Download - TITA
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Colour Technology
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Why use Colours?
IdentificationIdentificationBrandingBranding
Convey a Mood or a Style Convey a Mood or a Style ChoiceChoice
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Introduction
• Assessment and Measurement of Colour.
• Factors Affecting Colour Matching.
• Methods of Colouring.
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Colour Assessment
• Eyesight
• Light Source
• Size
• Background Colour
• Surface Finish
• Metamerism
observer
object
light source
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Human Visual System
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The Retina• Retina uses special cells called “rods” and
“cones”.• Rods “sees” in black, white & shades of grey and
tell us the form or shape. (Super-sensitive allowing us to see when it's very dark.)
• Cones “senses” colour but need more light. Three types and each is sensitive to one of three different colours - red, green, or blue. Together these can sense combinations of light waves. (To see millions of colours.)
• Rods and cones together process the light to give you the total picture.
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The Rods and Cones
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The Rods and Cones
5 million per eye (more L and M cones than S cones)
100 million per eye
Responsible for daylight (photopic) vision
Responsible for low-level (scotopic) vision
Cone function
Rod function
scotopic mesopic photopic
luminance
Cones Rods
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Eyesight
• Individuals perceive colour differently.Is “Sky Blue” the same as “Pale
Blue”?
• How many People are Colour-Blind?
• Experts are “turned in” to Colours.
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Light source• A red object in red light, appear red - as all
the red light is reflected.
• A red object in blue light, appear black - as no red light to reflect back.
The difference between say daylight and the Tungsten Lights used in homes, could be
significant!!
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Size
• A small area of colour may look very different to a large area of the SAME
colour.
Hence, it is important when decorating to Hence, it is important when decorating to paint a sufficiently large area.paint a sufficiently large area.
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Background colour
• Colours viewed against a strong, vivid coloured backgrounds, appears very different against a neutral or pastel coloured background.
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Surface finish
• A high gloss finish always appears darker than a matt finish of the same colour.
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Metamerism• Change in appearance of a colour under
different light source.• Describe the relative changes in colour between
two samples, i.e. Good match in day light, different in fluorescent shop lighting.
• Occurs when different colourants are used in each sample.
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Colour Measurement
• Colour Space• Colour Measuring Devices
Colorimeters Spectrophotometers
• Metamerism• Light Sources and Illuminates
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Colour
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Colour Space
• To measure colour objectively, to communicate differences in quantifiable terms.
• Principle is that all colours can be inside a “Colour Space” i.e. this space being a sphere.
• Each colour can then be give a position in the colour space.
• Differences between colours can be quantified by comparing the values of the co-ordinates.
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Colour Space
+a*
+b*
-a*
-b*
L*=100
L*=0
L*
L*
-a*
+a*
-b*
+b*
+ L = Lightness + L = Lightness - L* = Darkness- L* = Darkness+ a* = + a* = RednessRedness - a* = - a* = GreenessGreeness+ b* = + b* = Yellowness Yellowness - b* = - b* = BluenessBlueness
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Colour Space - Delta E
a*
b* C*H*
SC
SH
222***
H
ab
C
ab
L S
H
cS
C
lS
LE
is the difference between two points in colour space, often use to determine a colour tolerance or specification.
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Colour Space
• A different system (Yxy) is used for transparent colours.
Y = percentage of light transmitted
x = balance between blue and red light
y = balance between blue and green light
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More on Metamerism
• Two colours with the same L*,a*,b* values which are arrived at by a different route will show metamerism.
• Their colour will be different if the light source is changed.
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Colour Measuring Devices
• Colorimeters
Filters the reflected lights into Red, Green an blue lights and measure the relative amounts of each, then calculates the numerical lab value.
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Colour Measuring Devices
• SpectrophotometersMeasure the reflected lights at regular intervals across the visible spectrum, then produces a graph of the spectrum of light reflected by the colour called the Spectral Curve.
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Colour Measuring Devices• Spectrophotometers more accurate
than colorimeters.
• Spectrophotometers better at identifying metamerism than colorimeters.
Differences between the human eye and colour measuring devices > Possible to have an accurate reading but does not
visually look right!!!!
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Light sources and illuminants
• The three most widely used are;
D65 Simulation of Daylight (Artificial Daylight).
A Normal domestic tungsten light.
TL84 Standard fluorescent tube used in most shops and showrooms.
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Colour Matching
• Standard Colour Systems
• The Material
• Colorants
• Legal Restrictions
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Standard Colour Systems
• Colours presented as printed paper patterns books. (RAL and PANTONE system.)
• Building industry has its own set of BS colours.
• NCS (Scandinavian) a measuring system rather than a fixed set of colours.
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The Material• Processing temperatures and chemical
characteristics, means a colourant can work in one polymer, but degrade or discolour in another.
• The more different the standard material is to the match material, the less likely an accurate match.
• A painted sheet match to Nylon 6.6, the colourants used in paint will not survive 290oC.
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The colourants-Pigments• Very fine powdered chemicals dispersed in the
polymers.• Poor dispersion results in a weaker colour and
often a grainy surface.• Inorganic pigments are mineral based, i.e.
Metal Oxides & Sulphides.• Organic pigments are chemical compounds,
less heat stable and more difficult to disperse. Usually give richer and more vivid colours. Less pigment is required.
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The colourants-Dyes
• Chemical substances that dissolves in the polymer.
• Chemically interact, as such allows light to pass through. When use in transparent materials remain transparent.
• A limit how much dye can be added to a polymer, the dye can bleed out!
• Insoluble in Polyolefins.
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Legal Restrictions• Food, Medical and Toy - mainly base on purity and
inability to extract the colourant from the finish item.
• Cadmium pigments - base on the premise when the Plastic part is incinerated, they can release Cadmium metals. Applies mainly in the packaging industry.
• If a colour is required for safety purposes, then Cadmium pigments can be use regardless of the Cadmium legislation.
• Many companies have a “Cadmium Free” policy regardless of the details of the legislation.
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Methods of colouring
• Dry Colour
• Masterbatch
• Liquid Colour
• Fully Compounded Colour
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Dry Colour or Dry Blend
The colourants are mixed with the polymer. Some dispersion aids may be added and often a “wetting agent” to help bind the powder onto the surface.
Advantages : Cheap because the conversion cost is low. Quick to prepare. Very small lot.
Disadvantages : Can be very Messy, can affect drying, colour can vary with different machine due to dispersion.
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MasterbatchCompounds contains very high levels of colourants (up to 80%),
then mixed at a fixed ratio to give a specific colour.
“Polymer Specific” - Carrier is the same material as the base material.
“Universal” - Carrier will readily mix with a wide variety of polymers.
Advantages : Better colour control, cleaner and less drying problems than Dry-colour. A stock range of colours and specific colours can be develop.
Disadvantages : May not always be compatible with the base polymer. Accuracy depends on the Moulder with mixing.
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Liquid colourSimilar to masterbatches, contains a high level of
colourants, but the carrier is a liquid.
Advantages : Better colour control than Dry blend. More even distribution than masterbatch. Stock range of colours.
Disadvantages : Special dosing equipment is required. Spillage is messy. Colour can depends on processing. Properties can be affected by the liquid carrier.
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Fully compounded colourThe colourants are added to the base
Polymers, then extruded to encapsulate the colour into the polymer and is fully dispersed.
Advantages : Specific, accurate and controlled colours. The performance of the compounds is more predictable. Ease of handling.
Disadvantages : Less flexible than the other methods.
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A Polymer for Every Application