RO131882B1 - Composition and process for preparing nanocomposites from polyhydroxyalkanoates and bacterial cellulose nanofibres - Google Patents

Description

The invention relates to nanocomposites of polyhydroxyalkanoates and bacterial cellulose nanofibers and to the process for obtaining them, which are used in biomedical applications to obtain biodegradable medical devices but also in other fields such as obtaining packaging and automotive parts. Bacterial cellulose nanofibers and composites made of polyhydroxyalkanoates and bacterial cellulose nanofibers are materials from renewable resources, 100% biodegradable and biocompatible. The realization of these materials is part of the effort to replace materials based on synthetic polymers, which are obtained from petroleum raw materials, in order to protect the environment and use natural resources, theoretically unlimited, as sources of raw materials. The compounds of polyhydroxyalkanoates and bacterial cellulose are non-toxic, biodegradable and have good mechanical properties, being indicated in various fields, from; biomedical field, packaging up to technical fields such as automotive and electronics. Composites made of polyhydroxyalkanoates and 0.2 - 20% nanofibers from bacterial cellulose are obtained on melt processing plants specific to thermoplastic polymers and have better mechanical properties than polymers used as matrices.

Current environmental problems have led to an intensification of research for the partial or total replacement of materials from fossil resources, which are depletable in the near future and whose processing is harmful to the environment, with materials from natural resources.

Polyhydroxyalkanoates (PHA) are natural aliphatic polyesters, biosynthesized by bacteria and which accumulate as intracellular energy reserves. The plastic properties and biodegradability of PHA have led to their research as potential substitutes for non-biodegradable polyethylene and polypropylene, but also as biodegradable and biocompatible biomaterials for implants and other medical devices. Among PHA, the most advanced in terms of research and production are po I i - (3-h i d rox i b u t i r at) (PHB) and poly- (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The use of PHB and PHBV in practical applications is, however, hindered by the fact that they are brittle and have low thermal stability at high processing temperatures, the melting range overlapping over the beginning of their thermal degradation. Unlike PHB, PHBV has a higher elasticity and a lower range of processing temperatures, which allows only partial avoidance of degradation during processing. Several methods have been tried to overcome these disadvantages of commercial PHA. One of them proposes mixing PHB and PHBV with other biodegradable polymers, such as poly (ε-caprolactone) (PCL) or polylactic acid (PLA), to reduce scarring. Other ways suggest the use of fillers such as wood fibers, flax fibers, recycled cellulose fibers or other types of cellulose fibers to make composite materials. An aliphatic polyester obtained synthetically from a glycol and a carboxylic acid was mixed with coconut powder and carbon black and the mixture was processed in the melt (extrusion, injection or other process) to obtain various articles, being described in US 6380282 B1. Although among the polyester variants that can be used in these blends is PHB, the only examples in US 6380282 B1 refer only to Bionore, a polybutylsuccinate synthetically produced by Showa Kobunshi (Japan) and a non-biodegradable polypropylene. The filler proposed in this invention for use in composite is of micron size and aims only to increase biodegradability, no indication of mechanical or thermal properties being given in US 6380282 B1.

Several attempts to use cellulose nanofillers in PH B or PHBV and to make bionanocomposites can also be recorded (Jiang et al., 2008). However, all these nanocomposites are based on plant nanocrystalline cellulose, which requires

EN 131882 Β1 laborious intermediate work to be brought to the state of 100% cellulose nanofibers, with 1 high mechanical strength. In addition, PHBV / nanocrystalline cellulose bionanocomposites were made by the deposition method using N, N-dimethylformamide as solvent, and a slight increase in tensile strength and Young's modulus was observed. When the melt compounding method was tried to obtain the nanocomposites, a good dispersion of the nanofill could not be achieved, which agglomerated into micron-sized aggregates and, as a consequence, the obtained composites showed weaker properties than the polymeric matrix from which it was started ( Jiang et al., 2008). It should be noted that the solution deposition method used in this reference cannot be applied industrially and may raise some environmental issues.

