Evaluation of benzaldehyde and benzophenone derivatives as accelerators in NR.

Evaluation of benzaldehyde and benzophenone derivatives as accelerators in NR

Introduction In spite of the fact that some organic compounds used as accelerators are added in small amounts (1-2 phr), they play a very important role in determining the physicomechanical properties of the obtained rubber vulcanizates (refs. 1 and 2). According to Bedford and co-workers (refs. 3-5), aldehydeamino condensation products react with [H.sub.2] S and sulfur to give polysulphides, which are in turn able to react with zinc oxide to form complex zinc salts. Scott (ref. 6) regarded these complexes as the active forms of the accelerators.

In previous work (ref. 7), four pyridazine derivatives were synthesized and proved to be useful as vulcanization accelerators. In the present study, some organic compounds of the type "aldehyde, ketone-amine condensation products," namely benzophenone-aniline; benzophenone; and p-hydroxy benzophenone derivative with phenyl hydrazine; benzaldehyde and its p-methoxy derivatives with phenyl hydrazine, were synthesized and investigated in NR mixes as accelerators.

Materials and techniques

Materials

* Rubber: Natural rubber - ribbed smoked sheets (RSS1).

* Accelerators: Mercaptobenzothiazole (MBT); The investigated compounds are listed in table 1, other rubber ingredients were of grades customarily used in industry.

* Solvents and chemicals: All solvents and chemical reagents were of pure grade and were further purified, if necessary, by the usual techniques. Techniques Synthesis of the investigated compounds:

* Synthesis of aldehyde phenyl hydrazones (ref. 8): Equimolar amounts of the aldehyde and phenyl hydrozine were separately dissolved in methanol. The two solutions were heated to 50 [degrees] C, then mixed with shaking. Five drops of glacial acetic acid were added and the mixture was refluxed for three hours. The precipitated products were recrystallized from methanol. The products were benzaldehyde phenyl hydrazone (BaPH), m.p. 157 [degrees] C, 4-hydroxy benzaldehyde phenyl hydrazone (HBaPH), m.p. 170 [degrees] C, and anisaldehyde phenyl hydrazone (APH) m.p. 120 [degrees] C.

* Synthesis of benzophenone -Anil (BA) (ref. 9): A mixture of benzophenone (1 mole) and aniline (2 moles) were reacted in an oil bath, the temperature was raised to 160 [degrees] C within half an hour. Freshly powdered dry Zn [cl.sub.2] (1-2 gr.) was added, the reaction mixture diluted with ether or benzene and refluxed for 15 minutes. The yellow crystals that separated were crystallized from ethanol, m.p. 117 [degrees] C.

* Synthesis of benzophenone phenyl hydrazones (ref. 8): They were prepared by the same method used for the preparation of aldehyde phenyl hydrazones, using benzophenone instead of benzaldehyde. The reaction products were benzophenone phenyl hydrazone (BAH), m.p. 137 [degrees] C, and 4- hydroxy benzophenone (HBPH), m.p. 145 [degrees] C.

Preparation of rubber mixes and vulcanizates:

* All rubber mixes were prepared on a two-roller mill 470mm. in diameter, working distance 300mm., speed of slow roll 24 rev./min., gear ratio 1.4. The roller temperature was kept at about 50 [degrees] C during mixing. The compounded rubber was left overnight before vulcanization. The vulcanization press was operated at 142 [+ or -] 1 [degree] C under pressure of about 40kg/[cm.sup.2].

The rubber mixes and vulcanizates were tested according to the following standard methods: Monsanto Rheometer-100 for determining [t.sub.s2], [t.sub.c90], ML, MH, CRI (ref. 10); Zwick tensile testing machine for the mechanical properties (ref. 11); and equilibrium swelling (ref. 12).

Results and discussion

The effect of aldehyde phenyl hydrazone as vulcanization accelerators in NR mixes. The title compounds were incorporated into NR mixes in different doses and their efficiency as vulcanizing accelerators were compared with MBT, widely used in rubber goods. Table 2 includes the concentration of the prepared compounds, together with the rheometric characteristics of the rubber mixes. Other ingredients were: NR - 100; stearic acid - 0.5; zinc oxide - 5; processing oil - 3; Epc black - 50; sulfur - 3.

It is clear from the data that the three compounds (BaPH), (HBaPH) and (APH) show good accelerating efficiency which increases as the dose increases. It is also evident that substitution in the para-position of the benzene ring by hydroxy or methoxy group increases the accelerating efficiency of these compounds. But the cure rate index of these compounds is less than that of MBT. The effect of benzophenone-amine condensation products as vulcanization accelerators in NR mixes In the same manner, these compounds were incorporated in NR mixes. Table 3 contains the studied doses together with the rheometric characteristics of the compounded rubber. It is noticed that the investigated compounds show moderate vulcanizing efficiency. Here, again, it is clearly seen that the prepared compounds vulcanize NR with a slower rate than MBT. Physico-mechanical properties The modulus, i.e. the stress at 300% elongation, tensile strength and elongation at break were determined for NR vulcanizates with the investigated compounds. These data are given in table 4.

