WO1999061509A1

WO1999061509A1 - Synthesis of semi-crystalline polyphthalamides through reactive extrusion of hexamethylene terephthalamide oligomer with lower melting, semi-crystalline or amorphous polyamides

Info

Publication number: WO1999061509A1
Application number: PCT/CA1999/000415
Authority: WO
Inventors: Christian Leboeuf; David Alan Harbourne
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1998-05-21
Filing date: 1999-05-21
Publication date: 1999-12-02
Anticipated expiration: 2000-11-21
Also published as: WO1999061509B1

Abstract

This invention relates to the synthesis of semi-crystalline polyphthalamides (i.e. containing monomeric units of terephthalic acid and hexamethylene diamine) through reactive extrusion of hexamethylene terephthalamide oligomer, or the oligomer of another 6T-rich copolymer, with lower melting semi-crystalline or amorphous polyamides. This process is particularly useful since it is centered around the use of stirred autoclaves for the synthesis of the 6T oligomer and the semi-crystalline or amorphous polyamides which are subsequently combined in a twin screw extruder equipped for devolatilization of water of reaction. It can also be used for the modification of existing semi-crystalline or amorphous polyamides (not necessarily phthalamides) with low levels of 6T.

Description

SYNTHESIS OF SEMI-CRYSTALLINE POLYPHTHALAMIDES THROUGH REACTIVE EXTRUSION OF HEXAMETHYLENE TEREPHTHALAMIDE OLIGOMER WITH LOWER MELΗNG, SEMI-CRYSTALLINE OR AMORPHOUS POLYAMIDES

FIELD OF THE INVENTION

This invention provides a flexible and economical process for the synthesis of semi-crystalline polyphthalamides through reactive extrusion of 6T oligomer with lower melting, semi-crystalline polyamides. GLOSSARY OF ABBREVIATIONS

66 hexamethylene adipamide

10 61 hexamethylene isophthalamide 6T hexamethylene terephthalamide 6T/66 copolymer of hexamethylene diamine, adipic acid and terephthalic acid

6T/6 copolymer of hexamethylene diamine, terephthalic acid and ε-

15 caprolactam)

6T/DT copolymer of hexamethylene diamine, 2-methyl pentamethylene diamine and terephthalic acid

6T/612 copolymer of hexamethylene diamine, terephthalic acid and dodecanedioic acid)

20 6T/6I copolymer of hexamethylene diamine, terephthalic acid and isophthalic acid) 6T/6I/66 terpolymer of hexamethylene diamine, terephthalic acid, isophthalic acid and adipic acid

A/C autoclave 25 CO₂H acid

D 2-methyl pentamethylene diamine

DT 2-methyl pentamethylene terephthalamide

E/E electronic/electrical 30 HMD hexamethylene diamine

IA isophthalic acid

IV inherent viscosity

2-MPMD 2-methyl pentamethylene diamine MW molecular weight

NH₂ amine

NH₂-rich amine rich

PA66 hexamethylene adipamide PCV pressure control valve

TA terephthalic acid

SHP sodium hypophosphite

T_g glass transition temperature

T_m melting temperature TPP triphenyl phosphite

T/S twin screw

BACKGROUND OF THE INVENTION

For polyphthalamides in automotive or electronic/electrical connector applications requiring high temperature performance, polymer performance is very much influenced by the level of 6T in the copolymer or terpolymer compositions. Only very few "true" high temperature polyterephthalamides (T_m >

300°C) can be made by a single step synthesis process using autoclaves (A/C).

The A/C process is more or less limited to copolymers containing 50 mole% 6T or less because of phase boundary considerations under which high temperatures and pressures are required for processability. These A/C processing conditions usually result in long exposure to high temperatures, thus leading to polymer degradation.

But commercially available materials such as 6T/66 [55/45], 6T/6I [70/30],

6T/6I/66 [65/25/10], 6T/66 [65/35], 6T/6 [70/30] (numbers inside brackets refer to molar ratios) offer an attractive balance of properties, to a great extent linked to the higher level of 6T content. Those competitive, higher 6T-based polymers can only be manufactured by a 2-step or 2-stage process involving overall much shorter heat histories than those typical of A/C synthesis.

