STRENGTH OF GLASS FIBER F. 0. ANDEREGG Owens-Corning Fiberglas Corporation,
Newark, Ohio
HE strength of materials isof great practical importance. When it is realized that typical substances arc contribut-
further discontinuities. In addition, true homogeneity of atom distribution is difficult to obtain in such a system aa glass, and only occasionally is much attention paid to securing a high degree of purity or uniform dispersion in any technical process. Based on this analysis, progress has been made in obtaining more of the potential strength of glass so that strengths m a t e r than that of steel are now standard; this statement may seem remarkable until it is realized that only about 3 per cent of the atomic strength of glass is being utilized. Even to reach this fraction great care bas to be exeroised. The original hatch of glass must be carefully and thoroughly melted and freed as far as practicable from gas bubbles. Considerable refinement can be made to advantage in this stage of the process, the first essential of quality textiles being glass of very great uniformity. Care exercised here helps maintain uninterrupted spinning later. For convenience in dispensing, this glass is made into spheres or marbles 0.75 inch in diameter. The process of spinning the fibers was described by Plummer (7) and by Slayter (IO). As the fibers are pulled finer and finer, bubbles in the glass and other inhomogeneities are pulled out more or less lengthwise so that the surface diseontinuities become less serious, while the contribution to the propagation of the rupture from those within the body apparently becomes more important, and many breaks become jagged. In the coarser fibers some of these flaws are perceptible under the microscope. Because of the increasing importance of those discontinuities inside the glass, equations of the Karmasch type (G), as used by Griffith (.’?), need amplification, another term being required.
T
ing only a small fraction of their potential strengths, and when the reasons for such a state of affairs are understood, it may be possible Lo do something about it. Where tensile strengths should range in the million pounds per square inch for ceramic materials, resins, and metals, we encounter in practice only a few thousand pounds in most of these materials, or efficiencies of the order of 0.1 to 0 5 per cent. Glass is the typically brittle material, and all strength results reported in the literature have been characterized by great scatter ( 1 , 8 ) ; the same is true in this investigation where mean deviations of about 120 per cent are the rule. Griffith (3)made the plausible suggestion that discontinuities are the cause for the brittleness of glass and for the large range covered by the results. He pointod out the tremendous concentration of strcss a t the apex of each discontinuity and set up equations for it. Although some correctionswere made to his equations by Smekal (18), practically all the work carried out since has pointed strongly to this explanation as being correct. Most products are solidified from a melt or solution which always contains impurities, including dissolved or undissolved gas. When the system congeals, the gas or solvent apparently collects a t periodic intervals, probably to produce discontinuities in glass and to give rise to “slip planes” in metals. Although the solubility of mast gases in glass increases as the temperature falls, there is a tendency for dissolved gases to collect around any solid impurities in Ruid glass. Although any large gas bubbles would decrease in size as the melt solidified, they would still remain to cause 290
MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
We then have the tensile strength of a fine fiber equal to some constant, which may be regarded as the bulk strength of the glass of the given degree of refining, plus the Karmasch term, which is a constant divided by the diameter to take care of the effect of discontinuities a t the surface, plus another constant divided by the diameter squared. To prevent the corrections from becoming infinitely great as the fiber becomes very fine, another constant must be placed in the denominators of both corrections. This constant is about half a micron and is of the same order of magnitude as the distance between slip planes in metals. It is suggested that as the material solidifies, the dissolved gas is pushed out ahead until a certain amount accumulates in a pocket, which facilitates slip in ductile and fracture in brittle materials. This analysis leads to an equation for tensile strength, T: T = A + -d
B
+
e
+
C
291
physical theory has advanced only far enough to indicate roughly the order of magnitude of the ultimate strength, such a method of attack does not offer any improvement over that just developed. It is believed that the theoretical value is of the order of 14,000,000 pounds per square inch (10,000 kg. per sq. mm.). Then the highest value obtained in this investigation, 3,500,000 pounds per square inch (2,500 kg. per sq. mm.) would be about 25 per cent perfect; this is about as high a value as present methods can be expected to give.
m
Any effect of surface tension, which amounts to a prestressing, could be included in the Karmasch correction term. Based on an observation of Lillie (6) that pulling fibers fine enough for textiles resulted in a lowering of the softening point by about 300’ C., the calculation was carried through and indicates a correction for the surface tension of the order of 18,000 pounds per square inch (12.7 kg. per sq. mm.), which is well within the scatter usually obtained. Another way of setting up the equation would be to subtract from the full strength of the interatomic bond, suitable corrections for both surface and body discontinuities. As
The strength of glass is reduced by discontinuities but, by thorough melting and proper methods of attenuation, the effect is greatly reduced so that strengths of the order of 400,000 pounds per square inch are being produced commercially with fine fibers for textiles. Generally, the finer the fiber, the greater the strength, the discontinuities apparently being pulled out lengthwise. The strength may be summarized from the “bulk” strength plus corrections for the decreased cross section. The scatter in strength results of such a material as glass is considerable, resulting from the variations in the number and in severity of discontinuities. The shorter the fiber tested, the higher the average results. The breaking is also probably affected by thermal energies which vary from atom to atom. Glasses low in alkali or entirely free of it have extremely high electrical resistance and stand up well to repeated wetting and drying. On the other hand, glasses containing alkali seem to withstand most acids quite well. Fused silica has yielded strengths as high as 3,500,000 pounds per square inch.
