Breathable thermoplastic film with reduced shrinkage

Abstract

Methods for stretch-forming a thermoplastic film comprises: stretching the thermoplastic film between a first roller and a second roller, wherein the first the roller rotates at a first peripheral velocity and the second roller rotates at a second peripheral velocity that is higher than the first peripheral velocity; and advancing the thermoplastic film from the second roller to a third roller, wherein the third roller rotates at a third peripheral velocity that is lower than the second peripheral velocity, wherein a ratio of the third peripheral velocity to the second peripheral velocity is no larger than 0.95. The resulting breathable thermoplastic films may have a basis weight of no larger than 15 g/m.sup.2 and a shrinkage of no larger than 5%.

Claims

1. A composition comprising: a breathable thermoplastic film, formed by a process involving stretching in at least a machine direction, the film having a basis weight of no larger than 14 g/m.sup.2; a water vapor transmission rate of no smaller than 2,000 grams H.sub.2O/(24 h m.sup.2); and a residual shrinkage in the machine direction of no larger than 3%, wherein the shrinkage is determined by immersion of the thermoplastic film in a hot fluid bath at a predetermined immersion temperature for a predetermined immersion time, wherein the immersion temperature is no smaller than 60° C., and wherein the immersion time is no less than ten seconds.

2. The composition according to claim 1, the breathable thermoplastic film further having a water vapor transmission rate of no smaller than 3,000 grams H.sub.2O/(24 h m.sup.2).

3. The composition according to claim 1, wherein the breathable thermoplastic film comprises a filler.

4. The composition according to claim 3, the filler concentration being at least 30% by weight.

5. The composition according to claim 3, the filler concentration being at most 70% by weight.

6. The composition according to claim 1, wherein the thermoplastic film comprises polyethylene terepthalate.

7. The composition according to claim 6, wherein the thermoplastic film comprises a filler.

8. The composition according to claim 7, the filler comprising calcium carbonate.

9. A laminated article, comprising: a substrate; and at least one breathable thermoplastic film according to claim 1, wherein the breathable thermoplastic film is connected to the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features and numerous advantages of the techniques and devices according to the present disclosure will be best understood from a detailed discussion of examples with reference to the accompanying Figures, in which:

(2) FIG. 1 is a schematic illustration of an apparatus and method for producing a thermoplastic film according to an example;

(3) FIG. 2 is a schematic illustration of a stretching device for stretch-forming a thermoplastic film according to an example; and

(4) FIG. 3 is a flow diagram illustrating a method for stretch-forming a thermoplastic film according to an example.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(5) FIG. 1 is a schematic illustration of an apparatus 10 for producing breathable thin thermoplastic films, such as may be used as a back sheet of a diaper or other personal hygiene product. However, the breathable thermoplastic films according to the present disclosure have a wide array of applications across various industries. For instance, breathable thermoplastic films according to the present disclosure may likewise be employed for packaging food or consumer goods.

(6) The extrusion device 12 may comprise an extruder (not shown) that melts a thermoplastic polymer granulate to which a filler material, such as calcium carbonate has been added, and supplies the molten and homogenized composite to a foam blowing head (not shown). The foam blowing head may include a ring nozzle (not shown) from which the extruded composite material exits. Cooling air is supplied for inflating, from the composite material, a tubular thermoplastic film, which may be kept in shape by a calibration head. Examples for a blow extrusion device of this type are described in additional detail in US 2011/0006452 A1, or in US 2016/0151950 A1, both of which are incorporated herein by reference. Alternatively, the extrusion device 12 may be a cast extrusion device, as described in U.S. Pat. No. 9,492,332 B2, incorporated herein by reference.

(7) The resulting solid thermoplastic film 18 is passed along the machine direction X from the extrusion device 12 to the stretching device 14, which may be employed for stretch-forming the thermoplastic film 18 along the machine direction X and/or along a cross direction perpendicular to the machine direction X.

(8) The stretched thermoplastic film 18 is advanced along the machine direction from the stretching device 14 to the winder device 16, where the thermoplastic film 18 may be wound up to form a reel of thermoplastic film, which may then be stored or transported for further manufacturing.

