Editorial Type: Papers
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Online Publication Date: 01 Mar 2017

SILANIZATION CHARACTERIZATION AND COMPOUND PROPERTIES OF SILICA-FILLED RUBBER CONTAINING A BLOCKED MERCAPTO SILANE

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Article Category: Research Article
Page Range: 126 – 145
DOI: 10.5254/rct.16.83771
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ABSTRACT

The silanization of silica-filled compounds containing a blocked mercapto silane, 3-octanoylthiopropyltriethoxysilane (OTPTES), was characterized by evaluating the exposed silica surface covered by the silane. Measurements of silica flocculation upon heating were employed to estimate the exposed silica surface coverage for silanization calculation. Compounds were prepared with various mechanical and thermal histories by adjusting the mixing time and temperature. Dynamic hysteretic properties at 0 and 60 °C in conjunction with strategic mechanical properties were correlated with the degree of silanization for each compound. Well-balanced properties in processability, reinforcement, and performance were readily attained through the study of the silanization/structure/property relationship. Experimental data showed that an 84% degree of silanization was needed to attain the optimal properties of compounds containing OTPTES regardless of the mixing protocol. Consistent compound properties may be realized by using a designated degree of silanization as a key mixing parameter for effective quality control in tire production.

INTRODUCTION

The blocked mercapto silane 3-octanoylthiopropyltriethoxysilane (OTPTES), a commercially available thiocarboxylate functionalized silane [NXT* silane (NXT is a trademark of Momentive Performance Materials Inc.)], has been shown to improve rolling resistance performance in a silica-filled tire without the sacrifice of wet traction.1 The ease of compound processing using OTPTES in silica-filled rubbers has advantageously increased the production efficiency in tire manufacturing plants when compared with standard silanes such as bis[3-(triethoxysilyl) propyl] tetrasulfide (TESPT) and bis[3-(triethoxysilyl) propyl] disulfide (TESPD). Additionally, OTPTES can be mixed at temperatures as high as 170 °C without the concerns of scorch.1 OTPTES is typically recommended to be mixed at certain times and temperatures to attain the desired compound properties.1 The intention is to react OTPTES with the exposed silica surface (silanization) to optimize filler–polymer coupling. Owing to the difference in tire plant mixing equipment and the variation in mixing schemes adopted by tire manufacturers, it is difficult to ensure consistent results using a fixed mixing process. For the silane containing compounds, the degree of silanization achieved during mixing is one of the key factors to control the quality of processing, rubber reinforcement, and other compound properties. Being able to quantify directly or indirectly the extent of reaction between OTPTES and the exposed silica surface is essential to attaining consistent silanization during mixing. Therefore, a feasible and reliable method to quantify the silanization needs to be developed to realize the desired processing and compound properties.

Silanization can be quantified by directly measuring either the latent or the evolved alcohol released in mixed compounds.2–7 The measurement of the filler network by using a rheometer was also suggested.6,8–11 Since the degree of filler network developed is inversely proportional to the exposed silica surface coverage by a silane, the amount of silane reacted with the exposed silica surface can be calculated. However, such an indirect method is valid only when the exposed silica surface is covered by a silane such as OTPTES before curing.12 Materials other than silane that interact with the exposed silica surface are functionalized polymers designed to bond to silica,13–15 mercapto16 and standard silanes17 that prematurely couple to the rubber during mixing and processing, and additives including curatives that may be absorbed by the filler.14 Therefore, the indirect silanization measurement by rheology may not reflect the actual exposed silica-surface area covered by the silane, which affects the silanization calculation.

Based on the concerns expressed above, a measurement of the compound filler network formation is a reasonable approach to quantify the degree of silanization in an OTPTES/silica-filled rubber. In this paper, an indirect method has been devised using rheology, which may be more convenient and practical for typical tire plants. Specifically, the authors propose to measure a silane's capability to suppress silica filler network formation (flocculation) and to correlate those measurements to the resultant compound properties. The optimal mixing conditions can then be selected based on the silanization/property correlations to give desired compound properties. The degree of silanization quantified by the proposed method is used for controlling and predicting compound properties regardless of the mixing scheme and/or equipment. Providing tire manufacturers with the proposed quantification method in lieu of mixing instructions should be a more practical and feasible approach in rubber applications.

EXPERIMENTAL

Materials and formulation

The materials used for preparing the rubber compound are listed in Table I, and their compositions are expressed as parts per 100 parts of total diene rubbers (phr), where the “Common Ingredients A” and “Common Curative Ingredients” are listed in Tables II and III, respectively. Ten weight percent (wt%) of TESPT (Struktol, Stow, OH, USA) and 8 wt% of OTPTES based on 80 phr silica loading were compounded into their respective stocks. A dosage of 8.35 phr of n-octyl triethoxy silane (OTES; Sigma-Aldrich, St. Louis, MO, USA) was suggested by Lin et al.2 for the OTES stock, providing the same silicon concentration as the TESPT stock. The free sulfur phr for each compound was adjusted to equate the total sulfur content in each compound to 4.98 × 1022 atoms (theoretical number of sulfur atoms contained in the silane and curative phrs plus the free sulfur phr).

