10.1002/app.20681
Synthesis and characterization of acrylic rubber/silica hybrid composites prepared by sol-gel technique
Acrylic rubber (ACM, Nipol AR51, density at 25degC = 1100 kg/cm3, Mooney viscosity, ML1+4 at 100degC = 51) was obtained from Nippon Zeon Co. Ltd. (Tokyo, Japan). It was reported to have epoxy cure site and was made from ethyl acrylate monomer. Tetraethoxysilane (TEOS, density = 930 kg/m3) was procured from Acros Organics (USA). Tetrahydrofuran (THF, 99% pure) was purchased from Merck (India). The precipitated silica (Ultrasil VN3, particle size range = 40-100 nm, oil absorption = 2.4 g/kg, pH = 6) was supplied by Bayer AG (Germany). Benzoyl peroxide (BPO, 97% purity) was purchased from Aldrich Chemicals (USA). Hexamethylenediamine carbamate (HMDC, DIAK#1) was supplied by Nicco Corp. Ltd. (India). Ammonium benzoate (AmBz) was prepared in the laboratory by reacting ammonium hydroxide and benzoic acid in 1:1 molar ratio in a water bath at around 60degC for 30 min. The salt formation was confirmed by Fourier transform infrared spectroscopy as well as by studying its solubility. Deionized water and concentrated hydrochloric acid of laboratory grade were obtained from indigenous sources. 
The desired amount of ACM was dissolved in THF solvent. A proportion of the rubber to solvent was maintained at about 1 : 10 all throughout the experiment to retain the uniform viscosity of the reaction medium. TEOS, deionized water, and concentrated HCl as catalyst, in the molar ratio of 1 : 2 : 0.06,15 were throughly mixed by vigorous stirring for 15 min and then the mixture was added to the rubber solution under stirring conditions at ambient temperature. The proportion of TEOS was from 0 to 50 wt % of the ACM. Beyond 50 wt % TEOS, macrophase separation occurred. The formulations used in this study are given in Table I. The precursor solution for preparing the composites was stirred for 30 min and then poured over a uniform and thoroughly cleaned glass plate. The initial evaporation of the solvent was carried out under controlled conditions for 24 h, and then, in the next phase, further evaporation for 4 days was allowed to remove the residual solvent and byproducts (water and ethanol). The optimum gelling time was taken when practically no weight variation of the hybrid composites was noticed. All the films were transparent in appearance. For comparison, composites were also prepared with precipitated silica up to 30 wt % of its loading (beyond which the resultant film lost its homogeneity). The precipitated silica was initially dried at 120degC for 24 h and then dispersed in the ACM solution. All the composite films with precipitated silica were opaque and completely white in color. To crosslink the rubber phase, curatives were added to the solvated rubber. The two different curatives systems used in this study were BPO and a mixed crosslinked system comprising AmBz and HMDC, following an earlier work on acrylic rubber in our laboratory.16 Optimization of the doses of different curatives (Table I) was carried out by studying the maximum gel content in THF. 
Addition of curatives was made only after complete mixing of TEOS, water, and HCl with solvated ACM for 30 min. The crosslinkers were dispersed under ambient conditions and then stirred for another 30 min for homogenous mixing. The films were cast over a plain glass plate as before and kept for controlled solvent evaporation for 24 h. In the next phase, peroxide-containing composite films were kept in the oven at 70degC for 2 h for curing, whereas the samples having mixed crosslinked system were cured at 170degC for 30 min. The above cure times were optimized from the maximum gel content values.
Acrylic rubber
Mooney viscosity
ML1+4 at 100degC = 51
ML1+4 at 100degC
Acrylic rubber (ACM
density
1100 kg/cm3
kg/cm3
Tetraethoxysilane (TEOS
density
930 kg/m3
kg/m3
Tetrahydrofuran (THF
pure
99%
%
precipitated silica (Ultrasil VN3
particle size range
40-100 nm
nm
precipitated silica (Ultrasil VN3
oil absorption
2.4 g/kg
g/kg
precipitated silica (Ultrasil VN3
pH
6
Benzoyl peroxide (BPO
purity
97%
%
ammonium hydroxide and benzoic acid
molar ratio
1:1
ammonium hydroxide and benzoic acid
reacting
60degC
degC
ammonium hydroxide and benzoic acid
reacting
30 min
min
rubber to solvent
proportion
1 : 10
TEOS, deionized water, and concentrated HCl as catalyst
molar ratio
1 : 2 : 0.06
TEOS, deionized water, and concentrated HCl as catalyst
vigorous stirring
15 min
min
ACM
proportion of TEOS
0 to 50 wt %
wt %
TEOS
Beyond 50 wt %
wt %
precursor solution
stirred
30 min
min
solvent
evaporation
24 h
h
solvent
evaporation
4 days
days
precipitated silica
loading
up to 30 wt %
wt %
precipitated silica
dried
120degC
degC
precipitated silica
dried
24 h
h
TEOS, water, and HCl
complete mixing
30 min
min
crosslinkers
stirred
30 min
min
films
controlled solvent evaporation
24 h
h
peroxide-containing composite films
kept
70degC
degC
peroxide-containing composite films
kept
2 h
h
samples
cured
170degC
degC
samples
cured
30 min
min