Use of precipitated silica to improve brass-coated wire-to-rubber adhesion.

Amorphous precipitated silica is a high-quality, reinforcing particulate filler that is used in conjunction with carbon black in rubber components of tires and mechanical goods to improve performance. Wagner (ref 1) showed that silica use improves rubber tear strength, flex fatigue resistance, abrasion resistance, heat build-up, hardness, modulus, resilience and adhesion. Okel, Waddell and Evans (refs. 2 and 3) determined that surface area is the primary silica property which predicts rubber performance for use of silica in carbon black-filled compounds: natural rubber off-the-road tire tread, natural rubber wire coat and neoprene v-belt. Increasing silica surface area beneficially increased compound scorch protection, tensile and tear strengths, flex-fatigue life and aged elongation. Increasing silica surface area adversely increased compound viscosity, heat build-up and cure time, and adversely decreased compound abrasion index. The confidence limits of the predictions were dependent upon the relative concentration of silica used in the carbon black-filled compound.

Rubber coat stocks for brass-coated steel wire cord generally consist of natural rubber (or natural rubber blended with synthetic cis-1,4-polyisoprene) (ref. 4), carbon black, adhesion promoters such as resorcinol/formaldehyde-donor resins (refs. 5-9), organocobalt (refs. 4, 10, 11), or both adhesive systems (refs. 12-16), sulfenamide accelerators (refs. 5, 17, 18) and sulfur. Van Ooij and coworkers (refs. 19-21) have reviewed brass-coated wire-to-rubber adhesion showing the complexity of in-situ formation of the brass to rubber bond.. Use of silica in coat stocks to improve wire-to-rubber (refs. 5, 14, 17, 22-25) or fabric-to-rubber (refs. 22-24) adhesion have been important applications. In particular, silica has been used in conjunction with resorcinol/formaldehyde-donor resins to promote adhesion (refs. 5, 14, 22-25), the classical HRH system: Hi-Sil 233/resorcinol/hexamethylenetetramine (ref. 22). Tate (ref 14) examined the use of silica with a cobalt adhesion promoter finding significant improvements in steam-aged and humidity-aged adhesion along with an increase in unaged compound fatigue properties.

The present study examines the use of precipitated silica in a carbon black-filled natural rubber model wire coat compound containing an organocobalt as the adhesion promoter. The effect of silica physical properties on rubber cure and cured physical and adhesive properties are examined.

Experimental

Silicas 1 and 3 are the commercial precipitated silicas Hi-Sil 233 and Hi-Sil 233T, respectively. Silicas 5 and 8-10 are Hi-Sil 532EP, Hi-Sil ABS, Hi-Sil 135 and Hi-Sil 255T, respectively. Silicas 11 and 12 are powder forms of Hi-Sil 190 and Fe-Sil 2000, respectively. Silicas 2, 4, 6 and 7 are experimental materials. Silica properties were characterized by the procedures listed in table 1.

Table 1 - silica physical property measurements
Test                     Procedure       Properties analyzed
BET [N.sub.2]            ASTM D3037-92   Surface area
adsorption (1-point)      modified
BET [N.sub.2]            ASTM D1993-91   Surface area
adsorption (5-point)
pH                       ASTM D1512-90
DBP absorption           ASTM D2414-92   Surface area
Coulter counter          ASTM F662-86    Mean agglomerate
                                         particle size
Hg porosimetry           ASTM D4284-83   Mean pore diameter
                                         Mean pore volume
                                         Total pore surface area
[N.sub.2] porosimetry    ASTM D4222-83   Mean pore diameter
                                         Mean pore volume
                                         Micropore surface area
X-ray fluorescence       ASTM C575-86    Salt content
                          modified

The wire coat recipe studied is shown in table 2. It was established by selecting wire coat formulations of different tire manufacturers (refs. 15, 26-28) that used an organocobalt adhesion promoter, averaging the primary ingredients and omitting specialized ingredients.

