Applied Polymer Science - American Chemical Society

54 Paint Manufacture LOUIE F. SANGUINETTI

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 12, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch054

Manufacturing Committee, Golden Gate Society for Coatings Technology, San Francisco, CA 94107

Basic Operations in Paint Manufacture Oil Absorption and the Milling Process Bulk Properties of Paints Types of Mills for Paint Manufacture Stone and Colloid Mills Roller Mills Ball and Pebble Mills Sand Mills High-Speed Dispersers Horizontal Mill Kinetic Dispersion Mills Other Mills

Basic Operations in Paint Manufacture Paint is a mixture of pigment or combination of pigments and a liquid vehicle that is composed of binders and thinner (1). The formulation of paint is a highly developed combination of science, art, and technology. Since paints are required to meet a wide diversity of end uses, the formulations vary widely. The type and relative quantities of vehicle, pigments, extenders, additives, and volatile thinners determine the final film properties. The paint is usually applied by brush or r o l l e r when used in the home; commercially it is also applied by spray, dip, electrical deposition, screens, and curtain coaters. Normally applied in thin films on various surfaces such as wood, metal, concrete or plastic, paint is used for protection, decoration, identification, or some functional application such as an etch resist. C l e a r c o a t i n g s , while very important, are not considered as paints since there are no pigments i n these coatings. The manufacture of paint at f i r s t glance appears simple. F i r s t the pigment i s mixed with a p o r t i o n of the v e h i c l e t o form a paste that i s then m i l l e d to disperse the pigment i n t o the v e h i c l e . After 0097-6156/ 85/ 0285-1297506.00/ 0 © 1985 A m e r i c a n C h e m i c a l Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

APPLIED POLYMER SCIENCE


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the pigment i s dispersed to a predetermined standard and i n t o f i n e l y divided p a r t i c l e s , the balance of v e h i c l e and a d d i t i v e s are added to give the f i n a l product. Other steps are taken i n the manufacture of paints. T i n t i n g may be required to obtain the proper c o l o r . The paints are c l a r i f i e d i n order to s t r a i n out any foreign or large p a r t i c l e s . After passing q u a l i t y c o n t r o l , the p a i n t s are t r a n s f e r r e d i n t o c o n t a i n e r s , l a b e l e d , and stored. The layout of a plant u s u a l l y takes advantage of g r a v i t y flow. The paste or d i s p e r s i o n f l o w s from an e l e v a t e d f l o o r to letdown tanks where the operations of thinning, t i n t i n g , and blending with a d d i t i v e s are c a r r i e d out. Oftentimes, dispersing and letdown are done i n the same vessel. S l u r r i e s , which are pigments suspended i n l i q u i d s , are f r e q u e n t l y used i n p a i n t manufacture. Computers are being used to meter s l u r r i e s i n t o the tanks. O i l Absorption and the M i l l i n g Process V a r i o u s types of m i l l s are used to supply the energy necessary to d i s p e r s e the pigment i n the v e h i c l e . However, the pigment and v e h i c l e must be i n the correct r a t i o f o r a p a r t i c u l a r m i l l i n order for the m i l l to work e f f i c i e n t l y . The pigment manufacturers produce pigments h a v i n g an o p t i m a l p a r t i c l e s i z e (_2). These dry pigment p a r t i c l e s , however, are attracted to each other and tend to form c l u s t e r s of pigment known as agglomerates. In order to r e t u r n the pigment c l u s t e r s back to t h e i r inherent p a r t i c l e s i z e , the pigment c l u s t e r s are dispersed i n a s u i t a b l e m i l l to form s t a b l e suspended p a r t i c l e s i n a v e h i c l e . The paint manufacturer must u t i l i z e the m i l l i n g process so that there i s high throughput of m i l l base or pigment. To do t h i s , the pigment and v e h i c l e must be i n the proper r a t i o so that the l i q u i d f i l l s the voids, each agglomerate i n the dry pigment i s separated, and each p a r t i c l e i s wetted. In the m i l l i n g process enough energy i s necessary to separate the agglomerates and wet the p a r t i c l e s . The o i l absorption test i s used to measure the amount of v e h i c l e necessary to f i l l the v o i d s among the pigment p a r t i c l e s . The r e q u i s i t e quantity of v e h i c l e includes that which i s absorbed on the surface of the pigment. Depending on p a r t i c l e s i z e and shape of the pigment, the void volume u s u a l l y accounts f o r the greater portion of the o i l absorption. The o i l absorption of a pigment or combination of pigments for a given v e h i c l e combination of v e h i c l e s i s constant. The o i l absorption of a pigment i s the amount of v e h i c l e by weight t h a t i s absorbed by a g i v e n amount of dry pigment to form a paste. There are two ways t h a t t h i s property can be measured. One, the "rub out" method, i n v o l v e s the mixing of the pigment with increments of v e h i c l e with a s t i f f spatula u n t i l a s t i f f paste i s formed. This putty l i k e paste must not break or separate as d e s c r i b e d by ASTM method D-281-31. The second method i s l i k e the f i r s t method except the end p o i n t i s brought to a s o f t paste (ASTM D-1483-60). The tests are subjective but with a l i t t l e practice consistent r e s u l t s can be attained. Pigment volume c o n c e n t r a t i o n (PVC) of a p a i n t i s d e f i n e d as follows:


