IiVDUXTRIAL A N D ENGINEERING CHEMISTRY
242
VOl. 21, No. 3
Consistency of Animal Glue’ Don Brouse U. S. FOREST PRODUCTS LABORATORY, MADISON, WIS.
T
HE two factors of outstanding importance in making
under which flow takes place. Very often colloid solutions strong glue joints are the pressure and the consistency are not truly viscous. I n fact, Hatschek* says: Of the at the time Of the pressure* Increasing viscosity with decreasing shear gradient may now “‘st Of the many conditions affecting the Of glued be considered a general property of colloidal solutions. Since wood joints can be correlated by considering their influence the behavior is thus characteristic of a large and important on the consistency of the glue. When an animal glue solu- class of liquids, it is of great importance to find some explanation it is much for it. One suggested by several authors, including Professor tion, a t 60” C. for example, is applied to the Freundlich, is that these solutions, unlike normal liquids, possess too fluid for submission to ordinary gluing Pressures, but rigidity as well as viscosity. It can indeed be shown matheits consistency increases a8 the glue cools and gives up water matically by making the simplest assumption regarding rigidity that the results will be found to the wood. The increase experimentally, namely, deis gradual a t first but soon creasing v i s c o s i t y with inbecomes very rapid and the T h e U. S. Forest Products Laboratory has studied t h e creasing velocity gradient. glue finally forms a jelly. consistency of solutions of animal glue w i t h respect t o Furthermore, Humphrey Good gluing p r a c t i c e dethe influence of t e m p e r a t u r e , concentration, a n d glue and Hatschekg found that mands that the pressure be grade w i t h i n the r a n g e of wood-working practice. this property of rigidity or withheld until the glue a p T h e findings provide f u n d a m e n t a l principles for glueelasticity could be produced proaches the point of gelar o o m procedure. by the introduction of rigid tion.2 At 60” C. animal-glue solutions a r e t r u l y viscous. particles into a truly visI n spite of their direct The viscosity increases as the t e m p e r a t u r e falls, a t first cous liquid. But Hatschek p r a c t i c a l significance, the slowly, then very rapidly as the p o i n t of gelation is apalso found that many colconsistency-t em p e r a t u r eproached. Changing the glue-water ratio has m u c h the loids lose their property of concentration relations of s a m e effect o n gluing technic as changing t h e grade of rigidity when the temperaanimal glue solutions have the glue. ture of the solution increases never been studied with reWithin a very small r a n g e of t e m p e r a t u r e , solutions and become truly viscous spect to gluing technic. To of animal glue change f r o m viscous to plastic. This above a b o u t 40” C., l i k e be sure, the viscosity of relac h a n g e is a u n i q u e property of the gelation p o i n t a n d “pure” liquids or molecular tively dilute gelatin soluprovides a satisfactory m e a n s f o r defining a n d meassolutions. tions has been a favorite uring the gelation p o i n t independently of t h e appaBingham’o has suggested subject for research. The r a t u s used a n d t h e j u d g m e n t of the observer. The the following form of equagrading of animal glue is s u d d e n t r a n s i t i o n f r o m viscosity t o plasticity agrees tion for the flow of plastic based in part on measurew i t h the hypothesis of Hatschek a n d invites an exmaterials: ments of t h e viscosity of a m i n a t i o n by Herschel’s m a t h e m a t i c a l t r e a t m e n t of fairly dilute solution^,^.