Review Articles

This review will focus on recent progress regarding the mechanisms of light-induced discoloration of mechanical and chemimechanical pulps and on the proposed preventive treatments. It is evident that the mechanisms behind photoyellowing of lignin-rich pulps are complex and that several types of reaction pathways may coexist. Photoyellowing proceeds via one initial fast phase and a slower following phase. The fast phase has been ascribed to oxidation of free phenolic groups and/or hydroquinones and catechols to photoproducts of mainly quinonoid character. A multitude of reactions involving several lignin subunits are possible. Important intermediates are phenoxyl radicals, and to some extent ketyl radicals. The importance of the phenacyl aryl ether pathway might be more important than previously thought, even though the original content of such groups is low in lignin. Even though many preventive methods against photoyellowing have been suggested, no cost-efficient treatment is available to hinder photoreversion of lignin-containing paper permanently. Suggested methods for stabilization include chemical modification (etherification and esterification), coating the paper product, addition of radical scavengers, excited state quenchers, or ultraviolet absorbing compounds.

This review describes the role of wood extractives, especially fatty and resin acids, in papermaking, as well as the importance of their removal from process waters. One of the main aims is also to illustrate versatile analysis methods for this purpose and highlight recent developments in corresponding applications. Most of the current methods require time-consuming and laborious sample pretreatment procedures prior to gas chromatography coupled either with flame ionization or mass selective detection. However, some faster, even online techniques with minimum sample pretreatment, are also available, mainly including high performance liquid chromatography coupled with mass spectrometry. The advantages and disadvantages of all analytical procedures are briefly discussed.

This review summarizes the proposed mechanisms for irreversible coalescence of cellulose microfibrils within fibers during various common industrial treatments for chemical pulp fibers as well as the methods to evaluate it. It is a phenomenon vital for cellulose accessibility but still under considerable debate. The proposed coalescence mechanisms include irreversible hydrogen bonding. Coalescence is induced by high temperature and by the absence of obstructing molecules, such as water, hemicelluloses, and lignin. The typical industrial processes, in the course of which nano-scale coalescence and possible aggregation of cellulose microfibrillar elements occurs, are drying and chemical pulping. Coalescence reduces cellulose accessibility and therefore, in several instances, the quality of cellulose as a raw material for novel products. The degree of coalescence also affects the processing and the quality of the products. For traditional paper-based products, the loss of strength properties is a major disadvantage. Some properties lost during coalescence can be restored to a certain extent by, e.g., beating. Several factors, such as charge, have an influence on the intensity of the coalescence. The evaluation of the phenomenon is commonly conducted by water retention value measurements. Other techniques, such as deuteration combined with FTIR spectroscopy, are being applied for better understanding of the changes in cellulose accessibility.

Dissolved and colloidal substances (DCS) in the process waters of paper machine systems can interfere with the retention of fine particles, retard the drainage of water from the wet web, and generally hurt the intended functions of various polyelectrolytes that are added to the process. This review considers publications that have attempted to characterize the nature and effects of different DCS fractions, in addition to some of the ways that paper technologists have attempted to overcome related problems. The consequences of DCS in a paper machine system can be traced to their ability to form complexes with various polyelectrolytes. Such tendencies can be understood based on a relatively strong complexing ability of multivalent materials, depending on the macromolecular size and charge density. Continuing research is needed to more fully understand the different contributions to cationic demand in various paper machine systems and to find more efficient means of dealing with DCS.

In the production of paper, especially when using mechanical pulp or recovered wood-containing paper, a large amount of dissolved and colloidal substances (DCS), mainly composed of hemicelluloses, extractives, lignans, and lignin-related substances, are released from the pulp and dissolved or dispersed into the process water. The accumulation of DCS during the papermaking operations due to the closure of process water systems gives rise to various detrimental impacts on the papermaking process and the resulting paper products. Thus it is indispensible to remove or control the DCS in order to overcome or alleviate their negative influences. This review emphasizes recent advances in control of DCS by physical, chemical, and biological methods. The widely used fixatives such as aluminum sulfate, poly-(aluminum chloride), polyamines, polyvinyl amine, and highly cationic starch, as well as their functional effectiveness, mechanism, influencing factors, and influences on paper products are considered. Simultaneously, biological treatments including fungal treatment and enzymatic treatments with lipase, pectinase, laccase, and immobilized enzymes, are also assessed in detail. DCS control has been an important way to improve the runnability of paper machines and the quality of wood-containing paper and recycled paper products. Advances in DCS control are likely to create additional benefits to the papermaking industry in the coming years.

