Optimization of an organocobalt-containing wire coat compound using precipitated silica.

Wagner (ref. 1) reviewed the use of amorphous precipitated silica to improve rubber compound tear strength, flex-fatigue resistance, abrasion resistance, heat build-up, hardness, modulus, resilience and composite adhesion. Silica use in fabric (refs. 2-5) and wire (refs. 2, 3 and 5-16) coat stocks are important applications due to both the improved compound physical and composite adhesive properties, particularly when used in conjunction with resorcinol/formaldehyde-donor resins to promote adhesion (refs. 2-14), the classical HRH system: Hi-Sil 233 silica/resorcinol/hexamethylenetetramine (ref. 2). Silica has also been used with organocobalt complexes to promote brass-coated wire-to-rubber adhesion in the absence of resins. Tate (ref. 14) examined the use of silica with a cobalt-boron complex, finding significant improvements in steam-aged and humidity-aged adhesion and an increase in unaged compound fatigue properties with silica. Cochet and coworkers (ref. 15) showed a low surface area, silane-coupled silica with a cobalt-boron adhesive improved compound tear resistance, hysteresis and aged composite adhesion. Evans, Waddell and coworkers (ref. 16) showed that both compound tear strength and composite adhesion were significantly increased upon use of silica with an organocobalt salt. Since there was no correlation between these improvements in tear strength and adhesion, it was thought that the mechanism of silica action in improving wire-to-rubber adhesion was not simply a result of a physical effect within the rubber compound. Furthermore, the energy of adhesion values (ref.17) increased linearly with increasing silica levels (ref. 16), indicating a direct participation in the interfacial layer formed on the wire.

The effect of silica surface area on wire coat compound cure and cured physical properties was as expected (refs. 18-20). Increasing silica surface area increased compound cure time and tear strength, and decreased compound rebound, modulus at 100% elongation and elongation at break values. Waddell, Evans and coworkers (ref. 21) used surface spectroscopic techniques to directly characterize the interfacial layer formed on squalene-treated wire tire cord filaments to determine the mechanism of silica action. X-ray photoelectron spectroscopic analysis showed that silica use in a suspension containing squalene, carbon black, cobalt neodecanoate and curatives reduced the amount of carbon and copper and increased the amount of oxygen and zinc, thus increasing zinc oxide formation. The present study is an optimization of the physical and adhesive properties of wire coat compounds containing an organo-cobalt and/or a resorcinol/formaldehyde donor resin by using precipitated silica.

Experimental

The wire coat recipes studied are shown in tables 1 and 2. Formulation A was previously established (ref. 16) by selecting wire coat formulations of different tire manufacturers (refs. 22-25) that used an organocobalt adhesion promoter, averaging the primary ingredients and omitting specialized ingredients. Formulation B was used to optimize the adhesive and physical properties based on the inter-relationships from a five-variable, factorial design of carbon black, silica, cobalt neodecanoate, sulfur and accelerator levels (ref. 16). The non-nitrosamine generating accelerator tert-butyl-2-benzothiazole sulfenamide (refs. 26 and 27) (Santocure NS, TBBS) was selected for the follow-up rotatable central composite designed compounding studies. Formulation C is a model tire wire coat compound containing an organocobalt and resin adhesive system and is the control compound for formulation D, which simply replaces 15 phr of N-326 carbon black with 15 phr of precipitated silica.

Table 1 - silica/cobalt containing wire coat formulations

                                         A           B

Natural rubber, CV60                    75        75 phr
cis-Polyisoprene, Natsyn 2200           25            25
Carbon black, N-326                     55            55
Precipitated silica, Hi-Sil 233         10          0-25
Processing oil, Sundex 8125              3             3
Cobalt neodecanoate                    1.5         0-2.5
Antidegradant, Wingstay 100              1             1
Stearic acid                             2             2
Zinc oxide                               8             8
Santocure MOR                          0.8             0
Santocure NS                             0      0.25-1.5
Sulfur                                 4.5           2-7
Table 2 - silica/resin/cobalt containing wire coat formulations

                                        C          D

Natural rubber, CV60                   100      100 phr
Carbon black, N-326                     55           40
Precipitated silica, Hi-Sil 233          0           15
Cobalt naphthenate                     1.5          1.5
Flectol H                              3.5          3.5
Antidegradant, Wingstay 100            0.3          0.3
Stearic acid                           1.2          1.2
Penacolite B19S                          3            3
Zinc oxide                               8            8
Sulfur                                 3.8          3.8
Cyrez 963                                3            3
Santocure MOR                          0.7           0.7
Santogard PVI                          0.2             0

