GB1572996A - Coating compositions - Google Patents

Description

(54) COATING COMPOSITIONS (71) We, REED INTERNATIONAL LI MITED, a British Company of 82 Piccadilly, London, W.1 do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with improvements in or relating to the use of powder coating compositions.

Powder coating compositions are widely used, in the industrial finishing of articles such as consumer durables having a metallic, wood or plastics surface requiring the application of a decorative or protective wear surface. Such compositions comprise a mixture of resin and pigment and may be produced by grinding resin and pigment together in a suitable device.

Frequently efficient grinding is only achieved in a number of separate steps including, for example, (a) preliminary grinding of resin and pigment, (b) a plasticising step wherein the resin is rendered at least semi-fluid and the pigment is uniformly dispersed therein, and (c) a subsequent grinding and pulverising step in which the desired powder is obtained. In general a final seiving step will be introduced so as to grade the powder to a chosen particle size range which will, for most applications, be less than 120,u..

In the above discussion reference has been made only to the use of resin and pigment; various other components are however commonly used in the production of powder coating compositions. Thus fillers may be included as well as aids to the hardening of the resin such as cross-linking agents and the like although resins which are self crosslinking or mixtures of resins which react with one another may be used without additional cross-linking agents. Furthermore the compositions may include the usual flow promoting agents.

Powder coating compositions offer significant advantages to the industrial paint user. In particular one can achieve rapid build up a good film of the desired thickness on a surface using such compositions.

Moreover there are economies of storage and transportation costs to be gained from the absence of large quantities of solvent which are traditionally present in conventional paints. One is thus able to store the powder in bags (rather than much more costly drums) until just prior to use.

Powder coating compositions are, in general, applied by means of spraying or fluidized bed techniques in each instance using electrostatic means. A fine spray of dry powder is impinged upon the surface to be coated which may be heated or, alternatively, may be subjected to heating after spraying the dry powder. When the surface is heated the resin in the powder should soften then flow and finally cure on or shortly after impact so that a satisfactory film is quickly obtained. Because they can be easily pre- an post- heated metallic surfaces are preferred for the application of powder coating compositions.

In the heating of a surface coated as described above the flowing and curing of the resin are rarely ever totally separate steps. Thus, in general, curing commences before the resin has flowed completely and this produces a slight orange peel effect which is a known characteristic of films deposited as described above. Moreover, while films of a uniform thickness can be obtained one is not readily able to achieve a film thickness of less than 1 thousandth of an inch which may be desired in certain circumstances.

The spraying of a dry powder involves the use of specialised and expensive equipment. Thus, the user who intends to spray dry powders is put to considerable capital expenditure to acquire plant that is only of utility in the deposition of powder coating compositions. Furthermore, in the spraying process, very considerable quantities of dust are generated which can be a serious health hazard when inhaled by workmen or other persons using the spray shop as, frequently, the resin and pigment are toxic.

We have now invented a method of using powder coating compositions which enables one to preserve the advantages and convenience associated with a solid while avoiding the difficulties discussed above normally associated with such compositions.

According to the present invention therefore we provide a method of manufacturing a paint which comprises dissolving a powder coating composition in a volatile organic solvent having a boiling point below 250"C.

Paint manufactured by the method according to the invention may be applied using equipment normally used in the application of solution paints. Thus the substantial capital expenditure of installing equipment for spraying powders is eliminated as the paints may be applied using existing plant. Moreover, one gains other advantages nor normally associated with powder coating compositions when using paints manufactured according to the invention. In particular one is able to obtain smoother films and exercise greater control over the resulting dry film thickness. Film thicknesses of + thousandth of an inch may be readily achieved.

The process of applying paint manufactured by the method according to the invention may be linked to a solvent recovery plant thereby avoiding pollution of the environment and the recovered solvent may be re-used in the manufacture of more paint thereby avoiding waste. Solvent may thus be purchased in bulk and a given quantity may be constantly reused while making any small replenishments from stock as required.

The method according to the invention is primarily of application in the field of thermosetting resins since, in general, thermoplastic polymers that are normally applied as powders have limited solubility in most readily available organic solvents.

Moreover, the chosen solvent is preferably one which dissolves the resin at relatively low temperature thereby -eliminating the risk that, in the case of thermosetting resins, any reaction of the resin will occur during solubilisation.

