Patent Application
20080108772
A considerable demand exists for renewable and biodegradable plastics to be used for various purposes. Previous attempts to provide such plastics have met with mixed results, either not being sufficiently strong, or not being sufficiently bio-degradable. The present invention discloses a strong nanocomposite comprised mainly of cellulose compounds, and thus provides an environmentally beneficial solution satisfying both the above criteria.
Cellulose is the most abundant of naturally occurring organic compounds. As the main constituent of the cell walls of higher plants it comprises at least one third of the vegetable matter of the world. In spite of its wide distribution in nature, cellulose for chemical purposes is derived commercially from two sources, cotton linters and wood pulp. Evidently the exploitation of cellulose within the field of polymer manufacture would be beneficial if the produced polymer exhibited the desired properties.
Cellulose esters for instance are useful polymers when manufacturing plastics. The most important of the esters is cellulose acetate. Cellulose acetates are more costly than other commercial polymers like PVC, PS and polyolefins, but retain their small market share due to their advantageous characteristics. Since the early 1990s biodegradable cellulose acetates have been available. These systems are centred around the use of an additive which acts both as a plasticizer and biodegrading agent, causing the cellulose acetate to decompose within 6-24 months.
It is known that polymer properties can be modified and improved by reinforcement with fibres. For instance glass-, carbon-, aramid, natural fibres may be used to increase the mechanical properties of the pure polymer as is well known. In order to form a fully bio-degradable system use of natural fibres is a possible approach. The use of natural fibres to reinforce the polymer must per force involve compromises between preferred properties as the increase of one material property often comes at the expense of another. Typical trade-offs may be between stiffness and toughness or toughness and transparency.
The use of nanoscale reinforcements allows the avoidance of such trade-offs as the reinforcements are so small that they do not easily scatter light, thus allowing the addition of such reinforcing materials without reducing the transparency of the resulting plastic. The interfacial area of the reinforcing particles is very large due to the small size of the particles, and the reinforcing particle will thus to a great extent interact with the polymer further enhancing the effectiveness of the reinforcement.
The reduced particle size of the additive further eliminates the shortcomings of using larger scale particles, as for the lower scale particles there are little or no break initiators in the local vicinity of the reinforcing materials.
In comparison to conventional materials, which contain a multiplicity of grain boundaries, voids, dislocations and imperfections, single crystal whiskers approach structural perfection and have almost eliminated such defects. The resultant is a highly ordered structure showing exceptional properties. The mechanical strength of the whiskers may approach the binding forces of adjacent atoms.
If additionally the reinforcing material is comprised of a biodegradable material, and this reinforcement is mixed into a biodegradable matrix, it is possible to obtain a completely biodegradable plastic material having a combination of desirable properties.
It has however proven difficult to manufacture the amount of nanoscale reinforcements necessary for the large-scale production of such biodegradable plastics. The problems of upscaling the production process to achieve large amounts of reinforced plastics have additionally never been adequately solved. The production of such nanoscale reinforcements has been very costly and it has proven difficult to achieve an adequate dispersion of the reinforcing material in the matrix to be reinforced.
The above problems have resulted in the use of such plastics being of small industrial interest, however the present invention discloses means and materials for overcoming said problems.
Multiple attempts have been made to manufacture biodegradable polymers being strong, thermally stable, cheap, and being adapted for large scale production.
Grunert and Winter[1] describe a material comprising Cellulose Acetate Butyrate reinforced by cellulose nanocrystals, however the authors were not able to adequately disperse the cellulose nanocrystal reinforcement without performing a trimethylsilation of the nanocrystals. The individual crystals show a tendency to aggregate thus weakening the polymer, and no significant strengthening of the material was reported. Additionally the cellulose nanocrystals were extracted by means of sulphuric acid hydrolysis, thus probably weakening the crystals. Lastly the nanocrystals were obtained from bacterial cellulose, a very time consuming and costly process, suitable mainly for very small scale production. The characteristics of the resulting polymer were not substantially improved.
