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Polyethylene like polymers. Aliphatic polyesters of dodecanedioic acid
European Polymer Journal 39 (2003) 655–661 www.elsevier.com/locate/europolj
Polyethylene like polymers. Aliphatic polyesters of dodecanedioic acid 1. Synthesis and properties Giancarlo Barbiroli
a,*
, Cesare Lorenzetti a, Corrado Berti b, Maurizio Fiorini b, Piero Manaresi b,*
a
Dipartimento di Discipline Economico Aziendali, Area Tecnologia e Valorizzazione delle Risorse, Universit a di Bologna, Piazza Scaravilli 2, 40126 Bologna, Italy b Dipartimento di Chimica Applicata e Scienza dei Materiali, Universit a di Bologna, Via Risorgimento 2, Bologna, Italy Received 1 August 2002; received in revised form 11 September 2002; accepted 23 September 2002
Abstract A series of aliphatic polyesters has been synthesized starting from 1,12-dodecanedioic acid and aliphatic diols, bearing from 2 to 12 carbon atoms. These polymers, which were fully characterized in terms of chemical structure, molecular weight and thermal behaviour, were obtained as crystalline materials with melting points ranging from 70 to 90 °C and with a relatively high molecular weight. All the monomers used can be obtained from biomasses, as a consequence these materials can be an interesting alternative to synthetic polymers produced from petrochemical processes based on nonrenewable resources. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aliphatic polyesters; 1,12-dodecanedioc acid; Chemical recycle
1. Introduction A very important task for industrial polymer chemistry in the near future is represented by the preparation of easily recyclable (or biodegradable) polymers starting from biomasses sources. These materials are very attracting because they offer the opportunity for facilitated recovery of the starting monomers, thus avoiding the loss of nonrenewable resources on which is based the current polymer industrial chemistry. Interesting materials can be represented by polymer containing ester functionalities into the polymer backbone; these esters groups can be considered a structure weak point that can be subsequently cleaved by chemical or enzymatic
* Corresponding authors. Tel.: +39-51-2098053; fax: +39051-222949. E-mail addresses: [email protected], barbiroli@ economia.unibo.it (G. Barbiroli).
hydrolysis, allowing the recovery of the starting monomers or the biodegradation to harmless by-products. Fully aliphatic polyester match the characteristics above considered; these polymers have been known for many years since the pioneering work of Carothers [1] but up to now they are not extensively used. Considering that polyethylene (PE) is the most widely used synthetic polymer, being produced from petrochemical processes starting from nonrenewable resources, we considered the possibility of preparing aliphatic polyesters from long chain aliphatic acids and diols, with chemical and physical properties that allow them to be used as surrogate of PE or other polyolefins in some specific applications. In the following, for our polymers we use the expression ‘‘PE like polyesters’’ as proposed by other authors [2–5] for similar system. Since the 80s academic and industrial research has been dedicated to polymers having that set of properties. This research field led to the development of a few commercial products: ‘‘Bionolleâ ’’ [6], an aliphatic polyester
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 2 8 0 - X
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G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661
based on adipic acid and various diols is produced by the Japanese industry ‘‘Showa High Polymer Co. Ltd.’’; a similar product is commercialized under the trade name of ‘‘Sky Greenâ ’’ [7] and it is manufactured by a South Korean Industry ‘‘SK Chemicals Ltd.’’. Another important example is represented by poly(hydroxyalcanoates) developed by ICI and now commercialized by Monsanto under the trade name Biopolâ . The main features of these polyesters is the biodegradability in short time and the possibility of use in compost preparation, along with other organic wastes. Other important polyesters can be prepared starting from lactic acid or its dimer lactides. These polymers are finding some important applications as sutures and for temporary reabsorbable implants [8]; other biomedical applications have also been studied for aliphatic polyesteramides based on 1,12-dodecanedioic acid, 1,12-dodecanediol and L -alanine [9]. The aim of the work described in this paper was the preparation of easily processable thermoplastic materials having a low environmental impact, i.e. recyclable and possibly biodegradable polymers. For this reason, a wide series of aliphatic polyesters has been synthesized starting from 1,12-dodecanedioic acid and aliphatic diols bearing from 2 to 12 carbon atoms. A particular attention has been devoted to the optimization of the synthesis to minimize side reactions and to obtain high molecular weight polymers. Several catalytic systems have been tested and the use of a low boiling point diol for the synthesis with 1,12-dodecanediol has also been investigated, in order to reduce reaction time and the final polymerisation temperature. All the monomers used might be directly or indirectly obtainable from renewable sources at relatively low cost, especially considering biotechnology developments [10– 12]. 1,12-dodecanedioic acid was reacted with ethanediol, propanediol, butanediol, hexanediol, octanediol, decanediol and dodecanediol. These polymers, which were fully characterized in terms of chemical structure, molecular weight and thermal behaviour were obtained as crystalline materials with a high molecular weight. Hydrolysis tests, under basic conditions, were also performed in order to evaluate potential monomer recovery.
