FIGURE 1. TRASSPAREXT PLASTIC COVERIXG 07 TCRRET A S D COCKPIT OF .i ?V’IARTIS B O M B E R
nation of light transmission, haziness, scratch resistance, and indentation hardness of plastics. Data on the properties of various types of plastics obtained by the use of these methods are presented. The desirability of establishing uniform procedures for testing plastics in order to permit better correlation of test results from different sources is indicated.
Methods of
Testing Plastics
T
HE clarity and scratch resistance of transparent plastics are of prime importance in connection with their use as windshields on aircraft (Figure I). In a study of these materials now under way a t the Kational Bureau of Standards in cooperation with the Kational Advisory Conimittee for Aeronautics (Z) , special methods have been deyeloped for the measurement oi these properties. These methods and some of the test results obtained on various plastics are presented in this paper. I n order to show the relation between the scratch resistance and the more familiar indentation hardness, data on this latter property are also included. Clarity of Plastics The clarity of plastics is generally considered to be dependent upon two factors, transmission and scattering of light by particles or by surface imperfections. The military services generally specify that the plastic sheet shall transmit at least 68 per cent of white light when tested within 30 days of the date of manufacture and shall be free from wrinkles, bubbles, scratches, pits, or depressions. The light transmission is usually measured with a visual or photoelectric photometer, in the latter case with a photoelectric cell which is sensitive mainly to the visible portion of the spectrum. The transparent plastics now in use generally transmit more than 85 per cent of the incident light. The haziness has been measured by several methods, including visual inspection with or without a background of light from Cooper-Hewitt lamps, comparison of the plastic sheet with samples of varying degrees of haziness, attempts to count the ‘ U a r dust” visible in a small
G.M.KLINE A N D B. M.AXILROD National Bureau of Standards, Washington, D. C .
area under the microscope, and deteririination of a small portion of the light’scattered when the sample is placed at a suitable angle with relation to the light source and photometer. Akhough the latter method has the desirable feature of utilizing one inst’runient for measuring both light transmissicin and haziness, it presents some difficulty because of the posxibility of introducing considerable error in measuring only a small portion of the scattered light. A photometer of simple construction, called the “hazeoiiieter,” which can be used effectively to measure the haziness (Jf a plastic as well as the light transmission, was developed in the course of this investigation.
Description of Hazeometer A photograph of the photometer (built for us by E. E. Box-en, Bethesda, Maryland), is shown in Figure 2 : A &volt automobile-type bulb is mounted in front of a reflector. A cylindrical shield, blackened inside and with a circular aperture b in the center of the base, causes an approximately parallel beam of light to be radiated toward the photoelectric cell. A blackened disk with a circular aperture, a, is mounted in front of the photoelectric cell. In the instrument as built for experimental purposes the two openings, a and b, are each 1 inch (2.5 em.) in diameter and are spaced 18 inches (45.7 em.) apart. The 1170
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photoelectiic cell is of the We-ton pliotionic t>pe.l The meter has an inteinal resistance of 50 ohms and a iange of 100 microamperes. Spiing clamps aie fa.tened to the frame at each aperture to hold the plastic in place. -46-volt storage battery is used to operate the lamp. All the paitq aie housed within a Rooden box finished in a dull black nhich ieduceh ieflections to a minimum. The lid of t h e box is closed tiuiing meaqurements to keep rays nf light fiom outiidr wuice. fiom ieacliing the photoelectric cell
1171
TABLEI. LIGHT TRANSMISSION ASD HAZIXESS VALUES OF TR.,O~SP.AREST CELLULOSE ACETATE P L A S T I C S
Saniple So.
Test Procedure a n d Definitions
Thickne%
Dye
Light Transmission Sample 18 i n 3ample a t from photophotoelectric: electric cell
To obtain a measure of the light transmission of the plastic. the light is adjusted in intensity so that a current of 100 microamperes is obtained from the photoelectric cell. A flat sample is then placed in front of the cell at aperture a. Of the light incident on the sample, a fraction is transmitted undeviated, and a fraction is scattered by the surfaces and the interior of the sample. The photoelectric cell receives the unde-
7
1 5 9
53 67 68 65 67 67 85 31
15
35
16 3
35 105 105
4
Hlue Illue Sone Sone None None Sone Xone Blue Blue Blue Blue Blue
*53
Haziness Value
ilj
Mds 13 14 8 17 10
Differ.
