APPENDIX 111 LIST O F SUCCESSFUL APPLICATIONS IN THE

DAIRY FIELD

Static equipment Storage tanks, all shapes

Transport tanks, fixed and loose

Silo insulation covers Pipe lines, fittings Homogenizer covers Heat exchanger grids Farm tank outer casings Air hoses Filter bags, pads and gauzes Expanded polystyrene \ Expanded polyurethane j

PVC, GRP, Dual lami- nates

PVC, GRP, Dual lami- nates

GRP PVC, GRP GRP PVC coating GRP Nylon Nylon Insulation

Friction resisting parts Liners for valves Plug valves Flow splitters Ball feet Tape for threads Thrust washers Sliding parts, various Stuffing glands

Moving parts Pumps Bearings Slides

PTFE Impregnated PTFE Polypropylene Nylon PTFE, Nylon PVC, PTFE, etc. Nylon, PTFE Nylon, PTFE

PVC. PTFE. Nvlon ~. ~ ~- ~

PTFE, Nylon ' PTFE, Nylon

USES OF PLASTICS FOR DAIRY EQUIPMENT BY M . J . K E M P E R

Engineering Director, The A.P.V. Company Ltd.

Polyurethane and polystyrene for thermal insulation The main interest in polyurethane and polystyrene is for foamed plastics as a thermal insulating material. The conventional insulating material which has been used for many years for static and transport tanks, cold rooms and the like is cork. This is an economic material with quite good insulating properties, and it is also sufficiently hard to withstand local hammering and similar opera- tions which go with some copper-smithing work. However, i t does have certain specific disadvan- tages. To stick it to a surface it is necessary to use a hot setting bitumastic adhesive. This somewhat unpleasant and undesirable operation is expensive in man-power, particularly because of the relatively small sizes of the boards which have to be used, say 3 ft x I ft, and the fact that frequently two layers are needed to give the required insulating effect and to break joints to avoid preferential radial heat leakage paths. It is also difficult to select a bitumas- tic with a melting temperature sufficiently high to withstand steam sterilization temperatures without solidifying too quickly when applied to a tank before the cork can be placed in position. Pre- warming the tank is difficult to do. With thin cork sections, cases of mould growth on binding adhesives have occurred.

Polystyrene is generally cheaper than polyure- thane but its heat distortion temperature is about 90°C, and it is generally not recommended for use a t temperatures above 80°C. Thus, if steam sterilization is used, it is desirable to select polyure- thane which can be used up to 100°C which is normally adequate. The thermal conductivity of expanded polystyrene is generally quoted in the

range of 0.20 Btu in/h ftz "F whereas the figure for foamed polyurethane is about 0.13 and that for cork about 0-30. In many cases the excellent insulating properties are reduced by metallic heat conduction at points of support and fittings such as manways. Tests do show the effect of improved insulation but reduction in thickness is rarely justified. Both these materials provide excellent vapour barriers, do not absorb more than about 2 per cent water when immersed in it, are inert to the chemicals commonly used in the dairy, and do not assist mould growth. The density normally used is of the order of 1 to 2 Ib/ft3. The lower the density the lower the cost, but also the lower is the mechani- cal strength. Low density is of great advantage in transport applications.

The most convenient and economical method of using polystyrene and polyurethane is in moulded sections such as board, slab, or pipe half hollows. These sections can be readily cut and fitted by strapping or glueing with a suitable adhesive. To line the outside of tanks it is quite simple to use large board sections with a series of longitudinal slits part way through the thickness to permit shaping into an arc of a circle.

Foam-in-place polyurethane offers the advantage of completely filling a complicated shape and of the strengthening effect of a sandwich construction with the polyurethane stuck to say, inner and outer metal shells. Bending tests carried out on a stainless steel - 2 in polyurethane -mild steel sandwich gave a bond strength of 8.5 Ib/in*. The failure occurred at the bond between the mild steel and the polyure- thane. On the face of it, this should lead to a much more efficient design apart from the better insulation

20 Journal of the Society of Dairy Technology, Vol. 22, No. I , 1969

achieved, but a number of factors militate against it. First the metal surfaces must be really clean to obtain a good bond, which would be somewhat expensive to achieve; welding and forming techni- ques would set a limit to the minimum metal thicknesses that could economically be used, and the transmission of the shear loads due to the weight of tank and contents would demand support over a larger area than is normal.

