Structure and Physical Properties of Massive Nylon - Industrial

Note: In lieu of an abstract, this is the article's first page. Click to increase image size Free first page. View: PDF. Citing Articles; Related Cont...
0 downloads 7 Views 2MB Size
Download PDF
Structure and Physical Properties of Massive Nylon G. L. CLARK AND M. H. MUELLERI

L. L. STOW

University of Illinois, Urbana, Ill.

The Polymer Corporation, Reading, Pa.

Utilizing familiar metallurgical techniques, studies have been made of the structure and texture of massive nylon ranging in size from 0.011-inch strip to 3-inch rod. These techniques were x-ray diffraction, photomicrography, hardness and density measurements, heat treatment and recrystallization in various liquid media, tensile measurements, etc. Of principal interest is the fact that the two most intense x-ray diffraction rings vary in angular position depending upon the previous history of the sample, such as size, cooling rate, heat treatment, and aging. It is possible directly to correlate many of the properties of the nylon with these angular shifts. On the basis of this information nylon in massive form for gears and other uses may be used more intelligently and performance more adequately predicted and controlled.

In later articles Baker and Smyth ( 4 ) and Baker and Fuller ( 5 ) showed that quenching of all copolyamides caused extensive lateral disorder of the chains, including groups in the dipole layers. Annealing a t a sufficiently high temperature below the melting point caused rotation of segments in the solid state, resulting in an ordered crystalline form and a deflnite hardening of all samples. Brill (6) has a,lso made an investigation of annealing several polyamides. His method was to anneal fibers of the material a t fixed temperatures and take x-ray patterns of the material while a t the elevated temperature. I n the case of nylon Type 66 he found that the two equatorial interferences on the fiber diagram came closer to each other with increasing temperature until a t 160" C. they unite in one sharp interference. Bunn and Garner (8) found that ordinarily the x-ray patterns of nylon gave evidence of two crystalline phases of which one, designated a:, appeared only after a fiber of nylon Type 66 has been dipped in a phenol-water solution and then heated in boiling water for 2 hours. The 9, form which appears in small amounts has the same length along the c axis, but apparently there is a difference in the position of the molecules along the c axis, Correct intensity values were obtained if i t were assumed that the second chain is displaced upward 3.55 A. in relation to the first chain and then the third chain is displaced downward, so that it is on the same level as the first chain. I n the a: form each layer is displaced approximately 3.5 A. to each other but always in the same direction. This arrangement gives a two-molecule cell. A similar situation was reported by Bunn (7) in gutta-percha in which there were geometrically different molecules, by Fuller and Frosch ( 1 2 ) for polyesters, and by Schoon ( 1 7 ) for long-chain hydrocarbons and acids. The lattice constants for the a: and p forms of the Type 66 polyamide found by Bunn and Garner, are as follows:

S

INCE its introduction some 10 years ago nylon has become an important synthetic material. The largest use has been in thk form of a textile fiber and, as a result, most of the investigations have dealt with it in this form. Recently nylon has come into use as plastic for machine parts. It is stated ( 8 ) that in some applications, such as gears and bearings, this material outwears metal several times and requires no lubrication. This investigation began with the idea of finding a method for dktecting stress in the larger masses of nylon, inasmuch as difficulty had been encountered because of the fracturing of some pieces during fabrication. As the work progressed very interesting results were revealed. In the manufacture of nylon Type 66, solutions of adipic acid (or for Type 6.10 nylon, sebacic acid) and hexamethylenediamine are run together in measured amounts into stainless steel kettles. This then forms the nylon salt hexamethylene-diammonium adipate. The concentrated salt solution is then charged into a cylindrical autoclave where the polymerization is carried out. Stabilizers are added to control the molecular weight and the viscosity. The molten nylon is extruded as a ribbon on chilled rolls and the ribbon is cut into small chips. The material is then blended and stored. From this point the material is remelted and either extruded as fibers or molded into various shapes. It is also possible to cast or extrude the nylon.

1_

n . .Form ..

c

The large masses of nylon are opaque and very light cream colored. The material has very good machinability and can be polished to a very smooth finish.

p = 77'

y = 66.3'

a = 6.00 A. b = 4.17 A. c = 17.3 A.

