A diisocyanate will react wit a dihydric alcohol to give a linear polyurethane. If either of the reactants is trifunctional a crosslinked polymer such as the polyurethane from a dihydric compound and triphenylmethane-p-triisocyanate is formed. In an adhesive, formation of the crosslinked polymer must of course be completed in the gluline. The adhesive therefore normally consist of two parts which are mixed together immediately before use, one part being the polyisocyanate (or prepolymer), the other a polyester or polyalcohol with terminal hydroxyl groups. In calculating the amount of polyhydric compound to be added stoichiometric proportions of isocyanate and hydroxyl groups would theoretically appear to be important, but in practice the ratio is not at all critical, although uncombined isocyanate is undesirable.
A considerable excess of polyester may be used to produce a more flexible adhesive, with some sacrifice of heat and chemical resistance. Adhesives based on mixtures of tolylene diisocyanate prepolymer and saturated polyester resins can be formulated to give a range of pot-lives, from one or two hours up to several hours. Since the polyurethane reaction is base catalyzed, a small amount of a tentiary amine such as N-methyl morpholine may be used to accelerate the curing reaction, especially at room temperature. Although the curing of a polyurethane adhesive is accelerated by heat, the effect of heat is not as marked as in the curing of the condensation polymers.
One interesting adhesive system (reffered to as a polyurethane) is that in which a glycidyl ether is used as solvent for the solid amine component. The alleged purpose of such an unusual solvent is as a possible "reactive modifier" presumably rather on the lines of the reactive diluent used with epoxy resins.
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Wednesday, July 29, 2009
Friday, July 17, 2009
The Reactions of Polyisocyanates
The important reactions are those between polyisocyanates and substances containing active hydrogen atoms, addition polymers either linier or cross linked being formed. Recently homopolymers of polyisocyanates have been described.
There are three classes of compound containing active hydrogen atoms that are of interest in their reaction with polyisocyanates, namely compounds containing hydroxyl groups (polyols such as polyhydric alcohols, polyesters, polyesteramides, polyethers and castrol oil), carboxylic acids, and amines. The addition polymers formed with any of these are loosely refered to as polyurethanes, but this term is correctly applied to the product of reaction and amines (and also water) reacting to form polyureas.
In adhesive applications it is the polyurethanes that are of most importance. The hydroxyl containing component generally used a saturated polyester although unsaturated polyester have also been used, in one patent, for example, a metal to metal adhesive based on triphenyl methane 4,4,4-triisocyanate and the unsaturated polyester from maleic acid and diethylene glycol is claimed. A typical saturated polyester is polyethylene adipate, among many others the esters from phthalic unhydride, trimethylolpropane and 1,4-butandiol have been used or described.
In the preparation of the polyester, the ratio of alcohol to acid should be considerably in excess of equimolecular in order to ensure polyester chains terminated almost entirely by hydroxyl groups. Such hydroxyl groups in their reaction with isocyanate groups do not split off any other substances, whereas carboxyl groups split off carbon dioxide – as also does water. It is difficult to eliminate these sources of carbon dioxide totally from the polyester. The evolution of carbon dioxide creates bubbles in the glue layer that can be a source of weakness in the point – a shortcoming that has probably had some effect in limiting the development of polyurethane adhesive. With further reference to this disadvantage, it has been reported that when glued joints were prepared in an atmosphere of 190 psi nitrogen no bubble of carbon dioxide were formed and the bond strengths were 10-20% higher than when joints were prepared in atmosphere pressure; but their preparation in dry air would on feels, also produce bubble free glue films.
Whilst polyesters with hydroxyl end group such as polypropylene glycol are of great importance in the manufacture of polyurethane foams, they are of mine interest in adhesive systems; one of the reasons may be the readiness with which they foam, presumably due to traces of water. The opinion has, however, been expressed that the formation of bubble free films from solventless polyester may become possible through progress made with catalyst that, being specific for the polyurethane reaction, do not catalyze the reaction with carboxyl groups or water.
There are three classes of compound containing active hydrogen atoms that are of interest in their reaction with polyisocyanates, namely compounds containing hydroxyl groups (polyols such as polyhydric alcohols, polyesters, polyesteramides, polyethers and castrol oil), carboxylic acids, and amines. The addition polymers formed with any of these are loosely refered to as polyurethanes, but this term is correctly applied to the product of reaction and amines (and also water) reacting to form polyureas.
