1) Regular plastic is manufactured from non-renewable resources, such as oil, coal, and natural gas. These plastics will not biodegrade, because they contain long polymer molecules that are too large and tightly bonded with each other for decomposer organisms to break them down and absorb them (Christie 2002).
2) In an effort to overcome this environmental problem, biodegradable plastics made from renewable resources, like plants, are being used to manufacture biodegradable plastic. The definition for biodegradable is that “a substance is able to be broken down into simpler substances by the activities of living organisms, and therefore is unlikely to persist in the environment” (Christie 2002).
3) Plastics that are derived from natural polymers found in wheat, corn, or potato starch are composed of molecules that can be easily broken down by microbes. The reason plastic can be produced from starch is because “starch is a natural polymer… a granular carbohydrate produced by plants during photosynthesis”. Starch can be manufactured directly into a bioplatisc. However, because it is soluble in water, items manufactured from starch are limited in their use; since they tend to “swell and deform when exposed to moisture”.
4) This dilemma can be resolved by transforming the starch into a different polymer. To begin with, starch is collected from corn, potatoes, or wheat. After that, microorganisms convert the starch into lactic acid, a monomer. Lastly, the acid is treated chemically to rearrange the molecules of the acid into long chains or polymers, that bond together to form a plastic called polylactide (PLA). Another way to solve the problem is by mixing quantities of other biodegradable plastics with the starch, to create a waterproof product that does not degrade as rapidly (Christie 2002).
5) A different way to produce biodegradable plastic is from bacteria. The bacteria create a plastic called polyhydroxyalkanoate (PHA) within their cells. Then, the plastic is simply harvested from the bacteria. Scientists have also taken genes from this particular type of bacteria and “stitched them into corn plants, which then manufacture the plastic in their own cells” (Christie 2002).
6) Alexander Parkes was the inventor of plastic. He first introduced plastic to the world at London's Great International Exhibition in 1862. Parkes's plastic was made from “an organic derivative of cellulose that could be molded when heated and then maintained its shape upon cooling”. However, plastic was not very popular or widespread until after World War I. Before World War I, plastic was made from a substance created from coal tar. Petroleum, however, became easily accessible after the war, and proved to be a more easily processed material than coal. Today, the main ingredients in plastic are made from petroleum and natural gas (Lajeunesse 2004).
7) Plastics are made of polymers, large molecules of repeating units called monomers. In plastic bags, the strength of the plastic is the “degree of branching of the polymer chain”. Branching determines many physical properties of plastic, such as tensile strength and crystallinity. The more branched a molecule is, the lower its tensile strength and crystallinity are (Lajeunesse 2004).
8) Different types of plastics are made differently. One method is the Ziegler-Natta vinyl polymerization, invented by Karl Ziegler and Giulio Natta. It is a method that employs a transition-metal catalyst to instigate polymerization. Another method for producing polyethylene is the metallocene-catalyzed polymerization. It is similar to the Ziegler-Natta method, in that both methods use metal complexes as catalysts and create unbranched polyethylene. Branched polyethylene, is created using a different process called free-radical polymerization. It uses an initiator molecule, like benzoyl peroxide, as opposed to a metal complex (Lajeunesse 2004).
9) Plastic bags are mostly polymer. A polymer chemist at Occidental Chemical Corporation and president-elect of the American Chemical Society, William F. Carroll Jr. stated that "with the possible exception of a little lubricant to help in extrusion, plastic bags are pretty much just the native polymer" (Lajeunesse 2004).
10) Biodegradable bags can be made from starch acquired from corn or potatoes. The starch is changed into lactic acid, which is then polymerized to form the biodegradable plastic known as polylactide (Lajeunesse 2004).
11) Plastics are polymers, which are “simple molecular configurations of carbon and hydrogen atoms that link together repeatedly to form chains”. Natural polymers exist, such as spiders that spin their polymer fibers called silk, and trees that make cellulose and lignin (Weisman 2007).
