The present invention provides a retroreflective sheeting including: optical elements arranged in substantially a monolayer; a spacing layer in which the optical elements are at least partially embedded; a specularly reflecting layer underlying the spacing layer; and a bead bond layer in which the optical elements are at least partially embedded. The bead bond layer includes an aminoplast-crosslinked polymer comprising urethane groups, wherein prior to crosslinking the polymer has a glass transition temperature (Tg) of less than about 0.degree. C.

FIELD OF THE INVENTION 
This invention relates to retroreflective sheeting constructions comprising 
optical elements partially embedded in a bead bond layer. 
BACKGROUND OF THE INVENTION 
Heretofore, a number of retroreflective sheeting products have been 
marketed. A typical example is characterized by a single layer of tiny 
optical elements embedded in a polymeric bead bond layer and in a 
polymeric spacing layer. The spacing layer is typically backed by a 
specularly reflective layer and an adhesive with a strippable protective 
layer (i.e., a release liner). The bead bond layer is typically surfaced 
with a top film. Retroreflective sheetings such as this are known as 
"embedded" lens sheeting (or often as "enclosed" lens sheeting). The first 
example of an embedded-lens sheeting was taught by U.S. Pat. No. 2,407,680 
(Palmquist et al.). See also, for example, U.S. Pat. Nos. 3,551,025 
(Bingham et al.), 3,795,435 (Schwab), 4,664,966 (Bailey et al.), 4,530,859 
(Grunzinger, Jr.), 4,721,649 (Belisle et al.), 4,725,494 (Belisle et al.), 
and 4,808,471 (Grunzinger). 
Certain of the bead bond layers described in these patents are tacky prior 
to final cure when the optical elements are applied and set, whereas 
others are nontacky. Some of the layers, including the bead bond layers, 
are made of polyurethanes, which may or may not be crosslinked. Such 
sheeting has been sold commercially for many years in large volume and to 
the general satisfaction of its users. Despite this general satisfaction, 
there has been a desire for an improvement in certain properties of the 
sheeting as technology advances and as decentralized manufacturing 
expands. For example, it is desired to produce retroreflective sheeting 
which has greater conformability and embossability, particularly under 
high speed embossing forces, than retroreflective sheeting heretofore 
known in the art. 
SUMMARY OF THE INVENTION 
The present invention provides a conformable retroreflective sheeting that 
will not significantly crack under deformation, particularly high speed 
embossing conditions. Specifically, the present invention provides a 
retroreflective sheeting comprising: optical elements arranged in 
substantially a monolayer; a spacing layer in which the optical elements 
are at least partially embedded; a specularly reflecting layer underlying 
the spacing layer; and a bead bond layer in which the optical elements are 
at least partially embedded; wherein the bead bond layer comprises an 
aminoplast-crosslinked polymer comprising urethane groups, wherein prior 
to crosslinking the polymer has a glass transition temperature (Tg) of 
less than about 0.degree. C. 
Preferably, the present invention provides a retroreflective sheeting 
comprising: optical elements arranged in substantially a monolayer; a 
spacing layer in which the optical elements are at least partially 
embedded; a specularly reflecting layer underlying the spacing layer; a 
bead bond layer in which the optical elements are at least partially 
embedded; wherein the bead bond layer comprises an aminoplast-crosslinked 
polymer prepared from a polyester polyol and a polyisocyanate, wherein 
prior to crosslinking the polymer has a glass transition temperature (Tg) 
of less than about 0.degree. C.; and a thermoplastic top film disposed on 
the surface of the bead bond layer opposite the optical elements. 
Preferably and desirably, the sheeting of the present invention (including 
a top film) has a high speed impact resistance of at least about 0.69 
Kg.multidot.m when adhesively bonded to a 0.8 mm thick aluminum panel. 
The present invention also provides an article comprising a substrate 
supporting a retroreflective sheeting, wherein the retroreflective 
sheeting comprises: optical elements arranged in substantially a 
monolayer; a spacing layer in which the optical elements are at least 
partially embedded; a specularly reflecting layer underlying the spacing 
layer; a bead bond layer in which the optical elements are at least 
partially embedded; wherein the bead bond layer comprises an 
aminoplast-crosslinked polymer comprising urethane groups, wherein prior 
to crosslinking the polymer has a glass transition temperature (Tg) of 
less than about 0.degree. C.; and a layer of adhesive. Preferably, the 
layer of adhesive is disposed on the surface of the specularly reflecting 
layer opposite the spacing layer. 
Finally, a method for preparing retroreflective sheeting is provided. The 
method comprises: applying a layer of an uncured bead bond composition 
onto a release liner, the bead bond composition comprising a crosslinking 
agent and a crosslinkable polymer comprising urethane groups and 
unprotected functional groups, wherein the crosslinkable polymer has a 
glass transition temperature (Tg) of less than about 0.degree. C.; 
depositing optical elements onto the layer of uncured bead bond 
composition; heating the uncured bead bond composition to a temperature 
and for a time effective to crosslink the polymer; covering the exposed 
portions of the optical elements with a spacing layer such that the 
spacing layer forms an exterior surface cupped around the optical 
elements; and applying a specularly reflective layer to the cupped surface 
of the spacing layer.

DETAILED DESCRIPTION 
The present invention provides a conformable retroreflective sheeting that 
will not significantly crack under deformation. Thus, this sheeting can be 
applied to a substrate made of, for example, a metal such as aluminum or a 
plastic, and then embossed to make license plates, for example. This 
sheeting preferably has the advantageous properties of: (1) withstanding 
high speed embossing forces without significant cracking; (2) withstanding 
deformation without significantly lifting from the substrate around the 
deformed areas; and (3) not requiring heat during the application process. 
The sheeting is also capable of having graphic print under and/or on a 
protective top film. Although the sheeting is particularly useful for 
embossed articles, it can be used on nonembossed articles, which can have 
smooth or irregular surfaces, such as signage or the side of a car or 
truck. 
The desire to emboss license plates under various conditions has surfaced. 
In locations where the application of the sheeting is not centralized, the 
application equipment varies. Therefore, a sheeting that withstands 
various embossing conditions is desired. Typical embossing depths range 
from about 1.0 mm to about 2.0 mm and embossing speeds can vary from 
instantaneously to about five seconds. The faster the embossing speed, the 
more flexible the sheeting needs to be, which can be accomplished by a 
lower Tg material in the bead bond layer, as used in the sheeting of the 
present ivention. 
Furthermore, after embossing, the raised characters are typically inked, 
and then the construction may or may not be subjected to elevated 
temperatures. The elevated temperatures dry the ink and relax the sheeting 
around the characters. If the constructions are not subjected to elevated 
temperatures, however, the sheeting needs to be able to relax around the 
characters without the application of heat. Thus, the bead bond layer of 
the present invention provides a sheeting capable of being used in either 
situation (i.e., with or without the application of heat). 
The bead bond layer of the present invention also provides a sheeting 
(including a top film) that preferably has a high speed impact resistance 
of at least about 0.69 Kg.multidot.m, and more preferably at least about 
0.92 Kg.multidot.m, on a 0.8 mm thick aluminum panel. As used herein, 
"high speed" impact resistance refers to the amount of force used to 
deform the sheeting and aluminum panel with a free-falling weight without 
significant cracking of the sheeting. Typically, using a free-falling 
weight, the deformation occurs in less than about one second. Preferably, 
the embossing depth that occurs under such high speed conditions is at 
least about 2.0 mm, and more preferably at least about 2.25 mm without 
significant cracking of the sheeting of the present invention. 