Thus Japanese patent applications Kokai Hei 7-188409, Heisei 7-18072,

Hei 4-53827 and Hei 4-53825 describe a 2-step process involving, as first step, the synthesis of an amine-rich (or acid rich) oligomer, which is further polymerized in the melt using an extruder. An A/C reactor (e.g. one which is used primarily for the batch synthesis of PA66) can be used for the preparation of the

NH₂-rich primary condensate product. The balance of reactive ends is achieved during extrusion when molar amount of dicarboxylic acid, equivalent to the NH₂- ends excess, is added. Reaction times for the synthesis of the primary condensation product are long and can lead to the formation of significant levels of triamines. This process was applied to the synthesis of 6T/66, 6T/6I, 6T/6 copolymers with 6T contents up to 90 mole%. The typical number average MW for the primary condensation product is approximately 1350.

In a fashion similar to the one described in the previous paragraph, Japanese patent applications Kokai 3-296528, Hei 5-43681, European patent application No. EP 0744431 A2 describe a 2-step process involving, as first step, the synthesis of an essentially balanced oligomer which is further polymerized in the melt using an extruder. An A/C reactor (e.g. one which is used primarily for the batch synthesis of PA66) can be used for the preparation of the oligomer. The oligomer' s intrinsinc viscosity is typically 0.15 dL/g and it is finished to a polymer of IV > 1.0 dL/g by extrusion on a T/S extruder with vented barrel sections. Application to the synthesis of 6T/66 or 6T/6I copolymers, up to 100 mole% 6T, is claimed.

Another approach is described in US patents Re Nos. 34,447 and 5,550,208 whereby an oligomer is produced through polycondensation of a pressurized (balanced) salt solution using a preheater/downflow vapor phase reactor combination. The oligomer thus produced is melt fed continuously from the outlet of the vapor phase reactor to a T/S extruder to finish the polymer to the desired MW. Typically, the oligomer fed to the T/S extruder has an IV of 0.20- 0.24 dL/g and the finished polymer has an IV of 1.0-1.2 dL/g. No limits on mole% 6T content are set in the claims, but examples of 6T content as high as 65 mole% are cited.

Similarly, US patents Nos. 4,970,255 and 5,200,450 describe a process by which a superatmospheric salt solution is passed through a vaporization zone to yield a prepolymer which in turn is passed through a polycondensation zone, after which it is continuously removed from the polycondensation zone. In a preferred process, the polyamide thus obtained is passed as a liquid melt through a discharge zone with simultaneous removal of the residual water present in the melt. A suitable discharge zone is for example a devolatilizing extruder. Key to good polymer quality (i.e. low triamine content) is short residence time (< 60 sec) in the vaporizer, and the conversion at the exit of the evaporator zone is preferably

95-98%.

SUMMARY OF THE INVENTION

The present invention provides a process for the synthesis of semi- crystalline polyphthalamides of general formula 6T/y, wherein 6T represents units of hexamethylene terephthalamide and y represents units of other polyamide forming monomers, in which the proportion of 6T to y is increased by the steps of:

A. reacting in an autoclave portions of 6T and y to produce a polyphthalamide having up to 50 mole% of 6T units, and then

B. reacting in a reactive extruder the product of step A with further amounts of 6T oligomer to increase the proportions of 6T to y to the range of 55/45 to 99/1.

The present invention provides a process which comprises reactive extrusion of a 6T-based oligomer of inherent viscosity < 0.3 dL/g or of free amine ends and carboxyl ends essentially > 2000 equivalent/million grams of polymer, with a low-6T containing semi-crystalline polyphthalamide or other semi- crystalline polyamide to yield a high temperature high 6T-containing polyphthalamide, (i.e. 6T > 50 mole%) of inherent viscosity > 0.9 dL/g, and of residual free amine ends and carboxyl ends < 200 equivalents per million grams of polymer. The 6T-based oligomer may be the homopolymer oligomer of hexamethylene diamine and terephthalic acid (6T). The composition of the 6T- based oligomer may be 6T/x, where x is a repeat unit composed of hexamethylene diamine and an aliphatic or aromatic dicarboxylic acid of 6 to 18 carbon atoms, or ε-caprolactam. The low 6T- containing semi-crystalline or amorphous polyphthalamide, or semi-crystalline polyamide may be a copolymer consisting of at least hexamethylene diamine, terephthalic acid, with either i) one or two additional aromatic or aliphatic dicarboxylic acids of 6 to 18 carbons, or ii) one or two additional aromatic or aliphatic diamines of 6 to 18 carbons , or iii) one aliphatic or aromatic diamine of 6 to 18 carbons and one aliphatic or aromatic dicarboxylic acid of 6 to 18 carbons, or iv) an α,ω-amino acid of 6 to 18 carbons or its cyclic lactam.