The obtained data show that the NR vulcanized with BaPH, HBaPH and APH have higher tensile strength than both of MBT and benzophenone phenyl hydrazones. On the other hand, the modulus of NR vulcanized with the investigated compounds are lower than that of MBT vulcanizates. This can be explained on the basis of crosslinking density. This was confirmed by swelling measurements in toluene (table 4). Thus, it may be deduced that the crosslinking density of the prepared compounds' vulcanizates are less than that of MBT. Kinetic constants of vulcanization reaction in presence of aldehyde phenyl hydrazones and benzophonene amine derivatives in NR mixes As the vulcanization reaction, crosslinking can be considered as a first order chemical reaction. The obtained rheometric data can be utilized to calculate the vulcanization kinetic constants (ref. 13) and namely the vulcanization rate constant K [min.sup.-1] and the initiation time [t.sub.i] sec. The calculated kinetic constants for the prepared compounds are given in table 5. These data show that the prepared compounds are clearly different in their activity as accelerators for the vulcanization of NR and can be arranged in the following order: HBaPH [is greater than] BA [is greater than] BaPH, APH [is greater than] HBPH[is greater than] BPH 

Also, the kinetic data are in good agreement with the mechanical ones. It is worthy to notice that the efficiency of the prepared compounds are generally less than that of MBT used now in industry.

The effect of the prepared compounds as secondary accelerators

The prepared compounds (aldehyde phenyl hydrazones and benzophenoneamine condensation products) were used in combination with MBT as a binary accelerator system. The selected optimum doses were incorporated in the basic NR formula mentioned before. The rheometric characteristics of the compounded rubber, and the physico-mechanical properties of the rubber vulcanizates at the optimum cure time are given in table 6.

These data show that the combination of the prepared compounds with MBT (except BaPH) have almost the same cure index. But, on the other hand, the binary accelerator system greatly improves the tensile strength and elongation.

The rubber vulcanizates obtained by the accelerator binary system were subjected to thermal aging at 90 [degrees] C for different periods up to six days. The data represented at the international rubber conference, held in Kyoto 1985, depicts that the prepared compounds behave as good antioxidants comparable with phenyl B-naphthyl amine, the widely used antioxidant. The rubber vulcanizates can retain at least 90% and 85% from the tensile strength and elongation, respectively (ref. 14), while the MBT vulcanizates can retain 40% and 60% from the tensile strength and elongation, respectively. The antioxidant behavior of the prepared compounds can be attributed to their chemical structure; in other words, due to the presence of [Mathematical Expression Omitted] structure containing a labile hydrogen atom and the unshared pair of electrons on both nitrogen atoms (ref. 15).

Summary Three benzaldehyde phenylhydrazone derivatives and three benzophenone-amine derivatives were synthesized and evaluated as vulcanizing accelerators in NR mixes. The compounded rubbers were vulcanized in a hydraulic press at 142 [degrees] C. Rheological characteristics were determined by an oscillating disc rheometer. The kinetic constants for the vulcanization reaction in the presence of the prepared compounds were calculated using the rheometric data.

The prepared compounds showed good accelerating efficiency as compared with mercaptobenzothiazole (MBT), widely used in rubber industry. [Table 1-5 Omitted]

References [1]Morton, Maurice, "Rubber Technology," published by Nostrand Reinhold Company, U.S.A, 1973, p. 37. [2]Piotrofskii and Z.N. Tarasova, "Ageing and Stabilization of Synthetic Rubber and their Vulcanizates," Moscow "Chemistry Publishing House," 1980, pp. 26 and 208. [3]Bedford, C.W., and Scott, W., Ind. Engn. Chem. 12, (1920) p. 31, 13, (1921), p. 125. [4]Bedford, C.W., and Sebrell, L.B., Ind. Engn. Chem, 13, (1921), p. 1034; 14, (1927), p.

15. [5]Bedford, C.W. and Winkelman, Ind. Engn. Chem., 13, (1921) p. 32. [6]Scott, W., India Rubber Journ., 64, (1922), p. 476, p. 605. [7]Yehia, A.A., Doss, N.L., Ismail, M.N., Rubber World, Vol. 197, No. 1, p. 37-39, October 1987. [8]United States Rubber Company, Nech. Appl. 298, 728 (1965). [9]Reddelien, G., Ber., 46, 2720 (1913). [10]A.S.T.M. designation, D 2084 - 76T (1972). [11]A.S.T.M. designation, D 412 - 66T (1967). [12]Shvarts, A; Kautchuk Rezina, 7, 31, (1957). [13]Anand, R., Biack Ley, D.C. and Lees, K.S., "Correlation between Monsanto rheometer torque and concentration of crosslinks," Conference International du Caoutchouc., 2-4 June (1982). [14]Yehia, A.A. and Ismail, M.N., Proceeding of the International Rubber Conference, October, 1985, Kyoto, Japan. [15]Yehia, A.A. and Abdel Bary, E.M. European Polymer Journal, 9, 1417 (1973).

A.A. Yehia, N.L. Doss, National Research Centre, Dokki, Cairo, Egypt, and M.N. Ismail, Military Academy - Heliopolis, Cairo, Egypt