Several investigations have been made of thin films and fibers of nylon. Some of this work has direct application to nylon produced in more massive form. Fuller, Baker, and Pape (11) studied the effect of solidifying the polyamides a t different rates and also of annealing the completely quenched specimens. They found that the degree of polycrystallinity and the perfection of the lattice was dependent upon the rate of cooling, and that in annealing completely quenched specimens the crystalline lattice arrangement is improved with increasing annealing temperature. However, it never reaches the perfection of crystallinity obtained by slow cooling a t the same temperature from the molten state. Fuller (10) showed that the elasticity of copolyesters is due to the presence of an amorphous phase. 1

= 17.2 A.

a = 48.5'

a = 4.9 A. b = 8.0 A. c = 17.2 A.

a = 9@ = 77" = 67'

p

y

The lattice constants determined by Ecochard (9) for nylon Type 66 were as follows:

PRIOR WORK ON NYLON STRUCTURE *

R F o-.. r m.

~

a = 4.9 A. 6 = 5.4A.

Present address, Argonne National Laboratory, Chicago, Ill.

831

P

= 81O11'

S , =

76"23:

y = 63O08

A patent (IS)suggests the possibility that there are a t least two different crystalline forme existing in the polyamides based on the fact that the x-ray pattern of quenched massive nylon Type 66 showed two prominent rings an inner 100 reflection representing a d spacing of about 4.32 A. and an outer combined 010 and 110 reflection for a d of 3.82 A., while in the unquenched material the spacings were, respectively, 4.32 and 3.68 A. EXPERIMENTAL WORK WITH GEIGER SPECTROMETER

The main interest in the present investigation has been directed to these two principal peaks. For this purpose a Norelco Geiger spectrometer has proved to be most useful because of speed and ease of determining spacings from the recorded peak positions. EFFECTOF MASSAND COOLING RATE. There is a very definite difference in the 28 angular distance between these two peaks,

832

16

INDUSTRIAL AND ENGINEERING CHEMISTRY

18

20

22 24 -28-

26 28

A

16

18

20 22 -28C

24

26 28

16

18

20

22 24 -28-

26

28

D

Figure 1. Typical Spectrometer Curves for Massive Nylon Samples A. B. C. D.

2-inch rod as received 1-inch rod as received 1/4 X 4 X 10 inch slab as received 0.011 X 2 inch strip as received

depending upon the cooling rate and the mass of the various sizes. Of course, the mass has an effect on the cooling rate. In general, the faster the cooling rate the closer the peaks are to each other. In Table I are shown typical 28 differences obtained on various sizes after fabrication with no heat treatment. All specimens except the last one in the table were nylon Type 66. As an example of these data, there is a considerable difference between the '/4 X 4 X 10 inch and the I/? X 2 X 72 inch slabs explained by the considerably faster cooling rate from the molten state for the '/? X 4 X 10 inch slab. The differences found between the outside and inside portion are in line with what might be expected on the basis of differences in cooling rate. It is only logical that the outside portion should cool faster than the inside. With the 2- and 3-inch rods the mass is so relatively large that the whole piece cools down essentially a t the same rate. This results in practically the same spacing difference regardless of where in the rod the specimen is taken. In order to show the type of curve obtained with the Norelco Geiger spectrometer which was used for the information in Table I, reproductions of the curves of the 2-inch rod, 1-inch rod, l / d X 4 X 10 inch slab, and 0.011-inch strip, all with no heat treatment, are shown in Figure 1. The tvio dotted lines shown on the reproductions are for convenience in locating the approximate position of the peaks. These lines appear in every case at 20.00" and 23.50'. The peak a t the large angle was found to do most of the shifting in the 28 vtllue. € h aever, even the more constant inner 100 peak showed a slighr CiiEerence as between the thin pieces and the large rods. Table I1 sh(iws some typical results. The calrulatd ( I value for the other peak vaiied betvieen 3.92 A. for the 0.01 I - i r w h strip to 3.76 A. for the 2- :mcl 3-inch rods. .4 comparison of t h e latter figure s h o w that even in the case of relatively slow cooiiiig of the massive nylon the 010-110 spacing is greater than 3.72 A. found for nylon fibers. The average 28 of the more constant peak of over 300 curves