In addition to the advantage of a saving in storage space by virtue of the fact that the powder coating compositions may be stored in the absence of large quantities of solvent, the method according to the invention offers the further advantage that solutions of paint may be made up when required. Thus a complete range of paints need not be stored by the paint user; all that is necessary is that an appropriate range of powder coating compositions should be stored which may be blended (if necessary) and dissolved in an organic solvent to yield the desired paint when required.

Having regard to the fact that the resin itself may well be prepared in the presence of an organic solvent and that care is taken to separate the resin from the solvent prior to pigmentation it is not at all obvious to then dissolve the pigmented composition in an organic solvent. Nevertheless we have found that paints particularly suited for application by a variety of processes such as roller coating and spraying (manual or automatic) may be formulated by the method according to the invention. Moreover it is possible to obtain paints having a solids level substantially higher than it has normally been possible to obtain with conventional paint formulation techniques.

Thus, with conventional powder coating compositions we have achieved solids of greater than 60% by weight and even of the order of 70% by weight using the method according to the invention and this has resulted in a consequent improvement in operating efficiency. In particular a desired film thickness may be built up more quickly and with fewer operating steps using paints formulated by the method according to the invention than with conventional paints.

A still further advantage of the method according to the invention is in the preparation of the powder coating composition itself. Thus, a substantially simplified method of preparing the powder may be employed.

In particular, once the pigment has been dispersed in the resin the resulting material may be roughly ground and a final sieving step may be eliminated. If desired large particles may be separated off and any fine dust blown from the roughly ground material to hasten the process of dissolution. However, particles of relatively large or small size may be used with equal advantage provided that the paint user is not inconvenienced by the somewhat longer period of solubilisation in the organic solvent that may be necessary in these circumstances.

As stated above the method according to the invention is primarily of application in the field of thermosetting resins. The resin employed is conveniently a saturated polyester resin of relatively low molecular weight (e.g. from 400 to 1500). Such polyester resins may be formed by reacting an aliphatic glycol with an aliphatic dicarboxylic acid, e.g. adipic acid, if desired in admixture with an aromatic discarboxylic acid, e.g. terephthalic acid, to yield a product with residual reactive hydroxyl or carboxyl groups. The polyester resin may be used in admixture with a reactive material such as an approp riate amine resin for example hexamethoxymethylmelamine or an epoxy resin. The reactive material causes cross-linking of the polyester in the curing process through reaction with the reactive terminal hydroxyl and/or carboxyl groups.

Another convenient class of resins to employ are epoxy resins such as those commercially available products formed by reaction of materials such as Bisphenol A with epichlorohydrin. The epoxy resin should be cross-linkable in the curing step with the normal readily available crosslinking agents such as derivatives of amides such as dicyandiamide.

A still further class of resins which may be pigmented for use in the production of paints by the method according to the invention are those produced by polyaddition reactions such as acrylic resins. Acrylic resins may be formed by polymerisation of lower alkyl (Cl-C8) esters of acrylic and methacrylic acid e.g. methyl, ethyl and 2-ethylhexyl esters or copolymerisation of such monomers if desired, with the addition of other unsaturated monomers such as vinyl acetate, acrylic acid or methacrylic acid.

Another class of resin which may be produced by a polyaddition reaction are polyvinyl resins which may be formed by polymerisation of monomers containing a vinyl group such as vinyl acetate and/or vinyl chloride.

Pigmented polyurethane resins may also be used in the manufacture of powder coating compositions for use according to the invention. Such resins may be formed by reaction of an aliphatic and/or aromatic di- and/or tri-isocyanate with compounds containing active hydrogen atoms present as amino or hydroxyl groups for example aromatic amines and polymeric polyols. The polyurethane resins preferably contain blocked isocyanate groups so that they may be compounded with the chosen crosslinker. Curing of such resins will not occur until the blocking group has been splitvoff so as to make the isocyanate groups available for reaction with the cross-linking agent. The preferred blocking agents are those that are simply split off by raising the temperature and, in this regard caprolactam is a particularly preferred compound to use to block the free isocyanate groups.The cross-linking agent may be any suitable material such as a polyester having terminal carboxyl groups.

