Enhancement of
interfacial adhesion and
engineering properties
of polyvinyl alcohol/
polylactic acid laminate
films filled with modified
microfibrillated cellulose
Thorsak Kittikorn
1,
Wantani Chaiwong
1,
Emma Stromberg
2, Rosana M Torro
3,
Monika Ek
2and Sigbritt Karlsson
2Abstract
This work was done to improve the interfacial adhesion and engineering performance of polyvinyl alcohol/polylactic acid laminate film by altering the polyvinyl alcohol phase surface properties via incorporating microfibrillated cellulose modified by propionyla-tion. Incorporating the modified microfibrillated cellulose into polyvinyl alcohol film improved adhesion between film layers during the laminating process. Improved peel strength and tensile properties confirmed that modified microfibrillated cellulose can produce better bonding between polyvinyl alcohol and polylactic acid via mechanical interlocking and cohesive forces at the film interface. Modified microfibrillated cellulose (3 wt%) increased the peel strength by 40% comparing with the neat polyvinyl alcohol/ polylactic acid laminate film.The reduction of both moisture absorption and diffusion rate of the modified microfibrillated cellulose–polyvinyl alcohol/polylactic acid to 20 and
1Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Songkhla,
Thailand
2Department of Fiber and Polymer Technology, Royal Institute of Technology, Stockholm, Sweden 3Department of Bioeconomy, Research Institute of Sweden (RISE)—INNVENTIA AB, Stockholm, Sweden
Corresponding author:
Thorsak Kittikorn, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Songkhla 90110, Thailand.
Email: thorsak.k@psu.ac.th
0(0) 1–23 ! The Author(s) 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/8756087920915745 journals.sagepub.com/home/jpf
23%, respectively, also indicated that the modified microfibrillated cellulose could inhibit moisture permeation across the film. This was because the modified microfibrillated cellulose is hydrophobic. Furthermore, the addition of modified microfibrillated cellu-lose also increased the decomposition temperature of the laminate film up to 10% as observed at 20% of remaining weight, while the storage modulus substantially increasing to 72% relative to the neat laminate film.The superior interfacial adhesion between the polylactic acid and modified microfibrillated cellulose–polyvinyl alcohol layers, observed by scanning electron microscopy, confirmed the improved compatibility between the polyvinyl alcohol and polylactic acid phases.
Keywords
Polylactic acid, polyvinyl alcohol, microfibrillated cellulose, laminate film, thermal analysis, adhesion
Introduction
Due to environmental concerns, conventional plastics such as polyolefins or poly-ethylene terephthalate are being replaced with biodegradable plastics, especially polylactic acid (PLA). Although biodegradable plastics are less competitive econom-ically than conventional plastics, they can make valuable contributions to mitigating global warming and protecting the environment. PLA’s cost effectiveness can be increased by combining it with other low-cost polymers to expand its potential applications. This is an appropriate and effective method especially in packaging. As a result, the use of packaging based on bioplastic materials is increasing annually and entering the daily life of consumers. Recently, many research groups reported incorporating polyvinyl alcohol (PVA) with PLA by blending or laminating.
PVA is used widely1,2in the adhesive and film industries.3–5Due to its low cost, its solubility in water, and its biodegradability, PVA’s use as a component in PLA laminated film shows potential from both economic and environmental aspects. Since PVA can accelerate biodegradation through moisture absorption and pro-mote PVA/PLA laminate film hydrolysis, it could potentially be exploited as a disposal method for managing plastic waste.
However, although they are both biodegradable polymers, PLA and PVA are not compatible and their different characteristics must be overcome. PVA is very sensitive to moisture and is more polar than PLA due to the many hydroxyl groups along the chain molecules. Therefore, when laminating two film layers, poor bond-ing between the layers is likely to produce voids or gaps and moisture absorption excludes using the laminated film in moist environments. These failings are major drawbacks in laminate film products.
Previously, PVA film processing incorporated nanoclay to improve filler–matrix bonding6–8and chemical treatments to improve mechanical and thermal properties, along with the gas permeability.9–12By this route, it is possible to successfully produce
a PVA/PLA laminate film but these processes are quite complicated and difficult to commercialize. However, by adding modified microfibrillated cellulose (MFC), the PVA film surface free energy can be reduced, making it possible to improve the PVA/PLA compatibility. MFC is a very versatile, “green” nanomaterial with a very high aspect ratio, which makes it a very effective reinforcement for polymer compo-sites.13–15Because of its transparency and excellent gas barrier properties, the MFC was a logical choice of high-performance additive for use in packaging film.16–18
MFC is easily mixed with PVA due to their similar hygroscopic properties. Hence, a possible fabrication strategy is incorporating MFC into PVA to improve the PVA film performance before combining with PLA to produce a laminate film. The PLA film production cost can be reduced and the overall film performance can be improved. However, to increase the interfacial adhesion between PLA and PVA film layers lam-inated by hot compression, it is necessary to modify MFC by esterification, such as by propionylation. Propionylated MFC could decrease the PVA surface free energy to increase hydrophobicity which would lead to better bonding with PLA film.
