Foaming process kinetics
Table 3 shows foaming process of PU samples obtained from CG and CG/PEG based liquefaction. Foaming characterizations are typically followed by the cream time, rise time, and tack-free time . At the beginning of the foaming process, the color of mixture changes because of gas bubbles formation introduced cream time. The rise time is the time required for foam to reach to its maximum height. At the tack free time the outer surface of the foam loses its stickiness. The polyol viscosity decrease let to the foam mobility and kinetic rate increase. So, lower viscosity polyols had the longest foaming times. Foam 1 (bio polyols of CG based liquefaction) having polyols with the lowest viscosity, showed the shortest cream and tack free time. On the other hand, adding PEG to CG in liquefaction process declined foaming reaction times through the decreasing polyol viscosity caused more foaming efficiency. Therefore for foams derived from liquefaction by binary solvent, the foam prepared from bio polyols of higher ratio of PEG to CG showed lower reaction times. It could be ascribed to higher reactivity of hydroxyl groups of PEG than that of synthesized bio polyols.
Density and compressive strength of PU foams
With similar formulation of foaming, synthesized foams had different densities by various polyols (Fig. 1). It was observed that adding PEG increased liquefaction efficiency subsequently more bio polyol production. With the increasing bio polyol content, the foams density increased. This may be attributed to the liquefied biomass which acts as crosslinking agent having hydroxyl groups rather than as a chain extender in this polymerization . So, the foams produced from bio polyols with higher weight ratio of PEG to CG in liquefaction had higher density than foams prepared from liquefied sawdust by CG. On the other hand, bio polyols of foam 2 because of the lower reactivity in comparison to other synthesized polyols, partly act as adding weight to the foam and less participate in foaming process. A statistical analysis (ANOVA) showed that the effect of the type of polyol on density of synthesized PU foams at confidence level of 95% was significant.
The compressive strength of the PU foams was obtained from 200 to 311 kPa. The mechanical properties of PU foams depended on several parameters such as density, cross-linking density, cell geometry and blowing efficiency [13, 14]. The variation in compressive strength with foams density in Fig. 2 was illustrated. The minimum compressive strength was demonstrated at foam 1 with the lowest density. Foams density increase improved compressive strength. So foams obtained from CG/PEG based liquefaction with the higher density had more compressive strength. The similar results reported in previous studies [1, 4]. Besides density, the higher compressive strength of PU foams obtained from polyols of binary solvent based liquefaction could be attributed to higher reactivity of the produced polyols. Hu et al.  showed that in addition to density, the compressive strength of PU foams might be affected by factors such as biomass residues or the chemical structures of bio polyols. Having the highest density, foam 2 showed inconsistent behavior. It may be to the less reactive of some of hydroxyl groups of bio polyol of foam 2 that did not participate in forming urethane linkages. So, polyols without the increasing cross-linking density and subsequently improving the foams compressive properties, aggregated as adding weight to the foam. Statistical analysis demonstrated there was significant difference between the compressive strength with the density of foams (at confidence level of 95).
Water absorption of PU foams
Figure 3 shows the effect of density on the water absorption of the samples in volume percentage after soaking in water for 24 h. According to the figure, with the decreasing density the water absorption of foams increased whereas the highest water absorption was obtained for foam 1 with the lowest density. A statistical analysis (ANOVA) suggested that the effect of density on water absorption of the foam was significant in the confidence level of 98%. The cell structure and closed cell content of foams were the other effective factors on water absorption . Foam 2 having higher density and larger cell size had similar water absorption to foam 3 having lower density and smaller cell size. Also Duncan’s grouping put them in one category. This phenomenon was probably stated since, unlike the larger cell size of foam 2, most of the foam cells were closed. Meanwhile, foam 4 having smaller cell size and higher closed-cell percentage, demonstrated lower water absorption than the other foams.
Morphology of PU foams
Figure 4 shows scanning electron micrographs (SEM) images of polyurethane foams prepared from bio polyols of liquefied sawdust. Depending on the type of used polyol in foaming, the cell structure, cell size and closed or open cell content were varied in the synthesized foams. The average cell size was calculated from the SEM photographs (Fig. 5). The cell size of the PU foams decreased from 430 μm of foam 4 to 370 μm of foam 2 by adding PEG to CG in liquefaction process. So depending on foaming process and foaming materials the significant reduction of cell size was observed. Also the increasing ratio of PEG to CG in liquefaction decreased the cell size of foams. The results demonstrated that the higher ratio of PEG to CG as liquefaction contributed the smaller cell size of 170 μm of foam 4.
