WO2020007907A1

WO2020007907A1 - Post-treatment of shaped polyamide articles

Info

Publication number: WO2020007907A1
Application number: PCT/EP2019/067837
Authority: WO
Inventors: Jules Armand Wilhelmina Harings; Sanjay Rastogi
Original assignee: Universiteit Maastricht; Academisch Ziekenhuis Maastricht
Priority date: 2018-07-05
Filing date: 2019-07-03
Publication date: 2020-01-09

Abstract

The present invention relates to a process for post treatment of shaped polyamide articles. The process comprises exposing the articles to water having a ¹H NMR Chemical Shift of between 3.7 and 2.9 ppm. This can be achieved with superheated water having a temperature from 105 to 180 °C or with water containing certain salts. The process results in shaped polyamide articles with improved crystallinity and thus improved mechanical properties.

Description

POST-TREATMENT OF SHAPED POLYAMIDE ARTICLES

DESCRIPTION

The present invention relates to a process for the post-treatment of a shaped polyamide article and the polyamide article obtained by such a process.

Polyamides are known for their diverse roles in (bio-)engineering. Here, the mechanical performance relies on intra- and intermolecular hydrogen bonding between adjacent amide moieties. While hydrogen bonding in the amorphous phase mainly influences post-yield deformation, hydrogen bonding in the crystalline domains determines the initial stiffness. The stiffness scales with the strength of the hydrogen bond, which is realised by the most efficient electron exchange between the electron acceptor and donor and thus thermodynamically most stable state.

In the thermodynamically most stable crystalline structure of polyamides, the hydrogen bonds are uniplanar and spatially positioned at closest proximity, forming sheets that stack into a 3 dimensional lattice by Van der Waals forces. From crystallographic perspective this repetitive organization of chains demarks interchain/intrasheet and interchain/intersheet planes. Relative time-scales of the necessary conformational rearrangements during crystallization and processing determine firstly the crystal lattice unit cell with varying Gibbs free energy states, and secondly the degree of thermodynamic crystal perfection or deviations thereof. Note that the thermodynamically most stable unit cell may differ for different polyamides - e.g. monoclinic for polyamide 6 and triclinic for PA46.

The high cooling rates experienced in melt processing technologies limit the formation of the thermodynamically most stable crystalline state. It is common that melt processing yields thermodynamically non-optimized crystalline states, off which several unit cell classifications can co-exist. Denotations of crystalline unit cells by means of Greek symbols may deviate per polymer. For clarity, in the present invention the structure of the shaped article will be described by crystal lattice type, the degree of crystal perfection, and thermodynamical state.

Single difunctional monomers, such as in the case of polyamide 6, or poly-e- caproamide, induce multiple possibilities in spatial organization and thus extensive polymorphism that is largely affected by the time-scales and molecular deformation during processing.

At room temperature two distinct unit cells, from which the other polymorphisms are derived, are recognized for polyamide 6 and often co-exist after conventional melt processing. The first and thermodynamically most stable state is monoclinic obtained via slow crystallization either from the melt or from solution, commonly referred to as a. Here, linearly aligned hydrogen bonds are formed between antiparallel chains in an all-trans fully extended conformation. The less stable unit cell is the pseudo- hexagonal mesophase, denoted as g or b, where parallel chain packing of all-trans methylene segments with random hydrogen bonding planes is promoted during fast cooling or upon iodine treatment. It is important to realize that the theoretical modulus of the monoclinic structure, calculated by an X-ray method and linear compressability in three dimensions is a factor 6 times larger than in case of the pseudo-hexagonal phase (K. Tashiro and H. Tadokoro (1981 ), Macromolecules, 14, 781 -785).

In Vilas Bankar et al. (1977), J. Appl. Pol. Science, Vol. 21, 2341 -2358, the structure of melt-spun polyamide 6 fibers, as spun and after various treatments, is described. It is concluded that annealing treatment with hot water (up to 100°C) transforms the pseudohexagonal structure to monoclinic, but a classification according to the degree of perfectioning is to be made if one aims for maximum performance. The degree of crystal perfection of the thermodynamically most stable state, here monoclinic, is described by the angular distance of the scattering angle 2Q as measured by Wide Angle X-ray diffraction or more specifically the d-spacings derived thereof using the Crystal Perfection Index (CPI) as described by Pepin et al. (J. Pepin et al. (2016), Macromolecules, 49, 564-573).

