Stable aqueous microdispersions of finely divided polyamide resin particles and methods for the manufacture thereof are provided. The properties of these microdispersions are achieved by the addition thereto of an effective amount of a co-surfactant such as a lower alkyl alcohol. The provided microdispersions can be drawn into films which will form clear, continuous films at ambient or near-ambient temperatures regardless of the softening point of the polyamide resin. Dispersions according to this invention find use in hot melt adhesives, coatings, inks, and the like.

FIELD OF THE INVENTION 
This invention relates to stable, aqueous microdispersions of finely 
divided polyamide resin particles having the capacity to form clear, 
continuous films at ambient temperatures. 
BACKGROUND OF THE INVENTION 
Polyamide resins are well known as a class of resins, as are numerous 
methods for their preparation. Polyamide resins are typically manufactured 
by reacting a di- or polyfunctional amine with a di- or polyfunctional 
acid. Most of the commonly-employed diacids and diamines yield polyamide 
resins which are essentially linear. 
The properties of polyamide resins will vary considerably, depending upon 
the particular synthetic reactants employed. Polyamide resins which are 
prepared from relatively short chain diacids and diamines having, for 
example, 5-10 carbon atoms will tend to be relatively crystalline and have 
excellent fiber forming properties. These types of polyamide resins are 
typically referred to as nylons. 
Polyamide resins are also prepared from relatively long chain 
polyfunctional acids and diamines. A particularly important class of 
polyamide resins of this type are referred to as polymerized fatty acid 
polyamide resins. The polymerized fatty acid polyamide resins are 
especially useful in products such as hot melt adhesives, water resistant 
coatings, and binders for printing inks, because of their physical 
properties, including high strength, excellent flexibility, water and 
solvent resistance, and the ability to form smooth, nontacky coatings and 
films. 
The polyfunctional acids used in the preparation of polymerized fatty acid 
polyamide resins are derived from higher molecular weight unsaturated 
fatty acids by polymerization. In the polymerization process, the fatty 
acids having double bond functionalities combine to produce mixtures of 
higher molecular weight polymeric acids. 
The polymerized fatty acid polyamide resins are, in turn, typically 
prepared by reacting one or more suitable diamines--most commonly 
relatively short chain diamines--with the polymerized fatty acid. Often, 
another diacid is also reacted to increase the softening point, tensile 
strength, or other properties. The polymerized fatty acid polyamide resins 
which are obtained tend to be more amorphous than the nylon types of 
polyamides resins and are generally more flexible. The differences in the 
physical properties of the polymerized fatty acid polyamide resins as 
compared to the nylon types of polyamide resins are related to the long 
chain length and structural variations of the polymerized fatty acid 
component. 
One of the problems encountered with the polyamide resins--particularly the 
polymeric fatty acid polyamides--relates to the methods used to apply the 
resins to substrates. One method which has been used involves heating the 
polyamide resins above their melting point and then applying the molten 
resins to the substrate. This technique, however, has certain inherent 
problems. For example, polyamide resins typically have high melting 
points, often higher than the distortion temperatures of the substrates 
onto which they are to be applied. Accordingly, the hot melt method can 
only be used in certain limited applications which require relatively 
expensive application equipment. Thus, the use of molten polyamide resins 
is not practical in applications such as, for example, printing and 
coating. Molten polyamide resins are also impractical where the resin is 
to be applied as a latent hot melt layer to be activated at a later time. 
It has been recognized that certain of the problems associated with the 
polyamide resins might be solved if the polyamides could be applied at 
ambient temperatures as solutions or dispersions. For many applications, 
however, solutions of polyamide resins are unsatisfactory. Polyamide 
resins as a class have excellent resistance to solvents; even with respect 
to those solvents in which the polyamide resins are soluble, the 
solubility typically is relatively low. Furthermore, the solvents which 
have been used to make polyamide resin solutions often adversely react 
with the substrates to which the polyamide resin solutions are applied. 
Further problems associated with solvent solutions are that most solvents 
used are relatively expensive, often difficult or impossible to remove 
from the applied coatings, and present fire, toxicity, and environmental 
pollution problems. 
To overcome or at least reduce the problems associated with such 
solvent-based systems, it has been suggested to prepare emulsions or 
dispersions of the polyamide resins in water. Early emulsions were 
prepared by initially dissolving the polyamide resin in an organic solvent 
and then using selected emulsification agents to form an emulsion of the 
solvent solution and water. However, the resulting solvent/water/polyamide 
resin emulsions still had the problems associated with the presence of 
solvents and were relatively unstable. Those skilled in the art will 
appreciate that instability is manifested in aqueous resin emulsions or 
dispersions by phenomena such as phase separation, creaming, coalescence, 
flocculation, or gelation. Films formed from solvent-containing emulsions 
also tended to have an undesirable tackiness. 
