Chapter 1.Literature Review 1.1 Monomer Synthesis A synthesis of vinyl acetate monomer form syngas was researched by EASTMAN CHEMICAL COMPANY RESEARCH LABORATORIES. The overall objective of this project is to develop a commercially viable process for the generation of vinyl acetate monomer (VAM) based entirely upon coal-generated syngas. Previous attempts at this objective have generally involved the combination of acetic anhydride (generated by carbonylation of dimethyl ether or methyl acetate) with acetaldehyde (generated by hydrocarbonylation of methanol or a methyl ester) to generate ethylidene diacetate (EDA), which is subsequently cracked to form VAM and acetic acid in a separate step. Owing primarily to the need to recycle or otherwise dispose of huge quantities of the acetic acid byproduct, these efforts have failed to generate a commercially viable process to date. Eastman’s proposal centers around the generation of acetaldehyde from acetic acid (AcOH) rather than methanol or a methyl ester. The proposed linchpin technology was to develop and apply a proprietary method for generating transient, higher energy acetic acid species (referred to hereafter as activated acetic acid or AAA) thus permitting the normally endothermic hydrogenation of acetic acid to acetaldehyde (AcH) to proceed in high conversions, at acceptably low pressures, and moderate temperatures without subsequent hydrogenation to ordinarily thermodynamically favored ethanol. The acetaldehyde thus formed could be converted to EDA and subsequently to VAM by known methods (i.e. combining acetic anhydride with acetaldehyde to form EDA followed by subsequent cracking to VAM and AcOH.) Within the proposal, Eastman presented several methods for improving this known process. However, Eastman also proposed a very speculative application of the same (or modified) proprietary technology for the generation of activated acetic acid to the direct esterification of acetaldehyde to yield VAM. The normally endothermic esterification of acetic acid with acetaldehyde would also become favorable if the endothermic AAA generation process could be coupled with an exothermic reaction of AAA with acetaldehyde to produce VAM. If this overall speculative conversion came to fruition, the overall process would be represented by the following equations: Step 1. Step 2. AcOH + H 2→ AcH + H2O AcOH + AcH→ VAM + H2O and would represent a synthesis of VAM entirely from acetic acid. (The commercially preferred route to acetic acid, methanol carbonylation, is already based entirely upon syngas.) 1.2 Chemistry of the Process 1.2.1 Chemistry of the Polymerization Reaction Poly(vinyl acetate), or PVA for short, is one of those low-profile behind-thescenes polymers. It isn't blatantly obvious where it's found, as is the case with polyethylene or polystyrene. PVA likes to hide. But it's everywhere, if you're willing to look for it. It's a polymer that rewards one who is willing to look beyond the surface. One place PVA can be found hiding is between two pieces of wood that are glued together. PVA is used to make wood glues, as well as other adhesives. Paper and textiles often have coatings made of PVA and other ingredients to make them shiny. PVA is a vinyl polymer, as if you couldn't guess from the name. It's made by free radical vinyl polymerization of the monomer vinyl acetate. This is what the monomer looks like in 3-D: The development of the polymerization reaction, the obtained polymer structure and consequently the obtaining technology of the polymers depend on the structural particularities and reactivity of the monomer. Since the vinyl acetate monomer has no conjugation between the vinyl double bond and the substituting group, this monomer has low reactivity, meanwhile its deriving radical has a higher activity. These two effects are compensating on each other at the homopolymerization, but have a stronger effect in the copolymerization reaction. The most common and useful reaction for making polymers is free radical polymerization. It is used to make polymers from vinyl monomers that are from small molecules containing carbon-carbon double bonds. Polymers made by free radical polymerization include branched polyethylene, polystyrene, poly(methyl methacrylate), polybutadiene, and poly(vinyl acetate). But enough introduction. What is this reaction, and how does it work? The whole process starts off with a molecule called an initiator. This is a molecule like 2,2'-azo-bis-isobutyrylnitrile (AIBN) or benzoyl