CA1043930A - Process for pigment -hydroxyalkyl (meth) acrylate copolymer-amino resin thermosetting powder coating composition - Google Patents

Abstract

Abstract of the disclosure:
A process for preparing thermosetting powder coating composition which comprises the steps of:
A) dispersing a pigment in a solution of a copolymer dissolved in an organic solvent having a boiling point of 70 to 160° C, said copolymer comprising at least one of hydroxyalkyl acrylates and methacrylates having the formula

Description

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Tbis invention relate~ to a process for preparing novel powder coating compositions.
Conventional processes for preparing powder coating compo~itions by melt kneading method with the use of heating-type roll and extruder have the drawbacks that, as compared with the praparation o~ coating compositions of the solution type, they are extremely inefficient and that color matching takes a prolonged time and involve~
inaccuracies as well as technical difficulties. In fack the~e problem~ are encountered in ensuring stable availabilitg of powder coàting compositions in large quantitie~.
An ob~ect of this invention i9 to provid~ a novel process for preparing a powder coating composition ~r~m a solven~tgpe coating aompo~ition.
Another ob~ect of this invention i~ to provide a proaess for preparin~ a powder coatin~ composition Which i8 fea8ibl~ o~ a large ~ca~e and permitR easg and accurate color matching within a ~hort period o~ time.
- 20 Ano~her obJect Or this in~ention is to make it posslble to prepare a powder ~oating composition h~vlng a high pigmont ¢onoentration and a high hidin~ pow~r.
~tlll anobher ob~ct Or this invention is to pre~are fro~ a solution-t~pe coating composibion a powder coatin~ co~position which is ~ubstantially fre~ from .
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to remain in the form of fi.ne particles free of cohesion during storage) and gives smooth-surfaced coating films.
~his invention provides a process for preparing a thermosetting powder coat;ing composition which comprises the steps of:
A~ dispersing a pigment in a solution of a copolymer dissolved in an organic solvent having a boiling point of 70 to 160 C, said copolymer comprising (i) at least one of hydroxyalkyl acrylates and methacrylates having the formula : Rl l2 CH2= C-COOCH2CH
;~ OH
wher~in Rl is hydrogen or methyl and R2 is hydrogen, methyl or ethyl and (ii) at least one of ethylenically monoun~aturated compounds, B) heatin~ the resulting dispersion at a temperature of 170 to 220 C under a reduced pressure to separate the solvent from the solid component, and C) mixing an amino resin with the solid component at a t~mperature of lower than 140 C~
~he present process has the following advantages:
. ~1) Be~ause pigment and the like additi~eg are dispersed in the copolymer solution, powder coating .;;. . .
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compositions, like usual so:Lution-type coating compositions, can be prepared with the use of ball mill or sand mill which i9 suitable ~or large--scale production. Color matching, which can be done in the same manner as solution-type compositions, takes as short a period of time as is the case with solution-type coating compositions~ ~hus the present process eliminates the serious drawback of the conventional melt kneading method for preparing powder coating compositions that the color matching procedure require~ a greatly prolonged period of time and gives an inaccurate result.
With the present invention, removal of the solvent i9 carried out at a hi~h temperature of 170 to 220 C under a reduced pre~ure and therefore takes an extremely ~hortend period of time. Because of the low Vi8C08i`t~ of the system permitting thorou6h stirring with ea6e, it is possible to use for the removal of solvent a cylindrical container which has a small surface area relative to its volume. The efficient removal of solvent and effective dispersion of pigment and like additives lead to a remarkably improved overall efficisncy as compared with the conventional melt kneading method.

(2) The process of this invention easily gives a powder ooating oomposition having a high pigment ~- concentration [for example 120 Parts by weight of pigment - , . . .
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per 100 parts by ~eight of resin (copolymer plus ~nino resin)], whereas it has been almost impossible to prepare such composition by the con~entional melt kneadin~ method.
Recently there is a growing demand for powder coating compositions capable of forming 30-to 50-micron thick coating films which are as thick as those produced from solvent-type coating compositions. To satisfy this demand, the powder coating composition must have a high pigment concentration in view of hiding po~er. In this respect, this invention is very advantageous.

