Polymethylmethacrylate (PMMA) and its copolymers are used to make transparent, rounded “plastic” skylight covers for homes and businesses.  A number of fatal accidents have resulted from falls through those skylights, especially on warehouse roofs, despite the Occupational Safety and Health Administration (OSHA) building code requirement that mandates the support of at least a 200-pound weight.  Although some construction workers feel that significant degradation in mechanical properties occurs after exposure to the environment, others regard the devices as safe and sometimes stand on them despite the printed warnings.  Other accidents occur when roof maintenance personnel such as laborers installing caulking for leaks inadvertently lean on a skylight. Despite the widespread use of skylights and the history of injuries, relatively few studies have evaluated outdoor exposure effects on the mechanical properties of skylights.  Hence, we collected data to estimate the degree of mechanical property charges with time under Florida sun exposure for one or more decades and related those data to basic scientific studies published on this polymer.

Skylight removed from a roof after a 14 year old boy working on roofing repairs fell through
to a concrete floor 12 feet below.  He died of his injuries. (NIOSH 2004 report)

Those measurements provided strong support for a slow degradation process since the mechanical strength of older skylight covers showed sharply reduced strength over time.  This was consistent with peer-reviewed publications in the scientific literature as well as general information on the weathering of acrylic polymers. We conclude that more effort should be expended to reduce the consequences of this degradation process, especially by informing engineers of the extent of the problem.

PMMA is an excellent choice for applications that require transparent properties, such as skylights, automobile taillights, rigid contact lenses, and acrylic paints.  There are many varieties of this polymer in commercial use, and they vary in molecular weight, copolymer composition, stabilizer additives (especially ultraviolet stabilizers), and colorants.  However, they all consist mainly of a repeat unit with a methyl ester side group and a methyl group positioned along a saturated carbon-carbon chain.  Although this structure provides great stability in many adverse environments, the materials undergo slow degradation when exposed to heat, light, and moisture, all of which are common on rooftops in Florida.  At a high enough temperature, the polymer will “unzip” to form a monomer, and this reaction can be used to recover monomer in high yield from polymer.  The ceiling temperature (polymer in equilibrium with monomer) is relatively low at 220°C, and a very small amount of monomer is formed at slow rates at lower temperatures.  Light generally passes through this transparent polymer, but some ultraviolet (UV) light is absorbed and causes chemical changes such as cleavage of chemical bonds.  That cleavage leads to loss of side groups as well as main chain cleavage and is revealed by the decrease in molecular weight of thin films exposed to sunlight.  For thick sheets such as skylights, the molecular weight normally is reduced to a larger extent on the side facing the sun. Main chain cleavage forms radicals that can recombine and reverse the loss of molecular weight, but the presence of oxygen will allow a photooxidation reaction to occur that locks in the molecular weight loss.   Moisture can hydrolyze the ester bonds and release methanol.  This process is accelerated in the presence of heating or UV light or when acids or bases are present.

The net result of these changes is a slow progression of surface damage toward the center of the sheet, with more porosity on the side facing the sun.  Indeed, we have seen exactly this type of change on a cross section of a PMMA skylight that has been on a roof for over a decade (see Figure 1).

In terms of preventing falls through a skylight, the basic OSHA rules were designed for glass skylights.  Those structures were generally flat and brittle and thus required a “covering” to protect people on the roof.  The rules for the covering were designed largely to prevent falls through the glass underneath and basically consisted of “the ability to support a 200 pound weight applied perpendicularly to the covering at any point.”  For the 1/8-inch or ¼-inch sheets used to make skylights, this was no problem to achieve for new sheets of plastic.  This meant that any skylights (or covering) installed more than met the standard, and there are films of weights bouncing off new skylights that show an elastic bending deformation with full recovery of shape.  How do the mechanical properties of the plastic skylights compare to this when they have been sitting on a roof for over a decade?

We examined several batches of skylights that had been removed from a building’s roof for replacement after being on the roof for about 12 to 15 years.  They were translucent, and some had been painted to limit the amount of heat transferred to a building.  Three skylights of each type (painted and unpainted) were used, and five large pieces were cut from the sides and the center.  The center pieces were a bit thinner because of the way they are molded, but this was not a large difference and could be corrected for in calculating tensile strength in pounds per square inch (psi) of cross section. The ASTM D638M-91a method was followed for tensile testing.  Each section was cut into seven rectangular blanks (1 inch by 6.5 inches) with a band saw, and those sections were filed and sanded down to remove any surface cracks or obvious imperfections such as nicks.  Some of the blanks had significant visible damage upon microscopic examination and were discarded before testing. Final sanding was done with wet 400-grit sandpaper.  These sections were conditioned for at least 48 hours at 25°C and 52% relative humidity before being tested in an Instron tensile tester, using manual grips.  The gauge length was set at 3 inches, and the speed at 0.2 inches/minute.  Tensile strength and elongation at break were measured, and averages with standard deviations were calculated to compare with values for new material.  The results are shown in Table 1.

Figure 1: Cross Section of a PMMA Skylight. SEM photographs of fractured, sun-exposed skylight
(scale bars at 20 u for side images, and 2 mm for central image).

Table 1. Tensile Strength and Elongation at the Break of a Skylight

Where:
EB = Elongation at break, percent
EB(sd) = elongation at break, standard deviation
TS = mean tensile strength, psi
TS(sd) = tensile strength, standard deviation.

The mechanical testing showed a very much weakened material after exposure to the elements in Florida for about 12 to 15 years.  This is exactly what one would expect from the scientific literature but may not be appreciated by some workers on rooftops.  If one examines the area under a stress-strain curve, which is roughly the energy needed to cause a fracture, one can estimate that it takes less than a third of the value needed for a new unit.  The true reduction in strength is probably much greater since the tensile specimens had the rough surface removed by sanding.  This was where most of the damage occurred, and it also would be a site for stress concentration and fracture initiation.  It is likely that the impact strength also would be reduced sharply.  Many such skylights remain on homes and businesses in sunny climates and can be very unsafe.  One solution to the problem is to require safety screens.  In fact, OSHA does require such protection, but the regulation seems widely ignored, perhaps because the material can be safe when installed and does not become brittle until it ages.

This represents a common materials engineering consideration: Replacing a metal or glass part with a plastic part requires understanding of the new material and use conditions.  Often the new part is more fracture-resistant and cheaper to manufacture in a complex shape, but the properties may change unacceptably with time.  One only has to walk through a parking lot and look at automobile headlights to see how the plastics that have replaced glass no longer are as optically clear as they were when a car was new.

Acknowledgement:  The electron micrographs were taken by the Major Analytical Instrumentation Center at the University of Florida. Dr. Dow Whitney and Dr. Chris Sakezles helped with the mechanical property measurements.

For more information on Professor Batich and polymethylmethacrylate skylights, please read his interview on K Exchange.