Methods and apparatus for generating interfacial surfaces

Abstract: Interfacial surfaces in a fluid mass are generated by dividing a first stream of the fluid mass into a plurality of branch streams (14,16,18,20), redirecting or repositioning (40) the branch streams, expanding (48) and contracting (54) the branch streams in a symmetrical fashion, and recombining (56) the branch streams to form a second stream of the fluid mass. Alternatively, the branch streams are redirected or repositioned (62), recombined (64) to form a second stream of the fluid mass, and the second stream expanded and contracted (66) in a symmetrical fashion. The invention has particular application to the manufacture of multilayer films using a first stream of two or more layers.

Description: This invention relates to interfacial surface generators and to methods of generating interfacial surfaces in a fluid mass. Multilayer films have been made or suggested to be made previously through the use of complex coextrusion feedblocks, see, for example, US-A-3,773,882 and US-A-3,884,606 to Schrenk. These complex feedblocks are difficult and expensive to manufacture, however, and are not easily adapted to different applications. Multilayer films have also been made or suggested to be made by a combination of a simpler feedblock with one or more interfacial surface generators (ISG's) in series, see, for example, US-A-3,565,985 to Schrenk et al. This apparatus is easier and less costly to make and assemble, and by changing the type and/or number of ISG's used in series enables the economical manufacture of films having from a few to many layers. Previous ISG's have not been designed for the manufacture of multilayer films, however, but for the mixing of highly viscous materials such as melts of various thermoplastics. These known ISG's have operated generally by forcing the viscous fluids to be mixed through a series of passageways which repeatedly separate and recombine stream elements during the flow of a stream of viscous fluids through the apparatus, with the intent generally of creating interfacial surfaces by repeated divisions of layers or aggregations generally of a given material. Distortions and disruptions of the layers in a given stream of materials result from such divisions and recombinations, so that a second stream of materials exiting even a single ISG may depart significantly from the desired character and/or configuration of the layers in a coextruded first composite stream from the feedblock. These distortions are multiplied where several ISG's are used in series, as may be necessary for the creation of a second stream having the desired number of layers. FR-A-1308573 (corresponding to GB-A-941893) and US-A-3239197 disclose ISG's in which a stream is divided into a plurality of parts, the dimension of the parts in a plane non-coplanar with the plane of the dividing means is increased, and said dimensionally increased parts are recombined. Optionally, the cross-sectional area of the stream is reduced before said increase in dimension. These ISG's can be used to generate interfacial surfaces within a composite stream comprised of discrete layers of polymeric material, of which at least a pair of said layers define a planar layer interface therebetween which lies in an x-z plane of a Cartesian coordinate system, wherein the z-axis is taken as extending in the direction of flow of the composite stream, the x-axis extends transversely of the composite stream and defines a transverse dimension of the layer interface, and the y-axis extends perpendicularly away from said layer interface in the thickness direction of said pair layers. In an embodiment, said composite stream is divided along said x-axis such that said pair of layers and said interface are distributed among at least two branch streams; the branch streams are expanded along the x-axis and contracted along the y-axis, optionally after reduction of the cross-section of the stream; and the resultant streams recombined to form a second composite stream which comprises a greater number of discrete layers of polymeric material than the original composite stream. JP-A-55154127 discloses the preparation of a multilayer sheet by dividing a multilayer flow into equal upper and lower branch streams; repositioning said branch streams to lateral juxtaposition; dividing the juxtaposed branch streams into equal upper and lower combined branch streams; repositioning said combined branch streams to lateral juxtaposition; and extruding the juxtaposed combined branch streams. JP-A-55154128 discloses the preparation of a multilayer sheet by dividing a multilayer flow into a central main and two opposed side branch flows; laterally contracting and mutually spacing the side branch flows; and then expanding each of the main branch and both contracted side branch flows in the direction orthogonal to that in which the original multilayer flow was divided. The present invention, by the symmetrical expansion and contraction of the branch streams or the second stream limits the distortions and disruptions normally attendant to the division and recombination of the first composite stream of materials in a conventional ISG. Thus, while the ISG of the present invention can be used for the conventional purpose of intimately mixing viscous fluids, it is particularly designed to be useful in