Continuous casting of a polymerizable composition between moving endless belts

#The present invention relates to a process and apparatus for continuous casting of a liquid polymerizable composition comprising feeding the liquid polymerizable composition into a spacing defined between a pair of moving belts for the continuous polymerization into a plate thereof.

#As a process for preparing plates of polymers from polymerizable monomers, cell casting is in general known which comprises casting liquid monomer between a pair of glass sheets by the application of heat or light. For example, the cell casting process has been carried out for the preparation of plates of methacrylic resin from methyl methacrylate, thereby providing the plates thereof with desirable properties including transparency, gloss on the surface and weatherability. The cell casting process involves assembling a cell with a pair of sheets of reinforced glass which are provided with a peripheral gasket, filling the cell with methyl methacrylate monomer or a partially polymerized syrup thereof, immersing the assembly in a hot water bath or hot air oven to thereby polymerize the monomer and stripping the plates of polymeric material from the cell by separating the assembly. The size of the plates which may be produced by this process is restricted by the difficulty in handling large cells made out of sheets of reinforced glass. Furthermore, the manufacturing costs are high as a result of the considerable amount of manual work required during assembly of the cells, filling operations, separation of the cells and subsequent stripping of the plates of polymeric material. Thus, the development of a continuous casting process has been desired for rationalization.

#Some continuous casting processes are known, for example, by U.S. Pat. Nos. 2,500,728, 3,371,383 and 3,376,371, Dutch Patent Application No. 6900707 (corresponding to Canadian Pat. No. 895,406) and Japanese Patent Publication No. 41,602/1971 which each disclosed a process for preparing a plate of polymeric material which comprises filling with a polymerizable material a spacing between a pair of endless belts, the first belt being disposed above the second belt, and completing polymerization during the passage thereof through said spacing. These processes, however, cause non-uniformity of the thickness of the resulting plate thereof. Particularly, the difficulties involved in conventional continuous casting are a loss of control over the thickness of the final product in a widthwise direction. For example, Canadian Pat. No. 895,406 discloses that one of the two belts is so curved that the pressure applied by the other belt is made uniform, thus preventing the up-and-down movement of the belts. However, the process disclosed therein is not satisfactory in controlling the preciseness of thickness in the final product.

#Referring now to the drawings,

#FIG. 1 illustrates a side view of the continuous casting apparatus of the present invention wherein the path defined by the moving belts is curved twice; and

#FIGS. 2 and 3 illustrate each a side view of the continuous casting apparatus of the present invention wherein the belts are curved four times and three times, respectively.

#As shown in FIGS. 1, 2, and 3, these are examples of the continuous casting apparatus of the present invention wherein the lower belt is longer than the upper belt. These figures each indicate a relative location of point B at which the initial curvature starts and point C or an inflection point at which the adjacent curvatures change from one direction to the other alternate direction in the region beteen point A where the heating polymerization is caused to be carried out and point D where a mechanism capable of following the variation of volume of the polymerized composition ends. FIG. 1 illustrates the moving belts from point B to point C with one inflection point joining two curvatures; FIG. 2 shows the belts with three inflection points connecting four curvatures; and FIG. 3 illustrates the belts with two inflection points which are linked with three curvatures. FIG. 1 further illustrates the heating-polymerization region which is arranged in such a manner that the belts between point A and B are straightly disposed and inclined at an appropriate angle to the horizontal. In FIG. 2, there is illustrated the region defined between point points A and B which is arranged such that the path is inclined at an appropriate angle to the horizontal, as shown in FIG. 1, but it is defined by their natural curvatures. FIG. 3 illustrates a horizontal and straight region between points A and B.

#As further shown in FIGS. 1, 2, and 3, endless belts 1 and 2 are driven by pulleys 3, 4, 5 and 6 which provide the belts with tension. A pair of the pulleys 4 and 6 may be constructed such that they are driven at the same peripheral speed or such that either of them alone is driven, while the other is caused to move by the action exerted by the feed composition and the plate thereof and/or gaskets, thus permitting the belts 1 and 2 to move concurrently at substantially the same speed. The composition and the gaskets are fed continuously into the feed inlet between a pair of the pulleys 3 and 5 and the resulting plate is removed from the other end portion or the outlet between a pair of the pulleys 4 and 6.

#Referring to FIG. 1, the lower and upper runs of the respective upper and lower belts in the path between points A and B are held and maintained at a desired predetermined distance by a non-rotating support 7 having a frame structure. The lower and upper runs of the respective upper and lower belts are caused to follow a predetermined path having one inflection point, as at point C, of the curvatures between the points B and D in such a manner that the rollers of each pair as indicated generally by numeral 8 are positioned with the lower and upper runs of the individual belts 1 and 2 throughout the entire path between the points B and D. Thus, said runs are so arranged as to follow in response to variation of the volume of said composition during polymerization.