Phenol was provided by Nanjing Chemical Reagent Co., Ltd., China. Polyformaldehyde, urea and sodium hydroxide (analytical purified) were purchased from Shanghai Titan Co., Ltd., China. N-pentane (analytical purified) was supplied by Xilong Scientific Co., Ltd., China. Whisker silicon was purchased by Shanghai Huijingya Nano New Materials Co., Ltd. Twain 80 (chemically pure) was provided by Sino Pharmaceutical Chemical Reagent Co., Ltd. The PEG-12 polydimethylsiloxane was purchased from Saen Chemical Technology Co., Ltd., Shanghai. Calcium lignosulfonate was supplied by Guangxi Academy of Sciences. Alkaline catalyst and curing agent were self-made in laboratory.
Fourier transform infrared spectrometer (FT-IR, IS10, Nicolet, USA; SAT409PC/PG); thermogravimetric analyzer (Netzsch, Germany); Df-I heat-collecting magnetic agitator (Shanghai Weicheng Instrument Co., Ltd.); Intelligent limit oxygen index analyzer (Testech Instrument Testing Technology Co., Ltd.); CMT4000 computer controlled electronic universal testing machine (Shenzhen Xinsansi Material Testing Co., Ltd.); S-3000 N scanning electron microscope (Hitachi Co., Ltd., Japan).
After putting a condensing tube, a thermometer and a 500 mL four-mouth flask with stirrer in 60 ℃ water bath, the measured molten phenol, water, self-made alkaline catalyst and whisker silicon were added, raising the temperature to 75 ℃, and then polyformaldehyde (mole ratio of phenol to formaldehyde was 1.0꞉1.5) was added for three batches with an interval of 10 min. When the last batch was added, the temperature was slowly raised to 85 ℃, and the constant temperature reaction was maintained for 1.5 h. Then urea was added, after 0.5 h the materials were cooled to 50 ℃ to obtain whisker modified phenolic resin (PR1–PR2, Table 1) whose viscosity was 2500–3500 mPa s and temperature was 5 ℃. Volatile phenolic resins were obtained.
Resin Phenol (g) GX-Si (g) Lignin (g) PR1 200 0 0 PR2 200 1.80 0 PR3 180 1.80 20 PR4 170 1.80 30 PR5 160 1.80 40 PR6 170 0.72 30 PR7 170 2.52 30 PR8 170 3.60 30
Table 1. Formula of phenolic resin
After putting a condensing tube, a thermometer and a 500 mL four-mouth flask with stirrer in 60 ℃ water bath, lignin was added for pretreatment (Hu et al., 2013), then the measured molten phenol, water, self-made alkaline catalyst and silicon were added, raising the temperature to 75 ℃, and paraformaldehyde (mole ratio of phenol to aldehyde was 1.0꞉1.5) was added for three average batches with an interval of 10 min, and when the last batch was added, the temperature was slowly raised to 85 ℃, and the constant temperature reaction was maintained for 1.5 h. Then urea was added, after 0.5 h the materials were cooled to 50 ℃, obtaining the modified lignin base phenolic resin (PR3–PR8, Table 1) whose viscosity was 2500–3500 mPa s and temperature was 25 ℃. Phenolic resins were obtained.
The 100 g modified phenolic resin, 7 g surfactant between-80, and 3 g foam-homogenizer PEG-12 polydimethylsiloxane were mixed and stirred evenly at 2000 r/min. Then 7.5 g foaming agent n-pentane were added and stirred evenly. Finally, 22 g self-made acid curing agent were added and stirred for 20 s. Then these were quickly added into 20 cm × 20 cm × 5 cm mold, foamed and cured for 1 h in the oven at 80 ℃, then modified phenolic foam material (PR1−PR8) from resin (PF1−PF8) was obtained.
2.2.1. Whisker silicon modified phenolic resin
2.2.2. Whisker silicon modified lignin-based phenolic resin
2.2.3. Modified phenolic foam
Fig. 1 shows the FT-IR spectra of silicon whisker, common phenolic resin (PR1) and silicon whisker modified phenolic resin (PR4), respectively. The absorption peak at 796 cm–1 is the symmetric stretching vibration peak of Si—O bond. The absorption peak of 1058 cm–1 is the asymmetric stretching vibration peak of S—O bond. The absorption peak at 675 cm–1 is the out-of-plane deformation vibration peak of the terminal hydroxyl group (—OH) (Liu et al., 2008; Wang et al., 2009a).
