CA2140299A1 - Fluorinated powder coatings having improved gloss - Google Patents

Abstract

Pigmented powder coating products based on low melting point fluorine-based terpolymers, preferably on those made form a majority of vinylidene fluoride copolymerized with tetrafluoroethylene and hexafluoropropene, are capable of providing coatings with controllable gloss properties. The process for preparing coatings from the powders is essentially characterized by the selection of the temperature at which the desired gloss can be obtained, from low to high, without degradation of the mechanical properties.

Description

-- 21402~9 ~`

I~XPRESS MAIL #HB090~9~0~ PATENT
Docket No.: F 631 FLUORINATED POWDER COATINGS HAVING IMPROVED GLOSS

CROSS REFERENOE TO RELATED APPLICATION
S This application claims priority of previously filed cop~o.ndin~ Belgian application, Serial Number 93870243.8, ~ntitl~l "Fluorinated powder co~tin~ having regulable gloss", filed December 23, 1993; which previously filed appli~tiQn is hereby incorporated in its enlir~ly by referenc~

BACKGROUND OF THE INVENTION
The pr~selll invention related to fluorinated powder co~ting produc~;, based on certain vinylidene fluoride terpolymers (hereinafter referred to as VdFT), for preparing fluorinated co~tin~ having controllable gloss and to a process for co~tin~ a substrate with a pi~nente~l fluorinated c~ting having controllable gloss. In particular, the pr~se invention relates to the use of certain vinylidene fluoride terpolymers in pi~n~.nte~ powder lS co~tin~ products, for obt.~ining fluorinated pr~leclive co~tin~ having controllable gloss propertie~
C~o~tin~ based on vinylidene fluoride h~ mopolymers (hereinafter PVdF) are knownto be very useful as pro~liv~ co~tin~ for a variety of ~u~lla~es due to their good chemi~
and weather re~ nç~ and to the thermal stability of PVdF. The general known te~hni~lue used for plep~ing PVdF coA~ ~ is to plepale a dispersion of PVdF in a suitable solvent for application by known means onto the r~uired substrate, and thereafter subjecting it to heat treatment.

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The solvent used is generally known in the prior art as "latent solventn, and it is desclil~l therein as an (jlgdlliC solvent which has no si~nifir~nt action on PVdF at room l~lll~ralule, but which at an ele~ated lell~alule exerts a s -fficient solvent action.
Ho.._~_r, although the known ~ ellls may give good results, envir~nm~ont~l S pr~)teclion laws throughout the world, but mainly in Europe and in the US~, are m~king it increasingly d;ffl~`ult to utilize solvent-based ~y~ll~ Further, the r~co~.y of the solvent is a costly pr~lule. Accor~ gly, there is a need in the art for solventless PVdF-based c~tin~
British Patent GB 2194539-A, to Labofina ~i~l~s pigmented PVdF~ased powder 10 co~ting products c~n~ ing e~nti~lly of vinylidene fluoride homopolymer (or copolymer with at most 10 wt% of comonomer units), one or more comp~tible thermoplastic resins and one or more pigrnPnt~ It also ~ r1Qs~ a pr~ for p.ep~illg the pro~luc~. Hc..._~_r, the gloss obtained is not easily controlled at an ~ t~ble level.
Eur~ Patent No. 456018-A, to ~t~h~m North America d ~ pi~n~nte~
15 powder co~tin~ of vinylidene fluoride/h~fluQropropene copolymers, the resin component colllpl~ing 50 90 wt% of said copolymers having a melt vi~ ily of 1~ kPoise [100 400 Pa-s] at 100 s-l and 2320C, said copolymers being ch~acl~.i~ by a m~ltin~ point in the range of about 1600C to about 1700C (which closely appl ,~ches the m-o1ti~ point of PVdF), and 50 10 wt% of thermoplastic ac~lic resin. The resultin~ co~tin~ are said to be 20 chara.;l~i;æd by i~ ,r~t;d fle~nbility, crack r~~ ~ and surface ~moothnP~ (as oppose~l to surface rou~hn~ or "orange peel" ~ te 1 with the finish) while not requiring the inclusion of a flow~ ring agent. No information is reported about the gloss, not to 2~ 9 0299 --m~nti~-n gloss re~l~tion, except that the thermoplætic acrylic resin is re~o~ib!,~-for gloss durability.
U S. Patent S,229,460 to EvodeK -1:~1as~ fluoropolymer-bæed powder co~ ~ which may be based on PVdF or on a copolymer with at leæt 80 wt% of vinylidene fluoride and S up to 20 wt% of at leæt one other fluorine-bæed mon~ mer. No inform~tirn is reported about the gloss nor about gloss reg~llati~
There is, ho~.,er, a certain demand for co~tin&~ exhibiting very low gloss, which cannot be obtained pr~l.lly. Accor~dhlgly, there is a need for a pl~ for regulating the gloss of fluorinated powder co~tin~:.
An object of the plesell~ invention is to provide VdFT-bæed powder co~ting composition~ capable of providing easily controllable low gloss co~tin~ Controllable gloss co~tin~, as used herein, are co~tin~: whose gloss can be ~ ted from values from above 40, prefela~ly above 50 and most preferably above 60, down to values below 30, prer~ly below 20 (all values being me~ ed accor~hlg to ISO 2813 at an angle of 60 o), while still 15 having a good surface smoothnes~
Another object of the pr~selll invention is to provide VdFT-based powder co~ti~
co...~ilion~ c~pable of providing low gloss co~tin~- Low gloss c~tin~;, as used herein, are co~tin~ having a gloss below 30, plere.ably below 20 (all values being ~-.f~h..-~d accordhlg to ISO 2813 at an angle of 60 o)~ while still having a good surface sm~
A further object is to provide a proc~ for pl~h~g controllable gloss VdFT~ased CO~tin~

