METHOD FOR PREPARING VULCANIZABLE RETROREFLECTIVE SHEETING

Field of the Disclosure

This disclosure relates to methods for preparing vulcanizable retroreflective sheeting and retroreflective articles.

Background

Retroreflectivity is a phenomenon that has been successfully used to prepare safety related articles. Retroreflection may be defined as a phenomenon in which a large portion of luminous radiation is returned in the direction from which it originates. This may be achieved through the use of spherical retroreflectors, which may be glass beads or microspheres that are at least partially coated with a reflective material, directly or via an intermediate layer.

Among the articles that can benefit from the use of retroreflectivity are rubber articles such as tires. Bicycle and motorcycle tires are a particular example since visibility at night is a particular safety issue for cyclists. Frequently cycles come with attached reflectors on the frame and/or the tire rims, but often the tires themselves are not reflective. If the tires are reflective they are generally made retroreflective by the attachment of a reflector or the compounding of retroreflective elements with the tire when it is manufactured. However, often when a reflector is attached to a tire, the attached reflectors may not provide good visibility due to the limited area which is covered and the ease with which the attached reflectors can be damaged or fall off. An issue with compounding retroreflective beads with the tire is that such reflectors generally provide poor brightness.

Summary

Methods to incorporate retroreflectivity into rubber articles, such as tires, such that the retroreflectivity is retained in use is desirable. Accordingly, methods for preparing

vulcanizable retroreflective sheeting are provided. In some embodiments, the method of making vulcanizable retroreflective sheeting, comprises providing a reaction mixture comprising at least one diisocyanate, at least one polyisocyanate, at least one polyol, at least one unsaturated polyol and at least one chain extension agent, coating the reaction mixture onto a carrier web that comprises retroreflective elements, curing the reaction mixture, and removing the carrier. The retroreflective elements may be, for example, metal-coated microspheres. Polyurethane binders comprising the reaction product of at least one diisocyanate, at least one polyisocyanate, at least one polyol, at least one unsaturated polyol, and at least one chain extension agent are provided. The polyurethane binder may also comprise a polyol with a functionality greater than 2. In some embodiments the polyurethane binder also contains a silane coupling agent.

Brief Description of the Drawings

Figure 1 shows a schematic cross sectional view of a segment of vulcanizable retroreflective sheeting as described in the present disclosure.

Figure 2 shows a schematic cross sectional view of a segment of vulcanizable retroreflective sheeting as described in the present disclosure.

Detailed Description Methods for preparing vulcanizable retroreflective sheeting are provided. The retroreflective sheeting comprises retroreflective elements and a polyurethane binder. The polyurethane binder is co-vulcanizable with curable substrates to form retroreflective articles.

The retroreflective sheeting comprises a plurality of retroreflective elements at least partially embedded in a polyurethane binder. The polyurethane binder is the reaction product of at least one diisocyanate, at least one polyisocyanate, at least one polyol, at least one unsaturated polyol, and at least one chain extension agent.

The diisocyanate component of the polyurethane binder may be any aliphatic, cycloaliphatic, aromatic or heterocyclic diisocyanate, or any combination of such diisocyanates. Particularly suitable diisocyanates correspond to the formula:

Q(NCO) ₂

in which Q represents: an aliphatic hydrocarbon radical containing from 2 to 100 carbon atoms and zero to 50 heteroatoms; a cycloaliphatic hydrocarbon radical containing from 4 to 100 carbon atoms and zero to 50 heteroatoms; an aromatic hydrocarbon radical or heterocyclic aromatic radical containing from 5 to 15 carbon atoms and zero to 10 heteroatoms; or an araliphatic hydrocarbon radical containing from 8 to 100 carbon atoms and zero to 50 heteroatoms.

The heteroatoms that may be present in Q include non-peroxidic oxygen, sulfur, non-amino nitrogen, halogen, silicon, and non-phosphino phosphorous.

