CA1224616A

CA1224616A - Oriented polyethylene film and method of manufacture

Info

Publication number: CA1224616A
Application number: CA000450253A
Authority: CA
Inventors: Yasushi Itaba; Keichiro Saito; Masao Yoshizawa; Jyoichi Tabuchi
Original assignee: Toa Nenryo Kogyyo KK
Current assignee: Tonen General Sekiyu KK
Priority date: 1983-03-23
Filing date: 1984-03-22
Publication date: 1987-07-28
Also published as: EP0120672A3; EP0120672B1; US4590020A; US4705714A; DE3476980D1; EP0120672A2

Abstract

ABSTRACT OF THE DISCLOSURE

An oriented film is prepared by crosslinking a sheet of HDPE
to provide maximum crosslinking at the surfaces of the sheet and minimum orientation at its middle layer portion, and thereafter heating and stretching the film to obtain an oriented HDPE of high clarity film of improved moisture proof.

Description

~ 6 ~i 1 ~ALI~ROUNo OF THE I-NENTION

2 The present invention relates to an oriented polyethylene

3 film and method for producing the film. In one aspect, the

4 invention relates to high density polyethylene processed into a film by a method which imparts superior properties in 6 moistureproofness and clarity to the film 7 Heretofore, in spite of their many outstanding properties, 8 films made of medium- and low-pressure polyethylene have been 9 used very little in the application areas where a display effect is required, mainly because they are poor in clarity.
11 There have been proposed several kinds of polyethylene film 12 improved in clarity, which are produced by orienting a 13 radiation-cured polyethylene film. Such film is good in 14 clarity but is generally not satisfactory in moistureproofness. For example, for moistureproof packaging, 16 biaxially oriented polypropylene film ~referred to as OPP
17 hereinafter) is in general use at the present time. However, 18 OPP is not necessarily satisfactory in the packaging area where 19 high moistureproofness is required. Such requirements are met by increas1ng the thickness of OPP film or coating OPP film 21 with vinylidene chlor;de. This leads to an increase in cost.

23 It has been discovered that by processing the polyethylene 24 film to control the degree of cross linking across the thickness of the film, an improved film is obtained. The film 26 exhibits improved moistureproof properties without sacrificing 27 clarity. The invention resides in the film and the method of 28 manufacture.

.., ~22~L6~6 1 The method of the present invention broadly comprises (a) 2 crosslinking d layer (i.e. film9 sheet or tubular molding) of 3 polyethylene resin in such a manner that the degree of 4 crosslinking decreases across the thickness from both outer surfaces t~ward the middle, and (b) subsequently heating and 6 stretching the crosslinked layer at conditions to orient the 7 molecules in the layer producing an oriented film. The 8 stretching preferably is at a temperature lower than the 9 melting point of the polymer and in d direction(s) to produce 1~ uniaxial or bia~ial orientation. The amount of stretch in each 11 direction may vary within wide ranges but preferably is at 12 least 300~ in each direction. The oriented film thus produced 13 has a water vapor transmission ra~e (Y) lower than a numerical 14 value obtained from the following formula Y = 17 x-l/2 16 (where Y represents the water vapor transmission rate 17 (9/m2/24 hours) and X represents the thickness ( ~ ) of the 18 oriented film), and preferably has a haze value lower than 5%.
19 The polyethylene resin useful in the present invention is crystalline polyethylene produced by the medium- or 21 low-pressure process. It has a density higher than 0.935 g/cc, 22 preferably higher than 0.950 g/cc, and a melt flow index 23 (referred to as MI hereinafter) higher than 0.05 g/lO minutes, 24 preferably 0.5 to 20 g/lO minutes. It also includes a mixture of the above-defined polyethylene and a small quantity of 26 low-density polyethylene (i.e. LDPE or LLDPE) and/or other 27 polyolefins. The polyethylene resin may be incorporated with a 28 commonly used antioxidant, UY absorber, slip agent, 29 antiblocking agent, antistatic agent9 pigment, dye~ etc.

