Depolymerized lignin application for PU foam preparation

Extensive efforts have been made to explore high value applications of lignin, in

particular in polymeric materials, in particular PU (Pan and Saddler, 2013). Preparation

of low-cost polyols from abundant and renewable resources has long been an important subject in the PU industry. PU has rapidly grown to be one of the most widely used synthetic polymers with a continuously increasing global market. Rigid PU foam is a highly cross-linked polymer with closed-cell structure. KL was first incorporated into a

polyether triol, forming a cross-linked network of PU (Yoshida et al., 1990). Hatakeyama

et al. (2004) prepared rigid PU foam from Kraft lignin together with diethylene glycol (DEG), triethylene glycol (TEG) and polyethylene glycol (PEG) with molecular mass of 200 (PEG200). Sulfur-free lignin from straw steam explosion was also investigated for

polyurethane preparation (Bonini et al., 2005). Cinelli et al. (2013) liquefied Indulin AT lignin with polyols (glycerol and PEG 400) in a glass flask under microwave treatment in

oven at 180 oC for 3 min, and utilized the liquid product with in the preparation of

flexible PUs. However, the liquid products obtained from liquefaction could not be used for the production of flexible PUs due to excessive viscosity and very high OH values. Therefore PPG and castor oil was further added to reduce viscosity and glass transition temperature of the final materials while increasing the flexibility. Thus, the lignin

contents in the final PU foam product were very low. Hu et al. (2012) studied the

feasibility of using crude glycerol to liquefy soybean straw for the production of bio- polyols ad polyurethane foams. Biopolyols produced showed a hydroxyl number of 440-

540 mgKOH/g and the foams produced showed densities of 33-37 kg/m3 and

compressive strength from 148-227 kPa.

However, in spite of a lot of efforts for the incorporation of even depolymerized lignin with or without modification, the lignin contents are limited. Most of the research work presented in literature shows that the wt.% replacement of petroleum-based polyols with crude lignin in PU foams structure is not more than 30 wt.%. At the same time the molecular weight of lignin/depolymerized lignin is also an important factor responsible for deteriorated properties for the lignin-based PU foams. With the increase of replacement ratio glassy materials foam materials were resulted. To address this problem, the most viable route would be depolymerization of lignin prior to its application in the synthesis of PU foam. Depolymerization of lignin results in de-polymerized lignin (DL) products with lower molecular weights suitably high aliphatic and total hydroxy numbers, making them suitable for the preparation of PU foams at higher replacement ratios. The author’s group successfully depolymerized KL into low molecular weight products via direct hydrolysis using NaOH as a catalyst, without any organic solvent/capping

agent (Mahmood et al., 2013). At the best operating conditions (250 oC, 1 h, and

NaOH/KL ratio ≈0.28 (w/w) with 20 wt.% substrate concentration) yield of

depolymerized KL (DKL) was ~92 wt.% with solid residues <0.5 wt.% (Mw ≈3310

g/mole & aliphatic hydroxyl number ≈352 mgKOH/g). The molecular weight of DKL

rigid PU foams (Li and Ragauskas, 2012). Therefore, the author’s group carried out an

optimization study subjected to several constraints: (1) yield of DKL ≥ 75 wt.%; (2)

moderately high aliphatic-hydroxyl number (≥300 mgKOH/g) and; (3) lowest possible

Mw. The optimized reaction conditions determined for the hydrolytic depolymerization

are: 250 oC, 2 h, and NaOH/KL ratio ≈0.28 (w/w) with 10 wt.% substrate concentration,

under which the KL depolymerization produced DKL at a yield of ~77 wt.%, with Mw

≈1700 g/mole and aliphatic-hydroxyl number ~365 mgKOH/g. The produced DKL was

successfully utilized as bio-polyols replacing 50 wt.% of PPG400 and sucrose polyols for the preparation of rigid polyurethane foams. The bio-based foams prepared with DKL and sucrose polyols showed superior compression modulus (5152.0 kPa) and thermal

conductivity (0.032 W/mK) than those with DKL and PPG400. Mahmood et al. (2013)

also successfully depolymerized hydrolysis lignin (HL) in 50/50 (v/v) water-ethanol

mixture under N2 atmosphere at 250 oC for 1 h, and the depolymerized HL (DHL) was

also utilized as bio-polyols for the preparation of bio-based rigid PU (BRPU) foams, replacing at 30 wt.% and 50 wt.% of PPG400 and sucrose polyols. Again, BRPU prepared with DHL and sucrose polyols showed superior compression strengths at 10% (216±31 kPa) and lower thermal conductivities (0.036±0.001 W/mK) than those with DHL and PPG400.

2.4.10.2 Modified depolymerized lignin via oxypropylation and its

incorporation in PU foams

Depolymerized lignins after removing solvents are still in powder form and there exist less accessible hydroxyl groups in the molecular structure. Thus, chemical modification such as oxypropylation with alkylene oxide was found to be beneficial as it could improve the accessibility of the hydroxyl groups and convert lignin from solid form into a

liquid polyol with extended chain and exposed hydroxyl groups (Pan and Saddler, 2013).

To the best of the author’s knowledge, no study has been reported on the utilization of depolymerized KL and HL for the preparation of BRPU foams with satisfactory physical, mechanical and thermal characteristics, replacing 50 wt.% or more petroleum-based polyol and sucrose polyol. In the author’s group, DKL obtained from the hydrolytic de- polymerization of KL was used for the preparation of polyols via oxypropylation in a

unique medium consisting of propylene oxide (PO), glycerol+KOH (11 wt.% KOH) and acetone. The prepared polyols were employed as a single polyol in the preparation of BRPU with high bio-contents up to 70 wt.%. Similarly, oxypropylated DHL was also successfully incorporated in the BRPU foams at high percentage of bio-contents.