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Effect of carboxylic acids as compatibilizer agent on mechanical properties of thermoplastic starch and polypropylene blends
Accepted Manuscript Title: Effect of Carboxylic Acids as Compatibilizer Agent on Mechanical Properties of Thermoplastic Starch and Polypropylene Blends Author: Andr´ea Bercini Martins Ruth Marlene Campomanes Santana PII: DOI: Reference:
S0144-8617(15)00815-2 http://dx.doi.org/doi:10.1016/j.carbpol.2015.08.074 CARP 10277
To appear in: Received date: Revised date: Accepted date:
6-3-2015 13-8-2015 15-8-2015
Please cite this article as: Martins, A. B., and Santana, R. M. C.,Effect of Carboxylic Acids as Compatibilizer Agent on Mechanical Properties of Thermoplastic Starch and Polypropylene Blends, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.08.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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EFFECT OF CARBOXYLIC ACIDS AS COMPATIBILIZER AGENT ON
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MECHANICAL PROPERTIES OF THERMOPLASTIC STARCH AND
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POLYPROPYLENE BLENDS
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Andréa Bercini Martinsa; Ruth Marlene Campomanes Santanab
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Polymeric Materials Lab, Materials Engineering Department, Federal University of Rio
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Grande do Sul, Av. Bento Gonçalves, 9500, P.O. Box 15090, ZC 91501-970, Porto Alegre,
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RS, Brazil. ([email protected]; [email protected]) Tel.: +55 51 3308 9416
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ABSTRACT
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In this work, polypropylene/thermoplastic starch (PP/TPS) blends were prepared as an
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alternative material to use in disposable packaging, reducing the negative polymeric
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environmental impact. Unfortunately, this material displays morphological characteristics
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typical of immiscible polymer blends and a compatibilizer agent is needed. Three different
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carboxyl acids: myristic (C14), palmitic (C16) and stearic acids (C18) were used as natural
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compatibilizer agent (NCA). The effects of NCA on the mechanical, physical, thermal and
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morphological properties of PP/TPS blends were investigated and compared against PP/TPS
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with and without PP-grafted maleic anhydride (PPgMA). When compared to PP/TPS, blends
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with C18, PPgMA and C14 presented an improvement of 25, 22 and 17 % in tensile strength
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at break and of 180, 194 and 259 % in elongation at break, respectively. The highest increase,
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54 %, in the impact strength was achieved with C14 incorporation. Improvements could be
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seen, through scanning electron microscopy (SEM) images, in the compatibility between the
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immiscible components by acids incorporation. These results showed that carboxylic acids,
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specifically C14, could be used as compatibilizer agent and could substitute PPgMA.
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Keywords: thermoplastic starch, polymer blends, compatibilizer, mechanical and physical
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properties, carboxylic acids
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1. Introduction
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There has been an increased interest in producing environmentally friendly materials.
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Starch-based materials seems to be an attractive source for the development of biodegradable
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materials (St-Pierre, Favis, Ramsay, Ramsay & Verhoogt, 1997). Starch is a natural
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carbohydrate storage material accumulated by green plants. It is an important polysaccharide
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composed of a linear and branched chain of glucose molecules, named as amylose and
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amylopectin, respectively. It is an inexpensive, renewable and naturally biodegradable
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polymer (Yu, Prashantha, Soulestin, Lacrampe & Krawczak, 2013).
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Native starch itself does not show thermoplastic behavior, due to its intra and inter-
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molecular hydrogen bonds between hydroxyl groups of starch structures, which represent
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their crystallinity and, consequently, damaging the mechanical properties (Lu, Xiao & Xu,
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2009). In order to improve its properties, various methods have been developed to enhance
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starches positive characteristics, like physical or chemical modifications methods and the use
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of plasticizer such as glycerol (Carvalho, Curvelo & Gandini, 2005; Da Róz, Zambon,
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Curvelo & Carvalho, 2011; Morán, Cyras & Vázquez, 2013). The addition of plasticizers and
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exposition to thermo-mechanical energy leads to disruption of the semi-crystalline structure,
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resulting in an amorphous material called thermoplastic starch (TPS). This characteristic
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allows the use of traditional processing conditions (extrusion, blow and injection molding)
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(Lin, Huff, Parsons, Iannotti & Hsieh, 1995; Corradini, Carvalho, Curvelo, Agnelli &
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Mattoso, 2007; Liu, Xie, Yu, Chen & Li, 2009; Prachayawarakorn, Sangnitidej & Boonpasith,
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2010).
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The available hydroxyl groups on the starch chain makes the TPS, evidently, a very
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hydrophilic material with limited performance. This characteristic leads to a poor
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processability, dimensional stability and mechanical properties for its ends products.
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Therefore, thermoplastic starch is not used directly and blending with a synthetic 2
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thermoplastic is still necessary (Rodriguez-Gonzalez, Ramsay & Favis, 2003; Pedroso &
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Rosa, 2005; Ning, Jiugao, Xiaofei & Ying, 2007; Roy, Ramaraj, Shit & Nayak, 2011; Kahar,
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Ismail & Othman, 2012). However, an inconvenient issue is the incompatibility between hydrophilic TPS and
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hydrophobic synthetic polypropylene (PP). The resulting products of polymer/starch
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compound do not possess satisfying or favorable properties. The compatibility of starch filled
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polymer blending system has to be enhance, once it is utmost important to avoid loss of
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advance properties on materials. Compatibilizers reduce the interfacial energy and
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homogenizes the polar starch with synthetic polymer, improving the interfacial tensions
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between phases. One of the foremost approaches to improve compatibility was through
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maleated blending methods. The maleic anhydride (MA) is widely used to connect olefin
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polymers to dissimilar materials (Shujun, Jiugao & Jinglin, 2005; Rahmat, Rahman, Sin &
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Yussuf, 2009;; Taguet, Huneault & Favis, 2009; Huneault & Li, 2012; Kaseem, Hamad &
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Deri, 2012).
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Nevertheless, MA is expensive, difficult to manufacture and like petroleum based
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polymers, non biodegradable (Shujun, Jiugao & Jinglin, 2005). In order to find a
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compatibilizer agent from renewable source, the use of carboxyl acids has been a good
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possibility to replace synthetic agents, once they have a compatible chemical structure, high
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environmental availability and biodegradability (Poletto, Zattera & Santana, 2014).
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In the specific case of polypropylene (PP) and TPS blends, long hydrocarbon chain
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carboxylic acids has a polar end group (-COOH) that could reacts with the hydroxyl groups of
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starch through secondary bonds forces, as hydrogen bonding. While, its apolar fraction
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interacts with the PP matrix through secondary bonds forces, as Van der Waals forces. This
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allows the carboxylic acid to place itself at the interface and it acted as adhesion enhancer
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between PP/TPS during melt blending. Moreover, it has the advantage of being from
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renewable sources and been biodegradable. However, to our knowledge, detailed studies on
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the mechanical, thermal and morphological properties of using carboxylic acids on PP/TPS
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blends have not been reported in literature. Myristic acid (tetradecanoic acid) is used in the food industry as a flavor ingredient. It
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is found distributed in fats throughout the plant and animal kingdom, including common
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human foodstuffs, such as nutmeg, butterfat and coconut (Burdock & Carabin, 2007).
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Palmitic acid (hexadecanoic acid) is widely distributed, being found in practically all
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vegetable oils and animal (including marine animal). Its appearance ranges from a hard, white
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or faintly yellow, slightly glossy crystalline solid to a white or yellow-white powder or white
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crystals. Stearic acid (octadecanoic acid) is one of the useful types of saturated fatty acid that
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comes from many animal and vegetable fats and oils. It is a white solid with a mild odor.