2. Experimental section Ethanediol (ED, chemical purity 99%), 1,3-propanediol (98%), 1,4-butanediol (99%), 1,6-hexanediol (99%), 1,10-decanediol (98%), zinc acetate (ZnAc, 99.9%), dibutyl tin oxide (DBTO, 98%), antimony trioxide (99%), titanium (IV) tetrabutoxide (TBT, 97%) were purchased from Aldrich. 1,8-octanediol (98%), 1,12-dodecanediol (DD, 98%), 1,12-dodecanedioic acid (DA, 98%) were
purchased from Fluka. Low-density polyethylene (LDPE), RIBLENE FL34, was a commercial sample produced by Polimeri Europa. All the reagents were used as received without further purification. 2.1. Polyester syntheses Polycondensations were carried out by a two stage process in a 200 ml Pyrex glass reactor, equipped with a mechanical paddle stirrer and torque gauge which gives an indication of the viscosity of the reaction melt. As a typical procedure we report in the following paragraph the polymerization details of DA and DD. For this system the synthesis was also performed with small amounts of ethanediol in order to decrease the polymerization time and reaction temperature. In the following the polymers are labelled using a notation PEL accompanied by two digits, the first referred to the number of carbon atoms of the moieties deriving from the diol and the second from the acid. 2.2. Synthesis of PEL 12,12 DA (40.00 g, 0.1737 mol), DD (35.86 g, 0.1772 mol), DBTO (0.1892 g, 7.600 104 mol) were introduced into the reactor under nitrogen atmosphere and the reactor was then immersed into a silicone oil bath preheated at 200 °C. The first stage of the process was conducted at atmospheric pressure and the mixture was allowed to react for 220 min under stirring with continuous removal of water. After that time, the second stage was started by gradually reducing the pressure down to 0.02 mbar and the temperature was raised to the final value of 260 °C. These conditions were reached within 90 min, using a linear gradient of temperature and pressure, and maintained for 150 min. Then, the pale yellow polymer was unloaded from the reactor using a spatula. 2.3. Synthesis of PEL 12,12 in the presence of ethanediol DA (40.00 g, 0.1737 mol), DD (35.15 g, 0.1737 mol), ethanediol (1.615 g, 0.02606 mol), DBTO (0.1892 g, 7.600 104 mol) were introduced into the reactor and the process was carried out as described for the synthesis without comonomers. For this system the final temperature was 240 °C and after 150 min. at reduced pressure the polymer was unloaded. 2.4. Alkaline hydrolysis Alkaline hydrolyses were performed on polymers obtained from DD and DA (Mw 131,800) with the aim of studying the monomer recovery.
G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661
The depolymerisation was carried out as follows: 70 ml of 1 M solution of NaOH in methanol were poured in a 100 ml three-neck round-bottom flask equipped with a cooled condenser. 5.00 g of PEL 12,12 previously ground to 1–2 mm size, were added to the solution and heated under reflux with magnetic stirring. After 1 h the reaction was stopped; at this stage the reaction mixture looked very hazy and contained very fine suspended particles. After cooling to room temperature, the methanol volume was reduced to 30 ml in a rotary evaporator. 350 ml of distilled water were added to the suspension and the insoluble fraction was separated by filtration, washed with water until the filtrate was neutral and then dried overnight at 60 °C. The alkaline filtrate was neutralized with HCl 10% (w/w), then the precipitated formed was separated by filtration, washed with water and then dried. The two products were analysed with FT-IR and 1 H-NMR and were found to be 1,12-dodecanediol and 1,12-dodecanedioic acid, respectively. The amount of monomers recovered after alkaline hydrolysis was 2.73 g for the DD (94% yield) and 2.37g for DA (93%). The same procedure was repeated for PEL 2,12 and in this case the yield for the recovery of DA was equal to 97%; the ethanediol was not recovered for quantitative analysis. 2.5. Measurements 1 H-NMR spectra were recorded on a Varian XL-200 spectrometer in CDCl3 solutions. FT-IR spectra were recorded on thin films from chloroform solutions on NaCl disks with of a Perkin Elmer Spectrum One FT-IR spectrometer. Molecular weights were determined by GPC analysis using a calibration plot constructed with narrow molecular weight distribution polystyrene standards. The GPC measurements were carried out with a Hewlett Packard 1100 liquid chromatography instrument using a Polymer Labs PL Gel 5 lm mixed-C column (300/7.5 length/id., in mm). Chloroform was used as eluent at a flow rate of 1 ml min1 20 ll of a 0.2% polymer solution were injected. Differential scanning calorimetry (DSC)
657
analysis was performed using a Perkin Elmer DSC7, calibrated with high purity standards (indium and phenantrene). Dry nitrogen was used as purge gas. The samples (about 10 mg) were heated from room temperature to 130 °C at 20 °C min1 (1st scan) and then rapidly quenched to )30 °C at 100 °C min1 . The samples were then heated from )30 to 130 °C at 10 °C min1 (2nd scan). The melting temperature was determined during the 2nd scan as peak value; similarly DHm was determined as peak area in the same curve. Crystallization temperature was determined as peak value during the cooling step. All the measurements performed have shown excellent reproducibility. 2.6. Blends of polyethylene and PEL 12,12 Miscibility tests were performed on LDPE-PEL 12,12 blends with composition 75/25, 50/50 and 25/75 w/w. The component of each blend, total mass of about 1 g, were dry-mixed and introduced into a preheated cuprotor Minimax laboratory mixer preheated at 150 °C. Blends were mixed for 10 min at 100 rpm. At the end of the blending process, the blends were extruded through an orifice at the bottom of the mixing bowl into thread of about 3 mm diameter. Samples for SEM analysis were prepared by fracturing the thread in liquid nitrogen. The fracture surface of each sample was exposed to chloroform vapours to dissolve the PEL phase prior to metallization by sputtering with gold. SEM micrographs were recorded with a JEOL JSM-5 scanning electron microscope at 5.0 kV.
