Blend of Polypropylene/Poly(Lactic Acid) for Medical Packaging Application: Physicochemical, Thermal, Mechanical, and Barrier Properties

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Energy Procedia 56 ( 2014 ) 201 – 210

11th Eco-Energy and Materials Science and Engineering (11th EMSES) Blend of polypropylene/poly(lactic acid) for medical packaging application: physicochemical, thermal, mechanical, and barrier properties

Nalin Ploypetcharaa, Panuwat Suppakula,b, Duangduen Atongc, Chiravoot Pechyena,b,*

aDepartment of Packaging and Materials Technology, Kasetsart University, Chatuchak, Bangkok, 10900, Thailand bCenter for Advanced Studies in Agriculture and Food, KU Institute for Advanced Studies, Kasetsart University, Chatuchak, Bangkok, 10900, Thailand cNational Metal and Materials Technology Center, Thailand Science Park, Khlong Luang, Pathum Thani, 12120, Thailand

Abstract

The effect of polypropylene/poly(lactic acid) weight ratios on the properties of blend films compatilized with polypropylene- grafted-maleic anhydride were investigated with 100:0, 60:40, 50:50, 40:60, and 0:100 of weight ratio. The blend films were prepared by melt mixing technique and cast film extrusion. The results shown that the FTIR spectrum was confirmed the interaction between compatibilizer and polymers. Morphological investigation was distinctly seen a two phases system between polypropylene and poly(lactic acid). Increasing of PLA content from 40 to 60 wt.% resulting in decreased melting temperature and crystallinity from 158 °C to 154 °C and 38% to 31%, respectively. For tensile properties, modulus and tensile strength increased with increasing the PLA content, while elongation at break was drastically decreased from 500% (polypropylene) to less than 50% (blends). The barrier properties indicated that incorporation of poly(lactic acid) into polypropylene tend to increased water vapor permeability while oxygen permeability was decreased. From the morphology, thermal, mechanical, and barrier results, the polypropylene/poly(lactic acid) blends showed a typical immiscible polymer blend. ©© 2014 2014 Elsevier The Authors. Ltd. This Published is an open by access Elsevier article Ltd. under the CC BY-NC-ND license (Peer-reviewhttp://creativecommons.org/licenses/by-nc-nd/3.0/ under responsibility of COE of Sustainalble). Energy System, Rajamangala University of Technology Thanyaburi Peer-review(RMUTT). under responsibility of COE of Sustainalble Energy System, Rajamangala University of Technology Thanyaburi (RMUTT)

Keywords: Film; Polylactic acid; Polymer blend; Polypropylene

* Corresponding author. Tel.: +662-562-5295; fax: +662-562-5046. E-mail address: [email protected]

1876-6102 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala University of Technology Thanyaburi (RMUTT) doi: 10.1016/j.egypro.2014.07.150 202 Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210

1. Introduction

Blending polymer is a simple technique to enhance the property of pure polymer. The benefits of blending are providing new materials with desired properties at the low price, quick formulation changes, plant flexibility and high productivity and reduction of the number of grades that need to be manufactured and stored [1]. From those advantages, polymer blending is widely used in polymer consumption such as packaging, textile, and engineering industries. However, the properties of polymer blend are limited by immiscibility between neat polymers. To solve this problem, compatibilizers are often used as additives to improve the compatibility of immiscible blends. The compatibilization could achieve optimization of the interfacial tension, stabilize the morphology and enhance adhesion between the phases in the solid state. Graft copolymers with reactive functional compounds, such as polypropylene-grafted-maleic anhydride (PP-g-MA), are also used as compatibilizers. PP-g-MA has been established as an effective compatibilizer for blending polypropylene (PP) with polar polymers [2-4], which the PP part of the PP-g-MA is compatible with PP, and the anhydride part reacts with the polar component. In this work PP and poly(lactic acid) (PLA) were chosen as raw materials on fabrication of PP/PLA blend films due to tough-brittle properties of PP and PLA. Blends of PP and PLA may be an alternative for biodegradable materials because of the blends may have biodisintegration characteristics since PLA is biodegradable. The PP/PLA blend films were prepared with constant load of compatibilizer and investigated the effect of PP/PLA weight ratios on their properties.

2. Methodology

2.1. Materials

PP (EL-PRO™ P600F) was obtained from SCG Chemicals (Thailand). PP-g-MA (DuPont™ Fusabond® P613), as a reactive compatibilizer, was obtained from Chemical Innovation (Thailand). PLA (Ingeo™ Biopolymer 4043D) of Natureworks was supplied by BC polymers marketing (Thailand).