2 T~e preferred polyethylene is high density polyethylene 3 (HDPE) having a density greater than 0.940 g/cc and being 4 substantially free of long chain branching.
In ~he production of the orien~ed film of this invention~
6 the polyethylene resin is fed to an ordinary extruder and 7 formed into a flat or tubular stock sheet by melt extrusion, 8 followed by cooling and solidification. The stock sheet may be 9 obtained in the form of a flat sheet by extrusion from a conventional T-die or in the form of a tubular sheet by 11 extrusion from an annular die. The flat stock sheet may also 12 be obtained from a tubular stock sheet by cutting one edge or 13 both edges of the lay-flat tube. The th;ckness of the stock 14 sheet should be such that the sheet can be crosslinked so that the degree of crosslinking decreases across its thickness from 16 both surfaces toward the middle. It is determined according to 17 the draw ratio and the film thickness after stretching. It is 18 usually 2l0 to 2000 ~ , preferably 400 to lO00 ~ . The 19 thickness in this range is adequate to handle the stock sheet with ease and to accomplish the crosslinking as mentioned above.
21 According to this invention, the flat or tubular stock 22 shee~ of polyethylene resin should be crosslinked in such a 23 manner that the degree of crosslinking decreases inwardly fro~
24 each surface of the sheet reaching a minimum near the middle layer portion of the sheet. The degree of crosslinking is 26 expressed in terms of gel fraction. The object of this 27 invention is achieved when the degree of crosslinking in the 28 stock sheet is such that the gel fraction is 0 to 5% ;n the 29 middle layer portion where the degree of crosslinking is lowest and the gel fraction is 5X and up, preferably 20 to 70%, in the 31 opposite surface layer portions where the degree of 32 crosslinking is highest. The outer and middle layer portions 1 may be provided by separate layers wherein the middle layer 2 portion is not crosslinked ~or merely lightly crossl;nked) and 3 the outer layers are crosslinked. In the case where the middle 4 layer portion, in which the degree of crosslinking is lowest, (preferably a gel fraction of 0%), the thickness of each outer 6 layer port;on should be 0.1 to 10 times that of the middle 7 layer portion, and preferably 0.2 to 8 times the thickness of 8 the middle layer portion, and most pre~erably between 1:.5:1 to 9 1:6.5:1. It is particularly preferable that the outer layer portions have the same degree of crssslinking.
11 If crosslinking is performed, in such a manner that the gel 12 fraction in the midddle layer portion, in which the degree of 13 crosslinking should be lowest, is higher than 5X, the stock 14 sheet can be stretched uniformly and improved in clarity, but the film having improved moistureproofness, which is an object 16 of this invention, is not obtained. On the other hand, if 17 crosslinking is performed in such a manner that the gel 18 fraction in the outer layers is lower ~han 20%, the stock sheet 19 cannot be stretched uniformly and the resulting film is not improved in clarity and moistureproofness. If crosslinking is 21 performed in such a manner that the gel fraction in the outer 22 layers exceeds 70%, the stock sheet tends to break during 23 stretching and cannot be stretched smoothly. If crosslinking 24 is performed in such a manner that the stock sheet is crosslinked uniformly across the thickness, the stock sheet can 26 be stretched uniformly with improved clarity, but 27 moistureproofness is not improved. The preferred maximum 28 crosslinking produces gel fraction in the outer layer between 29 about 40 and 70%. If crosslinking is performed in such a manner that only one of the outer layers of the stack sheet is 31 crosslinked, the stock sheet tends to break during stretching.
32 If crosslinking is performed in such a manner that the degree 33 of crosslinking decreases unidirectionally across the thickness 34 from one surface to the other, the resulting film is not satisfactorily improved in clarity and moistureproofness.

1 The gel fraction is the quantity in percent of the fraction 2 that remains undissolved when a specimen is extracted in 3 boiling p-xylene.
4 The above-mentioned crosslinking can be accomplished by irradiating both surfaces of a stock sheet with electron rays 6 or by multilayer coextrusion in which the polyethylene for 7 individual layers contains a different amount of crosslinking 8 agent.
9 The dosage of irradiation with electron rays varies I0 depending on the thickness of the s~ock sheet and the type, 11 molecular weight, and molecular-weight distribution of 12 polyethylene resin; but it is usually 5 tO 50 Mrad, and 13 preferably 15 tO 30 Mrad. Both sides of the stock sheet may be 14 irradiated at one time or one after the other, or repeatedly several times. Both sides should be irradiated with the same 16 dosage. The penetrating power of electron rays should be 17 properly adjusted according to the thickness of stock sheet by 18 changing the applied voltage or by using a shield.
19 The dosage of electron rays can be adjusted in the following manner. Assume that a stock sheet, which is, say, 21 500-micron ~hick, is to be irradiated. Prepare a sample by 22 tightly laying 25 pieces of 20-micron thick film one on top of 23 another and irradiate the samp1e from both sides thereof with 24 the same dosage of electron rays. Take apart the crosslinked sample and measure the degree of crosslinking in the individual 26 films~ The result of measurements tells the degree of 27 crosslinking which varies across the thickness of the sample.
?8 The thus obtained data reveals the relationship between the 29 thickness of the stock sheet and the degree of crosslinking or the dosage or irradiation.
31 The irradiation with electron rays should preferably be 32 carried out in the atmosphere of nitrogen, argon, helium, or 33 other inert gas. Irradiation in the air is possible; but it 34 results in films which are not satisfactorily improved in clarity.

