Optical properties of plastics used for evaluating transparency

1. OPTICAL PROPERTIES

2.  Introduction
 Refractive Index
 Transmission & Haze
 Specular Gloss
 Colour
 Birefringence
 Clarity
 Photoelastic Property
CONTENTS
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3. INTRODUCTION
• The application of plastics is increasing day by day due to its unique
properties such as excellent clarity & transparency, good mechanical strength,
easy process-ability & low cost. The plastics are widely used in transparent &
translucent applications .The optical properties of a substance mainly depends
on the Molecular structure, morphological characteristics type and amount of
additives, processing conditions and post processing operations.
• Knowledge of these optical properties is needed for several reasons:
(a)Technical use is made of polymers as optical materials in lenses or light
fixtures.
(b)The relatively large change of refractive index with deformation
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4. Degree of transparency of commercial polymeric materials
may be classified according to the following:-
(a) Highly transparent materials
(b)Semitransparent materials
(c) Opaque materials
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5. REFRACTIVE INDEX
DEFINITION:-
It corresponds to the ratio of the speed of light in a vacuum (in practical terms
air) to its speed in the object (material) and can be calculated from the sine of the
angles of incidence (i) and refraction (r) beams, respectively.
n = sin i / sin r
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6. SIGNIFICANCE:
It is a fundamental property used for control the purity of the product and for optical
parts design. Its values are used in designing lenses for cameras, microscope & other
optical instruments.
TEST METHOD: -
* ASTM D 542
* ISO 489
SPECIMEN & CONDITIONING:-
• The size of the test specimen will correctly fit on the face of the fixed half of the
refractometer prisms. A specimen measuring 6.3 mm X 12.7 mm on one face is
usually satisfactory.
• Test conditioning at 23 + 2oC and 50 + 5% relative humidity.
REFRACTIVE INDEX
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7. EQUIPMENT:-
* The apparatus for this test method consists of an Abbe’ refractometer, a
suitable source of white light and a small quantity of a suitable contacting
liquid.
REFRACTIVE INDEX
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8. PROCEDURE:-
* Two procedures are normally used to measure the index of refraction of plastic
materials,one refractometer method and the other Microscopical method.
OBSERVATION, CALCULATION & RESULT:-
• The value of Refractive index is calculated by dividing the actual thickness of the
specimen to apparent thickness of the specimen.
• Index of refraction = Actual thickness of the specimen
Apparent thickness of the specimen
REFRACTIVE INDEX
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9. FACTOR INFLUENCING:-
• Medium used
• Wave length of light
• Temperature
SAFETY PRECAUTIONS:-
• Specimen surfaces must be parallel to each other and perpendicular to the
direction of orientation.
• Specimen must be free from dust & dirt.
REFRACTIVE INDEX
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11. DEFINITION:-
• Luminous transmittance is defined as the ratio of transmitted light to the
incident light. The value is generally reported in percentage of light
transmitted. Polymethyl methacrylate, for example, transmits 92 percent of
the normal incident light.
• The total transmittance light is consists of the direct transmitted and diffused
components which are depending on the angular distribution of light. There
are two types of angle distribution.
– Wide angle scattering (Haze)
– Narrow angle scattering (Clarity or Transparency)
LUMINOUS(LIGHT) TRANSMITTANCE
AND HAZE (ASTM D 1003)
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12. Corporate Training & Planning 12
• Haze is the cloudy appearance of an otherwise transparent specimen caused by
• light scattered from within the specimen or from its surface. Haze is defined as
the percentage of transmitted light which in passing through a specimen deviates
from the incident beam by forward scattering.
• It is generally accepted that if the amount of transmitted light is deviated more
than 2.5° from the incident beam, the light flux is considered to be haze.
• Haze is normally caused by surface imperfections, density changes, or inclusions
that produce light scattering. Haze is also reported in percentage
Transparency is defined as the light diffused in a small angle range
within 2.5° from the incident beam with high concentration.