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Table 1 – Physicochemical Properties of the Carboxyl Acids (Melting temperature Tm and
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Boiling temperature Tb)
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Empirical formula
Structure
Tm (ºC)
Tb (ºC)
Acid value
54-58.5
326
245.7
Myristic
C14H20O2
Palmitic
C16H32O2
63-64
351
218.0
Stearic
C18H36O2
69-71.2
383*
197.2
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*(decomposes at 360 ºC)
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The aim of this research was to explore the use of carboxylic acids as compatibilizer
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agents from renewable resources as an alternative to currently employed commercially.
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Hence, the influence of three different organic carboxylic acids as compatibilizer agent in the
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mechanical, thermal and morphological properties of blends of polypropylene (PP) and
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thermoplastic starch (TPS) were studied. 4
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2. Materials and methods 2.1. Materials Native regular cornstarch was purchased in local trade in the city of Porto Alegre, RS,
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Brazil. The plasticizer, glycerol and the natural compatiblizer agents (NCA): myristic acid
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(C14), palmitic acid (C16) and stearic acid (C18) were provided by Vetec Fine Chemicals.
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Pure polypropylene (PP), melt flow index 40 g/min (230 ºC, 2.16 kg) was supplied by
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Braskem (Triunfo, RS, Brazil) and the commercial compatibilizer agent used was the PP
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grafted with maleic anhydride (PPgMA) had a concentration equal to 1 % of maleic anhydride
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according to Polybond 3200 informations.
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2.2. Blend preparation
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For obtaining the thermoplastic starch (TPS), starch granules were plasticized with
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glycerol in a weight ratio of starch/glycerol 70/30, as shown in Figure 1, part I. The
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components were mixed at room temperature for 30 min. The obtained pulp was dried in an
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oven at 60 °C for 48 h.
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Figure 1 – Processing flow of PP/TPS blends
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The PP/TPS (70/30) blends with and without compatibilizer agent were manufactured
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in the twin-screw extruder (Haake Rheomex, model PTW16/25) with screw diameter of 16
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mm, L/D ratio of 25 and 120 rpm. The extrusion was conducted using a temperature profile
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from 160 to 180 °C (Fig.1, part II). Table 2 shows the composition of each blend developed.
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The compatibilizer agents were used at constant amount of 3% w/w in all cases throughout
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the study. This amount was chosen according to Liu, Wang & Sun (2003), who studied the
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critical interfacial concentration of a type of compatibilizer on starch blends with a polyolefin.
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PP
TPS
PP PP/TPS PP/TPS/MA PP/TPS/C14 PP/TPS/C16 PP/TPS/C18
100 70 70 70 70 70
0 30 30 30 30 30
PPgMA 0 0 3 0 0 0
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COMPATIBILIZER AGENT Myristic acid Palmitic acid Stearic acid 0 0 0 0 0 0 0 0 0 3 0 0 0 3 0 0 0 3
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Table 2 – Symbols and sample compositions (all values are in weight proportion, %)
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After mixing in the extruder, the materials were pelletized (SEIBT, model PS50) as
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showed in Fig.1, part III. The obtained samples were dried in an oven for 24 h at 60 °C.
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Thereafter, the blends in the pellets form were molded in an injection molding machine
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(Thermo Scientific Haake, MiniJet II). The temperature adopted was 190 °C, with a mold
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temperature of 40 ºC and 500 bar pressure, producing tensile and impact specimens test.
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2.3. Physical tests
The density of the samples was determined by the Archimedes method based on
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ASTM D792-1. The liquid medium used was ethanol. The extruded and injection molded
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specimens were used for this test (± 1 g). Analyzes were performed in triplicate. Density of
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the material was determined by the following equation:
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(1)
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Where
is the material density in g/cm3, a the weight of the specimen in air, b
apparent mass of specimen completely immersed and of the wire partially immersed in liquid,
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and w apparent mass of totally immersed sinker and of partially immersed wire, 0.7859 is the
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density of alcohol at 23 °C.
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The void content was based on ASTM D2374, the densities used were obtained experimentally. The equation for calculating the void content is (Eq. 3):
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(2)
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Where V represents the void content (%), Md the measured density (g/m3), r the
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fraction of resin (%), g the starch fraction (%), dr density of resin (g/m3) and dg the density of
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starch (g/m3).
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2.4. Mechanical tests
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The mechanical properties related to the materials tensile strength were analyzed
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according to ASTM D638 with crosshead speed of 5 mm/min in an universal testing machine
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(INSTRON, model 3382). The specimen dimensions were according to type V. Impact IZOD
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strength test was carried out as per ASTM D256 on IMPACTOR II (CEAST, Italy), using a
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2.75 J hammer. The specimens were not notched and dimensions were 63.5 mm x 12.5 mm x
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3.3 mm. It should be noted that the mechanical property results of different blends were
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obtained by averaging the measurement results of seven independent specimens. Results very
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far from the average were discarded. Hardness test was conducted according to ASTM D2240
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using Shore D durometer (Mainard). Ten different measurements were carried out for a
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sample to obtain average values of hardness. The equipment used was the meter thick, Shore
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D scale, with 3 seconds of compression. All the mechanical property measurements were
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performed at room temperature at 50 % relative humidity on injection moulded blends.
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2.5. Differential scanning calorimeter (DSC) Differential scanning calorimetry (DSC) thermograms were recorded by a TA
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Instruments model Q20. The samples (5 mg) were placed in an aluminum pan and were
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scanned from room temperature to 250 ºC at a heating rate of 10 ºC/min under nitrogen
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atmosphere. The degree of crystallinity of PP/TPS and the virgin PP was calculated using the
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following equation:
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(3)
Where ∆Hf is the heat of fusion for PP in the sample, ∆Hf◦ is the heat of fusion of
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100% crystalline PP which is 209 J/g (theorical melting entalphy) and w is the weight fraction
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of PP in the blend.
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2.6. Scanning electron microscopy (SEM) analysis
Fracture surfaces obtained after impact test were studied using a JEOL JSM 6060
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Scanning Electron Microscope (SEM) operating at an acceleration voltage of 15 kV. The
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samples were gold metalized.
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2.7. Statistical analysis
The statistical analysis of variance of obtained results has been carried out using
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commercial software. A one-way ANOVA and a Tukey’s test were used to check for
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statistical difference among groups (p≤0.05).
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3. Results and discussing
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3.1. Density and void content The density averages of the evaluated samples (molded by injection) are shown on
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Fig. 2. The densities of the blends were significantly (p ≤ 0.05) higher than of the pure PP.
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For the blends, there was not significant difference with 95 % of confidence between
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formulations. This effect can be attributed to the characteristic of thermoplastic starch, since it
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has higher density (Roy, Ramaraj, Shit & Nayak, 2011). As observed in Fig. 2b, the samples
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made with and without compatibilizer agent did not show significant void content differences.
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Voids are expected once the weak interfacial adhesion between phases can produced gaps. In
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this case, natural compatibilizer agents had the same behavior of the PPgMA presenting 3 %
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of void content, on average.
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(a)
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Figure 2 – (a) Density of polypropylene and its blends and b) Void content of PP/TPS blends
3.2. Mechanical proprieties
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To analyze the possibility of substituting the compatibilizer agent, PPgMA, by
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carboxylic acids, tensile strength tests were made. Hence, the results on Table 3 present
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enlightening results for this hypothesis. In the absence of any interfacial modifier, PP/TPS 9
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achieved a significate (p ≤ 0.05) equal Young’s modulus value compared to polypropylene
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(1326 and 1300 MPa, respectively). Others authors also found that the addition of starch into
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the polymer matrix caused rigid and less elastic blends. (Rodriguez-Gonzalez, Ramsay &
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Favis, 2003; Hoque, Ye, Yong & Mohd Dahlan, 2013).
