BACKGROUND Technical Field The disclosure relates generally to a solution casting system for producing thin plastic films. Particularly, the invention relates to a drying chamber having enhanced drying characteristics, for use with a solution casting system.

Brief Description of Related Technology Though the general technology for producing plastic materials has been used for decades, solvent-film casting is attracting increasing interest. One of the reasons is that specific requirements in the fields of water-soluble packaging and other related applications can only be met by this technology.

The development of a continuous process to manufacture thin plastic films was closely linked to the emerging photographic industry starting from the end of the 19th Century.

In those times, no other technology was available for industrial film forming, and polymer science was also still in its infancy. Two different technologies were soon developed: (1) casting on wheels or large drums; and, (2) casting onto endless flexible metal belts.

Surprisingly, both are still in use today, together with a third technology, casting onto moving plastic films. However, since the development of extrusion technologies for the production of thermoplastic polymer films, the importance of solvent casting methods has declined. Today, solvent casting is a specific manufacturing method which is used for niche markets and films with specific and high quality requirements.

Typical solvent casting systems utilize an organic solvent such as acetone, aniline, dimethyl sulfoxide (DMSO), benzene, dimethyl formamide (DMF), methyl ethyl ketone (MEK), ethyl acetate, ethylene dichloride, toluene, tetrahydrofuran, and the like. Such solvents usually necessitate a complex solvent vapor recovery and rehabilitation system.

Further, human and environmental exposure to these solvents is most undesirable.

The system described herein can overcome these disadvantages by using water as the solvent. No recovery and rehabilitation system is therefore necessary, and environmental and human exposure is not an issue.

There are many other processes for the formation of films, including calendering, extrusion, plastisol cast systems, and organosol cast systems. Extrusion and calendering are processes which melt the polymer and shape the plastic prior to freezing.

Plastisol and organosol casting processes involve the melting of the polymer in a plasticizer matrix, after which the solvent action of the plasticizer forms a film.

SUMMARY A solvent band casting system is provided. The solvent band casting system includes a band casting machine including at least first and second rotating drums about which a continuous band is tensioned and travels with the rotation of the drums, and a polymer applicator (e. g., die or other coating device) for applying a polymer solution to the band. The system also includes a drying chamber enclosing at least a portion of the band downline of the polymer applicator, wherein the drying chamber includes at least one heater for directing a flow of heated air counter-current to the travel of the band.

Further aspects and advantages will be apparent to those of ordinary skill in the art from a review of the following detailed description, taken in conjunction with the drawings.

While the solvent casting system and control system therefor are susceptible of embodiments in various forms, the description hereafter includes specific embodiments with the understanding that the disclosure is illustrative, and is not intended to limit the invention to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS For further facilitating the understanding of the invention, four drawing figures are appended hereto, wherein: Figure 1 is a schematic showing one possible embodiment of the solvent band casting system; Figure 2 is a perspective view illustrating the band casting components of one embodiment of the system; Figure 3 is a perspective view of one embodiment of the drying chamber; and Figure 4 is a schematic of a film winder for use with the system of Figure 1.

DETAILED DESCRIPTION It is desirable to provide a means for drying a solvent-cast film in a solvent band casting system. Described herein are embodiments of such drying chambers. A preferred drying chamber is adapted for a band casting system including a mixing system, a band casting machine comprising at least first and second rotating drums about which a continuous metal band is tensioned and travels with the rotation of the drums, a polymer applicator such as an adjustable sheeting die or other device used for applying the polymer solution from the tank to the metal band, a surfactant applicator in communication with a supply of surfactant and a portion of the metal band, and a buffer configured to pivotably abut the metal band, wherein the drying chamber encloses a least a portion of the metal band downline of the sheeting die and includes at least one heater for directing a flow of heated air counter-current to the travel of the metal band.

The solution cast process offers several unique features which conventional fusion processes lack. In solvent casting, film formation depends upon solubility, not melting.

Thus, a wide range of polymeric alloys can be produced by solvent casting. Because the flowability to form a film is provided by the solvent, a pure resin film can be manufactured without adulteration by heat, stabilizers, plasticizers or lubricants. Only additives which are beneficial to the finished product need to be incorporated with the polymer.

