J Polym Environ (2013) 21:450–460 DOI 10.1007/s10924-012-0531-5
ORIGINAL PAPER
Effect of Poly(Vinyl Acetate) on Mechanical Properties and Characteristics of Poly(Lactic Acid)/Natural Rubber Blends Wannapa Chumeka • Varaporn Tanrattanakul Jean-Franc¸ois Pilard • Pamela Pasetto
•
Published online: 12 August 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Natural rubber grafted with poly(vinyl acetate) copolymer (NR-g-PVAc) was synthesized by emulsion polymerization. Three graft copolymers were prepared with different PVAc contents: 1 % (G1), 5 % (G5) and 12 % (G12). Poly(lactic acid) (PLA) was melt blended with natural rubber (NR) and/or NR-g-PVAc in a twin screw extruder. The blends contained 10–20 wt% rubber. The notched Izod impact strength and tensile properties were determined from the compression molded specimens. The effect of NR mastication on the mechanical properties of the PLA/NR/NR-g-PVAc blend was evaluated. Characterization by DMTA and DSC showed an enhancement in miscibility of the PLA/NR-g-PVAc blend. The temperature of the maximum tan d of the PLA decreased with increasing PVAc content in the graft copolymer, i.e., from 71 °C (pure PLA) to 63 °C (the blend containing 10 % G12). The increase in miscibility brought about a reduction in the rubber particle diameter. These changes were attributed to the enhancement of toughness and ductility of PLA after blending with NR-g-PVAc. Therefore, NR-gPVAc could be used as a toughening agent of PLA and as a compatibilizer of the PLA/NR blend. NR mastication was an efficient method for increasing the toughness and
W. Chumeka V. Tanrattanakul (&) Bioplastic Research Unit, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Songkla 90112, Thailand e-mail:
[email protected] W. Chumeka J.-F. Pilard P. Pasetto De´partement Me´thodologie et Synthe`se, E´quipe Me´thodologies en Synthe`se de Polyme`res (MSP), Institut des Mole´cules et Mate´riaux du Mans, Universite´ du Maine, 72085 Le Mans Cedex, France
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ductility of the blends which depended on the blend composition and the number of mastications. Keywords Biodegradable polymer Natural rubber Poly(lactic acid) Poly(vinyl acetate) Toughness
Introduction In the last two decades polymers from renewable resources have received increasing attention from research workers and the industrial sector. Theses polymers could be classified into three categories based on their original source including (1) natural polymers, such as starch, protein, wood flour and cellulose; (2) synthetic polymers from natural monomers, such as poly(lactic acid) (PLA); and (3) polymers from microbial-metabolism, such as polyhydroxybutyrate (PHB) [1–5]. PLA is a promising biobased polymer that has already been commercialized. PLA is biocompatible and compostable, and can be readily broken down thermally by hydrolysis and is readily degraded in a soil environment. PLA has many advantages such as a high modulus and stiffness, thermoplastic behavior and good molding capability. The thermal properties and mechanical performances of PLA are comparable to those of other biodegradable aliphatic polyesters, such as poly(butylenes succinate) (PBS), poly(3-hydroxybutyrate) (PHB), and poly(e-caprolactone) (PCL) [1, 6]. The drawbacks of PLA are its low flexibility and low impact strength. General methods for improving the flexibility and the impact strength of PLA are blending with a toughening agent [7–11] and copolymerization [1, 12–14]. Because most of the PLA-based blends are immiscible, they may be made compatible by reactive blending techniques [15–19] in order to achieve better mechanical properties.
