High-yield Pulp Research Articles

Photoyellowing of peroxide-bleached lignin-rich pulps: a photochemical study on stilbene-hydroquinone chromophores issued from β-5 units of lignin during refining and (or) bleaching

The contribution of stilbene-parahydroquinone structures, formed from β-5 units during refining and bleaching, in photoreversion of peroxide-bleached high-yield pulps, was studied using 4'-benzyloxy-2,5-dihydroxy-3,3'-dimethoxystilbene (I), as a model compound. The various photoproducts formed during irradiation of I in solution or adsorbed on cellulose paper were identified. Isomerization of the stilbene double bond and oxidation of the hydroquinone into quinone were observed in solution, whereas in the solid state only the formation of the quinone was found to operate. Our attempts to detect and quantify stilbene-hydroquinones forming from β-5 units created on peroxide bleached mechanical pulp were not met with success.Key words: lignin, stilbene, hydroquinone, lignin model compound, pulp, photoyellowing.

From Seed to Paper

The productivity of a pulp and paper mill is dependent on the quality of wood chips entering the mill. Most kraft mills operate with a “bottleneck” and this is typically the recovery boiler. For these mills, using chips with “x%” higher pulp yield can result in a “3x%” increase in output of paper. This can represent a substantial increase in productivity and potential profit for the mill.In the case of plantation Eucalyptus wood, the quality of the chips from harvested trees is dependent on many factors, such as the climate and seed quality. but it is also dependent on the age of the trees at harvest. In most cases, allowing trees to grow for a longer time before harvest will cause the pulp yield to increase. These older trees would, therefore, be of great interest to pulp and paper mills. Unfortunately, the plantation owner may be reluctant to supply older trees as delaying harvesting will cost money.This paper will examine the conflict of interest between the pulp mill operator and the plantation owner. A mathematical model has been constructed which enables calculation of the impact to be calculated of varying the age at harvest on the overall economics for the whole fibre chain, “from seed to paper”. Input data used in the model was sourced from Eucalyptus plantations in Tasmania that are supplying Japanese integrated fine paper mills.It is proposed that, if a pulp and paper mill is going to use plantation chips, the management at the mill should take an active interest in the management principles used by the tree grower.

On the interaction of UV screens with the lignocellulosic matrix.

In this report, we describe our attempt to understand the photochemical interactions that occur between dihydroxybenzophenone (DHB)-based UV screens and lignin when high-yield pulps are treated with such materials. Milled wood lignin (MWL) and filter paper were used as models, and various irradiation protocols were carried out in the presence and absence of UV screens. After irradiation, the lignin and the UV screen were extracted and the products analyzed. These experiments showed that upon irradiation, fragments of MWL-containing chromophores were linked to cellulose via an acid-labile linkage. In the presence of UV screens, these reactions were minimized. Molecular weight measurements of the extracted lignin showed that the MWL is degraded upon solid-state irradiation. The samples that contained UV screens showed a reduced tendency to degrade. Using quantitative 31P NMR, it was possible to probe further the detailed structural changes that occurred in MWL during irradiation. In general, DHB-based UV screens and derivatives were found to interact actively with MWL when irradiated.

Photoyellowing inhibition of bleached high yield pulps using novel water-soluble UV screens.

To address the deficiencies of benzophenone UV screens for preventing brightness reversion in high yield mechanical papers, we synthesized a new series of such materials with enhanced water solubility and compatibility with the lignocellulosic substrate. A series of 2,4-dihydroxybenzophenones (DHB) were synthesized containing various Mannich bases at the C3 position of one of its rings. They possess the UV-screening ability of o-hydroxylbenzophenones, and they also contain tertiary nitrogen atoms that may function as radical scavengers. Aqueous solutions of the hydrochloride salt of 3-(dimethylaminomethylene)-2,4-dihydroxylbenzophenone (1), when applied on bleached chemithermomechanical pulp (CTMP) sheets, were significantly more efficient in preventing photoyellowing than the original DHB applied on the sheets from ethanol-water solutions. This confirmed our original hypothesis that increasing the compatibility of the UV screen with the lignocellulosic matrix would increase its efficiency in preventing photoyellowing. Compound 1, however, was found to be somewhat more effective than its hydrochloride salt toward preventing photoyellowing. This was attributed to the synergistic action of the free tertiary aminic center attached on the molecule with its UV-screening ability. To comprehend further the various parameters that influence the photoyellowing inhibition performance of these compounds and DHB with bleached CTMP pulp fibers, a series of handsheets were prepared at different pH. The interactions of the protonated compound 1 with pulp fibers were then evaluated by studying their kinetics of absorption and desorption to and from the fiber matrix. This part of our study found that the adsorption of protonated Mannich derivatives of DHB onto pulp is most likely governed by a cation-exchange mechanism involving the cationic amine group with the sulfonic and carboxylic acid groups located on the surface of the fibers. The pH the paper sheet was made from was also found to affect profoundly the adsorption and retention characteristics of these compounds onto the lignocellulosic matrix.

