US3872197A

US3872197A - Process and an apparatus for continuously casting a sheet and the like

Info

Publication number: US3872197A
Application number: US301716A
Authority: US
Inventors: Tetsuji Kato; Katsumi Tamai; Isao Kamada; Hiroshi Kichiji; Yoshio Nakai; Haruyoshi Kitahara; Yasuhiko Iwaoka; Kiyonori Okajima; Tadaomi Ueno
Original assignee: Mitsubishi Rayon Co Ltd
Current assignee: Mitsubishi Rayon Co Ltd
Priority date: 1968-12-30
Filing date: 1972-10-30
Publication date: 1975-03-18
Anticipated expiration: 1992-03-18

Abstract

A process for continuously casting a polymeric sheet and the like, comprising, running two feed belts positioned in face-toface relationship to each other, opposed belt surfaces of said belts running in the same direction at the same speed; running at least two gaskets together with the two belt surfaces at the edges thereof; maintaining the opposed surfaces rigid in the direction transverse to the running one and flexible in the running direction; supplying material between the belts from one end thereof; supporting apart the opposed belt surfaces by pressure of the material and retaining a gap of desired thicknesses between the belt surfaces at desired positions thereof; treating continuously the material between the belt surfaces and delivering treated sheet from the other end of the belts.

Description

Sates atent [191 Kato et a1. Mar. 18, 1975 PROCESS AND AN APPARATUS FOR [56] References Cited CONTINUOUSLY CASTING A SHEET AND UNITED STATES PATENTS THE LIKE 2,500,728 3/1950 Williams 264/175 Inventors; Tetsuji K3); Katsumi Tamai; [530 2,515,243 7/l950 Lyon 264/l75 Kamada; Hiroshi Kichiji; Yoshio e a. ggiz g xggig' jggf 3,422,178 1/1969 Ju hker et al. 425/150 Okajima; Tadaomi Ueno, all of Otake, Japan Primary Exammer-Jeffery R. Thurlow Attorney, Agent, or FirmKenyon & Kenyon Reilly [73] Assignee: Mitsubishi Rayon Co. Ltd., Tokyo, & Chapin Japan [22] Filed: Oct. 30, 1972 [57] ABSTRACT [21] Appl. No.: 301,716 A process for continuously casting a polymeric sheet and the like, comprising, running two feed belts posi- Related Apphcatm Data tioned in face-to-face relationship to each other, op- Continualion Of 5811 839,173, 9 1969, posed belt surfaces of said belts running in the same abandoneddirection at the same speed; running at least two gaskets together with the two belt surfaces at the edges l l Forelgn Application y Data thereof; maintaining the opposed surfaces rigid in the Dec. 30, 1968 Japan 43-96655 direction transverse to the running one and flexible in Apr. 17, 1969 Japan 44-29359 the running direction; supplying material between the belts from one end thereof; supporting apart the op- [52] 11.5. Cl 264/40, 264/166, 264/175, posed belt surfaces by pressure of the material and re- 264/216, 264/236, 264/331, 425/149, 425/224 taining a gap of desired thicknesses between the belt {51] int. Cl. B29d 7/14 surfaces at desired positions thereof; treating continu- [58] Field of Search 264/166, 40, 176 R, 212, ously the material between the belt surfaces and delivering treated sheet from the other end of the belts.

18 Claims, 32 Drawing Figures 7 7 6 o o t) o o O o 7 0 2 an a I/ PATENTEDHAR 1 8 1915 saw 0 1% 13 I OOQXUP H PATENTED 8 I575 sum 05 [1F 13 FEGIO HUENIEUHAR181975 SHEET, D70F 13 PMENTEU 1 81975 3,872,197

sum new 13 mimww 3 872 197 SHEET lEUF 13 R XXIX PRIOR ART PATENIEDHARIBISYE saw 130! 13 PROCESS AND AN APPARATUS FOR CONTINUOUSLY CASTING A SHEET AND THE LIKE This is a continuation of Ser. No. 889,178 filed Dec. 30, 1969, now abandoned.

The present invention relates to a process and an apparatus for continuously casting a sheet and the like and in particular relates to a process and an apparatus of the belt type for continuously manufacturing a sheet form product by continuously polymerizing a liquid material of a polymerizable compound and the like or successively cooling and solidifying a melted thermoplastic polymer.

In the manufacture of a sheet form polymer by continuous polymerization of a polymerizable compound such as methylmethacrylate or by solidification of melted thermoplastic compound such as polyvinylchloride a continuous casting process has been known heretofore which comprises feeding a polymer material between two endless belts located oppositely in upper and lower positions and running at the same speeds in the horizontal direction, to one end of said belts, polymerizing the polymeric compound by heating or the like the endless belts and producing a sheet form polymer product from the other end of the belts.

The continuous laminating process, in which between two opposing endless belts which are driven in the same direction at the same speed, film or sheet ma terial and other film or sheet material on both sides or one side of the material, are supplied to one end of the belts, whereby after these materials are stuck together with proper sticking material, or without using sticking material and only adding pressure and heat as the belts run, laminated sheet products are obtained from the other end of the belts, has been already proposed. In this process a lower run of an upper belt is not supported in a horizontal state and it is lowered by its own weight, so that a sheet of uniform thickness is not produced. When the belts are narrowly spaced apart the lower run of the upper belt would contact the upper run of the lower belt, making the production of thin sheet difficult.