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Paint Manufacture

1299

volume of pigment

PVC

=

_


volume of pigment + volume of binder In 1949, Asbeck, Laiderman, and Van Lee f i r s t defined the " c r i t i c a l pigment volume c o n c e n t r a t i o n " (CPVC) (3). CPVC i s t h a t l e v e l of pigmentation i n a dry p a i n t f i l m where j u s t s u f f i c i e n t binder i s present to f i l l a l l v o i d s among pigment p a r t i c l e s . The PVC of a paint a f f e c t s many properties such as hiding, corrosion resistance, scrub r e s i s t a n c e , e t c . The c r i t i c a l pigment volume concentration (CPVC) i s an important property i n that any increase or decrease of binder from the CPVC w i l l a f f e c t the f i l m p r o p e r t i e s ; i t i s more u s e f u l to know the volume pigment-binder r e l a t i o n s h i p than to know the weight r e l a t i o n s h i p . V a r i o u s m i l l s u s u a l l y do not " g r i n d " or reduce the pigment p a r t i c l e s i z e ; they d i s p e r s e the agglomerates. The m i l l i n g equipment must not o n l y wet and d i s p e r s e but a l s o be a b l e to move t h i s mass. Consequently, the v i s c o s i t y of the m i l l base i s important. Bulk Properties of Paints V i s c o s i t y can be defined as the r a t i o of shear stress to shear rate (4). The shear stress i s equal to a force measured i n dynes divided over an area upon which i t acts i n square centimeters. Shear stress has the dimensions of dynes per square centimeter. The shear rate i s equal to the v e l o c i t y of a l a y e r due to shear stress measured i n centimeters per second divided by the thickness i n centimeters; the unit of shear rate i s given i n r e c i p r o c a l seconds. V i s c o s i t y often i s expressed i n poises. The poise has the dimension of dyne seconds per square centimeter. Dyne seconds per square c e n t i m e t e r i s the dimension f o r shear s t r e s s d i v i d e d by shear r a t e and t h e r e f o r e i s equal to v i s c o s i t y . P a i n t s are r a r e l y Newtonian i n t h e i r f l o w behavior. They are u s u a l l y found to be t h i x o t r o p i c , p s e u d o p l a s t i c , and sometimes d i l a t a n t . Paints are t h i x o t r o p i c i f there i s i n i t i a l resistance to v i s c o s i t y l o s s by a g i t a t i o n , i f there i s a r e d u c t i o n i n v i s c o s i t y with continued a g i t a t i o n , and i f there i s a return to the o r i g i n a l v i s c o s i t y a f t e r the p a i n t i s no l o n g e r a g i t a t e d . A p s e u d o p l a s t i c paint i s s i m i l a r to a t h i x o t r o p i c paint except there i s no i n i t i a l resistance to v i s c o s i t y l o s s or no y i e l d factor. A d i l a t a n t paint d i f f e r s because the v i s c o s i t y increases or shows resistance to flow as the a g i t a t i o n i s i n c r e a s e d . M i l l bases are u s u a l l y s l i g h t l y d i l a t a n t and are intended to be that way. Types of M i l l s f o r Paint Manufacture M i l l s break up the agglomerates g e n e r a l l y by "smashing," "smearing," or a mixture of the two a c t i o n s c a l l e d " h y b r i d " (_5). Under these c l a s s i f i c a t i o n s , the "smasher" w i l l use a m i l l base of low viscosity. The "smearer" w i l l r e q u i r e a m i l l base of high viscosity.



