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Styrene-Butadiene Rubber: Complete Technical Guide on SBR & its Features

Styrene-Butadiene Rubber: Complete Technical Guide From tires, conveyor belts to seals or gaskets… Styrene-Butadiene rubber is a widely used general purpose rubber with an extensive range of applications.

The reason for the success of this synthetic rubber is, of course, its unique properties and diverse applications. Also, it is the only material which exhibits long-range elasticity, and which therefore fills a special need in modern technology.

Become an expert by learning some basic information about Styrene-Butadiene rubber along with the main properties, major benefits and popular applications in detail!

What is SBR - Styrene Butadiene Rubber?


Styrene-Butadiene rubber (SBR or Styrene-butadiene) is a synthetic rubber comprising of styrene and butadiene monomers. The random copolymer has characteristics like natural rubber and contains:

  • Styrene content in the range of 10-25% contributing to good wearing and bonding characteristics
  • While the butadiene unit is composed approximately 60 to 70% trans-1,4; 15 to 20% cis-1,4; and 15 to 20% 1,2 configurations for the polymer at 50°C.
Molecular Structures of Styrene and Butadiene - Monomers of SBR
Molecular Structures of Styrene and Butadiene - Monomers of SBR

Key benefits of SBR include:
  • Abrasion resistance 
  • Perfect impact strength
  • Good resilience 
  • High tensile strength

Also, when compared with polybutadiene rubber alone, styrene-butadiene rubber has improved strength, abrasion resistance, and blend compatibility. These properties are further enhanced with the use of additives.

Main applications of styrene butadiene rubber include tires and tire products, automotive parts and mechanical rubber goods.

Asahi Kasei, Lion Elastomers, Versalis (Eni Group), Kumho Petrochemical etc. are some of the key producers of SBR.

» View all SBR commercial grades and suppliers in Omnexus Plastics Database

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Structure and Properties of SBR


SBR manufacturing method was first developed in Germany in 1930s when IG Farben's Walter Bock and Eduard Tschunkur polymerized a synthetic rubber called Buna-S from butadiene and styrene in an aqueous emulsion. Then the first solution polymerized random SBR grades were produced commercially by Firestone and Phillips during 1960s.

Molecular Structure of SBR
Molecular Structure of SBR

Today, there are two major types of SBR with different properties based on their manufacturing process:

  • Emulsion SBR (e-SBR) - Hot SBR or Cold SBR
  • Solution SBR (s-SBR)

Emulsion SBR (e-SBR)


It can be produced by free-radical emulsion polymerization of styrene and butadiene either at 50 to 60°C (hot emulsion SBR) or at about 5°C (cold emulsion SBR).

  • The hot emulsion SBR process, which was developed first, leads to a more branched polymer than the cold emulsion process. SBR grades produced using this process have exception processing characteristics such as low mill shrinkage, good dimensional stability, and good extrusion characteristics
  • On the other hand, Cold SBR has a better abrasion resistance and, consequently, provides better tread wear and dynamic properties. It also exhibits superior mechanical properties such as tensile strength compared to grades produced by the hot emulsion polymerization route

Key Features
  • Green strength becomes low with increasing oil extension 
  • Low resilience and low tensile strength 
  • Outstanding resistance to abrasion 
  • Low resistance to oil, other hydrocarbon fluids and ozone 
  • Hot polymers are difficult to process with low green strength 
  • Poor tear strength 
  • High styrene resins have good low-temperature properties but stiffen 

Solution SBR (s-SBR)


Solution SBR is produced by termination-free*, anionic solution polymerization of styrene and butadiene with alkyl lithium initiator (e.g., butyllithium) in a hydrocarbon solvent, usually hexane or cyclohexane.

*It enables the synthesis of polymers with a very narrow molecular weight distribution and less chain branching.

S-SBR grades have improved flexibility, performance, superior mechanical properties like tensile strength, low rolling resistance etc. especially when used in tires.

Solution SBR has a narrower molecular weight distribution, higher molecular weight, and higher cis-1,4-polybutadiene content than emulsion polymerization SBR.

s-SBR rarely has more than 2% non-rubber materials in its finished form while e-SBR may have an emulsifier (soap) content of up to 5% and nonrubber materials sometimes in excess of 10%

Key Features:
  • Good resilience and tensile strength 
  • Outstanding resistance to abrasion and fatigue 
  • Low resistance to oil, other hydrocarbon fluids and ozone

Styrene Butadiene Rubber Vs Natural Rubber


While most of the properties of SBR are comparable with NR, but in some respects like heat build-up, tack and gum tensile strength make it inferior to natural rubber. Other disadvantages include:
  • Low elongation at break
  • Low hot tear strength
  • Hysteresis, resilience

But the addition of resins and reinforcing fillers adequately improve these properties.

However, there are properties which makes it superior over natural rubber. These include:

Also, scorch problems are less likely to occur with SBR than with NR.

Overall, the most important factors in the commercial viability of SBR making it material of choice over other rubbers are:

  • Wide availability
  • Low cost compared with those of all other synthetic rubbers, 
  • Ability to accept high filler levels, 
  • Relatively stable price compared with that of NR and 
  • Overall properties on a cost/performance basis

Processing and Compounding Synthetic Rubber - SBR


Unlike several other thermoplastics or thermosets (which are supplied in pellets or liquid resins form), SBR is available to rubber processors in the form of large bales. The processing of rubbers starts by mixing elastomers and additives. After that rubbers are shaped by using different kinds of processing methods.

