CA1064244A

CA1064244A - Method of tagging with color-coded microparticles

Info

Publication number: CA1064244A
Application number: CA243,661A
Authority: CA
Inventors: Pui K. Lee
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1975-02-19
Filing date: 1976-01-16
Publication date: 1979-10-16
Also published as: US4053433A; DE2607014C2; FR2301888B1; JPS51107800A; GB1536192A; DE2607014A1; FR2301888A1; JPS5912137B2

Abstract

ABSTRACT OF THE INVENTION
An improvement in the known method of tagging in-dividual units of production of a substance with microparticles for retrospective identification is disclosed. The improvement comprises the use of microparticles which are encoded with an orderly sequence of visually distinguishable colored segments.
Decoding of the microparticles can be accomplished with a microscope or other magnifying device.

Description

912,013 i4;~

METHOD OF TA~GING WITH COLOR-CODED MICROPARTICLES

This invention relates to an lmprovement ln a method of tagglng lndivldual unlts Or productlon Or a substance wlth mlcropartlcles ~or the purpose Or retrospectlve ldentirlcatlon of the substance. A rurther aspect o~ the lnventlon relates to mlcropartlcles userul ln retrospectlve tagglng according to the method Or the lnventlonu Tagglng lndlvldual unlts Or productlon Or bulk sub-stances with ldentlrylng mlcropartlcles ls known. U.S. Patent ~
No. 3,772,200 discloses a method Or tagglng utlllzlng re- -rractory mlcropartlcles contalnlng low levels Or elements such ;:
as manganese, cobalS, zlnc, cadmlum, and tln. The com-blnatlons o~ these elements wlthln the mlcropartlcle provlde the varlous ldentirylng codes. This method 18 especlally suited ror tagging exploslves such as dynamlte. The mloro-partlcles survlve detonatlon and are retrlevable rrom bla8tdebrlsO A continuation-in-part o~ this patent appllcatlon, U.Su Serlal NoO 398,569 flled September 18, 1973 descrSbes a slmilar method Or tagglng utlllzlng mlcropartlcles Or poly- -!~ .
.~ , merlc materlals, such as polypropylene, coded wlth low levels Or elementsO These mlcropartlcles are prererred ror tagglng dynamlte beoause Or thelr non-sensltlzlng errect on nltro-glycerlne The above-descrlbed methods Or tagglng substances utlllzing mlcropartlcles bearlng ldentlrylng codes are userul ` 25 rOr purposes Or retro8pectlve ldentlrlcatlon. They provlde - means Or labellng lndlvldual unlts Or productlon Or bUlk substances w1th mlcropartlalos which are not roadlly detectable by the user and whlch do not lnterrere wlth the proportlos Or - ;;
the ta8ged substancesO The8e methods do su~rer rrom a dl8-;' -:
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advantagc, however, in that they require rather sophisticated decoding pro-cedures. Where selected levels of elements are incorporatet into micro-particles, decoding is generally accomplished through the use of an electron microprobe analyzer at a location some distance from the retrieval site, necessarily requiring some delay between retrieval of the microparticles and reading of the code.
The present invention provides retrospective identification by means of microparticles which can be readily decoded by visual inspection of a single microparticle with a microscope or other magnifying means.
The known method of tagging individual units of production of a substance for the purpose of retrospective identification includes the steps of:
1. providing an inventory of batches of microparticles, each batch being uniquely coded;

2. maintaining a record of the unique code employed in each batch; and

3. incorporating microparticles from any one batch with only one unit of production of a substance.
The improvement in this method, according to the present invention, comprises providing microparticles which are coded by a particular orderly sequential arrangement of visually distinguishable colored segments.
According to one aspect of the invention, therefore, the invention provides in a method of tagging individual units of production of a substance with microparticles for the purpose of retrospective identification including ; -the steps of: ~1) providing an inventory of batches of microparticles, each batch being uniquely coded, ~2) maintaining a record of the unique code em-ployed in each batch and (3) incorporating microparticles from any one batch with only one unit of production of a substance, the improvement comprising the use of microparticles whose broadest dimension across the color sequence is from 1 to 1000 micrometers and which are encoded according to a particular orderly sequence of visually distinguishable colored segments, there being at least three segments in the color sequence, which inventory includes up to y ~ - 2 - ~
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(C)tC-l)n 1 uniquely coded batches of microparticles where C is the number of available colors and n is the number of segments in the sequence.
According to another aspect, the invention provides a plurality of batches of microparticles useful for tagging substances to permit retro-spective identification, the microparticles of each batch being uniformly and uniquely encoded with an orderly sequence of visually distinguishable colored segments, there being at least three segments in the color sequence and each microparticle being 1 to 1000 micrometers in its broadest dimension i ~
across the color sequence.
The inventory of microparticles may include up to [C][(C-l)n 1]
uniquely coded batches, where C is the number of available colors and n is i the number of segments in the color sequence. For example, using a library -of 12 colors in an eight-membered sequence, wherein no color is used adja-cent to itself, the number of codes would be determined as follows:

