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The curing retardation and mechanism of high temperature vulcanizing silicone rubber filled with superconductive carbon blacks.


Conductive polymer composites have a wide range of applications because of its electric resistivity variation with temperature (1-3), such as self-regulated or self-protection devices, electromagnetic interference shielding plastic, electrostatic charge dissipation materials, flexible force or conductometric chemical sensors, and environmentally sensitive membranes (3-7). For many industrial applications, conductive carbon blacks (CBs) are chosen as fillers for conductive composites because of their low cost, low density, high electric conductivity, and reinforcement action (8), (9). It is generally accepted that the reinforcement action of CBs comes from the interaction between the polymer matrix and CBs. Thus, the functional groups on the CB surface play an important role in the physical properties of CB conductive composites. Besides, the interactions between the polymer matrix and CBs also have a strong influence on the conductive network in the composite (9). Hence, the functional groups on the CB surface also have an impact on the electric properties of the conductive composites. The functional groups on the CB surface are generally carboxyl, hydroxyl, lactone, and carbonyl groups (8), (10). Their influences on physical properties and the cure kinetics of rubber/CBs have been widely studied. When CBs were modified by basic or nonpolar chemical solutions, an increase in the surface functional groups of the CBs was found, and the properties of styrene-butadiene rubber (SBR)/CBs composites were significantly improved, such as cure behaviors, tensile stress, and dynamic mechanical properties (11-13). Bandyopadhyay (14) studied the influences of surface oxidation of CBs on the interactions between nitrile rubbers and CBs and claimed that the oxidation of the CB surface increased the extent of the rubber-filler bonding, as the existence of hydrogen bonding and Van der Waal's forces between the CBs and the polar matrix. Yang et al. (8) stated that CB remarkably inhibited the polymerization of vinyl monomer in the preparation of the toner, which is used in electrophotography. However, few studies have been done on the influences of surface functional groups of CBs on the curing of conductive composites.

In our previous works on high temperature vulcanizing silicone rubber (HTV SR) (2), (15-18), several kinds of CBs with different physical properties were used as conductive fillers, BP2000 is one of them. As a kind of superconductive CB, BP2000 would endow composites with high conductivity. However, it was found that BP2000 would retard the curing of silicone rubber (SR). For example, when its loading was high, SR was difficult to shape even excess curing agents were used. BP2000 has high structure, small particle size, and high specific surface area. There are large amounts of functional groups on its surface, which have a great influence on the formation of conductive network. In addition, these functional groups also have a great influence on the reinforcement of SR. The curing of rubber is the key process in rubber manufacture. Therefore, in-depth understanding of chemical reactions in the curing process of HTV SR filled with BP2000 is essential to get conductive SR with excellent physical and electrical properties.

Matrix, fillers, and curing method are thought to be the deciding factors in the curing of conductive composite. In this article, three kinds of polymer matrices, three kinds of CBs, and two curing methods were used to discuss the effect of BP2000 on curing of conductive composites. The peroxide curing reactions were also studied by using density functional theory (DFT) at the MP2/6-31+G(d,p)//B3LYP/6-31G(d) levels.



Methyl vinyl SR gum (Mn = 5.8 X [10.sup.5], vinyl% = 0.16%), 107# SR gum (Mn = 0.2 X [10.sup.5]), and nature rubber (NR) were used as the matrices of conductive composites, obtained from Chenguang, Xinanjiang, and Xihuang Chemical Co. Ltd. (China), respectively. BP2000 and EC-300J were purchased from Cabot Co., USA. BP2000 was modified by mixing with 1.0 wt% of hexadecyltrimethyl ammonium bromide (CTAB) in ethanol media, ultrasonicated for 60 min at 80[degrees]C, and then filtrated and dried at 100[degrees]C for 24 h. BP2000, BP2000/CTAB, and EC-300J were used as fillers. The typical analytical properties of CBs were listed in Table 1.
TABLE 1. The typical properties of CBs.

