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published online 27 December 2012Journal of Elastomers and PlasticsMohd Bijarimi, Sahrim Ahmad and Rozaidi Rasid

epoxidized natural rubber blendsMechanical, thermal and morphological properties of poly(lactic acid)/

  

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Article

Mechanical, thermaland morphologicalproperties ofpoly(lactic acid)/epoxidized naturalrubber blends

Mohd Bijarimi1,2, Sahrim Ahmad1 and Rozaidi Rasid1

AbstractIn thiswork,we report a melt blendof poly(lactic acid)(PLA)/epoxidizednatural rubber (ENR)with liquid natural rubber (LNR). The LNR was synthesized by a photochemicaldegradation technique and used as a compatibilizer in the PLA/rubber binaryblending systems. The PLA/ENR/LNR blends were melt-blended in a Haake internalmixer at 180�C and mixing speed of 50 r. min�1 for 15 min. It was found that theaddition of LNR compatibilizer has improved the tensile strength and elongation atbreak for the compositions of the 40PLA/55ENR/5LNR blend system whencompared with a noncompatibilized system (40PLA/55ENR/5NR). The elongation atbreak for the blend with 5% LNR compatibilizer showed a twofold increment comparedwith the blend without LNR. The increase in tensile strength and elongation at breakwere associated with the ability of LNR to promote the uniform dispersion betweenthe natural rubber (NR) and PLA phases as observed in the scanning electron micro-scopic analysis. Moreover, the differential scanning calorimetric results indicated that the40PLA/55ENR/5LNR showed the highest degree of crystallinity and thus contributed toimprove their mechanical properties. Thermogravimetric analysis showed that twodegradation transitions for both compatibilized and noncompatibilized blend systemsdue to higher degradation temperatures of ENR50 and NR parts. Fourier transforminfrared spectroscopic analysis revealed that the PLA/ENR/NR and PLA/ENR/LNRblends were not miscible.

1 Faculty of Science and Technology, School of Applied Physics, Universiti Kebangsaan Malaysia, Bangi, Selangor,

Malaysia.2 Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Kuantan, Malaysia

Corresponding author:

Mohd Bijarimi, Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Kuantan,

Malaysia

Email: bijarimi@ump.edu.my

Journal of Elastomers & Plastics

1–17

ª The Author(s) 2012

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DOI: 10.1177/0095244312468442

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KeywordsPLA, ENR, rubber, blend, biodegradable polymers

Introduction

It is well-known that bio-based plastics are prepared using the renewable biomass that

offers sustainable alternative to the petroleum-based plastics such as poly(lactic acid)

(PLA). PLA is receiving considerable attention due to its biodegradability, renewabil-

ity and comparable properties with commodity plastics. Recent concerns with respect

to the environmental effects have fuelled the need to explore the new alternatives as a

potential candidate to compete with commodity plastics, especially from biomass. The

use of nondegradable plastics for consumer goods such as packaging material has

created a considerable disposal problem. Apart from the environmental issues, the cost

of production has been escalated due to its dependency on oil prices. As such, polymers

from renewable sources are on the verge of competing with the traditional plastics.

PLA is a typical biodegradable polyester obtained by the synthesis of lactic acid

(or lactide), which can be produced from renewable resources like corn or sugar-

cane.1 In general, PLA is the most promising plastic as its tensile strength and

stiffness are similar to polyethylene terephthalate, but it is too brittle to be used

commercially. Various types of modifications have been investigated in an effort to

improve the elongation and toughness properties of the PLA, and melt blending is one of

the most practical and economical methods. Therefore, it is not surprising that PLA has

been blended with both nondegradable2–4 and biodegradable polymers.5–8 Typically,

the blending of two different polymers leads to be incompatible blends and shows dif-

ferent phases of blend morphologies, that is, dispersion of one polymer in the matrix of

another polymer. Generally, a compatibilizer is used in order to improve the interfacial

adhesion that interacts chemically. The compatibilizer minimizes the separation of

phases, delamination, agglomeration or skinning and ultimate physical failure.9

Epoxidized natural rubber (ENR) is a chemically modified form of the cis-

1,4-polyisoprene rubber, where some of the unsaturation is converted into epoxide

groups, which are randomly distributed along the polymer chain. As such, it is a

desirable rubber impact modifier since both PLA and ENR have the same polarity.

