POLYMERIC FILM CASTING AND APPARATUS THEREFOR

Abstract: A method for casting polymeric film and apparatus suit­ able for use in the method are disclosed. In order to increase the rate of production of polyester film in cases where such film is produced by extruding a molten polymer onto the quenching surface of a rotating casting wheel, the shell thick­ ness (10) of the casting wheel (16) is reduced so that the maxi­ mum permanent local shell deformation at the point where the polymer first contacts the wheel surface does not exceed 0.002 inches (0.051 mm) in the radial direction.

Description: "POLYMERIC FILM CASTING AND APPARATUS THEREFOR" This invention relates to the casting of polymeric film, and to an improved process and apparatus therefor. Background Art In Alles and Saner, U.S. Patent 2,627,088, there is disclosed a process of preparing photographic film which includes casting a molten highly polymeric polyester into an unsupported sheet of film, the polyester being also capable of being spun into filaments, which when cold drawn, exhibit, by characteristic X-ray patterns, molecular orientation along the fiber axis. The properties of such film may be further enhanced by a post casting treatment of the type disclosed by Alles, U.S. 2,779,684 to provide a highly dimensionally stable polyester film suitable for use in the photographic industry. Traditionally, the casting procedure for such film involves extruding molten polymer onto a casting wheel at a temperature sufficiently high to assure good flow properties for the extrudate during the extrusion process, and then quenching said extrudate to provide a self-supporting web of film, which is further treated as shown in Alles above. The purpose of the quenching process is to cool the molten polymer to a temperature below its glass transition temperature through the bulk of the cast web to form a permanently self-supporting web which may be then stored or subjected to other treatment as desired. Commonly the casting wheel upon which the molten polymer is extruded also serves to quench the film. To that end, the casting wheel is hollow and a coolant fluid is circulated within said wheel to remove heat from the surface of the web supported on the wheel. Simultaneously the outer surface of the cast web may also be cooled, using air impingement, cooling baths, chilled rolls, or combinations thereof. U.S. Patent No. 3,853,447 discloses a combination casting and quenching apparatus which is useful in providing a high quality quenched web by employing a combination of the aforementioned cooling technques. While the use of combined casting and quenching wheels has proven satisfactory for the production of high quality polyester film, problems begin to surface when higher and higher production rates are desired. Since heat must be removed from the molten polymer while on the casting wheel, either (1) the size of the wheel must be increased as the production rate is increased, or (2) heat must be removed from the polymer at a faster rate, or both. At the same time, the casting wheel must be maintained true to a high degree of accuracy since it is the distance between the extruder lip and the outer wheel surface that determines the final product thickness and, even more important, the thickness uniformity of the cast web. The casting wheel is subjected to substantial stresses because of temperature fluctuations ranging from (a) over 2750C at the point where the molten polymer contacts the wheel surface, to (b) below 750C at the point where the polymer has been cooled to below its glass transition temperature, to (c) the coolant fluid temperature of about 150C. In addition to the above, an axial temperature gradient tends to form on the casting surface because the extreme ends do not carry any polymer and are exposed to the atmosphere and consequently to room temperature. The resulting distortion of the casting surface can be fatal to the success of the casting operation, a problem which is discussed in Pabo, U.S. 3,142,866. To prevent such distortion or deformation it is common practice either to go to casting wheels of'increased size and massive construction or to adopt complicated wheel structures as shown in Pabo above. However, the coolant fluid can only remove heat at a finite rate from the interior surface of the casting wheel. Thus, the rate of production as measured in terms of pounds of polymer per hour for a given installation appears limited by engineering factors related to the casting wheel and coolant heat-removing ability. BRIEF SUMMARY OF THE INVENTION We have now surprisingly discovered that the production capacity of a system which involves casting a molten web of thermoplastic film-forming polymer on the peripheral surface of a cooled casting wheel may be greatly increased without any loss in film thickness uniformity by reducing the shell thickness of the wheel rather than increasing its diameter. What is surprising is the discovery that reducing the shell thickness actually reduces the wheel deformation instead of increasing it. The overall process comprises cooling the molten web on the casting surface of the wheel to a temperature and depth sufficient to permit stripping of the web from said casting surface by a cooling, of the wheel and thereafter stripping the web from the casting surface, characterized in that the production capacity is increased by providing a shell thickness of the casting wheel such that the maximum permanent local shell deformation of the wheel at the point where the molten polymer first contacts the wheel surface does not exceed 0,002 inches (0.051 mm) in the