Bonding metals

A topic that sounds like something ordinary but that requires a great deal of know-how.

The fundamentals of bonding metals

Different metals

The question "have you got something that will bond metals?" is as mundane as breakfast.

But the questions fired back in reply are

· What sort of metal is it?

· What are its surface characteristics?

· What geometric bonding surface is available?

· Are there any specifications and constraints with regard to working?

· What type and level of stresses will the bond be subjected to?

· Where will the bonded metal be used afterwards?

In most cases, these responses will put the questioner on the spot. Only the few who are well prepared are able to answer these important bonding-specific questions straight away.

Let us now take a look at the aforementioned individual points. The same principle applies here, too: Bonding is a joining technique that "only" joins the surfaces of a material. The situation is different in the case of the typical mechanical joints such as those with screws, rivets, welds, sewing etc.

One standard question is always whether a bare metal is involved, or has the surface been refined in some way? When bare metals are involved, it is necessary to further distinguish between what type of metal is involved in that particular case. It is necessary to determine whether traditional metals such as iron, steel and stainless steels are to be bonded, or non-ferrous metals. Typical non-ferrous metals are aluminium, copper, nickel, lead, tin, zinc, brass and bronze.

Metal bonding with 2 component epoxy resin adhesives

The aforementioned metals display typical differences in their adhesive properties, and in each case, it is necessary to examine which of the many available adhesives provides the most successful bond here. In recent decades, the epoxy resin-based 2‑component adhesives have become the dominant solution for bonding these materials to either the same or a different type of metal. These 2‑component adhesives are also known as structural adhesives. The adhesives industry offers a wide variety of such adhesive systems. These adhesives have different adhesion and usage properties depending on the epoxy resins used in the recipes and on the additional materials used such as fillers and additives, with the different viscosities and other working properties also playing a role here.

Different adhesives for different jobs

It's already clear that there's not just one adhesive for metals. The extensive range of these adhesive formulations can be very confusing for the user. It is necessary to differentiate in terms of which ones are used for which jobs. The media like to cite the automobile industry and put it forward as a prime example of the efficiency of bonds on metallic parts. In vehicle construction, the recipes are normally developed specifically for a particular job, with the geometry of the bonding surface being specially configured for a bond, in other words, it is structurally adapted. Bolting the joint yesterday and bonding it today instead won't work.

As has already been pointed out above, the different metals have their own - often different - adhesive properties. Prior to use, it is always absolutely essential to carry out careful suitability tests that reflect actual practice. Corresponding ageing tests and load-bearing tests relating to future use are also on the list of specifications for selecting the adhesive. Extreme thermal stresses and the influences of the weather, water and other substances must also be included in the suitability assessment.

Mechanical stresses on the bonds

Any mechanical stresses that arise must also not be ignored. Whether the bonded joint is subjected to sudden, occasional or constant forces, and whether vibrations or peeling stresses occur - all of these parameters must be taken into account. The text above mentions 2‑component adhesives based on epoxy (EP). However, the adhesives industry is very innovative and has developed a very broad portfolio of suitable adhesives that do not necessarily have to be 2‑component or epoxy-based products. Depending on the job, hot-curing single-component epoxy resins, special formulations based on 2‑component polyurethane (PUR), and the acrylic adhesives (MMA) that have become more common in recent years can also be used. But even among those, there's no one-size-fits-all solution.

Mixing 2 component adhesives for bonding metals

Since metal is normally bonded with the aforementioned 2‑component adhesives, the user has no alternative but to achieve precise dosing and a homogeneous mixture of the components. Careful work is absolutely crucial. Small surfaces or strips and beads can be bonded easily and cleanly with structural adhesives from double-chamber cartridges (also known as side-by-side). These cartridges, which usually come in mixing ratios of 1:1, 2:1 or 10:1 parts by volume, are mixed by means of suitable dispenser guns and static mixing tubes. If larger volumes are needed, containers of the corresponding size are used. In such cases, the components are supplied in separate packages. Mixing is then done either manually (weighing and mixing the dose) or with suitable dosing and mixing systems. The machines industry supplies proven equipment for this. The adhesive is then mixed with a static mixing tube or a dynamic mixer, depending on the pot life.

Thick-film bonding as another possibility for bonding metal

Recent years have seen an increase in the use of what are known as sealing adhesives. These are systems that come originally from sealing applications but that are also very efficient as adhesives. Most of these preferably single-component adhesive sealants have as their basis either polyurethane (PUR) or what are referred to as silane-terminated polymers (STP, MS). The term hybrid adhesive is also often used. Individual 2‑component systems and so‑called booster systems also perform well.