peroxide. What is special about these molecules is that they have an uncanny ability to fall apart, in a rather unusual way. When they split, the pair of electrons in the bond, which is broken, will separate. This is unusual as electrons like to be in pairs whenever possible. When this split happens, we're left with two fragments, called initiator fragments, of the original molecule, each of which has an unpaired electron. Molecules like this, with unpaired electrons are called free radicals. Initiation C6H5 CO O O CO C6H5 60-800C C6H5 CO O . Now remember, these electrons still want to be paired, so any electrons they can find to pair up with, they will do so. The carbon-carbon double bond in a vinyl monomer, like vinyl acetate, for example, has a pair of electrons that is very easily attacked by the free radical. The unpaired electron, when it comes near those electrons, can't help but swipe one of them to pair itself with. This new pair of electrons forms a new chemical bond between the initiator fragment and one of the double bond carbons of the monomer molecule. This leaves another electron now without a partner, the remaining electron from the pair that had made up the carboncarbon double bond. This electron then, having nowhere else to go, associates itself with the carbon atom, which is not bonded to the initiator fragment. You can see that this will lead us back where we started, as we now have a new free radical when this unpaired electron comes to roost on that carbon atom. C6H5 CO . O + CH2 CH O C O CH3 C6H5 CO OH + CH3 . CH O C O CH3 Propagation CH3 . CH O C O + CH3 CH2 CH O C O CH3 CH3 CH CH2 . CH O C O CH3 OCOCH3 CH2 CH CH2 . CH O C O + CH2 CH3 CH O C O CH3 CH2 CH CH2 CH CH2 . CH O C O CH3 OCOCH3 OCOCH3 OCOCH3 Wouldn't you know it, this new radical reacts with another vinyl acetate molecule in the exact same way as the initiator fragment did. Of course, as we can see, this gets us nowhere as far as pairing electrons goes, because we still have yet another radical when this reaction takes place, and it just goes and reacts with another vinyl acetate molecule. But because we keep making a new radical, we can keep adding more and more vinyl acetate molecules, and in doing so build a long chain of them. Self-perpetuating reactions like this one are called chain reactions. So as long as the chain keeps growing, who really cares if a few electrons remain unpaired? Sadly, the electrons care. Radicals are unstable, and eventually they are going to find away to become paired without generating a new radical, and then our little chain reaction will come grinding to a halt. This happens in several ways. The simplest way is for two growing chain ends to find each other. The two unpaired electrons then join to form a pair, and a new chemical bond joining their respective chains. This is called coupling. Another way in which our unpaired electrons can bring a stop to our polymerization is called disproportionation. Termination CH2 CH CH2 . CH O C O + CH3 CH3 . HC C O O CH3 OCOCH3 coupling CH2 CH CH2 CH O COCH3 disproportionation CH2 CH2 + CH CH OCOCH3 OCOCH3 O COCH3 But radicals, being unstable react also with solvents, the monomer and even with the polymer in order to couple themselves. This way they give transfer reactions with benzene and toluene, with styrene (which acts as an inhibitor for vinyl acetate polymerization), with the polymer leading to longer chains and with the monomer leading to branched polymers. Transfer reactions with toluene, styrene, with the polymer and the monomer CH3 CH2 CH CH2 . CH O C O CH3 + CH2 CH2 O COCH3 + CH2 . O COCH3 CH2 CH2 CH CH2 . CH O C O + CH3 CH CH2 CH CH2 CH O C O CH2 CH3 CH . OCOCH3 O COCH3 CH2 . CH O C O 25% + CH3 CH2 CH O C O 75% CH3 CH2 CH2 O C O + CH3 CH2 . C O C O CH3 CH2 CH2 O + C O CH2 CH3 CH O C O . CH2 (CH2 CH2 . C O C O + nCH2 CH3 CH OCOCH3 CH2 C O C O CH )n-1CH2 OCOCH3 CH3 . CH OCOCH3 CH2 CH O C O + nCH2 . CH2 CH OCOCH3 CH2 C O C O CH2 ( CH2 CH )n-1 CH2 OCOCH3 . CH OCOCH3 1.2.2 Chemistry of the Hydrolysis Reaction One distinct characteristic of poly(vinyl alcohol) is that it is not obtained by direct polymerization of its monomer, but through a polymer-analogous reaction. This reaction is the hydrolysis of a vinylic ester – poly(vinyl acetate). The explanation lies in the fact that this monomer having the formula of the simplest enol could not be isolated because it gives a spontaneous tautomerization reaction in acetaldehyde. CH2 CH OH CH3 CH The tautomerization takes place through the migration of one