CHANGES IN POLYMER PROPERTIES DUE TO SOLAR LIGHT (UV LIGHT)
While plastic is used outdoors, its appearance and physical properties change due to photo oxidation/degradation. The UV light stabilizer used here dramatically extends the shelf life by controlling and preventing photodecomposition.
The various test items below are used to check the performance of light stabilizers and extension of their useful life when used externally on plastics.
Exterior
During external use, discoloration occurs in most aromatic polymers (PS, PC, PBT, TDI PU, polyacrylates, etc.) when exposed to UV. By measuring this color change, the degree of photolysis is quantified and photodecomposition is confirmed. Discoloration also occurs in many aliphatic polymers (PVC, Acrylic, Cellulose) and in polymers containing dyes and pigments. Other changes in appearance may also result in color fading.
Using a colorimeter, you can reduce experimental errors by measuring the initial YI (Yellow Index), L (Lightness), a (Red to Green), and b (Yellow to Green) values and measuring multiple samples. Other methods measure % Haze (opacity) and gloss. The % Haze value can be obtained by measuring the light transmittance, while the gloss can be obtained by measuring the amount of light reflected from various angles. A gloss level of 60 degrees is mainly used for plastics, and a gloss level of 20 degrees is mainly used for coatings.
In an accelerated weathering device (Weather-O Meter) or an actual weathering test, the difference is first confirmed with the naked eye after a certain period of time has passed, and weathering resistance can be evaluated by comparing the initial value of the sample and the color change value of the sample after a certain period of time. Delta YI mainly measures Delta YI (yellow discoloration) in white samples, and Delta E (Total Color Change, ) is used to check discoloration in other colored samples or when the YI value cannot represent the characteristics of discoloration. do. Usually, when the Delta E value exceeds 3, the difference in color can be clearly distinguished with the naked eye, and this value is used as a standard for judging weather resistance.
% Haze appears to cause whitening and lightening of color due to micro-cracks caused by oxidation of the product surface, but this has nothing to do with the weather resistance of the pigment contained in the polymer. This incontinence/whitening phenomenon lowers the degree of reflection of the surface, thereby lowering the gloss value of 60 degrees. In transparent samples, transparency decreases and opacity (% Haze) increases due to incontinence/whitening. Other changes in appearance may include migration of polymer-containing materials to the surface. Transfer of internal materials to the surface may occur immediately after completion of the manufacturing process or after a period of time, which may cause color changes and reduced gloss values.
(L, a, b Scale)
Properties
Changes in the physical properties of polymers occur along with changes in appearance, but the deterioration of the physical properties occurs slowly. This is because physical properties relate to the entire composition of a material, not just the surface. Among the many unique physical properties that polymers have, tensile properties and impact properties are important physical properties that must be maintained throughout the polymer's lifespan. Analysis of tensile strength is generally obtained by measuring 5-7 samples at Instron after a test period and calculating the average value. The % residual elongation value is also obtained. In order to measure various physical properties, the shape of the sample is tested as a tensile bar, film, fiber or filament depending on the end use of the polymer. In the case of polyethylene, Rotomolding grade of LD, UV 8, exposure to UV light for 8000 hours in Xenon Arc WOM. It must maintain its appearance and physical properties after use, which means it can be used for more than 8 years in actual climate conditions and is expected to maintain its physical properties and appearance.
