Emeline Prandato

Sébastien Livi

Michel Melas

Justine Auclair

Vincent Verney

Etienne Fleury

Françoise Mechin

Françoise Méchin

Effect of Bio-Based Monomers on the Scratch Resistance of Acrylate Photopolymerizable Coatings

Keywords: Photo-polymerization, micro-scratch, renewable, acrylate, photo-DSC, WAXD

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INTRODUCTION

Nowadays, many automotive components are made of thermoplastic in order for the vehicles to be lighter; headlights are for instance made of polycarbonate. However, an important drawback of this material is its low resistance to mechanical damage. Therefore the use of a coating is required, to protect the piece from impacts, abrasion or scratches. Protective coatings have to feature strong mechanical properties, such as a high elastic modulus, a good ability to recover their initial shape after deformation (i.e. a high elastic recovery) and a good resistance to indentation. The appearance of severe damage such as cracks is also expected to occur only for the highest stress as possible, since they can be the starting point of more severe damage.

In this prospect micro-or nano-scratch tests, which are used in the literature to assess the scratch resistance of coatings [1][2][3], are interesting. They consist in applying a constant or increasing load on an indenter in contact with the sample while it moves. The indented depth can be followed in real time and the elastic recovery can be calculated afterwards. The determination of the critical load, at which the first crack appears, also enables to compare the resistance of several materials. Different types of cracks can be observed, with or without chipping. Partial cone cracks are often obtained. More precisely when the test is carried out with an increasing load the cracks have the shape of a partial cone concave with respect to the direction of the scratch [1,4]. Indeed, the first crack appears when the tensile stresses (created at the rear of the indenter while scratching the sample with an increasing load) reach the ultimate tensile strength.

Protective coatings are most often polymer networks obtained by the polymerization of multifunctional compounds. Their properties are related to their composition, but also depend on the test conditions and on the temperature, since polymer materials display a viscoelastic behavior. Moreover during the buildup of a polymer network by radical (photo)-polymerization, numerous microgels are created and grow until they percolate to form a macrogel. From a morphological point of view, the resulting material is thus highly inhomogeneous at the micro or nanometer scale [5]; this can also have an impact on its macroscopic properties.

Protective coatings can be cured under UV-light. Photo-polymerization is indeed an interesting technology since it allows curing at high rate solvent-free mixtures deposited as low-thickness layers on a substrate. It is thus an economical and environmentally-friendly technology. However, 100%solids protective coatings are generally composed of petro-based monomers. Faced with the depletion of fossil resources and in a sustainable development concern, the use of bio-based monomers could represent a "green" alternative.

In the literature, numerous recent works concerning bio-based UV-curable materials can be found [6][7][8][9][10][11][12][13][14][15]. However many of the related acrylate compounds derive from triglycerides extracted from vegetable oils. Dillman et al. [7], Huang et al. [9] and Rengasamy et al. [15] (meth)acrylated respectively castor oil, tung oil and a commercial epoxidized soybean oil. Pietschmann et al. also acrylated several vegetable oils such as rapeseed, linseed, soybean, calendula or castor oil [14]. Such molecules feature long alkyl chains, thus lead to networks with poor mechanical properties that are not suitable for a protective application. Yet there exist other commercial alternatives, such as short, multifunctional bio-based acrylate molecules similar to common petro-based monomers. The aim of the present work is to evaluate photo-polymerizable protective coatings for polycarbonate substrates, based on such compounds. The influence of the increase in alkyl segment content, or the decrease in the crosslinking density was also studied. This work includes the investigation of photo-polymerization kinetics using photocalorimetric measurements, the examination of the nanomorphology of the bio-based photo-cured materials by X-ray diffraction (XRD), as well as the evaluation of their thermomechanical properties by dynamic mechanical thermal analysis (DMTA) and their scratch resistance by micro-scratch tests.

EXPERIMENTAL

Materials

DiPEPHA (mixture of dipentaerythritol penta-acrylate and dipentaerythritol hexaacrylate, 15% BBC), PETA (pentaerythritol tetraacrylate, 10% BBC), DDA (1,10-decanediol diacrylate, 60% BBC), THFA (tetrahydrofurfuryl acrylate, 60% BBC), LA (lauryl acrylate, 80% BBC) and IBOA (isobornyl acrylate, 75% BBC) are commercial partially bio-based acrylate oligomers and monomers provided by Arkema. TCDDA (tricyclodecane dimethanol diacrylate, SR833S) is a petro-based monomer also provided by Arkema. The structures of these compounds are detailed in Figure 1. The percentage of bio-based carbon (% BBC) is calculated following Eq. ( 1):

100 % × + = - - - based petro based bio based bio C C C BBC ( 1
)
Irgacure 184 (1-hydroxycyclohexyl phenyl ketone) and Lucirin TPO-L (ethyl-2,4,6trimethylbenzoylphenylphosphinate) were used as photo-initiators; both were provided by BASF. 3,4,5-trichloropyridine (purity 99%) was purchased from Aldrich. DMSO-d6 (deuterated dimethylsulfoxide) and tetramethylsilane (TMS) were purchased from Euriso-Top.

The polycarbonate substrates were injected Makrolon® AL 2447 plates with Tg = 145°C and weight average molar mass M w = 24 kg.mol -1 as determined by SEC with triple detection (LALS, refractometry and viscosimetry) [START_REF] Laurent | Développement et études de l'influence des procédés d'application et de réticulation de vernis acrylates photo-polymérisables (à 100% d'extrait sec)[END_REF].

Preparation of the coatings

Six different coatings were studied. Their compositions are summarized in Table 1. Bio 1 and Bio 3 differ by the nature of their oligomer, respectively DiPEPHA and PETA. Bio 2 was obtained by increasing the amount of DDA (+9.6%) in Bio 1 while keeping the mass ratio between DiPEPHA and TCDDA constant. Bio 2 allows to study the influence of the enrichment in long alkyl chains on the properties of the material. Bio 11, Bio 12 and Bio 13 were obtained by substituting in Bio 1 4.8 wt% of the multicyclic petro-based monomer TCDDA by a variable monoacrylate monomer. These 3 coatings allow to study the influence of the decrease in the cross-linking density on the material properties.

The mean percentage of bio-based carbon in all the resulting coatings is 25-30%. Note that a higher percentage could be reached if bio-based acrylic acid was used to provide the acrylic functionality to bio-based molecules, but it is not yet available at the industrial scale.

The raw materials were mixed with a Rayneri mixer at 700 rpm, first during 45 minutes (monomers alone), and for 15 more minutes after the addition of the photoinitiators. During this stage only a negligible increase in temperature (<1°C) was observed. 15 µm-and 150 µm-thick coatings deposited on polycarbonate (PC; Makrolon AL 2447) or glass panels were then prepared, with a motorized film applicator Elcometer K4340 equipped with a spiral bar coater. They were photo-polymerized through 3 passes under a Fusion F300S UV-lamp equipped with a conveyor belt. The mean total UV doses and irradiance peaks, measured with a Power Puck II (EIT), were respectively: UVA (320-290 nm): 3837 mJ.cm -2 , 1747 mW.cm -2 ; UVB (280-320 nm): 3997 mJ.cm -2 , 1822 mW.cm -2 ; UVC (250-260 nm): 683 mJ.cm -2 , 321 mW.cm -2 . The 150 µm-thick films prepared on glass panels were peeled off in order to get free-standing films for the DMTA analyses. The coatings on PC panels were especially designed for micro-scratch tests.

Determination of the acrylate double bond conversion by Fourier Transform Infra-Red spectroscopy (FTIR)

The conversion degree of the cured samples was determined by FTIR, using an Attenuated Total Reflectance (ATR) device equipped with a diamond crystal. To calculate the acrylate double bond conversion, the area of the acrylate band at 810 cm -1 was used. It was normalized using the carbonyl band (1720 cm -1 ), which is constant throughout the polymerization, as a reference in order to avoid any bias related to incorrect contact between the sample and the crystal. A comparison of the ratio of these areas for both the cured and the uncured sample (without photo-initiators) allows to calculate the acrylate conversion degree after polymerization (Eq. 2):

              = × -                 cm cm cured cm cm uncured A A Conversion (%) A A (2)
The analyses were carried out with a NicoletTM iS10 spectrometer (ThermoScientific), equipped with a "Smart iTR" ATR unit. The spectra were acquired after 32 scans with a resolution 4 cm -1 . The conversion could be determined for each side of the free standing films, but it could obviously be determined only on the upper side in contact with air during the polymerization of coatings on PC.

Photocalorimetry studies

Preparation of the samples

The photo-polymerization kinetics should not be too fast in order to be properly studied. Therefore, mixtures containing only one photo-initiator (Irgacure 184) were formulated, in a quantity such that the molar ratio I184/acrylate double bonds = 0.28%; the compositions are summarized in Table 2. The bio-based formulation Bio 1 was compared with each of its components alone. In this aim, the acrylate double bonds density (d ac ) was previously determined for each monomer and oligomer by 1 H NMR, using 3,4,5-trichloro pyridine as internal standard [START_REF] Rundlöf | [END_REF]. A known mass of standard together with a known mass of acrylate compound were placed in the NMR tube. After the analysis, the integration of the proton signals related to the acrylate and to the standard enabled to calculate d ac (Eq. ( 3)):

TABLE 2. Composition of the mixtures studied by photo-DSC (wt%); for each of them, the molar ratio between the photo-initiator and the acrylate double bonds equals 0.28%

ac s s s ac ac m M p m I I d 1 . 3 2 ⋅ ⋅ ⋅ = (3)
where I ac and I e are respectively the integrals of the acrylate and standard protons, p and M s respectively the purity and the molar mass of the standard, m ac and m s respectively the mass of acrylate compound and of standard.

The spectra were acquired at 300K in DMSO-d 6 (+TMS), with a Bruker Avance III 400 US+ spectrometer (400 MHz), 8 scans, 90 s relaxation delay and 128.103 points sampling.

Experimental conditions

A photo-calorimeter DSC 822e (Mettler Toledo) equipped with a LC6 (Hamamatsu) UV-lamp operating at 20% of its total power was used. The power received by the samples between 200 and 400 nm was 18.7 µW.cm -2 (measured with an Ocean Optics QE65000 spectro-photometer). The samples (2-3 mg) were placed in 40 µL aluminum capsules. The experiments were carried out at 25°C; using such small samples the increase in temperature due to the reaction exothermy was negligible. Nitrogen was flushed during 5.5 min before the start of the irradiation, and then during the whole experiment, in order for the kinetics not to be disturbed by O 2 inhibition. The samples were irradiated during 21.5 min. At the end of each experiment, vitrification was reached, but the signal did not return to zero. This is due to the fact that the reference and sample pans do not absorb the infrared component of the radiation in the same way [18]. To avoid this effect, a second experiment was run under the same conditions immediately after the first one. The resulting signal was then subtracted from that obtained during the first run. By integrating the resulting heat flow, and considering it is proportional to the number of reacted acrylate double bonds, it is possible to calculate the acrylate conversion (Eq. ( 4))

∫ ∆ = t dt t H H t x 0 % 100 ' ) ' ( 1 ) ( (4)
where x(t) is the acrylate conversion (%) at time t (s); H(t) is the heat flow versus time (W); ΔH 100% is the reaction enthalpy for 100% of the acrylate double bonds of the sample (J), calculated following Eq. ( 5):

Cacrylate C i i i H n m = 100% ∆ = ∆Η ∑ . . ( 5
)
where m i is the mass (g) of monomer or oligomer i in the sample, n i is the density of acrylate double bonds of the monomer or oligomer (mol/g), ΔH C=C acrylate is the reaction enthalpy for 1 mole acrylate double bonds (J/mol); we used ΔH C=C acrylate = 86 kJ/mol [19].

