DECORATING of plastics is required in the articles manufactured from any plastic/material, whether a thermoplastic or a thermoset, for aesthetic or functional purposes. Functional purposes include improved resistance to wear, scratching, light or heat etc.
The coloured parts can be obtained by getting coloured parts from the polymer manufacturer. Here the manufacturer mixes the colourants like organic or inorganic pigments and/or transparent dyestuffs, with the resin during fabrication. The colourants can be mixed using hot rollers or Banburry mixers or sometimes it is mixed when the resins are in a liquid or pasty state.
Dry powder colouring: (In-house colouring):
Here, the uncoloured, natural materials are mixed with powdered colourants in a tumbling barrel.
Advantages: This system allows the Moulder to stock a large amount of crystal pellets (uncoloured) and small stocks of many colours.
1. The parts either produced by extrusion or ram injection moulding machine do not always have the highest quality—streaks and non uniform colouration is common.
2. The dust produced by the process requires a separate mixing room.
3. Relatively high amount of labour is required for operation.
4. The process is used for the production of low cost parts.
Colour Concentrates/Masterbatch: Here 50-80% of pigment is dispersed in a polymer carrier and then converted into small pellets. The carrier can be any of the thermoplastic resin either produced as a stock concentrate/Masterbatch by the concentrate/Masterbatch manufacture or as a custom concentrate/Masterbatch for the Moulder. In both the cases, the pigment is thoroughly dispersed in the resin using hot rollers, Banburry mixers etc.
1. The Moulder has to stock the natural material in large quantities and small quantities of colour concentrate/Masterbatch.
2. Dust problem is eliminated because the pellets are dust free and automatic colour blenders are used to mix the virgin material and colourants.
1. The addition of pigments imparts opacity, though it is not its function because opacity can be increased by increasing the amount of carbon black and titanium dioxide in a colourant.
2. The colour concentrate/Masterbatch imparts the colour to the resin.
3. The concentrates/Masterbatch now also contain fire retardants, blowing agents, UV absorbers, anti oxidants and other additives in addition to the colourants. Before using these multifunctional concentrates/Masterbatch, it should be checked whether the carrier resin in the concentrate/Masterbatch is compatible with the matrix resin to be coloured e.g. LDPE matrix carrier resin is compatible with other Olefinics like HDPE or PP and lower melting range of LDPE assures thorough melting and dispersion in the matrix while LDPE carrier is not compatible with PS, Acrylics etc. matrix.
Universal colour concentrates/Masterbatches are also available, which can be blended in with most of the resins. However they also should be checked for compatibility before using.
Terms used in the colourant system:
1. Let-Down ratio: It is the number of Kg. of natural resin which can be satisfactorily coloured by 1 Kg. of the colourant.
a) L/D ratio of the press or extruder will determine the Let Down ratio that is possible with any given concentrate. The higher the L/D/ ratio, the greater will be the Let Down ratio.
b) Speed of the screw: Too fast a screw speed will lower the Let Down ratio.
c) The temperature profile of the barrel: Slight increase in the temperature at the feed section can result in a higher Let Down ratio.
2. Metamerism: It is the phenomenon exhibited by the two surfaces that appea to be of same colour when seen under the same light source, but have different colour when seen under different light source. To prevent this, the viewing light source should be specified when colours matching other colours are specified.
Liquid colour concentrates:
These can be metered directly in to the throat of the moulding press or extruder. Their Let Down ratio is 100:1. Their viscosity is approximately equal to that if a heavy oil. Usually the metering pump, which is mounted on a liquid reservoir, is tied directly with the screw rotation i.e. it pumps only on the return stroke of the injection screw, and when it is used on an extruder, it can be driven by the extruder screw to compensate for the fluctuations in the screw speed.
1. Power requirement is reduced due to the lubricating properties of the liquid carrier.
2. The change in the physical properties of the matrix resin is negligible because only a small quantity of carrier is to be absorbed by the resin.
3. It can also incorporate blowing agents, UV absorbers etc.
4. Though the cost/Kg. of the liquid colourants is considerably higher than the pellet concentrate/Masterbatch, the ultimate cost is much lowered due to the higher Let Down ratio.
Colouring of thermosets:
Basic problems in colouring of thermosets:
1. Thermosets undergo a chemical reaction during the moulding operation.
2. They are sometimes used at elevated temperatures.
3. Some of thermosets have a tendency of yellowing with age or from exposure to UV radiation from the Sun.
Colouring of various thermosets:
Moulding powder like Phenolics, Ureas, Melamines, Epoxies and Alkyds can be produced in coloured state by the raw material manufacturer. All other, except Phenolics can be made in various colours and most of them are available in stock colours from the manufacturer. The Phenolics are usually available only in brown, black or dark red, green or blue colours because of the natural tan or brown colour of the polymer and tendency of yellowing with age.
Liquid thermoset polymers like Epoxies, Polyesters and Urethanes are usually coloured by the paste colourants which can be added directly to the liquid resin.
The paste colourants are made by grinding the pigments into the appropriate carrier; by the companies specialized in the work because the simple mixing of the powdered pigment in the resin by the Moulder usually do not result in a uniform dispersion.
Selection criteria of colourant system:
Following factors should be considered while selecting a colourant system for a liquid resin system:
1. The colourant should be stable at the moulding temperature or at the service temperature of the part.
2. The colorant should not affect the polymerisation reaction.
3. The colourant should not affect the physical properties of the final part e.g. amount of colourant should not reduce the tensile and tear strength of the Urethane foam.
4. Inorganic pigments must be used as colourants for outdoor use. This is particularly important with Polyesters which are frequently exposed to the weather.