Aqueous adhesives as an alternative to conventional rubber-to-metal adhesives.

The object of this article is to provide you a short but up-to-date introduction into the current activities in modem rubber-to-metal adhesives. The focus will be on water-based bonding systems. However, the important developments made in the past years in this field of application should not dwarf simultaneous efforts to improve the range of solvent-based bonding agents. The new products developed in this sector are characterized by a considerably higher performance.

Different applications will be presented as examples to illustrate the main fields of development. A comparison between our proposed solutions involving conventional solvent-based adhesives and those with water-based adhesives will provide an impression of the high level of bonding system performance achieved while also showing the differences and framework conditions. A short introduction into the multitude of questions arising with regard to the structure of bonded components will be provided and the possibilities available for finding the corresponding answers will be examined.

Taking examples from three very different fields of application in the automotive sector, aqueous and solvent-based adhesive systems will be presented and described in detail, particularly with regard to their method of application.

Finally, a defective plastics-to-rubber component will be taken as an example to illustrate what kind of assistance and proposals can be offered in case of malfunction or defects.

Rubber-to-metal parts are an almost classical example for an entire sequence of boundary areas. A virtual trip through such a component will illustrate this, while also showing the complexity of this subject.

The elastomer itself raises a number of questions relating to the structure of the blend, the mixing cycle and the manufacturing conditions.

The next stop in the virtual trip is the first boundary at the interface between the rubber or elastomer and the covercement film. Here, questions concerning the coating thickness, type of adhesive polymer crosslinking system, application process and, of course, the vulcanization conditions arise.

The next boundary is just a few steps away at the interface with the primer. The primer must be specifically adjusted to the respective cover while simultaneously matching with the substrate below. Here, corrosion protection is the main topic.

This short trip has almost reached its final destination at the interface between primer and metal surface. Examples for an additional boundary could be a phosphate coat or, particularly in the case of aluminum substrates, a thin oxide or chromate coat which in turn must show excellent adhesion to the core of the metal substrate.

Examples for applications

Taking three very different fields of application as an example will provide an insight into the development of new high-performance bonding agents with an emphasis on the aqueous alternatives.

For each of the examples given, the following points will be covered:

* Field of application and requirements;

* typical elastomers;

* application method;

* suitable bonding agents;

* examples.

When dealing with these points one will immediately notice the complexity of the subject resulting from the demand to simultaneously fulfill all requirements in terms of corrosion resistance, metal adhesion and compatibility, not only among primer and cover, but also with regard to the respective elastomer and substrate.

All three examples are taken from the automotive supplier industry.

Vibration damper elements

The manufacture of engine mounts requires a vulcanized material with a high heat resistance, particularly when subjected to dynamic stress. At the same time, a high degree of corrosion resistance is required. For economic reasons, the selected vulcanization temperatures are constantly on the rise and the cycle times in the production process are reduced, while the geometries of the selected bushings become evermore complex.

To meet all the above requirements, mainly NR, SBR and - a little less frequently in Europe - IIR rubbers or blends of these elastomers are used. Given the above-mentioned complex part geometries, the products are preferably applied by spraying.

The optimum bonding system shall meet the above criteria. To achieve sufficient corrosion resistance, a two-coat system consisting of primer and covercement is required. The cover's crosslinking system must be adjusted to the rubber blend and show a sufficient preheating resistance during vulcanization in multi-cavity molds. Of course, the system must be suitable for spraying and painting.

A bonding system which meets the above criteria will be presented, taking the example of a laboratory examination of a 55 Shore A NR blend. They involve the use of the so-called standard covers, namely bonding systems with a very broad application spectrum. Figure 1 shows a comparison of the adhesion of natural rubber on sand-blasted pan head screws whereby the zinc phosphated screws remained without rubber coating in the vulcanization mold for varying periods of time and were only vulcanized afterwards. The test simulates a multi-cavity mold.

[Figure 1 ILLUSTRATION OMITTED]

Apart from the above standard system, the aqueous systems showed very different reactivities. At the fairly high temperatures selected, W 23800 showed a much earlier drop in adhesion. This is the sign for a different reactivity of the product's crosslinking system and is riot to be considered as a deficiency, but rather as the proof that this system is adjusted to less reactive elastomers. Too few C-C double bonds in the elastomer, for example, have to be compensated by a considerably increased reactivity of the crosslinking system in the bonding agent. Thus, a compromise between the chemical composition of the elastomer and the possible thermal impact on the adhesive film often has to be made.

