Lignin-Modified Materials and Their Applications

CHAPTE R 7

Lignin-Modified Materials and Their Applications Outline 7.1 Lignin-Filled Modified Rubber  182 7.2 Lignin-Modified Engineering Materials  184 7.3 Lignin-Modified Adhesives  186 7.4 Lignin-Modified Fibers  189 7.5 Lignin-Modified Nanofibers  191 7.6 Lignin-Modified Film Materials  195 7.7 Lignin-Modified Foaming Materials  199 7.8 Lignin-Modified Hydrogel  202 7.9 Lignin-Modified Material as Precursor for Carbon Materials  204 References  207

In the last two chapters, lignin-modified thermoplastic and thermosetting polymer materials were introduced. This chapter focuses on lignin-modified materials, including the most widely applied lignin-modified materials: rubber, engineering plastics, adhesives, and three other materials. Lignin as a filler instead of carbon black to modify rubber can enhance the mechanical properties of rubbers, while also improving the oxidation resistance and solvent resistance of the matrix. In engineering plastics and adhesives, the most common application matrixes for lignin modification are polyurethane (PU), phenolic resin (PF), epoxy resin, and urea resin. (The related studies of these materials were introduced in Chapter 6). To these materials, the introduction of lignin not only can reduce the cost of the material, but it also improves their mechanical and thermal properties. Lignin added to adhesives can improve water resistance and reduce the content of free formaldehyde molecules. With the deeper studies of lignin-modified materials and higher requirements of the applications, new technologies have been used to prepare lignin-modified materials, and have reported a variety of lignin-modified materials with interesting inner structures. Spinning technology can be used to make lignin-modified fibers, and nanofibers made by lignin-modified materials can be achieved by electrospinning technology. The introduction of lignin into polymer foams also can produce light and high-strength lignin-modified foams, which can be used in cushioning Lignin Chemistry and Applications. https://doi.org/10.1016/B978-0-12-813941-7.00007-2 Copyright © 2019 Chemical Industry Press. Published by Elsevier Inc.

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182  Chapter 7 packages and flame retardant or thermal insulation materials in the construction industry. Self-assembly technology can be applied in preparing lignin-based thin film material, with variety of technologies such as blow molding, spin-coating, and electrostatic spinning, ligninmodified blown film, spin-on film, and electrospun film can be produced. As lignin is a good carbon source, based on the lignin-modified materials, carbon fibers and carbon films can be prepared by using lignin-modified polymer fibers or films as precursors. Additionally, the introduction of lignin into hydrogels can increase their mechanical strengths and adsorption capacities to ions, which is another way to achieve high-value use of lignin. Therefore, considering the importance of lignin-modified materials for the studies of lignin-based highvalue applications, the preparation and application of lignin-modified fibers, nanofibers, films, foams, hydrogels, and carbon materials will be discussed.

7.1  Lignin-Filled Modified Rubber Lignin has rigid aromatic rings and flexible side chains in its molecules structure, while also containing a large number of reactive functional groups, and it is a kind of subpolymer with a fine particle shape and a high specific surface area. Therefore, lignin can be used widely to replace carbon black as a reinforcing agent to enhance the mechanical strength of rubber matrices. The hydroxyl groups of lignin not only can form hydrogen bonds with π-electron clouds in the rubber molecules, but they also can react with the functional groups in rubbers to achieve grafting or crosslinking structures. These interactions play an important role in the strengthening of rubber. By comparing the properties of lignin and carbon black-filled rubber materials, it has been found that lignin can be filled with high content, while the density of the composite material after filling is relatively low. The lignin-based composite rubber usually has better gloss, wear resistance, flex resistance, and solvent resistance. At the same time, sulfur-modified lignin can accelerate the curing rate of vulcanized rubber, and effectively prevent the sulfur blooming phenomenon [1]. Additionally, the combination of lignin and other fillers is more conducive to improve the comprehensive properties of modified rubber. For example, the new rubber filler (BL-MMT), which is prepared by dehydrating the mixture of lignin, NaOH, black liquor (BL), and montmorillonite (MMT), can be filled in butyronitrile rubber (NBR) to form new composite rubbers. The following studies show that the composite rubber consisting of lignin-containing BL and NBR has two glass transition temperatures (Tg1 and Tg2). The Tg1 comes from NBR was 27.4°C, which decreases by 2.5°C compared with the Tg of pure NBR. The Tg2 from lignin was 42.7°C (when the Tg of pure lignin was 46°C). Also, the tensile strength, elongation at break, 300% modulus, and hardness of the composite material are greatly improved compared with that of pure NBR, which is 25.9 MPa, 809%, 2.6 MPa, and 64, respectively. Such improvement is attributed mainly to the enhancement by lignin. When MMT is mixed with lignin-containing BL to modify NBR, the Tg1 increases gradually as the amount of MMT increases, and the Tg2 decreases after increasing to the maximum value of 50.9. When the mass ratio of MMT/BL is 1:1, best tensile strength (28.7 MPa) and elongation at break (813%) can be achieved [2].

Lignin-Modified Materials and Their Applications  183 The key problems in the production of lignin-based composite rubber are the improvement of the compatibility between lignin and rubber matrix and the optimization of the dispersion of lignin in rubber. That optimization currently is accomplished mainly through the improvement of technology and chemical modification of lignin. The resin-resin, resinrubber, and rubber crosslinked multinetwork structures also can be constructed by such modification strategy. Compared with common carbon black or other inorganic fillers, lignin has a large variety of active functional groups, which make the physical and chemical properties of lignin adjust easily by chemical modification, to optimize enhancement of lignin in rubber by increasing the molecular weight of lignin via chain extension, or to improve the surface compatibility between lignin and the matrix by forming special chain structure on lignin molecules. The surface decoration of lignin by formaldehyde can prevent lignin from forming supramolecular particles in rubber, caused by the π-π interaction between lignin molecules. Therefore, the simple modification of formaldehyde not only can enhance the bulk strength of lignin, but it also can improve the ability of lignin to disperse in the rubber matrix during the enhancement. Lignin has a special chemical structure, which consists of hydrolysable alkoxy groups and other reactive functional groups. Therefore, lignin molecules can be used as a bridge between rubber and inorganic fillers, and is suitable for modification with other inorganic fillers to improve the comprehensive properties of rubber. For example, the tensile strength and elongation stress can be improved significantly when the hardness and elongation at break of the materials are maintained after adding a certain amount of lignin powder into the natural rubber. This phenomenon indicates that the addition of lignin enhances the interface between rubber and inorganic fillers and forms a solid network throughout the material [3]. Melt blending butadiene rubber (SBR) and in situ produces lignin-layer double hydroxides (LDH), and the produced composite rubber exhibits improved tensile strength, elongation at break, 300% modulus, and hardness with a good dispersity of lignin-LDH in the rubber matrix [4]. Following transmission electron microscope (TEM) observation found that the existence of lignin promotes the dispersion of MMT in NBR matrix, and increasing the lignin/MMT ratio also increases the dispersibility of MMT [2]. The smaller the particle size of lignin and the more uniform the lignin particles are dispersed in rubber matrixes indicate better compatibility between lignin and rubber matrix, stronger physical and chemical interaction between the two compounds to achieve a better strengthening effect. Lignin-filled modified rubber usually is prepared by coprecipitation, dry mixing, or wet mixing. With the help of mixing and jet devices, shear force can be used to refine the lignin particles. Meanwhile, small molecules such as water can inhibit the aggregation caused by hydrogen bonding between lignin particles. In the separation and purification of lignin, however, strong surface interaction between particles will lead to agglomeration of lignin crystallite particles. Therefore, it is necessary to use chemical modification and alkali activation to make lignin particles have a loose granular structure, which is more favorable to shear thinning during mixing. The nanoscale dispersion (100–300 nm) of lignin particles in rubber matrix can be achieved by dynamic heat treatment, light methylation, and other technologies.

