Article pubs.acs.org/IECR
Strategy To Modify the Crystallization Behavior of EVOH32 through Interactions with Low-Molecular-Weight Molecules Micaela Vannini,*,† Paola Marchese,† Annamaria Celli,† and Cesare Lorenzetti‡ †
Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali, Via Terracini 28, 40131 Bologna, Italy Tetra Pak SA, Zone Industrielle la Maillarde 2, 1680 Romont, Switzerland
‡
S Supporting Information *
ABSTRACT: EVOH32 is a semicrystalline copolymer with high barrier performances and therefore suitable for packaging applications. In order to improve its processability through a decrement of its crystallization temperature from the molten state, EVOH32 was mixed with a wide range of low-molecularweight additives. The strategy is based on the possibility that the additives create hydrogen or covalent bonds with the OH groups of the EVOH main chains, thus influencing the crystallization process. Interesting correlations between the chemical nature of the functional groups present in the organic molecules and their effect on the polymer crystallization have been found. 3,5-Dihydroxybenzoic acid turns out to be the most effective molecule to decrease the crystallization temperature of EVOH32.
1. INTRODUCTION Ethylene vinyl alcohol (EVOH) copolymers are important semicrystalline polymers used in the food-packaging industry because of their low permeability to oxygen and their processing temperatures compatible with various commodity polymers. Indeed, the possibility of forming numerous hydrogen bonds between the OH groups present in every repeating unit greatly increases the interchain cohesion, the crystallizing capability, and the barrier performances.1,2 Moreover, the EVOH properties have been adapted to various applications by varying the content of ethylene in its structure or introducing other comonomers.3 An increment in the ethylene content significantly enhances processability (process window), flexibility, and transparency. However, such an increase has a general negative impact on the gas barrier properties, which vary according to humidity and crystallinity. As a general rule, a higher ethylene content improves the barrier properties only above 80% relative humidity (RH) with respect to EVOH grades with lower ethylene content. In any case, it is interesting to observe that above 80% RH the same performances obtained for high ethylene content (38−46%)4 can also be obtained at low ethylene content (24−32%) if the materials feature an extremely high crystallinity. In practical applications, the structures of packaging materials are designed to get the highest barrier performance from EVOH while minimizing its use compared to other cheaper polymers. If, on the one hand, such a goal could be achieved by using EVOH grades with the lowest oxygen transmission rate at the foreseen RH, on the other hand, the polymer will be exposed to several practical constraints that very often force © 2016 American Chemical Society
toward the use of EVOH grades with poorer barrier performances. Indeed, converting processes that require a stretching phase of the material, such as thermoforming or film orientation, need, for instance, EVOH grades with the higher ethylene content. In this article, EVOH with 32% of ethylene content (EVOH32) was chosen as a model for grades with low ethylene content and good barrier properties, while EVOH with 44% of ethylene content (EVOH44) was taken as a model for grades with high ethylene content and enhanced flexibility and stretchability. EVOH32 is, in fact, one of the most widely used grades in packaging film applications, although its high crystallization rate hinders the orientation of films in practical industrial processes (e.g., Tenter frame or double-bubble processes), commonly utilized for down gauging or to get shrinkable type packaging. On the other hand, EVOH44 shows good processability because of a lower crystallization temperature from the melt but much lower gas barrier properties. Such poor performances can be justified by considering that the crystalline phase of EVOH44 is characterized by a lower density compared to that of EVOH325 and the amorphous phase of EVOH44 by a higher free volume,6 both because of a lower vinyl alcohol content and then a lower density of intermolecular chain links. Received: Revised: Accepted: Published: 3517
November 5, 2015 February 5, 2016 March 8, 2016 March 8, 2016 DOI: 10.1021/acs.iecr.5b04191 Ind. Eng. Chem. Res. 2016, 55, 3517−3524
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Industrial & Engineering Chemistry Research Therefore, this paper aims at developing a material that combines the excellent barrier properties of EVOH32 with the good processability of EVOH44. To this purpose, it is necessary to modify the crystallization behavior of EVHO32 and confer it characteristics similar to those of EVOH44, i.e., a lower crystallization temperature during cooling from the melt. The literature reports numerous examples of EVOH modification, which results in a change of its crystallization behavior: (i) blends with polyolefins, polyesters, or polyamides;7−12 (ii) the introduction of ionomeric groups;13−15 (iii) derivatization of an OH group;16,17 (iv) preparation of composites and nanocomposites.18−20 In this work, the chosen strategy is based on mixing of the polymer with low-molecular-weight additives, which can interact with the −OH group of EVOH32. A recent example of this approach is described by Péter et al.21 In particular, the molecules used can be classified as (1) molecules with functional groups that are able to create covalent bonds with EVOH chains and (2) molecules with functional groups that are able to create hydrogen bonds with EVOH chains. We believe that this approach could be potentially implemented in practical industrial applications, aiming to provide a better compromise between the crystallization rate and barrier performances than currently exists by using EVOH grades with ethylene contents between 38 and 46%. This approach was put to work by blending small amounts of polymer and additive in the molten state in a Brabender mixer: the effects of the mixing are analyzed by 1H NMR, Fourier transform infrared (FT-IR), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The crystallization temperature and crystalline phase have been given particular attention.
