WO2007091778A1

WO2007091778A1 - Evironmentally-friendly packing materials for food decomposable by light, calcium and organisms

Info

Publication number: WO2007091778A1
Application number: PCT/KR2006/005591
Authority: WO
Inventors: Shun Lun He
Original assignee: Kim, Sun Bun; Liang, Jin Hua; Beijing Huaao Shunda Tech. Corporation
Priority date: 2006-02-10
Filing date: 2006-12-20
Publication date: 2007-08-16
Also published as: KR100748134B1

Abstract

Disclosed are food packing material with eco-friendly decomposable properties and a process of manufacturing the same. The food packing material comprises of CaSO, biodegradable fraction and polyolefin resin. The manufacturing process comprises steps of: treating surface of CaSO by using the coupling agent after grinding CaSO into the granular state with particle size larger than 800 mesh; mixing the surface treated CaSO granules with other materials in a high speed mixer for 5 to 10 minutes; and forming the mixture into final granules, in which the temperature for obtaining the final granules is setup to 120 to 16O0C in a step of adding individual materials, 180 to 22O0C in a step of mixing the materials, 180 to 2000C in a step of transporting the mixture, and 170 to 19O0C at the extrusion head of the extrusion granulator, re¬ spectively, in order to prepare the final granules from the mixture, and wherein the resulting granules are subjected to further processes to manufacture various packing products. The food packing material according to the present invention achieves enormous social and economic benefit by satisfying hygienic properties and converting nvert photoinduction degradation into biodegradation.

Description

ENVIRONMENTALLY-FRIENDLY PACKING MATERIALS FOR FOOD DECOMPOSABLE BY LIGHT, CALCIUM AND

ORGANISMS

Technical Field

[1] The present invention generally relates to material technology, and more particularly, to environmentally-friendly packing materials for food which is decomposable by light, calcium and organisms and process of manufacturing the same.

[2]

Background Art

[3] In general, decomposable plastics are classified into two classes; one of which is a composite material including inorganic powder, plastic and a variety of preparations; and the other is another composite material composed of modified starch material (including cellulose material), plastic and various preparations.

[4] At present, most of commercially available inorganic powder products are CaCO

(including marble and dolomite) and talc powder usually based on two reasons: that is, one of which is that both of the powders are obtained from a wide range of origins provided at relatively low price; and the other one is that raw materials are conveniently purchased since there are relatively large number of manufactories and/or workshops for the inorganic powder products.

[5]

Disclosure of Invention Technical Problem

[6] However, in view of the present technical knowledge, amount of the above inorganic powder to be loaded is about 30% and biodegradability of most of the powder products is not superior to other products according to ASTM test result. Even when "biodegradability" is determined by processes for CO production commonly available all over the world through the measurement of biodegradable performance of such powder according to conventional method, it is difficult to accomplish standard level for biodegradability desired in related art.

[7] In the case where a disposable tableware is manufactured by using CaCO loaded materials, acetic acid elution must be formed in amount of not more than 300mg/l in accordance with internationally available standards, and undergo chemical transition as follows:

[8] CaCO + 2HAc → Ca(Ac) + CO + H O [9] Therefore, amount of the acetic acid elution usually exceeds the standard level required for the same. As observed from experiments, the amount of the acetic acid elution exceeds the standard level when content of CaCO powder having diameter of about 5D among the composite materials is equal to or more than 5%, so that it becomes a technical problem not to be overcome in practical applications related to food packaging material production.

[10] In consideration of the above aspect, the composite materials containing talc powder as major material to be loaded or filler normally have dark gray color as well known in the art and extremely low visual effect, and restrict their application for the manufacture of packaging materials. At the same time, major ingredient of the talc powder is SO , which has difficulties in solubilization and/or melting in fire. Thus, talc powder ingredient may be accumulated in body of a person causing calculus and doing the person harm, after using the composite food packaging material for long term. Accordingly, many countries prohibit the use of such talc based composite materials for the manufacture of food packaging material.

[11] Although starch based biodegradable composite plastics can overcome various drawbacks of the above composite materials based on CaCO and/or talc powders, the starch plastics have considerably impaired physical properties of plastic ingredient itself contained in the plastics and may reduce strength of the food packaging material, thereby being under limitation in amount thereof to be added. Generally, content of starch in the composite plastics is not more than 15%. Alternatively, the starch plastics have relative sensitivity to processing temperature and are thus difficult to process, and residual fraction of the material from first processing is burnt out in secondary processing and impossible to be recycled. Also, if a final product is wet or damped during storage and delivery of the product, it is likely to get moldy and has unclear or impure color thus resulting in undesirable visual effect. Consequently, all of the starch based plastics, CaCO or talc powder based materials have significant obstacles to be widely applied in the food packaging material production.

[12] Accordingly, there is much research being undertaken to produce materials exclusively used in manufacturing disposable tableware by selecting suitable filler materials and combining two or more of the filler materials together in specifically desired composition, which exhibits excellent biodegradable performance and ideal visual effect of starch plastics while having advantages of inorganic powder such as CaCO₃.

[13]

Technical Solution

[14] Hereinafter, the present invention will be described in detail. [15] Therefore, in order to overcome the above conventional problems, it is an object of the present invention to provide eco-friendly food packing materials which are decomposable by light, calcium and organisms, and a process of manufacturing the same.

