Adhesive sticks can be divided by their performance into 3 categories: permanent bonding, removable bonding and temporary bonding.
* Permanent Bonding (Brennan, 1966; Muszik and Dierichs, 1971; and Ando and Yamazaki, 1974)
The adhesive is usually intended to give a permanent bond, i.e. when one surface has been bonded to a receiving surface and the adhesive has dried the two surfaces cannot be separated without tearing the material into pieces. It can be used to stick paper with paper or other materials such as textiles, aluminium foil and porous wood.
* Removable Bonding (Gollub et al., 1987)
This kind of adhesive can be used for temporary bonding purposes, such as attaching pieces of paper bearing messages to a notice board, desk window or other substrate, the paper being removable without difficulty and without leaving a film of adhesive on the substrate.
>I-Temporary bonding (Palm, 1989)
For temporary bonded adhesive, the adhesive coated surface can be adhered to a receiving surface and removed from this without leaving traces of adhesive on the receiving surface, and this process can be repeated a number of times.
2.6.1 Raw Materials in Glue Stick Formulation
Adhesive sticks can be produced from rubber, resins and waxes (Brennan, 1966). These components are combined in the form of a pressure sensitive, thermoplastic adhesive which is shaped into an elongated or crayon like body. Muszik and Dierichs (1971) found that with crayons made from rubber, resin and waxes the surface layer must be rubbed off these adhesive applicator crayons under relatively high pressures in order to carry out the spreading of the adhesive. This can cause the crayon itself to break and the paper is damaged during the rubbing of adhesive from the crayon. So they showed that self-supporting adhesive sticks can be formed from alkali metal salts of aliphatic carboxylic acids, water or water-miscible organic solvents and water-soluble or water dispersible adhesive materials.
Ando and Yamazaki (1974) used the reaction product of sorbitol and benzaldehyde as
the gel-forming agent. By using this as gel-forming agent, an adhesive which could maintain a constant hardness and adhesiveness in a wide temperature and humidity range was obtained. It could be readily spread by rubbing it on a surface to be bonded to form a thin and uniform film.
However, it was found that using alkali-metal salts of aliphatic carboxylic acids as gel forming agents had some disadvantages: no alkali-sensitive substances may be employed and no acid-reacting active components may be added because this would destroy the gel-like supporting structure of the stick. It was recommended that free long chain aliphatic carboxylic acids be used. Particularly preferred were wax acids obtained by oxidation of mineral or petroleum waxes, such as peat wax, montan wax or from ozokerite (Werke H.u.M. Fisher, G.m.b.H., 1974). Hence, adhesives which are active in the acid range, e.g. resin acids or esters of resin acids can be employed.
In general the raw materials used in adhesive sticks can be classified as follows:
>I- Adhesive
>I- Gel forming substance >I- Solvent
The following raw materials may also be added in the formulation in order to improve some properties of the adhesive stick.
>I-Plasticizers >I-Emulsifiers >I- Filler >I- Antifoam >I- Colouring >I- Perfume 2.6.2 Adhesive
A d hesive or fi l m for m i ng sllhstanl'l� is t i ll! primary componen t ilnd haH t he funct ion of
holding the substrates, particularly paper, together. The adhesive can be hard resins such as hydrogenated terpenes, polymerized terpenes, phenolic terpenes, polymerized rosin esters, and hydrogenated rosins with melting points of approximately 65 °c. Rubber ingredients which are used as adhesive are hydrocarbon polymers with molecular weights of from about 1,000 to 25,000 (Brennan, 1966).
Adhesive can also be any water- or alcohol-soluble adhesive either natural or synthetic, e.g. polyvinyl alcohol, polymethacrylic acid, polyacrylamide, copolymers of acrylamide, salts of polyacrylic acids, polyvinyl pyrrolidone, ethylene maleic anhydride copolymer, methyl vinyl ether copolymers, carboxylated polyvinyl acetate. The preference of many mani.l facturers is polyvinyl pyrrolidone (Adhesive Topes Limited, 1 974). Natural carbohydrate polymers or modified carbohydrate polymers, such as starch, dextrin, gum arabic, cellulose, methylcellulose, cellulose esters or carboxymethyl cellulose may also be employed. Mixtures of adhesives may be used. It is particularly preferred to employ polyvinyl pyrrolidone especially in conjunction with 2-6'1., by weight of a carbohydrate
or modified carbohydrate polymer (Werke H.u.M. Fischer, G.m.b.H., 1974). Adhesive Tapes Limited (1974) reported the use of ] 0 percent dextrin solution with polyvinyl alcohol as the adhesive substance in glue stick formulation.
