Mid Sweden University

Sulphonic acid analyses were performed on wing refiner chemithermomechanical pulps, low sulphite charge pulps, thermomechanical pulps and on wood samples using a new method referred to as the niacin method. This method gives sulphonic acid content comparable to those estimated with the quinoline method or the Schoniger method. The method gives low interference with carboxylic acids and dissolved substances in the pulp if the pH is kept sufficiently low and the pulp is well washed.

To reach maximum effect of different pretreatments we need to know how wood properties can be changed and how this can be related to both refining conditions and pulp characteristics. To understand how the material properties were affected, sulphonated wood samples were tested using several new testing techniques. The data was correlated to pulp properties of batch refined chips to learn more how the initial defibration mechanisms and pulp properties were affected by the pretreatments.

The chip-refining stage in the production of high yield pulps as TMP and CTMP determines a large part of the optical and mechanical characteristics of the pulp. Softening of the raw material influences where the fiber walls fracture sites will be located.

In the thermomechanical pulping (TMP) processes, refining is performed at temperatures close to the lignin softening temperature, which normally leads to fractures located in the in the primary wall and outer parts of the secondary wall. When wood material is chemically treated before chip-refining the position of the fracture is altered due to that the softening and swelling properties of the fiber walls are changed. In the chemi-thermomechanical (CTMP) process most of the fractures are, therefore, located in the middle lamella or in the primary wall due to introduction of charged groups in the lignin which facilitates the fiber separation. At alkaline pH phenolic lignin structures are sulphonated, at lower pH non-phenolic structures in the lignin are also sulphonated. In the mechanical pulping processes the wood material is subjected to both low strain rates (e.g. plug screw treatment) and high strain rates (refining). Since wood is a viscoelastic material it behaves differently at different strain rates.

In this study, sulphonation has been carried out using different sulphite concentrations and pH-levels and we have thereby influenced the sulphonation degree as well as where in the lignin the sulphonation takes place.

We used a hydraulic testing machine for low strain-rate testing and a Split-Hopkinson pressure bar device for high strain-rate testing to categorize pretreatments according to their material softening effect and the energy needed for fiber separation. This gives us increased fundamental knowledge of how the mechanical properties of wood are affected by the sulphonation in order to develop new/improved pretreatments.

The pH-profile in the hydrogen peroxide bleaching stage of a Norway spruce thermomechanical pulp was levelled out by using carbon dioxide in combination with sodium or magnesium based alkali sources. Addition of carbon dioxide when magnesium hydroxide was used as the alkali source increased the pulp brightness with up to 1.5 brightness units when the bleaching was performed at 70 °C. When the temperature was increased from 70°C to 90 °C the brightness was increased with about two brightness units, addition of carbon dioxide increased it further. The residual hydrogen peroxide and the chemical oxygen demand (COD) levels were not affected as a consequence of the carbon dioxide addition. In the experiments where sodium hydroxide was used as the alkali source, addition of carbon dioxide lowered the pH that resulted in a decreased COD level, a slightly lower brightness level and, to some extent, an increased residual hydrogen peroxide level.

To predict the brightness reduction in a chemithermomechanical process as well as the bleachability of pulps produced from different species of wood raw materials there are, at present, no shortcuts available. Pulps have to be manufactured in pilot or mill scale and bleached; evaluating and predicting the bleachability has consequently been very resource consuming and thus expensive - until now! Due to this, we have used a laboratory method based on 60 × 60 mm wood shavings making it possible to predict the bleachability of primarily chemithermomechanical pulps but also other mechanical pulps. Our experiments showed that retention time and impregnation temperature appears to have no impact on the final brightness of birch shavings. When being subjected to a simulated chemithermomechanical pulp process, eucalyptus exhibited an increase in chromophore content before the bleaching stage. In addition, eucalyptus suffered from severe alkaline darkening. After the bleaching stage, the alkaline darkening diminished. A higher retention temperature resulted in lower brightness of the eucalyptus shavings compared to when using a lower retention temperature. Birch reached the highest final brightness levels and had a higher overall yield too.