^ and sgd4 (P - p ) plasticity. more or less arbitrary meth= 128ql ods of measurine l‘settind’ ” where p = viscosity R = 3.1416 or “melting” points have g = the gravitational constant been described.6.6 Recently Kraemer and Fanselow have d = diameter of the capillary found sharp changes in optical rotation a t the “setting” P = total pressure point of dilute gelatin solutions.7 But the behavior of conp = yield value P - p = effective pressure centrated glue solutions can be inferred only roughly from p = volume extruded per second through the capillary such methods. Therefore direct observation of concentrated 1 = length of the capillary solutions is necessary before the fundamental principles of gluing technic can be placed upon a scientific foundation which may be reduced to instead of the empirical basis upon which they were first developed. The studies of glue solutions reported herein are of interest, in the general theory of consistency, especially that of emulsoid colloids. Most of the methods in common use for measuring viscosity, for example, observations of the rate of flow where K is a constant. According to this formula, the rate of flow is directly prothrough a capillary under a nearly constant hydrostatic head, portional to the pressure (upon which the shear gradient are applicable only when the material is truly viscous, that is, when the viscosity is independent of the shear gradient depends) in excess of a characterist,ic minimum pressure, or “yield value.” The slope of the flow-pressure curve is pro1 Received August 28, 1928. portional to the “mobility,” which is the reciprocal of the 1 Truax, Furniture Manuf. Aufison, May, 1924. viscosity, and the intercept on the axis of pressures is the 8 Bogue, “Chemistry and Technology of Gelatin and Glue,” p. 189, yield value. Mobility and yield value together define the McGraw-Hill Book Co., 1922. consistency of the plastic material. Theoretically there is 4 DeBeukelaer, Powell, and Bahlman, IND. ENG. CHEM., 16, 310 @
v
(1924). 6 Sheppard and Sweet, I b i d . , 19, 423 (1921). 6 Alexander, “Glue and Gelatin,” p. 182, Chemical Catalog Co. 7 Kraemer and Fanselow, J . Phys. Chem., 92, 894 (1928).
Hatschek, Nature, 119, 857 (1927). Humphrey and Hatschek, Proc. Roy. Soc., 28, 274 (1916). 10 Bingham, Bur. Standards, Sci. Paper 278 (1916). 8 @
March, 1929
INDUSTRIAL AXD ENGINEERlNG CHEiMISTRY
no flow unless the pressure exceeds the yield value; actual measurements usually reveal some flow a t lower pressures, which Bingham attributes t o an experimental error, “slipping” of the material through the capillary like a solid rod through a cylinder instead of tJhe “telescopic” flow of a fluid clinging to the walls of the capillary. The deviation of the flow-pressure curve from a straight line at low pressures may in some cases be due entirely to “slipping,” as Bingham maintains, but there are many materials in which the flow-pressure graph is certainly a curved line; that is, there is some flow a t any pressure, however small, but the rate of flow increases more rapidly than the pressure. Herschel and Bulkleyl’ find rubberbenzene solutions typical of this and suggest the following type of equation for expressing such consistency: in which c = a constant characteristic of the apparatus p = pressure q = volume extruded per second I and n = constants characteristic of the material
which describe its consistency
Herschel12 holds that emulsoids are plastic. The results reported herein for animal glue indicate that at temperatures below the point of gelation the consistency can possibly be expressed by the above equation. Materials a n d Methods
A satisfactory instrument for determining the consistency of concentrated glue solutions must be capable of (1) distinguishing clearly between viscosity and plasticity, (2) measuring consistencies ranging from 1 poise or less to a t least 1000 poises, and (3) operating a t precisely controlled temperatures between ordinary room conditions and 60 ” C. It should also measure viscosity in absolute units.