(3) Complete removal of the solvent achieved by the present process results in coating compositions having high amenabilit~ to storage, namely excellent blocking resistance.
~hese adv~ntages are assured by the unique procass of this invention comprisin~ ~he steps of dispersing a plgment in a copolymer solutlon containing no curlng agant (amino ~esin), thereafter heating the dlspersion at a h~h temperature of 170 to 220 C under a reduced pressure to remove the solvent, and mixing a curing agent with the resulting solid mass at a temperature of lowe~ than 140 C, ; One of the distinct features of this inventlon is to remove the solvent at 170 to 220 C under a reduced pressure. Conventionally, a copolymer solution having glycidyl groupa or llke functional groups is heated to remove .

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, ~ . . . - , : -i(;~43~30 the solvent there~rom. To avoid the possible reaction bet~lecn thc functional groups, in this ca~e~ the eolution is h~ated to as 10~7 as 150 C~ if hi6hest. We h~ve conducte~
various researches on the method of removing tho ~olvent and found the follo~nng. When a pi~ment-~ispersed copolymer solut~on usable in this invention is heated at a temperature of not higher than 150 C, under a reduced pressure, the system becomes markedly viscous and can not be stirred thoroughly in the later stage of the solvent removing step and takes a prolonged period of ti~e for the removal of solvent. In contrast, uhen the solvent is removed at 170 to 220 C under a reduced pressure according to this invention, the system exhibits a lo~er viscosity, permitting thorough stirring for the removal of solvent within a ~hort pcriod of ti~e. Moreover, the determination of molecular weight by gel permeation chromatography and determination of hydroxyl v~lue and of acid value have revealed that unexpectedly the resulting copolymer remains free of any reaction between the functional groups thereof. Furthermore, under the conditions of this invention, the amount of the solvent remaining in the resulting coating composition can be easily reduced to not more than 0.5% by welghtO
On the other hand, when the solvent i9 remo~ed at a temperature of not higher than 150 C under a reduced pressure, it is extramely difficult to reduce the amount .

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rosidual solvent crcatcs reMarkablc improvements in the blocl~ing resistancc of coating composition ~nd the - 5 smoothness of the coating prep~ Gd therefroM.

(4) The proccss of this invention can bc practiced batch~Jise. One of the advanta~es of batch systell is that evaporation and separation of the solvent can be conducted by one step, and evacuation can be effected efficiently and easily. The second advantage is that the pigment-disperscd copolymer solution prepared in advance can be charged into the hot copolymer solution as obtained from the copolymerization process. Therefore, the rerloval Or solvent involves smaller heat requirement and can be completed in short~r period of time, Useful as the binder resin in this invention is a eopoly~er of hydro:~yalkyl acrylate or methacrylate represented by the fore~oing formula and an ethylenically monounsaturated compound.
The amount of hydroxyl groups, the functional groups of the copolymer, is the same as in usual solution-type coating compositions. Preferably the copol~mer may contain about 0075 to 2.0 moles of hydroxyl groups per kilogra~ of the copolymer. Also usable in the presell~
process is a copolymer containing, as an intramolccular --.

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catal~st, up to 0.5 mole of carboxyl 6roups per ldlogram of the copolyr;ler~ as i5 usually the ca5C ~rith solution-type coatinG cor.~positions. Such a co~olyl~ler containill~ carboxyl Group~ is obtainablc by cr.lploying acrylic acid or ~:lcth-acrylic acid ~s an ethylenically Inonou~lsaturate(l cor.l~o~
Preferably the copolymer has a softenin~ point of at least 6~ C so that the coating composition prepared therefro~ may have good storage stability at room temperature.
However, the copolymer may have a softenin~ point of less than 65 C when the coating composition is storable with refrigerationO
To enable the coating composition to retain good flowability and to give smooth-surfaced coatings~ the softening point of the copolymer is ad~antageously up to 130 C.
Exemplary of the hydroxyallsyl acrylate or methacrylate represented by the formula Rl R2 CH2 = C - COOCH2CH
OH
~Yherein Rl iB hydrogen or methyl and R2 is hydrogen, methyl or èthyl are 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 2-hydroxybutyl acrylate and 2-hydroxybutyl methacrylate. The hydroxyalkyl acrylate or mcthacrylate can be usad singly or in admixture ~th one another.