radial direction. Typically, the thermoplastic film-forming polymer is a polyester, more particularly polyethylene terephthalate; the molten web is cast at a temperature 0 desirably of at least 265 C; and the undersurface of the molten web is cooled on the wheel to a temperature below the glass transition temperature of the polymer to a depth preferably not exceeding 25% of the polymer web thickness as cast. To practice this process we also provide a film casting wheel which comprises: a hollow body member having a tubular portion forming a shell of heat conductive material, the curved external surface of which constitutes a casting surface onto which in use a stream of molten thermoplastic polymer is to be deposited, and having end walls of heat conductive material substantially closing the ends of said tubular portion and directly connected thereto so as to be in a heat conductive relationship therewith; means for supporting said body member for rotation about the axis of said tubular portion; means for filling said hollow body member with a cooling liquid, and means for circulating said cooling liquid into and out of said body member while it is rotating in order to control the temperature of said casting surface; ; characterized in that the shell of said casting wheel has a thickness such that the maximum permanent local shell deformation thereof at a point where, in use, the molten polymer first contacts the wheel surface does not exceed 0.002 inches (0.051 mm) in the radial direction. It goes without saying that the total thermal stress-induced deformation should not exceed the elastic limit of the construction material of the wheel shell. The invention may be more clearly understood by reference to the following drawings, which are by way of example only and in which: FIG. 1 is a schematic representation of an end view, partly in section, of a casting wheel of this invention. FIG. 2 shows the casting wheel prior to the application of heat stresses. FIGS. 3, 4 and 5 illustrate the casting wheel deformation due to the combined effects of casting a hot polymer web on a portion of the surface area of the internally cooled hollow casting wheel. FIG. 6 is a graphic representation of the cooling capacity of a casting wheel as a function of shell thickness. In FIG. 1, a casting wheel 16 having a shell 10, illustrated in cross-section, is positioned under an extrusion device 18 which forms a nip 20 with the surface 22 of the casting wheel. The latter is supported along its axis 24 and a motor drive, not shown, rotates the wheel in the direction of the arrow. The wheel has end walls substantially closing the wheel (not shown). A coolant fluid 14 from a reservoir 30 is supplied to the casting wheel 16 through a pump 26 and a piping system generally denoted as 31, 27 and 25. Coolant thus enters the wheel, circulates within, and exits-, carrying away heat from the inner surface of the shell and controlling its temperature. In operation, a web of molten polymer 12, extruded through extruder 18 and nip 20, is carried by the rotating casting wheel 16 until it reaches a point 17 by which time it has formed a skin layer on the underside contacting the wheel, sufficient to allow stripping. The outer web surface may be also subjected to accelerated cooling by contact with a gaseous or liquid coolant, e.g., by air impingement through a system of air nozzles generally indicated at 32. A cool calendering roll 34 assures good contact between the molten web 12 and the casting surface 22 and also serves to smooth irregularities on the outer surface of the cast web. As the web reaches point 17 on the casting wheel, it is stripped by takeoff roll 36 and advanced for further treatment (not shown). The transfer of heat from molten polymer web 12 through the wheel shell 10 into the coolant 14 is schematically shown in FIG. 1. We have found the rate of heat transfer q to be dependent on two processes. First, heat is conducted through the wheel shell 1 , a conduction process, and then it is removed by the coolant through a convection process across the wheel coolant interface. For purposes of this invention the inter facial heat transfer between the polymer and the wheel outer surface 22 is ignored. The above described heat transfer process may be expressed in mathematical form as: q = UA#T where U represents the overall heat transfer coefficient, A represents the contact area between the polymer and the wheel and 4T is the mean temperature difference between the polymer surface and the coolant. It may be seen from this relationship that increasing the overall heat transfer coefficient will result in greater heat transfer q and therefore higher cooling capacity for a given wheel. The overall heat transfer coefficient is the composite of the conductive heat coefficient h for the castinq wheel, and the convective heat transfer coefficient between the wheel and coolant interface hc. This relatior- ship may be represented by the equation EMI7.1 In the past a great deal of effort has gone into increasing the convective heat transfer coefficient which is dependent upon the cooling passage design, the average coolant velocity, and the coolant fluid heat transfer properties. There comes a point, however, where further increases in that coefficient will only yield a small increase in the overall coefficient. This is because of the reciprocal addition relation between the overall coefficient and the individual coefficients given above in which increasing the value of h c while holding h w constant has a diminishingly small effect on the overall coefficient. Table I shows the effect of