Retroreflective films – constructions, history and applicationsDon McClure

Acuity Consulting and TrainingFt. Pierce, FL

Retroreflective films can be divided into two major classes based on whether they employ glass beads or cube-corner retroreflective elements. Within these two classes there are many variations. The films have found applications in highway signing, occupational and consumer high-visibility apparel, truck conspicuity films, security devices, among others. Some of these variations along with their histories and applications will be described, with an emphasis on the variations that depend on vacuum coated reflectors, especially those to which the author contributed while employed at 3M Company.

Introduction

Retroreflective films have been in use for many years in a variety of application areas. They have been brought to market by a number of providers. They can be divided into two major classes based on the type of optical elements used: glass beads or cube corners. Within each class there are many variations not only in optical design but also in the materials used. Device durability and optical performance depends on both design and materials. The history of these designs as used in signage, their relative optical performance, and their commercial suppliers have been described [1].

Glass-bead-based retroreflectorsThe simplest retroreflector is a glass bead, see Figure 1. For the most efficient light return the glass bead material would need a very high index of refraction. Most glass-bead-based retroreflective constructions use beads with a lower index and add a metal reflective layer on the back of the bead, shown in blue in Figure 2. The performance is often enhanced by the inclusion of a “spacer layer” between the bead and the metal layer, shown in yellow in Figure 2.

Figure 1. The basic concept for a glass-bead-based retroreflector.

Figure 2. A schematic of a retroreflective film based on metallized glass beads [2]. In this design the “carrier” and “paper” are removed and discarded before use.

Selective removal of the reflector layer (most often with a laser) can be used to add images of various kinds. Products based on this concept have been produced that give images which show up only at narrow viewing angles or that require a collimated-light viewing device; both have been used in security applications. Products can also be produced that display images which appear to float above or below the films and provide three-dimensional line images for a variety of special effects.

Glass bead constructions have been used in highway signing, consumer and work-zone high-visibility apparel, security, and other applications.

Cube-corner-based retroreflectorsFigure 3 is a two-dimensional schematic of a cube-corner retroreflector. The third dimension and third reflection (not shown in the schematic) are required for retroreflection. Many cube corner devices reflect light based on total internal reflection (TIR). In this design light approaches the polymer-air surface from within the polymer at high enough angles to insure high reflectivity.

In designs where the light cannot be reflected using TIR, a reflector, usually a thin metal layer, is added, usually by vacuum coating.

Figure 3. A two-dimensional schematic of a cube-corner retroreflector

A variety of designsOver time a variety of retroreflective film designs have been brought to market. Figure 4 shows four of the major designs used in signage. In each case the retroreflective element is “protected” from direct exposure to the atmosphere. Moisture, and in particular moisture droplets, condensed on an unprotected, top surface would scatter light in many directions, reducing or sometimes eliminating the retroreflective performance: a sign goes dark.

In the upper left image in Figure 4, the beads are covered with a “face coat.” In the lower left the beads are protected with a “face film” which is suspended above the beads with an intermediate air gap. This design requires a regular array of “bridges” to prevent the face film from contacting the beads. The design on the lower right is similar except here the beads are replaced with cube corners. Both of the designs on the bottom of the figure tend to be quite rigid. In the upper right the cube corner device has no air space available for TIR, and a metal reflective layer is used. The diagram erroneously suggests a very thick and space-filling “metallized layer.” A better diagram would show a thin metal layer with the “adhesive layer,” below it in the diagram, used to fill the space below the cube-corner array.

Figure 4. Cross-sectional views of four of the major retroreflective film designs used in signage [1].

So-called “exposed bead” designs would resemble the design in the upper left of Figure 4 if the face film were not present. “Exposed bead” designs have lifetimes that are far too short for highway signage but that can be more than adequate for many apparel applications.

More variations

My former employer, 3M Company, has been involved in supplying products in this market for many years. To do so, they have installed industrial-scale aluminum metallizing systems on four continents. My career in the company’s corporate research laboratory gave me opportunities to interact with the needs of this business several times. In each of the interactions described below, the presenting challenge was resolved but required making changes in the traditional aluminum metallization process.

Flexible Truck Conspicuity SheetingIn the early 90s the U. S. federal government mandated that retroreflective elements be added to truck trailers and tractors to make them more visible, particularly at night. For a variety of reasons, including durability and the flexibility needed to apply the materials over the corrugations and

rivets found on trailer sides, the choice was a metallized cube-corner design. The standard evaporated aluminum process did not provide adequate adhesion of the aluminum layer to the polymer of choice, a molding-grade polycarbonate resin. Most molding-grade resins include additives to enhance release of the cast polymer part from the mold; this same additive makes adhesion of the subsequently evaporated metal layers a challenge. Unbeknownst to the conspicuity sheeting team, a solution had already been demonstrated for a similar materials combination for a product in a quite different market, a linear lamp used on the front of a late-1980s-era Mercury Sable automobile. This same solution was quickly demonstrated for the conspicuity product in the lab. Getting the process into the appropriate manufacturing machine required machine modifications, but the process was soon working.

