Polylactic acid resin composition for injection molding

Abstract

To provide a polylactic resin composition that can increase the rate of production of an injection molded article using a polylactic resin composition and is suitable for mass production.SOLUTION: Injection molding is performed by using a polylactic resin composition containing cellulose nanofiber (CNF) as a crystallization agent, wherein the CNF is obtained by crushing hydrothermal-treated cellulose raw material and then subjecting the crushed wood chips to chemical treatment.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a polylactic acid resin composition for injection molding. More specifically, it relates to a polylactic acid resin composition with high mold release properties from a mold.

Background Art

[0002] Polylactic acid (PLA) is a plant-derived biodegradable resin, and even when incinerated, the amount of CO emissions is at the lowest level among resins with a large amount of emissions. Therefore, it has attracted attention as a material with a low environmental impact. In addition, since many resin molded products are manufactured by injection molding, manufacturing injection molded products using PLA has been widely studied. 2 The production of resin molded products by injection molding consists of the following six steps: (1) mold clamping; (2) injection; (3) holding pressure; (4) cooling; (5) mold opening; and (6) removal of the molded product.

[0003] In the injection step, generally, resin is charged into a hopper and melted resin is poured into a mold clamped through a heating cylinder. In this step, in particular, high fluidity of the resin is required when pouring it into the mold. In particular, it is an important factor when injecting into a mold with a complex shape. From the holding pressure step to the removal step, the entire internal structure of the mold is filled with resin, and while maintaining pressure for a predetermined time until the resin solidifies, it is cooled. After the resin solidifies, the mold is opened and the molded product is taken out. When taking out the molded product from the mold by ejecting with an ejector pin or by suction, the mold release property from the mold and the rigidity of the molded product are important factors so that the molded product does not deform or break. Therefore, the cooling and removal steps become the rate-determining steps for the production speed of molded products by injection molding.

[0004] In the case of injection molding using a conventional polylactic acid resin composition, the production speed is about 20 to 80 pieces per day. At this level of production speed, it cannot be applied to mass production.

[0005] ​Generally, when a resin crystallizes, its rigidity improves. However, since PLA is difficult to crystallize, it is difficult to utilize it as a heat-resistant resin molded product. Therefore, in order to improve the rigidity of the molded product, efforts have been made to enhance the crystallization of PLA so that deformation or damage does not occur when the molded product is taken out of the mold.

[0006] For example, Patent Document 1 discloses a molding method in which a polylactic acid resin composition with improved crystallization rate is supplied to a mold cavity with a surface temperature set at 90 to 140°C, and the surface temperature of the cavity is rapidly cooled to 40 to 80°C. On the other hand, Patent Document 2 discloses a method of improving crystallization by producing a polylactic acid resin under specific temperature conditions. The method of Patent Document 2 also describes using known crystallization nucleating agents for thermoplastic resins, such as synthetic mica, clay, talc, etc. Furthermore, methods of promoting the crystallization of PLA resin compositions by using trimesic acid triamide-based molecules (Patent Document 3), organic phosphonate compounds, amide compounds having a ring structure (Patent Document 4), etc. as crystallization nucleating agents have also been studied.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0008] An object of the present invention is to provide a polylactic acid resin composition that can increase the production rate of manufacturing injection-molded products using the polylactic acid resin composition and is applicable to mass production.

Means for Solving the Problems

[0009] By using the polylactic acid resin composition of the present invention, the production rate can be increased using an existing injection molding apparatus without newly introducing a temperature control system for achieving a sophisticated heating and cooling profile. In particular, even in a cooling process according to a conventional temperature setting and a conventional take-out process, the rigidity of a molded product of the polylactic acid resin composition can be improved to prevent deformation and breakage. More specifically, the polylactic acid resin composition of the present invention contains cellulose nanofiber (CNF) as a crystallization agent. When injection molding is performed using the polylactic acid resin composition of the present invention, it has been demonstrated that the production rate increases by as much as two digits to about 1500 to 10000 pieces per day without controlling the temperature of the mold cavity to prevent deformation and breakage in the take-out process. Both the resin and the crystallization agent of the polylactic acid resin composition of the present invention are derived from natural products, and as a result, a molded product with a low environmental load can be obtained.

Effects of the Invention

[0010] When the polylactic acid resin composition of the present invention is used for injection molding, molded products with a low environmental load can be mass-produced.

