Cavitated polyolefin films and methods of production thereof

Abstract

Provided is a feedstock comprising a polyolefin homopolymer such as polypropylene, polybutylene terephthalate (PBT) and a pigment having a refractive index of at least 1.5 complexed by maleic anhydride functionalized polypropylene (MAH-PP). Further provided are polyolefin films having a cavitated layer comprising the feedstock and methods of making such films.

Claims

1. A polyolefin film comprising a cavitated layer, said cavitated layer comprising a polyolefin homopolymer; polybutylene terephthalate (PBT) as a cavitating agent; and a pigment having a refractive index of at least 1.5 complexed by maleic anhydride functionalized polypropylene (MAH-PP), wherein said complexed pigment is not concentrated in the PBT domain and is dispersed evenly in said polyolefin homopolymer, wherein the pigment is titanium dioxide, further wherein said polyolefin film is biaxially oriented.

2. The polyolefin film according to claim 1, wherein said pigment is selected from the group consisting of: aluminum trihydrate, barium sulfate, calcium carbonate, calcium sulfate dihydrate, kaolin, zinc sulfide, magnesium carbonate, silicon dioxide, talc, and zinc oxide.

3. The polyolefin film according to claim 1, wherein said polyolefin homopolymer comprises polypropylene homopolymer (homo-PP).

4. The polyolefin film according to claim 1, wherein the polyolefin film is selected from the group consisting of: a three layer film, and a five layer film; wherein said cavitated layer is a core layer of the polyolefin film.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Some embodiments of the invention are described herein with reference to the accompanying figures. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments of the invention may be practiced. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the invention. For the sake of clarity, some objects depicted in the figures are not to scale.

(2) In the Figures:

(3) FIG. 1 is a schematic representation of the structure of a five-layered sheet in accordance with an embodiment of the present invention; and

(4) FIGS. 2A and 2B are electromicrographs showing surface topography (cross-section) of the core layer of two different five-layered cavitated polypropylene films of FIG. 1 by SEM at 6.25K magnification: a film according to the prior art (2A) and a film in accordance with an embodiment of the present invention (2B).

DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

(5) Some embodiments of the invention relate to cavitated films of a polyolefin, such as polypropylene, comprising titanium dioxide, methods of making such films and feedstock for use in the preparation of such films.

(6) It is known in the art to prepare cavitated polyolefin films, such as cavitated polypropylene films, by including PBT as a cavitating agent in the feedstock from which the film is prepared. In the case of a multilayer film, the cavitating agent is preferably present in the core layer. The PBT disperses inside the continuous polyolefin making up the core layer, in the form of small spherical particles, typically in the order of 0.5 m to 4.0 m microns.

(7) It is also known in the art to include solid white pigments, such as TiO.sub.2 particles as a coloring agent in the feedstock.

(8) It has been found that when white pigments such as TiO.sub.2 are present in the same feedstock as the cavitating agent such as PBT, the pigment is concentrated in the PBT domains (see FIG. 2A). This results in processing problems such as accelerated aging of the extrusion filters; accumulation of impurities in the die and consequent die build-up; and inefficient use of the white pigment, as well as lower optical properties of the film, all of which have a detrimental effect on the film quality and the plant productivity.

(9) The present inventors have surprisingly found that if a pigment having a refractive index of at least 1.5, such as TiO.sub.2, is complexed with maleic anhydride functionalized polypropylene (MAH-PP), instead of the pigment alone (in the form of a powder or dispersed within a polymer carrier as a masterbatch) is added to a feedstock comprising polyolefin and PBT, the pigment in the resulting film is not concentrated in the PBT domains but dispersed evenly in the polyolefin (see FIG. 2B). The resulting film has superior optical properties and is easier to process.

(10) Maleic anhydride is known to be an excellent ligand for metal oxides, it can be easily adsorbed onto the pigment surface by electron donation. The OH groups on the pigment surface are able to react with the MA functional groups to build up chemical bonds. Breaking up of the anhydride structure of the polypropylene-graft-maleic anhydride (PP-g-MA) leads to carboxyl groups which are able to build up complex structures via acid-base interactions.

(11) Without wishing to be bound by theory, the inventors hypothesize that while pigments such as TiO.sub.2 are attracted to PBT, MAH-PP is incompatible with PBT, such that a pigment MAH-PP complex is not attracted to PBT.

