Preparation and Characterization of Phenolic Foams Modified by Lignin-based Polyurethane Prepolymers

BO Caiying，HU Lihong,2，YANG Xiaohui，ZHENG Minrui，ZHANG Meng，ZHOU Yonghong*

(1． Institute of Chemical Industry of Forest Products，CAF; National Engineering Lab． for BiomassChemical Utilization; Key and Open Lab． of Forest Chemical Engineering，SFA; Key Lab． ofBiomass Energy and Material，Jiangsu Province，Nanjing 210042，China; 2． ResearchInstitute of Forestry New Technology，CAF，Beijing 100091，China)

Preparation and Characterization of Phenolic Foams Modified by Lignin-based Polyurethane Prepolymers

BO Caiying

BO Caiying1，HU Lihong1,2，YANG Xiaohui1，ZHENG Minrui1，ZHANG Meng1，ZHOU Yonghong1*

(1． Institute of Chemical Industry of Forest Products，CAF; National Engineering Lab． for BiomassChemical Utilization; Key and Open Lab． of Forest Chemical Engineering，SFA; Key Lab． ofBiomass Energy and Material，Jiangsu Province，Nanjing 210042，China; 2． ResearchInstitute of Forestry New Technology，CAF，Beijing 100091，China)

Lignin was liquefied with polyethylene glycol (PEG) in a solvent-free system and then directly used with suitable PEG as a co-monomer and co-solvent to prepare liquefied lignins (LLs)-based polyurethane prepolymers (LLPUPs). The structural changes that lignin undergoes after liquefaction were studied by Fourier transform infrared spectroscopy (FT-IR), the hydroxyl number of LL is 177-286 mg/g and decreases with the increasing of molecular mass of PEG. The structures of LLPUPs were characterized by FT-IR. Then, phenolic foams (PFs) modified with different contents of LLPUPs were prepared. Morphological properties, mechanical properties and thermal stability of PFs were assessed by scanning electron microscopy (SEM), universal test and thermogravimetric analysis (TGA). Results show that uniform cells can be prepared by incorporating 9% LLPUP1 and LLPUP2 into the network. The addition of LLPUPs into foam formulations improves the compressive strength and specific compressive strength of PFs, and the highest specific compressive strength is obtained by adding 9% LLPUP2 (4.44(Pa·m3)/g). Appropriate levels of LLPUPs could improve the specific flexural strength of PFs, and the highest specific flexural strength is obtained by adding 9% LLPUP1 (8.23(Pa·m3)/g). PFs modified with different contents of LLPUPs show similar thermal resistance as pristine foam.

liquefied lignin;phenolic foams;polyurethane prepolymer;characterization

Phenolic foams (PFs) are polymeric materials with unique physicochemical structures. PFs are increasingly used in the construction industry and sealing materials owing to their low density, high thermal insulation, low water absorption, and very high flame resistance[1-2].To further exploit the desirable properties of PFs, numerous studies over the past few decades focused on improving the toughness and reducing the brittleness and pulverization of PFs[3-6]. Particularly, chemical modification, in which flexible chains are introduced into the rigid backbone of phenolic resin and the toughening effect is notably improved, has attracted extensive attention. Among the existing modifiers, polyurethane prepolymer (PUP) is the most effective one and its high reactive isocyanate groups can improve the mechanical performance and reduce brittleness and pulverization by reacting with the resol resin at the end of the molecular chain of the toughening agent[7-9]. However, along with the increasing use of fossil fuels, there are growing concerns about the unavailability of some petrol fractions and human impacts on the environment. As a result, attention has been diverted to the renewable resources for the synthesis of prepolymers. Among all renewable resources, lignin is a low-value by-product of pulping[10]. It is considered as a macropolyol and used as a polyol precursor in synthesis of PUPs with isocyanate groups[11]. However, its reaction with isocyanateis restricted by the intrinsic properties, such as poly disperses molar mass and hyperbranched structure of lignin. To overcome such restriction, Kim et al[12]produced lignin biopolyols by the liquefaction of the sunflower stalk saccharification residues using crude glycerol as liquefaction solvent. The bio-PU was successfully synthesized from the biopolyol and methylene diphenyl diisocyanate (MDI). Li et al[13]liquefied alkaline lignin (AL) in PEG- 400/glycerol with sulfuric acid as catalyst. The resulting lignin-based polyether polyol (LPP) exhibited similar physicochemical properties to the commercial polyether polyol (PP). The polyurethane foams (PUFs) made from LPP presented better thermal stability and higher compressive strength than those obtained from commercial PP. However, to our best knowledge, few researchers have directly discussed the addition of lignin-based PUPs into preparation and characterization of PFs. In the present work, lignin was liquefied with polyethylene glycol (PEG) in a solvent-free system. Then the liquefied lignin (LL) was used directly to synthesize LL-based PUPs (LLPUPs) with 1,6-hexamethylene diisocyanate (HDI). This reduced the number of chemical reaction steps. The structure of LLPUPs was characterized by Fourier transform infrared spectrometry (FT-IR). Furthermore, PFs added with different contents of LLPUPs were prepared. The morphological properties, mechanical performance and thermal stability of PFs were assessed by universal test, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). This study provides scientific data for application of lignin-based PUPs into preparation of PFs.

