12.05 Finishing plastics
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Introduction
Early attempts to paint pvc-u window profiles failed due to problems of adhesion. Another major problem was the poor low impact strength after coating caused by the solvents within the paint. However, other techniques have been developed for the colouring and patterning of pvc-u surfaces. It should be noted that dark and coloured surfaces get hotter than white surfaces and coloured pvc-u profiles will get hotter than white ones. pvc-u has a high coefficient of thermal expansion and coloured frames will undergo large relative movements.
Four different approaches are used to achieve colouring and patterning of pvc-u profiles. These are:
Through-Frame Pigments
Both coloured and white finishes may be achieved by adding stable pigments to the polymer, resulting in an even distribution of colour and a smooth surface appearance. These should be colour stable and the pigment systems used should have been developed on the basis of natural weathering trials and artificial ageing studies. the most common colours produced are white, brown and grey.
The pvc-u profiles should still comply with BS7413 for all aspects of performance.
Co-extruded profiles
Co-extrusion is the extrusion of two different compounds through a single die to produce a composite profile. This technique can be used to co-extrude a pigmented pvc-u surface around a white pvc-u core. This is done to reduce the amount of pigment required and is done for highly coloured profiles rather than just brown or grey profiles. The method of heat welding the corners of a pvc-u frame means that the coloured surface is not continuous at the mitred corners and the white substrate may show through.
Woodgrain and other Foil Finishes
Different timber effect finishes may be printed on a metal foil and fused to the surface of the profile by means of a heated silicone wheel. The woodgrain foil is fused to the pvc in the absence of any adhesive using only heat and pressure. Plain colours can also be applied to the surface in this way. Some manufacturers have now developed acrylic films.
The method of heat welding the corners of a pvc-u frame means that the foil surface, which is applied to the bar length profiles is not always continuous at the mitred corners and timber patterned foils are normally used on a brown through-frame pigmented profile.
Foil finishes should meet all the requirements of the British Standard for surface coated pvc-u profiles, BS7722.
Specialist Paint Finishes
In recent times, specialist paint formulations have been introduced, which are designed to etch into the surface of the PVC. Some are capable of giving good long-term bonding and adhesion. The advantage of these finishes is the extended colour range possible.
Pvc-u is a suitable substrate for some printing processes. However printing should not be confused with painting. Printing on pvc-u is used mainly on profiles for internal decoration. This technique can be used to apply any kind of motif and colour to the profile.