What, No Diaphragm Coating Material ???


Much has been written and stated about the demise of the coating material that Quad were using on their electrostatic panels. It is true that ICI who originally made the coating no longer produce it. This does not mean other coatings cant be used. I suspect that the reason's for Quad stopping the production of new panels is more to do with economics rather than the coating not being available. The fact that the coating is no longer produced provided Quad with a good reason to stop producing new panels. I have seen emails from Ross Walker who has stated that they have tried the ESL 63 coating material on ESL 57 panels but this wasn't successful as the 63 coating is orders of magnitude higher resistance. I can only say that the coating on a diaphragm need only be slightly conductive for the panel to function properly. It really is surprising how little coating is actually needed. I don't underestimate when I say that you only need about 1 or 2 drops of coating material per panel. Based on this I find it surprising that the 63 coating wont work on a 57 panel.

So, if the original coating material is not available what can we use ? Other coatings which have been used include liquid soap, wallpaper paste, powdered graphite (very popular), anti static spray for VDU terminals etc.

The properties of any coating must include :

I personally don't use any of the above coatings. I use another product which is intended for another application. DO NOT ask what this coating is - I wont tell you. All I will say is that it contains tin oxide as the conductive part of the coating and has a polyester resin adhesive mixed in which adheres nicely to the polyester film. I will NOT reveal any more about this coating as I have been asked not to by the person who told me. Actually its more involved than just selecting a coating compound so obtaining some of the coating compound is only the beginning of the story.

COATING UPDATE - August 2001

Some time ago Gary Jacobson wrote to me and told me he had found a way of making a coating which was similar to Quad's original. I must admit that I looked at the process and decided it was too much trouble to use it for my panels. However on a recent surf round the internet I found Gary's website and was surprised to find that not only has he found some of the original Quad coating but has found a modern alternative.

The original coating was made by ICI and was called Calaton®. This is a trademark name for soluble nylon. It is a product which amongst other uses is used for making yarns more durable. Its easy to use - all you have to do is dissolve it in some alcohol - preferably methanol but you could also use ethanol or propanol. The only fiddly bit is you have to heat the alcohol to about 60ºC.

This is all very well but if ICI don't make it anymore, is there an alternative ? The answer to this is yes, there is. Its made by DuPont and is called Elvamide®. This product is the same as the ICI product. Its very easy to get hold of and its cheap. I phoned DuPont in the UK and they directed me to one of their sites in Hemel Hempstead. I phoned them (01442 218500) and asked for an engineering sample of Elvamide®. They asked what grade I wanted. There are several grades each with their own specific properties. I just went for the general purpose grade which is Elvamide® 8061. They sent me 1kg of the stuff for less than 20. Enough for a lifetime of panel rebuilding.

So you have some Elvamide® but how do you use it ? The first thing you need to do is find some alcohol to dissolve it into. You can use (in order of preference) methanol, ethanol or propanol. If none of these are available you can pop down your local hardware store and get some methylated spirits (which is methanol and ethanol together) and use that instead. Next problem is how to heat the alcohol. Gary suggests using a beaker in a hot water bath on a cooker. As I have a gas cooker this is potentially rather dangerous. Methylated spirits are highly combustable so for safety's sake I boil some water in a kettle and pour it into a basin. I then find a container which I can put in the boiling water and pour the meths in that. If I find the water going cold I just boil some more. Gary also seems to go to great lengths to exactly measure out the correct amount of alcohol to Elvamide®. I take a more cavalier approach. I use 9 parts meths to 1 part Elvamide®. Once you have the Elvamide® and meths in a heated container you need to stir it to make the Elvamide® dissolve. This will take some time. I reckon it takes me about half an hour and I still have some small un-dissolved pellets of Elvamide®. Actually it probably doesn't matter. Just so long as you have dissolved some Elvamide® you know it will be in solution.

It only remains to apply it to your diaphragm. I simply use a cotton wool ball although this isn't ideal as some fibres of cotton do inevitably get left on the diaphragm. Simply dip the cotton wool in the solution and wipe it over the surface of the diaphragm. Obviously you don't want too much such that it will run and you don't want too little otherwise the alcohol in the cotton wool will have all evaporated before you get to the end of the diaphragm. Take a little care when coating the diaphragm as you want the whole area covered. When you have finished the alcohol will evaporate and leave a thin coating of nylon. When I do it the coating initially turns white then goes clear.

