CA1077786A

CA1077786A - Magnetically orientable retroreflectorization particles

Info

Publication number: CA1077786A
Application number: CA279,777A
Authority: CA
Inventors: Terry R. Bailey; Randall L. Erickson
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1976-08-03
Filing date: 1977-06-03
Publication date: 1980-05-20
Also published as: JPS5521787B2; GB1576248A; FR2360344A1; BE857396A; JPS5317643A; SE433062B; US4055377A; DE2735170B2; DE2735170A1; FR2360344B1; SE7708711L; ATA569377A; AT380744B; DE2735170C3

Abstract

ABSTRACT

Novel magnetically orientable retroreflectoriza-tion particles permit application of highly oriented retro-reflective coatings. The retroreflectorization particles individually comprise at least one transparent microsphere, specular reflective means in optical connection with a first portion of said microsphere so as to provide retroreflection of light incident on the opposed portion of the microsphere, and a magnetic layer underlying said specular reflective means and having a magnetic axis parallel to the optical axis on which said first and opposed portions of the micro-sphere are aligned. When the particles are applied to a substrate in the presence of a magnetic field having flux lines of appropriate polarity perpendicular to the sub-strate, the particles tend to become aligned in a common direction that retroreflects light incident on the coating.

Description

~` ` F .N. 912, 957 :1~777~;
MAGNETICALLY ORIENTABLE
RETROREFLECTORI ZATION PARTICLES

Ever since retroreflective coating materials were first commercially provided in the late 1950's, there has been a desire for increased retroreflective brightness from the coatings. The problem has been that the retroreflective elements used in these coating materials, i.e. flour grain-sized hemispherically metal-coated glass microspheres dis-persed in a liquid paint vehicle ~see Palmquist et al, U.S.
Pat. 2,963/378), become randomly oriented when the coating material is applied to a substrate. Typically, only about one-third of the microspheres are aligned with their un-covered portion facing outwardly in an applied coating, and the result is a significant reduction in the intensity with which the coating retroreflects incident light.
The same deficiency exists to a lesser extent in a recently developed system for retroreflactorizing fabrics and other ~ubstrates using minuke retroreflectorization particles. The retroreflectorization particles used in this system comprise one or more microspheres arranged as a closely packed monolayer and supported and partially embedded in a binder material that may be softened to adhere the particles to the fabric. The particles are typically cascaded onto the fabric and adhered there in a sparse arrangem~nt. This sparse arrangement makes the ` coating i~conspicuous under ordinary daytime viewing con-; 25 ditions; but surprisingly the coating has a striking visi-hility when viewed under illumination by vehicle headlamps at night.
However, not all of the retroreflectorization particles cascaded onto a fabric in the described system ~ 777~6 become adhered in position to retroreflect light~ The result is that more particles must be applied than would otherwise be needed, and that increased number prevents further reductions in daytime conspicuity that would sig-nificantly widen the potential scope for such treatments.
The prior art has tried several approaches to increase orientation of retroreflective elements in applied retroreflective coatings. Probably the most successful approach has involved covering the whole surface of micro-spheres with metal; applying a coating of such microspheres~either by cascading them onto a previously applied, par-tially dried layer of binder material, or preferably by applying them in admixture in a paint vehicle, which is then dried; see Nellessen et al, U.S. Pat. 3,4~0,597);
and then etching away the metal from the top portions of the applied coating of microspheres. However, this method has several disadvantages -- it requires a presently ex-pensive whole-surface coating of microspheres; the extra step o applying etchant after formation of the coating;
and, as to sparse coatings on fabrics, a wasteful and ! possible damaging application of etchant to the large u~-covered areas of the fabric. Despite utility for some purposes, this approach has not been widely practiced.
No other approach has provided the necessary characteristics either, and there remains a need for retro-reflective coating materials by which retroreflective coat-ings may be formed in situ on a substrate and yet have a high degree o orientation of retroreflective elements that achieves brilliant retroreflection~
The present inve~tion provides new magnetically .. ; , ,. . ... : , (:1 777~i orientable retroreflectorization particles that may be applied directly to a substrate to form, in essentially one step, highly oriented retroreflective coatings. These new retroreflectorization particles eac:h comprise at least one transparent microsphere, specular reflective means in optical connection with a first portion of the microsphere so as to provide retroreflection of liyht incident on the opposed portion of the microsphere, and a magnetic layer underlying the specular reflective means and having a magnetlc axis khat is parallel to the optical axis of the retroreflectorization particles along which said first and opposed portions of the microsphere are aligned.
The terms used in the preceding paragraph are standard or self-explanatory, but for purposes of clarity are defined here as follows: "magnetic" mean~ magnetized or capable of being magnetized; "magnetic axis" is a north-south magnetic polarity produced by magnetization of the magnetic layer, or an axis of easy magnetization in case the layer is not presently magnetized7 "optical connection"
is preferably achieved by applying a specular reflective means, i.e. a hemispherical coating of metal, directly on the microsphere, or alternatively by separating the specular reflective means from the microsphere by only transparent material; and "optical axis" is established by the combina-tion of the specular reflective means with the microsphere(i.e. the optical axis for a microsphere is a line perpen-dicular to the applied specular reflective means; the first portion of the microsphere (i.e. the "back" portion) and the opposed portion (i.e. the "front" portion) are aligned along the optical axis, such that light incident on the front por-~77786 tion of the microsphere is transmitted through the micro-sphere to the specular reflective means, and then returned through the microsphere along substantially the same path that the light originally travelled to the microsphere).
The described retroreflectorization particles in effect actas miniature magnets when placed in an external or applied magnetic field. The particles tend to become oriented in the external field, such that their magnetic axes, and accordingly their optical a~es, become aligned with the magnetic lines of force of the field. The magnetic field is generally applied so that the lines of force are perpendicular to the substrate to be coated, and so that the polarity of the field will cause the particles to be -positioned in the desired "upright" position on the sub-, strate (an upright position is one in which the front or optically exposed surface of the microspheres facing up-wardly or away from a substrate being coated, ready to retroreflect light incident on the substrate). The magnetic field can be provided by a substrate that is itself magne-~0 tized, or more typically because the substrate is placedover a magnet of appropriate polarity.
In brief summary, the invention provides a mass of discrete retroreflectorization particles useful for forming a retroreflective coating on a substrate, the individual particles each comprising at least one trans-parent microsphere, specular reflective means in optical connection with a first portion of the microsphere so as to provide retroreflection of light incident on the opposed portion of the microsphere, and a magnetic layer underlying the specular reflective means and having a , , 1~7 78~

