COMPOSITE BIODEGRADABLE MATERIALS BASED ON POTATO STARCH AND MICRO/NANOFILLERS

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1 COMPOSITE BIODEGRADABLE MATERIALS BASED ON POTATO STARCH AND MICRO/NANOFILLERS Petr Duchek a, Jaromír Dlouhý and Petr Franče University of West Bohemia, Univerzitní 8, Plzeň, Czech Republic a duchekpe@kmm.zcu.cz Abstract Biodegradable composite materials based on pristine and modified potato starches with selected plastic/nonplastic mineral fillers and waste cellulose were prepared and characterized. There are two suitable preparation pathways of these materials casting and extrusion. The principal drawback of these biomaterials high degradation rate in aqueous media followed by total loss of mechanical strength is discussed as well as methods of reducing the hydrophilicity of these composites. Biological degradation of these composite materials proved to proceed rapidly both in water and soil environment. Keywords: potato starch, montmorillonite, mineral filler, waste cellulose, hydrophobization, biodegradability tests. Introduction Biodegradable polymers and composites are attracting research interest due to minimum impact of these materials on environment and life; many of applicable structures can be found in nature, or can be recovered easily from natural resources. Starch, biodegradable polymers from renewable forest resources, poly (lactic acid) and gelatine are several examples of such materials 1,2. There are many of biodegradable plastics merchandized nowadays nevertheless they are very rarely fully and easily biodegradable due to presence of a considerably high percentage (tens of per cent) of classic thermoplastics like PE. From the point of view of better mechanical properties and stability in water milieu it can be understood, but the rate of biodegradation process is seriously affected. Starch is a natural polysaccharide readily available all over the world 3. In its thermoplastic form (advantageously plasticized in glycerol/water environment) is one of the most used biodegradable plastics for many years. Principal disadvantages of this material that prevent from a more popular use as biodegradable plastic film involve high affinity to water and low strength. This bottlenecks can be overcome by reinforcing the matrix by other types of suitable materials (especially natural or derived from natural) like fibres, plastic and/or non-plastic fillers 4. Introduction of a small amount of layered silicates into the starch may increase the strength of the composite as well as to influence other properties in a positive way ( e.g. lower flammability, low gas permeability). For improving of properties for which it is a very common commodity, starch often undergoes chemical modifications that slightly change its chemical structure and behaviour. These changes involve e.g. starch cationization using 2,3 epoxypropyltrimethylammonium chloride when the ether 1-44

bond and terminal trimethylammonium groups are introduced in the macromolecule; acetylation using e.g. acetanhydride reagent, crosslinking of the polysaccharide framework or introduction of aldehyde groups into the starch polymer.

The maximum value for DS is 3 as each anhydroglucose unit has three available hydroxyl groups; the real DS is lower and typically ranges from 0.01 to 0.5.

2 bond and terminal trimethylammonium groups are introduced in the macromolecule; acetylation using e.g. acetanhydride reagent, crosslinking of the polysaccharide framework or introduction of aldehyde groups into the starch polymer. The degree of substitution (DS) of starch ethers and esters is then a measure of the average number of hydroxyl groups on each anhydroglucose unit that are derivatized. The maximum value for DS is 3 as each anhydroglucose unit has three available hydroxyl groups; the real DS is lower and typically ranges from 0.01 to 0.5. Montmorillonite (MMT) is a three layer aluminosilicate lamellar mineral occurring in bentonitic clays worldwide with applications in plenty industrial and agricultural areas. This type of clay minerals (smectites) consists of a central sheet of MO 4 (OH) 2 octahedral symmetrically bonded to two MO 4 tetrahedral sheets 5 (T:O:T layered structure, Fig. 1). The octahedral sites are occupied by aluminium and similar ions (iron, magnesium), while the tetrahedral centres accommodate silicon and aluminium ions. Due to possible isomorphic substitution of ions in this mineral structure resulting in charge disbalance (negative charge in the mineral framework), cations are present in the interlayer space thus neutralizing the negative charge of mineral sheets. The interlayer space plays an important role in properties of MMT as there are cations and water (solvents generally) placed within (Fig.1). By simple ion-exchange reactions we are able to introduce different organic/inorganic cations to the interlayer. Mineral unique microstructure - sheets of about nanometre thickness and hundred up to several hundreds nanometres length (Fig. 2), together with modification possibilities determine MMT as one of the most important clay materials used in nanomaterials. Figure 1. Structure of MMT Figure 2. Morphology of MMT. With respect to MMT ideal aspect ratio (Fig. 3) of particles (platelets), this material represents an ideal substrate both for theoretical (modeling) and synthetic studies in the nanoscale. We can distinguish a microcomposite with separated phases of the clay mineral and polymer, intercalated nanocomposite with more opened the lamellae, and finally its exfoliated form where the platelets are far enough to let the polymer chains freely penetrate troughout the composite. This type of nanocomposite is the most wanted one with the best mechanical properties (Fig. 4): 1-45