Haze

13. SIGNIFICANCE :-
• Haze and Luminous Transmittance values are used in Quality Control and
specification purpose and to measure colour and visibility in products.
TEST METHODS:-
• ASTM D 1003 for transparent plastics products
• ASTM D 1746 for transparent plastics sheets
• ASTM D 1494 for reinforced plastics panels
SPECIMEN & CONDITIONING:-
• Test specimen must be large enough to cover the aperture and small
enough to be tangent to the wall. The size of specimen is approximately
50mm in diameter.
• The test specimen is conditioned at 23 + 2oC and 50 + 5% relative humidity
for 40 hrs prior to test. .
LIGHT TRANSMITTANCE & HAZE
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SAFETY PRECAUTIONS:-
• Specimen size must be within range.
• Specimen must be free from dirt & dust.
• No hindrance should be there on Entry port.

16. EQUIPMENT:-
* The instrument used for measurement is hazemeter. A Schematic diagram of optics &
hazemeter is shown in the figure.
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17. SPECULAR GLOSS
DEFINITION:-
• Specular Gloss is defined as the relative luminous reflectance factor of a specimen
at the specular direction.
SIGNIFICANCE:-
• Specular gloss is used primarily as a measure of shiny appearance of films and
surfaces.
• All specular gloss values are based on a primary reference standard .A polished
black glass with an assign specular gloss value is 100.
TEST METHOD:-
– ASTM D 523
– ASTM D 2457
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18. SPECIMEN & CONDITIONING:-
• Specimen surfaces must have good planarity free from warpage, waviness, or
curvature.The test specimen is conditioned at 23 + 2oC and 50 + 5% relative humidity
for not less than 40 h prior to test.
EQUIPMENT :-
SPECULAR GLOSS
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19. SPECULAR GLOSS
PROCEDURE:-
• The instrument is turned on and placed on a black glass primary standard.
• The control knob is adjusted so that the meter indicates the value assigned to the
primary standard.
• Next, the sensor is placed on the specimen surface and the gloss value is read
directly from the analog or digital display.
• OBSERVATION, CALCULATION & RESULT:-
• The instrument is calibrated during the operation at sufficiently frequent intervals
to assure that the instrument value is practically constant. If at any time an
instrument fails to repeat readings of the standard to within 2 percent of the prior
setting the intervening result should be rejected. The gloss value is directly read
on the instrument.
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20. FACTOR INFLUENCING:-
• Surface property
• Temperature
• Humidity
• Polymer molecular weight, Melt pressure, injection pressure & mould
temperature also influences the gloss of a product.
SAFETY PRECAUTIONS:-
• Specimen surfaces must be free from dirt and other foreign particles.
The specimen surface must be free from surface warpage, waviness, or curvature.
SPECULAR GLOSS
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21. COLOUR
DEFINITION : -
• Colour is one small part of the appearance. The variables of appearance
include glossiness or matness, opacity or transparency, smoothness or
roughness, and metallic reflection or diffusion surfaces of parts of its rays.
Colour is measured by a particular wavelength of light reflect and producing
sensation in the observer (eye).
SIGNIFICANCE:-
• It used in part identifications, Quality Control, Product Development and
Classification of Products.
• There are following terminology to understand the colour theory.
• Value: - It is color range from dark to light in which black being darkest and
white being lightest.
• Hue:- Color has basic difference appearance such as red differ form blue
green or yellow these distinctions as called Hue.
• Chroma:- Chroma is defined as the attribute of color perception that
express the degree of departure from gray of the same lightness
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22. TEST METHOD:-
• ASTM D 2244
• ASTM D 1925
• ASTM D 1729
• ASTM E 308
• ISO 3557
• ISO 4582
SPECIMEN & CONDITIONING:-
• The test specimen is conditioned
at 23 + 2oC & 50 + 5% relative
humidity .
