OPTICAL MICROSCOPE MEASUREMENT OF THE SIZE OF PARTICLES OF CARBON IN RUBBER

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-04861A000400030017-8
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 12
• Document Creation Date: December 20, 2016
• Document Release Date: June 6, 2006
• Sequence Number: 17
• Publication Date: July 1, 1955
• Content Type: REPORT

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OPTICAL, MICROSCOPE MEASUREMENT OF THE
SIZE OF PARTICLES OF CARBON IN RUBBER
Kolloid Zeitschrift 110 (1948) 125 - 132
(From German)
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Optical Mi_crocone_IVIeasurement of the Size
of Par titles of Carbon in Rubber
J. Kruse
holloid Zeitschxift 110 (191,_8) uI2 -1, 2.
(From German)
Carbon black, dispersed in a tr_nsparent medium has - if the
concentration and. the thickness .of the layer are right - the
property of modifying the light passing through it in a characteristic
way. If, for example, rubber and carbon black mixtures are
preoarcd for examination according to the recomnend.ation of
H. P01-12 (1 ), i.e. about 2l by weight carbon black in rubber, pr--s _. ed
out thinly between the slide and the cover glass, the intensity of
colour of a carbon black can be seen by comparison, even with the
naked. eye by transmitted light. Looked at in this way highly
active blacks look brownish grey and inactive ones bluish grey.
It is very much easier to recognize definite kinds of blacks
prepared. in the same way under the microscope with high magnification
in the briht field and it is possible to see directly fairly large
particles of carbon black as well as to distinguish the colour,
In this way it is possible, by comparison with standard specilensas
to colour and characteristic shape of the particle, to recognize
most of the carbon blacks accordina to kind, or at least type.
In order to carry out more exact m.easure?ients of carbon particles
the author has used a method using the o;,Aical miscroscope, which
-Trill be briefly explained.
The smhoto',..,rc .f1 is to reproduce all the
p .rticle in iven zone of depth. Ab by ap.;ropriatc exposure
tunes so that the, appear on the photographic plate in a
concer,"-'tration low enough to permit satisf dory courting. To
analyze such ho Logra-i.h it is necessary to know the
magnification ,the carbon black concentration by volume per cent,
and tic value Qb. Every pE1rticle is countCd, irr spective of
the sharpness of definition of the image, provided it can be
recognized as such. This proce>s depends on the accurate
measurement of. Ob. In this connection it is helpful to kno-, how
far from the focal ,plane the carbon granules can:be:.and still be
x epr?oL +uuccd in the ho-tograph. A moment's reflection sill ;ho,;v
at once that all -L 1e particles cannot be evaluated alike because
the s smallest particles, i.e. those of lo;rruut light intensity
c._nnot be,: seen ;so far from the focal plane as larger ones, i.e.
the brighter particles. In the first case. the light intensity
of the dispersion rin drops more rapidly below the exposure
factor of the photographic plate - owing to the lack of focus -
than in the latter case. This situation is most clearly demonstrated
b7, an experiment u sin a carbon black preparation in which the
particles a.rticle will be reproduced but that further off only the brighter
ones , because hero the light intensity of the small. p articles,
being distributed over- a grater dispersion ring, no longer
reaches the exposure factor of the photographic plate.
The d-:cli;: ,e in definition up,,a:-nrds and downwards is not
symmetrical as i immedi=ately apparent from the laws of geometric
optics, For a1na:,lytl.cal :puraoo;su the photograph in Figure, 10 was
divided up into parallel strips, cor?res ondi . to a width in the
specimen of 5[L, and parallel to the focal lint;. The carbon
particles appearing on Lhese strips were counted and plotted
r ti hie, as as shown in figure 11.
Then determining the de ipth 6b to be photo : r ph d ~,7vith the
pr: t. ,r .lion pL-ced at an angle, it. is ?s,3urieci that the coacher of
particles actually present L ii ca.ch stria; h idth is, on the average,
the same and also ecn?al to the number obtained for the strip in
wr-~ich t,:' definition is gr eea"sest (B'ia ure 10 - stria) No. 4, number
of particles 150). The drop in the number of particles to the
left enc. ri,,ht i;, thus due only to the increasIn,= lack of definition
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in the photogrwe, h. As, unfortunately, it -,as not possible to
continue the photograph until the declining particle-count reached
zero, the missing values were obtained hh,y graphic extrapolation
(Figure 11), There is, thus, a reconstruction of the total of all the
particles which would appear if the photo ;r--,,ph in Figure 12 is imagined
to be extended to the left the right and. this sum 1055 is
divided by the average number of particles in each strip 150.
