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Condensation kinetics of polyphthalamides. II. polyesters and diamines.

INTRODUCTION

Polyethylene terephthalate (PET, poly[oxyethyleneoxyterephthaloyl]) is the success story of the last decade. Today, it dominates the markets of fibers and plastic bottles and is consumed in about the same quantity as polypropylene. PET waste is accordingly rich in supply. Its quality is usually high and so postconsumer PET bottles are much in demand. These bottles are either recycled into new bottles or, increasingly, processed into fibers. But PET waste can also be hydrolyzed [1, 2] to recover the terephthalic acid (TPA). Alternatively, PET could be amidated. This work deals with the amidation of PET at high temperatures to the polyphthalamide PA6T (poly [iminoterephtaloyliminohexane-1,6-diyl]) and derivatives thereof by diamines DA (Scheme 1).

This kind of polyester amidation seems attractive since it constitutes a low-price polymer-to-polymer upcycling process leading to semiaromatic or fully aromatic polyamides, both of which occupy special high-price markets. But this process is not used in industries, and only few publications are concerned with it, all on solution systems [3-6]. The reason is that all attempts to amidate PET in the melt resulted so far in crosslinked products.

Since the process is based on the amidation of ester functions, this reaction was investigated in all detail in the first part of this study on methyl esters [7]. The result is summarized in Scheme 2. The polycondensation of diamines and diesters stops early at the stage of oligoamides because it does not only yield the expected monoamide units BDA and, from those, the chain forming diamide units [B.sub.2]DA, but also methylated units BDAMe and BDAM[e.sub.2] and acid units B. These constitute inert chain ends. No catalysts were found to suppress this side reaction of amine alkylation.

It is easy to see why the amine alkylation leads to chain ends, when a methyl ester is amidated, but to crosslinks, when PET is being amidated. In the case of PET, the alkylating alcohol is glycol. The analogs of the methylated amides BDAMe and BDAM[e.sub.2] in Scheme 2 are the hydroxyethylated amides BDAGly and BDAGl[y.sub.2] in Scheme 3.

The units BDAGly and BDAGl[y.sub.2] are still reactive, because of the terminal OH functions. The ethyloldiamine can be built into linear chains just like the diamine itself. The resulting chains are not pure polyamide chains anymore but rather polyesteramide chains. This could be tolerated as a minor imperfection. However, the BDAGl[y.sub.2] units cannot be tolerated because they are crosslinkers: carrying two hydroxy functions, they will create branch points, which eventually can connect to a chain network that crosslinks the entire product.

In this article, a process will be presented where the formation of BDAGl[y.sub.2] is suppressed so linear polyphthalamides can be prepared from PET. The route towards this process will be described. First, it was established that the side reactions indicated in Scheme 2 occur not only with methyl esters but with other alkyl esters as well. Known was only that phenyl esters do not alkylate amines [8]. The model study on methylbenzoate (BMe) reported in Ref. 1 was repeated with ethylbenzoate (BEt), which was reacted at high temperatures with hexamethylene diamine (DA). The ethyl substituent is similar enough to the glycol group in PET to serve as a model. Then, bis-(2-dihydroxyethyl)terephthalate (TGl[y.sub.2]) was condensed with hexamethylene diamine to prove the existence of the units in Scheme 3 directly.

[GRAPHIC OMITTED]

[GRAPHIC OMITTED]

After attempts to amidate molten PET in the extruder, which predictably yielded crosslinked products, a process of amidating solid PET was finally developed. The idea of using solid PET granules came from calculations based on the results of the BEt-DA model study. They predicted that crosslinking can be avoided in systems containing amino functions in excess. When solid PET is being amidated while it dissolves in the diamine, the system reacts automatically under conditions of excessive amine. When all PET is in solution, this excess disappears again. Therefore, long, linear polyamide chains can grow at the end. The kinetics of this heterogeneous process will be discussed.