12) Chemists were able to break down the long, hydrocarbon chain molecules of unrefined petroleum into smaller ones, and mixed the smaller chains with other chemicals to produce different polymers. The addition of chlorine produced a strong, resilient polymer known as PVC. In another polymer, if gas was blown into it as it was forming yielded durable, linked bubbles called polystyrene, more commonly known as Styrofoam. The never ending quest for man-made silk led to the creation of nylon (Weisman 2007).
13) Plastics can biodegrade two different ways. One way is when the polymer molecules in plastic separate into carbon dioxide and water, their initial components. Another way is to photodegrade. The ultraviolet solar radiation shatters the long, chainlike polymer molecules found in plastic into shorter segments; which greatly deteriorates the tensile strength. Due to the fact that the strength of plastic is dependent on the length of its entwined polymer chains, the plastic begins to decompose when the UV rays break the polymer chains (Weisman 2007).
14) Plastic is an artificial macromolecular substance that is typically made from petroleum and regarded as non-degradable. However, since plastic is made from polymers, the use of natural polymers, such as starch, to manufacture plastic would result in biodegradable plastic. Starch “is regenerated from carbon dioxide and water by photosynthesis in plants”, and is completely biodegradable (Lu 2009).
15) “Starch is mainly composed of two homopolymers of D-glucose: amylase, a mostly linear α-
D(1, 4’)-glucan and branched amylopectin, having the same backbone structure as amylose but with many α-1, 6’-linked branch points (Figure 1). Evidently, starch is hydrophilic. The available hydroxyl groups on the starch chains potentially exhibit reactivity specific for alcohols. In other words, they can be oxidized and reduced, and may participate in the formation of hydrogen bonds, ethers and esters” (Lu 2009).
16) Starch is completely biodegradable in many different environments. Starch “can be hydrolyzed into glucose by microorganism or enzymes, and then metabolized into carbon dioxide and water”. In addition, the carbon dioxide can be recycled into starch again by plants and sunshine; so the starch is also considered a renewable resource. Starch is “poor in processability, also poor in the dimensional stability and mechanical properties for its end products”, which is why pure starch is not used directly in the making of plastics (Lu 2009).
17) The bioplastics commonly used are mostly starch based. There are starch bioplastics made with native or slightly modified starches that are either isolated or mixed with natural or synthetic molecules. There is also the lactic acid polymerization (PLA), in which plastic is created through the fermentation of starch (Vipoux 2009).
18) The production of biodegradable plastic derived from starch has one major drawback. Starch has a tendency to become deformed when in contact with moisture. This severely limits its use. To counteract this weakness, starch is used in mixtures with other biodegradable polymers and plasticizing elements (Vipoux 2009).
19) The biopolymers created from fermentation, such as PLA that is made from sugar or starch, have done remarkable well. Higher levels of moisture resistance have led to more possible uses for starch based biodegradable plastics (Vipoux 2009).
20) Starch is made from two macromolecules with the same structural units: “1, 4-D-glucopyranose in linear (amylose) and highly branched architectures (amylopectin)”. The two units are present in dissimilar proportions in starch, depending of the species that yields it. (Belgacem 2008).
21) Starch is made from energy of the sun during photosynthesis. It is a food reserve for plants, and widely abundant in many crops, especially corn and potatoes. Starch is completely biodegradable in various different environments. The “degradation or incineration of starch products would recycle atmospheric carbon dioxide trapped by starch-producing plants and would not increase…global warming” (Bastioli 2002).
22) Starch is composed mainly from two key components: “amylase, a mostly linear alpha-D(1-4)-glucan and amylopectin, and alpha-D(1-4) glucan which has alpha-D(1-6) linkages at the branch point” (Bastioli 2002).
23) In nature, starch is found in the form of crystalline beads. The starch beads in plastics can be utilized as fillers or transformed into thermoplastic starch that can be processed by itself or mixed with specific synthetic polymers. To manufacture starch thermoplastic, starch’s “crystalline structure must be destroyed using pressure, heat, mechanical work, and plasticizers such as water, glycerin, or other polyols” (Bastioli 2002).
