WILL H. SHEAROK, JR. Associate Editor
in collaboration with
URING the recent war there was considerable disagreement among interested groups concerning the relative importance of the paint industry in the war effort. h representative of a government agency, trying to maintain movement schedules on tank and box cars in the face of unloading delays experienced by paint plants, was quoted as saying that a war could be fought without paint, but not without railroad rolling stock. This s&me attitude was evident in other quarters, and as plant scale experiment the United States Xaritime Commission sent a n unpainted cargo ship to France and back. The ship was able to return to the United States, but its condition was so poor that it was promptly put on the junk pile. All through the war the importance of paints was demonstrated, for protection, particularly in the bot and humid South Pacific areas, and for camouflage. Actually, the widespread use of paint is a relatively ncw thing in the history of this country. It was frowned on by the early colonists and both paint making and painting were regarded as extravagances until Civil War days when, according to Toch (go), manufacture of ready-mixed paints began. Even then painters who had been accustomed to mixing their own paints were slow to accept the ready-mixed products; the use of inert fillers was considered an adulteration by painter and customer alike. Recognition of the advantages of using fillers-counterbalancing heavy pigments and prolonging durability-and that even the addition of water u p to a certain percentage is not adulteration but actually helps keep paint in suspension, has been B slow process. The paint industry has had remarkable growth,
as shown in Table I, and much progress has been made in developing new pigments and cheaper formulas. Paint chemist8 look at the cathedrals in Mexico, painted huridpeds of years ago with colors that still retain a large measure of their brilliance, and feel that with all their advances there may still be a missing secret, though part of that secret may have been simply the unhurried tempo of the age. Generally considered by the inexperienced as an industry of principally physical operations (mixing, grinding, tinting, and reducing), it is surprising to find that the manufacture of paints accounts foP 4% of all chemists and chemical engineers employed in various chemical fields, being surpassed only by those of industrial chemicals, petroleum, and drugs and pharmaceuticals (7). There are about 2000 raw materials available to the modern paint plant, and some of its formulas may contain as many as twenty ingredients. The common materials used in paints may seem inert, but in reality paints are colloidal materials whose properties initiate many problems in grinding, floating, and leveling. According to ILIattiello (16), particle size, occluded gases, electrostatic charge, and dispersing and wetting agents are all important factors. Reactions can occur when pigments, vehicles, et@.,are mixed, or as a result of heating or the purely mechanical operation of grinding. Occupied with the mdless routine of panel testing of products and with research and development, the paint chemist is also a trouble-shooter and a prescriptionist. Every paint plant has in essence two distinct lines of operation: mass production in large gallonage; and medium or small scale production of a variety of tailor-made products. Because a con-
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siderable amount of the paint manufactured is tailor-made, the chemist must know thoroughly the properties and reactions of all the materials with which he deals, singly and in combination, and the results obtained by various methods of milling and mixing. I n addition to these specialized requirements, hundreds of raw materials, in liquid and dry form, in various types of storage, in many measures of quantity, require an attention t o materials handling far more demanding than in most chemical industries. Further, rigid safety precautions must be maintained throughout all plant operations. Although all of the many paint manufacturers in the United States are working toward one end-the production of the best types of surface coatings for particular purposes-and essentially the same unit operations are employed for the same types of formulas, there is still a great deal of variation between individual manufacturers a s to type of equipment and the purposes for which it is used. The paint industry is known for the variety of products which are formulated and designed to meet practically any condition: house paint, interior and exterior enamels, undercoats, primers, trim colors, colors in oil, varnishes, stains, lacquers, and industrial finishes of all types.
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TABLEI. GROWTHOF
THE PAINT AND VARNISX INDCSTRY IX THE UNITEDSTATES
Yeara 1850 1860 1870 1880 1890 1899 1909 1919
Value of Productsb (Thousands of Dollars) 5,466 11,107 22,513 29,112 54,234 69,562 124,889 340.347
1943 568;621 1944 618,326 1945 644,429 1946 794,899 1 038 5 7 5 d 1947 1:050:682d 1948 Figures for 1850 to 1890 from Vol. X, P a r t I V , 12th Census of Manufartures 1900. figures 1899 to 1939 are for the census yeara indicated, 6 Figure; 1850 t o 1939 include value of colors and pigments Figures 1939 to 1948, from Facts for Industry Reports, Xl9b, Bureau of the Census, U. S. Department of Commerce, are total sales of paints, varnishes, lacquers, and fillers reported by 680 establishments (approximately 90% of the sales included in the 1939 census), d Preliminary figure.
