Adhesives and Adhesion Properties

02.11.2018

1.Introduction and General Information

The earliest examples for materials bonding with glues were dates back to 4.000 BC. However, archaeologists who conducted research on those bonded elements stated that the ones who died, he was buried with various resin repaired pottery and related items together. Although the adhesives have been known and used by mankind for thousands of years, we have used most of the adhesive technology was developed in the 20th century. In particular, the investigation of elastomers has also brought about rapid advances in adhesive formulations and gave chemists the opportunity to improve the properties of glues. However, the adhesive bonding between glue and substrate may occur either by physically that the adhesive works its way into small pores of the substrate, or by one of several chemical reaction mechanisms. In some cases, chemical interactions (electrostatic forces) or bonds (ionic, covalent) occur between adhesive and substrate.

2. General Features of Glue Combination

Nowadays, the production of cheap adhesives with the desired properties has brought about the rapid development of the glue industry for many different uses. However, the joining of materials with liquid or solid based glues has certain advantages over other bonding methods such as; sewing, welding, mechanical fastenings, etc. The use of synthetic or natural adhesives could offer some advantages over other binding techniques. Some of the important advantages can be briefly summarized as follows; 1. Substances that have different properties, could be effectively combined with glues. Such as; aluminum- paper, iron-wood, fabric-paper adhesions. 2. The thin films, fibers and small particles which are otherwise impossible to assemble in any other way, could be easily combined and glued. (i.e. glass wools, sand papers, brake pads, wood chips (particle board), wood fibers (fiber board). 3. Stronger, lighter and more effective bonding could be achieved with glues than other joins. Thus, it is possible to distribute the forces over a wide area and the breakage hence the weakening of the substance can be reduced (i.e. bonding floors elements, composite boards). 4. Strength/weight ratios and dimensional stability of heterogeneous materials could be improved by crosslinking of glues (i.e. bonding of wood fibers or particles with water resistant thermo-setting adhesives). 5. The glued line could be used in some cases to provide electricity, heat or water (moisture) insulation. Apart from the features that are tried to be explained briefly here, glued joints can have many advantages in different general or special usage areas. As a result,glues can be made of cheap, safe and robust joints for different materials. However, some disadvantages have also occurred due to stability problems at high temperatures and moisture environments and relative weakness in bonding large objects with a small bonding surface area with adhesives.

3. Classification of Glues

The classification of glues can be done in many ways. In general, glues can be classified as follows: • Application and solidification properties, • The chemical composition and the sources obtained, • The material they are bonded to, • End use specifications. Some glue types and classifications are briefly given below.

A. Natural Glues

• Protein Based Glues: Blood glue, soybean glue,casein glue, • Carbohydrate-Based Adhesives: Starch, cellulosic glues, dextrin, •Balsam and Rubber-Type Vegetable Glues: Natural rubber, arabic gum, •Natural Resins: Shellak.

B. Synthetic Adhesives (Organic Based)

•Ethylene (Vinyl) Polymers: Polyvinyl acetate (PVA), polyvinyl chloride (PVC), polyvinyl propionate (PVP), polyvinyl ether, polyvinyl alcohol •Butadiene Polymers: Rubber-Latex, rubber-chlorine, synthetic rubber, butadiene-stirol mixture polymers •Aminoplasts: Urea-formaldehyde resin, melamine formaldehyde resin, disian-diamide resin •Fenoplasts: Phenol-formaldehyde resin, resorcin-formaldehyde resin. •Others: Polyamide, polyester, epoxy, furan, silicone, polysulfides

C. Synthetic Adhesives (İnorganic Based)

•Linseed oil, lime, gypsum, magnesium oxide, zinc oxide, chloride, sulfur.

4. Basic Principles of Adhesion

Although various theories and practical approaches have been developed, the bonding of similar or different substances by gluing has still not been fully explained. According to the general results from the studies and the theories proposed so far, bonding process could be applied to the materials are closely related to; •Surface properties, •Adhesive to be used in chemical and physical states, •The surface interactions between materials and the adhesives. It is very important to determine the degree of adhesion. However, adhesion is the sum of the forces that allow the combination of the corresponding surfaces in the adhesion of the two substances. But it cannot bring two solid matter atoms together because of their microscopic surface properties. It is therefore necessary to use glue for joining the wood. Moreover, cohesion is the gravitational force of the same kind of matter. For example, glue or water molecules in their attraction forces. The resistance of the object to external influences and its deformation based on deformation depend on the size of the cohesion forces. In Figure 1 below, the interaction between the adhesion-cohesion and the surface to be bonded is shown schematically.

Studies on today’s adhesion can be examined under two main groups. One of them deals with the chemistry and physics of surfaces while the other deals with the fracture mechanics of the glued joints.

The first group is largely composed of bond formation; while the wetting is attempting to determine the magnitude of the convergence formed along the junction line, such as absorption-chemical bonding, the second group deals with the development and analysis of assay methods for measuring the resistance of glued objects along an interface. Chemical bonding with an adhesive such as; covalent, ionic, hydrogen or van der Waals those bonds between the surfaces of the glue layer is formed at direct interaction between molecules or atoms. But the physical bonding can be the result of mechanical coupling, when the adhesive is solidified on a coarse surface and the physical absorption forces formed between the molecules adhered to the adhesive molecules, or the diffusion of the adhesive molecules into the substance. Thus, the mechanism of the adhesion movement is completely different with respect to the different types of adhesives and bonded materials. However, physical or chemical adhesion occurs are very complex and still not fully explained. There are some theories about these subjects. Bond Type / Bond Energy (KJ / mol) Ionic Bonding / 600-1100 Covalent Bonding / 160-700 Lewis Acid-Base Interaction / up to 80 Bronsted Acid Base Interaction / up to 1000 Hydrogen Bonding / 10-25 Dipole Dipole Interaction / 4-20 Dispersion Force / 0.08-40 One of the main aims of the adhesion is to connect the two substances in the strongest possible way to each other, to ensure that the two surfaces are not separated from each other or that at least one of them is broken off.

This is the ideal form of adhesion. In general, the interaction (adhesion-adhesion) between two different substances along an interface can take place by physical or chemical bonding depending on the properties of the substances.

It is important to note that the following criteria’s must be considered to ensure a strong and durable bond between the two surfaces using adhesives; a) Glue should be used on the surfaces to be bonded, b) The selected adhesive should be covalently bonded with the material, c) The glue to be used should be easily spreadable on the surfaces, d) The surface properties of the materials to be adhered must be checked and they must be rough in microscopic sense, e) Surface deformations/dirt’s that could provide weakness on surfaces, should be removed, f) Check the suitability of the viscosity of the adhesive to be used for using It should be. In the above sections, the information about the basic principles and general characteristics of glues and adhesive bonding mechanisms are given as a summary. However, further information on this subject could be obtained elsewhere (Frihart ve Hunt 2010; Marra, 1992; Pizzi, 1994; Vick, 1999).