The Earth After Cellulose

Published Date: 02 Nov 2017

Ekaterina Korotkova

Vinay Kumar

Lignin-based biocomposites

Biocomposites course 2013

1. Introduction

Lignin is the second most abundant macromolecule on the Earth after cellulose. Tons of lignin

waste are produced annually by pulp and paper mills. Most of lignin is getting burned to

recover energy. But nowadays a lot of researchers are focused on new value added ways of

lignin utilisation. Lignin can play a role in replacing or partly replacing petroleum-based

components in a broad range of composite materials. The great number of publications

focuses on blending of lignin with natural and synthetic polymers. While properties of

homopolymer can be designed by controlling degree of polymerisation, of branching, and

crosslinking, blending of two or more polymers provides the ability to tailor material

properties to achieve specific goals with higher value. Blended with natural and synthetic

polymers lignin increases the modulus and cold crystallisation temperature but decreases the

melt temperature [Doherty et al 2011].

One of the interesting applications of lignin is using of lignin carbon fibres as a reinforcement

of composites for material applications. Carbon fibers are a relatively new breed of highstrength

materials, which have superior properties compared to traditional ones. Carbon fibers

have high stiffness, high tensile strength, low density, elevated temperature resistance, good

electrical conductivity, and low linear coefficient of temperature expansion [Foston et al.

2013, Chand 2000].

2. Physical properties of lignin

To be able to act as a part of composite material lignin should fulfil some thermal and

mechanical requirements. Melting temperature, or glass transition temperature in case of

amorphous materials, is one of important characteristics of the material as a lot of techniques

for composite production require thermal treatment. When the temperature of polymer

reaches Tg, stiffness of the material decreases rapidly. Glass transition temperature of lignin

can be greatly influenced by presence of other polymers for example hemicelluloses and

water which acts as a plasticizer. Tg of different lignin types can be found in Table 2.1.

Table 2.1 Tg of different lignin types [Doherthy et al 2011 adopted from Gargulak and Lebo,

2000]

Types of lignin Tg (°C)

Milled wood lignin

Hardwood 110 – 130

Softwood 138 – 160

Kraft lignin 124 – 174

Organosolv lignin 91 – 97

Steam explosion lignin 113 – 139

Tg generally increases with increasing molar mass. Studying of influence of lignin structure

on its thermal properties with no influence of its molar mass is challenging. Lately it was

investigated using transgenic trees with different levels of lignin content and syringyl to

guaiacyl ratios. Hovarth et al. (2011) used transgenic aspen wood with increased S/G ratio.

Despite the higher amount of methoxyl groups, which were believed to decrease Tg, an

increase in the S/G ratio did not affect the glass transition temperature of aspen lignin.

Baumberger et al. (2002) showed that increase in the glass transition temperature of lignin

isolated from genetically modified poplar trees can be correlated with the condensation degree

of lignins.

3. Properties of Lignin-based biocomposites

Lignin-based biocomposites form a relatively small group of biocomposites. Their properties

are controlled by various parameters like source of lignin, processing technique, modifications

etc. Different properties of lignin-based biocomposites are discussed in this section.

One of the main components of lignin-based biocomposites is lignocellulose-based fibers

which are prominently used as biodegradable filler [John and Thomas 2008]. These fibers

have a number of interesting mechanical and physical properties. Biocomposites are produced

by combining lignocellulosic fillers (LCF) which are by-products of an industrial fractionation

of wheat straw with biodegradable aromatic copolyester, polybutylene adipate-coterephthalate

(PBAT) [Avérous and Digabel 2006]. The amount of lignin affects the final

composite properties and a large range of thermal and mechanical properties can be obtained

with varying lignin amount in the filler [Digabel and Avérous 2006].

Silica/lignin biocomposites have also been recently synthesized (process shown in Fig. 3.1)

and their physiochemical and electrokinetic properties were thoroughly analysed

[Klapiszewski et al. 2013]. A highly functional material of new generation can be obtained by

using lignin in an inorganic matrix which can become important for general economic

development [Klapiszewski et al. 2012].