HOUSTON PLANT OF THE COOK PAINT AND VARNISH COMPANY
The Cook Paint and Varnish Company, with headquarters in Kansas City, Mo., has maintained a plant in Houston, Tex., for 22 years, and has just completed a new plant to replace its old one, now used as a warehouse and for offices and laboratory. This company pioneered along with the D u Pont Company in the use of titanium dioxide pigments, and is the only large paint 'manufacturer in the Southwest formulating its alkyd vehicles from alkyd resins made right in the plant; alkyd kettles were installed in 1927. The new plant is considered typical of modern paint manufacturing installations and this report will be concerned mainly with its operation. Raw Materials. The American Society for Testing Materials defines a paint as a pigmented composition, which is converted to an opaque solid film after application as a thin layer. Essentially a paint is nothing but a dry pigment carried in suspension in a suitable liquid (called the vehicle) which partially evaporates on application. If the vehicle is a drying oil or oil varnish, i t forms an oil paint; if it is a water emulsion, it is called a water paint. Varnish, according to the National Paint Dictionary (29) is: any homogeneous transparent or translucent liquid which, when applied as a thin film, dries on exposure to air as a continuous film. Varnishes may consist merely of a solution of a resin in a solvent, called spirit varnish, or a combination of resins, oils, driers, and solvents or thinners, called an oil varnish, Spirit varnishes dry by evaporation alone; oil varnishes dry by a combination of evaporation, oxidation, and/or polymerization. Shreve (17) gives a detailed analysis of paint constituents and their functions. Pigments fall into two general classes, white and colored, with a further breakdown including extender pigments-those of low refractive power added to produce certain desirable characteristics
of applicability and durability. Some pigments are chemically active, usually reacting by combination between a basic group in the pigment and an acid group in the vehicle. Extenders may vary from an acid china clay, through neutral calcium sulfate, to the alkaline precipitated calcium carbonates. In addition, it no% seems certain that many of the effects formerly attributed t o chemical reactivity are really the result of insufficient stability in a colloidal system. Titanium dioxide, introduced since World1 War I, is probably the most important white hiding pigment today. Others are white lead, zinc oxide, lithopone, zinc sulfide combinations, and combinations of titanium dioxides with other pigments. Table I1 shows the consumption of some of these pigments during the last century. Examples of color pigments are carbon black, earth colors (ochre, sienna, umber, etc.), inorganic blues such as ferric-ferrocyanide complexes, organic blues such as indanthrene and phthalocyanine, various chromium and molybdenum compounds for green and orange, and many others. Of the many drying oils available to the paint industry, the Cook Company generally, or on occasion, stocks as raw materials the principal ones-linseed, tung, oiticica, fish (menhaden or sardine), treated castor, treated soybean, and perilla (not currently available). A range of aliphatic and aromatic compounds, esters, ketones, and alcohols, and terpenes (principally turpentine) constitute solvent and thinner raw materials. Storage Facilities. Of the approvimately 300,000 gallon totat storage capacity a t the new Houston plant, 260,000 gallons are provided by 10,000-gallon tanks, thirteen vertical and thirteen horizontal (regular tank car type on foundations) ; varying types of small storage tanks including those for glycerol make up the
OF WHITEPIGMENTS IN PAINTS~ (SHORTT o m ) TABLE 11. CONSUMPTION
White lead Zinc oxide Leaded zinc oxide Lithopone Titanium dioxide (TiOz content of ilmenite used for pigments)
19486 48,830 33,132 57,381 112,720
1947 61,265 32,867 77,994 134,830
1946 60,943 34,785 64,816 123,279
1V45 46,418 28,014 58,852 109,398
1943 66,441 29,852 42,303 103,860
1941 100,665 30,304 67,472 132,691
1939 92,380 25,334 41,519 113,995
e
248,231
200,352
183,195
137,399
147,905
57,676/
1937 93,580 27,987 39,584 122,915 ,
.. .
1934 75,008 23,741 20,376 114,472 32,0008
1929 136.526 54,440 26,981 150,804 ,
., ,
Unless otherwise indicated data are from IMinerals Yearbook, Bureau of Mines U. 9. Department of Interior. oh Mines. Figures for 1900 are pigment production reported in 12th Census of Manufactures, U. 9. Department of Commerce d Total domestic zinc and lead pigments made directly from ore, including blue and white leads and oxides of zinc,' Bureau of Mines. a Preliminary reports indicate a possible 10% increase over 1947. f Titanium dioxide made for sale and plant transfer, 1939 Census of Manufactures. 0 Ti02 content of total titanium pigment production (estimated),
(1
b Figures for 1948 are estimates based on total shipments of pigments, Bureau C
1919
79,643
.....
19Oilc 30,148
.,.
, ,
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Plant Layout and Flow Sheet of Paint Manufacturing Operations a t Cook Paint and Varnish Company, Houston, Tex.
remainder. All are ordinary steel tanks and no particular storage problems are involved, other than safety requirements. I n compliance with the city fire code the 10,000-gallon tanks are surrounded by a concrete fire wall enclosing twice as much space as would be filled if all the liquid contained In the tanks at capacity were released. All the liquid storage is handled by remote control, with the pipes teed so that flow may be directed either to the paint or varnish plant; any pump in the central pump house can be started merely by opening the proper valve a t the headzrs on the operating floors. Some 550 t o 600 raw materials are stored, a majority of these being solids. Pigments, resins such as dammar, kauri, copal ( I S , 14) rosin, and various synthetics, chemicals for resin manufacture such as phthalic and maleic anhydrides, drier compounds, solid or flake caustic soda for equipment cleaning, and filter aids are stored in a variety of ways. Standard 50-pound cloth or jute bags, multiwalled paper bags, and wooden, cardboard, and metal barrels all are used as containers. The expensive colors (ranging from $4 to $5 per pound) are all stored in barrels. Some black pigments are received as pellets in small tins or paper bags. Handling of these solid raw materials constitutes one of the largest operations of the paint industry, and little scientific attention has been paid to it. Cook has not yet begun palletized storing and handling of dry solids, but a number of other paint manufacturers are finding it efficient. One disadvantage of palletization, in plants such as the one under discussion where pigments are stored on the same floor with varnishes and the mixing equipment, is the added precaution necessary to prevent sparking from the towmotors used for handling the pallets. Pittsburgh Plate Glass Company has found (8) in the case of finished product storage that semilive skids are preferable to pallets. P A I N T MANUFACTURIRG OPERATIONS
'
Unit operations in the manufacturing of paints fall into five general classifications: mixing, grinding, reducihg and tinting, filtering, and packaging. The types of equipment for all of these operations are legion. There are some plants which prefer to devote separate equipment to the mass production of large gallonage products and to small tailor-made batches. However, this practice entails large investment in equipment and physical facilities, and is not conducive to economical operation, as parts of the equipment will be idle over considerable periods of time. Cook's equipment from the old plant and additional equipment purchased for the new three-story plant has all been installed with flexibility in mind. A considerable portion of the movement of materials is by gravity (as shown on the cross section of operation flow), a scheme generally followed in the paint industry, although there are some plants which for one reason or another prefer one-story operation, relying more on pumps and other methods of horizontal transfer. Mixing. Probably not enough attention is paid to the problem .of paint mixing, which involves the addition of all of the pigment except the tint) and a part of the vehicle, to produce a paste .suitable for grinding. Every individual plant has its own ideas ,on the proportion of vehiclewhich should be added during mixing: ,depending on the paint to be made, the vehicle should be added in sufficient amount to produce the consistency and viscosity best ,adapted to the milling equipment to be used. The consistency should be such that interfriction will break up the lumps, thus re,ducing the power factor and the time required for milling. Pig.ment percentage is usually 60 to 70 by weight,; von Fischer (?')
Figure 1.