FIGURE1. APPARATUS FOR BREAKING FIBER PULLING GLASSBY STRAIGHT
It has been suggested that the high strength of these fibers is due to a tempering action similar to that given plate glass, but a little consideration excludes that possibility. The fibers are drawn so fine that cooling from the interior is so extremely rapid that no perceptible congealing can occur on the outside while the inside is still soft. Moreover, the best results have been obtained with fused silica, which has the smallest coefficient of thermal expansion and is therefore the poorest material for tempering in this way. Reduction in the effect of the discontinuities is the most logical explanation for the strength increases noted. In tensile experiments reported in the literature, considerable emphasis has been placed upon the “mirror” or the plane area which may happen to be left by the break; Smekal (13) apparently worked out a relation between the area of the mirror and the strength. In order to determine whether the same relation might continue into fibers running below 0.0005 inch (12.5 microns), a number of the breaks were classified as to type (straight, jagged, or intermediate), and no satisfactory correlation was found with strength. Early in the experiments all the fibers broken during one month were observed as to type of break and classified as straight, jagged, or intermediate. Of 320 fibers broken in this period, 194 broke square across, 84 were intermediate, and 42 quite jagged. The strengths of the 320 fibers were classified as 127 high, 64 medium, and 129 low strengths. The intermediate type of
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FIGURE 2. TENSILE STRENGTH AND DIAMETER OF GLASSTEXTILE FIBERS
PER SQUARE INCH
breaks were accompanied by 26 high, 16 medium, and 42 low. The straight breaks showed 70 high, 64 medium, and 60 low. Of the jagged breaks, 14 were high in strength, 1was medium, and 27 were low. For some time attention was paid to the type of fracture when especially high results were obtained; the frequency with which jagged ruptures were noted was surprising, and many of them accompanied high strengths.
Experimental Procedure During the development of fiber glass textiles, more than 6,500 fibers were broken in tension, and sufficient results were obtained to give a good indication of several of the relations. To secure reliable results, the first problem was to break the fibers by straight pulling; a special method of gripping the fibers was developed which has apparently been successful. The apparatus for this purpose is shown in Figure 1: After the fiber has been selected, it is held by the thumb and forefinger of each hand, and the heating element, H,is swung into close proximity to the wax, W , by means of a foot pedal; electrical contact is made b a Mercoid switch pressing against the left-hand post. After t i e wax has become softened by energy radiating from the heating coil, the element is allowed to swing back, and the fiber is placed in osition FF against pins TIand TS and held in position with a sliglt tension until the wax congeals. The wax soon cools so that loading can be applied by chain CH, the right-hand support, CS, being moved steadily downward. This support is su plied with a cork friction to facilitate a uniform rate of appEcation of the load. The breaking load is measured by the position of scale S with reference to one or the other pointer, P . The frequent breaks experienced by previous operators at the point where the fibers left the wax were seldom encountered during the course of this study, which indicated an application of true tensile loading. The whole apparatus is mounted on double rubber supports to eliminate vibration from outside.
The apparatus was calibrated by setting an ordinary chemical balance on top and measuring the load applied t o the lower movable arm as the right-hand support was lowered. Three different chains were calibrated for three separate ranges of strengths, but most of the work was done with the smallest chain. In order to measure the stretching of the fibers during loading, a fused silica rod, R, was fastened to the movable arm near T I ,the movement being read with the aid of a cathetometer. After being broken, the fibers are mounted on a microscope slide, and the diameters are measured to 0.00001 inch by microscopic projection onto an oiled paper on which a wedgehas been ruled, or onto a Euoscope background with vernier adjustment at the eyepiece; either method readily permits check readings t o within the desired degree. The square of the diameter is read
from a table, and the tensile strength is calculated by dividing
this square into the scale reading and multiplying by the constant for the apparatus.
