DE19830774A1 - Thermoplastic biodegradable molded article with good tensile strength - Google Patents

Abstract

The production of thermoplastic biodegradable molded articles involves using partially defibrillated and partially intact natural fiber components. Thermoplastic biodegradable molded articles are claimed which are made by compounding a mixture obtained during the preparation of starch esters, agglomerating, adding partially defibrillated and partially intact natural fiber components, and injection molding. The composition comprises: (i) 20-90 wt.% of a starch ester of degree of substitution below 3 and containing 0.5-5 wt.% acetic acid during the compounding of a mixture of starch esters; and (ii) 2-75 wt.% of a natural fiber component that has absorbed 2-25 wt.% polyalkylene glycol of mol. wt. 200-2000 during compounding. An Independent claim is also included for the preparation of the above articles by compounding the starch ester and natural fibers and injection molding at 100-500 Torr.

Description

The invention relates to a biodegradable material for molded bodies that consists of a biodegradable thermoplastic matrix and a partially defibril lated vegetable fiber material, as well as a process for the production of Objects by means of injection molding.

In the endeavor, through the use of renewable raw materials or biodegradation to bring about relief for the environment or at least partially Creating closed material cycles has been some biolo in the last 20 years Gisch degradable or compostable materials, such as thermoplastic cal starch (TPS), polyhydroxybutyric acid (PHB), polycaprolactone (PCL), poly lactic acid (PLA) starch diacetate and cellulose diacetate have been developed.

On the other hand, there have long been efforts to find the special properties to use natural fibers to modify the properties of plastics and, especially in the early days of the development of conventional plastics, thereby to make the material cheaper.

Both aspects, the strength of the natural fibers and the lowering of the price level, play a role in the attempts to compound biodegradable or compo adjustable polymers with wood flour and other natural fibers are essential Role. In addition to different variants of mixing starch or starchy half term components with fibers in the aqueous phase and subsequent Ver pressing the mixture, such as. B. in DE 43 17 692, are verar as thermoplastic machinable mixtures, among others the combinations of starch / EVOH (Mater-Bi) with wood flour (WO 94/03543), corn flour / natural resins with wood (WO 95/04111), polycaprolactone with Wood flour (US 3 850 863), as well as polyester amide (BAK) with wood flour (DE 195 35 715) has been described.

Similar solutions are provided for mixtures of starch acetate and natural fibers namely wood flour and cellulose fibers, in WO 97/28214, WO 97/03121 and in DE 195 00 283 indicated. What all three solutions have in common is that they are good Binding ability of starch acetate to natural fibers exploit the mechanical To improve the properties of the thermoplastic starch acetate compounds. In DE 195 00 283 also includes an unspecified adhesion promoter puts. In WO 97/03121 the use of various starch derivatives (esters, Ethers, grafted and degraded or oxidized starches) and plasticizer compounds nents used to vary the properties. In WO 97/28214 is also the Use of starch esters is described here as a decisive improvement the use of microfibers of 75-750 micrometers and an L / D ratio of 3 to 60 indicated. What all these variants have in common is that the natural fibers in their original form can be used as a filler.

Suitable for incorporating the organic fibers into the thermoplastic matrix according to the Technical Information Lignocel® (Rettenmaier & Söhne GmbH + CO) Single screw extruder, but better co-rotating twin screw extruder truder or co-kneader. Also pre-agglomeration using plastic stagglomerators or heating-cooling mixer is recommended. Sufficient due to the necessary degassing this step is not yet in preparation for injection molding processing. The conditions are chosen so that the fiber materials as possible we nig be sheared and their original shape is largely retained. Such polymer-fiber compounds can have significantly better properties can be achieved if it is possible to break open the corresponding fiber cells and at least partially defibrillate the contained cellulose fibers. One before course for defibrillation of natural fibers is in M. Avelle, E. Martuscelli, B. Pascuc ci, M. Raimo, B. Focher and A. Marzetti, Journal of Applied Polymer Science, Vol.49, 2091-2103.

The fiber material is broken down into its components, especially lignose and hemicellu loose and broken down cellulose fibers. The breaking down of the fiber bundles into individual microfi Eyewear increased in composite materials with polymers that have good bonding ability ben, the interactions between fibers and polymer matrix and thus leads to improved properties of the compound. On the negative side, it should be noted that a se A separate defibrillation step leads to a significant increase in the price of the end products leads.

