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Contact Adhesives . A fastest and easy way for Bonding

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What is Contact Adhesive ? …

What is Contact Adhesive ?
A bonding agent that is applied to two surfaces and allowed to dry before being pressed together or
A liquid adhesive which dries to a film that is tack-free to other materials but not to itself. The adhesive is applied to both surfaces to be joined and dried at least partially. When pressed together at light to moderate pressure, a bond of high initial strength results.

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  • 1. Let us understand what is meant by Contact Adhesive and how is it different from Pressure Sensitive Adhesive What is Contact Adhesive ? A bonding agent that is applied to two surfaces and allowed to dry before being pressed together or A liquid adhesive which dries to a film that is tack-free to other materials but not to itself. The adhesive is applied to both surfaces to be joined and dried at least partially. When pressed together at light to moderate pressure, a bond of high initial strength results. What is Pressure Sensitive Adhesive ? PSA is adhesive which forms bond when pressure is applied to marry the adhesive with the adherent. No solvent, water, or heat is needed to activate the adhesive. Pressure sensitive adhesives (PSA) adhere to most surfaces with very slight pressure. Pressure sensitive adhesives form viscoelastic bonds that are aggressively and permanently tacky at room temperature. They adhere with just finger or hand pressure and do not require activation by water, solvent or heat. Now let us know more about Contact Adhesives in this bulletin. Contact adhesive is widely used because of it’s easy application, fast setting properties, and excellent adhesion to many different substrates. The contact adhesive is coated on both surfaces to be joined. After the carrier/ solvent evaporates, the two surfaces are brought together under slight pressure, and the adhesive bonds to itself. A high degree of handling strength occurs immediately, and the joint strength increases further with time. Contact adhesives are available with solvent or water based Technology. The base polymer is generally poly Chloroprene, but Polyurethane, Styrene Butadiene Rubber, Natural rubber , Butyl Rubber and Acrylic polymers are also used. Rubber and elastomers are characterized by their high degree of 1 Contact Adhesive Date : 16 JAN 2014 Page No : PCP 003
  • 2. Bond Strengths Contact adhesives are available in a variety of bond strengths Specifications Specification for Contact adhesives include thermal properties, mechanical properties, and electrical and optical properties . However Service temperature, thermal conductivity, and coefficient of thermal expansion (CTE) are key thermal properties. Tensile strength and elongation are important mechanical properties. Electrical and optical properties for contact adhesives include dielectric strength, dielectric constant, index of refraction, and transmission. Process ability/ Coat ability Temperature and humidity are two important factors that affect the rate at which the cement( Contact Adhesive ) solidifies. High temperatures cause the adhesive to bond more quickly, while low temperatures reduce the rate at which the liquid evaporates, resulting in slower drying. High humidity causes contact cement to dry slowly, while low humidity speeds up the process. When precise placement is important, slower bonding is valuable, and allows plenty of time to position the objects before the cement is fully bonded. High temperatures and low humidity are ideal when time is short. Mechanism of Adhesion For Contact adhesives is based on intermolecular diffusion. Both surfaces are coated for bonding, and the carrier/ solvent is allowed to evaporate sufficiently so that the adhesive film becomes dry (non-tacky) to the touch. When the adhesive coated substrates are brought together auto adhesion occurs. Immediate diffusion of the polymers on each surface occurs across the interface, and the early bond strength is extremely high. After a relatively short period of time, the interface no longer exists and the full cohesive strength of the polymer prevails. 2 flexibility and elasticity (high reversible elongation). Date : 16 JAN 2014 Page No : PCP 003
  • 3. Sufficient drying time must be allowed before bonding to remove about 90% of the solvent or water . Generally for solvent-based contact adhesives the drying time is about 30 minutes under ordinary conditions of temperature and humidity and much longer for waterbased adhesives. Forced drying with heat can reduce this time significantly. After the adhesive is coated and dried on a substrate surface, there is a definite time span, known as the open time, during which diffusion may occur under specific conditions of temperature and pressure. During this open time, the adhesive film is changing in physical properties due to continuing loss of solvent, phase change, and crystallization of the base polymer. After the open time has been exceeded, the nature of the adhesive film is such that diffusion or auto adhesion will not occur and good adhesive properties will not be achieved. Characteristics of Contact Adhesive The contact adhesive can hold a load for substantially longer times than PSAs , because of very high the shear strength and peel strength of contact adhesives Therefore , contact adhesives are generally considered as "semi-structural adhesives" with properties falling between those of conventional structural adhesives (thermosetting epoxies, urethanes, etc.) and non-structural elastomeric pressure sensitive adhesives. 3 For auto adhesion to take place several important factors are involved. The polymer/ elastomer must be capable of diffusion. The degree of diffusion is influenced by the nature of the polymer and the choice of solvent . Therefore, only certain polymers and solvent combinations have auto adhesive characteristics. For maximum diffusion , the substrates must be in intimate contact. This will depend greatly on the elastic stress and plastic flow components of the adhesive composition. Thus, the coating uniformity and rheology of the adhesive composition is influential in achieving good bond strength. Also, the effects of pressure, time, and temperature on the joint area is also very important in achieving optimum diffusion. Date : 16 JAN 2014 Page No : PCP 003
