Formulation of Nitrocellulose Lacquers - American Chemical


tion rose enough to touch the drum, men started to brush the surface vigorously with steel wool to preventrusting until platingstarted. Then the surfa...
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July, 1928

I N D U S T R I A L A N D ENGINEERING CHEMISTRY

r. p. m. The generator was started and water run into the tank through a fire hose. As soon as the level of the solution rose enough to touch the drum, men started to brush the surface vigorously with steel wool to prevent rusting until plating started. Then the surface was brushed with steel brushes every 15 minutes for the first 24 hours and every half hour thereafter. By observing the surface during rotation, it was possible to detect any spots that were not plating and a little extra rubbing here soon gave a complete coat of zinc. Since about one-fifth of the drum's surface was submerged, it was only plating one-fifth of the time. Based on the amperes flowing, it was calculated that the zinc was plating a t the rate of 0.0001456 inch in thickness per hour. The specifications called for inch, so the electroplating was carried on for 100 hours. Of course, any shutdown would have been disastrous. To meet emergencies, pump connection to pump out the solution quickly, fire hose to wash the drum surface, etc., were provided, but fortunately these were not brought into use. During the plating, the solution was checked by taking pH readings and adjusted from time to time by the addition of salts. At the end of the 100 hours, the solution was pumped out of the tank and the drum was kept rotating. The amperage dropped as the level of the solution went down, and as soon as contact of the liquor with the drum surface was broken

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the ammeter indicated zero. The drum was immediately washed with water and steam, then quickly wiped and dried, and finally the finished surface was wire-brushed. Careful examination showed the galvanizing had been put on evenly, heavy, hard (not spongy) and, in general, was a satisfactory job. Calculation of Weight of Deposit

A final check on the thickness of the coating gave the following interesting figures: (1) The actual zinc deposited as determined by loss in weight of anodes less tares, etc. = 169 pounds ( 2 ) Area of cylindrical surface of drum = ?r (' 5 ) 2 lo 67 = 285squarefeet 4

(1) + ( 2

(3) Pounds deposited per square foot, i e

=

= 0.593 pound

285 Specific gravity of zinc = 7.14

Weight of 1 square foot of zinc l , =

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inch thick

=

x 62 12 X 64

7 14

0.582 pound

Therefore, the hourly depositing rate as figured by the current flow checked very closely with the actual deposit as calculated from the weight of zinc used.

Formulation of Nitrocellulose Lacquers' H. E. H o f m a n n a n d E. W. Reid MELLON IXSTITGTE

