Revealing weak points in paper web samples: identification and estimation of their strength-reducing effect.

Application: A new procedure has been developed for identifying weak points in a length of paper and estimating the effects that these weak points may have on the tensile strength of the sheet.

Weak points scattered in its structure detract fret the tensile strength of a paper sheet. Normally, these weak points are small and do not represent an risk of the sheet breaking at normal tensions. However, weak points may impair the sheet's ability to withstand tension peaks in a printing press.

Equipment and testing

The basic tool is a tensile testing machine specifically designed for making detailed visual inspections [TAPPI JOURNAL 1(84): 84(2001)]. The samples are taken from small-diameter paper rolls that are rewound and cut to width of 100 cm. A suitable length of the sample is wound at the top of the testing machine, and a 1.8n length is pulled down and clamped.

This sample is under tension control in front of ground glass plate through which the sheet is evenly illuminated from behind with a light of adjustable intensity. The detailed structure of the semi-translucent paper is clearly visible. A magnifying glass and a hand microscope are useful aids in identifying specks for further analysis in the breaking tests.

Four reels of newsprint and three reels of supercalendered magazine paper were used in this study. The breaking tension (or tensile strength) of narrow paper strips was measured according to the SCAN P38 method.

Evaluation of findings

Surprisingly, visible specks which one might guess to be the weakest points were usually harmless in relation to other weak points that are, more difficult to see. Harmless specks in newsprint included visible bark specks, small extreme dilutions in the fiber network, and sore ink residues from recycled paper. Shives turned in the cross machine direction were barely visible but were the dominating weak points.

Verifying that breaks start from the weak points

To verify that breaks start from the weak points identified, we placed a high-speed video camera in front of the tensile machine, where a sheet length was ready for a breaking test. On the left side of Fig. 1, a sample with its weak points marked is ready for testing. The eight frames show the sequence by which the break develops.

[FIGURE 1 OMITTED]

In the first frame, a crack has begun to form along a shive, and buckling can be seen around it. At Frame 4, a small expansion of the crack is still contained within the circle of the mark. In Frame 5, the crack has expanded in both directions beyond the circle. By Frame 8, the sample has broken through its whole width.

Results of breaking tests

For newsprint, the most common weakest points were shives, and the 5-10 most strength-reducing ones in each sample could be identified. For the magazine paper, the weakest points in each sample could be localized to be within a few centimeters of the rupture line, but no shives or distinct inhomogeneities could be seen.

Conclusions

A procedure has been developed for identifying and estimating the strength-reducing effect of the weakest point in each 1.8[m.sup.2] sample of paper. This method is based on special illumination, breaking tests, and an analysis of rupture lines. A purely visual test usually cannot reveal the weakest points, but visual inspections can provide hints about the types of defects that can lead to difficulties on a print run.

Age Hansen is with the Norwegian Pulp and Paper Research Institute, (PFI) Hogskoleringen 6B, Trondheim, Norge N-7034, Norway. Address correspondence to Hanson by email at: [e-mail]