Is the Principle of Reciprocity Valid?

Some people believe that reciprocity is a misguided notion in the relatively niche weathering industry. Despite this skepticism, the principle of reciprocity is a valid scientific premise that allows researchers to reduce testing times by increasing irradiance in the accelerated weathering test method in use.

Atlas Ultra Accelerated Weathering System – UAWS.

Atlas Ultra Accelerated Weathering System – UAWS. Image Credit: ATLAS Material Testing Technology

Observing reciprocity allows analysts to reduce their test time, but this must be done prior to making predictions about test duration at higher irradiances.

An article by Hugh Landes from the University of Virginia explores defines the principle of reciprocity:

“In the scientific sense, a theory that expresses various reciprocal relations for the behavior of some physical systems. Reciprocity applies to a physical system whose input and output can be interchanged without altering the response of the system to a given excitation.”

In a whitepaper titled “High Irradiance Weathering Testing”, Allen Zielnik expands on the role of reciprocity in weathering testing:

“The principle states that if reciprocity is observed, equivalent radiant exposures (radiant exposure = irradiance x time) will produce the same amount of photochemical change for different combinations of time and light intensity.”

“If reciprocity applies, a short exposure at higher irradiance would be exactly equivalent to that of a longer test at low irradiance, provided they delivered the same radiant exposure and produced identical results.”

In formulaic terms, reciprocity is observed in instances where an identical degree of radiant exposure (H) causes the same photochemical response (property change), irrespective of the irradiance (E) or time of exposure (t).

Hconstant  =  E  x  t

Exposures at different irradiance levels and test times must be executed to test for reciprocity, and these must all accumulate the same radiant exposure. Test parameters such as humidity and temperature should also remain constant between tests.

ISO Technical Specification ISO/TS 19022: ‘Plastics – Controlled acceleration of laboratory weathering by increased irradiance’ describes a procedure that can be used to validate reciprocity.

Photochemical principles state that the absorption of energy during exposure leads to the formation of molecules in their excited state. These excited molecules can initiate the degradation process.

Is the Principle of Reciprocity Valid?

Image Credit: ATLAS Material Testing Technology

Concentrations of molecules in their excited states are relatively low in natural solar exposure, so they exert minimal impact on other molecules. The principle of reciprocity may, therefore, be valid for photochemical (weathering) reactions.

The validity of the principle of reciprocity can be limited by secondary reactions, however. For example, oxidation processes occurring after the initial reaction are limited by oxygen diffusion, but oxygen diffusion is not affected by increased irradiance.

It should be noted that the principle of reciprocity applies for the photochemical initiation, but it may not be applicable to the entire weathering degradation process.

Resources

  1. Highly Predictive Accelerated Weathering of Engineering Thermoplastics (Pickett, 2005)
  2. Weatherable Copolyester Sheeting: A Study in the Use of Weathering Tools (Fagerburg, 2002)
  3. A New Approach to Characterizing Weathering Reciprocity (Scott, Hardcastle, 2004)
  4. Durability of Polymeric Glazing Materials for Solar Applications (Jorgensen et al, 2003)
  5. Reciprocity Law Experiments in Polymeric Photodegradation: A Critical Review (Martin et al, 2003)

This information has been sourced, reviewed and adapted from materials provided by Atlas Material Testing Technology LLC.

For more information on this source, please visit Atlas Material Testing Technology LLC.

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