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Preparation and characterization of LDPE/starch blends containing ethylene/vinyl acetate copolymer as compatibilizer.

INTRODUCTION

The ever-growing production and use of synthetic polymers has led to a deterioration of the waste disposal problem after their use, since most of them are not degraded when exposed to the environment (1). One possible solution is the recycling of used plastics. However, this method is faced with the difficulties of plastic removal, collection, and classification. Therefore, this method has not yet gained widespread acceptance, except in the case of soft-drink bottles made from PET or HDPE (2) and in the recycling of plastic parts in the automotive industry.

The use of unseparated recycled polymers is not a solution either, since it leads to the production of materials with very poor mechanical properties, and therefore unusable. In other cases such as packaging materials and plastic bags, recycling is simply impossible or impractical. As a result, the greatest part of plastic waste ends up either buried or incinerated.

Thus in the last few years an effort was undertaken for the production of fully biodegradable synthetic polymers such as aliphatic polyesters (3), poly(ethylene glycol), poly(vinyl alcohol) (4, 5), or the use of natural polymers such as starch, cellulose, and so on, with the main objective being the total replacement of the nonbiodegradable polymers.

Starch is the second largest biomass produced on earth after cellulose (6). In the last few years this material has attracted significant interest because it is abundantly produced by photosynthetic plants and therefore is renewable and relatively cheap. It is used for the production of various biodegradable materials (starch-based foamed materials, encapsulation, etc.) (7). Also, its structure can be easily altered in various ways. When processed at high temperatures and pressures in the presence of high amounts of water, a totally amorphous material can be produced that can be molded in various forms, even in films. This material, however, becomes brittle when the water content is less than 5 wt% (8). Thus, usually, water is replaced by plasticizers and especially glycerine (9). The material produced (plasticized starch) has a higher elongation ([approximately] 170%), but its tensile strength is relatively low 0.7 MPa (10). As a result, it cannot be used on its own as a thermoplastic material.

Starch modification in order to produce a thermoplastic material is known for several decades (11, 12). Lately there has been a renewed interest in the modification of starch, usually by esterification of its hydroxyls for the production of thermoplastic materials (10, 13, 14). The main objective is the replacement of nonbiodegradable thermoplastic materials. This method, however, is without much success so far because these materials have, in general, inferior mechanical properties. Another type of modification is the direct grafting of polymeric chains onto the starch backbone. Such examples are starch-g-poly(methyl acrylate) (15-18), starch-g-poly(methacrylic acid) (19) and starch-g-polystyrene (20, 21). A saponified starch-g-acrylonitrile copolymer (22-25) is commercially known as "Super Slurper." This material is capable of retaining several hundredfold times its own weight in water and is used in absorbent products such as disposable diapers, bandages, and hospital bed pads.

The greatest application of starch in the polymers field is as a component in various polymer formulations. These can range from totally biodegradable plastics such as polycaprolactone (26,27), poly(hydroxybutyrate-co-valerate) (28) and poly(vinyl alcohol) (29-31), to nonbiodegradable materials such as PVC (32), PP (33), SMA, EPMA (34-37), and polyethylene (38). In the first case, starch is added to reduce the cost (since it is very cheap) and in the latter to increase the biodegradability of the resulting material. Most of the applications are focused on polyethylene, which is widely used as packaging material. As a result of these efforts, in the last few years, several commercial products have been developed, but most contain at most 6-9 wt% starch (39). A greater amount of starch gives products with inferior mechanical properties since the two polymers are incompatible.

One way to increase compatibility, and thus the incorporated amount of starch in polyethylene blends, is to use a compatibilizer containing groups capable of hydrogen bonding with starch hydroxyls. The most frequently used compatibilizer is ethylene/acrylic acid copolymer (EAA) (40-42). It contains both polyethylene segments and acrylic acid units, which can form stable V-type complexes with starch as a result of hydrogen bond formation between the carboxylic groups of acrylic acid and hydroxyl groups of starch. The disadvantage is that EAA is four times more expensive than starch, and also, the strong complexation of starch with EAA retards the biodegradation of the former (43).