1 Experimental

1.1 Materials and equipment

Lignin from oxidatively-degraded enzymatic hydrolysis(Institute of Chemical Industry of Forest Products) was oven-dried at 50 ℃ for 24 h and then kept in a desiccator at room temperature until used. Resol-type phenolic resin (viscosity 5 000-6 500 mPa·s and solid content about 85% at 25 ℃) was prepared from phenol and paraformaldehyde at a molar ratio of 1 ∶1.6 based on our previous study[14]. The curing regent and modified silicon oil (H-203) were obtained from commercial sources and used as received. PEG400 (AR), PEG600 (AR), HDI (AR).All other chemicals in this study were reagent grade and used without further purification.

Automatic titrator ZDJ- 4A; Nicolet Magna-IR spectrometer (Nicolet Instrument Crop., USA); STA 409 PC/PG analyzer (Netzsch, Germany); CMT4000 universal testing machine; 3400N SEM meter (Hitachi Co.).

1.2 Preparation of samples

1.2.1 Liquefied lignins (LLs) Only PEG was used as the liquefaction solvent, while phosphoric acid was used as a catalyst during the liquefaction. The weight percent of phosphoric acid relative to the PEG-lignin mixture was kept constant at 2%. In each experiment carried out at 150 ℃, PEG, phosphoric acid and lignin were added at formula ratio into a four-necked glass flask equipped with a reflux condenser, a stirrer and a thermometer. Then the flask was immersed in an oil bath at 150 ℃, followed by liquefaction under nitrogen atmosphere for 1 h. The liquefied mixture was then diluted to ten times of their weight with dioxane-water (80 ∶20, volume ratio). The insoluble lignin residue was dried to a constant mass in oven at 105 ℃, and liquefied residue was calculated according to the previously reported literature[15]. The diluted solution was vacuum-stirred at 115-120 ℃ for 2 h to remove moisture, the flask was cooled down to stop the reaction and the LLs were collected for later use and analysis. LLs consist of lignin components dissolved through solvolysis and the solvent PEG.

1.2.2 Synthesis of LLPUPs LLs were characterized before synthesis of isocyanate-terminated PUPs. A standard method was developed to evaluate the amount of OH available in the reaction mixture synthesized in a typical procedure for preparation of isocyanate-terminated PUPs. LLs were put into a 500 mL four-necked kettle equipped with a mechanical stirrer. HDI was then added to the LLs with the molar ratio of —OH to —NCO 1 ∶2 to form isocyanate-terminated LLPUPs. The mixture was heated to 75-80 ℃ in an oil bath under mechanic stirring. Before and during the reaction, the installation was continuously flushed with N2to minimize oxidation and remove moisture from the reactor. The reaction started immediately after the addition of all the HDI and proceeded for 1 h. Then a black liquid product named LLPUPs was obtained. The NCO content in the LLPUPs was measured by the di-n-butylamine method. The synthetic route for preparation of LLPUPs is illustrated in Fig.1.

