WO2004089594A1

WO2004089594A1 - Buffing apparatus and method for solvent casting system

Info

Publication number: WO2004089594A1
Application number: PCT/US2004/010197
Authority: WO
Inventors: Alex Szopinski; Terry Stull; Andrew Szumlanski; John Steele; Larry Schest; William Shimkus
Original assignee: Monosol, Llc
Priority date: 2003-04-02
Filing date: 2004-04-02
Publication date: 2004-10-21

Abstract

A solvent band casting system is disclosed in which a tank (46, 48, 50) is used to mix and/or store a polymer solution for a band casting machine (14) having at least a first and a second rotating drums (16, 18) about which a continuous metal band (20) is tensioned to travel with the rotation of the drums. A sheeting die (22) applies the polymer solution from the tank to the metal b and where a drying chamber (24), enclosing at least a portion of the metal band downline of the sheeting die, is used to remove solvent from the polymer solution as it travels in a thin sheet on the metal band. A buffer (66) is disclosed which is usable on-line to reduce system downtime. This buffer, which may comprise a cylindrical, hollow-bristled metal brush (67), in one operation is normally at rest until it is needed, and may then be moved into position by an engagement actuator to clean and/or buff the band while the system is on-line. Brush speed, pressure, and direction can be adjusted in order to render the desired surface gloss to the band, which can affect product quality. A system controller (36) can be connected such that the operation of at least one of the band casting machine, the sheeting die, the drying chamber, and the buffer is monitored and/or controlled by the system controller.

Description

BUFFING APPARATUS AND METHOD FOR SOLVENT

CASTING SYSTEM

CROSS REFERENCE TO RELATED APPLICATION

The benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Serial No. 60/459,684 filed April 2, 2003, is hereby claimed. BACKGROUND

Technical Field

The invention relates generally to a buffing system for a solution casting system. Particularly, the invention relates to a buffing system for a band casting system which can be used to produce a substantially bubble-f ee, thin, water soluble film. The buffing system allows the band to be cleaned while the system is on-line, creating a more efficient process by eliminating the time and expense of taking the casting system off-line.

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, tetrahydrofαran, 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.

A brush in a continuous casting system that operates while the system is online is disclosed in U.S. Patent No. 4,819,710. A similar brush is disclosed in U.S. Patent Nos. 5,476,725 and 5,669,436.

SUMMARY

The disclosure provides an apparatus and method for cleaning and/or buffing a band surface in a polymer solution band casting machine, optionally while the machine is in operation. An apparatus for solvent casting is disclosed, the apparatus including a casting surface, a die for applying a polymer solution to the casting surface, and a buffer including a buffing surface disposed in removable contact with the casting surface. One embodiment of the apparatus includes a hollow-bristled metal brush adapted to be in contact with a band of a polymer solution band casting machine while the machine is on-line. Preferably, variables including brush rotation direction, speed, and pressure, are controllable to achieve the desired effects on film product type and quality.

A method of producing a film is disclosed, the method including the step of contacting a casting surface of a polymer solution casting machine with a buffer. Desirable optional steps include controlling and preferably also monitoring the operation of the buffer, controlling steps including adjustment of pressure, speed, and other parameters of the buffing process. The control can be automated.

An improvement in a solution casting method is disclosed, the method, preferably continuous, including applying a polymer solution to a casting surface to coat at least a portion of the casting surface, the polymer solution including a polymer dissolved in a solvent, removing solvent from the cast polymer solution to produce a polymer film, and removing the polymer film from the casting surface, the improvement including contacting the casting surface with a buffer prior to applying the polymer solution.

Further aspects and advantages may become apparent to those skilled in the art from a review of the following detailed description. While the methods and apparatus 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 disclosure, six drawings are appended hereto, wherein:

FIG. 1 is a schematic showing one possible embodiment of a band casting system including a buffer according to the disclosure;

FIG. 2 is a perspective view illustrating one embodiment of an endless flexible belt of a band casting system according to the disclosure;

FIG. 3 is a perspective view of one embodiment of a buffer according to the disclosure;

FIG. 4 is a is a perspective drawing illustrating embodiments of a take-up winder, camera, and scanner according to the disclosure; FIG. 5 is a cross-sectional view illustrating a pivoting engagement actuator for engaging a buffer with a band surface according to the disclosure;

FIG. 6 is a cross-sectional view illustrating a sliding engagement actuator for engaging a buffer with a band surface according to the disclosure. DETAILED DESCRIPTION

Solution casting may be done effectively through the use of a band casting system, such as that described below. 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 pmholes, 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.