Finally knowing the conversion vs. time relation, it is possible to calculate the polymerization rate (R polym ) versus time (Eq. ( 6)):

dt t dx t R polym ) ( ) ( = (6)
For each sample, 3 photo-DSC experiments leading to reproducible results were carried out. To compare the different studied samples, the maximal polymerization rate (R max ), its position in time and in acrylate conversion, and the final acrylate conversion (x max ) were simultaneously considered.

X-Ray Diffraction (XRD)

The diffractograms were collected with a Bruker D8 Advance X-ray diffractometer at the H. Longchambon Diffractometry Center. A bent quartz monochromator was used to select the Cu Kα radiation (λ r = 0.15406 nm). Experiments were run under operating conditions of 45 mA and 33 kV in Bragg-Brentano geometry. The angle range (2θ) was 4.5-50°. The diffractograms were analyzed and deconvoluted using the software Fityk. The apparent size of clusters of microgels was calculated using the Scherrer equation (Eq. 7, see Results & Discussion section):

θ β λ cos d r c k D = (7)
where β d is the width of the peak at the half-maximum height, and k is a constant depending on the direction of the lattice and the geometry of the clusters; here k=1 was used.

Dynamic mechanical thermal analyses (DMTA)

The DMTA were carried out with a TRITON apparatus, in tension mode, using the following experimental conditions: frequency: 1 Hz; temperature ramp: 3°C/min from -90°C to 300°C; deformation: 0.1%, spacing between clamps: 2.5 mm. 150 µm-thick free-standing films were analyzed (sample width: 5-10 mm), and at least 3 consistent analyses were carried out for each sample.

Micro-scratch tests

Micro-scratch tests were carried out at 23°C and 50% relative humidity, with a CSM Micro-Scratch Tester equipped with a diamond Rockwell indentor featuring a 120° angle and a 100 µm-radius sphere. The 4 mm-long scratches were implemented on 15 µm-thick coatings deposited on PC, applying an increasing load from 0.03 N to 10 N, at a speed 8 mm.min -1 . The indented depth (D i ) was followed in real-time during the test. A pre-scan and a post-scan, run with a constant load 0.03 N, allowed to determine the residual depth (D r ) and to calculate the elastic recovery (R e ), following Eq. 8.

(

)

100 i r e i D D R D - = ⋅ (8)
The elastic recovery expresses the ability of the sample to recover its initial shape. The load at which the first crack appears (critical load L c ) could be determined thanks to the observation of the scratch after the test with an optical microscope. For each sample, at least 5 micro-scratch tests were carried out. The results could not be statistically studied for each load between 0.03 and 10 N, therefore the focus was put on the results (D i , D r , R e and L c ) for two particular normal loads: 0.3 N and 1.5 N. The errors are given for a probability of 95%, and the reported values are averages of all the experimental data.

RESULTS AND DISCUSSION

The photo-polymerization of all the liquid mixtures (compositions in Table 1) led to clear tack-free coatings. In every case the mean acrylate conversion was about 95%, i.e. a rather high value.

Reaction kinetics estimated by photo-DSC

Photo-DSC experiments were run to study the photo-polymerization kinetics of the mixture Bio 1' (same polymer matrix as Bio 1 with less photo-initiator). Indeed Bio 1 could not be directly studied because of its high photo-initiator content that speeds up the reaction and results in a less precise kinetic study. The evolution of the polymerization rate and of the acrylate conversion versus irradiation time is illustrated in Figure 2 for Bio 1'. As the experiments were run under N 2 atmosphere, no induction period due to the oxygen inhibition of the polymerization could be noticed. As seen in Table 3, the maximal rate of polymerization was 2.1 s -1 , and was reached after 8 s irradiation, when the acrylate conversion was 9.5%. A high increase in the polymerization rate was initially observed, followed by a decrease until the polymerization stopped. This evolution is characteristic of the free-radical polymerization. The auto-acceleration, also named "gel effect", is due to the increase in the system viscosity as the polymerization proceeds. This decreases the mobility of the radicals located on the growing chains and thus slows the termination reaction. During this phase, the concentration of monomers decreases but they are still mobile. However, as the acrylate double bond conversion increases, the decrease in the mobility and concentration of monomers becomes important and the polymerization rate starts to decrease as well, leading to auto-deceleration. The polymerization finally stops because of vitrification, or when all the acrylate double bonds are consumed.

The final conversion for the mixture Bio 1' was 68.4%. So, even at 25°C, the conversion of the acrylate double bonds reaches a high value. The high conversion (~95%) noticed after 3 passes under the UV-lamp where the temperature can reach 90°C is thus not surprising.

To confirm the reactivity of the mixture Bio 1', the photo-polymerization kinetics of each acrylate compound used to formulate it was then studied separately (compositions given in Table 2).

For all samples, no induction period was ever noticed, and the characteristic auto-acceleration and auto-deceleration phenomena were always observed. The results are summarized in Table 3. It appears that the photo-polymerization kinetics of the oligomer DiPEPHA is much lower than that of the diacrylate monomers TCDDA and DDA. Indeed, the maximal rate is reached for a much longer irradiation time and a much lower conversion in the case of the oligomer. For this oligomer DiPEPHA, the final acrylate conversion is also quite low (~35.5%). These features are due to its high viscosity (~10 Pa.s at 25°C) [20] and high functionality, which both contribute to hamper the diffusion of the reactive species. On the contrary, the other two diacrylates have a lower viscosity (130 mPa.s for TCDDA and 10 Pa.s for DDA at 25°C, data given by Arkema). Their photopolymerization is thus faster, and their final conversion higher. In addition, for TCDDA the maximal photo-polymerization rate is reached at lower irradiation time and acrylate conversion than for DDA. What is more, its final conversion is also lower than for DDA (61.6% versus 87.9%). This can be attributed to the rigid structure of TCDDA, due to its 3 joined cycles. With this monomer the growing polymer network is rigid even at low acrylate conversion, and vitrification is quickly reached. To confirm this, DMTA was carried out for 99%-cured mixtures containing only photo-initiators and TCDDA or DDA, in order to determine the maximal temperature of the mechanical relaxation of "totally" cured monomers. For TCCDA, this value was about 252°C whereas it was approximately 107°C for DDA. These results confirm that vitrification is reached at a lower conversion degree for TCDDA than for DDA, and are thus in agreement with the respective final conversions observed for the two samples.

TABLE 3. Results of the photo-DSC experiments for Bio 1' and for the corresponding pure acrylate compounds; R max : maximal polymerization rate; x max : maximal acrylate conversion. For each sample, the indicated values are averages over 3 consistent experiments

Nanostructure of the coatings

As explained in the Introduction, the nanostructure of the studied coatings, obtained by the photopolymerization of multifunctional acrylate compounds, is supposed to be very inhomogeneous with agglomerated highly crosslinked microgels linked by a less crosslinked matrix. However, the chemical nature of the different domains must be very similar, and indeed traditional characterization methods such as Atomic Force Microscopy, AFM (regular and Peak Force tapping modes), or Transmission Electron Microscopy, TEM, performed on both neat and variously stained samples (using RuO 4 or phosphotungstic acid at pH = 4.0 or 7.2) did not allow the detection of distinctly separated nanophases in our materials. Quite recently, Barszczewska-Rybarek [21] analyzed the nano-morphology of a series of previously powdered polyacrylate materials based on 3 precursors with quite different chemical structures by X-ray diffraction. Diffractograms with 3 main broad peaks were obtained; these peaks were assumed to be associated with 3 representative sizes of microgel clusters. In other words, the D c value determined from the Scherrer equation (Eq. 7) was considered as a measure of the size of the various heterogeneities (agglomerated microgels) present in the polyacrylate network. It could objected that Scherrer's equation is usually considered applicable only to crystalline domains; however such amorphous microgel structures obtained from photopolymerization were already shown to display a definite local order [22]. This spatial organization was also experimentally observed by Rybarek on dimethacrylate networks [23]; WAXS and its classical equations were also used to study interchain spacing in amorphous homopolymers and blends [24]. In our case, a local organization is definitely detected by X rays, and all these arguments are finally in favor of similar size calculations using Scherrer's equation. Inspired by these works, we therefore attempted to analyze the nanostructure of the present photo-cured materials (150 µmthick free films) by XRD in the same way. The diffractograms (Figure 3) revealed several large "peaks" typical of amorphous materials. The results of the deconvolution of these peaks (angle and corresponding D c value) are summarized in Table 4. In this table, the parameter D tot (sum of all individual D ci for a given network) that is considered as an indicator of the network heterogeneity in Barszczewska-Rybarek's work, is also given. For all the samples except Bio 3, three "peaks" were detected and used for analysis, which led to two different apparent sizes of microgel clusters (D c ) which is the primary cause of the structural heterogeneity in networks [25]. The fact that two "peaks" can lead to a similar value of D c evidences the great inhomogeneity of the samples. All these samples display rather similar results, except for "peak" n°1, whose position in terms of angle 2θ and the corresponding calculated value of D c varies. This suggests that the size of the biggest clusters depends on the composition of the material.

TABLE 4. Analysis of the X-ray diffractograms of the bio-based materials

According to the literature, the D values are similar to those observed for dimethacrylate networks [23]. In fact, Rybarek studied the influence of different copolymers on the structural heterogeneity parameter and attributed the increase in this parameter to the strength of molecular interactions in the cured system. Compared to another study, this latter size is always much lower than the highest values depicted in ref [21] where D tot often reaches 11-14 nm. In this latter work, large clusters are said to be promoted both by the initial presence of strong hydrogen bonds between some monomer molecules, and by π-π stacking interactions between phenyl rings. Therefore the difference might be due to several factors: i) all the networks studied here were highly converted (~95%) contrary to the quoted literature; ii) the chemical structures of the various precursors used in the present work were maybe more alike than those used by Barszczewska-Rybarek, inasmuch as iii) they were not able to generate many hydrogen bonds (no urethane group); and finally iv) no aromatic monomer was used.

Sample Bio 3, which is the only one containing PETA, is slightly different from the others because the central peak could be deconvoluted into two peaks, leading to similar apparent sizes of clusters of microgels. Once again this tends to confirm that the composition of the material has some influence on its nano-structure.

In conclusion, the size of heterogeneities and the number of hydrogen bonds will have a significant influence on the mechanical properties of the final material, such as the impact strength.

Thermomechanical properties of the materials

A protective coating is expected to display a high elastic modulus and a large mechanical relaxation in order to prevent the properties of the coating from varying abruptly with a change in temperature. This is of particular importance when the protected piece has a broad range of operating temperature (outdoor applications).

Here all the studied materials display a rather similar high elastic modulus, lying between 4 and 6.10 8 Pa at the rubbery plateau ; its variation during the mechanical relaxation is lower than one decade (for all the samples E'glassy = 2 to 3.10 9 Pa). A finer comparison of the samples' moduli appears somewhat risky because of the insufficient sensitivity of the used rheometer and of the experimental difficulty in analyzing these extremely brittle materials.

