Biodegradation Of Polyethylene Starch Complex Biology Essay

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ABSTRACT

The thermoplastic Polyethylene (PE) is generally non-degradable under normal conditions. Polyethylene is a stable polymer consisting of long chains of ethylene monomers and has a high molecular weight. Therefore the polyethylene is blended along with an additive, in our case starch either by incorporating granular starch or gelatinized starch into plastic films, which reduces the molecular weight of the plastic by forming a bio film over the plastic and making it compatible for degradation by microorganisms. The microorganisms used for degradation of polyethylene are Rhodococcus rhodochrous, bacteria, Cladosporium cladosporoides, a fungus and Nocardia asteroids, bacteria. The bacteria and the fungi form a symbiotic relationship which easily breaks down the starch and enhances auto-oxidation which reduces the molecular weight of polyethylene and therefore making it biodegradable. The alternative method for degradation of polyethylene is by formulating the plastic with metal iron complexes and exposing them under Ultra-Violet radiations which will enhance photo-degradation by reducing the molecular weight thereby making the plastic compatible for degradation by the microorganisms.

CONTENTS

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APPENDIX............................................................................................................................19

INTRODUCTION:

BIODEGRADATION:

The breakdown of plastic materials by metabolising the molecular structure of the plastic by photo-degradation methods and microorganisms is known as biodegradation of plastics.

The biodegradable plastics generally form two types:

Bioplastics, whose components are obtained from renewable raw materials,

Petroleum based plastics, which uses an additive eg. Polyethylene (PE).

POLYETHYLENE:

Structure:

Polyethylene is a thermoplastic polymer which consists of long chains of ethylene monomers. The ethylene molecule C2H4 is CH2=CH2, each CH2 group is connected by a double bond.

Figure 1: Repeating unit of polyethylene.

Molecular weight:

The molecular weight of polyethylene differs widely and it is categorized as ultra high molecular weight polyethylene which has a molecular weight ranging from 3.1million to 5.8 million. They are categorized depending on the density as high density polyethylene (HDPE) and low density polyethylene (LDPE) which are of high concern when considering plastics from waste.

Polyethylene (PE) is as such not degradable because of its high molecular weight.

When it is blended with an additive (starch), metal complexes (iron), photo-initiators and chemical initiators they are biodegradable.

The additives reduce the molecular weight of the polyethylene enhancing oxidation which is acted upon by microorganisms making it biodegradable.

BRIEF MECHANISM OF BIODEGRADATION OF POLYETHYLENE:

Starch blended PE starch depleted plastic chemical oxidation

Physical disintegration low molecular weight microbial degradation biodegradable plastic (PE)

The synergetic action of the photo and thermo oxidation along with the biological activity releases various products like alkanes, alkenes, ketones, aldehydes, alcohols etc.

The biodegradation of Polyethylene can be further improved by adding compatibilizers and by copolymerization.

APPLICATIONS OF BIODEGRADABLE POLYETHYLENE:

The biodegradable PE is developed and commercialized to be used in medicine, packaging, agriculture, automotive industry.

The biodegradable plastic films can be used as garbage bags, plates and other cutlery, shipping and other related purposes.

They can be synthesised and used in drug delivery and bone replacement techniques in the medical field.

Efficient composting and field work.

2 MATERIALS AND METHODS:

2.1 MATERIALS

2.1.1 PLASTICS

The plastic film used in this study is polyethylene (PE), which is formulated with starch and another sample of polyethylene, which is formulated with metal complex ion.

2.1.2 MICROORGANISMS

Three microorganisms are used for the degradation of polyethylene mainly Rhodococcus rhodochrous (bacterium), Cladosporium cladosporoides (Fungus), and Nocardia asteroids (bacterium).

2.1.3 CULTURE

The microorganisms were cultivated in petri dishes containing mineral salt medium whose composition is as follows:

MINERAL SALT MEDIUM:

MgSO4.7H2O 0.5g

KH2PO4 0.5g

Na2HPO4.12H2O 2.25g

NH4Cl 1g

CaCl2 0.002g

MnSO4.7H2O 0.007g

FeSO4.7H20 0.001g

ZnSO4.7H20 0.007g

Distilled H2O 1000 ml

The mineral salt medium is used to grow these microorganisms because it maintains the optimum pH for each organism and its rich in minerals, which enhances the efficient growth of the microorganisms.

