Investigation on the application properties of epoxy resin and glass fiber in RTV mold rubber

2.1 Materials

RTV, two components (A:B, 100:1, 30% aerosilica, component A: hydrogenous silane, Si-(O-Si-(CH₃)₂-H)₄; component B: ethylene-terminated polysiloxane, CH₂=CH-Si(CH₃)₂O(Si(CH₃)₂O)_n-Si(CH₃)₂-CH=CH₂, and the organic platinum catalyst), industrial grade, were supplied by the Shanghai Australopithecus Stone Company (Shanghai, China). EP was obtained from the Shanghai Zhengrui Chemical Industry Company (Shanghai, China). GF was supplied by the Hangzhou Gaoke Materials Company (Hangzhou, China). Silane coupling agent, KH560, industrial grade, was obtained from the Nanjing Shuguang Chemical Industry Company (Nanjing, China).

2.2 Preparation of RTV

One hundred grams of the hydrogenous silane, one component of RTV, was placed in a 250 ml plastic beaker. Then, 1 g of the ethylene-terminated polysiloxane, the other component of RTV, was added into the beaker. The mixture was vigorously stirred at room temperature for 30 min. Afterwards, it was poured into a Teflon mold and cured at room temperature for 24 h. Finally, RTV elastic films were obtained.

2.3 Preparation of different RTV composites

EP, with different amounts, was added into 100 g of the hydrogenous silane. These mixtures were stirred for 3 min in a 50–60°C water bath. Then, 1 g of the ethylene-terminated polysiloxane was added into the above mixtures, and was vigorously stirred at room temperature for 15 min. The formulation of these composites is shown in Table 1. Then, the above mixtures were molded in a Teflon mold. Curing was conducted at room temperature for 24 h, after which different elastic films, RTV/EP-1,3,5,7,9 composites, were obtained.

After measurement, analysis, and selection of the above formulations, RTV/EP-3 was chosen as the best formulation. Different amounts of GF or MGF were added into 100 g of the hydrogenous silane with 3 g of EP. The formulation of these composites was shown in Table 2. Then, the above process was repeated, and after which different elastic films, RTV/EP-3/GF or RTV/EP-3/MGF composites, were obtained.

2.4 Characterization

3.1 Tensile properties

Figure 1 presents the effect of mass fraction of EP, GF, or MGF on the tensile properties of RTV composites. Pure RTV matrix possessed a certain tensile strength and elongation at break, 3.2 MPa and 930%, respectively (Figure 1A). This was ascribed to the addition of some amount of aerosilica. The tensile strength of RTV composite reached the maximum value, 4.2 MPa, with the addition of 5 phr of EP. It was improved about 31% compared with that pure RTV. In addition, the elongation at break reached the highest value of 960% with the addition of 3 phr of EP (Figure 1B). The main reason for such an increase may be resulted from the introduction of reactive groups such as epoxy and hydroxyl groups in the silicone rubber which may promote the formation of chemical or physical bonds in the blends (20). However, with further addition of EP, a phase separation occurred, and this led to the gradual decrease of their tensile properties. The separated phase may be resulted from unevenly distributed crosslinking points and a corresponding unevenly distributed stress. For example, the elongation at break of RTV/EP-5 was decreased to 810%, which was lower than that of pure RTV. Considering these tensile properties, RTV/EP-3 owned the best tensile performance.

Figure 1:

Tensile properties of different RTV composites (A) tensile strength, (B) elongation at break.

After the addition of GF or MGF in RTV/EP-3, the tensile strength and elongation at break were almost increased (Figure 1B). Obviously, RTV/EP-3/MGF exhibited better tensile properties. Compared with pure RTV, RTV/EP-3/MGF-3 showed the best tensile properties. The tensile strength and elongation at break were increased to 5.5 MPa and 1350%, which was 71% and 45% higher than that of pure RTV and 30% and 42% higher than that of RTV/EP-3. The main reason may be ascribed to the surface treatment of GF by a hydrolyzed silane coupling agent. The remaining hydroxyl groups in the silane coupling agent may be adhered to GF and react with the remaining hydrogen groups in the component of RTV, and silicon-oxygen bonds may be formed. This may improve the interface interactions between polymeric matrix and GF, and thus the mechanical properties of the blends can be improved. However, with the increasing amount of MGF, their mechanical properties were decreased due to the agglomeration of these fillers. This similar trend of reinforcing ability of GF was also reported by other researchers (21), (22). In addition, these fillers, GF and MGF, showed better reinforcing and toughing abilities than that of other fillers such as the pyrolysis char which was already reported by us (23).