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.: ~. . , i , : , ' ' .. :. ' ' ~ ~ ` ' ~(~;4'~44 Segment NoO l 2 3 4 5 6 7 8 Colors Avallable 12 ll ll 11 11 11 11 ll This system lncludes 233,846,052 posslble codes or one-halr that number ir the code can be read in elther directlonO
Each color-code should lnclude at least three color segments to provlde a de~lrably large number Or codesO
The broadest dlmension Or a mlcropartlcle across the color sequence may be l to lO00 mlcrometers, but upper llmits Or 250-300 mlcrometers are preferred ln order to pro-vlde large numbers Or micropartlcles per unlt welghtO The preferred mlcroparticles ror use ln the present lnvention range rrom 50 to 250 mlcrons at the broadest dlmenslon across the color sequenceO Below 50 mlcrons, the code may be dlfricult to read9 and more sophisticated magnirication apparatus may be requiredO
To facilltate separatlon rrom the bulk materlal lnto whlch they are lncorporated, lt 18 prererred that the shapes Or the mlcropartlcles be dlstlnctlveO Spherlcal, cyllndrlcal, polyhedral or other geometrically shaped mlcropartlcles are readlly recognizable and retrievable rrom common particulate substancesO
Understanding Or the various types Or color-coded mi¢roldentlriers and methods Or maklng thcm wlll be racilitated by rererence to the accompanylng dra~ings wherelnoO Flgs~ 1-4 are perspectlve vlews Or difrerent ;:
embodlments Or color-coded mlcroparticles; and FlgsO 5-6 schematlcally illustrate the manuracture Or the embodiment Or Flgo 40 Flgo 1 shows a spherical type Or micropartlcle 209 rererred to herein as the "onion mlcrosphere"9 whereln a pre~ormed, solld nucleu~ 22 18 pro~ontO Colored or dyed _ 3 _ -,; ~ .. .. . '' , .
. .

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10~4;1:4~li layers 24 Or polymerlc material are concentrlcally coated upon nucleus 22 in such a way that no ad~acent layers are o~
the same colorO Each layer provldes one segment of the identlrying codeO With conventlonal coatlng technlques the average thickness Or each layer 18 approxlmately 25 micrometersO
Arter retrleval Or the onlon mlcrosphere rrom the tagged substrate, lt ls cut ln halr or a ple-shaped sectlon cut out, and the color code read from the re~ultlng cross-sectlonO It ls customary to deslgnate a slngle color to be conslstently used to form the outer layer o~ the mlcro-partlcleO Thls assures that a mlcropartlcle bearlng a complete code wlll be read~ When the outer layer 18 an "lndlcator" segment lt 18 not read as part Or the codeD
Cholce Or core material ror the onlon mlcrospheres wlll depend, ln part, on the materlal to be tagged and its ultlmate use~ In addltlon to the ablllty to survlve further proce6slng Or the tagged materlal, the core material should be sultably s~all ln slze9 readlly sectloned, and capable Or formlng a strong bond wlth the surroundlng plg-mented layerO Sultable core materlals ror most purposes ~-lnclude plastlcs such as polyolerlns and polyacrylates, waxes, glass bubbles, and blodegradable macromolecules such as albumln, gum arablc, gelatln, and polyvlnylpyrrol- ;
ldone~ For descrlptlons relatlng to the preparatlon o~ ;
mlcrospheres ~rom a varlety substances ln a size range prererably 50 to 200 mlcrometers ln dlameter, sultable for use as ¢ore materials ln the onlon mlcrosphores> see UOSO
Patents 3,772,200 and 3,663,6B5~ `
The nucleus of the mlcropartlcle may be any : ~ ;
mono-fllament, nylon, whlch has a dlameter surrlclently ~mall ``