Vol. 42, No. 5

made of specimens ranging from 0.028-lnch strip to 2-inch rod gave a calculated d value of 4.38 A. This agrees perfectly with the spacing as calculated from Bunn's cr unit cell for the 100 planes and also with the value given by Ecochard. SHIFTCAUSED BY AXYEALING. Becausc ordinary temperatures did not seem to have much effect in causing shifting in the spacing, it was interesting to determine if the close peaks of the small sections of nylon could be shifted apart and a t least more nearly approach the angular separation found in the larger pieces. III order to prove this some of the smaller pieces were heated in an agitated oil bath a t various temperatures and for different times. Specimens were cooled to room temperature in air before making the curves. The general trend in the spectrometer curve for the 0.028-inch strip after heating 60 minutes at 80°, loo", 150°, 200", and 235 a C., respectively, is evident-namely, that the two peaks move apart from each other and also become somen hat sharper especially at the higher temperatures. Figure 2 shows the effect of time and temperature on the shiftc ing apart of the two principal peaks. In most cases 2.5 minutes are sufficient to cause the maximum movement which is l o take place at a particular temperature. This agrees with the evidence of previous workers (11). The curves were rerun after 6 months on the same samples. The results were identical with the original within experimental error. This Seems definitely to indicate that the spacings remain constant at room temperature but if enough energy is introduced in the form of heat the two principal peaks may move apart and then will remain in this position after cooling dorm. ' In contrast to the shifting observed with the 0.028-inch strip, the larger pieces showed little or no shifting even at the higher temperatures. Specimens of various sizes mere heated a t various temperatures for as long as 27 days. No greater change than that produced after 60 minutes seemed to be evident.

TIME-MINUTES

Figure 2. Effect of Time and Temperature on 28 Difference in Diffraction Peaks for 0.028-Inch Strip

ANNEALING m VARIOL-sMEDIA. I t was decided to check the effect of heating in other media, especially since Brill's work had shown that the presence of water had an affect on the shifting. Figure 3 shows the results of this portion of the investigation. Individual pieces of the 0.028-inch strip were heated for various times a t 100" C. in oil, water, air, and silicone fluid and then spectrometer curves were made. The 100 reflection remained constant but there is a different amount, of shifting in the other spacing, depending upon the fluid used for heating. The peaks are noticeably better defined and sharper after heating in water than in the case of the other heating media. Re-examination

INDUSTRIAL AND ENGINEERING CHEMISTRY

May 1950

IFFERENCE IN SIZESOF

Size, Inches

0.011 x

0.028 X 0.060 X '/a X 4 l/h X 2

2 itdp 2 strip 2 strip X 10 slab X 72 slab

sPACING) AND H~~~~~~~IN vARIoUs MASSIVENYLON

Peak Separation in Angular Superficial Degrees Rockwell (A281 15W 2.25 .. 2.70 47 2.85 43 2 50 48 3 05 62 2 30 51 3 15 3.25 3 30 67 3 40 66 3.20 69 3.40 70 3 25 66 3 40 67 3.00 65 3 30 68 3 35 72 3.45 71 3.45 74 3.45 73 3 45 75 3 45 75

.. ..

7/8

rod (outside) new

?/a rod (center) new t rod (owtsde)

I rod ce te ) 1 rod !ou%idel, 2nd sample

1 rod (center), 2nd sample

l'/a rod (outeide) I ~ / Prod (center) 2 rod (outside) 2 rod (center) 3 rod (outside) 3 rod (center) '/a

,

Knoop Hardness Number 4.8 6 2 6.1 5.1 7 6 4 9

.. ,.

8,9 9 9

7 1 8 2 7 5 8 7 12.9 12.2 13.3 12.4

..

X 2 X 72 strip (Type

6.10 nylon)

TABLE11. 20

2 60 AND

50

7.0

833

film cassette at a fixed distance. A satisfactory pattern was obtained in 4 hours by operating the tube as a self-rectifier a t 40 kv. and 15 ma. The strips and slabs of nylon showed no apparent orientation but some orientation was indicated in the rod material, owing to the method of manufacture. The degree of orientation varied within a single rod. In general, for most samples the average intensities of the two rings seemed to be about equal, but in a few cases the intensities were greatly different. This could not be accounted for by the usual evidences of fibering because the entire rings seemed to be of uniform intensity. A clue to the explanation of the above patterns was f by Bunn and Garner (8), who found that a double orientation of the crystals can be obtained if a nylon fiber is rolled or pressed. In such a case the c axis becomes perpendicular to the original fiber axis and the plane, which has been called the 010, becomes approximately parallel to the plane of the sheet. It may be that the molten material being fed into the central portion of the rod exerts a pressure against the solidifying material on the outside and thus causes a large number of the 010 planes of the crystals to become parallel to the outside surface.

d VALUESFOR 100 PLANEIN VARIOUS

for the preceding part of the investigation, the ordinary x-ray film method was superior for the determination of preferred orientation, beEause complete rings are registered.