Moreover, both PLA and ENR are derived from renewable sources and can be

processed by most conventional polymer processing. In view of their complementary

properties, blending PLA with ENR is a good choice to improve PLA properties,

such as toughness and elongation at break, without compromising its biodegradabil-

ity. The use of rubber as a toughening component in the hard thermoplastic has been

well reported in the literature.10–22

The use of liquid natural rubber (LNR) as a compatibilizer for various polymer blends

systems has been widely investigated.23–27 Nevertheless, until date, there have been

no studies in the literature that address the effect of LNR compatibilizer in the PLA

blends. This article reports the effect of LNR compatibilizer on the mechanical,

thermal and morphological properties of PLA/ENR/LNR blends. Furthermore, the

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chemical interaction of the compatibilizer with the constituent polymers was eval-

uated with Fourier transform infrared (FT-IR) spectroscopy.

Experimental

Materials

The PLA of Natureworks Ingeo™ Biopolymer 3251D grade with a density of

1.24 g cm�3 and a melt flow index of 30–40 g/10 min was used in this study. Natural

rubber (NR) of SMR L grade and ENR with 50% epoxidation (ENR) were obtained from

the Malaysian Rubber Board. Other chemicals were used as received.

Blend preparation and compounding

The NR was used in the preparation of LNR through the photochemical oxidation in our

laboratory according to the method described by Ibrahim and Zakaria in their previous

publications.24,28 The blend composition of the PLA/rubber was fixed at 40PLA/60

rubber throughout this study. The amount of NR or LNR was varied from 0 to 15 parts of

the dry weight that is 40PLA/55ENR/5NR, 40PLA/55ENR/5LNR, 40PLA/50ENR/

10NR and 40PLA/50ENR/10LNR as shown in Table 1. All melt blends were prepared in

a laboratory mixer (Haake PolyLab RheoDrive 4 from Thermo Electron Corporation,

Germany) at 180�C with a capacity of 60g. Blending was carried out with a rotor speed

of 50 r. min�1 for 15 min. The NR was initially melted for 60s, and, subsequently, LNR

was incorporated for another 120s. Finally, the PLA resin was added to the mixture for

another 12 min. The blend was removed from the internal mixer and then molded at

180�C under a pressure of 45 MPa for 13 min using a hot press to produce a sheet

measuring 150 mm width � 125 mm length with 1 mm thickness.

Stress–strain analysis

All compositions of blend were tested and compared in terms of their mechanical

properties. The tensile test was carried out according to ASTM D638 using a Testometric

M350-10 CT from Testometric Company, Ltd., UK, under ambient conditions with a

Table 1. Composition of PLA/ENR/NR and LNR blends

Sample Parts (wt%) NR or LNR

40PLA/60ENR 40/60 0% LNR40PLA/55ENR/5NR 40/55/5 5% NR40PLA/55ENR/5LNR 40/55/5 5% LNR40PLA/50ENR/10NR 40/50/10 10% NR40PLA/50ENR/10LNR 40/50/10 10% LNR40PLA/45ENR/15NR 40/45/15 15% NR40PLA/45ENR/15LNR 40/45/15 15% LNR

PLA: poly(lactic acid); ENR: epoxidized natural rubber; NR: natural rubber; LNR: liquid natural rubber.

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crosshead speed of 50 mm min�1. At least, five samples were tested in each case, and the

average value was taken as a result of determining the stress at peak, elongation at break

as well as the Young’s modulus.

Morphological analysis

Scanning electron microscopy (SEM) was performed on small pieces of the fractured

specimens after the tensile test using a Philips SL-30 SEM. The SEM morphology

characterization provides an insight into the distribution of the rubbery impact modifiers

to a polymer matrix, distribution of the components to binary blends, the effect of

interfacial addition to the particle size, the crystalline phase and dispersion/agglomera-

tion of the particles as well as the particle size.

