A polymer or resin can be defined as a solid or semisolid, water-insoluble, organic substance, with little or no tendency to crystallize. Resins are the basic components of plastics and are important components of surface-coating
formulations. There are two types of resins -- natural and synthetic. The natural resins are obtained directly from sources such as fossil remains and tree sap.
Synthetic resins can be classified by physical properties as thermoplastic or thermosetting. Thermoplastic resins undergo no permanent change upon heating.
They can be softened, melted, and molded into shapes they retain upon cooling, without change in their physical properties. Thermosetting resins, on the other hand, can be softened, melted, and molded upon heating, but upon continued heating, they harden or set to a permanent, rigid state and cannot be remolded.
Each basic resin type requires many modifications both in ingredients and techniques of synthesis in order to satisfy end uses and provide desired properties. Not all these variations, however, will be discussed, since not all present individual air pollution problems. Thermosetting resins are obtained from fusible ingredients that undergo condensation and polymerization reactions under the influence of heat, pressure, and a catalyst and form rigid shapes that resist the actions of heat and solvents. These resins, including phenolic, amino, polyester, and polyurethane resins, owe their heat resisting qualities to cross-linked
molecular structures.
Phenolic Resins: Phenolic resins can be made from almost any phenolic
HAZARDS IN THE CHEMICAL PROCESS INDUSTRIES 59
compound and an aldehyde. Phenol and formaldehyde are by far the most common ingredients used, but others include phenol-furfural, resorcinol-
formaldehyde, and many similar combinations. A large proportion of phenolic- resin production goes into the manufacture of molding materials. Phenol and formaldehyde, along with an acid catalyst (usually sulfuric, hydrochloric, or phosphoric acid), are charged to a steam-jacketed or otherwise indirectly heated resin kettle that is provided with a reflux condenser and is capable of being operated under vacuum. Heat is applied to start the reaction, and then the exothermic reaction sustains itself for a while without additional heat. Water formed during the reaction is totally refluxed to the kettle. After the reaction is complete, the upper layer of water in the kettle is removed by drawing a vacuum on the kettle. The warm, dehydrated resin is poured onto a cooling floor or into shallow trays and then ground to powder after it hardens. This powder is mixed with other ingredients to make the final plastic material. Characteristics of the molding powder, as well as the time and rate of reaction, depend upon the concentration of catalyst used, the phenol-formaldehyde ratio used, and the reaction temperature maintained.
Amino Resins: Among the most important amino resins are the urea- formaldehyde and melamine-formaldehyde resins. The urea-formaldehyde
reaction is simple: 1 mole of urea is mixed with 2 moles of formaldehyde as 38 percent solution. The mixture is kept alkaline with ammonia pH 7. 6 to 8. The reaction is carried out at 77 O F at atmospheric pressure without any reflux. The melamine resins are made in much the same manner except that the reactants must be heated to about 176 O F initially, in order to dissolve the melamine. The solution is then cooled to 77 O F to complete the reaction. The equipment needed for the synthesis of the amino resins consists of kettles for the condensation reaction (usually nickel or nickel-clad steel), evaporators for concentrating the resin, and some type of dryer. The amino resins are used as molding compounds, adhesives, and protective coatings, and for treating textiles and paper.
Polyester and Alkyd Resins: By chemical definition, the product obtained by the condensation reaction between a polyhydric alcohol and a polybasic acid, whether or not it is modified by other materials, is properly called a polyester.
All polyesters can then be divided into three basic classes: Unsaturated polyesters, saturated polyesters, and alkyds. Unsaturated polyesters are formed when either of the reactants (alcohol and acid) contains, or both contain, a double-bonded pair of carbon atoms. The materials usually used are glycols of ethylene, propylene, and butylene and unsaturated dibasic acids such as maleic anhydride and fumaric acid, The resulting polyester is capable of crosslinking and is usually blended with a polymerizable material, such as styrene. Under heat or a peroxide catalyst, or both, this blend copolymerizes into a thermosetting
resin. It has recently found extensive use in the reinforced plastics field where it is laminated with fibrous glass, It is also molded into many forms for a variety of uses. Saturated polyesters are made from saturated acids and alcohols. The polyesters formed are long-chain, saturated materials not capable of crosslinking.