2.2. Samples Preparation

Table 1. Material formulations in blends preparation.

Sample PP (wt.%) PLA (wt.%) PP-g-MA (phr) PP 100 - - PLA - 100 - PP/PLA (60:40) 60 40 3 PP/PLA (50:50) 50 50 3 PP/PLA (40:60) 40 60 3

PP and PLA were blended at different weight ratios with twin screw extruder (Labtech Engineering LTE 20-40). Before extrusion, all materials were dried at 80 ºC for 12 hr. The temperature and screw speed were set at 210 ºC and 25 rpm, respectively. The composition of the blends was given in Table 1. The blends films were prepared by using cast film extruder (5-layer-flat-film & sheet coextrusion line, COLLIN®). The die temperature, chill roll temperature, and screw speed were set at 170 ºC, 50 ºC, and 50 rpm, respectively.

2.3. Characterizations

Fourier transform infrared (FTIR) spectroscopic measurements were performed using a FTIR spectrometer (Bruker TENSOR 27). The interactions among the PP, PLA and compatibilizer were studied within the range 4000 - 400 cm-1. Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210 203

Morphological characterization of PP/PLA blend films was viewed under a scanning electron microscopy. Surface and longitudinal fractured surface were determined with Philip XL30 and JEOL JSM-6610LV, respectively. The samples were fractured in liquid nitrogen and gold-coated to avoid electrostatic charging during analysis. The spherulitic morphology of polymer and blend samples was performed on thin films using an optical microscope (OM). Thermal analysis was carried out using differential scanning calorimeter and thermogravimetric analyzer (Mettler Toledo TGA/DSC1 STARe, Switzerland). The average weight of sample approximately 8 mg. For differential scanning calorimetry (DSC), the samples were heated from room temperature to 200 ºC at 5ºC/min, held for 10 min to remove the thermal history. Subsequently, the samples were cooled to -30 ºC at a cooling rate of 5 ºC/min and heated again to 200 ºC at 5 ºC/min under constant nitrogen flow. The second heating scans were used for analysis and percentage crystallinity (Xc) was calculated by using equation 1. Where ¨Hm is the measured melting enthalpy, * * ¨Hc is the cold crystallization enthalpy, ¨H m,PP and ¨H m,PLA are the melting enthalpy of pure polymer with complete crystallinity (188.9 J/g and 93, respectively) [5], and w is the weight fraction of PP and PLA in the sample.

¨Hm - ¨Hc Xc ሺ% crystallinityሻ= ( * * )×100 (1) wPP¨Hm,PP + wPLA¨Hm,PLA

Thermogravimetric analysis (TGA) was used to detect the degradation temperature (Td) of neat PP, PLA, and their blends. Samples were heated from 40 to 600 ºC at a heating rate of 10 °C/min under a nitrogen atmosphere. The flow rate of nitrogen was maintained at 20 mL/min. Tensile properties of samples were measured at room temperature using a universal testing machine (Instron model 5965) according to ASTM D882 with a gauge length of 50 mm. Crosshead speed and load cell were 50 mm/min and 5 kN, respectively. The average values of at least five tests were reported. Barrier properties of blend films were studied in terms of oxygen permeability (OP) with Illinois Instruments 8500 oxygen permeation analyzer and water vapor permeability (WVP) by Desiccant Method according to ASTM E96. For WVP measurement, approximately 80 g of silica gel were used as the desiccant material, while the temperature was set to 23°C with a relative humidity of 50%.

3. Results and disscusions

3.1.FTIR studies

Fig.1. Comparison of the FTIR spectra of (a) PP; (b) PLA; (c) PP/PLA (60:40); (d) PP/PLA (50:50); (e) PP/PLA (40:60).

The transmittance bands corresponding to PP (Fig. 1(a)) at 2950-2838, 1455-1453, and 1376 cm-1 were assigned 204 Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210

to CH stretching, CH3 bending, and C-H bending, respectively [6]. Fig. 1(b) depicts the FTIR spectra of PLA, -1 transmittance bands at 1749, 1181, and 1080 cm referred to C=O stretching, symmetric COC stretching, and asymmetric CH3, respectively [7]. The transmittance bands that represents PP and PLA were observed in the PP/PLA blends (Fig. 1(c-e)) around 2950-2838, 1456-1454, 1376, 1183-1182, and 1086-1184 cm-1. Existence of PP-g-MA, which characteristic spectrum has the strong peaks between 1800 cm-1 and 1700 cm-1 [8-10], could improve the compatibility of PP/PLA blends through PP part of PP-g-MA is compatible with the PP and the active site in the anhydride group reacts with the carbonyl group of PLA that results in the ester linkage.