Solvent casting can provide a film which has excellent dimensional stability as well as reduction in or freedom from pinholes, gels and other imperfections. Due to the very low heat history which is inherent in a film produced by solvent casting processing, the process can also provide an extended service life to the film.

Additional advantages of the present method, apparatus and system relate to film quality. Film bubbles and pinholes can be detrimental to many film uses. Various aspects of the system disclosed herein not only allows for the reduction in the frequency and dimensions of such blemishes, but also can sense and index any such occurrences.

Referring generally to the appended Figures 1 to 4, the process of and system for manufacturing a substantially bubble-free film using the disclosed system can be more readily understood. The disclosed solvent casting system is generally referenced by the number"10"in the following disclosure and drawings. Other components are similarly and consistently numbered throughout the specification and drawings. While the embodiments disclosed herein are described for use with a particular continuous band casting machine, such as, for example, those designed and manufactured by Berndorf Belt Systems, Inc. of Carpentersville, Illinois, other such band casting machines are be capable of adaptation for implementation of the described system.

The general components of the system can be described with reference to Figure 1. The embodiment of a solvent band casting system 10 begins with a mixing system 12 for mixing and storing a polymer solution. The mixing system 12 can be a single tank, or in a preferred embodiment may comprise a plurality of tanks and attendant piping, pumps and valves to control the flow of the polymer solution among the tanks. Proximate the mixing system 12, a band casting machine 14 is shown including first and second rotating drums 16 and 18, respectively, about which a continuous loop of metal band 20 is tensioned and travels with the rotation of the drums 16,18. Operation of the mixing system 12 can be monitored and controlled by a control system 36.

A sheeting or casting die 22 (or other coating device) is used to apply the polymer solution from the mixing system 12 to the metal band 20 of the casting machine 14.

A feed line 13 connects the mixing system 12 to the die 22 and is used to feed the polymer solution from the mixing system 12 to the die 22.

A drying chamber 24 is shown enclosing a portion of the loop of metal band 20 downline of the sheeting die 22. The drying chamber 24 of the present embodiment comprises an upline zone 26 and a downline zone 28. Each zone 26,28 includes a heater (burner) 30 located near an air inlet 32 and an exhaust blower 34 located near an air outlet 38. The portion of the metal band 20 within the drying chamber 24 at any given time, travels over and is supported by a series of support rollers or idlers 40 (see Figure 3). The embodiment shown in Figure 1 includes a series of idlers 40 representing the combination of idlers and associated sensors for monitoring rotation of the idlers.

A take-up winder 60 (see Figure 4) is used to gather the film as it is removed at the end drum (tail drum) 18. Finally, a system controller 36 is shown, by which the overall operation of solution casting system 10 is monitored and/or controlled.

"Bubble-free"is a term applied to a film product having a bubble count (e. g., <BR> <BR> air bubbles) less than a given threshold based on an optical (e. g. , visual) inspection of a one square yard sample of the film. For the system described herein, in a bubble-free film the number of bubbles of less than 25 microns in diameter will not exceed 50 in the one square yard sample film. Optionally, the number of bubbles within the range of 25 to 40 microns will not exceed 10. Further optionally, there will be no bubbles of greater than 40 microns in the sample film. When a manual inspection method is used to determine when a film is bubble- free (e. g. , inspection of one square inch samples under magnification), then a method employing statistical sampling from a one square yard sample (e. g., 14 different one square inch locations) can give an approximation of the total number of bubbles in the full sample.

"Upline"refers to the chronological operating position of a component on the film production line which is prior to a reference point.

"Downline"refers to the chronological operating position of a component on the film production line which is after a reference point.

"Line"is the collective sequence of production components utilized by an embodiment of the system.

"On-line"is an operating condition of the casting system where film, though not necessarily a marketable product, is being produced.

"Polymer solution"refers to any substantially homogeneous mixture of a polymer dissolved in a suitable solvent. The disclosed system is ideally suited for a polyvinyl alcohol (PVOH) dissolved in water. The water content of the PVOH solution is preferably within the range of from about 60% by weight to about 85% by weight. While other polymer solutions are suitable for use with the disclosed system, the description of the embodiments herein is made with specific reference to the manufacture of PVOH film for packaging.

