Water dispersible, low molecular weight polyamide resin particles of uniform sizes, method of preparing same and coatings formed therefrom

Finely-divided, water dispersible, low molecular weight, linear polyamide resin particles having a unique morphology and of uniform particle sizes are formed by preparing a linear polyamide of a predetermined low molecular weight in the presence of water followed by rapidly quenching the reaction mass with an aqueous medium below the freezing point of the polyamide and continuing the cooling of the mass to a temperature sufficiently low so as to prevent particle growth and structural alteration while regulating the pH of the quenched mass to provide particles of predetermined, uniform size dispersed in the aqueous medium. The particles vary from ultimate flaky sheets or lamellae to loosely packed, randomly oriented, clusters of flaky sheets. Spray dried products may be agglomerates which disintegrate readily in water to the original particles or aggregates depending upon intended uses. The dispersions and dried products are useful in the coating art and the polyamide may be polymerized to high molecular weights after application to a substrate.

This invention relates to low molecular weight, linear polyamide resins in 
the form of finely-divided, water dispersible particles of sizes falling 
within controlled, very narrow size ranges, aqueous dispersions thereof 
and a method of preparing same. 
Finely divided polyamide resins have been proposed for use in the coating 
art. In powder form, they have been used by immersing a heated article in 
a bed, such as a fluidized bed, of the powder, or by a flame spraying 
procedure. In British Pat. No. 1,180,023 it is proposed to immerse the 
heated metal article into a fluidized bed of finely divided, low molecular 
weight polyamide particles containing an oxy-acid of phosphorus and 
subsequently subjecting the article to a further heat treatment so as to 
further polymerize the polyamide and thereby form a coating of a high 
molecular weight polyamide. Such method may be satisfactory for coating of 
rods and simple shapes but is not adaptable to the coating of complicated 
shapes or large articles, or articles or objects or low heat capacity such 
as, for example, wire screening, metal foil, etc. Alternatively, it has 
been proposed to use solutions of superpolyamides, however, the polyamides 
are generally soluble only in toxic and/or corrosive liquids such as 
phenol, cresol, formic acid, hydrochloric acid, or a limited number of 
organic liquids such as furfuryl alcohol or formamide. These solutions 
require special handling techniques and are highly disadvantageous in that 
normally the solvent presents pollution problems and must be recovered. 
As a further alternative, as, for example, in wire coating the 
superpolyamide resin may be in a molten state and the wire drawn through 
the molten resin. Both the solution and the molten techniques are 
undesirable because, in general, the synthetic, linear superpolyamides are 
degraded rather rapidly at the solution and at the molten temperatures 
which are required. Further disadvantages are associated with these 
methods of application utilizing the present commercially available high 
molecular weight polyamides. In the application of a coating from a 
solution of the polyamide, the coating is limited to a few tenths of a mil 
and, in general, several coatings are required with the necessity of 
drying and fusing each coat before the application of an additional coat. 
On the other hand, coatings applied by a molten technique are at least 10 
mils in thickness because of the high viscosity of the molten polyamides. 
Various methods have been proposed to form polyamide dispersions adapted 
for use in the coating art. The most common proposal is to dissolve the 
superpolyamide in a suitable solvent and then pouring the solution into a 
larger volume of a non-solvent with vigorous agitation. It has also been 
proposed to vigorously agitate water and a dispersing agent at 
temperatures sufficiently high to a melt a thermoplastic resin such as a 
polyamide and thereby form a dispersion of the resin. In these various 
prior art procedures the polyamides are the so-called superpolyamides 
which are fiber-forming and film-forming high molecular weight polyamide 
resins. 
In U.S. Pat. Nos. 3,299,011 and No. 3,536,647, a fiber- and film-forming 
superpolyamide is partially degraded so as to remove amorphous portions of 
the polyamide and the resulting partially degraded polyamide is subjected 
to mechanical attrition in the presence of a liquid swelling medium. The 
resulting product, which has been termed a microcrystalline, synthetic, 
linear polyamide, is dispersible in water and other liquid swelling media. 
As a result of the required mechanical attrition, the size of the 
particles extend over a very wide range. Thus a severely attrited product 
containing about 90% by weight of particles not exceeding 1 micron 
contains particles as small as 0.01 micron and as large as about 15 
microns. A moderately attrited material will contain particles as large as 
100 microns and particles under 0.2 micron and some as small as 0.01 
micron. Because of the presence of the minute particles, particularly 
those under 0.1 micron, upon drying, particles become bonded together into 
larger size particles or agglomerates to such a degree that it is almost 
impossible to reduce the particles to their original size and thus 
redisperse the material. 
One of the characteristics of these prior dispersible polyamide particles 
in their colloform or globular, granular form or shape. Because of this 
form or shape of particle which becomes deposited on the substrate being 
coated it is necessary to subject the substrate bearing the deposited 
particles to an elevated temperature for a sufficient period of time so as 
to permit the complete melting and flowing of the molten particles to form 
a continuous coating. 
One of the purposes of the present invention is to provide low molecular 
weight, linear polyamides in the form of finely-divided, water dispersible 
particles of sizes falling within controlled, very narrow size ranges by a 
simple and inexpensive method. 
A further purpose of the present invention is to provide a simple and 
inexpensive method for the preparation of aqueous dispersions of these 
finely divided, low molecular weight, linear polyamide resins. 
A further purpose of the present invention is to provide finely divided, 
low molecular weight, linear polyamide resin particles of a unique shape 
or form as will be described hereinafter. 
Another purpose of this invention is to provide a simple and inexpensive 
method for the production of finely divided, low molecular weight, linear 
polyamide resin particles of the unique shape or form which are extremely 
uniform in particle size. 
Another purpose of this invention is to provide finely divided, low 
molecular weight, linear polyamide resin particles which are readily 
dispersible in water. 
A further purpose of this invention is to provide aqueous dispersions of 
finely-divided, low molecular weight, linear polyamide resin particles 
within very narrow size ranges which may be applied directly to a 
substrate by any desired conventional method without the necessity of 
utilizing an intermediate or primer coating. 
Another object of the invention is to provide finely-divided, low molecular 
weight polyamide resin particles which may be utilized in conventional 
powder coating of substrates so as to provide adherent coatings without 
the necessity of utilizing an intermediate or primer coating. 
Other objects and advantages of the invention will become apparent from the 
following description and the claims. 
The present invention contemplates the preparation of low molecular weight, 
linear polyamide resin particles wherein water and a monomer or a mixture 
of monomers, or, alternatively, water and a high molecular weight, linear 
superpolyamide resin is heated in a closed or sealed system to a 
temperature above the melting point of the specific polyamide which is 
being formed followed by quenching rapidly in an aqueous medium to a 
temperature below the freezing point of the formed polyamide so as to 
obtain the unique particles of sizes and structure as will be described 
hereinafter and continuing to reduce the temperature of the mass to a 
point where particle growth is prevented and the initial particle size and 
structure is retained while vigorously agitating or stirring the entire 
mass and controlling or regulating the pH of the entire mass. The 
resulting product consists of a stable aqueous dispersion of the low 
molecular weight polyamide particles. The consistency will vary depending 
upon the solids content generally varying from a milk-like dispersion at 
low solids content to a paste-like product at high solids content. When 
practicing the method in a batch-wise manner, as illustrated in the 
examples, the rapid quenching and cooling may be effected conveniently 
under normal atmospheric conditions of pressure by discharging the heated 
mass into an open tank containing a sufficient quantity of a cold aqueous 
medium so that the temperature of the entire quench mass is brought to a 
temperature below the boiling point of water, as, for example, a 
temperature not exceeding about 90.degree. C to 95.degree. C. 
