Biodegradable
Background
The ever increasing global population is having a direct impact on consumption of natural and fossil fuels. Not only are we depleting these finite resources but our consumer driven lifestyle means that we are disposing of ever larger quantities of waste that will not degrade but remain indefinitely in landfill. The European Landfill Directive (1999) seeks to prevent or reduce as far as possible negative effects on the environment, in particular the pollution of surface water, groundwater, soil and air, and on the global environment, including the greenhouse effect, as well as any resulting risk to human health, from landfilling of waste, during the whole life-cycle of the landfill.
In order to reduce the volume and impact of landfill waste, a new generation of Biodegradable? products is required that will break down following disposal. However to minimise consumption of fossil fuels, water and energy this new generation of Biodegradable? products must also be sustainable and preferably renewable. The various issues relating to biodegradability and sustainability will be covered in this section.
Revision of the micro and macro structure of polymers and fibres
Degradable materials are all polymers. The properties of polymers depend on both the molecular structure (microstructure) and the arrangement of these molecules i.e. amorphous or semi-crystalline (macrostructure)
Molecular structure of polymers
Polymers (or macromolecules) are very large molecules made up of smaller units, called monomers or repeating units, covalently bonded together (Fig 1).
A is a monomer unit - Is a covalent bond
Figure 1A polymer chain
The chainlike structure of polymeric materials is responsible for their intriguing mechanical properties.
Macro structure of polymers
Many properties of polymeric materials depend on the microscopic arrangement of their molecules. Polymers can have an amorphous or semicrystalline (partially crystalline) structure (Fig 2).
- Amorphous polymers lack order and are arranged in a random manner
- Semicrystalline polymers are partially organised in orderly crystalline structures.
Figure 2(a) Amorphous polymer (observe the entanglements among the polymer chains) and (b)
semicrystalline polymer
More information can be found on polymer architectures : (click here)
The various types of Biodegradable? polymers/fibres and the processing conditions for their conversion into fabrics
While natural polymers are both Biodegradable? and sustainable, research is on-going to develop new synthetic polymers/fibres derived from renewable sources. For a material to fulfil the’ cradle to grave’ sustainability requirement, it must be both derived from a renewable source and be degradable (Fig. 3).
Figure 3: Degradable and sustainable polymers
Categories of Biodegradable? polymers/fibres
. Biodegradable? polymers can be classified into three main categories:
1) Natural polysaccharides and biopolymers; e.g. cellulose, alginate, wool ,silk, chitin and soya bean protein
2) Synthetic polymers, particularly aliphatic polyesters e.g. poly(lactic acid), Poly(ε-caprolactone)
3) Polyesters produced by microorganisms; e.g. poly(hydroxyalkanoate)s
Natural polysaccharides and biopolymers
Many natural polymers occur as fibres that are ready to be processed into yarns and fabrics, and which can ultimately be broken down by enzymes and metabolised in the ecosystem. Others such as Alginate, Chitin and Soya Bean Protein require processing to create useful fibres.
Cellulose Cotton processing includes dyeing, bleaching and spinning each with a negative environmental impact, so degradable and more sustainable man-made fibres maybe preferable.
Alginate is extracted from seaweed, and calcium alginate fibre is used in wound dressings and other wound management products, also for use in diagnostic swabs.
Wool Processing requires shearing, washing (generates lanolin), carding, and spinning to achieve a useful yarn. Woollen garments do decompose, but may produce methane and Nitrogen dioxide in some burial conditions, which may contribute to global warming and climate change.
Silk is documented to have been uses to suture wounds as far back as ancient Egypt and currently gaining much attention in the medical field as a bioresorbable material for tissue regeneration. (Click here)
Chitin (or chitosan) is a polysaccharide derived from crustacean (shrimps, crabs etc.). It has a structure similar to cellulose and can be combined with rayon to give a fabric with a soft feel and antibacterial properties.
