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Ever since papermakers made the great change from handmade sheets to continuous-web paper machines, they have been trying to recapture some of the isotropic properties of paper they lost in the process. Machine-direction tensile strengths are often over twice as high as cross-machine direction tensile strength. Cross-machine direction stretch, on the other hand, is in the order of 3 to 5 per cent, whereas machine-direction stretch is rarely over 2 per cent. For many years one of the largest uses for paper has been in the packaging industry. In 1957, almost 800,000 tons of kraft paper were used in the multiwall sack field alone, and here it has been observed that those papers with the highest tensile strength have not always been the strongest or best in actual use. Increased extensibility, even at the expense of tensile strength, will many times insure better end-use performance. Creped papers, despite their lower tensile strength, and rough surface not conducive to good printing, find widespread use in the multiwall sack field. It is an established fact that high humidity produces higher paper stretch capacity and lower tensile strength. Conversely, with low relative humidity, tensile strength increases while stretch capacity decreases. Yet despite the increased tensile strength, multiwall sack breakage is higher in winter months when the relative humidity is lower. Therefore, the paper industry has been interested in the effects of stretch on paper behavior and in a means for producing increased stretch capacity, or extensibility in paper as an “on machine” operation. Interest in the advantages to be gained from increased extensibility is attested to by the number of patents issued for various means of inducing both machine-direction and cross-machine direction stretch in paper webs.
West Virginia Pulp and Paper Co., Charleston, S. C.