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Recently, there have been publications showing that natural rubber (NR) is a good toughening agent for PLA and the optimal content of NR was established as 10 wt% [20–22]. Modified natural rubber can also be used such as natural rubber grafted with poly(methyl methacrylate) [22] and natural rubber grafted with poly(butyl acrylate) [23]. PLA blended with the polyisoprene grafted with poly(vinyl acetate) (PIP-g-PVAc) showed better mechanical properties than PLA blended with PIP [17]. It has been established that PVAc is miscible with PLA [24, 25]. We assumed that the presence of PVAc in the PLA/NR blend should improve compatibility or miscibility of this blend. The objective of the present study was to improve the toughness and ductility of the PLA/NR blend by using natural rubber grafted with poly(vinyl acetate) (NR-g-PVAc copolymer). The NR-g-PVAc will be used as a toughening agent and a compatibilizer. To be the toughening agent the PLA/NR-g-PVAc blend will be prepared and the PLA/NR/NR-g-PVAc blend will be prepared in order to investigate the compatiblization effect. Mastication of NR was also applied in the present study. Miscibility of the blends was characterized by using DMTA, DSC and SEM. Compatibility was observed from the Izod impact strength and the tensile properties of the blends. Although there have been two publications dealing with PLA/NR blends [20, 21], there are many different points between those studies and the present study, i.e., the PLA and NR grades, processing equipment, blending conditions, molding conditions, specimen thickness and tensile testing speed, etc. As a result, there are differences in experimental results obtained from both previous studies compared to the present study. Comparisons were made with the blend containing NR-g-PMMA produced in our previous work [22]. We expected that NR-g-PVAc should provide a higher toughness than NR-g-PMMA because PVAc has a lower glass transition temperature.
Experimental
451
technique [26]. VAc was purified by washing with an aqueous 10 % sodium bicarbonate solution, neutralized with distilled water until pH * 7, then removing the water with anhydrous sodium sulfate. A buffer, sodium bicarbonate (1 wt% of VAc), a surfactant, SLS (1 wt% of VAc) and a water-soluble initiator, PPS (0.8 mol % of VAc), were added into the natural rubber latex. The mixture was stirred thoroughly in a nitrogen atmosphere. Purified VAc was dropped into the latex mixture. The reaction was performed at 60 °C for 3 h. The mixture was precipitated with calcium chloride. The precipitant was washed with distilled water and vacuum dried at 60 °C until its weight was constant. The molar ratios of NR:VAc were 90:10, 50:50 and 60:40, and based on the isoprene unit which its molecular weight is 68 g/mol. Free PVAc (homo-PVAc) and free NR (ungrafted NR) were extracted, then the percentage of grafted PVAc (G) in the NR-g-PVAc copolymer was determined by 1H-NMR. Soxhlet extraction of free NR and free PVAc was carried out in petroleum ether at 60 °C for 36 h and in methanol at 40 °C for 24 h, respectively. The polymer conversion (X), grafting efficiency (GE), free PVAc and free NR was calculated according to the Eqs. 1–4, respectively: wdried product Xð%Þ ¼ 100 ð1Þ wNR þ wVAc GEð%Þ ¼
M3 100 M1
Free NRð%Þ ¼
M1 M2 100 M1
Free PVAc ð%Þ ¼
%mol of grafted PVAc ¼
Natural rubber grafted with poly(vinyl acetate) (NR-g-PVAc) was synthesized by using the emulsion polymerization
ð4Þ
where M1 was the sample weight before Soxhlet extraction, M2 was the sample weight after Soxhlet extraction with petroleum ether and M3 was the sample weight after Soxhlet extraction with petroleum ether and methanol. The grafted PVAc content (G%) in the sample was evaluated from the 1H-NMR spectrum according to the following equations:
Ò
Synthesis of the Graft Copolymer
ð3Þ
M2 M3 100 M2
Materials Poly(lactic acid) was Ingeo 2002D produced by Natureworks LLC. Block natural rubber (STR5 CV60) and high ammonium concentrated latex were produced by Jana Concentrated Latex Co., Thailand. The vinyl acetate monomer (VAc) was supplied by Merck. Sodium bicarbonate and anhydrous sodium sulfate were from Aldrich Chemicals. Potassium persulfate (PPS) and sodium lauryl sulfate (SLS) were from Ajax. All chemicals used were AR grade.
