Retroreflective sheeting in which a specularly reflective layer within the sheeting has extensive discontinuities which make the sheeting permeable to vapor, thereby allowing release of vapors from a substrate to which the sheeting is applied.

BACKGROUND OF THE INVENTION 
Retroreflective sheeting is sometimes adhered to painted surfaces, 
polymeric articles, or other substrates from which gaseous vapors evolve 
after the sheeting has been adhered in place. Such vapor evolution has 
caused blistering of prior-art reflective sheeting, especially when the 
vapor has evolved rapidly or in large volumes, leaving the sheeting with 
an unsightly appearance and creating a source of delamination, tearing, or 
other failure of the sheeting. 
Prior-art retroreflective sheeting is rather thick and comprises several 
layers, and all of these layers undoubtedly contribute to inhibiting 
migration of vapors. However, our experiments reveal that a metallic 
specularly reflective layer included in the sheeting is a primary cause of 
the blistering. Sheeting made without the metallic layer allows sufficient 
migration of vapors to avoid the previously experienced blistering. 
However, retroreflective sheeting made without a metallic specularly 
reflective layer underlying the transparent microspheres also provides a 
very low level of retroreflection. A specularly reflective layer is 
essential, and the blistering problem must be avoided while still 
retaining such a layer. Insofar as known, no one has previously taught how 
to do that. 
SUMMARY OF THE INVENTION 
The present invention provides a new vapor-permeable retroreflective 
sheeting. This new sheeting is similar to previous sheetings in that it 
includes a monolayer of transparent microspheres, a metallic specularly 
reflective layer underlying and in optical connection with the 
microspheres, usually a transparent spacing layer disposed between the 
microspheres and specularly reflective layer (to position the specularly 
reflective layer at the approximate focal point of light transmitted by 
the microspheres), one or more layers of transparent binder material to 
support the microspheres or form a flat top surface for the sheeting, and 
usually an adhesive layer for adhering the sheeting to a substrate. 
The new sheeting is distinctive from previous sheetings in that the 
metallic specularly reflective layer has an extensive array of minute 
discontinuities such as fracture lines formed by stretching the layer. The 
discontinuities in the layer are very small and constitute a small 
percentage of the total area of the layer, but it has been found that 
vapors migrate through such discontinuities rapidly enough to greatly 
reduce or avoid the blistering exhibited by conventional reflective 
sheeting products. Also, despite the discontinuities, the reflectivity of 
the sheeting is not noticeably affected, and the product remains 
physically strong and durable. 
Stretching of the metallic specularly reflective layer to fracture it is a 
preferred method for forming discontinuities, and preferred steps of such 
a stretching operation include (a) preparing a stretchable 
intermediate-stage product, which usually comprises at least a monolayer 
of transparent microspheres, a transparent spacing layer underlying and in 
optical connection with the microspheres, and a thin metallic specularly 
reflective layer carried on the bottom surface of the spacing layer; and 
(b) stretching the intermediate-stage product, as in tentering apparatus, 
so as to fracture the metallic specularly reflective layer and form the 
described array of discontinuities. 
Other components are typically added after the stretching operation. For 
example, one or more layers of transparent polymeric material can be added 
to the top of the product, forming a smooth top surface, and leaving the 
sheeting capable of reflecting when either wet (as with rain or other 
moisture) or dry; and one or more layers, typically including an adhesive 
layer, can be added at the bottom. 
After completion, the new retroreflective sheeting is sufficiently 
permeable that water vapor will pass through the sheeting at a rate of at 
least 15, and preferably at least 20, grams/square meter/24 hours. (In 
making this measurement, the test sheeting is placed as a membrane 
separating two sealed chambers, one of which is maintained at a 
temperature of 72.degree. F. and a relative humidity of 90 percent, and 
the other of which is maintained at a temperature of 72.degree. F. and a 
relative humidity of zero percent. The second chamber contains a water 
vapor sorbent, which is weighed before and after the period of testing, 
and the rate of water vapor transmission is calculated from the measured 
difference in weight.) By contrast, under the same conditions water vapor 
will pass through conventional retroreflective sheeting at a rate of about 
6 grams/square meter/24 hours.

DETAILED DESCRIPTION 
The sheeting 10 shown in the drawings comprises a layer of transparent 
microspheres 11; a layer 12 of transparent binder material in which the 
microspheres are supported essentially as a monolayer; a transparent top 
layer 13; a transparent spacing layer 14 having a bottom surface which 
generally follows the contour of the bottom of the microspheres, and which 
is spaced from the microspheres at the approximately focal point for light 
rays impinging on the front of the reflective sheeting and passing through 
the microspheres; a specularly reflective layer 16, which is carried on 
the contoured surface of the spacing layer, and which has an extensive 
array of minute discontinuities 17; and a bottom layer 18, which most 
typically is a layer of adhesive such as pressure-sensitive adhesive for 
adhering the sheeting to a substrate. 
Manufacture of the reflective sheeting shown in the drawing typically 
begins by coating material for forming the top layer 13 onto a carrier 
web, either from solution or from some other liquefied form, solidifying 
that material, and then coating material for the binder layer 12. 