In adhesive applications it is the polyurethanes that are of most importance. The hydroxyl containing component generally used a saturated polyester although unsaturated polyester have also been used, in one patent, for example, a metal to metal adhesive based on triphenyl methane 4,4,4-triisocyanate and the unsaturated polyester from maleic acid and diethylene glycol is claimed. A typical saturated polyester is polyethylene adipate, among many others the esters from phthalic unhydride, trimethylolpropane and 1,4-butandiol have been used or described.
In the preparation of the polyester, the ratio of alcohol to acid should be considerably in excess of equimolecular in order to ensure polyester chains terminated almost entirely by hydroxyl groups. Such hydroxyl groups in their reaction with isocyanate groups do not split off any other substances, whereas carboxyl groups split off carbon dioxide – as also does water. It is difficult to eliminate these sources of carbon dioxide totally from the polyester. The evolution of carbon dioxide creates bubbles in the glue layer that can be a source of weakness in the point – a shortcoming that has probably had some effect in limiting the development of polyurethane adhesive. With further reference to this disadvantage, it has been reported that when glued joints were prepared in an atmosphere of 190 psi nitrogen no bubble of carbon dioxide were formed and the bond strengths were 10-20% higher than when joints were prepared in atmosphere pressure; but their preparation in dry air would on feels, also produce bubble free glue films.
Whilst polyesters with hydroxyl end group such as polypropylene glycol are of great importance in the manufacture of polyurethane foams, they are of mine interest in adhesive systems; one of the reasons may be the readiness with which they foam, presumably due to traces of water. The opinion has, however, been expressed that the formation of bubble free films from solventless polyester may become possible through progress made with catalyst that, being specific for the polyurethane reaction, do not catalyze the reaction with carboxyl groups or water.
Thursday, July 9, 2009
Resin and Plastic
General people always called this compound as plastic, but chemist or chemical expert usually called this compound as resin. Plastic and resin can be said as the same material, this material called as resin because from the reaction process that involve a cross linking development on chemical reactions.
History of Plastic
The development of a commercial phenolic resin in 1900 by Backland was the star of the synthetic plastic industry. His discovery stimulated the search for other plastics and resulted in an industry that has grown to become one of the nation’s top ten in size. The first plastic of industrial significance was cellulose nitrate (Celluloid) and was discovered about the middle of the nineteenth century. It was first used in 1869 by Hyatt who was searching for an ivory substitute.
Cellulose acetate was developed in 1894 as a less flammable material and was used extensively as a base for photographic film and as dope for air plane covering during World War I. From that time on, the introduction of new polymer materials was rapid.
Classification
Plastics are often divided into thermosetting, thermoplastic, oil soluble, and protein products as presented in the table Type of Resin and Plastic. On the basic of derivation, they may be grouped as natural resins, cellulose derivatives, protein products, and synthetic resins. In general, except where noted, synthetic resins formed by condensation polymerization resins formed by addition polymerization are thermoplastic (heating soften and cooling hardens). These two polymerization reactions are fundamentally different.
Addition polymerization involves a series of conversions which produce a polymer having a recurring structural unit identical with that of the monomer from which it is formed. Condensation polymerization yields polymers whose recurring units lack certain atoms present in the original monomer. The reaction takes place by the combination of two or more units and the elimination of a small such as water, methanol or hydrogen chloride. During or after, the original polymerization, the long chains of polymer may react with each other to form a cross linked material which is usually harder and tougher than the straight chain polymer. Properties can be varied for special purpose by regulating the amount of cross linking.
Another variation in the type of final product is effected by the simultaneous polymerization of two or more types of monomers. By carefully regulating the relative amounts of the monomers and reaction conditions and initiators, the properties of the final polymer can be controlled. Three types of copolymers may be formed, depending upon conditions.
Random copolymer, M1 M2 M2 M1 M1 M1 M2
Alternating copolymer, M1 M2 M1 M2 M1 M2
Block copolymer, M1 M1 M1 M1 M2 M2 M2 M2
Properties of plastic can be change by reinforcement usually called as filler with various material, usually fibers of some sort. Common reinforcing fibers are cellulose fibers, fiberglass, carbon fiber, aramid fibers, and metal filaments.