EQUIPMENT:-
COLOUR
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23. PROCEDURE: -
(a) VISUAL ASSESSMENT OF
COLOUR & APPEARANCE: -
• The visual examination and
evaluation of colour is the most
commonly used technique in the
plastics industry.
• The two essential elements of
visual colour evaluation are: -
(1) The viewing of both the sample
and a standard by an experienced
observer.
(2) The use of a predetermined light
source at a fixed distance.
(b) INSTRUMENTAL METHODS OF
COLOUR MEASUREMENT:-
Two methods are used for colour
measurement: -
• Spectrophotometer
• Calorimeters
• The instrument is calibrated against
standard.
• A relatively flat, colored specimen is
placed in a specimen holder.
• A light source is illuminated.
• The CIE Lab Colour Scales are
displayed on the instrument.
• The L, a, b values are noted down.
• The L, a, b values are correlated
with L, a, b Chart.
COLOUR
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24. COLOUR
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25. COLOUR SCALES:-
• The mathematical relationship between these L scales and the CIE X, Y, Z
scale is as follows: -
L, a, b (Hunter – Square Root)
L=10Y½ ----- (1)
aL =17.5 (1.02X – Y) /Y½ ----- (2)
bL = 7.0 (Y – 0.8467Z) / Y½ ----- (3)
L, a, b (Glasser, et al – Cube Root)
L=25.29G1/3 ----- (4)
aL =106.0 (R1/3 – G1/3) ----- (5)
bL =42.34 (G1/3 – B1/3) ----- (6)
Where, G = Y R = 1.02 X B = 0.8467Z
COLOUR
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26. CIE L*, a*, b* (1976) (CIELAB)
L*= 116 (Y / Y0) 1/3 – 16 ----- (7)
a*= 500 (X / X0) 1/3 – (Y / Y0) -----(8)
b*=200 (Y / Y0) 1/3 – (Z / Z0) 1/3 -----(9)
Where, X / X0, Y / Y0, Z / Z0 > 0.01 & X0, Y0, Z0
define the colour of the nominally white
object-colour stimulus.
COLOUR
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27. OBSERVATION, RESULT & CALCULATION :
• Colorimeters & spectrophotometers calculate yellowness index as well as
whiteness index and is directly read by instrument.
FACTOR INFLUENCING:-
• Type of illuminant
• Wave length of light
• Particle size
• Angle of illumination
SAFETY PRECAUTIONS :-
• Observation zone area must be free from dust and dirt.
• Surface test areas shall be free from abrasion and warpage.
COLOUR
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28. BIREFRINGENCE
DEFINITION:-
• It is defined as the maximum algebraic difference between two refractive indices
measured in two perpendicular directions.
• Birefringence is referred to as stress birefringence and its contributions are:-
• Orientation of the amorphous chains between crystallites. Orientation of the
crystallites and the form birefringence arising from the interfaces between
crystalline and amorphous regions.
SIGNIFICANCE :-
• Birefringence is a conveniently handled tool for determining orientation in
materials. From the orientation of the molecules further conclusions regarding the
material properties can be derived.
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29. TEST METHOD:-
• ASTM D 4093
SPECIMEN & CONDITIONING:-
• The specimen is conditioned at 23 + 2
0C and 50 + 5% relative humidity.
EQUIPMENT:-
• The most common way to determine
birefringence is based on the
interference method, using a Babinet
compensator and polarised light.
PROCEDURE:-
• The birefringence of flat sample is
measured under uniaxial tension
using a Babinet compensator &
polarised light.
PROCEDURE:-
• The monochromatic parallel light
generated in the light source is plane
polarized in the polarizer (a Nicol prism or
Polaroid plate), passes through the
stressed sample, and then passes through
the Babinet compensator. Another
Polaroid plate, the analyzer, follows,
being rotated 900 with respect to the
polarizer.
• The light is travelling in the direction
perpendicular to the plane of the paper.
Since the refractive indexes nx and ny
indirections x and y of the stressed
polymer are different, the wavelengths of
the two wave components tx and ty are
also different, therefore the two waves
will be out of phase when leaving .