O F INDUSTRIAL

RESEARCH, UNIVERSITY O F

PITTSBURGH, PITTSBURGH, P A

I t is t h e purpose of this paper to develop a s y s t e m a t i c c o n s t i t u e n t s among themH E formulation a n d m e t h o d of lacquer formulation, based on the use of selves remains constant, but manufacture of nitroexperimental d a t a scientifically obtained a n d reprethe ratio of the volatile to the cellulose lacquers is a sented graphically by m e a n s of the t r i a n g u l a r con o n - v o l a t i l e varies as the part of our growing chemical o r d i n a t e chart. An a t t e m p t is m a d e t o show how the former evaporates. Likewise, industry which has not been properties of both the volatile a n d non-volatile porthe ratio between the solvents, subjected t o such accurate t i o n s of a lacquer m a y be studied, a n d their variation the alcohol, and the hydrochemical or scientific control w i t h changes in composition is recorded d i a g r a m carbon, changes during the as is warranted by its impormatically. It is also i n t e n d e d t o indicate how t h e s e drying period. For this reatance. As the industry has d a t a a n d charts m a y b e used to f o r m u l a t e new lacquers son it is essential, not only grown and the competition or to Predict Propertiesthat a proper balance of the has become keener, the need volatile constituents be presfor scientific formulation of ent initially, but that this balance be maintained throughout lacquers has become more urgent. A number of methods of testing lacquers have been given in the literature, but scientific the drying period. The various steps in lacquer formulation are not clearly procedures of formulation are lacking. The methods offered in this paper are given as suggestions for a systematic scheme defined since they are interdependent, and it is somewhat of lacquer formulation and are accompanied by a correlation difficult to show them in a logical or definite sequence. These of experimental data. It is believed that methods of this steps may be divided as follows: nature will aid in materially increasing the efficiency and (1) Determination of the properties desired in the finished decreasing the tedium and intricacy of lacquer formulation. product. A nitrocellulose lacquer consists essentially of two parts(2) Determination of the ratio of non-volatile constituents. (3) Selection of the non-volatile ingredients to be used viz., the volatile and non-volatile portions. The non-volatile (4) Selection of the volatile ingredients. part consists of nitrocellulose, resin, and plasticizer (the pig( 5 ) Determination of the composition of the volatile portion ment will be treated as an auxiliary component). The volatile portion consists of the nitrocellulose solvent, an Properties alcohol, and a hydrocarbon. This division includes all types of lacquers and is shown diagrammatically in Figure 1. The properties of a lacquer will depend upon how and for In the formulation of a lacquer, it must be considered that what purpose it is to be used. An automobile lacquer must the composition varies continuously from the moment it be hard but not brittle, durable. have good adhesion, and leaves the container until it is thoroughly dry as a film on the capable of being rubbed to a high gloss. The nature of the surface to n-hich it is applied. The ratio of the non-volatile solvent is of minor importance, except where applied over an old finish. Presented before the Division of Paint and Varnish Chemistry A household brushing lacquer should brush easily, have at the 75th Meeting of the American Chemical Society, St. Lours, hio April 16 t o 19, 1928 fair gloss and hardness, but need not have as great durability

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as an automobile lacquer. The solvent is of importance since a strong or disagreeable odor is undesirable. It should not "lift" old paint or varnish and the solvents should evaporate quite slowly since this factor contributes to the brushability. A furniture lacquer should possess a fair gloss, be somewhat flexible, yet hard enough to permit sanding and polishing. Lacquers for other special purposes will have different properties. It is not possible to obtain all of the desired properties in one-lacquer, and the ideal lacquer has not been found. 1

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Note-All materials used were of ordinary commercial grade. Unless otherwise specified, all solvent compositions are b y volume and all other proportions are by weight.

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widely. All the lacquers were pigmented, since the majority of commercial lacquers are used in a pigmented form. The pigment used in all tests was of the same nature and composition, and was present in the ratio of 1 : 2 (by weight) to the non-volatile vehicle. EXPERIMEKTAL PRocmwRE-In order to prepare a series of mixtures of varying composition, it was necessary to have available solutions or mixtures, each containing only one non-volatile ingredient. The solvent used throughout these experiments had the same composition, which was as follows: toluene, 70; Ansol, M.L., 5; ethyl acetate, 15; Cellosolve, 5; Cellosolve Acetate, 5 per cent.

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An illustration of this fact is shown in Figure 2. This is a triangular coordinate chart2 representing the system nitrocellulose-resin-plasticizer. It is known that an increase of the nitrocellulose content improves the toughness of the film but decreases the gloss; an increase of the resin improves the gloss but also produces a more brittle film; an increase of the plasticizer adds to the flexibility but detracts from the hardness. These facts are shown graphically in the diagram, where any composition of nitrocellulose, resin, and plasticizer may be represented by a point. It is obvious that this point cannot be in the region of maximum hardness and maximum adhesion or some other region a t the same time. With the materials a t present available to the lacquer technologist it is not possible to prepare a lacquer which will have the maximum of all the properties desired. Determination of Non-Volatile Composition The composition of the non-volatile part of the lacquer will depend primarily upon the properties selected above as being m o s t important. I n almost all cases a compromise must be made in certain respects, but t h e f i n a l composition s h o u l d represent the highest degree of each property attainable. I n order to explain clearly the method used to achieve this result, it will first be n e c e s s a r y t o t a b u l a t e the various P L * T I I r i s p r o p e r t i e s , or attributes, considered in lacFigure 2 q uer formulation and then to study the effect on each of variation in the composition of the non-volatile portion. For the purpose of this paper the following lacquer properties may be mentioned: gloss, adhesion (to metal), flexibility, hardness (printing), ease of sanding and polishing, durability (out-door exposure), blushing, lifting (of varnish) and brushing properties. The experimental data obtained to show the effect of the non-volatile composition on the properties are presented in Figure 3. They are obtained on a series of lacquers having the same solvent composition and the same solid ingredients, but the ratios of the latter to one another varied NlTRDCllLUiOIL