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Still another object is to provide a pr~s for pr~p~iug low-gloss VdFT-based c~,.~ s.
SUMMARY OF THE INVENTION
Accor~ ly, the pl~nl invention provides a pu..~ ng product c~n~ g 5 ~.nti~lly of:
(a) a resin c~n~pon~,nt c~ l;~ e~nti~lly of:
(i) from 60 to 90 wt% of one or more low m~lti~ point fluorine-based terpolyme~;
(ii) from 40 to 10 wt% of one or more co,ll~,a~le resins; and, (b) from 1 to 35 parts byweight of one or more pi~m~,nt~ per 100 parts byweight of the resin component.
The present invention further provides a proc~ for pr~p~illg ~lin&~ from the powder c~ p products of the invention, ~nti~lly chal~c~.i~ by a sq1~tinn of the baking ~ pe.alul~

DESCRIPTION OF THE PREFERRED EMBODIMENTS
The low m~.lting point fluorine-based terpolymers used herein made from t,hree monomers so1e~t~1 from the group OQ~ ;ng of vinylidene fluorine (VdF), lell alluor~e~-ylene (TFE), chlor~l irluoroe~lylene, vinyl fluoride, hexafluol ~pr~pene (HFP), and CF3-CF2-CF=CF2; and having a me1ting tempelalule below about 150C, preferably below 120 ~C and most plerelably below 100 oC Preferably the terpolymer is made from a majority of VdF copolymerized with TFE and HFP; the most preferred terpolymers are prepared from 65 to 85 wt% of VdF monom~r units with from 7.5 to 17.5 wt% of each of TFE and HFP. The melt vis~ily of the terpolymer should preferably be of from 9,000 to 15,000 dPa s, most pleÇelably from 11,000 to 13,000 dPa s, all ...f~ cd at 125 oc The VdFC is m~ed with one or several c~ ible resins, p.~fe.dluly of the acrylic S type. Acrylic resins are known in the art and need not be d~cril~l here; a descli~tion may be found in French Patent 2,636,959-A to ~to~h~om (see line 18 of page 3 to line 14 of page 4). As examples of thermoæfflng acrylic resin, there are those described in US 4,659,768 to DeSoto, particularly under the denomin~ti~n~ "Experirn~nt~l resin" and "control resin".
Huwevt:r, it has been found that it is advantageous to use a thermoplastic acrylic resin, and 10 most adv~nt~o~l~ to use a thermoplastic polymethylmeth~ylate (PMMA) resin. A
preferred thermoplastic PMMA resin is that obt~ined by (co)polymerization of at least 75 wt% of an alkyl meth~rrylate, the other c~monrrners being one or several o~^~inir~lly un~t-lrated comonomers, prerelably of the allyl (meth)acrylate type. These esters are formed by the re~tion of the acrylic or memarrylic acid with suitable alcohols, for ~mrl~, 15 methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol and 2-ethylhe~yl alcohol.
Generally, the larger the alcohol portion of the ester, the softer and more fl~bl~ the reslllt~nt resin. Also, generally, the meth~ylic esters form harder films than the c~ll~onding acry1ic este~ Examples of such resins inrlucle polymelllyl~ th?~glate, copolymers of methylmethacrylate with ethyl acry1ate, butyl m~h~ylatÆ, isol~utyl 20 methac~glate, acry1ic acid or methacry1ate acid, and the like. The most preferred PMMA
resins are those which exhibit a v~co~ity of from 7 to 17 P in 40% solution in a 95:5 by weight mixture of toluene and ethylene gly-col methyl ether.