Oksman et al.[2] describe a method for producing a reinforced polymer comprising a polylactic acid matrix material and a microcrystalline cellulose reinforcing material. The materials were mixed in an extruder using a melt compounding technique. The microcrystalline cellulose reinforcing material is swelled/separated by a N,N-Dimethyl acetamide (DMAc) and lithium chloride (LiCl) mixture. However the strength and the thermal stability of the resulting polymer are significantly lower than for the present invention. Furthermore LiCl can cause degradation of cellulose, and is thus undesirable for use with cellulose. The dispersion of the microcrystalline reinforcements was furthermore reported to be incomplete, thus the reinforcements provided insufficient reinforcement to the matrix material.
Peterson and Oksman[3] describe the use of either a swelled bentonite or a swelled microcrystalline material for the reinforcement of a polylactic acid matrix. However, the resulting polymer is weaker and less thermally stable than the polymer of the present invention. Furthermore the optical clarity of the reinforced polymer is inferior to the optical clarity of the disclosed polymer.
In a review of organic nanocomposites Peijs and Vilaseca[4] present several different cellulose based nanocomposites, such as nanocomposites based on bacterial compounds, chitin and paper. The authors point out the inherent difficulties of adequately dispersing the cellulose whiskers, and the polymeric materials to be reinforced have very low strengths, a tensile true stress of about 15 MPa for a reinforcement percentage of about 25 wt %, and very low glass temperatures.
In a second review of the state of the art at the time, Gacitua, Ballerini and Zhang[5] discussed various approaches to using cellulose nanocomposites as reinforcing materials in polymers. Reference is made to the above cited work of Grunert and Winter[1]. However the authors stress the inherent difficulties of both dispersing the nanocrystals in the polymer matrix and the difficulty of obtaining more than very small amounts of the nanocrystals. As has been discussed, the work of Grunert and Winter[1] did not provide any significant reinforcement of the polymer matrix.
Nair and Dufresne in a series of papers[6,7,8] describe the use of crab shell chitin whiskers as reinforcements for a natural rubber. The chitin whiskers were isolated by means of an acid hydrolysis before being swelled and mixed with the natural rubber. The resulting composite was in most instances vulcanized. However the resulting polymer exhibits quite low glass temperatures, and inferior mechanical properties when compared to the polymer of the present invention. The chemical properties and preparation of chitin whiskers also differ from the whiskers prepared according to the current invention.
Kim et al.[9] describe the difficulties of melt compounding nano-sized particles into individual functioning organic polymers, and conclude by stating that mixing blending agglomerated nanoparticles into polymers by conventional plastic melt/compounding methods does not work. This prejudice of the background art is overcome in the present invention.
Bondeson, Mathew and Oksman[10] describe a method for producing large quantities of nanocrystals from microcrystalline cellulose by acid hydrolysis, however sulphuric acid (H2SO4) is used for the acid hydrolysis. The use of sulphuric acid as the hydrolysing agent weakens the nanocrystals, and thus the resulting reinforced polymer. Additionally the resulting polymer has been shown to be more opaque when using nanocrystals produced by means of sulphuric acid.
Morin and Dufresne[11] describe an acid hydrolysis method for the isolation of Chitin whiskers, in which hydrochloric acid is used as the hydrolysing agent. However, the chemical properties of Chitin differ substantially from the chemical properties of cellulose and no polymer is proposed.
The present invention discloses a method for producing a polymer, said method being cheap and upscalable, and in which the resulting biodegradable polymer exhibits superior properties to the polymers as disclosed in the prior art. The uniform dispersion of the reinforcing nanowhiskers is a crucial improvement over the prior art resulting from the improved production method by mixing the nanoreinforcements into the polymer matrix by compound melting, a method previously thought to be impractical if not impossible.
The present invention discloses a method for producing a reinforced organic polymeric material comprising the following steps:
The invention further discloses an organic polymeric material in which the characterising features of the polymer comprise a carbon based matrix, said carbon based matrix mainly comprising cellulose acetyl butyrate and a reinforcing dispersion of cellulose nano whiskers, said cellulose nano whiskers being finely dispersed within said carbon based matrix.
The invention lastly describes a method for producing cellulose nano whiskers comprising the following steps:
Further new and advantageous features are given in the attached dependent claims.