ð2Þ
I4:8 =3 100 ¼ C1 ðI4:8 =3Þ þ I5:1
Gð%Þ ¼ %weight of grafted PVAc C1 MW1 100 ¼ ðC1 MW1 Þ þ ðC2 MW2 Þ C1 þ C2 ¼ 100
ð5Þ
ð6Þ ð7Þ
where I4.8 and I5.1 was the integrated area of the peak at 4.8 ppm (–CHO– of PVAc) and 5.1 ppm (=CH– of NR), respectively. C1 and C2 was the percentage mol of PVAc and percentage mol of NR in the graft copolymer, respectively. MW1 and MW2 was the repeating unit weight of PVAc and NR, respectively. The deuterated chloroform (d-CDCl3) was used as a solvent for 1H-NMR characterization. The FTIR
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452
spectra of the NR-g-PVAc were recorded in the range of 4,000–400 cm-1. The rubber sample was obtained by casting films of rubber solution on the Neat cell and chloroform was used as a solvent. Polymer Blending Method Blending was performed in a twin screw extruder (PrismÒTSE16TC) at 160 °C and a screw speed of 190 rpm, similarly to previous work [22]. The block NR was cut into small pieces and mixed with the antioxidant (1 phr of WingstayÒ L) prior to blending with PLA. The graft copolymer was used as received, without extraction of free NR and homo-PVAc. A 2 mm-thick sheet was molded by compression (Kao TiehTM KT7014) at 160 °C for 9 min and air cooled at room temperature under pressure for 10 min. Testing of Mechanical Properties Tensile properties and notched Izod impact strength were determined according to ASTM D638 and ASTM D256, respectively. Tensile properties were tested at a speed of 5 mm/min by using InstronÒ5569. Impact testing used ZwickÒ5102. Eight specimens were used for testing of every sample. An average value and a standard deviation were reported. Blend Characterization Scanning electron micrographs were recorded using a JEOLÒJSM5800LV and a QuantaÒ400 FEI. All specimens were immersed in liquid nitrogen for 6 h and immediately fractured prior to coating with gold. DSC thermograms were recorded from a Perkin ElmerÒDSC7 at a heating scan of 10 °C/min from 20 to 200 °C. DSC analysis was performed in 5 steps: the first heating scan, quenching (-100 °C/min), the second heating scan and a slow cooling scan (-10 °C/min). The last step was the third heating scan. The heat of fusion of pure crystalline PLLA (DHc) is 93 J/g [20, 22]. A Rheometric Scientific DMTA V was used for determination of the dynamic mechanical thermal analysis at a frequency of 1 Hz and a heating rate of 3 °C/min. DMTA was carried out in a similar way to previous work [22].
Results and Discussion Graft Copolymerization Table 1 shows the characteristics of graft copolymerization. Three graft copolymers were prepared with different PVAc contents: 1 % (G1), 5 % (G5) and 12 % (G12).
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J Polym Environ (2013) 21:450–460 Table 1 Characteristics of copolymerization Sample code
VAc (mol %)
X (%)
Free NR (%)
Free PVAc (%)
GE (%)
G (%)
G1
10
19.2
52.0
13.0
41.0
0.9
G5
40
19.6
40.3
23.8
45.5
5.2
G12
50
50.6
51.1
69.6
14.9
11.9
The percentage of conversion (X) increased with an increasing VAc content but the conversion was relatively low (B50 %). The grafted PVAc content (G) was increased with increasing VAc content. The graft copolymer was coded based on the value of the %G. The 1H-NMR spectra of NR, PVAc and NR-g-PVAc before and after Soxhlet extraction are shown in Fig. 1. The main characteristic peak of NR and PVAc are 5.1 ppm (C=CH2 proton) and 4.8 ppm (CHO proton), respectively. The FTIR spectra are illustrated in Fig. 2. After extracting free NR and homoPVAc, the graft copolymer (G5) showed a characteristic peak of PVAc at 1,738 and 1,241 cm-1 which was assigned for the C=O and C–O stretching of the vinyl acetate group, respectively. Effect of