Not sticking to tradition - a guide to adhesive bonding

TWI Bulletin, May/June 1991

Gareth McGrath joined TWI's Engineering Department in 1989. He has been involved with adhesives, composites and polymers for the last six years, after an initial career in metallurgy. His move to composite technology began during a research assistantship at Sheffield City Polytechnic where his thesis was devoted to reclamation prospects for advanced thermoplastic composites. This concluded with a predictive model and component fabrication. During this period he also completed a number of industrial research contracts on polymers.

Since joining TWI, Gareth has been developing techniques necessary to explain composite failure. These have included fracture toughness measurement techniques, impact damage analysis, and study of the time dependent response of joints in thermoplastics and composites. He has recently been appointed to a technical co-ordination function for the expanding adhesives activities at TWI.

A raised awareness of what adhesives can do in joining technology is changing the way structural and mechanical engineers think.

Traditional resistance to the use of adhesives is declining according to Gareth McGrath who has compiled this guide to adhesive bonding.

Adhesives form an integral part of a wide variety of fabricated products, and offer the potential to create new, challenging product designs. Structural and speciality adhesives account for about 30% of total adhesive and sealant sales, with uses in automotive, aerospace, domestic appliance, biomedical/dental, consumer electronics, 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 overwhelmingly complicated because of the many types of adhesives available. This guide aims to help overcome the confusion.

Terminology

First, it is helpful to understand the terms used in adhesive technology, as set out below.

Adhesion and cohesion

When surfaces are held together by interfacial forces, they are said to show 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 cleanly from the surface.

Adhesives, like other materials, when cured 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 as shown in Fig.1. Good adhesion (A) and cohesion (C) are required to achieve high performance joints.

Adhesives typically have a flow phase when they are applied, spread and wet the surface, followed by a hardening phase during which the cohesive strength develops.

Structural bonding

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.

Multi-resistant bonding

Not all adhesively bonded joints are solely structural and multi-resistant bonding is applied to use of adhesives where the joint also withstands other environmental forces. For example it could maintain its integrity and still be resistant to effects of salt water spray, temperature cycling, and vibration.

Modern adhesives

A bewildering variety of adhesives is available from a range of adhesive manufacturers. However, it is possible to simplify the choice by classifying the adhesive, and this can be done either by the way they are used or by their chemical type. The strongest adhesives solidify by a chemical reaction. Less strong types harden by some physical change. The major classifications are:

Anaerobics

Anaerobic adhesives cure when in contact with metal and air is excluded, e.g. when a bolt is home in a thread. They are often known as 'locking compounds', being used to secure, seal and retain turned, threaded, or similarly close fitting parts. They are based on synthetic acrylic resins.

Cyanoacrylates

Cyanoacrylate adhesives cure through reaction with moisture held on the surface to be bonded. They need close fitting joints and usually solidify in seconds. Cyanoacrylates are suited to small plastic parts and to rubber. They are a special type of acrylic resin.

Toughened acrylics

Toughened acrylics are fast curing and offer high strength and toughness. Both one and two part systems are available. In two part systems, no mixing is required because the adhesive is applied to one substrate, the activator to the second substrate, and the substrates joined. They tolerate minimal surface preparation and bond well to a wide range of materials.

Epoxies

Epoxy adhesives consist of an epoxy resin plus a hardener. They allow great versatility in formulation since there are many resins and many different hardeners. Epoxy adhesives can be used to join most materials. These materials have good strength, do not produce volatiles during curing, and have low shrinkage. However, epoxies have low peel strength and flexibility. Epoxy adhesives are available in one-part, two-part and film form and form extremely strong durable bonds with most materials in well designed joints.

Polyurethanes

Polyurethane adhesives are chemically reactive formulations which may be one or two part systems and are usually fast curing. They provide strong resilient joints which are impact resistant and have better low temperature strength than any other adhesive. Polyurethanes are useful for bonding glass fibre reinforced plastics (GRP). The fast cure usually necessitates applying the adhesives by machine. They are often used with primers.

