1. 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.

2. Typical adhesive systems for sandwich Structures
Lecture 3
Epoxy resin
Modified Epoxies
Phenolics
Polyurethanes (PU)
Urethane Acrylates
Polyester and Vinylester Resins

3. Epoxy resin Low temperature curing resin (20°C to 90°C)
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 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

4. Modified (Toughened) Epoxies
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

5. 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

6. 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 two-component adhesives which continue to cure when exposed to moisture

7. Polyurethanes continued
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-phenyl-diisocyanate, 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

8. Polyurethanes continued
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

9. 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

10. Polyester and vinylester resins
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

11. Polyester and vinylester resins con’td
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.

12. 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.