Method for producing a compound containing polyhydroxyalkanoate and cellulose

Abstract

A method for producing a biodegradable compound, comprising the steps of: providing an extruder having at least one rotating screw and at least two feeding units suitable for feeding raw materials and an extruder die; feeding a PHA polymer and maleic anhydride (MA) to the first feeding unit; feeding cellulose fibers to the second feeding unit; rotating the screw to mix the PHA and MA raw materials and grafting the MA onto the PHA molecules to form MA-grafted PHA (PHA-g-MA) in an amount of 1-10%, preferably 1-3%, of the total PHA content, and then mixing the PHA-g-MA with cellulose fibers to form a molten compound of PHA-g-MA and cellulose fibers; passing the molten compound through the extruder die and shaping the extruded compound into different compound formats.

Description

Detailed Description of the Invention

[0001] [Technical field] The present invention relates to a process for the preparation of a compound comprising cellulose fibers and a biodegradable polymer based on polyhydroxyalkanoates (PHA).The present invention further relates to the compound obtainable by such a process.

[0002] [Background technology] Recent environmental awareness has led the packaging industry to develop solutions to ensure that packaging materials do not utilize non-renewable resources and that such packaging materials are recyclable or biodegradable after use.

[0003] A material that has been considered promising is polyhydroxyalkanoate polymers (PHAs). PHA polymers are naturally produced by microorganisms and therefore are produced from renewable sources. Moreover, such PHAs are produced from lipids that can be derived from waste materials and therefore represent an excellent source of packaging material. It has been found that PHAs can be combined with cellulosic materials, such as cellulose fibers, to provide enhanced properties suitable for packaging, in particular improved mechanical properties.

[0004] However, compounding PHA polymers with cellulosic materials has proven itself difficult due to uncertainties in the chemical compatibility between these two raw materials.

[0005] Therefore, techniques have been developed to chemically modify either the PHA molecules or the cellulose fibers in order to compatibilize the PHA with cellulosic materials, allowing compounding with both as masterbatches, and thus making the resulting compound chemically and mechanically stable. The compound masterbatch is then processed in an extruder to form pellets.

[0006] The compound pellets are then melted and processed in liquid form into packaging articles by conventional packaging forming methods, such as injection molding for 3D articles, extrusion, film lamination for three dimensional articles, compression for three dimensional articles, and the like.

[0007] One of the successful methods to modify PHA for compatibility with cellulose fibers is to chemically react the PHA molecule with maleic anhydride (MA) to obtain maleic anhydride grafted PHA molecule (PHA-g-MA), which is relatively easy to compound with cellulose fibers and the resulting compound is stable when converted into pellets for further packaging production.

[0008] The techniques for grafting PHA with maleic anhydride to give PHA-g-MA are now well known and described in the prior art.

[0009] For example, Shengnan et al., in "Properties and structure of poly(3-hydroxybutyrate-co-4-hydroxybutyrate / wood fiber biodegradable composites modified with maleic anhydride" (published in Industrial Crops & Products), disclose a PHA polymer compounded with wood fiber and grafted with maleic anhydride to improve the interfacial adhesion of the compound. This document does not mention the optimization of the manufacturing process.

[0010] U.S. Patent Application Publication No. 2007 / 0287795 is a U.S. patent application to Huda et al. which discloses a composite composition including a synthetic polymer and corncob granules modified, such as with a chemical that reacts with the hydroxyl groups on the corncob granules. The corncob granules are modified to compatibilize with the polymer, particularly by grafting with maleic anhydride.

[0011] US Patent No. 2021079211 is a US patent that discloses that a highly compatibilized biodegradable composite having high impact strength, comprising (a) a polymer matrix having one or more biodegradable polymers, (b) one or more fillers, and (c) a free radical initiator, is produced by a one-step reactive extrusion process. The in-situ free radical reaction process for producing a biodegradable composite includes (a) (1) mixing one or more biodegradable polymers with a free radical initiator, (2) melting step (1), thereby producing a highly compatibilized biodegradable matrix, and (b) mixing the composite of step (a) with a filler or a second biodegradable polymer, thereby producing a biodegradable composite. Also, in this invention, nanoblends are successfully prepared by improving the compatibility of different components.