24) Starch can be utilized as natural filler in plastics. When polyethylene films are combined with starch beads, it biodeteriorates when exposed to a soil environment. Starch-filled materials, starch, and its derivatives are all biodegradable or partially biodegradable; due to the fact that starch granules in the materials increase the available surface area for microorganisms to break down (Bastioli 2002).
25) Polymers from renewable resources are “natural products that are polymeric in character as grown or can be converted to polymeric materials by conventional or enzymatic synthetic procedures”. Most of the petroleum based plastics have a long life span and are near indestructible. Plants and living organisms produce natural polymers, or biopolymers, through a biosynthetic process that involves carbon dioxide consumption. As a result, natural polymers are “ultimately degraded and consumed in nature in a continuous recycling of resources” (Bastioli 2002).
17) The bioplastics commonly used are mostly starch based. There are starch bioplastics made with native or slightly modified starches that are either isolated or mixed with natural or synthetic molecules. There is also the lactic acid polymerization (PLA), in which plastic is created through the fermentation of starch (Vipoux 2009).
18) The production of biodegradable plastic derived from starch has one major drawback. Starch has a tendency to become deformed when in contact with moisture. This severely limits its use. To counteract this weakness, starch is used in mixtures with other biodegradable polymers and plasticizing elements (Vipoux 2009).
19) The biopolymers created from fermentation, such as PLA that is made from sugar or starch, have done remarkable well. Higher levels of moisture resistance have led to more possible uses for starch based biodegradable plastics (Vipoux 2009).
20) Starch is made from two macromolecules with the same structural units: “1, 4-D-glucopyranose in linear (amylose) and highly branched architectures (amylopectin)”. The two units are present in dissimilar proportions in starch, depending of the species that yields it. (Belgacem 2008).
21) Starch is made from energy of the sun during photosynthesis. It is a food reserve for plants, and widely abundant in many crops, especially corn and potatoes. Starch is completely biodegradable in various different environments. The “degradation or incineration of starch products would recycle atmospheric carbon dioxide trapped by starch-producing plants and would not increase…global warming” (Bastioli 2002).
22) Starch is composed mainly from two key components: “amylase, a mostly linear alpha-D(1-4)-glucan and amylopectin, and alpha-D(1-4) glucan which has alpha-D(1-6) linkages at the branch point” (Bastioli 2002).
23) In nature, starch is found in the form of crystalline beads. The starch beads in plastics can be utilized as fillers or transformed into thermoplastic starch that can be processed by itself or mixed with specific synthetic polymers. To manufacture starch thermoplastic, starch’s “crystalline structure must be destroyed using pressure, heat, mechanical work, and plasticizers such as water, glycerin, or other polyols” (Bastioli 2002).
24) Starch can be utilized as natural filler in plastics. When polyethylene films are combined with starch beads, it biodeteriorates when exposed to a soil environment. Starch-filled materials, starch, and its derivatives are all biodegradable or partially biodegradable; due to the fact that starch granules in the materials increase the available surface area for microorganisms to break down (Bastioli 2002).
25) Polymers from renewable resources are “natural products that are polymeric in character as grown or can be converted to polymeric materials by conventional or enzymatic synthetic procedures”. Most of the petroleum based plastics have a long life span and are near indestructible. Plants and living organisms produce natural polymers, or biopolymers, through a biosynthetic process that involves carbon dioxide consumption. As a result, natural polymers are “ultimately degraded and consumed in nature in a continuous recycling of resources” (Bastioli 2002).
Andrea Curiacos Bertolini, renowned food scientist: 26) Biodegradable plastics are plastics that are derived entirely or partially from renewable raw materials. Typically, “bioplastics contain biopolymers as essential components, plasticizers, and other additives”. At first, bioplastics were substantially more expensive than their petroleum-based counterparts, but that gap is decreasing due to rising oil prices and environmental damage. Although there is a large variety of bioplastics available on the market, “the most important are those composed of starch-starch blends”. According to Bastioli, 75-80% of the global market for biopolymers is starch-starch blends (Bertolini 2009).
