Production of Wood Adhesive
Production of Wood Adhesive – It is not actually sure to say with precision when the technique of adhesive was first used. But it’s use can be traced as far back as 400BC when natural resins and bitman were being used. History made us to know that Egyptians used starch and cassava during the era of Pharaohs. The claims can be attested to because of the discoveries of glued cedre-wood chest and other glued objects which were found in the tumb of Tukankhanam (1356 BC)
The sixteenth century man witnessed the establishment of skin and bone glues for furniture making in Europe. While in England in the 18th century, fish glue was being used for book binding in England. All natural adhesive that are presently in use today were established in the 19th century with the exception of soya-bean glue which was developed in the U.S.A in the 1920s
The most important step forward in adhesive has taken place during this century with development of the synthetic resins resulting from the work of back in hand in U.S.A during 1900s – 1910 fallowed by urea-formaldely de resin 91920). Melamine Formaldelide (1939). Pressure sensitive adhesive tapes developed recently about 25 years ago (surgical plaster).
An important class of adhesive that has contributed greatly to the successful bonding of metal aeroplance and the manufacture of metal honey-comb sand starch structure is that which consists of a blend of thermosetting and thermoplastic resins
Starch-based adhesive has come of age; it was used by the ancient Egyptians for sticking sheets of papyrus together.
Starch adhesive, including degrading or converted starches such as dextrin, comprises the main classification “vegetable glues”. Which embraces water soluble or water dispensable adhesives derived from plants but excludes gum extracted from the back of trees, starch forms the bases of a large number of adhesive varying from a simple paste made by complex processes.
The principal component of a starch based adhesive is the polymer of dextrose synthesized by the plants and from which the adhesive derived its name. Other additives employed serves only to improve the working properties of the product. Starch commonly used as maize, potato and cassava. Starch adhesive could either be cold forming or hot forming and the method of production could either be acid converted or base converted. Though the working properties of starch adhesive are lower than those of synthetic resins, it is extremely cheap and easily sourced.
Starch adhesive required storage in dry and cool environment otherwise they are easily attacked by micro organisms. To prevent the attack of fungi and bacteria attack, preservatives such as zinc chloride or sulphide, paraformaidelhycle and other mercury salts are added.
Generally, starch adhesive is used for binding paper, holding woods, packaging materials, textiles, postage stamps bonding etc.
According to American Society of testing and materials (ASTM), Adhesive is a substance capable of holding materials together by surface attachment.
Adhesive is the general term which includes cement, glues, mucilage, paste etc. Various descriptives are often applied to the term adhesive to indicate certain characteristics for example physical form e.g liquid adhesive, wood adhesive, tape adhesive, chemical form e.g silicate adhesive, epoxy adhesive, materials bonded e.g paper & wood adhesive, metal plastic adhesive, conditions of use e.g cold salting adhesive, hot salting adhesive.
CLASSIFICATION OF ADHESIVE.
Adhesive can be conveniently classified into five main groups based on the chemical type or major ingredients from which the adhesives are formulated.
Vegetable glues made from plant starches are typical examples of natural adhesives. These types are used on postage stamps and envelops. However, such adhesives give bold with poor strength which are susceptible to fungal attack and are also weakened by moisture. They set a result of solvent evaporation.
Electrometric adhesives are based on synthetic rubbers. They also set as a result of solvent evaporation. Strong joints are not produced as they have low shear strength. The adhesive is inclined to creep. They are mainly used for unstressed joints and flexible bonds with plastics and rubbers.
These include a number of different setting types. An important group is that such as polyamides, which are applied hot, solidify and bond on cooling. They are widely used on metals plastics; wood etc. another group is the acrylic acid diesters which set when air is excluded. Cyanoncrylated the “super glue” set in the presence of moisture in the similar way the reaction taking place in seconds.
In general, the thermoplastic adhesives has low shear strength and under high loads are subjected to creep, so they are generally used in assemblies subject to low stresses and has good resistance to oil.
These set as a result of building up of molecular chains to glue a rigid cross-link. Eg. Epoxy resins and polyester resins such as Araddite, are one of the most widely used thermoses adhesives. These are two part adhesion in the setting only starts to occur when the components of the adhesives are brought together. They have good strength and resistance to water, oil and solvents and are widely used for bonding plywood (wood generally).