For polyacrylate networks, the crosslinking density and the elastic modulus at the rubbery plateau are related in a complex way [26]. Nevertheless, the substitution of 5 wt% diacrylate by a monoacrylate in the coating Bio 1 (Bio 11, Bio 12 and Bio 13) does not seem to be sufficient to induce a noticeable decrease in the elastic modulus. The enrichment in alkyl segments (Bio 2, compared to Bio 11) has no influence on the elastic modulus of the material.

As can be seen in Figure 4, for no sample can a secondary mechanical relaxation be noticed, and the principal relaxation is always broad which highlights the presence of many heterogeneities at the nanometer scale in the network and confirms the X-ray diffraction results [27,28]. The characteristics of the mechanical relaxations are summarized in Table 5. At room temperature, all the studied materials are in the glassy state. The variation of the width at half maximal height from one to another sample is negligible. The substitution of DiPEPHA by PETA in Bio 1 (leading to coating Bio 3) does not modify the position of the mechanical relaxation. However, a 20°C gap between the maximum of these relaxations is observed for Bio 1 and Bio 2. The enrichment in alkyl chains (Bio 2) shifts the mechanical relaxation toward lower temperatures. This is attributed to the C10 chain of DDA, which can favor longer relaxation times and consequently, following the equivalence principle between time and temperature, can favor the shift or the enlargement of the mechanical relaxation toward lower temperatures. Although the incorporation of a monoacrylate should decrease the crosslinking density of the polymer network, the observations differ following which monoacrylate is considered. The incorporation of IBOA (Bio 13) indeed shifts the maximum of the mechanical relaxation toward higher temperatures. This is due to its tricyclic hard structure: in order to provide it mobility, it is necessary to bring a high amount of energy. This effect prevails on the loosening of the polymer network through the incorporation of a monoacrylate. On the contrary, the maximum of the mechanical relaxation is logically shifted toward lower temperatures with THFA or LA, because the polymer network is loosened and this effect is not counterbalanced by the rigidity of the added monomer.

TABLE 5. Characteristics of the mechanical relaxation of the bio-based photo-cured materials, determined by DMTA at 1Hz

As a conclusion, every studied sample features thermomechanical properties in agreement with our expectations for a protective application. Other tests are thus necessary to reveal some difference between them.

Scratch resistance of the coatings

In Figure 4, the temperature used for the microscratch experiments (23°C) lies before or at the very beginning of the mechanical relaxation for all the samples. Therefore they should all be in a rather similar viscoelastic state. The results of the micro-scratch experiments for the bio-based coatings as well as for the uncoated PC substrate are presented in Figure 5. The coatings do not prevent the penetration of the indenter (Fig. 5a), but they all improve the elastic recovery (Fig 5c ) and thus notably limit the residual depth of the scratch (Fig. 5b). Yet, this residual depth is expected to be as low as possible, since the deeper it is, the more visible the scratch is. As for the critical load (Fig. 5d), the improvement is remarkable with respect to a classical petro-based formulation [29] that showed a 1.9 N critical load under the same test conditions, i.e. an increase ranging from ~ 60% for Bio 12 to ~ 175% for Bio 13. All the studied coatings are thus effective in terms of protection against scratches.

The replacement of the oligomer DiPEPHA in Bio 1 by PETA (coating Bio 3) slightly increases the elastic recovery, while the effect on the indented depth is negligible. Taking into account the experimental uncertainty, no change in the critical load can be noticed.

Comparing coatings Bio 1 and Bio 2, it appears that the enrichment in alkyl segments does not either influence the scratch resistance of the coatings (see Figure 5).

In coating Bio 1, the substitution of 5% TCDDA by a monoacrylate should lead to a decrease in the crosslinking density. In fact a slight increase in the elastic recovery can be observed as a result of this substitution (Figure 5c, Bio 11, Bio 12, Bio 13). Its influence on the critical load once again depends on the monoacrylate nature and is particularly noticeable in the case of IBOA that tends to increase Lc (Bio 13). In order to explain this, the acrylate double bond density of each mixture was first calculated. Values of 7.2.10 -3 mol.g -1 , 7.1.10 -3 mol.g -1 and 7.1.10 -3 mol.g -1 were found respectively for Bio 11, Bio 12 and Bio 13. As all the coatings have a similar conversion degree (95%), it can be assumed that their crosslinking density is similar. The observed differences can thus be ascribed to the chemical structure of the monoacrylate. As the critical load tends to be higher for Bio 13, IBOA must either restrain the increase in stresses in the polymer network, and/or shift the ductile-brittle transition toward higher temperature, certainly due to its bulky structure. On the contrary, LA must increase the stresses in the material since the critical load tends to be lower for Bio 12. Finally THFA has virtually no effect on the critical load.

Since cracks represent severe damage, the critical load is a very important parameter; it is obviously expected to have a high value. Taking this into account, Bio13 is a rather interesting coating, since it combines satisfying elastic recovery and critical load, as well as a slightly lower residual depth.

Scratches and cracks were finally observed with an optical microscope. As shown in Figure 6, no crack was created along the scratch in the case of the uncoated PC. For the other samples, all the cracks have the same shape; partial cone cracks can be recognized [1]. As the shape of the cracks is related to the spatial organization of the stress fields created in the material during the scratch [30,31], these fields can be assumed to be spatially organized in a rather same way in all the tested bio-based coatings.

CONCLUSIONS

Photo-polymerized clear tack-free coatings were obtained using bio-sourced monomers similar to classical petro-based products. The kinetics of the photo-polymerization reaction was shown to be quite comparable to that of usual systems, and is therefore fully compatible with industrial curing processes.

As usual microscopic techniques (AFM and TEM) did not allow to reveal possibly separated nanophases, the nano-morphology of the materials was investigated by XRD. Clusters of microgels of different apparent sizes were assumed to be present. These sizes were all very small (<5 nm), suggesting that the presence of the clusters could probably not be evidenced by SAXS either. Nevertheless, the results indicated an influence of the composition on these sizes.

All the studied samples feature a high elastic modulus and a wide mechanical relaxation, which is in agreement with our expectations for a material dedicated to a protective use. What is more, the 15 µm-thick coatings all efficiently protect the polycarbonate substrate from scratches.

The enrichment of the material in alkyl segments shifts the mechanical relaxation toward lower temperatures, and has no influence on the scratch resistance.

The decrease in the cross-linking density through the incorporation of a monoacrylate monomer slightly shifts the mechanical relaxation toward higher (IBOA) or lower (THFA, LA) temperatures. The micro-scratch experiments showed that this change in composition generally tends to increase the elastic recovery, but that the effect on the critical load depends on the monoacrylate. The incorporation of 5 wt% IBOA is particularly attractive since it allows a slightly higher elastic recovery along with a distinctly higher critical load.

Figure 1 :Figure 2 :Figure 3 :Figure 4 :

 1234 Figure 1 : Structure of the oligomers and monomers: DiPEPHA (a), TCDDA (b), DDA (c), PETA (d), IBOA (e), LA (f), THFA (g)

Figure 5 :Figure 6 :

 56 Figure 5 : Results of the micro-scratch tests for the bio-based 15µm-thick coatings deposited on PC: indented depth (a), residual depth (b), elastic recovery (c), critical load (d)

TABLE 1 . Composition of the coatings (wt%)

 1

	Coating	DiPEPHA	PETA	DDA	TCDDA	THFA	LA	IBOA	Irgacure 184	Lucirin TPO-L
	Bio 1	23.2	-	36.5	36.5	-	-	-	1.5	2.3
	Bio 2	19.5	-	46.1	30.6	-	-	-	1.5	2.3
	Bio 3	-	23.2	36.5	36.5	-	-	-	1.5	2.3
	Bio 11	23.2	-	36.5	31.7	4.8	-	-	1.5	2.3
	Bio 12	23.2	-	36.5	31.7	-	4.8	-	1.5	2.3
	Bio 13	23.2	-	36.5	31.7	-	-	4.8	1.5	2.3

ACKNOWLEDGEMENTS

This work was funded by the French FUI (Fond Unique Interministériel), under the aegis of Plastipolis and Mov'eo Competitivity Poles, as the Collaborative Project "3V" (Vernis Verts à très longue durée de Vie). We gratefully acknowledge all the partners from the 3V Project for fruitful discussions. We also thank Ruben Vera from the Centre de Diffractométrie H. Longchambon for the XRD experiments, and the NMR Polymer Center of the "Institut de Chimie de Lyon" (FR3023), for assistance and access to the NMR facilities. Finally Pierre Alcouffe (UMR5223) is gratefully acknowledged for his numerous attempts to visualize the nanophases by AFM and TEM.


J Peydecastaing
email: jerome.peydecastaing@ensiacet.fr

C Vaca-Garcia

E Borredon

S El Kasmi

Mixed acylation of Scots pine sawdust and impact on hydrophobicity

Keywords: Acetylation, fatty acylation, hydrophobicity, mixed anhydrides, wettability, wood esters

Acetic-fatty esters of Scots pine sawdust (SPS) were obtained by reaction of SPS with mixtures containing acetic-fatty anhydrides and no solvent or catalyst. Such mixtures were synthesized by reaction between a carboxylic acid and acetic anhydride. The global reaction of acetic anhydride and a fatty acid yields at equilibrium a mixture of five compounds: acetic-fatty anhydride, acetic anhydride, fatty acid, acetic acid and fatty anhydride. The influence of temperature, molar ratio, reaction time and length of the fatty chain on the esterification and on the ratio of grafted acetyl/fatty acyl was investigated. Static contact angles (CAs) with water were measured over 5 min. CA values were dependent on the fatty acyl content and independent of the acetyl content. The minimum ester content of the oleoyl group required to obtain permanent water repellency was 25 mmol kg-1 . Water vapour adsorption measurements indicated that in contrast to water repellency, hydrophobicity to water vapour was correlated with the total mass acyl content.

Introduction

Wood, even if it is the most important renewable material, presents limiting factors such as: dimen sional instability, susceptibility to biological degra dation and photodegradation. When the use of less durable species such as fast-growth softwoods is considered, these limitations are considerably increased. Chemical modification of wood with linear carboxylic anhydrides is one of the potential methods for improving these properties and many reviews have been published in this field (Kumar, 1994;N orimoto, 2001). U sing local non-durable species becomes attractive and the need for importing tropical wood is greatly reduced. The ecological benefit is evident.

The treatment of wood by acetic anhydride (acetylation) has been the most investigated and it has been shown that the dimensional stability (Stamm & Tarkow, 1947;Rowell et al., 1987;Militz, 1991), decay resistance (Stamm & Baechler, 1960;Rowell et al., 1987;[START_REF] Wang | Decay and termite resistance of planted tree sapwood modified by acetylation[END_REF]Hill et al., 2006) and photostability (Dawson & Torr, 1992;[START_REF] Chang | Comparisons of the photostability of esterified wood[END_REF][START_REF] Tolvaj | Light source dependence of the photodegradation ofwood[END_REF] of such treated wood were enhanced. Recently, the company Titanwood has started commercial pro duction of acetylated wood (Nashery & Durbin, 2005).

Work with other linear-chain anhydrides has been relatively limited. Symmetrical carboxylic anhy drides, presenting aliphatic chain lengths from C2 to C7, have been investigated and their reactivity has been revealed to decrease owing to steric hindrance with the increase in the aliphatic chain [START_REF] Hill | The dimensional stabilisation of Corsican pine sapwood by reaction with carboxylic acid anhydrides-The effect of chain length[END_REF][START_REF] Dawson | Reactivity of radiata pine sapwood towards car boxylic acid anhydrides[END_REF]Li et al., 2000;[START_REF] Chang | Moisture excluding efficiency and dimensional stability of wood improved by acylation[END_REF]. Improved dimensional stability has been highlighted as for acetylation, but it has been shown that the length of the fatty chain grafted on wood had no impact on the antishrink efficiency (ASE). Instead, ASE has been found to be a fonction only of the weight percentage gain (WPG; extent of chemical modification in mass) [START_REF] Hill | The dimensional stabilisation of Corsican pine sapwood by reaction with carboxylic acid anhydrides-The effect of chain length[END_REF]Li et al., 2000).