243
glycerol-water solutions, the viscosities of which are given by Her~che1.l~For the method of calculating viscosities with an instrument of this type, reference is made to Herschel’s work on consistency.12 A double check on the instrument was obtained by determining the viscosities of a glue solution by means of an Ostwald viscometer, independently calibrated and by the Herschel type consistometer. With the air pressure a v a i l a b l e (up to 600 mm.Hg) and with the 10 different capillaries for the modified Herschel instrument, it was possible to measure the consistency of any material ranging from viscosity of water to one with the properties of a concentrated glue soh, , ‘ I tion slightly below the gelation temperature. The first observations of consist e n cy I
,
,
lbk,-
were and were made then a trepeated 60” C. a t 5” intervals as the &---e 1 temperature was low‘*- - -*ered. When the gela%--* ___. ._ a* JO sd tionpointwasapTemperrture de9 C proached, consistencies F i g u r e %Relation between Consistd e t e r m i n e d at e n c y a n d T e m p e r a t u r e of T h r e e Differintervals of 1” C. The e n t C o n c e n t r a t i o n s of G l u e A temperature w a s controlled within =t0.lo C. by means of a thermostatically controlled water bath. The solution was poured into the reservoir of the instrument, which was then immersed in the bath. A stem thermometer extending through the wall into the reservoir indicated when the glue solution had reached the temperature of the bath. Consistency measurements were not made until this temperature equilibrium had been attained. Three glues were used: _L
‘-6
41
60
1’
GLUEA-A flake glue which had served in the past as a standard of comparison in purchasing glue for government aircraft work. The glue was rated by the Bureau of Standards in accordance with the system of the National Association of Glue Manufacturers as 93.0 millipoises viscosity and 298 grams jelly strength. GLUEB-A flake glue of lower grade than glue A, reported by its manufacturer to have a viscosity of 65 millipoises and a jelly strength of 190 grams in the National Association of Glue Manufacturers system. GLUEC-A flake glue from goat stock, which was reported by the manufacturer to have a viscosity of 115 millipoises and a jelly strength of 317 grams. The viscosity of this glue is high in comparison with its jelly strength. F i g u r e 1-Relation b e t w e e n T e m p e r a t u r e and Specific G r a v i t y of A n i m a l G l u e
The modified Herschel consistometer used by Browne and Brouse13 for a study of casein glues fulfils the requirements, provided capillary tubes of proper sizes are chosen. Ten capillary tubes were therefore constructed for use with this instrument. The inside diameter of the tubes varied from 0.04748 to 0.3099 cm. and the length of each tube was approximately 6.5 cm. The capillaries were calibrated against 1’
Herschel and Bulkley, Am. SOC.Testing Materials, 29th Ann. Rept.,
1926. Alexander, “Colloid Chemistry,” Chemical Catalog Co., 1926. Browne and Brouse, Colloid Symposium Monograph, Vol. V, p. 229 (1927). 1’
18
Glue A was studied in three concentrations, 1 part of dry glue (by weight) to 2, 2.25, and 2.50 parts of water, respectively. The other glues were observed in a ratio of 1part of glue to 2.25 parts of water. I n order to insure a constant moisture content of the “dry” glue as weighed out, it was stored in a room a t 60 per cent relative humidity and 27” C. for about 30 days before the experiments were begun. Bateman and Townel6 have shown that there is a relation between the relative humidity of the atmosphere and the moisture content of the dry glue in equilibrium with it. The moisture content of the “dry” glue remained approximately 17 per cent during the experiments, so that the actual concentration 14 16
Herschel, Bur. Standards, Tech. Paper 112 (1919). Bateman and Towne, IND.ENG.CHEM.,15, 371 (1923).
INDUSTRIAL AND E,VGINEERING CHEMISTRY
244
Vol. 21, No. 3
of the 1 to 2.25 mixture, for example, was about 25 per cent It is claimed that the specific gravity of a glue does not instead of 30.8 per cent. vary greatly with grade.l6 If this is true, the curve shown The "dry" glue was mixed with the desired amount of in Figure 1 might be used to determine the specific gravity water in an Erlenmeyer flask, which was then stoppered and of any grade of animal glue at the temperatures shown. allowed to stand overnight a t a temDerature between 10" and CONSISTEKCY-TEMPERATURE RELATIONSOF GLUE A 15" C. The next m o k n g the glie was melted a t 66" C., hIIXED I N THE PROPORTION O F 1 PART GLUETO 2.25 PARTS poured into the con- WATER-Glue A mixed in the proportion of 1 part to 2.25 s i s t o m e t e r , and al- parts of water is considered first because it approximates the lowed to reach the grades widely used for wood-joint work, and is representative temperature of t h e of commercial practice. water bath. The conThe variation of consistency with temperature is illustrated sistency was then de- by the middle curve of Figure 2. The values from which the termined. curve is plotted are given in Table 11. Except for the conbetween Consistency and Temperature of Glue A, centration, this proce- Table 11-Relation Mixed 1 Part Glue t o 2.25 Parts Water TEMPERATURE CONSISTENCY dure follows closely c. Poises that outlined in the 404.OOa National Association 51.45'~ 18.62b of Glue M a n u f a c 11.39b 9.39'~ turers directions for 7.27b the determination of 6.01'~ 3.61b vis~osity.