Aqueous adhesives for bonding NBR to metal.

Adhesives for bonding acrylonitrile-butadiene rubber (NBR) to metal have their largest class of users in the dynamic seal industry. Dynamic seals are made by vulcanization-bonding a thin layer of rubber to the inner diameter of a metal ring. This forms a rubber surface for sealing against a rotating shaft The purpose of the seal is to protect moving parts by keeping oil or grease on one side and dirt on the other side of the seal. NBR is used in this application because of its excellent oil resistance. Seal compounds were used in the development of the aqueous NBR adhesives described in this article, however, their use is not limited to seals.

Traditionally NBR has been bonded to metal using a single coat of solvent-borne adhesive. In recent years the use of solvent-borne adhesives has been increasingly restricted by legislation designed to control pollution. Most solvents which have been useful in rubber-to-metal adhesives are considered volatile organic compounds (VOCs) because they contribute to air pollution in the lower atmosphere. As such, they are subject to Title I of the U.S. Clean Air Act Amendments (CAAA) passed in 1990 which calls for reduction of VOC emissions. Other exempt solvents, such as 1,1,1 trichloroethane, will soon be banned from production due to their ozone depleting potential.

In addition to VOCs, many traditional rubber-to-metal bonding agents contain materials, such as lead compounds, that are considered hazardous air pollutants (HAPs) under Title III of the CAAA. Title III requires that emissions of these substances be controlled through the use of maximum achievable control technology (MACT), which refers to emission control equipment such as incinerators or carbon absorbers. Ideally, the emission restrictions would be met by using adhesives that contain no hazardous air pollutants.

Thus, the task of the adhesive formulator is to develop adhesives which allow users of adhesives to comply with emission restrictions while maintaining the performance of the traditional solvent-borne adhesives. Development work has focused on this task and has developed aqueous adhesives for bonding NBR to metal. As well as being waterdilutable, they contain no hazardous air pollutants or ozone depleting solvents. The performance of two of these adhesives is discussed in this article. Both adhesives rely on the same water-compatible technology developed at Lord for their bonding performance. However, they are formulated differently to handle varied requirements for cosmetics, recommended application methods and recommended film thickness.

Adhesive 8110 is formulated to behave much like a primer or a paint with regard to application and appearance. It can be applied to parts by spraying or dipping and the undiluted adhesive will give a film thickness of 7.6-12.7 (micro)(0.30.5 mil) if applied properly. It has been formulated to eliminate sweep and mold fouling at this thickness. For this investigation it was applied full strength, 35 weight percent solids, as well as diluted to 20, 10 and 5 weight percent solids for comparison purposes. Adhesive 8102 exists because of the need of many seal manufacturers to use a very thin coating of adhesive to accommodate tight mold tolerances. It is designed to be diluted to 10 weight percent solids or less and applied by dipping, which results in an adhesive coating of 1.3 (micro)m (0.05 mil) or less. A list of physical properties and recommended application methods for the two aqueous adhesives is given in table 1. The solvent-bome controls in this study were TyPly BN and Chemlok 205. TyPly BN is an NBR adhesive. Chemlok 205 is best known as a primer for natural rubber bonding but it is also an NBR adhesive.

Experimental

All vulcanization-bonded rubber-to-metal test assemblies were molded per ASTM D429, Method B 1. The results are the average of three test specimens per condition.

Acrylonitrile-butadiene rubbers

Six rubber compounds were used in this study, denoted as compounds A-F in this report. One was a Lord compound developed for testing. Five were acrylonitrile-butadiene rubbers obtained from seal manufacturers. Although they are all used for dynamic seals, they vary in acrylonitrile content, cure type, filler type, filler loading, and amount and type of various process aids. The exact compound formulas can not be disclosed here because of theft proprietary nature, however, the compounds are characterized in table 2 by acrylonitrile content, cure type, Shore A durometer and typical adhesion value where this information was available.

For most of the testing, the six NBRs were compression molded to form test assemblies using the cure conditions of 191 degrees C and 7 mins. Compound D was compression molded at a range of cure temperatures (177 degrees C, 191 degre es C and 204 degrees C) and cure times (two, four and six minutes).