Figure 3.1 Technological process for production of silica/lignin biocomposite (1–mixing

device, 2–pump, 3,4–reactor, 5–vacuum evaporator, 6–convection dryer) [Klapiszewski et al.

2012; Klapiszewski et al. 2013].

Fig. 3.2 shows particle size distribution and SEM images of silica/lignin biocomposites.

Separate primary particles can be observed from the SEM images that are gradually joined to

form aggregates (to 1 μm) and agglomerates (above 1 μm) [Klapiszewski et al. 2013]. Fig. 3.3

shows the electro-kinetic properties of silica/lignin biocomposites. The composites seem

electro-kinetically stable as they show high negative values of zeta potential in the entire pH

range studied. The amount of lignin used in obtaining the final biocomposites was also found

to affect the homogeneity of the biocomposites [Klapiszewski et al. 2013].

Figure 3.2: Particle size distributions by volume and SEM images of silica/lignin

biocomposites containing 20 weight parts of lignin per 100 weight parts of appropriate silicas

[Klapiszewski et al. 2013].

Figure 3.3: Zeta potential vs. pH for silicas preliminary modified with aminosilane, Kraft

lignin and SiO2/lignin biocomposites [Klapiszewski et al. 2013].

Enzymatic treatment of lignin has also been reported to improve its applicability in

manufacturing of eco-friendly composites as the performance of this type of biocomposites is

comparable to that of glass fiber-based composites [Boruszewski et al. 2011]. Recent results

from a study [Boruszewski et al. 2011] shown in Table 3.1 indicate that enzymatically

modified lignin when used as binder for wood fibers influences mechanical performance of

boards manufactured by injection molding. MOR and MOE are slightly affected; however the

highest increase is seen in water wetting (18%) and hardness of the material (12%) as shown

in Table 3.2.

Table 3.1: Properties of the studied boards [Boruszewski et al. 2011]

Table 3.2: Swelling, water absorption and surface water wetting [Boruszewski et al. 2011]

Methylene diphenyl diisocyanate (MDI) was used to compatibilize kraft lignin (KL)/soy

protein isolate (SPI) blends in a study [Huang et al. 2004]. The results showed that

compatibilization due to MDI resulted in graft copolymerization and a moderate degree of

crosslinking between KL and SPI, which favored the strengthening of the composite material

[Huang et al. 2004]. However, a high amount of MDI resulted in excess crosslinks which

hindered the interaction between SPI and KL leading to lower strength composite material. In

a recent study, it was found that the lignin could serve as fire retardant and toughening agent

for PP matrix [Chen et al. 2011]. In yet another recent research work, biocomposites

containing softwood Kraft lignin, poly-L-lactic acid (PLLA) and polyethylene glycol (PEG)

were developed by melt blending where lignin was reported to improve the stiffness of

biocomposites [Rahman et al. 2012]. Lignin has also been reported to show great potential as

biocomposite compatibilisers [Peltola 2010].

Lignin can also act as a curing agent in biocomposites. The thermal stability of epoxy and

lignin composite systems has been reported to increase because of relatively high chain

rigidity induced by the cross linking of epoxy and lignin as curing agent [Abdul Khalil et al.

2011]. The curing of epoxy composite with 25% lignin optimally enhances the mechanical

properties as shown in Table 3.3. The addition of lignin can also reduce the water absorption

of the composite. Lignin/epoxy composites are characterized by good dielectric, mechanical

and adhesive properties and therefore these composite materials can find their use in the

electronics industry as well [Simionescu et al. 1993].

Table 3.3: Mechanical properties of composites

The effect of temperature (T= 22°C, 30°C and 35°C) and relative humidity (RH=34% and

66%) on mechanical behaviour of natural fiber reinforced bio-based matrix composites

(lignin-based composites) subjected to tensile loading was also recently investigated [Rozite

et al. 2011] as shown in Fig. 3.4. The strength of composite deteriorated at high relative

humidity but the temperature did not seem to have a significant impact.