184  Chapter 7 In a lignin-filled rubber system, specific small molecules can be introduced to react with the functional groups of lignin, and crosslink lignin molecules to form networks in the matrices. These networks can corporate with the rubber network and form a double-network composite structure. For example, aldehyde and diamine molecules can conjugate the dispersed lignin in rubber matrix and form an integrated, stiff network throughout the rubber, which can improve the mechanical, wear and tear properties of the rubber. Meanwhile, such modification also gives composite rubbers excellent oil resistance and aging resistance [5]. Lignin also can improve the thermal stability of lignin-filled rubber by its special hindered phenolic hydroxyl structure. For example, in lignin-modified natural vulcanized rubber, the maximum thermal decomposition temperature (Tmax) of natural rubber modified with lignin increases from 358.3°C to 388.3°C when the lignin content reaches 20 phr (per 100 phr of rubber). With the addition of lignin to 30 phr, the Tmax of lignin-modified NR decreases, giving the rubber with 20 phr lignin the best thermal stability. It also is found that the combination of lignin with commercial rubber antioxidants (IPPD, for example) show a better antioxidant property than with lignin alone. The rubber is mixed with only 1 phr of IPPD and 1 phr of lignin, and then aged 1 day, 3 days, 7 days, 10 days, and 17 days in the open air at 80°C. The results show that the addition of 4 phr lignin has better thermal oxidative aging resistance and can maintain good tensile property after aged for 17 days [6]. Lignin also can be used as a flame retardant in rubber products. The flame-retardant material with oxygen index of lignin-containing rubber is better than more than 30% of flame-resistant elastic materials, while the smoke yield of the modified material decreases significantly [7].

7.2  Lignin-Modified Engineering Materials The reaction of lignin molecules with phenol and isocyanate is the basis for the development of lignin-modified PU and phenolic resin-based engineering plastics. (For the review of such two types of lignin-modified materials, see Chapter 6). The high-impact strength and heat resistance of lignin can meet the basic requirements of engineering plastics. In most of the engineering plastics made with lignin through chemical reaction and physical blending, the amount of lignin is limited to 25%–40% [8]. The amount of lignin is limited because the three-dimensional crosslinked network of these modified materials and the aromatic structure and three-dimensional network of the lignin molecules and the rigid domains of the urethane and PU groups formed by the reactions in composite materials usually make the materials brittle [9]. The modification of lignin with hydroxyalkyl groups can improve the viscoelasticity of the lignin and make it usable as a prepolymer of the thermosetting engineering plastics [10–22]. It also has been found that toughened thermosetting plastics could be obtained by introducing polyether and rubber-like soft segments into the network structure of lignin-modified materials. The soft segment structure, as a toughening unit, has a plastic response to the mechanical deformation of the material, which leads to a significant

Lignin-Modified Materials and Their Applications  185 decrease in brittleness and achieves a relatively low glass transition temperature. For example, polyethylene glycol (MW 400) is introduced into the hydroxypropylated lignin-modified PU (two types of isothiocyanates are used: hexamethylene diisocyanate [HDI] and toluene diisocyanate [TD]) and the effect of the amount of polyethylene glycol to the glass transition temperature and mechanical properties of the modified material are studied. With the increase of polyethylene glycol, the glass transition temperature of hydroxypropylated lignin-modified poly (vinyl acetate) (both HDI and TDI systems) decreases. For the HDI system, the increase of PEG content can lead the decrease of both Young's modulus and tensile strength of the hydroxypropyl lignin-modified PU material. When the PEG content (mass fraction) is 17.8%, the modules of hydroxypropyl lignin-modified PU material decreases by 3 to 4 times, and the elongation increases by about 5 times. The addition of polyethylene glycol to the TDI system has little effect on the mechanical properties of the modified material [12] because TDI has higher rigidity. These examples show that flexible polyol polymers can be used to solve the brittleness of the modified material caused by the rigidity of the lignin component. Such research provides new strategies and concepts for industrial application of lignin, enriches the regulation strategies of lignin-modified material properties, and enhances the competitiveness of lignin and other raw materials in the preparation of engineering plastics. Graft copolymerization modification can effectively combine the properties of lignin and modified grafted polymer chains to develop lignin-modified engineering plastics [23]. The properties of the graft copolymers mainly depend on the molecular weight of the lignin particles and the grafted polymer chain, the chemical structure and graft density of the grafted polymer chain, and the bonding type between lignin and the grafted chain. These grafted lignin copolymers not only can be used as compatibilizers during the blending of lignin with other thermoplastics, but they also can be applied to directly produce high-performance materials. Thermoplastic materials with high lignin content have been successfully prepared, with lignin consisting of 85% kraft lignin and 100% alkylated lignin. The kraft lignin is blended with polyvinyl acetate (PVAc, molecular weight 9.0 × 104) and diethylene glycol 3,4-benzoic acid, and indene is acting as plasticizer (kraft lignin, PVAc, and plasticizer mass ratio 16:2:1). After casting, a new composite material containing 85% craft lignin is achieved. The tensile strength and tensile modulus of the composites increase with the increasing of the average molecular weight of the lignin and can reach 25 MPa and 1.5 GPa, respectively, while the glass transition temperature of the material is 29.9°C. The alkylated lignin (including methyl and ethyl) prepared by etherification of kraft lignin can be used to prepare the 100% lignin material directly by casting with dimethyl sulfoxide as the solvent without any compatible aliphatic polyesters as plasticizers. The tensile strength and tensile modulus of the material are 37 MPa and 1.9 GPa, respectively. Comparing the properties of petroleumbased polymer plastics listed in Table 7.1, it is found that the two kinds of high lignin content materials have the same mechanical properties as the mechanical properties of currently used petroleum-based polymer plastics [24, 25].

186  Chapter 7 Table 7.1: Comparison of tensile strength and tensile modulus between high lignin content materials and general petroleum-based polymers (low density polyethylene, high-impact polystyrene, and polypropylene) [24, 25] Polymer Type

Tensile Strength/MPa

Tensile Modulus/GPa

Low density polyethylene High impact polystyrene Polypropylene 85% (mass fraction) kraft lignin material 100% (mass fraction) alkylated lignin material

14 28 35 25 37

0.22 2.1 1.4 1.5 1.9

7.3  Lignin-Modified Adhesives Because polar groups on lignin molecules can participate in the formation of physical interactions and perform good cohesiveness, lignin can be used directly as an adhesive. The lignin-containing waste liquor in the pulping industry also has relatively high viscosity and cohesiveness, and can be used as a raw material for adhesives. For example, the waste water containing sodium lignosulfonate can be coheated with formaldehyde in the presence of sodium hydroxide, and then react with phenol at 80–110°C for 1 h to prepare adhesives that can be used in particle boards, hardwood veneers, and plywood. The main advantage of developing lignin PF (L-PF) adhesive with lignin-containing waste liquor is that such strategy obviously can reduce the cost of preparation material. Practice has proved that the cost of using sulfite waste liquor and kraft BL as raw materials to make adhesives is lower than that of PF. Among the lignin-based adhesives, the lignin sulfonic acid in the sulfite waste liquor can be condensed with phenol and formaldehyde to produce an adhesive that is especially suitable for the production of fiberboard. In addition, the modification of lignosulfonate emulsion by nitrobenzene can improve its flexural strength. The results show that PFs can be replaced in the production of 20 mm thick single layer or three-layer particle board by lignincontaining sulfite waste liquor for about 25% and 35%, respectively, while the mechanical properties of the products are not changed [26]. Additionally, different sources and separation methods lead to the diversity of lignin chemical structure, thus affecting the comprehensive properties of lignin-based adhesives. The grafting copolymerization strategy usually is applied to eliminate the effect of lignin structure diversity and achieve uniform lignin surface structure. The most promising lignin/polymer adhesive is the thermosetting resin-based composite adhesives for the preparation of wood adhesives. It mainly consists of three types of resins: lignin urea formaldehyde resin (LU-F), lignin PU (L-PU), and L-PF. Low-cost urea formaldehyde (UF) resin is the most widely used wood adhesive. Such materials, however, usually have fatal defects, including poor water resistance and excessive residual formaldehyde content, which limit their scope of application. Using lignin instead of UF resin to make adhesives not only reduces the production cost, but it also improves the

