Mechanochemical Activation of Carboxy Methyl Cellulose and Its Thermoplastic Polyvinyl Alcohol/Starch Biocomposites with Enhanced Physicochemical Properties

Polyvinyl alcohol (PVA) / starch (S) /Carboxy methyl cellulose (CMC) composite films were prepared from PVA, three different loading of CMC and soluble starch aqueous suspensions by casting method in the presence of plasticize containing glycerol. The effects of the CMC addition on the some physical properties of the resulted blend films were investigated. Tensile tests demonstrated positive results from mechanochemical treatment. With the addition of 20% CMC was an increase in the tensile strength and percentage elongation at break. The thermal properties of polyvinyl alcohol (PVA)/Starch(S)/Carboxymethyl cellulose (CMC) blends were investigated by thermogravimetry (TGA) and derivative thermogravimetry (DTG). At least four degradation distinct stages were identified in the PVA/S/CMC blends by means of using TGA, being assigned to the mass loss due to the plasticizer leaching, and to the degradation of the starch, CMC and PVA fractions. The thermal stability of PVA/S/CMC blends is higher than that of PVA/S suggesting that the addition of CMC improved the thermal stability of PVA/S blend. The SEM (Scanning electron microscopy) indicates morphological structure of the polyvinyl alcohol/starch films in the different percent of CMC.


Introduction
The severe environmental problems, including the increasing difficulties of waste disposal and the deepening threat of global warming (due to carbon dioxide release during incineration) caused by the nonbiodegradablility of a number of polymers (used in packaging and agriculture field) have raised concerns all over the world [1]. In order to solve the problems generated by plastic waste, many efforts have been done to obtain an environmental friendly material. Most of the researches are focused on substitution of the petro-based plastics by biodegradable materials with similar properties and low in cost [2][3][4]. Among the natural polymers, starch has been considered as one of the most promising candidates for this regard because of its attractive combination of price, abundance and thermoplastic behavior, in addition to biodegradability. The main disadvantage of biodegradable starch-based films is their hydrophilic character, which leads to low stability when these materials are submitted to different environmental conditions [5]. In addition, starch-based materials have poor mechanical properties and particularly poor elongation at ambient conditions. Thus, the incorporation of a plasticizer is required to overcome the brittleness of these materials. Plasticizers reduced intermolecular forces and increase the mobility of polymer chains, decreasing the glass transition temprature and increasing permeability [6]. One approach to improve the functional properties of starch films is to blend starch with other polymers (natural and synthetic). Biodegradable starch-based plastics such as starch/cellulose and starch/PVA have recently been investigated for their great potential marketability in agricultural foils, garbage and composting bags, and food packaging, in the fast food industry, and in biomedical fields [7][8][9][10]. PVA is a versatile polymer with many industrial applications, and it may be the only synthesized polymer whose backbone is mainly composed of C-C bonds that is absolutely biodegradable. PVA is the most readily biodegradable of vinyl polymers. It is readily degraded in wastewater activated sludge. The microbial degradation of PVA has been studied, as well as its enzymatic degradation by secondary alcohol peroxidases isolated from soil bacteria of pseudomonas strain. The excellent chemical resistance, optical and physical properties of PVA resins, has resulted in its broad industrial uses [11][12][13]. Carboxy methyl cellulose is cellulose ether forms excellent films. Because of its polymeric structure and high molecular weight, it is capable to use as filler in 10 Mechanochemical Activation of Carboxy Methyl Cellulose and Its Thermoplastic Polyvinyl Alcohol/Starch Biocomposites with Enhanced Physicochemical Properties biocomposite films production. Carboxy methyl cellulose is able to improve the mechanical and barrier properties of starch-based films [14]. PVA, carboxy methyl cellulose and starch are polar polymers; thus a composite of PVA, carboxy methyl cellulose and starch is likely to produce a material having excellent mechanical properties [15]. This study reports on the effect of carboxy methyl cellulose content, on the properties of PVA/starch films, such as mechanical, thermal and morphological parameters. These studies were carried out to assess their potential uses as packaging films.

Materials
Starch (ST) was provided by Merck company, and polyvinyl alcohol (PVA) with Mn=72000 and glycerol (Mn= 92/10, 78% purity) purchased from Merck company. Carboxymethyl cellulose sodium salt, with an average molecular weight of Mn=295225 was purchased from Fluka company.

