CA1133156A - Process for preparing polyolefin powder coatings - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Powders of polyolefins such as low density poly-ethylene, high density polyethylene, ethylene-vinyl acetate copolymers and ionomer resins are dry blended with a slip agent-containing concentrate and a free flow agent to provide coating compositions having good lubricity charac-teristics. The blended polyolefin powders can be applied by electrostatic coating or fluidized bed coating to such objects as glass containers to improve their impact and shattering resistance.

Description

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This invention relates to the field of polyolefin powder coatings and more particularly~ to such powders 5 blended with slip agents to provide lubricated coatings when applied to various substrate surfaces.

The tendency of the surfaces of adjacent films of polyethylene~ ethylene-vinyl acetate (EVA) copolymer ~nd 10 ionomer resins (copolymer of ethylene and a vinyl monomer containing an acid group such as methacrylic acid, e.g.
Du Pont's SU~LYN) to adhere to each other or to resist separation is a well known and ~requently troublesome phenomenon~ especially for the packaging industr~ which is 15 a major user of such film material In order to subdue this so-called "blocking' tendency, it has heretofore been a practice to incorporate agents into films of polyethylene homopolymer and copolymer films wh~ch enhance slip and reduce blocking. Common slip agents which have been 20 employed are amides of long-chain fatty acids. These include amides of oleic, stearic and erucic acid~ The slip agents are generally effective at lo~r levels of appli-cation, the optimum quantity employed being a function of the amide, the density ofthe homopolymer/copoly~er and even 25 the process by ~hich the film resins are prepared. Thus, for e~ample, films made ~rom polyethylene powder prepared by way of water dispersion techni~ues usuall~ contain some residual sur~actant ~hich can affect the performance of a given slip agent.
According to U.S. Patent No. 2~956,035 to Mock, a 30 base polyet,hylene resin suitable for ~orming thin self-sustaining pol~ethylene ~ilms is provided with a slip agent,

-2 ~L33~56 1 an antioxidant for polyethylene, an inorganic siliceous material, a high melting point wax which migrates to the surface of polyethylene film at room temperature and a polymeric hydrocarbon having different flow or freeze characteristics than the base resin. This combination of materials is said to be necessary to give effective blocking resistance and slip in extruded films without degrading the optical properties of the film. Mock describes a suitable slip agent as being an oily liquid or soft pasty solld which will sweat-out to film surfaces at room temperatures~ as having a melting point within the range of 20C to 75C and a boiling point of at least 225C, as being readily dispersible in ;
polyethylene, and above all, as being capable of reducing the coefficient of friction (COF) of the film. The unsaturated fatty amides, particularly oleamide and erucamide, are identified as meeting these criteria. The slip agent and the other ingredients are first melt blended in the polymeric hydrocarbon, preferably selected to be the same polymer as the polyethylene base resin but mechanically dispersed by being worked in a high shear apparatus such as a sanbury mixer, to provide a concentra-te and the concentrate is thereafter melt blended in the polyethylene base resin which is then extruded into a film.
British Patent No. 915,589 to Huck describes poly-olefin films prepared from alpha olefins of at least three carbon atoms having improved slip and non-blocking properties achieved by the addition thereto of an unsaturated fatty ~..
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~33~56 amide such as erucamide. The amide can be incorporated onto the film surface from an organic solvent solution or aqueous dispersion. In a manner sirnilar to that described by Mock, supra, the amide can also be added to the polyolefin film 5by being melt blended therewith in the f'orm o~ a polymer con-centrateO The slip and non-blocking properties of the Huck films is said to be further improved by the addition of sucn inorganic powdered materials as calcium carbonate, magnesuim ~carbonate', dia~omaceous earth and silica.
, . ... -- . , Investigation has shown7 however, that the3e melt blending techniques of incorporating slip agents into bulk polyethylene homopolymer or copolymer are entirely unsatis-factor~ ~or resins subsequently reduced to powders for coating applications. The addition of erucamide to polyet'nylene 15at concentrations of 750 to 12~0QO ppm before reduction of the polyethylene to a fine powder gave inconsistent resul-ts ~ when the powder was electrostatically applied ~o glass containers. Although several trial runs gave an acceptable level of bottle-to-bottle lubricity, the overall results ~were inconsistent and non-reproducible. Accordingly, poly-ethylene and EVA copolymer powder which ha~e b~en provided with slip'agents using the aforedescribed melt blending procedures are not particularly suitable for commercial coating~application.

