R.
M. EICHHORN
Development Laboratories, Union Carbide Plastics Co., Bound Brook, N. J.
An Op ical M e
hod f o r .
..
Measuring Uniformity in Plastics Automatic measurements of uniformity and computation of uniformity indexes are made, using a simple analog computer and a microdensitometer
To
MEASURE r f i I F o R 5 r I T Y in multicomponent systems. a property should be selected, which varies with degree of mixing. The chosen property may be studied by three methods: A fairly large specimen can be examined-i.e.> a whole batch or a final product item; variance among many small specimens from the mixture can be evaluated; and minimum homogeneous volume within the system can be determined. (The minimum homogeneous volume is the smallest volume n.hich can be selected anyLvhere in the mixture and always have the same composition as the mixture.) In plastics. uniformity should be measured in the final product because each step in production involves some flow and shear. In a multicomponent system, shear involves redistribution of components, or mixing.
Method of Measurement
Optical absorbance is sensitive to uniformity of mixing in colored plastics. For example. consider a t\vo component system in tvhich one component is opaque and the other transparent. \Vhen mixing starts there are opaque regions in the system where the opaque component is concentrated and transparent regions where the transparent component is concentrated. As mixing progresses the components are distributed more and inore evenly throughout
Literature Bockground Subject Measurement of over-all properties to characterize degree of mixing Theoretical discussion of statistical uniformity indexes Practical applications of statistical uniformity indexes Use of minimum homogeneous volume as uniformity index
Ref. (11)
the system and the absorbance approaches a value determined by the optical density of each component and their relative concentrations. The method described here for studying and quantitatively characterizing the uniformity of such systems is b p e d on measurements of the uniformity of absorbance a t many points in the mixture. A specimen prepared in the form of a film or thin plaque is mounted on a microdensitometer so that a finelv
The specimen is rotated about an axis parallel to the optic axis of the microdensitometer
(7)
Optical System
The microdensitometer used is similar to that of Altman and Stultz (7) \vho. hoivever, obtained greater resolution c 2
< L:SOURCE L A M P CI: FIRST CONDENSER MI: ILLUMINATING MICROSCOPE S: S P E C I M E N
focussed light beam passes through it. T h e specimen is then scanned by moving it in a direction normal to the light path. This motion is equivalent to that of a flying spot scanner ( 4 ) . Thus. variation in absorbance modulates the intensity of the transmitted beam. This varying light signal is converted to a voltage signal by a photomultiplier tube. Fed
M 'PICKUP MICROSCOPE 2' CZSECON D CONDENSER
P: PHOTOMULTIPLIER
The specimen is positioned so that it includes the point of principal focus (PPF) of the optical system. The size of this spot can be varied but is nominally 0.1 mm. -4fter passing through the system. the light beam, diminished in intensity, is focussed on the cathode of a photomultiplier tube. To obtain the point-to-point variation in absorbance,
Example A common and difficult problem in the manufacture of plastics is the characterization of uniformity and dispersion in the carbon black-polyethylene system. This material is used as an outdoor protective coating and its useful service life is directly related to the protective properties of the carbon black dispersion. Poor coatings give little protection. A series of samples have been evaluated by the microscopic rating system used by Bell Telephone Laboratories ( 7 4 ) and the microdensitometer technique, Results obtained by the methods agree. 1A
IB
2
3
4
5A
5B
G
Microscope rating B+ Microdensitometer uniformity index* 2 . 1
B+
B+
B+
C
D
D
D
2.2
2.9
2.9
3.5
4.4
4.5
6.5
Sample No.= ( 3 , 8 . 9. 12. 13) (6,10, 15)
into a n electronic circuit, the uniformity computer, the voltage variation is analyzed into alternating current and direct current components which are used to compute a statistical uniformity indes.
*
l , A and B, and 5,A and B, are duplicate specimens. Uniformity index for themicrodensitometer is more quantitative and reliable because samples used were larger.
VOL. 53, NO. 1
JANUARY 1961
67
the alternating current and direct current components of the signal which are identifiable with its standard deviation and mean value. and reads them out on two meters. The two meter readings are used to specify uniformity index. The Uniformity Computer
In Figure 2, meter MI is a D'Arsonval movement which indicates the average value of the input current as
>-
! I:
t1:
I-
where T is the period of the signal (the time for one revolution), and z(l) is the photomultiplier tube current. The voltage drop across the load resistor R can be fed directly into a current amplifier. nominal gain of 100, or the high frequency components may be bypassed by condenser C. The amplifier detects and amplifies the alternating current variation in the voltage across R i t l ~is a thermocouple-type ammeter indicating root-mean-square (RhlS) value of a n alternating current signal:
k-Z -
I c3
-I
w
-I
W
e
or the standard deviation of an alternating current signal from its mean after the direct current component is separated : Figure 1. Here, recorder traces show the variation in light transmission through well mixed and poorly mixed d r y colored polystyrene Slow scanning speed with higll rpeed potentiometer
R.C .4.931 mounted in a cylindrical
the specimen, Mhich has a hole drilled or punched through its center, is rotated about a n axis parallel to the optic axis
shield with a small hole facing the photocathode.
2x2
X
X
A simple circuit provided the power needed for the photomultiplier
3
2 > "4
y
of the microdensitometer. The rotation is accomplished by a small motor. In this way, all points in a circular path of width 0.1 mm. are scanned and recorded. Optional optical elements include polarizer and analyzer for detecting orientation or strain in the specimen, monochromatizine: filters. and a neutral density filter wedie to limit the light intensity to a value below that which causes photomultiplier tube fatigue. The illuminator power supply is a storage battery continuously charged during operation to provide a stable, ripple-free voltage output. The photomultiplier tube is an
68
INDUSTRIAL AND ENGINEERING CHEMISTRY
Mixing Pattern Produced
The voltage output from the photomultiplier tube is a linear function of the transmission through the specimen. Thus the voltage signal has the form of the inverse of the absorbance variation along the path scanned. I n Figure 1 the curves have the form of random frequency noise. As the noise signal, although of random frequency, is regularly repeated with a period of one specimen revolution, it can be analyzed by conventional electronic techniques. A simple, special purpose analogue computer operates to separate
Meters M i and Mz read out values o f f and I,, respectively. The frequency response characteristic of the mixing computer circuit is reasonably flat from 10 to 600,000 cycles per second and the low frequency cut off is quite sharp. As the frequency response does not extend from zero to infinity, there are size limits on the inhomogeneities to which the apparatus responds. Simple calculation shows that at a scanning rate of 1 revolution per second, with path radius 2 inches, discrete and opaque particles between 2 times 10-5 and 1.25 inches in size can be detected. Obviously this calculated small size limit
f C
0 I
a
Figure 2. Main parts of the mixing computer. Voltage drop across the load resistor, I?, can be fed directly into a current amplifier, with a nominal gain of 100, or high frequency components may be bypassed b y condenser C
UNIFORMITY IN P L A S T I C S 6x4
CHICAGO
FULL SCALE
Complete circuit for the mixing computer. By increasing specimen rotation rate to 5 revolutions per second, the computer can respond i o any extended variation, even the most severe case which i s one-dimensional change in absorbance across the specimen
is meaningless and the true niinimum size restriction is imposed by the size of the scanning spot rather than the computer frequency response. The 1.25-inch maximum size can he increased, if inadequate: by faster rotation. The range in particle size to which the computer responds can be expressed in terms of the path radius r, the angular velocity a in revolutions per second, the frequency response limits ,fmin and fm,,,. and the size of the light spot S as follo\vs 2~i-u~
__ - S .fm,