Rotary-Mixers
gives several different typical formulations of which the Federal Specification TT-P-40 for exterior white house paint is an example: Lb. per 100 gallons Titanium dioxide Tyhite lead Zinc oxide Magnesium silicate Raw linseed oil
153 445 312 158 170
At least half a dozen different mixers are used in the paint industry to a considerable degree, but probably the most generally employed is the stationary rotary mixer (W7), Figure 1. httached to the motor-driven stirrer shaft is a crossbar with vertical arms. As the crossbar sweeps over the bottom of the mixer it forces the paint between these arms and a set of fixed vertical arms, resulting in a thorough mixing. All but the 300-gallon mixers used exclusively for emulsion paints are installed in pairs, with each pair of stirrers driven by one sparkproof induction motor from a single shaft; 10 hp. are required per set of 100-gallon mixers and 15 hp. per set of 200-gallon mixers. A clutch arrangement enables either mixer in a set to be thrown out of operation, and the usual practice is to fill one while the other is being emptied. Since cleaning of these stationary mixers is difficult, Cook finds i t desirable t o use them, in so far as possible, only for white paints. Cleaning is done with hot caustic when necessary. Various methods are employed over the country For charging mixers, ranging from completely hand operations to a t least semiautomatic. Some plants have a pigment chute from a floor above, feeding directly into the mixer, while the vehicle is introduced by pump from storage tanks. Pittsburgh Plate Glass, in its Houston plant, uses what are termed batch buggies. Tiieae are wooden platforms about 3 X 8 feet mounted on casters, wit!i an cllipticalshaped open metal tank mounted on rods above the platform. Bags of pigment are stacked on the platform, arid the vehicle is carried in the tank. The pigment is poured I)y limd into the mixer, and the vehicle drained by gravity tlirougll a valve. Cook places a tank on traveling scales (mounted 011 casters), weighs the required amount of vehicle into the tank from a header and transports it to the mixer to be used; the liquid is poured by hand into the mixer. A hand-operated gearlift is used to empty liquid into the weigh tanks from drums so designed that they must be kept vertical to empty. For certain uses there is also a 125-gallon sloping-bottomed tank on casters. Liquid put into this tank can be easily transported to and pumped into any mixer. Dry pigment is then added by hand and the stirrer turned on a t about 30 r.p.m. Ideal conditions require that the paste be mixed for
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Figure 2.
Water Paint Equipment
from 15 to 30 minutes, after the last of the dry pigment has been added. At the end of the mixing the paste flows from the mixer domn an open chute to roller mills on the floor below; discharge is accomplished through a gate in the bottom of the mixer side, controlled through a handwheel by the roller mill operators. Water paints are made in batches of 250 gallons in the single 300-gallon mixers. Casein is used in these paints but as an emulsion stabilizer only, as contrasted x i t h regular casein paints in which the casein actually acts as a binder. The casein is first dissolved in water in a vertical steam-jacketed kettle (Figure 2 ) with a motor-driven stirrer; solution takes about 30 to 35 minutes. The oil used is heated to about 180" F. in the mixer, which has a flat steam chest a t the bottom, and the casein solution is fed into the hot oil by gravity. An emulsifying agent is added; the mixture is cooked for 30 to 45 minutes at 180 O F. ; and finally the pigments, if any, are added. The resulting paste is processed on a three-roll mill and put directly into cans in paste form. For small color batches where i t is not desirable t o dirty large mixers, 30-gallon pony or change-can mixers ( 2 5 ) are used, filled to about 85% capacity. As the name implies the cans are movable and are dogged down to a gear driven a t about 40 r.p.m. from the bottom of a rotating pinion. Driven at about BO r.p.m. from the top of this pinion is a stirrer consisting of three slightly twisted vertical blades extending to the bottom of the can and raised and lowered by a handwheel and weights. A stationary doctor blade attached to the stirrer shaft scrapes the paint from the side of the can as it revolves. The same manufacturer now has a model in which a chain drive replaces the gears and a tilting head is provided. The major disadvantage of both models is that the stirrer is offset, resulting in a dead spot. A new type of pony mixer (22) is expected to offset these disadvantages. I n this the stirrer shaft is a t an angle, and one blade is parallel t o the bottom of the can
Vol. 41, No. A
while the other is parallel to the side. The shaft i s set with the, blades almost touching one side of the can, leaving no chance for a dead spot. Other mixers in use throughout the country are described in Table 111. Depending on the material processed, these may serve as both mixers and mills. Milling. Twenty-five years ago stone mills were in common usage, and Toch (20) stated that roller mills are often used for the manufacture of mixed paints and enamels, but their principal useis in the manufacture of printing inks. This further confirms the fact that most of the important equipment used by the paint in. dustry was borrowed from other industries. Today, however, the roller mill is used widely in paint manufacture and the stone mill only seldom. Ball and pebble mills also have widespread use I t is almost impossible to compare quantitatively the variow types of milling equipment because they are used for different purposes and in different ways by different paint manufacturers I t should be understood that the primary purpose of milling in the paint industry is not reduction in particle size, although some reduction does occur in the procees. Essentially the results desired are better dispersion, uniformity, and complete wetting of the pigment particles. The advantages, disadvantages, and opera^ tional efficiency of a piece of equipment will depend largely oa agreement between the chemist and the mill man iegarding the use of the equipment and the adaptation of products to best fit the equipment available. The same formula, with changes made i n the order of adding raw materials, etc., may be used with different types of equipment, and it is not uncommon for two manufac. turers to use distinctly opposite procedures in making perhaps their best grade product. For example, one plant states that roller mills are unsatisfactory for outside paints, whereas another uses these mills consistently to make its best grade outside paint. It can be stated, however, that in general hard pigments tend to score the rolls of roller mills (although some plants mill these b? rcpcated passes through the rolls), and that therc is a relative13 high volatiles loss. Ball and pebble mills are hard to clean and cannot satisfactorily handle milling where the final product is to be heavy bodied, due to difficulty of discharge, and steel ball mills are unsatisfactory with whites. Both pebble and ball mills however, accomplish mixing and milling in one step and have low labor costs. Table IV compares the constituent and produot types which may be handled in several common types of mills.
ROLLERMILLS. Roller mills consist in general of heavy steel frames in 4 hich are mounted hardened steel rolls, motor-driven in opposing directions and at successively increasing speeds, which are preset by the manufacturer and cannot be changed The rolls are cooled by circulating water inside, and they are
TABLE111. COMPARISON OF OTHER MIXERSUSED IS PAINT MANUFACTURE
Mixer
Description
Abbe-Lenart (89)
Disk with curved radial ribs rotates horizontally below an inner stationary cylinder with vertical slots and outwardly projecting baffles a t each slot Dispersion type: amall clearance between blades and mixer side
Banbury (89)
Types Available and Remarks 0 . 5 - to 330-gal. CBPPOity; 2800 to 300 r.p.m.; 0.25 t o 25 hp.