Several of these are used as plasticizers. A special type made from ethylene glycol and terephthalic acid is made into fiber and film. Still others of this type with lower molecular weights are used with di-isocyanates to form polyurethane resins. Alkyd resins differ from other polyesters as a result of modification by additions of fatty, monobasic acids. This is known as oil modification since the fatty acids are usually in the form of naturally occurring oils such as linseed, tung, soya, cottonseed, and, at times, fish oil. The alkyds, thinned with organic solvents, are used predominantly in the protective coating industry in varnishes, paints, and enamels. The most widely used base ingredients are phthalic anhydride and glycerol. Smaller quantities of other acids such as maleic, fumaric, and others and alcohols, such as pentaerythritol, sorbitol, mannitol, ethylene glycol, and others are used. These are reacted with the oils already mentioned to form the resin. The oils, as they exist naturally, are predominantly in the form of triglycerides and do not react with the polybasic acid. They are changed to the reactive monoglyceride by reaction with a portion of the glycerol or other alcohol to be used. Heat and a catalyst are needed to promote this reaction, which is known as alcoholysis. The resin is then formed by reacting this monoglyceride with the acid by agitation and sparging with inert gas, until the condensation reaction product has reached the proper viscosity. The reaction takes place in an enclosed resin kettle, equipped with a condenser and usually a scrubber, at temperatures slightly below 500 O F . The alcoholysis can be accomplished first and then the acid and more alcohol can be added to the kettle.
Polyurethane: The manufacture of the finished polyurethane resin differs from the others described in that no heated reaction in a kettle is involved. One of the reactants, however, is a saturated polyester resin, as already mentioned, or, a polyether resin. To form a flexible foam product, the resin, typically a polyether, such as polyoxypropylenetriol, is reacted with tolylene diisocyanate and water, along with small quantities of an emulsifying agent, a polymerization catalyst, and a silicone lubricant. The ingredients are metered to a mixing head that
deposits the mixture onto a moving conveyor. The resin and tolylene diisocyanate (TDI) polymerize and cross-link to form the urethane resin. The TDI also reacts with the water, yielding urea and carbon dioxide. The evolved gas forms a foam- like structure. The product forms as a continuous loaf. After room temperature curing, the loaf can be cut into desired sizes and shapes, depending upon required use. The flexible foams have found wide use in automobile and furniture upholstery and in many other specialty items for many years.
HAZARDS IN THE CHEMICAL PROCESS INDUSTRIES 61
Thermoplastic Resins: As already stated, thermoplastic resins are capable of being reworked after they have been formed into rigid shapes. The subdivisions in this group that are discussed here are the vinyls styrenes, and petroleum base resins.
Polyvinyl Resins: The polyvinyl resins are those having a vinyl (CH=CH,) group. The most important of these are made from the polymerization of vinyl acetate and vinyl chloride. Vinyl acetate monomer is a clear liquid made from the reaction between acetylene and acetic acid. The monomer can be polymerized in bulk, in solution, or in beads or emulsion. In the bulk reaction, only small batches can be safely handled because of the almost explosive violence of the reaction once it has been catalyzed by a small amount of peroxide. Probably the most common method of preparation is in solution. In this process, a mixture of 60 volumes vinyl acetate and 40 volumes benzene is fed to a jacketed, stirred resin kettle equipped with a reflux condenser. A small amount of peroxide catalyst is added and the mixture is heated, until gentle refluxing is obtained.
After several hours, approximately 80 to 90 percent is polymerized, and the run is transferred to another kettle, where the solvent and unreacted monomer are removed by steam distillation. The wet polymer is then dried. Polyvinyl acetate is used extensively in water-based paints, and for adhesives, textile finishes, and production of polyvinyl butyral. Vinyl chloride monomer under normal conditions is a gas that boils at -14 "C. It is usually stored and reacted as a liquid under pressure. It is made by the catalytic combination of acetylene and hydrogen chloride gas or by the chlorination of ethylene, followed by the catalytic removal of hydrogen chloride. It is polymerized in a jacketed, stirred autoclave. Since the reaction is highly exothermic and can result in local overheating and poor quality, it is usually carried out as a water emulsion to facilitate more precise control. To ensure quality and a properly controlled reaction, several additives are used. These include an emulsifying agent such as soap, a protective colloid such as glue a pH control, such as acetic acid or other moderately weak acid (2.5 is common), oxidation and reduction agents such as ammonium persulfate and sodium bisulfite, respectively, to control the oxidation-reduction atmosphere, a catalyst or initiator, like benzoyl peroxide, and a chain length controlling agent, such as carbon tetrachloride. The reaction is carried out in a completely enclosed vessel with the pressure controlled to maintain the unreacted vinyl chloride in the liquid state. As the reaction progresses, a suspension of latex or polymer is formed. This raw latex is removed from the kettle, and the unreacted monomer is removed by evaporation and recovered by compression and condensation.
Another form of the emulsion reaction is known as suspension polymerization. In this process, droplets of monomer are kept dispersed by rapid agitation in a water solution of sodium sulfate or in a colloidal suspension such as gelatin in water.