In British patent 1,491,136 there was disclosed a method for forming 
aqueous dispersions of various plastic powders, including polyamide resin 
powders. In the disclosed method, the polymer resin was first mechanically 
reduced to a powder form and then blended with water and a thickening 
agent. The method was less than satisfactory, The mechanical reduction of 
the resins to the required particle size was both expensive and difficult 
to control, especially for flexible polymers, and often caused thermal 
degradation of the polymers. Furthermore, the resulting thickened 
dispersions had limited utility in many applications because of the 
relatively high viscosity due to the thickening agent. 
It is also known to render a polyamide resin more readily dispersible in 
water by chemically modifying the resin so as to include solubilizing 
groups. This includes, for example, incorporating alkoxymethyl groups, as 
disclosed in U.S. Pat. No. 2,430,860 (Carirns) and U.S. Pat. No. 2,714,075 
(Watson, et al.). However, the incorporation of the additional groups into 
the polyamide resin increases the cost of the polymer and also typically 
reduces the desirable properties of the polyamide resins, especially in 
relation to water and solvent resistance. 
Another known method for increasing the water dispersibility of polyamide 
resins involves formation of a resin having a considerable excess of 
either free carboxyl or free amine groups. At least a portion of the free 
acid or free amine groups are then neutralized to form salt groups on the 
polyamide resin, which salt groups act as internal surfactants to 
facilitate the dispersion of the modified polyamide in water. In U.S. Pat. 
No. 2,811,459 (Wittcoff, et al.) there is disclosed a method for preparing 
polymerized fatty acid polyamide dispersions wherein the polyamide is 
formed from a substantial excess of a diamine. The resulting polyamide 
resins are then dispersed in an aqueous solution of an acid so that the 
acid forms ammonium salt groups which act as internal surfactants which 
allow formation of an aqueous dispersion. In U.S. Pat. No. 2,768,090 
(Wittcoff, et al.) a similar process is disclosed wherein the excess amine 
groups of a polyamide resin are reacted with an acid to form intrinsic 
ammonium salt groups and, hence, a cationic dispersion which is converted 
to an anionic dispersion by charge inversion. A similar salt forming 
process utilizing free amino groups was disclosed in U.S. Pat. No. 
2,824,848 (Wittcoff). In U.S. Pat. No. 2,926,117 (Wittcoff) there is 
disclosed a method wherein the polyamide resin formed with a deliberate 
excess of acid groups is then dispersed in an aqueous medium containing an 
alkaline substance to cause formation of carboxylate salt groups which act 
as internal surfactants. 
The discussed methods for preparing aqueous dispersions of polymerized 
fatty acid polyamides having salt groups are relatively effective in 
initially forming aqueous dispersions. However, the dispersions have 
limited stability and are not satisfactory for use in many applications, 
as their synthesis requires the presence of substantial amounts of free 
acid or free amino groups which adversely effect the performance 
properties of the dispersed polyamide resin. Optimal properties are 
typically achieved by conducting the amidations so as to cause as complete 
as a reaction as possible. This requires that approximately stoichiometric 
amounts of the starting diacid and diamine be employed and that the 
reaction be conducted so as to produce a final product having a low amine 
number and low acid number. The presence of substantial excesses of either 
reactant or an incomplete reaction--as required for the prior art salt 
forming polyamide material--inherently reduces the chain length and the 
resulting strength and flexibility of the polyamide resin. 
Furthermore, incorporation of polymers having substantial excess amounts of 
unreacted polymerized fatty acids typically results in unstable materials. 
The fatty acids can be liberated from the polymer and cause exceptional 
tackiness and undesirable degradation of the desired properties of the 
polyamide resin. These polyamide resins continue to react during 
application, which causes increases in molecular weight and coating 
viscosity, as well as changes in the melting point. A still further 
problem encountered with the method wherein the salt forms of the 
polyamide resins are used is that the salts tend to decompose during 
application and the resulting material becomes undesirably tacky when 
applied. This is particularly undesirable in many applications, such as in 
printing inks and protective coatings. 
Certain of the problems associated with aqueous polyamide resin dispersions 
can be obviated by the methods disclosed in U.S. Pat. No. 4,886,844 
(Hayes) for the preparation of stable aqueous dispersions of nonsolvated, 
un-neutralized, polymerized fatty acid polyamide resins having low acid 
and amine number. As disclosed therein, molten resin, water, and a 
surfactant are subjected to sufficient comminuting forces to form an 
emulsion in which resin droplets have a volume average size distribution 
of about 20 microns or less. 
However, even the aqueous polyamide resin dispersions according to Hayes 
are not without problems attendant to their use. For example, these 
aqueous dispersions can be drawn into films, but must be heated to within 
about 10.degree. C. of the resin's softening point for clear, continuous 
films to properly form. For example, in U.S. Pat. No. 557,649 (Wittcoff), 
the use of polyamide suspensions in heat-seal compositions requires a 
minimum temperature of 70.degree. C. Thus, it would be more desirable if 
such films could be formed at lower temperatures, preferably ambient 
temperatures. This is particularly true where resins having relatively 
high softening points are employed. 