In this study ethylene-co-vinyl acetate (EVA) copolymer was used as compatibilizer in order to increase the amount of plasticized starch that can be incorporated in LDPE matrix and still produce blends with satisfactory mechanical properties. An extensive study of the properties of these blends and especially the biodegradation rate was also undertaken.

EXPERIMENTAL

Materials

The LDPE (Borealis) used was appropriate for packaging applications. Ethylene-co-vinyl acetate copolymer containing 8 mol% vinyl acetate was supplied from Alcudia. The native corn starch was from Amylum, with 30 wt% amylose and 70 wt% amylopectin content, and was plasticized with 35 wt% glycerol. Glycerol and starch were premixed before the plasticization process. Plasticization was performed in a Haake-Buchler Reomixer by mixing the starch-glycerol mixture at 170 [degrees] C for 10 min. Placticized starch is referred to hereafter as PLST. The native starch used was previously dried at 150 [degrees] C under vacuum for 24 hours to remove moisture ([less than]1 wt%); it is referred to hereafter as FILST.

Melt-Blending

Starch was melt-blended with LDPE in a Haake-Buchler Reomixer model 600, with roller blades and a mixing head with a volumetric capacity of 69 [cm.sup.3] Prior to mixing, the polymers were dried by heating in a vacuum oven at 80 [degrees] C for 24 hr. The components were physically premixed before being fed in the Reomixer. Mixing was performed at 170 [degrees] C and 80 rpm for 15 min. For LDPE/PLST blends, four different levels of plasticized starch were used, namely, 5, 10, 20 and 30 wt%. In these blends EVA was used as compatibilizer at three different amounts, namely 10, 25 and 50 wt% based upon PLST. Blends of LDPE/granular starch as filler (FIT) with the same starch content and containing 25 wt% EVA (based on FILST) were also prepared, in order to compare the properties of the two systems. Melt temperature and torque were recorded during the mixing period. After preparation, the blends were milled and placed in tightly sealed vials to prevent any moisture absorption.

FTIR Measurements

FTIR spectra were acquired in a Biorad FTS-45A FTIR Spectrometer. For each spectrum 64 consecutive scans with 4 [cm.sup.-1] resolution were co-added. Samples were measured in the form of thin films [approximately] 70 [+ or -] 2 [[micro]meter] thick, which were prepared by hot press molding.

DSC Measurements.

DSC measurements of samples were performed in a Shimadzu DSC-50Q fast quenching differential scanning calorimeter. Samples were placed in sealed aluminum cells, using a quantity of about 10 mg for each sample. Samples were initially heated at 20 [degrees] C/min up to 200 [degrees] C and immediately quenched to remove any previous thermal history. The samples were subsequently rescanned with a heating rate of 20 [degrees] C/min. From these thermograms the melting temperature and heats of fusion were calculated.

TGA Measurements

TGA measurements were performed in a Shimadzu TGA-50 thermogravimetric analyzer. Measurements were performed under nitrogen atmosphere using a heating rate of 20 [degrees] C/min up to 650 [degrees] C. From the weight loss both the starch content and the thermal stability of the blends can be estimated.

Mechanical Properties of Blends

Measurements of the mechanical properties such as tensile strength and elongation at break, were performed according to the ASTM D638 method on an Instron mechanical tester, Model 1122. Measurements were done using a 5 mm/min crosshead speed. Prior to measurements, the samples were conditioned at 50 [+ or -] 5% relative humidity for 24 hr in a closed chamber containing a saturated Ca[(N[O.sub.3]).sub.2][multiplied by]4[H.sub.2]O solution in distilled water (ASTM E-104). Five measurements were conducted for each sample, and the results were averaged to obtain a mean value.

