Discussion on the Heat Resistance of Synthetic Resin Adhesive

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Heat-resistant polymers with high-temperature aromatic heterocycles in their molecular structure, which can maintain a certain service life at temperatures above 250°C in the air or inert gas, are referred to as heat-resistant polymers. Their main characteristic is high thermal stability, making them suitable for manufacturing composite materials, adhesives, fibers, and films. Currently, industrialized varieties include polyimide (PI), organosilicon, polybenzimidazole (PBI), and polyphenylene sulfide (PPS).

The aforementioned high polymers exhibit excellent heat resistance properties. However, there are still factors such as complex processing, high-temperature curing requirements, limited practicality, and high cost that restrict their widespread usage, making them applicable only in specific fields with special requirements. When exposed to heat, heat-resistant high polymers undergo two types of changes: chemical changes and physical changes. Chemical changes involve degradation, cross-linking, and oxidation reactions in the presence of oxygen as the temperature increases. Physical changes include softening or melting.

Methods for evaluating the temperature resistance of heat-resistant adhesives include thermal gravimetric analysis (TGA), differential thermal analysis (DTA), torsion braiding analysis (TBA), differential thermogravimetric analysis (DTG), and mass spectrometry thermal analysis. Among these methods, TGA is the most commonly used. Testing can be conducted in air, inert gas, or vacuum. Testing in air measures the polymer's resistance to oxidation at high temperatures while testing in vacuum or inert gas determines the decomposition temperature of the polymer itself under high temperatures.

Isothermal thermal gravimetric analysis (ITGA) is the most important method for determining the heat resistance and thermal stability of heat-resistant high polymers. It involves subjecting the polymer to thermal decomposition at a fixed temperature to record its weight changes. When evaluating heat-resistant adhesives, the shear strength is the primary factor tested, along with factors such as tensile strength, peel strength, compression strength, and cleavage strength, which also determine the adhesive's performance. These factors are influenced by the quality of the polymer, adhesive type, adherend, surface treatment, bonding conditions, aging, and testing conditions.

The temperature range and duration of heat-resistant adhesives are defined as follows:

- Usage for 1-5 years at 121-176°C or 20,000-40,000 hours at 204-232°C.

- Usage for 200-1,000 hours at 260-371°C.

- Usage for 24-200 hours at 371-427°C.

- Usage for 2-10 minutes at 538-816°C.

Adhesives that meet the above conditions can be referred to as heat-resistant adhesives. According to relevant literature, the shear strength of polybenzimidazole adhesive at 500°C for 0.5 hours is 10 MPa, and at 550°C for 10 minutes is 3 MPa. Organosilicon adhesive, cured at 270°C for 3 hours, exhibits shear strength of 9-10 MPa at room temperature and 3.7-4.2 MPa at 425°C. Epoxy-modified organosilicon (1:9) adhesive, after aging at 400°C for 5 hours, shows shear strength of 3.5 MPa.

In conclusion, heat-resistant adhesives play a crucial role in various industries that require materials to withstand high temperatures. Through advanced testing and evaluation methods, manufacturers can ensure the reliability and performance of heat-resistant adhesives in demanding applications. The continuous development of heat-resistant polymers and adhesives will drive progress in industries such as aerospace, automotive, electronics, and industrial manufacturing. These industries often encounter high-temperature environments where the stability and durability of adhesives are paramount.