Forests, fiber, and the environment – In view of the fiber supply to the pulp and paper industry

Forests play a vital role in the social, economic, and environmental development of any country. Paper, composed principally of wood fiber, is an essential commodity in promoting literacy, communication, documentation, and packaging. However, there is much concern about the world's forests being over-utilized. This has led to serious repercussions, not only to humanity, but also to the earth's biodiversity as a whole. It is now time that forests be used in a more responsible and ethical manner. Because of reduced forest area, increasing pulpwood costs, and an increasing demand for pulp and paper products, it might be expected that the focus would shift to high yield pulping processes or to the use of cellulosic non-wood raw materials. Non-woods are available in good supply all over the world, but are currently under-utilized. It is estimated that replacing 5–10% of wood pulp with non-wood pulp would have an important impact on the conservation of forests and the environment. This replacement of wood pulp by non-wood pulp could be environmentally and economically acceptable even in developed countries. By doing so, pulp and paper mills would have a lead role in reducing their dependency on forests for fiber. This determination and commitment would enhance the long-term sustainability of forests and the pulp and paper industry. Above all, it would be a sustainable step towards "our common future." Key words: forest, sustainable development, fiber supply, pulp and paper, non-woods

Brightness Reversion of Mechanical Pulps XI: Photostabilization of High-Yield Pulps by Thiosulfinates

A series of eleven cyclic and acyclic thiosulfinates was synthesized and applied onto hardwood BCTMP (bleached chemithermomechanical pulp) testsheets. Treated and untreated testsheets were photoaged and their optical property were monitored. The results of these photoaging experiments indicated that cyclic thiosulfinates were relatively ineffective at retarding brightness reversion, whereas most acyclic additives were effective photostabilization agents for BCTMP. The use of acyclic thiosulfinates in combination with UV absorbers provided a very effective means of retarding the photoaging process of mechanical pulps.

Evaluation of the contribution of lignin stilbene phenol units in the photoyellowing of peroxide-bleached lignin-rich pulps

In order to assess and quantify the contribution of stilbene phenols in photoreversion of bleached high-yield pulps, a method to reduce the stilbene double bond and quantify the formed diphenylethane was developed on 4-benzyloxy-3,3′-dimethoxy-4′-hydroxystilbene, as lignin monomer model, and also on a more sophisticated lignin polymer model constituted by a polystyrene framework containing 4,4′-dihydroxy-3,3′-dimethoxystilbene elements as pendent groups. The method used RhCl(PPh3)3 as soluble catalyst to get an efficient hydrogenation and AlCl3 in benzene to liberate the diphenylethane from the polymer framework. For the first time a semiquantitative evaluation of the content of the stilbene phenols formed from β-1 units was given for high-yield pulps. The value of 2 p-stilbene phenols for 1000 C9 lignin units found indicates that they are present in very small quantities after the peroxide bleaching. UV irradiation of the polystyrene model adsorbed on solid cellulose matrix had revealed an efficient stabilization after the hydrogenation treatment. This was not the case for the peroxide bleached pulp showing that the p-stilbene phenols formed from β-1 units are not the main contributors of the rapid yellowing of bleached lignin-rich pulps. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2517–2531, 1998

A mathematical model for neutral sulfite pulping of various broadleaved wood species

A mathematical model for neutral sulfite pulping process was derived. This model represented by a linear logarithmic relationship of the main pulping characteristics (yield and lignin content) to time and temperature parameters, both determined by the H-factor, is expressed by a logarithmic straight line equation. The relationship of yield to lignin content in the high yield pulps after neutral sulfite pulping is also expressed by a logarithmic straight line equation. A suitable way to calculate the main characteristics of neutral sulfite pulping is the power form of the logarithmic straight line equation for the dependence of the ratio yield/lignin vs. lignin content, yield/H-factor vs. H-factor, lignin/H-factor vs. H-factor and their mathematical equivalents (i.e. reciprocal ratio and/or product). A set of power equations characterizing the relationships yield vs. lignin content as well as yield and lignin content vs. H-factor for woodpulps prepared by neutral sulfite pulping of various broadleaved wood species are given.

