Accelerator activity of TBAB in NR vulcanization.

by V. Purohit, Arup K. Chandra and R. Mukhopadhyay, Harishankar Singhania Elastomer & Tyre Research Institute, and R. Mathur, M.L. Sukhadia University, India

Activity of quaternary ammonium salts in acceleration of two phase organic reactions is well documented (ref. 1). It has been shown that two structural features of these salts are mainly responsible for such acceleration. The first one is a large hydrophobic cationic part of the salt which makes it miscible with organic non-polar phase and second is the ionic structure which imparts miscibility with polar medium. Due to such acceleration effect in two phase systems, these salts are commonly called phase-transfer catalysts.

A survey of literature indicates application of quaternary ammonium salts in vulcanization of acrylic rubbers (ref. 2). Operating on the mechanism exactly identical with the low molecular weight organic substrates, octadecayl trimethyl ammonium bromide is reported as accelerator in crosslinking reactions of these rubbers. Buckler et. al. have reported a terpolymer of emulsion polymerized styrene-butadiene and a monomer containing tertiary amino group, which is subsequently crosslinked by reaction with a low molecular weight compound containing two reactive halogen atoms (ref. 3). This results in the formation of temporary crosslinks containing units of quaternary ammonium salt. The aim of this exercise was to improve green strength of SBR, which is a desirable property for every elastomer. It is reported that these temporary crosslinks have only minor effects on vulcanization and the properties of vulcanizate. Other reports of using quaternary ammonium salt as the vulcanization accelerator for acrylic rubbers, fluoroelastomers, epoxy containing acrylic polymers and as an additive for rubber processing are also available (refs. 4-6). However a literature survey reveals that quaternary ammonium compounds were not used for NR or any other non-polar rubber vulcanization prior to our earlier communication (ref. 7).

In our earlier study we have reported the effect of triethyl benzyl ammonium chloride (TBAB) in NR vulcanization. The quaternary ammonium salt, TEBA alone was not at all active as the accelerator. However, in combination with N-oxybenz thiazyl sulfenamide (NOBS), TEBA acted as a booster accelerator. Vulcanizates obtained were shown to possess better physical properties in comparison to vulcanizates obtained with NOBS . The major disadvantage of using TEBA is its effect to yield scorchy compound.

Since quaternary ammonium salts are obtained by simple nucleophilic substitution reaction of an alkyl/aryl halide with a tertiary amine, variety of quaternary ammonium salts can be obtained by changing either of the two components. Due to such variations salts of different activity towards biphasic systems can be obtained. Thus, one can select a salt of his own convenience for optimum level of desired property. In order to study behavior of variations of these salts in vulcanization of NR, TBAB has been selected for the present study for estimation of its accelerator activity.

Experimental

Materials

TBAB (L.R. Grade) was procured from Sisco-Chem, India. All other chemicals used in the experiment were received from standard Indian sources.

Mixing and curing

Mixing was carried out in a Brabender Plasticorder (PL 2000-3). The masterbatch was prepared at TCU of 90 [degrees] C and rotor speed of 20 rpm for six minutes. The final batch was prepared at TCU of 70 [degress] C for four minutes and sheeted out in a laboratory mixing mill. Cure characteristics were measured using a MDR 2000-E rheometer at 141 [degrees] C. Tensile slabs were cured to the corresponding optimum cure time (Tc90) of the mixes at 141 [degrees] C under 150 kg/[cm.sup.2] gauge pressure using a laboratory curing press (cure time was decided on the basis of Tc90 values of each mix).

Testing

The stress-strain measurements were performed on dumbbells punched out from cured sheets using die no. D following ASTM D412 in a Zwick UTS 1445.

Vr (volume fraction of rubber in the swollen vulcanizate) was taken as a measure of apparent crosslink density. Vr was determined following the procedure provided in our earlier communication (ref. 7). IR spectra were recorded on a Perkin-Elmer FTIR system 2000 with ATR attachment.

Results and discussion

Formulations, cure characteristics and physical properties of the mixes are presented in table 1. Mixes A-D are only parent mixes, without an activator system i.e., zinc oxide-stearic acid combination, while mixes E-H contain accelerator as well as activator.

Table 1-formulations and characterizations of mixes

Ingredients                    A          B          C          D
RMA-4                        100        100        100        100
Sulfur                      2.25       2.25       2.25       2.25
ZnO                         --          --          --        --
St. Acid                    --          --          --        --
NOBS                        --          0.6        0.6       0.6
TBAB                        --          --         1.0       1.0

Properties
Min TQ (lb-in)              0.99       1.17       1.25       1.44
Max TQ (lb-in)              1.53       2.82       3.08       3.08
Scorch time, Ts2 (min)       --    15.63(1)   35.31(1)    2.18(1)
Optimum cure time,
Tc90(min)                 106.91      69.34      48.42      12.72
100% modulus (MPa)           0.4        0.4        0.4        0.5
300% modulus (MPa)           0.8        0.8        0.9        1.3
Tensile strength (MPa)       3.5        8.5        4.6        8.2
Elongation at break %       88.5       1172        891        891
Energy to break (J/mm2)   913696    2139741    1111263    1863589
Hardness (Sh-A)               23         20         19         21
Tear (N/mm)                 11.9       14.1       12.1       19.5
Vr.                       0.0982      0.096    0.09986     0.1242

