SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Tri-(butanediol-monobutyrate) citrate (TBBC) as a new plasticizer for poly(lactic acid) (PLA) was synthesized via a two-step esterification. The chemical structure of TBBC was characterized by 1H-nuclear magnetic resonance. The studies on solubility parameters, transparence, and storage stability indicated the good miscibility between PLA and TBBC. The glass transition, crystallization, thermal, and mechanical properties of PLA plasticized by TBBC were evaluated. With an increase in TBBC content, the glass transition temperature (Tg), melting point (Tm), and the cold crystallization temperature (Tcc) of plasticized PLA gradually shifted to a lower temperature. The elongation at break and flexibility were greatly improved by the addition of TBBC. After 30 days of storage, PLA plasticized with up to 20 wt% of TBBC exhibited good storage stability and remained the original transparence and mechanical properties. The flexibility of PLA/TBBC films can be tuned by changing TBBC content. The corresponding crystalline morphology and structure were investigated by Polarizing optical microscope and X-ray diffraction as well. This study revealed that TBBC was miscible with PLA and may therefore be a promising plasticizer for PLA-based packaging materials. POLYM. ENG. SCI., 55:205–213, 2015. © 2014 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

The environmental and food safety problems have aroused people's concern about biodegradable materials [1]. Biodegradable polymers are considered as food packaging materials to solve serious ecological problems caused by the non-biodegradability of petrochemical-based polymers [2].

Poly(lactic acid) (PLA) is a biodegradable and thermoplastic polymer which can be produced from lactic acid obtained through fermentation of renewable resources [3]. PLA has played a major role for replacing polyolefins in “green” packaging applications, due to its good mechanical properties, moderate barrier properties, processability [4, 5], and transparence [6, 7]. To further improve the physical properties of PLA, many studies focus on blending and grafting with other biodegradable polymers such as thermoplastic starch [8], poly(ethylene oxide) [9, 10], poly(ɛ-caprolactone) [11, 12], poly(vinyl acetate) [13], poly(hydroxyl butyrate) [14, 15], cellulose [16], and poly(butylene succinate) [17, 18].

In general, the glass transition temperature (Tg) of neat PLA is 58°C which exhibits brittleness at room temperature [19, 20]. Thus its flexibility needs to be improved by selecting suitable plasticizers. The good plasticizers for PLA-based packaging materials can be embedded between the chains of PLA, spacing them apart, significantly lowering Tg, and thus giving this material improved flexibility and durability. Concerning food contact materials, plasticizers should be nontoxic and also show good compatibility with PLA and provide suitable thermal and mechanical properties. Besides, plasticizers should remain low volatility during melt blending and low migration during the shelf and service life. Plasticizers used for PLA have been reported such as oligomeric lactic acid [2], triacetin and tributyl citrate (TBC) [21], poly(ethylene glycol) (PEG) [22, 23], poly(propylene glycol) [24], or their copolymer poly(ethylene glycol-co-propylene glycol) [25]. The studies have found that some of these plasticizers efficiently reduced Tg and improved the flexibility of PLA films. However, the blends tend to phase separation, most likely because of the undergoing crystallization [26, 27]. As a consequence, the crystallization of PLA causes phase separation in the blend and forces the plasticizer to migrate to the film surface. TBC is an efficient plasticizer for PLA, but it migrates toward the film surfaces during storage period. A possible way to prevent the migration would be to increase the molecular weight of the plasticizers. However, if the molecular weight increases too much, it would eventually decrease the miscibility and cause phase separation [28-30]. By a relative increase in molecular weight of the plasticizer, the tendency for migration would decrease, and miscibility could be maintained through the polar interactions.

