Colouring plastics.pdf

7/27/2019 Colouring plastics.pdf

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White light, as we see it, is composed of a select group of special colours; each onecharacterized by a specific range of wavelengths, which it absorbs. These are the colours ofthe spectrum - red, orange, yellow, green, blue and violet.

Incandescence and luminescence are two common ways to create light. Incandescence islight from heat energy. Heating the filament of a light bulb to a high enough temperature will

cause it to glow. The stars and sun glow by incandescence. Luminescence is also known ascold light. It is light from other sources of energy independent of heating and can begenerated at room or even lower temperatures. Quantum physics explains luminescencethrough movements of electrons from their ground-state (lowest-energy level) into an

excited (higher-energy) state. Returning to its ground-state, the electron gives back theenergy in the form of a photon of light. If the interval between the two steps is short (some

microseconds), the process is called fluorescence; if the interval is long (some hours), theprocess is called phosphorescence.

The combination of these wavelengths in light can change depending on the light source.Therefore, colours look differently when compared under the influence of daylight,fluorescent light or sodium lamps. Natural sunlight varies widely. The character of sunlight

can be very blue, especially near midday, looking north. Direct sunlight usually is seen asgolden, but, near sunset, it can be bright red. Artificial light can be yellow, from sodiumvapour, blue-green from mercury vapour or it can be yellow, from an incandescent light bulb,or varying colours from fluorescent light. The graphs in Figure 2 show average north skydaylight (Illuminant D65), a cool white fluorescent light (Illuminant F), and an incandescentlight (Illuminant A).


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When light strikes an object, several phenomena can happen. Transmission occurs if the lightpasses through the object, which is the case with transparent colours. It is referred to asreflection if, for example, a green object reflects the part of the colour spectrum that

represents green and all other light is absorbed. The reflection curve of white will showroughly equal intensities close to 100% reflection in all wavelengths of the spectrum.Refraction or scattering is when light changes direction as it passes from one medium to

another, like from the polymer to a pigment or filler particle in a plastic part. Scattering isinfluenced by the difference in refractive index between a particle and its surroundings,particle size, and wavelength of light. An opaque colour shows a high level of scattering. Atranslucent colour shows a combination of transmission and scattering at the same time.

Absorption exists if most wavelengths of the visible spectrum are absorbed. Black surfacesabsorb nearly all light. Combinations of one or more effects exist.

The Object: A product appears in a specific colour because the light, which is reflected fromits surface, is made up exactly of wavelengths, which combine, to the colour observed. Theobject absorbs all other wavelengths. As an example, a blue object reflects the blue lightspectrum, but absorbs red, orange, yellow, green and violet, which are most of the otherwavelengths. A red object reflects the red spectrum but absorbs most of the orange, yellow,

green, blue and violet. The colours black and white differ in their absorption behaviour.Strictly following the rules they are not really colours.

A white object reflects all (or nearly all) colours while a black object absorbs most colourscompletely. In other words, whites represent a mixture of the colour spectrum and black theabsence of them.

Matching the colour of an object is complex and becomes more demanding with increasedneeds for appearance and effects. An object can be spherical or square, glossy or dull,transparent or opaque or translucent. It can appear metallic, fluorescent, pearlescent, or

phosphorescent. The target can be a completely different material than that of the currentmatch. Its colour can appear very different at various viewing angles. The gloss or texture of

a sample can change its appearance significantly. A good deal of a colour engineer's skill liesin getting around these problems and the limitations in materials.

The Observer: The defining observer is the human eye. No matter how carefully one matchesa colour, an observer nearly always bases acceptance of a colour on visual judgement. Mostof the time, this means a colour match can become highly subjective, since colour vision

varies widely from individual to individual. Age, gender, inherited traits, way of viewing asample - and even mood - can affect colour vision.