The result
7, indicates that, on the whole, th:;re are as many
particles reproduced as thc:re are in 7 ;Arips, In other words the
same result is obtained by counting all the particles which are
reproduced, taping into account the increasing lack of definition,
and by counting all the T articles in 7 ;3tri-os. Thus -one shall not go
wrong if, ,,when considering, a series of Thotograp>hic counting plates
(Figu.rel 2-16), we put t ',-e total number of particles counted equal to
that number of p~~ ticles actually fotuid to e;:ist in a space whose
depth is, as shown in Figure 10, the difference in depth of '7 strips.
This diff sror~oe can be calculated from the known inclination
a:b 1.4-:21.5 (Figures 9 and 10) giving 2.2 . 101-4 cm. This would
yield a value for the length Ab which would make possible the
calculation of the average particle volume on the basis of the
photographic pl~..e count.
The y1hhotoL,r?a-ohs 12-17 show _c.ho;;o -rayPhs with an increasing
carbon content. It ai:)Dcars from these that the rubber material in
which the black is dispersed itself contains foreigh bodies so that
the nurniber of the particles photo,,ra?-)hcd with a zero carbon concentration
does not drop to zero. With a few deviations there appears to be a
linear relation between the number of particles counted and the
cohcentrat=i'on - illustrated in Figure 18 - always keeping ,within the
range of small carbon concentrations. It is self-evident that when
evaluating photographic count -plates the, zero value must be deducted.
The photograph shorn in Figure 17, with a concentration of 0.1i
carbon black Inca in crepe, is not suitable for a quantitative
determination of particle sizes because the concentration is far too
high and it would be quite impossible to count all the particles
owing to the accumulations rich they form.
The actual evaluation is performed quite simply according to
the forcula - ,
F 6b . Vol. Vo
100 ' n
-;There Vm is the average particle volume
6b is the depth of the photograph as described above
Vol./., is the volume of carbon in the whole mixture
n is the number of carrion particles counted in the pkoto_raphed
surface F,
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Example: (evaluation of Figure 16
F 1215 . 10-8cm2
6b = 2.2 , 10 -4'cm
VD1,% = 3.09 . 10-5 (corresponds to 0,006 weight in rubber)
n = 102 (Figure 16 shows 156 particles minus the 54
particles counted in Figure 12 under the
same conditions without carbon black)
Vm = 8.10-16cm3
This result is based on the dispersion of carbon black in
rubber and it will be understood that such a number can only be obtained
for the particle volume which would correspond to an optically
perceptible dispersion in the rubber. Hence, if there should be, in
the rubber, particles of carbon formed, perh:~ps, by the conglomeration
of single p:.r.ticles at one spot - smaller than the resolving limit of
the microscope - a result mu;_t be expected which corresponds to the
volume; of the conglomeration as a whole rather than the particle
volume. Another important factor is the concentration of the
carbon-rubber mixture. If this is too high the points of light which.
have to be counted lie too close together so that the unavoidable
halations of the larger particles make an accurate count impossible
(Example Figure 17). It must also be noticed that in addition to the
light from. the particles which can be counted., light will be diffracted
into the objective from particles lying above and below those
actually to be reproduced. This stray light causes the dark field
to become brighter with the result that the particles of low light
intensity. no longer stand out from the background, with sufficient
clarity.:. It must therefore be expected. that if there is too high a
concentration of carbon false results will be obtained indicating too
great a volume, because all the particles will not have been included
in the count, The author's experience shows that good results can
always be expected if the concentration is such that in a well
maintained dark field there are no more than 500 particles in an area
of approximately 2000 , 10-8cm2 and a depth of Lb(magnification
2000 : 1). On the other hand it is not advisable to have fewer than
100 particles in this same space because this would affect the desired
average value of probabilities of particle accumulation to too great an
extent. There is doubtless a personal factor involved in the counting
of the photographs, but this would be partly compensated by the fact
that the same person would be measuring the value of the depth 6b by
counting. Another inaccuracy arises from the extrapolation
(Figure 11) which, howarever, remains within narrow limits in relation to
the total result because t;e decisive factor in calculation is the
centre of the picture which his actually been observed and the
extrapolation is confined to the cd;,,s where the numerical significance
is slighter. It should also be mentioned that the adjustment of the
microscope must be uniform, particularly in the case of the dark field.
