Proceeding Articles

The use of microfibrillated cellulose (MFC) in the paper industry has become established following many years of development by both academic and industrial researchers. Commercial installations typically use mechanical disintegration techniques such as refiners and grinders to convert aqueous suspensions of pulp fibres into a material consisting of fibrils and fibre fragments with diameters ranging from the nanometre to the micron scale. MFC suspensions of a few percent solids content show very high viscosity at low shear rates, but also very significant shear thinning behaviour, rapid viscosity recovery after shear and high filtration resistance. MFC added to paper furnishes at up to 5% by weight functions as a strength additive, enabling increases in mineral filler content, improvements in paper properties, reductions in weight and cost savings across a wide range of paper and board grades. As a complementary technology to pulp refining, addition of MFC offers process flexibility as well as improved wet web strength and runnability, reduced air permeability and increased z-direction strength. Although the fine fibrils of MFC do not dewater easily on their own, when added at low levels to paper their effect on machine drainage can be managed without loss of paper machine speed. In recent years, MFC has attracted much interest as a coating material. Layers or films of pure MFC show near-zero air permeability, high resistance to oil and grease and an effective barrier to organic vapours and oxygen. Mixtures of mineral particles and as little as 15% MFC provide an effective surface for water-intensive printing techniques such as flexography and inkjet. Application of MFC suspensions after the wet line of a papermachine has been demonstrated as a practical solution to obtain coatings, exploiting the rheological behaviour of the MFC to achieve excellent holdout onto a poorly-consolidated sheet, and using the vacuum and press sections of the machine to remove excess water. Further development and commercialisation of this technology, together with low cost MFC production and improved product characterisation, should ensure the continued growth of its use in the paper and board industry.

Mineral fillers are indispensable in many industrial branches and are used in a variety of different materials. In plastic technology they act as a classic extender to lower the costs of the production process but also as “active fillers” to improve the mechanical and optical property profile. In the paper industry, fillers are also used to reduce raw material costs and to adjust the optical as well as surface properties. However, fillers entail the problem that their application quantity is sharply limited. Inorganic fillers show no binding properties. They reduce the product strength with higher use and lead to complications in the further processing of the products (e.g. increased dust propensity during the packaging and printing processes). In order to expand the use of fillers and their positive effects on varying products and to prevent the negative effects of the material in parallel, mineralized cellulosic structures should be created for versatile applications in different branches of industry.

Cell and organoids culture in three-dimensions (3D) gel systems is important from a fundamental aspect for understanding the development and behaviour of body organs, and from a practical perspective for producing cells, tissues and even new organs for bio-medical applications. The cell culture requires a supportive network environment, biological or synthetic, which provides the suitable biological systems (proteins and co-factors), mechanical support (flexible morphology) and chemical composition for cells/organoids to grow, spread and migrate. Current naturally extracted matrices like, Matrigel and collagen, are expensive with poorly defined and variable composition; they are not reliable for common practice 3D organoids culture. To overcome issues with the naturally extracted matrices, researchers have been investigating and developing new synthetic and natural polymer gels as alternatives. Cellulose has emerged as an attractive matrix with strong potential for cell and organ culture in 2D and 3D networks. The inherent natural biocompatibility of cellulose fibres including non toxicity, low cost, and their ability to form flexible gels, provide a compelling alternative to the current limited and expensive animal-based matrices. This review focuses on the recent development of cellulose nano fibres (CNF) based gel matrices for 3D cell and organoids culture. The review highlights how functionalisation of CNF optimizes the gel structure, visco-elastic properties and composition for supporting cell growth, interactions, spreading and migration. The state-of-the-art characterisation methods are discussed to monitor CNF stiffness, strength, morphology and composition, and furthermore, cell culture and their stability in the CNF network. The knowledge gained from this review aims at supporting bioengineers in further developing the potential of CNF gels for different 3D organs culture and tissue engineering applications.

Today, simulation tools and digital twin models have taken a central role in product development at Tetra Pak. As a result, improved functionality and quality are secured in developing new packages, filling, and converting machines. First, an overview is presented on the development of paper models, followed by examples of how the paper models are used with simulation tools at Tetra Pak today. Such as the creasing and filling process related to material defects.

Paperboard is a common material for packages and other carriers of information. During rotary printing processes, the paperboard is subjected to rapid deformations in the out-of-plane direction as it passes through the nip between the rolls of the printer. Being viscoelastic in nature, the mechanical response of the material to high deformation rates differs from what is measured with conventional testing conducted at slower deformation rates. In this work, a device called the rapid ZD-tester is used to show the response of paperboards subjected to a rapid pressure pulse and compare this to measurements made at lower strain rates in a common universal testing machine. All the tested paperboards show complete recovery within 5 s when being rapidly compressed, while the slower compression to the same pressure leaves a deformation that remains after 5 s. The stiffness response differs between the paperboards, but does not consistently increase or decrease between slow or rapid compressions. The difference in response between slow and rapid compression appears larger for the low-density paperboard in the study. The time scales in the rapid ZD-tester are comparable to those in a printing press, and, therefore, evaluation of the material response of the paperboard measured by this device is relevant in the context of printing applications.