Table 1. Additives Mixed with EVOH32, Compositions, and Crystallization Temperatures of the Prepared Blends additive type
additive name
anhydride
succinic anhydride
acid
p-toluic acid
polyol
ionic salt multifunctional
benzoic acid isophthalic acid glycerol myo-inositol pentaerythritol dipentaerythritol D-sorbitol resorcinol sodium D-gluconate p-hydroxybenzoic acid bis(hydroxymethyl) propionic acid 3,5-dihydroxybenzoic acid
gallic acid
blend code
amount of additive (mol %)
SA06 SA2 PT5-L PT5-H BA5 IA2 GL5 IN5 PE5 DPE5
0.6 2.5 5.0 5.0 5.0 2.5 5.0 5.0 5.0 5.0
DS5 RE5 SG02 HB5
5.0 5.0 0.2 5.0
154 139 149 147 148 147 144 152 139 122 + 142 148 145 153 142
HP5
5.0
139
DHB1
1.5
152
DHB2 DHB5 GA5
2.5 5.0 5.0
144 127 142
Tc (°C)
of the prepared blends, the molecular structures, the molar and weight percentages of the additives used, and the complete thermal characterization is summarized in the Supporting Information (Table S1). Some blends were purified by dissolving the sample in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) and then dropping the obtained solution into methanol. The precipitated polymer was filtered, oven-dried, and then analyzed for comparison with the corresponding unpurified sample. In order to perform IR spectroscopy analyses, a few samples were transformed into thick films by compression molding. Therefore, the blend powder was scattered on a Teflon foil. The foils were placed between the plates of the Carver press and heated at 200 °C, under 5−6 bar over 4 min. Then the films were quickly cooled to 100 °C, allowed to reach room pressure, and finally separated from the Teflon foils. 2.3. Characterization. 1H NMR analysis was carried out using a Varian Mercury 400 MHz spectrometer. The samples were initially dissolved in HFIP and then diluted with deuterated chloroform in order to reach a final ratio equal to 80:20 (v/v) CDCl3/HFIP. Calorimetric analysis was carried out by means of a PerkinElmer DSC6 calorimeter, calibrated with high-purity standards. The measurements were performed under a nitrogen flow. The thermal treatments used were as follows: first scan, from 30 to 220 °C at 20 °C min−1 and 1 min of isotherm at 220 °C; cooling scan, from 220 to 10 °C at 10 °C min−1 and 1 min of isotherm; second scan, from 10 to 220 °C at 10 °C min−1. The X-ray diffraction measurements were carried out at room temperature with a Bragg/Brentano diffractometer system (Philips PW1050/81), equipped with a graphite monochromator in the diffracted beam. A copper anode was used as the Xray source. Data were collected in the range from 5 to 60° of 2θ. The analyzed samples were treated in a differential scanning calorimeter in order to control their thermal history.