2.2 METHODS:

2.2.1 BIOFILM PRODUCTION:

The polyethylene as such cannot be degraded therefore an additive like starch is to be blended along with it for it to degrade.

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Starch can be used in gelatinized form by heating the starch in the presence of water, which causes the formation of a biofilm around the thermoplastic material during blending.

Starch granules that have been plasticized with water and glycerol can also be used to blend along with the polyethylene, which forms a biofilm around it.

2.2.2 DETERMINATION OF TRANSISTION METAL CONTENT:

The other sample of polyethylene blended along with the metal complex ion is subjected to combustion.

The ash content of each plastic was determined and was solubilized with HNO3 and water was added and the sample solution was filtered.

Concentration of ion in the sample solution was analyzed with adsorption spectrophotometer.

2.2.3 DEGRADATION OF STARCH (Laboratory Methods):

Starch is completely degraded by the following methods and the starch free plastic is obtained with low molecular weight

2.2.3.1 Oven Treatment:

The polyethylene starch complex was cut into strips and was placed in an oven at 70 degree C.

The samples were removed after 2, 4, 6, 8, 10, 12, 16 and 20 days for evaluation of degradation.

By this method the molecular weight of polyethylene was determined and degradation of starch was done.

2.2.3.2 High-Temperature High-Humidity Treatment:

The plastic strips were placed in a steam chamber with continuous steam flow and harvested after 2, 4, 6, 8, 10, 12, 16 and 20 days.

The plastic strips were placed on vinyl-coated test tube rack.

The samples were washed with 70% ethanol and dried overnight at 45 degree C.

By this method the molecular weight of polyethylene was determined and degradation of starch was done.

2.2.3.3 UV Light Treatment:

Photo degradation is carried out by this method.

The plastic strips were placed in a UV box for about 8 weeks.

The samples were removed after 1 2 3 4 and 8 weeks.

By this method the molecular weight of polyethylene was determined and degradation of starch was done.

2.2.4 ANALYTICAL METHOD-FOURIER TRANSFORM INFRA RED SPECTROSCOPY (FTIR):

The polyethylene is cut into thin strips and these strips are inoculated into a suspension of each of the three microbial strains and it is incubated at 27dC and 85% humidity in an environmental cabinet.

The strips are then removed and air-dried and are examined by FDIR spectroscopy.

2.2.5 PHYSICAL DISINTEGRATION:

2.2.5.1 Photo Oxidation Of Polyethylene:

The polyethylene when subjected to UV facilitates the presence of oxygen and induces photo oxidation, which degrades the polymer.

The photo oxidation results in chemical changes and there by reduces the molecular weight of the polyethylene.

Mechanism:

When the polyethylene absorbs the UV light, a free radical is formed.

The free radical reacts with the oxygen and generates PE hydro peroxide (POOH) and PE alkyl racial (P.)

Photolysis occurs, which forms PE oxy radicals and hydroxyl radicals.

The free radicals crosslink with each other.

Polyethyleneï‚®UVï‚® P-+ P-

P-+ O2ï‚® POO- + PHï‚® POOH + P-

POOHï‚® P- + H2O

Photolysisï‚® PO-, HO-

Termination Step-Cross linking of free radicals.

2.2.5.2 GPC (Gel Permeation Chromatography):

This technique is carried out to determine the molecular weight of the polyethylene.

The experiment was carried out on samples before and after microbial attack by the microorganisms mentioned earlier.

The samples were incubated for 8 weeks.

The incubated samples were subjected to GPC analysis after removing the microorganisms to determine the molecular weight.

RESULTS:

The above described process like oven treatment,HT-HH and UV treatment majorly helps in

determinig film mechanical properties and

polyethylene molecular weight.

OVEN TREATMENT:

The degredation of plastics majorly depends on the transition metal iron .

Rapid degradation was observed in samples that were taken on day 2, 4 and 6.

Some plastic films took nearly 8 to 16 days for degradation due to the presence of starch.

3 to 5 days were required for transition metal combination to reach a molecular weight of less than 50,000.

HEAT TREAMENT AND HEAT HUMIDITY (HT-HH):

The rate of oxidative degradation differed from that of the oven treatment.

The degradation was elongated for 2 days and was variable and corresponded .

The major thing observed in such a treatment is when a plastic film molecular weight goes below 125,000 the film becomes brittle and these type of treatments resulted in longer degradation.

