LEATHER FINISHING BY PRECISION ROLL COATING 47 1

LEATHER FINISHING BY PRECISION ROLL COATING*

W. C. PRENTISS AND C. E. CLUTHE

Research Laboratories, Rohm and Haas co. Spring House, PA 19477

Abstract Coatings for film, sheet, o r strip metal and building products have

been applied by roll o r gravure for some time. I n recent years, the appli- caticn of these techniques to leather finishing has been studied to improve finish utilization and reduce solvent emission. These benefits are sub- stantiated by the results of this work. Finishing can be achieved with about one-half the solids add-on of spray application and without the over-spray waste. Further, the solids of applied coatings can be more than twice those of spray finishes, thus reducing solvent emission further. However, precision roll coating appears to be less than ideal, since a t best there is a compromise between face, fill and uniformity as in- fluenced by the tracking pattern. T h e same lacquers as applied by roll coatings, but reduced with thinner and sprayed generally produce ex- ccllent face, fill, and uniformity of leather appearance. I n general, the technology and mechanics of precision roll coating require, for trouble- free production, someone who is skilled in finish formulation and has a good understanding of the equipment.

Introduction Recmt work in the laboratory has been directed towards developing ;I work-

ing understanding of modern finishing techniques, one of which is the roll coating of leather. T h e special benefits derived from this approach are hoth economical and environmental in scope: reduction of finish consumption and, in the case of solvent-borne finishes, reduced emissions. These advantages are primarily derived from the fact that the finish is metered directly onto the leather and thus there is no finish wastage.

Although some machines have b x n used for printing and tipping specialty leathers, less has been done with the application of continuous coatings, specifically solvent topcoats, where the advantages of roll coating are potentially more attrac- tive. T h c leather best adapted to studying roll coating is smooth, corrected grain cowhide. I t is generally smoother and more uniform than other leathers, and lends itself to long production runs with solid colors, which are naturally suited

*Presented at the 74th Annual Meeting of the American Leather Chemists Associa- tion, Lake Placid, New York, June 21, 1978.

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172 LEATHER F I N I S H I N G BY PRECISION ROLL COATING

to this method of application ( 1 ) . Th i s type of finish requires the maximum in leather face (smoothness or shell) and fill (blanking out or hiding, particularly with darker colors which easily show surface defects). I t is natural, therefore, to focus attention on this substrate to study the application of full topcoats with a precision roll coater.

T h e dynamics of roll coating can be extremely sophisticated and have been modeled to a large extent by Gasken ( 2 ) , Meyer, et al. ( 3 ) , ( 3 ) . Thei r work states that velocity and pressure gradients initially c a w fluid cavitation which ultimately leads to the production of filaments as in Figure 1. These filaments split under tension at the exit of the roller nip and lie down in a’characteristic pattern, coinInonly known as “turkey tracks.” In our rather naive approach to

FIGURE !.-Photogiaph showiug coating filamentat~on a t exit of coater nip.

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LEATHER FINISHING BY PRECISION ROLL COATING 473

this problem, we viewed leather as an irregular substrate on which to apply relatively complex topcoat formulations. Th i s dictated a simplistic treatment. W e will equate dry solution add-on and an averaged track dimension, centerline to centerline distance, to leather face and fill. Th i s treatment assumes that (1 ) the finish systems considered have realistic viscosity limits and are nonleveling, ( 2 ) face improves with decreasing centerline to centerline track dimensions, and ( 3 ) fill improves with increasing finish dry add-on. T h e ideal situation is having the top coat level to a uniform film after application. In the case of leather, this ideal situation has not been realized, as we will show.

Experimental PRECISION GRAVURE ROLL COATER

Not all rotary coaters are mechanically alike, hut they do share basic princi- ples which allow transfer of the technology from one unit to another. First, finish is metered onto a rubber applicator roll with an engraved metal roll. 'These rolls may be driven independently or in tandem with a variable slip clutch arrange- ment. Leather is supported below the applicator by a steel feed roll (or a con- veyor belt on production units) which is also driven, normally at the same speed as the applicator roll. A gap is maintained between the feed and applicator rolls and, among other things, this prevents the transfer of finish to anything other than the leather n hich is passed under the applicator roll.