~%~

1 The crosslinking by multilayer coextrusion is accomplished 2 in such a manner that a polyethylene resin containing a 3 crosslinking agent likP an organie peroxide is fed to an 4 extruder which forms bo~h outer layers in the case of flat stock sheet or the inner and outer layers in the case of 6 tubular stock sheet, and another polyethylene resin containing 7 no organic peroxide or containing an organic peroxide just 8 enough to achieve the minimum degree of crosslinking is fed to 9 another extruder which forms the intermediate layer~ and the polyethylene resins are coextruded and crssslinked at a Il temperature higher than their melting point.
12 The crosslinked stock is then stretched uniaxially or 13 biaxially at a predetermined draw rati~ by the conventional 14 rolling~ tentering, or tubular method. Biaxial orientation may be accomplished by either simultaneous stretching or sequential 16 stretching.
17 The stretching temperature should be lower than the melting 18 point of polyethylene resin, and preferably at a temperature 19 higher than the softening point and lower than the melting point of polyethylene resin. Specifically, it is 70 to l35 21 degrees C, and preferably lO0 to 130 degrees C. Uniform and 2Z stable stretching is impossible at a temperature lower than the 23 softening point; and stretching at a temperature higher than 24 the melting point results in an oriented film with insufficient moistureproofness~ although uniform stretching is possible.
26 The draw ratio should be greater than 3 times (300%~, and 27 preferably greater than 4 times (400%)7 in one direction or in 28 both longitudinal and lateral directions. When the draw ratio 29 is lower than 3 times, uniform stretching is impossible and the resulting film is not suf~iciently improved in 31 moistureproofness and clarity. The film should preferably have 32 a thickness between about lO and about 50 microns.

1 The oriented film thus obtained has heat-shrinkable 2 properties. If the oriented film is to be used as the 3 substrate of composite film for packaging, it should be 4 subjected to heat setting at a temperature, e.g., 110 to 140 degrees C, lower than the melting point of the oriented film, 6 so that the heat-shrinkable in the lateral direction is reduced 7 to 1.5% or less, preferably 1.0% or less~
8 The oriented polyethylene film produced according to the 9 method of this invention exhibits improved moistureproof properties9 and preferably has a water vapor transmission rate 11 (Y) lower than a numerical value obtained from the formula Y =
12 17 X 1/2, Moreover, it preferably has a haze value lower 13 than 5.0%, and it has such a sectional structure that the 14 degree of crosslinking decreases inwardly across the thickness. It ls one w~lich has been stretched, preferably more 16 than three times, in at least one direction. The specific 17 value of the water vapor transmission rate is lower than 301 18 g/m /24 hours for a 30-micron thick film and lower ~han 5.4 19 g/m2/24 hours for a 10-micron thick film. Thus the method of this invention provides a highly moistureproof film which 21 cannot be obtained by the conventional methods. This high 22 moistureproofness provides an economical advantage, because it 23 is not necessary to thicken the film to obtain the desired 24 moistureproofness. Moreover, the thin film is suitable for packaging and good in clarity. The oriented film of this 26 invention has well-balanced strength and stiffness and a proper 27 degree of heat shrinkage. After heat-setting, it shrinks only 28 a little on heating and cuts like cellophane. These properties 29 make the oriented film versati1e. Particularly~ it will be suitably used as the substrate film for packaging such items as 31 medicines and laver which require a high degree of 32 moistureproofness.

1 The invention is now described in more detail with 2 reference to the following examples. The test methods employed 3 to evaluate the film performance in the examples are given 4 below.
(1) Water vapor transmission rate:
6 In accordance with JIS Z-0208? at 40 degrees C and 90%
7 RH~
8 (2) Ha~e: In accordance with ~IS K-6714.
9 (3~ Tensile strength: In accordance with ASTM D88Z.
(4) Heat shrinka~e: Measured after a specimen is placed 11 in a constant temperature bath (air) at 120 degrees C
12 or 140 degrees C for 10 minutes.
13 (5) Gel fraction: In accordance with ASTM D2765, Method A.