Fig.2. FT-IR spectra of samples in the region 1800-1700 cm-1 of (a) PP; (b) PLA; (c) PP/PLA (60:40); (d) PP/PLA (50:50); (e) PP/PLA (40:60).

The disappearance of the anhydride peaks of PP-g-MA and the appearance of ester linkage in the FTIR spectra of the blends indicated interaction between PP, PLA, and PP-g-MA [11]. Confirmation of the blend components -1 interaction in this study was observed by representing of the ester linkage at 1756-1755 cm (Fig. 2(c-e)). The level of attractive interaction can also be seen in an increase in the intensity of characteristic absorbance (A) bands [12], which A is defined as the negative logarithm of the transmittance (T) or A is inverse relation with T [13]. Intensity reduction of ester linkage transmittance band with increasing of PLA ratio in the blends corresponded to lower content of ester linkage formation [14]. The absence of this peak could explain the occurrence of thermal chain scission at CO bond. Hence, the ester hydrogen transfer of PLA was proposed. According to these observations, we believe that as PLA, OC=O hydrogen transfer reaction can also happened on the blends surface [15].

3.2. Morphological investigation

From Fig. 3, the surface of PLA films seem to looks smoother than any other samples. To compare SEM micrographs of pure polymer film with that of PP/PLA blend films, the surface of the blends become rougher after blending. The multilayer system, physically phase separation between PP phase and PLA phase, was clearly shown in SEM micrographs of longitudinal fractured surface that is a typical behavior of immiscible polymer blends [16]. Fig. 4 depicts the spherulitic morphology of pristine PP and PP/PLA blends, while crystallization of PLA was not observed (figure not shown). It was clearly illustrated that the blend composition has a significant effect on the nucleation density. The nucleation density decreased with increasing proportion of PLA in the blends by reason of non-crystallizable polymer disturbing on the spherulite formation [17]. This result was consistent with crystallinity data from DSC analysis (discussed in the next section).

Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210 205

(a) PP

(b) PLA

(d) PP/PLA (50:50)

(e) PP/PLA (40:60)

Fig.3. SEM micrographs of the surface (left) and longitudinal fractured surface (right) of (a) PP; (b) PLA; (c) PP/PLA (60:40); (d) PP/PLA (50:50); (e) PP/PLA (40:60). 206 Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210

(a) PP (b) PP/PLA (60:40)

Fig.4. Spherulitic morphology of (a) PP; (b) PP/PLA (60:40); (c) PP/PLA (50:50); (d) PP/PLA (40:60).

3.3. Thermal properties

Table 2. DSC Analysis of PP, PLA and PP/PLA blends.

Sample Tg (°C) Tc (°C) Tm (°C) ¨Hc(J/g) ¨Hm (J/g) Xc,blend (%) PP - 117.69 159.96 - 81.93 43.37 PLA 56.00 - 154.98 30.95 31.72 0.83 PP/PLA (60:40) 52.48 118.28 157.96 3.00 60.20 38.00 PP/PLA (50:50) 53.56 118.34 156.77 9.97 58.19 34.21 PP/PLA (40:60) 53.07 118.56 154.01 14.45 55.23 31.04

DSC measurements indicate that the melting temperature (Tm) values of the blends decrease with increasing of PLA content. In the crystalline/amorphous immiscible blend system, the presence of separate domains of PLA (amorphous component) in the molten PP (crystalline component) during the crystallization process may cause a depression of the observed Tm [16]. As shown in Table 2, glass transition temperature (Tg) of PLA in the blends slightly decrease compared with pure PLA, whereas no significantly changed for crystallization temperature (Tc). Since in non-compatible blends the phase were physically separated, the same heterogeneities that nucleate the homopolymer at Tc of pure polymer may nucleate the stability of crystalline matrix during cooling from melt phase [18]. Therefore, Tc values of the blends were similar to those of the pure component. The Xc value of the blends decrease with increasing of PLA content. Such a result was probably due to the decreasing in primary crystalline density of PP phase and the increasing in the interlamellar amorphous regions, which inhibited the crystallization process [19]. The thermal degradation of all the samples was shown in Fig. 5. Unlike the neat PP and PLA, two-step degradation process in immiscible polymer blends was clearly seen in PP/PLA blends [20]. The percentage weight loss in the first and second decomposition step related to the amount of PLA and PP. The onset temperature of thermal degradation (Td,onset) and decomposition temperature range for polymer are essential for evaluating its thermal stability and thermal sensitivity, respectively [21]. PLA was more thermal stable than PP, which Td,onset of PLA was 331 °C whereas Td,onset of PP was 318 °C. The incorporation of PLA lead to the increasing of Td,onset to 330 Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210 207