Figure 2 gave the tear strength of different RTV composites. The tear strength of pure RTV was 21 kN/m. After the addition of EP, a trend of first increasing and then decreasing was exhibited. The main reason may be ascribed to the mutual interactions and corresponding networks in the RTV matrix. With the increase of mass fraction of EP, phase separation occurred, and the tear strength was decreased. However, when GF or MGF were added, the tear strength showed increased behavior. The tear strength of RTV/EP-3/MGF-1 reached 32 kN/m, and the increase was about 52% and 23% compared with that of pure RTV and RTV/EP-3, respectively. This behavior was resulted from surface bonding between the organic glass fibers and the polymeric matrix. Also, the aggregates of MGF were formed with an increasing amount of this particle, and this may reduce the tear strength of these composites.

Figure 2:

Tear strength of different RTV composites.

Figure 3 presented the hardness of different RTV composites. The pure RTV possessed a hardness of 24.5. It was decreased after the addition of EP. This was resulted from the improved elasticity due to the interaction between RTV and EP. In the RTV/EP-3/GF blends, the hardness was obviously increased with the addition of GF due to the inorganic characteristics of this filler. Compared with the hardness of RTV/EP-3/GF composites, the hardness of RTV/EP-3/MGF composites were decreased. In RTV/EP-3/MGF composite, possible reactions between RTV, EP, and the silane coupling agent over the surface of MGF may occur, and this may influence the structure and the resultant hardness of the composites.

Figure 3:

Hardness of different RTV composites.

3.2 Thermomechanical analysis

COLE and COVE of different RTV composites at different temperatures were summarized in Figure 4. At low temperatures from 40 to approximately 110°C, the dimensional stability of pure RTV mold rubber showed little change. In the present case, the COLE and COVE of pure RTV was 1.28×10⁻⁴~1.25×10⁻³ and 4.91×10⁻³~3.75×10⁻², respectively. When the temperature was increased from 110 to 170°C, thermal expansion occurred in RTV, and a certain increase of size was exhibited. However, when the temperature was higher than 170°C, the silicone rubber was heated to a softened and shrinking state, and the COLE and COVE began to decrease.

Figure 4:

TMA of different RTV composites (A) COLE, (B) COVE.

After addition of 3 phr of EP, the dimensional stability of RTV mold was improved to a certain degree. The COLE and COVE of RTV/EP-3 was 0.99×10⁻⁴~1.18×10⁻³ K⁻¹ and 4.52×10⁻³~3.51×10⁻² K⁻¹, respectively. These were lower compared with that of pure RTV. Its COLE and COVE were increased rapidly before the temperature rose to 170°C. However, the mold rubber began softening when the temperature was higher than 170°C, and this led to the slight decrease of thermal expansion coefficient. In addition, the first shrinking and softening temperatures of RTV/EP composites were increased. The main reason for such an increase may be resulted from Si-O bonds that were formed by reactions between the molecular chains of RTV and EP. This may improve the dimensional stability of the RTV/EP composites.

With the addition of 3 phr of GF or MGF into EP modified RTV composites, they exhibited better dimensional stability. In RTV/EP-3/MGF-3 composite, its dimension stability was improved obviously, especially in the temperature ranges from 40 to 110°C. The COLE and COVE of this composite was decreased to 0.38×10⁻⁴~1.50×10⁻⁴ K⁻¹ and 7.92×10⁻⁴~9.56×10⁻⁴ K⁻¹, respectively. The main reason for such a decrease was resulted from the composition of these fillers which were inorganic and itself had a good dimensional stability. Thus, when they were added to the blends, a better dimensional stability was shown. In addition, these composites exhibited a first swollen and then soften phenomena as the temperature continued to increase (24), (25).

3.3 Thermal stability analysis

Figure 5 gave TG curves of different RTV composites. Overall, the RTV composites had higher thermal stability than that of pure RTV. Here, five parameters, T_−5%, T_−10%, T_max, T_end, and the residues (T_−5% and T_−10%, the temperatures at which weight loss is 5% and 10%; T_max, the temperature at which weight loss is the fastest; T_end, the ending temperature of weight loss) are shown in Table 3.

Figure 5:

TG curves of different RTV composites.

Table 3 shows that pure RTV possessed good thermal stability, and T_−5%, T_−10%, T_max, and T_end of which was 330°C, 418°C, 553°C, and 626°C, respectively. After addition of 3 phr of EP, the thermal stability of the blends was improved, and T_−5%, T_−10%, T_max, and T_end were increased by 35°C, 46°C, 40°C, and 7°C. The reason may be resulted from the reaction between EP and the RTV matrix, and thus more stable Si-O chains were formed together with an improved heat resistance (26). In addition, compared with that of pure RTV, T_−5%, T_−10%, T_max, and T_end of RTV/EP-3/GF-3 was increased with 42°C, 47°C, 39°C, and 13°C.However, after the addition of MGF, the thermal stability of the composites was decreased. T_−5%, T_−10%, T_max, and T_end of RTV/EP-3/GF-3 was decreased with 12°C, 37°C, 31°C, and 13°C. This was mainly due to the composition of MGF. A number of small organic molecules covered over the surface of the inorganic filler, and this may lead to the decrease of the thermal stability of the composite. However, in these composites, the amount of the residue was decreased compared to pure RTV. Furthermore, after comparison, the thermal stability of these composites filled with EP, GF, or MGF was better than that of the RTV composites filled with pyrolysis char and APP (25).

