A mixing apparatus and a filming plant comprising said apparatus

Abstract

A mixing apparatus (1) for a molten polymer composition comprising: a casing (11) delimiting a mixing chamber (120) having a chamber length (L); at least one screw (10), rotatably extending into the mixing chamber (120) and comprising a positive mastication element (101) and a negative mastication element (102) that are suitable for promoting the mixing and the advancement of the molten polymer composition (S1). Obtained within the casing (11) are at least one polymer material feeding opening (2) and at least one inorganic filler injection opening (3). The negative mastication element (102) closest to the inorganic filler injection opening (3) comprises a proximal end (15) at a distance of at least on third of the chamber length (L) from the inorganic filler injection opening (3).

Description

"A MIXING APPARATUS AND A FILMING PLANT COMPRISING SAIDAPPARATUS" DESCRIPTIONField of application

[0001] The present invention is in relation to a mixing apparatus for the mixing of a molten polymer composition, and in particular for the mixing of a polymer material in the molten state with powder inorganic fillers , in order to make a polymer composition that is suitable for being used with inline applications using thermoforming systems , preferably filming systems , for the production of transparent films .

[0002] It is known how the process of processing polymeric materials for the manufacture of arti facts involves a preparation step for the required composition, starting from single components , by means of the mixing of a polymer base whereto inorganic fillers are added ( in English, filler ) .

[0003] Sometimes , the mixing process occurs on the same thermoforming system line as that of the compound obtained ( in technical j argon, inline mixing) . In this case , the mixing of the polymer together with the filler is extremely critical .

[0004] The uni form mixing of thermoplastic polymer materials in the molten state (predominantly but notexclusively polyolefins such as polyethylene and polypropylene homopolymer and copolymer ) with inorganic fillers , for example , but not exclusively, Calcium Carbonate ( CaCO3 ) , commonly requires the use of a twin- screw mixer or mixers .

[0005] Also known in the prior art is the contemporaneous performing of the dual operation of mixing and extracting the material using a mixer of the type provided with two cooperating threaded shafts or screws , comprising, in di f ferent successive axial sections , a combination of positive , negative and neutral mastication elements with the aim of obtaining both the advancement of the product towards the exit and better mixing of the same .

[0006] Typically, such mixers are si zed as a function of the extruder that feeds them, in such a way as to allow for adequate filling of the screw channels ( filling factor ) of the screws .

[0007] The present invention is preferably applicable when the mixing is placed in line with a process for making filled plastic films .

[0008] Almost all filming processes involve the manufacturing of filled film in two steps : a first step, of making the compound ( filled melt ) , with a filler percentage that reaches at up to 80% and a second step of extrusion and filming of the compound itsel f up-and-untilobtaining a film with a thickness of 10 micrometers .

[0009] In the case of inline filming the criticality of the mixing step lies in the fact that the mixing times have to be reduced due to the need to have suf ficient film manufacturing productivity whil st at the same time maintaining good mixing uni formity in order not to compromise the manufacture of the film .

[0010] Such a need is greatly accentuated in the case of manufacturing microporous breathable films . In fact , a poor mixing of polymer and filler may negatively influence the manufacture of such products .

[0011] The manufacture of such products , in fact , requires a high filler content . The " stretching" of the film, by means of passing through a series of rotating cylinders with a progressively decreasing speed of rotation finally creating the microporosity, given the required high filler content , could generate , i f the mixing were not to be performed correctly, the generation of undispersed lumps of filler that would promote film breakages during the filming step .

[0012] Disadvantageously, the known mixing apparatus therefore suf fers with the following problems :- embedding of the volatile compounds such as moisture or gaseous residues resulting from the decomposition of the polymer, which, in expanding during the filming step,generate holes in the film;- thermo-oxidative decomposition and / or molecular crosslinking generated by the mechanical stress of the polymer phase and by an excessive residence time within the mixer .

[0013] With regard to the first point , the applicant has devised and implemented a speci fic degassing system described within the European patent EP1843888 , "Barrel processor having degassing means" , which allows for ef ficient removal of volatile , embedded or generated compounds .

[0014] The obj ect of the invention is instead the resolution of the second point .