In order to increase the molecular weight and retain the similar structure to TBC, a new plasticizer tri-(butanediol-monobutyrate) citrate (TBBC) is synthesized for plasticizing PLA. The study mainly involves on miscibility, crystallization, thermal properties, mechanical properties, and storage stability of PLA/TBBC blends. We expect that the results gain herewith could give more information toward a better understanding of the plasticizing effect of citrate esters for PLA.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Materials

PLA (Revode101), with l/d isomer ratio of 95/5, was supplied by Hisun chemical company, China. The melting temperature (Tm) is 152°C and the glass transition temperature (Tg) is 52°C. The polymer granules were dried for 24 h at 40°C in a vacuum oven and then stored in sealed PE-bags and placed in a desiccator. 1,4-butanediol (BDO; 99.5%), n-butyric acid (BA; 99.5%), citric acid (99.5%), methylbenzene (99.5%), and p-toluenesulfonic acid (99%) were supplied by Tianjin Kewei chemical company.

Synthesis of TBBC

Figure 1 shows a two-step esterification of TBBC. Butanediol-monobutyrate (BB) was synthesized as follow. BDO was mixed with excess BA in 1/1.2 mole ratio. Methylbenzene as water-carrying agent and p-toluenesulfonic acid as a catalyst were added. The mixture was heated at 120°C for 4 h to remove water until the removal of water approached 95% of theoretical yield. The mixture was purified with deionized water to remove residual BDO, BA, and catalyst. Methylbenzene was distilled by reduced pressure distillation on a rotatory evaporator at 95°C.

image

Figure 1. The synthesis path for tri-(butanediol-monobutyrate) citrate (TBBC).

Download figure to PowerPoint

TBBC was synthesized by esterification of BB with citric acid. Citric acid was mixed with excess BB in 1/3.4 mole ratio. Methylbenzene as water-carrying agent and p-toluenesulfonic acid as a catalyst were added. The mixture was gradually heated from 150 to 200°C for 4 h to remove water until the removal of water approached 95% of theoretical yield. A high vacuum was applied at 200°C for 2 h to further remove water and excess BB. After esterification completed, the mixture was purified with deionized water to remove residual BB, citric acid, and catalyst. Methylbenzene was distilled by reduced pressure distillation on a rotatory evaporator at 95°C.

Sample Preparation

Melt Compounding

PLA and TBBC were dried in a vacuum oven at 60°C for 24 h before compounding. PLA and TBBC were melt compounded at 30 rpm and 160°C for 6 min using a Hakke Rheomix-600 equipped with a pair of high-shear roller rotors. PLA/TBBC blends were blended at the weight ratios of 85/15, 80/20, 75/25, which were correspondingly denoted as PLA/TBBC-15, PLA/TBBC-20, and PLA/TBBC-25. In order to obtain the same thermal history, virgin PLA without TBBC has been melt compounded in the same procedure.

PLA/TBBC Film Preparation

The films were produced in a compression molding machine. Blends were placed in a square frame template with 140 mm × 140 mm × 0.5 mm to ensure a constant film thickness. The frame covered with teflon films to prevent sticking to the press plates. The compression machine was heat to 160°C and pressure was gradually increased to 10 MPa and maintained for 5 min. The compression mold was then cooled through a swift water cooling system, so that the sample approached ambient temperature quickly. The blended films were stored in sealed plastic bags to prevent moisture before analysis.

Sample Characterization

Gel Permeation Chromatography

The molecular weight (Mn) and polydispersity index (Mw/Mn) of PLA and PLA/TBBC blends were analyzed by a gel permeation chromatography PL-GPC220 using a refractive index detector. Chloroform was used as the eluent at a flowing rate of 1.0 mL min−1 at 40°C, and the concentrations of PLA/TBBC blends were about 2 mg mL−1. Polystyrene standards with low polydispersity were used to make a calibration curve.