Therefore, we investigated and developed MFC–PVA/PLA laminated film. The propionylated MFC was loaded into PVA film to reduce surface energy with an intent to improve the PLA film interfacial adhesion during the hot press process. However, the PVA film needed to be cross-linked using citric acid to prevent hydrolysis in the press and water dissolution. The fabricated laminate films were characterized by mechanical and thermal properties and moisture absorption. The MFC morphology and interfacial adhesion at the laminated layer boundary were analyzed by scanning electron microscopy (SEM). The surface free energy was determined by measuring contact angles.
Experimental
Materials
PLA, grade of Ingeo4043D (Mw¼ 158,000 g/mol, density ¼ 1.24 g/cm3) and PVA (Mw¼ 100,000 g/mol, density 1.15 g/cm3) were supplied as pellets by Nature Works (USA) and Loba Chemie (India), respectively.Citric acid (purity, 99.5%) and chloro-form (purity, 99.8%) were from RCI Lab. Northern bleached soft wood Kraft (long fiber pulp originated from pine and spruce) was kindly supplied by Siam Chemical Group (Thailand). Propionic anhydride (purity of 99%) was purchased from Sigma Aldrich. (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl (TEMPO) (purity of 98%) and sodium bromide (NaBr) (purity of 99.5%) were also supplied by Sigma Aldrich and Quality Chemical Reagent (New Zealand), respectively.
Preparation of microfibrillated cellulose
MFC was prepared according to the method proposed by Yano et al.19,20The pulp fiber was hydrolyzed by HCl and NaOH solution, respectively, for reducing the
cellulose to microcrystalline cellulose (MCC). Afterward, the MCC was further oxidized by TEMPO-oxidation reaction for converting alcohol groups on the cel-lulose molecules to carboxylate anionic groups. TEMPO and NaBr were used as reagents for this reaction. In sequence, the TEMPO-oxidized cellulose was disinte-grated by mechanical homogenization (Microfluidics M110P, USA) to produce MFC. The obtained MFC was suspended in water and kept as a feed stock for further processing.
Modification of MFC by propionylation
MFC in suspension was reacted with liquid propionic anhydride. The propionic anhydride to MFC suspension ratio was 1:1 by weight. The propionylation reac-tion was carried out at 120C. After 1 h, the reaction was terminated and the modified MFC was precipitated from the suspension using acetone and stored as a hydrogel.
MFC–PVA/PLA laminate film fabrication
Solution casting
The PLA film was fabricated via solution casting. PLA solution (5 wt% in chlo-roform) was poured into a mold plate and left overnight at ambient temperature. The solidified PLA film was separated from the mold and kept in a desiccator. A PVA solution was prepared by dissolving 10 wt% PVA powder in water. As a crosslinking agent, citric acid was added at 16 wt% of PVA. The PVA solution was poured into the mold plate and left to solidify at ambient temperature and then pre-cured at 60C for 1 h. PVA films were produced by adding 3 wt% (of PVA mass) of unmodified and modified MFC to PVA solutions and then casting the solutions as explained above.
MFC–PVA/PLA laminate film fabrication
The neat PVA, MFC–PVA, and modified MFC–PVA films were laminated with PLA film using a hot press at 165C for 3 min at 300 kg/cm2. Under these conditions, the PVA films were successfully cross-linked. All laminate films were kept in a desiccator to avoid moisture absorption before characterization.
Characterization
Tensile test
The mechanical properties were determined with an Instron 5566 universal electromechanical testing machine (Instron Corporation, High Wycombe,
UK) following the ASTM D638 standard. The analyses were performed at 25C and 55% relative humidity (RH), using a crosshead speed of 5 mm/min. The tensile modulus at 0.5% tensile strain was analyzed. The aver-age force in the measurement travel direction was monitored following the ASTM D638 standard. The test was repeated with five specimens for each sample.
