A Process For Forming A Coating On A Substrate And A Coated Substrate

Abstract

A process for forming a coating on a substrate, including the steps of effering tribostattc charging body of powdor by establishing a fluidisea-oed of the body powder ;n afluiding chamaer at least a part o~which :s conductive, appbang a voltaac :o the seconder cart of the fiuidis.ng cliamber. immersing substrate ■which :s essamally non-conductive or poo by conducive, eitherin to the continuous coating least part of the substrate.

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Specification

POWDER COATING PROCESS
The invention relates :o a process for the application of powder coating impositions to substrates.
Powder coatings are solid compositions which are useai!y applied cy ar. Electrostatic application p-ccess in wnich the powder coating particles are electrostaticaily charged and caused to adhere to a substrate which is usually metallic and electrically earthed. The charging of the powder coating partices is usually achieved by interaction of the panicies with ionised air 'corona charging) or by friction (triboelectric. triccstatic or "tribe11 charging) employing a spray gunThe charged particles are transported in air towards the substrate and their finai deposition is influenced, inter alia, by the electric field lines that are generated between the spray gun and the substrate.
A disadvantage of the corona charging process is that there are difficuities in coating substrates having complicated shapes, especially substrates having recessed portions, resulting from restricted access of the eieotric field lines into recessed locations in the substrate (the Faraday cage effect). The Faraday cage effect is less evident in the case of the tribostatic charging process but that process has other drawbacks.
As an alternative to electrostatic spray processes, powder coating compositions may be apciied by processes in which the substrate is preheated (typically to 200° C - 400° C) and dipped into a fluidised-bed of the powder coating composition. The pcwder particles that come into contact with the preheated substrate melt and adhere to the surface of the substrate. In the case cf thermosetting powder coating compositions, the initially-coated substrate -nay be subjected to further heating to complete the curing of the aoplied coating. Such post-heating may not be "necessany in the case of thermepiastic powder coating compositions.

Since the vc.tace applied to the fluidising chambe- is insufficient :c produce
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consequently, is uniikeiy to craw any electrical pewe" when the sucstrate is electrically isolated. The current drawn is expected to be less than 1 mA when the substrate is electrically earthed.
Where the substrate comprises a plastics materia! which' shews surface conductivity when at en elevated temperature, the process, preferably, includes the step of heating the plastics material to a temperature be;ow its melting point and below the class transition temesrature of the oowder ccatinc ccmccsiticn before immersing the substrate in the fluidised bed.
Where the substrate comprises a plastics material which shews no substantial surface conductivity even at an elevated temperature, the prccess. preferably, includes the step of pre-charging the substrate before immersing it in the fluidised bed.
Preferably, the prccess includes the step of equalising the charge on the pre-charged substrate at the point of immersion and then immersing the subsraie in the fluidised bed.
The charge may be equalised by heating the substrate to a temperature below its melting point cr by introducing surface moisture on the substrate or both.
The voltage a::.led to the*fiuiciising chamber in the cr::ess cf the present invention is: preferab'y. a direct voltage, either positive or negative, but the :se of an alternating voltage *s possible by, say, applying the voltage intermittent;/ at times when it is positive c at t:mes when it is negative. The- applied vo'tace may ,ar/ within

The process is aiso effective to powder coat a plastics substrate which does not include an electrically conductive additive. The substrate nay be heated in order to make it conductive. During heating the temperature remains below the melting point of the substrate and glass transition temperature of the powder coating.
The above observations, including those for MDF, apply to plastics substrates, except that there are no fibres and there is no requirement for moisture.
In the coating of the plastics substrates, referred to above, the substrate is, preferably, earthed a.though it may be electrically isolated, that is, without an electrical ccrmeciicr 'substrate electrically "floating", that is, its electrical potential is indeterminate).

0% to 5% by weight,
0% to 3% by weight, and
1% to 2% by weight.
In general, colouring agents, fillers/extenders and performance additives as described above will r.z: be incorporated by post-blending, but will be incorporated beiore and/or during ti-.e extrusion or other homogenisation process.
Alter applica-:c of the powder coating composition to a substrate, conversion or ihe res.;.'-g adherent particles into a continuous coating /including, y/here appropriate, cu'-g of the applied composition) nay be effected cy heat

The term post ended relation to any additive means tha: the additive has bee" 'incorporated arte" :he extrusion or other homoger.isadcn cr::sss used in the manufacture of the pcv.cer coating composition.
Post-biending z~ an additive may be achieved, for examc e. by any of the following dry-blending methods:
a) tumbling into the :hip before milling;
b) injection at the rill;
c) introduction at the s;age of sieving after milling;
d) post-production :iending in a "tumbler" or other suitable mixing device; cr
e) introduction into :he fluidised bed.
A general form :f fiuidised-bed triboelectric powder coating apparatus suitable for carrying out a process in accordance with the invention and several forms of process in accordance with the invention will now be described, by way of example only, with reference tc T.S accompanying drawings, in which:
Fig. 1 shews re genera! form of fluidised—bed tr.boeiectrc powder coating apparatus in diagramratic section,
Figs. 2A and 23 are perspective representations of first and second MDF substrates as used in Example 1 and-
Figs. 3A and are perspective views of a plastics substrate, as used in Example 3, which includes an electrically conductive adcitive making the substrate electrically poorly ccrrective.

