Weather testing of polymers
Weather testing of polymers is the controlled polymer degradation and polymer coating degradation under lab or natural conditions.
Just like erosion of rocks, natural phenomena can cause degradation in polymer systems. The elements of most concern to polymers are ultraviolet radiation, moisture and humidity, high temperatures and temperature fluctuations. Polymers are used in everyday life, so it is important for scientists and polymer producers to understand durability and expected lifespan of polymer products. Paint, a common polymer coating, is used to change the colour, change the reflectance (gloss), as well as forming a protective coating. The structure of paint consists of pigments in a matrix of resin. A typical example is painted steel roofing and walling products, which are constantly exposed to harmful weathering conditions.
Typical result of polymer surface after weathering
Figure 1.1: Weathered and unweathered samples of painted steel. Note the colour change.
Figure 1.2: Diagram showing paint pigments in a resin matrix. It can be seen that weathering only removes the smaller pigments, resulting in the colour change
Figure 1.1 shows typical weathering results of a sample of painted steel; the paint on the steel is an example of a common polymer system. The sample on the left had been placed in an outdoor exposure rack, and weathered for a total of 6 years. It can be seen that the sample has a chalky appearance and has undergone a colour change in comparison to the unweathered sample on the right.
Colour is determined by light-reflecting chemical particles, pigments, in the paint. These particles can have very different physical sizes, as shown in the diagram in figure 1.2. In this example, the black pigments are the small black dots; red pigments are larger spheres, while the yellow pigments are acicular. This combination of pigments produces the original brown colour. The upper diagram has had no weathering, and the surface is still smooth and undamaged. The lower diagram shows the painted surface after weathering has occurred. The surface has eroded with significant loss of the black and red pigments from the surface layer. The pitted surface scatters light, therefore reducing the gloss and creating the chalky affect. The larger acicular yellow pigments are more difficult to remove, resulting in a colour change towards a more yellowy appearance. Weather testing was paramount in discovering this mechanism. Pigment composition has recently been modified to help minimise this effect.
Types of weather testing
There are 3 main testing techniques; Natural Weathering, Accelerated Natural Weathering and Artificial Weathering. Because natural weathering can be a slow process, each of the techniques is a tradeoff between realistic weathering results and the duration of testing before results are collated.
Natural weathering
Natural Weathering involves placing samples on inclined racks oriented at the sun. In Northern hemisphere these racks are at an angle of 45 degrees in a southerly direction. In Southern hemisphere these racks are at an angle of 45 degrees in a northerly direction. This angle ensures exposure to the full spectrum of solar radiation, from infrared to Ultra violet. Sites used for this type of testing are usually in tropical areas as high temperature, UV intensity and humidity are needed for maximum degradation. Florida, for example is the world standard as it possesses all three characteristics. Despite the harsh conditions, testing takes several years before significant results are achieved.
Accelerated natural weathering
To speed up the weathering process while still using natural weather conditions, accelerated natural testing can be applied. One method uses mirrors to amplify available UV radiation. A device known as a Fresnel-reflecting concentrator uses photo-receptor cells to maintain alignment with the sun and 10 mirrors to reflect sunlight onto the test specimens. With the latest technology for ultra-accelerated exposure testing it is possible to simulate 63 years of UV radiation exposure in a single year.[1]
Such devices, which are known by trade names Acuvex, Q-Trac, and Emma, are typically used in Arizona and other desert locations with a high percentage of sunlight and low relative humidity. The Arizona desert typically provides 180 kilo-Langley per year. These exposures can be used with water spray to simulate a more humid climate. In addition, water containing up to 5% sodium chloride can be sprayed to create the conditions for corrosion to occur.
It is typical for this to accelerate weathering by a factor 5, in comparison to weathering in Florida.
An Atrac in Townsville Australia, uses follow the sun technology in which the samples are rotated so that they always face the sun. In 17 months this produced the equivalent of 2 years of weathering.
A variety of environmental chambers are also used in conjunction with industry standards.
Artificial weathering
The weather testing process can be greatly accelerated through the use of specially designed weathering chambers. While this speeds up the time needed to get results, the conditions are not always representative of real world conditions. Most of the commercialized devices are using Gas-discharge lamp (e.g. xenon arc lamps), electric arc (carbon) or fluorescent lamps to simulate/accelerate the effect of sunlight. Xenon, mercury, metal halide or carbon arc lamps have to be used with a careful elimination of shorter wavelengths usually by adding a borosilicate filter. In Fluorescent lamps the short UV is converted into visible light or long wavelength UV with fluorescent coatings.