O u r laboratory unit* has 26-inch long rolls with independent, variable-speed drives for both a 5-inch dia. feed roll and a 5-inch dia. 50-55 Durometer EP?' rubber covered applicator roll. I t uses a 3X-inch 55 quadrahelical gravure roll which is clutch protected and driven c;ff the applicator roll motor. T h e engraved roll is fully engaged by spring-abutting to the rubber roll. T h e coating gap can be regulated with a screw-operated carriage elevator for the entire coating head assembly.

FORMULATIONS AND FINISHING PROCEDURES

Basecoat Formulation; parts b y wcight

Primal@ Ochre Yellow Pigment 1 VV7a t e r 1 Primal@ AI(-350 1

Topcoat Formulations

Nitrocellulose concentrates were thinned and plasticized as indicated by ap- propriate solids. A solution of a black dye in alcohol was added to highlight tracking.

*Equipment purcha5ed from Black Bros., Co., Mendotta, Il l . @'Registered in I:. S. and principal foreign countries.

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474 LEATHER FINISHING BY PRECISION ROLL COATING

i. a

Finishing Procedure

Impregnated and buffed corrected leather was finished with two-swab basecoats, one spray basecoat (diluted) and satin plated. Leathers were cut into one-foot squares, weighed to the nearest 0.1 gm, rotary finished, and reweighed in the wet state.

ICleasuremmt o f Tracking

After drying, centerline to centerline distance between tracks was measured by use of a 7-power calibrated eye piece. T h e data reported are the averages of multiple measurements on each of a t least two test pieces.

Results and Discussion PERFORMANCE OF NITROCELLULOSE SOLUTIONS

T h e variables investigated were nitrocellulose molecular weight, plasticizer, solution solids, and ester and ketone solvents.

T h e results are presented in Figure 2 as dry finish add-on in gm/ft2 versus solution solids. Surprisingly, these results show that none of the variables, other than solution solids, significantly influence dry add-on. All the data approach a linear model with dry add-on increasing with lacquer solids. Th i s is a significant finding in that ( 1 j solution viscosity, presented in Figure 3 , does not play an active role in determining finish add-on and ( 2 j , the linear model implies a constant wet add-on which the slope defines as 1.1 gm/ft2.

If one equates dry add-on to fill, this analysis implies that maximum fill is achieved by roll coating with the highest possible lacquer solids the machinery can handle. Th i s limit is defined in that poor leather release from the application roll normally limits topcoat Brookfield viscosity to less than 500 cp.

A typical, spray applied topcoat lacquer is about six percent solids and results in a dry add-on in the range 0.25 to 0.50 gm/ft2. Extrapolation of the data of Figure 2 shows that unrealistically high lacquer solids, 23-45 percent, would be required to achieve the same add-on by roll coater. Since lacquers of this solids range are both technically and practically not feasible, as evidenced by extrapolat- ing the viscosity data of Figure 3 , one may question whether spray-like fill can be achieved by roll coating. I t is a production fact, however, that comparable fill is obtained, and this introduces the compensating factor, holdout, which we believe answers the high fill/low add-on riddle of roll-coated leather. Lacquers which we are prescribing for roll coating are more viscous and therefore ride higher on the leather surface (holdout j than those which are thinned for spray and tend to penetrate.

T h e centerline to centerline tracking behaviors for these systems are given in Figures 4 and 5. Tracking dimensions are shown to increase with increasing solids for all systems, but slopes for the data point plots diverge depending upon formula-

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LEATHER FINISHING BY PRECISION ROLL COATING 475

tion parameters. Increasing the lower molecular weight nitrocellulose solids has a lesser effect on increasing track dimension than increasing the solids of the higher molecular weight nitrocellulose. Adding plasticizer also decreases the slope. It is apparent that the choice of solvent will influence the slope; in this instance diacetone alcohol is a poorer choice than methylamyl ketone o r Cellosolve acetate.