Peel test
The adhesion strength between PVA and PLA layers of the laminate films was determined in terms of peel strength using the Zwick Roell testing machine (Model Z010, Germany). The laminate film sample with five specimens was tested using a separating rate of 254 mm/min at 25C and 55% RH following the testing method of ASTM D1876. The peel strength was expressed in N/m and was determined from the maximum load divided by the width of the laminate films. The results were recorded for statistical analysis.
Moisture absorption
To investigate moisture absorption, five specimens (dimension of 1 1 cm2) were kept in a closed container in the presence of KCl saturated salt (84% RH) and weighed after 2, 4, 6, 24, 48, and 72 h. Moisture absorption was calculated accord-ing to the followaccord-ing equation
Wab¼
Wt W0
W0
where
Wab ¼ absorbed moisture mass
W0 ¼ initial weight
Wt ¼ final weight
The diffusion coefficient was calculated using Fick’s law by the following cor-relation equation Mt M1 ¼ 4 L D p 0:5 t0:5
Where
Mt ¼ mass of absorbed water at any time
M1 ¼ mass of absorbed water at equilibrium
D ¼ diffusion coefficient L ¼ sample thickness (mm)
t ¼ time (min)
Differential scanning calorimetry (DSC)
The melting temperature and polymer crystallinity were determined using a Mettler Toledo DSC820 calorimeter (Schwerzenbach, Switzerland) calibrated with an indium standard. Both the first and the second heating runs were recorded, and the heating/cooling rate was 10C/min from 30 to 190C under N2
atmo-sphere. The samples were analyzed in duplicate, the results as obtained from the first and the second heating runs of glass transition temperature (Tg), crystalliza-tion temperature (Tc), melting temperature (Tm), and enthalpy change of fusion (DH) of PLA were analyzed to evaluate how MFCs affect PLA thermal properties.
Dynamic mechanical thermal analysis (DMTA)
The dynamic mechanical thermal behavior was analyzed using a Mettler Toledo DSC820 calorimeter (Schwerzenbach, Switzerland) in tensile mode. Samples were heated from 30 to 140C at 3C/min in multiple frequency strain mode. Storage modulus, loss modulus and Tan delta were recorded for analysis.
Thermal Gravimetric Analysis (TGA)
The thermal degradation of the laminate films was determined using a Mettler Toledo TGA/SDTA 851 (Greifensee, Switzerland). Samples were heated from 25 to 600C at 10C/min in N2 atmosphere. At least two samples of each specimen
were analyzed. Percent weight loss and derivative thermogravimetry (DTG) were also analyzed in order to evaluate how MFCs affect thermal decomposition.
Surface energy analysis
The surface energy was measured using contact angle apparatus from Dataphysics OCA 15EC. The measurements were at room temperature using a droplet of dis-tilled water, ethanol, and dimethylformamide on specimen samples measuring 1 cm 1 cm. Sessile drop volume was maintained at 5 ml using a microsyringe. The sample measurement was repeated six to nine times at different positions on the same specimen sample.
Atomic force microscopy
AFM images were obtained using a Digital Instruments Dimension 3100 AFM with a nanoscope III a controller. The system was operated in tapping mode with DI tapping mode tips having a resonant frequency of 280 kHz. The line speed was 1 Hz with an overall 512 s image integration time. The two-dimensional images were converted to three-dimensional images and a rough-ness analysis was done using WSxM Image Browser software v. 2.0, Develop 7.5 supplied by Nanotech Electronica S.L.
SEM
A field emission scanning electron microscope (SEM Quanta) was used for mor-phological analysis. Samples were dried at 60C for 24 h and then kept in a des-iccator for 24 h. The fiber surface was analyzed to study the treatment affects. Laminated film cross-sections were analyzed to study the interfacial adhesion between the PLA and PVA layers. The specimens were sputter-coated with a gold–palladium layer using a high resolution sputter coater, equipped with a thick-ness monitor controller.
Fusion emission SEM
The cellulosic fiber surface morphology was examined using field emission scan-ning electron microscopy (FE-SEM) (FEI Quanta 200F, Netherlands) with an accelerating voltage of 15–20 kV. Images showing surface morphologies of the MFC fibers were taken at various magnifications. Before observation, a fine layer of gold was sprayed on samples by an ion sputter coater with a low deposition rate. Energy dispersive X-ray diffraction spectra were included with FE-SEM images.
