, carbon-black content, and detectability.

29.4 Curing System

Specify where necessary:

• Sulfur cured

• Peroxide cured

• Platinum cured

• Bisphenol cured

• Amine cured

29.5 Food and Medical Compliance

Examples:

• Food-contact grade

• Drinking-water grade

• Low-extractables grade

• Medical grade

• Metal-detectable grade

Compliance must be verified for the finished compound.

29.6 Required Physical Properties

• Tensile strength

• Elongation

• Tear strength

• Compression set

• Heat aging

• Volume change

• Hardness change

• Low-temperature flexibility

• Ozone resistance

• Adhesion strength

• Electrical resistance

29.7 Service Environment

The drawing or purchase specification should include:

• Service fluid

• Fluid concentration

• Continuous temperature

• Maximum temperature

• Pressure

• Static or dynamic service

• Cleaning method

• Steam exposure

• Ultraviolet exposure

• Expected service life

30. Rubber Material Selection Procedure

30.1 Confirm the Function

• Is it a static seal?

• Is it a reciprocating seal?

• Is it a rotary seal?

• Is it a vibration-isolation component?

• Is it a shock absorber?

• Is it a roller or belt?

• Is it a hose?

30.2 Confirm the Fluid

• Water

• Steam

• Mineral oil

• Synthetic oil

• Grease

• Fuel

• Acids

• Alkalis

• Cleaning agents

• Solvents

• Food

• Pharmaceuticals

30.3 Confirm the Temperature

• Minimum temperature

• Continuous-use temperature

• Short-term peak temperature

• Thermal cycling

• Sterilization and cleaning temperature

• Frictional heating

30.4 Confirm the Mechanical Conditions

• Pressure

• Compression ratio

• Repeated deformation

• Speed

• Friction

• Extrusion gap

• Impact

• Vibration

• Clamping force

30.5 Confirm Long-Term Properties

• Compression set

• Stress relaxation

• Creep

• Heat aging

• Ozone cracking

• Fatigue cracking

• Plasticizer extraction

• Fluid swelling

30.6 Confirm Manufacturing Conditions

• Compression molding

• Transfer molding

• Injection molding

• Extrusion

• Calendering

• Cast polyurethane

• Two-shot molding

• Adhesive bonding

• Rubber-to-metal bonding during vulcanization

30.7 Confirm Regulations and Safety

• Food contact

• Medical use

• Flame resistance

• Hazardous substances

• Combustion gases

• Allergy

• Extractables

• Recyclability

• Worker exposure

31. Final Summary

Rubber and elastic materials are not simply soft and stretchable materials.

Their actual performance is determined by a combination of:

• Molecular structure of the base rubber

• Type and ratio of comonomers

• Curing system

• Crosslink density

• Type and amount of carbon black and silica

• Plasticizers

• Fillers

• Antioxidants

• Antiozonants

• Curing conditions

• Post-curing

• Service temperature

• Fluid and chemical environment

• Long-term compression and repeated deformation

NR has high elasticity, tensile strength, and fatigue resistance because of its cis-1,4-polyisoprene structure.

SBR balances wear resistance and economy through the combination of styrene and butadiene.

BR uses its low glass-transition temperature and high rebound resilience in low-temperature and dynamic applications.

NBR balances oil resistance and low-temperature flexibility according to acrylonitrile content.

HNBR improves heat resistance, ozone resistance, and mechanical durability by hydrogenating the unsaturated bonds in NBR.

EPDM uses the composition of ethylene, propylene, and diene to provide excellent performance in water, steam, ozone, and outdoor environments.

CR contains chlorine and therefore offers flame resistance, weather resistance, and moderate oil resistance.

IIR has very low gas permeability and good damping because of its high isobutylene content.

Silicone rubber has a silicon-oxygen backbone that provides an exceptionally wide temperature range and excellent weather resistance, but tear and wear performance require caution.

FKM and FFKM use high fluorine content to provide high resistance to heat and chemicals, but the exact grade must be selected according to fluid and temperature.

Polyurethane combines hard and soft segments to provide high strength, tear resistance, and wear resistance.

TPE combines rubber-like elasticity with thermoplastic processability, but long-term high-temperature creep and compression set must be reviewed.

Therefore, rubber materials should not be selected solely by generic names such as “NBR,” “EPDM,” or “silicone.”

The following must be specified and verified together:

• Base rubber

• Copolymer composition

• Hardness

• Curing system

• Fillers and plasticizers

• Service fluid

• Service temperature

• Pressure and strain

• Compression set

• Heat aging

• Fluid swelling

• Food, medical, and environmental compliance

• Supplier compound grade

Selecting rubber and elastic materials is not merely a matter of choosing a soft material.

It is a design process that selects the appropriate molecular structure and compound formulation according to sealing, vibration, friction, temperature, fluid, and service-life requirements.