with d₂₀₀

distance in nm,

^210/010 = interchain/intersheet distance in nm, and

W = is a constant, equal to 0.194 in order to

adjust the CPI value of the

thermodynamically most stable a phase to

1 . Aiming for improved performance various technologies have been additionally developed to promote the pseudo-hexagonal to monoclinic transition, including annealing at elevated temperature (G. Gurato et al. (1974), Makromol. Chem., 175, 953- 975) - solely or in combination with treatment in water and/or steam (US2004138344A1 , US5,262,099), aqueous phenol solutions (Y. Kinoshita (1959), Makromol. Chem., 33, 1 -20), or 20% formic acid solution (Bankar et al. (1977), J. Appl. Pol. Science, Vol. 21, 2341 -2358).

DE1258598 describes a process where polyamide articles can be post- treated in water to remove residual stresses introduced during shaping. The post-treatment takes place in water of 90 to 140 YD where the water is either in the liquid state for temperatures up to 97 °C or wherein water is saturated water vapor with temperatures below 140 °C.

GB 1 140906 describes a method of increasing the impact strength of a nylon 6 moulded article by immersing the article in a heated aqueous solution of an alcohol with a high boiling point, preferably a glycol. The temperature of the alcoholic solution should not exceed 120 °C to avoid swelling and cracking of the article.

CN107556504 describes a hydrothermal corrosion surface treatment of a nylon crystal film having the a crystal form by contacting the film with water. The water temperature is increased to a temperature below the dissolution temperature of the nylon in water. The crystal structure of the film is unchanged before and after the treatment

As shown later in the experimental section, some of the known processes may result in a relatively high CPI. However, these processes have different drawbacks, such as the use of acids like formic acid that are known to promote hydrolytic degradation at elevated temperature especially in the presence of water, lowering the average molar mass and consequently the mechanical properties.

Degradation is additionally promoted by the duration of the post-treatment, which in the above cases is longer than 1 hour and sometimes even 24 hours.

The described processes further have the drawback that due to the long duration, they are less suitable for industrial processes.

Polyamide articles are usually obtained by a process which comprises the steps of melting the polyamide and extruding the polyamide through a die. The process can be followed by a stretching step. In the known industrial processes of melt based processing the polyamides predominantly crystallize in the thermodynamically most unstable state, resulting in (mechanical and/or barrier) properties that are not optimal.

In the scientific literature it is described to crystallize polyamides in the presence of water at elevated temperature. See for instance J. Peng et al. (2016), Polymer, 84, 158-166. It is described in the conclusion that an increase in unsaturation was created by thermal hydrolysis during water-assisted compounding though the processing in the presence of water, acting as a solvent and cellulose nanocrystals induces crystal perfection of the thermodynamically most stable state, here the monoclinic phase. A similar route based on dissolution, or in fact supressed melting, was reported by Wang et al. (2015), Chinese Journal of Polymer Science, 33, 1334-1343).

However, the drawback of such processes is that water is trapped in the crystal lattice, thus shielding the interchain/intrasheet hydrogen bonding, weakening the crystal modulus and lowering the performance of the polyamide product, as discovered by Vinken and Harings using solid state NMR and FTIR spectroscopy (E. Vinken et al. (2008), Langmuir, 24, 6313-6326; J.A.W. Harings et al. (2008), Crystal Growth and Design, 8, 3323-3334).

Therefore, with the known processes it is not possible to obtain polyamide articles with an optimal crystal structure (as defined by the Crystal Perfection Index), having good mechanical and barrier properties, where the process can be carried out on an industrial scale.

The present invention thus provides a process for post treatment of a shaped polyamide article comprising exposing the article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm.

The advantages of the invention are that it is a relatively simple process, that can be carried out on an industrial scale and that the properties of the shaped polyamide article can be significantly improved.

It is surprising that the process of the invention provides such good results, since a person skilled in the art of polyamides would generally consider that the exposure of polyamides to water under these conditions is detrimental to polyamide performance by inducing hydrolytic degradation.

Moreover, in the process of the invention the shaped polyamide article is preferably exposed to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm for a time period from 5 seconds to 1 hour, more preferably from 1 minute to 45 minutes. The time of exposure will be determined by the precise process conditions, more in particular by the volume of the shaped polyamide article to be treated. At higher volumes, longer exposure times are applicable.

However, even at the upper range of these exposure times, the process is relatively fast, and thus applicable on an industrial scale.