Soya bean protein is derived from the `cake’ residue after extraction of oil from soya beans, giving a high polymer for spinning to a fibre
More information on biopolymers can be found in the following link; ( Click here)
Synthetic polymers
Degradable synthetic polymers (i.e. those created catalytically from monomers) include poly(lactic acid) (PLA) derived from a renewable ?monomer and Poly(ε-caprolactone) (PCL) from a petroleum product. A greater list of Biodegradable?polymers can be found at : (Click here)
Polymers from renewable ?aliphatic esters e.g. poly(lactic acid)
Poly(lactic acid) (PLA) is a linear aliphatic polyester derived from sugar or polysaccharide, a 100% renewable ?source, therefore sustainable and degradable. The monomer used to manufacture poly(lactic acid) is obtained from annually renewable ?crops (corn, sugarbeet, wheat, Fig. 3) in the agricultural carbon cycle which uses energy from the sun to convert carbon dioxide and water into starch or other fermentable sugar, which is fermented to lactic acid.
Figure 4: Potential feedstock for PLA production
PLA degrades by hydrolysis, and can be composted instead of being sent to landfill. Over the past few years NatureWorks LLC has developed large scale operations for the economic production of PLA polymer used for packaging and fibre applications. More information can be found in the following links
Polymers from synthetic monomers Poly(ε-caprolactone)
Poly(ε-caprolactone) (PCL) is a synthetic Biodegradable? polymer. It has mechanical properties similar to polyolefins (this is the polymer family to which polypropylene belongs), but unlike them is hydrolysable similar to polyester . However, it’s low melting point of around 60®C makes it unsuitable for many textile applications. Poly(ε-caprolactone) as a co-polymer with starch is probably best known under the trade name MaterBi produced by Novamont
Polyesters produced from microorganisms
Poly(hydroxyalkanoate)s
Poly(hydroxyalkanoates) (PHAs) form a group which includes poly(hydroxybutyrate) (PHB), a polyester produced from bacteria. Advantages include production from fully renewable ?resources and fast and complete biodegradability. PHAs also have excellent strength and stiffness. However several serious drawbacks hinder their wider application, including brittleness of the material resulting in low toughness (which increases further during storing due to physical ageing), and a high price. PHA’s are biocompatible so could be used in the medical sector. More information can be found on PHAs at the links below http://www.tjgreenbio.com/en/about.aspx?title=About%20GreenBio&cid=25
Manufacturing potential of Biodegradable? polymers
PLA is the most commercialised of all the Biodegradable? synthetics. One of the reasons for PLAs success is that it is it is the only melt-processable fibre from annually renewable ?natural resources. Both Filament? and spun yarns can be produced from PLA. Fabrics produced from spun yarns have a ‘natural’ hand and are considered to feel similar to cotton in this respect. Fabrics from Filament? yarns have a cool and soft hand and exhibit a high fluidity or drape with a degree of Elasticity?. Below is a list of the positive and negative attributes associated with the manufacturing processes and textiles produced from PLA. Although PLA is used as the example, other Biodegradable? textiles would be judged by similar standards-here the key issues to note are in Bold type.
Positive attributes of textiles manufactured from PLA
- The moisture management properties of good wicking, faster moistures spreading and drying mean that garments are comfortable.
- The elastic recovery and Twist? usually intended to be cut or stretch-broken for use in staple fibre or top form.">Tow?.">Crimp? retention properties provide excellent shape retention and crease resistance
- Thermosetting capabilities of the fibre provide for controlled fabric stability, with garments having low shrinkage through repeated washing
- PLA fabrics with no separate flame retardant treatments have passed the US tests 16 CFR 1610 and have also achieved the standards specific for children’s sleepwear, 16 CFR 1615 and 16 CFR 1616.
- The after-care properties of garments in washing are very positive. There is no damage in repeated laundering of PLA fabrics: testing has been carried out under simulated conditions in accordance with AA TCC standards, with no degradation observed.
Negative attributes of textiles manufactured from PLA
There are also certain factors in this relatively early stage of technical and commercial developments that are somewhat restrictive to the development across a full apparel spectrum.
- The melting point of the polymer that are commercially available today is relatively low at 170C. This means that consumer after-care of garments, garment pressing and ironing temperatures have to be lower than the popular fibres of cotton and PET.
