There is a growing demand for biodegradable plastics as a solution to the problems concerning the global environment and the solid waste management. This research project is about an in depth investigation of mechanical properties of starch based biodegradeable biopolymers. A thorough understanding is developed to achieve the additional functionality by converting the starch into thermo plastically processable form in combination with other polymers.An attempt is made to reduce the undesirable impacts of starch during thermal processing due to its thermally sensitive nature. A starch polymer sample of Biome plastics GF 106/02 was studied in terms of the decomposition mechanisms in degradation behaviour under shear and high temperatures. The experimental determination of temperature,pressure,dwell times,melting viscosity during degradation and their effect on product properties such as mechanical strength is carried out using a range of processing and characterization techniques. Rhelogical properties were demonstrated through graph of shear rates and viscosity. Various measurements were taken for mass loss under the impact of heating with the help of themogravimetric analysis. Chemical nature of starch was studied using techniques of evolved gas analysis and FT-IR study.
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Aims & Objectives:
The key objectives of the research were to provide a comprehensive analysis properties of bio resin compound against various processing conditions which include Temperature ,shear Dwell times including any changes in physical behaviour such as colour ,odour viscosity and other mechanical properties. The project gives a comprehensive awareness of nature of thermal degradation and decomposition mechanisms and experimental conditions which can be used as a guidelines for companies processing starch based polymers.
Key Tasks & Project Outline:
To collect the information about biodegradeable plastics in general and specifically on starch based polymers i.e structure, thermoplastic processing and applications.
Study the currently used experimental techniques for determining rheological and degradation behaviour of thermoplastic polymers.
Study the Injection moulding process in general and specifically the evaluation techniques for temperature,pressure,dwell times,melting viscosity and their effects on mechanical strength of product.
Study the thermogrivametric analysis techniques in general and specifically for measurements of changes in weight during heating or cooling ,absorbed moisture content and determination of extent of organic and inorganic compounds in a material.
Study the and Differential Scanning calorimetery techniques in general and specifically measurement of heat absorbed or liberated during heating or coolingand degree of crystallanity and melting temperatures.
Carry out experimental observation of GF 106/02 in the laboratory using above mentioned techniques and illustrate the data in terms of graphs ,bar charts and tables.
Comparison of results with experiments , discussion and conclusions.
Project Work Plan
Figure to be pasted from Presentation.
Biodegradeable polymers are polymers that under the action of biological enzyme breakdown into a biomass,CO2 and water in a given time period as defined by a biodegradation standard . In a broader perspective biodegradeable polymers are a subset of sustainable polymers . Sustainable polymers are defined as polymers that are produced in a sustainable way . This may involve using sustainable or renewable resources for ingredients or may involve polymers that are processed by the help of processing technique which decreases environmental impact of the process and product.these methods to increase sustainability are resusing the primary waste in polymer process,reducing the amount of polymers in a product in first place,recycling ,recovering the energy from the plastic waste.and use of biodegradeable polymers from renewable resources.These techniques are used by the plasics industry at present to reduce their environmentalimpact and ,maximize the long term sustainability for their business.Birodegradeable polymers can be further classified into two main areas.renewable and nonrenewable biodegradeable polymers . The renewable polymers utilize the renewable resource in the development of the polymer,rather than a non renewable resource. The long term research and development focus is on biodegradeable polymers .They have also been called as base polymers. There are also many opportunities to increase the strength of various polymer systems through the use of polymer alloys ,polymer blends and reactive compatibilisation strategies . For Example renewable starch based systems are low cost but suffer from poor processibility and final product properties. Whereas nonrenewable synthetic polymers traditionally are easier to process and have excellent properties.but are too costly .Thus synergizing the advantages of starch based polymers and synthetic biodegradeable polymers via new alloying and blending technologies provide a key strategy for creating more applications and larger markets for biodegradeable plastics. This chapter will focus on the development of these low cost base thermoplastic starch based polymers . It is important to introduce some background on starch itself.