Table I Materials Used for Preparing the Rubber Compounds
Table I
Table II The Common Ingredients A in Table I
Table II
Table III The Common Curative Ingredients in Table I
Table III
Table IV The Mixing Procedure for MB1 Stage
Table IV
Table V The Mixing Procedure for MB2 Stage
Table V
Table VI The Mixing Procedure for FM Stage
Table VI

Compound preparations

All compounds were mixed using a three-stage mixing protocol in a Farrell BR1600 Banbury (Ansonia, CT, USA). The mixing consists of two non-productive mixing stages (MB1 and MB2) and a productive mixing stage (FM), which contains the curatives. The mixer has a chamber net volume of 1.57 L, and the MB1, MB2, and FM mixing stages for each compound were conducted at fill factors of 72, 69, and 68%, respectively. The rubbers containing OTPTES were mixed at various mechanical and thermal histories by adjusting mixing time and temperature. Specifically, compounds were mixed to a designated maximum temperature for various time spans before being discharged. This part of the mixing procedure is called the maximum temperature heat history (MTHH) throughout this paper. When an OTPTES containing stock is mixed to a maximum temperature of 150 °C and is maintained at that temperature for 60 s, the stock has a MTHH of 60 s at 150 °C. The mixing temperature profile seen in Figure 1 is an example of MTHH. The total MTHH time for each test compound is the sum of the MTHH time performed in MB1 and MB2 stages.

Fig. 1. The exemplified mixing temperature profile used for rubber preparation.Fig. 1. The exemplified mixing temperature profile used for rubber preparation.Fig. 1. The exemplified mixing temperature profile used for rubber preparation.
Fig. 1. The exemplified mixing temperature profile used for rubber preparation.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

The OTES test compound was mixed with 180 s of MTHH time in MB1 and an additional 100 s of MTHH time in MB2 for a total of 280 s MTHH time at higher MTHH temperatures (170/160 °C for MB1/MB2) to maximize the completion of silanization. The TESPT stock (stock 1) was prepared as a reference for comparison to OTPTES stocks. Moderate MTHH temperatures of 140 °C (MB1) and 135 °C (MB2) were employed to prepare the TESPT stock following the recommended mixing conditions in the prior arts and the common practice in the industry.17–19 The mixing procedures for MB1, MB2, and FM are shown in Tables IV–VI, respectively. The MTHH times and temperatures of all rubber stocks are shown in Tables VII and VIII. All MB1 test stocks were allowed to cool overnight prior to starting MB2 mixing stage. All MB2 test stocks were allowed to cool for a minimum of 16 h prior to rubber process analyzer (RPA) silanization testing or mixing the productive stage to obtain property/silanization correlations. Selected FM stocks were sampled for RPA testing, and the remaining FM rubbers were sheeted out and cured at 160 °C for 20 min before they were tested.

Table VII The MTHH Times and MTHH Temperatures for Rubber Stocks
Table VII
Table VIII The MTHH Times and MTHH Temperatures for Rubber Stocks
Table VIII

TEST CONDITIONS

Strain sweeps before and after thermal annealing on mb2 stocks

Strain sweeps on MB2 stocks were conducted using an RPA from Alpha Technologies (Akron, OH, USA). MB2 samples were exposed and preconditioned to mechanical and thermal histories similar to the FM mixing before they were tested in the RPA. These preconditioning steps are described as follows:

  • 1.

    75 °C for 1 min at 1 Hz and 200% strain amplitude.

  • 2.

    93 °C for 1 min at 1 Hz and 200% strain amplitude.

  • 3.

    102 °C for 0.3 min at 1 Hz and 200% strain amplitude .

The conditions for testing strain sweep are a temperature of 45 °C, a frequency of 0.1 Hz, and strain amplitudes of 0.25 to 500%. The thermal annealing was conducted at 160 °C for 20 min, 0.5% strain amplitude, and 0.1 Hz frequency within the RPA. Note that the thermal conditions are the same as the curing condition to vulcanize the final stock rubbers. Strain sweeps were conducted on virgin samples before and after thermal annealing.

Strain sweeps on cured fm stocks

The RPA was used to measure the strain sweeps on the cured FM stocks. The conditions for strain sweep are a temperature of 60 °C, a frequency of 1 Hz, and strain amplitudes of 0.25 to 100%. The curing was conducted at 160 °C for 20 min, 0.5% strain amplitude, and 0.1 Hz frequency within the RPA die cavity.

Dynamic mechanical property measurements on vulcanized rubbers

Dynamic mechanical properties of the vulcanized rubbers were measured using a dynamic mechanical analyzer (DMA) Eplexor 500N manufactured by Gabo Qualimeter (Testanlagen, Germany) to perform temperature sweeps in compression mode. Samples for testing are cylindrical in shape, measuring 10 mm in diameter and 10 mm in height. Each sample was preconditioned at a static strain amplitude of 20% and was dynamically deformed at a strain amplitude of 12% for 2 min. The initial contact force on the sample was set at 2.0 N for the preconditioning. A frequency of 10 Hz was used for all temperature sweeps including the preconditioning.

