COMPOSITION CORK L. P. HART, JR., R. W. WORK', L. T. IRISH2, AND 3%. A. HOWE General Electric Company, Pittsfield, Mass.
The compressibility characteristics of composition cork are discussed. A simple method of determining the dynamic compressibility is established through the use of static measurements. Compressibility characteristics are related to density. Composition cork gaskets may be compressed rapidly during assembly rather than tediously, using two- and three-stage compression. Density of composition cork is studied and the optimum density for a gasket under compression established.
P
The effect of humidity changes on compressibility, dimensions, and weight is noted; equations and curves are derived to relate these changes. The variations that exist within composition cork are discussed and the probable causes of these variations pointed out. A method of quality control is proposed, based on statistical analysis of the material. A plan is suggested for a quantitative determination of the quality standard of each vendor. 50, 59,65); most of it consists of patents having t o do with particular adhesives or plasticizers, or combinations of them
OWER and distribution transformers, induction voltage regulators, and capacitors are examples of electrical equipment which are liquid-immersed in order to be properly cooled and adequately insulated. Such equipment usually depends upon gaskets throughout its expected life of twenty to twenty-five years to serve the double function of preventing the outward leakage of the liquid or the inward penetration of moisture. Either might cause electrical failures with all that would be attendant in the way of damage and discontinuity of service. One of the most important materials for gasketing purposes in this type of work is composition cork. . Composition cork has been widely and increasingly used in many other industries since its development about 1900, but in few cases does it need to possess such long life or be of such quality and uniformity as in its application in the electrical field. This industry depends almost entirely on resin-bonded composition cork rather than on the more common protein-bonded type. Both of them are manufactured in a similar manner, and each consists essentially of granulated cork, a plasticizer such as glycerol or glycol, and an adhesive, which have been mixed before being molded and baked. Not only must this material withstand in practically all cases the action of the weather on its outer surface and in most cases the hot insulating liquid on its inner surface, but it must also be flexible, strong, free of pores or capillaries connecting its two surfaces, compressible within certain limits, and capable of being die- or sawcut into intricate shapes and within closely held tolerances. The literature on composition cork is not large (19, $9, 43, ? Present address, Celanese Corporation of America, New York, N. Y. 2 Now in the United States Army.
(1-60). About forty-five United States patents and a correspondingly smaller number of foreign patents of this type have been issued in the last thirty-five years. Unfortunately other information concerning it is meager, and nowhere are there any data on its compression characteristics. Although it is generally known in the industry that changes in relative humidity mill vary its size, hardness, and compressibility, there are no published data on the subject. Yet these must be taken into consideration by the user who places composition cork gaskets in electrical equipment. If the cork is too hard and is used between steel and a porcelain bushing, the latter may be fractured during the tightening operation. If too soft, it may crack or fail to produce a liquid-tight seal. In a large gasket prepared from several pieces, lack of uniformity " mav " result in leaks a t the points where the gasket joints are madebetween hard andsoft pieces. To secure informationwhichwould make possible a more intelligent use of composition cork as a gasketing material in liquid-filled electrical equipment, this laboratory undertook a study of the subject. Compressibility Characteristics When composition cork is used as a gasket material, its compressibility characteristics are of great importance. Bolting structures must be so designed that they are capable of compressing the gasket to some degree. In view of this fact, it is essential that information be available concerning the load in pounds per square inch necessary t o give an amount of compression that will result in optimum density. Two methods of obtaining this information have been investiARROWS POINT TO SEVENOF THE TWENTY-SIX gated, the so-called dynamic and CORKGASKETS INVOLVED IN THE CONSTRUCTION static. OF THISTRANSFORMEH