The reference numeral 1, FIGS. 1-2, generally designates a license plate 
having a retroreflective sheeting, including a bead bond according to the 
present invention, thereon. The license plate 1 generally comprises a 
substrate 3, formed from metal or plastic, for example, having characters 
4 embossed therein. The characters 4 are generally embossed such that they 
are raised (i.e., projected outwardly) from surface 5 of license plate 1; 
that is, they project toward the viewer. This will be better understood by 
reference to FIG. 2. Typically, conventional embossed license plates carry 
characters thereon which are embossed, relative to surface 5 of substrate 
3, a total of at least about 0.15-0.20 cm. It is noted that license plate 
1 includes an outer border 6 debossed away from the viewer. Alternatively, 
referring to FIG. 3, the license plate 1' may include a raised rim 7' near 
the outer border 6' of substrate 3'. The raised rim 7' may or may not be 
embossed to the same height of the raised characters 4'. Although the 
present invention is primarily described with respect to applications 
concerned with embossed letters, numbers, symbols, rims, etc., it will be 
understood that similar concerns and problems are involved when debossed 
letters, numbers, symbols, rims, etc., are involved. 
In general, it is desirable that at least portions of surface 5 be 
substantially reflective, so that the license plate 1 will be very 
conspicuous, even at night and when viewed from a considerable distance. 
In general it is desirable to provide a license plate 1 which is very 
strongly retroreflective, so that it can be seen from a considerable 
distance, with only a small amount of light directed thereon. Further, an 
embedded lens arrangement is useful at least in part because good 
reflection is obtained under both wet and dry conditions. 
In general, what is needed is a retroreflective sheeting at surface 5 of 
license plate 1. A commonly used type of such a sheeting is an embedded 
lens retroreflective sheeting, which can be readily applied to, or 
laminated on, surface 5, which can then be embossed or debossed to provide 
the desired letters, symbols, numbers, etc. 
Referring to FIG. 4, an embedded-lens retroreflective sheeting utilizing 
the particular polymeric bead bond layer 14 is depicted. The structure of 
the sheeting 10 includes a top film 12, which typically forms the exterior 
front surface of the sheeting; a monolayer of optical elements 13, 
typically light transmissible glass beads, embedded in a bead bond layer 
14, which is also preferably light transmissible; a spacing layer 15, 
which is preferably light transmissible, applied to the back surfaces of 
the optical elements 13 in such a way as to follow the curved surface of 
the back of the optical elements; a specularly reflective layer 16 
typically vapor-deposited on the spacing layer 15; and a layer of adhesive 
17 covering the reflective layer 16, although a layer of adhesive could 
alternatively cover the bead bond layer 14 instead of the top film 12. 
Optional prime layers 18 and 19 can also be used. Prime layer 18 provides 
enhanced adhesion, for example, to decals or other adhesive-backed 
materials, as well as inks or other colorants that are applied to the top 
film 12 of the sheeting. Prime layer 19 provides enhanced adhesion of the 
top film 12 to the bead bond layer 14. Optional prime layers 18 and 19 are 
also preferably light transmissible. As used herein, light transmissible 
means that the material is able to transmit at least about 70% of the 
intensity of the light incident upon it at a given wavelength. Preferably, 
the light transmissible materials transmit greater than about 80%, and 
more preferably, greater than about 90% of the incident light at a given 
wavelength. 
Light rays incident on the sheeting travel through layers 12 and 14, and 
optional layers 18 and 19, to the optical elements 13, which act as lenses 
focusing the incident light through the spacing layer 15 and approximately 
onto the appropriately spaced specularly reflective layer 16. Thereupon 
the light rays are reflected back out of the sheeting along substantially 
the same path as they traveled to the sheeting. 
Except for the particular bead bond layer 14, the structure shown in FIG. 4 
is conventional, and methods for its formation are well known in the art. 
Polymeric Bead Bond Layer 
Bead bond layer 14 contributes to the illustrated retroreflective sheeting, 
in that it is the unique characteristics of this layer which enable the 
sheeting to possess improved conformability and embossability, 
particularly under high speed embossing forces. It comprises a conformable 
and embossable polymeric material, which is preferably light 
transmissible, that provides good adhesion to the optical elements (e.g., 
microspheres or beads). The optical elements are typically embedded 
sufficiently deep in the bead bond layer that they are securely anchored; 
however, they are not embedded so deep that the brightness or angularity 
of the sheeting is reduced. Typically, the optical elements are embedded 
to a depth of about 30-50% of their diameter. 
The polymeric material used to prepare the bead bond layer 14 is 
crosslinkable and can form a film with a thickness that can be readily and 
precisely controlled. The crosslinkable polymer is a polyurethane. That 
is, the polymer comprises urethane groups (--NH--C(O)--O--), although 
other groups may also be present such as urea groups (--NH--C(O)--NH--), 
for example. Preferably, the number average molecular weight of the 
crosslinkable polyurethane is less than about 30,000, and the weight 
average molecular weight is less than about 75,000, as determined by Gel 
Permeation Chromatography. 
This polymeric material is a crosslinkable polyurethane having a glass 
transition temperature (Tg) of less than about 0.degree. C., preferably 
less than about -5.degree. C., and more preferably within a range of about 
-5.degree. C. to about -25.degree. C. This low Tg crosslinkable 
polyurethane is sufficiently compliant such that upon crosslinking, it has 
a suitable elongation and tensile strength to provide a sheeting that can 
withstand high speed embossing forces without detrimentally impacting the 
sheeting. Typically, the addition of optical elements lowers the 
elongation of free-standing bead bond samples (i.e., layers of cured bead 
bond that are not supported by a substrate). This is because each bead 
acts as a fault point from which a crack can propagate. Significantly, 
this effect is not typically observed with the low Tg polyurethanes used 
in the bead bond layer of the present invention. Preferably, free-standing 
bead bond samples of the present invention with a monolayer of optical 
elements has an elongation of at least about 100%, preferably about 
200-1000%. Also, significantly, the low Tg polyurethanes used in the bead 
bond layer of the present invention provide a more impact resistant bead 
bond layer. Preferably, a bead bond sample of the present invention 
adhesively applied to a 0.8 mm thick aluminum panel has a high speed 
impact resistance (as defined above) of at least about 0.92 Kg.multidot.m, 
and more preferably at least about 1.6 Kg.multidot.m. 
The polyurethane can be any of a variety of crosslinkable polyurethanes 
prepared by combining one or more polyols with one or more polyisocyanates 
as long as the Tg is less than about 0.degree. C. prior to crosslinking. 
As used herein, crosslinkable means that the polymer has functional groups 
capable of reacting with a crosslinking agent. Preferably, the 
polyurethane has pendant hydroxyl groups free for reaction with a 
crosslinking agent, although other functional groups are possible for 
crosslinking, such as isocyanate groups and carboxyl groups. In the method 
of the present invention, the functional groups are unprotected (i.e., 
unblocked), allowing crosslinking to occur. 
A variety of polyols may be utilized in preparing the polyurethane. Also, 
mixtures of polyols can be used. The term "polyol" as used herein refers 
to polyhydric alcohols containing two or more hydroxyl groups. The polyol 
preferably has a hydroxyl functionality of 2-4 (i.e., diols, triols, 
tetraols). More preferably, the polyol is a diol, although higher 
functional polyols, such as triols and tetrols, can be used in combination 
with a diol. Most preferably, the polyol is a diol or mixture of diols and 
no higher functional polyols are used to prepare the polyurethane. 
The polyol can be a polyether polyol such as polytetramethylene glycol and 
polypropylene glycol; a polyester polyol such as the reaction product of 
adipic acid and neopentyl glycol or phthalic anhydride and hexanediol; an 
acrylic polyol; and the like. Preferably, the polyol is a polyester 
polyol. 