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to meet the prescribed slze reQulrements, preferably 25 to 200 mlcrometersO Concentrlc colored layers are bullt up around the monofilament nucleus and the resultlng mlcropartlcle may be used as a retrospective identlrier even though lt may be several mllllmeters in lengthO Alternatlvely, the coated monofllament may be cut lnto short lengths to form the mlcroparticle shown ln Figo 2, referred to hereln a~ the "mlcrodlsc~ Mlcrodlsc 26 contalns a monofllament nucleus 28, and colored layers 30 are concentrlcally coated thereon ln such a way that no two ad~acent layers are the same colorO
The dyes and pigments used to form the colored layers or segments Or the onion microsphere or the micro-disc are conventlanal ltems well known to those skllled in the art, and include, for example, lnorganic plgments such as sulrates, chromates, sulfides, oxides9 carbonates, etcO, and stable organic pigments such as phthalocyanine and Hansa YellowO
A list of suitable colors may includeO

Clear Red Blue Black Orange Violet Brown Yellow Fluorescent Red White Green Fluorescent Green An extensive 11st of dyes and pigments is round in Color Index, 3rd Edo~ Edited by Fo Mo Rowe, published by the Society of Dyers and Colorists, Yorkshlre, Unlted KlngdomO . :
The main requirements of the colored layers lnclude (1) the abillty to bond to the core material and each other 80 that the integrlty of the mlcroparticle 18 malntained and (2) the absence of ~color strlke" or bleeding between ~ ~
layersO The colored layers are generally applled as a liquid ~ .
resin system including dye or pigmentO An alternative method .`.

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ls to apply successlve colored layers Or non-solvent com-patlble reslnsO For example, one layer may conslst Or a ketone-soluble resln with the ad~acent layer belng a non-ketone soluble resln~ Alternating resln layers ln thls manner ellminates the need ror each layer to harden before the next layer i9 applledO Examples Or resln systems in whlch dyes and pigments can be incorporated to rorm the colored layers include celluloslc derlvatives, epoxy com-pounds, polyolefins and waxesO The most prererred materlals are polyacryllcs.
The concentrlc colored layers are applled to the core materlal by conventlonal processes lncludlng fluld or spoutlng bed, ball mlll, dipping, or pharmaceutical plll coatlng processesO A presently prererred method ror applylng the colored layers to spherlcal partlcles 18 through the use Or a Wurster coater descrlbed in.U.S. Patent No~ 3,241,520.
The color resln may be dlssolved or dlspersed ln a ~ugitlve : --solvent, or lr the plgment exlsts ln a liquld system Or low ~. -vlscoslty, lt may be applled wlthout the need rOr a solventO
The color coded mlcropartlcle lllustrated by Flgo 3, rererred to hereln as the "mlcrorosette", may be made by placlng monorllaments 34 Or dlr-rerent colors and pre~erably 5 to 50 mlcrometers ln dlameter, ln.an ordered arrangement lnslde a heat shrlnkable sheath 36 rormed rrom, ror example, heat shrlnkable plastlclzed polyvlnyl chloride, around a center mono~llament core 38~ Heat ls applled, and the sheath ~ :
ls drawn down to a smaller.dlameter~ The colored mono- -rllaments are retained ln.thelr ordered arrangement durlng drawn-down and form a rlng around the central coreO The sheath may then be sectioned to rorm mlcrorosettes Or the desired lengthO Ir longer sectlons are lncorporated lnto a :