In Fig. 1, the absorption peak at 3256 cm–1 is the peak of hydroxyl group, superimposed by the characteristic peaks of alcohol hydroxyl group and phenolic hydroxyl group (curves b and c). The absorption vibration peaks at 1598 cm–1 are aromatic ring framework vibration peaks, and the vibration peaks of C=C bond on benzene ring. The absorption peaks at 754 and 826 cm–1 are methylene —CH on ortho- and para-benzene rings, respectively, while the absorption peak at 754 cm–1 is markedly higher than that at 826 cm–1, indicating that the benzene ring is replaced with a high-ortho structure. The absorption peak at 1231 cm–1 is the stretching vibration of the phenol hydroxyl C-O bond. In Fig. 2, the absorption peak of Si-O-R at 1121 cm–1 can be clearly observed through local magnification, and the presence of silicon element in PR indicating that silicon is successfully grafted into the resin structure.
The degradation process of phenolic resin is divided into three stages: post-curing, thermal reforming and ring cracking (Hakkı Alma and Kelley, 2000). Figs. 3 and 4 show the effect of lignin content on thermal stability of resin when whisker silicon content is 1%. Figs. 5 and 6 show the effect of the addition of silicon whisker on the thermal stability of the resin when the lignin substitution amount is 30%. As can be found from the figures, there are three main degradation stages in the thermal degradation process. In the first thermal degradation stage (150–250 ℃), the mass loss was mainly caused by water evaporation. Only slight mass loss (about 2%) was observed in all resins due to the release of absorbed moisture at lower temperatures (< 120 ℃). As the temperature increased, the resin maintained a comparably high thermal stability below 150 ℃. When the temperature continued to rise, ordinary phenolic resin rapidly decomposed and lost about 20% mass. However, the modified resin still maintained high thermal stability under 200 ℃, and the mass loss was mostly about 6%. The second degradation stage (250–450 ℃) was mainly caused by water loss. During the temperature range of 200–350 ℃, the modified resin began to decompose slightly, with a mass loss of about 10%. Slight mass loss may be caused by further structural changes of solidified products to form a tighter cross-linking network and release small gaseous molecules such as water and formaldehyde (Zhang et al., 1997). During the third thermal degradation stage (450–600 ℃), the common phenolic resin (PR1) decomposed rapidly with the increase of temperature, and the carbon yield was only 48.9% at 600 ℃, while the carbon yield of the modified resin increased to different degrees, with the PR4 reaching 57.1%, and 16.8% higher than the ordinary resin. At 800 ℃, the PR8 increased by 20.2%–58.8%. At this stage, different resins exhibited similar decomposition behavior, demonstrating that they follow similar kinetic mechanisms. The weight loss at this stage was related to the relatively weak thermal cracking in the material system, releasing gaseous phenols, methyl derivatives and other products (Wang et al., 2009b).
Figure 3. Effect of lignin substitution amount on thermo gravimetric analyzer (TGA) of modified resin
Figure 4. The derivative thermogravimetric (DTG) curves of resins with different lignin substitutions
As can be found from Fig. 3, with the increase of lignin content, the carbon yield of the resin decreased to a certain extent, which attracted to the reason that lignin was a natural poly-molecular compound, and the hemicellulose in the raw material would degrade, but still had a more mass residues than ordinary phenolic resin. Fig. 4 shows resin thermal weightlessness rate curve changes with different lignin contents. The maximum thermal weight-loss rate of ordinary phenolic resin (PR1) was markedly greater than that of other modified resins, but the thermal weight loss rate of maximum temperature of the modified resin was high. This is due to the added lignin in modified resin under 200 ℃ has a slight mass loss. As can be seen from Fig. 5, the higher the content of silicon whisker is, the higher the carbon yield of the resin and the stronger the thermal stability are, which is mainly because silicon whisker itself has a relatively high thermal stability. The carbon yield of the PR1 at 800 ℃ was 43.8%, and that of the PR4 at 800 ℃ reached 57.6%, which was 31.5% higher than that of ordinary resin. In sum, the carbon yield of the modified resin was markedly increased, and the higher the carbon yield was, the better the heat resistance of the material was. All these showed that adding lignin and whisker silicon could remarkably improve the heat resistance of phenolic resin.
