LOW TEMPERATURE MANUFACTURING PROCESS FOR NYLON

Abstract: This invention concerns a low temperature process for the manufacture of nylon.

Description: TITLE LOW TEMPERATURE MANUFACTURING PROCESS FOR NYLON FIELD OF THE INVENTION This invention concerns a low temperature process for the manufacture of nylon. TECHNICAL BACKGROUND Some commercially important polyamides, referred herein to as dimonomeric polyamides, require starting monomers of two kinds, one monomer having two carboxylic acid functional reactive groups (diacid) and the other monomer having two amino functional reactive groups (a diamine) . In the most common method of preparing dimonomeric polyamides, the starting diacid and diamine components are mixed in stoichiometric proportions in a solution containing a large amount of water, typically up to as much weight of water as the combined weight of the diacid and diamine components. This water is subsequently removed by evaporation which requires a large amount of energy. The evaporation of water is usually done at elevated pressure in order to achieve a high enough boiling temperature to prevent the formation of solids. After the evaporation, there must be a pressure reduction step which requires heat to prevent the product from solidifying. The heating is known to cause discoloration and chemical degradation of the product. Attempts to produce dimonomeric polyamides without the use of water or other solvents have usually been unsuccessful. If one component is a solid, it is difficult to accurately proportion the solid component. If both components are supplied as liquids (melt) , these liquids may experience degradation, as a result of the high temperature required to keep the components in melt form. United States Patent No. 4,131,712 endeavors to overcome these difficulties. This patent teaches a process for the preparation of a high molecular weight polyamide, wherein a diacid-rich component and a diamine-rich component are prepared separately in non- stoichiometric proportions and then the diacid-rich component and the diamine-rich component are contacted in the liquid state at high enough temperature to prevent solidification. When the proportions of the total amounts of diacid and diamine are as much as possible stoichiometric, the molten components are further heated together to cause polycondensation to form a high molecular weight polyamide. The major utility of the process is in the manufacture of nylon 66. U.S. 4,131,712 identifies the low melting temperature for acid-rich and diamine-rich compositions for which substantial dehydration is avoided. U.S. 4,131,712 also identifies the temperatures required to prevent solidification at complete dehydration for each diacid-rich composition and each diacid-rich composition and for each combined proportion of diacid-rich component and diamine-rich component obtained by contacting portions of the diacid-rich component with portions of the diamine-rich component. For a given combined proportion of diacid-rich component and diamine-rich component, the melting temperature at complete dehydration is defined as the temperature which prevents solidification at complete dehydration. U.S. Patents 4,433,146 and 4,438,257, teach the use of a partial condenser to condense diamine out of vapor leaving the reaction mixture so as to return the diamine to the reaction mixture. However, the procedure, if used on a commercial scale, with stepwise addition of diamine, appears to require extended periods of time to recycle the diamine. SUMMARY OF THE INVENTION The present invention provides a process for the preparation of polyamides in the absence of added aqueous or organic solvents at low temperatures and low pressures. This process can achieve stoichiometric balance of non-stoichiometric components at temperatures which substantially avoid vaporization of diamine while the vaporization of water formed in the dehydration reaction is not avoided. A diacid-rich component is contacted with a diamine-rich component in the melt state at an intermediate degree of dehydration at temperatures which retain the melt state of the combined proportions of the diacid-rich component and the diamine-rich component and which are stable to solidification but which are below the high temperatures required to prevent solidification, at complete dehydration. The method steps comprise the following: 1. providing a diacid-rich component which may be one or more dicarboxylic acids, or a dicarboxylic acid and a carboxylic acid, which may be in the solid or in the melt state, or at least one dicarboxylic acid with at least one diamine in a total diacid to total diamine mole ratio greater than one, in the melt state; 2. providing a diamine-rich component which may be one or more diamines, or a diamine and an amine, which may be in the solid or in the melt state, or at least one diamine and at least one dicarboxylic acid in a total diamine to total dicarboxylic acid mole ratio, whether free or chemically combined, of greater than one, in the melt state; 3. contacting the diacid-rich component with the diamine-rich component in one or more steps such that the temperature is sufficient to retain the resulting mixture in a melt state but is at a temperature lower than that required to prevent solidification at complete dehydration, for the combined proportions of