the manufacture of multilayer films from a first composite stream. In a first aspect, the present invention provides a method of generating interfacial surfaces within a first composite stream comprised of discrete layers of polymeric material, of which at least a pair of said layers define a planar layer interface therebetween which lies in an x-z plane of a Cartesian coordinate system, wherein the z-axis is taken as extending in the direction of flow of the first composite stream, the x-axis extends transversely of the first composite stream and defines a transverse dimension of the layer interface, and the y-axis extends perpendicularly away from said layer interface in the thickness direction of said pair layers, said method comprising the steps of: dividing the first composite stream along the x-axis such that said pair of layers and said interface are distributed among at least two branch streams; expanding along the x-axis a transverse dimension of said branch streams and contracting along the y-axis the thickness of said branch streams; recombining the branch streams to form a second composite stream which comprises a greater number of discrete layers of polymeric material than the first composite stream; characterized in that said branch streams are stacked along the y-axis by repositioning along both of the x- and y-axes as they flow along the z-axis, said repositioning being prior to said expansion and contraction and being conducted without significantly altering their cross-sections; said expansion is symmetrical and is conducted on the individual branch streams prior to said contraction or on said second composite stream; and said contraction is symmetrical and is conducted on the individual branch streams after said expansion or on said second composite stream. In one embodiment of the method of the invention, the branch streams are stacked and then expanded and contracted before recombining. The invention provides an apparatus for generating interfacial surfaces by this embodiment, said apparatus comprising dividing means (34) for dividing the first composite stream into the branch streams along the x-axis; first conduits (40) for repositioning the branch streams along both the x-axis and the y-axis; second conduits (48) in fluid communication (44,46) with the first conduits (40), with each second conduit (48) having a progressively greater transverse dimension along the x-axis from an inlet opening (46) to an outlet port (52) thereof, the inlet opening (46) being positioned centrally in the apparatus; third conduits (54) in fluid communication (50,52) with the second conduits (48), with each third conduit (54) having a dimension in the y-direction which becomes progressively lesser from an inlet opening (50) to an outlet port thereof; and means (56) for recombining the branch streams from the third conduits (54) to form the second composite stream. In another embodiment of the method of the invention, the branch streams are stacked and then recombined before being expanded and contracted. The invention provides an apparatus for generating interfacial surfaces by this embodiment, said apparatus comprising dividing means (34) for dividing the first composite stream into the branch streams along the x-axis; first conduits (46) for repositioning the branch streams along both the x-axis and the y-axis; and a conduit wherein the branch streams are recombined and symmetrically expanded and contracted, said conduit defining a tetrahedral cavity through which the recombined branch streams flow, the cavity being formed by the intersection of sides (70) of the conduit in the general form of four truncated isosceles or equilateral triangles. Figure 1 is a schematic illustration of the practice of a method of the present invention for forming a multilayer stream from a first composite stream comprising two discrete and substantially continuous layers of diverse thermoplastic materials. Figure 2 is an isometric view of a first embodiment of an apparatus of the present invention as a whole. Figure 3-6 are views in isolation of conduits within a first portion or sub-element of the apparatus of Figure 2, taken from the front or upstream side of the first sub-element and showing the flow paths of fluids moving through the apparatus and through the conduits. Figure 7 is a rear or downstream face view of the first sub-element shown in Figures 3-6. Figure 8 is a front view of a second portion or sub-element of the apparatus of Figure 2, showing conduits within the second sub-element corresponding to the conduits in the first sub-element. Figure 9 is a top view of the conduits within the second sub-element of Figure 8, showing the direction of fluid flow through the conduits and through the second sub-element. Figure 10 is a side view of the conduits of the second sub-element, showing again the flow path of fluids through the conduits. Figure 11 is a rear view of the second sub-element depicted in Figures 8-10. Figure 12 is a front view of a third sub-element of the apparatus of Figure 2, showing conduits which correspond at their entrances to the exits of the conduits in the second sub-element, and which converge to a common exit. Figure 13 is a view in cross-section of the convergence of the conduits in the third sub-element of Figure 12 toward a common exit, showing by arrows the flow path of fluids