hydrogen atom and shifting of the double bond. The increased stability of acetaldehyde in comparison with poly(vinyl alcohol) is explained by the difference in the formation heat, which in the case of acetaldehyde is with 15 Kcal higher. The poly(vinyl alcohol) is therefore obtained from poly(vinyl acetate) through hydrolysis. The hydrolysis can take place either in acidic or in basic catalyst. Hydrolysis in acidic medium CH2 CH2 CH O: CH3 C + O H+ CH3 CH2 CH +O C H + CH3OH O CH3 H CH +O C O+ CH3 H O CH2 CH OH + CH3COOCH3 + H+ Hydrolysis in basic medium CH2 CH O: CH3 C + CH3O O - CH2 CH O CH2 OCH3 - CH O+ CH3 C O CH3COOCH3 CH2 CH O- + CH3OH CH2 CH OH + CH3O If there are traces of water in the reaction medium, the last phase is no longer reversible and acetate ions are obtained. These acetate ions lead to the catalyst consumption and forming of sodium acetate. CH3 HO + C O HO CH3 C O HO CH3 C O + CH3O - CH3OH + CH3COO - OCH3 - OCH3 CH3COO + NaOH CH3COONa + HO 1.3 Process Technology 1.3.1 Bulk Polymerization Bulk polymerization takes place in continuous installations or in reactors with anchor mixing device. The process takes place at 75-95 0C in nitrogen atmosphere. As initiator, the benzoyl peroxide is used and for molecular weight regulation we use propionic aldehyde. Finally, the temperature rises at 110-1200C to completion of the polymerization reaction. The polyacetate melt is then granulated. The main advantages of this method are the purity of the obtained polymer, its high molecular weight (the system does not contain other transfer agents than the monomer and the polymer itself) and the low cost (neither solvents nor surfactants are used, therefore the expenses these products’ utilization implies are null). But this procedure has also some limitations: the necessity of frequent stopping the installation to remove the reticular polymer crust from the reactor walls; unreproductable results concerning the molecular weights. 1.3.2 Solution Polymerization This procedure is used on a large scale in industry, the polyacetate solutions being used as lakes and adhesives or for obtaining the poly(vinyl alcohol). Solution polymerization is a homogenous process, where the solvent is selected to solve both the monomer and the polymer. The polymer’s molecular weight depends not only of the initiator, monomer and solvent type and concentration, but also of aldehyde content of the monomer. The solvents used in this procedure depend on the final destination of the product. Thus, for lakes we use ethyl acetate, benzene and acetone, meanwhile for poly(vinyl alcohol) fabrication alcohols like methanol, ethanol and isopropanol. Ethanol is used when the poly(vinyl alcohol) is used to prepare medical and cosmetic emulsions. The nature and concentration of the solvent influence the polymer’s viscosity and conversion. Solvent Toluene Ethanol Acetone Ethyl acetate Benzene Conversion (%) 28.0 22.6 68.5 89.3 55.0 Solution Viscosity (CP) 2.9 5.6 3.0 8.2 18.2 The polymerization process can be batch or continuous, the batch procedure being more used in industry. From the batch polymerization of vinyl acetate a poly(vinyl acetate) lake is obtained. This lake is further hydrolyzed to poly(vinyl alcohol). To obtain high molecular weight polymers a bulk-solution polymerization procedure is applied. According to this procedure the polymerization reaction is initially a bulk polymerization up to 30-40% conversion, allowing us to obtain a high molecular weight polymer. In the next phase polymerization takes place in solution. This procedure leads to a high molecular weight polymer and with few ramifications. The solution polymerization of vinyl acetate has the following disadvantages: low reaction rate, low molecular weight because of the transfer reactions, large molecular weight distribution, branching of the polymer, polymer crusts are formed inhibiting the heat transfer a.s.o., which all together limit the procedure applicability. 