The figures are intended for illustration purposes solely and shall not be in any way be construed to limit the invention which shall only be limited by the attached claims.
a illustrates the nanoreinforcements (3) being in an agglomerated state. There is little or no dispersion of the nanoreinforcements (3) in the carbon based matrix material (2), and the reinforcing effect of the nanoreinforcements (3) on the polymer (1) will be small.
b illustrates the nanoreinforcements (3) being in a swollen state. There is an increased degree of dispersion of the nanoreinforcements (3) in the carbon based matrix material (2), and the reinforcement of the polymer (1) by the nanoreinforcements (3) will be improved compared to the situation illustrated in
c illustrates the nanoreinforcements (3) being in a fully dispersed state. There is a very large degree of dispersion of the nanoreinforcements (3) in the carbon based matrix material (2), and the reinforcement of the polymer (1) by the nanoreinforcements (3) will be considerably improved compared to the situation illustrated in
The invention will in the following be described with reference to the attached figures. The invention discloses a method for manufacturing a reinforced organic polymeric material (1) comprising the following steps:
In a preferred embodiment of the invention said carbon based matrix material (2) is furnished at a first stage (51) of said extruder (5), and said dispersion (3, 4) is furnished into a second later stage of (52) of said extruder (5). This will allow the carbon based material (2) to be at least partially melted before pumping the dispersion (3, 4) into the material (2) allowing for an improved dispersion of the reinforcements (3) into the matrix (2). An illustration of the extruder (5) according to the present invention is shown in
The matrix (2) is fed into the extruder aperture (51) and is a least partially melted when the dispersion (3, 4) of plasticizer is added to the matrix (2) by liquid pumping of the dispersion (3, 4) into the extruder aperture (52). The mixture (2, 3, 4) is mixed and homogenized along the length of the extruder screw before being extruded at an extruder die at the stage (53) of the extruder (5). One or more apertures (54) are arranged for venting the mixture (2, 3, 4) and will serve several purposes. The apertures (54) are used to prevent the formation of voids and thus reduce the porosity of the mixture (2, 3, 4), and also for homogenizing and strengthening the resulting polymer (1) by removing fluids such as excess plasticizer (4) or possible solvents (7).
There are several major advantages to using an extruder in the production of nanocomposites as described in the present invention. The extrusion process is known as a melt compounding process. The main advantage of mixing said dispersion (3, 4) into said matrix (2) by means of a melt compounding process is that one is able to very finely disperse the cellulose nanowhiskers (3) within the carbon based matrix (2). This problem of adequately dispersing the reinforcements (3) has proven to be one of the major obstacles in previous attempts to produce reinforced polymers. This problem is solved by pumping the liquid dispersion (3, 4) directly into the at least partially melted carbon based matrix (2) allowing for an improved mixing of the whiskers (3) into the matrix (2). The dispersion (3, 4) being liquid resolves two different process problems:
Using the method according to the present invention in which the liquid dispersion (3, 4) is pumped directly into the at least partially melted matrix (2) will thus resolve these problems. The extrusion process will firstly at least partially melt the carbon based matrix (2) into a liquid plastic. The addition of the liquid dispersion (3, 4) into the extruder will result in the liquid dispersion (3, 4) being thoroughly homogenized with the at least partially melted carbon based matrix (2) due to the extrusion screw or screws kneading and working on the mixture (2, 3, 4). Specially designed extrusion screws may in some instances be necessary for the improved homogenizing of the mixture. Such screws are known to the person skilled in the art.
Use of a liquid plasticizer will ease the extrusion of the mixture (2, 3, 4) and will for many mixtures (2, 3, 4) be required in order for the extrusion to be possible. The plasticizer (4) must evidently be able to plasticize the matrix (2), and must further not degrade the whiskers (3) nor itself be degraded during the extrusion process. The plasticizer should additionally not adversely affect the resulting properties of the polymer (1) to a too large degree. Any plasticizer (4) fulfilling the above conditions may be used in the process as will be evident to a person skilled in the art.
As shown in
The extruder (5) may preferentially be specially adapted for nanocomposite processing, although most extruders (5) will probably adequately mix the dispersion (3, 4) into the matrix (2). Possible modifications of the extruder (5) for improving the mixing efficiency might include moving the kneading elements towards the extruder die at the extruder outlet (53) in order for building up pressure earlier in the extruder barrel. This may increase the degree of homogenization of the mixture (2, 3, 4).
