Pigmentation of plastics mould material
Method of producing pigmented extrudates or plastic mould from an unpigmented thermosetting plastic mould material, which comprises roughly blending a pigment component with an aminoplast plastic mould material substantially free from pigment and having an average particle size not greater than 150 microns.
Such plastic mould materials are produced in various forms determined by, inter alia, the intended method of plastic mould for example compression plastic mould or injection plastic mould.
Normally, these materials are pigmented to produce coloured plastic mould materials, the pigmentation process involving intimately dispersing the pigment particles with the particles of plastic mould compound. One method of achieving this is byball-milling a coarsely powdered plastic mould compound (hereafter referred to as `coarse powder`) with at least one pigment to produce a finely powdered pigmented intimate mixture (hereafter referred to as `fine powder`).
Since there are difficulties in compression plastic mould fine powder (owing to its low bulk density) and also in injection plastic mould fine powder (owing to its poor feeding and plasticizing effects in standard injection plastic mould machinery), the finepowder is, generally, converted into granular plastic mould material. Granular plastic mould material is thus widely used for both injection and compression plastic mould. However, owing to the additonal processing and capital investment necessary for its production,granular plastic mould material is more expensive than coarse powder or fine powder plastic mould material.
An object of the present invention is to obviate or mitigate the expense involved in producing pigmented thermosetting plastic mould material, particularly the injection plastic mould grades.
Thus, we have found that, by using an extruder or injection plastic mould machine having a screw housing grooved in accordance with the concepts set forth in our aforesaid Offenlegungschrift and U.K. and U.S. applications, it is possible to obtainhomogeneously pigmented thermoset artefacts from synthetic plastics plastic mould materials, without the necessity for granulating fine powder, and even without the necessity for ball-milling or otherwise pulverising a relatively coarse powder.
When the aminoplast plastic mould material is based on melamine-formaldehyde resin, the average particle is preferably not less than 75 microns, otherwise adequate compounding may not be possible.
The pigment component may be a pigment per se, or may be a masterbatch consisting of a pigment homogeneously blended or coated with a thermosetting plastics composition; whether a pigment per se or masterbatch is used, will depend on the ease ofdispersion of the pigment component in the thermosetting plastic mould material, difficultly dispersible pigments being preferably (and in some cases, necessarily) used in the form of masterbatches. The ease of dispersion may depend on the flowcharacteristics of the thermosetting plastic mould material.
Preferably the weight ratio of pigment to thermosetting plastics composition in the masterbatch is from 5 to 50, more preferably 5 to 20, times the weight ratio of pigment to total thermosetting material required for the resultant pigmentedthermosetting plastic mould material. Preferably the pigmented thermosetting plastic mould material comprises from 0.1 to 10% by weight of pigment, and more preferably comprises not more than 5% by weight of the pigment.
The masterbatch may be obtained by mixing pigment with a thermosetting plastics composition during production of the latter and whilst it is in the “wet” state, and drying the resultant mixture, or by mixing pigment with dry thermosettingplastics composition, to produce a homogeneous blend. Ball-milling or equivalent treatment may be necessary to obtain a sufficient degree of pre-dispersion of the pigment in the masterbatch, but the masterbatch may be produced in any one of variousphysical forms, preferably as powder. The thermosetting plastics composition of the masterbatch and the thermosetting plastic mould material will normally (but need not necessarily) comprise the same thermosetting resin.
The rough blending of the pigment component with the thermosetting plastic mould material may be effected by mixing pigment per se or masterbatch in a tumbler or blender prior to supplying the roughly blended material, without intermediate processing,to the hopper of an injection plastic mould machine. Alternatively the pigment component and thermosetting plastic mould material may be roughly blended during or after supply thereof to a stirred hopper machine and prior to injection plastic mould that portion of thematerial in the hopper.
It will be appreciated that the initial thermosetting plastic mold material is, prior to roughly blending with the pigment component, substantially unpigmented, and is in a substantially dry condition.
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