Fluorescent UV
Fluorescent UV Accelerated Weathering testing is a laboratory simulation of the damaging forces of weather for the purposes of predicting the relative durability of materials exposed to outdoor environments. Racks of samples are placed in the fluorescent UV chamber. Rain and dew systems are simulated by pressurized spray and condensation systems while damaging effects of sunlight are simulated by fluorescent UV lamps. The exposure temperature is automatically controlled. Cyclical weather conditions can also be simulated. Three types of fluorescent lamps are commonly used for UV testing. Two of these are of the type UVB (medium wavelength UV), while the third is UVA (longer wavelength UV similar to black light). All these lamps produce mostly UV as opposed to visible or infrared light. The lamp used, and therefore the wavelength of UV light produced will affect how realistic the final degradation results will be. In reality, natural sunlight contains radiation from many areas of the spectrum. This includes both UVA and UVB, however the UVB radiation is at the lowest end of natural light and is less predominant than UVA. Since it has a shorter wavelength, it also has a higher energy. This makes UVB more damaging not only because it increase chemical reaction kinetics, but also because it can initiate chemical reactions to occur which would not normally be possible under natural condition. For this reason, testing using only UVB lamps have been shown to have poor correlation relative to natural weather testing of the same samples.
SEPAP
The SEPAP 12-24 fr:Photovieillissement accéléré en SEPAP has been designed, in the late seventies, by the scientists of University Blaise Pascal , specialists of molecular photochemistry, to provoke in accelerated controlled conditions the same chemical evolutions as those occurring at long term under the permanent physicochemical stresses of the environment, i.e. UV - heat – atmospheric oxygen and water as an aggravating agent.
That ageing chamber based on fundamental concepts differs widely from the ageing units based on the simulation of environmental stresses in non-accelerated conditions like in some xenon based instruments.
In SEPAP 12-24 :
- the samples are rotating to insure an homogenous exposure ;
- the incident light is supplied by four medium-pressure Mercury-vapor lamps filtered by the borosilicate envelops of the lamps ; the incident light is not containing any radiations whose wavelength would be shorter than 300 nm. Although the spectral distribution is not simulating the solar light, the vibrational relaxations which occur from each excited states insure the absence of any wavelength effect under the mercury arc excitation, the light spectral distribution influencing only the rate of the photoreactions. That concept has been largely checked in the last 30 years ;
- since the photochemical activation (due to UV) and the thermal excitation cannot be deconjugated, it is essential to control the temperature of the samples surfaces directly exposed to light. A patented device insures that requirement in SEPAP 12-24 ;
- the water present in the polymeric blend exposed in SEPAP 12-24 is formed through the decomposition of the primary hydroperoxides ; no external water is brought on the exposed sample. The water effect on artificial ageing is handled either in SEPAP 12-24 H (when conjugated with the effects of UV, heat and oxygen), or through post-photochemical immersions in neutral water (in absence of conjugated effects') .
The durability control of polymeric formulations through the SEPAP 12-24 testing is currently required by some French and European standards and by many industrial companies (see list on )
The (French) National Center for Photoprotection Assessment (CNEP) is currently using SEPAP for industrial applications and is more generally involved in polymer and polymer failures analysis (National Centre for the Evaluation of Photoprotection) for plastic industries.
See also
References
- ASTM STANDARDS B117: Standard Method of Salt Spray (fog) Testing,
- ASTM D1014 (45° North): Test method for Conducting Exterior Exposure Tests of Paints on Steel
- ASTM G90: Standard Practice for Performing Accelerated Outdoor Weathering of Nonmetallic Materials Using Concentrated Natural Sunlight
- ASTM G154: Standard Practice for Operating Fluorescent Light Apparatus for UV Exposure of Non-metallic Materials
- Q.U.V Accelerated Weathering Tester operation manual, Q-Lab Corporation, Cleveland, OH, USA, www.q-lab.com.
- UV Weathering and Related Test Methods, Cabot corporation, www.cabot-corp.com
- G.C. Eastwood, A. Ledwith, S. Russo, P. Sigwalt, vol 6 ; "Polymer Reactions, vol 6" in Comprehensive Polymer Science, Pergamon press, 1989, ISBN 0-08-036210-9
- Olivier Haillant, "Polymer weathering: a mix of empiricism and science", Material Testing Product and Technology News, 2006, 36 (76), 3-12
- Jacques Lemaire,"Predicting polymer durability" in Chemtech, October 1996, 42- 47.