Temperature Sweep on Vulcanized Rubbers

Temperature sweeps were carried out on the cured FM stocks at a static strain amplitude of 10% and a dynamic strain amplitude of 0.5% over the temperature range of −80 to 100 °C. Hysteresis (tan δ) data at 0 and 60 °C were recorded and used as the predictions for tire wet traction and rolling resistance, respectively.

Tensile test

The rubber tensile mechanical properties were measured using a Zwick Ring Tester (Zwick/Roell, Ulm, Germany) following ASTM-D 412 test procedure at 25 °C on the cured FM stocks. Test specimens were rings with an inside diameter of 44.6 mm, an outside diameter of 52.6 mm, and a height of 6 mm. Tensile mechanical properties were measured as moduli at 50% (M50), 100% (M100), and 300% (M300) elongation. Total elongation was determined by the elongation at break, and tensile strength was recorded as the stress at break.

Din abrasion test

Abrasion resistance was evaluated by the Bareiss Abrasion Tester, AB6235 (Heirich Prufgeratebau GmBH, Oberdischingen, Germany), at room temperature following ASTM-D5963 test procedure on the cured FM stocks. The sample is a disk of 16 mm in diameter and 6 mm in height. The abrasion distance has a length of 40 m. Sample weights were recorded before and after testing for the abrasion calculation. Less material loss indicates a better abrasion resistance. The measurement of abrasion using the German standard Deutsches Institut für Normung (DIN 53516), DIN abrasion is reported as the volumetric loss calculated from the measured weight loss.

Shore a hardness test

Shore A hardness was performed using a Zwick durometer hardness tester (Zwick/Roell, Ulm, Germany) at ambient temperature following the ASTM-D2240 test method on the cured FM stocks.

Mooney viscosity measurement

Mooney viscosity was measured on the uncured FM stocks using a MV 2000 Mooney viscometer from Alpha Technologies following the ASTM-D1646 test method. The sample was preheated to 100 °C for 1 min. Mooney viscosity (ML1+4; large rotor) was recorded as the torque after the rotor had rotated for 4 min at 2 rotations per minute (rpm; average shear rate about 1.6 s−1).

Prevulcanization characteristics (mooney scorch)

Mooney scorch was measured in a similar fashion as Mooney viscosity on the uncured FM stocks but conducted at a temperature of 135 °C. The ASTM-D1646 test method was followed. Using a small rotor, torque versus time was recorded, and the time values were reported when the torque values increased 3 points (T3) and 30 points (T30) above the minimum. T3 was taken as the Mooney scorch, which is the measure of the safety margin of the compound viscosity during the processing before curing. Longer T3 time allows compounds to be processed with fewer processing issues related to compound viscosity change.

Curing-rheometer (mdr 2000)

Compound curing was characterized using a moving die rheometer (MDR 2000) from Alpha Technologies following ASTM-F5289 standard procedure at 160 °C on uncured FM stocks. Torque versus time was recorded for 30 min. The rheological responses upon curing for selected stocks are shown in Figure 2. Minimum torque (ML) and maximum torque (MH) were recorded. A cure point of 90% (T90) was calculated from the maximum torque value. Selected curing characteristics of all FM stocks are listed in Tables IX and X. Note that the curing time of 20 min used to prepare the rubber is greater than T90 of those TESPT and OTPTES stocks. No physical properties of cured stocks 17 and 18 were measured or reported throughout this study.

Fig. 2. The rheological responses upon curing measured by MDR 2000 for stocks 1 (solid line), 6 (dotted line), 11 (dashed line), 14 (dashed and dotted line).Fig. 2. The rheological responses upon curing measured by MDR 2000 for stocks 1 (solid line), 6 (dotted line), 11 (dashed line), 14 (dashed and dotted line).Fig. 2. The rheological responses upon curing measured by MDR 2000 for stocks 1 (solid line), 6 (dotted line), 11 (dashed line), 14 (dashed and dotted line).
Fig. 2. The rheological responses upon curing measured by MDR 2000 for stocks 1 (solid line), 6 (dotted line), 11 (dashed line), 14 (dashed and dotted line).

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Table IX The Curing Characteristics for Rubber Stocks
Table IX
Table X The Curing Characteristics for Rubber Stocks
Table X
Table XI The Summary of Physical properties of stocks 1, 11, and 12
Table XI

RESULTS AND DISCUSSION

The establishment of the silanization estimation

The degree of silanization of a silica-filled compound is estimated by calculating the difference in the degree of filler network developed before and after thermal annealing at 160 °C for 20 min with the MB2 stock containing no curatives. The thermal annealing condition was chosen to be the same as the one used for curing the rubber in order to simulate the occurrence of filler flocculation during vulcanization under minimum perturbation. It is essentially the measure of the silica flocculation upon heating. When the filler–polymer interaction is limited and remains relatively constant before curing, the degree of silanization can be estimated from the extent of the exposed silica surface coverage by the silane. Even upon heating, the premise is reasonable for compounds containing OTPTES because the filler–polymer interaction is limited due to the stable blocked mercapto moiety of OTPTES.12

The extent of silica flocculation upon heating is directly proportional to the amount of silica aggregates or agglomerates isolated from one to another. Therefore, the measure of silica flocculation suppression by the silane can be used to estimate the exposed silica surface covered by the silane, giving an estimation of silanization. The silica flocculation upon heating for various MB2 stocks was followed by monitoring the increase in G′ as a function of annealing time shown in Figures 3 and 4. The effectiveness of suppressing silica flocculation in compounds containing different silanes at a given dosage is in the order of OTES > OTPTES > TESPT > no silane. According to Lin et al.,11 it is reasonable to assume that nearly all of the exposed silica aggregate surfaces are covered by the OTES. In contrast, there are significant amounts of silica agglomerates with bare exposed surfaces in the stock containing no silane (stock 18). This information provides the baselines for calculating the degree of silanization of the OTPTES stocks.

