A Powder Coating Composition And A Process For Forming A Coating On A Substrate


Updated about 2 years ago

Abstract

The present invention relates to a powder coating composition which incorporates,by dry blending ,a wax-coated silica additive in finely divided form.The present invention also relates to a process for forming a coating on a substrate.

Information

Application ID IN/PCT/2000/899/CHE
Invention Field METALLURGY
Date of Application
Publication Number 17/2007

Applicants

Name Address Country Nationality

Specification

This invention relates to powder coating composition and a process for forming a coating on a substrate.
Powder coatings form a rapidly growing sector of the coatings market. Powder coatings are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate is earthed. Charging of the powder in the spray gun is effected by means of an applied voltage or by the use of friction (tribo-charging). Conversion of the adherent particles into a continuous coating (including, where appropriate, curing of the applied composition) may be effected by heat treatment and/or by radiant energy, notably infra-red, ultra-violet or electron beam radiation. The powder coating particles which do not adhere to the substrate can be recovered for re-use so that powder coatings are economical in use of ingredients. Also, powder coating compositions are generally free of added solvents and, in particular, do not use organic solvents and are accordingly non- polluting.
Powder coating compositions generally comprise a solid film-forming resin,
usually with one or more colouring agents such as pigments, and optionally also
contain one or more performance additives. They are usually thermosetting,
incorporating, for example, a film-forming polymer and a corresponding curing agent
(which may itself be another film-forming polymer), but thermoplastic systems
(based, for example, on polyamides) can in principle be used instead. Powder coating
compositions are generally prepared by intimately mixing the ingredients (including
colouring agents and performance additives) for example in an extruder, at a
temperature above the softening point of the film-forming

polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, for example by grinding, to the desired particle size.
In the case of powder coating compositions which are to be applied by electrostatic spraying, film thicknesses of 40 microns to over 100 microns are common. The parti¬cle size distribution of the composition will normally be in the range of from 0 to 120 microns, with a mean particle size in the range of from 15 to 75 microns, preferably 25 to 50 microns, more especially 20 to 45 microns.
Powder coating compositions of such conventional particle size are widely used in the industry. Some of the known drawbacks encountered with these materials are associated with the application characteristics of the powders. The ease with which the powder fluidises and is transported through the application equipment affects the uniformity of film weight distribution across the substrate and as a consequence the amount of powder needed to achieve "on average" the desired film thickness. With these conventional powder coatings the first-time deposition of the fine particles (especially powder particles of 10 microns diameter or less) is inefficient leading to an accumulation of fine particles (fines) in the over-sprayed material. In many instances, the over-sprayed material is collected and recycled for re-use. In such systems, as the powder application process continues, the percentage of fine powder particles in the recycled powder increases and the cohesive behaviour of the fines starts to influence the properties of the recycled powder. The major effect is that the powder loses fluidity and this causes increasing difficulties in transporting the powder through the recycle system and back to the spray gun.
Another well-known problem with using powder coating compositions of such conventional particle size

iistribution is the difficulty of applying thin film coatings of, say, 30 microns or less, for which there is an increasing demand in certain sectors of the powder roating market, whilst achieving a uniform opacity and an aesthetically pleasing appearance, particularly in a gloss white coating. Using compositions of conventional particle size distribution, the achievement of such results is possible only within a restricted range of powder chemistries, with the best performance generally being obtained with polyurethane powders using blocked isocyanates. To achieve coatings of less than 20 microns with a uniform opacity and an aesthetically pleasing appearance with compositions of conventional particle size distribution is very difficult if not impossible. The problems encountered ("orange-peel" imperfections, etc.) are considered to be attributable to the relatively large size of the majority of the particles in powder coating compositions of conventional particle size distribution.
In addition to the increasing demands within the powder coatings market itself, it has also been recognised that the inability of powder coatings reliably and routinely to achieve film thicknesses of 30 microns or less with aesthetically pleasing appearance is one of the factors that has inhibited further substitution of solvent-based "wet" paints by powder coatings.
It has been proposed that the problems of achieving satisfactory thin film powder coatings can in principle be alleviated with the use of powder coating compositions of a finer particle size. There are problems, however, in the fluidisation, handling and application of rela¬tively small particles, especially particles which are 10 microns in diameter or less. Such problems become more pronounced as the proportion of fine particles increases, and powder coating compositions have conventionally been manufactured so as to comprise not