The liquid pressure necessary for supporting the lower run of the upper belt when a liquid material is between the opposite belt surfaces, may be 1 mm. or less water column for a plastic belt and 25 mm. or less for a metallic belt. It is difficult to maintain the liquid pressure of such order under control and also to remove air bubbles in the material.

In order to produce adequate liquid pressure when a liquid polymeric compound is between the belt surfaces, the belts may be inclined or may be vertical with a supplying device in a higher position, but such a construction is practically undesirable as it needs an extremely tall building since generally the time forpolymerization is long and therefore a polymerizing apparatus is necessarily large in length.

Speaking from the viewing point of production, maintenance, and operation of the apparatus, it is preferable to position the endless belts horizontal; it is suit able to feed the liquid material forcibly by a constant flow pump and to press suitably the endless belts by a belt surface external holding mechanism. In any method for sealing between a material feeding device and the belt faces by direct contact therebetween, scratches are made on the smooth faces of the endless 2 belts by the relative sliding of the scaling portions, thereby the glaze and appearance of the belt faces and thus those of the product become injured. Even ila soft material would be selected as the sealing material, by dust or polymer growing gradually on the seal, the belt faces would be injured.

The gasket normally used for producing a methylmethacrylate sheet according to the cell casting pro cess has a compression strength of [.0Kg/cm. or more at the time of compression to a sheet thickness desired at the polymerizing temperature. To form a sheet using for the cell glass having a thickness of H) mm. or more in the cell casting method, the gasket is compressed by its weight markedly larger than the weight of a belt. Accordingly, use of a gasket smaller in compression strength will worsen the accuracy of sheet thickness. In the continuous casting process each of the belts interspaeed edge portions also require a sealing gasket, which in this case travels with the belts. The gasket is compressed while in used condition, the compression ratio gradually increasing with the contraction of volume of polymerizing material while the repelling force of the gaskets increases by compression. In case the repelling force due to compression of the gaskets is adequately large, there is produced deformation of the outer belt holding means owing to the repelling force due to compression of the gaskets, or the deformation of covering rubber of rollers, by which the space between the upper and lower belt surfaces loses uniformity in the transverse direction, that is, that the space between the upper and lower belt surfaces about the center in the transverse direction in the vicinity of the gasket on each side, becomes large. The polymer compound flows with the deformation of each belt surface. The viscosity rises as polymerization proceeds, with non-uniform distribution of thickness, finally to a degree that actually produces rigidity at polymerization temperature. That is, due to compression of the gaskets, the sheet thickness about the center in the transverse direction is small and there is produced a sheet polymer large in thickness in the vicinity of the gaskets on both sides.

To raise the fluid pressure, when using a gasket large in the repelling force due to compression, with respect to the repelling force of the gasket, means to make the pressure distribution large relative to the belt surface, and is therefore one proposal to make the sheet thickness of the polymer uniform. According to this process the upper and lower belt surfaces receive excessive pressure which makes the deflection of the outer belt holding means much larger. The excessive deflection of each surface destroys the sealing between the upper and lower belt surfaces and gaskets thereby causing leakage of the polymer compounds. In order to avoid this drawback it is necessary to make the outer holding means larger resulting in uneconomy and obstructions in the heat transmission at the polymerizing zone. Therefore, the fluid pressure may preferably be higher appropriately than the pressure equal to the weight of the upper belt surface or up to 50 mm. water column. However, the deformation of the outer holding device induced from the repelling force due to compression of the gasket cannot be solved by the method of raising the fluid pressure. To make the sheet thickness of a sheet form polymer therefore is not possible by making the repelling force (compression strength) due to compression of the gaskets, smaller. However, the gasket has a large function to seal the polymer compound between the upper and lower belt surfaces and prevent its leaking; the compression strength of the gasket must not be too small and the contact between the upper and lower beltsurfaces should not be lost. Furthermore, the gasket of small compression strength, that may not sufficiently perform its function because of deformation during its manufacture or safe keeping, is not practically adapted for use.

In the continuous casting method, it is essential to feed a corresponding amount of material polymer compound constantly between belt surfaces to obtain a sheet polymer of a predetermined thickness as a product, and in the course of running with the endless belts to secure the polymer compound against flow between the edges of the belts while the distance between the belt surfaces is strictly adjusted to follow the variation of volume of the polymer compound due to rise of temperature or polymerization. In this process, the time that the endless belt passes the polymerizing zone of the apparatus is the same in principle as the time needed for polymerizing a polymer compound. Accordingly, the production capacity of the apparatus is proportional to the product in the belt width and the length of the polymerizing zone. The industrial apparatus becomes large, using wide and long endless belts.

In this apparatus two endless belts are strictly required to be driven at the same speed. If the speed of two endless belts is different in some degree, for example, in the case of the above apparatus which manufactures sheets, it results in a bad condition of the optical feature of the products and retained internal stress in the products, and in the case of the latter half of the process it causes separation of the products from the belts. Also it gives undesirable force consumption to the apparatus especially to the endless belts.

For instance there is a method in which two main pulleys for each belt driving system are controlled automatically and strictly in order to get the same rotation speed. But in the case of this method the control apparatus and speed reduction gear box are expensive and complicated.