[0015] The present invention therefore proposes to resolve the technical problem of the possible thermo-oxidative decomposition and / or molecular cross-linking that may be generated due to mechanical stress during the polymer phase of the compound, and due to excessive residence time within the mixer .

[0016] In the state of the art , it is known to operate on the mixing speed ( twin-screw rpm) or on the residence time within the mixer . This allows to accelerate the mixing and to proceed with the filming with suf ficient productivity .

[0017] Nevertheless , disadvantageously, both thesesolutions are unfavorable and in certain cases deleterious , insofar as they may result in excessive processing of the thermoplastic material with resulting oxidi zation (visible by simple yellowing or carboni zation) or molecular cross-linking which may be identi fied by the formation of "gels" or " fish eye" i . e . , rigid, unmelted areas that are impossible to film and result in film breakage during stretching .Solution of the invention

[0018] Therefore , it is strongly felt the need to make available a mixing apparatus for a molten polymer composition and a filming plant capable of overcoming those drawbacks that are typical of the state o f the art .

[0019] In particular, the obj ect of the present invention is to implement a mixing apparatus for a molten polymer composition that allows for optimal mixing of the polymers and of the fillers within the same , starting from the basic components in order to make the mixture thereby obtained suitable for the subsequent continuous thermoforming and suitable to maintain the flexibility, adaptability, productivity features o f the mixers .

[0020] A further obj ect of the present invention is also to avoid thermo-oxidative decomposition and molecular crosslinking phenomenon within the mixture .

[0021] Within the context of such a technical problem afurther necessity lies in that said apparatus should also ensure the uni form distribution and homogeni zation of the inorganic fillers within the compound and optimal control of the mixture temperature during all of the mixing steps .

[0022] Such requirement is satis fied by a mixing apparatus and by a filming plant according to the attached independent claims . The dependent claims describe preferred or advantageous embodiments of the invention, comprising further advantageous features .Description of the drawings

[0023] The features and advantages of the mixing apparatus and filming plant will become apparent from the description below of some preferred embodiments , given by way of indicative and non-limiting examples , with reference to the attached figures , wherein :Figure 1 shows a block diagram of a filming plant according to one embodiment of the present invention;- Figure 2 shows a schematic form of a mixing apparatus according to one embodiment of the invention;Figure 3 shows a schematic view of a screw for the mixing apparatus according to one embodiment of the invention;Figure 4a shows a positive mastication element according to one embodiment of the present invention;Figure 4b shows a negative mastication element according to one embodiment of the present invention;- Figure 4c shows a neutral mastication element according to one embodiment of the present invention ;- Figure 5 shows a table wherein the experimental results are given of a test performed on the mixing apparatus according to one embodiment of the present invention . Detailed description

[0024] With reference to the above figures a mixing apparatus for a molten polymer composition has been indicated as a whole with the reference numeral 1 .

[0025] The term polymer composition refers , preferably, to a composition formed from a polymer material , preferably thermoplastic, preferably in the molten state , and additives such as inorganic fillers , powders , and / or other components that are commonly used for loading polymer materials that are intended for extrusion or thermoforming or filming .

[0026] According to the present invention, and with reference to the exemplary embodiment shown in Figure 2 , the mixing apparatus 1 comprises :- a casing 11 that extends longitudinally along a main extension axis X between a first end 21 and a second end 22 and delimiting a mixing chamber 120 extending over a chamber length L between said ends 21 and 22 , wherein, atthe second end 22 , an outlet opening 110 is obtained in order to allow the molten polymer composition to exit from the mixing chamber 120 ; at least one screw 10 rotatably extending into the mixing chamber 120 and comprising a mastication portion 4 comprising at least one positive mastication element 101 and at least one negative mastication element 102 .

[0027] Preferably, the outlet opening 110 is suitable for being operatively connected to a forming apparatus , such as for example a filming apparatus , in such a way that by means of said outlet opening 110 , such forming apparatus is fed with the molten polymer composition that is mixed in the mixing apparatus 1 .

[0028] According to the invention, the term positive mastication element 101 refers to a mastication element that is suitable for mixing the molten polymer composition and promoting the advancement thereof along the main extension axis X in an advancement direction S I . Such element is also known within the sector as a thrust element . One exemplary embodiment of a positive mastication element 101 is shown in Figure 4a .