Colour measurement

Consequently, there is a need for a less-biased observer in the form of a colour-measuring

device. This instrument is designed to provide an objective means of measuring, evaluatingand aiding in matching colours. These instruments measure the amount of light that anobject reflects in each part of the wavelength spectrum and develops a profile to describe

that colour. The information about light source and observer can be described mathematicallywith three variables, which describe how we see a colour. The measured values relate toamounts of primary colours needed for matching a colour appearance. Information oncharacteristics of the sample can be used in identifying type and amount of colour pigmentneeded to match its colour. Two types of commonly used colour measurement equipment area colorimeter and a spectrophotometer. Colour measuring equipment helps us to give anobjective definition of the colour. Each colour has a fingerprint reflectance pattern in the

spectrum. We look at the characteristics of a pigment where it absorbs light. The colorimetermeasures colour through three wide-band filters corresponding to the spectral sensitivity

curves and is therefore not very accurate, showing large differences between the variousinstrument manufacturers. However, colorimeters are less expensive thanspectrophotometers. Modern spectrophotometers contain monochromators and photodiodes


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that measure the reflectance curve of a product's colour every 10 nm or less. The analysisgenerates typically 30 or more data-points, with which an exacting colour composition can becalculated. The spectrophotometer can detect metamerism (matching under one light source,

but not another), perform colour matching recommendations and is often also utilized as aquality control tool.

Figure 3: CIEAB diagram and coordinate system

For the measurement of colour, standard values are used worldwide, for example asdetermined by an organisation called CIE. CIE was founded in 1913 and stands forCommission Internationale de l'Eclairage (International Commission on Illumination). CIE has

a technical committee, Vision and Colour, that has been a leading force in colorimetry since itfirst met to set its standards in Cambridge, England, in 1931.

The values used by CIE are called L*, a* and b* and the colour measurement method iscalled CIELAB. L* represents the difference between light (where L*=100) and dark (whereL*=0), a* represents the difference between green (-a*) and red (+a*) and b* representsthe difference between yellow (+b*) and blue (-b*). With these co-ordinates, any colour canbe defined as a place on the graph shown in Figure 3. Differences in L*, a*, b* or E* arerepresented as oL*, oa*, ob* or oE*, where oE* = o (oL*2+oa*2+ob*2). It represents the

magnitude of the difference in colour, but does not indicate the direction of the colourdifference.

Another colour measurement system often used is the Munsell Colour System developed byA. Munsell, an American artist in 1898. Munsell desired to create a 'rational way to describe


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colour' that would use clear decimal notation instead of colour names. In 1905 he publishedA Colour Notation, which has been reprinted several times and is still a standard forcolorimetry.

Munsell modelled his system as an orb around whose equator runs a band of colours. Theaxis of the orb is a scale of neutral grey values with white as the North Pole and black as the

South Pole. Extending horizontally from the axis at each grey value is a gradation of colourprogressing from neutral grey to full saturation. With these three defining aspects, any ofthousands of colours could be fully described. Munsell named these aspects, or qualities, hue,value, and chroma.Table 1: Difference between visual and computer controlled colour-matching procedureVisual ComputerColourist experience Preparation of calibration standards, reading and storage of

their reflectance curvesVisual assessment of the sample Measuring the sampleSelection of a combination of pigments Selection of possible pigmentsDesign of formulation, based on experience and on a libraryof previous matches Data entry and calculation of possible formulations. Selectionof a formulationPreparation of the formulation Preparation of the formulationVisual assessment Reading of the sampleEstimation of necessary corrections Calculation of the correctionSelection of a new combination of pigments (if necessary) Selection of a new combination of pigments (if necessary)Munsell defined hue as the quality by which we

distinguish one colour from another. He selectedfive principle colours: red, yellow, green, blue, andpurple; and five intermediate colours: yellow-red,green-yellow, blue-green, purple-blue, and red-purple. He arranged these in a wheel measured offin 100 compass points. The colours were identified

as R for red, YR for red-yellow, Y for yellow etc.Each primary and intermediate colour was allotted

ten degrees around the compass and then furtheridentified by its place in the segment. Munselldefined value as the quality by which wedistinguish light colours from dark ones. Value is a

neutral axis that refers to the grey level of thecolour. It ranges from white to black. The value ofa particular hue would be noted with the value

after the hue designation. Chroma is the qualitythat distinguishes the difference from a pure hue toa grey shade. The chroma axis extends from thevalue axis at a right angle and the amount ofchroma is noted after the value designation. Therefore, 7.5YR 7/12 indicates a yellow-redhue tending toward yellow with a value of 7 and a chroma of 12. However, chroma is notuniform for every hue at every value. Munsell saw that full chroma for individual hues might

be achieved at very different places in the colour sphere. In the Munsell System, reds, blues,and purples tend to be stronger hues that average higher chroma values at full saturation,

while yellows and greens are weaker hues that average fullest chroma saturation relativelyclose to the neutral axis.