(12) Before explaining at least one embodiment in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth herein. The invention is capable of other embodiments or of being practiced or carried out in various ways. The phraseology and terminology employed herein are for descriptive purpose and should not be regarded as limiting.

(13) Many modifications and variations are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention can be practiced in ways other than those specifically described.

(14) The specific embodiments listed below exemplify aspects of the teachings herein and are not to be construed as limiting.

(15) Throughout this application, various publications, including United States Patents, are referenced by author and year and patents by number. The disclosures of these publications and patents and patent applications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.

(16) Citation of any document herein is not intended as an admission that such document is pertinent prior art or considered material to the patentability of any claim of the present disclosure. Any statement as to content or a date of any document is based on the information available to applicant at the time of filing and does not constitute an admission as to the correctness of such a statement.

EXAMPLES

(17) In the experimental section below, all percentages are weight percentages (w/w).

(18) Materials and Methods

(19) Homo-PP was obtained from TOTAL Polymers (PPH 4030S05).

(20) A composition comprising 60% PBT in homo-PP was obtained from CONSTAB Polyolefin Additives GmbH (CONCAVITY 600 PP).

(21) A composition comprising 70% TiO.sub.2 in homo-PP was obtained from CONSTAB Polyolefin Additives GmbH (CONSTAB CC 18170 PP).

(22) A composition comprising 60% TiO.sub.2 in homo-PP was obtained from CONSTAB Polyolefin Additives GmbH (CONSTAB CC 18160 PP).

(23) MAH-PP was obtained from Arkema (Orevac CA100).

(24) Anti-blocking agent was obtained from CONSTAB Polyolefin Additives GmbH (CONSTAB AB 06064 PPR).

(25) Anti-static agent was obtained from CONSTAB Polyolefin Additives GmbH (CONSTAB AT 04082 PP).

(26) All materials were provided in granular form.

(27) Preparation of BOPP Films

(28) a) Cast Film Coextrusion of Films

(29) Some embodiments of the bioriented polypropylene (BOPP) films according to the teachings herein were produced by coextruding from 3 up to 9 layers. Typically the flat web consisting of 3-9 layers was extruded through a plane die at temperature T=230 C.-250 C.) and immediately cast on a cooling drum in which cooling water (T=30 C.-35 C.) was circulated.

(30) The screw speed of each individual extruder was set to provide an extruded layer having the desired thickness in the usual way. For multilayer films, a die having three, five, seven or nine ports, each fed by a dedicated extruder was used.

(31) The extruded film was then further cooled in a water bath at T=30 C.-40 C.

(32) b) Orientation of Extruded Films

(33) The above extruded films are biaxially oriented i.e. oriented in both the machine direction (in the direction of the extrusion) and transverse direction (perpendicular to the direction of extrusion).

(34) Biaxial orientation can be performed either simultaneous (i.e. stretching in both directions is carried out simultaneously) or sequential (i.e. stretching is carried out first in machine direction and then in transverse direction). For production of cavitated films, sequential orientation is preferred. Preferred orientation ratios are commonly from between about three to about six times the initial speed in the machine direction and between about four to about ten times the extruded width in the transverse direction.

(35) For biaxial orientation, the following steps were carried out:

(36) a) Machine Direction Orientation (MDO)

(37) Optional infra-red preheating of the extruded film; Preheating of the extruded film by means of a series of heated rollers (T=90 C.-130 C.); and Stretching of the heated, extruded film on the heated rollers (T=90 C.-130 C.), with a stretch ratio of 3:1-6.5:1, wherein the term stretch ratio refers to the ratio between the speed of the last heated roller to the speed of the first heated roller during stretching
b) Transverse Direction Orientation (TDO) Preheating of the film (T=165 C.-175 C.); Stretching of the film (T=150 C.-165 C.); and Annealing (T=140 C.-165 C.).

(38) TDO stretching is achieved using TD-Orienter, which essentially consists of two diverging rails running in an oven consisting of a series of connected heating and stretching zones.

(39) The TDO stretch ratio at a given location in the oven is the ratio between the width of the film at that location and the width of the film at the inlet of the oven. The TDO stretch ratio ranges from 4:1 to 9:1.

(40) Annealing is the last phase of the TDO stretching process, during which the rails slightly converge, while the temperature is optionally increased.

(41) Surface Treatment

(42) After stretching, one or both of the outer surfaces of the film is optionally treated in order to increase its surface energy to render the film receptive to metallization, coatings, printing inks and/or lamination. The desired value of surface tension is at least 36 dyne/cm.