Lignin-Modified Materials and Their Applications  187 water resistance of UF resin and reduces the content of free formaldehyde in the product. In most of the early studies, lignin is directly mixed with UF resin to produce lignin-based composite adhesives. Normally, lignin-containing sulfate waste liquid can be used to substitute 10%–50% of UF resin in the adhesive, reducing formaldehyde release by 10%–18%. This reduction occurs because of the chemical reaction between lignin with UF resin to form a more stable chemical structure, which can fix the formaldehyde. LU-F resin adhesive is prepared from sulfate waste liquor and the shear strength of the adhesive in dry and wet state is studied. The results show that it is suitable for plywood production. Although the direct mixing strategy to produce LU-F is simple and easy, the amount of waste liquid added in the production is too little, and the adhesion strength of the adhesive is low. Therefore, to overcome such problems, lignin is usually modified before mixing with UF to produce LU-F adhesives. The commonly used modification methods include hydroxymethylation, oxidation, and sulfonation. For example, the lignin is modified by formaldehyde, and then mixed with UF resin to prepare the LU-F adhesive. The free formaldehyde content in this kind of adhesive is less than 1%, which will not irritate eyes and skin, and has high bonding strength and stable properties [27, 28]. PU is a kind of reactive adhesive with good adhesion strength, chemical resistance, impact resistance, and low-temperature resistance. The cost of PU is comparably high, however, and traditional PU latex is usually difficult to degrade and recycle. This has caused a great pollution problem to the environment. Lignin-based PU adhesive has relatively high stability and has obvious advantages in environmental protection and human health. The addition of lignin not only can reduce the cost of PU, but also make the PU biodegradable, while the degradation rate is not too fast for the application. Lignin derivatives can be used to produce L-PU adhesives with polyester/polyether polyol and polyisocyanate (including cyclohexyl diisocyanate, methylene multiisocyanate, TD) [29]. Hydroxyalkylated kraft lignin, organic solvent lignin, steam explosion of lignin, and lignin sulfate can react with a crosslinking agent (such as a methylene phenylene isocyanate vinegar and methoxy methyl melamine) to prepare emulsion and solvent-based wood adhesive, respectively. The shear strength and the failure rate of wood can be the same as that of resorcinol formaldehyde resin and epoxy resin, which can be seen in Table 7.2 [20]. In addition, because the reaction between polyisocyanate and water at room temperature is slow, lignin-containing paper waste liquor can be mixed directly with polyisocyanate to prepare wood adhesives, and the porous structure of lignin Table 7.2: Comparison of properties between lignin polyurethane and other adhesives used in wood bonding [20] Adhesive Type

Shear Strength/MPa

Wood Failure Rate/%

Lignin polyurethane adhesive Resorcinol-formaldehyde resin Epoxy resin

16.0 15.9 16.1

60 ± 35 92 ± 2 30

188  Chapter 7 can absorb gases generated by the reaction between isocyanate with water, and will not affect the adhesion quality of the adhesive. Preparation of L-PU adhesives with lignincontaining papermaking waste liquor can be used to produce fiberboard, and the quality of the products can meet all the requirement of the standards of particle board. The properties of L-PU adhesive are similar to that of UF adhesives or PFs. However, in order to obtain sufficient adhesion strength and water resistance, before reacting with isocyanates, lignin must react with formaldehydes to obtain sufficient number of hydroxyl groups, which can ensure that a desired crosslinking structure can be produced via the reaction between lignin and isocyanates in the material. By this strategy, the quality of L-PU adhesives can reach the requirements of wood glue. PF adhesives have excellent properties such as high bonding strength, water resistance, heat resistance, and corrosion resistance, but the cost of PF adhesives is too high to be used in large-scale industrial production. Generally, lignin molecules that contain both phenolic hydroxyl groups and hydroxyl groups are used to modify PF adhesives, which not only can save the amount of phenol, but also reduce the residual amount of formaldehyde [30, 31]. To overcome the low reactivity of lignin, which hinders the normal polymerization of phenol and formaldehyde, lignin usually needs to be modified before the application. The main modification strategy of lignin is demethylation or methylation. By blending hydroxymethyl modified BL with low polymerization PF can be used to produce adhesives. It has been found that the content of free formaldehyde in the adhesive is only 0.007%–0.070%. Its excellent performance has reached the national standard, and without changing the traditional preparation process, the production cost is reduced by 28.69% [32]. Lignin also can be used for the preparation of epoxy resin adhesives and melamine formaldehyde resin adhesives. By blending lignin with epoxy resin for 2 h at 100°C, ligninmodified epoxy resin adhesive can be obtained. Compared with the unmodified adhesives, the adhesion strength of lignin-modified adhesive can be increased by 78%. In melamine formaldehyde adhesives, the addition of lignin can reduce the degree of crosslinking, increase the flexibility, and reduce the cost of the product. For example, the adhesive is prepared by copolymerization of lignosulfonate and melamine formaldehyde. The dosage of lignosulfonate is as high as 70% and this adhesive has excellent properties [33]. Enzymes released from white rot fungi can convert lignin into other compounds that have practical value, which make the lignin-modified adhesives perform biocatalytic effect during the bonding of particle plates. The curing of the adhesive is also achieved by the oxipolymerization of lignin by enzymes. Such bioadhesive process of lignin-based adhesives is suitable for the traditional molding process of particle board, and the board prepared this way will not release harmful gases during the application. A new type of biomimetic adhesives has been prepared successfully by the modification of kraft lignin and demethylation lignin with polyethyleneimine [34, 35]. The curing process consists of

Lignin-Modified Materials and Their Applications  189 two steps: Oxidation of phenolic hydroxyl groups to phenyl ketone and then reaction with polyethyleneimine. When the mass ratio of lignin to polyethyleneimine is 2:1, the adhesive has the highest shear strength and water resistance. That is, the adhesive bonded wood still has a high shear strength after immersion in water or even boiling water, as shown in Table 7.2. The application of lignin in biobased adhesives has attracted increasing attention, but the complex structure of lignin and its high molecular polydispersity, heterogeneous physical, and chemical properties limit its use. Two problems still need to be solved during the development of lignin-modified adhesives. First, the reactivity of lignin should be improved by chemical modification, especially by improving the activity of hydroxyl and phenolic hydroxyl groups. The most promising strategy to solve this problem is to phenolate or alcoholysis of the lignin. Second, the technological conditions and reaction mechanisms during the activation of lignin should be studied further. It is necessary to develop environmentally friendly biobased adhesives by using lignin and other types of derivatives and renewable biomass resources such as starches and proteins.