All the flat film patterns were taken with copper Ka radiation with a 0.025inch back pinhole and a 0.010-inch defined front pinhole. The sample-to-film distance was conveniently maintained at 5.12 em. by placing the sample directly against the front pinhole and holding with Scotch tape. The pinhole was fitted into a hole in an aluminum U-shaped casting which has slots for holding the

T I M E -MINUTES

Figure 3. Effect of Time and Heating Medium on 28 Difference of Diffraction Peaks in 0.028-Inch Strip

Figure 4. Pattern from 0.027-Inch Nylon Strip Cold Rolled to 0.011 Inch L e f t . No heat treatment Right. Heat-treated 30 minutea at 250' C.

PATTERNS AT ELEVATED TEMPERATURES. Inasmuch as Brill had found that if an x-ray pattern is taken of a nylon fiber a t an elevated temperature the two principal spacings came closer together, it seemed desirable to check the behavior of the material in more massive form. For this purpose transmittance patterns were taken as described above. The only change necessary was to provide a means of heating the sample. This was accomplished by surrounding the sample with a large metal container provided with openings for entrance of the primary beam and the exit of diffracted x-rays. The temperature in this container was then increased by a gas burner and the temperature near the sample noted. The same sample-to-film distance previously mentioned was maintained. Patterns were taken while holding a 0.010-inch section of the 3inch rod a t room temperature, looo, 150°, and 200" C. A comparison of these patterns with the fiber patterns shown in Brill's work disclosed that there is less shifting taking place in the large pieces than in a similar nylon fiber a t a given temperature. As shown by Brill, a temperature of approximately 160" C. was sufficient to cause the two main spacings for a fiber to fuseinto one, but this was by no means true for the massive specimens. This difference might be tied up with orientation, for the fiber is highly oriented and the rod has relatively little orientation, X-RAYEVIDENCE OF RECRYSTALLIZATION. Brill has also suggested the possibility that a recrystallization takes place in the nylon fibers upon heating. In this present study an x-ray diffraction pattern was made of a 0.027-inch strip after cold rolling

INDUSTRIAL AND ENGINEERING CHEMISTRY

834

Vol, 42, No. S

of this impression. The indentation is made by elevating the specimen against the diamond indenter under a load of 100 grams until the hardness of the specimen resists further indentation. When this condition is reached, an electronic circuit stops further elevation of the specimen and the load is applied for 20 seconds, under automatic control by a timing motor, after which the sprrimen is lowered. The length of the long diagonal is then measured by means of a microscope. The results are reproducible arid tlrpendable.

44) 0

I

10

I

x)

I

30

I

40

50

TIME- MINUTES

Figure 5. Effect of Time and Temperature on Superficial Rockwell Hardness for 0.028-Inch Strip

to 0,011 inch. This pattern was taken under the same conditions as mentioned previously with the x-ray beam perpendicular to the surface of the sheet (see Figure 4,left). This same piece was then heated in silicone fluid for 30 minutes a t 250" C. The x-ray pattern obtained after this treatment is shown in Figure 4,right. The increased number of spots obtained in this latter diagram indicates that the material has a t least been partially recrystallized. This is exactly parallel to the recrystallization of metals occurring upon annealing cold worked material. Further evidence of recrystallization of cold worked nylon is shown in the photomicrograph of Figure 11. HARDNESS