When two endless belts are driven by one driving motor, a big speed reduction is required, because of the speed of the main pulleys rotation, and as a power transmission system which gives rotation to the main pulleys, very small chains, belts and worm gears are put to use in order that the positions of the two pulleys may be easily changeable. When chains are used to drive, a proper gear box is necessary in order to rotate two sprockets in the reverse direction of each other.

Now to make two driving pulleys rotate with the same speed, two pulleys must be rotated with the same speed if the two driving pulleys have the same diameters, or two driving pulleys must have the number of rotations in inverse ratio to each other if the two driving pulleys have different diameters. But though driving pulleys are made very accurately, the diameters of these two pulleys have always some errors which are due to the process of making them, or there are also some errors elsewhere in the organization. So if this equipment is driven for a long time these errors will become accumulated errors and bring about an undesirable condition in the apparatus in the end.

A torque-limit-coupling which has a slipping system in the driving axle and a front free system like a ratchet system which goes freely ahead, can be inserted into the drive of the two belts type continuous sheet manufacturing equipment, to avoid a braking effect which comes from the condition that one pulley speeds ahead of the other pulley.

The object of the invention is to eliminate the above disadvantages.

Other objects and features of the invention will appear from the following description, reference being had to the accompanying drawings in which:

FIG. 1 is a side elevation of an apparatus according to the invention.

FIG. 2 is a perspective view of a material feeding device.

FIGS. 3 and 4 are a side elevation and a perspective view of another embodiment of a material feeding device.

FIG. 5 is a side elevation of another embodiment of the invention.

FIGS. 6 and 7 are a vertical section and a crosssection of still another embodiment of a material feeding device.

FIG. 8 is adiagram of backward flow.

FIGS. 9, l0 and 11 show another embodiment similar to FIG. 6, being an elevation, a perspective view and a bottom view, respectively.

FIGS. 12 and 13 are a vertical section and a cross section, respectively, of another embodiment of a material feeding device.

FIG. 14 is an elevation of a vertical type apparatus.

FIG. 15 is a perspective view of the feeder of the above device.

FIG. 16 a to e are cross-sectional view of various gaskets.

FIG. 17 is a cross-section showing an external belt holding arrangement.

FIG. 18 is a side elevation of a shaft used by the above.

FIGS. 19 and 20 are longitudinal and cross sections, respectively, of another embodiment.

FIG. 21 is a side elevation of still another apparatus.

FIG. 22 is a plan view taken on the line XXIIXXII in FIG. 21.

FIG. 23 is a transverse elevation view taken on the line XXIII-XXIII in FIG. 22.

FIG. 24 is a longitudinal elevation view taken on the line XXIV-XXIV in FIG. 22.

FIG. 25 is a cross section taken on the line XXVXXV in FIG. 21.

FIG. 26 is a bottom plan view taken on the line XXVIXXVI in FIG. 21.

FIG. 27 is a cross section taken on the line XXVIIXXVII in FIG. 26.

FIGS. 28 and 29 show a conventional type apparatus respectively in elevation and cross section.

FIGS. 30 and 31 are a side view and an end view of a belt driving device.

FIG. 32 is a view of an elevation of another apparatus.

In FIGS. 1 and 2 the endless belts l and 2 are provided with tension, respectively, by

main rollers

3 and 4 and 5 and 6 and driven by running the

rollers

4 and 6 at the same peripheral speeds. Each belt is held horizontally by the

idle rollers

7 and 8. The material feeding duct 9 forms an opening ABCDEFGHA with boundary surfaces A-B, B-C, D-E, E-F, F-G and H-A of the opening on which the belt surface slide. Two gaskets I0, 10 are respectively carried along E-D and F-G and slide with theedges C-D and OH of the duct for making a seal. The gaskets 10, may instead be delivered along A-H and B-C.

The material is fed from a reservoir or preparation tank 11 of the polymer material and fed to an upper feeding tank 9a of the material feeding duct 9 through a pipe 13 by means of a pump 12, flowing down in a thin layer on the inner surface of the duct, and forming a free surface at the location indicated at 14. The opening 15 at the upper part of the duct 9 is connected to a vacuum source device and the upper space in the duct is maintained at an adequately reduced pressure less than atmosphere.

lnto the duct 9, the liquid material is continuously fed from the reservoir or material preparation tank 11 through the upper feeding tank 9a, flowing down, and being fed by its weight between the two belt surfaces. The pressure in the upper space may be higher than the pressure at which the material is boiled in the tank 11 and lower than the atmosphere. For removing the gas dissolved in the material there may be adopted a pressure relatively low; otherwise there may be adopted a pressure approximate to the atmosphere but of an extent that the air bubbles floating from the material can be removed, and the relationship with the lower limit of liquid depth later described must be considered.

The liquid depth which may be preferred is in a range such that the liquid pressure caused from such liquid depth may be less than 1 kg/cm gauge pressure between the two belt surfaces and the free surface 14 of liquid can be formed in the duct. The object of the present invention may be attained if the force acting on the upper belt surface is slightly larger than the weight of the upper belt. Therefore an excessively large liquid depth is not required. The lower limit of the liquid depth is not specifically defined but generally a limit larger than the radius of the main roller holding the upper belt surface at the material feed end is preferable because of easy removal of air bubbles.

For supplying the material into the duct it may be directly injected in the vicinity of the free surface 14 of liquid in the duct, by means of a pump or it may be flowed downwardly to the free surface in a thin layer on the inner surface of the duct from the upper part since it is easy to remove the dissolved gas which is an advantage.