7.4  Lignin-Modified Fibers Spinning is a general technology for fiber preparation. The process involves spinning the polymer melt or concentrated polymer solution continuously, quantitatively, and evenly from the capillary pores of the spinneret into a liquid trickle by a spinning pump (or metering pump), then it solidifies into fibers in air, water, or a coagulating bath. Depending on the state of fiber's prepolymer, it can be divided into two types: solution spinning and melt spinning. Because the spinning materials containing lignin are similar to those of asphalt, phase transformation can occur at a large temperature difference during the spinning process. Therefore, the preparation of lignin-modified fibers usually is made by melt spinning, which has advantages of high winding speed, simple equipment requirement, and short process flow. From the rheology point of view, the shear flow at spinneret hole and the elongation flow on the melt trickle are the most important. They directly affect the diameter and unevenness of the spun fiber. In practice, the elements that can control the two kinds of flow behavior mainly include the solution composition, composition structure, spinning temperature, shear rate, cooling conditions of spinning line, winding rate, spinneret structure and size, and spinning length. These elements are interrelated and have synergistic effects. For melt spinning, the melt temperature is an important process parameter, and the rheological properties of the melt directly affect the formation of the fiber. When the temperature is too high, the viscosity of the melt is too small, and the stretch caused by its own weight is greater than spinneret and leads to filaments, breakage increase, and the failure of spinning. When the temperature is too low, the viscosity of the melt is too great, spinning is difficult,

190  Chapter 7 and fiber uniformity is poor. The shear rate is also a main factor affecting fiber formation. When the shear rate is too low, the flow rate of the spinneret hole is small, and is difficult to stretch long filaments. When the shear rate is too high, the elastic entrance effect is obvious and leads to unstable flow and the melt spinnability decreases, forming fiber surface defects. Therefore, the fiber stretching ratio is low, and the tensile orientation effect is not good. Spinneret structure and size are also factors affecting fiber forming. The main technical index of spinneret hole includes hole size, length/diameter ratio, and shrink flow structure shape. If the hole size is too large and the length is small, the shear effect of the melt is small, and the spinning fiber is too thick. If the spinneret hole size is too small, the length and diameter is relatively large, and the elastic energy storage is larger. This leads to an obvious elastic entrance effect. The cooling condition also affects the shaping of the fiber. A high-performance fiber with round section and consistent surface and internal structure can be obtained by selecting the cooling conditions corresponding to the spinneret section [36]. Therefore, the flow characteristics (especially the strength parameter) of the lignin-containing spinning materials are the basis for determining the technological conditions of melt spinning. In addition to the composition of lignin-containing spinning materials and the intrinsic factors of structure and molecular weight of each component, it is necessary to ensure the proper temperature conditions to achieve good fluidity and properties for fiber formation. Low temperatures during the spinning should be avoided, which can lead high viscosity and poor fluidity; high temperatures can cause spinning failure because of low viscosity. The increase of shear rate leads to a sharp decrease in viscosity of lignin-containing spinning materials. Therefore, it is necessary to consider the matching of temperature and shear rate in the melt spinning process. Hardwood kraft lignin (HKL) can be mixed with polyethylene terephthalate (PET), polypropylene (PP) [37], and poly ethylene oxide (PEO) [38] and melt spinning. Because of the good compatibility of HKL with PET and PEO, the fiber surface is smooth; the low compatibility of PP and hardwood sulfate lignin can be used to make porous fiber. Studies have found that the source of sulfate lignin directly affects the properties of the modified materials. Because of the poor heat transfer performance of cork sulfate lignin during the melt spinning process, HKL has better spinnability than that of the cork kraft lignin [39]. Compared with sulfate lignin, alkali lignin does not contain sulfur, so it will not pollute the environment and harm workers during spinning. Although soda hardwood lignin (SHL) has a good heat flux, the fibers directly produced by SHL are brittle. In order to solve this problem, PEO is used as a plasticizer in the melt spinning of SHL. The preparation process follows. First, SHL is dried to remove volatile substances. The dried SHL is mixed with PEO at a set temperature (170°C at a SHL/PEO mass ratio of 80:20; 180°C at a mass ratio of 95:5 and 90:10) and then the mixture is crushed into small particles. Finally, the mixture of particles spins through the rheometer (Rosand RH2000, the UK Worcestershire; radius 15 mm, roller length 250 mm) equipped with a spinneret. The spinning temperature is set to

Lignin-Modified Materials and Their Applications  191 190°C; the material is kept in the rheometer for 10 min by adjusting the rotation speed. The study shows that the SHL/PEO blended fiber has better spinnability than pure SHL fiber, and the higher the PEO content (quality score is 5%–20%), the easier the blended fiber spins. By increasing the stretch speed, the diameter of the SHL/PEO fiber can be reduced from 122 ± 17 μm to 15 μm. By optimizing the drawing process, the molecular orientation of the fiber is increased, and the tensile strength of SHL/PEO blended fiber is increased significantly, reaching about 20 MPa. However, the plasticization of PEO inevitably results in the decrease of modulus and tensile strength of blended fibers. When PP is used instead of PEO to blend SHL, the spinnability of SHL/PP blended fiber is poor. Compared to SHL/ PEO blended fiber under SEM (Fig. 7.1), it can be seen that the diameter of SHL/PP blended fiber is obviously greater. The mixtures with SHL have no spinnability when glycerol or PVA are used as plasticizers [40].

7.5  Lignin-Modified Nanofibers Electrospinning is a technique in which a polymer solution or a melt is used to form a charged jet by a high-voltage electrostatic field, then sprayed and stretched to produce a nanosized fiber. The diameter of fibers obtained by traditional spinning, template synthesis, and self-assembly is 5–500 μm, but the fiber obtained by electrospinning technology can reach nanometer scale, ranging from 3 nm to 5 μm. The nonwoven fabric produced by electrospinning has the advantages of high porosity, high specific surface area, high degree of

Fig. 7.1 SEM photos of blending lignin-based fibers. (A) SHL/PEO (80/20) wt% and (B) SHL/PP (80/20) wt%.

192  Chapter 7 fineness and homogeneity, and large aspect ratio. The mechanism of electrospinning begins when the droplets of polymer solution or melt at spinneret hole held by surface tension gather an electric charge on their surface via an external electric field. When the electric field is introduced, these droplets receive an electrostatic force opposite to the direction of surface tension. As the electric field is gradually increased, the droplet at the nozzle is elongated from sphere to taper cone to form a Taylor cone. When the electric field strength increases to a critical value, the electrostatic force can overcome the surface tension of the liquid and the droplets are ejected from the Taylor cone and solidified to form nanofibers [41]. The electrospinning schematic is shown in Fig. 7.2. Capillaries can be placed horizontally or vertically, the difference between two lies is in the formation mechanism of the droplets. When the capillary is placed horizontally, the piston is used to squeeze the fluid in the capillary to form droplets. When the capillary is placed vertically, the droplets can be formed by gravity or pump squeezing. Sometimes, the capillary is placed at different angles in order to control the fluid flow to form droplets. The nanofiber receiving plate is usually a fixed grounded metal plate or grid, and the spun nanofibers are deposited on a receiving plate. Three factors that influence the preparation of nanofibers by electrospinning are: (1) Fluid properties: Such as polymer molecular weight, fluid viscosity, solution concentration, elasticity, conductivity, surface tension, phase transition heat, and specific heat. (2) Process parameters: Such as the static voltage in the capillary, the electric potential at the capillary port, and the distance between capillary port and the collector. (3) Environmental parameters: Such as fluid temperature, air humidity, temperature, and airflow rate of the spinning environment.