Starch and its properties:
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Starch is the major polysaccharide reserve material of photosynthetic tissues and of many types of plant storage organs such bas seeds and swollen stems. Starch occurs in nature as water insoluble granules . The starch granule is essentially composed of two main polysaccharides,amylase and amylopectin with some minor components such as lipids and proteins . Starches from different botanical origin have different biosynthesis mechanisims and hence may exhibit distinct moleculer structure and characteristics as well as diversity in shape ,size ,composition and other macroscale constituents of the starch granules. Thus the ultimate processing and properties of starch are linked to starch genetics as well as various structure levels from granule structure ,macromolecular structure and crystalline macrostructures.
Different varieties of starch species generally have different granular sizes ,granule size distributions,granule structures ,starch compositions ,molecular sizes and degree of branching of amylase and amylopectin. Recent advances in genetic engineering have developed an understanding of metabolic pathways in starch synthesis. Figure â€¦. For starch synthesis was proposed by Rehman et al. (2000).
The extended starch polymer is branched through the action of starch branching enzymes and these de branching enzymes have also been known to be responsible in forming the final structure of amylopectin . By understanding the genes involved in the starch biosynthesis pathway ,prediction of starch structure and moleculer constituents from specific plants or biotechnologically modified starches can be made .Starch Granule Diversity:
The diversity in the starch granules from various botanical origins is illustrated in Table â€¦..
There is a wide variety of sizes and content of constituent macromolecules molecules within each type of granule . This is obviously relevant to the final macromolecular structure and morphology of starch based plastics derived from these various starch sources.
The major macromoleculer components of starch are amylase and amylopectin . Amylose is essentially a linear molecule of (1-4) linked Î±-D-glucopyranosyl units with some slight branches by (1-6)linkages. Typically amylase molecules have a moleculer weights ranging from 10^5 to 10^6 gmol^-1(Buleon,1998;Roger1996)
Amylopectin is a highly branched molecule composed of chains of Î±-D-glucopyranosyl residues linked together mainly(1-4)-linkages but with (1-6)linkages at branch points . Amylopectin consists of hundreds of short chains of (1-6)linked Î±-D-glucopyranosyl interlinked by (1-6)-Î±-linkages as shown in figure â€¦.. It is an extremely large and highly branched molecule (moleculer weights ranging from 10^6 to 10^8gmol^-1 and thus it is unique compared to the synthetic polymer (Thompson 20000. Moreover it has been established that the branching within amylopectin is not random (Parker and Ring 1998;Thompson ,2000)
The basic organization of amylopectin chains is usually explained in terms of A,B and C chains ,which is based on how side chains are linked to the backbone of the molecule . The concept of A,Band C types of chains in amylopectin was first introduced by Peat (1952)and is shown schematically in Fig â€¦
There are three types of crystallinaty in starch as observed in the X Ray diffraction pattern (Fig 6.5). They are A type mainly cereal starches such as maize ,wheat ,and rice ; B type such as tuber starches (potatoe,sago);and finally C type crystrallinity which is the intermediate between Aand B type crystallanity normally found in bean and other root starches (Blanshard,1987;French,1984)Another type of crystallinaty is the Vh type which is the characterstic of amylase complexed with fatty acids and monoglycerides.
Thermoplastic Starch Processing
Plasticised starch is essentially starch that has been modified by the addition of plasticizers to enable processing . Thermoplastics starch is plasticized starch that has been processed typically by using heat and pressure)to completely destroy the crystalline structure of starch to form an amorphous bthermoplastic starch . Thermoplastic starch processing typically involves an irreversible order disorder transition termed gelatinization within the starch granules and this process is effected by starch- water interactions .Due to its relevance in starch processing both in food and non food industry ,starch gelatinization has been extensively studied in the past decades . Most starch processing involves heating in presence of water and some other additives (for instance ,sugar and salt to control gelatinization in the food industry or glycerolas a plasticizer for biodegradeable plastics applications.)
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Understanding the mechanisim of starch gelatinsation and how the starch characterstics dictate the gelatinization behaviour is therefore necessary for better and more effective control of the structure development during processing and to allow the design of optimum processing conditions of starch polymer blends .