3. Results and discussion The polycondensations were carried out by a meltphase two-step process in the presence of an esterification catalyst (see experimental part). In Table 1 experimental conditions used for the synthesis of PEL samples are reported. These data refer to DBTO which resulted to be the best polymerisation catalyst. During the first step, the direct esterification occurred with elimination of water as
Table 1 Experimental conditions used for the synthesis of PEL samples Diol 1,2-Ethanediol 1,3-Propanediol 1,4-Butanediol 1,6-Hexanediol 1,8-Octanediol 1,10-Decanediol 1,12-Dodecanediol a
Acronym 2,12 3,12 4,12 6,12 8,12 10,12 12,12
Respect to 1,12-dodecanedioic acid.
Excess diol (mol%)a
1st stage t (°C)
time (min)
t (°C)
time (min)
40 30 20 8 5 3 2
200 200 200 200 200 200 200
120 140 140 180 220 220 220
240 240 240 250 260 260 260
140 150 180 210 210 230 240
2nd stage
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G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661
Fig. 1. Comparison of the catalytic activity of: DBTO (}); TBT (); ZnAc () and Sb2 O3 ().
by-product. The stoichiometric excess of the diol was decreased according to the reduction of the volatility. In other words, for more volatile diols we used a larger excess to reduce the reaction time; for less volatile diols the excess was reduced due to the difficult removal of the excess to reach the correct stoichiometry. For PEL 2,12 synthesis, 40% excess of ethanediol was used and the total time of first stage was 120 min; in the case of PEL 12,12 the excess of 1,12-dodecanenediol was 2% and the first stage was carried on for 220 min. The second stage of the reaction was completed at reduced pressure (0.02 mbar) and the temperature was raised up to 240–260 °C; depending on the used diol, these conditions were maintained for different times in order to complete the
Fig. 2. 1 H-NMR and FT-IR spectra of PEL 12, 12.
G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661
elimination of the water and of the excess of the diol (see Table 1). An important part of this study was dedicated to the optimization of the catalytic system in order to improve the reaction rate and minimize the side reactions. We tested several well known catalysts for esterification process, in particular we tested TBT, DBTO, ZnAc, and Sb2 O3 . The polymerization tests were conducted using 1,12-dodecanedioic acid and 1,12-dodecanediol as monomers, because the resulting polymer was the most difficult to prepare and also because its structure is the most analogous to PE. For the comparison of the results we carried out the reactions under the same conditions, specifically using the same catalyst concentration and following the same temperature and pressure gradient. The activity of the single catalyst was determined by measuring the molecular weight at the end of the first and of the second stage. The measured values are reported in Fig. 1. From the analysis of the data obtained the most active catalyst turned out to be DBTO followed by TBT and ZnAc which gave similar results, whereas Sb2 O3 was the least active. Following this procedure we obtained PEL samples of high molecular weight with a pale yellow colour. Polymers were characterised by FT-IR, 1 H-NMR, GPC and DSC. The FT-IR spectra were consistent with the expected structures with a strong band in the region of 1720–1730 cm1 typical of aliphatic ester groups; also the NMR spectra were consistent with the proposed structure. The NMR and FT-IR spectra of PEL 12,12 are reported in Fig. 2. The molecular weight of the polymers was determined by GPC using polystyrene standards for column calibration. We obtained PEL samples with Mw ranging from 64,000 to 125,000; the final value of the molecular weight depends on the final temperature and the total time of the process. The synthesis of polyesters deriving from long chain diols bearing from 8 to 12 carbon atoms can also be improved by the use of a second, more volatile diol. The presence in the reaction mixture of a second more volatile diol represents a process change which generally permits to obtain high molecular weight polyesters with shorter reaction time at lower temperature. In fact, at the beginning of the reaction, both the second diol and long chain diol react with DA forming hydroxyl-terminated oligomers. At the end of the polymerization, transesterification reactions free the second diol, which is almost completely removed by distillation. Following this approach we reacted DA with DD in 1:1 molar ratio with the addition of 15% mol excess of ED respect to the acid (see experimental part) obtaining PEL 12,12ED having molecular weight comparable to PEL 12,12 (see Table 2) but within shorter reactions times (150 compared to 240 min) with a significant lower temperature (240 vs. 260 °C). From the data reported in Table 2, it can be seen that the amount of ED inserted is
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Table 2 Molecular weight and thermal analysis data for PEL samples Polymer PEL
Mn
Mw
Tm (°C)
Tc (°C)
DHm (J/g)
2,12 3,12 4,12 6,12 8,12 10,12 12,12 12,12 EDa
42,950 37,570 56,730 47,390 23,520 39,050 45,580 77,900
75,550 81,600 95,820 84,910 64,490 85,420 81,000 125,400
85.0 66.7 77.4 77.4 76.1 83.1 88.7 86.4
15.0 9.7 24.3 24.3 33.0 29.7 36.3 37.1
66.7 66.9 106.7 106.7 96.7 110.7 130.0 110.8
a
0.9 mol% ethanediol inserted.
