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Modern Packaging Magazine - September 1958 - Return to Main Search
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Text Summary via OCR:

Table V: Testing film at various angles

Resin

Roll

speed

(f.p.m.)

Stretch distance (in.)

MD*

Angle (deg.)

30* 45* 60*

TD*

A

60

1

453/14

503/14

604/9

584/24

495/36

B

80

4

411/11

493/16

576/21

515/15

396/13

C

150

2

369/9

455/19

600/46

555/26

372/18

D

150

2

365/7

504/24

619/9

562/31

441/17

E

80

1

265/9

279/9

282/17

213/14

123/10

* 95%

confidence

interval.

difference between the impact strength of polyethylene-coated paper and plain kraft paper of the same thickness. This result, before further analysis, is unexpected because of the substantial difference in impact toughness between polyethylene and kraft paper (approximately 350 vs. 20 ft.-lb./cu. in.). Insight into its cause came from an inspection of the fracture obtained with the polyethylene-coated paper. Normally, polyethylene film will elongate before rupturing in tensile impact, while paper exhibits a brittle-type failure. But when polyethylene-coated paper is fractured, there is no extension of the polyethylene; e.g., it fails as if it were completely paper.

The last test sample listed in Table IV was a composite consisting of alternately spaced polyethylene film and kraft paper. With this arrangement, the polyethylene film experiences its expected or normal elongation and a comparatively high energy-to-rupture is required (which is a natural consequence, since toughness is actually a property

<ZBEI

EFFECT OF TESTING DIRECTION ON THE TENSILE IMPACT TOUGHNESS OF POLYETHYLENE FILM

determined by several interrelated factors, primarily tensile strength and ductility).

This analysis indicates that when polyethylene is coated on paper its ductility is completely restrained and its toughness advantage is lost. These results imply that if it is desired to design a multiwall bag with improved toughness as well as moisture resistance, etc., a completely separate and integral polyethylene bag should be provided rather than one of polyethylene-coated paper.

From a design point of view, the energy-to-rupture data in Table IV indicate that a separate polyethylene bag (1.5 mils) would provide approximately three times the impact strength of a polyethylene-coated paper and four times that of a kraft paper (60 lb), while a polyethylene bag inside a kraft paper (60 lb.) bag would provide a bag with approximately five and one-half times the toughness of the polyethylene-coated paper. Thus, the test results do not substantiate the often-expressed theory that polyethylene-coated paper is tougher than uncoated paper. However, the polyethylene coating-does impart other properties, such as moisture resistance. The net result may be a more durable bag.

Effect of testing direction upon the tensile impact toughness of polyethylene film. Advantage has been taken of the versatility and convenience of the tensile impact test to study the variation of polyethylene film toughness with testing direction. Thus, although film impact toughness is conventionally determined in the machine and transverse directions, it is of interest to know what function toughness is of film direction or orientation. This is accomplished with relative facility in the tensile impact test simply by cutting and testing specimens at any desired angle to the machine direction.

Results of this investigation are listed as the tensile impact toughness at various angles to the machine direction (Table V).

Curves revealing the variations of tensile impact toughness with angle from the machine direction are plotted in Cartesian coordinates (Figure 6) and polar coordinates (Figure 7).

The four upper curves of Figure 6 represent a variety of materials, different speeds of extrusion and different stretch distances. In both the machine and transverse directions, the impact strength averages approximately 400 lbs. per cubic inch. At 45 deg. to these directions, the tensile impact strength is a maximum and equal to approximately 600 7ta lbs. per cubic inch. These numbers are fairly representative of most of those encountered.

The lower curve illustrates an extreme condition which may be encountered because of the resin, the film speed or the stretch distance. In this case, the impact strength in the [Continued on page 207]