2. EXPERIMENTAL SECTION 2.1. Materials. EVOH32 (Kuraray EVAL F171B) and EVOH44 (Kuraray EVAL E105B) were supplied by TetraPak. The additives used are succinic anhydride (SA), coumarin (CO), D-(−)-pantolactone (PA), p-toluic acid (PT), benzoic acid (BA), isophthalic acid (IA), glycerol (GL), myo-inositol (IN), pentaerythritol (PE), dipentaerythritol (DPE), D-sorbitol (DS), resorcinol (RE), sodium D-gluconate (SG), p-hydroxybenzoic acid (HB), bis(hydroxymethyl)propionic acid (HP), 3,5-dihydroxybenzoic acid (DHB), and gallic acid (GA). The molecular structures of the additives are shown in the Supporting Information. All were purchased from SigmaAldrich. 2.2. Blend Preparation. Before blending, EVOH32 was dried overnight in an oven at 120 °C. Blends of EVOH32 and additives were prepared in a Brabender mixer, under nitrogen flux, feeding 45−50 g of charge and setting the screw speed at 50 rpm and the temperature at 190 or 220 °C. The mixing time was 10 min. For each additive, different blends were prepared with an amount of additive of up to 5 mol %, calculated with respect to the molar amount of the units of only poly(vinyl alcohol) (PVOH). The prepared blends are named with an abbreviation of the additive and its molar concentration, calculated with respect to PVOH units. For example, SA2 is the blend of EVOH32 and SA, in a concentration of 2 mol % with respect to PVOH. The H or L letter in some blend codes indicates that the blend has been prepared at the low temperature of 190 °C (L) or at the high temperature of 220 °C (H). The more interesting blends and their crystallization temperatures are reported in Table 1, while the whole list of all 3518
DOI: 10.1021/acs.iecr.5b04191 Ind. Eng. Chem. Res. 2016, 55, 3517−3524
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Industrial & Engineering Chemistry Research A Thermo Scientific Nicolet iN10 FT-IR Microscope equipped with a germanium Slide-On Micro-Tip attenuated total reflectance (ATR) instrument was used to collect the ATR spectra. Analyses were performed on samples filmed by compression molding at a resolution of 1.93 cm−1, and 16 scans were averaged for each spectrum in a range between 4000 and 700 cm−1.
3. RESULTS AND DISCUSSION 3.1. Characterization of EVOH Copolymers. The DSC curves of two EVOH samples (EVOH32 and EVOH44) are shown in Figure 1: considering the cooling scans from the melt,
Figure 2. WAXS patterns of EVOH44 cooled at 10 °C min−1 (a) and EVOH32 cooled at (b) 10, (c) 40 and (d) 1 °C min−1.
as indicated by the presence of the principal 101 and 200 reflections at about 19.7 and 22.9° of 2θ,25 while a fast cooling (curve c) allows formation of the orthorhombic phase as in EVOH44. Under the same cooling conditions of EVOH44 (curves b), EVOH32 shows a diffraction spectrum in which the two reflections typical of the monoclinic form are merging into one, indicating the presence of an intermediate state between the two modifications, in which probably crystallites of both phases are present.5 This behavior could justify the significant differences of the Tc values reported in Figure 1 for EVOH32 (Tc = 159 °C, corresponding to the intermediate state) and EVOH44 (Tc = 144 °C, corresponding to an orthorhombic phase). Accordingly, during the heating scans, the different Tm values for the two copolymers (Tm = 183 °C for EVOH32 and 166 °C for EVOH44) can be justified in terms of the different crystal phases. In particular, an interesting behavior is reported by Cerrada et al.5 for EVOH32, related to the melting of crystallites of different phases. A Tm value equal to 183 °C is obtained regardless to the cooling conditions without any appreciable differences in the melting temperatures of the monoclinic or orthorhombic modifications. Cerrada et al. discuss that the constancy of the melting temperatures can be justified as a consequence of a transformation, during melting, from the orthorhombic to the monoclinic lattice. Then, the Tm value of EVOH32 (183 °C) is due to the melting of the monoclinic form, even if the original crystalline form corresponds to an intermediate state, as shown in Figure 2, curve b. The Tm value of EVOH44 (166 °C) is due to the melting of the orthorhombic lattice. Therefore, it is clear that the crystallization conditions of EVOH32 must be accurately controlled to favor formation of the PVOH crystalline phase, that is characterized by better performances in terms of the barrier properties. 3.2. Strategic Selection of Organic Molecules for the EVOH32 Blends. In order to modify the crystallization behavior of EVOH32, the additives were chosen on the basis of certain criteria, i.e.:
Figure 1. DSC curves of EVOH32 and EVOH44 recorded at 10 °C min−1.
