Mechanics of Paper Products

Sören Östlund, Kaarlo Niskanen

• Publisher: De Gruyter
• Year: 2021
• ISBN:
  9783110619508
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Mechanics

This book focuses on the mechanical properties and performance of products made of fiber-based materials. It helps students to develop skills for solving problems of product performance and engineering challenges in product development. Organized with a problem-based approach - practical examples of product performance are presented and the relevant mechanics are analyzed to deduce which material properties control the performance. The new edition covers state-of-the-art and green technologies as modeling of fiber networks and applications of nanocellulose.

1 The challenge

Kaarlo Niskanen

Mid Sweden University, Sundsvall, Sweden

Sören Östlund

KTH Royal Institute of Technology, Stockholm, Sweden

This book discusses the mechanical properties of products made of wood fibres. Printing papers and office papers are the ubiquitous examples of such products. Packaging made of paper or board is another, and tissue papers are a third very common use of wood fibres. A little further from paper products, one has fibreboards that are used in furniture and construction. When compared with paper, fibreboards have similar structure but different physical properties. The renewable wood raw material used in all these products makes them attractive to the modern society, and therefore many believe that wood fibres will increasingly find also other uses. We have written this textbook in part to anticipate such developments.

Since – by definition – future products do not exist today, we use some of today’s paper and board products as concrete examples. The properties seen in these products today help one understand what may be required in other applications tomorrow. The cases we discuss in this book give a broad picture of the various challenges that face anyone working with materials based on wood fibres. We expect that the reader will learn how to solve analogous challenges in other situations.

The fact that we focus on the mechanical properties of paper and board products means that this textbook discusses practical applications of mechanical engineering. The reader is assumed to have good understanding of engineering mechanics, or to be able to acquire such as needed. One should also feel comfortable with mathematical concepts and be able to appreciate the power of numerical modelling methods. The reader is not expected to know much of paper or papermaking. We will describe the main features in Chapter 2, exemplifying what paper and board are as materials. Building a deeper understanding of materials based on wood fibres is in itself a goal of this book.

Before one can solve a practical performance problem or develop a better product, one has to understand what the problem really is. In an effort to underline the importance of problem definition, we have chosen a somewhat unorthodox and admittedly difficult approach. We have tried to build a book that starts from two practical cases (the “box” and the “web”), identifies what are the important process requirements and the related material or systems properties, before exploring the material properties, and ultimately discussing how the material properties or systems behaviour could be improved. The result is not perfect because, as it turns out, little work has been done on the really crucial issues.

The approach outlined means that this book overlooks lots of the material traditionally discussed in the context of the mechanical properties of paper and board. There are many excellent books on papermaking, paper physics, and the mechanical properties of paper products, explaining what is already known. We have been more interested in unveiling what should be known.

With all that said, we are confident that this book will be very useful not only when developing new products and materials based on wood fibres but also – and most readily – when solving performance problems of today’s paper and board products.

A short outline of the book contents is the following. After the introduction to paper materials in Chapter 2, the main body is divided into four parts that clarify the underlying logic. Part I considers the basic structural strength of paper products. Wood fibres are particularly beneficial in applications where stiffness at low weight is required – as in packaging applications. The first practical case introduced in Chapter 3 is the “box”, the manufacturing and usage requirements of paper-based containers and boxes. The performance of a box depends on the structure of the box and on the changes in the material that are created in the process of converting a board into a box. Chapter 4 illustrates the latter aspect by considering the corners of paperboard boxes. The quality of corners is crucial, for example, in liquid packaging applications where defects can allow leakage or trigger box fracture. Chapter 5 then provides the machinery that has been developed specifically for the analysis of fracture that is triggered by a defect or other discontinuity in a structure.