The extrusion of the mixture (2, 3, 4) as a homogenized polymer (1) at the outlet (53) of the extruder (5) may be performed through any well-adapted extrusion die as will be evident to a person skilled in the art. The shape of the extrusion die will depend on which post treatment of the polymer (1) is desired and the use for which the polymer (1) is intended. The extruded polymer (1) may for instance be compression moulded into films or injection moulded into the desired products.
In a particularly preferred embodiment of the invention the carbon based matrix material (2) will comprise cellulose acetate butyrate (CAB) (21). Experimentation has shown that the use of CAB (21) as the matrix (2) will greatly improve the dispersion of the cellulose nano whiskers (3) in the matrix (2). This is presumed to be due to the chemical properties of the whiskers (3) and the matrix (2) being quite similar, although the precise mechanisms for the improved dispersion are not yet been fully understood. Compared to background art in which many means have been used to achieve the required amount of dispersion of the nanoreinforcements in the matrix (2) without success, this solution uses the natural affinities of two compounds being chemically similar to promote the dispersion.
The use of CAB (21) as the matrix material (2) is advantageous from an environmental point of view as CAB (21) is manufactured by the chemical modification of cellulose, and is thus an entirely renewable resource. CAB (21) is relatively chemically stable and is usually furnished as a powder or granular and may thus easily serve in the industrial production of polymers. CAB (21) also presents the major advantages of being biodegradable and being cheap compared to other matrix materials (2) having comparable physical properties.
The use of CAB (21) in the extrusion process necessitates an appropriate plasticizer as CAB (21) is relatively brittle and little adapted to extrusion. In a preferred embodiment of the invention, triethylcitrate C12H20O7 (41) or TEC, is utilised as a well adapted plasticizer for CAB, albeit any plasticizer (4) being able to adequately plasticize CAB (21) may serve as is evident to person skilled in the art. TEC (41) however has the further advantage of being environmentally friendly.
When preparing the dispersion (3, 4) to be added to the matrix (2) the cellulose nanowhiskers (3) will thus in a preferred embodiment of the invention be suspended in TEC (41) wherein the TEC (41) will both serve as a plasticizer (4) for the extrusion process as well as serving as the liquid phase of the dispersion (3, 4).
In a further preferred embodiment of the invention, a solvent (7) may be added to the dispersion (3, 4) in order for rendering the dispersion more viscous and for reducing the required amount of plasticizer (4). As is evident, the solvent (7) must be soluble with respect to the plasticizer (4) and should not react with the matrix material (2) nor with the whiskers (3). The choice of solvent (7) will entirely depend on the choice of matrix material (2), whiskers (3) and the plasticizer (4).
In a preferred embodiment of the invention, a polar solvent (7) such as ethanol or methanol is chosen as the solvent (7). Ethanol is a cheap environmentally friendly solvent being inert with respect to most matrix materials (2). In the preferred embodiment of the invention in which TEC (41) serves as a plasticizer (4), TEC (41) will also serve to avoid the too fast evaporation of ethanol during the extrusion process. TEC (41) is not soluble in water, and the use of TEC (41)/ethanol as the liquid phase of the dispersion (3, 4, 7) is thus advantageous with respect to chemical affinities. In other embodiments of the invention, water and a compatible plasticizer may serve the same purpose.
In an alternative preferred embodiment of the invention, the proportion of TEC (41) in the dispersion (3, 4) also comprising the solvent (7) is about 30%. This use of a quite large proportion of plasticizer (4) allows increased process speeds thus allowing the process to be used industrially and not merely on a laboratory scale.
Although ethanol has served as example, it should be evident that other solvents (7) fulfilling the process requirements may serve equally well, and lie within the scope of protection of the present invention.
The addition of a solvent (7) may necessitate the removal of excess fluid before the polymer (1) is extruded. The arrangement of one or more vents (54) will in a preferred embodiment of the invention allow the easy removal of the excess liquids from the melt as described above.
