|Polyvinyl chloride (PVC), because of its sensitivity to heat and shear stress, is different from all the polymers like polyethylenes, polypropylene, etc. Besides, PVC is compounded with a very large amount of different additives. In fact, 70% of all polymer additives are used only in PVC. PVC compounds are also made under different processing conditions of heat and shear stress.
Before PVC can be made into products, it has to be combined with a range of special additives. The essential additives for all PVC materials are stabilisers and lubricants; in the case of flexible PVC, plasticisers are also incorporated. Rigid PVC is very shear sensitive while flexible PVC containing large amount of plasticizers is much more stable. Other additives which may be used include fillers, processing aids, impact modifiers and pigments. Additives will influence or determine the mechanical properties, light and thermal stability, colour, clarity and electrical properties of the product. The mixing of additives needs to be selected based on type and amount of these additives that are incorporated in PVC polymer to achieve different properties. Once the additives have been selected, they are mixed with the polymer in a process called compounding. The mixers can include batch mixers, continuous mixers, twin-screw compounding extruders or continuous kneaders. One method uses an intensive high-speed mixer that intimately blends all the ingredients. The result is a powder, known as a Ďdry blendí, which is then fed into the processing equipment. Additives are added in a mixer and first the dryblend is made. Dryblend is either directly processed into the finished products or is first melt blended and pelletized. The goal of dry-blending; the co-mingling of PVC pellets with several types of additives - is to achieve a homogeneous free-flowing powder without a phase change which is then fed into melt compounding equipment. The workhorse machine throughout the history of PVC compounding has been the high-intensity batch. Direct processing of dryblend is typically done for rigid PVC products like pipe & profiles. The dryblend is directly processed on a conical or parallel twin screw extruder into finished products. For direct extrusion of a finished part, there is no need to pelletize the dry-blend, which can be processed using a conical or parallel twin-screw extruder to extrude directly into the finished part. Sheet compounds, depending on the type, may need a pelletizing step first in some cases, especially if a single-screw extruder is used for producing the finished sheet.
Economics plays a greater role in selection of mixing process and its equipments. Bussís kneader technology and Coperionís co-rotating twin-screw compounder are found to be ideal for the production of highly filled compoundsóboth flexible and rigid. For flexible PVC, the mixing equipment is selected based on plasticizer levels like:
• For formulations with upto 30 parts of liquid plasticizer, a continuous turbo mixer, followed by a co-rotating twin-screw extruder combined with a discharge extruder and a hot die pelletizing system can be used.
• For formulations with 30-60 parts of liquid, a batch heater mixer with intermediate vessel and agitator, followed by a co-rotating twin-screw extruder combined with a discharge extruder, and a hot-die pelletizing system can be recommended.
• For formulations with more than 60 parts of liquid, a heater/cooler batch mixer combination for preparation and dispersion of dry-blend, followed by a co-rotating twin-screw extruder combined with a discharge extruder and a hot-die pelletizing system can be used
The key factors in determining the technology to be used should be based on quality requirements, including output rate, temperature control, mixing efficiency and investment cost. Temperature control is a key issue, since PVC has a temperature memory. The lower the processing temperature used in the compounding step, the lower the temperature for moulding or extruding the material, thus giving better flexibility and temperature stability. Quality is typically linked to dispersion of the components during premixing and melt compounding. Absence of gels on the extruded surface and improved processability are key parameters of good mixing.
Farrel Continuous Mixer (FMC) is regarded by many in the industry as the most effective compounder for all variants of PVC, but especially in the production of semi-rigid and rigid PVC compounds. Continuous Mixer or Combi machine of Coperion which has twin screw along with single screw for delivering the melt under pressure to pelletizer is now widely used in PVC compounding. Coperion has now developed the combi machines to incorporate almost 85% of filler in PVC.
For a high throughput vinyl compounding applications - such as at suppliers manufacturing pelletized compounds for the commercial market - the kneader technology is very effective for up to 4,600 kg/hr.