KatalinHalaszEtAl Application of nano and micro sized ...€¦ · poly(lactic acid) were investigated. 2 MATHERIALS AND METHODS Poly(lactic acid) was obtained from Esun (AI1031),

International Scientific Conference March 26-27 2012 on Sustainable Development & Ecological Footprint Sopron, Hungary

Application of Nano and Micro Sized Cellulose Crystals in

Poly(lactic Acid)

Katalin HALÁSZa* – Levente CSÓKAa – Rita RÁKOSAb aInstitute of Wood and Paper Technology, Faculty of Wood Sciences,

bInstitute of Chemistry and Soil Science, Faculty of Forestry, a b University of West Hungary, Sopron, Hungary

Abstract –In this research poly(ethylene glycol), microcrystalline cellulose (MCC) and ultrasound treated microcrystalline cellulose were used to modify the attributes of PLA film. The modification was carried out by compounding the reinforcements with the polymeric matrix in twin screw extruder. To achieve less brittle material and to help the fine dispersion of the particles and the compatibility between the cellulose and the poly(lactic acid) poly(ethylene glycol) (PEG) was used as well. In order to characterize the structure and properties of the composite/nanocomposite materials scanning electron microscopy (SEM), transmitting electron microscopy (TEM), UV-VIS spectrophotometry, differential scanning calorimetry (DSC), thermogravimetry (TG) and tensile measurement tests were applied.

Keywords: cellulose / poly(lactic acid) / composite

1 INTRODUCTION

Poly(lactic acid) (PLA) (Figure 1) is a biodegradable, linear, aliphatic polyester produced from lactic acid, which can be derived from the fermentation of different naturally available polysaccharides. The poly(lactic acid) is a versatile, natural based plastic which has got its potential in many applications such as in medical, drug delivery, textile or packaging applications. However, PLA is too brittle for many applications, softens at relative low temperatures - around 50-60°C, has worse water vapour and gas barrier properties compared to commercial polymers. To extend the application field the improvement of its properties (barrier, thermal, mechanical) is required. Focusing on the packaging applications the industry demands less brittle materials with high barrier and good thermal properties. One of the promising materials to improve the properties of the PLA can be the cellulose.

Figure 1. Chemical structure of PLA

Cellulose is one of the most abundant polymers on the Earth and its properties give a widespread industrial usage. Cellulose is renewable, biodegradable and it bears relatively reactive surface, thus it can be a good choice at producing PLA based green

* Corresponding author: Katalin Halász, [email protected], Bajcsy-Zsilinszky st. 4., Sopron, 9400


composites/nanocomposites. Microcrystalline (Figure 2) and nanocrystalline cellulose can be both applied for modifying the properties of PLA. Microcrystalline cellulose (MCC) is commercially available, it is produced by treating α-cellulose obtained as a pulp from fibrous plants with minerals acid, consisting of microcrystals together with amorphous regions. Nanocrystalline cellulose (NCC) on the other hand is not yet a commercially available product but it can be achieved from MCC via acidic hydrolysis and/or ultrasonical (US), hydrodynamical cavitation treatment (BELGACEM – GANDINI 2008, FILSON – DAWSON-ANDOH 2009, PINJARI – B.PANDIT 2010). It is necessary to mention that processing techniques have a remarkable impact on the adhesion properties of the resulting cellulose in composite/nanocomposite applications (GARDNER et al. 2008). Because of the small size NCC can provide unique properties of the matrix, it can improve the water vapour permeability, the mechanical properties and thermal stability without affecting the transparency of the matrix (AJAYAN et al. 2005, MAKITEV et al. 2009).

Figure 2. Microcrystalline cellulose (PETTERSON – OKSMAN 2005)

Processability and properties of composites or nanocomposites are strongly affected of

four main factors, namely: characters of the components, composition, interfacial interactions and structure (RENNER et al. 2009). Using cellulose has got its limitations because cellulose has a strong sensitivity to water and moisture and it display poor compatibility with the hydrophobic polymeric matrices.

Although PLA is an apolar plastic it contains polar groups like C=O as well, which can help the interaction - for example the hydrogen bond formation between cellulose OH and PLA C=O groups (NAKAYAMA 2007). Interfacial interactions can be improved by different kind of physical (corona, plasma, laser or γ-radiaton) and chemical treatments (BELGACEM – GANDINI 2005). Compatibilizers such as alcene ketene dimmer, maleic-anhydride, phenyltrimethoxysilican or poly(ethylene glycol). Poly(ethylene glycol) (PEG) can not only act as compatibilizer but as plasticizer as well thus reducing the brittleness of the PLA. In general, the lower the molecular weight the higher the plasticizing effect. In this study the effects of MCC, ultrasound treatment and PEG 400 on the properties of the poly(lactic acid) were investigated.

2 MATHERIALS AND METHODS

Poly(lactic acid) was obtained from Esun (AI1031), microcrystalline cellulose was supplied from Sigma Aldrich in particle size


the dispersion of the cellulose the MCC-PEG suspension was treated with ultrasound. To obtain nanosized cellulose no chemicals were used in order to keep the process “green”. Control samples (neat PLA and PLA with PEG400) were prepared in the same way.