A particularly preferred polyester polyol is a hydroxyl terminated polyol 
of the following formula: HO----R--O--C(O)--R'--C(O)--O--R--O--!.sub.n 
--H, wherein R is an aliphatic group having 2-10 carbon atoms, R' is an 
aliphatic or aromatic group having up to 14 carbon atoms, and n is at 
least 2. This polyester diol is typically formed from one or more types of 
aliphatic or aromatic acids/esters and one or more types of aliphatic 
diols. For example, a polyester diol of the above formula can be prepared 
from an aromatic acid or ester such as isophthalic acid or dimethyl 
isophthalate (or mixture thereof), and a diol such as neopentyl glycol, 
1,6-hexanediol, or 1,4-cyclohexane dimethanol (or mixture thereof). If 
both an aromatic and an aliphatic material are used, the weight percent of 
the aromatic material is generally less than that of the aliphatic 
material. 
The polyester diol, or other suitable polyol, preferably has a hydroxyl 
equivalent weight of about 90 to about 5000, more preferably about 200 to 
about 3000, and most preferably about 250-2000. The polyester diol, or 
other suitable polyol, preferably has an acid number of no greater than 
about 1.0, and more preferably no greater than about 0.7. Acid number can 
be determined in accordance with ASTM D 4662-93. 
An example of a commercially available polyester diol is FOMREZ 8056-146 
from Witco Corp., Melrose Park, Ill. This resin is believed to contain 
neopentyl glycol at approximately 26 wt-%, 1,6-hexanediol at approximately 
29 wt-%, adipic acid/ester at approximately 33 wt-%, and isophthalic 
acid/ester at approximately 12 wt-%. Other polyester diols are 
commercially available under the trade designations FOMREZ 55-112 
(believed to contain approximately 47 wt-% neopentyl glycol and 
approximately 53% adipic acid/ester) and FOMREZ 8066-120 (believed to 
contain approximately 49 wt-% hexanediol, approximately 33 wt-% adipic 
acid/ester, and approximately 18 wt-% isophthalic acid/ester) from Witco 
Corp, as well as LEXOREZ from Inolex Chemical Company, Philadelphia, Pa., 
and RUCOFLEX from Ruco Polymer Corp., Hicksville, N.Y. It should be 
understood that blends or mixtures of such diols can be used in 
preparation of the polyurethane used in the bead bond layer 14. 
A variety of triols may be utilized in the preparing the polyurethane. 
Suitable triols include, but are not limited to, polyether triols such as 
polypropylene oxide triol, polyester triols other than polycaprolactone 
triols, and simple triols such as trimethylolpropane and glycerol, and 
mixtures thereof. Preferably the hydroxyl groups in the triol are primary 
in order to facilitate crosslinking of the resultant polymer. Examples of 
triols include those commercially available under the trade designations 
FOMREZ 1066 (trimethylolpropane, hexanediol, and adipate) from Witco 
Corp., TONE 0305 (a polycaprolactone triol) from Union Carbide Corp., New 
Milford, Conn., and RUCOFLEX F-2311 from Ruco Polymer Corp. It should be 
understood that these materials could be used as blends or mixtures with 
other polyols to achieve a Tg of less than about 0.degree. C. 
Tetrafunctional or higher alcohols such as pentaerythritol may also be 
useful polyols. Other useful polyols are taught by E. N. Doyle in "The 
Development and Use of Polyurethane Products," McGraw-Hill, 1971. If a 
triol and higher functional polyol is used, the NCO:OH stoichiometry will 
need to be adjusted accordingly, although this would be understood by one 
of skill in the art. 
A wide variety of polyisocyanates may be utilized in preparing the 
polyurethane. "Polyisocyanate" means any organic compound that has two or 
more reactive isocyanate (--NCO) groups in a single molecule that can be 
aliphatic, alicyclic, aromatic, or a combination thereof. This definition 
includes diisocyanates, triisocyanates, tetraisocyanates, and mixtures 
thereof. Preferably, diisocyanates are utilized. These isocyanate groups 
can be bonded to aromatic or cycloaliphatic groups. Most preferably 
aliphatic isocyanates, including cycloaliphatic isocyanates, are used to 
improve weathering and eliminate yellowing. Useful diisocyanates include, 
but are not limited to, those selected from the group consisting of 
bis(4-isocyanotocyclohexyl) methane (H.sub.12 MDI, available from Bayer 
Corp., Pittsburgh, Pa.), diphenylmethane diisocyanate (MDI, available from 
Bayer Corp., Pittsburgh, Pa.), isophorone diisocyanate (IPDI, available 
from Huels America, Piscataway, N.J.), toluene 2,4-diisocyanate (TDI, 
available from Aldrich Chemical Co., Milwaukee, Wis.), hexamethylene 
diisocyanate (HDI, available from Aldrich Chemical Co., Milwaukee, Wis.), 
m-tetramethylxylene diisocyanate (TMXDI, available from Aldrich Chemical 
Co., Milwaukee, Wis.), and 1,3-phenylene diisocyanate. It is also noted 
that mixtures of diisocyanates can also be used. 
The stoichiometry of the polyurethane reaction is based on a ratio of 
equivalents of isocyanate to equivalents of polyol. The overall preferred 
NCO:OH ratio for the polyurethane is less than 1:1 to allow for residual 
hydroxyl groups in the resultant polyurethane. More preferably, the NCO:OH 
ratio is about 0.8-0.99 to 1. Most preferably, the NCO:OH ratio is about 
0.91-0.96 to 1. It will be understood by one of skill in the art that this 
ratio will vary depending upon the synthetic sequence when using triol(s) 
and/or tetrol(s). This is accomplished typically by varying the amount of 
isocyanate such that gellation is avoided and a soluble product is 
obtained. 
A catalyst may be added to the reaction mixture of polyol(s) and 
polyisocyanate(s) to promote the reaction. Catalysts for reacting 
polyisocyanate and active hydrogen containing compounds are well known in 
the art. See, for example, U.S. Pat. No. 4,495,061 (Mayer et al.). 
Preferred catalysts include organometallic compounds and amines. The 
organometallic compounds may be organotin compounds such as dimethyltin 
dilaurate, dibutyltin dilaurate, and dibutyltin dimercaptide. The 
preferred catalyst is dibutyltin dilaurate. The catalyst is used in an 
amount effective to promote the reaction. Preferably, it is used in an 
amount of about 0.01-2% by weight (wt-%), based on the total weight of 
solids. More preferably, the catalyst is used in an amount of about 
0.01-0.03 wt-% based on solids. 
The polymer may be prepared in the presence or absence of a solvent. 
Preferably, it is prepared in the presence of one or more organic 
solvents. Examples of suitable solvents include, but are not limited to, 
amyl acetate, aromatic hydrocarbons and mixtures thereof, butanone, butoxy 
ethoxyethyl acetate, 2-ethoxyethyl acetate, cyclohexanone, dioxane, 
4-methyl-2-pentanone, tetrahydrofuran, toluene, xylene, and/or mixtures 
thereof. Preferred solvents are xylene, 4-methyl-2-pentanone, and mixtures 
thereof. The polyurethane reaction mixture preferably includes about 30-75 
wt-% total solids, and more preferably about 40-55 wt-% total solids. 
An extensive description of some of the useful techniques for preparing 
polyurethanes can be found in J. H. Saunders and K. C. Frisch, 
"Polyurethanes: Chemistry and Technology," Part II, Interscience (New York 
1964), pages 8-49, and in the various references cited therein. The 
component polyol(s) and polyisocyanate(s) may be reacted simultaneously or 
stepwise. 
