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Synthesis of carboxylate complexes and evaluation of their catalytic activities for polyesterification of castor oil (Ricinum communis) with terephthalic acid/Sintese de complexos de carboxilatos e avaliacao da atividade catalitica para poliesterificacao de oleo de mamona (Ricinum communis) com acido tereftalico.

Introduction

Polymeric materials obtained from vegetable oils, particularly castor oil, have been used as binder in paints, coatings and inks for many years. Recently, the increasing cost of petrochemicals has raised the search for alternate raw materials to produce polymers (ERHAN; BAGBY, 1991; NIROOMAND et al., 1996) in special polymerization of vegetable oils (DUTTA et al., 2004; FERREIRA et al., 2007; SOMANI et al., 2003; SUAREZ et al., 2007; YEGANEH; MEHDIZADEH, 2004) or biomass processing residues (SUAREZ et al., 2008). These biopolymers have been described not only as binder for coatings (YEGANEH; REZA, 2007), but also for adhesives (SOMANI et al., 2003) and prosthesis (IGNACIO et al., 1997; REZENDE et al., 2001).

Vegetable oils are mainly composed by tri-esters of fatty acids and glycerin. The fatty acid chain of the ricinoleic acid contains 18 carbons and presents an hydroxyl group in the position 12 and a double bond in the position 9 and corresponds to ca. 90% of the composition of castor seeds oil (OGUNNIYI, 2006). Due to this composition, castor seed oil has a great potential as starting material to produce polymers (TRUMBO; TREVINO, 2002), which are suitable to be used as binder for paints, coatings and inks (PATEL et al., 2006; SOMANI et al., 2003; YEGANEH; REZA, 2007). The most of these studies describes the use castor oil as raw material for polyurethanes synthesis. However, interesting polyester materials have been reported using castor oil, such as: (i) resins after its transesterification with depolymerized post-consumer PET (PATEL et al., 2006) ; and (iii) co-polyesters suitable to use for drug delivery based on L- or DL-Lactic acid and castor oil (SOKOLSKY-PAPKOV; DOMB, 2008) or ricinoleic acid and poly-sebacic acid (KRASKO et al., 2003). Nonetheless, as far as our knowledge, no attempts have been done in order to use castor oil as tri-alcohol to obtain polyesters by its direct polyesterification with poly-acids.

In polycondensation is preferred catalyzed systems (ISHIHARA et al., 2002; LIGABUE et al., 1998; TAKASU et al., 2003). The most common methods for polyesterification use strong Bronsted or Lewis acid catalysts, usually using large excess of either carboxylic acid or alcohol monomer in order to achieve high reaction yields (OLAH et al., 1978). Because of their less corrosivity, Lewis acid catalysts are preferred. In this sense, several Lewis acid metal complexes have been utilized as catalysts for polyesterification (PARSHALL; ITTEL, 1992). An industrial examples are tin (IV) based compounds FASCAT[R] and LIOCAT[R], which are largely used for the alcoholysis of vegetable oils with poly-alcohols to prepare alkyd resins (SUAREZ et al., 2007) .

We have recently used castor oil fatty acids to obtain polyesters and also to produce composites using magnetic nanoparticules (PERES et al., 2014). Indeed, because of the presence of a hydroxi group and an acid group in ricinoleic acid, it was possible to obtain linear polyesters by its direct polyesterification. It is important to highlight that during this study we observed that there was a catalytic activity of the nanoparticles, probably due to the presence of iron cations.

In the present work, we describe the direct polycondensation of castor oil with terephthalic acid (TFA) catalyzed by different ricinoleate complexes using bi-valent Lewis metal cations in order to evaluate their catalytic activity. It was also studied some physical-chemical properties of the different polymeric materials obtained in order to evaluate their potential use in the paint and coating industry.

Material and methods

Solvents, terephthalic acid (TFA), metal halides and others chemicals were obtained either from Merck, Vetec or Acros, and used without further purification. Castor oil was obtained from Celtic LTDA (Sao Paulo State, Brazil) and used as received.

Synthesis of the complexes

A fatty acids mixture containing up to 90% of ricinoleic acid was obtained through saponification of castor oil followed by acidification with hydrochloridric acid. The mixture was washed three times with water, dissolved in dichloroetane, dried with magnesium sulfate, filtered and the final fatty acids isolated by flash distillation to remove the volatiles (MELLO et al., 2011).

