Post 218 –by Gautam Shah
An adhesive is a substance that as an intervening agent binds two similar or dissimilar surfaces (or objects) together and resists their separation. The term adhesive is used for substances such as the glue, gum, cement, mucilage, mastic compound, sealant or caulking compound.
Surfaces stick or remain together when forces that cause attraction are operative. A force of attraction is any type of force that causes objects to come together even if those objects are not close to each other or not touching each other. Gravity is one such force. The electrostatic force cause attraction due to differential electrical charge. The magnetic force affects objects that have magnetic properties.
Adhesive substances are many types. It is a solid that is dissolved by heat or solvent, a natural liquid that remains wet, a substance that in the presence of a reactant shows binding capacity, and a material that forms a longer chain (polymerization) over age or with chemical action.
Adhesives are joining substances, and offer unique advantages. The use of adhesives offers superior binding or joining in many situations. Its advantages are evaluated against other joining systems such as seaming, stitching, tying, knotting welding, forging, soldering, fusion joining, mechanical fastening, etc.
The adhesive like substances are valued, for non-invasive joining (without drilling, melting), ability to join similar and non similar materials, capacity to distribute stress across the joint surface, often without damaging the visual aspects of the objects, in some cases for demountable joining, functionality over wide range of temperature and other environmental conditions and facility to work without heavy duty equipment or energy use.
Adhesive are disadvantageous, in varied stress conditions due to environmental as well as structural loading conditions, require a large surface for adequate bonding and do not allow assurance inspection of joint integrity.
Earliest adhesives were plant exudates like gums and resins, and of animal origin like hide gums. The adhesives were perhaps used for joining, dissimilar materials for tools making, broken ceramics, and for waterproofing boats and canoes. Many of the primitive applications of binding materials were similar to use of painting or surface coatings materials.
Starches constitute largest and oldest form of adhesives. The starch is derived from cereals like, rice, wheat, and maize and vegetables like, potatoes, and agri. products like, corns, arrowroot, millet, sago, sorghum, bananas, barley, water chestnuts and yams, favas, lentils, mung beans, peas, and chickpeas. Starch grains have been detected in mill stone dating back to 30,000 years. Starch was used to join parts of adornments and as a hair fixer. Till the last century, Starch was used to size cotton fabrics and add weight to silk fibres. In modern age starch is used in paper making and for surface finishing. Starch is used as an eco-friendly gum for cigarette filter buds, forming corrugated paper boards, seaming tea bags. Starch has been traditional adhesive for forming kites and coat kite-strings. Starch is used as additive to gypsum for dry-wall boards.
Adhesive materials like, gums, glues, starches, egg whites, casein and other proteins, have been used in art work painting to fix various types of colourants. The Egyptians have extensively used animal-glues in tombs, furniture, ivory and papyrus items. Many societies worlds over have used adhesive materials to fix decorations on adornments, ornaments, etc. In Europe during the middle ages, egg whites were used to decorate parchments with gold leaves. Adhesives made of starch were used by Egyptians dating back 3,300 years, for bonding non-woven fabrics from fibres of reed plant -papyrus, and as a sizing material.
Limes and natural cementing materials like Pozzolana (volcanic ash), calcium carbonate and sulphates, were used in masonry work. Bitumen, tar pitches, and beeswax were used as caulk or sealants and also as adhesive for fixing statues and other repair work.
Wooden objects were bonded with glues from fish, horn and cheese. Hide glue was extensively produced in Holland and Fish glue was produced by the British, in 1750s.