The difference in molecular weight of polymer polyols directly affects their physical properties, reactivity and application areas, which is specifically reflected in the following aspects:
1. Relationship between molecular weight and hydroxyl value
Molecular weight and hydroxyl value (hydroxyl content) are inversely proportional, which can be calculated by the formula Molecular weight = (56.1 × 1000 × functionality) / hydroxyl value4. For example, under the same functionality, the higher the molecular weight, the lower the hydroxyl value, resulting in a reduction in the consumption of NCO groups that react with isocyanate14. This relationship directly affects the design of the ratio of polyols to isocyanates in polyurethane formulations.
2. Differences in physical properties
Melting point and viscosity: The larger the molecular weight, the higher the melting point and viscosity. For example, high molecular weight polyethylene glycol (such as PEG6000) has a harder texture and higher processing temperature requirements than low molecular weight varieties (such as PEG2000)27.
Solubility: Although all polyols are soluble in water and polar solvents, high molecular weight varieties may have a lower dissolution rate due to increased chain length27.
3. Effect of Molecular Weight Distribution
Polymer molecular weights are usually polydisperse, that is, the range of molecular weight distribution. Polyols with a wider distribution (such as a large weight average/number average molecular weight ratio) have better processing fluidity, but poor product uniformity; those with a narrow distribution have better mechanical properties (such as impact resistance) but are more difficult to process56.
4. Synergy between Functionality and Chain Structure
Functionality Difference: At the same molecular weight, 3-functionality polyethers have higher hydroxyl values and shorter side chains than 2-functionality polyethers, which are suitable for foam systems with high crosslinking density; while 2-functionality polyethers have longer chain segments and are more suitable for elastomers or adhesives34.
Chain structure characteristics: High molecular weight polyols have longer main chains, which can provide better flexibility and mechanical strength, but may reduce reactivity38. 5. Selection of application areas
Low molecular weight polyols (such as 1000-2000): commonly used in elastomers, coatings and other scenes that require fast curing and high reactivity34.
Medium and high molecular weight polyols (such as 2000-6000): mostly used in foam plastics, sealing materials and other fields that need to balance strength and processing performance38.
Ultra-high molecular weight varieties: may be used for special functional materials (such as slow-release carriers), but need to be adjusted in combination with specific functionality78.
In summary, the selection of polyol molecular weight needs to comprehensively consider multiple factors such as hydroxyl value requirements, processing conditions, and final product performance, and achieve specific application goals by adjusting the combination of molecular weight and functionality.