(9) In some examples, the apparatus 10 may comprise additional devices or components. For instance, the apparatus 10 may comprise an interdigitated roller device comprising tensioning rollers and/or interdigitating rollers for stretching the film 18 in the machine direction X and/or in the cross direction, resulting in additional thermo-activation and enhanced breathability. For instance, an interdigitated roller device may be located in between the stretching device 14 and the winder device 16.

(10) As described in the Background Art section above, the formed thermoplastic film may exhibit undesirable residual shrinkage, i.e. may shrink when exposed to heat during transport or further manufacturing of the thermoplastic film, such as when printing on the thermoplastic film. These problems are particularly pronounced for thin thermoplastic films, i.e. thermoplastic films having a low basis weight in the range of 15 g/m.sup.2 or less.

(11) The stretching device and stretching method of the present disclosure overcome these problems.

(12) The stretching device 14 may comprise at least a first roller, a second roller downstream of the first roller in the machine direction, and a third roller downstream of the second roller in the machine direction X. In many implementations, the stretching device 14 may comprise additional rollers for transporting and stretch-forming the thermoplastic film 18.

(13) Each of the rollers may be driven to rotate at a predefined peripheral velocity, i.e., a velocity measured along the periphery of the respective roller. This may include independent drive units and velocity control units for each of the rollers.

(14) The first roller may be adapted to advance the thermoplastic film, and may be adapted to be rotated at a first peripheral velocity. The second roller downstream of the first roller may be adapted to receive the thermoplastic film 18 from the first roller, and may be adapted to be rotated at a second peripheral velocity that is higher than the first peripheral velocity. The first roller and the second roller thereby form a stretching gap or a stretching nip that stretches the thermoplastic film 18 along the machine direction X.

(15) In some examples, the stretching device 14 additionally comprises a cross direction stretching unit (not shown) to stretch the thermoplastic film 18 in a cross direction perpendicular to the machine direction X.

(16) The third roller is located downstream of the second roller and is adapted to receive the thermoplastic film 18 from the second roller, wherein the third roller is adapted to be rotated at a third peripheral velocity that is lower than the peripheral velocity. The third roller thereby allows the thermoplastic film 18 to contract or shrink during manufacturing.

(17) The ratio of the third peripheral velocity to the second peripheral velocity may be chosen in the range of 0.95 or smaller.

(18) Each of the rollers of the stretching device may in addition be adapted to be temperature controlled, so that the temperature of the respective roller may be adjusted to heat or cool the thermoplastic film 18 as it is advanced by the respective rollers.

(19) For instance, the second roller and the third roller may each be provided with respective first and second heating devices, such as electrical heaters that are adapted to heat the second roller to a second temperature and to heat the third roller to a third temperature, respectively. A ratio of the third temperature to the second temperature may be chosen in the range of 1.03 or larger. The higher temperature of the third roller facilitates the reordering of the molecular structure of the thermoplastic film 18, and thereby allows the thermoplastic film 18 to contract, which reduces the risk of residual shrinkage in the final product.

(20) An example of a stretching device 14 according to the present disclosure is shown in greater detail in the schematic illustration of FIG. 2.

(21) The stretching device 14 of FIG. 2 comprises a preheating unit 20, a stretching unit 22 (sometimes also referred to as orientation unit) downstream of the preheating unit 20 in the machine direction X, an annealing unit 24 downstream of the stretching unit 22 in the machine direction X, and a cooling unit 26 downstream of the annealing unit 24 in the machine direction X.

(22) As further shown in FIG. 2, each of the preheating unit 20, stretching unit 22, annealing unit 24, and cooling unit 26 may comprise a plurality of rollers for advancing the thermoplastic film 18 through the stretching device 14, wherein each of the rollers of the stretching device 14 may be adjusted to rotate at a predetermined angular velocity and/or may be adapted to exert a predetermined tension force onto the thermoplastic film 18. Moreover, each of the rollers of the stretching device 14 may be temperature controlled to heat or cool the thermoplastic film 18 as it passes along the surface of the rollers through the stretching device 14.