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EXPERIMENTAL

A polyethylene terephthalate (PET) (Arnite A06, DSM) with a molar mass of [M.sub.w] = 31 kg/mol (GPC, PS scale) and a melting point of [T.sub.m] = 265[degrees]C was used, and hexamethylene diamine (Aldrich) and trimethylhexamethylene diamine (2,2,4- and 2,4,4-trimethyl mixed in the ratio 2:3, hub-Chemie). N,N'-Bis-(6-aminohexyl)terephthalamide (TD[A.sub.2]) was prepared from the dimethyl ester and an excess of hexamethylenediamine. Methanol was distilled off. The product was washed with water to remove the diamine ([T.sub.m] = 240[degrees]C). Polyisophthalamide (PEI, poly[oxyethyleneoxyisophthaloyl]) was prepared from dimethylisophthalate and an excess of glycol at 200-280[degrees]C by distilling off the alcohols. Actually, this PEI was an oligomer with a degree of polymerization of [P.sub.n] [congruent to] 4.

The procedures for the kinetic model study on mixtures BEt-DA of ethylbenzoate and hexamethylenediamine were described in Part I.

The diester TGl[y.sub.2] and hexamethylenediamine were reacted in a glass reactor. For the extrusion runs, a microextruder (Micro 5cc Twin Screw Compounder, DSM Research) and a miniextruder (Rheomex PTW, screw diameter 16 mm, L/D = 25, Haake) were used. The microextruder was actually run like a kneader: the melt flowing through the twin screws is guided back through a bypass. Therefore, the residence time in the extruder can be chosen at random. Samples are taken by opening a die. The melt flows in the extruder over a sensor where the exerted force (F) is measured, which reflects the melt viscosity. The miniextruder was equipped with a side stream dosage for the diamine and a distillation bridge to remove the glycol. In all extruder runs, the polymer was fed into the hopper and plastified, and then the diamine was added. The product was extracted with hexafluoropropanol to extract the soluble fraction.

For the reactor runs with solid PET, an electrically heated three-necked flask was used, equipped with a sturdy blade stirrer, a short reflux condenser with a distillation bridge, a temperature control, and a thermometer. To recover the final product, the flask was smashed in the end (saving the joints).

The concentrations of the ester, amine, and amide functions in the reaction mixtures were measured by determining the ester-amide ratio using FTIR spectroscopy (CO peaks at 1710 [cm.sup.-1] for the ester and 1635 [cm.sup.-1] for the amide) and the amine-amide ratio using [.sup.1.N]MR spectroscopy (N-C[H.sub.2] hydrogens at ([delta]) 3.3-3 ppm for the amine and [delta] = 3.8-3.5 ppm for the amide). Gel chromatograms of the oligoamides were recorded from samples dissolved either in hexafluoropropanol or, after trifluoroacetylation, in THF. The constituents were examined by MALDI spectrometry (matrix assisted laser desorption ionization). In later stages of the polycondensation, where all diamine was consumed, the degree of polymerization was determined by potentiometric titration of the amino functions. The glass transition temperature ([T.sub.g]) of the linear PA6.3T products was measured by DSC (differential scanning calorimetry). Samples for mechanical testing were prepared in the microextruder and the attached injection molding unit. Stress-strain curves were recorded with a tensile tester (Z020, Zwick).

RESULTS AND DISCUSSION

Model System BEt-DA

The reactions in the model system BEt-DA of ethylbenzoate and hexamethylene diamine were carried out, measured, and interpreted as in Part I. The pattern of reactions formulated in Scheme 7 of Part I turned out to be valid for the BEt-DA system as well. The GC curve in Fig. 1 shows peaks of the ethyl versions of all compounds corresponding to the groups shown in Scheme 2 for the methyl ester system. The evaluation of time series of GC curves yields conversion plots as in Fig. 2, which permits the determination of the rate constants for the basic reactions (Eq. A1, Part I). As indicated in Fig. 2, these are, first, the ester amidation leading to the amides BDA and [B.sub.2]DA, second, the amine alkylation leading to the ethylated amides BDAEt and BDAE[t.sub.2] and to the acid B, and third, the amidation of the acid B, again leading to the amides BDA and [B.sub.2]DA. The three rate constants are connected as follows in the BEt-DA system:

[FIGURE 1 OMITTED]

[k.sub.acid] [congruent to] 0.2k [k.sub.alk] = 0.1k. (1)

The first relation means that BEt is amidated five times faster than the acid. As shown in Fig. 3, BMe is even faster. The second relation is very important: it states that the ethylation of the amine, although it is weaker than the methylation (where [k.sub.alk] = 0.2k, Eq. 18, Part I), must be taken seriously. As shown in Fig. 2, it produces BDAEt and BDAE[t.sub.2] in nonnegligible quantities.

Assuming that the rates in the systems BEt-DA and PET-DA are similar, the danger of crosslinking in a stoichiometric PET system can be estimated. The byproduct BDAE[t.sub.2] is present in the final product of the BEt-DA system in a concentration of [c.sub.BDAEt.sub.2] = 3.7%. In the PET system, this byproduct is the crosslinker BDAGl[y.sub.2]. Generally, crosslinking occurs when each chain in a system carries two or more branch points. Therefore, polyamide chains produced by PET amidation should be crosslinked when their degree of polymerization exceeds

[P.sub.n] = 2/[c.sub.BDAGly.sub.2] (2)

which suggests an upper limit of [M.sub.n] [congruent to] 20 kg/mol for linear polyamide products. This estimate is not really encouraging.

But model calculations and test experiments revealed that the content of BDAE[t.sub.2] can be diminished. The concentrations of BDAEt and BDAE[t.sub.2] are shown in Fig. 4 as functions of the composition x of the BEt-DA feed:

[FIGURE 2 OMITTED]

x = [2[c.sub.DA]]/[[c.sub.BEt] + 2[c.sub.DA]]. (3)

BDAE[t.sub.2] is formed in high concentrations only in feeds with an excess of the ester (x < 50%). The reason is that the molecule BDAE[t.sub.2] comes from only one DA but three BEt molecules (two providing the ethyl groups). In a PET-DA mixture with insufficient DA, therefore, the dangerous BDAGl[y.sub.2] will crosslink the product. Unfortunately, this can happen easily also in stoichiometric systems when these have a locally varying composition due to imperfect mixing.

[FIGURE 3 OMITTED]

But in feeds with excessive amine (x > 50%), BDAE[t.sub.2] is almost or entirely absent. This suggests running the PET amidation in mixtures with an excess of DA, to avoid BDAGl[y.sub.2]. It will be demonstrated later that this excess, however only transient, comes about naturally in PET-DA systems with solid PET.

[FIGURE 4 OMITTED]

Crosslinked Polyamides

All stoichiometric systems of diesters or polyesters and diamines, which ended in crosslinking are collected in Scheme 4. In all cases, oligoamides with molar masses below 5 kg/mol could be extracted from the insoluble products. As in Part I, catalysts did not suppress the amine alkylation and thus did not prevent crosslinking.

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Before polyesters were investigated, the diester TGl[y.sub.2] was condensed with the diamine DA, in a reactor, because this mixture was particularly suited to analyze monomeric and oligomeric products from early stages of the condensation. The dialkylated BDAGl[y.sub.2] was detected in the MALDI spectrum in the form of the cyclic derivative BDAGl[y.sub.2cyc] shown in Scheme 5.

After these attempts with monomers, the polymers PEI and PET were amidated in the microextruder. The amidation of such polyesters is an involved process comprising many intermediate reactions, of which the first few are indicated in Scheme 6: the amidation leads to diamine (BDA) and glycol (BGly) chain ends while the side reaction of amine alkylation leads to ethyloldiamine (BGlyDA) and acid (B) chain ends. The chain ends BDA and BGlyDA condense to chain units [B.sub.2]DA and [B.sub.2]GlyDA. An endless sequence of steps follows, among them many isomerizations. Early intermediates still contain ester functions but these are successively converted into amide functions. An example is the isomerization of BGlyDA to BDAGly shown in Scheme 7 where an ethyloldiamine unit is simply turned around.