Table 2 lists the thermal mass loss of ordinary phenolic resin (PR1) and modified phenolic resin (PR2–PR6) at different temperatures, and clearly marks the carbon yield at different temperatures. As can be seen from the table, the carbon yield of the resin basically maintained a gradually increasing trend at different temperatures. At 200 ℃, the carbon yield was 80.1%, 83.4%, 84.5%, 83.6% and 85.4%, respectively. At 800 ℃, the carbon yield was 45.1%, 53.9%, 55.9%, 56.5% and 58.2%, and the carbon yield of the PR5 was 29.05% higher than that of the PR1. Therefore, carbon yield order was PR5 > PR4 > PR3 > PR2 > PR1, that was, with the increase of lignin content, carbon yield increased if whisker silicon contents remained the same. Similarly, for PR6, PR4, PR7 and PR8, with the same addition amount of lignin, the carbon yield at 400 ℃ was 71.2%, 72.9%, 73.8% and 75.5% respectively, showing a gradually increasing trend. Therefore, the carbon yield order was PR8 > PR7 > PR4 > PR6, that was, the carbon yield of the resin gradually increased with the increase of whisker silicon content when the lignin content remained the same. The reason was that the resin added with whisker silicon formed inorganic silicate at high temperature, which had a certain protective effect on the resin and prevents further mass loss (Arafa et al., 2004). In sum, the carbon yield of the modified resin was markedly increased, and the higher the carbon yield was, the better the heat resistance of the material was. All these indicate that addition of lignin and whisker silicon can markedly improve the heat resistance of phenolic resin.
No. Carbon yield at 200 ℃ Carbon yield at 400 ℃ Carbon yield at 600 ℃ Carbon yield at 800 ℃ PR1 80.1 69.1 50.9 45.1 PR2 83.4 70.7 57.8 53.9 PR3 84.5 73.2 59.9 55.9 PR4 83.6 72.9 60.4 56.5 PR5 85.4 74.3 62.0 58.2 PR6 83.9 71.2 56.3 53.9 PR7 84.7 73.8 61.1 56.9 PR8 86.2 75.5 62.5 58.3
Table 2. Thermogravimetric analysis data of phenolic resins (%)
Phenolic foam insulation material is a kind of closed cell foam material, and its thermal conductivity coefficient is low. Fig. 7 shows the effect of lignin content on the insulation performance of phenolic foam materials. From the figure it can be found that the thermal conductivity coefficient of phenolic resin modified by lignin changed little, which was within the range of 0.036–0.047 W/(m K). When the contents of lignin and whisker silicon reached to maximum, the thermal conductivity of foam was significantly improved. Combined with scanning electron microscope (SEM) figures, with the increase of the addition of lignin and whisker silicon, the structure in the foaming process of resin was destroyed, the phenomenon of pore opening was serious, and the solid adiabatic performance was decreased. But the thermal conductivity coefficient could still meet the requirements of insulation materials.
The curves of the effect of the substitution amount of lignin and the addition amount of whisker silicon on the thermal conductivity coefficient and obturation rate of phenolic foam are shown in Figs. 8 and 9 respectively. The obturation rate and thermal conductivity show a completely opposite trend. In Fig. 8, with the increase of lignin substitution rate, the obturator rate decreased and the thermal conductivity coefficient gradually increased. This is because the addition of too many lignin molecules reduces the uniformity of the resin structure. During the foaming process, the reactions between molecules led to the rupture of the vesicle, the increase of the vesicle diameter, and the increase of the obturation rate. When the lignin substitution rate was 40%, the obturator rate reached 90.7%, nearly 10% lower than the 99.8% of ordinary phenolic foam. The thermal conductivity increased from 0.028 W/(m K) of ordinary foam to 0.036 W/(m K). As shown in Fig. 9, the obturation rate decreased almost linearly with the increase of whisker silicon addition, and the descend range was significantly greater than that of lignin content on obturation rate. The reason is that whisker silicon particles are small with about 2−5 µm in size. The whisker silicon particles not involved in resin chemical reaction were scattered in the resin, affecting the nucleation of resin foam and led to the decrease of obturation rate. When the whisker silicon content reached 1%, the obturation rate was significantly reduced to 83.5%, which seriously affected the thermal conductivity coefficient of phenolic foam, 42.8% higher than that of ordinary phenolic foam. Therefore, the thermal conductivity coefficient was closely related to the obturation rate, and the content of lignin and whisker silicon should be controlled within a certain range, so that the influence of thermal conductivity coefficient of foam materials was small.