9 For the use of these charts in lacquer technology, see Hofmann and Reid, IND. END.CHEM.,20, 431 (1928).

The nitrocellulose solution consisted of 20 per cent (dry weight) of 'lrsecond R. S. nitrocellulose in the above solvent. The resin solution was 50 per cent dewaxed dammar in the above solvent. The dewaxed resin was prepared by dissolving Batavia dammar resin in 90 per cent commercial benzene, then adding a double volume of denatured alcohol ( c o m p l e t e l y denatured, formula No. 1) to precipitate the wax. When the wax had settled, the clear solution was placed in a d i s t i l l i n g flask and most of the solvent removed. The thick r e s i d u e was poured into shallow pans and placed in a v a c u u m oven for about 8 hours a t a temperature of 60' to 75' c. and a Figure 3-Properties of Nitrocellulose pressure Of to 2o In all lacquers theLacquers pigment equals 33I/a per cm. of mercury. cent of non-volatile content The plasticizer used was commercial dibutyl phthalate. Since nitrocellulose is the only ingredient common to all the mixtures, the pigment paste was ground in a nitrocellulose solution of the following formula: h src H 8 ~ a ~ i r r i u i a r r

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Parts b y wt. Nitrocellulose (dry), l/r-second Special solvent mixture Zinc oxide Prussian blue Bone black

150 2200

700 400

100

Per cent by wt. 4.3 62.0 19.7 11.2 2.8

This paste was ground in a pebble mill for 75 hours. A medium blue color was chosen as being fairly representative, and permitted such properties as gloss and blushing to be easily noted. The compositions of the test lacquers are given in Table I. Gloss. The gloss of all the panels, both bare and surfaced, coated with the above test lacquers, was practically the same throughout the series. Readings were made on the Ingersoll Glarimeter and all lacquers gave values between 55.5 and 57.0 on the Ingersoll scale. This may be expected from the fact that the percentage of pigment in the nonvolatile residue was the same in all cases. It is thought, however, that a number of commercial lacquers having the same pigment and other solids, but varying in the proportion of each, would give films of widely differing gloss. Besides giving the same readings with the Glarimeter, all the panels in the above series appeared to have practically the same gloss when observed visually. Adhesion. So far no satisfactory methods have been developed for the measurement of this property. The pro-

July, 1928

I N D U S T R I A L ALVDENGINEERING CHEMISTRY cedure used in this work was essentially that proposed by Sub-committee XXV of Committee D-1 of the American Society for Testing M a t e r i a l s . A moderately heavy double coat of each lacquer was sprayed on a steel panel, 10 by 15 em.; and, while still wet, a strip of good quality crepe de chine, 3.76 by 20 cm., was stretched and placed firmly upon the lacquer film, the ends of the cloth being pulled down slightly beyond the ends of the panel. The panels were allowed to dry in a horizontal position, and were observed after standing for 24 hours. The results of this preliminary test were negative, the strips of cloth being very easily lifted from the panels, having absorbed the lacquer, and leaving a well-defined bare strip on the metal. In a repetition of this test two methods were adopted. One coat of lacquer was sprayed on a clean bare metal panel as before, and allowed to dry. A second coat was sprayed on, and, while wet, a strip of the silk cloth placed on the film, similar to the procedure in the first series of tests. h second strip of cloth was then placed on the film when it was nearly but not quite dry, so that very little material was absorbed by the cloth, but the latter would yet adhere to the lacquer. I n the second series it was found that the strips of cloth which were placed on last-that is, those that were not saturated with the lacquer -gave the best results. The method needs considerable i m p r o v e m e n t , however, and the observations are only approximate. On Figure 3 the dotted lines show approximately the divisions containing the combinations exhibiting good, fair, and poor adhesion to bare metal (steel). As expected, the adhesion varies inversely as the amount of n i t r o c e l l u l o s e present. Flexibility. There are some objections to the method for testing the flexibility of the lacquers on the basis that the property tested may not be entirely flexibility but may also depend upon adhesion. The test is easily made, however, and the results are of interest. Two coats of each lacquer were sprayed on a piece of tin plate which had been previously sandpapered and cleaned thoroughly with solvent. After drying for 2 days, the tins were tested by quickly bending them double over a 0.5-cm. iron rod a t a temperature of 25" C. The lacquers were then divided into two groups, those that cracked upon being bent double and those that did