magnetic axis parallel to the optical axis on which said : :
first and opposed portions of the microsphere are aligned, whereby when the particles are coated onto a substrate in .
the presence of a magnetic field having flux lines of :
5 appropriate polarity perpendicular to the substrate, -: the particles tend to become aligned :in a common direction that retroreflects light incident on the coating.
Figures 1-4 are enlarged sectional views through portions of various intermediate-stage products formed in 10 the course of manufacturing retroreflectorization particles ~:~
of the invention;
Figure 5 is a perspective view of an illus-trative magnetic particle used in retrore1ectorization particles ~.
of the invention;
Figure 6 is a sectional view through representa-: tive retroreflectorization particles of the invention;
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~77~86 Figure 7 is a schematic view showing retrore-flectorization particles of the invention applied to a fabric;
Figure 8 is a sectional view through illustrative apparatus for preparing retroreflectorization particles of the invention;
Figure 9 is a schematic view of illustrative apparatus for applying retroreflectorization particles of the invention to a fabric; and Figure 10 is a sectional view through an inter-mediate-stage product prepared in the couxse of manufactur-ing an alternative retroreflectorization particle of the invention.
The invention will first be illustrated by the following examples, which are discussed with reference to the illustrative showings in the drawings, beginning with Figure 1.
Example 1 Transparent glass microspheres 10 of 1.92 refractive index and 50 micrometers average diameter were cascaded onto a preheated polyethylene-coated high-strength paper 11, and the paper passed for about one and one-half minutes through an oven heated to 250F (121C). Thereupon, the microspheres 10 sank to a depth of about 30 percent Oc their diameters in the polyethylene coating 12 to form a product 13 such as shown in Figure 1.
Aluminum was vapor-deposited onto the microsphere-coYered surface of the web to a thickness of about 250 angstroms, forming an approximate hemispherical coating 14 of aluminum on the microspheres as shown in Figure 2.