Modes for Carrying Out the Invention

[0011] The present invention provides a polylactic acid resin composition containing polylactic acid and cellulose nanofiber.

[0012] As a first aspect of the polylactic acid resin composition of the present invention, the cellulose nanofiber is produced by a production method including, in this order, a step of subjecting a cellulose raw material to hydrothermal treatment to obtain a swollen cellulose raw material; a step of crushing the swollen cellulose raw material to obtain pulp; and a step of chemically treating the pulp to obtain cellulose nanofibers (CNF).

[0013] In the polylactic acid resin composition of the present invention, the addition amount of the cellulose nanofiber is 0.1 to 50.0 parts by weight, preferably 1.0 to 10.0 parts by weight, based on 100 parts by weight of the polylactic acid.

[0014] The cellulose raw material used in the method for producing CNF according to the present invention may be any substance that can extract natural cellulose, for example, woody plants selected from the group consisting of coniferous trees, broad-leaved trees, and bamboo, or herbaceous plants selected from the group consisting of rice ears, scraps, and pampas grass, and further includes papers. These cellulose raw materials may not be new materials, but may be used waste materials of the above-mentioned woody plants and herbaceous plants, or old papers. Such cellulose raw materials are sized appropriately for handling and then subjected to the process. In the present invention, the size of the cellulose raw material when subjected to the process is preferably in the range of 0.5×0.5 cm to 2.0×2.0 cm, more preferably 0.7×0.7 cm to 1.5×1.5 cm, and most preferably 0.8×0.8 cm to 1.2×1.2 cm. If the raw material is larger than the above range, it is crushed into crushed materials in the form of chips or powders.

[0015] In the hydrothermal treatment step of the method for producing CNF according to the present invention, the cellulose raw material is immersed in water as it is and subjected to a subcritical to supercritical state under high temperature and high pressure conditions. More specifically, the hydrothermal treatment of chips or the like immersed in water is carried out at 400°C or lower under 1 to 300 atmospheres, preferably at 5 to 200°C under 2 to 250 atmospheres, more preferably at 100 to 380°C under 25 to 100 atmospheres, and most preferably at 150 to 250°C under 25 to 100 atmospheres for 60 to 180 minutes in a subcritical or supercritical state. By these hydrothermal treatments, the cellulose raw material becomes a soft and swollen crushed material.

[0016] In the conventional method for producing CNF, cellulose raw materials such as wood chips were first chemically treated with sulfuric acid or the like, and then solvothermal treatment was performed. However, in the production method of the present invention, it is characterized in that cellulose raw materials having a certain size such as wood chips are first hydrothermally treated, and then chemically treated using an acid or an alkali. When the CNF produced by the production method of the present invention is mixed with a resin, the physical properties of the composite material are enhanced.

[0017] Solvothermal treatment is a treatment in which an organic solvent is used instead of water used in hydrothermal treatment. Examples of such organic solvents include solvents such as methanol, ethanol, propanol, pyrrolidone-based solvents such as N-methylpyrrolidone, acetate-based solvents such as butyl acetate, glycol ether-based solvents such as diethylene glycol monomethyl ether, ketone-based solvents such as methyl ethyl ketone, aromatic solvents such as toluene and xylene, and hydrocarbon-based solvents such as paraffin.

[0018] Next, the obtained swollen cellulose raw material is subjected to a disintegration step to loosen the fibers and pulp them. In this disintegration step, a ball mill, a disk mill, a wet cutter mill, a pressure homogenizer, or the like can be used. By this disintegration step, the cellulose raw material becomes fibrous pulp of 0.05 to 0.5 mm.

[0019] Finally, the pulp obtained by disintegration is chemically treated. Examples of the chemical treatment include acid treatment, alkali treatment, or a combination thereof. For the acid treatment, acids such as sulfuric acid, nitric acid, hydrochloric acid, and acetic acid can be used. For the alkali treatment, sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, sodium hypochlorite, or the like can be used. By the chemical treatment, functional groups such as carboxy groups, hydroxy groups, ester groups, sulfo groups, ketone groups, and acetyl groups can be provided on the cellulose surface, and the dispersion state in the resin can be improved.

