Comparisons and contrasts in rubber-metal bonding using eight polymer types.

Bond studies often center on a single compound, or perhaps just a few related compounds (refs. 1-5). However, one question becomes how very different compounds, based on different polymers, might compare in bonding capacity. Since compounds of significantly different hardness and modulus can be expected to display contrasting bond strengths, the test recipes would need to be adjusted to fall within a reasonably narrow range of hardness. This will make the comparisons more valid and simplify the analysis.

Eight families of polymers were chosen, which included:

* Natural rubber (NR);

* styrene-butadiene rubber (SBR);

* polychloroprene rubber (CR);

* ethylene-propylene-diene rubber (EPDM);

* nitrile rubber (NBR);

* silicone rubber (Si);

* fluorocarbon rubber (FKM); and

* millable urethane rubber (AU).

All formulations were adjusted by filler and plasticizer levels towards a target hardness of 65 durometer A. As an additional facet of the experiment, three of the polymers were readjusted to a softer hardness range of 45-50 durometer. The descriptions of the formulae are provided in table 1 (detailed formulae are available in the reference table).

Test properties for evaluation were chosen to include original hardness, tensile properties, heat aging resistance and compression set, followed by original bond strength, heat aged bond strength and resistance by stressed specimens to extended exposure to a hot salt spray environment. Tensile testing was performed per ASTM D412. All bond tests were based on the ASTM D429 Method F specimen (the "buffer" specimen, figure 1), using a sample size of three. For all numerical data, the median is reported and used for analysis, the same practice required by ASTM for tensile testing. The amount of total elongation the specimen withstood prior to bond failure was also recorded. This is not part of the normal test procedure, but considered to be a possible response of interest, assuming any correlations with other properties could be detected.

[FIGURE 1 OMITTED]

Discussion

The comparison of rheometer responses and basic physical properties of the eight primary compounds is provided in table 2.

The low and high levels of each property are shown in table 3. The range of hardness is reasonably limited and close to the target of 65 +/-3. Other properties display considerably more variation, with the rheometer minimum viscosity covering more than an order of magnitude (0,19 to 3.0 dNm).

These broad variations in original properties, even at similar hardness levels, is only to be expected, given the contrasts in both the base polymers and the particular compounding methods applied to each of them. The wide range of 100% tensile moduli (M100) is noteworthy, since it demonstrates the lack of good correlation between hardness and stiffness, which engineers tend to find surprising. (Note: Compression set and heat aging comparisons are difficult to evaluate when the test temperatures are not the same; but it is less valid to compare exotic polymers to conventional ones at the same low test temperature than it is to compare them using temperatures that are reasonably comparable in aggressive tendencies for the different polymers.)

Heat aging is commonly used as a baseline criterion for long term aging. Contrasts between the compounds were expected, and the data in table 4 demonstrate that expectation was fulfilled.

In general, the compounds were affected by heat aging in the anticipated ways; that is, hardness and modulus increased, while elongation decreased (with the exception of the polychloroprene compound). The NBR formula contained a moderate loading of a volatile ester plasticizer, which led to its having the most severe heat aging effects. (See reference tables for formulation details.)

The key data in this study are the bond strengths, which are shown in table 5. The broad range of these results is not surprising, nor is the range of different levels of specimen elongation at break. The heat aging can also be seen to have contrasting effects on the bonds, ranging from the substantial lowering seen in the NBR specimens, to the dramatic increase of the FKM specimens.

The lack of change in the silicone bond strength demonstrates the robustness of that polymer and its bonding system, even when aged at 150[degrees]C. In stark contrast, the near-doubling of bond strength of the fluorocarbon specimens is due to the heat aging exposure becoming in effect a postcure process that produced a beneficial effect in the compound. (Note that while heat aging effects on the FKM compound reduced its elongation, there was an opposite effect on the bonded specimens.)

The appearance of the failed bond surfaces from their original testing reveals some interesting contrasts (figure 2). The NR, SBR, CR, NBR and Si all display large measures of cohesive failure in the rubber, with a pattern of torn material that indicates the initial rupture was in or near the center area and then propagated outwards. This is normally interpreted as demonstrating that an excellent bond has been achieved.

[FIGURE 2 OMITTED]

The FKM and AU specimens both display less torn rubber. In the case of the urethane, this does not appear to reflect anything about the bond strength, since it failed at a reasonably high level of force; but for the FKM rubber, it could be considered to imply less ideal bonding. The EPDM specimens all showed very little remaining rubber at the bond interface and the lowest bond strength as well, so the conclusion that this combination of compound and adhesive did not produce high strength bonds is well supported.

After heat aging, the FKM bond strength increased, as noted above, and the mode of failure of the specimens changed to leave substantially more torn rubber on the metal surface. This confirms that the heat aging in effect postcured the material and improved its properties. None of the other specimens from testing after heat aging changed in appearance.

The accelerated environmental test (salt spray) did not provide as graduated a series of material contrasts as was hoped. In essence, the various elastomers fell into four groups, which were the very short-term failures (NR and EPDM), the short-term failures (Si and AU), the medium term failures (CR and NBR) and the long-term survivors (SBR and FKM).

The strength of rubber-metal bonds might be expected to correlate with some basic properties of the compound, and bond robustness might be expected to correlate with resistance to heat aging. This was not observed. Figure 3 is an example of the possible relationships investigated, and demonstrates the lack of correlation between compound tensile strength and bond strength. None of the other measured properties, such as tensile modulus (M-100), original elongation, changes from heat aging of either tensile specimens or bonded specimens, etc., showed any correlation with salt spray life. [It must be noted that no primer coats were used in this experiment, and prior work (see reference 5) has demonstrated that use of a primer makes bonds more robust to environmental testing.]

[FIGURE 3 OMITTED]

Conclusions

These eight compounds, varying widely in polymer type and formulation details, demonstrate no major commonality in bonding characteristics. This illustrates the principle that rubber-metal bonding is a complex function of compound chemistry, adhesive system and substrate characteristics, and not readily predictable when dissimilar formulations are under consideration.

References

(1.) R.J. Del Vecchio and J.R. Halladay, "Rubber-metal bonding studies using designed experiments," meeting of the Rubber Division, ACS, October, 1997.

(2.) R.J. Del Vecchio, "Factors affecting rubber to metal bond strength and failure mode," 1998 Rubber Bonding Conference, Frankfurt, Germany.

(3.) J.R. Halladay and F.J. Krakowski, "The effect of cure system on natural rubber bonding," meeting of the Rubber Division, ACS, September, 1999.

(4.) R.J. Del Vecchio, "Compounding effects on physical properties and rubber-metal bonding," delivered June 12 2002, Rubber Chemicals, Compounding, and Mixing Conference, Munich, Germany.

(5.) R.J. Del Vecchio and Ernest Ferro, "Strength vs. durability of rubber-metal bonds: Factor effects from processing and chemistry," 2004 Polymer Bonding Conference, Munich, Germany.

R.J.Del Vecchio, Technical Consulting Services Ernest Ferro, Jr., Corry Rubber

Table 1--compound descriptions

Polymer

NR     A blend of natural and synthetic polyisoprene,
       reinforced primarily with N300 series black, using
       an EV cure system. RHC 57.3%
SBR    Oil and N339 black extended polymer, conventional
       sulfur cure. RHC 39.5%
CR     W-type polychloroprene, reinforced with a N700
       series black. RHC 49.6%
EPDM   Oil extended polymer, extended with N600 series
       black, EV cure system. RHC 25.8%
NBR    Medium ACN polymer, extended with medium
       particle blacks, combination of ester plasticizers,
       semi-EV cure system. RHC 37.7%
Si     Blend of VMQ bases, peroxide cured.
FKM    Cure incorporated fluorocarbon base, moderate
       reinforcement by thermal black. RHC 76.3%
AU     Millable polyester urethane, reinforced with a
       medium level of N600 series black, sulfur cure
       system. RHC 71.9%

Table 2--rheometer and physical property comparison

Polymer             NR     SBR      CR    EPDM     NBR      Si     FKM

ML, dNm           2.26    1.70    1.33    3.00    0.69    0.74    1.03
TS1 sec.           136     188     222     356     141      73     265
TC90 sec.          421     414     665     819     302     439     772
MH, dNm          16.92   14.44   20.40    9.06   13.71   22.60   13.31
Durometer         63.5    66.8    64.3    64.7    65.5    68.4    62.6
M100 MPa           3.0     2.6     4.5     1.9     3.1     2.5     2.3
Tensile Str.      25.1    15.9    16.2    12.5    10.7     7.1    10.3
Elongation %       501     468     218     687     373     329     425
Comp. set, %      13.4    21.8       9    29.3    20.1    13.4      49
22 hours at
Temp[degrees]C      70      70     100     100     100     150     150

Polymer             AU

ML, dNm           0.19
TS1 sec.           144
TC90 sec.          327
MH, dNm          16.52
Durometer         67.5
M100 MPa           3.3
Tensile Str.      18.6
Elongation %       481
Comp. set, %      44.9
22 hours at
Temp[degrees]C      70

Note: Compression set temperatures were adjusted to
common use for each polymer

Table 3--property ranges

              Low   High

ML, dNm      0.19      3
TS1, sec.    72.6    356
TC90, sec.    302    819
MH, dNm      9.06   22.6
Durometer    62.6   68.4
M100          1.9    4.5
Tensile       7.1   25.1
Elongation    218    687
Comp. set       9     49

Table 4--heat aging comparisons, % change (70 hrs.)

Polymer             NR     SBR     CR    EPDM     NBR      St     FKM

Aging temp.,        70      70    100     100     100     150     150
[degrees]C
M100              21.4    59.3   12.1    60.3     156    16.0    30.7
(% change)
Tensile           -4.1     1.0    8.1     4.8    18.3   -12.8     9.5
strength
Elongation        -9.8   -25.4    7.2   -32.5   -55.5   -19.2   -27.8
Durometer          6.6     6.7    3.2     4.8     9.6     2.8     2.7
(points change)

Polymer             AU

Aging temp.,        70
[degrees]C
M100              80.5
(% change)
Tensile            4.8
strength
Elongation       -29.5
Durometer          9.5
(points change)

Table 5--bond properties (ASTM D-429 Method F)

Polymer           NR      SBR      CR    EPDM      NBR       Si

Original        4.80     5.75    4.34    1.66     5.78     3.21
 strength
Heat aged        3.3%    -4.3%   38.0%    30.9%    -1.5%    -1.8%
 % change
 -strength
Elongation     124.4    147.4    83.6    57.0    129.9     90.6
 %
Heat aged       -5.4%   -10.6%   18.1%   10.0%   -16.7%   -17.0%
 % change
 -elongation
Salt spray         1       33+      7       1        7        4
 life, days

Polymer           FKM      AU

Original        2.97     4.77
 strength
Heat aged       86.3%    25.3%
 % change
 -strength
Elongation      97.2    116.0
 %
Heat aged       52.4%    -2.7%
 % change
 -elongation
Salt spray        33+       4
 life, days

Reference table--formulations

NR

SIR-10                50.00
Polysioprene SKI-3    50.00
Zinc oxide             5.00
Stearic acid           1.00
Proaid 9831            1.75
StanPlas 105           5.00
AgeRite Resin D        1.00
Antozite 67P           3.00
Nocheck 4607 wax       3.00
N-339 carbon black    50.00
N-650 carbon black    10.00
Sulfur                 0.51
Vanax A                1.55
BBTS                   1.55
Proaid 9802            3.00
Total phr            186.36

NBR

Nysyn 33-5           100.00
Stearic acid           1.00
Zinc oxide             5.00
Agerite superflex      1.33
 solid G
N-762 black           85.00
N-550 black           25.00
Sulfur                 0.75
Altax                  0.50
Methyl tuads           1.50
Santogard PVI          0.20
DOP                   35.00
Merrol 3810           10.00
Total phr            265.28

SBR

SBR-1847             225.00
Zinc oxide             5.00
Stearic acid           1.00
Antozite 67P           3.00
Nocheck 4607           3.00
wax
N-650 black           10.00
Sulfur                 2.25
ZBED                   0.20
BBTS                   1.55
Proaid 9802            2.00
Total phr            253.00

EU

Millathane E-76       96.00
SIR-10                 4.00
Zinc stearate          0.50
N-650 black           30.00
Altax                  4.00
MBT                    2.00
Sulfur                 1.50
Thanecure              1.00
Total phr            139.00

Si

Silastic NPC-940      25.00
Silastic NPC-980      75.00
Varox DBPH-50          1.00
Ultramarine blue       0.50
Thermax                1.50
Total phr            103.00

CR

Neoprene W           100.00
Agerite stalite S      2.00
Stearic acid           1.00
Stan mag bar           5.56
Sundex 790            20.00
N-762 black           65.00
NoCheck 4607           2.00
 wax
Zinc oxide             5.00
Vulkacit CRV/LG        1.25
Total phr            201.81

EPDM

Keltan 5531          200.00
Stearic acid           1.00
Sunpar 150            20.00
N-650 black          160.00
Zinc oxide             5.00
Sulfur                 0.45
Vanax A                0.40
Santocure              0.45
TMTM                   0.60
Total phr            387.90

FKM

Fluorel FC-2123      100.00
N-990 black           25.00
Maglite D              3.00
Calcium hydroxide      3.00
Total phr            131.00