Its then time to mount it into the panel you are repairing and try it out. Simple.

If you want some Elvamide® you can buy a small amount from Gary's website or phone DuPont in Hemel Hempstead. You could also ask me to send you a small amount. Gary also sells the diaphragm material which is very hard to find elsewhere.

An unfinished article about the Nylon coating

Since being introduced to the Quad ESL 57 in summer '97 they have continued to fascinate me. The design was revolutionary for its time (and still is) and has arguably outperformed any speaker since it was made. It was also years ahead in the way it used the materials it did to make the speaker work in the way it does.

I know these speakers very well. It amazes me that very little development work was done to the design in its 20+ years in production. Some areas of the design are revolutionary, Some are not.

Central to the way the speaker works is the way the high voltage is held on the diaphragm. At some point someone must have thought "Well we have a diaphragm but how are we going to get a voltage on there ?".

First some history. During the fifties it was the invention of MYLAR by DuPont which made the ESL 57's possible. MYLAR is a polyester film and before this time there was no product available which had the properties which Quad were looking for. They must have tried others but most would have failed due to stretching of the film and general instability issues. Once they had a diaphragm they had to find some way of charging it up. They couldn't just apply voltage to a diaphragm made of polyester as it wouldn't do anything. So they decided to coat the diaphragm in order to get a charge on there. Key considerations for the coating are :

  1. must have high surface resistance
  2. must not degrade over time
  3. must be cheap
  4. must be readily available
  5. must be electrically conductive *
  6. must adhere to diaphragm
* as I will show this isn't necessarily true

When I first started looking at Electrostatics I first thought "Why not just use some metalised film ?". This is a big NO NO. It fails big time on the high surface resistance requirement. High surface resistance is important as you cannot have the charge moving around the diaphragm's surface. If it does you will get high voltage hot-spots which will cause the diaphragm to move in a non-linear fashion. It may also cause the diaphragm to become un-stable at moderate listening levels. I then tried the method used by many people when building home made ESL's - graphite powder. This can be bought from hardware stores as a lubricant for household door locks. Its very cheap, fairly easy to apply with cotton wool (although you need to rub hard) but suffers from the problem of not knowing if you have a consistent uniform coating. Some areas of the diaphragm may be high voltage, some may be low. Also graphite rubbing for Quad was not a viable solution if they were going to build many speakers. Its OK for one off's but not for production. Other people have used a wide variety of different coatings including wallpaper paste, liquid hand soap, tin oxide, anti-static computer cleaner etc. None of these solutions quite matched the elegant simplicity of the Quad solution.

The brilliant thing about the Quad solution was its simplicity and the fact that it didn't make a compromise to any of the requirements listed above. What Quad realised is that we are not talking about a flowing electric current on the diaphragm. It should just sit there and allowing a little loss to atmosphere should not need much topping up. If there's no current flowing the coating doesn't necessarily need to be electrically conductive. Its a little difficult to get your head round but the voltage is just sitting there. Its not being drained away by anything. Think of it another way - the only reason the wiring in your home is insulated copper is that current needs to flow over the copper conductors. If you don't have anything plugged into your mains supply you just have voltage sitting on all the conductors but no actual current is flowing. If there's no current flowing it doesn't matter how conductive the conductor is. You could use water inside the insulation as water is conductive albeit at very high resistance. You don't see this resistance until you try to connect something to it. Whatever you connect to it won't work as the conductor is just too resistant to current flow. Therefore if there's no current flow the conductor can have almost infinite resistance.

Once this had been realised Quad approached the problem as a pure electrostatic problem rather than an electrical problem. What they needed was a material which could be applied to the diaphragm which would allow the diaphragm to be charged up. Now ALL materials (except cotton) can be charged up. Some materials take on a positive charge (they loose electrons) and some take on a negative charge (they gain electrons). The relative positions of all materials is known as the Triboelectric series. This is just a list of materials and their relative potential charge. This list is below :