Eventually, after all amidation reactions have occurred, the PET-DA system arrives at the stage formulated in Scheme 3, which is equivalent to the final stage of the BEt-DA model system. From this stage onwards, the esterification reactions take over [9] whereby the ethyloldiamine units BDAGly and BDAGl[y.sub.2] are built into the chains, the latter causing the crosslinking.

The amidation of the oligoisophthalate PEI by the terephthaldiamide TD[A.sub.2] in the microextruder at 270[degrees]C is characterized in Fig. 5 (TD[A.sub.2] was used instead of DA itself because DA is too volatile and the extruder was not airtight). The ester-amide conversion p was practically complete after 3 min. The molar mass [M.sub.n] of the product increased first, as expected, but then declined again at a time [t.sub.cross] where the system crosslinked, causing the viscosity (measured by the sensor force F in the extruder) to increase strongly. The decline of [M.sub.n] in the late stages is misleading: this [M.sub.n] does not describe the entire product but only the uncrosslinked part that could be extracted.

Microextruder runs with PET and TD[A.sub.2], with esteramine compositions varying from 2:1 to 1:2, confirmed the crosslinking. The PET was at first always strongly degraded but then the amidation proceeded basically as in Fig. 5. Samples taken in the late stages were always insoluble in hexafluoropropanol. Repetition of these experiments in the twin-screw miniextruder, where the throughput (1-5 kg/h) and the screw speed was varied (100-300 rpm), yielded the same results.

Even mixtures with an excess of amino functions yielded crosslinked products which was at odds with the prediction derived from Fig. 4. This raised the suspicion that the crosslinking might be favored by the procedure of mixing: as usual in reactive extrusion processes, the polyester was first plastified, and then the diamide BD[A.sub.2] was fed into the polyester melt. Probably, the polyester-diamine mixture in the feeding zone contained more ester than amine functions, which is exactly the situation to be avoided. Therefore, the procedure was changed entirely.

Linear Polyamides

Instead of molten PET, solid PET granules were used. The extruder was replaced by a reactor, made of glass so the system could be watched. The reaction temperatures were kept within 200-255[degrees]C, below the melting point of PET, to keep the granules from melting. Finally, hexamethylene diamine was replaced to avoid problems with the high crystallinity of the product PA6T (which melts at 370[degrees]C and tends to crystallize from solutions). Instead, trimethylhexamethylene diamine was used (notice: since these two diamines participate in all reactions equally, the trimethylated diamine will still be named DA, in this section). The trimethylated diamine leads to the polyamide PA6.3T (poly[iminoterephtaloylimino(trimethyl)hexane-1,6-diyl]), which is amorphous.

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[GRAPHIC OMITTED]

The description of a PET-DA run at 235[degrees]C may serve to describe the process. A stoichiometric amount of PET granules was poured into the hot, vigorously stirred DA. The PET was gradually dissolved in 30 min, in a peculiar fashion: the granules got smaller in time but remained white and solid until they disappeared. This indicates a mechanism of layer-by-layer dissolution of the granules from the surface. Moreover, the granules did not lump together, despite their high concentration in the system (which contained more PET than DA), and the viscosity remained low up to the late stages where long PA6.3T chains were formed. This is characteristic of a very thin swollen surface layer.

[FIGURE 5 OMITTED]

Under the same conditions, the PET granules behaved totally differently when dissolved in trichloro-benzene. The granules developed a swollen surface, got sticky, and aggregated to big lumps, and the viscosity increased rapidly. The conclusion must be that, in the PET-DA system, the dissolution of PET chains on the surface of the granules is aided by reactions with the diamine DA.