Results and discussions
Fiber analysis
Figure 1 shows the MFC and propionylated MFC (modified MFC) FTIR spectra. The carbonyl groups of carboxylate ion appeared around wavenumber 1600 cm1.21,22This result was similar to previous reports.23,24The new band of carboxylate groups strongly confirmed TEMPO-oxidation on cellulose. After modification of the MFC with propionic anhydride, a new peak, representing ester groups, appeared at 1730 cm1 and the peak intensity at 1450 and 1375 cm1 increased. These peaks were attributed to C–H bending of the CH2
and CH3 of propionate groups. Similarly, an increased peak at 1010 cm1 was
attributed to C–O–C bond after esterification. Furthermore, the new peak at 890 cm1 was identified as an ester group characteristic. These results confirmed that the MFC propionylation was achieved.
Surface morphology analysis
Figure 2 shows the SEM images of unmodified and modified MFC fibers. It shows that the esterification by propionic anhydride significantly altered the MFC surface morphology.
Table 1 presents that the modified MFC diameter was significantly reduced from 50 to 35 nm. The reduced modified fibers’ diameter increased the surface area. As a result, mechanical properties and resistance to gas permeability were
Figure 2. SEM images of the surface morphology: (a) modified and (b) unmodified MFC. Figure 1. FTIR spectra of MFC before and after propionylation. MFC: microfibrillated cellulose.
improved. The size reduction probably resulted from disrupted hydrogen bonding caused by the substituted OH groups with hydrophobic propionate groups in pyranose rings. Consequently, the modified MFC could easily split into smaller fibers with higher aspect ratio.
Mechanical properties
The stress–strain behavior showed that incorporating both unmodified and mod-ified MFC into the laminate films increased tensile strength compared with the neat laminate film (Table 2).
The increased tensile properties indicated that, due to hydrogen bonding between the MFC and PVA phases, the MFC fibers were well dispersed in the PVA phase. However, the modified MFC–PVA/PLA laminate film showed greater tensile strength and modulus while elongation at break is practically the same. As previously described in the “Surface morphology analysis” section, the reduced MFC fiber diameter is due to propionylation which increased the fiber aspect ratio. The modified MFC reinforcing effect in the PVA phase was improved by the higher aspect ratio and better interfacial adhesion between phases. The added modified MFC also reduced the PVA film surface free energy, which was con-firmed by the contact angle testing (Table 3).
Table 1. Diameter, length, and aspect ratio of the microfibrillated celluloses.
TEMPO-oxidized pulp fiber Average diameter(a) (nm) Average length(a) (nm) Average aspect ratio Unmodified MFC 50 570 11.4 Modified MFC 35 540 15.4
MFC: microfibrillated cellulose; TEMPO: (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl.
a
Diameter and length of MFCs measured using an Image JPG software.
Table 2. Tensile properties of all laminate films.
Film Tensile strength (MPa) Tensile modulus (MPa) Elongation at break (%) PVA/PLA 8 759 5 Unmodified MFC–PVA/PLA 11 721 4 Modified MFC–PVA/PVA 14 949 14
MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol. (Readers can see further in the supplemental materials)
The reduced surface free energy of the modified film by MFC propionylation was expected. Lowering the PVA composite film surface free energy was crucial when hot pressing with PLA film during lamination because reduced surface free energy improved cohesive bonding between PLA and modified PVA films.25 Improved stress transfer between the layers increased both modulus and strength. In addition, the substituting hydroxyl groups with propionate groups along the cellulose chains suppressed its hydrophilic property.
Table 3. Contact angle for films (surface energy analysis).
Film Dispersive Polar SE total
PLA 17 10 27
PVA 9 51 60
Unmodified MFC–PVA 8 57 65
Modified MFC–PVA 10 53 63
MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol; SE: surface energy.
Figure 3. Comparison of stress–strain behavior of all laminate films. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
Interestingly, the elongation at break of the modified MFC–PVA/PLA showed the most value of all laminate films as presented in Figure 3.
This confirms that PLA and the modified PVA films adhered strongly together. Conversely, the lower elongation at break of the unmodified MFC laminate film indicated poorer adhesion between the two layers. These behaviors were exactly as expected since surface free energy was increased by the MFC, as confirmed by contact angle testing. In unmodified MFC–PVA/PLA, bonding at the interface was less cohesive and broke up under greater elongation.
The results implied that the PVA–PLA film tensile properties were significantly affected by the nano-scale modified MFC. A high modulus, high aspect ratio, and lower surface free energy should improve compatibility and fiber dispersion in the PVA film layer and subsequently promote cohesive bonding between the laminate layers. The increased tensile strength also confirmed the better interfacial bonding between the PLA film and the modified MFC–PVA film.
