R E V I E WTECHNICAL I N F O R M AT I O N PLASTICS R2-02 Compounding Unplasticised PVC (The Mixing Process) Omya Applied Technology Services, Segment Plastics Mixing is an important operation in PVC compounding. Some of the problems that occur during subsequent processing can be avoided by optimising the mixing process. Below, we summarise the important aspects of mixing, some of the problems that often occur and suggested solutions. if dispersion problems occur frequently.0 2 . The mixer blade rotates slowly at first. especially at higher filler loadings. However. Optimum Mixing Process A common practice is to put all the components into the hot mixer at the beginning of the mixing process. Mature for 24 h In practice. 7]. 5]. ■ At about 50 °C. add the one-pack stabiliser system. but is speeded up subsequently to mix the compound and heat it up to about 110 to 130 °C. resulting in fisheyes. 7]. disperse homogeneously and bind to the PVC particles together with the stabilisers. for optimum mixing.). 2. lubricants. After the formulation has been heated to the preset temperature (about 110 to 130 °C).slow and fast. add them slowly at about 70 °C (inject them if possible). but. If the formulation contains liquids (Sn stabilisers. the compound is heated by intensive fluidisation at high circumferential speeds of 20 to 50 m/s of the mixing blade. Add fillers/pigments In the hot mixer (also called a high-speed mixer. Poor mixing of the components is difficult to correct subsequently and is often the cause of problems such as fisheyes and surface flaws in semi-finished products. fluid mixer or intensive mixer). the PVC powder and additives are first heated to about 120 °C in a hot mixer and then cooled to about 40 °C in the cooling chamber. in which the PVC is mixed with additives such as stabilisers. Mix Heat to 50°C The dry blend is cooled in a double wall. epoxidised soya bean oil. P a g e Charge PVC powder 2. Adding them all . greatly reducing the cooling efficiency of the mixer and generating agglomerates [7]. We consider the best procedure to be as follows (see Figure 1): ■ First charge the PVC into the mixer. most mixers can only operate at two speeds . in the ideal case. water-cooled mixer. 2].R e v i e w 1. surface flaws and poor mechanical properties. we recommend adding the additives step by step. this is satisfactory. it is best to add the lubricant at 50 °C and then the solid stabilisers at about 70 °C. we recommend mixers with infinitely variable speeds. ■ Figure 1: Optimum Mixing Sequence for Highly Filled PVC Formulations. As long as the compound produced is free from defects. a number of problems in processing can be traced back to agglomeration and separation resulting from an inadequately compounded dry blend. fillers and pigments to produce a dry blend. etc. In our experience. Low speeds are used to avoid generating frictional heat [4. while ensuring self cleaning of the mixing vessel and mixing blade [4]. Mix briefly and then start to add the other components one at a time. At lower temperatures. the hot dry blend must be cooled to below the melting point of wax (about 40 °C) to 2 prevent agglomeration and allow it to be stored. that all components should bind to the PVC particles. Mix Heat to 70°C Add stabilisers Mix Heat to 90°C Mix Heat to 120°C Charge dry blend to cold chamber Cool to 40°C It is important to match the filling volume (about 70 to 80 %). 2 . The low-melting lubricant that this contains will liquefy. Add lubricant It is particularly important to control the speed of the rotor in the cooling chamber to obtain optimum cooling times [6. the geometry of the mixing blades and the rotational speed to obtain optimum material flow with vortexing in the heating mixer.1. This often eliminates or at least reduces the problems. Hot/Cold Mixing The most widely used process for preparing the dryblend PVC is discontinuous compounding in a two-stage heating/cooling mixing process [3. Introduction PVC processing first requires a compounding process [1. Final compounding and processing of uPVC The cooling water should be at a temperature of between 15 and 20 °C. However. Remove dry blend from cold chamber The vortex assists the homogeneous distribution of additives. It is important not only that all components of the formulation should be homogeneously mixed. atmospheric moisture may condense on the walls of the chamber causing the dry blend particles to stick to them and form an insulating layer. In this case. If the stabilisers and lubricants are added separately. Often. At about 90 °C. MTI Mischtechnik Industrieanlagen GmbH. M. 2]. M. [10] Eigruber. pp.0 2 . 