it is notable that EVOH32 is characterized by a higher Tc value with respect to EVOH44 (159 vs 144 °C at 10 °C min−1 from the melt). On the other hand, the values of the crystallization enthalpies are very similar for the two copolymers. In order to understand the different thermal behaviors of the two EVOH samples, it is necessary to study the structure of the crystalline phase. In the literature, the existence of two types of crystalline phases has been proposed for EVOH copolymers containing 29, 32, and 44 mol % PE.5 Depending on the composition and cooling rate from the melt, EVOH is able to crystallize in the orthorhombic phase of low-density PE22,23 or in the monoclinic phase of PVOH.24−26 Moreover, an “intermediate” state between the two modifications can be present, where the two main reflections (characteristics of the monoclinic phase) are beginning to collapse. In particular, Cerrada et al.5 report an orthorhombic phase for the EVOH44 sample regardless of the cooling rate, whereas EVOH32 can crystallize under two different forms depending on the cooling conditions. Moreover, EVOH32 shows a continuous change of some of the lattice constants depending on the cooling rate. Figure 2 (curves a and b) shows the WAXD spectra obtained for the two samples cooled from the melt at 10 °C min−1 in DSC. It is evident that EVO44 (curve a) crystallizes in the orthorhombic phase, as indicated by the presence of the principal 110 reflection at about 21° of 2θ.23 The behavior of EVOH32 is more complex; in fact, a slow cooling (curve d) leads to formation of the monoclinic phase, 3519
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Industrial & Engineering Chemistry Research − additives should be solid in order to simplify their addition during blend preparation in an extruder at the industrial level; − their melting point should be lower than 190 °C, so they must be liquid during mixing, in order to improve their miscibility with EVOH32, or, in any case, additives must be miscible in the EVOH32 melt; − their thermal stability must be high enough to avoid degradation processes during blending at 190−220 °C; − their chemical characteristics must have a low toxicological profile. With the aim of a better understanding of the relationships between the chemical structures and the effects produced by the additives, in a few cases (RE, BA, CO, and SA), the low toxicological profile is not respected. Such results have been evaluated by considering modification of the Tc value of EVOH32 (159 °C) versus the Tc value of EVOH44 (144 °C). Moreover, the level of the crystallinity of EVOH32 (melting enthalpy of about 70 J g−1) and its high value of Tg (61 °C) must be maintained. All of these characteristics can improve the processability of EVOH32 without modifying its performance. 3.3. Blends with Molecules with Functional Groups That Are Able To Create Covalent Bonds with EVOH Chains. In order to verify the possibility of covalently linking the additives to EVOH32, the following molecules were mixed at 190 °C: anhydrides, lactones, and carboxylic acids. The effect of the addition of SA is reported in Table 1 for samples prepared with 0.6 and 2.5 mol %. The decrease of Tc of EVOH32 was very significant for an amount of SA equal to 2.5 mol % (Tc = 139 °C for SA2), but the high reactivity and the bifunctionality of this molecule caused cross-linking reactions with a consequent increase of the melt viscosity. That is why additives containing only one functional group able to react with the −OH group of EVOH were later chosen. Reactions between EVOH and the tested lactones (CO and PA) take place, but no sufficient effects on the thermal properties were recorded (see the Supporting Information). To verify the occurrence of reactions between the −OH and −COOH groups, EVOH32 was mixed at 190 °C with PT (L samples) and BA. PT was mixed also at 220 °C (H samples). The DSC data show a significant decrease of Tc after mixing at 190 °C with an amount of additives equal to 5 mol % (Tc = 149 °C for PT5-L and Tc = 148 °C for BA5), whereas no effects were recorded with lower amounts of additives. A similar result was obtained after mixing at 220 °C (sample PT5-H; Tc = 147 °C). To verify the presence of covalent bonds between −COOH groups of the additive and EVOH, the purification process was carried out on the PT5-L and PT5-H samples; 1H NMR and DSC analyses were performed before and after purification. For the sample prepared at 190 °C, the PT signals were not present in the 1H NMR spectrum after the purification process, suggesting that covalent bonds between the matrix and additive were not formed. Such behavior was confirmed by DSC analysis, and the Tc value of the purified sample was very close to that of EVOH32. Instead, at a mixing temperature of 220 °C, the PT signals were present in the 1H NMR spectrum after purification, but their intensity was lower than expected. The amount of additive covalently bonded to the polymer is very small, in agreement with the results of thermal analysis (indeed, Tc = 156 °C, with a reduction of only 3 °C).