With this in mind, water-based adhesives often achieve a very good level of adhesion while showing desired rubber failure.

However, it should also be mentioned in this context that a few particularities of aqueous systems have to be considered to achieve such results.

In the standard aqueous systems the polymers are present in the form of discretely emulsified droplets which do not react with the pigments. Compared to the standard covers whose dissolved polymers have already completely wetted the pigments prior to vulcanization, i.e., whose future film formation on the substrate is largely predetermined, the film formation of standard water-based systems only takes place when the latex particles coagulate on the substrate surface, during the drying stage. Hence, special requirements are stipulated for adhesive application and subsequent drying.

Freshly sand-blasted metal surfaces are particularly susceptible to corrosion. Thus, a primer is applied to the preheated (100-190 [degrees] F) metal part Forced drying at 190 [degrees] F for 5-15 minutes has also proven to be advantageous. The aqueous covercement can then be applied immediately afterwards to the still warm and primer-coated metal part. Here too, subsequent drying at 19 [degrees] F has proven to be efficient. However, to prevent the cover from losing its reactivity, drying times in excess of 10 minutes at 90 [degrees] C (190 [degrees] F) should be avoided.

Metal-reinforced profiles

Metal-reinforced profiles are another application in the field of automotive parts. Given the special conditions of their manufacturing process, the adhesives manufacturers have to meet special product requirements. One example would be the (roof drip moldings) in the automotive industry.

Suitable strip materials such as galvanized steel, stainless steel and aluminum are first subjected to a chemical and/or mechanical pretreatment. The primer and then the cover are applied by rollers in one single pass with downstream drying zones. The coated strip is then coiled and cut to the desired width. In the next step, the final part geometry is achieved by way of roller forming after which the sections are preheated. The preheated sections are then routed through an extruder where the rubber blend is applied. Vulcanization is completed in the downstream heating zone. Given its high weathering resistance and its particularly high ozone resistance, EPDM rubber is mainly used for this type of application. Occasionally, SBR rubbers are also used.

These production processes are very demanding with regard to the bonding system. Unlike a spray application which requires a comparably low-viscous bonding system, rollercoating requires highly viscous products.

For safety reasons, high oven temperatures (300-580 [degrees] F) in the drying zone require solvent compounds with a high flash point. At the same time, they are responsible for uniform coating formation. Furthermore, the adhesive film coat shall be highly flexible. Unlike in bushing manufacturing - with the exception of their re-calibration - the entire forming process of the metallic substrate is done after application of the bonding system. Hence, the adhesive layer must be highly flexible and scratch-proof. The application of the rubber blend by means of coextrusion and the subsequent pressureless vulcanization can only be achieved by way of perfectly matched crosslinking systems in the cover. These types of problems can be solved with combinations of primers and covers.

The adhesion values are of only secondary importance for the initial evaluation of the performance of coil coating systems. Since the mechanical stress of the components in actual application is rather low, a 100% R is almost a must.

Gaskets

Gaskets and shaft seals are used in the automotive industry. mainly for sealing against fuels, lubricants and/or brake fluids. Here, "oil resistance" is the main requirement. Hence, polar elastomers like ACM, NBR, H-NBR, ECO or FKM are usually added to the rubber blends. Mostly, these parts are comparatively small in size and manufactured at very short vulcanization times and high vulcanization temperatures. The adhesives are usually applied as a bulk material by immersion or in drums. The type of elastomer used and the optimum application method, as well as the required resistances, determine the selection of the bonding agent. Preferably, transparent adhesives, i.e. without pigments, are used. They are highly specialized blends of phenolic resins or others to promote the adhesion of products like NBR, ECO, H-NBR, etc., or mixes of reactive silanes and adhesion-enhancing additives for the vulcanization of FKM or silicone rubbers.

These systems must show excellent wettability on the substrates to be bonded, generate very uniform films and be absolutely free from contact-adhesion after thermal pretreatment, i.e., they shall have no tack.

Two new water-based bonding agents will serve as an example for solutions for NBR bonding. Under suitable conditions, they will meet all these criteria, as shown in figure 2.

[Figure 2 ILLUSTRATION OMITTED]

In some points, the aresults are better than those obtained with standard solvent-based systems. Apart from developing new water-based adhesives, our research activities with regard to creating new solvent systems have also shown good results. Megum 16513 was added to the existing products Megum 3290, 3290-1 and 3295.

The increased performance of this new product is demonstrated with figure 3.

[Figure 3 ILLUSTRATION OMITTED]