             Particle size    Nitrogen       Dibutyl    Surface OH
                 (nm)       surface area    phthalate  (/[nm.sup.2])
                            ([m.sup.2]/g)  absorption
                                           (mL/100 g)

BP2000             17           1403            330         2.26

CTAB/BP2000        18           1344            330         1.57

EC-300J            16            876            380         2.43


CBs and methyl vinyl SR gum were mixed with a two-roll mill and then dicumyl peroxide (DCP) was added as a curing agent. The mixture was vulcanized at 165[degrees]C for 20 min under 9.8 MPa (19-20). The vulcanizate samples were postcured at 180[degrees]C for 3 h. The mixture of NR and CBs was vulcanized at 165[degrees]C for 20 min under 9.8 MPa. The 107# SR gum and CBs were mixed by a stirring device. After standing for 2 h at 110[degrees]C, tetraethyl orthosilicate (TEOS) and dibutyl tin dilaurate were added, and the mixture was then molded at 25[degrees]C for 2 min under 9.8 MPa. The vulcanizate samples were postcured at 80[degrees]C for 3 h. All the vulcanizate samples were kept standing before testing [24 h for HTV SR and NR, 7d for room temperature vulcanizing silicone rubber (RTV SR) at room temperature]. The formulae of composites were listed in Table 2.
TABLE 2. The formulae of conductive rubbers (wt parts).

Sample No.  Rubber gum         CB         DCP  TEOS  Dibutyl tin

1-1          HTV(100)   BP2000 (20)        2    --        --
1-2          HTV(100)   BP2000/CTAB (20)   2    --        --
1-3          HTV(100)   EC-300J (20)       2    --        --
2            RTV(100)   BP2000 (20)       --     4         1
3-1          NR(100)    BP2000 (20)        2    --        --
3-2          NR(100)    BP2000/CTAB (20)   2    --        --
3-3          NR(100)    EC-300J (20)       2    --        --


Surface properties of CBs were analyzed by Fourier Transform Infrared (FTIR, Perkin-Elmer System 2000) and X-ray photoelectron spectroscopy (XPS, ESCA LAB 250 with Al K Alpha radiation used as an X-ray source). The HTV SR gum and the uncured sample of the HTV SR/BP2000 composite (the same weight content of HTV SR gum with the former) were dissolved in toluene, respectively. The former was dissolved totally and a mixture liquid was obtained; the upper clear liquid of the latter was extracted after the solution was centrifugated. FTIR spectrums were measured on the two liquids to provide structural information of related HTV SR gum. Curing dynamics were measured on a Moving Die Rheometer (MDR) 2000 (ALPHA technologies Corporation) under 165[degrees]C at the frequency of 100 cpm.

The physical properties of the vulcanizates were measured as described in the literature (19-20). The tensile testing of the samples was performed according to ASTM D 412 method and ISO 527 with dumb-bell shaped test specimens at a crosshead speed of 500 mm/min on a Universal Testing Machine (100CX, Lloyd Instruments Ltd., UK) at room temperature. The tearing testing was performed in the same condition but right-angled test specimens (QB/T 1130-1991). These tests provided the ultimate tensile strength (UTS), elongation at break, and tearing strength. Hardness (Shore A) was measured according to ASTM D 2240 method on a Shore A durometer (Laizhou Huayin Research Instruments Co., China).

Quantum Chemistry Calculation Methods

The peroxide curing mechanisms of SR and NR model molecules were studied by DFT at the MP2/6-31+G(d,p)//B3LYP/6-31G(d) levels (21-24). The structures of reactants, transition states (TSs), and products were optimized directly at the B3LYP/6-31G(d) levels. The frequency calculations were performed at the same level. Only one imaginary frequency for each TS was found. UB3LYP was used for the radical species. High-level energies were obtained using MP2/6-31+G(d,p) single-point calculations on the optimized structures. PMP2 energy was adopted for the radical species. The TS for H abstraction was verified by the vibration modes of imaginary frequency. The paths for addition reaction were examined by intrinsic reaction coordinate (IRC) calculation at the B3LYP/6-31G(d) levels. The Gaussian 03 series of programs were used in all calculations (25).


DCP is a universal curing agent for HTV SR. Generally, 1-3 phr of DCP is enough for HTV SR filled with [SiO.sub.2] (26). Thus, the DCP dosage for the rubber/BP2000 composites was fixed at 2 phr in experiments. According to our previous studies on HTV SR/BP2000 composites, the loading of 20 phr is already over the percolation region, and the moderate loading endows the composites with good and stable conductivity (17), (27). So, the loading of BP2000 in this article was fixed at 20 phr. The curing curve of HTV SR/BP2000 composite (Curve A) at 165[degrees]C is shown in Fig. la. There is no increase in the rheometer torque with time prolonging, indicating that curing reaction does not occur. In other words, the curing retardation of HTV SR/BP2000 composite is obvious.


