Polymers for Advanced Technologies 16 (2005) 310-317

POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2005; 16: 310–317

Published online 15 February 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.581

Effects of the compatibilizer PE-g-GMA on the

mechanical, thermal and morphological properties

of virgin and reprocessed LDPE/corn starch blends

A. G. Pedroso and D. S. Rosa*Laboratorio de Polımeros Biodegradaveis e Solucoes Ambientais, Programa de Pos-Graduacao Stricto Sensu em Engenharia e Ciencia dos

Materiais, Universidade Sao Francisco, Rua Alexandre Rodrigues Barbosa, no. 45, Centro, CEP 13251-900, Itatiba, SP, Brazil

Received 24 June 2004; Revised 17 September 2004; Accepted 6 October 2004

The effects of the compatibilizer polyethylene grafted with glycidyl methacrylate (PE-g-GMA) on

the properties of low-density polyethylene (LDPE) (virgin and reprocessed)/corn starch blends

were studied. LDPE (virgin and reprocessed)/corn starch blends containing 30, 40 and 50wt%

starch, with or without compatibilizer, were prepared by extrusion and characterized by the melt

flow index (MFI), tensile test, dynamic mechanical analysis (DMTA) and light microscopy. The

addition of starch to LDPE reduced the MFI values, the tensile strength and the elongation at break,

whereas the modulus increased. The decreases in the MFI and tensile properties were most evident

when 40 and 50wt% starch were added. Blends containing 3wt% PE-g-GMA had higher

tensile strength values and lower MFI values than blends without compatibilizer. Light

microscopy showed that increasing the starch content resulted in a continuous phase of starch.

Copyright # 2005 John Wiley & Sons, Ltd.

KEYWORDS: biodegradable; polyethylene (PE); blends; reactive processing; starch

INTRODUCTION

Low density polyethylene (LDPE), which is hard to degrade

in landfills, is currently one of the most commonly used ther-

moplastics for packaging material. For some short lifetime

applications such as bags, agricultural mulch films and

food packaging, it would be advantageous if plastics

degraded into safe by-products under normal composting

conditions.1 The resistance of polyethylene (PE) to biological

attack is related to its hydrophobicity, high molecular weight,

and lack of functional groups recognizable by microbial

enzymatic systems. All of these properties limit applications

in which biodegradability is a desirable attribute.2 The blend-

ing of biodegradable polymers, such as starch, with inert

polymers, such as PE, has received considerable attention

because of its possible applications in the waste disposal of

plastics. The reasoning behind this approach is that if the bio-

degradable component is present in sufficient amounts, and

if it is removed by microorganisms in the waste disposal

environment, then the plastic or film containing the remain-

ing inert components should disintegrate and disappear.3

Peanasky et al.4 investigated the accessibility of starch in PE

starch blends by computer simulation, percolation theory,

and acid hydrolysis experiments and concluded that the

accessibility of starch is highly dependent on an apparent

percolation threshold near 30% by volume or approximately

40% by weight of starch.

Starch is a natural polymer found in granular form in a

variety of plants, such as corn and cassava.5 Starch is a blend

of amylose and amylopectin, both of which are polysacchar-

ides composed of a-D-glucopyranosyl units, (C6H10O5)x.

Native starch granules swell after absorbing water through

hydrogen bonding with their free hydroxyl groups, but they

still retain their order and crystallinity. However, when

these swollen granules are heated, the hydrogen bonding

between adjacent glucose units is disrupted and the crystal-

linity is progressively destroyed. This process is called

gelatinization.6

Most synthetic polymers are immiscible with starch at the

molecular level. This thermodynamic incompatibility often

leads to poor performance of these blends.7 One way to

increase compatibility in starch blends is to use a compati-

bilizer containing groups capable of hydrogen bonding with

starch hydroxyls. An alternative approach is to form graft or

block copolymers in situ during the blend preparation by

using polymers containing reactive functional groups. This

method is commonly known as ‘‘reactive blending’’,7 in

which small amounts of block or graft copolymers formed

during the blending process, through reaction between the

two components, are generally enough to stabilize the

morphology and improve the properties of the blend.

Copyright # 2005 John Wiley & Sons, Ltd.