[0029] It is clear that the term advancement direction S I refers to a direction that proceeds from the first end 21 towards the second end 22 , i . e . , towards the outlet opening 110 .

[0030] According to the invention, furthermore , the term negative mastication element 102 refers to a mastication element that is suitable for mixing the molten polymer composition and promoting the return thereof along the main extension axis X in a return direction S2 , opposite to said advancement direction S I . Such element is also known within the sector as a counter-thrust element . One exemplary embodiment of a negative mastication element 102 is shown in Figure 4b .

[0031] It is also clear that the term return direction S2 refers to a direction that proceeds from the second end 22 towards the first end 21 , i . e . , from the opposite part with respect to the outlet opening 110 .

[0032] Such negative mastication element 102 is therefore suitable for mixing the material in pushing it backwards with respect to the advancement direction, in order to allow for better and more complete mixing thereof .

[0033] Advantageously, such negative mastication element 102 therefore makes it possible to perform mastication or mixing of the polymer composition that is more homogeneous .

[0034] According to one aspect of the present invention, said at least one positive mastication element 101 and at least one negative mastication element 102 are rotatably operable in the mixing chamber 120 and are suitable forpromoting, together, the mixing and the general advancement of the molten polymer composition into the mixing chamber 120 in the advancement direction S I .

[0035] It is therefore clear that such positive mastication element 101 and negative mastication element 102 are purposefully conformed and spaced apart in order to cause the general advancement of the polymer composition towards the outlet opening 110 , i . e . , in other words , the thrust generated by the positive mastication element 101 is such to overcome the thrust of the negative mastication element 102 and to generate a relative thrust that causes the polymer composition to advance in the advancement direction S2 .

[0036] With reference to Figure 2 , at least one polymer material feeding opening 2 and at least one inorganic filler inj ection opening 3 are obtained within the casing 11 in a mutually spaced manner along the main extension axis X, and so that the inorganic fillers inj ection opening 3 is located downstream of the polymer material feeding opening 2 in the advancement direction S I .

[0037] The polymer material feeding opening 2 is suitable for being operatively connected to polymer material feeding apparatus , for example an extruder, that is suitable for feeding polymer material , preferably thermoplastic, and preferably in the molten state , to themixing apparatus 1 by means of said opening 2.

[0038] Preferably, the polymer material feeding opening 2 is obtained within the casing 11 at the first end 21.

[0039] The inorganic fillers injection opening 3 is suitable for being operatively connected to a filler injecting apparatus, for example an injector, that is suitable for feeding inorganic fillers, preferably, but not exclusively, Calcium Carbonate (CaCO3) , to the mixing apparatus 1 by means of said opening 3.

[0040] In one preferred embodiment, the mixing apparatus 1 comprises at least one screw 10' rotatably extending into the mixing chamber 120 together with the screw 10 and comprising a mastication portion 4 comprising at least one positive mastication element 101 and at least one negative mastication element 102.

[0041] Preferably, the screw 10 and the second screw 10' are identical.

[0042] In the remainder of this disclosure reference will be made to the screw 10 and the second screw 10' , generically indicating them as screws 10, 10' or each screw 10, 10' or pair of screws 10, 10' .

[0043] It is also clear to an expert in the field that what is described in the following paragraphs with reference to the screw 10, is also applicable to the screw 10' , in the embodiments that include it.

[0044] In one preferred embodiment , the screw 10 extends into the mixing chamber for the entire chamber length L .

[0045] According to one aspect of the present invention, at least one negative mastication element 102 that is closer to the inorganic filler inj ection opening 3 comprises a proximal end 15 placed at a distance of at least one third of the chamber length L, along the main extension axis X, from said inorganic filler inj ection opening 3 .

[0046] Preferably, the term proximal end 15 refers to one face of the negative mastication element 102 that faces toward the inorganic filler inj ection opening 3 . For example , the proximal end 15 is such a face , or a plane that at least partially contains said face , substantially perpendicular to the main extension axis X .

