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The Effects of Natural Weathering on the Properties ofLinear Density Polyethylene (LDPE)/Thermoplastic SagoStarch (TPSS) BlendsR. Abdul Majid a , H. Ismail a & R. Mat Taib aa School of Materials and Mineral Resources Engineering USM Engineering Campus, NibongTebal, Penang, MalaysiaPublished online: 15 Sep 2010.

To cite this article: R. Abdul Majid , H. Ismail & R. Mat Taib (2010) The Effects of Natural Weathering on the Properties ofLinear Density Polyethylene (LDPE)/Thermoplastic Sago Starch (TPSS) Blends, Polymer-Plastics Technology and Engineering,49:11, 1142-1149, DOI: 10.1080/03602559.2010.482081

To link to this article: http://dx.doi.org/10.1080/03602559.2010.482081

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The Effects of Natural Weathering on the Properties of LinearDensity Polyethylene (LDPE)/Thermoplastic Sago Starch(TPSS) Blends

R. Abdul Majid, H. Ismail, and R. Mat TaibSchool of Materials and Mineral Resources Engineering USM Engineering Campus,Nibong Tebal, Penang, Malaysia

Linear density polyethylene (LDPE)/thermoplastic sago starch(TPSS) blends with and without the addition of compatibilizer(Polyethylene-grafted-Maleic Anhydride, PE-g-MA), with differentTPSS loadings (10, 20, 30, 40 and 50wt.%) were prepared. Theseblends were subjected to a natural weathering condition which wastropical climate in Penang, Malaysia, for 3 and 6 months. Resultsof mechanical tests showed that with an increase of weathering time,the tensile strength and elongation at break decreased for bothuncompatibilized and compatibilized blends. But the Young modulusincreased during 3 months and 6 months of exposure. With theincrease of TPSS loading, tensile strength and elongation at breakwere also decreased. The Young modulus however increased with theincrease in TPSS loading. Scanning electron microscopy (SEM) onblends surfaces after weathering showed that the degradabilityincreased with the increase of TPSS loading as well as exposuretime. This explained the reduction in tensile strength and elongationat break after weathering. Fourier transform infrared spectroscopyresults on the blends after exposure to natural weathering indicatedthe presence of carbonyl, vinyl, and hydroxyl groups, which formedfrom various chemical reactions taking place during weathering.

Keywords Chemical reactions; Mechanical properties; Naturalweathering; PE-g-MA

INTRODUCTION

In recent years, studies on the degradability of polymericmaterials during outdoor natural weathering, is rapidlygrowing due to the highly utilized polymer material tothe outdoor applications. Therefore a big effort is nowfocused on understanding the changes that occur at mol-ecular level and the degradation kinetics, causing deterio-ration of mechanical properties and leading to uselessmaterials, after a certain time[1]. All degradation phenom-ena are induced either by physical agents or by interactivesubstances present in the environment such as the rain,wind and the air[2]. For this reason, understanding the

different chemical processes and how they are related tothe molecular microstructure and morphology can helpimprove the knowledge about the lifetime of polymericmaterials.

Nowadays, the study on degradable polymeric materialshave become one of the major focus for researchers due tothe waste management problems. With some additional ofother additives or blending the common hydrocarbon poly-mer with other biopolymer might accelerate the degradablemechanism. In addition, low cost biopolymer such asstarch is very easy to be provided because it is naturalrenewable polysaccharide from a great variety of crops.

In the case of hydrocarbon plastic materials such aspolyethylene (PE), the degradability and the chemicalinteractions for degradation are very slow. El Rehimet al.[3] reported that adding certain additives such as starchto the PE increases polyethylene chain oxidation reactions.Microbial consumption of starch increases polyethylenesurface area by creating pores and enhances oxygen basedreaction. The radiation by UV or sunlight also reduces thepolymeric chain size of polyethylene and form oxidizinggroups such as carbonyl, carboxyl, and hydroxyl[3–6].

