Proceeding Articles

Traditional micromechanical theories for the tensile strength of paper do not account for the tensile stiffness of paper, even though in practice tensile strength is closely coupled with tensile
stiffness. Another problem is with the micromechanical input parameters, few of which have a precise meaning in real paper. Especially the interpretation of inter-fiber bonding is ambiguous.
None of the existing theories connects tensile strength with an independently measurable value of bonding degree or bond strength. As a result, the conventional interpretation of tensile strength data is unreliable.

In this paper we will present a “macromechanical” study that connects tensile strength with independently measured values of tensile stiffness and z-directional fracture energy (alternatively
Scott bond or z-directional tensile index). The model expression agrees well with many – but not all – of the experimental datasets that we had available. The disagreements demonstrate that z-directional measurements do not capture some aspects of inter-fiber bonding that contribute to in-plane tensile strength.

Fiber deformations and damage have a considerable influence on both fiber strength and network properties. Through their influence on the fiber network, they can also affect the way paper behaves during converting. Another aspect is their influence on end-use properties, again via their effect on the fiber network.

Separating the effects of fiber deformations and damage is often difficult. We prepared pulps in the laboratory and subjected them to different treatments that change the two relatively
independently in a controlled manner. We found that wet/dry zero-span fiber strength is not dependent on the deformation method or on the extent of the deformations. Neither pulp sheet
density nor Scott bond bonding was greatly affected by the type or method of deformation. In the case of deformed pulps, the pulp sheet tensile properties were dependent on the extent of fiber deformation via fiber segment activation. The pulp that was damaged instead of deformed had fiber deformations to about the same extent as the deformed pulp but lower single fiber strength measured with wet/dry zero-span. It also had lower bonding ability and sheet density. The tear index for the unbleached damaged pulp was 20–25% higher than that of the reference pulps. Fiber deformations (curl and kinks) affect fiber network properties via the lack of fiber segment activation. However, they do not significantly influence fiber shrinkage potential (WRV) or fiber strength.

Digital image correlation was used to measure the full-field deformation of paperboard and handsheet tensile specimens. The correlation technique was able to accurately measure strain in
regions 0.6 by 0.6 mm. Results showed the variation of strain to be much larger than has been previously reported. For machine-made paperboard tested in the cross-direction, the variation of strain increased throughout the tensile test and became erratic near failure, indicating many local failures. The measured strain distribution can be characterized by a Weibull function in agreement with weak-link failure theories. The analysis of a handsheet tensile specimen with a low-grammage region, approximately 4mm wide, showed large negative strains near the region’s edge.

We have exposed pulp handsheets to two different degradative treatments, and compared their effects on strength properties. In accordance with earlier research, tensile stiffness and the shape of the stress-strain curve were independent of cellulose chain length and the fiber defects caused by the degradative treatments. When evaluated at the same viscosity, we found acid vapor treatment to be more detrimental to axial fiber strength than ageing treatment at elevated temperature and humidity. At the same mean fiber strength, acid vapor-treated handsheets show higher tensile strength. This is because acid vapor treatment is more heterogeneous than ageing treatment. The fiber network is able to compensate for the local defects, but not for the general degradation in fiber strength. In both treatments the mechanism for cellulose cleavage is assumed to be acid hydrolysis, the difference in the effect on fibers coming from the treatment conditions. Acid vapor treatment induces a fast reaction at defect sites and discontinuities in a fiber, while ageing treatment induces a slow, more homogeneous hydrolysis in fibers. Unlike axial strength, the Z-directional strength of softwood decreases only after harsh degradation when viscosity has dropped below 400 ml/g. The Z-directional strength of hardwood is already compromised at a viscosity of around 700 ml/g. The differences probably arise from differences in fiber ultrastructure.