SUMMARY OF THE INVENTION 
The present invention provides stable, aqueous microdispersions of finely 
divided polyamide resin particles dispersed in water, which 
microdispersions have improved stability and film-forming properties. 
Specifically, the aqueous polyamide microdispersions of the present 
invention are stable against phase separation and gelation. Additionally, 
the microdispersions are capable of forming non-tacky, clear, continuous 
films at ambient or near-ambient temperatures. 
The microdispersions of the present invention are produced by first forming 
a water-in-oil emulsion by mixing together at a first temperature the 
polyamide resin, water, at least one surfactant, at least one cosurfactant 
and a neutralizing acid or base, wherein the water and surfactant are 
present in amounts effective to form the water-in-oil emulsion, the 
co-surfactant is present in an amount effective to form the aqueous 
microdispersion, the neutralizing acid or base is present in an amount 
effective to neutralize residual acid or base on the polyamide resin, and 
the first temperature is effective to liquify the polyamide resin and to 
maintain an oil phase of a water and oil emulsion as a liquid. The aqueous 
microdispersions are then formed by mixing together at a second 
temperature the water-in-oil emulsion and a second amount of water 
effective to form an oil-in-water emulsion. The oil-in-water emulsion is 
then cooled to a third temperature effective to form the aqueous 
microdispersion. At least one water soluble, dipolar chemical moiety is 
added in an amount effective to stabilize the aqueous microdispersion to 
either the oil-in-water emulsion at the second temperature or to the 
aqueous microdispersion. 
The stable, aqueous microdispersions thus produced, when drawn into a film, 
form a non-tacky, clear, continuous film upon drying at ambient or 
near-ambient temperatures.

DETAILED DESCRIPTION OF THE INVENTION 
Those skilled in the art will appreciate that emulsions of polyamide resin 
in water, more commonly known as oil-in-water emulsions, are to be 
contrasted with emulsions of water in resin, which emulsions are more 
commonly known as water-in-oil emulsions. Techniques for converting 
water-in-oil emulsions to oil-in-water emulsions are generally known to 
those skilled in the art as inversions. The water added to invert an 
emulsion is known as inversion water. The conversion of an oil-in-water 
emulsion to a water-in-oil emulsion is also known as an inversion. The 
term "oil phase" as referred to herein is understood to mean that phase of 
either the water-in-oil or the oil-in-water emulsion which includes the 
polyamide resin, at least one surfactant, and at least one co-surfactant. 
It will be appreciated that there exist numerous types of polyamide resins 
which can be employed to form aqueous dispersions according to the present 
invention. The terms "polyamide resin" or "resin" as employed herein are 
intended to include compositions comprising individual, chemically 
distinct polymerized fatty acid polyamide resins as well as blends 
thereof. Polyamide resins can be obtained commercially or can be prepared 
by generally well known methods. 
The term "polymerized fatty acid" is intended to be generic in nature and 
to refer to polymerized acids obtained from fatty acids. The term "fatty 
acids" refers to saturated, ethylenically unsaturated and acetylenically 
unsaturated, naturally occurring and synthetic monobasic aliphatic 
carboxylic acids which contain from about 8 to about 24 carbon atoms. 
While specific references are made in this application to polymerized 
fatty acid polyamide resins which are obtained from C.sub.18 fatty acids, 
it will be appreciated that the methods of this invention can likewise be 
employed with other polymerized fatty acid polyamides. 
The preferred starting acids for the preparation of the polymerized fatty 
acids used in this invention are oleic and linoleic acids, due to their 
ready availability and relative ease of polymerization. Mixtures of oleic 
and linoleic acids are found in tall oil fatty acids, which are a 
convenient commercial source of these acids. Fatty acids can be 
polymerized using various well known catalytic and noncatalytic 
polymerization methods. A typical composition of the polymerized C.sub.18 
tall oil fatty acids which are used as the starting materials for the 
polyamide resins used in the present invention is: 
______________________________________ 
C.sub.18 
monobasic acids (monomer) 
0-15% by wt. 
C.sub.36 
dibasic acids (dimer) 
60-95% by wt. 
C.sub.54 
(or higher) trimer acid 
0.2-35% by wt. 
or polybasic acids 
______________________________________ 
In preparing polymerized fatty acid polyamide resins for use in the present 
invention, it is preferable that the starting polymerized fatty acid 
contain as high a percentage as possible of the dimer (C.sub.36 dibasic) 
acid in order to obtain optimum physical properties in the final product. 
In addition to the polymerized fatty acids, a wide variety of dicarboxylic 
acids can be used to prepare polymerized fatty acid polyamide resins, 
including aliphatic, cycloaliphatic, and aromatic dicarboxylic acids. 
