THE DEVELOPMENT OF THE ISOPROPYL ALCOHOL/PFC CLEANING SYSTEM. presented by

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1 THE DEVELOPMENT OF THE ISOPROPYL ALCOHOL/PFC CLEANING SYSTEM. presented by B. H. Baxter Senior environmental Technolosist British Aerospace Defence DynamiGs Ltd Stevenage England abstract AD/= The European background to the development of the isopropyl alcohol/perfluorocarbon cleaning system is described. The currentstage afthe-design and its performance is summarised. 1.INTRODUCTION. The early stages of discussions and negotiations on the control and possible phase-out of the ozone depleting substances (ODS) started in the early 70's but did not result in concrete legislation until the signing of the Montreal Protocol in Since that time the original plan, namely the slow reduction of CFC production to 50% of 1986 levels has been replaced by a total phase-out of all the ODS including all the fully halogenated CFCs and methyl chloroform. The HCFCs, once thought of as the l*saviours*l of the ODS users, have now been included in the legislation'and will be phased out early in the next century. The main legislative changes are summarised in figure 1, which is based on EuroDean Communitv Regulations. Now in Europe the time has arrived when supplies of the Group 1 materials, particularly CFC 113 have begun to dry up. Buyers are beginning to experience difficulty in obtaining supplies. Production of the Group 1 materials will be reduced to 15% of 1986 production on against a predicted demand of about 35% across the whole EC. Some suppliers have indicated that they will not continue production at this low level. Progress toward phase-out of ODS in European Countries varies widely, Some countries with vigorous regulations, Sweden, Germany and Switzerland are reported as having phased out of ODs. Political and Economic factors, however, have had a considerable slowing effect on the rate of phase-out. The sudden changes in the World political scene have caused business to contract, not least in the Defence Industry. As a consequence investment is not buoyant and Companies are tending toward a "wait and seet1 policy, All of this taken together means that the next 2 to 3 years will see sudden panicked changes. This will be despite all our efforts to encourage industry to plan early for the introduction of non-cfc options. Despite all the lectures, paper presentations at Seminars and Conferences, and presentations to the Managements of our Companies we have arrived at the **end** of 526

2 the ODS and much of Industry has been caught unprepared. This pessimistic view arises in large part from the author's long experience in the precision engineering and instrument manufacturing sector. Of all the ODS application sectors, several in particular have always been "emissivet1 in nature. These are aerosols, flexible foams and solvent cleaning. Others such as auto air-conditioning and Halon fire suppression could be included, but only because of avoidable wasteful practices now largely eradicated. Of these emissive uses, aerosols were the first to stop the use of ODs, either by different technologies such as pump action sprays or the use of non-ods propellants. Flexible foam manufacture has continued either by changing to alternative chemistry or by the use of not in kind options. Solvent cleaning is a little more complex. It can be divided into several broad classifications:- 1. Metal degreasing. 2. Electronics defluxing. 3. Precision, or critical component cleaning. 4. Garmentiand industrial fabric dry cleaning. Of these, metal degreasing will revert to the older chlorinated solvents, trichloroethylene, perchlorethylene and methylene chloride (where allowed by local legislation). Alternatively aqueous or semi-aqueous methods will be used. Simple hydrocarbon cleaning may take up a small part of the sector market. Electronics defluxing is probably the great success story of ODS phase-out. The large scale commercial manufacturing sectors of this industry have developed new vvno-clean'v techniques such as inert gas soldering and no clean fluxes. Aqueous and semi-aqueous cleaning methods have been developed to a very high degree of efficiency, and as a result many manufacturers, particularly in the USA are virtually free of CFC. The remaining two present particular problems:- Both are largely dependent on CFC 113. There is no suitable direct substitute for this solvent. The basic High Street cleaners will continue with perchlorethylene or revert to it if they had made the change to CFC113. Unfortunately there is no suitable substitute for this solvent in the special fabric sector of the industry which has grown up mainly because of the availability of 113. It is possible therefore that radical changes may occur as a result such as loss of smaller operators and central large scale cleaner Plants operating through local collection and distribution outlets. Exactly similar problems exist in precision cleaning. There is no direct substitute (yet) for CFC113 and since the precision cleaning industry developed mainly because of the availability Of 113, then serious problems will arise. Of the current options HCFC 141b has the same ODP as methyl chloroform, it is already regulated and has a phase-out date in Its low boiling Point makes containment necessary and it is not a direct drop-in 527