Fig. 7.2 Schematic diagram of electrospinning device.

Lignin-Modified Materials and Their Applications  193 Lignin-modified nanofibers have been successfully prepared by electrospinning from aqueous of lignin and PVA blends. Fig. 7.3 shows the ternary phase diagrams of the spinnability of lignin-PVA-water blended system, establishing the relationship between the composition ratio and the fiber size morphology [42]. The ternary phase diagram is divided into three regions: the beaded fiber region, the nonbeaded fiber region, and the two-phase region. In the nonbeaded fiber region, the high content of lignin makes it easier to prepare nonbeaded fibers because pure PVA is a good fiber-forming polymer and mixing it with lignin can increase the fiber-forming effect. With the increase of the total concentration of spinning solution, the diameter of nanofibers increases. In the two-phase region, the viscosity of the solution is too high and is not suitable for spinning. The lignin-PVA-water ternary phase diagram can be used as a basis for predicting the formation of the lignin/PVA/water system, in which nanofibers

Fig. 7.3 Ternary phase diagram of the spinnability of lignin-PVA-water blended system and SEM photo. The diagram is separated into three sections: beaded fiber, nonbeaded fiber, and two-phase section. In the beaded section, the curves indicate the fibers with same diameter (100 nm, 200 nm, 400 nm, and 750 nm), with a half black circle for the beaded fibers, solid circle for no-beaded fibers, and circle for the phase separate fibers.

194  Chapter 7 have specific size and morphology, and further introduce cellulose nanocrystals (CNC) to enhance the lignin-modified PVA nanofiber material. The introduction of CNC directly affects the interaction between lignin and PVA and the apparent concentration of the whole dispersion system. The viscosity of lignin-PVA-CNC suspension increases with the increase of CNC content. What's more, lignin, PVA, and CNC form a physical network structure in the suspension system and present the gel state at lower shear rate. Taking a 75:25 ratio of lignin and PVA as an example, the surface tension and viscosity increase with the increasing of the content of the CNC system. In this system, the increase of surface tension leads to the formation of beaded fiber, and the increase of viscosity makes it easy to form nonbeaded fiber. When the content of CNC (mass fraction) is 5% and 10%, the surface tension takes the leading role, and the system forms beaded fiber. When the content of the CNC is 15%, the viscosity factor takes the leading role, and the system tends to form nonbeaded fiber. When the system of lignin and PVA has a ratio of 20:80, the introduction of CNC reduces the surface tension and increases viscosity, beaded fiber will be prepared. Thus, the influence of adding CNC on the morphology of nanofibers depends on many factors, such as the amount of CNC and the ratio of lignin and PVA. At a microscopic level, it relates to the viscosity-related balance effect of three kinds of interaction about CNC/lignin, CNC/PVA, lignin/PVA in the system [42]. The nanofiber membranes of lignin-modified polyacrylonitrile (PAN) are prepared by electrospinning with N,N-dimethylformamide as the blending solvent. Fig. 7.4 shows the SEM photographs of different content of lignin-modified electrospun nanofibers. When the lignin content (mass fraction) is 50%, the nanofibers have uniform size and average diameter is about 300 nm. With the increase of lignin content, the uniform fiber morphology changes

Fig. 7.4 SEM photo of lignin-modified PAN nanofiber produced films with different contents of lignin.

Lignin-Modified Materials and Their Applications  195 into nonhomogeneous bead shape. Lignin-modified PAN nanofiber electrospinning film irradiated by an electron beam can further improve its mechanical properties and thermal stability [43]. The highly soluble, environmentally friendly ionic liquid is used as solvent in the electrospinning system. For example, cannabis and lignin are dissolved in ionic liquids and prepare lignin-modified cannabis nanofibers by electrospinning. When the content of lignin is low, the spinnability of the solution is better, and nanofibers produced by this strategy are finer, with a uniform diameter and higher crystallinity [44]. In order to confer the more functional properties of lignin-modified nanofibers, the radical polymerization of N-isopropylacrylamide is initiated on the surface of the lignin-modified nanofibers to form shells with dual ion- and temperature-responsive properties, which is expected to be applied to thermal response separation and purification device [45]. In addition, lignin-modified nanofibers prepared by electrospinning can be used as precursors of lignin-based carbon nanofibers. Hollow micro/nanofibers are prepared by coaxial electrospinning with an ethanol solution of hardwood lignin, and smooth carbon fibers with a diameter of about 200 nm are formed by further carbonization treatment. The adsorption of carbon fibers on N2 and CO2 is almost zero, which implies that the nanofibers are nonporous structural fibers [46].

7.6  Lignin-Modified Film Materials Other than the traditional cast-molding strategy, new techniques that suit for industrial production also have been attempted for the preparation of lignin-modified film with various structures. Blown film, self-assembled film, spin-coating film, and electrospinning film prepared by blow-molding, self-assembly, spin-coating, and electrospinning technologies have shown a potential for further application. Cast molding strategy, one of the most common shaping technologies for film preparation, has been applied widely in the studies for the preparation of lignin-modified films (mentioned in Sections 5.1.5 and 6.1.1). Wolfgang G. Glasser prepared a series of lignin-modified PU film by cast molding and systematically studied the influence of the factors such as the source and type of lignin, lignin content, molecular weight, diisocyanate type, NCO/OH molar ratio, and the third component soft segment on the structure and properties of the modified material [10, 12, 14–21]. Studies on lignin-modified films prepared by cast-molding found that the solubility and stability in dispersion of lignin is crucial for the performance of film formation and its properties; also important is the compatibility of lignin in matrices and the distribution and dispersibility of lignin in the produced films. The solubility and stability of lignin in the dispersion in different solvents are related to the sources, separation, and extraction methods of the lignin, and can be controlled by chemical modification. Nitrification-modified lignin can be dissolved in tetrahydrofuran, acetone, and water, and can be cast with PU matrix in tetrahydrofuran solvent [47]. Such lignin also can be used to modify water-borne PU by cast-molding after chain extension, emulsification, and emulsion formation. For unmodified lignin, the extraction method of lignin will affect the network structure of the modified PU film material,