After the shifting of the peaks had been observed it became H. matter of interest to determine if the changes taking place upon annealing caused any appreciable change in the physical properties, Fuller, Baker, and Pape ( 1 1 ) had pointed out that modulus of elasticity was increased upon .heating, and others have mentioned increased hardness. The small sections were rather soft and flexible before heating but after heating in oil a t the higher temperature a piece of the material dropped on a table gave almost a metallic ring and was much less flexible. As a result, the first property investigated was the hardness. The first problem was to select a method of hardness testing which would give good reproducible results and could be applied to the various sizes of nylon. Some preliminary tests were made with the Rockwell hardness tester using a 1/8-inch ball with a 60-kg. load. However,, the load was much too large and the ball too small satisfactonly to test thin specimens, because there was a definite anvil effect in which the indentation could be observed on the opposite surface. The superficial Rockwell tester was then tried, because it is primarily designed for thin material, with a '/s-inch ball and a load of 15 kg. to give readingsTn the 15W scale. In testing many plastlcs owing to the load, shape of the peiietrator, and resistance of the plastic to deformation, the penetrator continues to indent the plastic indefinitely. Because of this it is necessary to specify a time factor for applying the loads. The major load was applied for 15 seconds and the hardness reading taken 15 seconds after removing the major load (16). Because of the inherent recovery in plastic it was thought advisable to check the hardness obtained with the superficial Rocliwell by a method which is less subject to the recovery of the material. The Knoop indenter and the Tukon tester (1, 16, 18, 19) make an impression which is not subject to SO much elastic recovery. The indentation made in.such a test is a n elongated diamond shape. In order to obtam the hardness number the length of the long diagonal is measured. A small amount of recovery of the material does not cause much change in the length

INITIAL HARDNESS. The first thing that was checked was tiit: initial hardness of the pieces of various size. Typical results ohtained by the two different methods of hardness testing are shown in Table I. I t is evident that,,in general, the material showing t h c closer spacings (the smallest difference in 20 values) is softer. HARDNESS AFTER ANNEALING. Figure 5 is a graph of the h:irtlne,% readings obtained on pieces of 0.028-inch strip heated in oil si various times and temperatures. These are the same samples which were used for the x-ray spectrometer curve shown in Figure 2. Each point on the graph was obtained by averaging t,hree 1,eadings taken on the same specimen. A comparison of this graph of hardness with the one showing the 20 differences shows a very good correlation. Because of this correlation, a check of the hardness of a larger specimen heated in oil a t corresponding times and temperatures was made. Small pieces Fere cut from the 2-inch rod in order to overcome the effect of mass in the heating of the samples. The hardness changed very little. Again there seems to be a good correlation b e k e e n the 20 difference and the hardness. However, even though the 28 difference is just as great, or perhaps a little larger, with the 0.028-inch strip heated a t 235" C. than the 2-inch rod, the hardness of the former upon reaching the maximum 20 difference is not so great. The final hardness with the 0.028 inch is approximately 62 (15w scale), whereas the 2inch rod has a hardness of 75. Therefore, a certain hardness with a particular 28 difference cannot' be expected. The loR-er hardness with the smaller piece may be accounted for by the pleselic.c? of more amorphous or less ci.J.stalline material even after heat,ing at the high temperatures. After the 0.028-inch specimens had stood for more than 6 months the hardness was again determined and the spechrometer curves were retaken. The results were not appreciably different from the original determination, which showed that the change t,hat took place in the material T T ~ Sapparently permanent'. The Knoop hardness numbers were also plotted for various times and temperatures. These values wehe obtained on the same 7

I

i

I

IO

!

I

I

20

I

30

I

40

50

J

60

TIME-MINUTES

Figure 6.

Effect of Time and Temperature on Knoop Hardness for 0.028-Inch Strip

May 1950

1

INDUSTRIAL AND ENGINEERING CHEMISTRY

I

I

I

T I M E - MINUTES

Figure 7. Effect of Time and Heating Medium on Superficial Rockwell Hardness for 0.028-Inch Strip

samples as used for Figure 5 . The same trend is indicated in Figure 6 as was shown with the superficial Rockwell hardness readings. ANNEALING IN VARIOUSMEDIA. Ileating in different liquids and in air seemed to produce various amounts of shifting in the x-ray pattern of the thin pieces. Superficial Rockwell hardness readings were taken on these specimens. The results showed that the hardness did not change appreciably when heating a t 100" C. in oil, silicone fluid, or air, but, as can be seen in Figure 7, the material became much softer after heating in water. This change was by no means permanent, as is shown in the same graph. MICROSCOPIC EXAMINATION