After considering all these factors the author thinks that errors due to
procedure should not cause deviations of more than 5M in the final
result. In the large majority of cases the results were contained
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within much narrower limits. At all events errors arising from
procedure are small by comparison with the diff er-nces caused by
differing levels of activity of the carbon blacks so that this can
still be regarded as a successful method for the characterization of
the technically interc;_3ting carbon-containing rubber's.
It will be particularly interesting now,to compare the results of
measurement made by the above procedure with those based on electron-
optical measurements and photographs. It must be borne in mind
while doing so that, on the basis of statistical measurements, the
procedure here described Lives the average volume of the particles
and gives no information about the shape of the particle. Electron-
optical methods on the other hand, yield highly magnified
"silhouettes" of the carbon particles from which much can be learnt
about the shape - but less about the volume - of the particles.
"There is a difficulty in connection with the electron-optical
photographs in that the results do not indicate the numerical relation
which the particles bear to each other and, as they show fairly large
differences in size it is difficult to gain a picture of the average
size, In order to obtain some guidance on this the author measured
the carbon particles shown in electron photographs and converted them
into volumes of the smallest and largest particles, using the, :Method
indicated. The results are given in Table I.
Comparison of the photo-optical results with those from the
electron pictures (as described in the previous paragraph) shows a
good measure of conformity in the case of the carbon black CK 4.
It is important to note here that this carbon consists entirely of
a highly active component .as is apparent from observation by
photo-optics in the bright field. This is not quite the case for
the black P 1250. There is no doubt that this carbon black consists
mainly of particles corresponding to the electron photographs
quoted. But it also contains other, larger particles - as can be
clearly seen, even with the bright field illumination of the photo-
optical microscope - which, obviously present to a smaller extent,
could not be seen with the electron photographs. . With the statistical
measurement of the procedure described it can be understood, therefore,
that the particle-volume iicasured photo-optically is a little higher
than would be expected from the electron photograph. On the other
hand, in the case of Inca black, the electron photograph also consists
of two distinct components, differing considerably from each: other,
and it is very difficult to gain any idea of the average volume of
the particles from the electron picture alone. Considered purely
from the aspect of rubber technology the photo-optical result seems
satisfactory, even when compared with the other carbon blacks,
With LW36, ho*vever, there is quite definitely a disparity between
the results from the two procedures. The smaller particles reproduced
in the electron picture, considered as spheres, agree with the photo-
optical values, but the larger particles exceed them by a considerable
amount. It is not possible here to answer the question whether perhaps
the black under investigation was not quite the same in both cases or
whether the shape is not spherical. All that need be said is that the
photo-optical result is satisfactory inasmuch as it conforms well with
the impression obtained by observing this carbon black under the optical
microscope in the bright field, particularly by comparison with P 1250
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and "Elan cic". In the case of flame-carbon black "Elastic" there
is reasonable agreement between the results from both procedures
provided the particles which cannot possibly be regarded as
spherical are thought of as cylinders or as a coherent, elongated
aggregation of spheres (similar to the inactive component of
Inca black:). They arc then of an order of magnitude which can
be directly resolved to some extent iith the optical microscope
the same impression of the shape is then obtained as in the case
of the electron photograph already mentioned.
A process is described which makes possible the measurements
of commercial rubber carbon blacks by means of the optical
microscope and gives the average volume of the carbon particles.
The process provides for the photographing of the carbon
particles dispersed in the rubber at high magnification on a dark
gro wed and for counting from the photographic plate. The
thickness of the photographed section is ascertained separately
by photographing at an angle a preparation 1with.particles of the
same oarbon black dispersed in one plane. The average volume of
the particles was then calculated from the thickness, the surface
count and the concentration by applying; the usual ultra-microscopic
f'ormulg.
