Green strength is of great weight when it comes to preventing rupture during the second stage of tyre production or when it comes to preventing the collapse of a complex extruded profile due to gravitational forces.
1. Influence of molecular weight
Generally speaking, the higher the molecular weight of the elastomer chosen, the higher the Green's strength. In the case of SBR, a high average molecular weight is used, but too high a molecular weight can lead to other processing problems.
2. Strain-induced crystallisation
Adhesives with strain-induced crystallisation tend to have a high Green's strength.
3. Natural rubber
Natural rubber has a high Green's strength. NR has a high Green's strength due to the fact that it crystallises when stretched. Natural glues with a higher content of fatty acid ester groups have a higher Green's strength due to a greater degree of crystallisation in tension, generally with a minimum content of fatty acid ester groups of about 2.8 mmol/kg.
4. Block polymers
The presence of small amounts of block styrene in random copolymer SBR adhesives can give the adhesive a good Green's strength.
5. Semi-crystalline EPDM
The choice of semi-crystalline EPDM with high ethylene content can give the adhesive a good Green's strength at room temperature.
6. Metallocene-catalyzed EPDM
The single active centre limited geometry metallocene catalyst technology enables the production of high ethylene content EPDM on a large scale. This EPDM with high ethylene content has a high Green's strength. With this technology the ethylene content can be regulated and the Green's strength of EPDM can be further increased.
7. Molecular weight distribution
NBR compounds with a narrow molecular weight distribution have a high Green's strength.
8. CR
High Green's strength can be obtained by choosing fast crystallising neoprene. The addition of SBR with high styrene content to CR can improve the Green's strength.
Among the various types of neoprene, Type T neoprene has the best resistance to collapse and deformation, i.e. the highest Green's strength, followed by Type W. Type G neoprene has the worst Green's strength.
9. Polytetrafluoroethylene
Teflon additives improve the Green's strength of the adhesive.
10. Carbon black
Carbon black with a high surface area and high structure improves the Green's strength of the rubber. n326 is often used in tyre wire coverings because it gives the rubber a high Green's strength while keeping the viscosity low enough for the wire to penetrate.
For a good Green's strength, a carbon black with a high structure and a low specific surface area should be used. This is because a low specific area carbon black allows for a higher filling volume, which in turn increases the Green's strength.
11. Mixing
In the mixing process, if the elastomer is over-plasticised, the Green's strength of the compound will be reduced.