Thermal analysis

The crystallization behavior of the blend components was characterized on the

compression-molded specimens. Differential scanning calorimetric (DSC) measure-

ments were performed under a nitrogen atmosphere on the samples of 5–8 mg using a

Mettler Toledo DSC 822 apparatus. The samples were added to the standard

aluminum pans with pierced lids. The first heating scan was at 250�C at a scanning rate

of 20�C min�1 to eliminate the influence of thermal and mechanical history. The samples

were then cooled from 250�C to 30�C at the same rate. Also the glass transition,

crystallization temperature (Tc) and melting temperatures were determined. Polymer

crystallinity was determined with DSC by quantifying the heat associated with the

melting (fusion) of the polymer. The degree of crystallinity (Xc) was calculated from the

following equation:

Xc ¼DHm � DHc

FPLADHom

� 100%

where DHm and DHc are the enthalpies of melting and cold crystallization during first

heating cycle, respectively, DHom is the melting enthalpy assuming 100% crystalline PLA

at 93.0 J g�1.29 The FPLA is the weight fraction of PLA in the blend.

Thermal degradation

Thermogravimetric analysis (TGA) is a technique that measures the mass and the change

in mass of the sample during heating as a function, time and/or temperature.

Decomposition of mass could be occurred due to chemical reactions or physical changes

during heating. The evaluation of thermal stability of PLA and blends was carried out

with a Mettler Toledo TGA/SDTA 851e apparatus. Samples (12 + 0.2 mg) were added

to the alumina crucibles. An empty alumina crucible was used as a reference. Samples

were heated from ambient temperature, that is, 25–600�C at a scanning rate of

20�C min�1 under nitrogen. The sample temperature, weight, derivative and its heat

flow were recorded.

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Fourier-transformed infrared spectroscopy

The chemical changes after blending were monitored by FT-IR spectroscopy. The

infrared (IR) spectra were recorded using a Spectrum 400 FT-IR from PerkinElmer Inc.,

USA spectrometer with 4 cm�1 resolution and 10 scans. All the spectra were recorded in

the absorbance mode at the 4000–600 cm�1 region.

Results and discussion

Tensile properties

The stress–strain properties of the PLA/ENR blends with different ratios of NR or LNR

compatibilizer are shown in Figure 1(a) and (b), accordingly. In general, the introduction

of the rubber component into the PLA decreases the tensile stress in the 40PLA/60ENR

and all blend systems in this study. The addition of NR causes a further reduction in

tensile stress in all the blend systems. For example, in the 40PLA/55ENR/5NR blend, the

addition of 5% NR decreased the tensile strength by 17% when compared with 40PLA/

60ENR blend. Conversely, the addition of LNR increased the tensile strength by 47%.

This remarkable improvement in blend with LNR suggested that LNR at this concen-

tration shows compatibility with PLA, with a more homogenous dispersion than the

blend without LNR. It can be concluded that the tensile strength obtained from 5% of the

LNR showed the highest value compared with 0, 10 and 15% compatibilizer in the PLA/

rubber binary blends, as shown in Figure 2.

The addition of rubber in the binary blend system had a significant impact on the

elongation at break in this blend, as shown in Figure 3. The blend containing 5% LNR

showed a twofold increment when compared with the blend without LNR. However,

with a further increase in the amount of LNR, the elongation at break gradually

decreased. These results indicate that without LNR, the interfacial between PLA and

rubber phase was poor and hence resulted in a low-tensile strength and elongation at

break. Such observation is associated with rubber, which is flexible in fracture, and can

act as a load-bearing component that favors the initiation and the growth of

shear-yielding deformation in the matrix.30 This finding suggests that blends changed

from brittle to ductile failure with the addition of LNR. The NR chains form separate

phases in the solid, typically 10–20 mm in diameter, so that when the material is strained,

crazes form on their surface, increasing the energy needed to break the material. This

phenomenon is known as rubber toughening and has been applied to a wide range of

brittle polymers. Furthermore, it was observed that during the tensile deformation, the

specimen underwent a process known as whitening at the necked zone suggesting a

cavitation process. In this study, it was observed that the tensile strength and elongation

at break increased when LNR was introduced into the blend system. Other researchers

have reported a similar trend in the NR/high-density polyethylene blends with LNR.24,31

Figure 4 shows the Young’s modulus for all blend compositions. As expected, the

Young’s modulus decreased with the addition of ENR in the 40PLA/60ENR blend

system. However, the addition of 5–15 parts of NR improved the Young’s modulus when

compared with the blend of 40PLA/60ENR alone. The effect of Young’s modulus was

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observed for noncompatibilized and compatibilized blend systems can be clearly seen in

Figure 4. The reductions in Young’s modulus for compatibilized systems (40PLA/

55ENR/5LNR, 40PLA/50ENR/10LNR and 40PLA/45ENR/15LNR) were associated

with plasticization effect of LNR.