Figure 2 shows the FTIR results of HTV SR gum (B) abstracted from the uncured sample of HTV SR/BP2000 composite by toluene. When compared with the original HTV SR gum (A), there is hardly any change in the typical peaks of the vinyl group between the two samples, indicating that the initiators definitely did not initiate the curing reaction of the HTV SR/BP2000 composite.


It is well known that curing is a chemical process in which rubber molecules are linked to each other by atomic bridges. HTV SR gum is methyl vinyl SR, which has a backbone of silicone-oxygen linkages with methyl and vinyl substitutions on the polymer chain. Does the special molecular structure of methyl vinyl SR lead to the curing retardation? To investigate the rubber matrix effect on the curing retardation, NR/BP2000 composites were compounded by using the same method as SR/BP2000 ones. Figure lb Curve A shows the curing process of NR/BP2000 composite at 165[degrees]C. The rheometer torque increases with time prolonging normally, showing that there is no curing retardation in NR/BP2000 composite. It seems that whether the curing reaction of rubber/BP2000 composites goes on normally or not depends on the polymer matrix of composites.

Thus, it is necessary to consider the difference in curing mechanisms between methyl vinyl SR and NR gums before understanding the retardation. Quantum chemistry calculation is a convenient and credible method to assist experimental study when reaction mechanism is investigated. So, the curing reaction of methyl vinyl SR and NR were studied theoretically. It is well known that vinyl groups mainly take part in the curing reaction in the peroxide curing of methyl vinyl SR, and pentamethylvinyldisiloxane was commonly used as the model of methyl vinyl SR in calculations. Meanwhile, 2-methyl-2-pentene was chosen as the model of NR (28). The addition reaction between C = C and the isopropylbenzene oxygen radical is considered to represent the curing reaction of methyl vinyl SR (26), (29). The studies on curing reaction of NR revealed that once heated DCP would dissociate to two isopropylbenzene oxygen radicals in neutral medium, and RO radical could not react with the C = C of polyisoprene. After the cleavage of DCP, the H abstraction of the isopropylbenzene oxygen radical from polyisoprene occurs as Scheme 1 (30-32). Although the H abstraction may occur at a, b, or C(c) atom, the reaction rates are actually in the order of a < b < c. This is due to the super conjunction effect of the radical (~[C.sup.b][H.sub.2] ([C.sup.a][H.sub.3])--C = C--[C.sup.c]H*~) with the methyl group. Considering the structural similarities between the repeated unit of polyisoprene and 2-methyl-2-pentene, the H-abstraction reaction of isopropylbenzene oxygen radical from C(c) atom of 2-methyl-2-pentene ([C.sup.b][H.sub.3]([C.sup.a][H.sub.3])C = [CC.sup.c][H.sub.2][CH.sub.3]) is considered to represent the curing reactions of NR gum. The curing mechanisms of pentamethylvinyldisiloxane and 2-methyl-2-pentene are shown in Scheme 2.

PhC[(CH.sub.3).sub.2]O * + ~[C.sup.b][H.sub.2] ([C.sup.a][H.sub.3]) C = [CC.sup.c][H.sub.2]~ ? PhC[(CH.sub.3).sub.2]OH + ~[C.sup.b][H.sub.2] ([C.sup.a][H.sub.3]) C = [CC.sup.c]H * ~


Bis-radical termination reactions are usually barrierless. Reactions I(a) and II(a) in Scheme 2 are, therefore, thought to be the rate-determining reactions of 2-methyl-2-pentene and pentamethylvinyldisiloxane, respectively. The B3LYP/6-31G(d) geometries of the main stationary points are shown in Fig. 3, along with the main vibrational modes of the imaginary frequencies in the transition states. As for 2-methyl-2-pentene, the vibration model of the imaginary frenquency of transition state TS1 corresponds to the moving of an H atom between 2-methyl-2-pentene (la) and isopropylbenzene oxygen radical (FR). Obviously, TS1 is responsible for the H abstraction by the isopropylbenzene oxygen radical from 2-methyl-2-pentene. As for pentamethylvinyldisiloxane, an intermediate complex (M2) between the isopropylbenzene oxygen radical and the pentamethylvinyldisiloxane (2a) forms first when isopropylbenzene oxygen radical approaches to pentamethylvinyldisiloxane and then the addition reaction occurs through TS2. The path of M2 to 2b was verified by IRC calculation, Fig. 4. The energy barriers for the two reactions are shown in Fig. 5. As it is shown, the barrier of the addition reaction of pentamethylvinyldisiloxane with the isopropylbenzene oxygen radical is only 1.879 kcal/mol, being 0.452 kcal/mol lower than that of the H-abstraction reaction of isopropylbenzene oxygen radical from 2-methyl-2-pentene. These results mean that the model molecule of methyl vinyl SR does not have a higher reaction barrier of peroxide curing reaction compared with that of NR.