The material is supplied between the belts by means of its own weight corresponding to the liquid material depth.

Main rollers for driving at both ends of the belt surface may be of a preferred curvature such that a tension within the limit of belt resiliency may be applied thereon. The surface of contact of the belt surface on the material mixture may be flat and smooth or it may be formed in a patterned sheet.

For maintaining a plane condition of the belt surface in the polymerizing zone there may be provided roller groups or other smooth belt engaging surfaces on which said belt surface may slide, and the lower belt surface may float on a fluid. In case an outer belt holding means is applied, designed so that it will deflect by pressure of the belt surface, contraction of volume by polymerization may automatically be compensated for by maintaining the distance between the belt surfaces by such means prior to deflection at a desired predetermined value of width greater than the thickness of the polymer when in sheet form and therefore it is more preferable. According to the present invention it is possible to change the fluid pressure desired between the belt surfaces, as well as the outer holding means.

Materials of the belt surfaces according to the invention may include various films, such as cellophane, polyester films and the like while, specifically, metal endless belts made of steel or stainless steel are more favorable. The films may be used as a laminate on said belts. Plastic belts for use may be generally of a thickness less than 1 mm. which are usually sold on the mar ket. Metallic belts may preferably be of a thickness of 0.1 to'3 mm., specifically of().5 to 2mm; the fluid pressure required for carrying the upper belt surface is less than 1 mm. water column for the plastic belts and less than 25 mm. water column even for the metallic belts. The belt surface separation distance is accomplished by slightly pressing both belt surfaces against the outer supporting device, such as roller groups, by liquid pressure. To this end a force acting on the upper belt surface by liquid pressure must produce a liquid pressure between the belt surfaces higher than the pressure equivalent to the weight of the upper belt surface. The upper limit of the liquid pressure is not specifically defined but it is generally preferable to have it in the range lower than the liquid pressure deforming the belts between the outer holding means more than 30% of the distance between the belt surfaces.

In the polymerizing area the polymerization is thermally controlled from the outside of the belt surfaces by heating and/or cooling. The heating systems include the method of applying hot air to the outside of the belt surfaces, method of dispersing warm water in a shower on the belts outsides, method of running the belts in a water bath, method of applying infrared ray radiation, etc.

The polymerizing temperature may be a constant outside temperature over the whole area of the polymerizing zone or it may be changed by stages or continuously, and the polymerizing temperature may be dependent on the polymerization catalyst in use but it must be kept below the boiling point of the material until the polmerization is almost completed.

Gaskets are generally made of plastics in a string shape. in case a low viscosity monomer is used for gasket material it is more advantageous to use gaskets of a square or rectangular cross section for the prevention of liquid leakage at the sliding part. However. when a partly polymerized polymer/monomer solution is used, a gasket of a hollow pipe shape can prevent the leakage since the solution is high in viscosity.

For the material of a gasket, there will be used for ex ample soft polyvinyl chloride as has been usually employed. Polyethylene and other flexible plastics, natural rubber and other rubber can be used for the gasket. By use of the gasket made of polyethylene, rubber and the like it is possible to recover the gasket and continuously reuse it. A monomer of low viscosity used as a material compound for the gasket of square or rectangular cross section is more advantageous for the prevention of leaking fluid at the sliding part. Partially polymerized polymer-monomer solution is high in its viscosity so that it can be used in a gasket of a hollow pipe shape with almost complete prevention of leakage. Flexible plastic rod or foaming plastics having individual air bubbles may be used for gaskets. The gaskets compression strength may be lessened for the hollow pipe type when using polyvinyl chloride having much plasticity by choice of outer diameter, and wall thickness of the material. As for foaming plastics it is possible to obtain a gasket smaller in compression strength readily by raising the magnification ratio of foaming. Therefore, material, shape and dimensions of the gasket may be desirably preferred in the range to fulfill the function of a gasket corresponding to the nature, production requirements of the polymer compound, thickness of the sheet form polymer, specific object of the product, etc.

The fluid material used in the polymerizing process of the present invention includes a mixture of one or more monoethylenic unsaturated compounds which is fluid under normal pressure and/or multi-functional polymerizable compounds. These monomers may be used as a mixture of polymer in solution or suspension or partially polymerized monomer polymer mixture. For monoethylenic unsaturated compounds there are used as for example methacrylates, styrene or its halogenated or alkyl substitute derivatives, vinyl acetate, etc., or a mixture of an essential amount of these compounds and acrylates, acrylonitrile or its derivatives. For multifunctional polymerizable compounds which can be used, there are, for example, glycoldimethacrylates, diallylmethacrylate, diallylphthalate, and diethylene glycol bis allylcarbonate. The invention is particularly advantageous for casting a polymer of methylmethacrylate and a copolymer of a major amount of methylmethacrylate and a comonomer copolymerizable therewith.

