Biodegradable and Sustainable Fibres

Richard Blackburn

• Publisher: Woodhead Publishing
• Year: 2005
• ISBN:
  9781845690991
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

With increasing concerns regarding the effect the textile industry is having on the environment, more and more textile researchers, producers and manufacturers are looking to biodegradable and sustainable fibres as an effective way of reducing the impact textiles have on the environment. The emphasis in Biodegradable and sustainable fibres is on textiles that are beneficial by their biodegradation and come from sustainable sources.Biodegradable and sustainable fibres opens with a discussion of microbial processes in fibre degradation. It then moves on to discuss the major fibre types, including bast fibres, alginates, cellulose and speciality biodegradable fibres, such as lyocell, poly(lactic acid) and poly(hydroxyalkanoate)s. The development of synthetic silks is covered along with biodegradable natural fibre composites, nonwovens, and geotextiles. The final chapter looks at the history and future of soya bean protein fibres.Biodegradable and sustainable fibres is a comprehensive monograph providing essential reference for anyone interested in the area and environmental issues relating to textiles including fibre and textile scientists and students, textile technologists, manufacturers, and forensic specialists in industry and academia.

• Indispensable new book on this hot topic
• Discusses the major fibre types, inlcuding bast fibres
• Looks at biodegradable and sustainable fibres as an effective way of reducing the harm disposed textiles have on the environment

1

Microbial processes in the degradation of fibers

Fedorak P.M. University of Alberta, Canada

1.1 Introduction

As society becomes more concerned about environmental quality, there are moves toward producing and using materials that will not accumulate in the environment. For example, many plastics that are made from petrochemicals persist indefinitely after being discarded. Similarly, synthetic fibers such as nylon or polypropylene are extremely stable in dark environments. Thus, there is a trend toward producing and using fibers that will break down after their disposal. Depending upon the fiber, their breakdown may occur by biotic or abiotic processes. Biotic processes involve biochemical reactions that are typically mediated by microorganisms such as bacteria and fungi. Abiotic processes include chemical oxidation and hydrolysis, and photodegradation. This chapter will focus only on biotic processes.

Of course, natural fibers like wool and cotton are broken down through biotic processes. Microorganisms have evolved enzymes that attack key bonds in these natural polymers, thereby releasing monomers that can be used as carbon and energy sources for microbial growth. In contrast, microorganisms lack enzymes to break down many synthetic fibers, thus these materials persist and accumulate in the environment.

This chapter provides an overview of microbial processes involved in the degradation of natural and synthetic fibers, starting with an introduction of relevant terminology. It discusses the methods used to assess the microbial breakdown of fibers and gives examples of the sources of microbial communities and the methods of incubation that are used in these studies. Finally, it provides examples of the types of bonds that are susceptible to microbial attack.

1.2 Background and terminology

Microbial processes, that change the structure or form of any material, always occur at the molecular level. In a general sense, microorganisms such as bacteria and fungi can be considered ‘bundles of enzymes’ or sources of enzymes that catalyze a diverse array of chemical reactions that break down or modify substrates. Microorganisms carry out these activities to provide energy and suitable smaller molecules for the production of new cellular material, and ultimately new cells. Thus, when considering the microbial attack on any substance, it is important to remember that the size and physical and chemical characteristics of the substance influence how the microbes attack it. In addition, because microorganisms are living entities, environmental conditions must be suitable for their survival and growth. In this section, the general characteristics of fibers, microorganisms, and microbial processes will be discussed and some important terms will be defined.

1.2.1 Fibers, textiles and films

Fiber is the basic element of fabrics and other textile structure [1]. A fiber is typically defined as a material having a length at least 100 times its diameter. These can be natural, such as cellulose or wool, or synthetic, such as nylon. A textile is any product made from fibers [1]. This includes nonwoven fabrics such as felt, in which wool fibers are physically interlocked by a suitable combination of mechanical work, chemical action, moisture, and heat [1]; and woven fabrics in which yarns are interlaced perpendicular to each other. Yarns are made of fibers twisted together in a continuous strand.

Many of the so-called thermoplastic, biodegradable natural polymers, known as poly(hydroxyalkanoates), and some synthetic polymers such as poly(lactides) can be made into fibers by cold drawing, melt spinning or thermal drawing [2, 3]. These polymers can also be extruded as sheets rather than fibers. These sheets are known as films which are not true textiles because they are not made of fibers [1]. Nonetheless, these films of poly(hydroxyalkanoates) or poly(lactides) have the same chemical properties as the corresponding fibers, so for convenience, films are often used in biodegradation studies.