The 6T oligomer may be obtained in powder form by a process comprising a) adding to an autoclave reactor an admixture of hexamethylene diamine, terephthalic acid and water equivalent to a 50-65 wt% hexamethylene terephthalamide salt solution in water, and in which there is excess diamine; b) heating the mixture under agitation to remove entrained oxygen; c) subsequently heating the reaction mixture to a higher temperature under pressure; d) maintaining the mixture under pressure and allowing volatile matter to vent; e) discharging the mixture by spray drying; f) collecting the oligomer powder and allowing separation of steam from the solids to yield an oligomer powder of low residual moisture.

In a preferred embodiment of this invention the oligomer may be either 6T or 6T/y, where y is hexamethylene adipamide.

In a most preferred embodiment of this invention the oligomer is 6T. DETAILED DESCRIPTION OF THE INVENTION

The process of this invention involves the use of conventional stirred autoclaves for independent synthesis of (1) a 6T oligomer powder and (2) 6T- based copolymers or terpolymers finished to IV > 0.90 dL/g and having a 6T content no higher than 50 mole%. The 6T oligomer and 6T-based polymers having a low 6T molar content are subsequently "coupled" during extrusion on a T/S extruder, in the presence of a coupling catalyst such as TPP (triphenyl phosphite) or SHP (sodium hypophosphite). The coupling reaction involves a condensation reaction, the by-product of which is water which is devolatilized during extrusion. One additional advantage of this invention over the conversion of oligomers to finished polymer via finishing on T/S extruders is the significant reduction in moisture removal requirement, which originates only from the coupling reaction of the 6T oligomer to the main polymer chain. In principle, this process is applicable to the synthesis of high temperature polyphthalamides such as, but not limited to: 6T/66 (copolymer of hexamethylene diamine, adipic acid and terephthalic acid), 6T/6 (copolymer of hexamethylene diamine, terephthalic acid and ε-caprolactam), 6T/DT (copolymer of hexamethylene diamine, 2-methyl pentamethylene diamine and terephthalic acid), 6T/612 (copolymer of hexamethylene diamine, terephthalic acid and dodecanedioic acid), 6T/6I (copolymer of hexamethylene diamine, terephthalic acid and isophthalic acid), and 6T/6I/66 (terpolymer of hexamethylene diamine, terephthalic acid, isophthalic acid and adipic acid). Thus according to the proposed invention, the final polymer composition is achieved through the extrusion finishing of an oligomer (e.g. 6T), and a semi- crystalline polyamide of inherent viscosity (IV) > 0.90 dL/g, as measured from a solution of 0.5g/L polymer in m-cresol at 25°C. Neither the oligomer, nor the polyamide is of the composition of the target polymer. In the references cited in the "background" section above, oligomers or low order condensates which are converted to a high MW polymer by extrusion finishing on twin-screw (T/S) extruders are of the same monomeric composition as the final polymer. The 6T oligomer of this invention has a very low MW (300 - 600). Both the 6T oligomer and the low 6T-based polymer of our invention are essentially balanced, i.e. NH₂ and CO₂H ends are close in value.

The 6T oligomer powder may be obtained by a process comprising: a) adding to an autoclave reactor an admixture of hexamethylene diamine, terephthalic acid and water equivalent to a 65 wt% hexamethylene terephthalamide salt solution in water, and in which there is 2-4 mole% excess diamine; b) heating the mixture under mechanical agitation to a temperature of approximately 130degC and venting vapors for 5 minutes to remove entrained oxygen; c) subsequently heating the reaction mixture to 226degC at an autogenous pressure of 2.20 MPa; d) with the vessel pressure maintained at 2.20 MPa, venting the volatile matter up to a melt temperature of 240degC over a period of approximately 65 minutes; e) then closing the autoclave reactor's pressure control valve and discharging the reactor through a spray drying nozzle, using the autoclave's internal steam pressure; f) collecting the oligomer powder in a sufficiently large enclosure to allow separation of steam from the solids and yielding an oligomer powder containing no more than 5 wt% residual moisture. EXAMPLES

