Date of Award

4-1997

Degree Name

Bachelor of Science

Department

Paper Science and Engineering

Abstract

As society stresses the need to recycle to reduce the amount of waste sent to our municipality landfills every year, the effect of secondary fiber on the papers that we produce becomes greater and greater. One of the largest effects that these fibers have is to change the strength properties in the sheets which contain them. Most studies have shown a decrease in the tensile strength of paper as fibers pass repeatedly through a series of recycle. The reason of this tendency is believed to be a loss of bonding potential between fibers as they become shorter and stiffer through a process known as irreversible hornification.

Four grades were chosen to observe the effects upon sheet strength when passed through a series of five recycles. The grades covered a range of softwood to hardwood ratios, of filler content, and of degree of refining in the initial sheet. In all cases, tensile strength was shown to increase over the first three to four recycles before beginning to decrease with further recycle. Tests indicate that the leaching of filler from these relatively highly filled sheets was the main reason for these unusual, but not unprecedented results. As the filler is removed from the sheet, more bonding sites between fibers become available. This allows for higher strength within the sheet until this effect becomes increasingly offset by the hornification of the fibers, at which point strength begins to deteriorate.

The hypothesis of filler loss is strengthened by general decreases in brightness and opacity through successive stages of recycle. As these fillers are used expressly to enhance these sheet properties in these grades, it is apparent that they are being removed during the sheet formation process. Tear strength was shown to decrease through the course of this investigation, a result which is consistent with a rise in tensile strength.

Fiber length analysis and ash testing of sample testing of sample sheets would further aid in evaluation as to the extent to which fiber shortening and the loss of filler affect the strength properties of sheets made from repeatedly recycled fibers.

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