diacid and diamine, and for which the evaporation of diamine is substantially avoided; and . after substantial stoichiometric balance is achieved, applying heat to the resulting mixture to cause polycondensation to a high molecular weight polyamide. DETAILED DESCRIPTION OF THE INVENTION In the process above, the diamine-rich component and the diacid-rich component can be contacted continuously or in one or more discrete steps until sufficient amounts of the diacid-rich and the diamine- rich components have been added to achieve substantial stoichiometric balance. Contacting the diacid-rich and the diamine-rich components at melt temperatures can result in dehydration of the components. Prior to complete dehydration, there is an intermediate degree of hydration. The term "intermediate degree of hydration" describes a state between a first state where no reaction has occurred, i.e. no dehydration, and a second state at which dehydration is substantially complete. Prior to achieving stoichiometric balance, temperatures which retain the combined proportions of diacid and diamine components in the melt state will also cause dehydration of the components. Temperatures lower than those which will prevent solidification at complete dehydration for the combined proportion of diacid and diamine are sufficient to retain the combined proportion of diacid and diamine in the melt state while the rate of dehydration is slow enough so that substantial dehydration is avoided and solidification will not occur over reasonable periods of time sufficient for commercial processing. These temperatures are low enough so that diamine vaporization is substantially avoided. For non-stoichiometric and for substantially stoichiometric combined proportions of the diacid and diamine components, lower processing temperatures are possible which retain the mixture in the melt state and keep the mixture stable to solidification even at temperatures which surprisingly are below the low melting temperatures where substantial dehydration is avoided. At the low temperatures and low pressures possible, water formed by dehydration will vaporize and only a small amount of water will be present in the liquid melt. Also, the temperatures taught herein can substantially avoid the vaporization of diamine. When substantially stoichiometric balance has been achieved, further dehydration is required to form a high molecular weight polyamide. The further dehydration can be achieved by conventional or other means. This process is based on the discovery that a diacid-rich component can be contacted with a diamine- rich component in the absence of water or organic solvent to form a melt possessing an intermediate degree of dehydration at surprisingly low temperatures. Before stoichiometric balance required to form high molecular weight polyamide is achieved, these temperatures may be lower than those required to prevent solidification at complete dehydration and even below those low melting temperatures for which substantial dehydration is avoided. When substantial stoichiometric balance of the diacid-rich component and the diamine-rich component required to achieve high molecular weight polyamide is achieved, the mixture can be held in the melt state at temperatures below the melt temperature where substantial dehydration is avoided. That is, for example, a substantially balanced mixture of adipic acid and hexamethylenediamine, nylon 66 salt, at an intermediate degree of dehydration can be held in the melt state at temperatures below that of the melting temperature where substantial dehydration is avoided, as defined in U.S. 4,131,712. Because the melting temperature for a given combined proportion of diacid and diamine components at an intermediate degree of dehydration can be surprisingly low and because the rate of reaction to increase the degree of dehydration can be relatively slower at lower temperatures, a process disclosed as operating at temperatures lower than those required to prevent solidification at complete dehydration can still be a process stable to solidification. The process of the present invention operates at temperatures lower than those required to prevent solidification at complete dehydration. Process temperatures which are significantly lower than those required to prevent solidification at complete dehydration can reduce thermal exposure and can be low enough to avoid significant diamine vaporization at low pressure, even as the proportions of diacid and diamine components approach stoichiometric balance. Low diamine loss helps to control compositional balance without the need for additional condensation or high pressure equipment, and avoids prolonged process time, prolonged thermal exposure, and possible increased degradation. The process involves contacting a diacid-rich component and a diamine-rich component in one or more steps so that the combined proportion of the diacid-rich and diamine-rich components approaches stoichiometric proportion. The temperature for one or more combined proportions of diacid and diamine components is lower than the temperature disclosed as required to prevent solidification at complete dehydration for that combined proportion and the pressure for that combined proportion of diacid and diamine components may be atmospheric or near atmospheric. The temperature needed to retain a