The discussion in Part I concerned primarily the “static” or “instantaneous” strength of products. Often in reality, the dynamics of the process is crucial. This extension to structural strength is discussed in Part II. A major advantage of paper materials is the high speed at which the web can be manufactured and converted – but only provided that the web can be held stable in the fast process. This is the second practical case, the “web”. Chapter 6 explains what determines the short-time dynamic stability of “running” paper or board webs. The requirements posed by the fast dynamical loading are different from the requirements on boxes. A stack of boxes should hold through the entire logistic chain that may last for months. The rheological properties of paper materials – applicable in both short-time web stability and long-time box endurance – are then explored in Chapter 7.

From Chapters 6 and 7, we conclude that strength problems of paper products are also a systemic issue. Product performance cannot be comprehensively explained by just the average material properties. Spatial and temporal variability in the material properties and process conditions is often crucial. Therefore, Chapter 8 frames out the general methodology for obtaining statistically reliable information about box failures and web breaks.

After tackling the “static strength” and “dynamic strength” of paper products, it is time in Part III to consider the dimensional instability caused by changes in the moisture content of paper. The moisture sensitivity of paper is a direct result of the biological origin of the wood fibres. It enables the preparation of paper even without any added adhesives, and equivalently makes it possible to recycle paper by simply soaking it in water. Thus, moisture sensitivity is a challenge in all uses of wood fibres. One just has to learn to minimize the problems. This is the motivation for Chapters 9 and 10.

Moisture changes are practically impossible to avoid in normal usage of paper products. Chapter 7 discusses the effect of moisture on the creep rate of boxes. In Chapter 9, we explain how changes in moisture can lead to permanent out-of-plane deformations, especially if the paper surface is exposed to liquid water. Chapter 10 then explains the mechanics of a printing nip where water and other liquids are applied on paper or board.

Finally, having illustrated some of the requirements that usage poses on the paper material, we are ready to ask how the relevant material properties could be controlled. This is considered in Part IV. In Chapter 11, we discuss the relations between the stiffness properties of paper, the papermaking fibres and the papermaking process using micromechanical concepts and numerical simulations. Chapter 12 continues from there, asking what functionality can be achieved if one goes beyond paper, using wood as an ingredient in bio-composites.

The book is by no means a comprehensive account of the performance of paper products or materials based on wood fibres. We have not intended to give such an account. What we hope to have accomplished is a book that illustrates how one can systematically tackle challenges in the development of better products. Finding out the right questions is critical if one wants to give relevant answers. We have certainly not uncovered all questions!

2 Paper as an engineering material

Per-Johan Gustafsson

Lund University, Lund, Sweden

Kaarlo Niskanen

Mid-Sweden University, Sundsvall, Sweden

2.1 Introduction

This textbook concerns the mechanical properties of products that are made of paper or paperboard. Throughout the book, therefore, it is important to know the properties of paper as an engineering material. In this chapter, we describe the general mechanical properties of paper and the papermaking process. The manufacturing process, including the wood raw material used, naturally governs these mechanical properties, but here we limit the discussion to what is most important to know when tackling problems of product performance. An overview of the underlying mechanisms and raw material effects is presented in Chapters 11 and 12. Those interested in learning more are referred to the many textbooks on papermaking, such as the series Papermaking Science and Technology, published by the Finnish Paper Engineers’ Association.

Paper is a thin, almost two-dimensional material. Everyday papers, such as office paper and newsprint, have a thickness of about 0.1 mm. The mass per unit area of such papers, called the basis weight, is usually between 40 and 100 g/m² depending on the type of paper. Specially prepared paper can have a thickness as low as 0.01 mm and a basis weight of a few grams per square meter. On the other end, paper material used for book covers or fixtures to display products in stores can be more than 1 mm thick. Thick paper grades are called board or paperboard, and typically they have a basis weight between 150 and 500 g/m². The trade terminology for paper and board grades refers primarily to the applications where the materi...

Table of contents

• Title Page
• Copyright
• Contents
• Preface
• 1 The challenge
• 2 Paper as an engineering material
• Part I: Structural strength
• Part II: Dynamic stability
• Part III: Reactions to moisture and water
• Part IV: Material properties
• Index