SBR is often compounded with additives such as:
  • Sulfur for vulcanization
  • Reinforcing or non-reinforcing fillers to enhance its mechanical properties or to extend the rubber to reduce cost

Thanks to compounding, the styrene butadiene rubber further enhanced to satisfy a given application in terms of properties, cost, and processability.

Vulcanization

It is a process to obtain cross-linking of elastomer molecules to make rubber stiffer and stronger as well as retains extensibility at the same time. All types of SBR are vulcanized using the same vulcanization agents as for natural rubber. Styrene-butadiene rubber can be vulcanized using sulfur, sulfur donor systems and peroxides. Sulfur is added in slightly smaller amounts than to natural rubber and in tire compounds.

On a submicroscopic scale, the long-chain molecules of rubber become joined at certain tie points, the effect of which is to reduce the ability of the elastomer to flow.
  • A typical soft rubber has 1 or 2 cross-links per 1000 units 
  • As the number of cross-links increases, the polymer becomes stiffer and behaves more and more like a thermosetting plastic (hard rubber)

Effect of Vulcanization on Rubber Molecules
Vulcanization of Rubber
1. Raw rubber - long-chain molecules
2. Vulcanized/crosslinked rubber - a) Soft rubber (Low Degree of crosslinking); b) Hard rubber, high degree of cross-linking

Among several additives used in SBR today, some of the key classes include:

Fillers

  • Carbon Black – Reinforcing Filler - Carbon black is a colloidal form of carbon obtained by thermal decomposition of hydrocarbons (Soot). It:
    • Increases tensile strength and resistance to abrasion and tearing
    • Provides protection from ultraviolet radiation
    • These enhancements are especially important in tires
  • China Clays (e.g. - hydrous aluminum silicates) - It is less reinforcing than carbon black, however, it is used for non-black rubber applications
  • Calcium carbonate is a non-reinforcing filler and mainly added to reduce cost. Due to large particle size, it does not 'bond' to the polymer in the same way as reinforcing fillers.
  • Silica can serve both reinforcing and non-reinforcing functions. It provides dimensional stability, improved thermal conductivity, and good electrical insulation properties at a low cost.
  • Reclaimed (Recycled) rubber is also added as a filler in some rubber product
  • Fiberglass and steel are also used as reinforcements.
  • Filament reinforcement – It is used to reduce extensibility but retains the other desirable properties. For example, extensively used in tires and conveyor belts

Other additives compounded with rubber include:
  • Antioxidants – to reduce aging by oxidation. 
  • Antidegradants – to provide protection during service. 
  • Coupling Agents - to provide a stable bond
  • Pigments – to develop colored rubber compounds to add appeal to consumer products. Organic pigments give brighter shades of color than inorganic pigments. However former is more sensitive to heat and chemicals & can also fade in long-term sunlight exposure
  • Plasticizers – to reduce hardness with a given level of filler. They also help improve low-temperature flexibility. However, plasticizers can cause problem by leaching out at high temperatures

More key additive types very often used in SBR processing include fatigue- and ozone-protective chemicals; blowing agents in the production of foamed rubber; flame retardants; curatives; processing aids; mold release compounds etc.

After compounding, the shaping of rubber is further done by extrusion, calendaring, coatings, compression molding, injection molding or casting.

The processing of rubbers is quite difficult. Rubber has high viscosity and that is why high shear forces are needed in the processing. Vulcanization poses restrictions too. The processing temperature of rubbers is typically 70-140°C.

Optimizing SBR Properties – Rubber Blending


SBR often blended or copolymerized with other polymers or chemically modified to further enhance its basic properties. The addition of small amounts of suitable rubber may improve properties such as oil or ozone resistance or improve processing behavior.

However, at times there are certain properties which are adversely affected by non-compatible rubber blends such as tensile strength, low-temperature behavior and covulcanizability.

SBR is compatible with NR, BR, EPDM, NBR, and CR.

Major Applications of Styrene Butadiene Rubber


SBR - Widely Used to Manufacture Tires The major use of SBR is in the production of tires, particularly car and lightweight vehicles tires. Today, most of the automotive OEMs are developing vehicles with high performance and durability, resulting in stringent tire performances specifications. Benefits offered by SBR such as low rolling resistance, high traction during braking, low abrasion etc. makes it an ideal material for tire applications.

Cold SBR is used for lighter duty tires and other mechanical goods. SBR is neither suitable for truck tire treads nor for elastic thread for golf ball manufacture.

Solution SBR is generally more expensive than the e-SBR and is used in Specialty applications. s-SBR consists of rubber with different styrene and vinyl levels both of which raise the Tg and hence the wet grip. This is particularly useful in high-performance radial car tires and motorcycle treads.

  » View All Available SBR Grades Suitable for Tires

Other automotive applications where SBR is blended with other elastomers include belts, hoses, seals, and various extruded and molded items.

Non-tire and non-automotive uses of SBR are in industries that require hoses, belts, gaskets, or seals. SBR fulfills the key requirements for these applications like end use and service, trouble-free processability as well as volume cost & availability of raw material.

Uses of Styrene Butadiene Rubber (SBR)

Other applications of SBR include various kinds of solid wheels; roll covers; coated fabrics; and electrical (wire and cable) insulation. The rubber hose is used for lining, insulation or cover which protects from deteriorating. Properties which make SBR suitable material for choice are its abrasion and degradation resistance.

SBR is also widely used in footwear (shoe soles). The main requirements for rubber sole compounds are abrasion and flex resistance, hardness, traction, shape retention etc.

Commercially Available SBR (Styrene Butadiene Rubber) Grades





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