Theme: Our Earth Resources

Topic 5. Plastics as a useful material from petroleum

A. Plastics

Since the discovery of plastics in the late 19th century, plastic material has integrated into

every corner of our lives. It has been used for containers, packing materials, clothing,

furniture, as well as shelters.

The characteristics and advantages of plastics:

Flexibility:

— Various kinds of additives could be added to broaden plastic uses and application. For

example, solvents could be added to make an adhesive or paint.

Good insulators to heat and electricity

— Plastics do not contain free electrons and atoms in the polymer chain. Therefore, it

cannot conduct heat and electricity

Resistant to chemicals

— Usually do not have reactions (or low reaction rate) with air, water, and most of the

common chemicals, such as acid and bases.

Tailor-made to suit requirements

— At some stages of plastic processing, heat and pressure are used to soften plastic and

made into desired shape.

Unbreakability

— As compared to glass, plastics are relatively tough. As there are strong intermolecular

forces holding the polymer chain in plastic structures.

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Other properties of plastics include:

Transparent and lightweight

— Making them the useful types of materials in the modern world. Can replace natural

materials such as cotton, wool, wood and leather to satisfy the human demand.

There is an increase of average mass of plastics in a new car form 1960 to 1993.

B. Plastics are Polymers from Petrochemicals

Plastics are important products of the petrochemical industry.

They are man-made polymers of hydrocarbons produced mostly from oil and natural gas.

C. What is a polymer?

The word polymer is derived from the Greek. Poly means “many” and Meros means

“parts”.

It is used to denote substances of high molecular mass formed by the polymerization of

small molecules (monomers).

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For example: Under ~200OC, >2000 atmospheres and the presence of oxygen, ethylene is

polymerised to low – density polyethylene.

C CH

H

H

H+ +

ethylene

C C C C C C

H H H HH

HH H H H

H

H

polyethylene

C CH

H

H

HC C

H

H

H

H

Examples of polymers:

Polyethylene Soft bottles, plastic bagsPolypropylene Plastic toys (more rugged)Teflon Nonstick cookwareNylon Ropes, nylon stockingsDacron Clothing fabric

Plastics are generally classified into two categories according to its property towards

heating.

1. Thermoplastics

They can be reshaped, as intermolecular forces among long flexible polymer chains

(Figure 1) can be weakened by high temperature, and the plastic soften again.

After being softened, they can be recycled (remoulded) for some other use.

— E.g. Polyethylene plastic milk containers.

Examples of thermoplastics:

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Suggested activity:

Production of nylon

Food-storage bags Drink containers

Low-density polyethylene (LDPE). High-density poly ethylene (HDPE)

2. Thermosetting plastics

They are polymers being shaped through irreversible chemical processes

Composed of rigid and hard giant network (cross – links formed between the chains at

different points hold them tightly, Figure 1).

Under strong heating, the plastics decomposes rather than softens.

Therefore, they cannot be reshaped readily.

Figure 1. Structures of thermoplastic and thermosetting plastics

D. Environmental problems with plastic disposal

The great usage of plastic materials also brings the issue of plastic waste disposal, which

is one of the major environmental problems.

Plastic wastes could be buried in landfills, burned in incinerators or recycled.

1. Disposal in landfill

Most of the plastics used today are non-degradable or take 200-400 years to degrade

in landfill sites.

Biodegradable plastics take decades to be partially decomposed in landfills. Oxygen

and moisture are insufficient for the microorganisms to survive and degrade the

wastes in landfill.

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Moreover, toxic chemicals such as cadmium and lead compounds that are used as

binders, colorants and stabilizers in making plastics.

— Leakage of toxic compounds resulting in contamination of surface water and

underground water in landfills.

— The leachate collected in landfills required specific treatment to lower the

toxicity.

There is limited land available for constructing landfill sites

This method of disposing plastics cannot last for long time.

2. Incineration

Combustion of plastic wastes in incinerator leads to air pollution.

Nitrogen, halogens (F, Cl, etc.), benzene and other chemicals are the constituents of

monomers of plastics.

Toxic products are formed when plastic wastes are burned.

— Burning of Poly(vinyl chloride) (PVC) forms toxic hydrogen chloride and

chlorine gases, both of which irritate respiratory systems.

— Combustion of benzene-containing plastics, such as polystyrene, results in

polycyclic aromatic hydrocarbons (PAHs), a group of chemicals proved to induce

cancers.

— Dioxins: the most toxic family of chlorinated organic compounds released in

incineration.

One of the dioxins, 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD), is of high

acute (<24hrs) toxicity. Within 24 hrs, its LD50 (dosage that cause 50% death

of the tested organism) is only 0.001mg/kg for rat.

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Suggested Student Activity：Survey the waste-generating capacity of school/household and the types of wastes and propose waste-reduction strategy.Design a leaflt on recycling of plastic waste.Design a questionnaire on waste recycling and survey in neighborhood.