Glass transition (Tg) and melting temperature (Tm) were determined by differential scanning calorimetry (DSC). Each sample was heated at a rate of 5°C/min under inert condition. The thermal decomposition temperatures were detected by using thermo gravimetry (TGA) with the heating rate of 5°C/min. The TGA measurements were carried out in nitrogen atmosphere. Tensile properties were performed in cross and production direction according to EN 527 on an Istron tensile tester, at 23°C and 50RH%, with 50 mm gauge length, 2 kN load and 50mm/min crosshead speed. Young-modulus, stress and strain at peak and break were measured. Scanning electronmicroscopy (SEM) photographs were taken of the fracture surface of the samples (gold coating was needed). Transmission electronmicroscopy (TEM) of the ultrathin PLA based samples was carried out with Jeol transmission electron microscope. The acceleration voltage was 120 keV. To characterise the transmittancy of the samples UV-VIS spectrophotometry was used.

3 RESULTS AND DISCUSSIONS

DSC and measurement was used to characterize the thermal properties of the PLA based composites. As the results show (Table 1, Figure 3) glass transition temperature (Tg) was slightly decreased in every cases. The melting temperatures did not show notable difference except the difference of the two melting peaks. While the neat PLA bears two well defined individual peaks, the modified samples first peak is not so strong and they shifted to lower temperatures. The two melting peaks of neat PLA can be due to the coexistence of two kind of crystalline modification or because of the melting behaviour with melt recrystallization model (YASUNIWA et al. 2003). The smaller peaks can indicate that the additives changed the crystal structure of the PLA (in case of C3PPLA_US no other peak can be observed). The amorphous part of the modified poly(lactic acid ) samples started to form organized structure at lower temperatures (Tcc), the exothermal cold crystallization peaks (Tcc) are smaller and thinner, but as one can see the melting peaks are more wider. According to the enthalpies and the calculated crystallinity the cellulose acts as nucleating agent especially when untrasonical treatment was used. This indicates that US treatment modified the cellulose characteristics and significant size reduction even appeared.

Table 1. Thermal properties measured by DSC

Tm melt peaks (°C) Tg (°C ) Tcc (°C )

xc calc. cryst. (%)

a neat PLA 142,7 151,9 52 102,6 9,63 b PPLA (PLA + PEG) 135,6 150,6 47,7 85,3 25,99 c C1PPLA 138,9 150,8 46,4 92,6 17,98 d C3PPLA 135,1 150,1 42,6 83,9 24,25 e C5PPLA 137,8 150,6 44,2 89,2 18,52 f C1PPLA_US 137,6 150,7 44,6 89,8 17,61 g C3PPLA_US - 150,7 44,4 90,5 53,43 h C5PPLA_US 138,9 150,9 46,6 91,8 65,87


Figure 3. DSC curves (second heating) of PLA based samples

The thermogravimetry analysis (TGA) gave the thermal decomposition temperatures of

the plastic samples (Table 2). Comparing to the neat PLA the highest improvement was observed in case of C5PPLA_US. The onset temperature of the composite shifted to higher temperatures with 8,8°C. On the other hand lower onset temperature and temperature at 100% weight loss appeared using 3 wt% US treated cellulose.

Table 2. TGA data

onset T of the

weight loss (°C) T at 50% weight

loss (°C) T at ~100%

weight loss(°C) residual weight at

500°C (t%) neat PLA 315,3 345,2 375,5 2,00 PPLA 318,0 346,2 371,7 0,04 C1PPLA 316,1 350,6 376,1 0,01 C3PPLA 313,1 346,2 376,4 0,02 C5PPLA 316,3 341,5 369,9 0,07 C1PPLA_US 319,0 347,7 368,9 0,04 C3PPLA_US 306,3 330,7 355,8 0,03 C5PPLA_US 324,1 349,7 371,7 0,09

The mechanical performance of the composites strongly depends on the dispersion and distribution of the particles in the matrix and also on filler–polymer interactions. If the compatibility is poor between the components the reduction of the mechanical properties will appear (DUCHEMIN et al. 2009). According to the results the maximal improvement at production direction in strain at peak was 186,77% (strain was 5,730 %) in case of C3PPLA_US, at cross section the C1PPLA_US showed the highest improvement with 212,84% (strain was 4,178%). More remarkable improvement was observed in case of elongation at break, where samples with US treated MCC showed significant enhancement at production direction (possibly du to the orientation of the cellulose during the sheet extrusion process), where improvement with 717,83 (strain:303,5%); 558,18 (236,0%) and 433,77% (183,4%) occurred in case of C1PPLA_US, C3PPLA_US, C5PPLA_US, respectively. In cross direction the same tendency can be observed, the highest improvement is 553,37% (sample C1PPLA_US). Although elongation at break was improved almost in every case, the ultimate strength was reduced. Young-modulus decreased too.