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Vol. 34, No. 6
into terms of per cent static compressibility, which is defined as the compression, expressed as per cent of the original thickness, which the sample undergoes under a pressure of 300 pounds per square inch for one minute. Typical observations follow: Origins Thickness, In. 0.250 0.252
0.373
Thickness after 1 Min., Deflection, % StaFiq In. In. Compressibility 0.175 0.075 30.0 0.136 0.115 45.0
0.322
0.051
13.7
The term “dynamic compressibility” is readily usable in engineering calculations but dynamic observations are time consuming. Such observations necessitate expensive equipment, are tediously made, and require laborious calculations. Static compressibility deFIGURE 1. DYNAMIC CO~IPRESSIBILITY TESTING MACHINE terminations are relatively simple and raDid. requiring only about 2 kdnuies per odserl vation. However, the figure “static compressibility” was To determine the dynamic compressibility characteristics only empirical until the work described in this report was of composition cork, a Baldwin Southwtirk hydraulic testing machine of 100,000pounds capacity \vas used. Becauscof the carried out, and was not readily interpreted into engineering design data. lurge size of this press, a subpress \vas plwed in it (Figure 1). For this reason a study was made to correlate the two types Thc platens were muchincd and ground in order that the latof compressibility. Dynamic and static compressibilities eral movement of t.he piecc under compression \~ouldbe were determined on similar samples without regard to the unrestricted except by the naturally high coefficient oi friction density of the material, its age, its vendor, or the relative of the cork. Tlie amount of compression is read from a dial humidity with which it had been in equilibrium. The regage graduated in thousandths of an inch, and the pressure sults are plotted in Figure 4. A straight line is drawn through is read directly from the dials on the control panel of the these data, which is fitted by the equation: press. The load is applied at a rate of 100 pouiids per square inch per minute. The data are calculated to per cent compression at a load, in pounds per square inch. Figure 2 gives log,, X = 1.888 - 5.34 >< loF4D (1) typical pressure-compression curyes. From such curves as This may be written these, plotted on coordinate paper, the loaci necessary to give 50 per cent compression is interpolated. The pressure, exs = i~g,,-l (1.888 - 5.34 x 10-4 D ) (2) pressed in pounds per square inch, necessary to compress the cork to hali of its original thickness is culled, in this report, tlie LLdynamic conipresuibility” of the iiiatcrial. It has been found that this degree of Compression produccs a satisfactory where = static compressibility and D = dynamic compressibility gasket from the kind of composition cork under study. Based on the data obtained, proper bolting structures may be designed.
LOAD, LBS. /SO IN
FIGURE2.
TYPICAL DYNAMIC COMPRESSIBILITY CURVES
The static compressibility was determined by a press which is a duplicate of one in the laboratory of the Armstrong Cork Company (Figure 3); a 300-pound dead weight may be applied to an area of one square inch of the sample at one time or in increments of 50 pounds on a circular foot. The compression is measured by a dial gage calibrated in thousandths of an inch. It has been found advantageous to lower the weight onto the sample, allow compression to take place for one minute, and then secure a reading, in spite of the fact that equilibrium is not reached. Readings are calculated
THREE OF THE TWELVE CORKGASKETS PRESENT I N THIS15-Kv. Anfp. C l P A C I T O R ARE VISIBLE
INDUSTRIAL AND ENGiNEERING CHEMISTRY
June, 1942
651
An attempt was made to relate durometer hardness to compressibility through the use of an ordinary Shore type A hardness tester. The correlation was very poor, unless a great number of observations were made.