through the conduits of the third sub-element. Figure 14 is a rear or downstream face view of the conduits of the third sub-element as previously shown in Figures 12 and 13. Figure 15 depicts an alternate embodiment of an apparatus of the present invention, with the conduits and cavities defined therein for fluid flow being shown in solid outline for clarity of viewing. Figure 16 is a front view of the second sub-element of the apparatus of Figure 15. As suggested in the summary description above of the drawings, an apparatus or element of the present invention in a first embodiment comprises three functionally distinct but not necessarily physically separable sub-elements - one for the dividing and redirecting or repositioning steps of the method, one for expanding the branch streams so created and repositioned, and one for contracting the branch streams and recombining them into an integrated whole. While these steps may be discretely and sequentially performed, it will be understood that for example the expanding and contracting steps may be combined or overlap if desired in a second sub-element, with the third sub-element being primarily adapted for accomplishing the recombination step while perhaps also performing some or all of the contracting function as well. Similarly, the third element may complete the expanding step before or while contracting and recombining the branch streams. The performance of the dividing, repositioning, expanding, contracting and recombining steps by these three sub-elements will typically and preferably be as shown schematically in Figure 1. As may be seen by reference to that drawing, a layered first composite stream of two discrete and continuous layers 10 and 12 of diverse thermoplastic materials, such as might proceed from a conventional coextrusion feedblock, enters the ISG and is divided into four branch streams 14, 16, 18 and 20 within the first sub-element. It is to be understood that a "conventional coextrusion feedblock" as used in the preceding paragraph embraces a multilayer feedblock, such as disclosed in US-A-3,773,882 to Schrenk. The first composite stream can thus comprise a number of layers and materials substantially exceeding the two layers shown in Figure 1. The illustrated first composite stream has been selected, however, in the interest of simplifying as much as possible the explanation and description of the present invention. The layered first composite stream defines a planar layer interface 22 between the layers 10 and 12 which lies generally in the x-z plane of an x-y-z coordinate system, wherein the z-direction is taken as corresponding to the direction of fluid flow generally through the apparatus (shown by the arrows 24 in Figure 1), the x-direction defines a transverse dimension of the interface 22 and layers 10 and 12, and the y-direction is in the thickness direction of the layers 10 and 12 and extends generally perpendicularly away from the planar layer interface 22. The division of the first composite stream is generally along the x-axis, so that the planar layer interface 22 is divided up generally among two or more of the branch streams 14, 16, 18 and 20, depending on the extent of the layers 10 and 12 and thus of the layer interface 22 in the x-direction. Where the layers 10 and 12 span the first composite stream as shown in Figure 1, the layer interface 22 is thus divided among each of the branch streams 14, 16, 18 and 20. At least two branch streams between which the interface 22 was divided, and preferably all of the branch streams 14, 16, 18 and 20, are then redirected and repositioned with respect to both the x- and y-axes and with respect to one another in flowing through the conduits of the first sub-element, so that at the exit of the first sub-element the streams 14, 16, 18 and 20 are stacked along the y-axis. Or, expressed differently, the branch streams 14, 16, 18, and 20, are in flowing through the first sub-element, divergently displaced along the "y" axis from their original positions while being convergently displaced along the "x" axis. In the second sub-element, at least two branch streams and preferably each of the stacked branch streams 14, 16, 18 and 20 independently undergo a progressive symmetrical expansion along the x-axis (in the plane of the layer interface 22 of greatest interest) in their transverse (or x) dimension, preferably until the transverse width of a branch stream is substantially as great as the width of the first composite stream entering the apparatus. This expansion is initiated from a central location in the apparatus along the y-axis, and occurs uniformly with respect to each of the layers 10 and 12 in a given branch stream as shown. Preferably the branch streams 14, 16, 18 and 20 also expand uniformly with respect to one another as shown. At the exit of the second sub-element, the branch streams 14, 16, 18 and 20 enter into the third sub-element wherein at least two branch streams, and preferably each of the branch streams 14, 16, 18 and 20, independently undergo a similar progressive symmetrical contraction or reduction of their thickness along the y-axis or in the y-direction and are recombined to form a layered second composite stream. The contraction of each branch stream occurs evenly along the x-axis (that is, in the plane of the layer interface 22) with respect to the layers 