Adhesives

All adhesives and all dilutions were applied by flow coating onto zinc phosphatized steel coupons. Because seal manufacturers are interested in using low film thicknesses, the adhesives were compared at several dilutions, down to five weight percent solids. Dilution was the controlled parameter instead of adhesive film thickness because the thickness is difficult to measure accurately below 2.5 (micro)m. Film thicknesses of the dilutions used in this study are shown in table 3.

* Adhesive 8102 -- The adhesive is manufactured at 20 weight percent solids and designed to be infinitely dilutuble with water. It was applied undiluted and diluted with deionized water to ten and five weight percent solids for this study.

* Adhesive 8110 --The adhesive is manufactured at 35 weight percent solids. It is designed to be used at full strength to 20 percent solids. For comparison purposes, it was applied undiluted and diluted to 20, 10, and 5 weight percent solids with deionized water in this study.

* TyPly BN -- The adhesive is manufactured at 37 weight percent solids. It was diluted to 20, 10, and 5 weight percent solids with MIBK for this study.

* Adhesive 205 -- The adhesive is manufactured at 24 weight percent solids. It was diluted to 20, 10, and 5 weight percent solids with MIBK for this study.

Tests

* Primary adhesion -- Bonded ASTM D429-B assemblies were pulled at a rate of 20 inches per minute at a 45 degrees angle. The percent rubber retention on the metal coupons was recorded.

* Boiling water resistance -- The rubber-tometal bonds of the ASTM D429-B assemblies were slightly stressed by holding back the rubber flaps with clips. The assemblies were then submerged in boiling water for two hours. These were pulled by hand and the percent rubber retention was recorded. (Parts were not wired back in the usual manner because NBR can fracture in this test if stressed too much.)

* Hot oil resistance ASTM D429-B assemblies were molded with 3.2 mm (.125 in.) thick rubber. These were submerged in ASTM 0il #3 for 72 hours at 135 degre es C. They were pulled by hand and percent rubber retention was recorded.

Results and discussion

Figure 1 shows the effect of dilution of 8110 on primary adhesion for six different rubbers cured for seven minutes at 191 degrees C. Compounds B and C, the two peroxide cured NBRs, gave 100% rubber tearing bonds at all dilutions tested. The other four compounds showed different degrees of dilution dependence. All NBR compounds gave 100% rubber tearing bonds with the most concentrated adhesive, 35 weight percent solids. This result demonstrates that rubber compounds which are nominally the same, i.e., NBR compounds for dynamic seals, can be very different in their bondability.

Although all of the compounds could be successfully bonded with 35% solids adhesive, or an adhesive film 7.6-12.7 (micro)m thick, some NBRs did not give 100% rubber tearing bonds as the adhesive film thickness was decreased. This result suggests the possibility of compounding rubbers for better adhesion properties at lower adhesive film thickness, although more work is required to understand the relationship between bondability and compound formulas.

Figures 2 and 3 show the effect of dilution on boiling water resistance and hot oil resistance, respectively, for Adhesive 8110 and rubber compounds A, B, C and D cured for seven minutes at 191 degrees C. (Rubber compounds E and F were not included in these tests because of lack of availability.) The results were much the same as the primary bonding results, with variability between rubber compounds at low solids and good environmental resistance with all compounds at high solids. Comparing these data with the primary adhesion data in figure 1 reveals that, for the adhesive and rubber compounds tested, when good primary adhesion was obtained by a particular dilution/rubber combination, boiling water resistance and hot oil resistance were also good. If anything, the bonds improved in the hot oil test, possibly because extended heat treatment increases the crosslinking of the adhesive.

Figures 4, 5 and 6 show the effects of cure temperature and adhesive dilution on primary adhesion, with two, four and six minute cures, respectively. The rubber for this testing was compound D (sulfur-donor cure, 80 durometer) and the adhesive was Adhesive 8102. Compound D was chosen because it had proven to be difffcult to bond in other testing. For this stock, both high temperature and higher film thickness were required to achieve good bonding. The combination of the highest cure temperature, 204 degrees C, and the highest solids level, 20%, gave primary bond results of greater than 90% rubber retention. Conversely, at either 177 degrees C cure temperature or 5% solids adhesive, the bonding dropped to less than 20% rubber retention. Cure time had almost no effect on adhesion as seen by the similarity of figures 4, 5 and 6. This suggests that the kinetics of the adhesive cure and/or the bonding chemistry are more affected by temperature than time. In some ways this is a fortuitous property because in a manufacturing environment it is usually more desirable to use high cure temperatures than long cure times because it allows cycle time to be decreased.

