Rubber-to-metal bonding agents.

Rubber and metal are two materials differing greatly in their chemical structure and mechanical properties. Metals such as steel are hard, have a very regular structure and feature high tensile strength but poor elongation at break. In contrast, rubber is soft, considerably lower in tensile strength but higher in elongation at break and has a chemical structure similar to a long and intertwined chain.

Vulcanization, the conversion from plastomer to elastomer, provides a certain fixation and the typical flexibility characteristics associated with rubber. Figures 1 and 2 elucidate the difference in stress - elongation behavior and chemical structure of the two different materials.

By means of rubber-to-metal bonding agents, these two materials can be combined to durable composites suitable for use as structural components without additional fasteners. This opens the door to new technologies for designers.

The main field of application for rubber-to-metal components is the automotive industry. Typical examples include:

* Engine mounts;

* axle suspensions;

* shock absorbers;

* flexible couplings;

* shaft seals;

* friction and torsion bearings;

* metal-reinforced profiles.

Rubber-to-metal components have also found broad application in other industries, for example in the manufacture of all kinds of suspension systems and bearings for the mechanical engineering and building industries. Typical examples are vibration damping elements, machine and bridge bearings or rubber-lined rollers for the paper, printing, paint, steel and food industries. Rubber-to-metal parts are also used for the manufacture of solid tires and track pads for caterpillar vehicles as well as in the military industry. Further examples include rubber linings for tanks, piping, flexible tubing and cable manufacture.

The typical film thickness of the bonding system used is of approx. 20 [mu]m (0.8 mil). Compared to the rubber layer to be bonded, this film thickness is relatively small.

In the manufacture of the above components, rubber-to-metal bonding agents only play a minor role from a quantitative point of view but they are essential in terms of function.

Unlike the adhesive technology, in rubber-to-metal bonding no prefabricated material but unvulcanized rubber is used. The bond is produced under conditions specific-to., rubber processing. During vulcanization of the elastomer, crosslinking also occurs in the adhesive film. This chemical reaction together with the corresponding intermediate reactions generates suitable reaction partners for the bonding process. As a result, this co-vulcanization creates extraordinary strength properties. Rubber-to-metal components joined by adhesives typically have tear resistance values of 1 to 4 MPa. Vulcanized components, on the other hand, may well reach strengths of 10 to 12 MPa depending on the type of elastomer used and other parameters.

Apart from the bond strength there are of course a number of other criteria that play a decisive role such as shelf life, range of application, processability and chemical resistance.

Against the ever more exacting demands of the automotive industry in-terms of temperature and chemical resistance, adhesive technology as well as standard bonding agents soon reach their practical limits.

The manufacturing methods of die rubber-to-metal component producers have resulted in a drastic change in the requirement profile to be met by bonding agents, to mention only application methods such as hot or airless spraying.

The introduction 6f injection molding processes has led to new requirements for bonding agents such as improved sweep resistance or higher reactivity.

The development of high-performance elastomers or improved crosslinking systems confronts bonding agents producers continuously with new challenges.

So the emission of solvents during the; application of bonding agents increasingly poses problems.

History

Changing requirements make it necessary to develop ever more efficient and universal bonding systems. Thus it comes as no wonder that rubber-to-metal bonding technology can look back on a long and eventful history that includes:

* Ebonite;

* brass bonding;

* RFL dip systems;

* phenolic resins;

* latex/albumin;

* isocyanates;

* chlorinated polymers/isocyanates;

* halogenated polymers/crosslinking agents;

* aqueous bonding agents.

Ebonite process

The very beginnings of rubber-to-metal bonding technology date back to the middle of the last century. The ebonite process, patented around 1850 and still used today for tank linings, is the oldest bonding system. It uses vulcanizates with high sulfur contents (35 to 40%) which are almost thermoplastic in nature. Due to these thermoplastic properties, the ebonite technology is restricted to applications not requiring special temperature resistance.

When using suitable natural rubber qualities, ebonite coatings on steel reach bonding strengths of approx. 6 MPa. At temperatures above 60[degrees]C, the bond weakens noticeably and fails completely above 100[degrees]C.

Brass method

Compared to the ebonite method, the brass method provides higher temperature resistance and also a stronger initial bonding strength (see figure 3).

This technology dates back to a patent of Sanderson in 1862. However, it was not before 1911 that this method was used on a commercial scale in the production of rollers. In the tire industry, this method is still employed today to produce steel cord-to-rubber bonds. The limitations of this technology are that it requires meticulously matched mix formulations to provide the desired bond. In addition, the brass layer, i.e. the copper concentration and the crystal structure, are of decisive importance to the quality of the bond.

Moreover, this bonding technology makes exacting demands on the preparation of the metal surface, from degreasing right through to the final deposition of the brass layer. Continuous bath monitoring is imperative, irrespective of the kind of bath used. Up to a temperature limit of 120[degrees]C, this method is superior to the ebonite technology in temperature resistance, but is still outperformed by latex-albumin bonding systems.