[0012] US Patent Application Publication No. 2013225761 is a US patent application to Whitehouse et al. that discloses a method for making an aqueous PHA emulsion or latex that includes a primarily amorphous PHA polymer or copolymer together with a polymeric dispersant or surfactant.

[0013] US Patent Publication No. 2018127554 is a US patent application to Mohanty et al., which discloses a biodegradable composite that includes (a) a polymer matrix having a biodegradable polymer, (b) a filler, and (c) an anhydride-grafted compatibilizer that includes one or more biodegradable polymers modified with an anhydride group. The composite may also include (d) a polymeric additive, such as a polymer chain extender or a plasticizer.

[0014] Cheng Chen et al., in "Synthesis and characterization of maleated poly(3-hydroxybutyrate)" (published in the Journal of applied polymer science on February 11, 2003), disclose that graft copolymerization of maleic anhydride (MA) onto poly(3-hydroxybutyrate) (PHB) was carried out by using benzoyl peroxide as an initiator.

[0015] Another exemplary publication that discloses PHA-g-MA formation is Shengnan et al., "Properties and structure of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) / wood fiber biodegradable composites modified with maleic anhydride," published October 5, 2017 in Elsevier-Industrial Crops and Products, Volume 109, 15 December 2017, Pages 882-888. The publication discloses that wood flour and P34HB composites were via a hot pressing process, and maleic anhydride (MAH) was added as a coupling agent to increase the interfacial adhesion.

[0016] Furthermore, US Patent Publication No. 2005225009(A1) is a US patent application that discloses a method for preparing a moldable compound containing cellulose fibers and thermoplastics for automotive, aerospace, furniture and other structural applications. The method includes mixing cellulose fibers, surfactants and molten thermoplastic resins in a high shear mixing device. After subsequent processing, the compound is subjected to heat and pressure by compression and injection to obtain a molded article of complex shape. The thermoplastic raw material can be a PHA and the surfactant can be a maleic anhydride grafted polymer.

[0017] Thus, such processes in which a compound is formed by mixing a cellulosic raw material with a PHA involve the use of a compatibilizer, which may be a thermoplastic grafted maleic anhydride, or such processes involve directly compounding the cellulosic material with a PHA that has already been grafted with maleic anhydride.

[0018] In all cases, but particularly when PHA is selected as the preferred polymer raw material, two steps are carried out to obtain compound pellets suitable for later forming into a finished product.

[0019] The first step is the preparation of a compatibilized PHA for compounding with cellulose, more precisely PHA-g-MA. The production of this material involves chemical and thermal treatment of the PHA to ensure grafting with maleic anhydride.

[0020] The second step involves heating the PHA-g-MA in an extruder until it reaches a molten state, mixing it with a certain amount of cellulose fiber in the extruder to obtain a PHA-cellulose compound, and extruding the compound into pellets. The pellets can then be used as material to be processed into packaging articles by various packaging forming techniques such as injection, extrusion blow, film lamination, compression, etc.

[0021] The inventors have discovered that although the above-mentioned techniques allow the production of excellent compounds ready for use in the manufacture of packaging materials, said compounds having the desired recyclability and biodegradability, the treatment of the PHA for grafting and subsequent extrusion into pellets degrades the PHA molecules and produces crotonic acid. Crotonic acid has been found to be particularly detrimental to the organoleptic properties of the compounds. In particular, such crotonic acid has been found to give an undesirable off-taste to products contained in packages made from such compounds. Attempts have been made to reduce the content of crotonic acid in the final compounds, but the levels achieved have been found not always compatible with the packaging of edible items, in particular those edible products with a neutral organoleptic profile, such as, for example, mineral water. For other types of food products, crotonic acid presents the risk of substantially modifying the organoleptic properties of the product in an unacceptable manner.

[0022] It is therefore an object of the present invention to provide a process for preparation which provides an improved PHA-cellulose compound, obviating the drawbacks of the known processes mentioned above and resolving the sensory problems of the known compounds.