:10~6 A solution of binde~r material was prepared by dissolving 50 parts of a thermoplastic polyester resin (understood to be the reaction product of terephthalic acid, 1,2-cyclohexanedicarboxylic acid, ethylene glycol, diethylene glycol, and cyclohexanedimethanol) in 25 parts of toluene and 25 parts of another commarcial solvent, and then adding 0.16 part of a solution prepared by mixing 40 parts of para-toluenesulfonic acid, 30 parts xylol, and 30 parts isopropanol.
To 100 parts of the solution of binder material (the binder material is denominated 15 in Figures 3, 4, 6 and 7 of the drawings) was added 3 parts of barium ferrite magnetizable pigment particles 16. The particles 16 had a somewhat hexagonal platelet shape, as represented in Figure ; 15 5, with an average length across the large-area surface (the dimension 17 in Figure 5) of about 1 micrometer, and an axis of easy magnetization (represented by the arrow 18) perpendicular to their large-area surface dimension 17. The particles were mixed into the solution of binder material using an air mixer or approximately 10 minutes. The resulting coating material was knife-coated onto the metal-coated surface of the pre~iously described web using a gap of 150 micrometers. The web was passed under the knife-coater at a rate of approximately 3 meters/minute and then through an oven heated to 93C for about one minute, leaving a product as shown in Figure 3.
At the end of the oven the web passed through a magnetic orienting device 19 such as shown in Figure 8, which produced a uniform magnetic field having flux lines perpen-dicular to the plane of the web. This device included . .

~77786 stacked sheets 21 of flexible permanent magnets composed of barium ferrite platelet particles in a flexible or elasto-meric matrix, and having a north pole on one large-area face and a south pole on the opposite face. The polymer-based magnet sheets 21 were surrounded by a xing 22 of steel/
which returned the magnetic flux and minimized any reverse field beyond the edges of the magnet sheets. In this example, the device exhibited a magnetic field of 850 oersteds in the center of the device and a maximum reverse field of 20 oers~eds.
The heating of the web prior to passage through the magnetic orienting device 19 had left the binder material 15 in a highly viscous condition that permitted the magnetic field of the device to rotate the barium ferrite platelets, but prevented the particles from becoming unoriented by the reverse field or by vibrations of the web during subsequenk drying steps. The magnetic field oriented the particles so that their axis of easy magnetization 18 was perpendicular to the web. After OrientatiQn of the magnetizable particles, the web was run through a series of three additional con-nected ovens to further dry the web and evaporate solvents, the oven temperatures and residence times for the three ovens were 53C and 1 minute; 88C and 1 minute; and 105.5C
and 5 minutes, respe~tively.
The polyethylene-coated paper 11 was stripped away from the dried web, and the resulting sheet material 24 shown in Figure 4 was placed in a blender having a four-blade impeller which was run for about 30 seconds, resulting in a chopping and breaking of the film into small particles 25 as shown in Figure 6. All the particles between 80 and ~77~

200-mesh (UOS~ Standard) screens were collected: the nom-inal thickness of the particles in theix smallest dimension was between 74 and 180 micrometers.
A fabric 27 was retroreflectorized with the particles 25, to prepare a product as schema~ically shown in Figure 7. This retroreflectorization was achieved by positioning the fabric over a polymer-based magnet sheet 28 in the manner shown in Fi~ure 9, and cascading a sparse coating of the particles onto the fabric. The flux lines 29 emanating from the magnet 28 were parallel and of the same magnetic polarity as thé magnetic polarity the par-ticles 25 would exhibit when oriented in the desired manner on the fabric. For example, if the retroreflectorization particles 25 had a north pole on their exposed-glass (front) side, the magnet 28 should have a north pole on the large-area face disposed against the fabric 27 in Figure 9. Conversely, if the particles 25 had a south pole on the exposed-glass side, the magnet 28 should have a south pole on the large-area face disposed against the fabric 27. ~he oriented particles were adhered to the fabric by heating both the fabric and the particles in a forced-air oven while under the influence of the magnetic field from the magnet 28, and allowing the fabric to cool, either within or outside the presence of the magnetic field.
In order to determine the extent to which retro-reflectivity was improved by magnetic orientation of par-ticles, two experiments were run.
Experiment 1 ;
Retroreflectorization particles 25 as described above were dropped onto a sheet of paper which had been : ;

