14 minutes reading time (2841 words)

    Basic Aerosol Technology

    Lindsay Showyin

    History

    The invention of the aerosol as we know it is attributed to Eric Rotheim of Norway in the late 1920s. It was not however until the early 1940s when two Americans, Goodue and Sullivan, produced a portable insecticide "bomb" that the evolution of a consumer product started. After World War II developments in the container and valve occurred and the birth of the commercial industry occurred in USA in early 1948. My aim is to now give a broad overview of aerosol technology so that a picture of the industry, products and opportunities can emerge.

    Conventional Aerosol Containers

    Metal cans have generally been used with tinplate most used although in some countries aluminium is dominant. Price and/or available manufacturing facilities are usually the determining factors. Internal lacquer may be used with the characteristic of the formulation determining its necessity or not. With tinplate the lacquer is put on whilst the metal is flat; with aluminium it is sprayed inside the formed container.

    Tinplate containers initially were tab soldered with the tabs being external or internal. Welded side seam cans ar€e now predominant. The welding process also allowed necking in the can body to allow usage of smaller diameter tops and ends. This can is variously known as Necked in, Trimline, Slimline, etc. in contrast to the conventional can which is known as straight walled. The can is constructed from a flat piece of metal and after the body is formed tops and bottom are put on. Printing if present occurs in the flat with tinplate. Of course with welded cans there is more area to print upon.

    Tinplate used in the aerosol industry in Australia is generally made by BHP at Port Kembla, the industry using about 13,000 tonnes annually. Steel is electrolytically coated with tin.

    The temper or steel grade used in the aerosol industry differs for the different components. The aerosol top and valve cups are produced by highly working the metal so usually tempers 1 to 3 are used.

    Aerosol bottoms have a symmetrically shaped shallow dome profile and so a harder grade (temper 5) is usually used.

    The can body has to be high speed welded and temper 3 is normally used.

    The tin level is low and usually EO5 (2.8g/m2) and/or D10/05 (5.6/2.8g/m2) are used.

    Note: E = equally coated; D = differentially coated

    In contrast the aluminium can has no seam and is generally made by impact extrusion of a slug.

    Aerosol can size is usually described by two numbers, eg. 65 x 198. The first is the diameter (in mm) and the second is the height measured from the base to the top of the rim.

    In the imperial system figures are eg. 211 x 713, the first being the diameter, 2 11/16", the second the height, 7 13/16". Such terminology is still used in America. As would be expected there is a small number of diameters available and a relatively large number of different heights available. One size and measurement which is consistent around the world for all can sizes and types is the opening where the valve is crimped.

    In Australia there are two major tinplate aerosol manufacturers - Containers Packaging which is a member of the Amcor Limited Group and National Can, a publicly listed company. Some years ago Containers purchased the G.E. Crane aluminium can business and so dominate both metal can segments.

    Plastic has long been investigated for aerosols and in the last few years in the UK PET aerosols have been marketed.

    Valves
    Aerosol valves to dispense almost any type of product are available - foams, dry sprays, space sprays, streams, upside down, etc.

    Basically the valve operation is either push down or tilt.

    The vast majority of aerosol valve technology lies in the hands of a relatively small number of valve manufacturers. As expected the majority of these are American. The major valve manufacturers in the world are:

    • PRECISION VALVE CORPORATION
    • SEAQUIST - VALOIS
    • SUMMIT PACMGING SYSTEMS INC.
    • COSTER TECHNOLOGIE
    • AEROSOL RESEARCH AND DEVELOPMENT (COPE ALLMAN)
    There are of course a number of smaller specific country manufacturers. In the majority of instances there are generally affiliations with the companies listed above.

    The basic components of a valve are illustrated in Figure 1. some of the terminology is unique to the aerosol industry.

    Generally the nozzle fits over the stem but in some instances it fits into the stem - in this case it is called a female valve.

    The latest developments are sleeve gaskets to replace the flowed in compound and laminated mounting cups.

     Aerosol Propellants

    In chemical type propellants can be divided into two - liquefied (i.e. they are present as a liquid inside an aerosol can), or compressed (where the gas is basically compressed in the headspace above the liquid level within the can).

    An important technical difference is that when a liquefied propellant is used the pressure within the can stays basically the same through the life of a can declining only at the very end of the can provided it is used properly whereas with a compressed gas the pressure begins to decline as soon as some of the gas is used. Additionally with a liquefied propellant as it comes out of the valve it changes from liquid to gas and thus has the capability of producing a much finer spray than a compressed gas. Therefore the best and predominant use of compressed gases is in surface sprays.

    The most common liquefied propellant is hydrocarbon (butane/propane blend) whereas compressed gas propellant are nitrous oxide, nitrogen and carbon dioxide.

    CFC's (Chlorofluorocarbons) are now used only in certain medical and safety aerosols and are of the liquefied propellant type. A DME (Dimethyl Ether) plant has recently been built in Australia by CSR. This is a liquefied propellant, is flammable but has the advantage of mixing in a single phase with water. There is growing interest and use of this propellant.