Transparent microspheres are cascaded onto the coated binder layer while 
the layer is still wet, whereupon the microspheres become partially 
embedded in the layer. After the coated layer has been dried or otherwise 
solidified, material for the spacing layer is coated over the 
microspheres, again either from solution or from some other liquefied 
form, and solidified. Thereupon the specularly reflective layer is applied 
to the spacing layer, typically by vapor-deposition of metal. 
In contrast to the product shown in the drawings, some products of the 
invention include no spacing layer. Such products include so-called 
"exposed-lens" sheeting in which the front surfaces of the microspheres 
are not embedded in polymeric material but are exposed to air, and 
sheeting in which the microspheres have a high index of refraction. In 
these products the specularly reflective layer is directly applied to the 
rear surface of the microspheres (such an application may be accomplished, 
for example, while the front surfaces of the microspheres are temporarily 
held in a removable carrier sheet), and a binder layer is applied over the 
specularly reflective layer to support the microspheres. 
As a next step in the manufacturing process, products as described in the 
two preceding paragraphs can be subjected to a stretching or tentering 
operation. Conventional tentering equipment, which stretches the sheet 
product transversely as it proceeds along the length of the tentering 
apparatus, is particularly useful. A five-percent expansion of the 
transverse width of the sheet product is usually sufficient to develop the 
described array of minute discontinuities, although we prefer to stretch 
the sheet product 10 percent. Except for the specularly reflective layer, 
the layers of the stretched product generally elongate and remain intact, 
and the materials and structure of the product are chosen to achieve that 
result. 
Following the stretching operation, the sheet material is typically allowed 
to retract, or heated to cause it to retract, so that it usually is no 
more than about two percent greater than its original dimensions prior to 
the stretching operation. The sheeting is then completed, as by 
application of a layer of adhesive, which is typically a 
pressure-sensitive adhesive, but alternatively can be a heat-activated or 
solvent-activated adhesive. 
Alternative procedures for forming discontinuities in the specularly 
reflective layer include applying solvent to the described 
intermediate-stage product so as to cause swelling of the spacing layer, 
which thereupon results in cracking of the specularly reflective layer; or 
drawing the intermediate-stage sheet product over a sharp edge so as to 
fracture the specularly reflective layer; or passing the intermediate 
stage product through nip rolls under high pressure. 
Also, deposition of thinner specularly reflective layers leaves 
discontinuities sufficient for the noted migration of vapor, and can 
provide adequate reflection. However, such a procedure is less preferred, 
since it is difficult to control the operation to reproducibly achieve the 
desired balance of discontinuities and reflection; and reflection is 
reduced. 
The discontinuities formed in the specularly reflective layer are most 
often a network of narrow lines, which tend to be concentrated between the 
microspheres. For most uses of the sheeting the discontinuities should be 
small in width, so that they are not normally visible to the unaided eye 
from typical viewing distances of one meter or more. Typically they are 
less wide than the average diameter of the microspheres in the sheeting. 
The various layers in retroreflective sheeting of the invention can be made 
from conventional materials. For example, a binder layer 12, top layer 13, 
and spacing layer 14 in a retroreflective sheeting as shown in the 
drawings are generally made of polymeric materials such as alkyd, vinyl, 
or acrylic resins; the layer 10 can be a pressure-sensitive-adhesive 
acrylate copolymer; and the specularly reflective layer can be 
vapor-deposited aluminum or silver. 
The invention will be further illustrated by the following example, which 
is described with reference to the drawing. An extensible plasticized 
vinyl resin containing ultraviolet- and heat-stabilizers was coated from 
solution onto a paper carrier web presized with an alkyd release agent, 
and the coated layer was heated to fuse it into a 55-micrometer-thick film 
useful as the top film of the ultimate sheeting. A solution comprising an 
uralkyd resin and melamine crosslinker was then coated onto the fused top 
film. After partial drying of the latter layer, transparent glass 
microspheres having an average diameter of 57 micrometers and an index of 
refraction of 2.26 were cascaded onto the coated layer as a dense 
monolayer. The microspheres became partially embedded in the coated layer 
and partially extended above the coated layer. After curing of the binder 
layer by heating (leaving a 34.2-micrometer-thick binder layer), a 
solution comprising a polyvinyl butyral resin and a butylated melamine 
hardener was coated onto the microspheres to provide the transparent 
spacing layer 14, which after drying and curing was approximately 19 
micrometers thick. Aluminum was vapor-deposited onto the exposed surface 
of the dried and cured spacing layer to form a metallic specularly 
reflective layer. 
The resulting assembly was stripped from the carrier web and passed through 
a tentering apparatus at a rate of 10 meters per second, with the assembly 
being stretched 10 percent at a rate of 4.7 percent per meter forward 
travel of the sheeting. The assembly was then heated and allowed to return 
to approximately its original dimensions. The aluminum specularly 
reflective layer was found to have an extensive array of fractures along 
lines that generally extended between the microspheres. 
The sheeting was then completed by laminating a layer 18 of 
pressure-sensitive acrylate adhesive onto the discontinuous aluminum 
layer. The sheeting exhibited a reflectivity of 90 candella per square 
meter per lux of incident light, and transmitted water vapor through the 
sheeting at a rate of 24.2 grams per square meter per 24 hours.