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Preparation and properties of heat curable blended methylfluorosilicone rubber.

INTRODUCTION

Silicone rubbers, based on high molecular weight poly-diorganosiloxane, have been widely used because of their unique characteristics endued by Si-0 chains (1-3). For example, methylsilicone rubber (MSR) exhibits superior heat resistance, low temperature flexibility, and weather resistance. Fluorosilicone rubber (FSR) has outstanding fuel and chemical resistant nature (4). To provide a flexible property-cost balance and combine the hot oil resistance of FSR with the economics of MSR, methylfluorosi-licone rubber (MFSR) has been developed. MFSR is currently regarded as the best materials for low temperature use (5) with no crystallization and glass transition temperature as low as -115[degrees]C (6).

Poly(methylsiloxane-co-fluorosiloxane) with high molecular weight is usually used to prepare heat curable MFSR. This type of copolymers includes random copolymers (7), block copolymers (6), and gradient copolymers (8). They are prepared via equilibrium or controlled anionic living copolymerization of hexamethylcyclotrisiloxane ([D.sub.3]) or octamethylcyclotetrasiloxane ([D.sub.4]) and 1,3,5-tris[(3,3,3-trifluoropropyl)methyl]cyclotrisiloxane ([D.sub.3]F). The polymerization condition is critical and special semi-batch process, addition [D.sub.3]F to the polymerization of less reactive [D.sub.3] (9), is usually adopted. However, it is difficult to synthesize the copolymers with high molecular weight (7), (8). The truly block and gradient copolymers are difficult to obtain because of the difference of the apparent reactivity ratios between [D.sub.3]F and [D.sub.3], namely [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8), respectively.

Polymer blending is an easy and effective method to develop new polymeric materials which combines the properties of different polymer or overcomes a particular drawback of one of the polymer components (1), (10-12). The main advantage of this method lies in the fact that it employs simple process that is attractive economically and less time-consuming. Furthermore, the properties of materials can be tailored by merely adjusting the polymer components.

To meet the need of new materials or make high performance materials, several polymers have been blended with MSR or FSR, such as fluororubber (13-16), ethylene vinyl acetate (17), vinylidene firoride polymers (18), and fluoroether rubber (19), and so on. The properties of this type of blends, such as processability, dielectric property, flammability, thermal stability, hot oil resistance, and mechanical properties have been investigated. However, little attention has been paid on the blends of MSR and FSR. Accordingly, the properties of blended MFSR, such as the curing characteristics, morphology, mechanical properties, and hot oil resistance have not been studied in details.

In this study, heat curable blended MFSR was prepared by combination of MSR and FSR. DBPMH was chosen as a curing agent. Poly(methylsiloxane-co-fluorosiloxane) with low molecular weight was synthesized and used as an interfacial agent. Curing parameters of blended MFSR such as curing reaction rate, reaction rate constant (k), and activation energy (E) were calculated. Effect of interfacial agent on morphology, mechanical properties of blended MFSR was studied in details. And then the hot oil resistance and low temperature performance with different FSR content were investigated. It was found that this was an alternative method to prepare high performance heat curable MFSR.

EXPERIMENT

Materials

Methylsilicone rubber (MSR) was obtained from Hesheng Zhejiang, China. Fluorosilicone rubber (FSR) and poly(methyltri fl uoropropylsilicone-co-dimethylsiloxane) with low molecular weight were prepared by controlled anionic living copolymerization of hexamethylcy-clotrisiloxane ([D.sub.3]) and 1,3,5-tris[(3,3,3-trilluoropropyl)methyl]cyclotrisiloxane ([D.sub.3]F). (6), (8), (9), (20-24). The fumed silica, TS-530, with specific surface area of 220 [m.sup.2]/g, was purchased from Cabort. 2,5-B is(tert-butylperoxy)-2,5-dimethylhexane (DBPMH) was obtained from Akzo Nobel Peroxide, Tianjin, China. ASTM 1# oil, ASTM 2# oil, and ASTM 3# oil were provided by Sinoham Chemical.