	Human Hands      <----        Most Positive
	Asbestos
	Rabbit Fur
	Glass
	Mica
	Human Hair
	Nylon
	Wool
	Fur
	Lead
	Silk
	Aluminum
	Paper
	Cotton           ZERO
	Steel
	Wood
	Amber
	Sealing Wax
	Hard Rubber
	Nickel, Copper
	Brass, Silver
	Gold, Platinum
	Sulphur
	Acetate, Rayon
	Polyester
	Styrene (Styrofoam)
	Orlon
	Saran
	Polyurethane
	Polyethylene
	Polypropylene
	Vinyl (PVC)
	Silicon
	Teflon         <-----     Most Negative

The above list is from NATURE'S ELECTRICITY, p63 by Charles K Adams (c)1987 Tab Books, #2769

We can see polyester 2/3rds down the list. As polyester is the diaphragm material we want a coating which has a more positive electrostatic charge i.e. it looses electrons and so therefore keeps the charge close into the diaphragm. Basically any of the materials which appear above polyester will fit this requirement. As it turned out Quad used Nylon but they could have used any material above polyester.

Quad eventually chose a product made by ICI called Calaton. This is an alcohol dissolvable Nylon. It dissolves in methanol, ethanol, propanol etc. by just heating the alcohol a little to help the dissolving process. Once the nylon is in solution its a simple mater of wiping the liquid over the surface of the diaphragm and waiting a few minutes for the alcohol to evaporate off leaving the nylon deposited on the diaphragms surface. This product has been used in the textile industry for many years to improve the abrasion resistance of many yarns. Run a yarn through the alcohol/Nylon liquid, let it dry and it instantly becomes more hard wearing. Quad continued to make new panels even after they discontinued making the speaker in 1980. That was until 1997 when they announced that the original coating was no longer available. ICI stopped making Calaton. This provided Quad (who were under new management at the time) with a good reason to stop making replacement panels. Ross Walker has since stated that they tried the ESL63 coating but it was unsatisfactory. All the original machines/jigs/fixtures were then sold to the Quad distributor in Germany. They are now making new parts for the ESL57 should people need them. They must have found a replacement coating.....

Some months ago I got an e-mail from Gary Jacobson in Australia. At the time he was just another fan of the ESL57 who had found my web site and wanted more information. I heard nothing from him until I got an e-mail saying he had found a way to make a coating which was equivalent to the original Quad coating. He also told me about the triboelectric series and explained why Quad used the materials they did. Many months later during a surf of the Internet I found Gary's web site. He has a page called Area 51 where he is selling the original Calaton coating AND a DuPont equivalent. When I saw this I couldn't quite believe it. This is my equivalent of the Holy Grail - a coating which is exactly the same as Quad's original. I emailed Gary immediately and he was kind enough to send me some samples of both ICI original product and DuPonts equivalent called Elvamide.

How do I mix up the coating ???

There are a few tools/materials which I use to mix up the coating. These are :

The first things to get are the basic materials which are Methanol (or an alternative) and some Elvamide. When I do a fresh batch of coating material I mix up 50ml of Methanol and about a palmful of Elvamide. Now I know this isnt too scientific in terms of proportions but I reason that so long as a reasonable amount of nylon has dissolved in the alcohol you will be OK.


About the right amount of Elvamide for 50ml of Methanol.

Next thing you have to do is dissolve the Elvamide into the Methanol. The first time I tried this I stood for what seemed like hours stirring the mix by hand. I decided there and then that there had to be a better way. So the engineer in me looked around for an old small motor I could use to stir the mixture. I found an old CPU fan out of a PC which I butchered. I basically cut all the fans off and was left with a small stubby motor. To this I stuck a bent paper clip to stir the mixture and I stuck the whole thing to the inside of a small jam jar. It all sounds a bit Heath Robinson but it works.


Motor with stirring attachment. Stuck to the inside of a jam jar lid.

Its then just a simple matter of connecting the motor to a bench power supply, pouring the Methanol and Elvamide into the jar, screwing the lid on, filling the outer container with nearly boiling water and turning the power supply on.


Everything ready.


Sit back and watch it dissolve.

Several times during the stir I unscrew the lid and make sure the Elvamide pellets are moving around. The Elvamide has an adhesive built in to bond it to whatever its being applied to. This tends to make it clump together or stick to the bottom of the jar. What I try to do is get all the pellets moving round in the Methanol. When its all dissolved I transfer the liquid to a sealed polypropelene container so it cant evaporate.


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