polyethylene blends

polyethylene blends

Polymer Degradation and Stability 87 (2005) 395e401 www.elsevier.com/locate/polydegstab Preparation and characterization of compatible thermoplastic ...

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Polymer Degradation and Stability 87 (2005) 395e401 www.elsevier.com/locate/polydegstab

Preparation and characterization of compatible thermoplastic starch/polyethylene blends Wang Shujun, Yu Jiugao*, Yu Jinglin School of Science, Tianjin University, Tianjin 300072, China Received 8 April 2004; received in revised form 19 August 2004; accepted 27 August 2004 Available online 16 December 2004

Abstract Compatible thermoplastic starch (TPS)/linear low density polyethylene (LLDPE) blends were prepared by one-step reactive extrusion in a single-screw extruder and characterized by means of mechanical properties, thermogravimetry (TG), scanning electron microscopy (SEM), rheological properties and Fourier transform infrared spectroscopy (FTIR). The thermal plasticisation of starch and its compatibilising modification with polyethylene were accomplished simultaneously in a single-screw extruder. The FTIR analysis showed that maleic anhydride (MAH) could graft onto the polyethylene chain using the same operational conditions as the preparation of the blends. The blends with MAH had better mechanical properties than the blends without MAH. From TG analysis, we concluded that the thermal stability of the compatible blends was higher than the incompatible blends. The FTIR results also indicated that with the addition of MAH, the compatibility between thermoplastic starch and polyethylene in the blends was truly improved. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Thermoplastic starch (TPS); Polyethylene (PE); Maleic anhydride (MAH); One-step reactive extrusion; Compatibility

1. Introduction Starch is a highly hydrophilic macromolecule, while polyethylene is non-polar and hydrophobic. Starch is often used as the degradable additive in the preparation of biodegradable polyethylene film. However, the great difference in their properties results in poor compatibility of starch/polyethylene blends. To date, many researchers have done much work to improve the compatibility of the blends, such as the modification of starch [1e4], the modification of polyethylene [5,6] and/or the introduction of compatibiliser [7e12] into the blends of starch and polyethylene. These compatibilisers

* Corresponding author. Tel.: C86 22 27406144; fax: C86 22 27403475. E-mail address: [email protected] (Y. Jiugao). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.08.012

contain ethyleneeacrylic acid (EAA) copolymer, ethyleneemaleic anhydride (MAH) copolymer, ethylenee vinyl alcohol (EVA) copolymer etc. Of all these compatibilisers, EVA is a highly hydrophilic polymer, which restricted its application in moist environments. EAA is a very effective compatibiliser between starch and polyethylene, but only a high content of EAA could result in good compatibility. However, the higher the EAA content was, the lower was the biodegradability. So far, PE-g-MAH was considered as the most effective compatibiliser between starch and polyethylene. But, its price is high and the manufacture is difficult. In the present study, we adopted one-step reactive extrusion [13,14] to prepare compatible thermoplastic starch and polyethylene blends. The thermal plasticisation of starch and its compatibilising modification with polyethylene were accomplished simultaneously

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by one-step reactive extrusion in a single-screw extruder. We prepared compatible thermoplastic starch/polyethylene blends with good properties. The main objective was to prepare and characterize the compatibility of the blends.