3.3 UV LIGHT TREATMENT:

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The major principle behind Ultra violet light treatment is the photosensitivity of each film.

certain films initial showed low initial degradation and then after about 2 weeks the degradation increases gradually and then after 8 weeks there is steady increase in degradation.

The transition metal Fe and starch was easily degraded through this treatment.

Photo degradation of polyethylene by UV rays reduces molecular weight and supports microbial growth.

3.4 FOURIER TRANSFORM INFRARED SPECTROSCOPY(FTIR)

The major prinicple underlying this procedure is the chemical bond in a molecule produces infrared absorption spectrum.the result is obtained as graph . Different plastics have different absorbance levels based on the frequency and intensity of infra red radiations at which the material absorbs.

Below is a graph between wavenumber and absorbance and the graph explains about chemical bonds in a molecule by producing an infra red absorption spectrum in various polyethylene plastic films.

Figure 2: graph showing production of infra red absorption spectrum in various plastic films.

A material's absorbance of infrared light at different frequencies

3.5 GEL PERMEATION CHROMATOGRAPHY(GPC):

The molecular weight of the polyethylene strips on incubation for 8 weeks are found using GPC and are illustrated in the table below.

The reduction in the molecular weight on incubation with the molecular weight is due to the photo-oxidation mechanism.

samples

Molecular weight(MW)

Before incubation

Molecular weight(Mn)

After incubation

Polydispersivity

(IP )

1

62,600

10,300

6.08

2

12,300

3500

3.51

3

153,300

8800

17.42

4

111,700

7300

15.30

Eg. Table 1: molecular weight data on degradable polyethylene before and after exposure to UV light.

Sample

MW

Mn

IP

5

14,000

3700

3.78

6

12,700

3600

3.52

7

14,400

3700

3.89

8

14,500

3600

4.03

Eg. Table 2 : molecular weight distribution data for samples incubated with three microorganism for 8 weeks.

DISCUSSION:

BIOFILM FORMATION:

The starch incorporated either in the gelatinized form or as granules adheres to the plastic and forms a bio film over the plastic by making the double bonds weak.

DEGRADATION OF STARCH - OVEN TREATMENT, HT-HH TREATMENT, UV LIGHT METHOD:

The above methods reduce the molecular weight of the polyethylene by degrading the starch. The low molecular weight of the polyethylene makes it compatible for degradation. The chemical structure of the polymer is also changed. The thick plastics are converted to thin plastic strips that are easily degradable by placing them in oven at 70 degree Celsius or by increasing temperature and humidity or by exposure to UV radiations.

FTIR SPECTOMETRY:

Different absorbance level of the different plastic strips inoculated in the microbial samples of Rhodococcus rhodochrous (bacterium), Cladosporium cladosporoides (Fungus), and Nocardia asteroids (bacterium) are determined. The wavelengths determine the degradation of starch or transition metal in the polymers. The chemical bonds of the polymer absorbs the infra red radiations and produces different wavelengths which determines the degradation of the polymer as such.

PHYSICAL DISINTEGRATION:

PHOTO-OXIDATION:

The plastic strips inoculated in the culture of the 3 microorganisms are subjected to photo-oxidation where free radicals are formed which results in photolysis suitable for further degradation of plastic by tremendous decrease in the molecular weight.

GPC:

The GPC analysis determines the molecular weight of the polyethylene before and after the action of microorganisms. The comparative study between the molecular weight before and after the action of microorganisms on the polymer clearly defines the biodegradation of the polymer by the three microorganisms thereby showing reduction in the molecular weight after the action of microorganisms.

CONCLUSION:

The polyethylene is a non biodegradable thermoplastic polymer. The polymer is blended with an additive starch and a transition metal complex iron. This forms a film over the polymer which decreases the molecular weight of the plastic and enhances degradation. The molecular weight and the degradation of starch can be carried out either by oven treatment, high temperature- high humidity treatment or by UV radiation method. The UV radiation method is found to be more effective when compared to the other two methods. This decreases the molecular weight of the polymer to below 50,000 which can undergo degradation by microorganisms. The three microorganisms form a symbiotic relationship which reduces the molecular weight by inducing photo-oxidation and further reducing the molecular weight which can be analysed by GPC method. The FTIR spectroscopy determines the absorbance levels of different plastic strips placed in the culture of these microorganisms. The low molecular weight polyethylene obtained from the above method is bio-degradable and has various applications ranging from agriculture, waste production to medicine.