The following examples are meant to illustrate the present invention. This concept was applied successfully in the following examples: Example A: Synthesis of 6T/DT (60/40 molar ratio), from reactive extrusion of 6T/DT (50/50 molar ratio) 80 wt% with 6T oligomer, 20 wt%. Example B: Synthesis of 6T/DT (50/50 molar ratio) from reactive extrusion of 6T/DT (30/70 molar ratio), at 70 wt%, with 6T oligomer, at 30 wt%. Example C: Synthesis of 6T/66 (50/50 molar ratio) from reactive extrusion of 6T/66 (38/62 molar ratio), 80 wt%, with 6T oligomer, 20 wt%. Example D: Synthesis of 6T/66 (65/35 molar ratio) from reactive extrusion of 6T/66 (55/45molar ratio), 80 wt%, with 6T oligomer, 20 wt%.

Referring to the examples above, these examples reflect the fact that 20 - 30 wt% 6T oligomer powder can be conventionally compounded with 6T-based polymers (low 6T content) in a single pass. In that fashion, the 6T content of a variety of autoclave produced polymers, not limited to the examples cited above, can be increased beyond the level practical with an autoclave process. Additives and reinforcements may be added at the reactive extrusion stage. This offers the additional advantage of accomplishing reactive extrusion and compounding with filler and/or other reinforcing materials and/or additives in a single step. Synthesis of 6T Oligomer

A 12 liter reaction vessel equipped with a helical ribbon agitator and a thermowell to measure reaction temperature, was charged with 1907g (16.44 moles) of hexamethylene diamine as a 77.67% (wt) aqueous solution (2393g of solution), 2659g terephthalic acid (16.02 moles), 3.94g of a 1% water emulsion of Dow Corning B antifoam, and 1800g of demineralized water. With the reactor agitator rotating at 50 rpm, the mixture was heated to 130°C and then vented to remove entrained oxygen. Subsequently, the reaction mixture was heated to 226°C at an autogenous pressure of 320 psig. With the vessel pressure maintained at 320 psig, volatile matter was vented over a period of 67 minutes up to a melt temperature of 240°C. Then the reactor's pressure control valve (PCV) was closed and the contents of the reactor was discharged through a spray-drying nozzle, using the autoclave's internal steam pressure. In that fashion, an oligomer powder was collected in a sufficiently large enclosure to allow separation of the steam from the solids and yielding an oligomer containing no more than 5%(wt) residual moisture.

The oligomer thus obtained had an inherent viscosity (IV) of 0.17 dL/g; in these instances, inherent viscosity was measured on a 0.5g/L solution in m-Cresol at 25°C. The oligomer had a melting point of 356°C, as measured by differential scanning calorimetry (DSC; ASTM D3418) from the remelt curve. The DSC showed also a fairly broad melting peak at about 276°C, which is evidence of the presence of unreacted salt. The oligomer was dried overnight at 85°C, under nitrogen, then used in reactive extrusion runs on a co-rotating non-intermeshing twin-screw extruder. The water of reaction generated during the reactive extrusion step was removed through a vacuum port. Synthesis of 6T DT [50/50] Copolymer A 12 liter reaction vessel equipped with a helical ribbon agitator and a thermowell to measure reaction temperature, was charged with 1271g (8.588 moles) of hexamethylene diamine as a 78.53% (wt) aqueous solution, 998g (8.588 moles) of 2-methyl pentamethylene diamine, 2659g terephthalic acid (16.006 moles), 12g of 47% (weight basis) of aqueous sodium phenyl phosphinate solution, 6 ml of 10% (wt) Carbowax^® 3350 polyethylene glycol in water and 1200g of demineralized water. With the reactor agitator rotating at 50 rpm, the reaction mixture was heated to 232°C. With the vessel pressure maintained at 1.71 MPa, volatile matter was vented over a period of 43 minutes up to a melt temperature of 275°C. The pressure in the reactor was then reduced to atmospheric pressure over a period of 48 minutes, the temperature of the reaction mixture rising to 320°C; the rate of agitation was reduced to 5 rpm when the temperature had reached 318°C. The reaction mixture obtained was maintained under a vacuum (pressure reduction) of 40 kPa for 15 minutes. The polymer obtained was then discharged from the reactor and quenched in a water bath. The copolyamide obtained had an inherent viscosity (IV) of 0.98 dL/g; in this instance, inherent viscosity was measured on a 0.5g/L solution in concentrated sulfuric acid at 25°C. The polymer had a melting point of 301°C, as measured by differential scanning calorimetry (DSC). Synthesis of 6T DT [30/70] Copolymer