10 and 12 in a given branch stream as shown, and preferably also occurs evenly or uniformly from branch stream to branch stream. An apparatus for performing the steps schematically illustrated in Figure 1 may be understood by reference to Figures 2-14, wherein a first embodiment of the ISG apparatus as a whole is denoted by the numeral 26. In Figure 2, the ISG 26 comprises first, second, and third sub-elements 28, 30, and 32. A first composite stream comprised of discrete overlapping layers of diverse polymeric or thermoplastic materials, of which at least one pair defines a generally planar layer interface therebetween lying generally in an x-z plane, enters the ISG 26 through regularly-shaped inlet openings 34 in the first sub-element 28 and flows through the elements 28, 30 and 32 as indicated by the arrows 36. Referring now to Figures 3 through 6, it can be seen that these inlet openings 34 have preferably substantially identical rectangular or square cross-sections and are aligned generally along the x-axis in the previously-defined x-y-z coordinate system. The four inlet openings 34 are defined by three dividing wall portions 38 (Figure 2) which can serve as dividing means for dividing the first composite stream of Figure 1 into the four separate branch streams 14, 16, 18, and 20. The branch streams 14, 16, 18, and 20 in the first sub-element 28 proceed from the inlet openings 34 through first conduits 40 (Figures 3-6). These first conduits 40 act as means for redirecting or repositioning each of the streams 14, 16, 18, and 20 with respect to both of the x and y axes, the general direction and degree of displacement associated with each conduit 40 being indicated by the arrows 42 in Figures 3-6. Preferably these conduits 40 are substantially of the same length. The streams 16 and 18 entering through the inner two inlet openings 34 are displaced to the greatest extent along the y-axis and the least along the x-axis (see Figures 4 and 5). Similarly, the branch streams 14 and 18 are displaced least along the y-axis, and most along the x-axis (see Figures 3 and 6) so that at the exit of the first sub-element 28, the outlet ports 44 associated with the conduits 40 are in a preferred embodiment stacked generally along the y-axis (Figure 7). Preferably, the repositioning described above is effected without substantially changing or distorting the cross-sections of the various branch streams, such as rotating the laminae within a given branch stream with respect to the laminae of another branch stream. The conduits 40 and outlet ports 44 are preferably also substantially identical in true cross-section to one another and to the square or rectangular inlet openings 34. As may be seen in Figure 2, and now more particularly in Figure 8, the outlet ports 44 from the first sub-element 28 communicate with second inlet openings 46 of the second sub-element 30, which second inlet openings 46 are also preferably substantially identically square or rectangular in cross-section to the outlet ports 44 and to one another. After passing through the outlet ports 44 and second inlet openings 46, the streams 14, 16, 18, and 20 are preferably progressively and symmetrically expanded in their transverse (x) dimension by passage through respective second conduits 48 (see Figures 2, 8, 9 and 11). The second conduits 48 are preferably substantially identically configured to one another in cross-section so as to accomplish a uniform expansion of the branch streams 14, 16, 18 and 20. This common configuration in its preferred form gives the impression of a flattened funnel having a rectangular end portion (Figures 8 and 9), so that the branch streams in passing through the second conduits 48 each possess a gradually progressively greater transverse dimension along the x-axis. Referring now to Figure 12, the streams 14, 16,18, and 20 are then communicated to the third inlet openings 50 of the third sub-element 32 via the outlet ports 52 (Figures 8 and 9) of the second sub-element 30, the third inlet openings 50 thus communicating with the downstream ends of the conduits 48 and being preferably substantially identically square or rectangular in true cross-section therewith. The branch streams 14, 16, 18, and 20 are then conveyed through third conduits 54 which symmetrically and preferably uniformly contract the dimension of the various branch streams in the y- or thickness direction (Figure 13), and recombine the streams in overlapping relationship so that the second composite stream leaving the device 26 possesses more of the layers 10 and 12 than the first composite stream entering the device 26. The third conduits 54 are preferably of a basically constant transverse or "x" dimension (Figures 12 and 14) and convey or direct the streams passing therethrough toward a common exit 56. This common exit 56, where the apparatus 26 is but one of several such devices in series, will preferably be of substantially the same overall dimensions in the x- and y-directions as the combined inlet openings 34 (Figure 2). An alternate embodiment of an apparatus for producing multilayered streams of this character from a first composite stream is shown in dashed outline in Figures 15 and 16 and designated by the numeral 60. The embodiment 60 is comprised in part of a first sub-element 62 which, like the first sub-element 28 of the embodiment