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Stone and C o l l o i d M i l l s . The o r i g i n a l stone m i l l , used f o r centuries, consisted of t h i c k large stone disks that rotated s l o w l y (j>). These m i l l s were i n common use up to the l a t e 1940s. These m i l l s were replaced by the c o l l o i d m i l l and high-speed stone m i l l . While these m i l l s are not the same, they d i f f e r c h i e f l y i n the shape of t h e i r r o t o r / s t a t o r c o n f i g u r a t i o n ( F i g u r e 1). The m i l l s c o n t a i n two stones of which one r o t a t e s ( r o t o r ) and the other i s a stationary stone (stator). There i s a l s o an a l l metal r o t o r / s t a t o r . A premix i s required f o r these m i l l s . The m i l l paste normally has a c o n s i s t e n c y of 90-140 Kreb u n i t s . The m i l l base should be smooth and free from lumps, since the residence or d w e l l time i n the m i l l i s very short. The v e h i c l e s o l i d s f o r the paste can be as low as 20% f o r low o i l a b s o r p t i o n pigments and up to as high as 75% v e h i c l e s o l i d s f o r high a b s o r p t i o n pigments. The c a p a c i t y of the m i l l w i l l vary with v i s c o s i t y of the paste. The paste or m i l l base i s u s u a l l y fed by g r a v i t y to the m i l l (Figure 2). As the paste reaches the r a p i d l y r e v o l v i n g rotor, i t i s impelled to the outer edge by c e n t r i f u g a l force. This force pushes the material through the narrow gap i n which the clearance between the rotor and s t a t o r has been preset. The material i s subjected to high s t r e s s as i t passes through the gap. T h i s s t r e s s (smearing) causes the agglomerate p a r t i c l e s to disperse. The gap between the rotor and stator can be adjusted even while the m i l l i s operating. Adjustments are u s u a l l y necessary since the m i l l w i l l generate heat and expand the r o t o r and s t a t o r . The adjustment i s q u i c k , a c c u r a t e , and s i m p l e . T h i s can be done manually with a c a l i b r a t e d adjustment ring. Care must be taken that enough paste passes through so t h a t the m i l l w i l l not wear e x c e s s i v e l y or cause o v e r h e a t i n g . The gap between the s t a t o r and rotor i s i n a nearly closed p o s i t i o n when s t a r t i n g to operate, and the m i l l i s then a d j u s t e d immediately. The m i l l temperature s t a b i l i z e s r a p i d l y and another adjustment may not be necessary. The stone or c o l l o i d m i l l , which i s e a s i l y cleaned, i s u s u a l l y used f o r a r c h i t e c t u r a l p a i n t s . Pigments t h a t are d i f f i c u l t to disperse are not u s u a l l y used on these m i l l s . R o l l e r M i l l s . The most popular types of r o l l e r m i l l s have been the three- or f i v e - r o l l e r m i l l (Figure 3). The t h r e e - r o l l e r i s mostly used i n the paint industry while the f i v e - r o l l e r m i l l i s preferred i n the i n k i n d u s t r y . There are, a l s o , one-, two-, and f o u r - r o l l e r m i l l s . A l l work on the same p r i n c i p l e and are of the smearer type. The r o l l s on a t h r e e - r o l l e r m i l l each r o t a t e i n o p p o s i t e d i r e c t i o n to the one or two adjacent r o l l s . The speeds between the r o l l s a l s o d i f f e r . While d i f f e r e n t manufacturers may have d i f f e r e n t speeds for t h e i r r o l l s , t y p i c a l RPM values are as f o l l o w s (7): R o l l Number Three-roll m i l l Five-roll mill

1 rpm rpm

2

3

35

115

345

25

50

100

4

5

200

300

In a r o l l e r m i l l , the paste must be tacky so that i t adheres to the surface of the r o t a t i n g r o l l s . The v e h i c l e portion must be high



54.

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Figure

1.

Figure

Paint Manufacture

1301

Metal and carborundum rotor and stator f o r c o l l o i d m i l l . Courtesy Premier M i l l Corp.

2.

Inside of c o l l o i d m i l l .

Courtesy Premier M i l l Corp.



1302


i n r e s i n s o l i d s . The m i l l s have tremendous s h e a r i n g a c t i o n t h a t requires that the paste has f l u i d i t y and that the v e h i c l e has high viscosity. Since the premix i s a v i s c o u s composition, an i n t e n s i v e heavy duty mixer i s required. The v e h i c l e and then the pigment are added to the mixer. S u f f i c i e n t v e h i c l e to wet the e n t i r e pigment l o a d must be used, and there must be enough v e h i c l e so that the paste i s f l u i d enough to charge the m i l l . The pigments must be t h o r o u g h l y wet and free from lumps. The paste i s fed i n t o the m i l l i n the space between r o l l one and r o l l two whereupon the material flows i n t o the narrow gap. Part of the material i s rejected and returned to the feed bank. Part of the m i l l base continues on through where i t i s s p l i t between the feed r o l l and the center r o l l . The material on the center r o l l continues to the apron r o l l where i t i s again s p l i t between the center r o l l and the apron r o l l . The material on the center r o l l returns back to the feed bank w h i l e the m a t e r i a l on the apron r o l l i s k n i f e d o f f onto the apron. The r o l l s r o t a t e at d i f f e r e n t speeds. C l e a r a n c e s between the r o l l s must be a d j u s t e d c a r e f u l l y and are r e l a t e d to the r e l a t i v e speed of each r o l l . The c l e a r a n c e of the back r o l l s must be s u f f i c i e n t so that enough material i s received by the front r o l l so that output at the front r o l l i s sustained. The clearance between the back r o l l s determines the amount of paste that i s d e l i v e r e d . R o l l e r m i l l s have c e r t a i n advantages. They can handle viscous m a t e r i a l s and produce h i g h - q u a l i t y d i s p e r s i o n s of f i n e p a r t i c l e s i z e ; these are necessary i n s p e c i a l t y c o a t i n g s such as i n k s . D i f f i c u l t - t o - d i s p e r s e pigments are handled by these m i l l s . However, these m i l l s are c o s t l y because the m i l l base throughput i s at best moderate and a premixed paste i s required. While the r o l l e r m i l l s are used u s u a l l y f o r s p e c i a l t y c o a t i n g s , other m i l l s can handle easy-to-disperse pigments more e f f i c i e n t l y . B a l l and Pebble M i l l s . B a l l m i l l s are hardened s t e e l s h e l l s with closed ends that use s t e e l b a l l s as the grinding medium (Figure 4). Pebble m i l l s have s t e e l s h e l l s and ends but they are l i n e d w i t h burrstone or s y n t h e t i c stone ( p o r c e l a i n ) and they use n a t u r a l or p o r c e l a i n b a l l s as the grinding media (8) (Figure 5). B a l l m i l l s d i s p e r s e f a s t e r than pebble m i l l s due to the f a c t t h a t the s t e e l b a l l s are more dense than the p o r c e l a i n or n a t u r a l stone pebbles. Since the s t e e l b a l l s g i v e s a g r e y i n g e f f e c t they are not used i n white or l i g h t c o l o r s . No premixing i s needed. Since i t i s a closed m i l l , there i s no v o l a t i l e l o s s . They can be operated w i t h minimal s k i l l r e q u i r i n g l i t t l e a t t e n t i o n d u r i n g operation. These m i l l s can be used overnight with the batch being ready i n the morning; a timer can be used to s t a r t and stop the m i l l . The m i l l should be vented when necessary or when experience d i c t a t e s (9). The e f f i c i e n c y of the m i l l i s a f f e c t e d by the f o l l o w i n g f o u r main factors: 1. 2. 3. 4.

