WO2008005415A1

WO2008005415A1 - Manufacture of polyamides

Info

Publication number: WO2008005415A1
Application number: PCT/US2007/015326
Authority: WO
Inventors: David N. Marks; Marion Glenn Waggoner
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2006-07-05
Filing date: 2007-06-29
Publication date: 2008-01-10
Also published as: EP2035481A1; JP2010501646A

Abstract

Polyamides are manufactured from their monomers by adding the monomers, such as a diamine and a dicarboxylic acid, to an extruder and passing these materials through the extruder while heating and removing volatile byproducts. The process is simple and fast, and allows the use of relatively uncomplicated process equipment.

Description

MANUFACTURE OF POLYAMIDES

FIELD OF THE INVENTION Polyamides can be synthesized to high molecular weight by introducing the polyamide monomers into an extruder and progressively heating the extruder contents to high temperatures before the contents exit as a polyamide polymer.

TECHNICAL BACKGROUND Polyamides are important items of commerce, many thousands of tons being used each year for a myriad of uses including fibers and molding resins. Many of these polyamides are made by a "classical" condensation polymerization involving the reaction of a diamine with a dicarboxylic acid or a diester, usually a dicarboxylic acid. In this reaction, when a dicarboxylic acid is used, the byproduct is typically water. As more and more water is removed the mo- lecular weight of the polyamide produced is increased. These reactions have been carried out in a wide variety of equipment, such as batch and continuous autoclaves/melt polymerizers. Final adjustment of the molecular weight of the polyamide produced has optionally been carried out by solid state polymerization and/or in an extruder. The properties of the polyamide produced depend in large measure on the monomers used. Thus the polyamides can range from amorphous polymers (melting point below room temperature) to polyamides with melting points in excess of 300⁰C. Typically the higher melting polyamides have monomer units (in the diamine and/or dicarboxylic acid) which include aro- matic rings. Commercial polyamides produced by this reaction include nylon- 6,6 and so-called (partially) aromatic polyamides made from diamines such as 1 ,6-hexanediamine (HMD), 1 ,4-diaminobenzene or 1 ,3-diaminobenzene and dicarboxylic acids such as adipic acid, terephthalic acid or isophthalic acid, or mixtures thereof. A partially aromatic polyamide has aromatic rings, such as phenyl rings, included in the main chain of the polyamide. Such aromatic rings may be part of monomeric diamines such as 1 ,4-diaminobenzene or 1 ,3- diaminobenzene, and/or monomeric dicarboxylic acids or their diesters such as terephthalic acid or isophthalic acid. However in a partially aromatic poly- amide there must be some amide groups having aromatic rings between them, preferably at least about 10 mole percent, more preferably at least about 20 mole percent of amide groups have aromatic rings between them. It is known that the molecular weights of (already polymeric) polyam- ides can be increased by passing the polyamide through an extruder under certain conditions, see for instance German Patent 4,329,676, Japanese Patent Application 05230204, and U.S. Patent 5,079,307. None of these references describes a polymerization in an extruder starting from monomers.

SUMMARY OF THE INVENTION This invention concerns, a process for the manufacture of polyamides, comprising, agitating in an extruder, at elevated temperature, one of the following monomer compositions:

(a) a diamine and a dicarboxylic acid or a diester;

(b) a diamine, a dicarboxylic acid or a diester, and an aminocar- boxylic acid or ester of an aminocarboxylic acid; or

(c) an aminocarboxylic acid or ester of an aminocarboxylic acid; provided that during said contacting water and/or alcohol byproduct is removed from said extruder.

DETAILS OF THE INVENTION Herein certain terms are used and are defined below:

By a "monomer composition" herein is meant composition which is or is derived from one or more monomers such as a diamine, dicarboxylic acid or an aminocarboxylic acid, a so-called amine "salt" which is a salt formed by a diamine and a dicarboxylic acid, as well as very lower molecular weight oli- gomers, with an average degree of polymerization of about 3 or less. For the purposes of determining the degree of polymerization one "monomer" unit includes a diamine and a dicarboxylic acid, or a single aminocarboxylic acid derived repeat unit. Preferably the monomer composition is the monomer(s) themselves, a diamine, dicarboxylic acid or an amine carboxylic acid, and/or a so-called amine "salt" which is a salt formed by a diamine and a dicarboxylic acid. More than one diamine and/or dicarboxylic acid and/or aminocarboxylic acid (or the carboxylic acid esters) may be used in the polymerization to form a copolyamide. By a polyamide is meant a polymer in which at least 80 mole percent, more preferably at least about 90 mole percent, especially preferably at least about 95 mole percent, and very preferably essentially all of the repeat unit connecting groups are amide groups. Other connecting groups which may be present include ester, imide and ether groups.

