WO1999065978A1

WO1999065978A1 - Reactive extrusion process for the conversion of certain polyesters to a resin suitable for powder coatings applications

Info

Publication number: WO1999065978A1
Application number: PCT/US1999/011965
Authority: WO
Inventors: Robert Boyd Barbee; Brian Steven Phillips
Original assignee: Eastman Chemical Company
Priority date: 1998-06-15
Filing date: 1999-06-01
Publication date: 1999-12-23

Abstract

Provided is a process for preparing amorphous polyesters useful as binder resins in thermosetting powder coating compositions. In particular, the process provides a hydroxyl functional polyester suitable for powder coating compositions which is prepared via a reactive extrusion process.

Description

REACTIVE EXTRUSION PROCESS FOR THE CONVERSION

OF CERTAIN POLYESTERS TO A RESIN SUITABLE FOR

POWDER COATINGS APPLICATIONS

Field of the Invention

This invention belongs to the field of polymer chemistry. In particular, it relates to a reactive extrusion process for converting certain amorphous polyesters to a resin which can be used in thermosetting powder coating compositions.

Background of the Invention

Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together to form a continuous film. Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, 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 have created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. Such a coating lacks the gloss and luster of coatings typically obtained from thermoplastic compositions.

In addition to exhibiting good gloss, impact strength and resistance to solvents and chemicals, coatings derived from thermosetting coating compositions must possess good to excellent flexibility. 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 .

Thermosetting powder coating compositions are typically comprised of one or more curable resins, crosslinking agent (s) , and other additives, as more fully described below. The present invention provides a process for preparing a resin suitable in such compositions which can be prepared from certain scrap or recycled non-crystalline polyesters. U.S. Patent No. 3,808,302 describes a process for spinning (i.e., extrusion) of poly (ethylene terephthalate) fiber containing alumina trihydrate at a temperature sufficient to cause the release of a major portion of the available water in the alumina trihydrate and thereby reduce the inherent viscosity of the polymer by hydrolytic degradation. The product thus generated would not be suitable for use in thermosetting powder coating compositions.

U.S. Patent No. 4,359,557 describes a process for preparing low molecular poly (ethylene terephthalate) by admixing ethylene glycol with poly (ethylene terephthalate) and extruding, i.e. heated, to lower the inherent viscosity. These low molecular weight polyesters would not be suitable for use in thermosetting powder coating compositions. U.S. Patent No. 5,451,611 describes a process for the conversion of poly (ethylene terephthalate) -waste to poly (alkylene terephthalate) useful as engineering plastic, which comprises reacting poly (ethylene terephthalate) powder with a diol containing 3-12 carbon atoms in the liquid phase in the presence of a catalyst at a temperature in the range of 180°C-270°C under reduced pressure and inert atmosphere .

Summary of the Invention

This invention provides a process for preparing amorphous polyesters useful as binder resins in thermosetting powder coating compositions. In particular, the process provides a hydroxyl functional polyester suitable for powder coating compositions which is prepared via a reactive extrusion process . In a preferred embodiment of the invention, an amorphous polyester comprised of residues of 1,4- cyclohexanedimethanol is converted, via a reactive extrusion process, to a hydroxyl functional resin suitable for use in a thermosetting powder coating composition.

Detailed Description of the Invention

The present invention provides a process for preparing a hydroxy-functional polyester having a number average molecular weight of 500 to 5000 and a hydroxyl number of 20 to 300, which comprises:

extruding an amorphous polyester having a number average molecular weight of 10,000 to 30,000, at a temperature of 250 to 300°C, with a residence time in the extruder of 1 to 5 minutes, in the presence of a polyol .

In the above process, the amorphous polyester is preferably one which is comprised of from about 10 to 50 mole percent of a polyol modifier. Suitable polyol modifiers include cyclohexanedimethanol, diethylene glycol, 2, 2-dimethyl-1, 3-propanediol, 1, 4-butanediol, 1- 6-hexanediol, and the like. Specific examples of preferred amorphous polyesters include those polyesters with a melting point of less than about 200°C, and having about 15 to 40 mole percent of residues of cyclohexanedimethanol . A preferred example of such resins are those sold by Eastman Chemical Company under the SPECTAR^® Copolymer trademark. The higher melting point of polyesters such as poly (ethylene terephthalate) precludes their use as a resin in thermosetting powder coatings. Introduction of a second polyol to reduce crystallinity results in a resin possessing a molecular weight which is too low to be useful as a resin. Consequently, a second step s required to remove excess glycol and increase the molecular weight of the resin. This second step is difficult to do on a practical scale in existing extruders. Amorphous polyesters, however, can be simply melt extruded with a polyol in a single step to provide a resin having the desired molecular weight and hydroxyl number for a thermosetting powder coating.