PVAc Content on the Mechanical Properties and Characteristics of the Polymer Blends In this section polymer blends consisted of 90 wt% PLA and 10 wt% rubber. The unnotched Izod impact strength of PLA was 19.55 ± 2.67 kJ/m2 whereas all the blends did not break during testing. The notched Izod impact strength of PLA and the blends are displayed in Fig. 3. Considering the binary blends (90/10/0 and 90/0/10), all rubbers enhanced the Izod impact strength of PLA, particularly G5 had a four-fold increase of the impact strength of the PLA. The impact strength of the blends were ranked based on the toughening agents as following: G5 [ G12 [ NR [ G1. These results indicated that NR-g-PVAc is a good toughening agent for PLA and it is better than NR and NRg-PMMA [22]. The impact strength of PLA and the PLA/ NR blend were similar to those reported by Suksut and Deeprasertkul [20] and slightly higher than those reported by Zhang et al. [23]. In order to determine the effect of the NR-g-PVAc in the PLA/NR blend, the ternary blends, a mixture of PLA/NR/ NR-g-PVAc, was employed, i.e., 90/5/5 and 90/7.5/2.5, and compared with the 90/10/0 binary blend. The ternary blends displayed different results depending on the grafted PVAc content as shown in Fig. 3. G5 improved the toughness of the PLA/NR blend for both blend compositions. The impact strength of the PLA/NR blend increased from 6.36 to 12.23–12.49 kJ/m2 after adding G5. G12 exhibited a positive effect only in the 90/5/5 blend that had a toughness of 11.57 kJ/m2 while G1 provided little increase in the 90/7.5/
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453
Fig. 1 1H-NMR spectrum of NR, PVAc and NR-g-PVAc (G5) before and after Soxhlet extraction
Izod impact strength (kJ/m2)
16 14
G1
G5
G12 12.01
12 10
12.49
8.48
8
7.08
6.36 5.42
6 4
12.23 11.57
4.25
4.02
2.85
2 0 100/0/0
90/10/0
90/0/10
90/5/5
90/7.5/2.5
PLA/NR/NR-g-PVAc
Fig. 2 FTIR spectrum of NR, PVAc and NR-g-PVAc (G5) after Soxhlet extraction
2.5 blend, i.e., 7.08 kJ/m2. The maximum impact strength obtained in the present study was higher than that reported previously [20, 22–24]. The results indicated that the NR-g-PVAc could be used directly or mixed with NR to enhance the toughness of PLA. A similar comparison has been made for the tensile properties as shown in Fig. 4. All the blends exhibited lower tensile properties than PLA, except that the elongation at break of some blends was higher than for PLA. It is common to obtain lower tensile properties of PLA after blending with NR [20–23]. PLA and all blends showed a
Fig. 3 Notched Izod impact strength of PLA/NR/NR-g-PVAc blends (10 % rubber)
yield point before failure. The Young’s modulus of all blends was in the range of 1.3–1.4 GPA. It seemed that the grafted PVAc had an insignificant effect on the modulus of the blends. In the binary blends only G12 showed a higher yield stress than NR. In the ternary blends only G5 and G12 in the 90/5/5 blends revealed a higher yield stress than NR. PVAc decreased the stress at break of the PLA/NR blends but the elongation at break of the blends containing G5 and G12 was relatively high for both binary and ternary blends, especially for G12 in the 90/5/5 blend that showed a higher value than the PLA and PLA/NR blend by approximately
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(a) 2,000
(b) 80 1,638
Modulus (MPa)
1,600 1,345
G5
1,422 1,360 1,350 1,305 1,330 1,284
G12 1,330 1,295 1,258
1,200 800 400
70
Yield stress (MPa)
G1
G1
G5
G12
62.08
60 50
43.51
44.20 35.34 33.80
38.69
40
40.93
39.67 37.26
35.13
33.79
30 20 10
0
0 100/0/0
90/10/0
90/0/10
90/5/5
100/0/0
90/7.5/2.5
90/10/0
PLA/NR/NR-g-PVAc
70
90/5/5
90/7.5/2.5
(d) 20
80 G1
61.88
G5
G12
