1. Field Of The Invention

This invention relates to retroreflective articles, and more particularly to fencing made of a high density polyethylene substrate wfth reflective glass beads, sometimes referred to as microspheres embedded therein.

2. Description Of The Related Art

The use of glass beads embedded in plastic is known in the art. U.S. Patent 4,766,032 disclosed "Reflective Transfer Sheeting and Method of Making the Same" wherein glass beads are embedded in a thermoplastic composite sheet. However, they are not embedded directly into the substrate as in the case of the present invention nor are they embedded 50-70% of the diameter of the beads. The extent of embedding is referred to only in general

terms (See columns 3, lines 50-55).

U.S. Patent 4,950,525 relates to "Elastomeric Reflective Sheeting" in which beads are embedded in a polymeric sheet. However, the sheet is then covered with a covering layer 19c (see column 5, lines 25-32 for a description of the final product). U.S. Patent 4,104,102 is somewhat similar to 4,950,525 in that the glass beads are covered with a sealing layer.

U.S. Patent 3,873,390 makes use of glass beads (column 13) which are adhered with an adhesive (Fig. 5).

U.S. Patent 4,505,967 describes retroreflectrve sheeting in which

the microspheres are embedded into a first polymeric layer to less than one- half the average diameter of the microspheres and a film is laminated to the

microsphere covered surface of the first layer.

U.S. Patent 4,569,857 describes a method of preparing

retroreflectϊve sheeting in which a binder material is required.

U.S. Patent 4,648,932 also relates to retroreflective sheeting

wherein microspheres are embedded into a first polymeric layer to less than one-half their diameter and a covering film is placed over the microspheres.

U.S. Patent 4,721 ,649 is similar to this patent.

U.S. Patent 4,876,141 relates to a technique in which a mixture of

materials including the glass beads or microspheres are formed into a sheet, the beads having a variety of exposed surfaces.

U.S. Patent 4,897,136 describes retroreflective sheeting wherein a cover film is utilized over microspheres.

U.S. Patent 4,921 ,754 is of interest in that microspheres are

embedded as to their lower portions only in a base layer and then covered

with a surface layer.

U.S. Patent 5,039,557 describes the use of preheated glass

beads in pavement marking material. The beads are heated to a temperature below their melting point but above the melting point of the existing

thermoplastic pavement marking material into which they are to be

embedded. The beads are then dropped onto the marking material and heat

from the beads is transferred to the plastic material causing it to melt and the

bead to adhere. The beads are then embedded by rolling them into the plastic material. No specific teaching is provided concerning depth of embedment. The invention specifically relates to the provision of reflective

beads in pavement marking. U.S. Patent 5,055,347 disclosed partially embedded microspheres having a binder layer applied over them after they have been

embedded into a substrate (see column 8).

Japanese Patent 57-186702 is of general interest regarding embedded glass spheres in which an adhesive is utilized. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is "prior art" with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.

Summary Of The Invention

The present invention relates to a process and the resultant article in which retroreflective plastic articles, namely plastic fencing in the preferred embodiment, are prepared by embedding glass beads in plastic or

thermoplastic materials such as high density polyethylene (HDPE). The glass beads are heated and the thermoplastic material is softened by contact with the heated beads. Pressure is applied to the beads to embed them into the substrate at least 50% of their diameter but no greater than 70% of their diameter. The resulting product has superior durability and abrasion

resistance. The 50-70% embedment of the beads in the present invention offers an advantage over the existing art in that no covering layer is necessary to retain the beads in their embedded position in the thermoplastic substrate.

Since no covering layers, adhesives or binders are required to hold the glass beads in place, material costs are reduced and greater service temperature range, ultraviolet stability and chemical resistance are provided.

Brief Description Of The Drawings

Fig. 1 is a fragmentary perspective view of the plastic reflective

fencing of the present invention; Fig. 2 is a greatly enlarged perspective detail view;

Fig. 3 is a side view of a schematic of the inventive embedment process as applied to plastic fencing;

Fig. 4 is an overhead view of a schematic of the inventive

embedment process; Fig. 5 is a schematic detail of a portion of Figs. 3 & 4, showing the glass bead hopper in more detail; and

Fig. 6 is a conceptual diagram of the embedment of a glass bead into a plastic substrate according to the invention. Description Of The Preferred Embodiments While this invention may be embodied in many different forms, there is shown in the drawings and described in detail herein a specific

preferred embodiment of the invention. The present disclosure is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiment illustrated.