Representative of such acids--which may contain from 2 to 22 carbon 
atoms--are oxalic, glutaric, malonic, adipic, succinic, suberic, sebacic, 
azelaic, pimelic, terephthalic, isophthalic, dodecanedioic and phthalic 
acids, naphthalene dicarboxylic acids, and 1,4-or 1,3-cyclohexane 
dicarboxylic acids. Preferred dicarboxylic acids employed in the invention 
are straight chain aliphatic diacids having at least 6 carbon atoms and 
more preferably 6 to about 22 carbon atoms such as azelaic, sebacic, and 
dodecanedioic dicarboxylic acids. It should be understood that use of the 
corresponding acid anhydrides, esters, and acid chlorides of these acids 
is included in the term "dicarboxylic acid". These acids and anhydrides 
are readily available from commercial sources and methods for their 
preparation are well known. 
Monocarboxylic acids may be added to control molecular weight. Preferred 
monocarboxylic acids are linear and have 2 to 22 carbon atoms. Most 
preferred are stearic, tall oil fatty and oleic acids. 
The diamines used in the preparation of the polymerized fatty acid 
polyamide resins employed in the present invention may be one or more of 
the known aliphatic, cycloaliphatic or aromatic diamines having from about 
2 to about 20 carbon atoms. Preferred are the alkylene diamines, such as 
ethylene diamine, 1,3-diaminopropane, 1,4-diaminobutane, terephthalyl 
diamine, known as p-xylene diamine, 1,6-hexamethylene diamine, 
bis-(4-cyclohexylamine)methane, 2,2-bis-(4-cyclohexylamine)propane, 
polyglycol diamines, isophorone diamine, isophthalyl diamine, known as 
m-xylene diamine, cyclohexanebis(methylamines), 
1,4'-bis-(2-aminoethyl)benzene, dimer diamine, polyether diamines, methyl 
pentamethylene diamine, and piperazine. These diamine compounds are all 
prepared by well known methods and many are commercially available. 
Particularly preferred are the straight chain aliphatic diamines of 2 to 
about 20 carbons atoms, especially ethylene diamine and hexamethylene 
diamine, and cycloaliphatic diamines, especially 
4,4'-methylenebis(cyclohexylamine) and piperazine. 
In the method of the present invention, it is desirable to use as the 
polymerized fatty acid polyamide a material which is the result of as 
complete an amidation reaction as possible between the starting 
polymerized fatty acid and the diamine. Those skilled in the art will 
recognize that the degree of completion of the amidation process can be 
determined by evaluating the acid number and the amine number of the final 
polymer. Ideally, the amine or the acid numbers, depending upon which is 
in lower stoichiometric amounts, of the polyamide resin employed should be 
zero (0). However, it is often difficult, if not impossible, to reach 
complete reaction, and this value should be one or less. It has been 
found, however, that polymerized fatty acid polyamide resins having 
relatively low amine numbers of, for example, up to about 10 and 
relatively low acid numbers up to about 12 are especially useful in the 
present invention. 
The number of free acid groups and free amine groups present in the 
polymerized fatty acid polyamide resin are directly related to the 
relative amount of the polymeric fatty acids, dicarboxylic acids and 
diamines involved in the polymerization reaction and the degree of 
completion of the reaction. For the above reasons, approximately 
stoichiometric amounts of the polymerized fatty acids plus the 
dicarboxylic acids and the diamines based on the total number of available 
acid and amine groups should be used to prepare the polyamide resins for 
this invention and the reaction conditions should be selected to ensure 
completion or substantial completion of the amidation reaction. The 
reaction conditions required for the amidation reaction are generally well 
known in the art, with the reaction being generally conducted at 
temperatures from about 100.degree. C. to about 300.degree. C. for from 
about 1 to about 8 hours. The use of acid catalysts, such as phosphoric 
acid, and vacuum can be used, especially in the latter part of the 
reaction, to yield a more complete amidation reaction. 
It will be appreciated that a wide variety of water soluble surfactants can 
be employed successfully in preparing microdispersions of the present 
invention, in part because of the relative neutral charge of most 
polymerized fatty acid polyamide resins. The surfactant or combination of 
surfactants which are preferred in the process of this invention are ones 
which will promote the emulsification of the molten polyamide resin and 
the water and which will also act to stabilize the final microdispersion 
of the polyamide resin particles in the water. Those skilled in the art 
will recognize that the choice of a surfactant will depend intimately upon 
the particular polyamide resin employed. The surfactants which are 
selected are those which are capable as acting either as oil-in-water or 
water-in-oil emulsifying agents for the polyamide resin-water mixture. The 
surfactants include well known anionic, polar and non-polar nonionic, 
amphoteric, and cationic surfactants. 
Among the surfactants which have been found to be especially useful are the 
non-ionic surfactants Tergitol.TM. NP-40 and Tergitol.TM. 15-S-40 (Union 
Carbide, Danbury, CT) and Igepal.TM. C0-850 and Igepal.TM. C0-870 (GAF 
Corporation, Wayne, NJ). 