Fig.1 Synthetic route for preparation of LLPUP

1.2.3 LLPUPs modified PFs Resol-type phenol resin 100 g was premixed with different contents of LLPUPs at room temperature. Each mixture was stirred with propeller stirrer at about 2 000 r/min for 30 s, which guaranteed the complete reaction between —NCO and —OH. Then after addition of 4 g surfactants (mass ratio of Tween-80 and modified silicon oil 1 ∶1), the reaction systems were stirred at 2 000 r/min for 20 s to a homogeneously mixing. The resulting mixtures were added with 8.5 g foaming agent (n-pentane) and 8.8 g curing agent, and stirred at 2 000 r/min for 30 s. Then each resulting viscous mixture was poured into foaming mould quickly and cured at 80 ℃ for 30 min to obtain LLPUPs-modified PFs.

1.3 Analysis and measure of samples

1.3.1 Measurement of acid value of LLs The acid value of LLs were determined according to GB/T 12008.5—2010. Specifically, the LL sample was dissolved in isopropanol and titrated with 0.02 mol/L KOH solution in methanol to the equivalence point. The acid value (A) of the sample was calculated as follows:

(1)

where:A—the acid value, mg/g;V—the volume of the KOH solution used in titration to the equivalence point, mL;C—the normality of the KOH solution,mol/L;m—the weight of LL, g.

1.3.2 Measurement of hydroxyl number of LLs The hydroxyl number of LLs was determined according to GB/T 12008.3—2009. Briefly, LLs and blank samples were each refluxed at (115±2)℃ in 25 mL acetylation reagent solution. The solution was prepared freshly everyday from 116 g phthalic anhydride, 700 mL dry pyridine and 16 g imidazole (catalyst). After the refluxing, the flasks were left to cool down at room temperature. Thereafter, the excessive phthalic anhydride was hydrolyzed with distilled water. The resulting phthalic acid was titrated with 0.5 mol/L NaOH. The hydroxyl number (H) of each sample was calculated as follows:

(2)

where:H—hydroxyl number, mg/g;V1—the volume of the NaOH solution required to titrate LL sample, mL;V0—the volume of the blank solution, mL;C′—the normality of the NaOH solution, mol/L.

1.3.3 FT-IR analysis FT-IR spectra were recorded in the range of 400-4000 cm-1with a nominal resolution of 4 cm-1.

1.3.4 Mechanical tests of PFs Compressive and flexural properties were tested according to GB/T 8813—2008 and GB/T 8812.1—2007, respectively. Compressive strength and compressive modulus were determined as the maximum value and the slope of the stress-curve (strain<10%), respectively. At least three samples were tested to obtain an average value.

The density of PFs was measured according to GB/T 6343—1995. The average value of five samples was recorded.

1.3.5 TG analysis of PFs Specifically, a small amount of a sample was placed in an Al2O3pan, which was put in a furnace. Then the furnace was heated at 20 ℃/min from 35 to 800 ℃ under nitrogen atmosphere.

1.3.6 SEM analysis of PFs The samples were adhibitted on a copper plate and sprayed with a thin gold layer to prevent electrostatic charging during examination.

2 Results and Discussion

2.1 Characterization of LLs

Fig.2 FT-IR spectra of lignin and residue liquefied by PEG

2.1.1 FT-IR analysis Fig.2 shows the FT-IR spectra of lignin, residue liquefied by PEG400 (mPEG400∶mlignin10 ∶1), and residue liquefied by PEG600 (mPEG600∶mlignin10 ∶1). The broad absorption band at 3000-3500 cm-1, is attributed to the —OH group in lignin. After liquefaction, this absorption decreased significantly, which indicated that the —OH group would react with PEG during liquefaction[16].The peaks at 2933 cm-1(stretching vibration of methyl), 2844 cm-1(stretching vibration of methylene); 1323, 1212 cm-1(syringyl ring breathing with C—O stretching vibrations), 1114 cm-1(syringyl-type aromatic C—H in plane deformations)[17], 1600, 1511 cm-1(the stretching of aromatic rings in lignin) were not obvious in the spectra of lignin residues and these indicated that some inner structural units of lignin were remarkably varied and lignin was destroyed by liquefaction. However, a significant new absorption band appeared at 1059 cm-1(the ether band), this could be assumed that lignin would be reacted with PEG by a new ether linkage to form a derivative. After the liquefaction, most of the peaks of the lignin have significantly modified. This reveals the decomposition of lignin components.