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However, PP/TPS blends were characterized by the lower tensile strength at break
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(15.5 MPa) and elongation at break (73.4 %). As expected, the hydrogen bonds between
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starch and PP were not produced and the mechanical properties of PP/TPS blends were rather
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poor. Moreover, TPS acts as a non-reinforced agent and as a stress concentrator, reducing the
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resulting tensile strength, increasing the fragility and inducing cracks during tensile testing
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test (Iovino, Zullo, Rao, Cassar & Gianfreda, 2008).
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Table 3 – Tensile mechanical properties (Young’s modulus E, Tensile strength at break σb,
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Elongation at break εb, Toughness T and Hardness Shore D) for the all tested samples
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σb (MPa)
E (MPa)
PP PP/TPS PP/TPS/MA PP/TPS/C14 PP/TPS/C16 PP/TPS/C18
1301 ± 59c,d 1326 ± 29d 1155 ± 48b 1141 ± 33b 1082 ± 24a 1230 ± 30c
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30.4 ± 1.7d 15.5 ± 0.9a 18.9 ± 0.8c 18.1 ± 0.8b,c 16.9 ± 1.3a,b 19.5 ± 0.5c
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εb (%)
T (J)
H. Shore D
535.6 ± 51.4c 73.4 ± 26.5a 216.1 ± 34.6b 263.2 ± 32.2b 237.9 ± 20.1b 205.5 ± 20.1b
32.7 ± 4.2c 3.6 ± 1.0a 10.6 ± 1.4b 11.1 ± 1.8b 10.4 ± 1.0b 9.1 ± 0.9b
58.2 ± 3.0c 56.2 ± 3.2b,c 53.0 ± 3.6ª 50.9 ± 4.0a 50.4 ± 4.3ª 53.1 ± 4.4a,b
*Same superscript letters indicate no significant difference with 0.05% of confidence according to Tukey’s test.
Previous studies have shown that PPgMA has positive effect on mechanical properties
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of synthetic polymer and TPS blends promoting a better interaction between both phases
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(Kaseem, Hamad & Deri, 2012; Sabetzadeh, Bagheri & Masoomi, 2015; Rahmat, Rahman,
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Sin & Yussuf, 2009; Taguet, Huneault & Favis, 2009; Cerclé, Sarazin & Favis, 2013; Taguet,
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Bureau, Huneault & Favis, 2014). In this case, for blends made with C16, C14, PPgMA and
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C18 the values of the elasticity modulus were statistically lower and showed a decreased of
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18, 14, 13 and 7 % in comparison to PP/TPS samples. This tendency was also observed by
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(Samper-Madrigal, Fenollar, Dominici, Balart & Kenny, 2015), who showed that
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polyethylene and TPS blends when compatibilized had a plasticizing effect. As shown in Fig. 3, for tensile strength at break, there was a significantly increase for
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the three carboxylic acids tested compared to the PP/TPS blend without any compatibilization
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(about 15.5 MPa), with values ranging between 16.9 MPa for C16 to almost 19.5 MPa for
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C18. It should be noted that stearic and myristic acids probably promoted a better interfacial
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adhesion between phases and there was not significant differences compared to blends with
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PPgMA. A similar tendency was also observed for tenacity, so PP/TPS with carboxylic acid
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behave in a very close way than PP/TPS with PPgMA. This result is very interesting showing
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that the effect of the natural compatibilizer agent is analogous to the commercial one.
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Figure 3 – Tensile strength-strain curves of PP/TPS blends
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The elongation at break,
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εb,
is a high strain property, which is sensitive to the
interfacial interaction. Shifts in elongation were observed between blends with and without
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compatibilizer agent. For blends made with PPgMA, C14, C16 and C18, there was an
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increasing on the elongation value when compared to PP/TPS, which presented the highest
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decreased. This result suggested that the incorporation of acids improve the interaction,
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presenting the same behavior of PPgMA. (Taguet, Huneault & Favis, 2009) said that the
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decreasing of the interfacial tension is due to the reaction between the maleic anhydride of
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PPgMA copolymer and the OH of the starch. Probably the studied carboxyl acids have a
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similar mechanism, where the long carbon chain reacts with the apolar polymer and the -
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COOH group with the OH group from the starch.
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This trend was confirmed by the hardness test, whose results are also presented in
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Table 3. It indicates that TPS addition caused stiffer blends. Among blends, the ones with
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compatibilizer agent, from any nature, showed less resistant to permanent deformation. No
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statiscal differences were observed between blends with carboxylic acids and PPgMA. Once
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more, these acids showed to be as good as PPgMA. These results were consistent with the
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relationship between the modulus and rigidity of the materials.
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The results of impact strength of PP/TPs blends can be better understood by
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examining Fig. 4. It was noted that the lowest impact strength was obtained for PP/TPS
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samples. It was well known that the poor interfacial bonding due to the chemical structure
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induces micro spaces between starch and matrix polymer. Therefore, the agglomeration of
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starch particles in the interfaces of PP-starch induces crack propagation easily and
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collaborates for decrease the impact strength of the blends (Sabetzadeh, Bagheri & Masoomi,
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2012).
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Figure 4 – Impact energy results of PP/TPS blends
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In this test, the highest value between blends, with 95% of confidence, belong to
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PP/TPS/C14 (370 J/m). Indeed, lower modulus, higher elongation and higher toughness are
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expected to promote improvements in the impact energy, as seen on Table 3. However, the
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values obtained for PP/TPS/C16, PP/TPS/C18 and PP/TPS/PPgMA (320, 325 and 333 J/m)
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were higher when compared to PP/TPS (240 J/m). This result is most likely caused by new
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group’s formation between the multifunctional groups from acid and with the hydroxyls from
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starch. This fact represents new potential reactive points, promoting a greater interaction
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between phases and mobility, due to the intermolecular forces decreasing, resulting in more
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resistant samples. The action of the citric acid on the compatibility of starch and polymeric
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matrix is known and it has already been evaluated in other studies (Yun, Na & Yoon, 2006;
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Shi et al., 2008; Shi et al., 2007; Olivato, Grossmann, Bilck & Yamashita, 2012). Indeed the
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carboxylic acid has been incorporated into to formulation to interact with the polymer and the
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starch chain, reducing the intermolecular forces of the starch chains and enhancing the
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molecular mobility.
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3.4. Differential scanning calorimeter (DSC) 13
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The effects of different compatibilizer agents on DSC curve of PP were investigated.
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The thermograms are shown in Fig. 5 and the results are summarized in Table 4. Glass
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transition temperature was not determined as this transition is hard to discern in the DSC
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tracings. Also, the glass transition temperature is difficult to detect when the crystallinity of
289
the sample increases.
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Figure 5 – DSC thermograms of PP/TPS blends
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Table 4 - Thermal properties of PP in blends obtained by DSC (Melting temperature Tm, Heat
294
of fusion ∆Hf and Cristallinity Xc)
Formulations
Tm (ºC)
∆Hf (J/g)
Xc (%)
PP PP/TPS PP/TPS/MA PP/TPS/C14 PP/TPS/C16 PP/TPS/C18
165 164.3 163.5 163.3 163.4 163.4
91.55 58.31 66.87 71.73 69.79 67.40
43.80 39.85 45.70 49.02 47.70 46.06
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Based on DSC thermograms, only one endotherm peak was observed for all blends, it
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apparently indicates a certain miscibility. However, it was possible to note on DSC plots that
298
small endothermic peaks appeared for C14 (near 53ºC) and C16 (near 63ºC). It would indicate
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that 3% w/w is an excess of these acids, since these temperatures are the melting temperature
300
as can be seen on Table 1. Possibly smaller amount of C14 and C16 was required.
