Powder coating compositions

Novel powder coating compositions consisting of resinous polyhydroxyl compounds as film-forming agents, a cross-linking agent, an activator, and conventional additives are described. In contrast to the known powder coating compositions, the activator rather than cross-linking agent is blocked. The novel cross-linking agents employed are bisazlactones. Activators for use in cross-linking reactions are strong acids which are blocked, i.e they are employed in the form of their thermolabile esters or amine salts. The powder coating compositions of this invention are storable and they can be baked at predetermined temperatures to provide high gloss and elastic coatings.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to powder coating compositions consisting of 
resinous polyhydroxyl compounds as film-forming agents, a compound 
comprising at least two functional groups reactive with the hydroxyl 
groups of said resinous polyhydroxyl compounds, as the cross-linking 
agent, and further comprising conventional additives, such as levelling 
agents, etc. and pigments, if so desired. 
2. Description of the Prior Art 
To produce powder coating compositions the components must be thoroughly 
mixed so that, following the mixing and powderizing treatments, the 
individual powder grains are of substantially the same composition which 
can only be achieved when the mixing is done in a heated extruder. 
Thereafter, the powder is spread onto the surface to be coated by way of 
the elctrostatic powder spray coating technique and baked at the required 
temperature for the respective coating composition. The preparation of 
powder coating compositions and the processing of such materials to 
lacquers requires that the individual components be miscible with each 
other at elevated temperatures and that the cross-linking reation take 
place only at baking temperatures. In practice, mixing temperatures of at 
least 90.degree. C, preferably between 100.degree. and 120.degree. C, have 
been found to be indispensible. The baking temperatures of powder coating 
compositions range without exception from 180.degree. to 200.degree. C 
with baking periods of from 20 to 30 minutes. Reduction of the baking 
temperatures to as low as 170.degree. C has been achieved by means of 
suitable activators. 
Therefore, the powder coating compositions must include cross-linking 
agents that are not reactive with the hydroxyl groups of the polyhydroxyl 
compounds at mixing temperatures but which are reactive therewith at 
baking temperatures. This is achieved by employing cross-linking agents 
having functional groups protected (i.e., blocked) by compounds split off 
only at temperatures clearly ranging above said mixing temperatures. The 
most commonly used cross-linking agents today are diisocyanates or 
polyisocyanates like toluene diisocyanate, hexamethylene diisocyanate, 
isophorone diisocyanate; .omega.-lactams having from 4 to 12 carbon atoms, 
more especially .epsilon. caprolactam, proved to be suitable blocking 
agents. These blocked cross-linking agents, however, suffer from several 
disadvantages. For example, they escape while baking said lacquers which 
means not only establishing suitable hood means but also involves a 
considerable loss in material since the blocking agents have a relatively 
high molecular weight compared to the equivalent weight of the 
--N.dbd.C.dbd.O group to be blocked. Even though the baking temperature 
must range clearly above the melting temperature for achieving good 
processing prior to the cross-linking, the baking temperature should be 
the lowest possible for reasons of energy costs and in order that the 
substrate will be subjected only to low thermal stresses. 
It is an object of the present invention to provide powder coating 
compositions that contain an unblocked cross-linking agent in which all 
components thereof are miscible with each other in the extruder at 
temperatures above 90.degree. C without any cross-linking reaction taking 
place, and which can be baked at the lowest possible temperatures. 
DETAILED DESCRIPTION OF THE INVENTION 
Accordingly, powder coating compositions have been discovered consisting of 
polymers containing hydroxyl groups as the film-forming agents, a 
cross-linking agent and the conventional additives such as levelling 
agents, and pigments if so desired, characterized by a bisazlactone of the 
general formula I: 
##STR1## 
wherein R.sub.1 and R.sub.2 are selected from the group consisting of 
hydrogen, alkyl having 1 to 8 carbon atoms, cycloalkyl having from 4 to 16 
carbon atoms, aryl having from 6 to 16 carbon atoms, wherein only one of 
R.sub.1 and R.sub.2 may be hydrogen, or wherein R.sub.1 and R.sub.2 
together with the C in 4-position form a cycloaliphatic ring having from 4 
to 12 carbon atoms; wherein R.sub.3 is selected from the group consisting 
of alkylene having from 2 to 6 carbon atoms or arylene having from 6 to 16 
carbon atoms, as the cross-linking agent, and wherein a blocked acid is 
used as the activator. In formula I aryl and arylene may also be 
substituted by alkyl of from 2 to 8 carbon atoms or halogen such as 
chlorine, bromine, or fluorine. It is unpredictable that bisazlactones 
would prove to be suitable cross-linking agents for resinous polyhydroxyl 
compounds in the powder coating compositions. In German Auslegeschrift No. 