The decrement of Tc observed for the sample prepared with PT and BA can therefore be ascribed to the effect of physical interactions between the polar groups of the polymer and additives. Then, the molecules containing carboxylic groups can be included in the second group, and the effect of −COOH groups will be discussed in these terms in the following paragraph. 3.4. Blends with Molecules Bearing Functional Groups That Are Able To Create Hydrogen Bonds with EVOH Chains. 3.4.1. Effect of the Hydroxyl Groups. This section will only take the samples containing 5 mol % additives into consideration, in order to simplify the high amount of data (see the Supporting Information for the complete list). To understand the effect of the −OH group, it is interesting to compare the thermal data of BA, PT, and HB. These molecules are aromatic acids, characterized by no substituting groups (BA), a methyl group (PT), or a hydroxyl group (HB) in the para position. The DSC curves for the samples containing 5 mol % of such acids are reported in Figure 3. It
Figure 3. Thermal properties of EVOH32 containing 5 mol % BA5, PT5-L, and HB5.
is worth noting that PT and BA induce a significant Tc decrement (Tc = 148 °C), indicating good interaction of aromatic acids with the matrix, independent of the presence of a substituting group in the para position. Anyway, the contribution of the −OH group (HB sample) produces a further decrease in Tc, up to 142 °C, indicating the importance of hydrogen bonds. Therefore, it is possible to assume that the chain folding during the crystallization process is slightly hindered if the −OH groups of the matrix are engaged in hydrogen bonds and then the crystallization temperature decreases. By considering all of the additives tested, even a correlation between the number of −OH groups and the decrement of Tc can be observed. Regarding the aliphatic acyclic structures, a comparison among the thermal data of GL and PE can be carried out. Indeed, these molecules are characterized by an increasing number of −OH groups (three for GL and four for PE). It is worth noting that the corresponding Tc values for the 3520
DOI: 10.1021/acs.iecr.5b04191 Ind. Eng. Chem. Res. 2016, 55, 3517−3524
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Industrial & Engineering Chemistry Research 5 mol % blends decrease proportionally (Tc = 144 and 139 °C for GL5 and PE5, respectively). A similar behavior can be observed in the presence of aromatic structures, by comparing the thermal data of the samples containing 5 mol % BA, HB, and DHB (see Figure 4 and Table 1). In this case, the number of −OH groups increases from 0 to 2 and the Tc proportionally decreases from 148 to 142 °C (HB5) and 127 °C (DHB5).
Tc value that is very similar to that of the sample prepared with 5 mol % BA5. The addition of another −COOH group imparts a remarkable decrease in Tc. This trend is confirmed by comparing the Tc decrement in the blends with RE5 and DHB5, as shown in Figure 5. In this
Figure 5. Thermal properties of EVOH containing 5 mol % of some additives based on aromatic rings with or without COOH group: RE5 and DHB5.
case, the additives are characterized by the same chemical structure and differ only for the bearing of one −COOH group. Such a presence produces a decrement of Tc equal to 18 °C, thus proving the huge effect of the carboxylic functionality on the crystallization behavior. The effect of the −COOH group in combination with the −OH group is also apparent in aliphatic molecules. For example, the addition of GL, containing three −OH groups, produces a decrement of Tc equal to 15 °C (GL5), whereas the addition of HP5, containing two −OH groups and one −COOH group, produces a decrement of 20 °C. 3.4.3. Effect of Ionic Interactions. The effect of ionic groups was studied by using sodium gluconate as the additive (1 wt %). The ionic interactions produced by the addition of this molecule induce a certain Tc reduction (Tc = 153 °C, SG02 sample) but some degradation reactions as well. Consequently, blending with higher amounts of this ionic molecule was not studied. 3.4.4. Effect of the Additive Amount. Figure 6 shows, by way of example, the cooling scans of three blends obtained with increasing amounts of DHB. It is notable that the crystallization process is moved to low temperatures as a function of the additive amount. A similar trend was also verified with other additives (see the Supporting Information). A particular behavior was observed for the samples prepared with DPE, as reported in a previous paper.28 The crystallization temperature decreases with the DPE content, and a second crystallization process, due to the DPE crystals, is also present in the sample containing 5 mol % DPE5. It is notable that DPE crystallization can be hindered by the addition of a nanoclay.28 Finally, it is interesting to observe some results of this study in Figure 7, where the Tc values of some blends are reported as a function of the weight amount of the additives. The
Figure 4. Thermal properties of EVOH32 containing 5 mol % BA5, HB5, and DHB5.