A different method is proposed in CN102493021 B and consists in obtaining 11 nanofibers composed of PHBV and cellulose nanocrystals from the solution. The cellulose nanocrystals were dispersed in N, N-dimethylformamide and slowly added to a solution of 13 PHBV in chloroform. Although good mechanical results are specified for such composite fibers, the method uses large amounts of solvents, some of which are very toxic. 15

Bacterial cellulose has clear advantages over plant cellulose because it is pure and therefore does not require expensive procedures to separate hemicelluloses, lignin, proteins, waxes and other impurities as in the case of plant cellulose fibers. Therefore, attempts were made to make nanobiocomposites from PHA and bacterial cellulose nanowiscers by the method of pouring from the solution, but no improvement of the mechanical properties was obtained (Martinez-Sanz et al., 2014). Another pathway that was tried 21 was to immerse the whole bacterial cellulose film in a solution of PHB in chloroform and remove the solvent by lyophilization to obtain a layer. Although the obtained cornice presented better mechanical properties than the matrix polymer, however, by this method can only be obtained laminates with irregular surface and composition difficult to define and reproduce.

Obtaining nanocomposites by casting from solution is not a viable method for the industrial scale manufacture of bio-nanocomposites, due to the use of huge amounts of toxic solvents to be purified, recirculated and the harmful effects they have on the health of personnel who handle these solvents and the environment. Only melt processing techniques such as melt mixing, extrusion or injection can be effective on an industrial scale, and not harmful to the environment and staff health. With these methods it is possible to obtain new materials with 33 modulus of higher elasticity and mechanical strength, as well as better thermal stability. 35

There are few attempts to obtain composites based on biodegradable polymers, which specify that the polymer matrix can be obtained microbially, synthetically or can be a natural polymer such as starch, and the filling is defibrillated bacterial cellulose, these tests being described in US 6410618; US 6274652; 39

PCT / JP99 / 00847. These biodegradable composites are intended for injection molding of video and acoustic equipment. Defibrillated bacterial cellulose was obtained by macerating the aqueous bacterial cellulose film with a rotary mixer, pressing and dispersing in organic solvent (acetone) for dehydration and then changing the organic solvent acetone with cyclohexane. Freeze-drying of dehydrated bacterial cellulose resulted in dry bacterial cellulose. It is stipulated in these inventions 45 that the cellulose obtained is in powder form, but its dimensions are not specified. It is also mentioned in these inventions that the composite material is obtained by melting the biodegradable polymer and cellulose 'powder', preferably in

RO 131882 Β1 concentration above 30% in a screw device. The dimensions of the 'bacterial cellulose powder' are not specified and no indication is given as to the mixing process. Correlating with the literature data, it can be assumed that the bacterial cellulose obtained by this process has micron dimensions. In the two examples of these inventions, only polylactic acid and a Bionore-type polyester are specified as biodegradable matrix polymers, which is a synthetic polyester (not microbial) of the polybutylene succinate type and not a polyhydroxyalkanoate.

A composition for interior car parts, containing aliphatic polyester and bacterial cellulose, is described by JP 2007112859. The composition contains two different types of polyhydroxyalkanoates and bacterial cellulose. The type of polyhydroxyalkanoates used is not specified, but probably differs in the number of carbon atoms in the side chain. The form of bacterial cellulose used in the mixture (dry or wet) is not specified but only that it is in the form of micro / nanofibers with a diameter of up to 1000 nm and a form factor between 1 and 1000 and that it is used in a proportion of 0.1 % to 50%, as a nucleating agent. Nothing is mentioned regarding the strengthening function of bacterial cellulose in this invention.