During the course of the development work numerous fibers were broken in tension, and the results were used in determining the conditions governing the attenuation needed for the flexibility required in the various textile processes. Therefore, most of the results plotted in Figure 2 are with fibers larger in diameter than are now produced. At present the diameters, as determined by the yards per pound for continuous strand and confirmed by frequent microscopic projections, are held quite close within the range 0.00020 to 0.00023 inch (5 to 6 microns). For the tensile strengths given in Figure 2, T = 20,000
550,000 + d+ 0.2
+
600,000 (d 0.2)2
+
where d = diameter, ten-thousandths of an inch T = tensile strength, pounds/square inch Curve A A illustrates the effect of glass temperature on fiber diameter and tensile strength. It shows that an increase in glass temperature will cause both the diameter and the tensile strength to increase, assuming that the same size of orifice or rod is used in the production of the fiber for all cases. Dots, according to size, represent averages of from ten to twenty data. Crosses are for individual results. More than four thousand tensile strengths are summarized in Figure 2.
Fiber Strength and Rate of Loading Among the variabIes which may affect strength results, the rate of loading needs careful consideration. Bailey (2) reviewed the published results on testing bulk glass, such as bottles, and discussed the effect of the rate of loading in reducing strengths. A logical explanation of this effect was developed by Smekal (II), combining Griffith’s discontinuities (S),where the atoms a t the apex are under very high stress, with the random distribution of thermal energies among the atoms. If a load (for example, of the order of 60 per cent of the breaking load obtained under rapid loading) is applied continuously, rupture will occur sooner or later when the atoms at the apex become highly agitated thermally. Thus, superimposed upon the uncertainty caused by the
Thnm: I.
Av. Fibci Dism.
Mm.
In. x
10'
5
0 5
0
0.197 10 0.394 20 0.787 1.72 45 3.54 90 7.20 183 01.5 1500 Infinity
5.1
5.3
4.9
5.1 5.0 5.0
5.1 5
Mhona
..
13 13.5 12.5 13 12.7 12.7 13 12.7
variability among discontinuities, is the added uncertainty of thermal vihrations. These two in combination produce a scatter among the results of typically *20 per cent. Such scatterings seem to be characteristic of brittle materials; the greater the brittleness, the greater the scatter, as a rule. Fibers varying in diameter one hundred times have been broken, and the rate of stress application per unit area has varied considerably. An experiment was made to see if the rate of loading had appreciable effect. A single fiber was divided into thirty parts, which were broken alternately a t three speeds, and the time required to move support CS downward a given distance was measured with a stop watch. The results show that the strengths were constant well within the usual scatter of ahout 20 per cent; variations in speed of lowering CS were within * 10 per cent: Fiber
I".
x
106
Micronr
4.2
10.6 10.6 10.6
4.2 4.2
-
INFLnENCn OF LENGTE oP SPAN O N TENBILE STKENGTW OF
Length of Spsn Inchca
Diameter
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Rete of Loading
LL./sn. in. x K d e q . iOl/min. mm./mzn. 14.0 200 i(i 10 26.3 375 * 10 53.7 765 * IO#
Temiie Strength LL./aq. in. X K g k 10'
241 268 265
* * d
9
11 168
16.9 18.8 17.9
GLASSFIBERS
Tensile Strength -.-AveraEe---r--C~i~"i~t~d.~ Lb./sn. UL. Lb./sp. in. x 2 v K ~ P Pmm. . x 10' Xo./sa. mm. 228 157 147 213 2'1077 iii 210 173 * 1 5 8 122 200 140 130 121 185 115 162 114 76 142 100 10s * 14 90 87 124 103 72 100 70 ... ..
From the calibration cnrve for each chain, the following results were calculated, the movement of support CS being a t a constant rate. The smailest is used for fibers below 0.0006 inch, the next chain up to about 0.0012 inch, and the heaviest for larger fibers. The range in which each chain is applied seldom exceeds diameter ratios greater than 4:1, and, in view of the results in the above table, it seems unlikely that variation in the rate of load application is of any appreciable importance in these resulta: Fiber Diameter I n . x 104 Microna 1 2.5 2 5.1 5 12.7 in 28.4 20 50.8 60 127 100 254
-
aste of stresains No. 60 chain No. 120 chain Lb./aq. in./min. ( k d s p . nm./min.) 113oMI (801
E chain
2s:oMI (201 4,300 ( 3 . 1 )
.... .... .~..
....
ao,oooii.r~
7,600 (5.271
....
....
....
....
....
....
400% ?is1 10'000 17) 1:600 (1.111 400 1n.281
The length of fiber to he tested, however, is important. According to Griffith's theory of discontinuity, the longer the fiber, the greater the chance to encounter severe discontinnities and the lower the avcrage tensile strength. The resultv obtained with fibers ranging in length from 5 to 1,560 mm. are given iaTahle I. A soda-lime glass was used in this work. These results indicate that the length of the fiber broken has some influence upon the strength. The last two columns of Table I were obtained from the follo~ingequation:
T = 100 +
(
-
T
ahere T S
-
= (70
A) +
+ sx6) 38.5
1,wO lh./sq.
in.
kg./sq. rnm.
tensile strength, 1,WO pounddsquare inch (kg./aq. mm.)
length of span, inches (mm.)
