Adhesive classification and. properties

2.1 Engineering and non-engineering adhesives

Adhesives may be classified as either organic or inorganic materials in a number of different ways; for example by origin, by method of bonding, by end use or on a chemical basis (1). Table 2.1 gives a broad classification of the organic adhesives based upon origin under the general headings of animal, vegetable, mineral, elastomeric, thermoplastic and thermosetting adhesives.

Animal glues are generally based on protein either in the form of mammalian collagen, from fish or from milk. They tend to be ‘sticky’ which is useful for applications requiring an instant grab or bond.

The common vegetable glues are based on either starch or cellulose. Unmodified starch dispersed in water is used to form paper pastes but a large proportion is now used in modified forms such as dextrin. Cellulose-based glues are produced by reacting hydroxyl groups present in the polymer chain with different reagents to form a variety of adhesives.

Mineral adhesives include silicates and phosphates for high temperature use and naturally derived products such as bitumen and asphalt.

The elastomeric group of adhesives is based on natural rubber latex and its derivatives or totally synthetic rubber known as SBR (styrene butadiene rubber). There is now a wide range of synthetic rubber adhesives based upon SBR including nitrile and butyl rubber. Another elastomeric adhesive is the versatile polyurethane rubber group.

Thermoplastic adhesives are so called because they may be softened by heating and rehardened on cooling without undergoing chemical changes. There is a wide range of such adhesives many of which contain the vinyl group. The PVA (poly vinyl acetate) group of general purpose adhesives which are formulated in aqueous emulsions tend to be moisture sensitive. Others are derived from

Group

Type

Source

Use

Animal

gelatin

mammals, fish

can labels

casein

milk

plywood, blockboard

albumen

blood

Vegetable

starch

corn, potatoes, rice

paper, packaging

cellulose acetate 1 cellulose nitrate )

cellulose

leather, wood, china

Mineral

asphalt/bitumen

earth’s crust

road pavements

Elastomeric

natural rubber

tree latex

carpet making

SBR

synthetic

tyre vulcanising

nitrile rubber

synthetic

PVC solvent glue

polyurethane rubber

synthetic

fabrics, bookbinding

silicone rubber

synthetic

Thermoplastic

PVA

synthetic

wood and general

polystyrene

synthetic

model making

cyanoacrylates

synthetic

plastics, metals, glass, rubber

liquid acrylic

synthetic

structural vehicle assembly

Thermosetting

phenol-formaldehyde 1

synthetic

chipboard and plywood

urea-formaldehyde j unsaturated polyesters

synthetic

glass fibre, resin mortars

epoxy resins

synthetic

structural, especially metal to metal

polyurethane

synthetic

semi-structural uses with plastics, metals, wood and sandwich panel construction

polyesters, nylons and, importantly, the cyanoacrylates. The latter group set very quickly when squeezed out in thin films. These monomers are liquids of low viscosity which polymerise very easily on contact with traces of moisture invariably present on the surface of the adherend. Their main disadvantages with regard to potential structural use, apart from the very rapid set, are a lack of moisture resistance when used to bond metal surfaces and their restriction to thin bondlines.

The cyanoacrylates are only part of the wider range of acrylic adhesives now on the market. The development of liquid methacrylates and acrylates followed from the introduction of anaerobic gasketing sealants and thread couplants. Two-part liquid acrylic adhesives are now available in forms which allow the liquid to be applied to one surface and the initiator (activator or catalyst) to the other. Contact during assembly produces a strong bond although the glue-line thicknesses necessary to achieve intimate mixing on contact may be rather too thin for civil engineering use. Pre-mixing the two components can result in a rapid set unless specially formulated. Some reservations have also been expressed regarding durability against moisture when used on metal surfaces, although there is conflicting evidence on this point. Nevertheless, the acrylics show potential for providing an alternative source of structural adhesive to the epoxy resin in the future, particularly as they are believed to be less toxic.

The molecular chains present in thermosetting adhesives undergo irreversible cross-linking on curing. Unlike thermoplastics they do not melt or flow on heating but become ‘rubbery’ and lose strength. Synthetic resins formed from a reaction between urea, phenol, resorcinol or melamine and formaldehyde are common adhesives of this type used in the production of glulam. However, for hot and moist exposure conditions phenol or resorcinol formaldehyde products only are favoured. The other very important thermosetting adhesives come from the epoxy and unsaturated polyester groups.

As structural adhesives, epoxies are the most widely accepted and used. They typically contain several components, the most important being the resin. To the base resin is added a variety of materials, for example hardeners, flexibilisers, tougheners and fillers. These all contribute to the properties of the resulting adhesive. Formulations may be further varied to allow for curing at either ambient or elevated temperatures. The epoxies and polyesters, together with acrylics, polyurethanes and synthetic polymer lattices will be discussed in greater detail in the sections which follow. However, it will become evident that epoxy resins have several advantages over other polymers as adhesive agents for civil engineering use, namely:

(1) high surface activity and good wetting properties for a variety of substrates

(2) may be formulated to have a long open time (the time between application and closing of the joint)

(3) high cured cohesive strength; joint failure may be dictated by adherend strength (particularly with concrete substrates)

(4) may be toughened by the inclusion of a dispersed rubbery phase

(5) lack of by-products from curing reaction minimises shrinkage and allows the bonding of large areas with only contact pressure (in stark contrast to the phenolics commonly used in the aerospace industry)

(6) low shrinkage compared with polyesters, acrylics and vinyl types; hence, residual bondline strain in cured joints is reduced

(7) low creep and superior strength retention under sustained load

(8) can be made thixotropic for application to vertical surfaces

(9) able to accommodate irregular or thick bondlines (e. g. concrete adherends)

(10) may be modified by (a) selection of base resin and hardener (b) addition of other polymers (c) addition of surfactants, fillers and other modifiers.

In practice, most commercially available epoxide resins are blended with a variety of materials to achieve desirable properties, as outlined above; indeed, the possibilities for useful new combin­ations are numerous. A major disadvantage of epoxides is that these various modifications and the materials concerned make them relatively expensive when compared with other adhesives.

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