Sample Biodegradability

The biodegradation of the blends was followed during soil burial for six months. The samples were placed in a form of thin films ([approximately] 130 [+ or -] 5[[micro]meter] thickness) in a well 60 x 60 m wide and 35 cm deep in positions depending on the sample composition and scheduled time of sampling. Samples were also tagged for easier identification. After sample placement, the site was covered with soil from which all extraneous materials (stones, weed, litter) had been previously removed to avoid any interference with the samples. The films remained at the site for six months with a sample removed each month in order to follow their biodegradation.

Measurements performed to follow the biodegradation of the samples were the following:

* Sample weighting to measure the starch loss.

* Scanning electron microscopy (SEM).

* Mechanical measurements such as tensile strength and elongation at break.

RESULTS AND DISCUSSION

Starch contains three hydroxyl groups in each repeating 1,4-a-D-glucopyranosyl units and as a consequence is a polar hydrophilic polymer. On the contrary, nonpolar polyethylene is hydrophobic. Because of this totally different polar character of the two polymers, they are immiscible. As a result of this immiscibility, PE/starch blends have mechanical properties inferior to those of pure polyethylene. To increase the interfacial adhesion between the two polymers, the use of a compatibilizer is necessary.

Poly(ethylene-co-vinyl acetate) (EVA) copolymer was chosen as a potential compatibilizer. EVA contains carbonyl groups, which can form hydrogen bonds with the hydroxyl groups of starch, and thus its use is expected to increase the compatibility between LDPE and starch. Such hydrogen bonding formation has been detected in polymer blends consisting of polymers containing carbonyl groups and poly(vinyl alcohol) (44) or poly(vinyl phenol) (45, 46). The polymers were miscible because of hydrogen bending between the proton accepting group (carbonyl groups) and the hydroxyl groups.

Torque measurements

Torque measurements for LDPE/PLST with 10 wt% EVA blends during mixing are presented in Fig. 1. After an initial torque increase, there is a continuous decrease as soon as the materials start to melt. The torque finally stabilizes after approximately 6 min of mixing, suggesting that good mixing has occurred within this time period.

The final torque decreased with increasing PLST content in the blends. This phenomenon can be explained by the lower melt viscosity of plasticized starch compared with that of LDPE. Glycerine plasticizes the starch and reduces its melt viscosity. The same behavior was observed in blends of esterified starches (10, 14), where the ester groups act as internal plasticizers for starch. The effect of glycerine as plasticizer is clearer in Fig. 2 where the torque curves of blends with two different starch types are presented. The blend with FILST has a higher torque that the respective blend with PLST. One reason for this is that FILST does not melt during mixing with LDPE at this temperature, and it remains in a granular form, thus increasing the viscosity of the blend. Extrapolation data from model systems has shown that starch has a melting point of 257 [degrees] C (47), which is higher than the decomposition temperature of starch. On the contrary, PLST is a totally amorphous material, since during plasticization all its crystal structure is destroyed.

The amount of EVA copolymer has also a very small effect on melt viscosity of the blends. The torque decreases by [approximately] 1-2 Nm with increasing amount of EVA from 10 to 50 wt%. This behavior is mainly due to the lower viscosity provided by the EVA copolymer and their lower melt temperature.

FTIR Measurements

In the native starch spectrum, the characteristic broad peak of starch appears at 958-1190 [cm.sup.-1]. This peak is attributed to C-O bond stretching (48). A strong broad band due to hydrogen bonded hydroxyls appears at 3380 [cm.sup.-1] and the symmetric C-H vibration band at 2850 and 2920 [cm.sup.-1]. Figure 3 depicts the FTIR spectra of native corn starch used as filler, plastisized starch, and their blends with LDPE containing also 25 wt% EVA as compatibilizer.