Compression during Assembly
It has been the practice in many cases to compress gaskets
F I G ~ R3. E STATIC COMPRESSIBILITY TESTINQ MAcHINB
That the observed points in Figure 4 scatter badly from the average is probably caused by a variation in samples. I n some cases dynamic compressibilities were run on samples obtained from different sheets from those for static determinations. That the corpelation can be better is evidenced by Figure 5, where more care was used to select uniform samples. By virtue of Equations 2 and 3 or the curve in Figure 4, the dynamic compressibility of composition cork can be determined accurately by means of a static determination in one tenth the time formerly required.
stepwise by allowing one or two relaxation periods during the operation of tightening the bolts. This was thought to reduce the possibility of destroying the cork structure and make easier the compressionto apredetermined thickness. In Figure 6 data are plotted to show the result of applying the pressure in one, two, and three stages to reach one half of the original thickness of the gasket. Although exactly 50 per cent compression was not secured, about 500 pounds per square inch or slightly more is required to secure that degree of compression whether carried in one, two, or three steps. These diagrams show that the pressure exerted by the compressed cork tends to drop away to about 50-65 per cent of its original value. These observations lead to the conclusion that composition cork gaskets may be compressed rapidly without allowing relaxation periods to interfere and delay the operation. Careful studies of factory gasketing practice indicate that splitting results from the gaskets being compressed between surfaces not in the same plane. The causes of such a condition may be either an improperly designed structure or, in the case of large gaskets, tightening of the bolts un-uniformly.
Compression Set To obtain information on the long-time set characteristics of cork under compression, circular samples having an area of one square inch were compressed between steel platens. In the compressed condition the samples were exposed to 70' * 2' C. for 22 hours. They were then released and their thicknesses determined after one hour. Results were as follows : Vendor A
Density under Compression
Lb./Cu. Ft:
Per Cen! Compresaion
Per Cent Set
Per Cent Return
66 64 56
51 54 51
45 62 43
12 11 17
B C
lo
DYNAMIC
0
p 0 o 0 o 0 o 0 o 0 o 0 o 0 o 0 o 0 o 0 o ~
m
0
0 ~
b
~
w
o
r
0 0 ~
0 0
0
0 0
-
m
m
o
=
COMPRESSIBILITY
4. DYNAMIC COMPRESSIBILITY ver8w STATICCOMPRESSIBILITY, INDEPENDENT OF AGE, DENSITY, VENDOR,AND HUMIDITY STATE
FIG-
It is possible that the figure for dynamic compressibility determined this way may more nearly represent the true value than that of an actual observation. This follows because each dynamic compressibility observation is open to an error of =+=4per cent due t o the uncertainty of measuring the thickness of 12 square inches of cork, to an error introduced from differences in loading rates, and to a further error from the inherent inaccuracies in plotting the points and interpolating from a ourve.
lo
0 ~
DYNAMIC
0 0 0
0 0
m , o =
~
COMPRESSIBILITY
FIGURE 6. DYNAMIC COMPRESSIBILITY versus STATIO COMPRESSIBILITY DETERMINED UPON UNIFORM SAMPLES
0
0
0
I N D U S T R I A L A N D ENGINEERING
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crowded together. At higher densities the cork granules are broken; a t lower deneties the granules are not tightly fitted. In either case they are particularly susceptible to liquid penetration. Two methods of density determination were investigated. One consisted of weighing a sample, determining its dimensions, and calculating its volume. The second method utilized a Jolly balance, using distilled water as the immersion liquid. The latter method was found more satisfactory for this investigation because it was less time consuming and possessed greater accuracy when used on small samples. To indicate the correlation which may be obtained between the two methods when sheets 18 X 18 X inch were weighed and measured, or when two samples 1.5 X 2 X " 8 inch were measured on the Jolly balance, the following data are listed: Sheet No.