Z5 lt has very surprisingly been discovered that if slip agent containing concentrates are dry blended with powder coatings of polyolefin homopol~mers and copolymers, the powders reliably provide coatings with good lubricity

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1characteristics. Broadly stated, the process for preparing the polyolefin powder coatings of this invention comprises pre-paring a melt blended concentrate of at least one polyolefin selected from the group consisting of low density polyethylene, shigh density polyethylene~ ethylene-vinyl ac~tate copolymer and ionomer resin with a fatty acid amide slip agent, reducing the concentrate to a ~ine powder, and thereafter dry blending the concentrate into a larger ~uantity o~ powdered polyolefin selected from the aforesaid group together with, prior to, lOor following the addition of a ~ree flow agent to the larger quantity of powdered polyole~in if the ~ree flow agent is not already a component o~ the concentrate.

The polyolefin powders of this invention can be applied to many different types o~ substrates, both rigid and flexible~ and are especially useful as shatter-resistant protective coatings when appliecl to glass tubing, glass bottles and other similarly ~ragile objects. The powders can be applied in a variety of ways, for exa~ple, as a dry powder by fluidized bed, elec~rostatic fluidized bed~
or electrostatic spraying, or as an aqueous or orga~c 20dispersion by spraying. In the case of a dispersion, the liquid portion o~ the dispersion ~ormulation serves as ~ree flow agent for the polyole~in powder. The coatings applied in accordance with this invention have good surface smoothness and exhibit excellent lubricity per~ormance.

The polyolefin coating powders which are suitable for use herein include any o~ the known and conventional polyethylene and ~thylene-vinyl acetate copolyrner powders which are or can be ernployed in coating applications. Such powders may be 3Dspherical or irregularly shaped and can have an average 3L133~

particle size of from less than 20 microns to greater -than 1,200 rnicrons. An especially useful class of polyolefins are the MICROTHENE F microfine polyolefin powders ol U.S.I.
Chemicals Co., Division of National Distillers and Chemical 5Corp. These low density polyethylene (I,DPE), high density polyethylene (HDPE) and ethylene-vinyl acetate (~VA) copolymer powders possess a regular and substantially spherical shape and typically range from about 8 to about 30 microns.
The characteristic properties of five corLmercially available MICROTHENE F powders are given in the Table below:

TABLE
Properties o~ MICROTHENE F Powders . _ _ . ~ . , .
. . FN 500 Fl`l 51~ FN ~4 IFA 520 F~ ~32 _ _ _ . - 'I
Polyolefin Typelow densitylow densitylow densi~yhigh d~nsily et;~ylene-vinyl_.__ _ p~lyethylenepolyeth~,~lenepolyelhylene polyelhylene aceta~e ~opoly.
t~Ael~ Index (9/10 min.~ 22 5 55 17 9 i~ D~nsi~y(g/cu.cm~0~915 0.924_ -0.925-- L 0.96 ~0.928 .
. . Parti~le Shape .~phericalsphe~ical sphericalsphericzl spherical ._ __ _ _ Avera~e Particle e(micron) <20 <20 <:20 _<20 <20 Volatils3 (m~x. %) 0.1 0.1 0.1 0.1 0.1 Color white white white wnl~ whi~e 2 0 : ~ ~ ~ ~ ~ ` - ~ ~ ~
The slip agents employed in the powder coating o~ !
this invention are advantageously selected from the unsaturated fatty acid amides having at least about 14 carbon atoms in the chain. The amides can be supplied as powders and as such, can usually be incorpora~ed into the polyolefin powder coatings without further processing. Erucamide ls an especially preferred slip agent herein. Other unsaturated ~33~6 fatty acid amides which can also be used are the amldes of' palmi-toleic~ rincinoleic, petroselenic, vaccenic, linoleic, linolenic, licanic, gadoleic, arachidonic, cetoleic and similar fa~ty acids. It is also within the scope of this Sinvention to employ a combination of unsaturated fatty acid and from about 0.01 to about 1.0 percent by weight thereof of a saturated fatty acid amide, e.g., an alkylene distear-amide such as methylene .dis~earamlde or ethylene distear-amide, distearamide and palmitamide. Hydroxylated fatty l~acid amides, e.g. hydroxy stearamide can also be used in combination with unsaturated ~atty acid amide at the afore-said level.
The quantity of slip agent required to provide acceptable l~b.ricity properti.es in coatings prepared from the : 15powders of this invention will, of course, dep~nd upon the .. . particular polyolefin homopolymer~copolymer involved. Generally, it is sufficient to employ from about 0.001 to 0.15 weight percent of slip agent by weight of polyole-fin powder to provide coatings having an ef~ective degree of lubricity. Thus~ for 20example, 0.10 weight percent of erucamiae by weight of polyethylene powder has been found to provide excellent results.
The slip agent is not i~corporated directIy ln~o Gne powder coating mass but is first melt blended in a known and conventional manner with a suitable polyole~in,~ith'the result-25~,,,n,~ conc,entrate being reduced to a fine_powder? and.