*Correspondence to: D. S. Rosa, Laboratorio de PolımerosBiodegradaveis e Solucoes Ambientais, Universidade SaoFrancisco, Rua Alexandre Rodrigues Barbosa, no. 45, Centro,CEP 13251-900, Itatiba, SP, Brazil.E-mail: [email protected]

Synthetic polymers with functional groups such as

carboxylic acid, anhydride, epoxy, urethane, or oxazoline,

can react with the hydroxyl or carboxyl group (in modified

starch) to form a blend with a stable morphology.7 Sailaja

et al.8 observed that the mechanical properties of LDPE/

tapioca starch blends improved significantly with the

addition of poly(ethylene-co-glycidyl methacrylate), app-

roaching values close to those of virgin LDPE. Jang et al.9

showed that high-density polyethylene (HDPE) modified

with glycidyl methacrylate (GMA) enhanced the miscibility

of HDPE/starch blends. In addition, according to Jane et al.,10

the carboxylic groups and the ketone groups of oxidized PE

react with the hydroxyl groups of starch to form bonds

among the components of the LDPE/starch blend.

In this study, the mechanical and thermal dynamic

mechanical properties, melt flow index (MFI) and morphol-

ogy of LDPE/starch blends containing high starch contents

(30, 40 and 50 wt%), with or without the compatibilizer PE

grafted with GMA (PE-g-GMA).

EXPERIMENTAL

MaterialsVirgin LDPE, PB 526, MFI 0.25 g/10 min (1908C, 2.16 kg), was

supplied by Braskem (Triunfo, RS, Brazil). Pre-gelatinized

corn starch (RD 337) was supplied in powder form by Corn

Products Brazil-Ingredientes Industriais Ltda. (Jundiaı, SP,

Brazil), and contained 27 wt% amylose and 73 wt% amylo-

pectin. Ethylene-co-glycidyl methacrylate copolymer con-

taining 8 wt% GMA (PE-g-GMA), Lotader AX8840-Elf

Atochem, MFI 5.0 g/10 min (1908C, 2.16 kg), was supplied

by Atofina (Sao Paulo, SP, Brazil).

LDPE reprocessingVirgin PB 526 LDPE, in pellet form, was extruded with

extruder CARNEVALLI CHD to obtain films 40mm thick.

The thermal profile was 175–191–198–198–190–201–2008C(zones 1–7). The film obtained was ground and subsequently

agglutinated using equipment with a rotating cylinder that

heated the ground plastic through friction transforming it

into a paste and then, a small quantity of water was added

to cause a sudden cooling.

Blend preparationPure virgin and reprocessed LDPE and their blends contain-

ing 30, 40 and 50 wt% starch were prepared with a co-rotating

twin-screw extruder (Werner & Pfleiderer, model ZSK 30).

The LDPE and starch were dried in an air-circulating oven

at 708C for 1 hr before extrusion. For the blends containing

compatibilizer, PE-g-GMA content was fixed at 3 wt% rela-

tive to the LDPE content and the compatibilizer was mixed

to the LDPE before extrusion. The LDPE was fed in the first

zone and the starch was fed in the third zone. The thermal

profile was 140–160–160–160–160–1508C (zones 1–6,

respectively) and the screw speed was 200 rpm. The extruded

materials were granulated.

MoldingVirgin and reprocessed LDPE and the blends were compres-

sion molded into sheets (180 mm� 180 mm� 2 mm) using a

model MA 098 Marconi Press (Marconi-Equipamentos e

Calibracao para Laboratorios, Piracicaba, SP, Brazil). The

mold containing the desired material was initially placed in

the press and heated for 3 min without applying any pres-

sure, in order to ensure uniform heat flow throughout the

material. For all of the blends, the temperature was kept at

170� 58C for 3 min at 10 t. The resulting sheets were cooled

to room temperature.

Melt flow index (MFI)MFI measurements of the LDPE and blends were obtained

using a model MI-1 plastometer (DSM Instrumentacao Cien-

tıfica Ltda., Sao Paulo, SP, Brazil), according to ASTM D 1238

(procedure A, 1908C/5 kg).