Figure 8. Effect of lignin substitution on thermal conductivity coefficient and obturation rate of phenolic foams
The LOI is an important parameter to measure the flame retardancy of insulating materials. Phenolic foam materials have attracted more and more attention due to their flame retardancy. However, traditional phenolic foams cannot meet the demand for the application of high flame-retardant materials in building insulation. The side chain of lignin benzene ring contains alkyl chain, and the degradation products contain long-chain alkyl fatty acid methyl ester which can improve the properties of phenolic foam. Whisker silicon can be used as an inorganic flame retardant additive, not only retarding flame, but also preventing smoke without producing dripping and toxic gasses. Figs. 10 and 11 show the effects of lignin content and whisker silicon content on limit oxygen index (LOI) of foams respectively. It can be found that with the increase of lignin or whisker silicon content, the LOI of phenolic foam increases, and the flame retardancy improves. Because of the low activity of the lignin phenolic resin, it is difficult to fully cure under the same curing condition with the increase of lignin substitution rate of lignin to phenol. The more black particle carbides were produced in the foam, the higher the solid oxygen index was. As a flame retardant, whisker silicon could further improve the flame-retardant phenolic foam and relatively reduce the content of phenolic foam itself during smoke combustion. However, when the content of whisker silicon and whisker silicon increased, the viscosity of the foaming system increased, and the foaming process was difficult to control, which was not conducive to the foaming operation. In addition, the increase of the addition amount of whisker silicon and whisker silicon would destroy the overall structure of the foam body, making the mechanical properties decline. The LOI of PF5 was 53.3, and 49.7% higher than that of ordinary PF, and the LOI of PF8 was 53.1 increasing by 49.16%. The test results showed that the flame-retardant performance of modified foam was significantly improved.
Therefore, under the same foaming solidified process, when the amount of curing agent was the same, the more lignin and whisker silicon were added, the more particles there were in the foam and the higher the LOI was within a certain range.
The SEM is the most intuitional way to observe the microstructure of materials which can clearly observe the compatibility of the material interface. It is a prerequisite to ensure the properties of materials. The shape, pore size distribution and size of foam are related to the foaming conditions and directly determine the properties of foam. The SEM analysis results of common phenolic foams and modified lignin-based phenolic foams (PF4 and PF8) are shown in Fig. 12. Phenolic foam is a kind of spherical obturator structure with a certain degree of openness, which is difficult to achieve complete opening or obturation. It can be found from the images that the ordinary phenolic foam is a relatively uniform and compact foam with a prismatic shape and an aperture of about 150 µm. Some cell walls have undergone deformation, partial rupture and collapse with clear fracture interface and no adhesion from which we can infer that phenolic foam is a brittle material. Comparatively, modified lignin-based phenolic foams, PF4 (Fig. 12c) has more uniform pores, of which the diameter is about 50 µm, a hexagonal shape, and higher obturation rate. However, when the silicon content of whisker increases significantly, the cellular structure is destroyed and the aperture ratio increases.
