CA2115763A1 - Thermosetting powder coating compositions - Google Patents

Abstract

Provided are novel thermosetting powder coating compositions comprised of an aliphatic polyester, an aromatic polyester, and a self-blocked polyisocyanate. Also provided are shaped or formed articles coated with these compositions and cured.

Description

~093J04102 PCT/US92/07083 - ~

- 1 - 2 1 1 ~ 7 6 ~

THERMOSETTING POWDER COATING COMPOSITIONS

Field of the Invention S This invention relates to certain novel thermo-setting powder coatin~ compositions. More particularly, this invention provides a composition comprising a blend of an aromatic polyester, and aliphatic polyester, and a self-blocked polyisocyanate.
Background of the Invention Thermosetting powder coating compositions are u~ed extensively to produce durable protective coatings on various-materials. Thermosetting coatings, when compared to csatings derived from thermoplastic composi-tions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal -~
~ substrates, and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable :~
characteristics, good smoothness and flexibility.
Coatings prepared from thermosetting powder composi-tions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough or non-uniform finish. Such a coating :`
surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The rough or non-uniform surface problem has caused thermo-setting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems sometimes occasioned :
by the evaporation of the solvent system. Solvent-based . ~:
coating compositions also suffer from the disadvantage ~``

W093/04102 2 1 1 :3 7 ~ ~ PCT/US92tO7083 ~

of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does not contact the -~
article or substrate being coated obviously cannot be reclaimed. -To produce smooth, glossy,uniform coatings, the -polymeric materials constituting powder coating composi-tions must melt within a particular temperature range to permit timely and ample flow of the polymeric material - -prior to the occurrence of any significant degree of curing, i.e., cross-linkin~. Powder coating composi-tions which possess the requisite melting range provide smooth and glossy coatings upon being he~ted to cure the -compositions. In addition to b~ing smooth and glossy, , coatings derived from thermosetting coating compositions should exhibit or possess good impact strength, hard-ness, flexibility, and resistance to solvents and chemicals. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles wherein the sheet metal is flexed or bent at various angles.
It is essential that powder coating compositions remain in a free-flowing, finely divided state for a i reasonable period after they are manufactured and packaged. Thus, amorphous polyesters utilized in powder coating formulations desirably possess a glass transi- `
tion temperature (Tg) hi~her than the storage tempera- ~ ;
tures to which the ~ormulations will be exposed. Semi-crystalline polyesters and blends thereof with amorphous polyesters also may be utilized in powder coating -. ,.-2 i I ~-s 7~

formulations. For this application, semi crystalline polyesters desirably possess a significant degree of crystallinity to prevent caking or sintering of the powder for a reasonable period of time prior to its application to a substrate. Semi-crystalline polyesters used in powder coating formulations also must have melting temperature low enough to permit the compounding of the powder coating formulation without causing the cross-linking agent to react prematurely with the polyesters. The lower melting temperature of the semi- ;
crystalline polyester also is important to achieving good flow of the coating prior to curing and thus aids the production of smooth and glossy coatings.
Finally, the production of tough coatings which are ~ ~-resistant to solvents and chemicals requires adequate ;
cross-linking of the powder coating compositions at curing temperatures and times c0mmonly employed in the industry. In the curing of powder coating compositions, a coated article ty~ically is heated at a temperature in the range of about 325 to 400F (163-204C) for up to about 20 minutes causing the coating particles to melt and flow followed by reaction of the cross-linking (curing) agent with the polyester. The degree of curing may be determined by the methyl ethyl ketone rub test described hereinbelow. Normally, a thermosetting coating is considered tv be complet~ly or adequately cross-linked if the coating is capable of sustaining `;
200 double rubs. It is apparent that the use of lower temperatures and~or shorter curing times to produce adequately cross-linked coatings is very advantageous since higher production rates and~or lower energy costs can be achieved thereby.
Powder coating systems based on hydroxyl polyesters and caprolactam-blocked polyisocyanate cross-linking agents have been used extensively in the coatings WO93/04102 PCT/US92/0708~ ~