Comparing the absorbances of hydroxyl groups of FILST and PLST, it can be seen that the hydroxyl groups of FILST absorb at a higher frequency (3600 [cm.sup.-1] than those of PLST (3569 [cm.sup.-1]). This shift is due to hydrogen bonding between the hydroxyls of starch and glycerin in PLST. When starch is plasticized with glycoles, the hydrogen bonds between the hydroxyls of starch are destroyed, and new ones are formed with glycol hydroxyls. As expected, in the blend spectra there are no significant differences observed (e.g. peak shifts) compared with the spectra for pure components, an indication that there are no strong intermolecular interactions between PLST or FILST and LDPE. The only intermolecular forces expected to develop between LDPE and PLST are the weak van der Waals interaction forces (dispersion forces).

Even after using the EVA as compatibilizer there are no significant changes in the spectrum. It was anticipated that EVA, which contains carbonyl groups, could develop hydrogen bonds with the hydroxyl groups of PLST. However, it must be realized that the concentration of vinyl acetate groups in the blends is very low. For example, for a blend containing 30 wt% starch and 50 wt% EVA (based on starch), the total concentration of EVA is 15 wt%. Taking into account that EVA contains 8 mol% vinyl acetate groups, the total concentration of the later in this blend will not exceed 1,2 mol%. With such a low concentration, the degree of hydrogen bonding is very limited and mainly occurs at the interface. Nevertheless, it can still affect compatibilization.

It is well known that starch can form a V-type complex when mixed with low molecular weight molecules such as fatty acids and monoglycerides (49-51) as well as with macromolecules like ethylene/acrylic acid copolymer (52-56). That is why starch can be blended in higher amounts with EAA and still produce blends with satisfactory mechanical properties (40, 41). Complex formation was confirmed by X-ray diffraction, CP/MAS-NMR, and FTIR measurements (52), In the case of poly(vinyl alcohol)/starch blends, second derivatives of the IR spectra have been used to detect such a complex formation (31). The characteristic peak, which was attributed to the V-type complex, is at 947 [cm.sup.-1] and was detected only in starches with an amylose content higher than 20 wt%. To study whether this type of bonding also occurs in our blends, the second derivatives of the IR spectra were calculated and are presented in Fig. 4.

As can be seen in the area of interest (1000-880 [cm.sup.-1]), the pure PLST has two peaks at 937 and 953 [cm.sup.-1]. When it is blended with LDPE using EVA copolymer, the peak at 937 cm-1 disappears and the peak at 953 [cm.sup.-1] shifts to lower values at 948 [cm.sup.-1]. The same was also observed for LDPE/FILST blends. The peak at 952 [cm.sup.-1] of pure starch shifts at 947 [cm.sup.-1] in the blends. This shift may be due to the ability of EVA to form such complexes with starch. Nevertheless, as was previously mentioned, the number of vinyl acetate groups is very low. Thus, for a more definite study of hydrogen bonding, blends containing EVA copolymers with a higher vinyl acetate content, or pure EVA/starch blends must be produced and studied.

Thermal analysis of the blends

DSC Measurements

The DSC thermograms of LDPE/Starch/EVA blends presented no significant differences compared with those of pure components. In all blends, a clear melting peak of LDPE can be observed and in starch-rich blends an additional very weak endothermic peak appears near 89 [degrees] C, as can be seen in Fig, 5.

In these blends the amount of EVA copolymer increases with the amount of starch since it is added at a 25 wt% level based on starch. Thus this second peak was attributed to melting of EVA copolymer, which is immisible with LDPE and has a melting point of about 89 [degrees] C.

The melting points of LDPE have no significant differences among the blends. Only a small decrease in melting temperatures of LDPE in blends (1-1.5 [degrees] C) compared with that of pure LDPE is observed, This is, however, well within experimental error. Similar observations were made for blends containing granular starch in a filler form.

Similar conclusions were drawn from the heat of fusion data. There is an apparent decrease in heat of fusion as the amount of LDPE decreases in the blends. However, when the heats of fusion are corrected taking into account the LDPE content in the blends, it is evident that there are no significant changes in the crystallinity of LDPE. It can be concluded than LDPE is hardly miscible with starch even in the case where EVA is added as compatibilizer.