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previously placed over a polymer-based magnet sheet. The polarity of the magnet sheet caused the particles to orient with the exposed surface of the microspheres up and the binder material down and against the paper. Particles that were the same as described in Example 1 except that no magnetic particles were present in the binder material were also cascaded onto the sheet of paper. A microscopic count o~ the particles facing upwardly was then made, with results as follows:
- 10 Upright Other Directions No. of No. of par- percent- par- percent-; ticles a~e _ ticles a~e Magnetically orientable 768 98.1 15 1.9 ;
retroreflectorizationparticles (70-180 micro-meter size range) Retroreflectorization 369 41 527 59 particles with no magnetic properties (70-180 micro-meter size range) As seen, approximately 2.4 times as many magnetically orient-able retroreflectorization particles were positioned correctly as nonmagnetized retroreflectorization particles.
Experiment 2 __ _ Two pieces of denim fabric were retroreflectorized using magnetic retroreflectorization particles of Example 1, one of the pieces o abric (Sample A in the following table) being prepared in the presence of a magnetic field, and the other piece (Sample B) in the absence of such a field. The particles were applied in an amount of 0.6 gram per 387 sq.
cm. area o~ the fabric. The fabrics were heated after cas-cading of particles using a 600-watt heat lamp until the particles were set in place; then placed in an oven heated :
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to 204C for 25 seconds; and then rolled with a rubber- :
surfaced hand-roller.
The prepared samples were then tested for retro-reflective brightness, with results as follows (in candellas per square meter per lux):

Average After Average Before Average After Heating and Sample Heating With ~amp Heating With hamp Rolling in Oven A 11.0 6.3 6.5 B 2.65 3.2 3.95 The change in the measured brightness levels after heati~g is primarily due to heat distortion of the applied retrore-flectorization particles, which caused some of the magneti-cally oriented retroreflectorization particles to assume a less than perfect orientation (due to conformation of the particles after heating to the uneven surface of the fabric and to a spheroidization of the particles produced by sur-face tension of the heated binder material in the particles), ::
and also caused some of the incorrectly positioned retrore-flectorization particles on the samples prepared without amagnetic field to turn over into at least a partially correct position.
Example 2 Retroreflectorization particles were satisfactorily prepared in the manner described in Example 1 except that the knife-coater was set at a gap of 125 micrometers; the .
coating was dried for 10 minutes at 65C and for 20 minutes at 93C prior to passage through the magnetic orienting ;^
- device; and the magnetic orienting device was replaced with a magnetic saturating device (an electromagnet producing a field of about 15,000 oersteds). Prior to application , - 10 - .

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1~77~7~36 of the magnetic field, the binder material had dried to , lock the barium ferrite particles into a randomly oriented position. The saturating field magnetized the particles in a direction perpendicular to the plane of the web, even though for most particles that was not the axis of easy magnetization; the magnetization of a particle was lessened by the extent to which the axis o easy magnetization for the particle was at an angle of less than 90 to the plane of the web. The direction of the applied field was perpen-dicular to the plane of the web and the web was passedthrough the field enough times to assure that the entire area of the web had been exposed to the magnetic field at least once.
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Retroreflectorization particles were satisfactorily prepared in the manner described in Example l except that the barium ferrite particles were replaced with 3 parts of acicular particles of gamma iron oxide (a~eraging about 0.7 micrometer in length and 0.2 micrometer in diameter); the coating material was applied with a gap of 125 micrometers;
the magnetic orienting device comprised two 6.4-millimeters-thick polymer-based magnet sheets separated by a distance of about 1.25 cm, positioned so that the two sheets mag-netically attract each other; the coated web was dried for about 10 minutes at 93C while held between the two magnet sheets; and the web was heated for an additional 10 minutes at 93C after removal from between the magnet shee~s.
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Retroreflectorization particles were satisfactorily prepared in the manner described in Example 3 except that a .~
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thermoplastic acrylic resin consisting of a copolymer o ethyl acrylate and methyl methacrylate in an approximately 50-50 weight ratio was used instead of the polyester resin, and barium ferrite particles were used instead of gamma iron oxide particles.
Example 5 A coating material was prepared by mixing the following ingredients in an air mixer until all of the resin was dissolved and the barium ferrite particles dis-persed:
Parts by Weight Polyester resin of Example 1 50 Toluene 25 Commercial solvent 25 Barium ferrite 3 This solution was coated onto the back of a sheet material as shown in Figure 10, which is generally prepared according to the teachings of Palmquist, U.S. Pat. 2,407,680. This sheeting consists of a liner 32, a transparent top film 33 of alkyd resin, a transparent binder coating 34 of alkydresin, microspheres 35 having an index of refraction of

