High-visibility retro-reflective materials

In this article, SATRA looks at one of the essential components of high-visibility clothing – retro-reflective materials.
by Mark Gamble
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Testing Personal Protective Equipment (PPE)

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Figure 1: A scanning electron microscope image showing the mixture of glass beads that makes up the surface of a retro-reflective tape.

Figure 2: The geometrically-shaped embossed surface of a micro-prismatic tape as revealed by a scanning electron microscope.

Figure 3: SATRA’s photometer is used to measure retro-reflective material under different viewing conditions, as defined in high-visibility standards.

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High-visibility materials are used to make an object, animal or person visually stand out from the surrounding environment. SATRA’s laboratories test different types of high-visibility products and materials, particularly those that are used in personal protective equipment (PPE).

As a European Notified Body, SATRA is also responsible for certifying different types of high-visibility products to show that they meet the requirements of the EU Directive (89/686/EEC) on PPE. This activity permits PPE to be CE marked and sold in Europe – and includes the type examination and certification of professional use (occupational), non-professional use (leisure) garments and accessories. Firefighters’ clothing and gloves, which also optionally incorporate high-visibility materials, are also certified.
 

Definition of personal protective equipment (PPE)
Any device or appliance designed to be worn or held by an individual for protection against one or more health and safety hazards.

Effective high-visibility PPE works by combining different materials that produce a high level of contrast between the user of a product and the background against which they are set. The specifications for these products take account of the nature of the hazard that is posed to a person and the risk of injury that may be encountered. For example, working on a motorway entails more risk than walking on a pavement in an urban area.

Under daylight conditions, coloured fluorescent high-visibility materials are used to produce a contrast of appearance against a particular environment – fluorescent yellow, orange-red or red materials being those most commonly used. Low-light or night-time high-visibility materials are those that can be said to have retro-reflective properties. A large proportion of light that is shone onto these materials is reflected back towards the origin of the light source.

The current European standard for occupational-use high-visibility PPE specifies the application of retro-reflective materials in the form of tapes that are placed on a garment according to specific design criteria. The main requirement is that these tapes encircle the torso and limbs – the primary components of the human body. The application of tapes to a garment in this manner is intended to make a body visible when it is viewed in different orientations and postures. Leisure-wear high-visibility PPE, which is intended for use under circumstances which are not as hazardous as those for professional-use garments, permits retro-reflective materials to be applied in a freer manner – for example, patches and logos can be used.

The standards for high-visibility garments, and those for accessories, set out the minimum performance requirements for retro-reflectivity. These include retro-reflectivity testing after potentially destructive pre-treatments have been applied. To maintain an effective level of retro-reflectivity, materials must retain their performance after any treatment which is prescribed for them. For example, if a retro-reflective tape is claimed to be effective after ten dry cleaning cycles, then evidence that the minimum specified level of retro-reflectivity can be maintained after cleaning must be provided by testing.

The retro-reflectivity of tape is achieved by different means. The most commonly used type is one that uses bead technology (figure 1). Microscopic glass beads of different diameters are distributed over the surface of a substrate layer, which is usually, but not exclusively, a textile material. These beads are stuck to the substrate by a silvered adhesive layer which forms a mirror around that part of the bead that is embedded. The performance of the tape is determined by several factors, including bead sizes, how spherical they are, their distribution over the surface of a substrate layer and the depth to which they are embedded in the adhesive layer. The durability of a tape depends not only upon how well individual beads are held in position on the tape, but also the flexibility of the adhesive layer and the inherent strength of the substrate material. Any loss of beads will, of course, lead to a reduction in the retro-reflective performance of the tape.

Another type of retro-reflective tape often used on high-visibility PPE incorporates micro-prismatic structures. This type of material is embossed with many reflectors that are shaped liked the corners of a cube (figure 2). These may be arranged in different orientations to provide different paths of retro-reflection. Each micro-prismatic shape functions by reflecting light from the three sides of its structure. The geometry of these micro-prisms is such that light is reflected back towards its source. The collective effect of the many micro-prisms embossed on a tape provides the required level of retro-reflection.

A retro-reflector does not return light shone onto it exactly to the point of origin. Rather, it returns a cone of retro-reflective light towards the source of illumination. Retro-reflected light can therefore only be seen if a viewer is within the cone of retro-reflection. For example, a car driver can see a pedestrian wearing a retro-reflective garment because the driver sits more or less behind the headlights of the vehicle. The laboratory measurement of retro-reflective materials is undertaken to define the effective cone of retro-reflection of a material (figure 3).

At present, the following standards are applied to PPE high-visibility products sold within Europe:

  • EN471:2003 + A1:2007 – High visibility warning clothing for professional use
  • EN1150:1999 – Visibility clothing for non-professional use
  • EN13356:2001 – Visibility accessories for non-professional use.

The standard for professional use high-visibility garments permits the manufacture of three classifications of garment. Each classification is based on minimum allowances for both fluorescent material and tape that can be used in making a garment. Additionally, two performance levels of tape are permitted. In many instances, buying authorities will only purchase garments that are manufactured using the highest performing ‘level 2’ tape which provides greater visibility, particularly at a distance (see table 1).
 

Table 1: EN471:2003+A1:2007 retro-reflective performance of new material requirements
Level 1 – Minimum coefficients of retro-reflection in cd/(lx·m2) for separate-performance material
Observation angle Entrance angle
  20° 30° 40°
12' 250 220 135 50
20' 120 100 75 30
25 15 12 10
1°30' 10 7 5 4
Level 2 – Minimum coefficients of retro-reflection in cd/(lx·m2) for separate-performance material
Observation angle Entrance angle
 
20° 30° 40°
12' 330 290 180 65
20' 250 200 170 60
25 15 12 10
1°30' 10 7 5 4

Therefore, manufacturers should always make sure that the retro-reflective material they intend to use meets their market’s demands.

Buyers of high-visibility garments may also specify that retro-reflective tapes are able to withstand specific cleaning treatments, usually appropriate to the type of contamination that their clothing may experience. For example, clothing that becomes contaminated with oil may require dry cleaning. It is, therefore, important to determine whether a retro-reflective material has the ability to withstand repeated solvent immersion.

SATRA can carry out independent checks on retro-reflective materials to ensure that they meet the minimum performance levels that are demanded by standards. Additionally, testing can be arranged to determine whether materials have the durability requirements that may be demanded by individual manufacturers or users of high visibility garments.

SATRA's Essential Guide to PPE is available at www.satrappeguide.com

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