increasing h from a nominal value to infinity for a given value of he in terms of % change in the overall coefficient. TABLE 1 U hw h c % Change 95 130 .350 110 130 700 16% 130 130 Infinity 36% The term hw which characterizes the conductive heat transfer through the shell of the wheel is dependent upon the heat transfer properties of the material of construction and the thickness 1 of the wheel. If k is the thermal conductivity of the wheel shell material, then EMI7.2 Thus a thinner shell would change h . The effect of this coefficient change is w shown in Table II. TABLE II U hw hc % Change 1 95 130 350 - 1.3 inches (33.02 mm) 142 240 350 498 0.7 inches (17.78 mm) The aforesaid determines the important elements in maximizing the rate of heat removed from the polymer through the casting wheel shell. However, none of the above considerations is of great importance when the heat transfer through the polymer bulk is taken into account, as the thermal conductivity of a molten polymer such as a polyethylene terephthalate is the major bottleneck in rapidly cooling the cast web. However, it is not necessary according to the process of this invention to completely cool the cast polymer to below its glass transition temperature. The complete cooling process may be divided into two distinct heat transfer processes: stripping and bulk cooling. Maximum casting capacity may therefore be defined as the point where either of those becomes limiting. At the stripping limit the polymer sticks to the casting wheel. To break the adhesive forces which develop between the polymer and the casting wheel, a thin layer of polymer in contact with the casting wheel should be cooled below the glass transition temperature. When thus cooled, this layer begins to contract, generating a stress which breaks the adhesion forces holding the polymer on the wheel and allows the as-cast sheet to release from the wheel. When the stripping temperature becomes a limitation to the dwell time of the polymer on the casting wheel the heat transfer coefficient of the casting wheel shell becomes significant since the cooling of the polymer bulk is of no further interest. It is only under these circumstances that the shell thickness becomes a critical element, and it is here that we have found that a reduction in shell thickness offers substantial improvement in casting rates. This is illustrated in FIG. 6 in which the casting rate in pounds of polymer per hour is plotted against the shell thickness of a five foot (1.52 m) diameter casting wheel. Two curves are developed for the case where a 0.090 inch (2.29 mm) thick film is cast. Curve No.l is the theoretical maximum rate, calculated for the case where stripping occurs when an infinitesimal layer has cooled to below the glass transition temperature. Curve No. 2 is calculated for the case in which the film is stripped at the point when a 0.0045 inch (0.114 mm) layer has formed at a temperature below the glass transition temperature. Both curves are for a polyethylene terephthalate film cast at a temperature of 2850C and having a glass transition temperature of 750C. It is clear from the above that thinner shell casting wheels greatly increase the rate at which a given casting wheel may operate. However, as already mentioned before, the casting wheel is subjected to a variety of thermal stresses. This is depicted in FIGS. 2-5. FIG. 2 illustrates a cylindrical casting wheel 16 upon which is to be cast a molten polymer web which does not cover the full length of the wheel; but only the annular band 40. In the situation depicted in FIG. 3 (immediately following casting) the end portions 16a, 16b, shown as severed from the wheel are still cold and want to retain their original size, but the hot annular band 40 has expanded to a larger diameter. As heat diffuses radially through the shell to the coolant it causes annular band portion 40 to assume a barrel shape (bulge), depicted in FIG. 4. This results because the inside wall of the shell is cooler than the outside and expands to a lesser degree. This, combined with the distortion illustrated in FIG. 3, produces the deformation illustrated (greatly exaggerated) in FIG. 5. Of course, such distortions would interfere with maintaining a uniform casting nip between the extruder and the casting surface, which is necessary in order to produce a uniform thickness web. Conventional casting wheels have therefore been massive in order to provide the structural integrity necessary. The exaggerated deformation shown in FIG. 5 is acceptable only if it is limited such that it will not exceed the elastic limits of the materials of construction of the casting wheel. Otherwise, permanent deformation of the casting surface will result, because of the thermal stresses illustrated in FIGS. 2-5. If that occurs, the wheel will no longer expand into a repeatably predictable shape, but instead the distortions will be accentuated and product uniformity will suffer. But as long as the deformation remains below the elastic limit of the materials of contruction, the wheel will return to its original shape at the cessation of operations. This makes it possible to accurately predict the shape of the wheel as distorted under thermal stress and to compensate for it through changes in the design of the extruder lips or the subsequent web treatment following the quenching process or both. What is important is that the deformation be predictable and repeatable, and, in order for that to occur, the thickness of the material must be sufficient so that when stressed under the thermal variations discussed above, it will not stretch beyond its