The speed of r o t a t i o n of the m i l l . The r e l a t i v e volume of grinding media and m i l l base. The s i z e and d e n s i t y of the g r i n d i n g medium. The c o n s i s t e n c y of v i s c o s i t y of the m i l l base.



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Figure

3.

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T h r e e - r o l l h o r i z o n t a l m i l l . Courtesy Lehmann/Thropp, D i v i s i o n of Paxson Machine Co.

Figure

4.

Pebble m i l l .

Courtesy Paul 0. Abbe, Inc.



1304


The speed of r o t a t i o n i s i n the range of 50-60% of the c r i t i c a l speed (10). The c r i t i c a l speed i s the speed a t w h i c h the c e n t r i f u g a l f o r c e s a c t i n g on the charge are balanced by the c e n t r i p e t a l forces. The volume of the grinding media as w e l l as the volume of paste i n the m i l l a f f e c t s the g r i n d i n g time. The pebble m i l l s h o u l d c o n t a i n about 45-50% pebble charge by volume. From 33% to 40% by m i l l volume of s t e e l b a l l s should be charged. The m i l l base to be added should cover the grinding media; t h i s i s approximately 20-25% of the m i l l volume. This volume should f i l l the void spaces that are l e f t by the g r i n d media. The l o a d of g r i n d i n g media, the m i l l paste, and the v i s c o s i t y may vary, but these v a r i a t i o n s w i l l change the d i s p e r s i n g time and/or c r e a t e e x c e s s i v e wear of the m i l l or grinding media. I t i s best to stay within the general recommended range of volume of grinding medium, v i s c o s i t y of m i l l base, and m i l l base volume to o b t a i n the best r e s u l t s . O v e r l o a d i n g of the m i l l increases the dispersing time, and the desired r e s u l t s may never be attained since the m i l l base may continuously r i d e over the grinding media. Since l a b o r a t o r y m i l l s g i v e good c o r r e l a t i o n to f a c t o r y m i l l s , p r a c t i c a l r e s u l t s can be obtained i n the laboratory. The c o n s i s t e n c y of the paste must be adjusted to the type and s i z e of g r i n d i n g media. The v i s c o s i t y of the m i l l base g e n e r a l l y w i l l vary w i t h the d e n s i t y of the g r i n d i n g media; the denser the grinding media, the higher v i s c o s i t y the m i l l base. The v i s c o s i t y of the paste used i n m i l l i n g i s i n the range of 75-120 Kreb u n i t s . Since these are r e l a t i v e l y low v i s c o s i t i e s , i n practice low v e h i c l e s o l i d contents are used with a high r a t i o of pigment to v e h i c l e . Various methods have been used to determine the c r i t i c a l speed of the b a l l m i l l (11). C o g h i l l and De Vaney c a l c u l a t e d the constant i n terms of the radius as f o l l o w s : 54.18 S = / R ; a 6-ft diameter m i l l would then rotate 54.18 S = / 3

