WO2015057557A1

WO2015057557A1 - Process for making semi-aromatic terpolyamide

Info

Publication number: WO2015057557A1
Application number: PCT/US2014/060247
Authority: WO
Inventors: Annakutty Mathew
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2013-10-16
Filing date: 2014-10-13
Publication date: 2015-04-23

Abstract

A process is provided for making 6T/6I/6 semi-aromatic terpolyamide, the semi-aromatic terpolyamide having a melting point of at least 290 C and not more than 320 C, including: heating a polymerization salt mixture comprising water, a1) 55 to 62 mole % hexamethylene diammonium terephthalate, a2) 13 to 29 mole % hexamethylene diammonium isophthalate, and a3) 9 to 26 mole % ε-caprolactam, in at least two constant pressure stages; and optionally extruding said molten semi-aromatic terpolyamide through a die.

Description

Process for Making Semi-Aromatic Terpolyamide

FIELD OF THE INVENTION

The present invention relates to a process for making a semi-aromatic terpolyamide.

BACKGROUND OF INVENTION

Polyamides, having melting points of 290 °C or higher (high melting polyamides) are very desirable for injection molded articles that may experience high temperatures under normal operation. Such applications include many under hood- applications in the automotive area. One common approach to high temperature polyamides is using a copolymer comprising repeat units of poly(hexamethylene terephthaiamide/ hexamethyiene isophthalamide) (PA6T/6I), and adjusting the 81 repeat unit concentration to obtain the desired melting point and crystallinity.

However, 6T/6I copolyamides having high melting points are difficult to make in batch reactors (autoclaves) without significant branching. The extent of branching is evident in the melt viscosities and the bishexamethylene triamine (BH T) content of copolyamides. BHMT in the copolyamide acts as a site of branching. High melt viscosity and high BHMT indicate high degrees of branching. The difficulty in downstream processing and risks associated with having to remove cross-linked materials from an autoclave make extensive branching in high temperature polyamides very undesirable.

There is therefore a need for processes providing high melting polyamides in batch reactors (for instance, autoclaves) that have low melt viscosities and low levels of BHMT branch sites.

US 3,787,373 discloses a process for providing PA 6T/6I/6 in various weight per cents of repeat units, but with high levels of ε-caprolactam providing terpolymers with melting points less than 280 °C.

EP 0592996 discloses processes, including an autoclave process, for providing PA 6T/6I/6 in various weight per cents of repeat units. However the autoclave process did not include two constant pressure stages and did not provide a polymer demonstrated to be extrudabie through a die.

US 8,324,307 discloses high melting polyamides including PA 6T/6I/6 in various weight per cents of repeat units and recommends a one-step batch process, described in US 3,843,61 1 and US 3,839,296, for their manufacture. i US 1 ,528,329 discloses PA 6T/6I/66 terpolyamides prepared in a one-step constant pressure process.

JP 2012-122066 discloses terpolyamides including PA 8T/8I/8.

SUMMARY OF THE INVENTION

Disclosed is a process for making 6T/6I/6 semi-aromatic terpolyamide, said semi-aromatic terpolyamide having a melting point of at least 290 °C and not more than 320°C, comprising: a) charging a reactor with a polymerization salt mixture comprising water, a1 ) 55 to 62 mole % hexamethylene diammonium terephthalate, a2) 13 to 29 mole % hexamethylene diammonium isophthalate, and a3) 9 to 28 mole % ε-caprolactam, wherein the sum of a1 ), a2) and a3) equals 100 mole % of polymer components;

b) heating said polymerization mixture, in at least two constant pressure stages comprising: b1 ) first heating with venting of volatiles at a first constant pressure of 180 psia to 285 psia to a temperature of 225 °C to 240 °C; followed by b2) second heating and venting of volatiles at a second constant pressure of 320 psia to 450 psia to a temperature of 285 °C to 310 °C; followed by

c) heating with venting to a final temperature of 315 °C to 335 °C, while reducing the pressure to atmospheric pressure; and, followed by d) heating to maintain the temperature of 315 °C to 335 °C, while reducing and holding the pressure at 5 to 12 psia, to provide a molten semi-aromatic terpolyamide.

