Application of Enzymatically Treated Lignin Oligomers as Lignopolyols For A Full Replacement of Commercial Polyols in Polyurethane Foam Formulation

Lignin has shown to have a great potential in replacing oil in many applications, including resins ...

reactivity enhancement [5,6]. This is due to harsh conditions for lignin isolation together with the prevalence of highly recalcitrant intra-molecular lignin linkages, and repolymerization tendency of phenoxy radicals. Enzymatic depolymerization of lignin is envisaged as one of the potential breakthrough applications for lignin valorization [7]. However, matching the optimal operation conditions of the enzyme with the optimal processibility of lignin has been a challenge. Recently, alkaliphilic laccase enabling the enzymatic operations at the conditions to better match the optimal processibility of crude lignin, i.e. alkaline aqueous conditions (pH 10-11) has been demonstrated [8]. In addition, a novel lignin valorization technology utilizing alkaliphilic laccases for the enzymatic oxidation of lignin in alkaline aqueous conditions (pH 10-11) combined with cascading membrane operations has been recently introduced by MetGen Oy [9].
This technology, called METNIN™, has shown to able to produce different molecular size lignin fractions (from oligomeric down to a mixture of tri-, di and monomeric units) with a distinct molecular weight distribution and a low polydispersity together with more favourable physicochemical properties. METNIN™ process enables the utilization and potentially better applicability of lignin in a variety of application areas, including polyurethane foams. The current case study describes the application of a selected lignin oligomeric fraction produced by METNIN™ process as a suitable lignopolyol to completely replace a commercial polyol in PUR foam formulation.
More specifically, the preparation of liquid lignopolyols by oxypropylation of METNIN™ oligomeric lignin fraction and its full utilization in the polyurethane rigid foam formulation is demonstrated.
In addition, selected technical specifications of PUR demonstrators are characterized.

Lignin Characteristics
A crude lignin extracted from a woody (birch) biomass was supplied by SWEETWOODS project [10]. The selected lignin oligomeric fraction was obtained by the fractionation of the crude lignin by METNIN™ enzymatic lignin valorization process (MetGen Oy, Finland) [9]. METNIN™ process produces an aqueous alkaline solution of lignin fractions (pH 10-11). For this study, METNIN™ lignin fraction was extracted as a dry powder using acid precipitation protocol adapted from Sameni et al. [11]. An example of the sample is shown in Figure 1A. Selected physicochemical properties of MET-NIN™ lignin oligomer fraction after extraction are collected in Table   1. METNIN™ lignin oligomer shows high purity (>95%) with low ash and carbohydrate content. Average molecular weight (Mw) and polydispersity index (PDI) were determined by HPLC chromatographer 1200 CompactLC with UV detector (Agilent Technologies), equipped with size exclusion column set MCX 1000 Å + 100 000 Å   was determined by a gas chromatographic analysis with modified alditol acetate method [12]. Acetylated OH group (OH acet ) content was determined by the modified Verley and Bolsing method [13].
Methoxy (OCH 3 ) content was determined by Zeisel-Viebock-Schwapapch method [13]. Total hydroxylic group (OH total ) content was determined by the wet chemistry method (acetylation by acetic anhydride followed by free acid potentiometric titration by 0.1 M NaOH) [13].
Carboxylic group (OH COOH ) content was determined by wet chemistry chemisorption method using calcium acetate. The total acidic groups (phenolic + carboxylic) content was determined by the acid-base back conductometric titration method [13]. The content of phenolic groups (OH ph ) was calculated from the difference between the values of conductometric and chemisorption methods.
The aliphatic OH (OH aliph ) content was calculated by the following relation: OH aliph = OH acet +OH COOH -OH ph . analysis (TGA) to determine the temperature at which 5% of weight loss is observed (T5%) was conducted in following conditions: sample weight 8.0-8.5 mg, heating rate 10˚C/min, atmosphere nitrogen. The differential scanning calorimetry (DSC) used to determine the glass transition temperature (Tg) was performed in the temperature range of 0-180˚C with a heating rate of 10 ˚C/min.  Table   2. Oxypropylation parameters were adjusted based on the lignin content in the reaction mixture Table 3. Adjustable parameters included maximal pressure in the reactor (P max ), maximal temperature (T max ), the temperature at which the exothermic process was started (TP, max ) and time necessary for a two-fold decrease of maximal pressure in reactor (t 0.5 ) [15].  Table 3: Dependence of oxypropylation parameters on lignin content in the reaction mixture. Values for the maximal pressure in the reactor (P max ), maximal temperature (T max ), the temperature at which the exothermic process was started (T P,max ) and time necessary for a two-fold decrease of maximal pressure in the reactor (t 0.5 ) are presented. assumed that in the density range of 32-52 kg/m 3 dependence of foams characteristics on their density was directly proportional for each PUR matrix [16]. Normalisation will remove the dependence of the density on compression characteristics of material.