In general herein substituents (especially functional groups) present in the monomers should not interfere with the reactions to form the polyamide, nor cause the resulting polyamide to degrade (for example thermally) during . the polymerization process. By an extruder is meant an apparatus that is similar in function to a typical single or twin screw extruder used, for instance, for melt processing thermoplastics or for processing foods. The extruders useful herein typically have some or all of the following characteristics:

- It acts as a modified plug flow reactor (sometimes also called a tubular reactor). Plug flow reactors are well known in the art; see for instance

J. I. Kroschwitz, et al., Ed., Kirk-Othmer Encyclopedia of Chemical Technology, 4^th Ed., Vol. 20, John Wiley & Sons, New York, 1996, p. 1007-1059, which is incorporated by reference herein. In a plug flow reactor the ingredients enter a tube or pipe at one end and flow to the other end with little or no backmixing, while for instance a reaction is taking place. The time needed to flow from one end to the other is the residence time. The present extruders are usually "modified" plug flow reactors because they often do have some localized backmixing within the length of the extruder.

- It has one or more vents along the length of the extruder to re- move volatile byproducts formed during the polymerization reaction. These vents may be at any pressure, but typically are at atmospheric pressure or below..

- Different sections along the length of the extruder may be heated to different temperatures. - It has a high surface to volume ratio, the surfaces in this case being the outer vessel (in an extruder usually called the barrel) and the surface area of the agitators (in a typical extruder the screws or kneaders).

- It is capable of rapidly generating new surface or changing the surface of the liquid iπgredient(s). By this is meant the ingredients in actual contact with the surfaces of the extruder (barrel and screw) are constantly changing.

- It has enough power and structural strength to push through the ingredients to the discharge end, in this case the polymer formed. - It melts at least some of starting ingredients and/or some of the starting ingredients are molten, and usually maintained in the melt. Also the oligomer and polymer formed are in the melt until exiting the extruder, unless it is desired to freeze (solidify) the oligomer or polymer before exiting the extruder (for example to form a particulate of the polymer or oligomer). Thus for the most part a melt polymerization is taking place.

- Usually the agitator elements, such as the screws or kneaders, rotate in a circular motion.

- Usually the cross section of the vessel (barrel) perpendicular to the long axis of the extruder is circular (as in a single screw extruder) or over- lapping circles (as in a twin screw extruder).

Examples of extruders (as used herein) include single screw extruders, such as those supplied by Davis-Standard, LLC, Pawcatuck, CT 06379 USA, under various tradenames including Sterling ®, and Coperion USA, Ramsey, NJ 07446, USA supplied under the tradename Werner-Pfleiderer®, and Davis-Standard LLC, and kneaders such as those supplied by Coperion USA under the tradename Buss®.

As mentioned herein the screw configuration of the extruder may be used to (in part) control the process, such as the molecular weight of the product produced and/or the time required for the process. Such variables are discussed in M. H. Mack, NPE'88, Vol. 2 Conference Papers, p. 131 et seq. (1988), and M. H. Mack et al., Annual Technical Conference Society of Plastics Engineers 1987, p. 136-139(1987).

The monomers may be added, preferably at or near the back of the extruder, in a variety of ways. The solid and/or liquid monomers may be me- tered into the extruder by volume (preferably for a liquid) or by weight (for a solid, as by using a weight loss feeder) individually or a premixed mixture (in the correct proportions) may be added. As is well known in the art, the molar amounts of diamine and dicarboxylic acid should be approximately equal to form a higher molecular weight polyamide. However if one of these is lost preferentially, as by volatilization from the extruder, then it may be desirable to add an excess of this ingredient initially. Simple experimentation will determine if and how much of such an excess is needed. The monomers may also be added in the form of a salt (see above). Using any of these forms, the monomers may be added neat or in solution, for example the salt may be added as an aqueous solution in many instances.