The strengths calculated from this equation are well within the scatter among the individual results. The longer the span is, the greater the probability of encountering severe discontinuities.
Stretch and Modulus of Elasticity Organic fibers differ from fine glass filaments in the amount they will stretch under load; the former lengthens 20 per cent or more before breaking. For many purposes such stretch is desirable, but for others, a fiber of smaller stretch and of freedom from distortion is desirable. It is -of interest to know the amount of stretching that glass fibers will undergo before breaking. A large number of &hemwere loaded stepwise, and the elongation was carefully measured. I n every instance after loading (even up to two thirds of the breaking load) and releasing, the pointer returned to its original position as closely as could be read. In no single instance in the eoume of this investigation was any evidence developed of plastic flow in glass. Stress-strain cuwes were all perfectly straight, as far as the methods used would indicate.
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TABLE11. INFLUENCE OF LENGTH OF SPANUPON ELONGATION AT RUPTURE AND MODULUS OF ELASTICITY Fiber Diameter
In. X 104 (;:)
5.2 5.4 4.9 5.2 5.2
Mm.
(g:;) 13.2 13.7 12.4 13.2 13.2
*4.
10 4 4 4 6
In. 0 0.197 0.394 0.79 1.77 3.54 7.20
Tensile Modulus of Strength Elastioity Lb./sq. in. Kg./sq. Lb./sq. in. Kg./sq. Mm. X 103 mm. X 108 mm.
0 5 10 20 45 90 183
204 219 182 160 112 107
The values obtained from stretching 360 glass fibers over a length of 1 cm. averaged 4.24 per cent elongation at rupture and 2,830,000 pounds per square inch (2,000 kg. per sq. mm.) for Young’s modulus, each with a scatter of about 25 per cent. Comparison of modulus of elasticity and tensile strengths showed that the former increased slowly as the strengths increased; this confirmed a similar observation by Reinkober (8). The elongation of the fibers and the modulus of elasticity are also affected by the length of the span under load. The extent of this effect is shown in Table 11. The single continuous fiber used for this series of measurements was similar to those on which the results in Table I were obtained, but the fibers were loaded in steps instead of continuously, and the rate of loading was very close to 100,000 pounds per square inch per minute (70 kg. per sq. mm. per minute). Variations in this rate of loading had no perceptible effect on the result. The scatter in the results is shown only for the data on elongation. For the other quantities the scatter is similar to that given in Table I. The calculated values for the elongation were obtained from the equation: e = 1.5
+ s36.3 +1 430.11 = (1.5 f -) S + 2.9
where e = elongation, per cent S = length of span, inches (mm.)
No explanation suggests itself for this interesting relation, although the maximum elongation of 14.5 per cent calculated
25
Calod.
No. of Determinations Length of Span
50
100
NUM~ER OF CYCLESOF WETTING& DRYING
FIQIJRE 3. EFFECTOF REPEATED WETTINQ AND DRYINQ ON GLASSFIBERS
...
143 154 128 112 79 75
2:8
4.9 5.2 6.3 4.7 6.3
2,060 3,500 3,700 4,400 3300 4:400
ElongaElongation tion ( X 102) ( X 10%)
...* 28%
6.1 4.5 4.1 2.6 2.3 1.7
* * * * *
19
4
21 15 10
14.5 6.1 4.3 3.1 2.3 1.9 1.7
for zero length is very close to the increase in the separation of the ions in a rock salt crystal required for rupture, as calculated by Zwicky (16).
Glass Composition and Fiber Strength The effect of the composition of the glass upon the strength of the filaments appears to be only secondary, principally by determining the viscosity range over which satisfactory attenuation takes place. Even with short-range glasses considerable experience has demonstrated that very fine fibers can be obtained by the choice of proper conditions. The most important factor is to have a thoroughly melted, highly homogenized glass heated to as high a temperature as may be handled. For resistance to weathering and to high temperatures, the composition of the glass has considerable importance.
Strength of Fine Fibers and Surface Treatments A large number of experiments was made with coatings and various surface treatments. Coating a fiber or strand with any of the usual waxes or resins did not produce any perceptible improvement in strength, because the tensile strength of the best of these is only about 20,000 pounds per square inch, and the coating placed on the fiber has usually only a fraction of area in cross section of the fibers. With glasses containing alkali it was found, however, that coatings of some of the better resins, acrylates, methacrylates, chlorinated rubber, and a few others gave protective action against alkaline solutions. An example is shown in Figure 3. On two separate occasions apparent improvement in strength resulted from washing with various chemical reagents. Since these increases might well have been included in the natural scatter of rupture values, it seemed logical to make trials under more carefully controlled conditions. For such comparisons single fibers were selected and short lengths subjected to the different treatments. Thus, samples 1, 6, 11, etc., would be saved for control; samples 2, 7, 12, etc., would be given one treatment; samples 3,8, 13, another, and so on; and all would be broken at the same time. Any fibers which had been immersed in solutions were rinsed in distilled water and dried first, since it had been found that whereas wetting tended to reduce strength about 20 to 25 per cent, redrying brought it back. In this way a large number of chemical reagents was tested; it was found that reagents (including distilled water) which were nearly neutral produced no harmful effect when the fibers were immersed for a few hours. Alkali glasses were apt to be attacked by alkaline solutions but seemed quite resistant to acids; alkali-free glass stood up better to the attack of alkalies than of acids. No evidence was detected as to any base exchange between metallic ions in the solution and in the . glass.
