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Temperature Effect on Mechanical Properties of Toughened Silicone Resins

Posted on: Tuesday, 22 November 2005, 03:02 CST

By Wu, Yuhong; McGarry, Frederick J; Zhu, Bizhong; Keryk, John R; Katsoulis, Dimitris E

The temperature dependence of mechanical properties of two families of toughened silicone resins was investigated. The first family was representative of hydrosilylation reaction curable silicone resins, and the second representative of condensation reaction curable ones. The hydrosilylation curable resin was cross- linked with a variety of cross-linkers, including 1,4- bis(dimethylsilyl) benzene, 1,1,3,3,5,5,-hexamethyltrisiloxane, diphenylsilane, and their mixtures. The condensation reaction curable resin and its toughened versions were cross-linked by silanol condensation. Properties studied included flexural strength, flexural modulus, and fracture toughness K^sub Ic^. Temperature effect on these properties of the first family of resins was substantial and varied strongly with the type of cross-linkers. For this family of resins the flexural strength and modulus decreased with a rising temperature. Fracture toughness K^sub Ic^ showed a peaking behavior with the peak appearing at approximately 62C below the α transition peak. This was explained by the effect of the plastic zone size, and the effect of the network resistance to plastic deformation. The second family of resins also showed decreases in modulus and strength with a higher testing temperature, but the fracture toughness changed little with temperature. POLYM. ENG. SCI., 45:1522-1531, 2005. 2005 Society of Plastics Engineers

INTRODUCTION

The question of mechanical property dependence on temperature has been brought up repeatedly when the toughened silicone resins are used for a range of applications, including structures in space. Those structures experience possible temperature fluctuations from - 100 to +100 C on a daily basis. Any high temperature and radiation resistant polymer also has to resist low temperature cracking. Other applications such as structures in aircraft and on ships take advantage of the low fire/smoke/toxicity of silicone resins. But they have to maintain mechanical integrity at both subambient and elevated temperatures.

The toughened silicone resins have been reported before. They include condensation reaction curable ones [1-8], and hydrosilylation reaction curable ones [9-16). These resins exhibit much improved fracture toughness as compared with the traditional silicone resins. For example, the condensation cured ones have a K^sub Ic^ of 0.55 MPam^sup 1/2^, compared with 0.25 MPam^sup 1/2^ for the nontoughened resins. The hydrosilylation cured ones have a K^sub Ic^ improved from 0.31 MPam^sup 1/2^ to 1.08 MPam^sup 1/2^ by choice of cross-linkers, and further to 1.8 MPam^sup 1/2^ by incorporation of nanometer-sized particles. The purpose of this study is to gain an understanding of how the K^sub Ic^ and other mechanical properties of these resins vary with temperature. The dependence of mechanical properties on rate is not to be addressed here, although we realize it is also very important and often it is not separable from the temperature effect. Two families of resins will be included in the investigation. One family is a hydrosilylation curable resin (Resin I, [(PhSiO^sub 3/2^)^sub 0.75^ (ViMe^sub 2^SiO^sub 1/2^)^sub 0.25^]), cross-linked with a variety of SiH functional cross-linkers and their mixtures. The other family is a silicon-bounded hydroxyl condensation curable silicone resin (Resin II [(PhSiO^sub 3/2^)^sub 0.40^(MeSiO^sub 3/2^)^sub 0.45^(Ph2SiO)^sub 0.10^ (PhMeSiO)^sub 0.05^]), and its toughened version (Resin III).

BRIEF REVIEW OF RELEVANT WORK AND THEORIES

Temperature greatly affects both deformation and fracture behaviors of polymers. Because of the viscoelasticity of polymers, temperature effect on mechanical behavior of polymers is more complex compared with that on metals and ceramics. Most studies on temperature effect on polymers are concerned with the flow characteristics of the network and chains. Studies on temperature effect on mechanical strength and toughness have found that results are material dependent [21-27]. Interpretations of these results are also material specific and there is no general theory obtained so far. The most frequently studied thermoset resin is epoxy resin and temperature effect on deformation and fracture behavior is usually manifested through parameters like strength, plasticity, and fracture toughness.