As the monomer(s) enter the extruder they will gradually be heated, often to higher and higher temperatures, as they pass through the extruder and are converted to the polyamide. The heat may serve two purposes. Higher temperatures increase the rate of reaction to form the polyamide. In the case of polyamides with relatively high melting points, or which have no melting point but a high glass transition temperature, the higher temperatures prevent the polyamide which is formed from solidifying inside the extruder. Thus it can be seen that higher temperatures in the extruder barrel are desired for good reason. However the temperature should not be so high that it causes degradation of the monomers or polyamide formed. The decomposition temperatures of such compositions are well known, so avoiding temperatures which are too high is relatively easy. Especially in the initial stages of the process, i.e., when the monomers have just been placed into the extruder, the molecular weight of the process materials is relatively low and so relatively speaking they may be somewhat volatile at elevated temperatures. The temperatures at this stage should not be so high that excessive amounts of monomers and/or relatively low molecular weight oligomers are volatilized. The temperatures needed and the overall desirable temperature profile of the extruder may be readily determined by experimentation. It will vary somewhat depending on the monomers used and the desired molecular weight of the final polyamide (see below).

As noted above the formation of the polyamide is a condensation po- lymerization in which a byproduct is water and/or alcohol. In order for the formation of the polyamide to proceed this byproduct must be removed during the reaction (passage through the extruder). Typically this is done with vent ports, one or more "openings" along the length of the extruder to allow vola- tiles to escape. These vent ports may be operated at elevated pressure, at- mospheric pressure, or subatrnospheric pressure. Typically they are operated at atmospheric and/or subatrnospheric pressure, and the pressure they are operated at determines to some extent how fast the polyamide is formed (the lower the pressure the faster the reaction), and what molecular weight is achieved in the final product (see below). Also typically, if the pressure at which the various vents are operated vary, the pressures of the various vents will decrease as one proceeds from the rear of the extruder (where the ingredients) are added] to the front of the extruder (where the product emerges). The pressure of any particular vent should not be so low that substantial amount of the liquid ingredients are blown up or foamed into the vents or vent lines. Again simple experimentation will determine the useful pressures.

These vents may be equipped with devices to prevent foaming into the vent and vent lines, and the vent lines may be equipped with condensers or other devices for collecting and/or disposing of the byproduct water and/or al- cohol.

The molecular weight of the final polyamide is dependent on the temperature profile of the extruder, the efficiency of removing the byproduct, and the residence time in the extruder. All other factors being equal the longer the residence usually the higher the molecular weight the polymer produced is. Residence time is affected by the length of the extruder (longer is longer residence time), the pitch of the conveying elements (shorter pitch is longer), the speed (rpm) of the conveyor (higher rpm is shorter), and the presence of items such as reverse elements or kneading blocks (usually longer).

On vapor ports lower pressure ("higher vacuum") will usually increase molecular weight. Higher effective surface (of the ingredients) to volume ratios, and/or the rate at which new surface is produced will normally increase molecular weight. A.gas sweep may also be used to increase molecular weight, that is gas is introduced into the extruder space and removed through a (usually nearby) vent. Typically this gas is inert, such as nitrogen. All of these factors affect the efficiency of removing the byproduct.

Higher temperatures, assuming they can be used, will also increase the molecular weight of the polyamide formed. The presence of a catalyst for the condensation reaction may also increase the molecular weight. However in one preferred form, such a (polymerization) catalyst is not deliberately added to the process.

Useful diamines include 1,6-hexanediamine, 1 ,4-diaminobutane, 1,12- diaminododecane, 1 ,4-diaminobenzene, 1 ,3-diaminobenzene, and 1,5- diamino-2-methylpentane. Useful dicarboxylic acids include adipic acid, terephthalic acid, and isophthalic acid. Useful aminocarboxylic acids include 3-aminobenzoic acid, and 4-aminobenzoic acid. One or more of the above diamines may be combined with one or more of the above dicarboxylic acids (or their esters), and this combination may optionally be combined with one or more aminocarboxylic acids. Also one or more aminocarboxylic acids may be used by themselves.