While all surfactants are not suitable for use in the method of the present 
invention, it has been found that a wide range of surfactants are 
suitable. It is relatively simple to screen suitable surfactants for use 
in the presence of this invention. It was found for certain embodiments, 
for example, that the preferred surfactants are those which exhibit 
outstanding ability to cause the emulsification of the water in the 
liquified polymerized resin. These surfactants must also have the ability 
to form emulsions of the liquified resin in the water upon inversion of 
the water-in-oil emulsion. These surfactants are typically also highly 
effective for imparting long term stability to the final dispersion. The 
relative amount of the employed surfactant added is based upon the amount 
of the polymerized fatty acid polyamide resin which is to be present in 
the final dispersion and upon the particular surfactant used. It has been 
found, however, that optimum results are obtained when the surfactant is 
used in an amount from about 0.05% to about 10% by weight, based on the 
weight of the polymerized fatty acid polyamide resin. 
The employment of a co-surfactant in accordance with this invention is 
important in that it aids in the formation of aqueous microdispersions 
having resin particles of a sufficiently small size regime as described 
hereinbelow. The small dispersed particles formed, in turn, provide 
improved performance properties. For example, small polyamide resin 
particles facilitate the formation of non-tacky, clear, continuous films 
at ambient. or near ambient temperatures when the dispersion is drawn down 
into a film with a blade or by some other well-known technique. The 
co-surfactants must be sufficiently volatile so as to be substantially 
absent from the films formed from the aqueous microdispersions. 
Additionally, the co-surfactants must be able to hydrogen bond and must be 
soluble in both the water and liquified polyamide resin phases in the 
emulsion stages of the process. Non-limiting examples of co-surfactants 
suitable for use in this invention are lower alkyl alcohols having up to 
10 carbon atoms. Preferred alcohol co-surfactants include ethanol, 
pentanol, hexanol, n-propanol, butanol, and isopropanol. Isopropanol is 
particularly preferred. 
The role of the surfactant/co-surfactant component in the microdispersion 
is to control the phase behavior of the system. To better understand the 
phase behavior, a ternary phase diagram (FIG. 1) has been constructed. As 
the microdispersions of the present invention generally comprise 6 
components, the ratio of several of the components have been fixed for 
ease of illustration. 
Specifically, Point A represents a concentration of 100% of component A 
(water), Point B represents 100% of Component B (45 parts polyamide to 1 
part mixture of a neutralizing acid or base and an amino acid) and Point C 
represents 100% of Component C (0.4 parts surfactant to 1 part 
co-surfactant). All percentages are weight percentages based on the total 
weight of the microdispersion. Region 1 represents that region in which 
microdispersions, those having a volume average particle size distribution 
of less than about 1000 nanometers are formed. Region 2 represents that 
region in which "regular" dispersions, those having a volume average 
particle size distribution of greater than about 1000 nanometers are 
formed. In practice, this means that a microdispersion is either 
translucent or transparent while a "regular" dispersion is opaque or 
cloudy. 
Due to the surfactant/co-surfactant component, finely divided polyamide 
resin droplets having a size less than about 1000 nanometers are formed in 
the emulsion stage. Upon solidification, the droplets form finely divided 
polyamide particles dispersed in the water, thereby forming the 
microdispersions of the present invention. 
It is believed that the surfactant/co-surfactant component allows either 
the homogeneous mixture of liquified polyamide resin, surfactant and 
co-surfactant or the water-in-oil emulsion to be cooled below the ring and 
ball softening point of the neat polyamide resin, while still maintaining 
the oil phase of the emulsion in the liquid state. The "melting point" of 
the oil phase is that temperature below which the resin droplets solidify 
to form the aqueous microdispersion. This allows the inversion of the 
water-in-oil emulsion to the oil-in-water emulsion to occur below the ring 
and ball softening point of the neat polyamide resin. 
While it is essential to add an amount of co-surfactant effective to form 
the aqueous microdispersion as described hereinabove, typically, the 
effective amount of the co-surfactant will be less than would be required 
to completely dissolve the resin. In fact, amounts of the co-surfactant 
sufficient to completely dissolve the resin result in the dissolved resin 
being incorporated into the water phase of the microdispersion, which 
results in excessive viscosity in the resulting aqueous polyamide 
microdispersion. This effective amount usually will be between about 10% 
and about 40% by weight, based on the weight of polyamide resin. 
It will be appreciated that polyamide resins typically contain residual 
acid or base attributable to the synthetic source of the resin. While it 
is preferred that aqueous microdispersions be formed from polymerized 
fatty acid polyamide resins which have relatively low (i.e., less than 
about 12) acid or amine numbers, aqueous microdispersions have been formed 
from polyamide resins with acid numbers up to about 45 and amine numbers 
up to about 250. It will be appreciated that acid number represents the 
titratable acid present in a gram of resin expressed in terms of 
milligrams potassium hydroxide required to neutralize that amount of acid. 