Compounds were mixed according to ASTM D3182-89 using a two-stage mix in an internal mixer. Elastomer, fillers, processing aids, organocobalt salt, antidegradants, stearic acid and zinc oxide were added during the first stage mix, as was the partially reacted resorcinol resin when used. Sulfur and accelerator were added in the second stage, as was the melamine resin when used. 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 (refs. 28-31), 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 a&Bred to the pulled-out wire. TCAT test specimens were cured for a time corresponding to 2 x [T.sub.90] + 5 minutes. Determining the energy of adhesion (ref. 17), 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, heat-aged (five days at 90 [degrees] C in a circulating air oven), humid-aged (five days at 90% R/H and 90 [degrees] C) and salt-aged (five days at 32 [degrees] C and 5% NaCl fog) specimens.

Procedures and equipment used to characterize rubber cure properties, and cured physical and dynamic properties have been reported (refs. 18-20). Statistical analyses were performed using the statistical analysis system software (ref. 32).

Results and discussion

Silica/cobalt adhesive

A series of statistically designed compounding studies was performed to followup the exploratory study of the effect that material changes had on composite physical and adhesive properties (ref. 16). Compounds were prepared using formulation B with the centerpoint replicated and varying the silica levels from 0 to 25 pier, total filler levels from 40 to 90 pier, cobalt neodecanoate levels from 0 to 2.5 phr to afford approximately 0 to 0.5 phr of elemental cobalt, TBBS accelerator levels from 0.25 to 1.5 pier, and sulfur levels from 2.0 to 7.0 pier. From the analysis of these designs, it was possible to determine the effect that use of these four material variables had upon the energy of adhesion of the original sample and after the three aging conditions studied. Table 3 is a summary of the significant predictors obtained at the 95% confidence level of how the energy of adhesion is affected per phr of each material variable. The effect of using silica in a cobalt-containing wire coat compound is to linearly increase the original, heat-aged, humid-aged and salt-aged energy of adhesion values for each phr of silica used. The increase in aged adhesive properties upon using silica with a cobalt adhesive is in agreement with Cochet et al (ref. 15). Ishikawa (ref. 33) showed that formulations which gave high humidity-aged adhesion also produced more zinc oxide on the cord surface during bonding. Zinc oxide inhibited the formation of excessive and loose bonding products. Waddell and coworkers (ref. 21) used x-ray photoelectron (XPS) and proton induced X-ray emission (PIXE) spectroscopic analyses to quantitatively show that silica had a pronounced effect upon the concentration of the elements formed in the interfacial layer upon treatment of wire filaments in squalene suspensions. PIXE showed that the total sulfur concentration was reduced, thereby increasing the copper/sulfur and zinc/sulfur ratios, and XPS showed that the oxygen concentration was increased, thus promoting zinc oxide formation.

Table 3 - significant predictors for energy of adhesion using silica/cobalt adhesives

                   Optimization coefficients(*)/phr

       Original                     Aged
                          Heat       Humid       Salt
Silica         +0.014    +0.143      +0.087      -0.134
Cobalt         +0.213    -0.150      -1.248(+)   +0.168
Sulfur         +0.149    +0.171      +0.283(+)   +0.137(+)
Accelerator    -0.628    +0.131(#)  -12.312(+)   +0.233(#)

(*) All two-way interaction terms are negative, and ail three-way interaction terms; are positive.

(+) Quadratic term is significant.

(#) Interaction with silica is significant.

The effect of using cobalt neodecanoate is to increase the original and salt-aged energy of adhesion values, but also to reduce the heat-aged and humid-aged energy of adhesion values. The quadratic term is also important for the humid-aged energy of adhesion, thus the cobalt concentration is a very important variable. This result is in agreement with van Ooij (ref. 34) who reported that at high levels of cobalt the presence of moisture caused metallic cobalt to form, accelerating dezincification and destroying the integrity of the interfacial film. Increased sulfur use is predicted to significantly increase all energy of adhesion values; however, the quadratic terms are important for both the humid-aged and salt-aged energy of adhesion values. Thus, an optimum sulfur level exists. Increased use of the TBBS accelerator had a very negative effect on the original and, in particular, the humid-aged energy of adhesion values. The quadratic term is important for the humid-aged energy adhesion, and the silica/accelerator interaction terms are important for the heat-aged and salt-aged energy of adhesion values. Finally, all two-way interactions between the four material variables are negative, adversely affecting the energy of adhesion values, and all three-way interactions have a positive effect upon the energy of adhesion values.