Steam exploded biomass for the preparation of conventional and advanced biopolymer-based materials

Lignocellulosic materials treated by the steam explosion technique alone, or in the presence of chemical reagents, undergo remarkable modifications at the level of both the chemical composition and supramolecular structure, developing features suitable for the production of conventional and advanced materials with improved properties. Experience in the production of high yield pulps, composite materials with microbial (PHB) and synthetic (polypropylene) thermoplastic matrices and high modulus lignocellulosic fibers is reported.

Ozone in the production of softwood and hardwood high-yield pulps to save energy and improve quality

Article Ozone in the production of softwood and hardwood high-yield pulps to save energy and improve quality was published on January 1, 1998 in the journal Nordic Pulp & Paper Research Journal (volume 13, issue 1).

Sterculia villosa Roxb — A potential source of wood-fibre for pulp and paper making

Sterculia Villosa Roxb, is one of the fast-growing plant species abundantly available in the North Eastern Region of India. It is a medium-sized tree with grey bark about 2.50–2.65 cm thick in the mature plant. A full-grown tree attains a height of about 15–18 metres with a girth about 1.4–1.6 metres. The average wood fibre length is 1.2 mm. A high pulp yield of 55±3% is achievable from this plant material. Experiments for preparing pulp and paper were carried out adopting the Sulphate and Soda-anthraquinone pulping processes. Standard laboratory handsheets were made from the pulp stock of bark, debarked woody portion and their blend (30:70) and exhibited good strength properties. The results indicated that this fibrous material may be considered as one of the potential sources of raw materials for making pulp and paper either alone or in combination with the conventional fibrous materials.

Homolytic cleavage of the P-ether bond in phenolic P-0-4 structures and its significance in high-yield pulping and lignin analysis.

Homolytic cleavage of the P-ether bond in phenolic P-0-4 structures and its significance in high-yield pulping and lignin analysis.

Brightness reversion of mechanical pulps VIII. Investigation of synergistic photostabilization methods for high-yield pulp

Recent advances in the photostabilization of mechanical pulp suggest that the use of additive mixtures may provide optimal protection against brightness reversion. Some of the most promising additive mixtures reported in the literature include the use of a UV absorber, such as 2,4-dihydroxybenzophenone, and a radical scavenger. The mechanistic basis of this synergistic effect was explored by monitoring the photostability of benzophenone during brightness reversion. These studies demonstrated that the benzophenone additives were stable under the initial photolysis conditions. The use of mercapto derivatives of benzophenone did not enhance the photostabilization effect, suggesting that the thiol additive and UV absorber need not be in close association with each other for the observed photostabilization effects to occur.

Brightness Reversion of Mechanical Pulps VIII: Fate of Dienic Additives During Brightness Reversion of High-Yield Pulp

The photostabilization mechanism of 1,4-dihydro-2-methylbenzoic acid was examined with a lignin model compound in solution and on BCTMP pulp. Photodegradation of 3,4-dimethoxy-α-(2′-methoxyphenoxy)-acetophenone (1), dissolved in benzene, was shown not to be influenced by the addition of the diene additive, suggesting that the additive does not influence the initial photochemistry of lignin model compound 1. Product analysis indicated that the addition of the diene additive enhanced formation of 3,4-dimethoxyacetophenone, one of the principal products formed from photofragmentation of compound 1. These results were attributed to hydrogen donation from the diene to the phenacyl radical. Photolysis of 1,4-dihydro-2-methylbenzoic acid impregnated on BCTMP test sheets revealed a gradual conversion of the diene additive into o-toluic acid; this reaction was attributed to a hydrogen donation mechanism.

The Bleaching of Mechanical Pulps with Oxygen and Borohydride

Mechanical pulps are bleached by treatment with oxygen gas and alkali in the presence of borohydrides. The combined treatment gives brightness gains of 15-20 points, comparable with those achieved using stabilized alkaline 4% hydrogen peroxide. By contrast the reaction of oxygen and alkali alone darkens high-yield pulps, and treatment with borohydride in the absence of added oxygen bleaches only partially, to give about half the brightness gained if oxygen is added. Evidence is presented that the bleaching occurs via the conversion of oxygen molecules (O 2 ) to hydroperoxide ions (HOO - ) by reaction of the oxygen with ionized phenolic groups in the lignin. The hydroperoxide ions are thus generated in situ in the lignin where they bleach by reacting with coloured lignin quinone and coniferaldehyde chromophores. Without the addition of borohydride the lignin phenolic groups are irreversibly oxidized by oxygen to coloured products. The primary role of the borohydride is to regenerate the phenolic groups and maintain an oxygen hydroperoxide [(O 2 ) → (HOO - )] cycle. In principle other reducing agents could be used as alternatives but in practice only sodium borohydrides and substituted borohydrides have been found to be sufficiently stable to the reaction conditions. In a secondary role. the borohydride also contributes to the bleaching by reducing radicals, organic peroxides and carbonyl compounds present in the pulps.