These are adhesives consisting of composite materials derived from the above three (3) synthetic adhesives. Thermosets by themselves give brittle joints, but combined with thermoplastic or elastomers a more flexible joint can be produced. For example, phenolic resins with brittle or neoprene rubbers have high shear strength, excellent peel strength, good resistance to water, oil and solvents and good creep properties.
In conclusion, adhesives may be classified under two broad headings:
- Natural adhesives
- Synthetic adhesives.
FACTORS CONSIDERED WHEN SELECTING A WOOD ADHESIVE.
Several factors must be considered or taken into consideration in the selection of an adhesive (wood adhesive):
- The wood adhesive must wet th substrate.
- If the substrate are impervious and non-absorbant the adhesive must be free from water or organic solvent.
- Low cost may be needed not only in adhesive itself, but also in the method of application.
- The wood adhesive should be more rigid than the adherents, otherwise stresses will concentrate in the adhesive layer.
- After setting, the wood adhesive joint must be able to withstand environmental conditions; resistance to high temperature, to prolonged stress, to water and to various chemicals and low electrical conductivity may be required depending on the intended use.
ADVANTAGES AND LIMITATIONS OF THE ADHESIVES WITH RESPECT TO MECHANICAL FASTENERS.
Adhesives offer many advantages over other method of joining materials in the sense that:
- Thin films and small particles that could not be joined by other techniques are readily bonded with adhesives. Typical of such products are labels, abrasive wheels, sandpaper etc.
- Dimensional stability of amistropic materials can be improved by cross-bonding as in plywood (wood generally)
- Stresses are distributed over a wider area making possible assemblies lighter than could be achieved by mechanical fasteners.
- Most important of all, in many application is the processing cost.
- The layer of glue acts also as an insulator in electrical equipment a moisture barrier in laminates and corrosion-inhibiting barrier when two dis-similar metals are bonded together.
- Joining can be carried out at room temperature.
- A smooth finish is obtained.
The following are the limitations on the use of adhesive for mechanical fasteners. Although many synthetic resins adhesive have excellent chemical resistance. Some are attacked or dissolved by some substances, in many cases however, they are chemically not encounter.
Optimum bond strength is usually not produced immediately. A curing time has to be allowed. The bond can be affected by environmental factors such as beat, cold and humidity. These adhesives generally cannot be used as temperatures above about 2000c. On some applications, the electrical resistant of adhesives is a disadvantage. Finally, it is difficult to access the quality of the bond because the inside of the joint cannot be seen.
ADHESION AND COHESION.
Adhered in a term generally used to refer to the body held together by an adhesive. The process of attacking one adherent to another by means of anhesive are primary forces resulting from the joining together of atoms to form molecules in making soldered joint or as in bond a sulphur- vulcanisable rubber to metal via-brass-plated surface or copper. These forces of adhesion are van-der-waals forces of attraction which act between the atoms in the two surfaces. Interfacial strengths based on this forces alone, far exceed the red strength of one or other of the adhering materials. As a result, interfacial separation is lively when mechanical forces are applied to separate a pair of materials which have achieved complete interfacial contract.
Van-der-waals forces are operative over small distances; hence for effectiveness of the bond, the atoms in two surfaces must be brought close enough for these forces to become effective this necessitates the coating of adherents with thin film adhesive. Excessively, thick film reduces the holding power of the adhesive and should be avoided for instance, with certain plastic, thin films give joint strength of high magnitude and this falls progressively with the thickness of the film which laten remains approximately constant.
As for good adhesion, the adherends surfaces must be free from dirt, dust, grease, moisture, carrosion products and other contaminants. Surface uniformity should also be ensured. Adequate wetting of the adhered by the adhesive plays an important role in bonding strength. This is as a result of the displacement of air absorbed on the surface of the adherends by a solution of the adhesive and allows adhesion to have fuller play than they would if the materials were cushioned with layers of air.
The time required for an adhesive bond joint to develop strength varies from few seconds to 4hours or higher. While pressure required during bonding varies from full contact between the parts from minimum pressure to 500ps.
Adhesive can be applied by spraying, rolling, brushing, dusting and toweling, setting temperature ranging from room temperature of 250c and above, available in all forms, while structural adhesive usually set as 20 – 750c.
COMPONENTS OF ADHESIVES
BINDER OR BASE.
This is the compact of the adhesive responsible for the adhesion forces which hold the two bodies together. The name of the adhesive is usually derived from the binder for example, starch adhesive and wood adhesive.