Scanning electron microscopic (SEM) photos were taken of the facture surfaces after the


tensile tests. As the photos demonstrate the neat PLA shows rigid broken surface, no plastic regions can be observed. The sample containing PEG 400 on the other hand shows plastic deformation as well such as the other modified samples. MCC can be clearly seen in the fracture surfaces, the dispersion and the distribution is optimal. Although samples with US treatment contain micro sized particles, thus the size reduction was not complete, but other results indicate that the samples contain smaller particles as well.

Figure 4. SEM micrographs of the tensile fractured surfaces

After cutting ultrathin films of the composites transmission eletronmicroscopy (TEM)

was carried out. In the two pictures below (Figure 5.) cellulose particles can be seen in the poly(lactic acid) matrix, the less the initial concentration of the MCC-PEG suspension was the more effective the US treatment became. There are small particles under 200 nm in the composites. Because of the sensitivity of the PLA for the relative high acceleration voltage closer images could not be taken.

Figure 5. TEM photographs of the modified samples

UV-VIS spectrophotometry was used to characterize the transmittance of the foils in the

range of ~ 400-700nm. In the table the average thicknesses and transmittances are shown. As expected the more cellulose the samples contained the lower their transmittance became. Even though the US treated cellulose containing samples are a slightly thicker (d) their transmittance in the visible range is higher. This is due to the small particle phase formed during the ultrasonic treatment which - because of the small size - does not affect the transmittance of the PLA.

Table . Transmittance of the samples

neat PLA PPLA C1PPLA C3PPLA C5PPLA C1PPLA_US C3PPLA_US C5PPLA_US

d (mm) 0,11 0,078 0,094 0,097 0,111 0,112 0,126 0,107

T (%) 87,15 90,19 66,21 41,74 38 79,42 65,37 47,02

400nm 400nm

c


4 CONCLUSIONS

During this research MCC, ultrasound treated MCC and PEG 400 was used to modify the properties of the PLA. According to the results small amount (1wt%) of US treated cellulose in PEG 400 was enough to reduce the brittleness of the PLA. Slighter improvement occurred when PEG400 or MCC with PEG400 were only used. The thermal properties of the foils did not change significantly, but the presence of the MCC and especially the US treated MCC indicated higher crystallinity. Although SEM images showed that micro sized cellulose particles still remained (despite the US treatment), the TEM photos and the UV-VIS spectrofotometry proved that the size of the particles were reduced remarkably too. Possibly a hybrid of micro and nanocomposite was formed. Although further research is needed to improve the compatibility between cellulose and PLA, materials in micro and nano size derived from these renewable biomaterials could play a large role in the improvement of PLA. Acknowledgements: The research was supported by the Social Renewal Operation Programme TÁMOP 4.2.1.B-09/1/KONV-2010-0006 research project co-funded by the European Social Fund.

REFERENCES

AJAYAN, P. –SCHANDLER, L. S. – P. V. BRAUN, P. V. (2005): Nanocomposite science and

technology, WILEH-VC, Mörlenbach, 231 p. BELGACEM, M. N. – GANDINI, A (2005): The surface modification of cellulose fibres for use

as reinforcing elements in composite materials, Composite Interfaces 12 (1-2): 41-75. BELGACEM, M. N. – GANDINI, A (2008): Monomers, Polymers and Composites from

Reneawable Resources, Elsevier Ltd., Oxford, 414 p. FILSON, P.B. – DAWSON-ANDOH, B. E. (2009): Sono-chemical preparation of cellulose

nanocrystals from lignocellulose derived materials, Bioresource Technology 100: 2259-2264.

GARDNER, D. J. – OPORTO, G. S. – MILLS, R. – SAMIR, M. A. S. A. (2008): Adhesion and surface issues in cellulose and nanocellulose, Journal of Adhesion Science and Technology 22: 545-567.

LIM, L. T. – AURAS, R. – RUBINO, M. (2008): Processing technologies for poly(lactic acid), Progress in Polymer Science 33: 820-852.

MAKITEV, A. – KOZLOV, G. – ZAIKOV, G. (2009): Polymer nanocomposites, Nova Science Publishers, Inc., New York, 207-251 p.

PETTERSON, L – OKSMAN, K. (2006): Biopolymer based nanocomposites: Comparing layered silicates and microcrystalline cellulose as nanoreinforcement, Composite Science and Technology 66: 2187-2196.

PINJARI, D. V. –PANDIT, A. B. (2010): Cavitation milling of natural cellulose nanofibrils, Ultrasonics Sonocemistry 17 : 845-852.

RENNER, K. – MOCZÓ, J. – Pukánszky, B. (2009): Micromechanical deformations in particulate filled polymers: The effect of adhesion, Conference Proceeding of the 17th International Conference on Composite Materials

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KatalinHalaszEtAl Application of nano and micro sized ...€¦ · poly(lactic acid) were investigated. 2 MATHERIALS AND METHODS Poly(lactic acid) was obtained from Esun (AI1031),