In producing polymer film using a band casting system, it is desirable to occasionally clean the band, as it may become contaminated with film residue or other foreign substances, which may inhibit film quality or cause adhesion problems. In addition, the ability to alter the surface gloss of the band is also desirable. The texture of the band surface is imparted directly to the film and, therefore, affects film quality. A smoother surface can produce a more transparent film, while a rougher surface can produce a more opaque film, hi addition, a smoother surface can aid in the reduction or elimination of bubbles from the film, because a rougher surface traps air more easily, which may lead to the formation of bubbles as or before the solution forms a film. Film bubbles and pinholes can be detrimental to many film uses. One or both of these problems may be addressed by buffing a band surface to strip away contaminants and/or alter the surface gloss of the band. In known band casting systems, buffing may only be done while the system is off-line. Thus, the system must be shut down before buffing and restarted again after buffing is completed. Shutting down and restarting the system is time consuming and expensive. The apparatus and method described herein preferably buffs the band while the system is on-line, preventing all the required procedures which must be followed for a line shut-down and start-up, and thus achieving greater efficiency. Essentially, the time savings can be as much as half the time originally necessary to buff the band off-line.

While this method and apparatus are susceptible of embodiment in many different forms, this disclosure will describe in detail preferred embodiments of the invention. The present disclosure is to be considered as examples of the associated principles, and is not intended to limit the broad aspect of the invention to the embodiments illustrated.

Referring generally to the appended FIGS. 1-6, the operation of the band casting system and buffer can be more readily understood. The disclosed solvent casting system is generally referenced by the element number 10 and the buffer is generally referenced by the element number 66 in the following disclosure and drawings. Other components are similarly and consistently numbered throughout the specification and drawings.

"Bubble-free" is a term applied to a film product having a bubble count less than a given threshold based on a full- width optical (e.g., visual) inspection of film sample measuring approximately 4 inches by 55 inches. For the present invention, to qualify as "bubble-free" the number of bubbles of less than 25 microns in diameter should not exceed 50 in the sample film. Optionally, but preferably, the number of bubbles within the range of 25 to 40 microns should 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 sub-samples under magnification), then a method employing statistical sampling can be used to approximate the total number of bubbles in the full sample. In a preferred embodiment, 14 different locations within the film sample, each measuring approximately 4" x 0.25", will be inspected to determine if the "bubble-free" threshold has been met.

"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 present invention. "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 homogeneous mixture of a polymer dissolved in a suitable solvent. The method and apparatus are particularly 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 may be suitable for use with the present invention, 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, the PVOH resin, plasticizer system and other ingredients vary and 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.

With reference to FIG. 1, the general components of a band casting system can be described. The present embodiment of the 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. In the embodiment shown, the mixing system 12 comprises a bulk handling station 44, a mixer 46 having a mix tank 72, a hold tank 48, and a run tank 50. Each of said tanks or vessels is in flow communication with the mixer 46. 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.^'

Proximate the mixing system 12, a band casting machine 14 is shown. The band casting machine may be seen in greater detail in FIG. 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. In a preferred embodiment, the drums are approximately 65 inches wide and 48 inches in diameter, and the band 20 is approximately 61 inches wide with a circumference of approximately 325 feet. A suitable band casting machine is available from Berndorf Belt Systems, Inc. of Carpentersville, Illinois. At any given position, the band has a production or upper portion 21 and a return or under portion 23. The upper portion 21 of the band is used to support the applied polymer solution during drying. Several idlers (not shown) may be spaced along the underside of upper portion of band 20 to provide support 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 should be avoided. As the dimensions of the band 20 change ~ even incrementally due to heating or cooling — the band 20 can begin to run off one end of a drum. Accordingly, the band preferably is made of stainless steel to address the varying thermal gradient of the system existing between the lead drum 16 and the end drum 18. Other metals or alloys having the proper or desired thermal expansion parameters may also be suitable for construction of a band 20.

Referring again to FIG. 1, sheeting or casting die 22 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 is used to feed the polymer solution from the mixing system 12 to the die 22. The die 22 coats a continuous curtain of polymer solution across the width of the band 20. A vacuum box blower and vacuum box (not shown) can be positioned adjacent the automated die 22 to create a pull on the film solution to be more perpendicular onto the band 20 to counteract the natural tendency of the rotating band to pull the solution out of or away from the die 22. A drying chamber 24 (see FIG. 1) is shown enclosing a portion of the metal band 20 downline of the sheeting die 22. The drying chamber 24 of the present embodiment comprises an up-line zone 26 and a downline zone 28. 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 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. The embodiment shown in FIG. 1 includes a series of idlers 40 representing the combination of idlers and associated sensors for monitoring rotation of the idlers.