Tensile propertiesThe tensile properties were determined with an EMIC model

DL 2000 universal testing machine (EMIC Equipamentos e

Sistemas de Ensaio Ltda., Sao Jose dos Pinhais, PR, Brazil)

using specimens (type IV) cut from compression molded

sheets, according to the ASTM D-638 standard. The control

program used was Mtest LBP version 3.00 and the load cell

had a capacity of 200 kgf, at a rate of 20 mm/min. The average

and standard deviations of the tensile strength, elongation at

break and Young’s modulus were determined for each for-

mulation.

Dynamic mechanical thermal analysis (DMTA)Dynamic mechanical thermal analysis (DMTA) was done

using a dynamic mechanical analyzer, model MK III

(Rheometric Scientific, Inc., Piscataway, NJ, USA) over

the temperature range of �408C to 1108C at a frequency

of 1 Hz. The heating rate was 58C/min. The analyses were

done in duplicate using specimens 30 mm� 10 mm�1.5 mm.

Light microscopySpecimens were fractured after freezing in liquid nitrogen

and micrographs of the fractured surfaces were obtained

using a light microscope, model XP-500 (LABORANA, Sao

Paulo, SP, Brazil).

RESULTS AND DISCUSSION

Melt flow index (MFI)Figure 1 shows the MFI values for virgin and reprocessed

LDPE and their blends with starch, and Fig. 2 shows the effect

of compatibilizer on the MFI values for these blends.

The MFI Values for virgin and reprocessed pure LDPE

(1908C/5 kg) did not differ significantly. When starch was

added to the LDPE, the MFI values decreased with increasing

starch content, independently of the LDPE used in the blends.

Since the MFI is an indirect measurement of viscosity, it

follows that the starch must act as a rigid filler, because the

main effect of rigid fillers is to increase the elastic modulus of

a composite or the viscosity of a fluid suspension.11 For

blends containing 30 wt% starch, the decrease in the MFI was

more significant for virgin LDPE. For the blends containing

40 and 50 wt% starch, the MFI values decreased more

significantly when reprocessed LDPE was used, perhaps

Copyright # 2005 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2005; 16: 310–317

LDPE/corn starch blends 311

because of the higher interfacial interaction between repro-

cessed LDPE and starch. Since reprocessed LDPE underwent

previous processing, it was more susceptible to degradation

during processing. As a result, there was a greater tendency

to form carboxylic groups and ketone groups which could

react with the hydroxyl groups of starch, thereby promoting

bonds among the components of the LDPE/starch blend.10

The MFI values decreased as PE-g-GMA was added, which

implies that PE-g-GMA increased the extent of crosslinking

between the blend components. Jang et al.9 showed that the

modification of HDPE by the grafting of GMA was an

effective method for enhancing the compatibility of the

blend, and that the MFI of the blend decreased with the

content of grafted GMA, which implies that the degree of

crosslinking increased with the GMA content.

Tensile testsFigure 3 shows also evidence of possible degradation of

reprocessed LDPE since pure reprocessed LDPE and blends

with reprocessed LDPE had lower tensile strengths. For all

LDPE/starch formulations, the tensile strength decreased

with increasing starch content, indicating that corn starch

behaved as a non-reinforcing filler. For blends containing vir-

gin LDPE, the tensile strength decreased by 57, 60 and 74% for

the blends containing 30, 40 and 50 wt% starch, respectively,

in relation to pure LDPE. For blends containing reprocessed

LDPE, tensile strength decreased by 50, 59 and 76%, for the

blends containing 30, 40 and 50 wt%, respectively, compared

to pure reprocessed LDPE. A possible explanation for the

decrease in tensile strength after the addition of starch could

be the heterogeneous distribution of starch in LDPE and the

low interfacial interaction between components of the blend,

which resulted in mechanical rupture at the blend interface.

Figure 4 shows that the addition of PE-g-GMA increased

the tensile strength of all the blends, indicating that there was

a better interaction between the phases of the blend.

According to Jang et al.,9 the compatibilizer induces the

formation of a network, causing crosslinking that increases

the tensile strength of the blend. These results corroborate

with findings for the MFI, in which blends containing

compatibilizer showed lower MFI values (another indicator

of increased compatibility between the phases of the blend).

