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The Principle of Reciprocity – Its Value as a Tool in Weathering

Atlas Weathering Blog Reciprocity Cover UAWS
Atlas Ultra Accelerated Weathering System - UAWS

In our niche weathering industry, there are some who believe that reciprocity is a misguided notion. Well, the principle of reciprocity is a valid scientific premise, that when observed, allows researchers to reduce test times by increasing the irradiance of the accelerated weathering test method used. If one wishes to reduce their test time, testing if reciprocity is observed must be performed before making predictions about test duration at higher irradiances.

The principle of reciprocity is described in an article by Hugh Landes from the University of Virginia. He defines reciprocity as “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 his White Paper titled “High Irradiance Weathering Testing”, Allen Zielnik clarifies the use of reciprocity for use in weathering: “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.” Reciprocity is observed if the same radiant exposure (H) causes the same photochemical response (property change), no matter the irradiance (E) or time of exposure (t).

Atlas Weathering Blog Reciprocity Formula

To test for reciprocity, exposures at different irradiance levels and test times must be executed, all accumulating the same radiant exposure. All other test parameters such as temperature and humidity must remain constant from test to test. ISO Technical Specification ISO/TS 19022 – Plastics – Controlled acceleration of laboratory weathering by increased irradiance, which is summarized below describes the procedure to validate reciprocity.

Atlas Weathering Blog Reciprocity FOW Reciprocity Slide

Based on photochemical principles, during exposure, the absorption of energy leads to the formation of molecules in their excited state, which can initiate the degradation process. In natural solar exposures, the concentration of these molecules in their excited states is relatively low, so their influence on other molecules is minimal. For this reason, the principle of reciprocity may be valid for photochemical (weathering) reactions. However, what can limit the validity of the principle of reciprocity are secondary reactions, following these initial processes. This, for example, can be subsequent oxidation processes, which are limited by oxygen diffusion. Oxygen diffusion is not influenced by increased irradiance. Even though the principle of reciprocity applies for the photochemical initiation, it might not be valid for the complete weathering degradation process.

More Information

Examples of research highlighting increasing irradiance by validating reciprocity can be found at:

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

Atlas offers several recorded seminars on technical and standards-related topics which touch upon reciprocity, for example:

• Fundamentals of Weathering: Part 5 – Acceleration
Accelerated Outdoor Weathering

For more information and a deeper dive into various aspects and applications for weathering and lightfastness testing, have a look into our online library.

Click here for recorded online seminars or upcoming live educational classes.