~ fi~ - 4 -industry. The most widely used caprolactam-blocked polyisocyanates are those commonly referred to as ~-caprolactam-blocked isophorone diisocyanate, e.g., those described in U.S. Patents 3,822,240, 4,150,211, and 4,212,962. However, the products marketed as ~-caprolactam-blocked isophorone diisocyanate may consist primarily of the blocked, difunctional, monomeric isophorone diisocyanate, i.e., a mixture of the cis and trans isomers of 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional trimer thereof or a mixture of the monomeric, dimeric and~or trimeric forms. For example, the blocked polyisocyanate compound used as the cross-linking agent may be a mixture consisting primarily of the ~-caprolactam-blocked, ~ `
difunctional, monomeric isophorone diisocyanate and the ~-caprolactam-blocked, trifunctional trimer of isophorone diisocyanate. The reaction of the isocyanato `
groups with the blocking compound is reversible at -elevated temperatures, e.g., about 150C and above, at which temperature the isocyanato gxoups are available to ~ ;
react with the hydroxyl groups present on the polyester to form urethane linkages, thereby cross-linking or curing the coating composition.
During the curing process using an ~-caprolactam-blocked polyisocyanate as described above, ~-caprolactam is liberated from the powder coating compositions. To eliminate th~ presence of ~-caprolactam from the work-place, adducts of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and diols have been developed for use as cross-linking agents in powder coating compositions. Such adducts and powder coating composi- `
tions containing the adducts are described in the literature such as, for example, U. S. Patent 4,413,079, German OLS 3,328,133, and the Journal of Chromatography, WO93/04102 PCT/US92/0708~
7 ~ 3 472 (1989) 175-195. While these oligomeric cross-linking agents avoid the liberation of ~-caprolactam, they possess the disadvantage of not being as reactive as the ~-caprolactam-blocked polyisocyanates when used in combination with commercially-available, amorphous polyesters. Thus, powder coatinqs based on amorphous polyesters commonly used in the powder coating industry and adducts of the 1,3-diazetidine-2,4-dione dimer of -isophorone diisocyanate must be heated at higher temperatures, e.g., 400F (204C) as compared to 350F
(177C) for caprolactam-blocked isophorone diisocyanate, and~or for longer periods of time to provide adequately cured coatings. However, the use of such higher temperatures does not produce the degree of cross- :~
linking necessary to impart to the cured coating a --~
satisfactory combination of properties, especially resistance to chemicals and solvents.
Further, the use of these self-blocked isocyanates also generally provides a coating having poorer physical properties, such as impact resistance, than do the corresponding compositions cross-linked with caprolactam-blocked isophorone diisocyanate.
We have discovered that an aliphatic polyester derived from 1,4-cyclohexanedicarboxylic acid and 1,4-butanediol can be blended with an aromatic polyester and formulated into a powder coating with a self-blocked isocyanate and which has better cure and physical properties than coatings not containing the aliphatic I polyester.
Brief Descri~etion_of~the Drawing . . ~
Figure 1 is a graph of Q W weathering of the powder coatings of Comparative Example 1, and Examples 2 and 3, all of which are described in the Experimental Section `~"
; -'` ~
, ~ .

W O 93/04102 PC~r/US92/07083 2 ~ ~ 3 7 ~JY~

below. ~Q W " weathering is performed by exposing the coating to high intensity ultraviolet radiation, thereby simulating performance of the coating in the presence of sunlight, albeit on an accelerated basis. The plot indicated by bold dots is the coating of Comparative Example l; the plot indicated by open-circled points is ` ;
the coating of Example 2; and the plot indicated by "triangle-points" is the coating of Example 3. Percent gloss retention at 60~ is plotted versus time in hours.
This plot illustrates the unexpectedly superior weathering properties of the compositions of the present -`~
invention relative to compositions containing no aliphatic polyester component. ~ ~

-:: :` `
_ummary of the Invention ~-. .
., ~.
The present invention provides a novel therm~
setting powder coating composition comprised of a blend of an aromatic polyester and an aliphatic polyester, poly(tetramethylene trans-1,4-cyclohexane-dicarboxylate) and a self-blocked crosslinking agent. The cross-linking agent is an adduct of 1,3-diazetinine-2,4-dione and a diol. The powder coating compositions of the `~
present invention and the coatinqs derived therefrom were found to possess superior cure and physical properties, especially weather resistance.
,. ~
Detailed Description of the Invention !
The present invention provides a thermosetting powder coating composition which comprises an intimate blend of (1) A novel blend of polymers containing free hydroxy groups comprised of:
, :
.: . .' W O 93/04102 PC~r/US92/07083 - 7 - 2~ ~ j7~

(a) about lO to about 80 weight percent of an aromatic polyester having a glass - -~
transition temperature (T~) of greater than about 40C., a hydroxyl number of about 20 to 200 and an inherent viscosity of about 0.1 to about 0.5; and ;;~:

(b) about 90 to about 20 weight percent of ~ ;
poly(tetramethylene trans 1,4-cyclo- ~ :
hexanedicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.5;
and `-(2~ a cross-linking effective amount of an adduct of the ~,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and a diol having the structure :

ocN-Rl~x-Rl~NH-~-o-R2-o-~-NH-Rl3x-Rl-Nco n wherein Rl is a divalent l-methylene-1,3,3-trime~hyl-5-cyclohexyl radical, i.e., a radical having -:~
the structure CH3\ ~-~
CH3f!, !
4~
c~3 CH2 .. :
R~ is a divalent aliphatic, cycloaliphatic, araliphatic or aromatic residue of a diol; and : :~

~ 1~ .j, t~ ~.