1.313" ~. -.- - - Constant -...-..- -
An investigation was made to determine whether any relation exists between the dynamic compressibility and density of composition cork. The material was supplied through the courtesy of one of the vendors, mho furnished a series of sheets of composition cork inch thick, the density of each of which differed from the next by a definite increment. From a random point a sample was cut which was 7 . 5 inches long and 2 inches wide. This, in turn, was divided into two pieces 6 x 2 and 1.5 X 2 inches. They \\?ere placed in a humidity cabinet a t 50 * 3 per cent relative humidity and 75" * 5" F.; after conditioning, densities were determined on the small pieces and dynamic compressibilities o n the large pieces. By plotting density against dynamic compressibility, the curve of Figure 7 was obtained. This curve is of the type y = A bx cx2,and the constants derived give
Thickness)
400
0
r
300
,246':
%
Density&Lb./Cu. Ft. B y measurement By Jolly balance 19.2 18.7: 19.0 20.8 2 0 . 7 ; 20.8 24.5 23.6: 24.1 25.9 2 5 . 4 ; 26.1 27.8 26.6; 2 7 . 8
Conslant Thickness1
200
+
+
a
. l
+
IO0
D = 1693 - 1 6 3 . 4 ~ 4 . 4 7 ~ ' where a = density, lb.,'cu. f t . TIME IN MINUTES
FIGURE 6 . RESILIENCYCURVE OF COMPOS~TION CORK ( T o p ) One compression, sample 6 X 2 X 0.245 inch; (center) two compressions, sample 6 X 2 X 0.375 inch: (bottom) throe compressions, sample 6 X 2 X 0.371 inch.
The per cent return is very small, and the per cent set approaihes closely the per cent compression. This indicates that composition cork gaskets take a permanent set under service conditions. If there should be any movement of the gasketed surfaces apart from each other, such as might be encountered in the breathing of a transformer, the strength of the seal, which is dependent upon pressure alone, would be materially affected. It is generally necessary, then, to use an adhesive to fasten the cork and gasketed surfaces. If this is done, the seal \+ill remain intact during movement of surfaces.
(4)
This equation is valid only between densities of 19 and 29 pounds per cubic foot. When x is known, the solution of D is easily calculated. Khen D is known, 2 may be found by the ordinary solution to a quadratic equation. The density-dynamic compressibility function for all resinous-bonded composition cork products is represented by an equation similar to 4 except that the constants will vary because of the different amounts of plasticizer and binder em-
Density The density of composition cork is expressed in the industry in pounds per cubic foot. Work in this laboratory has established that composition cork functions best as a gasket material, for the purpose to which it is put in this plant, when it is under such compression that its density is between 55 and 65 pounds per cubic foot. I n this range the cork granules remain intact and are closely
THREE O F AT LIGAST NINE CORK GASKETS LOCATED I N THISLOWVOLTAGE
CONCENTRIC-TYPE BUSHINGFOR A DISTRIBUTION TRANSFORMER ARE VISIBLE
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1942
ployed in different compositions. Constants observed for different vendors are as follows: Constant" b 1695 -163.4 980 -94.8 935 -99.2 accurate t o 1 5 in the last place.
653
ingredients and their ratios to one another in the product are not varied.
7
a
Vendor
A B C
a Figures
Humidity
C
4.47 2.79 3.13
Observations were made to determine the relation of static compressibility to density. On samples conditioned a t 50 * 3 per cent relative humidity and 75" * 5" F. which were cut from the same sheets referred to above, static compressibilities were determined. The data are plotted in Figure 8. The solid line represents the average of the observations. Utilizing the equation which expresses the relation between dynamic compressibility and density, D = 1695
-
163.42
+ 4.472'
(4)
and the equation which expresses the relation between dynamic compressibility and static compressibility,
