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«Unmodified versus organo-modified clays - their effect on thermoplastic cellulose and starch esters M. Hassan Nejad, J. Ganster, H.-P. Fink ...»

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Unmodified versus organo-modified clays - their effect on thermoplastic cellulose

and starch esters

M. Hassan Nejad, J. Ganster, H.-P. Fink

Fraunhofer Institute for Applied Polymer Research, Geiselbergstraße 69, 14476 Potsdam

Abstract

Starch derivatives (starch acetate (SA), starch propionate (SP), starch acetate propionate

laurate (StAcPrLau) and starch propionate acetate laurate (StPrAcLau)) and cellulose acetate (CA) / clay

nanocomposites were prepared through melt antercalation. Three organo-modified clays and two unmodified clays with different percentage of triacetin (TA) as the plasticizer (with starch derivatives) were used. The effect of clays on the tensile, dynamic mechanical (DMA) and impact properties of nanocomposites were studied. The dispersion of silicate layers in the polymer matrix was characterized by transmission electron microscopy (TEM).

It was observed that certain unmodified clay (Dellite LVF) significantly improved the elongation at break of plasticized SA and StAcPrLau. Adding organo-modified clay (Dellite 67G) into the plasticized SP and StPrAcLau not only performed the tensile strength and Young’s modulus but also restrained the elongation at break and consequently boosted the impact strength of StPrAcLau considerably.

Incorporating the Dellite LVF, unmodified clay into the plasticizer free cellulose acetate (CA) improved the mechanical properties significantly with a novel core/shell structure. The high molecular orientation in this kind of structure is likely to be the reason for the unusually high mechanical properties. So that, in optimum percentage –5 wt%- of this clay, tensile strength and young’s modulus increased by 335 % and 100 %, to 178 MPa and 8.4 GPa, respectively.

Gel permeation chromatography (GPC) results show that the use of this kind of clay reduces molecular degradation compared to the unfilled system. The heat distortion temperature HDT-A of these claysystems reached values as high as 180 °C.

Keywords: Starch derivatives, Cellulose acetate, nanocomposite, Mechanical properties, Molecular degradation Introduction Recently, there has been considerable interest to develop bio-based polymers, which can be substitute of synthetic plastics for environmental enhancement.

Prominent examples for these so-called bioplastics are cellulose esters, which are mainly used in fiber, film and filter tow industries, thermoplastic starch (TPS) used as film and packaging materials, polylactic acid (PLA) for bottle applications and woven shirts, and polyhydroxyalkanoates (PHA) for packaging materials.

Although various types of nano reinforcements are currently being developed, the main focus in this field is on the use of layered silicate clay for its easy availability, low cost and more importantly environmentally friendliness. There are several attempts to improve the properties of starch1-12 and CA13, 14 by using layered silicate clay.

By compounding organo-modified MMTs and unmodified MMTs with plasticized SP, SA, StAcPrLau and StPrAcLau and plasticizer free CA via a melt extrusion, we hoped to improve the properties. This paper reports the results of optimized processing conditions varying the amount of plasticizer and different types of organo-modified and unmodified MMTs. Tensile properties, DMA, charpy impact, TEM, scanning electron microscopy (SEM) and GPC of the resulting plasticized starch derivatives and CA nanocomposites were used to evaluate the successful preparation.

Materials SP with a degree of substitution (DS) of 2.45, SA with DS = 2.60, StAcPrLau (DSAc DSProp DSLAu) with DSAc = 2.27, DSProp = 0.63 and DSLau = 0.1 and StPrAcLau (DSProp DSAc DSLAu) with DSAc=0.59, DSPr=2.31 and DSLau=0.1 were synthesized in this institute from high amylose (50 wt%) maize starch and CA (DSoverall=2.63, DS2=0.82, and DS6=0.79), without additives in powder form was used. 1,2,3triacetoxypropane (triacetin or glycerin triacetate) was used as the plasticizer purchased from Aldrich with 99 % purity. Five kinds of MMT were purchased from LAVIOSA S.p.A.; tree organically modified MMT (Dellite 43B, Dellite 67G and Dellite 72T) and two unmodified MMT (Dellite LVF, cation exchange capacity (CEC) = 105 milliequi/100 gram and Dellite HPS, CEC = 128 milliequi/100 gram). The ammonium cations of the organo-modified clays are respectively dimethyl-benzyl-dihydrogenated tallow ammonium for the Dellite 43B, dimethyl-dihydrogenated tallow ammonium for Dellite 67G and Dellite 72T.