Figure 4 presents the peel strength for the laminate films. The MFCs increased the lamination bond elucidated in terms of peel strength. Incorporating the modified MFC improved the strength up to 40% versus the neat laminate film. The interfacial properties were significantly different when MFC is added. The differentiation was influenced by two factors: the PVA film surface roughness and surface energy.
The atomic force microscope images in Figure 5 show that adding unmodified and modified MFC made the PVA film surfaces rougher.
Figure 4. Peel strength for the laminate films. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
Each laminated film was separated and the PVA surface that had been in contact with the PLA film was scanned. The PVA film separated from PVA/PLA laminate film without MFC was the smoothest of all the PVA films (10 nm RMS) and the PVA film peeled from unmodified MFC–PVA/PLA laminate film was the roughest (705 nm RMS) as presented in Table 4. Although the surface film of the unmodified MFC–PVA was rougher than the surface film of modified MFC–PVA (544 nm RMS), the peel strength of the modified MFC–PVA/PLA laminate film was higher. This result indicated that adhesion between modified MFC–PVA and PLA was predominantly affected by the PVA film polarity, which was determined by the contact angle method. The neat PVA/PLA film peel strength was anticipated to be the lowest since bonding by mechanical interlocking was weakest.
The surface energy was analyzed using contact angle apparatus and the unmodi-fied MFC in PVA film considerably increased the PVA film polarity as previously described. Substituting OH groups with propionate groups in modified MFC altered their surface property, which decreased the PVA film polarity. However, PVA and modified MFC–PVA polar values were nearly the same while the
Figure 5. The AFM images show (a) surfaces of neat PVA, (b) unmodified MFC-PVA and (c) modified-PVA film after hot-pressed lamination with PLA film. The surfaces scanned had been in contact with the PLA film.
Table 4. AFM analysis of surface roughness.
Sample Root mean
square (RMS) (nm)
PVA 10
Unmodified MFC–PVA 705
Modified MFC–PVA 544
AFM: atomic force microscope; MFC: microfibrillated cellulose; PVA: polyvinyl alcohol.
unmodified MFC–PVA polarity was significantly higher. According to our hypothesis, tuning the surface energy of each film layer to be closer could improve the laminate adhesive bond. However, as confirmed by peel testing, the peel strength was as follows: modified MFC–PVA/PLA> unmodified MFC–PVA/ PLA> PVA/PLA. Therefore, it could be concluded that the best interfacial adhe-sion observed in the modified MFC–PVA/PLA film was mainly governed by cohe-sive bonding due to the improved hydrophobicity.
Moisture absorption
The moisture absorption test showed that, after 72 h, moisture uptake by the modified MFC–PVA/PLC film was the least. Meanwhile, unmodified MFC– PVA/PLA showed the highest moisture absorption (Figure 6).
The lower moisture uptake for the modified MFC laminate film was due to the improved hydrophobicity and the smaller modified MFC diameter compared with unmodified MFC. Thus, the modified MFC, well dispersed in the PVA phase, acted like a barrier to prevent water molecules penetrating the film. Therefore, this could reduce film swelling and lead to prolong shelf life in the long term. However, both unmodified and modified MFC–PVA/PLA films tended to exhibit two equilibrium stages of moisture uptake. The first equilibrium stage films occurred within 24 h. After that, moisture absorption increased again from 24 to
Figure 6. Moisture absorption versus time for laminate films. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol. (readers can see further in the supplemental materials)
48 h and then remained constant. It could be that unmodified and modified MFC behaved like solid fillers and produced in homogeneities which differentiated mois-ture transfer across the PVA film.
Considering the initial equilibrium stage, the modified MFC–PVA/PLA had the lowest diffusion rate of all the films, while the unmodified MFC–PVA/PLA diffu-sion rate was the highest (Table 5).
The increased moisture diffusion rate in unmodified MFC–PVA/PLA indicates that the inherent MFC hydrophilicity could accelerate water uptake in the laminate
Table 5. Diffusion rates of moisture in all laminate films at 30C.
Sample Diffusion coefficient
(cm2/min)
PVA/PLA 5.47 108
Unmodified MFC–PVA/PLA 5.94 108
Modified MFC–PVA/PLA 4.19 108
MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
Figure 7. Moisture diffusion rate versus t1=2in all laminate films. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
film. When the lower modified MFC–PVA/PLA film diffusion rate is considered, it confirms that the improved MCF hydrophobicity is due to propionylation.