melt for a combined proportion of diacid and diamine will depend on the particular proportion of diacid and diamine and on the degree of dehydration. The temperature needed to retain a melt may change as the combined proportion of diacid and diamine or as the degree of dehydration changes so that the temperature may not be held constant, but for at least a single combined proportion of diacid and diamine at some point in time will be below that required to prevent solidification at complete dehydration. The "degree of dehydration" is the fraction of the maximum potential reactive ends of the minority diacid or diamine component which have undergone a water forming reaction to form a chemical combination with the majority component. The number of potential reactive ends can be calculated from the mass of the diacid or diamine component added, and their respective molecular weights. The number of chemically unreacted ends can be found by titration. EXAMPLE 1 Molten, anhydrous hexamethylenediamine (HMD) was added to an agitated reaction vessel containing 225 grams of a molten acid rich mixture consisting of 81% by weight of adipic acid and 19% by weight of hexamethylenediamine at 145°C. The reactor pressure was 9 psig, maintained by a slight nitrogen flow to the reactor. While adding HMD to the reactor, the temperature never exceeded 184°C. After 102 grams of HMD were added to the vessel, a sample was taken from the vessel. The vessel temperature at the time the sample was taken was 168°C, and the vessel contents were clear and boiling was occurring. Since vaporization of HMD at this temperature is known to be minimal, the boiling is indicative of reaction (dehydration) occurring which liberates water as a by-product and must be removed. Testing of the sample taken indicates a substantially balanced composition with a water content of 0.73% by weight and with a degree of dehydration of 0.39. The reactor contents were subsequently heated to high temperatures to form a high molecular weight polyamide. This example illustrates the ability to balance an acid-rich component with a diamine-rich component at low pressures and at tempreatures low enough to avoid substantial diamine vaporization en route to forming high molecular weight polyamide. The temperatures for non-stoichiometric, combined proportions of the diacid- rich component and the diamine-rich component are lower than those required to prevent solidification at complete dehydration, while for substantially stoichiometric proportions and a degree of dehydration of 0.39, the temperature of 168°C is significantly lower than that where substantial dehydration is avoided which is about 195-200°C. EXAMPLE 2 An acid rich mixture of 158.3 grams comprising 81% adipic acid and 19% HMD by weight was melted in the reaction vessel of Example 1. Molten anhydrous HMD at 50°C was added to the reaction vessel. The vessel pressure was atmospheric. When a substantially balance proportion was reached, the temperature was 180°C and a sample was taken. Twenty minutes later, the vessel was clear, the vessel temperature was 170°C, and another sample was taken. The first sample indicates a degree of dehydration of 0.26 and the latter sample indicates a degree of dehydration of 0.47. This example illustrates the ability to balance an acid-rich component with a diamine-rich component at atmospheric pressure and at temperatures low enough to avoid substantial diamine vaporization. The temperatures for non-stoichiometric, combined proportions of the diacid-rich component and the diamine-rich component are lower than those required to prevent solidification at complete dehydration, while for substantially stoichiometric proportions at an intermediate degree of dehydration, the temperature is slightly lower than that where substantial dehydration is avoided. This example also illustrates the ability to hold the combined proportions of diacid and diamine components for a length of time of 20 minutes at low temperatures without solidification occurring. EXAMPLE 3 Molten, anhydrous hexamethylenediamine was added to an agitated reaction vessel containing a molten acid rich mixture consisting of 81% by weight of adipic acid and 19% by weight of HMD at atmospheric pressure. Addition of HMD was stopped before stoichiometric proportions were reached. A sample was taken 10 minutes after HMD addition was discontinued. The vessel temperature at the time the sample was taken was 162°C. The sample indicates a combined molar proportion of 54.8% adipic acid and a degree of dehydration of 0.48. Comparison of this example with Example 4 of U.S. 4,131,712 at a combined molar proportion of 57.1% adipic acid and a temperature of 250°C which is disclosed as the temperature required to prevent solidification at complete dehydration illustrates the ability to operate at temperatures significantly lower than those disclosed as required to prevent solidification at complete dehydration and the benefit of operating at lower temperatures since HMD will vaporize when added to a mixture at a temperature of 250°C. This example also illustrates that for a non- stoichiometric proportion of 54.8% adipic acid and an intermediate degree of dehydration, operating temperatures stable to solidification can be lower than the melt temperatures where substantial dehydration is avoided, i.e. about 185°C.