The bead bond layer comprises a crosslinked polyurethane prepared from a 
crosslinkable polyurethane and a crosslinking agent. A variety of 
crosslinking agents may be used with the crosslinkable polyurethane of the 
present invention. In general, what is required is a crosslinking agent 
that will readily react with the reactive moieties of the polyurethane, 
which are preferably hydroxyl groups, and which will provide for a 
substantial amount of crosslinking. The crosslinking agent is at least 
difunctional. Preferably, it will react at relatively low temperatures 
(e.g., 90-180.degree. C.), but not at room temperature (i.e., 
25-30.degree. C.), although this is not required if in-line mixing is used 
(i.e., if the crosslinking agent and crosslinkable polyurethane are 
combined during the manufacturing process immediately prior to coating). 
Thus, a storage stable composition may be prepared with both the 
crosslinkable polyurethane and the crosslinking agent therein without 
having to protect the crosslinkable moieties of the polyurethane. 
It is further preferred that the crosslinking agent be such that reaction 
with the crosslinkable polyurethane can be efficiently carried out to 
substantial completion in a relatively short period of time, often in less 
than about three minutes. As used herein "completion" means that the 
polyurethane cures to a stage whereat relatively little further reaction 
causing change in volume or conformation will occur. That is, the 
polyurethane is cured to greater than about 50% gel fraction, and 
preferably greater than about 80% gel fraction. Thus, advantageously and 
preferably, the crosslinkable polyurethane includes unprotected functional 
groups, preferably hydroxyl groups, and can be cured upon crosslinking in 
a relatively short amount of time at a relatively low temperature. 
Preferably, the crosslinkable polyurethane is provided in substantial 
excess relative to the crosslinking agent. Typically, polyurethane resin 
to crosslinker ratios (based on solids) vary from about 2:1 to about 20:1 
depending on the crosslinker. Preferably, the polyurethane resin to 
crosslinker ratio is about 5:1 to about 6.5:1. 
The crosslinking agents that exhibit the above, preferred, qualities 
include: aminoplast resins (i.e., the reaction product of an aldehyde and 
an amine or urea) such as urea-formaldehyde resins, melamine-formaldehyde 
resins, glycouril-formaldehyde resins; acrylic copolymers containing 
etherified adducts of the reaction product of acrylamide and formaldehyde; 
polyfunctional aziridines; epoxy resins; aldehydes; azlactones; and/or any 
other polyfunctional material whose functional groups are reactive with 
the functional groups of the crosslinkable polyurethane. Aminoplast resins 
are preferred, and melamine-formaldehyde resins are particularly 
preferred. Although the inventors do not wish to be held to any theory, it 
is believed that these resins contribute to the high impact resistance of 
the crosslinked polyurethane and weatherability in the bead bond layer. 
Examples of commercially available melamine-formaldehyde resins are the at 
least partially butylated melamine-formaldehyde resins available under the 
trade designations RESIMENE 881, RESIMENE BM 5901, RESIMENE 750, and 
RESIMENE 7512 from Monsanto, St. Louis, Mo., as well as URAMEX CP 1132 MF 
from Dutch State Mine Resins, The Netherlands. Other suppliers of 
melamine-formaldehyde resins are described in Table 2 at page 136 of 
Ullmann's Encyclopedia of Industrial Chemistry, Fifth, Completely Revised 
Ed., Volume A2, VCH Publishers. As used herein, resin refers to mixtures 
of monomer, oligomers, and/or polymers. 
Additionally, weathering additives such as UV absorbers, hindered amine 
light stabilizers, antioxidants, etc., can be added to the bead bond to 
improve the overall durability of the retroreflective sheeting of the 
present invention. 
Top Film 
The top film 12 is typically an abrasion resistant, polymer coating that 
provides a hard, weatherproof exterior to the retroreflective sheet. The 
top film 12 is preferably made of a light transmissible polymeric material 
that is substantially thermoplastic and nonelastomeric, and more 
preferably extrudable. Examples of suitable such materials include 
polyethylene, or preferably, one or more copolymers of monomers comprising 
by weight a major portion of at least one of ethylene or propylene, and a 
minor portion of at least one polar monomer (e.g., a monomer that contains 
an oxygen or a nitrogen, or combination thereof). Examples of suitable 
such polar monomers include acrylic acid, methacrylic acid, ethyl 
acrylate, and vinyl acetate. Alternatively, the top film can be made of a 
blend of a major amount by weight of one or more of these ethylene- or 
propylene-containing copolymers with polyethylene and/or other polymers 
such as vinyl acetate and/or methacrylic acid polymers or copolymers. The 
melt index of the polymeric material suitable for preparation of the top 
film is typically less than about 500, preferably less than about 150, and 
more preferably less than about 20. Polymeric materials having lower melt 
indices are typically easier to extrude and more resistant to softening at 
elevated temperatures. 
A number of suitable polymers are commercially available, including those 
available under the trade designations PRIMACOR 3440 (copolymer of 
ethylene with 9 wt-% acrylic acid, melt index=10) from Dow Chemical Co., 
Midland, Mich., as well as NUCREI 035 (containing 20 wt-% methacrylic 
acid, melt index=35), ELVAX 230 (containing 28 wt-% vinyl acetate), and 
SURLYN 1706 (containing methacrylic acid) from E. I. duPont de Nemours, 
Wilmington, Del. Each of these polymers has excellent flexibility, 
strength, and toughness at temperatures as low as -40.degree. C. 
The top film 12 is preferably preformed, such as by extrusion or solvent 
casting, and then adhered to the bead bond layer 14 by a variety of 
techniques, such as heat lamination, or priming by known surface 
modification treatments such as corona treatment and/or with an added 
layer of a chemical prime, etc. Alternatively, the top film 12 can be 
formed directly on the bead bond layer 14, either by extrusion or solvent 
casting, although extrusion is preferred. Typically, the top film 12 is 
about 0.025-0.05 mm thick, and preferably, about 0.031-0.036 mm thick. 
Prime Layers 
As stated above, the top film 12 and/or the bead bond layer 14 can be 
primed, e.g., by corona treatment and/or with an added layer of a chemical 
prime, to improve the adhesion therebetween, even if the top film and bead 
bond layer are of different polymer families. Also, the top film 12 can be 
primed to improve the adhesion of inks, other colorants, decals, other 
adhesive-backed materials, etc., to the outer surface of the 
retroreflective sheeting. Such priming treatments are generally known to 
one of skill in the art. 
For example, aziridine can be added to urethane-based prime layers that are 
applied as the inner surface of an olefin-based top film to provide good 
adhesion and surface receptivity to a urethane-based bead bond layer. The 
aziridine-containing prime layer adheres well to the olefin-based top film 
and provides a receptive surface to which the urethane-based bead bond 
would not typically adhere well, thereby providing the desired high 
adhesion between the top film and the bead bond layer. 
Optical elements 
Optical elements 13 are typically in the form of glass beads (also referred 
to as microspheres or microsphere lenses) that are preferably light 
transmissible. For the embedded-lens retroreflective sheeting of the 
present invention, the refractive index of the optical elements is 
preferably about 2.2-2.3, and is more preferably about 2.23. 
Generally, the optical elements do not exceed about 200 microns in 
diameter. Preferably, the optical elements are about 20-120 microns in 
diameter, and more preferably about 60-90 microns in diameter. The 
narrower the range of diameters, the more uniform and better the 
properties of the sheeting. The preferred size distribution from the mean 
diameter should be plus or minus 10 microns. The most preferred size 
distribution from the mean diameter is plus or minus 7.5 microns. 
