Rubber-to-metal bonding agents.

Solvent emissions, methods to reduce such emissions

Method of application and formulation

The use of solvents is increasingly restricted by legal regulations and costly due to requirements made by the authorities. Conventional bonding agents are still mixes of reactive polymers and pigments in organic solvents. Hence, the manufacturers have to develop environment-friendly solutions for the customers.

Just like paint, the polymeric components of rubber-tometal bonding agents are diluted in solvents. After evaporation of the solvent, they form a matrix for the pigments and the crosslinking agents thus enabling the formation of a uniform film on the metal surface. The polymers necessary for adhesives often exhibit high molecular weights leading to correspondingly high dissolution viscosities which in turn set the limits for the formulation of bonding agents.

When processing standard bonding agents, a considerable amount of solvent is set free during drying. In the as-supplied state, a bonding agent usually exhibits a concentration of 25%, the rest being the solvent. Depending on the type of application, the product has to be additionally diluted with solvents to adjust the required viscosity. Table 1 shows the typical parameters for spray application, the most commonly used method of application in the rubber industry. A concentration of 15% means that 85% of the mix has to evaporate thus putting a strain on the environment.

Table 1 - typical dilution of bonding agents
required for spray application
Product           Concentration   Diluent   Concentration
                  as-supplied               for application

Primer            approx. 24%     MIBK*     approx. 15%
Bonding agent 1   approx. 24%     xylene    approx. 16%
Bonding agent 2   approx. 26%     xylene    approx. 16%

*Methyl isobutylketone

Reliable processing is only ensured when the product exhibits the viscosity required for the specific method of application. The viscosity depends on the product concentration but it can also be adjusted by increasing the temperature. Figure 15 shows the temperature and dilution graphs of a bonding agent.

Both dilution and heating up can be used to adjust commercial-grade bonding agents to the viscosity required for spray application. Hence, a suitable combination of these parameters allows a reduction in the amount of solvent required. Special equipment is needed for such hot spray applications. Figure 16 shows an example. The bonding agent is routed with a pump via the flow heater to the spray gun. In the heater, it is heated to the specific temperature required for the system in question. Unused material is recirculated to the pump.

Since bonding agents are heat-reactive systems, certain maximum temperatures may vary according to the product. Usually, primers are processed at temperatures ranging between 60-80[degrees]C and cover cements at 70-90[degrees]C.

Hot spraying is one possibility to reduce solvent emission. Another step in this direction are the so-called high solid systems where suitable formulations allow a considerably higher concentration of active ingredients at nearly the same viscosity. Table 2 shows the concentrations and viscosities of such high solid products as compared to conventional bonding agents.

Table 2 - comparison of concentration and viscosity
Product         Concentration   Viscosity   Viscosity
                 as-supplied      DIN4*     Brookfield
Primer
  conventional        24%         22 s      170 mPa*s
  high solid          35%         34 s      420 mPa*s
Bonding agent 1
  conventional        26%         50 s      200 mPa*s
  high solid          35%         60 s      280 mPa*s
Bonding agent 2
  conventional        24%         75 s      400 mPa*s
  high solid          34%         70 s      620 mPa*s
Table 3 - bonding systems
Product
Primer         Substrate for cover cements and one-coat
               bonding agent for polar elastomers
Cover cement   Universal bonding agent for polar and
               non-polar elastomers; used as a one-coat
               system, but preferably on a primer
               as a two-coat-system
One-coat       Bonding agent for polar elastomers
bonding agent 1
One-coat       Bonding agent for special rubbers
bonding agent 2

Unlike hot spraying, the high solid technology has so far not established itself on the market. Practical experience has not shown any differences in terms of processability or applicability as compared to bonding agents with a higher solvent content. Figure 17 shows the reduction in solvents obtained with alternative application methods. The figure shows the amount of solvent released for the application of 1 kg of bonding agent, i.e. in conventional systems, 7 kg of diluted adhesive are required to apply 1 kg of solid substance on a surface thus releasing 6 kg of solvent.

Exhaust air cleaning

Alternatively to the reduction of emissions by changing the type of application or increasing product concentration there is also the possibility of removing the solvent from the waste air. Various methods and plant units are currently being discussed. Some of the different concepts are: Condensing; adsorption/absorption; membrane permeation; exhaust air incineration; catalytic decomposition; and biological oxidation. However, some of the above-mentioned processes cannot be so readily used in practical applications. Only postcombustion and biological oxidation are suited for industrial-scale application. Nevertheless, all methods for solvent reduction can only be regarded as an interim solution. The objective is to avoid the use of solvents all together.

Waterbased bonding agents

Figure 17 already showed that solvent emission is reduced to zero when using water-based bonding agents.

Waterbased adhesives can look back on a long history only to mention the products based on latex/albumin and the corresponding patent issued in 1930.

This subject has become a matter of topical interest particularly with regard to the current requirements for substituting conventional bonding agents. The existing systems show excellent bond strength and chemical resistance to a large variety of substances. And this is why other bonding methods were substituted by these universally applicable adhesives. Table 4 shows solvent-borne bonding agents covering all major fields of application.