14 Example 1 High-density polyethylene (density: 0.960 g/cc, MI: 5 9/lO
16 minutes, melting point: 135 degrees C) was formed into a 17 500-micron thick stock sheet by using a T-die extruder. Both 18 sides of the stock sheet were irradiated with electron rays 19 (165 kV - 8 mA) at a dosage of 20 Mrad in the nitrogen gas atmosphere by using ESI's apparatus.
21 The gradient of the degree of crosslinking across the 22 thickness of the shPet was estimated in the following manner.
23 Twenty-five pieces of 20-micron thick films were laid on top of 24 another to make a 500-micron thick test specimen. The specimen was irradiated under the same conditions as mentioned above.
26 After irradiation, the specimen was taken apart and the degree 27 of crosslinking of individual films was measured~ The maximum 28 gel fraction was 50% in the outer layers, and the minimum gel 29 fraction was OX in the inner layer. The irradiated stock sheet was found to be composed of crosslinked outer 31 layer/uncrosslinked inner layer/crosslinked outer layer, the 32 ratio of their thicknesses being 1:0.5:1.

~ 6 ~

1 After heating to l27 degrees C, the irradiated stock sheet 2 was stretched 4 times in the longitudinal direction and 4 times 3 in the lateral direction sequentially by using a tenter-type 4 biaxial stretching machine. Thus there was obtained a 30-micron thick oriented film having the properties as shown in 6 Table l.

7 Exam~les 2 to 7 8 Polyethylene resins of different density and MI were formed 9 into stock sheets, which were then crosslinked and stretched in the same way as in Example l under the conditions shown in 11 Table l. There were obtained different kinds of oriented films 12 having the properties shown in Table l.

~2~L6 S-.. ~ o r ~
L- I V K ~ K K ~C K K
O ~ ~1 ~ O
O ~ I t~
c o r o~ o~ r o 1~ 4 C _I 1~ ~ ~ ~ ~ ~ ~C v ~f ~ r r 1~ o --E 5 ~ ~
Ql E I O O O O O O O ~ ~I V
O K_ ~7 ,t, ul r~ Ul U'~ It) 1 a e'0 :~:
c E ~¦ O o O ~ L~
v 0 ~--I _,c o Q ~ ~ ~ r~
E I ~ o v '7-' -~1cooooo :~ oO o o o o o ol v~ ~ O c 0 ~ ~ _i E ~ c O K X K X # X )t .111 0 L- ~ 1C Gl C ~
.0 1~ ~ C ~ ..., .. .~ .. ~ ~ V ~C ~ ~ 0 ~ Ul ~ O
E-~ uo~ _~ ~0 ,. _~ u~ ._l _ O ~ O ~ O _l ~D O O_~I V Cl C tl ~ C ~ N ~l ~ o _1 O
t~ _ C
.X V v O U~ o o r v h ~1 u~ r ~ v O
1~ ~0'0 g C ~
0 K ¦ 0- ~ V 0 ~ ~ N _I ~ 0 ~ 1-7 ~, v I u~ o o ou~ o u~ :~ E--C ~ 00 c 3 lo ,~ o u~ o Q- oo Q o o ~r~ o .. 0 0 0~ ~
O t:l ~P o o Q' O O O O 1~5 E ~ o r .

~2~6~g~

1 ~ote ~o Table l~

2 * Longitudinal direction X lateral direction 3 ** In lateral direction, at 140 degrees C
4 **~ In lateral direction, at l20 degrees C
Heat-setting was performed at 132 degrees C, with l~
6 relaxation in Examples l, 4, and 5; with 5% relaxation 7 in Example 7.

8 Comparative E~e_e 1 9 High-density polyethylene (density: 0.957 g/cc, MI: 0.8 g/lO minutes, melting point: 134 degrees C) was formed into a 11 500-micron thick stock sheet by using a T-die extruder. The 12 stock sheet was irradiated with electron rays at a high applied 13 voltage that provides sufficient penetrating po~er. Thus there 14 was obtained a stock sheet which is uniformly crosslinked across the thickness thereof and has a gel fraction of 55%.
16 The crosslinked stock sheet was stretched at 135 degrees C 4 17 ~imes in the longitudinal direction and then 4 times in the 18 lateral direction sequentially. Thus there was obtained a 19 30-micron thick oriented film having a water vapor transmission rate of 4.0 g/m2t24 hours and a haze value o~ l.S%. The 21 improvement of moistureproofness was not sufficient.