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Examples of ethylenieally monounsaturated compounds are a) acrylic compounds represented by the formula '3 H2C = CX wherein R3 is hydrogen or methyl and X is -COOH, -COOR4 or -CN, R4 being alkyl. haYing 1 to 12 carbon atoms or cyclohexyl~ and b) styrene compounds having the formula ~_- R6 = CH2 wherein R5 is hydrogen or methyl and R6 is hydrogen or alkyl having 1 to 4 earbon atoms. Examples of the acrylie eompounds are aerylie aeid, methaerylie acid, and methyl, ethyl, propyl~ i-propyl, n-butyl, i-butyl, t-butyl, ~ee-butyl~
2-ethylhexyi~ n-oetyl~ lauryl~ cyelohe~yl and llke ester~
of aerylie aeid and methaerylie aeid~ and aerylonitrile and methaerylonitrile, Example~ of the styrenQ compounds are ~tyr~ne; a-~methylstyrene~ vinyltol~ene~ t-butylstyrene and the like. ~he acrylic ¢ompound~ can be u~ed singly or in admixture with one another.
~ he copolymer can be prepared by various polymeri-zation mqthods such as solution polymerization, block polymerization, emulsion polymerization, s~spension polymeriza~:ion and lilse methods. Because of the necessity to prepare a eopolymer &olution and for the ease of control of the polymerization reaction~ solution polymerization method i8 preferable in fulfilling the ob~eets of this - .
:` ' ~'"'`"' - '' . ' ... ' ~, . , ' ~ ~ 3~ ~0 invention. Useful solvents for solution polymerization are those having a boiling point of 70 to 160 C and include, ~or example, benzene, toluene, xylene, ethyl acetate, butyl acetate, amyl acetate, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether, methyl ethyl k~tone, methyl butyl ketone~ methyl isobutyl ketone~ dioxane, etc. Generally solution polymerization may preferably be conducted under reflux. The polymeriæation is carried out in the presence of a radical polymerization initiator widely used for the polymerization of vinyl compounds~ such as benzoyl peroxide, lauroyl peroxide~
tert-butylhydroperoxide, t-butylperoxy benzoate, acetyl-cyclohexanesulfonyl peroxide, isobutyroyl peroxide~ di~
(2~ethylhexyl) peroxydicarboxylate~ dii~opropyl peroxydicar~oxylate~ tert-butyl peroxypivalnte~ decanoyl peroxide~ ~zobisisovaleron~trile~ azobisisobutyronitrilc~
et¢, According to this invention, a pigment is dispersed - in a solution o~ the copolymer in the first step, When the copolymer is obtained by solution polymerlzation, the copolymer solution is usable as it is or a8 suitably diluted or concentrated. When it is prepared by some other poly-merization method, the resulting copolymer is dissolved in a solvent to prepare a copolymer solution. Uséful solvents are simllar tothoseusAble for solution polymerization.

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The concentration of the copolymer solution, although vlidely variable, is suitably 40 to 70 ,' by ~eight.
~mployable as pigmcnts are a ~Jidc variety o~
those used in solution-type coatin6 composltions and po~der coating compositions. DepcndinG on the particular color deslred, the pigment can be used in an amount of 3 to 150 parts by weight per 100 parts by weight o~ the copolymer and amino resin combined. Furthermore, copolymer solutions containin~ di~ferent pigments dispersed therein may be mixed together in this step, whereby color matching can be done às easily as with general solution-type coating composltions. In addition to pi~ment, also usable arc various additives such as levellin~ agent, antistatic agent and the lilte as desirod, It is also possible to suitably add to tho dispersion the ~oregoing solven~ ~or viscosity ad~ustment or the copolym~r solution for the ad~ustment of the plgment concentration.
In the follo~ring second step, the dispersion is placed into a container equipped with a stirrer, a Z0 concurrent condenser and evacuating means and i8 preferably preheated with stirring to about 170 C at atmospheric pressure to remove ~he solvent. Subsequently, the residual solvent is removed at a temperature of 170 to 220 C at a reduced pressure. In this step, the solvent is evaporated and separated at the same time. Thus the solvent can be . . . . . .

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~0 ~ 39 30 substa~tiall~ completcly separated from thc solid component by a sin~lc step. Because thc present proccss can be practiced batchwise, evacuation c~n bc effected efficiently and easily. In this step, it is critical to heat the dispersion at a temporaturc of 170 to 220 C uncler a reduced pressure. This enables the sol~ent to be separated from the solid com~onent substantially `complctely. ~t temperaturcs lower than 170 C, incomplete se~aration of the sol~ent ~ill result to reduce the blocl~ing resistance of the coatin~ co~lposition, whereas at temperatures hi~her than 220 G similarly lead to lo~Jer blockin~ resistance due to the decomposition of thc copolymer. Espocially pr¢ferable tempernture range is from 175 to 210 C. Preferable pressur¢ is lowcr than about 70 nln~Ig, mo~t prefel'~ble being 10 to 40 mmHg.
When sufficiently freed of the solvent, the solid component is placed, for the third stepj into a heatin~
type mi~er such as heating kneader, e.truder or the like, in which it is uniformly mi.~ed ~ith an amino resin in a ~ `
molten state. Catalyst can be added thereto, if requiredO `~
The temperature is higher than that necessary for melting the solid component, but lower than 140 C. Mixing at temperaturcs higher than 140 C is objectionable since the copolymer tends to react ~th the amino resin.
Preferable temperature is in the ran~e of 100 to lZ0 C. ~`