elastic limit and be permanently deformed. As mentioned above again, the casting wheels of the past have therefore been massive to provide the structural integrity necessary. As pointed out earlier, it has now been surprisingly found that making the casting wheel with a thinner shell does not result in greater deformation, such as illustrated in FIGS. 2-5, but instead the degree of deformation and the thermal stresses appear to actually lessen as the shell thickness is reduced. Of course, the reduction in shell thickness cannot exceed the safety limits established by the ASME pressure vessel design code. Another important criterion in selecting the minimum shell thickness is local transient deformation, such as would occur if, for instance, some impurity or debris were present in the cast polymer, or a minor temperature fluctuation created a small amount of higher viscosity polymer melt. As such harder material is extruded it presses onto the shell surface at the extrusion point with more pressure than normal and locally depresses the shell at that point to a greater degree than if normally would. Hence, the shell thickness must be selected so that this localized deformation will not produce a permanent radial deformation in excess of 0.002 inches (0.051 mm). In a specific embodiment employing a polyester film, molten polyethylene terephthalate was extruded onto a conventional 5-foot (1.52 m) diameter casting wheel having a shell thickness of 1.3 inches (33.02 mm). The molten polymer had an "as cast" thickness of 0.090 inches (2.29 mm) at a temperature of 2850C, and was cooled to a wheel side surface temperature of less than 75 0C at the output of the casting wheel. When the shell thickness of the casting wheel is reduced from 1.3 inches (33.02 mm) to a thickness of 0.7 inches (17.78 mm) the wheel adequately satisfies the several criteria disclosed above, providing a three-fold safety margin. At the same time, as shown in FIG. 6, when a 0.0045 inch (0.114 mm) skin is allowed to form on the casting surface prior to stripping, the rate of production increases by almost two-fold. While the invention has been described in connection with the casting of a polyester film, it should be understood that the invention is not limited to this but is also applicable to the casting of analogous thermoplastic film-forming polymers, such as for example polycarbonates and polyolefins.

Claims: Claims. 1. A method of forming a polymeric film comprising casting a molten web of a thermoplastic film-forming polymer on the surface of a rotating casting wheel, cooling the molten web on the casting surface of the wheel to a temperature and depth sufficient to permit the stripping of it from said casting surface by a cooling of the wheel, and subsequently stripping the web from the castingasurface; characterised in that the production capacity is increased by providing a casting wheel with a shell thickness such that the maximum permanent local shell deformation of the wheel at the point where the molten polymer first contacts the wheel surface does not exceed 0.002 inches (0.051 mm) in the radial direction. 2. A process as claimed in claim 1 wherein ih the cooling step, the surface of the molten web in contact with the casting wheel is cooled to a temperature below the glass transition temperature of the polymer to a depth not exceeding 25% of the polymer web thickness as cast. 3. A process as claimed in claim 1 or claim 2 wherein the polymer is polyethylene terephthalate and the molten web is cast at a temperature of at least 2650C 4. A process as claimed in any of claims 1 to 3 wherein the surface of the molten web remote from the casting wheel is cooled and hardened whole on the casting surface by contact with a gaseous or liquid coolant. 5. A process as claimed in any of claims 1 to 4 wherein the casting wheel has a shell thickness of about 0.7 inches (17.8 mm) and a diameter of about 5 feet (1.5 m). 6. A process as claimed in claim 1 substantially as hereinbefore described. 7. A process as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings. 8. A wheel (16) for use in forming a self-supporting film of thermoplastic polymer for cooling to its setting point and comprising: a hollow body member having a tubular portion forming a shell (10) of heat conductive material, the curved external surface of which provides a casting surface (22) onto which in use a stream of molten thermoplastic polymer is to be deposited and having end walls of heat conductive material substantially closing the ends of said tubular portion and directly connected thereto so as to be in a heat conductive relationship therewith; means for supporting said body member for rotation about the axis (24) of said tubular portion, and means for filling said hollow body member (27, 31, 25) with a cooling liquid and for circulating said cooling liquid (26) into and out of said body member while it is rotating in order to control the temperature of said casting surface; characterized in that said casting wheel (16) has a shell thickness such that the maximum permanent local shell deformation thereof at a point where in use the molten polymer first contacts the wheel surface does not exceed 0,002 inches (0.051 mm) in the radial direction. 9. A wheel as claimed in claim 8 having a shell thickness of about 0.7 inches (17.8 mm) and a diameter of about five feet (1.5 m). 10. A wheel as claimed in claim 8 substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: STEINBERG, NEIL IRA
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1982
Main classification: B29C41
IPC codes: B29C41, B29C31, B29C47, B29L7, B29B7