Mixed anhydrides, i.e. with two different car boxylic acid radicals, are reactive molecules that have seldom been investigated for reactions with wood constituents [START_REF] Vaca-Garcia | Cellulose esterification with fatty acids and acetic anhydride in lithium chloride/N,N-dimethylacetamide med ium[END_REF][START_REF] Vaca-Garcia | Solvent-free fatty acylation of cellulose and lignocellulosic wastes. Part 2. Reactions with fatty acids[END_REF][START_REF] Chemeris | Synthesis of cellulose esters in a trifluoroacetic acid medium. Effry Tsellyulozy i Krakhmala: Sintez, Svoistva, Primenenie, Materialy[END_REF]. These unstable molecules are very reactive and have the advantage of permitting the grafting of two acyl groups on wood. The acetic-fatty anhy drides enable a dimensionally stable treated wood to be obtained, showing better water repellency than acetylated wood (Magne et al., 2003).

Because of their instability, the use of mixed anhydrides must be done in situ. Such reactive mixtures or media are obtained by reaction between a symmetrical anhydride (acetic) and a carboxylic acid yielding at equilibrium a mixed anhydride, two symmetrical anhydrides and two fatty acids (Peydecastaing et al., 2009a).

The aim of this paper was to study the reactivity of mixed acetic-fatty anhydrides mixtures on wood and to evaluate the impact of the fatty chain grafted on the wettability of the mixed wood ester. Fatty acids from CS to C 18 were used as the aliphatic long chain. In contrast to previous studies, no solvent or catalyst was used. The heterogeneity of wood blocks is a handicap when focusing on the reactivity of the constituents of wood, and the reactivity can be limited by diffusion phenomena. Therefore, the reactivity of wood constituents was investigated using Scots pine sawdust (SPS). This method will yield information on the straight reactivity of mixed anhydrides towards wood constituents. Results obtained with SPS may be compared with results obtained in previous works on cellulose (Peydecastaing et al., 2009b, c). The hydrophobie properties ofthese novel wood derivatives will be described by measur ing the wettability of sawdust moulded pellets and by measurement of the sorption and desorption iso therms of the treated SPS.

Materials and methods

Chemicals and standards

Acetic anhydride and carboxylic acids: propionic (C3:0), octanoic (C8:0), capric (C 10:0), lauric (Cl2:0), myristic (Cl4:0), palmitic (Cl6:0) and oleic (Cl8:l), of 99%+ purity were purchased from Sigma-Aldrich (France). Pentadecanoic acid (Cl5:0) was used as an internai standard in gas chromatography and was obtained from Fluka (France) (99% purity). Phosphoric acid, 85% solu tion in water, was purchased from Acros (France). Trimethylsulphonium hydroxide (TMSH) was obtained from Macherey-Nagel (France) as a 0.2 mol 1 -1 solution in methanol. Tert-butyl methyl ether (MTBE), high-performance liquid chromato graphy (HPLC) grade was purchased from Scharlau (Spain). All the chemicals were stored at 4 °C before use.

Synthesis of the reaction media

Reaction media were prepared on a 300 ml scale. The appropriate amounts (molar ratio varying from 1 :2 to 2: 1) of fatty acid and acetic anhydride were added to a 500 ml glass reactor equipped with a condenser. Reactions were carried out at 100 °C with mechanical stirring at 500 rpm for 1 h. The reaction media were analysed by reversed-phase HPLC (Peydecastaing et al., 2008).

Scots pine sawdust esterification

SPS was Soxhlet extracted with ethanol during 8 h, dried overnight at 103 °C, and then conditioned at 25 °C and 60% relative humidity (RH) for 2 weeks to reach its equilibrium moisture content (EMC): 8.8%. Reactions were performed in 50 ml reactors equipped with a condenser. One gram of condi tioned SPS was stirred at 350 rpm in 15 ml of reaction medium without catalyst at the desired temperature and for the required duration. After cooling down to about 80 °C, 20 ml of ethanol were added to recover the soluble fraction. SPS esters were separated by filtration over fritted glass and purified by Soxhlet extraction with ethanol for 8 h. The purified product was then dried under vacuum at 70 °C at least for 24 h and to constant weight.

Determination of the f atty acyl content

The method used was based on the transesterifica tion of ester functions with TMSH followed by gas chromatography (GC) analysis. This technique has been developed for the determination of the ester content on cellulose esters and has been described in a previous paper (Peydecastaing et al., 2009d). The conditions were optimized for wood and the ester content was determined as follows.

Sample preparation.

A precise quantity of about 20 mg of treated SPS was introduced into a 2 ml vial. Tuen, 500 µl of 0.5 mmol 1-1 pentadecanoic acid in MTBE and 250 µl of TMSH were added. The vial was hot-stirred for 60 min in a VorTemp 56 shaking incubator at 1200 rpm and 75 °C. Once the sample had cooled down and the solid had been decanted, the formed fatty acid methyl esters (FAMEs) in MTBE were quantified by GC.

Gas chromatography. The GC analysis was carried out using a Varian 3900 gas device equipped with a Varian 8400 autosampler, a split/splitless injector, a flame ionization detector (FID) and a Varian r, CP7 419 capillary column (50 m, 0.25 mm i.d., 0.25 µm film thickness). Helium was used as carrier gas, at a flow rate of 1.2 ml min -1 • The temperature of the injector used was set at 260 °C and the split ratio at 1 :20. The oven temperature was pro grammed as follows: 110 °C for 4 min, then increased to 230 °C at a rate of 15 °C min -1, then 230 °C for 7 min. This enabled the separation of the corresponding FAMEs within 19 min. The tempera ture of the FID detector was set at 260 °C. From the GC analysis the concentration of fatty acyl groups, C can be determined.

Calculation of the ester content off atty a cyl groups in mol g-1 •

• C f : concentration of the FAME in MTBE determined by GC (in mol 1-1 )

• Vi: volume of interna! standard (pentadeca noic acid) solution added to the analysed sample (in litres) • mi: mass of the SPS sample analysed m MTBE with TMSH (in grams).

Determination of the acetyl content. The acetyl content was determined by performing an alkaline hydrolysis followed by acidification and GC analysis of the acetic acid formed. Conditions were performed on commercial cellulose acetates and acetylated SPS and the optimal conditions are as follows.

Sample preparation. A precise amount of about 20 mg of treated SPS was weighed and introduced into a 2 ml vial. Tuen, 1000 µl of aqueous sodium hydrox ide (0.5 N) and 500 µl of a 0.5 mmo11-1 propionic acid solution in water were added. The vial was hot stirred for 5 h in a VorTemp 56 ™ Shaking incubator set at 1200 rpm and 100 °C. The sample was cooled down, 45 µl of H 3 P0 4 was added and the vial was stirred at 1200 rpm for 10 min. Once the solid had been decanted, a small sample of the supernatant in the reaction mixture was analysed by GC.

Gas chromatography

The GC analysis was carried out as described above.

Separation was achieved in a CP-Select CB for FFAP fused silica capillary column (CP7845, Varian; 25 m, 0.32 mm i.d., 0.25 µm film thickness. Helium was used as carrier gas, at a flow rate of 2.6 ml min -1 • The temperature of the injector used was set at 250 °C and the split ratio at 1: 100. The oven temperature was programmed as follows: 80 °C for 1 min, then increased to 145 °C at a rate of 20 °C min -1 , then 250 °C for 2 min. This enabled the separation of the acetic and propionic acids within 12 min. The temperature of the FID detector was set at 270 °C. From the GC analysis the concentration of acetyl groups, C 2 , can be deter mined.

Calculation of the aoetyl content in mol g-1 •

• C 2 : concentration of the acetic acid after saponification and acidification determined by GC (in mol l-1 )

• v;: volume of interna! standard (propionic acid) solution added to the analysed sample (in litres) • mi: mass of the treated SPS sample analysed (in grams).

Contact angle measurements

Pellets (10 mm diameter) of the esterified products were obtained using a laboratory press (10 t) and a conventional pellet mould at room temperature.

Metal surfaces in contact with the sample were carefully cleaned to avoid pollutant sources. A drop (3 µl) of Milli-Q water was placed on the surface of the pellet and the static contact angle (CA) was measured with a goniometer (GBX Instruments, France), equipped with an automatic camera regis tering still images every 0.1 s. CAs were measured automatically using the triple-point calculation method. Three specimens were used for each sample. Two CA measurements were made per specimen.

Dynamic vapour sorption analysis

All experiments were performed on a dynamic vapour sorption (DVS) automated gravimetric vapour sorption analyser (Surface Measurement Systems, London, UK). The DVS measures the uptake and loss of vapour gravimetrically using a Cahn D200 recording ultra-microbalance with a mass resolution of ±0.1 µg. The RH around the sample was controlled by mixing saturated and dry carrier gas streams using mass flow controllers. The temperature was maintained constant (± 0.1 °C) by enclosing the entire system in a temperature controlled incubator. The samples were stored in a desiccator. For each experiment SPS ester was immediately placed in the DVS under a continuous stream of dry (<0.1 % RH) air. A sample size between 5 and 8 mg was used. Before being exposed to any water vapour the samples were dried at 0% RH to remove any surface water present and establish a dry, baseline mass. Next, the samples were exposed to the following RH profile: 0%, 10%, The individual acetyl and oleoyl contents obtained at 160 °C are around two times and four times higher than those obtained at l00 °C. These results highlight the fact that the esterification reaction is highly dependent on the temperature. FTIR analysis was performed of the SPS samples treated at various temperatures. The ratio was calculated and plotted of the transmission intensities of the ester band at 1733 cm -1 and of a constant band characteristic of cellulose at 1641 cm -1 (Figure 4). The trends for the acetyl and oleoyl ester content followed the same tendency. These FTIR analyses also allowed confirmation that no residual carboxylic acids were present after extraction.

RCOOH + CH 3 C(O)OC(O)CH 3 CH 3 C(O)OC(O)R CH 3 COOH + RCOOH + CH 3 C(O)OC(O)R RC(O)OC(O)R CH 3 COOH +
::---1.4 E � 1.2 m r-- � 1.0 • -• -• -•-• -• -• -• -• -• -• -•Untreated SPS .c
Even though Figure 3 may give the impression that the oleoylation and the acetylation follow parallel trends, in reality, the increase in temperature causes an enhancement of the global grafting but with amplified proportions of fatty chains (Figure 5). When changing the temperature from l00 °C to 175 °C, the ratio .6.EC 2 !.6.ECr diminishes, with values of 112 and 41, respectively.

One hypothesis that may explain the decrease in this ratio is that the composition of the reaction medium is dependent on the temperature. Never theless, as demonstrated in a previous report (Peydecastaing et al., 2009a), the composition of the medium at equilibrium was scarcely dependent on the temperature. For the considered temperature, the variation in the composition would only be 3% at most. This small variation cannot account for a decrease of 50% in the substituents ratio.

