Transcript EBB 324 /3 - Universiti Sains Malaysia

Adhesive and Their Properties
Lecture 3
•There exists variety of adhesives, far too many and beyond the
context of this subject
•Most 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 interest
•The 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.
Typical adhesive systems for sandwich
Structures
•Epoxy resin
•Modified Epoxies
•Phenolics
•Polyurethanes (PU)
•Urethane Acrylates
•Polyester and Vinylester Resins
Lecture 3
Lecture 3
Epoxy resin
•Low 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 shrinkage
•The lack of solvents makes epoxies usable with almost every type
of core material
•Available in the forms of paste, powder, films, or solid adhesives.
•Generally quite good mechanical properties with shear strength at
room temperature of about 20-25 MPa.
•The bond to metal is greatly improved by pre-treating the metal
surface with primer
•A major drawback is that it is highly allergic when in contact with
human skin
Lecture 3
Modified (Toughened) Epoxies
•Similar 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 flow
•These 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 temperature
•Shear strength approaches of about 35 MPa
•Toughened epoxy adhesive films are the most common material
used when bonding honeycomb sandwich parts
Lecture 3
Phenolics
•Phenolic adhesives have excellent strength, high-temperature
mechanical properties, and durability
•The main drawback is that they give off some water when curing
making venting essential
•The viscosity is also quite high and adhesive films must thus be
used
•These 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 required
•The out-gassing makes phenolic unsuitable for use in bonding
sandwich constructions, apart from applications where venting is
possible
•Phenolic adhesives are often modified with synthetic rubber to
improve the toughness
Lecture 3
Polyurethanes
•Polyurethane (PUR) adhesives are probably the most widely used
adhesives for bonding sandwich elements
•This 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
resistance
•PUR adhesives contain virtually no solvents and thus
environmentally friendly and the least toxic of all resin
•There exist two different types of PUR-adhesives, one-component
moisture-cured and two-component systems
•One-component PUR adhesives are in fact pre-reacted twocomponent adhesives which continue to cure when exposed to
moisture
Lecture 3
Polyurethanes continued
•Moisture necessary for curing is simply provided by spraying water
on the surfaces prior to bonding
•Onset of curing can vary between minutes and several hours
depending on the choice of adhesive
•Two-component PUR adhesives consist of various polyols, water
scavengers, catalysts, fire-retardants, fillers, etc.
•The curing agent is usually a polymeric methylene-di-phenyldiisocyanate, which is the least volatile of all isocyanates
•The 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
brushes
•Curing must take place under pressure, preferably mechanical
pressure but vacuum tables are commonly used in the making of
sandwich elements
Lecture 3
Polyurethanes continued
•Curing must take place under pressure, preferably mechanical
pressure but vacuum tables are commonly used in the making of
sandwich elements
•Heat drastically increases the cure time
•PUR adhesives are mainly used in bonding of foam or balsa core
sandwich structures
Lecture 3
Urethane acrylates
•Urethane acrylate is a resin which is compatible with polyesters
and vinylesters
•In 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
shrinkage
•A 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 core
•The 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
Lecture 3
Polyester and vinylester resins
•Polyester and vinylesters are the most commonly used matrix
materials used for reinforced plastic composites outside the
aerospace industry
•Prefabricated 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
Lecture 3
Polyester and vinylester resins con’td
•A problem with these resins is their curing volume shrinkage
creating very high interface shear stresses
•A 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.
Lecture 3
ASSIGNMENT
Identify 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.