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    Colour your thinking on the Optical Properties of Thermoplastics

    In a previous blog we looked at what determines transparency in thermoplastics but what about colour?

    Very few commercial polymers have chemical groups (chromophores) which absorb light in the visible region of the spectrum. Consequently the base polymers are colourless.  If you need colour, you have to add dyes (soluble in the polymer phase) or pigments (insoluble).  Dyes fell out of favour because of problems of migration out of the polymer into other materials (‘bleeding’). Also because of the development of a wide range of efficient organic pigments, with appropriate heat and weathering resistance.  Mixing of pigments can match any specific combination of hue (shade), chroma (intensity) and lightness.

    To generate colour in a natural thermoplastic, the preferred method is to add granules of the same polymer (or a compatible polymer) loaded up with a high concentration of pigment (colour concentrate or masterbatch).  The secret of consistent, efficient colour is to ensure the pigment is broken down into very small particles (dispersion) and evenly mixed (distribution). Both require careful application of shear in the melt state.

    Colourants are added in such small amounts that they do not affect mechanical properties significantly.  The exception is impact resistance.  Different shaped pigment particles can affect crack initiation.  It was a standing joke with a colleague that, after impact test results from one particular consultancy project, we politely declined green hard hats on factory visits.

    Although the base polymer is generally colourless, a slight yellowish tinge can appear from polymerisation residue, polymer decomposition during processing and service and, more commonly, from other additives.  This yellowness can be cancelled out by optical brighteners, a special class of colourant, which absorb uv light from the invisible part of sunlight or artificial light and emit it in the blue region, giving a cleaner transparency or more intense white and sharper colours. To check for optical brightener, just observe the thermoplastic under a UV lamp.

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    What determines friction between thermoplastic components?

    Friction is an important property for thermoplastics in bearings and gears but also has a part to play in assembly of plastic parts (snap-fit and interference-fit) and ejection during moulding.

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    Will Styrenic thermoplastics evolve further to meet design demands ?

    Styrenic thermoplastics?

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    What makes medical grade plastics so special?

    Toughness and transparency are important properties for the constituents of intravenous lines.

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    What causes mouldings (and moulders) to be off-colour?

    When mouldings are not the intended colour, the first thing to check is the raw material, particularly the dosing rate, if you are using masterbatch, and the quality of regrind.

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    Where are Engineering Thermoplastics Blends going ?

    The timeline of appearance of materials for the plastics industry can be viewed as several overlapping phases.

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    Understanding the difference between SBS & SEBS thermoplastic elastomers?

    The difference between SBS and SEBS thermoplastic elastomers explained.

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    Is MFR really much help to moulders these days ?

    Often a moulder has to change material grades. One of the first properties to be consulted on the new data sheet is usually the MFR. This is to establish if the new grade has the same melt viscosity performance.

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    What gives Moulders a Warped View on Life?

    Warping is one of the most frustrating injection moulding faults. This is because it does not always appear immediately, the causes are complex and prevention is difficult.

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    How can a thermoplastic elastomer be both thermoplastic and elastomer ?

    Thermoplastic elastomer? The class of polymers known as thermoplastic rubbers (TPR), or thermoplastic elastomers (TPE), describes pretty well what it says.  As thermoplastics these materials soften with heat. They can be melt processed, by extrusion or injection moulding, but regain their soft solid properties when cooled.  As elastomers they conform to the ASTM definition of […]

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    Understanding the causes of silvering in injection mouldings .

    Silvering. Water is essential for human and plant life but can prove inconvenient for thermoplastics.

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