One of the unique characteristics of the products is the uniformity of size 
of the low molecular weight polyamide particles achieved by a control of 
the pH value of the quenched mass; that is, the particles are within a 
very narrow range of particle sizes of not more than several microns. For 
example, in those instances where the polyamide resin is a low molecular 
weight polycaprolactam and the pH of the final quenched mass is above 
about 7 and up to about 9, or where an alkaline material has been added, 
if necessary, to provide a desired predetermined pH value within this 
range, the mean size of the particles will be within the range of between 
about 2 microns and about 6 microns. Where the pH of the final mass is 
about 3.5 to about 7, the particles will be of a mean size of about 0.5 
micron to 2 microns. As an alternative, a soap such as sodium, potassium 
or ammonium oleate, stearate, etc., may be included in the initial charge 
and in such instance where the quenched mass has a pH of about 8.5, the 
particles will have a size between about 0.1 micron and 0.5 micron. In 
those instances where a soap is present and the pH of the quenched mass is 
about 3, all of the particles are appreciably below 0.1 micron in maximum 
size and of approximately the same size, approximately 0.03 .times. 0.03 
.times. 0.005 micron, a ratio of thickness to maximum dimension of 
approximately 1 to 6. 
The monomers satisfactory for the purpose of the present invention include, 
for example, .omega.-aminocarboxylic acids, such as 6-aminocaproic acid, 
or their corresponding lactams or cyclic amides, such as, 
.epsilon.-caprolactam, and salts of diamines with dicarboxylic acids, such 
as, hexamethylene diamine with hexanedioic (adipic) acid, and mixtures of 
.omega.-aminocarboxylic acids, such as, 6-aminocaproic acid and 
11-aminoundecanoic acid, or .epsilon.-caprolactam and 12-aminododecanoic 
acid, and mixtures of .omega.-aminocarboxylic acid or lactam and a salt of 
a diamine with a dicarboxylic acid, such as, a mixture of 
.epsilon.-caprolactam and hexamethylenediammonium adipate (nylon-6,6 
salt). 
High molecular weight, linear polyamide resins satisfactory for the 
purposes of the invention include such high molecular weight resins as 
derived from the foregoing monomers and mixtures. 
In U.S. Pat. No. 2,241,322 a cyclic amide such as .epsilon.-caprolactam and 
water (the water content in the mixture varies from 1.6% to 61.5%) are 
heated under high pressure to a temperature between 180.degree. to 
300.degree. C. to effect a partial polymerization of the lactam. The 
pressure is then reduced to atmospheric pressure and the water and 
unchanged monomer gradually distilled from the mass and polymerization 
allowed to proceed to form a superpolyamide or fiber-forming resin. Where 
the initial polymerization step is arrested and the mass is allowed to 
cool, the product will vary from a wax-like solid where the water content 
in the mixture is about 1.6% to a cheese-like mass where the water is at 
an upper limit of about 61.5%. The polymer which is formed crystallizes in 
a dendritic-like structure and forms large aggregates of the dendritic 
structures. These aggregates as formed are too coarse to permit direct 
dispersion to form stable and useful dispersions. The cheese-like masses 
that are formed from charges wherein the percentage of water is in the 
upper portion of the above stated range even when subjected to at least 3 
passes through a roll mill contain particles over a large size range up to 
particles as large as 1 mm. 
In accordance with the present invention, a mixture of the polymer forming 
monomer or monomers, or a high molecular weight polyamide and water is 
introduced into a pressure vessel or an autoclave that is provided with 
means for agitating the mass during the heating period. The mixture or 
charge contains from about 30% to about 80%, preferably between 50% and 
65%, by weight of the monomer or polyamide with the balance water. If 
desired, the charge may also include an acidic or alkaline substance to 
serve to regulate the pH of the final quenched mass. The pressure vessel 
is also provided with a dip tube extending to a position just above the 
bottom of the vessel to serve as a means for discharging the mass at the 
termination of the heating period. The dip tube is provided with a valve 
externally of the vessel and the external end of the dip tube extends into 
a quench tank. After sealing the pressure vessel, the mass is heated to 
about 230.degree. C. to 235.degree. C. or other applicable elevated 
temperature and maintained at such temperature for from 4 to 24 hours, 6 
to 10 hours being generally sufficient. Under these conditions the 
pressure will rise to about 400 to 500 psi. 
The higher the proportion of monomer or high polymer in the charge, the 
higher the number average molecular weight of the polymer which is being 
formed and the lower the proportion of monomer and oligomers; that is, 
polymer having a D. P. (degree of polymerization) not exceeding about 4. 
Roughly, using .epsilon.-caprolactam as the monomer, the number average 
molecular weight of the polymer formed and under the above stated 
conditions will vary between about 1300 and about 7,300, corresponding to 
a D. P. of between about 12 and about 65. The reduced viscosity as 
measured at 20.degree. C. of a m-cresol solution containing 1 gm. of 
polymer per deciliter of solution will vary from about 0.1 to about 0.4. 
In order to form the unique dispersible particles of the present invention, 
it is essential and critical that following the heating step the entire 
mass be quenched in an aqueous medium as rapidly as possible to reduce the 
temperature of the mass below the freezing point of the formed polyamide, 
to continue the cooling of the mass so as to prevent the growth of the 
particles and prevent an alteration of the structure of the particles and 
to bring the entire mass to a uniform predetermined pH value. In 
batch-wise procedure, quenching and cooling may be effected in a suitable 
tank open to the atmosphere and provided with agitating means, such as a 
Lightnin Mixer having a baffle plate, wherein the quench bath and 
discharging mass is subjected to good agitation so as to reduce the 
temperature of the discharging mass below about 90.degree. C. to 
95.degree. C. as rapidly as feasible and to bring the entire mass to the 
predetermined pH. 
In batch-wise procedure, the valve on the dip tube is opened at the 
termination of the heating period thereby allowing the autogenous pressure 
in the autoclave to force the reaction mass through the tube into the 
quench bath. The quench bath conveniently may consist of water and crushed 
ice. Obviously, the quench bath may consist of water cooled by means of 
suitable cooling coils. The weight ratio of the quench bath to the 
reaction mass may vary from about 2:1 to about 6:1, generally being about 
4:1. In those instances where the initial charge does not include a 
substance to provide the predetermined desired pH of the final quenched 
mass, the pH controlling substance is incorporated in the quench bath. As 
the reaction mass issues from the dip tube into the agitated quench bath, 
the reaction mass is instantaneously brought to a temperature below the 
freezing point of the polymer, is cooled to a temperature sufficiently low 
so as to prevent particle growth and structural alteration of the 
particles and brought to the predetermined pH thereby forming uniformly 
sized particles of loosely packed, randomly oriented, bonded lamellar 
sheets. In a typical example using .epsilon.-caprolactam and water as the 
reaction mass, such as will be described in detail in Examples 1 to 9 set 
forth hereinafter, the total time to discharge 6 gallons of reaction mass 
and cool it to a temperature below 90.degree. C. was approximately 90 
seconds, although, as stated, each increment of the discharging mass as it 
leaves the dip tube becomes cooled instantaneously. 