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When TPS was added in PP, the melting enthalpy (∆Hf) decreased and also
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crystallinity, indicating that the starch granules are distributed in the crystalline region of PP
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matrix and the crystalline order is broken as a consequence of the filler aggregates (Roy,
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Ramaraj, Shit & Nayak, 2011). As was reported by Liu, Wang & Sun, 2003, the interfacial
305
tension between starch and a polymer limited the migration and diffusion of molecular chain
306
to the crystal surface, thus depressing the crystallinization.
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The neat PP, which has melting temperature (Tm) at 165 ºC (Fig. 5, Table 4), shows a
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decrease in Tm to 163 ºC with insertion of a compatibilizer. It is also noted that
309
crystallinization increases. This observation indicates when starch was added with carboxylic
310
acid, its presence did not hinder the migration of the nucleus of PP, on the contrary, the NCA
311
acted as a nucleating agent, increasing the crystallinity. Moreover, enthalpy peak was
312
extended and the initial temperature decreased. We believed that a major quantity of smaller
313
crystals were formed. Samples made with C14 were responsible for the highest value of
314
crystallinity, it indicates the best performance among formulations, because it is assumed that
315
C14 interacted with starch during extrusion and reduced the interfacial tension between starch
316
and PP.
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3.3. Scanning electron microscopy (SEM) analysis
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The incorporation of TPS particles in a PP matrix leads to important changes in blends
320
morphology. The fracture surface morphology of the blends was evaluated by scanning
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electron microscopy (SEM) and the resulting images are presented in Fig. 6. Fig. 6a shows a
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rigid surface and a weak adhesion between phases, it could explain low values in PP/TPS
323
blends mechanical properties (Table 3).
324
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Figure 6 – Scanning electron microscopy (SEM) of PP/TPS blends, 15 kV and x400. (a)
326
PP/TPS; (b) PP/TPS/MA; (c) PP/TPS/C14; (d) PP/TPS/C16; (e) PP/TPS/C18.
327 328
All of the tested compatibilizers provide a significant decrease of the starch particle
329
size. For blends with PPgMA (Fig. 6b), a coarse morphology was visualized. This means that
16
Page 16 of 21
the size of the phases and the lack of adhesion (gaps between TPS and PP) were possible to
331
observed. Furthermore, it should be noted that some starch particles were “pulled-off” from
332
PP matrix during specimen fracture, leaving cavities in the fracture surface. It could explain
333
the unexpected decrease in the impact strength following the MA addition. On the other hand,
334
in the samples made with C14, C16 and C18 (Fig. 6c, 6d and 6e) the phases and the typical
335
morphology of dispersed domains immersed in a continuous matrix and a dimension
336
reduction and irregularity of starch particles could be seen.
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Other authors had used organic acids as citric acid, maleic acid and tartaric acid (Yun,
338
Na & Yoon, 2006; Shi et al., 2007; Ning, Jiugao, Xiaofei & Ying, 2007; Shi et al., 2008;
339
Olivato, Grossmann, Bilck & Yamashita, 2012; Chabrat, Abdillahi, Rouilly & Rigal, 2012)
340
with the purpose to increase the plastification phenomena, by the acidification of starch and
341
leading to its fragmentation and dissociation. Differently to these authors, in this work, the
342
carboxylic acids were added to act as a compatibilizer agent, improving the interfacial
343
adhesion between both phases. The improved interfacial adhesion was attributed to the strong
344
chemical interaction between the polar phase of acids and the hydroxyl groups of starch while
345
the long hydrocarbon chains interact with the PP matrix. This allows those carboxylic acids to
346
place itself at the interface of PP/TPS during melt blending.
348 349
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4. Conclusions
351
In this paper cornstarch plasticized with glycerol (TPS) was incorporated in a
353
polypropylene matrix. TPS addition increased the density of the blends. According to test
354
void content, carboxylic acids or PPgMA insertion had the same behavior. Blends without
355
compatibilizer agent had a poor interfacial adhesion, thus resulting in reduced mechanical
356
properties. Blends made with C14, C16 & C18 carboxylic acids of myristic, palmitic and
357
stearic acid proved to be equivalent or better compatibilizers when compared to maleic
358
anhydride polypropylene and no compatibilizer. The adhesion between both polymers phases
359
was improved and the tensile strength, elongation and impact strength increased by the
360
addition of carboxylic acids. Among blends, PP/TPS/C14 blend presented the highest impact
361
energy, showing a slightly better performance. Results from DSC, SEM, impact and tensile
362
strength analyses suggest the improved compatibility was attributed to a chemical reaction
363
between hydroxyl groups in starch and carboxyl groups in acids and the interaction between
364
long hydrocarbon chain in acid and PP. Further works will be carried out to confirm the
365
increased biodegradability of these blends.
367 368
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ACKNOWLEDGMENTS
This work was supported by the National Council for Scientific and Technological
369
Development (CNPq) and the Federal University of Rio Grande do Sul (UFRGS). The
370
authors acknowledge the Polymeric Materials Lab (LAPOL), the Chemical Lab (K212) and
371
the post graduated program of Materials Engineering (PPGEM). Also the authors
372
acknowledge to Braskem.
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Hoque, M. E., Ye, T. J., Yong, L. C., & Mohd Dahlan, K. (2013). Sago Starch-Mixed Low-Density Polyethylene Biodegradable Polymer: Synthesis and Characterization. Journal of Materials, 2013, 7. Huneault, M. A., & Li, H. (2012). Preparation and properties of extruded thermoplastic starch/polymer blends. Journal of Applied Polymer Science, 126(S1), E96-E108.
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Rodriguez-Gonzalez, F. J., Ramsay, B. A., & Favis, B. D. (2003). High performance LDPE/thermoplastic starch blends: a sustainable alternative to pure polyethylene. Polymer, 44(5), 1517-1526. Roy, S. B., Ramaraj, B., Shit, S. C., & Nayak, S. K. (2011). Polypropylene and potato starch biocomposites: Physicomechanical and thermal properties. Journal of Applied Polymer Science, 120(5), 3078-3086.
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Sabetzadeh, M., Bagheri, R., & Masoomi, M. (2012). Effect of corn starch content in thermoplastic starch/lowdensity polyethylene blends on their mechanical and flow properties. Journal of Applied Polymer Science, 126(S1), E63-E69.
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Samper-Madrigal, M. D., Fenollar, O., Dominici, F., Balart, R., & Kenny, J. M. (2015). The effect of sepiolite on the compatibilization of polyethylene–thermoplastic starch blends for environmentally friendly films. Journal of Materials Science, 50(2), 863-872.
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Shi, R., Bi, J., Zhang, Z., Zhu, A., Chen, D., Zhou, X., Zhang, L., & Tian, W. (2008). The effect of citric acid on the structural properties and cytotoxicity of the polyvinyl alcohol/starch films when molding at high temperature. Carbohydrate Polymers, 74(4), 763-770. Shi, R., Zhang, Z., Liu, Q., Han, Y., Zhang, L., Chen, D., & Tian, W. (2007). Characterization of citric acid/glycerol co-plasticized thermoplastic starch prepared by melt blending. Carbohydrate Polymers, 69(4), 748755.
