146
A similar trend was also observed by Zlatanić et al. (2015) for the tensile strength
of the BPU foam. The tensile strength dropped from 80.76 kPa to 31.2 kPa with
biopolyol replacement increased from 30% to 50%. However, the tensile strength of
the PU foam prepared with 30% biopolyol was slightly lower than the industrial
requirement for tensile strength (82 kPa) (Zlatanić et al. 2015).
7.3 Biopolyols Derived From Liquefaction and PUs
Biopolyol, derived from thermal liquefaction of biomass, is a mixture with multiple
reactive hydroxyl groups of degraded cellulose, hemicellulose and lignin, which can
substitute petroleum polyol in the preparation of PU foams (Li et al. 2017a; Li
et al. 2018).
7.3.1 Biopolyols Derived From Liquefaction
The biopolyols derived from liquefaction have been commonly obtained from
hydrothermal liquefaction processes, which are carried out at a high temperature
(between 250 °C and 290 °C) with water or a water/alcohol mixture with or without
catalyst (Cheng et al. 2010; Wang et al. 2013; Li et al. 2017a).
Temperature generally plays an important role in the liquefaction process. Li
et al. (2017a) reported that the yield of biopoliol increased by approx. 51% to 72%
as the temperature increased from 250 °C to 270 °C in the liquefaction of wheat
straw. A similar trend was observed by Cheng et al. (2010), who reported a biopolyol yield increased from 22% to 66% when the temperature increased from
200 °C to 300 °C. However, temperatures that are too high should be avoided, since
copolymerization and condensation reactions can be given.
The application of catalysts could commonly suppress carbon formation and
improve the biopolyol yields in liquefaction processes (Kishida et al. 2005).
According to Li et al. (2017a), the yield of biopolyol in liquefaction of wheat straw
increased from 45.7% to 72.1% when the catalyst (NaOH) concentration increased
from 10% to 15%. Wang et al. (2013) also found that the biopolyol yield in the liquefaction of sawdust from pinewood increased from 7% to 17% by using an alkaline
catalyst. It should, however, be noted that the addition of a high concentration of an
alkaline catalyst is harmful in the liquefaction process, since it promotes the repolymerization of biopolyol products or intermediates, thus forming more carbon
(Li et al. 2017a).
Residence time also has a significant influence on the biopolyol yield in a liquefaction process. For example, Li et al. (2017a) found that the biopolyol yield in
liquefaction of wheat straw decreased drastically as residence time increased, likely
due to the re-polymerization reactions or other side reactions (such as dehydration/
cracking reactions) which form water and low boiling point compounds for a longer
residence time (Cheng et al. 2013; Ye et al. 2014).
H. Li et al.
A similar trend was also observed by Zlatanić et al. (2015) for the tensile strength
of the BPU foam. The tensile strength dropped from 80.76 kPa to 31.2 kPa with
biopolyol replacement increased from 30% to 50%. However, the tensile strength of
the PU foam prepared with 30% biopolyol was slightly lower than the industrial
requirement for tensile strength (82 kPa) (Zlatanić et al. 2015).
7.3 Biopolyols Derived From Liquefaction and PUs
Biopolyol, derived from thermal liquefaction of biomass, is a mixture with multiple
reactive hydroxyl groups of degraded cellulose, hemicellulose and lignin, which can
substitute petroleum polyol in the preparation of PU foams (Li et al. 2017a; Li
et al. 2018).
7.3.1 Biopolyols Derived From Liquefaction
The biopolyols derived from liquefaction have been commonly obtained from
hydrothermal liquefaction processes, which are carried out at a high temperature
(between 250 °C and 290 °C) with water or a water/alcohol mixture with or without
catalyst (Cheng et al. 2010; Wang et al. 2013; Li et al. 2017a).
Temperature generally plays an important role in the liquefaction process. Li
et al. (2017a) reported that the yield of biopoliol increased by approx. 51% to 72%
as the temperature increased from 250 °C to 270 °C in the liquefaction of wheat
straw. A similar trend was observed by Cheng et al. (2010), who reported a biopolyol yield increased from 22% to 66% when the temperature increased from
200 °C to 300 °C. However, temperatures that are too high should be avoided, since
copolymerization and condensation reactions can be given.
The application of catalysts could commonly suppress carbon formation and
improve the biopolyol yields in liquefaction processes (Kishida et al. 2005).
According to Li et al. (2017a), the yield of biopolyol in liquefaction of wheat straw
increased from 45.7% to 72.1% when the catalyst (NaOH) concentration increased
from 10% to 15%. Wang et al. (2013) also found that the biopolyol yield in the liquefaction of sawdust from pinewood increased from 7% to 17% by using an alkaline
catalyst. It should, however, be noted that the addition of a high concentration of an
alkaline catalyst is harmful in the liquefaction process, since it promotes the repolymerization of biopolyol products or intermediates, thus forming more carbon
(Li et al. 2017a).
Residence time also has a significant influence on the biopolyol yield in a liquefaction process. For example, Li et al. (2017a) found that the biopolyol yield in
liquefaction of wheat straw decreased drastically as residence time increased, likely
due to the re-polymerization reactions or other side reactions (such as dehydration/
cracking reactions) which form water and low boiling point compounds for a longer
residence time (Cheng et al. 2013; Ye et al. 2014).
H. Li et al.