1. By Shrikant Athavale Presentation No : A.002.A ADHESION THEORY

2. A002.A.Adhesion Theory

3. Dear Participants We Welcome you all to attend this Innovative, Workshop on Pressure Sensitive Adhesives and Coatings. We will be here, together for the next 4 Days, to Understand, What is Adhesive ? Here it worth to mention. This is not a Class Room. I am not going to teach you. We are going to learn through a lot of inter action. I will be asking you some questions time and again, related to our day to day work. You will understand the subject automatically, through your own answers

4. What is Pressure Sensitive Adhesive ? https://www.youtube.com/watch?v=ogXR4k-UWxE https://www.youtube.com/watch?v=9MiRYdHnmTA How is it applied or coated ? Why it is coated and on what substrates or Backing Materials ? What is the end application ? How is the Testing done ?

5. What is Adhesive ? Can anybody say what is Adhesive ? In your day to day life are you using any adhesive, can you give me some examples ? 1. Adhesion and Adhesion Theory

6. This is really a very goodInteraction so now all of you know what is Adhesive. Right ?

7. Now we have some Idea about what is a Adhesive, or Stickiness We all know for any such Chemistry there is Chemical reason or Theory behind. Now we will see what is that Theory ?

8. What is Adhesive ? adhesiveis able to An adhesive, also known as glue, cement or paste, is able to stick fast to a surface or object; it is sticky, a substance used for sticking objects or materials together; glue.

9. What is Adhesive ? "adhesive" Adjectives may be used in conjunction with the word "adhesive" the substance's physical or chemical form, the type of materials joined, or conditions under which it is applied.

10. What is Adhesive ? "adhesive" Adjectives may be used in conjunction with the word "adhesive" to describe properties based on the substance's physical or chemical form, the type of materials joined, or conditions under which it is applied.

11. What is Adhesive ? The use of adhesives offers many advantages over binding techniques such as sewing, mechanical fastening, thermal bonding, etc.sewingmechanical fasteningthermal bonding

12. is the craft of fastening or attaching objects using stitches made with a needle and thread. Sewing is one of the oldest of the textile arts Sewing Sewing https://www.youtube.com/watch?v=x9yn2mqLjjY

13. A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together mechanical fastening In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or dismantled without damaging the joining components. https://www.youtube.com/watch?v=1uJfGIyrdbs

14. mechanical fastening Welding is an example of creating permanent joints. There are three major steel fasteners used in industries: stainless steel, carbon steel, and alloy steel https://www.youtube.com/watch?v=qZv6DCxTfXI

15. Thermal bonding https://www.youtube.com/watch?v=KyRaxcD5 6c4

16. What is Adhesive ? These include the ability to bind different materials together, to distribute stress more efficiently across the joint, the cost effectiveness of an easily mechanized process, an improvement in aesthetic design, and increased design flexibility.

17. What is Adhesive ? Disadvantages of adhesive use include decreased stability at high temperatures, relative weakness in bonding large objects with a small bonding surface area, and greater difficulty in separating objects during testing.

18. What is Adhesive ? Adhesives are typically organized by the method of adhesion. These are then organized into reactive and non-reactive adhesives, which refers to whether the adhesive chemically reacts in order to harden. Alternatively they can be organized by whether the raw stock is of natural or synthetic origin, or by their starting physical phase.chemically reactsphysical phase https://www.youtube.com/watch?v=EMEy-txEdRE

19. What is Adhesive ? Adhesives may be found naturally or produced synthetically. The earliest human use of adhesive-like substances was approximately 200,000 years ago, when Neanderthals produced tar from the dry distillation of birch bark for use in binding stone tools to wooden handles.

20. What is Adhesive ? The first references to adhesives in literature first appeared in approximately 2000 BC. The Greeks and Romans made great contributions to the development of adhesives. In Europe, glue was not widely used until the period AD 1500–1700. From then until the 1900s increases in adhesive use and discovery were relatively gradual. Only since the last century has the development of synthetic adhesives accelerated rapidly, and innovation in the field continues to the present.

21. is a attraction process between dissimilar surfaces that stick to one another and cohesion takes place between similar molecules. (cohesion refers to the tendency of similar or identical particles/surfaces to stick to one another). The forces that cause adhesion and cohesion can be divided into several different types.

22. The most important theories of adhesion are As all adhesive bonds involve molecules in intimate contact, physical adsorption must always contribute.

23. https://www.youtube.com/watch?v=UrAkujZsf4w

24. https://www.youtube.com/watch?v=026KzA6pFAY

25. https://www.youtube.com/watch?v=vgBnbem19cI

26. https://www.youtube.com/watch?v=KaUh_v5t6YY

27. https://www.youtube.com/watch?v=xkaAv636A9A

28. Physical Absorption Theory The adhesion results from molecular contact between two and the surface forces that develop, usually designated as secondary or van der Walls forces. For these forces to develop, the adhesive must make intimate, molecular contact with the substrate surface.