However, foam-in-place polyurethane used to replace cork on normal tanks may be attractive because the extra cost of the insulation can be more than offset by the reduction in man-hours involved, but to achieve this it is necessary rigidly to control the density achievement and hence the amount of polyurethane used. In one case of a 3,000 gal horizontal milk store tank, the weight of foam used was about 3 that of the normal cork. To ensure complete filling of all voids it is necessary to use a number of carefully spaced process holes which can subsequently be welded up without much damage since polyurethane does not support combustion. Also the rise in pressure during foaming must be high enough to force the rising foam to fill the whole of the space. If this is not done quite large cavities will result after the foam sets. The rise in pressure required can well be in excess of 2 Ib/in* and care must be taken to guard against collapse of the inner vessel under external pressure.Avital point is that the equipment must be thoroughly cleaned after use if its operation is to be satisfactory. A variety of foam systems is available using two or three components consisting of isocyanate, polyether, blowing agent, catalyst and stabilizer. The system to be used will depend upon the work to be done or the duty required, for example fire retardant spray applications, and it should be worked out with the specialist supplying the foam. Finally care should be taken to observe the specialist suppliers’ advice on storage and handling of the components and on ventilation during foaming.

The application of expanded polystyrene mould- ings for transport of refrigerated materials should not be ignored, such as the transport of block ice cream and frozen foodstuffs. Here both the light- ness and insulating properties are the advantages sought, and there seems little doubt that the use of these materials for such purposes will extend. Also the adequate rigidity together with shock absorp- tion properties of both polystyrene and polyure- thane are of great help, particularly for transport and packing applications.

Covers Quite a large number of plastics applications are really centred around covers of one sort or another. Generally speaking, the advantages of plastics material for this type of duty are: their lightness, formability into a wide variety of shapes in quite

thin sections, resistance to the normal dairy atmos- phere and most of the cleaning materials, trans- parency in some cases, electrical insulation, and other specialized properties. It is not really possible to generalize about the type of application or the plastics which may be suitable for a particular type of duty. However comment on certain recent applications of plastics materials will illustrate many of the benefits and problems involved.

Let us first consider a Flo-Splitter which is an automated Flo-Verter or key piece arrangement enabling pipes to be connected alternately to product and cleaning solutions. The requirement here was to shroud the device containing the product pipes which could carry products over a fairly wide range of temperatures, say 0 - 135°C. It was also necessary for the cover to be cheap, tough, and to resist the action of leakage and washing fluids at temperatures up to 110°C.

In most dairy applications the appearance must be good, and there was some benefit in this case as the plastics concerned could be either transparent or, at least, translucent so that the operator could quickly see the position of the mechanism at any given time.

In all plastics applications the batch size is of considerable importance because of its influence on acceptability of mould costs and the moulding machinery which could be considered. Generally speaking the methods which may be considered for batch sizes between 100 and 300 gal are as follows: (a) fabrication, (b) blow moulding, (c) vacuum moulding, (d) rotational moulding, and (e) dip moulding.

A few comments can be made on these methods in this particular application. Fabrication is a very convenient method where a strictly limited number of products is to be made and avoids the cost of the mould. In general it does not give anything like the good appearance which can be obtained using moulds and it tends to give a high unit cost. In addition to this, distortion due to welding up can be a problem and in many cases the dimensional repeatability of products is substantially less than in the case of the moulded product.

Blow moulding is a very useful technique which unfortunately could not be used in this particular case because the depth to diameter ratio of 1 : 1 was considered too great. However in the end it was found possible to use vacuum moulding, and one can only say that many of these applications do require a certain amount of persistence in the development of the correct moulding technique. The particular dimensions of this moulding are 14 in diameter x 124 in deep x & in thick.