Latex-albumin bonding agents

Based on latices and hemoglobine, these adhesives feature the dual advantage of producing high-strength bonds while affording at the same time improved temperature resistance. Their development dates back to the thirties. The product Megum SK, as an aqueous system at that time trend-setting for adhesives, provided a standard for rubber-to-metal bonding agents for many years. However, the demands on chemical resistance, especially resistance to hot water, oil and fuel, soon showed the practical limits of this system.

Isocyanates, resins, chlorinated polymers

Apart from the systems described above, there are a number of other systems based on heat-reactive resins, halogenated polymers, isocyanates or mixes thereof. While resinous systems and chlorinated polymers are mainly restricted to,applications where polar elastomers, e.g. acrylonitrile butadiene rubber (NBR), are bonded to metals or other substrates, isocyanates, either alone or in combination with substances such as chlorinated polymers, offer a broader range of applications.

However, the sensitivity of isocyanates to humidity, their slipping in the vulcanization mold (resistance to shifting) and their susceptibility to amine accelerators and anti-aging additives do not allow universal applications. Already a few hours of storage prior to vulcanization led to a distinctive deterioration in their adhesion values resulting from a reaction of the isocyanate with atmospheric humidity (see figure 4).

Consequently, modem bonding systems had to be developed whose requirements can be described as follows:

* Simple, robust processes;

* universally applicable;

* high quality of the finished part.

Modern bonding systems

Normally, modem bonding systems are used as two-coat systems. Since they were introduced more than 30 years ago, they can almost be said to be standard methods. The first coat, the so-called primer, is applied to a suitably treated surface. After drying, the second layer, also called,cover cement, is applied. Once this coat has completely dried, vulcanization can be started.

Primers

The commercially available primers are combinations of halogenated polymers with heat-reactive resins and functional pigments, either dispersed or dissolved in organic solvents. The metal bond is achieved by physical actions (van der Waal's forces) and is described by the so-called adhesion theory. In the case of heat-reactive phenolic resins, a certain percentage of direct chemical bonds (free methylol groups) can be assumed.

Silane bonding agents

One special type of adhesives are those based on reactive silanes. Here, a chemical bond is supposed to form by reactions of the silane alcoxy groups with the hydroxyl groups on the metal surface. In addition, the reactive substitutents of the silane react with the elastomer to be bonded to the metal (see figure 5). However, applicability of such systems is limited to such special elastomers as fluorocarbon elastomers or silicone rubber.

Cover cements

The most commonly used rubbers for rubber-to-metal bonding are natural rubber, ethylene propylene diene rubber (EPDM) and styrene butadiene rubber (SBR). They require more complex bonding systems.

Such systems generally include chlorinated, brominated or chlorosulfonated polymers or mixes thereof. Given the varying structure of the polymer matrix of the bonding agent and the crosslinking system, the cover cements made from the above base materials differ greatly in terms of their mechanical properties.

Depending on the polymer system used, differences both in terms of tensile strength and resistance against external influences are found with a specific test mixture. A universally applicable system offering highest strength values as well as excellent chemical and thermal resistance has not yet been found. This is why several different cover cement systems exist and are applied as a function of the type of application.

Combined with special crosslinking agents, e.g. polyfunctional nitrogen compounds or isocyanates, these polymers form the bonding agent whose properties are adjusted by adding functional pigments such as carbon black or lead compounds. In other words, the adhesive itself is already a crosslinking system which is activated prior to starting the vulcanization process and which should remain active until the crosslinking process of the rubber matrix has been completed.

Analytical methods

Methods to characterize the activity

Up to now, the activity or suitability of a bonding agent for a specific application can only be determined by practical tests on a specimen or finished part.

Different analytical methods may provide slight differences but cannot replace testing of the bond strength.

Solubility following thermal pretreatment 

As the crosslinking of the adhesive film advances, adhesion of this coat diminishes and drops to zero upon completion of the crosslinking process. On the other hand, crosslinking reduces the solubility of the adhesive in organic solvents. This method can be used to quantify the degree of crosslinking. In this test, metal parts are coated with bonding agent and this film is cured at a defined temperature and varying periods of time. Following this thermal treatment, some parts are subjected to adhesion tests and some to treatment with organic solvents to determine the degree of crosslinking.

Figure 6 shows the dissolution graph and the corresponding results of the adhesion test. The values determined at a temperature of 170[degrees]C clearly show that the bond strength decreases the higher the degree of crosslinking of the film which is equal to a reduction in the amount of soluble bonding agent.

The two graphs almost coincide at a vulcanization temperature of 190[degrees]C (see figure 7). Compared to figure 6, it also shows that the speed of the crosslinking reaction is dependent on the vulcanization temperature.

The resulting graphs somewhat give a picture of the crosslinking isotherms and are thus similar to rheometer graphs. They can be used to describe the reactivity of the adhesive film.

However, they are of only limited suitability to describe the interaction between the matrix of the elastomer and the bonding agent. The adhesion graphs may vary considerably as a function of the elastomer and crosslinking system used. The result of the bond adhesion test of a given compound cannot be predicted on the basis of the dissolution graph.