1.3.3 Suspension Polymerization This polymerization is essentially a bulk polymerization in which the reaction mixture is suspended as droplets in an inert medium (the reaction mass is heterogeneous). Suspension polymerization of vinyl acetate takes place in aqueous media, in the presence of initiators soluble in the monomer like benzoyl peroxide, azoisobutyrodinitrile or mixtures of initiators. As suspension agents poly(vinyl alcohol) (with 88-92% hydrolyses degree), methylcelulose, oxyethylcelulose a.s.o. are used. This procedure allows the synthesis of a product with high molecular weight and it is usually used for fabrication of poly(vinyl alcohol) for fibers. Its major difficulty lies in the fact that besides the main polymer a gel (20% against the polymer) is formed. This occurs due to the transfer reactions with the polymer in the final stages of the polymerization process. Its advantages are that the polymerization temperature might be higher than in solution polymerization, the system’s viscosity is low, approaching water viscosity. Also water’s high caloric capacity improves the heat removal and the suspension can be easily separated by filtration. 1.3.4 Emulsion Polymerization A distinct characteristic of the emulsion polymerization of vinyl acetate is the fact that its polymerization rate does not depend of the emulsifier concentration. This might be explained knowing that the monomer exhibits a 5% solubility in water. In this case, the polymerization starts almost exclusively in an aqueous solution of vinyl acetate. The such formed polymer precipitates and forms particles, which absorb the emulsifier. Then the polymerization takes place at the surface of these poly(vinyl acetate) particles formed. From the practical point of view the most used emulsifier is the poly(vinyl alcohol). The emulsions realized with poly(vinyl alcohol) have a high stability even at very high concentration of polymer (50%). It was established that during the polymerization process grafting of poly(vinyl acetate) on poly(vinyl alcohol) takes place. The following structure will be obtained: Polymerization results in a latex phase: particles of about 100µ m protected by an emulsifier layer, which do not separate because of their superficial charge. The separation can be made by spray drying or coagulation with appropriate electrolytes. 1.3.5 Poly(vinyl acetate) hydrolysis in solution The poly(vinyl acetate) hydrolysis can take place either in acidic catalyst (with H2SO4, HCl) at approximately 800C, either in basic catalyst (with NaOH, CH3ONa) at almost 350C. Acidic hydrolysis has a series of disadvantages that stopped it to be industrially applied, as follows: the minimum limit of temperature is of 800C at which the finished product can suffer thermal degradation; low reaction rate, which leads to high reaction time, making this method uneconomical; high reagents consumption; the obtained poly(vinyl alcohol) is unpurified with salts resulting from the sulfuric acid; side reactions, like sulfonation of the poly(vinyl alcohol). The basic hydrolysis eliminates all these disadvantages, therefore being applied industrially using two basic technological procedures: 1. The direct method or procedure consisting of introduction of the catalyst in a methanolic solution of poly(vinyl acetate); 2. The indirect method or procedure consisting in step-by-step introduction of poly(vinyl acetate) in the catalyst solution. The main disadvantage of the first procedure consists in the appearance of a hard gel in a certain phase of the reaction (at a hydrolysis degree of almost 65%), which in reactors with normal mixing, will block the reactor. But because of the powerful mixing employed here the mechanical degradation of the macromolecules appears. So our product will be nonhomogenous from the molecular weight point of view. Its main advantage is that the hydrolysis degree is easily controlled. In order to make this procedure functional, methyl acetate or a few drops of water are introduced in the reaction mass to shift the reaction equilibrium at a higher reaction rate or to slowly deactivate the catalyst after passing the gel phase. In this case regular reactors can be used. Introducing both methyl acetate and water (in well predetermined ratios) we can obtain a product at a desired granulation and hydrolysis degree. Usually, the working concentration for PVAc is of 20-30% in methanol depending of the molecular weight of PVAc. At these concentrations the gel phase cannot be avoided, its consistency and duration being ameliorated only by introduction of water or methyl acetate. The gel phase can be completely avoided by poly(vinyl acetate) hydrolysis in suspension. The second procedure has the advantage that during the reaction the gel phase is no longer present. Therefore we need no more special technological measures, but the hydrolysis degree is no longer controllable. During the reaction the small quantities of PVAc introduced in the catalyst solution are rapidly and totally hydrolyzed (98-99%). Thus, this method is applied when we need a maximum hydrolysis degree. Technologically the poly(vinyl alcohol) can be obtained either by batch or continuous procedure. 