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10~3930 The mixture is thereafter pulveri~ed ~nd then screened to obtain a powder coatin~ composition in the form of fine particles passing through a 150-mesh screen, Exemplc~ry of the c~ino resin are alcohol-modi~ied melamine-~ormaldehyde condensation products such as hexc~nethylolmel~line derivatives.
Thc he.camethylolmelamine derivatives include, for ex~lple~ hexakismethoxymethyl melamine and etherified products of hexamethylolmelamine which have CII30- and R0- groups substituted for the hyAroxyl group in the methylol group of hexamethylolmelamine~ the average total number of the CEI30- and R0- groups being 5.5 to 6~0 per : melamine nucleus, the avera~e number o~ the R0- group bein~ 0.5 to 3.0 per mel~line nucleus, wherein R is R" C~I~-CH2 Rl0CHCH20- group or C~I2 C~-0- group, Rl being an CH2~CH2 . alkyl group having 1 to 4 carbon atoms and Rll being hydrogen atom or methyl group.
The copolymer/amino resin ratio is preferably 96/4 to 80/20 by weight. The kno~m acid catalysts to be used when the copolymer and amino resin are mixed together as above include, for example, amine salts of paratoluene-sulfonic acid, phosphoric acid~ dichloroacetic aGid~ etc.
This invention will be described below ~th reference to Reference Examples and Examples, in ~/hich parts are by weight.

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Reference ~xamPle 1 ~ oluene (800 parts) i 9 placed into a 2-liter four-necked flask equipped with a reflux condenser,a stirrer and a dropping funnel and is heated to 90 to 100 C while introducing nitrogen gas into the f}ask.
To the toluene maintained at the same temperature is added dropwise over a period of 2 hours a mixture of 120 PartS of 2-hydroxypropyl methacrylate, 40 parts of butyl acrylate, 62~ parts of ethyl methacrylate, 16 parts of acrylic acid and 10 parts of benzoyl peroxide, the mixture being pre~iously placed into the dropping funnel. To the resulting mixture maintained at the same temperature, a solution of 1 part of benzoyl peroxide in 10 parts of toluene i9 added dropwise in three portions at an interval of 1 hour. (~he catal~st thus added dropwise Will be hereinafter referred to as "additional aatalyst".). ~he mixture is maintained at 90 to 100 C for further 3 hours and then cooled to obtain a copolymer solution (hereinafter referred to as "Copolymer solution-A"). ~he copolymer has a so~tening point of 86 C and an acid value of 18.5 and contains 1.04 moles of free hydroxyl groups per kilogram of the copolymer.

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Reference Example 2 Styrene 248 parts Methyl methacrylate2~0 partq Butyl methacrylate206 parts 2-Hydroxyethyl methacrylate78 parts Methacrylic acid 8 parts Azobisisobutyronitrile16 Parts ~he above mixture is copolymerized in 800 parts of toluene at a temperature of 100 to 110 C.
In the same manner as in Reference Example 1, a mixture ~ of 30 parts of toluene and 3 parts of azobisdimethyl-- valeronitrile serving as an additional catalyst is added in three equal portions to the reaction mixture.
The other reaction procedures and conditions are similar to those of Reference Example 1 to obtain a copolymer solution (hereinafter referred to as "Copolymer ~olution-B"). ~he resulting copolymer has a softeniDg point of 101 C and an acid value of 7.6 and contains 0.922 mole of free hydroxyl groups per kilogram of the copolymer Reference ExamPle 3 -Styrene 305 parts ~thyl acrylate 200 parts Ethyl methacrylate135 parts 2-Hydroxyethyl methacrylate160 parts t-Butyl peroxybenzoate3 parts 10~3~30 The above mixture is copolymerized in 800 parts of butyl acetste at a temperature of 110 to - 120 C, using ~s an additional catalyst a`mixture of 12 parts of t-butyl perox~benzoate and 30 parts of butyl acetate. The other reactio~ procedures and conditions are exactl~ the same as those of Reference Example l to obtain a copolymer solution (hereinsfter referred to as "Gopolymer solution-C").
The resulting copolymer has a softening point of 78 C and an acid value of 0.5 and contains 1.54 moles of free hydroxyl groups per kilogram of the copolymer.
Reference Example 4 Ethyl ~ethacrylate 592 parts 2-Hydroxyethyl methacr~late 80 parts 2-Hydroxypropyl methacr~late 120 parts Acrylic acid 8 Parts Benzoyl peroxide 3 parts `
~he above mixture is copolymerized in 800 parts of toluene at a temperature of 110 to 120 C, - `
using as an additional catalyst a mixture of 12 parts of azobisisovaleronitrile and 30 parts of toluene.
- The other reaction procedures and conditions are - -exactly the same as those of Reference Example 1 to obtain a copolymer solution (hereinafter referred to as "Copolymer solution-D"). ~he resulting copolymer .