Utilizing the foregoing conditions and the conditions as set forth in the 
examples which are included hereinafter; that is, the composition of the 
initial charge and the reaction conditions of time, temperature, and 
pressure, the reaction mass attains approximate equilibrium composition. 
Obviously, the method will be practiced in a manner and under conditions 
so as to produce a product having a predetermined desired molecular 
weight. Hence, as will be recognized, a higher proportion of monomer may 
be included in the initial charge so that the desired molecular weight may 
be attained in a shorter time period by arresting the reaction or 
polymerization prior to reaching an equilibrium composition. In such 
practice, the quenched mass will contain higher proportions of monomer and 
oligomers than under equilibrium conditions and, advantageously, the 
quenched mass may be processed as by centrifugation or electrodeposition 
so as to separate the solids as a wet cake and recover a liquid phase 
containing some of the monomer and dissolved oligomers which may be used 
in the preparation of subsequent charges. 
The resulting product is a dispersion and the consistency will vary 
depending upon the solids content and the particle size of the polymer 
particles will be dependent upon the pH of the original quenched mass. 
Where the particles of this invention do not exceed about 2 microns and 
the solids content is in the useful range, the particles will remain in 
suspension or in a dispersed state for an indefinite period of time. Where 
the particles exceed about 2 microns and the pH of the dispersion is above 
about 6.5 some settling of the particles may take place with time. 
However, the particles do not pack into a dense mass even over extended 
periods and they may be redispersed by merely shaking the container or 
stirring the mass. The product may be used directly as a coating 
composition. In contrast to the prior art dispersions of superpolyamides 
which do not dry to form self adherent coating on metals, for example, the 
dispersions of the present invention when applied to a substrate such as a 
metal or glass and dried form self-adherent coatings on the substrate. 
As stated hereinabove, a common proposal to form dispersions of high 
molecular weight polyamides is to dissolve the resin in a solvent and 
precipitate the resin in a nonsolvent as described in U.S. Pat. No. 
2,265,127. The resulting particles are usually of an irregular size and 
fibrillar in structure. In the partial degradation of high molecular 
weight polyamides followed by mechanical disintegration, the particles are 
colloform or globular, granular in shape or structure and random in size. 
If the initial polymerization step using the higher proportions of water 
as described in U.S. Pat. No. 2,241,322 is arrested and the mass is 
allowed to cool, the low molecular weight polyamide resin particles are 
randomly sized and consist of densely packed, oriented lamellar sheets. 
In contrast to these prior products, for example, the low molecular weight 
polycaprolactam resin particles of this invention, wherein the reaction 
mass is quenched rapidly and the pH of the quenched mass is controlled, 
the particles are uniform in size and consist of loosely packed, randomly 
oriented, bonded lamellar sheets or lamella or clusters of flaky sheets. 
When examined by the electron microscope, these particles appear much like 
a mass of wet cornflakes where the flaky sheets drape over each other into 
loose clusters. It is believed that this flake-like structure aids in a 
spreading of the flakes over the surface of the substrate in much the same 
manner as the so-called "metal" paints, that is, paints or coatings 
containing metal flakes such as aluminum, bronze, etc. 
The dispersion as resulting from the quenching of the reaction mass may be 
used directly as a coating composition providing the quenched mass 
possesses the desired solids content. If desired, the solids may be 
separated from the quenched mass in the form of a wet cake as by 
centrifugation or electrodeposition so as to reduce the monomer and 
oligomers content and then redispersed in water to form a desired 
concentration of dispersed solids. Alternatively the wet cake may be 
washed with water so as to remove additional monomer and oligomers which 
may be present in the wet cake prior to redispersing the solids in water. 
The proportion of monomer and oligomers present in the quenched mass will 
vary inversely with the proportion of monomer (assuming equilibrium 
conditions are used), or of high molecular weight polyamide in the initial 
charge. Inasmuch as the monomer becomes volatilized during subsequent heat 
treatment of the coatings, the monomer content is preferably reduced by 
one or more of these processing steps particularly when non-equilibrium 
conditions are used in forming the polyamide. In reference to the 
production of the polyamide from .epsilon.-caprolactam as outlined above, 
the proportion of monomer and oligomers will vary from about 42% to about 
14%, based on the solids content of the quenched mass. 
In the production of a dried product, the recovered quenched mass is 
preferably spray dried and the monomer content will be lowered due to the 
temperatures employed. If desired, both washing and spray drying may be 
utilized to effect a reduction in the monomer and oligomers content. An 
outstanding characteristic of the spray dried products is the ease with 
which the product may be redispersed in water. A dispersion which remains 
stable for extended periods of time may be formed by adding the spray 
dried powder to water and shaking the mass or by subjecting the mass to 
agitation. 
Since in most instances the completed coating desired is a high molecular 
weight polyamide, a polymerization catalyst is preferably included in the 
dispersion. Compounds which are or which upon heating are converted to 
non-volatile, strong acids may be used as catalysts and include such 
compounds as orthophosphoric acid, monoammonium orthophosphate, diammonium 
orthophosphate, orthophosphorous acid, metaphosphoric acid, 
p-toluenesulfonic acid and the ammonium salt of benzyl phosphite. In view 
of the range of pH of solutions of these catalysts, they may be 
advantageously incorporated in the initial charge or in the quenching 
medium to control the pH of the final quenched mass. In general, the 
proportion of catalyst desired in the final coating composition is between 
about 0.15% and about 0.8%, preferably 0.3% to 0.6%, based on the weight 
of the polymer. Where lesser amounts are used in the initial charge or 
quenching medium so as to obtain a predetermined desired pH, an additional 
amount of catalyst may be mixed into the dispersion prior to its 
application to a substrate. 
The aqueous dispersion, recovered quenched mass or reconstituted 
dispersion, is applied to a desired substrate by any conventional method, 
such as brushing, dipping, spraying, electrostatic spraying, etc. In the 
subsequent heating step, water is evaporated and heating is continued so 
as to melt the polyamide flakes, allow the liquid phase to flow and permit 
molecular polymerization, thereby forming the adherent continuous coating. 
In the instance where the polymer has been prepared from 
.epsilon.-caprolactam, and the dispersion contains a catalyst, after 
application of the coating to the substrate and the coating is subjected 
to the required heat treatment, it has been found that there occurs about 
a 15-fold to about a 45-fold increase in the weight average molecular 
weight and the film properties are those of the usual high molecular 
weight polycaprolactam or nylon-6 film. As an alternative, the dried 
powder product containing a catalyst may be applied to the substrate by 
any conventional powder coating technique, such as, for example, fluidized 
bed coating, flame spraying, electrostatic spraying, etc. As a further 
alternative, the resin particles may be deposited from an aqueous 
dispersion on conducting substrates by electrophoretic techniques. The 
specific temperature utilized in the heat treatment will be dependent upon 
the specific low molecular weight resin, generally being at the melting 
point of the resin. The period of heat treatment will vary directly with 
the thickness of the coating. Again referring to a low molecular weight 
polycaprolactam as produced as described herein, the coating will be 
heated to a temperature of about 235.degree. C. for a short period, such 
as 10 to 15 minutes where the coating thickness is between about 2 and 5 
mils. 