196  Chapter 7 while the order of the lignin types that facilitate the network structure formation of the film is steam explosion lignin > organic solvent lignin > kraft lignin > acid hydrolysis lignin, and the increase of network crosslinking corresponds to the increase of glass transition temperature of the film. This indicates that the solubility of the prepolymerized lignin is the key parameter that determines the consistency of the lignin in the thermosetting network and the properties of the modified material [48]. The low-crosslinking density lignin-modified PU film was prepared by low molecular weight kraft lignin and different molecular weights of polyethylene glycol and TD. When the NCO/OH molar ratio is 2:1, the maximum Young's modulus and stress of the materials can be achieved, which is 1.25 GPa and 48 MPa, respectively [49]. Except for cast films, lignin-modified films can be prepared by blow molding method. Lignin-modified PVA film material can be prepared by lignin, PVA, and glycerol blends through blow-molding [50]. The spin-coating method is the earliest and most widely used strategy to prepare membranes. Taking the three kinds of lignin―softwood kraft lignin, cork mill lignin, and hardwood mill lignin―as raw materials, combining with spin-coating technology, the lignin-based spincoated film is prepared. The process of spin-coating cork craft lignin film follows: 1.5% lignin solution is prepared by dissolving lignin in 1 mol/L ammonia solution for 12 h. Then the lignin solution is spin-coated (time 60 s, rotation speed 1500 r/min) on silicon wafer to prepare smooth and ultrathin lignin film (thickness 50–60 nm) and was placed in ultrapure water for 2 h. Finally, the film is dried under nitrogen to achieve the final product. The results show that the mean square root roughness of all the films is in the nanometer size (kraft lignin is 0.93 nm; hardwood lignin is 1.38 nm; cork lignin is 1.31 nm) and has no crack or hole. The surface energy of lignin and kraft lignin is 53–56 mJ m−2 by contact angle test. Because of the formation of polar groups (carbonyl and phenolic hydroxyl groups) on lignin molecules during pulping, the content of the polar component of the surface energy of the spin-coated film of lignin kraft paper is higher than that of the lignin spin-coated film. The surface energy of lignin-based spin-coating is similar to that of cellulose, but the contact angle of cellulose is lower than that of lignin spincoated film. This indicates that the difference in solid-liquid interfacial energy determines the wettability of water to cellulose and lignin [51]. This conclusion has important implications for understanding the transport mechanism of water in plants. Self-assembly refers to when the molecules in the system spontaneously assemble into highly ordered mesostructures with specific physical and chemical properties without external interference. Self-assembly technology can be used to prepare lignin-modified film materials. LB (Langmuir-blodgett) membrane is an ultrathin ordered membrane that disperses amphiphilic molecules at two different interfaces (gas/liquid interface), is driven by a certain pressure, and relies on self-assembly capacity between film-forming molecules, arranged in a highly ordered and relatively dense monolayer. The monomolecular layer is transferred to a solid substrate by vertical or horizontal pulling to obtain a monolayer film, repeated pulling

Lignin-Modified Materials and Their Applications  197 also can be used to produce a multilayer film. LB film is ultrathin, with uniform thickness, and a precisely controllable molecular layer, high anisotropy, and no damage to the substrate. Lignin (BL), extracted from bagasse by n-butanol-supercritical CO2 strategy, was transferred onto the substrate to prepare a Y-type LB film under the water subphase at a pressure of 25 mN m−1. The effect of the concentration of metal ions in the subphase and the temperature of different water subphase on the surface pressure (Π) is determined and the average molecular area (A) curve is studied. The average molecular area decreases with the increase of temperature. On the other hand, the average molecular area increases with the increase of metal ion concentration, suggesting that the LB film is expected to be used in detecting metal ions in water [52]. By using layer-by-layer strategy to self-assemble lignosulfonate (LS) and Cu2+ at the surface of paper pulp, a thin film with hydrophobicity can be formed. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and dynamic contact angle are used to characterize the surface of the assembled film, and the results show that the content of S and Cu increased with the alternating assembly of LS and Cu2+, which indicated that LS and Cu2+ could be self-assembled on the surface of the fiber. It is found that the initial contact angle of the fiber after assembly of five layers of lignin increased from 0 to 104.8 degrees, and then decreased to 78.9 degrees after 0.08 s, which indicates that the surface changes from highly hydrophilic to hydrophobic. Therefore, through controlling the number of LS self-assembled layers (as shown in Fig. 7.5), hydrophobic modification of pulp fibers can be achieved efficiently and controllably [53]. The nanofibers prepared by electrospinning are deposited on each other and form an electrospinning material with a porous structure. The characteristics of the porous structure are particularly suitable for the preparation of tissue engineering scaffolds, drug delivery, surface dressings, and suction masks. This is because the electrospinning film has good biocompatibility, high porosity of fiber membrane, high specific surface area, and good fiber uniformity. For tissue engineering scaffolds, the high porosity of the electrospinning film can provide more growth space, and high specific surface area is conducive to cell adhesion and reproduction. Good porosity permeability is suitable for mass exchange between the scaffold and the environment. For drug-loaded materials, the high specific surface area of the electrospinning film can slowly decompose the drug, which the human body has difficulty absorbing so it can play a role in protecting sensitive drugs and controlling the release rate. These lignin-modified PVA nanofibers creates a typical electrospun film. For example, by using electrospinning, lignin and polyvinyl can be weaved into electrospun films and the radius of the nanofiber in the film increases with the increase of lignin content. The average radius of nanofibers was 89 ± 2 nm for lignin/polyvinyl alcohol (mass ratio of 20:80), and when the ratio of lignin/polyvinyl alcohol changed to 75:25, the average radius of the fibers will increase to 148 ± 4 nm. The water contact angle was closely related to the chemical composition (surface energy), surface roughness, and morphology of the surface of electrospun film. Compared with the electrospun film and its corresponding spin-coated

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Fig. 7.5 AFM phase diagram of the layer-by-layer self-assembly of lignosulfonate on pulp fibers [53]. (A) 0 layer; (B) 1 layer; (C) 2 layer; and (D) 5 layer.

film, the water contact angle of both films decreased with the increase of lignin content (mass fraction), and the trends were basically the same (as shown in Fig. 7.6) because the primary determinant is their similar surface chemical composition, while surface roughness and morphological differences have no effect on the water contact angle [54]. Water-responsive lignin-based films can be prepared via electrospinning by kraft lignin with different fractions (according to the differences between molecular weights of lignin). During the oxidative heat stabilization stage, the differences in the thermal fluidity of different fractions of lignin affect the degree of interfiber fusion, leading to different material shapes, including submicron fibers, bonded nonwovens, porous membranes, and smooth films. The results show that the relative content of the different lignin fractions and the degree of fiber flow and thawing could affect the tendency of electrospinning fibers to transform into water-responsive materials.

Lignin-Modified Materials and Their Applications  199

Fig. 7.6 The water contact angle of lignin/PVA electrospinning (A) and spin-coat (B) films and their SEM and AFM photos [54].

The regulation of lignin film morphology can be adjusted by changing the relative content of different lignin fractions and the rate of heating. When the film is exposed to moisture, the material deforms immediately. It takes 30–60 s to reach the maximum deformation, while larger deformation can be observed at the first 10 s (as shown in Fig. 7.7A–D). When the film is transferred to a dry environment, the material is gradually reduced to the original shape, and shape recovery is a slow process (as shown in Fig. 7.7E–H) for about 60–120 s [55].

7.7  Lignin-Modified Foaming Materials As an enhancer or main reactant in a reaction, lignin can be added to various existing foaming systems to prepare lignin-modified foams, and can use lignin-containing BL to prepare lignin-modified PU foams, which provides a feasible way for the rational use of BL and reduces environmental pollution. Foams produced by pure lignin rarely have been reported. The addition of lignin can improve the mechanical properties (compressive strength, tensile strength, and Young's modulus), thermal stability, pore size and uniformity, foam density and porosity of the foam. What's more, it can save costs of foam production. Lignin structural units have a large number of hydroxyl groups on the benzene ring and the side chain. Therefore, lignin can be used instead of polyols as raw materials in synthetic

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(A)

Wet surface

(B)

t=0

(C)

t=2s

(D)

t = 10 s

(E)

Dry surface, t = 0

(F)

t = 10 s

(G)

t = 30 s

(H)

t = 45 s

Fig. 7.7 Heat-stabilized methanol extracted lignin fraction/methanol extracted lignin fraction blended produced film and its shape change after being put on a wet paper (A–D) and dry paper (E–H) (heating rate 5°C/min) [55].