Rotational moulding is of considerable interest but in this particular case an adequate guarantee of dimensional stability of the final product was not possible but was necessary because of the need for a close fit over the metallic parts of the unit.

Journal of the Society of Dairy Technology, Vol. 22, No. I , 1969 21

TABLE I Suitability of various plastics for cover type applications

- Air bubbles

1 : 1 toogreat Good Good Good* -

I

Material 4 in sheer mailable

Yes 0.12 in

Yes

Yes Yes Yes

Polyethylene type :

Astralite

PVC Polypropylene Polycarbonate TPX

(Ei

Permanency

Good Good Good

Fair Good Good

Clarity

Opaque Clear Clear

Clear Opaque

Clear Clear

Nature

Tough Tough Brittle

Tough Tough Tough Tough

Temp. limit

100°C 100°C 100°C

100°C 150°C 150°C 200°C

cost relation

Results

*Polycarbonate required careful preheating before moulding.

Finally, dip moulding was rejected since this is really only suitable for low temperature plastics.

Apart from the problems in moulding, those of selecting the appropriate plastics to use were quite considerable. These resulted from the need to withstand temperatures of at least 135°C for periods of up to one hour and of obtaining

in sheet material reasonably economically so that satisfactory mouldings could be made. Table 1 indicates the qualities of the seven different plastics materials which were considered for this duty.

The current mouldings are being made in poly- propylene and appear to be eminently fit for the job except that they are translucent only and not transparent. It is not yet certain how great an advantage transparency is because even if efficient there must be some considerable loss in its pro- perties in use.

A completely different type of application is the micro-switch casing. This again is a cover type application used to enclose the micro-switch assembly of the air operated Zephyr valve. The requirement is to protect the switches from dirt, moisture, etc. and mechanical damage; to establish as far as possible reasonable insulated conditions and to allow rapid inspection to be made. In addi- tion there is a certain amount of dispersion of heat but in this case it is not a particularly severe problem because of the operating air and surrounding atmos- phere. A particular requirement was for very good dimensional tolerances and repeatability, and a good surface to the interior of the moulding so that the mounting of the micro-switch components, and their adjustment could be satisfactorily carried out. Most operators object to screw fixings particularly where these involve loose screws which can be lost down drains, etc. In this case a cylindrical box has been selected with a snap-on cover which eliminates any special tools or loose fixings. The casing itself is made in glass-filled phenolic resin and has moulded in it screwed inserts which hold the micro-switch actuating mechanism. It has mounting holes and a hole drilled in the side to insert the rubber gromett for the electrical leads. The reason for drilling this

side hole was to reduce the tooling cost in this particular case but in quantity it would be justified as a mould feature.

An idea of the accuracy required of the com- ponents can be obtained by considering a micro- switch assembly. The micro-switches themselves are on sliding delrin carriers with a simple screw adjustment. By this method of moulding it has in fact been possible to hold tolerances of 0.003 in repeatedly, and there is no doubt that close tole- rance mouldings of this typedo lend themselves to a number of moving part applications. The lid is made in nylon 1 1 to obtain flexibility and the snap- on cover arrangement. The lid is not intended to be fully air-tight and, in any case, this would be difficult to arrange because the spindle of the valve moves up and down during its actuation and tends to displace air inside the box. However the nylon does give flexibility and ease of handling to the component. The use of plastics such as delrin for a combination of their insulating and low co- efficient frictional properties is interesting. The setting of the micro-switch is done normally to an accuracy of 0.005 in to be sure that the switch is operated when the valve is fully closed or open. It is therefore necessary that the mechanism used to adjust the setting remains absolutely fixed once it has been set, and is not affected by vibration, expansion, distortion, etc. Use has been made of the self-locking properties of delrin for moulding in an undersized screw thread in the carrier, and for the actuating arm, which carries a moulded in screwed insert by which it can be mounted on its mounting plate. It i s appropriate to remember that plastics can be used for parts of the micro-switch itself and particularly for encapsulation to prevent the ingress of moisture, etc. All those concerned with electrical maintenance in dairies will realize the corrosion problems which arise in the dairy atmos- phere because of its moisture and fumes of hypo- chlorite vapour. There is no doubt that encapsula- tion of such electrical devices is probably the most satisfactory way of protecting them during their operational life.