[0047] Preferably, the chamber length L is between 1000 and 3000 millimeters .

[0048] In one embodiment , the screw 10 comprises at least on first neutral mastication element 103 that is suitable for mixing the molten polymer composition and not to impart additional motion in the forward direction ( S I ) or in the return direction ( S2 ) to the molten polymer composition . One exemplary embodiment of a neutral mastication element 103 is shown in Figure 4c .

[0049] In other words , preferably, such element is purposefully conformed to perform the mixing of thematerial without generating, thereupon, an advancement or return thrust , i . e . , without af fecting the relative general advancement thrust .

[0050] In one preferred embodiment , the at least one positive mastication element 101 and the at least one negative mastication element 102 are rotatably operable within the mixing chamber 120 in a mutually integral manner .

[0051] Preferably, such positive 101 and negative 102 mastication elements are part of the respective screws 10 , 10 ' as a single piece .

[0052] Furthermore , preferably, in the embodiment that also comprises the neutral mastication element 103 , such element is also operable in rotation within the mixing chamber 120 in a manner that is integral with the positive 101 and neutral 102 mastication elements .

[0053] In one embodiment , the proximal end 15 of said at least one negative mastication element 102 is placed at a distance between a third and two thirds , inclusive , of the chamber length L from said inorganic filler inj ection opening 3 along the main extension axis X .

[0054] In one embodiment , the proximal end 15 of said at least one negative mastication element 102 is placed at a distance between a third and a hal f , inclusive , of the chamber length L, from said inorganic filler inj ectionopening 3 along the main extension axis X.

[0055] In one further embodiment, the proximal end 15 of said at least one negative mastication element 102 is placed at a distance between a half and two thirds, inclusive, of the chamber length L, from said inorganic filler injection opening 3 along the main extension axis X.

[0056] Preferably, the screws 10, 10' of said pair are identical and rotatable within the mixing chamber 120.

[0057] In one embodiment, the screws 10, 10' are rotatable within the mixing chamber 120 in a mutually co-rotating manner .

[0058] In one embodiment, the screws 10, 10' are rotatable in the mixing chamber in a mutually counter-rotating manner .

[0059] Furthermore, in a preferred embodiment, for example shown in Figure 3, the screw 10 comprises a first conveying portion 5 upstream of the mastication portion 4 and a second conveying portion 5' downstream of the mastication portion 4. Each of said first and second conveying portions 5, 5' comprises a plurality of positive conveying elements 104 that are suitable for promoting the advancement of the molten polymer composition without mixing it.

[0060] In a preferred embodiment, the polymer materialfeeding opening 2 is obtained in the casing 11 in so that the polymer material is fed into the mixing chamber 120 at the first conveying portion 5 .

[0061] In one advantageous embodiment , the at least one neutral mastication element 103 is interposed between the at least one positive mastication element 101 and the at least one negative mastication element 102 .

[0062] Preferably, the screw 10 comprises a number of positive mastication elements 101 , at least twice that of the number of negative mastication elements 102 .

[0063] Preferably, the screw 10 comprises negative 102 and neutral 103 mastication elements in equal number .

[0064] Preferably, each screw 10 , 10 ' comprises six positive mastication elements 101 .

[0065] Preferably, each screw 10 , 10 ' comprises three negative mastication elements 102 .

[0066] Preferably, each screw 10 , 10 ' comprises three neutral mastication elements .

[0067] In one embodiment , the second conveying portion 5 ' extends into the mixing chamber 120 over a second conveying length L2 and the first conveying portion 5 extends into the mixing chamber 120 over a first conveying length LI which is grater than said second conveying length L2 .

[0068] In a preferred embodiment , the length of the firstconveying length LI is at least twice that of the said second conveying length L2 .

[0069] Preferably, the first conveying portion 5 comprises six positive conveying elements 104 .

[0070] Preferably, the second conveying portion 5 ' comprises three positive conveying elements 104 .

[0071] The obj ect of the present invention also relates to a filming plant 8 . As shown schematically, according to a particular embodiment , in Figure 1 , the filming plant 8 comprises a mixing apparatus 1 according to the present invention and a filming apparatus 6 that is operatively connected to said mixing apparatus 1 at the outlet opening 110 of the same in order to receive the molten polymer composition and to perform the filming thereof .