- Hydrolytic degradation of polymer can occur, particularly under combined aqueous high temperatures and alkaline conditions; the degree of hydrolysis is influenced by the time, the temperature and the pH. This is particularly significant in in the dyeing and finishing processes, because if the appropriate finishing conditions are not observed it will cause a reduction of the molecular weight of the polymer and therefore the strength of the yarn of fabric.
The range of applications for PLA can be found at: (Click here)
The applications of another Biodegradable? polymer. (Click here)
The mechanisms by which polymers/fibres degrade vary depending on the material. These degradation mechanisms include biodegradation, hydrolysis and photo degradation. Some materials degrade solely by one mechanism alone, while others degrade by a combination of mechanisms. The environment where the polymer/fibre is disposed has a great effect in the types of degradation that can occur. A ‘degradable’ material disposed in an inappropriate environment may not degrade at all.
Hydrolytic degradation
Most heterochained polymers are susceptible to aqueous acid or base degradation referred to as hydrolysis. This susceptibility is due to a combination of the chemical reactivity of heteroatom sites and to the materials being at least wetted by the aqueous solution, allowing contact between the protons or hydroxyl ion to occur.
Photo-degradability
Synthetic polymers can be degraded by the absorption of ultra-violet (u.v.) light. This applies also to natural polymers, but these tend to be more rapidly degraded biologically, by the attack of micro-organisms. More on photodegradability can be found at : (Click here)
Biodegradation,
In the broadest sense, biodegradation is the biologically catalysed reduction in the size and complexity of a molecule. This breakdown is carried out by Microorganisms ?which, because they are living entities, require suitable conditions such as optimum pH and temperature in the composting process. (Fig. 4). With reduction in landfill more and more food waste is being composted, and the new generation of degradable products made from PLA is also designed to degrade in the compost system.
Figure 5: Biodegradation
Microorganisms
The two major groups of microorganisms associated with the breakdown of organic matter are bacteria and fungi (Fig. 5). Bacteria are typically simple, unicellular organisms, 1 to 5 µm in size, and invisible to the naked eye. Some bacteria are aerobes and require oxygen (O2) for growth, whereas others are anaerobic and are killed in the presences of oxygen. Most bacteria are heterotrophs, deriving energy and carbon from degrading their substrate and therefore play a major role in nutrient cycling in the environment.
Fungi are more complex microorganisms. They are typically multicellular, making them much larger than bacteria. They can often be seen with the naked eye (an example is the mould growing on bread). Most fungi produce spores that are spread by wind and can remain dormant until conditions are favourable for germination and growth. Fungi are heterotrophs and most prefer aerobic conditions. The ability of fungal hyphae to rapidly penetrate into plant tissue and other organic materials makes these organisms important decomposers.
Figure 6: Microorganisms ?associated with the breakdown of organic
Growth requirements for microorganisms
The habitats, activities and metabolism of bacteria and fungi are extremely diverse; however there are certain requirements and conditions needed by all microorganisms. Moisture is essential, also a number of macro and micro nutrients required for cell growth including compounds of carbon, oxygen, nitrogen, hydrogen. Microorganisms ?absorb these nutrients from the material they are degrading, but to be taken into the cell the nutrients must be soluble and small enough to pass through the cell wall and cytoplasm membrane. Polymers (macromolecules) are too large to be taken into cells, and must be broken down outside the cell into subunits or monomers that can be absorbed. Fungi and bacteria therefore produce enzymes that hydrolyse soluble or insoluble polymers, for example cellulose can be broken down by cellulase into glucose which can be readily transported into the cell, where metabolism by the cell provides energy and carbon for microbial growth and reproduction.