Fig. 3. The filler flocculation observed as G′ increased with annealing time at 160 °C for various MB2 stocks. (A) Red line, stock 1 with TESPT; (B) Blue line, stock 11 with OTPTES; (C) Green line, stock 17 with OTES.Fig. 3. The filler flocculation observed as G′ increased with annealing time at 160 °C for various MB2 stocks. (A) Red line, stock 1 with TESPT; (B) Blue line, stock 11 with OTPTES; (C) Green line, stock 17 with OTES.Fig. 3. The filler flocculation observed as G′ increased with annealing time at 160 °C for various MB2 stocks. (A) Red line, stock 1 with TESPT; (B) Blue line, stock 11 with OTPTES; (C) Green line, stock 17 with OTES.
Fig. 3. The filler flocculation observed as G′ increased with annealing time at 160 °C for various MB2 stocks. (A) Red line, stock 1 with TESPT; (B) Blue line, stock 11 with OTPTES; (C) Green line, stock 17 with OTES.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Fig. 4. The filler flocculation observed as G′ increased with annealing time at 160 °C for the MB2 stock 18.Fig. 4. The filler flocculation observed as G′ increased with annealing time at 160 °C for the MB2 stock 18.Fig. 4. The filler flocculation observed as G′ increased with annealing time at 160 °C for the MB2 stock 18.
Fig. 4. The filler flocculation observed as G′ increased with annealing time at 160 °C for the MB2 stock 18.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Silanization calculation

The procedure for calculating the degree of silanization of the OTPTES green compound is described in the following sections

Calculation of the flocculation suppression upon heating in uncured rubbers (δ(Δg′))

The capability of suppressing the filler flocculation by a silane is calculated by measuring the difference in the Payne Effect values (δ(ΔG′)) of a MB2 stock from the strain sweep taken before and after thermal annealing where

Typical G′ strain dependences of OTPTES MB2 stocks prepared with different MTHHs before and after thermal annealing are shown in Figure 5 and used for the δ(ΔG′) calculations. Note that less silica flocculation occurred in stock 6, which was mixed with longer MTHH time (220 s), compared with stock 2 with a MTHH of only 33 s. Longer mixing at a given MTHH temperature appears to increase silanization for stocks containing silane.

Fig. 5. The G′ strain dependences of MB2 stocks 2 (square data points, MTHH time = 33 s) and 6 (triangular data points, MTHH time = 220 s) before (open symbols) and after (solid symbols) thermal annealing.Fig. 5. The G′ strain dependences of MB2 stocks 2 (square data points, MTHH time = 33 s) and 6 (triangular data points, MTHH time = 220 s) before (open symbols) and after (solid symbols) thermal annealing.Fig. 5. The G′ strain dependences of MB2 stocks 2 (square data points, MTHH time = 33 s) and 6 (triangular data points, MTHH time = 220 s) before (open symbols) and after (solid symbols) thermal annealing.
Fig. 5. The G′ strain dependences of MB2 stocks 2 (square data points, MTHH time = 33 s) and 6 (triangular data points, MTHH time = 220 s) before (open symbols) and after (solid symbols) thermal annealing.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Calculate the silanization

Based on the reasoning and assumptions described in the previous sections, the degree of silanization of OTPTES containing compounds can be calculated using the following equation:

where

  • δ (ΔG′)no coverage is obtained from a silica-filled compound with no silane (stock 18).

  • δ(ΔG′)partial coverage at a given mixing is obtained from an OTPTES compound prepared at a given mixing condition (stocks 2–16).

  • δ(ΔG′)complete coverage is obtained from octyl silane compound (stock 17).

The OTES dosage and mixing used to prepare stock 17 for δ(ΔG′)complete coverage calculation were estimated by Lin et al.11 The data shown in Figures 3 and 4 confirm a reasonable approximation of covering most of the exposed silica surface by OTES.

The silanization curves of the otptes stocks

Silanization values calculated from Eq. 3 are shown in graphs as a function of total MTHH time and temperature in Figure 6. The degree of silanization increased as compounds were mixed at either higher MTHH temperature and/or longer MTHH time. The degree of silanization shown in Figure 6 appears to be greater than the maximum silanization of 66.7%20,21 possible attained through the normal reaction process, or 83.3% with the catalytic effect from the presence of water or in an acidic or alkaline enviroment.7,22 Since the degree of silanization calculated in this study is based on the exposed silica surface coverage, the silane molecular orientation will affect the coverage depending on the size, configuration, and silica-associated moiety of the silane.6 It has been shown that at minimum eight carbons in the linear alkyl chain of a triethoxy silane monomer are required to effectively cover the silica suface and shield it from other reactants.11 Therefore, the increase in the degree of silanization may be due to the extra exposed silica surface shielded by the OTPTES' aliphatic tail. Regardless of the cause and effect, the silanization values calculated in this study are quantitatively proportional to the actual extent of OTPTES reaction. Since the comparsions among compounds were made based on OTPTES reaction with 80 phr of silica, the findings are significant.