Anyway, the effects of the −OH groups present in the additive are complex enough. Indeed, when the additives contain too many −OH groups, the decrement of Tc is less important. For example, both HB5 (having only one −OH group) and GA5 (bearing even three −OH groups) cause Tc = 142 °C. Both GL5 (characterized by the presence of three −OH groups) and DS5 (characterized by a double amount of −OH groups compared to GL) cause Tc = 147 °C. In these cases, a reduced interaction between the −OH groups of the additive and matrix could be due to self-association of the −OH functionalities present in the molecule or between molecules. In any case, the phenolic structures can be an added value to improve the barrier properties of the matrix.27 The hypothesis is realistic for DS and IN5 (Tc = 152 °C), where the −OH groups are located along an aliphatic chain and therefore characterized by a reasonable mobility. The unusual trend is even more pronounced for the samples containing 5 mol % DHB5 (two −OH) and GA5 (three −OH): the Tc decrement is higher in the DHB5 sample. Probably, the hydroxy group in the para position for GA is capable of interacting with the other two −OH functionalities, by reducing the possibility of forming hydrogen bonds with the matrix −OH groups. Finally, the addition of 5 mol % DPE5 produces a significant decrease of Tc, but also crystallization of DPE occurs. 3.4.2. Effect of the Carboxylic Group. By a comparison of the Tc values obtained with the addition of BA and IA, it is possible to highlight the effect of the −COOH groups. As indicated in Table 1, the sample containing 2.5 mol % IA2 has a 3521
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some blends were analyzed using a FT-IR microscope. The samples chosen were HB5, DHB5, and GA5, i.e., the blends containing 5 mol % of various BA derivatives, bearing different numbers of OH groups. It is well-known that the fundamental stretching of a “free” hydroxyl group, not engaged in hydrogen bonding, produces a sharp band around 3600 cm−1. Instead, when hydrogen interactions are present, the O−H bond is almost lengthened by the other attractions and therefore slightly weakened. Consequently, the resulting stretching frequency decreases as a function of the intensity of the matrix hydrogen bonds. In our case, the maximum OH vibration band for EVOH32 is located at 3337 cm−1, already indicating the presence of hydrogen interactions (see Figure 8). Such interactions become
Figure 6. Thermal properties of EVOH32 containing different amounts of DHB: 1.5 mol % DHB1, 2.5 mol % DHB2, and 5.0 mol % DHB5.
Figure 8. Expansion of the IR spectra of EVOH32 and its blends containing 5.0 mol % HB5, DHB5, and GA5.
more intense in the HB5 blend, and correspondingly the maximum shifts to a lower wavenumber (3326 cm−1), and even more in the DHB5 blend, where the peak moves to 3320 cm−1. Interestingly, the GA5 sample, bearing three OH groups, is characterized by a different trend: as we already discussed, by considering its unexpectedly moderate reduction of Tc, the extent of hydrogen bonding with the EVOH matrix is low, probably because of self-association of the hydroxyl functionalities present in GA. This speculation is now confirmed by IR spectroscopy, which shows the maximum of the OH stretching band at 3326 cm−1, the same value determined for the HB5 blend (having also the same Tc). Finally, a similar trend was observed for the samples containing different amounts of the same additive: by increasing the quantity of DHB, the consequent increment of hydrogen bonds results in a shift of the OH band to lower wavenumber. Particularly, for the sample DHB1 (1 mol % additive), the maximum of the OH stretching band was located at 3329 cm−1, whereas for the blend DHB2 (2 mol % of additive), it is positioned at 3327 cm−1. These two values are perfectly consistent with the other stretching vibrations of EVOH32 and DHB5, already discussed (3337 and 3320 cm−1, respectively). 3.6. XRD Analyses. In order to understand the modification of the crystallization behavior of the EVOH32 matrix as a function of the presence of links with functional additives, a study on the crystalline phases is quite significant. Examples of the results of WAXD analyses on some samples are reported in Figure 9. At the top, the PVOH phase (in
Figure 7. Tc versus weight percentages for some additives tested.
correspondence between the molar and weight percentages for every additive is reported in the Supporting Information. The dotted horizontal line, corresponding to Tc = 144 °C, represents the Tc target value. It must be pointed out that a linear decrement of Tc is obtained for all of the considered additives. This linear correlation makes it possible to calculate the requested amount of a single molecule necessary to reach the target Tc value. For example, for DHB, which is the most effective additive in reducing Tc, an amount of 6 wt % is sufficient to achieve a Tc value of 144 °C for the EVOH32 matrix. Instead, 9 wt % HP or 19 wt % DPE is necessary to obtain the same goal. 3.5. IR Analyses. It is frequently reported in the literature that the presence of hydrogen bonds in the EVOH matrix can be observed by IR spectroscopy.29−32 Therefore, in order to confirm that the decrement of Tc depends on hydrogen links, 3522
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Industrial & Engineering Chemistry Research EVOH32, cooled at 1 °C min−1) and the PE phase (in EVOH44, cooled at 10 °C min−1) are shown.
