The development of new materials—with their wide-range of application potential—poses major challenges for modern bonding technology. In production, adhesive technology enables the optimum use of specific material properties and the implementation of new construction methods such as lightweight construction. The use of structural adhesives has also become essential to the widely-used mixed construction method, as well as the application of all kinds of metal, synthetic and fibre composite materials.
In recent years, the use of modern adhesive technologies has become increasingly prominent in the industry. There is a good reason for this, as high-performance adhesives offer numerous advantages for constructors and processors alike. In particular, the property of bonding all kinds of materials quickly and securely is one of the reasons why adhesives are now regarded as an established part of many production processes. Almost all types of metals—conventionalstandard steels, and stainless steel and aluminium—as well as synthetic and fibre composite materials and glass or ceramics can be combined adhesively with professional application. In this way, the specific material properties can be optimally utilised for a component in product manufacture.
Structural bonding has now become an essential joining method in all industrial sectors, especially where different materials have to be bonded together, explains Shilip Kumar – Business Director (Automotive & Metals), Henkel Adhesive Technologies, India.
Structural adhesives are used in the manufacture of domestic appliances, building facades, filters and wind turbines, in addition to a wide array of other industrial applications. In truck construction, nearly all modern box bodies are now bonded and sealed at the same time. The sandwich design of modern wall and ceiling elements facilitates lightweight structures that can only be fitted by means of bonding.
Advantage over Conventional Joining Methods
In addition to their all-round versatility, there are other factors that contribute to the increasingly important role of adhesives in trade and industry. On a like-for-like basis, traditional connection technologies have familiar disadvantages. Mechanical processes such as riveting or bolting only enable transfer of force at specific points. The flux results in peak stresses at the ‘hole’ or spot edges, which increase the risk of fatigue fracture, and thus adversely affect the dynamic and static strength of the bond.
In contrast, when bonding connections are effected correctly, the stress or load transfer is more evenly distributed across the entire bonding surface. Compared with traditional joining methods, bonds are superior in the presence of dynamic stress. In addition, the parts to be bonded are not weakened by the drilling of holes. Compared with welding and soldering, the use of adhesives chiefly stands out by virtue of its flexibility.
Heat-sensitive materials such as aluminium or plastic, as well as very thin components can therefore be securely bonded. There is no negative impact on the strength of the material here. The low heat build-up is another key criterion when processing joining parts that already have their final surface finish (for example; chrome-plated steel). As a nonconducting material, cured adhesives also have an insulating effect, thus preventing contact corrosion.
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Henkel provides solutions for a wide range of structural applications with its Loctite and Teroson adhesives. As the world's leading manufacturer of adhesive and sealants, it provides a generalpurpose selection of structural adhesives for all kinds of industrial applications through these brands. Covering entire spectra of adhesive technologies—including epoxy, acrylics and polyurethane adhesives and silane-modified polymers, along with silicone-based products. Henkel is on hand with extensive advice and services for industrial customers who want to trial adhesives for their specific applications.
Depending on the field of application, various adhesive technologies with specific product properties are available for structural bonding. This chiefly involves one to two component reaction adhesives that cure at room temperature or as a result of heat. For selecting the most suitable adhesive, expert advice from the manufacturer is extremely important. Mr Kumar also believes that to make the right adhesive choice, it is essential to know and take into account the subsequent everyday requirements for bonded connections.
One distinguishing criterion is the difference between rigid and flexible bonds, both of which have different advantages. Conventional rigid bonds are mainly used for high mechanical transfers of force, and replace traditional joining methods in this case. They enable a tough adhesive bond on the surfaces to be joined, and meet all practical requirements in terms of their strength properties, as demonstrated by numerous examples in the aircraft and vehicle industry. In contrast, elastic adhesives are increasingly being used in the presence of high dynamic stress resulting from temperature differences or component movements.
From Theory to Practice
Adhesive technology can be practically integrated into existing production processes in all industrial sectors. Depending on the application, the coating is applied manually, semi-automatically or fully automatically by robot. For this purpose, Henkel provides a range of dispensing systems that ensure secure and constant production.
Based on the quantity and type of adhesive used, a suction device is sometimes necessary— workers are advised to wear protective clothing and equipment in each case. The principal factor in the quality of the bond is the surface quality of the joining parts. For example, inadequate cleaning of oil or grease from fitted parts to be bonded is a common source of faults in practical application. Another essential condition is the correct application of the adhesive; the effectiveness of bonded structures is heavily dependent on the production conditions.
These include preparation of the fitted parts, correct mixing in the case of twocomponent adhesives and their application, as well as fixing of and adherence to curing times. This is why adhesive manufacturers regularly arrange seminars and workshops to train specialist workers.
The extent to which the use of structural adhesives at metal-processing firms pays off commercially must be assessed on a case-by-case basis. In the design and manufacture of composite materials, the joining method of bonding provides significant freedom of design and the ability to integrate many functions in one component. Optimising the materials used not only reduces the weight of the component, but also enables a more cost-effective design. In addition, the possible increase in process speed compared with many mechanical joining technologies also delivers commercial benefits. In terms of visual aspects, one advantage is the prevention of surface pitting due to spotwelded connections.
Before adopting structural adhesives for industrial use, experts recommend tests under realistic conditions—prior to the product going into series production. Shanghai-based Henkel has state-of-the-art centres for development and application technology, where tests and analyses through to customer-specific modification and product processing can be carried out.
A Structurally Sound Future
In view of the prevailing trend towards lightweight design and the targets of high productivity and economic efficiency in manufacture, the use of structural adhesives opens up attractive future prospects for small and large firms alike. They will come increasingly to the fore, as resource-conserving lightweight design is hard to implement without adhesives. There is a clear trend towards composite materials and alloys.
In future, the economic success of a metalprocessing firms will increasingly depend on the extent to which the high innovation potential is utilised in the manufacture of composite materials. In this context, structural bonding is an important factor, which is why it is constantly referred to as a key technology of the 21st century.