D =
(1.888
- lOgiwS')
X lo4
(3)
5.34
it is possible to calculate an equation which expresses the relation between static compressibility and density, as follows : S
=
1ogIo-l (0.983
+ 8.7 X lo-% - 2.39 X
(5)
The curve of this equation is represented by the dotted line in Figure 8. The same procedure was followed for samples obtained from other sources, and a similar correlation found. Constants observed are as follows:
-
Vendor A B C
a 0.983 1.389 1.365
Constant b 8 . 7 x 10-2 5 . 3 x 10-2 5 . 1 X 10-2
0
- 2 . 3 9 x 10-5 - 1 . 6 7 x 10-8 - 1 . 5 0 X 10-8
From the derived curves and equations, it is possible t o calculate with a fair degree of a'ccuracy the compressibility characteristics of composition cork materials from a density observation. Once the equation constants have been determined for any composition cork having a given binder and plasticizer, that equation will remain useful as long as the
As would be expected of a material having the origin and composition of cork, it is affected by changes in the moisture content of the air. There are changes in weight, dimensions, and compressibility characteristics. These changes are complicated by the presence of a hygroscopic plasticizer in composition cork with the result that the material is slow and irregular in coming to equilibrium with a change in relative humidity. To determine the amount and rate of moisture absorption, a study was carried out as follows: Samples of composition cork, 2 x 2 inches square and '/E to ' / p inch thick, were dried over anhydrous calcium chloride until their weights were constant. The figures so obtained were taken to be the weights a t 0 per cent relative humidity. The samples were then exposed to a given humidity, controlled to * 3 per cent a t 75" * 5" F., and allowed t o remain there until their weights had again become constant. The observed data, giving the moisture contents a t different humidities based on dry material, aretas follows: Vendor
A A A A A A
A
B
B
C
Per Cent Moisture a t Humidity of: -49%-77%-85%-
-31%2.2 2.1 2.1 2.2 2.0 2.0 2.2 , 1.3 1.2 1.0 Per 36% 1.3
2.1 2.2 2.2 2.1 2.2 2.3
3.4 3.4 3.3 3.6 3.3 3.4 3.5 i:i 2.2 1.4 1.5 1.1 1.1 Cent Moisture 43 % 2.2
3.4 3.5 3.4 3.4 3.4 3.7
9.7 9.7 11.3 11.3 9.9 9.9 11.4 11.4 8.5 9.5 11.6 12.0 10.3 10.5 12.5 12.9 9.8 9.7 12.1 12.1 10.2 11.1 12.2 13.1 10.0 12.2 ... 2.9 5.2 5.2 5.4 5.2 1.6 5.3 5.7 5.3 5.7 1.2 4.4 4.4 4.6 4.3 of Granulated Cork at Humidity of: 52 % 66% 74 % 3.5 4.9 6.1
...
...
Typical data are plotted for each of these vendors in Figure 9. The amount of moisture contained by the cork varies with the source of the material. The rate of absorption was studied on these samples as they picked up moisture in a humidity of 31 * 3 per cent after having been previously dried over anhydrous calcium chloride (Figure 10). Taking 2 per cent as the equilibrium point, the following data may be tabulated: Sample 1 2 3
4
5 6
Thickness, In. '/a '/a '/P
l/a 1/8
=/4
Density Lb./Cu. Pt. 28.9 29.0 28.6 29.0 19.6 20.2
Hr. t o Equilibrium 102 87 67
44 44 30
Thickness us. time to equilibrium is plotted in Figure 11; if it takes x hours to condition a sample of composition cork '/E inch thick, then 3 x / 2 hours will be required for a sample '/4 inch thick, 22 for a a/8inch sample, and 5x/2 for a samplel/zinch thick. A 10-pound density increase (in the range 19 to 29 pounds per cubic foot) approximately doubles the time necessary for equilibrium to be reached. Using the information given above, i t is posW a sible, for example, to estimate that a sample D'1695-163.4 X + 4.47 X2 inch thick, having a density of 24.5 pounds per 16/- I cubic foot, will be at equilibrium with moisture in (z 2/4) 2/a hours or in the conditions of these observations in about 33 hours. STATIC COMPRESSIBILITY,Z DYNAMIC COMPRESSIBILITY, LBS /SO IN Samples of cork were exposed to increments in humidity for 48 hours. Dimensional changes FIGURE 7. RELATION OF DENSITY FIGURE 8. RELATION OF DENTO DYNAMIC COMPRESSIBILITY SITY TO STATIC COMPRESSIwere determined by measuring the distance FOR ONE VENDOR BILI'IY FOR O N E VENDOR between two lines on the cork. The cork was I