~ 3.39b 2.726 At each tempera2.44b ture a t which consista Solution plastic. b Solution viscous. ency was measured four observations of Because the solution was viscous a t temperatures between r a t e of flow were 60" and 30" C., each figure givenin Table I1 for these temperamade, each one a t a tures is an average of the four observations made at different different pressure beFigure 3-Relation between Consistency pressures, the viscosity being independent of pressure in a n d T e m erature for Equal Concentra- t w e e n 100 and 600 tions of 'TPhree Different Glues mm. Hg, correspond- viscous liquids. At 29" C. the golution was no longer viscous, ing to 135 and 816 grams per sq. cm. Four points for a the apparent viscosity decreasing as the pressure increased flow-pressure diagram were thus obtained to indicate whether (Figure 4),and the figure given in the table is the apparent the solution was truly viscous or plastic, as well as to give a viscosity a t 473.5 grams per sq. cm. pressure for the particular capillary tube employed. precise measure of the consistency. The viscosity of the solution changes comparatively little Hatscheks states that some materials that seem truly in cooling from 60" C. to about 38" C. (Figure 2), but on viscous a t higher rates of shear, in reality may possess a degree of plasticity revealed only a t very low rates of shear. further cooling the viscosity increases very rapidly. The change from viscosity to plasticity takes place within Such plasticity would not be disclosed a t the pressures obtaining in these experiments. For the present, solutions will a very small range in temperature. Figure 4 presents the be called viscous if the four points of the flow-pressure dia- flow-pressure diagrams a t 29" and 30" C. At 30" C. it is gram, within the limits of experimental error, lie on a straight line through the origin. If the diagram departs from a straight line in the sense of more rapid increase in rate of 0 06 flow than in pressure, or if the viscosity decreases with inci creasing rate of shear, the solution will be called plastic. % 005 Results
SPECIFICGuvrTY-Specific
gravity enters into the computation of viscosity from the rate of flow under known pressure. For purposes of grading animal glue, the specific gravity is taken as 1.020, because 12.5 per cent solutions a t 60" C. vary but little from that value.'6 However, when the temperature is changed over a range of 30" to 60" C. and the solutions are concentrated, the specific gravity can no longer be assumed to be constant. The specific gravity of the 1 to 2.25 mixture of glue A was measured a t temperatures from about 30' to 65" C. The results appear in Table I and Figure 1. Gravities of Glue A Mixed i n t h e Proportion of 1 Part Glue t o 2.25 Parts Water TEMPERATURE SPECIFIC GRAVITY
Table.-1-Speci5c
c. 65.56 60.00 60.00 56 46 54.44 48.80 43.33 37.78 32.22 29.44
1.0680 1.0680 1.0750 I1,0795 n79.5 1.0835 1.0865 1.0890 1.0910 1.0920
18 Nat. Assocn. Glue Manufacturers, "Calibration and Use of Viscosity Pipette."
k 004
Q
8
b 0 03
3
0 002
G
0 00 200 300 400 500 BOO 7 W Pressure - gms per sq cm Figure 4-Flow-Pressure Diagram a t t h e Gelation Point for Glue A, Mixed 1 t o 2.25
0
(00
a straight line through the origin within the limit of experimental error, but a t 29' C. it is curved markedly and is roughly similar in shape to the flow-pressure graphs for the rubber-benzene solutions studied by Herschel and Bulkley.ll There is apparently a sharp change in consistency of animal glue solutions when the temperature falls below a point characteristic of the solution, which may therefore be defined as the gelation point. Above the gelation temperature the
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1929
glue solution behaves like a true liquid; that is, varying the rate of shear does not vary the calculated viscosity. Below the gelation temperature the property of rigidity or elasticity is evident and the higher the rate of shear the lower is the calculated viscosity. If the consistency were to be completely expressed below the gelation temperature, an equation would be required which would indicate the entire flow-pressure curve. The curve seems to be of the type y
f
a = b(x
*
TEMPERATURE
C)"
c.