of Figures 2-14, operates to divide a first composite stream into branch streams and reposition the branch streams. A first composite stream flowing in the direction indicated by the arrow 36 is thus divided into four branch streams by inlet openings 34 which are aligned along the x-axis. These branch streams are then, in flowing through the conduits 40, repositioned with respect to one another and with respect to the x- and y-axes, so that at the inlet opening 64 of a second sub-element 66 the branch streams are centrally stacked along the y-axis. Second sub-element 66 of embodiment 60 thereafter symmetrically expands and contracts the recombined branch streams from the inlet opening 64 to an outlet opening 68 aligned with the x-axis. Second sub-element 66 shown in Figures 15 and 16 defines a tetrahedral cavity through which the recombined branch streams then flow, the tetrahedral cavity being formed by the intersection of sides 70 in the general form of four truncated isosceles (or equilateral, depending on the dimensions of the inlet opening 64 and outlet opening 68 in the y- and x-directions, respectively) triangles. A feature of common significance in both embodiments is the symmetrical expansion and contraction of the divided and stacked branch streams in the apparatus 26, or of the second composite stream comprised of the recombined branch streams in the apparatus 60. A symmetrical expansion in the x- or transverse dimension of the branch streams or of the second composite stream from along the y-axis, rather than unevenly from a peripheral portion of the given apparatus, prevents the bulk of the materials in a given branch stream or in the second composite stream from being funneled or channeled along the near wall of the apparatus relative to such peripheral portion. Distortions in individual layer thicknesses because of this channeling effect (due to the flow path being shorter along the nearer wall as opposed to the far wall) are in this manner discouraged from occurring. In the same manner, the symmetrical (along the x-axis) reduction of the thickness of the layers in a given branch stream or in the second composite stream is designed to avoid causing distortions in the thickness of such layers along the x-axis or in the transverse direction, as well as to avoid creating flows in the y-direction which might tend to disrupt the thinly-layered character of a branch stream or the second composite stream. By further contracting the branch streams or the second composite stream uniformly as is preferred, the layers 10 and 12 in a first composite stream may be evenly divided into a large number of thin but continuous layers in the second composite stream whereas a non-uniform contraction of the branch streams could result in some thicker layers of a given material in one branch stream with some discontinuous layers in another branch stream. In some multilayer film applications, for example in the manufacture of multilayer barrier films, it will be particularly advantageous to utilize layers of a material which are both uniformly thin and continuous. The second composite stream emerging from an apparatus 26 or 60 may itself become the feed stream (that is, the first composite stream) to a second ISG of the present invention and so on, or may itself be formed into a desired configuration by some suitable method for forming a layered stream of thermoplastic materials into an article, such as by a thermoforming process. In designing the ISG's of the present invention, it will normally be preferable to have the branch streams or second composite stream manipulated in a gradual manner. But where space for the apparatus is limited, or where there is the potential for degradation of some of the materials in the first composite stream in passing through a feedblock apparatus and one or more of the present ISG's, for example, the overall length of the various elements required in combination to create the desired number of layers may be limited also. There are a number of options with regard to the design and employment of the ISG's of the present invention under such circumstances, including employing elements constructed according to the teaching of the present invention but with a greater number of channels than the four exemplified herein, shortening one or more of the sub-elements 28, 30 and 32 or 62 and 66, changing the feedblock, or a combination of one or more of these options. The person of ordinary skill in the art is well-equipped to choose from among these options for particular applications of the present invention. While preferred embodiments of the method and apparatus of the present invention have been described, it will be appreciated that numerous changes in these embodiments can be made. For example, a first composite stream could be divided into any number of branch streams, rather than just the four which have been exemplified. Additionally, while it is preferred that an apparatus of the present invention perform both of the expansion and contraction steps of the inventive method, other ways of performing the method could be devised. As an example, in a feedblock-apparatus-die combination the apparatus could perform the expansion step, and the contraction step could occur in the die. Or, the contraction step could occur in the apparatus, with the expansion step occurring in the die.