The fluid material is mixed with polymerization catalysts. For polymerization catalysts there can be used for example azobisisobutyronitrile, azobisdimethylvaleronitrile, azobiscyclohexanenitrile, benzoylperoxide, lauroyl peroxide, acetyl peroxide, caprylylperoxide, 2,4-dichlorobenzoyl peroxide,v diisopropyl peroxy dicarbonate, isobutylperoxide, and acetylcyclohexylsulphonyl peroxide as free radical catalysts. Polymerization catalysts of oxidized reducing systems such as peroxides and amines may also be used in combination. The fluid raw material may be mixed with various additives such as stabilizers, plasticizers, polymerization controlling agents, fillers, dyestuffs, pigments, mold releasing agents, etc. Example 1 A viscous liquid of l poise at 25C consisting of a solution of methylmethacrylate monomer containing 20 weight methylmethacrylate polymer of a mean degree of polymerization of about 900, mixed with 0.05 weight azobisisobutyronitrile as a polymerization catalyst is delivered from the reservoir 11 in FIG. I to the feed duct 9. Height from the free surface 14 of the mixture material in the feed duct to the upper level of the belt 2 is 3 mm, and the pressure in the upper space 9a is about 460 mmI-lg abs. The pressure decrease is controlled so as to have the liquid pressure on the same level of the belt 2 equal lcm water column. The belts l and 2 are the endless belts of flat and stainless steel having a thickness 1mm and a width 1,200mm. The distance between the upper and lower belts is maintained such that the polymerizing area may have a thickness of about 3 mm. by

roller groups

7, 7 and 8, 8. Gaskets 10 are of polyvinylchloride hollow pipe of a wall thickness 0.3mm. and an outer diameter 10mm. The entire length of the polymerizing area is 60m, and the front part of 40 m. is heated with warm water of 85C from the outside of belts and the rear part is heated in the air furnace at 120C. The belts l and 2 are driven at the speed of 1m per minute. There is obtained a flat and smooth transparent sheet of mean degree of polymerization of about 5,000, continuously.

FIG. 1 shows an apparatus in which the lower belt has a larger length than the upper belt. FIG. 3 shows an apparatus in which lengths of the upper and lower belts are equal and the material feeding duct does not have the belt surfaces sliding thereon. In FIG. 4, l6 denotes a material feeding tank, which is connected to a mate rial feeding duct 9b at qrts and the fluid material is delivered into said feeding duct by weight of the fluid material due to fluid depth. The feeding duct 9b opens at abdc between the upper and lower belts. The upper surface amnc and the lower surface brtd of the feeding duct 9b respectively have sliding contact with the belts on the upper and lower surfaces. The right surface edts and the left surface abrq of the injecting duct 91; form passages l7, 17 to the outside between these surfaces and the gaskets l0, l0. Fluid material discharged from the opening abdc of the feeding duct 9b flows reversely by fluid pressure through said passage l7, l7 balancing dynamically with the running speed of the belt, and apparently stops at ijlk and efhg; therefore, the fluid ma terial will not leak to the outside but will produce constant fluid pressure between the upper and lower belts, it thus being possible to generate fluid pressure between and on the belt surfaces by use of a material feeding duct inserted in a proper length into the space between the belt surfaces from an opening at the material feed end and thereby prevent the leaking of polymer compound at the feeding part.

From a reservoir of a controlling tank of raw material is fed the material by a pump through pipe 18 into a material feed tank 16 to form the previously described free surface 14. The feed tank 16 is connected at qrts to the material feeding duct 9 b and by weight due to fluid depth of the fluid material in the feed tank 16, the material is delivered into the material feeding duct 9b. It was found possible to have the material feeding duct between the upper and lower belt surfaces free from sliding contact by the upper and lower belt surfaces and to use the spaces between the upper and lower and left and right surfaces of the feeding duct and between the upper and lower belt surfaces and the left and right gaskets as passages to the outside. Also it is possible to permit sliding between the upper and lower surfaces of the duct and the upper and lower belt surfaces, thus to provide a space of adequate width between either one or both of the left and right surfaces to use for the passage to the outside, as well as to provide a passage having an adequate space between either one or both of the upper and lower surfaces of the duct and the upper and lower belt surfaces. The size and sectional area of each said passages defines the reverse flow speed of the fluid material which in turn is affected by the viscosity and fluid pressure of the fluid material, these being considered to determine the size and sectional area of the passage in the range controllable without leaking of the reverse flow fluid raw material to the outside. The sectional area of the passage may preferably be small, possibly when the reverse flow fluid material might adversely affect the quality of a sheet form polymer by contacting with the atmosphere.

Example 2 Methylmethacrylate monomer is mixed with methylmethacrylate polymer of about 900 mean polymerization degree, 20 weight and is dissolved into a solution of 1 poise of 25C in viscosity, which is mixed with 0.05 weight of azobisisobutyronitrile as a polymerizing catalyst. The resultant raw material mixture is delivered from the reservoir to the material feed tank 16. Height of the free surface 14 of the material in the feed tank 16 to the level of the belt 2, for the material mixture, is 10 cm. The upper and lower surfaces of the material feeding duct 9 b inserted between the upper and lower belts and connected to the feed tank 16, slides on the upper and lower belts running at the speed of 1m, per minute along a length of 40 cm. The fluid pressure at the opening of the feeding duct 9b is about 2 cm water column and the raw material mixture is delivered to the polymerizing zone between the belts, a part of which material flows reversely through passages of 5 cm. width provided between the left and right surfaces of the feeding duct 9b and the left and

right gaskets

10, 10 and the back end of the reversely flowing material apparently stops in a state of rest after 10cm reverse flow.