In the practice of the process of the invention, amorphous pellets or chips are fed into an extruder, such as twin-screw Warner & Pfleider, with sufficient barrel length and heated to a sufficient temperature to allow depolymerization of the amorphous polyester by a polyol . A polyol is pumped into the extruder barrel at such a rate that depolymerization will generate a resin with the desired number average molecular weight of 500 to 5000 and hydroxyl number of 20 to 300.

Reactive extrusion may be done on a single-screw or twin-screw extruder, although the latter is preferred. Extruder conditions, such as feed rate of the amorphous polyester and polyol, screw speed, and temperature of individual barrels may be controlled as required.

Suitable polyols used for the depolymerization of the amorphous polyester are 1, 4-butanediol, 1, 6-hexanediol, 1, 4-cyclohexanedimethanol,

2,2-dimethyl-l, 3-propanediol, trimethyolpropane, and the like.

Additional catalysts may be used. Such catalysts may be based on tin or alkyloxides of titanium. Inherent viscosities are determined at 25°C in a

(60/40 by weight) mixture of phenol/tetrachloroethane at a concentration of 0.5 g/100 ml. Acid and hydroxyl numbers are determined by titration and are reported herein as mg of KOH consumed for each gram of polymer. The glass transition temperatures (Tg) are determined by differential scanning calorimetry (DSC) on the second heating cycle at a scanning rate of 20° per minute after the sample has been heated to melt and quenched to below the grass transition temperature of the polymer. The molecular weights were determined by gel-permeation chromotography (GPC) on a Perkin-Elmer instrument with tetrahydrofuran as a mobile phase and solvent . Values are reported in polystyrene equivalents . Resins prepared by reactive extrusion as described herein are hydroxyl terminated and can be formulated into powder coatings using typical powder coating procedures. Accordingly, the present invention also provides a thermosetting powder coating composition comprising (a) a hydroxy-functional polyester having a number average molecular weight of 500 to 5000 and a hydroxyl number of 20 to 300, wherein said polyester is prepared by extruding an amorphous polyester having a number average molecular weight of 10,000 to 30,000, at a temperature of 250 to 300°C, with a residence time in the extruder of 1 to 5 minutes, in the presence of a polyol; and

(b) a crosslinking agent,

Examples of crosslinking agents include the blocked polyisocyanate cross-linking compounds; such compounds include those which are based on isophorone diisocyanate blocked with e-caprolactam, commercially available as HULS 1530 and CARGILL 2400. The internally blocked polyisocyanate crosslinker are those that are self-blocked as uretidione group, such as those based on isophoronediisocyanate, for example HULS BF 1540. Other examples of blocked isocyanates are taught, along with conventional additives and application methods are taught in U.S. Patent No. 5,160,792, incorporated herein by reference.

Conventional ultraviolet light stabilizers, such as TINUVIN 234, and hindered amine light stabilizers, such as TINUVIN 144 may also be used. Also, conventional dyes or pigments such as R960 titanium dioxide pigment marketed by Du Pont may be used as well as catalysts.

The components of the compositions according to this invention may be mixed by dry blending in a Henschel mixer, followed by compounding in a ZSK-30 Extruder (Werner & Pfleider) or APV twin screw extruder at 100-130°C, grinding, and screening to obtain powder with average particle size of about 35 microns. The powder is electrostatically deposited on the substrate by use of a powder gun. After deposition, the powder is heated to a temperature sufficient to cause its particles to flow and fuse together to form a smooth, uniform surface. Coatings were prepared on 3 inch by 9 inch panels of 20-gauge, polished, cold roll steel, the surface of which has been iron phosphated (The Q-Panel Company) .

The 20 and 60 degree gloss are measured using a gloss meter (Gardner Laboratory, Inc., Model GC-9095) according to ASTM D-523.

The pencil hardness of a coating is that of the hardest pencil that will not cut into the coating according to ASTM 336374 (reapproved 1980) . The results are expressed according to the following scale: (softest) 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H (hardest) .

Experimental Section Example 1

Depolymerization of SPECTAR^® Copolymer 6763 with

10 pph Trimethyolpropane in a Reactive Extruder

Eastman Chemical Company's SPECTAR^® Copolymer 6763 pellets were depolymerized in a 30 mm ZSK 30 twin-screw extruder with a barrel length of 900 mm. the barrel temperature was maintained at about

270°C and the screw speed was held at 250 rpm. The pellets were fed into the extruder at 100 pounds per hour while molten trimethylolpropane was metered at 10 pounds per hour into a port in the extruder barrel located just downstream from the feed hopper containing the starting material copolymer. The resulting extrudate had an I.V. of 0.25, a Tg of 52°C, a number average molecular weight of 6,000, a hydroxyl number of 90 and an acid number of 3.

