Recycling of plastics – A Solution to waste Management

Dr. Mrs.G.D.SHAH (Ph.D. Chemical Engg., M.E. PolymerTech.)  

I/C HEAD OF PLASTICS ENGG.DEPT.
GOVT. POLYTECHNIC, AHMEDABAD
GUJARAT, INDIA.

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ABSTRACT
Plastics consumption has grown at a tremendous rate over the past two or three decades. In the 'consumer' societies scarce petroleum resources are used for producing an enormous variety of plastics for an even wider variety of products. Many of the applications are for products with a life-cycle of less than one year and then the vast majority of these plastics are discarded. In most instances reclamation of this plastic waste is simply not economically viable. In industry (the automotive industry for example) there is a growing move towards reuse and reprocessing of plastics for economic, as well as environmental reasons, with many praiseworthy examples of companies developing technologies and strategies for recycling of plastics. Plastics are made from a non-renewable resource and it is generally non-biodegradable (or the biodegradation process is very slow). Hence plastics litter is often the most objectionable kind of litter and will be visible for weeks or months, and waste will sit in landfill sites for years without degrading. There is a much wider scope for recycling in developing countries like India.
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Plastics are unique, remarkable versatile material having fantastic properties to fulfill the need of our modern society. The plastic age arrived in 1868 with the invention of “CELLULOID” by Alexander Park to replace billiard ball previously made of ivory. In 1909 Leo Backeland derived “BACKELITE” i.e. phenol formaldehyde – a thermo set material. The other plastics like PP, PS, PC, PVC, NYLON, TEFLON POLYSULFON, and other engineering plastics were derived in the last 100 years.
Virtually every known plastics material is used in some aspect of packaging. After using packaged item the plastics wrapper, bag, or container is merely a waste. During plastics processing also some portion of material is in the form of waste along with the finished products. But such fresh waste is reprocessed or converted in to scrap form and is reused along with virgin materials up to 15%. But the huge amount of waste generated by industry and the consumer can not be ignored. Plastics waste becomes nuisance for human beings. Environmentalists and regulatory authority are thinking that plastics are not environment friendly and cause health hazards.
THIS IS NOT TRUE:
Actually implementation of eco friendly waste disposal management system should be developed to solve this problem. Plastics waste itself is not at all dangerous to human being but the unscientific manner of waste disposal creates the danger.
WASTE DISPOSAL MANAGEMENT SYSTEMS:
To reduce the problem of ecological imbalance, due attention must be given to waste managements systems like:

1. Recycling
2. Combustion or incineration (5%)
3. Land filling (40%)
4. Source reduction
5. Repairs
6. Reuse
7. Degradation of polymers
8. Use as fillers

TYPES OF WASTE:

1. Waste products – defective products
2. Industrial waste
3. Post consumer waste- packaging films, damaged moldings etc.
4. Nuisance waste- thermo set waste
5. Scrap waste- feed system, flash, finishing scrap etc.



Fig. 1 use of plastics in various sectors Fig. 2 plastics waste through various industries

DEFINITIONS:
REGRIND MATERIAL: The plastics scrap obtained from grinding machine in form of coarse powder is known as regrind material.
REGRIND MATERIAL: the regrind material obtained in the form of pellets from granulation line of extruder is known as reprocess material.
RECYCLE MATERIAL: The regrind material compounded with suitable additives so that the quality of reprocess material approaches to that of virgin material.
PROCESS OF RECYCLING:
The manufacture of recycle material involves five steps:

1. Separation from Municipal solid waste
2. Grinding the plastics waste material
3. Reprocessing material
4. Testing for quality reduction
5. Compounding to attain virgin quality


SEPARATION FROM MUNICIPAL SOLID WASTE:
The municipal waste is reduced in size mechanically by tension, compression, shear using crushers, shears, chippers, drum pulverizers, disc mills, pulpers and hammer mills.
After making small particles each individual particles are separated by:

1. density-floating in water
2. magnet-ferrous particles
3. Manual- glass, paper etc.
4. gravity- fluidized bed separation
5. electrostatic- charged particles by attraction to a charged roll
6. color- different colored particles
7. air separation- paper and light weight material by blowing air
8. heat separation- hot roll or doctor blade
9. solvent extraction- dissolution in solvent


GRINDING:
The plastics waste material is obtained in various forms viz. the feed system, flash, film, molded product etc. all these forms must be sorted out and possibly cleaned. These materials are now cut to suitable size so that they can be fed to the grinding machine. The regrind material obtained from various forms of scrap is collected separately.
REPROCESSING:
The regrind material is then fed to the hoper of an extruder which is previously prepared for processing. While using the film material as regrind material, care should be taken for positive feeding because the film has high bulk density.
After the material is melted to the required melt temperature, the material extruded from the granulation die of the extruder in the form of a long, continuous strands. These strands after cooling are cut to the required pellet size. The pellet size i.e. length and diameter can be varied to some extent by varying the take off speed of the cutter.
TESTING:
The reprocess material always involves quality reduction. The reason is:
The processing of plastics involve the melting of material to required processing temperature and then cooling this melt so as to obtain required shape. While melting the material, the heat is imparted to that material and while cooling the material, the heat is extracted from the material.
While heating the material the bond in the polymer chain expand and the molecular distance is increased. While cooling the bonds contract and the molecular distance is reduced. While reprocessing these steps are repeated. Due to these expansion and contraction, some of the heat energy is absorbed by the bonds so the bond strength is reduced. Due to this reduction this reduction in bond strength, now less energy is required to break this bonds i.e. the strength of the material or resistance towards load is reduced. Also during these heating and cooling process, some of the bonds break. Thus there is reduction in molecular weight and corresponding strength.
This reduction is tested by measuring melt flow index of the reprocess material, because MFI is inversely proportional to the molecular weight of the material and corresponding melt viscosity.