Engineering plastic are high strength high performance materials that can be substituted for many metal uses. There are a wide variety of engineering plastics available. Each one has its own special properties. These material are often the usual plastics but have been carefully manufactured to process extra quality properties. These materials show better resistance to wear, impact and corrosive chemicals and have excellent electrical properties. Some of the uses are automobile bumpers and dashboards, pumps, valves, and gears, and driveshaft and transmission heavy-duty equipment. Many of the common resins are in use as engineering plastic such as acetal, fluoroplastics, nylon, polyphenylene oxide, polycarbonate, polyphenilene sulphide, polysulfone, polyesther-imide, polyethersulfone and nylon polyester block amides and several other copolymers.
The common names of plastics are usually the common or even the principal trade names and often are referred to by abbreviations.
Types of Resins and Plastics:
Thermosetting Resins
History of Plastic
The development of a commercial phenolic resin in 1900 by Backland was the star of the synthetic plastic industry. His discovery stimulated the search for other plastics and resulted in an industry that has grown to become one of the nation’s top ten in size. The first plastic of industrial significance was cellulose nitrate (Celluloid) and was discovered about the middle of the nineteenth century. It was first used in 1869 by Hyatt who was searching for an ivory substitute.
Cellulose acetate was developed in 1894 as a less flammable material and was used extensively as a base for photographic film and as dope for air plane covering during World War I. From that time on, the introduction of new polymer materials was rapid.
Classification
Plastics are often divided into thermosetting, thermoplastic, oil soluble, and protein products as presented in the table Type of Resin and Plastic. On the basic of derivation, they may be grouped as natural resins, cellulose derivatives, protein products, and synthetic resins. In general, except where noted, synthetic resins formed by condensation polymerization resins formed by addition polymerization are thermoplastic (heating soften and cooling hardens). These two polymerization reactions are fundamentally different.
Addition polymerization involves a series of conversions which produce a polymer having a recurring structural unit identical with that of the monomer from which it is formed. Condensation polymerization yields polymers whose recurring units lack certain atoms present in the original monomer. The reaction takes place by the combination of two or more units and the elimination of a small such as water, methanol or hydrogen chloride. During or after, the original polymerization, the long chains of polymer may react with each other to form a cross linked material which is usually harder and tougher than the straight chain polymer. Properties can be varied for special purpose by regulating the amount of cross linking.
Another variation in the type of final product is effected by the simultaneous polymerization of two or more types of monomers. By carefully regulating the relative amounts of the monomers and reaction conditions and initiators, the properties of the final polymer can be controlled. Three types of copolymers may be formed, depending upon conditions.
Random copolymer, M1 M2 M2 M1 M1 M1 M2
Alternating copolymer, M1 M2 M1 M2 M1 M2
Block copolymer, M1 M1 M1 M1 M2 M2 M2 M2
Properties of plastic can be change by reinforcement usually called as filler with various material, usually fibers of some sort. Common reinforcing fibers are cellulose fibers, fiberglass, carbon fiber, aramid fibers, and metal filaments.
Engineering plastic are high strength high performance materials that can be substituted for many metal uses. There are a wide variety of engineering plastics available. Each one has its own special properties. These material are often the usual plastics but have been carefully manufactured to process extra quality properties. These materials show better resistance to wear, impact and corrosive chemicals and have excellent electrical properties. Some of the uses are automobile bumpers and dashboards, pumps, valves, and gears, and driveshaft and transmission heavy-duty equipment. Many of the common resins are in use as engineering plastic such as acetal, fluoroplastics, nylon, polyphenylene oxide, polycarbonate, polyphenilene sulphide, polysulfone, polyesther-imide, polyethersulfone and nylon polyester block amides and several other copolymers.
The common names of plastics are usually the common or even the principal trade names and often are referred to by abbreviations.
