Polymeric Composites with Rice Hulls

An Introduction

Chris Defonseka

• Publisher: De Gruyter
• Year: 2019
• ISBN:
  9783110639766
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Polymeric composites with rice hulls have superior properties compared with other polymer composites and are suitable for a wider range of applications. This book explains what it is that makes polymeric composites with rice hulls ideal substitutes for natural wood and how they may help ease global environmental concerns. It is an ideal source of information for researchers, resin-pellet manufacturers, processors and end users.

1 Introduction

1.1 History of composite materials

In ancient times, natural wood from trees and plants (e.g., palms and bamboo) were the ‘natural’ composite materials used by mankind, followed by straw and mud in the production of bricks and wattle by the Egyptians for the construction of buildings. As the needs of humans widened, other composite materials (e.g., papyrus; plywood; volcanic sands; lime mortars; and combinations of paper and glues) were used. Concrete began to be used 25 BC and, as time progressed, the natural polymer shellac began to be employed until the birth of the first ‘plastic’: bakelite.

Many ‘developing’ countries produce huge quantities of agricultural residues and plastic wastes but their uses have been small compared with the large volumes produced, thus causing extensive pollution to the environment. Some of the major agricultural residues are rice husks (hulls); coffee husks; coir pith; jute sticks; bagasse; groundnut shells; mustard stalks; rice stalks; and sawdust. The major plastic wastes are films, bottles, parts and containers. Apart from the problems of collection, transport, storage and handling, the direct burning of loose biomass in conventional grates is associated with very low thermal efficiency and widespread pollution. To some extent, plastic wastes are recycled and re-used but two of the major constraints have been the limited number of times they can be recycled and the lack of bio-degradation in landfill sites.

Historically, agricultural biomass wastes have been used as important energy sources in industrial and domestic settings. These have been used for heating; cooking; lighting; insulation; composites; and fuel for steam boilers. Plastic wastes have been recycled and fed back into production on a limited basis. Agricultural wastes are being used ideally as briquettes; logs; chips; and ash. In the manufacture of versatile briquettes in various shapes and sizes, two main methods have been used: ram/piston press and extrusion screw press. Both processes have their merits and demerits but, over the years, it has become universally accepted that screw-pressed briquettes are far superior in terms of storability and combustibility. Worldwide, both technologies are being used for the briquetting of sawdust and other agricultural residues. The importance of biomass briquettes as a substitute fuel for wood, coal and lignite is well recognised, but numerous limitations with regard to machinery and production may be the reasons for the slow expansion in this sector.

Briquetting and composite technology is yet to get a strong foothold in many developing countries because of technical constraints and the lack of knowledge for obtaining higher efficiencies to match the end requirements. Overcoming the many operational problems associated with this technology and ensuring the quality of raw materials at the time of use are critical factors for determining its commercial success. In addition to this commercial aspect, the importance of composite technology lies in conserving wood, a commodity used in all countries extensively and which can lead to the widespread destruction of forests. The availability of wood plastic composites (WPC) with bio-wastes over the past few years has lessened the need for using natural wood. Also, the arrival of polymeric composites with rice hulls (PCRH) as ideal substitutes for natural wood with a greater range of applications will help to ease the need for wood but on a much larger scale.

1.2 What are composite materials?

Composites are materials made from two or more constituent materials with significantly different physical and chemical properties that, if combined, produce a material with characteristics different from the individual components while remaining separate and distinct within the structure. A composite material can also be described as a macroscopic combination of two or more distinct materials having recognisable interfaces between them, the properties of which can be optimised by the addition of additives to achieve a balance of properties to meet the requirements of a given range of applications. Composites are used not only for their superior structural strengths but, among others, for electric, thermal, biological and environmental applications.