3 replacement in existing equipment. HCFC 225cb, with a much lower ODP ( ~0.05)~ is a virtually direct replacement for 113 based on compatibility and physical properties. However, it will not be available in large quantities for a few years and will be more expensive than 113 currently. The proposed HFC cleaning solvents are also some time away. Since aqueous and semi aqueous methods can give some problems, particularly corrosion and difficulty with drying, the remaining available practical contender is isopropyl alcohol. This solvent is ideally suited to precision cleaning having suitable moderate solvent power and compatibility with most materials, but it is flammable. It is however finding a wide - range of applications in the precision cleaning industry DEVELOPMENT HISTORY. lly the project to develop a safe industrial cleaner based on IPA envisaged the electronics industry as the main market. However as that sector quickly developed its wide range of new options it became clear that precision cleaning was much the most important sector and this has shaped the design considerations of the system. The main considerations which controlled the current design of the equipment were:- 1. Safety. 2. Minimum vapour loss. 3. Rapid drying of the IPA after cleaning. 4. Low energy consumption. 5. Low volume waste stream. A diagram of the system is shown in figure two. The flammability of the alcohol has been overcome by a process developed by David Slinn of ISC Chemicals, now part of the Rhone Poulanc group. By combining the alcohol with a perfluorocarbon he showed that the two liquids would ftco-evaporatelf when heated, forming a mixed vapour which was non-flammable. Furthermore the system operated at about 50 deg C, very close to the boiling point of 113 and a convenient temperature to handle finished product. At 80 deg C, the boiling point of IPA, parts can be uncomfortable to handle and might need a cooling period. The drying of the parts is achieved by an ingenious process described as a superheated PFC spray. PFC is pumped through the hot water heat exchanger and boils rapidly. The.resulting mix of vapour and hot PFC liquid is then directed at the cleaned parts. As the superheated spray contains no alcohol it can evaporate the alcohol very rapidly from the wet parts. It is equivalent to drying water wet parts in a very low humidity air stream. The low emission of the system is achieved by the combination of a horizontally operated pneumatically sealed lid, and a flexible storage bag which traps the vapour laden air as the system heats up to operating temperature. Because the system is brought up to temperature with the lid closed and is not operated until the temperature is stabilised, then opening and closing the lid does not result in vapour loss because the pressures are equalised 528

4 inside and out. Introduction of cold work load to the cleaning tanks is also carried out with the lid closed so that as the vapour layer collapses temporarily, vapour is drawn into the tank fromthe storage volume and then replaced as operating conditions are re-established. These two features, the sealed lid and the balancing volume greatly reduce two of the major causes of loss in vapour degreasers, namely vapour loss as the system heats up to operating temperature and loss due to the "piston effect" of introducing cold parts into the vapour layer. Excessive energy consumption is avoided by the use of the low specific heat PFC which boils at about 50 deg C and is also used as the drying agent. This gives a better energy use than raising IPA to its boiling point and then having to evaporate the liquid alcohol from the cool parts by hot air drying. The low waste stream levels are achieved by an alcohol concentrator. This operates under the same PFC blanket and is therefore safely non-flammable and delivers a low volume of concentrated dirty alcohol for easy disposal. 3. DEVELOPMENT TRIALS AND INDUSTRIAL APPLICATIONS A prototype alkohol PFC machine was used as the alcohol cleaning part of the UK investigation programme into non-cfc defluxing methods. This was a joint project between The Department of Trade and Industry the Ministry of Defence and three UK Companies, GEC, BNR and BAe Dynamics. Units have been evaluated for bearing final cleaning, gyro component final assembly cleaning and hydraulic component cleaning. Production units are in operation in the manufacture of computer disc drives, electronic component cleaning, bearings and optical components. 4.SuMMARY. The experience to date with the alcohol PFC machine indicates that the design is viable. There was no doubt that the alcohol would be an effective cleaner and the use of efficient ultrasonic agitation greatly enhances the cleaning effectiveness. The low emission features are effective and reliable and will allow the design to be used to great advantage when the newer HFC and HCFC solvents become available commercially. In the interim the design allows for the safe industrial application of a flammable liquid with very low emission rates. 529

5 Changes in Regulatory Legislation since w 0 06 OM 099A 0893 Year BRITISH AEROSPACE DEFENCE DYNAMICS

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