The examples which follow illustrate the practice of the present invention 
but are not to be considered as limitations. Where reference is made to 
percentages of various substances, the percentages are by weight unless 
stated otherwise. In the case of references to percentages of additive or 
catalyst, the percentage is based upon the weight of the resin forming 
monomer or constituents. Stated pressures are pounds per square inch (psi) 
gauge. Reduced viscosities were determined at 20.degree. C. on m-cresol 
solutions of the resins containing 1 gm. of resin per deciliter of 
solution.

EXAMPLES 1-9 
In the preparation of the resins of these examples, a 10 gal., electrically 
heated, stainless steel autoclave was utilized. The autoclave was fitted 
with a stirrer and a dip tube having a valve externally of the autoclave. 
The end of the dip tube internally of the autoclave extended to a position 
just above the bottom of the vessel. The external end of the dip tube was 
positioned near the bottom of a 30 gal. stainless steel tank which was 
provided with a Lightnin Mixer. In each example, about 6 gals. of a 
solution of .epsilon.-caprolactam in distilled water, after filtration to 
remove any possible foreign matter, with or without phosphoric acid, were 
charged into the autoclave and the autoclave sealed. The stirrer was 
operated at about 270 RPM. Heat was applied for the stated period of time. 
In general, 21/2 to 3 hours were required to bring the reaction mass to 
about 235.degree. C. and the mass was maintained at this temperature to 
the end of the stated periods of time. The autogenous pressure reached 
about 460 to 500 psi. At the termination of the heating period, heating 
was discontinued and the dip tube valve opened whereby the autogenous 
pressure in the autoclave forced the mass through the dip tube into the 
quench tank. The quench tank contained a mixture of about 160 lbs. of 
water and about 40 lbs. of crushed ice. Generally, the mass was discharged 
in about 90 seconds and the temperature of the entire mass reduced to 
about 50.degree. to 60.degree. C. within this period of time. 
Specifically, in Example 6, 33.38 lbs. of .epsilon.-caprolactam was 
dissolved in 22.26 lbs. of distilled water. About 3 hours were required to 
heat the mass to 235.degree. C. and the pressure reached about 470 psi. 
The total heating period was 8 hours. Heating was then discontinued and 
the mass discharged in 90 seconds into a bath (temperature about 0.degree. 
C.) consisting of 167 lbs. of water and 39 lbs. of crushed ice and at the 
end of the discharge period the temperature of the entire mass was about 
51.degree. C. 
In this group of examples, the proportions of monomer, 
.epsilon.-caprolactam, and water varied from 30% monomer and 70% water to 
80% monomer and 20% water. Samples of the dispersions (adjusted to about 
14% to 15% solids) were poured into soft, thin gauge aluminum pans to 
provide coatings of about 5 mils in thickness. In those examples where no 
phosphoric acid catalyst had been incorporated in the initial charge, 0.3% 
phosphoric acid (about 0.35% of 85% phosphoric acid) was added to and 
mixed into the dispersion before pouring a sample into the aluminum pans. 
The coatings were then heated to 230.degree. to 240.degree. C. for about 
15 minutes. Subsequently the coatings were stripped from the aluminum and 
the reduced viscosities of the film determined. 
Samples of the dispersions were also tested in a thermobalance (Perkin 
Elmer TGS-1). In this test a sample of known weight was first dried by 
passing dry helium at room temperature over the sample until no change in 
weight was noted. The temperature of the sample was then increased at a 
rate of 10.degree. C. per minute to 80.degree. C. while continuing the 
flow of helium over the sample. The weight of the sample was noted and the 
loss in weight from the original sample was considered as the water 
content of the dispersion. The heating rate was then continued until the 
sample had been heated to 210.degree. C. and the weight of the sample 
noted. The loss in weight of the sample between 80.degree. C. and 
210.degree. C. was considered an approximation of the monomer and oligomer 
content of the low molecular weight polymer produced. The properties of 
the products of this group of examples are tabulated in Table I. The 
molecular weight of the resins and the film were determined by a 
combination of the gel permeation chromatography procedure and solution 
viscosity measurements. Samples of the dispersions were air dried and the 
melting points of the recovered resins were determined by the use of the 
Fisher-Johns Melting Point Apparatus. 
TABLE I 
__________________________________________________________________________ 
Reaction 
Reduced Resin 
Reduced*** 
Additive 
time viscosity 
M-- .sub.w 
M-- .sub.n 
MP viscosity 
M-- .sub.w 
Wt. loss 
Example 
% CL 
% H.sub.2 O 
%* hours 
resin 
(.times.10.sup.3) 
(.times.10.sup.3) 
.degree. C. 
film (.times.10.sup.3) 
% 
__________________________________________________________________________ 
1 30 70 -- 24 0.115 
2.5 1.3 115-120 0.874 
&lt;60 41.5 
2 40 60 0.3 24 0.128 
2.8 1.5 128-132 1.16 
63.1 31.9 
3 50 50 0.3 8 0.160 
3.0 1.6 150-153 1.20 
64.6 24.2 
4 55 45 0.3 10 0.177 
4.5 2.4 153-155 1.32 
74.1 22.2 
5 60 40 -- 8 0.200 
5.2 2.7 ** 1.32 
74.1 22.0 
6 60 40 -- 10 0.203 
5.3 2.8 155-158 1.46 
87.0 20.9 
7 60 40 -- 24 0.199 
** ** ** ** 
** ** 
8 70 30 0.3 10 0.231 
6.5 3.4 193-196 3.53 
303 17.1 
9 80 20 0.3 10 0.397 
13.8 
7.3 197-200 5.04 
&lt;500 13.6 
__________________________________________________________________________ 
CL - .epsilon.-caprolactam 
*"Dash" indicates no additive used 
**Not measured 
**After addition of 0.3% H.sub.3 PO.sub.4 to Examples 1, 5 and 6. 
EXAMPLES 10,11,12 
In this group of examples, the equipment and general procedure as described 
above were followed utilizing .epsilon.-caprolactam as the monomer and 
water, however, no catalyst was added to either the initial charge or the 
quench bath. The quantity of the quench bath was adjusted so as to provide 
a final quenched mass containing approximately 14% solids. These examples 
illustrate the reduction of monomer and oligomer content of the product by 
filtration, as by centrifugation, by washing with water and by spray 
drying. In each instance, the monomer and oligomer content of the quenched 
mass or product was determined as described above. The quenched mass was 
then subjected to centrifugation in a Komline-Sanderson general purpose 
centrifuge, Model CL-10, fitted with a solid bowl attachment. The 
centrifuge was operated at 1800 RPM to provide a filter cake of about 28% 
solids. The monomer and oligomer content of the centrifuged mass was 
determined. The filter cakes were then washed by redispersing the solids 
in water to provide dispersions containing about 14% solids and the 
dispersions again subjected to centrifugation and the monomer and oligomer 
content measured. The filter cakes were again redispersed in water to form 
dispersions containing about 14% solids and 85% phosphoric acid added in 
an amount sufficient to provide 0.3% phosphoric acid based on the weight 
of the solids. The dispersions were subsequently spray dried by spraying 
the dispersions at room temperature into a spray drying chamber, the 
introduced air having a temperature of about 350.degree. F. (177.degree. 