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Reference ~@~e~
Ethyl methacrylate 512 parts Isobutyl acrylate 100 parts 2-Hydroxyethyl methacrylate170 parts Methacrylic acid 18 parts t-Dodecylmercaptan 12 part~
Benzoyl peroxide 3 parts The above mixture i~ copolymerized in 800 parts of methyl isobutyl ketone at a temperature of 110 to 120 C, using as an additional catalyst a mixture of 16 parts of benzoyl peroxide and 30 parts Or methyl isobutyl ke~one. The other reaction procedur~s and conditions are exactly the ~ame a~ those of Reference Exa~ple 1 to obtain a copolymer solution (hereinafter referred to as "Copol~mer solution-E"). ~he resulting copolymer has a softening poinb Or 70 C and an acid value of 18.2 and contains 1.63 moles of free hydroxyl groups per kilogram Or the copolymer.
Reference Exam~le 6 Styr~ne 720 parts 2-Hydroxyethyl methacrylate78 part~
Acrylic acid 16 parts t-Butyl peroxybenzoate 3 parts .
, . .

The above mixture is copolymeri~ed in 800 Parts of toluene at a temperature of 110 to 120 C, using as an additional catalyst a mixture of 12 parts of t-butyl perox~benzoate and 30 par-ts of toluene.
The other reaction procedures and conditions are exactly the same as those of Reference Example 1 to obtain a copolymer solution (hereinafter referred to as "Copolymer solution-~").
~he resulting copolymer has a softening point of 116 C and an acid value of 18.~ and contains 0.922 mole of free h~droxyl groups per kilo~ram of the copolymer.
Exam~le 1 A 150 g quantity of Copolymer ~olution A prepared in Refarence Example 1, 300 g of rutile-type titanium dioxide, 75 g of toluene and 6 g of polylauryl methacrylate (leveling agent) are charged into a l-liter pebble mill and dispersed for 15 hours to prepare a disperson, to which 150 g o~ Copolymer solution-A
(hereinafter referred to as "First additional copolymer solution-A"~ is added. ~he mixture is similarly disper~ed for further 30 minutes to prepare a pigment dispersion.
~ he pigment dispersion (681 g~ and 240 g of Copolymer solution-A (hereina~ter referred to as "Second additional copol~mer solution-A") are placed .

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into a 1.5-liter container equipped with a concurrent condenser, a stirrer and a thermometer and are heated with stirring under atmospheric pressure to distil off the solvent. The temperature of the contents rises with the outflow of the solvent, and when the temperature reaches 170 C, the system is maintained at a reduced pressure of 30 mm of mercury for 15 minutes to remove the solvent. ~he pigment-dispersed solid resin obtained contains 0.3~' by weight of residual solvent.
~o 384 g of the solid resin is added 20 g of melamine resin in which one of the methoxy groups of hexakismethoxymethyl melamine is replaced by ~ CH2 CH2 CH2 ~CH- 0- group (hereinafter referred to as "Amino resin-A"), and the mixture is kneaded by kneader of the heating type at 110 C for 10 minutes.
~he resulting mass is pulverized and then screened to obtain a coating composition in the form of fine particles passing through 150-mesh screen. The coating composition has a pigment/resin ratio by weight of 100/100 and a copolymer / amino resin ratio by weight of 90/10.
Examples~ 2 to 6~ and ComE~arison Examples 1 and 2 Pigment-dispersed solid resins are prepared in the same manner as in Example 1 except for the conditions listed in ~able 1 below.

