4 PROJECT 2: EFFECTS OF ACCELERATED AGING ON SELECTED CONTEMPORARY TRACING PAPERS

4.1 RESEARCH PROCEDURES

4.1.1 Method of Aging

Samples of all the papers were subjected to accelerated aging for 4 weeks in the dark at 90�C and 50% RH in an Associated Environmental Systems HK-4116 Temperature/Humidity chamber. These conditions have been chosen as suitable for artificial aging studies (Erhardt 1988). Sets of transparent papers were sewn with cotton thread into Plexiglas frames so that all four corners were anchored and the samples did not touch one another, although the samples did vibrate in the oven draft.

4.1.2 Measurement of Properties

Evaluation of the effects of accelerated aging on the properties of the papers (except the vegetable parchment paper, which is still being tested) was undertaken by measuring changes after aging in color, opacity, gloss, mechanical strength, and pH, using the apparatus and systems outlined previously (table 2 and section 3.1.3).

4.2 FINDINGS AND DISCUSSION FOR PROJECT 2

As noted previously (section 3.2), U.S. Federal Specifications for Tracing Papers (No. UU-P-561H 1972) uses dry oven accelerated aging at 100�C for 72 hours. Our accelerated aging conditions of 90�C, 50% RH for 4 weeks employs higher humidity levels and longer exposure times that may engender more extensive hydrolysis and oxidation. This effect may account for the change, exceeding the Federal Specifications, that occurred in some properties of some of the samples tested under our more severe conditions (van der Reyden et al. 1992a, 1992b).

Following aging at 90�C, 50% RH for 4 weeks, all the papers underwent subjective and measurable change in color (fig. 2a). Opacity increase exceeded the Federal specification of 9 percentage points for the chemical pulp natural tracing paper and 3–7 percentage points for the rag pulp vellum sample (fig. 2b). Mechanical properties, as measured by tensile strain to break, decreased considerably for the natural tracing paper (fig. 2c). Accelerated aging appeared to have less effect on gloss (fig. 2d).

The natural tracing paper sample showed the greatest change in CIE L∗a∗b∗ values under our conditions, perhaps because of the sulphamic acid used during processing or the modified starch added to surface sizing. The natural tracing paper also underwent the greatest increase in opacity. With regard to strength, the tracing papers that could be measured had far greater strength than paper processed the standard way, possibly as a result of a combination of exceptionally strong sizes, coatings, and impregnants and overbeating. After accelerated aging, these papers underwent a reduction in elongation and tensile energy absorption (TEA) characteristic of embrittlement, perhaps from a breakdown in sizing and coating material. In addition, overbeating, as in the case of the natural tracing sample, which is responsible for substantial increases in strength before aging, may also result in a decrease in strength after aging, since the process produces shorter fibers and weakens fibrillar attachment (Yates 1991–92). The pH of all but one of the papers dropped after accelerated aging. The pH of the natural tracing sample paper, which was already acidic at 4.7, dropped to 3.8 after aging, as did the pH of the imitation parchment (from 6.1 to 5.4) and the prepared tracing (from 5.8 to 5.1). The vellum paper sample had a pH (4.3), which increased after aging (5.6).

4.3 CONCLUSIONS FOR PROJECT 2

Accelerated aging under the conditions used for this project caused dramatic differences in the properties of all the samples (table 2), sometimes in excess of the standards considered acceptable by the U.S. Federal Specifications for Tracing Papers (UU-P-561H 1972). There were significant differences in mechanical properties, before and after aging, depending on whether the samples were transparentized by overbeating or by transparentizing agents (fig. 2). In general, the heavily coated transparent paper samples seem to have better retention of optical and strength properties and dimensional stability after aging as compared with the overbeaten natural transparent and imitation parchment paper samples (van der Reyden et al. 1992a, 1992b).