Another type of cover for which plastics have

22 Journal of the Society of Dairy Technology, Vol. 22, No. I , 1969

been tried is the homogenizer cover. These are large covers of fairly intricate shape which need to be readily removable. Glass reinforced plastics (GRP) are suitable for this type of application when hand laid up in the mould and they do have the advan- tages of lightness, self-colouring and mouldability. They also do not rattle which can be important with unbolted covers. However it should be clearly realized that to absorb sound it is necessary to have an adequate weight of material for plastics are not as good absorbers of sound as are heavier materials such as masonry or steel.

The main disadvantage of GRP for this type of duty is the tendency of its surface to become damaged over the years as a result of handling and contact with the fluids normally encountered. A certain amount can be done to overcome this difficulty by careful selection of the gel1 coat type and thickness but it must be accepted that the general resistance of plastics to handling .and operating conditions is never as good as a suitably selected metal. In some cases this type of cover can be cheaper than a cast metal one but this depends entirely on the size, shape, and detailed design of the individual cover. The main overall saving is possibly in subsequent machining operations but no firm comment can be made which would apply to a wide range of cases.

Outer casings for tanks At present this is the largest application of plastics as a cover. It is quite possible to mould very large GRP shapes which can be used as covers. Anyone who has seen the very large GRP moulds used in small sea-going pleasure craft will realize the possibilities as far as size and intricacy of mould is concerned. However in many cases this is either uneconomic or there may be problems arising from the need to transmit a load through such a cover, and also the question of fittings such as manways, instrumentation, and the like. The most satisfac- tory methods to date of using GRP for outer casings, e.g. for tanks, have used GRP of about fi in thick. Such sheets can be used as flat sheets which may be wrapped around either the correct insulation or structural rings containing the inner vessel, which ever is the more convenient. Sheets can then be screwed to the rings using, for example, stainless steel screws and the screw holes can be sealed without any great difficulty. This can be done by means of a sealant or a sealing washer under the head of a screw and a cover strip can then be applied over the top. In this way a very high degree of vapour-tightness and leak-tightness can be given to the entire casing. Various alternative methods of construction such as the use of rivets or adhesives can confidently be used. The self-colouring nature of the plastics, its resistance to atmospheric attack and the normal dairy cleaning fluids, the smooth and pleasing appearance that results from the gel1

coat, and the considerable economy resulting from the fact that no significant amount of maintenance is required during the life of the casing are all advantages of this type of construction. In addition it can be applied equally to horizontal or vertical, round or flat-sided tanks and it is quite possible to arrange for the transmission of substantial loads through the normal supporting members.

The construction of conical or dished ends is a little more difficult and does require suitable moulds for the production of hand lay up GRP mouldings. These can be made in a number of sections depending on the general dimensions and shape and the amount of money which can be afforded for moulds. It is possible to obtain end covers in this way which are quite attractive and have the same general properties of the GRP sheet used on the cylindrical shell.

Polytetra fluoroethylene impregnation All users of dairy equipment will be very familiar with the very bad galling and self-welding properties of stainless steel when used in rubbing applications, particularly when both rubbing members are made of stainless steel. It is not possible to avoid this, particularly in severe cases, by selecting stainless steels of widely different hardness such as 304 and precipitation hardening stainless steels. Severe applications are those where the load is high and lubrication is either minimal or absent. Polytetra fluoroethylene (PTFE) is a valuable means of overcoming this problem where dissimilar materials such as gun-metal or mild steel cannot be used for other reasons. It is quite possible to use PTFE in solid form as washers, tape trapped in screw threads or machined components, but this is not always acceptable. PTFE tends to flow under load even when reinforced, differential thermal expansion can be a problem, because of its intrinsic anti-friction properties it cannot be lapped as is done with say, cock plugs, and it does not have the mechanical strength or adaptability of the metals normally used.