[0072] Preferably, furthermore, the filming plant 8 comprises a polymer material feeding apparatus 7 , for example an extruder that is operatively connected to the polymer material feeding opening 2 in order to feed the mixing apparatus 1 .

[0073] Furthermore , preferably, the filming plant 8 comprises an inorganic fi ller inj ection apparatus 9 that is operatively connected to said inorganic filler inj ection opening 3 .

[0074] In a preferred embodiment , the screw 10 has a mixing diameter D2 and the polymer material feeding apparatus 7comprises an extrusion screw having a nominal extrusion diameter DI , wherein the ratio between the mixing diameter D2 and the extrusion diameter DI is of between 1 . 5 and 2 . 0 .

[0075] Preferably, the extrusion diameter DI is of between 50 mm and 100 mm .

[0076] Furthermore , preferably, the mixing diameter D2 is of between 75 mm and 200 mm .

[0077] Advantageously, such preferred si zing allows for working at appropriate mixing chamber fil ling factors , and for keeping them higher and close to 1 .

[0078] Furthermore , advantageously, such si zing of the diameters of the screws , with respect to the extrusion screw, ensures a dispersive and distributed mixing of the inorganic fillers within the polymer material , and therefore ensures the formation of polymer composition that is homogeneous and substantially devoid of defects .

[0079] Innovatively, the present invention solves the disadvantages of the typical inline mixing apparatus of the prior art .

[0080] According to a first advantage , the mixing apparatus according to the present invention allows for the elimination of any thermo-oxidative or mechanical stress degradation .

[0081] This ef fect has been experimentally found by theApplicant by means of visual analyses and infrared spectroscopy . Such analysis revealed the total absence of gels .

[0082] According to a further advantage , the mixing apparatus ensures uni form distribution of the inorganic fillers and the manufacturing for a homogeneous polymer composition .

[0083] The Applicant has experimentally observed homogeneity of the composition of a film produced in a filming plant according to the present invention by means of the measurement of the "Water Vapour Transmission Rate" ( or WVTR) , according to the ASTM D6701 method and by means of Fourier Trans form Infrared Spectroscopy (" Fourier Trans form Infrared Spectroscopy" , or FTIR) performed over the entire extruded fi lm.

[0084] Advantageously, furthermore , the mixing apparatus according to the present invention is purposefully designed for maintaining an elevated mixing chamber filling factor .

[0085] Advantageously, the mixing apparatus of the present invention is versatile and easily adaptable in order to mix various types of compound, allowing for the control and adj ustment of the mixing temperature and high productivity .

[0086] According to a further aspect , furthermore , themixing apparatus allows the residence time of the polymer composition to remain essentially unchanged (within a range of variation of within 10% ) within the mixing chamber, as the input flow rate of polymeric material and inorganic fillers changes up to an increase of 60% . Innovatively, such advantage is obtained while maintaining constant the mixing speed, i . e . , the angular speed of rotation of the screws .

[0087] In particular, such technical ef fect is shown in a schematic form in the table of Figure 5 , wherein the variation in residence time within the mixing chamber is listed as the input flow rate , respectively for two di f ferent values of mixing speed .

[0088] It is clear that , to the embodiments of the aforesaid mixing apparatus and the aforesaid filming plant , a person skilled in the art , in order to meet speci fic needs , could make variations or substitutions of elements with functionally equivalent ones .

[0089] These variants are also contained within the scope of protection as defined by the following claims . Moreover, each variant described as belonging to a possible embodiment may be implemented independently of the other variants described .