Table: Effect of molecular structure on various properties of plastics material

Sr. No. Property Linear chain Polymer Branched chain Polymer
1 Density More Less
2 Permeability Less More
3 Tensile strength More Less
4 % elongation Less More
5 Stiffness More Less
6 Crystallinity More Less
7 HDT Less More
8 Softening temperature Less More
9 Hardness More Less
10 Creep resistance More Less
11 Flow ability Less More
12 Compressibility Less More
13 Impact strength Less more
COMPOUNDING:
This step involves the addition of suitable additives to improve particular quality of the material. The material and additives are then mixed into a homogeneous mass with the help of suitable compounding methods. The selection of compounding method depends upon the additives added and the mixing quality required.
MODES OF RECYCLING:
Depending on the products obtained after recycling it can be explained as:

1. Primary recycling
2. Secondary recycling
3. Tertiary recycling
4. Quaternary recycling
PRIMARY RECYCLING:
It is the processing of scrap plastics into the same or similar type of product from which it has been generated using standard plastics processing methods.
Uniform, uncontaminated plastics waste reprocessed directly after scrapping particularly thermoplastic waste.
Plastics waste is ground to a particle size close to that of virgin material. The size reduction is done in scrapping machine consisting of a hoper, cutting chamber (rotor knives), fixed and rotating cutters, screen and drive. The knife arrangement may be parallel, slanted, V type, etc. Counter rotating rotors or two step rotors are also designed for scrapping. Cryogenic grinders are available for difficult material. For film type material waste plunger type or screw type stuffer is used.
SECONDARY RECYCLING:
Secondary recycling utilizes plastics waste unsuitable for direct reprocessing using standard plastics processing equipments.
Plastics waste feedstock from:
1. Post consumer plastics waste recovered from municipal refuse.
2. Post consumer plastics waste obtained from returnable packages (milk pouches, jars, soft drink bottles etc.)
3. Industrial plastics waste consisting of a single type of plastics material.
Reverser machines are used to manufacture products from contaminated and/or mixed thermoplastic waste. It can employ as feed stock such as PVC, PE, Nylon and used bottles and drums. It can handle contaminated scrap- pieces of copper, sand, glass etc. Waste plastics are pulverized in a crusher, dried and conveyed to large extruder and from extruder the material is delivered to accumulator; from accumulator a vertical screw plunger discharges the material into the mould. Final products can be formed by intrusion (flow molding), compression molding or extrusion.
TERTIARY RECYCLING:
It is the pyrolysis of post consumer waste. In this process the physical and chemical decomposition of organic materials caused by heating in an oxygen free atmosphere. This process is used for manufacturing charcoal, acetic acid and turpentine.
Pyrolysis is an endothermic reaction and heat must be supplied to distill off the volatile components.
PRIMARY REACTIONS:
Organic material → gases + liquid + char
SECONDARY REACTIONS:
CO2 + H2O → CO2 + H2 + 
C + H2O → CO + H2 + 
C+CO2 → 2CO + H2 + 
C + O2 → CO2 + 
C + 2H2 → CH4 + 
As shown, the products of pyrolysis of solid waste are in the form of gases, liquids and char.
The solid residue from the process was in the form of a light weight, flocky char, which could be coarsely sieved to remove extraneous materials such as bottle caps and lids.
The liquid portion of the pyrolysis products consists of tar, light oils and liquor.
QUARTERNERY RECYCLING: (Energy from plastics waste)
Incineration pf refuse is done to reduce combustible waste to inert residue by controlled high temperature combustion. The main reason for incineration is reduction in the volume of waste. Incineration is capable of reducing the weight of refuse by 80% and the volume by over 90%. The residue from the refuse is inert and may be disposed off in land fill.
ENERGY RECOVERY:
1. By burning refuse in steam generating incinerators to generate electricity.
2. Pouring refuse in existing heat exchangers as a fuel in power boilers.
3. Pyrolysis of refuse can be employed to produce a transportable fuel.
4. Refuse can be converted to heavy oil by hydrogenation by heating under pressure in the presence of carbon monoxide and steam.
5. In anaerobic digestion the organic portion of the refuse is decomposed in the absence of oxygen. Methane produced has the potential of being used as a natural gas substitute.
At present time, burning refuse in steam generating incinerators and its use as supplemental fuel are the most advanced waste energy utilization technologies.
An incinerator consists of:
1. Scale → To measure the weight of feed
2. Storage pit → To collect feed stock
3. Cranes → To handle waste from storage t o furnace
4. Charging/ Feeding mechanism/ Hoper → For furnace
5. Furnace → Where combustion occurs
6. Boilers → For generating steam
7. Steam turbines → To convert heat energy to mechanical energy
8. Generators → To convert mech. energy to electrical energy
Lastly, the environmental attack on the plastics industry is not justified because of the small contribution of solid waste (2-4% only). Also the large portion of waste generated by industry is reprocessed or recycled in the industry it self.
Not all plastics waste can be easily recycled. At present a considerable portion of plastics waste is still being disposed off without the recovery of cost incurred in production. However rising material and energy costs, govt. regulations and awareness of the consumer exert a pressure on the industry to change the situation. New separation techniques, incinerators and technology will make more recovery of energy and chemically possible scientific waste disposal management.