°C in the blends. Similarly, an increasing of decomposition temperature ranges from 318-438 °C in PP to 330-476 °C in the blends were observed. According to the above mentioned, blending of PP and PLA resulted in an enhancement in the thermal stability and thermal sensitivity. However, there were no significant changes in Td,onset and decomposition temperature range with varying of PP/PLA blend ratios.

Fig.5. TGA curves of (a) PP; (b) PLA; (c) PP/PLA (60:40); (d) PP/PLA (50:50); (e) PP/PLA (40:60).

3.4. Tensile properties

Fig. 6 displays the effect of PLA content on Young’s modulus and tensile strength. An increasing trend of Young’s modulus was observed with increasing of PLA content. In general, tensile strength could be improved by incorporation of PLA in PP matrix [22]. The reduction of tensile strength in the blends, compared to the pure polymers, may caused by the incompatibility between nonpolar PP and polar PLA. Non-compatible polymer blend eventuating in sharp boundaries between the two polymeric phases, which the applied stress could not be efficiently transferred through the interface of blended component [11, 23]. This kind of results that the tensile strength values of the blend are lower than pure polymer could be seen in typical immiscible polymer blend [24]. Percentage elongation at break (Fig. 7) decrease when increase PLA content to the blend film which revealed the change from ductile to brittle deformation. This is due to the fact that PLA has low percentage elongation compared to PP and the incompatibility between PP and PLA. As a result, the PP phases in PP/PLA blends are not completely stretched compared to the pure polymers which lead to a decrease of the elongation at break [25, 26].

Fig.6. Modulus and Tensile strength of PP, PLA, and difference ratio of PP/PLA blends. 208 Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210

Fig.7. Elongation at break of PP, PLA, and difference ratio of PP/PLA blends.

3.5. Barrier properties

Permeation studies on PP/PLA blend films have shown that no significant improvement in oxygen and water vapor barrier properties. There were not much difference in oxygen permeability (OP) and water vapor permeability (WVP) between PP and PP/PLA blends (Fig. 8), compared to the values of pure PP and PLA. Nevertheless, in case of PP/PLA blend (50:50), OP and WVP values were lower than any other blend formulations. This result was probably due to well-dispersed structure of PP/PLA blend (50:50) as shown in Fig. 3(d). In immiscible system, well- dispersed structure could provide barrier properties by increasing the pathway length for permeants diffusing through the blend. The longer pathway causes a depressing of the concentration gradient of the permeates, thus reducing the permeability of material [27].

Fig.8. Barrier properties of PP, PLA, and difference ratio of PP/PLA blends.

4. Conclusion

The PP/PLA blend films compatibilized with PP-g-MA prepared by melt mixing technique and cast film extrusion. Increasing the PLA content lead to decrease Tm and crystallinity of blend films. For tensile properties, modulus and tensile strength increased with increasing the PLA content, but elongation at break was extremely decreased. The interestingly results from water vapor permeability testing observed that PLA could improved water vapor permeability of blend films. This indicated that biodegradability of PP might be improve with incorporation of PLA. From the morphology, thermal, mechanical, and barrier results, PP/PLA blend showed a typical behavior of Nalin Ploypetchara et al. / Energy Procedia 56 ( 2014 ) 201 – 210 209 immisible polymer blend. The optimal blend ratio for medical packaging application was PP/PLA blend 50:50 due to its highest barrier properties, compared to other blend, that could be reduce contamination of environmental microorganisim. PP/PLA blend in ratio of 50:50 are ideal for bacterial prevention outside of the medical industry as well. Blends film acts as a barrier which inhibits mildew and bacteria growth in duct work for hospital and medical system. Additionally, antimicrobial protection can be incorporated in various industries.

Acknowledgements

The authors gratefully acknowledge the financial support from the Graduate School Kasetsart University Grant; Kasetsart University Research and Development Institute (KURDI) and NSTDA-University-Industry Research Collaboration (NUI-RC). We also acknowledge the Active-Intelligent Packaging & Advanced Polymer Composite Materials Research Group.

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