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1() ~ 3~ 3~) Seven kind of coating compositions are prepared using the above pigment-dispersed solid resins in the same manner as in Example 1 except for the conditions listed in Table 2 below.
~ Table 2 ~ ' Pigment- Pigment~ Copolymer/
di~persed Amino resin resin amino resin resin tg) (wt.ratio) ~wt. ratio) EXample 2 Resin-2 20 100/100 90/10 A ~
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Example 3 Res n-3 0.2 5/100 95/5 Example 4 Re n-4 10 10/100 90/10 ;
Resin-5 Example 5 408 A 120~100 80/20 Example 6 Re n-~ 60 25/lQ0 80/20 Comp.Ex.l Re38n_7 2oA 100/100 90/10 Comp Ex 2 Re n-8 20 100/100 90/10 . . . :
Note: ~ : Morpholine salt of paratoluenesulfonic acid.
: Amino resin prepared by substituting CH3CH-OCH2-CH2-0- group for l mole of methoxy - group of hexakismethoxymethyl melamine;

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~ 3 Comparison E:~ampl~ ~
A 681 g quantity of the pi~ment dispersion prepared in Example 1, 240 g of Copolymer solution-A
and 30 g of Amino resin-A are mixed to6ether and placed into a flat-bottomed container to a depth of about

5 cm. The mixture is dried in a vacuum dryer at 30C
for 25 hours. The dried mass contains 1.8,' by weight of resi~ual solvent. The solid mass is pulverized and then screened to obtaina powdery coating composition ;
passing through a 150-mesh screen.
Com~arison Example The same mixture as used in Comparison Example 3 is dried under reduced pressure at 60C.
In 5 hours ~t~r the initiation of vacuum dryin6~
the mixture ~els. The gel contains 12~ o by weight of residual solvent ~ Com~ari 50n ~ample ~
-Exactly in the same manner as in Example 1, 1,000 g of Copolymer solution-A is freed of the solvent to ohtain a solid resin containing 0.2~ by weight of residual solvent. The solid resin (180 g), 200 g of rutile-type titanium dio.{ide, 20 g of Amino resin-A
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and 4 g of polylauryl methacrylate are mixed togecher by heating-type roll at 100C ~or 30 minutes to prepare a dispersion~ which is pulveriæed and then screened to obtain a powdery coating composition passing through a 150-mesh screen, The composition has a pi~ment/resin ratio by ~veight of 100/100 and a copolymer/amino resin ratio by weight of 90/10. `
Exam~le 7 This is an example on a larger scale, A 2,000 kg quantity of Copolymer solution_A, 4,000 kg of rutile-type titanium dioxide, 60 ~g of polylauryl~ methacrylate and 750 kg of toluene are placed into a lO~cl pebble mill and treated for 15 hours to prepare a dispersion, to which 1,000 kg of Copolymer solution-A is thereafter added. The mixture is further treated for 30 minutes for disperslonO A 7,310 k~ portion of the resultlng pigment dispersion and 4,200k g of Copolymer solution-A
are charged into a 15-kl cylindrical container tl m in diameter and 5 m in height) equipped with a stirrer, a thermometer and a concurrent condenser, and the solvent is removed from the mixture exactly in the same manner as~ in Example 1. The removal of the solvent takes 2 hours. The mass obtained in a yield of about -21~-~ ,, .
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:-~043930 90~ by weight contains 0.2,h by wei~ht of residual solvent~
A 6,894 kG quantity of the pigment-dispersed solid resin - thus prepared and 360 k6 o~ Amino resin-~ are placed into a kn~ader of the heating type and mi~ed together with heating for 10 minutes. The mi~ture is then pulverized and screened to prepare about 7,200 kg of a powdery coating composition passing through a 150-mes~ screen and having a pigment/resin ratio by ~eight of 100/100 and a copolymer/amino resin ratio by ~reight of 90/10, 10 Example 8r Toluene (800 parts) is placed into a 2-liter `
four-necked flask equipped ~vith a reflux condenser, a stirrer and a dr~pping funnel and is heated to 90 to 100C while introducing nitro~en gas into th3 flask, To the toluene maintained at the same temperatur~ is added drop~se` o~er a period of 2 hours a mixture of 120 parts of 2-hydroxypropyl methacrylate, 40 parts of butyl acrylate, 624 parts of ethyl methacrylate, 16 parts of acrylic acid and 10 parts of benzoyl peroxide, the mixture being previously placed into the dropping funnel. To the resulting mixture maintained at the same temperature, a solution of 1 part of benzoyl peroxide in 10 parts of toluene is added dropwise in three portions -at an interval of 1 hour. The mixture is maintained ~ .
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at 90 to 100C for further 3 hour6~ to ~Yhich is added at the same temperature 5664 parts of a pigment dispersion comprising 1600 parts of Copolymer solution-~ prepared in Reference Example 1, 3200 parts of rutile-type titanium dioxide, 800 parts of toluene and 64 parts of polylauryl methacrylate.
The resultant mixture is heated with stirring under atmospheric pressure to distil: off the solvent.
: The temperature of the contents rises with the outflow of the solvent, and when the temperature reaches 170C, the system is maintained at a reduced pressure of 30 mm of mercury for 15 minutes to remove the solvent, The pigment-dispersed solid resin ontained contains Q.3 by weight o~ residual solvent.
To 384 g of the sol~d resin is added 20 g of melamine rosin in which ono of ~he methoxy groups of hexa~ismethoxymethyl melamine i~ replaced by ~' H2 H2 C C