2~4~293 The weight ratio of the VdFC to the ~r~ .le resin may vary widely from 90:10 to 60:40, pref~ly from 75:25 to 6535, most p.~rel~dbly of about 7030.
The resin co,llpollent may A~lition~lly c~>ntAin minor ~m~ln~ of the usual addi~ive~, such as an antioxidant, a flow promoting agents, a W absorber, and/or an lhe~sion S promoter. As s~ ~l in U S. Patent 5,Z2'9,460, an ~nti~ YidAnt may be utili7f~1 to çnh,~n~
heat stability and provide pr~tion against thermal degrA-1~tion and ~li~o~oration of the co~ting film due to e~iv~ heat at high curing le",~latur~ also, the use of Kynar ADS
as s~ lu(e for flow promoting agent is ~lis~lc)sed The use of pigments in the coAting colllposilion is ~ ~.able. Indeed, if no pigment is used, a clear~oat or varnish can be obtained; h<,~ r, the r~sulting ooA~ing is unevenly milky, which is undesirable. Fur~er, such cl~d~ have a lesser ~ nc~ to high tempe. atul~s, and most impo. ~Illy, they in~llffl.~ien~ly absorb ultraviolet light which would contribute to the degradation of the primer (if any). The use of ultrafine ~ niu... dioAide to absorb ultraviolet light is known commercially.
When pi~,æl,ls are Ic~lui~d, any pig,llelll or co,lll,.-l~tiQn of pigments may be utili7f~fl, and the choice of pigments should plerelably be made in accor~lce with what is known in the art for PVdF-based coAtin~- The amount of pigment used may vary widely accor~ ; to its hiding power. For example, a white topco~l pr~pa~ lu~i~,ely with li~liulll dioYAide may r~ up to about 35 w~% of pigment Other pi~n~.n~ with better 20 hiding properties may r~uir~ lower amounts.
The process of pr~pal ing the powder co~ting product CO1ll~ es the steps of:
(a) melt-mixing the VdFC, the compatible resin components and the pigments;

~1~029g (b) fG~ ingpellets; and, (c) grinding the pellets.
Melt-mixing is normally carried out by extrusion. E~truding and gr~n~ tin~ the reslllting I~ ure is performed by the usual procedures which are easily determined by one S skilled in the art. In particular, a single or double screw extruder is utili7fxl at a p,~ g tempelalu-e of from 100 oc to 130 oc The ~im.on~ ns of the pellets are usually about 3 mm in ~ r and 2 mm in length.
Grinding the pellets is done by any suitable means which allows suitable si_ed particles to be obtained, which techniques are known to those skilled in the art and need not be described in detail. For example, U.S. Patent 5,229,460 contains a ~ ~;on of the state of the art of grinding fluoropolymer-based thermoplastic blends and the influence of the cooling step thereon.
The finished powder should colll~,lise particles of si_e and shape suitable for achieving col~lL flow through the application e~uipllællt, mainly in order to obtain a 15 con~l~n~ thickness of co~ting. It is preferred that the shape of the particles be as spherical as possible and their si_e as uniform as possible so that the rçsl~lting powder will have better flow pr~,bel Li~ Rega~di~lg the size of the particles, the grinding step is co~ ed with a sieving step for elimin~ting the largest si7e particles, i~, those particles ~y.~ling about t~hree times the desir~d thir1~nf~ of the co~tin~ On the other hand, ~i~ly small 20 particles are to be avoided bec~ e they are a hazard to health and they tend to block transport lines during application.