1.4 Poly(vinyl alcohol) Properties Poly(vinyl alcohol) is a white-yellowish product with density 1.21-1.31 g/cm3 and a vitrifiation temperature of about 800C. It is a crystalline material due to the high number of hydrogen bonds between the polymer chains. Solubility. It is soluble in water, its solubility being dependent on the hydrolysis degree (HD). At HD higher than 80%, the solubility in water decreases due to the fact that the water molecules are hindered to penetrate the PVA chains because of the high number of hydrogen bonds established between the macromolecular chains. For example, at HD 100% PVA is soluble in water only at high temperatures. But when HD is 20% PVA is soluble in water even at room temperature. Besides water, the poly(vinyl alcohol) is also soluble in polar solvents like glycols, glycerin, dimethylformamide, monoethanolamine, phenol, urea, dimethylsulfoxide. From aqueous solutions the poly(vinyl alcohol) can be separated either through water evaporation, when transparent thin layers of film are obtained, or through precipitation with methanol or saturated solutions of inorganic salts. PVA is hardly soluble in organic solvents. Thermal Properties. Due to the high number of hydrogen bonds between the chains, PVA does not melt but at increasing the temperature the polymer begins to decompose. Up to 1200C no change is noticed. Over this temperature, the etheric bonds are formed, leading to the alcohol insolubilization. At even higher temperatures(1200C) its color turns yellow and double bonds appear due to water elimination. Mechanical Properties. It exhibits high mechanical resistance, especially high fraction strength (σ n=3000 kg/cm2). Subjected to elongation, PVA crystallizes and its mechanical resistance increases. Mixing Capacity. The best plastifier for poly(vinyl alcohol) is water. Its increased viscosity limits its uses. Because of its high higroscopicity, the poly(vinyl alcohol) has always small quantities of water, which decreases its vitrifiation temperature. In industry phosphoric acid, ethylene glycol, butylenes glycol and glycerin are used. Chemical Properties. PVA presents all characteristic reactions of polyalcohols. The hydroxilic groups of this alcohol can give etherification reactions, esterification reactions, acetalization reactions or they can react with metallic sodium, ethylene or propylene oxide. By heating with anhydrides we can obtain polymers with esteric substituting groups. Still PVA has the big disadvantage of being easily attacked by water, so it should be made totally insoluble to water. This can be realized by reaction with aldehydes or metal salts resulting in the PVA insolubilization. Another good insolubilization method consists in treatment with epychlorhydrine or by thermoforming at temperatures higher then 140 0C, with acidic catalyst. 1.5 Poly(vinyl alcohol) Uses Poly(vinyl alcohol) is used to obtain fibers, synthetic leather, hoses, fittings stabile at gasoline action, rubber like products, transmission belts, surgeon threads; or used as adhesive or emulsifier. From PVA are made especially short fibers to replace cotton(when PVA has a hydrolysis degree of 100%). In this case the polymer is dissolved, spinned, stretched, dried, thermoforming and acetalized. In industry is most used wet spinning, but fibers of PVA can be obtained also by dried spinning. Stretching is done in 2 or 3 steps at 1:5 ratio. Drying and thermoforming are done to increase the product crystallinity and its stability towards water. Drying is done at 90-110 0C and thermoforming is a thermal treatment at 210-2300C in organic solvents’ vapors or water. Acetalizing is realized with formic aldehyde and sulfuric acid as catalyst at 65-700C for 30-40 minutes achieving an acetalization degree of 35-40%. This way the poly(vinyl alcohol) fibers are totally insoluble. Acetalization is followed by water washing at temperature, drying at 90-1000C and cutting at the desired sizes. The usage of poly(vinyl alcohol) as thermoplastic implies its mixing with the plastifiers. The most used plastifier is water, which reduces the polymer’s vitrifiation temperature, increases compatibility with a higher number of plastifiers and improves processing. Phosphoric acid has a series of advantages like: low volatility, small tendency towards esterification and the polymer is not decomposing. The indirect plastifiers