Chemistry Investigatory

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Text of Chemistry Investigatory

CONTENTS

1. Acknowledgement2. Why I Chose This Project?3. Introduction4. History of Adhesives5. Characteristics of Adhesives6. Classification of Adhesives7. Theory of Adhesion8. Adsorption9. Types of adsorption10. Characteristics of Adsorption11. Adsorption and Absorption12. Factors affecting Absorption13. Experiment14. Bibliography

WHY I CHOSE THIS PROJECT?The role played by colloidal and surface chemistry which we see in everyday life is due to surface phenomenon. The attraction between surfaces of two materials can be related to their strong adhesion. This attractive adhesion is a direct result of the adsorption existing between the surfaces in contact. Greater the surface attraction, stronger is the adhesion. Therefore studying adsorption ultimately provides explanation for adhesion action. So I chose to do a project connecting these two topics by preparing an adhesive and studying its properties.

INTRODUCTIONAdhesives are substances which are capable of holding materials together in a functional manner by surface attachment that resists separation. We use adhesives in our lives to a large extent. The term adhesives include cement,mucilage, glue and paste terms that are often used interchangeably for any organic material which forms an adhesive bond. In this the nature, cause and mechanism of adhesion is studied and an adhesive is prepared.

HISTORY OF ADHESIVES

The first synthetic adhesive was produced in 1869. This material was incorrectly termed nitrocellulose and was created by a reaction between nitric acid,sulfuric acid, andcellulose. Today, this product is known as cellulose nitrate. In 1912,Leo Baekelandproduced phenol-formaldehyde resins, a basic material for many of todays adhesives. High strength, elastomeric adhesives were available in 1928 when a reaction that produced polychloroprene was developed. Later in the 1930s, pressure sensitive tapes were developed.The first metal bonding adhesive was developed by Nicholas de Bruyne in 1941. This material was used in the construction of aircraft. Later in the decade, epoxy resin adhesives were introduced. During the 1960s the extremely strong cyanoacrylate adhesives were developed. These products, called super glues, became adhesive when exposed to moisture in the air. Other adhesives that were developed during this time include silicones and anaerobic adhesives. Since that time, most of the advances in adhesive technology have been the result of formulation modifications using varied polymers. In the late 1980s Post-It notes were introduced using a micro structured adhesive.

CHARACTERISTICS OF ADHESIVES

For a material to be a good adhesive it must has a variety of characteristics. It should have a liquid surface tension that is lower than the wetting tension of the substrate. It should be applied to a surface that is significantly rough to improve adhesion. For polymeric substrates, the adhesive should be somewhat mutually soluble allowing diffusion between the two to occur.

CLASSIFICATION OF ADHESIVES

Adhesives can be classified by the type of delivery of the adhesive or by the polymer used in the adhesive. In general, there are five categories of adhesives including structural, natural, pressure-sensitive, hot melt, and solvent-based adhesives.

Structural adhesivesStructural adhesives are some of the strongest adhesive materials available. They are based on resin systems, typically thermosets, and are meant to serve as permanent bonds. They are supplied as low-molecular-weight polymers that solidify when polymerized. They are sold in a variety of forms including two-part systems, pastes, and films.

Examples of Structural Adhesivesa.Epoxy resins: Epoxy resins are typically based on the polymerization reaction of bisphenol A with epichlorohydrin. The product is sold as a two-part system in which the user applies the epoxy resin, then an amine hardener that causes the resin to cure. Epoxy resins are used in the construction of aircraft and automobiles. They are also a component of plastic cement.

b.Phenolic resins:Phenolic resins have been used as adhesives since the early 1900s. They are produced by the polymerization reaction of phenol and formaldehyde. To cure, heat is typically required to drive off excess solvent. Thistype of adhesive is usedin the production of plywood. They also are noted as the most adhesive material to aluminum.

c.Acrylic adhesives:A variety of acrylic adhesives are available. Anaerobic adhesives are unique reactive adhesives. They consist of a mixture of hydro peroxides and dimethacrylates that polymerize in the absence ofoxygen. This type of material is useful in anchoring screws and bolts. This adhesive is based on cyanoacrylates which spontaneously polymerizes in moist air.