2.1.2 Hydroxyl number and acid value The properties of LLs are summarized in Table 1. LLs are acidic due to the acid catalyst used during liquefaction, and the acid value of LLs is similar. The hydroxyl number is a measure of OH concentration in polyol. This important parameter should be monitored during polyol production. The hydroxyl numbers of LLs are 177-286 mg/g, while the results of LLs with PEG600 as the solvent are lower than those with PEG400 as the solvent. This was caused by the fact that PEG600 has fewer hydroxyl groups per unit weight than PEG400. Thus, PEG400 is better effective as liquefaction solvent to improve lignin conversion. This comparative result is consistent with the observation of Jin et al[18]who liquefied lignin with PEG and glycerol.

Table 1 Properties of each LLs

2.2 Characterization of LLPUPs

Fig.3 FT-IR spectra of LL2, HDI and LLPUP2

2.3 Characterization of PFs

2.3.1 Mechanical properties Given the significant application of PFs as foam materials, compressive strength and flexural strength of PFs are critical mechanical properties, while apparent density must be considered in the analysis of mechanical behaviors of foams[3,20].The mechanical properties and apparent density of pristine foam and LLPUPs-modified PFs are summarized in Table 2. Results show that these properties are significantly influenced by the addition of LLPUPs. Higher contents of LLPUPs result in higher density of LMPUPs-modified PFs. As expected, the additions of LLPUPs into the foam formulations improve the compressive strength and specific compressive strength (compressive strength/density). The reason is that the increased crosslink density leads to the shortening of network strands and thus the improvement of both strengths. The specific compressive strengths of PEG400-based LLPUP-modified PFs (PF1-1#-PF2-3#) are increased by 14.47%, 19.50%, 22.96%, 0%, 21.07% and 39.62% compared to the pristine PFs, respectively. Thus, LLPUP2 outperforms LLPUP1 in enhancing compressive properties. The partial reason is that the content of hard segments increases with further addition of lignin, whose aromatic rings improve the chain stiffness. The mechanical properties achieved increasing with the high amount of lignin in LLPUPs is similar to the observations on lignin in composites[21-22].

The specific compressive strengths of the PEG600-based LLPUP PFs (PF3-1#-PF4-3#) are increased by 5.66%, 14.15%, 21.07%, -2.52%, 0.94% and 18.87% compared to the pristine PFs, respectively. Thus, LLPUP3 outperforms LLPUP4 in enhancing compressive properties. It can be explained on basis of that PEG600 is not a good liquefying solvent for lignin. Thus, some unreacted lignin causes the formation of irregular foam structure and reducing the specific compressive strength.

Table 2 Mechanical properties of PFs

1) PF1: PFs modified by LLPUP1; PF2: PFs modified by LLPUP2; PF3: PFs modified by LLPUP3; PF4: PFs modified by LLPUP4

However, the changes of flexural behavior are distinct from those of compressive behavior in LLPUPs-filled PFs. The specific flexural strengths of the PF1-1#-PF1-3#are 7.50, 7.60 and 8.23(Pa·m3)/g with increase of about 6.38%, 7.80% and 16.74% compared to the pristine PFs, respectively. The specific flexural strengths of the PF2-3#were enhanced by 1.99%. These results indicate that appropriate addition of LLPUPs into the resin matrix can improve the specific flexural strength of PFs. This improvement may be attributed to the isocyanate groups in LLPUPs, which can react with hydroxymethyl from resins, and introduce the flexible chains from PEG400 into the rigid backbone of phenolic resin[7,23]. However, the specific flexural strength of LLPUPs-filled PFs is weakened with increasing of PEG when PFs contain the same content of LLPUPs. These facts are dependent on the decrease of crosslinking degree with increasing of PEG and also on the increase of lignin content[24]. The toughening mechanism is illustrated in Fig.4. Based on the above results, the addition of LLPUPs into foam formulations improves the compressive strength and specific compressive strength of PFs, and the highest specific compressive strength is obtained by adding 9% LLPUP2 (4.44(Pa·m3)/g), appropriate levels of LLPUPs could improve the specific flexural strength of PFs, the highest specific flexural strength is obtained by adding 9% LLPUP1 (8.23(Pa·m3)/g).