THINNERS ON DILUENTS.
These are volatile liquids added to an adhesive to modify the consistency or other properties. They are required for diluting or thinning the composition as might be required for spray application. Hence helps to provide the desired degree of volatility compatible with open assembly condition of use and aid in avoiding busters and blow-ups in subsequent hot pressing.
This is added to reduce the amount of primary binder required per unitarea and this reduce cost of the actual joint.
These are added to improve the permanence of the resultant bonds. These include resorcinol and melanine formaldelyde resins. These are added along the harders during formulation and the time of use.
Starch is the major raw material in the production of the wood adhesive in this research project, and act as a binder, hence the name starch adhesive. It should therefore be of the primary concern to know a lot of facts about it with regards to their sources, chemical and physical properties, application as well as industrial production.
It exist as an accumulated required material in certain plant parts, particularly in tubers as in cassava, potatoes and seeds or grains as in corn (maize) rice etc. they serve as raw materials for its commercial production. The project is only interested in those starches produced from corn and cassava tubers.
COMMERCIAL STARCHES APPROXIMATES DATA AND RANGE
|5 – 12
Starch is used primarily as feed. It is used in industries to produce ethanol and glucose, in textile industries, laundry, dye, wood and paper adhesive industries.
Since starch derived products are used in many industrial products and because the application in these areas is so large, the starch industry is being vigorously researched to produce some synthetic products. Hence in the food industry, there is no doubt that the future for starch-based products is very bright due its availability and cheapness in cost. Products are being developed with may replace some natural gums at a much lower cost and the growth these products means the for more specialized starch compounds.
Summing up, the future looks bright provided the technical efforts continue to expand. With the recognition that new synthetic reagents are not to be made use of , the starch industry has eliminated the obstacle that really threatened its profitable existence.
PHYSICAL PROPERTIES OF STARCH
Pure starch is a soft, shinning powder which is sparingly soluble in cold water. It is more soluble in hot water. Accompanied by swelling, a viscous solution is obtained which does not reduce Fehling’s solution and gelatinizes on cooling (starch paste). It can exist as a white, tesok paste as in cold water, hence the white colour of starch adhesive.
Starch contains 0.022 phosphorous or phosphoric acid which probably plays a apart in enzymatic degradation. The photomicrograph of the starch types show that starch shows characteristics differences in shape, size and structure of the granules, hence easier determination of the origin of such starch. The starch granule essentially consists of radially arranged, needle shaped, very small crystals. The ability to swell is explained by the chain structure.
Different sizes and shapes are associated to various starch granules. Diameter varies from 2 to 150 microns. Rice granules are the smallest of the grains of starch, they have diameter at the average of 3 to 8 microns and they are polygonal in shape. Cassava and corn starch granules have diameter 12 to 25 microns. The former is round or polygonal in shape. Data show that one pound weight of corn starch contains an estimated 800 billion starch grains.
Mullon and Pascu found that the heat of gelatinization of starch varies with the granule size from 5,700 call glucose unit for the relatively large potato starch granules. The value for corn starch 117,100 call glucose unit.
Heat of combination is between 4.183 and 4.228 cal for starch.
HEAT OF COMBINATION TEMPERATURE FOR SOME STARCH SAMPLES
|70 – 80
|59 – 68
|62 – 72
|62 – 78
CHEMICAL PROPERTIES OF STARCH
Starch consists of the two groups of polysaccharide carbohydrate namely: the Amylase and Amylopectin. When starch is heated with water. It smells and gives an oporlesent solution. It is hydrolyzed by acid to dextrins, maltose and finally glucose.
Acid starch Dextrins Maltose Glucose
(C6H10O5)n + nH20 nCoH1206.
Amylase is a linear polymer composed of short chains of 1.4& linkages and gives a deep blue colour with iodine.
Amylopectin is a branched polymer composed of short chains of 1:4 and linked glucose units, the chains being joined by cross linkages, the majority of which are of the 1:6 types. This yield a red violent colour with iodine.
The major components of starch can be enzymatically hydrolyzed in two different ways. Amylase can be hydrolysed and Ø-amylase which is also present in the saliva and pancreatic juice and participate in the digestion of starch in the gestro intestinal test. It hydrolyzes Ø (1-4) linkages to yield a mixture of glucose and free maltose. The latter is not affected.
Under enzymatic reactions, starch can undergo oxidation to yield energy and carbondioxide (Co2). Most starch have a pH between 4.7 to 5.3.