Preparation of Blends

MSR and FSR were blended in a Haake Torque Rheometer (Rheomix 600 OS, mixer; Thermo Fisher Scientific, German) at room temperature with a rotor speed of 40 rpm. Then TS-530 was mixed step by step. After being compounded uniformly, DBPMH was mixed. Thin sheets of ~2 mm thickness were compression molded at 160[degrees]C under a pressure of 10 MPa and post-cured at 180[degrees]C for 2 h. The formulations were listed in Table 1.
TABLE 1.  The formulations of blends.

No.      MSR (a)  FSR (a)  TS-530 (a)  Interfacial agent (a)  DBPMH (a)
FSR            0      100         50                       1          1
MFSR-85       15       85         50                       4          1
MFSR-80       20       80         50                       4          1
MFSR-70       30       70         50                       4          1
MFSR-50       50       50         50                       4          1
MFSR-40       60       40         50                       4          1
MSR          100        0         50                       1          1

(a.) Parts by weight.


Curing Characteristics

The curing characteristics of blended MFSR were studied by a Monsanto moving disk rheometer (MDR-2000, Alpha Technologies) at 160[degrees]C. To determine the activation energy (E) of curing reaction, the curing characteristics were also measured at 170[degrees]C and 180[degrees]C.

Low Temperature Performance

To evaluate low temperature performance of blended MFSR and low temperature effects, such as crystallization, or those effects that were introduced by low temperature incompatibility of MSR and FSR, differential scanning calorimetry (DSC) measurements were performed on a Pheometric scientific DSC SP instrument at a heating rate of 10[degrees]C/min from -150[degrees]C to 20[degrees]C.

Morphology

Morphological observations were made by a scanning electron microscope (SEM; JSM-7600F, JEOL) on the tensile broken surface of blended MFSR. The surfaces of the specimens were sputter coated with gold.

Mechanical Properties

Mechanical properties of blended MFSR, including tensile strength, tear strength, and elongation at break were measured according to ASTM D412 and D624 procedures at a crosshead speed of 500 mm/min using dumbbell-shaped test pieces in an electronic rubber tension tester (XLD-A, Second Experimental Machine Factory, Changchun, China) at room temperature. Testing of hardness was carried out using a Shore type A Durometer in accordance with ASTM D2240.

Hot Oil Resistance

The hot oil resistance of blended MFSR was determined according to ASTM D471-1998 in ASTM 1# oil, ASTM 2# oil, and ASTM 3# oil. The specimens were immersed in oils at 150[degrees]C for 70 h.

RESULTS AND DISCUSSION

2,5-B is(tert-butylperoxy)-2,5-dimethylhexane (DBPMH) was chosen as a curing agent in this study, because it produced volatile by-products that could be removed from the vulcanizates during press and post-cure treatment (25-28). Figure 1 showed the curing curves of MSR, blended MFSR and FSR at 160[degrees]C. The curing curve of MFSR-50 was taken as an example of blended MFSR because the curing curves of other samples with different blending ratios were similar to MFSR-50. For all the samples, an initial decrease in torque was observed because of the softening of the matrix followed by an increase due to the forming of new carbon-carbon bonds. The plateau region indicated the completion of the curing process. The results showed that blended MFSR could get the co-vulcanization by DBPMH.

[FIGURE 1 OMITTED]

The curing characteristics of blended MFSR, expressed in terms of the minimum and maximum values of torque, [M.sub.L] and [M.sub.H], the scorch time, [t.sub.SI] and the optimum cure time, No, were compiled in Table 2. At a fixed temperature, the minimum torque ML which was proportional to the viscosity (29) of the uncured blended MFSR increased with increasing of FSR content.[M.sub.L] was also related to the processability of blends. The increasing of [M.sub.L] indicated the poor processability of blended MFSR. The maximum torque [M.sub.H] values increased following by the increase of FSR content. The scorch time [t.sub.SI] decreased with the increase of MSR content. [t.sub.SI] Reflected the premature vulcanization of the materials (30). It meant the scorch safety of MSR and FSR blends increased with the increase of FSR content. This may be due to the diffusion difference (31) of DBPMH between FSR and MSR phase. The time corresponding to 90% of maximum torque, [t.sub.90], also increased with the increase of FSR content which meant that MSR was the cure-activating component in MSR and FSR blends.
TABLE 2. Curing characteristics of MSR and FSR blends at 160[degrees]C.