    Aerosol Formulatlons
    Very broadly these can be categorised into:
    • solvent based or
    • water based.
    Alternatively they can be separated into
    • single phase or
    • two phase - which require shaking of the product before spraying.
    Generally water based products are two phase although with the propellant DME (Dimethyl Ether) single phase water based products can be produced. Another way to categorise aerosol products is space spray or surface spray. Generally this is related to the amount of propellant present.
    • Space spray - 50-90% propellant
    • Surface spray - 5-15% propellant

    Technical Considerations

    Corrosion

    This is probably the major concern for all formulators. During the development of a formulation there are a number of basic technicalities which have to be observed. For example, in anhydrous formulations the moisture level of raw materials must be checked.

    Shelf testing is obviously a necessity - if possible shelf test samples should be filled on a production line. It must be remembered that corrosion requires the presence of an electrolyte, two electrodes and a potential difference between the electrodes. The electrolyte is the solution of the product and the two electrodes (in a tinplate container) are the tin and iron. However, as tin and iron are very close in their electrochemical behaviour, slight variations in their environment such as the level of oxygen can determine which metal is the anode and which is the cathode. Normally corrosion occurs at the anode.

    As the area of tin which is exposed is large compared to the amount of iron, you can see why the polarity is critical. If the large area (i.e. tin) cathode is coupled to the small area anode (iron) the corrosive attack is concentrated and pinholing results. If the system was reversed general detinning occurs rather than pinholing.

    Aluminium can be rather unpredictable in terms of corrosion, particularly as it is a very active metal which readily takes on a tenacious film of inert aluminium oxide when exposed to air, giving rise to a rather unique and complex set of corrosion properties.

    Products can be successfully shelf tested and even be manufactured satisfactorily for many years yet occasionally corrode. Unfortunately in many cases the reason for the failures is never determined despite considerable time and expense being spent on investigation of the problem.


    General
    The presence of air (oxygen) in an aerosol product, apart from causing corrosion problems, can also significantly increase the pressure inside should large amounts be present. Water quality should be at least considered as presence of too many or different metal ions or other impurities may affect compatibility with the container.

    It should be acknowledged that internal can linings (usually epoxies) and side stripes on the weld are never perfect and should not be relied upon to protect a container. In some instances "concentration" can occur through the pinholes in a lacquer, thus allowing a lacquered can to corrode or pinhole faster than a plain can.


    Flammability

    In Australia, flammability of an aerosol is measured by the flame test (see Figure 2), whilst in Europe, the percentage of flammable materials governs the classification. In the latter case, any adjustment by way of spray rate and/or valve configuration does not affect the flammability classification which determines such items as wording on labels, warehousing, transport etc.

     Aerosol Manufacture
    The specific step of air removal is known as purging and this can take the form of drawing a vacuum or addition of a small amount of liquid propellant before the valve is placed in the can so that it vaporises rapidly and displaces the air.

    The product concentrate is usually filled first - in certain products there is a two stage concentrate fill. There are various methods of filling the propellant and process of attachment of the valve to the container is known as crimping, clinching or sometimes swaging.

    • Pressure filling - The valve is crimped on first and the propellant is filled in through the valve (with or without the nozzle on).
    • Undercap filling - The machine in this instance has a 3-way action - draws a vacuum, injects in the propellant (as a liquid) and crimps on the valve.
    • Cold filling - is a very old method which is rarely used today. In this instance the propellant is chilled and filled as a liquid. This is of course an expensive and relatively inefficient method.
    • Gasser/Shaker - Where compressed gas is used usually shaking is necessary to ensure all the gas is put in and to obtain what little dissolution of the gas in the concentrate is maximised.
    • Impact Gassing - This is the more modern method of filling compressed gas and special valves are used.

    Safety
    As well as the fact we are dealing with a pressurised product in the case of hydrocarbon propellants we are dealing with a highly flammable material. The general manufacturing caution that is used is to have the gassing machines isolated in a separate room and to pass all filled products through a heated (50°C) water bath for around 2-3 minutes to check for can or valve leakers or incorrectly crimped cans. It is also very important to have a reasonable ullage in a can because the volume expands as the temperature rises and a liquid full situation, which would lead to the can exploding, must be avoided at temperatures the can may reach. Temperatures in rear parcel shelves of cars can be extremely high. Generally a fill at 20°C of 80% by volume should not be exceeded.Quality ControlUsually checks are carried out on all raw materials used in aerosol formulations, as well as on the concentrate once it is manufactured. Tests (eg. pH, emulsion stability etc.) obviously depend on the product. If blending of propellants occurs the blend must be checked at least for pressure.

    Considerable setting up of filling lines is necessary to ensure product and propellant fills are within a narrow range. Tests on filled cans usually are for weight, spray rate, spray characteristics and pressure. In addition, crimp height and diameters are checked.