Also disclosed is a process by which the aforementioned molten semi-aromatic terpolyamide is extruded, optionally through a die to provide polymer strands. Also disclosed is a process by which the aforementioned polymer strands can be cooled and cut into pellets, cubes, and the like.

Also disclosed is a terpolyamide made by any of the processes of the invention.

BRIEF DESCRIPTION OF FIGURES

Figure 1 illustrates a batch process profile for the process of the invention including two constant pressure stages. DETAILS OF THE INVENTION

All references, patents and publications, cited in this description to more fully describe the state of the art to which this invention pertains, are incorporated by reference.

The following definitions apply to the terms as used throughout this specification, unless otherwise limited in specific instances.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the specification, including definitions, will control.

As used herein, the terms "comprises," "comprising," "includes," "including," "containing," "characterized by," "has," "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

The transitional phrase "consisting of excludes any element, step, or ingredient not specified in the claim, closing the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase "consists of appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

The transitional phrase "consisting essentially o limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. A 'consisting essentially of claim occupies a middle ground between closed claims that are written in a 'consisting of format and fully open claims that are drafted in a 'comprising' format. Optional additives as defined herein, at levels that are appropriate for such additives, and minor impurities are not excluded from a composition by the term "consisting essentially of.

In one embodiment the semi-aromatic terpolyamides consists essentially of the ranges of repeat units, including the stated preferences, as disclosed above. The term "consist essentially of means the embodiment necessarily includes the listed repeat units and is open to unlisted repeat units that do not materially affect the basic and novel properties of the invention. Herein, for instance, the term as applied to the semi-aromatic terpolyamides, means the semi-aromatic terpolyamides includes the repeat units 6T/6I/6, and may include additional repeat units in small amounts, so long as the additional repeat units do not materially affect the basic and novel properties of the invention. The basic properties of the semi-aromatic terpolyamides include a melting point of at least 290 °C, preferably in the range of 290 °C to 320 °C and more preferably 290 °C and 310 °C; and preferably a melt viscosity in the range of 80 Pa-s to 160 Pa-s; and, independently a

bishexamethylene triamine content of 60 mequiv/Kg, and preferably 52 mequsv/Kg or less.

"Additional repeat units in small amounts" means less than 5 mole percent repeat units, and preferably, less than 3 mole percent, less than 2 mole percent, and 1 mole percent or less, other than 6T/6I/6.

When a composition, a process, a structure, or a portion of a composition, a process, or a structure, is described herein using an open-ended term such as "comprising," unless otherwise stated the description also includes an embodiment that "consists essentially of or "consists of the elements of the composition, the process, the structure, or the portion of the composition, the process, or the structure.

The term "substantially free", as used herein with respect to a composition and a component, refers to a composition that includes no more than an adventitious amount of the component. Stated alternatively, the composition includes no added amount of the component, only the amount that is commonly present in the raw materials from which the composition is produced. In some commercially available materials, the level of adventitious components is less than less than 2.5%, 1 .0%, less than 0.5%, or less than 0.1 % by weight, based on the weight of the

commercially available material.

The term "neat", as used herein, refers to a composition or a component that is substantially free of all other materials.

The articles "a" and "an" may be employed in connection with various elements and components of compositions, processes or structures described herein. This is merely for convenience and to give a general sense of the compositions, processes or structures. Such a description includes "one or at least one" of the elements or components. Moreover, as used herein, the singular articles also include a description of a plurality of elements or components, unless it is apparent from a specific context that the plural is excluded.

The term "or", as used herein, is inclusive; that is, the phrase "A or B" means "A, B, or both A and B". More specifically, a condition "A or B" is satisfied by any one of the following: A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present). Exclusive "or" is designated herein by terms such as "either A or B" and "one of A or B", for example.

The term "about" means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. In general, an amount, size, formulation, parameter or other quantity or characteristic is "about" or "approximate" whether or not expressly stated to be such.

In addition, the ranges set forth herein include their endpoints unless expressly stated otherwise. Further, when an amount, concentration, or other value or parameter is given as a range, one or more preferred ranges or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether such pairs are separately disclosed. The scope of the invention is not limited to the specific values recited when defining a range.