[0023] [Summary of the invention] The object of the present invention is a method for producing a biodegradable compound suitable for the manufacture of packaging articles, said compound comprising a mixture of cellulose fibres and at least one polyhydroxyalkanoate polymer (PHA), said method comprising the steps of: (i) providing an extruder comprising a heater, at least one rotating screw, at least two feeding units suitable for feeding raw materials, and an extruder die, the temperature of the extruder being set at 130°C to 190°C, preferably 130°C to 175°C; (ii) feeding a PHA polymer and maleic anhydride (MA) to a first feeding unit, the ratio of maleic anhydride to PHA being comprised between 0.1 and 10, preferably between 0.1 and 5; (iii) feeding cellulose fibres to a second feeding unit, the cellulose fibres having a length comprised within the range of 15 μm to 150 μm, preferably within the range of 20 μm to 120 μm, and a density of at least 1.0 g / cm 3 , preferably at least 1.5 g / cm 3 Step 2: A hardwood cellulose fiber having a density of (iv) rotating the at least one screw to mix the PHA and maleic anhydride raw materials and graft the maleic anhydride onto the PHA molecules to form MA-grafted PHA ("PHA-g-MA") in an amount of 1-10%, preferably 1-3% of the total PHA content, and then mixing the PHA-g-MA with cellulose fibers to form a molten compound of PHA-g-MA and cellulose fibers; (v) passing the molten compound through an extruder die and extruding the compound; by cutting the extrudate compound cord with a knife system into compound pellets, or into a compound film or compound plate (in which case the extruder die takes the form of a cast line), or molding the extruded compound into a mold, either by injection molding or extrusion blow molding, into a compound three-dimensional article; This is achieved by a method which in turn includes:

[0024] Using this method, the inventors have realized that PHA can be grafted with maleic anhydride and simultaneously form a stable compound with PHA-g-MA and cellulose fibers by a one-step approach in an extruder, thus obtaining a desired fiber content by a one-step process, and in particular, the final compound thus obtained is not only chemically very stable but also contains a very large amount of cellulose fibers per weight of the total compound, which makes the entire material suitable for either recycling through standard paper recycling processes or biodegradability.

[0025] Furthermore, the inventors have discovered that the resulting compounds of PHA-g-MA and cellulose fibers are characterized by excellent mechanical properties, especially with respect to stiffness (Young's modulus), tensile strength, and elongation at break.

[0026] In a preferred embodiment of the present invention, the extruder is a twin-screw extruder. The rotation speed of at least one screw is preferably from 10 revolutions per minute (rpm) to 300 rpm, and preferably the screw rotation speed is about 100 rpm.

[0027] Advantageously, a catalyst is added together with the PHA and maleic anhydride in the first feed unit, such catalyst being selected from the list of dicumyl peroxide (DCP), benzoyl peroxide, dibenzoyl, hydroperoxide and ketone peroxide, or combinations thereof.

[0028] More precisely, in the above preferred embodiment, the ratio of catalyst to PHA is comprised within the range of 0.01% to 5%, preferably a ratio of about 1%.

[0029] Also advantageously, a plasticizer can be added in the second supply unit together with the cellulose fibers, said plasticizer being chosen within the list of beeswax (BW), stearic acid (SA), glycerol monostearate (GMS), or combinations thereof.

[0030] In such cases, the ratio of plasticizer to cellulose fibres is preferably comprised within the range of 0.1% to 10%, preferably said ratio is about 3%.

[0031] Generally, within the scope of the present specification, the polyhydroxyalkanoate polymer used is preferably selected from the list of poly 3-hydroxybutyrate-co-3-hydroxyhexanoate (PHBH), poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV), poly-3-hydroxybutyrate (PHB), poly-3-hydroxyvalerate (PHV) or poly-3-hydroxyhexanoate (PHHx), and derivatives thereof, or combinations thereof.

[0032] In a highly preferred embodiment of the invention, the method includes the step of quenching the extrudate compound in a quench bath.

[0033] The temperature of the quench bath is advantageously chosen between 5° C. and 50° C., preferably between 15° C. and 30° C., and the duration of contact between the extrudate compound and the quench bath is a few seconds, in accordance with normal quenching practice. As a rule, the quench bath is water.

[0034] In one embodiment of the present invention, the cellulose fibers are advantageously modified with a coupling agent to enhance their chemical compatibility with PHA-g-MA, preferably said coupling agent being alkyl ketene dimer (AKD).