HEATEDMATERIAL.When some of the samples were heated for days a t 200" C. in an air furnace it was noticed that the lines of flow, which had taken place during fabrication, could be seen on some of the rod samples. Figure 8 shows such a condition. This specimen had been finely polished with a 3/0 emery polishing paper. I t was observed later that if the pieces were polished with a polishing compound, such as aluminum oxide, similar to the technique for metallurgical examination of a metal specimen, the flow of the material in the rods could be observed with the eye. This can be explained by the relief polishing obtained because of differences in hardness of the material in different portions of the material The material which had been heated a t 200" C. for a long time was then examined under the microscope, using incident light. This showed a "grainlike" structure. The surface was very uneven and showed a great deal of relief. The individual "grains" were more or less equiaxed and showed curvature indicating that these grains were probably in the form of spheres. There was a good deal of difference in the size of the grains, depending upon the size of the piece. Figure 9, left, was taken near the outside surface of a 1.25-inch rod after heating for 7 days a t 200" C. Figure 9, right, is a photomicrograph of a 0.060-inch strip of nylon which was treated in the same way. There is a tremendous dif-

.

835

ference in size of the grains between the two samples. The reason for this difference is discussed below. POLISHED MATERIAL.It was felt that there might be a chance of showing up this structure by merely polishing the specimen and perhaps etching. In other words, the material was to be treated as a metal specimen which is to be used for microscopic examination. The procedure used in polishing all the specimens was as follows: A flat surface was obtained on the specimen and it was ground down with increasingly fine emery paper until the final paper used was a 3/0. The piece was then polished on a olishing disk covered with a well-napped, soft velvet cloth. either finely levigated alumina or Micro-polish, a product of Buehler, Ltd., was used as an abrasive on the cloth. Usually the best results were obtained b rotating the 10-inch disk at approximately 1150 r.p.m. If tXe samples were observed with the microsco e immediately after polishing, the grain structures were usuafy only faintly visible, but upon standing for 15 to 30 minutes the structural details were very pronounced, as the result probably of oxidation. Some typical photomicrographs obtained after polishing for ?/pinch rod are shown in Figure 10. From the photomicrographs it would seem that there is a correlation between the cooling rate and the size of grain structure obtained. This is logical from a consideration of nuclei formation. As the material cools from the molten state nuclei are formed and crystallization takes place around each. At the point of junction the material is probably less crystalline and more nearly amorphous. The smaller sizes of nylon rods and sheets are cooled more rapidly, hence would allow more nuclei to form, which would result in a h e r structure than observed in larger pieces which are cooled more 9lowly. The photomicro-

Figure 8. Flow of Material in T/g-Inch Rod (X2)

observation was the fact that the largest grains looked as if they might be made up of small grains. Evidence of cracks could be seen within some of the larger grains. Considerable relief polishing was noted with the material. The material between the grains seems to be polished away more than a t the center of the grains, which would indicate that the boundary material is softer. If it is assumed that the less crystalline material surrounds the grain, it would follow that the material with the smaller grain would contain less crystalline material. A comparison of the intensities of diffraction lines from the larger and smaller masses seems to indicate more crystallinity in the former. Another bid of evidence in favor of greater amount of crystallinity in the larger pieces is given by the density. Hardness values seem to indicate that the less crystalline material is softer, which nicely accounts for the relief polishing observed in polishing the specimens. Bunn has stated that the opaque character of unoriented nylon is due to optical discontinuities which are caused by the boundaries of spherulitic aggregates of crystals. This spherulitic structure has been observed in thin film by an examination b e tween crossed Nicols. Apparently the structure observed in this present investigation is the result of a very large aggregation of these small spherulites, in full agreement with a recent investigation on spherulite formation (14).

INDUSTRIAL AND ENGINEERING CHEMISTRY

836

Vol. 42, No. 5 fibers. The difference which was found between sizes probably does not indicate much tlifference in the amount of amorphous m a t e r i a l found in different sized masses. A a l i g h t 1 y 1 o PI' e r density was found for the strip material, which may indicate a slightly greater amount oP amorphous material. SUMMARY

Figure 9.

