Plastic is a group of natural or manmade materials that can be heated and molded into different shapes which are retained after cooling. Plastic has been in use since the mid 19thcentury. Plastic materials are used for different applications which range from utensils to packaging materials among many other uses. Plastic has been produced from many derivatives. The success of any of these derivatives depended on how fit the product was when in use. Cost was another factor which determined whether the plastic made from a given derivative would be acceptable for widespread use by the public or not.
With advancement in human knowledge, attention has shifted to the effect the use of any product would potentially have on the environment. This is because people are becoming more and more environmentally conscious. In addition, the general public, as well as manufacturers are considering the long-term availability of a given resource, especially in day to day use. Everybody would like to get a form of assurance that substitutes to plastic products will be available after depletion of the natural reserves of crude oil.
The most common use of plastic materials is, perhaps, their use as packaging material, because they are tough and can be airtight. Additionally, they can preserve packaged materials for a long time as they are not affected by the environment. The traditional plastic materials are made from nonrenewable sources and derived from petrochemical sources. This creates a concern relating to their availability in the future.
A renewable source of plastic has to be a sort of guarantee of the future supply
Another issue arising from the use of traditional plastic is its ability to persist in the environment, leading to its degradation. The remedy to this is collection and recycling which can be expensive. Recycling is not always done. Even when recycling is done, not all products can be recycled. Furthermore, recycling does not eliminate the need of use of petrochemicals for production of plastic. The plastic that has been in use by the public is non-biodegradable according to various standards for assessing biodegradability. Most of the plastic packaging materials have been seen to persist without much alteration when left in common composting environment for around six months, and this has been the basis of labeling them as non-biodegradable.
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What is observed above happens due to these materials comprising unusually long polymers composed of closely associated atoms, which cannot be broken down. On the other hand, production of biodegradable plastic has been known for a long time. However, the cost of this production has been exceptionally high compared to the non-biodegradable ones, and this has not provided a relief to the environment, especially regarding the adverse effects caused by the non-biodegradable material (Chiellini & Solaro, 2003). There can be identified many ways through which biodegradable plastic has been made over time. The emphasis is on getting the cheapest method for commercial production of packaging bags and other plastic products, to replace the non-biodegradable packing materials.
The earliest attempts to make biodegradable plastic involved the use of starch. Starch is a natural white crystalline polymer that is made by green plants during photosynthesis. It is a medium through which plans store energy for future use. Cereals and tubers have a lot of carbohydrates stored in them. Starch was sometimes converted into bioplastic and found several uses; one of the greatest limitation of this plastic was that it absorbed water, and this limited its use. Later on, scientist came up with methods of breaking starch into lactic acid monomer through the use of some microorganisms. The product of this process was called polylactide (PLA) and was later converted into starch. This was achieved by causing the lactic acid monomer to link into long chains of polymers that could be used as plastic. This product found various applications among them the use for pots for plants, as well as, nappies. The products of polylactide have been commercialized for the last 30years and have found significant use in several fields, e.g. medicine. The major disadvantage of these materials is that they are made from expensive raw material (polylactide).
Deeper and more refined research into biodegradable plastic found out that bacteria can be used to manufacture biodegradable plastic
This research, as well as its finding was inspired by the need to come up with cheaper and more adaptable bioplastic that could be used to replace traditional one. In this method, the bacteria are made to produce small granules of biodegradable plastic called polyhydroxyalkanoate (PHA).The bacteria are cultured and accumulate the above compound inside their cells. Both the PLA and the PHA are still hugely expensive and have not yet served the purpose of successfully replacing the various petrochemical plastic materials.As plants store their energy and carbon in the form of carbohydrates, bacteria store them in the form of polyhydroxyalkanoate (PHAs). This compound is produced and accumulated only given favorable conditions by microbes.