Example 2

Depolymerization of SPECTAR^® Copolymer 6763 with 5 pph Trimethylolpropane

SPECTAR^® Copolymer 6763 pellets were depolymerized as described in Example 1 to produce extrudate with an I.V. of 0.30, a Tg of 62°C, a number average molecular weight of 8,000, a hydroxyl number of 39, and an acid number of 0.3. Example 3

A powder coating composition was prepared from the following materials:

414 g Polyester of Example 1; 186 g HULS 1530 crosslinker; 6 g Benzoin;

9 g MODAFLOW III flow aid; 6 g Dibutyltin dilaurate; and

240 g Ti0₂, R960.

The above material was melt-blended in a ZSK-30 twin screw extruder at 110°C, ground in a BANTAM mill to which a stream of liquid nitrogen is fed, and classified through a 170 mesh screen on a KEK centrifugal sifter. The finely-divided, powder coating composition obtained had an average particle size of about 50 microns. This powder coating composition is applied electrostatically to one side of the 3 inch by 9 inch panels described hereinabove . The coating is cured (cross-linked) by heating the coated panel at 177°C in an oven for 20 minutes. The coating on the panel had a 20 degree gloss value of 49, a

60 degree gloss value of 92, pencil hardness of 3H, Gardner Impact of 140 inch-pounds, and T-bend of 1.

Example 4

Using the procedure described in Example 3 , a powder coating composition was prepared from the following materials:

503 g Polyester of Example 2; 97 g HULS 1530 crosslinker; 6 , g Benzoin;

9 g MODAFLOW III flow aid; 6 g Dibutyltin dilaurate; and 240 g Ti0₂, R960.

Using the procedure of Example 3, panels are coated with his powder coating composition and the coatings are cured and evaluated. The coatings have a 20 degree gloss value of 80, a 60 degree gloss value of 100, pencil hardness of F, Gardner Impact of 160 inch-pounds, and a T-bend of 0.

Example 5

Using the procedure described in Example 3, a powder coating composition was prepared from the following materials :

503 g Polyester of Example 2;

97 g HULS 1540 crosslinker;

6 g Benzoin;

9 g MODAFLOW III flow aid;

6 g Dibutyltin dilaurate; and 240 g Ti0_2/ R960.

Using the procedure of Example 3, panels are coated with this powder coating composition and the coatings are cured and evaluated. The coatings have a 20 degree gloss value of 57, a 60 degree gloss value of 97, pencil hardness of 3H, Gardner Impact of 160 inch-pounds, and a T-bend of 1.

Claims

CLAIMSWe Claim:

1. A process for preparing a hydroxy-functional polyester having a number average molecular weight of 500 to 5000 and a hydroxyl number of 20 to 300, which comprises :

extruding an amorphous polyester having a number average molecular weight of 10,000 to 30,000, at a temperature of 250 to 300┬░C, with a residence time in the extruder of 1 to 5 minutes , in the presence of a polyol .

2. The process of claim 1, wherein said amorphous polyester is comprised of 10 to 50 mole percent of one or more polyols selected from the group consisting of 1, -cyclohexanedimethanol, diethylene glycol, 2, 2-dimethyl-1,3-propanediol, 1,4- butanediol, and 1-6-hexanediol .

3. The process of claim 1 or 2, wherein the polyol is 1 , 4-cyclohexanedimethanol .

4. The process of claim 1 or 2 , wherein the polyol is 2, 2-dimethyl-1,3 -propanediol.

5. The process of any one of claims 1 to 4 , wherein the amorphous polyester is comprised of 15 to 40 mole percent of residues of 1,4- cyclohexanedimethanol and wherein said amorphous polyester possesses a melting point of less than 200┬░C.

6. The process of any one of claims 1 to 5 , wherein the polyol is selected from the group consisting of 1, 4 -cyclohexanedimethanol, diethylene glycol, 2, 2-dimethyl-l, 3 -propanediol, trimethylolpropane, 1, 4-butanediol, and 1, 6- hexanediol .

7. The process of claim 1, wherein the polyol is trimethylolpropane .

8. A thermosetting powder coating composition comprising

(a) a hydroxy-functional polyester having a number average molecular weight of 500 to 5000 and a hydroxyl number of 20 to 300, wherein said polyester is prepared by extruding an amorphous polyester having a number average molecular weight of 10,000 to 30,000, at a temperature of 250 to 300┬░C, with a residence time in the extruder of 1 to 5 minutes, in the presence of a polyol; and

(b) a crosslinking agent.