No.       Scorch time  Optimum cure  Minimum torque  Maximum torque
         ([ts.sub.1])          time      ([M.sub.t]     ([M.sub.H])
                (min)  ([t.sub.90])    (d[N.sub.m])    (d[N.sub.m])
                              (min)

FSR              1.29          9.35            2.64           11.42

MFSR-85          0.88          8.48            2.45           10.70

MFSR-80          0.87          8.35            2.00           10.57

MFSR-70          0.82          7.98            1.73           10.53

MFSR-50          0.81          7.00            1.50           10.40

MFSR-40          0.80          5.78            1.15            9.30

MSR              0.79          4.40           E0.95            6.71


To understand the kinetics of vulcanization, curing parameters such as reaction rate constant (k), activation energy (E), and curing reaction rate were determined according to the followed equations (10), (32), (33):

ln([M.sub.H] - [M.sub.L]/[M.sub.H] - [M.sub.t]) = kt (1)

where [M.sub.t] was torque at a time t; and k was the rate constant for the vulcanization.

log[t.sub.90] = logA + E/2.303RT (2)

where E was the activation energy, R the gas constant, and T the absolute temperature.

Curing rate = (Curing time torque - Scorch time torque)/(Curingtime - Scorchtime) (3)

The reaction rate constant k values tabulated in Table 3 were obtained from the plot of ln([M.sub.H] - [M.sub.L]/[M.sub.H] - [M.sub.t]) against t and the obtained graphs were straight lines, which indicated the cure reaction of blended MFSR followed first order kinetics. And higher FSR contents lowered k values which meant FSR was less reactive than MSR in blends.
TABLE 3. Kinetics parameters of MSR and FSR blends.

No.            Cure rate  Reaction rate  Activation
         (160[degrees]C)   constant (k,  energy (E,
                            [s.sup.-1])     kJ/mol)

FSR                 0.80          0.255       127.1

MFSR-85             0.87          0.262       117.5

MFSR-80             1.02          0.263       114.8

MFSR-70             1.06          0.292       111.5

MFSR-50             1.12          0.300       104.4

MFSR-40             1.17          0.307       102.3

MSR                 1.25          0.344        91.7


The activation energy E values were calculated from the plots of log [t.sub.90] against 1/T using regression analysis and the results were shown in Table 3. It is clear that FSR exhibited higher E values than MSR. It is also observed the E values for vulcanization decreased with a decrease of FSR content, which also meant that MSR was the cure-activating component in MSR and FSR blends.

The values of curing reaction rate calculated by Eq. 3 were also given in Table 3. The higher curing reaction rate was obtained followed by a decrease of FSR content. The difference in curing reaction rate was due to the solubility of DBPMH between MSR and FSR and also due to the difference in the rate of vulcanization of MSR and FSR.

To evaluate low temperature performance of blended MFSR, differential scanning calorimetry (DSC) measurements were performed. MFSR that had the same compositions as the blended MFSR was also included as a control. Figure 2 showed the DSC curves of blended MFSR and MFSR. MFSR-50 was taken as an example. As showed in Fig. 2, MFSR-50 exhibited a melting temperature ([T.sub.m]) at -47.9[degrees]C and a glass transition temperature ([T.sub.g]) at -85.6[degrees]C. However, MFSR only exhibited Tg at -88.2[degrees]C, lower than the [T.sub.g] of MFSR-50. It proved the incompatibility and immiscibility of MSR and FSR in blended MFSR which was attributed to the difference of the solubility parameters of MSR and FSR (6). It also meant that MFSR had better low temperature flexibility than MFSR-50.