Claims: A method of generating interfacial surfaces within a first composite stream comprised of discrete layers of polymeric material, of which at least a pair of said layers define a planar layer interface therebetween which lies in an x-z plane of a Cartesian coordinate system, wherein the z-axis is taken as extending in the direction of flow of the first composite stream, the x-axis extends transversely of the first composite stream and defines a transverse dimension of the layer interface, and the y-axis extends perpendicularly away from said layer interface in the thickness direction of said pair layers, said method comprising the steps of: dividing the first composite stream along the x-axis such that said pair of layers and said interface are distributed among at least two branch streams; expanding along the x-axis a transverse dimension of said branch streams and contracting along the y-axis the thickness of said branch streams; recombining the branch streams to form a second composite stream which comprises a greater number of discrete layers of polymeric material than the first composite stream; characterized in that said branch streams are stacked along the y-axis by repositioning along both of the x- and y-axes as they flow along the z-axis, said repositioning being prior to said expansion and contraction and being conducted without significantly altering their cross-sections; said expansion is symmetrical and is conducted on the individual branch streams prior to said contraction or on said second composite stream; and said contraction is symmetrical and is conducted on the individual branch streams after said expansion or on said second composite stream. A method as defined in Claim 1, wherein the branch streams are stacked and then expanded and contracted before recombining. A method as defined in Claim 1, wherein the branch streams are stacked and then recombined before being expanded and contracted. A method of making a multilayered plastic article, comprising molding into a desired configuration a product of a method as defined in any one of the preceding claims. An apparatus for generating interfacial surfaces by a method as defined in Claim 2, said apparatus comprising: dividing means (34) for dividing the first composite stream into the branch streams along the x-axis; first conduits (40) for repositioning the branch streams along both the x-axis and the y-axis; second conduits (48) in fluid communication (44,46) with the first conduits (40), with each second conduit (48) having a progressively greater transverse dimension along the x-axis from an inlet opening (46) to an outlet port (52) thereof, the inlet opening (46) being positioned centrally in the apparatus; third conduits (54) in fluid communication (50,52) with the second conduits (48), with each third conduit (54) having a dimension in the y-direction which becomes progressively lesser from an inlet opening (50) to an outlet port thereof; and means (56) for recombining the branch streams from the third conduits (54) to form the second composite stream. An apparatus for generating interfacial surfaces by a method as defined in Claim 3, said apparatus comprising: dividing means (34) for dividing the first composite stream into the branch streams along the x-axis; first conduits (46) for repositioning the branch streams along both the x-axis and the y-axis; and a conduit wherein the branch streams are recombined and symmetrically expanded and contracted, said conduit defining a tetrahedral cavity through which the recombined branch streams flow, the cavity being formed by the intersection of sides (70) of the conduit in the general form of four truncated isosceles or equilateral triangles.

Inventors: AYRES RALPH E, GOSEN DANIEL J, SCHRENK WALTER J, SHASTRI RANGANATH K, AYRES, RALPH E., GOSEN, DANIEL J., SCHRENK, WALTER J., SHASTRI, RANGANATH K.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: B29C47
IPC codes: B01F5, B29C47, B29L9