PTFE coatings are now familiar, and it is generally realized that their life is dependent on the quality of the initial work done in applying the coating as well as the care with which the coated item is subsequently used. Serck Audco have per- fected a process for embodying a dry lubricant into the surface structure of metals to reduce friction and overcome galling known as LoMu. This process is applied to a range of taper plug valves. The PTFE is physically bonded into the surface layers of the metal by pre-treatment to produce re-entrant cavities in the metal surface in which subsequent application of PTFE produces a metal plastics matrix with a much reduced coefficient of friction and elimination of self-welding properties in stainless steel. Subsequent use may well remove PTFE from the surface but sufficient remains in the

Journal of the Society of Dairy Technology, Vol. 22, No. 1, 1969 23

interstices of the metal to retain the low friction properties. Possibly the physical effect is similar to that of the familiar plain bearing, which relies on a matrix of the hard and soft components of the bearing metal, say lead and bronze for example.

Be this as it may, extensive trials have shown that this process does substantially reduce the co- efficient of friction between metals, and it is particularly helpful in the case of stainless steel with its undesirable galling and self-welding properties and where lubrication is absent. Unfortunately, the process hides the normal shiny surface of stainless steel, and it cannot be lapped so that maximum sealing pressures are not improved, but they will not be greatly reduced, by scoring. Another point is that a component cannot be effectively re-treated later in its life, but this may not be very important if it is by then damaged anyway. The process must be carried out with care but it does offer con- siderable benefit on rubbing components parti- cularly where conditions are severe and fine tole- rances are essential.

Conclusion A number of varied examples of the application of plastics have been given without .going into the field of packaging which is being dealt with by another lecturer. In talking about applications one can never exhaust the subject and can only seek to illustrate some of the points involved. I hope that the selected examples have done this satisfactorily. It is of course fair to point out that no plastics available at present have the durability, formability and many other desirable advantages of a suitably selected metal, such as stainless steel. The out- standing properties which plastics have and metals do not have are related to their lightness, their low co-efficient of friction, and their low electrical and thermal conductivities. These are specific advan- tages and it is in this type of application that plastics undoubtedly can be very useful.

When these properties are not involved one is always concerned with the selection of the most suitable material, bearing in mind all factors, including cost. I believe that in the UK we are not using plastics nearly as widely as they are being used on the Continent, and I feel this situation is bound to change.

DISCUSSION Mr. R. E. Suggars: We have listened to two interesting papers on the general characteristics of plastics, with some applications. However, these have not even scratched the surface in outlining the needs and problems of applying plastics to industrial dairy plant. Without question the dairy industry as a whole must work in close conjunction with the manufacturers, moulders, and fabricators of plastics to specify the precise needs of this industry in terms of en- gineering materials, and these requirements must be met by plastics where technical and financial advantage can be shown.

This of course involves development, and only by demon- strating to the plastics manufacturer that he has a whole industry to serve, and thus a good market, will the necessary effort be made to produce the precise materials and fittings we require.

The relatively small, isolated developments of the types described today, important as they may be to the particular manufacturer, will not by themselves create any radical changes in the attitudes and approaches of dairy engineers to the use of plastics.

Mr. Dodson referred to lack of understanding of the properties of plastics by engineers. This may be so, but knowledge of the general types of plastics and their charac- teristics does not really help the engineer to determine whether or not a specific duty can be met by using a plastics material. Everyone of thematerials mentioned can bemodified either by the formationof co-polymers, reinforcement by glass, ceramic or metal fibres, or combination as a lining to other materials. What is needed is a schedule of materials specific- ally suited to the dairy industry with their characteristics, uses and limitations detailed.

One notes that PVC, polyethylene and polypropylene are suitable for process piping. Are they all suitable for milk or its by-products, or is each qualified in its use in some respect? In the reference to PVC it is not sufficiently stressed that PVC invariably contains plasticisers and stabilizers which are unsuitable for food applications. Standard PVC pipe for water mains is unplasticised and suffers the disadvantage of being rather brittle at low temperatures.