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substance ror retrospectlve identlrlcatlon, cross-sectlonlng may be necessary in order to read the color coded segments provlded by the fllaments surrounding the core. One Or tho~e rilaments may be of a preselected color such as black or clear as a startlng polntO
Materials useful in maklng mlcrorosettes include colored monoPilaments Or polyamides, polyesters, polyolerins, polyacrylics, and modirled celluloses.
A preferred type of color-coded micropartlcle ror use in practicing the lnvention conslsts of mlcroscoplc pieceæ o~ colored plastic films rused together to form a rectangular "microsandwlch" as shown in Fig. 4O The micro-sandwlch 40 ls a generally rectangular hexahedron which has ten color segments 42 ln sequence, wlth no segments o~ the same color belng ad~acent to one another~ The code may be read ~rom le~t to right or rlght to le~tO However, a key segment 44 of a specirled color such as black may be bullt lnto the sequence to designate the direction in whlch the code ls to readO End segment~s are generally clear or a designated color ln order to readily determlne whether a complete or partial mlcrosandwlch has been retrleved. The outer "indicator~ segments are not read as part of the code.
The mlcrosandwlch identi~iers may be prepared by a doùble sklving technlque as rollowsO Indlvldual colored plastic rilms o~ polyethylene or other solvent-resistant plastic are stacked in the particular color sequence of the ~`~
desired-identi~ying codeO The thickness o~ each rilm is generally between 12 and 200 micrometersO The outer-~aclng surface of each of the top and bottom ~ilms o~ the ~taak i8 coated with a release material which may be any resin material 10~i4Z44 whlch 18 heat fusable wlth the plastlc rllm and solublo in a solvent whlch wlll not attack the plastlc fllm. Sultable -: release materlals include polystyrene, polyvlnyl alcohol, and the preferred materlal ls Unlon Carblde VAGH vlnyl polymer (a mlxture of about 91% vlnylchlorlde, 3S vlnyl acetate and 6% other ingredlents) whlch 18 soluble ln methylethyl ketoneO
Dlscs havlng a center hole are cut rrom the ~llm stackO These colored dlscs are stacked on a mandrel wlth a large number Or clear dlscs Or the solvent reslstant plastlc above and below the colored dlscs. Thls lay-up ls heated (eOgO 120-125 C for two hours ln the case Or polyethylene film) to fuse the fllm layers and the release materlal lnto a bllletO Thls blllet ls then sklved to produce a rlbbon 52 as lllustrated ln Flg~ 5 The blllet consists-of clear plastlc layers 48 fused to colored layer 500 Colored layer 50 is normally made up Or a multipllcity of.code sequences separated from ..... -each other and from-the clear layers by release material~
Release materlal.is coated onto both sides Or ribbon 520 Dlscs having a center hole are cut from the coated rlbbon and stacked on,a mandrel as shown in Flgo 6~ . ~
The lay-up is-heabed to fuse.,the dlscs into a blllet, which :,:. , ' ls sklved in the manner shown ln Flgo 5 to produce a ribbon .~
havlng repeatlng clear and colored areas. Thls rlbbon 18 ,. .
soaked in a solvenb.which will selectlvely dlssolve tho ` ;
release materlal, thus generatlng the indlvidual micro~
sandwlchesO ~.
, . ~ .
m e overall dimensions of the mlcrosandwiches wlll be determlned by the thickness Or the colored rilms usod and `' `.

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the thlckness Or the rlbbons whlch can be cut wlth the sklvlng devlce. For general descrlption Or the skiving technlque used, see UOS~ Patent Re 27,6170 For most materials, heatlng is not requlred during the sklving process~ Generally sklvlng to produce a ribbon about 1205 micrometers thlck is adequateO
Thus, by using plastlc film about 65 mlcrometers thick and skiving to rorm rlbbons about 12.5 mlcrometers thlck a mlcro-sandwich having individual segments approxlmately 65 x 120 5 x 1205 mlcrometers can be manu~actured. The overall length Or the sandwich will, Or course, depend on the number Or segments presentO Ib is prererred that the microsandwlch contain at least three segments constituting the color code wlth an indicator segment at each endO
The selection Or materlals comprising the micro-particles depends upon the propertles Or the substance into whlch they are to be lncorporatedO In certaln cases .....
micropartlcles must be "tallor made" to rlt the individual requlrements of the substance.to be taggedO Survlval Or the m~cropartlcles arber rurther processlng Or the substance ls Or prlmary concernO
The method o~ the present lnventlon ls partlcularly ~ ~
*ell sulted ror.the tagglng or.bu}k materlals such as gralns . ~ -`
and chemlcals, anlmal and rowl reeds, and orally-administered drugs for human and veterinary useO Liquid products such as non-opaque lacquer.s.and..realns, can be tagged with low concentrations Or microparticles~ Microsandwich identiflers have been incorporated into dynamlte and success~ully retrleved rrom blast.debris rollowlng detonation~ Mlcro- ; .
sandwiches have also been.used.to tag paper and other solld products such as concreteO
The color coded mlcropartlcles should be homo-_ g _ . .