Claims

CLAIMS1. A mixing apparatus (1) for a molten polymer composition comprising: a casing (11) extending longitudinally along a main extension axis (X) between a first end (21) and a second end (22) and delimiting a mixing chamber (120) extending over a chamber length (L) along said main extension axis (X) , wherein, at one of said first and second end (21, 22) at least one outlet opening (110) for the molten polymer composition is obtained; at least one screw (10) rotatably extending into the mixing chamber (120) and comprising a mastication portion (4) comprising at least one positive mastication element (101) , suitable for mixing said molten polymer composition and promoting the advancement thereof along said main extension axis (X) in an advancement direction (SI) , towards the outlet opening (110) , at least one negative mastication element (102) suitable for mixing said molten polymer composition and promoting the return thereof along at least one portion of said main extension axis (X) in a return direction (S2) , opposite to said advancement direction (SI) , said at least one positive mastication element (101) and at least one negative mastication element (102) being suitable, together, for causing the mixing and thegeneral advancement of said molten polymer composition into said mixing chamber (120) in said advancement direction (SI ) ; wherein at least one polymer material feeding opening (2) and at least one inorganic filler injection opening (3) are obtained in said casing (11) in a mutually spaced manner along said main extension axis (X) , and so that the inorganic filler injection opening (3) is located downstream of the polymer material feeding opening (2) in the advancement direction (SI) ; said mixing apparatus (1) being characterized in that the at least one negative mastication element (102) closest to the inorganic filler injection opening (3) comprises a proximal end (15) placed at a distance of at least one third of the chamber length (L) from said inorganic filler injection opening (3) .

2. Mixing apparatus (1) according to claim 1, wherein the screw (10) comprises at least a first neutral mastication element (103) , suitable for mixing the molten polymer composition and not imprinting a further movement in the advancement direction (SI) or in the return direction (S2) to the molten polymer composition.

3. Mixing apparatus (1) according to claim 1 or 2, wherein said at least one positive mastication element (101) and at least one negative mastication element (102)are rotatably operable in the mixing chamber (15) in a mutually integral manner.

4. Mixing apparatus (1) according to any one of the preceding claims, wherein the proximal end (15) is placed at a distance between a third and two thirds, inclusive, of the chamber length (L) from said inorganic filler injection opening (3) .

5. Mixing apparatus (1) according to any one of the preceding claims, wherein the proximal end (15) is placed at a distance between a third and a half, inclusive, of the chamber length (L) from said inorganic filler injection opening (3) .

6. Mixing apparatus (1) according to any one of claims 1 to 4, wherein the proximal end (15) is placed at a distance between a half and two thirds, inclusive, of the chamber length (L) from said inorganic filler injection opening (3) .

7. Mixing apparatus (1) according to any one of the preceding claims, wherein the screw (10) comprises a first conveying portion (5) upstream of the mastication portion (4) and a second conveying portion (5' ) downstream of the mastication portion (4) , wherein each of said first and second conveying portions (5, 5' ) comprises a plurality of positive conveying elements (104) suitable for promoting the advancement of themolten polymer composition without mixing it.

8. Mixing apparatus (1) according to the preceding claim, wherein the polymer material feeding opening (2) is obtained in the casing (11) so that the material is fed at the first conveying portion (5) .

9. Mixing apparatus (1) according to claim 2, wherein the at least one neutral mastication element (103) is interposed between the at least one positive mastication element (101) and the at least one negative mastication element (102) .

10. Mixing apparatus (1) according to any one of the preceding claims, wherein the screw (10) comprises at least twice the number of positive mastication elements (101) with respect to the number of negative mastication elements (102) .

11. Mixing apparatus (1) according to claim 2 or 9, wherein the screw (10) comprises negative (102) and neutral (103) mastication elements in equal number.

12. Mixing apparatus (1) according to claim 7, wherein the second conveying portion (5' ) extends into the mixing chamber (120) over a second conveying length (L2) and the first conveying portion (5) extends into the mixing chamber (120) over a first conveying length (LI) which is at least twice said second conveying length (L2) .

13. Mixing apparatus (1) according to any one of thepreceding claims, comprising a second screw (10' ) rotatably extending into the mixing chamber (120) together with the screw (10) and also comprises a mastication portion (4) comprising at least one positive mastication element (101) and at least one negative mastication element (102) .

14. Mixing apparatus (1) according to the preceding claim, wherein the screw (10) and the second screw (10' ) are identical and rotatable in the mixing chamber (120) , preferably co-rotatably with each other.

15. A filming plant comprising a mixing apparatus (1) according to any one of the preceding claims, and a filming apparatus operatively connected to said mixing apparatus (1) at the outlet opening (110) .

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