When materials, methods, or machinery are described herein with the term "known to those of skill in the art", "conventional" or a synonymous word or phrase, the term signifies that materials, methods, and machinery that are conventional at the time of filing the present application are encompassed by this description. Also encompassed are materials, methods, and machinery that are not presently conventional, but that will have become recognized in the art as suitable for a similar purpose.

Unless stated otherwise, all percentages, parts, ratios, and like amounts, are defined by weight. Herein melting points are as determined with differential scanning calorimetry (DSC) at a scan rate of 10 °C/min in the first heating scan, wherein the melting point is taken at the maximum of the endothermic peak.

The process for making 6T/6I/6 semi-aromatic terpo!yamide disclosed herein provides polyamide resins having a high melting point, in the range of 290 °C to 320 °C, preferably 290 °C to 310 °C. as determined with differential scanning calorimetry (DSC) at a scan rate of 10 °C/min in the first heating scan. Preferably the

terpolyamide resins have a melt viscosity in the range of 80 Pa-s to 160 Pa-s, as determined at 1000 s^"1 shear rate and 325 °C with terpolyamides having a moisture content of 125 to 550 parts per million (ppm). Preferably and independently, the terpolyamides have a bsshexamethylene triamine content of 60 mequiv/Kg or less, and preferably 52 mequiv/Kg or less, as determined with hydrolysis of the

terpolyamides and gas chromatography determination of amine content.

The semi-aromatic terpolyamides are identified by their respective repeat units. The following list exemplifies the abbreviations used to identify monomers and repeat units in the terpolymer polyamides (PA) disclosed herein:

HMD 1 ,6-hexamethylene diamine (or 6 when used in combination with a diacid)

AA Adipic acid

T Terephtha!ic acid

I Isophthaiic acid

66 polymer repeat unit formed from HMD and AA

6T polymer repeat unit formed from HMD and T

6I polymer repeat unit formed from HMD and I

6 ε-caprolactam

Note that in the art the term "6" when used alone designates a polymer repeat unit formed from ε-caprolactam. Alternatively "6" when used in combination with a diacid such as adipic acid, for instance 66, the "6" refers to HMD. In repeat units comprising a diamine and diacid, the diamine is designated first. Furthermore, when "6" is used in combination with a diamine, for instance 66, the first "6" refers to the diamine HMD, and the second "6" refers to adipic acid. Likewise, repeat units derived from other amino acids or lactams are designated as single numbers designating the number of carbon atoms. The semi-aromatic terpolyamide repeat units are separated by a slash (that is, /). For instance poly(hexametbyiene terephthalamide/ hexamethylene

isophthalamide/caprolactam) is abbreviated PA 6T/6I/6 (62/23/15), wherein the values in parentheses are the mole % repeat unit of each repeat unit in the

copolymer.

The semi-aromatic terpolyamides provided by the process disclosed herein comprise a1 ) 55 to 62 mole % hexamethylene terephthalamide (6T), a2) 13 to 29 mole % hexamethylene isophthalamide (6i), and a3) 9 to 26 mole % caprolactam (6), wherein the sum of a1 ), a2) and a3) equals 100 mole % of polymer components. Preferably the semi-aromatic terpolyamides provided by the process disclosed herein comprise 58 to 62 mole % 6T, 20-24 mole % 6!, and 16 to 20 mole % caprolactam.

One embodiment is a process for making 6T/6I/6 semi-aromatic terpolyamide, said semi-aromatic terpolyamide having a melting point of at least 290 °C and not more than 320 °C, comprising:

a) charging a reactor with a polymerization salt mixture comprising water, a1 ) 55 to 62 mole % hexamethylene diammonium terephthalate, a2) 13 to 29 mole % hexamethylene diammonium isophthalate, and a3) 9 to 26 mole % ε-caprolactam, wherein the sum of a1 ), a2) and a3) equals 100 mole % of polymer components;

b) heating said polymerization salt mixture, in at least two constant pressure stages comprising: b1 ) first heating with venting of volatiles at a first constant pressure selected in the range of 180 to 265 psia to a

temperature of 225 °C to 240 °C; followed by

b2) second heating and venting of volatiles at a second constant pressure selected in the range of 320 to 450 psia to a temperature of 285 °C to 310 °C;

followed by

c) heating with venting to a final temperature of 315 °C to 335 °C. preferably 320 to 330 °C, while reducing the pressure to atmospheric pressure; and, followed by

d) heating to maintain the temperature of 315 °C to 335 °C, preferably 320 °C to 330 °C, while reducing and holding the pressure to 5 to 12 psia, preferably 5 to 9 psia and 8 to 9 psia, to provide a molten semi-aromatic terpoiyamide.