29. Physical Absorption Theory The process of establishing continuous contact between an adhesive and the adherent "wetting. is known as "wetting. " Wetting can be determined by contact angle measurements.

30. This is the formation of covalent, ionic or hydrogen bonds across the surface. The covalent bonds are formed with silane coupling agents ( or adhesion promoters), and it is possible that adhesives containing isocyanides groups react with active hydrogen atoms, such as hydroxyl groups, to form a permanent bond.

31. diffusion Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile ( free to move ) and soluble in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. When polymer granules are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.

32. ADHESIVE In Liquid Form Substrate / Web PaperFilmFabricsFoam Is now Coated on Substrate or web Now heat is applied and the liquid adhesive is dried

33. ADHESIVE In Liquid Form Substrate / Web PaperFilmFabricsFoam Now heat is applied and the liquid adhesive is dried

34. Dry ADHESIVE Dry ADHESIVE Due to heating

35. Now What Change you have seen here Immediately after coating Wet adhesive thickness Web thickness Immediately after drying But dry adhesive thickness is reduced to almost half Web thickness Remains the same Adhesive molecules are now Diffused into the web

36. This theory is true when both the adhesive and adherent are polymers, capable of movement and are mutually compatible and miscible. Parameters affecting the diffusion process are

37. Polarity generally increases adhesion. Adhesive Inter Diffusion Substrate Adhesive Substrate has limitation The diffusion theory, has limitation ( or limited application ) where the polymer and adherent are not soluble or the chain movement of the polymer is restricted due to cross linking, crystalline structure, or when it is below its glass transition temperature. Adhesive Substrate Adhesive Substrate Inter Diffusion

38. Adhesion theory – Electrostatic The basis of the electrostatic theory of adhesion is the difference in electro negativities of adhering materials. Adhesive force is attributed to the transfer of electrons across the surface creating positive and negative charges that attract one another. For example, when an organic polymer ( of Conductive Nature) is brought into contact with metal, electrons are transferred from metal into the polymer, creating an attracting electrical double layer (EDL).

39. Adhesion theory – Electrostatic The electrostatic theory tell us that these electrostatic forces at the surface ( i.e. in the EDL), are responsible to non-separation of the adhesive and the substrate.

40. Mechanical adhesion Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking. Adhesion can take place by adhesive flow The adhesive must wet the substrate, and fill micro cavities on the substrate Adhesive must have the right rheological properties to penetrate pores and openings in a reasonable time.

41. Mechanical adhesion The surface of a substrate is never truly smooth but consists of a maze of peaks and valleys. The adhesive then hardens, the substrates are held together mechanically. Adhesive

42. An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials. Adhesives cure (harden) by either evaporating a solvent or by chemical reactions that occur between two or more constituents.

43. An adhesive, or glue, Adhesives are advantageous for joining thin or dissimilar materials, minimizing weight, and when a vibration dampening joint is needed. A disadvantage to adhesives is that they do not form an instantaneous joint, unlike most other joining processes, because the adhesive needs time to cure

44. Now a day's adhesives are used in all types of manufacture, A range of adhesives (hot melt, vegetable glues and emulsions) are used in making cardboard boxes, with rarely a staple to be seen. Apart from expensive handmade shoes, footwear is now adhesively bonded using hot melt adhesives for the basic construction, natural rubber latex for linings, and solvent based polyurethanes or poly chloroprene's for sole attachment Bookbinding is by hot melt adhesives. Adhesive bonding is increasingly used in the construction of aircraft, cars, buses etc.

45. Basic Properties What is an adhesive and what are its basic properties? A definition is Adhesive is a material which when applied to the surfaces of materials can join them together and resist separation. The terms adherent and substrate are used for a body or material to be bonded by an adhesive. Other basic terms are shelf-life, the time an adhesive can be stored before use, and pot-life, the maximum time between final mixing and application.

46. Basically an adhesive must do two things: 1. It must wet the surfaces, that is it must spread and make a contact angle approaching zero, as is illustrated in Figure 1.1. Intimate contact is required between the molecules of the adhesive and the atoms and molecules in the surface. When applied the adhesive will be a liquid of relatively low viscosity. 2. The adhesive must then harden to a cohesively strong solid. This can be done by chemical reaction, loss of solvent or water, or by cooling in the case of hot melt adhesives. There is an exception to this, and that is pressure-sensitive adhesives which remain permanently sticky. These are the adhesives used in sticky tapes and labels.

47. liquid droplets making n high and low contact angle on a flat, solid surface. high contact angle leading to no spreading on a rough surface. wetting on a rough surface.

48. Basic Chemistry All adhesives either contain polymers, or polymers are formed within the adhesive bond Polymers give adhesives cohesive strength, and can be thought of as strings of beads (identical chemical units joined by single covalent bonds), which may be either linear, branched or cross linked as illustrated in Figure 1.2. Linear and branched polymers have similar properties and it is not easy to distinguish them, and they will flow at higher temperatures and dissolve in suitable solvents.