very low (0.9% mol); therefore this approach seems to be useful and very efficient. Properties of PEL samples were also characterized by DSC. The data obtained, reported in Table 2, are in good agreement with those reported in the literature for similar materials [2–4]. The chemical recovery of the monomers was carried out by hydrolysis in alkaline media (see experimental part); the depolymerisation reactions were carried out on PEL 12,12 suspended in 1 M NaOH in methanol.
Fig. 3. Proposed life-cycle analysis of PEL polyesters prepared from renewable resources.
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Under these conditions the two monomers were recovered in very high yield (94–96%). These results open up the opportunity of the proposed closed-loop life cycle represented in Fig. 3. Starting from biomass, monomers can be synthesized, polymerized and after the use they can be recovered by depolymerisation and recycled back to the polycondensation step. Even if we have not carried out biodegradation tests, it is reported that aliphatic polyesters are generally known to be susceptible to biological attack [4]. Therefore after their use PEL disposed off into the environment could be biodegraded by biological processes. All these concept are summarized in Fig. 3. We have investigated the miscibility of PEL 12,12 with LDPE in different compositions. It has been reported
that long chain aliphatic polyesters give rise to the formation of solid phases with morphological similitudes to PE [2,3]. Blends were prepared by melt mixing PEL with LDPE and the morphology of the surfaces of criofractured samples was analyzed by SEM (see experimental part). The micrographs, reported in Fig. 4, clearly show immiscibility for all tested compositions similarly to blends of PE and commercial aliphatic polyesters (Bionolle) [13]. In particular, in Fig. 4A, the dispersed PEL phase in the LDPE matrix has been removed by etching with chloroform vapours leaving several holes onto the surface. At LDPE richer compositions, Fig. 4B and C, the PEL 12,12 forms the continuous phase with almost spherical PE domains irregularly distributed.
4. Conclusions A series of aliphatic polyesters based on 1,12-dodecanedioic acid and aliphatic diols bearing from 2 to 12 carbon atoms were obtained with high molecular weight. Among the tested catalysts the dibutyl tin oxide turned out to be the most active. The synthetic process was improved by the use of a volatile diol which allowed shorter reaction times and lower polymerization temperature. The alkaline hydrolysis of PEL 12,12, which can be considered in our opinion the most interesting polyester of the series, gives rise to the recovery of the starting monomers in almost quantitative yields confirming the potentially easy chemical recycle of this class of polymers. In our opinion, the results of this work confirm the interest on aliphatic polyesters. The easy and relatively low cost polycondensation coupled with the simple and almost quantitative recovery of the monomers make this materials quite interesting. Future work will be dedicated to this class of polymers, particularly to their chemical modification using different monomers and to their application.
References
Fig. 4. SEM micrographs of LDPE-PEL blends of different compositions (wt/wt): A: 75/25; B: 50/50; C: 25/75.
[1] Mark H, Whitby GS. In: Mark H, Whitby GS, editors. Collected papers of WH Carothers on high polymeric substances, vol. 1. New York: Interscience; 1940. [2] Le Fevere de Ten Hove C, Jonas A, Penelle J. J Polym Prep (Am Chem Soc, Div Polym Chem) 1998;39:156–7. [3] Le Fevere de Ten Hove C, Jonas C, Penelle A. J Proc Am Chem Soc: Div Polym Mater Sci Eng 1997;76:158–9. [4] Cho I, Lee K. Macromol Chem Phys 1997;198:861–9. [5] Skoglund P, Fransson A. Polymer 1998;39(10):1899–906. [6] Scherer TM, ClintonFuller R, Lenz RW, Goodwin S. Polym Degrad Stab 1999;64:267–75. [7] Kim M, Lee Ae-Ri, Yoon J, Chin I. Eur Polym J 2000;36: 1677–85. [8] S€ oderg ard A, Stolt M. Prog Polym Sci 2002;27(6):1123–63.
G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661 [9] Rodriguez-Galan A, Fuentes L, Puigallı J. Polymer 2000; 41:5967–70. [10] Jetter R, Riederer M. Phytochemistry 1999;52:907–15. [11] Jetter R. Phytochemistry 2002;55:169–76.
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[12] McKiernan RL, Gido SP, Penelle J. Polymer 2002;43(10): 3007–17. [13] Kim J, Kim JH, Shin TK, Choi HJ, Jhon MS. Eur Polym J 2001;37:2131–9.