TGA Measurements

The blends of LDPE with PLST showed two decomposition stages, as presented in Fig. 6. The first one (250-400 [degrees] C) is due to starch decomposition as it is similar to those of pure starches. The second stage, appearing at higher temperatures, is due to LDPE decomposition.

Two well separated decomposition steps appear also in blends with FILST. The only difference in these blends is that a slightly higher thermal stability is observed compared with blends with PLST. In the latter the weight loss starts after 200 [degrees] C because of volatilization of glycerine contained in PLST, while in the former no weight loss is observed up to 250 [degrees] C, where the decomposition of starch begins.

The amount of EVA copolymer does not affect the thermal stability of the blends, as shown in Fig. 7.

Mechanical Properties

Tensile strength and elongation at break values were determined from stress-strain curves of each sample of LDPE/starch blends.

The tensile strength vs. composition graph of blends is presented in the Fig. 8. From the diagram we can conclude that as the amount of starch increases, the tensile strength is decreased. The blends containing plasticized starch have satisfactory tensile strength, especially those containing 5 and 10 wt% starch. The blends with FILST have lower tensile strengths than the respective blends containing PLST. Plasticized starch produces more homogeneous blends with LDPE than FILST because the dispersion is finer. This was verified by optical microscopy after staining the starch with iodine solution in water, as can be seen in Fig. 9.

In the blend containing FILST, dispersed granules of corn starch can be observed ranging in diameter from 8 to 12 [[micro]meter]. Some aggregates with diameters of 30-40 [[micro]meter] are also visible. Such aggregates are not observed in blends containing PLST. On the contrary, there is a finer dispersion of starch. The maximum domain size of PLST does not exceed 12 [[micro]meter], whereas much smaller phases in the order of 1-2 [[micro]meter] are also visible.

The introduction of the compatibilizer in the blends increases the tensile strength compared with the uncompatibilized blends. However, the addition of compatibilizer seems to have a small negative effect on the tensile strength of the blends since the tensile strength decreases slightly with increasing amount of EVA. It is well known that the mechanical properties in polymer blends are mainly affected by the interfacial adhesion between the polymers. LDPE and starch are incompatible and their interfacial adhesion is very poor. EVA is expected to reduce the interfacial tension and, thus, to increase the adhesion. This was verified by SEM micrographs shown in Fig. 10.

In all compatibilized blends, the size of the PLST domains is reduced, which is further evidence that EVA acts as a compatibilizer. Some of the domains become less than 1 [[micro]meter] in diameter, and they are visible only with higher magnification. This reduction in domain size was expected to lead to a dramatic increase in tensile strength. However, this was not observed. The slight decrease in tensile strength with increasing amount of compatibilizer could be attributed to the lower tensile strength of EVA copolymer (about 7.1 MPa) compared with that of LDPE (9.5 MPa).

Significant differences, however, appear in elongation at break measurements, as clearly shown in Fig. 11. The elongation at break of blends increases with increasing amount of EVA. It is remarkable that all the blends with 50 wt% of EVA have elongation at break higher than pure LDPE. This is observed even for the blend containing 30 wt% of PLST. This behavior is mainly due to the higher elongation at break of EVA copolymer (900%) compared with that of LDPE (about 580%). FILST has a negative contribution in elongation at break, and thus its blends have lower elongation at break, compared with that of the respective blends containing PLST, which, as a thermoplastic material, has a higher elongation at break (about 170%).