1,645,214 there is described a solvent lacquer system wherein the 
film-forming component consists of a copolymer of acrylic or methacrylic 
acid ester and azlactone having in the 2-position a substitutent 
comprising a polymerizable double bond, and of a compound having at least 
two functional groups such as amino groups or hydroxyl groups, as the 
cross-linking agent. Hardening takes place at normal (about 20.degree. C) 
or slightly elevated temperatures under the influence of acidic or 
alkaline catalysts. Such film-forming systems which cross-link at such low 
temperatures are completely unsuitable for use in the production of powder 
coating compositions. In the Journal American Chemical Society, Vol. 71 
(1955), pages 1541 to 1546, Cleaver and Pratt describe the synthesis of 
bisazlactones, the properties thereof, and the preparation of long-chain, 
i.e., non-cross-linked, polyamides from these bisazlactones. In this 
acticle it is disclosed that although the reaction of lactones and 
alcohols (leading to ester amide linkage and thereby opening the ring, as 
is well known) is catalyzed by acids as well as bases, these acids and 
bases are incapable, in the case of bisazlactones, of catalyzing the 
reaction sufficiently and, as a result, only relatively low molecular 
weight i.e., short-chain, compounds are formed. 
Surprisingly, it has been found that bisazlactones with resinous 
polyhydroxyl compounds cross-link completely in the presence of strong 
acids (but not in the presence of bases) at baking temperatures of between 
155.degree. to 185.degree. C although the reactivity of the hydroxyl 
groups of resinous polyhydroxyl compounds is much less than that of low 
molecular weight compounds, such as alcohols and glycols, as is well 
known. Furthermore, it has been found that the cross-linking reaction can 
be controlled in that the said reaction starts only at an elevated 
temperature, as desired, when blocked acids in catalytic amounts as 
aforementioned are used as activators. 
Thus, a novel system of powder coating compositions has been discovered. 
This system comprises a nonblocked cross-linking agent easily miscible 
with said polyhydroxyl compound and the activator in an extruder wherein 
the temperature is more than 90.degree. C. Since the activator is employed 
only in an amount of 2 percent by weight at the maximum (basis polyhyroxyl 
compound) with only a fractional amount of said 2 percent by weight 
pertaining to said blocking agent, the loss in material in the baking 
operation is exceptionally low. In addition, the formation of the lacquer 
surface is, of course, much less adversely affected by virtue of the fact 
that only a minor amount of material escapes from the surface during 
baking as compared with those powder coating compositions which utilize 7 
percent and more of a blocking agent of which the major portion is 
released from the lacquer surface during the baking step. With the powder 
coating compositions of the invention, high gloss and elastic films are 
obtained. Resinous polyhydroxyl compounds suitable for use in the powder 
coating compositions of this invention are any polymers comprising 
hydroxyl groups generally used for lacquer resins, for instance, 
polyesters, copolymers, acrylic resins, etc. having free hydroxyl groups. 
Useful resinous polyhydroxyl compounds suitable for preparing the novel 
powder coating compositions of this invention include polyesters of 
glycols and terephthalic acid modified with a certain amount of triols 
with a hydroxyl number of about 95 or copolymers consisting of styrene and 
unsaturated mono- or dicarboxyic esters with an amount of 
.beta.-hydroxyalkyl groups with a hydroxyl number of about 100. 
Suitable bisazlactones are those falling within the scope of the 
aforementioned Formula I. The substituents in the 4-position, R.sub.1 and 
R.sub.2, are preferably methyl groups but they may also be alkyl groups 
having up to 8 carbon atoms, with one of the substituents preferably being 
a lower alkyl group. For example, either one of the two substituents, 
R.sub.1 and R.sub.2 can be hydrogen with the other one being an aromatic 
or a cycloaliphatic radical. Substituents R.sub.1 and R.sub.2 have, among 
other things, an influence on the melting point of said bisazlactone. 
Radical R.sub.3 may be an alkylene having from 2 to 6 carbon atoms or 
arylene of from 6 to 16 carbon atoms. The use of arylene radicals is, 
however, preferred in the production of colourless or white lacquers since 
the bisazlactones wherein the two azlactone rings are coupled to each 
other by means of saturated aliphatic chains, are responsible for slight 
discolourations. The arylene radical can also carry many different 
substituents, e.g. alkyl, aryl and acyl radicals having 2 to 8 carbon 
atoms or halogens such as chlorine, bromine or flourine. The choice of 
substituents permits one to vary not only the properties of the 
bisazlactone with respect to its solubility in the polyhydroxyl compound 
and with respect to its melting point but also the properties of the 
lacquer resin, as, for instance, levelling during the baking operation. 
The preparation of bisazlactones is known (see the previously mentioned 
Cleaver & Pratt reference, especially pages 1544 to 1547). Generally, a 
dicarboxylic acid dichloride is reacted with an alpha-amino acid according 
to the Schotten-Baumann reaction in the presence of alkali and, 
subsequently, the ring closure is effected by splitting off water. 
