1.  Field of Invention
This invention relates to retroreflective sheeting which is useful for road signs, construction signs, vehicle number plates such as for automobiles, motorcycles, etc.,  safety goods such as apparel to prevent disasters, survival equipment, or for marking signboards, etc. 
2.  Description of Prior Art
Retroreflective sheeting which reflexes reflexed light to the light source is well known, and the sheeting has been widely used in aforesaid fields where its retroreflectivity is utilized.  In particular, such retroreflective sheeting as encapsulated lens-type retroreflective sheeting whose retroreflective performance of light is enhanced by providing a layer of air between light-transmittable protective film and retroreflective glass beads, making use of low refractive index of air, has been finding continually expanding field of utility due to its excellent retroreflective performance. 
Generally such an air layer-containing retroreflective sheeting is composed of a light-transmittable protective film and a support sheet, which face each other across a narrow air layer, and bonding portions for binding said two, which are formed by locally thermofusing and forming the fused parts of support sheet.  In the support sheet, retroreflective glass beads are embedded substantially as a monolayer, in such a manner that the non-reflective portions of the glass beads are exposed on said support sheet. 
The most important function desired of such retroreflective sheeting is high retroreflective performance.  Besides, high weatherability to allow its use under severe conditions which are encountered in outdoor use, etc.  while maintaining the excellent retroreflective performance over prolonged periods, is also one of the important requirements therefor. 
Deterioration in the retroreflective performance during long-term outdoor use of an air layer-containing retroreflective sheeting is in most cases caused by breakdown of the cells in which air is sealed and subsequent infiltration of rain water or the like into the broken cells.  Thus, hermetic property of sealed air cells is lost by, for example, cracks in the protective film caused by repetitive expansion-contraction cycle of the air in the sealed air cells accompanying ambient temperature variation or repetitive extension and constriction of a substrate such as metal plate onto which the retroreflective sheeting is adhered, which again is caused by ambient temperature variation; adhesion breakdown at the interface of the protective film and bonding portions; destruction of the bonding portions themselves or the support sheet itself.  Then rain water or the like infiltrates into the cells to vary the refractive index condition in the cells which is an important factor of retroreflective performance of the sheeting, or the vacuum-deposited metal film or the like serving as the light-reflecting membrane deteriorates and loses its ability to reflect light, resulting in decrease of retroreflective performance. 
A support sheet which constitutes the main part of such a retroreflective sheeting, serving as the bonds, the glass beads-holding layer as well as the walls of sealed air cells, plays an important role for achieving good weatherability to maintain the retroreflective performance over a prolonged period.  Accordingly, various excellent properties are required for the support sheet, such as not only thermofusibility and formability sufficient to form the bonding portions (bonds), high adhesive strength to the protective film as thermofused and formed into the bonds, and high tensile strength as the bonds as well as the support sheet itself; but also pliability to withstand repetitive stretching and good weatherability to maintain the high performance level over prolonged outdoor use.  It is still another important requirement that the bonds and the support sheet should be free from residual strain which is apt to cause breakdown of individual parts of the retroreflective sheeting. 
Numbers of attempts have heretofore been made about the support sheet, to improve those various properties.  For example, U.S. Pat.  No.  4,025,159 (Japanese Patent Publication No.  13561/86) proposed to apply radioactive rays, after the support sheet is thermofused and formed into bonds (bonding structure), to the bonds to cause their crosslinking and improvement in their strength.  However, according to this proposal the bonds are crosslinked to become hard and brittle, which causes such problems that pliability sufficient to withstand repetitive stretching can hardly be obtained, and the retroreflective sheeting itself also becomes hard.  There is still another defect that crosslinkage of the bonds by radioactive irradiation causes volume contraction of the bonds leading to serious strain.  Furthermore a crosslinkable component used in the crosslinking reaction under radioactive irradiation, such as one having unsaturated bonds, seldom completes the reaction under the irradiation but in most cases the unreacted component remains as it is in the support sheet or the bonds in the finished retroreflective sheeting.  During use of such a sheeting, therefore, undesirable crosslinking progresses under external factors such as heat, sunshine, etc.  to which the sheeting is exposed.  It is found that this progress in crosslinking deteriorates adhesion between the protective film and the bonds or embrittles the support sheet or bonds, rendering the sealed air cells more apt to break. 
U.S. Pat.  No.  4,897,136 [Japanese Laid-open (KOKAI) Patent Application, KOKAI No.  121043/87] has proposed to use a high molecular weight thermoplastic resin as the material for the support sheet, whereby imparting toughness and flexibility to said sheet, and eventually imparting high toughness and adaptability to irregular surfaces to the retroreflective sheeting. 
In this proposal, however, the improvements in toughness and flexibility of the support sheet rely on an increase in molecular weight of the material resin used for making same sheet.  In consequence, attempts to secure the high toughness desirable for the bonds or the support sheet itself by increasing molecular weight of said material resin generally lead to impairment in thermofusibility and forming property of produced support sheets, which tends to decrease adhesive strength between the bonds and the protective film.  Furthermore, pliability of the sheet also is impaired.  Whereas, when the molecular weight of said sheet-forming resin is maintained at a low level to retain satisfactory thermofusing and forming property or pliability, the toughness is impaired and the object of said proposal cannot be accomplished.  Moreover, resins used in the proposal such as aliphatic urethane polymers or copolymers of ethylene or propylene cannot provide sufficient weatherability, and the products are unsuitable for utilities requiring weatherability durable over prolonged periods. 
Aside from above-cited prior art, U.S. Pat.  No.  4,653,854 (Japanese Laid-open Patent Application KOKAI No.  194405/85) and Japanese Laid-open Patent Application KOKAI No.  196653/90 have proposed a two-layered structure for the support sheet, i.e., a support sheet composed of a surface layer (binder layer, upper side) and a reinforcing layer (lower side, support layer), with the view to solve the above-described problems.  According to the proposals, the reinforcing layer is made of a crosslinked resin to impart tensile strength to the support sheet, which is combined with a surface layer formed of a crosslinked resin or non-crosslinked resin. 
These proposals are useful for improving tensile strength of the support sheet by the presence of said reinforcing layer.  However, when the surface layer is formed from a crosslinked resin, similar problem to the one as described as to U.S. Pat.  No.  4,025,159 (Japanese Patent Publication No.  13561/86) is caused.  Whereas, when the binder layer is formed from a non-crosslinked resin, again the problem as described as to U.S. Pat.  No.  4,897,136 (Japanese Laid-open Patent Application KOKAI No.  121043/87) cannot be avoided. 
Claim 1 of said Japanese Laid-open Patent Application KOKAI No.  196653/90, furthermore, discloses addition of an elastomer to the surface layer-forming thermofusible (thermoplastic) resin; and Example 2 of same Application, addition of a minor amount of cellulose acetate butyrate resin besides an elastomer.  However, according to our experiments, no retroreflective sheeting having satisfactory physical properties and excelling in weatherability can be obtained, using elastomers satisfying the requirements set in this reference, i.e., an elongation at normal temperature of at least 50% and a tensile resistance at 70.degree.  C. of at least 120 kg/cm.sup.2.