C. Zhijiang, H. Chengwei, Y. Guang published a study, Poly (3-hydroxybutyrate-co-4-hydroxybutyrate) / bacterial cellulose composite porous scaffold: Preparation, characterization and biocompatibility evaluation, in Carbohydrate polymers, 2012, on lyophilization preparation, characterization and evaluation of biocompatibility of compounds based on poly (3-hydroxybutyrate-co-4-hydroxybutyrate) and bacterial cellulose used for the preparation of scaffolds, using trifluoroacetic acid as cosolvent; the characteristics of the scaffolds obtained from the poly composite (3hydroxybutyrate-co-4-hydroxybutyrate) and bacterial cellulose were evaluated, the biodegradation capacity but also the biological activity, the composite being evaluated by cell adhesion studies that proved a better biocompatibility of the composite than scaffolds obtained only from poly (3-hydroxybutyrate-co-4-hydroxybutyrate). Also, CJ Brigham, AJ Sinskey showed in “Applications of polyhydroxyalcanoates in the medical industry”, published in the International Journal of Biotechnology for Wellness Industries, 1.53-60, the results of studies of biodegradability and biocompatibility of polyhydroxyalkanoates in cell cultures or host animals, which suggest that polyhydroxyalkanoates are suitable materials for the manufacture of resorbable medical devices.

Patent application JP 2012087207 describes a biomass resin composition, biodegradable and with sufficient thermal and mechanical strength to replace synthetic polymers for various applications, based on lactic acid and 3-hydroxybutyric or 3-hydroxyvaleric acid.

F. Torres et al., In the article “Biocompatibility of bacteria! cellulose based biomaterials ”, published in J. Funct. Biomater. 2012.3 (4); 864-878 refers to the possibility of using bacterial cellulose in nanocomposites along with other materials for specific applications; In vivo and in vitro studies have shown that bacterial cellulose can be used in various medical applications as the treatment of wounds, burns, as artificial skin, obtaining artificial vessels, scaffolds.

Known solutions have a number of disadvantages due to the high toxicity and corrosivity of the solvents used or the lack of biodegradability of the polymeric components in the resulting composite materials.

The technical problem solved by the invention consists in the capitalization of renewable natural resources for obtaining composite materials usable in biomedical applications.

RO 131882 Β1

The nanocomposite based on polyhydroxyalkanoates and bacterial cellulose according to 1 of the invention eliminates the disadvantages mentioned in that it consists of 0.2 to 20 parts nanofibers of bacterial cellulose, up to 20 parts plasticizer and 60 to 99.8 parts 3 polyhydroxyalkanoate, which may be poly- (3-hydroxybutyrate) or poly (3-hydroxybutyrate-co-3-hydroxyvalerate). 5

The process for obtaining the nanocomposite according to the invention consists in mixing the components in a sigma rotor mixer, with counterclockwise rotation, at a speed of 7 to 20 to 60 min ' ¹ , for 3 to 15 min, at a temperature of 140 and 200 ° C, depending on the polyhydroxyalkanoate and the amount of plasticizer, the resulting material being profiled on 9 rolls and finally the grains in a mill with rotary knives or in a rotary mixer for 10 to 30 minutes at a temperature between 20 and 35 ° C, followed by extrusion on an 11 twin-screw extruder with L / D 20-30, at a temperature ranging between 140 and 200 ° C depending on the type of polyhydroxyalkanoate and the amount of plasticizer, the resulting material being finally 13 granulated.

In one embodiment of the process, the dry powdered bacterial cellulose nanofibers are obtained by defibrillating wet bacterial cellulose films in an amount of up to 20% relative to the wet film in a high power blender for 17 15 ... 60 min, the suspension of defibrillated cellulose being subsequently passed through a colloidal mill, in a number of up to 5 passes, then lyophilized and ground using a centrifugal mill 19 at a speed of at least 5000 min ' ¹ .