E. Solving plastic disposal problems

There are ways to solve the plastic disposal problems:

1. Technologies applied preventing generation of toxic by - product

Specific technology, such as base-catalysed decomposition (BCD) is required for

dehalogenation of compounds, including dioxins and PAHs.

Hydrogen donor + chlorinated compounds

Base catalyst, > 300OC

Donor + dechlorinated compounds (+H) + salt (chlorides)

— Incineration of the resulting products will not produce chlorinated compounds.

2. Pyrolysis

The process that plastics are heated in the absence of air at about 700OC

Involves the break down of large molecules to smaller molecules

Sorting of plastic wastes by their types is not required.

Produces hydrocarbons with small molecular mass (e.g. ethane) that can be separated

by fractional distillation and used as fuels and chemicals.

However, pyrolysis of plastics is uneconomical and still at an experimental stage.

F. Recycling of plastics

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Suggested Student Activity：Debate ‘Incineration is a good method for handling solid waste in Hong Kong’.

Suggested Student Activity：Display some plastic products and comment on the degradability and recycling potential

of the plastics.

Recycling of plastics is a technically difficult process, as compared to the recycling of metals.

Thermoplastics can be heated and reformed for multiple times. In other words, they are

generally amenable to recycling.

Thermosetting plastics often decompose when they are heated. Therefore, they cannot

be simply remoulded and reformed into new products. As a result, they are not very

amenable to recycling.

Before plastics can be recycled, they must be sorted into different type of resins* as they

require different recycling methods. Unseparated plastic refuse has little value because

its properties depend on the type of plastic materials that make up the waste mixture.

*Resin: An organic substance of natural or synthetic origin, polymeric in structure and

predominantly amorphous. Most of them but not all are of high molecular weight and

consist of long chain or network molecular structure.

In this regard, industry does not want to recycle plastic waste mixture because it may plug

up its machinery due to its different melting temperatures. Separating plastics by polymer

type is very costly because machines cannot do the separation process.

This discourages the recycling of plastics.

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Moreover, the additives such as plasticisers and pigments are still present, so the recycled

plastics can only be used to make cheap products with few uses (e.g. insulating bricks).

G. Plastic Coding System

A logo is often indicated at the bottom of a plastic container.

The number in the middle of the logo and the abbreviation below it indicate the kind of

polymer from which the container is made.

These

logos

facilitate

the sorting

of plastic containers by their composition

Separation of plastics into different kinds is essential before heating and re-moulding.

The lower the number, the easier with which the material can be recycled.

One effort on recycling of plastics focuses on the conversion of plastic products with

short service lives (e.g. foam and wrap) into products with long service lives (e.g.

construction materials and plastic pipe). For example:

— Plastic soft drink bottles (polyethylene terephthalate) are being recycled to make

carpets and insulation for ski jackets.

— Used polystyrene coffee cups and throw-away plates are being converted to plastic

“lumber”, which are resistant to termites and water.

Outdoor furniture made of plastic lumber.

H. Environmental Degradation of Plastics

No. Abbreviation Polymer1 PETE Polyethylene terephthalate2 HDPE High density polyethylene3 V Poly(vinyl chloride) (PVC)4 LDPE Low density polyethylene5 PP Polypropylene6 PS Polystyrene7 OTHER Others

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In addition to commonly used methods such as recycling, land filling and incineration;

degradation of plastic materials to smaller and less harmful molecules has held a

promising future in minimising the negative impact of plastic materials to environment.

Environmental degradation of plastics can be classified into two categories, namely,

biodegradation and photodegradation.

1. Biodegradation

A classical degradation method for environmental degradation of plastics.

Biodegradable plastics are broken down by the action of naturally occurring

microorganisms, such as fungi, bacteria and algae.

However, biodegradation of high molecular weight plastic materials is a slow process.

This remains to be a limitation of this degradation method.

2. Photodegradation

Photodegradable plastics undergo “chain scission” upon absorption of ultraviolet (UV)

radiation from the sun.

High molecular weight polymer molecules of the plastic materials are broken down into

smaller fragments. The smaller fragments eventually degraded by various

microorganisms.

An example of photodegradable plastic:

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A photodegradable six-pack ring

Suggested Student Activity：Perform investigation on the claimed ‘degradable plastic products’.

References:

1. Curlee, T. R. and Das, S. (1991) Plastic Wastes: Management, Control, Recycling, and

Disposal. New Jersey: U.S. Environmental Protection Agency.

2. Hegberg, B. A., Brenniman, G. R. and Hallenbeck, W. H. (1992) Mixed Plastics -

Recycling Technology. New Jersey: U.S. Environmental Protection Agency.

3. Hoyle, W. and Karsa, D. R. (1997) Chemical Aspects of Plastics Recycling. Cambridge:

Royal Society of Chemistry.

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