The reference to high density polythene is a little mis- leading with respect to temperature, as polythene pipe is now obtainable which withstands temperatures a little above boiling point and is used for hot water services.

We are all familiar with the use of expanded plastics for cold storage and the unquestioned economic advantages of these materials. Where preformed sections for pipcwork, vessels, etc., are concerned the finishing off may leave a lot to be desired. No preformed sections are made to cover standard pipe flanges or valves, and pipe laggers usually bodge up a box to cover such items. Wrap-round outer coverings of cambric or PVC are untidy. Improvements in these aspects should be emphasized. Foam-in-place techniques for insulation purposes should be considered with great care. As Mr. Kemper rightly stated very rigid control is necessary and the spacing of filler holes is critical. If the voids are not filled with foam (and this is not easy to determine during the filling operation) nothing can be done after the foam sets.

The electrical industry has pioneered all major uses and developments of plastics and encapsulation, and micro- switch applications as described are commonplace. One of the newest applications, which may be of interest, is the use of irradiated PVC sleeving which when heated will shrink about 4 diameters. This is extremely useful in forming water tight joints over cable glands and joints, and can also be used for sleeving shafting, repairing split hoses, etc.

Finally, I would like to hear more about the relative costs and value of using fibreglass tanks against the equivalent in stainless steel.

Mr. C. Dodson: Mr. Suggars seems to have hit upon a number of points which are not easy to answer.

First of all, I think that the plastics fabricator, the raw material manufacturer, the resin manufacturer, and in fact all the people interested in plastics are well aware that the dairy industry may offer a fine field. But as I pointed out, the acceptance of plastics for handling raw milk is very much in the hands of the user and it would seem that it is really up to the two sides, those making plastics and those who are likely to use them, to get together. I think they are doing this to some extent, and that in another few years plastics will be used quite extensively in the dairy industry.

The requirements, as I see it, are not very well defined as yet. They may be well defined in people’s minds, but they

24 Journal of the Society of Dairy Technology, Vol. 22, No. I, 1969

might not have been so well defined in discussion or colla- boration between the two sides.

As to modified plastics, I only tried to give a very broad outline of what plastics are, but this list could be increased tenfold or more. Plastics can be modified, but of course making specials can become very expensive. In general, the plastics fabricator tends to use established raw materials that can be supplied at a relatively low cost.

I did not mention irradiated PVC or anything of this nature since I felt that the audience must know something about plastics and must already have used plastics pipes.

Any one of the plastics available can be used for vessels and tankage for holding milk. Obviously stainless steel remains the first choice: I do not see how it could be other- wise as plastics are relatively new, whilst stainless steel is very well known, and I believe that its cost is being reduced. The relative advantages of stainless steel and plastics involve price. I have established that a 500 gal vessel is cheaper in stainless steel, but above say 1,OOO gal it could be cheaper in plastics. It depends upon who the manufacturer is, how many he makes, where he is located, and what technologies and knowledge he has of the purchasing industry.

There are, of course, no standards in plastics yet. There are attempts to develop a British Standard in terms of the design of plastics equipment, particularly on the reinforced glass- fibre side. This may take a long time, but nevertheless committees have been established to work out standards.

Mr. M. J. Kemper: With regard to costs, I was deliberately vague. They depend to a very large extent on how much money can be afforded for tools, moulds, or whatever is involved in the particular case. With a line of standard tanks, in very many cases the GRP type of tank will be more economical. With the larger typz of tank, with a good deal of variation - particularly the peculiarities that we get with our instrumentation,vents and so forth - the problem is much more difficult. Another thing to be borne in mind is un- doubtedly operating temperature in a number of cases. People will steam tanks and therefore generally speaking the cheaper types of plastics are a bit suspect. People will even walk in a tank with hob-nailed boots on to see whether the bottom is clean. There are problems to be solved on the design side from the point of view of the collapse under vacuum, etc., and all these can be met, but there is no one answer that can be given ‘off the cuff’. I f you want, for example, to have stainless steel fittings on a plastics tank, it becomes quite difficult. Where vessels are under pressure, as Mr. Dodson said, the question of standards, not only for the reinforced plastics themselves, but also for the vessels is under discussion and methods of design are certainly under development. All we could do this afternoon was to illustrate what could be done, and in the future the picture will undoubtedly change. The prices of the newer plastics with better high temperature propxties will eventually come down.