If desired, polyamides with very high molecular weights may be prepared without the limitations posed by some present manufacturing methods, where, because of the high melt viscosities of higher molecular weight poly- amides, the polymerization apparatus can't handle the high melt viscosity.

Typical residence times for this process are about 30 seconds to about 5 minutes, depending on the raw materials used, process conditions, apparatus and the desired molecular weight of the polyamide produced.

Other materials which are normally found in polyamide compositions may also be present during the process. These may be added with the ingredients or downstream in the extruder. However these other ingredients should not interfere substantially with the polymerization process, unless they are added near the exit of the extruder where the polymerization is essentially complete. These other materials may be added in conventional (for polyam- ide compositions) amounts. These materials include reinforcing agents, fillers, pigments, antioxidants and other stabilizers, lubricants, crystallization nu- cleators such as plasticizers, and flame retardants. Specific useful materials which usually do not interfere with the polymerization include carbon black, TiO2, glass fiber, glass flake, milled glass fiber, carbon fiber, polyethylene wax (in minimal amounts as a lubricant), clay, talc, and wollastonite. The addition and dispersion of these types of ingredients during the polymerization allows the preparation of these types of compositions without the need for a separate step to mix them in after the polyamide is formed. Melting point- Melting points were determined by ASTM Method D3418. Melting points are taken as the maximum of the melting endotherm. Melting points are measured on the second heat, using a heating rate of 10°C/min. Molecular weights - Measured by Gel Permeation Chromatography using hexafluoroisopropanol as the solvent.

In the Examples certain abbreviations are used. They are: DSC - Differential scanning calorimetry Mn - number average molecular weight Mw — weight average molecular weight

Examples 1-2

A solid salt was prepared as a starting material for the polyamidation. 1 ,5-Diamino-2-methylpentane (20.6 g) and 20.6 g 1 ,6-diaminohexane were dissolved in 110 g of deionized water. Terephthalic acid (58.8 g) was added slowly with stirring. Some additional water was added and the mixture was heated to dissolve all of the solids. The solution cooled to room temperature and was filtered to collect the solid salt. The solid was washed with ethanol and dried. A total of 37.7 grams was collected. Another batch of salt was made using the same method.

For Example 1 a mixture of 30 g of salt and 0.4 grams of 1 ,6- diaminohexane was made up. For Example 2 the solid salt was used alone. Since the salt of the 1 ,6-diaminehexane is less soluble than salt of the 1 ,5- diamino-2-methylpentane, it is likely that the isolated solid contains a higher 1 ,6-diaminohexane content than the expected 50 mole percent.

The polymerization was run in a Prism Model TSE-16-TC twin screw extruder (Prism Engineering, Staffordshire, W5B 6PW, England) which had a nominal L: D of 25:1. From the rear of the extruder, the feed section was about 4.6 cm long, the first mixing section was about 17.5 cm long, a knead- ing section was about 5.1 cm long, the next mixing section was about 5.6 cm long, the next kneading section was about 4.1 cm long, and the final screw section was about 7.4 cm long. The product was discharged into an aluminum pan. The approximate free internal volume of the extruder was about 10 mL. Downstream of the first mixing section equipped with kneading elements, the process was vented via two atmospheric pressure vent ports to a cold trap to capture any volatiles from the process. The extruder feed throat was blanketed with nitrogen gas and a local exhaust evacuated the nitrogen from the area. The extruder discharge was into an enclosure blanketed with nitrogen gas and equipped with local exhaust ventilation. The product exiting the extruder was dropped into water in the pan to freeze the polyamide. The extruder was run with barrel temperatures ranging from 320 to 340⁰C. The screw was run at 100 rpm to give a residence time of approximately one minute. Example 1 was started by slowly adding the well mixed mixture into the feed throat of the extruder. Steam was observed exiting the extruder at the die and at the feed throat. The viscous product exiting the extruder at the die had some gas bubbles. After the mixture of Example 1 had been run through the extruder and isolated, Example 2 was started. For Example 2, a new col- lection pan was used and the second mixture was added to the extruder. Steam was again observed exiting at the feed throat and at the die.