At the end dram 18, the dried film material is removed (in any conventional manner) from the band 20. A take-up winder 60 can be used to spool the finished film product, as shown in FIG. 4. Material may be trimmed from the edges of the film and spooled on a trim rewinder (not shown). Also, quality inspection devices such as a camera 62 and a gauge scanner 42 may be positioned to monitor the film as it is being removed from the band 20.

Referring again to FIG. 1, at the underside or return portion of the metal band 20, a surfactant applicator 31 is positioned to apply surfactant to the metal band 20. Also at the return of the metal band 20, a buffer 66 is configured to pivotably abut the metal band 20 in the embodiment shown. The buffer is preferably positioned at the return portion of the band, but practically can be positioned in any location, more preferably upline of the die 22 and downline of film removal. Finally, a system controller 36 is shown, wherein the operation of at least one of the mixing system 12, the band casting machine 14, the sheeting die 22, the drying chamber 24, the take up winder 60, the trim winder (not shown), and the buffer 66 is monitored and/or controlled by the system controller 36.

Referring to FIG. 3, the buffer may be seen in more detail. In the embodiment shown, the buffer 66 is covered with hollow metal bristles 67 to form a brush. The band 20 passes above the buffer 66, which may be raised and lowered to engage or disengage the band 20. The brash of bristles 67 is approximately the same width as the band 20. The brush is preferably a hollow-bristled, metal brush manufactured by Osborn International of Cleveland, Ohio. However, several alternative bristle designs were found to be suitable, including those manufactured by Power Brushes, Inc. of Toledo, Ohio. The buffer 66 is initially "off-line" as film product is being made. However, at some point the band 20 may become contaminated with film residue or the like which may inhibit the quality of the film, or it may be desired to buff the band to alter its surface characteristics. At this time, the buffer can be brought on-line whereby, preferably with a continuous quenching of water (e.g., with an irrigator, such as, but not limited to, a spray nozzle), the buffer rotates in contact with the band surface to strip away any such residue and/or otherwise modify the surface of the band. While film can still be produced at this time, it may not be suitable for commercial use and, therefore, may be sent to trim or scrap or tagged for some non-commercial use. Preferably, the buffer 66 is positioned to abut the band adjacent to the underside of an end drum 16, 18, as shown in FIGS. 1, 5, and 6. This positioning is believed to be advantageous for one or more of three reasons. First, it allows the rigidity of the drum to provide support for the band 20 and aid in maintaining a constant pressure, when desired, between the buffer 66 and the band 20. Second, the positioning prevents the pressure exerted by the buffer 66 from substantially affecting band tension. Third, the positioning allows the band casting system to produce film while the buffer is engaged, because this positioning assures that the buffer 66 will contact the band 20 after the film product has been removed.

The buffer 66 may be manually moved into and out of position using an actuating mechanism 68 once the brush is positioned near the band 20. FIGS. 5 and 6 illustrate two possible configurations of an actuating mechanism 68. Preferably, the actuating mechanism includes a pneumatic or hydraulic cylinder or cylinders. Alternatively, one of many different mechanical or electronic mechanisms may function as the actuating mechanism, such as a motor-driven mechanism or even a simple lever. The actuating mechanism configuration shown in FIG. 5 functions pivotably. The actuating mechanism 68 raises the buffer 66 vertically (shown by arrows) to contact the band 20 by revolving about a pivoting joint 69, and lowers it to disengage the buffer 66. The actuating mechanism configuration shown in FIG. 6 functions slidably. The actuating mechanism 68 slides the buffer 66 horizontally (shown by arrows) to contact the band 20, and slides in the opposite direction to disengage the buffer 66. Such a sliding mechanism need not operate horizontally, as it could function equally well by sliding in any direction, if adequately supported. These are by no means the only possible mechanisms for engaging and disengaging the buffer, and those skilled in the art could easily envision a multitude of other possibilities.

Preferably, the brash 66 is rotated using an AC motor (not shown) and a variable frequency drive to control the speed and direction of the brush. In one method, the rotation speed can be adjusted (manually or automatically) in relation to the line speed, i.e., the brush speed will follow the line speed to maintain a constant ratio between brash speed and line speed.

The preferred irrigator is a series of water nozzles (not shown) providing a continuous fine mist of fluid (e.g., water) to the brushing surface. This continuous quenching can serve one or more of three purposes, hi addition to preventing overheating, water can lubricate the brash to prevent overly aggressive brushing and can dissolve and/or carry away debris as it is brushed off the band surface. Water is a preferred irrigation fluid, but the method and apparatus are not so limited.