A 12 liter reaction vessel equipped with a helical ribbon agitator and a thermowell to measure reaction temperature, was charged with 585g (5.05 moles) of hexamethylene, 1392g of 2-methyl pentamethylene diamine (12.00 moles), 2659g terephthalic acid (16.01 moles), 3.94g of a 25% aqueous solution of sodium hypophosphite, 7.88g of a 1 % water emulsion of Dow Corning B antifoam, and 1200g of demineralized water. With the reactor agitator rotating at 50 rpm, the mixture was heated to 130°C and then vented to remove entrained oxygen. Subsequently, the reaction mixture was heated to 231°C, at an autogenous pressure of 265 psig. With the vessel pressure maintained at 265 psig, volatile matter was vented over a period of 99 minutes up to a melt temperature of 280°C. The pressure in the reactor was then reduced to atmospheric pressure over a period of 40 minutes, the temperature of the reaction mixture rising to 296°C. The reaction mixture obtained was at atmospheric pressure for 10 minutes, while the melt temperature was maintained essentially constant. The polymer obtained was then discharged from the reactor and quenched in a water bath over a period of 73 minutes; the final melt temperature was 301°C. The copolyamide obtained had an inherent viscosity (IV) of 0.97 dL/g; in this instance, inherent viscosity was measured on a 0.5g/L solution in m-Cresol at 25°C. The polymer had a melting point of 244°C, as measured by differential scanning calorimetry (DSC). Synthesis of 6T/66 [38/62]

A 12 liter reaction vessel equipped with a helical ribbon agitator and a thermowell to measure reaction temperature, was charged with 1959g (16.888 moles) of hexamethylene diamine, 1118g terephthalic acid (6.735 moles), 1475g of adipic acid (10.103 moles), 0.79g of a 25% aqueous solution of sodium hypophosphite, 0.26g of a 25% solution of Carbowax 3350 in water and 1500g of demineralized water. With the reactor agitator rotating at 50 rpm, the mixture was heated to 130°C and then vented to remove entrained oxygen. Subsequently, the reaction mixture was heated to 221°C, at an autogenous pressure of 275 psig. With the vessel pressure maintained at 275 psig, volatile matter was vented over a period of 117 minutes up to a melt temperature of 274°C. The pressure in the reactor was then reduced to atmospheric pressure over a period of 57 minutes, the temperature of the reaction mixture being at 298°C. The reaction mixture obtained was maintained at atmospheric pressure for 2 minutes, while the melt temperature was maintained essentially constant. The polymer obtained was then discharged from the reactor and quenched in a water bath over a period of 54 minutes; the final melt temperature was 307°C. The copolyamide obtained had an inherent viscosity (IV) of 1.30 dL/g; in this instance, inherent viscosity was measured on a 0.5g/L solution in m-Cresol at 25°C. The polymer had a melting point of 276°C, as measured by differential scanning calorimetry (DSC). Example A: Synthesis of 6T/DT (60/40), from reactive extrusion of 6T/DT (50/50) 80 wt% with 6T oligomer, 20 wt%.

The experiment was conducted on a 20mm non-intermeshing/corotating T/S Welding Engineers extruder. Processing Conditions: Temperature Profile (°F)

Screw pee s: rpm Throughput: 10 lb/hr

The preblended mixture of polymer pellets/oligomer powder (80/20 weight ratio) was directly charged into the hopper and fed at the feed zone of the extruder (zone 1). The temperature of the first zone of the extruder is kept at 490°F to prevent the oligomer powder from reacting too quickly and giving off steam which causes the oligomer powder to bridge at the feed port.