Illustrative examples of suitable diisocyanates include ethylene diisocyanate, 1,4- tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, trimethyl hexamethylene diisocyanate, 1,12-dodecane diisocyanate, cyclobutane- 1,3 -diisocyanate, cyclohexane-1,3- and - 1 ,4-diisocyanate, 1 -isocyanato-3 ,3 ,5 -trimethyl-5 -isocyanotomethy lcyclohexane (isophorone diisocyanate, IDPI), 2,4- and 2,6-hexahydrotolylene diisocyanate, perhydro-

2,4'- and -4,4'-diphenylmethane diisocyanate (H ₁₂ MDI), hexahydro-1,3- and -1,4- phenylene diisocyanate, 1,3- and -1,4-phenylene diisocyanate, 2,4- and 2,6-tolylene diisocyanate, diphenylmethane-2,4'- and -4,4'-diisocyanate, mixtures of 2,2,4- and 2,4,4- trimethyl hexamethylene diisocyanate (TMDI), naphthylene-l,5-diisocyanate, including mixtures of these isomers, as well as oligomers thereof, and any combination of the above diisocyanates.

Diisocyanates that are commercially available and which impart good processability to the urethane prepolymer are preferred. Illustrative examples of such diisocyanates include hexamethylene diisocyanate, methylene -bis-(4- cyclohexylisocyanate), isophorone diisocyanate, naphthalene 1,5-diisocyanate, toluene diisocyanate, isomers of diphenylmethane diisocyanate, or a mixture thereof. Isophorone diisocyanate, such as DESMODUR I commercially available from Bayer MaterialScience, Leverkusen, Germany, is particularly useful.

The polyisocyanate component of the polyurethane binder is similar to the diisocyanate component described above but is generally a higher molecular weight

analog prepared by chain extension of diisocyanates. These higher molecular weight analogs may be aliphatic, cycloaliphatic, aromatic or heterocyclic diisocyanate, or any combination thereof. In addition the functionality of the polyisocyanates may be greater than 2. An example of a useful polyisocyanate is the commercially available DESMODUR VL from Bayer MaterialScience, Leverkusen, Germany which is an aromatic polyisocyanate based on diphenylmethane diisocyanate.

Generally the amount of polyisocyanate is 5-30 weight percent of the amount of diisocyanate. In some embodiments, the amount of polisocyanate is 10-20 percent by weight of the amount of diisocyanate. The polyol component of the polyurethane binder may be in a liquid form, or may be an oligomeric difunctional alcohol. The polyol preferably has a number average molecular weight (M _n ) ranging from about 90 to about 5,000 or even about 90 to about

1,000 g/mole. Illustrative examples of suitable polyols include the CARBOWAX 400, 600, 800 and 1000 series of poly(ethylene oxide) compounds (commercially available from Dow Chemical, Midland, MI), caprolactone polyols such as the TONE 200, 201,

210, 230, 240 and 260 series of polyols (commercially available from Dow Chemical), poly(tetramethylene oxide) polyols such as the Poly THF 250, 650, 1000 and 2000 series of polyols (commercially available from BASF Corp., Parsippany, NJ), polypropylene oxide polyols, polycarbonate polyols, such as KM-10-1667 and KM-10-1733 polycarbonate diols (commercially available from Stahl USA, Peabody, Mass.) and the DESMOPHEN series of polycarbonate diols such as DESMOPHEN C 1200 and DESMOPHEN X 2501 polycarbonate diols (commercially available from Bayer MaterialScience, Leverkusen, Germany), polyurethane polyols, such as K-flex UD-320- 100 polyurethane diols (commercially available from King Industries, Norwalk, CN), aromatic polyether polyols, such as SYNFAC 8024 polyols (commercially available from

Milliken Chemical, Spartanburg, SC), and random copolymers of poly(tetramethylene oxide)polycarbonate, such as the Poly THF CD series of polyols (commercially available from BASF Corporation, Mount Olive, NJ). Polyester polyols include the FOMREZ family (commercially available from Chemtura Corporation, Middlebury, CT), such as FOMREZ 11-112, 22-55, 33-56, 44-58, 55-112 polyols or the RUCOFLEX family

(commercially available from RUCO Polymer Corporation, Hicksville, N.Y.) such as

RUCOFLEX S-IOl, S-102, S-105, S-107, S-1014, S-1021, S-1028 and S-1034 diols. Polycaprolactone polyols, polycarbonate polyols, polyurethane diols and polyester polyols are generally preferred for weatherability reasons.

