Description
Methylene chloride (dichloromethane (DCM)) is a volatile, colorless liquid with a sweet odor, has seen use as a solvent for many applications, including coating of photographic films, in aerosol formulations, and in paint processes where high volatility is desirable, and as a common “blowing” agent for synthetic foams. This volatility may result in high concentrations in poorly ventilated areas. Methylene chloride was used as a hair spray propellant, but this use was discontinued in 1989. It is a powerful solvent that is effective in dissolving cellulose esters, fats, oils, resins, and rubber.
DCM is the most water soluble and among the least toxic of the chlorinated methanes, with a predominant toxic effect of CNS depression, expressed as narcosis. Reported systemic effects following nonlethal exposures to methylene chloride include headache, giddiness, stupor, irritability, numbness, psychomotor disturbance, and increased blood carboxyhemoglobin. Prompt removal from exposure typically results in complete recovery. Methylene chloride is mildly irritating to the skin, and dermal absorption is not considered a significant threat to human health. Eye contact may be painful but is not likely to cause serious injury. Adaptation to methylene chloride vapors occurs with repeated contact, decreasing the ability to detect exposure.
Chemical Properties
Methylene chloride, a chlorinated solvent, is a volatile, colorless liquid with a sweet-smelling odor. It is often referred to as dichloromethane. Methylene chloride has many industrial uses, such as paint stripping, metal cleaning and degreasing.
Methylene chloride does not occur naturally in the environment. Methylene chloride is used as an industrial solvent and as a paint stripper. It may also be found in some aerosol and pesticide products and is used in the manufacture of photographic film.
Uses
Methylene chloride is used as a solvent, especially where high volatility is required. It is a good solvent for oils, fats, waxes, resins, bitumen, rubber and cellulose acetate and is a useful paint stripper and degreaser. It is used in paint removers, in propellant mixtures for aerosol containers, as a solvent for plastics, as a degreasing agent, as an extracting agent in the pharmaceutical industry, and as a blowing agent in polyurethane foams. Its solvent property is sometimes increased by mixing with methanol, petroleum naphtha, or tetrachloroethylene.
Methylene chloride is the active ingredient in many formulations of paint removers including industrial paint and commercial furniture strippers, home paint removers, and products used for aircraft maintenance. The chemical has a unique ability to penetrate, blister, and lift a wide variety of paint coatings. Formulations of the chemical are used extensively in both flow-over and immersion (dip) tanks in furniture refmishing operations. For the maintenance of military and commercial aircraft, a methylene chloride-based product is often required to inspect the surface for damage.
Since the mid-1990s methylene chloride has replaced 1,1,1-trichloroethane in nonflammable adhesive formulations for industrial applications, including fabrication of upholstery foam. It provides adhesive formulations with strong, instant bonding characteristics and efficacy under extremes of temperature and humidity. In foam applications, use of methylene chloride eliminates the possibility of hard seams and allows for ready compliance with flammability requirements for upholstered furniture.
Methylene chloride is used in aerosols as a strong solvent, a flammability suppressant, vapor pressure depressant, and viscosity thinner. Current aerosol uses of methylene chloride include spray paints and lubricants.
Methylene chloride is a leading auxiliary blowing agent used in the production of slabstock flexible polyurethane foams for the furniture and bedding industries. Methylene chloride is used as an extractant in the recovery and purification of a wide variety of materials including oils, fats, and waxes. The chemical is used for the decaffeination of coffee and tea, oleoresin extraction from a variety of spices, and for the extraction of hops. As with tablet coatings, little or none of the chemical remains in the finished product.
Preparation
Methylene Chloride is derived from the chlorination of methane during which other chlorinated methane derivatives may be formed. Propylene oxide, cyclohexane, and/or 2-methyl-2-butene are added as stabilizers. Purity depends on the amount of C2 and higher hydrocarbons in the methane and the extent of chlorination. Small amounts of several other chlorinated compounds may be present. Methylene Chloride is commonly recovered from extraction processes and several grades are commonly found in commerce. Methylene Chloride is stable when dry but hydrolyzes in the presence of water.
Health Hazard
When methylene chloride enters the human body, it affects brain function, such as not being able to concentrate. At high enough levels, it can stop breathing. At lower levels, methylene chloride exposure causes dizziness, fatigue, headaches, and nausea. Methylene chloride breaks down into other chemicals in the body, such as carbon monoxide. In addition, methylene chloride can displace the oxygen in a worker’s environment because of its high vapor pressure. Skin contact with methylene chloride causes burning and redness of the skin.
Incompatibilities
Methylene chloride reacts with strong oxidizers, caustic substances, chemically active metals such as aluminum and magnesium powders, potassium, sodium, and concentrated nitric acid.
Physical properties
A common synonym for methylene chloride is dichloromethane.
Methylene chloride is a colorless liquid with a sweetish odor.
The chemical formula for methylene chloride is CH2Cl2, and the molecular weight is 84.93 g/mol.
The vapor pressure for methylene chloride is 349 mmHg at 20 °C, and it has an octanol/water coefficient (log K ow ) of 1.30.
Methylene chloride has an odor threshold of 250 parts per million (ppm).
Methylene chloride is slightly soluble in water and is nonflammable.
Uses
Methylene chloride is predominantly used as a solvent in paint strippers and removers; as a process solvent in the manufacture of drugs, pharmaceuticals, and film coatings; as a metal cleaning and finishing solvent in electronics manufacturing; and as an agent in urethane foam blowing.
Methylene chloride is also used as a propellant in aerosols for products such as paints, automotive products, and insect sprays.
It is used as an extraction solvent for spice oleoresins, hops, and for the removal of caffeine from coffee. However, due to concern over residual solvent, most decaffeinators no longer use methylene chloride.
