Polyurethane soft foam formula summary book
01
Preface
Polyurethane flexible foam series mainly include block. Continuous. Sponge, high resilience foam (HR), self-crusting foam, slow resilience foam, microporous foam and semi-rigid energy-absorbing foam, etc.. This type of foam still accounts for about 50% of the total polyurethane products. A large variety of applications is expanding day by day, and it has been involved in various fields of national economy: home appliances, automobiles, home decoration, furniture, trains, ships, aerospace and many other fields.
PU soft foam since its introduction in the 1950s, especially after entering the 21st century, both technically and in terms of variety and product output have a leap forward. Prominent among them are.
Environmentally friendly PU flexible foam, that is, green polyurethane products; low VOC value PU flexible foam; low atomization PU flexible foam; full water PU flexible foam; full MDI series flexible foam; flame-retardant, low smoke, full MDI series foam; reactive high molecular weight catalysts, stabilizers, flame retardants and antioxidants and other new varieties of additives; low unsaturation, low mono-alcohol content of polyols; ultra-low density of excellent physical properties of PU flexible foam; low resonance frequency, low transmission PU soft foam; polycarbonate diol, polyε-caprolactone polyol, polybutadiene glycol, polytetrahydrofuran and other special polyols; liquid CO2 foaming technology, negative pressure foaming technology, etc. –
In short, the emergence of new varieties and new technologies to promote the further development of PU soft foam.
02
Foaming principle
To synthesize the ideal PU soft foam to meet the requirements, we must understand the chemical reaction principle of the foam system, in order to choose the appropriate main and auxiliary raw materials and manufacturing process. Polyurethane industry has developed to today, is not the imitation stage, but according to the performance requirements of the final product, through the raw material structure, synthesis technology means, in order to achieve, for this reason, a good grasp of the principle of foam formation is essential.
Polyurethane foam is involved in chemical changes during the synthesis process, and the change factors affecting the structural properties of the foam are more complex, which involves not only the chemical reaction between isocyanate and polyether (ester) alcohol, water, but also the colloid chemistry of foaming, whose chemical reactions include chain expansion, foaming and cross-linking processes. It is also influenced by the structure, functionalities and molecular weights of the substances participating in the reaction.
In general, the total reaction of polyurethane foam synthesis can be expressed by the following equation.
However, the actual situation is more complicated, and the important reactions are summarized as follows.
01 Chain expansion
The chain expansion of polyfunctional isocyanate and polyether (ester) alcohols, especially difunctional compounds, is carried out according to the following formula: Picture
In the foaming system, the amount of isocyanate is generally greater than the amount of compounds containing active hydrogen, that is, the reaction index is greater than 1, usually taken as 1.05, so the end product of chain expansion in the foaming process should be the end of the isocyanate group.
Picture
The chain expansion reaction is the main reaction of PU foam, which is the key to the physical properties: mechanical strength, god length rate, elasticity, etc.
02 Foaming reaction
Foaming is very important in the preparation of soft foam, especially in the synthesis of low-density products more prominent. Generally there are two kinds of foaming: the use of reaction heat vaporization of low boiling point hydrocarbon compounds, such as HCFC-141b, HFC-134a, HFC-365mfc, cyclopentane, etc. to achieve the purpose of foaming, the other is the use of water and isocyanate chemical reaction between a large number of CO2 gas foaming: in the presence of no catalyst, the reaction rate of water and isocyanate is slow.
And amines and isocyanate reaction rate is quite fast, for this reason, water as a foaming agent brings a large number of rigid chain, polar urea compounds, it affects the feel of foam products, resilience and heat resistance, in order to produce excellent physical properties and low density foam, must improve the molecular weight of polyether (ester) alcohol and the softness of the main chain.
03 Gelation
The gelation reaction is also called cross-linking and curing reaction, and the gelation is very important in the process of foaming, and the quality of foam products will be reduced or turned into scrap if the gelation is too early or too late. The ideal state is the balance of chain expansion, foaming reaction and gel reaction, otherwise there will be foam density or collapse foam.
There are three types of gelation in the foaming process.
01 Gelation of multifunctional compounds
Generally, the reaction of compounds with three or more functionalities can form a bulk structure compound. We use polyether polyol with more than three functionalities in the production of polyurethane flexible foam. Recently, the all-MDI system is also mixed with polyisocyanates with fn≥2.5 to improve the load-bearing capacity of low-density foams, which are the basis for the formation of three-phase cross-linked structures.
It is worth noting that the size of the molecular weight between the crosslinking points directly reflects the crosslinking density of the foam. That is to say, the cross-linked density is high, the hardness of the product is high, the mechanical strength is good, but the softness of the foam is poor, the resilience and elongation is low.
The molecular weight (Mc) between the cross-linking point of soft foam is 2000-2500, and the semi-rigid foam is between 700-2500.
02 shrinker urea formation when water is used as a foaming agent, correspondingly produce urea bond compounds, the more water, the more urea bonds, they will further react with excess isocyanate at high temperatures to generate three-phase structure of shrinker urea bond compounds:.
3), the formation of urea-based formate
Another type of cross-linking reaction is the hydrogen on the main chain of carbamate, which further reacts with excess isocyanate at high temperature to form triphasic structured ureidoformate bonds.
The formation of condensed diurea compounds and ureidoformate compounds is not ideal for foaming systems because of the poor thermal stability and high temperature decomposition of these two compounds. So it is crucial for people to control the temperature and isocyanate index in the production.
03
Chemical calculation
Polyurethane synthetic material is a kind of polymer synthetic material that can be synthesized from raw materials in one step to polymer products, that is to say, the physical properties of the products can be directly adjusted artificially by changing the raw material specifications and composition ratios. Therefore, it is very important to apply the polymer synthesis principle correctly and establish a simple calculation formula to improve the quality of polyurethane products.
01 Equivalent value
Equivalent value (E) refers to the molecular weight (Mn) of a compound molecule per unit functional degree (f);
For example, if the number average molecular weight of a polyether tertiary alcohol is 3000, then its equivalent value.
The commonly used cross-linking agent MOCA, i.e. 4,4′-methylenebis(2chloroamine), has a relative molecular mass of 267, and although there are four active hydrogens in the molecule, only two hydrogen atoms are involved in the isocyanate reaction, so its functional degree f = 2
In the product specifications of polyether or polyester polyol, each company only provides the hydroxyl value (OH) data, so it is more practical to calculate the equivalent value directly by the hydroxyl value.
It is worth reminding that the actual measurement of product functionality is very time-consuming, coupled with many side reactions, often the actual functionality of tertiary alcohol polyether (ester) is not equal to 3, but between 2.7-2.8, so it is recommended to use the (2) formula, that is, the hydroxyl value calculation is good!
02 Amount of isocyanate required
All active hydrogen compounds can react with isocyanate, according to the principle of equivalent reaction, accurate calculation of the amount of isocyanate consumed by each component in the formula is a common thing in PU synthesis.
