Recycling Of Plastics

Recycling of Plastics

INTRODUCTION

Recycling of plastics not only helps the conservation of raw materials but also

reduces

the

pollution

of

the environment. Different processes for

recycling of plastics are described in this topic. The method of recovery of plastics from variety for wastes are also outlined in this topic. Plastic in solid wastes are a great problem in the environment for their indestructible nature. It is not surprising not learn that the overall growth of solid wastes. Presently it is estimated that the generation of solid waste per capita is around 1/2 kg which is likely to increase to I kg by the year 2000. Plastics as wastes constitute around 4 wt %-5 wt % of solid waste. Almost 80 % of this recyclable waste is collected by the rag pickers at source from the garbage dumps. The balance 20 % goes to land filling. In 1993, consumption of virgin plastics was 1.44 Mt (production being 0.92 Mt and imports were 0.52 Mt) Besides 0.13 Mt plastics wastes was reprocessed (amounted to 0.53 Mt)

in India during 1993. This, the total

consumption of plastics in India during 1993 was around 2 Mt. It is predicted that by the year 2000, consumption of virgin plastics will be 3.3 Mt and reprocessed plastics will be 1.3 Mt (thus total consumption will be 4.6 Mt).

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Recycling of Plastics

Over a million rag pickers including plastics wastes collector are involved and around 20000 units all over the country are engaged at present in reprocessing around 0.4 Mt of plastic waste. It is predicted that I Mt of plastics waste will be available by 2000. The polymer - wise share of plastics waste is PVC - 45%, LDPE - 25%, HDPE-20%\ PP - 7.6% and others like PS-2.4%. The two important environmental problems are how to reduce pollution of air, water and land, and how to conserve raw materials. The planned application of existing technologies would be sufficient to avoid any serious risks of direct or transferred pollution by plastics wastes. Recycling of plastics is currently believed to be the most acceptable route to solve the above problems in the long run. In addition to these,

recycling of plastics will supplement the

requirement of plastics raw materials and reduce the import of plastic raw materials in the country.

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Recycling of Plastics

SOURCES OF PLASTICS WASTE

Creation of waste occurs in three main areas : a) During manufacture of the polymer; b) During manufacture of the product; and c) During the disposal of the product after use. Figure 1 shows the flow of materials from the polymer producer until it reaches the waste dump. This shows that primary as well as secondary raw materials are absorbed, resulting from in-plant recycling and reclaiming. Plastics reclaimers obtain their raw materials in the form of wastes from polymer producer, plastic processor and industrial end users and converters.

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Recycling of Plastics

Primary Raw Material

Polymer Producer Reprocessing Reclalmers

Plastic Processors

Commercial and Industrial Waste

Reprocessing Industrial End Users and converters Repeated Reuse

Private and Official Households

Domestic Waste

Flow of Material From Polymer Producer to the Waste Dump

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Recycling of Plastics

PROCESSES OF RECYCLING OF PLASTICS

Recycling describes the actual process for usefully employing the recovered raw materials. There are three processes available for recycling of plastics. a) Direct recycling This would be the use of recovered material for the manufacture of identical or similar products; b) Indirect recycling This would imply the use of the recovered or salvaged material for the manufacture or use of a product of less critical specification. c) Thermal recycling This implies that conversion of waste under conditions which generate either energy or chemical products or both.

Direct Recycling of Plastics This is mainly applicable to the wastes which arise during the production processes within the plastics industry.

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Recycling of Plastics

For direct recycling, waste plastics must satisfy three requirements : a) b)

Satisfactory from (granules, pellets, powders etc.), Homogeneity,

because

mixed

grades

of

plastics will not possess the

required properties for most processes or products; and c)

Cleanliness, because contaminants such as dust, stray matter, oil etc. would degrade the quality of the product. Clean process waste may be recycled by a relatively simple route as

shown in Fig. 2

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Recycling of Plastics

Virgin Material

Cooling

Regular Granules

Processing Machine

Product

Processing Waste

Rejects

Good Product

Rechip

Extrude

Recompound

Recycling Direct On Processing Machine

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Recycling of Plastics

Irregular shaped waste from production must be converted to a consistent shape by a regranulation technique which involves a minimum of temperature degradation. The introduction of irregular chips into a hopper containing regular virgin granules causes preferential feeding of one component in the hopper due to different flow-characteristic. The alternative to regranulation lies in remixing or reextruding the rechipped material, so that it may then be cut into regular granules like the virgin compounds. The recycling of thermoplastic scrap from extrusion, injection moulding, blow moulding, calendering, thermoforming and moulding operations has always been a regular part of industrial practice and is one of the main reasons from economical point of view for plastics with respect to other materials. In-plant recycling is easy, the nature of the material is known an any defective products can be internally recycled in the works provided that mixing of different wastes does not occur.

Indirect Recycling of Plastic Indirect recycling was defined earlier as the use of plastics wastes for the manufacture of products having a less demanding specification than that of the original product. In the case of plastics wastes the importance of

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Recycling of Plastics

downgrading the reuse may be the possibility of avoiding the requirements of homogeneity and cleanliness which are fundamental to direct recycling. In commence

with

general,

a

shredding

process and

of

mixing

indirect of

recycling

would normally

the thermoplastic

wastes

and

incorporation at this stage of compounding ingredients. This product would then be compacted into granules and fed to a processing machine for the production of the final product. The general sequence can be represented by flow diagram as shown in fig. 3.