22 Comparative Example 2 23 The stock sheet as obtained in Example l was irradiated in 24 one direction across the thickness. The gel fraction was 50%
in the outer layer facing to the radiation source, and the gel 26 ~raction was 0% in the opposite outer layer. The ratio of the 27 thicknesses oF the crosslinked layer to the uncrosslinked layer 28 was l:l.5. The stock sheet was stretched under the same 29 conditions as in Example l, but no oriented film could be obtained due to breakage.

~ L~i~L6 1 Example 8 2 An oriented film was prepared in the same manner as in 3 Example l, except tha~ the dosage of irradiation was reduced so 4 that the gel fraction in the irradiated outer layer was 5%0 The resulting oriented film had a water vapor transmission 6 rate of 2.4 g/m2l24 hours and a haze value of 3~%. In other 7 words, the film was very poor in clarity.

8 Example 9 g An oriented film was prepared in the same manner as in Example l, except that the irradiation was performed in the air.
11 The resulting oriented film had a water vapor transmission 12 rate of 2.2 g/m /24 hours and a haze value of 5.5%. In other 13 ~ words~ the film was not sufficiently improved in clarity.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for producing high clarity oriented polyethylene film which comprises (a) crosslinking opposite surfaces of a sheet of polyethylene selected from HDPE and blends of HDPE and a small quantity of at least one other polyolefin, said polyethylene having a density greater than 0.940 g/cm3, in such a manner that the degree of crosslinking decreases from said surfaces inwardly to produce a sheet wherein opposite outer layer portions of the sheet each has gel fractions of between 20 to 70% and the middle layer portion of the sheet has gel fractions between 0 to 5%, said outer and middle layer portions having substantially the same polyethylene composition; and (b) heating and stretching the crosslinked sheet under conditions to orient the polyethylene molecules thereby producing an oriented film having a thickness between 10 and 50 microns.

2. The method as defined in claim 1 wherein the polyethylene is HDPE.

3. The method of claim 1 wherein the sheet is a monolayer and the crosslinking step comprises exposing opposite sides of the sheet to irradiation in an inert environment under conditions which result in the inward decreasing crosslinking, each of said outer layers portions achieving highest degree of crosslinking and said middle portion achieving the lowest degree of crosslinking.

4. The method as defined in claim 3 wherein the outer layer portions and the middle layer portion are contiguous and the thickness ratio of the outer layer portion:middle layer portion:outer layer portion thereof ranges between 1:.1:1 to 1:10:1.

5. The method as defined in claim 4 wherein said ratio is between 2:1:2 and 8:1:80

6. The method as defined in claim 4 wherein the sheet has a thickness of from 210 to 2000 microns and the stretching at least in one direction is at least 300 percent.

7. A method of producing an oriented film which comprises:
(a) crosslinking by irradiation in an inert environment a monolayer sheet of a polyethylene selected from HDPE or blends of HDPE and a small quantity of at least one other polyolefin, said polyethylene having a density higher than about 0.940 g/cc and a thickness of between 210 and 2000 microns to produce opposite outer layer portions each having a gel fraction between 20% to 70% and a middle layer portion having a gel fraction between 0 and 5%.
(b) stretching the crosslinked sheet in at least one direction at least 300% its original length to produce a clear oriented film of improved moistureproofness.

8. The method as defined in claim 7 wherein at least a portion of the middle portion has a gel fraction of 0%.

9. The method as defined in claim 7 wherein the oriented film has a haze value of less than 5%.

10. The method of claim 1 wherein the outer layer portions and the middle layer portion are separate layers laminated together by coextrusion and wherein the outer layer portions are crosslinked by use of crosslinking agent prior to coextrusion.

11. The method of claim 7 wherein the polyethylene is HDPE.

12. A high clarity oriented polyethylene film of HPDE or HPDE blend having a thickness between 10 and 50 microns and consisting of (a) outer surface layers crosslinked to have a gel fraction of between 20 to 70%; and (b) a middle layer portion having a gel fraction of between 0 and 5%.

13. The oriented film of claim 12 wherein the polyethylene is HPDE.

14. The oriented film of claim 12 wherein the outer layer portions and the middle layer portion are contiguous and the thickness ratio of the outer layer portion:middle layer portion:outer layer portion thereof ranges between 1:.:1:1 to 1:10:1.

15. The oriented film of claim 14, wherein said ratio is between 2:1:2 and 8:1:8.

16. The oriented film of claim 14 or 15, being heat oriented stretched by at least 300%.

17. The oriented film of claim 12, having a haze value of less than 5%.

18. The oriented film of claim 12, wherein at least a portion of the middle layer portion has a gel fraction of 0%.