Wood-Polymer Composites

K O Niska, M. Sain

• Publisher: Woodhead Publishing
• Year: 2008
• Print ISBN: 9781845692728
• eBook ISBN: 9781845694579
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Wood-polymer composites (WPC) are materials in which wood is impregnated with monomers that are then polymerised in the wood to tailor the material for special applications. The resulting properties of these materials, from lightness and enhanced mechanical properties to greater sustainability, has meant a growing number of applications in such areas as building, construction and automotive engineering. This important book reviews the manufacture of wood-polymer composites, how their properties can be assessed and improved and their range of uses.After an introductory chapter, the book reviews key aspects of manufacture, including raw materials, manufacturing technologies and interactions between wood and synthetic polymers. Building on this foundation, the following group of chapters discusses mechanical and other properties such as durability, creep behaviour and processing performance. The book concludes by looking at orientated wood-polymer composites, wood-polymer composite foams, at ways of assessing performance and at the range of current and future applications.With its distinguished editors and international team of contributors, Wood-polymer composites is a valuable reference for all those using and studying these important materials. - Provides a comprehensive survey of major new developments in wood-polymer composites - Reviews the key aspects of manufacture, including raw materials and manufacturing technologies - Discusses properties such as durability, creep behaviour and processing performance

1

Raw materials for wood–polymer composites

C CLEMONS, USDA Forest Service, USA

Publisher Summary

This chapter highlights that to understand Wood-Plastic Composites (WPCs) adequately, one must first understand the two main constituents. Though both are polymer-based, they are very different in origin, structure, and performance. Polymers are high molecular weight materials whose performance is largely determined by its molecular architecture. In WPCs, a polymer matrix forms the continuous phase surrounding the wood component. Those matrix polymers are typically low-cost commodity polymers that flow easily when heated, allowing for considerable processing flexibility when wood is combined with them. The polymers tend to shrink and swell with temperature but absorb little moisture and can be effective barriers to moisture intrusion in a well-designed composite. The chapter explores the basic structure and properties of polymers and wood individually to lay a foundation for a greater understanding of the composites made from them. Basic concepts and properties are briefly summarized in the chapter with emphasis on materials common to current commercial technology.

1.1 Introduction

To understand wood–plastic composites (WPCs) adequately, we must first understand the two main constituents. Though both are polymer based, they are very different in origin, structure, and performance. Polymers are high molecular weight materials whose performance is largely determined by its molecular architecture. In WPCs, a polymer matrix forms the continuous phase surrounding the wood component. These matrix polymers are typically low-cost commodity polymers that flow easily when heated, allowing for considerable processing flexibility when wood is combined with them. These polymers tend to shrink and swell with temperature but absorb little moisture and can be effective barriers to moisture intrusion in a well-designed composite.

Wood itself contains polymers such as lignin, cellulose, and various hemicelluloses but has very different properties from the synthetic polymers with which it is most often combined. Wood is less expensive, stiffer, and stronger than these synthetic polymers, making it a useful filler or reinforcement. Though wood does not shrink and swell much with temperature, it readily absorbs moisture, which alters its properties and dimensions and can lead to bio-degradation if not protected.

In this chapter, we explore the basic structure and properties of polymers and wood individually to lay a foundation for a greater understanding of the composites made from them. Basic concepts and properties are briefly summarized with emphasis on materials common to current commercial technology. Sources of further information are listed at the end of the chapter.

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by US Government employees on official time, and it is therefore in the public domain and not subject to copyright.

1.2 Polymers: structure and properties

Polymers are high molecular weight substances consisting of molecules that are, at least approximately, multiples of simple units (Carley, 1993). The word polymer comes from the Greek poli, which means many, and meros, which means parts (Osswald and Menges, 1996). Polymers can be natural (e.g. cellulose, collagen, keratin) or synthetic (e.g. polypropylene, polyethylene) in origin. A polymer is called a plastic when it has other materials such as stabilizers, plasticizers, or other additives in it.

Owing to the low thermal stability of wood flour, plastics that can be processed at temperatures lower than about 200 °C are usually used in WPCs. In North America, the great majority of WPCs use polyethylene as the matrix, though polypropylene, polyvinyl chloride, and others are also used (Morton et al., 2003). The large use of polye...

Table of contents

• Cover image
• Title page
• Table of Contents
• Related titles
• Copyright
• Contributor contact details
• Introduction
• Chapter 1: Raw materials for wood–polymer composites
• Chapter 2: Additives for wood–polymer composites
• Chapter 3: Interactions between wood and synthetic polymers
• Chapter 4: Manufacturing technologies for wood–polymer composites
• Chapter 5: Mechanical properties of wood–polymer composites
• Chapter 6: Micromechanical modelling of wood–polymer composites
• Chapter 7: Outdoor durability of wood–polymer composites
• Chapter 8: Creep behavior and damage of wood–polymer composites
• Chapter 9: Processing performance of extruded wood–polymer composites
• Chapter 10: Oriented wood–polymer composites and related materials
• Chapter 11: Wood–polymer composite foams
• Chapter 12: Performance measurement and construction applications of wood–polymer composites
• Chapter 13: Life-cycle assessment (LCA) of wood–polymer composites: a case study
• Chapter 14: Market and future trends for wood–polymer composites in Europe: the example of Germany
• Chapter 15: Improving wood–polymer composite products: a case study
• Index