Report on adhesive
The first adhesives were natural gums and other plant resins or saps. It was believed that the Sumerian people were the first to use it until it was discovered that Neanderthals as far back as 50,000 years made adhesives from birch bark. The finding of 6000-year-old ceramics brought astounding evidence to archaeologists about the first practical uses and ingredients of the first adhesives. Most early adhesives were animal glues made by rendering animal products such as the horse teeth. The eastern United States (or the Native Americans) used a mixture of spruce gum and fat as adhesives to add waterproof seams in their birch bark canoes. During the times of Babylonia, tar-like glue was used for gluing statues. Also, Egypt was one of the most prominent users of adhesives. The Egyptians used animal glues to adhere furniture, ivory, and papyrus. The Mongols also used adhesives to make their short bows. In Medieval Europe/Eurasia, egg whites were used as glue to decorate parchments with gold leaves. Holland, in the early 1700s, founded the first ever glue factory. Later, in the 1750s, the British introduced fish glue. As life and the modern world evolved, several other patented materials, such as bones, starch, fish, and casein, were introduced and was come to know as alternative materials from glue manufacture. Modern-time glues have improved beyond recognition. Such improvements are noticeable in its flexibility, toughness, curing rate, temperature, and chemical resistance. The bond between two items depends on the shape of the adhesive.
From the earliest day( the materials that we, later called cements, glues, gum, mucilage, resins, pastes, and finally adhesives were used interchangeably.
Historically, the first thermoplastic synthetic adhesive was the cellulose ester, cellulose nitrate, often called nitrocellulose, and it is still one of the most important.
There is little agreement in the literature about the dates when various adhesive were first development or used in a specific application. This is due to simultaneous developments in many parts of the world and the fact that references in the literature are almost exclusive from the more developed countries.
Synthetic rubber, a dimethylbutadine, was developed as a substitute for natural rubber and saw limited use as an adhesive but today SBR adhesives are the most important adhesive in the USA. Neoprene rubber adhesive are available as both thermoplastic and cross-linking systems in both solvent and emulsion formulation. Neoprene rubber is the major base resin for contact adhesives.
Most of the thermoplastic resins were soluble in organic solvents and were used as solvents adhesives for different materials. PVC, a thermoplastic developed in 1927, is used today in solvent formulation to bond PVC article such as coated fabrics, films, foams and pipe.
PVC was used as a solvent-based adhesive in the 1930s, and later as a hot melt, but was not of commercial importance until its introduction in the 1940s as an emulsion adhesive. Today, in emulsion form as a white glue, it is the most widely thermoplastic adhesive worldwide. Today acrylic adhesive appear in many forms; as both pressure sensitive and non-pressure sensitive formulation in organic solvents and emulsion forms.
Polyurethane adhesives are polymers produced by addition reactions between polyisocynates and hydroxyl-rich compounds (at least two hydroxyl groups per molecule) such as glycol, polyesters, polythene. Today, PU adhesives are available as solvent-based moisture adhesives, thermoplastic, hot melts, thermosetting systems and emulsions.
Today even in the most developed countries, natural adhesives dominate the market because they are less expensive than synthetic based materials, and they perform the intended function. Natural rubber is still the most widely used based materials in pressure sensitive adhesives. Natural rubber solvent solution adhesives are widely used throughout the world as general purpose-adhesives.
Adhesion began to create interest in scientific circles only about 50 years ago. At the time adhesion became a scientific subject in its own right but still a subject in which empiricism and technology are slightly in advance of science, although the gap between theory and practice has been shortened considerably.
An adhesive may be defined as a material which when applied to surfaces of materials can join them together and resist separation. This definition was proposed not normally considered as adhesives such as mortar and solder. There are other substances which show the phenomenon of adhesion; these include paints and printing inks.
In the phraseology of adhesive technologists a material possesses tack when it will wet another surface and the two different films will combine with each other in such a way that the initial interfaces will be completely lost and a considerable force is required to rupture the film. May sticky substances are not worth while adhesives because the solid film has no great cohesion strength and may be ruptured by a very small separating force so that it is incapable of holding two components together. In terms of the substances involved the principal component of an adhesive is an organic polymer, or one or more (usually two) compounds which can chemically react to produce a polymer.
CONCEPT OF ADHESIVE: An adhesive is a compound that adheres or bonds two items together. Adhesives may come from either natural or synthetic sources. Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry.
CONCEPT OF ADHESION: The adhesion phenomenon is relevant to many scientific and technological areas and has become in recent years a very important field of study. The main application of adhesion is bonding by adhesives, this technique replacing, at least partially more classical mechanical attachment such as bolting or riveting. Adhesion began to create real interest in scientific circles only about 50 years ago. At that time adhesion became a scientific subject in its own right but is still a subject in which empiricism and technology are slightly in advance of science, although the gap between theory and practice has been shortened considerably.