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INDUSTRIAL AND ENGINEERIfiG CHEMISTRY
295
Strength of Fine Fibers and Their Age Griffith’s results were obtained with a hard potash glass, and he noted indications of loss of strength with standing. His glass apparently reached a steady state in an hour or two, resulting from mme arrangement in molecules near the surface, according to the explanation offered by Griffith. Sosman (14) noted a similar effect with silica fibers. Such a change would have an important bearing on the strength of commercial glass fibers; therefore numerous attempts were made to determine whether any similar decrease in strength might occur. Fibers were drawn and broken within a very few minutes. Others were stored for months in dry air, in humid air, or in water, or were alternately sprayed with moisture and dried. The strength was not affected exceut under the last of these conditions and &en only with typical sodalime glasses. By eliminating the alkali, or removing most of i t in certain cases, several glass compositions have been developed which have withstood as many as two hundred cycles of alternating spraying and drying with no perceptible loss in strength. Baking has been bard on some of the alkali glass, but when other fluxes were substituted for alkali, glass compositions were prepared which have not been perceptibly affected by prolonged heat treatment at several hundred degrees Fahrenheit. In the exueriments reoorted here on fused silica fibers, no evidence was observed of any loss in strength on standing, but no attempt was made to get a decisive indication concerning strength loss. F i y r e 3 shows results obtained when a fine mist of moisture was sprayed repeatedly 011 glass fibers of different compositions with complete evaporation between sprays.
Fiber Flexibility
or bending. The lower practical diameter of the fibers is determined by the cost and by the total strength of the strand. For instance. the cost of fabricatine and weavinr a bundle of 102 fibers drawn down to 0.00015&h (3.8 mioiks) is practically 8 s much when drawn to 0.00020 inch, but the load-carrying capacity is only about three fourths as great, even when allowance is made for the increase in tensile strength. Therefore, in practical fabrication, the range of diameters is maintained between about 0.00020 and 0.00023 inch (5 to 6 microns), which represents 80,000 to 100,000 yards of strand per pound.
ANOTHERVIEW
OF
STAPLE-FIBER OPERKPION
(Seeillurtration on psg* 290.)
For satisfactory handling it is essential that the surface of the glass be luhricatcd. Clean glass seizes clean glass with great ease, and union between the atoms takes place on contact; therefore to loosen them, actual rupture with more or less shattering occurs. It is for this reason that the friction of glass on elass is so high. Where the glass is to be subiected t o m u c h &ding or rnechanical actio;, a thin coat& of tough, properly plasticized resin bas been found invaluable. Continuous strands so treat.ed have given outstanding resistance to rubbing and flexing, in comparison with other materials available for electric insulation.