Differential Scanning Calorimetry

Thermal properties of PLA/TBBC films were measured using differential scanning calorimetry (DSC), DSC 204 F1, NETZSCH. Before measurements, all samples were melted and then quenched in liquid nitrogen in order to ensure that all samples are fully amorphous state. PLA/TBBC blends were weighed between 5 and 10 mg, accurate to 1 μg. The first heating thermogram was performed from −30 to 200°C with 10°C min−1 of heating rate. The temperature of crystallization obtained during heating thermogram is defined as cold crystallization temperature (Tcc). In the cooling thermogram, samples were held at 200°C for 5 min to get rid of thermal history, and then cooled down from 200 to −30°C at a cooling rate of 10°C min−1. The temperature of crystallization obtained during the cooling process is defined as hot crystallization temperature (Thc). The second heating thermogram was performed from −30 to 200°C at a heating rate of 10°C min−1. Glass transition temperatures (Tg), cold crystallization temperatures (Tcc), hot crystallization temperatures (Thc), melting temperatures (Tm), and the melting enthalpy (ΔHm) were recorded from the scans.

The degree of crystallinity (Xc) was calculated from the melting enthalpy in the second heating thermogram by the following equation:

  • display math(1)

where Φ is the weight percentage of TBBC in the blends, ΔHm is the melting enthalpy (J g−1) that was calculated from the melting peak in the DSC curve. inline image is the melting enthalpy for completely crystallized PLA (94 J g−1) [31].

Polarizing Optical Microscope

A Nikon polarizing optical microscope (POM) equipped with a hot stage was used to observe crystalline morphology of PLA/TBBC blends under the isothermal crystallization process. A small sample weighed about 5 mg was sandwiched between two glass cover slips and placed on a digital hotplate. The hotplate was rapidly heated above Tm of the sample and kept for 3 min. The molten sample was gently pressed between glass cover slips, and then was rapidly cooled to 100°C and kept at this temperature for 2 h. The crystalline morphology of PLA/TBBC blends was observed under POM.

X-ray Diffraction

X-ray diffraction (XRD) patterns of PLA/TBBC blends were taken on a Philips X′pert Pro PW3040160, using flat plate geometry with Cu Kα radiation at 30 kV and 20 mA. In order to confirm the amorphous state of neat PLA and PLA/TBBC films, all samples were quenched from melt state to room temperature by a swift water cooling system. The samples were placed into the plate aperture before performing the XRD experiments. Data were collected as step scans from 5° ≤ 2θ ≤ 50°, with a step of 0.02° and a time/step of 0.25 s.

Tensile Measurement

Tensile test was performed on the dumbbell shape specimens using an Instron-5582 Machine at room temperature according to British Standard BS2782-3:326F:1997. During measurement, the crosshead speed was set at 20 mm min−1 to record tensile strength (σ) and elongation at break (ɛ). At least five specimens were tested for each batch to obtain the mean value and the standard deviation values. The standard deviation values (S) were obtained by the following equation:

  • display math(2)

where S is the standard deviation, N is the number of values, xi is the values of each measurement, and inline image is the mean value. Before measurements, all samples were placed in a programmable constant temperature humidity oven, at which the condition was set at with 23 ± 2°C and 53 ± 2% relative humidity.

Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was performed with a TA instrument TGA Q500. PLA/TBBC blends weighed approximately 7 mg were placed in alumina crucibles and heated at a heating rate of 10°C min−1 from room temperature to 500°C under nitrogen atmosphere at a flow rate of 50 mL min−1.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

The Chemical Structure of TBBC

The 1H-nuclear magnetic resonance (1H-NMR) spectrum for the synthesized TBBC was recorded using a Bruker Advance 400 MHz spectrometer. The spectrum was obtained at 25°C with CDCl3 as a solvent, and tetramethylsilane was used as the internal reference. The chemical shift signals of the protons are analyzed and shown in Table 1. The peak at δ = 0.95 ppm (t, H1), δ = 1.70 ppm (m, H2), and δ = 2.28 ppm (t, H3) are assigned to the proton in the methylene and methyl group of image. The chemical shift signals of the protons in methylene groups of image correspond to δ = 4.09 ppm (t, H4) and δ = 1.66 ppm (m, H5). The signal at δ = 2.85 ppm (s, H6) corresponds to the chemical shift of the proton in image.