+
Vol. 34, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
654
On samples exposed to differinghumidities until they were a t equilibrium, static compressibilities were determined in duplicate. Data on four of these, which are representative of all ten samples, are shown by the solid lines in Figure 14. Obviously the per cent static compressibility change from 0 per cent relative humidity is dependent upon the per cent compressibility a t 0 per cent relative humidity. However, the actual change in units appears to be independent of the original compressibility. The following tabulations were made independently of density and vendor, and illustrate this unusual and unexplained phenomenon : sample No. 1
FIQURE9. VARIATION OF MOISTURE CONTENT OF COMPOSITION CORKWITH HUMIDITY
6
7 8 9 10
protected with several layers of cellulose acetate while measurements were made with a cathetometer. The dimensional changes were practically the same in each direction. The average of the observed points is plotted in Figure 12. The points on line A include measurements on natural cork and on composition cork samples from two different vendors. C represents the changes found for composition cork from a third vendor. B is the average of all observations. It is supposed that the use of a different binder formulation in the composition accounts for the marked difference. Curve B is fitted by the equation: y = 0.015 H (6) where H = relative humidity, yo y = dimensional increase over that at 0% relative humidity, % For example, from Equation 6, it can be calculated that a gasket 16 inches in diameter will increase in diameter about inch with a humidity increase of 70 per cent. This type of change must be taken into consideration during the design of gasketed parts. Samples of cork were exposed t o various constant relative humidities in the range 36-74 per cent for 48 hours. The average weight change of six samples is plotted in Figure 13 and approximated by the dotted line. This line is represented by the equation,
- 3.7
20.9 26.3 27.3 18.9 34.3 24.9 19.2 38.0 40.8 40.9
2 3 4 5
46 RELATIVE HUMIDITY
p = 0.14 H
Units Change in Static Compressibility Humidity Changes from 0% to: ~ ~ ~ E $ g iWhen l i Relative ~ ~ at 0% 31% 49 % 77 % 85% 11.9 13.0 10.9 14.4 10.9 13.0 14.6 15.0 15.6 15.6
13.6 16.6 12.9 19.0 13.3 16.1 19.5 17.8 19.8 19.1
20.0 20.2 19.1 21.9 17.3 20.1 22.7 21.3 21.3 21.4
21.7 21.3 20.2 21.7 19.7 18.3 21.3 17.0 17.8 18.5
Average 1 3 . 5
16.7
20.5
19.9
The average of these points is plotted in Figure 15, and the following equation is derived: dS
n7hei-e dH dS
= =
16.2 log,, d H - 10.3
(11)
% change of relative humidity from 0 yo change of static compressibility from 0
In a more convenient form the equation is: S H ~ SHI - 16.2 (log10 HI
- loglD Hz)
(12)
which is usable for a11 points except 0. The dotted lines in Figure 14 give the functions calculated for the four representative samples mentioned above. It is possible to calculate the change in dynamic compressibility from that a t 0 per cent by the relation derived from Equation 1: dS = log,~-~(1.888 - 5.34 X low4d D ) (13) so that from Equations 8 and 10,
(7)
where p
= weight increase over that at 0% relative humidity,
and H
= humidity increase over that at 0% relative hu-
%.
midity, %
which becomes d p = 0.14 ( H I - Hz)
(8)
and from Equation 6 Z = 3y = 0.045 H
where2 = volume change, % dZ = 0.045 ( H I - HI) For a change of 10 per cent in relative humidity, dz = 0.045 (10) = 0.45
d p = 0.14 (10) = 1.4 dp dz S 1.0
-
Therefore a change of 10 per cent in relative humidity should be accompanied by a change of 1 per cent in density.