2. Experimental 2.1. Materials The linear low density polyethylene (LLDPE), 7042, was purchased from Jilin Petrochemical Filiale (Jilin, China). The native corn starch (ST, 11% moisture) containing 30 wt% amylose and 70 wt% amylopectin was obtained from Langfang Starch Company (Langfang, Hebei, China). The plasticiser, glycerol, was purchased from Tianjin Chemical Reagent Factory (Tianjin, China). Maleic anhydride (MAH), was purchased from Tianjin Chemical Reagent Factory (Tianjin, China), and was recrystallised twice from CHCl3 before use. Dicumyl peroxide (DCP) was obtained from Shanghai Chemical Reagent company, China Pharmacy Group (Shanghai, China), and was recrystallised from absolute alcohol before use. 2.2. Sample preparation Blending was carried out by the use of a GH-100Y mixer (made in China) at room temperature. The rotor rate was maintained at 3000 rpm for 2 min, adding polyethylene and starch first, then adding glycerol, MAH and DCP. The mixtures were manually fed into a laboratory scale single-screw extruder (SJ-25(S), screw diameter, d Z 30 mm, length-to-diameter ratio, l/d Z 25:1, made in China). The extrusion conditions were as follows: temperature profile along the extruder barrel: 140e145e150e130  C (from feed zone to die); the screw speed was 15 rpm. The die was a round sheet with 3 mm diameter holes. For TPS (blend of starch with glycerol)/PE blends, five different levels of TPS were used, namely, 50, 60, 70, 80 and 90 wt%. MAH and DCP were used as monomer and initiator at a 1 wt% and 0.1 wt% level based on polyethylene. Some extrudates were heat pressed into thin films at 110e120  C and 10 kN for 15 min, in order to test the dynamic thermal mechanical properties and FTIR properties. The level of glycerol in the blends was 30 wt% based on the native corn starch. The detailed constituents of samples are listed in Table 1. 2.3. Characterization 2.3.1. Fourier transform infrared (FTIR) spectroscopy The IR spectra were measured with BIO-RAD FTS3000 IR Spectrometer. The extruded TPS strips

Table 1 Sample code and constituents Sample code 1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11# 12# 13# 14# 15#

Raw materials DCP

PE

ST

GL

MAH

50 40 30 20 10 50 40 30 20 10 50 40 30 20 10

38.5 46.2 53.8 61.5 69.2 38.08 45.81 53.59 61.37 69.15 37.65 45.51 53.36 61.22 69.07

11.5 13.8 16.2 18.5 20.8 11.37 13.75 16.08 18.41 20.74 11.30 13.65 16.01 18.36 20.72

e e e e e 0.5 0.4 0.3 0.2 0.1 1 0.8 0.6 0.4 0.2

e e e e e 0.05 0.04 0.03 0.02 0.01 0.05 0.04 0.03 0.02 0.01

were pressed to the transparent slices with a thickness of around 0.2 mm and tested in transmission.

2.3.2. Mechanical properties of blends Measurements of the mechanical properties such as tensile strength and elongation at break were performed according to ASTM (D828-88) (ASTM, 1989) method on a WD-5 electronic tester. Measurements were done using a 100 mm/min crosshead speed. Prior to the measurement, the samples were conditioned at a relative humidity of 50 G 5% for 48 h at an ambient temperature in a closed chamber containing 33.46 wt% CaCl2 solution in a beaker. Ten measurements were conducted for each sample and the results were averaged to obtain a mean value.

2.3.3. Themogravimetric analysis (TGA) The thermal properties of the blends were measured with a ZTY-ZP type thermal analyser. The sample weight varied from 10 to 15 mg. The samples were heated from ambient temperature to 500  C at a heating rate of 15  C/min. The derivatives of TGA thermograms were obtained using origin 6.0 analysis software.

2.3.4. Scanning electron microscopy (SEM) Specimens were fractured after freezing in liquid nitrogen and the exposed surfaces were observed with environmental electron microscope (ESEM, Philips XL-3). All surfaces were coated with gold to avoid charging under the electron beam. The electron gun voltage was set at 30 kV. The micrographs of samples were taken at magnifications !500 to identify cracks, holes and other changes on the surface of the samples in the presence of or in the absence of MAH.

