DARKENING OF PAPER FOLLOWING EXPOSURE TO VISIBLE AND NEAR-ULTRAVIOLET RADIATION
S. B. Lee, J. Bogaard, & R. L. Feller
4.1 Preparation of Paper at Various pH Values
THREE TYPES OF WATER-LEAF TEST SHEETS—based on a supply of groundwood pulp (GP) and unbleached pulp (50% hardwood kraft/50% softwood kraft) (UBP), as well as unaged (FP) and aged filter paper (A·FP, Whatman No. 42 filter paper, previously aged 150 hours in an oven at 90°C, 50% RH)—were pretreated in various buffer solutions and air-dried overnight to produce test sheets having a wide range of initial pH values as measured by cold-water extract, Tappi Standard T509 Su-68.
Table III presents the list of the buffers used to produce the sheets at various pH values. The resultant water-extracted pH values, measured on the sheets that had been dipped into these solutions and air dried, were not the same pH as that of the buffer solutions used to prepare the test sheets. The cold-water-extract pH values of the test sheets as finally achieved in the sheets are those employed in Figures 4 and 5.
4.2 Preparation of Pulps With Various Contents of Gamma Cellulose
The unbleached pulp stock was subjected to a brief chloriting treatment followed by an extraction with different strengths of alkali solution to remove the lignin and gamma cellulose.15 Twenty-five grams of sodium chlorite and 12.5 ml of glacial acetic acid were reacted at a temperature of 75 to 80°C for 30 minutes at a pulp consistency of 10% using 105g air-dried pulp. This treatment reduced the lignin content of the pulp. Caustic extractions were subsequently performed at a pulp consistency of 2%, with the concentrations of alkali, reaction temperature, and reaction time varied, as noted in Table IV.
After the caustic extractions, the pulp was filtered off (except the 17.5%-alkali treated pulps, which were first diluted with water and then decanted), 500 ml of 10% acetic acid added and the mixture allowed to soak for 1 minute before removal of the pulp. Treatment with acetic acid was repeated with a second 500 ml portion. The pulp was then washed several times until the wash water tested acid-free with pH paper. Handsheets were made from the resulting pulps at a basis weight of 75 g/m2, as other types of handsheets.
Alpha, beta, and gamma cellulose in the resultant pulps was determined by standard Tappi methods as noted in a previous publication.14
4.3 Pulp Treatments to Produce Variable Lignin Content and HAS Matter
To prepare the pulp at various contents of lignin, groundwood pulp (designated as NC, not chlorited) was subjected to chloriting, using the technique of Wise, et al. 15 for 1, 2, and 4 hours, respectively (yielding stock samples 1HC, 2HC, and 4HC). Further treatments were carried out with hot-1%-alkali solution in order to remove the degradation products arising from chloriting, and with 17.5% caustic soda to remove the hemicellulose component. Thereafter, these pulps were treated with 2% sodium borohydride (SBH), at 2% pulp consistency and room temperature for 20 hours with occasional stirring, to convert aldehyde groups to hydroxyl. Lignin content and HAS matter were determined on the resultant pulps by Tappi methods T222 os-74 and T212 os-76, respectively.
4.4 Exposure to Light Sources
Three different light sources were used: (a) a bank of six General Electric High-Output (HO) “daylight” fluorescent lamps, (b) a bank of six fluorescent BLB blacklights, and (c) an Atlas 6500-W xenon-arc Fade-ometer¯ with Pyrex¯ filters. Data on the irradiance under the three lamps is given in Table V, information provided primarily to indicate the percentage of ultraviolet emitted.
TABLE V Irradiance From Different Light Sources (mw/cm2)
Both types of fluorescent lamps were mounted about 3 1/2″ above the samples in a room maintained at 50% relative humidity (RH) and 23°C. The temperature of the handsheets reached about 27°C. In the 6500-W Fade-ometer¯ the air was maintained at 31°C (±1.0°) and 27% RH; the black-panel temperature reached 65°C.
4.5 Storage Conditions
Following exposure to the various light sources, samples were kept in the closed steel cabinet in the paper-testing room maintained at 23°C and 50% RH.