0.73- to 1000-lb. capac. i t y ; for stiff pastes (borrowed from rub ber industry)
Baker-Perkins (84) T w o spiral blades (available 0.25- to 2660-gal. workin a number of types) ?eing capacity~ 2 t o volving each passing 150 hp.; rbr stif3 throuah' path of the other; pastes (borrowed tank Fan-be tilted and disfrom bread induscharged with mixer running try). Dispersion and easy wetting make further grinding unnecesswy. High output per unit requirement power Beken (81)
Sixteen curved intermeshing 30 t o 15 r.p.m.; 0.6 t o blades in two sets: 2 to 1 500 gal. speed ratio between sets
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mounted so that they can be moved toward or away from TABLE Iv. MATERIALS BEST HaNDLED B Y VARIOUS MILLS each other by means of worm Type of Mill gears actuated by handwheels. Combination Here again the paint manucolloid stone and (SO) facturer borrowed equipment Stone Roller Steel ball Porcelain not only from the printing ink Any A n y color Dark Any color; white Bny color Pigment color manufacturer, but elsewherecolor particularly the 2-roll mill came from Hard Soft and nonabraHard Soft and medium Soft and medium Pigment type sive the rubber and cocoaindustries. Medium: Medium to heavy; Medium to heavy; Light t o medium; IIedium; soft Paste The mill floor a t the Cook tough tough tough soft plant is divided by a partition Volatiles (evaporat- Slow Medium t o slow Fast to slow Fast to slow Slow ing charactgrseparating the roller mills from istics) the ball and pebble mills in order that dry pigment from the latter will have no chance to settle on the rolls and ruin batch of paint. The roller mills are set along the outside and inside walls of the room with the reduction mixers in two rows between them. Tables IV and V detail and compare the operation of various roller mills. Since the rolls operate in opposite direction and with increasing speeds, two adjacent rolls are in contact along their entire length a t any given moment, and any given amount of surface on one roll will slide past a greater amount on the next roll; this results in a rubbing action and a shearing and smearing of paste, between any two rolls, t o a thinner film on the next roll. The space be tween individual rolls probably averages about 1 mil and is adjusted by the operator, according to the degree of milling desired, by turning handwheels mounted on the frame. Depending on the consist.ency and character of the paste and the setting of the mill, roller mills may grind from 200 to 1000 gallons per day. In addition, there is a combination stone and colloid mill (SO), Figure 3, in which the paste is ground between carborundum stones revolving a t high speed. Initial investment for this type of mill is less than one tenth that of a large roller mill and power consumption is low; yet it can grind to a fineness of 3, as measured on a fineness gage ($8)with a 1000-gallon-per-day capacity, equivalent to that of the roller mill, or t o a 5 to 5 l / ~fineness with somewhat lower capacity. PEBBLE AND BALLMILLS. The pebble mills are simply steel cylinders mounted on trunnions and rotated with a gear drive. They are filled approximately half full of steel balls or pebbles, and their action may be pictured as that of an elevator carrying material to the top of an incline and dumping it, the material tumbling with a speed dependent on the slope of the dump. Thus the balls do not merely slide down the walls of the cylinder counFigure 4. Pebble and Ball Mill -Mounting tercurrently as it rotates. They actually go with the mill concurrently until the top of the path of travel is almost reached, a t which point they cascade down. I t is generally considered necessary that the balls be of greater density than the paste, but it has Steel ball mills process about twice as fast as pebble mills, but been found that in milling certain pastes of high density and vissteel balls have a shorter life than the pebbles, and the cost of opcosity where the balls actually float on the paste, it is actually poseration per unit of time is higher because a greater weight of sible to obtain what is in effect a reverse cascade. This operation equipment is involved. The best practice dictates that the user is utilized a t the Beaumont, Tex., plant of the Socony Paint run the balls through a grader every 6 t o 8 months, removing flat Products Company. and misshapen balls and those reduced below a minimum size. Cook’s steel ball mills are lined with chrome-manganese steel containing balls whose initial size is 0.3125 inch. One mill of 50gallon capacity, t a o 100-gallon, one 350-gallon, and one 1000-galIon mill are installed ( 2 7 ) . Ball mills of 2000- to 3000-gallon capacity are not uncommon in the paint industry, Tne lining is not smooth but ribbed with either triangular or rectangular ribs. The pebble mills are lined with synthetic porcelain (one 50-gallon, two 100-gallon, and one 1000-gallon mills) and are about half full of 1-inch balls of this same material, called Porox ( $ 7 ) . Figure 4 is a part of the pebble and ball mill area silowing method of mounting on steel beams independent of the building structure, permitting new arrangements without altering tne building structure. A similar type of construction was installed a t Pittsburgh’s Springdale, Pa., plant (8). The Mobile (Ala.) Paint Company uses buhrstone-lined mills with flint pebbles, and Socony Paint Products a t BeaJmont, Tex., mixes and mills its entire line of products with this latter type of equipment. Pittsburgh a t Houston uses both tilese and also steel mills with flint pebbles, As a special safety feature, Cook’s 1000-gallon mill and many other pieces of equipnent have a stop switch which may be used to lock tKe equipment in position when work is being done on it. Another type of mill (21) used by some paint manufacturers consists of a stationary water-jacketed cylinder in waich grinder blades, attached to a central rotor by means of heavy springs, travel under spring pressure over the inside working sirface of the cylinder. Low maintenance, little operating attention, and easy cleaning are claimed by the manufacturer. Figure 3. Hy-R-Speed Mill and Reducing Tank Pebble and ball mills are charged by hand from the floor above
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Vol. 41, No. 6