Mineralisation and microbial biomass formation
Mineralisation can be considered to be complete biodegradation, i.e. the conversion of organic material to mainly inorganic molecules. The molecules produced depend on the environmental conditions (aerobic or anaerobic ) and on the microbes present e.g. carbon dioxide (CO2), NO3-, PO3- and methane (CH4 an organic compound) (Fig. 6 left). An important product of microbial metabolism and biodegradation is microbial biomass or new cell material. New cell material is formed by the incorporation of some of the carbon from the substrate. A portion of this organic carbon is mineralised (turned into inorganic carbon) to yield energy for synthesis of biomass. More energy is produced from the oxidation of an organic substrate in the presence of oxygen (aerobic metabolism) than in the absence of oxygen (anaerobic metabolism), therefore under aerobic conditions more energy is available for biosynthesis. Microbial biomass will release its energy if it is burnt. It is therefore being promoted as a renewable ?energy source which could be used instead of fossil fuels because it maintains a closed carbon cycle with no net increase in atmospheric CO2 levels. More information can be found at: (Click here).
Figure 7: Products of biomineralisation (above- upper) and sources of Biomass (above lower) (refs)
Incubation conditions used for evaluating biodegradability of fibres and textiles
In the natural world, fibre or textiles can find their way into aerobic or anaerobic environments. Many aquatic environments and the top few centimetres of the soil contain sufficient oxygen to be aerobic. In contrast anaerobic conditions exist in the deeper soils, water-logged soils, aquatic sediments and landfill. Thus biodegradation studies have assessed the fates of fibres and textiles under both aerobic and anaerobic conditions.
Aerobic incubations
Experiments involving aerobic incubations are much easier to set up and maintain, simply ensuring that the fibres are accessible to the Microorganisms ?and there is an ample supply of oxygen. One of the simplest aerobic cultures is to add the fibre and the microbial inoculum to a container with liquid medium. This is covered in some manner to allow air (oxygen) to get into the container while preventing foreign Microorganisms ?from contaminating the culture. The formulation of the liquid medium depends on the goal of the aerobic experiment, often only inorganic salts (including phosphate and ammonium or nitrate), leaving the polymer as the sole source of carbon for the heterotrophic microorganism.
Anaerobic incubation
The methods required to create and maintain conditions that are suitable for growing anaerobic microbes are more difficult than those required for culturing aerobic microbes. Nonetheless use of the methods such as the serum bottle modification of the Hungate technique is now routine in many laboratories when assessing anaerobic degradation. For more information, (click here)
Standard test methods used for sustainability, ecology and degradation tests on textiles
The textiles industry is responding to legislation such as the EU Landfill Directive (1999) by developing new fibres that can be properly regulated and certificated.
UK Kitemark for textiles
The UK Kitemark is a registered certification mark owned and operated by the British Standards Institute (BSI), a recognised symbol of quality and safety offering assurance to consumers and businesses. More information on the Kitemark for Energy Reduction Verification can be found at: (Click here)
CE mark for textiles
CE marking is a declaration by the manufacturer that the product meets all the appropriate provisions of the relevant legislation implementing certain European Directives. CE marking gives companies easier access into the European market to sell their products without adaptation or rechecking. The letters CE stand for "Conformité Européenne" which means "European Conformity". More information on the CE mark for textiles can be found by: (Clicking here.)
More information on the european standards in Ecodesign and Energy Labelling (relating to the CE mark can be found by (Clicking here)
Chemical tests for heavy metals and ecological toxicity
Textile testing helps to protect the interest of both manufacturers and consumers. The growing list of contolled chemicals includes: Formaldehyde; Chlorinated phenols (PCP, TeCP) and Orthophenylphenol (OPP); Banned Amines from Azo dyes; Allergenic Disperse Dyes; Carcinogenic Dyes; Heavy Metals; Organotin compounds; Alkylphenol Ethoxylates (APEOs); Chlorinated Organic carriers; Phthalates; Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonate (PFOS); Pesticide residue; Flame retardants; Short chain chlorinated paraffins (SCCP); Polychlorinated biphenyl (PCBs); Polyaromatic hydrocarbons. For an updated list: (Click here) (and here) www.eco-label.com.