i he dimensions of the substrates ranae as fallows: Width = 7 to 11 om Length = 5 to 15 cm Depih = 1.5 :c 2.5 cm.
Two powder coating systems designated A and 3 were used in Example 1, both made up by the same formulation and differing in particle size cistributlcn (PSD) and -;ie manner cf preparation. The powder coating systems were crecarec bv conventional powder coating mining.
The formulaticn common to the systems is given below:
Parts bv welaht

Rutile Titanium Dioxide 321
Filler (dolomite; 107
Carboxylic Ac'c-Functional Polyester Resin 374
Epoxy Resin Coring Agent 152
Catalyst ■30
Wax 3
Flow modifier 10
Benzoin •• - -s
TOTAL 1000
In addition, the following.:- additive formulation for post-blending was prepare--;

the bed: 30 min at 3 bar
Standard bake of deposited
. material 30 min. at 120 CC
The results obtained are summarised in the following table:

The values reported in the "thickness" column are the average value of 12 film thickness measurements performed for each substrate. Each panel was measured at 6 different points on each face.
STDEV is the standard deviation of the film thickness measurements.
The substrate can be either electrically earthed or electrically isolated. The substrate exhibited moderate electrical conductivity and the process was more effective when the substrate was earthed rather than when electrically isolated.
The polarity and the magnitude of the applied voltage influence the performance (speed of coating process and uniformity and evenness film pattern thickness) of the pcwder coating system used. The powder coating system has a set of process conditions (applied voltage, dip-time, air-pressure) for the best performance.

Example 3
Referring to Figs. 3A and 33 of the accomparying drawings, the substrate used in Example 3 was a section of a motor vehicle wrsei cac, Fig. 3A shewing the front face of the section and Fig. 3B showing the back face of the section. The wheel cap had a diameter of 7.7 cm. and the section used was about a quarter of the wheei cap. The material in which the wheel cap was fabricated is available under the name Poiyamide 66 and exhibits measurable, but significantly poor, electrical conductivity.
Referring to Fig. 3A, the substrate 30 had the form of a quadrant of a disc bearing edge formations 31 and inner formations 32 extending across its front surface in addition to isolated depressions 33 and 34 on its front surface.
Referring to Fig. 3B, the substrate 30, having the form of a quadrant of a disc, bore edge formations 36 and inner formations 37 extending across its back surface and, in addition, isolated depressions 40 and 41 and isolated projections 38 and 39-on its back surface.
Only one powder coating system was used in Examcie 3 and was the System B powder used in Examples 1 and 2.
The general operating conditions were as follows:
Weight of the powder loaded in the bed: 750 g
Free fluidisation time for equilibrating
the bed: 30 min at 3 bar
Standard bake and cure of deposited
material 30 min. at 120 CC


The substrate was of a relative!y complex forn including a plurality of curved and recessed areas. Taking film thickness measurement difficult. A measurement of the Deposited Mass was used as a measured the film thickness built up.
The coverage was assessed visually.
The evenness of the film thickness pattern v/as assessed visually, the va!ue:0 indicating very bad an: the value 5 indicating very good.
Eetter results v.ery obtained when the substrate was earthed than when ;t was electrically isolated.
In the case of Example 3, it was found that the coverage was enhanced by heating the substrate to a temperature T °C which lay below the melting point cf the plastics substrate material and the transition point (Tg °C) of the powder composition prior to dipping. The :emperature of the substrate at the moment of the dipping was less than Tg °C in crder that the powder would adhere to the substrate only by a electrostatic process and not by a kind of sintering process. The heating process was carried out in an air-circulating oven.
The results obtained through heating of the substrate are summarised in the following Table:


Example 4
The substrate usee In Example 4 was a transparent polycarbonate (non-filled) ectangular panel c:47 mm x '01 mm.
Only one powder System was used in Example 4. It was the System B powder jsed in Examples 1. 2 a~d 3.
The aenera! oceraiina cceditions were as follows:
VVeiaht of the ccv.ber leaded in the bed: 750 g
Free fluidisaticn time fcr equilibrating
the bed: 30 min at 3 bar
Standard bake arc cure of deposited
material 30 min. at 120 °C
Coating of the Substrate was achieved. The uniformity of the coating was improved by heating the plastics,material to a temperature beiow its melting point and below the transition point of the powder coating composition before immersion..
Further improvement was obtained on pre-charging the substrate before immersion and still further improvement was obtained by equalising the charge on .the substrate before immersion. Charge equalisation was achieved either by heating the substrate to a temperature below its melting point or moistening the surface of the substrate.
Example 5
The substrate used in Example 5 was a rectangular block of MDF board of dimensions 10cm x 15cn x 13mm.
The formulation given above in relation to a System A powder was used, but
was milled to a smaller cariicie size distribution as follows, this being identified as a
>
System E powder: System E d(v)c9l pm 10 d(v)5-Jt um 5.5

% < 5 jjm -2
in addition, :hs following additive formulation, "or cost-blending, was prepared:
Additive formulation 2
Aluminium Cxice - 15 parts by weight
Aluminium Hydroxide - 45 cans by weight
Silica (Wacker HDK H3004) - 40 parts by weight
Silica HDK H3CC4 is a hydrophobic silica available from Wacker-Chernie. The term hydrophobic siiica denotes a silica of which the surface has been modified by the introduction of sily! groups, for example, polydirr.ethylsiloxane, bonded to the surface.
The general operating conditions were as fciicws: Weight of the powder in the fluidised bed - 500g Free fluidisation time for equilibrating the bed - 30 min at 3 bar Fluidisation pressure during coating - 3 bar Standard bake of deposited material - 30 min at 120 °C
Two MDF boards were dipped in 500g of the System E powder with 2% of additive 1 and 2% of additive 2, respectively. The dipping time was 60 seconds in each case, 3kV was applied to the fluidising chamber and the panels were heated at 120 °C for 30 minutes. The results are set out below and show that the System E powder with additive 1 post-blended has a relatively poor coating performance whereas, when additive 2 is used post-blended, the coating performance is considerably improved.

Pest additive ; Potential Gradient I Coverage Film thickness
Additive 1 A 2 kV/cm 30% 9 \im
Additive 2 :1.2kV/cm 100% 41 (jrn
— '—I.- „_—^_

Example 3
The substrate used in Example 3 was a CONAMIDE R5 plastics slab details of which are set cut in Example 2 abcve. The genera! operating ccncit:-:ps were as for Examole 5 above.
Two CONAMIDE R6 siaos were dipped in 500g of the System E pcwder with 2% of additive 1 and 2% of additive 2, respectively. The dipping time was 6C seconds in each case: 3kV was applied to the fluidising chamber and the slabs were heated at -120 3C for 30 minutes. The results are set out below and show that the System E powder with additive 1 post-blended has a relatively poor coating performance whereas, when additive 2 is used post-blended, the coating performance is considerably improved.
Example 7
The substrate used in Example 7 was MDF board as for Example 5 above.
A second powder formulation and a third additive formulation for pos: blending, as set out below, were prepared.
Powder Formulation 2 Parts by weight
Titanium dioxide 252
Filler (Dolomite) 161
Carboxylic acid functional Polyester Resin 400
Epoxy Resin 147
Catalyst ' " 24
Wax 3
Benzoin 3
Flow Modifier - 10

Additive formulation 3
Aluminium Oxide -40 parts by weight
" luminium Hydroxide - 43 parts by weight TFE modified wax - 12 parts by weight
The above Powder Formulation 2 was used and the particle size distribution /as as for a System A powder used in Example 1 above. The general operating onditions were as for Example 5 above.
Two MDF boards were dipped in 500g of the Formulation 2 System A powder with 0.6% of additive 1 and 0.6% of additive 3, respectively. The dipping time was 60 ieconds in each case, 3kV was applied to the fluidising chamber and the panels were leated at 120 -°C for 30 minutes. The results are set cut below and show that the ;oating performance-can be radically improved for a particular substrate by careful selection of the post-blended additive.

Example 8
The substrate used in Example 8 was a CONAMIDE R6 plastics slab details of which are set out in Example 2 above.
The general operating conditions were as for Example 5 above.
Two CONAMIDE R6 slabs were dipped in 500g of the Formulation 2 System A powder with 0.6% of post-blended additive 1 and 0.6% of post-biended additive 3: respectively. The dipping time was 60 seconds in each case, 3kV was applied to the fluidising chamber and the slabs were heated at 120 QC for 30 minutes. The results are set out below and show that the improved coating performance can be maintained, even when the substrate changes, by careful selection of the post-biended additive.