Compound properties with silica/cobalt adhesive

Examples of the energy of adhesion values of five selected compounds are presented in table 4. Note that the 55 phr carbon black control compound that does not contain silica (denoted 0/55) has an extremely low humid-aged energy of adhesion value, with no rubber coverage remaining on the pulled-out wire. Use of increased percentages of silica in the compound served to significantly increase the humid-aged energy of adhesion and rubber coverage values. A compound containing 25 phr of silica, 42.5 phr of carbon black, 1.2 phr of cobalt neodecanoate, 4 phr of sulfur and 0.5 phr of TBBS accelerator afforded the highest energy of adhesion values for the four conditions studied. Figures 1-4 show the response surfaces of these designed experiments.

[Figure 1-4 ILLUSTRATION OMITTED]

Table 4 - adhesion properties of selected silica/cobalt wire coat compounds

Compound      Energy adhesion of (% rubber coverage)
phr silica/   Original              Aged
black                          Heat        Humid         Salt

0/55(*)       1.64 (30%)     0.46 (40%)    0.09 (0%)     0.63 (25%)
13.5/46.5(*)  2.00 (60%)     1.22 (60%)    0.81 (40%)    0.94 (50%)
20/35(*)      1.81 (90%)     2.86 (100%)   4.00 (90%)    2.62 (80%)
20/50(*)      1.53 (80%)     1.99 (100%)   2.19 (70%)    2.19 (80%)
25/42.5(#)    2.53 (100%)    3.07 (100%)   5.33 (100%)   2.83 (90%)

(*) 1.5 phr cobalt neodecanote, 4.5 phr sulfur, 0.8 phr TBBS

(+) 1.2 phr cobalt neodecanote, 3.8 phr sulfur, 0.8 phr TBBS

(#) 1.2 phr cobalt neodecanote, 4.0 phr sulfur, 0.5 phr TBBS

Compound cure and cured physical properties measured were minimum and maximum torque, [TS.sub.2] scorch, [T.sub.50] and [T.sub.90] cure times, Shore A hardness, rebound, stress/strain, trouser tear using a molded groove specimen, DeMattia cut-growth resistance and dynamic properties using a strainsweep. Results at the 95% confidence level indicated that use of silica improved rubber compound properties, particularly tear strength and cut-growth resistance. Tables 5 and 6 summarize the cure and physical properties of the five selected compounds. The increased [T.sub.90] cure times observed for the compounds containing the 20/50 and 25/42.5 phr silica/black levels indicated that these two compounds were not optimized and probably require additional accelerator. The highest tear strength and cut-growth resistance values were obtained for those compounds containing the highest percentage of silica, namely the compounds containing 20/35 and 25/42.5 phr silica/black levels, which is consistent with previous results (refs. 18-20). Modulus at high strain was dependent upon the total filler level and the percentage of carbon black, with the highest level obtained for the carbon black control compound. A comparison of selected properties of compounds prepared with 20/35 and 20/50 phr of silica/black are shown in figure 5 relative to properties of the carbon black compound (0/55). Thus, it is possible to adjust the hardness and modulus values of a compound by adjusting the total level of silica plus carbon black, with a high silica level providing for increased energy of adhesion values. Hysteresis, as measured by G", was lowest for compounds having lower total filler levels with the highest percentage of silica.

[Figure 5 ILLUSTRATION OMITTED]

Table 5 - cure properties of selected silica/cobalt wire coat compounds

Test                           Compound, phr silica/black

MDR @ 1 50 [degrees] C       0/55  13.5/46.5  20/35   20/50  25/42.5
-[TS.sub.2] Scorch, min.     3.93     4.23    4.20    3.96    3.25
-[T.sub.50] Cure time, min.  6.49     7.76    8.50    7.97    7.37
-[T.sub.90] Cure time, min.  9.45    10.19   14.00   27.81   21.99
-Minimum torque, dNm         2.92     3.22    3.04    4.59    4.50
-Maximum torque, dNm        27.90    27.26   24.80   27.61   28.46

Table 6 - physical properties of selected silica/cobalt wire coat compounds

                                 Compound, phr silica/black
                                 0/55    135/     20/35
Test                                     46.5