Figure 4: The Munsell System


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Colour matching and appearance

A colour match is often a combination of the colour engineer's visual perception and the

measurement results of a PC-controlled measurement tool. Both have advantages, butneither one should be used alone. The information a colour engineer receives before theexperiment is critical for the successful outcome of the match.

Therefore, it is important to understand the objective and means of the plastic manufacturer.For example, the chemical nature of the specific resin to be coloured and the processingconditions will determine the correct masterbatch formulation. The main performance

requirements that affect the selection process include:1. Obtaining information on resin and type of processing equipment. This includes what type

of polymer(s) are used and what is the end product. The process the customer intends to useis also important, such as injection or blow moulding, sheet or film extrusion.

2. Understanding the chemical resistance requirements of the plastic product. For example, ifis it to be used in contact with acids, bases or organic solvents, the colourant of choice must

be resistant to these chemicals.

3. Knowing the heat exposure during each processing step. The degree of heat the colouranthas to withstand is predominantly determined by the maximum processing temperature and

duration of exposure.

4. Protecting against ambient conditions. The long-term resistance of the final productagainst humidity, light, temperature and combinations of these can be greatly influenced bychoosing suitable raw materials, which protect the polymer. For measuring the life of a

certain product with regard to the colour, most colour engineers use the grey scale. The greyscale is a tool for describing the colour changes of an object that might occur under the

influence of temperature, light (in particular UV radiation), and other weather conditions,

during a certain period of time. This scale contains a range of 1 to 5, where 5 means that thecolour of the object has not changed and 1 that a severe colour change has occurred.

5. Enabling usage in food approved products. The legislation in many countries bans specificraw materials in products and requires special approval for plastic ingredients used in contactwith food or pharmaceuticals. For example, most heavy metals are highly regulated in food

packaging. When developing masterbatches for food packaging colour engineers are notallowed to use pigments containing heavy metals. A close co-operation between customer

and colour engineer is imperative to ensure compliance.

6. Offering the ability to reduce waste. Another legislative restriction found in many countriesaims at reducing waste. Reducing the thickness of the plastic packaging achieves this goal.

The colour engineer has to create a concentrate that can obtain the same level of opacitywith a thinner product, by either adding more pigment to the concentrate or by usingdifferent types of pigments with higher colour strength.

7. Providing optimal colour quality and desired effects. The stability of the final productdepends largely on the correct ingredients within the formulation, their degree of dispersion,resistance to migration, bleeding, crocking or bronzing. Effects like metallescent orluminescent looks will give the plastic the desired appearance. The more complex the desiredeffect becomes in the plastic product, the higher the cost of the raw materials.If any (or all) of the above questions cannot be answered properly, the chances of getting a

correct match decrease considerably. For example, if the colour engineer is not informed that

the customer wants to use the end product in warm climates for ten years, then sufficientUV-stabilisation will not be added to the colourant. Alternatively, if the colour engineer is notinformed that a product has a life cycle of only six weeks, more expensive, but highly stable


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pigments may be used to get the right colour, and protect against weatherability, whilemaking the final formulation cost prohibitive.

Over the past decade consumers have become much more demanding. For example, in thepackaging industry requirements are longer shelf life, fresher products and packaging that iseasier to handle and easier to recognize. Demands in this industry are high UV-stability,

improved barrier properties, and improved tear strength, antistatic and antifoggingproperties and a colour that helps differentiate their packaging from their competition. In theautomotive industry looks, scratch resistance and cost reduction have become importantissues, while in the appliance industry it is appearance, design and advice about colour

trends.

The tools - inorganic and organic colours

Colourists work with a variety of pigments to reach the desired colour and effect. Typically,two main pigment classes, inorganic and organic, exist because of their chemical differences.Another group referred to as polymer-soluble dyes are organic colourants that dissolve incertain polymers when melted.Table 2: Differences between inorganic and organic pigments and dyesinorganic Organic DyeLess expensive Moderate to expensive More expensiveHigh stability Low to high stability Low stabilityHigh opacity Low to medium opacity Low opacityLow colour strength High colour strength Very high colour strengthDull colours (often non-toxic) Bright colours Bright coloursBright colours (lead,cadmium Seldom found Seldom found - more toxic)Easy to process Difficult to process Difficult to mix at high concentrationEasy to disperse Difficult to disperse (small particle size) No dispersion required (dissolve in polymers)Suitable for allthermoplastics More or less suitable, depending on the pigmentand polymer Not suitable for olefins - at very low levels inPS, PA, etcInorganic colours are based on oxides, salts, or complexes of metals in various oxidationstates. Most inorganic colour pigments have a simple molecule structure, except for some

mixed metal oxides, which have more complex shapes. The metals often require a special

pre-treatment to make them suitable for use in plastics. Organic colours are based on carbonchemistry and typically do not contain any metals. These colours can have complex molecularstructures, are produced through multi-stage synthesis and are often easier to mix with thepolymer. Based on the requirements of the customer, the proper pigments are chosen toachieve the desired performance. Often, a combination of organic and inorganic pigments isused to obtain the desired colour and effect. The main difference between both classes is

summarized in Table 2.