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Shujun, W., Jiugao, Y., & Jinglin, Y. (2005). Preparation and characterization of compatible thermoplastic starch/polyethylene blends. Polymer Degradation and Stability, 87(3), 395-401. St-Pierre, N., Favis, B. D., Ramsay, B. A., Ramsay, J. A., & Verhoogt, H. (1997). Processing and characterization of thermoplastic starch/polyethylene blends. Polymer, 38(3), 647-655. Taguet, A., Bureau, M. N., Huneault, M. A., & Favis, B. D. (2014). Toughening mechanisms in interfacially modified HDPE/thermoplastic starch blends. Carbohydrate Polymers, 114(0), 222-229. Taguet, A., Huneault, M. A., & Favis, B. D. (2009). Interface/morphology relationships in polymer blends with thermoplastic starch. Polymer, 50(24), 5733-5743. Yu, F., Prashantha, K., Soulestin, J., Lacrampe, M.-F., & Krawczak, P. (2013). Plasticized-starch/poly(ethylene oxide) blends prepared by extrusion. Carbohydrate Polymers, 91(1), 253-261. Yun, Y.-H., Na, Y.-H., & Yoon, S.-D. (2006). Mechanical Properties with the Functional Group of Additives for Starch/PVA Blend Film. Journal of Polymers and the Environment, 14(1), 71-78.
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HIGHLIGHTS
495
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• Long carboxylic acids (C14-C18) were used as compatibilizer agent on PP/TPS blends.
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• Mechanical properties of PP blends decrease with starch addition.
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• PP/TPS/C14 had mechanical properties similar or even higher than PP/TPS/PPgMA.
500
• Myristic acid (C14) could be used as compatibilizer agent on PP/TPS.
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• Mechanical performance of blends with C16 and C18 were always higher than
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PP/TPS.
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S0144-8617(15)00815-2 http://dx.doi.org/doi:10.1016/j.carbpol.2015.08.074 CARP 10277
To appear in: Received date: Revised date: Accepted date:
6-3-2015 13-8-2015 15-8-2015
Please cite this article as: Martins, A. B., and Santana, R. M. C.,Effect of Carboxylic Acids as Compatibilizer Agent on Mechanical Properties of Thermoplastic Starch and Polypropylene Blends, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.08.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
EFFECT OF CARBOXYLIC ACIDS AS COMPATIBILIZER AGENT ON
2
MECHANICAL PROPERTIES OF THERMOPLASTIC STARCH AND
3
POLYPROPYLENE BLENDS
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Andréa Bercini Martinsa; Ruth Marlene Campomanes Santanab
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Polymeric Materials Lab, Materials Engineering Department, Federal University of Rio
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Grande do Sul, Av. Bento Gonçalves, 9500, P.O. Box 15090, ZC 91501-970, Porto Alegre,
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RS, Brazil. ([email protected]; [email protected]) Tel.: +55 51 3308 9416
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ABSTRACT
11
In this work, polypropylene/thermoplastic starch (PP/TPS) blends were prepared as an
12
alternative material to use in disposable packaging, reducing the negative polymeric
13
environmental impact. Unfortunately, this material displays morphological characteristics
14
typical of immiscible polymer blends and a compatibilizer agent is needed. Three different
15
carboxyl acids: myristic (C14), palmitic (C16) and stearic acids (C18) were used as natural
16
compatibilizer agent (NCA). The effects of NCA on the mechanical, physical, thermal and
17
morphological properties of PP/TPS blends were investigated and compared against PP/TPS
18
with and without PP-grafted maleic anhydride (PPgMA). When compared to PP/TPS, blends
19
with C18, PPgMA and C14 presented an improvement of 25, 22 and 17 % in tensile strength
20
at break and of 180, 194 and 259 % in elongation at break, respectively. The highest increase,
21
54 %, in the impact strength was achieved with C14 incorporation. Improvements could be
22
seen, through scanning electron microscopy (SEM) images, in the compatibility between the
23
immiscible components by acids incorporation. These results showed that carboxylic acids,
24
specifically C14, could be used as compatibilizer agent and could substitute PPgMA.
25
Keywords: thermoplastic starch, polymer blends, compatibilizer, mechanical and physical
26
properties, carboxylic acids
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1. Introduction
28
There has been an increased interest in producing environmentally friendly materials.
30
Starch-based materials seems to be an attractive source for the development of biodegradable
31
materials (St-Pierre, Favis, Ramsay, Ramsay & Verhoogt, 1997). Starch is a natural
32
carbohydrate storage material accumulated by green plants. It is an important polysaccharide
33
composed of a linear and branched chain of glucose molecules, named as amylose and
34
amylopectin, respectively. It is an inexpensive, renewable and naturally biodegradable
35
polymer (Yu, Prashantha, Soulestin, Lacrampe & Krawczak, 2013).
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Native starch itself does not show thermoplastic behavior, due to its intra and inter-
37
molecular hydrogen bonds between hydroxyl groups of starch structures, which represent
38
their crystallinity and, consequently, damaging the mechanical properties (Lu, Xiao & Xu,
39
2009). In order to improve its properties, various methods have been developed to enhance
40
starches positive characteristics, like physical or chemical modifications methods and the use
41
of plasticizer such as glycerol (Carvalho, Curvelo & Gandini, 2005; Da Róz, Zambon,
42
Curvelo & Carvalho, 2011; Morán, Cyras & Vázquez, 2013). The addition of plasticizers and
43
exposition to thermo-mechanical energy leads to disruption of the semi-crystalline structure,
44
resulting in an amorphous material called thermoplastic starch (TPS). This characteristic
45
allows the use of traditional processing conditions (extrusion, blow and injection molding)
46
(Lin, Huff, Parsons, Iannotti & Hsieh, 1995; Corradini, Carvalho, Curvelo, Agnelli &
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Mattoso, 2007; Liu, Xie, Yu, Chen & Li, 2009; Prachayawarakorn, Sangnitidej & Boonpasith,
48
2010).
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The available hydroxyl groups on the starch chain makes the TPS, evidently, a very
50
hydrophilic material with limited performance. This characteristic leads to a poor
51
processability, dimensional stability and mechanical properties for its ends products.
52
Therefore, thermoplastic starch is not used directly and blending with a synthetic 2
Page 2 of 21
53
thermoplastic is still necessary (Rodriguez-Gonzalez, Ramsay & Favis, 2003; Pedroso &
54
Rosa, 2005; Ning, Jiugao, Xiaofei & Ying, 2007; Roy, Ramaraj, Shit & Nayak, 2011; Kahar,
55
Ismail & Othman, 2012). However, an inconvenient issue is the incompatibility between hydrophilic TPS and
57
hydrophobic synthetic polypropylene (PP). The resulting products of polymer/starch
58
compound do not possess satisfying or favorable properties. The compatibility of starch filled
59
polymer blending system has to be enhance, once it is utmost important to avoid loss of
60
advance properties on materials. Compatibilizers reduce the interfacial energy and
61
homogenizes the polar starch with synthetic polymer, improving the interfacial tensions
62
between phases. One of the foremost approaches to improve compatibility was through
63
maleated blending methods. The maleic anhydride (MA) is widely used to connect olefin
64
polymers to dissimilar materials (Shujun, Jiugao & Jinglin, 2005; Rahmat, Rahman, Sin &
65
Yussuf, 2009;; Taguet, Huneault & Favis, 2009; Huneault & Li, 2012; Kaseem, Hamad &
66
Deri, 2012).
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Nevertheless, MA is expensive, difficult to manufacture and like petroleum based
68
polymers, non biodegradable (Shujun, Jiugao & Jinglin, 2005). In order to find a
69
compatibilizer agent from renewable source, the use of carboxyl acids has been a good
70
possibility to replace synthetic agents, once they have a compatible chemical structure, high
71
environmental availability and biodegradability (Poletto, Zattera & Santana, 2014).
