Mechanism of brass adhesion.

The past 20 years have seen considerable commercial and laboratory evidence which testifies to the beneficial effect of precipitated silica on brass adhesion. In particular, the inclusion of 10 to 18 parts of a 150 BET surface area silica in wire belt compounds has been widely adopted for use in radial passenger and truck tires. The development of this technology was described in a recent publication (ref. 1). In the technical history of the rubber industry it has often been noted that the pragmatic precedes the academic. The relationship between silica and brass adhesion is certainly an appropriate example of this proposition.

Long after the commercial development an explanation of the mechanism by which silica improves the adhesion between natural rubber and brass remains a matter of speculation. An early review by Wagner (ref. 3) stated that "the interaction between silica and zinc oxide is important in obtaining maximum adhesion." This conclusion was based, in part, on the observation that addition of zinc oxide with silica early in the mix cycle improved static pull-out values and percent coverage. Other theories involved compatibility (wetting effects), water affinity and chemical reactivity of the silanol surface.

This article describes a compounding approach to explore the mechanism by which precipitated silica enhances the adhesion of natural rubber to brass.

Procedures

The compounds described in the following sections were mixed in a laboratory internal mixer in two stages. The use of peroxide crosslinking in the initial part of this investigation made it possible to isolate the adhesion effects of various cure system components.

Brass adhesion evaluation was generally conducted in the dynamic mode by the disc fatigue procedure. This method and its merits have been described in previous publications (refs. 1 and 2). Replicate assemblies (50 x 13 x 13 mm) of rubber and wire (6 x .35 mm + 3 x .20 mm) coated with 63% copper brass are flexed in the cord compression fatigue test apparatus for six hours at 16% strain, compressed and extended. Original, unaged specimens are flexed at 90[degrees]C; for humid aged evaluations a lower temperature (80[degrees]C) is used. Humid aging conditions involve exposure to 100% relative humidity either three days at 50[degrees]C for uncured (green) assemblies or three days at 90[degrees]C after curing. Exceptions to these procedures are noted in the data summaries. After flexing the specimens are bisected and wire extracted. Wire separations which occur during flexing leave a bare metal surface with little or no adhering rubber. These separations, noted as failures in the data tables, are the primary basis of adhesion evaluation. Pull-out values are also involved, particularly in the initial experiments where he level of bond strength is too low to warrant dynamic testing. Commercial wire belt compounds generally show from 0-2 separations in the original state.

An important precaution in any adhesion evaluation, static or dynamic, is to maintain low strain modulus (M20) at a constant level. In most of the experiments reported here, varying the plasticizing oil accomplished this objective.

Interfacial compatibility

Silica surface silanols would appear to offer increased compatibility (wetting) to the oxide sites of metals. This theory has supplied a satisfactory explanation in respect to textile or organic cords where bond strength increases in proportion to the amount of silica filler in skim or friction compounds. Contact angle measurements verify that replacement of carbon black by silica produces a significant improvement in wetting, much of which is lost where the silica surface is modified by the addition of methyl silane.

However, peroxide cured NR compounds without sulfur in which silica was the only potential source of adhesion showed no improvement in brass wire bond strengths when silica content was raised to 20 and 40 phr (table 1). At the 40 part level adhesion after humid aging actually declined. A second series, in which the added presence of sulfur brought overall bond strengths to normal values, demonstrated that in the static mode, 10 to 40 parts of silica had a profound influence on both pull-out and rubber cover (table 2). However, disc fatigue dynamic testing after humid aging reversed the static findings. Under dynamic conditions silica concentrations above 20 phr severely reduced bond fatigue life. This effect is also seen at the 10 phr silica level when the uncured wire assemblies are aged at 50[degrees]C (table 3). Here the obvious benefits of silica on both pull-out and rubber cover are mitigated by an increase in disc fatigue separations after three and seven days green aging.

Table 1 - brass adhesion of silica compounds in peroxide cured NR without

sulfur Formulas NR-100; DCP-2; Hi-Sil 210; N330; Naphthenic oil

Variables
Hi-Sil 210                      -     20    40    40
Methyl silane                   -     -     -     2
N330                            50    30    -     -
Oil                              4    -     11     4
Cure; 20'/160[degrees]C
Durometer                       59    60    59    57
M20, MPa                       .64   .70   .74   .65
Static wire adhesion, N/25mm
6+3 Wire original               80   100    80    90
Humid aged(*)                   75    74    23    30
5+1 Wire
Original                        50    37    44    15
Humid aged(*)                   45   51     37    29

(*)24 hours 90[degrees]C, 100% R.H.