Table 1. Characteristic 1H-NMR shifts.
Type of proton*PositionChemical shift signals, δ (ppm)
image10.95
image21.70
image32.28
image44.09
image51.66
image62.85
image

The Molecular Weight of PLA/TBBC Blends

Table 2 shows the effect of different TBBC content on the weight-average molecular weight (Mw) and polydispersity index (Mw/Mn) of the PLA/TBBC blends. The weight-average molecular weight obtained from GPC obviously decreases from around 129,696 to 46,256 g mol−1 by the introduction of TBBC and polydispersity indexes of the blends are in the range of 2.18–1.96. Due to thermal unstable of PLA and transesterification between PLA and TBBC during the melt compounding process, it is inevitable to show a clear downward trend for their molecular weight. After 30 days of storage, the Mw of PLA plasticized with TBBC shows either a slight decrease or a stable Mw compared to that of the fresh blends.

Table 2. The molecular weight and polydispersity index of PLA and PLA/TBBC blends.
Sample inline image (g mol−1) inline image
Days of storage030030
PLA129,696121,8362.182.18
PLA/TBBC-1596,25695,6342.091.98
PLA/TBBC-2060,67460,7481.961.85
PLA/TBBC-2546,25642,8721.961.78

Solubility Parameter Calculation

In order to predict the miscibility between TBBC and PLA matrices, the solubility parameter ( inline image) of the materials is determined by calculation of their group contribution, according to the following equation:

  • display math(3)

where inline image represents the solubility parameter, inline image represents the density, inline image represents the molecular mass, and F represents the group molar attraction constants according to the method of Hoy [26]. The chemical groups of TBBC and the group molar attraction constants (F) are shown in Table 3. The density of PLA and TBBC are 1.25 and 1.06 g mL−1, respectively. The solubility parameters, which are calculated from Eq. (3) according to Table 3, are 19.93 and 18.40 (J cm−3)1/2 for PLA and TBBC, respectively. The similar value of solubility parameters indicates the miscibility between TBBC and PLA.

Table 3. The group molar attraction constants.
GroupF (J cm−3)1/2 mol−1
image303.4
image269.0
image65.5
image668.2
image462.0

Transparence and Storage Stability

Neat PLA and PLA/TBBC blends in the form of films are shown in Fig. 2. All films exhibit amorphous state due to a quench process from melts. TBBC is homogeneously plasticized among PLA, so these films exhibit high transparence similar to neat PLA and good flexibility. After 30 days of storage, PLA/TBBC-15 and PLA/TBBC-20 keep the original transparence because of good miscibile stability between PLA and TBBC. However, the transparence for PLA/TBBC-25 slightly decreases due to slow cold crystallization during storage. The corresponding cold crystallization during storage is further studied via XRD in Fig. 5.

image

Figure 2. Comparison on optical transparence of PLA/TBBC blends before and after 30 days of storage: (a) neat PLA, (b) PLA/TBBC-15, (c) PLA/TBBC-20, and (d) PLA/TBBC-25.