T I M E IN HOURS
FIGURE10. INCREASE IN WEIGHT OF COMPOSITION CORKEXPOSED TO 31 PERCENTRELATIVE HUMIDITY AFTER HAVINGBEEX DRIEID OVER CALCIUM CHLORIDE (Abooe) Samples of uniform density but variable thickness. (Below) Samples of variable density but uniform thickness.
June, 1942
65s
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY Density, Lb./Cu. Ft. Av. deviation from mean 24.0 0.8 25.7 0.9 26.2 0.9
Mean
Pad 1 2 3
The average deviation amounts to about 3 per cent of the mean density. This lack of uniformity in the vertical dimension of the pad is probably caused by the inherent difficulty of uniformly baking a material such as cork during the stage when the resin is curing. Since the heat must be applied from the ou,tside, a softening of the material adjacent to the outer surfaces first occurs, 'The inner core of cork under compression then tends to relieve the pressure on itself by compressing the softer material on the outside. The resin in the outer shell starts to cure in this condition before the temperature on the inside is raised; and when finally the temperature of the entire mass is uniformly high, the cured resin in the outer part of the pad does not allow the cork freedom to recompress the inside oore. To determine the density variatiom which may exist within a sheet-in other words, laterally within a pad-samples were systematically cut from each of two sheets, selected a t random from a pile of one hundred and fifty, and density determinations made. The results are recorded in Figure 18 and are summarized as follows: Density,
Sheet
ABOUTTWENTY-EIQHT C O R K GASKETS ARE USED I N THE ASSEMBLYOF THIS150-Kv. AMP. THREE-PHASE
No. 1
TRANSFORMER
logLo-1 (1.888 - 5.34 X lO-'dD) = 16.2 loglo dH
dD =
[1.888 - 10gio(16.2 loglo dH
2
- 10.3
- 10.3)110'
5.34
(14)
96.6
0.5
27.7
0.6
1
z
THIGKNESS
% RELATIVE WMIDITY
FICUF~E 12. DIMENSIONAL CHANGES versw HUMIDITY
FIGURE 11. THICK-
NESS versus TIMETO
c
APPROACH EQUILIBRIUM
60
2
Material Variations As previously mentioned, composition cork is made by pressing a volume of granulated cork which has been premixed with resin and binder into a mold, and heating the mold until the resin is cured. The product is a pad usually about 24 X 36 X 3 inches thick. These pads are then split laterally into sheets by a knife which resembles a band saw. Samples were taken a t uniform increments from a pad 2.5 inches thick; the density of each sample was determined. The results for three pads are plotted in Figure 17. They may be summarked as follows:
Mean
In. Av. deviation from mean
The shaded areas indicate that part of the sheet whose deviation was greater than the average. The average deviation from the mean is, roughly, 2 per cent. This variation in density is probably caused by uneven filling of the mold prior to pressing. The distribution of the cork granules, after being dumped into the mold, is commonly r
Figure 16 shows the relation between dynamic compressibility and humidity for a typical sample. Curve 1 represents the relation when it is calculated from the observation at, 49 per cent relative humidity. Curve 2 represents actual observations. Curve 3 is calculated from a density observation. It is seen that the correlation is reasonably good between humidities of 5 and 85 per cent. By means of the equations derived, it is possible to calculate changes in density, dimensions, weight, and compressibility which are brought about by a change in relative humidity (between 5 and 85 per cent). The state of any of these characteristics can be calculated from a single density observation a t a known humidity.
La./&.
50
% R E L A T I V E HUMIDITY
FIGURE13. PER CENTWEIQHT INCREASE (AVERAGE OF FIVE SAMPLES EXPOSED FOR 48-HonR INCREMENTS)
u 3
RELATIVE HUMIDITY
Frcuar: 14. EFFECTOF RELATIVE ON STATIC COMPRESSIBILITY HUMIDITY
INDUSTRIAL AND ENGINEERING CHEMISTRY
656
done by hand, and it is inevitable that a certain lack of uniform distribution should result. As is well known, cork has a high coefficient of friction; therefore, the unevenly distributed cork does not flow as the mass is compressed into the mold, nor does it flow during the subsequent baking operation.