Hardness @ 23 [degrees] C         72       69        59
Heat aged                         76       76        71
Humid aged                        76       77        72
Salt aged                         72       67        63
Hardness @ 100 [degrees] C        69       68        58
Rebound @23 [degrees] C           51.8     52.0      57.8
Rebound @ 100 [degrees] C         68.8     69.2      69.2
Breaking strength, MPa            29.16    28.83     28.42
Elongation @ break, %            494.0    523.5     641.8
Modulus @ 20%, MPa                 1.32     1.08      0.78
   @100%, MPa                      4.25     3.43      1.95
   @300%, MPa                     16.56    14.44      7.34
Tear strength, kN/m                9.18    11.14     16.87
Cut growth, mm @ 36kc             23.61     4.3       2.89
Rheometrics @2%
   strain, 27 [degrees] C
   [G.sup.I]                       5.257    4.163     3.101
   [G.sup.II]                      0.773    0.579     0.322
   Tan delta                       0.147    0.139     0.104

Test                             20/50     25/42.5

Hardness @ 23 [degrees] C         68          68
Heat aged                         82          80
Humid aged                        81          79
Salt aged                         74          72
Hardness @ 100 [degrees] C        62          62
Rebound @23 [degrees] C           44.2        44.0
Rebound @ 100 [degrees] C         57.8        58.6
Breaking strength, MPa            24.31       23.94
Elongation @ break, %            544.4      540.1
Modulus @ 20%, MPa                 1.10       1.00
   @100%, MPa                      2.82       2.80
   @300%, MPa                     11.24       11.15
Tear strength, kN/m               11.74       16.11
Cut growth, mm @ 36kc             10.19        4.79
Rheometrics @2%
   strain, 27 [degrees] C
   [G.sup.I]                       5.617       5.488
   [G.sup.II]                      1.015       0.902
   Tan delta                       0.181       0.164

Silica/resin/cobalt adhesive

Wire coat formulation C containing a resin/cobalt adhesive system was studied without and after simple replacement of 15 phr of N-326 carbon black with 15 phr of precipitated silica, see formulation D, table 2. The original, heat-aged, humid-aged and salt-aged energy of adhesion values were significantly increased, see table 7. Tear strength and cut-growth resistance values were also significantly increased. As a follow-up, a three-variable central composite compounding design was performed using formulation B to investigate the effects of materials: silica (0 - 24 pier), resin as a system consisting of equal amounts of hexamethoxymethylmelamine and a pre-reacted resorcinol precursor (0-7.2 pier), and cobalt neodecanoate (0-2.0 pier). The carbon black level was set at 55 pier, sulfur level was set at 3.8 phr and the TBBS accelerator level was set at 0.8 pier. Table 8 is a summary of the significant predictors obtained at the 95% confidence level. Optimum levels of each ingredient were predicted to afford good original, heat-aged and humid-aged energy of adhesion. Increasing the level of silica used in the compound linearly increased the energy of adhesion for all three conditions. Peterson and Dietrick (ref. 13) showed that silica use with either a resin or resin/cobalt adhesive system improved original, heat-aged and saltwater-aged adhesion regardless of the chemical composition of the resin, and also improved steam-aged and humidity-aged adhesion when used with the resorcinol/hexamethylenetetramine resin. Tate (ref. 14) reported that silica use with a resin/cobalt adhesive system gave significant improvements in steam-aged and humidity-aged adhesion and increased unaged compound fatigue properties. Increasing the level of the resin system and, in particular, of cobalt neodecanoate afforded quadratic effects upon the energy of adhesion values obtained. A total resin level of approximately 5 phr and a cobalt neodecanoate level of 1.25 phr appeared to afford optimums for the three energy of adhesion values studied. Basically, the primary effect of using the resin system was to provide for minimum energy of adhesion values, regardless of the amounts of silica and cobalt used. The quadratic effects on the energy of adhesion values obtained by adding cobalt still requires careful optimization of the amount used in a resin-containing formulation. Silica use linearly increased the original and aged energy of adhesion values, suggesting use of high levels in the wire coat compound. Silica use beneficially increased compound tear strength and cut-growth resistance at the same hardness of the carbon black control, but also increased compound cure time, see table 7.