X is a 1,3-diazetidine-2,4-dionediyl radical, i.e., a radical having the structure ;~

. ~ .
wherein the ratio of NCO to OH groups in the formation of the adduct is about 1:0.5 to 1:0.9, the mole ratio of diazetidinedione to diol is from 2:1 to 6:5, the content of free ;~
isocyanate groups in the adduct is not greater than 8 weight percent and the adduct has a .
molecular weight of about 500 to 4000 and a .
melting.point of about 70 to 130C. `~`~

The powder coatings of the present invention ~ !
30 ' provide coatings with improved impact streng~h, improved ~."
flexibility, improved weatherability, anG a higher degree of cure at a lower cure temperature than a system -without the aliphatic polyestex. :~
In the above composition, both the aromatic poly-ester and the aliphatic polyester may be produced using well known polycondensation procedures. ~ .
Poly(tetramethylene tra~s-1,4-cyclohexanedicar~
boxylate) may be prepared from 1,4-butanediol and the :. -acid or diester of trans-1,4-cyclohexanedicarboxylic --acid. When the diester is used, it is preferred that some excess glycol is utilized during the ester interchange reaction and is removed under reduced pressure until the desired viscosity is obtained. . ;:
The preferred aliphatic poly(tetramethylene trans- .
1~4-cyclohexanedicarboxylate~ polyester has a Tm in the -~
range of about 110 to 160C, a hydroxyl number in the ~ . , 2 1 1 t'i 7 ~

range of about 25 to 65, an acid number of not more than 10, and an inherent viscosity of about o.lo to 0.40. ~ `~
The aliphatic polyester component may also contain a polyol branching agent such as trimethylolpropane, to increase the crosslink density of the coating. As a preferred embodiment of the present invention, up to -~
about 10 mole percent of the 1,4-butanediol is replaced ~-~
with a glycol having 2 to 12 carbon atoms. Examples of such glycols include ethylene glycol, propylene glycol, ~ -1,3-propanediol, 2,4-dimethyl-2-ethylhexane-1,3-diol, ?~
2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, thio~
diethanol, 1,2-, 1,3- and 1,4-cyclohexanedimethanol, ~;
15 ~ 2,2,4,4-tetramethyl-1,3-cyclobutanediol, 1,4-xylylene-dio~l and the like. When trans-1,4-cyclohexane~
dicarboxylic acid is referred to herein, it is intended to denote a mixture comprised of at least 70% trans , ~ ~
isomer.
The preferred aromatic polyester component of the -composition provided by this invention has a ~ greater than 55C., a hydroxyl number in the range of about 25 ~`
to 80, an acid number of not more than 15, and an inherent viscosity of about 0.15 to 0.4. The aromatic polyester component preferably is comprised of (1) diacid residues of which at least 50 mole percent are terephthalic acid residues, (2) glycol residues of which at least 50 mole percent are derived from 2,2-dimethyl-~l 1,3-propanediol and (3) up to 10 mole percent, based on the total moles of t2) and (3), of trimethylolpropane residues. These pre~erred aromatic polyesters are comercially available under the tradenames Rucote 107 ~ "
and Cargill Resin 3000. `
A further preferred aliphatic poly(tetramethylene ~' ' ;`','' '~`''.

WO93/04102 PCT/US92/0~083 ~

2 l 1.3 7 ~ 3 - lo : ~

trans-1,4-cyclohexanedicarboxylate) has a hydroxyl number of about 20 to 200, and an inherent viscosity of about O.l to 0.5; wherein up to lO mole percent of the polyol residues are comprised of trimethylolpropane residues. ~ ;
The relative amounts of the aromatic polyester and the aliphatic polyester can be varied substantially ~;
depending on a number of factors such as the particular polyesters employed, the cross-linking agent and the amount thereof being used, the degree of pigment loading, the properties desired from the cured coating, etc. As described above, the compositions of the present invention comprise a blend of about lO to 80 -~
weight percent of the aromatic polyester and about 20 to `
90 weight percent of the aliphatic polyester. The blend of polymers containing free hydroxy groups provided by this invention is preferably co~prised of about 20 to 75 ~ ;
weight percent of the aromatic polyester and 25 to 80 weight percent of the aliphatic polyester. It should be thus appreciated that components (l)(a) and (l)(b) will always total lO0 percent. ~`
The adducts of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and a diol are prepared according to the procedures described in U S. Patent 4,413,079, incorporated herein by reference, by reacting the diazetidine dimer of isophorone diisocyanate, preferably free of isocyanurate trimers of isophorone diisocyanate, with diols in a ratio of reactants which gives as isocyanto:hydroxyl ratio of about l:0.5 to l:0.9, preferably l:0.6 to l:0.8. The adduct preferably has a molecular weight of 1450 to 2800 and a melting point of a~out 85 to 120C. The preferred diol residue (R2) is a C2-C8 diol, most preferably the residue of l,4- -butanediol. Such an adduct is commercially available under the name Huls BFl540. ~;~
'-~:' .~:

WO93/04102 PCT/US92/07083 ~

The amount of the cross-linking adduct present in the compositions of this invention can be varied -depending on several factors such as those mentioned hereinabove relative to the amounts of aromatic polyester and aliphatic polyester utilized. Typically, the amount of ~ross-linking adduct which will `~;
effectively cross-link the hydroxy-containing polymers to produce coatings having a good combination of properties is in the range of about 5 to 30 weight percent, preferably 10 to 25 weight percent, based on the total weight of the aromatic polyester, the aliphatic polyester and the cross-linking compound.
The cross-linkin~ component of the compositions may ~ -contain a minor amount, e.g., up to about 30 weight percent based on the total weight of the cross-linking ::~
component, of another blocked polyisocyanate such as ~ ~
those which are based on isophorone diisocyanate blocked --~ `
with ~-caprolactam, commercially available as ; `~`
H~ls B1530, Ruco NI-2 and Cargill 2400, and phenol- ~ -blocked hexamethylene diisocyanate. The presence of minor amounts of such blocked polyisocyanates has been found to provide good cross-linking at temperatures as low as 325F ~163C) with the liberation of very minor amounts of the blocking agent, e.g., ~-caprolactam.
The powder coating compositions of our invention may be prepared from the compositions described herein by dry-mixing and then melt-blending the aromatic ~;
polyester, the aliphatic polyester and the cross-linking adduct, along with other additives commonly used in powder coatings, and then grinding the solidified blend to a particle size, e.q., an average particle size in the range of about 10 to 300 microns, suitable for `
producing powder coatings. ~or example, the ingredients ~ ~
of the powder coating composition may be dry blended and ~-then melt blended in a ZSX twin-screw extruder at 90 to WO93/04102 PCT/US92/070~ -2 1 1 :~ 7 ~ 12--130C, granulated and finally ground. The melt blending should be carried out at a temperature sufficiently low to prevent the conversion of the cross-linking adduct to a reactive form and thus avoid premature cross-linking.
To minimize the exposure of the cross-linking adduct to elevated temperatures, the amorphous and semi-crystalline polyesters may be blended prior to the incorporation therein of the cross-linXing agent.
Typical of the additives which may be present in the powder coating compositions include benzoin, used to reduce entrapped air or volatiles, flow aids or flow control agents which aid the formation of a smooth, glossy surface, catalysts to promote the cross-linking reaction between the isocyanate groups ~f the cross-linking agent, and the hydroxyl groups on the polymers, stabilizers, pigments, and dyes. Although it is possible to cure or cross-link the composition without the use of a catalyst, it is usually desirable to employ a catalyst to aid the cross-linking reaction, e.g~, in an amount of about 0.05 to 2.0 weight percent cross-linking catalyst based on the total weight of the amorphous and semi-crystalline polyester~ and the cross-linking agent. Suitable catalysts for promoting the cross-linking include organo-tin compounds such as dibutyltin dilaurate, dibutyltin dimaleate, dibutyltin oxide, stannous octanoate and similar compounds.
Conventional ultraviolet light stabilizers such as TIN W IN 234, and hindered amine light stabilizers such as TINUVIN 144, may also be utilized.
The powder coating compositions preferably contain a flow aid, also referred to as flow control or leveling agents, to enhance the surface appearance of cured coatings of the powder coating compositions. Such flow aids typically comprise acrylic polymers and are available from several suppliers, e.g., M~DA~LOW from WO93/04102 PCT/US92/070~3 `