3. Aufbereitung von PVC für die Fensterprofilextrusion – Verfahren und aktuelle Trends. leading to inhomogeneities in the final compounding [1. This prevents colour discrepancies (greying) by metal particles. Table 1 lists some typical problems that arise from inadequate mixing. In both cases. and some of them remain in the dry blend as separate fractions. Grossefehn. In such cases. If CaCO3 and TiO2 are added earlier. The rest of the PVC for the formulation is then put into the cooling chamber.74-77. particularly of TiO2. 395-405. [2] cpm GmbH.2 Double Batching In the double batching process [9].. Problems must therefore always be dealt with on a specific case-bycase basis. we recommend a two-stage heating/mixing process in which PVC. conveying and metering [6]. 1998. Late addition. 2000. the additives are not completely adsorbed by the PVC grains (because the available surface area is too small). differences in density between the particles may lead to separation during storage. H. H. In the best case. stabiliser and additives are first loaded into the mixer and then subsequently the CaCO3 and TiO2 pigments. variations in the gelation of the PVC. Mischen von Schüttgütern. Dissertation. this brings the following specific advantages ■ Free flowing compound ■ No segregation during storage. pp. 2 . . Grundsätzliches und Problemlösungen für die PVC-Extrusion. a homogeneous dryblend is obtained. the numerous problems that may occur can have entirely different causes. 3rd Intern. 1998. Detmold.. M. where it is mixed with the premix from the hot mixer. Dusseldorf. The additives begin to stick to the surface of the PVC particles. Optimisation of the PVC Mixing Process for Window Profiles. ■ ■ At 75 to 80 °C. pp. Dissertation.. VDI-Gesellschaft Kunststofftechnik. Where the optimum mixing process is used. they become almost completely bound to the PVC particles. Kunststoffe/Plast Europe 76 (1986) 5.. M. H. Aufbereiten von PVC. when the temperature reaches about 90 °C. M. Double-Batching in der Profilherstellung. [7] Heine. [4] Pahl.. [5] Pahl. P a g e 3 [6] Grosse-Aschhoff.. For processors. Troubleshooting during Mixing In uPVC processing. Aachen. PVC-Aufbereitung. Literature [1] Krüger. Paderborn. pp. in which the additives are satisfactorily bound to the PVC. transport or metering ■ Low dust ■ Improved dispersion of the formulation components ■ Reduction of wear on the mixing blades ■ Reduced residual moisture content in the dry blend Not all processors are able to adapt their compounding process to meet optimum mixing criteria.. 1999. [9] Grosse-Aschhoff. 2. The molten lubricant promotes this process. 4.. and therefore to poorer quality of the semi-finished product. J. Plastics Window Congress 2000.R e v i e w at once runs the risk of agglomeration and inhomogeneities. Solid-State Mixing of Plastics. [8] Grosse-Aschhoff. in: Schüttguttechnik in der Kunststoffindustrie. some (usually 50 %) of the PVC is mixed with the total amount of additives in the hot mixer. some of the low-melting lubricant may adhere to the surface of the TiO2 or CaCO3 particles. 38-44. this yields a mixture of virgin PVC particles without additives and PVC particles containing twice the amount of additives than that produced in a conventional (single batching) mixing process. so that as the processing continues. H. the CaCO3 and TiO2 pigment are added. Kunststoffe/Plast Europe 76 (1986) 5. Kunststoffe/Plast Europe 89 (1999) 2.. [3] Langenberg. Design of Heating Mixers for Plastics Compounding. Plastics Special 7-8 (1999). there is no obvious relationship between the processing problems and the PVC compounding process. E. In the worst case. 406-409. 