Another possible explanation for the decrease in the ratio could be that the vaporization of acetic anhydride and acetic acid, whose boiling points are l l6 °C and 145 °C, respectively, could also affect the composition of the reaction medium and shift the equilibrium. Nevertheless, this is not the case because the decrease in the ratio is linear in the range of temperature concerned. The third and most likely hypothesis would be that the energy of activation for the grafting of acetates is lower than for the grafting of oleates.

The values of the ratio of grafting tend to reach a plateau at l 60 °C. More acetyl groups are grafted than oleoyl groups on SPS because the mixture still contains acetic anhydride and the acetic---oleic anhy dride can lead to the formation of both acetates and oleates. Moreover, the steric hindrance of the fatty chain can account for such a difference; reasonably, the acetyl group would be more able to reach hydroxyl functions in the cellulose microfibrils than the oleoyl group from the acetic---oleic and oleic anhydrides.

Comparing these results with the values of ratio obtained when treating cellulose in the same condi tions in a previous work (Peydecastaing et al., 2009b), the ratio of grafting is more important in the case of cellulose. The reactivity of cellulose pulp is lower than that of SPS; the accessibility of the hydroxyl groups towards the fatty acyl groups is more difficult because of the crystallinity of cellu lose. Hydrogen bonds limit considerably the reactiv ity of cellulose without a pretreatment or swelling agent. The reactivity of lignins and hemicelluloses present in SPS is much higher than the reactivity of cellulose, which explains the higher reactivity of SPS compared with the reactivity of cellulose pulp.

Reaction time is also an important factor (Figure 6). When a mixture with a molar ratio of 1.5 is treated at 140 °C, the grafted acetate and oleate contents are multiplied by two when passing from 30 min to 4 h of reaction. But, surprisingly, the ratio of grafting (acetate/oleate) remains constant at a value of about 73 ±3 for the duration of the reaction (Figure 7). Since the ratio of grafting is dependent on the temperature only and not on the reaction time, the hypothesis formulated above concerning the energies of activation is confirmed. Fatty acyl groups need more energy to react with hydroxyl groups. A limit to this ratio is reached regardless of the temperature. The limitation is due only to the steric hindrance encoun tered by the fatty compounds.

In the following experiments, the temperature and reaction time were kept constant (140 °C, 1 h) and only the molar ratio reagents was varied. The plot of 6.EC 2 and 6.EC 18 , 1 as a fonction of the molar ratio shows an increase of 6.EC 18 , 1 with the molar ratio accompanied by a decrease in 6. EC 2 (Figure 8). It is ---------------------2 3 4 Time (h) important to note that a sample treated with pure acetic anhydride, i.e. a molar ratio of zero, presents a .6. EC 2 of 3664. This is the same order of magnitude as for acylation of SPS with a molar ratio of 0.5 with a .6.EC 2 of 3005. When the molar ratio increases, the concentration of reactive molecules tends to dimin ish to reach eventually the value of O for an infinite molar ratio (pure oleic acid). .6. EC 2 / .6. EC 18 , 1 de creases when the molar ratio of reagents increases (Figure 9). The theoretical value of .6.EC 2 / .6.EC 18 , 1 for a molar ratio of O (pure acetic anhydride) is infinite. This is why .6. EC 2 decreases with the molar ratio as the total concentration of molecules suscep tible to acetylate SPS globally decreases. In contrast, .6.EC 18,1 increases despite the diminishing of the total anhydride content in the mixture. This could et al., 2009a). The concentration of oleic anhydride also increases, thus increasing in the mixture the concentration of molecules susceptible to forming fatty esters of SPS.

o Plnc sawdust -----Cellulo5e --
Finally, the nature of the fatty chain was varied by preparing different mixtures from acetic anhydride and fatty acids bearing saturated aliphatic chains from C8 to C16 at a molar ratio of 1.5 (Figure 10).

The plot of the ester contents as a function of the chain length (Figure 10) shows a decrease in the number of grafted fatty chains ( .6. ECr) when the number of carbon atoms of the aliphatic chain increases. This can be explained by the steric hindrance encountered by the bigger molecules and by the presumed increase in the activation energies of the reaction of esterification with the length of the fatty chain. !',. EC 2 keeps approxima tel y the same value and seems to be independent of the fatty acid used for the mixture synthesis. However, a slight increase in its value with fatty chain length was noted; this was due to the higher reactivity of the mixed anhydride. Indeed, the increase in the fatty chain length makes the mixed anhydride more asymmetrical and therefore more reactive [START_REF] Ami | Chemical modification of wood. I. Use of trifluoroacetic anhydride in the esterification of wood by carboxylic acids[END_REF]. The ratio of grafting (Figure 11) is therefore correlated with the difficulty in grafting fatty molecules; the steric hindrance of the fatty chains favours the grafting of acetyl groups. Water repellency prevents or slows down the rate at which liquid water is absorbed by a material. It may therefore be correlated with the water CA measured at a certain point in time. In the case of SPS, it was necessary to make an object with a regular smooth surface. In this work, static CAs with water were measured on pellets moulded from the treated SPS samples.

First of all, the CAs of untreated SPS samples were determined, and an average value of 39 ±4 ° was found at the initial time (0 s). The drop after deposition was totally absorbed by the material in less than 3 s. Untreated SPS can therefore be considered highly hydrophilic. The CAs of water drops deposited on the pellets prepared from SPS samples, treated at various temperatures with a molar ratio of 1.5 for 1 h, were then measured. The evolution of the CA as a function of time is shown in Figure 12. Although all the samples show high CA values (>95 °) at the initial time, some of them saw their CA value decrease after a few seconds. The sample treated at 100 °C, for example, can be considered as wettable compared with the samples treated at temperature higher than 120 °C. From a treatment temperature of 140 °C, the moulded pellets of SPS esters present high water repellency with CA values of 105 ° and 109 ° at 140 °C and 160 °C, respectively. The CA values were mea sured for 5 min, and for the samples treated at 140 °C and l60 °C there was a slight decrease in the CA values, but no absorption of the water drop. Evaporation of the water drop due to the heat from the light may explain this decrease. Therefore, an experiment was set up with a non-porous tablet of Teflon. As the slope of the line was identical to those of SPS esters, it was concluded that this decr ease was effectively induced by the measurement conditions.

A plot of the CA values measured after 1 min of contact with the pellets as a function of the temperature is also indicated in Figure 12. Con sidering the ester content values of these SPS esters, their water repellency is dependent on the grafting. Since the acetyl and fatty contents both increase, it is difficult to determine whether the increased water repellency is dependent on the fatty content or on the total grafting. The same observation was made with the samples treated at 140 °C with acetic---oleic anhydride mixture (molar ratio of 1.5) during different reaction times (Figure 13).

The water repellency of the sample treated at molar ratios between 0 and 2 (at 140 °C for 1 h) was studied. A molar ratio of 0 correspond to acetylation. Figure 14 shows the CA of these samples as a function of time. The acetylated sample (molar ratio = 0) did not show water repellency even if its total ester content was higher than that of all the other samples. Its CA at the initial time was 52 ± 2 ° and the water drop was absorbed in less than 4 s, the same as for untreated sample. The high acetyl content does not give water repellency to the treated SPS. The literature reports acetylated wood blocks presenting CA values of 98 ° [START_REF] Hill | The dimensional stabilisation of Corsican pine sapwood by reaction with carboxylic acid anhydrides-The effect of chain length[END_REF]. However, these values were measured just after the deposit of the water drop and no information is given on the "permanent" character of this property. Moreover, the reported sample has an acetyl content of about 5700 meq kg-1 , which is higher than the value in the present study(3500 meq kg-1 ).

The wettability of the samples decreases with the molar ratio of the mixture used for the treatment. The SPS samples treated with mixtures with molar ratios from 1.25 to 2 can be qualified as highly water repellent. Their initial content angles are between 105 ° and 110 ° and no absorption of the water drop by the material is observed in 5 min of measurement. These results are interesting as there is no correlation between the wettability of the samples and their total ester contents. 1::,, EC 2 decreases with the molar ratio, while 1::,,EC 18 , 1 increases. This seems to indicate that the water repellency of the treated samples depends only on the fatty acyl content. The acetate content seems therefore to have no influence on the water repellency of the pellets. To confirm this hypothesis the CA values after 1 min of measurement were plotted as a fonction of the fatty content added by the treatment (Figure 15). A good correlation can be clearly observed (R 2 =0.961). In contrast, no corre lation was found with the acetyl content. This indicated that by grafting fatty chains to a low extent on SPS, an important water-repellent character can be given to the material. A threshold at 25 mmol kg-1 fatty acyl content for the SPS esters is highlighted in Figure 15.

The impact of the aliphatic chain on the water repellency of the treated SPS is shown in Figure 16. Although the grafting is higher for the short chains, with ratios acetyl/fatty acyl of 6 in the case of CS and 7 6 in the case of C 18: 1, the CA values are more or less identical.

1-füter vapour adsorption

Hydrophobicity is a concept related to the affinity of a material with water. There is no absolute scale for hydrophobicity. However, there are quantitative parameters directly related to these concepts, e.g. the EMC. In this work, hydrophobicity was considered as the capacity of SPS to adsorb water in vapour form.

Eight of the treated samples were selected to be analysed by DVS with water. The aim was to eval uate the influence of the extent of grafting and of the nature of the fatty chain. Samples representative of the whole range of variation of these parameters were selec ted (Table I). The sorption isotherms of these samples are gathered in Figure 17. The values of EMC are spread in a wide range owing to the significant differ ences among ail the samples. The sample that shows the highest hydrophobicity is the acetylated sawdust (SPSl). Any of the samples containing grafted fatty chains are less hydrophobie than this sample containing only C2 aliphatic chains. This surprising fact seems to be in contradiction with the values of CA.

3664 SPS2 C z-C 8 1981 314 SPS3 C 2 --C 1 0 2075 194 SPS4 C 2 --C 12 2108 121 SPS5 C 2 --C 1 6 2245 41 SPS6 C 2 --C 1 s: 1 2018 27 SPS7 C 2 --C 1 s : 1 1649 28 SPS8 C 2 --C 1 s: 1 1079 10
Indeed, all the acetic-fatty SPS samples exhibited permanent hydrophobicity, whereas the acetylated sample did not. Remember that the water repellency is dependent only on the !}.EC f . In this case, the hydrophobicity measured with the DVS device does not depend on the fatty acyl content. Instead, it depends exclusively on the total acyl content ex pressed on a mass basis (Figure 18). Other correla tions were explored (in function of the molar acetyl content, or the molar fatty acyl content), but none of them was satisfactory. This is not first time that this type of correlation was been expressed. [START_REF] Hill | The dimensional stabilisation of Corsican pine sapwood by reaction with carboxylic acid anhydrides-The effect of chain length[END_REF] (2003) was due to remaining free fatty acids is false. As demon strated in the present study, a small amount of grafted fatty chains compared to acetyl groups enables permanent water repellency to be attained.

Conclusions

Mixed SPS esters bearing acetyl and fatty acyl groups can be synthesized by reaction in a medium prepared from acetic anhydride and a fatty acid without the use of any solvent or catalyst. The relative proportion of the acetyl and fatty acyl groups grafted on SPS can be controlled by an appropriate selection of the molar ratio of the initial reagents, the The EMC has been demonstrated to be depen dent on the mass acyl proportion, in contrast to the water repellency. The nature of the substituent had no impact in the studied domain.

The mixed anhydride mixture use here enabled chemically modified substrates to be obtained that showed both hydrophobicity and water repellency.