C.) and the air leaving the chamber having a temperature of about 
210.degree. F. (99.degree. C.). The spray dried powders contained 
approximately 0.3% phosphoric acid thus exhibiting no loss of phosphoric 
acid in the spray drying. Samples of the quenched masses, the washed 
products and the spray dried products were tested in the thermobalance so 
as to determine the monomer and oligomer content of the products at the 
various stages. The properties were as reported in the following table: 
TABLE II 
__________________________________________________________________________ 
Oligomer & Monomer Content 
Reaction 
Quenched mass % based on solids 
period Final Particle Centrif. 
Washed 
CL/H.sub.2 O 
Hours Temp. size Quenched 
Quenched 
& Spray 
Example 
% 235.degree. C. 
% Solids 
.degree. C. 
pH microns 
Mass Mass Centrif. 
Dried 
__________________________________________________________________________ 
10 50/50 
10 14.1 53 8.43 
5-6 29.3 9.8 7.7 4.4 
11 60/40 
10 14.1 51 8.22 
5-6 21.2 7.4 5.3 3.2 
12 70/30 
5 14.1 49 8.07 
5-7 19.6 4.8 2.4 1.7 
__________________________________________________________________________ 
CL - .epsilon.-caprolactam 
EXAMPLES 13-35 
This group of examples illustrates the effect of the pH of the quenched 
mass upon the size and the uniformity of size of the produced polyamide 
resin particles. The monomer used was .epsilon.-caprolactam. The equipment 
and preparatory method used were as described in Examples 1-9. The 
quantities of the various substances were selected so as to provide 
quenched masses whose pH values varied over a pH range of between about pH 
3 and about pH 9. The heating periods were varied and the maximum 
temperature was about 235.degree. C., except where noted. The particle 
sizes of the products were determined by microscopic examination. The 
reduced viscosities were determined as described above. In those instances 
where no catalyst was present in the product as produced, 0.3% phosphoric 
acid (about 0.35% of 85% phosphoric acid) was added to a sample of the 
product before pouring the sample of the dispersion into an aluminum pan 
for the preparation of fused films. The properties of the products are 
tabulated in Table III which follows: 
TABLE III 
__________________________________________________________________________ 
Additive Quenched Mass 
Particle 
% & Reaction Final Size 
CL/H.sub.2 O 
Location 
time Temp. (mean) 
Reduced Viscosity* 
Example 
% ** Hours 
% Solids 
.degree. C. 
pH microns 
Resin Film 
__________________________________________________________________________ 
13 60/40 
-- 8 12.8 54 8.58 
5 0.2295 
-- 
14 60/40 
-- 8 12.4 53 8.52 
5 0.2050 
-- 
15 60/40 
-- 8 14.2 37 8.48 
4 0.2004 
1.3240 
16 60/40 
-- 8 15.2 53 8.46 
6 0.2390 
-- 
17 50/50 
-- 10 14.1 53 8.43 
5 0.1876 
-- 
18 65/35 
-- 8-220.degree. 
13.2 40 8.32 
3 0.2126 
-- 
19 65/35 
-- 8 13.3 40 8.32 
6 0.2557 
-- 
20 60/40 
-- 10 14.1 51 8.22 
6 0.2193 
-- 
21 60/40 
0.43 (A)C 
24 27.0 85 7.98 
6 -- -- 
22 50/50 
0.3 (B)Q 
10-265.degree. 
11.0 54 7.12 
2 0.1632 
-- 
23 60/40 
0.3 (B)Q 
10 12.2 34 6.73 
1 0.1862 
1.5403 
24 40/60 
0.3 (B)C 
24 12.4 72 6.70 
2 0.1281 
1.1593 
25 50/50 
0.3 (B)C 
10 14.1 55 6.63 
1 0.1596 
1.2266 
26 70/30 
0.3 (B)Q 
10 14.0 36 6.61 
2 0.2308 
3.5315 
27 50/50 
0.3 (B)C 
10 18.5 76 6.57 
1 0.1381 
1.8160 
28 50/50 
0.3 (B)C 
8 12.7 53 6.53 
1 0.1604 
2.7475 
29 60/40 
0.55 (B)Q 
24 23.8 87 6.32 
1 0.2305 
-- 
30 60/40 
0.58 (B)Q 
24 23.7 80 6.30 
0.5 0.2256 
1.8725 
31 60/40 
7.7 (B)C 
18 13.0 45 4.52 
1 0.1835 
0.6062 
32 60/40 
7.7 (B)Q 
18 13.0 45 3.58 
1 0.2016 
-- 
33 70/30 
1 NH.sub.4 St.C 
5 13.4 45 8.88 
&lt;1 0.2528 
-- 
34 60/40 
1 NH.sub.4 St.C 
24 12.7 43 8.48 
&lt;0.1 0.2171 
1.0812 
35 50/50 
(1 NH.sub.4 St.C 
10 3.7 43 3.05 
&lt;0.1 0.1576 
-- 
(10 (B)Q 
__________________________________________________________________________ 
CL -- .epsilon.-caprolactam 
A -- Diammonium orthophosphate 
B -- Orthophosphoric acid 
C -- Charge 
Q -- Quench bath 
NH.sub.4 St. -- Ammonium stearate 
*"Dash" indicates value not measured 
*"Dash" indicates no additive used 
From the foregoing data it will be noted that, in the absence of a soap in 
the initial charge, the mean size of the resin particles are within the 
range of between about 2 microns and 6 microns in those instances where 
the pH value of the quenched mass is within the range of about pH 7 and 
about pH 9. Where the pH value of the quenched mass is within the range of 
about pH 3.5 and about pH 7, the mean size of the resin particles are 
within the range of between about 0.5 micron and about 2 microns. In the 
presence of a soap in the initial charge the mean size of the particles 
will be under 1 micron where the pH of the quenched mass is within the 
range of about pH 3 and about pH 9. In Example 35 where ammonium stearate 
was included in the initial charge and sufficient orthophosphoric acid was 
present in the quench bath to form a quenched mass of pH 3.05 the 
particles were of a size of approximately 0.03 .times. 0.03 .times. 0.005 
micron. Thus, conditions of preparation may be controlled so as to provide 
particles of a size adapted for specific uses dictated by the desired 
thickness of a particular coating. 
It will be noted that in Example 31, although the resin particles as formed 
possessed a low molecular weight as measured by the reduced viscosity, the 
fused film did not exhibit a typical increase in molecular weight. Failure 
to attain the high molecular weight was due to the presence of an amount 
of orthophosphoric acid (7.7%) beyond the useful range for catalyzing a 
low molecular weight polycaprolactam to the desired high molecular weight. 
The application of the present invention to the preparation of other 
polyamides from other monomers and the preparation of finely-divided 
particles of copolymers is illustrated by the examples which follow: 
EXAMPLE 36 and 36A 
Hexamethylenediammonium adipate (nylon 6,6 monomer salt) was prepared as 
described in Preparative Methods of Polymer Chemistry, 2nd edition, by 
Wayne R. Sorenson and Tod W. Campbell, page 74. The preparation of low 
molecular weight resin particles followed the general procedure as 
described above. In this example, a 2 liter stainless steel laboratory 
autoclave was fitted with a stirrer and dip tube in a manner as described 
above. In each instance, a 1080 gram mixture of the monomer salt and water 
was used. In the first case the mixture contained 60% of the monomer salt, 
while in the second case the mixture contained 80% of the monomer salt. In 
each case, after introducing the mixture into the autoclave, the autoclave 
was sealed and heat was applied for 24 hours. The maximum temperature was 
280.degree. C. In the first case, the reaction mass was discharged into an 
agitated quench bath consisting of 1620 grams of water and 1620 grams of 
crushed ice while in the second case, 2340 grams of water and 2340 grams 
of crushed ice were used. Both quenched masses were creamy dispersions, 
the first containing 11.6% solids and having a pH of 9.4, while the second 
contained 12.1% solids and had a pH of 9.3. The particles of the first 
product had a bimodal distribution of sizes with two peaks; that is, one 
group of particles having a mean size of 2 microns with the other group 
having sizes between 10 and 60 microns. The mean size of the particles of 
the second product was about 15 microns. 
