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Polyurethanes (PUs) are a major family of polymers displaying a wide spectrum of physico-chemical, mechanical and structural properties in coatings, elastomers, adhesives, and foams widely. As a result of rising and widespread environmental concern, as well as attempts to minimize hazardous waste volume flow and expand the use of sustainable raw materials, the focus has turned to the development of a number of bio-based polyurethanes.
Synthesis of Bio-based Polyurethanes
According to the structures, polyurethanes can be divided into two principal families which are (i) thermoplastic polyurethanes (TPUs) and (ii) thermoset polyurethanes. Thermoplastic polyurethanes are mostly obtained by a two-step process. The first step is based on the synthesis of a prepolymer with –NCO ending chains by reaction of a long diol such as a polyester or a polyether-diol, with an excess of diisocyanate. In the second step, long thermoplastic polyurethane chains are obtained by using a chain extender and the previous prepolymer. In opposite, thermoset polyurethanes are mainly prepared by mixing polyols and polyisocyanates in a one pot process such as for polyurethane foams.
Based on the knowledge above, it’s clear that polyols and isocyanates play important roles in the synthesis of polyurethanes. Thus, the synthesis of these two components from renewable resources becomes particularly important.
The polyol precursors may have at least two hydroxyl groups, and variation of the polyol side of polyurethanes has been one means to vary the resultant polyurethane properties. Many types of agriculture and forestry industry biowastes have been used to create polyols. These biowastes include wood, wheat straw, corn bran, sugar cane, and bamboo, which can be treated as rapidly renewable bio-resources.
Renewable isocyanates can be categorized into seven classes: (1) amino acid-based polyisocyanates, (2) sugar-based polyisocyanates, (3) furan-based polyisocyanates, (4) lignin-based polyisocyanates, (5) cashew nutshell liquid-based polyisocyanates, (6) vegetable-based polyisocyanates, and (7) algae-based polyisocyanates, which now provide the opportunity for production of sustainable polyurethanes with high bio-based content.
For example, isocyanate derivatives can be obtained by the phosgenation of dianhydrohexitols. And the dianhydrohexitols can be made by hydrogenolysis of hydrolyzed starch or double dehydration of sorbitol from reduced glucose.
Applications
Because of their superior properties, such as a broad palette of hardness, abrasion, and tear resistance; flexibility and elasticity; and bonding ability, Bio-based polyurethanes have found applications in all manner of commercial products. For example, they can be used in surface coating, self-healing, shape memory materials, and biomaterials, etc., as shown in figure 4 [3]. Further, different types of modifications widen their applications.