2~a299 The pler~r~ method of grinding utiliæs a h~ .-Pr mill ~.he.eill a r~la~lg shaft carries ha"l~ which break the pellets on fixed shapes in the casing of the hqmmer mill, and h~ Pr them ~ U~h a sieve lining at the bottom of the mill. Sieve openin~ of about 0~ mm have been found particularly appr~pliate.
SThe re~l-lting powder may be applied on the ~ul~llate by any means suitable forachieving an even d~LIibulion of the particles, such as, for example, an elP~lic spray applic~tiorl app~alu~, whereby the particles reo~ e an electrical charge and then are ay~d onto an oppositely charged ~ul~l~ale. Alternative t~hni~lues include cloud chambe~, fluidiæd beds, even triboelectric co~tin~, and the like. Such terhniques are well 10known in the industry and need not be described in great detail.
The VdFC-based p~wder is p.c:fielably applied over a suitable primer co~tin~, commercially available for similar fluorinated co~tin~, in~ in~ solvent-based "flash" or powder primers.
After the co~ting has been applied to the substrate, it must be subject to heat 15treatment. The coated ~ul~llale is passed into a heated oven wherein the co~ting is baked.
An ~;enti~l fealule of the ~)r~c~ of the invention is the sol~tion of the t~ alul~
enabling control of gloss. The ~Illpe.aLule to be se1~cted for obl~ining a s~i~lr gloss (within experimt-nt~1 variations) is easily d~l,l,lilled by one skilled in the art using an oven with a tempelalule gradient such as is ~;ull~lllly used in the c~tin~: industry and need not 20be described further. This feature is based on the ol~~ lion that the gloss obtained with the co~ osilions of the invention first increase, then decrease to a low value (as definecl herein).

21~0~99 Finally, the co~ting and its ~ ale can either be cooled slowly in air or qu~on~h~d in water.
In order to further illu~l-dle the invention, the following example is given which;is intended to be illu~llalive only and not to limit the scope of the invention. The example shows that the desil~d gloss, from high values to low values, can be obtained by s~ecting a baking tempeld~ure between about 140C (ie~ about 50oC above the melting tempel alur~) to about 220 oC (i.e., well below the hi~h~t tenlperdlule at which the polymer is stable). It also shows that better me~h~nic~l properties can be obtained that with PVdF.

Example 1 The following white-pigmented powder co~ting composition was pr~pdred:
- fluorinated polymer 54.9 pbw (parts by weight) - acrylic polymer 243 pbw - li~liulll dioxide 21.1 pbw 100.0 The fluorinated polymer is a terpolymer con~ g of about 75 wt% of VdF monomer units and about 12.5 wt% of each of HFP and TFE monomer units (as ~lelell,lilled from NMR data by the method of Pianca et al. in Polymer ~, æ4, 1987); said polymer having the following properties accolding to its te~hnir~l d~t~heet--melting point about90 C ASTM D-3418 -specific gravity 1.846 g/cm3 ASTM D-792 -refractive inde~ (nD25) 1384 ASTM D-524 -tensile ~ng~l @ yield 8 MPa ASTM D~82 -tensile strength @ break 15 MPa ASTM D-882 ~lon~tion @ break 990 % ASTM D 882 S -tensile modulus 130 MPa ASTM D-882 -solution vi~i~y 20 mPa-s Bro~ 1 LV
(20 wt% in MEK at 25 oC) #1 spindle @ 30 rpm -stability up to 316 C
The acrylic polymer is a commercially available thermoplastic copolymer of 70 wt%
10 methyl methacrylate monomer and 30 wt% of ethyl acrylate monomer, having a vi~cosi~y of about 12 P in 40% solution in a 9S S by weight mixture of toluene and ethylene glycol methyl ether, cont~ining 3 wt% (relative to the total acrylic polymers) of a low molecular weight copolymer which colllprised 30 wt% ethyl acrylate monomer and 70 wt% 2-ethylhexyl acrylate monomer, having a viscosi~y of about 1.06 Pa-s at 98.9 oC.
The r~l.lltin~ mixtures were extruded under the following conditions, giving pellets of 3 mm in ~ meter and about 2 mm in length:
- twin screw extruder - screw rotation: 200 rpm - load: 85%
- tempe~a~ul~ profile: 100C at hopper exit, rising to 1300C in the middle of the screw, then llS oC up to the end of the screw, - tempelaLul-e of the material at the exit: 115~