60 50 38.49
40 30
34.94 33.46 33.31 33.63 33.38 33.34 30.95 29.94 28.46
20 10 0
Elongation at break (%)
Stress at break (MPa)
(c)
90/0/10
PLA/NR/NR-g-PVAc
18
G1
G5
G12
16.10
16 14 12 10
8.40
6
7.12 6.05
7.15
8 5.44 4.25
4
3.83 3.73
3.95
4.07
2 0
100/0/0
90/10/0
90/0/10
90/5/5
90/7.5/2.5
PLA/NR/NR-g-PVAc
100/0/0
90/10/0
90/0/10
90/5/5
90/7.5/2.5
PLA/NR/NR-g-PVAc
Fig. 4 Tensile properties of PLA/NR/NR-g-PVAc blends (10 % rubber)
threefold and fourfold, respectively. The results indicated that PVAc raised the yield stress and the elongation at break of the PLA/NR blend. The fractured surface of the tensile tested specimens (Fig. 5) agreed with the values of the elongation at break. A brittle fracture was observed in PLA (Fig. 5a) and the 10 % G5 blend (Fig. 5c). A ductile fracture was indicated by a yielding of the PLA matrix found in the blend containing 10 % NR (Fig. 5b) and 10 % G12 (Fig. 5d). Crazing might be a major deformation mechanism in PLA and the 10 % G5 blend while shear yielding occurred in the other blends. Figure 5e showed more yielding than Fig. 5b, d, and this corresponded to the highest elongation at break. The tensile properties of the present PLA/NR blend were not comparable with those reported by Bitinis et al. [21] perhaps because of the different sample thickness (2 vs. 0.4 mm) and different testing speed (5 vs. 10 mm/min). The first reason seems to be more important. Typically PVAc is a soft and weak amorphous polymer; therefore, PVAc itself is not a good toughening agent (no results shown here). The objective of adding PVAc to the PLA/NR blends in the present study was to use PVAc as a compatibilizer in term of NR-g-PVAc which was used directly or mixed with NR. The results showed that the impact strength and tensile properties of PLA and the PLA/ NR blend increased with the addition of PVAc in the form
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of NR-g-PVAc. G5 became the best impact modifier of the blend and was the best compatibilizer for impact strength, whereas G12 seemed to be the best compatibilizer for enhancing the elongation at break of the blend. Based on both mechanical properties, the 90/5/5G12 blend should be the best blend. NR-g-PVAc was better than NR-g-PMMA [22] because NR-g-PMMA did not increase the mechanical properties of PLA and the PLA/NR blend. The rubber particle size in the blends was observed by SEM. All blends showed spherical rubber particles (Fig. 6). The average particle diameter of all blends is listed in Table 2. It was found that PVAc decreased the particle size of NR but some blends had a higher diameter, i.e., 2.47 and 2.60 lm, due to coalescence of the rubber particles. The size of the dispersed phase implies miscibility between the continuous and the dispersed phase. High immiscibility induces coalescence of the dispersed phase because phase separation is preferred in the blend. Miscibility between PLA and NR was poor; therefore, the NR dispersed particles tried to combine in its phase causing the coalescence. In contrast, the miscibility between PLA and NR-g-PVAc was higher due to the miscibility between PLA and PVAc; consequently, PVAc part acted as an emulsifier leading to higher stability in the PLA matrix. Theoretically, smaller particle indicates higher miscibility. Preparation with a rubber diameter larger than 2.1 lm exhibited low impact
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Fig. 5 Tensile fractured surfaces of a PLA, and PLA/NR/NR-g-PVAc: b 90/10/0, c 90/0/10G5, d 90/0/10G12 and e 90/5/5G12