EXAMPLES 37-40 
In this group of examples, copolymers were prepared from mixtures 
consisting of 85% .epsilon.-caprolactam (nylon 6 monomer) with 15% 
hexamethylenediammonium adipate (nylon 6,6 salt), or 15% amino-undecanoic 
acid (nylon 11 monomer) or 15% .omega.-lauryl lactam (nylon 12 monomer). 
The general preparatory procedures were as described hereinbefore. In 
Examples 37,38 and 39, the products were prepared utilizing the 2 liter 
stainless steel autoclave, while in Example 40, the 10 gal. autoclave was 
utilized. After introducing a mixture of monomers and water into the 
autoclave and sealing the autoclave, heat was applied for the stated 
period. In each example, the maximum temperature was about 235.degree. C. 
After the heating period, the reaction mass was discharged into an 
agitated quench bath consisting of water and crushed ice. The properties 
of the products were as tabulated in Table IV which follows: 
TABLE IV 
__________________________________________________________________________ 
H.sub.3 PO.sub.4 Particle 
Resin 
M/H.sub.2 O Quench 
Time 
Quenched Mass 
Microns MP 
Example 
% CL M2 H.sub.2 O 
Bath Hours 
% Solids 
pH Mean Max .degree. 
__________________________________________________________________________ 
C 
37 50/50 
342 g 198 g 540 g none 24 21.7 8.58 
1 2 95-100 
(N66) 
38 60/40 
551 g 97 g 432 g 0.6% 18 10 5.68 
0.5 1 115-120 
(N11) 
39 50/50 
459 g 81 g 540 g 0.3% 10 12 6.48 
1 1 105-110 
(N12) 
40 50/50 
23.65 lbs 
4.17 lbs 
27.82 lbs 
0.3% 10 8.2 6.62 
1 1 105-110 
(N12) 
__________________________________________________________________________ 
M - Monomer mixture 
CL - .epsilon.-caprolactam 
M2 - Comonomer 
N66 - Nylon-6,6 salt 
N11 - nylon-11 monomer 
N12 - Nylon-12 monomer 
Finely-divided, water dispersible, low molecular weight polyamide particles 
as described herein may be produced from high molecular weight (fiber- and 
film-forming) polyamides, such as scrap or waste fibers, film scrap, etc., 
as illustrated by the following example: 
EXAMPLE 41 
The high molecular weight polyamide utilized was a commercial 
polycaprolactam product marketed as Plaskon 8200 Nylon 6 Molding Pellets, 
Natural Grade (Allied Chemical Co.). The pellets, as received, were ground 
in a Wiley Mill to pass a 20 mesh screen (841 microns openings). A charge 
consisting of 500 gms. of the ground polycaprolactam, 500 gms. of water 
and 5 gms. of ammonium stearate was introduced into the 2 liter autoclave. 
After sealing, heat was applied for 35 minutes, the maximum temperature 
reaching 235.degree. C. The heated mass was discharged into an agitated 
quench bath consisting of 1400 gms. of water, 1400 gms. of crushed ice and 
2.75 gms. of 85% H.sub.3 PO.sub.4. The quenched mass had a temperature of 
50.degree. C., a pH of 6.72 and contained 13% solids. The particles had a 
size of 1 to 3 microns. The reduced viscosity of the original 
polycaprolactam was 2.6670. The product had a reduced viscosity of 0.2533. 
A sample of the quenched mass after conversion to a fused film in a manner 
as described above exhibited a reduced viscosity of 1.5280. 
Aqueous dispersions of the finely-divided, water dispersible, low molecular 
weight, linear polyamide particles, either as the formed quench mass, or 
after concentration to a desired solids content, or after washing to 
reduce the monomer and oligomer content and redispersed to a desired 
concentration, or after redispersing a spray dried product, may be applied 
to a desired substrate. Obviously, in those applications to substrates, 
such as, for example, metallic substrates, glass structures, such as, 
glass fibers, etc., where it is desired to convert the coating to a high 
molecular weight polyamide, the dispersion should contain a polymerization 
catalyst as set forth hereinbefore. Where the dispersion is intended for 
uses not requiring a high molecular weight polyamide as desired in the 
coating art, a polymerization catalyst may be omitted. For example, where 
the low molecular weight polyamide is intended as a cross-linking or 
curing agent for water-dispersible epoxy resin compositions, the polyamide 
catalyst may be omitted. 
It is obvious that the dispersions may contain a variety of additives, such 
as, for example, coloring materials (dyes, pigments, etc.), thermal 
stabilizers, antioxidants, ultra-violet light stabilizers and biocides. As 
is obvious, certain of the additives which are non-reactive with the 
polyamide or monomer and are not affected by the temperatures involved in 
the heating step may be incorporated in the initial charge. Where the 
additive is heat sensitive, it may be added to the quench bath or to the 
dispersion prior to its application to the substrate. 
The application of the coatings to metallic substrates may be illustrated 
by the examples which follow. 
EXAMPLE 42 
A portion of the quenched mass of Example 15 was centrifuged and the filter 
cake dispersed in water to form a dispersion containing approximately 25% 
solids. Sufficient orthophosphoric acid was mixed with the dispersion to 
provide 0.3% H.sub.3 PO.sub.4, based on the solids content. The metallic 
substrates were 3 .times. 6 inches, 24 gauge, cold rolled steel panels 
with Bonderite 37 treatment. The panels were provided with a 10 mil wet 
coating of the dispersion by means of a doctor blade. The panels were 
placed in an oven through which nitrogen was circulated and heated to 
230.degree. to 240.degree. C. for 15 minutes. The finished coatings were 
approximately 2 mils in thickness. 
Coated panels were subjected to an impact test both directly (on coating) 
and indirectly (on reverse side of panel) in a Gardner Heavy Duty Impact 
Tester, Model lG-1120, having 4 pound weight terminating in a 5/8 inch 
ball head, the weight being dropped from a 40 inch height. The coatings 
withstood the maximum impact; namely, a 160 inch-pound impact, both direct 
and indirect without separation from the panels nor was there any evidence 
of a cracking or crazing of the coatings. 
Coated panels were also subjected to a flexibility test by the use of a 
Gardner Mandrel Set, Model MG 1410, commonly employed in the testing of 
paint coatings. In this test, the coated panel is bent around a 1/8 inch 
diameter rod, the uncoated side being in contact with the rod. In this 
test, the coatings exhibited no separation from the panels and no cracking 
or crazing of the coatings. 
In a third test, known as the "3M Scotch Tape Cross-Hatch Adhesion Test", a 
grid is scored through the coating to the metal with a knife edge, the 
grid consisting of 11 .times. 11 lines, the lines being spaced 1/16 inch. 
