Comparison of Toughening Effects of Various Additives on Phenolic Foam

Phenolic foams (PFs) are considered excellent insulation materials owing to their flame retardancy and low thermal conductivity. However, their mechanical properties often lag behind those of other polymeric insulation materials. To fully exploit their properties and broaden end-use applications, the mechanical properties of PFs must be enhanced. In this study, various modifications were introduced into the PF matrix with the aim of enhancing its properties. The toughening effects of four additives: urea (U), nano clay (NC), sodium silicate (SS), and lignin (Li) were studied and compared. Changes that occurred in the density, cell morphology, thermal conductivity, compressive strength, and thermal stability after the addition of these fillers were analyzed. Both compressive strength and thermal stability increased with the inclusion of all additives, and the SS-toughened foam shows the biggest improvement. Li and NC addition resulted in a 34% improvement in compressive strength, while SS and U addition displayed increases of 52 and 11%, respectively. SS-toughened PF shows greater improvements in all of the important properties compared with those of the other toughened foams. Several PFs were prepared by changing the SS concentration to optimize the formulation, which yielded improvements in properties. The effects of SS concentration on density, thermal conductivity, and compressive strength were studied. The formulation with 0.37% sodium silicate concentration (PF-SS1) shows a 15% improvement in mechanical properties.

Preparation of a Ceramifiable Phenolic Foam and Its Ceramization Behavior

Ceramifiable phenolic foam (GC-PF) with a low ceramization temperature has been prepared by incorporation of low melting point glass frits (LMG) containing B₂O₃ and Na₂O as main components into a phenolic resin matrix. Fourier transform infrared spectrometry, X-ray diffractometry, and scanning electron microscopy were used for assessment of the structure, phase composition, and morphology of GC-PF before and after combustion analysis, respectively. A glassy ceramic protective layer is formed when GC-PF is exposed to flame or a high temperature environment. The presence of LMG not only reduces the level of defects in the phenolic foam cell wall (gas escape pore), but also promotes the generation of a glassy ceramic protective layer that could inhibit heat feedback from the combustion zone and reduce the rate of formation of volatile fuel fragments. Thermogravimetric analysis and differential scanning calorimetry were used to establish that GC-PF exhibits excellent thermal stability. Limiting oxygen index (LOI) determination suggests that GC-PF displays good flame retardancy. The LOI of GC-PF was as high as 45.6%, and the char residue at 900 °C was six times greater than that for ordinary phenolic foam (O-PF). The area of the raw material matrix of GC-PF after combustion for 60 s was about 1.7 times larger than that for O-PF. A possible mode of formation of glassy ceramics has been proposed.