strength. This reflects that it is not only the compatibilization but also the optimal rubber particle diameter that controlled the impact resistance of the present blends. The submicron size of the rubber particle diameter in the 90/5/ 5G12 blend might be a key factor in the improvement in the elongation at break besides the increment of compatibility. As stated earlier, it is believed that the PLA/PVAc blend is a miscible blend. Therefore, our aim was to increase miscibility of the PLA/NR blend by using NR-g-PVAc. It was expected that PVAc in this graft copolymer would act as a compatibilizer and promote interfacial adhesion
between the PLA matrix and the rubber particle. The increase in impact strength, yield stress and elongation at break as well as the reduction in the rubber particle diameters due to the presence of PVAc in the blends indicated the enhancement of miscibility of the blends. DMTA was used as a tool to identify miscibility between PLA and the NR-g-PVAc in the blends. It was found that the temperature of maximum tan d of the blends decreased with increasing grafted PVAc content (Fig. 7). This value is equivalent to Tg obtained from DSC and normally it is higher than that obtained from DSC due to the nature of its
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Fig. 6 SEM micrographs of freeze-fractured surface of PLA/NR-g-PVAC blends: a 10 % NR, b 10 % G1, c 10 % G5 and d 10 % G12
Table 2 Average diameter of rubber particles in the blends PLA/NR/G
Diameter (lm)
90/10/0
2.5 ± 1.2
90/0/10G1
2.2 ± 0.7
90/0/10G5
1.7 ± 0.7
90/0/10G12
1.9 ± 0.8
90/5/5G1
2.4 ± 0.9
90/5/5G5
1.9 ± 0.9
90/5/5G12
0.9 ± 0.3
90/7.5/2.5G1
2.0 ± 0.7
90/7.5/2.5G5
1.9 ± 0.7
90/7.5/2.5G12
2.6 ± 1.1
testing method. The maximum tan d of PVAc and PLA appeared at 48.90 and 71.20 °C, respectively. The temperature at the maximum tan d of the 10 % NR blend was not significantly different from that of PLA whereas this temperature of the 10 % G12 blend shifted to the lower temperature by 8 °C, showing at 63.03 °C. This means PVAc increased the miscibility between PLA and NR. The thermal properties of PLA and the blends are illustrated in Fig. 8 and Table 3. The addition of NR and
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NR-g-PVAc enhanced the crystallizability of PLA by inducing cold crystallization in the heating scan. The crystallization behavior of compression molded PLA was similar to that of the PLA pellet, except the degree of crystallinity of the first heating scan of the PLA sheet was 10.89 % while that of PLA pellet was 35.40 %. The lower crystallinity in the PLA sheet might be one factor causing a lower impact strength when compared with the blends that showed higher crystallinity. Cold crystallization did not appear in the extruded PLA and PLA sheet (not shown here). Thus, NR and NR-g-PVAc acted as a nucleating agent of PLA. The Tg, Tm and degree of crystallization in the first heating scan of the blends differed slightly from those of PLA. All the blends showed similar thermal properties in the first heating scan and Tg tended to decrease with an increasing PVAc content. The double melting peak in the first heating scan of all blends disappeared in the second and the third heating scan. In the second heating scan, PLA and the blends displayed a lower Tg. This may be due to thermal degradation of PLA during the first heating scan. PLA became amorphous after the first heating scan but the blends remained crystallized in the second and the third heating scan. The Tm of the blends also decreased in the second heating scan and there was no
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2.5
(a) Heat Flow Endo Up (W/g)
2
Tan delta
151.67 °C
PVAc (48.90°C) PLA/G12 (63.03°C) PLA/G1 (66.82°C) PLA/G5 (64.32°C) PLA/NR (69.80°C) PLA (71.20°C)
1.5
1
0.5
60.64 °C
1st Heating
58.33 °C
2nd Heating
52.83 °C