The control of moisture penetration is a necessary concern for applications of pack-aging films and the low rate of moisture absorption in the modified MFC film (Figure 7) also confirms that modified MFC enhanced the moisture resistance of the film.
Thermal properties of laminate films
The thermal properties of the laminate films are presented in Table 6. The Tg, Tc, and Tm in Table 6 were representative of the PLA phase during the second heating scan of DSC.
Table 6. DSC analysis of all laminate films.
Film Tg (C) Tc (C) Tm (C) DHm (J/g) Crystallinity (%) PVA/PLA 56 121 150 13 12 Unmodified MFC–PVA/PLA 57 121 150 20 19 Modified MFC–PVA/PLA 57 116 149 29 27
DSC: differential scanning calorimetry; MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
Figure 8. Comparison of DSC spectra of all laminate films. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
The high value of the crystalline domain, calculated from the enthalpy change of melting (referred 93.1 J/g for 100% crystal of PLA26), confirmed the induced crystallinity due to modified MFC (Figure 8).
The thermal decomposition of all laminate films was analyzed using TGA as presented in Figure 9.
As observed TGA spectra of all films, it could be seen four ranges of thermal decomposition. The first decomposition is from 50 to 190C and this action was occupied by water evaporation. The second from 190 to 275C could be the dis-integration of citric acid27while decomposing at temperatures from 300 to 380C and 400 to 500C was occupied by PLA and PVA films, respectively.
Taking into consideration weight remaining, the decomposition temperature of neat PVA/PLA laminate film at 80, 50, and 20% weight remaining was approxi-mated around 305, 328, and 398C, respectively, as presented in Table 7.
It was clear that the modified MFC–PVA/PLA resisted thermal degradation the most. As comparing with the neat laminate films, incorporating modified MFC in the PVA phase remarkably improved the thermal property as rising temperature resistance. Furthermore, the modified MFC–PVA/PLA laminate film exhibited the lowest rate of decomposition in the DTG spectrum (Figure 10), indicating that modified MFC slowed down thermal decomposition. Then, the added modified MFC not only made the greatest effect to increase decomposition temperature, but it also suppressed the decomposition rate.
Figure 9. Comparison of % weight loss by TGA of all laminate films. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
DMTA
The thermal–mechanical behaviors were analyzed using DMTA (Figure 11). The modified MFC–PVA/PLA film storage modulus indicated the greatest stiff-ness. This was attributed to the higher aspect ratio of modified MFC and the excellent adhesion between the modified MFC–PVA and PLA films. These results confirmed that the modified MFC is a reinforcing filler and surface modifier that controls the PVA/PLA laminate film properties.
Table 7. Analysis of thermal decomposition of all laminate films.
Percent weight remaining Decomposition temperature (C) PVA/PLA Unmodified MFC–PVA/PLA Modified MFC–PVA/PLA 80 305 307 310 50 328 330 348 20 398 446 441
MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
Figure 10. DTG spectra of all laminate films. DTG: derivative thermogravimetry; MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
Figure 11. DMTA spectra of all laminate films. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
Figure 12. Tand versus temperature. MFC: microfibrillated cellulose; PLA: polylactic acid; PVA: polyvinyl alcohol.
The glass transition profiles were distinct from the neat laminate film. Both MFC laminate films show a prominent tan d peak at 58–60C and a minor peak at 70C (Figure 12).
Generally, PLA Tg occurs around 60C seen as a tan d peak; however, the small second peak at around 70C might be attributed to tight interfacial binding between the partial molecular chains in the PLA amorphous phase and the MFC–PVA films. More energy was needed to mobilize the chains and loosen the binding.
SEM images
SEM was used to prove and evaluate the interfacial adhesion between PLA and PVA layers in the laminate (Figure 13).
The neat PVA/PLA laminate film (Figure 13(a)) shows voids at the boundary layer interface. The voids indicate poor bonding between the PLA and PVA layers. The voids were remarkably reduced in the unmodified MFC–PVA/PLA laminate
Figure 13. SEM cross-sections: (a) PVA/PLA, (b) unmodified MFC–PVA/PLA, and (c) modified MFC–PVA/PLA. PLA: polylactic acid; PVA: polyvinyl alcohol.
film (Figure 13(b)), which confirmed better bonding between the two layers. The seamless interface in the modified MFC–PVA/PLA laminate film (Figure 13(c)) evidenced the improved compatibility between the two layers. As described earlier, the better interfacial adhesion between PLA film and modified MFC–PVA film was due to the improved mechanical and thermal properties of the PVA film surface. These improvements were caused by alterations due to modified MFC. During the lamination process, the improved surface properties of the PVA film facilitated cohesive bonding. Furthermore, it was possible that modified MFC in the PVA surface layer might become entangled in the PLA surface.
