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Physical and chemical investigation on Starch Based bio-plastic.

1. Starch Based Bio-Plastics 1.1. Introduction:-

Plastics are one of the most important petrochemical based products and they are used in all aspects of life. Most polymers are oil based and are made by either addition or condensation polymer techniques. Most are considered to have a detrimental effect on the environment as they do not readily biodegrade. The consumption of oil-based plastic in 2007 was 260 million tonnes.1 This is a significant issue as oil resources will dwindle rapidly over the next thirty years. Although polymer collection for recycling is widely carried only a small proportion is actually remade into materials. The majority is incinerated to reclaim energy .There are many other reasons that motivate industries and researchers to find alternatives to non-renewable resources; however it is noted that all replacements for current plastics should meet some important conditions, they need to be low cost, renewable, sustainable and biodegradable. However, environmental issues have led researchers to study the degradation of plastics. The development of short-lived bio-degradable plastic is seen as a major goal and so the use of starch as a natural, raw material for degradable thermoplastics is seen as an important goal. Starch alone cannot, however form plastics with satisfactory mechanical or chemical properties, thus using a modifier is important to improve these properties.

1.2. Polymers The word polymer is derived from the Greek poly meaning many and meros meaning parts. Macromolecule is another term that can be used where a polymer is made up of repeating structural units linked by covalent bonds.Polymer science saw its most significant developments in the 1930 when the first natural polymers were produced.Today polymer science is a very important research subject and plastic production is one of the biggest industries. A large theme of polymer research is modifying the chemical and physical properties and trying to produce environmentally friendly polymers.

Polymers can be categorised in many different ways including their: 1- Source: natural, semi- synthetic and synthetic polymers. 2- Polymer chain type: linear polymers, branched chain polymers and cross linked (network polymers). 3- Mode of polymerisation: addition polymers and condensation polymers. 4- Properties: elastomers, fibres, thermoplastic polymers and thermosetting polymers. 5- Polymerisation method: relying on the polymerisation mechanism e.g. bulk, solution, suspension and emulsion. 6- Monomer composition: homopolymers and copolymer (hetropolymers): block and graft. 7- Functional group type: polyester, polyamide, polyether, polyolefin, etc. 8- The degradability: degradable in nature, degradable in industry and non-degradable polymers. Natural polymers or biopolymers are those formed in the natural environment by living organisms, they are usually extracted from plants or animals. Proteins,natural rubber, and polysaccharides are all examples of natural polymers that are widely used in modern life. There is currently a significant interest in biopolymers as an alternative to oil-based plastics in some applications. The most important advantages of using natural polymers especially those based on proteins and polysaccharides is that they can degrade biologically in the environment much more easily than oil-based polymers, and that they also offer the potential to be renewable.Mankind has used natural biopolymers for millennia and these include wood, leather, wool, cotton and silk. The most common polymers that are used are polyolefin, polyester and polyamide due to their ease of manufacture and their advantageous chemical and physical properties. Polyolefins are the most common (approximately 60% by mass) synthetic oil-based-polymers. They are produced by addition polymerisation of olefin such as ethylene or propylene, and they can be classified as homopolymers, copolymers and terpolymers.

1.3. Thermal Properties:-

Polymeric materials differ from most inorganic or metallic materials because of their unusual thermal properties. At low temperatures, polymers are mostly crystalline because the motion of one polymer chain that relative to another is difficult. Polymers are usually brittle but strong and they have a short elastic region. As the temperature increases rotation around the C-C bond is possible so the crystalline regions dissolve, and slip of neighbouring chains is possible. The temperature at which a polymer changes from a rigid to a flexible material is known as the glass transition temperature (Tg), that can be determined by measuring the specific volume of a sample as a function of temperature. Many other properties change markedly around Tg such as refractive index, stiffness, thermal conductivity and hardness. Very few materials are made up of just polymeric components. Usually a variety of additives are included to achieve the required properties of the material. These can include; fillers such as carbon black to reduce cost; plasticisers such as alkyl phthalates to change mechanical properties; lubricants to aid processing; pigments to change the optical properties; stabilisers to stop the degradation of the polymer by heat; anti-wear agents; antistatic agents to spread the charge on materials introduced by friction; curing agents to increase rigidity; flame retardants and blowing agents to produce foams. Some additives fulfil many roles e.g. carbon black is a filler, stabiliser and pigment. In many cases the additives can be the major component of the material. The combination of polymer and additive is known as a plastic. Polymers can be classified into 2 types depending on their thermal response. Thermoplastics make up 90% of all processed polymers; they become soft and fluid on heating and set on cooling. This cycle can be repeated i.e. the polymers can be re-cycled. Thermosets are highly cross-linked polymers generally formed by condensation polymerisation, where heating leads to decomposition and so they cannot be recycled.