Adhesives and Adhesion Properties

1. Introduction and General Information

The earliest examples of materials bonding with adhesives date back to 4,000 BC. However, archaeologists who conducted research on those bonded elements stated that the deceased were buried with various resin-repaired pottery and related items together. Although adhesives have been known and used by mankind for thousands of years, 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 physically, with the adhesive working 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 inexpensive adhesives with desired properties has brought about rapid development of the adhesive industry for many different applications. However, the joining of materials with liquid or solid-based adhesives has certain advantages over other bonding methods such as sewing, welding, and mechanical fasteners. 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. Materials with different properties could be effectively combined with adhesives, such as aluminum-paper, iron-wood, and fabric-paper bonds. 2. 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, sandpapers, brake pads, wood chips (particle board), wood fibers (fiberboard)). 3. Stronger, lighter and more effective bonding could be achieved with adhesives than with other joints. Thus, it is possible to distribute forces over a wide area and reduce breakage and weakening of the material (i.e., bonding of floor elements, composite boards). 4. Strength-to-weight ratios and dimensional stability of heterogeneous materials could be improved by crosslinking of adhesives (i.e., bonding of wood fibers or particles with water-resistant thermoset adhesives). 5. The glued line could be used in some cases to provide electrical, thermal or water (moisture) insulation. Apart from the features briefly explained here, glued joints can have many advantages in different general or special usage areas. As a result, adhesives can create inexpensive, safe and robust joints for different materials. However, some disadvantages have also occurred due to stability problems at high temperatures and in moisture environments and relative weakness in bonding large objects with a small bonding surface area using adhesives.

3. Classification of Adhesives

The classification of adhesives can be done in many ways. In general, adhesives can be classified as follows: • Application and solidification properties, • Chemical composition and source of origin, • The material they are bonded to, • End use specifications. Some adhesive types and classifications are briefly given below.

A. Natural Adhesives

• Protein-Based Adhesives: Blood adhesive, soybean adhesive, casein adhesive, • Carbohydrate-Based Adhesives: Starch, cellulosic adhesives, dextrin, •Balsam and Rubber-Type Vegetable Adhesives: Natural rubber, gum arabic, •Natural Resins: Shellac.

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, chloroprene rubber, synthetic rubber, butadiene-styrene copolymers •Aminoplasts: Urea-formaldehyde resin, melamine-formaldehyde resin, dicyanodiamide resin •Phenoplasts: Phenol-formaldehyde resin, resorcinol-formaldehyde resin. •Others: Polyamide, polyester, epoxy, furan, silicone, polysulfides

C. Synthetic Adhesives (Inorganic 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 adhesives has still not been fully explained. According to general results from studies and theories proposed so far, the bonding process in materials is closely related to: •Surface properties, •The chemical and physical states of the adhesive to be used, •The surface interactions between materials and 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 two substances. But adhesives cannot bring two solid matter atoms together because of their microscopic surface properties. It is therefore necessary to use adhesive for joining materials. Moreover, cohesion is the gravitational force within the same type of matter. For example, the attraction forces between glue or water molecules. The resistance of the object to external influences and its deformation depend on the magnitude of the cohesion forces. In Figure 1 below, the interaction between adhesion-cohesion and the surface to be bonded is shown schematically.

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

The first group is largely composed of bond formation; wetting attempts to determine the magnitude of the convergence formed along the junction line, such as absorption-chemical bonding, while the second group deals with the development and analysis of test 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 bonds is formed at the direct interaction between molecules or atoms across the surfaces of the glue layer. But physical bonding can be the result of mechanical coupling, when the adhesive solidifies on a rough surface and physical absorption forces form between the molecules adhered to the adhesive molecules, or the diffusion of adhesive molecules into the material. Thus, the mechanism of adhesion movement is completely different with respect to different types of adhesives and bonded materials. However, physical or chemical adhesion 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 adhesion is to connect two substances in the strongest possible way to each other, to ensure that the two surfaces do not separate from each other or that at least one of them does not break 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 must be considered to ensure a strong and durable bond between two surfaces using adhesives: a) Adhesive should be used on the surfaces to be bonded, b) The selected adhesive should be covalently bonded with the material, c) The adhesive to be used should be easily spreadable on the surfaces, d) The surface properties of the materials to be bonded must be checked and they must be rough in the microscopic sense, e) Surface deformations and dirt that could cause weakness on surfaces should be removed, f) The suitability of the viscosity of the adhesive to be used should be checked. In the above sections, information about the basic principles and general characteristics of adhesives and adhesive bonding mechanisms is given as a summary. However, further information on this subject could be obtained elsewhere (Frihart and Hunt 2010; Marra, 1992; Pizzi, 1994; Vick, 1999). Prof. Dr. H. Turgut Şahin Department of Forest Products Engineering Forestry Faculty Isparta University of Applied Sciences  

Literature Frihart, C. R., Hunt, C.G. 2010. Adhesives with Wood Materials: Bond Formation and Performance. In: Wood Handbook: Wood as an Engineering Material: USDA, Forest Service, General Technical Report FPL, GTR-190. Madison, WI. Gillespie, R.H., Countryman, D., Blomquist, R. (1978). Adhesives in Building Construction, USDA Agriculture Handbook No. 516, Washington D.C. USA. Kubler, H. 1980. Wood as Building and Hobby Material, John Wiley & Sons, NY. Marra, A. A. 1992. Technology of Wood Bonding: Principles in Practice, Van Nostrand Reinhold, NY. Pizzi, A. 1994. Advanced Wood Adhesives Technology, CRC Press, NY. Şahin, H.T. 2005. Kontrplak Üretiminde Kullanılan Formaldehit Esaslı 'Termo- Setting' Tutkallar (in Turkish), Laminart, Haziran-Temmuz, Sayı 38, 128-133. Şahin, H.T. 2012. Ağaç Malzeme Tutkalları, SDU Orman Fakültesi Ders notları (Basılmamıştır) 101s. Isparta. Şahin, H.T. 2017. Odun Kompozit Malzemelerin Üretiminde Kullanılan Tutkallar ve Özellikleri, Putech & Composites, Temmuz-Ağustos 2017: 22-28. Tank, T. 1993. Tutkallar ve Yapıştırma Tekniği, IU Orman Fakültesi Ders notları, (Basılmamıştır), İstanbul. Vick, C. B. 1999. Adhesive Bonding of Wood Materials. In: Wood Handbook: Wood as an Engineering Material. Madison, WI. USDA, Forest Service, General Technical Report FPL, GTR-190. Madison, WI.