27) Whenever starch is used in bioplastics, with the exception of when it is used as filler in reinforced plastics, it “requires the transformation of semi crystalline starch granules into a homogeneous, essentially amorphous matrix, in order to enhance the processability as compared to granular starch”. The disturbance of the molecular order of the granules can be achieved by using thermal/mechanical energy and adding plasticizers. Starch in this form is referred to as destructurized starch and thermoplastic starch (Bertolini 2009).
28) The moisture induced plasticization of a polymer results in “an increased intermolecular distances, decreased local viscosity, and increased backbone chain segmental mobility”. Because of the hydrophilic nature of starch, the water content of starch-based substances depends on air relative humidity during the processing and storage, which directly affects the physical properties (Bertolini 2009).
29) The early forms of thermoplastic’s uses were restricted since the starch was mixed with water as the plasticizer and the starch degraded as a result of water loss at higher temperatures. To solve this dilemma, a starch-based composition was made by blending starch with “an appropriated addititive or plasticizing agent, such as glycerol”. The added additive or plasticizer reduces the melting point of starch, and brings it under the decomposition temperature. They are also used to augment the “flexibility and processability of a polymeric compound by decreasing the hydrogen bonding of the polymeric chains…[leading] to an increasing free volume or molecular mobility of polymers”. The type and quantity of plasticizer greatly influences the physical properties of the processed starch, because the plasticizer controls the starch’s destructuration and depolymerization (Bertolini 2009).
30) Generally, the surface of starch films is made up of a network structure. This structure is most likely composed of crystalline or co-crystallized amylose and amylopectin strands. For such a network to form, high moisture content or slow cooling rate of melt during processing is needed to supply adequate molecular mobility (Bertolini 2009).
31) A major drawback of starch films is that although they have high tensile strength, they are extremely brittle. At low relative humidity, the film is too brittle; while at righ relative humidity, the film is too soft. To solve these problems, different strategies are employed “such as chemically modified starch in formulations; suitable plasticizers; association of biodegradable, renewable, or synthetic polymers to thermoplastic starch; and surface modifications of starch plastics” (Bertolini 2009).
32) Plasticizer can affect the aging through crystallization and water sensitivity. Therefore, the choice of plasticizer is crucial. The plasticizer must “impart flexibility…and suppress retrogradation during the ageing time”. Many different kinds of plasticizers have been integrated with pure thermoplastic starch, such as water, glycol, sorbitol, urea, amide, sugars, and quaternary amine. In addition, starch is often mixed with biodegradable polymers (Bertolini 2009).
Glycerol is produced from the fermentation of sugar, or as a byproduct of soap. At least one biopolymer and one plasticizer are required in order to make plastic. The more plasticizer plastic contains, the more flexible the plastic. However, the less plasticizer plastic contains, the stronger the plastic will be; but it will be substantially less flexible (Stevens 2002).
Glycerol is a commonly used plasticizer. It makes the plastic flexible, even at colder temperatures. However, if too much glycerol is added to plastic then the resulting plastic “turns to gum” at higher temperatures; like those of a microwave. More importantly, over time, glycerol has a tendency to lose its usefulness as a plasticizer; which leads to an increase in brittleness in the plastic.
Sorbitol is the opposite of glycerol. It is more resistant at warmer temperatures, but quickly becomes brittle at colder temperatures. However, as a plasticizer, it lasts longer than glycerol.
Ager passes on its good properties when it is made into plastic. In addition, in plastics constructed from ager and glycerol, the glycerol’s effectiveness is increased; because the “ager seems to slow down the increase to brittleness”. Though ager is a favorable ingredient to use in the making of biodegradable plastics, compared to starch, it is significantly more expensive. The higher cost greatly restricts its large scale use. But, plastics made from blends of ager and starch is stronger and more affordable.Plasticizers are a crucial ingredient in the making of plastics. The addition of plasticizer makes plastics softer and more flexible; which also makes plastics easier to mold and deform. Plasticizers are “organic compounds with relatively low molecular weight” that are physically incorporated into the polymers; they are not connected to the macromolecules with chemical bonds. The role of plasticizers in plastics is similar to that of hinge-joints. Plasticizers act as ‘hinge-joints between the molecular chains and lead to slight deformation, respectively, displacement under mechanical stress” (Habenicht 2009).
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