Chemical treatment of bead surfaces, such as with an organochromium 
compound, may be utilized as known in the art to enhance resin to glass 
adhesion. Additionally, fluorocarbon treatment of the glass beads can aid 
in achieving hemispherical bead sinkage and obtaining uniform bead 
sinkage, as disclosed in U.S. Pat. No. 3,222,204 (Weber et al.). 
Reflective Layer 
As illustrated in FIG. 4, reflective layer 16 underlies the spacing layer 
15. Suitable underlying reflecting means include uniformly-thick metallic 
deposits such as silver, aluminum, etc. However, instead of forming the 
reflective layer from metal, dielectric coatings taught in U.S. Pat. No. 
3,700,305 (Bingham), can be used. The thickness of reflective layer 16 
depends upon the particular metal used and is preferably about 80-100 nm. 
As an alternative to providing a separate reflective layer 16, specularly 
reflective or pearlescent pigment may be added to a layer such as the 
adhesive layer 17. 
Spacing Layer 
The spacing layer 15 is typically formed from a polymeric material of a 
suitable rheology to form a coating that follows the curved surface of the 
back of the optical elements. Typically, this requires a polymer having a 
weight average molecular weight of greater than about 30,000, and often 
greater than about 50,000. Spacing layer 15 preferably includes a 
crosslinked polymeric material, and more preferably a blend of a 
crosslinked reactive polymer for dimensional stability and a nonreactive, 
extractable, plasticising polymer. Examples of the reactive polymer 
include polyvinylacetals, acrylic copolymers, polyurethanes, polyesters, 
polyamides, polyester-amides, and acrylic block and graft copolymers. 
Examples of crosslinkers include at least difunctional materials that 
react with the reactive polymer, which can include other crosslinking 
polymers, such as aminoplast resins, including urea-formaldehyde resins, 
melamine-formaldehyde resins, and glycouril-formaldehyde resins; epoxy 
resins; isocyanates; aldehydes; polyfunctional aziridines; and azlactones. 
Examples of the nonreactive, extractable, plasticising polymer include 
polyesters, polyethers, polyamides, polyurethanes, and polymers of certain 
ethylenically unsaturated monomers. Preferred such spacing layers include 
aminoplast crosslinked resins such as polyvinyl butyral, acrylic resins, 
or polyester resins. Examples of these preferred spacing layers are 
disclosed in, for example, U.S. Pat. No. 5,262,225 (Wilson et al.). It 
should be understood, however, that other materials can be used in the 
spacing layer of the retroreflective sheeting of the present invention. 
A preferred spacing layer 15 comprises a polyvinyl butyral crosslinked with 
a urea-formaldehyde crosslinking agent blended with a polyester 
plasticizer. A suitable polyvinyl butyral includes that available under 
the trade designation BUTVAR B-76 (a random copolymer of vinyl alcohol, 
vinyl butyral, and vinyl acetate) from Monsanto, St. Louis, Mo. A suitable 
urea-formaldehyde crosslinking resin is that available under the trade 
designation BECKAMINE 21-510 from Reichold Chemicals, Research Triangle 
Park, N.C. A suitable polyester plasticizer is that available under the 
trade designation AROPLAZ 1351 from Reichold Chemicals. Preferably, the 
polyvinyl butyral is used in an amount of about 55-70 wt-%, the 
urea-formaldehyde resin is used in an amount of about 10-35 wt-%, and the 
polyester plasticizer is used in an amount of about 5-20 wt-%. These 
values are weight percents of solids, based on the total solids content of 
the composition. 
The thickness of the spacing layer 15 typically depends on the ratio of the 
index of refraction of the optical elements to the index of refraction of 
the top film 12, the bead bond layer 15, and the diameter of the optical 
elements 13. The spacing layer must be sufficiently thick so as to 
position the specularly reflective layer 16 at the approximate focal plane 
for light rays passing through the optical elements. In some cases, 
through an appropriate combination of high-index optical elements and low 
index top film layer, no spacing layer is needed, and a specularly 
reflective layer may be applied directly to the optical elements. However, 
a spacing layer 15 is normally present, and is generally between about 
0.005 and 0.035 mm thick. 
Adhesive Layer 
Adhesive layer 17 can be either a pressure sensitive or a heat or 
solvent-activated adhesive. Preferably, adhesive 17 is a 
pressure-sensitive adhesive, between about 0.01 mm and about 0.06 mm 
thick. The adhesive is generally coated from solution on a silicone 
release-coated paper backing (not shown in FIG. 4), dried, and then 
laminated to reflective layer 16 (or bead bond layer 14). Alternatively, 
the adhesive may be applied directly to the reflective layer 16 (or the 
bead bond layer 14) and the release-treated paper backing laminated to the 
adhesive layer 17, to complete the retroreflective product. 
Preferably, the pressure sensitive adhesive is an acrylic-based adhesive. 
Suitable acrylic polymers that can be used to prepare the pressure 
sensitive adhesive include those formed by polymerizing polar monomers 
with acrylic monomers. Suitable acrylic monomers include (C.sub.4 
-C.sub.12)alkyl acrylate monomers and (C.sub.6 -C.sub.12)alkyl 
methacrylate monomers, such as, for example, isooctyl acrylate, butyl 
acrylate, 2-ethyl hexyl acrylate, 2-ethyl hexyl methacrylate, octyl 
acrylate, octyl methacrylate, and mixtures thereof. Suitable polar 
monomers include acidic monomers such as ethylenically unsaturated 
carboxylic acids, ethylenically unsaturated sulfonic acids, and 
ethylenically unsaturated phosphoric acids. Specific examples include 
acrylic acid, methacrylic acid, itaconic acid, styrene sulfonic acid, 
maleic acid, fumaric acid, crotonic acid, citraconic acid, 
beta-carboxyethyl acrylate, sulfoethyl methacrylate, and mixtures thereof. 
The acrylic and polar monomers typically are present in a weight percent 
ratio of about 87.5-95 acrylic monomers to 5-12.5 polar monomers. 
The acrylic-based adhesive is preferably crosslinked using a crosslinking 
agent that is chosen depending on the monomers employed. Acrylic pressure 
sensitive adhesives can be crosslinked using radiation (e.g., electron 
beam, ultraviolet, etc.), moisture, or heat. Bisamide crosslinking agents 
are examples of a thermal crosslinking agent that utilizes heat to provide 
a chemical crosslink. 
Colorants or Graphics 
Retroreflective sheeting made according to the method of the present 
invention returns the most incident light to the source when the top film 
12, bead bond layer 14, and spacing layer 15 are uncolored and clear, in 
which case the sheeting will generally have a silver or gray appearance 
caused by the metallic appearance of the reflective layer 16. However, 
colored sheeting can be prepared by placing dyes or pigments, which are 
preferably light transmissible, in the spacing layer 15, bead bond layer 
14, top film 12, and/or optional prime layers 18 and 19. Alternatively, 
images, such as graphics, can be applied to the bead bond layer 14 before 
the top film 14 is applied, or they can be applied to either major surface 
(i.e., inner or outer surface) of the top film 14. When images are 
embedded within the sheeting, the images are generally more durable. 
Images can also be formed in the reflective layer 16 through the use of 
lasers as disclosed in U.S. Pat. Nos. 4,634,220 (Hockert et al.) and 
4,688,894 (Hockert). 