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3. Results and discussion 3.1. Grafting reaction In order to verify the occurrence of the grafting reaction (MAH grafting onto polyethylene chain) during the blending extrusion, we performed an experiment where, at the same extruder parameters (temperature profile along the extruder barrel:140e145e150e130  C (from feed zone to die); the screw speed was 15 rpm), polyethylene and MAH were extruded in the presence of DCP in the single-screw extruder. The extrudate was purified in order to remove the unreacted MAH and small molecules. The purification method [15] was by dissolution of modified polyethylene in xylene followed by precipitation in acetone. The FTIR spectra of the purified product and pure polyethylene are shown in Fig. 1. By comparing the pure LLDPE with the grafted product, we observed the symmetrical and asymmetrical stretching vibration bonds of anhydride groups at 1869 cmÿ1 and 1791 cmÿ1, respectively. This verified the fact that MAH has grafted onto LLDPE in the presence of DCP, and could be used as compatibiliser in the TPS/ LLDPE blends during the extrusion. 3.2. Mechanical properties Figs. 2 and 3 show the effect of TPS content on the tensile strength and elongation at break of samples. The variation in the tensile strength and elongation at break of samples with MAH content is also shown in these figures. As observed from Figs. 2 and 3, tensile strength and elongation at break decreased with increase in TPS

a

1869cm-1

b

1791.87

1791cm-1 2400

2200

2000

1800

1600

Wavenumbers(cm-1) Fig. 1. FTIR spectrum of (a): PE; (b): PE-g-MAH.

content for all samples, regardless of the presence of MAH. Generally speaking, the starch granule is highly hydrophilic, containing hydroxyl groups on its surface, whereas LLDPE is mostly non-polar. Therefore, the strong interfacial bonds such as hydrogen bonds between starch and LLDPE are not produced and the mechanical properties of the TPS/LLDPE blends were rather poor. However, the blends with MAH had better mechanical properties than the blends without MAH. Moreover, the mechanical properties of the blends decreased with the increase in the MAH content. In the presence of DCP, the introduction of MAH into the TPS/LLDPE blends really strengthened TPS and LLDPE interfacial adhesion and improved the mechanical properties of the blends. The improvement in the mechanical properties was attributed to the formation of PE-g-MAH during one-step extrusion. However, further increase in the MAH content reduced the tensile strength and the elongation at break. With the content of MAH increasing, it would destroy TPS and LLDPE

9.0

Sample1-sample5 Sample6-sample10 Sample11-sample15

8.5 8.0

Tensile strength / MPa

2.3.5. Rheology The extruded strips were cut into small pieces, which were tested by XYL-II capillary rheometer. The radius of the capillary was 1 mm and l/d was 40. The small pieces were placed into the barrel through a funnel and then packed down with the plunger until the first extrudate appeared at the capillary exit. The samples were allowed to come to temperature (balancing for 10e15 min), and were then forced through the capillary by the plunger at pre-selected velocities. The next velocity in the measure schedule begins when the load versus extension curves reaches a slope close to zero. The load on the plunger and plunger speed provide the total pressure drop through the barrel and capillary and the volume flow rate. Shear rate (g) and shear stress (t) were calculated by stand methods. In order to understand the TPS/LLDPE blends processing properties, the rheology experiments were carried out at 125  C, which covered the processing temperature range.

7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5

50

60

70

80

TPS content / Fig. 2. Tensile strength of the blends.

90

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W. Shujun et al. / Polymer Degradation and Stability 87 (2005) 395e401 700 100

Sample1-sample5 Sample6-sample10 Sample11-sample15

600

Weight loss/

Elongation at break /

80

500 400 300 200

1

1:sample 9

60

2:sample 14

2

3:sample 4

3

40

20

100 0

50

60

70

80

0

90

100

200

300

400

500

600

Temperature / °C

TPS content / Fig. 3. Elongation at break of the blends.

Fig. 5. The TG curves of the blends.

heavily under high temperature and high pressure, thus impairing the mechanical properties of the final product.