Melt compounding and injection molding The starch derivatives, CA and clays were dried in oven at 80 °C for at least 24 h before use. Certain amounts of different kinds of clays were immerged into the TA corresponding to the weight of starch derivatives for 24 h. The starch derivatives and mixture of TA and clays were mixed mechanically with a high-speed mixer for about 5 min and then stored in sealed polyethylene bags for 24 h prior to further processing. The preplasticized mixture of starch derivatives or solid mixture of CA and clays was then homogenized in a HAAKE kneader at a temperature of 140 to 170 °C (for starch derivatives) and 230 °C (for CA) and a speed of 50 to 100 rpm. The mixtures were then melt compounded in a HAAKE minilab twin-screw extruder at a temperature of 160 to 180 °C and a speed of 250 rpm in starch derivatives cases and 235 °C and a speed of 100 rpm in CA case. Finally, dumb-bell shape standard test specimens were injection moulded with a HAAKE minijet according to ISO 527, type 5 A. Prepared dumb-bell specimens were put in a climate room with a temperature of 23 °C and a relative humidity of 50 % for 24 h prior to tensile testing.





Characterization Tensile properties of samples were measured using a Zwick 1445 universal testing machine at 23 °C and 50 % relative humidity. Initial clamp separation and crosshead speed were 44 mm and 22 mm/min, respectively. All measurements were performed at least for six replicated dumb-bell shaped specimens and averaged. DMA was carried out by using a TA Instrument DMA 2980 in a single cantilever mode from

-30 to 200 °C (250 °C for CA) with a frequency of 1 Hz and a heating rate of 2 K/min. During testing dynamic mechanical property parameters of storage modulus and loss factor (tan δ) were recorded as a function of temperature. Charpy impact tests were carried out on unnotched specimens with pendulum impact tester PSW 1 Joule type, from the company Wolfgang Ohst Rathenow, Germany. HDT measurements have been done by using a TA Instrument DMA 2980,with a load of 1.8 MPa according to EN ISO 75 (without oil bath).

TEM images were taken from cryogenically microtomed ultrathin sections (60 nm) using a Phillips CM 200 at an acceleration voltage of 120 kV. SEM images were taken from JSM 6330 F (Jeol, Japan).

The number-average (Mn) and weight-average (Mw) molecular weights were determined by GPC with three columns in series and calibrated with polystyrene standards. The standards and the columns were both supplied by polymer Laboratories, Ltd. The composites were eluted with dichloromethane/Methanol 90/10 (vol%/vol%) and were detected with a dual absorbance detector 2487, Waters.

Results and discussion

Starch derivative nanocomposites In this part the effect of nano-clays on the mechanical properties of plasticized SA and StAcPrLau are presented. Being the best performing compositions, SA with 20 wt% TA and StAcPrLau with 15 wt% TA were selected to discus here.

Figure 1 shows the tensile properties of plasticized SA with 20 wt% of TA compounded with 5 wt% of Dellite 43B, Dellite 72T, Dellite 67G, Dellite HPS, and Dellite LVF. Obviously, all organo-modified clays improved the tensile strengths and modulus. For example, by adding 5 wt% of Dellite 43B, tensile strength and modulus increased by 40 % to 33.0 MPa and 2 GPa, respectively, in agreement with very good dispersion and partial exfoliation that can be seen from TEM micrographs (see below). However, tensile strengths and modulus enhanced but elongation remained at low values of about 3 %. Dellite HPS, unmodified MMT with higher CEC, improved a little all properties yet did not show considerable improvement for elongation at break. The best result was achieved in the composition of SA / 20 wt% TA / 5 wt% Dellite LVF.