more than 10% by volume of particles which are 10 microns in diameter or less.
WO 94/11446 discloses powder coating compositions which incorporate, by dry-blending, various combinations of two or more additives, the preferred combination being aluminium oxide with aluminium hydroxide. By using combinations of dry-blended additives in accordance with WO 94/11446, it is possible to alleviate the problems of fluidisation, handling and application of fine particles (especially those of 10 micron diameter or less) as outlined above and also to alleviate other problems attributable to differential and premature electrostatic charging of the powder particles. WO 94/11446 is directed to powder coating compositions of which at least 95% by volume has a particle size not exceeding 50 microns.
Although good results are achievable using the additive combinations disclosed in WO 94/11446, it has been found inter alia that the optimum blend of aluminium oxide/aluminium hydroxide, in terms of the fluidity, transport and handling characteristics of the powder coating composition, tends to detract from the utility of the composition in tribostatic application processes. Attempts to compensate for that effect by increasing the proportion of aluminium oxide in the additive combination tend to result in powder coating compositions having less good fluidity and transport properties.
EP-A-0 300 818 suggests that tribo-charging charac¬teristics can be conferred on powder coating compositions of conventional particle size distribution by the use of an additive which comprises one or both of aluminium oxide and/or hydroxide, which has been sheared (by high¬speed shearing or milling) such that the additive resulting therefrom is a fine, essentially aggregate-free powder which comprises at least 5% by weight of particles of maximum size 0.2 microns. According to EP-A-0 300 818, a preferred way of achieving the

specified particle size requirement is to add to the aluminium oxide/hydroxide, prior to shearing, a proportion of a fine particle extender of maximum particle size 0.2 microns. The preferred extender disclosed in EP-A-0 300 818 is fumed silica, but it has been found that the inclusion of fumed silica tends to detract from the efficiency of an aluminium oxide/ aluminium hydroxide additive (as proposed in WO 94/11446) in terras of fluidity and transport characteristics, so that the optimum benefit of the additive combination in terms of those characteristics is not achieved.
The present invention provides a powder coating composition which incorporates, by dry blending, a wax-coated silica in finely-divided form.
The term "coating" as used herein in relation to silicas for use according to the invention includes impregnation of porous silica materials, and the expression "coated silica" is to be understood accordingly.
The term "silica" as used herein includes materials obtained by pyrogenic and, preferably, wet processes leading to precipitated silicas or silica gels, as well as, in principle, mixed metal-silicon oxides and naturally-occurring materials such as, for example, diatomaceous earth. Silicas for use according to the invention will in general have an amorphous structure. The term "silica" includes silicic acid materials. Silicates also come into consideration.
A preferred material comprises micronised silica
gel
The term "wax^" as used herein includes:
i) Natural animal waxes (for example, beeswax,
lanolin); ii) Natural vegetable waxes (for example,
carnauba wax); iii) Natural petroleum waxes (for example,
paraffin wax, microcrystalline wax);

iv) Synthetic waxes (for example, ethylenic polymers and polyol ether-esters).
Mineral waxes other than petroleum waxes may also come into consideration.
An important group of waxes for use in accordance with the invention comprises esters of long-chain aliphatic alcohols (typically Cig and above) with long-chain fatty acids (typically CK, and above) . Such esters and acids are preferably straight-chain compounds, and may be saturated or unsaturated. Examples of acids which may be used include stearic acid, palmitic acid and oleic acid and mixtures of two or more thereof.
Waxes derived from long-chain aliphatic compounds as described above may include hydrocarbons.
In addition to esters of the long-chain acids as described above there may be mentioned salts such as, for example, aluminium stearate.
Preferred wax materials for use in accordance with the invention are materials which have good compatibility with the polymer component(s) of the powder coating composition, that is to say, materials which can be mixed homogeneously with the polymers without significant phase separation. It will be found that some wax materials (for example, halogenated waxes) are in general not compatible in this sense with the powder coating polymer(s). The use of such materials would be expected to give rise to defects in the surface appearance of the finished applied coating, and is accordingly not recommended.
Wax-coated silicas suitable for use in accordance with the invention include commercially available materials such as, for example, GASIL 937 £2 Crosfield (a silica gel coated with microcrystalline paraffin wax) and OK 607 £2 Degussa (a similar material with a coating which also includes a short-chain [C^] saturated amine or alkyl ammonium component).