54,18 = 1.732 = 31.3 c r i t i c a l speed

at 50% of 31.3 = 16 rpm; at 60% of 31.3 = 19

rpm

In the B e r l i n e r r e p o r t , the c r i t i c a l speed i n rpm of any m i l l can be determined by d i v i d i n g by a constant 76.6 by the square root of the m i l l diameter i n f e e t (12). In a 6 - f t diameter m i l l , the c r i t i c a l m i l l speed would be 31.2 rpm. I t has been determined that a m i l l with l i f t e r bars with a 6-ft diameter should have an optimum speed of about 20 rpm. M e l l o r ' s f o r m u l a g i v e s s i m i l a r r e s u l t s (13). The f o r m u l a i s rpm = 43.3 / 1/(D - d ) ^ where D = i n s i d e diameter of the m i l l i n feet and d = average diameter of pebbles i n feet. The l i f t e r bars are used f o r v a r i o u s reasons ( F i g u r e 6). The bars are important f o r l i f t i n g the grinding media to the high point of the cascade, producing more effectiveness. In addition, a l a y e r of the grinding media i s locked against the w a l l , thereby reducing slippage against the s h e l l and r e s u l t i n g i n l e s s wear of the s h e l l .



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Figure

5.

Inside of pebble m i l l . Courtesy Epworth Manufacturing Co., Inc., and Coors P o r c e l a i n Co.

Figure

6.

C o n t r o l of g r i n d i n g media w i t h b a f f l e bars i n c l o s e d m i l l . Courtesy Paul 0. Abbe, Inc.


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The b a l l or pebble m i l l i s considered to be a hybrid. The cascading action of the grinding media causes the pigment to be both impacted and i n t e n s i v e l y sheared by the tumbling b a l l s .


Sand M i l l s . In a b a l l m i l l , i f the speed of the m i l l i s increased, the grinding media i s immobilized by c e n t r i f u g a l force. In a sand m i l l , the design i s such that the grinding media i s not immobilized The sand m i l l i s i n essence a u n i t t h a t c o n s i s t s of a waterjacketed c y l i n d r i c a l s h e l l containing disks mounted on a c e n t r a l l y l o c a t e d s h a f t i n the s h e l l and a screen a t the top of the m i l l to a l l o w the base to flow through while r e t a i n i n g the media i n the m i l l (Figure 7). The base i s pumped through the m i l l media by a v a r i a b l e pump. The sand m i l l can be an open or c l o s e d m i l l . The o p e r a t i o n of the sand m i l l i s not complicated; d e t a i l e d i n s t r u c t i o n s are given by the manufacturers. The c o n s i s t e n c y of the m i l l base may vary from 70 to 140 Kreb u n i t s . T h i s i s i n the same range as the v i s c o s i t y used i n b a l l m i l l s so that b a l l m i l l bases are e a s i l y applied to the sand m i l l . However, the v e h i c l e s o l i d s i n the sand m i l l base i s u s u a l l y higher than i n the b a l l m i l l formulation. The m i l l , when o p e r a t i n g , i s i n the temperature range of 120150 °F, but f o r s p e c i a l cases the temperature may be as h i g h as 300 °F. C a r e must be t a k e n f o r h e a t - s e n s i t i v e m a t e r i a l s . Temperature i s c o n t r o l l e d by means of the water flowing through the water jacket. The rate of flow of water i s u s u a l l y adjusted so that the water coming out i s s l i g h t l y warmer than the water t h a t i s coming i n t o the i n l e t . S i n c e the temperature of the m i l l i n o p e r a t i o n i s h i g h , the c o n s i s t e n c y of the paste s h o u l d be f o r m u l a t e d f o r these temperatures. Too t h i n a base r e s u l t s i n excessive wear of the m i l l . Too t h i c k of a base may keep the media from c i r c u l a t i n g , g i v e excessive d w e l l time, and cause the m i l l to s t a l l or the media to b l o c k the screens. The rate of base entering i n t o the m i l l i s c o n t r o l l e d by pumps. Once the pumps are s e t , the m i l l s p r a c t i c a l l y run by themselves. The pump together with the temperature and v i s c o s i t y a l l are f a c t o r s that i n f l u e n c e the e f f i c i e n c y of the m i l l . In some cases, the paste may not a c h i e v e the proper d i s p e r s i o n on the f i r s t pass due to the nature of the pigment and v e h i c l e i n v o l v e d . The m i l l base must then be put through the m i l l again. However, before doing t h i s , i t i s necessary to ensure that there i s no "kick out" or that the grinding media i s not breaking down. Once the m i l l i s put i n t o operation, i t s h o u l d not be a l l o w e d to run dry or the m i l l media w i l l s t i c k to i t s e l f and the m i l l w i l l become a s o l i d mass. The m i l l i s c a l l e d a sand m i l l because of i t s o r i g i n a l use of 20-30-mesh Ottawa sand. In r e c e n t y e a r s , new media have been developed for the sand m i l l . Some of the types of media used are as follows:


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54. SANGUINETTI

Figure

7.

Sand m i l l .

Courtesy Chicago B o i l e r Co.


