|Though masterbatches for adding colour to flexible PVC have been available for many years, they have never worked reliably in rigid PVC (PVCu). There are many reasons for this. PVC is a complex material that is highly sensitive to the introduction of additives. A universal masterbatch may be unsuitable in PVCu because the carrier lubricates the compound. Processors of the material have always had to find just the right grade for their particular plant and application. Consequently there are literally thousands of PVC formulations on the market with varying amounts of additives and different levels of compounding; all developed to meet specific performance and processing requirements, or both. Conversely, polymer specific masterbatches for flexible PVC contain plasticisers. Although this does not present compatibility problems when used in PVCu, plasticisers are being added to a polymer chosen specifically for its unplasticised characteristics. For over 30 years now, colour has been added to PVCu by using concentrates such as:
• liquid colour
• wax dispersions colour
• flexible PVC based masterbatches
All of these systems introduce additional components into the compound which, depending on circumstances, may present processing difficulties and/or lead to application failures due to plasticiser migration and changes in physical properties in the final product. Typical processing problems could be lamination, plate out or screw slip. In the final product, faults could manifest such as 'hazing' (due to optical density compatibility) or migration.
Like most masterbatches, PVCu polymer specific types have two distinct component parts; the active ingredients (colour pigments, additives) and the carrier (but unlike most masterbatches this is in constituent parts before compounding the masterbatch). Both these parts are manufactured simultaneously during the compounding of the masterbatch. The masterbatch is generally supplied to two types of processors:
Those who add colour to resin (compound suppliers, food packaging film manufacturers and producers of pipe, etc), and
Those using pre-compounded polymer (injection moulders, profile extruders, etc). Ultimately the application and the process used to manufacture the product determine the formulation of both parts of the masterbatch. New patented masterbatch technology focusing on processing aids and lubricant packages has lead to the development of products for rigid PVC grades without any of the inherent compatibility and homogenisation problems. During development, the primary considerations were to design a masterbatch that would work consistently in PVCu and that:
• Contained a realistic amount of pigment (i.e. could be used at low addition rates and was not just 'strong compound')
• Had to be processable on twin screw co-rotating masterbatch extruders
• Would homogenise easily into PVCu by a variety of subsequent processing methods acknowledging that the level of mixing in these processes might be less than that usual for other polymers
• Must not introduce any �foreign� ingredients to the final PVCu compound.
One of the major problems associated with PVCu processing is its resistance to flow, which is not uniform. When filled with large quantities of pigment this problem is increased significantly and, coupled with its heat sensitivity, the problem becomes a major hurdle to overcome. With flexible PVC, the incorporation of plasticisers negates this tendency, but for PVCu other ways are required to overcome these difficulties. The essence of the new technology for producing masterbatches that allow colour to be added successfully to PVCu is a combination of two additive systems used in the formulation:
High molecular weight acrylate processing aids are used to help the material flow smoothly. They also reduce melt viscosity, overcome melt fracture, and reduce shear burning. Selection of these aids can be dependent on application (e.g. when manufacturing tints for crystal compounds as some grades impart a haze or opalescence into the PVCu).
Lubricants fall into two types: internal and external. Internal lubricants help reduce frictional forces between the polymer molecules and/or the pigment. This generally aids dispersion and lubricates the melt flow (increase 'flowability' at lower temperatures).
: Internal lubricants: Selection and levels utilised are governed by the pigment type (organic, inorganic etc.) and pigment level. While individual lubricants work well, combination packages are generally used to achieve optimum properties. Internal lubricants used with success include stearates, montan ester waxes and amide waxes.
External lubricants: Prevent the masterbatch melt from sticking to hot processing equipment. This reduces the likelihood of shear burning and goes some way towards obtaining uniform flow. Care has to taken with the overall loading as excessive levels of external lubricant can reduce processing efficiency and give plate out (a condition where lubricants and pigments migrate to the surface of the extrudate and transpose onto processing equipment) problems in subsequent processing.
A number of lubricants can be used including low molecular weight PE wax, calcium stearate, oxidised PE wax and a commercial three wax blend designed to improve die lubrication. All of these worked in formulations but again, similar to internal lubricants, when used in combination to overcome specific problems, depending on pigment type and loading, the overall benefits were greater than using any one lubricant.
Masterbatches produced using the Vynacol technology makes it as easy to colour PVCu as any other plastic material. What makes this technology different is that it produces masterbatches that work both in rigid and flexible grades without any of the inherent compatibility and homogenisation problems - overcoming all the traditional difficulties associated with adding colour to rigid PVC via the masterbatch route. Tests proved that the ideal processing temperatures for processing these masterbatches were between 160°C and 180°C. Contributing to the success of the masterbatch is the fact that grades can be tailored to suit individual manufacturing processes. This is important because PVC has never been the easiest of materials to work with - a formulation that runs well on one machine may present processing difficulties on another, even identical, manufacturing plant. The ingredients used have been selected for good weatherability and excellent colour ageing characteristics. Further enhancements can be added to outdoor performance by adding UV stabilisers and absorbers. Use of approved pigments allows the masterbatches to meet all European food contact, packaging, and EN71 (toy) criteria. In practice the masterbatches are easily dispersed at a recommended dosage of 1-2% -although in thin film applications 8% is usual. All formulations are designed for use in filled or unfilled grades with no effects on processing or physical properties. In fact, the product contains additives that can improve and enhance the characteristics of the base PVC. Ease of processing, improved weld strength, increased ductility, and impact strength (especially at low temperatures), plus high gloss finishes are benefits derived from using the masterbatch.
(Author: Tony Gaukroger, ColourTone Masterbatch Ltd.)