Coating of the silica material may be effected by methods known in the art, for example, by co-milling of the silica with a solid wax material, or by admixing the silica material with a wax material dissolved in a suitable solvent which is then evaporated.
The amount of wax coated onto the silica may, for example, be in the range of from 2 to 10% by weight, based on the weight of the silica.
Further information concerning wax-coated silicas which may be used in accordance with the invention may be found in U.S. Patent Specifications Nos. 3 607 337 and 3 816 154, and in WO 97/08250. In addition to wax-coated silica, a powder coating composition of the invention may incorporate, also by dry blending, aluminium oxide and/or aluminium hydroxide, preferably aluminium oxide or aluminium oxide and aluminium hydroxide. Aluminium oxy-hydroxide may be used in addition to or instead of aluminium hydroxide. It is believed that any of the main structural types of these materials may be used, that is to say:
a - AI2O3 Corundum a - AlO(OH) Diaspore a - A1(0H)3 Bayerite 1 - AI2O3
Y - AIO(OH) Boehmite y - A1(0H)3 Gibbsite Preference may be given to y-structural types. The proportion of wax-coated silica incorporated in a powder coating composition of the invention may in general be in the range of from 0.002 to 2.0% by weight, based on the total weight of the composition without the additive, advantageously from 0.02 to 1.5% by weight and preferably from 0.04 to 1.0% by weight, more especially at least 0.2% by weight, especially 0.3 to 0.7% by weight, for example, 0.3 to 0.5% by weight.
The total content of the wax-coated silica

additive (s) and, if present, the other dry-blended additive (s) specified above, incorporated in a powder coating composition of the invention, may in general be in the range of from 0.1 to 5% by weight, based on the total weight of the composition without the additive(s), advantageously 0.1 to 2% by weight, preferably at least 0.2% by weight, especially 0.2 to 1.5% by weight, and more especially 0.3 to 1% by weight.
In the case in which the powder coating composition includes dry-blended additives comprising wax-coated silica and aluminium oxide, the relative proportions of silica to aluminium oxide may in general be in the range of from 99:1 to 1:99, advantageously from 80:20 to 20:80, and preferably 70:30 to 30:70, for example 50:50.
In the case in which the dry-blended additives comprise wax-coated silica and aluminium hydroxide, the relative proportions of silica to the aluminium hydroxide may in general be from 99:1 to 30:70, advantageously from 90:10 to 40:60, preferably from 80:20 to 50:50, for example 65:35.
In the case in which the dry-blended additives comprise wax-coated silica, aluminium oxide and aluminium hydroxide, the relative proportions of the additives may in general be as follows:
SiOj AI2O3 Al (OH) 3
1 to 98% 1 to 98% 1 to 70%
advantageously 5 to 50% 10 to 90% 1 to 60%
preferably 10 to 30% 20 to 85% 1 to 55%
In preferred forms of composition according to the invention, the dry-blended additive(s) consist solely of wax-coated silica or, as the case may be, wax-coated silica with aluminium oxide and/or aluminium hydroxide.
As a generality, the greater the proportion of particles of below 10 microns in diameter in the composition, the higher the proportion of dry-blended additive(s) that is preferred for use in accordance with

the invention. This relationship may be illustrated by the following table, which also includes (as independent variables) typical ranges for d(v)5o and d(v)99 for the compositions:
% fay volume d(v)5o d(v)99 % additive (s)
below 10 microns by weight

< 18%
< 12%
< 8%
18-22 microns 52-58 microns 0.6 - 1.0
25-30 60-80 0.4 - 0.6
30-40 80-120 0.2 - 0.4
Advantageously, in the case in which there is more than one dry-blended additive, the additives are pre-mixed, preferably intimately and homogeneously by a high-shear technique, before being incorporated in the powder coating composition. In the case of three dry-blended additives, all three may be blended together in a single mixing operation, or any two of the three may first be blended together with the third additive being mixed in afterwards.
Although any additive or mixed sub-combination of additives may in principle be incorporated separately in the powder coating composition, pre-mixing of additives is generally preferred.
The additive(s) of the invention may be incorporated in the powder coating composition by any available dry-blending method, for example:
(a) injection at the mill, with the chip and additive(s) fed into the mill simultaneously;
(b) introduction at the stage of sieving after milling; and. .,
(c) post-production blending in a "tumbler" or other suitable mixing device.
The particle size of each dry-blended additive may be up to 5 microns, or even up to 10 microns in some cases. Preferably, however, the particle size is not greater than 2 microns, and is more especially not

