Cutting the cost of colouring rigid PVC

Modern masterbatch technology utilises intensive mixing in a masterbatch extruder to produce highly pigmented products. Unfortunately PVC is extremely sensitive to shear which makes it a poor carrier for masterbatches. Plasticising somewhat overcomes this and PVCp based masterbatches can be made on conventional masterbatch equipment.

However, PVCp masterbatch in PVCu adds plasticiser into an unplasticised polymer. Traditionally, PVCu polymer specific based masterbatches were produced on PVC extruders and generally featured lower levels of pigmentation compared with other polymer specific types. A combination of resistance to flow and being prone to degradation under shear make it difficult to produce highly loaded PVCu masterbatches.

Twelve years ago Colour Tone developed the Vynacol polymer specific colouring system for PVCu that made it as easy to colour PVCu as any other polymer by eliminating compatibility and processing issues historically associated with adding colour to PVC. This technology employs innovative methods of modifying and manipulating the additive systems commonly used in PVCu compounds.

Where and when to add colourants

Liquid based colouring systems are usually pumped directly into the polymer melt. 'Dry' colourants can be added to pre-compounded pellet or dry blend, prior to compounding. Dye packs require premixing and a liquid dispersant will help to reduce dust.

However, introducing colourants into a dry blend is pointless unless the colourant [dye packs, liquid colourants and some wax dispersions] can be wetted out. Benefits can be offset by the need for additional cleaning of colour contaminated equipment which will impact subsequent batches. While masterbatches and wax dispersions can also be treated in this fashion, it is more common to dose these materials directly into the polymer stream during processing.


How much colour is required?

Some applications require lots of colourant. This may be for a number of reasons, for example strong polymer colour, exterior performance, poor coverage of the pigments or opacity specification on thin wall products.

Processors should quantify the amount of dispersed pigment or dyes required because they will be mixed with a relatively large quantity of colourant carrier. It would be foolish to decide on a colouring method which met colour requirements but introduced processing issues or reduced the physical performance of the base polymer.

Whatever the colour carrier media there are technical merits in using pre-dispersed pigments. Pigments and dyes are dusty, 'dirty' products and present a risk of contamination. Conversely, dispersed pigments are dust free, have no health and safety issues and the dispersing process breaks down the pigment agglomerates into primary particles which give better pigment development. The net result is better dispersion, enhanced quality and increased cost efficiency.

Can it be done?

In-house colouring is not difficult. There may be some issues but they are entirely manageable with the assistance of a colourant supplier. PVC suppliers can take up to six weeks to deliver product while masterbatch suppliers offer immediate technical support, colour matching and delivery of usable product within 24 hours.

With the freedom to avoid extended delivery times, processors can negotiate more favourable natural polymer prices and reduce stocks of coloured compounds.

The bottom line is improved production flexibility and better service to customers.



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