  • 4. As a matter of fact , sometimes difficulties are encountered in working with contact adhesives due to their immediate grab or "green strength" which prevents any repositioning due to misalignment. Contact adhesives, have very good toughness and elongation . As a result, they withstand rapid and wide changes in temperature, moisture, and considerable vibration The chemical and heat resistance of contact adhesives are much similar to structural and non-structural adhesives. The hot strength of the assembled component is generally good, depending on the adhesive formulation. In many cases the hot strength is sufficient to permit post forming of thermoplastic substrates without the adhesive excessively softening or releasing due to the temperature of the process. The chemical and moisture resistance of contact adhesives are generally sufficient for using them in many exterior and interior industrial applications in the building and construction, automotive, and footwear industries. The tensile shear strength, heat and chemical resistance lie between thermoplastic PSAs and structural adhesives. The Advantages and Disadvantages of Contact Adhesive Advantages Immediate joint strength development Easy application by hand and application can be made on-site Very good adhesion to many materials, even non-porous Flexible bonds due to elastomeric base polymer Good resistance from –40°C to +130°C depending on formulation 4 Contact adhesives offer very high bond strengths immediately after the joints are made. The assembled components can immediately undergo additional finishing operations without clamping or fixturing. Date : 16 JAN 2014 Page No : PCP 003
  • 5. Dis advantages Immediate set does not allow for repositioning Generally flammable solvents are required, and some solvents are toxic Waterborne systems have long drying times and somewhat poorer performance than solvent borne systems Not a gap filler (substrates should be smooth and parallel) Critical drying and open times depending on the formulation. Application Methods Contact adhesives are generally applied by spraying, curtain coating, roller coating, or brushing. Spraying is by far the most important commercial application method as it permits rapid coverage of adhesive and minimizes the drying time by breaking-up the adhesive as it is applied. Roller and curtain coating are used to apply adhesive to flat sheets such as the cores and skins of panels. Adhesives with relatively slow drying solvents usually are used with roller or curtain coating methods. The slow drying adhesives are necessary to prevent their drying out or building-up in viscosity on the roller or curtain coater. Solvents are removed by prolonged airdrying or by forced drying with heat. Certain sprayable contact adhesives, known as dry spraying adhesives, lose so much solvent during spraying that the adhesive film is practically ready for bonding immediately after application. This type of adhesive has been used successfully when very fast assembly times are required or when bonding a substrate, such as polystyrene foam, that may be susceptible to solvent. Another way of achieving this effect is to use hot spraying. In this method the adhesive passes through heaters as it travels along the fluid line to the spray gun. When the adhesive leaves the spray gun, the elevated temperature combined with the large volume of air passing over the adhesive tends to evaporate the solvent rapidly. 5 Good aging resistance indoors; aging outdoors will depend on base polymer and formulation Date : 16 JAN 2014 Page No : PCP 003
  • 6. No : PCP 003 Date : 16 JAN 2014 Types of Contact Adhesives Disadvantages Neoprene Rapid strength development High shear strength Good temperature and chemical resistance Limited open time Limited formulation capability SBR Good bond to LSE substrates Properties dependent on styrene to butadiene ratio Less heat and solvent resistance than others Requires tackifier addition Polyurethane Resistance to plasticizer migration Adhesion to many substrates Freedom in formulation Adhesive must be in the amorphous state when substrates are mated Low temperature resistance and chemical resistance Acrylic copolymers Tack and ultimate strength is less than others Good bond to LSE substrates Good environmental resistance Mainly latex emulsions Less moisture resistance than other contact adhesives Contact adhesives based on these polymers are available in either solvent solution or water dispersion. Although the solvent-based adhesives generally 6 Advantages Page Polymer Natural rubber was the first polymer used in contact adhesive formulations, but it has been almost entirely displaced by synthetic polymers. Today, the main polymer used in contact adhesive formulation is polychloroprene. Polyurethanes, acrylic copolymers, and styrene-butadiene copolymers are also used in certain formulations.
  • 7. Polychloroprene By far, the majority of contact adhesives are formulated with polychloroprene (Neoprene) elastomers. Rapid strength development coupled with high ultimate strength is typical of polychloroprene contact adhesives. They also have auto adhesion characteristics, high shear strength, and resistance to oils and chemicals. Selection of the grade of polychloroprene will be based on a compromise between strength, strength development, and open time characteristics. The adhesive properties directly affected by the type of polychloroprene used are: Initial tack, open time Bond strength development Hot bond strength (heat resistance) Application properties and Adhesive viscosity. Polychloroprene elastomers prepared for solvent-based adhesive formulation can be dissolved in a wide variety of polar and non-polar solvents. This provides a range of solution viscosities and drying times as well as specific adhesion to many porous and nonporous substrates. The choice of solvents used in the manufacture of contact adhesive depends not only on the solvent’s dissolving power, but also on its 7 have greater strength and durability, environmental and legislative pressures have encouraged the development of water-based systems. Water borne contact adhesives often have problems with slow evaporation rate, less tack, and microbe buildup in the wet adhesive. Often a small amount of solvent, called a coalescing aide, which is soluble in both the elastomer and water is used to aid in wetting as well as to improve the coalescence of the latex particles. Contact adhesives can be further classified into curing and non-curing types. The curing types have a crosslinked molecular structure and provide greater resistance to heat and chemicals. However, they are generally supplied as two component systems with a limited pot life. In addition to the base polymer and carrier, contact adhesive formulations may include tackifiers, stabilizers, pigments and fillers, and crosslinking agents.The main commercial types of contact adhesives are described below Date : 16 JAN 2014 Page No : PCP 003
  • 8. Polychloroprene contact adhesives have also been formulated from water dispersed elastomers. Polychloroprene lattices have not widely replaced solvent-based contact cements as originally thought. It takes a water borne adhesive much longer to dry, and if force dried the energy used is an added cost. The auto adhesion properties of water-based polychloroprene adhesives provide lower initial strength and make bonding at low pressures difficult. Also the heat and moisture resistance of these adhesives are generally lower than their solvent-based counterparts because of the presence of surface-active ingredients. However, significant development work has occurred over the last several years to minimize these problems. Polychloroprene or chloroprene rubber (CR), introduced by DuPont in 1931, was the first synthetic rubber developed that exhibited the elastomeric properties of natural rubber. The new rubber was given the trade name “Duprene” and then renamed Neoprene. It remains one of the most important specialty elastomers with an annual consumption of 300,000 tons worldwide. The modern chloroprene process is based on the conversion of butadiene into the chloroprene monomer (2chlorobutadiene-1,3). With the aid of free radical initiators, chloroprene in the form of an aqueous emulsion is converted into homo polymers or, in the presence of co-monomers, into copolymers. In the production of solid-grade poly chloroprene, the latex is coagulated, drawn into a thin sheet, and then chopped and dusted to form chips or granules. Poly chloroprene is also produced and sold in latex form, which is used in waterborne adhesives. Structure Key roles in changing the molecular structure of polychloroprene are played by the polymerization conditions (polymerization temperature, monomer 8 evaporation rate. Solvent selection can also influence the rate of crystallization. Generally, the more efficient the solvent and the lower the volatility, the longer the open time for the adhesive. Date : 16 JAN 2014 Page No : PCP 003
  • 9. Another classification of commercially available chloroprenes is the linearity of the backbone. The polychloroprene grades used in solvent-based adhesives are highly linear and soluble. Lowermolecular-weight chloroprenes can be dissolved by simply stirring in solvent, while higher-molecular-weight materials often have to be masticated before easy dissolution is possible. Latex-based polychloroprenes have varying degrees of gel structure and are marginally solvent-soluble. Latex polychloroprenes are often anionically stabilized with rosin acid-based emulsifiers. Applications Some 60% of CR production is used in the rubber industry for products such as molded goods, cables, transmission belts and conveyor belts. About one third of CR production is used as a raw material for adhesives (both solvent-based and water-based). The balance of CR production has different latex applications such as dipped articles (e.g., gloves), molded foam and as a rubber modifier for improved performance in asphalt products. Excellent adhesive properties and formulating latitude at competitive prices vs. other adhesives technologies have contributed to the dominant position of polychloroprene solvent borne adhesives in many market segments. Historically, polychloroprene was used as a replacement for natural rubber, which was in short supply during World War II. However, synthetic polychloroprene demonstrated clear performance advantages over adhesives formulated with natural rubber. Polychloroprene contact adhesives are used for bonding high-pressure laminates, automotive trim, roofingmembrane attachment, furniture, kitchen cabinets, custom display cabinets, interior and exterior panels, wall partitions, shoe soles, and many other applications where quick, high-strength, permanent bonds are needed. 9 conversion, polymerization process), polymerization aids (co-monomers, type and amount of molecular weight modifier and emulsifier) and conditions during finishing. The molecular weight of polychloroprene is a function of the conversion level and initiator content. Date : 16 JAN 2014 Page No : PCP 003
  • 10. No : PCP 003 Date : 16 JAN 2014 In the furniture and related industries, a wide range of substrates must be bonded, including wood; metal, such as cold-rolled steel and aluminium; fabric; paper; foam; plastic, such as polyvinyl chloride, polystyrene, polyethylene, polypropylene and acrylonitrile butadiene styrene (ABS); fiberglass; and materials used to construct high-pressure laminates, for example, for countertops. Essentially all furniture made from a laminate of Formica on a less expensive wood base is fabricated with contact adhesives. Adhesive Properties A significant performance advantage of contact adhesives is auto-adhesion. Polychloroprene contact adhesives provide immediate green strength and high ultimate bond strength for efficient assembly. The substrates are coated with adhesive, allowed to dry and then combined under light to moderate pressure. This results in an instant bond. The mechanism is not a simple diffusion process but diffusion with solubility. Therefore, the residual solvent concentration is an important factor. Sufficient green strength is achieved so that the assembled components can undergo finishing operations without additional clamping or fixturing. In the past, furniture manufacturing has been dominated by one-part contact adhesives dissolved or dispersed in organic solvents, such as chlorinated solvents and low-flash-point organic solvents. Such one-part contact The two main criteria that influence the polymer selection when designing a 10 adhesives are convenient to apply. Recently, however, there has been a desire to shift from organic, solventbased adhesive compositions to aqueous-based or aqueous-dispersed adhesive compositions for environmental, health and safety reasons. Page The U.S. shoe industry was the first major market penetrated by polychloroprene contact adhesives. Although this market has declined in the United States during the past 15 years, the shoe industry worldwide still is a major consumer of polychloroprene adhesives.
  • 11. No : PCP 003 Date : 16 JAN 2014 formula for a contact adhesive are the crystallization rate and viscosity. losing their strength. This can already be noticed at 40 to 50 degrees C, and as the temperature rises, the thermo plasticity increases. If the bonds are simultaneously subjected to a load, this will eventually result in bond destruction. Crystallization Polychloroprene adhesives that have been de-crystallized at elevated temperatures exhibit auto-adhesion. This property is exploited in the production of heat-activated adhesives. Viscosity influences the inherent strength of the adhesive film, the solution viscosity and solids content. The higher the polymer viscosity or molecular weight, the higher the film strength, the higher the adhesive viscosity, or the lower the solids at a given adhesive viscosity. Polarity The polarity of the polychloroprene gives versatility. Polychloroprene rubbers can be dissolved in a wide variety of polar and non-polar organic solvents to provide a range of solution 11 Unlike chemical crosslinking, crystallization is reversible. The crystallization of polychloroprene is temperature-dependent, being very pronounced at low temperatures, while higher temperatures result in a gradual loss of crystallinity. Adhesives that have crystallized at room temperature decrystallize at elevated temperatures, Viscosity Page Crystallization determines the rate of initial strength development. Fastcrystallizing polychloroprene grades give rapid bond-strength development. Crystallinity in a polymer inhibits the diffusion process and hence reduces auto-adhesion. A balance has to be achieved between auto-adhesion characteristics and ultimate mechanical performance.
  • 12. No : PCP 003 viscosities and drying times, all with good solution stability. Good specific adhesion to a wide range of porous and non-porous substrates can be achieved. Open Time Polychloroprene contact adhesives can be formulated to provide a long open time before the adhesive-coated substrates are combined. This time span, commonly referred to as “tack life” or “open time,” is the period within which a satisfactory bond can be formed under contact pressure. Long open times enable assembly operations to pre-coat or stage the preparation of substrates prior to bonding. Date : 16 JAN 2014 polychloroprene rubber, a tackifying resin (most commonly a tertiary-butyl phenolic resin) and antioxidants/acid scavengers, such as metal oxides. Various grades of solid polychloroprene, which generally differ in level of crystallinity, are available for use in solvent-based adhesives. Solvent selection and adhesive solids will vary depending upon end use and method of application. The selection of the grade of polychloroprene and the tackifier allows the adhesives formulator to design products having specific properties, such as long or short tack or open time, controlled development of ambient- and/or elevated-temperature bond strength, and ultimate peel and shear strength. The cured adhesives are generally resistant to heat, oxidation, water, solvents and other chemicals. Solventborne Adhesive Formulations Solvent-based polychloroprene contact adhesives are formulated using a Solvent blends are normally used in polychloroprene contact adhesives. Nonflammable chlorinated solvents, once popular, have been replaced due to environmental restrictions. The choice of Page The shoe industry is a major consumer of polychloroprene adhesives. 12 Solvent Selection
  • 13. No : PCP 003 Date : 16 JAN 2014 solvents or a solvent mixture used in the manufacture of adhesives depends not only on their dissolving action but also on evaporation rate. Non-solvents are often added to solvents that dissolve polychloroprene without impairing the dissolving action of the resulting mixtures. The actual rate at which an adhesive dries is also influenced by the solvent retention of the formulation as well as the absorption and permeability of the substrates to be bonded. PROPERTIES OF COMMONLY USED SOLVENTS FOR NEOPRENE RUBBER 58.08 84.16 88.10 100.20 86.70 60.09 72.10 72.15 92.13 106.16 * SOLUBILITY PARAMETER 10.00 8.20 9.10 7.40 7.30 11.50 9.30 7.00 8.90 8.80 7.60 Antioxidants and Acid Acceptors Another common characteristic of adhesive formulations based on polychloroprene is that they require the use of antioxidants and acid acceptors. The adhesives are commonly modified RELATIVE EVAPORATION RATE 1160 720 615 386 1000 300 572 2860 240 63 275 FLASH POINT 15 40 45 25 -25 70 35 < -50 44 80 48 by the incorporation of magnesium oxide and zinc oxide to inhibit dehydrochlorination. An example of this is the use of zinc oxide, which is often used in a dispersed form in 13 ACETONE CYCLO HEXANE ETHYL ACETATE HEPTANE HEXANE IPA MEK PENTANE TOLUENE XYLENE P. NAPHTHA MOL. WT Page SOLVENT
  • 14. No : PCP 003 Historically, the metal oxides were milled into the polychloroprene to ensure a fine dispersion that would stay suspended in the adhesive. With the direct dissolving process that is mainly used today, they have to be dispersed differently. Generally, these products have to be used in a dispersed form, which has the overall effect of reducing the stability of the formulation and increasing its cost. The use of metal oxides with extremely small particle size Resins Resins are also used in polychloroprene contact adhesives, the most common being the alkyl phenolic resins. These substances have little effect on the final crystallizing of polychloroprene but improve adhesion characteristics. They are normally added at about 10-50 phr. Tertiary-butyl phenolic resins improve both the auto-adhesive and the oxidative stability of the adhesive. The free methylol groups on the resin react with the magnesium oxide present to form a high-melting-point resinate that also improves the heat resistance. Adhesive formulations containing these resins have high initial strength, are rapidsetting and have excellent final bond 2 Magnesium oxide and zinc oxide are both acid acceptors for trace amounts of hydrogen chloride evolved during aging. The presence of metal oxides improves the storage stability of the adhesives and the resistance to aging of the bonds. Magnesium oxide will also prevent scorching during mastication. Magnesium oxide and zinc oxide are generally used at about 4 parts per hundred parts rubber (phr) of each. makes incorporation easier. Pre-reacting the magnesium oxide with reactive alkylphenolic resins can also improve the incorporation into the adhesive. Alternatively, the metal oxides can be dispersed in a master batch (containing the metal oxides at about the 10% to 20% level) that is milled into the polychloroprene. Page Polychloroprene-based formulations. The zinc oxide serves to act as an acid acceptor and as a crosslinking agent with carboxyl functional groups to improve heat resistance. Date : 16 JAN 2014
  • 15. As alternatives to the alkyl phenolics, terpene phenolic resins, rosin-modified phenolic resins, rosin esters, and hydrocarbon and coumarone resins can be added to improve the tack and adhesion. Coumarone resins and rosin esters with a low melting point can improve the auto-adhesive characteristics but only by sacrificing heat resistance and oxidative stability. Less tack but greater strength is obtained with terpene phenolic resins, rosinmodified phenolic resins and rosin esters with a relatively high melting point. The aging properties of bonds produced from polychloroprene adhesives are influenced by the resins incorporated. Resins sensitive to oxidation, such as rosin esters and coumarone resins, cause embrittlement or softening in adhesive films within a relatively short period of time. These resins must therefore only be used in combination with suitable antioxidants. It is likewise advisable to use antioxidants when terpene phenolic resins are employed. Alkyl phenolic resins have no undesirable aging effect on bonds obtained with polychloroprene adhesives. The thermal stability of bonds can be considerably improved by adding a polyisocyanate crosslinker to the adhesive. The crosslinking of the adhesive reduces its thermoplasticity. The addition of polyisocyanate crosslinker not only raises the thermal stability but also improves the cohesive strength and in many cases significantly increases the adhesion to certain substrates while improving solvent resistance. Polyisocyanates must be added to the adhesive solution immediately before use. Resins reacting with polyisocyanate, such as phenolic resins, should be avoided. The pot life of an adhesive to after-addition of a polyisocyanate crosslinker is usually several hours before the reaction proceeds to the point where gelling occurs. The pot life depends largely on the composition of the adhesive solution. Good heat resistance is important in footwear for tropical countries, in flooring materials in the vicinity of 3 strength, but their tack range is limited compared to other resins. Date : 16 JAN 2014 Page No : PCP 003
  • 16. Polychloroprene adhesives are used to attach roofing membranes. Waterborne Adhesive Formulations Historically, water-based systems were sensitive to operating conditions. As a result, their bonding properties were inconsistent. The stability of polychloroprene latices in combination with other commonly used ingredients necessary in adhesive compositions has been a general problem. In recent years, latex compositions with improved stability have become available. They are generally based on polychloroprenemeth acrylic acid copolymers stabilized with partially hydrolyzed polyvinyl alcohol. Many factors have limited the use of waterborne adhesive formulations. They have a major disadvantage, namely the slow evaporation of the water. It takes a waterborne adhesive much longer to dry at ambient conditions than a solventbased adhesive. Attempts to provide them in one-part form have achieved only gradual industry acceptance since they have had longer dry times than conventional, solvent-based adhesives, as well as a relatively slow rate of bondstrength development. Quick drying of a latex-based system can only be achieved by an investment in drying equipment. The contact bond properties of polychloroprene latices are not as good as solvent-based polychloroprene adhesives in several respects. The difference in auto-adhesion properties gives lower initial bond strength and makes bonding at low pressures difficult. The contact ability of the adhesive films is often insufficient after 4 radiators or dark flooring materials that are exposed directly to sunlight, and in laminated kitchen counters (hot pots and pans, vicinity of stoves). Heat resistance is also important in many automotivetrim applications that must maintain bond integrity through heat-age cycles (often in excess of 80 degrees C). Date : 16 JAN 2014 Page No : PCP 003
  • 17. Adhesive Strength To overcome such limitations, two-part aqueous-dispersed adhesive systems have been developed that demonstrate high adhesive strength within seconds of spraying. The adhesive composition is one part of the two-part system with an external coagulant as the second part in a predetermined ratio. Coagulants such as citric acid, lactic acid, acetic acid or zinc sulfate are typically used. The two parts are combined in a spraymixing process that involves the addition of the coagulant at the spray head. Both parts are conveyed separately into a spray gun and become mixed in the spray. Since the components only come into contact with each other in the spray, there is no need to be concerned about pot life. Heat Resistance The heat resistance of adhesives based on polychloroprene latices is generally lower than solvent-based adhesives because of the presence of surface-active ingredients used to emulsify the latex, tackifiers and other compounding ingredients. Also the high heat resistance achieved with solventborne CR contact adhesives through the addition of an alkylphenol-magnesium chelate complex is not possible in water-based systems. New grades of homopolymer polychloroprene latex have recently been introduced with increased chain crosslinking that are relatively low in crystallinity and exhibit improved heat resistance. When compounded with a terpene phenolic tackifier, they give a high softening point comparable to that of solventborne polychloroprene adhesives. Peel Strength Many water-based adhesives formulated from polychloroprene latices do not have good initial peel strength. Adhesives based on polychloroprene latex with increased chain crosslinking blended with low-crystallinity polychloroprene 5 drying. Waterborne systems lack the crystallization-inhibiting and tackifying effect of a solvent remaining in the film. Heat activation prior to bonding is often necessary. Date : 16 JAN 2014 Page No : PCP 003
  • 18. No : PCP 003 Date : 16 JAN 2014 latex containing increased chain branching have demonstrated both improved heat resistance and excellent peel strength. short time. Bond strength and heatresistance properties comparable to isocyanate-crosslinked solventborne systems can be achieved. Improving Properties by Crosslinking Improving Properties With Acrylics Many of the commercially available water-based contact adhesives are based on polychloroprene latex and acrylic/vinyl acrylic type emulsions. Blends of polychloroprenes with soft vinyl acrylics produce desirable synergistic effects. The acrylate dispersion provides some initial tack and also a destabilizing effect that accelerates coagulation. Two-part waterborne adhesives utilizing the spray-mixing system have improved the application and efficacy of waterborne adhesives. However, they are not entirely satisfactory. The co- 6 Status of Waterborne Systems Recent developments in polychloroprene homopolymer and copolymer latices with varying molecular structure have demonstrated improved performance approaching that of solventborne systems. Page Increasing the heat resistance of waterborne systems to the level achievable with two-component, crosslinked, solventborne CR adhesives is another problem recently addressed. Aromatic polyisocyanates and hexamethylene diisocyanate (HDI) trimers are unsuitable for crosslinking alkaline polychloroprene latices because they cause coagulation of the latex. With the availability of dispersions of isophorone diisocyanate (IPDI) trimer, a monomeric cycloaliphatic diisocyanate, a sufficiently long pot life without coagulation has been demonstrated in a waterborne system. In combination with polychloroprene latex with a significantly higher amount of hydroxyl groups in the polymer chains, crosslinked adhesives can be made. The use of such isocyanates allows a crosslinking reaction to take place at room temperature within a relatively
  • 19. Polyurethane Urethane polymers for solution contact adhesive are generally prepared from bulk polymerization of difunctional hydroxyl compounds with difunctional isocyanates. These polymers are usually supplied in bead or granular form and can be dissolved in common ketonic solvents. Like polychloroprene, polyurethane resins are available with a wide range of crystallization, solution viscosity, and cohesive properties. Polyurethane contact adhesives can be applied similarly to polychloroprene contact adhesives. Polyurethane adhesives can also be applied as a heat sealable contact adhesive. The adhesive is exposed to temperatures (70 deg -90ged C) above the crystalline melting point before joining pressure is applied. While the adhesive is in the amorphous state, the substrates are mated and the adhesive on each substrate diffuses into the other. Strength develops as the adhesive recrystallizes. This type of adhesive and processing are commonly used in the footwear industry. Polyurethane adhesives are often chosen over polychloroprene adhesives when greater resistance to plasticizer migration is required or difficult to bond polymeric substrates are involved. One of the largest applications is in footwear where high bond strengths to PVC compositions are required. Acrylic Copolymers Contact adhesives have been developed based on acrylic emulsions. These emulsions provide high bond strength to various surfaces, good environmental resistance, high strength, and relatively fast setting times without having to use a solvent system with resulting environmental and safety restrictions. Acrylic emulsions are prepared by conventional emulsion polymerization processes. A blend of several monomers 7 spraying equipment is expensive, the equipment requires maintenance and the ratio of the two parts (the coagulant and the adhesive composition) must be monitored during application. Thus, there is a need for a true, one-part, aqueous-based, fast-setting contact adhesive. Date : 16 JAN 2014 Page No : PCP 003
  • 20. No : PCP 003 Contact Adhesive Formulations FORMULATION NO: 1 ADHESIVE BASED ON GRAFT POLYMERS OF NEOPRENE AND METHYL METHACRYLATE Material 1 2 3 4 NEOPRENE AD- G METHYL METHACRYLATE TOLUNE MEK Sub Total Quantity 20.000 20.000 80.000 20.000 140.000 SLOWLY HEAT THIS SOLUTION IN A CLOSED REACTOR BY INDIRECT HEAT / OR ON A WATER BATH , TO 60 deg C ADD BENZOYL PEROXIDE TO INITIATE POLYMERISATION as follows. 5 BENZOYL PEROXIDE 50% DISP CONTINUE HEATING TO 80 deg C ADD 1.000 8 Styrene Butadiene Rubber (SBR) SBR elastomer is a random copolymer of styrene and butadiene formed through emulsion polymerization. As with polychloroprene, SBR can be coagulated and re dissolved in a solvent or used as a latex. The application and end-use properties are very much dependent on the ratio of the styrene to butadiene along the molecular chain. Styrene butadiene polymer systems have relatively poor auto adhesive characteristics and require compounding with a tackifying resin to provide a contact adhesive. High strength is achieved by strong intermolecular forces and entanglements. However, SBS polymers are susceptible to oxidative degradation, especially when stressed. An antioxidant, such as a hindered phenol, is necessary for bond durability. Because of their good solubility in many different solvents, SBR contact adhesives have been used in applications where the substrate may be sensitive to a certain type of solvent, such as the bonding of polystyrene foam. In this application SBR contact adhesives are used where the solvents are either short chain linear hydrocarbons or alcohols. Page is used to achieve the desired copolymer characteristics such as glass transition temperature, flexibility, ultimate strength, and autohesion. They are often the same acrylic emulsions that are used in pressure sensitive adhesives or sealant formulations. Date : 16 JAN 2014
  • 21. No : PCP 003 6 A/O WINGSTAY L TO TERMINATE THE REACTION A/O WNGSTAY L Total Date : 16 JAN 2014 FORMULATION NO: 3 2.000 143.000 FORMULATION NO: 2 NEOPRENE CONTACT ADHESIVE Material 1 2 3 4 NEOPRENE - AD MgO ZnO A/O Quantity 20.000 1.600 1.000 0.400 23.000 Bayprene 320 Bayprene 233 MgO ZnO Heat Reactive Alkyl Phenol Resin Sub Total Solvents Ethyl Acetate White Spirit Boiling Point 65/95 deg C CycloHexane MEK Sub Total Total 10.000 10.000 1.400 0.600 20.000 42.000 7.000 14.000 28.000 20.000 69.000 111.000 Mix the above on a Rubber Mixing Mill t- BUTYL PHENOLIC RESIN WATER 9.000 0.200 add Solvent as follows 7 TOLUENE + HEXANE+ ACETONE Add Solvent in the Ratio of 2 Parts/ 4 Parts / 4 Parts 120.000 Total 152.200 Alternative for Neoprene Rubber SBC 's is a alternative to substute Chloroprene Rubber in Solvent borne Contact Adhesives. Advantages of SBC over Chloroprene Narrow mol weight distribution than CR No pre mastication required Lower viscosity 9 5 6 23.000 Page Mixed Stock as above
  • 22. No : PCP 003 Excellent solubility in a wide Range of organic solvents Thermoplastic nature Excellent cohesive performance Excellent Tack Date : 16 JAN 2014 variety of additional applications where quick, high strength, permanent bonds are required. Contact Adhesive Applications Page Contact bonding is preferred when bonding two nonporous surfaces since most of the solvent or carrier is removed from the adhesive film during the open time period. Any number of materials can be bonded with contact adhesives. Contact adhesives can be used for furniture, kitchen cabinets, custom display cabinets, interior and exterior automotive trim, roofing membraneattachments, and a wide 10 There are five principal markets for contact adhesives. These are footwear, automotive, do-it-yourself (DIY), building and construction, and furniture. Contact adhesives are generally used in applications where the type of substrate is likely to vary (decorative laminates to wood for countertops, shoe compositions, etc.) and where fast setting speeds and high ultimate strength are required.
  • 23. No : PCP 003 Date : 16 JAN 2014 How much cement should be applied Now Just to Sum up How is contact cement applied By brush, roller coater, notched trowel, air spray (hot or cold), airless spray (hot or cold ), curtain coater. How soon should bonding be done This may be done as soon as solvent is completely gone. If you can press a finger to the surface and pull away cleanly and with no sign of “legginess” the surface is dry. If there is any doubt at all, give it a few extra minutes. If any puddling has occurred, be sure the puddles are dry. After dryness has been reached, you have an hour to make the 11 Contact cement is an adhesive, which, when coated on two mating surfaces and allowed to dry, provides an instant permanent bond when brought into firm contact. Page What is a contact cement Enough to coat the surface uniformly and thick enough that, when completely solvent-free, a tacky, some what resilient film is left.
  • 24. No : PCP 003 Date : 16 JAN 2014 bond. This is called “open time”. soaking-in porous core materials. How is drying accomplished Drying is accomplished by ordinary air exposure, use of fans, passage under infra-red bulbs, or passage through a heated tunnel. It all depends on the production speed desired. What is the solids percentage of a contact adhesive The solid portion of the adhesive is dry adhesive content in it. The solvent is only a carrier to provide an easy method of application. Therefore, adhesives with a higher solids percentage generally contain more usable adhesive per Kg. Does temperature affect the drying time of contact adhesives Can any precautions be taken to prevent “blush” Two courses of action are possible:  What does viscosity mean Viscosity is a measure of the thickness of the adhesive: the higher the viscosity, the thicker the adhesive. Spray adhesives have low viscosity to allow proper atomization by the spray gun. Brush adhesives have a higher viscosity to provide easy spreading and prevent  Install a drying heat tunnel and use it for all panels immediately after the adhesive is applied. This will also decrease the drying time and increase productivity. Apply the adhesive by the hot spray method. Using this method, the adhesive is applied at about 135 degrees F and the cooling 2 Does humidity affect the drying time of contact adhesives When humidity is low, drying is accelerated. When it is high, drying is slower. Page Drying takes place more slowly at lower temperatures and is faster at higher temperatures. What is “blush” “Blush” is moisture condensing on freshly applied adhesive due to the rapid solvent evaporation cooling the air below the dew point.
  • 25. No : PCP 003 Date : 16 JAN 2014 effect of solvent evaporation is not sufficient to cause “blush.” What are the prime causes of bond failure   3  Coating too thin (not enough glue) Bonding too soon Not enough pressure Exposure to high heat Page 