greater than 1 micron. In general, the lower the thickness of the coating that is to be applied, the smaller the particle size of the additives. The preferred minimum additive particle size is 0.1 microns.
The particle size distribution of the powder coating composition may be in the range of from 0 to 120 microns, with a mean particle size in the range of from 15 to 75 microns, preferably 25-50 microns, more especially 20 to 4 5 microns.
In the case of relatively fine size distributions, especially where relatively thin applied films are required, for example, the powder coating composition may be one in which one or more of the following criteria is satisfied:
a) 95-100% by volume < 50 \im
b) 90-100% by volume < 40 )im
c) 45-100% by volume < 20 |j.m
d) 5-100% by volume < 10 p.m preferably 10-70% by volume < 10 \vxe) 1-80% by volume < 5 (xm preferably 3-40% by volume < 5 ^.m
f) d(v)5o in the range 1.3-32 |im preferably 8-24 |am
Powder coating compositions generally comprise a solid film-forming resin, usually with one or more colouring agents such as pigments, and optionally also contain one or more performance additives.
A powder coating composition for use according to the invention will in general be a thermosetting system (incorporating, for example, a film-forming polymer and "a corresponding curing agent which may itself be another film-forming polymer), but thermoplastic systems (based, for example, on polyamides) can in principle be used instead.
The film-forming polymer used in the manufacture of a thermosetting powder coating composition for use

according to the invention may be one or more selected from carboxy-functional polyester resins, hydroxy-functional polyester resins, epoxy resins, and functional acrylic resins.
The composition may, for example, be based on a solid polymeric binder system comprising a carboxy-functional polyester film-forming resin used with a polyepoxide curing agent. Such carboxy-functional polyester systems are currently the most widely used powder coatings materials. The polyester generally has an acid value in the range 10-100, a number average molecular weight Mn of 1,500 to 10,000 and a glass transition temperature Tg of from SO^C to 85°C, preferably at least 40°C. The polyepoxide can, for example, be a low molecular weight epoxy compound such as triglycidyl isocyanurate (TGIC), a compound such as diglycidyl terephthalate or diglycidyl isophthalate, an epoxy resin such as a condensed glycidyl ether of bisphenol A or a light-stable epoxy resin. Such a carboxyfunctional polyester film-forming resin can alternatively be used with a bis(beta-hydroxy-alkylamide) curing agent such as tetrakis(2-hydroxyethyl) adipamide.
Alternatively, a hydroxy-functional polyester can be used with a blocked isocyanate-functional curing agent or an amine-formaldehyde condensate such as, for example, a melamine resin, a urea-formaldehyde resin, or a glycol ural formaldehyde resin, for example, the material "Powderlink 1174" supplied by the Cyanamid Company, or hexahydroxymethyl melamine. A blocked isocyanate curing agent for a hydroxy-functional- polyester may, for example, be internally blocked, such as the uret dione type, or may be of the caprolactam-blocked type, for example, isopherone diisocyanate.
As a further possibility, an epoxy resin can be used with an amine-functional curing agent such as, for example, dicyandiamide. Instead of an amine-functional

curing agent for an epoxy resin, a phenolic material may be used, preferably a material formed by reaction of epichlorohydrin with an excess of bisphenol A (that is to say, a polyphenol made by adducting bisphenol A and an epoxy resin). A functional acrylic resin, for example a carboxy-, hydroxy- or epoxy-functional resin can be used with an appropriate curing agent. Mixtures of binders can be used, for example a carboxy-functional polyester can be used with a carboxy-functional acrylic resin and a curing agent such as a bis(betahydroxyalkylamide) which serves to cure both polymers. As further possibilities, for mixed binder systems, a carboxy-, hydroxy- or epoxy-functional acrylic resin may be used with an epoxy resin or a polyester resin (carboxy- or hydroxy-functional). Such resin combinations may be selected so as to be co-curing, for example, a carboxy-functional acrylic resin co-cured with an epoxy resin, or a carboxy-functional polyester co-cured with a glycidyl-functional acrylic resin. More usually, however, such mixed binder systems are formulated so as to be cured with a single curing agent (for example, use of a blocked isocyanate to cure a hydroxy-functional acrylic resin and a hydroxy-functional polyester). Another preferred formulation involves the use of a different curing agent for each binder of a mixture of two polymeric binders (for example, an amine-cured epoxy resin used in conjunction with a blocked isocyanate-cured hydroxy functional acrylic resin).
Other film-forming polymers which may be mentioned include functional fluoropolymers, functional fluorochloropolymers and functional fluoroacrylic polymers, each of which may be hydroxy-functional or carboxy-functional, and may be used as the sole film-forming polymer or in conjunction with one or more functional acrylic, polyester and/or epoxy resins, with appropriate curing agents for the functional polymers.
Other curing agents which may be mentioned include