Studies have been made of massivo nylon ranging in size from 0.011.inch strip to &inch rod. An x-ray investigation was made using the Geiger spectrometer and filii3 methods. Hardness measurements were made with a superficial Rockwell and a Tukon tester. A microscopic study of tho polished specimens was made using incident light, and densitv measurements were made with a pycnometer. For nylon as received the position of the peak from the 100 plane remained relatively constant, varying approximately 0.05 A. between the two extreme sizes studied; whereas for the other peak, arising from the 010 and the 110 planes, the d value varied from 3.92 A. for the least massive to 3.76 A. for the most massive sections. The faster the cooling rate the greater the value of ae of the 100 reflection and the smaller the value of 28 from the I10 and 010 planes. When the thin strip material, such as the 0.028-inch strip, waj: heated in an oil bath uu to 235°C.. the aeak caused bv diffraction from the 110 and 010 ;lanes shiftid in ;elation to thc ot'her peak and both peaks became sharper, being greater the higher the temperature. The change is rapid and remains permanent. I t would seem from the range of differences in 28 between the peaks found in the material as received and after heating that a, gradual movement of the chains in relation to each other must be possible. This may mean that there is a permissible variation in the dimensions of Bunn's unit cell rather than a change from the 6 t o the 01 form. The effect of heat,ing in various media was determined by heating 0.028-inch strip in oil, water, air, and silicone fluid a t the same temperature. There was considerable difference in the amount of shifting taking place, with the greatest amount after heating iu water. Hardness readings made on the different sizes of nylon as received by both methods of testing indicated, in general, that tho material with the initially closer spacing is softer. Heating the strip nylon caused a considerable increase in hardness. The final hardness obtained, like the amount of shifting of the peaks, is dependent upon the annealing temperature. Again there is a ver,y good correlation between the lack of a change in the hardness and spacing when heating the larger masses such as the 2-inch rod. The hardness did not change appreciably on heating a t 100" C~ in oil, silicone fluid, or air, but the material became much softcr

Nylon after Heating 7 Days at 200" C. (X100) Left. 1.25-inch rod R i g h t . 0.060-inch strip

STRUCTURE O F LARGh SIXGLE GRAIX. Sfter observing these large grains in some of the specimens it was of interest to determine whether the arrangement of the unit cells was similar to that found uithin the grain obseived in metals. In order to determine the orientation a large grain w-nb located near the center of a longitudinal section of 7/8-inch rod (Figure 11). A thin section, approximately 0.002 inch, was then prepared a t this point. This portion was then positioned, by means of a microscope, in front of a 0.010-inch pinhole. An x-ray beam was passed through the pinhole and the pattern recorded on a film a t a distance of 5.14 cm from the sample. Should this large grain be similar to that found in metals, a series of Laue spots could be expected. However, because fairly smooth rings (Figure 11) were observed it is certainly an indication that there are within this grain a number of randomly oriented crystallites with a slight amount of preferred orientation. It is definitely not a single grain in the sense that exists in metals. EFFECT OF HEATING ON GRAINSIZE. The grain size of the nylon plastic seems to be a function of the cooling rate from the molten state and cannot be changed upon heating. Even the small grain size exhibited by the rapidly cooled specimens showed no change or increased size nhen heated a t 235" C. However, this is not surpiising. In the case of metals it is necessary to have a phase change or prior cold work in order to obtain a change in grain size. Because apparently there is no phase change and in moit samples studied there was no cold work, no change would be expected t o take place. In addition to this, the grains in nylon ale different from those in metals. DENSITY

Bunn has calculated the theoretical density of nylon according to his 01 unit cell as 1.24. The density a t 25" C. of fibers of this material has been reported as 1.1339 (undrawn), 1.1384 (drawn), and 1.156 (annealed) (6). This lower value can be accounted for by the presence of amorphous material along with the erystallinr material. The density of the nylon plastic was investigated to determine if there was any great difference in density of the various size pieces. Density determinations in this investigation were made by means of a pycnometer. In Table I11 the density values, as determined for the various sizes, are given. The results of the determination indicate that the density of large sections of nylon is approximately the same as that found in

TABLE

111. DENSITYO F VARIOUS Size, Inches strip strip 4 X 10 slab 4 X 10 slab X 2 X 10 slab

0.028 0.060 '/a X 11s X 1/4 >/a

7);

;,rod a

1 rod (center)