Fibers are composed of polymeric molecules with different arrangements. These can be random or parallel [4]. Amorphous regions of a fiber are due to random or unorganized arrangement of the polymers. In contrast, a parallel, highly ordered arrangement of the polymers is referred to as a crystalline region [4]. Fibers generally contain both types of polymer arrangements (for example, cotton is about 30% amorphous and 70% crystalline) and, typically, fibers of a particular type display greater strength with an increasing proportion of crystalline regions. These different arrangements also affect the biodegradability of fiber; the amorphous regions are more susceptible to biodegradation than the crystalline regions (for example, amorphous cellulose is biodegraded more rapidly than crystalline cellulose [5]).

Hydrogen bonding between chains of polymers is the major force that contributes to crystallinity [4]; the sum of the myriad of weak hydrogen bonds between adjacent polymers yields a strong, tight structure in the crystalline region of a fiber. Covalent cross-linking also occurs in some fibers, most notable are wool and silk in which disulfide bonds form between amino acid residues; the greater the number of disulfide bonds, the tighter the fiber structure. ‘Tightness’ imparted by hydrogen bonding and cross-linking reduces the susceptibility of the fiber to biodegradation.

1.2.2 Biodegradation, mineralization and biomass formation

The term biodegradation may have different connotations for people in different situations. In the broadest sense, biodegradation is the biologically catalyzed reduction in the complexity of chemicals [6]. A simple example is the conversion of glucose to ethanol during yeast fermentation; ethanol is a less complex molecule than glucose. Mineralization can be considered complete biodegradation, leading to the conversion of organic forms of elements to inorganic forms, as shown in Table 1.1. In some cases, two products are shown Table 1.1, for example, under aerobic conditions, organic-N may be converted to NH₃ or NO₃⁻, depending upon the environmental conditions and the structure of the microbial community, that is, if conditions are favorable for nitrifying bacteria, NH₃ (the first product of mineralization) may be oxidized to NO₃⁻. An anomaly in Table 1.1 is the appearance of CH₄ (an organic compound) as a mineralization product of organic-C. This occurs in methanogenic environments (see Section 1.3.2), where CH₄ production occurs along with CO₂ production; however, it is generally accepted that CH₄ is a product of mineralization in these environments. In non-methanogenic environments, CO₂ is the product of mineralization (Table 1.1).

Table 1.1

Major products of microbial mineralization under aerobic or anaerobic conditions

Conditions	Substrate	Inorganic products
Aerobic	Organic-C	CO2
	Organic-H	H2O
	Organic-N	NH3, NO3^−
	Organic-S	SO4^2−
Anaerobic	Organic-C	CO2, CH4
	Organic-H	H2, H2O
	Organic-N	NH3
	Organic-S	H2S

An important product of microbial metabolism and biodegradation is biomass or new cell material. In heterotrophic microorganisms, new cell material is formed by the incorporation of some of the carbon from the biodegradable organic substrates, while a portion of the organic carbon is mineralized to yield energy for biosynthesis of biomass. More energy is produced from the oxidation of an organic substrate in the presence of O₂, than in the absence of O₂ (see Section 1.2.5), thus, under aerobic conditions, more energy is available for biosynthesis and more substrate carbon is incorporated into biomass in the presence of O₂. In general, about 50% of substrate carbon is assimilated into new biomass under aerobic conditions, whereas only about 10% of substrate carbon is assimilated into new biomass under anaerobic conditions.

1.2.3 Microorganisms

The two major groups of microorganisms associated with the breakdown of organic matter are bacteria and fungi, both groups are extremely diverse in form, habitat and activity. Bacteria are typically simple, unicellular, prokaryotic (having no nucleus) organisms. They are commonly 1 to 5 μm in size, and are invisible...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• Introduction
• 1: Microbial processes in the degradation of fibers
• 2: Bast fibres (flax, hemp, jute, ramie, kenaf, abaca)
• 3: Alginate fibers
• 4: Cellulosic fibres and fabric processing
• 5: Lyocell fibres
• 6: Poly(lactic acid) fibers
• 7: Poly(hydroxyalkanoates) and poly(caprolactone)
• 8: The route to synthetic silks
• 9: Biodegradable natural fiber composites
• 10: Biodegradable nonwovens
• 11: Natural geotextiles
• 12: Conversion of cellulose, chitin and chitosan to filaments with simple salt solutions
• 13: Soya bean protein fibres – past, present and future
• Index