Method of Preparing Embedded Lens Sheeting 
A preferred procedure for making the embedded-lens structure of FIG. 1 
comprises the steps of: 
(1) preparing a crosslinkable polymer comprising urethane groups, 
preferably a crosslinkable polymer comprising urethane groups and 
unprotected functional groups, and more preferably a hydroxyl-terminated 
polyurethane, for the bead bond layer; 
(2) applying a layer of a mixture of the crosslinkable polymer and a 
crosslinking agent onto a smooth-surfaced release liner, such as an 
acrylic-coated release liner and optionally heating this to a temperature 
of about 45.degree.-90.degree. C. to flash off any solvent used to prepare 
the polyurethane; 
(3) applying a monolayer of optical elements 13 to the uncured bead bond 
layer 14; 
(4) applying slight pressure to the optical elements to aid resin 
capillation and embed the lenses in the bead bond layer to about 30-50% of 
their diameter and then thermally curing the bead bond layer 14 by heating 
to a temperature of about 90.degree.-180.degree. C., preferably about 
140.degree.-175.degree. C.; 
(5) covering the exposed portions of the optical elements 13 with spacing 
layer 15 having an exterior surface cupped around the optical elements; 
(6) applying the specularly reflective layer 16 to the cupped surface of 
the spacing layer 15, typically by vapor-deposition techniques; 
(7) stripping away the release liner from the bead bond layer 14; 
(8) applying, in typical embodiments, the adhesive layer 17 over the 
specularly reflective layer 16; 
(9) optionally, corona treating and/or applying a prime layer to the bead 
bond surface; and 
(10) applying a top film 12 to the top surface of the retroreflective film. 
It may be desired in some instances to apply a priming layer to the outside 
surface of the top film 12, such a layer being receptive to the 
application of marking compositions, e.g., inks, to enable the application 
of legends to the face of a retroreflective sheeting. Such a priming layer 
is preferably light transmissible. An example thereof is a water-borne 
urethane with an aziridine crosslinker. 
The following examples are offered to further illustrate the various 
specific and preferred embodiments and techniques. It should be 
understood, however, that many variations and modifications may be made 
while remaining within the scope of the present invention. 
EXAMPLES 
The following exemplify preparations of the bead bond polyurethane 
compositions in accordance with the invention. All parts, percentages, and 
ratios throughout the specification, including the examples, are by weight 
unless otherwise indicated. The terms "equivalent weight" or "Eq. Wt." as 
used herein with respect to a functionality or moiety, refers to the mass 
of polymer per mole, or equivalent, of the functionality. 
Testing 
Inherent Viscosity: The inherent viscosity of each composition was measured 
to provide a relative comparison of the molecular weights. The inherent 
viscosity was measured by conventional means using a Cannon-Fenske 
viscometer (available from Jupiter Instrument Company, Jupiter, Fla.) in a 
water bath controlled at 25.degree. C. to measure the flow time of 10 
milliliters of a polymer solution (0.8 grams per deciliters (g/dl) of 
polymer in tetrahydrofuran solvent) and the flow time of the 
tetrahydrofuran solvent. In each experiment, inherent viscosity is 
reported in deciliters per gram. 
Glass Transition Temperature: The glass transition temperature (Tg) of the 
materials of the present invention were determined by differential 
scanning calorimetry. A small amount of dried film was placed in the DSC 
chamber of a Perkin-Elmer DSC-7 (Norwalk, Conn.) differential scanning 
calorimeter under a nitrogen atmosphere. The sample was cooled from room 
temperature to -100.degree. C. and then heated to 150.degree. C. at a 
scanning rate of 20.degree. C. per minute, and finally cooled at a rate of 
40.degree. C. per minute. The Tg was taken as the midpoint of the curve in 
the glass transition region (the temperature at which amorphous material 
changes from a glassy state to a ductile state). 
Tensile and Elongation: The tensile strength and total elongation of some 
of the coatings were tested under ASTM Test Method D882-83 (1986). Results 
are reported in Table A, below. 
Impact Resistance: The impact resistance of bead bond film, bead bond film 
with beads, and finished sheeting were tested at 25.degree. C. according 
to ASTM D2794 (1986). The bead bond films and bead bond films with a 
monolayer of beads were prepared and removed from the acrylic-coated 
release liners. These "free-standing" films were then laminated to a 0.8 
mm thick aluminum alloy 3003-H12 panel using a pressure sensitive adhesive 
having a 90:10 ratio of isooctyl acrylate-acrylic acid copolymer and 
radiation cured with an E-beam at 4.0-6.0 Mrads having an inherent 
viscosity of 0.6-1.0 dl/g prior to crosslinking measured as described 
above (referred to herein as 90:10 IOA:AA adhesive). Finished 
retroreflective sheeting was directly laminated to the aluminum using the 
same adhesive. The panels were impacted on the side of the panels opposite 
the films or sheetings with a punch having a diameter of 15.9 mm. The test 
was performed with increasing force to a maximum of 1.6 kg.multidot.m. The 
results are reported in Table B, below. 
License Plate Embossibility: With the retroreflective sheeting mounted on 
an aluminum alloy 3003-H12 panel having a thickness of 0.8 mm, a variable 
embossing pattern was impressed using male/female dies to form a series of 
five alphanumeric symbols (.OMEGA.) 7.06 cm high and 5.16 cm across and a 
stroke width of 0.95 cm. The symbols embossed the blank to depths of 1.5 
mm, 1.75 mm, 2.0 mm, 2.25 mm, and 2.5 mm. The slope of the edges varied 
from 0.8 at 1.5 mm to 1.3 at 2.5 mm. The embossing depth was reported, 
which was the greatest depth of embossing where no visible cracking was 
evident. Pop-off resistance was reported as the greatest depth of 
embossing where there was no visible lifting of the sheeting from the 
areas around the symbols after 24 days. The results are reported in Table 
B, below. 
Top Film Adhesion: Retroreflective sheeting was applied to a 0.8 mm 
aluminum panel and submerged in boiling water for two minutes. A sharp 
razor blade was used to carefully start peeling the top film away from the 
base retroreflective sheeting. Tensile force for peeling the top film away 
from the base retroreflective sheeting was measured on an INSTRON or 
SYNTEX tensile testing instrument, pulling the cover film away from the 
base sheeting at a 180.degree. angle. The results are listed in Table B, 
below. 
Simulated Weathering Resistance: Bead bond films from Example 2 were 
laminated to a 90:10 IOA:AA pressure sensitive adhesive which had been 
applied to aluminum. The panels were tested for weatherability in a 
weathering machine for 500, 1000, and 1500 hours under ASTM Test Method 
G23, Condition E. The results are listed in Table C, below. 
Preparation of Materials 
Top Film: A light transmissible top film comprising: 97.4 parts of PRIMACOR 
3440 (an extrusion grade, thermoplastic, high molecular weight copolymer 
believed to comprise a major portion of ethylene monomer and a minor 
portion of acrylic acid monomer, available from DuPont de Nemours, having 
a melt flow index of about 10); and 2.6 parts of a stabilizing system (1.0 
part of ultraviolet absorber, 1.5 parts of a hindered amine, and 0.1 parts 
of an antioxidant, the stabilizing system is not believed to affect the 
strength of the top film or its adhesion to other materials), was extruded 
as follows. The stabilized copolymer was extruded onto a 
biaxially-oriented polyethylene terephthalate (PET) carrier using a 
single-flighted screw with a compression ratio of 3:1 to form a 0.033 mm 
thick film. The extruder temperatures were ramped up from about 
190.degree. C. to about 275.degree. C. The extruder screw speed was 30 rpm 
while the film takeaway speed was adjusted to provide a film having a 
desired thickness. The extruded film was wound upon itself into roll form. 