Because there are so many chemically different types of products to be packaged, packaging films are formulated in different ways. That is, a PVOH resin, plasticizer system and other ingredients can vary and can provide a range of films with different product compatibility characteristics. One or more different films may be suited to a particular application, with a suitable film grade easily predictable based upon compatibility testing.

"Water soluble"refers to a film which, when exposed to water, begins to dissolve or disintegrate to its smallest components. Polyvinyl alcohol (PVOH) is a hydrophilic polymer and the plasticizers typically used in its manufacture also have an affinity for water.

PVOH will absorb moisture from a wet atmosphere and give up moisture to a dry atmosphere.

As moisture content increases (even with humidity), a PVOH film will tend to quickly become softer and more elastic, losing tensile properties and increasing in ultimate elongation. Also, the coefficient of friction of a PVOH film will increase with increased moisture content.

The components and operation of a preferred solvent band casting system and a drying chamber 24 are further described below.

MIXING SYSTEM The polymer solution that is eventually cast onto the band 20 must first be mixed. The mixing takes place in the mixing system 12 (see Figure 1). In the embodiment shown, the mixing system 12 includes a bulk handling station 44, a mixer or mix tank 46 having a mix tank 72, a hold tank 48 and a run tank 50. The bulk handling station 44 (shown schematically in Figure 1) is used for holding the raw ingredients for the desired solutions.

These ingredients can include various resins, polymers, plasticizers, and other additives.

Accordingly, the bulk handling station 44 can include a number of vessels or tanks, each corresponding to one or more different ingredients. Each of the tanks or vessels is in flow communication with the mixer 46 for transporting the desired ingredients into the mix tank 72.

Additionally, the various ingredients may be manually fed into the mix tank 72.

Once mixed, the finished batch is then pumped out of the mix tank 46 to a hold tank 48 or a run tank 50, but typically to a hold tank 48.

The hold tank 48 is typically used to hold the solution to allow bubbles (e. g. , air bubbles) and other imperfections (such as gels or affects due to temperature variation) to rise to the top and be separated from the solution. A feed line 13 runs from the hold tank 48 to the run tank 50, from where it is pumped to the extrusion die 22 for casting onto the band 20. A filter 47 may be placed between the hold tank 48 and the run tank 50, or between the run tank 50 and the die 22, or both places.

BAND CASTING MACHINE The band casting machine 14 is further understood with reference to Figure 2.

The casting machine 14 is comprised of a first or lead drum 16 and a second or end drum 18 around both of which is wrapped a continuous metal band 20. The drums 16 and 18 travel in the direction indicated by the arrows, imposing a similar revolution of the band 20. A suitable band casting machine is manufactured by Berndorf Belt Systems, Inc. of Carpentersville, Illinois.

Extending about lead drum 16 and end drum 18 is a continuous loop of metal band 20. As configured in the figure, the loop has a production or upper portion 21 and a return or under portion 23. The outer surface 25 of the band is used to support the applied polymer solution during drying. A plurality of idlers 40 (see Figure 3) may be spaced along the underside of upper portion of the band 20 to provide support of the band 20. The idlers 40 <BR> <BR> may also be monitored by the control system (e. g. , by position sensor for monitoring rotation), to determine movement of the band 20. As the band 20 can be a very expensive piece of equipment, any complications of production which might tend to damage the band 20, such as an idler that stops rotating (e. g. , resulting in the band being dragged across the idler or guiding the band off the edge of the drums 16,18) can be avoided by monitoring and taking appropriate control action.

The band is preferably made of stainless steel; however, other metals, alloys, plastics, or rubber having the proper or desired thermal expansion parameters may also be suitable for construction of a band 20.

DIE The process of solvent casting begins with the application of a layer of polymer solution onto the band surface 25. This is accomplished by the use of an extrusion die 22 or other applicator or coating device. A suitable die 22 is commercially available from Extrusion Dies Inc. of Chippewa, Falls, Wisconsin or Cloeren Incorporated of Orange, Texas. The die 22 coats (deposits) a continuous curtain of polymer solution across the width of the band 20.

The die 22 includes an internal channel (not shown) through which the solution flows. At the end of the channel, is a slot-shaped orifice (not shown) which extends across the width of the die 22.