2 rod

3 rod

SIZES O F X Y L O N h A S ' I ' I C

Density 1,138 1,138 1,142 1,140 1.172 1.155 1.161 1,145 1,148 1,145

May 1950

'

83'1

INDUSTRIAL AND ENGINEERING CHEMISTRY

A slightly lower density was found for the thin strip material than for massive rods, which may indicate a slightly greater amount of amorphous material in the more rapidly cooled nylon. This investigation has indicated there are some entirely unsuspected structural phenomena in polymers, even as common and apparently well known as nylon, as functions, of mass, previous Figure 10. Grain Structure of Portion of '/s-Inch Rod (X100) history, h e a t L e f t . Central portion treatment, agR i g h t . Outer portion i n g , gold a n d hot work, imafter heating in water. This change was not permanent and can purities, etc. A similar separate study therefore is required for be explained by a plasticizing effect. each type of polymer. On the basis of this information A heat-etching effect brings out the flow of the material which certainly nylon in massive form for gears and a variety of took place during fabrication. This heating also makes it possiother uses may be used more intelligently and performance ble to observe a grainlike structure in the material. After the may be more adequately predicted and controlled. metallurgical polishing technique was applied to the specimens of nylon the grain structure became visible, after standing in air for a few minutes, without etching, In general, the smallest grains LITERATURE CITED appeared in the thin sections and the outermost portions of specimens of larger sizes and the largest grains appeared in the (1) ~ m SOU. . Testlng Materials, Bull. 138 (January 1946). center of the s ecimens of maximum dimensions, Some of these ( 2 ) Anon., C%m. E%'. New81 26, 1928 (1948). larger grains $owed evidence of internal cracks. The size of (3) Baker, W. O., and Fuller, C. S., J . Am. Chem. Soc., 64, 2396 the grain is believed to be the result ot the cooling rate. These (1942). grains are made evident partially by a relief polishing in which the material surrounding the grains is polished away more (4) Baker, W. O., and Smyth, C. P., Ibid., 60, 1229 (1938). rapidly, which is probably due to a smaller degree of Crystallinity (5) Black, c. E., and Dole, M., J . Polymer Sei., 3, 358 (1948). and hence softer material. Even the small grain size exhibited (6) Brill, R., J. prakt. Chem., 161, 49 (1942). by the rapidly cooled specimens showed no change when heated (7) Bunn, C. W., Proc. R o y . SOC.(London), A180, 40 (1942). up to 235" C. (8) Bunn, C. W., and Gamer, E. V., Ibid., A189,39 (1947). When an x-ray beam was allowed to pass through a single large (9) Ecochard, F., J . chim. phys., 43, 113 (1946). grain the pattern obtained had two rings of very uniform intensity, indicating practically complete random orientation. (10) Fuller, C. S.,IND.ENQ.CREM.,30, 472 (1938). Hence these graills are probably aggregates of spherulites and (11) Fuller, C. S., Baker, W. O., and Pape, N. R.,J . ~ mChem. . SOC., different from the grains found in metals. 62,3275 (1940). Recrystallization after cold working was confirmed by x-ray (12) Fuller, C. S.,and Frosch, C . J., Ibzd., 61, 2575 (1939). arid inirroscopir methods. (13) Graves, G. D. (toE. I. du Pontde Nemours & Co.), U. S. Patent 2,212,772 (Aug. 27,1941). (14) Langkammerer, C. M., and E., J. Polymer Catlin, R7. Sci., 3, 305 (1948). (15) Lysaght, V. E., Materials and Methods, 22, 1079 (1945). (le) Ibid., 2 7 , 8 4 (1948).

n

(17) Sohoon, T., 2. physik. Chem., 39B, 385 (1938). (18) Tarasov, L. P., and Thibault, N. W., Am. SOC. Metals Trans., 38, 331 (1947). (19) Thibault, N. W., and Ny-

quist, H. I,., Ibid., 38, 271 (1947).

Figure 11. Large Grain in '/*-Inch Rod Right.

X-ray pattern of grain on left showing it to be not a single crystal but spherulitio aggregate

-

RECEIVEDAugust 5 , 1949. Presented at the Symposium on the Solid State, before the Division of Physical end Inorganic Chemistry, AMERICAN CHEMICAL SOCIETY, Pittsburgh, Pa., June 22, 1949.