2.26, a transparent spacing layer 36 of polyvinyl butyral resin, and a layer 37 of vapor-deposited aluminum. The coating material 38 was applied using a knife-coater with a gap of 125 micrometers, after which the coated web was ~;placed in a magnetic field establishea by polymer-based magnet sheets as described in Example 3 and the coating material then dried under the influence of that magnetic field for 10 minutes at 66C and 10 minute~ at 93C. The ;30 coated web was then removed from the magnetic field and .:
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~C~777~36 dried an additional 10 minutes at 93C. The liner 32 was removed and the resulting film placed in a blender with dry ice and chopped for about 30 seconds. Particles between 20 and 80 mesh (180-850 micrometers) were then collected. The particles were dropped on a sheet of paper, placed over a magnet sheet and the orientation of the particles visually e~amined in a microscope and determined to be 98 percent correctly oriented for retroreflection.
A primary use for retroreflectorization particles as described in the above examples is to reflectorize fabric. For such a use, the retroreflectorization par-ticles should be as small as practical. Preferably the retroreflectorization particles of the invention include on the average no more than about 10 microspheresv prefer-ably no more than about 5 microspheres, and most preferablyno more than about 3 microspheres. Retroreflectorization particles containing more than 10 microspheres can be satisfactorily used, even for reflectorizing fabrics (e.g., by coloring the retroreflectorization particle a color that ; 20 matches the fabric); generally such particles will average no more than about 50 microspheres. Larger particles have several advantages: e.g., they require less chopping, which makes them less expensive and also causes less disruption o the retroreflec~ive structure of the particles.
Over the whole treated surface of a fabric of the invention there should be on the average less than ` about 2,000 microspheres per square centimeter; preferably .:j there are less than about 500, and more preferahly less than about 300 microspheres per square centimeter. To achieve uniformity of effect, these numbers preferably also apply " .

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~al77'7~6 to any area of the treatment occupying one square centimeter.
Generally sufficient microspheres are included to provide at least one candella, and preferably at least 2 candellas, ; of reflected light per square meter of a treated surface per lux of light that is incident on the surface. On the ~; other hand, to minimize daytime visibility the treatment usually provides less than 20, and more often less than 10 candellas/square meter of trea~ed surface per lux of inci-dent light.
10A variety of binder materials can be used in the retroreflectorization particles. Preferably, the binder material is a heat-activatable adhesive, softening upon ; exposure to elevated temperatures so as to wet and de~elop adhesion to the fabric. Examples of useful binder materials of this kind are polyesters, acrylics, polyurethanes, and polyamidesO The binder material may also be activated or softened in other ways, such as by application of solvent.
Following application of particles to a fabric, the binder material hardens as by cooling, loss of solvent 2Q or other volatiles, or by chemical reaction including cross-linking or polymerizing. Illustrative chemically reactive materials include thermosetting resin compositions such as epoxy-based resin compositions, melamine-formalde-hyde resin compositions, and acrylic-based resin composi-tions.
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~` The layer of binder material in a retroreflector-ization particle may comprise two or more sublayers. For ~ ~ .
example, in some embodiments the microspheres are embedded in one isublayer of binder material~ and a second sublayer ~-; 30 of magnetic-particle-filled binder material is used to bond ~ - 14 -:' ~, .. . .