Al : 6T/DT (50/50) as prepared A2: 6T/DT (50/50), after single pass on extruder A3 : 6T/DT (50/50) + 0.5% TPP catalyst - single pass

A4: 6T/DT (50/50) + 6T oligomer (80/20 weight ratio)

A5: 6T/DT (50/50) + 6T oligomer (80/20 ratio) - single pass on extruder

A6: 6T/DT (50/50) + 6T oligomer (80/20 ratio) + 0.5% TPP - single pass

(1) 0.5g/L solution in m-Cresol

(2) Mathematical Average

Example B: Synthesis of 6T/DT (50/50) from reactive extrusion of 6T/DT (30/70), at 70 wt%, with 6T oligomer, at 30 wt%.

Throughput: 10 lb/hr or 7.5 lb/hr

The preblended mixture of polymer pellets/oligomer powder (70/30 weight ratio) was directly charged into the hopper and fed at the feed zone of the extruder (zone 1). The temperature of the first zone of the extruder is kept at 490°F to prevent the oligomer powder from reacting too quickly and giving off steam which causes the oligomer powder to bridge at the feed port.

Bl: 6T/DT (30/70) as prepared B2: 6T/DT (30/70), single pass on extruder B3 : 6T/DT (30/70) + 6T oligomer (70/30 wt ratio) - single pass B4: 6T/DT (30/70) + 6T oligomer (70/30 wt ratio) - double pass B5: 6T/DT (30/70) + 6T oligomer (70/30 ratio) + 0.5% TPP catalyst single pass at 290rpm

B6: 6T/DT (30/70) + 6T oligomer (70/30 ratio) + 0.5% TPP catalyst single pass at

150 rpm

(1) 0.5g/L solution in m-Cresol

Example C: Synthesis of 6T/66 (50/50 molar ratio) from reactive extrusion of

6T/66 (38/62 molar ratio), 80 wt%, with 6T oligomer, 20 wt%.

Screw pee s: rpm Throughput: 10 lb/hr

The preblended mixture of 6T/66 (38/62) polymer pellets/oligomer powder (80/20 weight ratio) was directly charged into the hopper and fed at the feed zone of the extruder (zone 1). The temperature of the first zone of the extruder is kept at 490°F to prevent the oligomer powder from reacting too quickly and giving off steam which causes the oligomer powder to bridge at the feed port.

Cl : 6T/66 (38/62) start material

C2: 6T/66 (38/62) + 6T oligomer (80/20 weight ratio) - single extruder pass C3: 6T/66 (38/62) + 6T oligomer (80/20weight ratio) - double extruder pass

(1) 0.5g/L solution in m-Cresol

(2) in HFIP

Example D: Synthesis of 6T/66 (65/35 molar ratio) from reactive extrusion of

6T/66 (55/45molar ratio), 80 wt%, with 6T oligomer, 20 wt%.

Throughput: 10 lb/hr

The preblended mixture of 6T/66 (55/45) polymer pellets/oligomer powder (80/20 weight ratio) was directly charged into the hopper and fed at the feed zone of the extruder (zone 1). The temperature of the first zone of the extruder is kept at 490°F to prevent the oligomer powder from reacting too quickly and giving off steam which causes the oligomer powder to bridge at the feed port.

Dl: 6T/66 (55/45) start material D2: 6T/66 (55/45) + 6T oligomer (80/20 weight ratio) - single extruder pass D3: 6T/66 (55/45) + 6T oligomer (80/20 weight ratio) - double extruder pass

(1) 0.5g/L solution in m-Cresol (2) in HFIP

Claims

1. A process for the synthesis of semi-crystalline polyphthalamides of general formula 6T/y, wherein 6T represents units of hexamethylene terephthalamide and y represents units of other polyamide forming monomers, in which the proportion of 6T to y is increased by the steps of:

A. reacting in an autoclave 6T and_y to produce a polyphthalamide having up to 50 mole% of 6T units, and then

B. reacting in a reactive extruder the product of step A with 6T oligomer to increase the proportions of 6T to y to the range of 55/45 to 99/1.