Straining and Packaging. When the paint, has been a~pRoller Top proved finally by the IaboSO. Sizes, Speed, ratory it is strained before beRollers In. Hp. R.P.M. Position Fed Discharge Adjustment ing packaged. To remove very Three ( 3 2 ) 1 5 . 7 3 - X 30 300 Horizontal From mixer chute All mills h a v e sim- Outer rolls move 36 D directJy beilar discharge; toward or a w a y small lumps Cook has installed ($6) 12 X 30 tween first a n d doctor blade from center sta9 X 24 second rolls scrapes paint off tionary roll b y a nei7- supercentrifuge ($8) with final roll i n t o inhandwheel variable speed of input and two clined chute. Discharged t o speeds of bowl rotation, 8500 adjacent reduction mixers f o r t,o 15,000 r.p.m. If centrifugalarge batchcs; tion is not necessary, bai,ches 100 gallon tanks for small may be passed through a gyraFive (91) 13 X 3 2 20 300 Inclined Froinmixer chute Third roll stationi n t o adjustable a r y ; rolls adtory riddle, simply a galvanized hopper (regujusted in pairs; iron tub with a motor suspended lator guide one a n d t w o or hlade) on feed f o u r a n d five apmore or less freely ahove it. side between proach eacb first a n d sccother a n d roll Theshaft, which has a t its lower end a circu1e.r frame holding a sieve (40, 80, or 120 mesh), is made end-.heavy by a weight', and is agitated violently when the motor is in operat,ion. A hatch of paint i s poured into the sieve from a ohange-can t'hrough holes cut in the floor. The pigment is charged directly agitator tank, passes through the sieve into the tub, and thencc: from its original container through a hopper placed over bhe maninto cans, hole of the mill. Vehicle is delivered in 100-gallon tanks on rollers Used interchangeably with the gyratory riddle is a small sieve and empt,ied by gravity into the manhole in t,he mill. During the which can be a,itached directly t o 1he reduction mixer outlcts 01' grind a solid plate called the grind plate is bolted o n ; after the grind is over and the reducer is added, the discharge plate is put the change cans for large and sinall batchcs, respectively. This on in place of the grind plate, and the mill is turned TTith the plate consist,s of a cylinder about 7 inches in diameter and 13 inches on the bottom. Thi5 plate has a valve and hose attachment, and long containing a cylindrical sieve rotated very slow1~-by a motor. the t,hinned past,e flows into reduction tanks on the floor belor for Paint adniit,ted to t,he out,er cylinder passes through a sieve and further reduction andior t,inting. out into cans, while the skins are scraped off the sieve by a, s1a.Reducing and Tinting. Regardless of the method of milling tionnry knife blade attaxhed t o thc wall of the outer cylinder. used, the paste must be reduced andlor tinted. Reduction. mixers Such a machine can handle several thousand quarts a day, a t are used for large hatches, and 100-gallon change-can agitator least three Limes the quantity that, can be hand-packaged by a tanks for small batches. It is a t this point t,hat the remainder of skilled operator, Sonle plants feed paint from the thinning txnks, the vehicle, which up to this point has been held out, is added t,hrough screens, directly into t,he hopper of an automatic filling while the paste is being fed from the mill into the miser. T h e vemachine. hicle is pumped into the mixer from a traveling tank rvhich has Ratches of 100 gallons or less are packagcd from a special handbeen filled, on floor scales, from the header connected to the storfilling block on the first floor; change cans are brought t'o t'he age area. block by a special automatic elevator. Aft,er filling, the cans are The Cook Company's new plant, is so laid out that each roller placed on a conventional belt hoister and roller conveyer, labeled mill serves two stationary reduction mixers arid one change-can automatically, and hosed. agitator tank. The reduction mixers are closed cuhical steel tanks, 5 X 5 X 5 feet., with rounded corners, and were designed VARNISH MANUFACTURE and fabricated especially for the plant. Welded and braced and Varnish manufacture is much older t,han the making of r r d y . set into the floor a t their mid-point, they are equipped with a 5mixed paints. According t o the census of 1900 the iirst varnish hp. motor and motor reducer ( 8 6 ) and a stirrer consisting of two factory in this country mas founded in 1828 in New 'Yorli by 1'. H. arms of thick strap iron turned a t a 45" angle so that the lower Smith. The simplest description of ordinary varnish manufacture blades turn the paint up t,oward t,he center of the mixer and the is the solution of a suitable resin in a hodied oil, followed by thin-, upper blades force it down. ning and the addition of a drier. The resin may be a natural one Tinting pigments are ground into a paste with an oil, like the such as rosin or shellac, or a synthetic such as ester gum, phenolvehicle used in making the paint, and are added by hand. This formaldehyde, or coumarone-indene. The driers, added to reduce operation is in a sense a n art rather than a science, and it usudrying time, are usually napht,henates or fatty acid salt,s (linoally takes a year or longer t o train a tinter. A tiny floor scale leic, oleic, etc.) of lead, manganese or cobalt, inorganic chlorides, or which weighs accurately to 0.01 pound is used in weighing small aromatic amines. quantit,ies of these t>intingpigments and has proved to be very Varnish malting involves more rt:actions than does paint manuvaluable. The batches remain in the reducers until finally apfacture. Among these are polymerization and isomerization of proved by laboratory test (from 2 hours to a like number of days oils, est,erification, and distillation of drying components. To obin extreme cases), and then are discharged t,hrough the bottom of tain a light-colored oil with better drying properties it is custonithe tanks by gravity or pump to can filling. ary to body the raw oil before use. St,rictly speaking, the paint The change-can agitator tanks are simply 100-gallon cans, on chemist uses the term, body, t,o refer to consistency or viscasters, which can be transported easily. A rotary spiral stirrer cosity, although such usage appears t o be gradually dying out. (31) driven by a 3-hp. motor is handled by a handwheel and The bodying process is essentially one of heat polymerization, and weights. I n addition a new type of stirrer ( S q ) , Figure 5 , has been unless carried out in vacuum or a n inert atmosphere, oxygen may installed with a 5-hp. motor; movement of the stirrer is controlled be considered as an activator, Von Fischer ( 7 )gives an excellent by compressed air. Four additional reduction tanks placed on review of the chemistry of drying oils, and suggests the formation the first floor are reserved for paste from the pebble and hall mills of cyclic dimers in the bodying process. Addition of synthetic and these inills are discharged by gravity into them. TaBLE
r'.
COMPARIsOK O F
ROLLER ATILLS
AT COOK P L A N T
,
I
*
Tune 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
resins has B favorable inHueiice on the heat bodying of oils (18). A chemical treatment, mhich involves a structural change together with some polymerization, is also practiced. This is generally an alkaline isomerization of fatty acids or a direct conjugation of oils by catalvsts such as the nickel-cobalt catalyst developed by the Korthern Regional Research Laboratory (16). Other methods of polymerization include heat, radiation, and electrical discharge.
J
Figure 5.