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geneously incorporated into the substance to be tagged, prererably ln an amount ranglng rrom 0.0001 to lo O part by weight rOr every lOO parts by weight of bulk materialO To facilitate homogeneous dlstrlbutlon, the mlcropartlcles should be tack-free at room temperature. To know whether bulk material has been subsequently diluted, carerul control Or concentration Or the micropartlcles is necessary.
Retrieval of the microidentiflers from the manuractured product can be.acco~plished in a number Or ways, depending upon the type of mlcropa~tlcle used and the nature Or the material tagged. With bulk materials in powder rorm, separation can generally be accomplished by visual means.
The microparticles, because Or thelr characteristic shape and color, can be distinguished.readily under magniricationO
A flucrescent color in the code may aid in retrieval Or the microparticleO
With some bulk materials it may be more convenient to separate the microparticles.by their density characteristics ln a liquidO For example, microsandwiches Or polyethylene have been retrieved rrom debris collected at the site Or an explosion by addlng the debrls to a lo 2 specirlc gravlty ZnC12 solutionO- The-microsandwlches will rloat ln the ZnCl2 solutlon and can be easlly lsolated~
To racllltate the retrieval Or the microparticles, -.
lt ls prererred9 in most cases,.to lncorporate magnetic : .
lron oxlde plgments or lron powder lnto the ~lcroparticle.
Mlcropartlcles can then be magnetically separated from surroundlng material. This method ls particularly userul in séparating micropartlcles rrom the blast debrisO
When maklng mlcrosandwlches rrom extruded poly-ethylene rllm, dlfrlculty was encountered ln extrudlng resin 4~

whlch had a hlgh (about 30% by welght) magnetlc lron oxlde contentO It was round that by uslng lron powder (e~sentlally spherlcal) dlspersed in small amounts (iOeO about 12 to 30%) ln the polyethylene, extrusion problems were elimlnatedO The iron powder did not mask the color of the fllm and the re-sulting microldentiflers were retrlevable wlth a magnetO
When uslng a magnet to retrleve mlcroldentiriers from bulk materlal, lt ls convenlent to place the magnet on the inside Or an open plastlc bagO The particles will adhere to the outer surrace of the bag, and the bag can be turned inside-out to trap the mlcropartlclesO
The followlng nonllmltlng examples ln whlch parts are glven by welght, wlll ~urther lllustrate the color-coded mlcroparticles useful ln the practice Or the lnvention:

..
A bundle Or red, green, amber and blue polypropylene monofilaments, each having a diameter Or about 600 micrometers, - -were assembled in a preselected color sequence around a center monofilament oore wlthln a sheath o~ heat shrinkable plastl-cized polyvlnyl chlorlde tublng about 30 cm long and 3200 mlcrometers ln diameter with a wall thlckness Or 400 mlcro-metersO Whlle under tenslon the sheathed fllament bundle was sub~ected to a blast Or hot~alr and the sheath drawn down to a diameter Or about 1600 mlcrometersO The result was a fused bundle of filaments rixed within a matrix o~ polyvinyl chloride tublngO Sectlons about 400 micrometers in length were cut rrom the rused bundleO Microscopic examination Or these ~.
"microrosettes" revealed lndlvidual colored ~ilaments, rlxed ~ :
in an orderly sequence around a center coreO In order to be more use~ul the rused bundle should be drawn down to a diameter _ 11 --- ~:

24'1 less then 1000 mlcrometersO

Mlcroporous polymerlc ion exchange resln beads (Amberlite No D A-26, a quaternary form Or a polystyrene resin avallable from Mallinckrodt Company) about 500-750 mlcro-meters ln dlameter were used as nuclel for spherical colorcoded mlcroparticles, io eO ~ ~onlon microspheresO~
Coating solutlons were made by mlxlng 005 parts Or General Mllls "Versamld" 125, a polyamide resln with an amine value of 290-300, with 5 parts of Shell Chemlcal Company "Epon" 828~ a blsphenol A epoxy wlth an epoxlde equlvalent of l90o To the polymerlc mlxture were added flve parts methylethyl ketone and two parts of chrome yellow plgmentO
A simllar coating mixture was prepared using red lead ~97% ~ . -grade) plgment TWG parts Or the yellow colored resin-pigment mix-ture were added to 25 parts of the lon exchange resin beads ln - .
a tumbler, and tumble-mixed until tackyO Two parts Or dry .~ . :
yellow plgment were added wlth continued tumble-mixlng until the tacky surface o~ each bead was well coated with the dry pigmentO The yellow plgment-coated beads were drled at 88 C ~. -for 1/2 hour to gel the resln.coatlngO The unused pigment was screened from the coated beadsO
The same process was repeated wlth the red resin-plgment mlxture and the yellow until four coats had been placed on the beadsO The lot Or color coded beads was sized by-screening through No~ 18, 20 and 25 American Standard screens (1000, 841 and 707 micrometer openings, respectively) to remove agglomerates Or beads and dry pigment residues~
Microscopic examination Or a sectioned bead re-d~ ~na~l~