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lthereafter being dry blended with the powder coa~ing mass.In a preferred mode, from 1 to 10 parts by weight of a concentrate containing from 1 to 10~ slip agent by weight is dry blended with a correspondin~ 99 to 90 parts by weight sof polyolefin powder. This manner of dry blending the slip agent concentrate fine powder into the polyolefin powder coatings herein insures uniform distributio~ of the slip agent and consistent coating performance.

In addition to the afo~redescribed unsaturated fatty acid amide slip agents, the powder coating~ of this invention loal~ contain at least one inorganic finely divided ~ree flow ; agent, e.g.~ of an average particle size be~ween abou-t ; O.Olto about 7 microns. Such finely divided inorganic materials include diatomaceous earth~ colloidal silica, silica ge~, calcium carbonate, magnesi~m carbonate, and like materials.
~sThe free flow agent is suitably employed at from about 0.01 to -- . .
about 1.0 weiOht percent ~y weight o-f polyolefin powder and can be incorporated therein as part of the slip agent-containing concentrate or it can be separa~ely dry blended with the powder coating mass prior to, simultaneously with~ or ~ollowing~ the 20dry blending `of the concentrate with the po~lder coating mass.
When the polyolefin;homopolymer/copolymer is to be foxmulated as an aqueous or organic dispersion, the resin and slip agent-containing concentrate is admixed with such known and conven-tional dispersing coating ingredients as surfactants~ thickeners, 25dispersing agents, etc. Such ingredients perform a function analogous to the aforesaid finely divided i~o~anic materials of a dry powder coating composition, i.e., they provide free-flowing characteristics ~or the polyolefirl polymercoating powder.

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1The polyolefin coating powders prepared in accordance with this invention can, if desired, be ~ormulated to contain such optional ingredients as antiblock agents and anti-oxidants, numerous representatives of which are well ~nown sin the art.
The pol~olefln powders of this invention can be applied to rigid surface o~jects such as glass containers by any known technique, however, it is pref'erred to U6~ ~ny one of several electrostatic co~ting techni~ues in current e. This includes electrost~tic spray and cloud coating methods.
In the electrostatic spray coating method, coatîng powdex and air are introduced into a high voltage spray "gun". As the powder particles are ejected ~rom the gun, -they pass a negative electrode. The part to be coated i.s 1~grounded, and is therefore positively charged. As the negatively charged powder particles approach the positi~ely - charged ob~ect, the~ are attracted to the~sur~aces of the obvect and adhere tenaciously thereto. As the coating forms 7 the substrate ~ecomes insulated and the powder;in the applied 20coatin~ repels excess po~der. The uni~orm coa~ing that results must then be heated to the fusion temperature of the polyolefin to complete the process. The thickness of the f'ilm that can be applied can vary from l.5 mils to approxi-mately 15 mils arld even higher, depending on the temperature 250f the part, electrical potential diff'erence a~d duration of spraying ti~è. The use of the preferred polyolefin powder coatings, the MICROT~ENE F resins, permits thinner films ~o ~3~i6 1be applied. Higher substrate temper~tures allow the deposition of thicker coatings.
In the electrostatic cloud coating method, a low volume of dry air or gas, passed throu~h a porous 5plate, suspends and ~luidizes the powder. Upon application of high voltage to an electrode grid in the bottom of the bed, the powder becomes charged and is dispersed into.a fine cloud within the top portion of the bed. The powder is then attracted to the grounded ob~ect which has been passed through the 10cloud. An object coated by this method will be coated evenly over its entire sur~ace due to the ~elf-insulating e~fect of the powder. The "Pherostatic" coatin~ method (Electrostatic Equipment Corporation, New Xaven, Co~necticut) which is a variation o~ the aforedescribed ~luidized bed coating method 15involves the use of two electrostatic beds which are placed i- next to each other with a po~der collector below and common to both ends of the beds. A grounded object is inser-ted between the beds so that the charged p1astic particles that o~erflo~, creating a cloud in between the beds, will 20be attracted to the grounded object.
- Fluidized bed coating with powdered thermoplastic polymers can also be used. This method is ~nalogous to the dipping process used with liquid coatings. In using this method the powdered plastic is expanded by passing an air stream up~
25wardly through it. The parts to be coated are preheated to a temperature above the melting point of the powder and then lowered into the expanded bed of material. Each portion of the heated part that touches the powder causes the powder to melt and adhere. A~ter the part is remo~ed from the ~luidized bed, 3Dit is post heated to produce the desired ~low and coating properties.