Composites are formed by individual materials, which may occur naturally or be made artificially. There are two main categories of constituent materials: a matrix and reinforcements. At least one portion of each is required to make a composite. The matrix material surrounds and supports reinforcement materials by maintaining their relative positions. Reinforcement materials provide the special mechanical and physical properties to enhance the properties of the matrix. Synergism produces material properties that are not available from the individual constituent materials, whereas the wide variety of matrices and additives allow the designer/producer of composite products to choose the optimum combinations and to structure them to achieve the desired end result.

Several commercially produced composites use a polymer matrix material often called a ‘resin solution’. Many natural polymers and modified polymers are available for use and depend upon the planned composite. Selection is highly dependent upon the compatibility of the reinforcement materials to be used. There are several broad categories, each with numerous variations. Some of the most common polymers are polyester; vinyl ester; epoxy, phenols; polyimide; polypropylene (PP); polyethylene (PE); high-density polyethylene (HDPE); and polyvinyl chloride (PVC). Reinforcement materials are very often fibres but also ground minerals. In polymeric composites, the polymer matrix can also be considered a ‘carrier’ for processing an end product, as in extrusion.

The ratio of the resin matrix to reinforcements used is important to the end product and the targeted special properties. As a rule of thumb, a 60% resin and 40% fibre may be considered a good starting point. The main aim is to reduce the resin content in the final product while achieving the special properties required, and additives go a long way in helping in this area. There is a limit to the reduction of resin content because the strength of the final product may be dependent upon the ratio of the constituents. Also, too little resin may also cause problems or completely prevent smooth processing of the composite. Polymeric composites can be made with a wide range of natural fibres, such as rice hulls, wood flour, palm-fibre wastes, flax, rice straw and other biomass wastes. This emerging ‘family’ of composites with such great possibilities can be categorised as ‘thermoplastic bio-composites’.

1.2.1 Engineered composites

Engineered composites are, in general, made to be shaped. The matrix material can be introduced before or after the reinforcement materials are placed into a mould cavity or onto a mould surface, as in the production of fibre-glass products. The matrix material undergoes melting and then the part shape is set. Depending on the nature of the matrix material, this melting phase can occur in various ways, such as chemical polymerisation or solidification from the melted state.

Various moulding methods can be used according to the design of the end-product. The principal factors affecting the methodology are the nature of the matrices chosen. Another important factor is the quantity of composite material to be produced. Large volumes may justify high capital investment for rapid and automated manufacturing. This chapter is mainly about the new technological field of PCRH and the methodology that can be used to produce composite resin pellets as well as how to process them to make composite profiled boards.

1.3 Types of composites

Many types of materials are suitable for combination with a range of polymers for making composites for different applications. Due to technological advances, polymers can be used singly, in combination, or as modified polymers, depending on the final objective in regard to a particular application(s).

1.3.1 Composite resins with rice hulls

Rapid developments are taking place in the production of composite resins in pellet form by combining a polymer matrix with rice hulls ground to a fine powder. These resins can be made in small volumes using manual operations but, for large-volume and quality resins, it is imperative that advanced technological equipment is employed. This process can be done in two stages: the rice hulls are ground, dried, compounded with the polymer matrix and then put through an extrusion process, resulting in a pellet form. Alternatively, this process can be set up with a continuous processing line, whereby the entire process is completed in one operation. These composite resins can be made with various plastics (e.g., PE, PP and polystyrene). Also, these WPC are just one category of an emerging family of materials termed thermoplastic bio-composite resins. Although these bio-composite resins can be made with fibres such as flax, palm-fibre wastes, wood flour, and rice-straw wastes, laboratory testing and practical applications have shown that PCRH elicit better overall properties (e.g., better structural strength) and capabilities.

These resins are slightly transparent to yellow in colour. Properties are decided by the types of constituents and additives used, whereas pellet sizes can be varied during pelletising. Colouring is an option but can also be carried out during conversion to the final product. The second option has benefits in that, whichever process is used, different finishes can be obtained by adding master-batches, dyes, pigments, or combinations of dyes/pigments to get the desired effects.