72
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In the specific case of polypropylene (PP) and TPS blends, long hydrocarbon chain
73
carboxylic acids has a polar end group (-COOH) that could reacts with the hydroxyl groups of
74
starch through secondary bonds forces, as hydrogen bonding. While, its apolar fraction
75
interacts with the PP matrix through secondary bonds forces, as Van der Waals forces. This
76
allows the carboxylic acid to place itself at the interface and it acted as adhesion enhancer
77
between PP/TPS during melt blending. Moreover, it has the advantage of being from
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Page 3 of 21
78
renewable sources and been biodegradable. However, to our knowledge, detailed studies on
79
the mechanical, thermal and morphological properties of using carboxylic acids on PP/TPS
80
blends have not been reported in literature. Myristic acid (tetradecanoic acid) is used in the food industry as a flavor ingredient. It
82
is found distributed in fats throughout the plant and animal kingdom, including common
83
human foodstuffs, such as nutmeg, butterfat and coconut (Burdock & Carabin, 2007).
84
Palmitic acid (hexadecanoic acid) is widely distributed, being found in practically all
85
vegetable oils and animal (including marine animal). Its appearance ranges from a hard, white
86
or faintly yellow, slightly glossy crystalline solid to a white or yellow-white powder or white
87
crystals. Stearic acid (octadecanoic acid) is one of the useful types of saturated fatty acid that
88
comes from many animal and vegetable fats and oils. It is a white solid with a mild odor.
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Table 1 – Physicochemical Properties of the Carboxyl Acids (Melting temperature Tm and
91
Boiling temperature Tb)
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Empirical formula
Structure
Tm (ºC)
Tb (ºC)
Acid value
54-58.5
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245.7
Myristic
C14H20O2
Palmitic
C16H32O2
63-64
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218.0
Stearic
C18H36O2
69-71.2
383*
197.2
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Acids
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*(decomposes at 360 ºC)
94
The aim of this research was to explore the use of carboxylic acids as compatibilizer
95
agents from renewable resources as an alternative to currently employed commercially.
96
Hence, the influence of three different organic carboxylic acids as compatibilizer agent in the
97
mechanical, thermal and morphological properties of blends of polypropylene (PP) and
98
thermoplastic starch (TPS) were studied. 4
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2. Materials and methods 2.1. Materials Native regular cornstarch was purchased in local trade in the city of Porto Alegre, RS,
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Brazil. The plasticizer, glycerol and the natural compatiblizer agents (NCA): myristic acid
103
(C14), palmitic acid (C16) and stearic acid (C18) were provided by Vetec Fine Chemicals.
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Pure polypropylene (PP), melt flow index 40 g/min (230 ºC, 2.16 kg) was supplied by
105
Braskem (Triunfo, RS, Brazil) and the commercial compatibilizer agent used was the PP
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grafted with maleic anhydride (PPgMA) had a concentration equal to 1 % of maleic anhydride
107
according to Polybond 3200 informations.
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2.2. Blend preparation
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For obtaining the thermoplastic starch (TPS), starch granules were plasticized with
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glycerol in a weight ratio of starch/glycerol 70/30, as shown in Figure 1, part I. The
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components were mixed at room temperature for 30 min. The obtained pulp was dried in an
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oven at 60 °C for 48 h.
115 116
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Figure 1 – Processing flow of PP/TPS blends
117 118
The PP/TPS (70/30) blends with and without compatibilizer agent were manufactured
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in the twin-screw extruder (Haake Rheomex, model PTW16/25) with screw diameter of 16
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Page 5 of 21
mm, L/D ratio of 25 and 120 rpm. The extrusion was conducted using a temperature profile
121
from 160 to 180 °C (Fig.1, part II). Table 2 shows the composition of each blend developed.
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The compatibilizer agents were used at constant amount of 3% w/w in all cases throughout
123
the study. This amount was chosen according to Liu, Wang & Sun (2003), who studied the
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critical interfacial concentration of a type of compatibilizer on starch blends with a polyolefin.
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PP
TPS
PP PP/TPS PP/TPS/MA PP/TPS/C14 PP/TPS/C16 PP/TPS/C18
100 70 70 70 70 70
0 30 30 30 30 30
PPgMA 0 0 3 0 0 0
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COMPATIBILIZER AGENT Myristic acid Palmitic acid Stearic acid 0 0 0 0 0 0 0 0 0 3 0 0 0 3 0 0 0 3
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SAMPLES
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Table 2 – Symbols and sample compositions (all values are in weight proportion, %)
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After mixing in the extruder, the materials were pelletized (SEIBT, model PS50) as
129
showed in Fig.1, part III. The obtained samples were dried in an oven for 24 h at 60 °C.
130
Thereafter, the blends in the pellets form were molded in an injection molding machine
131
(Thermo Scientific Haake, MiniJet II). The temperature adopted was 190 °C, with a mold
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temperature of 40 ºC and 500 bar pressure, producing tensile and impact specimens test.
134 135
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2.3. Physical tests
The density of the samples was determined by the Archimedes method based on
136
ASTM D792-1. The liquid medium used was ethanol. The extruded and injection molded
137
specimens were used for this test (± 1 g). Analyzes were performed in triplicate. Density of
138
the material was determined by the following equation:
139
(1)
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140
Where
is the material density in g/cm3, a the weight of the specimen in air, b
apparent mass of specimen completely immersed and of the wire partially immersed in liquid,
142
and w apparent mass of totally immersed sinker and of partially immersed wire, 0.7859 is the
143
density of alcohol at 23 °C.
145
The void content was based on ASTM D2374, the densities used were obtained experimentally. The equation for calculating the void content is (Eq. 3):
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(2)
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Where V represents the void content (%), Md the measured density (g/m3), r the
148
fraction of resin (%), g the starch fraction (%), dr density of resin (g/m3) and dg the density of
149
starch (g/m3).
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2.4. Mechanical tests
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The mechanical properties related to the materials tensile strength were analyzed
153
according to ASTM D638 with crosshead speed of 5 mm/min in an universal testing machine
154
(INSTRON, model 3382). The specimen dimensions were according to type V. Impact IZOD
155
strength test was carried out as per ASTM D256 on IMPACTOR II (CEAST, Italy), using a
156
2.75 J hammer. The specimens were not notched and dimensions were 63.5 mm x 12.5 mm x
157
3.3 mm. It should be noted that the mechanical property results of different blends were
158
obtained by averaging the measurement results of seven independent specimens. Results very
159
far from the average were discarded. Hardness test was conducted according to ASTM D2240
160
using Shore D durometer (Mainard). Ten different measurements were carried out for a
161
sample to obtain average values of hardness. The equipment used was the meter thick, Shore
162
D scale, with 3 seconds of compression. All the mechanical property measurements were
163
performed at room temperature at 50 % relative humidity on injection moulded blends.
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2.5. Differential scanning calorimeter (DSC) Differential scanning calorimetry (DSC) thermograms were recorded by a TA
166
Instruments model Q20. The samples (5 mg) were placed in an aluminum pan and were
167
scanned from room temperature to 250 ºC at a heating rate of 10 ºC/min under nitrogen
168
atmosphere. The degree of crystallinity of PP/TPS and the virgin PP was calculated using the
169
following equation:
cr
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165
170
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(3)
Where ∆Hf is the heat of fusion for PP in the sample, ∆Hf◦ is the heat of fusion of
173
100% crystalline PP which is 209 J/g (theorical melting entalphy) and w is the weight fraction
174
of PP in the blend.
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2.6. Scanning electron microscopy (SEM) analysis
Fracture surfaces obtained after impact test were studied using a JEOL JSM 6060
178
Scanning Electron Microscope (SEM) operating at an acceleration voltage of 15 kV. The
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samples were gold metalized.