1985. has the added benefit of reducing wear on the mixer blade and vessel. This results in poor homogeneity and has a negative effect on the subsequent process [10]. especially since these minerals have a larger specific surface area than PVC [8]. Omya also offers technical support. the softening point of the PVC is reached. [email protected] 2 . the mixer blades and mixer blade speed to one another ➞ Check mixer blades for abrasion Table 1: Processing Problems that often Occur as a Result of Poor Mixing.1) ➞ Check the material flow in the mixer (vortexing assists the distribution of formulation components and promotes self-cleaning of the mixer vessel and blades) ■ Mixer must be frequently cleaned ▲ Caked material on the mixer wall and mixer blades (see above) ▲ Material deposits in mixer as a result of inefficient material flow ➞ Use the optimum mixing process ➞ Check the material flow (see below) ■ Irregular gelation of the PVC [cf. These agglomerates cause fisheyes in semi-finished products. fisheyes ▲ Inhomogeneities in dry blend ➞ Use the optimum mixing process ➞ Check the metering equipment and lines (has separation occurred leading to irregular. higher speed of the mixer blade. 1. higher ultimate mixing temperature [up to 130Þ C]) ➞ Check the material flow in the mixer (does vortexing occur?) ■ Hard agglomerates in the dry blend cause fisheyes in the semi-finished product ▲ Caked material on the mixer wall and mixer blades (see below) ▲ Insufficient dispersion of the one-pack stabiliser ➞ Check the raw materials ➞ Is the mixer too full? ➞ Check the material flow in the mixer (see below). and it is the user's responsibility to assess the material's suitability (including safety) for a particular purpose prior to such use (March 2009).com www.com . at 90 °C (see 2. does vortexing occur? ➞ Use the optimum mixing process ■ Caked material on the mixer wall and mixer blades ▲ Additives melt on the hot wall and mixer blades and stick fast ➞ Charge the PVC first. Recipients receiving this information must exercise their own judgement as to the appropriateness of its use. losses. or damages of any third party. inhomogeneous dosing?) ➞ Avoid double batching ■ Inhomogeneities/agglomerates during the feeding of liquid components ▲ Sudden addition of liquids runs the risk of agglomeration/inhomogeneities ➞ Add liquids slowly while the motor is running. then the additives (see 2.) ➞ Check regularly for wear of the mixer blades ■ Inefficient material flow in the mixer (poor vortexing) ▲ Mixer is too full ▲ Material flow altered by wear to mixer blades ➞ Match the amount of material in the mixer. Omya International AG Baslerstrasse 42 4665 Oftringen Switzerland Telephone: Telefax: E-Mail: Internet: +41-62 / 789 29 29 +41-62 / 789 26 36 plastics. inject if possible ■ Dry blend generates dust ■ Separation of the dry blend on storage. 2]. however Omya makes no representation or warranty as to the completeness or accuracy thereof and Omya assumes no liability resulting from its use or for any claims. ▲ Soft agglomerates can form in the raw material as a result of transport and storage Moisture absorption promotes the formation of such agglomerates ➞ Increase the energy input to the mixer (longer mixing time.1.R e v i e w Problem Causes 2 . conveying or metering ▲ Inadequate binding of the additive to the PVC particles ➞ Check the material flow in the mixer (does vortexing occur?) ➞ Increase the energy input to the mixer (see above) ➞ Use the optimum mixing process ➞ Avoid double batching ■ Colour deviations/greying Dryblends of the dry blend ▲ Abrasion on the mixer blades ➞ Only add the TiO2 later in mixing process. The information provided herein is based on technical data that Omya believes to be reliable. P a g e 4 Solutions ■ Dry blend contains soft agglomerates that can be crumbled between the fingers. The information contained in this Technical Documentation relates only to the specific material designated herein and does not relate to use in combination with any other material or in any process.