In the process of the invention a shaped polyamide article is treated containing at least 30 v/v% based on the total volume of polyamide, of polyamide having the pseudo-hexagonal (b or y) crystal phase. The polyamide article obtained by the process contains at least 80 v/v%, preferably at least 90 v/v%, based on the total volume of polyamide, of polyamide having a monoclinic a phase. Moreover, the article has a high crystal perfection index.

As described above it has been found that it is advantageous to expose polyamide articles to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm. Compared to the known post-treatment conditions, water molecules within this specific range of the chemical shift possess high diffusivity and yet is fluidic in nature and thus possesses relatively high density and high wettability at the given temperature. This reduces the time-of the transformation of the crystal lattice into the thermodynamically stable state while minimizing hydrolytic degradation.

Besides, since the temperature of the treatment is below the dissolution temperature of polyamide in water, i.e. from 140°C onwards, the shaped article retains its shape over the structural perfectioning.

With ¹ H NMR chemical shift is meant the chemical shift of the bulk water proton, as measured with in-situ NMR (Nuclear Magnetic Resonance) under HRMAS (High Resolution Magic-Angle Spinning), operating at 500 MHz ¹ H Larmor frequency, a MAS spinning frequency of 5.0 kHz and a 4.0 ps p/2 pulse, calibrated using ²⁰⁷Pb MAS NMR, reporting the ¹ H chemical shifts relative to tetramethylsilane (TMS) using adamantane as an external reference.

In detail, with ¹ H NMR chemical is meant the chemical shift of the bulk water proton, which appears as the second highest chemical shift, as measured by in-situ Nuclear Magnetic Resonance under High Resolution Magic-Angle Spinning (HRMAS) to describe the physical state of water. The highest ¹ H NMR chemical shift belongs to the amide proton typically in the range of 8.5 to 7.0 ppm under these conditions. In situ variable-temperature ¹ H HRMAS NMR experiments can be performed on a solid state NMR spectrometer, e.g. a Bruker DSX spectrometer, operating at 500 MHz ¹ H Larmor frequency using a commercial MAS double-resonance probe (¹ H-X) for rotors with 4.0 mm outside diameter. A MAS spinning frequency of 5.0 kHz and a 4.0 ps p/2 pulse, corresponding to 62.5 kHz rf nutation frequency, is chosen for ¹ H. Samples are prepared by placing immersion of the sample in water or ionic solutions in glass capillaries (from Wilmad Glass), which are successively sealed using a LPG gas flame. The filling degree of the capillary is approximately 90% by volume. Reported temperatures are corrected for sample rotation induced temperature changes. The calibration was performed using ²⁰⁷Pb MAS NMR where the chemical shift of Pb(N03) as a function of temperature is known. ¹ H chemical shifts are reported relative to tetramethylsilane (TMS) using adamantane as an external reference.18 calibrations of 1 H rf-field strength and shimming were performed at ambient conditions.

Further details are described by Y. S. Deshmukh et al. (2013), Macromolecules, 46, 7086-7096.

Preferably, the water has a ¹ H NMR Chemical Shift of between 3.6 and 3.2 ppm, more preferably of between 3.6 and 3.4 ppm.

The ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm can be obtained by using water in the fluidic state having a temperature higher than 100 °C and lower than the melting temperature of the polyamide.

The invention thus uses superheated water in the fluidic state. This is a fluid with a unique behavior. It is fluidic water where the mobility of water molecules is higher than liquid water below 100 °C. Further, superheated water has a lower density than liquid water below 100 °C but higher than saturated steam, thus securing high wettability.

Without wishing to be bound by any theory, it is believed that this specific state of water causes high diffusivity of water, dense contact of the polyamide article with water which reduces the time scale of contact considerably.

The melting temperature of the polyamide is determined by the onset of the melting process as measured by Differential Scanning Calorimetry. The onset is defined by intersection of the extrapolated baseline before melting and the tangent of the positive slope the endothermic event that accompanies the melting.

The heated water in the first embodiment will normally be in a closed vessel creating an elevated pressure, typically the corresponding water vapor pressure of the temperature applied, i.e. less than 20 bar, in particular 1 .5 to 5 bar (0.15 to 0.5 MPa). Thus, in general, the polyamide article is exposed to water at a temperature of at least 105, preferably at least 120 °C. The maximum temperature is limited by the melting and/or dissolution temperature of the polyamide and is generally at most 180 °C, preferably at most 160 °C and most preferably at most 140 °C.