--13-- 2 1 ~ j 7 ~

Monsanto Company and Acronal from BASF. Other flow control agents which may be used include Modarez MFP ~
available from Synthron, EX 486 available from Troy -Chemical, BYK 360P available from BYK Mallinkrodt and Perenol F-30-P available from Henkel. A specific flow aid is an acrylic polymer having a molecular weight of about 17,000 and containing 60 mole percent 2-ethylhexyl ~ ~-methacrylate residues and about 40 mole percent ethyl acrylate residues. The amount of flow aid present may be in the range of about 0.5 to 4.0 weight percent, ``~
based on the total weight of the amorphous and semi-crystalline polyesters and the cross-linking aqent.
The powder coating compositions may be deposited on ``~
various metallic and non-metallic substrates by known -`
techniques for powder deposition such as by means of a .. ..
powder gun, by electrostatic deposition or by deposition from a fluidized bed. In fluidized bed sintering, a preheated article is immersed into a suspension of the powder coating in air. The particle size of the powder ~--~20 coating composition normally is in the range of 60 to `
300 microns. The powder is maintained in suspension by passing air through a porous bottom of the fluidized bed chamber. The articles to be coated are preheated to about 250 to 400F (about 121 to 205C) and then brought into contact with the fluidized bed of the powder coating composition. The contact time depends on the thickness of the coating that is to be produced and typically is from 1 to 12 seconds. The temperature of I the substrate being coated causes the powder to flow and thus fuse together to form a smooth, uniform, continuous, uncratered coating. The temperature of the preheated article also affects cross-linking of the ;
coating composition and results in the formation of a ~`
tough coating having a good combination of properties.

....
"`~

W093/04102 PCT/~S92/07083 2 1 1 ~ b ~ -- 14 --coatings having a thickness between 200 and 500 microns may be produced by this method.
The compositions also may be applied using an electrostatic process wherein a powder coating composition having a particle size of less than 100 microns, preferably about 15 to 50 microns, is blown by means of compressed air into an applicator in which it is charged with a voltage of 3~ to 100 kV by high-voltage direct current. The charged particles then are ;~
sprayed onto the grounded article to be coated to which the particles adhere due to the electrical charge ~
thereof. The coated article is heated to melt and cure ~ ;
the powder particles. Coating of 40 to 120 microns -thickness may be obtained.
Another method of applying the powder coating compositions is the electrostatic fluidized bed process which is a combination of the two methods described above. For example, annular or partially annular .
electrodes are mounted over a fluidized bed so as to produce an electrostatic charge such as 50 to 100 kV. -The article to be coated, either heated, e.g., 250 to -400F, or cold, is exposed briefly to the fluidized powder. The coated article then can be heated to effect cross-linking if the article was not preheated to a ~`
temperature sufficiently high to cure the coating upon contact of the coating particles with the article.
The powder coating compositions of this invention may be used to coat arkicles of various shapes and sizes constructed of heat-resistant materials such as glass, ceramic and various metal materials. The compositions are especially useful for producing coatings on articles constructed of metals and metal alloys, particularly steel articles.
The compositions and coatings of our invention are ;~
further illustrated by the following examples. The ;~

W093/04102 PCT/US92/07083 ~

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inherent viscosities (I.V.; dl~g) referred to herein were measured at 25OC using 0.5 g polymer per loo mL of a solvent consisting of 60 parts by weight phenol and 40 parts by weight tetrachloroethane. Melt viscosities (poise) were determined using an ICI melt viscometer according to ASTM D42~7-83. Acid and hydroxyl numbers `
were determined by titration and are reported herein as mg of KOH consumed for each gram of polymer. The glass -transition temperatures (Tg) and the melting tempera- ~`
tures (Tm) were determined by differential scanning ~ ~-calorimetry (DSC) on the second heating cycle at a ` ;
scanning rate of 20C per minute aft~er the sample was heated to melt and quenched to below the Tg of the polymer. Tg values are reported as the midpoint of the transition and Tm at peaks of transitions. The weight ` -average molecular weight (Mw) and number average molecular weight (Mn) were deter~ined by gel permeation ~ `
chromatography in tetrahydrofuran (THF) using a poly- -~
styrene standard and a W detector.
Coatings were prepared on 3 inch by 9 inch panels of 24-gauge, polished, cold roll steel, the surface of which has been zinc phosphated (BONDERITE 37, The Parker Company). Impact strenqths were determined using an impact tester (Gardner Laboratory, ~nc.) according to ASTM D2794-84. A weight with a 5~-inch diameter, hemispherical nose was dropped within a slide tube from a specified height to drive into the front (coated face~ ~ ;
or back of the panel. The highest impact which did not crack the coating was recorded in inch-pounds, front and reverse.
The solvent resistance and the degree of cure (cross-linking) of the coatings were determined by a methyl ethyl ketone (MEK) rub procedure in which coated panels were rubbed with a two-pound ball peen hammer ~` `
wrapped with cheese cloth approximately 0.5 inch thick.
`''`' Wo93/04102 PCT/US92/070X3 ~ 7 ~ 3 - l6 - !