7%
70
86.3 86.2 90.7 91.2 91 0 90.7 90.7 89.7 4.2 86 7 86.7
83.9 83.7 87.6 87.5 85.8 85 3 85.2 84.0 3 . ax 78.4 77.6 64.3 63 S
75.0 75.1
% 2.4
2.8 2.9
2.5 :3 1 1 7 5.2 5.4
:3 4 4.1
0.37 8.3 9.1 10.7
6 1 6.4 8.8 9.6 10.5 14.3 15.0
;,;
ll..?J
.
5.7 6 0
ante in the microammeter used. Haziness is defined ab the percentage of the total light transmitted a t position a which is *tattered when the sample is placed a t position b.
Results Results obtained in tests of cellulose acetate plastics with the hazeometer are shown in Table I. The light transmission icolumii A ) of the samples containing blue dye is in all cases less than the materials free from dye. Because of this pronounced effect on light transmission, the practice of adding blue dye to cellulose acetate transparent sheet has been discontinued by some manufacturers. For undyed materials approaching the transparency of polished glass, most of the loss is due to reflection a t the two surfaces and can be calculated from the refractive index. It might appear that the relative haziness of a sample could be judged merely by noting the difference between the light transmission at positions a and b in the hazeometer (column Ai - B ) . However, the haziness in a material which has a
FIGCRE 2. HAZEOMETER
viated fraction of the light and that part of the scattered light which is not deflected at angles greater than about 90". The light transmission of the plastic is defined as the fraction of the original light received by the photoelectric cell which continues to reach the sensitive element when the plastic is placed over the aperture a a t the photoelectric cell. To obtain a measure of the haziness of the plastic, the sample is placed a t aperture b, 18 inches distant from aperture a. The photoelectric cell still receives the undeviated fraction of the light but collects only that small portion of the scattered light which is confined to the small solid angles subtended by aperture a a t points in opening b. The difference between the photoelectric current with the sample a t a and a t b is a measure of the light scattered, assuming that a linear relation exists between the photoelectric current and the total light incident upon the sensitive element of the cell. The current is practically proportional to the tot'al incident light for no external r e s i s t a n c e ; the deviation is onlv slight for the 50-ohm resistT h e spectral response of this cell deviates considerably from t h e luminosity curve of the eye, a n d hence the light transmis8ion obtained is not t h e same as would be measured with a visual photometer b u t is a c l o s ~approximstion t o such value for aamples with little or no dye. 4 filter can be obtained from t h e manufacturer which will approximately correct the responae of the cell t o t h a t of the eye.
FIGURE
3.
BIERBAGY
SCR4TCH-
RESISTANCE APPARATUS Above.
Diamond tool
qiviine
&de
and
VOL. 28, 30. 10
1172
niuunted so that the diagonal oi the cube i s noriiial to the test surface and one edge is in line with the direction of the scratch. The diam01~11is suspended from a balanced am pivoted on sappliire bearings. One end of the arm is provided wibli a spirit level and t.he ot.her with a 3-gram weight which constitutes the standard load for the dianiond point. The diamond is mounted a t t,lre small end of a tapered steel spring; the other end of the spring is fastened to the lower side of the suspension area. A vertical rack is provided for raisiiig and lowering the hracket that carries the cutting t,ool. This bracket. is attnched t o a microscope inechanical stage pnivided with suitable clamps and scrcw niechanism t o permit the specimen to he moved slowly under the diamond point. The width of the scratch (Figure 4) is measured with a microscope having a calibrated filar micrometer eyepiece. The plastics were conditioned at 21’ C. and 65 per cent relative humidity for at least 24 hours, and the test was conducted UIIder these same conditions. The scratch resistance is reported iiuinerically as the quotient of the loWt in kilograms divided by thesquare of thercratch a.idt,h in millimeters.
Results
FIOUI
