TWI Knowledge Summary
Adhesive bonding - chemically curing adhesives
by Gareth McGrath
Chemically reactive adhesives are subdivided into two groups: one-component systems and two-component systems (mix and no-mix). The most widely used chemically reactive structural adhesives include:
- Acrylics
- anaerobics
- cyanoacrylates
- modified - Epoxies
- Phenolics
- Polyurethanes
- Silicones
- High temperature adhesives
- polyimides
- polybenzimidazoles
Cyanoacrylates and one-part polyurethanes are one-component systems that cure by moisture from the atmosphere, or adsorbed moisture from the substrate surface. Non-porous surfaces may not hold sufficient moisture to develop an adequate bond.
One-component systems that cure by heat include epoxies, polyurethanes, polyimides and usually consist of two pre-mixed components, which eliminates the need for metering and mixing. However, special storage conditions are required and these adhesives generally have a limited shelf life.
Anaerobics are also classified as chemically reactive one-component adhesives, They are not, however, included in either the moisture or heat activated cure categories because they cure by free radical polymerisation upon elimination of oxygen.
Two component systems come in several forms but usually consist of one part called the adhesive or resin, and a second part called the hardener, catalyst or accelerator. The two components are brought together, i.e. mixed, in a variety of ways, with the mixing starting a chemical reaction, which leads to hardening of the adhesive. Some two component (two-part) systems do not require careful mixing, such as modified acrylics, in which accelerator is applied to one surface, adhesive to a second and the surfaces are joined. Other two-part adhesives, for e.g. some epoxies, often need more accurate mix ratios. Different methods have been developed to help with this mixing, such as pre-measured packs, or cartridges and mixer nozzles. Two-part adhesives can cure at room temperature, although some need an elevated temperature to achieve full cure.
The Table highlights the essential characteristics of each item.
Fault finding
- Adhesive selection is an important step in the design process. The aspects that must be considered are:
- Adherends to be joined
- Performance requirements
- Production processes
- Cost
- Good joint design is essential: bonded joint design must not be treated in the same way as welded joint design for example. Stress distribution is the most important aspect since stresses are not evenly distributed over the joint
area. For example, designs can give good shear, tensile and compressive strength and yet fail because of peel stresses.
- Two-part adhesives must be mixed according the manufacturer's recommendations.
- Curing must be performed at the recommended curing conditions (e.g. time, temperature, relative humidity, etc).
- Design must account for cure schedule, fatigue loads, temperature of operation, surface preparation and gap filling.
- Adhesives must be stored in accordance with recommendations and not used beyond the shelf life.
- Personnel should always be trained in use of adhesives.
| Type | Form | Application method | Cure | Service temp | Benefits | Restraints | Environment resistance | Health & safety | Applications | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Water | Solvent | Oil | ||||||||||
| Anerobic | One-part liquid or paste | Container or automatic applicator | Polymerisation on exclusion of air, cures in 24h at 20°C or 10 min at 120°C | -55 to 120 | Versatile method for co-axial joints, with limited surface preparation
Gasketing |
Usually limited to co-axial joints | Good (cold) | Depends on formulation | Good | No major toxicity problems and their general level of physiological activity appears to be low | Small assembly work, joining cams, shafts, screws subject to torsion,
Gasketing |
|
| Cyanoacrylate | One part liquid | Container or automatic applicator | Polymerisation in presence of moisture on substrate cures in seconds or minutes at 20°C | -30 to 80 | Cure speed | The cure rate prevents joining large areas. The cured adhesive is brittle. Poor gap filling | Poor (cold or hot) | Fair/Good | Good | The rapid cure rate, presents difficulties particularly bonding to skin | Rapid assembly of light structures comprising small components; optical & electronic information, poor gap-filling properties | |
| Modified Acrylics | Two part cold cure (could be mixed) [must be mixed] | Container, static mixer, fully automated systems | Pseudo-one part systems are catalysed by an initiator to cure very quickly | -40 to 120 | Potential for rapid cure and ability to bond poorly prepared surfaces. Good environmental resistance | Strength and modulus are lower than for epoxies | Good | Good | Good | No major toxicity problems and their general level of physiological activity appears to be low | Rapid assembly of small components through to load bearing structures where gap-fill and extended cure are necessary | |
| Epoxies | Heat cure | Film, paste | Manual, Film application | Polymerisation under heat & pressure | -40 to 120 | Strength, durability | Storage & restrictions Heat cure | Excellent | Excellent | Excellent | Dermatitis
Respiratory problems |
A vast range of applications eg aircraft, cars and drive shafts |
| Two part | Two part plastics | Manual static mixing
Fully automated mixing & dispersing |
Polymerisation at room temperature, may be accelerated by heat | -40 to 100 | Strength, durability | Cure rates are usually slow unless the joint is heated. Rapid cure systems tend to be brittle | Good | Good | Good | Dermatitis
Respiratory problems |
A vast range of applications eg aircraft, cars and drive shafts | |
| Phenolics | Solvent solutions, powders & films | Manual, brush spray & film application | Polymerisation under heat & pressure | -30 to 100 | Good shear strength in demanding environment | Brittle.
High forming pressure |
Excellent | Good | Good | Dermatitis
Toxic Irritant Respiratory problems |
Plywood applications, glass to metal adhesive for electric bulbs; component of two polymer structural adhesive | |
| Polyurethanes | One part | Solvent dispersions | Cartridge based | Reaction with atmospheric moisture | -200 to 80 | Versatile formulations give good gap filling & sealing with a semi-structural performance | Handling strength can take up to eight hours. Moisture sensitive particularly bare metal | Fair | Fair/good | Good | Care is needed to avoid physiological hazards | Non-structural applications subject to low loads; cryogenics and footwear industry |
| Two part | Pastes or liquid | Dual cartridge systems to fully automated systems | Rapid polymerisation that can include a foaming element | -200 to 80 | High reaction speed plus versatile formulations give good gap filling & sealing with a semi-structural performance | Moisture sensitive, particularly on bare metal | Fair | Fair/good | Good | Care is needed to avoid physiological hazards | Wide range of materials and the ability to foam | |
| Silicones | One or two part viscous liquids or pastes | Spatula and extrusion gun | Polymerisation at room temperature | -70 to 250 | High flexibility and temperature resistance | Low strength | Good | Fair | Good | Low overall toxicity, mild skin and eye irritation upon exposure to some bi-products of cure | Adhesive sealants for welded joints; silicone rubber gaskets; heat resistant seals; moisture and corrosion resistant coatings; aircraft & electronic assembling | |
| Polyimides | One part resin on glasscloth support | Sandwich assembly | Polymerisation at 250°C under pressure | -40 to 280 (short term at 300) | High temperature resistance | Difficult processing during manufacture | Excellent | Excellent | Excellent | No major toxicity problems | Structural adhesive for high temperature use; aerospace industry | |
Also, you can use the
Weldasearch literature database to supplement what you find in JoinIT.
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