2009 Volume 2

Despite the technical importance of mechano-sorptive creep in paper, the exact mechanism behind this phenomenon is still not fully understood. In this study it was shown that the mechanosorptive creep of paper sheets can be significantly reduced by chemical cross-linking through periodate oxidation. The mechanism behind this reduction has been examined through creep measurements of both sheets and individual fibres. For sheets the creep acceleration due to varying humidity was significantly reduced by the chemical cross-linking. For single fibres, however, the creep acceleration was not affected by the chemical crosslinking. In fact the absolute creep rate for the periodate oxidised fibres were higher than that of the reference fibres. This clearly showed that the improvement in mechano-sorptive creep found on a sheet level does not originate from an improved creep resistance for individual fibres but rather from mechanisms operating at the fibre network level. Hygroexpansion and moisture sorption of the sheets during the humidity cycling used for creep testing have also been measured, and the results showed that both was reduced by the periodate oxidation. Reduced moisture sorptivity and hygroexpansion probably minimises stress concentrations at the fibre network level and thereby also the creep acceleration.

A smooth web transfer in a paper machine requires sufficient tension. It is well known that excessive tension leads to web breaks. However, too low a tension can also be fatal. Particularly with wet web, the maintenance of the tension is challenging due to the fast relaxation of the tension. Some factors that affect the tension and relaxation of wet web were studied in this paper.

The initial tension and the tension after constant relaxation time, called residual tension, was found to depend on factors such as the applied strain, straining rate, dry solids content, fibre and fines properties, substances in the white water, and the dry strength chemicals. The residual tension was reduced by increased straining rate, addition of TMP filtrate, and addition of cationic starch. The tension and residual tension seemed to be dependent on both the properties of the fibre fraction, such as in- and out-of plane stiffness, and on the factors affecting the stress transfer conditions at the inter-fibre contact areas.

The orientation of the fibres in a paper directly influences many of its properties. The focus of this work was to predict the fibre orientation distribution and tensile stiffness distribution of a paper. The predictions were based on a proposed link between the two distributions and physical parameters measurable on the paper, no fitting parameters.

The fibre orientation distribution in paper was approximated by a probability density function. Both curve fitting type of distribution functions earlier used in paper physics and physical based functions derived from Fluid mechanics, Orthotropic analysis and a simple Stress/strain analysis were evaluated. The physical based functions used one measurable physical parameter, the fibre orientation anisotropy. The tensile stiffness distribution was predicted with a distribution function from the literature and functions derived from the Fluid mechanics and Orthotropic analysis approach. The predictions needed two measurable physical parameters, the MD and CD tensile stiffness.

Predictions of fibre orientation distribution and tensile stiffness distribution for restrained dried papers were compared with experimental data from restrained dried oriented handsheets with varying fibre orientation anisotropy. General approaches valid for all papers were compared with experimental data from pilot made papers with different drying restraint history. Both the predicted results for fibre orientation distribution and tensile stiffness distribution showed good agreement with experimental data.

Results in the literature disagree regarding the effect of drying temperature, final drying time, and drying constraint history, respectively, on the in-plane tensile properties of paper materials. Furthermore, it is debated whether the in-plane tensile properties are controlled by the final drying stress or by the total strain during drying.

In this work, the drying mechanics of a pilot machine-made paper grade was studied. Wet paper sheets were collected after the wet press section. The sheets were dried in a laboratory dryer using different drying constraints. The supplied heating power, the ambient climate, and the ventilation of the paper sheets were controlled during the drying trials, which made it possible to independently alter the drying temperature and the final drying time. The dried sheets were conditioned in 23°C and 50% RH before tensile testing.

The results showed that the tensile stiffness, tensile strength, strain at break, and tensile energy absorption of the dried sheets, respectively, were linearly related to the total strain during drying of the sheets. These linear relations were shown to be unaffected by drying temperature, final drying time, and drying constraint history. On the other hand, the corresponding relations between the in-plane tensile properties and final drying stress were found to be both non-linear and greatly dependent on the drying constraint history.

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The thermal stability of Alkaline phosphatase enzyme (ALP) adsorbed on paper was measured using a colorimetric technique quantifying the intensity of the product complex. ALP adsorbed on paper retains its functionality and selectivity. Adsorption of ALP on paper increased the enzyme thermal stability by 2 to 4 orders of magnitude compared to the same enzyme in solution. Complex patterns of enzyme were also printed using a thermal inkjet printer on paper. Microfluidic channels were printed on paper to demonstrate the concept of paper-based bioassays as diagnostic devices. Paper is an ideal material for functional materials for functional bioactive surfaces.

Interactions between a hydrophobic probe particle and surfaces with nanoscopic surface features have been investigated. Such surfaces were prepared by polishing or by spin-coating of nanoparticles. The surface topography was characterized by AFM, using the methods of high-resolution imaging, low-resolution imaging using the probe particle, and by the rolling ball method. The polished surfaces display sharp nanoscopic peaks and hardly any crevices. In contrast, the spin-coated surfaces can be characterized as nanostructured, due to the high density of nanoparticles that on a short length scale provides a regular pattern of crevices and hills. On all surfaces a larger waviness is also distinguished. In all cases the dominant force at short separations was found to be a capillary attraction due to the formation of an air/vapour condensate. Our data show that the large-scale waviness of the surface does not significantly influence the range and magnitude of the capillary attraction, but large local variations in these quantities are found. The large variation in adhesion force corresponds to a small variation in local contact angle of the capillary condensate at the surfaces. The report discusses how the nature of the surface topographical features influences the capillary attraction by influencing the local contact angle and by pinning of the three phase contact line. The effect is clearly dependent on whether the surface features exist in the form of crevices or as extending ridges.

When the percentage of filler is increased in paper, the optical properties are improved and the production cost lowered. However, fillers weaken paper strength by decreasing the fibre-fibre bonded area. Little is known about the optimum filler floc size or filler floc properties to allow developing optimum paper characteristics. Consequently, the floc structure and strength of precipitated calcium carbonate (PCC) aggregates was studied using various polymers (flocculants and dry strength agents): by static light scattering/diffraction (SLS), real time fluorescent video imaging, image analysis, photometric dispersion analysis (PDA) and scanning electron microscopy (SEM).

It was found that PEO/cofactor induced PCC aggregates were weaker at high shear and far more irreversible than those induced by the partially hydrolysed polyvinyl formamide copolymerised with acrylic acid (PVFA/NaAA) or C-starch. Flocs produced at low polymer dosages were smaller and weaker than those produced at higher dosages. The number of discrete PCC particles in aggregates was measured using real time fluorescent video imaging combined with image analysis.

Fundamental material science investigations of superhydrophobicity in recent years has evolved toward industrial applications and recently to papermaking and packaging. The present study concerns both fundamental and applied aspects of superhydrophobicity. An industrially viable process for a one-step waterborne superhydrophobic coating was developed. It is shown that different measures of the degree of superhydrophobicity are needed depending on the final application whether this may be self-cleaning or stain repellent action. Fundamental aspects of superhydrophobicity were investigated using silica wafers roughened by a particulate formulation containing nanosize silica particles, which were fixed to the substrate by calcination. After hydrophobization by silylation, the forces between a colloidal superhydrophobized silica probe, made according to a similar procedure, and these surfaces were measured by Atomic Force Colloidal Probe Microscopy. The results show an extremely long range interaction force and a large influence of surfactant and surfactant concentration. The results would prove useful in designing robust superhydrophobic application in the papermaking and packaging industry and also imply that coating and printing technique could be used for controlled deposition of superhydrophobized layers or areas.

Thermal conductivity of paper coating structures can be regarded as an important property for many processes involving the application of thermal energy on coated papers. This work analyses the thermal conductivity of coatings in terms of their structure. A Monte Carlo simulation-based particle deposition was used to create idealised two-dimensional coating structures. They acted as a master template for the superimposed parameters of a Lumped Parameter Model for the calculation of thermal conductivity, in which pigment and binder are treated as separate solid phases within a fluid (air). Binder alone was initially assumed to provide the necessary thermal connectivity. Comparison of the numerically calculated conductivities with corresponding experimental results, obtained from ground calcium carbonate pigment structures, showed generally lower calculated conductivities and clear differences in the change of conductivity when increasing latex binder content. Two different mechanisms are suggested as the cause of this lack of correlation. Firstly, it is shown that both the simulation and the current Lumped Parameter Model do not account sufficiently for pigment connectivity. This is the reason for the underestimation, especially evident when no binder is present. The nature of pigment connectivity is related to polymer dispersant on the pigment surface and the surface crystallite planar structures, if present, mostly related to larger particles. Secondly, it is confirmed that surface and colloid chemistry factors cause binder to accumulate first at pigment nodal points, which causes a disruption of the pigment packing already at 6 w/w% binder. This creates in homogeneity in the real coating structure which is not accounted for by the homogeneous assumption of the model. It could be shown that an introduced parameter of pigment connectivity becomes lower for the binder concentrations for which pigment disruption occurs. It is shown that the method is sensitive enough in respect to refinement of both pigment and latex connectivity factors to allow identification and parameterisation of the subtleties occurring in real colloidally interactive particulate systems that are reflected in the thermal conductivity response of the dried coating structure.