Types of Resins and Plastics:
Thermosetting Resins
- Phenolic resins; Bakelite, Durez, Catalin, Formica, Indur
- Amino resins; Plaskon, Beetle, Cymel, Micarta, Melmac
- Alkyd resins; Glyptal, Rezyl, Becksol, Dulux
- Epoxy resins; Epon, Araldite, Ren, Epocast, Marblette
- Polyester (unsaturated) and allyl resins; Aropol, Atlac, Dapon
- Silicone resins; Pyrotex, Dow Corning
- Cellulose nitrate; Celluloid, Pyralin, Nitron
- Cellulose acetate; Kodapa, Tenite, Plastacele
- Cellulose propionates; Forticel, Reed
- Cellulose acetate-butyrates; Tenite II, Kedapak II
- Ethyl cellulose; Ethocel, Soplasco, Campco
- Acrylate or polyacrylates; Plexiglass, Lucite, Acryloid
- Vynils; Vinylite, Gelva, Butacite, Kroseal, Alvar, PVA
- Polyvinylydenes; Saran
- Styrene; Styron, Lustrex, Laolin
- Polyamides; Nylon, Zytel, Kevlar, Nomex
- Polyethers; Penton, Calcon, Delrin
- Polyethylene; Polyethylene, Poly-Pro, Pro-fax
- Fluorocarbons; Kel-F, Teflon, Fluorosint
- Polyesters; Mylar, Celanex, Ekonol
- Polycarbonates; Lexan, Merlon
- Plysulfones; Udel, Astreel 360, Victrex, Radel
Wednesday, July 8, 2009
Preparation of Polyisocyanates and Prepolymers
Isocyanates can be made in a number of ways, but the commercial method is by phosgenation of an amine. As an example, in the preparation of tolylene diisocyanate (TDI), phosgene is passed into a heated solution of tolylene-diamine. The solvent commonly used is ortho-dischlorobenzene, an excellent solvent for both the phosgene and amine. A mixture of 2,4- and 2,6-tolylene-diamine isomers is preferred because the tolylene diisocyanate prepared from this is less likely to crystalline than that from the pure isomer. After distillation tolylene diisocyanate is obtained as a mobile liquid having a viscosity of less than one poise.
Tolylene diisocyanate is the most important polyisocyanate and its low viscosity would be a great advantage in adhesive applications were it not for the fact that in this (“pure”) form the substance is toxic and has a relatively high vapor pressure, making it unpleasant to handle. Furthermore, it reacts inconveniently rapidly rapidly with many hydroxyl compounds. These defects can be overcome to some extent by using a prepolymer made by combining the “pure” isocyanate with a small amount of hydroxyl compound in order to reduce the vapor pressure and reactivity. Unfortunately, the prepolymer inevitably has a higher viscosity, making it necessary to add a solvent to facilitate application.
The prepolymer is made by reacting the diisocyanate with polyhydric alcohol, such that the ratio of isocyanate to hydroxyl group is about 2:1, thus leaving one half of the isocyanate group available for subsequent reaction. To ensure a smooth reaction, an alcohol in which both monomer and prepolymer are soluble such as trimethylolpropane, is used. The formation of such a prepolymer is shown in the following equation:
A triisocyanate of interest as an adhesive is triphenylmethane p,p,p-triisocyanate. This is produce by phosgenation of para-triminotriphenylmethane. In the pure state it is a solid, whereas the technical grade is a viscose liquid, frequently used as a solution in methylene chloride.
Tolylene diisocyanate is the most important polyisocyanate and its low viscosity would be a great advantage in adhesive applications were it not for the fact that in this (“pure”) form the substance is toxic and has a relatively high vapor pressure, making it unpleasant to handle. Furthermore, it reacts inconveniently rapidly rapidly with many hydroxyl compounds. These defects can be overcome to some extent by using a prepolymer made by combining the “pure” isocyanate with a small amount of hydroxyl compound in order to reduce the vapor pressure and reactivity. Unfortunately, the prepolymer inevitably has a higher viscosity, making it necessary to add a solvent to facilitate application.
The prepolymer is made by reacting the diisocyanate with polyhydric alcohol, such that the ratio of isocyanate to hydroxyl group is about 2:1, thus leaving one half of the isocyanate group available for subsequent reaction. To ensure a smooth reaction, an alcohol in which both monomer and prepolymer are soluble such as trimethylolpropane, is used. The formation of such a prepolymer is shown in the following equation:
A triisocyanate of interest as an adhesive is triphenylmethane p,p,p-triisocyanate. This is produce by phosgenation of para-triminotriphenylmethane. In the pure state it is a solid, whereas the technical grade is a viscose liquid, frequently used as a solution in methylene chloride.
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