Fig.4 The schematic of LLPUP toughening of PF

2.3.2 Cell morphology Cell morphology affects the physical-mechanical properties of PFs. Fig.5 presents the morphologies of 9% LLPUPs-modified PFs at the same magnification, and we also calculated the sizes of 100 cells in SEM images on Nano Measurer 1.2.

Fig.5 SEM micrographs of pristine PF and modified PFs

The microstructure of pristine foam (PF0#in Fig.5) shows large and very non-uniform cells. The cell sizes range from 83 to 287 μm with the mean of 147 μm. The situation is distinctively different in the PFs modified with PEG400-based LLPUP, since the cells are approximately polyhedral and symmetrical as a whole (The cell sizes of PF1-3#range from 95 to 182 μm with the mean of 136 μm, the cell sizes of PF2-3#range from 95 to 215 μm with the mean of 148 μm). The regular cell shapes suggest that LLPUPs act as a nucleating agent during cell formation[7，9]. The cell sizes of PFs modified with PEG600-based LLPUP become obviously heterogeneous and the bubbles collapse (The cell sizes of PF3-3#range from 92 to 266 μm with the mean of 167 μm, the cell sizes of PF4-3#range from 104 to 266 μm with the mean of 156 μm). The reason is that the addition of PEG600-based LLPUP enhances the viscosity of the resin matrix (PEG600-based LLPUP is stickier than PEG 400-based LLPUP) and results in lower liquidity. This means the imperfection of cure process and will cause a considerably non-uniform cell structure and a gradual increase of hole collapse[25]. The irregular cells, large bubbles, and poorly-dispersed lignin are likely responsible for the low mechanical properties of the foams. Therefore, PEG400-based LLPUP can be incorporated into the network to obtain uniform cells by adding 9%.

Fig.6 TGA and DTG curves of pristine foam (PF0#) and modified foams with different content of LLPUP1

2.3.3 TGA of PFs The thermal stabilities of pristine PFs and PFs modified with different contents of LLPUPs were investigated by TGA and derivative thermogravimetry (DTG). Typical TGA and DTG profiles of LLPUP1-modified PFs are shown in Fig.6. And results are summarized in Table 3. Clearly, all of the composites show quite similar three-stage degradation behaviour[26]. The first relatively low-mass stage within 40-130 ℃ is attributed to the release of excessive phenol, formaldehyde, short oligomers and water. The second stage within 130-430 ℃ results from pyrolysis and the polymerization of pyrolysis products[27]. The third stage within 430-800 ℃ is the main process of thermal degradation, where chain scission and most of polymer decompositions occur and form low-molecular-mass products[28]. In fact, this main process of thermal degradation of isocyanate prepolymers depends on the molar mass of PEG[29].

Table 3 TGA date of pristine Foam (PF0#) and modified foams with different content of LLPUPs

The studied parameters are the temperatures corresponding to 5% of weight loss (T-5, initial decomposition), 30% of weight loss (T-30), and maximum degradation (Tmax), as well as char yield at 800 ℃. As shown in Table 3, a similar thermal resistance behavior was observed from PFs with different compositions of LLPUP1, LLPUP2, LLPUP3 and LLPUP4. Compared with pristine foam, the modified foams show slightly lowerT-30andTmax(slight shift to lower temperature), while residue char yield of PFs at 800 ℃ gradually decreases with the increasing content of LLPUPs. Indeed, further incorporation of LLPUPs into PFs would reduce the thermal stability of polymers. As reported, the thermal stability of polyurethane-modified PFs is reduced by further addition of polyurethane[30].