Likewise, amine number represents the acid titratable amine groups present 
in a gram of resin expressed in terms of equivalent milligrams potassium 
hydroxide. 
It is preferred in accordance with this invention that a resin's residual 
acid or base be neutralized to some empirically determined level prior to 
formation of aqueous microdispersions. The preferred degree of 
neutralization will vary from resin to resin and will be manifested by 
incremental improvement in the performance properties of aqueous 
dispersions prepared therefrom. Preferred neutralizing bases are potassium 
hydroxide, sodium hydroxide, ammonium hydroxide, and ethanolamines. 
Preferred neutralizing acids are acetic acid, hydrochloric acid, sulfuric 
acid, and phosphoric acid. 
A wide variety of water soluble, dipolar chemical moieties, such as amino 
acids, may be incorporated into aqueous microdispersions of polyamide 
resin in accordance with this invention, so long as they possess 
sufficient amphoteric character to stabilize the microdispersions. It will 
be appreciated that the amphoteric character of an amino acid relates to 
the degree to which its constituent molecules possess points having 
opposite charges. 
The water soluble, dipolar chemical moieties used in the process of this 
invention are selected from the group consisting of amino acids of the 
formula 
##STR1## 
anionic and cationic salts derived therefrom and mixtures thereof, wherein 
R represents an alkyl, alkenyl or aryl group having up to 10 carbon atoms 
and Y is in a polar or non-polar ionic, or non-ionic substituent. Examples 
of such amino acids are p-aminobenzoic acid, glycine, lysine, arginine, 
phenylalanine and serine. Most preferred are glycine and p-aminobenzoic 
acid. 
In preferred embodiments of the methods for preparing the stable, aqueous 
microdispersions of the present invention the solid polyamide resin is 
heated substantially in the absence of oxygen to a temperature at least as 
high as its melting point to liquify the resin. 
This liquification process is preferably conducted in a closed vessel under 
a protective blanket of nitrogen. The melting temperature of the 
polymerized fatty acid polyamide resin will vary considerably depending 
upon the particular starting reactants employed to prepare the polyamide 
resin. Typically, however, polyamides will melt in the temperature range 
from about 100.degree. C. to 200.degree. C. If the temperature to which 
the molten polyamide resin will be heated for liquification is above the 
boiling point of water, the process equipment used in the method of the 
present invention must be capable of being operated at elevated pressures 
and temperatures. 
A homogeneous mixture of the liquified polyamide resin, water, surfactant 
and co-surfactant is then formed by mixing together the liquified 
polyamide resin, a first amount of water effective to form the homogeneous 
mixture, at least one surfactant in an amount effective to form a 
water-in-oil emulsion and at least one co-surfactant in an amount 
effective to form the aqueous microdispersion, at a temperature effective 
to maintain the oil phase of the emulsion as a liquid. The surfactant may 
be anionic, cationic, non-ionic or amphoteric and in an amount from about 
0.05 to 10% by weight of the polyamide resin. The co-surfactant preferably 
is an alcohol having up to 10 carbon atoms and is used in an amount from 
10 to 40% by weight of the polyamide resin. 
The water, surfactant and co-surfactant may be preheated to a temperature 
above the melting point of the polyamide resin in a separate vessel and 
then added to the liquified polyamide resin. Preferably, the water, 
surfactant and co-surfactant are added to the liquified polyamide resin 
without preheating, at a rate sufficiently slow such that the temperature 
of the mixture is maintained above the effective temperature as described 
above. Preferably the water and surfactant are added simultaneously in the 
form of an aqueous surfactant solution, while the co-surfactant is added 
thereafter. 
In other embodiments, a mixture of polyamide resin, water, surfactant and 
co-surfactant is formed by mixing the solid polyamide resin, a first 
amount of water effective to form the mixture, at least one surfactant in 
an amount effective to form a water-in-oil emulsion upon liquification of 
the resin and at least one co-surfactant in an amount effective to form 
the aqueous microdispersion. The mixture is then heated to a first 
temperature above the melting point of the oil phase, which temperature is 
effective to liquify the polyamide resin in the mixture to form a 
homogeneous mixture of the liquified polyamide resin, water, at least one 
surfactant and at least one co-surfactant. 
A water-in-oil emulsion is then formed by mixing with the homogeneous 
mixture a second amount of water effective to form the water-in-oil 
emulsion comprising from about 5 to 50% by weight of the emulsion, said 
second amount of water including an amount of acid or base effective to 
neutralize residual acid or base on the polyamide resin. 
In the most preferred embodiment, the temperature of the homogeneous 
mixture is cooled to a temperature less than but not more than about 
50.degree. C. below the ring and ball softening point of the polyamide 
resin, said temperature also being above the melting point of the oil 
phase. A solution of the neutralizing acid or base and an amount of water 
effective to form the water-in-oil emulsion is then added to the 
homogeneous mixture at a rate sufficiently low so that the temperature 
remains substantially unchanged. 