Figure 3.4: Normalized properties of lignin-based composite with 30% flax fibers

(lignin/FL30). (a) Elastic modulus and (b) maximum stress (normalized with respect to

properties at room temperature (RT) and relative humidity (RH) = 34%) [Rozite et al. 2011].

The effect of a coupling agent Maleic anhydride grafted to polyethylene (MAPE) on lignin-

HDPE composites was studied recently [Mariotti et al. 2012] and MAPE (6%) was found to

enhance the tensile strength as shown in Fig. 3.5. Young’s modulus of the composite

increased by the addition of lignins into HDPE, while the addition of esterified lignin had

slightly less effect on the modulus as can be observed from Fig. 3.5.

Figure 3.5: Tensile properties of lignin based composites (Young’s modulus calculated

without extensometer [crosshead], gage length of 7.62mm) [Mariotti et al. 2012].

4. Lignin based carbon fibres

The global production of carbon fiber was estimated to be around 45 thousands tone in 2009.

It is expected to increase up to 110 thousand ton by 2018 with an annual growth rate of 7% or

more (Fig. 4.1). Strongest demands come from aircraft and aerospace, wind energy, as well as

the automotive industry [Acmite Market Intelligence 2010].

Figure 4.1. Carbon fibre production by region [Acmite Market Intelligence]

There are primarily three types of precursor materials of commercial significance; coal or

petroleum pitch [Shin et al. 1997], rayon [Li et al. 2007, Plaisantin et al. 2001] and

polyacrylonitrile (PAN) [Wojcik et al. 2012; Xue et al. 2013, Yusof and Ismail 2012]. Among

all the available precursors, PAN is one of the best materials for preparing high performance

carbon fiber for structural applications.

The desirable development of an alternative precursor for carbon fibres, based on renewable

materials, was identified a long time ago. In the 1960’s, Kayocarbon fiber, a commercial

lignin-based carbon fiber, was produced by Nippon Kayaku Co in Japan, but it was

discontinued in 1973. In this process, the fiber was produced from an alkaline solution of

lignosulfonates, using polyvinyl alcohol added as the plasticizer, and then dry spun.

(Gellerstedt 2010). Within the past decades, a considerable amount of research has shown

various types of lignin to be appropriate precursors for carbon fibers. Melt spinning and

electrospinning were applied to prepare lignin-based carbon fibres. In melt-spinning process

hardwood lignins were generally used as a raw material with a good thermal fusibility. In case

of softwoods it was necessary to modify isolated lignin or to remove the high molecular

weight structure to improve fusibility of material. One of the explanations of the difference in

behaviour of hardwood lignin and softwood lignin might be the frequency of the presence of

condensed 5-5 and β-5 structures in softwood lignins (Lin et al. 2012).

Various techniques were applied to analyse carbon fiber production from lignin; FTIR (Shen

et al. 2010 Kadla and Kubo 2004), XPS (Braun et al. 2005), NMR (Foston et al. 2013), GPC

(Qin and Kadla, 2012), SEM.

Not only the source and native structure of lignin but the way of its isolation might influence

the properties of lignin and therefore its fusibility. Various types of lignin were studied by

different researchers in respect of its ability to form carbon fibre.

4.1 Kraft lignin

Nowadays kraft pulping is the most common wood pulping process in the production of paper

globally. Hence a lot of kraft lignin is readily available for utilisation, but if it is to be utilised

as a precursor for carbon fibres production, it usually needs to be treated to increase its purity.

All early attempts to produce carbon fibres from kraft lignin failed. Softwood kraft lignin was

reported to only degrade and produce char on heating. The first carbon fibres based on

industrial hardwood kraft lignin were first produced by Kadla et al. (2002). No chemical

modification of lignin was applied, but extensive purification was needed.

To enable fiber formation, the hardwood kraft lignin was first heat-treated under vacuum at

145°C for 60 min. This was needed to remove volatile components in the lignin. Increase in

molecular mass was observed after heat-treating. Addition of a small amount of

polyethileneoxide (PEO) as a plasticizer was found to improve the spinnability of the lignin.

