More On Solvents
Solvents for cleaning
A satisfactory solvent for film cleaning should have the following characteristics:
- Should be able to dissolve oil and grease, which are the major chemicals by which dust and loose particle adhere to the film. A solvents ability to act as a solvent is measured by the KB value; the higher the number the greater the "dissolving ability" of the solvent. KB stands for Kauri Butyl gum and the solvent power number is a function of the amount of this gum that a given volume of the solvent will dissolve. The greater the quantity of gum dissolved, the greater the solvent ability. A KB above 125 is ideal for film cleaning and anything much above that is known to affect the film base detrimentally.
- A satisfactory solvent should also cavitate under the effect of ultrasonic energy.
- Should not leave any unwanted residue or photographically active chemical on the film.
- Should not affect the film emulsion or base, or cause changes to the life of either. Evaporation rate should be such that drying is fast and reclamation by distillation relatively easy.
- Solvents with a low boiling point are not ideal because the film pre-dries between the liquid surface of the cleaning tank and the air knives, which results in a tendency to produce streaking under high humidity conditions. The boiling point preferred is low enough to allow the liquid blown off by the air knives and to evaporate in the air stream easily. Low boiling points also result in the rapid evaporation causing atmospheric temperature depression of the dew point, which will result in droplets of water being deposited on the film. The boiling point is best when slightly lower than that of water so that the liquid blown off by the air knives will evaporate in the air stream and provide efficient extraction. Much lower than this and the process of evaporation causes atmospheric temperature depression of the dew point, this will result in droplets of water depositing on the film.
- Toxicity should be low with a TLV of at least 300 ppm is preferred. The Threshold Level Value [TLV] is a recommended level of concentration in the room air, in parts per million, above which an operator is not permitted to work for an 8 hour day.
- Electrostatic elimination ability is desirable.
- Flammability should be low
- It should have no effect on the ozone layer
There is no doubt all the solvents mentioned below all have some value as cleaners and from time to time each will be found to do a job the others cannot. It is recommended that small quantities be kept under safe conditions for use in local hand cleaning using the technique mentioned above. The operation must be carried out under safe conditions in such a manner that all vapour is extracted away from the operator.
The operator must wear a mask specifically devised to absorb this vapour, although nearly all these solvents will be satisfactorily absorbed by the facemasks designed for use with a broad spectrum of organic solvents. If there is any doubt whatsoever the local Health and Safety Executive (in the UK) or their equivalent in other EU countries must be contacted and the manufacturer of the masks.
CFC's and the ozone layer
Solvent cleaning with 1:1:1 Trichloroethane [1:1:1 TCE] has been the mainstay of all film laboratories for over 30 years; it is effective and fast, and replaced solvent cleaning with other more toxic solvents, notably Trichloroethylene and Carbon tetrachloride. The Montreal Convention has removed 1:1:1 TCE for the future and a new solvent is being sort. There are a number of other solvents that archive laboratories have used for specialised cleaning purposes and these are mentioned later.
The Montreal Protocol on Substances that Deplete the Ozone Layer is the international agreement that is designed to protect the environment with the power to ban substances, which may be harmful to the ozone layer. The ozone layer is found in the stratosphere, between 10 to 25 miles above the ground. Ozone is a gas whose molecules have three atoms of oxygen [O3]. The molecules are spread very sparsely throughout the stratosphere but are vital to the environmental system because ozone is capable of absorbing harmful UV-B ultraviolet radiation from the sun. Ozone is continually being formed and destroyed naturally because ultra-violet radiation from the sun breaks down oxygen molecules O2 into two atoms and these atoms combine with other oxygen molecules O2 to form ozone, O3. Ozone molecules in turn break down to produce O2 and the process begins again.
It is now known that chlorofluorocarbons (CFC's) released into the atmosphere are a major cause of ozone depletion. Because of their stability, CFC's do not break down in the lower atmosphere but eventually reach the upper atmosphere where they are broken down by ultraviolet radiation to produce two chlorine atoms for each molecule of chlorofluorocarbon. The chlorine acts as a catalyst for destruction of ozone. The effect of this is a depletion of the ozone layer which in turn causes an increase in the levels of UV-B reaching the earth’s surface which would give rise to increased levels of skin cancer.
There is evidence that the ozone layer is thinning all over the world and holes in the layer over the Antarctic have been visible for many years.
Trichloroethylene and Perchlorethylene are the two of the substances under consideration. Trichloroethylene has a lifetime of only seven days if released into the atmosphere that prevents the molecules reaching the upper atmosphere. It also has no global warming potential. It is however more toxic. Perchlorethylene is more toxic and less efficient as a cleaner but has been selected by several film cleaning machine manufacturers as the successor to 1:1:1 TCE. In Sweden another solvent is being used as the alternative
Safety Rules
Considering the extreme volatility of the three solvents most widely used in the cinematographic laboratory (Perchlorethylene, 1,1,1 TCE and acetone, for polishing and cements) it is imperative to guarantee an efficient ventilation system, a monitoring method for their concentration in the atmosphere, a health and safety policy and ensure staff are trained to handle them. The machines that use these solvents must be connected with the correct ventilation systems. Respiratory-protection devices such as masks with active carbon filters are essential during tank cleaning and accident spills.
Solvents for cleaning