The water used in this embodiment of the process in the invention is relatively pure water, e.g. not containing any minerals (deionized water) or acids such as formic acid. In particular the water is more than 90% pure, thus containing more than 90 wt.% H₂0, preferably more than 95% pure, most preferably more than 99%, or even 99.5% pure.

According to a second embodiment the water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm is water containing one or more ions, selected from the group of anions consisting of Cl , Br, L, N0₃ , CI0₃ , Br0₃ ^_, I0₃ , CI0₄ and SCN- and the group of cations consisting of K⁺, Na⁺, Li⁺, Ca²⁺ and Ga³⁺, wherein the concentration of ions in the water is preferably from 0.5 to 9 M.

Preferably, the anions are selected from the group consisting of Br, L, N0₃- ,CI0₃-, Br0₃- and CICV. More preferably the anions are selected from the group consisting of Br, I-, CI0₃-, and CI0₄ .

Preferably, the cations are selected from the group consisting of K⁺, Na⁺, Li⁺ and Ca²⁺.

More preferably, the ratio of ions to polyamide is determined by the effect of the ions on the ¹ H NMR Chemical Shift. The relationship was demonstrated for Lil. Without ions the preferred maximum chemical ¹ H NMR Chemical Shift of 3.7 ppm is achieved at 105 °C. When 1 M Lil is used, the¹ H NMR Chemical Shift of 3.7 ppm is achieved at 105 °C, whereas at 8 M Lil, the ¹ H NMR Chemical Shift of 3.7 is already established at 94 °C (Y.S. Deshmukh et al. (2013), Macromolecules, 46, 7086-7096).

As described above, the process according to the invention is a post treatment. With post-treatment is meant that the article is already shaped before being exposed to the water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm.

Thus, the process of the invention comprises the steps of:

- melting polyamide

- shaping polyamide obtaining a shaped polyamide article,

- optionally cooling the shaped article to room temperature,

- exposing the shaped polyamide article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm. After exposing the shaped polyamide article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm, the shaped polyamide article is dried and, if needed, cooled to room temperature. The drying can be carried out at a temperature above the Tg of the polyamide, preferably under reduced pressure or even vacuum.

According to a particular embodiment of the invention, after exposing the shaped article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm, the article is heated to a temperature of at least 160 °C, preferably at least 170 °C. The temperature is at most 215 TD, preferably at most 200 °C, most preferably at most 190 °C.

The effect of this treatment is that any trapped water residing in the polyamide crystal lattice is released thus further increasing the mechanical properties of the article.

With shaping of the polyamide is meant any known technology, for instance extruding the polyamide through a die, thermoforming or injection molding the polyamide.

Thus, the article to be treated is a shaped polyamide article obtained after melting and shaping the polyamide. The article is generally a semi-crystalline article. This means that the article is not a gel . Due to the linearity, high molar mass and entangled nature of thermoplastic polymers, thermoplastic polymers - and thus the polyamides used in the invention, only partially crystallize. The resulting structure is hence referred to as semi-crystalline, where the term crystallinity demarks the volume percentage of crystals and where the remaining volume percentage is assigned to the amorphous, non-ordered phase. The crystallinity of the article is derived from Wide Angle X-ray Diffraction pattern by the following equation.

where X_c is the crystallinity in %

A_t the total area of the background corrected,

azimuthally integrated intensity against the

scattering angle, 2Q, curve

A_a the area of the scattered intensity of the amorphous

phase against the scattering angle, 2Q, curve

that is modelled by a Gaussian fit. Examples of articles that can be treated are polyamide fibers, polyamide foams, polyamide films, thermoformed polyamide articles, compression molded polyamide articles, or injection molded polyamide articles. In the process of the invention it is possible to expose a bobbin of polyamide fiber or a roll of polyamide film to the water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm.

The polyamide that can be treated according to the process of the invention is preferably an aliphatic polyamide.

The polyamide is preferably selected from the group consisting of polyamide (PA) 6, PA5, PA4, PA66, PA46, PA410, PA1 1 or PA12.