With the addition of low-molecular-weight plasticadditives like plasticizers and fillers make the polymer moresusceptible to microbial attack. This leads to physicalembrittlement of the polymer, leaving a porous andmechanically weakened polymer. The microbes, in turn,release nonspecific oxidative enzymes that could attackthe synthetic polymer. Also, the gradual degradation ofthe natural polymer leads to increased surface area byerosion and pitting. This will accelerate the degradationof the synthetic polymer by diffusion of O2, moisture,and enzymes into the porous polymer matrix[4–6].

During the weathering of blends, cross-linking andscission of molecular chain take place at the same time;therefore, it is very difficult to find an index to characterizethe weathering extent of the weathered blends, becausemany properties of the weathered blends do not showmonotone trend with the weathering time. In this article,

Address correspondence to R. Abdul Majid, School ofMaterials and Mineral Resources Engineering USM EngineeringCampus, 14300 Nibong Tebal, Penang, Malaysia. E-mail:aniemajid@yahoo.com

Polymer-Plastics Technology and Engineering, 49: 1142–1149, 2010

Copyright # Taylor & Francis Group, LLC

ISSN: 0360-2559 print=1525-6111 online

DOI: 10.1080/03602559.2010.482081

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LDPE=TPSS blends were subjected to natural weathering.Tensile test, surface morphology and FTIR, were appliedto characterize the change of tensile properties, carbonylindex and melting point with weathering time. Differencebetween the uncompatibilized and compatibilized blendswas also discussed in this article.

EXPERIMENTAL

Materials

Sago starch was obtained from the Land CustodyDevelopment Authority (LCDA), Sarawak. Its moisturecontent was 13wt.%. The granules sizes ranges from 9 to35 mm, with an average granule size of 20 mm. Sago starchdecomposes at 230�C. Pellets of low density polyethylene(LDPE) with melting temperature of 138�C were obtainedfrom Titan Chemical. Polyethylene-grafted-Maleic Anhy-dride (PE-g-MA) was obtained from Sigma-Aldrich (M)Sdn. Bhd. Glycerol was obtained from Ajax Chemicals.

Preparation of TPSS

Sago starch was dried in a vacuum oven for 24 hours at80�C. TPSS was prepared by mixing the sago starch inpowder form with 35wt.% liquid glycerol in a kitchenblender with a capacity of 200 g. After the process, thecompound was melt-mixed using a heated two roll mill at150�C for 10 minutes.

Preparation of LDPE/TPSS Blends

TPSS was melt-blended with LDPE in a HaakeRheomix Model R600=610. Mixing was performed at150�C and 50 rpm for 10 minutes. LDPE was first addedinto the mixing chamber followed by TPSS after 3 minutesof mixing. The total mixing time was 10 minutes. TPSSloadings were varied from 10, 20, 30, 40 and 50wt.%.PE-g-MA was used at an amount of 10wt.% based onTPSS weight. Melt temperature and torque were recordedduring the mixing periods.

Compression Molding

LDPE=TPSS blends were compression molded in anelectrical heated hydraulic press. Hot press proceduresinvolved preheating at 150�C for 6 minutes, followed bycompression with a pressure of 1000 kg=m3 for 3 minutesat the same temperature. All compression molded sheets(150� 150� 1mm) were cold pressed for 2 minutes.

Natural Weathering Study

The maximum and minimum temperature, meanhumidity and mean rainfall in Penang, Malaysia is com-piled for each month and plotted in Figures 1 and 2. Theweathering period was from Mac 2008 to August 2008.The natural weathering test was conducted according toASTM D1435-94. LDPE=TPSS blends with and without

the addition of compatibilizer were attached to a rack witha rack holder. The rack was placed on the roof of ourresearch building and adjusted to face the equator at anangle of 45�. The rack was located at an open area whereit was free from being overshadowed by other objects.The samples were exposed to all environmental effects,such as rain, sunlight, wind, etc., and collected after 3months and 6 months to determine the degradability.Moisture at the surface of the samples was removed witha clean towel and left in air for 24 h at room temperaturebefore mechanical tests were conducted.