T h e nonleveling assumption is amply supported by the observation that these systems show dark stripes contrasted against the ochre colored base in both the wet and dry state. Whi le individual track widths are an important parameter

i n n a

7 .IO-

8 .08- 2-

I I I I 8 IO 12 14 16

ROLLER GAP =30 MILS APPLICATOR SPEED = 6 0 f pm FEED SPEED = 75 t p m A

METHYLAMYL KETONE 0 5-6'NC/PARAPLEX G-60 ( I l l ) DIACETONE ALCOHOL 0 5-6'NC/ PARAPLEX 6-60 ( I l l )

0 A 5-6" NC A 5-6'NC/PARAPLEX G-60 ( 1 / 1 ) 0

J

7. SOLIDS

FIGURE 2.-Effect of solids concentration on dry add-on.

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LEATHER FINISHING BY PRECISION ROLL COATING 476

360 40

34

280 - a

>. - 29 c 240- v) 0 0 2 >

0

m 0 0 u

200-23 ”- # z I U 8 160- N

P - I7

2 w IL Y

-

m

120 - -12

5 -6’ NC, CELLOSOLVE ACETATE

4

5-6.NC/G-60(1/1), METHYLAMYL

KEToNEF I 5-6’NC/G-60 (1/1) I CELLOSOLVE ACETATE

I f l

I 0

% SOLIDS .-

% SOLIDS

FIGLRE 3.-Effect of solids content on viscosity of lacquers.

in the subject of leather face, they were just too difficult to measure accurately. T rack widths can be considered roughly half the center line dimensions and would not appear to disrupt our relating leather face to centerline measurements.

Since fluid add-on has been established as a near constant, the explanation of the observed tracking data is likely related in some manner to systeIn rheology, and this is partially verified by the order of the track results. T h e lowest viscosity systems - represented by the sec. nitrocellulose and the plasticized 5-6 sec. nitrocellulose in methylamyl ketone or Cellosolve acetate - have a flatter track response vs. solids than either the unplasticized high molecular weight nitro- cellulose or the plaqticized version in diacetone alcohol. Some preliminary, high

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LEATHER F I S I S H I N G RY PRECISIOS ROLL CO.4TING 477

shear rheology studies do not predict the precise ordering of results, so the nature of the rheology is not clear. T h e complication is further compounded, since we confirmed with Cellosolve acetate and dye alone the value 1.25, the intercept of all the lines at zero percent solids.

Since in this paper we presume that the finest track produces the most satis- factory leather face, w e are led to conclude that both optimum face and optimum fill cannot be simultaneously satisfied by controlling only the parameters dis- cussed in this paper. It is possible, however, to approach an agreeable compromise

ROLLER GAP = 30 M I L S APPLICATOR SPEED = 60 fpm FEED SPEED = 7 5 f p m

8 METHYLAMYL KETONE 0 5 - 6 ' N C / P A R A P L E X 6-60 ( 1 / 1 ) DIACETONE ALCOHOL 0 5-6" N C I P A R A P L E X @ G - 6 0 ( 1 / 1 )

QD CELLOSOLVE ACETATE G-60(1/1)

0

0 A

0

o.l t I I I I - 2 4 6 8 IO 12 14 16

7e SOLIDS

FIGURE 4.-Effect of solids concentration on tracking.

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LEATHER FIS'ISHING BY PRECISION ROLL COATING 479

dimensions. A n argument based on finish exclusion at the roller nip seems to ex- plain rezsonably the observed add-on results, while the tracking is likely reflective of a changing pressure gradient across the roller nip. Table I illustrates this be- havior on 60 to 70 mil leather for Cellosolve acetate thinned 5-6 sec. nitro- cellulose.

TABLE I EFFECT OF GAP DIMENSION BETWEEN FEED' A N D APPLICATORZ ROLLS

ON FINISH ADD-ON A N D TRACKING

Solution Solids8 Roller Gap Percent Mils

Tracking Dry Add-on mm, centerline

gm/ft* to centerline

15 30 50

50

.005

.007

.oos 0.10 0.11 0.12

0.15 0.25 0.35

0.4 0.45 0.5

'Feed Speed = 60 fpm 2Applicator Speed =I 75 fpm 35-6 sec nitrocellulose in Cellosolve acetate.