As can be seen from the above figures, the thermal stability of the blends with MAH was higher than the blends without MAH. Take Fig. 5 for example, the total weight loss of sample 4 at 400  C was 78 wt%. The weight loss data were as much as in agreement with TPS content in the blend. However, the weight loss data reduced to 68 wt% and 49 wt%, corresponding to sample 14 and sample 9. The decrease in the weight loss was due to the improvement in thermal stability of the blends with MAH. In the presence of DCP, the improvement in the thermal stability of the blends with MAH was due to the occurrence of the grafting reaction during the extrusion in a single-screw extruder. The graft copolymer produced in one-step reactive extrusion brought about a steady interfacial layer and improved the interfacial adhesion between TPS and LLPE. But then, the thermal stability of the blends decreased with the increase in MAH content. Maybe this was due to the

3.3. Thermal stability The TG thermograms of samples 5, 10, 15, samples 4, 9, 14, samples 3, 8, 13 and samples 2, 7, 12 are presented in Figs. 4e7. From the above figures, there were three well-defined shifts in the TG curves of the blends. The first shift, at around 100  C, was produced by water evaporation or the unreacted MAH sublimation; the second shift started at 180  C, and it was due to the evaporation of glycerol. This process continued gradually up to 300  C where the thermal degradation of starch occurred; the last shift, at around 400  C, was caused by the thermal decomposition of LLDPE.

100

100

1 2 3

60

80

Weight loss/

Weight loss/

80

1:sample 10 2:sample 15 3:sample 5

40

1:sample8

60

2:sample13

1

3:sample3

2

40

3

20 20 0 0

100

200

300

400

Temperature / °C Fig. 4. The TG curves of the blends.

500

0

0

100

200

300

400

Temperature / °C Fig. 6. The TG curves of the blends.

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showed that the morphology of the blends with MAH was obviously improved due to the improvement in the compatibility between TPS and LLDPE. With increasing content of MAH, the morphology of the blends (Fig. 8c,f,i) was somewhat changed. But the morphology of the blends (Fig. 8c,f,i) was still better than that of the blends without MAH (Fig. 8a,d,g).

100

Weight loss/

80 1

1:sample 7

60

2 3

2:sample 12 3:sample 2

3.5. Rheology

40

20

0

0

100

399

200

300

400

500

Temperature / °C Fig. 7. The TG curves of the blends.

destruction of MAH on TPS and polyethylene at high temperature and pressure during one-step reactive extrusion. Overall, the addition of MAH improved the compatibility of the TPS/LLDPE blends in the presence of DCP. 3.4. Blend morphology Morphology of the polymer blends plays an important role in the properties of the final product, especially their mechanical properties depend on it. In most cases, the major components of the blends form the continuous phase, whereas the minor component is the dispersed phase. However, as the volume fraction of the minor component increases to a certain volume, it will transfer from the dispersed phase to the continuous phase [16]. In our present study, for a blend with high starch content, starch was expected to be the continuous phase and LLDPE was the dispersed phase. To see the interfacial structure between the matrix and the dispersed phase, SEM photographs of fracture surfaces were obtained and are presented in Fig. 8. Fig. 8a,d,g shows the SEM photographs of TPS/ LLDPE blends without MAH. The starch particles were not completely demolished and some of them were removed from the surface of the blends during the fracture of the specimen, leaving some cavities in the fracture surface. The LLDPE was dispersed in a disorderly way on the surface of the starch particles. The interfacial adhesion between TPS and LLDPE was poor, and there was obvious phase-separation. With the addition of MAH (Fig. 8b,e,h), the starch particles in the blends were almost completely destroyed and plasticised very well. TPS and LLDPE combined a continuous phase in which starch particles and LLDPE fibre disappeared. There was also no apparent phase interface between TPS and LLDPE. The result