Conclusions
Modifying MFC by propionylation significantly reduced the MFC fiber diameter and led to improved PVA/PLA laminate film in several aspects.
Incorporating modified MFC into the PVA layer in a PVA/PLA laminate film successfully altered the PVA surface energy. Consequently, compatibility between the PVA and PLA layers was improved due to better cohesive bonding.
The modified MFC as a filler in the PVA layer also restrained moisture perme-ation determined by the rates of moisture absorption and diffusion.
The better mechanical properties of modified MFC–PVA/PLA analyzed by tensile and peel testing along with DMTA were crucial confirmation of the improved performances of the modified MFC–PVA/PLA laminate film.
Acknowledgement
The achievement of this project is under the collaboration between Prince of Songkla University (PSU), Thailand and Royal Institute of Technology (KTH). The authors would like to gratefully acknowledge Prince of Songkla University, Thailand and the Royal Institute of Technology (KTH), Sweden for any support in this project.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, author-ship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, author-ship and/or publication of this article: Funding received from Prince of Songkla University under project no. SCI591062N.
ORCID iD
Supplemental materials
The authors would like to state that the data as presented in this manuscript including the supplemental materials can be publicly accessed.
References
1. Lu DR, Xiao CM and Xu SJ. Starch-based completely biodegradable polymer materi-als. Express Polym Lett 2009; 6: 366–375.
2. Chiellini E, Corti A, Antone SD, et al. Biodegradation of poly (vinyl alcohol) based materials. Prog Polym Sci 2003; 28: 963–1014.
3. Kaboorani A, Bernard R, Pierr B, et al. Nanocrystalline cellulose (NCC): a renewable nano-material for polyvinyl acetate (PVA) adhesive. Eur Polym J 2012; 1829–1837. 4. Kumar A, Negi YS, Choudhary V, et al. Characterization of cellulose nanocrystals
produced by acid-hydrolysis from sugarcane bagasse as agro-waste. J Mater Phys Chem2014; 2: 1–8.
5. Kausar A. Polymeric nanocomposites reinforced with nanowires: opening doors to future applications. J Plast Film Sheet 2018; 35: 65–98.
6. Gaume J, Rivaton A, Therias S, et al. Influence of nanoclays on the photochemical behaviour of poly(vinyl alcohol). Polym Degrad Stabil 2012; 97: 488–495.
7. Dhawal PD and Sadhan CJ. Nanoclay-induced morphology development in chaotic mixing of immiscible polymers. J Appl Polym Sci 2005; 43: 3638–3651.
8. Tripathi S, Mehrotra GK and Dutta PK. Preparation and physicochemical evaluation of chitosan/poly(vinyl alcohol)/pectin ternary film for food-packaging applications. Carbohydr Polym2010; 79: 711–716.
9. Grunlan J, Grigorian A, Hamilton C, et al. Effect of clay concentration on the oxygen permeability and optical properties of a modified poly(vinyl alcohol). J Appl Polym Sci 2003; 93: 1102–1109.
10. Ray SS, Bousmina M and Maazouz A. Morphology and properties of organoclay mod-ified polycarbonate/poly(methyl methacrylate) blend. Polym Eng Sci 2006; 46: 1121–1129. 11. Mandal A and Chakrabarty D. Characterization of nanocellulose reinforced semi-interpenetrating polymer network of poly(vinyl alcohol) & polyacrylamide composite films. Carbohydr Polym 2015; 134: 240–250.
12. Qua EH, Hornsby PR, Sharma SHH, et al. Preparation and characterization of poly (vinyl alcohol) nanocomposites made from cellulose nanofibers. J Appl Polym Sci 2009; 3: 2238–2247.
13. Fujii K, Nakagaito AN, Takagi H, et al. Sulfuric acid treatment of halloysite nanoclay to improve the mechanical properties of PVA/halloysite transparent composite films. Compos Interfaces2014; 21: 319–327.
14. Das K, Ray D, Bandyopadhyay NR, et al. Physico-mechanical properties of the jute micro/nanofibril reinforced starch/polyvinyl alcohol biocomposite films. Compos Part B 2011; 42: 376–381.
15. Kausar A. A review of fundamental principles and applications of polymer nanocom-posites filled with both nanoclay and nano-sized carbon allotropes – graphene and carbon nanotubes. Plastic Film Sheet, Epub ahead of print 21 October 2019. DOI: 10.1177/2F8756087919884607.