“Improved durability” work-zone-safety productsProducts to add retroreflective features to apparel are often based on a glass-bead product. The latter tend to be more flexible than cube-corner products. For many applications the standard beaded product is more than adequate. As time progressed the commercial work-zone-safety products were seeing failures that were traced to the increased use of high pH detergents; the exposure to high pH solutions degraded the aluminum reflector layers.

Many types of additions and variations to the aluminum reflective layer were tried without success. While silver is often thought of as less durable than aluminum, silver films can be used in these constructions with very good durability. Again this process did not fit conveniently into the production metallizers, but a good work-around was found.

Gold metallized cube corner sheetingThis was a once-and-done effort based on a customer request. The customer was sponsoring the sprinter, Michael Johnson, who, for a time, was called "World's fastest man" [3]. The sponsor wanted to provide Mr. Johnson with a 24K-gold-coated running shoe to be used in the 2000 Olympic games and decided a retroreflective feature was also desired. This required only a short length (~100’) of narrow and flexible material to satisfy the need. It was handled quickly and successfully in one of our pilot-scale, roll-to-roll coating tools. The construction of the shoes was left to the customer/sponsor; see Figure 5.

Figure 5. Gold-coated, retroreflective-fabric-based running shoes.

Other retroreflector variations

These next paragraphs are added for general interest; the author was not involved in any activities related to these efforts.

Transparent reflectorsIt is well known that alternating layers of high and low index materials can fabricated so as to produce controlled levels of reflection and transmission. Glass-bead-based constructions as described above with the aluminum layer replaced by layers of zinc sulfide (high index) and cryolite (low index) have been used and sometimes combined with selective removal of the reflector to generate a variety of security films used in passports, ID cards, driver’s licenses, etc.

Fluorescent reflectorsDuring daylight hours you may occasionally see highway signage that seems brighter than expected. They stand out due to the use of fluorescent dyes which convert some of the UV light in sunlight to visible light. The “extra” visible light catches the viewers attention in ways that are valued by signage designers.

3M™ Diamond Grade™ DG³ Reflective Sign SheetingThis is another brighter than expected design, one that works at night. Cube-corner reflectors have areas near the edges of each cell where, by simple geometry, the light arrives but does not undergo the three reflections needed for retroreflection. The complement to that statement is that there are areas where all the light entering (within a reasonable acceptance angle) is retroreflected. A clever engineer figured out how to build sheeting that is made up of only these higher efficiency areas. It is commercially available as “DG3” sheeting. It has been reported to return “about double what HIP reflective sheeting reflects” [4] (HIP refers to the standard “high intensity prismatic sheeting). Figure 5 shows the relative performance of three types of sheeting. High brightness signage is gaining in importance for several reasons but especially as automotive headlights are changing to reduce the amount of upward-directed light. For more quantitative relative brightness values in signage applications and rough guides for the expected lifetimes of the various commercial offerings see reference [4].

Figure 5. A side-by-side photo of three types of retroreflective signage taken to illustrate the different light return capabilities [5].

The performance of most of the highway signage products and many of the workplace apparel products are regulated by various government agencies. The author has not been involved with any of those types of activities.

SummaryThis article has introduced and summarized a variety of retroreflective film constructions and applications. It highlighted three projects involving changes in the traditional metallization process. It concluded with mention of a device made with a non-metallic reflector and two recent advances in highway signage constructions.

[1] John Lloyd, “A brief history of retroreflective sign face sheet materials”www.rema.org.uk/pub/pdf/history-retroreflective-materials.pdf as captured September 20, 2018.

[2] From a late 1990s era 3M product brochure.[3] en.wikipedia.org/wiki/Michael_Johnson_(sprinter) as captured September 20, 2018.[4] shannonbaum.com/the-difference-between-engineer-grade-high-intensity-prismatic-and-

diamond-grade-reflective-sheeting/ as captured September 20, 2018.[5] www.3m.com/3M/en_US/company-us/all-3m-products/~/All-3M-Products/Safety/Road-

Safety/Traffic-Vehicle-Safety/Sign-Sheeting/?N=5002385+7567192+8709322+8709397+8710677+8710896+8711017+3294857497&rt=r3 as captured September 20, 2018.