1.3.2 Composite profiles with rice hulls

A wide range of profiles can be produced using an extrusion process in which the composite resins of choice can be converted to profiled length boards. A specially designed die at the end/head of the extruder will determine the profile shape of the extrudate (i.e., whether the cross-section is round, rectangular, octagonal, or hollow). In general, the thicknesses of these boards can range from 15–25 mm and widths ≤1,000 mm and almost any desired lengths which are determined by a pre-set vertical cutting saw. Addition of extra additives during processing will enable a producer to achieve any special properties desired and incorporation of single colours or in combination to obtain aesthetically pleasing finishes.

1.3.3 Wood polymer composites

WPC have been available for some time and very popular for outdoor applications, especially for decking. The two main constituents are a polymer matrix and wood flour (finely ground wood waste). Some producers are also using other bio-mass wastes. These combinations produce strong boards, and have been replacing traditional materials used for outdoor applications such as decking, fencing, and park benches. These products are, in general, available in solid rectangular or hollow profiles and other pre-determined shapes with varying dimensions, and can be ≤5 m in length.

1.3.4 High-density polyethylene resin composites with rice hulls

These eco-friendly composites are made from HDPE polymer resin with ground rice hull powder as the reinforcement fibre in the matrix. The mix can be 30–40% HDPE resin and 70–60% rice hull powder plus additives to achieve any special properties desired as required in the end application. An extrusion process is used to produce board lengths of desired profile sections. Additives are used singly but very often in combinations of two, three or more. Some of the common additives used are lubricants, binders, and crosslinking agents. It is also possible to use small percentages of other biomass wastes with rice hull content but this is dependent upon the producer.

1.3.5 Polymer composites with recycled plastics

This is a class of composites made with the wastes of recycled plastics, rice hull powder or other biomass and made into extruded profiled products. Here, the important factor is compatibility between the different grades of plastic wastes. Though most thermoplastic grades are compatible, a few grades may not be. A combination of thermoplastic and thermosetting grades is to be avoided because they will not be compatible and pose problems when being processed. With these composite mixtures, achieving the desired properties and finishes may be difficult unless they are used as a small percentage of the matrix comprising virgin polymers. However, products made from 100% recycled material may be suitable for certain low-end applications.

1.3.6 Fibre-reinforced polymers

Fibre-reinforced polymers include carbon-fibre reinforced plastics and glass-reinforced plastics (GRP). If classified by matrix, then they are termed ‘thermoplastic composites’, ‘short-fibre thermoplastics’, ‘long-fibre thermoplastics’ or ‘long fibre-reinforced thermoplastics’. There are several thermoplastic composites but advanced systems usually incorporate aramid fibre and carbon fibre in an epoxy resin matrix. Good examples of carbon-fibre composites are aircraft parts, whereas glass-reinforced composites include marine boats manufactured using a lay-down production method.

1.3.7 Concrete

Concrete is probably the most common artificial composite material and typically consists of loose stones (aggregate) mixed with a matrix of cement. Concrete is a very robust material (much more robust than cement) and will not shatter even under quite a large compressive force. However, concrete cannot survive tensile loading (will breakdown under stretching). Therefore, to give concrete the...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• Preface
• Acknowledgments
• A note on sources
• 1 Introduction
• 2 Basic technology of plastics
• 3 Composite technology
• 4 Common polymers and additives used for processing
• 5 Rice hulls for composites
• 6 Polymeric composites with different constituents
• 7 Important stages of the production process
• 8 Colouring of polymers and composites
• 9 Manufacturing of polymeric composite resins
• 10 Polymeric composites with rice hulls for extrusion
• 11 Polymeric composite resins in injection moulding
• 12 Polymeric composite resins in compression moulding
• Abbreviations
• Glossary
• Appendix 1 Conversion table of units commonly used in industry
• Index