The cleavage of lignin bonds in a wood matrix is an important step in the processes employed in both the biorefinery and pulp and paper industries. β-O-4 ether linkages are susceptible to both acidic and alkaline hydrolysis. The cleavage of α-ether linkages rapidly occurs under mildly acidic reaction conditions, resulting in lower molecular weight lignin fragments. Acidic reactions are typically employed in the biorefinery industries, while alkaline reactions are more typically employed in the pulp and paper industries, especially in the kraft pulping process. By better understanding lignin reactions and reaction conditions, it may be possible to improve silvicultural and breeding programs to enhance the formation of easily removable lignin, as opposed to more chemically resistant lignin structures. In hardwood species, the S/G ratio has been successfully correlated to the amount of β-O-4 ether linkages present in the lignin and the ease of pulping reactions.

UV weathering, a process initiated primarily by the ultraviolet portion of the solar spectrum, causes surface degradation of wood. Additionally, the wetting and drying of wood through precipitation, diurnal and seasonal changes in relative humidity, abrasion by air particulates, temperature changes, atmospheric pollution, oxygen, and human activities, all contribute to the degradation of wood surfaces. Photo-oxidation or photo-chemical degradation affects only the wood surface, starting immediately after exposure to sunlight. Understanding the chemistry of UV degradation of wood requires knowledge of the chemical nature of wood components, the UV spectrum, and the interactions of UV radiation with various chemical structures in wood. Chemical changes can be evidenced by FTIR spectroscopy. Previous study has shown that wood chemical modification with succinic anhydride makes it slightly more stable to the artificial light action than non-modified wood, which might be due to a slight increase in lignin stability to the polychromatic light action. Analysis of color changes on coated wood surfaces for modified wood treated with epoxidized soybean oil (ESO) has shown that lightness (ΔL*) decreases, whereas a*, b*, and ΔE* increase with increasing irradiation time.

Cellulose derivatives, i.e. cellulose functionalized in a solvent state with various side groups, are an important source of biomaterials for food packaging. This review considers the following materials: i) cellophane, ii) cellulose acetate, iii) methylcellulose, and iv) carboxymethylcellulose. Mechanical and barrier properties are important for freestanding packaging films as well as for coatings. The potential of the selected cellulose derivatives and cellophane is thus examined from the viewpoint of their tensile properties as well as their moisture and oxygen barrier properties. The capacity of microcrystalline cellulose and nano-sized celluloses to reinforce the films and to help impede gas diffusion is examined for microfibrillar celluloses, nanocrystalline celluloses, and whiskers. Very good oxygen barrier properties have been reported for cellophane. Nanocellulose fillers have regularly been shown to enhance the tensile properties of several cellulose derivatives, but the effects on the water vapor permeability (WVP) have been studied less often.

Water-insoluble oils, including crude petroleum and a wide variety of refined organic liquids, can cause major problems if spilled or leaked to aqueous environments. Potential environmental damage may be reduced if the spilled oil is promptly and efficiently removed from the water. This article reviews research that sheds light on the use of cellulose-based materials as sorbents to mitigate effects of oil spills. Encouraging results for oil sorption have been reported when using naturally hydrophobic cellulosic fibers such as unprocessed cotton, kapok, or milkweed seed hair. In addition, a wide assortment of cellulosic materials have been shown to be effective sorbents for hydrocarbon oils, especially in the absence of water, and their performance under water-wet conditions can be enhanced by various pretreatments that render them more hydrophobic. More research is needed on environmentally friendly systems to handle oil-contaminated sorbents after their use; promising approaches include their re-use after regeneration, anaerobic digestion, and incineration, among others. Research is also needed to further develop combined response systems in which biosorption is used along with other spill-response measures, including skimming, demulsification, biodegradation, and the use of booms to limit the spreading of oil slicks.

In this review, fundamental wood machining research is evaluated to determine the general cutting mechanics of simple, orthogonal, and oblique cutting tools. Simple tool force trends and chip formation characteristics are indentified here, along with the cause and effects of tool wear. In addition to this, specific methods of evaluating sawing processes have been investigated. These include the use of piezoelectric dynamometers to record tool forces and high speed photography to evaluate chip formation. Furthermore, regression analysis has been previously used to identify tool force trends with respect to both tooth geometry parameters and work-piece properties. This review has identified the original findings of previous research. This will allow for further original research to be conducted.