Fig. 6. The silanization graphs of compounds as a function of MTHH time and temperature. (diamonds for stocks 2 to 6; squares for stocks 7 to 11; and triangles for stocks 12 to 16).Fig. 6. The silanization graphs of compounds as a function of MTHH time and temperature. (diamonds for stocks 2 to 6; squares for stocks 7 to 11; and triangles for stocks 12 to 16).Fig. 6. The silanization graphs of compounds as a function of MTHH time and temperature. (diamonds for stocks 2 to 6; squares for stocks 7 to 11; and triangles for stocks 12 to 16).
Fig. 6. The silanization graphs of compounds as a function of MTHH time and temperature. (diamonds for stocks 2 to 6; squares for stocks 7 to 11; and triangles for stocks 12 to 16).

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Procedure for determining a mixing condition to attain optimal processing and compound properties

The goal of this work is to establish a method to quantify the degree of silanization in order to determine the mixing conditions for attaining the optimal processing and compound properties. The method is then used as a guide for using OTPTES effectively for tire manufacturing. The silanization characterization is chosen as the method to achieve this goal, and the procedure is described in the following:

  • •

    Establish the mixing silanization curves as those described in the preceeding sections.

  • •

    Correlate the degree of silanization to various compound properties.

  • •

    Determine the degree of silanization that gives optimized properties of OTPTES stocks compared with those of TESPT stock.

  • •

    Correlate the silanization with the mixing scheme.

Correlation between the degree of silanization and compound processing

OTPTES is known for providing excellent processing characteristics such as lower compound Mooney viscosity (ML1+4) compared with standard silanes.1 Lower ML1+4 and longer Mooney scorch (T3) of the compounds are generally desirable for better processing and tire build processes in a tire plant. The ML1+4 and T3 values from the OTPTES stocks are shown in Figure 7. Both ML1+4 and T3 improved as the degree of silanization increased in the OTPTES stocks. In the absence of a significant increase in filler–polymer interaction, the greater the degree of silanization, the more exposed silica surface that is covered by OTPTES yielding less filler–filler interaction. This results in lower ML1+4 and longer T3. The ML1+4 of all OTPTES stocks is lower than that of TESPT (ML1+4 = 83), while equal or longer T3 values (11.5 min of TESPT stock) are found for OTPTES stocks with the silanization greater than 84%.

Fig. 7. The Mooney viscosity (ML1+4) and Mooney scorch (T3) of OTPTES compounds are plotted as a function of degree of silanization. (squares for ML1+4; triangles for Mooney scorch (T3 at 135 °C)).Fig. 7. The Mooney viscosity (ML1+4) and Mooney scorch (T3) of OTPTES compounds are plotted as a function of degree of silanization. (squares for ML1+4; triangles for Mooney scorch (T3 at 135 °C)).Fig. 7. The Mooney viscosity (ML1+4) and Mooney scorch (T3) of OTPTES compounds are plotted as a function of degree of silanization. (squares for ML1+4; triangles for Mooney scorch (T3 at 135 °C)).
Fig. 7. The Mooney viscosity (ML1+4) and Mooney scorch (T3) of OTPTES compounds are plotted as a function of degree of silanization. (squares for ML1+4; triangles for Mooney scorch (T3 at 135 °C)).

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Correlation of the degree of silanization with shore a hardness and with abrasion resistance

Hardness is one of the important physical properties for tire performance evaluation. It is usually an indication of the cure state for the styrene-butadiene rubber (SBR) compounds,23 and a direct relationship to the material low-strain elastic modulus,24–26 which can be used to correlate with traction and handling performance. Usually a Shore A hardness greater than 65 (ASTM D2240 at 23 °C) is desired for SBR passenger tread compounds.27 Although there is no precise and accurate method to predict rubber wear resistance, abrasion resistance testing is most commonly used for wear evaluation in the tire industry. Moderate Shore A hardness (65–70)27 and higher abrasion resistance are required for performance improvements in passenger tire tread applications.

The Shore A hardness and abrasion resistance from all of the cured OTPTES stocks are shown in Figures 8 and 9, respectively. Correlations between Shore A hardness and abrasion resistance as a function of the degree of silanization are in conflict. An increase in the degree of silanization improved abrasion resistance but reduced hardness. Increased silanization by OTPTES decreased filler–filler interaction, giving a reduction in low-strain modulus (shown in Figure 10), and may result in lowering compound hardness. In contrast, increased silanization yielded more filler–polymer coupling, improving reinforcement and hysteresis. Depending on the desired rubber compound properties, a compromise between these two properties needs to be made with acceptable hardness and abrasion resistance performances. A good balance of Shore A hardness and abrasion resistance of OTPTES stocks 11 and 12, which are comparable to those of TESPT stock (65.7 and 102 mm3 in Shore A hardness and DIN abrasion volumetric loss, respectively), can be attained at the 84% silanization level. Note that similar Shore A hardness and DIN abrasion were found in OTPTES stocks 11 and 12, whose silanizations were approximately 84%. In this study it was determined that the optimized OTPTES compounds at a silanization level of 84% needed a comparable Shore A hardness to TESPT. Validation of this compound optimization may need additional consideration of other properties at this chosen silanization level.

