Presentation on theme: "Adhesive and Their Properties"— Presentation transcript:
1 Adhesive and Their Properties Lecture 3There exists variety of adhesives, far too many and beyond the context of this subjectMost are for special purposes, e.g. special PUR for bonding to stainless steel, a toughened epoxy for aluminium alloys in high temperature applications.The choice of adhesive is primarily set to find an adhesive that satisfies the mechanical requirements of the structure for providing a good bond between the material components in the environment where the structure is to work, and considerations like fatigue, heat resistance, strength, ageing and creep are of primary interestThe adhesive must also meet the requirements of the environment in which it is supposed to be used. Thus issues like health considerations, manufacturing technique, curing time and temperature, special tooling requirements and etc.
2 Typical adhesive systems for sandwich Structures Lecture 3Epoxy resinModified EpoxiesPhenolicsPolyurethanes (PU)Urethane AcrylatesPolyester and Vinylester Resins
3 Epoxy resin Low temperature curing resin (20°C to 90°C) Lecture 3Epoxy resinLow temperature curing resin (20°C to 90°C)However, some are made for high temperature curing (130°C to 220°C)Advantage: Used without solvent and therefore without volatile by product during curing and thus low volume shrinkageThe lack of solvents makes epoxies usable with almost every type of core materialAvailable in the forms of paste, powder, films, or solid adhesives.Generally quite good mechanical properties with shear strength at room temperature of about MPa.The bond to metal is greatly improved by pre-treating the metal surface with primerA major drawback is that it is highly allergic when in contact with human skin
4 Modified (Toughened) Epoxies Lecture 3Modified (Toughened) EpoxiesSimilar to normal epoxy but mixed with synthetic rubber, like polysulfide elastomers, which greatly improves the peel resistance.The greater the portion of elastomers the greater ductility but the creep tendencies increase at the same time and the heat resistance decreases.Other modifications are the inclusion of Nylon to improve filleting and controlled flowThese types are, however, sensitive to humidity.By mixing the epoxy with nitrile instead of nylon the same advantages are gained but with maintained resistance to humidity.Limited to approximately 150°C service temperatureShear strength approaches of about 35 MPaToughened epoxy adhesive films are the most common material used when bonding honeycomb sandwich parts
5 Lecture 3PhenolicsPhenolic adhesives have excellent strength, high-temperature mechanical properties, and durabilityThe main drawback is that they give off some water when curing making venting essentialThe viscosity is also quite high and adhesive films must thus be usedThese characteristics have limited the use of phenolics to mainly the process of making honeycombs, where venting is no obstacles and a high temperature bond is requiredThe out-gassing makes phenolic unsuitable for use in bonding sandwich constructions, apart from applications where venting is possiblePhenolic adhesives are often modified with synthetic rubber to improve the toughness
6 Lecture 3PolyurethanesPolyurethane (PUR) adhesives are probably the most widely used adhesives for bonding sandwich elementsThis is because they provide excellent adhesion to most materials, can be used as paste or liquid in a wide range of viscosities, may have long or short cure times, and can be fire-retardant and water resistancePUR adhesives contain virtually no solvents and thus environmentally friendly and the least toxic of all resinThere exist two different types of PUR-adhesives, one-component moisture-cured and two-component systemsOne-component PUR adhesives are in fact pre-reacted two-component adhesives which continue to cure when exposed to moisture
7 Polyurethanes continued Lecture 3Polyurethanes continuedMoisture necessary for curing is simply provided by spraying water on the surfaces prior to bondingOnset of curing can vary between minutes and several hours depending on the choice of adhesiveTwo-component PUR adhesives consist of various polyols, water scavengers, catalysts, fire-retardants, fillers, etc.The curing agent is usually a polymeric methylene-di-phenyl-diisocyanate, which is the least volatile of all isocyanatesThe pot-life when mixed can be made to vary between 5 minutes and several hours, and the consistency from liquid to paste.PUR adhesives can be applied by spraying, rolling or even by brushesCuring must take place under pressure, preferably mechanical pressure but vacuum tables are commonly used in the making of sandwich elements
8 Polyurethanes continued Lecture 3Polyurethanes continuedCuring must take place under pressure, preferably mechanical pressure but vacuum tables are commonly used in the making of sandwich elementsHeat drastically increases the cure timePUR adhesives are mainly used in bonding of foam or balsa core sandwich structures
9 Lecture 3Urethane acrylatesUrethane acrylate is a resin which is compatible with polyesters and vinylestersIn fact, acrylates are so compatible that they can be incorporated in, e.g. a wet polyester laminate.Urethane acrylates are very tough, and exhibit almost no curing shrinkageA way to drastically increase the face-to-core bond in foam core GRP-sandwich structures is to use urethane acrylate resin for the first reinforcing layer, that closest to and bonding to the coreThe rest of the laminate can then be laminated wet using, for example, polyester resin on top of the acrylate layer and still provide a perfect interlaminar bond
10 Polyester and vinylester resins Lecture 3Polyester and vinylester resinsPolyester and vinylesters are the most commonly used matrix materials used for reinforced plastic composites outside the aerospace industryPrefabricated laminates can be bonded to, e.g. foam or balsa cores using the same resin as in the laminate, and it will in most cases prove to be an adequate bond.Usually, however, the laminate is built-up directly onto the core and hence the first layer of the laminate is laid wet onto the core and bonds to it.It is essential that enough resin is used to fill all the surface cells otherwise dry areas a left in bond line
11 Polyester and vinylester resins con’td Lecture 3Polyester and vinylester resins con’tdA problem with these resins is their curing volume shrinkage creating very high interface shear stressesA way of decreasing the effect of shrinkage is to prime the core surface by applying a thin layer of resin to it which fills only the surface cells, and which is allowed to cure before rest of the laminate is applied.
12 Lecture 3ASSIGNMENTIdentify one typical application of sandwich structure and consequently, identify the manufacturing processes, skin and core materials and also adhesive system used in that particular structure.