Ross Change-Can Agitator
Steps in linseed oil bodying and the preparation of a typical linseed-based varnish are shown in Table VI. The source of heat for the cooking step may be direct firing, electrical, or a jacketed reaction vessel using Dowtherm. Cook uses direct firing for regular varnishes and Dowtherm for alkyd vehicles only, although the latter is adaptable to manufacture of varied types of varnishes. For versatility of operation in the direct-fired method, both stack and set kettles are employed. The stack kettles are 250-gallon capacity, open copper kettles mounted on four-wheel dollies in a space enclosed on three sides and having a stack a t the top. Iron kettles cannot be used due to reaction between iron and the hot oil; some reaction may occur between copper and the oil, but the resulting compounds are either colorless or very light. Barnes ( 2 ) discusses the use of nickel and Monel in such applications. Generally no more than 200 gallons are cooked at a time because of the ever-present danger of foaming and the fires which might result. This foaming is due either to the formation of gas by chemical reaction during the cooking or formation of steam from water present in the original resin or oil. Most varnishes that can be cooked in set kettles can be cooked also in stack kettles, but some that polymerize quickly can be cooked only in stack kettles or in set kettles with cooling coils. Cook's set kettles are not so equipped, and these varnishes therefore must be cooked in the stack kettles so that they can be moved outside quickly and cooled down with a water spray when necessary. Even when a varnish can be made in either a set or stack kpttle, varnishes of the same composition often may have somewhat different properties when made in the two types of kettles. As varnish cooking really involves a rather large number of physical and chemical reactions, the relative importance of these reactiops and, frequently, the properties of the varnishes, may be changed by varying such factors as the time schedule or atmosphere used for the manufacturing operation. At one time Cook used fuel oil for firing, but natural gas has proved t o be much easier to handle and currently all kettles are gas-fired. Because of the danger of fire, the flame under the set kettles is shielded carefully from these kettles and even the light-
1095
ing of the fire is done from the other side of the partition in back of the kettles. As their name implies the hemispherical set kettles are stationary and are mounted in a concrete foundation on a firebox lined with refractory brick. A target breaks up the flame and refractory brick baffles ensure uniform distribution of the flame. At the Cook plant, two of these 750-gallon kettles (usually loaded to 500 gallons) are stainless steel, which does not react with oil, and a third iron kettle is used for blacks such as asphaltum. Conical hoods with doors are lowered over the kettles and in the event of fire these hoods can be closed and the kettles flooded with an inert gas, such as carbon dioxide, by means of a jet passing through the hood and operated by a remote control valve. T'arious materials may be vaporized into the stacks during cooking (such as formaldehyde in the case of phenol- or urea-formaldehyde resins or some type of ether in the case of linseed oil) and some modern electrically-heated systems contain both condensers and scrubbers. Cook has not found such recovery economical, however, and the vapors are exhausted through tall stacks into the atmosphere. The relatively small volume, the height of the stacks, and dilution a i t h air make pollution no problem, but one disadvantage is condensation of material on the walls of the stack. Removal of this material when it has built up after a long period of time is a considerable problem. Because of handling problems, the use of mechanical stirring has been only slowly adopted in varnish manufacture. There is a tendency at present toward the use of mechanically-agitated continuous processing equipment with built-in cooling coils. The plant of the Pacific Paint and Varnish Company a t Berkeley, Calif., is a good example of installations of this type, Agitation is provided by a special two-speed agitator having push-button control and by bubbling a mixture of nitrogen and carbon dioxide through the batch. (This is primarily for fire protection). When the cooking process is concluded the varnish must be thinned, and the old method, still practiced t o some extent, was t o add the thinner t o the varnish under a hood. Since the thinners used are quite volatile, a portion will volatilize immediately on hitting the hot varnish, producing a safety hazard and requiring t h a t the varnish be cooled to below the flash point of the thinner used. The method practiced by Cook and other progressive manufacturers is just the opposite, as indicated in the steps of Table VI. A measured amount of thinner is introduced into the thinning tank, and then blanketed with carbon dioxide. The hot varnish then is fed by gravity into the thinner, forcing the gas out as i t fills the tank. Possibilities of formation of a n explosive mixture are thus minimiaed. The thinning tank is agitated with a
TABLE VI. Admit
VARNISH
hTAKUFACTURE
(250-GALLONBATCH)
Linseed Oil Bodying Air a t 100 lb. per sq. in. to raw linseed oil in bodyine tank Cooling water through t a n k jacket
36 to 42 hours
I
Circulate
During exothermic reaction (approx. 3 hours) During endothermic reaction (approx. 36 hours)
Admit
Steam at 40 lb. per sq. in. t o t a n k jacket
Discharge
(by gravity to chemical bodying tank) Chemical bodying agents
Stir f o r 1 hour
Varnish Manufacture 630 lb. resin (with some oil) i n kettle T o 500' F.
1.25 to 2 hours
Add Melt Heat Add Cook
Remainder of oil, to make 157 lb. total Without fire a t 500' F.
Cool
To 400' F.
Discharge
I n t o thinner, through ine r t gas blanket
..........
.
.
.
.
.
.
I
.
.
.
3 hours (exothermic reaction) Overnight
(.........
