Glue refers to a material that is applied between two other materials in order to join them. The glue thus forms a joint that through its inner continuity (cohesion) and its adherence (adhesion) causes the joined material to form a unit.
There have been many theories as to how glue works, i.e. what adhesion actually is. Today, gluing capacity is considered a physical mechanism, namely adsorption (not to be confused with absorption, which means soaking up). Adsorption entails that molecules are drawn to one another by so-called van der Waals forces.
These forces are greatest when the distance between the molecules are of the size 3–4 angstrom (an angstrom [A] is equal to ten millionths of a millimetre). When the distance increases over 5 angstrom, the force is in principle infinite. In practice, this means that if it were possible to press together two surfaces so tightly that the distance was under 5 angstrom, a bond would occur. However, such smooth surfaces do not exist. A liquid, however, can form itself to the substrate and come sufficiently close. If the liquid can later be converted into solid form, without shrinkage, a glued joint occurs. A requirement for the liquid (the glue) coming sufficiently close is that it has a surface tension that is lower than the substrate’s. The surface tensions for epoxy glues are in the range 35–45 mN/m.
Good wetting to the left and no wetting to the right
Materials that have lower surface tension than epoxy are difficult to glue. Examples of such materials are Polytetrafluoroethylene (PTFE [Teflon]), with a surface tension of 18.5 mN/m, and polyethene (PE), which is at 32 mN /m. Wood, however, with a surface tension of about 200 mN/m, and metals, which are between 200 and 2000 mN/m, are easily glued.
If the glue’s surface tension is right, i.e. lower than the substrate’s, one can say that the glue has the capability to wet the substrate. This wetting capability is also related to the glue’s consistency.
The need to join together various things has existed since time immemorial. In nature, there are a number of gummy substances that have been used, but when it comes to strength, they naturally have their limitations.
Many other ways to join materials have been developed, for example, welding, soldering, riveting, bolted joints, etc. These methods also have many disadvantages. Welding and soldering are limited to joining metals, rivets and bolted joints are difficult to make tight; there is also always the risk for galvanic elements and corrosion.
There is, and there has always been, a certain scepticism to glues. This is because previously, the common glue types did not have the right properties pertaining to strength, filling the joint, durability, etc. The situation today is entirely different. Glues have now been developed that comply with the requirements, and gluing techniques have been refined. Modern epoxy glues are presently replacing both welding and riveting in many cases.
To be able to choose the right glue for a joint, it is important to perform a requirements analysis.
Examples of requirements factors are:
- Which forces will affect the joint?
- Within which temperature ranges shall the joint function?
- Are the materials that will be joined absorbing?
- Do the materials require pre-treatment prior to gluing?
- Does the glue wet the material sufficiently?
- Is the joint such that it shall be filled?
- What are the hardening conditions?
- Shall the glue be mixed manually or with a machine?
A glued joint’s capacity to transfer forces depends on how the forces shall be transferred. In principle, there are four basic cases, namely, tensile force, shearing force, tearing and splitting.
Various forces in glued joints
Splitting force. Difficult for all kinds of adhesives.
Tearing force. Almost as a splitting force.The most common force when checking if the laminate are stuck to the surface. And fail.
Tensile force. The only force you actually want when using an adhesive.
Epoxy glues have very high tensile strength and therefore withstand shearing forces well.
Resistance to tearing and splitting are, however, less favourable. It is thus important that joints be formed so that they are subjected to as little tearing and splitting as possible. Heat hardened epoxy glue of the single-component type generally has better resistance to tearing than room temperature hardened epoxy glue.
The joint’s working temperature is also important to know. All glues have temperature limitations. In glue specifications, the glue’s HDT (Heat Deflection Temperature) or Tg is often specified.
At HDT, the glue quickly loses strength. Room temperature hardened epoxy has an HDT of between + 40° and + 70°C. In exceptional cases, it is somewhat higher.
Heat hardened glues can withstand temperatures up to +250°C.
Furthermore, one should be aware of the differences in thermal expansion between the glue and the glued surfaces. If the difference is large, the glue must be able to withstand the stress that arises with temperature variations.
If the material that shall be glued is absorbent, i.e. can absorb a liquid, consideration must be taken to this when choosing glue.
Wood is an example of an absorbent material. A low viscosity glue with long open assembly time can entirely vanish into the wood, leaving the joint empty. The right type of filler can eliminate this risk. In some cases, it is necessary to saturate the wood with glue before actual gluing.
Very often, it is necessary to pre-treat the materials that will be glued. Examples of pre-treatments can be degreasing, sanding, sandblasting, etching, priming and blazing. At the end of this book, a number of suggestions are provided for pre-treatments that can be applicable.
Surface wetting is directly related to the pre-treatment that to a large extent consists of removing contaminants from the surface to be glued.
In addition to this, the glue’s limitation is in its surface tension in relation to the substrate’s surface tension.
When uneven surfaces are to be glued, the glue must be able to fill the gap that arises without the glue running out. An example is gluing of steel plate to concrete. Here the glue gap can be up to several millimetres.
The glue’s consistency, or its rheology, is very important. When applying the glue, the consistency shall be such that the glue wets the surface. After application and attachment of the materials, the glue shall quickly build up a thixotropic consistency so as to not run out of the joint.
In regards to hardening conditions, it can be said that glue intended for room temperature hardening should be permitted to harden at +20°C. At this temperature, it is easy to achieve reasonable hardening times, at the same time as one has a somewhat long potlife.
Epoxy can be made to harden down to temperatures of about ±0°C, but when it comes to glues, the limit should be set at about +10°C. All chemical reactions are temperature dependent. For epoxy, what approximately applies is that an increase in temperature with 10°C doubles the reaction speed.
A post-hardening at increased temperature (+50 – +100°C) is often required. It is namely so that at room temperature, polymerisation is nearly linear, i.e. with few cross linkings. At increased temperature, the reaction is complemented with several cross linkings and the end result is a stronger glue.
Heat hardened glue is often of the single-component type with long potlife at room temperature. These glues are normally hardened at temperatures between + 150°C and + 180°C. A hardening cycle with certain times at specified temperatures is often needed.
For continual gluing, mechanical equipment is often used to obtain well-mixed dosages of the components. It is important that such glues be developed in close collaboration with the glue manufacturer for final properties to be optimal.