49. All structural adhesives are cross linked because this eliminates creep (deformation under constant load). Automotive tires are cross linked natural or synthetic rubber, and if they crept they would permanently deform during parking, and a rough ride would follow.

50. Linear polymer Branched polymer Cross linked polymer

51. Factors that influence the adhesion - Introduction The stronger adhesion of bonds between mechanically or chemically roughened surfaces is based on the enlargement of the effective surface (contact surface between the adhesive and the substrate), and an increase in the number of active centers, e. g. edges, corners, increase the interactive forces in the interface adhesive/surface. The following factors have a predominant importance in the adhesion process:

52. Wetting of the surface Surface treatment Structure of the materials to be bonded (incl. Adhesives and substrates) Design of the joint (incl. stresses applied on the bonded materials) The Glass Transition Temp Factors that influence the adhesion - Wetting of the surface To enable the adhesive bonds between the adhesive and the surface, the adhesive must first wet the surface; in other words, it must be applied in the liquid form (as a solution, dispersion, or hot-melt).

53. A measure for the wet ability of a surface is the angle of contact that forms between a drop of liquid and a smooth, plain surface A good wetting occurs when the angle of contact () between the adhesive and the substrate is inferior to 90? Complete wetting occurs when the molecular attraction between the liquid and solid molecules is greater than that between similar liquid molecules. Whether or not a given liquid will wet a solid depends on the surface tension of both substances, e.g. polymer and substrate.

54. The contact surface formed during wetting depends on the surface tension and the viscosity of the adhesive, and also on the structure (shape and size of the pores) of the surface. The size of the effective surface is generally smaller than the true surface of the substrate, because the pores and uneven parts of the surface are not completely filled by the adhesive. Pressure may also help enhance the adhesion. Generally, bonds that have been set under pressure have higher adhesive strength Pressures imparts better wetting and consequently a more complete interfacial contact.

56. Factors that influence the adhesion – Surface treatment All surfaces exposed to the normal atmosphere undergo gas and water adsorption in the molecular range; the surface condition can be also changed by oxidation processes. To ensure a good adhesion it is sometimes necessary to carry out, particularly on metals, expensive mechanical and/or chemical pre-treatment (e.g. sandblasting and pickling). On the other hand, inert (too little reactive) plastics surfaces are activated by subjecting them to specific surface treatment for plastics (e.g. flame treatment, corona discharge). In principle, these processes serve to form active centers and polar, reactive groups, which favor the wet ability and the chemi absorption of suitably pretreated surfaces.

58. Glass Transition Temperature The mechanical properties of polymers radically change at the glass transition temperature (TJ; molecular motion is the underlying cause of the change. Below Tg there is no translational or rotational motion of the atoms that make up the polymer backbone, but these motions are present above Tg. Below Tg, polymers are relatively hard, inflexible and brittle, whilst above it they are soft and flexible. The terms glassy, and rubbery or leathery are used to describe properties in the two temperature regions.

59. Both glassy and rubbery polymers are used as adhesives, examples being the use of glassy adhesives for structural bonding in engineering and bone cements in surgery, and rubbery ones as pressure-sensitive adhesives and for bonding flexible substrates. It is unacceptable for an adhesive to pass through the glass transition during service.

61. Viscoelastic Properties Polymers are described as viscoelastic in that they show a combination of the properties of a spring, and a dashpot filled with a viscous liquid (an automotive shock-absorber). A spring will deform instantaneously when loaded, and will recover fully and instantaneously when the load is removed. The deformation of a dashpot will increase with time, and it will not recover when the load is removed. A model which contains two springs and two dashpots, and which describes the qualitative behavior of polymers and adhesives is shown in Figure 1.7. On loading, spring B will instantly deform and dashpot A will begin to flow interminably.

62. The response of spring C will be delayed by dashpot D. When the load is removed, spring B will recover immediately and fully; the recovery of spring C will be total but delayed by dashpot D. The deformation of dashpot D is irreversible. Clearly the properties of an adhesive that might be used in engineering will be dominated by the spring-like properties, and such adhesives are cross- linked to eliminate the viscous element. In contrast, pressure sensitive adhesives for tapes and for sticky solids such as Blu-Tak'R have a large viscous component, and the transverse stripes which form on clear tapes at the point where they are left to dwell on the roll is due to viscous flow.

63. Application A pplicators of different adhesives are designed according to the adhesive being used and the size of the area to which the adhesive will be applied. The adhesive is applied to either one or both of the materials being bonded. The pieces are aligned and pressure is added to aid in adhesion and rid the bond of air bubbles. C ommon ways of applying an adhesive include brushes, rollers, using films or pellets, spray guns and applicator guns (e.g., caulk gun). All of these can be done manually or can be automated into a machine.