What is the relationship between the hydroxyl number and molecular weight of polyether polyols?

RELATIONSHIP BETWEEN HYDROXYL VALUE AND MOLECULAR WEIGHT OF POLYETHER POLYOLS

Polyether polyol is an important chemical raw material, which is widely used in polyurethane, coatings, adhesives and other fields. Its performance is mainly determined by the hydroxyl value and molecular weight in the molecular structure. In this paper, the relationship between the hydroxyl value and molecular weight of polyether polyols will be analyzed in detail, and its influence on material properties and the control methods in practical application will be discussed.

1. hydroxyl value and molecular weight of the basic concept

The hydroxyl value is an important indicator to measure the hydroxyl content in polyether polyols, usually expressed in mg KOH/g. It reflects the number of hydroxyl groups in the polyol molecule, which determines its ability to participate in chemical reactions. Molecular weight is the total mass of polyether polyol molecules, usually in g/mol, and is one of the key parameters that determine the physical and chemical properties of the material.

The molecular structure of polyether polyols determines the relationship between their hydroxyl value and molecular weight. The larger the molecular weight, generally means that the longer the molecular chain, the number of hydroxyl groups is relatively increased, resulting in an increase in the hydroxyl value. Therefore, there is a certain positive correlation between the hydroxyl value and the molecular weight.

2. RELATIONSHIP BETWEEN HYDROXYL VALUE AND MOLECULAR WEIGHT

The hydroxyl value of polyether polyols is closely related to their molecular weight. As the molecular weight increases, the molecular chain grows and the number of hydroxyl groups increases, resulting in an increase in the hydroxyl value. This relationship can be understood by molecular structure analysis: polyether polyols with larger molecular weight usually have more repeating units, each repeating unit contributing one or more hydroxyl groups, so the hydroxyl value naturally increases.

For example, common polyether polyols, such as polypropylene glycol (PPG) and polyethylene glycol (PEG), gradually increase the hydroxyl value as the molecular weight increases. This is because the growth of the molecular chain increases the number of hydroxyl groups, and the increase in molecular weight also reflects the depth of the polymerization reaction.

3. Hydroxyl Value and Molecular Weight on Material Properties

The hydroxyl number and molecular weight have an important influence on the properties of polyether polyols, especially in the preparation of polyurethanes. The hydroxyl value determines the reactivity of polyol with isocyanate, which directly affects the crosslinking density and mechanical properties of the product. The molecular weight affects the flexibility, strength and permeability of the material.

In particular, low molecular weight polyether polyols generally have lower hydroxyl values, lower reactivity, but shorter molecular chains, and the materials exhibit better flexibility and lower glass transition temperatures. The high molecular weight polyether polyol has a higher hydroxyl value, enhanced reactivity, and a longer molecular chain, and the material exhibits higher strength and better heat resistance.

In practical applications, polyether polyols with specific properties can be prepared by adjusting the molecular weight and hydroxyl value to meet the needs of different fields. For example, in the preparation of high-performance polyurethane elastomers, the selection of polyether polyols with high molecular weight and high hydroxyl value can improve the mechanical strength and durability of the material.

4. hydroxyl value and molecular weight control methods

During the synthesis of polyether polyols, the hydroxyl value and molecular weight can be controlled by adjusting the synthesis conditions. For example, the length of the molecular chain and the number of hydroxyl groups can be controlled by changing the ratio of the starting monomers, the polymerization time and the reaction temperature.

The use of molecular weight regulators (e. g., chain terminators) can also effectively control the molecular weight and hydroxyl value of the polyether polyol. Selecting the appropriate catalyst and reaction conditions can further optimize the performance of the product to meet the specific application requirements.

5. summary

There is a close relationship between the hydroxyl value and molecular weight of polyether polyols, and the hydroxyl value increases with the increase of molecular weight. This relationship not only affects the reactivity of the material, but also determines its physical and chemical properties. In practical applications, polyether polyols with specific properties can be prepared by adjusting the synthesis conditions and selecting appropriate polyols to meet the needs of different fields.

The relationship between hydroxyl value and molecular weight of polyether polyol is an important basis for its performance optimization. Future research can further explore the influence of molecular structure on performance and develop higher performance polyether polyol materials.