Table 2 - wire coat formulation    75 phr
Natural rubber, CV60               25
cis-polyisoprene, Natsyn 2200      55
Carbon black, N-326                10
Precipitated silica, Hi-Sil 233     3
Processing oil, sundex 8125       1.5
Cobalt neodecanoate                 1
Antidegradant, Wingstay 100         2
Steariic acid                       8
Zinc oxide                        0.8
Santocure MOR                     4.5
Sulfur

Compounds were mixed according to ASTM D3182-89 using a two-stage mix either in an internal mixer or in combination with a two-roll mill. Polymer, fillers, zinc oxide, stearic acid, process aids, antidegradants and cobalt neodecanoate were added during the first stage mix. Sulfur and accelerator were added in the second stage. When the combination of internal mixer and mill was used, a polymer/carbon black masterbatch was made in the internal mixer and all other additions were made on the two-roll mill. Specimens were cured at 150[degrees]C. Stress/strain, hardness, dynamic and tear strength test specimens were cured for a time corresponding to [T.sub.90]+5 minutes. All other test specimens were cured for a time corresponding to [T.sub.90]+10 minutes.

Brass-coated (64% copper) wire (5 + 1 x 0.25) composites were constructed and tested according to the tire cord adhesion test (TCAT) procedures (ref. 29-32), since TCAT gives genuinely adhesive failure, and was found to be independent of (i) the rubber tearing energy, (ii) the observed penetration into the cord bundle interstices, and (iii) the amount of rubber remaining adhered to the pulled-out wire. Determining the energy of adhesion (ref 33), the true interfacial wire-to-rubber adhesion, from the measured pull-out force values is possible because of the construction of the TCAT test specimen. The TCAT specimen appears to be of particular relevance to adhesion associated with belt and ply end areas. TCAT testing was performed on original, humid-aged (five days at 90% R/H and 90[degrees]C) and salt-aged (five days at 32[degrees]C and 5% salt fog) specimens.

Procedures and equipment used to characterize rubber cure properties, and cured physical and dynamic properties have been reported (ref 2). Statistical analyses were performed using the Statistical Analysis System SAS) software (ref 34).

Results and discussion

Statistically designed evaluation ingredients A five-variable, factorial statistical design was used to explore the effect that material, changes had on composite physical properties in order to obtain a general understanding of their importance and inter-relationships in the precipitated silica/organocobalt adhesive system. Eighteen compounds were prepared using the formula in table 2 as a centerpoint (duplicated), and varying the N-326 carbon black levels from 40 phr to 70 phr, silica levels from 0 to 20 phr, cobalt neodecanoate levels from 0.5 phr to 2.5 phr to afford approximately 0.1, 0.25 (centerpoint) and 0.5 phr of elemental cobalt, accelerator levels from 0.4 phr to 1.2 phr, and sulfur levels from 2.0 phr to 7.0 phr. These ranges were established to cover the range of carbon black, elemental cobalt, accelerator and sulfur levels of the various tire manufacturer formulations (refs. 15, 26-28).

Composite properties measured were minimum torque, [TS.sub.2] scorch, [T.sub.90] cure time and maximum torque, Shore A hardness, trouser tear using a molded groove specimen, dynamic properties using a strain-sweep and original TCAT adhesion. Results at the 90% confidence level indicate that use of silica improves rubber composite properties. Tear strength linearly increases with silica use up to 20 phr (figure 1). Original TCAT pull-out force and energy of adhesion (a) values also linearly increase with silica use up to 20 phr figures 2 and 3). There is no relationship between compound tear strength and TCAT pull-out force values (figure 4), hence the mechanism by which precipitated silica improves wire-to-rubber composite adhesion is not a simple mechanical effect dependent upon rubber tear strength.