The pellets were cooled in liquid nitrogen down to about -150 oC, then ground at a tempeldlul~ of about -100 oC in a h~mtn~r mill and sieved to çlimin~te particles larger than about 150 ,~n. In the hammer mill, a rotation shaft carries hammers which break the pelle~ts on fixed shapes in the casing of the h~mm.o.r mill and hammer them through a sieve lining S at the bottom of the h~mm~r mill. The particle size di~LIiluuLion was me~ red and 99%
of the parhcles had a size below 90 ,~an, with 40% below 32 ,~n. Only 5% had a size below 15 ,~n.
The rçs~llting ~wd~l~ were applied by an ele~;l,~laLic spray process on both sides of a 1 mm thick rolled steel plate previously covered by a 50 ,um layer of an epoxy primer.
10 The epoxy primer had been prepaled and applied accordillg to Example 1 of Applicants' European Patent, No. 404752A
Two coated ~ul~LIaLes were then heated 20 ll~inll(~ in a tempe,alure-gradient oven, giving top c~tin~ of 80 ,~n thickness The following results were obtained in the two separate determin~tiorl~:
lS L
TeL4~ uu~ ( C) 121 126 131 138 144 157 164 171 179 Gloss (60-CISO-2813) 49 56 59 62 60 61 SS 51 44 30 II.
Temperature ( C) 149 158 168 177 188 199 210 æ 235 249 Glcss (60 C IS0-2813) 59 50 41 26 20 18 18 16 16 16 21~0299 A further coated sul~Llale was baked at 200OC for 10 minutes and the following properties were m~ ~l after cooling:
- direct impact > 100 kg.cm ASTM D-2794 e impact > 100 kg cm ASTM D-2794 S - flexibility (conical mandrel) 0 mm ISO 6860 - flexibility (Erichsen) 7.7 mm ISO 1510 -- adherence 0 GT ISO 2409 In the fo,eg~ g examples and written de~.i~tion, ~e following units were abbreviated as in ~ tef1 ~1scosities Energy Poise - P Joules - J
Pascal seconds - Pa-s Joules per gram - J/g ;p~l seconds - dPa-s Pr~ure and Force ~lillir~ l seconds - mPa-s Pascals - Pa Me~ l-seconds - MPa-s Megapascals - MPa The following p~tent~ are hereby incorporated by l~rerellce into this appli~ti- n FR 2,636,959 - Atochem US 5 ~29,460 EP 404752~

Claims (11)

1. A powder coating product consisting essentially of:
(a) a resin component consisting essentially of:
(i) from 60 to 90 wt% of at least one low melting point fluorine- based terpolymer, (ii) from 40 to 10 wt% of at least one compatible resin; and, (b) from 1 to 35 parts by weight of at least one pigment per 100 parts by weight of the resin component.

2. The powder coating product of claim 1, wherein the terpolymers have a melting temperature below about 150°C.

3. The powder coating product of claim 1 wherein said terpolymers have a melting temperature below about 100°C.

4. The powder coating product according to either of claims 1 and 2 or 3 wherein the terpolymers are made from three monomers selected from the group consisting essentially of vinylidene fluoride (VdF), tetrafluoroethylene (TFE), chlorotrifluoroethylene, vinyl fluoride, hexafluoropropene (HFP) and CF3CF-2-CF=CF2.

5. The powder coating product according to claim 4 wherein the terpolymers are prepared from 65 to 85 wt% of VdF monomer units with from 7.5 to 17.5 wt% each of TFE
and HFP.

6. The powder coating product according to claim 5 wherein the terpolymers have a melt viscosity of from 9,000 to 15,000 dPa-s measured at 125°C

7. The powder coating product according to claim 6 wherein the compatible resin is thermoplastic polymethylmethacrylate.

8. The powder coating product according to claim 7 wherein the weight ratio of the terpolymer to the compatible resin is about 70:30.

9. A process for preparing coating from the powder coating product according to any one of claims 1 to 8, essentially characterized by a selection of the baking temperatures.

10. The use of the powder coating product according to any one of claims 1 to 7 to produce regulable-gloss coatings.

11. The use of the powder coating product according to any one of claims 1 to 7 to produce low gloss coatings.