3.4 Differential scanning calometry analysis

Figure 6 shows the DSC curves of different RTV composites. Pure RTV matrix exhibited an endothermic point at −38.6°C, and this was ascribed its crystalline melting. This was similar to the results obtained in the previous reports (27). After addition of 3 phr of EP, its melting point was decreased to −41.2°C. The reason was ascribed to the decreased crystal structure of silicone rubber matrix by the molecules of EP resin. In addition, with the addition of GF or MGF, its melting point was increased to −38.1 and −37.9°C, respectively. This was mainly resulted from the reinforcing ability of GF, which may increase the amount of networks in the RTV matrix. Thus, a greater amount of energy was needed to turn the polymer from the solid to the liquid state.

Figure 6:

DSC curves of different RTV composites.

3.5 X-ray diffraction analysis

Figure 7 presented the crystalline structure of different RTV composites. The diffraction pattern of RTV showed intensive and characteristic crystalline peaks at 12.5°, 22.5°, 32.5°, 32.5°, and 37.5°. This was in good accordance with the description of XRD for silicone rubber in other reports (28). As for the crystalline structure of RTV/EP-3, the characteristic peaks of EP exhibited at 24.51°. However, RTV/EP-3/GF-3 showed other diffraction peaks at 21.68°, 22.76°, 25.83°, 28.31°, 42.89°, and these were the characteristics of GF. In addition, compared with that of RTV/EP-3/GF-3, novel peaks appeared at 16.54°, 18.23°, and 21.39° in RTV/EP-3/MGF-3. These were ascribed to the composition of KH-560 which covered over the surface of MGF.

Figure 7:

XRD of different RTV composites.

3.6 Scanning electron microscope analysis

In order to obtain better compatibility between GF and RTV, a silane coupling agent, KH-560, was used to modify the addition filler, GF. The preparation process of MGF is given in Figure 8. In this process, two reactions occurred, that is, hydrolysis of KH560 (Figure 8A) and some physical and chemical reactions over the surface of GF (Figure 8B). First, KH560 hydrolyzes in the water environment, and hydroxyl groups may be produced after the release of methanol. Second, condensation reaction occurs between the hydroxyl groups both in KH560 and over the surface of GF, and thus covalent bonds may be formed (29), (30). In addition, some physical reactions occur during the modification of GF, and thus hydrogen bonds may be formed over the surface of MGF.

Figure 8:

Preparation process of MGF (A) hydrolysis of KH560, (B) reactions occurred during surface modification of GF.

Figure 9 shows the surface morphology of GF and MGF by SEM. Figure 9A shows that these fibers exhibit rod morphology, and the surface is almost smooth. However, in Figure 9B, the surfaces of these fibers show the difference after treatment with a siliane coupling agent. Portions of the surfaces were smooth, but most of which were rougher. That is, there was a small amount of powder adhered over the surface of MGF. This phenomenon may be resulted from the adsorbed materials by the chemical and physical reactions on the surface of the fibers (31).

Figure 9:

SEM of (A) GF, (B) MGF.

The surface morphology of different RTV composites is presented in Figure 10. Figure 10A shows that some small particles are dispersed uniformly over the surface of pure RTV, and these were ascribed to the additives such as aerosilica in the polymeric matrix. Figure 10B shows that after the addition of EP, more flat sheets are formed and shown in the surface of this composite. Figure 10C, D show that the fracture surface of RTV added with GF before and after treatment. As can be seen from Figure 10C, more holes are shown on the surface of GF filled matrix, and this is due to the poor compatibility between the organic matrix and the inorganic fillers. However, enhanced compatibility appears due to the treating of GF by a silane coupling agent KH-560 (Figure 10D). This silane coupling agent contains similar molecular chain groups as that of the two types of polymers, RTV and EP. Thus, the interface tension between two polymers is reduced, and a good compatibility occurs (29). More networks were formed in this matrix, and better physical and mechanical properties were thus obtained.

Figure 10:

SEM of different RTV composites (A) pure RTV, (B)RTV/EP-3, (C)RTV/EP-3/GF-3, (D) RTV/EP-3/MGF-3.