Considering the little difference on curing reaction barriers between SR and NR model molecules, the curing rate of SR gum may not be much lower than that of NR gum. The curing curves of unfilled SR and NR at 165[degrees]C are also shown in Fig. 1 (D in (a) and B in (b), respectively). It can be seen that the [T.sub.90] of unfilled SR is much shorter than that of unfilled NR. This is consistent with the deduction made from the calculational results. In other words, the curing retardation of HTV SR/BP2000 composite should not be attributed to the curing reaction dynamics of SR molecules.

To investigate the effect of fillers on the curing retardation of HTV SR/BP2000 composite, BP2000 modified by CTAB (BP2000/CTAB) and another superconductive CB EC-300J, which has similar physical properties with BP2000, were used as conductive fillers. The curing curves of conductive SR filled with the two kinds of CBs were shown in Fig. la. When compared with HTV SR/ BP2000 composite (Curve A), HTV SR/BP2000/CTAB composite (Curve B) behaves with a higher curing degree, whereas HTV SR/EC-300J composite (Curve C) has the highest curing degree among the three samples. The retardation neither shows up in HTV SR/BP2000/CTAB composite nor in the HTV SR/EC-300J composite. The difference performed in the three curing curves strongly suggests that CBs play an important role in the curing reaction of HTV SR/CBs composites. Because the three kinds of CBs have similar physical properties (Table 1), the surface functional groups were thought to be related to this difference. So, FTIR and XPS analyses of the CB surfaces were carried out, Fig. 6.


Figure 6a is the FTIR spectra of CBs. The water absorbed by CBs and KBr leads to a broad peak of OH group at 3320-3500 [cm.sup.-1], which may lap over other OH peaks, such as those from phenol, carboxylic acid, or alcohol. The strong absorption peak at 1630 [cm.sup.-1] should also be attributed to the absorbed water (33). In Curve A of Fig. 6a, at 1224 [cm.sup.-1] exists the typical stretching vibration peak of the phenol-OH group, indicating the presence of phenol-OH on the surface of BP2000 (34). When compared with BP2000 (Curve A), no absorption peaks of phenol-OH appear in BP2000/CTAB (Curve B) and EC-300J (Curve C). Figure 6 also shows the expanded scale of the [C.sub.1S] XPS spectra of BP2000 (b), BP2000/CTAB (c), and EC-300J (d). The [C.sub.1S] spectra of BP2000 reveals the presence of six peaks, corresponding to C-C groups (284.6 eV), C-OH groups (284.9 eV), C-O groups (285.9 eV), C = O groups (287.6 eV), O-C = O groups (288.11 eV), and COOH groups (289.6eV) (13). Curve B shows that when BP2000 was modified by CTAB, OH, C = O and COOH groups failed to be detected by spectra. This is because the positive charge groups of CTAB [[CH.sub.3][([CH.sub.2]).sub.15] [([CH.sub.3]).sub.3][N.sup.+]] induce a physical absorption, especially with the -OH and -COOH groups on the surface of CBs (35). As a result, OH and COOH groups were shielded by short chains of CTAB molecules. As for EC-300J, there are only C-OH and C-O groups in its XPS spectra, combining with the FTIR analysis, it can be concluded that the OH group at 3320-3500 [cm.sup.-1] of the FTIR spectra of EC-300J should be attributed to alcohol-OH groups.