DEGRADATION OF STARCH
Degradation can be affected by heating, bacteria, enzymes or by oxidizing agent. In starch, it can be used to improve the viscosity and adhesion characteristic of starch-based adhesive systems.
When starch is treated with little, at temperature between 170 and 2500c, it change to dextrin (C6H10O3)n n(C6H10O5)
When heated up to 2000c without water, charring with result and if in the presence of air, auto-ignition may result, dextrin can be divided into white and yellow dextrins. British gums depend on this kind of method for its production.
ISOLATION OF STARCH
Starch is commonly extracted by wet method by breaking down the plant cells by grinding and washing with water. Then the extracts are passed through a sieve leaving the other material (fibre) behind. These starch granules are collected and dried under the sun.
GELATINIZATION OF STARCH.
Starch grain can be induced to swell enormously and irresistibly. This change called gelatinization is brought about by hot water. The principle states that the initially thin, opaque starch suspension becomes at certain point viscous, semi-opaque and finally transparent. These are the three changes associated to gelatinization. When the kinetic energy of the water molecule becomes great enough to overcome the attraction between the starch molecule within the granule water molecule can penetrate the starch grains. Thin then causes the granules to swell. Gelatinization temperature for starches from different sources can be seen on table 2.
as the name implies, it is starch that is produced from cassava plant. The compund granules of the starch break up during the process of extraction and purification of the starch, often showing one or more flat surface. The component fragments range 12 – 25A/m and an average of 15a/m. Many of the granules are egg-shaped with one end cut off leaving a concave surface.
Cassava starch granules rapture completely (gelatinizes) between the range of temperature 70 – 800c. It is this property that is put into use during adhesive formulation from cassava starch. Also the high amyloptin content of cassava (75-85) is often desirable interest. This is also responsible for the adhesive action of starch.
COMPOSITION OF CASSAVA STARCH TABLE 3
Maize grain normally contains about 15 – 20% moisture content and about 70% dry basis starch. The maize consists of four distinct parts.
- Crown region.
- Glutinous region.
Other components outside starch and moisture content comprises of protein, oil fibre, mineral matter and water soluble constituents of relatively low molecular weight. The starch is separated from these other components by physical process called mixing. The hydration and disintegration of this protein in order to lessen the starch, the separation of the starch from the finely disposed protein and the accomplishment of the resistant granules in the crown region constitute the chief problem in milling.
COMPOSITION OF MAIZE STARCH TABLE 4
|7 – 7.5
|1.2 – 1.5
|1.2 – 1.3
COMPONENTS OF STARCH ADHESIVES AND THEIR FUNCTIONS
It is very essential to know the components of the starch adhesives and their various functions since it is this components that are varied in different percentages of compositions that help in the various formulation that would be carried out in the research project. These additives are:
- 1. WATER.
In starch adhesive formulation, water is used as the base solvent. It is therefore used to dispersed the starch to a spreadable form. Water is often used for the preparation of paper and wood adhesives. In synthetic adhesive, organic solvents are used such as kerosene etc.
These are components of adhesive forces which hold two bodies together. This research project used a base binder of corn and cassava starch.
3. SODIUM HYDROXIDE (NAOH)
This is a vital addition in the formulation of a base adhesive as it reduces the geletinization temperature of starch. Sodium hydroxide helps to increase the working life of the adhesive. However, it reduces the speed of setting since it becomes less viscous and flows more, thus it enhances spreadability.
4. CALCIUM CARBONATE (CaCo3)
This additive is used as filler in starch adhesive formulation and processing. It aids in the increase of the speed of setting adhesives (tack time). It is relatively a non-adhesive substance added to improve their working properties and strength.
5. FORMALDEHYDE (HcHo)
This substance acts as a preservative to the adhesive. Since starch is easily degradable, especially when in contact with water formaldehyde is needed to prevent or retard decomposition by the effects of micro-organisms either while the adhesive is being stored or during the service life. Other example of preservatives are zinc II chloride mercury etc.
This is the trade name for sodium tetraoxodecahydrate. Borax helps in the maintenance of the strength of an adhesive by increasing its viscosity. It is mainly used when water in the solvent used in the adhesive formulation and acts best in the production of paper and wood adhesives.
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This article was extracted from a Project Research Work Topic “PRODUCTION OF WOOD ADHESIVE”
Production of Wood Adhesive