Belts 1 and 2 are smooth stainless endless belts of the thickness 1mm. and the width 1,200 mm. The upper and lower belts are held by

roller groups

7, 7, 8, 8 such that an obtained sheet form polymer may have a thickness of 3mm. For the gaskets 10 a polyvinylchloride hollow pipe of a wall thickness 1.3 mm. and outer diameter 10mm. is used. The whole length of the polymerizing zone extends 60mm; a zone of 40m. in the front part is heated with warm water of 85C in a shower form on the outer surface of each belt and a zone of m in the rear part is heated in an air furnace to 120C.

Thus there is obtained a smooth and transparent sheet of a very uniform thickness and a mean degree of polymerization of about 5,000 continuously, in a completely sealed space between the belts, without leakage. Example 3 Methylmethacrylate polymer of a mean polymerization degree of about 900 was dissolved in a methylmethacrylate monomer to obtain a solution containing about 20 weight of the polymer, the viscosity of which is 1 poise at C, and to. which is mixed 0.05 weight azobisisobutyronitrile. This fluid material is delivered from the reservoir 11 in FIG. 1 to the feeding duct 9. Height from the free surface 14 of the fluid material in the feeding duct to the level of the belt 2 is 3 m. and pressure in the upper space of the duct 9 is about 460mmHg abs. The pressure decrease is adjusted so that the fluid pressure at said level of the belt 2 is 1cm water column. The belts 11 and 2 are smooth stainless steel endless belts of thickness 1mm. and width 1,200mm. The internal space between the upper and lower belts is maintained by

roller groups

7 and 8 such that the sheet form polymer may be of a 3mm. mean thickness. The whole length of the polymerization zone covers 60 m; the area at the front part of 40 m. being heated by dispersing warm water of 80C in shower form on the outside of each belt and the area at the rear part of 20m. heated in the air furnace of 120C. Gaskets 10 are made of a hollow pipe of polyvinylchloride containing dibutylphthalate equivalent to 60 weight of a polymer as a plasticizer, the wall thickness being 0.6mm. and outer diameter 6mm. The gasket has a compression strength of 0.07 kg/cm. when compressed to 3mm. at 80C. 1f the belts l and 2 are driven at a speed of 1m. every minute there is obtained a smooth transparent sheet having a mean degree of polymerization of about 5,000 and accuracy of thickness of 10 3-H).3mm., continuously. Reference Data re the Above:

For gaskets, a hollow pipe of polyvinylchloride containing dibutylphthalate equivalent to 44 weight of a polymer as a plasticizer, having a wall thickness of 1.3mm. and an outer diameter of 6mm. and exhibiting a compression strength of 0.97kg/cm. when compressed to 3mm. at C. Other sheets were manufactured under the same polymerizing conditions by use of the same continuous polymerizing apparatus as in the above embodiment. The accuracy of thickness of the obtained sheet was 3+0.5mm.

Example at compression load of less than 0.01 kg/cm The sheet was produced under the same condition of polymerization by using the same continuous polymerizing apparatus as the above embodiment except for the gasket. As the gasket a hollow polyvinylchloride tube having an outer diameter of 9mm. and a compression intensity (or resistance) of 0.008 kg/cm when compressed up to 3mm. at 80C was used.

Since the compression load was too small in spite of the gasket having a large outer diameter, movability of the liquid material was high and leakage of the liquid material occurred between the belts and the gasket in the front half of the polymerization zone in which the liquid pressure between the belts was high. The preci' sion of sheet thickness was reduced and at the same time the leaked liquid material was polymerized and set in the hot water; thereby smooth operations of the rollers and the hot water system were disturbed.

RESl-LIENT FORCE OF GASKET Compression loads measured with some of gasket used recently were as follows.

Nowadays it is optional to use a double gasket system. In this case combinations such as 9d) 0.4t/6d 0.4t, 9d: 0.4t/6 0.6t and 10 0.6t/6 0.4t may be used. In a case of a single gasket system a gasket having an outer diameter of more than 8mm, is used.

If a gasket of 9 0.4t 44 part is used and load of roller is improperly set, then syrup may be leaked between the belts and the gasket. Therefore, it is sufficiently considered that the leakage of syrup will occur under the compression load of less than 0.01kg/cm.

When a gasket of 84 0.4t 44 part is compressed up to 3mm. at 80C, the compression load is 0.02kg/cm. and considerably near the above minimum permissible 0.01 kg/cm. However, a gasket of less than 0.01kg/cm. has not been used. Therefore, estimation of value of the compression load with outer diameter, wall thickness and amount of DB1 plasticizer specified deflnitely, is impossible at present.

An embodiment of the present invention will be described with reference to FIG. 5 in which case approximately horizontal endless belts are used.

Two endless belts 1 and 2 disposed upwardly and downwardly respectively are provided tension by

main pulleys

3, 4 and 5, 6 and driven to run at the same speed.

Idle rollers

7 and 8 in a group and upwardly and downwardly forming a pair, carry the running endless belts horizontally and control the thickness of the distance between the belt surfaces, or the thickness of the polymer compound. The polymer liquid material is pumped by a pump 19 and fed between the belts by a material feedingdevice 20. Both sides between the belt surfaces are sealed with gaskets having resiliency. The polymer compound is heated and polymerized by warm

water spray systems

21, 22 during the running of the belts and the compound is subsequently heat treated by an infrared

ray heater system

23, 24 to complete the polymerization, and a sheet polymer product is taken out.