Also addition of PEO allowed using lower temperatures for spinning compared to that for a

pure lignin sample, but at PEO additions >10%, self-fusion of the lignin fibers occurred. After

thermo-stabilization of the lignin fibers at 250°C for 60 min in air and followed carbonization

at 1000°C, carbon fiber was obtained. Without addition of PEO, but using more carefully

controlled thermo-stabilization conditions, including a very slow temperature increase to

250°C, the subsequent carbonization resulted in improved strength characteristics of obtained

carbon fibre (Gellerstedt et al.2010; Kadla and Kubo 2004).

Poor processability through spinning appears to be the reason why no neat softwood kraft

lignin based carbon fibres have been produced in the past. This behaviour may have been

caused by the absence of a major lignin fraction with softening properties, able to act as a

plasticizer. It was suggested to add a small fraction of hardwood fraction to softwood kraft

lignin to improve its spinnability. Addition of 10% hardwood fraction to softwood kraft lignin

allowed increasing its spinnability and producing a carbon fibre (Nordberg et al. 2012).

Nordström (2012) has shown that the thermal stabilisation and carbonisation of carbon fibres

produced from softwood kraft lignin can be performed as a one-step process which can help

to reduce production costs. The thermal stabilisation allowed a complete stabilisation of

softwood kraft lignin fibers in 85 minutes, whereas the addition of hardwood lignin extended

the time required up to 255 minutes, thus confirming the difference in thermal reactivity

between softwood and hardwood lignin. Oxidative stabilisation could be achieved even faster,

reaching complete stabilisation in 45 minutes for softwood kraft lignin fibers, and 105

minutes for the softwood-hardwood blend fiber. This is much faster than commonly reported

stabilisation times for hardwood based carbon fibres.

4.2 Organosolv lignin

Organosolv pulping is a treatment of wood with organic solvents, e.g. ethanol, methanol,

acetone, acetic acid etc.

So far one of the most interesting organosolv processes is the ethanol pulping according to the

Alcell technology. Lignin isolated in this process was purified, modified and subjected to melt

spinning followed by carbonisation. Alcell lignin has suitable properties for being a precursor

for carbon fibers. In the presence of polyethyleneoxide as a plasticizer, hardwood Alcell

lignin had an improved spinnability, but these blends were thermally unstable and the

thermostabilization in air had to be done at a very low heating rate to avoid inter-fiber fusing

of the fibers. After carbonization under the nitrogen atmosphere at 1000°C, the carbon fibres

had a superior mechanical properties compared to those of other types of carbon fibres from

organosolv lignins (Kadla et al. 2002).

Electrospinning allows producing of carbon nanofibres from Alcell lignin solutions with

lignin ethanol weight ratio 1:1. Thermostabilisation was performed under an oxidizing

atmosphere at 200°C for 24-36 hours. The stabilised fibres were carbonised at temperature of

600-1000°C. this type of carbon fibres showed a high oxidation resistance and microporous

structure (Lavalle et al. 2007, Ruiz-Rosas et al. 2010).

Pulping with acetic acid in the presence of a small amount of sulphuric acid or hydrochloric

acid was studied recently. Carbon fibers were produced from acetic acid lignin by melt

spinning. The spinnability of acetic acid lignin, obtained through acetic acid pulping of birch

wood, was improved after a thermal treatment under reduced pressure to modify the lignin

structure. Thereby, an increase in average molecular mass was achieved while the content of

methoxyl and acetyl groups was kept the same. After thermostabilization in air at 250°C for 1

hour and carbonization at 1000°C carbon fibres were obtained. (Kubo et al. 1998). Similar

approach using softwood did not yield fusible lignin, and carbon fibre could only be obtained

after fractionation of lignin to remove the highest molecular mass material. However, the

strength of the resulting carbon fibre was inferior to that of the hardwood-based fibers (Kubo

et al. 1998).