Synthetic polyamides are made by reacting a) two difunctional monomers with each species possessing one type of reactive group, namely dicarboxylic acids or diacyl chlorides and diamines, or b) one difunctional monomer with two deviating functional groups of identical origin as in (a), including b1 ) cyclic monomers that upon ring opening provide two deviating functional groups. For instance, polyamide 6 with the aliphatic spacer C5H10 that covalently links the two functional groups, is mostly synthesized via hydrolytic polymerization where the presence of 5-10 wt-% water initiates the polymerization via hydrolysis of e-carpolactam into e-amino-caproic acid or via anionic polymerization in which strong bases such as alkali metals initiate via the lactam anion (G. Odian, Principles of Polymerization, 4th ed. John Wiley & Sons, Inc., Hoboken, New Jersey 2004).

Following IUPAC guidelines in the nomenclature of aliphatic polyamides, polyamide m represents a homopolymeric polyamide obtained from the polymerization of an amino acid or a lactam comprising m carbon atoms. Polyamide m,n stands for a copolymeric polyamide made by step polymerization of an diamine containing m carbon atoms and an alkanedioic acid containing n carbon atoms. For as well the homo- as copolymeric polyamide the number of carbon atoms includes those in the carbonyl moiety (C=0) (IUPAC, Pure and Applied Chemistry 81 , 2009, 1 131-1 186).

More preferably, the polyamide is polyamide 6.

The polyamide to be treated according to the invention may contain the usual additives, such as flame retardants, fillers, UV stabilizers, pigments and processing aids.

The present invention also relates to an article obtained with the process. Such an article contains at least 80 v/v.%, based on the total volume of polyamide, polyamide having a monoclinic a phase. Moreover, the article has a Crystal Perfection Index (CPI) of at least 0.85, preferably higher than 0.90 and preferably higher than 0.95. The Crystal Perfection Index (CPI) of the thermodynamically most stable state is based on stacked hydrogen bonded sheets and thus defines interchain/intrasheet and interchain/intersheet distances.

The v/v % polyamide having a monoclinic a phase can be determined by Wide Angle X-ray Diffraction after subtraction of the background and scattering intensity of the amorphous phase by first dividing the total area of the monoclinic diffraction signals by the area of the all crystalline diffraction signals and successive multiplication by 100.

To distinguish the article of the invention from laboratory (incidentally) produced articles, the article has preferably a weight of at least 500 grams, preferably at least 1000 grams. The article is thus obtainable on an industrial scale.

The process of the invention may be carried out continuously or batch-wise. In a batch process the polyamide article is placed in a closed chamber in the presence of water. In the batch wise process the shaped polymer article is typically an injection moulded article, a compression moulded article, a thermoformed article or an entire bobbin of fibers. The water is then heated to an elevated temperature where the ¹ H NMR Chemical Shift is between 2.9 and 3.7 ppm, e.g. 130 °C, for a short period of time depending on the smallest dimension but no longer than 1 hour, preferably less than 30 minutes, even more preferred less than 15 minutes The pressure rises to the corresponding water vapor pressure. According to a preferred embodiment of the invention, in a next step, the pressure is release from the chamber and the article is further heated to a temperature of 160 to 200 °C for 5 to 90 minutes.

After completion of the posttreatment, which may be preferably performed under tension for morphology and shape retention, the article is cooled and successively dried under vacuum above the glass transition temperature determined by Dynamic Mechanical Analysis on dried polyamide reference samples.

Another way of carrying out the process is in a continuous fashion, where the short time-scales of the invention are of great benefit. Typical processes are filament, film, tape, fiber and continuous foam extrusion, where the shape continuous article is in-situ exposed to water with a ¹ H NMR Chemical Shift of between 2.9 and 3.7 ppm that can be achieved either by operating in a closed environment to increase the pressure, or by the addition of the salts that dissociates in the ions of the second embodiment above to realize ¹ H NMR Chemical Shift of between 2.9 and 3.7 ppm. After exposure to water with a ¹ H NMR Chemical Shift of between 2.9 and 3.7 ppm, the article can be continuously moved to an oven where it is heated to 160 to 200 °C.

According to the above, the present invention in particular relates to a process for making a polyamide article comprising the steps of

a) melting polyamide;

b) shaping the melted polyamide to obtain a polyamide article; wherein the polyamide article obtained in step b) contains at least 30 v/v% based on the total volume of polyamide, of polyamide having the pseudo-hexagonal (b or y) crystal phase;

c) exposing the polyamide article to water in the fluidic state at a temperature of 120 to 140 °C for a period of 5 seconds to 1 hour;

d) removing the polyamide article obtained in step c) from the water and heating the article to a temperature of 170-190 °C;

e) cooling the polyamide article obtained in step d) thereby obtaining a polyamide article containing less than 5 v/v% based on the total volume of polyamide, of polyamide having the pseudo-hexagonal (b or g) crystal phase.