Polyethylene like polymers. Aliphatic polyesters of dodecanedioic acid 1. Synthesis and properties Giancarlo Barbiroli
a,*
, Cesare Lorenzetti a, Corrado Berti b, Maurizio Fiorini b, Piero Manaresi b,*
a
Dipartimento di Discipline Economico Aziendali, Area Tecnologia e Valorizzazione delle Risorse, Universit a di Bologna, Piazza Scaravilli 2, 40126 Bologna, Italy b Dipartimento di Chimica Applicata e Scienza dei Materiali, Universit a di Bologna, Via Risorgimento 2, Bologna, Italy Received 1 August 2002; received in revised form 11 September 2002; accepted 23 September 2002
Abstract A series of aliphatic polyesters has been synthesized starting from 1,12-dodecanedioic acid and aliphatic diols, bearing from 2 to 12 carbon atoms. These polymers, which were fully characterized in terms of chemical structure, molecular weight and thermal behaviour, were obtained as crystalline materials with melting points ranging from 70 to 90 °C and with a relatively high molecular weight. All the monomers used can be obtained from biomasses, as a consequence these materials can be an interesting alternative to synthetic polymers produced from petrochemical processes based on nonrenewable resources. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aliphatic polyesters; 1,12-dodecanedioc acid; Chemical recycle
1. Introduction A very important task for industrial polymer chemistry in the near future is represented by the preparation of easily recyclable (or biodegradable) polymers starting from biomasses sources. These materials are very attracting because they offer the opportunity for facilitated recovery of the starting monomers, thus avoiding the loss of nonrenewable resources on which is based the current polymer industrial chemistry. Interesting materials can be represented by polymer containing ester functionalities into the polymer backbone; these esters groups can be considered a structure weak point that can be subsequently cleaved by chemical or enzymatic
* Corresponding authors. Tel.: +39-51-2098053; fax: +39051-222949. E-mail addresses: [email protected], barbiroli@ economia.unibo.it (G. Barbiroli).
hydrolysis, allowing the recovery of the starting monomers or the biodegradation to harmless by-products. Fully aliphatic polyester match the characteristics above considered; these polymers have been known for many years since the pioneering work of Carothers [1] but up to now they are not extensively used. Considering that polyethylene (PE) is the most widely used synthetic polymer, being produced from petrochemical processes starting from nonrenewable resources, we considered the possibility of preparing aliphatic polyesters from long chain aliphatic acids and diols, with chemical and physical properties that allow them to be used as surrogate of PE or other polyolefins in some specific applications. In the following, for our polymers we use the expression ‘‘PE like polyesters’’ as proposed by other authors [2–5] for similar system. Since the 80s academic and industrial research has been dedicated to polymers having that set of properties. This research field led to the development of a few commercial products: ‘‘Bionolleâ ’’ [6], an aliphatic polyester
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 2 8 0 - X
656
G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661
based on adipic acid and various diols is produced by the Japanese industry ‘‘Showa High Polymer Co. Ltd.’’; a similar product is commercialized under the trade name of ‘‘Sky Greenâ ’’ [7] and it is manufactured by a South Korean Industry ‘‘SK Chemicals Ltd.’’. Another important example is represented by poly(hydroxyalcanoates) developed by ICI and now commercialized by Monsanto under the trade name Biopolâ . The main features of these polyesters is the biodegradability in short time and the possibility of use in compost preparation, along with other organic wastes. Other important polyesters can be prepared starting from lactic acid or its dimer lactides. These polymers are finding some important applications as sutures and for temporary reabsorbable implants [8]; other biomedical applications have also been studied for aliphatic polyesteramides based on 1,12-dodecanedioic acid, 1,12-dodecanediol and L -alanine [9]. The aim of the work described in this paper was the preparation of easily processable thermoplastic materials having a low environmental impact, i.e. recyclable and possibly biodegradable polymers. For this reason, a wide series of aliphatic polyesters has been synthesized starting from 1,12-dodecanedioic acid and aliphatic diols bearing from 2 to 12 carbon atoms. A particular attention has been devoted to the optimization of the synthesis to minimize side reactions and to obtain high molecular weight polymers. Several catalytic systems have been tested and the use of a low boiling point diol for the synthesis with 1,12-dodecanediol has also been investigated, in order to reduce reaction time and the final polymerisation temperature. All the monomers used might be directly or indirectly obtainable from renewable sources at relatively low cost, especially considering biotechnology developments [10– 12]. 1,12-dodecanedioic acid was reacted with ethanediol, propanediol, butanediol, hexanediol, octanediol, decanediol and dodecanediol. These polymers, which were fully characterized in terms of chemical structure, molecular weight and thermal behaviour were obtained as crystalline materials with a high molecular weight. Hydrolysis tests, under basic conditions, were also performed in order to evaluate potential monomer recovery.