  Figure 3. Influence of the reaction temperature on the acetyl and fatty acyl contents.

Figure 4 .

 4 Figure 4. Ratio oftwo Fourier transform infrared bands transmis sion intensities of solvent extracted SPS treated at various temperatures. SPS = Scots pine sawdust.

. 175 TemperatureFigure 5 .

 1755 Figure 5. Influence of the reaction temperature of the treatment on the grafting ratio acetate/oleate. Molar ratio of 1.5.

Figure 6 .

 6 Figure 6. Influence of the duration of the treatment on the acetyl and oloeyl content. Reaction at 140 °C.

Figure 7 .Figure 8 .

 78 Figure 7. Influence of the reaction time on the grafting ratio acetate/oleate.

  be explained by the fact that in a mixture of acetic anhydride and oleic acid the concentration of acetic fatty anhydride remains constant, whereas the con centration of acetic anhydride diminishes (Peydecastaing

Figure 1

 1 Figure 9. Influence of the molar ratio of the mixture on the grafting ratio acetate/oleate.

  Scots pine sawdust esters

Figure 11 .Figure 12 .

 1112 Figure 11. Influence of aliphatic chain length on the grafting ratio acetate/oleate.

Figure 13 .Figure 14 .

 1314 Figure 13. Water contact angles measured at 60 s ofmixed acetic oleic Scots pine sawdust esters treated at various reaction times.

Figure 15 .

 15 Figure 15. Water contact angles of mixed acetic---oleic Scots pine sawdust esters as a function of the oleoyl content.

  Figure 16. Water contact angles of mixed acetic-fatty Scots pine sawdust esters measured at 60 s. A synthesized SPS acetate sample is also shown.

  demonstrated that the ASE of Corsican pine was dependent on the WPG. The latter is a parameter equivalent to the mass acyl content. These results indicate that water repellency and the ability to adsorb water vapour are two different concepts.Finally, comment is made on recent works on the esterification of guiacol and glucose as lignin and cellulose model compounds by mixed acetic-oleic anhydride mixtures.Lyon et al. (2008) did not find any evidence of the grafting of fatty chains and concluded that the process claimed by Magne et al.

Figure 17 .

 17 Figure 17. Dynamic vapour sorption analysis of samples described in Table I. SPS = Scots pine sawdust.

  Figure 18. Influence of the acyl groups content on the hydro phobicity of esterified sawdust. RH =relative humidity. nature of the fatty acid and the conditions of treatment. The CA of SPS acetate is null, indicating that water repellency depends only on the fatty acyl content in the studied range of esterification. Mixed acetic-fatty SPS esters shaped into pellets could exhibit water repellency at low fatty contents. The CA values of such mixed SPS esters lay between 90 ° and 110 °. In the case of acetic--oleic SPS esters the minimum fatty acyl content to reach water repel lency was 25 mmol kg-1 •The EMC has been demonstrated to be depen dent on the mass acyl proportion, in contrast to the water repellency. The nature of the substituent had no impact in the studied domain.The mixed anhydride mixture use here enabled chemically modified substrates to be obtained that showed both hydrophobicity and water repellency.

Table I .

 I Acy! contents of treated samples selected for dynamic vapour sorption analyses.

	Sample	tillC 2	l:,ECr
	SPSl C 2

  /::,. Acy! fraction (total) (g kg-1 ) was reduced to a pure acetylation of the substrates. The analytical method used to character ize the chemically modified model compounds (MAS-DEC, [ 1 3 C]NMR) is evidently not adapted to the characterization oflow degrees of substitution. In such cases, specific analytical methods are required(Peydecastaing et al., 2009d). The daim byLyon et al. (2008) that the water repellency of modified wood observed by Magne et al.

	3664 2295 2269 2229 2286 2045 1677 1089	154.0 166.3 138.5 120.6 105.2 91.9 76.7 48
	(2003)

Acknowledgements

We thank LAPEYRE (France) for a research grant and Mr Michael Charton for his valuable technical help.

Forster, S. C. (2006). Decay resistance of anhydride modified Corsican pine sapwood exposed to the brown rot fungus Coniophora puteana. Holzforschung, 60, 625---629. Kumar, S. (1994). Chemical modification of wood. Wvod and Fiber Science, 26, 270--280. Li, J.-Z., Furuno, T., Katoh, S. & Uehara, T. (2000). Chemical modification of wood by anhydrides without solvents or catalysts. Journal of Wvod Science, 46, 215-221. Lyon, F., Thevenon, M. F., Pizzi, A., Tondi, G., Despres, A., Gril, J. & Rigolet, S. (2008). In: Wvod preservation by a mixed anhydride treatment: Using simple models of polymeric wood compounds for an 13C NMR investigation. International Research Group on Wood Protection IRG 39, Istanbul, Turkey, May 25-29, 2008. Magne, M., El Kasmi, S., Dupire, M., Morard, M., Vaca-Garcia, C., Thiebaud-Roux, S., et al. (2003). Method for treating lignocellulosic materials, in particular wood and material obtained by said method. WO084723. Militz, H. (1991). The improvement of dimensional stability and durability of wood through treatment with non-catalyzed acetic-acid anhydride. Holz als Roh-und Werkswff, 49, 147-152. Nashery, K. & Durbin, G. J. (2005). Impregnation process. WO2005077626. Norimoto, M. (2001). Chemical modification ofwood. In D. N. S Hon & N. Shiraishi (Eds.), Wbod and Cellulosic Chemistry (2nd ed.) (pp. 573-598). New York: Marcel Decker. Peydecastaing, J., Vaca-Garcia, C. & Borredon, E. (2008). Quantitative analysis of mixtures of various linear anhydrides and carboxylic acids. Chromawgraphia, 68, 685---688. Peydecastaing, J., Vaca-Garcia, C. & Borredon, E. (2009a). Consecutive reactions in an oleic acid and acetic anhydride reaction medium. European Journal of Lipid Science and Technology, 111, 723-729. Peydecastaing, J., Vaca-Garcia, C. & Borredon, E. (2009b). Reactivity of mixed acetic-fatty anhydride mixtures toward cellulose. Cellulose (in press). Peydecastaing, J., Vaca-Garcia, C. & Borredon, E. (2009c). Hydrophobicity of mixed acetic-fatty cellulose esters. Cellu lose (in press). Peydecastaing, J., Vaca-Garcia, C. & Borredon, E. (2009d). Accurate determination of the degree of substitution of long chain cellulose esters. Cellulose, 16, 289-297. Rowell, R. M., Simonson, R. & Tillman, A. M. (1987). A process for improving dimensional stability and biological resistance of lignocellulosic material. European Patent EN 0213252 Bl. Stamm, A. J. & Baechler, R. H. (1960). Decay resistance and dimensional stability of five modified woods. Forest Products Journal, 10, 22-26.


Sandrine Waig Fang

Pascale De Caro

Pierre-Yves Pennarun

Carlos Vaca-Garcia

Sophie Thiebaud-Roux
email: sophie.thiebaudroux@ensiacet.fr

Synthesis and characterization of new polyesters based on renewable resources

Keywords:

A series ofnon-crosslinked biobased polyesters were prepared from pentaerythritol and aliphatic dicar boxylic acids, including fatty acids grafted as side-chains to the backbone of the polymer. The strategy utilized tends to create linear polymers by protecting two of the hydroxyl groups in pentaerythritol by esterification with fatty acids before the polymerization reaction. The solvent-free syntheses were performed under vacuum and catalyzed by the ion-exchange resin Amberlyst 70. The maximum yield was around 98%. Pristine polyesters had average molecular weights of about 10 4 g/mol according to SEC-MALLS analysis. Melting temperatures and extent of crystallinity were determined by differential scanning calorimetry. By using relatively short fatty acids, such as lauric acid, soft materials were obtained with low crystallinity and a melting point below room temperature, whereas longer side-chains, such as behenic acid, gave brittle polymers with higher melting temperatures and crystallinity. The use of a short chain dicarboxylic acid, such as succinic acid, resulted in doser side-chains and promoted higher crystallinity and melting temperatures. In order ta improve the thermal properties of these materials, a series of copolyesters were designed by developing synthetic methods to approach a randomand a block-copolymerization. A wide range of properties was thus obtained according ta the composition of these navel copolyesters.

Introduction

ln the general context related to the dwindling of fossil resources, it is urgent to find new materials based on biomass and renewable resources to replace petroleum-based counterparts. A large number of monomers and polymers are available for a more sustainable chemistry, including poly(lactic acid), lignins, furan derivatives, sugars (Belgacem and Gandini, 2008;Gandini, 2011), and fatty acids derivatives (Montera de Espinosa and [START_REF] Montera De Espinosa | Plant oils: the perfect renewable resource for polymer science[END_REF]Xia and Larock, 201 O;[START_REF] Meier | Plant oil renewable resources as green alternatives in polymer science[END_REF]. Sorne polyesters based on sucrase fatty acid esters or ricinoleic acid-lactic acid copolyesters are being developed as mild and non-toxic emulsifiers for food, cosmetics and pharmaceuticals [START_REF] Akoh | Lipid based synthetic fat substitutes[END_REF][START_REF] Slivniak | Hydrolytic degradation and drug release of ricinoleic acid-lactic acid copolyesters[END_REF]. Other biodegradable polyesters, based on glycerol, sorbitol or pen taerythritol have been widely studied for their adhesive properties [START_REF] Kricheldorf | Biodegradable hyperbranched aliphatic polyesters derived from pentaerythritol[END_REF] or for medical applications [START_REF] Kline | One-step biocatalytic synthesis of linear polyesters with pendant hydroxyl groups[END_REF][START_REF] Kumar | Versatile route ta polyol polyesters by lipase catalysis[END_REF] or for their good water solubility [START_REF] Zhao | Synthesis and application of water-soluble hyperbranched polyesters from maleic anhydride and glycerol[END_REF]. Given the multifunctional nature of such polyols (glycerol or pentaerythritol), the preparation of homogeneous non-crosslinked polyesters by polycondensation using diesters or dicarboxylic acids as monomers is a complex task because their numerous hydroxyl groups result in the creation of networks [START_REF] Kricheldorf | Biodegradable hyperbranched aliphatic polyesters derived from pentaerythritol[END_REF][START_REF] Murillo | Characterization of hyper branched polyol polyesters base don 2,2-bis (methylol propionic acid) and pentaerythritol[END_REF][START_REF] De Meireles Brioude | Synthesis and characteri zation of aliphatic polyesters from glycerol, by-product of biodiesel production, and adipic acid[END_REF]. Therefore, such syntheses may require a multistep reaction pathway including protection and deprotection of monomer units. In this context, some researchers reported a simple strategy to prepare aliphatic poly(butylene succinate-co butylene maleate) and polybutylene tartrate [START_REF] Zhang | Synthesis and character ization of poly(butylene succinate-co-butylene malate): a new biodegrad able copolyester bearing hydroxyl pendant groups[END_REF][START_REF] Hao | New facile approach ta navel water soluble aliphatic Poly(butylene tartarate )s bearing reactive hydroxyl pen dant groups[END_REF]. [START_REF] Shibata | Synthesis of polyester having pendent hydroxyl groups via regioselctive dehydration polycondensations of dicarboxylic acids and diols by low temperature polycondensation[END_REF] proposed a one-step bulk synthesis of polyesters with pendent hydroxyl groups under mild conditions using rare-earth catalysts. Other chemical routes to produce polyester by condensation polymerization were devel oped from vegetable oils derivatives: ring-opening polymerization of lactones, polycondensation between a diacid (or diester) and a diol ( or monoglycerides) or polycondensation of hydroxy-acids or esters (Xia and Larock, 201 O;[START_REF] Bakare | Polyesters from renew able resources: preparation and characterization[END_REF]. For example, high molecular weight linear polyesters can be obtained by three steps: ozonolysis and reduction of castor oil followed by methanolysis and then, transesterification of the obtained hydroxy methyl ester for polymerization [START_REF] Petrovic | A chemical route ta high molec ular weight vegetable oil-based polyhydroxyalkanoate[END_REF].