In fact the term “adhesion” covers a wide variety of concepts and ideas, depending on whether the subject is broached from a molecular, microscopic or macroscopic point of view. Adhesion is therefore ambiguous, meaning both the establishment of interfacial bond and the mechanical load required to break an assembly.
THE PRINCIPLE OF ADDITION: At the time of application the adhesive must be a liquid, as this enables it to make intimate molecular contact with the adherents; that is it must wet the surfaces. It must then harden (cure) to a cohesive solid, pressure sensitive adhesives are an exception in that they do not harder, but remain permanently sticky.
In simple terms, materials are held together by attractions (‘magnetic forces’) between the atoms and molecules from which they are built up. The same forces might reasonably be expected to cause materials brought into contact to stick together.
Unfortunately, just as a toy magnet will only attract a pin at close range, intermolecular forces only operate over very short distances (one millionth of a mm!), so the necessary close contact is never achieved even with highly-polished surfaces.
MECHANISMS OF ADHESION
There are two main mechanisms by which an adhesive (cement) sticks to a material. These are referred to as Mechanical Adhesion and Specific Adhesion.
This is the more common of the two, being effective to some extent in almost all examples of bonding. Many materials have visibly rough surfaces,
or have a fibrous structure which is porou, close examination shows that even the smoothest surfaces contain microscopic pores. When adhesive is applied in liquid form to the surface, some of it flows into these pores. After drying, the adhesive layer will be `keyed’ to the material surface rather like two pieces of a jig-saw puzzle are joined together.
The effectiveness of the bond will depend on the strength of the material, and on the size, depth and shape of the pores. Deep and undercut pores will lead to stronger bonds than shallow indentations.
On a molecular scale, bonding occurs when adhesive molecules diffuse into and become intertwined with the molecules in the material surface. For this to happen the attractive forces between adhesive molecules and material molecules must be at least as strong as the attraction of adhesive molecules for each other. i.e. the adhesive is more specific in what it will bond to.
The diffusion process is helped by the presence of solvents in the adhesive which swell or partially dissolve the material surface. Heat performs a similar function when using hot melt cements, and when causing one cement film to coalesce with another after reactivation.
In another type of specific adhesion the molecules in the cement become bound by strong chemical bonds to molecules in the material. For this to happen there must be specific chemical structures present in the two bonding surfaces. Frequently the material surface is made chemically reactive to the cement just before spreading by applying a special primer. An example is the use of a halogenations agent on thermoplastic rubber before applying PU cement.
Two materials may form a compound at the join. The strongest joins are where atoms of the two materials swap (ionic bonding) or share (covalent bonding) outer electrons. A weaker bond is formed if oxygen, nitrogen or fluorine atoms of the two materials share a hydrogen nucleus (hydrogen bonding).
Water droplets are flatter on a Hibiscus flower which shows better adhesion.
Five mechanisms have been proposed to explain why one material sticks to another:
In dispersive adhesion, also known as adsorption, two materials are held together by van der Waals forces: the attraction between two molecules, each of which has a regions of positive and negative charge. In the simple case, such molecules are therefore polar with respect to average charge density, although in larger or more complex molecules, there may be multiple “poles” or regions of greater positive or negative charge. These positive and negative poles may be a permanent property of a molecule (Keesom forces) or a transient effect which can occur in any molecule, as the random movement of electrons within the molecules may result in a temporary concentration of electrons in one region (London forces).
Some conducting materials may pass electrons to form a difference in electrical charge at the join. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials.
Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and soluble in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. It is also the mechanism involved in sintering. When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.
WHAT MAKES AN ADHESIVE BOND STRONG?
The strength of the adhesion between two materials depends on which of the above mechanisms occur between the two materials, and the surface area over which the two materials contact. Materials that wet against each other tend to have a larger contact area than those that don’t. Wetting depends on the surface energy of the materials. Well-known examples of adhesion are tape, glue, stickers, and rubbing dirt on golf balls.
TERMINOLOGY DEFINITIONS AND GLOSSARY OF TERMS
These are materials used for attaching by means of an adhesive. Shoe uppers, soles, toe puff etc. are a few examples for adherend or substrates.
A substance which speeds up a chemical reaction during the curing up of an adhesive bond.
A material responsible for improving a few properties of an adhesives and also bringing down the cost.
A chemical to an adhesive to slow down the chemical reaction.
These chemicals in an adhesive prolong the storage life.
Chemicals when added to an adhesive promote storage life.
An ingredient chemically inert but changes the property of an adhesive without taking part in the reaction. These may be fillers thinners, plasticisers, stabilisers or wetting agents. Plasticizer:
Plasticizer in an adhesive reduces the viscosity & brings down the temperature required for heat reactivation, increases theflexibility of an adhesive.
It is a substance which increases the activity of an adhesive. Tackifier:
Tackifier resins are of low molar mass & have a higher glass transition temperature than the base polymer. They used in pressure sensetive adhesive & hot-melt adhesive.
