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Influence of dumping and vulcanisation temperature on silica filled ENR compound properties.

INTRODUCTION

The alteration of natural rubber (NR) vulcanizates during service at elevated temperatures or under anaerobic conditions, i. e. long cure times at high temperatures, continues to be a major problem in the tire industry. Reversion is the anaerobic aging of a polysulfidic rubber network which takes place not only at high vulcanizing temperatures and during long vulcanization times, but also in use if the vulcanizate is subject to dynamic stress. Reversion reduces the crosslink density of the vulcanizate which causes deterioration in the mechanical and dynamic vulcanizate properties. Hence the requirements in terms of heat and reversion resistance of truck tire compounds are also becoming more demanding. The thermal degradation of polysulphidic crosslink is generally referred to as reversion. Reversion leads to changes in crosslink types and crosslink density along with the main chain modifications. The reversion of Natural Rubber is dependent on the nature of the curing system, choice of accelerator temperature and time [1]. He also concluded that a reversion process in an accelerated sulphur vulcanisation is associated with the formation of a trans-methane structure which is generated by the main chain modification through desulphuration process. In the present study, the effect of curing temperature on silica filled ENR compound was determined via rheometer curve and Fourier Transform Infrared (FTIR) spectroscopic.

Experimental and sample preparation:

ENR 25 having 25 mol% of epoxidation was used as the base elastomer. The filler used was Silica: Zeosil[R] 1165 MP with 165 (m2/g) BET surface area. Other compound ingredients such as zinc oxide, stearic acid, sulphur, calcium stearate and poly-2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) were of commercial grades and used without purification. The compound formulations are listed in Table 1. The compounds mixing were carried out in internal mixer, Banbury 1600. The mixing of the compound was carried out in three stages. Two types of dumping temperature was used at a first stage namely, 150[degrees]C and 180[degrees]C. All the ingredients except the curatives were added in the first and second stage mixing. The third mixing was carried using two roll mill for addition of curatives. In the first stage of mixing, the starting temperature and fill factor was 90[degrees]C and 0.7 respectively, for all compounds. Later the curing characteristics were assessed by Mosanto Rheometer at various temperatures curing according to ASTM D2084-95 (1994). The effect of thermal ageing on ENR was analysed using Thermo Scientific FTIR Nicolet 6700 for FTIR analysis. FTIR spectra was recorded with a resolution of 4 cm-1 and the wavenumber range of 4000-400 cm-1. Quantitative analyses of ENR were made by using the methyl C-H deformation at 1373cm-1 as internal standard. The effect of thermal ageing on ENR was analysed using Fourier Transform Infrared (FTIR) spectroscopic. On the other hand, the swelling test or crosslink density is done to measure the total number chemical link between polymer chain per unit volume of each compound according to ASTM D 471-97(1998).

RESULTS AND DISCUSSIONS

Figure 1 and 2 shows the rheographs of the silica filled ENR compounds. It was found that reversion resistance decreases with vulcanization temperature for both compounds. Indeed greater reversion was observed with compound dumped at 180[degrees]C as compared to 150[degrees]C figure 3. Poor reversion properties of compounds dumped at high temperature which believed due to the thermal and mechanical degradation. It is thought that the natural rubber molecules can easily be broken down by excessively high temperatures and shearing action because it has reactive double bonds in every repeating unit along the rubber chain. Hence under excessive mixing conditions, the NR chains are broken down, leading to lower physical linkages in the compound. This phenomenon corresponds well with decreased of crosslink density at high dumping temperature Figure 4.

Results showed that that the network structure varied with dumping and vulcanization temperature. This observation is associated with the gradual decomposition of di and polysulphidic cross link and also ether network. Such polysulphidics bonds are easily cleavage during a prolonged heat/oxygen and high temperature ageing environment. Higher reversion above 160[degrees]C may be attributed to the additional thermal breakdown from monosulphidic crosslinks and ether crosslinks which are formed by a ring opening reaction of the epoxide group in ENR [2]. Changes in the chemical structure of the silica filled ENR 25 upon thermal oxidation can be seen from the ATR-FTIR spectrum in Figure 5. In general peaks at 2960, 2924 and 2854 [cm.sup.-1] are assigned to C-H stretching vibrations. The absorption band at 1636 [cm.sup.-1] is characteristic of C=C stretching. The C-O-C stretching vibration of epoxy groups that should be at 1252 [cm.sup.-1] disappeared after addition of silica, suggesting all epoxy groups has been used up to react with silanol groups which produce chemical filler-to-rubber bonds in the silica-filled system. A new shoulder peak appeared at 3416 [cm.sup.-1], which is ascribed to -OH stretching of C-OH which is the result of the reaction of silanol groups and ENR. Moreover, there was a new peak at 1716 [cm.sup.-1] which could be assigned to -C=O groups, which were introduced by some of the C-OH being oxidised because there is no -C=O groups in silica and raw ENR. It has also been observed that with increasing moulding time the intensity of the peak at 875[cm.sup.-1] which is due to the epoxy ring vibration decreases.