~77786 the particles to fabricO FurtherJ one sublayer such as a sublayer of magnetic-particle-filled binder material, may be preformed and then applied to a previously built microsphere-carr~lng sheet. Use of a binder materlal from the same chemical class as the fabric being treated is often advantageousO
The microspheres should average less than about 200 micro-meters in diameter to achieve the least conspicuous treatment on a abric, and preferably they average less than about 100 micrometers in diameter. However, larger microspheres may be used in retro-reflectorization particles to be used for other purposes, and in any event they are generally at least 25 micrometers or larger in average diameter.
While metal layers provide useful specular reflective means, dielectric mirrors prepared in the manner taught in Bingham, United States PatO 31700,3~5~ are also quite useful~ As described in that patent, these dielectric mirrors comprise at least one transparent thin coating or layer having a refractive index -1' the faces of said transparent layer being in contact with ma*erials of reractive index n2 and n3, both n2 and n3 being at least 0.1 either higher or lower than nl, and the transparent layer having an optical thickness corresponding to an odd-numbered multiple of about one-quarter wavelength of light in the wavelength range o~ a~out 3800 to 10,000 angstromsO Such coatings, which can be visibly transparent while reflecting sufficient light to provide good retroreflection, may improve the color or appearance of a reflective treatment by letting an underlying color, e.g., a colored binder material in the retro-,: il reflectorization particles, or the color of a garment, be visible through the reflective treatment. Reflective means can also be pro-~i vided by use of a specular reflective material in the binder material :; 30 underlying ~he microspheres, for example, a reflective pigment such as aluminum flakes or nacreous pigment ~see Bingham, United States Pat. 3,758,192) may be dispersed in the binder material.
Barium ferrite particles are preferred magnetiz-~617~77~6 able particles for use in retroreflectorization particles of the invention because of their high coercivity and their anisotropic character. When such particles are used and physically oriented with their axis of easy magnetization -parallel to the optical axis for the particles, the retro-reflectorization particles will become aligned in a magnetic field even if there is little i any magnetization of the particles. Since the retroreflectorization particles are unmagnetized, they axe easier to handle and flow more freely during application to a substrate. Other useful magnetiz-able particles include strontium ferrite, acicular gamma iron oxide, and acicular iron particles. The magnetiæable particles can be used in rather low amount, since only a low amount of magnetic energy is needed to rotate the very small and light weight retroreflectorization particles.
However, generally they will be used in an amount of at least 0.5 weight~percent of binder material coated on the particles and preferably in an amount of at least l weight- ;
percent of the binder material.
While physical orientation of magnetizable pig-ments within retroreflectorization particles, when used, is preferably achieved by a magnetic field, it may also be achieved by mechanical means, such as taught in Blume, U.S.
Pat. 2,999,275.
; 25 ~esides use of retroreflectorization particles of the invention to reflectorize fabric, they may be used :.
~ to reflectorize other sheet materials. Further, retrore-,..:
flectorization particles of the invention may be mixed into a liquid paint vehicle ~comprising a film-forming binder material, which advances to a nontacky adherent ~17778~ :
paint film when applied to a substrate a~ a thin layer and exposed to an appropriate environment, and typically a volatile thinner in which the binder material is dissolved or dispersed) to form a coating composition of the type described in Palmquist et al, U.S. Pat~ 2,g63,378. Such a coating composition can be applied to a substrate in the presence of a magnetic field to achieve a high degree of orientation of retroreflective particles.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A mass of discrete retrorefletorization par-ticles useful for forming a retroreflective coating on a substrate, the individual particles each comprising at least one transparent microsphere, specular reflective means in optical connection with a first portion of the microsphere so as to provide retroreflection of light in-cident on the opposed portion of the microsphere, and a magnetic layer underlying the specular reflective means and having a magnetic axis parallel to the optical axis on which said first and opposed portions of the microsphere are aligned, whereby when the particles are coated onto a substrate in the presence of a magnetic field having flux lines of appropriate polarity perpendicular to the sub-strate, the particles tend to become aligned in a common direction that retroreflects light incident on the coating.

2. Retroreflectorization particles of claim 1 which each include up to an average of about ten trans-parent microspheres arranged as a closely packed monolayer and supported in a layer of binder material.

3. Retroreflectorization particles of claim 2 in which said magnetic layer comprises magnetic pigment dispersed in at least a portion of said binder material.

4. Retroreflectorization particles of claim 3 in which said magnetic pigment comprises barium ferrite particles.

5. Retroreflectorization particles of claim 2 or 3 in which at least a portion of said binder material softens in the presence of heat to an adhesive state whereby the retroreflec-torization particles will adhere to a substrate to which they are applied.

6. Retroreflectorization particles of any of claims 1-3 in which said specular reflective means comprises a transparent thin layer having a refractive index n1, the faces of said trans-parent layer being in contact with materials of refractive index n2 and n3, both n2 and n3 being at least 0.1 either higher or lower than n1, and the transparent layer having an optical thickness corresponding to an odd-numbered multiple of about one-quarter wavelength of light in the wavelength range of about 3800 to 10,000 Angstroms.

7. A coating composition comprising a liquid paint vehicle and retroreflectorization particles of any of claims 1-3 dispersed in the vehicle.

8. A method for reflectorizing a substrate comprising applying retroreflectorization particles of any of claims 1-3 to the substrate in the presence of a magnetic field having flux lines of appropriate polarity perpendicular to the substrate.

9. A method of reflectorizing a fabric substrate compris-ing applying retroreflectorization particles of any of claims 1-3 to the substrate in the presence of a magnetic field having flux lines of appropriate polarity perpendicular to the substrate.

10. Fabric carrying a sparse coating of retroreflectoriz-ation particles of any of claims 1-3.