Adhesives are used at home, at the workplace , school and in industry. The adhesives vary with the purpose for which they are intended for. Adhesives are essential to the manufacture of footwear, automobiles, nonwoven fabrics, plywood and timber articles, paper and packaging materials, textiles, and other products based on plastics.
Natural adhesives have been replaced in many uses by synthetics; but animal glues, starches, gums, cellulose, caesium, albumen bitumen, and natural rubber cements continue to be used in large volumes.
Synthetic adhesives, used either alone or as modifiers of natural adhesives, perform better and have a greater range of application than natural adhesives. Synthetic adhesives bring together the thermoplastic and thermosetting resins and elastomers. Adhesives having special properties are prepared from synthetic resins. Most synthetic adhesives form polymers, huge molecules incorporating large numbers of simple molecules to form strong chains and nets that link surfaces in a strong bond. There are three types of synthetic adhesives: thermoplastic adhesives , thermosetting adhesives and elastomeric adhesives.
Thermoplastic adhesives are fusible and harden without chemical change on cooling. It is used to bond wood, glass, rubber, metal and plastics.
Thermosetting adhesives are essentially infusible, insoluble materials formed from long-chain polymers by heat, catalysis, or both. They are made into, tough and heat-resisting solids by the addition of a catalyst or the application of heat, are used in such structural functions such as bonding metallic parts of aircraft and space vehicles.
Elastomeric adhesives are used for bonding flexible materials to rigid materials. Examples of elastomeric adhesives comprise of solvent-based adhesives, aqueous dispersions and pastes.
Adhesives vary in the way they are applied and in the manner in which they form a bond. For example, adhesives made from the synthetic materials polyethylene and polyvinyl acetate, called hot melt adhesives, are heated to a liquid state before being applied. The bond forms as the adhesive cools and hardens. Epoxy adhesives are usually sold as two substances in separate tubes. The two substances must be mixed before being applied to the surfaces to be joined.
Sticky Tapes
Sticky tapes or “adhesive tapes” is cotton or other fabric coated with an adhesive substance and is used for various purposes.
A pressure sensitive tape is any adhesive tape that will stick to a wide variety of clean dry surfaces with only the minimum of pressure applied. It is ready to use and does not need to be activated by water, solvent or heat.
A pressure sensitive tape consists of a relatively thin, flexible backing or carrier, coated with an adhesive, which is permanently sticky at room temperature. The tapes can be manufactured with the adhesive coating on one side (single-sided) or both sides (double-sided) of the carrier.
Single sided tape Double-sided tape
Adhesive tapes are made up of two components: a carrier which is usually paper, plastic, cloth or rubber and an adhesive which is either water or solvent based. Many modern adhesive tapes use pressure sensitive adhesives. The adhesive is coated on the backing and bonds to the target, when you apply pressure to the tape with your finger, a strong adhesive bond is formed. One or both of the sides of the backing is specially treated to provide controlled release or adhesion.
Pressure-sensitive adhesives include a variety of natural and synthetic materials, including natural rubber, synthetic rubber, block copolymers, acrylics, silicones, etc., modified in various ways through molecular weight selection, resin additions, oils, fillers, cross-linking agents, stabilizers, pigments and other additives.
The majority of today’s pressure-sensitive adhesive tapes consist of plastic film backing coated with natural rubber, synthetic rubber or acrylic pressure-sensitive adhesive. Polypropylene, polyester and polyvinyl chloride constitute most of the films currently used. These films are normally formed by extrusion and do not require the use of solvents in forming the film. Other common materials include films (which are typically made from regenerated cellulose, polypropylene or PVC), paper tissues, cloth or even metal foils and can be clear, coloured or printed. The choice of material used as the carrier depends on what the tape is intended to do.
Some tapes are made of poor quality materials which will deteriorate quickly. Paper carriers may become yellow and brittle causing the repair to break open again. Plastic carriers may shrink, yellow and become brittle. Rubber based carriers will oxidise, harden, crack and shrink.