Other polyols suitable for use in the invention include the hydroxyalkyl ethers obtained by the addition of optionally substituted alkylene oxides, such as ethylene oxide, propylene oxide, butylene oxide and styrene oxide, onto the abovementioned polyols. Preferred examples of such hydroxyalkyl ether polyols include diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, dibutylene glycol, l,4-bis-(2- hydroxyethoxy)cyclohexane and 1 ,4-bis-(2-hydroxyethoxy-methyl)-cyclohexane, 1 ,4-bis- (2-hydroxyethoxy)-benzene. These materials have relatively low molecular weights and help incorporate rigidity into the urethane prepolymer backbone.

In addition to the above, in some embodiments one or more polyols with a functionality greater than 2 such as trifunctional or greater- functional polyols may be added to the polyurethane binder at a level which will not hamper the thermoformability of the binder. Examples of trifunctional or greater- functional polyols include glycerol, trimethylol propane, trimethylol ethane, 1,2,6-hexane triol, 1,2,4-butanetriol, pentaerythritol, mannitol, sorbitol, formitol and mixtures thereof. In addition, polymeric multifunctional polyols may be used. Examples of such polyols include, for example, some of the DESMOPHEN polyols such as DESMOPHEN 670 and DESMOPHEN 800 commercially available from Bayer MaterialScience, Leverkusen, Germany.

The unsaturated polyol component of the polyurethane binder may be any aliphatic, cycloaliphatic, aromatic or heterocyclic polyol, or any combination of such polyols that contain unsaturated groups. Particularly suitable polyols correspond to the formula: Z(OH) _n

in which n is 2 or greater and Z represents an n valent, hydrocarbon-based, radical which contains unsaturated groups along or pendant from the hydrocarbon chain. The hydrocarbon-based group may or may not contain heteroatoms.

A variety of unsaturated polyol materials are commercially available including, for example, hydroxy-terminated polybutadiene materials, such as the POLY BD series of

polyols (commercially available from ATOFINA Chemical, Philadelphia, PA), including POLY BD R-45HTLO, POLY BD R-20LM, POLY BD 600 and POLY BD 605. Another class of commercially available unsaturated polyols are the HTBNs (hydroxy-terminated poly(butadiene-co-acrylonitrile)) liquid rubbers. Generally, the amount of unsaturated polyol is 1-99 weight percent of the total polyol content of the polyurethane binder composition, more typically 40-80 weight percent of the total polyol content.

The chain extension agent component of the polyurethane binder is typically a diol. The diols are typically low molecular weight, short chain diols well known in the polyurethane art. In some embodiments the chain transfer agent has a number average molecular weight of 120 or less. Chain extension agents are generally incorporated into polyurethane backbones, to improve ductility or strength characteristics. These diols may be aliphatic, aromatic, cycloaliphatic or combinations thereof. Among the chain extension agents useful in the present disclosure are, for example, ethylene glycol, 1 ,4-butanediol, diethylene glycol, 1 ,2-propanediol, 1,3-propanediol, 1,6-hexanediol, hydroquinone bis(2- hydroxyethyl) ether (HQEE), bisphenol A, bisphenol F, 2,2,4-trimethyl-l,3-pentanediol, dipropylene glycol, 1,5-pentanediol, 3-methyl-l,5-pentanediol, 1,4- cyclohexanedimethanol, cyclohexanediol, and the like.

The polyurethane binder may additionally comprise other materials typically used in the generation of preparation of polyurethane polymers. Among these are catalysts, fillers, leveling agents, defoamers, colorants, antioxidants, UV light stabilizers and the like.

Catalysts for the reaction of polyisocyanates 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, dibutyltin dimercaptide, dimethyltin dithioglycolate, and dioctyltin dithioglycolate. The amine catalysts preferably are tertiary amines such as triethylene diamine, dimorpholinodiethyl ether, and tris(dimethylamino ethyl)phenol. Generally, the catalyst is present in the reaction mixture at 50 ppm (parts per million by weight of the total composition) to 50,000 ppm, or 100 ppm to 2,000 ppm, or even 200 to 1,000 ppm.