(43) Commonly used treatments include: corona discharge, plasma, and flame (including polarized flame).

(44) Winding

(45) The bioriented and treated film was then optionally wound for final use on mother-rolls or jumbo rolls having internal diameter between 400-700 mm and external diameter between 900-1200 mm.

(46) Metallization

(47) Outer surfaces of the film (i.e. the side facing away from the core) may optionally be metallized by deposition of a very thin layer of a metal, such as aluminum, copper, silver, chromium or mixtures thereof on an outer surface. Metallization may be carried out using conventional methods, such as vacuum metallization.

(48) Additional Coating Layers

(49) Additional coating layers may optionally be applied subsequent to metallization, includes priming layers, (e.g. to increase printability), antifog layers or layers to improve sealability.

(50) A primer coating may be applied to any surface of the multilayered films by applying a continuous coating of a primer material. Such primer materials are well known in the art and include, for example, epoxy and poly(ethylene imine) (PEI) materials.

Example 1: Five-Layered Bioriented Cavitated Film Comprising Titanium Dioxide

(51) Referring now to FIG. 1, there is shown a five-layered sheet 10, comprising a core layer 12; a first and second skin layer 16a and 16b, respectively, positioned one on either side of core layer 12; and a first and second tie (intermediate layer) 14a and 14b, respectively, wherein first tie layer 14a is positioned between core layer 12 and first skin layer 16a, and wherein second tie layer 14b is positioned between core layer 12 and second skin layer 16b.

(52) The layers were co-extruded from a 5-twin screw extrusion system. Core layer 12 were extruded from main extruder (I); skin layers 16a and 16b were extruded from a first and a second satellite extruder, (II and III) respectively, positioned on opposite sides of the main extruder; and tie layers 14a and 14b were extruded from a third and a fourth satellite extruder, (IV and IV) respectively, wherein the third satellite extruder was positioned between the main extruder and the first satellite extruder, and the fourth satellite extruder was positioned between the main extruder and the second satellite extruder. The first satellite extruder was at the casting side, while the second satellite extruder was in proximity to a water bath.

(53) The resultant sheet was biaxially oriented, using methods as known in the art.

Reference Sample

(54) A reference core feedstock and a reference tie layer (intermediate layer) feedstock were prepared, each comprising the following mixture: i) 6% (w/w of feedstock) of a composition comprising 60% TiO.sub.2 in homo-P; ii) 10% (w/w of feedstock) of a composition comprising 60% PBT in homo-PP; iii) 2% (w/w of feedstock) of anti-static composition; and iv) 82% (w/w of feedstock) homo-PP.

(55) Each of the core and tie layer comprised 6% w/w PBT and 3.6% w/w TiO.sub.2.

(56) A skin layer feedstock was prepared, comprising the following mixture: i) 99% (w/w of skin layer feedstock) of a terpolymer of polypropylene; and ii) 1% (w/w of skin layer feedstock) of anti-blocking agent.

(57) The components for each feedstock were either blended in the solid phase prior to introduction into an extruder or introduced substantially simultaneously into an extruder.

(58) The feedstocks were introduced into the 5 twin screw extruder system as follows: Extruder IV: First skin layer Extruder II: Frist tie layer Extruder I: Core layer Extruder III: Second tie layer Extruder V: Second skin layer.

(59) Extrusion conditions were as specified in Table 1 below.

(60) TABLE-US-00001 TABLE 1 Extruder Main Sat 1 Sat 2 Sat 3 Sat 4 Layer core skin Skin tie tie Output 187.7 6.8 6.5 14.6 13.8 (kg/h) Layer 32.9 1.0 1.0 2.5 2.5 thickness (m) Barrel temp 25 240 240 240 240 ( C.)

(61) The casting conditions were as follows: Cast roll speed: 14.3-16.0 m/min Cast roll temperature 35 C./water bath 30 C.

(62) The extruded films were then biaxially oriented according to the following conditions:

(63) Machine Direction Stretching (MDO)

(64) Stretch ratio 5.26:1

(65) Roller temperature: preheat 125 C. /stretch 120 C./annealing 125 C.

(66) Transverse Direction Stretching (TDO)

(67) TD stretching ratio (calculated according to the local width of the rails)=7.89 max (maximum/inlet) 7.05 outlet (outlet/inlet) TDO oven temperature: preheat 176-174-171-165 C./stretch 158-158 C./annealing 165 C.