The production and accumulation of the compound occurs in the presence of excess carbon. (PHAs) are gaining popularity as a potential replacement for the Traditional plastics, due to the current concerns that require the replacement of non-biodegradable plastic with degradable ones. This is because they have exceedingly similar properties to petrochemical-derived plastics. In addition, they are fully biodegradable in the environment upon disposal. This makes them environmentally friendly in use and also disposal(Xu, 2010).
PHAs is polyesters composed of hydro alkanoates (HAs) this compound was first discovered around the year 1926 in a certain bacteria referred to as Bacillus magisterium. A wide range of bacteria have been found to synthesize and accumulate PHAs in their bodies. They do so as a way of storage of carbon and energy or in order to get rid of some reducing power, especially under the conditions of inadequate nutrients and excess carbon. The stored PHAs can be broken down to provide energy, as well as, carbon when such are needed. The breakdown takes place inside the cells to supply these requirements. The breakdown happens when there is an adequate supply of nutrients after PHAs have been formed.
PHAs are made up of carbon chains, which are bound to hydroxy alkanoic acids on the one hand, and other molecules on the other. The carbon chains have between 3-14 carbon atoms. The chains are extremely varied in characteristics, with vast variations occurring in relation to branching, as well as the level of saturation. The molecular mass of PHAs is varied depending to the microorganism that produces them, and conditions of their growth. The PHAs are accumulated in the cells in the form of granules and their occurrence, and their arrangement is dependent on the organism that produces them. The size of the granules and the number of granules per cell will depend on the different species of bacteria. The average diameter of the granule is about 3.5 micrometers ant the number of these granules per cell is about 7-14. The PHAs, when observed under the electron microscope, appear like pigments with extremely high refractive index. The PHAs is a pigment that aid in staining and is useful in microscopy as it makes organelles visible.
Roughly, the number of bacteria that have been proved to be able to produce and accumulate PHAs is around 250. All these bacteria have different morphology, as well as cell wall composition. There are two broad categories of PHAs depending on the length of the carbon chain one polymer.
The PHAs with the carbon chain having 3-5 carbon atoms are described as short-chained. On the other hand, the PHAs that have 6-14 carbon atoms are said to be medium-length PHAs. Many samples of actinomycetes were collected in Saudi Arabia for trials that examined their ability to produce biodegradable plastic molecules(Ghaffar, 2002). Around 80% of all the samples that were collected were found to be able to produce and accumulate polyhydroxyburate (PHB) in quantities of between 0.5-9percent of their body’s dry weight.
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The optimum accumulation rate of (PHB) was found to take place at conditions of 30degrees Celsius, and the total period of around 7 days. The isolation medium was a combination of actinomycetes isolation agar and starch nitrate agar. Addition of actinomycin was optional and did not have effects on bacterial growth. NaCl was added at a concentration of around 10%, which is crucial for production and accumulation of PHBs. Of all the bacterial isolates, one isolate Streptomyces (NM10) accumulated the biggest percentage of its dry weight, and this further attracted the attention of researchers to use it for further research.
The compound accumulated here was PHB. The best isolate was to be subjected to further biochemical, physiological and morphological tests to find the reasons behind its performance, as well as, potential for commercial exploitation. Further work of identification was done which established that this isolate was Streptomyces species NM10.The optimum conditions for this type of bacteria were found to be a pH of 6.5, incubation period of 5 days, a temperature of 25degrees Celsius and rotation at a rate of 120 rpm. These enhanced the bacterial growth, production and accumulation of PHB from 9% to above 15%. This was not the final step in attaining biodegradable plastic. The product of this was not yet bioplastic but polymer that can be used to manufacture biodegradable plastic.
Synthesis of PHAs can be done by a wide range of bacteria, but the bacterium with the highest yield is the Streptomyces NM10. The production and accumulation in the bacteria cell occurred when the bacteria culture was made to grow under stress conditions. The stress required for the formation of PHAs was the one caused by lack of nutrients such as phosphorus, nitrogen, sulphur among others.
