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Polypropylene Handbook Nello Pasquini ISBN 3-446-22978-7
Leseprobe
Weitere Informationen oder Bestellungen unter http://www.hanser.de/3-446-22978-7 sowie im Buchhandel
7
Fabrication Processes
A. ADDEO, EDWARD P. MOORE, JR.
7.1
Introduction
Polypropylene (PP) is transformed into useful products by a wide variety of processes, which has been, together with a suitable cost/performance balance, a major factor in its commercial success. Figure 7.1 shows the breakdown of global sales among the major processes. The ease of molding and the attractive strength, stiffness, and high use temperatures of articles molded of PP have made injection molding the largest consumer of PP among the processes used. A unique aspect of the PP processes, compared to the other major plastics, is the use of orientation to develop enhanced properties, principally in fibers and films, constituting nearly one-half of the consumption. None of the other major plastic materials uses orientation to any appreciable extent, except PET, which is considered a major plastic, where orientation is used in fibers, biaxially oriented films, and soda bottles, and nylon, which is oriented into fibers. Lesser but significant quantities of PP are used in unoriented film, sheet, and blow molding. 7% 5%
20%
10%
7%
4%
19% 21% 7%
Homo IM
Fibre
Copo IM
IPP Film
Sheet
BOPP
Other
Nonwoven
Raffia
Figure 7.1 Breakdown of PP demand by end-use in 2003 (Source: Jacobs consultancy – Phillip Townsend Associates Inc.)
382
7 Fabrication Processes
7.1.1
[References on Page 448]
Introduction to Extrusion Processes
Extrusion-based processes may consist of the following steps: • Extrusion, • Forming of the extrudate within and outside of a die, • Quenching the extrudate and crystallizing it, • Reheating the extrudate for forming, or • Orienting the crystallized extrudate. Not all of these operations occur in every extrusion-based PP process, but they are basic to the processing of PP. Table 7.1 shows the variety of products made using these process steps. All employ extrusion and quenching, and some use one or more of the downstream processes in forming the end-use product. Extrusion Processes and Products
Table 7.1
Shape of die opening
Extruded product
Oriented product
Melt formed product
Slot
Sheet Cast film Melt formed Extrusion coated
SPPF* items Slit tape Biaxially oriented film (tentered) Decorative ribbon
Thermoformed items
Annular
Pipe Water quenched blown film Air quenched blown film Foamed sheet Wire and cable insulation
Biaxially oriented film (bubble) Twine
Blow molded items
Individual openings
Special profiles *
Strapping Slit tape Continuous multifilaments Staple multifilaments Spun-bonded nonwovens Melt blown nonwovens Corrugated board Profiles
Solid phase pressure forming
7.1.1.1
Extrusion
The purpose of extrusion is to deliver a molten polymer, uniform in temperature, molecular weight (MW), and output rate, and free of contaminants or faults such as bubbles or unmelted polymer, to the forming die. A whole separate science has been built around the process of accomplishing these easily stated aims. Over the years, many papers and books have been published on extrusion or, in general on plastic processing science, but only some of them,
7.1 Introduction
383
although dated, still remain fundamental for the study of the relevant theory [1, 2, 3, 4]. Rather than attempt to present that science, we will mention some of the more important points that govern the selection, design, and operation of processing equipment and the consequences to the end-products. When the physical volumes of the PP products are considered, along with the fineness of some of the extrudates, such as fibers, the requirements on cleanliness and uniformity of viscosity in the extruded melt (whether from temperature or MW variations) are extremely demanding. Yet, they are met on a regular basis. The processor must know the limitations of his equipment with regard to the above needs, as well as chose the appropriate polymer grade, to achieve the desired goals. The supplier, in turn, must recognize the sensitivity of the processor’s operation to each of the polymer variables under his control. With any given system, the capabilities of the equipment can be exceeded, leading to quality fluctuations. It is the task of the processor, with the assistance of the polymer supplier, to reach the best balance of polymer, equipment, operating conditions, and end-use properties for the intended application. Because of the large volume, critical PP processing operations usually require close co-operation between supplier and processor to achieve the best balance of these factors. Of course, cost is always part of the equation, as well. To aid in producing the most uniform output rate and melt quality, metering pumps are often used in extruding PP, and melt filters are almost universally employed. Although continuous filters are often used, most filters are batch-operated, and a shutdown is needed to replace or renew them. This results in gradually changing conditions as the filter plugs and the back pressure increases. While this can be mitigated to some degree by using larger filters, the size can quickly become cumbersome. In spite of attempts to improve the filter designs and operation, filtration remains a problematic but essential step for reliable extrusion operations. The designs of extrusion dies are highly varied; they are often custom designed for the particular process or customer. Fortunately, PP is far less prone to disastrous thermal breakdown in the die than, for example, PVC, where minor hang-up points can lead to major degradation of the polymer, with the release of toxic and corrosive gases. In such a situation, PP merely degrades to a somewhat lower MW, and proceeds on through the die. Consequently, the design of dies for extrusion of PP is simpler than for the more sensitive polymers. In general, for thin extrudates, high melt flow rates are used to reduce the pressure drop through the die. This normally results in some compromise in toughness properties, which is less of a problem in oriented items. Where the melt must retain its shape for some time before crystallization occurs, or where high toughness is crucial, low melt flow rates are used. Enhancement of the melt shape retention can be provided with a special type known as “high melt strength PP” [5]. For thin extrudates, a large drawdown from the die is usually employed to allow a larger die opening and lower pressure drop through the die, and, in the case of slot and annular dies, more practical die opening adjustments for controlling thickness distribution across the extrudate. The smallest die openings are used for thin films, extrusion coating, and fibers, where 0.3 mm to 0.5 mm (10 mil to 20 mil) are typical openings, although the final extrudate is often a small fraction of that. If the drawdown ratio is high, it is possible to encounter a phenomenon known as drawdown surge. This is a rheological response to the drawing stress that can give a large periodic variation
384
7 Fabrication Processes
[References on Page 448]
in the thickness of the drawn item. It is most commonly found in fiber processing but may also occur in thin film or extrusion coating operations. It is alleviated by lowering melt viscosity, reducing the draw stress, or more rapid cooling during the draw, which solidifies the drawn item more quickly, thus stabilizing it. Depending on the particular process, changes in the rheology of the polymer may also help.