According to the invention, the nanocomposite materials are obtained using both matrix polymer 21 (polyhydroxyalkanoates) and the reinforcing material (bacterial cellulose nanofibers) from renewable natural resources, which can be used both as substitutes for composite materials 23 with glass fibers, black smoke and other fillers harmful to the environment but also in biomedical applications for obtaining scaffolding and medical devices. These nanocomposite materials can be obtained on melt processing plants, specific to thermoplastic polymers, which allow to obtain with good productivity some forms of high complexity. The nanofibers of bacterial cellulose are obtained in powdery, dry form starting from wet bacterial cellulose films, without using any organic, toxic solvents. By mixing in the melt using the processes of kneading, extrusion, pressing or injection of polyhydroxyalkanoates in powder form or granules with bacterial cellulose nanofibers 31 in powder, dry form, whether or not having surface treatment agents, new nanocomposite materials are obtained. , 100% biodegradable, characterized by better mechanical properties 33 compared to the polymer from which it started. The nanocomposite material can be obtained in the form of plates, wires or granules, being possible to use it to obtain various benchmarks for the biomedical field or industry.

According to the above, there are no processes for obtaining bacterial cellulose nanofibers in powder form, dried from wet bacterial cellulose films or bacterial cellulose nanofiber polymer nanocomposites 39.

The production processes according to the invention make it possible to obtain bacterial cellulose nanofibers 41 in powder form, dried starting from wet bacterial cellulose films and nanocomposites with controlled characteristics, characterized by a uniform dispersion 43 of the bacterial cellulose nanofibers in the polyhedral matrix.

The process for obtaining bacterial cellulose nanofibers in powder form, 45 dry, with a section size between 10 and 200 nm, according to the invention, starts from wet bacterial cellulose films which are defibrillated in a small amount of water, of maximum 47 20% reported to wet film, using a high-power blender, for 10 to 60 minutes,

RO 131882 Β1 the suspension of cellulose defribiled with the help of a blender being passed through a colloidal mill (0 ... 5 passes), then lyophilized and ground using a centrifugal mill (at a speed of at least 5000 min ' ¹ ).

The process for obtaining polymeric nanocomposites with bacterial cellulose nanofibers in powder form according to the invention consists in that 0.2 ... 20 parts of bacterial cellulose nanofibers or not treated with organosilanes, 0 ... 20 parts plasticizer and 60 ... 99.8 parts of polyhydroxyalkanoate are mixed 1) directly in a mixer equipped with sigma rotors, with counterclockwise rotation, with a speed between 20 and 80 min ' ¹ for 3 ... 15 min, at a temperature between 140 and 200 ° C, depending on the type of polyhydroxyalkanoate used as the polymer matrix and the amount of plasticizer, then profiled on a roller in the sheet, which is crushed in a mill with rotating knives, resulting in nanocomposite A or 2) first in a mixer rotary for 10 .. .30 min at a temperature between 20 and 35 ° C and then in a double-screw extruder with L / D = 20 ... 30 at a temperature ranging between 140 and 200 ° C, depending on the type of polyhydroxyalkanoate used as the polymer matrix and edges plasticizer, then granulated and resulting in granules of nanocomposite B.

Nanocomposites made of polyhydroxyalkanoates and bacterial cellulose are extremely useful for many fields of application because they are 100% biodegradable and biocompatible, being useful both in the biomedical field and in other fields, such as packaging and automotive parts. Bacterial cellulose is pure cellulose that does not require additional processes to remove hemicelluloses, lignin, proteins, waxes and other impurities, which is usually applied to obtain plant nanocellulose. in addition both components of the composite material (cellulose and polyhydroxyalkanoate) are obtained from renewable resources and are not harmful to the environment. Nanocomposites made of polyhydroxyalkanoates and bacterial cellulose nanofibers are obtained on melt processing plants specific to thermoplastic polymers, which eliminates the problems posed by the use of toxic solvents, specific to the process of obtaining composites by pouring from solution or the process of changing the solvent. . In addition, melt compounding processes are extremely advantageous in terms of productivity, environmental impact and the wide possibility of obtaining extremely complex shapes with very short processing time. It should be noted that objects made of polyhydroxyalkanoate nanocomposites / bacterial cellulose nanofibers are not harmful to the environment at the end of their life, because, as specified, they are made of 100% biodegradable materials.