Figures

In the figures, figure 1 shows the defected monoclinic a₂ (4.38 and 3.75 A) and pseudo-hexagonal g (4.13 A) diffraction signals in water while heated (above 100°C where the chemical shift is smaller than 3.8 ppm). Despite the time component in the figure it is evident that at 130°C all crystallographic transformations, yielding the thermodynamically most stable eg phase, have been initiated and completed.

Figures 2A, 2B and 2C show X-ray results of filaments as spun and treated with a process according to the invention.

Figures 3A, 3B and 3C show X-ray results of the Izod bars as injection molded and treated with a process according to the invention.

Figures 4A and 4B show X-ray results of the tensile bars as injection molded and treated with a process according to the invention.

Figure 5 shows representative tensile curves patterns of polyamide 6 post- treated under various conditions. Figure 6 shows representative wide angle diffraction of polyamide 6 post- treated under various conditions.

Examples

Water treatment of polyamide articles

Dried Polyamide 6, Akulon obtained from DSM, was melt processed at 250 °C in an Explore co-rotating mini extruder from where either (i) monofilaments of 0.2 mm diameter, or (ii) injection moulded izod impact bars or tensile test specimens were made employing an Explore micro-injection moulding setup. Filaments, izod and tensile bars were placed in 20 ml special Biotage glass vials, immersed in demineralised water and sealed with aluminium caps withstanding pressures up to 30 bar. The expanded state of water treatment was realised by immersing the vials for 15 or 30 minutes in an oil bath that was pre-heated at 130°C, a temperature at which according to the time-resolved studies, the entire pseudohexagonal phase (b or g) instantly, and defected monoclinic crystals (a_å) gradually reorganise into the thermodynamically most stable perfectioned monoclinic packing (on) (Figure 1 ). After the time lapse completed the vials were cooled in an ice bath and dried with a series of non-expanded water treated samples (as made) in a vacuum oven at 75 °C and 20 mbar for 16 hours.

Despite the time component in Figure 1 , it is evident that at 130°C all crystallographic transformations, yielding the thermodynamically most stable ai phase, have been initiated/completed.

Characterization

Crystallography

The crystallographic changes of were followed by 2D Wide Angle X-ray Diffraction (WAXD) using a SAXSLAB Ganesha instrument equipped with a divergence source producing X-ray photons with a wavelength of 1 .54 A. The beam center and q range were calibrated using the diffraction peaks of silver behenate. Conversion of 2D into 1 D data was carried out with Saxsgui V2.13.01 using an automated beam stop mask and successive azimuthal integration of the 2D diffraction patterns. The scattering angle 2Q was converted to d-spacing using Bragg’s law: hl = 2dsin9. Mechanical testing

Impact strength of the injection moulded izod bars of 4 mm x 10 mm cross sectional dimensions was tested using a Zwick/Roell impact tester under ISO 180/A defined conditions. Tensile tests were performed on a Zwickk/Roell Z020 tensile tester equipped with a 1 kN load cell and dedicated fiber or tensile specimen clamps. The speed of deformation was 50mm/min. The tensile specimens were tested under DIN and EN ISO 527-1 conditions.

Results

Example 1 Filaments

X-ray results of the filaments are shown in Figures 2A, 2B and 2C: as spun filaments (figure 2A), after 15 minutes at 130 °C (figure 2B) and after 30 minutes at 130 °C (figure 2C).

Example 2 Izod bars

X-ray results of the Izod bars are shown in Figures 3A, 3B and 3C: As injection molded (figure 3A), after 15 minutes at 130 °C (figure 3B) and after 30 minutes at 130 °C (figure 3C).

Example 3 Tensile bars

X-ray results of the tensile bars are shown in Figures 4A and 4B. As injection molded (figure 4A) and after 30 minutes at 130 °C (figure 4B).

Table 1 : Mechanical testing

a> did crack, but not completely broke apart.

As described above, another way to characterize the polyamides obtained, is the Crystal Perfection Index (CPI). In table 2, the CPI is calculated for the experiments of the invention and also of the prior art products.