During the reaction, the droplets of monomer are converted to beads of polymer that are easily recovered and cleaned. This process is more troublesome and exacting than the emulsion reaction but eliminates the contaminating effects of the emulsifying agent and other additives. Other vinyl-type resins are polyvinylidene chloride Saran polytetrafluoroethylene (fluoroethene), polyvinyl alcohol, polyvinyl butyral, and others. The first two of these are made by controlled polymerization of the monomers in a manner similar to that previously described for polyvinyl chloride. Polyvinyl alcohol has no existing monomer and is
prepared from polyvinyl acetate by hydrolysis. Polyvinyl alcohol is unique
among resins in that it is completely soluble in both hot and cold water.
Polyvinyl butyral is made by the condensation reaction of butyraldehyde and polyvinyl alcohol. All have specific properties that make these materials superior for certain applications.
Polystyrene: Polystyrene, discovered in 183 1, is one of the oldest resins known.
Because of its transparent, glasslike properties, its practical application was recognized even then. Two major obstacles prevented its commercial
development, the preparation of styrene monomer itself, and some means of preventing premature polymerization. These obstacles were not overcome until nearly 100 years later. Styrene is a colorless liquid that boils at 145 "C. It is prepared commercially from ethylbenzene, which, in turn, is made by reaction of benzene with ethylene in presence of a Fridel-Crafts catalyst such as aluminum chloride. During storage or shipment the styrene must contain a polymerization inhibitor such as hydroquinone and must be kept under a protective atmosphere of nitrogen or natural gas. Styrene can be polymerized in bulk, emulsion, or suspension by using techniques similar to those previously described. The reaction is exothermic and has a runaway tendency unless the temperature is carefully controlled. Oxygen must be excluded from the reaction since it causes a yellowing of the product and affects the rate of polymerization. Polystyrene is used in tremendous quantities for many purposes. Because of its ease of
handling, dimensional stability, and unlimited color possibilities, it is used widely for toys, novelties, toilet articles, houseware parts, radio and television parts, wall tile, and other products. Disadvantages include limited heat resistance,
brittleness, and vulnerability to attack by organic solvents such as kerosine and carbon tetrachloride.
Manufacturing Equipment and Inhalation Exposures
Most resins are polymerized or otherwise reacted in a stainless steel, jacketed, indirectly heated vessel, which is completely enclosed, equipped with a stirring mechanism, and generally contains an integral reflux condenser. Since most of
HAZARDS IN THE CHEMICAL PROCESS INDUSTRIES 63
the reactions previously described are exothermic, cooling coils are usually required. Some resins, such as the phenolics, require that the kettle be under vacuum during part of the cycle. This can be supplied either by a vacuum pump or by a steam or water jet ejector. Moreover, for some reactions, that of polyvinyl chloride for example, the vessel must be capable of being operated under pressure. This is necessary to keep the normally gaseous monomer in a liquid state.
The size of the reactor vessels varies from a few hundred to several thousand gallons capacity. Because of the many types of raw materials, ranging from gases to solids, storage facilities vary accordingly: ethylene, a gas, is handled as such;
vinyl chloride, a gas at standard conditions, is liquefied easily under pressure. It is stored, therefore, as a liquid in a pressurized vessel. Most of the other liquid monomers do not present any particular storage problems. Some, such as styrene, must be stored under an inert atmosphere to prevent premature polymerization. Some of the more volatile materials are stored in cooled tanks to prevent excessive vapor loss.
Some of the materials have strong odors, and care must be taken to prevent emission of odors to the atmosphere. Solids, such as phthalic anhydride, are usually packaged and stored in bags or fiber drums. Treatment of the resin after polymerization varies with the proposed use.
Resins for moldings are dried and crushed or ground into molding powder.
Resins, such as the alkyd resins, to be used for protective coatings are normally transferred to an agitated thinning tank, where they are thinned with some type of solvent and then stored in large steel tanks equipped with water-cooled condensers to prevent loss of solvent to the atmosphere. Still other resins are stored in latex form as they come from the kettle.
The major sources of possible air contamination are the emissions of raw materials or monomer to the atmosphere, emissions of solvent or other volatile liquids during the reaction, emissions of sublimed solids such as phthalic anhydride in alkyd production, emissions of solvents during thinning of some resins, and emissions of solvents during storage and handling of thinned resins.
Table 1 lists probable types and sources of air contaminants from various operations.
