EVA热熔胶配方设计

Jan 10, 2007>> All articles Formulating Ethylene Vinyl Acetate Hot Melt Adhesives Edward M. Petrie, Member of SpecialChem Technical Expert TeamIntroductionKey Material Properties Required for Hot Melt AdhesivesFormulation of EVA Hot Melt SystemsAdvances in EVA Hot Melt AdhesivesIntroductionHot melt adhesives are increasingly becoming popular because of their fast setting speed, relatively low cost (materials and processing), and low environmental impact. Hot melt adhesive systems also have broad formulation latitude so that a multiplicity of products can be produced for specific end-uses and substrates.Primary hot melt applications are high-speed applications such as packaging, attaching labels and wood veneer, bookbinding, and textiles. Hot melt adhesives can also be manufactured in various forms including pellets, slugs, and blocks for bulk application or stick and continuous rope forms for heated gun applicators. Films are also available for high area assembly and continuous laminating.Hot melt adhesives are primarily made from thermoplastic polymers including ethylene vinyl acetate (EVA), block copolymers such as styrene butadiene styrene (SBS) or styrene isoprene styrene (SIS), polypropylene (atactic), polyethylene, polyamide, polyester, and polyurethane. Of these, EVA is the most popular due to their high versatility.EVA resins are highly flexible products, compatible with many other polymers and additives, and easy to process. They have high cohesive strength and excellent adhesion to a wide range of substrates. EVA copolymers can be used in soft, permanently tacky pressure sensitive adhesives or in tough rigid hot melt compositions used for semi-structural applications. Table 1 identifies the major advantages and limitations associated with EVA based hot melt adhesives.Broad formulating latitude necessary for many differentapplications and adhesion to a wide variety of substratesCold flow (creep)Quick setting - no cure or fixturing required Attacked by some greases, oils, and solventsRetention of properties at low temperatures High viscosity needed for maximum performanceNo water or solvent removalLow equipment and operating costsPressure sensitive systems can be formulatedMany formulations have approval for food contactRelatively low costTable 1: Advantages and Disadvantages of EVA Based Hot Melt AdhesivesThis article first reviews the key materials properties that are required generally for hot melt adhesives. The formulating principles and potential product possibilities available with EVA hot melt adhesives are then reviewed, and starting formulations are offered for specific applications.Key Material Properties Required for Hot Melt AdhesivesHot melt adhesives facilitate fast production speeds because the adhesives set simply by cooling from a molten liquid to a cohesive solid. Hot melts do not require processing time for evaporation of solvents or water carrier or for a chemical reaction to occur. This also generally results in lower energy and maintenance costs.Hot melt formulations contain polymers to provide strength and hot tack, and tackifiers and oils or plasticizers to reduce melt viscosity, adjust glass transition temperature, and improve wetting. Some formulations also contain waxes to speed setting and further reduce viscosity. These additives are generally high concentrations (comparable to the base polymer) in hot melt adhesive formulations. Fillers are also used to increase melt viscosity to reduce excessive penetration into porous substrates and to modify cohesive properties. Antioxidants are generally added to prevent thermal oxidation during processing and service. Additives often consist of blends of two or more materials to optimize certain properties. A typical EVA hot melt formulation for packaging is indicated in Table 2.EVA copolymer 20-50Tackifying resin 20-50Synthetic wax 0-20Plasticizer 0-20Filler 0-20Antioxidant 0.1-1.0Table 2: Starting Formulation for an EVA Hot Melt Packaging AdhesiveIn order to obtain adequate strength and heat resistance from a nonreactive formulation, hot melt adhesives generally require some component to separate out into a dispersed but interconnected hard phase network on cooling. For adhesives based on styrene block copolymers, the hard phase consists of glassy styrene domains. For adhesive based on ethylene copolymers, waxes, or olefinic copolymers, the hard phase consists of organic crystallites (Figure 1).Figure 1: Crystalline and amorphous regions in a polymer structureFigure 2 illustrates the mechanical spectrum of amorphous and crystalline polymers. The glassy region is indicative of a brittle polymer; in the flow region the polymer is in its melt form and lacks strength. The rubbery region is indicative of high strength and viscoelastic deformation to absorb stress and impact.Figure 2: Mechanical spectrum of two hot melt polymers1Because hot melt adhesives are applied in melt form and achieve their bond strength on resolidification on cooling, there are two important physical properties: glass transition temperature and melt temperature. These will vary significantly depending on the type of base polymer used and the additives or modifiers present in the formulation.Generally, most hot melt adhesives and sealants have a glass transition temperature, Tg,below room temperature. The melt temperature, Tm, should be low enough to conveniently provide for application but not so high as to result in a safety or fire issue. Most hot melt adhesives have a Tm so that typical application temperatures are 150-200°C, and they become fully solid at temperatures below 80°C.The viscosity of the hot melt at elevated temperature is also an important criterion. The hot melt must be capable of flow to be applied and wet the substrate, but it must not have such a low viscosity so as to flow out of the processing equipment or joint. The viscosity in the melt form will also determine how the product can be applied (Figure 3).Figure 3: Viscosity of molten hot-melt adhesives as a function of temperature2Viscosity of hot melt adhesives or sealants is generally not measured in centipoises. The viscosity at elevated temperature is usually present as a melt flow index (MFI), which is the weight of polymer that can be extruded from a nozzle at a given temperature, pressure, and time. Polymers with a high MFI have low viscosity at elevated temperatures. The melt flow index test method is specified in ASTM D1238. The melt viscosity of most polymers is an exponential function of the molecular weight, but the relationship is not simple.The hot melt system must achieve a relatively low viscosity when in the molten state to achieve wetting, but it must not cool too rapidly or it will not have time to completely wet the molecular roughness of the substrate. Fast cooling can be the result of applying the hot melt to a substrate with high thermal conductivity (e.g., metals) or polymers, which crystallize rapidly. The problem of achieving high bond strength with metals can be resolved by preheating the metal prior to applying the adhesive. This will provide a longer time for the adhesive to be in the melt form and wet the substrate. The problem of rapid crystallization with certain polymers must be balanced against the higher cohesive strength that usually results from these systems.The semicrystalline polymers generally exhibit a greater temperature range for application and a higher modulus at elevated temperatures as was shown in Figure 2. This is due to the pseudo-crosslinking characteristics of hydrogen bonding. Amorphous polymers would require significant molecular weight and molecular entanglements to have properties equivalent tosemicrystalline polymers. This would result in an unrealistically high melt flow index for the amorphous materials. Therefore, the majority of hot melt adhesives and sealants are formulated from semicrystalline polymers, although amorphous polymers are often used as modifiers and additives.The application, performance, and cost properties of hot melt adhesives and sealants can vary significantly depending on the base polymer and the specific formulation employed. Typical properties of several common fully formulated hot melt adhesives are shown in Table 3.Softening point, °C 40 100 --- ---Melting point, °C 95 --- 267 137Crystallinity L L H H or LMelt index 6 2 5 5Tensile strength, psi 2750 2000 4500 2000Elongation, % 800 300 500 150Cost L to M M H LTable 3: General Comparison of Common Hot Melt Adhesives3Formulation of EVA Hot Melt SystemsLike other adhesive formulations, hot melt adhesives require a delicate balancing of constituents relative to the performance and processing properties. The adhesive components as well as the base polymer play a very significant role in the formulation process. The major components of a hot melt adhesive or sealant include base polymers, tackifiers, processing oils and waxes, filler, antioxidants, and UV inhibitors.•The base polymer is the molecular backbone of the systems, and it is used to provide the inherent strength and chemical resistance as well as the applicationcharacteristics.•Tackifiers are added to improve initial adhesion and to modify the base polymer.•Processing oils and waxes are used to adjust viscosity and set times. Both tackifiers and processing materials will affect the Tg and Tm of the final product.•Fillers are used to fine tune certain properties such as melt viscosity, thermal expansion coefficient, set time, etc.•Antioxidants are used to provide oxidation resistance - more for the polymer in the application state rather than in the final joint.•UV inhibitors are to provide stability against exposure to light.The selection of formulation components is determined primarily by the chemical compatibility of the components both during the formulation stage and during the service life stage. Optimal properties and performance can only be exhibited by a formulation where all the componentsare compatible.Certain thermoplastic resins do not require modification by way of additives to achieve good hot melt adhesive properties. These are primarily polyurethanes, polyamides, and polyesters. But the remaining thermoplastic resins will require compounding with the additives mentioned above.Ethylene vinyl acetate (EVA) copolymers are perhaps the most widely used base polymer in general-purpose hot melt adhesives. The material is essentially a random, amorphous copolymer with regions of crystallinity. Melt viscosity is very dependent on molecular weight for this material. Melt flow indices ranging from 2 to 200 are possible. Suppliers of EVA resins for adhesive systems along with information regarding their technical properties can be found in the SpecialChem4Adhesives polymers database.EVA copolymers can be made in continuous bulk, solution, or emulsion processes. The emulsion process produces resins with vinyl acetate content greater than 60% and is more appropriate for water-based adhesives than for hot melt systems. These high vinyl acetate resins are more usually described as vinyl acetate ethylene (VAE) copolymers.Generally, for hot melt adhesives a resin with vinyl acetate concentration of 18-40% is utilized. The vinyl acetate content can be a significant parameter in varying the properties of the adhesive. The materials with high vinyl acetate concentration exhibit reduced crystallinity and increased polarity. The physical property changes as a function of vinyl acetate content is shown in Table 4. At about 50% vinyl acetate content, all crystallinity is lost. Recrystallization rate or setting speed is greatly influenced by the choice of specific EVA resin.Stiffness modulus DecreasesSurface hardness DecreasesCrystalline melting (softening) point DecreasesTensile yield strength DecreasesChemical resistance Decreases (generally)Impact strength (especially at lowIncreasestemperatures)Optical clarity IncreasesEnvironmental stress crack resistance IncreasesCoefficient of friction IncreasesRetention of mechanical strength at high fillerIncreasesloadingsCompatibility with other polymers, resins,Variableetc.Dielectric loss factor IncreasesCompatibility with polar resins andIncreasesplasticizersSpecific adhesion IncreasesSurface printability IncreasesTable 4: Changes in Physical Properties of EVA Due to Increasing Vinyl Acetate Content4The higher vinyl acetate copolymers provide better adhesion to polar substrates such as vinyl, aluminum, and steel, while the lower vinyl acetate copolymers are often used for bonding low energy surfaces. Table 5 illustrates the effect that vinyl acetate content has on the adhesion of EVA to a number of substrates.Kraft paper Little effectGlassine paper Little effectWood Little effectABS Slight trendAluminum Major improvementSteel Major improvementPlasticized vinyl Major improvementRigid vinyl Major improvementPolypropylene Major improvementHigh density polyethylene Major improvementTable 5: Effect of Vinyl Acetate Content in EVA Copolymer Hot Melts on Adhesion to VariousSubstrates5EVA resins normally used in hot melt adhesives have a vinyl acetate content less than 40%. However, a new product line, Levamelt, from Bayer provides vinyl acetate contents in the40-80% range. This polymer backbone consists of fully saturated methylene units with acetate groups attached. Thus it is referred to as a EVM copolymer. Levamelt is particular well suited for pull-off protective film such as that used for automobiles, furniture, and liquid crystal displays.Melt index or melt viscosity is another important criterion in choosing the correct EVA resins for adhesive formulations. Low melt index EVA grades provide high viscosity, strength, and hot tack. In contrast, high MI grades enable higher polymer content and low application viscosities. Mid-range MI grades provide formulation flexibility.EVA resins are considered to be relatively safe to use in food applications. Certain adhesive manufacturers offer EVA hot melt formulations that are approved for indirect food contact (FDA 21 CFR.175) applications. This allows the adhesive to be used not only in food packaging but also in food cooking and other food preparation applications.EVA resins exhibit miscibility in the melt with a wide range of modifying resins, tackifiers, and waxes. This provides the adhesive formulator with a wide latitude of compounding possibilities. Often a modifier resin is incorporated along with the EVA copolymer to provide specific adhesion characteristics and substrate wetting. Common modifying resins include:•Wood rosin derivatives (adhesion to vinyl and other plastics),•Hydrocarbon resins (adhesion to Kraft paper),•Polyterpene resins (adhesion to metals and plastics),•Low molecular weight styrene and styrene copolymers (adhesion to metals at elevated temperatures), and•Phenolic resins (improved temperature resistance and adhesion to wood).Tackifiers are added to EVA copolymers to reduce viscosity and improve wetting. These include C5 - C9 hydrocarbon resins, polyterpenes, and rosin esters of pentaerylthritol and glycerol. Waxes are added to lower cost and reduce viscosity. Fillers such as calcium carbonate lower cost and increase viscosity. Antioxidants are needed to protect the adhesive during application and service life. The idealized formulation ranges for an EVA hot melt adhesive was shown in Table 2.Specific EVA hot melt formulations for certain bonding applications can be found in the SpecialChem4Adhesive's Ethylene Copolymers Center. Selected formulations from this site are given in Table 6.Evatane 28.420 EVA 20Evatane 28.05 EVA 5Evatane 33.45 EVA 20Evatane 28.40 EVA 5Evatane 33.400 44Hydrogenated rosin ester 40Rosin ester (mp 105°C) 34Rosin ester (mp 85°C) 40 7.5Microcrystalline wax (mp3070°C)Acid functionalized wax 4.5 8Polyethylene wax 25Paraffinic oil 9.5Antioxidant 0.5 0.5 0.5Table 6: Starting Formulation for Several EVA Hot Melt Adhesives6Advances in EVA Hot Melt AdhesivesEVA hot melts are generally susceptible to creep as any thermoplastic. However, they can be crosslinked by adding peroxide and heating or by electron beam (EB) radiation. EVAs exhibit outstanding viscosity retention on aging at elevated temperature; however, they slowly release acetic acid. This can cause corrosion problems with adhesive applications equipment. Formulations based on ethylene n-butyl acrylate have been developed to address this deficiency.Current trends favor fast setting hot melts to improve line speed while keeping a "long" open time in order to reduce quality problems when the line is running at low speed. New EVA formulations can be formulated to offer relatively long open times (40-60 secs) and excellent adhesion to many substrates including metal, plastics, wood, and elastomers. Pressure sensitive EVAs that stay tacky even after cooling are another recent advancement in hot melt technology7.References1. Pocius, A.V., Adhesion and Adhesives Technology, Hanser / Gardner Publications, New York, 1997, p. 247.2. Petrie, E.M., Handbook of Adhesives and Sealants, 2nd ed., McGraw-Hill, New York, 2006, p. 686.3. Gauthier, M. M., "Types of Adhesives", in Adhesives and Sealants, vol. 3, Engineered Materials Handbook,ASM International, 1990.4. Henderson, A.M., "Ethylene Vinyl Acetate (EVA) Copolymers: A General Review", IEEE Electrical InsulationMagazine, January/February, 1993, p. 31.5. Domine, J.D., and Schaufelberger, R.H., "Ethylene Copolymer Based Hot Melt Adhesives", in Handbook ofAdhesives, I. Skeist, ed., van Nostrand Reinhold, New York, 1977.6. Ethylene Copolymers Center, SpecialChem4Adhesives.7. Tremblay, S., "Advances in Hot Melt Technology", Loctite Technical Brief, Henkel Corporation.This document was provided by SpecialChem's editor. If you want to share your technical expertise in a specific area related to adhesives and sealants or if you want to submit press releases, please send it to james.brown@. SpecialChem reserves the right to refuse any article or news item.。