Effects of monomer structures on the foaming processes and thermal properties of polyimide foams

Improved synthesis and properties of BTDA-TDI/MDI ternary copolyimides via microwave-assisted polycondensation

Polyimides are usually synthesized over a long period of time, and the process is complex. In order to shorten the synthesis time and simplify the process, we developed an improved synthesis method, namely, microwave-assisted polycondensation. The 3,3′-4,4′-benzophenone tetracarboxylic dianhydride (BTDA)–2,4-tolylene diisocyanate (TDI)/1,1′-methylenebis(4-isocyanatobenzene) (MDI) ternary copolyimides prepared in this work have been synthesized using microwave heating at rather short reaction times (15 min), without the need for either the once-off addition of TDI/MDI or the drop-wise addition of that mixture. The structure of the desired ternary copolyimide was confirmed by proton nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy. Its glass-transition temperature is above 400°C, and the temperature for 5% thermal loss under nitrogen atmosphere is 508.8°C. The tensile strength is 141 MPa, which shows that the microwave-assisted copolyimide has excellent heat resistance and mechanical properties.

Improved mechanical, thermal and flame resistant properties of flexible isocyanate-based polyimide foams by graphite incorporation

In order to improve the mechanical, thermal and flame-resistant properties of polyimide foams (PIFs), in the present study, flexible polyimide (PI)/graphite composite foams were prepared with dianhydride and isocyanate as the starting materials and graphite as the filler. The experimental results showed that most cells of PIFs possessed an open cell structure, and the open cell content decreased by graphite incorporation. While with the increase in graphite, the distribution of cellular size became uneven and the size distribution became broad. The compressive strength increased initially and then decreased and reached the maximum value of 26.4 kPa when the graphite content was 1.98 wt%, but all the composite foams exhibited higher strength than the neat PIF. In addition, the limiting oxygen index increased from 31% to 34.8% with the increase in graphite from 0 wt% to 3.25 wt%. The peak heat release rate of composite foams was 2.3% to 24.7% lower than the neat PIF and reached a minimum value of 42.36 kW m−2 with the graphite content of 1.98 wt%. Considering the above analysis, it is feasible to improve mechanical properties, thermal stability and flame-resistant properties of PIFs by graphite addition.

Synthesis and properties of polyimide foams containing benzimidazole units

A series of novel polyimide foams containing benzimidazole units were prepared from polyester ammonium salt (PEAS) precursor powders, which were synthesized by co-polymerization of BTDA with two diamines, BIA and ODA, in various molar ratios.

Thermal properties of the polyimide foam prepared from aromatic dianhydride and isocyanate

A new type of polyimide foam (PIF) was prepared successfully based on a one-pot process by the reaction of a first solution and a second solution. The first solution comprised pyromellitic dianhydride (PMDA), N, N-dimethyl formamide (DMF), methanol, polyethylene glycol and water. The second solution contained polyaryl polymethylene isocyanate (PAPI). The thermal properties of the polyimide foam were thoroughly investigated by thermogravimetry–Fourier transform infrared spectroscopy (TG–FTIR) and high-resolution pyrolysis gas chromatography–mass spectroscopy (PyGC–MS) at various temperatures. The results of TG–FTIR indicated that polyimide foam was thermally stable and experienced a weight loss of less than 2% before the scanning temperature reached 280.7 °C. It showed a two-stage degradation process during the whole decomposition. The major decomposition products were carbon dioxide (CO2), carbonic oxide (CO) and methyl formate. The results of PyGC–MS indicated that no pyrolysis products were indentified when the test temperature was 250 °C. Certain pyrolyzates such as oxygen, carbon dioxide and methyl formate was identified as the pyrolysis temperature increased. According to the degradation behaviors and the structure and relative content of pyrolysis products, a proposed pyrolysis mechanism of polyimide is presented in this article.