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Type of
carbon black
Average
Volume Vm
photo-optic
0.3x10-16cm3
P 1250 5.10-16cma
8.10-16 cm
10x10-6cm width
Luv 36 23x10-16cm 10x10~6cm diameter
of smallest particles
40x10-6 cm diameter
of largest particles
Dimensions of
particles (3)
(electron-.optically)
2.5x10 'cm diameter
of smallest particles
5.Ox10-6cm diameter of
largest particles
5.7x10-6cm diameter
of smallest particles
10.Ox10-6cm diameter
largest particles
4.otive component:
1.5x10-6 cm diameter
of smallest particles
7.Ox10-6cm diameter
of largest particles
Inactive component:
elongated form
80x10 cm length
Elastic
126x10-16cm3
Elongated form
66x10-6 cm length
16x10-6 cm width
Volume calculated on
basis of electron-
optical measurements.
Spherical shape
0.08x10-16cma for
smallest particles
0.65x10-16cm3 for
largest particles
Spherical shape
0,97x10 -16cma for
smallest particles
5.25x10-16cm3 for
largest particles
Active component:
Spherical shape
0.018x10-1 6cm3 for
smallest particles
1.0x10-i6 cma for
largest particles
Inactive component:
As cylinder
14Ox10-16cm3
or as elongated,
coherent aggregation of
spheres with single
spheres of
17x10-16 cm
100to 150x10-16cm3
Spheri al shape
5.10-1 ~cm"I for
smallest particles
335x10-16cma for
largest particles
As cylinder
170x10-16cma
or as elongated,aggregation
of spheres
100 to 200x10-16cma
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M
b e
N
Figs.1-6: The photographs show the same area of a prepared film of rubber
containing 'Inca' carbon black. All the carbon black particles are
located in the objective plane. Magnification 2000:1. Objective, 1112
Leitz Fluorite, Ocular, Periplan 10x, Extension 46.5 cm. Arc lamp without
filter. Leitz dark field condenser D 1.20. Figs. I-5 Agfa Isochrome plate,
Fig.6 Monlalamp 5 amp. Leitz 2 stop condenser, Aperture stop fully open,
Green filter, Perutz silver eosin plate. Exposures Figure 1,2 seconds,
Figure 2,8 seconds, Figure 3, 32 seconds, Figure 4, 128 seconds, Figure 5,
512 seconds, Figure 6, 15 seconds. The magnification given is for the
original negative.
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Photographed particles
Exposure time seconds
Fig.7: Graph Illustrating the number
of particles reproduced In relation
to the exposure time. Inca black,
dark ground series (Figs.l-5)
Carbon black
particles J Qb depth of
dispersed in Optical axis 11 the microscope
the rubber imag
Thickness of f Cover glass
layer m 10/1 Ea',.. 4+. ,
Fig.8: Section illustrating the
arrangement for taking a photo-
graph for counting
Micro-objective
Immersion oil
Prepared carbon black film
/1 Pecimen holder thickness a
Fig.9: Section illustrating the method of
tilting the prepared film in order to
determine the depth of the microscope image
Fig.10: Plane film prepared for Inca carbon black
at a known inclination. Objective, ocular as in
Figs. 1-6, Inclination a:b - 1.34:21.5, Agfa
Isoehrome plate, exposure 8 minutes.
Magnification-2000:1.
Strips
Fig.ll: Graphic evaluation of Figure 10 showing
the number of particles with respect to successive
strips. The curve is extrapolated both to the
right and the left beyond the limits of the
photograph.
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0
Ftg.12: Counting plate for white crepe
without carbon black. Counted
particles 50.
Fig.14: Counting plate for white crepe
with 0.002% Inca carbon black. Counted
particles 67.
Fig.16: Counting plate for white crepe
with 0.006% Inca carbon black. Counted
particles 156.
Fig.13: Counting plate for white crepe
with 0.001% Inca carbon black. Counted
particles 75.
with 10. 4% Inca carbon fblack. white pe
particles 124.
1Inca Counting carbon plate blafor ck~htte crepe
with 0.1%
Figs.12-17: Microscope data. Exposure and plate material as for Figure S. The
carbon black concentrations given indicate that in each case the given percentage
of carbon black in grams was added to 100 grams of rubber. Counting was done
from the plate. It is to be expected that the large number of weak light spots
will not show up in the reproduction.
% concentration of carbon black in the rubber
Fig.18
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