3M Scotch Tape, approximately 3/4 inch in width, is then applied over the 
area of the grid and rubbed so as to effect good adhesion over the area of 
the grid, leaving an unadhered portion of the tape beyond the area of the 
grid. The unadhered tab is then grasped and the tape is pulled off 
rapidly. The results of this test are expressed in the number of the 1/16 
.times. 1/16 inch areas of the coating which are removed. In such tests of 
the coated panels, no areas of the coating were removed. 
For the production of finish decorative coatings where a plane surface of 
high smoothness and a high degree of uniformity in thickness is desired, 
that is, a surface free of minute depressions or surface craters and free 
of an orange-peel effect and the like, a flow promoter may be included in 
the dispersion. The flow promoter may be included in the quench bath, or 
may be added to the dispersion before spray drying, or may be added to a 
dispersion prior to its application to the substrate. The amount of flow 
promoter may vary from about 1% to about 8%, preferably 3% to 5%, by 
weight, based upon the solids content of the dispersion. Flow promoters 
satisfactory include n-butylurea, nylon 6,6 salt, nylon 6,9 salt, nylon 
6,10 salt, intermediate molecular weight, water soluble 
polyethyleneimines, such as the commercial product marketed as NC-1612 by 
Dow Chemical Co., and water emulsifiable epoxy resins, such as the 
commercial modified bisphenol A epichlorohydrin-based epoxy resin marketed 
as Genepoxy M205 by General Mills Chemicals, Inc. 
In the electrophoretic coating of conducting surfaces, the polyamide 
particles may be provided with either a positive charge whereby the 
particles will be deposited on a cathode, or with a negative charge 
whereby the particles will be deposited on an anode. Thus where the 
dispersion has an acidic pH, as resulting from the presence of phosphoric 
acid catalyst, for example, the particles will be deposited on the 
cathode, whereas, if the dispersion has a basic pH, as resulting from the 
presence of diammonium phosphate catalyst, the particles will be deposited 
on the anode. The electrophoretic coating may be illustrated by the 
following example: 
EXAMPLE 43 
A portion of the spray dried product of Example 14 was dispersed in water 
by means of a Waring Blendor to form a 5% solids dispersion. Sufficient 
orthophosphoric acid was added to provide 0.6% H.sub.3 PO.sub.4 based on 
the solids content. A sufficient amount of a 50% water emulsion of 
GenEpoxy M205 was added to provide 2% of the flow promoter based on the 
solids content. The pH of the final dispersion was 6.7. The dispersion was 
then transferred to a stainless steel tank which was subsequently made the 
anode. The cathode consisted of aluminum alloy (Gardner PG 1304A) panels, 
3 .times. 6 inches .times. 20 mils. The panels were immersed in the 
dispersion and 50 V.D.C. applied for approximately 10 seconds. The panels 
were subsequently placed in an oven through which nitrogen was circulated 
and heated to 230.degree. to 240.degree. C. for 10 minutes. The coating 
thus formed had a thickness of approximately 1 mil. The continuity of the 
coatings was tested by the use of a 2.2% hydrochloric acid solution 
containing about 1% copper sulfate, a copper deposit being indicative of a 
"pin hole" in the coating. Areas of the coating were covered with drops of 
the acidic solution and observations were made by the use of a microscope 
covering a period of 10 minutes. No copper deposits were observed, thus 
indicating the coatings to be free of "pin holes." 
The electrostatic powder spraying of the low molecular weight polyamide 
produce is illustrated by the following example: 
EXAMPLE 44 
Example 16 was repeated to provide a dispersion of the low molecular weight 
polycaprolactam particles. Sufficient 85% orthophosphoric acid was added 
to the quenched mass so as to provide 0.6% H.sub.3 PO.sub.4, by weight, 
based on the weight of the polycaprolactam. A sufficient amount of a 50% 
water emulsion of GenEpoxy M205 was added to provide approximately 3%, by 
weight, based on the polycaprolactam, of the flow promoter. The mixture 
was blended for about 15 minutes by use of a Lightnin Mixer. The 
resulting aqueous dispersion was spray dried by spraying the dispersion at 
room temperature into a spray drying chamber, the introduced air having a 
temperature of about 350.degree. F. (177.degree. C.) and the air leaving 
the chamber having a temperature of about 210.degree. F. (99.degree. C.). 
The spray dried product recovered was free flowing and had a mean particle 
size of approximately 20 microns. 
The metallic substrates were as described in Example 42. Conventional 
electrostatic powder spraying apparatus was used wherein the steel panels 
were grounded. The spray dried powder was blown through the spray gun 
where the particles were given in a high-voltage low-amperage negative 
charge as they left the spray gun and thus were attracted to and deposited 
on the grounded panels. The coated panels were subsequently heated in an 
air oven to about 205.degree. C. for 10 minutes. The resulting coating was 
approximately 2 mils in thickness. The coatings were subjected to the "3M 
Scotch Tape Cross-Hatch Adhesion Test" as described in Example 42. In such 
tests, no areas of the coating were removed. 
In the foregoing example reference is made to the use of a spray dried, 
free flowing powder having a mean particle size of about 20 microns. The 
unique characteristic of the spray dried products is that when such a 
product is added to an aqueous medium and the mixture is subjected to 
agitation as by use of a Lightnin Mixer or Cowles Dissolver, the product 
reverts to the approximate particle size of the material before spray 
drying and the resulting dispersion is almost indistinguishable from the 
dispersion from which the dry powder was derived. 
Glass fibers and filaments have a harsh hand or feel and poor resistance to 
abrasion when rubbed together and necessitate specialized handling to 
convert them into textile products. Because of the non-hydrophilic nature 
of glass, the conventional yarn finishes have not been satisfactory. The 
coatings formed from dispersions of the low molecular weight polyamides of 
the present invention overcome these inherent disadvantageous 
characteristics. In the production of glass filaments, they may be passed 
over or between rolls so as to apply a dispersion by roller coating. The 
thickness of the coating may be controlled by the solids content of the 
dispersion and by the particle size of the dispersed polyamide particles. 
The filaments are then passed through a suitable heating zone so as to 
fuse the coating. The coating is flexible, tough and is resistant to 
abrasion and permits the use of yarn finishes as conventionally utilized 
in the nylon textile industry. 
Furthermore, as indicated hereinbefore, the polyamide dispersion may 
contain coloring materials such as pigments or dyes so as to provide glass 
based filaments of any desired color. Alternatively, the coated filaments 
or textile products formed from such filaments may be dyed to a desired 
color. 
Although in some of the preceding examples and in the foregoing discussion 
the dispersions of the polyamide particles have been applied directly to a 
substrate, in certain instances it may be desired to apply to a substrate 
a different coating or polymer which may not have a sufficiently high 
toughness and/or abrasion resistance. In such instances, the use of the 
present dispersions may be advantageously used to form an overcoat of high 
toughness and abrasion resistance. For example, in the wire coating art, 
in many instances, the wire may first be provided with an enamel coat such 
as a polyester coat deposited from a solution of the polyester. The 
dispersions of the present invention may be applied over the base coat and 
cured as described above to provide an outer coating of greater toughness 
necessary for subsequent winding operations. 