The ricinoleic acid (0.01 mol) and sodium or potassium hydroxide (0.01 mol) were added to a 30 mL of distilled water to produce an aqueous solution of the corresponding salt. This mixture was dropped to distilled water solutions of tin (II), iron (II), nickel (II) or cobalt (II) chlorides (0.0125 mol) and kept stirring for 2h at 343 K producing different insoluble complexes. These compounds were filtered and washed with distilled water to take out the starting soluble salts and then dried until constant weigh. The resulting reaction yields for ricinoleate metal complexes were: 65.10% for Sn, 47.43% for Fe, 55.85% for Ni and 40.43% for Co. The metal content of the final complexes were checked by Thermogravimetry (Shimadzu DTG-60) and were in very good agreement with the theoretical value. Infrared analysis (KBr pellets): (i) Sn: 3397.06 (vO-H), 1550 (vC[??]O), 3009.85 (v = C-H); (ii) Fe: 3448.57 (vO-H), 1560.51 (vC[??] O), 3010.45 (v =C-H); (iii) Ni: 3448.57 (vO-H), 1561.13 (vC[??]O), 3009.42 (v =C-H ); (iv) Co: 3417.63 (vO-H), 1563.12 (vC[??]O), 3009.41 (v = C-H).

Polymerization reactions

Polymerization was carried out in a 1 L reactor system consisting of a five necked round-bottom flask. The flask was equipped with a mechanical stirring, a thermocouple, nitrogen inlet tube and adjacent partial reflux condenser. The monomers TFA and castor oil in the proportion 3:2, respectively, and the ricinoleate metal complex were added to the flask and heated using an electrical heating mantle connected to the automatic controller setted at 523 K and connected to a Pt 100 thermocouple inserted in the reaction bulk. It is important to highlight that according to the literature, the thermal degradation of the castor oil occurs above 623 K (LIMA et al., 2004). The stirrer is adjusted to 100 RPM. Samples were collected every two hours and their acid index (AI) was determined according to ASTM D 465-9 (1996). The reaction was stopped when the acid index stabilized and then dropped to a glass tray.

Material analysis

Fourier Transformer Infra-Red (FTIR): The FTIR-ATR, spectra was obtained on an Equinox 55 Fourier transform instrument from Bruker. The FTIR-ATR spectra were recorded using a NaCl cell, using a DTGS detector. Each spectra data were obtained using a 4 [cm.sup.-1] of nominal resolution and it was a averaging of 32 scans.

Nuclear Magnetic Resonance (NMR): The [sup.1]H (300 MHz) NMR spectra was recorded in a Varian Mercury Plus M300 spectrophotometer using CD[Cl.sub.3] as solvent.

Differential scanning calorimetry (DSC): Determinations were performed in a TA Instruments DSC-2010 ([T.sub.star]: - 100[degrees]C, [T.sub.end]: + 100[degrees]C, heating rate: 20[degrees]C [min..sup.-1] in [N.sub.2]).

Size exclusion chromatography (SEC): The molecular weights were measured using the equipments Waters 410 Differential Refractometers, operating at 45[degrees]C, equipped with a ultra tyragel linear column (7.8 x 300 mm) from Waters using THF as solvent with 1.0 mL min.-1 flowing rate.

Results and discussion

Catalytic polymerization

Four complexes of the type [M.sup.+2] [([ricinoleate.sup.-1]).sub.2] were prepared and their catalytic activity in cartor oil polyesterification with TFA was evaluated, and the results are summarized in Table 1.

As can be depicted from Table 1, in the absence of metal complex (entry 1 of Table 1), it was observed the formation of esters in low reaction yield, evidencing a self-catalyzed reaction promoted by TFA probably because of its weak BronstedLowry acidity. Nonetheless, except for the nickel complex, the use of metal complexes leaded to better yields when compared to the auto-catalyzed reaction, indicating their catalytic activity. This results can be better visualized in Figure 1, where is plotted the acid values for different times during the polymerization. It is worth to mention that the observed catalytic activity of the metal increases in the same order of their Lewis acidity (PARSHALL; ITTEL, 1992).

[FIGURE 1 OMITTED]

Resin characterization

A comparison of the infrared spectra of castor oil, TFA and the different obtained materials is showed in Figure 2. TFA spectra shows broad band in the region between 2500 and 3300 [cm.sup.-1], characteristic of carboxylic acids and attributed to O-H stretching (LIMA et al., 2004), and a band in 1701 [cm.sup.-1] related to the C = O stretching. After the polyesterification, this broad band disappears and the band in 1701 [cm.sup.-1] was dislocated to 1720 [cm.sup.-1]. It is also possible to note that the O-H stretching related to the hydroxyl group of castor oil observed in 3422 [cm.sup.-1] seems to have also disappeared from the spectra after the polymerization. Other important new band in the polymer spectra appears in 1266 [cm.sup.-1], probably related to C-O stretching in aromatic esters (SILVERSTEIN et al., 2000). These observations strongly indicate the esterification of TFA with hydroxyl groups from castor oil. In the castor oil spectra appears in 1750 [cm.sup.-1] a strong stretching of the C = O related to fatty acids and glycerol ester groups. After the reaction, this band is maintained and a shoulder appears in low wavenumber, suggesting that the original acil esters are maintained and a new different ester group was formed.

[FIGURE 2 OMITTED]

The formation of the polyesters was also confirmed by [sup.1]H-NMR analysis. It was observed a peak at [delta] = 8.0-9.0 ppm, indicating the presence of aromatic hydrogens related to TFA. It was also observed a peak at [delta] = 1.25-1.30 ppm and at [delta] = 5.33-5.35 ppm aliphatic C-H stretching and at [delta] = 4.1-4.3 ppm corresponding to glycerol C-H stretching, usually observed for vegetable oils [4].

