143
are usually flexible with favorable flexibility at low temperature but relatively low
mechanical strength (Kraitape and Thongpin 2016).
Polyisocyanates are polymers containing monomeric units of isocyanates
(-NCO-) in their structure. Isocyanates products, along with polyols, are the two
main materials in PU industry. Frequently, polyisocyanate compounds are synthesized by the reaction of primary amine compounds (amine hydrochloride oramines,
etc.) with phosgene (COCl 2 ). The most common isocyanates used for the preparation of PU are divided into two main categories: TDI and phenyl isocyanate, commonly refers to diphenylmethane diisocyanate (MDI), PMDI and polymethylene
polyphenyl isocyanate (PM200) (Li et al. 2015). TDI containing two isomers (2,4TDI and 2,6-TDI) commonly applied to produce flexible PU foams, while MDI and
PM200 are more reactive than TDI, and are usually used in the rigid PU foam industry (Li et al. 2017c; Li et al. 2017b; Yang et al. 2019), while polymeric MDI (PMDI)
is a mixture obtained after distillation process of MDI/MDI oligomers, which has an
average functionality of 2.7 and is also commonly used to produce rigid and flexible
PU foams due to the high reactivity and low volatile and toxicity (Li et al. 2017a;
Li et al. 2018; Scott et al. 2019).
Catalysts also play an extremely important role in the preparation of PU foams,
since they adjust the rate of foaming reactions (Yu and Lee 2014). The catalysts
commonly used are: (1) organometallic compounds, including dibutyltin dilaurate
and stannous octoate (SnOct 2 ) (Simón et al. 2013) and (2) tertiary amines, which
refer to alcohol, alicyclic, aliphatic and aromatic amines, and ammonium compounds (Zhao et al. 2014) In general, two or more catalysts are used to adjust the
chemical reaction rate of the PU foaming process to meet the requirements of the
different manufacturing processes (Xu et al. 2014; Zhao et al. 2014).
Surfactants are also known as foam stabilizers, and is an indispensable component in the preparation of PU foam materials. This plays an important role in the
emulsion of foaming ingredients, adjusting the solubility of each component and
preventing the bubble body from collapsing. It also has a great impact on the foam
cellular size and the mechanical properties of PU foam materials (Dounis and
Wilkes 1997a). At present, most of the foam surfactants used are polyether- modified
silicone compounds (commonly known as non-ionic surfactants), which consist of
a polydimethylsiloxane skeleton with polysiloxane and oxyalkene copolymer grafts
(Zhang et al. 1999).
Often in PU foam systems, chemical (e.g. water) or physical (low boiling point
compounds) foaming agents are used to aid the foaming process (Park et al. 2013).
Water, as a chemical blowing agent, can react with the -NCO groups to produce
CO 2 , thus leading to the formation of a network in foam structure. Water due to low
cost and toxicity, water, as a chemical blowing agent, has become more popular in
the PU foam industry (Li et al. 2017a; Li et al. 2018). The physical blowing agents
typically used are difluorodichloromethane (CFC-11), monofluorodichloroethane
(HCFC-141b), monofluorotrichloromethane (CFC-12), etc. These compounds contain chlorine in their chemical structure, and are the main substances that cause
damage to the ozone layer, global warming and other environmental problems.
7 Synthesis of Biobased Polyurethane Foams From Agricultural and Forestry Wastes
are usually flexible with favorable flexibility at low temperature but relatively low
mechanical strength (Kraitape and Thongpin 2016).
Polyisocyanates are polymers containing monomeric units of isocyanates
(-NCO-) in their structure. Isocyanates products, along with polyols, are the two
main materials in PU industry. Frequently, polyisocyanate compounds are synthesized by the reaction of primary amine compounds (amine hydrochloride oramines,
etc.) with phosgene (COCl 2 ). The most common isocyanates used for the preparation of PU are divided into two main categories: TDI and phenyl isocyanate, commonly refers to diphenylmethane diisocyanate (MDI), PMDI and polymethylene
polyphenyl isocyanate (PM200) (Li et al. 2015). TDI containing two isomers (2,4TDI and 2,6-TDI) commonly applied to produce flexible PU foams, while MDI and
PM200 are more reactive than TDI, and are usually used in the rigid PU foam industry (Li et al. 2017c; Li et al. 2017b; Yang et al. 2019), while polymeric MDI (PMDI)
is a mixture obtained after distillation process of MDI/MDI oligomers, which has an
average functionality of 2.7 and is also commonly used to produce rigid and flexible
PU foams due to the high reactivity and low volatile and toxicity (Li et al. 2017a;
Li et al. 2018; Scott et al. 2019).
Catalysts also play an extremely important role in the preparation of PU foams,
since they adjust the rate of foaming reactions (Yu and Lee 2014). The catalysts
commonly used are: (1) organometallic compounds, including dibutyltin dilaurate
and stannous octoate (SnOct 2 ) (Simón et al. 2013) and (2) tertiary amines, which
refer to alcohol, alicyclic, aliphatic and aromatic amines, and ammonium compounds (Zhao et al. 2014) In general, two or more catalysts are used to adjust the
chemical reaction rate of the PU foaming process to meet the requirements of the
different manufacturing processes (Xu et al. 2014; Zhao et al. 2014).
Surfactants are also known as foam stabilizers, and is an indispensable component in the preparation of PU foam materials. This plays an important role in the
emulsion of foaming ingredients, adjusting the solubility of each component and
preventing the bubble body from collapsing. It also has a great impact on the foam
cellular size and the mechanical properties of PU foam materials (Dounis and
Wilkes 1997a). At present, most of the foam surfactants used are polyether- modified
silicone compounds (commonly known as non-ionic surfactants), which consist of
a polydimethylsiloxane skeleton with polysiloxane and oxyalkene copolymer grafts
(Zhang et al. 1999).
Often in PU foam systems, chemical (e.g. water) or physical (low boiling point
compounds) foaming agents are used to aid the foaming process (Park et al. 2013).
Water, as a chemical blowing agent, can react with the -NCO groups to produce
CO 2 , thus leading to the formation of a network in foam structure. Water due to low
cost and toxicity, water, as a chemical blowing agent, has become more popular in
the PU foam industry (Li et al. 2017a; Li et al. 2018). The physical blowing agents
typically used are difluorodichloromethane (CFC-11), monofluorodichloroethane
(HCFC-141b), monofluorotrichloromethane (CFC-12), etc. These compounds contain chlorine in their chemical structure, and are the main substances that cause
damage to the ozone layer, global warming and other environmental problems.
7 Synthesis of Biobased Polyurethane Foams From Agricultural and Forestry Wastes