The process according to the present invention is effectively utilized in the polymerizing zone where the thickness of a sheet. polymer is determined, i.e., in a portion heated by the warm

water spray systems

21, 22 in the drawing.

In case of the conventional continuous casting process, the distance between the belt surfaces in the polymerizing zone was previously set to hold a thickness of material corresponding to the thickness as predetermined for the product to be obtained. It is based on the idea inherently provided from the cell casting process, which is to facilitate the flow of the polymeric compound between belt surfaces during polymerization. In

such cases also the distance between belt surfaces is set larger in the former half of the polymerizing zone than in the latter half, which has followed the contraction of volume accompanying the polymerization of the polymer compound.

According to the process of the present invention,

the distance between the belt surfaces is set such that in the former half of the polymerization area where the viscosity of the polymer compound is still low, having fluidity, the distance is held larger than the thickness of the liquid material required to obtain a sheet polymer having the thickness as predetermined for the final product, and in the latter half of the polymerizing zone where the viscosity of the polymer compound becomes high as the polymerization advances, losing the fluidity, and the thickness of the polymer on the sheet is substantially determined, the distance between the belt surfaces has a thickness of a sheet form product as finally produced, whereby the time required for the passing of the endless belts through the polymerizing zone is made shorter than the time required for the polymerization of the polymeric compound, or the time of staying in the polymerizing zone is reduced, so as to elevate the productivity of the apparatus.

In the process according to the present invention, the

- range to the end the half of the polymerizing zone where the distance between the belt surfaces is kept large or, inversely speaking, the range to start the latter half of the polymerizing zone to set the distance between the belt surfaces so that the thickness of the sheet form polymer may become as predetermined for the sheet form product, can be respectively and experimentally obtained according to the terms of manufacturing a continuous polymer sheet, the apparatus, and, further, the precision desired for the thickness of a sheet-form product.

Where in the above described so-called range of the latter half of the polymerizing zone, irregular variation sometimes occurs in the thickness of the sheet form product, it is for the reason that it is caused from the variation in the advance of polymerization in the former half of the so-called polymerizing zone by which if some part is advanced in polymerization excessively the viscosity in said part becomes high and in the latter half of the polymerizing zone it becomes thick without being fully levelled off. Accordingly, experiments for deciding the former half and the latter one of the socalled polymerizing zones is carried out by measuring the accuracy of thickness of the sheet form product.

Needless to say, it is an effective contrivance to continuously change the distance between belt surfaces continuously and moderately without exactly distinguishing the former half and the latter one of the socalled polymerizing zones to mitigate the influence upon the accuracy of thickness of the sheet form product. Example 4 Two flat and smooth stainless steel endless belts respectively of thickness 1mm; width 800mm, and lengths 15.5m and 16.5m are tensioned horizontally on the upper and lower stages by use of main pulleys of a diameter 1,000mm. and driven such that the opposite surfaces thereof will run at the same speed in'the same direction. The polymerizing area of this apparatus extends about 6m,,4m. in the fore part of which are disposed idle rollers of a diameter mm. and having flexibility, in 21 opposite sets at intervals of 200mm, in pairs upwardly and downwardly, for adjusting the distance between the positions of the endless belts and the distance between the belt surfaces. This fore part is temperature controlled by spraying the warm water of approximately 80C on the outside of the opposite belt surfaces. In the latter half of 2m. said outside of the belt surfaces is heated with an infrared ray heater to C or more for heat treatment. A viscous liquid of about 5 poise in a solution of methylmethacrylate monomer to which is added about 20 weight methylmethacrylate polymer as a material polymer compound, was prepared as a mixture with a proper amount of azobisisobutyronitrile as a polymerization catalyst. The solution wasdelivcred in the material injection device by use of a pump. For sealing both sides of the belt surfaces a polyvinylchloride tube of wall thickness 0.6mm. and an outer diameter 8mm. and containing dibutylphthalate 60 weight was provided for gaskets running at the same speed of running as the endless belts.

1. First, the distance between belt surfaces was adjusted such that the liquid material would have a thickness adapted to obtain a product of 2mm. thickness. The endless belts were run at 10cm. per minute. The liquid material was supplied at a rate of cc. in every minute so that the liquid material would have a thickness of 2mm. Thus there was obtained a smooth and transparent sheet of 2+ 0.2mm. thickness.

2. Then the running speed of the endless belts was changed to 12cm in aminute and the amount of material feed to cc. in a minute with the .distance between belt surfaces being the same and there was obtained a polymer having small bubbles dispersed in the interior. These bubbles were produced because the polymer was I transferred to high temperature heat treatment without completion of polymerization at about 80C and too rapidly polymerized.

