3 General Characteristics of Oligo-Polyols
3.12 Renewable Content
As a general definition, renewable content is the concentration of the natural-origin part (derived from living-world raw materials) in the structure of an oligo-polyol.
Renewable content or bio-based content of a polyol is determined by ASTM 6866, and is calculated as function of the ratio between the content of two isotopes of the carbon atom: 14C/12C [44]. Bio-based compounds (or compounds of natural origin) always have a measurable content of 14C but petrochemical compounds do not. Bio- based content is the ratio between ‘young’ carbon atoms (carbon atoms of natural compounds) and the total carbon atoms of a polyol. Thus, oligo-polyols based exclusively on petrochemical raw materials have no renewable content. Oligo-polyols based on natural compounds (or mixtures of natural compounds) and petrochemical raw materials have measurable renewable content (e.g., castor oil, sucrose polyols, sorbitol polyols, vegetable oil polyols). The ratio of carbon-atom isotopes 14C/12C is determined by liquid scintillation and ‘isotope ratio’ mass spectrometry.
In summary, irrespective of their chemical structure, oligo-polyols have general and common characteristics:
• All oligo-polyols are low-MW polymers, with oligomers having MW <10,000 Da.
• All oligo-polyols have terminal hydroxyl groups; they are telechelic, low-MW polymers (hydroxyl-terminated telechelic oligomers).
• All oligo-polyols have primary or secondary hydroxyl groups, but not tertiary hydroxyl groups.
• All oligo-polyols have functionality (the number of hydroxyl groups/mol) in the range 2–8 OH groups/mol.
• Transformation of all oligo-polyols in high-MW PU polymers is based on reactive processes as a consequence of a chemical reaction.
• All oligo-polyols are liquid at RT or at low temperatures (40–60 °C) and, due to their low viscosities, are very easy to process to high-MW PU.
• All oligo-polyols are characterised by general and common physico-chemical characteristics determined by standard test methods.
• A common characteristic of oligo-polyols is renewable content (the content of bio-based raw materials present in the structure of oligo-polyols). As a function of the content of natural-origin raw materials, the renewable content of oligo- polyols varies from 0% (100% petrochemical polyols) to 100% (100% bio-based polyols).
In this book, as mentioned previously, for practical reasons oligo-polyols are divided into two groups: oligo-polyols for elastic PU and oligo-polyols for rigid PU.
The main types of oligo-polyols described in detail in this present book are presented in Table 3.2.
Table 3.2 Main types of oligo-polyols
Oligo-polyols for elastic PU Oligo-polyols for rigid PU 1. Polyalkylene oxide polyols (polyether
polyols)
2. Polymer polyols (filled polyols) 3. Polytetrahydrofuran polyols 4. Polyester polyols
5. Polybutadiene polyols 6. Acrylic polyols 7. Other oligo-polyols
1. Polyether polyols 2. Aminic polyols
3. Polyols based on condensates 4. Polyester polyols
5. Polyols by thiol-ene reaction 6. Polyols from renewable sources 7. Flame-retardant polyols 8. New oligo-polyol structures
9. Polyols by chemical recovery of PU waste
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The oligo-polyols for elastic polyurethanes (PU) are characterised by a high molecular weight (MW), usually situated in the range of 2,000–6,500 daltons (Da), and by a low functionality, of approximately 2–3 hydroxyl groups/mol. The high elasticity is given firstly by the high mobility of the oligo-polyol chains, which permits free rotation around the bonds of the main chain and, as general rule, these polymers have a low value for the glass transition temperature (Tg) in the range of -50 °C to -86 °C. At room temperature (RT), the PU based on high-MW polyols are situated in the highly elastic domain and conserve this important characteristic at lower temperatures (of course at negative temperatures higher than Tg). It is clear that the high elasticity of the resulting PU is given by the high-MW and high mobility oligo-polyol segment and due to the low crosslink density, as a consequence of the low functionality of the oligo-polyols.