The foam 1 having lower density showed high percentage of open cells (Fig. 6). The open cell content of PU foam decreased from 89.5% of foam 1 to around 64.3% of foam 2 because of the adding of PEG to CG as liquefaction and the bio polyols formation thereby the varying of foaming material properties. With the increasing ratio of PEG to CG as liquefaction, open cell content of foam 4 decreased more to 10.3%. The addition of chemical solvent as liquefaction process improved foams cell structure and qualify. So foam 4 compared to other foams had more uniform and regular cell structure, higher the number of cells with smaller diameter. In this relation, Xu et al.  indicated that with an increasing usage of PEG to polyol produced from sawdust liquefaction by glycerol, the cell structure of the foams became more regular.
FT-IR spectra of PU foams
The FTIR spectra of the PU foam samples produced from biopolyols are shown in Fig. 7. Spectra for all the foams formulation did not show significant differences among them. In all samples the formation of urethane linkage was confirmed by FTIR and showed approximately similar signals. The absorption band at 3320 cm−1 is related to N-H groups which are making hydrogen bonds stretching . As can be seen, more band width at foam 2 indicated the presence of the more free OH groups compared to the other foams. With regard to constant ratio of polyol to isocyanate in the foams formulation, this phenomenon can be attributed to less reactivity of polyols and isocyanates during foaming of foam 2, resulting in less activity hydroxyl groups left in the system. The OC=O vibration at 1730 cm−1and the CO-NH vibration at 1600 cm−1 confirms the formation of urethane linkage. Also, the intensities of transmission band at 1530 cm−1 are assigned to the N-H stretching and bending vibrations. Other strong absorption bands at 2932 and 2894 cm−1 are CH2 bridges. The weak peak of unreacted NCO group was at about 2270 cm−1. The band at 1900.0 cm−1 represents the stretching vibration of free carbonyl groups of urethane linkages and the bands at 1510 and 1600 are derived from the aromatic rings of lignin.
Thermal analysis of PU foams
Figure 8 (A) shows thermal conductivity of PU foams. The thermal conductivity depends on the foam density, cell size, cell structure (percentage of closed and open cell) and the thermal conductivity of the blowing agent . The value of thermal conductivity is observed ranging from 0.031 to 0.040 W/m K at foams with densities of 0.08 to 0.042 g/cm3. Foam 1 with the lowest density and more open cell content had the maximum thermal conductivity due to higher heat transfer in foam cells. However, by increasing the density of the foam the thermal conductivity decreased foam 2 showed different thermal behaviors. This evidence contributed to the bigger pore diameter and thereby higher thermal conductivity. Then having the high density, well-defined cell wall, smaller cell size and the lower open cell content, foam 4 demonstrated the lowest thermal conductivity. Because the considerable amount of air can be trapped that led to an increased passive insulation.
Polyurethanes were considered thermally unstable because of the existence of urethane bonds, so TGA measurements were carried out to obtain information on the thermal stability of the foams. The thermal degradation depends on the nature of substituent on the isocyanate and polyol. Figure 8 (B) depicts the TGA of PU foams. In the TGA studies, it was found that, polyurethane degraded in two weight loss stages. The first stage decomposition (5% weight loss of the samples) can be contributed to the decomposition of pyranose rings and isocyanate which usually starts between 150 and 220 °C [18, 19]. One study reported that PU foams were thermally stable at 191.9 °C . The second stage decomposition (50% weight loss) occurred at 400–500 °C as a marker for structural decomposition of foams  and also the decomposition of lignin and other difficulty parts. The onset of degradation (5% weight loss) of foams in this study occurred at around 212 °C of foam 1 and 217, 226, 237 °C of foams 2, 3 and 4 respectively. So depending on the type of polyols used in produced foams, the degradation temperature was different. The reason for the increasing of decomposition temperature of the foams produced from CG/PEG based liquefaction may be seen as the result of the existence of polyethylene glycol. Adding PEG to CG lead to an increase in liquefaction efficiency subsequently more bio polyols production. Increasing the percent of bio polyol in the produced foams may cause branching and more cross linking which require more thermal energy to initiate chain movements [22, 23]. Xu et al.  indicated that by adding 30% of PEG as high molecular petroleum based polyols to bio-polyols thereby the formation of three dimensional cross-linked polyurethane groups the first stage decomposition temperature of foams increased. The onset of the second stage degradation (about 50% weight loss) occurred at approximately 393 °C for all foams. Previous researches indicated that the second degradation observed at 389 and 400 °C resulted in depolymerization of polyol components such as derivatives of cellulose or lignin [24,25,26].