Thermoset Fully biobased
Thermoset Fully biobased epoxy resin from lignin Thermosets Thermosets are polymeric materials generated by irreversible crosslinking of multifunctional monomers or oligomers thus forming a three dimensional network out of the initial resin system. Obviously, thermoset properties depend both on the monomer structure and on the architecture of the network. For the latter, the network density is a decisive parameter: the shorter the links, the more resistant the thermoset against mechanical, thermal, and chemical impacts. Thus the spectrum of crosslinked polymers reaches from flexible elastomers to firm resin systems for high performance composites. Lignin structure When looking for alternatives to petroleum-based resin components, lignin is a particularly interesting candidate. It is synthesized biochemically from three aromatic monomeric units, i. e. Cumaryl, Coniferyl, and Sinapyl alcohol (Fig. 1). With its highly cross-linked structure in combination with its specific functional groups (Fig. 1 B), lignin is suited as a building block in phenol formaldehyde (PF), polyurethane (PU), and epoxy (EP) resin systems. While for PU and EP systems the OH functionalities play the major role. PF resins take advantage of the free ring positions as reactive centres. Lignin is synthesized in all vascular plants and represents, after cellulose, the second most frequently occurring polymer on earth. There are three main types: hardwood (e. g. eucalyptus, birch, beech), softwood (e. g. pine, spruce), and annual plant lignin. Lignin sources Technically lignin is a by-product of the pulp and paper industry and is used almost exclusively as a fuel, in particular for running the pulping processes. Two processes dominate by far the chemical pulping to obtain cellulose: the Kraft (or sulfate) process with 90 % market share [2] and the sulfite process. Both generate sulphur containing lignins but with different chemical bonding patterns to the lignin skeleton. While the socalled lignosulfonates from the sulfite process have been available on the market for decades, this is not the case for lignins from the Kraft process. Only recently big pulping companies like Domtar, Stora Enso or Suzano began to isolate lignin from their black liquors. An important input to this development was the market introduction of the Ligno-Boost technology by Metso, which is applicable to both hard and soft wood and works with supercritical CO 2 for lignin precipitation. Sulphur may cause olfactory problems in final applications of lignin as a material. Therefore sulphurfree pulping processes such as Alcell ® [3], Organocell [4], or Soda [5] could gain some importance in this respect. Also enzymatic bio-ethanol production from annual plants generates sulphur-free lignins with quite a high molecular weight. This is a disadvantage for resin formulations since it impairs the solubility of the lignin in general. Figure 1 Structure of lignin monomers (A) and a lignin fragment (B) according to Freudenberg [1] By: Gunnar Engelmann Johannes Ganster Fraunhofer-Institute for Applied Polymer Research IAP Potsdam-Golm, Germany CH 2 OH HO H 2 C C HO HO O CH OCH OH 2 3 OH CH HO H 2 OCH HOH 3 2 C C C O CH O H OH HC OH OCH 3 H 3 CO OCH 3 OH OH Cumaryl- OCH 3 H 3 CO OCH 3 HOH 2 C O Coniferyl- OH HC HC OH A Sinapyl- O O H 3 CO OH H 3 C O OH HOH 2 C H O HOH 2 C O C C HO OH O H HC OCH CH 3 OH 2 OH OCH 3 HC HO H CH HC O HO C O O CH CH OCH 3 H 3 C OH H 3 CO O B O H 3 CO OCH 3 HO 30 bioplastics MAGAZINE [03/15] Vol. 10
Thermoset Lignin utilization Apart from the use of the caloric value of lignin for energy generation mostly by directly burning the spent liquor, lignin is used in comparatively small quantities for thermoplastic processing with lignocelluloses reinforcing fibres [6] and more recently as a blend component in derivatised form in biobased packaging films in combination with biodegradable petro-based polyesters [7]. On the other hand lignosulfonates from the sulphite process have a broad spectrum of applications, e. g. as additives for briquettes, animal feed, or concrete [8]. The possibility of using lignosulfonates for PF resin formulations, substituting the increasingly expensive phenol has been known for a long time, but is not exploited commercially on a larger scale. However, detailed investigations were performed for products like plywood, oriented strand boards, and medium density fibre boards. For PU and EP resin formulations lignosulfonates are less suited owing to the different chemical structure compared to sulphur-free lignins or lignins from the Kraft process. With regard to synthetic EP resins made of bisphenol-A, (Kosbar et al.) in cooperation with IBM demonstrated the possibility to use 50 % lignin in a resin formulation for the manufacture of printed circuit boards [9]. However, the demonstrator never went into production. For resin producers the use of Kraft lignins isolated from the black liquor would be the economically most viable way. However, these lignins have a relatively high molecular weight (not to mention organosolv or enzymatic lignins) and thus impede the lignin solubility in the reactive resin formulations. To avoid an additional technological step to degrade the lignin separately, a modification of the cooking process such that a more severe degradation takes place in situ, might be an option. Biobased epoxy resins The advantages of using low molecular weight lignins can be demonstrated for a fraction of a softwood Kraft lignin in a completely biobased, bisphenol-A-free epoxy resin formulation [10]. To achieve this goal, besides the low molecular weight lignin fraction, glycerol-1,3- diglycidyl ether (1) and, as a co-cross-linker, pyrogallol (2) are used (Fig. 2). Here the glycidyl ether can be traced back to glycerol which is (also) a by-product of bio-diesel production. Pyrogallol can be prepared by thermal decarboxylation of gallic acid, a biobased building block of hydrolysable tannins [11]. Optimum compositions lead to thermosets with a tensile strength of 82 MPa, a stiffness of 3.2 GPa, and a glass transition temperature of 70 °C. These resins are suited for manufacturing fibre reinforced composites. Using 50 % of (bio-based) cellulose regenerated fibres in unidirectional composites; a bending strength of 210 MPa, a modulus of 12.5 GPa, and a heat distortion temperature of 160 °C were achieved. Further improvements can be obtained by abandoning the claim of being completely biobased and using carboxylic acid anhydrides as hardener but still being bisphenol-Afree. Approximately 65 % of biobased formulations give values of 85 MPa strength, 3.5 GPa modulus and a glass transition temperature of 80 °C, still somewhat below petroleum-based bisphenol-A containing formulations (Fig. 3). O O OH 1 HO 2 Figure 2: Main components (besides lignin) for a completely biobased epoxy resin Figure 3: Comparison between plant oil [12], lignin-, and bisphenol‐A-based [13] resins in terms of selected mechanical and thermal properties Values 120 100 80 60 40 20 0 Tensile strength (MPa) E-Modulus*10 (GPa) Figure 4: Prototype of light element Prachteck by Alfred Pracht Lichttechnik using lignin-based resin O O Waste vegetable oil Lignin Bisphenol-A-based OH T g (°C) OH bioplastics MAGAZINE [03/15] Vol. 10 31