Useful fillers that may be used in the polyurethane binder composition include, for example, carbon black, metal oxides such as silica, alumina, titanium oxide and the like. Any suitable filler may be used as long as it doesn't interfere with the preparation or use of the polyurethane binder composition. In some embodiments, a silane coupling agent may be used in the polyurethane binder composition. Silane coupling agents are reagents well known in the polyurethane art. Typically the silane coupling agents are bifunctional having a silane or substituted silane group on one end and a reactive group on the other end. Examples of silane groups include silane, alkyl silanes and alkoxy silanes. Reactive groups suitable for use with polyurethane systems include any group which may react with an isocyanate or a polyol. Such groups include, for example, isocyanate groups, primary or secondary amine groups, hydroxyl groups, epoxy groups, thiol groups, acetic acid groups and the like. Examples of particularly suitable silane coupling agents include, for example, such compounds as KH- 560 a commercially available coupling agent of (3 glycidyloxypropyl) trimethoxy silane or Z 6020, Z 6040, KH-550, commercially available from Dow Corning, Midland, MI. If used, the silane coupling agent is generally present in the amount of 0.1 to 7 weight percent based on the total weight of the polyurethane binder composition. In some embodiments, the amount of silane coupling agent is 1-3 weight percent.

The retroreflective sheeting comprises a plurality of retroreflective elements at least partially embedded in the polyurethane binder. In some embodiments, the retroreflective elements are retroreflective beads. Among the retroreflective beads useful for this application are microspheres that, generally, are substantially spherical in shape in order to provide the most uniform and efficient retroreflection. The microspheres preferably also are substantially transparent so as to minimize absorption of light so that a large percentage of incident light is retroreflected. The term "transparent" is used herein to mean capable of transmitting light. The microspheres often are substantially colorless but may be tinted or colored in some other fashion. The microspheres may be made from glass, a non-vitreous ceramic composition, or a synthetic resin. In general, glass microspheres are preferred because they tend to be less expensive, harder, and more durable than microspheres made from synthetic resins.

The microspheres may have any suitable size. Typically, the microspheres have an average diameter in the range of about 10 to 200 micrometers, or about 25 to 80 micrometers. Microspheres used in some embodiments typically have a refractive index of about 1.91, although values in the range of about 1.5 to 2.5, or any other suitable values, may be useful as well, depending on the type of sheeting desired.

The microspheres may have a reflective metal layer disposed beneath the embedded portions of the microspheres. Generally, the reflective layer is disposed on the embedded or rear portions of the microspheres. The reflective layer may be disposed directly on the microspheres or it can be disposed on the microspheres via an intermediate layer. The term "reflective layer" is used herein to mean any suitable layer capable of reflecting light, and preferably it is capable of specularly reflecting light. In one exemplary embodiment, the reflective layer may be a layer comprising elemental metal. The metal may be a continuous coating produced by vacuum-deposition, vapor coating, chemical-deposition, or electroless plating. A variety of metals may be used to provide a reflective layer. These include aluminum, silver, chromium, nickel, magnesium, and the like, in elemental form. Aluminum and silver are the typically used metals in the reflective layer. It is to be understood that in the case of aluminum, some of the metal may be in the form of the metal oxide and/or hydroxide. Aluminum and silver metals are desirable because they tend to provide good retroreflective brightness. The reflective layer should be thick enough to reflect incoming light. Typically, the reflective layer is about 50 to 150 nanometers thick. Although the reflective color of a silver coating can be brighter than an aluminum coating, an aluminum reflective layer is typically used.

In lieu of a metal layer, a dielectric mirror may be used as a specularly reflective layer. The dielectric mirror may be similar to known dielectric mirrors disclosed in U.S. Pat. Nos. 3,700,305 and 4,763,985 to Bingham.