Also known as deluent or extenders, a volatyl liquid added to an adhesive to modify the consistency or working properties. Stabiliser:
A substance which increases the resistance to light, heat, radiation & so on.
The expected storage life in unopened containers. It is determined by the stability of the adhesive at the storage temperature to chemical and physical changes.
With 1-part cements it refers to the maximum usable life after opening the container. With solvent cements it is almost impossible to reseal containers efficiently and solvent will be lost by evaporation more or less slowly depending on volatility. It can of course be replaced, but if this is not done the liquid becomes too thick and lumpy to reconstitute effectively. Rate of solvent loss when a container is in use depends on its dimensions. In general the pot life will be longer if the container is narrow and deep rather than wide and shallow. There are applicators available designed to reduce evaporation to a minimum. Some adhesives are affected by exposure to atmospheric moisture.
With 2-part cements, pot life refers to the maximum usable life after mixing the two components. The mixing initiates a chemical reaction which results in the adhesive eventually becoming solid. Rate of reaction depends on the chemical nature and amount of ‘hardener’ used, temperature, and total amount mixed. In general the greater the proportion of hardener and the higher the temperature the faster is the reaction and the shorter the pot life. The reaction produces heat. The greater the amount mixed the harder it is for the heat to escape and the higher the temperature rises. Consequently the faster the reaction proceeds, the shorter the pot life.
This is the time required for a spread cement film to lose sufficient water or solvent for a successful bond to be made. It applies mainly to water or solvent-based cements using a two-way dry stick method. The times are normally given for natural drying (at room temperature) but can be reduced considerably by passing coated components through hot air tunnels. The higher the temperature and the better the air circulation the faster the drying rate. Drying times are shorter for films on porous materials like leather and fabrics than on non-porous materials like PVC, VR, TR etc. The thicker the film the longer the time Failure to allow sufficient drying time will result in bonds with poor green strength and likely to ‘spring back’. With hot melt cements, there is no drying time because there is no solvent. For liquid curing adhesives, drying time refers to the time taken for the film to develop sufficient tackiness (spotting tack) for efficient bonding using a minimum press time.
As a general rule the shorter the drying time the shorter is the pot life but the more cost-effective is the bonding process.
This is the time after drying that a cement film retains enough tackiness for a bond to be made by applying pressure alone to the components. Dry tack of many adhesives can be increased by the addition of various resins.
The pressure-sensitive life of cement films is reduced by the presence of dust in the air. Much of the cements have no pressure sensitivity once dry.
Tack retention time (reactivation life or tack life):
This refers to the maximum time after drying that a cement film can be successfully reactivated by heat for good bonding. For some time after the pressure-sensitive life of an open film has expired, tackiness can be recovered (or generated) by heating the surface of the film to the correct activation temperature. Tack life is reduced by dust and ageing, and can sometimes be extended wiping the film.
This term is ‘used by some people to mean tack life. Here it refers to the actual time which elapses between applying the adhesive and making the bond in the shoe factory. It depends on the work organization. Maximum open time should not exceed the tack life of the cement.
This refers to bond strength developed when two cement films are lightly pressed together. It should be sufficient for the components to remain in place while they are transferred to the press; but not so high that the components cannot easily be re-positioned if necessary before applying full pressure.
This refers to the joining of two cement films. If coalescence is not efficient (due to expiry of tack life or failure to heat activate properly) the cohesive strength of the bond will be poor.
This is the minimum time for which the components must be under pressure in the bonding press.’ It is determined by the rate of set-up.
This is the recommended pressure to be applied to the particular combination of materials and adhesive in. the press to achieve the best results. It controlled by the coalescent properties of the cement and the compressive properties of the materials e g. Expanded solings should not be subjected to as high a pressure as harder solid soling because of the risk of permanent deformation. Up to a point, the higher the pressure the stronger the bond.
Rate of set-up
This is the speed with which green strength develops in the press. It is influenced mainly by the chemical and physical properties of the adhesive system. For dry stick methods the set-up should be rapid to reduce “press time. With wet stick ‘methods the set-up needs to be slow to allow components to be positioned correctly after making the bone (e.g. stiffener attachment and socking).
THE MAIN CLASSIFICATION OF ADHESIVES:
Adhesives may be classified in many ways. One of the ways adhesives can be classified is by the manner in which they harden. This can be by loss of solvent, loss of water, cooling or chemical reaction. Once hardened the polymer in an adhesive can be linear or cross linked. All structural adhesives are cross linked.
Other than this, adhesives may be classified by mode of application and setting, chemical composition, cost and suitability for various adhered and end products. They are:
3. Radiation curable, and
4. Pressure sensitive adhesives.
Adhesives can be also classified according to their origin:
1. Natural adhesives: Starch, animal and vegetable proteins, natural rubber and shellac.
2. Semi synthetic: Cellulose materials, phenolic resins, polyamide, PU from castor oil.
(a) Vinyl type: Polyvinyl alcohol, acrylic, unsaturated polyester, NBR, SBR, Chloroprene, butyl rubber.