The cloth was wetted with MEK every 50 double strokes.
The rubbing was continued until bare metal is observed or until 200 double rubs are completed. The result of each MEK rub procedure is reported as the number of double rubs required for the observation of bare metal ~
or 200, whichever is less. -The flexibility of the coatings was determined in "`~'J, accordance with ASTM 4145-83 at ambient temperature by bending or folding a coated panel back against itself, usinq a hydraulic jack pressurized to 20,000 pounds per -~
square inch (psi), until the apex of the bend is as flat as can be reasonably achieved. This bend is referred to as~OT meaning that there is nothing (zero thicknesses) ~between the bent portions of the panel. The bend is -~
examined using a lOX magnifying glass and a pass is recorded if no fractures of the coating are observed.
If~fraotures of the coating are observed, the panel-is bent a se¢ond time (lT) to form a three-layer sandwich. ~ -~The~second bend is inspected for coating fracture and ~ `
20 ~ this procedure is repeated, forming 4-, 5-, 6-, etc.
layer sandwiches, until a bend exhibits no fracture of `~
the~coating. The result of each bend test is the min~imum thi~kness (minimum T-bend) of the bend which ~ ;
does not give any fractures of the coating. Although the bend test used is excessively severe for most purposes for which coated articles are used, it pro~ides a means to compare the flexibilities of different powder coating compositions.
The 20 degree and 60 degree gloss are measured using a gloss meter (Gardner Laboratory, Inc~, Model GC-9095) according to ASTM D-S23.
The acid numbers and hydroxyl number are determined by titration and reported as mg of KOH consumed for each gram of resin.

~, ' `-`':`'`

-, 17 -- 2 ~ ~ r ~3 ~j tr The pencil hardness of a coating is that of the hardest pencil that will not cut into the coating according to ASTM 3363-74 (reapproved 1980). The results are expressed according to the following scale: -~
5 ~ (softest) 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H (hardest).
The conical mandrel test is performed by bending the panel over 15 seconds using a Gardner Laboratory, Inc., conical mandrel of specified size according to ASTM-522-85. A pass or fail is recorded.
The artificial weatherability of the coatings was ~ `
determined by exposure of the coated panels in a Cyclic Ultraviolet Weathering Tester (Q W) with 313 nm fluorescent tubes. The test condition was 8 hours of light at 70C and 4 hours of condensation at 45 C.

Experimental Section ' Example 1 ;
This example illustrates the typical procedure for preparing the aliphatic polyesters of the invention. A
3000mL, 3-necked, round-bottom flask equipped with a stirrer, a short distillation column, and an inlet for nitrogen, is charged with dimethyl cyclohexane-dicarboxylate (1259.7 g, 6.29 mol), 1,4-butanediol (997.5 g, 11.08 mol), trimethylolpropane (73.9 g, 0.55 ~ ;
mol) and 10 mL of titanium tetraisopropoxide~2-propanol solution(100 ppm Ti). The flask and contents are heated under nitrogen atmosphere to a temperature of 170C at which point methanol begins to distill rapidly from the flask. A~ter the reaction mixture is heated with stirring at this temperature for about 1 hour, the temperature is increased to 200C. for 2 hours, raised ~`
to 215C. for 4 hours, and then to 2350C. After 3 hours 2 1 1 ~ 7 ~ t ;, .`
at this temperature, a vacuum of 10 mm of mercury is applied over a period of 18 minutes. Stirring is :
continued under 10 mm of mercury at 235C. for about 3 ~ `:
hours to produce a low melt viscosity, colorless ;
polymer. The polymer has an inherent viscosity of 0.30, ~-a melting point of 130C., and a hydroxyl number of 30.

Example 2 ~ .
A powder coating composition is prepared from the following materials~
~` `'`''` .

141.0 g Polyester of Example l;
423.0 g RUCOTE 107, a polyester based primarily lS on terephthalic acid and 2,2-dimethyl~
1,3-propanediol;
136.0 g Self-blocked isophorone polyisocyanate ~ ~
- (Huls BF 1540) : .
280.0 g Tio2 ` : ~
: 20 7.0 g Dibutyltin dilaurate; `~ ;
7.0 g Benzoin; `. ~;
10.5 g MODAFLOW II~ (flow aid Monsanto);
7.0 g TIN W IN 144; and 7.0 g TIN W IN 234.
``
The above materials were melt-blended in an APV
twin screw extruder at 110~C, ground in a Ban'am mill to which a stream of liquid nitrogen was fed and classified -I through a 170 mesh screen on a KEX centrifugal siftar. ~-The finely-divided powder coating composition thus obtained had an average particle size of about 50 microns.
The powder coating composition prepared in Example 2 was applied electrostatically to one side of the 3 `~ :
inch by 9 inch panels described above. The coating was .~
, i. .. .
:

WO93/04102 PC~/US92/0708~

-- 19 ~

cured (i.e., cross-linked) by heating the coated panels ; ~
at 177C. in an oven for 20 minutes. The cured coatings . ~:
are generally about 50 microns thick.
The coatings on the panel have both front and back impact strengths of >160 inch-pounds, 20C and 60 gloss :
values of 84 and 94, respectively, and a pencil hardness of F. The coated panels pass a 0.125 inch conical .
mandrel test and have a T-bend flexibility value of 1. .
After 520 hours of Q W exposure, the coating retains 50% .~.
of the 60O gloss.