The temperature gradient within the extruder (5) should be adapted to the materials used during the process. In the particular embodiment of the invention in which CAB serves as the matrix material, the extrusion temperature will preferentially vary between about 130° C. and about 210° C. Using other matrix materials (2) other temperature ranges will be used as is evident to a person skilled in the art.
The invention further discloses an organic polymeric material (1) comprising a carbon based matrix (2) mainly comprising cellulose acetyl butyrate (21) and a reinforcing dispersion of cellulose nano whiskers (3), said cellulose nano whiskers (3) being finely dispersed within said carbon based matrix (2). The particular advantages of the reinforced organic polymeric material (1) according to the invention are multiple, it being strong, transparent, and has a substantially improved temperature resistance compared to the non-reinforced CAB material (21).
The improvement in the softening temperature from 117° C. to about 148° C. is a very large improvement, and results from the addition of only about 5% nanowhiskers. As is evident to a person skilled in the art, the addition of larger amounts of reinforcing material may potentially further increase the tan δ peak temperature thus further increasing the strength of the material. With respect to the previously reported work in the field, this increase in the tan δ peak temperature as well as the increase in the storage modulus is remarkable. Neither Grunert and Winter[1] nor Oksman et al[2] were able to furnish similar or indeed significant improvements in the material properties.
An E-modulus of about 3.2 GPa has never been reported for biodegradable polymers of this kind. The result is in the orders of magnitude above what has been previously achieved.
It should be clear that other proportions of nano-reinforcements are possible, and that these also lie within the scope of the invention. Although experimental values for the addition of just 5% of nanoreinforcements are given, these are given as examples only, and should not be considered to limit the invention. The polymer may be tailor-made to specifications, in which the percentage of nanowhiskers may range from less than 0.1% to more than 30% according to the desired polymer characteristics.
A second possible explanation for the improved characteristics of the produced polymer (1) may lie in the improved method for the production of whiskers (3) according to the invention. Previous experimentation in the field has concentrated on the acid hydrolysis of the cellulose base material by means of sulphuric acid. However the resulting whiskers (3) have not proven to be adequate for the reinforcement of CAB (21). Thus a new and inventive method is proposed according to the invention in which cellulose nanowhiskers (3) are produced by the acid hydrolysis of microcrystalline cellulose by means of hydrochloric acid HCl. An example of the production methodology for the whiskers is given below.
Microcrystalline cellulose (MCC), is used to prepare the cellulose whiskers. In an example according to the invention 26 g of MCC is added to 900 ml 4 N HCl and heated at 80° C. for 225 min. The suspension is centrifuged repeatedly at 12000 rpm for 10 min. After each centrifugation step the supernatant is removed and replaced by distilled water, arranged on a magnetic stirrer and stirred until the sediment is blended. When the pH reaches ≧4, usually after approximately six centrifugations, the centrifugation speed is decreased to 3800 rpm for 10 min to obtain a turbid supernatant. The turbid supernatant is collected and replaced by distilled water. This step is repeated three times.
In order for ensuring that the suspension contains whiskers, it may be placed between a filter and stirred. If the suspension shows birefringence, which is the decomposition of a ray of light into two rays when it passes through crystals, this is an indication of that the microcrystalline cellulose has successfully been separated into single crystals or whiskers. The turbid suspension is put in dialysis tubes and placed in distilled water for approximately one week to remove excess acid. In order to concentrate the whisker suspension, the dialysis tubes may be placed in a polyethylene-glycol (PEG) bath for approximately one week.
This method according to the invention will result in cellulose nano whiskers (4) in the size range approximately 100 nm length to approximately 1000 nm length and having a diameter in the range of approximately 5 nm to approximately 15 nm. These are so-called one dimensional nanoparticles.
A comparison of the physical properties of the whiskers prepared by using HCl and the properties of the whiskers prepared by using H2SO4 is given in
The resulting polymer (1) is very clear and transparent as may be seen in
The resulting polymer (1) has many potential areas of use including packaging film, food packaging, electronics, biomedical applications etc. As the resulting polymer is cheap, biodegradable as well as strong, it presents multiple advantages compared to previously disclosed polymeric materials.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20065147 | Nov 2006 | NO | national |