3 Figure 3. Lamellar structure of MMT. Figure 4. Morphology of composites with MMT reinforcement. The aim of this paper is to present choise of natural and waste materials for reinforcement of thermoplastic starch matrix in the biodegradable green material prepared without any synthetic man-made component, and problems related to its high hydrophilic character. Abbreviations and Acronyms ` `` DS degree of substitution, MMT montmorillonite, TPS thermoplastic starch, XRD X-ray diffraction, NA not available. Experimental Materials. Native potato starch as well as modified starches (cationic, acetylated, dialdehyde) were kindly supplied by Lyckeby Amylex a.s., Czech Republic. Sodium montmorillonite (CloisiteNa ) was supplied by Southern Clay Products Inc., USA. Samples of bentonite (Sabenil, Lutila ), kaolin, diatomite and precipitated micronized chalk were commercial samples. Micro- and nanosilica (LUDOX ) were obtained from Sigma-Aldrich, cellulose was used in the form of waste disintegrated newspaper mass. Measurements. X-ray diffraction (XRD) experiments were conducted using X Pert PRO MRD diffractometer (PANanalytical B.V., The Netherlands) using CuK radiation. Mechanical testing was performed in accordance with ČSN EN ISO standard using Zwick/Roell Z005 device. Preparation of thermoplastic starch and starch/filler hybrids. Thermoplastic starch (TPS) was prepared on the basis of solid starches (70% w/w) with glycerol (30% w/w). Sodium montmorillonite was dispersed in distilled water (1% dispersion) and stirred 24 hrs; the resulting well-matured dispersion was poured into the starch/water/glycerol solution (w/w ratio matrix/clay 97.5/2.5 or 95/5, respectively). The thermoplastic mixture of the pure matrix or the composite with MMT was poured on the flat PVC surface and let to cool (solution synthesis). Hydrophobization. Films prepared by casting (stripes 6 x 15 cm) were surface hydrophobized by spraying the material with water diluted hydrophobization media. Results 1-46

4 For us so called cationic starches ( modified by 2,3 epoxypropyltrimethylammonium chloride) have been most challenging due to their structure friendly for entering the interlayer area of MMT (they are polycations!!). During reaction with the native starch, the epoxy ring opens introducing the ether bond and terminal trimethylammonium groups to the starch structure (Fig. 5).: Figure 5 : Structure of cationized starch segment Preparation of thermoplastic starch matrices based on modified starches of various structure and DS provided interesting relationship between the mechanical strength of matrices and their structure. Cationic starches plasticized with glycerol generally provided matrices with poor material strength compared to native potato starch; products based on acetylated and dialdehyde starches were even stronger (Table I): TABLE I Influence of starch modifications in TPS prepared by solution synthesis on mechanical properties of the starch matrix. TPS Sample Tensile strength (MPa) Elongation at break (%) Native starch Cationized, DS = Cationized, DS = Cationized, DS = Acetylated, DS = Acetylated, DS = Dialdehyde starch 3.81 NA (Corn starch) 0.48 NA Reinforcement of the matrix based on plasticized starches has been done by both plastic and non-plastic fillers. Table II gives results of mechanical testing of casted composite plastic films with reinforcement by clays. Generally, the strongest composite films were prepared with the unmodified potato starch. 1-47

5 TABLE II Influence of starch modifications in TPS nanocomposites (with plastic clays as reinforcement) on mechanical properties (solution synthesis). Composite: TPS/clay Tensile strength (MPa) wt.% +5.0 wt.% Montmorillonite Native potato starch/mmt Cationized, DS = 0.04/MMT NA 4.33 Cationized, DS = 0.08/MMT NA 3.65 Cationized, DS = 0.17/MMT NA 1.73 Acetylated, DS = /MMT Acetylated, DS = /MMT Table II continued: Bentonite Native starch /SABENIL Native starch /LUTILA Acetylated, DS = /SABENIL Acetylated/Crosslinked, DS = /SABENIL Corn Starch/SABENIL Corn Starch/LUTILA Kaolin Native potato starch/kaolin Non-plastic minerals as micro-and nanofillers provide similar results to clay minerals and clays. Application of waste cellulose as a filler in slightly higher amounts compared to minerals gives the best mechanical results. Materials with various combinations of fillers exhibit tensile strength in wide range nevertheless chalk and waste cellulose provided the worst/best results (Table III, IV).: TABLE III Mechanical properties of some TPS nanocomposites with non-plastic reinforcement (solution synthesis). Composite: TPS/non-plastic reinforcement Tensile strength (MPa) wt.% +5.0 wt.% Chalk Diatomite Microsilica Nanosilica LUDOX AM Nanosilica LUDOX AS Nanosilica LUDOX TM Cellulose 11.6 (4 wt.% 19.2 (11.6 wt.% addition) 1-48