Combustion calorific value is the natural property of materials unrelated to the appearance size and usage state of materials. It is an important parameter to characterize the potential fire risk of building materials, and also an index to evaluate the classification of combustion performance of materials. Previous researches showed that only one in seven people died from burning in fires, and the vast majority died from asphyxiation caused by smoke and toxic gasses. Therefore, reducing the calorific value and smoke emission during combustion has become the focus of flame-retardant research. Reducing the calorific value of combustion can reduce the release of heat and minimize the impact on the environment. The combustion calorific value of phenolic foam is reduced by modifying phenolic foam. In Table 3, PF1, i.e., the combustion calorific value of ordinary phenolic foam, was up to 31.3 MJ/kg, significantly higher than that of various phenolic foam materials after modification. Lignin and whisker silicon could not only improve flame retardancy, but also reduce smoke emission and combustion calorific value. With the increase of lignin and whisker silicon content, the modified foams had lower combustion calorific value. The combustion calorific value of PF8 decreased to 16.8 MJ/kg, and 46.33% lower than that of ordinary phenolic foams. The data showed that the phenolic foam modified by lignin and whisker silicon could significantly reduce the fire risk, improve the flame-retardant property of phenolic foam, and greatly reduce the combustion calorific value of the material.
Phenolic foam Combustion phenomenon Calorific value of combustion (MJ/kg) PF1 Dense smoke 31.3 PF2 Dark smoke 25.8 PF3 White smoke 23.1 PF4 No smoke 20.7 PF5 No smoke 18.2 PF6 No smoke 21.2 PF7 No smoke 17.9 PF8 No smoke 16.8
Table 3. Effect of different compositions on combustion calorific value of phenolic foams
Compression properties reflect the ability of foam to withstand external pressure, which can directly reflect the mechanical properties of phenolic foam materials. Bending strength can reflect the bending resistance of foam materials, which is used to measure the bending properties of materials. Table 4 shows the data about compression and bending properties of phenolic foam materials with different lignin substitution rates and silicon whisker addition amount. The compressive strength and bending strength of ordinary phenolic foam were 0.192 MPa and 0.231 MPa respectively, which were significantly lower than other modified foams. When whisker silicon content reached 1%, compressive strength and bending strength were significantly lower than other modified foams even lower than ordinary phenolic foams. This is due to the addition of whisker silicon was too much and whisker silicon particles in the resin structure formed into a group, affecting the internal structure of the resin, increasing foam opening rate in the foaming process, and deceasing mechanical properties.
Phenolic foam Replacement percentage of lignin (%) Content of whisker silicon (%) Compressive strength (MPa) Bending strength (MPa) PF1 0 0 0.192 0.231 PF2 10 0.5 0.249 0.302 PF3 20 0.5 0.280 0.351 PF4 30 0.5 0.311 0.412 PF5 40 0.5 0.272 0.343 PF6 30 0.2 0.283 0.321 PF7 30 0.7 0.221 0.288 PF8 30 1.0 0.183 0.226
Table 4. Mechanical properties of whisker-modified lignin-based phenolic foams
Fig. 13 shows the effect of lignin substitution amount on the compressive strength and bending strength of phenolic foam at a whisker silicon content of 0.5%. With the increase of lignin substitution quantity, the compressive strength and bending strength have the same trend, increasing firstly and then decreasing. The value of compression and bending strength reached the maximum of 0.311 MPa and 0.412 MPa respectively when the lignin substitution amount was 30%, which was 62% and 78% higher than that of ordinary phenolic foam (PR1). The reason is that the proper number of lignin molecules participate in the phenolic condensation reaction, and can improve the hydroxymethyl content of the resin, leading to more powerful bubbles during the foaming process and higher degree of curing of foams with the increase of the hydroxymethyl content. When the substitution amount of lignin continued to increase, the resin activity decreased, the curing foaming degree decreased, and the mechanical properties of foamed materials decreased.
Fig. 14 shows the curves of compressive strength and bending strength of foam with the addition of whisker silicon under 30% of lignin substitution amount. The compressive strength and bending strength of the foam increased firstly and then decreased with the increase of whisker silicon content. The maximum silicon content of whisker was 0.5%. The reason is that the whisker silicon particles are evenly distributed in the resin, playing a connection role in the formation of a cross-linked network, and can better resist external force. When the whisker silicon content was high, the stress of particles would cause agglomeration and affect the stability of foam. The effect of whisker silicon on foam mechanical properties is greater than that of lignin, because whisker silicon is a kind of inorganic rigid particle, lignin has a polymer structure, and whisker silicon is more likely to cause the instability of foam structure in the process of resin foaming curing.