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and the mixture is kneaded by kneader of the heating type at 110C ~or 10 minute6. ~he resulting mass i8 pulverized and then screened to obtain a coating , _26-: '. - ' ' ~ .. ., ' ~ . :, . - .
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10~3~30 composition in the form of fine particles passing through a 150-mesh screen. The coating compositiOn has a pigment/resln ratio by wcight of 100/100 and a copolymer/amino resin ratio b~ weight of 90/10.
Comparison E am~lQ ~
This is an example on a larger scale, Exactly the same procedure as in E~ample 7 is ollo~ed except that the vacuum treatment is conducted at a temperature of up to 150C. Removal of the solvent takes 6 hours to give, in a yield of about 75% by weight, a pigment-dispersed solid resin containing 0.9,' by weight of residual solvent. The powdery coating composition obtained in a yield of 6,000 kg has a pigment~resin ratio by weight of 100/100 and a copolymer/amino resin ratio by weight of goho.
Comparion Example 7 This is an example on a larger scale. A 6,300 kg quantity of solid resin ~ containing 0.2% by waight of residual solvent ) prepared using tha same apparatus as employed in Example 7 and removing the solvent under the same conditions as in Comparison Example 4, 7,000 kg of rutile-type titanium dioxide, 700 kg of ~27-.

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' ' ` ~: ' ~0 ~ 3 ~ 30 A~ino resin-A and 105 kg of polylauryl methacrylate are mixed togethcr by Icncader of the heatin~ type at 100C
for 30 ~inutcs, and thc mi~ture is theroaftcr trcatcd at 100C at a rate of 500 li~/hr, in Buss ICo-ICneader ~Iodel P~-14, product of Buss Ltd., S~t~crlan~, olle of the lar~cst mclt ~ncader presently available to pre-pare a dispcrsion. The resultinG mass is pulvcrizcd and then screened to obtain about 14,000 kg of a po~vdery coatin~ composition passing throu~h a 150-rn~sh screen and having a pi~ment/resin ratio by wei~ht of 100/100 and a copolymer/amino resin ratio by weight of - 90/10.
There are ~de differences between E.cample 7 and Comparison Example 6 in thc time required for removin~ the solvent and in the amount of residual sol-ventS indicatin~ tho superiority of the former.
Simple calculations show that the preparation of 1 ton of coating composition takes about Z.5 hours according to Example 7 and about 2.2 hours accordin~
to Comparison Example 7, seemingly indicating no -~
substantial difference in productivity betwcen the two. ~nlereas dispersion by ball mill rcquirea no - labor durins operation and involves low clectric -~
powor conswnption, removal of solvcnt and the stel) of .. .