The increase in tensile strength was highly significant for

blends containing reprocessed LDPE, suggesting that the

carboxylic and ketone groups, that possibly formed during

the reprocessing and preparation of these blends, also

contributed to the greater compatibility of the blends.

Figure 5 shows Young’s modulus (E0) values for virgin and

reprocessed LDPE and their blends with starch LDPE. There

was no significant difference between the values for virgin

and reprocessed LDPE. When starch was added to LDPE, the

modulus showed high standard deviations and increased

when 30 wt% starch was added to virgin LDPE and when 30

and 40 wt% starch was added to reprocessed LDPE. In

general, the modulus is closely related to the hard domain of

the material.9 For starch contents higher than 30 wt% added

to virgin LDPE and for starch contents higher than 40 wt%

added to reprocessed LDPE, the modulus showed a tendency

Figure 1. MFI for virgin and reprocessed LDPE and LDPE/

starch blends.

Figure 2. MFI for LDPE and LDPE/starch blends, with and

without compatibilizer. (a) Virgin and (b) reprocessed.

Figure 3. Tensile strength for virgin and reprocessed LDPE

and LDPE/starch blends.

312 A. G. Pedroso and D. S. Rosa


to decrease, which could be attributed to a higher starch

agglomeration, where the lower rigidity of these agglomer-

ates prevailed.

Figure 6 shows that the addition of PE-g-GMA to blends

containing virgin LDPE, increased Young’s modulus only

when 50 wt% starch was added, while for blends containing

reprocessed LDPE, the addition of PE-g-GMA was very

effective at all starch contents. Again, this effect may reflect

the interaction of carboxylic and ketone groups that possibly

formed during reprocessing and preparation of the blends

with the starch, also increasing this compatibility.

Figure 7 shows the values of elongation at break for virgin

and reprocessed LDPE and their blends with starch. The

elongation at break was lower for pure reprocessed LDPE,

compared to pure virgin LDPE (Fig. 7a), since the degradation

induced by radical chain reactions caused crosslinking and

branching along polymer chains during reprocessing, which

led to a decrease in this property.12,13 Figure 7(b) shows that

the elongation at break decreased as the starch content

increased, and that blends containing reprocessed LDPE

had lower elongation at break values than those containing

virgin LDPE. The addition of starch granules to LDPE

produced the general trend for filler effects on polymer

properties, i.e. the modulus increased through stiffening of the

granules and elongation decreased as the starch content

increased.6 In synthetic polymer blends, the addition of a

second immiscible phase to a ductile matrix material usually

significantly diminishes the elongation properties at break. In

many cases when 20 wt% of the dispersed minor phase has

been added, highly deformable matrix materials are trans-

formed into fragile materials. In synthetic polymer blends

with a ductile matrix, the elongation at break is therefore

considered to be highly sensitive to the interfacial interaction

between the phases of the blend.6 In this work, starch contents

higher than 20 wt% were used which led to catastrophic

decrease in the elongation when the starch was added.

Figure 8 shows that the addition of PE-g-GMA increased the

elongation at break only for virgin LDPE/30 wt% starch and for

reprocessed LDPE/40 wt% starch blends. However, for all

starch blends, the values of elongation at break fell markedly

when compared to those of pure PE (Fig. 7a), and the small

differences among blends with different starch contents, with

and without compatibilizer, were insignificant.

Figure 4. Tensile strength for LDPE and LDPE/starch

blends, with and without compatibilizer. (a) Virgin and (b)

reprocessed.

Figure 5. Young’s modulus for virgin and reprocessed

LDPE and LDPE/starch blends.

Figure 6. Young’s modulus for LDPE and LDPE/starch


reprocessed.



Dynamic mechanical thermal analysis (DMTA)Based on the DMTA, the glass transition values (Tg) can be

defined as the temperature at which: (1) the loss tangent

(tan d) is maximum, (2) the loss modulus (E00) is maximum,

and (3) the inflexion point at which a significant drop in the

storage modulus (E0) occurs.14 The viscoelastic behavior of

the PEs and their blends was studied by DMTA.