The data for the shear stress and the apparent viscosity as a function of shear rate are shown in Figs. 9 and 10. As obtained from Figs. 9 and 10, the apparent viscosity of all the three samples decreased with the increase in shear rate, and such flow behaviour is called shear thinning [17]. The samples exhibited power-law behaviour, which was ascribed to the gradual destruction of the intermolecular action of starch and LLDPE. The apparent viscosity of samples 8 and 13 was lower than that of sample 3 at the same shear rate, which was closely related to the molecular realignment. For sample 3, the compatibility of TPS and LLDPE was poor. The rigid starch particles which were not plasticised and the molecular orientation prevented the melt blends from flowing smoothly at the experimental temperature. However, for samples 8 and 13, the arrangement of molecules was more orderly than in sample 3 because of good plasticisation of starch and good compatibility between TPS and LLDPE. Consequently, the flow friction of the blend melt reduced, thus producing the lower apparent viscosity. The viscosity of sample 8 was higher than that of sample 13 as a result of the heavy destruction of MAH acting on the TPS and LLDPE. 3.6. Fourier transform infrared (FTIR) spectroscopy The compatibility of the polymer blends was also characterized by FTIR. For the compatible polymer blends, the FTIR spectrum band would deviate greatly or obviously displace (the band’s frequency shifting and the peak style asymmetrically widening) compared with that of the single polymer component. However, for the completely incompatible polymer blends, the characteristic absorption spectrum would overlap perfectly compared with the single polymer component [18]. As can be seen from Fig. 11, there were three characteristic peaks of starch between 1019 cmÿ1 and 1156 cmÿ1, attributed to CeOeC bond stretching, and the peak near 1019 cmÿ1 was a characteristic peak of the anhydroglucose ring OeC stretch [19]. Within the spectrum, the peak shape and location of the blend without MAH (sample 1) changed less. However, with the addition of MAH into the blend (sample 6), the peak shape and location of the blend changed distinctly. The

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Fig. 8. The SEM photographs of the TPS/polyethylene blends (a): sample 1; (b): sample 6; (c): sample 11; (d): sample 3; (e): sample 8; (f): sample 13; (g): sample 5; (h): sample 10; (i): sample 15.

peak situation shifted from 1019 cmÿ1 in starch spectrum to 1012 cmÿ1 in the blend spectrum, the peak style was also sharper. According to the mechanism of compatibility, the better the compatibility of the poly-

mer blend, the more the correlative peaks shifted and the peak shapes changed. This phenomenon indicated that the blends with MAH had better compatibility than those without MAH. 50000

0.16

sample 3

sample 3 sample 8

0.14

sample 8

40000

sample 13

sample 13

0.12 30000

/ MPa

0.10 0.08

20000

0.06 10000

0.04 0.02

0

0

20

40

60

80

100

120

/ S–1 Fig. 9. The tuegu curves of samples at 125  C.

140

160

0

20

40

60

80

100

120

/ S–1 Fig. 10. The haegu curves of samples at 125  C.

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properties and rheology properties compared with the blends without MAH. The improvement in the properties of the blends was attributed to the increase in the compatibility of TPS/LLDPE. The characteristic IR peaks of starch and LLDPE in the blends with MAH shifted more greatly than the blends without MAH. All of these demonstrated that in the presence of DCP, the introduction of MAH into the TPS/LLDPE blends improved the compatibility between TPS and LLDPE.

PE

sample 1 sample 6

817

1263 1261 1146

starch

1089 1019

401

798

1258 1078

790

1012

References

1156 1081 1019

1400

1200

1000

800

600

Wavenumbers/ cm-1 Fig. 11. FTIR spectra of the PE, starch and blends.

In addition, the two peaks of starch in 1081 cmÿ1 and 1156 cmÿ1, attributed to CeOeH bond stretching, also changed greatly. In the blend without MAH (sample 1), there was only one characteristic peak at 1089 cmÿ1. Maybe this was due to the weak hydrogen bond between starch and glycerol. However, in the blend with MAH (sample 6), these two peaks moved to lower wave number due to the strong hydrogen bonds between starch and glycerol (or MAH/PE-g-MAH). This was also the result of compatibility improvement in the blends with MAH.

4. Conclusion As can be concluded from the above discussion, we could prepare compatible TPS/LLDPE blends by one-step reactive extrusion in a single-screw extruder. The TPS/LLDPE blends with MAH showed better mechanical properties, morphology properties, thermal

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