Preparation of Polyurethanes for Bead Bond Layer 
Polyurethane Preparation 1: The following materials were charged to a one 
liter three neck round bottom flask: a polyester diol available from Witco 
Corp., Melrose Park, Ill., under the trade designation FOMREZ 8056-146 
(220 grams; 0.560 equivalents) and xylene (440 grams). Because the water 
content of this diol solution was greater than 500 ppm, a total of 220 
grams of xylene was azeotropically distilled to achieve a water content of 
less than 500 ppm. Bis(4-isocyanotocyclohexyl) methane (67.48 grams; 0.515 
equivalents; Bayer Corp., Pittsburgh, Pa.), xylene (67.5 grams) and two 
drops of dibutyltin dilaurate (Aldrich Chemical Co., Milwaukee, Wis.) were 
added to the reaction vessel. The reaction was heated under reflux for 
five hours, then maintained at 110.degree. C. (approximately 12 hours) 
until there was no free isocyanate observed in the infrared spectrum. The 
material had an inherent viscosity of 0.44 dl/g when measured in 
tetrahydrofuran. The calculated hydroxyl equivalent weight was 6418. The 
Tg was -11.5.degree. C. 
Polyurethane Preparation 2: The following materials were charged to a one 
liter three neck round bottom flask: polyester diols available from Witco 
Corp. under the trade designations FOMREZ 55-112 (150.07 grams; 0.295 
equivalents) and FOMREZ 8066-120 (150.00 grams; 0.326 equivalents), and 
xylene (381.59 grams). Because the water content of this diol solution was 
greater than 500 ppm, a total of 153.94 grams of xylene was azeotropically 
distilled to achieve a water content of less than 500 ppm. 
Bis(4-isocyanotocyclohexyl) methane (74.93 grams; 0.572 equivalents; Bayer 
Corp., Pittsburgh, Pa.), xylene (147.5 grams) and three drops of 
dibutyltin dilaurate were added to the reaction vessel. The reaction was 
heated under reflux for four hours, then maintained at 110.degree. C. 
(approximately 12 hours) until there was no free isocyanate observed in 
the infrared spectrum. The material had an inherent viscosity of 0.40 dl/g 
when measured in tetrahydrofuran. The calculated hydroxyl equivalent 
weight was 7539. The Tg was -21.1.degree. C. 
Polyurethane Preparation 3: The following materials were charged to a one 
liter three neck round bottom flask: the polyester diol FOMREZ 8056-146 
(283.5 grams; 0.721 equivalents) and 4-methyl-2-pentanone (366 grams). 
Because the water content of this diol solution was greater than 500 ppm, 
a total of 306 grams of 4-methyl-2-pentanone was azeotropically distilled 
and replaced with 306 grams of dry 4-methyl-2-pentanone to achieve a water 
content of less than 500 ppm. Diphenylmethane diisocyanate (82.97 grams; 
0.664 equivalents; Bayer Corp., Pittsburgh, Pa.) and two drops of 
dibutyltin dilaurate were added to the reaction vessel. The reaction was 
heated under reflux for ninety minutes until there was no free isocyanate 
observed in the infrared spectrum. The material had an inherent viscosity 
of 0.29 dl/g when measured in tetrahydrofuran. The calculated hydroxyl 
equivalent weight was 6349. The Tg was -12.0.degree. C. 
Polyurethane Preparation 4: The following materials were charged to a one 
liter three neck round bottom flask: the polyester diol FOMREZ 8056-146 
(275.00 grams; 0.700 equivalents) and 4-methyl-2-pentanone (550 grams). 
Because the water content of this diol solution was greater than 500 ppm, 
a total of 275 grams of 4-methyl-2-pentanone was distilled to achieve a 
water content of less than 500 ppm. Isophorone diisocyanate (84.35 grams; 
0.644 equivalents; Huels America, Piscataway, N.J.), 4-methyl-2-pentanone 
(85 grams), and two drops of dibutyltin dilaurate were added to the 
reaction vessel. The reaction was heated under reflux for three hours, 
then maintained at 105.degree. C. (approximately 14 hours) until there was 
no free isocyanate observed in the infrared spectrum. The material had an 
inherent viscosity of 0.29 dl/g when measured in tetrahydrofuran. The 
calculated hydroxyl equivalent weight was 6418. The Tg was -9.6.degree. C. 
Comparative Polyurethane Preparation 1: A polyester polyol extended with 
H.sub.12 MDI to form a hydroxyl terminated polyurethane (Tg=27.degree. 
C.), which was prepared from a polyester polyol (Tg=2.6.degree. C.) in a 
manner described above for Preparation 1. 
Example 1 
In the lab, the bead bond resin from Preparation 1 was crosslinked with a 
partially butylated melamine formaldehyde available from DSM Resins, 
Zwolle, the Netherlands, under the trade designation URAMEX CP 1132 MF at 
a resin to crosslinker ratio of 4.1:1 (based on solids). The sample was 
mixed for approximately 5 minutes with an air mixer. The resulting 
solution was coated to 0.075 mm thick (wet) onto an acrylate-coated paper 
release liner with a knife coater. Three samples were produced in this 
manner. The first sample (1A) was air dried for 1 minute, baked at 
90.degree. C. for 2 minutes, and then baked at 150.degree. C. for 3 
minutes. The thickness of the dry coating was 0.025 mm. The two remaining 
samples (1B and 1C) were dried for 1 minute, glass beads (60 micron mean 
diameter with a .+-.10 micron distribution, surface treated with an 
organochromium complex and a fluorocarbon) were cascaded over the samples, 
the samples then were baked at 90.degree. C. and 150.degree. C. for 2 and 
3 minutes, respectively, without the application of pressure to embed the 
beads. Sample 1C was then coated with a spacing layer containing 63 wt-% 
BUTVAR B-76 polyvinyl butyral (Monsanto, St. Louis, Mo.), 23 wt-% 
BECKAMINE 21-510 urea-formaldehyde crosslinking resin (Reichold Chemicals, 
Research Triangle Park, N.C.), and 14 wt-% AROPLAZ 1351 polyester 
plasticizer (Reichold Chemicals). These weight percentages were based on 
solids. The spacing layer was then cured at 90.degree. C. and 150.degree. 
C. for 2 and 3 minutes, respectively, resulting in a dry coating thickness 
of 0.013 mm. The acrylic release liner was removed from the bead bond 
surface. The 90:10 IOA:AA pressure sensitive adhesive described above was 
laminated to the spacing layer. A 0.033 mm thick top film prepared as 
described above was heated laminated to the bead bond layer of the 
sheeting. 
Example 2 and Comparative Example 1 
Several bead bond compositions were prepared with differently butylated 
melamine-formaldehyde crosslinkers at different polyurethane resin to 
crosslinker ratios. These may be found in the following table: 
______________________________________ 
Ratio 
% Amount % (Resin 
Poly- Amount Solids 
Cross- Solids to 
Sple. urethane Resin of linker.sup.1 
of Cross- 
Cross- 
No. Prep. (g) Resin (g) linker linker) 
______________________________________ 
2A 1 100 48.5 14.26 68 5 
2B 1 50 48.5 4.04 60 10 
2C 1 100 48.5 2.54 95 18 
2D 4 45 51.3 5.22 68 6.5 
2E 3 45 53.1 5.41 68 6.5 
2F 2 45 52.3 5.32 68 6.5 
2G 1 40 37.3 7.06 68 3.1 
Comp. Comp. 44.5 50 5.84 68 5.6 
1 1 
______________________________________ 
.sup.1 All samples contained a butylated melamineformaldehyde resin as th 
crosslinker: Sample 2B contained RESIMENE 881 (partially butylated); 
Sample 2C contained RESIMENE 7512 (fully butylated); all others contained 
URAMEX CP 1132 MF (partially butylated). 
