Adhesives form an integral part of a wide variety of fabricated products, offering the potential to create new and challenging designs. Structural and specialty adhesives account for about 30% of total adhesive and sealant sales, with uses in automotive, aerospace, domestic appliance, biomedical, dental, consumer, electronic, construction, general industrial, industrial machine, marine and sports equipment applications.
Synthetic adhesives have good adhesion to a variety of substrates, can be applied quickly, have excellent properties, and are cost effective. However, the adhesive selection process can be overwhelming due to the many types of adhesives available. This guide can help to overcome the confusion.
Adhesion and Cohesion
When surfaces are held together by interfacial forces, they are said to show good adhesion. Good adhesion requires very close contact. In the case of adhesive bonding this is achieved by flow of the adhesive and wetting of the substrate. Adhesion strength is the force required to pull the adhesive clearly from the surface.
Cured adhesives, like other materials, can also be characterised by their internal strength, or the force required to cause permanent deformation. To differentiate from adhesion, cohesive strength of adhesives and substrates is used for this internal strength as shown in Figure. 1. Good adhesion and cohesion are required to achieve high performance joints. Adhesives typically have a flow phase when they are applied, building up adhesion, followed by a hardening phase during which the cohesive strength builds up.
Figure 1. The types of forces in an adhesive joint.
Structural bonding is the name given to a bond where it forms a joint that performs a load bearing function. This means that forces in a structure may be transmitted from one member to another through the joint. This force would be taken up by the adhesive and spread or transmitted to the next member.
Multiresistant Bonding: Not all adhesively bonded joints are solely structural and multiresistant bonding is the use of adhesives where the joint also withstands other environmental forces. For example it could maintain its integrity and still be resistant to the effects of salt water spray, temperature cycling and vibration.
Why Use Adhesives?
The exact combination of reasons will vary from case to case, but advantages should be sought in the following areas:
• increased production speed. In comparison with other fabrication methods, adhesive assembly is essentially fast. Even if curing is required, this can often be accommodated "off line", or combined with other processing stages such as paint curing;
• wider material selection. In contrast to welding, adhesives allow a wide freedom of choice during material specification. It is possible to mix and match material combinations to suit product function and save production costs in ways which have been impossible in the past. Traditional materials may be combined with new metal alloys, plastics, composites and ceramics to give distinct product advantages. Absorbing the full potential of this new freedom is perhaps one of the biggest challenges in finding significant market opportunities;
• design for manufacture. Adhesive assembly offers significant cost savings if material costs can be reduced and production operations can be simplified. There is ample evidence that this can be the case if the design and manufacturing functions co-operate to design or redesign the product with manufacturing in mind. New approaches can be taken to the manufacture of sub components, and castings may be combined with extrusions, sheet components and parts produced in a variety of other ways;
• better production sequences. Traditional assembly methods such as welding impose fairly rigid sequences during production, and frequently demand intermediate processing to remove contamination or rectify distortion. As adhesives may be applied more flexibly within the production sequence (after finish painting for instance), they allow new approaches to production planning, shop layout and work flow. Bottlenecks can be removed, unnecessary operations can be eliminated, and work in progress can be reduced;
• low capital costs. Many adhesive operations involve manual application and the use of adhesive packages which have built in applicators. Even when mechanised or automated application is justified for high volume work, the equipment is usually lighter than would be used for welding;
• low production costs. This is a source of significant confusion. Costs "per tube" of some adhesives may be high, but this should not be confused with all in production costs or cost per product. Cost comparisons should be based on the costs of the whole joining process, including plant, preparation and other pre-assembly costs, production and rectification expenses. All in cost assessments of this type provide a basis for accurate comparisons, and adhesive assembly may often give significant benefits.
Advantages of Using Adhesives
The advantages of using adhesives are:
• dissimilar materials can be joined;
• the bond is continuous;
• stronger and stiffer structures can be designed;
• on loading there is a more uniform stress distribution (Figure 2);
• local stress concentrations are avoided;
• porous materials can be bonded;
• adhesives prevent catalytic corrosion;
• adhesives seal and join in one process;
• no finishing costs;
• improved fatigue resistance;
• vibration damping;
• reduced weight and part count;
• large areas can be bonded;
• small areas can be bonded accurately;
• fast or slow curing systems available;
• easy to combine with other fastening methods;
• easily automated/mechanised.
Figure 2. Stress distributions in a riveted joint and an adhesively bonded joint. Note areas of high stress concentration in the riveted joint.
All these advantages may be translated into economic advantages: improved design, easier assembly, lighter weight (inertia overcome at lower energy expenditure) and longer life in service.
Limitations of Using Adhesives
Among the limitations of adhesives are:
• not as strong as metals;
• increasing the service temperature decreases the bond strength;
• short term handleability is poor;
• bonded structures are usually difficult to dismantle for in service repair;
• need to prepare the surface;
• environmental resistance depends on the integrity of the adhesive;
• need to ensure wetting;
• unfamiliar process controls;
• health and safety responsibility.
Nevertheless, even materials which traditionally are difficult to join can be bonded, although some substrates may give rise to lower bond strengths or durability.
Future Product Areas
One of the most demanding areas for the use of adhesives is in civil engineering. Other areas ripe for development include microelectronics, leisure goods, domestic appliances and furniture.
Adhesives have been used to strengthen and repair concrete structures for more than twenty years and in principle this technique could be used for other materials, for example steel. With steel structures there are three additional considerations:
• the strength is no longer limited by the structure material (as with concrete);
• the local temperatures may exceed 80°C;
• projecting attachments such as web stiffeners must be able to withstand accidental impact and consequent peel stresses in the joint.
These factors govern the required performance of the chosen adhesive. One effective method of strengthening a steel girder is to attach additional vertical stiffeners to the web plate. Traditionally these have been either bolted or welded in place. However, a bolted detail results in a loss of cross-sectional area, whilst welding produces stress concentrations in the parent material. Research in the UK has shown that the use of adhesively bonded stiffeners can avoid both of these problems and produce a superior fatigue detail.
Another potential structural application of adhesives is in the construction of bridges and their use to make the necessary shear connection between the steel girders and the concrete deck of composite bridges, in place of the conventional mechanical fasteners. Two approaches are possible:
• Post-bonding may be used to bond precast concrete slabs to the steel surfaces and this technique has already been used with success in East Germany;
• pre-bonding by pouring fresh concrete structure material (as with concrete); onto uncured adhesive spread on the surface of the steel. This may be used in the construction of external plate reinforced open sandwich slabs, with the potential advantage of reduced depth of construction compared with conventional reinforced slabs.
To avoid some of the conceivable on-site fabrication problems, off-site developments are an exciting area for potential developments in the field of bonded assemblies and examples are already in evidence for building construction, e.g. sandwich panels, glazing and composite structural elements. As the use of lightweight composites continues to grow, adhesives will have an indispensable role to play in joining these materials.
The resistance of traditional engineering practice to the use of adhesives is being broken down. Adhesives have now been developed which clearly demonstrate the contribution they can make to the work of both the structural and mechanical engineer. Perhaps of even greater significance is the growing understanding of adhesives as materials in their own right and the need to consider joint design based on the properties of adhesives selected as an integral part of product development.