Adhesives and Bonding Properties

Adhesives and Bonding Properties The earliest examples of using substances for bonding purposes date back to 4,000 B.C. Archaeologists conducting research have noted that in prehistoric times, earthenware and pottery found buried with the deceased were repaired using various tree resins with adhesive properties after breaking. Although adhesives have been known and used by humans for thousands of years, much of the adhesive technology we use today has been developed over the last century. Particularly, the discovery of elastomers brought rapid advances in adhesive formulations and gave chemists the opportunity to improve adhesive properties. The bonding phenomenon between adhesive and material involves quite complex reactions and many variables. Bonding can occur physically through the adhesive's penetration into small pores or through chemical reaction mechanisms. Chemical interactions between adhesive and material can be related to weak forces (electrostatic forces) or strong chemical bonds (ionic, covalent).

2. General Characteristics of Adhesive Joining

Today, the ability to produce inexpensive adhesives with desired properties for many different applications has brought about rapid development in the adhesive industry. Joining materials with liquid or solid adhesives has certain distinct advantages compared to other mechanical joining methods such as welding, soldering, fastening, etc. These advantages can be briefly summarized as follows. 1. Materials with different properties can be effectively joined with adhesives, for example: aluminum-paper, iron-wood, fabric-paper. 2. Thin films, fibers and small particles that are otherwise difficult or impossible to join can be easily bonded with adhesives, for example: glass wool, sandpaper, brake linings, wood particles (particleboard). 3. Adhesives provide stronger, lighter and more effective bonding than other joining methods. This allows forces to be distributed over a wide area and reduces fractures resulting from material weakening. Examples include adhesive flooring and composite boards. 4. The strength-to-weight ratio and dimensional stability of different materials can be improved through cross-linking. For example, water-resistant wood composite products (plywood, particleboard, fiberboard) joined with thermoset adhesives. 5. The adhesive line can in some cases provide electrical, thermal or moisture insulation. Beyond the properties briefly explained here, adhesive joints have many advantages in general or specific applications. In conclusion, adhesives enable inexpensive, safe and durable joints for various materials.

3. Classification of Adhesives

  Adhesives can be classified in many ways. Generally, adhesives can be classified as follows: • Application and curing properties, • Chemical composition and source, • The material being bonded, • End-use application.

Below is a brief summary of some adhesive types and their classifications.

  A. Natural Adhesives • Protein-Based Adhesives: Casein adhesive, blood adhesive, soy adhesive, • Carbohydrate-Based Adhesives: Starch, cellulosic adhesives, dextrin, • Balsam and Rubber-Type Plant Adhesives: Natural rubber, gum arabic, • Natural Resins: Rosin, shellac.

B. Synthetic Adhesives (Organic-Based)

  • Thermoplastic Polymers: Polyvinyl acetate (PVA), polyvinyl chloride (PVC), polyvinyl alcohol, • Butadiene Polymers: Rubber-latex, rubber-chlorine, synthetic rubber, • Aminoplasts: Urea-formaldehyde resin, melamine-formaldehyde resin, dicyanodiamide resin, • Phenoplasts: Phenol-formaldehyde resin, resorcinol-formaldehyde resin, • Others: Polyamide, polyester, epoxy, silicone. C. Synthetic Adhesives (Inorganic-Based) • Linseed oil putty, lime, gypsum, magnesium oxide-chloride. Some of the adhesive classes mentioned here can also be further subdivided into more specific groups.

4. Fundamental Principles of Bonding

Although various theories and practical approaches have been developed, joining similar or different materials through bonding still covers many topics that are not fully explained. Based on research conducted so far, proposed theories and the general conclusions reached; the bonding process depends on: • The surface properties of materials to be joined, • The chemical and physical conditions of the adhesive to be used in the joining, • The surface relationships that can be created between the materials to be joined and the adhesive to be used.   The terms adhesion and cohesion, which are widely used in adhesive bonding, should not be confused with each other. Adhesion is defined as the sum of forces that enable the corresponding surfaces of two materials to bond. In this respect, atoms of two solid materials cannot be brought closer together more than a certain level due to their surface characteristics, and adhesive molecules help establish bonding along this interface. Cohesion is the attractive force between molecules of the same material. For example, the attractive forces between adhesive molecules or water molecules among themselves are referred to as cohesion. A material's resistance to external effects and the resistance of its shape to deformation depend on the magnitude of cohesive forces. Figure 1 below schematically shows the interaction between adhesion-cohesion and the bonded surface.

Figure 1. Bonding phenomenon and adhesion-cohesion interactions

Current research on bonding or adhesive joining can be examined under two main groups. One deals with the chemistry and physics of surfaces while the other is concerned with the fracture mechanics of bonded joints. The first group largely attempts to determine bond formation; the magnitude of interactions occurring along the bond line such as wetting, absorption and chemical bonding; while the second group is involved in developing testing methods and analyses to measure the resistance of bonded objects along an interface. Some bond energies of certain chemical bonds that should be considered in adhesive bonding are briefly shown below. Chemical bonding occurs as a result of direct interactions between the molecules of the adhesive and the molecules of the bonded material through covalent, ionic, hydrogen, van der Waals and similar bonds at the surfaces and the adhesive layer. Physical bonding, on the other hand, can result from physical absorption forces between adhesive molecules and bonded molecules when the adhesive solidifies on a rough surface, or from the diffusion of adhesive molecules into the bonded material, resulting in mechanical interlocking. Thus, the mechanism of the bonding process is completely different depending on the type of adhesive and material being bonded. How bonding occurs physically or chemically is quite complex and still not fully explained. There are some theories related to this subject. Bond Type / Bond Energy (kJ / mol) Ionic Bond / 600-1100 Covalent Bond / 160-700 Lewis Acid-Base Interaction / up to 80 Bronsted Acid-Base Interaction / up to 1000 Hydrogen Bonds / 10-25 Dipole-Dipole Interaction / 4-20 Dispersion Forces / 0.08-40 The fundamental purpose of adhesive bonding is to bind two materials together as strongly as possible, keeping the two surfaces from separating. This creates the most ideal form of bonding. Generally, the interaction between two different materials along an interface (bonding-adhesion) can be achieved through physical or chemical bonding, depending on the properties of the materials. In order to ensure a firm and durable joint between two surfaces using adhesives, the following criteria should be observed: a) An adhesive that is soluble on the surfaces to be bonded should be used, b) The selected adhesive should be capable of forming covalent bonds with the material to be bonded, c) The adhesive to be used should spread/apply easily on surfaces, d) The surface properties of the materials to be bonded should be controlled and be microscopically rough, e) Surface deformations/contaminants that cause weakness on surfaces should be removed, f) The viscosity of the adhesive to be used should be checked for suitability in application.   In the sections above, the most fundamental information about the basic principles and general characteristics of adhesive bonding has been provided in summary form. Detailed information on this subject can be obtained from other sources (Frihart and Hunt 2010; Marra, 1992; Pizzi, 1994; Vick, 1999).

Prof. Dr. H. Turgut Şahin - Department of Industrial Engineering / Faculty of Forestry - Isparta University of Applied Sciences

 

References

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.