1096
INDUSTRIAL A N D ENGINEERING CHEMISTRY
rotary stirrer and is provided with a condenser t o recover volatiles. From the tank the varnish is pumped t o storage tanks on the top floor. T h e top surface of the varnishes and alkyds in these 480-, 720-, and 960-gallon steel tanks IS kept blanketed with carbon dioxide during storage. This practice allows addition of drier compounds t o the varnish without the danger of the formation of skins. Coli0 ( 5 ) has developed a laboratory method and equipment for quantitatively studying this thinning step under simulated plant conditions. Alkyd resins are synthesized in 300-gallon batches in stainless steel 500-gallon autoclaves (Type 347 or 321 A.I.S.I. is common), designed and fabricated especially for the Cook plant. A Dowtherm type heating unit with gas-fired vaporizer is employed; temperature of the heating liquid usually ranges from 350 t o 400 O C.at a pressure of 22 to 26 pounds per square inch. Heat input is controlled carefully by manual adjustment. Plants using a fatty acid in alkyd manufacture usually mix the phthalic anhydride and fatty acid with enough glycerol or other polyhydric alcohol to esterify the fatty acid and react with all the phthalic, cook t o boil off all the water of reaction, then add the oil and cook again. If a fatty acid is not used, the oil and some of the glycerol first must be cooked together with a catalyst to form the monoglyceride. When the reaction has procecdcd to thc point where the product shows thc desired solubility in methyl alcohol (time varies between batches, average is 2 hours j, then the remainder of the glycerol and all of the phthalic or maleic anhydride are added and the batch cooked a t about 350" C. for 5 hours. This methyl alcohol solubility test is a simple control test which measures the degree to which the reaction must proceed before the dibasic acid can be added. Both kettles have reflux condensers, for totally refluxing t h e glycerol, and a stack for exhausting the water vapor produced during the reaction. Since some of the materials used are toxic, operators are required t o wear chemical masks a t all times. Foaming is a considerable problem in alkyd manufacture and i t is important t h a t the operator attempt to p r w e n t it, or a t least be aware of and stop i t when it does occur. Each kettle has two sight glasses; by putting a light source a t one the operator can see inside the kettle a t the other. .4n atmosphere of carbon dioxide is used t o control foaming. The gas is introduced a t the top of the autoclave, forming a blanket, or bubbled through the batch from t h e bottom of the vessel through two four-pronged perforated pipes on t h e bottom of the autoclave just clear of t h e agitator. Passing carbon dioxide through the reaction mixture speeds up removal of water in addition to providing thorough agitation. When foaming occurs or is about to occur the operator can either lower the heat or increase t h e inert gas pressure. Ordinarily t h e operations are conducted a t atmospheric pressure, but in stopping foaming or in discharging the kettle a pressure of 4 t o 6 pounds per square inch is applied. T h e extent of the cook is determined by the operator on the basis of acid number, viscosity, and color. If the acid number is too high the operator can reduce it by blowing carbon dioxide through the resin a t a rapid rate. Thus some of the acid is actually blown out the stack. If the viscosity is high he can reduce the blow and decrease the heat. When t h e cook is completed $he kettle is discharged by gravity into t h e thinning tank. T h e operator opens a valve and watches a manometer until a slight drop in pressure is evident, indicating that t h e resin f l o ~has almost stopped and inert gas is entering the bottom of the kettle. H e then closes t h e valve for about 30 seconds until an additional quantity of resin has collected at the bottom and opens i t again. This is repeated three or four times, or until the kettle is emptied. 'Varnish can filling is done semiautomatically, from a stainless steel trough or corn fitted with multiple float-controlled siphons. O
TESTING
Exhaustive tests are made in the selection of raw materials. After materials are selected, elaborate routine testing is not re-
Vol. 41, No. 6
quired, particularly for materials such as white pigments which are received from established manufacturers. However, man! of the color pigments, which are manufactured batchwise, r+ quire complete tests as there may be considerable variat,ion between batches. Tests, when made, are for strength, cleanness. tone, ctc. Oils are tested on receipt principally for water content and gelation properties; tung oil particularly is tested as most of it is received from China amd its composit,ion is not reliable. When paints are made to rigid and reproducible specifics.. tions, as are many industrial finishes and formulas covered b: government specifications, t,esting of raw materials becomes wpecially important. Laboratory tests on finished paints and varnishes usually involve determination of application characteristics (spraying, brushing, dip, etc.), drying time, texture, viscosity, weight pe? gallon, and surface characteristics (gloss, reflectance, etc.) In some finishes other tests such as water (and reagent) resistance, flexibility, and wear resistance are import'ant. Metal test, panels are used in the Cook laboratory; where adhesion to metal surfaces is not one of the characteristics t o be tested, some other paint manufacturers use glass panels orj for purposes of easy fil. ing, paper strips. FUTURE PROSPECTS
There is always a battle, though i t may not be evident, between the paint manufacturer and his customers. This is partly the fault of the consumer, but goes back further t o the failure of the paint industry t o pursue a n adequate educational program. The manufacturer spends hundreds of thousands of dollars on new equipment and on research and development to give t h e customer the very best for his money and for the particmlar conditions under which he will use the paint. The customer, on the other hand, takes a paint whose formulation for certain conditions has been developed painstakingly, a,dds some turpentine or linseed oil to i t strictly by instinct, and then accuses the manufacturer of putting out a cheap product when it fails. The paint indust'ry has failed t o inform the public of the work behind what t8he customer thinks is essentially a bucket andi paddle field; it has not stressed the fact t h a t a paint designed to dry in 4 hours on a hot summer day in desert country may n a dry at all on a cold and humid mid-January afternoon on the sea coast. Industry it,self, as a consumer, is receiving a better education through the technical products engineer in the paint field. However, considerable attention could be paid by both manufac. turer and consumer to the concept t h a t the paint manufactures is not selling a finished product. A good portion of the chemistry ob paint occurs after the paint has been applied; in other words, the manufacturer supplies a n intermediate whose properties he guarantees under cerbain conditions. The end product attained by 81 customer depends on the physical or chemical changes which he makes in the paint himself; these changes depend largely os. the conditions (such as surface, humidity, light, etc.) under whhh he applies it. The last few years have shown remarkable advances in paint manufacturing. Pigments have been greatly improved; the classical pigment's show higher color purity and uniformity; white pigments are more durable and opaque; and there are B host of new organic and met,allic pigments as well as those which are functional, such as anticorrosive, antibiotic, and luminescent pigments. Kew drying oils from esterification of linseed fatty acid with such polyhydric alcohols as sorbitol ( 8 ) and polypentaerythritol (4)have shown considerable promise; use of milkweed seed' oil also has been suggested (10). Petxoleum resins, drying oils, solvents, and thinners have taken prominent places in the indust r y ( 1 1 , I,$)* Where turpentine once domina,ted the thinner mar. lret, it amounts t o only about 20% now; others are alcohols, ketones, esters, and aromatic hydrocarbons, most of which are made from petroleum products. The new light-colored hydro-, carbon drying oils, predominantly cyclic in structure and wit8h