Download figure to PowerPoint

Thermal Properties

The storage stability and miscibility of PLA/TBBC blends are related to TBBC content and their thermal histories. Miscibility is generally stated when only one glass transition temperature is recorded on the DSC traces and also yields some changes in crystallization and melting behaviors [32]. The thermal properties of PLA/TBBC blends are shown in Fig. 3 and their DSC data are summarized in Table 4. The melting and cold crystallization behaviors of PLA/TBBC blends in the first heating thermogram are presented in Fig. 3a. All samples exhibit only one glass transition temperature, so they can be considered as miscible ones and no phase separation is expected. Tg of PLA/TBBC blends shifts to lower temperature with an increase in TBBC content, around 54, 36, 28, and 24°C for neat PLA, PLA/TBBC-15, PLA/TBBC-20, and PLA/TBBC-25, respectively. Therefore, the flexibility of their films is great improved by adding TBBC due to Tg reduction. It should also be emphasized that the experimental Tg values obtained for PLA/TBBC blends is comparable to the results obtained for PLA plasticized with oligomeric lactic acid (OLA) and PEG [29, 32], that show a downward tendency and the similar Tg value and in all cases for the same plasticizer concentrations. Nevertheless, the crystallinity of PLA might influence the miscibility of this system. A cold crystallization and a subsequent melting peak are observed for PLA/TBBC blends. The similar enthalpy values for the cold crystallization and subsequent melting is an indication of the amorphous nature of the virgin samples. The neat PLA exhibits two melting points (Tm), respectively, at 141.8 and 151.6°C, and the cold crystallization temperature (Tcc) at 105.4°C, which are consistent with previous studies [28]. The lower melting peak corresponds to the melts of the original crystalline grains, while the higher melting peak corresponds to the melts of more stable grains because recrystallization occurs during thermogram scan. With an increase in TBBC content, Tm and Tcc shift toward the lower temperature. Table 4 shows the first peak of Tm is located at 131.7, 126.9, and 128.6°C and the second peak of Tm is located at 144.8, 141.5, and 139.2°C, respectively, whereas Tcc is located at 68.5, 61.9, and 60.1°C. TBBC effectively reduces Tg and promotes the PLA chains mobility, which induces cold crystallization to start at an earlier temperature in the first heating thermogram [33]. Since the cold crystallization reduces the transparence of the blends and enhances the surface migration of plasticizer, so it is expected that Tcc should much higher than the storage temperature. From Table 4 and Fig. 3a, it concludes that the PLA/TBBC blends can keep good transparence during shelf storage, because the onset of cold crystallization of them is at least 20°C above the room temperature. PLA/TBBC-15 is more stable than PLA/TBBC-20 and PLA/TBBC-25, which is consistent to the results of Fig. 2.

image

Figure 3. The thermal properties of PLA/TBBC blends under various thermal thermograms in differential scanning calorimetry: (a) under the first heating thermogram, (b) under the cooling thermogram, and (c) under the second heating thermogram.

Download figure to PowerPoint

Table 4. Cystallinity and thermal properties of PLA and PLA/TBBC blends.
 Tg (°C) inline image (°C) inline image (J g−1) inline image (°C) inline image (J g−1) inline image (%)
  1. First H represents the first heating thermogram; second H represents the second heating thermogram; Tg represents the glass transition temperature; inline image represents the cold crystallization temperature; ΔHcc represents the cold crystallization enthalpy; Tm represents the melting temperature; inline image represents the melting enthalpy; and Xc represents the degree of crystallinity of PLA calculated from the melting enthalpy in the second heating thermogram.

PLA
First H54.0105.4−22.39141.8; 151.623.03
Second H55.0121.3−29.77153.630.0932
PLA/TBBC-15
First H36.568.5−32.85131.7; 144.833.01
Second H132.4; 147.631.1539
PLA/TBBC-20
First H28.261.9−28.77126.9; 141.529.54 
Second H132.0; 147.530.4548
PLA/TBBC-25
First H24.360.1−30.65128.6; 139.231.45--
Second H128.0; 145.530.0852

Figure 3b shows that PLA does not exhibit the hot crystallization temperature (Thc) in the cooling thermogram, whereas PLA/TBBC blends exhibit Thc appearing at around 90°C with an addition of TBBC due to the promotion of crystallization of PLA. The peak width of the hot crystallization becomes broader by adding TBBC. It is in agreement with previous studies and similar results about other plasticizers such as acetyl tributyl citrate, and PEGs for plasticizing PLA [29].