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Comparison o~ the results o~ prlor art E~xamples 1 -to ¦
1 3 and E~arr.ple ~ hich is illustrative of the process o~ pre-paring polyolefin coatings herein dernonstrates the rnarked improvement to be gained by utillzation o~ the present inven-tion.
EXA~PLE 1 ' This example illustrates the slip characteristics o~
a polyolefin coating containing no slip agent. A low density polyethylene (00916,density~ having a melt index of about 20 and an average (spherical) particle size of less than 20 10 micxons was dry blended with appro~-~imately 0.5~ by weight o~ colloidal silica (Cab-0-Sil M-7 of ~he Cabot Corporation) to provide the resin with the free flow and fluidizable properties re~uired by powder coating processes.

4 Oz Boston Round glass bottles were coated with this 15 po~der using an electrostatic spray gun under the ~ollowing conditions:
1. bottle preheat temperature - 250F
2~ powder output - 90 grams per minute 3. bottle rotation during coating - 60 rpm 4.,postheat temperature - 500F

5. postheat time - 1-1/2 minutes

6. cooling cycle - three minute~ air ~ollowed by immersion in tap water

7. coating thickness - 6 to 8 mil The coatings were smooth, glossy and clear with acceptable shatter resistant properties.
Coating lubrici~y as determined by slip angle was approximately 45. Slip angle repres~n~s the angle at which the top bottle of a three-bottle pyramid begins to slip o~

3athe botto~ two bottles which are restrained from moving.

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This example illustrates the prior art technique of melt-blending a slip agent concentrate containing erucamide in a polyole~in coating resin prior to the reduction of the 5polymer to a ~ine spherical powder.
A slip agent - containing concentrate in pellet form was prepared by blending 5.5~ weight parts o~ erucamide with gL~.5 weight parts o~ a low density polyethylene ~0.915 density and 8 melt index). 24 grams of the resulting slip lOagent - containing concentrate were melt blended with 576 ; grams o~ the low density polyethylene of Example 1 on a 6"
two roll mill at 225F for approximately 10 minutes (erucamide content o.~ about 2200 ppm). The relativel~ low melt temperature was selected in order to minimize the volatilization of the 75erucamide (m.p. 75-80C). The melt bl~nded resin / e~ucamide concentrate was then reduced to a fine powder using the process disclosed in U.S. Patent 3,422,049, incorporated herein by ~e~erence.
A ~ree ~low powder was prepared and bottles were 20 coated as in the same manner as described in Example 1.
Slip angle was approximately the same as the non-modi~ied base powder o~ Example 1, indicating that erucamide was volatilized out of the coating resin during melt blending and during the reduction to a fine powder.

Example 2 was repeated except that 48 grarns o~
erucamide-containing concentrate and 552 grams of resin were melt blended together. The erucamide content was about 4400 ppm. Again, slip angle was approximately the same 3~ as the base powder o~ Example 1.