Effect of High Temperature One of the most important applications of glass textiles is for electric insulation, where the behavior of the glass under fairly high temperatures is important. Several series of tests were run to determine the effect of baking. The resistance to baking depends greatly upon the chemical compcsition; those glasses most resistant to weathering and to alkalies are most resistant to baking. Several of the glasses tested showed little, if any, loss in strength after 48 hours at 570* F. Practical trials in motors running a t very high temperatures and in turbine blankets have confirmed these laboratory results. Fibers of Large and Medium Diameter LARGE(0.0034.015 INCH). The coarsest fibers are made by pulling hot glass through relatively coarse boles by a small amount of steam. During this pulling a small amount of lubricant is applied and the fibers are then collected into the mat. The mat is sprayed with a suitable binder to hold
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VOL. 31, NO. 3
drawing them down to about 25 microns in an oxyacetylene flame. The filament melts in this flame and is blown out and carried away by the draft. The fibers were smooth IM FIGURE 4. TENSILE but all of them possessed a slight curl, the effect of which I20 STRENGTH OF COARSE was to increase the stress in the fiber under test by an GLASSFIBERS indeterminate amount. In spite of this curliness some of Ell0 X Square root of mean posiz these fibers (indicated in Figure 5 by crosses) broke under -100 tions loads in excess of 1,000,000 pounds per square inch, and 0 0 TXd=275 one fiber required a load of 2,250,000 pounds per square 3 80 550 0 T=20+inch. These fibers were all broken with B chain the same (d + 0.2) -k 600 day they were produced. (d + 0.2)2 In all but one case, the type of break found for the fibers where T is expressed in with a diameter of about 0.0001 inch (2.5 microns) was smooth thousands of pounds per and at right angles to the length of the fiber. This one fiber square inch and d in tenbroke in such a way that the jagged end spread a t the break; thousandths of a n inch. Althe distance across this spreading was approximately three though the simpler expression covers these results times the diameter of the fiber. Seven of the fibers with a 6 ?lo adequately, they fall within 5 diameter of 0.00039 inch (10 microns) broke with very jagged L LU the usual scatter range of ends; the other six broke straight across to form smooth ends. the general curve for glass. 10 Ch No correlation between type of break and strength was noted. If fine curly fibers could be produced with tensile strengths in excess of 1,000,000 pounds per square inch (700 kg. per sq. mm.), the question arose as to what strength could be obtained with fine straight fibers drawn from very hot silica. it together. Fibers were collected both before and after the binder had been applied and broken in tension. Each To obtain such fibers, a crossbow was set up which would shoot an arrow vertically into a target. A latch, operated by point in Figure 4 represents a single break. The square root a foot lever release, was set to hold the bowstring in the mean positions for any group fall close to the curve for the drawn position. A fused silica fiber of about 0.001 inch equation diameter (25 microns) was fastened by one end to the arrow T X d = 275 and by the other to a rigid support. This fiber was heated by an oxyhydrogen flame. Just as the fiber reached the but the data also fall well within the normal experimental proper softness, the flame was removed and the latch released. scatter of the general equation for the “standard” curve: Long uniform fibers were obtained; some were over 30 feet in 550 length. T = 20+- d 0.2 (d 6oo0.2)a thousand pounds/square inch These fibers were stored under room conditions for 24 hours and then broken with No. 50 chain. The portions of where d = ten-thousandths inch
-
-
I
+
o r T =14+-
982
d
+ 0.5
+
+
+
(d
2’720 kg./sq. mm.
+ 0.5y
where d = microns
MEDIUM(0.0003-0.001 INCH). T h e medium-size fibers are also steam-blown and lubricated as produced. They are formed into a bat. Fibers were pulled from a batting by hand and broken in tension. The 20 broken ranged in length from 3.5 to 15.4 inches (9 to 39 cm.) and in diameter from 0.00029 to 0.00094 inch (7 to 24 microns), and gave a tensile strength averaging 129,000 pounds per square inch (9,100 kg. per sq. cm.), which is well within the usual scatter of the “standard” curve. Fibers of Fused Silica The first experiments were made by drawing down fibers of very clear, fused silica by hand. The tensile strength of these fibers is shown in Figure 5 by the dots. These fibers were rather coarse and were broken with No. 120 chain the same day they were made; when examined under a microscope, they were rough and the breaks were uneven and jagged. In only about one third of the breaks was there any evidence of a mirror surface. Fine fibers were then blown from an oxyhydrogen flame. The method consists in
0.3
a4
0.6
aa
I Z 3 4 5 6 810 DIAMETER OF FIBERS IN 0.0001 INCH
to
30 AO
FIGURE 5. TENSILE STRENGTH OF FUSEDSILICA FIBERS
60
a0
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INDUSTRIAL AND ENGINEERING CHEMISTRY
297
the fibers tested were selected at random, and no effort was made to eliminate weaker sections. I n spite of this random selection, the results showed less scatter than any other series of measurements. In practically all cases the broken ends of these fibers were rough and jagged. The results obtained are indicated in Figure 5 by the circles. Figure 5 also gives the maximum results reported by Reinkober (8) and by Shurkov (9). A comparison of the data obtained with the fibers made by the three different processes shows that the flame-blown and the hand-drawn fibers possess strengths of the same magnitude as those reported by Reinkober and Shurkov. The results obtained on fibers drawn by the bow and arrow are characterized by higher strengths and by low scatter. Fnrthermore, the slope of the curve drawn through these points is different from thr curves of previous investigators as it is necessary to add to the Karmasch equation a term, as before, correcting for the effect of internal flaws. The solid line in Figure 5 was obtained by plotting the equation: 7,600
34 450
where T = tensile strength,' 1,OOO'pounds/square inoh (kg./sy. mm.) d = diameter of fiber, ten-thousandths inch (microns)
It is believed that the long viscosity range of fused silica was helpful in this experiment in drawing fibers with fewer and less severe discontinuities than in the case of some of the other glasses. ~~~~~~~~~
AND Monukus TABLE111. TENSILESTIZENGTH OF FLAME-ULOJVN SILICAFIBER?