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2.7. Statistical analysis
The statistical analysis of variance of obtained results has been carried out using
183
commercial software. A one-way ANOVA and a Tukey’s test were used to check for
184
statistical difference among groups (p≤0.05).
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186
3. Results and discussing
187
3.1. Density and void content The density averages of the evaluated samples (molded by injection) are shown on
189
Fig. 2. The densities of the blends were significantly (p ≤ 0.05) higher than of the pure PP.
190
For the blends, there was not significant difference with 95 % of confidence between
191
formulations. This effect can be attributed to the characteristic of thermoplastic starch, since it
192
has higher density (Roy, Ramaraj, Shit & Nayak, 2011). As observed in Fig. 2b, the samples
193
made with and without compatibilizer agent did not show significant void content differences.
194
Voids are expected once the weak interfacial adhesion between phases can produced gaps. In
195
this case, natural compatibilizer agents had the same behavior of the PPgMA presenting 3 %
196
of void content, on average.
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198 199 200 201 202 203
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(a)
(b)
Figure 2 – (a) Density of polypropylene and its blends and b) Void content of PP/TPS blends
3.2. Mechanical proprieties
204 205
To analyze the possibility of substituting the compatibilizer agent, PPgMA, by
206
carboxylic acids, tensile strength tests were made. Hence, the results on Table 3 present
207
enlightening results for this hypothesis. In the absence of any interfacial modifier, PP/TPS 9
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achieved a significate (p ≤ 0.05) equal Young’s modulus value compared to polypropylene
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(1326 and 1300 MPa, respectively). Others authors also found that the addition of starch into
210
the polymer matrix caused rigid and less elastic blends. (Rodriguez-Gonzalez, Ramsay &
211
Favis, 2003; Hoque, Ye, Yong & Mohd Dahlan, 2013).
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However, PP/TPS blends were characterized by the lower tensile strength at break
213
(15.5 MPa) and elongation at break (73.4 %). As expected, the hydrogen bonds between
214
starch and PP were not produced and the mechanical properties of PP/TPS blends were rather
215
poor. Moreover, TPS acts as a non-reinforced agent and as a stress concentrator, reducing the
216
resulting tensile strength, increasing the fragility and inducing cracks during tensile testing
217
test (Iovino, Zullo, Rao, Cassar & Gianfreda, 2008).
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Table 3 – Tensile mechanical properties (Young’s modulus E, Tensile strength at break σb,
220
Elongation at break εb, Toughness T and Hardness Shore D) for the all tested samples
223
σb (MPa)
E (MPa)
PP PP/TPS PP/TPS/MA PP/TPS/C14 PP/TPS/C16 PP/TPS/C18
1301 ± 59c,d 1326 ± 29d 1155 ± 48b 1141 ± 33b 1082 ± 24a 1230 ± 30c
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Formulations
30.4 ± 1.7d 15.5 ± 0.9a 18.9 ± 0.8c 18.1 ± 0.8b,c 16.9 ± 1.3a,b 19.5 ± 0.5c
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εb (%)
T (J)
H. Shore D
535.6 ± 51.4c 73.4 ± 26.5a 216.1 ± 34.6b 263.2 ± 32.2b 237.9 ± 20.1b 205.5 ± 20.1b
32.7 ± 4.2c 3.6 ± 1.0a 10.6 ± 1.4b 11.1 ± 1.8b 10.4 ± 1.0b 9.1 ± 0.9b
58.2 ± 3.0c 56.2 ± 3.2b,c 53.0 ± 3.6ª 50.9 ± 4.0a 50.4 ± 4.3ª 53.1 ± 4.4a,b
*Same superscript letters indicate no significant difference with 0.05% of confidence according to Tukey’s test.
Previous studies have shown that PPgMA has positive effect on mechanical properties
224
of synthetic polymer and TPS blends promoting a better interaction between both phases
225
(Kaseem, Hamad & Deri, 2012; Sabetzadeh, Bagheri & Masoomi, 2015; Rahmat, Rahman,
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Sin & Yussuf, 2009; Taguet, Huneault & Favis, 2009; Cerclé, Sarazin & Favis, 2013; Taguet,
227
Bureau, Huneault & Favis, 2014). In this case, for blends made with C16, C14, PPgMA and
228
C18 the values of the elasticity modulus were statistically lower and showed a decreased of
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18, 14, 13 and 7 % in comparison to PP/TPS samples. This tendency was also observed by
230
(Samper-Madrigal, Fenollar, Dominici, Balart & Kenny, 2015), who showed that
231
polyethylene and TPS blends when compatibilized had a plasticizing effect. As shown in Fig. 3, for tensile strength at break, there was a significantly increase for
233
the three carboxylic acids tested compared to the PP/TPS blend without any compatibilization
234
(about 15.5 MPa), with values ranging between 16.9 MPa for C16 to almost 19.5 MPa for
235
C18. It should be noted that stearic and myristic acids probably promoted a better interfacial
236
adhesion between phases and there was not significant differences compared to blends with
237
PPgMA. A similar tendency was also observed for tenacity, so PP/TPS with carboxylic acid
238
behave in a very close way than PP/TPS with PPgMA. This result is very interesting showing
239
that the effect of the natural compatibilizer agent is analogous to the commercial one.
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241 242
Figure 3 – Tensile strength-strain curves of PP/TPS blends
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The elongation at break,
243
εb,
is a high strain property, which is sensitive to the
interfacial interaction. Shifts in elongation were observed between blends with and without
245
compatibilizer agent. For blends made with PPgMA, C14, C16 and C18, there was an
246
increasing on the elongation value when compared to PP/TPS, which presented the highest
247
decreased. This result suggested that the incorporation of acids improve the interaction,
248
presenting the same behavior of PPgMA. (Taguet, Huneault & Favis, 2009) said that the
249
decreasing of the interfacial tension is due to the reaction between the maleic anhydride of
250
PPgMA copolymer and the OH of the starch. Probably the studied carboxyl acids have a
251
similar mechanism, where the long carbon chain reacts with the apolar polymer and the -
252
COOH group with the OH group from the starch.
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This trend was confirmed by the hardness test, whose results are also presented in
254
Table 3. It indicates that TPS addition caused stiffer blends. Among blends, the ones with
255
compatibilizer agent, from any nature, showed less resistant to permanent deformation. No
256
statiscal differences were observed between blends with carboxylic acids and PPgMA. Once
257
more, these acids showed to be as good as PPgMA. These results were consistent with the
258
relationship between the modulus and rigidity of the materials.
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The results of impact strength of PP/TPs blends can be better understood by
260
examining Fig. 4. It was noted that the lowest impact strength was obtained for PP/TPS
261
samples. It was well known that the poor interfacial bonding due to the chemical structure
262
induces micro spaces between starch and matrix polymer. Therefore, the agglomeration of
263
starch particles in the interfaces of PP-starch induces crack propagation easily and
264
collaborates for decrease the impact strength of the blends (Sabetzadeh, Bagheri & Masoomi,
265
2012).
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Figure 4 – Impact energy results of PP/TPS blends
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In this test, the highest value between blends, with 95% of confidence, belong to
270
PP/TPS/C14 (370 J/m). Indeed, lower modulus, higher elongation and higher toughness are
271
expected to promote improvements in the impact energy, as seen on Table 3. However, the
272
values obtained for PP/TPS/C16, PP/TPS/C18 and PP/TPS/PPgMA (320, 325 and 333 J/m)
273
were higher when compared to PP/TPS (240 J/m). This result is most likely caused by new
274
group’s formation between the multifunctional groups from acid and with the hydroxyls from
275
starch. This fact represents new potential reactive points, promoting a greater interaction
276
between phases and mobility, due to the intermolecular forces decreasing, resulting in more
277
resistant samples. The action of the citric acid on the compatibility of starch and polymeric
278
matrix is known and it has already been evaluated in other studies (Yun, Na & Yoon, 2006;
279
Shi et al., 2008; Shi et al., 2007; Olivato, Grossmann, Bilck & Yamashita, 2012). Indeed the
280
carboxylic acid has been incorporated into to formulation to interact with the polymer and the
281
starch chain, reducing the intermolecular forces of the starch chains and enhancing the
282
molecular mobility.