Claims: _]_____ What is claimed is: 1. A process for manufacturing nylon comprising: (a) providing a diacid-rich component in solid or in the melt state; (b) providing a diamine-rich component in the melt state; (c) contacting the diacid-rich component with the diamine-rich component in one or more steps such that the temperature is sufficient to retain the resulting mixture in a melt state but is at a temperature lower than that required to prevent solidification, at complete dehydration, for the combined proportions of diacid and diamine, and at a temperature at which the evaporation of diamine is substantially avoided; and (d) after substantial stoichiometric balance is achieved, applying heat to the resulting mixture to cause polycondensation to a high molecular weight polyamide. 2. The process of Claim 1 wherein the diacid component is selected from one or more dicarboxylic acids and a dicarboxylic acid and a carboxylic acid, provided that the dicarboxylic acid is a polymethylene compound having 6-12 carbon atoms. 3. The process of Claim 1 wherein the dicarboxylic acid component is adipic acid. 4. The process of Claim 1 wherein the diacid-rich component consists of a dicarboxylic acid and hexa- methylenediamine in mole proportion greater than 1. 5. The process of Claim 1 wherein the diacid-rich component consists of adipic acid and hexamethylene¬ diamine in molar proportions greater than 1. 6. The process of Claim 1 wherein the diacid-rich component consists of adipic acid and hexamethylene- diamine in molar proportion of about 77:23 which corresponds to a proportion by weight of about 81:19. 7. The process of claim 1 wherein the diamine- rich component is selected from the group consisting of one or more diamines and at least one diamine and at least one dicarboxylic acid wherein the total diamine to total dicarboxylic acid mole ratio of greater than one, 8. The process of Claim 1 wherein the diamine- rich component consists of hexamethylenediamine and a dicarboxylic acid in proportion greater than 1. 9. The process of Claim 1 wherein the diacid-rich component and the diamine-rich component are contacted in non-stoichiometric combined proportion at temperatures lower than those required to prevent solidification, at complete dehydration 10. The process of Claim 1 wherein the diacid-rich component and the diamine-rich component are contacted in non-stoichiometric combined proportion at temperatures lower than the melt temperatures for the combined proportion where substantial dehydration is avoided. 11. The process of Claim 1 wherein the diacid-rich component or the diamine-rich component at an intermediate degree of dehydration are at temperatures lower than the melt temperature for the component where substantial dehydration is avoided. 12. The process of Claim 1 wherein the diacid-rich component and the diamine-rich component are contacted in substantially stoichiometric combined proportion at temperatures lower than the temperature for the combined stoichiometric proportion where substantial dehydration is avoided. 13. The process of Claim 1 wherein adipic acid and hexamethylenediamine are contacted in substantially stoichiometric combined proportion for degrees of dehydration between about 0.15 and 0.60 and at temperatures lower than the melt temperature of about 195-200°C for the nylon 66 salt where substantial dehydration is avoided. 14. The process of Claim 1 wherein adipic acid and hexamethylenediamine are contacted in substantially stoichiometric combined proportion for degree of dehydration between about 02.5 and 0.50 and at temperatures lowered than about 180°C where an atmospheric pressure substantial vaporization of diamine is avoided.

Inventors: LANG JAMES JOSEPH, LANG, JAMES, JOSEPH
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1995
Main classification: C08G69
IPC codes: C08G69