When being used adhesives may yellow and stain documents. Pressure sensitive adhesives may lose their fluidity, harden and lose adhesion. Adhesives may also become more fluid at higher temperatures and leach into the paper surface causing that familiar translucence.
Masking tapes have a paper carrier, however other heavy-duty tapes for other DIY uses such as sealing or heating duct work are made from cloth.
Common sticky tape (also known as pressure-sensitive tape) has varying strengths in stickiness and the ability to stay bonded for a longer time and be more durable. The bond strength is dependent on the degree of contact area, a rough surface, such as concrete, rough wood, or an open fabric, which may be offering a very poor contact, will result with a resulting poor bond. Adhesion strength is also dependent on temperature, the lower the temperature, the harder it is to make a tape stick, until eventually, there will be no tack to the tape whatsoever, and a bond will never be formed. The tape can also be affected by the contamination from oil, grease and moisture which affect the bond strength.
There are 2 main types of adhesives used for pressure sensitive tapes - rubber/resin and acrylic.
1.) Resin adhesive is a mixture of a rubbery material and a hard resin. The rubber and the resin can come from natural or synthetic sources. The natural materials are usually extracted from trees while the synthetics come from the oil industry. Resin adhesives are relatively cheap but, unless they are specially formulated through curing or stabilising, do not have very good resistance to heat, ageing or exposure to light.
2.) Acrylics are fully synthetic polymers, which are manufactured to give the specific adhesive properties required. Acrylic adhesives are more expensive but generally have better resistance to ageing and exposure to light.
How is tape produced?
There are 2 processes in the production of pressure sensitive tape; coating and slitting. Coating is the application of the adhesive on to the carrier. Slitting is the cutting up of a huge 'jumbo' roll of coated carrier film into the large and small rolls of tape as you see them in the shops.
Coating
Pressure sensitive adhesives are not runny liquids, so they are difficult to coat on to the thin flexible carriers, therefore they are made less thin to make coating possible. There are 3 main ways to do this:-
1. Solvent Coating
The adhesive materials are dissolved into a suitable solvent. The resulting relatively runny solution is coated onto the carrier and the solvent removed by passing it through a heated oven.
2. Water-based Coating
The adhesive materials are mixed with water to form an emulsion or dispersion. This is then coated onto the backing and the water removed through a heated oven in the same way as for solvent coating.
3. Hot-melt Coating
The adhesive materials are heated until they melt and coated as a hot liquid. The coated tape is then simply cooled to give the finished tape.
These 3 coating techniques are frequently used to define pressure sensitive tape i.e. Solvent, Water-based or Hot melt. This is a description that refers to the technique used to coat the adhesive onto the carrier. It does not take account of the adhesive type and it is this that can have a considerable influence on the tape's performance.
The Hot Melt method
To make sure the pressure-sensitive tape does not stick to itself , the back of carrier is coated with a release coating, this release coating is needed to make all tapes function properly.
The finished products
The earliest reference to sticky tape being used can be found in Thomas Mace's "Music's Monument" published in 1676 where lute makers used 'little pieces of Paper, so big as pence or two pences, wet with Glew' to hold the thin strips of sycamore in place during construction of their musical instruments.
The first use of “pressure sensitive” tape was in 1845 when a US patent was taken out by Shecut and Day where they used rubber, pressure sensitive adhesive was used in the making of sticky plasters which are put onto sticky bandages to be used during the American Civil War.
Major progress in pressure-sensitive adhesive tapes was achieved in the 1920s, when Henry Ford introduced industrial uses of tapes. Masking tapes were used in the spray-painting process, where different coloured cars were assembled. In the 1930s the first cellophane plastic were produced which had a rubber-cement based adhesive.
Cellophane
Cellophane is the common name for flexible, transparent film made of regenerated cellulose and used principally as a wrapping material. Cellophane is produced by dissolving wood pulp or other cellulose material in an alkali with carbon disulfide, neutralising the alkaline solvent with an acid, extruding the precipitate into a sheet, infusing it with glycerine, and then drying and cutting the sheets to the desired size. Cellophane was invented about 1910 by the Swiss chemist Jacques Brandenberger.
