Most modern adhesives grip and set hard because of chemical reactions within the adhesive. The trick is to persuade these ingredients to stay liquid in the tube but set quickly when the components are assembled.

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There are two approaches to this problem. One is to separate the reagents until just before they are applied: two-tube epoxy resins are of this type. The other is to pre-mix the reagents but shield them from factors that will trigger their reaction, such as heat, light, air or moisture. Superglue belongs to this second group.

Two-Component Adhesives

Adhesive families with similar chemistry can include both two-component and single-component products. However, most epoxies, silicones, urethanes and methacrylates come in two-component packages for mixing before use.

Two-component adhesives are widely used in construction and manufacturing but have several weaknesses. To obtain maximum strength they must be mixed thoroughly in exact proportion, but their viscosity begins to increase the moment mixing begins. If the reaction is too hasty, there is a risk of inadequate preparation or improper distribution. If reaction speed is too slow, curing times are delayed and surfaces must be held in position for extended periods.

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Each class of chemicals comes with its own additional limitations. For example, epoxies tend to be inflexible and brittle, silicones rather too flexible and methacrylates are best suited to plastics.

Single-Component Adhesives

These are often classified according to the factor that triggers them to cure, such as heat, moisture, radiation (usually light) or the presence or absence of oxygen. Cyanoacrylates (superglues) and moisture-triggered silicones fall into this group.

Chemicals are often added to block the reaction between the reagents under certain conditions and accelerate it under others. Formulas may also contain bridge building ingredients with an affinity for specific substrates Рsuch as metals. An example is CT1, a metal bonding adhesive from http://www.ct1ltd.com/product-applications/metal-to-metal-adhesive/ which contains an acetylacetonate ingredient with an affinity for metallic ions (see https://wikivisually.com/wiki/Metal_acetylacetonates).

For applications like machinery assembly or boat building, a metal bonding adhesive must have a wide range of properties. These include rapid grab, quick cure times, stability across a wide temperature range and the flexibility to absorb expansion and impact stresses.

To secure rough surfaces like stone cladding or wood as well, some bulk is also required. Formulating the ideal all-round adhesive is therefore a complex feat of juggling.

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