The molten semi-aromatic terpoiyamide made by this process can

subsequently be extruded, optionally through a die. When extruded through a die, polymer strands can be formed. The strands made by this process can be optionally cooled with water, and subsequently cut to provide pellets, cubes and other like materials. The reactor is preferably an autoclave capable of a safe operating pressure range of 1 to 500 psia, and an operating temperature range of 23 °C to 350 °C.

Other than water and a1 } - a3), the polymerization salt mixture may comprise additional reagents including excess hexamethylenediamine, acetic acid or other organic monocarboxylic acids, amine capping agents, thermal stabilizers, and other functional agents known in the art to be useful in polyamide polymerization. For instance, excess hexamethylenediamine, acetic acid or another organic acid, and/or amine capping agents can be added to manipulate the type of end group and the amount of end group in the polymer which ultimately affects the melt viscosity.

"Venting of volatiles" means the release of volatiles from the pressurized reactor. Volatiles include water vapor, amines such as HMD, organic acids, monoamines and other capping reagents which may be used to control end group concentration.

Figure 1 shows the batch process profile in graphic terms indicating the temperature and pressure of process. The polymerization salt mixture is heated in at least two constant pressure stages: b1 ) (corresponds to Stage 2-1 in Fig 1 ) a first heating with venting of volatiles at a first constant pressure of 180 to 265 psia to a temperature of 225 °C to 240 °C; and b2) (corresponds to Stage 2-3 in Fig 1 ) a second heating and venting of volatiles at a second constant pressure of 320 to 450 psia, preferably 330 to 450 psia, and more preferably at 330 to 400 psia, to a temperature of 285 °C to 310 °C. As the salt mixture is heated, water boils, and the pressure is raised to a first constant pressure selected between 180 and 265 psia. The salt mixture is held at the first constant pressure at +/- 2 psia as volatiles (steam, and small amounts of HMD and/organic acids, if present) are vented until the temperature reaches 225 °C to 240 °C. Then the venting is terminated or reduced to allow the pressure to rise to the second constant pressure. The polymerization salt mixture is held at the second constant pressure as volatiles (steam, and small amounts of HMD and/organic acids, if present) are vented until the temperature reaches 285 °C to 310°C. The selected two constant pressure stages allow the polymerization to progress without encountering a phase boundary.

In another embodiment step b) heating may comprise three or more constant pressure stages.

Heating steps c) and d) (corresponding to Stage 3 and Stage 4 in Fig 1 ) drive the polymerization to provide a molten semi-aromatic terpolyamide with the desired molecular weight, as evidenced by the melt viscosity range. Preferably step c) is performed over a period of 30 to 80 minutes; and step d) is performed over a period of 15 to 45 minutes, and more preferably 20 to 30 minutes.

The steps of extruding said molten semi-aromatic terpolyamide, optionally through a die to produce polymer strands, and the optional steps of cooling with water and cutting the cooled strands into pellets, cubes and the like, are

conventional steps used in manufacturing of pelletized resins. However, if the branching in the terpolyamide is too high, the terpolyamide will not flow sufficiently to allow its efficient removal from the reactor through a die to provide solid material such as pellets, cubes, and the like.

The 6T/6I/6 semi-aromatic terpolyamide provided by the process disclosed herein may be melt-blended with reinforcing agents, polymeric tougheners, flame retardants and/or additional functional additives to provide thermoplastic

compositions useful in injection molding to provide thermoplastic molded articles.

Methods

Melting point

Herein melting points were as determined with DSC (TA Instruments Q2000, TA Instruments, New Castle, Delaware, USA) at a scan rate of 10 °C/min in the first heating scan, wherein the melting point is taken at the maximum of the endothermic peak per ASTM D3418.