The buffing process can performs one or both of two beneficial functions in the film manufacturing process. First, it can be used to remove residue from the belt surface. Second, it can be used to provide desired surface characteristics (e.g., gloss level) on the band, which is then imparted to the film. The desired gloss level for a belt surface may be product dependent, and can play a part in bubble reduction. A low gloss level is indicative of a rougher surface, and can be used to produce a somewhat opaque, matte-looking film. A higher gloss finish will produce clearer, more transparent material. In addition, a smoother band surface provides a better surface for bubble elimination since air is easily trapped by a rough surface. Different gloss levels can obtained through combinations of brash speed, band speed, type of brash, pressure and brush direction, through routine experimentation by those skilled in the art. Buffing the band 20 on-line prevents all the required procedures which must be followed for a line shut-down and start-up. Essentially, the time savings can be as much as half the time originally necessary to buff the band off-line.

As a means for coordinating all the various components of the present invention, a system controller 36 maybe 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. It should be generally understood that the system controller 36 would require the initial input of operating parameters (e.g., film gauge, temperatures, and the like) and then would operate to maintain such parameters within the acceptable preset ranges.

The system controller 36 is capable of automatically regulating the brash pressure during the buffing process by maintaining a constant amp load on the buffing brash motor. The amp load is proportional to the pressure of the brush 66 against the band 20. This can accomplished by establishing a set point on the amp load of the buffing motor. In a preferred method, the system controller 36 will monitor the set point and adjust the actuating mechanism 68 to bring the brash 66 closer to the belt 20 when amp load drops and to move the brush 66 away when amp load increases. In a preferred embodiment, the system controller 36 uses PC-based

WONDERWARE software and programmable logic controllers to monitor and control such process parameters as temperatures, speeds, pressures and others mentioned elsewhere in the present application. The process data is also stored in a database as an archive of specific running conditions for later use. 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 will be apparent to those having ordinary skill in the art. Throughout the specification, where the apparatus and method are 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.

Claims

What is claimed is:

1. An apparatus for solvent casting comprising a rotating casting surface, a sheeting die for applying a polymer solution to the casting surface, and an on-line buffer comprising a buffing surface disposed in removable contact with the casting surface.

2. An apparatus according to claim 1, wherein the rotating casting surface comprises a metal band tensioned about at least first and second rotating drums.

3. An apparatus according to any one of the preceding claims, wherein the on-line buffer comprises a plurality of buffing surfaces.

4. An apparatus according to claim 3, wherein the on-line buffer comprises a rotating cylinder covered with hollow metal bristles.

5. An apparatus according to any one of the preceding claims, wherein the on-line buffer further comprises a pivoting joint and the on-line buffer is disposed in reversible pivoting contact with the casting surface.

6. An apparatus according to any one of the preceding claims, wherein the on-line buffer comprises an engagement actuator.

7. An apparatus according to claim 6, wherein the engagement actuator comprises at a pneumatic or hydraulic cylinder.

8. An apparatus according to claim 6, wherein the engagement actuator operates to slide the buffer to abut the buffing surface to the casting surface.

9. An apparatus according to any one of the preceding claims, further comprising a take-up winder, wherein the buffer is disposed upline of the die and downline of the take-up winder.

10. An apparatus according to any one of the preceding claims, further comprising an electronic system controller connected to the on-line buffer and adapted to monitor/and or control the buffer.

11. An apparatus according to any one of the preceding claims, further comprising a irrigator disposed in operational relationship to one or more of the casting surface and the buffer.

12. An apparatus according to any one of claims 2 to 11, wherein the online buffer is disposed to contact the casting surface of the band adjacent to a drum.

13. An apparatus according claim 12, wherein the on-line buffer is disposed to contact the casting surface of the band in an area where the band is supported on its opposite surface by a drum.

14. In a continuous method of producing a polymer film comprising, applying a polymer solution to a rotating casting surface to coat at least a portion of the casting surface, said polymer solution comprising a polymer dissolved in a solvent; removing solvent from the cast polymer solution to produce a polymer film; and removing the polymer film from the casting surface, the improvement comprising: contacting the casting surface with a buffer prior to applying the polymer solution.

15. A method according to claim 14, wherein the casting surface is continuous.

16. A method according to claim 15, wherein the casting surface comprises a continuous metal band in a band casting machine comprising at least first and second rotating drums about which the metal band is tensioned and travels with the rotation of the drams.

17. A method according to any one claims 14 to 16, comprising irrigating one or both of the casting surface and the buffer during the contacting of the casting surface with the buffer.

18. A method according to any one claims 14 to 17, comprising cleaning the casting surface by said contacting of the casting surface with the buffer.

19. A method according to any one claims 14 to 18, comprising modifying the surface texture of the casting surface by said contacting of the casting surface with the buffer