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dispersing pigment by melt kneader need much labor and high electric power consumption. For these steps, the preparation of 1 ton of~ow~cr coating composition takes about 10 minutes according to E~ample 7, in sharp con-trast ~th about 120 minutcs accordin~ -to Co~lparison Example 7. Thus there exist wide differences between the two in the preparation of powder coating composition, one bein~ superior to the other in pro-ductivity.
Steel panels treated with phosphoric acid and havin~ a thickness of o.8 mm are electrostatically coated in usual manner with the powder coating com-positions o~ Examples and Comparison ~xamples to stepwise varying thicknesses and to a curcd film thicl;ness of 80~. The coated panels are bal~ed at 180C for ~0 minutes to prepare test piec~s.
Properties of the powder coating compositions and coating films are determined in accordance with the following methods with the results shown in Table 3 belo~r (1) Blocking resistance:
The powder coating composition is subjected to a load of 30 g/cm2 vlhile being maintained at 40~0.5C for 120 . - . . .
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hours and is thereafter inspected~
(2) Gloss of film :
JIS K-5400 6.7.
(3) Erichsen test :.
, The coated plate is placed in a constant temperature and humidity chamber l;ept at 20C and a humidity of 75/,' for one hour.
Thereafter, the plate is set on Erichsen testin~ machine u/ith the coatin~ positioned outside. A punch ha.ving a radius of 10 mm - is pushed outward predetermined distancesin contact ~th the rear face of the plate at as uniform speed as possible of about 0,1 mm/sec. The pushed out portion of , I the plate is checlced by the nalced eye .
~or crac~ing or ~Qeling immedi.ately after , pushing out to determine the maximum distance (mm) of stroke of the punch causing no changes on the coat~n~.
(4) Poppin~ film thickness : -Coating film is observed ~lith the naked ~ -~
- ` eye to determine the maximum coatinG
- . f:Llm thickness free of popping. -~ -~ . . . ~- .
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: . , Ir (~lo~s Specir,len value Erichsen Blochin~ Popping~ :Eilm No. (60 ~loss) test(nnm? rcsist~nce t~ ckncsst~) Example 1 92 ~ 7 Good 150 2 93 ~ 7 lL~5 3 97 ~7 " 135 4 95 ~7 " 1l~0 > 7 " 150

6 93 >7 " 150

7 93 ~7 " 150 93 '7 " 150 Comp.Ex. 1 92 ~7 Poor 85 2 . 89 .~ 7 " 9 3 92 >7 " 7 17 2,5 Good 70 6 91 ~ 7 Poor 80 7 15 205 Good 75 - . . . .

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~ 043Y~30 lith solvent kne.ldin~ matllod, compositions ~th a hi~h pi~ment concentratlon fail to ~ive any 610ssy coatin~ surface as seen in Comparison E~amples 5 and 7. Comparison Examples 1,2,~ and 6 reveal that when the solvent is removed at low tem~peratures and e~cessi~ely high te~peratures, the resultin~ coating compositions have poor blockin6 resistance.

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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A process for preparing thermosetting powder coating composition which comprises the steps of:
A) dispersing a pigment in a solution of a copolymer dissolved in an organic solvent having a boiling point of 70 to 160° C, said copolymer comprising at least one of hydroxyalkyl acrylates and methacrylates having the formula wherein R1 is hydrogen or methyl and R2 is hydrogen, methyl or ethyl and at least one of ethylenically monounsaturated compounds, B) heating the resulting pigment dispersion at a temperature of 170 to 220° C under a reduced pressure to separate the solvent from the solid component, and C) mixing an amino resin with the solid component at an elevated temperature of lower than 140° C.

2. The process according to Claim 1, in which said copolymer contains about 0.75 to 2.0 moles of hydroxyl groups per kilogram of the copolymer.

3. The process according to Claim 1, in which said ethylenically monounsaturated compound is at least one of acrylic compounds and styrene compounds, said acrylic compounds having the formula of wherein R3 is hydrogen or mehtyl and X is -COOH, -COOR4 or -CN, R4 being alkyl having 1 to 12 carbon atoms or cyclohexyl, said styrene compounds having the formula of wherein R5 is hydrogen or methyl and R6 is hydrogen or alkyl having 1 to 4 carbon atoms.

4. The process according to Claim 3, in which said ethylenically monounsaturated compound is at least one of acrylic compounds having the formula of wherein R3 is hydrogen or methyl and X is -COOH, -COOR4 or -CN, R4 being alkyl having 1 to 12 carbon atoms or cyclohexyl.

5. The process according to Claim 3, in which said ethylenically monounsaturated compound is at least one of styrene compounds having the formula of wherein R5 is hydrogen or methyl and R6 is hydrogen or alkyl having 1 to 4 carbon atoms.

6. The process according to Claim 1, in which said temperature for heating the pigment dispersion is in the range of 175 to 210° C.

7. The process according to Claim 1, in which said reduced pressure is lower than about 70 mm Hg.

8. The process according to Claim 7, in which said reduced pressure is in the range of 10 to 40 mm Hg.

9. The process according to Claim 1, in which said pigment dispersion is preheated to about 170° C at atmospheric pressure to remove the solvent and further heated at 170 to 220° C under a reduced pressure to remove the solvent substantially completely.