The equation of Herschel and Bu!kley (P - k)" I=c-
245
CONSISTENCY AT A
1 to 2.5
Poises
.
GLUE-WATER RATIOOF:
1 to 2.25
1 to 2
Poises
Poises
.....
404.00" 51.451, 18.62b 11.39b 9.39s 6.016 3.61b
?!
might be used to express the consistency (where I and n are constants characteristic of the solution).
.....
2.72b 2.448
a
b
..... .....
1260.OOa 364.0Ob 25.45b 20.56b 11.13b 5.921, 4.81b 4.26b 3.41b
Solution plastic. Solution viscous.
IKFLUENCE OF GLUE GRADEON CONSISTENCY-TEMPERARELATIONS FOR GLUE-WATER RATIOSOF 1 TO 2.25As stated previously, glue B grades higher and glue C grades lower than glue A. The temperature-consistency relations are very similar to those of glue A. Figure 3 and Table I V show that the higher the grade the higher the viscosity a t any temperature; that is to say, increasing the grade of a glue serves to shift the viscosity-temperature curves farther from the axes. The shape of the curve remains nearly the same. A comparison of Figures 2 and 3 indicates that decreasing the grade of a glue has much the same effect as decreasing the concentration of the solution. I n both cases the viscositytemperature curves are shifted nearer the axes. TURE
Pressure - 9ms. per
q. CM.
Fipure 5-Flow-Pressure Diagram a t the Gelation Point for Glue A, Mixed 1 to 2.5
The bchnic of the gluing operation is in harmony with this ides of a decided change in consistency a t the temperature of gelption. It has been established2 that if the pressure is applidd too soon after spreading the glue, that is, while the glue i#3 still in the sol form, an excessive amount of the glue may be squeezed from the joint or forced into the wood and a "8tarved joint" may result. If a good joint is to be produced the pressure should be so regulated that a slight movemerit of the film takes place but no excessive extrusion occurs. We can imagine that the glue, when it is ready for pressing,
has the consistency characteristics similar to those shown in t h e lower curve of Figure 4 and that the pressure to be applied i.8 of such a magnitude that it w ill cause a slight movement of the glue but not a rapid flow. INFLUENCE OF WATERRATIOON THE CONSISTEKCY-TEMPERATURE RELATIONS OF GLUEA-When glue A was mixed in the proportions of 1part glue to 2.5 parts of water or 1 part of glue to 2 parts of water, the consistency-temperature relations were found to be very similar to those for the 1 to 0 100 200 300 400 500 600 700 2.25 concentration. The data are given in Table I11 and Pressure - qrns.per sq cm. Figure 2. The curves for different water ratios are of the Figure 6-Flow-Pressure Diagram a t t h e Gelation Point for Glue A. Mixed 1 t o 2 same general shape, but they are shifted closer to the axes by increasing the proportion of water. I n other words, the The temperature of gelation increases as the grade of the more water in the solution, the more it must be cooled in order to reach a high viscosity. Figures 5 and 6 and Table glue increases (if equal concentrations are considered). I11 show that there is the same sharp change from viscosity Figures 4, 7, and 8 show that the different grades exhibit the to plasticity as the gelation temperature is passed as was same sharp change from viscosity to plasticity. With glue found a t a glue-water ratio of 1 to 2.25, but that the point of B the change is between 31" and 32" C.; with glue A begelation is higher the less water the solution contains. tween 29" and 30" C.; and with glue C between 28" and Here too is substantial agreement between theory and good 29' C. gluing practice, for it is common knowledge among craftsmen Consider conditions a t 29' C., for example. Glue C is that a more dilute solution should be allowed to stand for viscous, glue A is just beginning to display elasticity or a longer time before applying pressure, or, in other words, rigidity, and glue B, being well below its gelation temperature, the temperature must fall to a lower degree before the point exhibits marked rigidity. Consistency may be observed well of gelation is reached. Furthermore, it is customary for the beyond the point of gelation, at least roughly, in terms of competent glue-room foreman to increase the concentration jelly strength. On this basis glue C has zero strength! as of his glue mixture when his operation requires that he shorten it is purely liquid; glue A displays a marked degree of rigidity;
INDUSTRIAL AND ENGINEERIh'C; CHEMISTRY
246
and glue B has such a marked degree of rigidity that it might be measured by a Bloom gelometer.
ence in the fundamental properties underlying the gluing technic.18 Summary
Table IV-Relation
between Consistency a n d Temperature for C l u e s A. B. a n d C. Mixed 1 to 2.25 ~~~
TEMPERATURE
c.