There are several ways a polymer may degrade in the environment . These include biodegradation ,photodegradation,oxidation and hydrolysis.The vbreakdown of a plastic into small ,invisible fragments is biodegradation which in reality these fragments may remain in the environment over a significant period of time.Standard test procedures are available to evaluate the biodegradeability of the plastics.The absence of polymer and residue in the environment indicates complete biodegradation ,whereas incomplete biodegradation may leave polymer and residue as a result of polymer fragmentation or metabolism in the biodegradation process.Faliure in one test doesnot necessarily exclude biodegradation,it only indicates environmental conditions and the time frame where the experiment was conducted ,no biodegradation occurred.The requirement of the biodegradation process is a well defined environment where the desired microorganisms thrive . Factors that effect the growth of microorganisms include appropriate temperature range,moisture level,oxygen ,trace materials,pH,redox potential,environmental stability and pressure. If any of these factors are not in the suitable range ,the rate of biodegradation may be reduced .
Biodegradeable polymers can be either natural or synthetic. Similarly,biodegradeable polymers can be synthesized from renewable or petrochemical resources. It should be noted that the origin of the material and biodegradability are not related . Natural polymers like polysaccharides(starch and cellulose ) are biodegradeable.
Biodegradation Mechanism :overview
Biodegradeable polymers are generally degraded through two steps of primary degradation and ultimate biodegradation.Primary degradation is the main chain cleavage forming low moleculer weight fragments that can be assimilated by the microbes. Moleculer weight reduction is mainly caused by the hydrolysis or oxidative chain scission . Hydrolysis occurs using environmental water with the aid of an enzyme or under non enzymatic conditions. In this case ,autocatalysis,heat or catalytic metals are often responsible for the hydrolysis. Oxidative session occurs mainly by oxygent ,a catalytic metal ,UV light or an enzyme .
This chapter highlights the progress of determination of rheological properties of polymers with the help of capillary rheometers. Various concepts regarding effects of various parameters including temperature,pressure ,thermal background ,shear rate and additives on the rheological behaviour of polymer are explained in relation to viscosity,shear rate and shear stress.
Polymers normally shows viscoelsatic properties during processing . The rheological behaviour depends on the way polymer is being deformed..Flow behaviour of polymer is studied in order to measure the melt viscosity for a particular processing condition ,to investigate the processability and stability of materials and to evaluate the effects of processing variables such as temperature pressure and changes in viscosity.The rheological properties of polymer varies in injection moulding and extrusion process.In injection moulding viscosity,elasticity,changes in molecular structure ,mechanical behaviour of moulding are determined.During process of cooling the elastic characterstics are obvious . As shear rate or shear stress increases the viscosity of a polymer increases.The viscosity varies during the processes of injection moulding,filling,packing and cooling.The rhelogical properties are very significant because they depend on pressure and time during processing process of injection moulding.In order to avoid melt instability and thermal degradation the residence time and volumetric flow rate are to be controlled.The thermosplastic flow behaviour is complicated by various factors of flow rate,temperature and pressure.This complicated flow behaviour is usually demonstrated with the help of graphs between shear rate and shear stress and viscosity and shear rate.These properties are measured with the help of capillary rheometer.
The capillary rheometer consists of a barrel best described as heated barrell at the bottom with the capillary die.The piston is moved with a constant speed and extrusion of molten polymer takes place through a capillary die.Aconstant shear and flow rate is achieved.At the bottom of the barrel above the die face is the transducer which measures the melt pressure as a result of moving piston.The polymers exhibit pseudoplastic behaviour because viscosity of polymer is a decreasing function of shear rate.The flow characterstics for a limited range of shear rates are demonstrated with the help of power law equation as follows:
Ï„ = Î³â¿
where Ï„ is the shear stress N/m2,Î³ is the shear rate (s^-1) and n is the power law index.
In case of polymer melts different plots of shear stress and shear rate are obtained by using different dies.
The degradation most commonly occurs at a processing temperature of a polymer.The change in molecular mass is determined by shear and temperature effects.The molar mass changes occur due to chemical and mechanical processess.The viscosity of polymer depends on temperature and pressureTemperature increase will result in decrease of melt viscosity of polymer.The measurements of rheological characterstics of polymer melts are influenced by the equipment design because of the pseudo plastic behaviour of viscosity. The rheological behaviour change with the varion in processing time.Shear and thermal background behaviour of polymer involve variations in molecular mass of polymers thus significantly effect rheological properties.The melt viscosity of polymer decrease with temperature and increase with pressure.