epoxy phenol novolacs and epoxy cresol novolacs; isocyanate curing agents blocked with oximes, such as isopherone diisocyanate blocked with methyl ethyl ketoxime, tetramethylene xylene diisocyanate blocked with acetone oxime, and Desmodur W (dicyclohexylmethane diisocyanate curing agent) blocked with methyl ethyl ketoxime; light-stable epoxy resins such as "Santolink LSE 120" supplied by Monsanto; and alicyclic poly-epoxides such as "EHPE-3150" supplied by Daicel.
The following ranges should be mentioned for the total film-forming resin content of a powder coating composition according to the invention (including curing agent, where appropriate, but disregarding dry blend additives):
40% to 100% by weight,
47% to 100% by weight,
47% to 90% by weight,
53% to 99% by weight, and
53% to 74% by weight.
As already explained, there may be more than one film-forming resin binder and curing agent, as appropriate.
A powder coating composition for use according to the invention may be free from added colouring agents, but usually contains one or more such agents (pigments or dyes). Examples of pigments which can be used are inorganic pigments such as titanium dioxide, red and yellow iron oxides, chrome pigments and carbon black and organic pigments such as, for example, phthalocyanine, azo, anthraquinone, thioindigo, isodibenzanthrone, triphendioxane and quinacridone pigments, vat dye pigments and lakes of acid, basic and mordant dyestuffs. Dyes can be used instead of or as well as pigments.
The composition of the invention may also include one or more extenders or fillers, which may be used inter alia to assist opacity, whilst minimising costs, or more generally as a diluent.

The following ranges should be mentioned for the total pigment/filler/extender content of a powder coating composition according to the invention (disregarding dry blend additives):
0% to 55% by weight,
0% to 50% by weight,
10% to 50% by weight,
0% to 45% by weight, and
25% to 45% by weight
Of the total pigment/filler/extender content, a pigment content of

Documents

Name Date
IN-PCT-2000-899-CHE-RELEVANT DOCUMENTS [10-02-2020(online)].pdf 2020-02-10
IN-PCT-2000-899-CHE-RELEVANT DOCUMENTS [08-01-2019(online)].pdf 2019-01-08
IN-PCT-2000-899-CHE-RELEVANT DOCUMENTS [03-01-2018(online)].pdf 2018-01-03
Form 27 [05-01-2017(online)].pdf 2017-01-05
Other Document [01-07-2016(online)].pdf 2016-07-01
Assignment [28-04-2016(online)].pdf 2016-04-28
Form 16 [28-04-2016(online)].pdf 2016-04-28
Power of Attorney [28-04-2016(online)].pdf 2016-04-28
Other Document [13-04-2016(online)].pdf 2016-04-13
Form 27 [11-01-2016(online)].pdf 2016-01-11
ipindiaonline.gov.in_epatentfiling_online_frmPreview.asp.pdf 2015-03-12
in-pct-2000-0899-che abstract-duplicate.pdf 2011-09-05
in-pct-2000-0899-che abstract.pdf 2011-09-05
in-pct-2000-0899-che claims-duplicate.pdf 2011-09-05
in-pct-2000-0899-che claims.pdf 2011-09-05
in-pct-2000-0899-che correspondence-others.pdf 2011-09-05
in-pct-2000-0899-che correspondence-po.pdf 2011-09-05
in-pct-2000-0899-che description(complete)-duplicate.pdf 2011-09-05
in-pct-2000-0899-che description(complete).pdf 2011-09-05
in-pct-2000-0899-che form-1.pdf 2011-09-05
in-pct-2000-0899-che form-19.pdf 2011-09-05
in-pct-2000-0899-che form-26.pdf 2011-09-05
in-pct-2000-0899-che form-3.pdf 2011-09-05
in-pct-2000-0899-che form-5.pdf 2011-09-05
in-pct-2000-0899-che others.pdf 2011-09-05
in-pct-2000-0899-che pct.pdf 2011-09-05
in-pct-2000-0899-che petition.pdf 2011-09-05

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