As for PFs modified with LLPUP1(PF1-1#) and LLPUP3(PF3-1#) at 1% content compared with the pristine foam, theT-30(554.6,558.8, and 550.5 ℃ respectively) and residual mass at 800 ℃(55.54%, 56.23%, and 54.45%, respectively) are slightly higher. These phenomena could be logically explained by the lower LLPUPs content and the high thermal resistance of lignin, as its chemical structure contains ether linkages and aromatic groups[29]. Based on the above results, PFs modified with different contents of LLPUPs show similar thermal resistance as pristine foam.

3 Conclusions

3.1 Lignin was liquefied with PEG in a solvent-free system and then directly used to synthesize LLPUPs with suitable PEG as a co-monomer and co-solvent, and the hydroxyl number of LL is 177-286 mg/g and decreases with the increasing of molecular mass of PEG. PEG400 was better effective as liquefaction solvent to improve lignin conversion.

3.2 PFs modified with different contents of LLPUPs were prepared. 9% LLPUP1 and LLPUP2 could be incorporated into the network to form uniform cells.

3.3 The addition of LLPUPs into foam formulations could improve the compressive strength and specific compressive strength of PFs, and the highest specific compressive strength is obtained by adding 9% LLPUP2 (4.44(Pa·m3)/g); Appropriate levels of LLPUPs could improve the specific flexural strength of PFs, and the highest specific flexural strength is obtained by adding 9% LLPUP1 (8.23 (Pa·m3)/g).

3.4 PFs modified with different contents of LLPUPs show similar thermal resistance as pristine foam.

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2016- 06-15

国家自然科学基金资助项目(31470613)；江苏省生物质绿色燃料与化学品重点实验室开放基金(JSBGFC14002)

薄采颖(1984— )，女，江苏东海人，助理研究员，博士生，主要从事生物质资源转化与利用研究；E-mail：newstar2002@163.com

薄采颖，胡立红，杨晓慧，等.木质素基聚氨酯预聚体改性酚醛泡沫的制备及表征(英文)[J].林产化学与工业，2017，37(1)：63-72.

木质素基聚氨酯预聚体改性酚醛泡沫的制备及表征

薄采颖1， 胡立红1,2， 杨晓慧1， 郑敏睿1， 张 猛1， 周永红1

(1.中国林业科学研究院 林产化学工业研究所；生物质化学利用国家工程实验室；国家林业局 林产化学工程重点开放性实验室；江苏省 生物质能源与材料重点实验室， 江苏 南京 210042；2.中国林业科学研究院 林业新技术研究所， 北京 100091)

以聚乙二醇(PEG)为液化剂液化木质素，得到液化产物直接合成木质素基聚氨酯预聚体(LLPUPs)，并用来改性酚醛泡沫。采用FT-IR对木质素液化产物(LLs)和LLPUPs的结构进行了表征和分析,通过SEM、万能试验机结合热重分析仪研究了LLPUPs对酚醛泡沫的形态，机械性能和热稳定性的影响。结果表明：木质素液化产物的羟值为177～286 mg/g，并随着PEG相对分子质量的增加而下降；添加LLPUPs可以增强酚醛泡沫的压缩强度和比压缩强度，当LLPUP2添加量为9%时，比压缩强度最高，为4.44(Pa·m3)/g；LLPUP1 和LLPUP2添加量为9%时，酚醛泡沫泡孔均匀，当LLPUP1添加量为9%时，比弯曲强度最高，为8.23(Pa·m3)/g； LLPUPs改性酚醛泡沫与纯酚醛泡沫相比，具有相似的热稳定性。

液化木质素；酚醛泡沫；聚氨酯预聚体；表征

10.3969/j.issn.0253-2417.2017.01.008

*通讯作者：周永红，研究员，博士，博士生导师，主要从事生物质化学转化与应用的研究；E-mail：yhzhou777@163.com。

TQ35 Document code:A Article ID:0253-2417(2017)01- 0063- 10