2005 Volume 2

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.

Paper is a complex three-dimensional network of fibres, pores and often fillers. The main goal of this study is to characterise its structure in a non invasive and non destructive way. In order to overcome the limitations of 2D measurements, Synchrotron Radiation microtomography is used to visualise the samples. Coupled with appropriate processing tools, it allows a quantifica-
tion of some structural characteristics on the samples. This is the main aim of this paper. Different articles relate the feasibility of such a study for paper samples: imaging at the European Synchrotron Radiation Facility (ESRF, France) in phase contrast [1] or in absorption mode [2] gives the opportunity to reach a pixel size smaller than a micron which can not be obtained with classical tomographs [3]. A first step is to visualise the structure then, structural parameters are extracted from these 3D data [4]. However, this requires a segmentation technique adapted for application to typical paper samples that are constituted of three phases. The first step consists in the segmentation of the different phases, namely, air, fibres and fillers. The amount of each component may be evaluated. This was validated for both the porosity and the filler content. Furthermore, structural parameters were calculated from the binarised volumes. The comparison with the published results validates the calculation.

A new experimental technique is presented that allows the direct observation of fibre deformation during wet pressing. Pulp fibres were wet pressed onto a glass slide and the region where two fibres crossed was examined microscopically through the glass. While the underlying fibre was in contact with the glass slide down its length, the overlying fibre must span from the top of the fibre to the glass slide. The geometry of the intersection is controlled by both the local conformability of the overlying fibre and the deformability of the underlying fibre. It is not primarily con-
trolled by the longitudinal flexibility of a fibre. The method provides new opportunities to investigate the effect of mechanical and chemical treatment on the papermaking properties of pulp
fibres in both the dry and wet state.

We provide a simulator for a range of bivariate stochastic processes of various application in the physics of stochastic fibrous networks. We illustrate the effects of local correlation on the statistics of voids in the bulk and the surface of fibre mats in general and paper in particular. The reference case of random isotropy has an inherent ‘ground-state’ correlation of adjacent free-fibre-lengths; this explains the classical observation of Corte that pores seem mainly ‘roundish’ in real paper samples. In the isotropic case, the mean pore radius can be reduced from that in a random network by 20% through structural changes associated with increased flocculation. The mean eccentricity of pores seems to give a measure of the variability in free-fibre-length distributions that is not due to local correlation. We find a uniform effect of local correlation on mean pore eccentricity over a range of stochastic network structures; at a given correlation, increased flocculation increases mean eccentricity slightly.

This paper describes the development of a method for mapping the apparent density of paper, nonwovens and other fibrous webs based on non-contact laser profilometry and β-transmission radiographic imaging. The method is applied in three complimentary studies that examine the in-plane non-uniformity of thickness, grammage and apparent density in printing papers. The first section focuses on the development and verification of the analytical method for mapping thickness. Through simultaneous scanning of the topography of both sides of the specimen by opposing range sensors, the surface contour of each side, the local thickness, and the out of plane deformation were measured. It was demonstrated that the laser based method provides results that closely approximate the intrinsic thickness that is independent of paper formation. The method was then used to examine the structural differences in laboratory sheets pressed by soft and hard nip calenders. The well known difference in web densification mechanisms of the two was reaffirmed by mapping discrete changes in thickness and statistically comparing these with corresponding points on grammage maps. Densification was shown to be dependent on grammage for the hard calender, and independent of grammage for the soft calender. The final study used the thickness mapping method to monitor the hygroexpansivity of representative printing papers as equilibrium humidity was varied between 9% and 80%. Thickness maps were obtained for newsprint, SC-A (calendered and uncalendered), bulk offset and office copy. Differential thickness maps were used to compare the in-plane non-uniformity of hygroexpansion. The inplane hygroexpansion was characterized and corrected for using a recently developed algorithm known as Enhanced Digital Image Correlation (EDIC). Thickness change did not appear to correspond to grammage maps. The results suggest that a significant irreversible increase in thickness occurs for papers that are heavily calendered.