Figure 1. Stress–strain curves of (a) PLA/ENR/NR and (b) PLA/ENR/LNR blends. PLA: poly(lacticacid); LNR: liquid natural rubber; ENR: epoxidized natural rubber; NR: natural rubber.

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In general, comparatively, the overall tensile and elongation properties of the blend

with LNR were higher than the blend without LNR. This result was supported by the

morphological properties because the mechanical properties of the blend were closely

related to their morphology.

Thermal analysis

Differential scanning calorimeter. Table 2 shows the details of DSC data for all the blend

compositions. The DSC curves for neat polymers and blends are shown in Figures 5 and

6, respectively. The individual traces show that the Tm for PLA and the Tgs for NR,

ENR50 and PLA are shown in Figure 5. The exothermic crystallization peak indicates

the Tc, and the area under the crystallization curve indicates the heat of crystallization

values, which depend on the crystallinity of the material. The degree of crystallinity is an

underlying property that affects the properties, such as melting temperature and mod-

ulus. As shown in Figure 5, the Tgs of individual components vary greatly, that is, from

�65.2�C (NR), �22.0�C (ENR50) to 62.8�C (PLA), correspondingly.

The Tg of ENR50 is higher than NR due to the insertion of oxygen atoms in the

polyisoprene, which decreases the rotational freedom of the polymer backbone. The Tg

of ENR can be increased by increasing the level of epoxide due to the intermolecular

interaction between polar groups in the ENR chains.

Figure 2. Tensile strength of 40PLA/60ENR blends with and without LNR. PLA: poly(lactic acid);ENR: epoxidized natural rubber; LNR: liquid natural rubber.

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It is evident that Table 2 shows the Tg of PLA corresponds to 54.9�C and further heating

exhibits a mobility of PLA molecules above Tg that results in a crystallization peak at

101.1�C for the 40PLA/55ENR/5NR blend. However, when the LNR was used in the

system, that is, 40PLA/55ENR/5LNR, the Tc is reduced slightly as depicted in Figure 6. It is

worth noting that the Tc is the temperature where ordering and production of the crystalline

regions occurred. As such, the exothermic peak was observed at 99.9�C due to the ability of

the PLA chains having sufficient mobility to crystallize. It appears that increasing the

amount of LNR does not have a significant effect on the Tc for all blend compositions for

40PLA/60 rubber. The degree of crystallinity for blends with 5% NR or LNR were found to

be 38.1% and 39.1% respectively. These values are quite consistent for all the blends with

NR or LNR. Similar behavior was also reported by Yokohara and Yamaguchi32 for the blend

of PLA and poly(butylenes succinate). Recently, Bitinis et al.33 reported that the addition of

10% and 20% of NR enhanced the crystallization ability of PLA/NR blends.

The Tgs of blends with and without LNR at 40% PLA shows a reduction in

approximately 8�C. Similarly, a downward shift of the Tm at 167.9�C was observed for

all the blend compositions as shown in Table 2. The phenomenon of Tm reduction was

also reported by Dahlan et al. in their 60/40NR/linear low-density polyethylene

compatibilized with LNR blends.25 The change in Tm is thought to be associated with

the interference of NR/ENR during the formation of crystalline part of PLA. The fact

that the presence of Tg and Tm in all blend compositions indicated that the blend is

semicrystalline. Other studies have also reported the lower values of Tg and Tm in the

Figure 3. Elongation at break of 40PLA/60ENR blends with and without LNR. PLA: poly(lacticacid); ENR: epoxidized natural rubber; LNR: liquid natural rubber.

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blends containing poly(L-lactide) and dendritic polylactide copolyesters.34 Generally,

these DSC results confirm that the blends of PLA/ENR/LNR are not miscible.