The aim of the present invention is to achieve the advantageous properties properties of defibrillated natural fiber materials for the production of biodegradable ones To use injection molded articles, but the defibrillation in the manufacturing pro Zeß of the natural fiber compound to be integrated so that while minimizing the Ar steps the defibrillation at least partially during the injection molding process takes place and thus the costs of production compared to normal compounds are not should be increased, but should be reduced if possible. The expenses for the Aufbe The preparation of the compound components should be as low as possible than with the steps known so far for conventional plastics.

The compounds according to the invention consist of starch esters, predominantly of Starch acetate with a degree of substitution <3, preferably 1.8-2.6, biological degradable plasticizers, preferably polyalkylene glycols with molecular weights of 200 to 2000, preferably from 300 to 1000. The additional use of sliding agents such as alkoxylated fatty alcohols or alkoxylated natural resins is also possible, such as the use of straight-chain fatty acids with 8 to 30 carbon atoms preferably with 10 to 20 carbon atoms. As a natural fiber component, wood flour is used in egg A concentration of 10 to 80%, preferably 25 to 50%, is used. Just so is the use of flax, hemp, jute fibers in the same concentration area possible.

A particular advantage of the procedure described below is that thanks to the defibrillation integrated in the last step of the injection molding process the fibers used have the same or similar properties even with straw, Flax shives, hemp, reeds, nettles, ramie or similar agricultural products Products or waste products can be obtained and thus in addition to the Ver simplifying the process and improving the quality of the products essential liche economic and ecological effects can be achieved. A gentle one Product processing is guaranteed by the fact that an agglomeration of the starch sters and the fiber component is already sufficient to then in one step To produce good quality injection molded parts.

The following processing steps are essential for the production of the aforementioned compounds according to the invention:

• 1. Production of the starch acetate:
  Starch is activated with a mixture of acetic acid and acetic anhydride and then esterified with a stoichiometric amount of acetic anhydride to the desired degree of acetylation of 1.5 to 2.8. It is essential for the production of the natural fiber compounds according to the invention that the use of any catalyst is dispensed with. This is the only way to work up the polymer by directly drying the acetic acid reaction solution, please include. The necessary residual acetic acid content can thus be set, which is essential for the further course of the compounding process. The residual acetic acid content should be 0.5 to 5% by mass, preferably 1 to 2% by mass. The resulting powder with a relatively high bulk density and low internal surface is contained in the compound at 20 to 90% by mass, calculated on the basis of a dry starch acetate. The residual acetic acid remaining in the granulate has the function of a temporary plasticizer for the starch acetate and, in the next processing step, it serves as an aid to defibrillate the natural fiber component. Complete drying of the starch peace and subsequent enrichment of the mixture with the specified acetic acid concentrations does not lead to the same result due to irreversible structuring processes in the starch acetate granules.
• 2. Prepare the natural fiber component:
  The natural fiber component with a proportion of 2 to 75 wt .-%, preferably 10 to 65 wt .-%, with the polyalkylene glycol (2 to 25 wt .-%, preferably 4 to 20 wt .-%), preferably polyethylene glycol with a Molar mass from 200 to 2000 g / mol, preferably from 300 to 1000 g / mol, mixed. The known absorption capacity of the fiber components is used for such substances.
• 3. Mixing the components:
  These two components are mixed, if necessary, with the addition of further auxiliaries for thermoplastic processing such as lubricants and stabilizers. These can be various known additives, but they should be biodegradable. In the simplest case, a long-chain saturated carboxylic acid can fulfill both functions and the biodegradability of the compound is preserved.
• 4. Agglomeration of the mixture:
  The premixed components of the compound are pressed into agglomerates. Their original structure is retained. Before the injection molding process, drying to <1% residual moisture (H ₂ O content) is required.
• 5. Injection molding:
  Surprisingly, it has been found that the starch acetate wood compound agglomerates can be processed directly by means of injection molding. The defibrillation is already set in motion by the thermoplastic processing.