Unusually small and large roller openings can lead to production problems. Large gaps prevent compression of the leather so that with variations in thickness some areas are not coated. Narrow openings, on the other hand, insure that the finish is regularly deposited even in surface depressions, hut this is at the expense of fill. Additionally, narrow openings are more difficult to maintain without caus- ing some finish transfer to the conveyor, or the leather may "hang-up" at the roll entrance. O u r experience is that a roller opening comparable to half the average leather thickness affords a good balance of face and fill and avoids production problems.

Roller specds

This may be more of academic interest In that both production economics and labor place firm limitations on roll coater speeds. Increasing both the applicator and conveyor speed from 30 to 120 fpm causes both finish add-on and tracking to increase. Th i s is illustrated by the results in Table 11. W e feel that 60 fpm represents a reasonable compromise on leather quality and production.

Differential feed/applicator roll speeds not only effect cavitation and filamen- tation (tracking) as reported by Myers but can also affect finish add-on with only a minor effect on track dimensions. Add-on has been increased as much as 50 percent by coating a t a roll speed ratio of 2.0. Th i s phenomenon is thought

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480 LEATHER FINISHINC BY PRECISION ROLL COATING

TABLE I1 INFLUENCE OF ROLL1 SPEED ON FINISH ADD-ON AND TRACKING

( 5 % 5-6 sec nitrocellulose in Cellosolve acetate)

Applicator Rr Feed Speed fPm

Dry Add-on qm/ft?

Tracking mm, centerline to centerline

30 0.05 0.30 60 0.06 0.45

120 0.08 0.50

‘Roller Gap = 30 mils

to be due to a combination of less finish exclusion at the roll nip and an increase in coating shear forces. W h e n leather is coatrd at ratios below 1.0 there is also a gain in add-on, but the gain is less than a t ratios greater than 1.0.

Of interest, ratios greater than 1.0 correspond to reverse roll coating, and ratios less than one correspond to direct roll coating. Reverse and direct roll coating modes are often used to coat yard goods such as paper o r sheet metal, but in these cases, the gap between feed and applicator roll is relatively much larger than for leather. For leather, since compression of the leather between the feed and applicator rolls occurs, and leather is a piece rather than yard goods, coating at extreme ratios can create problems such as rejection of the leather at the fiip or distortion of the leather. Since leather tends to release more readily from the applicator roll a t ratios greater than one, we have preferred a ratio of 1.25 for our work.

Differential applicator-gravure roll speeds can also affect add-on and tracking performance of the finish. W e have investigated only one possibility : gravure roll surface speeds which are slower than the applicator roll. Predictably this causm less finish to be picked up by the applicator and therefore less is applied to the leather. W e find that considerable gravure surface speed reduction is necessary before measurable effects occur. Unless the machinery is designed to perform in this manner, i.e., independent gearing, it is not advisable to attempt this operation as irregular add-ons and “chattering” can result.

Cracure Rolls

Two basic patterns are commonly used, trihelical and quadrahelical. ’The tri- helical appears as spiral “V” grooves and the quadrahelical as helically arranged, indented, truncated pyramids. These rolls are available in a variety of screen s iza , to effect a change in the volume applied. Full leather topcoats demand heavier engravings (fewer screen counts/inch) in order to achieve a reasonable level of fill whereas finer patterns are better for effect znd dress coats. Relative to the

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LEATHER FINISHING BY PRECISION ROLL COATING 48 1

55 quadrahelical used in this paper, the amount of add-on expected with other engraved rolls is calculated in Table 111.

W e believe minimum fill requirements are satisfied with an 85 quadrahelical or a 95 trihelical roll. Fo r some unexplainable reason the quadrahelical pattern is used somewhat more successfully than the trihelical.