Dispersion and distribution of colorants

Besides matching the colour, the masterbatch needs to ensure dispersion of pigment andfacilitate the distribution of the colour in the plastic. Dispersion is the separation of particles

of pigment from each other through wetting the surface of the pigment with the polymer


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carrier to form the colour concentrate. Distribution is the spreading of this concentrate evenlythrough the polymer without streaks.Dispersion: Pigment particles easily form

aggregates, where the particles are firmlyadhered to each other. It generally requiresaggressive grinding to break up theseaggregates. Agglomerates, on the other hand,can be broken fairly easily. Figure 5 shows thegeneral problem. The strength of a pigmentdepends on dispersion, in particular. The

number of colour sites largely determines thestrength of a pigment. Breaking agglomerates of

two pigment particles doubles the strength ofthe system. A 40-micron agglomerate of one-micron particles can contain up to 64,000particles. A dye is usually stronger than apigment, since a dye is dissolved, and each

molecule becomes a colour site. Dispersion isgenerally a two-step process. A pigment agglomerate is broken up by physical force. The

agglomerate can be crushed, pulled apart or shattered by an impact. The pressure ofcrushing can also create new agglomerates by squeezing particles together. Many organic

pigments show this tendency of compaction, which can occur at different stages of theproduction process. In order to prevent particles of the same chemical make adhering toeach other they have to be coated (wetted) by a different substance. Only then will theyhave a lower tendency to stick together. This other substance can be a wetting agent orpolymer. The wetting material coats the pigment and allows pigment particles to slide over

one another instead of sticking to each other.

Figure 5: Pigment dispersion and agglomeration

One popular type of wetting agent is a

surface-active agent (surfactant),which has a molecule with a polar endto bind to pigments, since they areoften polar molecules, and a non-polarend to bind the polymer. A commonexample is stearic acid, which has a

polar acid group attached to ahydrocarbon backbone. Wetting agentsare usually lubricating. Too muchlubricant can prevent the necessaryforce from reaching the pigment. Theagglomerates roll and are not sheared

apart. One consideration indetermining how much surfactant touse is the surface area of the pigment.

Inorganic pigments typically have small surface areas, about 4m2 per gram. Carbon black canhave more than 300m2 per gram. Obviously, carbon black would need more surfactant.Figure 6 shows the surface area as a function of particle size.

Distribution of colour is a shared responsibility between the colour concentrate producer andthe plastic manufacturer. For example, a hard colour concentrate will not flow as well as thepolymer it is being letdown into. It will mix poorly, leaving specks and streaks. The letdownratio is a consideration in good mixing. If the letdown is 1%, and the pellets of concentrate

and resin are roughly equal in size, there is only one pellet of concentrate for each hundred

pellets of resin. The one pellet must break down and spread among the hundred pellets ofnatural resin. Consequently, a soft resin that can flow easily, and ideally, melts before theletdown resin, is best for the carrier. A pigment dispersed in the concentrate can increase the

Figure 6: Relation between surface area and particle size


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viscosity, making mixing even more difficult. UV additive formulations can also make mixingdifficult and cause streaks. Conventional pellet concentrates are usually used at 2% to 4%,to make the product look more even. New trends are towards smaller concentrate pellets.

Pellets half the size of the resin pellets at 1% have one colour pellet for every 50 and not 100resin pellets.

Designing appearanceThe colour concentrate allows the manufacturerto produce plastics with desired colour, opticaleffects and specific physical properties.

Appearance is critical for many products and akey differentiator for plastic producers and their

clients. For example, designing interference orflip-flop colours, which change depending on theviewing angle requires a good understanding ofthe matter. In order to match an interferencecolour, the colour engineer works in two steps.