The invention has the following advantages:

- elimination of the consumption of raw materials deriving from oil processing, by using both the polymer matrix and the nano-fillers from renewable resources; avoidance of by-products and waste, which pose problems of separation, recovery or destruction;

- making materials with new characteristics through processing operations on high productivity installations, specific to thermoplastic polymers;

- The materials of polyhydroxyalkanoates and bacterial cellulose nanofibers according to the invention can be used to obtain scaffolds and medical devices but also as substitutes for traditional composites, with glass fibers, harmful to the environment, in technical applications in which these polymeric composites are currently used.

The bacterial cellulose nanofibers obtained according to the process of the invention, as well as the bacterial cellulose film from which it was derived, were characterized by AFM atomic force microscopy to determine the fiber size and the degree of defibrillation using a Multi Mode 8 microscope equipped with a Nanoscope V converter and an untreated silicon cantilever at room temperature with a scanning speed of 1 Hz.

RO 131882 Β1

The polymeric composites with bacterial cellulose nanofibers obtained according to 1 invention were characterized by:

- maximum tensile strength and elasticity module using a 3 universal Instron 3382 machine with video extensiometer, 2KN force cell, test speed of 2 mm / min, type 5A specimens according to SR EN ISO 527; 5

- AFM atomic force microscopy to determine the size and dispersion of nanofibers in the polymer matrix using a MultiMode 8 microscope equipped with a 7 Nanoscope V converter and an untreated silicon green cantilever at room temperature with a scanning speed of 1 Hz. 9

The following are 10 non-limiting examples of the invention.

Example 1 11

200-300 g of films of bacterial cellulose obtained by biosynthesis in static culture, such as according to the invention R0126940, are placed in a vessel and rinsed with sufficient quantities of distilled water to remove any impurities from biosynthesis or post-treatment . With the help of a high power blender (minimum 600 W) and a speed of 15 minimum 15000 min ' ¹ , the bacterial cellulose films to which a maximum amount of 20% distilled water is added, compared to the wet film, are defibrillated a total time of 15 min 17 at room temperature in cycles of 4-5 min each. The defibrillated cellulose is lyophilized using a 2.5 L FreeZone lyophilizer. The cellulose obtained in the form of foam is then crushed 19 in a ZM 200 type centrifugal mill (Retsch), resulting in dry cellulose nanofibrils in powder form. Defibrillation of the bacterial cellulose film was highlighted by 21 AFM compared to the initial film (Fig. 1 a - AFM image of the bacterial cellulose film; Fig. 1b - AFM image of the defibrillated film according to Example 1). 2. 3

Example 2

We work as in Example 1 with the difference that the total defibrillation time with the blender is 30 min. The AFM image of the fibers (fig. 1 c) confirms the more advanced defibration and their smaller size. 27

Example 3

We work as in Example 1 with the difference that between the defibrillation of the bacterial cellulose films with the help of the blender and lyophilization, an additional phase of homogenization is interposed using a colloidal mill (1 pass). 31

Example 4

We work as in Example 3 with the difference that the defibrillated bacterial cellulose with the help of a blender is passed 5 times through the colloidal mill. Microscopic investigation of the fibers confirms a more advanced defibrillation. 35

Example 5

A quantity of 0.25 g of dry powdered bacterial cellulose nanofibers, 37 obtained according to Example 2, is mixed with 49.75 g of PHB powder in a mixer equipped with sigma rotors at 170 ° C at a rate of 40 ° C. rpm for 10 min. The obtained composite 39 (CI) is profiled on the roller. For characterization, plates with dimensions of 150 x 150 x 2 mm are pressed, from which specimens for mechanical tensile tests 41 with dimensions according to SR EN ISO 527 are stamped. The results obtained are presented in table 1 compared to simple PHB, processed under the same conditions (M1 ). 43

Example 6

Work as in Example 5 with the difference of 1.00 g of bacterial cellulose 45 nanofibers in powdered, dry form, obtained according to Example 2, mixed with 49.00 g of PHB powder, obtaining composite C2 with the mechanical characteristics in Table 1. 47

RO 131882 Β1

Example 7

Work as in Example 5, with the difference of 2.50 g of dry powdered bacterial cellulose nanofibers, obtained according to Example 2, mixed with 47.50 g of PHB powder, obtaining composite C3 with the mechanical characteristics in Table 1.