Temile Strength
DiSmster In.
x
1.1 1.2 1.2
1.3
1.6 2.0
2.6 3.3
1.2 1.2 1.2 1.3
1.3 1.3 1.6
2.0
Kdan.
tion
EwsrIcrTY
Mdmo*azbi IO*
FiaEao1,As INSULATENO CEIMENT
Modulue of Elantioity
Lb./sq. in. mm. % x 106 Loaded Steowiae snd Released Eaoh Time A. 3.6 13.7 350 2460 2.8 11.95 3:670 4.4 3.1 522 5 . 0 7.15 300 2,630 3.1 5.1 8.77 449 3.150 3.3 4.5 5.5 248 1,740 4.1 5.3 1.840 3.6 5.1 191 1.4 12.6 172 1.210 6.6 2.09 370 5.9 124 8.4 B . Loaded Stepwise without Release 273 1,970 3.8 7.2 3.1 345 2,480 3.6 9.7 3.1 3.1 236 1.708 3.4 7.0 547 3,920 6.3 8.7 3.3 1,460 2.6 7.8 2w3 3.3 278 1.890 7.4 3.8 3.3 73 525 3.7 2.0 4.1 235 1.630 5.8 4.1 5.1 A
IO'
. ail. in.
Elonm-
OF
K"./SC.
mm.
x
10'
9.6 8.4 5.0 6.2 3.9
3.7 6.8
1.47
5 2 7.0 5 0 6.3 5.6 2.7 1.4 3.0
MODULUS OF ELASTICITY. The modulus of elasticity for a number of flame-blown silica fibers was determined one week after the fibers were produced. The results vere obtained with two d i e r e n t methods of loading. In the first case (Table 111-A) a load of 0.39 gram was placed on the fiber, the elongation was noted, and the load was removed. In all trials the fiber contracted to its original length BS soon as the load was removed, as closely as it was possible to read; this proved that the fiber was perfectly elastic within the accuracy of the measurements and that the fiber was not slipping in the wax used to hold it in place. Then a load of 0.78 gram was placed on the fiber, the elongation noted, and the load removed. This stepwise loading with removal of load each time was continued until the fiber broke. In all cases the fibers proved to be elastic up to their breaking point. The results shown in Table 111-B were obtained in a similar way but the load was nel'er released.
The modulus of elasticity of fused silica fibers was also measured by Reinkober, who found the same inconsistencies in individual results as those in Table 111. However, by taking a large number of measurements he was able to show that the maximum value of the modulus of elasticity for a given fiber diameter increases as the fiber diameter decreases. The highest value which Reinkober found for Young's modulus was 1.55 X lo'pounds per square inch (1.11 X 104 kg. per sq. mm.). EFFECTOF REAGENTS. Three spceimens of bow-andarrow fibers were placed in each of several reagents for 5 hours with the following result,s: nosgent I N HCI 1 N NaOH watei .4laohol
h". niain.
W e t Tenskie
Strength
Indl 0.00067
Lb./so. . . in.
0.00065
132.000
0.000s9 0.000s0
87.000 69,000
ss.noo
These reagents apparently had a deleterious effect on fused silica fibers, confirming previous observations of Joffe (4).
Other Fibers PYREXGLASS. The tensile strength of Pyrex glass was determined with fibers attenuated both by bow-and-arrow and by flame drawing. The results lie in the region between those for fused silica and glass fibers; the average is close to the curve drawn through Griffith's results for a hard potash glass and not very far from lbinkober's results for fused silica. The degree of purity and homogeneity of the glass and the temperature a t which it is worked are extremely important in determining tensile strength.
INDUSTRIAL AND ENGINEERING CHEMISTRY
298
FIBERS FROM GLASSDRESS. Fibers taken from the dress of the Spanish Princess Eulalia, made in 1893 and preserved in the Toledo Museum of Art, were broken in tension with the following results: Diameter Fibers Blue
Bluieh green
Average
In. X 100
Mm.
1.35 1.54 1.69 2.02 1.72 1.56 1.59 1.34 1.60
0.034 0.039 0.043 0.051 0.044 0.040 0.040 0.034 0.041
Tensile Strength Lb./sq. in. Kg./sq. em. 11,000 770 4,000 280 8,000 560 5,000 710 9,000 630 7,000 490 12,000 770 11,000 840 8,400 590
Eight other fibers of each color were measured for diameter; the average was 0.00164 inch (42 microns). These results are appreciably lower than those with the coarsest fibers for air filters; but when it is remembered that they were obtained by pulling out softened glass rod and were probably never properly lubricated, the results seem reasonable. RAYON.A sample of 75-denier rayon, which had been spun under tension, was obtained and divided into two groups
VOL. 31, NO. 3
according to thickness. Seven fibers averaging 0.00066 inch had a strength of 92,000 pounds per square inch *13 per cent, and seven averaged 0.00078 inch with a strength of 64,000 pounds per square inch * 13 per cent.
