Method for improving tear resistance of stretching film

Abstract

A method for improving tear resistance of stretch films in which two stretch films are bonded together with an adhesive by hot rolling or UV curing such that their primary stretch directions cross each other. Composite films resulting from the invention possess significantly improved slit tear strength and tear resistance over biaxially stretched films with the same thicknesses. The invention can be widely used to improve tear resistance of stretch films fabricated from various existing materials and processes. In addition, it allows simple operations and is readily applicable to mass production.

Claims

1. A method for improving tear resistance of stretch films, in which two stretch films are bonded together with an adhesive by hot rolling or UV curing such that their primary stretch directions cross each other.

2. The method of claim 1, wherein the primary stretch directions of the two stretch films form an angle of 30-150°.

3. The method of claim 2, wherein the primary stretch directions of the two stretch films form an angle of 60-120°.

4. The method of claim 3, wherein the primary stretch directions of the two stretch films form an angle of 90°.

5. The method of claim 1, wherein the stretch films are asymmetrically stretched films comprising uniaxially stretched films and asymmetrically biaxially stretched films.

6. The method of claim 5, where in ratios of primary stretch ratio to secondary stretch ratio of the asymmetrically stretched films range from (1.2:1) to (4:1).

7. The method of claim 5, wherein the asymmetrically stretched films are formed using a flat-film method.

Description

DETAILED DESCRIPTION

[0022] The subject matter of the present invention will be described in further detail below with reference to several embodiment examples so that it can be fully understood.

[0023] In the examples, each stretch film sample was prepared using a flat-film method including: making a cast sheet with a thickness of about 140-560 mm from a plastic material or a resin (a commercially available PET resin was used in the examples) using a multi-layer extruder (manufactured by Nanjing Chuangbo Machinery Co., Ltd.); placing the cast sheet on a film stretcher (manufactured by Bruckner Maschinenbau GmbH and used at Beijing Research Institute of Chemical Industry (BRICI)), clamping it with clips and stretching it according to a preset program (stretch ratios and stretching temperatures); and finishing the resulting film by rapid cooling in the air.

[0024] In the examples, tear resistance related tests were carried out using a CH-9002A-20 multi-functional tensile tester (manufactured by Suzhou Baoyuntong Testing Equipment Co., Ltd.). In each test, an approximately 20-mm-long slit was made in the film sample along a tear direction under test, and both ends of the slit were then clamped with clips. Afterward, the test was initiated under the control of an automatic program, followed by data collection.

Example 1

[0025] An adhesive (e.g., an TS9002A, TS9002B or TS9002C adhesive manufactured by Yantai) was uniformly applied to one side of a stretch film, and another stretch film was then bonded thereto after evaporation of a solvent contained in the adhesive such that a primary stretch direction (with a greater stretch ratio) of the second stretch film crossed that of the first stretch film at angles of 90°). The composite film was then subjected to hot rolling performed on a 60-85° C. roller (the hot rolling temperature might depend on the adhesive used) and then placed at room temperature until it completely cured.

[0026] Table 1 shows a tear resistance comparison among composite films formed from respective stretch films with different ratios of primary stretch ratio to secondary stretch ratio using the method according to this Example.

TABLE-US-00001 TABLE 1 Sample Single Slit Tear Resistance Primary Secondary Layer Sample Tear Stretch Stretch Thickness Thickness Tearing Strength ratio ratio (μm) (μm) Force (kg) (N/mm) 3.9 3.8 100 100 0.07 7.18 3.6 3.1 150 150 0.30 19.86 4 1 75 150 1.03 67.49 3.5 1 50 100 3.00 294.00 2.7 1 50 100 3.44 337.27 4 2.8 70 140 3.47 242.94