In recent years, enzyme-catalyzed syntheses have emerged as interesting green alternatives [START_REF] Meier | Plant oil renewable resources as green alternatives in polymer science[END_REF][START_REF] Kelly | Lipase-catalyzed synthesis of polyhydric alcohol-poly(ricinoleic acid) ester star polymers[END_REF][START_REF] Uyama | Enzymatic synthesis of polyesters via polyconden sation[END_REF], allowing to limit branching and obstruct cross-linking reactions [START_REF] Yang | Poly(oleic diacid-co-glycerol): comparison of polymer structure resulting from chemical and lipase catalysis[END_REF]. Thus, immobi lized lipases (Novozyme 435) were used for a one pot synthesis to prepare both linear and hyperbranched copolymers containing glycerol [START_REF] Kulshrestha | Glycerol copolyesters: contrai of branching and molecular weight using a lipase catalyst[END_REF]. In the presence oftoluene, lipase catalyzed polycondensation of unsaturated dicarboxylic acids with diols Ied to polyesters containing carbon-carbon double bonds [START_REF] Yang | Two-step biocatalytic route ta biobased functional polyesters from ù-carboxy fatty acids and diols[END_REF]. In similar conditions, the polymerization of bio-based diols and di-carboxylic acids via cyclic oligomers was performed by [START_REF] Yagihara | Enzymatic synthesis and chemical recycling of navel polyester-type thermoplastic elastomers[END_REF]. However, these syntheses were carried out under a reduced pressure or nitrogen atmosphere and required very long reaction times (between 42 and 48 h), as the temperature should not exceed 60 °c to preserve the activity and the stability oflipases. Therefore, [START_REF] Korupp | Scale-up of lipase-catalysed polyester synthesis[END_REF] have mentioned that the mechanical stability of enzyme towards energy input brought by the mixture might be a major problem for a scale-up.

To synthesize biobased polyesters according to a green approach, we have considered esterification methods using hetero geneous catalysis through ion-exchange resins [START_REF] Park | Esterification of free fatty acide using water tolerable Amberlyst as a heterogeneous catalyst[END_REF][START_REF] Pouilloux | Reaction of glycerol with fatty acids in the presence of ion-exchange resins: preparation of monoglycerides[END_REF]. Thus, [START_REF] Pouilloux | Reaction of glycerol with fatty acids in the presence of ion-exchange resins: preparation of monoglycerides[END_REF] esterified oleic acid with glycerol at 90 °C with different Amberlyst resins that can be readily removed from the final product. Amberlyst 31 gave the highest yield (50%) and the selectivity to monooleyl glyceride was over 90%. Under some conditions, ion-exchange resins suffer from poor thermal stability, which restricts their use to 150 °C and a rel atively low acid strength. Amberlyst 70 shows a higher thermal stability compared with that of other resins, and a high resistance to desulfonation, even at 190 °C [START_REF] Siri | New polystyrene sulfonic acid resin cata lysts with enhanced acidic and catalytic properties[END_REF].

The purpose of this work was to synthesize polyesters using pentaerythritol, a dicarboxylic acid and different length fatty acids as pendant chains to obtain selectively bifunctional compounds and to promote the synthesis oflinear polymers. Solvent-free syn thesis of the polyesters was carried out in order to obtain either statistical or block co-polyesters in two or three steps. In order to limit crosslinking, esterification was first conducted between fatty acids and polyalcohol to produce expected bifunctional monomers ( depending of the ratio of alcohol and acid functions ). The obtained polyol fatty acid esters were then used as monomers in a second step to be polymerized with dicarboxylic acids. An experiment design was applied to reach the optimum yield and selectivity. Then, the influence of the nature of the fatty acid in the side-chains and of the dicarboxylic acid in the polymer backbone on the thermal properties of the ensuing polyesters was studied.

Materials and methods

Materials

Pentaerythritol (POH), glycerol (GOH) and lauric acid (AL, C12:0) were purchased from Sigma Aldrich, stearic acid (AS, C18:0) and adipic acid (AD, HOOC(CH 2 ) 4 COOH) from Acros, sebacic acid (ASeba, HOOC(CH 2 ) 8 COOH) from Merck and succinic acid (ASuc, HOOC(CH 2 )zCOOH) from Carlo Erba. Ali chemicals were obtained in the highest purity available. Technical grade (purity over 80%) triglycerol (TriGOH) and behenic acid (AB, C22:0) were purchased from Sigma Aldrich. Ali monomers were used without any further purification. Amberlyst 70 was kindly provided in the form of wet beads by Rohm and Haas. Prior to any esterification, the resin was washed and dried under vacuum at 40 °C for 24 h.

Measurements

FfIR spectra were obtained with KBr disks using a ]ASCO FfIR-460 plus spectrometer. The 1 H NMR spectra were performed on a Bruker Advance 300 MHz spectrometer. The molecular weights of the polyesters were measured by size exclusion chromatography multiangle light scattering (SEC-MALLS) using a Dionex HPLC system equipped with a PS80 pump, an ASI 100 autosampler, a Crococil oven, a Iota-2 refractometer and a multiangle laser light scattering photometer Wyatt-miniDawn TREOS. A three-column set PLgel 5 µm, from Polymer La bora tories, of pore size 1000 A, 500 A and 100 A in series, was used to analyze the samples. A polystyrene standard (Polymer Laboratories) with a molecular weight of 30,000 g/mol were used to calibra te the Wyatt minidawn TREOS. The relative molecular weight of the pre-polymers were measured using polyethylene glycol standards (Polymer Laborato ries) with molecular weight range from 600 g/mol to 20,000 g/mol to calibrate the refractometer. Tetrahydrofuran (THF), at 30 °C, with a flow rate of 0.8 mL min-1 was used as eluent. The specific refractive index increments (dn/dc) in THF at 30 °C were measured with a Onde 2010 PSS refractometer at 620 nm. Differential scan ning calorimetry (DSC) was performed on a Perkin Elmer Pyris 1. In order to provide the same thermal history, each sample was preheated from room temperature to 100 °c at a heating rate of 10 °C min-1 then cooled down to -40 °C at the same rate. The DSC scan was then recorded by heating from -40 °C to 150 °C at 4 °c min-1 in a nitrogen atmosphere. The cation exchange capacity of the dried resin (2.34 mmol g-1 ) was determined by percolating an aqueous NaCI solution, followed by titration with a 0.1 N NaOH solution.

Preparation of polymers

Ali polymers were prepared by bulk polymerization, catalyzed with the ion exchange resin Amberlyst 70 (0.05 eq W /mol pen taerythritol). In order to prevent crosslinking or gelation, synthesis were conducted in two or three steps. The two-steps reaction led to random copolymers, whereas the three-steps process tends to yield block-like configuration.

To produce random copolymers, fatty acids were esterified with pentaerythritol to give the corresponding esters. In order to enhance the miscibility of pentaerythritol in the fatty acid, a small amount of triglycerol (10 mol% compared to pentaerythri tol) or glycerol (33 mol%) had to be added. In a typical formulation, a mixture of pentaerythritol (1 g, 7.35 mmol), triglycerol (0.176 g, 0.74 mmol), stearic acid (3.07 g, 10.80 mmol), Iauric acid (0.24 g, 1.20 mmol) and 0.1 7 g of Amberlyst 70 was introduced in a cylin drical glass reactor equipped with a magnetic stirrer, gas-inlet and gas-outlet tubes. The system was heated at 160 °C and atmospheric pressure for 1 h and at reduced pressure ( 40 mbar) for one more hour. It was then heated to 180 °C and the pressure Iowered to 20 mbar for 4 more hours. After 30 min under these conditions, the mixture became a clear monophasic liquid. In the second step, a dicarboxylic acid was added in stoichiometric conditions and polymerization occurred. For example, 1.18 g (8.09 mmol) of adipic acid was added and the bulk polymerization continued for 5 h. The ensuing product was cooled to approximately 60 °C and filtered to remove the resin. The light-yellow random copolymer was homo geneous and showed no gel particles.

To prepare block-copolyesters containing long fatty acids in one block (i.e. behenic and stearic acid) and short fatty acids (i.e. Jau rie acid) in the other block, we proceeded in three steps. The first step was identical to the synthesis of random copolyesters and pro vided blends of pentaerythritol (and glycerol, or triglycerol) fatty acid esters; some blocks are obtained from lauric acid and oth ers from a mixture of stearic and behenic acids. Then, 60% of the total amount of dicarboxylic acid was first added and the remaining quantity after 4 h of pre-polymerization. This last step of polymer ization was carried out for 3.5 h. The ensuing product was filtered and the copolymer was analyzed without further purification.

o==( a=< )=o )=o R R R R R R rO 0 � H0 � O � O� O�O �O � O �o �oH �l --0 a a a J O a m o==( a=< )=o )=o R R R R R = (CH 2
) n CH 3 with n = 10 for lauric acid or n = 16 for stearic acid F ig . 1. Examples oftarget ideal structures ofrandom copolymers from pentaerythritol, glycerol, fatty acids (lauric or stearic acids) and adipic acid.

Results and discussion

appeared in very low concentration. The highest contents of di substituted alcohols were expected to favor linear polyester in the next step of the polymerization. These compositions were assessed by SEC. Fig. 4 shows the SEC chromatograms of blends arising from several experiments involving a molar ratio pen taerythritol :glycerol of 1 :0.33 and variable amounts of stearic acid, according to reaction time. With this experimental design, we estimated by SEC that the highest yield (94%) and selectivity (around 40% of di-substituted alcohol) were obtained with a fatty acid/polyol molar ratio of 1 .45 and a reaction time of 5.25 h, as sum marized in Table 1. These conditions were then applied to ail the other syntheses discussed here.

The synthetic strategy used to prepare the polyesters as much linear as possible by polycondensation from dicarboxylic acids and pentaerythritol is the protection by esterification with fatty acids of some hydroxyl groups in pentaerythritol before the polymeriza tion reaction itself. Due to the insolubility of pentaerythritol in the fatty acid medium, the former was mixed with glycerol or triglyc erol. The branched statistical or block co-polyesters targeted are represented in Figs. 1 and2. They were produced by polyconden sation between diacids (such as adipic acid) and fatty acid esters of pentaerythritol and (glycerol or triglycerol), which were firstly synthesized by polyol esterification (Fig. 3).