(23) In the example illustrated in FIG. 2, the preheating unit 20 comprises four pre-heating rollers 28a to 28d for pre-heating the extruded thermoplastic film 18 in stages to a pre-heating temperature. In other examples, the pre-heating unit 20 comprises a larger number or a smaller number of pre-heating rollers.

(24) The stretching unit 22 comprises a first roller 30a adapted to advance the thermoplastic film 18 supplied by the pre-heating unit 20, wherein the first roller 30a is adapted to be rotated at a first peripheral velocity.

(25) The stretching unit 22 further comprises a second roller 30b that is located downstream of the first roller 30a and is adapted to receive the thermoplastic film 18 from the first roller 30a. The first roller 30a and the second roller 30b define a stretching nip 32 or stretching gap in between, whose size may be adjusted by moving the first roller 30a and/or the second roller 30b relative to one another in the stretching unit 22.

(26) The second roller 30b may be adapted to be rotated at a second peripheral velocity that may be considerably higher than the first peripheral velocity of the first roller 30a, so to stretch the thermoplastic film 18 in the stretching nip 32.

(27) As explained in the Background section, the stretching in the stretching nip 32 leads to an orientation of the polymeric molecular chains along the machine direction X, and to the formation of micropores at the boundary of the filler particles. The resulting thermoplastic film is thin and has a high breathability.

(28) The ratio of the second peripheral velocity of the second roller 30b and the first peripheral velocity of the first roller 30a is usually referred to as a stretching factor, and may be in the range of 2 to 5, depending on the composition of the thermoplastic film 18 and its intended application. A higher stretching factor may generally lead to thinner thermoplastic films 18, and hence a reduced basis weight.

(29) The annealing unit 24 downstream of the stretching unit 22 comprises a first annealing roller 34a downstream of the second roller 30b along the machine direction X, and a second annealing roller 34b downstream of the first annealing roller 34a along the machine direction X. In some examples, the annealing unit 24 may comprise only one annealing roller, or more than two annealing rollers.

(30) In the context of the present disclosure, the first annealing roller 34a and/or the second annealing roller 34b may be referred to as a third roller, thereby referring to their location downstream of the first roller 30a and the second roller 30b of the stretching unit 22.

(31) The first annealing roller 34a and the second annealing roller 34b may be adapted to be rotated at respective third peripheral velocities that are lower than the second peripheral velocity of the second roller 30b. In some examples, the peripheral velocity of the first roller 30a and the second roller 30b is adjusted to be identical, whereas in other examples the peripheral velocity of the second annealing roller 34b differs from the peripheral velocity of the first annealing roller 34a. In particular, the peripheral velocity of the second annealing roller 34b may be lower or higher than the peripheral velocity of the first annealing roller 34a.

(32) Similar to the pre-heating rollers 28a to 28d, the first annealing roller 34a and the second annealing roller 34b may each be equipped with a heating unit so as to adjust a temperature of the first annealing roller 34a and the second annealing roller 34b.

(33) In some examples, the temperature of the first annealing roller 34a and the temperature of the second annealing roller 34b may both be adjusted to a temperature that is higher than a temperature of the first roller 30a and/or second roller 30b of the stretching unit 22.

(34) The lower peripheral velocity of the first annealing roller 34a and/or the second annealing roller 34b compared to the second velocity of the second roller 30b in combination with the increased temperature allows for an efficient partial relaxation of the molecular orientation of the thermoplastic film after the stretching. As a result, the thermoplastic film 18 is less susceptible to shrinkage after fabrication. In particular, the techniques of the present enclosure allow to achieve a residual shrinkage of 5% or less in the fabricated film, even for very thin films with a basis weight of no larger than 15 g/m.sup.2 and while still maintaining a high breathability.

(35) In an example, the first annealing roller 34a is adjusted to rotate at a third peripheral velocity that is no larger than the peripheral velocity of the second roller 30b, and is heated to a third temperature that is at least 1.03 times higher or at least 5° C. higher than the temperature of the second roller 30b.