[0035] In one embodiment, the cellulose fibers are compatibilized before being introduced into the second feed unit. Alternatively, in a second embodiment, the native cellulose fibers are introduced into the second feed unit together with a coupling agent, and the compatibilization reaction is carried out in situ directly inside the extruder. In this latter case, the compatibilized cellulose fibers preferentially combine with the PHA-g-MA to form the PHA-cellulose compound.

[0036] The polyhydroxyalkanoate polymers (PHAs) suitable for the present invention are biodegradable polymers, preferably home compostable polymers. Home compostability is now well defined at national level and is mainly based on the international standard EN13432. Therefore, they do not need to be defined in more detail here. Materials or products that comply with these standards can be recognized by a conformity mark indicating their home compostability. Some examples of home compostability at national level include, but are not limited to: the certifier TUV AUSTRIA BELGUIM provides a certification scheme for such home compostability, and DIN CERTCO provides certification for home compostability according to the Australian standard AS 5810. Italy has a national standard for composting at ambient temperature, UNI11183:2006. In November 2015, the French standard "NF T51-800 Plastics - Specification for plastics suitable for home composting" was introduced. This standard is covered by the DIN CERTCO scheme.

[0037] Additional features and advantages of the present invention are described in, or will be apparent from, the following description of the presently preferred embodiments which take place in conjunction with the drawings. [Brief description of the drawings]

[0038] [Figure 1] FIG. 1 is a schematic diagram of a production facility suitable for producing a compound in the method according to the invention. [Diagram 2] FIG. 1 compares the mechanical properties of different compounds, including mixtures of unmodified PHA and unmodified cellulose (50% of the total), mixtures of unmodified PHA and modified cellulose (50% of the total), and mixtures of unmodified cellulose and PHA, where a specific fraction of the PHA has been grafted with maleic anhydride (PHA-g-MA). [Diagram 3]FIG. 1 compares the mechanical properties achieved by different compounds including a mixture of unmodified PHA (PHBH) with grafted polypropylene (PP-g-MA) and cellulose fibers, and a mixture of cellulose fibers and PHA (PHBH) in which a portion of the PHA has been grafted with maleic anhydride in soluble or powder form. [Figure 4] FIG. 1 illustrates comparative mechanical testing of three alternative compounds formed by a method according to the present invention (including mean and standard deviation values).

[0039] [Mode for carrying out the invention] The present invention relates to compounding PHA polymers with cellulosic fibers, particularly hardwood fibers, having the preferred length and density characteristics set forth herein and in the claims.

[0040] The inventors have discovered that the so-called "reactive compounding" process, in which a PHA polymer is first fed into an extruder together with maleic anhydride to graft the two to produce PHA-g-MA, and then cellulose fibers are fed into the same extruder to be compounded with the freshly produced PHA-g-MA, is particularly advantageous not only in terms of industrial and economic efficiency, but also in terms of improved chemical and mechanical properties of the compounds thus obtained.

[0041] As shown in the embodiment of Figure 1, the present invention involves a single extrusion process with an extruder 1 comprising a casing 2 and a screw 3 disposed therein. In the embodiment of Figure 1, the extruder is a twin screw extruder having two screws that rotate in opposite directions or that rotate in the same direction as indicated by the arrows in Figure 1.

[0042] The extruder further comprises a first feeding unit 4 and a second feeding unit 5. The two feeding units 4, 5 are preferably located apart from each other along the length of the extruder by a sufficient predetermined distance so that the raw materials introduced into the first feeding unit are properly mixed and chemically reacted completely inside the extruder before reaching the position of the second feeding unit. The sufficient "time for reaction" of the raw materials fed in the first feeding unit can be appropriately determined and adjusted in advance according to the amount of the raw materials. An example of compound preparation is provided in more detail below.

[0043] In the embodiment shown in Figure 1, the second feed unit comprises a pair of screws 6 to facilitate the introduction of the raw materials into the extruder. This is particularly useful when the raw materials are dry or in a solid particulate state and therefore difficult to flow, in which case the pair of screws will facilitate the flow of said raw material components into the extruder.