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A P P L I E D POLYMER SCIENCE

Material


S1O2 (sand) S1O2 (glass beads) Ceramic Ceramic

Density (g/cm^)

Media Diameter ( i n . )

2.8 2.8 2.7 3.6 3.5 5.4 7.1

0.028 various s i z e 0.050 0.020 various s i z e 0.026 various s i z e

With a l l these new types of g r i n d i n g media, the sand m i l l i s more appropriately c a l l e d a small media m i l l . The r a t i o of 1:1 of m i l l base to sand i s u s u a l l y d e s i r a b l e . With d i f f e r e n t types of grinding media a v a i l a b l e , the r a t i o can and does vary. In a d d i t i o n , the s i z e of the media and the d e n s i t y of the media a f f e c t the grinding rate (15). S a t i s f a c t o r y r e s u l t s can be obtained with any media by a l t e r i n g the paste formulation. With higher density grinding media, the v i s c o s i t y of the m i l l base can be higher with consequent greater throughput of paste. With the higher d e n s i t y media, the s i z e of the media can be s m a l l e r because of the l a r g e r surface f o r grinding. While higher density and smaller s i z e media may g i v e b e t t e r r e s u l t s , a study s h o u l d be made to i n c l u d e cost, wear of m i l l , and the degree of dispersion needed. A r e l a t i v e l y good q u a l i t y premix i s required for t h i s m i l l , but a l l types of pigments can be used. I t i s d i f f i c u l t to clean because the paste can dry on the screen and the screen can become c l o g g e d w i t h media. These m i l l s d i s p e r s e by s h e a r i n g and by some impingement. High-Speed Dispersers. The high-speed disperser consists of a disk t h a t r o t a t e s at high speed i n the c e n t e r of a v e r t i c a l tank (Figure 8). The d i s k , mounted on a s h a f t , r o t a t e s at a speed of 4500-5500 ft/min. Although there are s t a t i o n a r y s h a f t d i s p e r s e r s , most high-speed dispersers are designed so that the shaft can go up or down; the shaft i n some cases can be moved sideways. The disk i s driven e i t h e r by a variable-speed or a two-speed motor. The d i s k ( i m p e l l e r ) i s t h e key f e a t u r e of a h i g h - s p e e d disperser. The o r i g i n a l i m p e l l e r was a sawtooth disk (Figure 9); i t and v a r i a t i o n s of the sawtooth disk are s t i l l the most popular. The teeth are formed by bending the edges of the disk a l t e r n a t e l y up and down. The s l a n t and height of the teeth can vary. The e f f e c t i v e n e s s of the high-speed m i l l i s dependent on the v i s c o s i t y of the m i l l base. The m i l l base must be t h i c k enough to a v o i d t u r b u l e n c e s i n c e s h e a r i n g and a t t r i t i o n e f f i c i e n c y i s necessary f o r the e f f e c t i v e use of the m i l l . There i s a need f o r some d i l a t a n c y of the m i l l base. The m i l l base must be p u l l e d i n from the edge of the tank i n t o the blade. The s i d e of the tank and the shaft should be scraped to make sure that no pigment i s caked on the side a f t e r a l l the pigment has been added. In determination of the m i l l base, s e v e r a l p o s s i b l e procedures may be used. One of the e a r l i e s t methods of determining the m i l l base was given by Taylor (16). The o i l demand i s determined by the Gardner-Coleman method, and the amount of o i l needed i s m u l t i p l i e d by 2. T h i s i s the amount of v e h i c l e needed f o r d i s p e r s i n g the pigment i n the disperser when the Cowles d i s s o l v e r i s used.



54. SANGUINETTI

Figure

Figure

8.

9.

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Variable high-speed disperser.

Courtesy Jaygo, Inc.

Type of i m p e l l e r f o r high-speed d i s p e r s e r . Courtesy Morehouse-Cowles, D i v i s i o n of Morehouse Industries, Inc.


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Another method was presented by Guggenheim (17). The " s t i r r i n g rod" method of determining the o i l absorption was used. The formula as c a l c u l a t e d by Guggenheim i s the f o l l o w i n g : F/C = 0.90 +