Results and Discussion
The physicochemical characteristics of synthesised lignopolyols are summarised in Table 5 mg KOH/g and viscosity should be below 300000 mPa·s (at 25˚C) [17,18]. Lignopolyol samples containing 30% and 40% of lignin meet these requirements. Thus, these two lignopolyol samples were selected to be applied in the production of PUR demonstrators.
Compared to reference formulation, a significant decrease both in the start time and the gel time of PUR foaming was observed for formulations containing lignopolyols (Table 6). This indicates that lignopolyols were more reactive towards isocyanate compared to the commercial polyethers presented in reference formulation. In addition, the lignopolyol with the higher lignin content showed higher reactivity in the PUR foam system.  The higher reactivity of PUR foam compositions containing lignopolyols compared to reference formulation resulted to a higher rate of heat release, a higher temperature in foaming block and as a sequence the higher volume that occupied the volatiles formed in the result of water reaction with PMDI (i.e. carbon dioxide) and physical blowing agents (i.e. Freon) evaporation. As the result, the apparent density of Formulation A was lower in comparison with less reactive reference PUR foam composition. In the case of Formulation B, the lower density was achieved by the increased water content (0.5 versus 0.65) in the formulation (Table 4). Lower density is beneficial as it is known that heat insulation properties of PUR foams are deteriorating with with increased foam density [18].
No traces of foams shrinkage were observed with any of the formulations, even with Formulation B with very low density.
Results show that over 90% closed cell content was achieved with all PUR foam formulations ( Table 6). The high closed cell content indicates that crosslinking reactions leading to the polymeric network strength development and subsequent process of gaseous phase liberation were in the balance to reach optimal closed cell structure [18]. This is one of the major requirement influencing the heat isolation properties of PUR foams.
Water absorption of PUR foams is another parameter influencing the level of heat isolation characteristics, with a lower water uptake enabling prolonged exploitation duration to moist conditions [18].
The water absorption values for PUR foams containing a lignopolyol were lower than those of the reference sample, indicating a more hydrophobic character of the former ( Table 6). In addition, the water uptake decreases with the increasing lignin content in the polyol. Compression characteristics of PUR foams are shown in Table 7. The deformation characteristics of the PUR foam are dependent on its density as well as on the chemical composition and structure of polymeric matrix [18].  One explanation for anisotropy behaviour could be the lower NCO conversion at the gel moment in lignopolyol due to its high functionality. As the result, a significant amount of heat was evolved after gel formation when the fluidity of the foaming system decreases dramatically and the growing of foam proceeded predominantly in a vertical direction. This process leads to deformation of closed cell form from spherical to ellipsoidal.
Results show that the compression characteristics of PUR foams on the basis of lignopolyols with 30% of lignin content (Formulation A) in both directions were lower in the comparison with reference formulation-, whereas for the higher lignin content (Formulation B) the compression characteristics exceeded those of the reference -. In any case, the density and compression characteristics of both lignopolyol containing foams meet the requirement for PUR foams suitable for producing heat isolation in a building.
In conclusion, the case study presented here demonstrates the successful synthesis of lignin based polyols with high lignin content using METNIN™ lignin oligomer and the subsequent application of the lignopolyol as a full replacement of commercial polyol in PUR foam demonstrator. In addition, the selected technical specifications of the rigid PUR foams are within the typical industrial requirements. In some cases, lignopolyol shows improved performance. However, for the full commercial validation, additional properties such as dimension stability at enhanced temperature, heat resistance, thermo-oxidative stability and fire resistance have to be determined. This is in work in progress and will be presented in further communications.