[0044] When adding fibers to the second hopper, it is alternatively possible to add the fibers either in the form of fibers or powder, but also in the form of compressed or pelletized fibers. Furthermore, it is also possible to envisage the use of compatibilizers and / or wetting or sizing agents.

[0045] The extruder 1 further comprises an extruder die 7 through which flows a cord of hot extrudate material 8. In this embodiment, the extrudate cord is a compound of PHA-g-MA and cellulose fibers prepared inside the extruder.

[0046] After flowing through the die 6, the cord of hot extrudate compound is cooled in a quench station 9, which includes a water quench bath (not shown). The quench bath is thermostated to a temperature of about 20° C., so that the cord of hot extrudate compound, which flows out of the extruder in a molten state (i.e., above the melting point of the compound), reaches a temperature below the melting point of the compound within a few seconds. As a result, the compound cord leaves the quench bath in a solid state and is then conveyed to a pelletizing station 10. In the pelletizing station 10, the extrudate cord 8 is cut into small pellets 11. The pellets are then packaged and can be used as a compound for producing packaging materials using conventional packaging manufacturing processes (injection, extrusion blow molding, lamination, compression, etc.).

[0047] In this exemplary embodiment, PHBH-g-MA is prepared as follows: 0.5 g maleic anhydride (MA), 0.1 g dicumyl peroxide (DCP), and 9.4 g poly 3-hydroxybutyrate-co-3-hydroxyhexanoate (PHBH) are mixed as ground powders or mixed in acetone, after which the acetone is evaporated. The powder mixture is then fed into the extruder (first feeding unit 4, as explained above) and held there for 5 minutes (starting when all materials have been fed). The extruder temperature is set to 175°C.

[0048] After 5 minutes, the extruder is cooled to room temperature. At this stage, a clear color change of the polymer from colorless to yellow can be observed, indicating the formation of PHBH-g-MA (which is yellow in color).

[0049] The resulting PHBH-g-MA is ground with liquid nitrogen before compounding with cellulose fibers. Then, cellulose fibers (FC) are introduced into the extruder through a second feeding unit 5, and PHBH, FC, and PHBH-g-MA are mixed. A lubricant additive ("Add."), i.e., beeswax (BW), is added, the amount of which is selected between 3% and 8% by weight of the total compound.

[0050] The processing temperature for extrusion and hot pressing is set to 175°C to 180°C.

[0051] Turning to Figure 2, the inventors conducted a series of comparative mechanical tests for elongation at break to compare various combinations of raw materials and the mechanical properties obtained in the resulting final compounds. The various alternative formulations tested and reported in Figure 2 are as follows:

[0052] On the far left side of the figure, a PHA not modified with maleic anhydride is compounded with 50 wt % cellulose fibers, and the resulting compound exhibits brittle mechanical behavior, as indicated by the low % strain compared to the stress experienced by the material.

[0053] The replacement of cellulose with chemically modified cellulose, in particular cellulose modified by the AKD reaction according to techniques known in the art, makes it possible to improve compatibility with the polymer, so that the resulting compounds exhibit increased elongation at break.

[0054] However, the best results are observed for reactive compounding of PHA with maleic anhydride (see the family of curves on the right side of the figure). In this case, PHA-g-MA compounded with 50% by weight of cellulose fibres in the total compound shows the highest strain (%) for the stress applied to the compounded material, i.e. 2-4% strain for an applied stress reaching 300 N.

[0055] As shown in Figure 3, a comparison of strain-stress tests of various compounds in tensile mode reveals that PHBH-g-MA strongly improves the mechanical behavior of the final compound. For example, by adding 3% by weight of PHBH-g-MA of the total compound, the mechanical properties of the compound thus obtained are increased compared to a compound containing only unmodified PHBH (not grafted to maleic anhydride) and cellulose fibers. Whether the beeswax lubricant (BW) is added in solid or solution form does not substantially modify the results.

[0056] From the experiments, it is concluded that the interface between PHBH and cellulose is fully mediated by PHBH-g-MA.