Each bead bond composition was mixed for approximately 5 minutes with an 
air mixer. The resulting solution was coated to 0.05 mm thick (wet) onto 
an acrylic-coated paper release liner with a knife coater. Two samples 
were produced in this manner for each bead bond composition. The first 
sample for each was air dried for 1 minute, baked at 90.degree. C. for 2 
minutes, and then baked at 150.degree. C. for 3 minutes. The thickness of 
the dry coating was 0.025 mm. The remaining sample for each composition 
was air dried for 1 minute and glass beads described in Example 1 were 
cascaded over the sample, which was then baked in an oven at 90.degree. C. 
and 150.degree. C. for 2 and 3 minutes, respectively. 
Example 3 and Comparative Example 2 
The polyurethane resin from Preparation 1 (Sample 3A) and from Comparative 
Polyurethane Preparation 1 (Comp. 2) were mixed with DSM's URAMEX CP 1132 
MF and Monsanto's RESIMENE 881 melamine-formaldehyde crosslinkers, 
respectively, at a resin to crosslinker ratio of 6.36:1 (based on solids). 
The solutions were coated with a knife coater onto an acrylic-coated paper 
release liner, with a resulting coating thickness of about 0.025 mm when 
dried. A monolayer of glass beads described in Example 1 were applied to 
the uncured bead bond composition. Samples 3A and Comp. 2 were exposed for 
3.3 minutes and 2.3 minutes, respectively, through an oven with an initial 
temperature of 90.degree. C., which was ramped up to a final temperature 
of 170.degree. C., using a line speed of 10.7 meters/minute and 15.2 
meters/minute, respectively. 
Next, a spacing layer as described in Example 1 was applied to the bead 
bond layer of the sheeting to form a 0.008-0.031 mm thick spacing layer. 
The space-coated films were exposed for 2.3 minutes through an oven with 
an initial temperature of 75.degree. C., which was ramped up to a final 
temperature of 170.degree. C., using a line speed of 15.3 meters/minute. 
To the spacing layer, a reflective layer of aluminum metal about 100 nm 
thick was applied by vapor deposition. The acrylic release liner was then 
stripped away from the bead bond layer. A 0.038 mm thick layer of the 
90:10 IOA:AA adhesive described above was then applied to the reflective 
layer. 
Next, a layer of a priming solution was applied to the top surface of the 
bead bond layer, which had been corona treated at 0.39 
kilowatts/meter/minute. The priming solution included 75.0 parts of a 
water-borne aliphatic urethane available under the trade designation 
NEOREZ R960 from Zeneca Resins, Wilmington, Mass., 14.9 parts water, 0.2 
parts bubble breaker available under the trade designation WITCO 3056A 
from Witco Corp., Melrose Park, Ill., 7.5 parts ethyl alcohol, 0.1 part of 
a fluorocarbon leveling agent available under the trade designation FC-120 
from 3M Company, St. Paul, Minn., and 2.3 parts of a 100% active 
polyfunctional aziridine liquid crosslinker available under the trade 
designation CX-100 from Zeneca Resins, Wilmington, Mass. applied with a 
175 line quadrangular knurl coater. The coating was then dried at 
65.degree. C. to yield a priming layer having an approximate dry thickness 
of 2 microns. 
Next, a 0.033 mm thick top film prepared as described above was laminated 
to the bead bond surface of the retroreflecting base material. Prior to 
lamination, each surface of the top film was corona treated using the 
following conditions: 2.4 kw/meter width; surface of hot can, 150.degree. 
C.; hot can diameter,61 cm; nip roll hardness, 70 shore A; speed, 15.3 
meters/minute; and length of composite heating, 60 cm. The PET carrier 
film was stripped and the resulting retroreflective sheeting was wound 
upon itself for storage. 
Results 
The results of tensile and elongation, impact resistance, embossability, 
weathering resistance, and top film adhesion are reported in the following 
tables. 
TABLE A 
______________________________________ 
Tensile Tensile Impact 
at Yield at Break Elongation 
Resistance.sup.1 
Example (MPa) (MPa) (%) (Kg-m) 
______________________________________ 
1A nt 6.32 430 &gt;1.37 
1B nt 0.43 710 &gt;1.37 
1C nt nt nt &gt;1.37 
2A 0.73 0.67 490 &gt;1.6 
2A with Beads 
1.30 0.48 536 &gt;1.6 
2B 4.00 0.19 998 &gt;1.6 
2B with Beads 
0.88 0.66 505 &gt;1.6 
2C with Beads 
** ** ** &gt;1.6 
2D 1.90 1.90 217 &gt;1.6 
2D with Beads 
2.06 2.06 236 &gt;1.6 
2E 0.51 0.19 999 &gt;1.6 
2E with Beads 
1.16 1.01 541 &gt;1.6 
2F 0.26 0.14 1,008 &gt;1.6 
2F with Beads 
0.89 0.86 581 &gt;1.6 
2G nt 5.84 440 &gt;1.37 
2G with Beads 
0.75 0.29 585 &gt;1.37 
Comp. 1 nt 58.23 5 &lt;0.46 
3A with Beads 
nt nt nt &gt;1.6 
3A Finished 
nt nt nt 0.91 
Sheeting 
Comp.2 with 
nt nt nt &lt;0.11 
Beads 
Comp.2 nt nt nt &lt;0.23 
Finished 
Sheeting 
______________________________________ 
.sup.1 For values listed as &gt;1.37 or &gt;1.6, there was no failure (i.e., no 
significant cracking) at the maximum force tested. 
**Not able to test because not able to strip enough sample from release 
liner. An alternative release liner could be used to make this sample 
work. 
nt = not tested. 
TABLE B 
______________________________________ 
Slow Speed License 
Pop-Off Top Film 
Example Plate Embossibility 
Resistance Adhesion 
______________________________________ 
1C No failure at 2.5 mm 
No pop-off of 2.5 
nt 
mm embossed 
symbol 
3A Finished 
No failure at 2.5 mm 
No pop-off of 
No Separation 
Sheeting 2.25 mm emboss- 
ed symbol 
Comp. 2 Failure at 1.75 mm 
nt No Separation 
Finished 
Sheeting 
______________________________________ 
The results in Tables A and B demonstrate the high impact resistance and 
embossibility of the sheeting of the present invention, relative to 
conventional sheeting. The embossibility is reported as the greatest depth 
of embossing where no visible cracking was observed. The pop-off 
resistance is reported as the greatest depth of embossing where there was 
no visible lifting of the sheeting from the areas around the symbols after 
24 days. The results in Table C, below, demonstrate the improved 
weatherability of the sheeting of the present invention. 
TABLE C 
__________________________________________________________________________ 
500 Hours 1000 Hours 1500 Hours 
60.degree. Gloss 
Crack 60.degree. Gloss 
Crack 60.degree. Gloss 
Crack 
Example 
% Ret. 
Size 
Color 
% Ret. 
Size 
Color 
% Ret. 
Size 
Color 
__________________________________________________________________________ 
2G 112.8 
none 
clear 
124.8 
none 
very pale 
99.7 none 
pale 
yellow yellow 
Comp. 1 
102.0 
none 
clear 
97.2 1-3 mm 
pale 39.6 1 to 3 
pale 
yellow mm yellow 
__________________________________________________________________________ 
All patents, patent documents, and publications cited herein are 
incorporated by reference as if individually incorporated. The foregoing 
detailed description has been given for clarity of understanding only. No 
unnecessary limitations are to be understood therefrom. The invention is 
not limited to the exact details shown and described, for variations 
obvious to one skilled in the art will be included within the invention 
defined by the claims.