3. The running speed of the endless belts was lowered to 10cm. per minute and the amount of material feed

Claims

1. A METHOD FOR CONTINUOUSLY CASTING A LIQUID HAVING A VISCOSITY RENDERING IT FLOWABLE UNDER THE FORCE OF GRAVITY AND WHICH HARDENS WITH CHANGING TEMPERATURE AND TIME, IN A TRAVELING CASTING SPACE HAVING CASTING AND DISCHARGING ENDS, SAID SPACE BEING DEFINED BETWEEN TWO OPPOSED, VERTICALLY INTERSPACED, EXTERNALLY RESTRAINED, HORIZONTAL UPPER AND LOWER, ELONGATED, FLEXIBLE BELT SPANS HAVING SIDE SEALS AND TRAVELING AT THE SAME SPEED AND IN THE SAME DIRECTION AND FORMED BY ENDLESS FLEXIBLE BELTS EACH RUNNING AROUND HORIZONTALLY INTERSPACED CYLINDRICAL ROLLS FORMING EACH BELT INTO A LOOP HAVING SEMI-CYLINDRICAL ENDS, WHEREIN THE IMPROVEMENT COMPRISES CONTINUOUSLY CASTING SAID LIQUID INTO SAID CASTING END BY FLOWING THE LIQUID INTO CONTACT WITH SAID BELTS WHILE CONTROLLING THE TRAVELING SPEED OF SAID BELTS SO THAT BY THE BELTS'' ENGAGEMENT

2. The method of claim 1 in which said liquid is flowed into contact with said belts via a column of the liquid having a free upper surface.

3. The method of claim 1 in which said lower belt span is extended backwardly from said upper belt span to form a forwardly moving horizontal belt surface on which the liquid forms backwardly extending body of liquid terminating where said forwardly moving force received from this belt surface is adequate to hold the liquid body against extending further backwardly.

4. The method of claim 3 in which said liquid body is maintained in a laminar condition.

5. The method of claim 3 in which said liquid is flowed into contact with said belts in the form of a flow of less width than the width of said casting space so that this flow spreads laterally while moving into said casting space under said forwardly moving force, said flow being at a rate filling said space.

6. The method of claim 3 in which said liquid is flowed down a surface declining towards said casting end of the casting space and terminating above said forwardly moving horizontal belt surface, said surface being spaced from the semi-cylindrical end of the upper belt at said casting end.

7. The method of claim 6 in which said liquid is flowed onto said surface with a flow width less than the width of said casting space.

8. The method of claim 3 in which said body of liquid is confined in a space formed between said traveling horizontal surface and a static non-traveling, flat surface spaced thereabovE free from contact with said belts.

9. The method of claim 8 in which said liquid is cast in the form of a laterally confined column of continuously fed liquid having a free upper surface and a forward side contacting the adjacent cylindrical end of said upper belt, said column extending above the level of said lower span and communicating with said space in which said body of liquid is confined and by its height above said level hydrostatically placing said casting liquid under pressure.

10. The method of claim 1 in which said belt spans are externally restrained by resilient forces.

11. The method of claim 1 in which one of said belts is driven by a power source and the other belt is driven by this powered belt via said side seals.

12. The method of claim 10 in which said resilient forces permit said belt spans to separate different distances with changes in the rate said liquid is cast in the casting end of said casting space.

13. The method of claim 1 in which the viscosity of said liquid is not materially less than 1 poise.

14. The method of claim 1 in which said liquid is flowed into contact with said belts at said casting end from a level below the axis of the roll around which the belt forming said upper belt span runs at the casting end.

15. The method of claim 10 in which said liquid is flowed into contact with said belts at said casting end from a level below the axis of the roll around which the belt forming said upper belt span runs at the casting end.

16. A method for continuously casting a liquid having a viscosity rendering it flowable under the force of gravity and which hardens with changing temperature and time, in a traveling casting space having casting and discharging ends, said space being defined between two opposed, vertically interspaced, externally restrained, horizontal upper and lower, elongated, flexible belt spans having side seals and traveling at the same speed and in the same direction and formed by endless flexible belts each running around horizontally interspaced cylindrical rolls forming each belt into a loop having semi-cylindrical ends; wherein the improvement comprises continuously casting said liquid into said casting end by flowing the liquid into contact with said belts while controlling the traveling speed of said belts so that by the belts'' engagement with the liquid the latter receives therefrom a forwardly directed force, said liquid hardening as it travels through said space and the latter being long enough to permit said hardening, said force being greater than the force of gravity on the liquid to a degree preventing the latter force from causing said liquid to flow reversely out of said casting end far enough to fall from said lower belt, and the total of said forces producing a pressure on the liquid cast in said casting space, prior to the liquid hardening therein, which is at least sufficient to hydraulically support said upper span; said lower belt span being extended backwardly from said upper belt span to form a forwardly moving horizontal belt surface on which the liquid forms a backwardly extending body of liquid terminating where said forwardly moving force received from this belt surface is adequate to hold the liquid body against extending further backwardly; said liquid being flowed into contact with said belts in the form of a flow of less width than the width of said casting space so that this flow spreads laterally while moving into said casting space under said forwardly moving force, said flow being at a rate filling said space; said belt spans extending from said casting end and towards a location within said casting space where said liquid has substantially hardened, being externally restrained by resilient forces reacting against the hydraulic force of said liquid between said spans, said spans being held against separation to provide the casting thickness desired at least while adjacently approaching said location, and said resilient forces being proportioned relative to saiD hydraulic force to permit the latter to hold said belt spans apart for a distance greater than said thickness and controlling the lateral spread of said flow to cause the latter to completely fill said cast space at a location within said casting end but adjacent thereto.

17. The method of claim 16 in which said liquid is flowed down a surface declining towards said casting end of the casting space and terminating above said forwardly moving horizontal belt surface, said surface being spaced from the semi-cylindrical end of the upper belt at said casting end.

18. The method of claim 6 in which said liquid is flowed onto said surface with a flow width less than the width of said casting space.