In the formulation of polyurethane foam, a slight excess of tolylene diisocyanate is usually added. Some of this is vaporized and emitted along with carbon dioxide during the reaction. The TDI fumes are extremely irritating to the eyes and respiratory system and are a source of local air pollution. Since the vapor pressure of TDI is small, the fumes are minute in quantity and, if exhausted from the immediate work area and discharged to the outside atmosphere, are soon
diluted to a nondetectible concentration. No specific controls have been needed to prevent emission of TDI fumes to the atmosphere.
The finished solid resin represents a very small problem
-
chiefly some dust from crushing and grinding operations for molding powders. Generally the material is pneumatically conveyed from the grinder or pulverizer through a cyclone separator to a storage hopper. The fines escaping the cyclone outlet are collected by a baghouse type dust collector. Many contaminants are readily condensable.In addition to these, however, small quantities of noncondensable, odorous gases similar to those from varnish cooking may be emitted.
These are more prevalent in the manufacture of oil-modified alkyds where a drying oil such as tung, linseed, or soya is reacted with glycerin and phthalic anhydride. When a drying oil is heated, acrolein and other odorous materials are emitted at temperatures exceeding about 350 O F . The intensity of these emissions is directly proportional to maximum reaction temperatures.
Table 1. Principal Air Contaminants and Sources of Emissions.
Resin
outlet, vacuum pump discharge Possible sources of emission Air contaminant
Phenolic
Uncontrolled resin kettle Oil-cooking odors
Polyester and
Storage, leaks Aldehyde odor
Amino
Storage, leaks, condenser Aldehyde odor
alkyds
1
Phthalic anhydrideI
dischargefumes, Solvents Kettle or condenser discharge Polyvinyl acetate Vinyl acetate odor, Storage, condenser outlet
Solvent during reaction, condenser outlet during steam distillation to recover solvent and
unreacted monomer Polyvinyl chloride
Emission from finished foam Tolylene
Polyurethane resins
Leaks in storage and reaction Styrene odor
Polystyrene
Leaks in pressurized system Vinyl chloride odor
resulting from excess TDI in diisocyanate
equipment
formulation
I
Control of monomer and volatile solvent emissions during storage before the reaction and of solvent emissions during thinning and storage after the polymerization of the polymer is relatively simple. It involves care in maintaining
HAZARDS IN THE CHEMICAL PROCESS INDUSTRIES 65
gas-tight containers for gases or liquefied gases stored under pressure, and condensers or cooling coils on other vessels handling liquids that might vaporize.
Since most resins are thinned at elevated temperatures near the boiling point of the thinner, resin-thinning tanks, especially, require adequate condensers. Aside from the necessity for control of air pollution, these steps are needed to prevent the loss of valuable products. Heated tanks used for storage of liquid phthalic and maleic anhydrides should be equipped with condensation devices to prevent losses of sublimed material. One traditional device is a water-jacketed, vertical condenser with provisions for admitting steam to the jacket and provisions for a pressure relief valve at the condenser outlet set at perhaps 4 ounces of pressure.
During storage the tank is kept under a slight pressure of about 2 ounces, an inert gas making the tank completely closed. During filling, the displaced gas, with any sublimed phthalic anhydride, is forced through the cooled condenser, where the phthalic is deposited on the condenser walls. After filling is completed, the condensed phthalic is remelted by passing steam through the condenser jacket.
The addition of solids, such as phthalic anhydride to other ingredients that are mentioned above, the sublimation temperature of the phthalic anhydride causes temporary emissions, that violate most air pollution standards regarding opacity of smoke or fumes. These emissions subside somewhat as soon as the solid is completely dissolved, but remain in evidence at a reduced opacity, until the reaction has been completed. The emissions can be controlled fairly easily with simple scrubbing devices. Various types of scrubbers can be used. A common system that has been proved effective consists of a settling chamber, commonly called a resin slop tank, followed by an exhaust stack equipped with water sprays. The spray system should provide for at least 2 gallons per 1,000 scf at a velocity of 5 fps. The settling chamber can consist of an enclosed vessel partially filled with water, capable of being circulated with gas connections from the reaction vessel and to the exhaust stack. Some solids and water of reaction are collected in the settling tank, the remainder being knocked down by the water sprays in the stack.
Many resin polymerization reactions, for example polyvinyl acetate by the solution method, require refluxing of ingredients during the reaction. Thus, all reactors for this or other reactions involving the vaporization of portions of the reactor contents must be equipped with suitable reflux- or horizontal-type condensers or a combination of both. The only problems involved here are proper sizing of the condensers and maintaining the cooling medium at the temperature necessary to effect complete condensation.
When noncondensable, odor-bearing gases are emitted during the reaction, especially with alkyd resin production, and these gases are in sufficient concentration to create a nuisance, more extensive air pollution control