Crack growth under dynamic loading in silanised silica filled rubber vulcanisates

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

Developing ethylene-propylene-diene rubber compounds for industrial applications using a sulfur-bearing silanized silica nanofiller

The loading of a sulfur-bearing silanized silica nanofiller in ethylene-propylene-diene rubber with 4.5 wt % of ethylidene norbornene diene content was increased progressively to 60 parts per hundred rubber by weight. The rubber compounds were cured via the tetrasulfane groups of the silane by adding sulfenamide accelerator and zinc oxide. The hardness, tensile strength, elongation at break, stored energy density at break, tear strength, Young’s modulus, M50-M300, compression set, cyclic fatigue life and bound rubber content of the rubber vulcanizates were measured. With the exception of the elongation at break and compression set which deteriorated, the remaining properties improved and the rate of cure, optimum cure time and crosslink density benefitted also when the loading of silica was increased in the rubber. The bound rubber content was unchanged and the cyclic fatigue life of the rubber vulcanizate enhanced considerably when silica was added. Optimizing the chemical bonding between the rubber and filler via the tetrasulfane groups of TESPT reduced the chemical curatives in the rubber. This was a major improvement in health, safety and environment

Measuring effect of the blooming of chemical curatives on the rate of cyclic fatigue crack growth in natural rubber filled with a silanized silica nanofiller

Two rubber compounds with different amounts of chemical curatives were prepared by mixing natural rubber with a high loading of a sulfur-bearing silanized precipitated amorphous white silica nanofiller. The chemical bonding between the filler and rubber was optimized via the tetrasulfane groups of the silane by adding a sulfenamide accelerator and zinc oxide. The rubber compounds were cured and stored at ambient temperature for 65 days before they were tested. One compound showed extensive blooming as a function of storage time. Thin tensile strips of the rubber vulcanizates containing an edge crack were repeatedly stressed at constant strain amplitude and test frequency at ambient temperature and crack length c was measured as a function of the number of cycles n. The cut growth per cycle, dc/dn, was calculated and plotted against the tearing energy, T. The blooming of the chemical curatives increased dc/dn by up to an order of magnitude at a constant T. This was due to the reagglomeration of the chemical curatives in the rubber and also within a thin layer approximately 15 to μm in size beneath the rubber surface. Under repeated stressing, cracks grew through the relatively weak agglomerated areas in the rubber and this caused the rate of crack growth to increase at a constant T

Using a sulfur-bearing silane to improve rubber formulations for potential use in industrial rubber articles

The availability of the coupling agent bis (3-triethoxysilylpropyl)-tetrasulfide (TESPT) has provided an opportunity for enhancing the reinforcing capabilities of precipitated amorphous white silica in rubber. Styrene-butadiene rubber, synthetic polyisoprene rubber (IR), acrylonitrile-butadiene rubber, and natural rubber (NR) containing the same loading of a precipitated silica filler were prepared. The silica surface was pretreated with TESPT, which is a sulfur-bearing bifunctional organosilane to chemically bond silica to the rubber. The rubber compounds were subsequently cured by reacting the tetrasulfane groups of TESPT with double bonds in the rubber chains and the cure was optimized by adding sulfenamide accelerator and zinc oxide. The IR and NR needed more accelerators for curing. Surprisingly, there was no obvious correlation between the internal double bond content and the accelerator requirement for the optimum cure of the rubbers. Using the TESPT pretreated silanized silica was a very efficient method for cross-linking and reinforcing the rubbers. It reduced the use of the chemical curatives significantly while maintaining excellent mechanical properties of the cured rubbers. Moreover, it improved health and safety at work-place, reduced cost, and minimized damage to the environment because less chemical curatives were used. Therefore, TESPT was classified as "green silane" for use in rubber formulations

Two advanced styrene-butadiene/polybutadiene rubber blends filled with a silanized silica nanofiller for potential use in passenger car tire tread compound

Styrene-butadiene rubber (SBR) and poly- butadiene rubber (BR) were mixed together (75:25 by mass) to produce two SBR/BR blends. The blends were re- inforced with a precipitated amorphous white silica nano- filler the surfaces of which were pretreated with bis(3- triethoxysilylpropyl)-tetrasulfide (TESPT). TESPT is a sul- fur-bearing bifunctional organosilane that chemically bonds silica to rubber. The rubbers were primarily cured by using sulfur in TESPT and the cure was optimized by adding non-sulfur donor and sulfur 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 d of the cured blends were measured. The blend which was cured with the non-sulfur 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 d at low tem- peratures to obtain high-skid resistance and ice and wet- grip. Optimizing the chemical bonding between the rubber and filler reduced the amount of the chemical curatives by approximately 58% by weight for passenger car tire tread. This helped to improve health and safety at work and reduce damage to the environment

Marine Bioactive Compounds: Innovative Trends in Food and Medicine

Marine Bioactive Compounds: Innovative Trends in Food and Medicin

Effect of the blooming of chemical curatives on the dynamic behaviour of silanised silica-filled natural rubber-to-metal bonded bobbins

Rubber is viscoelastic in nature and used in a variety of industrial applications. Rubber mounts are used to dampen vibration and shock. Damping, fatigue and dynamic properties of rubber mounts depend to a large extent on chemical ingredients mixed with the rubber. Natural rubber is the most widely used polymer for conventional mounts. Apart from natural rubber, different fillers and rubber chemicals are also present in conventional formulation of rubber mounts. Conventionally, five different classes of chemical curatives are used in rubber industries, which include curing agents, primary and secondary accelerators as well as primary and secondary activators. When chemical curatives are present in excessive amounts in rubber, they migrate to the rubber surface and form a bloomed layer. In this work, two rubber formulations were used for preparing rubber-to-metal bonded bobbin mounts. The formulations were primarily based on natural rubber with 60 parts per hundred rubber by weight (p.h.r.) precipitated amorphous white silica nanofiller. The surface of silica was pre-treated with bis(3-triethoxysilylpropyl)-tetrasulphane (TESPT) coupling agent to chemically bond silica to the rubber. The rubber was cured primarily by reacting the tetrasulphane groups of TESPT with the rubber chains using a sulphenamide accelerator and the cure was then optimised by adding zinc oxide as an activator. The ratio of the accelerator to activator in one compound was 6 p. h.r./0.3. p.h.r. and the compound showed extensive blooming of the accelerator on the rubber surface when stored at ambient temperature for up to 60 days. However, the blooming was reduced significantly by changing the ratio of the accelerator to activator to 3 p.h.r./2.5. p.h.r., which was subsequently used to prepare a second compound. Dynamic and static properties of the bobbins were subsequently measured. Both compounds showed very low phase angle (δ) and spring rate ratio K d /K s (K d : dynamic spring rate; K s : static spring rate). Notably, the compound with the high accelerator to activator ratio had superior aforementioned properties, but the dynamic fatigue life of the bobbin reduced noticeably due to a gradual deterioration of the bond caused by the migration of the accelerator to the bonded interface

Curing rubber compounds efficiently and cost-effectively

Curing rubber compounds efficiently and cost-effectivel