All about Adhesives

An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials. Adhesives cure (harden) by either evaporating a solvent or by chemical reactions that occur between two or more constituents.

Adhesives are advantageous for joining thin or dissimilar materials, minimizing weight, and when a vibration dampening joint is needed. A disadvantage to adhesives is that they do not form an instantaneous joint, unlike most other joining processes, because the adhesive needs time to cure.

The earliest known date for a simple glue is 200,000 BC and for a compound glue 70,000 BC.

NATURAL ADHESIVES

Natural adhesivesNatural adhesives are made from organic sources such as vegetable matter, starch (dextrin), natural resins or from animals e.g. casein or animal glue. They are often referred to as bioadhesives. One example is a simple paste made by cooking flour in water. Animal glues are traditionally used in bookbinding, wood joining, and many other areas but now are largely replaced by synthetic glues. Casein are mainly used in glass bottle labelling. Starch based adhesives are used in corrugated board production and paper sack production, paper tube winding, wall paper adhesives. Another form of natural adhesive is blood albumen (made from protein component of blood), which is used in the plywood industry. Animal glue remains the preferred glue of the luthier. Casein based glues are made by precipitating casein from milk protein using the acetic acid from vinegar. This forms curds, which are neutralized with a base, such as sodium bicarbonate (baking soda), to cause them to unclump and become a thicker plastic-like substance.

SYNTHETIC ADHESIVES.

Synthetic adhesivesSynthetic adhesives are based on elastomers, thermoplastics, emulsions, and thermosets. Examples of thermosetting adhesives are: epoxy, polyurethane, cyanoacrylate and acrylic polymers. See also post-it notes.

Mechanisms of adhesion:Adhesion, the attachment between adhesive and substrate may occur either by mechanical means, in which the adhesive works its way into small pores of the substrate, or by one of several chemical mechanisms. The strength of adhesion depends on many factors, including the means by which it occurs.

In some cases, an actual chemical bond occurs between adhesive and substrate. In others, electrostatic forces, as in static electricity, hold the substances together. A third mechanism involves the van der Waals forces that develop between molecules. A fourth means involves the moisture-aided diffusion of the glue into the substrate, followed by hardening.

Glue sticks are solid adhesives in twist or push-up tubes. The user can apply glue by holding the open tube, thus keeping their fingers clean. Because of their ease of use, and their economic value, glue sticks are a popular school and office supply.

A Post-it note is a piece of stationery with a re-adherable strip of adhesive on the back, designed for temporarily attaching notes to documents and other surfaces. Although now available in a wide range of colors, shapes, and sizes, Post-it notes are most commonly a 3-inch (76 mm) square, canary yellow in color. A unique low-tack adhesive allows the notes to be easily attached and removed without leaving marks or residue, unless used on white boards.