Literature Cited (1) Anderegg, F. O., Keram. Rundschau, 44, 255 (1936). (2) Bailey, James, and Lyle, A. K., paper presented before 1937 meeting of Am. Ceramic SOC. (3) Griffith, A. A., Trans. Roy. SOC.(London), A221, 163 (1920). (4) Joffe, A., Intern. Conf. Physics, London, 11, 80 (1935). (5) Karmasch, S., Mitt. gew. Ver. Hannover, 1858, 138. (6) Lillie, H. R., private communication. (7) Plummer, J. H., IND. EYG.CHEM.,30, 726 (1938). (8) Reinkober, O., Physik. Z . , 32, 243 (1931); 33, 32 (1932); 38, 112 (1937). (9) Shurkov, S., Physik. 2. Sowjetunion, 1, 123 (1932). (10) Slayter, Games, J. Am. Ceram. Soc., 19, 335 (1936). (11) Smekal, Adolf, Glastech. Ber., 13, 141, 222 (1935); Z. Physik, 91, 336 (1934); 103, 495 (1937). (12) Smekal, Adolf, J. SOC.Glass Tech., 20, 432 (1936). (13) Smekal, Adolf, 2. Physik, 103, 495 (1936). (14) Sosman, R. B., private communication. (16) Zwicky, P. W., Physik. Z.,24, 131 (1923). RECEIVBID Ootober 14, 1938.
MOLYBDENUM ORANGE PIGMENTS ARTHUR LINZ Climax Molybdenum Company, 500 Fifth Avenue, New York, N. Y.
M
OLYBDENUM. orange pigments, characterized by enormous covering power and tinting strength, bril’ liance of color, and extreme fastness to light, are becoming increasingly important in the printing ink and paint industries. Commercially these pigments are made by adding a solution of a bichromate, a sulfate, and a molybdate to a solution of a soluble lead salt. The original yellow precipitate is converted through orange to a brilliant red in successive stages under proper conditions. The properties of the pigment depend entirely upon the conditions existing in the solution during and after precipitation. Because the effects of variations are not clearly understood and have been subjects of sharp disagreement, this study was undertaken to learn what takes place during the change of the pigment from yellow to red and to determine methods which would produce uniform results in its manufacture. The first observation of the formation of red normal lead chromate was that of Schultze (12)who noticed that yellow lead ore (wulfenite) was sometimes strongly colored by the red lead ore (crocoite) found near by. Since wulfenite, or native lead molybdate, crystallizes in the tetragonal system and crocoite or lead chromate ore is monoclinic, it occurred to him that the strong red coloration might be due to crystallization of lead chromate in the tetragonal system. He conducted (and published in 1863) experiments showing that it was possible to obtain homogeneous crystals in the tetragonal form from mixtures of lead molybdate and lead chromate containing as much as 42 per cent of the latter. Such crystals were a deep, dark red in color, much darker than any crystals of pure lead chromate he had ever obtained. I n 1921 Jaeger and Germs (4,in a study of binary systems of lead sulfate, chromate, molybdate, etc., corroborated the work of Schultze and showed that lead chromate could exist
in the tetragonal form a t room temperature in the presence of lead molybdate. On August 30, 1930, Lederle (6) applied for a patent in Germany, covering a yellow to red pigment. In this patent Lederle cIaims “as a ‘new’ article of manufacture, mixed crystals suitable as yellow to red pigment coloring matters comprising lead chromate and a t least one salt of lead with an acid selected from the group consisting of molybdic and tungstic acids.” The remainder of the claims cover mixed crystals of lead molybdate, lead chromate, and lead sulfate or tungstate with or without the use of barium or strontium chromates, molybdates, or sulfates. On August 2,1932, Wagner, Haug, and Zipfel(l4) submitted for publication a short survey of their work on the modifications of lead chromate in which they discuss, briefly, tetragonal lead chromate. They refer to the work of Jaeger and Germs in producing this form by the pyrogenic method (S), but no reference is made to the pertinent work (4) by these same authors published in the same periodical in the same year, or to that of Schultze ( I d ) . They state: “One obtains a red tetragonal mixed crystal modification if one also adds PbMoOd to the chromate in addition to PbSOc by the simultaneous use of ammonium molybdate.” They also give the crystal lattice of a “compound,” 5PbCr04.3PbMo04.10PbSO4. The impression which they seem to be trying to produce is that red tetragonal lead chromate is stable a t room temperature only in the presence of both molybdate and sulfate, which supports Lederle’s patent. Lederle’s second patent (6) was applied for in Germany on August 30, 1932, just 28 days after the article by Wagner, Haug, and Zipfel was received for publication. This is a process patent claiming methods for production of mixed crystals of lead chromate, molybdate, and sulfate in acid