4. CONCLUSIONS In order to decrease the crystallization temperature of EVOH32 and then improve its processability, numerous additives, characterized by aliphatic or aromatic structures and different functional groups (OH, COOH, etc.), were blended with the polymer in a Brabender mixer. The efficacy of the additive addition has been evaluated mainly by DSC analysis. It was found that some molecules are able to interact with the macromolecular chains and interfere with their crystallization ability, influencing the crystallization temperature from the molten state. Indeed, the occurrence of hydrogen bonds, between the functional groups present in the additives and the EVOH matrix, was observed by IR spectroscopy and was considered as the main cause for the observed decrement in the crystallization temperature. Moreover, not only the kind of functionality but also its position and its amount have an evident effect on the observed Tc value. Considering the DSC data, the more promising additives seem to be DHB, HP, and IA, really effective also at concentrations lower than 10 wt %. However, because the purpose of this work is to decrease the crystallization rate of EVOH32 without compromising its barrier properties, the developed crystalline phase must also be evaluated. The crystalline phase, indeed, is influenced by the filler addition. By observation of the X-ray diffractograms, the additives are able to induce the monoclinic, orthorhombic, or mixed phases. Finally, the linear relationships between Tc and the amount of a specific additive have been found. This result makes it possible to predict the percentage of each additive necessary to induce a specific crystallization behavior to EVOH32 in order to maximize its processability in addition to its good barrier performances for packaging applications. Before these finding at the industrial level can be exploited, it will be necessary to perform tests related to the additive migration. Indeed, considering that the additives tried are dispersed in the EVOH matrix and not covalently linked to it, the possibility that the additive can migrate toward the free surface is not negligible. In any case, to bypass the migration keypoint, because better results have been obtained with molecules containing acidic functionalities, it will be possible to use them to covalently link the additives to the matrix.
Figure 9. WAXD spectra of EVOH32 cooled from the melt at 1 °C min−1, EVOH44 cooled from the melt at 10 °C min−1, and other interesting blends.
In Figure 9, the EVOH-based blends are presented in order (top-down) from those that present the only monoclinic phase (identified by label M), passing through those in which there is the presence of both phases (monoclinic and orthorhombic, indicated as the intermediate state, evidenced by the letter I) and the last two containing only the orthorhombic phase (marked with the letter O). Upon a comparison of the DSC data of Table 1, it can be noted that the blends with only the monoclinic phase have Tc very similar to that of EVOH32. Tc tends (and sometimes is even lower than) to that of EVOH44 when the orthorhombic form starts to be detectable, a clear sign that the additive has a deep influence on the crystallization temperature and crystalline structure formed. Blends with intermediate phases (I) are interesting because they are characterized by significantly low crystallization temperatures (and, then, good processability). In any case, because the kind of crystalline phase notably influences the barrier properties of the materials, a more indepth investigation on the relationships between the crystalline phases and barrier performances will be carried out in a future work. Finally, EVOH appears as a very interesting material because its crystalline phases, and then properties, change as a function of the cooling conditions: therefore, it is possible to tune the crystallization process (Tc) and crystalline phases in order to match the requested performances according to the final applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04191. Detailed thermal characterization of all of the blends prepared (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +39-0512090349. Fax: +39-0512090322. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Coleman, M. M.; Yang, X.; Zhang, H.; Painter, P. C. Ethylene-covinyl alcohol blends. J. Macromol. Sci., Part B: Phys. 1993, 32, 295−326. (2) Lagaron, J. M.; Catalá, R.; Gavara, R. Structural characteristics defining high barrier properties in polymeric materials. Mater. Sci. Technol. 2004, 20, 1−7.
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Industrial & Engineering Chemistry Research
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DOI: 10.1021/acs.iecr.5b04191 Ind. Eng. Chem. Res. 2016, 55, 3517−3524