CONSISTENCY OF:
Glue A
Glue B
Poises
Poises
..... .....
Poises 155,00" 20.68b
..... ..... ..... ..... 449.00" ..... 38.44)
404.00" 51.45b 18.62b 11.39b 9.39b 6.01b 3.6lb 8.39b
8.59b 6.53b 4.89b 4.98b,e 3.50s 2.07bto
..... 2.11)
5:03b 4.71b 3.891,
2.44,
To summarize the work, the change from viscosity to plasticity gives a definite and exact measure of the gelation point of a glue solution. The findings agree with the hypothesis of Hatschek and the data of Kraemer and Fanselow to the
11.15s
16.07s 5.39s
.2.72s ... .
I
Glue C
Vol. 21, No. 3
1.95b 1.59b
Solution plastic.
b Solution viscous. e
Irregularities believed to be due to experimental error.
The similarity in consistency-temperature relations between the higher grade glues and the low-grade glue, when allowance is made for their difference in water requirement, is in harmony with the results of numerous investigators who could find no relation between the grade of glue and the tensile strength of the glue film or between grade and the ~ ~ good joints joint strength. It has been d e m o n ~ t r a t e dthat can be produced from low-grade glues by a proper regulation of the gluing conditions, which means, essentially, allowing the glue to reach the plastic condition and then applying such a pressure that the glue film will move slightly but not flow from the joint. The fact that a higher-grade glue behaves so much like a more concentrated solution of the lower-grade glue indicates another interesting point. If the gluing operation is so fixed by production factors that the glue must reach the gelation
Pressure
- 9ms. per sq cm.
I
Figure 8-Flow-Pressure Diagram of the Gelation Point for Glue C, Mixed 1 to 2.25
effect that below certain temperatures gelatin or glu6 displays distinct elasticity. The point a t which the change ltakes place is the temperature called the "gelation temperature," and its importance in the gluing operation has been long recognized. In this paper no attempt is made to derive an expressXon for the consistency of the glue after it has fallen below t h e gelation temperature. Above the gelation temperature, the viscosity may be expressed by lrgd4Pt
p = - = -w
128 7 1
*gd'P 128qL
The shape of the flow-pressure diagram for temperature below the gelation temperature indicates that it would be difficult to employ the intercept type of expression
- P) '= *gd4(P 128qL
as suggested by Bingham for certain types of materials. It is believed that the expression suggested by Herschel .g"d4(P - 9)" 0
IO0
200
300
4m
Joo
600 700
-
Pressure 9ms. p e r 9 cm. Diagram at t h e Gelation Point for Glue B, Mixed 1 to 2.25
Figure 7-Flow-Pressure
temperature quickly, this desired effect may be produced by using either a higher grade glue a t one concentration or a more concentrated solution of a lower grade glue. Likewise, the converse is true; that is, if the assembly periods must be long, either a low grade glue or a dilute solution of a highgrade glue may be used. The choice must rest on a purely economical basis, for the experiments have shown no differ1'
Truax, Browne, and Brouse, IND. END.C H B M . , 21, 74 (1929).
=
128qL
might be applicable. The ease of preparation and the reproducibility suggest that these jellied glue solutions might well serve as the experimental basis for the derivation of an expression for plasticity. S I n making these comparisons between the higher and the lower grades of glue, reference is made only to those properties discussed in this article, that is, the temperature-viscosity relations and their influence on the gluing technic. It is not to be inferred from this that there are no differences between a high-grade and a low-grade glue, even though the glue-water ratio has been adjusted. The water-resistance of joints will vary somewhat depending upon the grade of animal glue used, and presumably the permanence under certain conditions is not the same for different grades.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
March, 1929
Conclusions
The viscosity of the animal glue solutions tested is a hyperbolic function of temperature. As the grade of the glues tested or the concentration is changed the position of the viscosity-temperature curve may change with respect to the axes, but its shape remains the same.