Freezing point

Herein freezing points were as determined with DSC (TA Instruments Q2000, TA Instruments, New Castle, DE, USA) at a scan rate of 10 °C/min in the cooling cycle as per ASTM D3418.

Inherent Viscosity Inherent viscosity (IV) was measured on a 0.5% solution of terpolyamide in m~ cresol at 25 °C, using a Schott AVS310 viscosity measuring unit, a Schott CK300 cooling unit, and a Schott CT52 constant temperature bath according to the method described in ISO 307.

Melt Viscosity

Samples were dried in a vacuum oven at 95 °C for approximately 2 days to achieve moisture content 125 to 550 ppm. The moisture contents were measured by a coulometric Karl Fisher automatic titrator. Metrohm 831 , available from Metrohm USA, Riverview, Florida, USA.

Melt viscosity of the polymer samples were measured using a Dynisco capillary rheometer, Model LCR7001 , available from Dynisco, Franklin,

Massachusetts, USA. Polymer samples were loaded into a heated barrel kept at 325 °C and melted for 300 seconds. A motor driven piston, in series with a force transducer, pushed the molten polymer through a die of specified internal diameter of 0.04 inch and length of 0.8 inch. The force required to extrude the sample at a shear rate of 1000 per second was used to calculate its melt viscosity.

Amine End Determination

Amine ends were determined by titrating a 1 percent solution of polyamide in a phenol/methanol mixture (90:10 by volume) with 0.025 N perchloric acid solution. The end point was determined potentiometrically, using a Metrohm "Tiamo" operating system with a Titrando 809 and Dosino 800 burette, along with a Metrohm pH electrode, all available from Metrohm USA, Riverview, Florida, USA.

Acid End Determination

Polymer samples were dissolved in a blend (55:45 by volume) of two solvents: solvent 1 = 95:5 o-cresol / o-dichlorohenzene and solvent 2 = 20% lithium chloride in methanol; followed by addition of 1 wt % perchloric acid in methanol, in slight excess of the amount required to react with the amine ends until the solution is acidic. The polymer solutions were titrated with 0.04 N tetrabutylammonium hydroxide in benzyl alcohol, through the potentiometric endpoint for the excess perchloric acid, to the end point for the carboxyl ends. A Metrohm "Tiamo" operating system with a Titrando 809 and Dosino 800 burette, along with a Metrohm pH electrode, all available from Metrohm USA, Riverview, Florida, USA, was used. The difference in the titres for the two endpoints was used to calculate the carboxyl ends concentration. BHMT determination

BHMT content was determined by hydrolysis of the polyamide samples, liberation of free amines (diamines, triamines) by basification, followed by GC analysis of the resulting amine mixture using the following procedure.

The hydrolysis of the polyamide sample was carried out as follows: a polymer sample was hydrolyzed in concentrated hydrochloric acid solution for 2 hours in a sealed tube at 175 °C. The resulting solution of hydrolysis products was diluted with water and then saturated with potassium hydroxide. The basic and neutral fractions of the hydrolysis products were then extracted from the diluted hydrolysate into an organic layer. It was then analyzed by using a Bruker 450 gas chromatograph with a DB-5 column (10 m long, 0.25 mm ID, 1 .0 μηι film), available from Brucker Corp., Billerica, Massachusetts, USA. The oven temperature was raised from 50 °C to 235 °C at 15 degrees per minute and a hold time of 8 minutes (total run time of 18.3 minutes). The concentrations of degradation products in the orginal polymer sample was calculated using the appropriate dilution factors, accounting the proportion of the sample that was hydrolyzed.

Materials

All chemicals were purchased from commercial sources.

Example 1

The following process was used to provide PA6T/6I/6 (55/27/18):

Salt Preparation: A 10L autoclave was charged with terephthalic acid (1459 g), isophthalic acid (729 g), ε-caprolactam (331 g), an aqueous solution containing 79.2 weight % of HMD (1982 g), an aqueous solution containing 28 weight percent acetic acid (97 g), an aqueous solution containing 1 weight percent sodium hypophosphite (SHP, 88 g), an aqueous solution containing 1 weight percent Carbowax 8000 (10 g), and water (2250 g).