2.6.3 Gel or Substance
Gel forming substances are blended with the binder in an adhesive stick to give a stick which is easy to apply without deformation. It also gives a uniform coating, free from lumps of adhesive on the substrate. However the gel forming agent reduces the tack of the adhesive so the type and the amount of this ingredient must be chosen carefully (Palm, 1989). The following are gel forming agents commonly used:
* alkali or ammonium salts of aliphatic carboxylic acids with 8 to 36 carbon atoms such as: sodium stearate (Muszik and Dierichs, 1971; Gollub et aI., 1987 and Palm, ] 989)
* reaction products of sorbitol and benzaldehyde (Ando and Yamazaki, 1974) * wax which can be one or more partly saponified or fully saponified esters of 10 to 30 carbon atom acids with various alcohols, both natural and synthetic, oxidised polyethylene wax, paraffin wax, chlorinated paraffin wax, glycerides of fatty acids, hydrogenated oils and fatty alcohols. The preference is for ester waxes (Adhesive Tapes Limited, 1974).
2.6.4 Solvent
Solvent can be water or water miscible organic solvent or both. Water miscible organic solvents used are one or more mono or multivalent alcohols, e.g. methanol, ethanol, isopropanol, glycerol, ethylene glycol and may include water as in water/organic solvent mixtures, e.g. water/acetone, ester/ethanol (Adhesive Tapes Limited, 1974). Some water is used in most formulations to dissolve the adhesives and other water soluble ingredients
2.6.5 Plasticizers and Emulsifiers
Plasticizers or softening or moisture retaining agents can be used to tackify the adhesive and to control drying out on the substrate to which the adhesive is applied. These substances are also conducive to an easy, soft rubbing. Tri- and tetra-ethylene glycol, sorbitol, mannitol, glucose, glycerol, propylene glycol, polyethylene glycols and polypropylene glycols of molecular weight 400 to 1500 can be used. The preference is for glycerol - up to 25% by weight (Werke H.u.M. Fischer, G.m.b.H., 1974).
The emulsifier can be non-ionic, cationic, anionic or mixtures of these emulsifiers provided that, in combination with the other ingredients, it produces a water in oil emulsion. The preference is for non-ionic emulsifiers, particularly the polyethylene oxide esters of phenols and fatty esters. The emulsifier can also act as a lubricant in the adhesive stick. These substances may be used to increase transparency of the product. The following are examples of the types of emulsifiers which can be used (Adhesive Tapes Limited, 1974):
Nonionics:
Cationics: Anionics:
polyoxyethylene esters of a phenol, polyoxyethylene esters of a fatty acid, polyoxypropylene esters of phenols and fatty acids, polyoxyethylene esters of phenols and fatty acids, glycerides of fatty acids, sorbitan fatty acid esters, fatty acid amides, natural occurring emulsifiers - lanolin and cholesterol derivatives.
ethoxylated amines, quaternary ammonium compounds. soaps, sulphonates, phosphates.
Fillers are nonadhesive substances added to the adhesive to improve its working properties, strength, permanence, or other qualities. Fillers are also used to reduce materials costs. Considerable changes can be made in the properties of the adhesive by selective use of fillers such as clay, chalk (Pletcher and Wong, 1978).
Antifoam Nopco 8034, an antifoam agent containing silica, mineral oil and a methacrylate copolymer can be added (Palm, 1989). Dye-stuffs, e.g. phthalocyanine blue, may be used to add colour to glue stick (Pletcher and Wong, 1978). Odour improving compounds such as, pine-needle oit eucalyptus oil, aniseed oil, benzaldehyde, may be used (Muszik and Dierichs, 1971).
The composition of the adhesive sticks must be so chosen, that a not too soft mass is formed which may be readily rubbed off. This is usually achieved when the content of the gel forming substance is between 3 and 60% preferably from 10 to 40%. The volatile liquid components of the adhesive sticks normally amount to about 20-80%, particularly 30 to 65% and the proportion of adhesive components normally lies somewhere between 5 and 50%, usually between 20 and 40% (Werke H.u.M. Fischer, G.m.b.H., 1974).
The tougher and more cohesive the formulation, the more difficult it is to separate a film of the material from the crayon body. It has been found that the higher the initial strength of a formulation tested, the more difficult the film of adhesive will be to separate from the crayon (Brennan, 1966). Brennan also added that the formulation with the lower softening temperature will apply a film with less physical effort than the formulation with the higher softening point. For this reason the softening point should be generally kept as low as practical.
Processing methods vary according to the raw materials used in glue stick. The adhesive sticks are usually prepared by mixing the individual components while heating them to �0-95 °C until homogeneous under reflux conditions, the process taking about 7 hours. Then the mixture is allowed to cool in a mould or after extrusion in the desired shape form (Muszik and Dierichs, 1971).
The stick may be conveniently mounted in an applicator of the lip stick holder type. In such a holder it may be readily carried among personal possessions or kept in an office desk for use in light gluing applications such as the sticking together of sheets of paper or cardboard or thin layers of flexible plastic materials. The stick can also be placed in