2. The process of claim 1 wherein the 6T oligomer, used in Step B, is obtained by the process comprising: a) adding to an autoclave reactor an admixture of hexamethylene diamine, terephthalic acid and water equivalent to a 50-65 wt% hexamethylene terephthalamide salt solution in water, and in which there is excess diamine; b) heating the mixture under agitation to remove entrained oxygen; c) subsequently heating the reaction mixture to a higher temperature under pressure; d) maintaining the mixture under pressure and allowing volatile matter to vent; e) discharging the mixture by spray drying; f) collecting the oligomer powder and allowing separation of steam from the solids to yield an oligomer powder of low residual moisture.

3. The process of claim 1 wherein the proportions of 6T and y in the product of step A are in the range of 20/80 to 50/50 and the proportions of 6T and y in the product of step B are in the range 55/45 to 99/1.

4. The process of claim 3 wherein the proportions of 6T and y in the product of step A are in the range of 30/70 to 50/50 and the proportions of 6T to y in the product of step B are in the range 55/45 to 70/30.

5. The process of claim 1 wherein y represents units of polyamide forming monomers consisting of i) aliphatic or aromatic diamines and aliphatic or aromatic diacids, or ii) α,ω-amino acids or their corresponding cyclic lactams.

6. The process of claim 5 wherein: i) the aliphatic or aromatic diamines are selected from the group comprising hexamethylene diamine, 2-methylpentamethylene diamine, l,4-bis-(aminomethyl)cyclohexane, 4,4'-methylene- bis(cyclohexylamine), m-xylylene diamine, p-xylylene diamine, and the aliphatic or aromatic diacids are chosen from a list comprising adipic acid, 1,10-decanedioc acid, 1,12-dodecanedioc acid, 1,4-cyclohexane dicarboxylic acid, isophthalic acid and 2,6- naphthalene dicarboxylic acid. ii) the α,ω-amino acids or their corresponding cyclic lactams are selected from the group comprising ε-caprolactam, 12- aminododecanoic acid or laurolactam.

7. The process of claim 5 wherein: iii) the aliphatic or aromatic diamines are selected from the group comprising hexamethylene diamine, 2-methylpentamethylene diamine, and the aliphatic or aromatic diacids are chosen from a list comprising adipic acid, 1,12-dodecanedioic acid or isophthalic acid. iv) . the α,ω-amino acids or its corresponding cyclic lactam is ε- caprolactam.

8. The process of claim 1 wherein the 6T-based oligomer of Step A has an inherent viscosity of from about 0.05 to about 0.3 dL/g and free amine-ends and carboxyl ends essentially > 2000 equivalent/million grams of polymer, and the 6T oligomer of Step B is a low-6T containing semi-crystalline polyphthalamide or other semi-crystalline polyamide.

9. The process of claim 1 wherein the resulting semi-crystalline polyphthalamides of general formula 6T/y are high temperature high 6T- containing polyphthalamides of inherent viscosity > 0.9dL/g with residual free amine ends and carboxyl ends < 200 equivalents per million grams of polymer.

10. The process of claim 8 wherein the low-6T containing semi-crystalline or amorphous polythalamide, or semi-crystalline polyamide may be a copolymer consisting of at least hexamethylene diamine, terephthalic acid, with either i) one or two additional aromatic or aliphatic dicarboxylic acids of 6 to 18 carbons, or ii) one or two additional aromatic or aliphatic diamines of 6 to 18 carbons , or iii) one aliphatic or aromatic diamine of 6 to 18 carbons and one aliphatic or aromatic dicarboxylic acid of 6 to 18 carbons, or iv) an α,ω-amino acid of 6 to 18 carbons or its cyclic lactam.

11. The process of claim 1 wherein the 6T-based oligomer may be 6T/x, where x is a repeat unit composed of hexamethylene diamine and an aliphatic or aromatic dicarboxylic acid of 6 to 18 carbon atoms or ε-caprolactam.

12. A process for the synthesis of semi-crystalline polyphthalamides which comprises independently synthesizing in stirred autoclaves a 6T oligomer powder and 6T-based copolymers or terpolymers finished to IV > 0.90 dL/g, and having a 6T content no higher than 50 mole% and subsequently coupling by a condensation reaction, the 6T oligomer and the 6T-based copolymers or . terpolymers during extrusion on a T/S extruder, in the presence of a coupling catalyst.

13. The process of claim 12 wherein the coupling catalyst is selected from triphenyl phosphite and sodium hypophosphite.