4.3 Steam-explosion lignin

The concept of steam explosion was introduced in the 30-s. Steam explosion is a technique

whereby wood chips or other types of biomass are treated with high pressure steam followed

by a rapid release to lower pressure. The liquid inside the wood expands, leading to rupture of

wood structure (Jedvert et al. 2012).

Generally softwoods are more difficult to defiber than hardwoods. SO2 impregnation was

found to be necessary in aiding this process (Li et al. 2009). Impregnation of wood chips with

NaBH4 before steam explosion was reported to stabilize galactoglucomannan in softwoods

without influence on lignin content (Jadvert et al. 2012).

From hardwoods a subsequent extraction with organic solvents or aqueous alkali can be used

to isolate lignin from steam-exploded samples. Modification of lignin is required in some

cases. By hydrogenolysis of the steam-exploded birch lignin, subsequent extractions with

chloroform and carbon disulfide followed by heat treatment and melt spinning, a filament was

obtained by Sudo and Shimizu (1992). After thermostabilization in air at 210°C, the fibers

were carbonized at 1000°C to yield carbon fibre of general purpose grade (Sudo and Shimizu

1992). The same type of lignin subjected to phenolysis rather than hydrogenolysis followed

by purification and heat treatment under vacuum at 280°C was also melt spun, and the

resulting fibers were stabilized in air at 300°C and further carbonized at 1000°C. The strength

of obtained carbon fibres was comparable to the previous ones, but the yield of lignin based

carbon fibre by phenolysis was 23% higher than that by hydrogenolysis (Sudo et al. 1993).

4.4 Pyrolytic lignin

Carbon fibres prepared from pyrolytic lignin isolated from commercial bio-oil were compared

to hardwood kraft lignin based and Alcell lignin based ones. Pyrolytic lignin is similar to

other technical lignins in terms of its chemical structure and composition, but it has higher

carbon content and lower average molecular mass (Qin and Kadla 2012). These

characteristics make it an interesting source of carbon fibers. It is known that higher carbon

content increase carbon-fiber yield and lower molecular mass facilitate fiber processing by

melt spinning.

Pretreatment was a critical step in the successful preparation of carbon fibers from pyrolytic

lignin. Higher temperatures and longer periods of time led to poor fiber spinning of pyrolytic

lignin, whereas lower temperatures and shorter times resulted in the lignin fibers fusing

together in the subsequent thermostabilization process, but disadvantages of pyrolytic lignin

were effectively eliminated by pretreatment at elevated temperatures under reduced pressure.

The pyrolytic lignin treated under these conditions produced hollow carbon fibers with

mechanical properties and yields comparable to those based on the Kraft and Alcell lignins

(Qin and Kadla 2012).

4.5 Solvolytic lignin

Polyethilene glycol (PEG) treatment on the presence of sulphuric acid as a catalyst was

developed as a liquefaction method of wood. As a by-product of the solvolysis a lignin

fraction could be recovered, and it was found to be chemically modified with PEG through

ether bonds. This product was found to be thermally fusible.

Cedar chips were used as a lignin source. Fibres were obtained by melt spinning at 145-

172°C. Traditional oxidative thermostabilisation at elevated temperatures was not viable as

very slow heating rate is required; it might take days to get infusible fibres. Chemical

thermostabilisation with concentrated HCl aqueous solution for 2 h was suggested for the

fibres obtained from solvolytic lignin. Carbonization at 1000°C for 1 h under nitrogen flow

was performed after stabilisation (Lin et al. 2012).

5. Applications of Lignin-based biocomposites

Lignin is seen as a promising biopolymer for producing biocomposites; however, the

utilization of polymeric lignin in solid material systems is prohibited by the extensive crosslinking

and its strong intra-molecular interactions. The viscoelastic properties of lignin can be

altered by polymer blending and the intra-molecular interactions can thus be disrupted [Kadla

and Kubo 2004]. Various new processing techniques like the one mentioned above have made

it possible to produce lignin which is suitable and compatible for composite formation.

Lignin-based biocomposites have wide ranging applications from general purpose to high

performance. Potential application areas include automotive, construction, packaging,

consumer goods and manufacturing. Two of these important applications are discussed in this

section.

