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

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

Auswirkung der Paraffinmigration auf die freie Oberflächenenergie von Naturkautschuk

Die Auswirkung der Migration von Paraffinwachs auf die freie Oberflächenenergie (Oberflächenspannung) von Naturkautschuk (NR) wurde untersucht. Dazu wurde das Wachs in den Kautschuk eingemischt und die Proben anschließend bis zu 168 h bei Raumtemperatur gelagert, bevor die Oberflächenspannung durch Kontaktwinkelmessungen bestimmt wurde. Mit Hilfe der Sekundärionen-Massenspektrometrie wurden”chemische Fingerabdrücke” der Kautschukoberflächen erstellt. Die Oberflächenspannung verringerte sich mit zunehmender Lagerzeit aufgrund der Wanderung des Wachses an die Oberfläche. Die rascheste Abnahme erfolgte innerhalb der ersten 3 h. Danach verringerte sich die Oberflächenspannung sehr viel langsamer und erreichte nach 48-stündiger Lagerung ein Plateau. Durch die Migration des Wachses an die Oberfläche verringerte sich die Oberflächenspannung insgesamt um ca. 46 %. Die Verringerung der Oberflächenspannung beeinflusst nachteilig die Möglichkeit des Kautschuks zur Bindung mit sich selbst oder mit anderen Elastomeren. The effect of the migration of paraffin wax on the surface free energy of natural rubber (NR) was investigated. The rubber was mixed with the wax and then stored at ambient temperature for up to 168 hrs before its surface free energy was measured using contact angle measurement. Static secondary ion mass spectrometry was also used to provide a chemical fingerprint of the rubber surfaces. The surface free energy decreased as a function of storage time because of the migration of the wax to the rubber surface. The highest rate of reduction was recorded up to 3 hrs and thereafter, the surface free energy decreased at a much slower rate, reaching a plateau after 48 hrs in storage. In total, the surface free energy reduced by approximately 46 % as a result of the migration of the wax to the rubber surface. The reduction in surface free energy could adversely affect ability of the rubber to stick to itself and to other dissimilar elastomers

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