DRYING CHAMBER Downline from the sheeting die 22, and encasing a portion of the band 20, as shown in Figure 3, is a drying chamber 24. The drying chamber 24 includes two zones 26,28, either of which can be represented by Figure 3. Each zone 26,28 includes a heater 30 located near an air inlet 32 proximal to the downline end of the zone 26,28. The heaters 30 are adapted for introducing heated air into the drying chamber 24. Each zone 26,28 also includes an exhaust blower 34 located near an air outlet 38 proximal to the upline end of the zones 26, 28. The heater 30, air inlet 32, air outlet 38 and blower 34 all combine to produce a heated air flow within the drying chamber 24 in each zone 26,28.

The heated air flow is directed in a counter-current flow to the band direction, as described and shown in the figures, though other arrangements may be useful in other embodiments. Preferably, the air inlets 32 or one or more baffles therein are configured disrupt laminar air flow, and more preferably to create and control a turbulent air flow as the heated air is discharged. Baffles, either fixed or adjustable, can be installed to further control air flow. Such features can provide a more efficient drying of the film. Preferably, the heated air flows across both the upper surface 25 and the under surface of the production portion 21 of the band 20 as it moves through the drying chamber 24, e. g., to further increase efficiency of drying of the film. Finally, infrared heating elements, either inside the drying chamber or outside, can be used to further increase the drying capacity of the system.

The drying chamber 24 preferably covers a substantial length of the band 20 between the lead and end drums, 16 and 18. The chamber 24 preferably is made of an insulated sheet metal body, and can have temperature sensors (such as thermocouples 29) and moisture vapor sensors, to monitor chamber temperatures and humidity (e. g. , relative humidity). The sensors are preferably disposed in positions to measure inlet air and outlet air, and more preferably throughout the length of chamber, such as in periodic fashion. The sensors (e. g., thermocouples 29) preferably are be connected to the control system 36 to allow automatic monitoring. For maintenance purposes, trap doors 39 preferably are provided on the top of the chamber 24. The idlers 40 are seen to extend from the chamber 24 to provide a visual guide regarding band movement. A suitable drying chamber is commercially available from Feco of Cleveland, Ohio.

WINDERS At the end drum 18, the dried film material is removed (in any conventional manner) from the drum 18. A take-up winder 60 can be used to spool the finished film product, as shown in Figure 4. For quality assurance purposes, the formed film roll should be of good uniform size with flat edges and no gauge bands. A film scanner 42 may be used to monitor film thickness or gauge. The scanner 42 preferably is implemented just after the point of film removal from the end drum 18. A preferred film scanner 42 is commercially available under the trade name ADVANZ from ADVANZ Measurement and Control.

Another inspection component of the system is the camera system 62, shown in Figure 4. The camera system 62 is designed to alert the operator to the presence of holes or other defects (such as gels, bubbles, dark spots, slits, and"fish eyes") in the film greater than 0. 015" inch in diameter.

Still referring to Figure 4, the take-up winder 60 has several available options.

For commercial film product approximately one inch is trimmed from each edge of the film.

This trim can be taken up on a separate trim rewinder (not shown) to be later dissolved and recast.

The final film preferably is substantially free of bubbles. If the number of bubbles is excessive (see definition above), the process can be adjusted to reduce the frequency. Adjustments to reduce or eliminate bubbles include vacuum box blower speed, line speed, drum temperature, amount of band coating applied, drying chamber temperatures, solution temperatures, and the like.

SYSTEM CONTROLLER As a means for coordinating all the various components of the casting system, a system controller 36 may be used. From the tank mixing, drum speed, drying chamber temperatures, die gaps and temperatures and pressures, winder speed, and on-line buffing to the adjustment of the tensioning mechanisms, vacuum box blower speed, thermocouple sensors, film camera, and film gauge scanner, the system controller 36 may be employed to control each component to produce the highest quality film possible. In one embodiment, the system controller 36 is provided the initial input of operating parameters (e. g., set points or <BR> <BR> ranges for film gauge, temperatures, etc. ) and then senses and acts to maintain such parameters at or within the acceptable preset ranges.

The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications within the scope of the invention may be apparent to those having ordinary skill in the art. Throughout the specification, where the system is described as including components or process steps, it is contemplated that they can also consist essentially of, or consist of, any combination of the recited components or steps, unless described otherwise :