So, if you've read some of the other materials on this site, then you already know that Quad used a form of nylon (CALATON) to coat the diaphragms in the Quad Electrostatic Loudspeaker. This was N-methoxy-methyl Nylon, and is soluble in alcohol solutions, and thus quite easy to handle in the factory. Alas, it is no longer made. If you want a Quad returned to its original condition, this is the coating to use...but why is it so good? Well, resistivity for one thing, is very high, and can be very uniform. However, there are also properties such as those I'll discuss below that make it a unique substance in a unique application. See Carothers 1939 Nylon, for the original U.S. Patent.
The family of nylons consists of several different types. Nylon 6/6, nylon 6, nylon 6/10, nylon 6/12, nylon 11, nylon 12, and nylon 6-6/6 copolymer are the most common. The numbers refer to how many methyl units (-CH2-) occur on each side of the nitrogen atoms (amide groups). The difference in number of methyl units influences the property profiles of the various nylons. Moisture absorbance is decreased due to reduced polarity with further separation and less regular location of the very polar amide roups. Resistance to thermal deformation is lowered due to more flexibility and mobility in these methyl unit sections of the main chain. As these units increase in length, making the molecules appear more like polyethylene, the properties of the nylon shift slightly toward those of polyethylene. Not considering the effects of moisture, Nylon 6/12 has lower modulus, higher elongation, lower strength, lower thermal distortion temperature, lower hardness and lower melting point than nylon 6/6. The property which gives nylon 6/12 its utility is moisture absorption which is approximately half of that of nylon 6/6. This means the properties are much more consistent and experience less fluctuation due to ambient humidity levels in the end application. The latter is not something we are interested in (particularly) when coating a speaker diaphragm. We would in fact like a material that absorbs atmospheric moisture to some extent.
Moisture absorption by nylon has been a source of great study for many years. Although all polymers absorb some amount of moisture, the effect with Nylon is particularly significant. The table shown below illustrates the moisture absorption levels of various types of nylons.
Absorption of Moisture by Nylons by Weight % |
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Type of Nylon |
Equilibrium @50% R.H. |
Equilibrium @ Saturation |
6 |
2.7 |
9.5 |
6/6 |
2.5 |
8.0 |
6/10 |
1.5 |
3.5 |
6/12 |
1.3 |
3.0 |
11 |
0.8 |
1.9 |
12 |
0.7 |
1.4 |
From this data and the electrical properties highlighted at the bottom of this page we can quickly appreciate that Nylon (especially Nylon 6/6 and 6) is an excellent material for diaphragm coatings in electrostatic speakers.
Water molecules produce polar bonds with the amide groups in the nylon molecules. Although small, water molecules take up space and displace the nylon molecules. This results in the nylon molecular matrix swelling. Dimensional changes of 0.7% can result in nylon parts from the "as-molded" state to equilibrium at 50% R.H. environments. This change occurs in approximately 1 day for a 0.5 mil layer. Molecular mobility is increased through the absorption of water. There is less resistance to applied stress from the decrease in intermolecular friction, and conductivity increases slightly. From this sort of mechanical data we can deduce that the lifetime of a membrane coating which is exposed to frequent, wide fluctuations in humidity (e.g. air conditioning) could be considerably reduced due to constant flexure of the coating caused by absorption and subsequent loss of water which is causing mechanical stresses in the coating.
The absorption of moisture by nylon is a completely reversible physical reaction. Drying in an oven will drive off all but a small percentage of the water molecules which can only be removed through dissolution of the nylon molecular matrix. Keeping a little water in the DIY Nylon mixture is therefore essential to avoid absolute "dryness" of the coating and a reduced charging rate. The rate of absorption/desorption varies with type of nylon as well as temperature and relative humidity. Addition of fillers reduces the effect of moisture both due to volume reduction of the amount of nylon polymer in the mixture, and by sharing the attraction of the molecules, somewhat reducing polarity and the available space for moisture molecules. In addition to the mechanisms which take place with fillers, the adhesion of the nylon molecular matrix to dimensionally stable reinforcements is stronger than than polar bonding of the water molecules and it dominates.
Properties |
A.S.T.M. Test Method |
Nylon |
Nylon |
Nylon |
Nylon |
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Specific Gravity |
D792 |
1.1 |
1.14-1.1 |
1.06 |
1.15 |
Water Absorption Method A |
D570 |
2.9 |
1.24 |
0.25 |
-- |
Tensile strength at yield, 1000 psi |
D638 |
9.4 |
12 |
8.8 |
11-14 |
Elongation at yield, % |
D638 |
25 |
>150 |
7 |
10 |
Elastic Modulus in Tension, 10~5 psi |
D638 |
-- |
4.4 |
-- |
3.5-4.5 |
Flexural Strength at yield, 1000 psi |
D790 |
NO YIELD |
16 |
NO YIELD |
16-17.5 |
Elastic modulus in flexure, 10~5 psi |
D790 |
1.50 |
4.1 |
2.95 |
-- |
Rockwell Hardness (Method A) |
D785 |
R104 |
88 |
R114 |
R112 |
Izod impact strength, ft-lb/in. notch 1/8 in. specimen |
D256 |
2.2 |
1.2 |
1.5 |
-- |
Deform. under load(2000 psi; 122f), % |
D621 |
-- |
0.8 |
1.6 |
0.5-1.0 |
Deflection temperature, F at 66 psi fiber stress |
D648 |
340 |
450 |
356 |
400 |
Max recommended service Temp., F continuous use |
-=- |
175 |
270 |
290 |
200-225 |
Coeff. of Linear Thermal Expansion, F |
D696 |
4 x 10-5 |
4.5 x 10-5 |
5 x 10-5 |
5.0 x 10-5 |
Underwriters' Lab Rating (Subj. 94) |
-=- |
HB |
V-2 |
V-2 |
-- |
Dielectric strength, v/mil, short time |
D149 |
-- |
555 |
650 |
500 |
Dielectric constant at 60 Hertz |
D150 |
7.2 |
4.0 |
4.0 |
3.7 |
Dielectric constant at 1 MegaHertz |
D150 |
3.7 |
3.5 |
3.5 |
3.7 |
Dissipation factor, at 60 Hertz |
D150 |
-- |
0.02 |
0.02 |
-- |
Dissipation factor, at 1 MegaHertz |
D150 |
0.12 |
0.03 |
0.2 |
-- |
Volume resistivity, ohm-cm |
D257 |
1012 |
1015 |
1015 |
-- |
Arc resistance (SS Electrode), sec. |
D495 |
-- |
123 |
-- |
-- |
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These values are representative of those obtained under standard ASTM conditions. |