The strength of this process, often referred to as sealing bonding or thick-film bonding, is that the bonded joint maintains a certain degree of elasticity and thereby compensates for stresses, expansions or vibrations. This property is popular in the construction of vehicles, machines and ventilation systems. In other words, applications in which vibrations and oscillations place stress on a bond. So that such sealing adhesives deliver the expected levels of performance, it is in most cases necessary to maintain a defined minimum joint coating thickness in these applications. Due to the cross-linking mechanism (also known as curing), these products tend to be applied more in streaks and beads and rarely on large surface areas.

Bonding of larger metal surfaces

The previous paragraphs deal mainly with what are known as structural bonds. However, it is often necessary to bond larger metal surfaces to each other or to other materials using adhesives. Both single-component and 2‑component reactive adhesives can be used in such applications. Depending on the surface sizes, other working parameters must also be taken into account besides the suitability characteristics of the adhesives. Since the so-called "pot life" is an important criterion in the case of 2‑component products, this often greatly restricts the choice. It must be noted that when used on large bonded surfaces, reactive systems require an appropriate pot life. Similarly, long pot lives are associated with long curing times. Until an adhesive layer has hardened, the parts or surfaces must be carefully fixed so as to ensure that they are completely wetted by the adhesive. For this, it is best to select an adhesive with an appropriate curing time in order to achieve an optimum work flow.

So far, we have mainly discussed the reactive systems with regard to metal bonding. With adhesives of this type, curing normally takes place via a chemical reaction. It is essential for both adhesive components to be precisely dosed in the specified mixing ratios and homogeneously mixed. The adhesive can only be used within the pot life that is determined by the recipe. The hardening time that starts after the adhesive is dispensed onto the surface is also controlled according to the recipe. A proportional and controlled application of heat can accelerate the chemical hardening process. The respective information, i.e. the times in the data sheets, normally apply in the temperature range of between +20 °C and 23 °C, so‑called room temperature. Lower temperatures slow down these processes.

Bonding of larger metal surfaces with contact adhesives

Other types of adhesives are also used in addition to the reactive systems, particularly on large surface areas. Contact adhesives based on polychloroprene (also popularly known as neoprene) as the raw material dominate here. The raw materials based on polychloroprene (CR) display good adhesion properties on bare metals as well as on metals with treated surfaces. In most cases, these are dissolved in solvents, occasionally dispersed in water, and, depending on the application, are rolled, sprayed, brushed or squeegeed on, or are applied in other processes. Contact adhesives must be applied to both sides of the materials, then degassed and joined immediately within the open period. Adhesives of this type provide an immediate initial strength, and the bonded parts can be worked on immediately afterwards. This bonding technique is a preferred solution for use in lamination applications such as sheet metals on timbers etc.

Surface preparation prior to bonding

It was said at the beginning that "bonding" connects surfaces. It was also clearly explained that these different surfaces vary in terms of adhesion properties. Some bare metals show changes on their surfaces. This is oxidation which is often (but not always) recognisable by discolouration. Since, like oils and greases, such layers create separation layers onto which the adhesives can bond well but this layer does not in turn bond onto the metal with sufficient strength, they must be removed carefully. Another obstacle in the case of metals can be the formation of a condensation film. On metals, this symptom is often seen when they are stored at cold or lower temperatures and are moved to a warmer, heated location for bonding. In such cases, it is essential to first allow the metals being bonded to get to the same temperature.

Cleaning the surfaces

Since metallic surfaces are rarely sufficiently clean for a bond, careful cleaning of the bonding surface is almost always necessary. As already mentioned, metals can have oxide layers but often also have corrosion inhibitors such as oils, greases and other substances, along with dusts and various impurities sticking to them. All of these surface conditions that can interfere with or damage a bond must be removed. Perfect cleaning of the bonding surfaces is absolutely essential. Surface cleaning easily removes light, non-sticking impurities. Greases, oils and separating agents can only be removed by suitable cleaning agents. Various solvents and similar substances are effective for this. The respective cleaning effect must be tested on a case-by-case basis. Mechanical cleaning has proved to be successful in the case of oxide layers.

Procedures such as abrasion and sandblasting are very popular surface treatment methods. It is important to remember here that cleaning and degreasing with suitable substances are necessary first. Only then may abrasion or sandblasting be performed. Separating media such as oils and greases that are present can be ground (blasted) into the metal during the abrasive process and can then continue to have a separating effect on the adhesive. Pretreatment by abrasion or blasting is a popular method for metals, because it increases the size of the bonding surfaces by creating peaks and valleys. Depending on the abrasive grit, this can be equivalent to several times the surface area.

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