Figure 1 Effect of different types of MMT on tensile properties of plasticized SA.

This unmodified MMT with lower CEC indicated significant improvement especially in elongation at break.

This clay improved properties compared to plasticized SA with 20 wt% TA by 30 %, 40 %, and 1000 %, tensile strength, modulus, and elongation, respectively. It means that in this composition and with this clay not only the tensile properties were improved but also a tougher and more ductile starch-based nanocomposite was produced. It seems that there is a specific composition to achieve this unexpected behavior. Since, incorporating lower and higher amount of TA and also lower amount of LVF did not show the same effect. These results are not in agreement with TEM micrographs that showed no intercalation and exfoliation.

As it can be seen from the mechanical properties of SA nanocomposites, organo-modified Dellite 43B and unmodified Dellite LVF indicated better mechanical bahavior. For this reason, the effect of these two clays on the mechanical properties of StAcPrLau was investigated.

As it is shown in Figure 2, incorporating of nanoclays to plasticized mixed ester starches resulted in higher young’s modulus especially in the presemse of organo-modified clay, Dellite 43B. This is because of better affinity of hydrophobic clay and hydrophobic matrix, which result in good dispersion and partially exfoliation in the polymer matrix.

Existences of nanoclays in nanocomposites normally diminish the elongation at break for their high aspect ratio, which lead to higher matrix rigidity.15 Unexpectedly, adding 5 wt% of Dellite LVF into the StAcPrLau / 15 wt% TA composite not only improved the tensile strength and young’s modulus by 10% and 9%, respectively but also increased the elongation at break by 371%. This behavior was also observed with SA. It seems there is a certain concentration of TA in rich acetylated starch, which by incorporating Dellite LVF not only tensile strength and young’s modulus increase but also elongation at break considerably improved. This tendency shows that extent of plasticization leads to greater chains mobility of rich acetate starch.

Figure 2 Effect of different types of MMT on tensile properties of plasticized StAcPrLau.

Pandey16 explained this improvement in strain happens when plasticizer was mixed after starch diffusion inside the clay gallery, it can migrate throughout the system owing to its smaller size and retaining the plasticizer efficiency. In this case the explanation cannot be accepted because this effect does not happen in lower or higher plasticizer concentration with the same clay (the results are not presented here). Also, the effect does not agree with TEM micrographs that show no intercalation and non-broken particles and therefore nanocomposite is not formed. Based on this behavior it suggests that in a certain concentration of TA the rigid and non-exfoliated Dellite LVF particles act like internal mixing elements, improving homogeneity and bringing out the proper potential of the matrix material.

In this part the impact of nanoclays on the mechanical properties of plasticized SP and StPrAcLau are given. For better performance, SP with 5 wt% of TA and StPrAcLau with 10 wt% of TA were chosen to be present in this paper.

Figure 3 shows the mechanical properties of SP / 5 wt% TA / MMTs (5 wt% of Dellite 67G, Dellite 72T, Dellite 43B, Dellite LVF, and 2.5 wt% of Dellite 67G). As it can be seen from Figure 3, all organo-modified MMTs did not show any improvement in terms of tensile strength, while a little improvement in modulus was observed. In all cases, elongation decreased significantly. Adding 5 wt% of unmodified Dellite LVF did not show any significant change in properties compared to plasticized SP. Better results were achieved by adding 2.5 wt% of Dellite 67G to plasticized SP. For this composition, tensile strength and modulus almost remained constant, but elongation increased by 20 % to 18.3 %. This corresponds to the good dispersion and partial exfoliation that is seen from TEM micrographs.

Figure 3 Effect of different types of MMT on tensile properties of plasticized SP.



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