Process Conditions: The polymerization was carried out in five consecutive stages illustrated in Figure 1 . The autoclave agitator was set to 5 rpm and the contents were purged with nitrogen at 25 psia for 10 minutes to remove head space oxygen.

Stage 1 : The agitator was then set to 50 rpm, the pressure control valve was set to 55 psia and the autoclave was heated. When pressure reached 55 psia, a small amount of steam was allowed to vent (-50 grams of condensate). The pressure control valve was then set to 200 psia and heat Input to the autoclave was raised and the pressure was allowed to rise.

Stage 2-1 (corresponds to step b1 ): At a constant reactor pressure of 200 psia steam began to vent at a rate determined by the heat input. This stage was terminated after 30 minutes.

Step 2-2: The pressure control valve was then set at 345 psia and pressure was allowed to rise to 345 psia in 25 minutes.

Stage 2-3 (corresponds to step b2): The pressure was then maintained 345 psia during which steam was vented and the temperature of the contents were allowed to rise further to 285 °C.

Stage 3: At 285 °C, the pressure was reduced to 14.5 psia over about 45 minutes while the heating continued to approximately 315 °C.

Stage 4: The autoclave pressure was reduced to 5 psia by applying vacuum and held there for 20 minutes.

Stage 5: The autoclave was then pressurized with 65 psia nitrogen and the molten polymer was extruded into strands, quenched with cold water and cut into pellets.

Examples 2-7

Examples 2A, 3 to 7 were performed by charging the reactor with the materials listed in Table 1 . The process conditions were the same as Example 1 except that the temperature at initiation of pressure reduction was raised by 5 to 20 °C and the final finish temperature of the polymer melt was raised by 5 to 7 °C higher than that used in Example 1 . Example 2B was made in a manner similar to that used in Examples 3 to 7, except the first constant pressure hold was at 285 psia. Tables 2 and 3 list the properties of the terpolyamides prepared in Examples 1 -7.

Table 1

Aqueous SHP (1 wt %, 71 g) and aqueous Carbowax 8000 (1 wt %, 14 g) were each added to each polymerization salt mixture.

HMD = hexamethyiene diamine, T= terephthalic acid, l= isophthaiic acid, SHP = sodium hypophosphite.

Table 2

Table 3

Comparative Examples C1 to C3

The ingredients used for examples C1 to C3 are listed in Table 4. The process conditions for making comparative examples C1 and C2 were similar to those described in Example 1 . The process conditions for C3 were the same as Example 1 except that the temperature at Initiation of pressure reduction was raised by 15 °C and the final finish temperature of the polymer melt was raised by 5 °C higher than that used in Example 1 .

Comparative Example C1 had a melting point lower than the desired 290 °C - 320 °C and a BHMT level within the range of Examples. C2 and C3 show that as the level of 6T is increased, the melting point increased as expected, but also the level of BHMT increased to undesirable levels. The process of the invention, using the recited mole % salt mixtures (corresponding to mole % repeat units in

terpolyamide), provided the desired high melting points, and low BHMT levels consistent with low branching of the terpolyamide.

Table 4

Aqueous SHP (1 wt %, 68 g) and aqueous Carbowax 8000 (1 wt %, 10 g) added to each

polymerization salt mixture.

HMD = hexamethyiene diamine, T= terephtha!ic acid, l= isophthaiic acid, SHP = sodium

hypophosphite.

Comparative Example 4

Comparative Example 4 is identical to Example 1 in composition but the process conditions included a single constant pressure stage, rather than the at least two constant pressure stages of Example 1 .

PA6T/6I/6 (55/27/18) salt mixture was prepared in the lab as follows: A 500 ml 3-necked round bottomed flask was equipped with a high efficient condenser, nitrogen purge, thermocouple, heating mantle and magnetic stirring. The flask was first charged with 88 grams of water and 87.24 grams of an aqueous solution of hexamethyiene diamine of 79.2% concentration. The flask was magnetically stirred and was kept under nitrogen purge. The flask was then slowly heated to 40 °C.