Thin sections of wood, i.e., shavings, were used in spectroscopic measurements studying the formation and elimination of chromophores under mechanical pulping conditions. Light absorption coefficients, light scattering coefficients, and brightness were monitored.The method was applied to study how temperature in the absence of mechanical energy affects wood brightness. An increase in absorption coefficient at 420 nm indicates formation of ortho-quinone structures. By applying mechanical pulp processing conditions, the light absorption coefficient increased in the 400-500-nm wavelength region, probably due to the formation of chromophores in the lignin. Comparison of heat treated wood shavings with mechanical pulps indicated that additional chromophores were created during the mechanical pulping processes.The method was also used in studying how high-temperature hydrogen peroxide bleaching of the wood shavings affects the light absorption coefficients. The light absorption spectra reveal information about chromophore elimination in wood chemical components during hydrogen peroxide bleaching at slightly pressurized conditions. This is relevant to processes, such as the alkaline peroxide mechanical pulp process, in which the wood chips are impregnated with bleaching chemicals before refining. The easy penetration of chemicals into the wood shavings ensures rapid and even distribution of the treatment chemicals. A change in light absorption coefficient at shorter wavelengths (400-500 nm) was observed.

When processing wood to obtain pulp, the fibers are subjected to high temperatureat moisture levels and to the influence of metal ions. Many process developmentshave resulted in utilization of higher temperatures that induce chemical reactionsin the wood fibers. This thesis deals with some aspects of using high temperaturesin pulping processes. The first part of this work concerns how carbohydrates areaffected, in terms of discoloration and degradation, by high temperature, moisture,and the presence of metal ions. The next part deals with the metal ion profiles inthermomechanical pulp (TMP) processes and the effects of the high temperaturetreatment of native wood under TMP processing conditions. The third sectionconcerns processing conditions present in the high‐temperature hydrogenperoxide bleaching of TMP.In experiments using cotton linter sheets impregnated with glucuronic acid anddifferent metal ions, increased 5‐(hydroxymethyl)furfural and 2‐furaldehydeconcentrations were accompanied by decreased viscosity and increaseddiscoloration when Mg2+ ions and, to some extent, Ca2+ ions were present. Thisindicates that under certain conditions, in this case, dry heat‐induced ageing underslightly acidic conditions and with access to oxygen, the presence of alkaline earthmetals can cause substantial degradation and discoloration of cellulose. Thisindicates that Mg2+ ions can promote the Lewis‐acid‐catalyzed degradation ofcellulose. When glucuronic acid was added to the cotton linter sheets, the presenceof Ca2+ or Mg2+ ions decreased the discoloration caused by the dry heat‐inducedageing of glucuronic acid and also led to a smaller decrease in viscosity comparedto that in cotton linter sheets containing only Ca2+ or Mg2+ ions. This may explainwhy the negative effect of Ca2+ or Mg2+ ions on viscosity is less pronounced incertain pulps.