(36) The second annealing roller 34b may be heated to approximately the same temperature as the first annealing roller 34a, and may be adjusted to rotate at a lower peripheral velocity than the peripheral velocity of the first annealing roller 34a. This allows for a relaxation of the microstructure of the thermoplastic film in stages. For instance, a ratio of the peripheral velocity of the second annealing roller 34b to the peripheral velocity of the first annealing roller 34a may be chosen to be no larger than 0.95.

(37) The cooling unit 26 downstream of the annealing unit 24 comprises a first cooling roller 36a and a second cooling roller 36b, but may in other examples comprise either a single cooling roller or a larger number of cooling rollers. In the context of the present disclosure, the first cooling roller 36a and/or the second cooling roller 36b may be referred to as a fourth roller, due to their location downstream of the third rollers 34a/34b.

(38) The first cooling roller 36a and/or the second cooling roller 36b may be driven to rotate at respective fourth peripheral velocities. In some examples, the peripheral velocity of the second cooling roller 36b is identical to the peripheral velocity of the first cooling roller 36a. In other examples, the peripheral velocity of the second cooling roller 36b is higher or lower than the peripheral velocity of the first cooling roller 36a.

(39) In some examples, the peripheral velocity of the first cooling roller 36a and/or the second cooling roller 36b is lower than the peripheral velocity of the first annealing roller 34a and/or the second roller 34b.

(40) Each of the first cooling roller 36a and the second cooling roller 36b may be equipped with cooling units, which may comprise a cooling liquid adapted to cool a surface of the cooling rollers 36a, 36b, and thereby cool the thermoplastic film 18.

(41) The cooled thermoplastic film 18 can be passed on to the winder device 16 for winding of the thermoplastic film 18.

(42) FIG. 3 is a flow diagram illustrating a method for stretch-forming a thermoplastic film, such as by means of the stretching device 14 of FIG. 2.

(43) In a first step S10, the thermoplastic film is stretched between a first roller and a second roller, wherein the first roller rotates at a first peripheral velocity and the second roller rotates at a second peripheral velocity that is higher than the first peripheral velocity.

(44) In a second step S12, the thermoplastic film is advanced from the second roller to a third roller, wherein the third roller rotates at a third peripheral velocity that is lower than the second peripheral velocity, wherein a ratio of the third peripheral velocity to the second peripheral velocity is no larger than 0.95.

EXAMPLES

(45) Several examples of a PET film comprising calcium carbonate as a filler were produced by blow extrusion and stretch-formed in a stretching device as described above with reference to FIG. 2.

(46) The resulting thermoplastic film was analyzed in accordance with DIN 55543-4 to determine its residual shrinkage below its crystallite melting point. To this effect, strips of material having a length of 100 mm and a width of 15 mm were cut from the thermoplastic film 18, and were immersed into a hot bath of polyethylene glycol at 80° C. for 20 seconds.

(47) The difference between the original length of the strip and the length after immersion was measured and compared to the original length. The resulting quantity (in percent) is the residual shrinkage.

(48) However, other techniques of determining the residual shrinkage may likewise be employed. For instance, the residual shrinkage may be determined based on an area sample rather than a linear sample. The immersion temperature or immersion time may likewise be varied.

(49) The residual shrinkage may alternatively be determined by heating samples in an oven, such as in a liquid polyethylene glycol bath.

(50) The water vapor transmission rate of the samples was likewise analyzed according to the standard ASTM D-6701-01.

Example 1

(51) A polymeric composite comprising PET and 85% calcium carbonate was blow extruded with a blowhead diameter of 400 mm. The resulting thermoplastic film was pre-heated in stages by a sequence of four pre-heating rollers 28a to 28d from 55° C. to 80° C. The speed of the film at the pre-heating rollers 28a to 28d was between 51.5 m/min at the first roller and 54.8 m/min at the last roller 28d. The first roller 30a and the second roller 30b of the stretching unit were adjusted to the same temperature of 80° C., with a stretching factor (peripheral velocity of the second roller 30b divided by the peripheral velocity of the first roller 30a) of approximately 4. The speed of the film was increased to appr. 224 m/min at the second roller 30b, resulting in a web tension of appr. 430 N. The temperature of the first annealing roller 34a and the second annealing roller 34b were set to 100° C. A velocity ratio of the peripheral velocity of the first annealing roller 34a to the peripheral velocity of the second roller 30b was set to 0.93. The peripheral velocity of the second annealing roller 34b was further reduced, with a velocity ratio of again 0.93 respective to the first annealing roller 34a.