The Low-Bake and Cure formulation was milled to a System A particle size distribution:
The general operating conditions were as for Example 5 above.
The MDF board was dipped in 500g of the low-bake and cure formulation powder system with 0.5% of additive 1. The dipping time was 60 seconds in each case.. 3kV was applied :o the fluidising chamber and the panels were heated at 120 °C for 30 minutes. Bake and cure were achieved at 120 °C in the time normally required for bake alone. The results, which are set out below, show that good coating performance can also be obtained by using a low-bake and cure formulation in a powder System with a rcrmal mean particle size.

effecting tr:bcs:at;c charing cf the powder coating composition, the f!uidised-bed irciudinc a -fluidising chamber at ieast a cart cf which is conductive.,
applying a veltage to the conductive cart of the fiuidising chamber. -immersing a substrate which is either electrically r.cn-ccnductive or poorly conductive whcilv or partly in :he fiuidised bed. wherebv tribcstaticaiiv charged particles of the powder coating composition adhere to the substrate, the substrate being either electricaiiy isolated or earthed, vvithdrawina the substrate from the fluidised-bed and
w
forming the adherent particles into a continuous coating over at least part of the substrate,
the process being conducted without ionisaticn or corona effects in the fiuidised bed.
2. A process as claimed in claim 1, wherein the substrate comprises a medium density fibrebcard (MDF).
3. A process as ciaimed in claim 1 or claim 2, wherein the substrate comprises wood.
4. A process as z aimed in claim 1 or claim 2, wherein the substrate cc-prises a weed product.
5. ■ A process as eaimec in claim 1, wherein the substrate comprises a plastics
material.

I
plastics material including a- electrically conductive additive.
7. A process as claimed in claim 5. wherein the plastics materia! comprises polyamide.
3. A process as claimed ir. clain 1 or ciaim 5, wherein the substrate comprises a highly insulating plastics material.
9. A process as claimed in claim 3, wherein the plastics material comprises polycarbonate.
10. A process as claimed in any one of claims 1 to 4, wherein the surface resistance of the substrate is of the order of at least 103 ohms/square.
11. A process as claimed in any one of claims 1 to 4 or ciaim 10, wherein the surface resistance of the substrate is of the order of from 1 03to 10-ohms/square.

12. A process as claimed in any one of claims 1 to 4 or claim 10, wherein the surface resistance of the substrate is of the order of at least 10° ohms/square.
13. A process as claimed in anyone of claims 1, 5 or 5, wherein the surface resistance of the substrate is of the order of from 103 to 10 : ohms/scuare.
14. A process as c'aimed in any one of claims 1 or 7 to 9, wherein the s-rface resistance of the substrate is of the order of at least 10"' chms/squs'e.

A process as claimed in anyw ere of claims 1 zoo"., wherein the sucstrate is immersed with the fluidising chamber in a charge- condition fcr a period of up to 30 minutes, 20 minutes. 10 minutes. 5 minutes or 2 minutes.
A process as claimed in any one of ciaims 1 to 22. wherein the substrate is immersed with the fluidising chamber in a charged condition for a ceriod of ai least 10 milliseconds, 500 milliseconds or 1 second.
A process as claimed in any one of ciaims 1 to 32, wherein a coating of thickness of up to 500 microns, or up to 200, 150. 1C0 cr 80 microns is applied.
A process as claimed in any one of claims 1 to 24, wherein a coating of thickness of at least 5 microns, or at least 10, 20, 50, SO or 30 microns is ... applied
A process as claimed in claim 35, wherein a coating of thickness in the range of from 20 to 5C microns, 25 to 45 microns or 50 to 60 microns is applied.
A process as claimed in any one of claims 1 to 36, including shaking or vibrating the substrate to remove loose particles.
A process as claimed in any one of claims 1 to 37, wherein the powder coating composition is a thermosetting system.
A process as c.aimed in claim 33, wherein the film-forming polymer In the or each powder coating component of the powder coating composition is one cr more selected Tom carboxy-functional polyester resins, hydroxy-furctional polyester resins, epoxy resins and functional acrylic resins.

A prccess as claimed in any one of claims 1 to 37T .vherein :he pcwce- ccstlng composition is a thermcciasilc system.
A process as claimed in any one of claims 1 to 40. wherein the pcv/cer coating comccsiticn irccrocrates, bv cost-blending, one or more flulcity-assis:ina additives.
A process as claimed in ciaim 41, wherein the powder coating comccsiticn incorporates a combination of alumina and aluminium hydrcxide as a "uicity-assis:ing additive.
A process as claimed in claim 42, wherein the fluidity-assisting additive includes hydrophobic silica.
A process as claimed in ciaim 42, wherein the fluidity-assisting additive includes a PTrE modified wax.
A process as claimed in any one of claims 1 to 44, wherein substantially all of the powder panicles are no larger than 10 |jm.
A prccess as claimed in any one of claims 1 to 45, wherein the powder coating composition is a low-bake composition.
A prccess as calmed in any one of claims 1 to 46: wherein the subsi'ate is wholly immersed within the fluidisecf bed.
A coated subsua:te obtained by a process as claimed in any one cf c alms 1 to

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