2. Experimental section Ethanediol (ED, chemical purity 99%), 1,3-propanediol (98%), 1,4-butanediol (99%), 1,6-hexanediol (99%), 1,10-decanediol (98%), zinc acetate (ZnAc, 99.9%), dibutyl tin oxide (DBTO, 98%), antimony trioxide (99%), titanium (IV) tetrabutoxide (TBT, 97%) were purchased from Aldrich. 1,8-octanediol (98%), 1,12-dodecanediol (DD, 98%), 1,12-dodecanedioic acid (DA, 98%) were
purchased from Fluka. Low-density polyethylene (LDPE), RIBLENE FL34, was a commercial sample produced by Polimeri Europa. All the reagents were used as received without further purification. 2.1. Polyester syntheses Polycondensations were carried out by a two stage process in a 200 ml Pyrex glass reactor, equipped with a mechanical paddle stirrer and torque gauge which gives an indication of the viscosity of the reaction melt. As a typical procedure we report in the following paragraph the polymerization details of DA and DD. For this system the synthesis was also performed with small amounts of ethanediol in order to decrease the polymerization time and reaction temperature. In the following the polymers are labelled using a notation PEL accompanied by two digits, the first referred to the number of carbon atoms of the moieties deriving from the diol and the second from the acid. 2.2. Synthesis of PEL 12,12 DA (40.00 g, 0.1737 mol), DD (35.86 g, 0.1772 mol), DBTO (0.1892 g, 7.600 104 mol) were introduced into the reactor under nitrogen atmosphere and the reactor was then immersed into a silicone oil bath preheated at 200 °C. The first stage of the process was conducted at atmospheric pressure and the mixture was allowed to react for 220 min under stirring with continuous removal of water. After that time, the second stage was started by gradually reducing the pressure down to 0.02 mbar and the temperature was raised to the final value of 260 °C. These conditions were reached within 90 min, using a linear gradient of temperature and pressure, and maintained for 150 min. Then, the pale yellow polymer was unloaded from the reactor using a spatula. 2.3. Synthesis of PEL 12,12 in the presence of ethanediol DA (40.00 g, 0.1737 mol), DD (35.15 g, 0.1737 mol), ethanediol (1.615 g, 0.02606 mol), DBTO (0.1892 g, 7.600 104 mol) were introduced into the reactor and the process was carried out as described for the synthesis without comonomers. For this system the final temperature was 240 °C and after 150 min. at reduced pressure the polymer was unloaded. 2.4. Alkaline hydrolysis Alkaline hydrolyses were performed on polymers obtained from DD and DA (Mw 131,800) with the aim of studying the monomer recovery.
G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661
The depolymerisation was carried out as follows: 70 ml of 1 M solution of NaOH in methanol were poured in a 100 ml three-neck round-bottom flask equipped with a cooled condenser. 5.00 g of PEL 12,12 previously ground to 1–2 mm size, were added to the solution and heated under reflux with magnetic stirring. After 1 h the reaction was stopped; at this stage the reaction mixture looked very hazy and contained very fine suspended particles. After cooling to room temperature, the methanol volume was reduced to 30 ml in a rotary evaporator. 350 ml of distilled water were added to the suspension and the insoluble fraction was separated by filtration, washed with water until the filtrate was neutral and then dried overnight at 60 °C. The alkaline filtrate was neutralized with HCl 10% (w/w), then the precipitated formed was separated by filtration, washed with water and then dried. The two products were analysed with FT-IR and 1 H-NMR and were found to be 1,12-dodecanediol and 1,12-dodecanedioic acid, respectively. The amount of monomers recovered after alkaline hydrolysis was 2.73 g for the DD (94% yield) and 2.37g for DA (93%). The same procedure was repeated for PEL 2,12 and in this case the yield for the recovery of DA was equal to 97%; the ethanediol was not recovered for quantitative analysis. 2.5. Measurements 1 H-NMR spectra were recorded on a Varian XL-200 spectrometer in CDCl3 solutions. FT-IR spectra were recorded on thin films from chloroform solutions on NaCl disks with of a Perkin Elmer Spectrum One FT-IR spectrometer. Molecular weights were determined by GPC analysis using a calibration plot constructed with narrow molecular weight distribution polystyrene standards. The GPC measurements were carried out with a Hewlett Packard 1100 liquid chromatography instrument using a Polymer Labs PL Gel 5 lm mixed-C column (300/7.5 length/id., in mm). Chloroform was used as eluent at a flow rate of 1 ml min1 20 ll of a 0.2% polymer solution were injected. Differential scanning calorimetry (DSC)
657
analysis was performed using a Perkin Elmer DSC7, calibrated with high purity standards (indium and phenantrene). Dry nitrogen was used as purge gas. The samples (about 10 mg) were heated from room temperature to 130 °C at 20 °C min1 (1st scan) and then rapidly quenched to )30 °C at 100 °C min1 . The samples were then heated from )30 to 130 °C at 10 °C min1 (2nd scan). The melting temperature was determined during the 2nd scan as peak value; similarly DHm was determined as peak area in the same curve. Crystallization temperature was determined as peak value during the cooling step. All the measurements performed have shown excellent reproducibility. 2.6. Blends of polyethylene and PEL 12,12 Miscibility tests were performed on LDPE-PEL 12,12 blends with composition 75/25, 50/50 and 25/75 w/w. The component of each blend, total mass of about 1 g, were dry-mixed and introduced into a preheated cuprotor Minimax laboratory mixer preheated at 150 °C. Blends were mixed for 10 min at 100 rpm. At the end of the blending process, the blends were extruded through an orifice at the bottom of the mixing bowl into thread of about 3 mm diameter. Samples for SEM analysis were prepared by fracturing the thread in liquid nitrogen. The fracture surface of each sample was exposed to chloroform vapours to dissolve the PEL phase prior to metallization by sputtering with gold. SEM micrographs were recorded with a JEOL JSM-5 scanning electron microscope at 5.0 kV.