Tensile Properties

Tensile tests for the unexposed and exposed sampleswere carried out with a universal testing machine Instron3366 according to ASTM D638. Dumbbell specimens of1mm thickness were cut from the compression moldedsheets with a Wallace die cutter. A crosshead speed of5mm=min was used and the test was performed at25� 3�C. Five specimens were used to obtain averagevalues for tensile strength, elongation at break andYoung’s modulus.

Morphology Study

Studies on the surface morphology of the exposed sam-ples of LDPE=TPSS blends were carried out using a fieldemission scanning electron microscope, SUPRA36VP.

FIG. 1. Maximum and minimum temperature from Mac 2008 to August

2008.

FIG. 2. Mean humidity mean rainfall from Mac 2008 to August 2008.

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Surfaces of the sample were mounted on aluminum stubsand sputter coated with a thin layer of gold to avoid elec-trostatic charging and poor resolution during examination.

Fourier-Transform Infrared Spectroscopy

Fourier-transform infrared spectroscopy (FTIR; PerkinElmer System 2000) was used to obtain some qualitativeinformation about the functional groups and chemicalcharacteristics of the LDPE=TPSS blends before and afterthe natural weathering exposure. For each spectrum, 52consecutive scans with 4 cm�1 resolution were recorded.Samples were measured in the form of sheets about 1mmthickness, which were prepared by hot press molding.The extent of the blends oxidation was monitored by mea-suring the levels of keto carbonyl (1713 cm�1) absorptionby Fourier transform infrared (FTIR) spectroscopy. Car-bonyl Index (CI) was calculated with following equation:

CI ¼ Area at 1713 cm�1

Area at 1465 cm�1ð1Þ

Carbonyl index was used as an index to monitor theextent of the blends oxidation.

RESULTS AND DISCUSSION

Mechanical Properties

Figure 3 shows the tensile strength for control andweathered LDPE=TPSS and LDPE=TPSS=PE-g-MAblends. The tensile strength of pure LDPE (0% TPSSloading), LDPE=TPSS and LDPE=TPSS=PE-g-MA blendsdecreased with the exposure time, as seen in Figure 3. Asexpected, the increase in TPSS content decreased the tensilestrength and as a function of time as well. The percentageof decrease in the tensile strength after 3 months and 6months of weathering for pure LDPE (0% TPSS loading),LDPE=TPSS and LDPE=TPSS=PE-g-MA blends can beseen in Table 1.

From the table it can be seen the percentage of reductionin tensile strength increase with the increase in TPSS load-ing as well as time. This is due to the starch particle thatcontaining abundance of hydroxyl group, which couldprone to moisture absorption due to its hygroscopicnature. The rainfall is relatively frequent during the naturalweathering tests. This may enhance the susceptibility of theTPSS to absorb rain water. It is believed that the watermolecules will be absorbed by the TPSS particles, whichwere located in the outer surface, followed by the pen-etration of the water molecules into the inner layer of theexposed LDPE=TPSS blends. In addition, some TPSS par-ticles on the skin layer of weathered LDPE=TPSS blendswere leached out due to the effects of rainfall. But fromthe table, the reduction rate in the tensile properties of pureLDPE was greater than the blends at lower TPSS loading.

This result is due to the higher amount of LDPE in thesamples, which is the part that reacts directly to sunlight[5].

From Figures 3 and 4, it can be seen that the compati-bilized blends show higher tensile strength and Youngmodulus for the control samples compared to the uncom-patibilized blends. However, as the exposure timeincreased, the tensile strength of both compatibilized anduncompatibilized blends decreased continuously. However,for Young modulus, some of the weathered samples showan increase with the increase in exposure time. These resultscan be attributed to the enhanced stiffness of the blends

FIG. 3. Tensile strength for control and weathered LDPE=TPSS and

LDPE=TPSS=PE-g-MA blends; (a) 3 months of weathering and (b) 6

months of weathering.

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caused by photo-oxidation which the increase in crosslink-ing that happened due to the high rate of radical termin-ation in the bulk of the polymer where O2 cannot gainaccess. Thus, there was an observed increase in the Youngmodulus. But from Figure 4, after 6 months of weathering,the Young modulus value still increasing for both compa-tibilized and uncompatibilized blends. This might due tothe rate of radical termination in the bulk of the polymerwas still increasing. But this radical termination wouldeventually decrease due to the predominance in chainscission[7].

