By Sahely Mukerji

 Christopher C. White, researchchemist, at the Materials & Construction Research of the Engineering Lab at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., made a presentation: Measuring and Predicting the Change in Polymer Used in Protective Glazing Films. He was speaking at the Protective Glazing Council International’s (PGCI) Annual symposium at NIST on October 26.

White talked about the tornado that ripped through Joplin, Mo., in May, and said that it blew away all the glass in St. John’s Hospital and killed five people in the hospital. The level 2 trauma center had to shut down, 182 patients had to be transported, because “glass shards exploded from every window and nearly every patient was splashed or covered with blood from all the glass.” Protective glazing would have been the solution to flying glass, he said. The unique properties of polymeric materials – such as elasticity, toughness and clarity — could have prevented glass from flying. “We wanted to investigate how the unique properties of polymers used in window film applications are changed by controlled indoor and monitored outdoor exposure,” he said. “So, a request was sent out by PGCI to its members, and NIST received many samples, out of which two rolls of clear film were randomly selected for testing.”

The test was done on mechanical properties, chemical properties and physical appearance of the films, White said. “The mechanical properties test involved tensile testing; the chemical properties measured attenuated total reflectance fourier transform infrared spectroscopy [ATR-FTIR], and the physical appearances measured ultraviolet-visible [UV-Vis] spectroscopy and yellowness index,” he said.

The test was done in controlled indoor exposure and monitored outdoor exposure. In the indoor exposure “we precisely controlled three aspects: temperature, humidity and UV,” White said. “For the outdoor exposure, we put the films on the roof of the Engineering Lab in a chamber covered with borofloat glass. All the sides were enclosed with vortex materials that allow water vapor, but prevent dust from entering. There was a weather station nearby that recorded 27 elements of the weather in one minute increments.”

The first thing the researchers learned was that the films absorb almost 100 percent of UV radiation, White said. “This means that a significant amount of energy was being absorbed by the film,” he said. “Secondly, the films were oxidized when exposed to UV; heat and humidity were secondary. They were oxidized by the same mechanism indoor and outdoor.”

The ATR-FTIR showed that carboxyl end groups are formed at the surface layers after a short period of irradiation and reached a plateau region, or saturation, in about ten days, White said. “A longer exposure time led to a reduction in the elongation at complete failure, and strain hardening moduli,” he said. “There were no apparent changes in Young’s moduli – below the elastic limit – suggesting that the elastic and plastic deformation did not act in series; and the environments affected only the mechanical properties in the post-yield region, i.e., between the elastic limit and the ultimate failure.”

Degradation of the films was dominated by UV radiation, White said. “The effect of temperature and moisture were minimal. Films exposed to high temperature showed no change in chemical and mechanical properties even after 90 days,” he said. “None or little yellowing was seen in the specimens after prolonged exposure, although chemical and mechanical properties were degraded significantly. UV is required to initiate the degradation because it has sufficient energy to break polymer chemical bonds, which leads to secondary reactions promoted by weather factors.”

Since degradation is such a strong function of UV, calculate UV incident on any window using software, and the total UV incident on the window as a function of time, White said. “Our work can relate that total UV to change in non-linear mechanical properties, and it can be verified with field FTIR measurement.”

In conclusion, “we can say that the unique properties of polymeric materials used in window film applications change with exposure, and the changes can be predicted,” White said.