Preparation of Films
Films were obtained by the casting method [16]. First, PVA (2.5 g) was solubilized in 50 ml of distilled water at 75°C for 15 min and starch (2.5, 2, 1.5 g) was mixed (25°C for 10 min) with distilled water (15 ml) and glycerol (40 ml/100 g starch) then added to 20 ml of boiling water to obtain a clear solution. Carboxy methyl cellulose (0, 0.5, 1g) was solubilized in 75 ml of distilled water at 75°C for 15 min. Then, PVA, starch and CMC solutions were mixed together and stirred with using a magnetic bar at 75°C for 90 min with a reflux condenser. Then, about 70 ml of the sample was poured into a Teflon casting tray and then dried at 60°C in oven to cast the films. The thickness of the films was 0.08±0.01 mm. The thickness of the films measured with an Alton M820-25 hand-held micrometer having a sensitivity of 0.01 mm.

Solubility in Water
Solubility in water was defined as the percentage of the dry matter of film which is solubilized after 24 h immersion in water [17].
Film specimens were kept in a desiccator containing dry calcium sulphate till they reached constant weight. Afterward, about 500 mg of each film were immersed in beakers containing 50 ml of distilled water at 23 °C for 24 h with periodical gentle manual agitation. The Films were removed from the water and were placed back in the desiccator until they reached a constant weight to obtain the final dry weight of the film. The percentage of the total soluble matter (%TSM) of the films was calculated using the following equation: %TSM = [(initial dry Wt-final dry Wt) / initial dry Wt] × 100. (1) TSM tests for each type of film were carried out in three replicates.

Swelling Studies
The extent of swelling was determined by a conventional gravimetric procedure [18]. In brief; a preweighed dried piece of a blend sample (0.1 g) was immersed in distilled water, and allowed to swell. By recording the weights of the swollen blends at desired time intervals, it was possible to monitor the extent of swelling. The swelling process was expressed in terms of the swelling ratio, as follows: Swelling ratio (SR) = Weight of swollen blend/ Weight of dry blend (2)

Mechanical Properties
Ultimate tensile strength (UTS) and strain to break (SB) of the films were determined at 21°C±1°C using a tensile tester (Zwick/Roell model FR010, Germany) according to ASTM standard method D882-91 [19]. Three dumbbelly forms films (8 cm×0.5 cm) were cut from each of samples and were mounted between the grips of the machine. The initial grip separation and cross-head speed were set to 50 mm and 5 mm/min, respectively.

Thermal Properties
Thermogravimetry was carried out using a Perkin Elmer pyris diamond TG-DTA instrument at heating rate (10 0 C min-1) under inert atmosphere (nitrogen environment) in order to avoid thermoxidative degradation. Temperature programs for dynamic tests were run from 0 to 500 0 C.

Scanning Electronic Microscopy (SEM)
The morphology of the surface of the films was investigated using a scanning electronic microscope of XL30 type (Netherland). The films were covered with pure metallic Au. The laying down of Au was carried out using evaporation of the metal under a high vacuum, to give a thickness of around 100 °A.

Solubility in Water
The water solubility of the PVA/S/CMC films as a function of CMC content is shown in Fig. 1.
Addition of CMC, in all concentrations, increased the water solubility of films. The %TSM was 27.41% for the samples without CMC, which increased to 33.77%, 43.71% and 46.27% for the films containing 5, 10 and 20% W/W CMC, respectively. This is in agreement with [20] that investigated the effect of CMC on the solubility in water of the thermoplastic starch/CMC and reported that when filler content increased, the solubility in water values increased gradually. The observed results may be explained by the fact that an increased CMC content in the blend renders the network more hydrophilic and henceforth, the degree of water sorption increases, which, in turn, increases the solubility in water.