In the illustrative examples, the coatings after application to a substrate 
were heated to temperatures between about 205.degree. C. The specific 
temperature used to melt the low molecular weight polyamide particles and 
permit the molten material to flow and to polymerize the polymer to a 
desired high molecular weight must be, obviously, at least the melting 
point of the specific low molecular weight polyamide. Higher temperatures 
may be used so as to reduce the require polymerization period provided 
that the temperature is not sufficiently high to adversely affect the 
polymer, as by decomposition. 
In the foregoing discussion, reference has been made to "superpolyamides" 
and "fiber- and film-forming polyamides" and these terms have been used in 
the sense first enunciated by Carothers. The simple test usually used to 
define fiber-forming polymers has involved dipping an end of a rod into 
the molten polymer and withdrawing the rod so as to determine whether or 
not a self-supporting filament could be drawn from the molten polymer. As 
indicated by Carothers, fiber-forming polymers (superpolymers) generally 
require a molecular weight of at least 10,000 for minimum fiber 
properties. The term "superpolymer" was coined by Carothers to describe 
polymers having molecular weight above 10,000 (Textbook of Polymer 
Science, 2nd Ed., 1971, by Fred W. Billmeyer, Jr.). For practical 
fiber-forming purposes the molecular weight should substantially exceed 
10,000. For example, in Encyclopedia of Polymer Science and Technology, 
1st Edition, in the section entitled Polyamides by W. Sweeny and J. 
Zimmerman (page 542), it is pointed out that commercial nylon fiber has a 
number average molecular weight of from about 12,000 to 15,000. 
The term "low molecular weight polyamide" as used herein and in the claims 
is intended to designate polyamides having a number average molecular 
weight of from about 1,300 to not exceeding about 7,300. The polyamide 
products of the present invention preferably have a reduced viscosity 
within the range of from about 0.15 to about 0.26. As is apparent from the 
foregoing discussion the molecular weight or reduced viscosity may be 
controlled by the relative weight proportions of the polyamide forming 
constituent and the water. The specific particle size and uniformity of 
particle sizes in a specific product is controlled by an instantaneous 
quenching of the heated reaction mass and the pH of the quenched mass. 
It is obvious that the examples illustrate the preparation of the products 
by batch procedures. Conveniently, in such preparations the instantaneous 
cooling of the reaction mass is effected by discharging the reaction mass 
into an agitated quench bath. It is obvious that other means may be 
utilized, particularly in a continuous method. For example, the quenching 
medium may be pumped through a conduit positioned so that the quenching 
medium impinges on or collides with the reaction mass as it is discharged 
from the dip tubes and the resulting quenches mass then collected in a 
suitable vessel. Alternatively, the dip tube may discharge the reaction 
mass and the quenching medium may be simultaneously pumped into a mixing 
chamber from which the quenched may be withdrawn continuously. In the 
latter procedures, any desired additive, such as catalysts, flow 
promoters, stabilizers, pigments, etc., may be conveniently metered into 
the conduit through which the quenching medium passes so as to aid in 
providing an intimate mixture or blend of these agents throughout the 
quenched mass. 
As stated hereinabove, one of the unique characteristics of the 
water-dispersed polyamide particles is the very narrow size distribution 
of the particles. In general, the size distribution of the particles of 
any specific product will vary directly with the means particle size; that 
is, the lower the mean particle size, the narrower the size distribution. 
The particle size distribution is about d .+-. 0.8d, where d is the mean 
particle size; in other words, the sizes of the particles in any specific 
product will be within a range of between about d - 0.8d and about d + 
0.8d. Thus in Example 35 where soap was present in the initial charge and 
the quenched mass has a pH of about 3, the mean particle size was 
approximately 0.03 .times. 0.03 .times. 0.005 micron and the particles 
ranged in the larger dimension from about 0.006 micron to about 0.054 
micron. 
Upon spray drying of a dispersion of the polyamide resin particles, the 
particles become bound together loosely into agglomerates and form a free 
flowing powdery product. The size of the dry, free flowing powder 
particles may be within a range of between about 3 microns and about 150 
microns, the mean particle size of any specific product being dictated by 
the intended use. The unique characteristic of these loosely bound 
agglomerates is that when subjected to agitation in water, the 
agglomerates readily disintegrated of crumble into what appears to be the 
same particles as were present in the original dispersion prior to spray 
drying. For example, when the dispersions of Examples 14, 18 and 19 were 
spray dried, the dried products were free flowing and consisted of 
agglomerates of sizes within the range of 3 microns to about 35 microns. 
Upon agitation in water by the use of a Cowles Dissolver, the agglomerates 
had broken up into dispersed particles having mean sizes of about 5-6 
microns, 3-6 microns and 5-6 microns, respectively. Where the powder 
product is intended for use in electrostatic and flame spraying 
techniques, the mean particle sizes are preferably in the lower portion of 
the range, for example, between 20 microns and 40 microns. Where the 
product is intended for fluidized bed techniques, coarser particles are 
preferred such as products having mean particle sizes between about 75 
microns and 150 microns. 
The particles as formed from .epsilon.-caprolactam and copolymer 
predominating in .epsilon.-caprolactam are loosely packed, randomly 
oriented, bonded lamellar sheets, or clusters of flaky sheets which are 
plate-like in structure with two dimensions roughly equal and having 
irregular or crenulated edges. In the case of the particles produced from 
the nylon-6,6 monomer salt (Example 36), the ultimate particles are 
similarly lamellar sheets but are more bladed-to-fibrous in structure. 
In the foregoing description and discussion, references have been made to 
particle sizes. In Example 35, the particles are ultimate particles and 
consists of flaky sheets or lamella, the particle size (d) designating the 
maximum dimension of the lamella. The particles, for example, as formed in 
Examples 13-32, consist of loosely packed clusters of flaky sheets and the 
particle size (d) designates the maximum dimension of approximate diameter 
of the clusters. The spray dried products consist of loose agglomerates of 
the clusters of flaky sheets and, as stated above, readily separate into 
the original clusters of flaky sheets when subjected to agitation in 
water. The particle size of the spray dried products has reference to the 
maximum dimension or diameter of the agglomerates of the clusters. 
Aggregates designate materials which consist of tightly bonded particles 
which do not separate into the original particles when subjected to 
agitation in water. 
In the production of spray dried products, the dispersion which is to be 
spray dried should not contain ultimate particles such as produced when 
soap is included in the initial charge. The presence of such submicron 
particles results in the formation of aggregates wherein the particles are 
tightly bound and up agitation in water the aggregates do not disintegrate 
into the original particles. It is essential that the dispersion to be 
spray dried contains particles consisting of loosely packed, randomly 
oriented, clusters of flaky sheets so that the dried product consists of 
loosely bound agglomerates which disintegrate readily upon agitation in 
water to form the particles as were present in the dispersion prior to 
spray drying. 
However, if desired, spray dried products may be prepared from the soap 
containing dispersions (Examples 33-35) or form dispersions containing 
sub-micron particles. Such spray dried products will consist of aggregates 
that are capable of withstanding abrasion such as occurs in fluidized bed 
coating techniques. In such techniques where the coating powders are in a 
continuing swirling motion, the aggregates because they contain the 
original particles tightly bonded together do not crumble or break apart 
into minute particles which will escape or be blown from the fluidized bed 
chamber. The aggregates, however, will deposit on the heated substrate, 
melt and flow and the polyamide may be polymerized as described above.