247
The gelation temperature of the glues tested is marked distinctly by a change in the material from a viscous to a plastic state. These tests indicate that at temperatures below the gelation point the usual methods for expressing consistency do not apply, as the viscosity decreases as the shear gradient increases.
Lifting of Varnishes by Lacquer Solvents' H a r r y E. H o f m a n n and E. W. Reid MELLONINSnTUrS
OF
INDUSTRIAL -SEARCH,
HE introduction of nitrocellulose brushing lacquers brought about comparisons of this novel surface
T
covering with the materials previously used-vie., varnish and varnish enamels. One difference of special notice was the tendency of lacquers to swell or lift the old varnish or enamel already on the object or surface to which the lacquer was being applied. This did not occur when varnish or enamels were applied over old coatings of a similar nature, since the old varnish had, by oxidation and polymerization, been rendered insoluble in the varnish oils. This property, commonly called "lifting," is due to the absorption of lacquer solvents, causing a swelling of the oxidized gels of drying oils or oleoresinous mixtures producing a wrinkled surface, as shown in Figure 1. The lifting of varnishes by a lacquer is dependent upon the type and age of the varnish and the composition of the lacquer solvents. I n view of the complexity of the subject and the absence of published data, no definite rules can be given regarding the effect of the different factors. It is hoped that the observations made in this paper may serve to further additional study of the subject. The following factors are of importance in the study of the lifting of varnishes by lacquers and lacquer solvents: (1) Composition of the oleoresinous film. It is usually conceded that short oil varnishes do not lift so easily as varnishes longer in oil; that linseed oil varnishes do not lift so easily as China wood oil varnishes; and that pigmented lilms do not lift quite as readily as the same films unpigmented. (2) Age, or degree of oxidation and polymerization in the oleoresinous film. The older and more highly oxidized and polymerized the paint or varnish film, the less readily is it affected by lacquer solvents. Indeed, it is noteworthy that the majority of oleoresinous undercoatings for nitrocellulose lacquer work are of little value until baked at a relatively high temperature. (3) Character of the solvent, or composition of the solvent m i x t u r e . This refers t o the chemical nature of the solvent whether ester, alcohol, h y d r o carbon, ether, etc., or a mixture of two or more. (4) Amount of solvent or solvent mixture present.
-
1 Presented before the Division of Paint and Varnish Chemistry at the 78th Meeting of the American Chemical Society, Swampscott, Mass., September 10 to 14, 1928.
UNIVERSITY
OF
PITTSBURGH, FTl"l%BURGH, PA.
(5) Speed of evaporation of the solvent or mixture. The longer a solvent is in contact with the oleoresinous film, the greater will be the opportunity for lifting. In other words, if an excessive amount of a certain lacquer were applied to a varnished surface, lifting might ensue, which would be avoided by the application of a thin coat of the lacquer. Furthermore, it is possible that a highly viscous lacquer material might not lift varnish, whereas one containing the same ingredients, but lower in viscosity, would cause lifting, owing t o the great amount of solvent mixture present. Experimental
The various varnishes used in the tests were sprayed (one coat) on clean bare steel panels, 6 by 12 inches, and allowed to dry standing in a vertical position. When dry, the lifting tests were made by dropping 2 to 3 drops of the solvent on the panel in a horizontal position and noting the result. The test was made in this way to conserve space; and since only mobile liquids were used, the results are comparative. Four commercial varnishes of widely different types were used-rubbing varnish, floor varnish, spar varnish, and long oil enamel (white). All were standard products of wellknown manufacturers. This series was intended primarily as a study of the lifting effect of different solvents and certain combinations of solvents, although some points were observed in connection with the varnishes used. The results of tests on these varnishes with a large number of typical TABLE I LIFTING OF VARNISHES