Thermogravimetric analysis. The TGA is a technique that measures the mass and the

change in mass of the sample during heating as a function of time and/or temperature.

Decomposition of mass could be occurred due to chemical reactions or physical changes

during heating. TGA was performed to determine the thermal stability of the PLA,

ENR50, NR and blends as shown in Table 3. The onset temperature is the beginning of

Figure 4. Young’s modulus of 40PLA/60ENR blends with and without LNR. PLA: poly(lactic acid);ENR: epoxidized natural rubber; LNR: liquid natural rubber.

Table 2. DSC data of the neat PLA, NR, ENR50 and the blends

Sample Tg1, Tg2 (�C) Tc (�C) Tm (�C) Xc (%)

Neat PLA 62.8 – 167.9 –ENR 50 �22.0 – – –NR �65.2 – – –40PLA/55ENR/5NR �19.9, 54.9 101.1 154.5 38.140PLA/55ENR/5LNR �20.2, 54.8 99.9 159.0 39.140PLA/50ENR/10LNR �22.2, 54.0 101.2 160.7 37.7

DSC: differential scanning calorimetry; PLA: poly(lactic acid); NR: natural rubber; ENR: epoxidized natural

rubber; LNR: liquid natural rubber.

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weight loss while the final temperature is the end of degradation. It was found that the

decomposition of PLA starts at around 338.5�C and completed at 388.9�C, while the

decomposition of ENR starts at around 366.2�C and completed at 451.8�C. However, for

the blends with and without LNR, it started earlier than the individual polymers, as

shown in Figure 7. It is worth mentioning that the blends showed the two degradation

transitions, which are shown in the derivative plot of Figure 8. This can be explained by

higher degradation temperatures of ENR50 and NR which resulted in the increment of

the blend 40PLA/55ENR/5LNR system as depicted in Table 3.

Figure 5. DSC thermograms of (a) Neat PLA, (b) NR and (c) ENR50. DSC: differential scanningcalorimetry; PLA: poly(lactic acid); NR: natural rubber; ENR: epoxidized natural rubber.

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Fourier transform infrared spectroscopy

The FT-IR spectra of PLA, ENR, NR, LNR and blends are shown in Figure 9. The IR

bands at 2995 and 2945 are assigned to the C–H stretching regions. The carbonyl group

C¼O stretching region is located at about 1748 cm�1 as a broad asymmetric while the

Table 3. TGA onset, peak and final degradation temperature data of the neat PLA, NR, ENR50and the blends

SampleTemperature(�C) (onset)

Temperature(�C) (peak)

Temperature(�C) (end)

Neat PLA 338.5 368.9 388.9ENR 50 366.2 389.4 451.8NR 348.7 380.0 409.540PLA/55ENR/5NR 276.7, 346.9 307.3, 423.1 330.1, 464.540PLA/55ENR/5LNR 277.3, 399.8 310.9, 428.6 332.9, 485.640PLA/50ENR/10LNR 282.3, 364.2 315.4, 428.8 336.2, 482.4

TGA: thermogravimetric analysis; PLA: poly(lactic acid); NR: natural rubber; ENR: epoxidized natural rubber;

LNR: liquid natural rubber.

Figure 6. DSC thermograms of (a) 40PLA/55ENR/5NR and (b) 40PLA/55ENR/5LNR. DSC: dif-ferential scanning calorimetry; PLA: poly(lactic acid); ENR: epoxidized natural rubber; LNR: liquidnatural rubber.

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CH bending appears at 1382 and 1359 cm�1. Also, it has been reported that the

peaks at 868 and 751 cm�1 are related to the amorphous and crystalline region. The

peak at 1180 cm�1 belongs to C–O–C stretching of PLA, which is clearly visible in

the IR spectra.

It can be seen in Figure 9 that the spectrum for ENR50 absorption peaks at 873 and

1250 cm�1 correspond to half ring and whole ring stretching of the epoxide ring. The

stretching of C¼O and OH may be related to the peaks in 1750 and 3400 cm�1,

respectively. It was observed that there were no changes in the carbonyl group of PLA

observed in this case, thus suggesting that there were no interactions between the PLA/

ENR/NR components. There was no difference between the FT-IR spectra of blends with

LNR content. Since there were no changes, this further confirms that the blend between

PLA/rubber is not miscible.