Ingredients                     E          F          G          H
RMA-4                         100        100        100        100
Sulfur                       2.25       2.25       2.25       2.25
ZnO                           5.0        5.0        5.0        5.0
St. Acid                      2.0        2.0        2.0        2.0
NOBS                          --         0.60       --          --
TBAB                          --          --        1.0        1.0

Properties
Min TQ (lb-in)               1.06       1.07       1.27       1.22
Max TQ (lb-in)               2.17       6.01       5.47       6.88
Scorch time, Ts2 (min)   99.33(1)      11.37      17.56       3.43
Optimum cure time,
Tc90(min)                   99.29      19.37      52.03      15.69
100% modulus (MPa             0.4        1.0        1.1        1.4
300% modulus (MPa)            0.8        2.7        3.0        4.5
Tensile strength (MPa)        4.2       21.2       20.6       24.5
Elongation at break %         861        867        779    4329286
Energy to break (J/mm2)    964646    4415997    3761845         44
Hardness (Sh-A)                19         35         35       42.0
Tear (N/mm)                  12.4       38.1       33.7     0.2295
Vr.                        0.0844     0.2027     0.2048

The cure characteristics and physical properties for mixes A-D reveal that TBAB possesses accelerator activity comparable with NOBS.

Rheographs of mixes E-H are presented in figure 1. With the incorporation of TBAB in the control compound containing accelerator -- activator (mix E and G), the Tc90 value has reduced substantially. This shows that TBAB not only functions as a booster accelerator (as reported for TEBA), but it acts as primary accelerator. This is further confirmed by the values of Vr, i.e., apparent crosslink density (table 1).

[Figure 1 ILLUSTRATION OMITTED]

Synergistic effect of TBAB with NOBS is evident from the optimum cure time (Tc90) of mix H. All physical properties of the vulcanizate also improved considerably with a corresponding increase in Vr value. However, it can be observed that process safety time has reduced significantly.

It is evident from the cure characteristics of mixes A-D that the process safety of TBAB is less only in a presence of NOBS. For mixes C and G, containing only TBAB without NOBS, scorch safety is considerably high.

Stress-strain curves for mixes E-H are presented in figure 2. Although percent elongation at break reduces for the mixes containing TBAB, other physical properties viz. modulus, tensile strength, tear strength and hardness are either comparable or marginally higher for these mixes (table I and figure 2).

[Figure 2 ILLUSTRATION OMITTED]

Infra-red spectra of Vulcanizates E-H, presented in figure 3 reveal that in vulcanizates G and H there are clearly defined sharp bands in the region 1,400-1,460 cm- 1, whereas for vulcanizates E and F shows only one little band in the above mentioned region. Since this region accounts for absorption of carboxylate ion, it is to be concluded that stearate ion is free in mixes G and H, while it is not so in mixes E and F (the only source of carboxylate ion is stearic acid, hence it must be stearate ion).

[Figure 3 ILLUSTRATION OMITTED]

In our earlier communication (ref. 8) we have reported that bulky triethylbenzyl cation forms a tight and stable ion pair with stearate anion, due to which the free stearate ion is undetectable in the IR spectra of the vulcanizate. Further, results clearly highlighted that TEBA acts only as a secondary accelerator and not as primary. Since both of these observations are quite different for TEBA and TBAB, it can be concluded that the presence of a free stearate ion is an important parameter for determining accelerator activity of quaternary ammonium salts. This is in conformity to the general conception about quaternary ammonium salts that they should form a loose ion-pair for an effective role in catalyzing organic reactions (ref. 8).

Conclusion

Quaternary ammonium salts can form a new class of accelerator for NR vulcanization.

References

[1.] E.V. Dehmlow and S.S. Dehmlow, "Phase transfer catalysis" Verlag Chemie, Basel 1983.

[2.] E. Giannetti, R. Mazzocchi, L. Fiore and E. Crespi, Rubber Chemistry and Technology, 56 (1983) 21.

[3.] E.J. Buckler, G.J. Briggs, J.R. Dunn, E. Lasis and Y.K Wei, Rubber Chemistry and Technology, 51 (1979) 872.

[4.] Chemical Abstracts 258106 k (1991).

[5.] Chemical Abstracts 138074 b (1991).

[6.] Chemical Abstracts 182683 f (1993).

[7.] B. Ellis and G.N. Welding, Techniques of Polymer Science, Society of Chemical Industries. London. 1964, p. 46.

[8.] R. Mathur, A. Biswas, A. K. Chandra and R. Miakhopadhyay, Rubber World, 213, No. 5 (1996) 34.

[9.] J. Dock, Synthesis, 441 (1973: