THEORIES OF ADHESION
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INTRODUCTION
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This chapter will explore the various theories that relate to adhesion and have withstood the test of time. There is, unfortunately, no universal theory of adhesion on which to accurately model the interactions that take place between the adhesive and the adherend. Instead, sev¬eral theories exist that provide a means by which one can rationalize certain observations.
They are generally useful in understanding why adhesives stick and why, at times, they fail. Adhesion theories also allow us to make predictions and even obtain a semi-quantitative realization of joint strength. By being familiar with these theories, one can develop a knowl¬edge base and an awareness of how adhesives and sealants work in practical situations. It is necessary to investigate adhesion and cohesion as separate topics, but both are fun¬damental to adhesive and sealant technology. Theories that are associated with cohesive strength of materials have been well established in many textbooks. These cohesive theo¬ries will be summarized here as they pertain to adhesive and sealant joints and in Chap. 4 as they pertain to bulk properties. The primary focus in this chapter will be on adhesion.
FORCES INVOLVED
The forces involved in holding adhesives and. sealants to their substrates or in holding mol¬ecules together as a bulk material arise from the same origins. These forces are all around us in nature, and they are both physical and chemical. To understand what is happening in an adhesive or sealant joint, we must first understand the forces that bind atoms and mole¬cules together.
Adhesive and Cohesive Forces
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Bond strength is not only the result of adhesion forces. Other forces also contribute to the strength of a joint. For example, molasses may have good adhesion, but it is a poor adhe¬sive or sealant. Its failure is usually cohesive. The cohesive strength of an adhesive or sealant is at least as important as its adhesive strength.
Adhesive forces hold two materials together at their surfaces. Cohesive forces are those forces that exist between molecules of the same material. They are the forces that provide a material with its bulk physical properties (e.g., tensile strength, elongation, etc.). Adhesive or sealant joints may fail either adhesively or cohesively as was shown in Fig. 1.1.
Adhesive failure is a failure at the interface between the adherend and the adhesive. An example would be the peeling of cellophane tape from a glass surface if the adhesive film .
separates cleanly from the glass. Cohesive failure is failure within the adhesive or one of the adherends. Cohesive failure would result if two metal substrates held together with grease were pulled apart. The grease would be found on the two substrates after the joint failed. The grease would have failed cohesively. Another example of cohesive failure is if two wooden panels were bonded together with strong wood glue and then pulled apart. Most likely, the resulting failure would show that particles of wood fiber were left embed¬ded in the adhesive. In this case, the wood or adherend failed cohesively. Both adhesion and cohesion playa part in determining the strength of a bond. The joint will fail adhesively if the adhesive or adherend is stronger than the interfacial bond. The joint will fail cohesively if the interfacial bond is stronger than the adhesive or the adherend. A general goal is to have the adhesive strength sufficiently high as to cause cohesive fail¬ure of the adhesive or adherend when the joint is stressed to its ultimate. This is the case when the internal strength of the substrate or adhesive material, and not the adhesion, is the limiting factor (weakest link) when the joint is stressed. Cohesive and, to a certain extent, adhesive properties of adhesives or sealants are generally determined by their manufacturers. Cohesive properties will be dependent on the chemistry, reinforcements, cross-linking mechanism, and such that are used to produce the material. Adhesive properties will be determined by the surface chemistry of the adhe¬sive relative to the substrate. The adhesive properties, however, are also often determined by the end-users. The end-user must provide a clean substrate and optimum conditions to provide wetting and cure.
Both adhesive and cohesive forces are the result of charge attractions existing between atoms or molecules. The positive portion of one molecule attracts the negative portions of adjacent molecules. The more positive or negative the charged sites and the closer together the molecules, the greater will be the forces of attraction.
Adhesive or cohesive forces can be attributed to either short or long range molecular inter¬actions. These are also referred to as primary or secondary bonds. Table characterizes.
Forces at the Interface or Within the Bulk of the Material Table
Description Bond energy
(Kllmol) Bond
length (nm) Source of force Type of force
Diamond or cross linked polymers. Highly directional 150-950 0.1-0.2 Covalent Chemical:
Crystals. Less directional than covalent 400-800 0.2-0.3 Ionic or electrostatic Metallic Primary or short range forces
Forces in welded joints Interactions between temporary dipoles. Forces fall off as the sixth power of the distance 100-400 0.3-0.5
0.4-0.5 Van der Waals- dispersion forces Intermolecular:
Secondary long range forces
Interactions of permanent dipoles. Forces fall as the third power of the distance 0:1-15
Sharing of protons between two atoms
possessing loaned pairs
of electrons 4-15 0.4-0.5 Van der Waals- polar forces
20-30 0.2 Hydrogen bonds
these forces. The exact types of forces that operate at the adhesive - adherend interface are thought to be the following:
. Van der Waals forces (physical absorption)
. Hydrogen bonding (strong polar attraction)
. Ionic, covalent, or co-ordination bonds (chemisorptions)
Short-range molecular interactions include covalent, ionic, and metallic forces. Covalent forces result from chemical reactions such as provided by some surface treatments on glass fiber. Welding, soldering, or brazing processes form metallic bonds. However, these forces generally are not at work in the more common, everyday adhesive applications.
The excellent cohesive strength of polyamides compared to other common polymers at equivalent molecular weights is due to the presence of interchain hydrogen bonding. Excellent adhesion of epoxies to aluminum, of rubber and other polymers to glass, and of polymers to cellulosic s are also attributed to interfacial hydrogen bonding. Hydrogen bond¬ing can be considered a special type of cross-linking.
The most important forces relative to adhesion are the van der Waals forces. These forces are dependent on the distance between the adhesive and adherend. The bond forces fall off exponentially as this distance increases. The exact nature of these forces and their influence on adhesive or cohesive strength are discussed in the following sections.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
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