3. Results and discussion The polycondensations were carried out by a meltphase two-step process in the presence of an esterification catalyst (see experimental part). In Table 1 experimental conditions used for the synthesis of PEL samples are reported. These data refer to DBTO which resulted to be the best polymerisation catalyst. During the first step, the direct esterification occurred with elimination of water as
Table 1 Experimental conditions used for the synthesis of PEL samples Diol 1,2-Ethanediol 1,3-Propanediol 1,4-Butanediol 1,6-Hexanediol 1,8-Octanediol 1,10-Decanediol 1,12-Dodecanediol a
Acronym 2,12 3,12 4,12 6,12 8,12 10,12 12,12
Respect to 1,12-dodecanedioic acid.
Excess diol (mol%)a
1st stage t (°C)
time (min)
t (°C)
time (min)
40 30 20 8 5 3 2
200 200 200 200 200 200 200
120 140 140 180 220 220 220
240 240 240 250 260 260 260
140 150 180 210 210 230 240
2nd stage
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Fig. 1. Comparison of the catalytic activity of: DBTO (}); TBT (); ZnAc () and Sb2 O3 ().
by-product. The stoichiometric excess of the diol was decreased according to the reduction of the volatility. In other words, for more volatile diols we used a larger excess to reduce the reaction time; for less volatile diols the excess was reduced due to the difficult removal of the excess to reach the correct stoichiometry. For PEL 2,12 synthesis, 40% excess of ethanediol was used and the total time of first stage was 120 min; in the case of PEL 12,12 the excess of 1,12-dodecanenediol was 2% and the first stage was carried on for 220 min. The second stage of the reaction was completed at reduced pressure (0.02 mbar) and the temperature was raised up to 240–260 °C; depending on the used diol, these conditions were maintained for different times in order to complete the
Fig. 2. 1 H-NMR and FT-IR spectra of PEL 12, 12.
G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661
elimination of the water and of the excess of the diol (see Table 1). An important part of this study was dedicated to the optimization of the catalytic system in order to improve the reaction rate and minimize the side reactions. We tested several well known catalysts for esterification process, in particular we tested TBT, DBTO, ZnAc, and Sb2 O3 . The polymerization tests were conducted using 1,12-dodecanedioic acid and 1,12-dodecanediol as monomers, because the resulting polymer was the most difficult to prepare and also because its structure is the most analogous to PE. For the comparison of the results we carried out the reactions under the same conditions, specifically using the same catalyst concentration and following the same temperature and pressure gradient. The activity of the single catalyst was determined by measuring the molecular weight at the end of the first and of the second stage. The measured values are reported in Fig. 1. From the analysis of the data obtained the most active catalyst turned out to be DBTO followed by TBT and ZnAc which gave similar results, whereas Sb2 O3 was the least active. Following this procedure we obtained PEL samples of high molecular weight with a pale yellow colour. Polymers were characterised by FT-IR, 1 H-NMR, GPC and DSC. The FT-IR spectra were consistent with the expected structures with a strong band in the region of 1720–1730 cm1 typical of aliphatic ester groups; also the NMR spectra were consistent with the proposed structure. The NMR and FT-IR spectra of PEL 12,12 are reported in Fig. 2. The molecular weight of the polymers was determined by GPC using polystyrene standards for column calibration. We obtained PEL samples with Mw ranging from 64,000 to 125,000; the final value of the molecular weight depends on the final temperature and the total time of the process. The synthesis of polyesters deriving from long chain diols bearing from 8 to 12 carbon atoms can also be improved by the use of a second, more volatile diol. The presence in the reaction mixture of a second more volatile diol represents a process change which generally permits to obtain high molecular weight polyesters with shorter reaction time at lower temperature. In fact, at the beginning of the reaction, both the second diol and long chain diol react with DA forming hydroxyl-terminated oligomers. At the end of the polymerization, transesterification reactions free the second diol, which is almost completely removed by distillation. Following this approach we reacted DA with DD in 1:1 molar ratio with the addition of 15% mol excess of ED respect to the acid (see experimental part) obtaining PEL 12,12ED having molecular weight comparable to PEL 12,12 (see Table 2) but within shorter reactions times (150 compared to 240 min) with a significant lower temperature (240 vs. 260 °C). From the data reported in Table 2, it can be seen that the amount of ED inserted is
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Table 2 Molecular weight and thermal analysis data for PEL samples Polymer PEL
Mn
Mw
Tm (°C)
Tc (°C)
DHm (J/g)
2,12 3,12 4,12 6,12 8,12 10,12 12,12 12,12 EDa
42,950 37,570 56,730 47,390 23,520 39,050 45,580 77,900
75,550 81,600 95,820 84,910 64,490 85,420 81,000 125,400
85.0 66.7 77.4 77.4 76.1 83.1 88.7 86.4
15.0 9.7 24.3 24.3 33.0 29.7 36.3 37.1
66.7 66.9 106.7 106.7 96.7 110.7 130.0 110.8
a
0.9 mol% ethanediol inserted.
