Basics Of Gluing Explained: Prerequisite, Adhesion, Cohesion

2023 Author: Hannah Pearcy | [email protected]. Last modified: 2023-06-01 01:49

To create a good adhesive bond, the user has to take many factors into account. The adhesive itself naturally has a significant influence on the quality or strength of an adhesive. Therefore, the optimal type of adhesive and the ideal type of adhesive for an adhesive application must first be selected - which is a challenge in itself due to the approximately 30,000 adhesives available on the market. With regard to the joining part materials, the quality of the adhesive surfaces is also very important for the adhesive.

Factors important for the quality and strength of the bond

Constructive design:

- Adhesive-friendly construction

- Geometric design of

adhesive:

- Selection

- Transport

- Storage

- Processing

- Adhesive layer

stress:

- Practice- related testing

- Further processing

- Use

manufacturing:

- Location

- Production equipment

- Suppliers

- Manufacturing parameters

- Training level of the employees

Joining material:

- Type

- surface

- pretreatment

- preparation

- manufacturing parameters

It is therefore not important which materials are to be glued together, but rather what the adhesive surfaces are like in terms of quality, roughness, surface energy, etc. Decisive for a connection are the boundary layers on the adhesive side and the part to be joined, which are sometimes only one or a few molecular layers thick, as well as the phase interface between the adhesive and the part material to be joined. In the case of non-permeable or non-absorbent parts to be joined, it is obvious that deeper substrate layers cannot have any significant influence on the bond.

A prime example of adhesion to supposedly “difficult” surfaces can be found in nature: mussels, for example, are able to hold themselves on very smooth surfaces. With the help of their extremely strong biological "mussel glue" made of proteins and proteins as well as their so-called byssus threads, mussels achieve an extremely good hold even on Teflon - and Teflon is not least known as an ideal non-stick coating, which at first is in conflict with the adhesiveness. This example shows that in principle everything can be glued - it depends on the "right" surface. If necessary, the adhesive surfaces of substrates have to be prepared or pretreated beforehand in line with the adhesive.

Definitions: What is surface energy, surface tension and wetting?

Surface energy: In a liquid adhesive, the forces acting between the molecules inside the liquid cancel each other out, since every molecule is surrounded all around by molecules of the same type. On the other hand, the outward forces are missing on the surface, so that a resulting force results in the interior of the liquid.

Note

This article is an excerpt from the specialist book "Adhesive Bonding Technology", which, in addition to the basics of bonding described here, also describes various types of adhesive, the technology of bonding and the application of bonding technology.

More info

Surface tension: The surface tension is the reason why liquids always endeavor to reduce their surface area and thus assume the state of the lowest possible potential energy. For this reason, liquid surfaces are always minimal surfaces. The spherical shape offers minimal surface with maximum volume.

As a result, water assumes the energetically favorable drop shape when no other forces act on the water. Even heavy liquids, such as mercury, are known to be able to form small and stable drops on most surfaces. The surface tension of the water also prevents, for example, the sinking of water runners or other insects when they are on water surfaces.

Wetting: The wetting of the joint surfaces with adhesive is a necessary criterion for the development of adhesive forces. Ideally, a liquid adhesive must completely wet the joining surfaces of the substrates in order to create the conditions for a maximum of adhesion points in the joining zone. Adequate wetting of the surfaces of the parts to be joined is therefore the most important prerequisite for the development of adhesive forces. The ratio of surface energies of the substrates to be wetted and the surface tension of the adhesive is decisive for the wettability.

glue

Basics of gluing: When glued connections fail - and why

The surface energy of a solid can be determined with the help of the wetting angle, often referred to as the contact or contact angle. The wetting angle is the angle that a horizontal liquid drop forms on a solid surface to this surface (see figure). This lying drop method is a standard arrangement for the optical measurement of the wetting angle. The drop of adhesive rests on the substrate surface.

In order to enable the good wetting of the substrate surfaces with liquid glue, which is required for the gluing, the surface tension of the liquid glue must always be lower than the surface energy of the joining part. Materials with high surface energy are therefore relatively easily wetted by liquid adhesive. Low-energy solid surfaces such as plastics, on the other hand, are often poorly or only partially wetted by the liquid adhesive.

Note

If you want to delve deeper into the calculation of the wetting angle and the determination of the surface tension, we recommend the specialist book "Adhesive Technology", from which this excerpt comes. The most important calculations and formulas are taken into account here.

More info

What is surface roughness?

The substrate surfaces also play a significant role in good adhesion. Therefore, in addition to the surface energies of the joining partners, the geometric structure of the substrates also has a fundamental influence on the bond.

It is easy for the CAD designer to create an ideal surface with the CAD system used. For example, surfaces are usually constructed that are usually free of any shape deviations. This ideal surface is called the geometric surface and does not take into account surface roughness.

Most surfaces, however - at least microscopically - are as uneven as a mountain. Technical surfaces therefore have a more or less pronounced surface topography and can be divided into shape deviations of different orders (see box text).

The different shape deviations

The surface roughness is determined by the shape deviations of the 3rd to 5th order. The shape deviations of the 1st to 4th order overlap to the actual surface (according to DIN 4760: 1982-06 or H. Hoischen / A. Fritz).

• Shape deviations of the 1st order: shape deviations. Examples are: straightness, flatness and roundness deviations.
• Shape deviation of 2nd order: ripple. Examples are: waves.
• Shape deviation of 3rd order: roughness. Examples are: grooves, scratches.
• Shape deviation 4th order: roughness. Examples are: grooves, scales, crests.
• Shape deviation of the 5th order: roughness (can no longer be depicted in a simple manner). Examples are: structure.
• Shape deviation of the 6th order: can no longer be depicted in a simple manner. Examples are: Lattice structure of the material.

In reality, the effective substrate surface is much larger than the surface that can be seen with the naked eye. This results in the so-called true surface.



As a result, the true surface has a significantly enlarged surface for gluing. Nevertheless, the true surface is more of a theoretical term, since depending on the surface energies of the substrates, the surface tension of the adhesive and the viscosity of the adhesive, the entire true surface can never be wetted with adhesive.

That is why one speaks in adhesive technology of the effective surface. The effective surface is to be understood as the sum of the contact areas between the substrate and the adhesive. However, certain areas of the true surface are not wetted with adhesive. These non-wetted areas therefore do not contribute to increasing the adhesive strength.



Bonding forces: what is adhesion and cohesion?

The consideration of the binding forces in bonds leads to two important standard terms in adhesive technology - adhesion and cohesion. Together they form the determining binding forces in an adhesive.

Adhesive forces are decisive for the strength of the boundary layer between the part to be joined and the adhesive layer. Cohesive forces, on the other hand, are responsible for the internal strength of the adhesive layer itself, the cohesive forces being the much stronger forces. They are about 20 to 100 times stronger than the individual adhesive forces.

definition

What is adhesion and cohesion?



The adhesion (Latin: adhaerere = adhere) denotes the surface of adhesion, which is effective on the surface of the adhesive and add portion-side boundary layer as a result of general attractive forces between different substances.

Under cohesion (Latin: cohaerere = associated) is generally the action of attractive forces between atoms or molecules similar understood one and the same substance.

A distinction is made between specific adhesion (adhesion due to the formation of intermolecular forces), mechanical adhesion (adhesion due to the adhesive and the surface of the part to be joined) and auto-adhesion (adhesion when the same elastomers are combined). These terms are explained in more detail below:

Definition: What is specific adhesion, mechanical adhesion and auto-adhesion?

Under specific adhesion the totality of intermolecular forces is understood that work in the interface adherend surface-adhesive layer. The range of action of the specific adhesion is approximately in the range of 0.2… 1 nm. The specific adhesion is fundamentally more important than the mechanical adhesion.

The mechanical adhesion is the adhesion through mechanical or positive locking and anchoring of the hardened adhesive layer in pores, capillaries or undercuts on the substrate surfaces. The mechanical adhesion can be positively influenced by a suitable surface pretreatment - such as compressed air jets (with oil-free compressed air).

In the case of very smooth or only slightly roughened surfaces, the proportion of the so-called mechanical adhesion is relatively insignificant, especially for metal bonds. In contrast, mechanical adhesion plays an important role in the bonding of absorbent, very porous and / or open-pore surfaces (paper, wood, foams, textiles, …).

glue

Gluing starts before gluing (part I)

The term auto-adhesion is used almost exclusively in connection with the joining of the same rubber-elastic polymer layers. The prerequisite is a great mobility of the macromolecules, as is the case with elastomers that are capable of mutual diffusion with the application of pressure with subsequent clamping of chain segments. If two samples of the same elastomer are brought into contact, adhesion (= auto-adhesion) occurs.

Adhesion and cohesion exemplified

Adhesion is a necessary, but not a sufficient, criterion for bonding. On a surface of one square centimeter there are about a billion sticking points or adhesive bonds. But only the combination of adhesive and cohesive forces offers an important prerequisite for durable and resilient bonds. This fact can be clearly explained using the example of water:

A uniformly thin layer of water between two glass plates, foils or convex and concave ground lenses initially ensures that the substrates are held together due to the extremely high number of adhesive bonds in the joint surface. However, this cohesion is relatively weak and can easily be canceled by low forces and an unfavorable direction of loading. Thus water is rather unsuitable as an adhesive, since the liquid water completely lacks the property of cohesion.

DIN 2304-1

Gluing starts before gluing (part II)

The cohesion usually refers herein to the internal strength of the cured adhesive. Of course, each substrate also has an internal strength or cohesive strength. The cohesive strength is essentially a material and temperature dependent variable. Metals generally have a much higher cohesive strength than plastics. In the case of plastics, thermosets (for example epoxy resin) in turn have a much higher cohesive strength than thermoplastics (for example hot melt adhesive), which is due to the spatially closely meshed molecular structure of the thermosets.

Molecular mobility increases with increasing temperature (Brownian molecular movements). The already weak secondary valence bonds dissolve with increasing temperature - the molecular cohesion decreases. As a result, the cohesive strength decreases with increasing temperature. With adhesive layers, the cohesive strength is above all a characteristic feature for the retardation of plastics or adhesives. Under retarding will creep or flow, so the change in length, understood under constant mechanical load.

Note

This article comes from the specialist book "Adhesive Bonding Technology", which, in addition to the basics of bonding described here, also describes various types of adhesive, the technology of bonding and the applications of bonding technology

More info

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Adhesive technology in criticism

Bonding: insufficient

* Prof. Dr.-Ing. Tim Jüntgen, professor at the Amberg-Weiden University of Applied Sciences (plastics technology / plastics processing technology, construction, tool construction, adhesive technology)