Morphological characterization

Figure 10 shows the SEM micrographs examined from the tensile fracture surfaces for

neat PLA and blends. It can be seen that Figure 10(a) and (b) illustrates the micrographs

of the neat PLA and blend without LNR, that is, 40PLA/55ENR/5NR correspondingly.

Typically, the blend without LNR showed a large void measuring about 20 mm compared

with much smaller voids in the blend with LNR, as shown in Figure 10(b) and (c),

respectively. Large voids were visible in the surface where PLA particles were pulled

away during the tensile testing, indicating that the fracture occurred at the interface

Figure 7. TGA curves of (a) Neat PLA, (b) NR, (c) ENR50, (d) 40PLA/55ENR/5NR and (e) 40PLA/55ENR/5LNR. TGA: thermogravimetric analysis; PLA: poly(lactic acid); NR: natural rubber; ENR:epoxidized natural rubber; LNR: liquid natural rubber.

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Figure 8. DTG curves of (a) Neat PLA, (b) NR, (c) ENR50, (d) 40PLA/55ENR/5NR and (e) 40PLA/55ENR/5LNR. DTG: derivative thermogravimetric; PLA: poly(lactic acid); NR: natural rubber;ENR: epoxidized natural rubber; LNR: liquid natural rubber.

Figure 9. FT-IR spectra of (a) LNR, (b) NR, (c) ENR50, (d) Neat PLA and (e) 40PLA/55ENR/5LNR. FT-IR: Fourier transform infrared spectroscopy; LNR: liquid natural rubber; NR: naturalrubber; ENR: epoxidized natural rubber; PLA: poly(lactic acid).

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between the two phases. In contrast, a uniform dispersion of the PLA particles could

be due to a good interaction between PLA and ENR in the presence of LNR. The

uniform dispersion of PLA/LNR in the blend altered the crack path, which leads to

more resistance for crack propagation and, hence, higher tensile strength. Further-

more, the presence of LNR increased the interaction between the phases, thus

improving the compatibility and enhancing the tensile strength. Moreover, LNR is a

chemically modified NR generated by oxidative degradation of NR. Such groups

like OH and C¼O are formed during the degradation of NR and these groups form

the active sites in LNR.24,28. The terminal hydroxyl group (–OH) in the PLA could

be possibly interacting with ether groups (C–O–C) of ENR by hydrogen bonding. It

is interesting to note that other studies have suggested that there could be hydrogen

bonding formed between the ester group of PLA and oxirane group of ENR.35

According to the mechanical properties, as previously described, the addition of

LNR led to an increase in the tensile strength and elongation at break properties

compared with the blends without LNR.

Figure 10. SEM micrograph taken from the tensile fracture surfaces at �1000 magnification (a)neat PLA, (b) 40PLA/55ENR/5NR with large voids and (c) 40PLA/55ENR/5LNR blends with smallvoids. SEM: scanning electron microscopy, PLA: poly(lactic acid); ENR: epoxidized natural rubber;NR: natural rubber; LNR: liquid natural rubber.

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Conclusion

The structure and properties for immiscible binary blend of PLA/ENR with and without

LNR compatibilizer were investigated. It was found that the addition of 5% LNR

compatibilizer has improved the tensile strength and elongation at break for the blend

composition of 40PLA/55ENR/5LNR due to the ability of LNR to promote uniform

dispersion between the NR and PLA phases. It was also found that the incorporation of

5–15 parts of NR improved the Young’s modulus when compared with the blend of

40PLA/60ENR alone. On the contrary, the observed reductions in Young’s modulus in the

PLA/ENR/LNR blend systems were due to the plasticization effect of LNR. The addition

of LNR or NR part in the 40PLA/60ENR has also enhanced the crystallization ability of

the blend system. Compared with the neat PLA, the presence of ENR and NR/LNR

contributed to a two-step degradation transitions for both compatibilized and noncompa-

tibilized blend system due to higher degradation temperatures of ENR50 and NR parts.

Acknowledgements

The authors would like to thank Universiti Malaysia Pahang (UMP) and Universiti Kebangsaan

Malaysia for sponsoring this research project under FRGS grant (UMP-RDU100114).

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