High-intensity laser generation of ortho-quinones from 2-methoxyphenols; relevance to the question of color reversion in pulp and paper

High-intensity photodecomposition of 2-methoxyphenols using 248 nm laser excitation leads to the formation of small yields of ortho-quinones through the intermediacy of phenoxyl radicals, which undergo photocleavage of the O C bond. This process was studied using the laser-drop technique and the products were characterized by fluorescence spectroscopy following derivatization with 1,2-phenylenediamine. It is suggested that, given the long lifetime (several days) of related lignin-derived phenoxyl radicals in paper, similar processes may be partially responsible for quinone-mediated color development in papers manufactured from high-yield pulps.

Attempt to photostabilize paper made from high-yield pulp by application of UV screens in conjunction with thiols

The stabilization against photoinduced brightness reversion of peroxide-bleached stoneground wood pulp by two phenolic UV screens (methyl 3-hydroxy-4-benzoylphenoxy acetate (DHBE) and methyl 3-(2″-benzotriazinyl)-4′-hydroxy-5′-terbutylphenyl propanoate HBTE) and two polyethyleneoxythiols (2-(2′-methoxyethoxy)ethanethiol (MSH) and 2,2′-oxydiethanethiol (DSH), individually or in combination (DHBE + MSH and HBTE + DSH), was investigated. A synergistic photostabilization effect was observed between the thiol and the UV absorber, but was not retained during irradiation due to thiol consumption probably by Michael-type addition on the quinones, generated by light. Moreover, during the irradiation, the HBTE derivative was found to be very stable on paper in contrast with the DHBE compound. This effect was related to the presence in HBTE of a bulky substituent (tert-butyl) adjacent to the phenolic group which locks the molecule in a favorable conformation for proton transfer in the ground and excited states.

The photodegradation of some naturally occurring polymers

This review contains a discussion of how chemical and spectroscopic methods have been used to unravel the complex photodegradation processes that occur when wool and paper (made from high yield pulps) are exposed to light. Both materials yellow when exposed to UV light and are bleached by visible light. In addition, both materials are bleached when irradiated in solutions of reducing agents. The photoyellowing of wool is, in the main, associated with tryptophyl residues and the degradation of the oxidation products of this amino acid. Tyresine also contributes to the yellowing process, whereas cystine offers some protection and retards degradation. Degradation involves both singlet oxygen and radical species. Many of the degradation products are fluorescent and fluorescence spectroscopy can be used to gain an idea of the extent of degradation. Fluorescence microscopy shows that the tips of wool fibres (i.e. the weathered region) are more highly fluorescent than the roots. The degradation of lignin (an important constituent of paper made from high yield pulp) is due to photochemical reactions of carbonyl groups (including quinones), phenols, stilbenic derivatives, etc. Fluorescence microscopy shows that the paper exhibits a photochromic effect and that bleaching the paper with reducing agents leads to an increase in the fluorescence emitted at 400 nm. The most effective way of retarding the photoyellowing process is to apply a UV screen plus a reducing agent.

The Bleaching and Photostabilization of High-Yield Pulp by Sulfur Compounds. I. Reaction of Thioglycerol with Model Quinones

The reaction of thioglycerol with quinones selected to model the chromophores present in high-yield pulp has been investigated. The reaction is rapid and complex, involving initial Michael addition, aromatization of the adducts to hydroquinones, and reoxidation of those hydroquinones to new chromophores. The Michael addition is responsible for the bleaching effect of thioglycerol upon pulp, and the reoxidation is responsible for the eventual development of more intense color by treated pulp. Semi-empirical and ab initio molecular orbital calculations have been used as an aid to rationalizing the experimental results.

Optimization and fundamentals of high-yield pulping with ethanol

A new high-yield pulping process based on ethanol pretreatment and steam cooking was studied. The effect of processing conditions (cooking temperature at six levels and ethanol concentration at five levels) was determined. Optimum results appeared at 30% ethanol concentration and with cooking at 195 °C. The cooking liquor was analyzed by GC-MS and low-molecular lignin fragments were identified. Alcohol in alkaline medium destroys the three-dimensional lignin structure by cleavage of ether bonds, increases the penetration of the impregnation solution (higher sulfonation) and solubilizes lignin. Ethanol treatment facilitates fiber separation in the middle lamella. Pulp prepared under optimal conditions has higher flexibility, conformability and better bonding ability when compared with other samples. Higher concentrations of ethanol (above 30%) might cause precipitation of lignin on the fiber surface.