Fig. 8. The Shore A hardness of cured OTPTES compounds are plotted as a function of degree of silanization.Fig. 8. The Shore A hardness of cured OTPTES compounds are plotted as a function of degree of silanization.Fig. 8. The Shore A hardness of cured OTPTES compounds are plotted as a function of degree of silanization.
Fig. 8. The Shore A hardness of cured OTPTES compounds are plotted as a function of degree of silanization.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Fig. 9. The DIN abrasion volumetric losses of cured OTPTES compounds are plotted as a function of degree of silanization.Fig. 9. The DIN abrasion volumetric losses of cured OTPTES compounds are plotted as a function of degree of silanization.Fig. 9. The DIN abrasion volumetric losses of cured OTPTES compounds are plotted as a function of degree of silanization.
Fig. 9. The DIN abrasion volumetric losses of cured OTPTES compounds are plotted as a function of degree of silanization.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Fig. 10. The G′s at 2% strain amplitude of OTPTES compounds are plotted as a function of degree of silanization. The measurements were conducted on the cured FM stocks by the strain sweep using the RPA.Fig. 10. The G′s at 2% strain amplitude of OTPTES compounds are plotted as a function of degree of silanization. The measurements were conducted on the cured FM stocks by the strain sweep using the RPA.Fig. 10. The G′s at 2% strain amplitude of OTPTES compounds are plotted as a function of degree of silanization. The measurements were conducted on the cured FM stocks by the strain sweep using the RPA.
Fig. 10. The G′s at 2% strain amplitude of OTPTES compounds are plotted as a function of degree of silanization. The measurements were conducted on the cured FM stocks by the strain sweep using the RPA.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Correlation of the degree of silanization with rolling resistance and wet traction indicators

In tire tread compounds, wet traction performance is related to the compound high-frequency hysteresis,28–30 which is normally predicted based on the viscoelastic properties measured at reduced temperatures.31 Hysteresis is typically measured by tan δ [the ratio of dynamic loss (G′′) and storage (G′) modulus] and is related to the energy loss of the material. These properties, referred to as tire performance predictors, can be conveniently measured by dynamically deforming a rubber compound (frequency equal to 1–10 Hz) at certain strain levels as a function of testing temperature ranging from −100 °C to 100 °C.31 Thus, the desired rubber compound should give a lower tan δ value at 50 °C to 80 °C to reduce the rolling resistance (RR) and improve fuel economy. Ideally, this rubber compound should also exhibit higher tan δ in the temperature range of −20 °C to 0 °C to obtain improved wet grip. In this paper, tan δ measured at 0 °C and 60 °C are used for predicting the wet traction and RR, respectively.

The tan δ values measured at 0 and 60 °C of the cured OTPTES stocks are shown in Figure 11. Data were collected by dynamic mechanical analysis (DMA) in compression mode. As the degree of silanization increased in OTPTES compounds, tan δ at 0 °C increased with comcomitant decreasing of tan δ at 60 °C. The wet traction and RR of the OTPTES compounds are predicted to improve with increasing silanization. At 84% silanziation, the tan δ values at 60 °C of OTPTES stocks 11 (0.144) and 12 (0.142) were 20% lower than TESPT stock 1 (0.188), which is indicative of lower RR. Equivalent tan δ at 0 °C (0.542, 0.537, and 0.543 for stocks 11, 12, and 1, respectively) suggests similar wet traction. Note that OTPTES stocks 11 and 12 were prepared using significantly different mixing time and temperature as described in Table VIII and Figure 12. Similar properties in Shore A hardness, DIN abrasion, and dynamic viscoelastic tan δ were attained in OTPTES stocks with equivalent degrees of silanization regardless of the mixing conditions employed.

Fig. 11. The tan δ at 60 °C (circles, RR predictor) and 0 °C (diamonds, wet traction predictor) of cured OTPTES compounds are plotted as a function of degree of silanization.Fig. 11. The tan δ at 60 °C (circles, RR predictor) and 0 °C (diamonds, wet traction predictor) of cured OTPTES compounds are plotted as a function of degree of silanization.Fig. 11. The tan δ at 60 °C (circles, RR predictor) and 0 °C (diamonds, wet traction predictor) of cured OTPTES compounds are plotted as a function of degree of silanization.
Fig. 11. The tan δ at 60 °C (circles, RR predictor) and 0 °C (diamonds, wet traction predictor) of cured OTPTES compounds are plotted as a function of degree of silanization.

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Fig. 12. The mixing temperature profiles for preparing stocks 11 (solid line) and 12 (dashed line) (red and black lines for MB1 and MB2 mixing stages, respectively).Fig. 12. The mixing temperature profiles for preparing stocks 11 (solid line) and 12 (dashed line) (red and black lines for MB1 and MB2 mixing stages, respectively).Fig. 12. The mixing temperature profiles for preparing stocks 11 (solid line) and 12 (dashed line) (red and black lines for MB1 and MB2 mixing stages, respectively).
Fig. 12. The mixing temperature profiles for preparing stocks 11 (solid line) and 12 (dashed line) (red and black lines for MB1 and MB2 mixing stages, respectively).

