Styrene Butadiene Rubber: How to select the right grade?

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

 

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

Monomeric units of Styrene (L) and Butadiene (R)

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

How is SBR manufactured?

The SBR manufacturing method was first developed in Germany in the 1930s by IG Farben's Walter Bock and Eduard Tschunkur. They polymerized a synthetic rubber called Buna-S from butadiene and styrene in an aqueous emulsion.

 

Molecular structure of SBR

Then the first solution polymerized random SBR grades were produced commercially by Firestone and Phillips during the 1960s.

Types of SBR

Based on the polymerization, styrene butadiene rubbers can be classified as: emulsion or solution.

Emulsion SBR (e-SBR)

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

1. Hot emulsion SBR process - SBR grades have exception processing characteristics such as:
   • low mill shrinkage,
   • good dimensional stability, and
   • good extrusion characteristics

2. Cold SBR process - SBR grades have better abrasion resistance. Consequently, they provide better tread wear and dynamic properties.

The hot emulsion process leads to a more branched polymer than the cold process. Whereas the cold process exhibits superior tensile strength than the hot process.

 

Let's look at the features of emulsion SBR:

• 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). This takes place in a hydrocarbon solvent, usually hexane or cyclohexane.

 

Benefits of solution SBR include:

• Improved flexibility and performance
• Good resilience, tensile strength, and low rolling resistance when used in tires
• Outstanding resistance to abrasion and fatigue
• Low resistance to oil, other hydrocarbon fluids, and ozone

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

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


 

SBR has improved strength, abrasion resistance, and blend compatibility compared to polybutadiene rubber. Most of the properties of SBR are comparable with NR. But in some respect 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

Also, scorch problems are less likely to occur with SBR than with NR. But the addition of resins, additives, and reinforcing fillers adequately improves these properties. 

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

• Wide availability
• Better processability
• Perfect impact strength
• High tensile strength
• Slightly better heat aging
• Better abrasion resistance and resistance to degradation (under heat)
• Low cost compared with those of all other synthetic rubbers
• Ability to accept high filler levels
• Relatively stable price compared with that of NR
• Properties on a cost/performance basis

How to optimize SBR material properties?

SBR is often blended or copolymerized with other polymers or chemically modified to enhance its properties. The addition of small amounts of suitable rubber may improve: 
 

• oil or ozone resistance or
• improve processing behavior

However, other properties are adversely affected by non-compatible rubber blends. These include tensile strength, low-temperature behavior, and covulcanizability. 

Check out SBR grades that are compatible with the polymers available in our Master Catalog. Get access to technical datasheets and request samples with ease.
 

• NR (Natural Rubber)
• BR (Butadiene Rubber - Polybutadiene)
• EPDM (Ethylene Propylene Diene Rubbers)
• NBR (Nitrile Butadiene Rubber)
• CR (Chlorinated Rubber)

Thermoplastics or thermosets are supplied in pellets or liquid resin form. However, 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 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. This makes 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 natural rubbers. 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. This reduces 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
 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

Use of reinforcing or non-reinforcing additives

Among several additives used in SBR today, some of the key additives are discussed below.

 

Fillers

• Carbon Black (Reinforcing filler): Carbon black is a colloidal form of carbon. It is obtained by thermal decomposition of hydrocarbons (Soot). Benefits of carbon black include:
  • Increases tensile strength and resistance to abrasion and tearing. Explore how to improve tear strength.
  • Provides protection from ultraviolet radiation
  • Important for 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 is mainly added to reduce cost. Due to the 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

• Antioxidants to reduce aging by oxidation.
• Anti-degradants 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, the former is more sensitive to heat and chemicals. It 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 problems by leaching out at high temperatures

Other additive types are very often used in SBR processing including: 
 

• 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 a 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.