Figure 7 shows primary adhesion of the two aqueous adhesives, 8102 and SILO, compared to the solvent-borne adhesives, 205 and TyPly BN. For comparison purposes, all adhesives were applied by flow-coating at 20 weight percent solids. Although 100% rubber retention was not always achieved at this dilution, the fi/m thickness was within the range where bonding for all of the adhesives was sensitive to the film thickness, giving a more meaningful comparison between the aqueous and solvent-borne adhesives. The two aqueous adhesives performed essentially the same or better than the two solvent-borne adhesives for all six rubber compounds tested. All four adhesives gave greater than 80% rubber retention with all six of the NBRs, with the exception of compounds E and F with TyPly BN and compound F with Adhesive 205. It should be remembered from figure 1 that 100% rubber retention was observed with all of the NBR compounds with Adhesive 8110 at 35 weight percent solids.

Figure 8 shows results of boiling water testing. All four adhesives were applied by flow-coating at 20 weight percent solids. Again, the aqueous adhesives compared favorably with the two solvent-borne adhesives for the four rubbers tested. Both aqueous adhesives gave excellent boiling water resistance, essentially 100% rubber retention, with NBRs B, C and D. Adhesive 205 did outperform the other three adhesives with compound A, but they all had greater than 50% rubber retention which is considered acceptable in this test because seals are usually in an oil environment and NBRs themselves have limited water resistance. However, a degree of confidence is gained by good water resistance because of the possibility of exposure to water in the atmosphere.

Figure 9 shows results of hot oil immersion testing. All four adhesives were applied by flow-coating at 20 weight percent solids. In this critical environment for dynamic seals, the aqueous adhesives performed excellently, as did the solvent-borne controls, for the four rnbbers tested.

Conclusions

The two aqueous nitrile adhesives, 8102 and 8110, were as effective as solvent-borne adhesives for bonding six NBR compounds. Both aqueous adhesives gave excellent performance in primary adhesion, boiling water resistance and hot oil resistance. The adhesives have very low VOC content and they are dilutable with water which enables users of the adhesives to reduce VOC emissions to extremely low levels. There are no hazardous air pollutants present in the adhesives so the issue of controlling emission of these substances through incineration or other control methods is avoided altogether. And, of course, the removal of flammable solvents from the work place is desirable for safety reasons. Thus, the advantages of aqueous adhesives, in regulatory compliance and improved safety, can be gained without any penalty in bond performance.

As well as showing that the aqueous adhesives were effective in bonding all of the NBRs tested at normal film thicknesses, this work showed that some NBRs can be successfully bonded to metal with an extremely thin film of adhesive. For compounds B and C, the two peroxide cured NBRs, 5% solids, or 0.3-0.8 (micro)jm film thickness, was sufficient for excellent primary adhesion as well as excellent environmental resistance. Bonding at dilutions ranging from 5 to 35 weight percent solids, or 0.3 to 12.7 (micro)m film thickness, showed that Adhesive 8110 was effective at different dilutions depending on the NBR compound. This is a benefit to those who require a very thin adhesive film and have an easy-to-bond NBR. It also suggests the possibility of compounding NBR to increase its bondability if a very thin adhesive film is desired. The other variable which may have a strong influence on the performance of the two aqueous adhesives tested is cure temperature. For compound D, a sulfur-donor cured NBR, adhesion was enhanced by increasing the vulcanization temperature from 177 degrees C to 204 degrees C and increasing the adhesive film thickness from 0.3 to 3.0 (micro)m. This suggests that adhesion to difficult-to-bond NBR compounds may be improved by increasing cure temperature as well as increasing adhesive film thickness.

References

1. ASTM D429-81, Method B, Annual Book ASTM Stand., volume 09.01, p. 56 (1991).

Acknowledgements

"Aqueous adhesives for bonding NBR to metal," is based on a paper given at the May, 1993 Rubber Division meeting. "Elastomers in automotive fuels, oils and fluids at high temperatures," is based on a paper given at the March, 1993 SAE meeting.