Figure 9(a)–9(c) shows the E0 versus T, E00 versus T and tan

d versus T, respectively, for virgin and reprocessed PEs.

Figure 8. Elongation at break for LDPE and LDPE/starch


reprocessed.

Figure 9. DMTA curves for virgin and reprocessed LDPE.

(a) E 0 versus T, (b) E 00 versus T, (c) tan d versus T.

Figure 7. Elongation at break for virgin and reprocessed

LDPE and LDPE/starch blends.



Figure 9(a) shows that the E0 values for virgin and

reprocessed LDPE were practically the same, indicating that

reprocessing did not significantly change the material

rigidity, as also observed for Young’s modulus.

Figure 9(b) shows that virgin LDPE had a higher E00maximum

value compared to reprocessed LDPE, in contrast to the E0

versus T curves which showed no difference between virgin

and reprocessed LDPE. TheE00 value is related to loss of energy

as heat, i.e. to viscous behavior (irreversible deformation).

Therefore, the lower E00maximum value for reprocessed LDPE

implied a higher elastic recovery, which could be attributed to

the greater polymer rigidity probably caused by the cross-

linking that resulted from the degradation processes.

Considering Tg values as the temperature where E00 was

maximum, the PEs had a transition in the range of�13 to 08C,

which was attributed to b relaxation (relaxation of

branches).15 Although the PEs showed similar relaxation

ranges, theb relaxation of reprocessed LDPE tended to have a

lower temperature which was attributed to a decrease in

molecular weight caused by reprocessing.

Figure 10. DMTA curves for virgin and reprocessed LDPE and LDPE/starch blends. (a) E 0

versus T for virgin LDPE and virgin LDPE/starch blends, (b) E0 versus T for reprocessed LDPE

and reprocessed LDPE/starch blends, (c) E00 versus T for virgin LDPE and virgin LDPE/starch

blends, (d) E 00 versus T for reprocessed LDPE and reprocessed LDPE/starch blends, (e) tan dversus T for virgin LDPE virgin LDPE/starch blends, (f) tan d versus T for reprocessed LDPE and

reprocessed LDPE/starch blends.



Figure 9(c) shows the tan d versus T curves. The

temperature at which tan d was maximum was approxi-

mately 908C for both virgin and reprocessed LDPE. This

temperature is related to a relaxation, which has been

interpreted as relaxation of the constrained molecules with

reduced mobility located near crystallites.15

Figure 10(a) and 10(b) show that, in general, the addition of

starch to virgin and reprocessed LDPE resulted in a shift in

the E0 curves to higher temperatures, indicating that the

starch caused a decrease in polymer chain mobility. The E0

values increased with starch contents up to 40 wt%, whereas

at 50 wt%, theE0 values decreased. These results corroborated

the Young’s modulus results obtained in the tensile tests.

The E00 value decreased slowly up to �258C, after which, a

peak appeared (Fig. 10c and 10d). This peak was attributed to

b relaxation, which has properties of the glass–rubber

transition.15 In general, the addition of starch shifted the

E00maximum value to a higher temperature, indicating that

there was an increase in the relaxation temperature and that

this was caused by a decrease in chain mobility.

Figure 11. E 0 versus T curves for virgin and reprocessed LDPE and LDPE/starch blends, with

and without compatibilizer. (a) Virgin LDPE and virgin LDPE/30 wt% starch blends, (b)

reprocessed LDPE and reprocessed LDPE/30 wt% starch blends, (c) virgin LDPE and virgin

LDPE/40 wt% starch blends, (d) reprocessed LDPE and reprocessed LDPE/40 wt% starch

blends, (e) virgin LDPE and virgin LDPE/50 wt% starch blends, (f) reprocessed LDPE and

reprocessed LDPE/50 wt% starch blends.



The peaks where tan d was maximum showed broadening

and a small shift to the right when starch was added to LDPE

(Fig. 10c and 10f).

Figure 11(a) and 11(b) show theE0 versusT curves for virgin

and reprocessed LDPE and their blends with starch, with and

without PE-g-GMA. For blends containing virgin LDPE, the

addition of PE-g-GMA increased the E0 value only in the

presence of 30 wt% starch, indicating that there was an

improvement in the interfacial interaction between the phases

of the blend. For blends containing reprocessed LDPE, the

addition of PE-g-GMA was efficient for all starch contents,

indicating an improvement in the interfacial interaction

between the LDPE and starch.