Combination               Time(h)   Delamination(%)

Primer     Cover cement
Aqueous    Aqueous           1,000          <5
Solvent    Solvent           1,000          <5

The table clearly shows that even solvent-based systems cannot cover all types of application with one single bonding agent. Consequently, the same wide range of application has to be covered by the new systems to be developed. In the following, the alternative systems will be presented and evaluated by looking at selected examples.

Requirements

As mentioned above. modern bonding agents are mainly applied as two-coat-systems combined with a primer. The primer gives the rubber-to-metal composite better resistance against environmental influences and dynamic stress.

The same applies to aqueous bonding agents. Hence, the tests presented in the following always refer to a primer plus cover cement system.

Several criteria are essential for the determination of the suitability of a bonding agent. Figure 18 gives an overview of the properties to be checked during product development.

The first test phase includes the following steps:

* Adhesion is determined as a criterion for the bond strength between the elastomer and the substrate;

* The pattern of rupture is evaluated in terms of compatibility between metal, primer, cover cement and elastomer;

* Resistance to hot water is checked as a first criterion to evaluate the resistance to high temperatures and chemicals;

* Film formatting and spraying properties are determined as a criterion for processability, particularly for conventional methods of application.

After successful completion of these tests, a second test phase is started to check e.g. the shelf life of the bonding agents. This is only one of the main test requirements to be met successfully prior to industrial-scale trials. For aqueous systems additionally the freeze-thaw-stability is a criterion.

Further technical tests shall allow an assessment of the product's suitability for the different types of elastomers and processing methods. Chemical resistance such as resistance to corrosion is tested in a salt spray test. Another test shall identify the product's temperature stability.

Following the successful completion of this test phase, the suitability of a system for industrial application is tested in a third phase in cooperation with the customer.

Primer

The primer presented herein is an aqueous system with grey pigments and containing no organic solvents. Just like many aqueous paint systems, the product exhibits a high structural viscosity and requires thorough stirring (avoid foaming) prior to application. The product is easily applied by spray, immersion or brushing after adjusting to the required viscosity.

Adhesion tests show that the aqueous primer is equivalent to solvent-borne systems in terms of efficiency. Figure 19 shows a comparison of a standard primer with an aqueous system, both combined with a solvent borne cover cement.

Concerning the main criterion - adhesion to metals - no differences were found between the two systems. They both did not show any signs of metal defect when subjected to destructive tests.

Apart from processability and adhesion, chemical resistance is also of great importance. When subjected to salt spray testing under tensile stress, a completely aqueous system did not show any sign of coming off even after 1,000 hours (table 4). In this test, the composite is subjected to tensile stress of 0.8 N/mm at an angle of 180[degrees].

These positive results were confirmed by the boiling water test described in table 5. In this test, the composite is exposed to water heated to 95[degrees]C and subjected to a tensile stress of 0.8 N/mm at an angle of 90[degrees].

Table 5 - boiling water test with tensile stress
Combination               Time(h)   Delamination(%)

Primer     Cover cement
Aqueous    Aqueous            24          0
Solvent    Solvent            24          0

All these results underline the fact that aqueous primers represent a viable solution to the conventional solvent-borne systems. This is also illustrated on figure 20. Aqueous primers exhibit good to very good adhesive values, patterns of rupture and resistance. Compared to solvent-based primers, they are slightly inferior in terms of shelf life, film formation and processability.

This is can be attributed to the fact that the properties of water are different from those of organic solvents, e.g. the drying properties are influenced by the different evaporation enthalpies, surface wetting is influenced by the different surface tensions and frost resistance by the different melting points. These differences can be compensated by changes in the equipment and improved logistics.

Cover cements

Together with the primer, the cover cement completes the aqueous system. At present, Chemetall offers two commercial-grade cover cements exhibiting different characteristics. In the following, they shall be treated as cover 1 and cover 2.

The adhesive value and the pattern of rupture represent two important criteria for the suitability of a bonding agent. Table 6 shows laboratory test results with natural rubber. Tests are performed with two aqueous cover cements combined with a solvent-borne and an aqueous primer. The measured values are compared to a solvent-based reference system.

Table 6 - adhesion tests with a natural rubber
compound (hardess shore A 65)

Bonding agent combination    Adhesion value   Fracture
                                 MPa            %Rubber
Primer     Cover cement
Solvent    Solvent              10.3-11.5         100
Aqueous    Solvent               9.6-10.4         100
solvent    Cover 1               8.0-10.2          98
Aqueous    Cover 1               8.2-10.5         100
Solvent    Cover 2              10.0-11.5          97
Aqueous    Cover 2              10.2-11.8         100

Initial adhesion, i.e. the tensile strength prior to aging, of both the purely aqueous systems and the hybrid systems (combination of aqueous and solvent-based products) is comparable to that of the reference system.