    Chlorofluorocarbons (CFC) / Ozone
    No discussion on aerosols is possible without reference to the ozone issue and the Greenhouse effect.Ozone Issue

    The gas ozone provides the earth with an effective UV (ultraviolet) shield. The majority of ozone is located in the stratosphere, about 10 to 50 km above the earth's surface. It is in constant movement because it is determined by a balance between photochemical processes that produces it and other processes that destroy it. The quantity therefore varies with latitude, longitude, night and day, seasons, year by year and possibly with cycles of solar activity.

    The concern about CFC is that prolonged release might deplete the stratospheric ozone layer. This hypothesis was raised in 1974 by Rowland and Molina and current evidence is that global ozone levels are decreasing very slightly. The ozone problem arises because CFCs, released at the Earth's surface predominantly in the major cities of the northern hemisphere, are sufficiently stable to resist breakdown by oxidation in the lower atmosphere. Thus they are transported by the weather systems into the southern hemisphere and also into the upper atmosphere where they are broken down by intense ultraviolet radiation. Not only does this photolysis of CFCs release chlorine which is capable of enhancing the natural ozone destruction processes, but it does so in a region of the atmosphere close to where most of the ozone is found. Atmospheric models indicate that continued use of CFCs at current levels will cause small but significant losses of ozone over the entire globe, with substantial losses in Antarctica and possibly in the Arctic as well. The Montreal Protocol (under the auspices of the United nations Environment Program) was agreed to in September 1987. it called for, using 1986 consumption as base figures, for CFCs,

    • freeze at 1986 levels from mid 1989.
    • reduce these levels by 20% by mid 1993.
    • further reduction by 30% (i.e. total 50%) by mid 1998.
    The Australian aerosol industry response was,
    • in December 1987 endorse the protocol but phase down at a faster rate.
    • in April 1988 to phase out CFC (except for essential use Government defined products) by the 31 December 1989.
    In any event it is important to note that at the end of 1987 around 80% of Australian aerosols did not contain any CFC.

    There was a meeting in London in June 1990 to review the Montreal Protocol. It revised the Protocol for CFCs (in total) to now be,

    • 1995 50% phase out
    • 1997 85% phase out
    • 2000 100% phase out
    At present overall Australia has achieved a phase out level of 50%, thanks predominantly to the aerosol industry action.Methyl Chloroform (1,1,1-Trichloroethane)This material was officially added to the Montreal Protocol list of controlled substances in June 1990. The phase out timetable was:
    ​Year ​% Phase Out
    ​1993 ​Freeze at 1989 levels
    ​1995 ​30
    ​2000 ​70
    ​2005 ​100

     To be reviewed at third meeting of parties in 1992.Again the Australian aerosol industry had well recognised the potential problems with this material and had begun reformulating some years ago. Therefore it was able to take a proposal to the Federal Government in October 1990 with the following timetable.

    ​Date ​% Phase Out
    ​December 1991
    ​60% reduction on 1989 level
    ​December 1992
    ​100%*

    * with provision for essential use products and permitted sale of aerosols containing MCF produced prior to 31.12.92.

    Comparison of the timetables shows again how far we are in advance of the Protocol timetable. In this case Australia is, as far as I am aware, well ahead of any other country either in aerosol or any industry sector.

    Greenhouse Effect
    This is an important property of the natural atmosphere that results from the ability of certain components such as water vapour, carbon dioxide, methane, CFCs and nitrous oxide to absorb infrared radiation emitted by the Earth's surface. It it was not for this effect the Earth would be too cold to sustain life. However, the use of fossil fuels to generate energy and the rapid expansion of agriculture has caused the concentrations of carbon dioxide and methane to grow to an extent that they are calculated to have already caused a global warming.

    CFCs can also contribute to the Greenhouse effect by absorbing infrared radiation.

    Carbon dioxide is the major gas of concern - it is expired by man, it is also formed when forests and other items are burnt.

    The natural gas (hydrocarbons) used in aerosols, do not persist in the atmosphere for very long and the contribution to the Greenhouse Effect is, in effect, so small as to be of no consequence.

    To put the issue in perspective, the greenhouse contribution through the per capita use of 10 aerosol products per year is the equivalent of driving an average family car an extra 15 km in a whole year.

    Further Reading
    1. Fraser, P.J. (Ed), Environmental, Health and Economic Implications of the use of Chlorofluorocarbons as Aerosol Propellants and and Possible Substitutes, 1987.
      A joint Australian Environment Council/National Health and Medical Research Council Document.
    2. Australian Environment Council, "Strategy for Ozone protection", August 1989.
    3. Johnsen, M.A., "The Aerosol Handbook", Second Edition, 1982.
    4. Herzka, A.,(Ed), "International Encyclopaedia of Pressurized Packaging (Aerosols)" 1966.
    Western Australia Branch Bayliss Youth Lecture Che...
    IN SEARCH OF MORE SOLUTIONS 2000
     

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