Stress cracking and aging of the hottest thermopla

Stress cracking and aging of thermoplastic powder coatings (2)

aging

another important indicator that affects the service life of polyethylene powder coatings is aging resistance. In the absence of stress cracking, prolonging the aging of polyethylene powder coatings is to prolong the service life of polyethylene powder coatings

the light radiated by the sun to the outer atmosphere of the earth is a continuous spectrum, and its wavelength is between 0.7 ~ 3000Nm. However, the whole solar radiation cannot completely irradiate the earth's surface. The solar spectrum with a wavelength of more than 1400nm will be absorbed by the water vapor scattered on the earth's surface, that is, only the infrared radiation with a short wavelength can irradiate the earth's surface. As for the ultraviolet radiation with a wavelength of less than 175 nm, it will be absorbed by oxygen, and the radiation with a wavelength of 175 nm and 290 nm will be absorbed by ozone. Only the solar radiation with the wavelength between 290 nm and 1400 nm irradiates the earth's surface, which leads to the aging of polyethylene powder coating. The radiation in this band can be subdivided into:

the effect of the percentage of wavelength (nm) in the total radiation on aging

infrared 780~140042~60% heat

visible light 380~78039~53% heat and photochemical reaction

ultraviolet 290~3801~5% is mainly photochemical reaction

although UV line only accounts for 1~5% of the total radiation, it plays a key role in the aging of polyethylene powder coating. UV can be divided into UV-A (315~400nm) and UV-B (290~315nm). Because UV with a wavelength between 290~315nm is blocked by the highest energy potential, it is the main factor leading to the aging of plastics, while the maximum aging wavelength of polyethylene powder coating is 300nm

activity spectrum of different polymers

maximum value of polymer activity spectrum, Nm

polyacrylate 290325

polyethylene 300

polypropylene 310370

polyvinyl chloride 320

polyester 325

※ maximum active spectrum: the wavelength that leads to the maximum deterioration of the polymer

the aging of polyethylene powder coating is mainly reflected in the following aspects:

(I) the bright color is also an important factor why polyethylene powder coating is favored, Colorful colors can play an excellent decorative effect on the guardrail. However, due to the fact that polyethylene powder coating manufacturers choose different pigments and additives, and excessively pursue profits regardless of quality, the guardrail changes color in a short time during the use process, which seriously affects the beauty of the guardrail. It is the guardrail manufacturers that have been deceived, resulting in serious consequences. In order to maintain the bright color for a long time, it is very important to strictly select pigments. Another working state is "online working state", which can be operated by computer. The weatherability grade of pigments should reach or approach the top level. Some pigments must undergo a series of special treatments to meet the requirements of high weatherability. The best way to test the discoloration of polyethylene powder coating is to artificially accelerate aging and observe the change of color and the size of color difference

(II) the decline of plastic properties. Other kinds of aging, such as pulverization, loss of light, cracking, peeling and other phenomena, are mainly the decline of plastic properties. When polyethylene powder coating is exposed to sunlight, its light absorbing groups are excited to generate free radicals, which is related to the energy of ultraviolet light, the structure of polyethylene and some impurities in it. Because of the existence of oxygen, it is oxidized at the same time, and, The thermal oxidation process may be superimposed with the photooxidation process (the total process is usually regarded as the photooxidation process). Under the action of solar ultraviolet light, with the influence of oxygen, ozone, NO2, H2S and water in the air, the plastic properties of polyethylene 3.175mm steel globule powder coating slowly decline, mainly reflected in the decline of elongation at break and tensile strength. With the passage of time, the phenomena of pulverization, loss of gloss, cracking, peeling and so on slowly appear. A good polyethylene powder coating is very strict in the selection of raw materials. At the same time, adding corresponding series of additives can greatly delay the aging of the coating. The aging resistance of polyethylene powder coatings from different manufacturers varies greatly. Guardrail manufacturers should carefully screen when selecting materials. Powder coating manufacturers should at least have the artificial accelerated aging test of authoritative departments. When choosing polyethylene powder coatings, guardrail manufacturers, Artificial accelerated aging can be compared, and then the retention of elongation at break and tensile strength can be tested. Under the same conditions, the higher the retention of elongation at break and tensile strength, the better