Example 3 Using the procedure described in Example 2,~a powder coating composition was prepared from the : .
following materials~
.
281.8 g Polyester of Example lj 281.8 g RUCOTE 107, a polyester descri~ed in Example 2;
136.4 g Self-blocked isophorone diisocyanate (Huls BF 1540);
280.0 g Tio2;
7.0 g Dibutyltin dilaurate;
7.0 g Benzoin;
10.5 g MODAFLOW III;
7.0 g TIN W IN 144; and .
7.0 g TIN W IN 234.

Using the procedure of Example 2, panels were coated with this powder coating composition and the coatings were cured and evaluated. The coatings have both front and back impact strengths of >160 inch-pounds :
and 20 and 60 gloss values of 82 and 94, respectively, -~
and a pencil hardness of F. The coated panels pass a `:
'~ :

WO93/04102 P~T/US92/070X3 2 1 l S ~ ~ c ; ~
0.125 inch conical mandrel and have a T-bend flexibility value of 0. After 500 hours of Q W exposure, the :
coating retains 50% of the 600 gloss.

Example 4 This example illustrates the typical procedure for preparing the aliphatic polyesters of this invention using 1,4-cyclohexanedicarboxylic acid. A 3000 mL, 3-nec~ed, round-bottom flask equipped with a stirrer, a .
short distillation column, and an inlet for nitrogen was ::-charged with 1,4-cyclohexanedicarboxylic acid (1100.3 g, 6.39 mol), 1,4-butanediol (629.77 g, 6.68 mol), 1.5 g of butanestannoic acid (FASCAT 4100) catalyst. The flask and contents are heated under nitrogen atmosphere at 200 C during 60 minutes and maintained at 200 C for 2 hours. The temperature was then increased to 215 C -for 2 hours and then to 235 C for 8 hours to produce a low melt viscosity, colorless polymer. The polymer has an inherent viscosity of 0.25, a melting point of 155~ C, and a hydroxyl num3ber of 36. ~

Example 5 .

Using the procedure described in Example 2, a powder coating was prepared from the following material : -.` '.
204.2 g Polyester of Example 4;
612.5 g RUCOTE 107, a polyester described in Example 2;
183.4 g Self-blocked i50phorone diisocyanate (Huls BF 1540);
10.0 g Dibutyltin dilaurate; ~:
10.0 g Benzoin;
:' ;''`
''`' ;'''`
" `' ``'~'.

WO93/04102 l'~T/US92/07083 ~1:157~
- 21 - ~ :

15.0 g MODAFLOW III; ~ .
400.0 g TiO2; `~
10.0 g TINUVIN 144; and -:
10.0 g TINUVIN 234.
Using the procedure of Example 2, panels were ~, coated with this powder coating composition and the coatings were cured and evaluated. The coatings have both front and back impact strengths of ~160 inch-pounds `-:~
and 20 and 60 gloss values of 84 and ~6, respectively, "-and a pencil hardness of F. The coated panels pass a 0.125 inch conical mandrel and have a T-bend flexibility : .
value of 0. After 600 hours of QW exposure, the :
coating retains 50% of the 20~ gloss. -~:
!.
Comparative Example 1 ::
' A powder coating composition was prepared from the following materials~
.
968.0 g RUCOTE 107, a polyester described in Example 2;
232.Q g Self-blocked isophorone diisocyanate (Huls BF 1540~;
12.0 g Dibutyltin dilaurate;
12.0 g Benzoin;
18.Q g MODAFLOW III
480.0 g Tio2;
j 12.0 g TINUVIN 144; and 3Q 12.0 g TIN WIN 234.

Using the procedure of Example 2, panels were coated with this powder coating composition and the coatings were cured at 177C for 20 minutes and evaluated. Coated panels have front impact of 60 inch-WO93/04102 PCT/US92~0708~

~ I 1 5 G ~ 3 - 22 -pounds and back impact of 10-inch pounds. The 20O and 60 gloss values were 86 and 96, respectively, and the pencil hardness is F. The coated panels fail a 0.125 inch conical mandrel test and have a T-bend flexibility value of 6. After 220 hours of Q W weathering exposure, the coating retains 50% of the 60~ gloss. :

.:

Claims (15)

Claims We claim:

1. A thermosetting powder coating composition which comprises an intimate blend of (1) A novel blend of polymers containing free hydroxy groups comprised of:
(a) about 10 to about 80 weight percent of an aromatic polyester having a glass transi-tion temperature (Tg) of greater than about 40°C., a hydroxyl number of about 20 to 200 and an inherent viscosity of about 0.1 to about 0.5; and (b) about 20 to about 90 weight percent of poly(tetramethylene trans-1,4-cyclo-hexanedicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.5;
and (2) a cross-linking effective amount of an adduct of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and a diol having the structure wherein R1 is a radical having the structure R2 is a divalent aliphatic, cycloaliphatic, araliphatic or aromatic residue of a diol; and X is a 1,3-diazetidine-2,4-dionediyl radical having the structure wherein the ratio of NCO to OH groups in the formation of the adduct is about 1:0.5 to 1:0.9, the mole ratio of diazetidinedione to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is not greater than 8 weight percent and the adduct has a molecular weight of about 500 to 4000 and a melting point of about 70 to 130°C.

2. The thermosetting powder coating composition of claim 1, wherein the aromatic polyester component has a Tg greater than 55°C, a hydroxyl number in the range of about 25 to 80, an acid number of not more than 15, and an inherent viscosity of about 0.15 to 0.4.

3. The thermosetting powder coating composition of claim 2, wherein the aromatic polyester component is comprised of (1) diacid residues of which at least 50 mole percent are terephthalic acid residues and (2) glycol residues of which at least 50 mole percent are derived from 2,2-dimethyl-1,3-propanediol.

4. The thermosetting powder coating composition of claim 1, wherein the aliphatic polyester component has a Tm in the range of about 110° to 160°C, a hydroxyl number in the range of about 25 to 65, an acid number of not more than 10, and an inherent viscosity of about 0.10 to 0.40.

5. The thermosetting powder coating composition of claim 4, wherein in the aliphatic polyester component, up to about 10 mole percent of the 1,4-butanediol residues are replaced with a glycol residues having 2 to 12 carbon atoms.

6. The thermosetting powder coating composition of claim 1, wherein in component (2), R2 is the residue of a C2-C8 diol.

7. The thermosetting powder coating composition of Claim 6, wherein in component (2), R2 is the residue of 1,4-butanediol.

8. A shaped or formed article coated with the cured composition of claim 1.

9. A thermosetting powder coating composition which comprises an intimate blend of (1) A novel blend of polymers containing free hydroxy groups comprised of:

(a) about 10 to about 80 weight percent of an aromatic polyester having a glass transi-tion temperature (Tg) of greater than about 40°C., a hydroxyl number of about 20 to 200 and an inherent viscosity of about 0.1 to about 0.5; wherein the aromatic polyester component is comprised of (1) diacid residues of which at least 50 mole percent are terephthalic acid residues, (2) glycol residues of which at least 50 mole percent are derived from 2,2-dimethyl-1,3-propanediol and (3) up to 10 mole percent, based on the total moles of (2) and (3), of trimethylol-propane residues, and (b) about 20 to about 90 weight percent of poly(tetramethylene trans-1,4-cyclo-hexanedicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.5;
wherein up to 10 mole percent of the polyol residues are comprised of tri-methylolpropane residues; and (2) a cross-linking effective amount of an adduct of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and a diol having the structure wherein R1 is a radical having the structure R2 is a divalent aliphatic, cycloaliphatic, araliphatic or aromatic residue of a diol; and X is a 1,3-diazetidine-2,4-dionediyl radical having the structure wherein the ratio of NCO to OH groups in the formation of the adduct is about 1:0.5 to 1:0.9, the mole ratio of diazetidinedione to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is not greater than 8 weight percent and the adduct has a molecular weight of about 500 to 4000 and a melting point of about 70 to 130°C.

10. The thermosetting powder coating composition of claim 9, wherein the aromatic polyester component has a Tg greater than 55°C, a hydroxyl number in the range of about 25 to 80, an acid number of not more than 15, and an inherent viscosity of about 0.15 to 0.4.

11. The thermosetting powder coating composition of claim 9, wherein the aliphatic polyester component has a Tm in the range of about 110° to 160°C, a hydroxyl number in the range of about 25 to 65, an acid number of not more than 10, and an inherent viscosity of about 0.10 to 0.40.

12. The thermosetting powder coating composition of claim 11, wherein in the aliphatic polyester component, up to about 10 mole percent of the 1,4-butanediol residues are replaced with a glycol residues having 2 to 12 carbon atoms.

13. The thermosetting powder coating composition of claim 9, wherein in component (2), R2 is the residue of a C2-C8 diol.

14. The thermosetting powder coating composition of Claim 6, wherein in component (2), R2 is the residue of 1,4-butanediol.

15. A shaped or formed article coated with the cured composition of claim 9.