Mr. J. F. Hunter: This morning Dr. Horner told us of a publication - which incidentally, he recommended highly - dealing with the packaging aspects of plastics materials. Can either Mr. Dodson or Mr. Kemper tell us of any similarly informative publications dealing with their particular aspects of plastics technology? Any easily readable yet informative publications would be particularly welcome.

Mr. C. Dodson: Easily readable publications covering the whole range of plastics just do not exist. Books on one or two specific subjects such as polyester resins are available, however, and in my opinion the most interesting publications are issued by the product manufacturers themselves. I would suggest The Polyester Handbook, by Scott Bader 8c Co. Ltd., publications by British Celanese Ltd. regarding

their Celmar (polypropylene/glass fabric laminate material), and ICI who offer a number of very interesting publications on PVC.

Mr. H. R. Lovell: Do the speakers consider that epoxy-resin or alkyl resin surfacings could be employed with a cheaper base material than stainless steel for spray dryers, cyclones, and milk storage tanks?

Mr. C. Dodson: Linings of metal vessels are subject to scratching and marking, and our experience in the chemical field has revealed a lot of trouble with this type of lining. As far as milk is concerned, the original two spray driers which my company installed were in galvanized mild steel, and I gather that they operated for about twenty- five years and one of them may still be in part operation, having been modified so much that it is no longer the original machine. The coating or lining of a large metal vessel is a rather expensive process which would not compare with the cost of a stainless steel fabrication today.

The President: Mr. Dodson touched on a point which con- cerns all of us in the dairy industry; the cleanability of plastics-type tanks. At Auchincruive we recently installed a reinforced polyester resin tank for milk storage, and on the first day that milk was in it a phznol taint was found. We cannot be certain whether the milk came in with a phmol taint or whether it developed the phenol taint after it had been in the tank, but the taint was s? severe that it was present in the butter and cheese made from the supply and the produce was ruined. We have not encountered the taint in subsequent use over several months.

Mr. C. Dodson: This is, of course, quite a point. On the Continent a very clear pattern of operation has been estab- lished in the manufacture of glass-fibre tanks. To set off the resin a certain amount of styrene is used. The excess comes off as a free styrene which can impart aroma or taste. The Continentals impose a very lengthy process of steam leaching the tank before it is sent out for handling foodstuffs of any type, including milk. My own feelings on this are that where milk products are concerned a dual laminate should be employed until such time as the fabricator has established a clear pattern of operation for making a straight GRP tank.

The newer resins that arecoming in, mostly from the USA or Germany, do tend to cut out the possibility of aroma, but even so if personally asked to recommend a tank for the milk industry, I would at present play safe with a dual laminate, that is a polypropylene tank with a glass fibre reinforcement on the outside.

VOTE OF THANKS The President: We have heard this afternoon from our speakers and opener of the discussion of the many interesting challenges which lie ahead in the applications of plastics, and 1 am quite certain that there will be plenty in the future. I think Mr. Suggars was very right to point out some of the particular difficulties which do exist in relation to the application of plastics in our industry. I t would seem to me that there is a great need to keep effective liaison between the manufacturers and fabricators of plastics materials, the manufacturers of dairy equipment, and the ultimate users. It is good to know that British Standards Committees are in existence and we can only hope that in some way the Society can play an active part in making certain that plastics can be used to advantage in the dairy industry.

Mr. Dodson and Mr. Kemper knew of their task for some time, but Mr. Suggars took his on at the last minute and I am quite certain he did it very well. I would simply ask you to show our appreciation of what they have done for us this afternoon in the normal way and accord these gentlemen a hearty vote of thanks. (Applause.)

Journal of the Society of Dairy Technology, Vol. 22, No. I, 1969 25