[0027] As can be seen from Table 1, the composite stretch films obtained from the inventive method all had significantly increased tear strength over those of the biaxially stretched single-layer films having the same thicknesses. Although tear strength increased with sample thickness, compared to the 150-μm-thick biaxially stretched single-layer film whose tear strength was only 19.86 N/mm, the tear strength of the inventive 100-μm-thick composite film (with a ratio of primary stretch ratio to secondary stretch ratio of 2.7) was up to 337.27 N/mm. That is, a significant increase in tear resistance was obtained. In addition, the higher the asymmetry, the greater the slit tear strength. Ratios of primary stretch ratio to secondary stretch ratio of (1.4:1)-(3.5:1) corresponded to slit tear strength of 240-340 N/mm. However, at a ratio of primary stretch ratio to secondary stretch ratio of 4, tear strength tended to decrease even when the thickness was increased to 150 μm.

Example 2

[0028] An adhesive (e.g., an 6062A or 7725B adhesive produced by Henkel) was uniformly applied to one side of a stretch film, and another stretch film was then bonded thereto such that a primary stretch direction (with a greater stretch ratio) of the second stretch film crossed that of the first stretch film at angles of 90°). The composite film was subjected to hot rolling performed on a 60-85° C. roller (the hot rolling temperature might depend on the adhesive used) and then placed at room temperature until it completely cured.

[0029] Table 2 shows a tear resistance comparison among composite films formed from respective stretch films with different ratios of primary stretch ratio to secondary stretch ratio using the method according to this Example.

TABLE-US-00002 TABLE 2 Sample Single Slit Tear Resistance Primary Secondary Layer Sample Tear Stretch Stretch Thickness Thickness Tearing Strength ratio ratio (μm) (μm) Force (kg) (N/mm) 3.9 3.8 100 100 0.07 7.18 3.6 3.1 150 150 0.30 19.86 4 1 75 150 1.48 96.45 3.5 1 50 100 6.22 609.79 4 2 50 140 3.57 349.86

[0030] As can be seen from Table 2, the composite stretch films obtained from the inventive method all had significantly increased tear strength over that of the biaxially stretched single-layer films having the same thicknesses. Although tear strength increased with sample thickness, compared to the 150-μm-thick biaxially stretched single-layer film whose tear strength was only 19.86 N/mm, the tear strength of the inventive 100-μm-thick composite stretch film (with a ratio of primary stretch ratio to secondary stretch ratio of 3.5) was up to 609.79 N/mm. That is, a significant increase in tear resistance was obtained. In addition, the higher the asymmetry, the greater the slit tear strength. Ratios of primary stretch ratio to secondary stretch ratio of (2:1)-(3.5:1) corresponded to slit tear strength of 340-610 N/mm. However, at a ratio of primary stretch ratio to secondary stretch ratio of 4, tear strength tended to decrease even when the thickness was increased to 150 μm.

[0031] It is revealed from a comparison between Tables 1 and 2 that when an adhesive containing a solvent is used in the bonding by hot rolling, the solvent may cause loose molecular orientation in the stretch films and hence lower tear strength.

Example 3

[0032] With reference to Example 2, a tear resistance comparison was made between a composite film resulting from asymmetrically biaxially stretched films obtained by simultaneous stretching and a composite film resulting from asymmetrically biaxially stretched films obtained by successive stretching.

[0033] The test results are presented in Table 3.

TABLE-US-00003 TABLE 3 Sample Slit Tear Resistance Primary Single Layer Sample Tear Stretch Secondary Thickness Thickness Tearing Strength ratio Stretch ratio (μm) (μm) Force (kg) (N/mm) 4 2.8 70 140 0.78 54.81 (simultaneous stretching) 4 2.8 70 140 0.61 42.95 (successive stretching)

[0034] As can be seen from the results in Table 3, the slit tear strength of the composite film made from the simultaneously biaxially stretched films was slightly higher than that of the composite film made from the successively biaxially stretched films, although the difference was significant.

[0035] It is noted here that, the above embodiments are presented merely to describe the subject matter of the present invention in further detail and are not to be construed as limiting the scope of the invention. Non-substantive improvements and modifications made by those skilled in the art in accordance with the above disclosure all fall within the scope of the present invention.