VS


145

+

Ρ 40

where f/c = o i l absorption f a c t o r f o r high-speed i m p e l l e r equipment, VS = percent v e h i c l e s o l i d s , and Ρ = v i s c o s i t y i n poises. S t i l l another method f o r determining the m i l l base f o r the high speed disperser i s the wet point/flow. I t was developed i n 1946 f o r b a l l and pebble m i l l s , but since then i t has been s u c c e s s f u l l y used on the sand m i l l and on the high-speed d i s p e r s e r . The method developed by Daniel i s quite simple, but some experience i s needed to obtain the correct i n t e r p r e t a t i o n (18). The wet p o i n t , the f i r s t - s t a g e c o n s i s t e n c y , i s determined by kneading a c e r t a i n amount of pigment with j u s t enough l i q u i d to form a s t i f f coherent paste. T h i s end p o i n t i s s i m i l a r to the r e s u l t s obtained from the Gardner-Coleman rub out o i l absorption procedure. The end point w i l l vary with d i f f e r e n t combinations of pigments and with d i f f e r e n t v e h i c l e s . The flow point requires two end points. F i r s t , the paste e i t h e r f l o w s or drops from the s p a t u l a . Second, the s p a t u l a i s dragged over the paste. There s h o u l d be no s t r o n g drag and no permanent ridges. The s u r f a c e must r e t a i n i t s g l o s s y a p p e a r a n c e or immediately regain i t . The d i l a t a n c y of t h e p a s t e i s i n d i c a t e d by the p e r c e n t d i f f e r e n c e needed to go from the wet p o i n t to the f l o w p o i n t . C l o s e l y spaced wet and flow points i n d i c a t e a good base. The gap i s u s u a l l y i n the order of 15-30%. The degree of d i l a t a n c y can be c o n t r o l l e d by the c o n c e n t r a t i o n of the s o l i d s of the v e h i c l e . In aqueous systems, a d d i t i v e s , such as t h i c k e n e r s , can be added to control dilatancy. The p o s i t i o n of the b l a d e w i t h i n the tank p l a y s an important r o l e (19). The h e i g h t of the b l a d e from the bottom s h o u l d be a t l e a s t h a l f the diameter of the blade. The h e i g h t can be a d j u s t e d f o r the l e v e l of paste up to 2-3 times the diameter of the tank. When the paste i s moving i n a doughnut shape p a t t e r n , about 1/3 of the b l a d e can be seen. The i m p e l l e r s h o u l d be moved up or down to obtain the proper vortex. Other u s e f u l equipment on the disperser includes a tachometer, p a r t i c u l a r l y f o r variable-speed m i l l s (20). Also, an ammeter should be used to measure power transfer. With the advent of pigments t h a t are c o n s i s t e n t i n p a r t i c l e s i z e , the high-speed d i s p e r s e r g i v e s h i g h throughput of pigment. The batch can be made and completed i n the same tank. However, heat-sensitive pigments and v e h i c l e s may cause problems due to the heat generated when m i l l i n g . Horizontal M i l l . The h o r i z o n t a l m i l l i s a newly designed micromedia m i l l with a h o r i z o n t a l grinding chamber as shown i n Figure 10. The g r i n d i n g chamber i s f i l l e d t o 70-85% volume w i t h g r i n d i n g media, which i s evenly d i s t r i b u t e d i n the chamber. S u i t a b l e types of media are made of g l a s s , ceramic, or s t e e l c l o s e l y graded i n s i z e . The



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SANGUINETTI

Paint Manufacture

Figure 10. Horizontal m i l l .

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Courtesy Premier M i l l Corp.



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d e n s i t y of media i s not a c r i t i c a l f a c t o r . U n l i k e the sand m i l l , the h o r i z o n t a l m i l l does not work a g a i n s t g r a v i t y . The premix, adjusted to the type of media used, i s pumped into the chamber. In the m i l l , the premix i s s u b j e c t e d to both impact and shear by the grinding media. A h i g h l y e f f i c i e n t c o o l i n g system a l l o w s the use of heats e n s i t i v e products. T h i s c o o l i n g system a l s o a l l o w s f o r l o n g e r residence time i n the grinding chamber. With increased quantity of grinding media and longer residence time i n the m i l l , the need f o r m u l t i p l e passes can be eliminated. As the paste moves through the m i l l , i t e x i s t s through a s l o t t e d c y l i n d r i c a l s c r e e n t h a t i s r o t a t i n g w i t h the g r i n d i n g s h a f t . The c e n t r i f u g a l f o r c e p r e v e n t s the g r i n d i n g media from c l o g g i n g and jamming the screen. The s c r e e n i s s e l f - c l e a n i n g , and as a consequence the paste does not cake on the screen, the media does not block the screen, and there i s unrestricted product removal from the m i l l . K i n e t i c Dispersion M i l l s . Another important m i l l i s the Kady m i l l d e v e l o p e d by C h a r l e s Kew of the K i n e t i c D i s p e r s i o n Corp. (21). By many i t i s considered another high-speed d i s s o l v e r because both use a high-speed i m p e l l e r even though t h e r e are b a s i c d i f f e r e n c e s between the two m i l l s . The Kady m i l l uses a paste of low v i s c o s i t y and an i m p e l l e r rim speed of 8700 f t / m i n (22). The m i l l g i v e s l i t t l e or no s h e a r i n g action and depends p r i m a r i l y on impact f o r dispersion. Most of the work i s done i n the s l o t t e d area of the s t a t o r . The agglomerates leave the rotor and smash i n t o the stator at high speed where they r e c e i v e a s e r i e s of impacts t h a t break up the agglomerates (23). The m i l l base leaves the stator i n a j e t stream where some shearing may take place. The m i l l base should always cover the i m p e l l e r to a c h i e v e the best e f f i c i e n c y and t o a v o i d the i n t r o d u c t i o n of a i r i n t o the base. The r o t o r i s a s s i s t e d by two p r o p e l l e r s , one of which i s above the r o t o r and the o t h e r below. These p r o p e l l e r s function as a pump, drawing the batch i n t o the head from the top and the bottom. No premix i s needed i n the Kady m i l l . The v e h i c l e i s f e d i n t o the tank, the motor i s s t a r t e d , and the pigments a r e added a t a r e l a t i v e l y f a s t r a t e . The m i l l i s a l l o w e d t o run f o r about 15-45 min u n t i l the r e q u i r e d degree of d i s p e r s i o n i s a t t a i n e d . The d i s p e r s i o n i s r a p i d , the m i l l i s e a s i l y c l e a n e d , but pigment throughput i s u s u a l l y low. S i n c e the m i l l depends on impact f o r dispersion, the more e a s i l y dispersed pigments are reserved f o r t h i s mill. Other M i l l s . There are many types of m i l l s that have been t r i e d i n industry. Two a d d i t i o n a l types worthy of mention are the A t t r i t o r m i l l and the SWMill. The A t t r i t o r m i l l (24) has bars that rotate, l i f t the grinding media, and drop i t upon the paste to break up the agglomerates while at the same time the paste i s c i r c u l a t e d by a pump. The m i l l i s designed f o r pigments that are d i f f i c u l t to disperse. I t i s claimed that t h i s m i l l i s more e f f i c i e n t than a b a l l m i l l . In the case of the S W M i l l ( F i g u r e 11), the i n g r e d i e n t s are placed d i r e c t l y i n the m i l l because no premixing i s required. The