Referring now to Figures 1 & 2, a fragment of the retroreflective fencing material comprising glass beads embedded in a plastic substrate is shown generally at 10. The fencing material 12 has panels 50 separated by rows of perforations 52. The perforations are separated by strips 54. Glass beads 30 are embedded in panels 50 to form reflective strips 56, imparting reflective quality to fencing material 12. Although the preferred use of the

present inventive process is on fencing material made of high density polyethylene (HDPE) blended with polypropylene, for example 90% HDPE and

10% polypropylene, the process may be utilized with any plastic or thermoplastic substrate.

Referring now to Figures 3-6 which illustrate the embedment process, fencing material 12 comes off unwind stand 14 and is propelled to coater 24 by driven roll 26, held in place by rollers 16,18,20 and 22. As fencing material 12 enters coater 24 it passes over driven roll 26 and is subjected to the glass embedment process. Glass bead hopper(s) 28 contains glass beads 30. More than one hopper may be utilized as can be seen in Fig. 4. The most preferred type of glass beads are size 11 , sign

quality, silica glass beads with a refractive index of 1.91 , commercially available from Flex-O-Ute, P.O. Box 4366, St. Louis, MO 63123. As shown in

Figure 5, glass beads 30 descend through neck 29 of hopper 28 and are heated by electrical heaters 32 situated on opposite sides of neck 29. Although any temperature well above the melting point of the plastic substrate and below the temperature of the glass beads may be used, a temperature of

approximately 900°F is preferred in the case of the HDPE/polypropylene

substrate. Electrical heater 32 is set at a temperature between 920°-960°F to produce a bead temperature of about 900° F. A simple mechanical valve 27 located in neck 29 of glass bead hopper 28 controls the flow of glass beads

30 from hopper 28. It may be remotely activated. Beads 30 may therefore be released continuously or intermittently, controlled by valve 27.

Heated glass beads 30 flow through nozzle 34 and meet deflector(s) 36, which deflects the beads so they evenly disperse over the width of reflective strip 56. When the inventive process is utilized on fencing material 12, a plurality of glass hoppers 28 continuously release glass beads 30 onto panels 50 of fencing material 12, forming reflective strips 56. Fig. 4 shows three glass bead application units which apply reflective glass bead strips 56 to fencing material 12. The apparatus shown will accommodate fencing material of up to 48" wide. The process may, however, be used on plastic material of any width. Although the fence is shown with three reflective strips 56 on alternating panels, the process may employ as many as ten or more application units, depending on the configuration of the fencing material used.

The thermal energy of glass beads 30 is used to melt the plastic substrate as it comes in contact with glass beads 30. A load is applied to the

glass beads by nip roll 38 (best seen in Fig. 3) to displace molten plastic and increase the surface area in contact to promote heat transfer. The thermal energy and load applied must be set to embed the glass bead to approximately 50% of its diameter into the original substrate surface, and no more than 70%. As shown in Figs. 6a & 6b, the molten plastic flows up

around the glass beads to provide a physical lock of the beads into the plastic surface. a

The embedment depth is important for two reasons, first to provide a positive lock of the beads and second, for the retroreflectrvity performance of the glass bead. Embedment depth must be greater than 50% for physically locking the beads into the surface, producing product durability.

Embedment depth determines the maximum angle at which the product (substrate surface or plane) can be rotated away from 90° of the light source and remain retroreflective. A maximum embedment depth of 70% of the beads' diameter is critical to maintain product performance.

After application of the glass beads, fencing material 12 travels

through coater 24 to winder 40, where the finished product 10 is wound into a roll. This application method of embedding the glass beads into the plastic substrate surface uses no adhesives or binders to hold or retain the reflective glass beads onto the surface.

Advantages of the present invention over the prior art are many. Elimination of the use of adhesives or binders to hold the glass beads in place reduces the number of material interfaces to one, which reduces the number of possible failure modes. This interface is mechanical and does not rely

solely on shear strength or bond strength to hold the glass beads in place. Therefore, long term retroreflective life characteristics of the product come from only the glass beads and plastic substrate, giving the product superior durability and abrasion resistance. The elimination of adhesives or binders

also reduces material costs, and provides greater service temperature range, greater U.V. stability and better chemical resistance.

This completes the description of the preferred and alternate embodiments of the invention. Those skilled in the art may recognize other equivalents to the specific embodiment described herein which equivalents are intended to be encompassed by the claims attached hereto.