There are many processes for the preparation of precipitated silica, such as water-soluble silicate precipitation and the reaction of sodium silicate solution with sulfuric acid. Silane and silanol chemical functional groups are found on the surface of silica, and silanol or hydroxyl groups are acidic. In addition, because the surface of the filler is polar and hydrophilic, it has strong moisture absorption. When silica is used in tires and even any rubber products, the unique properties mentioned above bring numerous problems. For example, when a large amount of silica is added, the viscosity of the compound increases greatly, which has a negative impact on the processing performance of the compound, and also causes excessive wear and tear of the processing equipment. Relevant studies have shown that the increase of viscosity of NR compound may be attributed to the strong interaction between silica and silica. In addition, acidic silica delayed the sulfur curing reaction rate, prolonged the curing time and reduced the crosslinking density of sulfur curing system. In order to promote the wider application of silica in rubber products, these processing problems must be eliminated. Bifunctional organic silane is designed for sulfur curing system. The addition of these materials into the formulation can help to reduce the interaction between silica and silica, reduce the viscosity of the compound, and improve the vulcanization performance by preventing acid silica from interfering with the reaction mechanism of sulfur-yellow vulcanization system. Bis (3-triethoxysilylpropyl) tetrasulfide (tespt), also known as Si69 coupling agent, is a silane suitable for sulfur curing system. The purpose of this study was to investigate the effect of increasing tespt silane content on the Mooney viscosity and vulcanization properties of some silica NR compounds with high filling content.

Five common accelerators/sulfur vulcanizates were tested with raw materials. These compounds contain 60 copies of precipitated silica Ultrasil VN3, 0, 3, 5, 7 and 10 copies of TESPT, respectively. Before use, silica was parked for 8 weeks at 18 C and 76% relative humidity. The moisture content was 5.7%, the pH value was 5.9, and the average particle size was about 19 nm. Silane is sprayed on glass containers at ambient temperature (23 C) before silica is added to the rubber. The rubber used is L-grade standard Malaysian NR. In addition to raw rubber and fillers, some complexes were added.

2. Shaw Mark 4 K1 internal mixer with counter-rotating meshing rotor is used for mixing. In this study, the temperature of rotor and mixing chamber was maintained at 40 C by water cooling system. ChronosRichardson SOW 1000 software was used to control mixing conditions and store data. The volume of mixing chamber is 5.5L, and the filling factor of preparing test compound is 0.5. The mixing conditions of each compound and the relationship between temperature and time in the mixing process.

3 Rubber Properties

The Mooney viscosity of rubber compound at 100 ([ML(1+4)]) was measured by a single-speed rotating Mooney viscometer. According to the following formula, the coke burning time TS2 (the time required to raise the torque to two units above the minimum torque) and the positive curing time T95 (the time to reach 95% of the maximum torque) are calculated. In the Mn=0.95(MH-ML)+ML formula, Mn is the new torque reading corresponding to the 95% vulcanization of the compound determined by the vulcanization curve, while the vulcanization curve is obtained by the oscillating disc vulcanizer at (140+2), +3 degree swing angle and 1.7Hz. The vulcanization rate index is proportional to the average slope of the vulcanization rate in the steep zone of the vulcanization curve. The formula 100/(t95-ts2) is used to calculate the vulcanization rate.

The results of Menny viscosity measurements and vulcanization tests.

4. Test results

4.1 The relationship between Mooney viscosity and silane content. When the amount of silane increased to 7 phr, the viscosity of the compound decreased from 234 to 121 Mooney units. Interestingly, when the amount of silane was further increased to 10 phr, it had little effect on the viscosity, which remained basically about 120 Mooney units.

4.2 The relationship between the scorch time TS2 and the sulfuration time T95 and the amount of silane.

When three parts of silane were added to the compound, the scorching time of the compound decreased from 34 minutes to 7.5 minutes. However, when the amount of silane was gradually increased to 10 phr, the scorching time was significantly prolonged by 7 min to 14.4 min.

The addition and gradual increase of silane content had different effects on the curing time. When the amount of silane increased to 7 phr, the curing time decreased from 178 min to 82 min. It is noteworthy that the amount of silane is further increased to 10 phr, and the curing time is prolonged by 13 min to 95 min.

4.3 The addition of silane and the increase of the curing rate have similar effects on the curing rate of the compound. The vulcanization rate of rubber without silane was the slowest, and the vulcanization rate index was 0.69min-1. 

However, when the silane content increased to 7 phr, the vulcanization rate index increased from 0.69 min-1 to 1.43 min-1. It is noteworthy that the vulcanization rate of the compound will be slowed down slightly from 1.43 min-1 to 1.24 min-1 when the silane content is increased by 3 phr to 10 phr.

5. The results of this study showed that when TESPT was added to the NR compound containing 60 phr precipitation silica Ultrasil VN3, the viscosity and vulcanization properties of the compound were significantly affected. The Mooney viscosity decreases when 7 parts of silane are added to 60 parts of silica rubber, the scorching time shortens when 3 parts of silane are added to 60 parts of silica rubber, and the curing time shortens when 7 parts of silane are added to 60 parts of silica rubber, and the curing speed accelerates when 7 parts of silane are added to 60 parts of silica rubber. The main advantages mentioned above can be obtained by adding only 7 preferred silanes to 60 silica compounds, and there is no benefit to increase the amount of silane.