Figure 3c shows the melting behaviors of PLA/TBBC blends in the second heating thermogram. The neat PLA exhibits Tm at 153.6°C and Tcc at 121°C. The PLA/TBBC blends do not exhibit any crystallization peak because they are well crystallized during the previous cooling process. PLA/TBBC blends shows double endothermic peaks. The melting points (Tm) decrease correspondingly with an increase in TBBC content. The first peak is located between 128 and 132°C while the second peak is located between 145 and 148°C. The double melting peaks of plasticized PLA could attributed to the melt of coexistence of disorder (α′) and order (α) crystalline phase of PLA, which are promoted by adding TBBC [30, 34].

The melting enthalpy (ΔHm) of neat PLA is about 30 J g−1, which corresponds to the crystallinity of 32.2%. Even though the melting enthalpies (ΔHm) of PLA/TBBC blends are in the range of 30–32 J g−1, after deducting the weight percentage of TBBC, these values correspond to the crystallinity of 39, 48, and 52%, respectively, with an increasing of TBBC content. The increase in crystallinity of PLA in the blends should be attributed to the plasticizing effect of TBBC which increases the chain mobility and promotes the formation of a higher crystallinity.

Crystalline Morphology

The crystalline morphology of PLA/TBBC blends was observed under a POM in an isothermal crystallization state. In Fig. 4a, neat PLA presents small-sized spherulites with Maltese cross which are smaller than 10 μm and cover all surface area. The spherulite size gradually increases with an addition of TBBC and accordingly the number of spherulites decreases. The addition of TBBC enhances the flexibility of PLA chains, so their crystallizability and the size of spherulites is promoted, which is consistent with the previous DSC analysis. The results from DSC and POM measurements demonstrate that the properties of PLA/TBBC blends is greatly related to the content of TBBC and thermal history. The good flexible, transparent, and stable PLA/TBBC blends can be produced, if only a large amount of amorphous state can be kept in the blends and their crystallization are suppressed during storage period.

image

Figure 4. The crystalline morphology of PLA/TBBC blends under the isothermal crystallization process at 100°C: (a) neat PLA, (b) PLA/TBBC-15, (c) PLA/TBBC-20, and (d) PLA/TBBC-25.

Download figure to PowerPoint

XRD Analysis

The effect of storage period on their crystallization is studied by XRD patterns, which is shown in Fig. 5. XRD patterns confirm that the fresh PLA/TBBC films, which are produced by a quench process from melt state to room temperature, exhibit a fully amorphous state, so only the amorphous halo can be observed. After 30 days of storage at room temperature, PLA/TBBC-25 exhibits the characteristic diffraction peaks located at 15.0, 16.5, and 18.9°, corresponding to the planes (010), (200)/(110), and (203)/(113) of orthorhombic crystalline phase [35, 36]. Such peaks correspond to the typical α crystalline phase of PLA, which is described as pseudo-orthorhombic or orthorhombic unit cell. However, the amorphous halo is still observed in PLA/TBBC-15 and PLA/TBBC-20 after 30 days of storage, because the crystallization at room temperature is not raised during storage period. They remain amorphous state and do not appear phase separation and plasticizer exudation. It is also found that the addition of TBBC up to 20 wt% does not reduce the transparence and mechanical properties of the films, which can keep the original flexibility and plasticizing effect.

image

Figure 5. X-ray diffraction patterns of neat PLA and PLA/TBBC blends before and after 30 days of storage.

Download figure to PowerPoint

Mechanical Properties

The corresponding mechanical properties of the blends are measured via tensile tests. The stress-strain curves for PLA/TBBC blends are shown in Fig. 6 and the tensile strength (σ) and elongation at break (ɛ) are plotted in Fig. 7. Neat PLA shows a brittle fracture and very low elongation at break (∼3%) and a high tensile strength (∼44 MPa), which is in agreement with the results previously mentioned [2, 18, 25]. Even though the weight-average molecular weight of PLA obviously decreases from around 129,696 to 46,256 g mol−1 by melt compounding with TBBC, as expected the elongation for PLA/TBBC blends is still significantly enhanced with the addition of TBBC. It indicates a great enhancement in ductility for PLA/TBBC blends, which is well correlated with the decrease in Tg observed in DSC curves. PLA/TBBC blends exhibit a higher elongation at break (∼350%) and a lower tensile strength (∼18 MPa) with an increase in TBBC content.

image

Figure 6. The stress-strain curves of PLA/TBBC blends.