7.1.1.2
Quenching
The cooling, or quenching, of the PP extrudate is easily separated into two categories: slow and rapid. With thick extrudates, thicker than approx. 2 mm (80 mil), no amount of rapid outside cooling can speed up the removal of heat held inside the PP section. So at least the center of the extrudate will be slowly cooled, with the high crystallinity and large spherulites associated with high crystallization temperatures. In water-cooled thick sections, such as profiles, it is also necessary to provide enough heat transfer on the water side, through agitation or flow, to prevent the stagnation of the water movement and the occasional evaporation of the water, causing bubbles and visual faults on the surface of the extrudate. In thin sections, thinner than approx. 0.3 mm (12 mil), the heat transfer from the inside of the molten extrudate is rapid enough that the external cooling rates control the PP form obtained. When the cooling rate exceeds 80 °C/s, lower crystallinities and the clearer, tougher mesomorphic (smectic) form is obtained. This usually means making sure that the cooling device, such as a chill roll, intimately contacts the melt. In the case of chill roll cast film, an air knife is usually used to insure that the melted web is in contact with the metal roll, not floating on an insulating layer of air between the film and roll. Layers of air as thin as 25 µm (1 mil) can significantly reduce the cooling rate. The most rapid cooling results, of course, from the coldest cooling device. However, practical considerations usually limit the coldness of the quench device; in the case of a chill roll, the condensation of moisture from the atmosphere on the exposed section of the roll would lead to difficulties in obtaining uniform cooling of the thin film. When air is used for cooling, as with a bundle of fibers, the uniform flow of the air over the molten strands is usually the factor limiting the rate and the uniformity of the cooling. Therefore, considerable attention is given to the design of air distribution systems. Air-cooling is not rapid enough to achieve the smectic form of PP in conventional grades. Therefore, PP is not conventionally used in air-cooled films, while this has been a major outlet for PE. However, with the development of more rubbery, less crystalline forms of PP, known as high alloy copolymers, air-cooled films have become a reality with PP. Further details appear in Section 7.2.3.3.
7.1.1.3
Reheating
When reheating PP, such as in a thermoforming operation, a new set of questions arises. While in theory the most rapid heating comes from the highest temperature source, such as radiating red-hot heating elements, the uniformity of heat becomes a problem with PP. Because PP does not absorb infrared rays well, variations in the temperature reached can be quite high, depending on minor differences in thickness, crystallinity, composition, or proximity to
7.1 Introduction
385
the heaters. In addition, the sharp melting point of PP usually means that the behavior of the partially melted form changes rapidly as the temperature rises, requiring a quite precise temperature to process well in the next process step. It is for this reason that, for processes such as thermoforming and foaming, PP has been regarded to have a narrow processing window. That view has been revised with the development of high melt strength PP. Heating with hot air provides more precise temperature control, but larger installations and accordingly higher investments are needed. This approach is taken with the tenter frames (see Section 7.2.2.2) used for making biaxially oriented films. In large volume operations, some advantage can be realized by conducting the downstream operation in-line with the extrusion. In that instance, the quenching operation need only remove enough heat to establish the dimensional stability of the extrudate, and reheating may begin while considerable heat remains.While this reduces the amount of reheating needed, the condition of the form entering the final operation now depends on the consistent operation of all three upstream steps: the extrusion, the quench, and the reheat. Therefore, this approach is attractive only for large volume products. The crystalline nature of PP requires more heat to melt it, more cooling to quench it, and again more heat for reheating operations, than required for amorphous polymers. In this respect, comparisons with, for example, polystyrene are often disappointing, as some 40% more heat must be transferred to achieve similar processing rates, resulting in slower throughput or greater investment in heating and cooling equipment.
7.1.1.4
Coextrusion
Often, special properties can only be achieved by a significantly different polymer in a coextruded structure. In PP, the properties typically achieved by coextrusion are improved oxygen barrier, typically using an ethylene vinyl alcohol (EVOH) polymer or a wider heat seal temperature range, although embedding colored or reclaimed polymer have also employed coextrusion. Coextrusion has been applied to sheet, blow molding, and biaxially oriented film. The achievement of uniform distribution of a polymer such as EVOH, differing sharply from PP in composition, preferred process conditions, and rheology, is a major technological challenge. Whether a slot or annular die is used, special designs are needed to distribute the included non-PP layer uniformly across the sheet or around the tube. Multichannel dies, fed by separate extruders for each polymer, offer the most reliable, but also the most expensive, approach. When the polymers involved are reasonably well matched rheologically, a feedblock approach can be a less expensive alternative [6]. In this design, layers of polymer are joined at a relatively large cross section and are designed to distribute uniformly as the melt flows into a relatively wide, thin extrudate. In non-critical situations, this approach can provide satisfactory distribution, but maintenance of the distribution also requires careful control of process variables. Even with multichannel dies, other problems arise. Adhesion between polymer layers can be a problem; therefore special polymers are introduced to provide adhesion between the dissimilar polymers. Consequently, the original three-layer structure, PP/barrier/PP, becomes five layers: PP/adhesive/barrier/adhesive/PP.