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

The tan δ temperature dependences of these compounds are seen in Figure 13, where the two OTPTES stocks 11 and 12 were practically identical. The similar tan δ temperature dependencies of stocks 11 and 12, in concert with lower Mooney viscosity and longer scorch safety T3, appears to support the premise of consistant filler–polymer interaction regardless of the mixing temperature employed to prepare rubber stocks.

Fig. 13. Tan δ temperature sweeps: Stock 1 (TESPT, dotted line); Stock 11 (OTPTES, dark green dashed line) mixed with condition (a); Stock 12 (OTPTES, light green solid line) mixed with condition (b).Fig. 13. Tan δ temperature sweeps: Stock 1 (TESPT, dotted line); Stock 11 (OTPTES, dark green dashed line) mixed with condition (a); Stock 12 (OTPTES, light green solid line) mixed with condition (b).Fig. 13. Tan δ temperature sweeps: Stock 1 (TESPT, dotted line); Stock 11 (OTPTES, dark green dashed line) mixed with condition (a); Stock 12 (OTPTES, light green solid line) mixed with condition (b).
Fig. 13. Tan δ temperature sweeps: Stock 1 (TESPT, dotted line); Stock 11 (OTPTES, dark green dashed line) mixed with condition (a); Stock 12 (OTPTES, light green solid line) mixed with condition (b).

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Mixing control to attain the optimal compound properties

By use of the correlations of properties and the degree of silanization, the optimal silanization level that gives the best balanced compound properties with 20% reduction in tan δ at 60 °C and 10% reduction in ML1+4 was 84%. The mixing conditions used to give 84% silanization were mapped from the silanization curves (Figure 6). Surprisingly, there are two mixing conditions that provide similar, almost identical, solutions as described in the preceding sections:

  1. Stock 11 prepared by MTHH temperatures of 150/140 °C (MB1/MB2) for 120/100 s (MTHH times for MB1/MB2) with total MTHH time of 220 s.

  2. Stock 12 prepared by MTHH temperatures 160/150 °C (MB1/MB2) for 20/13 s (MTHH times for MB1/MB2) with total MTHH time of 33 s.

The physical properties of TESPT stock 1 and OTPTES stocks 11 and 12 are summarized in Table XI. For comparison of predicting tire performances, TESPT stock 1 was used as the reference, and properties in Table XI were translated into performance indices according to Eqs. 4 and 5. Equation 4 was used to normalize ML1+4, DIN abrasion, and tan δ at 60 °C performance indices. The rest of the indices were calculated by Eq. 5.

where

  • P OTPTES is a property of the OTPTES stock. (2 to 16).

  • P TESPT is a property of the TESPT stock (1).

Data normalization showed improved performance indices in the desired properties. Predicted tire performance indices of stocks 11 and 12 with 84% silanization are shown in Figure 14, where significant reductions in tan δ at 60 °C (20%) for RR and Mooney viscosity (10%) for processing with comparable tire performances in wet traction, abrasion resistance, Shore A hardness, and Mooney scorch (T3) are seen. The spider chart in Figure 14 and the data in Table XI show how similar the tire performance indices and physical properties of stocks 11 and 12 were, although the mixing conditions were significantly different. The lack of sulfur donation or premature coupling to the polymer during mixing allows greater flexibility in mixing schemes for compounding with OTPTES within the normal temperature range (100–170 °C). This is in contrast to TESPT due to the complexity of sulfur donation32 and low temperature coupling (≥150 °C)17 during the mixing and processing.

Fig. 14. Comparison of compound performance indices for TESPT stock 1(dotted line), OTPTES 11 mixed with condition (a) [dashed dark green line], and OTPTES stock 12 mixed with condition (b) [solid light green line].Fig. 14. Comparison of compound performance indices for TESPT stock 1(dotted line), OTPTES 11 mixed with condition (a) [dashed dark green line], and OTPTES stock 12 mixed with condition (b) [solid light green line].Fig. 14. Comparison of compound performance indices for TESPT stock 1(dotted line), OTPTES 11 mixed with condition (a) [dashed dark green line], and OTPTES stock 12 mixed with condition (b) [solid light green line].
Fig. 14. Comparison of compound performance indices for TESPT stock 1(dotted line), OTPTES 11 mixed with condition (a) [dashed dark green line], and OTPTES stock 12 mixed with condition (b) [solid light green line].

Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83771

Therefore, once the silanization level of 84% was achieved, regardless of the mixing scheme, similar compound properties from hardness, abrasion resistance, and the viscoelastic properties described in this section and others could be achieved in OTPTES stocks (see Table XI, Figures 13 and 14). A quantative guideline can be followed concerning the best use of OTPTES for compoundings regardless of the mixing equipment and procedures employed by different users. The excellent reproducibility in compound properties should provide greater flexibility in daily tire plant operations and may generate increased productivity and throughput. It may also be useful for quality control by reducing the standard deviation between rubber batches and producing consistent performances in the final product.