Table 2: Published post-treatment conditions and CPIs in comparison

a) FA is formic acid This table shows, that with the method of the invention a high CPI can be obtained, in a relatively fast process, while retaining, or improving the mechanical properties of the polyamide.

Example 4

Injection molded tensile bars with 4 x 2 mm² cross sectional gauge dimensions, consisting of both pseudohexagonal crystals as defected monoclinic with low crystal perfection index, were exposed to varying post-treatment protocols,

namely: P1 (comparative). MC AN135 - Under inert atmosphere the Melt

Crystallized injected molded tensile bars were exposed to 145°C for 1 hour and cooled to room temperature.

P2 (comparative). MC AN180 - Under inert atmosphere the Melt Crystallized injection molded tensile bars were exposed to 180°C for 1 hour and cooled to room temperature.

P3 (invention). WC 135 - melt crystallized injection molded tensile bars were immersed in de-ionized water and heated in a sealed container to 135 °C. The samples were exposed to the superheated state for 30 minutes and cooled to room temperature.

P4 (invention). WC AN180 - the superheated water treated samples from protocol 3 (WC 135) were exposed to 180 °C for 1 hour in inert atmosphere and successively cooled to room temperature.

The generated tensile bars were characterized by wide angle X-ray diffraction and tensile testing as described above. The results are shown in Table 3 (below) and in Figures 5 and 6.

Figures 5 and 6 and Table 3 conclusively demonstrate that with respect to the“only temperature post-treated” melt-shaped polyamide 6 articles (P1 (me AN145) and P2 (me AN185), comparative tests), the superheated water treatment at (i) 135 °C induces a purely monoclinic crystalline phase with extremely high crystal perfection index and mechanical ductility (large strain), whereas (ii) the additional heating to 180 ^qC increases the E-modules and max stress to well above the best“only temperature” treated polyamide 6 samples. The samples retain the high crystal perfection index. Table 3: Mechanical properties of polyamide 6 post-treated under various conditions

The values presented are the averages of 5 measurements.

Claims

1 . A process for post treatment of a shaped polyamide article comprising exposing the article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm.

2. The process according to claim 1 , wherein the water has a ¹ H NMR Chemical Shift of between 3.6 and 3.2 ppm, preferably of between 3.6 and 3.4 ppm.

3. The process according to claim 1 or 2 , wherein the water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm is water in the fluidic state having a temperature higher than 100 °C and lower than the melting temperature of the polyamide.

4. The process according to claim 3, wherein the water has a temperature of between 105 and 180 TD, preferably of between 120 and 160 °C, more preferably of between 120 and 140 °C.

5. The process according to claim 1 or 2, wherein the water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm is water containing one or more ions, selected from the group of anions consisting of Cl , Br, h, NCV, CIO3 , BrCV, IO3 , CICV and SCN- and the group of cations consisting of K⁺, Na⁺, Li⁺, Ca²⁺ and Ga³⁺, wherein the concentration of ions is from 0.5 to 9 M.

6. The process according to claim 6, wherein the water contains one or more ions selected from the group of anions consisting of Br, h, CICV, and CICV and the group of cations consisting of K⁺, Na⁺, Li⁺ and Ca²⁺.

7. The process according to any one of the preceding claims, wherein the shaped polyamide article is obtained by melting and shaping the polyamide, followed by the post-treatment of the polyamide article by exposing the article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm.

8. The process according to any one of the preceding claims, wherein exposing the article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm is followed by heating the article to a temperature of 160 to 215 °C, preferably 170 to 200 °C.

9. The process according to any one of the preceding claims, wherein the polyamide is polyamide (PA) 6, PA5, PA4, PA66, PA46, PA410, PA1 1 or PA12, preferably polyamide 6.

10. The process according to any of the preceding claims, wherein the time of exposure of the shaped polyamide article to water having a ¹ H NMR Chemical Shift of between 3.7 and 2.9 ppm is between 5 seconds and 1 hour.

1 1 . A shaped polyamide article obtainable with the process according to any one of the preceding claims, wherein the article contains at least 80 v/v.% polyamide having a monoclinic a phase.

12. The shaped polyamide obtainable with the process according to any one of the claims 1 to 10, wherein the article has a Crystal Perfection Index of at least 0.85, preferably at least 0.90 and most preferably at least 0.95.

13. The shaped polyamide obtainable with the process according to any one of the claims 1 to 10, wherein the article has a modulus of at least 3 Gpa.