Table 2 - silica effect on brass adhesion in peroxide cured NR with

sulfur

Variables

                    1      2     3     4      5

Hi-Sil 210          -     10    20     30    40
HAF N330           50     40    30    20     10
Oil                 4      5     6     8     10
Durometer          53     52    52    56     58
MG trouser tear,
 KN/M              3.9   4.6   7.2   10.3   19.1
Wire (6+3) adhesion
Static: aged 3 days 50[degrees]C before curing, N/25mm
 Pull-out          315(*) 650(*)  670   675   775
 % Cover            20     90      90    90    80
(*)Unchanged with 0 aging
Dynamic disc fatigue flexed 3 hours 70[degrees]C
 humid aged 3 days 50[degrees]C before curing
 Separations        0     0     0     4     4
 Pull-out          505   490   585   400   135
Dynamic disc fatigue flexed 6 hours 70[degrees]C
 humid aged 5 days 90[degrees]C before curing
 Separations        0     0     3     6     6
 Pull-out          730   720   690   310   85

Formula (1347): NR-100; DCP40-5, Sulfur-2

Table 3 - brass adhesion of silica compounds in peroxide cured NR with

sulfur

Formula (1347)
NR-100; DCP-2; Sulfur-1.5; Silica; N330; Naphthenic oil-
4/5
Variables
 Hi-Sil 243LD                      -          10
 N330                             50          40
Static wire adhesion, N/25mm; cure 25'/160[degrees]C
 Uncured 50[degrees]C   Pull-out   % Cover   Pull-out   % Cover
  0 Days                  525        40       799         80
  1 Day                   469        25       627         75
  3 Days                  455        30       579         70
  7 Days                  427        20       659         60
Disc fatigue dynamic wire adhesion:
 Uncured 50[degrees]C   Failures   Pull-out   Failures   Pull-out
  0 Days                   0        440           0        563
  1 Day                    1        430           1        582
  3 Days                   1        383           4        406
  7 Days                   2        409           6        -

It appears that large amounts of silica or extended uncured aging produced a degrading change at the interface which, with dynamic testing, was sufficient to over-power the improvement in static pull-out strength. These data indicate that while wetting effects are probably involved, they do not provide a full explanation of the silica-brass adhesion mechanism.

Zinc oxide and acceleration effects

In addition to silica and sulfur, the accelerator TBBS and zinc oxide (with stearic acid) are also likely sources of influence on the brass bonding reaction. To obtain an indication of the relative importance of these materials a series of eight compounds in which each variable was present or absent was mixed and tested (table 4). The presence of peroxide assured that crosslinking was adequate in all compounds. Comparison of average pull-out values of the three variables reveals severe bond deterioration where soluble zinc is present, for both original and aged conditions. Accelerator TBBS shows a negative effect in aged samples, and the benefit of silica is apparent in the unaged group. The combined presence of soluble zinc and TBBS produced an almost complete lack of resistance to disc fatigue wire separation, as well as the lowest pullout values (table 5).

Table 4 - brass adhesion of silica, zinc oxide and accelerator in

peroxide cured NR

Variables

                   1     2     3     4    5     6     7     8

N330              50    50    50    50   40    40    40    40
Hi-Sil 233         -     -     -     -   10    10    10    10
Zinc oxide         -     3     -     3    -    3      -     3
Stearic acid       -     2     -     2    -    2      -     2
TBBS               -     -    1      1    -     -     1     1
MDR 160[degrees]C
 T75, min.        14     9     2     3    7     7     2     4
MDR crosslinks,
 dN-M             12    12    11    24   10    11    14    20
Durometer         50    54    55    68   50    50    52    64
M20, MPa         0.4   0.5   0.5   1.1  0.4   0.5   0.5   0.9
M300, MPa          8     9    10    22    7     8    8     17
Wire adhesion by disc fatigue
Original:
 Separations       0     0     0     6    0     0     0     4
 Pull-out, N     530   185   470   320  785   430   570   375
Humid aged 5 days 90[degrees]C;
 Separations       0     0     1     8    0     0     1     6
 Pull-out, N     720    550  580   235  780   560   530   185
Formula (1488): NR-100; LP-oil-4; Sulfur-4; DCP40-5

Table 5 - brass adhesion of silica, zinc oxide and accelerator in

peroxide cured natural rubber

Average pull-out of variables, N/25mm
Variable     Present   Absent   Change
Humid aged 5 days at 90[degrees]C
Silica        515       520        -5
Zinc oxide    385       655      -270
TBBS          385       655      -270
Original
Silica        540       375      +165
Zinc oxide    330       590      -260
TBBS          435       485       -50

Silica and zinc oxide effects

The strong negative response of soluble zinc in the peroxide systems described above led to the investigation of zinc concentration effects. This work was carried out with response surface designs in sulfur cure systems with and without the bonding resins usually present in wire belt formulas.