Suitable dicarboxylic acid chlorides include, for example, succinic acid 
dichloride, adipic acid dichloride, suberic acid dichloride, fumaric acid 
dichloride, unsubstituted or substituted isophthalic acid dichloride or 
terephthalic acid dichloride. Suitable alpha-amino acids are, for 
instance, glycine, alanine, leucine, phenylalanine, 
alpha-aminoisovalerianic acid. Depending on the respective compounds from 
which said bisazlactone is prepared, the melting point and other 
properties of said bisazlactone can be predetermined. As a rule, a 
bisazlactone is selected which has a melting point ranging from 
100.degree. to 140.degree. C. however a bisazlactone compound melting at a 
higher temperature may be utilized provided it possesses good solubility 
in said polyhydroxyl compound with which the higher melting bisazlactone 
is mixed. As mentioned before, especially useful bisazlactones are derived 
from aromatic di-carboxylic acid dichlorides in case transparent varnishes 
of from white to light-colored varnishes are desired. Where minor 
discolorations does not matter, bisazlactones from saturated carboxylic 
acids may be employed. A second especially preferred class of 
bisazlactones are those prepared from isophthalic acid dichloride and 
alpha-aminoisobutyric acid or alpha-aminoisovalerianic acid because of 
their favourable melting points (129.degree. to 103.degree. C, 
respectively) and because they do not show any tendency to discolouration. 
As mentioned before, the cross-linking reaction is surprisingly activated 
by means of a strong acid. Such acids are, for example, hydrochloric acid, 
phosphoric acid, sulfuric acid, organic sulfonic acids, such as benzene 
sulfonic acid, p-toluene sulfonic acid, naphthalene sulfonic acid, etc. If 
these acids were added unblocked to the mixture of the polyhydroxyl 
compound and cross-linking agent, an immediate cross-linking reaction 
would occur. According to the present invention, these difficulties are 
overcome by employing as the catalyst thermolabile compounds of the acids 
which are preferably crystalline, as, for example, alkylamine comprising 
alkyl groups having from 2 to 8 carbon atoms, alkylene diamines having 
from 2 to 8 carbon atoms, etc. Representatives of the thermolabile amine 
salts useful in this invention are pentylamine sulfonate, 
hexamethyleneditolyl sulfate, etc. Suitable ester components according to 
the invention are secondary terpene alcohols like menthol, borneol, 
terpineol, and also other alcohols which form defined crystalline esters 
with the said acids. Especially well suited are the esters from 
bromobenzene or toluene sulfonic acid with borneol and menthol. Such 
esters can be easily recrystallized and can therefore be conveniently 
purified to remove free acids. The complete removal of the acid is a 
prerequisite so that there is no premature start of the cross-linking 
reaction. It is surprising that only crystalline esters or amines from the 
organic sulfonic acids which are not easily volatilized among them benzene 
sulfonic acid, toluene sulfonic acids, halogenated organic sulfonic acids, 
and the like, are all around activators suitable for varnishes and for 
pigmented varnishes, as well. The phosphoric acid glycidyl ester (which is 
not crystalline), for instance, activates well in varnishes formulations 
(see Example 6). In contrast thereto, in formulations containing 
commercially available titanium white pigments, the cross-linking reaction 
which takes place under the same baking conditions is insufficient. 
The splitting or decomposition temperatures of the activator according to 
this invention are in the range of between 120.degree. and 150.degree. C. 
Depending on the powder resin mixtures melting behavior, the activator 
selected may either be one of the more active type or one of the type more 
difficult to be decomposed. For instance, the decomposition point of said 
borneyl toluene sulfonic acid ester is 135.degree. C. 
The coatings according to the invention are composed of from 65 to 98 
percent by weight of binder, from 0 to 35 percent by weight of pigments, 
and from 0 to 2 percent of levelling agents and other additives. The 
binder is composed of from 83 to 93 percent by weight of resinous hydroxyl 
compounds (basis the binder), from 5 to 15 percent by weight of 
bisazlactone and from 0.25 to 2 percent by weight of activator. 
Mixing of the compositions of this invention is done at temperatures of 
about 100.degree. to 120.degree. C in, for example, a heatable screw 
extruder, followed by powderizing and recovering by screening the grain 
fraction having a size smaller than 100.mu.. The powder does not form 
lumps at storing temperatures of up to 40.degree. C, and it does not show 
any tendency to cross-linking when being heated up to a temperature of 
120.degree. C. The powder coatings compositions are applied in the usual 
way, i.e., by the electrostatic powder spray coating technique, and the 
baking is preferably done in a heated circulating air drier. Baking 
temperatures may be adjusted within certain limits by the choice of the 
respective activator. 
The invention is illustrated by way of the Examples 1-8 as set out below 
which are to be considered not limitative. All of the examples are carried 
out in the same way.