Table 7 - cure properties of silica/resin/cobalt wire coat compounds

Test                           Compound, phr silica/black
                                  0/55      15/40
MDR @150 [degrees] C
   [TS.sub.2] scorch, min.         4.4        5.2
[T.sub.90] cure time, min.        25.0       31.2
   Minimum torque, dNm             3.1        3.3
   Maximum torque, dNm            36.8       31.5
Hardness @23 [degrees] C          76         75
Rebound @100 [degrees] C          53.6       54.4
Breaking strength, MPa            24.3       23.6
Elongation @ break, %            506.0      532.0
Modulus @ 100%, MPa                3.3        2.5
Tear strength, kN/m                9.6       12.2
Cut growth, mm @ 100 kc           11.5        8.8
Energy of adhesion
(% rubber coverage)
Original                           3.7 (90)   5.8 (90)
Heat-aged                          2.5 (90)   2.8 (95)
Humid-aged                         2.2 (90)   3.8 (90)
Salt-aged                          3.9 (90)   8.6 (95)

Patented wire coat formulations

A number of patented wire coat formulations (refs. 12, 22-26 and 35) was studied in order to establish acceptable ranges of composite cure, adhesive and physical properties. Different adhesive systems are used in these formulations including silica/resin (ref. 12), resin/cobalt (ref. 22), silica/resin/cobalt (ref. 35), silica/resin/cobalt with lead oxide (ref. 26) and cobalt (refs. 23-25) as an adhesive. Adhesive properties of these compounds differed widely, with the highest overall energy of adhesion values obtained by using precipitated silica in either a silica/ resin (ref. 12) or silica/resin/cobalt (ref. 35) adhesive system. Compound physical properties also differed widely. The three compounds containing silica (refs. 12, 26 and 35) had the highest tear strengths with good cut-growth resistance values, while the three compounds prepared using only a cobalt adhesive (refs. 23-25) had the lowest cut-growth resistance values.

General considerations

The design of a tire wire coat compound requires careful balancing of compound cure and cured physical and adhesive properties. The cure rate of the wire coat compound must be compatible with the surrounding rubber components of the tire, yet provide for formation of the required thickness of copper sulfide in the interfacial layer on the brass-coated wire (refs. 34 and 36). Tire design parameters require a compatible hardness and modulus to allow for the efficient transfer of force from the tire cord to the rubber compound. Finally, the energy of adhesion of the brass-coated wire/natural rubber composite must be sufficient to prevent debonding and failure of the tire during service. van Ooij, Giridhar and Ahn (ref. 37) studied the mechanism of adhesion degradation in a truck tire wire carcass and concluded that the major cause of adhesion degradation during service of the tire was the formation of small cracks in the rubber compound between adjacent filaments. Independent of the nature of the adhesive, use of precipitated silica in wire coat compounds significantly improve tear strength and cut-growth resistance, see tables 6 and 7.

Precipitated silica serves a unique role in balancing wire coat compound physical and adhesive properties, particularly for the aged energy of adhesion values. Increased interfacial adhesion is obtained by using increasing levels of silica, which is not the case for use of the resin or cobalt adhesives. A wide range of precipitated silica types and reinforcement potential have all been shown to improve the wire to rubber adhesion (refs.15 and 16).

Summary

Use of precipitated silica in a carbon black-filled, natural rubber wire coat compound containing an organocobalt adhesion promoter is beneficial to composite performance. Results of statistically designed compounding studies showed that original, heat-aged, humid-aged and salt-aged energy of adhesion values increased linearly with increasing silica levels, and that compound tear strength and cut-growth resistance values were significantly increased upon use of silica. A central composite compounding design showed that increased use of precipitated silica in a carbon black-filled wire coat formulation containing a resin/cobalt adhesive system also linearly improved original, heat-aged and humid-aged energy of adhesion values and increased compound tear strength and cut-growth resistance. The primary effect of resin use is to provide for minimum energy of adhesion values, regardless of the amounts of silica and cobalt used. The quadratic effects on the energy of adhesion values obtained by using cobalt neodecanoate requires a careful optimization of the level used. Results on patented wire coat formulations showed that the highest overall energy of adhesion values were obtained using a silica/resin or silica/resin/cobalt adhesive system and that these compounds had the higher tear strengths and cut-growth resistance values.

References

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Larry R. Evans and Walter H. Waddell, PPG Industries, Since this was written, both authors have changed companies. Larry Evans is now with J.M. Huber Corp. in Havre de Grace, MD and Walter Waddell is with Exxon Chemical in Baytown, TX