In Fig. 6, the amount of phenol-OH on BP2000 is the highest among the three groups. Does the phenol-OH make a major influence on the peroxide curing of HTV SR/BP2000 composites? To get a better understanding of the question, phenol equal to 20 phr of BP2000 with OH groups was added to the HTV SR and NR gums, and the resulting composites were cured under the same condition with conductive composite samples. The results are shown in Fig. la also. Compared with the blank sample of HTV SR (D), the sample with phenol (E) had a much lower rheometer torque. This means that phenols do retard the curing of HTV SR, just like the cases of HTV SR/BP2000 composite (A). This is because that both phenol-OH groups on BP2000 surface and in phenol work as radical scavengers of initiator radicals (36), (37). However, the rheometer torques of NR with the same amount of phenol (Fig. lb, Curve C) are slightly lower than that of the blank one (Curve B). Phenol-OH has a slight effect on the peroxide curing of NR in comparison with SR. However, the reason is not the study focus of this article.

Based on the aforementioned results, it is reasonable to conclude that among the functional groups on BP2000 surface the phenol-OH group works mostly on the peroxide curing of HTV SR/BP2000 composites. If the amount of the phenol-OH groups on the CB surface is up to a certain extent, the effect of radical scavenging of phenol-OH groups in the peroxide curing reaction would result in curing retardation of HTV SR/CBs composites.

If the aforementioned analysis is reasonable, the curing reaction of SR should go on well in the nonperoxide curing. Thus samples of condensation-cure SR filled with 20 phr of BP2000 were compounded, followed by the condensation cure between the gum and TEOS at room temperature (26). Table 3 shows the comparison of the physical properties between RTV SR/BP2000 and HTV SR/BP2000 composites. Obviously, the former always presented better properties than the latter in the tensile strength, elongation at break, tearing strength, hardness, and permanent deformation. This is because perfect crosslink networks are formed in the RTV SR/BP2000 composite, meaning that there is no retarding curing phenomenon to occur in the RTV SR/BP2000 composite. The factors which effect the peroxide curing of HTV SR/BP2000 composites does not work in the condensation mechanism. This result further confirms our conclusion that BP2000 retards the curing of HTV SR because the phenol-OH, COOH, and C = O groups, especially the phenol-OH groups, on its surface consume the initiator radicals during the peroxide curing.
TABLE 3. Physical properties comparison between RTV SR/BP2000 composite
and HTV SR/BP2000 composite filled with 20 phr BP2000.

                Tensile  Elongation  Tearing   Hardness    Permanent
               Strength   at break   Strength  (Shore A)  deformation
                 (MPa)       (%)      (kN/m)                   (%)

HTV SR/BP2000    3.72        612       23.32       36           36

RTV SR/BP2000    8.76       1540       28.75       39            6


The effects of rubber matrix on the curing retardation of conductive composites filled with superconductive CBs BP2000 have been studied theoretically and experimentally. The results reveal that the curing retardation of HTV SR/BP2000 composite should not be attributed to the curing dynamics of SR molecules. In addition, the studies on the effects of surface functional groups of CBs on curing retardation show that the phenol-OH groups existing on BP2000 surface have a primary influence on the peroxide curing of HTV SR/BP2000 composites. The effect of radical scavenging of phenol-OH groups is the main reason for the curing retardation.

If vulcanization method involving nonradical reaction is adopted, the curing retardation of SR filled with BP2000 would be avoided. This result further confirmed the conclusion that the effect of radical scavenging of phenol-OH groups on BP2000 surface results in the curing retardation.


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Correspondence to: Jie Zhang; e-mail:

Contract grant sponsor: The Postdoctoral Scientific Research Item of Shandong Province; contract grant number: 200603086; contract grant sponsor: the Key Natural Science Foundation of Shandong Province of China; contract grant number: ZR2009BZ006; contract grant sponsor: the National Natural Science Foundation of China; contract grant numbers: 20574043, 20874057; contract grant sponsor: the Key Natural Science Foundation of Shandong Province of China; contract grant number: Z2007B02.

Published online in Wiley Online Library (

[C] 2010 Society of Plastics Engineers

Yuanyuan Zhang, (1) Minglei Pang, (2) Qiang Xu, (1) Haifeng Lu, (1) Jie Zhang, (1) Shengyu Feng (1)

(1) Key Laboratory of Special Functional Aggregated Materials, Ministry of Education; School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People's Republic of China

(2) Institute 53 of China North Industries Group Corporation, Jinan 250100, People's Republic of China

DOI 10.1002/pen.21793
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Author:Zhang, Yuanyuan; Pang, Minglei; Xu, Qiang; Lu, Haifeng; Zhang, Jie; Feng, Shengyu
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jan 1, 2011
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