Tensile Strength of Starch
Starch granules are partly crystalline and will upon heating decompose before they melt.If on the other hand starch is heated in an extruder in the presence of a suitable plasticizer ,for example glycerol ,a thermoplastic melt will be formed .This melt can be processed by injection moulding ,film blowing or pressing into true biodegradeable fims ,coatings and articles which could replace petrochemical thermoplasts in application with short durations. Thermoplastic starches will typically contain 70 -90% starch ,the rest being plasticizer or other biodegradeable additives.They are food- and bio- compatible compatible with natural fibresand polar polymers,of a competitive priceand of reasonable strength without addition of synthetic polymers.The physiochemical properties of the thermoplastic materials vary strongly with the type of starch as shown in Figure 1.
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A clear strength on the amylase- amylopectin can be noted ;the higher this ratio ,the stronger the materials usually are . However despite the even lower amylase /amylopectin ratio can be noted ;the higher this ratio the stronger the materials usually are .The mechanical properties are influenced by the type and concentration of the plasticizer and the extrusion parameters .
An investigation of structure function relationships concerning starch and plasticizers will help to specifically modify the properties of the product. During the extrusion process the partly crystalline granules have to be transformed into a homogenous melt during which the starch will fully or partly destructuralized . It is essential to optimize the process of low temperatures ,low water contents ,predictable crystallinity and crystal size and a minimal degradation of the molecules in order to obtain the plastics of maximal strength . This process is studied by means of Differential Scanning Calorimetery.Biodegradation properties are also studied in this research. The study covers the relationships between raw material composition and structure ,processing history and how this leads to the development of a microstructure which in turns effect biodegradation .
Starch filled polymers Processing Technology
Problems arise in the conversion of starch filled polymers when using conventional techniques of thermoplastic processing and difficulties reported are usually traceable to the careless handling of the master batch leading to excessive moisture pickup.The problems can usually be solved by adding a few percent of calcium oxide during processing.
The pyrolysis temperature of all the common starches is close to
265Ù’ C and because most of the packaging thermoplastics are processed at indicated melt temperatures between 170 and 230 Ù’C ,starch pyrolysis is not usually a problem.Care should be taken in assessing the suitability of particular extrusion process for running starch materials because on very fast operatin production lines ,the use of rather narrow die gaps can cause the elevation of melt back pressure which in turn can give rise to melt temperatures at the screw tip.
Cyclic Conversion Process
A care needs to be taken because if the machine size is not well matched to the cavity volumes then it is possible for the melt to be maintained at high temperature in the barrel and possibly in a hot runner system for excessive time periods which may lead to yellowing the melt .
Thermoplastic starch polymers have been widely investigated since the 1970's and are attractive in that they start from a low cost base and are able to be modified or blended with other polymers in order to 'engineer-up'their processing and properties.
The review of initial research on processing ,rheology and properties of thermoplastic starch is provided by Lai and Kokini (1991)and concentrates on effects of starch constituents and moisture on gelatinization,rheological properties and fragmentation during extrusion of thermoplasticised starch. Effects of temperature and moisture content and additives on the rheological properties of thermoplastic starch have also been examined by Willet(1995a,b)They generally find a power law behaviour of the viscosity -shear rate profile and a reduction in viscosity with increasing moisture ,temperature and plasticizers,with the exception of glycerol monostearate(GMS)which increases viscosity (they propose,due to unmelted helical complexes wof starch - GMS)More detailed studies of effects of structural changes and rheological properties ( Della Valle et al ,1998;Dintzis et al ..1995) highlighted the importance of semi- crystalline gel like structure of starch on the rheological properties and the ability of strong shear conditions to disrupt this structure .
Jane et al,1993 also examine the effects of the addition of various salts on the breakdown of starch structure where the salts interact with free water to effect plasticization . They find that there are both water structure effects (that depend on a salt charge density as to whether salt increases (high charge density ;structure maker with water ,ex NaCl)or decreases (low charge density ;structure breaker with water ,ex NaCl)gelatinization temperature 0and electrostatic effects .Onteniente et al.(1998)examines the extrusion blending of starch plastics with epoxidised linseed oil and found some improvement in water resistance offorded by the oils
In 1989 studies on EAA- thermoplastic starch films ,containing 40% by weight of EAA ,processed at water contents lower than 2%,led to the improved processability and film properties with elongation at break upto 200%. By microscopic analysis it was possible to observe atleast three different phases.one consisting of destructured starch ,one consisting of synthetic polymer alone and the third one described as interpenetrated characterized by a strong interaction between the two components . As a confirmation phase changes observed by DSC ,Nuclear magnetic resonance(NMR)for starch -EEA-PE films showed atleast four phases.It was demonstrated that a portion of the starch forms complexes with EEA when EAA is salified by ammonium hydroxide or other salts during extrusion cooking,providing partial miscibility between two polymers.