Phenolics

Phenolics were the first adhesives for metals, and have a long history of successful use for joining metal-to-metal and metal-to-wood. They require heat and pressure for the curing process.

Polyimides

Polyimide adhesives are based on synthetic organic chains. They are available as liquids or films, but are expensive and difficult to handle. Polyimides are superior to most other adhesive types with regard to long-term strength retention at elevated temperatures.

The following adhesives undergo a physical change and are less effective at forming an adhesive bond.

Hot melts

Hot melts are based on modern plastics and are used for fast assembly of structures designed to be only lightly loaded.

Plastisols

Plastisols are modified PVC dispersions which require heat to harden. The resultant joints are often resilient and tough.

Rubber adbesives

Rubber adhesives are based on solutions or latexes and solidify through the loss of the solvent. They are not suitable for sustained loading.

Polyvinylacetate (PVA)

Vinyl acetate is the principal constituent of the PVA emulsion adhesives. They are suited to bonding of porous materials, such as paper or wood, and to general packaging work.

Pressure-sensitive adhesives

Pressure-sensitive adhesives are suited for use as tapes and labels and although they do not solidify they are often able to withstand adverse environments. This type of adhesive is not suitable for sustained loading.

Application

How adhesives are applied and cured depends on:

• Whether there is a facility to change the manufacturing process;
• Whether it is possible to heat cure an adhesive;
• The curing mechanism of the adhesive;
• Time factors in the assembly process.

The adhesive can be applied by an automated robotic system, a bulk dispensing system or a portable hand held dispensing cartridge which allows the system to be mobile. Adhesives can even be applied by hand with a spatula. Which application method is chosen really depends on the volume of adhesive being used. With respect to the two part epoxies and polyurethanes, mixing and metering of exactly the right amount of component parts are vital for optimum performance of the adhesive. Equipment exists which can provide both exact metering and full mixing, as well as dispensing in exactly the right position for manufacturing the product. In the case of heat curing either one-part paste or film adhesive, platens, autoclaves and vacuum presses may be required.

Advantages and limitations

The many advantages of adhesives include:

• Dissimilar materials can be joined;
• The bond is continuous;
• Stronger and stiffer structures (Fig.2)
• On loading there is a more uniform stress distribution (Fig.3);
• Local stress concentrations are avoided;
• Porous materials can be bonded;
• Adhesives prevent cathodic corrosion;
• Adhesives seal and join in one process;
• No finishing costs;
• Good 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.

All these advantages may be translated into economic benefits: for example improved design, easier assembly, lighter weight (inertia overcome at lower energy expenditure), and longer life in service.

Among their limitations are:

• Increasing the service temperature decreases the bond strength;
• Short term handleability is poor;
• Bonded structures are usually not easily dismantled 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 with adhesives, although some substrates may give rise to lower bond strengths or durability.

Joint design

When designing an adhesively bonded joint it is not advisable to assume that the design should be the same as for traditional fixing methods like welding and brazing. There are characteristics of adhesives which mean that they perform better in compression, shear and tension than in peel and cleavage (Fig.4). When designing the joint it is necessary to:

• maximise tension, shear and compression;
• minimise peel and cleavage forces;
• maximise the area over which the load is distributed.

The strength of a joint is a complex function of the stress concentrations set up by the load. In a simple lap joint made from thin metal sheet there are two types of stress: shear and peel. The shear stress varies along the length of the joint with concentrations at the ends. The peel stress acts at right angles to the lap joint, and is likewise a maximum at the ends (Fig.5). The peel stress tends to distort the joint and consequently weakens it. Alternative joint designs are presented in Fig.6. In these the stresses are more evenly distributed, resulting in joints of greater strength. These joints can be applied to more complex geometries including the stiffening of large thin sheets, strengthening around apertures, bonding of multilayer structures, and joints using profiles.

Other specific types of joint are:

Butt joints

A straight butt joint has poor resistance to cleavage and Fig.7 shows recessed butt joints which are recommended.