[FIGURE 6 OMITTED]

The PET-DA system is divided into a solid PET phase (containing ester functions [E.sub.sol]) and a liquid DA phase where the dissolved PET (with ester functions [E.sub.liq]) reacts with amino functions N to amide groups A:

[E.sub.sol] [right arrow] [E.sub.liq] [+N.[right arrow]] A.

The evolution of these components in time is shown in Fig. 6a. The ester concentration [c.sub.Esol] was estimated visually, by observing the disappearing PET granules. At the time t*, all PET is dissolved and the system turns homogeneous. The concentrations of [E.sub.liq], N, and A in the liquid phase were determined from samples using FTIR and [.sup.1.H] NMR spectroscopy (Fig. 7).

Only the dissolved ester functions [E.sub.liq] can react, and these are present in the DA phase in quite a low concentration (Fig. 6a). In Fig. 6b, the content x of amine functions in the liquid phase is shown:

x = [c.sub.N]/[[c.sub.N] + [c.sub.E.sub.liq]]. (4)

The diagram demonstrates that, as long as the system is heterogeneous (t < t*), the amine functions N exceed the ester functions [E.sub.liq] (x > 50%). This excess is supposed to prevent crosslinking, according to Fig. 4, and indeed crosslinking was never observed.

The curves in Fig. 6 were calculated assuming that the diameter of the PET granules decreases linearly in time, because of swelling and dissolution. This yields for the volume of the granules and thus for the concentration of the solid ester [E.sub.sol]:

[c.sub.E.sub.sol] = (1 - [t/t*])[.sup.3]. (5)

The whole process, involving the solid-liquid transition [E.sub.sol] [right arrow] [E.sub.liq] and the ester-amide conversion [E.sub.liq] [right arrow] A, is described by the following set of rate equations, which is controlled by the time t* and the rate parameter q for the amidation:

[d[c.sub.E.sub.sol]]/dt = -[3/t*](1 - [t/t*])[.sup.2] [d[c.sub.E.sub.liq]]/dt = -[[d[c.sub.E.sub.sol]]/dt] - [[d[c.sub.A]]/dt]

[d[c.sub.N]]/dt = -[[d[c.sub.A]]/dt] [d[c.sub.A]]/dt = q[c.sub.E.sub.liq][c.sub.N]. (6)

The GPC curves in Fig. 8 reveal more details. The PET entering the liquid phase is not polymeric. In the first few minutes, the DA phase contains only short PET oligomers (Fig. 8a, PE[T.sub.liq]). This proves that the PET chains on the surface of the granules are not just dissolved but are actually split to bits by DA right on the surface. Only these bits enter the DA phase where they are amidated rapidly as witnessed by a new series of PA6.3T oligomers in the GPC curve (Fig. 8b, PA6.3T). The PA6.3T oligomers become later dominant and turn eventually into long PA6.3T chains. Test bars prepared from final products PA6.3 yielded typical values: a glass transition of [T.sub.g] = 150[degrees]C, a Young's modulus of E = 2.5 GPa and a tensile strength of [sigma] = 75 MPa.

The two series of oligomers in Fig. 8 were confirmed by MALDI spectrometry. As expected, all oligomers carried two DA end groups. Notably absent from the MALDI spectra were BDAGl[y.sub.2] end groups, which could have crosslinked the product.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The mechanism is shown schematically in Fig. 9. The chain splitting in the surface of the granules is important because it keeps the surface so thin that the granules do not lump together. For kinetic as well as thermodynamic reasons, the short PET oligochains are dissolved much faster than the long original chains. Therefore, surface swelling is followed immediately by dissolution. A second effect helps keeping the swollen surface layer thin: thermoplastics are generally swollen and dissolved by solvents via a special mechanism of "case II diffusion" where the swelling front between the surface and the interior of the solid remains fairly sharp [10, 11].