[FIGURE 2 OMITTED]

Table 4 showed the effect of FSR content on the low temperature properties of blended MFSR. It was found that MSR has a melting temperature [T.sub.m] at -43[degrees]C, so the low temperature flexibility of MSR reached the limit at -43[degrees]C. On the other hand, FSR exhibited the glass transition temperature [T.sub.g] at -73[degrees]C, which meant that FSR had better low temperature performance than MSR. It is also observed that [T.sub.g] of blended MFSR decreased with an increase of MSR content. This may be due to the higher low temperature flexibility of MSR and the crosslinking between FSR and MSR phases. As the presence of vinyl groups in the MSR and FSR chains, crosslinking may be generated due to the formation of new carbon--carbon bonds between MSR and FSR phases, the higher MSR lowered the [T.sub.g] of blended MFSR. When MSR content was lower than 15%, we thought MSR phase could disperse in FSR phase uniformly and the blended MFSR exhibited only a [T.sub.g], lower than FSR. This also proved blended MFSR could get the co-vulcanization by DBPMH.
TABLE 4. Low temperature performance of blended MFSR.

No.          Melting temperature        Glass transition
         ([T.sub.m] ([degrees]C)  temperature ([T.sub.g]
                                            ([degrees]C)

FSR                            /                   -73.8

MFSR-85                        /                   -78.0

MFSR-80                    -46.9                   -81.3

MFSR-70                    -47.9                   -85.6

MFSR-50                    -46.6                   -92.8

MFSR-40                    -47.5                   -93.4

MSR                        -43.4                       /


Generally, the widespread way to improve the compatibility of blended components was the use of interfacial agent (34). In this study, poly(methylsiloxane-co-fluorosi-loxane) with low molecular weight was synthesized and used as an interfacial agent. The effect of interfacial agent on the morphology of MFSR-50 was shown in Fig. 3. It could be noticed from Fig. 3a that the dispersion of MSR and FSR was not uniform and the microcosmic incompatible or immiscible appearance was observed. Similar results could be found for MFSR-85, MFSR-80, MFSR-70, and MFSR-40 which were not mentioned one by one. This proved the incompatibility or immiscibility of MSR and FSR in blended MFSR. However, the use of interfacial agent decreased the domain size of MFSR-50, shown in Fig. 3b. It meant that the heterogeneous nature was improved and blended MFSR became more homogeneous. It also meant that the interfacial agent could limit the phase separation and improve the thermodynamically stability of blended MFSR.

[FIGURE 3 OMITTED]

The effect of interfacial agent on the mechanical properties of blended MFSR with different FSR contents was shown in Fig. 4 and Table 5. As expected, the phase separation phenomenon could lead to the mechanical loss of blended MFSR. However, it was found that for all the blended MFSR samples, the tensile strength could exceed 7.6 MPa and the tear strength could arrive at 20.0 kN/M. This proved that the crosslinking between FSR and MSR phase was formed which was in agreement with the DSC analysis. The crosslinking network structure limited the phase separation of MSR and FSR in blends and increased the mechanical properties. It could also be found that interfacial agent did not directly influence the hardness of blended MFSR, but the tensile strength and the tear strength were elevated following by the decrease of the elongation at the break. For example, the tensile strength and the tear strength of MFSR-50 were brought up to 8.0 MPa and 23.0 kN/m.

[FIGURE 4 OMITTED]
TABLE 5. Effect of interfacial agent on the mechanical properties
of blended MFSR with different FSR contents.

                      Mechnical properties

                Hardness   Tensile       Tear  Elongation
                  (ShA)   strength    strength         at
                             (MPa)    (kN/m)    break (%)

                               Interfacial agent(a)

FSR content (%)   /   4    /     4     /     4    /    4

40               60  60  7.8  8.11  21.4  24.6  650  583
50               59  60  7.9   8.3  20.6  23.3  636  550
70               59  59  8.3   8,9  21.5  26.9  566  530
80               60  60  8.7   9.1  20.6  25.4  534  501
85               59  60  9.3   9.5  21.1  28.8  515  485

(a.) Parts by weight.