Positive results were also obtained when testing the adhesion of EPDM, SBR, NBR and CR compounds.

Figure 21 shows an example of adhesion tests with a sulfur-cured EPDM compound applied to different substrates. In all cases, destructive testing of the rubber-to-metal composite resulted in a break in the rubber matrix.

However, results were less uniform when testing the chemical resistance. Here, strong dependence on the type of rubber compound was found thus showing the limited applicability of at least one of the two aqueous cover cements tested. E.g., elastomer EPDM does not come off even after 10 days of exposure to hot water. And a test of the EPDM/steel bond resistance against glycol at 120C for 10 days equally showed good results. Tests of NR with cover 1 showed considerable delamination following the hot water test and storage in a salt spray climate.

Figure 22 shows the results of the boiling water test where both aqueous cover cements were combined with an aqueous primer. A hybrid system was used for reference, waterbased primer and solvent-based cover cement. While cover 2 and the reference sample were not affected, cover I showed clear signs of delamination. The same holds true for the salt spray test shown in figure 23.

Here too, cover 1 already fails after a few days exposure. Hence, universal applicability of this system can be excluded according to our present knowledge. Cover 2 showed excellent results in all tests: just like the pattern of rupture, the adhesion values were good irrespective of the primer used; the product quality was not affected by either boiling water or the salt spray testing.

One objective which still remains to be attained is to give the system sufficient processing reliability. In this context, process reliability means sufficient stability during preheating, because at present the system still reacts too fast. When the coated metal parts are placed in the hot vulcanization mold, they heat up as a function of the specific volume of the part and the mold temperature. The cover cement starts reacting at a certain temperature thus losing activity. Prevulcanization of cover 2 is so quick that the product quickly loses all reactivity. Hence, manual loading of multiple-part molds may take more time than allowed by the quick pre-vulcanization of the cover 2 (figure 24).

The coated metal parts are placed in the mold and the rubber compound is injected after a previously defined preheating time. Both the waterbased cover cement 1 and the solvent-based reference system showed sufficient reactivity to ensure adhesion of the rubber compound even after 10 minutes prebaking at 155[degrees]C.

Cover 2, however, exhibited a 60% reduction in adhesion value already two minutes after exposure. Hence, this system is not suited for a large number of vulcanization processes due to the restricted process reliability. Figures 25 and 26 show an evaluation of the two waterbased cover cements.

One-coat bonding agents

The above bonding systems allow coverage of many applications. However, a number of special applications and elastomers pose entirely different demands on the bonding agents to be used. Both NBR as well as acrylate (ACM), epichlorohydrine (ECO) and fluorinated rubber (FKM) compounds showed excellent results when combined with aqueous systems. This shall be demonstrated taking the example of FKM with three crossluking systems which are typical for this elastomer.

In the past, amine crosslinking did not pose a great problem for solvent-based bonding agents. As shown in figure 27, there are no significant differences in terms of adhesion value and pattern of fracture when comparing the two solventbased systems. The same holds true for bisphenolic crosslinkage. Here too, sufficient bonding strength was achieved with standard bonding agents. It is now also achieved with the aqueous product.

During the development of this solvent-free product, it was also tested in numerous compounds with varying formulations. Figure 28 shows a comparison of the product with solvent-based systems using different rubber compounds. The deviations are negligible. Performance of the aqueous bonding agent is equivalent to that of the reference system.

The bonding of peroxide cured compounds is associated with more problems. Here, in the past, the limits of solvent based products have been demonstrated very clearly. Figure 29 shows the results of an adhesion test with a standard compound. Concerning adhesion value and pattern of rupture the water-based product is equivalent to the solvent-based products. In the case of problematic compounds however, the adhesion resulting from solvent-based systems is far excelled by the bond obtained with the aqueous product (figure 30). With the present results it can be concluded that a fully viable alterative can be offered in the field of FKM bonding.

Summary

Conventional bonding agents were introduced a long time ago. With continuous improvements, they have now set the standard to be met by any new product. These bonding agents involve the emission of organic solvents. The objective of bonding agent manufacturers and processors is to reduce or even avoid such emissions all together.

Possible procedures include more effective application methods and exhaust gas cleaning. However, the reduction of solvent emissions can merely serve as a short-term interim solution. The objective of R&D activities should focus on the development of solvent-free products, namely aqueous bonding systems.

Promising attempts have already been made. It should, however, not be expected that a complete product range ready for marketing is developed within a brief period of time.

The new systems have to be subjected to practical trials and the experience gained has to be used for product optimization. This presupposes close cooperation between manufacturers and customers. R&D departments need the feedback from their customers to be able to meet market requirements.

The systems currently available exhibit a wide range of properties to cover many but not all types of application.

In this context, the question arises whether it would not be better to redefine existing specifications. A situation should be avoided where promising innovations are dismissed because they do not meet requirements set up at times when everything was new and no experience was available. This will require intensive dialogue between all parties involved so as to ensure technological progress.

[FIGURES 15 to 30 ILLUSTRATION OMITTED]