Macroscopically, strength of polymers, including yield stress and ultimate stress, generally decreases with temperature [21]. The most general explanation of this is that thermal energy enhances deformation of the material. At a higher temperature, an otherwise brittle material exhibits stress-strain curves that show substantial deformation. Pink and Campbell [22] have studied the tensile behavior of an epoxy resin. With increasing temperatures (in the range of -196C to 108C), the stress strain curves change from linear, to partly bent over, to substantially curved shapes. Young's modulus of the resin also decreases with temperature, showing less resistance to load with increasing temperature. This behavior is typical of thermoset resins, which are often brittle at room temperature but demonstrate ductility at higher temperatures. Harismendy et al. [23] have studied the strain rate and temperature effects on the mechanical behavior of two epoxy mixtures with different cross-link densities. Their experiments suggest that testing temperature is an important factor for flexural modulus and flexural strength of the epoxy resin and its effect is more obvious than that of the rate. Others have observed an obvious decrease of fracture toughness with epoxy resins as the testing rate is increased [24].

In glassy polymers (including both thermosets and thermoplastics), with temperature change fracture toughness can experience a monotonic increase or decrease, a constant, or a peaking mode. Hashemi and Williams [21] have found that the fracture toughness changes with temperature can be described by three distinct regions: brittle, semibrittle, and ductile, which are closely related to the shape of the load-displacement curve from a single edge notched bending test to measure fracture toughness. The nonlinearity portion of the load-displacement curves is observed to increase as ductility increases and is believed to relate to the plastic deformation occurring at the crack tip during loading. Researchers have found that rubber modified epoxy resins demonstrate a greater temperature dependence of toughness than the unmodified ones [25, 26]. With these resins the fracture toughness increases with temperature, accompanied by observed shear plasticity at the crack tip [26]. Crack propagation has been observed to be continuous at low temperatures but become unstable at high temperatures. An increase in temperature will promote the stick-slip crack propagation mode, which suggests the material become tougher [17, 27]. Another interesting observation in epoxy is that an increasing yield stress has led to the transition from ductile tearing to brittle unstable to brittle stable crack growth [25].

TABLE 1. Materials and their abbreviations in the text.

MATERIALS AND EXPERIMENTAL PROCEDURES

Materials

The materials used in this study are listed in Table 1. Their abbreviations appearing in the text, as well as material compositions, are also included in the table. Mixtures of cross- linkers for Resin I were also used in addition to the single cross- linkers. The cross-linkers are referred to as B, D, and P. The mixed cross-linkers of D and P are designated as DP. Two numbers following DP indicate the relative ratio of SiH groups from each type of cross- linker. DP37, for example, means a mixed cross-linker containing 30% (mole) SiH from D and 70% SiH from P.

Experimental Procedures

Curing. Resin I was mixed with cross-linker(s) so that the SiH/ SiVi = 1.1/1, and the Pt catalyst so that the Pt concentration for the systems cross-linked with 1,4-bis(dimethylsilyl) benzene (B) or hexamethyltrisiloxane (D) was 1 ppm, and for the systems containing diphenylsilane (P) was 30 ppm. The mixture was poured into a Teflon coated mold and degassed in a vacuum oven without heat for about 20- 30 minutes. After this, the mixture was moved to an aircirculating oven to cure. The curing temperature cycle was: 85C/24 hours. 150C/ 24 hours, and 200C/24 hours. After cure the oven was switched off. Castings were cooled in the oven from 200C to room temperature and removed from the Teflon coated mold.

To prepare condensation reaction curable resin castings, both Resin II and Resin III as 60 wt% solutions in toluene were mixed with 0.05 wt% of the catalyst Y-177(TM). The mixture was poured into a Teflon mold and degassed inside a heated (~60-70C) vacuum oven to remove toluene and trapped air. Then it was placed in another oven to cure. The temperature cycle used was: 70C/24 hours, 75C/24 hours, 80C/24 hours, 85C/24 hours, 90C/24 hours, 95C/48 hours, 110C/24 hours, 120C/24 hours, 130C/48 hours, 150C/4 hours, 175C/4 hours, 200C/12 hours, 230C/6 hours, and 260C/8 hours.

Testing. Flexural and fracture toughness test specimens were cut from the 15-cm 15-cm cured castings. Final samples were ground with SiC papers and/or polished with alumina polishing \dispersions. Samples were conditioned at room temperature for two days before being tested on an Instron 4505 machine. Flexural test was performed according to ASTM standard D790-92 [28]. The size of the specimen was 12.7 mm 2.5 mm 51 mm and the span was 38 mm. The loading rate was 1 mm/min.

FIG. 1. Flexural modulus vs. temperature for Resin I cross- linked by cross-linkers B, D, and P, and their combinations.

For tests done at temperatures other than room temperature, an Instron environmental chamber 3111 was used. For testing at elevated temperatures, the samples were heated to the testing temperature in the environmental chamber and kept for 30 min before force was applied. For low temperature testing, the samples in the chamber were cooled with liquid nitrogen with a temperature controller. The samples were held at the test temperature for 15-20 min before testing.

FIG. 2. Maximum flexural stress vs. temperature for Resin I cross- linked by cross-linkers B, D, and P, and their combinations.

FIG. 3. Maximum flexural strain vs. temperature for Resin I cross- linked by cross-linkers B, D, and P, and their combinations.

Dynamic Mechanical Analysis (DMA). DMA was performed on a Dynamic Mechanical Rheology Station DMS200 (Seiko Inc.). Specimens had a rectangular cross-sections of 0.5 mm 4 mm, with a clamping distance of 20 mm. Data were collected at a frequency of 1 Hz, in a temperature range of -150C to 350C with a ramping rate of 2C/min. The samples were loaded in tension mode, with a nitrogen flow rate of 200 ml/min.

RESULTS AND DISCUSSION: HYDROSILYLATION REACTION CURABLE RESIN

The hydrosilylation reaction curable resin (Resin I) was cured with D, P, and B cross-linkers (see Table 1 for meanings of these abbreviations). For the DP system, variations in combination of the cross-linkers were: D100 (100%D), DP37 (30%D, 70%P), DP55 (50%D, 50%P), DP73 (70%D, 30%P) and P100 (100%P). The flexural properties and fracture toughness are presented in Figs. 1-4.

FIG. 4. Fracture toughness vs. temperature for Resin I cross- linked by cross-linkers B, D, and P, and their combinations.

FIG. 5. Tan δ and fracture toughness vs. temperature for Resin I cross-linked by cross-linker B.

Mechanical Properties

At the same testing rate, generally the maximum flexural stress decreased with a rising temperature for all cross-linked systems. The same was true for the flexural modulus. Fracture toughness, however, demonstrated a peaking pattern in all systems.

The resin cross-linked with the B cross-linker, B100, was the toughest system at room temperature. The B cross-linker offered both rigidity with the backbone benzene ring and flexibility from its limited number of functionality. In the testing temperature range, its fracture toughness peaked at about 30C. At both extreme ends, away from 30C. toughness values dropped, with the high-temperature end illustrating a gradual decrease while the low-temperature end had a steep slope.

Resin I cross-linked with the D cross-linker, D100, had the lowest modulus but the largest flexural deformation before fracture among all systems. The fracture toughness of D100 had a unique pattern, showing a peak at 0C with a lowered temperature, and then increasing again with a further lowered temperature.

The resin cross-linked with the P cross-linker, P100, exhibited the highest maximum flexural stress and flexural modulus among the three types of cross-linkers. It was also the least flexible, showing the smallest flexural strain to failure. P100 had the lowest fracture toughness values at all temperatures. Its curve was also very flat compared with other systems because toughness change with temperature was small. Its value stayed almost unchanged at 0.44 0.03 MPa m^sup 1/2^ for temperatures of 0C, 20C, 40C, and 60C. There was, however, a barely observable maximum at approximately 35C.

DP73, DP55, and DP37 are Resin I cross-linked with different combinations of D and P. The purpose of using mixtures of cross- linkers was to combine the attractive properties of more than one cross-linkers while hopefully avoiding the shortfalls of any single one. The P cross-linker added rigidity, while the D cross-linker provided network flexibility. Combination of D and P cross-linkers resulted in systems more similar to the P100 than to the D100. Compared with D100, these resin systems had high strength and were more rigid at all temperatures. DP73, DP55 and DP37 showed consistent changes in strength and modulus in relation to temperature: a decreasing trend with a rising temperature, and an increasing trend with more P cross-linker at the same testing temperature. At room temperature, fracture toughness values of DPs decreased with increasing P content. Fracture toughness versus temperature curves flattened as the P cross-linker content increased. But a fracture toughness vs. temperature peak was clearly seen. The peak temperatures were around 10C, 20C and 30C for DP73, DP55, and DP37, respectively, showing a trend to move up as the P content increased.

FIG. 6. Tan δ and fracture toughness vs. temperature for Resin I cross-linked by cross-linker D.

DMA (tan δ vs. T) and Fracture Toughness (K^sub Ic^ vs. T)

Tan δ curves from Dynamic Mechanical Analysis and fracture toughness vs. temperature curves are plotted together. Examples are shown in Figs. 5 and 6 for B100 and D100. The two peak temperatures (tan δ and K^sub Ic^) and their differences for every cross- linked system are listed in Table 2. As P cross-linker content increased (from DP73 to DP37), the tan δ peak temperature shifted to the higher temperature end. At the same time, the peak temperature of the toughness curve moved similarly to a higher temperature. The differences in peak temperatures of tan δ and fracture toughness were approximately 62C and they were strikingly consistent except for the D100 where the K^sub Ic^ peak position was less obvious. The fracture toughness peak occurred almost always at about 62C below its a transition peak. This consistent difference suggested that for the cross-linked Resin I, the fracture toughness peak was closely associated with the glass transition temperature, which determined to a large extent what segmental/ network motions were activated al a certain testing temperature and strain rate.

TABLE 2. Positions of fracture toughness curve peaks and DMA tan δ peaks of Resin I cross-linked with various cross-linkers.

FIG. 7. Log (Rp) vs. Temperature for Resin I cross-linked by cross-linker B, D, and P, and the mixtures of D and P. Rp: size of the plastic zone.

FIG. 8. A sketch of how toughness is affected by deformability and resistance to plastic deformation vs. temperature.

TABLE 3. Summary of load-displacement curves in K^sub Ic^ tests for different crosslinked Resin I.[dagger]

TABLE 4. Mechanical properties of cured Resin III at different temperatures.

Plastic Zone Size

Using the fracture toughness and flexural strength data obtained in previous sections, the plane strain plastic zone size Rp was estimated by Equation (4) [17]. The maximum flexural stress and the fracture toughness, both obtained experimentally, were used as σy and Kq in the equation. The calculated plastic zone size increased with temperature at all circumstances. D100 had the largest plastic zone size over all temperatures. The linear lit of the curves showed that D100 had a smaller slope compared with B100, DPs and P100 (Fig. 7). P100 had the lowest lying curve, and the combinations of D and P had curves in between those of the D100 and P100, generally in the order of mixing ratios. B100, showing the highest fracture toughness but the greatest change in toughness vs. temperature, also had the steepest curve of plastic zone size vs. temperature.

TABLE 5. Mechanical properties of cured Resin II at different temperatures.

Load-Displacement Curves of Fracture Toughness Test

The load-displacement curves from a fracture toughness test provide further insights to the deformation process. A summary of these curve features is listed in Table 3. In summary, the load- displacement curves show differences before, at, and past the maximum load point. A straight line before and a sharp transition at the maximum load point indicate brittle fracture involving little plastic deformation. To the contrary, a rounded transition at the maximum load point demonstrating non-linearity suggests plastic deformation. Past the maximum point, a sharp drop indicates an unstable fracture, and a gradual drop signals ductile tearing. In the D/P cross-linked Resin I, as the content of P cross-linker increased, the curves started to show non-linearity at higher temperatures. With the B100, non-linearity began to appear at about 0C. It seemed there was a general trend that the fracture toughness was maximized at a temperature where non-linearity appeared indicating plastic deformation, but before the fracture became ductile tear.

A Proposed Mechanism

The peaking behavior of fracture toughness with temperature in cross-linked Resin I suggests that there are competing aspects of the deformation process. Although experimental evidence is less than sufficient, we propose that the two most important competing factors here are the network deformation ability, and the material resistance to plastic deformation. The former determines the size of the plastic zone in front of a crack tip that is involved in the energy consumption process. And combined with it, the latter determines how much energy is needed to develop the plastic zone. A graph in Fig. 8 demonstrates how deformability and plastic deformation resistance change with temperature in a limited temperature range. As the temperature increases, the network resistance to plastic deformation is reduced, manifested by a decreased yielding stress as seen in the cases where relatively extensive plastic deformation is observed, such as \the B100. On the other hand, the network deformability increases with a rising temperature and should level off at a point where the theoretical draw ratio of the inter cross-link segments is reached. While an enhanced deformability enlarges the size of the plastic zone, thus increasing the fracture toughness, this is simultaneously met with the reduced yielding stress, which tends to decrease the amount of energy needed to develop a plastic zone of certain size. At an optimum point, the product of the two is a maximum and the fracture toughness peaks. On both sides of this point, the fracture toughness drops. It is then not surprising that the fracture toughness peak seems to be related to the tan δ from a DMA experiment.

FIG. 9. Flexura modulus change with temperature for cured Resin II and Resin III.

FIG. 10. Maximum flexural stress change with temperature for cured Resin II and Resin III.

FIG. 11. Maximum flexural strain change with temperature for cured Resin II and Resin III.

RESULTS AND DISCUSSION: CONDENSATION REACTION CURABLE RESIN

Mechanical Properties

Both the toughened resin (Resin III) and the nontoughened resin (Resin II) were cured and tested for mechanical properties at different temperatures. Testing temperature ranges were from -40C to +80C for Resin II and from -40C to +60C for Resin III. More detailed compositional information for Resins II and III is not included here but can be found in Reference 6.

The mechanical properties are listed in Tables 4 and 5 and plotted in Figs. 9-12. In general, an increase in temperature resulted in a drop in modulus and strength of both resins. The change in strength of both Resin II and Resin III was very similar and the two curves overlapped each other. It was also seen that scattering in the stress data was relatively large. Both resins were brittle and thus were sensitive to scratches/defects present on the test samples. The non-loughened resin had a consistently higher modulus value than the toughened resin at all temperatures. Maximum flexural strains of both resins went up with temperature. For the cured Resin III, it increased at a faster rate.

FIG. 12. Fracture toughness change with temperature for cured Resin II and Resin III.

TABLE 6. Summary of load-displacement curves in K^sub Ic^ (MPa m^sup 1/2^) tests for cured Resin II and Resin III.

In Fig. 12, Resin II showed an almost constant fracture toughness value with changing temperature. However, fracture toughness of Resin III experienced an increase with a decreasing temperature. The K^sub Ic^ value was also constantly higher than the nontoughened resin. The toughness of Resin III was more sensitive to temperature change.

The calculated plastic zone size of both resins did not change much with temperature. For the non-toughened resin, the size increased from about 1 micron to 4 micron when temperature was raised from -40C to 80C. For Resin III. it remained almost constant at 3-4 micron in the entire temperature range.

Load-Displacement Curves From the Fracture Toughness Text

A summary of load-displacement curve features of the fracture toughness test for the silanol condensation reaction cured resins is listed in Table 6. Resins II and III were cured as described in the Experimental section. In the test temperature range, both resins showed very limited plastic deformation during the test, as compared with the hydrosilylation cured Resin I. Nonlinearity in the load- displacement curves almost completely disappeared at all temperatures for the cured Resin II. which was strongly associated with its brittleness and lack of chain mobility. The curves for the cured Resin III showed slightly increased nonlinearity, and more indication of ductile tear past the maximum load at several temperatures. These features correkited with the higher fracture toughness the cured Resin III exhibited than the cured Resin II.

The drastically different temperature dependence of the fracture toughness of the two families of resins is derived from the network structure as well as the segmental structure along the polymer chain. Containing 85% trifunctional monomers and no monofunctional end terminating agents, Resin II is much more highly cross-linked than Resin I. The cross-link density of Resin III is slightly reduced by the incorporation of 10 wt% polydimethylsiloxane but is still much higher than that of Resin I. Resin I contains 75% trifunctional monomers but roughly one third of them are end terminated by a monofunctional group -SiMe^sub 2^Vi. The difunctional cross-linkers establish a network by chemically linking these end blockers and do not contribute to increasing the density of cross-links. The other major difference between those two cross- linking mechanisms is the final bridging segments in the resin network. A silanol condensation reaction results in a bridging segment of =SiOSi= (non network forming bonds not shown), while a hydrosilylation reaction produces a bridging segment of -SiCH2CH2Si- which is much longer and more flexible. Due to the difference in network and molecular structure these two families of resins have very different thermal mechanical properties. Resin I cured with a difunctional cross-linker usually shows a very sharp and distinct glass transition [15], while the cured Resins II and III normally show a very gradual, hard-to-detect glass transition spanning a temperature range of more than 100C [8]. The different temperature dependence of fracture toughness is merely another manifestation of the different network structure.

CONCLUSIONS

The temperature dependence of mechanical properties of two families of silicone resins was investigated. The first family was representative of hydrosilylation reaction curable silicone resins, and the second representative of condensation reaction curable silicone resins. The hydrosilylation curable resins were the Resin I [(PhSiO^sub 3/2^)^sub 0.75^ (ViMe^sub 2^SiO^sub 1/2^)^sub 0.25^] cross-linked with a variety of cross-linkers, including 1.4- bis(dimethylsilyl)benzene, 1,1,3,3.5,5,hexamethyllrisiloxane, diphenylsilane. and their mixtures. The condensation reaction curable resins were the Resin II [(PhSiO^sub 3/2^)^sub 0.40^(MeSiO^sub 3/2^)^sub 0.45^(Ph2SiO)^sub 0.10^(PhMeSiO)^sub 0.05^] and its toughened version Resin III. Properties studied included flexural strength, flexural modulus, and fracture toughness K^sub Ic^. Temperature effect on these properties of the first family of resins was substantial and depended strongly on the type of cross-linkers used. Generally for this family of resins the flexural strength and modulus decreased with a rising temperature. Fracture toughness K^sub Ic^ showed a peaking behavior with the peak appearing at -62C below the a transition peak. This was explained by the effect of the network deformability, which determined the plastic zone size, in association with the effect of the network resistance to plastic deformation. The second family of resins also showed decreases in modulus and strength with a higher testing temperature, but the fracture toughness changed little with temperature. This was probably due to the high cross-link density that resulted in a limited ability of deformation.

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Yuhong Wu, Frederick J. McGarry

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Bizhong Zhu, John R. Keryk, Dimitris E. Katsoulis

Dow Corning Corporation, Midland, Michigan 48686

Corresponence to: B. Zhu; e-mail: bizhong.zhu@dowcorning.com

DOI 10.1002/pen.20423

Published online 23 September 2005 in Wiley InterScience (www. interscience.wiley.com).

2005 Society of Plastics Engineers

Copyright Society of Plastics Engineers Nov 2005

Source: Polymer Engineering and Science

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