Corner joints

Corner joints can be assembled with adhesives by using simple supplementary attachments. Typical designs are right angle butt joints, slip joints and right angle support joints. These joints increase the structure's rigidity. With this technique, thin gauge metals or sandwich panels can be easily formed.

When larger gauge materials are to be joined, end lap joints are the simplest design type although they require machining. Mortise and tenon joints are excellent from a design stand point but may also require machining if a standard section is not available. A mitred joint with spline is the best if members are hollow extrusions, in this case a void filling adhesive is recommended.

Surface preparation

Bonding advantages gained through joint design can be meaningless if the surfaces to be bonded are not adequately prepared. The amount of surface preparation depends on the required bond strength, desired environmental ageing resistance and economic practicalities. For maximum strength structural bonds, paints, oxide films, oils, dust, mould release agents and all other surface contaminants must be completely removed. There are four principal ways or preparing surfaces:

• Solvent degreasing;
• Abrasion, including emery paper, sand, shot or grit blasting;
• Chemical etching and anodising;
• Use of surface primers.

Given the correct adhesive and the appropriate surface preparation almost any substrate can be bonded, provided that the operating conditions are not too extreme. However, creation of the optimum surface can be expensive and may not be practical in many manufacturing situations. Fortunately, most applications do not require preparation at this level, and usually an acceptable compromise can be found at some point in the sequence:

• No treatment at all;
• Solvent wash;
• Solvent wash, abrade, solvent wash;
• Solvent wash, abrade, solvent wash, chemically etch;
• Any of the above plus an appropriate primer.
  

Adhesive selection

Selection of an adhesive appropriate for a job depends on many things including:

• Substrates;
• Forces in the joint;
• Environmental conditions;
• Required durability;
• Application/process factors.

With respect to the substrates being used, different adhesives suit different substrates. The issue of application and curing of the adhesives depends on:

• The adhesive selected;
• Flexibility of the manufacturing process;
• Type of manufacturing process;
• Substrate.
  

Product development

There are many opportunities for development using adhesives, and many reasons why companies may wish to develop a new or existing product. A new use of adhesives might help to revitalise a product or re-establish market share, or make it possible to use better or more economic production methods. Adhesives may facilitate use of improved materials, or entry into a new market. This has led to many exciting uses of adhesives:

Aluminium cylinder heads

The studs used to secure inlet manifolds to cylinder heads may be retained in position with an anaerobic adhesive. Although of high strength, the grade chosen allows maintenance and removal of the studs when necessary. Because the studs are a loose fit, and retention is ensured by the adhesive, no stresses are generated in the aluminium cylinder head. This means that sections may be of minimal thickness.

Railway carriage window panels

Some window panels of railway carriages are made from cold-press moulded GRP. They are stiffened with both aluminium and steel formers which are bonded directly on to the panel with a toughened acrylic adhesive.

Hammer heads

Hammer heads may be bonded to their shafts using a rubber toughened, two part cold-curing epoxy. This form of assembly prevents the loosening so often associated with conventional wedging techniques.

Tractor bonnets

The steel bonnets and stiffeners of selected tractors are bonded with a rubber toughened single-part heat cured epoxy. No special surface preparation is required. The adhesive chosen remains in place as the temperature is raised to induce cure.

Stereo speaker assembly

Cyanoacrylate adhesives are used in the manufacture of loudspeakers. Manufacturers have used adhesives to stay ahead in a competitive market, using the vibration damping properties of adhesives. Using adhesive to bond a coil to the core has improved the handling performance of a speaker, increasing it from 100 to 140W whilst being subjected to high stress and vibration.

Honeycomb panels

Perhaps one of the most significant materials to be developed recently has only been possible with the development of adhesive bonding. Lightweight honeycomb panels give the designer another capability. The face skins are bonded to a honeycomb core, which may be Nomex or aluminium; one example is the rudder of Concorde.

Conclusion

The resistance of traditional engineering practice to 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 of the need to consider joint design based on the properties of the adhesive selected as an integral part of product development.