(b) Condensation & other: Polyurethane, phenol formaldehyde urea-formaldehyde etc.
Adhesives can be also classified based on their performance,
1. Temporary work
2. Permanent work.
Based on the hardening adhesives types are described here
1. Adhesives which harden by loss of water! Solvent based types:
The adhesive is basically a solution of natural or synthetic polymer in organic solvents, which is applied to both surfaces to be bonded. Some time is allowed for the solvent and the surfaces are then press tighter. Polychloroprene (Neoprene) adhesives are prominent examples. Here are three types commonly used in leather goods making:
(a) Rubber solutions
(b) Polychloroprene (Neoprene) solutions
(c) Polyurethane solutions.
2. Adhesives which harden by loss of water/Water-based adhesives: This adhesive is basically a solution of water, which is applied to the surfaces to be bonded for temporary purposes as stitching is required for the perpetual use of the product. Those which are commonly used in leather making:
(a) Latex solution
(b) Vegetable paste
(c) Synthetic lattices
3. Adhesives which harden by cooling
4. Adhesives which harden by chemical reaction
5. Adhesives which do not harden-pressure sensitive adhesives
DESCRIPTION OF THE DIFFERENT TYPES OF ADHESIVES:
1. Homemade Casein adhesives
Is an adhesive prepared at home, or by one’s own efforts with normal household products. There are many types of glue that can be made. Unlike commercial glue, for example Elmer’s glue, homemade glue can have the ability to stick or hold better depending on the ingredients that are used. It is best when the glue is kept in an airtight container in the refrigerator. See more at: Homemade glue
2. Natural adhesives
Adhesives were developed by a kid scientist named Daryl from inorganic mineral sources, or biological sources such as vegetable matter, starch (dextrin), natural resins, animal skin, and bioadhesives. A simple paste can be made by mixing flour and water.
3. Synthetic adhesives
4. Drying adhesives
These adhesives are a mixture of ingredients (typically polymers) dissolved in a solvent. Glues such as white glue, and rubber cements are members of the drying adhesive family. As the solvent evaporates, the adhesive hardens. Depending on the chemical composition of the adhesive, they will adhere to different materials to greater or lesser degrees. These adhesives are typically weak and are used for household applications. g for use by small children are now made non-toxic.
5. Contact adhesives
Contact adhesive is one which must be applied to both surfaces and allowed some time to dry before the two surfaces are pushed together. Some contact adhesives require as long as 24 hours to dry before the surfaces are to be held together. Once the surfaces are pushed together, the bond forms very quickly, hence, it is usually not necessary to apply pressure for a long time. This means that there is no need to use clamps, which is convenient.
6. HOT MELT ADHESIVE
A hot glue gun loaded with a glue stick
Hot melt adhesive (or hot glue) is a form of thermoplastic adhesive that is commonly supplied in solid sticks designed to be melted in a special gun. The glue comes in cylinders of various lengths, and is pushed into an electric hot glue gun. The gun contains a heating element to melt the plastic glue, which operates all the time the gun is plugged in. Squeezing the trigger pushes the stick through the heating element, ejecting molten glue which is initially hot enough to burn and blister skin. The glue is tacky when hot, but hardens and stops being sticky in a few seconds—a minute at most.
However, the most common application for hot melt glue is industrial. Hot melt can be found in many products that we purchase each day.
Hot Melt Adhesives Importance of hot melt adhesives has grown steadily in shoe construction as an alternate bonding, method to solvent and solvent-free adhesives specifically in shoe lasting. These are of cent percent solid content and are free from solvents. The availability of a variety of polymers with a broad spectrum of properties tailored for a specific end use is an added advantage. To-day hot melt formulations are available for temporary as well as permanent attachment in shoe manufacture.
Hot Melt Adhesives are usually supplied to footwear industry in the form of granules and rods (in coils). When these adhesives are heated to higher temperatures, they melt rapidly into a fluid, flow freely on bonding sites, wet the surfaces, quickly cool, solidify to a crystalline solid and thereby adhere to give good bond strength between the two surfaces.
Performance of Adhesive Bond is controlled’ by – Quantity of adhesive – Temperature of application
– Open time (the time interval between the application of adhesive on first surface and bringing the second surface to come into contact with the former surface).
– Pressure applied on the surface of a material.
To establish effective bond strength between any two surfaces, the adhesive should posses
– Low melting point
– Wetting characteristics
– Low viscosity
– Cohosive strength
– Adhesive strength
– Resistance to moisture
Hot Melt Adhesive Formulations are products of a blend of
– Base polymer
(Provides cohesive or internal strength of the adhesive)
– Tackifying resins
(Responsible for reducing viscosity, wetting the surfaces and increasing the open time)
(Decreases viscosity, controls setting time and by softening prevents blocking of nozzles, and lowers the cost)
(Contribute to wetting, adhesion and flexibility)
– Antioxidant and stabilizers
(Provides colour stability, develops resistance to charring, and guards against U V radiation).
Base Polymers of hot melt adhesives used in shoe construction usually consists of polyamides, polyesters, ethylene- vinyl acetate and cellulosic polymers.
Polyesters Polyesters are synthetic resins having ester linkages in the main chain. These are produced by the reaction between dihydric alcohols and dicarboxylic acids.
Preparation of adhesive In the preparation of polyester hot melt adhesive, methyl esters of terephthalic acid and isophthalic acid are mixed with 1,4 -butanediol and are heated to about 140°C in an atmosphere of nitrogen, lead peroxide
Is added, stirred and heated at 140°C. Displaced methyl alcohol is collected. The temperature is increased to 220°C to distill butanediol and the heating is continued to, ?600C. The molten mass is extruded as rod cement through a die into cold water where it solidifies, then rolled on to .pools and ready for use.
Polyester adhesive for shoe lasting At the time, of lasting, the adhesive rod is heated to a temperature above the melting point and deposited on the substrate The adhesive softens at 180°C melts at 200°C and when deposited on the substrate has a temperature ‘of 120°C. At the same time the wiper plates pushes the upper on to the insole with the molten adhesive in between. The molten copolyester has low surface tension and viscosity which is responsible for the molten adhesive to wet flow and spread on the surface, in between the substrates. The molten adhesive is super cooled, solidifies into a oriented crystalline state forming a very strong bond between the surfaces.
Adhesive for out sole attachment hot melt adhesives are also used for the sole attachment to the lasted uppers. Depending upon the composition and concentration of various ingredients the adhesive is intended, either for leather soles or -m-leather soles the adhesive should have low viscosity set rapidly and activated at a low temperature. Low viscosity enables the behaviour of flowing adhesive to wet the surfaces lasted margin of uppers). Low sole activation temperature enables the operator to handle the soles, safely without burning his Inrgers. The adhesive should set rapidly in between the upper .end the sole. Rapid setting time prevents the sole lagging, away from the upper after the shoe is taken out from sole, a1tMChing press.
7. PRESSURE SENSITIVE ADHESIVES
Pressure Sensitive Adhesive (PSA) The Adhesive is spread uniformly over a backing material usually on one side and occasionally on both the sides depending upon the bond requirements. The bond between the adhesive and the substrate is achieved by light pressure applied by hand or figure.
Such adhesives are soft, permanently tacky and exhibit a balance between adhesive and cohesive strength.
The performance of adhesive depends on surface tack, peel, and adhesion and shear resistance.
Composition of adhesive consists of :
– Natural rubber, acrylates, SBR,
– Tackifying resins
8. THERMAL ADHESIVE
Thermal adhesive is a type of thermally conductive glue used for electronic components and heatsinks. Thermal adhesive is similar to thermal paste in that it is used for transferring heat from one surface to another, it usually consists of two separate parts that are mixed together and applied to the surfaces to be bonded. This is commonly used to bond heatsinks to motherboard chipsets and video card processors where there are no mounting holes to clamp a heatsink down.
Thermal adhesive is usually applied by the manufacturer of the part to which it is applied, although it is for sale to the public.
9. NATURAL RUBBER/LATEX
Natural rubber is a product existing as a milky substance known as LATEX obtained from “Heveabrasiliensis”. Latex is a collodial dispersion of rubber particles (hydrocarbons) in water. In addition there are traces of proteins, fatty acids, resin like substances and mineral salts acting as emulsifiers, antioxidants, accelerators providing adhesive properties.
Adhesives of natural rubber are prepared by
– dispersing latex in water (water based latex)
– mixing in suitable solvents – by grafting with a polymer.
These adhesives usually find their use in shoe manufacture mostly for temporary attachment before stitching of various components. The commercial names being water based latex and solvent based rubber solution.
Water Based Latex Natural rubber is centrifuged to get 60% concentrated latex. A solution of ammonia in water is prepared and added. The concentration of ammonia depends on the material to be attached. Depending upon the end use, casein and or phenolic resins are also added. Such adhesives are used for outsole channel closing of welted constructions, outsole attachment to the upper where the sole is stitched to the upper, top line binding, upper to lining attachment and so on.
Non-curing cement for temporary bonding may consists of natural rubber latex, ammonia caseinate solution and zinc oxide solution of 60%, 10% and 60% concentrations respectively.
Rubber Solution is prepared by milling latex, together with the compounded rubber (crepe or smoaked sheets) and dissolving the same in a solvent such as benzene or gasoline. A resin tackifier is also used. Rubber solution is used for establishing temporary bonding for edge folding, upper to lining attachment and sock lining attachment to insole and so on.
Modified Rubbers: Latex is modified by grafting with acrylic monomers (e g.) methyl methaciylate. This graft copolymer is lightly milled and dissolved in solvents. This formulation could be tried for establishing good bond between varieties of soles with leather uppers.
Formulations for the establishment of a permanent bond are ported. Various ingredients for such a formulation may -i of natural rubber latex with 60% solid content, n of potassium hydroxide, zinc oxide, sulphur, sulphur an antioxidant, resin emulsion and ammonia caseinate solution.
Natural Rubber: Adhesive which has curing properties is prepared from natural latex 60% (solids), 10% potassium hydroxide solution, 60% zinc oxide, 6% sulphur dispersion, antioxidant, 40% resin emulsion and 10% ammonia caseinate soultion.
Latex Adhesive : (Reduced solvent or solvent free): The adhesive formulations are prepared with a view to reducing the solvent content or eliminate the solvent completely as suggested by Arendth. These adhesives use plasticizers which perform the properties similar to the solvents, provides the viscosity response, improve wetting and normally affect adhesive application properties favourably. The plasticizers are Benzoate ester plasticizers.
10. POLYCHLOROPRENE ADHESIVE
Polychloroprene is a synthetic elastomer with many of the properties of natural rubber. These are prepared in number of grades depending on the crystallisation rate.
Adhesives of polychloroprene also known as neoprene adhesives and have the advantages of
– film flexibility
– high bond strength,
– easy handling
– bonds quite a number of materials
– application through a brush or spraying
– resistant to deterioration caused by chemicals, oils and heat.
In addition to the above advantages the adhesives
– readily crystallizes even at room temperature
– posses high polarity
– dissolve in a number of solvents
– can control tack retention time
Ingredients used in polychloroprene adhesives are
– antioxidants – solvents
– cross linking agents.
11. POLYURETHANE ADHESIVE
The chemistry of POLYURETHANES has revolutionized the science and technology of shoe manufacture as an alternate material to the natural product namely leather. The contribution of polyurethane to shoe industry is tremendous. To day one is able to fabricate a full shoe out of polyurethane. Polyurethanes are used for shoe uppers, shoe soles, adhesives and shoe finishes.
Polyurethane: Polyurethane (PU) is produced when a diisocyanate having two isocyanate groups is reacted with a diol having two hydroxyl groups.
Polyurethane Adhesive: Various ingredients used in the formulation of PU adhesive are
(a) Polyols having a molecular weight more than 1000. The polyols may be a polyether, polyester or polybutadiene based.
(b) Diisocyanate : This is either toluene diisocynate (TDI) or methylene bis (4,4′- phenyl isocyanate) (MDI)
(c) Curing agent: The curing agent may be a short chain did or trio for MDI and an aromatic diaminer for TDI.
Polyols: Polyols are classified into polyether diols, polyester diols and polycarbonate diols.
Polyether polyols are addition products derived from cyclic ethers (ethylene oxide or propylene oxide), pylene oxide), using proper initiators such as water, alcohols or amines. Polyester polyols are produced by reacting di-functional alcohol or tri-functional alcohol with carboxylic acids.
Polyols are hygroscopic, because of this the ingredients used in adhesive manufacture are kept dry. If necessary, moisture absorbing chemicals or chemicals to prevent hydrolysis of polyols are added.
LEATHER PRODUCTS PREPARATION AND USED ADHESIVES:
Adhesives form an important part of leather product chemicals in general and shoe chemicals in particular. Adhesives used in leather goods are for two purposes-Temporary and Permanent Joint. Most of the leather goods require temporary joint while assembling; and they are stitched later to be permanent for the end use. So water based latex and rubber solutions serve the best result for this purpose. Other kind of products is made only by using adhesives; so permanent joint is achieved with it. Solvent based polychloroprene and polyurethane serve the best result in this respect as they are permanent types.
Natural rubber makes strong adhesives which have proved their suitability time and again for shoe trade operations; but of the synthetic materials can give as strong an adhesion and are not as susceptible to be attacked by grease and oxidizing agents.
SURFACE PREPARATION AND BONDING
In order to obtain good adhesion and bond strength between the materials, a few procedures are followed before, during and after the attachment of the two bonding surfaces. These procedures in steps being Surface preparation Coating of the surfaces by adhesive Drying of the adhesive coating Attachment of surfaces together Setting.
Surface Preparation Surface preparation is pretreatment carried out on the bonding surfaces before the application of an adhesive. The bonding surfaces are generally uneven contains contamination like oil, dust and chemicals. Further, during lasting pleats are formed by the upper on the lasted margin and these are more prominent across the toe region. Non-uniform spreading of cements between the insole and the upper results an uneven surface.
During the pretreatment, depending on the surface of the material, surfaces are subjected to mechanical or chemical or both mechanical and chemical treatments.
Surface Preparation Consists of
– Scouring the surfaces using 24 to 40 grit energy paper..
– Roughening of the surfaces using 24 to 36 swg wire brush.
– Dusting and cleaning.
– Solvent wiping.
One or more above operations are required depending on the nature of the material or the substrate. During scouring or roughening, light touch of the material with rotating wire brush or emery paper is required to prevent heat generation.
Adhesive application: Polyurethane adhesive (one or two coats) as suggested by the manufacturer is applied. PU adhesive when applied on the lasted margin of the uppers ensures good bonding even on 3 mm of unroughed surface of the uppers. After the solvent is evoparated, a coating of PU is given. It takes 10-15 min. for the solvent to evoparate. Then another coating of PU adhesive is given and allowed to dry. The adhesive on the uppers are heat reactivated at about 110°C under infrared radiation for 10-15 sec. before attaching Use soles.
Sole to upper Bonding: The soles are correctly positioned on the lasted uppers and compressed on sole attaching press.
Design of adhesive joints
A general design rule is a relation of the type: “Material Properties > Function (geometry, loads)”
The engineering work will consist in having a good model to evaluate the “Function”. For most adhesive joints, this can be achieved using fracture mechanics. Concepts such as the stress concentration factor K and the energy release rate G can be used to predict failure. In such models, the behavior of the adhesive layer itself is neglected and only the adherents are considered.
Failure will also very much depend on the opening “mode” of the joint.
Modes of failure
- Mode I is an opening or tensile mode where the loadings are normal to the crack.
- Mode II is a sliding or in-plane shear mode where the crack surfaces slide over one another in direction perpendicular to the leading edge of the crack. This is typically the mode for which the adhesive exhibits the higher resistance to fracture.
- Mode III is a tearing or antiplane shear mode.
As the loads are usually fixed, an acceptable design will result from combination of a material selection procedure and geometry modifications, if possible. In adhesively bonded structures, the global geometry and loads are fixed by structural considerations and the design procedure focuses on the “material properties” of the adhesive (i.e. select a “good” adhesive) and on local changes on the geometry.
Increasing the joint resistance is usually obtained by designing its geometry so that:
- The bonded zone is large
- It is mainly loaded in mode II
Stable crack propagation will follow the appearance of a local failure.
Failure of the adhesive joint can occur in different locations
Other types of fracture
Beside these two cases, other types of fracture are
- The “mixed” fracture type which occurs if the crack propagates at some spots in a “cohesive” and in others in an “interfacial” manner. “Mixed” fracture surfaces can be characterized by a certain percentage of “adhesive” and “cohesive” areas.
- The “alternating crack path” fracture type which occurs if the crack jumps from one interface to the other. This type of fracture appears in the presence of tensile pre-stresses in the adhesive layer.
- Fracture can also occur in the adherent if the adhesive is tougher than the adherent. In this case the adhesive remains intact and is still bonded to one substrate and the remnants of the other. For example, when one removes a price label, adhesive usually remains on the label and the surface. This is cohesive failure. If, however, a layer of paper remains stuck to the surface, the adhesive has not failed. Another example is when someone tries to pull apart Oreo cookies and all the filling remains on one side. The goal in this case is an adhesive failure, rather than a cohesive failure.
Surface Preparation Shoe uppers: Materials used for Shoe uppers are 1.Leathers, 2. Textiles, 3. Polyvinyl Chloride (Solid PVC, Expanded PVC and PVC coated fabrics).4. Polyurethane (Expanded PU and PU coated fabrics) 5. Rubbers.
Leathers Pretreatment on lasted shoe uppers consists of roughening of the material on the lasted margin using rotating wire brush. The grain layers with the finish as well as the grain layer are removed until the corium layer is exposed. At the same time caution is exercised that, when the sole is bonded, roughened surface should not be seen. An Inadquate roughening lead to bond failure, whereas, strong roughening may cause mechanical failure, peels the material on the lasted margin. For vulcanizing construction, the upper roughening extends over the feather edge as, the moulds grip the shoe around the side of the shoe. When soles are attached by the Direct Injection Moulding (DIM), same roughening procedure is followed.
Fat Content in Leather Adhesion The bond strength between the leathers as well as leathers with other materials depends on the fat content in leathers. The fat in leather is tested for fatty acid content or grease content. Depending upon the fat content either neoprene based adhesive or polyurethane adhesive is selected. The fat content is to be limited to less than 7% for grease and 2.5% for fatty acids when neoprene based adhesive is used. In the case of polyurethane adhesive these figures are 11% for grease and 4% for fatty` acids.
Leather soles Sole leather is prepared by vegetable, inning. Soles are attached to the upper on flesh side. Sole surface on flesh side is roughened until contamination is moved and fibrous bundle is exposed. The surface is loaned by removing dust. The fat content in sole leather layer an important role in deciding adhesion between sole and upper. The fat content is kept within the limits as mentioned above.
Coating of adhesive: Proper adhesive selection for leather sole to leather upper is neoprene based adhesive i.e., phychloroprene cement, as this is cheap and establishes good bond strength. The lasted margin of the leather upper is
given a coating of neoprene adhesive by a brush. The leather sole is, also given a coating of neoprene based adhesive either by a brush or by spraying. Since leather is a porous materials sufficient quantity of adhesive should be applied as poor adhesion may result in the failure of soles. A second Coat of adhesive over the first coat is also given after drying.
