B.T. Poh

Effect of silane coupling agents on the mooney scorch time of silica-filled natural rubber compound

Abstract Mooney scorch time and cure index of silica-filled natural rubber ( Hevea Brasiliensis type) compound with and without the presence of silane coupling agents were studied in the temperature range 110–160°C using a Mooney Shearing Disk Viscometer. Two types of silane coupling agents, viz γ -mercaptopropyltrimethoxysilane (A-189) and bis(triethoxysilylpropyl)tetrasulphide (Si-69) of various loadings were used to study the effect of silane coupling agents on the scorch property of natural rubber. Results indicate that the Mooney scorch time and cure index decreases with increasing A-189, but increases with increasing Si-69. This observation is attributed to the catalytic effect of the thiol group in A-189 which enhances the rate of vulcanization, whereas in the case of Si-69, cure retardation is associated with the steric hindrance effect of its bulky end groups. The apparent activation energy for various loadings of silane coupling agents was determined and discussed.

FTIR study of thermal oxidation of ENR

Abstract Thermal oxidation of two grades of purified epoxidized natural rubber (ENR 25 and ENR 50) and one grade of unmodified natural rubber (purified SMR L grade) were studied using Fourier Transform Infrared Spectroscopy (FTIR). Changes in the characteristic absorption band, particularly the hydroxyl and carbonyl peaks of the degraded rubbers are carefully monitored. Results indicate that thermal oxidation occurs in the double bond region for SMR L to form oxidative products which mainly consist of hydroperoxide, hydroxyl and carbonyl compounds. In the case of ENR, thermal oxidation occurs not only in the double bond region, but also involves epoxide ring-opening reaction to yield oxidative products having similar functional groups as that of SMR L. For the three rubbers studied, increasing temperature of thermal oxidation increases the rate of oxidation. From a time-dependent study, it suggests that thermal oxidation of ENR is an autocatalytic process. No attempt, however, is made to ascertain the exact chemical structures of the oxidative products formed in this study.

Effect of Coumarone-Indene Resin on Adhesion Property of SMR 20-Based Pressure-Sensitive Adhesives

The viscosity, tack, and peel strength of a natural rubber (SMR 20)–based pressure-sensitive adhesive (PSA) was studied using coumarone-indene resin as the tackifier. The resin loading was varied from 0–80 parts per hundred parts of rubber (phr). Toluene was used as the solvent throughout the experiment. The viscosity of PSA was measured using a Haake Rotary Viscometer whereas loop tack and peel strength were determined using a Lloyd Adhesion Tester. PSA was coated onto the substrates using a SHEEN hand coater to give a coating thickness of 60 μm and 120 μm. Results show that the viscosity and tack of the adhesive increases with resin content due to the concentration effect of tackifier resin. However, for the peel strength, it increases up to 40 phr of resin for both coating thickness, an observation that is attributed to the wettability of substrates.

The effect of γ-mercaptopropyltrimethoxysilane coupling agent on t90, tensile strength and tear strength of silica-filled ENR vulcanisates

Abstract The cure times (t90), tensile strengths and tear strengths of silica-filled epoxidised natural rubber (ENR), with or without the presence of γ-mercaptopropyltrimethoxysilane (A-189) and bis(triethoxysilylpropyl) tetrasulphide (Si-69) and cure using a semi-efficient vulcanisation (semi-EV system) were studied. Three grades of ENR having 10, 25 and 50 mole % of epoxidation were used. Mixing was done using a Brabender Plasti-Corder attached with a mixing can. A semi-EV system was found to be better than either a conventional vulcanisation (CV) or an efficient vulcanisation (EV) systems when ENR is mixed with silica, with or without the addition of silane, 2% A-189 was found to be the optimum amount of coupling agent for wetting out the silica. The t90 of all three ENR grades without the presence of A-189 show cure retardation. When 2% A-189 was added, cure enhancement occurs in all three ENR grades. All three grades of ENR show improvements in the tensile strengths and tear strengths.

The effect of accelerator/sulphur ratio on the scorch time of epoxidized natural rubber

Abstract The Mooney scorch time of epoxidized natural rubber (ENR) was studied with natural rubber (SMR L) as a reference. The effect of sulphur to accelerator ratio on scorch behaviour in the presence of the more common types of accelerators was investigated. Mooney scorch time was studied by using the Mooney Shearing Disk Viscometer in the temperature range 100–180 °C. The results indicate that Mooney scorch time decreases exponentially with increasing temperature for ENR and SMR L. At a fixed temperature, ENR with 25 mol % epoxidation shows shorter scorch time compared with SMR L. This has been attributed to the activation of the double bond by the oxirane group. The investigation involving different types of accelerators shows that Mooney scorch time decreases in the order; N-tert-butyl-2-benzothiazylsulphenamide (TBSS) > tetramethylthiuram disulphide (TMTD) > diphenylguanidine (DPG) > 2-mercaptobenzothiazole (MBT) > zinc dimethyldithiocarbamate (ZDMC). This accelerator dependence becomes less significant as temperature increases. Studies of the effect of varying amounts of MBT at a fixed sulphur concentration show that scorch time decreases as the accelerator concentration increases. Thus, scorch time was observed to decrease in the order; conventional vulcanization system > semi-efficient vulcanization system > efficient vulcanization system for MBT accelerated compounds. However, no such predictable patterns were shown by the other types of accelerators. It was also found that the influence of accelerator/sulfur ratio becomes less significant as vulcanization temperature increases.