The average molecular weight values were determined for the different materials by SEC and were listed in Table 2. A high polydispersity were observed in all cases and an important [M.sub.w] variation was observed according to the catalyst used. Indeed, the [M.sub.w] varied from 24300 Da when no catalyst was used to 36400 Da for the polymer prepared in the presence of Co[(ricinoleate).sub.2].

The polymers were also analyzed by Differential Scanning Calorimetric (DSC). For all polymers were obtained similar profiles and, as an example, Figure 3 shows the results obtained for the materials prepared in the presence of Ni[(ricinoleate).sub.2] (Figure 3A) and in the absence of any complexe (Figure 3B). As can be depicted from Figure 3, no melting point were observed, but only glass transitions in the temperature range from -80 and -40[degrees]C, suggesting a no-crystalline structure.

The glass transition temperatures for the different materials are listed in Table 2. It becomes clear from Table 2 that, as observed for the average molecular weights, the catalyst used in the process has a great influence in glass transition temperature. It is also clear that there is a good correlation between the glass transition temperature and the average molecular weights and polydispersity of the polymer, indicating that the noncrystallinity of the material increases with the average molecular weight and the dispersity of the chains size. A good explanation for these results is the relation between the polymeric chain growth and its molecular organization. The smaller and less polydisperse polymers are easily organized and for a better molecular organization of the biggest chains is necessary more energy release. On the other hand, polymeric chains will have less movement with less temperature. This result was particularly expected due to the crosslink in the chains during the polyester formation using a tri-alcohol and a di-carboxylic acid.

[FIGURE 3 OMITTED]

Conclusion

We prepared polyesters from castor oil and terephthalic acid in the presence and absence of different ricinoleate metal complexes and the ester formation was confirmed by IR and NMR spectroscopy. It was observed that in the presence of the complexes the reaction velocity was increased and that the iron II complex has more active. The materials prepared showed none crystallinity, high molecular weight and a high polydispersity and only a glass transition ([T.sub.g]). It is important to emphasize that these properties make these polymers suitable to be used when no rigid materials are desired, such as coatings, sealings, and others.

Doi: 10.4025/actascitechnol.v37i3.25056

Acknowledgements

Procad-CAPES and INCT-Catalise for financial support. PAZS thanks CNPq for research fellowship.

References

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Received on September 24, 2014.

Accepted on January 23, 2015.

Eduardo Ulisses Xavier Peres (1), Alexandre Perez Umpierre (2) and Paulo Anselmo Ziani Suarez (2), *

(1) Instituto Federal de Educacao, Ciencia e Tecnologia de Brasilia, Campus Taguatinga, Brasilia, Distrito Federal, Brazil. (2) Universidade de Brasilia, Instituto de Quimica, Cx. Postal: 4478, 70919-970, Brasilia, Distrito Federal, Brazil. *Author for correspondence. Email: psuarez@unb.br
Table 1. Castor oil and terephthalic acid direct and assisted by
different [M.sup.+2] [([ricinoleate.sup.-1]).sub.2] complexes
polyesterification.

Entry   Catalyst                            Reaction   Acid    Water
                                              time     value    (g)
                                             (min.)      *

1       No catalyst                           840       145     5.0
2       [Ni.sup.+2][[[C.sub.17][H.sub.34]     480       143     5.2
          (OH)CO[O.sup.-1]].sub.2]
3       [Sn.sup.+2][[[C.sub.17][H.sub.34]     360       132     5.1
          (OH)CO[O.sup.-1]].sub.2]
4       [Co.sup.+2][[[C.sub.17][H.sub.34]     240       124     3.9
          (OH)CO[O.sup.-1]].sub.2]
5       [Fe.sup.+2][[[C.sub.17][H.sub.34]     240       118     4.6
          (OH)CO[O.sup.-1]].sub.2]

* Starting acid value (at t = 0 min.): 245.22. Reaction conditions:
temperature 523 K; Castor oil/terephthalic acid/complex = 200 g/
50 g / 0.57 g.

Table 2. The average molecular weights, polydispersity and glass
transition temperature of the different materials prepared after
polyesterification of TFA with castor oil in the absence (A) and
presence of [Ni.sup.+2] (B), [Sn.sup.+2] (C), [Co.sup.+2] (D)
and [Fe.sup.+2] (E) ricinoleate complexes.

Material    Mn     [M.sub.w]   Polydispersity     [T.sub.g]
           (Da)      (Da)                       ([degrees]C)

A          2,700     24,300         9.07           -49,23
B          2,700     33,700        12.43           -79,83
C          3,600     32,500         8.95           -57,47
D          2,900     36,400        12.42           -79,54
E          2,500     34,100        13.60           -79,09
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Date:Jul 1, 2015
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