5.1 Application of Lignin-based carbon fiber in transportation

The fuel consumption can be reduced by lowering the weight of vehicles. Carbon fiber

composites seem a viable option to achieve that as they are lightweight and possess high

stiffness and high strength. Kraft lignin blends are a good alternative for carbon fiber

feedstock and have been studied for more than 50 years. Advantages of lignin for the

production of commercial carbon fiber are that it is naturally occurring, readily available,

inexpensive, and structurally rich in phenyl propane groups that are high in carbon (60%)

[Luo et al. 2011]. The first lignin–based carbon fiber was produced using thiolignin, alkali

lignin, and lignosulfonates [Schmidl 1992]. The ability to produce small tows of (10-20μm

diameter) fibers shown in Fig. 5.1 with properties approaching those needed for transportation

applications has been established using the melt spinning process [Compere et al. 2004].

These fibers are composed primarily of lignin from Kraft pulping process.

Figure 5.1: Scanning electron micrograph of carbonized lignin blend fibers produced during

multifilament spinning [Compere et al. 2004].

However, the main challenge is to increase fiber production and remove the contaminants to

permit economic commercial use of carbon fiber composites. New processing technologies

and techniques for modification of the fiber surface to enhance resin adhesion are needed. For

example, fiber epoxy resin compatibility has been reported to improve by a combination of

plasma treatment and silanation with a conventional agent as shown in Fig. 5.2 [Compere et

al. 2004]. It is very critical to develop such cost effective methods for purification of lignin in

order to improve fiber properties. Replacement of heavy door panels and other similar parts in

vehicles with these lignin-based biocomposites can offer several benefits in terms of less fuel

costs and environment friendliness.

Figure 5.2: Small composite test specimen (left) made from lignin-based fibers showing good

resin-fiber adhesion along fracture edge (right) [Compere et al. 2004].

5.2 Application of lignin as surface treatment agent in biocomposites

Lignin as a surface treatment agent for natural fiber composites seems to have positive

outcome. The irregular shape of the lignin particles provides mechanical interlocking with the

matrix in composites as lignin bonds to the surface of natural fibers [Thielemans et al. 2002].

The fiber–matrix interfacial strength is improved due to this mechanical interlocking,

resulting in improved properties for the composites as shown in Table 5.1 [Thielemans et al.

2002]. However, it can be observed from Table 5.1 that the improvement in interfacial

strength is more when a lower concentration of lignin at the interface is used. One of the

possible reasons for that can be that the lignin particles plausibly impede the flow of resin and

subsequent wetting of the fibers, resulting in a weak interface at higher lignin concentrations

[Thielemans et al. 2002].

Table 5.1: Mechanical Properties of Hemp Composites with Lignin Surface Treatment

[Thielemans et al. 2002]

6. Conclusions

The interest in natural, renewable resources-based and compostable materials has been

renewed due to ecological concerns such as environmental safety etc. The natural fibers,

biodegradable polymers can be considered as ‘‘interesting’’ – environmentally safe –

alternatives for the development of new biodegradable composites (biocomposites) [Avérous

and Digabel 2006]. Although, the driving force for various applications of biocomposites is

their biodegradability, however, one need to understand clearly the processing, performance

and especially price of these biocomposites compared to synthetic petrochemical-based

composites [Imam et al. 1999]. Removal of contaminants, salts, and inclusions, as well as

preparation of a consistent byproduct material, can present a significant challenge [Compere

et al. 2004]. The current research seems to address the main issues regarding the processing of

lignin-based biocomposites. Lignin-based biocomposites show a large range of properties and

thus are able to compete with non-bio-degradable polymers for different applications.

Natural/Biofiber composites (Bio-Composites) are emerging as a viable alternative to glass

fiber reinforced composites especially in automotive and building product applications

[Mohanty et al. 2002]. Composites based on lignin reinforced with NF have established

themselves as engineering materials used in various industrial branches [Rozite et al. 2011].

Lignin biocomposites seem to present strong advantages, especially for short-term

applications.