Reversion behaviour of epoxidized natural rubber

Abstract The reversion behaviour of two grades of epoxidized natural rubber (ENR 25 and 50) was investigated in the temperature range from 140 to 180 °C in an accelerated sulphur vulcanization system. A Monsanto Oscillating-Disk Rheometer was used to determine the reversion property of the rubber compound. Results indicate that the time-dependent reversion behaviour of ENR is similar to that of unmodified natural rubber. Below 160 °C, reversion of ENR is associated with the decomposition of di and polysulphidic crosslinks, whereas above 160 °C, additional breakdown of mono- and ether crosslinks accounts for a higher reversion observed in an ENR network. ENR 25 exhibits higher reversion than ENR 50 indicating a higher density of polysulphidic crosslinks in the former. Effects of accelerators and fillers on the reversion behaviour of ENR were also studied and discussed.

Effect of γ-mercaptopropyltrimethoxysilane coupling agent on t90, tensile strength and tear strength of silica-filled NR, NBR and SBR vulcanizates

Abstract A study has been made of the cure times (t90), the tensile strengths and the tear strengths of silica-filled NR, NBR and SBR, with and without the presence of γ-mercaptopropyltrimethoxysilane, (MPTS) and cured using the semi-efficient vulcanization system (semi-EV). Mixing was done using a Brabender Plasti-corder having a mixing cam attached and a two-roll mill, under approximately similar conditions. The t90 of the Brabender mixed silica-filled NBR and NR, in the absence of MPTS display a cure retardation phenomena, while those of SBR exhibit a cure enhancement phenomenon. When MPTS is present, all Brabender mixed rubbers display a cure retardation phenomenon which can be attributed to a direct involvement of the MPTS in the sulphur vulcanization. Generally, the tensile strengths and the tear strengths of the Brabender mixed NBR and SBR are greater than those of the two-roll mill, whereas NR seems unaffected by the mixing methods. Addition of MPTS gives rise to marked improvements in the physical properties of NBR and SBR. However, the physical properties of NR are only slightly affected by the incorporation of MPTS.

Dependence of viscosity of polystyrene solutions on molecular weight and concentration

Abstract Viscosities of solutions of polystyrene in toluene were measured for concentrations up to 400 kg m−3 at 298 K. Polymers of molecular weights ranging from 8.7 × 103 to 2.4 × 106 were used. It is observed that viscosity of the polymer solution increases with increasing concentration and molecular weight; the rate of increase is greater at higher values of the two parameters. A master curve for the system is constructed by using the experimental data for viscosity, concentration and molecular weight of the polymer. Regions of various polymer interactions in solution are identified.

Cure index and activation energy of ENR 25 compared with SMR L in various vulcanization systems

Abstract Cure index of standard Malaysian rubber (SMR L) and epoxidized natural rubber (ENR 25) was determined in the temperature range from 100 to 180°C using a Monsanto Automatic Mooney Viscometer MV2000. Five accelerators, viz. 2-mercaptobenzothiazole (MBT), tetramethylthiuram disulphide (TMTD), zinc dimethyldithiocarbamate (ZDMC), N - tert -butyl-2-benzothiazyl sulphenamide (TBBS) and diphenylguanidine (DPG) were used in the study and the vulcanization systems used were conventional vulcanization (CV), semi-efficient vulcanization (semi-EV) and efficient vulcanization (EV). Results indicate that the cure index and apparent activation energy for SMR L is greater than that of ENR 25 and it decreases exponentially with increasing temperature. The lower values of the cure index and apparent activation energy in ENR 25 is attributed to the activation of a double bond by the adjacent epoxide group in ENR 25. The cure index also depends on the accelerator and vulcanization systems. Highly substituted accelerators (e.g. TBBS) tend to give a higher cure index and apparent activation energy which suggests that steric hindrance plays an important role in accelerated sulphur vulcanization of rubbers.

Effect of filler loading on the mechanical properties of epoxidized natural rubber (ENR 25) compared with natural rubber (SMR L)

Abstract The mechanical properties viz., tensile strength, elongation at break, tear strength and fatigue life of epoxidized natural rubber (ENR 25) was studied using a semi-efficient vulcanization system. Carbon black and calcium carbonate were chosen as the fillers with loading up to 60 phr. SMR L grade natural rubber was used for comparison purposes. Tensile property, tear strength and fatigue life were determined by using a Monsanto Tensometer T10, Instron Universal Tensile Tester and Fatigue To Failure Tester respectively. Results indicate that the tensile property increases with filler loading up to a maximum value after which it drops with further loading of filler. This observation is attributed to the dilution effect of filler after the maximum level of reinforcement. However, the fatigue life decreases steadily with increasing filler content, a finding which is associated with the chain immobility with increasing filler concentration. Carbon black and calcium carbonate show varying effects on th...