BIREFRINGENCE
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30. OBSERVATION, CALCULATION & RESULT: -
Different refractive indexes nx & ny of the sample shows the principle stresses x &
y at x & y plane. So retardation, R’ of one wave in the respect to another wave
determined by
R = d (nx – ny) / no
FACTOR INFLUENCING:-
* Humidity
* Degree of cross linking
* Temperature
SAFETY PRECAUTIONS:-
* Incident light direction must be perpendicular to the plane.
* Proper intensity of light must be present.
* Equipment must be free from dust and dirt.
BIREFRINGENCE
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31. CLARITY
DEFINITION:-
• Clarity is defined as ‘Light flux deviating less than 2.50 from the normal, on the
average’.
SIGNIFICANCE:-
• The Clarity is the distinctness with object appears when viewed through a film.
SPECIMEN & CONDITIONING:-
• Specimen may be in form of sheet, or film.
• The specimen is conditioned at 23 + 2oC & 50 + 5% relative humidity.
EQUIPMENT:-
• It consists of clarity sensing unit and photometric unit. Clarity sensing unit consist
of a light source, focusing lens, specimen holder & test cell.
SAFETY PRECAUTION:-
• Specimen must be fitted properly in specimen holder.
• Specimen must be free from dust and dirt.
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32. PROCEDURE:-
• The equipment is turned on. With no sample in position the unit is standardized
until a reading of 100.0 relative to air is obtained.
• If a calibrated standard is to be used, it is inserted in the specimen holder, and the
instrument brought to balance upon the value assigned to the standard.
• The beam is blocked completely with a piece of block flet of paper. ‘Zero adj’ knob
is then turned until reading of zero is obtained.
• The specimen is placed in the specimen holder.
• The reading is recorded as percentage clarity directly from the measurement unit.
FACTOR INFLUENCING:-
• Humidity
• Wavelength
• Temperature
CLARITY
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33. PHOTOELASTICITY
DEFINITION:-
• Photoelasticity is the property exhibited by
some transparent isotropic solids of becoming
doubly refracting when subjected to the stress.
• The difference in the two velocities shows up a
birefringence. Stress optical sensitivity is define
as the ability of some materials to exhibit
double refraction of light when placed under
stress.
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34. SIGNIFICANCE: -
• Photo elastic properties of the
transparent materials have been
used by design engineers for stress
analysis and by process engineers for
determining residual stress as well as
the degree of orientation in molded
parts.
SPECIMEN & CONDITIONING :-
• The test specimen is conditioned at
23 + 2oC and 50 + 5% relative
humidity .
EQUIPMENT:-
• Basically, two types of photoelasticity
polariscopes are employed.
PHOTOELASTICITY
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35. PROCEDURE:-
• The object to be examined is placed in the center of the polarizing medium.
• The object is viewed from the opposite side of the light source
• The polarized light indicates the number of fringes or rings, which is related to the
amount of stress, if light is used.
• Several additional techniques are used in conjunction with the basic photoelastic
concept, which make it possible to analyze complex or unusual problems. Briefly,
these techniques are-
– Stroboelasticity: use of stroboscopic light to observe photoelastic stress patterns
on vibrating or rotating parts. This technique is most often used on photoelastic
coatings.
– Thermophotoelasticity: measurement of thermal stresses by photoelastic
means.
– Photoviscoelasticity: analysis of rheology of plastics by photoelastic means.
– Scattered – light photoelasticity: observation of laterally illuminated transparent
– Models to determine stresses inside the model without slicing; integration
– Techniques are used.
PHOTOELASTICITY
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36. FACTOR INFLUENCING:-
• Mould design, Gate size & location, weld
line etc.
• Temperature & Pressure
SAFETY PRECAUTIONS:-
• Incident light direction must be
perpendicular to the plane.
• Proper intensity of light must be present.
• Equipment must be free from dust & dirt.
PHOTOELASTICITY
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