(68) The reference film formed had a density of 0.65 g/cm.sup.3

(69) Optical Properties

(70) Transmission and optical density were measured according to ASTM D1003.

(71) Gloss was measured on the chill roll side and the air side according to ASTM D2457.

Results

(72) A photomicrograph of a cross-section of the core layer of the reference film is presented in FIG. 2A, in which the particles of TiO.sub.2 are seen to reside preferentially in the PBT domains. As already discussed, this preferential agglomeration of TiO.sub.2 in the PBT domains may cause processing problems (such as shorter life of the filters, die build-up etc.) during production.

(73) Film #1

(74) The skin layer and tie layer feedstocks were prepared as described above for the reference sample.

(75) A composition comprising TiO.sub.2 complexed by maleic anhydride functionalized polypropylene (MAH-PP) was prepared by combining 90% of a composition comprising 70% TiO.sub.2 in homo-PP with 10% MAH-PP.

(76) A core layer feedstock in accordance with the principles of the present invention was prepared comprising the following mixture: i) 6.0% (w/w of feedstock) of the composition comprising TiO.sub.2 complexed by MAH-PP; ii) 10% (w/w of feedstock) of the composition comprising 60% PBT in homo-PP; iii) 2% (w/w of feedstock) of antistatic composition; and iv) 82% (w/w of total feedstock) homo-PP.

(77) The components were blended in the solid phase prior to introduction into the extruder.

(78) The core layer feedstock comprised 6% w/w PBT and 3.7.8% w/w TiO.sub.2 and 0.6% MAH-PP.

(79) The film #1 had a density of 0.70 g/cm.sup.3

(80) The film was prepared and optical properties measured as described above for the reference sample.

Results

(81) A photomicrograph of a cross-section of the core layer of film #1 is presented in FIG. 2B. It can be seen that, contrary to the prior art reference film of FIG. 2A, TiO.sub.2 particles do not accumulate in the PBT domains but instead exist primarily as an individual dispersed phase. The increased uniformity of distribution of TiO.sub.2 results in better optical properties, such as higher gloss of the film as compared to the reference sample.

(82) The optical properties of film #1 as compared to that of the reference film are shown in 30 Table 2. As seen in Table 2, the gloss of test film #1, prepared in accordance with the principles of the present invention, is significantly higher than those of the reference sample.

(83) Optical properties are shown in Table 2 below.

(84) TABLE-US-00002 TABLE 2 Gloss Thickness Optical Air side Chill roll side (m) Transmission density 20 60 85 45 20 60 85 45 Reference 39 19.3 0.714 11.5 46.6 82.9 52.4 8.4 40.8 81 45.3 Test 19.2 0.717 13.4 66.2 88.3 62.3 8.6 47.3 84.4 47.8

Example 2: Three-Layered Film Comprising Titanium Dioxide

(85) A three-layered film is prepared comprising a core layer as described in film #1, and further comprising two skin layers, each comprising homo-PP or a polypropylene polymer.

(86) The three-layered film is prepared by cast film coextrusion of: homo-PP or a polypropylene polymer (extruder II); the core layer feedstock as for Film #1 (extruder I) homo-PP or a polypropylene polymer (extruder III).

(87) Each skin layer may (extruder II and III) optionally comprise at least one additive selected from the group consisting of a matte compound, an anti-blocking agent, a slip agent, an anti-fog agent, an antistatic agent, a blend of polyolefin homopolymers and or copolymers with high-density polyethylene (HDPE) and/or medium-density polyethylene (MDPE), or combinations thereof. The three-layered film produced has a core layer of thickness in the range of from about 29 to about 63 m and each skin layer has a thickness in the range of from about 1 to about 2 m.

Example 3: Five-Layered Film Comprising Calcium Carbonate

(88) A five-layered film is prepared as described in Example 1, except that titanium dioxide is replaced by the same amount of calcium carbonate.

Example 4: Three-Layered Film Comprising Barium Sulfate

(89) A three-layered film is prepared as described in Example 2, except that titanium dioxide is replaced by the same amount of barium sulfate.

Example 5: Five-Layered Film Comprising Zinc Oxide

(90) A five-layered film is prepared as described in Example 1, except that titanium dioxide is replaced by the same amount of zinc oxide.

Example 6: Three-Layered Film Comprising Barium Sulfate

(91) A three-layered film is prepared as described in Example 2, except that titanium dioxide is replaced by the same amount of zinc sulfide.