TABLE I11 INFLUENCE OF GRAVURE ROLL ENGRAVING ON RELATIVE ADD-ON

Screen Count Gravure Pattern No./Inch Trihelical Quadrahelical

35 55 85 95

110

1.1 1 .o 0.8 0.7 0.6

2.4 1.6 1.0 0.8 0.7

Conclusions T h e benefits which result from precision roll coating of leather are reduced

material consumption and reduced solvent emission for control of air pollution. These benefits are substantiated by the results of this work. Finishing can he achieved with about one-half the solids add-on of spray application and without the over-spray waste. Further, the solids of applied coatings can be more than twice those of spray finishes, thus reducing solvent emission further. However, precision roll coating appears to be less than ideal, since a t best there is a com- promise between face, fill and uniformity as influenced by the tracking pattern. The same lacquers as applied by roll coating but reduced with thinner and sprayed generally produce excellent face, fill and uniformity of leather appearance. I n general, the technology and mechanics of precision roll coating requires, for trouble-free production, someone skilled in finish formulation and with a good understanding of the equipment.

Acknowledgment W e wish to thank Rohm and Haas Co. for permission to submit this paper.

O u r particular thanks go to Mr. John Sweriduk, Mr. Raymond Porter, and Mr. Robert Dieckmann for conducting much of the experimental work and mak- ing lacquer preparations.

References 1. Emerson, E. K. 2. Gaskell, R. E.

Wisconsin Tanners Production Club Symposium, March 18, 1978. Trans. ASME, 72, 334 (1950).

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182 LEATHER FINISHING BY PRECISION ROLL COATING

3. Miller, J. C. and Myers, R. R. 4. Myers, R. R. and Hoffman, R D.

Trans. Sot. Rheology 1 , 77 (1958). Trans. Sot. Rheology 5, 317 (1961).

Received June 16, 1978.

Discussion DR. S T E P r i E N FEAIRHELLER (Eastern Regional Research Center) : T h a n k

you, Rill, for this very fine paper. I t is indeed a very thorough and scientific evaluation of this new technique for applying finishes. I know that this technique is creating considerable interest in the tanning industry-. I have some questions, but in view of the time, I will save these and see if there are any from the floor. Do we have any questions?

MR. HARRY BRACE (Ellithorp Tanning Co.) : W h a t is the effect of this roll coating process in regards to the drying time of the application? Also, does it produce any tackiness because of the slow drying solvent which, if not evaporated at the end of the conveyor, would cause production problems?

DR. P R m " s s : Actually, we were under the assumption that to make an effec- tive roller coating would require a slow drying solvent, because with fast drying solvents in the system, it is possible that the finish on the applicator rolls would tend to dry out. I t would be difficult to resolubilize the lacquers on the rolls, and further, we thought with a slower drying solvent we might achieve leveling on the leather. As it turns out, even going with the slowest solvent, and that is butyl Carbitol acetate, the tracking patterns are about the same as we observed with the Cellosolve acetate o r with the methyl isobutyl ketone. Now, in terms of the drying rate, using Cellosolve acetate or ketone solvents, the weights that we obtain immediately after the leather goes through the roll coater are the exact wet weights as close as we can measure; yet, within a very short period of time the lacquers are dry to touch. I do not know of anyone who has difficulty drying roll coated leather, because part of the solvent is taken up right in the leather it- self on the base coat.

MR. BRACE: T h a n k you.

DR. FEAIRHELLER: W e have time for one more question.

MR. ANTHONY PILAR (A. J. and J. 0. Pilar, Inc.) : I was wondering whether you must have the leather in a certain condition to use the roll system properly, and whether you can only use this on side leather? Could you use it on garment leather ?

T h e second question is: W h e n you operate the equipment, what would you suggest is the proper way to maintain a constant solids content in your pumping system?

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for all kinds of leather. Tony, it is not. T o be able to achieve a given amount of add-on for any kind of leather requires a certain viscosity in the lacquer as it is used, and there is a certain amount of clinging of the leather to the applicator roll. With very light weight and very flexible leathers, such as garment, there is much difficulty in removing them from the applicator roll. So actually, the procedure is much more applicable to the smooth, corrected grain men’s upper- weight leather than it is for garment.

Now, to answer your second question, as you know, our laboratory unit does not have a recycle device, but commercial machines do have recycling devices on them. T h e finish material overflows into a sump, and by viscosity control, the solids can be maintained. It’s a simple measuring device.

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