(52) The film was cooled at the first cooling roller 36a and second cooling roller 36b, which were water-cooled to temperatures of appr. 50° C. and 40° C., respectively.

(53) The resulting film had a basis weight of approximately 12.9 g/m.sup.2 and a water vapor transmission rate of at least approximately 7,500 g/(24 h m.sup.2). The shrinkage in machine direction was determined at 3.5% (immersion temperature 80° C./immersion time 20 seconds).

Example 2

(54) A similar polymeric composition having 70% calcium carbonate was blow extruded with a blowhead diameter of 400 mm, and the resulting film was pre-heated in stages by the pre-heating rollers 28a to 28d from appr. 55° C. to a temperature of up to approximately 90° C. The speed of the film at the pre-heating rollers 28a to 28d was between 57.8 m/min at the first roller and 61.3 m/min at the last roller 28d. The temperature of the first roller 30a and the second roller 30b was likewise adjusted to 90° C., and the stretching factor (peripheral velocity of the second roller 30b divided by the peripheral velocity of the first roller 30a) was chosen at approximately 3.5. The speed of the film was increased to appr. 220 m/min at the second roller 30b, resulting in a web tension of appr. 375 N.

(55) The temperature of the first annealing roller 34a and the second annealing roller 34b was set to approximately 100° C. The peripheral velocity of the first annealing roller 34a was set at 0.9 times the peripheral velocity of the second roller 30b, and the peripheral velocity of the second annealing roller 34b was set to 0.9 times the peripheral velocity of the first annealing roller 34a.

(56) The film was cooled at the first cooling roller 36a and second cooling roller 36b, which were water-cooled to temperatures of appr. 50° C. and 40° C., respectively.

(57) The resulting film had a basis weight of 13.8 g/m.sup.2, a water vapor transmission rate of approximate 4000 g/(24 h m.sup.2), and a residual shrinkage in machine direction of approximately 2% (immersion temperature 80° C./immersion time 20 seconds).

Example 3

(58) A similar PET compound having 85% by weight calcium carbonate was blow extruded with a blowhead diameter of 400 mm. The resulting film was pre-heated by pre-heating rollers 28a to 28d from 55° C. to 80° C. The speed of the film at the pre-heating rollers 28a to 28d was between 54.1 m/min at the first roller and 57.4 m/min at the last roller 28d. The temperature of the first roller 30a and the second roller 30b was likewise adjusted to 80° C., and the stretching factor (peripheral velocity of the second roller 30b divided by the peripheral velocity of the first roller 30a) was set to approximately 4. The speed of the film was increased to appr. 234.4 m/min at the second roller 30b, resulting in a web tension of appr. 408 N.

(59) The temperature of the first annealing roller 34a and the second annealing roller 34b was set to 100° C. each. The peripheral velocity of the first annealing roller 34a was set at 0.92 times the peripheral velocity of the second roller 30b, and the peripheral velocity of the second annealing roller 34b was set at 0.93 times the peripheral velocity of the first annealing roller 34a.

(60) The film was cooled at the first cooling roller 36a and second cooling roller 36b, which were water-cooled to temperatures of appr. 50° C. and 40° C., respectively.

(61) The resulting film was measured to have a basis weight of approximately 10.3 g/m.sup.2, a water vapor transmission rate of approximately 4000 g/(24 h m.sup.2), and a residual shrinkage in machine direction of approximately 3.5%.

(62) The description and the Figures merely serve to illustrate the invention and its advantages, but should not be understood to imply any limitation. The scope of the invention is to be determined from the appended claims.