In the series without resins, zinc oxide concentrations ranged from 1.5 to 10 phr and silica from 0 to 15 phr (table 6). A contour plot of disc fatigue separations (figure 1) reveals the anticipated effect of low zinc oxide concentration on bond fatigue life. Complete resistance to bare wire separations is achieved with a reduction to 1-3 phr. Higher concentrations of both silica and zinc oxide produce a rapid increase in separations. Pull-out data confirm bond deterioration at normal zinc concentrations in both original and humid aged compounds.

[TABULAR DATA OMITTED]

Accelerator TBBS was increased in the silica compounds to counter the usual cure retarding effects of silica. In this case, a significant increase in rheometer crosslinking indicated that undesirable effects in bond chemistry could be involved. Additional testing of four of these formulas in which TBBS remained unchanged confirmed this supposition. Without the added TBBS the beneficial effect of 10 phr silica became more apparent. However, humid aging before flexing reversed matters at low zinc oxide (1.5 phr) concentrations. Bond deterioration due to overcure appeared only in aged samples.

Continued exploration of zinc oxide and silica effects was carried out with a bonding resin system which included resorcinol-formaldehyde and hexamethoxy methylmelamine resins (table 7). Contour curves again illustrate the beneficial effect of low zinc content. Unaged disc fatigue date (expressed in pull-out terms in the absence of wire separations) show that highest bond strengths are obtained with 1-2 phr zinc oxide and 5-12 phr silica (figure 2). These concentrations also apply to wire assemblies which were aged at 100% relative humidity for three days at 90[degrees]C (figure 3).

[CHART OMITTED]

Bond deterioration due to aging allows evaluation in terms of dynamic fatigue separations rather than static pull-out values, which are often compromised by a large rubber cohesion component (rubber cover). Positive slopes of the curves indicate that replacement of black by 5-10 phr silica provides complete resistance to wire separation under these test conditions. When wire assemblies were exposed to high humidity before curing, an optimum silica concentration appears at 8 phr (figure 4). In all cases zinc oxide at lower than normal concentrations produces the best fatigue resistance.

[CHART OMITTED]

Although these data offer convincing proof that zinc oxide is a controlling variable in brass adhesion, practical application of this knowledge requires assurance that reduced concentrations do not produce undesirable effects in vulcanizate properties. In both black and silica compounds there is a trend to faster cure rate and lower rheometer crosslinks at 1 phr zinc oxide. Except for a slight decrease in hardness, original stress-strain properties remain unchanged, and only a slight loss in aged tensile occurs at 1 phr (table 7).

It can be concluded that practical optimum concentrations will range from 8 to 12 phr for silica and from 1 to 1.5 for zinc oxide.

Discussion

The relationship between silica and zinc oxide in which soluble zinc (i.e. zinc oxide plus fatty acid) is strongly attached to the silanol surface was described by Wartmann (ref. 4). Almost all compounding aspects of silica reinforced elastomers reflect the altering of zinc metabolism by this reaction. Crosslinking, viscosity and reinforcement are all affected. It now appears that brass adhesion can be included in this list.

Results of the experiments described here demonstrate that lower than normal concentrations of zinc oxide produce improved resistance to bond fatigue in all-black compounds. Further improvement appears when relatively small amounts of silica replace black. The role of silica is evidently its ability to repress available zinc, and ultimately the formation of excess zinc sulfide and zinc oxide at the interface.

One of the latest confirmations of the adhesion degrading influence of interfacial zinc oxide is found in a recent paper by Pieroth et. al. (ref. 5). Analysis (TEM/ SNMS) of their control compound which, like the compounds described in this article, contained no cobalt or boric ester showed a high zinc oxide content at the delaminated surface. The zinc oxide layer was considerably reduced in the presence of cobalt naphthenate and boric acid ester in compounds which provided the highest aged adhesion. In respect to the use of cobalt, it is relevant to note that previous work by Wagner and Hewitt found no positive influence from cobalt in compounds which contained silicas (ref. 2).

Summary

Bond fatigue life of brass coated wire to rubber adhesion assemblies is greatly improved at reduced concentrations of zinc oxide. Further improvement occurs when carbon black is partially replaced by precipitated, fine particle silica. The mechanism of brass adhesion enhancement by silica involves a reduction of interfacial zinc oxide when soluble zinc is removed from the system by attachment to silica surface silanols.

References

1. N.L. Hewitt, Rubber World, December, 1991.

2. M.P. Wagner and N.L. Hewitt, Kautschuk and Gummi, October 1984.

3. M.P. Wagner, paper presented to the Akron Rubber Group, December 1972.

4. H. Wartmann and C. Strauss, "Analysis of cure parameters to define vulcanization and reinforcement," paper presented at Spring meeting, Rubber Division, Miami, 1965.

5. M. Pieroth, D. Holtkamp, A. Elschner, "The influence of boric acid ester on the structure and composition of the rubberbrass interface," paper presented at Rubber Division Meeting, Louisville, 1992.