Additional measures to improve the possibilities of direct processing:

• - By a mixing nozzle and / or shearing elements in the screw structure In the injection molding machine, additional friction is introduced into the melt brings, so that the defibrillation of the wood fibers takes place to more than 50%.
• - Gases that are produced can be extracted in the tool at the moment of opening a negative pressure can be applied during spraying. This also makes filling the Tool favored with the relatively difficult-to-flow mass.
• - Addition of 0.1 to 2% water-binding minerals such as CaCO ₃ or NaCl.

The advantage of the procedure described is that one optically largely homogeneous mass is created, which has a pleasant wood-like handle. The mass of the individual fibers is no longer separate even in SEM images recognize. With the proposed procedure, only part of the company defibrillates sern. The rest can still be seen optically as a whole fiber. The homogeneity that resulting from the defibrillation of the natural fiber components leads to a ver improvement of essential mechanical parameters such as flexural strength and tensile strength speed. The fiber components that have not been macroscopically destroyed provide additional benefits effect for the strength of the material.

The good mechanical characteristics are both when using High Amylose starch acetates as well as when using starch acetates from starch ken with normal amylose content of 20 to 30%. Also the heat form persistence, measured e.g. B. as HDT A, is much higher than normal strength keacetat-Compounds, but also higher than many other biodegradable compounds ren materials. Further advantages are that the material has an increased Has water resistance, but at the same time a defined water absorption shows. This means there are many options for an eco-friendly color and de Corative replica given. The remaining whole fibers bring in The direction of flow of the injection molding compound provides additional stability in the resulting Injection molded parts. Thus, ne results from the proposed method of processing In addition to the more economical design of the process, there is also an additional Ef fect in terms of the quality of the products. These are in addition to the one already mentioned additional stiffening of the products also have a lighter color resulting from lesser thermal stress in the entire manufacturing process results.

By adding a small amount of hydrophobic biodegradable polymers such as Po lyhydroxyalkanoic acids (e.g. PHB), polyesteramides (e.g. BAK), polyesters, polyca prolactone or polyethylene carbonate, the mixture described can if necessary also an additional hydrophobization and a certain flexibility given who the. For this purpose, 2 to 20% of such an additive is sufficient. These additions can be made before simply added to the agglomerate for injection molding.

The following examples illustrate the manufacturing procedure and the benefits of the resulting compounds.

The starch acetate used for this was in a one-pot synthesis in the above be written way made. This synthesis variant was based on catalysis ren and additional solvents are omitted. The following recipes each with Stoichiometric amounts of acetic anhydride resulted in the starches tested stars with different degrees of substitution from DS 1.5 to 2.6. So that became Procedure according to the invention enables.

For a comparison mixture, a starch acetate according to one of Mark and Mehltretter described method with a DS of 2.3.

In Table 1 are the starting material, degree of substitution and production method of starch acetates used for the compounds.

Table 1

Hylon VII is a high amylose corn starch from National Starch & Chemi cal S.p.A.

The type PT 20002 from Cerestar was used as the wheat starch.

The compound production took place in the following steps:

option A

• 1. Mixing the fiber component with the respective amount of polyalkylene glycol and Heat treatment at 50 to 100 ° C
• 2. Mixing the pretreated natural fiber component with the starch ester and the remaining recipe ingredients in a high-speed mixer
• 3. Compression of the mixture into agglomerates
• 4. Processing by injection molding in the manner described above

Variant B

Up to point 3 like variant A, but with

• 1. Extrusion of the agglomerates in a twin-screw kneader
• 2. Processing of the granules by means of injection molding in the manner described above

Variant C

The starch ester was mixed with all ingredients in one step and extruded.

The following exemplary embodiments are intended to demonstrate the effectiveness of the inventive show the following procedure:

Example 1 (comparative example)

The following recipe was processed according to variant C:

40.5% starch acetate 1
8.5% PEG 400
1% stearic acid
49% Lignocell S 150 TR (wood flour type from J. Rettenmaier & Söhne GmbH + Co)
1% CaCO ₃

The granules obtained were injection molded with a shear nozzle and at work vacuum (100-400 Torr) applied to shoulder bars according to DIN 53 455 processed, on which the following parameters were determined:

Flexural modulus DIN 53 457 Flexural strength DIN 53 452 tensile strenght DIN 53 455 Heat resistance (HDT A) DIN 53 461 Water absorption DIN 53 495

As expected, the elastic modulus of the compound was due to the filler effect of the wood flour is very high, an increase in flexural strength compared to the un however, the filled material was hardly recognizable. The tensile strength became drastic reduced. That, too, was within the range of expectations of such a mix.