[0057] As shown in Figure 4, a comparative test of three different compounds, viz. PHBH and cellulose fibers, and maleic anhydride grafted polypropylene ("PP-g-MA") PHBH and cellulose fibers, and maleic anhydride grafted PHBH (PHBH-g-MA), and beeswax in powder form as a lubricant. PHBH and cellulose fibers, and maleic anhydride grafted PHBH (PHBH-g-MA), and beeswax in solution form as lubricant. The comparative studies show that compounding PHBH and cellulose fibers with modified PHBH (PHBH-g-MA) improves all the mechanical properties of the final compound, especially Young's modulus, tensile strength, and elongation at break.

[0058] Working Example A typical example of a formulation for compounding modified PHA (particularly PHBH) with cellulose fibers is shown below, where the amount of each raw material is shown for each part of the extruder where it is introduced: the first feeding unit ("Feeder 1") and the second feeding unit ("Feeder 2"). [Table 1]

[0059] It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. Accordingly, such changes and modifications are intended to be covered by the appended claims.

Claims

1. A method for producing a biodegradable compound, wherein the compound comprises a mixture of cellulose fibers and at least one polyhydroxyalkanoate polymer (PHA), and the method is (i) A step of preparing an extruder (1) comprising a heater, at least one rotating screw (3), at least two supply units (4, 5) suitable for supplying raw materials, and an extruder die (6), wherein the temperature of the extruder is set to 130°C to 190°C, preferably 130°C to 175°C. (ii) A step of supplying a PHA polymer and maleic anhydride (MA) to a first supply unit (4), wherein the ratio of maleic anhydride to PHA is between 0.1 and 10, preferably between 0.1 and 5. (iii) A step of supplying cellulose fibers to a second supply unit (5), wherein the cellulose fibers are hardwood cellulose fibers having a length in the range of 15 μm to 150 μm, preferably in the range of 20 μm to 120 μm, and having a density of at least 1.0 g / cm³, preferably at least 1.5 g / cm³. (iv) Rotating at least one screw to mix the PHA and maleic anhydride raw materials, grafting the maleic anhydride onto the PHA molecules to form MA-grafted PHA ("PHA-g-MA") in an amount of 1 to 10%, preferably 1 to 3%, of the total PHA content, and then mixing the PHA-g-MA with cellulose fibers to form a molten compound of PHA-g-MA and cellulose fibers, (v) The molten compound is passed through the extruder die (6), and the extruded compound is The extruded compound cord (8) is cut by a knife system to form compound pellets (11), or To a compound film or compound plate (in this case, the extruder die takes the form of a cast line), or The steps include: forming the extruded compound into a three-dimensional compound article by injection molding or blow extrusion molding in a mold; A method that includes the following in order.

2. The method according to claim 1, wherein the extruder (1) is a twin-screw extruder.

3. The method according to claim 1 or 2, wherein the catalyst is added together with the PHA and the maleic anhydride in the first supply unit (4), and the catalyst is selected from the list of dicumyl peroxide (DCP), benzoyl peroxide, dibenzoyl, hydroperoxide and ketone peroxide, or combinations thereof.

4. The method according to claim 3, wherein the ratio of the catalyst to PHA is within the range of 0.01% to 5%.

5. The method according to claim 1 or 2, wherein a plasticizer is added to the second supply unit (5) together with the cellulose fibers, and the plasticizer is selected from the list of beeswax (BW), stearic acid (SA), glycerol monostearate (GMS), or a combination thereof.

6. The method according to claim 5, wherein the ratio of plasticizer to cellulose fibers is within the range of 0.1% to 10%.

7. The method according to claim 1 or 2, wherein the polyhydroxyalkanoate polymer is selected from the list of poly-3-hydroxybutyrate-co-3-hydroxyhexanoate (PHBH), poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV), poly-3-hydroxybutyrate (PHB), poly-3-hydroxyvalerate (PHV), or poly-3-hydroxyhexanoate (PHHx), and derivatives thereof, or combinations thereof.

8. The method according to claim 1 or 2, wherein the rotational speed of at least one screw is between 10 rpm and 300 rpm.

9. The method according to claim 1 or 2, further comprising the step of rapidly cooling the extruded compound in a rapid cooling bath (9).

10. The method according to claim 9, wherein the temperature of the rapid cooling bath (9) is between 5°C and 50°C, preferably between 15°C and 30°C.

11. The method according to claim 9, wherein the rapid cooling bath is water.

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