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vealed a sequence o~ ~our coatlngs, each about rlve mlcro-meters thlcko Microdlsc ldentirlers were prepared by applylng con-centric coatlng~ to a nylon monorllament uslng the coatlng technlque o~ Example 20 The nucleus monorllament having a dlameter Or 25 micrometers was pas~ed through a serles Or resln-plgment coatlng solutlonsO The color sequence used was yellow, redj whlte and black (chrome yellow, red lead, tltanium dio~lde and lamp black)0 Between each coatlng bath, the mono-filament was passed through a 100 C forced air drier whlch evaporated the solvent and gelled the epoxy resln surriciently to prevent color bleeding.
After the rourth color coat had been applled and .-.drled to provide an overall.dlameter Or about.60 micro-meter~, the coated monorilament.was.passed through a hi8h speed mlcrotome to produce microdiscs about 50 micrometer~ ln thlckness~
When a relatlvely.large monorllament, about 25 :
.mlcrometers ln diameter, .18. used.as a nucleus, heat and tenslon may be applled.to the monofllament after.the colored layers have been appl$ed.to.permlt.reduction in the dlameter Or the coated monorilamenb~ When coating a monorilament and drawing .
it- down to a smaller diameter,.tbe same polymeric material -.
.should be used.ror both.the nucleus.and the coatlng 80 that all parts Or the.-coated monorilament are matched in stretch characteristicsO ~he lndividual coatings should be thinly .
applled and rlash.drled.to..prevent bleedlng.between color coatsO When color coats Or rlve.mlcrometers or less are applled, a dark co}or may show through a light color over-..,... , ~ - .: , . ~

... . . ..

10~i4;~44 coatO However, slnce the code 18 read rrom a cros~ section, thin coat tran~parency 1~ not a problemO

Thls example lllustrates the preparatlon Or mlcro-sandwlch microparticle~ using the double ~kivlng technlque~
Colored polyethylene rllms each havlng a thlcknes~
Or 65 mlcrometers were stacked in a selected color sequence, eOg~, blue, redJ green, yellow and purple. The outer surra¢e of the two outslde rllms Or the stack had been corona-treated and subsequently coated wlth.a release materlal conslsting Or 10 parts by weight Or Unlon Carblde vlnyl resln VAaH, 10 parts by weight o~ ethyi alcohol and 30 parts by weight Or methyl ethyl ketone (MEK)~ . .
The ~ilm stack was punched into discs havlng a ; ..... ... ... . .
diameter Or about 15 cm with a center hole about 6 cm ln .
dlameterO The dlscs were stacked on a mandrel against a butt plate to a height Or about.308.cm,.which repre~ented about 40 repeating color-codesO Clear polyethylene disc~ ~
were stacked on both sides or.. the colored discs to provide .-an overall lay-up Or about 15 cmO
This lay-up was heated to 120-125 C ror two hours to achleve rusion Or the layers and rorm a plastic billetO
The blllet was s~ived as.shown in Figure 5 to form ~-a ribbon approximately.l25 mlcrometers thicko The VAGH vinyl . separation coat was applied to..both the.upper and lower sur-races Or the ribbon~ Dlscs.. appro~imately.15 cm ln dlameter ` :.
havlng center holes Or about 6.cm were cut rrom the ribbonO
The dlscs were stacked..on a mandrel to a helght Or about 15 cmO
The lay-up was heated rOr two.hours at 120-125 C to ~uso the layers and form a plastlc billet as shown ln Figure 6 tr~ole rnA~ -- 14 --'~-- ~ ' ' 10~i4Z44 The blllet was sklved to produce a rlbbon approxl-mately 125 mlcrometers thlcko By washlng thls rlbbon ln MEK9 the VAGH release layer was dlssolved, generatlng the lndividual microsandwlches which were alr drled~
The microsandwiches can be made magnetlc by lncor-porating iron powder lnto the resln mlxture prior to ex-trudlng the colored fllms~ Concentratlons Or lron powder up to a maximum Or about 30 percent by weight may be added to the resln wlthout afrectlng the color of the fllmO I~
lron powder i8 added to each layer, a concentratlon o~ about -12 percent by weight ls sufflclent to make the particles readily retrievable by a magnet~