The degree of polymerization ([P.sub.n]) of the oligomers and, finally, polymers is shown in Fig. 10 as a function of the ester-amide conversion p. To obtain true [P.sub.n] values, the oligomers in Fig. 8 were used to recalibrate the [M.sub.PS] axis. The curve in Fig. 10 was calculated with an extended version of Eq. 6 based on the difunctional constituents, namely the chain units [B.sub.2]Gly and [B.sub.2]DA, the chain ends BDA and BGly (neglecting BDAGly) and the diamine DA. These constituents are related to the monomeric units by

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[E.sub.sol] = [c.sub.[B.sub.2]Gly,sol] [E.sub.wet] = [c.sub.[B.sub.2]Gly,liq] + [[c.sub.BGly]/2]

N = [c.sub.DA] + [[c.sub.BDA]/2] A = p = [c.sub.[B.sub.2]DA] + [[c.sub.BDA]/2] (7)

[FIGURE 11 OMITTED]

and the degree of polymerization results from the ratio of chain ends and chain units:

[P.sub.n] = 1 + [[[c.sub.[B.sub.2]Gly,liq] + [c.sub.[B.sub.2]DA]]/[[c.sub.BDA] + [c.sub.BGly]]]. (8)

The curve in Fig. 10 fits the data quite well. This curve looks very much like that in Fig. 6a of Part I for the polycondensation of the TPA. Nothing in Fig. 10 hints to the fact that the PET-DA mixture contains at the start a long-chained polymer. This suggests that polyphthalamides can be produced either from TPA or from solid PET in much the same process, perhaps even in the same factory.

Also, the rates of the TPA and the heterogeneous PET amidation are similar. In Fig. 11, the half-life times [[tau].sub.amid] (from Eq. 6 with Eq. 11 of Part I) for the amidation of PET and TPA are compared. PET is inherently faster, in fact even a little faster than BEt (Fig. 3) but the PET amidation is slowed down because the granules must be dissolved first.

Fig. 11 also shows the half-life time [[tau].sub.sol-liq] of the dissolution process. At temperatures >200[degrees]C, the amidation is faster. This ensured thin swollen surface layers and prevented aggregation of the granules. But at 200[degrees]C, the amidation is too slow with disagreeable consequences.

As PET granules were poured into DA at 200[degrees]C, the granules lumped together and the viscosity of the liquid phase increased rapidly. This suggests that the PET chains were dissolved at 200[degrees]C without being directly attacked by DA on the surface of the granules. Because the swelling and lumping reduces the overall surface of the granules, the amidation was slowed down.

Since one big advantage of the amidation of solid PET is that it proceeds basically like the polycondensation of TPA, this means that the temperature range for the process is limited to 210-255[degrees]C (above which PET melts).

CONCLUSIONS

PET cannot properly be amidated by diamines in the melt. Because of a side reaction of amine alkylation, the reaction tends to yield only crosslinked polyphthalamide products. A systematic investigation of this side reaction led to the prediction that the crosslinking can be avoided in PET-DA mixtures with an excess of amine. Therefore, solid PET granules were amidated, at temperatures not far below the melting point. In the heterogeneous PET-DA dispersions, the amidation proceeds on the surface of the solid granules and in the liquid DA phase. In this situation, the reactions take place under conditions of amine excess, even in stoichiometric PET-DA mixtures. When the PET granules are completely dissolved, the system is stoichiometric. Long-chained, linear polyphthalamides could be prepared in this manner from PET.

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Edith Hellmann, Jan Malluche, Goetz Peter Hellmann

Deutsches Kunststoff-Institut (DKI), Schlossgartenstrasse 6, D-64289 Darmstadt, Germany

Correspondence to: G.P. Hellmann: e-mail: ghellmann@dki.tu-darmstadt.de

Contract grant sponsor: Federal Ministry of Economics and Labour through the Federation of Industrial Cooperative Research Associations (Arbeitsgemeinschaft industrieller Forschungsvereinigungen "Otto von Guericke" e.V.); contract grant number: AiF-No 13563
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Author:Hellmann, Edith; Malluche, Jan; Hellmann, Goetz Peter
Publication:Polymer Engineering and Science
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Date:Oct 1, 2007
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