Table 3 summarizes compound data. Silica use beneficially increases tear strength and energy of adhesion, but also adversely increases compound [T.sub.90] cure time and hysteresis (as determined by G" at 2% strain). For example, for compounds 2 and 3 which contain 40 phr of carbon black, silica use as the only change increases tear strength by approximately 150% and composite adhesion by over 200%; however, [T.sub.90] cure time is also increased by 40% and hysteresis is increased by almost a factor of two. The magnitude of the effect of silica use upon compound properties also depends upon the other material variables. Compound hysteresis values are increased to a greater extent by use of carbon black or sulfur. For example, the only change between compounds 4 and 8 is that carbon black is increased from 40 phr to 70 phr, with a factor of four increase in hysteresis resulting. This is compared to compounds 4 and 5 or compounds8 and 9 which use zero and 20 phr of silica, respectively, and have hysteresis values that increase by less than a factor of two. For compounds containing 70 phr of carbon black, lower adhesion is generally obtained and use of 20 phr of silica only provides limited improvements. In general, high silica and low sulfur containing compounds exhibited the best tear strength and adhesive properties. Compound 1, the centerpoint compound, provides for good overall performance.

[TABULAR DATA OMITTED]

Cobalt does not have a statistically significant effect on TCAT adhesion; however, keep in mind that cobalt neodecanoate is always present. Follow-up studies show that cobalt does have a statistically significant effect on adhesion with an optimum level within the range presently studied. Carbon black, accelerator and sulfur show expected behaviors. Table 4 summarizes the influence of using 1 phr of a material on composite cure, physical and adhesive properties. For example, use of 1 phr of silica increases compound cure time by 0.47 min, original TCAT adhesion by 0.28 J, tear strength by 0.38 N/mm, and hysteresis by 0.11 MPa. Relationships appeared linear over the ranges studied.

Table 4 - material effects on composite properties
Property          Materials       Effect    Probability
                   (phr)                        >f
Cure              Silica             0.47     0.02083
                  Cobalt            -20.3     0.03769
                  Silica*Cobalt     -1.59     0.08679
                  Accelerator        -7.8     0.09207
Hysteresis (G")   Black              0.11     0.00000
                  Sulfur             0.28     0.00031
                  Silica            0.066     0.00047
                  Silica*Sulfur     0.010     0.05838
Tear              Silica             0.38     0.00641
                  Silica*Black     -0.028     0.00385
TCAT              Silica             0.28     0.01076
                  Silica*Sulfur    -0.105     0.01389
                  Black*Sulfur     -0.044     0.08410
                  Silica*Black      0.010     0.09743Effect of salt type: chloride versus sulfate Precipitated silicas that were prepared by different commercial processes were studied. Silica 1 was prepared by using carbon dioxide to neutralize sodium silicate and precipitate the silica aggregates followed by hydrochloric acid to adjust to the desired final pH. Silica 3 was prepared using sulfuric acid throughout the process. Silica 2 is an experimental silica prepared by using sulfuric acid to precipitate and hydrochloric acid to adjust the final pH. Silica 4 is an experimental silica prepared using carbon dioxide to precipitate and sulfuric acid to adjust the final pH. All silicas had surface areas of approximately 150 [m.sup.2]/g. Testing in the wire coat compound of table 2 showed no significant effect on compound cure, and cured physical properties. Original, salt-aged and humid-aged TCAT adhesions were evaluated and showed no statistically significant differences in adhesion (table 5). This is also evident from figure 5 which shows that energy of adhesion values for precipitated silicas made using hydrochloric acid (circles) and sulfuric acid (pyramids) are equivalent. Thus, the residual salt in precipitated silica has no measurable effect upon wire composite physical or adhesive properties.  [TABULAR DATA OMITTED]