In alternate embodiments, the aqueous neutralizing solution of acid or base 
is added to the homogeneous mixture before cooling the mixture. The 
aqueous acid or base solution preferably is added at a rate sufficiently 
slow so as to maintain the temperature of the mixture greater than the 
melting point of the oil phase. In other embodiments, the aqueous 
neutralizing solution of acid or base is heated in a separate vessel to a 
temperature which is at least as high as the melting point of the oil 
phase. More preferably, the acid or base solution is heated to a 
temperature at least slightly higher than the melting point of the oil 
phase. Under these conditions it may be required to heat the solution and 
maintain it under pressure in order to reach a temperature higher than the 
melting point of the oil phase. Alternatively, the acid or base solution 
is heated to a temperature somewhat below the temperature of the 
homogeneous mixture and the homogeneous mixture is heated to a temperature 
significantly above the melting point of the oil phase, such that the 
resulting blend of aqueous base or acid and homogeneous mixture will have 
a temperature above the melting point of the oil phase. The blend of the 
homogeneous mixture and aqueous neutralizing acid or base is then cooled 
to a temperature less than but not more than about 50.degree. C. below the 
ring and ball softening point of the polyamide resin, said temperature 
also being above the melting point of the oil phase. 
The mixture is then subjected to comminuting forces sufficient to form an 
emulsion in which the droplets of the molten polymerized fatty acid 
polyamide resin preferably have a volume average size distribution of less 
than about 1000 nanometers. The particular type of apparatus used for 
applying the comminuting force to the blend of the polyamide resin, water, 
surfactant, cosurfactant, and neutralizing acid or base is to some extent 
a matter of choice and can include apparatus which operates on the basis 
of shear, impact, or a combination of these process steps. The equipment 
includes commercially available apparatus such as homogenizers, submicron 
dispersers, emulsifiers, colloid mills, ultrasonic sound mixers and the 
like. In general it is preferable for process purposes to run the blend 
through the comminuting equipment for one pass in that this facilitates 
the manufacturing process. It should be appreciated, however, that the 
blend may be sent through the comminuting equipment for a number of passes 
in order to obtain sufficiently small droplets. In general, the smaller 
the size of the liquid droplets of an emulsion, the more desireable the 
dispersion made therefrom. This is true for dispersions prepared by 
inversion techniques as well. 
An oil-in-water emulsion is then formed at a temperature above the melting 
point of the oil phase and less than but not more than about 50.degree. C. 
below the ring and ball softening point of the polyamide resin by mixing 
with the water-in-oil emulsion a third amount of water effective to form 
the oil-in-water emulsion, said third amount of water including at least 
one water soluble, dipolar chemical moiety, as described hereinabove, in 
an amount effective to stabilize the aqueous microdispersion. The 
oil-in-water emulsion comprises between about 20% and 60% by weight 
polyamide resin. The amount of water soluble, dipolar chemical moiety 
effective to stabilize the aqueous microdispersions is preferably from 
about 0.25% to about 3.0% by weight, based on the weight of the polyamide 
resin. In preferred embodiments the inversion water and water soluble, 
dipolar chemical moiety are heated to a temperature just below the 
temperature of the water-in-oil emulsion prior to mixing with the 
water-in-oil emulsion so that the emulsion is not "thermally shocked" such 
that the liquid polyamide resin droplets prematurely solidify. 
The oil-in-water emulsion is then cooled to a temperature below the melting 
point of the oil phase to cause the finely divided droplets in the 
emulsion to solidify into finely divided dispersed particles, thereby 
forming the aqueous microdispersions of the present invention. This 
cooling step is preferably conducted rapidly so as to prevent coagulation 
of the particles during the stage of solidification, wherein the droplets 
become semi-solid and highly adhesive. Cooling of the oil-in-water 
emulsions prepared at pressures above atmospheric pressure can be rapidly 
performed by pumping the emulsion through a heat exchanger or the like. 
Alternatively, the cooling can be effected by rapidly reducing the 
pressure to cause evaporation of the water. A combination of these 
techniques can also be employed. 
The microdispersions of this invention do not require that the starting 
polymerized fatty acid polyamide resin initially be completely solvated in 
a solvent or that the polyamide resin be formed with excess amine and acid 
groups to allow for salt formation as is required in the prior art methods 
of forming dispersions. 
It is preferred in accordance with this invention that polyamide resin 
microdispersions have volume average particle size less than about 1000 
nanometers, more preferably between about 10 and about 400 nanometers, 
most preferably between about 100 and 150 nanometers. Those skilled in the 
art will appreciate that particle size can be determined by a number of 
methods, such as sedimentation or laser light scattering techniques. 
Determination of particle size by photon correlation spectroscopy is 
preferred. 