Effect of oils on rubbers depended on a number of fractions that included type of the rubber compounds, composition of the oils, temperature, and time of exposure (13). Figure 5 and Table 6 showed the effect of oils on the hot oil resistance of blended MFSR with different FSR contents. It was found that the increment of FSR content elevated the mechanical properties and the hot oil resistance of the blended MFSR. No matter which oils the specimens were immersed, the higher FSR content was, the better mechanical properties could reach. So FSR had the best hot oil resistance and MSR had the worst. It was also observed that all the mechanical properties of blended MFSR, including the tensile strength, the tear strength, and elongation at break, decreased in various degrees following the hardness decreased, particularly in the ASTM 1# oil. This was due to the degradation or cracking of blended MFSR chains after aging in oils. It meant that the degradation or cracking of blended MFSR in ASTM I# oil was serious. The retention of mechanical properties in the ASTM 2# oil was always higher than in ASTM l# oil and ASTM 3# oil which meant ASTM 2# oil influenced the mechanical properties of blended MFSR least.

[FIGURE 5 OMITTED]
TABLE 6. Effect of oils on the hot oil resistance of blended MFSR
with different FSR contents.

FSR content (%)             0    40    50    70    80    85   100
Hardness (ShA)             45    60    59    59    60    59    60
Tensile strength (MPa)    9.1   8.0   8.3   8.9   9.1   9.5    11
Tear strength (kN/m)     24.8  24.6  24.3  26.9  25.4  28.8  25.3
Elongation at break (%)   817   583   550   536   500   485   449
ASTM 1# oil
Hardness (ShA)             35    56    57    58    59    59    60
Tensile strength (MPa)    1.5   1.9   2.5   3.5   4.5   4.6   5.5
Tear strength (kN/m)       10  11.4  11.9  12.5  14.0  17.0  18.0
Elongation at break (%)    92   125   138   136   202   220   260
ASTM 2# oil
Hardness (ShA)             30    40    45    46    51    53    60
Tensile strength (MPa)    6.6   6.8   7.2   8.0   8.1   8.1    11
Tear strength (kN/m)     15.2  19.3    21  22.1    23  23.3  22.8
Elongation at break (%)   709   576   518   355   394   387   438
ASTM 3# oil
Hardness (ShA)             22    40    45    49    51    51    60
Tensile strength (MPa)    2.9   3.7   3.8   3.9   4.9   5.4   7.4
Tear strength (kN/m)     10.6  12.9  13.5  14.4  14.8  15.2  16.9
Elongation at break (%)   566   327   320   295   267   227   250


CONCLUSIONS

Heat curable blended methylfluorosilicone rubber (MFSR) was successfully prepared by combination of methylsilicone rubber (MSR) and fluorosilicone rubber (FSR). Curing characteristics, mechanical properties, low temperature performance, and hot oil resistance of blended MFSR were studied in details. It was found that higher FSR content could elevate mechanical properties and hot oil resistance of blended MFSR. FSR had the best hot oil resistance and MSR had the worst.

Curing curves showed that co-vulcanization could be reached between MSR and FSR by 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (DBPMH). The cure characteristics revealed that higher FSR content could improve the scorch safety, lower the rate of cure reaction and lead to the poor processability of blended MFSR. MSR was the cure-activating component in the blends.

SEM study confirmed the incompatibility of MSR and FSR in blended MFSR. Poly(methylsiloxane-co-fluorosiloxane) with low molecular weight as an interfacial agent could limit the phase separation of MSR and FSR, make the blended MFSR more thermodynamically stable and hence increased the mechanical properties of blended MFSR.

It was noted that higher MSR content lowered the glass transition temperature ([T.sub.g]) and improved the low temperature flexibility of blended MFSR. However, the low temperature performance was damaged due to the incompatibility of MSR and FSR when MSR was >15%.

Correspondence to: Shengyu Feng; e-mail: fsy@sdu.edu.cn, zhonchuanjian@sdu.edu.cn

Contract grant sponsor: The Key Natural Science Foundation of Shandong Province of China; contract grant number: ZR2011BZ001.

DOI 10.1002/pen.23230

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

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Yuetao Liu, (1) Hongzhi Liu, (1) Rong Zhang, (2) Chuanjian Zhou, (1), (3) Shengyu Feng (1)

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

(2) Hangyu Lifesaving Equipment Co., Ltd., Xiangyang City 441003, Hubei Province, People's Republic of China

(3) School of Material Science and Engineering, Shandong University, Ji'nan 250061, Shandong Province, People's Republic of China
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Publication:Polymer Engineering and Science
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Date:Jan 1, 2013
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