addition) TABLE IV Mechanical properties of some TPS nanocomposites with mixed reinforcement (solution synthesis). Tensile Composite: TPS/mixed reinforcement strength (MPa) MMT 5 wt.

6 addition) TABLE IV Mechanical properties of some TPS nanocomposites with mixed reinforcement (solution synthesis). Tensile Composite: TPS/mixed reinforcement strength (MPa) MMT 5 wt.5% + Cellulose 5 wt.5% 15.8 Bentonite 5 wt.5% + Chalk 5 wt.5% 5.8 Bentonite 5 wt.5% + Chalk 10 wt.5% 3.7 Silica 5 wt.5% + Diatomite 5 wt.5% 8.8 Bentonite 5 wt.5% + Diatomite 5 wt.5% + Chalk 5wt.% 2.9 MMT 2.5 wt.% + bamboo fibres load parallel to fibre 14.7 reinforcement MMT 2.5 wt.% + bamboo fibres load perpendicular to fibre 9.8 reinforcement The composite plastic films based on thermoplastic starch matrix reinforced by different natural fillers (individual or combination) have comparable strength to synthetic plastic products but are ideally biodegradable. Nevertheless they all exhibit high affinity to water (all components are miscible with) which is a principal problem of this biodegradable material that hinders its utilization as packaging film. There are several methods of supressing hydrophilic properties linked often to polysaccharide chemistry. One of the most applied principle is crosslinking of polysaccharides and a great variety of physical crosslinking (by ionic interactions, crystallization, hydrophobisation) and chemical crosslinking methods has been used nowadays 6. In addition to crosslinking, blending starch with synthetic polymers with also improve the performance of pure bioplastics 7. We were interested in such methods only that were not toxic and did not otherwise jeopardize our environment. Several chemical methods has been tested use of organosilicon compounds, natural oils etc 8. Linseed and tung oil are examples of drying natural oil that contain esters of unsaturated acids. In the atmosphere, they slowly polymerize and this system has been found very good for conditioning the biodegradable composite materials. Hydrofobization of the material has been performed by dipping the biodegradable composite into the commercial product at laboratory temperature - boiled linseed oil. All the applied systems (linseed and tung oil; boiled linseed oil) created an impermeable film on the surface of composite film. Figure

7 Contact angles of biodegradable material before (left) and after (right) hydrophobization An important issue characterizing ability of the biodegradable material to withstand contact with water is the mechanical strength after certain time of contact. As given in Table V, the material after hydrophobization still has considerably good mechanical properties that drop down strongly after one hour full exposition to water. It is positive that tensile strength values 2 3 MPa can be sufficient for many packaging applications. TABLE V MECHANICAL PROPERTIES OF SOME TPS NANOCOMPOSITES AFTER HYDROPHOBIZATION AND WATER EXPOSITION (SOLUTION SYNTHESIS) Composite: TPS/reinforcement hydrofobized Tensile strength (MPa) MMT 5 wt.5% + Cellulose 5 wt.5% 15.8 MMT 5 wt.5% + Cellulose 5 wt.5% hydrofobized: boiled linseed oil 14.6 MMT 5 wt.5% + Cellulose 5 wt.5% hydrofobized: boiled linseed oil +1 hour exposition to water 2.9 Bentonite 5 wt.5% 7.4 Bentonite 5 wt.5% hydrofobized: polydimethylsiloxane water emulsion 5.9 Silica 5% 8.8 Silica 5 wt.5% hydrofobized: boiled linseed oil 6.8 Silica 5 wt.5% hydrofobized: boiled linseed oil +1 hour 2.2 exposition to water Cellulose 11.6% 19.2 Cellulose 11.6% hydrofobized: boiled linseed oil 19.0 Cellulose 11.6% hydrofobized: boiled linseed oil +1 hour exposition to water 3.1 Biodegradation Biological degradation of selected samples of plastic films has been determined both in soil under aerobic conditions (the biologic material substrate was a general agricultural soil with compost and in water aerobic milieu in presence of the activated sludge from municipal sewage treatment plant. As reference readily biodegradable natural materials have been selected: corn starch for both testing and glucose for aqueous biodegradation. It was proved that the decomposition in aqueous environment proceeded after 8-9 days with the lag period less than 8 hours. All materials showed very similar behaviour during degradation process. 1-50

