CS257709B1 - Catalytic system for expanded polyurethane production - Google Patents

Abstract

Catalytic system for lightweight production polyurethane, particularly suitable for production lightweight polyurethane the weight at which it is used methylene chloride or a mixture thereof with chlorofluorinated hydrocarbons as a foaming agent. The essence is based on compounds piperidine. Delayed catalyst effect the system has a favorable effect on the speed of the sieve reaction.

Description

The invention relates to a catalyst system for the production of expanded polyurethane, in particular suitable for the production of expanded polyurethane of low bulk density.

The production of expanded polyurethane is already well developed and utilized. The basic principle of production and technological details are given, for example, in the publication Dombrow - Polyurethanes (SNTL, Prague 1961) or in the publication Becker - Polyurethane (VEB Fachbuchverlag,

Liepzig 1973).

It is known that the polyol diisocyanate, water diisocyanate and amine diisocyanate reactions are of particular importance in the preparation of expanded polyurethanes. The polymer formed by the polyol-diisocyanate reaction is foamed by carbon dioxide resulting from the water-diisocyanate reaction. Thus, from the foaming point of view, the most important reaction is water-diisocyanate, in which carbamic acid is formed as an unstable intermediate which decomposes spontaneously into the primary amine and carbon dioxide.

The amino group further reacts with another diisocyanate to form a disubstituted urea bond. In this reaction sequence, water is considered to be a difunctional monomer involved in the extension of the polymer chain as well as in the formation of a foaming medium. The concentration of water in the formulation of the recipe not only determines the weight of the resulting lightweight mass, but also affects some of its physical-mechanical properties.

From the theoretical point of view, the whole reaction system can be simply represented as follows: after the start of the reaction, the viscosity of the resulting polymer increases and simultaneously develops carbon dioxide, which foams the adduct to the point where its pressure exceeds the cell wall strength. The pressure of the carbon dioxide is equal to the pressure of the atmosphere and the volume of the expanded mass is stabilized. If the walls break when the strength of the cell structure is sufficient, we will get a good product. Thus, the reaction must proceed at a rate such that the strength of the cell walls of the polymer, the amount, and the pressure of the gas increase in a consistent manner. The correct reaction rate of both main reactions can be considered as the main prerequisite for the preparation of a quality expanded mass. This means that the strength of the polymer must initially rise before the pressure of the gas produced, but so that the pressure in the cells differs little from atmospheric pressure.

This ensures that the volume will increase in proportion to the amount of gas produced and the temperature increasing. From the moment when the strength of the polymer reaches a value such that the skeleton of the expanded structure does not collapse or otherwise break, the gas pressure should increase to perforate the cell walls. However, if at this stage the gas pressure is slow, the cell strength may exceed the maximum gas pressure, the walls will not break and the gas will remain under pressure at the elevated temperature in the cells. Cooling then reduces the temperature in the closed cells.

Cooling then reduces the pressure and volume of the gas and thereby shrinks the expanded structure.

Conversely, if the gas pressure in the initial phase exceeds the strength of the polymer, the gas escapes from the cells and the foam collapses because the cell backbone does not yet have sufficient strength.

A low density polyurethane expanded polyurethane can be achieved in several ways.

Less efficient methods include the use of special polyols and the choice of a very low isocyanate index. Especially in the case of soft expanded polyurethane, the method by means of the index is very problematic as it can cause the formation of cross-links of the polymer. This crosslinking is possible to an appreciable extent with an excess of diisocyanate and at a sufficient temperature. Biuret and allophanate bonds then increase the strength of the polymer, especially in the final foaming phase, increasing the pressure required to expand the gas and increasing the bulk density.

As already mentioned, the increased amount of water in the recipe is one of the main ways to reduce bulk density. However, there are also some limitations, which are related in particular to the high temperature profile during the exothermic reaction and the related technological and safety problems. When dosing larger amounts of water, it is also necessary to dispense higher amounts of diisocyanate. Due to the reaction of the polyol diisocyanate, a higher reaction temperature is produced, which results in an increase in the reaction rate. This is the case when working in the area of bulk weights below 25 kg.m \, ie when dosing water in 4 to 7 masses. d. per 100 wt. d. a polyol corresponding to a diisocyanate consumption of 50 to 80 wt. d.

Thus, the combination of chemical foaming - formed by carbon dioxide - with physical foaming using low boiling, inert organic compounds is widely used in practice, in particular using chlorofluorinated compounds such as monofluorotrichloromethane.

These compounds are added to the reaction mixture up to 20 to 30 wt. d. per 100 wt. d. of the polyol, here the polyol diisocyanate is evaporated by the liberated heat of the reaction and, together with carbon dioxide, foams the reaction mass.

The heat required to evaporate these chlorofluorocarbons is so great that it strongly affects the reaction rate of the polymerization reaction, which is particularly temperature sensitive. During foaming, it can be seen that at the beginning of the gas evolution is considerably slower.

This phenomenon can be explained by the fact that the heat of reaction is consumed in phase 1 to evaporate the chlorofluorocarbons and hinders the development of carbon dioxide at the beginning of the process. The water-diisocyanate reaction does not start sufficiently until the heat produced is sufficiently large, i.e. the polyol-diisocyanate reaction has already largely been carried out. Due to the high heat dissipation during evaporation of chlorofluorocarbons, the risk of closed cell formation is reduced, but other defects, such as cracks in the lightweight structure, may occur.