Methylene chloride is also approved for use as a postharvest fumigant for grains and strawberries and as a degreening agent for citrus fruit.
Preparation
Methylene Chloride was first prepared by Regnault in 1840 by the chlorination of methyl chloride in sunlight. It became an industrial chemical of importance during the Second World War. Two commercial processes are currently used for the production of Methylene Chloride—hydrochlorination of methanol and direct chlorination of methane (Rossberg et al., 1986; Holbrook, 1993).
The predominant method of manufacturing Methylene Chloride uses as a first step the reaction of hydrogen chloride and methanol to give methyl chloride. Excess methyl chloride is then mixed with chlorine and reacts to give Methylene Chloride, with chloroform and carbon tetrachloride as co-products. This reaction is usually carried out in the gas phase thermally but can also be performed catalytically or photolytically. At low temperature and high pressure, the liquid-phase process is capable of giving high selectivity for Methylene Chloride (Rossberg et al., 1986; Holbrook, 1993).
The older and currently less used production method for Methylene Chloride involves direct reaction of excess methane with chlorine at high temperatures (400–500°C), or at somewhat lower temperatures either catalytically or photolytically. Methyl chloride, chloroform and carbon tetrachloride are also produced as co-products (Rossberg et al., 1986; Holbrook, 1993).
World production of Methylene Chloride increased from 93 thousand tonnes in 1960 to an estimated 570 thousand tonnes in 1980 (Edwards et al., 1982) and is believed to be still several hundred thousand tonnes. Production in the United States has shown a steady decline from 1981 to 1993, as shown by the following figures (thousand tonnes): 1981, 404; 1984, 275; 1987, 234; 1990, 209; 1993, 160 (Anon., 1994, 1997). The total amount produced in western Europe ranged from 331 500 tonnes in 1986 to 254 200 tonnes in 1991 (WHO, 1996).
Definition
ChEBI: Dichloromethane is a member of the class of chloromethanes that is methane in which two of the hydrogens have been replaced by chlorine. A dense, non-flammible colourless liquid at room temperature (b.p. 40℃, d = 1.33) which is immiscible with water, it is widely used as a solvent, a paint stripper, and for the removal of caffeine from coffee and tea. It has a role as a polar aprotic solvent, a carcinogenic agent and a refrigerant. It is a member of chloromethanes and a volatile organic compound.
Health Hazard
Methylene chloride is predominantly used as a solvent. The acute (short-term) effects of methylene chloride inhalation in humans consist mainly of nervous system effects including decreased visual, auditory, and motor functions, but these effects are reversible once exposure ceases. The effects of chronic (long-term) exposure to methylene chloride suggest that the central nervous system (CNS) is a potential target in humans and animals. Human data are inconclusive regarding methylene chloride and cancer. Animal studies have shown increases in liver and lung cancer and benign mammary gland tumors following the inhalation of methylene chloride.
Chemical Reactivity
Reacts vigorously with active metals (lithium, sodium, potassium) and with strong bases (potassium tert-butoxide) (Sax, 1984)
Industrial uses
Methylene chloride has been suggested, and indeed is already in commercial use, as a substitute for chlorofluorocarbon auxiliary blowing agents. Methylene chloride is highly volatile (boiling point 39.8°C) and inert in polyurethane-forming mixtures. However, methylene chloride is a suspected carcinogen and has other deleterious effects on workers exposed to it. Accordingly, the concentration of methylene chloride in the air inside a foam plant must be kept low. The American Conference of Governmental Industrial Hygienists recommends that workers not be exposed to more than 50 ppm of the chemical, while the Occupational Safety and Health Administration’s Permissible Exposure Limit is 500 ppm. Keeping the levels of methylene chloride in a foam plant below 50 ppm may require additional ventilation equipment, with an associated increase in costs. In addition, the chemical is a recognized environmental pollutant, and both the federal and state governments are beginning to limit releases of the chemical from plants that employ it. Thus, in the near future, plants using methylene chloride as an auxiliary blowing agent may be faced with the substantial additional expense of installing scrubbers or similar equipment to remove methylene chloride from air and/or other gases discharged from the plant. In addition, California has recently proposed that emissions of methylene chloride in that state be subject to a heavy “pollution” tax, and other states are likely to follow a similar course.
Potential Exposure
The principal route of human exposure to methylene chloride is inhalation of ambient air.
Occupational and consumer exposure to methylene chloride in indoor air may be much higher, especially from spray painting or other aerosol uses. People who work in these places can breathe in the chemical or it may come in contact with the skin.
Methylene chloride has been detected in both surface water and groundwater samples taken at hazardous waste sites and in drinking water at very low concentrations.
storage
Prior to working with Methylene Chloride you should be trained on its proper handling and storage.
A regulated, marked area should be established where Methylene Chloride is handled, used or stored as required by the OSHA Methylene Chloride Standard (29 CFR 1910.1052).
Methylene Chloride reacts violently With OXIDIZING AGENTS(such as PERCHLORATES, PEROXIDES PERMANGANATES CHLORATES NITRATES CHLORINE, BROMINE and FLUORINE CHEMICALLY ACTIVE METALS (such as POTASSIUM, SODIUM MAGNESIUM and ALUMINUM) and STRONG BASES(such as SODIUM HYDROXIDE and POTASSIUM HYDROXIDE).
Methylene Chloride is not compatible with liquid oxygen;Titanium and Amines.
Store in tightly closed containers in a cool, well-ventilated area away from Metals and Ligh.
Methylene Chloride attacks some forms OFPLASTIC RUBBER and COATINGS and will corrode iron. some STAINLESS STEELS COPPER and NICKEL in the presence of WATER.