d.High-temperatureresistant adhesives:Many structural adhesives that are used for aerospace purposes are lightweight and high-temperatureresistant. These are typically resins that have high glass-transition temperatures. An example of this type of adhesive is polyimide that is formed by the polymerization reaction of an aromatic amine with an aromatic anhydride.

e.Elastomeric Adhesives:Since many of the structural adhesives become brittle after they cure, adhesives formulators developed elastomeric adhesives. These materials contain compounds that reduce brittleness without significantly reducing adhesion. This property is useful in applications where vibrational forces can stress adhesives.

f.Urethane adhesives:The final class of structural adhesives is the urethanes. These materials are based on the polymerization reaction of a diol with a diisocyanate. They are primarily sold as two-part structural adhesives. An important application of these kinds of adhesives is in the automotive industry. Polyurethanesare used to bondpolyestercords in rubber tires.

Natural-Based Adhesives:Many natural-based adhesives are available. In general, these adhesives are not as strong as the synthetic adhesives. Natural rubber has been used as an adhesive for over 100 years. In 1825, the Macintosh raincoat, which consisted of two layers of cotton bonded by a layer of natural rubber, was introduced.Proteinadhesives are used as structural adhesives. They are based on polyamino acids obtained from various animal and plant sources. Curing is typically dependent on heat. Starch-based adhesives are derived from plants. They are primarily used for binding paper and as envelope adhesives. Cellulose is another natural material that provides adequate adhesion. Cellulose nitrate was one of the first modified adhesives produced. It is a general all-purpose adhesive which is waterproof and flexible. Modified methylcellulose is used to make wallpaper paste. Other natural adhesives are tariffing resins that are derived fromcoal, petroleum or wood tar.Pressure sensitive adhesives:Pressure sensitive adhesives are polymeric-based adhesives that melt at room temperature. When pressure is applied to the adhesive, they become flow able thereby covering the substrate. As the pressure is removed adhesion takes place. Many tapes use this kind of adhesive material. Pressure-sensitive tape uses a blend of glycerol and abiotic acid esters with natural rubber on cellophane. In addition to pressure-sensitive adhesives, hot melt adhesives have been developed. Since thermoplastics melt when heated and reform when cooled, they make good adhesives. This is the principle behind glue guns. Nylon polymers are often used for their formulation. Solvent-based adhesives:Solvent-based adhesives work through the action of the substrate or adhesive. The material is put on the substrate and when the solvent evaporates bonding occurs. Adhesion is aided if the solvent interacts with the substrate. An example is model airplane glue that tends to dissolve some of theplastic. This helps to create a solid weld. Latex adhesives are another type of solvent-based adhesive. They are polymeric materials that provide adhesion after their aqueous solvent evaporates. These are the same types of polymers that are used for latex paints. Latex adhesives are used for bonding pile to carpets.THEORY OF ADHESION

A variety of theories about how adhesives work have been proposed. While none of these satisfactorily describes all aspects of adhesion, they do attempt to explain observed phenomena. The leading theories of adhesion include thediffusion, electrostatic, surface energetics, and mechanical and adsorption theories.

Diffusion theory:The diffusion theory states that adhesion is a result of thesolubilityof the adhesive to the substrate. When the adhesive is applied to the substrate, it is suggested that asolutionof the substrate and adhesive is formed if the solubility characteristics are equal. This creates a stable phase that bonds the two surfaces together. While this theory provides some insights, it is mostly applicable to situations in which polymers adhere to each other. It is not applicable to systems where the substrate and adhesive are radically dissimilar.

Electrostatic theory:The electrostatic theory suggests that adhesion is the result of differences in theelectro negativitiesof adhering materials. According to the theory, when two materials are brought into contact there is an amount of electron transfer that occurs. This results in a charge layer being formed which causes the materials to stay together. An example of this theory can be seen when using staticelectricityto make a balloon stick to a wall.

Surface energetics theory:The surface energetics theory describes adhesion in terms of intermolecular and interatomic forces. For these forces to have an effect, the adhesive must come into close contact with the substrate. According to this theory adhesion is a result of bonding across the interface.An example o