Vol. 34, No. 6
Under normal conditions, then, it is possible to have a total variation of 165 pounds per square inch in the dynamic compressibility of composition cork, due to the causes cited here.
Quality Control Because of the wide variations which occur in composition cork and the importance of keeping them at a minimum, a system of quality control has been established. This system ensures, as far as possible, that only those sources of variation discussed previously in this article mill occur and that these will not increase.
*
- 0 2
N
UNIT CHANGE
N
IN STATIC COMPRESSlElLlTY
FIGURE 15. RATEOF CHANGE OF STATIC COMPRESSIBILITY WITH CH.4NGE IN HUMIDITY
The preceding data indicate that the total of average variations within a pad of composition cork amount to * 2 plus *3 per cent, or *5 per cent. The pads measured would serve as good gasket material under a compression of 50 per cent, which means that the *5 per cent is doubled and so becomes *10 per cent. This means, then, that a variation of 5 pounds per cubic foot from the mean of the established 55-65 pounds per cubic foot will often be exceeded by virtue of the fact that such a large amount of the sheet had a deviation larger than the average deviation. The same holds true for the deviations in the vertical dimension of a pad.
DENSITY IN POUNDS PER CUElC r'007
FIGURE 17. DESSITYWITH RESPECTTO POSITION O F 1 , / 1 - 1 SHEET ~ ~ ~IS .i 2.5-IKCHS1a.r
All characteristics of composition cork are affected by humidity. Compressibility determinations cannot be used as a criterion of quality unless samples are conditioned to some standard humidity. This is a slom procedure, consuming as much as five days; consequently it is not adaptable for use in regular routine quality-control operations. On the other hand, density variations are small, amounting to a maximum of 1 per cent per 10 per cent change in relative humidity. If, then, a density range is established at 50 per cent relative humidity and 5 per cent is added to this in each direction, density determinations may be used as a quality control instrument.
X RELATIVE HUMIDITY
FIGURE 16. EFFECTOF RELATIVE HUMIDITY o s DYNAMIC COMPRESSIBILITY
These figures, therefore, serve to show the necessity of adequate gasketing procedures to prevent leakage in liquid-filled electrical equipment. By means of the equation,
D
=
935 - 99.22
+ 3.132'
hich is the expression that relates density and dynamic compressibility of this product, it is possible to calculate the compressibility variations of a typical pad from the density variation :
TT
Variation Minus Mean P l us
Density Lb /Cu E't. 24.7 26.0 27.3
Dynamio Compressibility
(joyo Comprrssmn),
Statio Compressibility,
L b / S q In. 390
47.9
465
555
% 43.7 39.1
LOC.4TIONS
O F S O M E O F THE
80-100 GASKETS
USED I N T H I S
25,000-Kv. AMP. TRANSFORMER ARE SHOWN
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1942
657
From careful consideration of existing observations, a preliminary density range of 25 to 27 pounds per cubic foot was established for a certain grade of cork. Over a period of seven months incoming shipments were randomly sampled and densities determined for each sample; 245 samples had a mean of 26.20 pounds per cubic foot and the standard deviation of the samples was
4; -
S =
0.971 from the mean
(16)
=
where d = deviation of sample N = number of samples
The standard error of the mean was S, = S< /N = 0.062. The range of the mean with an assurance of 99.7 in 100 was 3 s / d % = 0.186; with an assurance of 95 in 100, it was 2 s / d N = 0.124. Using an assurance of 95 in 100--that is, 19 in 20-it is possible to find the number of samples necessary t o locate the mean within the range, for example, of 1.00 unit for an infi0.12 and a nitely large population having a mean of 26.20 standard deviation of 0.971 as follows: 2s 1 94
z='=-, N = 4N
