The article "Analysis of Research Progress on Aging Mechanism of Polymer Coatings" written by Zhao Yuan and Li Xin from Guangzhou Synthetic Materials Research Institute Co., Ltd. is mainly aimed at the aging phenomenon of polymer coatings under the influence of various factors under spontaneous combustion conditions , summarizes and analyzes several main environmental factors that cause the aging of polymer coatings, including solar radiation, temperature, humidity, oxygen and ozone, and pollutants. Moreover, the aging mechanism of several typical polymer coatings such as polyurethane coating, acrylic resin coating, epoxy resin coating, polycarbonate coating and alkyd coating are analyzed.
The following extracts part of the article and introduces several common polymer coatings:
1. Polyurethane coating
Polyurethane resin has the characteristics of foamability, high elasticity, abrasion resistance, light resistance, solvent resistance, etc., and is widely used in the field of coatings. The structural formula of polyurethane is -R'OOC-NH-R-NH-COO-, and its degradation under natural light conditions is mainly due to the cleavage of carbamate bonds. There are two main forms of cleavage: one is the cleavage of NC bonds, Generate amino radicals and alkyl radicals, and release CO 2 , the reaction equation is shown in formula (1). Another form is that the CO bond is broken to generate carbamoyl radicals and alkoxyl radicals, and the carbamoyl radicals are decomposed into amino radicals and CO2. The reaction equation is shown in formula (2).
2. Acrylic resin coating
In the presence of oxygen, the acrylic resin coating can undergo photooxidation reaction under sunlight (ultraviolet rays), causing the -CC- bond in the polymer chain to break, destroying the internal structure of the coating film, and causing aging. The process is as follows .
start react:
X - X → 2X ·
X + RH (polymer) → XH + R
Generating reaction:
R + O 2 → RO 2 ·
RO 2 + RH → RO 2 H + R
R or ROOH → main chain breaks
The basic form of main chain scission is:
Additionally, moisture and pigments can cause acrylic coatings to age. Moisture will participate in the breakage of the secondary bond of the hydrogen bond and the hydrolysis of the primary bond of the ester bond, while some pigments such as titanium dioxide will undergo oxidation and catalytic reactions, which will promote the aging of the coating film. Titanium dioxide absorbs light of a certain wavelength (rutile type 415nm, anatase type 385nm), and converts water and oxygen into hydroxyl (OH) and hydrogen peroxide (OOH). The two free radicals have high activity and will The internal structure of the coating film is destroyed and aging is accelerated.
3. Epoxy resin coating
Epoxy resin is a kind of composite material resin matrix with excellent comprehensive performance, which is widely used in the aerospace industry, but the molecular structure of ordinary epoxy resin contains a large number of polar groups such as hydroxyl groups, and the moisture absorption rate is high, so that The mechanical properties of its composites decreased significantly under hot and humid conditions.
Experiments show that after the matrix resin absorbs water, some water molecules occupy the free volume, resulting in a decrease in the size of the free volume, and the other part of the water molecules are distributed in the resin matrix, playing a plasticizing role, the number of holes formed increases, and the number of free volumes in the system increases. increases, while causing more microcracks to form in the resin matrix, resulting in more interface formation. Especially in the case of high temperature, the mechanical properties of the polymer matrix are reduced, and the degree of reduction is related to the amount of water adsorption. When a sample is moved from a humid high-temperature environment to cold water, the adsorption capacity increases and the mechanical properties further decrease.
4. Polycarbonate coating
Jie Ganxin et al. studied the aging behavior of polycarbonate under natural conditions through the infrared characteristic index. They found that under natural exposure conditions, polycarbonate would undergo Fluoris rearrangement reaction and photooxidation of gem-dimethyl Both reactions will lead to changes in the color difference of polycarbonate. Among them, under hot and humid climate conditions, the color change is mainly caused by the Fluoris rearrangement reaction, while under dry and hot climate conditions, the color change is caused by the photo-oxidation of gem-dimethyl The reaction leads, but the dominant reaction mechanism for the decrease of tensile strength is the Fluoris rearrangement reaction.
5. Alkyd coating
Under the action of sunlight (ultraviolet rays), the oxidation ability of the alkyd resin coating is closely related to the ability to absorb hydrogen atoms (H in RH) connected with the double bond to the methyl group. Therefore, Norrishtype II breakage is easy to occur: R 2 - CR 2 - CR 2 - CO - R' - → R 2 C = CR 2 + HR 2 C -CO - R'. During the aging process, the peroxy group decomposes into small molecular products such as acetaldehyde and ketone by recombination and fragmentation.
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