Crack growth under dynamic loading in silanised silica filled rubber vulcanisates
2011-11-23T10:05:23Z (GMT) by
Rubbers are widely used to manufacture industrial articles such as tyres, conveyor belts, hoses, and engine mounts. During flexing, these articles fail in service due to initiation and subsequent growth of cracks leading to catastrophic failure. The failure is due to either environmental ageing by ozone and oxygen or mechanical failure due to crack initiation and growth. The unexpected failure in service is due to mechanical crack growth and may cause danger to life and property. Therefore, rubber articles are designed for long durability and low fatigue damage. To achieve these requirements, reinforcing fillers such as colloidal carbon black and synthetic silica are added to raw rubbers. In recent years, silica has been replacing carbon black in many industrial rubber articles. Some studies have investigated crack growth behaviour in unfilled and carbon black filled rubbers. But limited data is available for crack growth behaviour in silica-filled rubber vulcanisates and their effects on the durability and service life of industrial rubber articles has remained uncertain. When partly soluble chemical curatives are mixed with raw rubber, they migrate to the rubber surface, which can be detrimental to the rubber properties. Two rubber compounds with different amounts of curatives were prepared by mixing natural rubber with a high loading of precipitated amorphous white silica nanofiller. The silica surfaces were pretreated with bis(3-triethoxysilylpropyl)-tetrasulphane (TESPT) coupling agent to chemically adhere silica to the rubber. The chemical bonding between the filler and rubber was optimised via the tetrasulphane groups of TESPT by adding accelerators and activators. The rubber compounds were cured and stored at ambient temperature for up to 65 days before they were tested. One compound showed extensive blooming as a function of storage time. The cyclic fatigue life of the rubber vulcanisates was subsequently measured at a constant strain amplitude and test frequency at ambient temperature using standard dumbbell test pieces. The crack length, c, was also measured as a function of the number of cycles, N, at a constant strain amplitude ranging from 15% to 40% using tensile strip test pieces and the crack growth rate, dc/dn, was then calculated. The rate was subsequently plotted against the tearing energy, T, to determine correlation between the two. In storage, the chemical curatives migrated to the rubber surface and formed bloom. Blooming of the chemical curatives had detrimental effects on the cyclic fatigue life, crack growth rate and internal structure of the rubber. Blooming reduced the cyclic fatigue life of the rubber vulcanisate by more than 100%. The migrated chemical curatives produced thin layers approximately 15-20 µm in size beneath the rubber surface. When the rubber was stressed repeatedly, cracks initiated in these layers and subsequently grew, causing the cyclic fatigue life of the vulcanisate to decrease. At a given value of the tearing energy, the rate of crack growth also increased due to the re-agglomeration of the chemical curatives within the rubber which produced regions of low resistance to crack development. There was evidence that migration of the chemical curatives to the rubber surface had significantly damaged the internal structure of the rubber, creating voids and cracks which weakened the rubber mechanically. Styrene-butadiene rubber (SBR) and polybutadiene rubber (BR) were mixed together (75:25 by mass) to produce two SBR/BR blends. The blends were reinforced with a precipitated amorphous white silica nanofiller the surfaces of which were pre-treated with TESPT. The rubbers were primarily cured by using sulphur in TESPT and the cure was optimised by adding non-sulphur donor and sulphur donor accelerators and zinc oxide. The hardness, Young s modulus, modulus at different strain amplitudes, tensile strength, elongation at break, stored energy density at break, tear strength, cyclic fatigue life, heat build-up, abrasion resistance, glass transition temperature, bound rubber, and tan δ of the cured blends were measured. The blend which was cured with the non-sulphur donor accelerator and zinc oxide had superior tensile strength, elongation at break, stored energy density at break and modulus at different strain amplitudes. It also possessed a lower heat build-up, a higher abrasion resistance and a higher tan δ at low temperatures to obtain high-skid resistance and ice and wet-grip. Optimising the chemical bonding between the rubber and filler reduced the amount of the chemical curatives by approximately 58% by weight for passenger car tyre tread. This helped to improve health and safety at work and reduce damage to the environment.