Cooling
58.75 °C
3rd Heating
0 30
40
50
60
70
80
30
90
50
70
Temperature (°C)
significant difference in the Tcc and Tm among the blends. The effect of rubber on the thermal properties of PLA could be identified from the degree of crystallinity (Xm2). The higher the PVAc content the lower was the degree of crystallinity. It is expected that PVAc interrupted the crystallizability of PLA. Effect of NR-g-PVAc as a Compatilizer in the PLA/NR Blends
Effect of NR Mastication on Mechanical Properties of the Ternary Blends In our previous work we found that mastication of NR increased the toughness of the PLA/NR blend [22]. As a result, the present study also focused on the effect of NR mastication in the PLA/NR/NR-g-PVAc blends. Only the NR was masticated by a two-roll mill for 100, 140 and 180
130
150
170
190
144.33 °C 152.17 °C 66.72 °C
1 st Heating
106.33 °C 148.87 °C
56.34 °C
2nd Heating
125.87 °C
52.52 °C
Cooling 148.53 °C
57.29 °C
30
50
70
3 rd Heating
124.53 °C
90
110
130
150
170
190
Temperature (°C)
(c)
145.50°C 64.45 °C
Heat Flow Endo Up (W/g)
In this section the blends contained 90 % PLA, 10 % NR and different NR-g-PVAc contents. The aim of this experiment was to investigate the effect of G5 and G12 as a compatibilizer of the PLA/NR blend. The addition of G5 and G12 lowered the Young’s modulus, the yield stress and the stress at break (Fig. 9). It was expected that the modulus and the stress of the blends should decrease with increasing rubber content because of the higher content of the soft and weak component. The elongation at break of the blends significantly increased and these blends had a higher ductility than the 90/5/5G12 blend. However, the standard deviation of the elongation at break was relatively high compared with other properties. The notched Izod impact strength was increased when adding 5 and 10 % of G5 but it was still lower than the blends displayed in Fig. 3. The compatibilization effect of the NR-g-PVAc was dominant in the elongation at break more than in the impact strength.
110
(b) Heat Flow Endo Up (W/g)
Fig. 7 Tan d versus temperature curves of PLA and the blends containing 10 % rubber
90
Temperature (°C)
151.50°C
107.50 °C
1st Heating 149.53°C
56.12 °C
2nd Heating
129.70°C
Cooling 50.89 °C
149.53°C
56.30 °C 3rd Heating
129.87°C
30
50
70
90
110
130
150
170
190
Temperature (°C)
Fig. 8 DSC thermograms of a PLA, b the 10 % NR blend and c the 10 % G12 blend
passes before melt blending with PLA and NR-g-PVAc. The Young’s modulus, the yield stress and the stress at break of the blends containing G5 were slightly changed by the mastication of NR (not shown here). In contrast, rubber mastication increased the elongation at break (Table 4). The number of mastications had different effects on the elongation at break in each blend. The elongation at break significantly increased after NR mastication at 100 and 140 passes for the 90/5/5 and 90/7.5/2.5 blend, respectively. The notched Izod impact strength of the blends decreased with the
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(a) 1,600 1,345 1,215
Modulus (MPa)
(b) 45
G12
1,205 1,213
1,200
1,151 1,102
1,000 800 600 400 200
33.43 32.71
35
28.38 28.05
30 25 20 15 10
2.5
5
0
10
2.5
NR-g-PVAc content
5
10
NR-g-PVAc content
(d)
45
G5
38.49
G12
35 30 25
22.84
23.49
24.07 21.69
20
40
Elongation at break (%)
Stress at break (MPa)
32.38 32.47
0 0
40
G12
5
0
(c)
G5
38.69
40
1,251
Yield stress (MPa)
1,400
G5
20.87 19.59
15 10 5
35 30
G5 G12
25
26.48
25.38
23.37 20.34
18.24
20
17.56
15 10 4.25 4.25
5 0
0 0
2.5
5
10
0
(e)
5
10
10
G5
9
G12
8.42
8.06
Impact strength (kJ/m2)
2.5
NR-g-PVAc content
NR-g-PVAc content
8
6.99 7
6.36
6.03
6
6.79
5.91
5 4 3 2 1 0 0
2.5
5
10