Rheological studies were performed on a product consisting of 60% of starch and natural additives and 40 % of EEA copolymer,containing 20 % by mole of acrylic acid . A strong non Newtonian behaviour was shown by the viscosity curves at high shear rates;at intermediate shear rates and the material seemed to approach a Newtonian plateau,while at low shear rates a viscosity upturn was observed.as shown by fig â€¦
Pg 120 degradeable polymers principles and applications
It suggests the presence of yield stress.Breaking Stretching data for the same material are also reported in the literature
A general study of shear flow characterstics was performed on a material containing about 60% of starch and natural additives and 40 % of ethylene- vinyl alcohol copolymer 40/60 mol/mol. A strong pseudoplastic behaviour at high shear stresses as well as yield stress at lower ones was detected.High levels of melt viscosity were detected from steady shearing tests,whereas recoverable fraction was almost negligible,at least for a reasonable timescale.The peculiar viscous and elastic behaviour has been explained on the basis of the droplet-like morphology generated by the ability of starch to form V complexes in the presence of EVOH. Not withstanding the rheological behaviour shown by starch /EVOH systems,traditional processing techniques such as film blowing can easily be applied. .
The products based on starch /EVOH show mechanical properties good enough to meet the needs of specific industrial applications . Their mouldability is comparable with that of traditional plastics such as polystyrene(PS) and acrylonitrile-butadiene -styrene copolymer (ABS) They are sensitive to low humidities especially when in film form.
However,Although these studies are instructive in understanding thermoplastic starch processing it should be noted that thermoplastic starch polymers based only on starch are extremely water sensitive ,can suffer from a significant moleculer weight change in extrusion (Davidson et al,1984;sugar and Merrrill,1995a;Gomez and Aguilera,1983)and are thus of limited practical value.Therefore most of commercial research on the thermoplastic starches has involved modified starches and /or blends with additives and other polymers .Early work on thermoplastic starch blends includes the study by Otley(1974) who investigated castable degradable mulch film derived from starch polyvinylacetate(PVCc)blends with Polyvinyl chloride (PVC)coatings.Additionaly Otley et al (1980.1987)investigated the development of blown starch based agricultural mulch films based on gelatinized corn starch ,Polyethylene(PE)and polyethylene-acrylic-acid(PEAA)polymers and various additives.Good film performance was achieved with increased ammonia and urea (to improve starch -EAA interactions)Bastioli et al (1994a,b)and Shogren et al (1993)also investigated starch -ethylene-vinyl alcohol (EVOH)blends and Novamont MaterBi(Starch-EVA)materials looking at the ratio of amylase and amylopectin and moisture content in the starch . However note that these studies are symptomatic of early biodegradable plastics work ,where the material was not entirely biodegradeable and did not meet biodegradeability standards(De Kesel et al ,1997;Breslin,1993).But ofcourse it should be noted that much of this earlier work has led to the science behind modern biodegradeable blends.
Mao et al. (2000) examined the extrusion of thermoplastic corn starch-glycerol-Polyvinyl alcohol (PVOH) blends and noted the effect of PVOH to improve mechanical properties and slow biodegradation .Much debate on PVOH degradability has been well reviewed by Chiellini et al . (2003) which summarises that molecular weight and type of PVOH may effect its biodegradability,which although much slower than starch ,appears to show degradability with specific enzymes.Doane(1992) has reviewed thermoplastic starch /biodegradeable blends research and highlighted work at USDA focusing on developing either starch/biodegradable blends,starch Polyactic acid (pla),starch/Polyhydrooxybutrate(phb) or starch with grafted thermoplastic side chains.
