NEW MASTERBATCH TECHNOLOGY FOR COLOURING RIGID PVC
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Traditionally colour has been added to PVCu by using concentrates such as liquid colour, wax dispersions colour and flexible PVC based masterbatches. All of these systems introduce additional components into the compound and can present processing difficulties and lead to application failures due to changes in physical properties.
New patented masterbatch technology focussing 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.
Universal and polymer specific masterbatches for adding colour to flexible PVC have been available for many years but 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. For example, a universal masterbatch may be unsuitable in a 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, you are, in effect, adding plasticisers 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.
Formulation and processing
Like most masterbatches PVCu polymer specific types have two distinct component parts:
- The active ingredients (colour pigments, additives)
- The carrier (but unlike most masterbatches this is in constituent parts before compounding the masterbatch)
Both of these parts are manufactured simultaneously during the compounding of the masterbatch.
The masterbatch is generally supplied to two types of processor;-
- Those who add colour to resin (compound suppliers, food packaging film manufacturers and producers of pipe for example)
- 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.
Clearly a specific colour requirement would affect the selection of the active ingredients (pigments) and the ratio at which these are utilised to obtain the required shade. However, the formulation of the carrier, even into a single application, e.g. food packaging film, would be different if the subsequent manufacturing process was, for example, either calendering or twin screw extrusion. This not surprising when one considers that the compound formulation would be different for each of these processes.
Opacity requirements also affect the choice of pigments and hence are factors to consider in formulation.
Continuing with the food packaging film example mentioned earlier, a colour masterbatch to be used in a calendering line would tend have a higher pigment loading and be less lubricated than a colour masterbatch for use in the same film produced by the extrusion process.. This would result in different levels of masterbatch being used in each process to produce the same result, and this too has to be considered in the masterbatch formulation.
In developing this masterbatch technology 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.
Two key target customer types were identified as those who add colour to a process starting from PVC resin and those who add colour to pre-compounded PVCu.
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 we have to look for other ways to overcome these difficulties.
The essence of our new technology for producing masterbatches that allow colour to be added successfully to PVCu is a of combination of two additive systems used in the formulation;-
- Processing aids
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). We have found that processing masterbatch in the temperature range 160 – 180ºC is ideal and it is critical to obtain uniform flow of the melt at this point.
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.
It is worth noting that 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.
During experimentation we also found that lubricant efficiency can only be realistically evaluated in a production environment. Our 19mm co-rotating laboratory extruder was simply not powerful enough to process the range formulations and provide qualitative data.
Processing temperaturesTests proved that the ideal processing temperatures for processing these masterbatches were between 160ºC and 180ºC. Early in the development programme it was found that the material tended not to flow in the area in between the end of the extruder screws and the die. This area is without positive cooling on the Leistritz ZSE compounders we use for masterbatch production. When producing a material containing organic pigments at 15% and greater the flow became almost static.
We noted that the temperature of the melt 'hanging up' in this uncooled area tended to slowly rise. After 20 minutes the temperature would reach 200ºC with catastrophic results - burn up!
The problem was solved by introducing positive liquid cooling to this transfer section. Subsequently lubricant development overcame the problem but we still order extruders with this modification because it has proven to offer a useful extra measure of control.
Putting it all together
Let us now look at some examples of colour masterbatches produced utilising the process aids and lubricant packages discussed.
A relatively simple formulation based on rutile TiO2. Incorporating 5% calcium stearate, 5% low molecular weight PE wax and 8% of a polyacrylate process aid is enough to ensure uniform running extrudate.
2) 30% SRF Carbon Black
Because of the stiffening effect of the carbon black on melt rheology more has to be done to obtain even flow.
Like the white, 5% calcium stearate is included but a combination 5% low molecular weight PE and 2% of a fatty acid bis-amide wax gives a good result with uniform flow.
3) Green masterbatch for profile extrusion
This masterbatch presents a different lubrication task because it contains 19% organic pigment and 9% inorganic pigment.
TiO2 – 9%
C.I. Pigment Yellow 17 – 11.9%
C.I. Pigment Blue 15.3 – 5.1%
Carbon Black – 2.1%
To facilitate processing it is necessary to increase the acrylate processing aid to 14% and a introduce a mixed lubrication package consisting of 2.5%Calcium stearate and 7.5% of a partly saponified ester wax based on montanic acids.
4) Gold masterbatch for extruded film
Even more demanding is this masterbatch which is used at high loading to achieve opacity in PVCu film. Its pigment package includes:
Titanium coated mica – 32%
C.I, Pigment Yellow 62 – 4.2%
C.I. Pigment Orange 64 – 0.3%
This formulation has certain aspects that make it an interesting example of how processing aids and lubrication packages have to be combined to ensure optimum processing and end use properties:
Acrylate processing aid – 10%
Calcium stearate – 2.5%
Amide wax – 5.0%
Commercial blend of waxes – 2.0%*
Oxidised ethylene wax – 2.0%**
* The problem of material becoming static between the screw tips and the die (as mentioned earlier) is reduced considerably by using this commercial product
** Mica based formulations suffer extreme melt fracture problems around the die which is improved significantly by using oxidised waxes.
Note that care has to be taken with this type of application not to over-lubricate the product and cause gelation problems, screw slip or plate out during subsequent processing.
Employing innovative masterbatch formulation techniques and focussing on processing aids and lubricant packages we believe we have developed the world's first polymer-specific masterbatch for PVCu.
This technology, which we have called Vynacol, is now patented world-wide. Masterbatches produced using this 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.
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.
Of course, the exact formulation and screw configuration of our compounding extruders are closely guarded secrets but the all the work is based on established PVC processes and materials combined with specially modified manufacturing plant.
The ingredients we use have been selected for good weatherability and excellent colour ageing characteristics. If required we can add further enhancements 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.
Looking ahead we are working on the development of a new range of additive package formulations that include antimicrobial, uv, antistatic, process aids, flame retardant, and specific problem-solver characteristics.
ColourTone Masterbatch Limited
Pant Glas Farm Industrial Estate
Telephone: 01222 888910
Fax: 01222 868487
Telephone: 01428 723211
Fax: 01428 722371