16. Gunes IS and Jana S. Chemical and engineering aspects of morphology development and processing of multiphase polymer blend nanocomposites. In: Isayev AI and Pasule
S (eds) Encyclopedia of polymer blends 2011, Vol. 2, pp.357–394. Chichester; Weinheim: Wiley-VCH, 2011.
17. Fortunati E, Peltzer M, Armentanoa I, et al. Effects of modified cellulose nanocrystals on the barrier and migration properties of PLA nano-biocomposites. Carbohydr Polym 2012; 90: 948–956.
18. Held A, Kowalska J and Nowin K. Propane-to-propene oxide oxidation on silica-supported vanadium catalysts with N2O as an oxidant. J Catalysis 2016; 336: 23–32. 19. Yano H and Nakahara S. Bio-composites produced from plant microfiber bundles with
a nanometer unit web-like network. J Mater Sci 2004; 39: 1635–1638.
20. Yano H, Sukiyama J, Nakagaito AN, et al. Optically transparent composites reinforced with network of bacterial nanofibers. Adv Mater 2005; 17: 153.
21. Henriksson M, Henriksson G, Berglund LA, et al. An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers. Eur Polym J2007; 43: 3434–3441.
22. Kumar A, Singh Negi1 Y, Choudhary V, et al. Characterization of cellulose nano-crystals produced by acid-hydrolysis from sugarcane bagasse as agro-waste. J Mater Phys Chem2014; 2: 1–8.
23. Dongping S, Lingli Z, Qinghang W, et al. Preliminary research on structure and prop-erties of nano-cellulose. J Wuhan Univ Technol Mater Sci Ed 2007; 22: 677–680. 24. Singh M, Kaushik A and Ahuja D. Surface functionalization of nanofibrillated cellulose
extracted from wheat straw: effect of process parameters. Carbohydr Polym 2016; 150: 48–56.
25. Fanga JM, Fowlera PA, Escrigb C, et al. Development of biodegradable laminate films derived from naturally occurring carbohydrate polymers. Carbohydr Polym 2005; 60: 39–42.
26. Gil-Gastell O, Badia JD, Kittikorn T, et al. Hydrothermal ageing of polylactide/sisal biocomposites – study of water absorption behavior and physic-chemical performance. Polym Degrad Stabil2014; 108: 212–222.
27. Wang S, Ren J, Li W, et al. Properties of polyvinyl alcohol/xylan composite films with citric acid. Carbohydr Polym 2014; 103: 94–99.
Biographies
Thorsak Kittikorn is deputy head of Department of Materials Science and Technology, assistant professor in Polymer Science Program in the Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hatyai, Songkhla, Thailand. He is interested in biodegradable poly-mers, lignocellulose polymer biocomposites, nanocellulose and its derivative, anti-microbial additive from natural product, polymer degradation in environment, plastic processing, injection molding, and extrusion process.
Wantani Chaiwong is an assistant researcher in Polymer Science and Technology Division at Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hatyai, Songkhla, Thailand. His interest is in bio-degradable polymers for packaging film.
Emma Stromberg is an associate professor in Division of Polymeric Materials at Department of Fiber and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology, Stockholm, Sweden. His interests are in polymer materials, characterization of the material properties of virgin and recycled polymeric materials, biomedical materials, biopolymers, natural compos-ite materials, nanocomposcompos-ites, biofilm formation on the materials surfaces, and the prevention of biofouling. The environmental interactions of the polymeric materi-als and the adhesion of microorganisms provide the fundamentmateri-als for the design of new materials with antimicrobial properties. Special significance in the research is aimed at the release of low molecular weight compounds and nanoparticles during the degradation of the material.
Rosana M Torro is a researcher in Division of Nanocellulose in the Department of Bioeconomy, Research Institute of Sweden (RISE)—INNVENTIA AB, Stockholm, Sweden. She is interested in biobased polymers and composite, mod-ification and characterization to design sustainable materials and composites. Monika Ek is the head of division of Wood Chemistry and Pulp Technology and professor in Wood Chemistry, at Department of Fiber and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology, Stockholm, Sweden. She is interested in energy-efficient cellulosic insulation prod-uct, green nanocomposite from forest waste, and biointeractive fibers.
Sigbritt Karlsson is rektor of Royal Institute of Technology, professor in Division of Polymeric Materials at Department of Fiber and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology, Stockholm, Sweden. She is interested in environmental interaction of polymeric materials.
