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High performance precipitated silica equivalent to fumed silica in silicone reinforcement.

Silicone rubber is used in a number of applications where its unique properties provide a substantial benefit (ref. 1). Many of these properties are highly dependent on the type and quantity of filler used in the compound. Particular silicone rubber applications have certain physical strength requirements for example: wire and cable, medical and surgical, belting, hose, tubing and various fuel-resistant rubber uses (ref. 2). When physical strength is a primary concern, a reinforcing silica is the filler of choice (refs. 3 and 4). When choosing a reinforcing silica the effect on a number of silicone rubber performance properties has to be considered (ref. 5) (table 1). Note that the amount of reinforcement per unit cost is a key consideration. The amount of reinforcement is defined by a strength-to-hardness ratio. Commercial precipitated silicas currently offer the best cost advantage, but perform only in the low to the medium reinforcement range. Fumed silicas currently provide the highest reinforcement, but also at a premium in cost (refs. 3 and 5).

Fumed and precipitated silica production processes differ significantly (figure 1). The fumed silica process produces a finer and higher purity product, but is more energy intensive than the precipitated silica process. These differences result in a significant premium in fumed silica cost. Both processes rely on proper manipulation of various parameters for production of silicas with unique properties. Silica properties such as surface area, structure and polymer adhesion have been related to the reinforcement of silicone rubber (refs. 3 and 6). By careful control of process parameters a unique precipitated silica has now been produced which possesses a certain balance of these properties. The properties of this precipitated silica (BXS-245) and results of statistically designed studies for BXS-245 and a fumed silica reinforced silicone rubber are summarized. Mathematical models were developed and are used to predict formulations for maximizing the compound strength at a number of hardness levels.

Experimental

Silica physical properties

BET surface was determined using an automatic surface area analyzer which employed a single point dynamic method using nitrogen adsorption.

Oil (dibutyl phthalate) adsorption was determined following a modified ASTM D2414 procedure. The modifications were: (1) the sample was dried at 105-degrees-C for two hours, (2) the test weight was 10 grams of dried silica, (3) dampening was set at one second, and (4) the adsorption volume was determined by the intersection of tangents drawn on the baseline and curve.

Particle size was determined using a Coulter Counter Model TAII instrument, which employs an electrical zone sensing method to cause a resistance change (voltage spike) proportional to particle volume.

105-degrees-C weight loss was determined on equilibrated samples by drying overnight at 105-degrees-C then exposing to 50% RH at 23-degrees-C for 17 hours. 105-degrees-C loss was then determined by measuring the percent change in weight after exposure to 105-degrees-C for 30 minutes at atmospheric conditions and 30 minutes at 760 mm pressure.

Physical properties are summarized in table 2. The 105-degrees-C loss test results indicate that BXS-245 precipitated silica has the same low moisture affinity as does a fumed silica.

Statistically designed experiments

To achieve the best performance, optimum formulations were determined using a Box-Wilson statistically designed experiment for both the BXS-245 precipitated and fumed silicas. The initial combinations investigated were augmented with additional experimental points. Process aid and silica loadings were varied from 0.0 to 11.5 and 15.9 to 78.0 parts per hundred of rubber (phr), respectively. Several points in the design were replicated. Compounding was based on a standard formulation consisting of silicone gum (100 phr), Lupersol 101 (0.5 phr) and a heat stabilizer (1 phr). The various combinations tested are summarized in tables 3 and 4. Experimental design and statistical analysis were performed using the computer software package "X-Stat: Statistical Experiment Design/Data Analysis/Nonlinear Optimization" published by Wiley Professional Software and copyrighted by Softpower Inc.

Compounding

Compounding was performed in a Baker-Perkins mixer. The gum and process aid, carried on 10 grams of the silica to be used, were mixed for 10 minutes. Ten percent of the total silica weight was added allowing thorough massing to occur before subsequent additions. A maximum of 75 minutes was allowed for incorporation and massing of silica. A few batches required massing on a two roll mill. After massing, a one-hour heat treatment at approximately 150-degrees-C was performed via application of high pressure steam to the jacket on the mixer. A vacuum was applied to the mixer chamber during this heat treatment. The batch was removed and aged at room temperature overnight. The initiator (2,5-bis(t-butylperoxy)-2,5-dimethylhexane, 100% liquid) and heat stabilizer were added on a two-roll open mill. Six 3/4 cuts and six end rolls were performed after addition. The batch was then sheeted out and pressed into test slabs, about 2.03 millimeters thick, and cured for 10 minutes at 170-degrees-C under 10.4 MPa pressure following ASTM D3182. Post-curing was performed in an air circulating oven at 250-degrees-C for one hour. All mixing, milling, molding equipment and post-curing oven were used only for silicone rubber compounding to minimize any possible contamination from organic materials.

Rubber properties

Hardness was measured following ASTM D2240 using a Type A durometer. Tensile strength and ultimate elongation were measured following ASTM D412 Method A. Tear resistance was determined following ASTM D624 Die B. Compression set was determined following ASTM D395 Method B. These tests were performed on post-cured specimens. Plasticity was determined following ASTM D926 (five minute reading) on samples aged one day after final milling.

The data are summarized in tables 3 and 4. The durometer hardness (Shore A) ranged from 21 to 80, tensile strength ranged from 1.8 to 9.4 MPa, ultimate elogation ranged from 230 to 586%, tear resistance ranged from 5.1 to 26.6 kN/m, compression set ranged from 3 to 54% and plasticity ranged from 0.47 to 5.67 mm.

Photomicrographs

Thin sections were prepared from representative silicone rubber compounds by cryo-ultramicrotomy using a RMC model MT 6000 XL at temperatures of -126 to -140-degrees-C with a diamond knife. The sections were then examined by optical microscopy using transmitted light and differential interference contrast on a Nikon FXA.

Results and discussion

A review of the raw data (tables 3 and 4) shows that variations in silica and process aid loadings significantly impact a number of silicone rubber performance properties. Note that BXS-245 precipitated silica could be compounded at all combinations of silica and process aid loadings; however, fumed silica could not be compounded at high silica and low process aid loadings. The range in durometer hardness, tensile strength and tear resistance was equivalent between BXS-245 precipitated and fumed silica reinforced silicone rubber. Use of BXS-245 generally showed lower plasticity and higher ultimate elongation and compression set.

In a number of applications the amount of physical strength achieved at a particular hardness level is critical. Both tensile strength and tear resistance can be indicative of physical strength. Current commercially available precipitated silicas do not match fumed silica's physical strength/hardness relationship.

Data were used to build mathematical models for predicting silicone rubber performance, tables 5 and 6. Note that rubber performance properties, other than plasticity, are for post-cured conditions. A combination of >98% confidence and >80% correlation (R2) indicate that most of these equations have a relatively high degree of accuracy. Durometer hardness, tensile strength, ultimate elongation, compression set and plasticity were affected by the levels of silica and process aid. However, tear resistance was affected only by the silica loading. Compression set for both silica types and ultimate elongation for BXS-245 precipitated silica showed lower degrees of predictability. This suggests that these performance properties may be affected by additional parameters not contained in the design.

Mathematical models were used to predict optimum formulations which would provide maximum tensile strength at given hardness levels. These optimum formulations were predicted for both BXS-245 precipitated and fumed silica reinforced silicone rubber compounds. These predicted optimum formulations were confirmed via follow-up compounding and testing (tables 7 and 8).

Graphical comparison was made of the predicted tensile strengths achieved at these various hardness levels. This comparison indicated that BXS-245 precipitated silica was essentially equivalent to fumed silica's reinforcement performance. This is further supported when comparing predicted tear resistance over a similar range of hardness levels.

This conclusion was verified by re-examining the raw data. Note that a number of the predicted optimum formulations are similar to those used in the design. The performance data for both sets compare favorably. Graphical comparisons show the same strength-to-hardness relationship, as seen in the optimized formulas, for both BXS-245 and fumed silica reinforced compounds. This provides further support that BXS-245 precipitated silica has the unique ability of matching fumed silica in reinforcement of silicone rubber. This, in combination with the follow-up confirmation study, also indicates that the predicted optimum formulas are a good representation of actual performance.

The predicted optimum formulas (tables 7 and 8) indicate that a slightly higher loading of the BXS-245 precipitated silica, but at a significantly lower loading of process aid, is required to match the reinforcement obtained using fumed silica. BXS-245 also provides additional advantages in higher ultimate elongation and lower plasticity, particularly at higher hardness levels. Lower plasticity should readily translate into improved processing efficiencies. This combination indicates that a significant cost savings should be realized when using BXS-245 precipitated silica.