A hardener or cross-linking agent appropriate to the resin system employed may be present in the powder coating composition.

Alternatively the hardener or cross-linking agent may be added during manufacture of paint by the method according to the invention.

The organic solvent must be volatile at the conditions chosen for completion of the film. Thus, when film formation is completed by curing the solvent should be readily volatilisable at the stoving temperature. In general we prefer the solvent to have a boiling point from 70-200"C.

The organic solvent should be chosen according to the character of the pigmented resin. Thus, in the case of polyester and/or epoxy ester resins, the organic solvent may conveniently be a glycol ether or a mixture of a glycol ether with an aromatic hydrocarbon solvent.

Suitable solvents include esters such as ethyl acetate or isopropylacetate; glycol ethers such as ethylene glycol monomethyl ether; glycol esters such as 2-ethoxyethyl acetate and 2-butoxyethyl acetate; aliphatic and alicyclic ketones such as methyl ethyl ketone, methyl isobutyl ketone or cyclohexanone; and aliphatic and alicyclic ethers such as tetrahydrofuran. The solvent may be employed in admixture with a diluent such as a hydrocarbon for example benzene, toluene or xylene. It may be convenient to add an unsaturated monomer with the organic solvent and such a monomer may itself be a solvent for example styrene.

Paints manufactured by the method according to the invention may be applied by any conventional process used in the surface coatings industry for the application of a liquid paint. Thus roller coating; spraying for example hot spray, airless spray or electrostatic spray; sheet, dip or curtain coating may all be employed with equal advantage.

The following example illustrates the invention. Parts referred to are, unless otherwise stated, parts by weight.

Example A dry pigmented resin powder was prepared as follows: 100 parts of an acrylic resin ("Bayer" (registered Trade Mark) EPS Polyacrylic Development Product L 2269), 20 parts of cross-linking agent ("Bayer" EPS Crosslinking Agent Development Product L 2272), 60 parts of titanium dioxide and 1 part of polymeric acrylic based flow promoting agent (MODAFLOW (registered Trade Mark) (Powder form)) were given a preliminary grinding and then thoroughly mixed in a screw extruder. The extrusion temperature was maintained at about 90"C (at which temperature the resin was rendered semi-fluid) and the residence time was about 60 seconds. The resulting product was ground and pulverised to yield a powder and subject to a final seiving step so as to grade the powder to particles of a size less than 120 ,(t.

7 parts of the dry pigmented resin powder were added gradually to 30 parts of stirred 2-butoxy ethyl acetate to yield a solution paint. The paint was applied to a degreased metal panel by conventional roller coating and stoved to a temperature of 1800C. for 20 minutes to yield a completed film possessing the desired properties.

WHAT WE CLAIM IS: 1. A method of manufacturing a paint which comprises dissolving a powder coating composition in a volatile organic solvent having a boiling point below 250"C.

2. A method as claimed in claim 1 wherein the powder coating composition comprises a thermosetting resin.

3. A method as claimed in claim 1 or claim 2 wherein the solvent is one in which the resin dissolves at relatively low temperatures.

4. A method as claimed in any of the preceding claims wherein the powder coating composition is manufactured by dispersing pigment in a resin and roughly grinding the resulting material with a final sieving step or other step of grading particle sizes.

5. A method as claimed in any of the preceding claims wherein the powder coating composition comprises a saturated polyester resin of relatively low molecular weight.

6. A method as claimed in any of the preceding claims wherein the powder coating composition comprises an epoxy resin formed by reaction of Bisphenol A with epichlorohydrin, the said epoxy resin being capable of undergoing cross-linking reactions.

7. A method as claimed in any of the preceding claims wherein the powder coating composition comprises an acrylic resin.

8. A method as claimed in any of the preceding claims wherein the organic solvent has a boiling point of 70-200"C.

9. A method as claimed in claim 1 substantially as described herein.

10. A method as claimed in claim 1 substantially as described herein with reference to the Example.

11. Paint whenever manufactured by a method as claimed in any of the preceding

Claims (1)

1. claims.

   12. A paint as claimed in clahn 11 having a solids content of at least 60% by weight.

   13. A paint as claimed in claim 12 having a solids content of about 70% by weight. ~