[0020] Also, lignin can be added to the cellulose raw material before subjecting it to hydrothermal treatment. By adding lignin, the surface of the generated CNF is hydrophobized (hydrophobized CNF). In a composite material produced by mixing hydrophobized CNF and a resin, the tensile strength is higher than that of a composite material of non-hydrophobized CNF without adding lignin, which is more preferable. The mixing ratio of the cellulose raw material and lignin is preferably about cellulose raw material / lignin (weight ratio) = 0.5 to 2, more preferably 0.7 to 1.5, and even more preferably 0.8 to 1.2.

[0021] Various additives that can be added to a resin composition for injection molding can be added to the polylactic acid resin composition of the present invention as long as the object of the present invention is not impaired. Examples of additives that can be added to the polylactic acid resin composition of the present invention include lubricants, plasticizers, mold release agents, fluidity improvers, and the like.

[0022] Examples of the above lubricants include fatty acid esters such as sucrose fatty acid ester and glycerin fatty acid ester; hydrocarbons such as liquid paraffin, paraffin wax, and synthetic polyethylene wax; fatty acids and higher alcohols such as stearic acid and stearyl alcohol; fatty acid amides such as stearic acid amide, oleic acid amide, and erucic acid amide, and aliphatic amides such as methylene bisstearic acid amide and ethylene bisstearic acid amide; metal soaps such as zinc stearate, calcium stearate, and magnesium stearate; and fatty acid esters of ester alcohols such as monoglyceride stearate and stearyl stearate. Among them, from the viewpoint of reducing environmental impact, natural product-derived components such as sucrose fatty acid ester, glycerin fatty acid ester, stearic acid, stearyl alcohol, zinc stearate, calcium stearate, and magnesium stearate are preferable.

[0023] Examples of the above plasticizers include phthalic acid esters such as dioctyl phthalate, diisononyl phthalate, diisodecyl phthalate, and dibutyl phthalate; adipic acid esters such as dioctyl adipate and diisononyl adipate; trimellitic acid esters such as trioctyl trimellitate; dibasic acids (such as adipic acid, sebacic acid, and phthalic acid) and glycols (such as polyesters of 1,2-propanediol and butanediol); phosphate esters such as tricresyl phosphate; citric acid esters such as tributyl acetyl citrate, triethyl citrate, and tributyl citrate; epoxidized vegetable oils such as epoxidized soybean oil and epoxidized linseed oil; mono- or diesters of dicarboxylic acids such as sebacic acid esters, azelaic acid esters, and maleic acid esters; esters of aromatic carboxylic acids such as benzoic acid esters; fatty acid methyl esters such as methyl myristate, methyl palmitate, methyl stearate, methyl oleate, methyl animal fatty acid, methyl beef tallow fatty acid, methyl vegetable fatty acid, methyl soybean fatty acid, and methyl linseed oil fatty acid; and fatty acid butyl esters such as butyl myristate, butyl palmitate, butyl stearate, butyl oleate, butyl animal fatty acid, butyl beef tallow fatty acid, vegetable fatty acid, butyl soybean fatty acid, and butyl linseed oil fatty acid. Among them, from the perspective of reducing environmental impact, natural product-derived components such as tricresyl phosphate, triethyl citrate, tributyl citrate, epoxidized soybean oil, epoxidized linseed oil, and sebacic acid esters are preferred.

[0024] Examples of the above mold release agents include glycerin fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, and higher alcohol fatty acid esters. These compounds are preferred from the perspective of reducing environmental impact.

[0025] In addition, inorganic material fillers such as powders of carbonates, sulfates, silicates, phosphates, borates, oxides, or hydrates of calcium, magnesium, aluminum, titanium, zinc, etc. also have effects as lubricants, mold release agents, and fluidity improvers. Specifically, for example, calcium carbonate, silica, clay, feldspar, magnesium carbonate, zinc oxide, titanium oxide, silica, alumina, kaolin clay, talc, mica, wollastonite, aluminum hydroxide, magnesium hydroxide, aluminum silicate, magnesium silicate, calcium silicate, aluminum sulfate, magnesium sulfate, calcium sulfate, magnesium phosphate, barium sulfate, silica sand, zeolite, diatomaceous earth, sericite, shirasu, calcium sulfite, potassium titanate, bentonite, graphite, ferrite, etc. can be mentioned.

[0026] The present invention provides a method for manufacturing a molded article by injection molding the above polylactic acid resin composition. The manufacture of a resin molded article using the polylactic acid resin composition of the present invention includes the following six steps: (1) mold clamping; (2) injection; (3) holding pressure; (4) cooling; (5) mold opening; and (6) taking out the molded article. The set conditions for each step may be conventionally known conditions.

[0027] Using the polylactic acid resin composition of the present invention, various molded articles such as tableware and containers such as cups, bowls, trays, parts of electrical appliances such as televisions, computers, washing machines, mobile phones, and parts of automobiles are manufactured by injection molding.

Example

[0028] Example 1 1 kg of wood chips (1.0 x 1.0 cm) and 10 L of N-methylpyrrolidone were mixed and subjected to solvothermal treatment in an autoclave (200 °C, 25 atm) for 2 hours. The obtained wood pulverized product was heat-treated in an aqueous solution of 10% sodium hypochlorite at 90 °C for 1 hour to obtain solvothermal treatment / chemically treated CNF. To 95 g of polylactic acid (PLA; manufactured by Natureworks), 5 g of the CNF produced above was blended and mixed using a twin-screw extruder to prepare a biodegradable composite material containing 5 wt% of CNF.

[0029] [Tensile property test] Using a desktop mini injection molding machine (HAAKE MiniJet Pro manufactured by Thermo Fisher Scientific), in accordance with JIS K7139, strip test pieces (80 mm x 10 mm x 4 mm) were created. Using a precision universal testing machine (Autograph AG-X plus manufactured by Shimadzu Corporation), in accordance with JIS K 7161:1994, the tensile strength of the created strip test pieces was measured (test speed: 20 mm / min), and data processing was performed using "TRAPEZIUM LITE X" equipped with a macro function attached to the tester. The results are shown in Table 1.

[0030] [Injection moldability test] Using a normal production line for manufacturing tableware such as spoons, forks, and knives by plastic injection molding, spoons, forks, and knives were manufactured from the biodegradable composite material. The mold cavity temperature was left at ambient temperature, and no particular temperature control was performed.

[0031] Comparative Example 1 A biodegradable composite material containing only polylactic acid (PLA; manufactured by Natureworks) was prepared, and in the same manner as in Example 1, tensile property tests and injection moldability tests were performed. The results are shown in Table 1.

[0032]

Table 1

[0033] From the biodegradable composite material containing only PLA without CNF, only 80 spoons, forks, and knives could be manufactured per day. On the other hand, from the biodegradable composite material containing 5 wt% of CNF, 1800 spoons, forks, and knives could be manufactured per day. It was confirmed that just by adding 5 wt% of CNF, the mold release property was significantly improved without setting the mold cavity temperature to a high temperature or quenching to promote the crystallization of PLA.

Industrial Applicability

[0034] If injection molding is carried out using a polylactic acid resin composition containing polylactic acid and the cellulose nanofiber manufactured by a production method including, in this order, a step of subjecting a cellulose raw material to hydrothermal treatment to obtain a swollen cellulose raw material; a step of crushing the swollen cellulose raw material to obtain pulp; and a step of chemically treating the pulp to obtain cellulose nanofiber (CNF), molded products with low environmental impact can be mass-produced.

Claims

1. The method comprises producing cellulose nanofibers by a production method including, in this order, a step of subjecting a cellulose raw material to hydrothermal treatment to obtain a swollen cellulose raw material; a step of disintegrating the swollen cellulose raw material to obtain pulp; and a step of chemically treating the pulp to obtain cellulose nanofibers (CNF), A method for producing a molded product, comprising injection molding a polylactic acid resin composition containing polylactic acid and the cellulose nanofibers.

2. The method for producing a molded article according to claim 1, wherein the amount of the cellulose nanofiber added is 0.1 to 50 parts by weight per 100 parts by weight of the polylactic acid.

3. The method for producing a molded article according to claim 1, wherein the hydrothermal treatment is carried out in a critical or subcritical state.

4. 2. The method for producing a molded product according to claim 1, wherein the cellulose raw material is a woody plant selected from the group consisting of conifers, broad-leaved trees and bamboo, or a herbaceous plant selected from the group consisting of rice ears, kudzu and pampas grass, or waste materials thereof, or paper or waste paper.

5. 2. The method for producing a molded product according to claim 1, wherein the chemical treatment is an acid treatment using an acid selected from sulfuric acid, nitric acid, hydrochloric acid, and acetic acid, or an alkali treatment using an alkali selected from sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, and hypochlorous acid.

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