very low (0.9% mol); therefore this approach seems to be useful and very efficient. Properties of PEL samples were also characterized by DSC. The data obtained, reported in Table 2, are in good agreement with those reported in the literature for similar materials [2–4]. The chemical recovery of the monomers was carried out by hydrolysis in alkaline media (see experimental part); the depolymerisation reactions were carried out on PEL 12,12 suspended in 1 M NaOH in methanol.
Fig. 3. Proposed life-cycle analysis of PEL polyesters prepared from renewable resources.
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Under these conditions the two monomers were recovered in very high yield (94–96%). These results open up the opportunity of the proposed closed-loop life cycle represented in Fig. 3. Starting from biomass, monomers can be synthesized, polymerized and after the use they can be recovered by depolymerisation and recycled back to the polycondensation step. Even if we have not carried out biodegradation tests, it is reported that aliphatic polyesters are generally known to be susceptible to biological attack [4]. Therefore after their use PEL disposed off into the environment could be biodegraded by biological processes. All these concept are summarized in Fig. 3. We have investigated the miscibility of PEL 12,12 with LDPE in different compositions. It has been reported
that long chain aliphatic polyesters give rise to the formation of solid phases with morphological similitudes to PE [2,3]. Blends were prepared by melt mixing PEL with LDPE and the morphology of the surfaces of criofractured samples was analyzed by SEM (see experimental part). The micrographs, reported in Fig. 4, clearly show immiscibility for all tested compositions similarly to blends of PE and commercial aliphatic polyesters (Bionolle) [13]. In particular, in Fig. 4A, the dispersed PEL phase in the LDPE matrix has been removed by etching with chloroform vapours leaving several holes onto the surface. At LDPE richer compositions, Fig. 4B and C, the PEL 12,12 forms the continuous phase with almost spherical PE domains irregularly distributed.
4. Conclusions A series of aliphatic polyesters based on 1,12-dodecanedioic acid and aliphatic diols bearing from 2 to 12 carbon atoms were obtained with high molecular weight. Among the tested catalysts the dibutyl tin oxide turned out to be the most active. The synthetic process was improved by the use of a volatile diol which allowed shorter reaction times and lower polymerization temperature. The alkaline hydrolysis of PEL 12,12, which can be considered in our opinion the most interesting polyester of the series, gives rise to the recovery of the starting monomers in almost quantitative yields confirming the potentially easy chemical recycle of this class of polymers. In our opinion, the results of this work confirm the interest on aliphatic polyesters. The easy and relatively low cost polycondensation coupled with the simple and almost quantitative recovery of the monomers make this materials quite interesting. Future work will be dedicated to this class of polymers, particularly to their chemical modification using different monomers and to their application.
References
Fig. 4. SEM micrographs of LDPE-PEL blends of different compositions (wt/wt): A: 75/25; B: 50/50; C: 25/75.
[1] Mark H, Whitby GS. In: Mark H, Whitby GS, editors. Collected papers of WH Carothers on high polymeric substances, vol. 1. New York: Interscience; 1940. [2] Le Fevere de Ten Hove C, Jonas A, Penelle J. J Polym Prep (Am Chem Soc, Div Polym Chem) 1998;39:156–7. [3] Le Fevere de Ten Hove C, Jonas C, Penelle A. J Proc Am Chem Soc: Div Polym Mater Sci Eng 1997;76:158–9. [4] Cho I, Lee K. Macromol Chem Phys 1997;198:861–9. [5] Skoglund P, Fransson A. Polymer 1998;39(10):1899–906. [6] Scherer TM, ClintonFuller R, Lenz RW, Goodwin S. Polym Degrad Stab 1999;64:267–75. [7] Kim M, Lee Ae-Ri, Yoon J, Chin I. Eur Polym J 2000;36: 1677–85. [8] S€ oderg ard A, Stolt M. Prog Polym Sci 2002;27(6):1123–63.
G. Barbiroli et al. / European Polymer Journal 39 (2003) 655–661 [9] Rodriguez-Galan A, Fuentes L, Puigallı J. Polymer 2000; 41:5967–70. [10] Jetter R, Riederer M. Phytochemistry 1999;52:907–15. [11] Jetter R. Phytochemistry 2002;55:169–76.
661
[12] McKiernan RL, Gido SP, Penelle J. Polymer 2002;43(10): 3007–17. [13] Kim J, Kim JH, Shin TK, Choi HJ, Jhon MS. Eur Polym J 2001;37:2131–9.