Indeed, the oxidation competitively occurs on the carbon-carbon double bonds and the epoxy groups in ENR raw rubber. Figure 6 shows that at the temperature below than 100[degrees]C, there is no obvious destruction on carbon-carbon double bonds (840cm-1 and 1662cm-1). However, as the temperature increases at 140[degrees]C, there is significant destruction on the carbon-carbon double bonds at 840cm-1. This could be due to the labile [alpha]-H adjacent to carbon-carbon double bonds being converted into radicals under the effects of heat and oxygen, and thus, the carbon-carbon double bonds are firstly destructed. After 140[degrees]C, the numbers of carbon-carbon double bonds are less than that of epoxy groups [3]. Therefore, the destruction starts to occur on epoxy groups with decrease in the absorbance as shown in Figure 5.

It is also possible that the low crosslink density of the silica-filled compound, can be explained by adsorption of TBBS (cure accelerator) on the silica surface Figure 6. Since silica has lots of hydroxyl groups which are acid, hydrogen bonds between silica and polar materials, especially basic organics, formed with ease. TBBS has an amine group and a sulfur linkage which can hydrogen bond with hydroxyl groups of silica with ease. Due to the adsorption, the availability of accelerator will be decreased. As for that, pH is another factor in determining the cure characteristic of a filled ENR compounds [4].

It was also speculated that the situation becomes worse when the adsorption on the silica tends to accelerate the dissociation of the N-S bond of CBS and to be available for any crosslinking reaction to take place as shown in figure 7 [5]. He also discovered that, for a silica filled compound without coupling agent, reversion increased with TBBS accelerator content [6].

Conclusions:

Reversion increases with an increasing of temperature curing. A new peak is discovered at a region of 1720cm which could be assigned to c=o due to the oxidation at high temperature. The process would believe to be accelerated due to the adsorption of acidic silica on the basic accelerator available which contributed to low cross linking density.

ARTICLE INFO

Article history:

Received 18 February 1014

Received in revised form 15 May 1014

Accepted 6 June 1014

Available online 10 June 1014

REFERENCES

[1] Chen, C.H., J.L. Koenig, J.R. Shelton and E.A. Collins, 1981. Rubb. Chem. Tech, 54: 734.

[2] Poh, B.T., C.P. Kwok and G.H. Lim, 1995. European Polym. J. 31: 223.

[3] Yu, H., Z. Zeng, Lu, Guang, Q. Wang, 2008.Processing Characteristics and Thermal Stabilities of Gel and Sol of Epoxidized Natural Rubber. Eur. Polym. J., 44: 453-464.

[4] Gelling, I.R., Epoxidised Natural Rubber, J. Nat. Rubb. Res., 6(3): 184.

[5] Sung-Seen Choi, Byung Ho Park and Hanjong Song, 2004. Influence of filler type and content on properties of styrene-butadiene rubber (SBR) compound reinforced with carbon black or silica, Polymers for Advanced Technologies., 15: 122.

[6] Sung-Seen Choi, Changwoon Nah and Byung-Wook Jo, 2003. Properties of natural rubber composites reinforced with silica or carbon black: influence of cure accelerator content and filler dispersion, Polym. Int 1382-1389.

(1) Mazlina Mustafa Kamal, (2) Rohani Abu Bakar, (3) Muhammad Zahid Zakaria

(1) Malaysian Rubber Board, Technology Centre, RRIM Research Station Sg. Buloh, 47000 Selangor, Malaysia.

(2) Malaysian Rubber Board, Technology Centre, RRIM Research Station Sg. Buloh, 47000 Selangor, Malaysia.

(3) University of Technology MARA (Perlis) 01600, Arau, Perlis Malaysia.

Corresponding Author: Mazlina Mustafa Kamal, Malaysian Rubber Board, Technology Centre, RRIM Research Station Sg. Buloh, 47000 Selangor, Malaysia.

E-mail: mazlina@lgm.gov.my

Table 1: Formulation of silica ENR compounds.

Material                             (phr)

Epoxidised Natural Rubber (ENR 25)   100
Silica 1165/Carbon Black N234        75
Carbon Black, N234                   5
Naphtenic Nytex 840                  16
Zinc Oxide                           3
Stearic Acid                         3
Antioxidant, 6PPD                    1
Anti oxidant TMQ                     1
Calcium Stearate                     2
Sulphur                              1.8
TBBS                                 2.1
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Author:Kamal, Mazlina Mustafa; Bakar, Rohani Abu; Zakaria, Muhammad Zahid
Publication:Advances in Environmental Biology
Article Type:Report
Date:Jun 5, 2014
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