In addition, photo-oxidation only concentrated near thesurface of the blends and the process mainly takes place inthe amorphous region because of higher permeability tooxygen. Longer exposure times cause the development ofmore surface cracks and allow chemical degradation andwater absorption to take place. The chemical degradationand water absorption can be caused by exposure of theblends to acidic rainwater and moisture, which can seepthrough the cavity of the surface cracks, causing chemicalattack to the polymer bonds and might enhance somecross-linking reaction to happen due to radical recombi-nation during exposure[6–8]. In case of LDPE=TPSS andLDPE=TPSS=PE-g-MA blends, 6 months of exposure stillnot decrease the Young modulus value.

For elongation at break in Figure 5, the trend is similarwith the tensile strength. The decrease in the elongation atbreak was drastic at the 6th month, because the starchgranules of the blends had no interfacial adhesion withthe LDPE matrix as shown later in SEM study. This gapfacilitated the penetration of oxygen and light into theinner part of the LDPE matrix.

FTIR Study

From Figures 6 and 7, the infrared spectroscopy of theblends shows few changes due to the various chemical reac-tions taking place during weathering. First, the increase in

peak area of carbonyl groups is observed in the region of1712–1860 cm�1, with small peaks and shoulders detectedin the spectra of the blends. Second, the existence of moresmall peaks in the region of 800–1000 cm�1 as can be seenin Figure 6 after weathering vs. before weathering representvinyl groups. The vinyl groups and hydroxyl groups (3200–3600 cm�1) show the presence of oxidation products ofweathered blends[9,10].

FTIR analysis also shows that in natural weathering, thecarbonyl peak of LDPE=TPSS blends start to develop, andwith the increase in weathering time, the carbonyl peakbecoming broader because the volume of carbonyl groupbecome higher. It is worth noting that the carbonyl groupof the weathered blends forms more carboxylic groups, butless ester and vinyl alkenes. Thus, the enrichment of the

FIG. 4. Young’s modulus for control and weathered LDPE=TPSS and

LDPE=TPSS=PE-g-MA blends; (a) 3 months of weathering and (b) 6

months of weathering.

TABLE 1Percentage of decrease of the samples compared to control

samples

Percentage of decreasecompared to control samples

Samples 3 months (%) 6 months (%)

Pure LDPE 50 70LDPE=TPSS 30% 29 57LDPE=TPSS 50% 45 79LDPE=TPSS=PE-g-MA 30%

73 75

LDPE=TPSS=PE-g-MA 50%

68 88

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weathered blends with carboxylic groups may themselvesaccelerate further the degradation process, producing anautocatalytic effect. Furthermore, there is some evidencethat the carboxylic groups may promote microbiologicalgrowth within the system. When this takes place, the fungal

FIG. 5. Elongation at break control for control and weathered LDPE=

TPSS and LDPE=TPSS=PE-g-MA blends; (a) 3 months of weathering and

(b) 6 months of weathering.

FIG. 7. FTIR for control and weathered samples of LDPE=TPSS=

PE-g-MA.

FIG. 6. FTIR for control and weathered samples of LDPE=TPSS.

FIG. 8. Carbonyl Index for (a) LDPE=TPSS and (b) LDPE=TPSS=

PE-g-MA.

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metabolites may further increase the rate of oxidation ofthe blends[7]. Both Figures 6 and 7 show an increase inthe carbonyl peak which means that the addition ofPE-g-MA did not affect the degradability of the blends.

Figure 8 illustrates the carbonyl index of LDPE=TPSSblends with and without PE-g-MA as a function of time.The increase in the carbonyl index in LDPE and TPSSchains resulted from the increase in the amount of smallfragments of LDPE and TPSS chains with carbonylgroups. This mechanism makes the polymer chains shortenand simultaneously decreases the molecular weight andtensile properties of the blends.