Light microscopyLight microscopy showed that increasing the starch content

resulted in a continuous phase of starch. This is important

when producing biodegradable blends since, according to

Zuchowska et al.,16 an increase in the content of the continu-

ous starch phase improves the accessibility of the polymer

matrix to different degrading factors. It was not possible to

observe any difference among blends wthout compatibilizer

and blends with compatibilzer by light microscopy analysis

(Fig. 12).

CONCLUSION

Blends of virgin or reprocessed LDPE containing 30, 40 and

50 wt% starch were prepared. The addition of starch to

LDPE reduced the MFI values, the tensile strength and the

elongation at break, whereas the modulus increased. The

decreases in the MFI and tensile properties were most evident

when 40 and 50 wt% starch was added. The addition of the

compatibilizer PE-g-GMA decreased the MFI values and

increased the tensile strength in all blends, indicating an

improved interfacial interaction between the phases of

the blends. However, the effect of the compatibilizer on the

tensile strength was more efficient for blends containing

reprocessed LDPE.

AcknowledgementsThe authors thank Brasken, EcoQuali and Corn Products

Brazil-Ingredientes Industriais Ltda. for supplying the virgin

LDPE, recycled LDPE and starch, respectively. This work

was supported by FAPESP (grants 99/10716-4 and 02/

06803-3), CNPq (grant 303500/2002-6) and Universidade

Sao Francisco.

REFERENCES

1. Aburto J, Thiebaud S, Alric I, Borredon E, Bikiaris D, PrinosJ, Panayiotou C. Carbohydr. Polym. 1997; 34: 101–112.

2. Chiellini E, Corti A, Swift G. Polym. Degrad. Stab. 2003; 81:341–351.

3. Chandra R, Rustgi R. Biodegradable polymers. Prog. Polym.Sci. 1998; 23: 1273–1335.

4. Peanasky JS, Long JM, Wool RP. J. Polym. Sci., Part B: Polym.Phys. 1991; 29: 565–579.

5. Carioca JOB, Arora HL, Selvam PVP, Tavares FCA,Kennedy JF. Starch/Starke 1996; 48: 322–326.

6. St-Pierre N, Favis BD, Ramsay BA, Ramsay JA, Verhoogt H.Polymer 1997; 38: 647–655.

7. Vaidya UR, Bhattacharya M. J. Appl. Polym. Sci. 1994; 52:617–628.

8. Sailaja RRN, Reddy AP, Chanda M. Polym. Int. 2001; 50:1352–1359.

9. Jang BC, Huh SY, Jang JG, Bae YC. J. Appl. Polym. Sci. 2001;82: 3313–3320.

10. Jane JL, Schwabacher AWN, Ramrattan SN, Moore JA. USPatent 5,115,000, 1992 (cited in ref. 6).

11. Nielsen LE. Mechanical Properties of Polymers and Composites,vol. 1. Marcel Dekker: New York, 1974; 255.

12. Kartalis CN, Papaspyrides CD, Pfaendner R. Polym. Degrad.Stab. 2000; 70: 189–197.

13. Dintcheva NT, La Mantia FP, Acierno D, Di Maio L, CaminoG, Trotta F, Luda MP, Paci M. Polym. Degrad. Stab. 2001; 72:141–146.

14. Nielsen LE. Mechanical Properties of Polymers and Composites,vol. 2. Marcel Dekker: New York, 1974; chapt. 7.

15. Bikiaris D, Aburto J, Alric I, Borredon E, Botev M,Betchev C, Panayiotou C. J. Appl. Polym. Sci. 1999; 71:1089–1100.

16. Zuchowska D, Steller R, Meissner W. Polym. Degrad. Stab.1998; 60: 471–480.

Figure 12. Light micrographs for virgin LDPE/starch blends:

(a) 70/30; (b) 60/40; (c) 50/50 (30�).



Documents

Polymers for Advanced Technologies 16 (2005) 310-317