Drying of Adhesive: The solvent in the adhesive evoparates. The bonding surfaces (upper and bottom/soles) should be attached only when sufficient solvent evoparates. Then the adhesive on the lasted upper and the soles are heat reactivated by exposing them to infrared heat radiation50°C to 800C. The adhesive film develops tackiness. The sole is accurately positioned on the bottom of lasted shoe. The shoe M positioned in the sole attaching press and pressed under correct pressure (usually 60-80 lbs/Sq. in.) for 5 to 30 sec.
Poly Vinyl Chloride (PVC) PVC is used in both shoe tippers and shoe soles. Shoe uppers are available as PVC coated fabrics and solid PVC.
Surface preparation Lasted uppers of coated PVC fabric sin solvent wiped. Scouring could be attempted on solvent resistant uppers. Solid or cellular PVC uppers are also pretreated by wiping the solvent. Scouring may weaken the uppers.
PVC Soles PVC soles are manufactured by injection moulding. The plasticizer in PVC soles gradually migrates to the surface. When it comes in contact with the adhesive, the adhesive becomes soft resulting in sole failure. The surfaces of the unit soles are prepared by:
– degreasing with trichloroethane followed by roughening and solvent wiping (or)
– roughening the surface without any solvent wiping (or)
– removing the surface contamination by roughening and then solvent wiping.
Coating with adhesive: A thin uniform coating of adhesive is applied over the lasted margin of PVC uppers and also on the soles and dried. A second coat of adhesive is also given over the first coat after drying. The adhesive on the uppers and the soles are dried. The adhesive on the uppers and soles are heat reactivated under infrared heat radiation. As the PVC soles soften at higher temperatures, the heat should be concentrated on to the adhesive for specific time.
Sole failures are quite common with neoprene based adhesives. As such, polyurethane adhesive is preferred. Alternately it is reported in literature, a formulation consisting of neoprene and chlorinated rubber adhesives give better results with good bond strength.
Polyurethane (PU) Uppers Polyurethane uppers are also known as poromerics (leather like materials). They are available as PU coated fabric and solid PU. Surface preparation on uppers consists of scouring until the base fabric is exposed. The material is scoured up to 3mm from the feather edge to prevent peeling back of the upper luring wear.
Polyurethane soles: PU soles are manufactured using injection moulding process. These are called moulded unit Soles are also directly attached to the lasted uppers by injection moulding (DIM process). During the sole fabrication, sole releasing agents are used. This results in silicone contamination on the soles. Again the soles are coloured either by dipping in lacquor solutions or spraying. This lacquor spray usually extends over the lasted margin.
The surface preparation/pretreatment of PU soles or uppers .ire carried as follows:
– Vapour degreasing of soles are preferred to increase the adhesion of lacquor to the soles.
– The silicone and lacquor contamination on the bonding surface are got rid off by roughening.
– PU rigid soles are heavy. Roughening may weaken the cellular structure. They are primed by wiping with a mixture of an isocyanate and adhesive.
– The surface of lasted upper is cleaned by Metheyl Ethyl. Ketone or a suitable solvent.
Thermo Plastic Rubber (TPR) Soles: Pre-treatment of TPR soles consists of roughening on 1 to 2 mm wire brush and cleaning. The sole surface is then treated by chlorination using 2% haloginating solution giving a uniform coat by a brush. The soles are dried for 20 min, polyurethane adhesive is applied by an automatic machine or a brush. To increase thermal stability, a hardner may be used; but not necessary for an adhesive having a heat resistance of 60°C. Sole surfaces are dried for the solvent to evaporate and then heat reactivated (70°-80°c). The lasted upper is attached to sole by pressing at a pressure of 2 to 4 Kg./cm2.
Styrene Butadiene Rubber (SBR) The sole surface is scoured, cleaned and halogenated. Soles are wiped by a solution of isocynate, dried and attached to the upper using single component PU adhesive as quickly as possible, otherwise the adhesive weakens.
Soles of ethylene vinyl acetate (EVA): Two types of EVA soling materials are available as
– Cross linked microcellular material
– Thermoplastic material
The surfaces of EVA soles get contaminated due to the migration of plasticizer which drastically reduces adhesion upon ageing. Different sole preparations and the adhesive applications are suggested, by any one of the following methods
– The bonding surface is roughened and polychloroprene adhesive is applied.
– The surface is wiped by MEK solvent, polychloroprene adhesive is applied.
– The surface is roughened, wiped by isocyanate solution and bonded with single component polyurethane adhesive.
– Rough the surface and bond with two component polyurethane adhesive.
Thermoplastic EVA: soles are processed by injection moulding. The surface is pre-treated and adhesive is applied as follows:
– The soles are heated in alcoholic alkali.
– Washed in water and dried.
– Primed with dilute solution of isocyanate
– Polyurethane adhesive is applied.