Example 2 (comparison)

The same proportions as in Example 1 were used. The ver Used starch acetate was produced according to variant 2. The residual acetic acid content was 4%. The processing was carried out according to variant B.

Examples 3 to 13 (according to the invention)

For Examples 3-13, the same proportions were used as for Example 1.

According to the invention, processing was carried out according to variant A.

Table 2

The samples were processed and tested as in Example 1.

The test results of all examples are compiled in Table 3.

Table 3

As expected, the modulus of elasticity of the compound from example no. 1 was due to the The wood flour has a very high filler effect, increasing the flexural strength compared to the unfilled material, however, it was hardly recognizable. The tensile strength was drastically reduced and the heat resistance increased compared to a comparable unfilled material from approx. 50 ° C to 75 ° C. That too was in the He maintenance area of such a mixture. In all manufactured according to the invention Compounds, the high modulus of elasticity was more or less retained. That improved significantly Behavior of the material in terms of flexural strength or tensile and tear strength. The same applied to the reduction in water absorption after storage in water for 1 hour and the heat resistance. Since these property improvements with ei to improve the flowability of the material at this high filler content ten go hand in hand, the procedure according to the invention broadens significantly Applicability of such compounds. The comparison with an additional granu lated material (Example 2) shows that the shearing effect of a two-shaft kneader and the associated complete destructuring of the wood particles can still bring minor improvements in terms of flexural strength and tensile strength, overall the significantly higher effort compared to the proposed method rensweise the direct processing of pressed agglomerates in injection molding is justified according to the specified conditions.

Another advantage of the production method described is that to a large extent homogeneous appearance of the material. It's a smooth material with a wood-like handle.

The fibers that have been preserved here and there can be related to the optically lighter appearance of the parts from the direct processing of the Agglomerates can be used for additional optical effects.

Claims (10)

1. Thermoplastic biodegradable molding compositions based on starch esters and natural fiber components, characterized in that they are made from

• 1. 20 to 90 wt .-% of a starch ester with a degree of substitution <3, which contains an amount of acetic acid resulting from the manufacturing process of the starch ester of 0.5 to 5% during the compounding and
• 2. 2 to 75% by mass of a natural fiber component which has absorbed 2 to 25% by mass of a polyalkylene glycol with a molar mass of 200 to 2000 during compounding,

exist and that the natural fiber component is present in the compound constituents after agglomeration and direct injection molding partly in defibrillated form, partly in undestroyed form. 2. Molding compositions according to claim 1, characterized in that the starch ester egg have a degree of substitution of 1.5 to 2.6. 3. Molding compositions according to claims 1 and 2, characterized in that the Residual acetic acid content is 1 to 2%. 4. Molding compositions according to claims 1 to 3, characterized in that 4 to 20 Mass% polyalkylene glycol with a molecular weight of 300-1000 can be used. 5. Molding compositions according to claims 1 to 4, characterized in that as Po lyalkylene glycol polyethylene glycol is used. 6. Molding compositions according to claims 1 to 5, characterized in that as Na Turf fiber material flax, hemp, wood, reed, straw, nettle, ramie and the like can be used. 7. Process for the production of thermoplastically processable molding compositions according to An Claim 1 characterized in that the starch acetate used during the Compounding an Es resulting from the manufacture of the starch ester Acetic acid content of 0.5 to 5% by mass and the natural fiber component 2 to 25% by mass of a polyalkylene glycol and the mixture mechanically ag glomerized and in the directly following injection molding step a shearing nozzle se and / or shearing elements in the screw structure of the injection molding machine and a vacuum of 100 on the ejector side of the tool to 500 Torr is applied. 8. The method according to claim 7, characterized in that further lubricants, such as Stearic acid, abietic acid resins and their derivatives are used. 9. The method according to claims 7 and 8, characterized in that inorganic cal fillers, such as chalk, for neutralization with a content of 0.5 to 5% by mass, preferably 1-2%, can be used. 10. The method according to claims 7 to 9, characterized in that further Blend components to increase flexibility and water repellency, such as Po lyalkylene carbonate, polyester amides, polyesters or polyhydroxy fatty acids with a proportion of 2 to 20 wt .-% can be used.