Figure 7 Biodegradation test in water environment (plastic fillers) Figure 8 Biodegradation test in water environment (non-plastic fillers) The percentage of biological degradation amounts up to 76

For non-plastic mineral fillers the biodegradation amounts to 60 70% with the best results for micro- and nanosilica fillers (Fig.

8 Figure 7 Biodegradation test in water environment (plastic fillers) Figure 8 Biodegradation test in water environment (non-plastic fillers) The percentage of biological degradation amounts up to 76 85% for plastic mineral fillers and cellulose.this value is comparable well with easily degradable material natural corn starch (Fig. 7). For non-plastic mineral fillers the biodegradation amounts to 60 70% with the best results for micro- and nanosilica fillers (Fig. 8): In soil aerobic environment, the percentage of biodegradation was deteremined as biochemical oxygen demand (BOD) necessary for biological decomposition of material under testing. The samples were decomposed by normal soil microflora during 13 days with the lag phase shorter than 24 hours. According to filler characteristics the percentage of biological decomposition reaches 60 65%, i. e. value of well biodegradable substance such as natural corn starch (Fig. 9): It should be mentioned that despite of referenced biodegradability of all finally used hydrophobization agents (linseed and tung oils; boiled linseed oil), according to preliminary tests these materials considerably reduce the biodegradation rate both in soil and water environment. This is the subject of ongoing testing. Summary Figure 9 Biodegradation tests in soil This contribution deals with green chemistry composite products based on plastified starch and various reinforcement based on pure natural raw materials. Resulting biocomposites have good mechanical properties compared to commercial synthetic 1-51

9 products and very high degree of biological decomposition both in aqueous and soil environment. Major problems include very high hydrophilic affinity of these materials badly influencing their strength causing final destruction of the product after not a long time. From plenty of hydrophobization systems the advantageous surface protection has been chosen from natural unsaturated oils that undergo crosslinking when in contact with atmosphere. Resulting hydrophobized materials have good mechanical properties and withstand contact with water for 1 hour period without visible destruction. Mechanical properties of such material though with hydrophobic surface properties drop down quickly but are sufficient enough for some applications (packaging). Utilization of natural abundand raw materials for green products that do not harm our environment, are not toxic and difficult to decompose is the right way to go. On the other side our everyday requirements for better material properties of these green biomaterials may raise other material and environmental problems not easy to solve. The authors wish to thank the Ministry of Education, Youth and Sports of the Czech Republic (project SGS ) for financial support. References 1. Biodegradable polymers for industrial applications. R. Smith, Ed. Woodhead Publishing Ltd, Cambridge, England Richard P. Wool, Xuizhi S. Sun: Bio_Based Polymers and Composites. Elsevier AP, USA, ISBN E. S. Stevens: Green Plastics. An Introduction to the New Science of Biodegradable Plastics. Princeton University Press, Princeton, New Jersey, USA, Brydson J. A.: Plastic Materials, 3 rd Ed. Butterworth London Grim, R. E.: Clay Mineralogy (2 nd ed.), McGraw-Hill, New York, Patil S., Jadge D. R.: Pharmaceutical Reviews 6 (2), Reddy N., Yang Y.: Food Chemistry 118(3), (2010). 8. Petr Duchek, Jaromír Dlouhý, Petr Franče: Hydrophobization of starch-based biodegradable materials. 8th International Conference on Polysaccharides- Glycoscience, Prague, November ISBN:

INFLUENCE OF CLAY ON MECHANICAL PROPERTIES OF POLYVINYL(ALCOHOL)/ MONTMORILLONITE MEMBRANES

INFLUENCE OF CLAY ON MECHANICAL PROPERTIES OF POLYVINYL(ALCOHOL)/ MONTMORILLONITE MEMBRANES Maria C. Carrera 1*, Eleonora Erdmann 1, Hugo A. Destéfanis 1 Marcos L. Dias 2, Victor J. R. R. Pita 2 1 Instituto