Analysis ofthefirst step product

The first step of esterification using fatty acids and polyalco hols as starting materials led to a mixture of mono-, di-, and tri-substituted alcohols assessed by SEC. The tetra-derivatives

AS AS AS AL AL
In order to compare the influence of the fatty acid, pen taerythritol laurate esters, pentaerythritol stearate esters and pentaerythritol behenate esters were synthesized. Fig. 5 shows the 1 H NMR spectrum of the behenic acid esterified by pentaerythri tol and triglycerol (10:1 mol). The chemical shifts of the protons of the CH 2 groups in ex-positions of ester or alcohol functions are dif ferent for mono, di and tri-substituted pentaerythritols. Therefore, the proportions of mono-, di-and tri-substituted alcohols could be calculated by either 1 H NMR or SEC. For the chromatographie anal yses, we considered that the response factors are similar for the mono-, di-and tri-substituted pentaerythritols, which can explain the slightly different results compared to those obtained by 1 H NMR (Table 2). The NMR yield was the lowest for pentaerythri tol monolaurate, which means that lauric acid reacted faster than the longer fatty acids. These results are explained by lower steric hindrance and viscosity of Cl 2 chains promoting their reactiv ity. Tetra-substituted pentaerythritols were found in very small amounts, obviously because of steric hindrance. Tri-substituted pentaerythritol may constitute dead chain ends when they react with the dicarboxylic acid in the second step. The mono-substituted a Yield = [1 -(area of the residual monomer/total area)] x 100 (areas were mea sured by SEC analysis ).

b Selectivity = [ area ofbifunctional alcohol/total area] x 100 ( areas were measured by SEC analysis after the esterification offatty acid with alcohol).

pentaerythritol may cause branching on the polymer backbone and even cause cross-linking between the polymer chains. Therefore, equimolar ratio polyol/dicarboxylic acid was used for the polymer ization step in order to prevent the polyol tri-substitution and the gelation as described by [START_REF] Kricheldorf | Biodegradable hyperbranched aliphatic polyesters derived from pentaerythritol[END_REF] for the polycondensation of pentaerythritol and dimethyl sebacate.

Table2

Percentages of mono-, di-and tri-substituted alcohols determined by 1 H NMR and SEC analyses under the following conditions: molar ratios pentaerythri tol:triglycerol = [ 1 O: 1 ], fatty acid: polyol = [ 1.45: 1] with lauric, stearic or behenic acid as fatty acid.

Laurie acid

Stearic acid Behenic acid SEC 1.00 Fig. 5. 1 H NMR spectrum of the compound resulting from the esterification ofbehenic acid with a mixture of pentaerythritol and triglycerol [10:1] (mol) in CDC!s.

Characteristics of the random copolymer

After esterification of the fatty acid by pentaerythritol and glycerol (or triglycerol), polymers and copolymers were synthe sized by the addition of the dicarboxylic acid such as adipic acid (HOOC(CH 2 ) 4 COOH), succinic acid (HOOC(CH 2 )zCOOH) or sebacic acid (HOOC(CH 2 ) 8 COOH).

Influence of the fatty acid

Table 3 gives the results of experiments based on the polycon densation of the polyol fatty acid esters and diacids. The average content of unreacted hydroxyl groups was measured by 1 H NMR and ranged between 9 and 13%. This result was confirmed by FTIR spectroscopy: a peak around 3400 cm-1 corresponds to the stretching vibrations of hydroxyl groups and the other characteris tic signais of polyesters were found at 1731 cm-1 for the stretching vibrations of C=O bonds and at 1150-1050 cm-1 for C -O-C link ages. Pentaerythritol grafted with long side-chains, like those from behenic acid, were less reactive than pentaerythritol grafted with lauric acid, anticipating a lower yield for the behenic-based poly mer. This was indeed confirmed by the results (Table 3, entry 1 to 3), which indicated that when the length of the hydrocarbon side chain increased, both reaction yield and average molecular weight decreased.

Concerning the thermal properties, polymers with long side chains showed higher melting enthalpy and melting temperature. No glass transition temperature was observed for these polyesters. For the copolymers, increasing the amount of long fatty acids (i.e. behenic or stearic acid) enhanced the melting temperature and the crystallinity. In Table 3, polymer 9, with the lowest stearic acid content ([AS:AL] = [6:4] mol), showed lower melting temper ature (27 °C), compared with those of polymers 7 and 8. When behenic acid (C22:0) (5% molar compared to the total amount of fatty acids) was added, the melting temperature rose from 28 °C to 30 °C (Table 3, polymers 10, 11 ). When behenic acid completely replaced stearic acid in the formulation, the melting temperature increased to 50 °c (Table 3, entry 12). The obtained melting tem peratures are most likely due to side-chain crystallization if these branched chains are long enough. This phenomenon was shown to depend on the degree of chain branching and the branch length [START_REF] Watanabe | Rigid-rod polyesters with flexible side chains. 4. Thermotropic behavior and phase structures in polyesters based on 1,4-dialkyl esters of pyromellitic acid and 4,4-bisphenol[END_REF][START_REF] Jin | Properties of aliphatic polyesters with n-paraffinic side branches[END_REF]McI<ee et al., 2005). Above a given branch length depending of the polyester, the melting tem perature tends to increase, due to ordering and crystallization of the side-chains. Crystals can also incorporate overlapping or inter digitated side-chains from neighboring main chains. It has been demonstrated in other type of polymers, especially in cellulose derivatives with fatty chains longer than octanoic [START_REF] Sealey | Novel cellulose derivatives. IV. Preparation and thermal analysis of waxy esters of cellulose[END_REF]. The global consequence is an increase of crystallinity and a higher melting temperature.

Influence of the dicarboxylic acid

In the backbone of the polymer, sebacic acid, adipic acid and succinic acid as dicarboxylic acids were used. The increase of the hydrocarbon chain length resulted in a lower melting tempera ture. The melting temperature was 31 °C for succinic acid, 28 °C for adipic acid and reached 21 °C for sebacic acid (Table 3, entries 14, 1 O and 13 ). Fig. 6 shows the DSC curves of the three polymers. The main peaks reflect the melting temperature of the polymers, white secondary peaks were attributed to fraction of oligomers or tri and tetra-substituted pentaerythritols. Once again, crystallization of the side-chains is involved. A shorter dicarboxylic acid makes the side-chains cl oser, and thus crystallinity increases. As expected, polymers built with succinic acid were brittle, whereas sebacic acid based counterparts were soft. 3, for Run 14, 10 and 13).

Characteristics of the block copolyesters

The block copolyesters were prepared by copolymerization of two blocks synthesized separately with one of the three diacids. Each black was obtained in a previous step by pre-polymerization of the same dicarboxylic acid and one kind ofpentaerythritol fatty acid esters (prepared from polyols mixture and lauric acid for the first block, or stearic acid, associated or not to behenic acid, for the second block). The results relative to the black copolyesters are summarized in Table 4.

Table4

Formulation and characterization of black copolyesters. 3, entry 9, and Table 4 entry 16, shared the same monomers in the same proportions. Yet the black copolyester showed a melting temperature of 34 °C and that of the random one was only 27 °C. The random copolymer of Table 3, entry 6, showed a melting temperature of 32 °c, while that of the corresponding black copolymer of Table 4, entry 19, was 36 °c. When succinic acid was used in the backbone instead of adipic acid, the melting tempera ture increased from 33 °C to 39 °C (Table 4, entries 17 and 18). These results showed that the micro-crystallinity is higher in these black structures because of the intra-and inter-molecular stackings of side-chains involved in these polymers. Thus, the synthesis ofblock copolyesters is a performing method to increase the melting point.

Polymers of Table

Conclusion

In this study, we described the bulk synthesis of polyesters with fatty acid side-chains, using an ion exchange resin as cat aiyst. Amberlyst 70 allowed to obtain a maximum yieid as high as 98% and was easy to remove from the final product. The co polyalcohol, i.e. glycerol or trigiycerol, prevented the use of organic soivent and enabled a better contact between pentaerythritoI and the fatty acids in a one-pot system with sequential addition of the reactants. We showed that longer side-chains increased the crystallinity and rose the melting temperature, due to side-chain crystallization. Shortening the length of dicarboxylic acid in the backbone produced an increase in both crystallinity and meiting temperature. In ail the cases, the block copolyesters showed higher melting temperature than the random ones.

F

  ig . 2. Example of a block-copolymer (with AL lauric, AS stearic or AB behenic esters as side-chains).

  Fig. 3. Mono-and polysubstituted polyols (pentaerythritol and glycerol based fatty acid esters) likely ta be obtained at the first reaction step.

Fig. 4 .

 4 Fig. 4. Size exclusion chromatograms of the polyesters from stearic acid obtained with a molar ratio pentaerythritol:glycerol = 1 :0.33. (r = molar ratio fatty acid/polyol).

aFig. 6 .

 6 Fig.6. DSC thermograms ofrandom copolyesters based on succinic acid, adipic and sebacic acid (obtained under the experimental conditions given in Table3, for Run 14, 10 and 13).

Table 1

 1 Results

	Run	AS:polyol	Time(h)	Yield"%	Selectivity b %
		0.5	3.25	75.7	22.7
	2	0.5	5.75	82.8	12.1
	3		2	80.7	22.5
	4		4.5	92.1	37.8
	5		7	95.7	31
	6	1.5	3

of the design of experiments from stearic acid and a mixture of pentaerythritol and glycerol [1:0.33] (mol).

.25 92.2 38.2 7 1.5 5.75 96.2 39.5 8 1.4 5.8 96.2 39.7 9 1.45 5.25 94.0 40.6

Table3

  Composition and characterization ofrandom copolyesters: average Mn and Mw measured by SEC-MALLS analysis, melting temperature and melting enthalpy, measured by DSC analysis.

	Run	Polyols a	Fatty acid b	Dicarboxylic acid'	Yield%	Mn (g/mol)	Mw (g/mol)	Tm ( °C)	L',.HU/g)
	1	POH:TriGOH [10:1]	AB	AD	89	3.84 X 10 3	7.46 X 10 3	63	129
	2	POH:TriGOH [10:1]	AS	AD	90	2.28 X 10 5	2.85 X 10 6	41	79
	3	POH:TriGOH [10:1]	AL	AD	96	3.77x104	1.48 X 10 5	25	51
	4	POH:TriGOH [10:1]	AS:AL [9:1]	AD	92	1.41 X 104	2.02 X 10 5	41	77
	5	POH:TriGOH [10:1]	AS:AL[7:3]	AD	90	1.47x104	1.01 X 10 5	37	57
	6	POH:TriGOH [10:1]	AB:AS:AL [0.5:6:3.5]	AD	89	3.39 X 104	1.96 X 10 5	32	36
	7	POH:GOH [10:3]	AS:AL [9:1]	AD	95	3.35 X 104	1.71 X 10 5	36	60
	8	POH:GOH [10:3]	AS:AL[7:3]	AD	94	1.40x10 4	1.01 X 10 5	31	47
	9	POH:GOH [10:3]	AS:AL[6:4]	AD	96	1.67 X 104	6.68 X 10 4	27	44
	10	POH:GOH [10:3]	AS:AL [6.5:3.5]	AD	98	1.61x104	6.66 X 10 4	28	54
	11	POH:GOH [10:3]	AB:AS:AL [0.5:6:3.5]	AD	90	1.78x104	1.03 X 10 5	30	52
	12	POH:GOH [10:3]	AB:AL [6.5:3.5]	AD	89	1.62x104	1.19 X 10 5	50	124
	13	POH:GOH [10:3]	AS:AL [6.5:3.5]	Aseba	94	9.37x103	2.43 X 10 4	21	43
	14	POH:GOH [10:3]	AS:AL [6.5:3.5]	Asuc	93	1.3 X 10 4	4.6 X 10 4	31	40
	15	POH:GOH [10:3]	AS:AL[7:3]	Aseba	92	2.92 X 103	8.62 X 103	23	51

Acknowledgements

The authors gratefully acknowledge the ADEME for financiaI support and the IMRCP for the measurements of dn/dc.