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EXAMPLE 1~
This example is illustrative o~ the process according to this inventîon wherein a slip-agent containing concentrate and free flow agent are dr~ blended with polyolef'in coating resin~
~ n erucamide-containing.concen~rate was prepared as in Exampie 2 and reduced to a fine powder o~ a size and shape s~milar to that o~ the resin powders of the preceding examples~ r~he concentrate was dry blended at varying levels with the same resin and ~ree flow agent as in Example 1 to prov~de a free flowing and ~luidizable powder. The ~ottle coating procedure o~ Example 1 was repeated and slip angle was measured. The dry blended ~ormulations were as ~ollows:
. A ~ C D E
Erucamide concen~rate~ grams 12 24 48 72 5 Resin, grams 588 576552 528 600 Free Flow Agent, grams 3 3 3 3 3 Erucamide, ppm 1100 22004400 6600 The ~ollowing slip angles were observed:
SliP Angle A B C D E
After one hour 45 32 13 181~5~
A~ter 24 hours 22 14 12~ 15 45 ~ ~33~LS~

1 These data show -that after the short period of one hour~ each of the bottle coatings containing dry blended erucamide in accordance with this invention significantly out-performed the bottle coatings o~ Examples 1 to 3 in slubricitY characteristics and that such improvement was in several cases even more dramatically improved after 24 hours.

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Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing polyolefin coatings for application on a substrate which comprises preparing a melt blended concentrate of at least one polyolefin selected from the group consisting of low density polyethylene, high density polyethylene, ethylene-vinyl acetate copolymer and ionomer resin with a slip agent which is an amide of a fatty acid selected from the group consisting of erucic, palmitoleic, rincinoleic, petroselenic, vaccenic, linoleic, licanic, gadoleic, arachidonic and cetoleic acids, reducing the concentrate to a fine powder, and dry blending the concentrate into a larger quantity of powdered polyolefin selected from the aforesaid polyolefin group together with, prior to, or following, the dry blending of a free flow agent with the larger quantity of powdered polyolefin if said free flow agent is not already a component of the concentrate, said coating when applied to the exterior surface of a glass bottle imparting superior bottle to bottle lubricity as determined by the slip angle test com-pared to that of a coating composition prepared by melt blending all of the slip agent into all of the polyolefin.

2. The process of claim 1 wherein the polyolefin powder possess an average particle size of from less than about 20 microns to about 1200 microns.

3. The process of claim 2 wherein the polyolefin powder possesses an average particle size of from about 8 to about 30 microns and the powder particles possess a regular and substantially spherical shape.

4. The process of claim 1 wherein the slip agent is present in the concentrate in an amount sufficient to con-stitute from about 0.001 to 0.15 weight percent of the entire quantity of polyolefin powder.

5. The process of Claim 4 wherein the concentrate also contains a saturated fatty acid amide or a hydroxylated fatty acid amide in an amount from about 0.01 to about 1.0 percent by weight of the unsaturated fatty acid amide.

6. The process of Claim 1 wherein the slip agent is erucamide.

7. The process of Claim 1 wherein the free flow agent is a finely divided inorganic material selected from the group consisting of diatomaceous earth, commercial silica gel, silica gel, calcium carbonate and magnesium carbonate, said free flow agent being incorporated into the powder at a level of from about 0.01 to about 1.0 weight percent by weight of the entire amount of polyolefin powder.

8. The process of Claim 1 wherein the free flow agent is an aqueous or organic dispersion medium.

9. The process of Claim 1 wherein said application of said polyolefin powder coating to a substrate is by electrostatic deposition.

10. The process of Claim 9 wherein electrostatic deposition is by spray coating.

11. The process of Claim 9 wherein electrostatic deposition is by cloud coating.

12. The process of Claim 1 wherein said application of said polyolefin powder coating to a substrate is by fluidized bed deposition.

13. The process of Claim 9 wherein said substrate is a glass bottle and wherein said coating is deposited from about 1.5 to about 15 mils in thickness, said bottle demonstrat-ing superior bottle-to-bottle lubricity as determined by the slip angle test compared to that of a bottle having a coating composition thereon containing said olefin resin, free flow agent and slip agent but in which the coating composition is prepared by melt blending all of the slip agent into all of the olefin resin.

14. The process of Claim 13 wherein said free flow agent is colloidal silica, and said fatty acid amide is erucamide.

15. The process of Claim 14 wherein said erucamide is present in said coating in an amount of 750 to 1100 ppm.