Differential scanning calorimetry 

Another method to characterize the activity of bonding agents is with differential scanning calorimetry (DSC). Differences between the bonding systems are found both in progressive (figure 8) and in isothermal temperature control (figure 9). In the former, the reactivity of a system can be characterized by the location and the intensity of the registered peaks.

The location of the peaks serves as an indicator for the starting point of a reaction. The peak intensity is dependent on the reaction enthalpy thus acting as an indicator for the thermal heat released in the course of the crosslinking reaction.

Figure 8 shows that bonding agent 2 is the more reactive system. Reaction already starts at low temperatures and there is more heat released during crosslinking.

The same applies to measurements at constant temperatures. During the test in question, the energy released during the crosslinking reaction is recorded as a function of time.

Figure 9 shows that the crosslinking reaction of bonding agent 2 starts at a much earlier point in time thus having an impact on process reliability. In addition, it proves that this type of adhesive is suited for quick-acting vulcanization systems.

However, the two methods only describe the reaction within the adhesive film. Similar to what was stated for the dissolution curve, these measurements are of only limited indicative value for the cor-responding bond strength.

Examination of the phase boundary using x-ray methods

Scanning electron microscopy (SEM) represents a method of analysis allowing for an insight into the interface which plays a decisive role in adhesion.

Combined with EDX (energy-dispersive x-ray analysis) and microprobe analysis (analysis of the wave length dispersion of x-rays), SEM equally allows for an insight into complex systems such as the phase boundary between rubber-and bonding agent.

The topography is shown by means of secondary electrons. In addition, the element distribution within a defined sample section can be shown by x-ray distribution pictures which also allow for quantitative measurements. Hence, x-ray emission can be used to prepare concentration profiles for different elements by energy dispersion (EDX) and wave length dispersion (microprobe).

In order to use these analytical methods for a description of the phase boundary between rubber, bonding agent and metal, this boundary first has to be set free.

Taking the example discussed herein, the boundary phase was set free by mechanically destroying the specimen at 180[degrees]C for subsequent analysis.

To facilitate rupture and obtain an even fracture surface at the bonding agent/metal interface, a standard bonding agent was applied to a ground metal surface.

No primer was used because the main objective-of the test was to analyze the interaction between the rubber and the bonding agent. Fractures were made after bonding a sulfur cured natural rubber compound to the metal substrate during vulcanization.

Figure 10 shows the secondary electron picture of such a surface, respectively an enlarged section thereof. For a quantitative analysis of the element concentration, element-specific x-ray emissions were recorded at different points of measurement (points 0-16; spacing between the points of measurement: 4 microns).

Figure 11 shows the result of this examination. The elements determined by analysis are typical components of a bonding agent, namely chlorine and bromine, as well as zinc and sulfur which are typical for the rubber matrix. As expected, concentration of the halogens chlorine and bromine is highest within the bonding agent layer. These elements were also detected in the rubber matrix whereby their concentration continuously decreases the greater the distance to the adhesive/rubber interface.

In the rubber layer, the elements sulfur and zinc were found at rather constant concentrations. Concentration of these two elements was considerably higher at the interface between bonding agent and rubber.

X-ray emission pictures showed the same results. Figures 12 and 13 show the quantitative distribution of the elements zinc and sulfur. The white spots indicate that the element in question is detected. A significant concentration of zinc and sulfur is clearly visible around the interface between rubber and bonding agent.

Polyfunctional-nitrogen compounds play a decisive role in the crosslinkage of bonding agents. Hence. nitrogen is a typical element found in rubber-to-metal bonding agents. Microprobe analysis allows a quantitative determination of this element.

Figure 14 shows the concentration profile of nitrogen as well as those of zinc and sulfur. The accumulations of the latter two determined by EDX analysis was confirmed. In addition, an accumulation of nitrogen was detected at the interface adhesive/rubber.

The above-described effects indicate that diff-usion from the rubber to the bonding agent occurs during vulcanization (zinc and sulfur). Furthermore, components of the adhesive diffuse towards the adhesive/rubber interface (nitrogen) and even into the rubber matrix (chlorine and bromine). Diffusion of zinc and sulfur explains a certain degree of hardening at the rubber boundary.

This mechanism is comparable to the bonding mechanism of the ebonite system. The compounds diffusing from the bonding agent may cause a local overcure at the interface and also in the rubber matrix next to the rubber.

But also the formation of chemical bonds by co-reaction of the polymer of the adhesive with the rubber matrix or crosslinkage by polyfunctional nitrogen compounds cannot be excluded.

However, the statements made on the basis of the above phenomena can only be used as working assumptions. Even with the highly efficient methods of analysis available it is still not possible to determine the impact of physical, chemical and other reactions on the bonding strength by quantitative measurements. We are still very far from being able to give an exact account of the bonding mechanisms which occur in rubber-to-metal bonding.

[Figures 1-14 ILLUSTRATION OMITTED]

(Part two of Rubber-to-metal bonding agents will appear in the April issue).

Acknowledgements

"Advances in silcone engine gasket sealing" is based on a paper given at the October, 1995 Rubber Division meeting. "Dynamic properties of elastomers as related to Vibration isolator performance" is based on a paper given at the May, 1995 Rubber Division meeting.

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