Bleached mechanical pulps

Peroxide Stage. Peroxides in the form of both hydrogen and sodium peroxide have been used to bleach mechanical pulps since about 1940. 

It is often possible to use different process chemicals to accomplish the same goal. For example, both zinc hydrosulfite and sodium hydrosulfite can be used to bleach mechanical pulps. The substitution of the use of sodium hydrosulfite for zinc hydrosulfite was prompted, at least in part, by the establishment of effluent limitations controlling the discharge of zinc. Other opportunities exist to minimize the discharge of toxic and nonconventional pollutants through chemical substitution. 

The purpose of this work was to investigate the nature of lignin in bleached mechanical pulp and sulfonated chemithermomechanical pulp (CTMP). Specifically, the purpose was to identify structures that may be responsible for photoyellowing. 

Fomier de Violet, P., Nourmamode, A., Colombo, N., Zhu, J. and Castellan, A., "Photochemical Brightness Reversion of Peroxide Bleached Mechanical Pulps in the Presence of Various Additives", Cellulose Chem. Technol., 1990, 24, 225. 

The maximum that was observed at 360 nm for alkali darkening of peroxide or borohydride treated pulps was attributed to the formation of stilbenes [70], Stilbenes, which are most likely formed by a retro-aldol condensation when ()-l or phenylcou-maran lignin structures are treated with alkali (Figure 3.3), have been isolated from peroxide-bleached mechanical pulps [110]. The action of alkali and oxygen removes such phenolic stilbenes [111]. 

Although coniferaldehyde model compounds reactrapidly and quantitatively with peroxide [120], the reaction in pulp fibers is much slower. Residual coniferaldehyde has been detected in peroxide-bleached mechanical pulps [87,110], and can have a profound effect on the absorption difference spectra for irradiation. Figure 3.13 shows difference spectra for ultraviolet irradiation of peroxide-bleached softwood TMP and CTMP [121]. The spectrum for CTMP has a broad absorption increase... 

Y Ni, A Ghosh, Z Li, C Heitner, and P McGarry. Photo-Stabilization of Bleached Mechanical Pulps with DTPA Treatment. J. Pulp Paper Sci. 24 259-263, 1998. 

Y Ni, Z Li, and ARP van Heiningen. Minimization of the Brightness Loss Due to Metal Ions in Process Water for Bleached Mechanical Pulps. Pulp Paper Can. 98 T396-T399, 1997. 

Alkaline hydrogen peroxide in the presence of sodium silicate bleach mechanical pulps (SOW, TMP, CTMP). In order to further stabilize the bleaching liquor against metal ion induced decomposition, the pulp must be pretreated with a chelating agent, usually EDTA or DTPA. Typically, the bleaching is carried out at a temperature of 60-70°C during 60-120 min. Under these conditions, the yield loss is low,... 

Lee and Sumimoto [47] suggested that peroxide-bleached mechanical pulps yellow readily because the hydroquinones they contain are leucochromophores, which may be subsequently oxidized further to colored compounds. In particular they showed that peroxide treatment of phenolic stilbene (XIII), which was formed from phenylcoumaran (XII) by mechanochemical action, gave rise to a labile hyd-roquinone that was easily oxidized by air to an intensely colored stilbenequinone (XIV) (Figure 12.4). It should be noted that these conclusions were based on experiments with model compounds. Conversely, Zhu et al. [48] found that mechanochemical action had no significant effect on the photosensitivity of mechanical pulps. 

P Eomier de Violet, A Nourmamode, N Colombo, J Zhu, A Castellan. Photochemical brightness reversion of peroxide bleached mechanical pulps in the presence of various additives. Cell Chem Technol 24(2) 225-235, 1990. 

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