In general, the sheeting material is prepared by embedding substantially a monolayer of retroreflective elements such as glass microspheres into a carrier web to a depth not exceeding 50% of the diameter of each microsphere; depositing specularly reflecting material over the retroreflective element-bearing surface of the carrier web; coating the binder composition of this disclosure over the specularly reflecting deposit; applying thermal energy to the binder composition to form a thermoplastic or

thermosetting binder layer; and stripping away the carrier web while leaving the retroreflective elements partially embedded in the binder layer.

Referring to Figure 1, the exemplary retroreflective sheet article, 100, has a carrier web, 10, which may be paper, fabric, film or other material. Carrier web 10 has a coating, 20, which may be polyethylene, for example, in which are at least partially embedded beads, 30, which may be any transparent beads such as glass beads. Beads 30 have a reflective layer, 40, such as a metal coating or a polymeric reflector, functionally disposed on the side of the beads opposite to the coating 20 and the carrier web 10. The reflective coating may be disposed on the beads and it may at least partially cover a portion of the bead surface. The beads 30 are at least partially embedded in the binder, 50,which may comprise the polyurethane composition. The reflective layer 40 may be disposed between the binder 50 and the beads 30. Other suitable intermediate layers may also be disposed therebetween. The binder 50 typically is 100-600 micrometers thick, or even 200-400 micrometers thick. Figure 2 shows the exemplary retroreflective sheet article, 200, which is the same construction as in Figure 1 in which the carrier web, 10, with coating, 20, has been removed.

The binder composition may be either a mixture of the diisocyanate, polyisocyanate, polyol, unsaturated polyol, chain extension agent and any desired additives such as catalysts, pigments and the like; or it may be a polyurethane prepolymer mixture. If a prepolymer mixture is used, generally the mixture contains 2 parts, Part A and Part B. Typically Part A contains the reaction product of a diisocyanate, polyisocyanate, polyol and unsaturated polyol, and Part B contains one or more chain extension agents. Typically Part A and Part B are combined and mixed prior to application. The binder composition can be discharged or coated onto the beaded support web. The thermal curing process for the polyurethane is generally carried out at temperatures of about 70 to about 180° C, using an oven or other heating techniques. In some instances heating may be carried out at temperatures of about 70 to about 150° C. The cure rate may also be accelerated by using a catalyst as above described, if desired.

It is well understood in the polyurethane art that the properties of a polyurethane polymer can be varied by varying the composition and content of the isocyanate and hydroxyl reactive species used to make the polyurethane polymer. Typically the binder

composition contains a mole ratio of isocyanate groups: hydroxyl groups of from 0.7 : 1 to 1.5 ; 1. More typically the binder composition contains an isocyanate: hydroxyl ratio of from l : 1 to 1.3 : 1.

Typically the polyurethane binder comprises 30-60 percent by weight hard segment. The "hard segment" and "soft segment" concept is one that is well understood in the polyurethane art. In the present disclosure, hard segment elements are the diisocyanate, polyisocyanate, chain extension agents and any polyols with functionality greater than 2 while the soft segment elements are the polyols which are diols. Retroreflective articles are prepared by co-vulcanizing or co-curing the retroreflective sheet which comprises a plurality of retroreflective elements at least partially embedded in a polyurethane binder with a curable substrate. The polyurethane binder comprises unsaturated groups which may co-vulcanize or co-cure with the curable substrate. Examples of curable substrates include rubber and/or rubber-coated articles as well as other substrates that have curable groups. EXAMPLES

These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims. All parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight, unless noted otherwise. Solvents and other reagents used were obtained from Sigma-Aldrich Chemical Company; Milwaukee, Wisconsin unless otherwise noted.

Table of Abbreviations

Test Methods

Retroreflective Brightness

Retroreflectivity measurements for each of the following sheet materials were obtained using a retroluminometer according to the test method ASTM E808-81. The data are reported in candelas per lux per square meter (cpl).

Adhesion to Rubber

The adhesion of the retroreflective sheeting to rubber was tested by covering half of the surface area of the urethane side of the sheeting with a polyethylene terephthalate (PET) film. The PET film is to prevent that portion of the sheeting from bonding with the rubber surface. The resulting sheet was contacted with unvulcanized rubber and the resulting laminate was placed in a plate mold where it was cured at 18O ⁰ C with a pressure of 15 Newtons for 5 minutes. After vulcanization the PET film was removed and the sample was cut into strips. The peel strength was measured according to ASTM D 1876- 95 T-Peel Test using the unbonded portion of the strip to start the peel front.

Example 1 :

In a vessel DESMOPHEN C 1200, DESMOPHEN X 2501, HTPB 2800, and 1,4- BDO in the amounts shown in Table 1 were mixed for 30 minutes at 500 rpm. To this mixture was added DESMODUR I, DESMODUR VL, FOMREZ UL 29 in the amounts shown in table 1 and the resulting mixture was stirred for 30 minutes at 500 rpm. The mixture was coated onto a web containing aluminum-vapor-coated glass beads embedded in a temporary support film, at a coating thickness of 300 micrometers. The coated sheet was placed in a 125 ⁰ C oven for 30 minutes and then allowed to anneal for 2 days in a 65 ⁰ C oven. The sheeting was stripped from the temporary support film and the Retroreflective Brightness was measured (Initial Retroreflective Brightness), the Adhesion to Rubber test was carried out, and the Retroreflective Brightness test was again carried out (Post Vulcanization Retroreflective Brightness) using the test methods described above. The results are shown in Table 2.

Example 2:

The same procedure used for Example 1 above was followed with the amounts of reagents shown in Table 1. The results are shown in Table 2.

Table 1

Comparative Example Cl :

The same procedure described for Example 1 was followed except instead of the experimental retroreflective sheeting prepared in Example 1, the commercially available product 3M SCOTCHLITE 8150 was used. The results are shown in Table 2.

Table 2

Example 3 : Part A Preparation:

In a vessel DESMOPHEN C 1200, HTPB 2800, DESMODUR I, DESMODUR VL and REGAL 99R in the amounts shown in Table 3 were mixed for 3 hours at 1 ,000 rpm.

Part B Preparation: In a vessel 1,4-BDO, BYK 359 and BICAT Z in the amounts shown in Table 3 were mixed for 30 minutes at 500 rpm.

Mixing, Coating and Curing:

Part A and Part B prepared above were mixed in an online mixer and coated to a thickness of 400 micrometers on a web containing aluminum-vapor-coated glass beads embedded in a temporary support film. The coated sheet was placed in a 165 ⁰ C oven for 30 minutes and then allowed to anneal for 2 hours in a 15O ⁰ C oven. The sheeting was stripped from the temporary support film and the Retroreflective Brightness was measured (Initial Retroreflective Brightness), the Adhesion to Rubber test was carried out, and the Retroreflective Brightness test was again carried out (Post Vulcanization Retroreflective Brightness) using the test methods described above. The results are shown in Table 4.

Table 3

Table 4

Examples 4-9: Part A Preparation:

In a vessel DESMOPHEN C 1200, DESMOPHEN X 2501, HTPB 2800, DESMODUR I, DESMODUR VL, DESMODUR NZl and FOMREZ UL 29 in the amounts shown in Table 5 were mixed for 5 hours at 500 rpm.

Part B Preparation:

In a vessel 1,4-BDO, Glycerol, FOMREZ UL 29 and KH-560 in the amounts shown in Table 5 were mixed for 15 minutes at 100 rpm. Mixing, Coating and Curing:

Part A and Part B prepared above were mixed and stirred for 15 minutes at 500 rpm and then was coated to a thickness shown in Table 6 on a web containing aluminum- vapor-coated glass beads embedded in a temporary support film. The coated sheet was placed in a 125 ⁰ C oven for 30 minutes and then allowed to anneal for 2 days in a 65 ⁰ C oven. The sheeting was stripped from the temporary support film and the Retroreflective Brightness was measured (Initial Retroreflective Brightness), the Adhesion to Rubber test was carried out, and the Retroreflective Brightness test was again carried out (Post Vulcanization Retroreflective Brightness) using the test methods described above. The results are shown in Table 7.

Table 5: Part A

Table 6: Part B

Table 7