Paint Manufacture

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Figure 11. I n t e r n a l view of SWMill. Courtesy Epworth Manufacturing Co., Inc.


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grinding media consists of miniature beads of b a l l s . While moving at high speed, rotators develop a deep vortex of paste and grinding media. Grinding occurs very r a p i d l y i n the SWMill; the v i s c o s i t y of the paste i s the same as i n the sand m i l l and b a l l m i l l . After the paste i s properly dispersed, letdown with more v e h i c l e or s o l v e n t can be accomplished i n the m i l l or a f t e r transfer to another tank. S o l v e n t used t o c l e a n the m i l l i s saved t o be used i n subsequent batches or as a reducer f o r v i s c o s i t y c o n t r o l .


Literature Cited 1. Definitions Committee of the Federation of Societies for Coatings Technology. "Paint/Coatings Dictionary"; Federation of Societies for Coatings Technology: Philadelphia, 1978. 2. Ν L Industries, Titanium Pigment Division. "Pigment Dispersion," No. P-3, p. 1. 3. Ibid., p. 2. 4. Baker Castor Oil Co. "Fundamentals of Viscosity and Thixotropy"; 1971, Technical Bulletin 83c, p. 2. 5. Patton, Temple C. "Paint Flow and Pigment Dispersion," 2nd ed.; Wiley: New York, 1979; p. 380. 6. Payne, H. F. "Organic Coating Technology"; Wiley: New York, 1961; Vol. II, p. 991. 7. Ibid., p. 993. 8. Brown, Harry M. Off. Dig. Fed. Soc. Coat. Technol. 1948, No. 284, 668. 9. Paul O. Abbel, Inc. "Handbook of Ball Mill and Pebble Mill Operation"; Paul O. Abbe, Inc.: Little Falls, NJ; p. 21. 10. Ibid., p. 7. 11. Payne, H. F. "Organic Coating Technology," p. 998. 12. Berliner, J. J., et al. "Pigment Dispersion"; J. J. Berliner Research: New York; No. 4716, p. 10. 13. Baker, Chester, P.; Vozzella, Joseph F. Off. Dig. Fed. Soc. Paint Technol. 1949, No. 294, 435. 14. Payne, H. F. "Organic Coating Technology," p. 1002. 15. Wahl, E. F. J. Paint Technol. 1969, 41(532), 345. 16. Taylor, J. J. "Mathematical Approach Toward Proper Pigment-toVehicle Ratios"; Los Angeles, CA, 1956. 17. Guggenheim, Stanford Off. Dig. Fed. Soc. Paint Technol. 1958. 18. Daniel, Frederick, K. J. Paint Technol. 1966, 38(500), 535. 19. Guggenheim, Stanford Off. Dig. Fed. Soc. Paint Technol. 1958. 20. Myers, C. K. "Bud" "Fundamentals of High Speed Dispersion"; Myers Engineering, Inc.: Bell, Calif. 1976; p. 11. 21. Payne, H. F. "Organic Coating Technology," p. 1007. 22. Zimmerman, O. T.; Lavine, Irvin Cost Eng. 1967, 12(1), 4-8. 23. Behrns, L. S. paper presented to the Golden Gate Society for Coatings Technology, Manufacturing Committee Conference, June 1970. 24. Patton, Temple. "Paint Flow and Pigment Dispersion," 2nd ed.; Wiley: New York, 1979; p. 439.


Applied Polymer Science - American Chemical Society

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