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Recycling of Plastics

Waste Material

Granulator or Shredders

Compounding Ingredients

Mixture

Extruder

Regular Granules

Processing Machine

Product

Flow Diagram of Indirect Recycling

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Recycling of Plastics

Thermal Recycling of Plastics In thermal recycling, heat is generated and used (of course, once only), so that the consumption of fossil fuels can be reduced. The

following three methods are used in thermal recycling of

plastics: Incineration The first method of thermal recycling of plastics waste is incineration.

Incineration

is

the

term

used

for

the combustion and

reduction to an inert residue of any combustible waste. Incineration can be used as a means of plastic waste disposal and incinerators may be used to produce steam for domestic heating and the generation of electricity. In the incinerator the combustion takes place at high temperature (around 7000 C) in the presence of the requisite amount of oxygen. Since the plastics present in waste consist mainly of carbon and hydrogen, the product obtained after combustion should consist mainly of carbon dioxide and water. Incineration is not usually favored as a method of disposal because of very high capital, maintenance and operating costs, unreliability and adverse environmental impact. Problems are added when incineration is combined with generation of steam and the output to be controlled to suit customer demand.

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Recycling of Plastics

PVC has received considerable criticism since HCI gas is evolved on combustion for long term use, the incineration may be a favored and economical method to dispose of all the plastics exclusive of PVC from the incinerator to safeguard it from the problems associated with evolution of HCI, the other plastics could be readily utilized as a high energy fuel, because they have considerably high caloric value. Pyrolysis The second method of thermal recycling to plastics waste is heated to a higher temperature in an inert atmosphere, possibly under pressure. This process is sometimes called destructive destination where the decomposition products formed may be used for producing fuels, energy and primary chemicals. Plastics

are

considered as

assets

in pyrolysis. Pyrolysis is not yet entirely

satisfactory, because there is not yet, sufficient information about the reactions which take place during pyrolysis. It has, therefore, not yet been possible to control the processes accurately.

Hydrolysis The third method of thermal recycling of plastics waste is hydrolysis. The breaking down of plastics into their constituent raw materials

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Recycling of Plastics

by hydrolysis, is, of course, possible if the plastic contains that groups which can be

hydrolysed. Since hydrolysis is the reverse of condensation, it is clear that

plastics which can be hydrolysed are all those made by condensation. Whether hydrolysis is practical or not, is solely a question for the reaction conditions. The produced

by

polycondensation

polyaddition,

polycarbonates,

polyureas

products

encompass

such

which

also

include plastics

as polyamides,

polyesters,

and polyurethanes. Hydrolysis involves the specific

reaction of the water molecule at the point of linkage with the starting materials (this is the reverse of the reaction that produced the polymer originally), so that the starting materials used originally are recovered directly. The groups of plastics mentioned above as being hydrolysable are all resistant to hydrolysis under normal conditions use. Hydrolysis must be induced under extreme conditions.

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Recycling of Plastics

PROCESSES OF RECOVERY OF PLASTICS Recovery describes the process of collecting and/or segregating homogeneous wastes for reuse, mainly in-plant or within the organization. Depending on the types of plastics waste,

the

processes

available

for

the

recovery are the following two :

Recovery of Plastics from Relatively Pure Waste Relatively uncontaminated plastics waste is available from the polymer producer as well as from the plastic processor. The waste is homogeneous in nature unless it is allowed to mix with other types of plastics. Recovery at this stage is rather simple. It is a traditional technique to recover the wastes produced during polymer production or plastics processing and blend them with virgin material. If these waste are allowed to become mixed and/or contaminated, they may be of little value and find their way into industrial wastes. Where such wastes are available in bulk quantities, it may be more economical to sort and grade them and sell to reclaimers.

Recovery of Plastics from Mixed Plastics Wastes

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Recycling of Plastics

Mixed plastics waste, thermoplastics packaging waste mainly consist of the five most important thermoplastics -LDPE, HDPE, PP, PS and PVC. A scheme of separation for the five components has been shown based on the sink-float method. This scheme relies on the use of fluids differing in composition and density. A diagram of the scheme is shown in Fig. 4. the process requires chopping /shredding or grinding of mixed plastics waste in a shredder or a grinder and also washing throughly with running water to eliminate dust and external contamination.

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Recycling of Plastics

A Sink Float Method For Isolating the Thermoplastics in Four Liquids Differing In Density d(g/ml)

CONCLUSION

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Recycling of Plastics

Recycling of plastics is possible when the recycled plastic is equal in quality and not higher in cost than the virgin raw material. These two constraints which effectively limit recycling at present, due to the cost of reclaiming or salvaging followed by the further cost of sorting, grading and cleansing. Thus, the waste occurred as a result of primary production is to be recycled because recovery cost will be small and it is less likely to be contaminated, while at the end of the user chain, the reverse will be true. At every stage of production, distribution and consumption, there is a decline in quality of the waste, for the purpose of recycling and a rise in the cost of reclaiming.

REFERENCES 1) Engineering Chemistry

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Recycling of Plastics

R. Gopalan D. Venkappayya S. Nagarajan 2) Organic Chemistry

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