The data shows that, as the exposure time and starchcontent increased, the carbonyl index increased. This isattributable to the microstructure of the starch granulesthat are embedded within the LDPE matrix. Our previousstudy revealed that there is a gap between the TPSS gran-ules and the LDPE matrix for the uncompatibilizedblends[15]. And for the compatibilized blends, the compati-bilizer was strongly functioning to reduce the agglomer-ation between TPSS granules. Therefore, the higher TPSScontent led to an increase in porosity of the LDPE matrix,which then easily allowed the permeation of light andoxygen throughout the inner part of the LDPE, resultingin the higher carbonyl index.

Figure 8 also shows that there is no significant dif-ferent of the carbonyl index for compatibilized and

uncompatibilized blends as a function of the exposure time.Both set of samples show that the addition of compatibili-zer inside the blends did not affect the degradability of theblends.

Morphological Study

The surface morphology of the compatibilized anduncompatibilized LDPE=TPSS blends after weatheringfor 3 and 6 months is illustrated through SEM micrographs(Figures 9 and 10). From Figure 9, SEM micrographsillustrate that the surface of the LDPE=TPSS andLDPE=TPSS=PE-g-MA blends after 3 months weatheringis not smooth, with cracks and holes. As can be seen inFigure 9a, the blend without the compatibilizer shows theholes cannot be seen clearly because at 10% TPSS loading,the amount of TPSS is very small. But Figure 9b provedthe agglomeration of TPSS occurred inside the blends at50% TPSS loading. Most of the starch had leached outdue to the environment effect such as UV sunlight, rainand wind. However, for the blend with the compatibilizerin Figure 9d, the micrograph shows the small holes canbe seen clearly but with minimum agglomeration.

As seen in Figure 10, the surfaces of LDPE=TPSS andLDPE=TPSS=PE-g-MA blends after 6 months of weather-ing were remarkably changed to rough surfaces with largecontinuous crack formation, and detachment of starch onthe surfaces. And the exposed surfaces of the blends

FIG. 9. Surface morphology for LDPE=TPSS and LDPE=TPSS=PE-g-MA weathered for 3 months; (a) 10% TPSS uncompatibilized, (b) 50% TPSS

uncompatibilized, (c) 10% TPSS compatibilized, and (d) 50% TPSS compatibilized.

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showed deeper crack formation compare to 3 months ofweathering. There were remarkable matrix losses especiallyfor the 50% TPSS for the uncompatibilized blends due todegradation occurring during weathering. The agglomer-ation of the TPSS contributed to higher weight lost becausemost of the TPSS were leached out as a bundle. All thesedata suggest that the attack of outdoor agents takes placemainly on the exposed surface.

As a result, most of the starch granules can be seen easilyafter the exposure for the uncompatibilized blends andreduced the interaction between the LDPE and TPSS. Thelost of interaction between TPSS and LDPE and the weightlost in the blends might be one of the factors of the decreasein tensile properties beside the cracks that occurred on sur-faces of the blends. But for the compatibilized blends, goodinterfacial adhesion and interaction between LDPE andTPSS reduced the gap between the blends. Therefore, theamount of starch that had leached out was reduced andbecause of that, the weight lost was lower than the uncom-patibilized blends. And as results tensile properties ofweathered LDPE=TPSS with compatibilizer were highercompared to the uncompatibilized blends.

CONCLUSION

After exposured to natural weathering, tensile testresults indicated that LDPE=TPSS=PE-g-MA blends

exhibited higher tensile strength and Young modulus thanLDPE=TPSS blends due to good interfacial adhesionbetween LDPE and TPSS although both blends showreduction in mechanical properties with the increase inweathering time. FTIR results also indicate that the car-bonyl group increase with the increase in TPSS loadingas well as the weathering time. Although cracks were evi-dent in the exposed surfaces of both blends, the exposedsurface morphology of LDPE=TPSS=PE-g-MA showlower TPSS leached out than LDPE=TPSS due to loweragglomeration of TPSS in LDPE matrix.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financialsupport of the Research University Grant (GrantNo. 1001=PBAHAN=814008).

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