In order to obtain a perfect lightweight structure, it is necessary to assemble the recipe in such a way that increasing the volume over time is an inverse function for increasing the pressure. This purely theoretical requirement is, of course, influenced in practice by the strength of the polymer, the non-ideal behavior of the gases, the external conditions and their influences. However, it is apparent that the dosage of both water and chlorofluorocarbons is limited by a certain limit.

Among the reported and recently discussed foaming agents is methylene chloride. Its use is not new, and many producers have been returning their application solutions.

This is because of economic reasons, but also because of boycott of chlorofluorocarbons for environmental reasons. The technical problems lie in its higher boiling point and lower vapor pressure than chlorofluorocarbons and in the browning of the foam, especially its foam.

The current situation with the catalytic systems of expanded polyurethanes is adapted to foaming with carbon dioxide and chlorofluorocarbons, so that their effect occurs simultaneously. However, the reaction profile using methylene chloride must be adapted to its physical properties. For example, as opposed to monofluorotrichloromethane, which has a mole. wt. 137.38 and a boiling point of 23.8 ° C has methylene chloride moles. wt. 84.94 and boiling point 40 ° C. In particular, this leads to efforts to solve new catalyst systems that would sweeten the foaming and crosslinking reactions as described in the introduction. Reducing the browning of the core of the expanded mass can then be solved on the one hand by determining the critical dose of methylene chloride for the system under consideration, by dosing the antioxidants, respectively. mixtures thereof and addition of some other substances described in the literature. In any case, it should be noted that the use of methylene chloride as a foaming agent is highly variable - requiring accurate adjustment of the entire reaction and a comprehensive solution to the entire recipe.

The present invention is based on such a catalyst system which gives a wide range of applications in the production of expanded polyurethanes, which, however, is particularly suitable for the delayed action of methylene chloride or mixtures thereof with chlorofluorocarbons as a foaming agent. A substantial part of this catalysis system consists of piperidine compounds, namely 2,2,6,6-tetramethylpiperidine and its homologues in admixture with conventional tertiary amines.

The catalytic action of the tertiary amines, as already mentioned, is based on the reaction of the amine diisocyanate. According to the mechanism of Baker, one could expect that the more alkaline the catalyst, the more active the effect should be, because the strength of the base is a measure of the ease with which an amine gives its free proton electron pair. However, it is now known that the activity of a tertiary amine catalyst is also determined by its structure, and the less steric amine inhibition, the more active the amine catalyst. On the other hand, the tertiary amine varies sterically with its properties according to the size and functionality of the substituents.

The aromatic rings of said tertiary amines are characterized by substitutions with particularities that result from the effect of the heteroatom, in this case nitrogen, on the state of the electrons in the aromatic ring. The positive charge of nitrogen attracts the valence electrons on all bonds that come from it. Thus, a portion of the positive charge is transferred to adjacent atoms because the common electrons are drawn closer to the nitrogen atom. The radical groups react with the NCO groups / reactions are separated and the exposed nitrogen determines the further course of the reaction.

In this amine mixture, the effect of free nitrogen in the second phase of the reaction is emphasized, and thus, after the first phase, which represents a slow rise in the second phase, a rapid conclusion of the reaction is applied after the temperature of the reaction mixture increases. This has very positive consequences in the production of expanded polyurethane using methylene chloride or a mixture thereof with chlorofluorocarbons as a foaming agent.

Example 1

Foam mixture containing 100 wt. d. polyetherpolyol (mol. wt. 3,000, hydrox. no. 56) based on glycerin, propylene oxide and trimethylopropane; d. water, 2.0 wt. d. polysiloxane surfactant, 0.5 wt. d. tin octoate, 0.1 wt. d. triethylenediamine, 0.08 wt. d. bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate, 8.0 wt. d. Methylene chloride at the 105 index of toluylene diisocyanate had reaction times - beginning of foaming 25 sec, end of foaming 102 sec, end of sieving 170 sec.

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Example 2 A foam composition containing 100 wt. d. a prepolymer formed by the reaction of polytetramethyleneterglycol and toluylene diisocyanate, characterized by an NCO content of 6.3%, 2.0 wt. d. polysiloxane surfactant, 3.9 wt. d. phenylenediamine, 3.2 wt. d.

2,2,6,6-tetramethylpiperidine, 8.0 wt. d. methylene chloride, 2.0 wt. d. The pigment black composition had reaction times - beginning of foaming 60 s, end of foaming 117 s, end of sieving 200 s. The reaction times corresponded to the technological requirements for the preparation of semi-hard microporous foam.

Claims (1)

Catalyst system for the production of expanded polyurethane, particularly suitable for the production of expanded low density polyurethane prepared with methylene chloride or a mixture thereof with chlorofluorocarbons as blowing agent, characterized in that it consists of 0.01 to 3.5% by weight. d. tetramethylpiperidine or homologues thereof, and from 0.01 to 3.5 wt. d. a tertiary amine, all based on 100 wt. d. polyether polyol and / or polyester polyol.