June 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
iodine numbers of 180 to 250, indicating three to five double bonds per molecule, constitute a great improvement over the black or dark-colored petroleum drying oils heretofore available and make possible an expanded field of utility in light-colored finishes. They are less expensive than linseed oil or the cheapest vegetable or animal oils. A number of petroleum resins have shown considerable promise in air-drying and baking varnish formulations. Although petroleum derived product$ have their limitations, good quality products can be manufactured economically by use of the hydrocarbon oils in conjunction with linseed and other drying oils. Although the field of petroleum-derived protective coatings for pipe lines, etc., is rapidly enlarging, the heaviest impact will be felt in displacement of coal tar coatings rather than paint. The chemist is making progress in supplying temperature resistant paints. Work done by the Houston Paint and Varnish Production Club on a formula to resist temperatures up to 800” F. and to withstand exterior exposure has indicated that a paint based on an organo-silicon oxide polymer vehicle, and pigmented with any of the commonly used metallic pigments and/or zinc oxide will come closest to fulfilling these requirements. Work is being done also on the use of silicates for high temperature resistance, and the Australians have developed a butyl titanate pigment which looks promising for such applications. Gardner ( 1 ) stated in 1935 that it had been suggested that within a few years natural resins would almost entirely disappear from industry and their place be taken by synthetic resins produced in chemical factories under strict control; he predicted that water dispersions of resins might prove useful for many special coatings. Water emulsions of resins, known under such brand names as Kern-Tone, dpred, and Texolite are showing increasing popularity. Some of these are now available in semigloss and gloss products. Up until the last few months these paints were sold in paste form and thinned for use, because of instability during storage, but announcement has just been made of their availability in readymixed form. The new vinyl resins, although somewhat hard to handle, have had excellent reception in the paint industry, and water emulsions of these and of the butadiene-styrene copolymers itre being used for specialty paints although they must be applied under carefully controlled conditions to give the best service. Of greater importance, as far as volume is concerned, are other resins such as nitrocellulose, ethyl cellulose, urea, melamine, and phenolic (nonvarnish type). Hovey (9) has shown why alkyd resin manufacture has remained largely a batch process, but predicts that future emergencies may cause standardization of alkyds which will encourage continuous processes, and greater tendency toward larger size kettles, even to 10,000-gallon capacities. Earhart (6) predicts the use of oil-modified alkyds in increasing volume for many years, but states that development of new varieties of alkyds has slowed down awaiting developments in other fields. Increasing tendency on the part of paint manufacturers to make their own alkyds and the fact that existing plants are already overbuilt in capacity is causing alkyd manufacturers to take a serious view of their own situation. Some of the things that Gardner saw 15 years ago are still in the future-overcoming humidity differences by the development of a paint which will permit escape of moisture from the inside but prevent its entrance from the outside; surface coating of pigments to case-harden them and retard fading and chalking; and a tendency toward varnish bases of low viscosity, consisting largely of nonvolatile matter.
109’1
(5) Colio, W. P., Phelps, N. T., Harvey, W. T., Kurtz, 8.S., Jr., et al., Ibid., 33, 1413-15 (1941). (6) Earhart, K. A., Ibid., 41, 716-25 (1949). (7) Fischer, W. yon, “Paint and Varnish Technology,” New York Reinhold Publishing Corp., 1948. (8) Freeman, S. E., Oflcial Digest, Federation P a i n t & V a r n i s h Production Clubs, 287, 995-1012( 1948). (9) Hovey, A. G., IND. ENG.CHEW,41, 730-7 (1949). (10) Lanson, H. J., Habib, D., and Spoerri, P. E., Zbid., 37, 179-81
.--
I1 94.5). --,
(11) Lazar, A., I b i d , 28, 658-61 (1936). (12) Lee, R. J., Adams, L. M., and McSweeney, E. E., P a i n t Oil & Chem. Rea., 3, No 1 , 16-25 (1948). ENG. (13) Mantell, C. L., Allen, C. H., and Sprinkel, K. M., IND. CHEM.,27, 1369-73 (1935). (14) Zbid., 30, 262-9 (1938). (15) Mattiello, J . J . , “Protective and Decorative Coatings,” Tol. 3. New York, J6hn Wiley & Sons, 1945. (16) Radlove, S. B., et al., IND.ENG.CHEM.,38, 997-1002 (1946). (17) Shreve, R. N . , “Chemical Process Industries,” 484-5, 503, Kew York, McGraw-Hill Book Co., 1945. (18) Shuey, R. c.,I N D . E N G .CHEM., 32,921-30 (1940). (19) Stewart, J. R., “Natl. Paint Directory,” Washington, D. C., Stewart Research Laboratory, 1948. (20) Toch, M., “Chemistry & Technology of Paints,” New York. D. Van Nostrand & Co.. 1925. PROCESSING EQUIPMENT
(21) Bramley Machinery Corp., New York, N. Y., Brarnley Mill. Beken duplex mixer. (22) Brasington Corp., Cincinnati, Ohio, Brasington angular mixer. (23) Chemical Engineering Catalog, New York, Reinhold Publishing Corp., Paul 0. Abbe, Inc., Abbe-Lenart mixers. (24) Zbid., Baker-Perkins, Inc., Universal mixers. (25) Ibid., J. H. Day Co., single speed pony-type mixer, Model 2; three-roll mixer 12 X 30 inches and 9 X 24 inches. (26) Ibid., Falk Corp., Falk Motoreducer, size 5402RZX. (27) Ibid., Patterson Foundry and Machinery Co., Type C agitator: Patterson Porox balls; Patterson mills. (28) Ibid., Sharples Corp., Super centrifuge, Type M-59-P-32. (29) Farrel-Birmingham Co., Inc., Ansonia, Conn., Buffalo, N. Y . , B u l l . 180, Banbury internal mixers. (30) Hy-R-Speed Inc., Los Angeles, Calif., Hy-R-Speed mixer, Model A-400. (31) Kent Machine Works, Inc., Brooklyn, N. Y . , senior Super-Five roll mill; change-can agitator. (32) J. M . Lehmann Co., Lyndhurst, N . J., Five-Roll mill, No. 812A: Three-Roll mill, Model 1A. (33) Precision Gage and Tool Co., Dayton, Ohio, fineness gage. (34) Charles Ross and Son Co., Brooklyn, N . Y., motor-driven change-tank mixpr. Model 30-H.
LITERATURE CITED
(1) Am. Soc. Testing Materials, Symposium on Paint and Paint Materials (1935). (2) Barnes, R. T., Jr., IND. ENG.CHEM.,31, 847-9 (1939). (3) Brandner, J. D., Hunter, R. H., Brewster, M . D., and Bonner, R. E., I b i d . , 37, 809-12 (1945). (4) Burrell, H., Zbid., 37, 86-9 (1945).
Tank Farm and Supply Lines at Cook Paint and Varnish Company