Several theories on the dependence of composite properties on filler volume fraction [Phi] have been developed. One of the simplest is that of Nielsen (57) relating elongation and [Phi]:

[[Epsilon].sub.c] = [[Epsilon].sub.0] (1 - [[Phi].sup.1/3]) (1)

where [[Epsilon].sub.c] and [[Epsilon].sub.0] are the elongations at break of the composite and the unfilled polymer respectively. In this model, the adhesion between the filler and the polymer matrix is assumed to be perfect. Another simple formula is that of Nicolais and Narkis (58) relating tensile strength [Sigma] and [Phi]:

[[Sigma].sub.c] = [[Sigma].sub.0] (1 - 1.21[[Phi].sup.2/3]) (2)

The subscripts have the same meaning as in Nielsen's formula. Both of the above-mentioned theories have been recently applied by Willett (42) in LDPE/granular starch blends. A good agreement with theoretical predictions has been found, though the proportionality constants were less negative than those predicted by theory. This was attributed to imperfect adhesion between the starch and the LDPE matrix. Applying the above-mentioned theories in our blends gave a poor fit for elongation properties (Nielsen's formula) whereas the fit for tensile strength was generally satisfactory. The poor agreement between theory and experimental values for elongation properties must be attributed to poor adhesion between the two phases. As mentioned before, Nielsen's theory assumes perfect adhesion of the two phases. The Nicolais/Narkis formula on the other hand does not make such an assumption, and thus the agreement with the experiment is much better.

Biodegradation

Numerous studies of biodegradation of polyethylene-starch blends have shown that microbes consume starch, creating pores in the plastic that increase the surface area of the polyethylene matrix and provide opportunities for its degradation. Starch consumption by microorganisms results in weight loss of the blends. These differences are shown more clearly in Fig. 12, which illustrates the weight loss of blends during soft burial with different amounts of PLST and 10 wt% EVA.

It can be seen that in the blends with low amounts of starch, e.g. 5 and 10 wt%, the weight loss is very small, and even after six months of soft burial does not exceed 0.2 and 0.5 wt%, respectively. The biodegradation rate increases for the blend with 20 wt% starch, whereas in the blend with 30 wt% starch there is a rapid weight loss even after the first month. Assuming that all this weight loss is due to starch consumption, then for blends containing 5 wt% starch, the starch consumption is approximately only 1-1.5%. For blends containing 10% starch the respective loss is three times as high, reaching 5% of initial starch. For blends containing 30% starch, the highest losses are observed reaching 32-34% of initial starch. As a conclusion, consumption of starch by microorganisms depends on the blend's starch content. For blends containing low mounts of starch, the latter is apparently almost completely covered by LDPE and thus not accessible to microorganisms. It must be noted that the coiled starch diameter is about one tenth the film thickness, and thus microorganisms will consume only the starch that is located at the surface of the film. On the contrary, in blends with higher starch content, starch is more exposed, and as result, a greater portion of it is consumed by microbes.

The scalar percolation theory can be applied to LDPE/granular starch blends (59) Percolation theory concerns the connectivity of one component (in our case starch) randomly dispersed in another. Computer simulations have shown that starch accessibility generally increases with increasing concentration of starch. Near a critical concentration of -31.17% by volume (percolation threshold), the accessibility increases dramatically. The computer simulation findings were in good agreement with acid hydrolysis experiments on LDPE/starch blends. These experiments showed that below an apparent percolation threshold of 30% by volume (about 40% by weight) of starch, only small amounts of it are accessible for removal. These starch areas lie mostly on the surface. Above this threshold, however, starch could be very effectively extracted. As seen in Fig. 10, the weight loss increases dramatically for blends containing 30 wt% starch. These findings are in good agreement with percolation theory.

An examination of the weight loss curves for all blends shows that the greatest part of the weight loss occurs between the first and the second month of burial, with a lower loss rate thereafter. Though starch is fully biodegradable, a time period greater than six months is required for total consumption since it is covered and protected from the LDPE matrix. In blends containing the same amount of starch but different amounts of compatibilizer, the weight loss decreases with increasing amounts of EVA, especially for blends containing 50% EVA [ILLUSTRATION FOR FIGURE 13 OMITTED]. This inhibiting effect of EVA copolymer may be due to the hydrogen bonding between the carbonyl groups of EVA and hydroxyl groups of PLST. A similar inhibition effect was also observed in LDPE/starch blends containing ethylene/acrylic acid copolymer as compatibilizer (40).