Example 8

A quantity of 1.00 g of dry powdered bacterial cellulose nanofibers, obtained according to Example 1, is mixed with 49.00 g of PHBV (12% polyhydroxyvalerate) containing 10% of tributylacetyl citrate plasticizer in a mixer equipped with sigma rotors at temperature of 160 ° C with a speed of 40 rpm for 8 min. The composite obtained (C4) is profiled on a roller, pressed into plates, from which specimens for mechanical tensile tests are stamped. The results obtained are presented in Table 1 compared to PHBV without bacterial cellulose fibers, processed under the same conditions (M2).

Example 9

A quantity of 1.00 g of bacterial cellulose nanofibers in powder form, obtained according to Example 1, is dispersed in 50 g of a 10% solution of aminopropyltriethoxysilane in a mixture of 90:10 ethanol: water, stirred at room temperature for 2 h, leave in a vessel uncovered for 24 h at room temperature and then in an oven at 110 ° C for 1 h, resulting in bacterial cellulose nanofibers treated with aminosilane mixed with 49.00 g PHBV (12% polyhydroxyvalerate ) containing 10% tributylacetylcitrate plasticizer in a mixer equipped with sigma rotors at 160 ° C at 40 rpm for 8 min. The composite obtained (C5) is profiled on a roller, pressed into plates, from which specimens for mechanical tensile tests are stamped (Table 1).

Example 10

A quantity of 5.50 g of cellulose nanofibers in powdered, dry form, obtained according to Example 1, is mixed with 269.50 g PHB powder in a rotary mixer for 20 min at room temperature and then in a double screw extruder with L / D = 20 at a temperature varying between 140 and 170 ° C from the feed in the extrusion head, the obtained wires being granulated and resulting granules of composite material. The composite material obtained shows no signs of degradation.

Table 1

Composite Maximum tensile strength MPa Tensile modulus of traction MPa M1 264 2301 C1 281 2538 C2 29.9 2715 3 258 2638 M2 22.8 1560 4 239 1758 C5 245 1651

Claims (3)

1. Nanocomposite based on polyhydroxyalkanoates and bacterial cellulose, characterized in that it consists of 0.2 to 20 parts nanofibers of bacterial cellulose, up to 20 parts plasticizer and 60 to 99.8 parts polyhydroxyalkanoate, which can be poly- (3- 5 hydroxybutyrate) or poly (3-hydroxybutyrate-co-3-hydroxyvalerate). Process for obtaining the polymeric nanocomposite according to claim 1, 7, characterized in that it consists in mixing the components either in a sigma rotor mixer, with rotation in the opposite direction, at a speed of between 20 and 60 min ¹ , time 9 3 to 15 min, at a temperature between 140 and 200 ° C, depending on the polyhydroxyalkanoate and the amount of plasticizer, the resulting material being profiled on a roller and in the final 11, the grains in a mill with rotating knives, either in a rotary mixer for 10 to 30 minutes at a temperature between 20 and 35 ° C, followed by extrusion on a 13-screw twin-screw extruder with L / D 20-30, at a temperature ranging from 140 to 200 ° C depending on the type of polyhydroxyalkanoate and the amount of plasticizer, the resulting material being finally granulated. 15 Process for obtaining the polymeric nanocomposite according to claim 2, characterized in that the bacterial cellulose nanofibers in dry powder form 17 are obtained by defibrillating the wet bacterial cellulose films in an amount of maximum 20% relative to the wet film, in -a high power blender for 15 .. .60 min, 19 the defibrillated cellulose suspension being subsequently passed through a colloidal mill, in a number of up to 5 passes, then lyophilized and ground using a centrifugal mill at a speed of 21 minimum 5000 min ' ¹ .