Firstly, the base colour is matched. Theinterference effect is then added by typically

using mineral-coated mica. By looking at themoulded part from different angles, the light will

be reflected in different ways from the micaparticles, causing a change in colour. Actually,the colour does not change, but the reflectioneffect of the mica particles will be more or less visible, causing the interference effect. Thereare a number of new effects and recent trends in designing colour masterbatches. Figure 7

below shows an overview of the different optical effects.

Opaque plastics: The opacity of an object is determined by measuring how much light is

transmitted through the object. Opacity of a colour is critical to its appearance. The usualway to measure opacity is to measure reflectance over white and over black in order todetermine the contrast ratio. A contrast ratio of 98% is considered one-coat hiding in paints.A colour engineer needs to know the desired opacity and thickness of the plastic part. Withthis information the colourist can formulate the required pigment concentration in themasterbatch.

Pearlescent plastics: These contain pigments that are applied to generate a pearl-like,

iridescent effect. Typically, the pigments are thin, metal oxide coated mica platelets, usuallyless than one micron thick and between 5 to 100 microns long with a high refractive lightindex. The metallic oxide coatings have the ability to reflect or suppress specific wavelengthsof incident light causing a soft, silky appearance. The platelets can change colour depending

on the viewing angle. Which wavelengths are reflected or suppressed depends on thethickness of the metallic oxide coatings. The pigments typically have to be well dispersed inthe plastic matrix to prevent agglomeration. Light scattering, either by the resin of the

compound or by other pigments, needs to be avoided since this interferes with the reflectionof the platelets. Usually, oxides of iron- or titanium-coated mica are the leading pearlescentpigments, which contain superior chemical and processing stability. The automotive industry,for example, is always looking for new colour shades and effects like pearlescent finishes.

Figure 7: overview of different optical effects


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Figure 8: Moulded plastic parts with knit-line (right) and without knit-line (left)

Fluorescent plastics: These materials contain pigments that possess the unique capability to

absorb light energy in the visible and UV range and re-emit it at different wavelengths. Thisresults in a bright glowing plastic in daylight, which is dark without light. This is also the maindifference when compared with phosphorescent plastics, which contain an afterglowgenerating effect pigment. Fluorescent pigments typically consist of an organic moiety. Somemolecules with fluorescent capabilities also show phosphorescent behaviour. The pigment isusually mixed into the plastic matrix and shows highest fluorescent intensity when applied

over white surfaces.

Phosphorescent plastics: These contain pigments that have the capability of absorbing lightenergy at one wavelength and releasing it in packets at a lower wavelength. The energyrelease is delayed and the re-emission process varies by pigment type and can last forseveral hours depending on length and size of the excitation process. Most common areinorganic oxides, like doped zinc sulphide complexes. The ZnS crystal lattice containsimplanted metal-ions such as Sr+, Ca2+, Li+, Cd2+ or other metals in low concentrations.

Organic pigments are also known to provide similar effects. Products containing these kindsof pigments are known for their special effects like glow-in-the-dark. Red, green and yelloware the most common colours observed in industries focusing on toys, safety, highway androad markings, and related industries.

Metallescent plastics: Manufacturers often have to use metal or follow a two step paintprocess, applying a primer and a liquid coating in a post fabrication process to generate ametallic look. Over the past decade the automotive, household goods and electronicindustries have looked at a number of ways to replace metal or paint in an attempt to reduce

weight and organic solvents, while maintaining a metallic appearance. Polymers containingmetallic flakes are designed to mimic metal. A metallic look is typically generated through theuse of aluminium, copper or mixtures of these metals. The actual size of the flakes will vary,but typically range from 0.1 m to 2.0 m in thickness and 0.5m to 200m in diameter.When incorporated into a resin, the flakes have a tendency to orient themselves in a multi-layered position. The metallic effect varies depending on how parallel the flakes are to thesurface, flake size, and shape. The characteristic colour can be described as whiter, brighter

or greyer, and darker, and is again related to particle size distribution. The lightness orgreyness is provided by the amount of light reflected from the surface of the flake. Generally,

as the particle size distribution becomes finer, the colour becomes darker. However, throughparticle size separation techniques, it has become possible to provide grades with a verysmall average particle size while maintaining a high degree of brightness. Smaller particlesprovide more opacity and hiding power, while larger particles cause higher reflectivity andtherefore brightness of the object. Colours that vary with angle are particularly difficult tomatch. A metal looks light in colour when viewed head-on (face angle), but darker at anangle (flop angle).


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Colouring plastics.pdf