Comparing weight loss in blends containing PLST and FILST (with 25 wt% EVA) it can be seen that PLST blends have higher weight losses [ILLUSTRATION FOR FIGURE 14 OMITTED]. This means that PLST is more accessible to microorganisms than FILST. The reason is that PLST produces more homogeneous blends with LDPE, and thus, possibly, a higher amount of starch is available to microorganisms. Another possible reason is the glycerine contained by PLST is very soluble in water, and its leaching contributes additionally to the weight loss.

Electronic microscopy confirms starch consumption, as clearly shown in Figs. 15 and 16 for blends with PLST and FILST, respectively.

Starch grains can be seen on the sample before burial. In the same sample, after only two months of burial these grains start to disappear because they are consumed by microorganisms. This leaves a film with a surface full of cavities, which become more abundant as the burial time increases. These film imperfections lead to a deterioration in mechanical properties and make the film fragile. This can be easily seen in elongation measurements of samples exposed to soft presented in Figs. 17-18.

It is observed that all samples, especially those showing higher elongation at break, exhibit a significant decrease in elongation within the first two months of soft burial with a much smaller decrease afterwards. This is due to starch consumption by microorganisms and the creation of holes. It is remarkable that even in the blends with lower amounts of PLST, the decrease in the elongation is very high. On the contrary, tensile strength properties are not seriously affected during this period for the same blends. A higher decrease is observed only in the blends containing 20 and 30 wt% of PLST, which lose [approximately]25-30% of the initial tensile strength [ILLUSTRATION FOR FIGURE 19 OMITTED].

From the above measurements it seems that starch consumption affects mainly the elongation-at-break properties. The tensile strength properties are affected to a much lower degree. As mentioned before, in blends with smiler mounts of starch, the stare consumption is restricted only to the film surface. Holes covering the whole mass of the films appear only in starch contents above 30 wt%, in good accordance with percolation theory (59).

CONCLUSIONS

From the experimental study we can conclude the following:

* Ethylene-co-vinyl acetate can be used as compatibilizer for LDPE/starch blends. The advantage is that when it is used in high concentrations, the elongation of the blends is superior even to that of LDPE. This means that higher amounts of PLST (20 and 30 wt%) can be used for blend preparation with satisfactory mechanical properties.

* LDPE/PLST blends exhibit mechanical properties superior to those with granular starch as filler.

* EVA copolymer has a small inhibiting effect on the biodegradation rate of these systems.

LIST OF NOMENCLATURE

LDPE Low Density Polyethylene

PLST Plasticized starch

FILST Granular starch

EVA Ethylene/vinyl acetate copolymer

FTIR Fourier transform infrared spectroscopy

DSC Differential scanning calorimetry

TGA Thermogravimetric analysis

PET Poly(ethylene terephthalate)

HDPE High density polyethylene

PVC Poly (vinyl chloride)

PP Polypropylene

SMA Styrene/maleic anhydride copolymer

EPMA Ethylene/propylene/maleic anhydride copolymer

EAA Ethylene/acrylic acid copolymer

SEM Scanning electron microscopy

PE Polyethylene

CP/MAS-NMR Cross polarization/magic angle spinning nuclear magnetic resonance

[[Epsilon].sub.c] elongation at break of the composite

[[Epsilon].sub.o] elongation at break of unfilled polymer

[[Sigma].sub.c] tensile strength of the composite

[[Sigma].sub.o] tensile strength of the unfilled polymer

[Phi] Volume fraction of filler

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Title Annotation:low-density polyethylene
Author:Prinos, J.; Bikiaris, D.; Theologidis, S.; Panayiotou, C.
Publication:Polymer Engineering and Science
Date:Jun 1, 1998
Words:5962
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