When the temperature was above 40 °C, the flask was charged with 50.4051 gram of terephthalic acid, 25.22 grams of isophthaiic acid and 1 1 .48 grams of ε- caproiactam. Then, an additional 88 grams of water were added to the flask. The temperature of the flask was then slowly raised to approximately 90 °C. The salt was completely dissolved at this point. Then, 42.2 grams of hot salt solution was transferred to a quartz tube of approximately 60 mL volume. A magnetic stirrer bar was also added to the quartz tube. The quartz tube was then sealed with a plastic film and allowed to cool overnight to room temperature during which time the salt froze.

The quartz tube was installed in the quartz tube reactor. The pressure controller of the quartz tube reactor was set at 285 psia. The head space of the quartz tube was purged with nitrogen. The magnetic stirring was turned on and the heating started. The temperature of the salt solution gradually rose and the salt completely melted at 80 ⁰C. The heating was continued and the pressure of the reactor was allowed to rise to 265 psia. When the pressure was at 265 psia steam began to vent at a rate determined by the heat input. The pressure was kept constant at 265 psia and the temperature of the contents was allowed to rise further. When the temperature was about 237 °C, the solution started to become hazy. The venting continued and temperature increased. At about 246 °C, the stirred bar started to move slowly, at 250 °C, the stirrer bar stopped and at 251 °C, the solution was completely frozen. The temperature/pressure conditions at which the phase separation occurred was noted as the phase boundary point. This demonstrated that a single constant pressure stage at 265 psia does not allow the polymerization to proceed sufficiently without crossing a phase boundary wherein further heating would lead to significant decomposition.

Claims

0 L j jVi 3 What is claimed is:

1 . A process for making 6T/6I/6 semi-aromatic terpolyamide, said semi-aromatic terpolyamide having a melting point of at least 290 °C and not more than 320 °C, comprising:

a) charging a reactor with a polymerization salt mixture comprising water, a1 ) 55 to 82 mole % hexamethylene diamrnoniurn terephthaiate, a2) 13 to 29 mole % hexamethylene diamrnoniurn isophthaiate, and a3) 9 to 26 mole % ε- caprolactam, wherein the sum of a1 ), a2) and a3) equals 100 mole % of polymer components;

b) heating said polymerization salt mixture, in at least two constant pressure stages comprising: b1 } first heating with venting of volatiles at a first constant pressure of 180 psia to 265 psia to a temperature of 225 °C to 240 °C;

followed by b2) second heating and venting of volatiles at a second constant pressure of 320 psia to 450 psia to a temperature of 285 °C to 310 °C;

followed by

c) heating with venting to a final temperature of 315 °C to 335 °C, while reducing the pressure to atmospheric pressure; and followed by

d) heating to maintain the temperature of 315 °C to 335 °C, while reducing and holding the pressure at 5 to 12 psia; to provide a molten semi-aromatic terpolyamide.

2. The process of Claim 1 , further comprising extruding said molten semi- aromatic terpolyamide.

3. The process of Claim 2, wherein said molten semi-aromatic terpolyamide is extruded through a die to provide polymer strands.

4. The process of Claim 3, further comprising cooling the polymer strands with water.

5. The process of Claim 4, further comprising cutting the polymer strands to provide pellets or cubes.

6. The process of Claim 1 , wherein the molten semi-aromatic polyamide has a bishexamethylene triamine content of 60 mequiv/Kg or less.

7. The process of Claim 1 wherein the molten semi-aromatic polyamide has a bishexamethy!ene triamine content of 52 mequiv/Kg or less.

8. The process of Claim 1 wherein the molten semi-aromatic polyamide has a melt viscosity in the range of 80 Pa-s to 160 Pa-s, as determined at 1000 s^"1 shear rate and 325 °C with terpolyamsdes having a moisture content of 125 to 550 parts per million (ppm).

9. A semi-aromatic terpolyamide made according to the process of Claim 1 .

10. A semi-aromatic terpolyamide made according to the process of Claim 2.

1 1 . A semi-aromatic terpolyamide made according to the process of Claim 3.

12. A semi-aromatic terpolyamide made according to the process of Claim 4.

13. A semi-aromatic terpolyamide made according to the process of Claim 5.