Download figure to PowerPoint

image

Figure 7. The tensile strength (σ) and elongation at break (ɛ) for PLA/TBBC blends before and after 30 days of storage.

Download figure to PowerPoint

After 30 days of storage, the elongation at break (<4%) of PLA/TBBC-25 dramatic declined due to the cold crystallization of PLA at the room temperature. The presence of low Mw (42,872 g mol−1) and Tg (24.3°C) of PLA provides an increase in the crystallization rate, thus allowing the crystallization of PLA to occur at a room temperature. On the contrary, PLA/TBBC-15 remains the original mechanical properties, because the cold crystallization is depressed at room temperature due to relatively high Mw (95,634 g mol−1) and Tg (36.5°C) during the storage period which can be confirmed from XRD measurements. In this experiment, an addition of 15 wt% TBBC to PLA is an effective approach to obtain the long shelf-stable and flexible film. The corresponding mechanical properties of PLA/TBBC-20 are fallen in between them. Therefore, the mechanical properties of PLA/TBBC films can be tuned by changing TBBC content.

Thermogravimetric Analysis

Thermal degradation of PLA and PLA/TBBC blends are evaluated by TGA and the corresponding derivative thermogravimetric analysis (DTG) curves shown in Fig. 8a and b, respectively. Neat PLA is decomposed in a single step, which is characterized by a single peak at 359°C on DTG curves. The peak on DTG curve characterizes the maximal rate ( inline image) of thermal degradation. This is consistent with conclusions previously reported on PLA [2, 31, 32]. The PLA/TBBC films show a similar degradation process. With the addition of TBBC, inline image decreases slightly compared to neat PLA. By comparing the initial decomposition temperature ( inline image), a significant decrease with increasing TBBC content can be observed. Residual weights of all samples are lower than 5 wt% and have no significant differences. It demonstrates that TBBC plasticizing PLA does not promote char formation. inline image and inline image of TBBC are located at 251 and 332°C, respectively, which are much higher than the melt-compounding temperature (160°C). Therefore, no plasticizer volatilization was created during melt-compounding process. inline image of PLA/TBBC blends is located between that of TBBC and PLA. The improved thermal stability of the plasticizer in PLA blends could be due to the well infusion of TBBC between PLA chains and the enclosure effect of the PLA matrix on TBBC.

image

Figure 8. Thermogravimetric analysis of PLA/TBBC blends in a nitrogen atmosphere: (a) thermogravimetric curves and (b) derivative thermogravimetric curves.

Download figure to PowerPoint

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

TBBC as a new plasticizer was synthesized. The molecular structure of TBBC was confirmed by 1H-NMR. The similar value of solubility parameters indicated the compatibility between PLA and TBBC. The present work demonstrated that PLA was effectively plasticized by adding TBBC, being miscible up to 25 wt%. With an increase in TBBC content, the glass transition temperature (Tg), the melting temperature (Tm), the cold crystallization temperature (Tcc), and the hot crystallization temperature (Thc) are decreased. After 30 days of storage, PLA/TBBC blends containing up to 20 wt% of TBBC kept good flexibility, transparence, and remained the good mechanical properties without phase separation. In summary, the good flexible, transparent, and stable PLA/TBBC blends can be produced, if only a large amount of amorphous state can be kept in the blends and their crystallization are suppressed during storage period. TBBC may be a promising plasticizer for PLA-based flexible packaging materials. The physical properties of PLA/TBBC films can be tuned by changing TBBC content.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES