Multifunctional elastomeric coating for use on rubber components.

Elastomeric materials are utilized in numerous industrial and aerospace applications, including hoses, seals, tires, mountings, dampers and insulating devices found in automobiles and other vehicles. In many applications, the elastomeric materials of choice are not resistant to certain hostile environments to which they are exposed during service. One method of rendering molded elastomeric parts resistant to degrading environments is to apply a protective coating over the elastomeric material. Room temperature curable elastomeric coatings have been developed that combine heat resistance, environmental protection, adhesion to both rubber and metal, and the flexibility to withstand dynamic motions. These new coatings are highly elastic and can also be used as decorative coatings for rubber, since they can be made in black, clear or colored versions. The coatings are suitable for use in diverse applications like vibration isolators, hoses, tires and other rubber goods.

The HPC-5 series coatings with Enduralast technology (HPC) are based on a hydrogenated nitrile butadiene elastomer, and they form tough, high-strength coatings with good ozone and weather resistance coupled with excellent oil and fuel resistance. A "B" suffix refers to a black coating (i.e., HPC-5B) while a "C" suffix refers to a clear base coating. A clear base coating may have pigments added to color it, and such addition is noted when applicable.

Procedures

Testing was conducted using a natural rubber/polybutadiene (NR/BR) blend as a substrate material because the coatings were developed for use on vibration isolators. The NR/BR blend is a nominal 55 hardness (durometer A), carbon black reinforced, sulfur-cured elastomer. All specimens to be coated were treated to enhance adhesion with a solvent-based surface treatment prior to the application of the coating.

Fatigue was tested using DeMattia flex specimens tested in accordance with ASTM D 430 Method B (un-pierced), with observations made at various intervals to determine if the coatings were cracking or delaminating. Uncoated specimens were used as controls.

Fluid resistance was evaluated using standard ASTM D 471 volume swell specimens. In some fluids, tensile and elongation were tested both before and after oil aging using the procedures documented in ASTM D 412.

Dynamic ozone testing was performed using the procedure described using ASTM D 3395 at 40[degrees]C with 50 pphm of ozone. Instead of using the standard 12.2 mm wide specimens, 25.4 mm wide specimens were used to better evaluate surface cracking.

Weather-Ometer testing was conducted on specimens in accordance with SAE J 1960--light and dark cycles with an intermittent specimen and rack spray (.55 irradiance). Gloss measurements were made per ASTM D 523 using a BYK Gardner micro-tri-gloss meter with measurements taken at a 60[degrees] angle both before and after Weather-Ometer testing.

The ultimate tensile strength and elongation-to-break of the unsupported films were measured by curing films and pulling them using the procedures documented in ASTM D 412, except that the film thickness was 0.05 to 0.08 mm.

Discussion

A good coating material for rubber should have a number of specific characteristics. The coating should have significantly better heat and oxidation resistance than the underlying rubber substrate, since the coating has a high surface area exposed to heat and oxygen. The coating should exhibit adhesion to both rubber and metal substrates, and it should be room temperature curable for ease of use. The coating should be easy to apply using common application methods such as spraying, dipping and brushing. The coating, when used in dynamic applications, needs to have a high degree of both elasticity and fatigue resistance. The coating needs to possess ozone and UV resistance to survive aging outdoors. In some applications, it needs to impart environmental protection against exposure to petroleum oils and fuels. Finally, the coating should have an aesthetically pleasing appearance.

Both fatigue and adhesion were evaluated using DeMattia flex specimens. The strain created at the flexure is quite severe, and it was felt that this test would screen out less than adequate coatings. A commercial fluorocarbon coating was cracked in less than 4,000 cycles and was delaminating from the rubber substrate (figure 1). Once the coating cracks and delaminates, most of the protection afforded by the coating is lost. The uncoated specimens showed cracking starting at around 80,000 cycles (figure 2). HPC-5B (figure 3) looked quite good at 80,000 cycles, with little or no cracking evident.

[FIGURES 1-3 OMITTED]

The specimens continued to run to 150,000 cycles, by which time the uncoated specimens were severely cracked (figure 4). HPC-5B showed only minimal cracking at 150,000 cycles (figure 5). It is well known that atmospheric ozone initiates cracking in natural rubber, and it is speculated that the coating may delay fatigue cracking because it protects the substrate from ozone cracking (as will be shown later).

[FIGURES 4-5 OMITTED]

Since one purpose of the coating was to protect the substrate against the deleterious effects of fuel and oil, both coated and uncoated volume swell specimens made from an NR/BR blend were exposed to various fluids (table 1). The data provide considerable experimental support for the conclusion that the coating protects the rubber substrate against a variety of oils and fuels used in both industrial and aerospace applications.

Further work was done to determine the effect of coating thickness on the fluid resistance (table 2). It was not surprising that the effectiveness of the coating is directly related to the thickness that is applied.

Testing was conducted to see how the coatings protected the substrate from loss of physical properties after fluid aging (table 3). Coating the specimens had negligible effect on the original tensile and elongation properties, but the coating clearly reduced volume swell as well as loss in ultimate tensile strength and elongation at both room temperature and elevated temperature.

For coatings intended for use on highly extensible substrates such as cured rubber, it is important that the coatings have high elongation so that they do not crack when the substrate flexes. The ultimate tensile strength and elongation at break of the unsupported films (table 4) were measured by curing films and by pulling them using the procedures documented in ASTM D 412, except that the film thickness was 0.05 to 0.08 ram. The data showed that the cured coatings possessed moderate-to-high strength and excellent elongation.

Although the elongation of the unsupported films is around 400%, when the HPC-5 films were applied to a natural rubber substrate, they did not show signs of cracking, even when the substrate was strained to 600% elongation.

Dynamic ozone testing was run using ASTM D 3395 at 40[dynamnic]C with 50 pphm of ozone. The failure criterion for dynamic ozone specimens is cracking, typically first observed on the edge of the specimens. Because of the sharp edge on the specimen, it is uncertain how thick the coating actually is on the edge. The coating thins out as it approaches the sharp edge of the die-cut specimen, so there is minimal coverage at the edge. Figure 6 shows the cross section of an ozone specimen with a silver reflective coating that was applied at a nominal thickness of 0.02 mm.

[FIGURE 6 OMITTED]

In order to determine the effect of the coating film thickness in preventing ozone cracking, a wider specimen (25.4 mm) was utilized to evaluate surface cracking and minimize edge effects. The coating thickness was measured in the center of the flat strip and specimens were run until they exhibited cracking in the center (table 5).

Figures 7-12 show the uncoated and coated specimens at various time intervals during the ozone aging tests. Coated specimens were recorded as being center-cracked as soon as a single center crack was observed. HPC-5C showed a significant improvement in ozone protection, and the specimens with three coats did not crack even after 275 hours.

[FIGURES 7-12 OMITTED]

The one and two coat samples of HPC-5C looked good at 275 hours (figures 10 and 11), although it often seemed that a single center crack developed in some instances at a flaw site in the coating. The black coating did not protect quite as well as the clear coating.

[FIGURES 10-11 OMITTED]

The Weather-Ometer simulates the environmental parameters of sunlight, temperature and moisture in the form of rain, dew and humidity. The xenon long arc light source simulates UV and visible solar radiation. Weather-Ometer testing was conducted to determine the effect of weathering on gloss. A BYK Gardner micro-tri-gloss meter was used to measure gloss of the specimens. The measurement results of a gloss meter are related to the amount of reflected light from a black glass standard with a defined refractive index. The measurement value for this defined standard is equal to 100 Gloss Units. Table 6 shows the difference between initial gloss readings and gloss after aging for each of the coatings. Higher numbers indicate a more glossy surface.

The coating can also be used as a cosmetic or decorative coating (either clear, black or colored) for various molded rubber products. Special compounding is not required for adhesion as has been demonstrated by coating commercially available tire sidewall compounds (figure 13). Applying the coating to tire sidewalls gives a much longer lasting appearance than can be achieved with commercially available tire dressings, while colored versions of HPC-5C can be used to create designer rubber parts (figures 14 and 15).

[FIGURES 13-15 OMITTED]

Summary and conclusions

Through innovative compounding, stable one-part elastomeric coatings that crosslink at room temperature have been developed. These coatings bond well to elastomer substrates and exhibit excellent strength and fatigue resistance. These coatings can be used to improve the resistance of natural rubber and polybutadiene to ozone or to hydrocarbon oils and fuels. They can also be used to improve the cosmetic appearance of molded rubber parts.

Table 1--fluid resistance of coated and
uncoated rubber

Coating designation                          Control    HPC-5B   HPC-5C

Base elastomer: NR/BR blend                  Uncoated
Fluid age, 2 coats applied by dipping

Jet A Fuel
22 hrs. at 21[degrees]C: volume change (%)     219.8       1.7      --
70 hrs. at 21[degrees]C: volume change (%)     210.6      13.8      --
168 hrs. at 21[degrees]C: volume change (%)    204.6       --      35.0

Unleaded gasoline
22 hrs. at 21[degrees]C: volume change (%)     103.6       1.6      1.1

Mineral spirits
22 hrs. at 21[degrees]C: volume change (%)     181.1       1.7      1.7

IRM-903 oil
70 hrs. at 21[degrees]C: volume change (%)      62.5       0.1      0.5
70 hrs. at 70[degrees]C: volume change (%)     184.0      27.4     14.8
70 hrs. at 100[degrees]C: volume change (%)    256.2      50.8     41.5
1,000 hrs. at 21[degrees]C: volume change
  (%)                                         180.1       0.5      --

Citgo type F automotive transmission fluid
70 hrs. at 21[degrees]C: volume change (%)      43.4       0.0      --

Mil-PRF-23699F Exxon turbo oil 2380
70 hrs. at 21[degrees]C: volume change (%)      37.8      -0.5      --
70 hrs. at 100[degrees]C: volume change (%)    105.7       --      54.6

Mil-PRF-5606 AeroShell fluid 41
70 hrs. at 21[degrees]C: volume change (%)     188.7       0.4      0.4

Table 2--effect of coating thickness on fuel resistance

Properties                            HPC-
Fluid age, jet A fuel, 168 hrs. @      5C
  rm. temp. brushed coating
0 coats (thickness in [micro]m)
Change in volume (%)                   0.00
1 coat (thickness in [micro]m)       204.6
Change in volume (%)                  13
2 coats (thickness in [micro]m)       77.5
Change in volume (%)                  25
3 coats (thickness in [micro]m)       35.0
Change in volume (%)                  44

Table 3--resistance to IRM-903 reference oil

Coating designation                  Control     HPC-5B    HPC-5C

Base elastomer: NR/BR blend          Uncoated
Original physical properties of substrate
Hardness (durometer A)                 58         57        55
Tensile (MPa)                          23.33      21.57     22.48
Elongation (%)                        575        560       570

Aged 70 hrs. at 70[degrees]C in IRM-903 oil
Hardness (durometer A)                 28         38        43
Tensile (MPa)                           3.27      13.20     17.06
Elongation (%)                        145        360       435
Change on hardness (points)           -30        -19       -12
Change in tensile (%)                 -86.0      -38.8     -24.1
Change in elongation (%)              -74.8      -35.7     -23.7
Change in volume (%)                  184.0       27.4      14.8

Aged 1,000 hrs. at 21[degrees]C in IRM-903 oil
Hardness (durometer A)                 26         55         --
Tensile (MPa)                           3.79      19.58      --
Elongation (%)                        145        530         --
Change on hardness (points)           -32         -2         --
Change in tensile (%)                 -83.7       -9.2       --
Change in elongation (%)              -74.8       -5.4       --
Change in volume (%)                  180.1        0.5       --

Table 4--film properties

Properties        HPC-5B    HPC-5C

Tensile (MPa)      27.61     27.40
Elongation (%)    385       430

Table 5--effect of coating thickness on ozone
resistance

Dynamic testing in 50 pphm ozone, 40[degrees]C
Brushed coating, 25.4 mm wide strips

HPC-5B               Un-        1 coat/       2 coats/       3 coats/
                    coated    6 [micro]m     19 [micro]m    32 [micro]m
Edge cracking
  (hours)             4            7             31             58
Surface cracking
  (hours)             5           48             86             185
HPC-5C               Un-        1 coat/       2 coats/       3 coats/
Edge cracking       coated    13 [micro]m    25 [micro]m    44 [micro]m
  (hours)             4           26             48             129
Surface cracking
  (hours)             5           185            223           >275

Table 6--weathering resistance

                                     Control     HPC-5B    HPC-5C
                                     uncoated

Initial gloss (60[degrees] angle)      6.9        59.9      63.3
Weather-Ometer testing:
  Gloss after 1 week aging             4.0        49.1      52.3

James R. Halladay and Frank J. Krakowski, Lord