Triple Threat: Light, Temperature, and Moisture

Most weathering damage is caused by three factors: light, high temperature, and moisture.

Any one of these factors may cause deterioration. Together, they often work synergistically to cause more damage than any one factor alone.

Light

Spectral sensitivity varies from material to material. For durable materials, like most coatings and plastics, short-wave UV is the cause of most polymer degradation. However, for less durable materials, such as some pigments and dyes, longer wave UV and even visible light can cause significant damage.

High Temperature

The destructive effects of light exposure are typically accelerated when temperature is increased. Although temperature does not affect the primary photochemical reaction, it does affect secondary reactions involving the by-products of the primary photon/electron collision. A laboratory weathering test must provide accurate control of temperature, and it usually should provide a means to elevate the temperature to produce acceleration.

Moisture

Dew, rain, and high humidity are the main causes of moisture damage. Our research show that objects stay wet outdoors for a
surprisingly long amount of time each day (8-12 hours daily, on average). Studies have shown that condensation in the form of dew is responsible for  most outdoor  wetness. Dew is more damaging than rain because it  remains  on the material  for a  long time, allowing significant moisture absorption. Of course, rain can  also be very  damaging  to some  materials. Rain  can  cause  thermal  shock  , a  phenomenon  that occurs , for example, when  the  heat  that builds up in an automobile over  the  course  of  a  hot summer day is rapidly dissipated by a sudden  shower. Mechanical erosion  caused  by the scrubbing action of rain can also degrade materials such as wood coatings because it wears away the surface, continually exposing fresh material to the damaging  effects of sunlight. For  indoor  materials , the major  effect  of  humidity is often  the physical stress  caused   by   the material   trying   to  maintain   moisture  equilibrium  with  its surroundings. The greater  the  range  of humidity  the  material  is exposed to, the greater the overall stress. Although  indoor products, such as textiles and inks, may only be exposed to moisture in the form of humidity, it can also be an important factor in the degradation of outdoor materials. Outdoors, the  ambient  relative  humidity  (RH)  will  affect the  speed  at  which a wet material dries. The UV-3 and the XTC chamber each reproduce light temperature, and moisture in different ways.