This article examines the literature pertaining to the creep behavior of paper. The basic concept of creep, the terminology used to describe creep, and the various ways to present creep are introduced. This is followed by a historical overview of creep in paper. Using this framework, discussions centered on tensile, compressive, and accelerated creep are presented. For years,
research efforts have focused on accelerated creep. Because of this diversion, an acknowledged fundamental understanding of paper creep is lacking. Using previous data for tensile creep in constant humidity conditions, a rudimentary model of creep in paper is developed. The model clearly demonstrates that the role of bonding is accounted for simply with an efficiency factor that acts to magnify the stress. In addition to the impact of inter-fiber changes, intra-fiber effects resulting from hardening and wet-straining are demonstrated. It is suggested that compressive creep differs from tensile creep due to material instability. Accelerated creep is taken to be the result of moisture-induced load cycling. The result of this discourse is that to increase understanding, fundamental studies of creep behavior in constant conditions are and will be more fruitful than studies in cyclic humidity.

In this study, two sets of sheets were made with differing levels of specific bond strength and relative bonded area. One set of sheets were wet pressed using a high press load and the other set of sheets were wet pressed using a low press load. Within each set, the sheets were treated with either a debonder or a bonder or received no treatment. Creep compliance data showed that creep curves for the debonder, bonder and untreated sheets were the same for the sheets wet pressed at the high press load and different for the sheets wet pressed at the low press load. Creep failure time was influenced by the treatments in both the high and low load wet pressed sheets; sheets treated with debonder failed first and the sheets treated with bonder failed last. It was concluded that at high levels of bonding as is the case with the high load wet pressed sheets, differences in specific bond strength due to the treatments do not influence creep deformation because fiber-fiber bonding is at a level where the sheets are efficiently loaded structures. The low load wet pressed sheets showed differences in creep deformation when specific bond strength was changed with treatments because fiber-fiber bonding was at a lower level where the sheets were inefficiently loaded. As the loading efficiency of the paper structure is improved through increased fiber-fiber bonding (either by increasing specific bond strength or relative bonded area), an efficiently loaded structure can be achieved where fiber-
fiber bonding no longer affects deformation. This allows creep compliance to reach a minimum level which is dictated solely by the fibers. An efficiency factor can be used to describe deformation behavior where an efficiency of “1” indicates an efficiently loaded structure and lower values indicate a less than fully efficient structure, one in which fiber-fiber bonding influences deformation behavior. In this study, efficiency factors were used to scale the low load wet pressed sheet results and several sets of lesser refined and pressed sheets (thereby “removing” fiber-fiber bonding influence) and the data superimposed onto the high load wet pressed sheet results.

The purpose of this investigation was to study the effect of fibre shape and fibre distortions on the creep properties in constant and cyclic humidity and compare these data to other standard paper properties. The fibre shape and magnitude of fibre distortions were varied by low consistency beating and high consistency treatment of the pulp. The term virgin were used for these pulps. Furthermore the effect of drying history was investigated where papers from straight fibres were dried under restrained- and freely-dried followed by reslushing. The term re-dried were used for these pulps.

Fibre shape and fibre distortions of the virgin pulps had different effects on the creep stiffness. Straight fibres with few distortions had the highest stiffness values. By making the fibres curlier, the creep stiffness decreased significantly. The presence of fibre distortions in the straight fibres had no effect on the creep stiffness.

The creep stiffness of papers from re-dried pulps depended on the drying method that produced the fibres. Without beating, fibres of the freely-dried paper gave a lower stiffness than fibres of the restrained-dried paper. The fibres had a memory of the distortions that were introduced by the free drying. After beating, the difference due to the drying method disappeared.

Runnability of the paper web during production and converting is a topic which has always concerned the pulp and paper industry. Good runnability at the lowest possible production cost is of primary importance to paper producers, converters and printers. This paper will review the literature on runnability and fracture of dry printing paper webs.