The perception of mechanical rigidity when touching a package is important for purchasing decisions. This perception will depend both on the material and geometry of the product packaging, but also on the position where the package is grasped. Both kinaestethic (globally) and cutaneous cues (locally around the fingertip) play a role in the perception of compliance, but cutaneous cues are more important. We therefore use a tactile sensor to investigate the mechanical interaction between the tactile sensor and a cartonboard package; we study the changes depending on the measuring position and the material. Using linear discriminant analysis (LDA) on the measurement result we show that we can separate these two changes for separate analysis.

Tissue is a low-density paper product distinguished by a microscale crepe structure. We investigate the relationship between the macroscale tissue tensile response and crepe structure. We propose a parameter called the Crepe Index (CI) that can be measured from edge images of the creped sheet. Crepe Index correlates very well with the measured tensile failure strain (“stretch”), but its correlation with the measured initial elastic stiffness is unclear. A discrete elastoplastic model (DEM) is developed to explain the experimental results and understand the nonlinearity in the tensile curve. The model accounts for both material nonlinearity through a bilinear elastoplastic constitutive law for the sheet material, and the geometric nonlinearity arising from large deformations. The creped sheet is idealized as a triangular wave of prescribed wavelength and waveheight, with nonlinear bending and stretching effects. The model results show that the tensile response is governed by both the nonlinearity of the sheet material (fibre network) and crepe structure (geometry). The yielding in stretching and bending gives rise to an inflection in the tensile response. It is found that the initial stiffness depends not only on CI, but also on parameters such as sheet thickness to crepe-wavelength ratio, and stiffness of sheet material after creping. Thus, the variability in above parameters can be one of the reason for unclear correlation between measured initial stiffness and CI. For CI range of tested commercial tissues, both experiments and model show that stretch varies linearly with CI, with an almost unity slope and a positive intercept (i.e, stretch> CI). Thus, the overall stretch of creped tissue is a sum of CI and network stretching.

We discuss a new mean-field theory to describe the compression behaviour of thick low-density fibre networks. The theory is based on the idea that in very large systems, the statistics of free segment lengths causes the stress-deformation behaviour to be quantitatively predictable. The theoretical ideas are supported by several different experimental characterisations. Firstly, we have carried out single-fibre buckling tests using hemp fibres, which indicate a maximum level of axial stress before deformation localization, after which the load carrying ability of a fibre decays. Secondly, the stress-compression behaviour of over 130 different foam-formed lightweight fibre materials were measured. For kraft pulps with low fines content, the average stress compression behaviour closely follows the theoretical prediction as described in terms of a universal s-function. Moreover, the acoustic emission can be described by the same function until collective phenomena cause deviations from the predicted behaviour. Similar deviations at smaller compressive strains are seen with furnishes with high fines content or added nanocelluloses together with samples with large voids. The localized buckling deformations lead to rapid stress re-distributions and subsequent fibre displacements in a fibre network as shown with in-situ CCD imaging.

When coating lightweight grade paper with aqueous coating color, wrinkles could appear on roll to roll coating system. This study is conducted from fundamental and theoretical point of view to laboratory experiments. The impact of water content will be taken in count. It has been experimentally observed that the appearance of wrinkles during a coating process on a roll to roll pilot depends on different parameters: web tension; misalignment angle; the coefficient of friction between the roll and the side of the paper in contact; the water content in the paper that appears to favour the wrinkles formation. Moreover, this work highlights the impact of discretisation of the variable represented wrinkles number i on the prediction model. Initial tests of the use of DIC during a tensile test have been carried out. These initial tests are very promising and the development of this technique is currently being developed. The perspective of this work is the improvement of the DIC study on the formation of out-of-plane displacement during the coating process. It can reduce the cost of non-conformities and thus reduce the waste of raw materials.

The larger hygro-expansivity of freely compared to restrained dried handsheets has been intensively studied during the past decades. To investigate the role of the fibers forming the sheets on this complex phenomenon, in this work, the hygro-expansivity of fibers picked from freely and restrained dried handsheets is characterized. To do so, a versatile, highly accurate, fiber hygro-expansion methodology based on Global Digital Height Correlation is proposed, which enables identification of the transient full-field hygro expansivity of single paper fibers. It was found that the longitudinal, transverse and shear hygro-expansivity of fibers picked from freely dried handsheets is significantly larger than fibers picked from the restrained dried handsheet. Furthermore, a restrained dried fiber can yield the hygroexpansivity of a freely dried fiber after being subjected to a sufficiently long wetting period, implying that the moisture-induced release of dried-in strain drives the hygro-expansivity differences. Finally, the sheet-scale hygro-expansivity is comparable to longitudinal fiber hygro-expansivity for both handsheet types. The presented results are of key importance for understanding the paper hygromechanics and improve their applicability.