Groundwood pulp

Principal Uses. Groundwood pulp contains a considerable proportion (70—80 wt %) of fiber bundles, broken fibers, and fines in addition to the individual fibers. Principal uses of paper-grade pulps are in newsprint, magazine papers including coated pubHcation grades, board for folding and molded cartons, wallpapers, tissue, and other similar products. 

The TMP process is similar to the RMP process except that after chip washing, a steaming vessel is inserted to achieve the thermoplasticization of the lignin in the wood. The production of thermomechanical pulps increased dramatically after the introduction of this method in the early 1970s. Because these pulps can be substituted for conventional groundwood pulps in newsprint blends to give a stronger paper, lower quantities of the more expensive, lower yield chemical pulps are required. 

This has historically involved only minimal use of chemical additives. The product is usually unsized and, because of its low cost, the use of polyelectrolytes for retention is not usually cost-effective. However, this situation is changing as a result of the trend towards limited filler inclusion and also the increased use of deinked waste paper. Newsprint is usually made in an acidic system due to the naturally low pH of groundwood pulp but, as more recycled fibre is used, the process may eventually become neutral. 

Hewitt, L.M. Pryce, A.C. Parrott, J.L. Marlatt, V. Wood, C. Oakes, K. Van der Kraak, G.J. 2003, Accumulation of ligands for aryl hydrocarbon and sex steroid receptors in fish exposed to treated effluent from a bleached sulfite/groundwood pulp and paper mill. Environ. Toxicol. Chem. 22 2890-2897. 

Singh, R.P., Bleaching Groundwood Pulp "Investigation of Methods Based on Chemical Modification of Pulp", Tappi, 1966, 49(7), 281. 

Figure 4. Raman spectra in spectral region 850 to 1850 cm-1 A, unbleached stone groundwood pulp B, peroxide-bleached C, hydrosulfite-bleached. Sharp band at 1550 cm-1 is due to molecular oxygen.

The peroxide-bleached groundwood pulp was also severely oxidized with Fremy s salt Although the pulp was almost stabilized against any further brightness decrease during irradiation, die photochromic effect remained (Table HI). 

Unbleached pulps and groundwood pulps have poor permanency properties. Both pulps retain a high percentage of natural wood resin residuals independent of cellulose which readily decompose in the sheet. This affects color, strength, and sizing. It is generally specified that permanent paper must not contain either unbleached or groundwood fibers. 

Note 2 The test may also be performed on lignocellulosic materials, for example, groundwood pulp (Singh 1966)... 

Extraction of HemiceUuloses from Various Pulps. The extraction procedure followed was that of Giertz and McPherson (6). Fifteen grams of unaged pulps [four kinds in all bleached kraft pulp (BP), unbleached kraft pulp (UBP), unbleached groundwood pulp (GP), and chlorited UBP] was treated with 290 mL of 10.9 lithium hydroxide solution for 1 h at room temperature and was stirred intermittently (see Table I for the characteristics of the pulps). After the samples were diluted to about 1 L, the insoluble material was filtered off. The extract (filtrate) was neutralized to pH 6 with 2 M phosphoric acid (300 mL) and was allowed to stand overnight. The precipitated lithium phosphate and /3-cellulose were separated by filtration. The filtrate was then concentrated to about 175 mL under reduced pressure at 30-40 °C. Salts present in the system were further precipitated with methanol and separated by filtration. The filtrate was again concentrated to about 60 mL. The filter paper was immersed into this concentrate to saturate the test sheets. 

Figure 1. Changes in postcolor number of filter-paper sheets with 15% add-on of hemicellulose fractions derived from various pulps. For comparison, the thermal discoloration of a test sheet of groundwood pulp, gp, is shown. The initial postcolor number represents the increase in color over that of the untreated filter paper a, daylight fluorescent lamps b, black-light fluorescent lamps c, heat (90 °C and 50% rh). Key O, blank filter paper , BP A, GP V, UBP 0, chlorited UBP.

In contrast, marked and very similar darkening occurred upon the thermal aging of added-on test sheets at 90 °C and 50 rh, regardless of the particular hemicellulosic extract (Figures lc and 2c). For comparison, the behavior of handsheets made solely from groundwood pulp (gp), notorious for its tendency to discolor, is shown in each figure. 

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