Silica property correlation

Nine precipitated silicas were studied in order to obtain a wide range in silica physical properties (ref. 2). Averages for various determinations of surface area and structure, and pore diameter, salt impurities, particle size and pH values are shown in table 6. The variation in silica properties impacted wire coat processing and cure properties, and cured physical properties as expected (ref. 2). Performance characteristics are shown in table 7. Data in table 8 indicate that silica surface area and porosity have the highest correlation with performance ref. 2). The nearer the absolute correlation coefficient is to 1.00, the higher is the degree of association. Correlation coefficient values > 98% are considered to be predictive values > 90% are considered to rep resent good relationships, while values between 80% - 90% are considered to be indicative of genera trends. Silica. BET nitrogen (single point) surface area has the highest correlations with compound [T.sub.90] cure time, rebound at 100[degrees]C, modulus 100% elongation, elongation and tear strength. Increasing silica surface area increased cure time and tear strength, and decreased rebound, modulus at 100% elongation and elongation values. Silica nitrogen pore volume had the highest correlations with rebound and tear strength, and silica mercury pore diameter with hardness, elongation, rebound at 23[degrees]C and tear strength. Data also suggest that use of a silica with high BET nitrogen surface area and high dibutylphthalate absorption values could further increase compound tear strength. These results are in agreement with the relationships reported by Voet, Morawski and Donnet (ref. 35), Wagner (ref 36), Morawski (ref. 37), and Evans and Waddell (ref 38) for rubber compounded without carbon black.

[TABULAR DATA 6 to 8 OMITTED]

Energy of adhesion values showed considerable variation, but did not statistically correlate with any silica physical properties. In a plot of energy of adhesion versus silica BET nitrogen surface area and dibutylphthalate absorption values, no correlation has been reported between silica properties and compound to cord adhesion. Again, the mechanism of precipitated silica improvement in wire-to-rubber adhesion is not a physical mechanism simply related to improved compound physical properties.

The design of a wire coat compound requires careful balancing of compound properties. Tire design parameters require the correct hardness and modulus, for instance, to allow for the efficient transfer of force from the wire reinforcing member to the surrounding rubber compound. The cure rate of the coat compound must be compatible with the surrounding rubber components of the tire. Finally, the interfacial adhesion of the wire/rubber composite must be sufficient to prevent debonding and failure of the tire during service. These data have shown that precipitated silica can play a unique role in balancing wire coat compound physical and adhesive properties, particularly for aged adhesion. Unlike the other compounding ingredients studied which decreased interfacial adhesion (particularly aged adhesion) when the hardness of the compound was increased by increasing its phr level, precipitated silica shows increased interfacial adhesion with increasing level. A wide range of precipitated silica types and reinforcement potential have all been shown to improve adhesion of brass-coated wire to natural rubber in this model wire coat compound. The design of a superior compound with die correct cure rate, engineering properties and original and aged wire adhesion containing precipitated silica is possible for the tire designer and rubber compounder.

Summary

Use of precipitated silica in a carbon black-filled, natural rubber model wire coat compound containing an organocobalt adhesion promoter is beneficial to composite performance. Results of a five-variable statistically designed compounding study showed that compound tear strength and composite adhesion are significantly increased upon use of silica. Since there was no correlation between tear and TCAT values, the mechanism of silica improvement of wire-to-rubber adhesion is not a simple physical effect of increased tear strength. Energy of adhesion values were determined and increase linearly with increasing silica levels, indicating that silica use has an effect on the interfacial layer formed on the wire. The residual salt type in precipitated silicas prepared using different acids did not have a statistically significant effect upon compound properties or wire-to-rubber adhesion properties. The effect of silica surface area on wire coat compound cure and cured physical properties was as expected: increasing silica surface area increased cure time and tear strength, and decreased rebound, modulus at 100% elongation and elongation values. Energy of adhesion values did not statistically correlate with any silica physical properties, thus the mechanism of precipitated silica improvement in wire-to-rubber adhesion is not a physical mechanism simply related to improved compound physical properties.

[Figures 1 to 5 ILLUSTRATION OMITTED]

References

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