Eco-Textile Testing
A European and U.S. ecological requirements is that textiles are free from heavy metals. Heavy metals are constituents of some dyes and pigments, and may also be found in natural fibres due to absorption by plants through soil. Metals may also be introduced into textiles through finishing processes. More information on heavy metals : (Click here)
From more information on ecological toxicological testing for textiles and how it is helping to achieve sustainability: (Click here)
Complete biodegradation tests and composting
End products made with Biodegradable? polymers such as PLA are designed to compost in municipal/industrial facilities. They must therefore conform to certain regulations. For a British Standard (30 June 2012) in textile composting: (Click here)
Currently, DIN V 54900, EN 13432 and ASTM D 6400 are the relevant standards for the determination of compostability Europe. Each of these standards is being applied by a number of certification organisations in the testing and assessment of compostable products and materials. The standards are very similar in their general construction, the applicable tests and the necessary pass levels. As a rule, the assessment of compostable materials and products comprises five different parts:
Characterisation/chemical testing (including for heavy metals)Determination of ultimate biodegradability
Determination of compostability (disintegration)
Analysis of the quality of the compost
Determination of ultimate anaerobic biodegradability (voluntary)
EN 13432 is currently the most relevant standard because it is a harmonised, mandated European standard, which gives it a special legal relevance.
Figure 8: DIN CERTCO compostability mark
DIN CERTCO is responsible in Germany for testing, certification and awarding of the compostability mark (fig…). More information can found on Standard D6400 at:
Textile waste, Recycling? and life cycle analysis of degradable materials
Landfill Directive in relation textile waste
The Landfill Directive represents a step change in the way waste is disposed of in the UK. Its aim is to help drive waste up the hierarchy through waste minimisation and increased levels of re-use, Recycling? and energy recovery. The Directive's overall aim is "to prevent or reduce as far as possible negative effects on the environment, in particular the pollution of surface water, groundwater, soil and air, and on the global environment, including the greenhouse effect, as well as any resulting risk to human health, from the landfilling of waste, during the whole life-cycle of the landfill". It sets demanding targets to reduce the amount of Biodegradable? municipal landfill. These targets are:
• By 2010 to reduce Biodegradable? municipal waste landfilled to 75% of that produced in 1995
• By 2013 to reduce Biodegradable? municipal waste landfilled to 50% of that produced in 1995
• By 2020 to reduce Biodegradable? municipal waste landfilled to 35% of that produced in 1995.
More information can be found at the following link:Recycling?-guide.org.uk/targets.html"> (Click here)
Recycling? Textile Waste
Why recycle textiles?
Over 1 million tonnes of textiles are thrown away every year, mostly from domestic sources, of which only 25% are recycled.
Textiles represent between 3% - 5% of household waste.
Estimates for arising of textile waste vary between 550,000 - 900,000 tonnes each year.
Recycling? textiles can save up to 15 times the energy recoverable by incineration.
Textiles make up 12% of landfill sites.
Although there has been an increasing volume of clothes consumed and disposed of, many of these clothes are of decreased quality. Additionally Biodegradable? materials in the Recycling? system are seen as potential contaminates: (Click here) These factors have added to the complexities of Recycling?.
Life Cycle Analysis
Quantifying the overall impact of a product on its environment demands an account of all the inputs and outputs throughout the life cycle of that product, from its birth, including design, raw material extraction, material production, part production, and assembly, through its use, and final disposal.
Information on Life cycle analysis of Biodegradable? fibres:Biodegradable?-fibers">(Click here)
In the case of a textile in a global market, much of the variability will be in the production of the fibre, which is illustrated here:
Energy used in production of fibres
Fibre
|
energy use in MJ per KG of fiber:
|
flax fibre (MAT)
|
10
|
cotton
|
55
|
wool
|
63
|
Viscose
|
100
|
Polypropylene
|
115
|
Polyester
|
125
|
acrylic
|
175
|
Nylon
|
250
|
More on life cycle analysis: (Click here)
Combining Economic Input-Output Models and Life Cycle Assessment: (Click here)
Figure 10 - Life cycle analysis
In order for Lifecycle evaluation to lead to the development of sustainable waste management and Recycling? policy it is necessary to be able to evaluate the external costs of these schemes in comparison with alternative methods of waste disposal.
These issues are considered in the following link: (Click here)