The high reinforcement by BXS-245 precipitated silica was further evaluated in a number of silicone gum types that varied in manufacturing source and functionality. Equivalent formulations were employed and the results (figure 2) indicate that the high degree of reinforcement provided by BXS-245 appears to be independent of the silicone gum used.

Physical strength can be a function of the reinforing structural unit developed during processing (ref. 6). Silicone rubber mixing is a relatively low shear operation. The properties of BXS-245 precipitated silica are such that dispersion to the desired reinforcing unit is readily obtained under this low shear condition. Photomicrographs (figure 3) show that the dispersion of BXS-245 precipitated silica is significantly better than that of a precipitated silica currently used for silicone reinforcement. This is clearly indicated by the absence of large discrete islands generally associated with nondispersed particles. The dispersion of BXS-245 precipitated silica is essentially equivalent to fumed silica which provides additional evidence that BXS-245 gives reinforcement equivalent to fumed silica in silicone rubber.

Some high strength silicone rubber applications have the additional requirement of high thermal stability. One quantitative measurement of thermal stability is the percent loss in ultimate elongation when post cured specimens are exposed to 225[degrees]C for 70 hours (refs. 7 and 8). Non-stabilized silicone rubber reinforced with BXS-245 precipitated silica is superior to those reinforced with fumed silica. However, silicone rubber compounds reinforced with BXS-245 precipitated silica do not respond as favorably to oxidative stabilizers developed for fumed silica reinforced compounds. Silicone rubber compounds reinforced by precipitated silica are thought to thermally degrade by a non-oxidative mechanism. An appropriate means to provide the required heat stability for these particular applications is still required.

Conclusions

A unique precipitated silica, BXS-245, has been developed which is equivalent to fumed silica in the reinforcement of silicone rubber. This precipitated silica has an appropriate balance of physical properties to provide the proper reinforcing unit under the low shear mixing conditions encountered in silicone rubber compounding. This proper reinforcing unit provides a high degree of silicone rubber reinforcement. This high degree of reinforcement was independent of the type of silicone gum used.

Statistically designed experiments successfully determined formulations which would provide optimum strength at particular hardness levels for both BXS-245 precipitated and fumed silica reinforced silicone rubber compounds. Both raw data and predicted optimum formulations indicate that the strength/hardness relationship is equivalent for both BXS-245 precipitated and fumed silica reinforced silicone rubber. The optimized formulations for BXS-245 precipitated silica reinforced silicone rubber could also provide other additional performance benefits. The use of the optimized formulations containing the unique BXS-245 precipitated silica could produce significant cost savings to the silicone rubber compounder.

References

(1.) M.R. Toub, "Technical innovations enhance commercial value of silicone rubber," Elastomerics, 20 (August 1987).

(2.) J.F. Auchter, R. Mulach and K. Tsuchiya. "CEH data summary silicone elastomers," Chemical Economics Handbook - SRI International (June 1989).

(3.) M.A. Lutz. K.E. Polmanteer and H.L. Chapman, "Novel wet-process silica prepared from alkyl silicates. Part 1: Synthesis and Part 2: Performance in reinforcing silicone elastomers," Rubber Chem. Technol, 58, 939 and 953 (1985). 4. M.R. Toub "New developments in high-strength silicone rubber," presented at a meeting of the Rubber Division, ACS, Cleveland, OH (October 1987). "Electrochemical influence on carbon filled ethylene-propylene elastomers" is based on a paper given at the October, 1991 meeting of the ACS Rubber Division.

"Development and application of superfine tire powders for rubber compounding" is based on a paper given at the October, 1991 Tire Industry Conference.

"High performance precipitated silica equivalent to fumed silica in silicone reinforcement" is based on a paper given at the October, 1990 meeting of the ACS Rubber Division.

"Viscoelastic characterization of rubber with a new dynamic mechanical tester" is based on a paper given at the April, 1992 meeting of the Akron Rubber Group.
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Author:Wagner, M.P.
Publication:Rubber World
Date:Jun 1, 1992
Words:2206
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