PU foams. For example, alkali lignin-modified rigid PU foams are prepared by using alkali lignin extracted from papermaking BL to react with isocyanates instead of partial polyether polyols. The addition of lignin improves the mechanical properties of PU foams, and the tensile strength and flexural strength of the modified foams reaches 0.925 MPa and 0.36 MPa, respectively, when 15% of polyol is replaced. These properties are far superior to the tensile strength (0.147 MPa) and flexural strength (0.196 MPa) of polyether PU industrial foam [56]. The reinforced PU foam is prepared by using silicone resin as surfactant, a small amount of water as blowing agent, butyltin dilaurate as catalyst, and lignin and saccharide (lignin content was only 1%) as reinforcing agent. The results show that the density of modified PU foam increases with the increase of lignin content, while the compressive strength and elastic modulus increases linearly [57]. The amorphous structure of the PU matrix in the modified foams indicates that lignin serves as an enhancer in the network structure of the PU. Alkali lignin and sodium lingosulfonate are respectively melt mixed with corn protein in the presence of plasticizer polyethylene glycol (MW 400) to prepare thermoplastic biobased blends, then the lignin-modified corn protein foam is prepared at 50–60°C with the mixture of CO2 and N2 as foaming agents. When the lignin content (mass fraction) is 1%, the lignin-modified foam has more cells and more homogeneity than pure corn protein foam, the density of the foam decreases from 0.53 g/cm3 to 0.45 g/cm3. For the

Lignin-Modified Materials and Their Applications  201 high alkali lignin content (mass fraction of 10%) modified material and the lignosulfonate (1% and 10%) modified material, the overall foaming properties of the modified system were inhibited [58]. Highly active hydroxyl groups in the molecular structure of lignin can react with the diisocyanate component, which is essential for the construction of the PU system. Therefore, in the preparation of lignin-modified PU foam, hydroxyls on lignin molecules are considered to be the key factor for material molding. Sodium lignosulfonate is dissolved in a mixture of diethylene glycol, polymethylene polyphenyl isocyanate (PDMI), and triethylene glycol and poly (ethylene glycol) (MW 200) with plasticizer (silicone surfactant), catalyst (din-butyltin dilaurate), and foaming agent (water) to prepare the lignin-modified rigid PU foam. With the change of lignin content (mass fraction) from 0 % to 33%, the appearance of the modified foams changed from bright brown to dark brown, the apparent density ranged from 0.08 to 0.12 kg m−3, and the pore size of the foams was between 100 and 300 μm observed by SEM. The lignin-modified PU foaming system showed obvious glass transition behavior in the temperature range of 80–140°C. Increasing the lignosulfonate content or decreasing the ethylene oxide unit in the molecules can result in the increase of glass transition temperature (Tg) of the modified PU foam. Especially, for polyethylene glycol/lignin-modified foam, the relationship between Tg and lignosulfonate content is linear [59, 60]. Sodium lignosulfonate, honey, and polyethylene glycol are blended with PMDI to prepare thermoset PU foam. With the increase of lignosulfonate content, the Tg of the modified foams did not change significantly, but the apparent density increases slightly (range 0.06–0.09 kg m−3), the compressive strength, yield strength, and compressive elasticity also increase at 10% strain [61]. PU foams are prepared by the reaction of lignin dissolved with diisocyanate in ethylene glycol (MW 200) solution. The properties of the material can be changed by altering the content of lignin in the polymer. As the lignin content increases, the glass transition temperature and compression strength of the modified foam increases and the thermal decomposition temperature decreases [62]. Taking advantage of the microwave technique, using the water as the foaming agent, liquefied kraft lignin is used as a chain extender under the conditions of poly (propylene glycol) and castor oil, and controlling the molar ratio of NCO/OH to be less than 1, a highly elastic, flexible PU foam can be obtained. Such flexible lignin-modified foams are shown in Fig. 7.8 [63]. By using ethylene glycol instead of polyvinyl alcohol, dissolving lignin, and reacting with diisocyanate, L-PU foam is prepared successfully. In order to further improve the reactivity of the hydroxyl groups on the lignin molecules and match the viscosity of the polyol that is used in the preparation of the PU foam, the lignin can be reacted with ethylene oxide, propylene oxide, and alkyl sulfide to improve the reactivity of hydroxyl groups with diisocyanates to make rigid PU foams [64, 65]. The low-density foaming material containing 20% hydroxypropyl lignin prepared by such modified lignin and furan polyol performed at a moderate strength and excellent flame retardant [66].

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Fig. 7.8 Photo of high-elastic lignin-modified PU foams.

7.8  Lignin-Modified Hydrogel Hydrogel is a kind of polymer material with three-dimensional crosslinked network that can absorb water and swell and can keep large amount of water in its network. It has been proved that the introduction of lignin can change the lowest critical solution temperature (LCST) of temperature-responsive hydrogel, and give hydrogels solvent sensitivity, pH sensitivity, or adsorbability. The main strategies of preparing lignin-modified hydrogel are grafting and crosslinking lignin with hydrophilic monomers or lignin interpenetrating and semiinterpenetrating into the hydrogel matrix. The polymer chains in temperature-sensitive hydrogel usually have a proportion of hydrophobic and hydrophilic groups. When the ambient temperature is lower than the LCST of the gel, hydrophilic groups on the polymer chains of the hydrogel are bound to the water molecules through hydrogen bonds and swell. With the increase of temperature, the strength of hydrogen bonds decreases, the interaction of hydrophobic groups in the polymer chain is strengthened, and the gel shrinks gradually [67]. When the temperature rises to above LCST, the hydrophobic interaction becomes the main interaction between polymer chains, the polymer chains gather to each other through hydrophobic interaction, then the gel phase changes, and the swelling rate drops sharply. Lignin is used to modify the thermosensitive hydrogel, and its rigid three-dimensional network formed in hydrogel can improve the strength of the hydrogel without affecting its biocompatibility. The addition of lignin can give the hydrogel some special functions, which shows the high-value use of lignin. The reaction of lignin acetate with N-isopropylacrylamide is carried out in the presence of crosslinking agent (N, N′-methylenebisacrylamide) and initiator (H2O2) to produce thermosensitive lignin hydrogel. The LCST of the lignin-modified thermosensitive hydrogel is about 31°C and the rapid decomposition temperature is 400–410°C. With the increase

Lignin-Modified Materials and Their Applications  203 of lignin content, the pore size of lignin-modified hydrogel increases, ranging from 20 to 100 μm [68]. The thermosensitive hydrogel with porous network structure made of lignin acetate and N-isopropylacrylamide also can be prepared by UV irradiation. The pore size and temperature sensitivity of the gel are determined by the mass ratio of lignin acetate/Nisopropylacrylamide. The LCST of the lignin-modified hydrogels decreases with the increase of the mass ratio of lignin acetate/N-isopropylacrylamide [69]. The structural properties of the different types of lignin are expected to confer pH-sensitive and solvent-sensitive properties to the hydrogels. Lignin can be dissolved in alkaline solution or partially soluble in ethanol, so lignin-modified hydrogel can be swollen in alkali and ethanol solution. This property can be used to make a lignin-modified hydrogel to be the carrier of alkali-soluble or alcohol-soluble drugs. The lignin-modified hydrogel is prepared by dissolving the lignin in NaOH solution and then crosslinking it with polyethylene glycol glycidyl ether. The swelling ratio of the modified hydrogel in ethanol aqueous solution reaches its maximum when the volume fraction of ethanol and water is 50%. The swelling rate in its alkaline solution shows pH sensitivity, which is not possessed by hydrogels prepared by polyethylene glycol and glycidyl ether alone. This unique swelling behavior might be related to the amphiphilic nature of the lignin molecular structure [70]. In addition, acetate lignin can be chemically crosslinked with PU to prepare hydrogels, and the swelling ratio of hydrogel is related to pH value. The addition of lignin improves the thermal stability of the hydrogel, and it can be used as a material for sustained release of fertilizer [71]. First, the kraft lignin reacts with phenol, then it reacts with resorcinol under basic conditions to obtain lignin-phenol-resorcinol resin. Finally, a crosslinker, glutaraldehyde, is added to prepare the hydrogel. The lignin-modified hydrogels are immersed in water and ethanol alternately, and show a swelling and shrinking behavior alternately [72]. The hydroxyl groups, ether groups, carbonyl groups, and benzyl groups in lignin structure can form hydrogen bonds with the hydroxyl groups in alcohols. The aliphatic and aromatic groups can interact with the aliphatic groups in alcohols by van der Waals forces. These two interactions give the adsorption of lignin to alcohols [73]. The hydroxyl and carbonyl groups on the lignin can interact with the metal ions, showing that lignin has the ability to adsorb metal ions. When lignin is introduced into the hydrogel system prepared from starch/ acrylamide by interpenetration, the hydrogel is compared with peat-modified hydrogels. Fig. 7.9 shows SEM photographs of the interior of the lignin-modified hydrogel, indicating that the lignin component is well dispersed in the hydrogel matrix. The adsorption capacities of Cu (II) and Ni (II) by hydrogels are studied. The results show that the adsorption capacity of Cu (II) and Ni (II) is better than that of peat-modified hydrogels [74]. As a filler, kraft lignin can be added to the carboxymethyl cellulose hydrogel microspheres, and it has been found that the introduction of lignin can reduce the release rate of the aldicarb (a carbamate insecticide) loaded hydrogel microsphere. And the release rate of aldicarb decreases markedly with increasing lignin content [75].