Swelling Studies
The swelling ratio of the PVA/S/CMC films as a function of CMC content is shown in table 1.
Addition of CMC, in all concentrations, increased the SR of PVA/S/CMC films. This is in agreement with [20] that investigated the effect of CMC on the swelling ratio of the thermoplastic starch/CMC and reported that when filler content increased, the swelling ratio values increased gradually. Carboxymethyl cellulose (CMC), being a modified natural water-soluble polymer, contains hydroxy and carboxyl groups which impart hydrophilicity to the molecule. When the weight fraction of CMC is increased in the reaction mixture of the blend, the swelling ratio is found to increase substantially. The observed results may be explained by the fact that an increased CMC content in the blend renders the network more hydrophilic and henceforth, the degree of water sorption increases, which, in turn, increases the swelling ratio. Another plausible explanation rests upon the possibility that as the content of CMC increases in the blend matrix, there may be an increase in the mutual repulsion between the carboxylate groups of the CMC molecules, which relaxes the network chains of the blend and widens the mesh sizes of the blend. This ultimately results in enhanced swelling of the system.   It was observed an important increase of fourfold in the UTS when 5% W/W of CMC was added to the PVA/starch, but with increasing CMC concentration from 5% to 20%, the UTS decreased from 27.88 MPa to 18.36 MPa. Therefore, with increasing CMC concentration from 0% to 20%, the UTS increased significantly from 6.38 MPa to 18.36 MPa. This was probably due to the interfacial interaction between the matrix and filler due to the chemical similarity (polysaccharide structure) of starch and CMC. It was interesting that with increasing CMC concentration (from 0% to 20%), the UTS increased significantly. It was observed an important decrease of sixfold in the SB when 5% W/W of CMC was added to the PVA/starch, but with increasing CMC concentration from 5% to 20%, the SB increased from 4.03% to 35.56%. This is in agreement with [14] who reported a significant improvement mechanical property of starch films is achieved by adding relatively small amounts of CMC (till 10%). Therefore, with increasing CMC concentration from 0% to 20%, the SB increased significantly from 23.86% to 35.56%. It seems CMC could improve the films strength without depressing effect on flexibility.

Thermal Properties
The TGA and DTG curves for PVA50/S50 and PVA50/S30/CMC20 blends at heating rate 10 0 C/min are shown in Figures 4 and 5 respectively.  It can be seen that both PVA/S and PVA/S/CMC blends exhibited a four-step degradation pattern. As samples contain a small quantity of water (water of free), the first stages of degradation (up to about 90 0 C) represent the evaporation of the water of degraded components. The second stages of degradation (up to about 220 0 C) represent the volatilization of the easily degraded components such as glycerol [21]. The major mass loss took place in the third step (from about 250 to 360 0 C), and was followed by a further smaller mass loss in the fourth step (from about 380 to 440 0 C). The third step of weight loss was predominantly the characteristic degradation of a polymer structure (water elimination of PVA, decomposition of Starch and decomposition of CMC) [22,23]. Further heating broke down the polymer backbone (cracking of PVA) [24,25]. The non-volatile residues of approximately 5-10wt% were left for the studied samples. Affirmatively, the third step was the predominant degradation process.
The peaks, positions of the mass loss of all these three processes (exception first peak) in all the studied blends at the heating rate 10 0 C are shown in table 2. The thermal stability of PVA/S/CMC blends is higher than that of PVA/S suggesting that the addition of CMC improved the thermal stability of PVA/S blend. Fig. 6 shows the morphology of PVA/S/CMC films with different contents of starch and CMC. The addition of CMC significantly changes the morphology of PVA/S/CMC composite. Blends PVA/starch and PVA/S/CMC had a uniform morphology, with no evidence of phase separation. Homogeneous distributions of the CMC in the PVA/S/CMC matrix were observed in all composite films, implying good adhesion between fillers and matrix. This is attributed to the good compatibility resulting from the chemical similarities between starch and carboxymethyl cellulose and the hydrogen bonding interactions existing in the interface between PVA, starch and CMC. Such an event and uniform distribution of the fillers in the matrix could play an important role in improving the mechanical performance of the resulting composite films.

Conclusions
The present study shows the role of CMC on the physicmechanical properties of PVA/starch films. The addition of CMC to PVA/starch blends improved the mechanical properties of the resulted starch based composites. CMC (20% W/W film) could increase the UTS and SB of starch based composite films by more than 35% and 67% compared to the PVA/starch film respectively. Solubility of the films increased from 27.14% to 46.27%. At least four degradation distinct stages were identified in the PVA/S/CMC blends by means of using TGA, being assigned to the mass loss due to the plasticizer leaching, and to the degradation of the starch, CMC and PVA fractions. The thermal stability of PVA/S/CMC blends is higher than that of PVA/S suggesting that the addition of CMC improved the thermal stability of PVA/S blend. In addition, by increasing of CMC content, outward properties of the composite films improved appreciably. With considering these results, it seems that the PVA/S/CMC biocomposit films show better physicomechanical properties than PVA/starch films and can be potentially replaced of PVA/starch films.