In mechanical pulping processes, the addition of sodium silicate and kaolin clayand re‐circulation of water from the paper machines increased the levels of ironand aluminum ions. In a mill using dithionite bleaching, the levels of iron andaluminum ions in the final pulp were approximately ten times higher than in a millusing hydrogen peroxide bleaching. This was probably due to extraction of moreiron and aluminum ions from the clay to the water due to low pH and to thecorrosion of process equipment induced by thiosulfate ions derived fromdecomposed dithionite. In laboratory trials, it was impossible to completelyremove iron from these pulps using diethylenetriaminepentaacetic acid orextraction with hydrochloric acid. This was probably due to the strong binding ofiron to the wood fiber or its presence as oxide/hydroxide clusters. The mostimportant issue is to avoid contaminating the pulp with iron ions; process waterand process chemicals with low iron contents should therefore be used. Especiallyin the dithionite bleaching process, iron contamination increases thedecomposition of the dithionite, causing corrosion of the process equipment andadding further iron to the pulp.The shavings method is a straightforward method for studying the formation andelimination of chromophores in wood. Light absorption coefficients, lightscattering coefficients, and brightness can easily be obtained using this method.The method was used for studying how temperature in the absence of mechanicalenergy affects wood brightness. This provided useful information about the effectof process conditions in mechanical pulping and its relationship to chromophoreformation. An advantage of using thin shavings is the rapid penetration ofchemicals into the fibers. The shavings method was therefore also used in studyingthe kinetics of hydrogen peroxide bleaching at high temperature.The high‐temperature (HT) hydrogen peroxide bleaching of a thermomechanicalpulp was carried out in a pressurized laboratory‐scale wing defibrator. Theexperiments demonstrate two ways to obtain high brightness under HT conditions.One way is two‐stage peroxide bleaching with a low total alkali/peroxide ratio inthe first stage to avoid alkaline darkening. The second way is to pre‐impregnatethe pulp with hydrogen peroxide before adding alkali in two stages. Themaximum brightness was reached after only 2.5 minutes at low alkali charge.Thus, the bleaching time can be short and the total alkali charge low under hightemperatureperoxide bleaching conditions. To achieve very high brightness usingHT hydrogen peroxide bleaching, other measures than increasing the alkali chargeand prolonging the bleaching time are necessary.

The influence of Mg2+, Ca2+, Fe3+ ions and glucuronic acid on the dry and humid heat-induced (120180C) discoloration of cotton linter sheets was investigated. The quantity of the added metal ions was similar to that of metal ions found in wood and pulp. Heat-induced ageing (120150C, pH 5) under dry conditions caused only slight discoloration of the reference sheets and of the sheets impregnated with the metal ions. However, during ageing at 180C, the presence of Mg2+ ions accelerated the discoloration. An initial pH in the range of 38 had no great influence on the discoloration of the reference sheets, but a low pH in combination with Mg2+ ions increased the discoloration drastically. Under humid ageing conditions at 180C (<30 min), the sheets discolored approximately twice as much as under dry conditions; the difference in discoloration caused by the metal ions was less than in dry ageing. Adding glucuronic acid to the sheets caused extensive discoloration under dry ageing conditions (120180°C), while Ca2+ and Mg2+ ions decreased the glucuronic acid induced discoloration. Under humid conditions (180°C), however, Mg2+ and Ca2+ ions did not prevent the discoloration caused by glucuronic acid. An increase in the absorption coefficient was observed at 285 nm, most likely due to the formation of 5-hydroxy-2-methyl-furaldehyde (HMF) and 2-furaldehyde (furfural). GC-MS analysis reveals increased formation of HMF in the sheets impregnated with Mg2+ ions. Degradation of the cellulose in which the alkaline earth metals act as Lewis acids is proposed.

We investigated the influence of Mg2+, Ca2+, and Fe3+ ions and glucuronic acid on the viscosity, after dry or humid ageing, of two different cellulosic materials, cotton linters and dissolving pulp. The quantity of added metal ions was similar to that found in pulp. In the cotton linters sheets, especially in those containing Mg2+, increased 5-hydroxy-2-methyl furfural (HMF) and 2-furaldehyde (furfural) concentrations were accompanied by decreased viscosity under dry conditions at 180C (pH 5). This indicates that Mg2+ ions can promote the Lewis-acidcatalyzed degradation of cellulose. For cotton linters and dissolving pulp sheets, adding Ca2+ and Mg2+ ions is detrimental when ageing at 180C under dry conditions (pH 5). Adding glucuronic acid increases the viscosity of cotton linters sheets, especially when Ca2+ and Mg2+ ions are present. Due to formation of complexes between the Ca2+ and Mg2+ ions and glucuronic acid, these ions reduce pulp viscosity to a smaller degree.