The accumulation of PHAs cannot occur without a supply of excess carbon. There are considerations that are made while selecting a suitable type of bacteria for the commercial production of PHAs.Such a microorganism should be easy to culture and have the ability to produce large amounts of PHAs. In addition, the bacteria should be able to utilize cheap sources of carbon in order to achieve the low cost of the product. In order to reduce the overall cost and enhance productivity; several methods have been tested for their efficacy in this matter. Among these methods is the continuous cultivation and the fed-batch method. These methods have been effective in enhancing the production, as well as increasing the yields.
Among many PHAs that have been cultivated only three are the most prominent which include 3-hydroxyhexanoate-co-3hydroxyoctanoate.The last step of PHB production requires is the isolation of the compound from the bacteria’s body(Arshady, 2003). An economical method of extraction is required to do this. In most cases, a solvent is utilized for aqueous extraction process. This method involves breaking of the cell after which the compound is acquired for purification. The advantage of this method is that it is straightforward. Its chief disadvantage is its reducing effect on the molecular mass. The solvent is required in particularly large amounts because of the viscosity of PHAs hence making the method expensive.
During various work of research, the study of chemical, as well as physical characteristics of the plastics that is formed is done. Evaluations are done to determine how close this plastic is to traditional plastic in its use, as well as how unrelated they are regarding their effects on the environment. Most of the research work that is carried out to evaluate the possibility of large-scale production of bioplastic is done using a PHB; 3-hydroxybutyrate-co-3-hydroxyvalerate.
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The bacteria Streptomyces NM10 can accumulate above 15% per body weight of this polymer.
PHB, crystalline in nature, has a melting point of around 175 degrees Celsius. The temperature at which this polymer degrades is only ten degrees above the melting temperature. This makes its molding into various shapes a challenge especially through injection molding method. The important qualities of PHBs include its resistance to moisture, its insolubility in water and its outstanding optical purity. These two advantages are highly crucial as they isolate PHBs from other biodegradable plastics that are available. Another advantage of PHB is its impermeability to oxygen. Other kinds of bioplastic available are optically impure and others are affected by water. The second most serious limitation of PHB from low degradation temperature is its brittleness(Kraak, 1998). PHB shatters readily when its elasticity limit is slightly exceeded, an example of which is 3hydroxyvalerate.
Researchers are working to overcome this through various methods. The first method that is being tested for efficacy includes combining PHB with a comonomer. The second remedy is trying to use it in mixtures with other polymers. In relation to mechanical strength such as the Young’s module, as well as the tensile strength, PHB does not show any difference from polypropylene. However, it is outdone by polypropylene in terms of its resistance to breaking through elongation. There has been production of another improvement of PHB. This copolymer contains 3hydroxyvarerate molecule P (3HB-CO-3HV). This new polymer has more improved the mechanical properties than ordinary PHB. The flexibility, as well as toughness have substantially increased. In addition, it can be molded into various shapes exceedingly easily.
It can be seen that the future of having biodegradable plastic that is cheap and affordable to the public is attainable in the nearest future. Research work started from scratch and is showing enormous potential to provide adaptable solutions. Different alternatives have been discovered and some can be produced in large commercial quantities. The sole limitation of having these products replacing the traditional plastic is their cost of production(Chen, 2010).
The most notable cost in their production is the one of the solvent used to dissolve PHAs or PHB in order to extract them from the cells. Another important cost in their production is the source of carbon. This can also limit the production or make the product less preferred than the existing ones in the market, especially in terms of cost. The above two limitations can be solved over time. Alternatives to these extracting solvents should be taken into account. This will be beneficial to use solvents that are less expensive than the existing ones.
On the other hand, another method of extraction can reduce the cost. Among the steps that have been taken to minimize the cost of the source of carbon include the use of waste carbon source in the agricultural fields. This can go along the way in making the bioplastic produced to be a possible replacement to the traditional plastic. There has been an attempting simulate this biological process chemically, so as to have this process done in the lab. Intense research is underway to explore this possibility.
In conclusion, the dream of having the environment free from deleterious effects of non biodegradable plastic is no longer a mirage. It has been pursued, and the world is getting there in the near future.