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Claims

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

1. In a method of tagging individual units of production of a substance with microparticles for the purpose of retrospective identifica-tion including the steps of: (1) providing an inventory of batches of micro-particles, each batch being uniquely coded, (2) maintaining a record of the unique code employed in each batch and (3) incorporating microparticles from any one batch with only one unit of production of a substance, the improve-ment comprising the use of microparticles whose broadest dimension across the color sequence is from 1 to 1000 micrometers and which are encoded according to a particular orderly sequence of visually distinguishable colored seg-ments, there being at least three segments in the color sequence, which in-ventory includes up to (C)(C-1)n-1 uniquely coded batches of microparticles where C is the number of available colors and n is the number of segments in the sequence.

2. Method of tagging according to claim 1 wherein said microparticles are incorporated into said substance in a concentration of 0.0001 to 1 part by weight for every 100 parts by weight of the substance.

3. Method of tagging according to claim 1 wherein each said micro-particle is 50 to 1000 micrometers at its broadest dimension across the color sequence.

4. Method of tagging according to claim 3 wherein said microparticles comprise up to 30 percent by weight of a magnetic substance which permits re-trieval of said microparticles with a magnet.

5. Method of tagging according to claim 3 wherein each microparticle comprises a solid nucleus having coated thereon concentric layers of visually distinguishable colors, with no two adjacent layers of the same color.

6. Method of tagging according to claim 5 wherein said colored layers are formed from pigmented polymeric material.

7. Method according to claim 6 wherein the colored layers are formed from pigmented mixtures of polyamide and epoxy resins.

8. Method of tagging according to claim 5 wherein said nuclei are essentially spherical particles between 50 and 200 micrometers in diameter.

9. Method of tagging according to claim 5 wherein said nuclei are monofilaments 25 to 200 micrometers in dia-meter.

10. Method of tagging according to claim 9 wherein the monofilaments are polyolefin monofilaments.

11. Method of tagging according to claim 3 wherein the colored segments of each microparticle are arranged in a ring around a central core.

12. Method of tagging according to claim 11 wherein each colored segment is provided by a colored monofilament of 5 to 50 micrometers in diameter.

13. Method of tagging according to claim 12 wherein the monofilaments are fixed in a matrix comprising polyvinyl chloride.

14. Method of tagging according to claim 3 wherein each of said microparticles is a generally rectangular hexa-hedron and said colored segments are provided by a series of layers generally parallel to one face.

15. Method of tagging according to claim 14 wherein said microparticles are formed from colored polyethylene films.

16. A plurality of batches of microparticles useful for tagging substances to permit retrospective identification, the microparticles of each batch being uniformly and uniquely encoded with an orderly sequence of visually distinguishable colored segments, there being at least three segments in the color sequence and each microparticle being 1 to 1000 micro-meters in its broadest dimension across the color sequence.

17. A plurality of batches of microparticles according to claim 16 wherein each of said microparticles comprise a solid nucleus having coated thereon concentric layers of visually distinguishable colors which in section provide said sequence of colored segments.

18. A plurality of batches of microparticles according to claim 17 wherein said nucleus is essentially spherical.

19. A plurality of batches of microparticles according to claim 17 wherein said nucleus is a monofilament.

20. A plurality of batches of microparticles according to claim 16 wherein the colored segments of each microparticle are arranged in a ring around a central core.

21. A plurality of batches of microparticles according to claim 16 wherein each of said microparticles is a generally rectangular hexahedron and said colored segments are provided by a series of layers generally parallel to one face.

22. A plurality of batches of microparticles according to claim 16 wherein each of said microparticles comprises up to 30 percent by weight of a magnetic substance which permits retrieval of said microparticles with a magnet.