Several review articles have been written on this and related subjects. Niskanen [1] gave a thorough review of strength and fracture of paper. Niskanen reviewed the relationship between fibres, bonds and strength. He also discussed the relationship between web tension and fracture frequency before thoroughly describing the development of fracture mechanics methods. Kortshot [2] and Mäkelä [3] have also given excellent reviews of paper fracture and fracture mechanics. Roisum [4,5,6] has written reviews of the runnability of paper.

It is important to remember that many causes for paper web breaks are quite trivial. Paper rolls are damaged by transport and handling. Direct contact with water or condensation due to rapid temperature changes may give damage. Poor tape gluing may give web breaks during the flying splice. For many such problems the best procedure for improving runnability is to keep
the paper mill tidy, the floors clean and even. Further to follow and quality check the paper transport. Avoid gravel on the floor of transport containers, adjust the clamping pressure on the trucks used to handle paper rolls and so on. Yet even if the best precautions are taken, there will always be some damaged and weaker zones in the paper web. Thus, it is meaningful to use
fracture mechanics as a tool to investigate if such defects will develop to a web fracture at the web tension conditions used.

Much work on improving runnability of paper has been done based on the assumption that if the paper’s tensile strength, tear strength or fracture toughness is increased, then even the runnability will be improved. I will discuss this assumption and argue that the best way to improve runnability is to perform an engineering analysis of the converting or printing applica-
tion where the fractures occur. The important factors in such an analysis are web stress, defect size distribution and mechanical properties of the paper.

The ability of adhesives to bond paper and paperboard is critical for most packaging and converting operations. Despite the huge body of literature describing both paper and adhesives technologies, there are only a few research papers describing paper/adhesive interactions. Described herein are the results of a systematic investigation of pressure sensitive adhesive (PSA) peeling from paper. The peel force versus peel distance curve depends upon the failure mode. A constant force is observed when the PSA cleanly separates from paper (i.e. interfacial failure) at low peel rate. By contrast, at high peeling rates, in the paper failure domain, the peel force climbs to a maximum and then relaxes to a steady-state value. The maximum peel force, which we call the peak force, corresponds to the fracture of the top layer of fibres during the initiation of paper delamination whereas the steady-state peel force occurs during the propagation of paper delamination.

To characterize the range of behaviors it is necessary to conduct a series of peeling experiments over an extended range of peel rates. The results are best analyzed by plotting the peak peel
force versus the peel rate on logarithmic axes giving what we call a peel map. For a broad range of tape/paper combinations, peel maps have similar shapes. The interfacial failure domain consists of a linear segment with a positive slope. This line intersects with a horizontal line segment at higher peel rates, corresponding to the paper failure domain.

Principal component analysis, a multivariate statistical analysis, of a large set of peel maps was used to reveal the influence of paper properties on peeling. The peak peel forces in the paper
failure domain correlated with standard paper properties linked to z-directional strength. The slopes of the peel maps in the interfacial domain were independent of paper properties but were sensitive to adhesive rheology. The absolute location of the interfacial segment of the peel map mainly was sensitive to the chemical composition of the paper surface and secondarily related to surface roughness. Water contact angles on paper were not good predictors of adhesion. Finally, we illustrate the utility of peak peel force in the paper failure domain as a measure of paper surface strength.

This paper introduces a new concept for digitizing the three dimensional paper structure, based on a fully automated microtomy process and light microscopy. The microscope can be moved in all three directions of space with high accuracy in order to be able to digitize large samples with high spatial resolution. All components are controlled by a PC interface which enables an
automated digitization process.

The literature concerning 3D analysis of paper structure is reviewed. Non destructive and destructive techniques are compared.

Image analysis algorithms for creation of a detailed digital representation are described. This digital data set is analyzed, to derive characteristics of the paper structure.

As a first example of possible applications the analysis of the 3D coating layer formation is presented. The coating layer is detected by means of image analysis based on a 3D color segmentation concept. Initial experiments on analyzing coated paper samples prove the applicability of the concept.

The correctness of the implemented sample digitization process and following image analysis was validated.