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283 284
3.4. Differential scanning calorimeter (DSC) 13
Page 13 of 21
The effects of different compatibilizer agents on DSC curve of PP were investigated.
286
The thermograms are shown in Fig. 5 and the results are summarized in Table 4. Glass
287
transition temperature was not determined as this transition is hard to discern in the DSC
288
tracings. Also, the glass transition temperature is difficult to detect when the crystallinity of
289
the sample increases.
291 292
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Figure 5 – DSC thermograms of PP/TPS blends
293
Table 4 - Thermal properties of PP in blends obtained by DSC (Melting temperature Tm, Heat
294
of fusion ∆Hf and Cristallinity Xc)
Formulations
Tm (ºC)
∆Hf (J/g)
Xc (%)
PP PP/TPS PP/TPS/MA PP/TPS/C14 PP/TPS/C16 PP/TPS/C18
165 164.3 163.5 163.3 163.4 163.4
91.55 58.31 66.87 71.73 69.79 67.40
43.80 39.85 45.70 49.02 47.70 46.06
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Based on DSC thermograms, only one endotherm peak was observed for all blends, it
297
apparently indicates a certain miscibility. However, it was possible to note on DSC plots that
298
small endothermic peaks appeared for C14 (near 53ºC) and C16 (near 63ºC). It would indicate
299
that 3% w/w is an excess of these acids, since these temperatures are the melting temperature
300
as can be seen on Table 1. Possibly smaller amount of C14 and C16 was required.
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When TPS was added in PP, the melting enthalpy (∆Hf) decreased and also
302
crystallinity, indicating that the starch granules are distributed in the crystalline region of PP
303
matrix and the crystalline order is broken as a consequence of the filler aggregates (Roy,
304
Ramaraj, Shit & Nayak, 2011). As was reported by Liu, Wang & Sun, 2003, the interfacial
305
tension between starch and a polymer limited the migration and diffusion of molecular chain
306
to the crystal surface, thus depressing the crystallinization.
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The neat PP, which has melting temperature (Tm) at 165 ºC (Fig. 5, Table 4), shows a
308
decrease in Tm to 163 ºC with insertion of a compatibilizer. It is also noted that
309
crystallinization increases. This observation indicates when starch was added with carboxylic
310
acid, its presence did not hinder the migration of the nucleus of PP, on the contrary, the NCA
311
acted as a nucleating agent, increasing the crystallinity. Moreover, enthalpy peak was
312
extended and the initial temperature decreased. We believed that a major quantity of smaller
313
crystals were formed. Samples made with C14 were responsible for the highest value of
314
crystallinity, it indicates the best performance among formulations, because it is assumed that
315
C14 interacted with starch during extrusion and reduced the interfacial tension between starch
316
and PP.
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3.3. Scanning electron microscopy (SEM) analysis
319
The incorporation of TPS particles in a PP matrix leads to important changes in blends
320
morphology. The fracture surface morphology of the blends was evaluated by scanning
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electron microscopy (SEM) and the resulting images are presented in Fig. 6. Fig. 6a shows a
322
rigid surface and a weak adhesion between phases, it could explain low values in PP/TPS
323
blends mechanical properties (Table 3).
324
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Figure 6 – Scanning electron microscopy (SEM) of PP/TPS blends, 15 kV and x400. (a)
326
PP/TPS; (b) PP/TPS/MA; (c) PP/TPS/C14; (d) PP/TPS/C16; (e) PP/TPS/C18.
327 328
All of the tested compatibilizers provide a significant decrease of the starch particle
329
size. For blends with PPgMA (Fig. 6b), a coarse morphology was visualized. This means that
16
Page 16 of 21
the size of the phases and the lack of adhesion (gaps between TPS and PP) were possible to
331
observed. Furthermore, it should be noted that some starch particles were “pulled-off” from
332
PP matrix during specimen fracture, leaving cavities in the fracture surface. It could explain
333
the unexpected decrease in the impact strength following the MA addition. On the other hand,
334
in the samples made with C14, C16 and C18 (Fig. 6c, 6d and 6e) the phases and the typical
335
morphology of dispersed domains immersed in a continuous matrix and a dimension
336
reduction and irregularity of starch particles could be seen.
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Other authors had used organic acids as citric acid, maleic acid and tartaric acid (Yun,
338
Na & Yoon, 2006; Shi et al., 2007; Ning, Jiugao, Xiaofei & Ying, 2007; Shi et al., 2008;
339
Olivato, Grossmann, Bilck & Yamashita, 2012; Chabrat, Abdillahi, Rouilly & Rigal, 2012)
340
with the purpose to increase the plastification phenomena, by the acidification of starch and
341
leading to its fragmentation and dissociation. Differently to these authors, in this work, the
342
carboxylic acids were added to act as a compatibilizer agent, improving the interfacial
343
adhesion between both phases. The improved interfacial adhesion was attributed to the strong
344
chemical interaction between the polar phase of acids and the hydroxyl groups of starch while
345
the long hydrocarbon chains interact with the PP matrix. This allows those carboxylic acids to
346
place itself at the interface of PP/TPS during melt blending.
348 349
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350
4. Conclusions
351
In this paper cornstarch plasticized with glycerol (TPS) was incorporated in a
353
polypropylene matrix. TPS addition increased the density of the blends. According to test
354
void content, carboxylic acids or PPgMA insertion had the same behavior. Blends without
355
compatibilizer agent had a poor interfacial adhesion, thus resulting in reduced mechanical
356
properties. Blends made with C14, C16 & C18 carboxylic acids of myristic, palmitic and
357
stearic acid proved to be equivalent or better compatibilizers when compared to maleic
358
anhydride polypropylene and no compatibilizer. The adhesion between both polymers phases
359
was improved and the tensile strength, elongation and impact strength increased by the
360
addition of carboxylic acids. Among blends, PP/TPS/C14 blend presented the highest impact
361
energy, showing a slightly better performance. Results from DSC, SEM, impact and tensile
362
strength analyses suggest the improved compatibility was attributed to a chemical reaction
363
between hydroxyl groups in starch and carboxyl groups in acids and the interaction between
364
long hydrocarbon chain in acid and PP. Further works will be carried out to confirm the
365
increased biodegradability of these blends.
367 368
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ACKNOWLEDGMENTS
This work was supported by the National Council for Scientific and Technological
369
Development (CNPq) and the Federal University of Rio Grande do Sul (UFRGS). The
370
authors acknowledge the Polymeric Materials Lab (LAPOL), the Chemical Lab (K212) and
371
the post graduated program of Materials Engineering (PPGEM). Also the authors
372
acknowledge to Braskem.
373 374
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492 493 494
HIGHLIGHTS
495
497
• Long carboxylic acids (C14-C18) were used as compatibilizer agent on PP/TPS blends.
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• Mechanical properties of PP blends decrease with starch addition.
499
• PP/TPS/C14 had mechanical properties similar or even higher than PP/TPS/PPgMA.
500
• Myristic acid (C14) could be used as compatibilizer agent on PP/TPS.
501
• Mechanical performance of blends with C16 and C18 were always higher than
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PP/TPS.
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