The aqueous microdispersions of the present invention preferably comprise 
from about 20% to 60% by weight polyamide resin and from about 30% to 70% 
by weight water. The microdispersions also include at least one surfactant 
selected from the group consisting of anionic, cationic, non-ionic and 
amphoteric surfactants, said amount preferably being from about 0.05% to 
about 10% by weight, based on the weight of the polyamide resin. The 
aqueous microdispersions also comprise at least one co-surfactant selected 
from the group consisting of alcohols having up to 10 carbon atoms, most 
preferably isopropyl alcohol, in an amount effective to form the aqueous 
microdispersions, said amount preferably being from about 10% to about 40% 
by weight, based on the weight of the resin. The aqueous microdispersions 
further include an amount of acid or base effective to neutralize a 
residual acid or base on the polyamide resin. 
The stable, aqueous microdispersions of polyamide resin of the present 
invention comprise at least one amino acid, as described hereinabove, in 
an amount effective to stabilize the resulting aqueous microdispersions. 
Most preferred is from about 0.25% to 3.0% by weight glycine, based on the 
weight of the polyamide resin. Amino acids may be incorporated as a 
reactant during formation of the oil-in-water dispersion, or may be 
incorporated into the dispersion after the formation thereof. It is 
preferred that the aqueous microdispersions contain water soluble, dipolar 
chemical moieties upon formation. 
The polymerized fatty acid polyamide aqueous microdispersions of this 
invention can contain various additives in addition to the above-noted 
materials, such as water soluble alkali metal salts of polymeric organic 
acids and protective colloids such as lignin derivatives, proteins, water 
soluble cellulose derivatives, starch, alginic acid, and long chain 
alcohols and lecithin. The amount of such additives employed can vary in 
amounts from about 0% to about 5% by weight, based on the weight of the 
polyamide resin. 
The polyamide dispersion may likewise contain other materials such as 
viscosity modifiers, plasticizers, dyes, pigments and the like. In this 
regard, it should be noted that the excellent stability of the polymerized 
fatty acid polyamide resin dispersions of this invention allow substantial 
loadings of additives without adversely affecting the overall stability of 
the polyamide dispersion. 
The stable, aqueous microdispersions may be used in, for example, overprint 
varnishes and aqueous inks, as well as in structural and laminating 
adhesives. Additional objects, advantages, and novel features of this 
invention will become apparent to those skilled in the art upon 
examination of the following examples thereof, which are not intended to 
be limiting. 
EXAMPLE 1 
400 lbs. of polyamide resin, UNI-REZ.TM. 2620 (Union Camp Corp., Wayne, 
NJ), were charged to a 200 gallon reactor. The resin was heated to 
127.degree. C. under 20 psi of nitrogen until it became molten. At an 
agitator speed of 140 rpm, 28.6 lbs. of Tergitol.TM. NP-40 (70% aqueous 
solution) followed by 80.0 lbs. of isopropanol were added. The mixture was 
cooled to 82.degree. C. A solution of 10.3 lbs. KOH (45% aqueous solution) 
in 71.4 lbs. deionized water was added to form the initial water-in-oil 
emulsion. This was followed by the addition of a solution of 4.0 lbs. 
glycine in 801 lbs. deionized water over an 85 minute period in order to 
invert the water-in-oil emulsion to an oil-in-water emulsion. The emulsion 
was cooled to 40.degree. C. and the resulting microdispersion was 
discharged. 
The product was a yellow, translucent dispersion of 29% solids. Photon 
correlation spectroscopy showed that the volume average particle size was 
150 nanometers. The microdispersion formed a clear, continuous film at 
room temperature when drawn down on polypropylene film. The aqueous 
microdispersion product is stable against phase separation or gelation for 
periods in excess of one year. 
EXAMPLE 2 
200 g of polyamide resin, UNI-REZ.TM. 2641 (Union Camp Corp., Wayne, NJ), 
14 g of Tergitol.TM. NP-40 (70% aqueous solution), and 40 g of isopropanol 
were charged to a 2 liter Parr reactor and heated to 140.degree. C. with 
100 RPM stirring. The mixture then was cooled to 120.degree. C. A solution 
of 4 g KOH (85% aqueous solution) and 50 g deionized water was added to 
form the initial water-in-oil emulsion. This was allowed to equilibrate 
for 30 minutes at 120.degree. C. A solution of 2 g glycine and 600 g 
deionized water was added to invert the water-in-oil emulsion to an 
oil-in-water emulsion. The resulting microdispersion was cooled to 
40.degree. C. and discharged. 
The product was a purple translucent dispersion at 25% solids. Photon 
correlation spectroscopy showed that the volume average particle size was 
about 100 nanometers. The microdispersion formed a clear, continuous, 
non-tacky film at room temperature when drawn down on a polypropylene 
film. The aqueous microdispersion product is stable against phase 
separation or gelation for periods in excess of one year.