Ratto et al.(1990)has examined poly butylenes succinate (pbsa) terpolymer/granular starch composites for blown films and showed they could produce good film tensile properties and control biodegradation with granular starch additition .Thermoplastivc starch /cellulose fibre extrudates and injection moulded products were examined by Funke et al (1998)and they showed a reduction in water adsorption with increasing fibre content . Work by Halleyet al . (2001)examined the use of thermoplastic starch polyester blends for use in mulch film applications noting excellent field performance and biodegradability for those materials.
Modified processing techniques have also been useful for thermoplastic starch polymers . Recent work by Martin et al (2001) and Martin and Avernous(2002) has examined the use of coextruded sheet processing to produce polyester /thermoplastic wheat starch/Polyester multi layer films.It was found that adhesion strength between the layers and stability of the interface were crucial properties in controlling the final performance properties of the films. Wwork by Sousa bet al . (2000)has also examined use of the novel shear controlled orientation injection moulding (SCORIM)process to control morphologies and provide tensile property increases of thermoplastic starch /synthetic blends.
In summary this section has demonstrated that thermoplastic starch polymers and their blends provide an exciting foundation for developing low cost biodegradeable polymers .
Modified Thermoplastic Starch Polymers
In terms of modification of starch many laboratory approaches have been taken from acetylation /estrification of starch to starch acetates ,carbonilation of starch with phenyl isocyanates,addition of inorganic esters to starch to produce phosphate or nitrate starch esters ,production of starch ethers ,and hydroxyl-propylation of starches via propylene oxide modification (Gilliard,1984). Generally all these modifications involve hydroxyl group substitution on the starch will lower gelatinization temperatures ,reduce retrogradation and improve the flexibility of final products.
Takagi et al (1994)examined corn starch acetate/polycaprolactone(PCL) blends and showed that blends are able to maintain biodegradability and have stable viscosities with increasing acetylation . This is due to the increasing stability and thermoplasticity afforded by acetylation . Increasing acetylation also reduces retrogradation . Tomasik et al . (1995) examined the acetylation of starch via extrusion with succinic, maleic and phthalic anhydrides and found a decrease in water binding capacity of the extrudates. However they also noted the reactive extrusion process was difficult to control m. Fringant et al.(1996) examined the acetylation of starch via the pyriding -acetic anhydride procedure and produced modified starches with DOS of 1.7 that were easily processable but had detoriated in some mechanical properties . Of course acetylation levels can reduce biodegradation of the starch.polymer(Rivard et al 1995) so as a balance between property water resistance and biodegradation must always be maintained.Fatty acid ester modification of starches were also examined by Sugar and Merrill(1995b),however although thermoplastic processible starch-ester polymers could be produced ;it was stated that the additional cost of these modifications would limit the industrial use of these materials .The modification of thermoplastic starch based polymers ,its property and processing improvements will help thermoplastic starch polymers widen their application producta and markets.
One of the first thermoplastic starch polymer based products developed was the National starch expanded starch foam packaging material ECO-FOAM. ECO-FOAM material was derived from maize or tropioca starch and include modified starches.This relatively short term protected -environment packaging use is ideal for a range of applications including injection moulded toys ,extruded sheet and brown film applications.
Novamont has been developing thermoplastic starch based polymers since 1990. Mater- Bi polymers are based on thermoplastic starch-blend technologies and product applications include biodegradable mulch films and bags,thermoformed packaging products ,injection moulded items ,personel hygiene items and packaging foam.
Rodenberg Biopolymers produce Solanyl a thermoplastic starch based biopolymer which is focused on injection moulding applications . Solanyl is derived from patatoe peels.Earthshell are also developing thermoplastic starch based materials for thermoformed tray applications.
Very recently Plantic Technologies Ltd produced soluble Plantic thermoformed trays for confectionery packaging.
The research and development has focused on engineering,water resistance and better mechanical properties into starch based materials . However there is an existing and growing interest in soluble biodegradable polymer applications . An overview of environmentally biodegradable water soluble polymers is given by Swift(2002)Swift first characterizes the breakdown of soluble biodegradeable polymers as either single stage via an equatic environment or two stage via water treatment and thus subsequent treatment such as composting or digestion .
The engineering of more advanced properties into the low cost starch materials will continue to be the main technological drive into the future.