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not. These are indicated in Figure 3, and a line has been drawn separating the more flexible combinations from the less flexible. It must be understood that the values obtained in this series of experiments are only approximate, and are not applicable when a different resin or a different plasticizer (such as an oil) is used, but the general trend of variation will be similar. It is advisable for the individual investigator to study in this manner the properties of the nonvolatile ingredients he intends to use. When a series of charts similar to those being described in this paper has been prepared, a lacquer may easily and quickly be formulated for any given purpose with the assurance that it will have the desired properties when finished. Other properties, as humidity resistance, settling of pigments, resistance to extreme temperature changes, resistance to ultra-violet light, outdoor durability, and, in the case of data intended for brush lacquer formulation, the brushing properties, may be quantitatively determined and plotted on an appropriate triangular chart, against the non-volatile composition. Selection of Non-Volatile Ingredients

The choice of the non-volatile ingredients to be used in a lacquer does not appear to rest on any easily recognizable scientific basis, but seems to be more or less arbitrary. The chief considerations are price and availability. The lowviscosity, or 1/2-second, nitrocellulose is the most widely used member of its class, since it may be used in greater amounts without unduly increasing the viscosity of the lacquer solution. This results in greater film thickness, hence greater gloss and more protection per coat, which are desired in most present-day lacquers. I n certain cases where a thin film is particularly wanted, or where gloss and depth of film are not so important as strength, the higher viscosity types of nitrocellulose are used. It will be seen that the choice of the types of this fundamental ingredient is really restricted by the use to which the material is to be put. In the common use of the term "nitrocellulose lacquer," whether an automobile, a household brushing, or an architectural lacquer, it is generally understood that l/Z-second R. S. nitrocellulose is involved in the formulation. This type has become a fairly well-defined article of commerce. The choice of plasticizers is almost as limited as that of nitrocelluloses. From the standpoint of commercial availability, the dialkyl phthalates and the triaryl phosphates are the only ones of major importance. The properties of dibutyl and diamyl phthalates and tricresyl phosphate make them the most popular members of these groups. There seems to be little real difference in their merits and the price is usually the deciding factor. From time to time various compounds are proposed as lacquer plasticizers, some of which appear to have considerable merit, but their widespread adoption is usually restricted by their high price, inadequate supply, or some undesirable physical or chemical property. The oils now being used advantageously in nitrocellulose lacquers include castor oils, both raw and blown, rapeseed, and blown linseed oil. Of all the commercially available oils these seem to possess the most desirable properties for this use. Tung oil is objectionable on account of its odor and its tendency to "crystallize;" raw linseed and other ram oils, such as corn oil, cottonseed oil, etc., are too thin, and cannot be used except in very limited quantities. In general, the drying oils tend to become hard and brittle with age, thus changing the nature of the lacquer film. This leaves the non-drying oils of the castor oil type holding the preferred place for this particular use. Contrary to the relatively narrow choice in the cases of nitrocellulose and plasticizer, there is a bewildering array of resins from which to choose. A discussion of the properties

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Selection of Volatile Ingredients

The solvent ingredients to be used in any lacquer composition will depend chiefly upon the mode of application and the nature of the non-volatile components. For ex; Note-The authors prefer t o use the term “solvent” to include a n y liquid used in the volrtile part of a nitrocellulose lacquer. The fact t h a t “non-solvents” for nitrocellulose (also called “diluents”) are always good solvents ior some other ingredient of the lacquer renders the latter terms misleading and not scientifically accurate.

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