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Fig. 7.9 SEM photo of the internal structure of lignin-modified hydrogel.

7.9  Lignin-Modified Material as Precursor for Carbon Materials Carbon fiber is high strength, low density, corrosion resistant, aging resistant, and conductive, and is an essential new material for the development of aerospace, military, and other cuttingedge technologies, and is one of the basic materials for civil industry. Carbon film is an inorganic membrane that has developed rapidly in recent years. It has a high mechanical strength and superior resistance to high temperature, acid and alkali, and chemical solvents. Lignin and its modified materials can be used as precursors to prepare the carbon fiber and film-based materials. Organosolv lignin (AWL), which is extracted from acetic acid pulping without chemical modification, can be used as a precursor of carbon fiber materials by melt spinning. The spinnability of AWL is related to the polydispersity of lignin and the acetylation degree of the hydroxyl groups on lignin in the extraction process. The lignin fibers can be further oxidized and carbonized to obtain carbon fibers. It has been found that the mechanical properties of the carbon fiber prepared by AWL are related to its diameter, which usually is 14 ± 1.0 μm, while the elongation, tensile strength, and elastic modulus of the fiber are 0.98 ± 0.25%, 335 ± 53 MPa, and 39.1 ± 13.3 GPa, respectively, which can satisfy the general standard of carbon fibers [76]. Lignin-modified polymer fibers also can be used as precursors for the preparation of carbon fibers, while the diversity of polymer matrices can lead to the difference of resulting carbon fibers in structures and properties. For example, different contents of lignin can react with different contents of formaldehyde and phenol to prepare an L-PF, then the lignin-modified PF fiber can be prepared by wet spinning, and further preoxidation and carbonization to achieve carbon fiber. The addition of lignin can directly affect the size of the pores in carbon fibers (Fig. 7.10), and the addition of lignin can enhance the thermal stability of the fiber, reducing the degree of thermal degradation [77]. Polarized HKL can

Lignin-Modified Materials and Their Applications  205

Fig. 7.10 SEM photos of lignin/phenolic risen/carbon fiber composite material. (A) Lignin content (weight%) 8%; (B) Lignin content (weight%) 14%; and (C) Lignin content (weight%) 20%.

be blended with PP, then the lignin-modified PP with surface polarity and porous structure is prepared after thermal stabilization and carbonization. The carbon fiber produced by this strategy has similar adsorption/desorption isotherms to other activated carbon fibers, and the inner surface area of carbon fibers from lignin-modified PP is 499 m2 g−1 when the content of HKL is 62.5%. Although the inner surface area is less than commercial activated carbon fiber, this porous lignin-modified carbon fiber is inactivated carbon fiber and can be activated easily by steam, and it can be used as precursor of activated carbon fibers [78]. Softwood lignin obtained by the acetic acid pulping method also is used as a raw material (after removal of insoluble macromolecular distillates and unstable substances in lignin) for direct spinning at 350–370°C. The carbon fiber can be obtained directly by carrying out the carbonization treatment without preoxidation of this kind of lignin fibers. Although the performance of the product is lower than that of the carbon fibers after preoxidation treatment, the carbon fibers produced by this carbonization strategy still can meet the general level of carbon fiber standards. When mixing nickel acetate (as catalyst) with lignin prepared by acetic acid pulping to make carbon fibers, the structure and crystallinity of produced carbon fibers can be improved. However, the strength of the carbon fiber achieved by this strategy is low because of the remaining catalysts inside the fiber. The hardwood lignin is heated at 160°C for 30 min in a vacuum environment, then blended with PET and PP, respectively. The mixture was melt-spun at 130–240°C and treated at 250°C for 1 h to make lignin-based polymer fibers. The fibers can be carbonized at 1000°C in a nitrogen atmosphere to make carbon fibers. The results show that the lignin/PET blends have higher heating rate than the lignin/PP blends because the stability of the fibers is dependent on the thermal stability conditions and the content of the blended polymers. In fact, increasing the rate of heating above 120°C/h causes the lignin/PP blends to fuse together during the process. Air oxidation makes lignin fiber deformation, and increasing the PP, especially PET content, will improve the stability of the fiber. Increasing the amount of mixed polymer, however, will reduce the carbon fiber yield. If the carbonation process is accompanied by thermal decomposition of the polymer, the ideal yield is only about 34.3% when the mass ratio of lignin and polymer is 73:25 [37, 79].

206  Chapter 7 Except to make carbon fibers by spinning the composite material, lignin-modified PFs can be used as a membrane precursor to produce carbon films with high absorbability [80]. The content of lignin in the modified PF can be used to control the micro/nano pore structure of carbon film. The preparation process follows. First, phenol is heated to liquid, then mixed with formaldehyde at a molar ratio of 6:7, a small amount of NaOH solution is added as catalyst, and lignin is added to mass fraction of 8%, 14%, and 20%, respectively, and the mixture reacts at 90°C for 2 h. The product of the reaction is coated on the glass plate and placed in an oven for 12 h to dry the film. Finally, the film is carbonized at 800°C for 1 h in a carbonizing furnace with nitrogen gas to achieve carbon films. As the lignin content in the film precursor is different, and lignin is the main component that can lead carbon film to have microporous structures, the porosity of the prepared carbon film is different. When the lignin content is 8% (as shown in Fig. 7.11A and B), the microscale pore size in the carbon film is between 1.1 and 2.6 μm and the nanoscale pore size is between 120 and 320 nm.

Fig. 7.11 SEM photos of two types of lignin produced carbon films. (A) 8% weight fraction of lignin, 5000×; (B) 8% weight fraction of lignin, 20,000×; (C) 14% weight fraction of lignin, 5000×; (D) 14% weight fraction of lignin, 20,000×.

Lignin-Modified Materials and Their Applications  207 When lignin content is 14% (as shown in Fig. 7.11C and D), the 80–830 nm nanopores will be formed in the carbon film. When the lignin content is 20%, the pore size of the carbon film is polarized, but the adsorption performance of the carbon membrane is best among the three samples. The absorption abilities of the 8% and 14% lignin-containing carbon film are similar to each other.

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