Information on Chlorofluorocarbons                   

The Chemical Nature of CFCs

Control of Ozone Depletion

Possible CFC Alternatives

Chlorofluorocarbons (CFCs) are a family of compounds containing chlorine, fluorine and carbon. Owing to their unreactive nature, low flammability and low toxicity, CFCs have a wide range of uses: 

• As aerosol propellants 

CFCs such as trichlorofluoromethane(CCl3F) and dichlorodifluoromethane (CCl2F2) are normally packed with materials like paint, insecticide or cosmetic preparations in pressurized canisters. Upon depressurization by opening the valve, the propellent vaporizes and expels the materials inside the canister in the form of aerosol spray. 

• As solvents in cleaning electronic components and metals 

CFCs such as trichlorotrifluoroethane (CCl2FCCLF2) can dissolve grease and are widely used as solvents in cleaning electronic components and metals. 

• As refrigerants 

Freon is a series of CFCs, of which dichlorodifluoromethane (CCl2F2) is perhaps one of the most important. Freons absorb heat of vaporization on evaporation resulting in cooling of the surroundings. They are widely used as refrigerants in refrigerators and air conditioning units. 

• As blowing agents in foam plastic manufacturing

In making foam plastic, a volatile CFC such as trichlorofluoromethane 

Free radical chain reactions involved with chlorofluorocarbons leading to the depletion of the ozone layer

Ozone in the stratosphere undergoes photodissociation by absorbing ultra-violet radiation: 

O3 + UV Light => O2 + O - - - - - - - - (1)

The free O atom further reacts with another molecule of ozone: 

O + O3 => O2 + O2 - - - - - - - (2)

This has the effect of reducing the concentration of ozone in the stratosphere. However, ozone is being made at the same time. Under natural equilibrium, the rate of producing ozone is somewhat equivalent to the rate of destroying ozone, and thus a fairly constant concentration of ozone is maintained. 

During the past few decades, there has been a drastic change in the concentration of ozone in the stratosphere. Ozone concentration has fallen over the Antarctica and Arctica. Evidence shows that ozone depletion is related to the release of chlorofluorocarbons (CFCs) to the atmosphere. The destruction of ozone by CFCs is a complex chemical process. Very much simplified, an outline of this process is illustrated below by CCl3F and CCl2F2: 

CCl3F + UV Light => CCl2F + Cl - - - - - - - - (3) 
CCl2F2 + UV Light => CClF2 + Cl - - - - - - - - (4)
The Cl  from the above initiation steps readily depletes ozone via a sequence of reactions (5) and (6) 

Cl + O3 => ClO + O2 - - - - - - - - (5) 
ClO + O => Cl + O2 - - - - - - - - (6)

The depletion of ozone by reaction (5) is considerably faster than that by reaction (2). This has the effect of disturbing the balance in the production and destruction of ozone. What worries environmentalists more is the fact that since the reactive species Cl consumed in (5) is actually regenerated in (6), the presence of one Cl can effectively destroy many ozone molecules. 

Among other adverse effects on aquatic and terrestrial ecosystems, ozone depletion leads to reduced crop yield, and higher incidence of skin cancer and eye cataract. There is therefore an urgent need to minimize the use of CFCs and to develop suitable CFC substitutes which do not adversely affect the environment. 

Control of the ozone depletion problem

As early as 1978, CFC aerosol propellants were banned in some countries like Canada and United States. The first international effort to protect the ozone layer came in January 1978 when 24 countries signed the Montreal Protocol on Substances That Deplete the Ozone Layer which called for a cut back of the CFC production in stages. The Protocol was amended in 1990 when 93 countries pledged to stop all production of CFCs and other ozone depleting substances by 2000. Since then, many industrialized nations have agreed to a complete phaseout as early as 1995.

Hong Kong joined the international effort to protect the ozone layer by becoming a party to the Protovol and introduced the Ozone Layer Protection Ordinance(OLPO) in 1989. The ordinance enpowers the governmnet to control the consumption of CFCs and other ozone depleting substances through import/export licensing control.

Installation of recovering and recycling units to recycle CFCs from used refrigeration systems and motor vehicle air conditioners is an important practice to control the venting of CFCs into the atmosphere. Using substitutes for CFCs is another possible solution to the depletion problem.

Possible alternatives for chlorofluorocarbons 

Attempts have been made to develop compounds which have low ozone depletion potential (ODP) to replace CFCs as refrigerants, aerosol propellants and solvents. Possible alternatives for CFCs include: 

• Hydrochlorofluorocarbons (HCHCs) such as CF3CHCl2 

They break down more quickly in the atmosphere. Though they have a lower percentage of chlorine and hence a lower ozone depletion potential than CFCs, they could damage ozone if overuse. 

• Hydrofluorocarbons(HCFs) such as CF3CH2F 

They have no chlorine and thus are "ozone safe". However, safety question on toxicity is still unsolved. 

• Hydrocarbons such as butane and propane 

They are cheap and readily available. As they contain no chlorine, they are "ozone safe". However, they are flammable and poisonous. 

• Water and steam 

They are effective for some cleaning applications and thus can replace some CFCs as solvents in cleaning. 

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