The product of Example 1 had a melting point of 303⁰C and the melting point of the product of Example 2 was 341⁰C. A typical melting point for this polyamide prepared by melt polymerization (not in an extruder) is about 303⁰C. The product of Example 1 had an Mn of 5160 and an Mw of 19,900. The product of Example 2 had an Mn of 5480 and an Mw of 16,860.

Example 3

In this Example the extruder used in Exampte 1-2 was fed. using an aqueous solution of the salt. Terephthalic acid (143 g) was weighed out into a 1 L bottle containing a magnetic stir bar. Deionized water (400 g) was added to the bottle. The slurry was stirred and 108 grams of a solution containing 47.5 wt. % 1 ,6-diaminohexane, 47.5 wt. % 1 ,5-diamino-2-methylpentane and 5 wt. % water was added. The mixture heated during the addition and all of the solid went into solution. An additional 50 g of deionized water was used to rinse in the diamine solution. The solution was warmed at 50⁰C to keep all solids dissolved and it was fed into the extruder at 0.68 kg/h using a peristaltic pump. The extruder was run with all barrel temperatures set at 340⁰C with the screw at 55 rpm. The product had a melting point of 291 "C as determined by DSC. The inherent viscosity measured in a 0.5% m-cresol solution at 25°C was 0.23.

Example 4

This example illustrates the preparation of a partially aromatic copoly- amide having the nominal molar composition 6T/DT [65/35]. (6T is the designation for a repeat unit derived from 1,6-diaminohexane and terephthalic acid, and DT is the designation for a repeat unit derived from 1 ,5-diamino-2- methylpentane and terephthalic acid.) A 35% solution of the salt was prepared in water by mixing 203 g of terephthalic acid, 650 g of deionized water, 42 g of anhydrous 1 ,6-diaminohexane and 108 grams of a solution containing 47.5 wt% 1 ,6-diaminohexane, 47.5 wt% 1 ,5-diamino-2-methylpentane, and 5 wt% water. The solution was heated by the exothermic salt formation reaction and all of the solids went into solution. The solution was warmed at 54°C to keep all solids dissolved and it was fed into the extruder (as described in Ex- ample 3) using a peristaltic pump. The extruder was run at temperatures of 320-340⁰C with a feed rate of approximately 0.7 to 1.4 kg/h and the screw at 55 rpm. The product had a broad melting point of 320-330⁰C as determined by DSC.

Claims

1. A process for the manufacture of polyamides, comprising, agitating in an extruder, at elevated temperature, one of the following monomer com- positions:

(a) a diamine and a dicarboxylic acid or a diester;

(b) a diamine, a dicarboxylic acid or a diester, and an aminocar- boxylic acid or aminocarboxylic acid ester

(c) an aminocarboxylic acid or aminocarboxylic acid ester; and provided that during said contacting water and/or alcohol byproduct is removed from said extruder.

2. The process as recited in claim 1 wherein in said polyamide at least about 95 percent of connecting groups are amide groups.

3. The process as recited in claim 1 or 2 wherein said extruder is a sin- gle screw extruder, a twin screw extruder, or a kneader.

4. The process as recited in any one of the preceding claims wherein said water and/or alcohol byproduct is removed through vent ports.

5. The process as recited in any one of the preceding claims wherein said diamine comprises one or more of 1,6-hexanediamine, 1,4- diaminobutane, 1,12-diaminododecane, 1 ,4-diaminobenzene, and 1,5- diamino-2-methylpentane, and/or said dicarboxylic acid or a diester comprises one or more of adipic acid, terephthalic acid or isophthalic acid or a diester thereof, and/or said aminocarboxylic acid or aminocarboxylic acid ester comprises one or more of 3-aminobenzoic acid and 4-aminobenzoic acid or an ester thereof.

6. The process as recited in any one of the preceding claims wherein a residence time is about 30 second to about 5 minutes.

7. The process as recited in any one of the preceding claims wherein one or more of a reinforcing agent, filler, pigment, antioxidant and/or other stabilizer, lubricant, crystallization nucleator, or flame retardant is also present.

8. The process as recited in any one of the preceding claims wherein a partially aromatic polyamide is produced.

9. The process as recited in any one of the preceding claims wherein a gas is swept through at least part of said extruder and/or a vacuum is applied to at least part of the contents of said extruder.