Influence of polyols on spandex production

The influence of polyol on spandex production is determined by the properties of polyol itself.

The quality of polyether polyol is evaluated by the following indicators: hydroxyl value, acid value, moisture, viscosity, color, molecular weight distribution, unsaturation, etc. The following is a review of their importance and significance.

1. Hydroxyl value: Hydroxyl value is the basis for calculating the amount of substance and the number of average molecular weight, which is essential for accurately controlling the mass ratio of isocyanate and polyol in the formulation (--NCO/-OH), predicting the viscosity of PET and its prepolymer, determining the content of soft segments and getting stable performance of finished polyurethane foam, in addition, it is also an important indicator for monitoring the reaction degree of polyester polyol.

2.Acid value: the acidity of polyol directly affects the reaction activity of isocyanate, and may have a high or low impact on the measured value of hydroxyl value, in the determination of hydroxyl value, the measured value of acidic polyol should be corrected, that is, the corrected hydroxyl value is equal to the sum of the measured hydroxyl value and acid value. Generally, the acid value of polyether polyol is very low, so it can be ignored when correcting the hydroxyl value; while polyester polyol, due to the use of organic acid as raw material, often has a small amount of acidic compound residue in the product, so that the product has a certain acid value. The acid value in polyester polyol must be less than 2.0mgKOH/g, otherwise it will bring adverse effects to the processing and product performance, so it is especially important to determine the acid value of polyester polyol.

In addition, in the synthesis process of polyol, due to the use of catalysts may also bring and produce alkaline impurities, polyol should try to avoid alkaline residues, because it can promote many unpredictable side reactions in the synthesis of polyurethane materials, so the PH value can be measured to quickly determine the approximate degree of polyol tainted by acid and alkaline substances.

3. Moisture: Moisture control in polyols is also extremely important. Isocyanate is extremely sensitive to water, a small amount of water will consume nearly 10 times the amount of isocyanate, but also affect the viscosity of prepolymer and product properties; determination of hydroxyl value, water in polyol will consume part of the acylating agent, but also affect the accuracy of the determination of hydroxyl value; because of the presence of water, it is difficult to accurately calculate the ratio of isocyanate to hydroxyl in the formulation; usually polyol production polyurethane requires water content of 0.2%, above this value, the hydroxyl value must be dehydrated first when measuring the hydroxyl value.

In addition, in the polyol synthesis process, trace amounts of water can also be used as a starting agent to react with the epoxy compound monomer, so that the number of starting agents in the specified formula increases, resulting in a decrease in the designed molecular weight of synthetic polyol, that is, when the moisture exceeds the standard, the molecular weight may also be different from the conventional.

4. Viscosity: Viscosity is an important indicator to evaluate the performance of different batches of products, because in polyol synthesis, even if the raw material ratio is the same, if there is a slight difference in the process, the viscosity may have a large difference. Conventional analysis, with experience, through the viscosity value can also be estimated by the hydroxyl value of the general range, for polyester polyol is generally large viscosity, low hydroxyl value; polyether polyol viscosity, hydroxyl value is generally large. Viscosity is also an indicator to characterize the degree of polymerization and molecular weight of the product. In the ratio of polyol combination material, but also consider the processing of the combination of material flow requirements, if the polyol viscosity is too large, will make the combination of material flow decreased.

5. color: the color of different polyols and the type of starting agent and production process, the color of different types of polyol varies greatly, the same type of polyol also exists a certain color difference, the polyol often used, the color can help qualitative analysis of polyol category, compare the decolorization of the batch product. Manufacturers, to record the color of each batch of products, raw materials use manufacturers can only do a comparative analysis of the color of polyols from different manufacturers.

6. unsaturation: unsaturated double bond in polyether polyol exceeds the standard will directly affect the distribution of polyether molecular weight, this is because in the synthesis process of polyether, the reaction temperature will directly affect the unsaturation of polyether products, polyether end groups in the reaction process once the formation of unsaturated double bond, it will lose reaction activity, chain growth termination, thus affecting the molecular weight of polyether: in the polyurethane synthesis reaction process, as polyol unsaturated bonds have instability, double bonds are easy to break during the reaction process, forming part of the low molecular weight molecules, which will make the physical properties of the product decline, so polyether polyol unsaturation is also an important indicator to measure the quality of polyether polyol, especially for small-scale manufacturers who do not have the ability to detect molecular weight distribution, it is particularly important.

7. Molecular weight distribution: The structure (linear or polyfunctional) and molecular weight of polyether polyol or polyester polyol used in various polyurethane products are different due to their different properties and applications, the molecular weight distribution of polyether of different manufacturers and the same grade may have large differences when the technical indexes such as hydroxyl value, acid value, moisture and viscosity are all in line with the standard range, and whether the molecular weight distribution is uniform will directly affect the good and bad physical properties of polyurethane plastics, especially the impact on compressive strength and dimensional stability.Therefore, determination of the molecular weight distribution of polyols is essential for both raw material selection and formulation design.

In summary, the evaluation of the quality of polyol, is a comprehensive multivariate examination, these indicators are affected by each other, hydroxyl value can help calculate the molecular weight, acid value and moisture may affect the hydroxyl value, too much moisture, along with the distribution of molecular weight may not be uniform, the size of viscosity and hydroxyl value, the size of the molecular weight has a certain relationship, molecular weight distribution or not will directly affect the physical properties of the foam good or bad. Therefore, when we choose polyol, to test a variety of aspects, never from a simple few indicators of qualified, define the quality of polyol qualified, and sometimes even through the performance of downstream products to evaluate (but this may be a long-term experience and lessons learned, and sometimes pay a huge price).