like ethylene glycol, glycerin, some amides and alcohols, .a.s.o., make the polymer gonflate. At a percentage of 65% (molar) of plastifier, the products based on PVA can be sometimes used instead of rubber. For a smaller percentage of 65, the materials obtained are leather like. Having a high mechanical resistance, PVA is used to manufacture transmission belts. To obtain thin films of PVA the water is evaporated from PVA solutions deposited on shiny metallic surfaces. As plastifiers glycerin or ethylene glycol are used with max. 35% concentration. The aqueous solution of PVA is mixed with the plastifier, filtrated at 1000C and deposited under pressure on transporting bands and then dried. At one cycle a thin film of 0.1 mm is obtained. On it a new layer of solution is deposited and so on, the max. number of layers being of 10. The such obtained films are subjected to mechanical processing. The obtained belts have higher mechanical resistance: 2200 kg/cm2 comparing with 480 kg/cm2 for leather belts. Beside they have higher usage resistance. The PVA films are also used as covering material, when insolubilization is done with dimethylol urea. The PVA with 18-20% acetate groups is used as adhesive for gluing rubber on metal. PVA solutions with formaldehyde are used as adhesives for paper, textiles and leather. Poly(vinyl alcohol) has very good tensio-active properties. Because of this is used as protection colloid in suspension polymerization and anionic emulsifier in emulsion polymerization. The best emulsifying properties are the one for PVA with 88-92% hydrolysis degree. Besides all the above mentioned, the poly(vinyl alcohol) can be also used in pharmaceutical industry, at printing inks, in photography for light sensitive layers of films. But one interesting place where the polymer likes to hide is in paint cans. So what is PVA doing hiding in your paint? PVA is the latex in acrylic latex paint. So what does that mean? Let me explain. Before PVA can hide in a can of paint, we have to react it with NaOH and methanol like this: When we do this we clip off all those acetate groups to end up with another polymer, poly(vinyl alcohol). But for paint we don't really want to clip all of the acetate groups. We can control this reaction so that when we're done we still have about 20% of the acetate groups left on the polymer. What we have then is a copolymer of poly(vinyl alcohol) and poly(vinyl acetate) called poly(vinyl alcohol-co-vinyl acetate), appropriately enough. It's a random copolymer, that looks like poly(vinyl alcohol), except with vinyl acetate repeat unit every now and then, like this: But why do we want to do this? It has to do with how acrylic paints work. Acrylic paints in them a compound called methyl methacrylate. Now this is some reactive stuff, and after the paint is brushed on it polymerizes to form poly(methyl methacrylate). Its structure is similar to PVA, but it's different. Look closely. Can you see the differences? Poly(methyl methacrylate) is a hard, tough, and shiny plastic, and if when it forms in the wet paint, it makes the paint surface, hard, tough and shiny. This is good. We want paint to do this. But there's a problem. Methyl methacrylate is hydrophobic. It doesn't dissolve in water, and a lot of paints are water based. This is where poly(vinyl alcohol-co-vinyl acetate) comes to the rescue. You see, this copolymer has an identity crisis. The alcohol groups are hydrophilic. They love water, and want to dissolve in it. But the acetate groups are hydrophobic. They hate water and don't want to dissolve in it. So when you put the copolymer in water, it forms into a ball. The alcohol repeat units are on the outside of this ball, happily embracing the water, while the acetate groups are on the inside of the ball, hiding from it. Now ask yourself this question. If you were a molecule of methyl methacrylate, and as a methyl methacrylate molecule you hated water, where would you go? Would you go out into the water, or would you go into the inside of that coiled polymer, away from the water? You'd go to the inside of the coil of course! And that's just what the real methyl methacrylate does. The methyl methacrylate hides in the hydrophobic center of the coiled polymer. By doing this, it can stay suspended in water based paints! A suspension of an insoluble substance, like methyl methacrylate, held in suspension by being wrapped in another kind of molecule, like our copolymer, is what we call a latex. That's where we get the name latex paint.