CONCLUSIONS

A method of characterizing the percentage silanization of silica-filled rubbers containing a blocked mercapto silane 3-octanoylthiopropyltriethoxysilane (OTPTES) was successfully established. The degrees of silanization of the rubber compounds prepared by various thermal and mechanical histories were calculated by following the process of silica flocculation suppression upon heating. Increasing mixing temperature and time effectively enhanced the silanization of OTPTES containing compounds without the concern of premature filler–polymer coupling during mixing and extrusion.

After establishing silanization-compound property correlations, 84% silanization was found to give the following best balance of properties in OTPTES containing compounds as compared with those of TESPT: (1) 20% reduction in tan δ at 60 °C, which can be beneficial to rolling resistance improvement in tires; (2) 10% reduction in compound Mooney viscosity, which could facilitate more efficient downstream processing such as extrusion for increased throughput; (3) and comparable properties such as tan δ at 0 °C (predictor of wet traction), Shore A hardness, DIN abrasion resistance (predictor of wear resistance), and tensile mechanical properties. Mixing the non-productive batches (MB1/MB2) at a lower heat history temperature (MTHHs) of 150/140 °C for 120/100 s and at higher MTHHs of 160/150 °C for 20/13 s produced similar silanization levels of 84%. Compound properties from stocks prepared by these two conditions were nearly identical, indicating little to no filler–OTPTES–polymer coupling during mixing regardless of temperature and time. Therefore, optimal OTPTES compound properties can be attained through silanization quantification and mixing scheme selection. The finding suggests increased mixing flexibility using OTPTES. Also, using the degree of silanization as a guideline for compounding OTPTES may be more convenient than proposing mixing instructions because of the vast differences in mixer types, equipment, and mixing protocols used by different manufacturers. In addition, the designated degree of silanization can be used as a key mixing parameter for effective quality control in tire production by providing consistent compound properties.

Copyright: 2017
<sc>Fig</sc> . 1.
Fig . 1.

The exemplified mixing temperature profile used for rubber preparation.

<sc>Fig. 2</sc> .
Fig. 2 .

The rheological responses upon curing measured by MDR 2000 for stocks 1 (solid line), 6 (dotted line), 11 (dashed line), 14 (dashed and dotted line).

<sc>Fig</sc> . 3.
Fig . 3.

The filler flocculation observed as G′ increased with annealing time at 160 °C for various MB2 stocks. (A) Red line, stock 1 with TESPT; (B) Blue line, stock 11 with OTPTES; (C) Green line, stock 17 with OTES.

<sc>Fig</sc> . 4.
Fig . 4.

The filler flocculation observed as G′ increased with annealing time at 160 °C for the MB2 stock 18.

<sc>Fig</sc> . 5.
Fig . 5.

The G′ strain dependences of MB2 stocks 2 (square data points, MTHH time = 33 s) and 6 (triangular data points, MTHH time = 220 s) before (open symbols) and after (solid symbols) thermal annealing.

<sc>Fig</sc> . 6.
Fig . 6.

The silanization graphs of compounds as a function of MTHH time and temperature. (diamonds for stocks 2 to 6; squares for stocks 7 to 11; and triangles for stocks 12 to 16).

<sc>Fig</sc> . 7.
Fig . 7.

The Mooney viscosity (ML1+4) and Mooney scorch (T3) of OTPTES compounds are plotted as a function of degree of silanization. (squares for ML1+4; triangles for Mooney scorch (T3 at 135 °C)).

<sc>Fig</sc> . 8.
Fig . 8.

The Shore A hardness of cured OTPTES compounds are plotted as a function of degree of silanization.

<sc>Fig</sc> . 9.
Fig . 9.

The DIN abrasion volumetric losses of cured OTPTES compounds are plotted as a function of degree of silanization.

<sc>Fig</sc> . 10.
Fig . 10.

The G′s at 2% strain amplitude of OTPTES compounds are plotted as a function of degree of silanization. The measurements were conducted on the cured FM stocks by the strain sweep using the RPA.

<sc>Fig</sc> . 11.
Fig . 11.

The tan δ at 60 °C (circles, RR predictor) and 0 °C (diamonds, wet traction predictor) of cured OTPTES compounds are plotted as a function of degree of silanization.

<sc>Fig</sc> . 12.
Fig . 12.

The mixing temperature profiles for preparing stocks 11 (solid line) and 12 (dashed line) (red and black lines for MB1 and MB2 mixing stages, respectively).

<sc>Fig</sc> . 13.
Fig . 13.

Tan δ temperature sweeps: Stock 1 (TESPT, dotted line); Stock 11 (OTPTES, dark green dashed line) mixed with condition (a); Stock 12 (OTPTES, light green solid line) mixed with condition (b).

<sc>Fig</sc> . 14.
Fig . 14.

Comparison of compound performance indices for TESPT stock 1(dotted line), OTPTES 11 mixed with condition (a) [dashed dark green line], and OTPTES stock 12 mixed with condition (b) [solid light green line].

Contributor Notes

*Corresponding author. Email: Linjeffrey46@yahoo.com
Received: 01 Apr 2016
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