NR-g-PVAc content Fig. 9 Effect of G5 and G12 as a compatibilizer on mechanical properties of the PLA/NR (90/10) blends
number of NR mastications, excluding the 90/7.5/2.5 blend masticated at 100 passes (Table 4). A similar behavior was observed in the ternary blends containing G12 (Table 5). All blends showed higher elongation at break when used masticated NR. Rubber masticated in the 90/5/5 blend caused a decrease in the impact strength, similar to that presented in Table 4. On the other hand, the 90/7.5/2.5 blend showed an
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increase in the impact strength when the masticated rubber was employed. As reported earlier [22], mastication of NR caused a decrease in the molecular weight and Mooney viscosity. The viscosity of the matrix and the dispersed phase is one of the important factors that can control the morphology of the polymer blends and affect their mechanical properties. These experimental results confirmed that rubber
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Table 3 Thermal properties from DSC analysis PLA/NR/G
Tcc1 (°C)a
Tg1 (°C)a
Tm1 (°C)a
Xm1 (%)a
Tg2 (°C)b
Tcc2 (°C)b
Tm2 (°C)b
Xm2 (%)b
PLA pellet
–
60.6
151.6
35.4
58.3
–
–
–
90/10/0
106.3
60.7
152.1
31.9
56.3
125.9
148.9
18.9
90/0/10-G1
106.3
60.0
144.3, 152.1
30.7
55.3
126.4
148.9
21.7
90/0/10-G5
104.8
59.0
144.3, 151.5
30.2
55.2
123.0
147.4
13.2
90/0/10-G12
107.5
58.5
145.5, 151.5
31.2
56.1
129.7
149.5
7.6
a
data obtained from the first heating scan
b
data obtained from the second heating scan after quenching
Table 4 Effect of rubber mastication on the elongation at break and the impact strength of the blends containing G5
No. of mastication
0
Table 5 Effect of rubber mastication on the elongation at break and the impact strength of the blends containing G12
Elongation at break (%)
Impact strength (kJ/m2)
90/5/5
90/5/5
90/7.5/2.5
6.1 ± 1.3
12.2 ± 0.7
13.7 ± 1.7 16.2 ± 2.5
90/7.5/2.5
8.4 ± 1.4
100
29.0 ± 3.2
12.3 ± 2.7
11.9 ± 0.8
140
10.9 ± 2.8
24.0 ± 2.1
8.3 ± 0.7
7.9 ± 0.9
180
7.0 ± 1.3
8.6 ± 2.6
8.9 ± 0.8
10.5 ± 1.0
No. of mastication
Elongation at break (%)
Impact strength (kJ/m2)
90/5/5
90/5/5
90/7.5/2.5
90/7.5/2.5
0
16.1 ± 1.41
7.1 ± 0.9
11.6 ± 0.8
4.0 ± 0.8
100
16.9 ± 1.
15.9 ± 3.6
8.9 ± 0.7
8.1 ± 0.9
140
23.9 ± 2.5
10.3 ± 2.0
8.6 ± 0.9
8.3 ± 0.9
180
23.5 ± 4.3
26.5 ± 3.6
10.6 ± 0.4
13.4 ± 1.4
mastication could be used as a compatibilization technique for the PLA/NR/NR-g-PVAc blend.
Acknowledgments This work is financially supported by The Royal Golden Jubilee PhD Program, Thailand Research Fund (Grant no. PHD/0253/2551) and Garduate School, Prince of Songkla University.
Conclusions References The presence of PVAc in the NR-g-PVAc copolymer increased the miscibility of PLA and NR by decreasing the temperature of the maximum tan d of PLA in the blends. The higher the grafted PVAc content, the lower the temperature of the maximum tan d. NR and NR-g-PVAc acted as a nucleating agent for PLA by inducing the cold crystallization and increased the crystallinity in the second heating scan. Although PVAc reduced the particle size of rubber, it seemed that coalescence of the rubber particles occurred, to produce relatively larger sized rubber particles in some blends. The higher miscibility and smaller rubber particle diameter in the PLA/NR-g-PVAc blends was attributed to the higher impact strength and elongation at break than the PLA/NR blends. NRg-PVAc could be used directly as a toughening agent of PLA or a compatibilizer of the PLA/NR blend. NR mastication could be applied to the blends containing NR-g-PVAc for improvement of the impact strength and elongation at break.
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