STUDIES ON SELF-VULCANIZING FLUOROELASTOMER/PHENOL HYDROXY SILICONE RUBBER BLENDS *

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1 Chinese Journal of Polymer Science Vol. 27, No. 3, (2009), Chinese Journal of Polymer Science 2009 World Scientific STUDIES ON SELF-VULCANIZING FLUOROELASTOMER/PHENOL HYDROXY SILICONE RUBBER BLENDS * Ya-ming Wang, Lan Liu **, Yuan-fang Luo and De-min Jia College of materials science and Engineering, South China University of Technology, Guangzhou , China Abstract Self-vulcanizing blends of phenol hydroxy silicone rubber (PHSR) and fluoroelastomer (FPM) were prepared. Vulcanized rubbers with lower glass transition temperature (T g ) were successfully obtained. The results of dynamic mechanical analysis (DMA) show that the vulcanized FPM/PHSR (10 phr) blend has only one T g temperature, demonstrating the well compatibility between FPM and PHSR. The thermogravimetric analysis (TGA) demonstrates that the PHSR do little damage to the thermal stability of FPM. The vulcanization characteristics of the FPM/PHSR blends were analyzed by using oscillating disc rheometer (ODR). The results show that FPM/PHSR blends have smaller S min values and longer scorch time than that of FPM with the same level of bisphenol AF curing agent. It means that FPM/PHSR blends have better processability and curing security. Better mechanical properties can be gained for FPM/PHSR blends at appropriate level of PHSR. Keywords: Fluoroelastomer; Phenol hydroxy silicone rubber; Glass transition temperature; Self-vulcanizing. INTRODUCTION Fluoroelastomers (FPM) are widely used in many industrial applications due to their excellent heat, oil and solvent resistance properties. The use of such polymers in automobile, aerospace, offshore, and energy-related industries implies their according with the stringent product performance standards under elevated temperatures and in hostile chemical environments [1]. Recently, their use in cold districts is especially increasing. However, one of the disadvantages of fluoroelastomers is their lack of low temperature resistance [2], and the conservation of elasticity is necessary for elastomer materials, even in low-temperature environments [3]. The elastomeric materials with lower glass-transition temperatures tend to have more excellent lowtemperature characteristics. Blending of the fluoroelastomer with rubbers having low glass transition temperature (T g ) might enhance its low temperature properties. However, there were few literatures on this aspect in recent years [1, 4]. The reason was that the specific properties of fluoroelastomer, such as hightemperature resistance, solvent and oil resistance, etc. were reduced after blending. Enhancing the lowtemperature resistance of fluoroelastomer without damage of its specific characteristics is the key for fluoroelastomer blending modification. Silicone rubber is a special material that excels in low temperature flexibility, high temperature stability, chemical resistance, weather-ability, electrical performance and sealing capability [5]. Materials with better overall properties may be prepared by blending silicone rubber into fluoroelastomer. However, it has been reported that the blends of fluoroelastomer/silicone rubber are thermodynamically immiscible [2, 6]. In this paper, phenol hydroxy silicone rubber (PHSR) was prepared and used as a curing agent of the fluoroelastomer. Thus the molecular chains of fluoroelastomer and PHSR were connected through chemical bonds, and FPM and PHSR were compelled to be miscible. It is expected the fluoroelastomer vulcanized by PHSR may have better low-temperature resistance. * This work was financially supported by the National Natural Science Foundation of China (No ). ** Corresponding author: Lan Liu ( 刘岚 ), E-mali: psliulan@scut.edu.cn Received January 24, 2008; Revised May 20, 2008; Accepted May 21, 2008

2 382 Y.M. Wang et al. EXPERIMENTAL Materials Details of the materials used are given in Table 1. The other materials were purchased from the chemistry market. The PHSR was made from methyl ethylene silicon rubber (110-2) in author s laboratory [7]. The M n of the silicon rubber is about and the value of M w /M n is about The glass transition temperature of methyl ethylene silicon rubber and PHSR is about 109 C and 98 C, respectively. Materials Copolymer of vinylidene fluoride (VF2) and hexafluoropylene (HFP) Table 1. Details of the materials used Abbreviated Specification names/symbols Fluoroelastomer, FPM-2603 Specific gravity , about 66% F, Mooney viscosity, ML (1 + 10) at 121 C: 80 Source Chenguang Chemical Co. Ltd. Sichuan, China Bisphenol AF Industrial grade Dupont Dow, USA Benzyltriphenylphosphonium chloride BPP Industrial grade Dupont Dow, USA Preparation of Rubber Blends Rubber blends were prepared in an φ open two-roll mill. The rotors operated at a speed ratio of 1 (front):1.22 (back). The materials were charged as follows: fluoroelastomer, Ca(OH) 2, MgO, PHSR, ultrafine silicone dioxide and BPP. The formulations of the blends were: fluoroelastomer 100 g, Ca(OH) 2 6 g, high activity MgO 3 g, ultrafine silicone dioxide 7.25 g, BPP 0.4 g and the variable PHSR of 5, 10, 15, 20 g. 2 g bisphenol AF was used instead of PHSR for 100 g fluoroelastomer vulcanized by bisphenol AF. The blends were vulcanized in a hydraulic press at a pressure of 5 MPa for 10 min (10 min was in excess of the optimum cure time about 3 min, which was determined from the oscillating disc rheometer (ODR)). The molded samples were post-cured at 200 C for 6 h. Characterization The vulcanization time and curing behavior of the blend gums were determined from the oscillating disc rheometer (ODR). Thermogravimetric analysis (TGA) was carried out in a NETZSCH 209 (NETZSCH Instrument, Germany) thermogravimetric analyzer over a temperature range from room temperature to 900 C at a heating rate of 20 K/min. Nitrogen flow (80 ml min 1 ) was utilized in order to remove all corrosive gases involved in the degradation and to avoid thermoxidative degradation. The dynamic mechanical spectra of the samples were obtained by using a NETZSCH242 (NETZSCH Instrument. Germany) dynamic mechanical analyzer (DMA). The specimens were analyzed in tensile mode at a constant frequency of 1 Hz, dynamic force 4 N, static force 0.5 N, in a temperature range from 150 C to 100 C for FPM with 10% PHSR and 100 C to 100 C for FPM cured by bisphenol AF at a heating rate of 5 K/min. Mechanical loss factor (tanδ) was measured as a function of temperature for the samples. Tensile stress-strain properties were measured according to ISO specifications on an U-can dynamometer at 25 C with crosshead speed of 500 mm min 1. Tensile strength, elongation at break, tear strength and modulus at 100% elongation were determined. For swelling tests the specimens were weighed accurately before immersing into butanone kept at 30 C. After a specific time over 48 h of swelling, the specimens were removed from the solvent and weighed after removing surface fluids by blotting with filter paper. Volume swelling in percentage was calculated using the following equation [1, 4] : W q 1 = ( ρs W 2 1) ρc / 1 where q is the ratio of swollen volume to original volume; (q 1) is the percentage of volume swell/100; W 1 and

3 Studies on Self-Vulcanizing Fluoroelastomer/Phenol Hydroxy Silicone Rubber Blends 383 W 2 are the weights of the specimen before and after swelling, respectively; and ρ c and ρ s are the densities of the specimen and the test solvent, respectively. RESULTS AND DISCUSSION Vulcanization Characteristics The vulcanization curves of FPM/PHSR are graphically presented, as obtained from the ODR measurements, in Fig. 1. The curing characteristics, expressed in terms of the vulcanizaiton times, t S2 (scorch time) and t 90 (optimum cure time), as well as the maximum and minimum values of the torque, S max and S min, respectively and delta torque ΔS (ΔS = S max S mix ), were deduced from the curves. These parameters, along with the cure rate index, CRI expressed as CRI = 100/(t 90 t S2 ), are compiled in Table 2. Fig. 1 Vulcanization curves of the blends at 170 C Table 2. Curing characteristics of various systems at 170 C t S2 (min) t 90 (min) S min (dn m) S max (dn m) ΔS (dn m) CRI (min 1 ) FPM cured by bisphenol AF FPM/PHSR(5 phr) FPM/PHSR(10 phr) FPM/PHSR(15 phr) FPM/PHSR(20 phr) The effect of PHSR loading on cure characteristics of FPM was studied. The effect of PHSR on the rheographic profile obtained at 170 C is shown in Fig. 1. The increase of PHSR loading in FPM blends decreased both minimum and maximum torques noticeably and increased the scorch time and the optimum cure time except for FPM with 5 phr PHSR. Upon addition of 5% PHSR, there was very little rise in torque (Fig. 1) with almost uncured state of the resulting blend. Higher proportion of PHSR induced slower curing reaction of FPM because of the depleting of BPP when more phenol hydroxyl existed [8]. Upon increasing the amount of PHSR, there was a considerable decrease in S max values from dn m for 10 phr to dn m for 20 phr. Similar reduction in ΔS and CRI was also observed for the blends with higher level of PHSR. It deserves to be mentioned that the FPM with PHSR has smaller S min values and longer scorch times than those of FPM with the same level of bisphenol AF curing agent. It means that FPM with PHSR may have better processability and curing security. Mechanical Properties Effects of blend ratio on the stress-strain behavior of FPM/PHSR blends are summarized in Table 3. Although there are marginal changes in tensile strength for different blend ratios, the blends show synergism in modulus, elongation at break, tear strength and hardness. This is believed to be attributed to the curing agent rule of the PHSR. The blend of FPM/PHSR (5 phr) vulcanized rubber has weak mechanical properties because of

4 384 Y.M. Wang et al. deficiency of vulcanization. However, the mechanical properties for the vulcanized blend of FPM/PHSR(20 phr) were not good because of the low modulus and aggregation of PHSR. Only the FPM/PHSR blends with appropriate PHSR contents have better mechanical properties. Table 3. Mechanical properties of FPM/PHSR Properties FPM/PHSR FPM/PHSR FPM/PHSR FPM/PHSR (5 phr) (10 phr) (15 phr) (20 phr) Modulus, 100% (MPa) Modulus, 200% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kn/m) Hardness, Shore A TGA Characteristics The TGA and DTG thermograms of the FPM cured by bisphenol AF and FPM/PHSR blends obtained at a heating rate of 20 K/min are shown in Fig. 2. Onset degradation temperatures (T 0 ), weight loss (ΔW), as well as DTG maximum temperatures (T max ) corresponding to the maximum degradation rate are presented in Table 4. From Fig. 2, it is observed that the FPM/PHSR (10 phr, 15 phr, 20 phr) blends degraded in a single stage with 86.29%, 85.47% and 86.95% weight loss, respectively. And the FPM/PHSR blends have almost the same weight loss (ΔW) as the FPM cured by bisphenol AF irrespective of the PHSR content. From Fig. 2 and Table 4, it s evident that the onset degradation temperature (T 0 ) and DTG peak temperature (T max ) of FPM/PHSR blends decrease little compared to those of FPM cured by bisphenol AF. The reason is the interaction between PHSR and FPM molecules. Fig. 2 Thermogravimetric (TGA and DTG) curves of FPM cured by bisphenol AF and FPM/PHSR blends Table 4. Thermal characteristics of FPM cured by bisphenol AF and FPM/PHSR blends Onset deg. temp., T 0 Weight loss, ΔW (%) DTG peak temp., T max FPM cured by bisphenol AF FPM/PHSR (10 phr) FPM/PHSR (15 phr) FPM/PHSR (20 phr) Dynamic Mechanical Properties Dynamic mechanical analysis (DMA) measures the responses of a given material to a cyclic deformation as a function of temperature. Figure 3 shows the temperature dependence of loss tangent (tanδ) of the fluoroelastomer cured by bisphenol AF and FPM/PHSR (10 phr) blends over the temperature range from 100 C to 100 C and from 150 C to 100 C, respectively. The loss tangent peaks, corresponding to glasstransition temperature of FPM cured by bisphenol AF and FPM/PHSR (10 phr) blend were observed at 19.4 C

5 Studies on Self-Vulcanizing Fluoroelastomer/Phenol Hydroxy Silicone Rubber Blends 385 and 23.3 C, respectively. The FPM/PHSR (10 phr) blend showed lower glass-transition (T g ) temperature than that of fluoroelastomer cured by bisphenol AF, because of the internal plasticization effect of PHSR. The FPM/PHSR (10 phr) blend showed a single peak in tanδ, because of the specific interaction, as we explained in Fig. 1. Fig. 3 Temperature dependence of tanδ Swelling Studies Figure 4 represents the influence of level of PHSR on equilibrium volume swelling of FPM/PHSR blends. It is clear from Fig. 4 that the FPM/PHSR blends have big equilibrium volume swelling than that of FPM cured by bisphenol AF. An imaginabale explanation is that the fluoroelastomer molecular chains were connected through pliant molecular chains of PHSR for FPM/PHSR blends. The long pliant PHSR chains help the movement of fluoroelastomer molecules and induce the big equilibrium volume swelling in solvent. The fluoroelastomer was not vulcanized absolutely when 5 phr PHSR was used, so that the blend showed a very big equilibrium volume swelling in butanone. The FPM/PHSR blends also have bigger equilibrium volume swelling when 15 phr and 20 phr PHSR were used because of the aggregation of PHSR. The equilibrium volume swelling result is in coincidence with that of mechanical properties. CONCLUSIONS Fig. 4 Effect of PHSR contents on volume swell The results of vulcanization characteristics show that the PHSR can be used as the curing agent of FPM. The FPM/ PHSR blends may have better processability and curing security. The mechanical properties of FPM can be improved by adding an appropriate amount of PHSR depending on the vulcanization density and aggregation of PHSR. The FPM/PHSR blends have lower glass transition temperature than that of FPM cured by bisphenol AF, and the FPM/PHSR blends have big equilibrium volume swelling than that of FPM cured by bisphenol AF.

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