[16] Such materials can be prepared from the following raw materials in specific composition by weight:

[17] CaSO 4 (gypsum powder anhydrite) 10 to 70%

[18] Composite light sensitizer 0.1 to 5%

[19] Biodegradable fraction 0.1 to 5 %

[20] the balance of residual ingredients such as polypropylene or polyethylene.

[21] Preferably, amount of CaSO (gypsum powder anhydrite) is ranged from 30 to 60% by weight. Amount of the composite light sensitizer is preferably ranged from 0.5 to 2% by weight. Also, the residual ingredients are preferably contained in amount of 2 to 3% by weight.

[22] Typically, CaSO (gypsum powder anhydrite) is prepared by steps in series including ore-dressing, washing the treated ore material, calcining the washed material and grinding the calcined material, and has whiteness not less than 90% and particle diameter not less than 800 mesh. Before the melting process, CaSO 4 should be necessarily subjected to surface treatment using any coupling agent. [23] The coupling agent comprises 0.3 to 1.2% by weight of aluminate as a primary coupling agent and 0.4 to 0.6% by weight of rare-earth based soap as a coupling aid, relative to total weight of CaSO . [24] The above aluminate may comprise known aluminate coupling agents named DL-

411- A, DL-411-AF or DL-411-DF. [25] The surface treatment is typically conducted by heating CaSO powder up to 80 to

4

9O⁰C in a high speed mixer while stirring to remove moisture from surface of the powder, adding the aluminate and the rare-earth based soap in amounts of 0.3 to 1.2% and 0.4 to 0.6%, respectively, to the heated CaSO powder and homogeneously mixing

4 the mixture. [26] The biodegradable fraction is a mixture of sodium stearate, fatty acid amide and sodium alginate, with the relative ratio by weight of individual ingredients of 1 : 1 : 2.

Both of sodium stearate and fatty acid amide cover surface of sodium alginate to make the final product have improved hydrophobic properties. [27] Sodium stearate used in the present invention is preferably selected from calcium stearate or iron stearate. [28] Fatty acid amide used in the present invention is preferably selected from a group consisting of oleamide, linoleamide and stearate amide. [29] The composite light sensitizer used in the present invention is prepared by mixing the rare-earth based soap as well as organic light sensitizer, with the relative ratio by weight of the rare-earth based soap and the organic sensitizer to the composite sensitizer ranging from 1:1 to 1:2. The composite light sensitizer increases absorption of UV rays by about 20% and, simultaneously, extends the range of absorbance spectrum of the sensitizer.

[30] The rare-earth based soap is preferably selected from a group consisting of: stearate rare-earth chlorides; salicylic rare-earth chlorides; lauric rare-earth chlorides; olein rare-earth chlorides; linolein rare-earth chlorides, etc. Among them, more preferably selected are stearate rare-earth La, salicylic rare-earth Ce and linolein rare-earth Pr.

[31] The above organic light sensitizer may comprise iron dimethyldithiocarbamate, iron diethyldithiocarbamate and iron dibutyldithiocarbamate, and more preferably, iron dibutyldithiocarbamate.

[32] Polypropylene used in the present invention is preferably a mixture of homo- polypropylene and copolymerized polypropylene in a mixing ratio of 1:1.

[33] The process of manufacturing the food packing material product according to the present invention comprises several steps of: treating surface of granulated CaSO by using the coupling agent after grinding CaSO into the granular state with particle size larger than 800 mesh; mixing the surface treated CaSO granules with other materials in the high speed mixer for 5 to 10 minutes; and forming the mixture into final granules by an extrusion granulator. The temperature for obtaining the final granules is controlled in steps, more particularly, setup to 120 to 16O⁰C in a step of adding individual materials, 180 to 22O⁰C in a step of mixing the materials, 180 to 200⁰C in a step of transporting the mixture, and 170 to 190 ⁰C at the extrusion head of the extrusion granulator, respectively, in order to prepare the final granules from the mixture. The resulting granules are subjected to further processes to manufacture various packing products such as food bag, disposable food box, etc.

[34] The coupling agent comprises 0.3 to 1.2% by weight of aluminate as the primary coupling agent and 0.4 to 0.6% by weight of rare-earth based soap as the coupling aid, relative to total weight of CaSO .

[35] The surface treatment is carried out by heating CaSO powder up to 80 to 9O⁰C in the high speed mixer while agitating to remove moisture from surface of the powder, adding the aluminate and the rare-earth based soap in amounts of 0.3 to 1.2% and 0.4 to 0.6%, respectively, to the heated CaSO powder and homogeneously mixing the mixture.

[36] The extrusion granulator is preferably a parallel twin-screw type extruder.

[37] From a result of experiments for the present material, it was demonstrated that the composite light sensitizer used in the present invention has no adverse effect on mechanical performance of the present inventive food packing material, and has pho- todegradable performance sufficient to meet requirements described in GB 18006.1-1999. After light induction, the packing material is continuously oxidized and decomposed to enhance complete decomposition under a condition of excluding light. Since plastic waste containing the present inventive packing material is deteriorated by photodegradation after land-filling and/or composting the waste, it increases hy- drophilic properties of the plastic to give a condition liable to grow bacteria or microorganisms and facilitate progression of the biodegradation of the plastic waste. Such bacteria or microorganisms act together with the biodegradable fraction contained in the packing material to increase the biodegradability of the packing material, so that it practically achieves conversion of photooxidation degradation into biodegradation of the composite material contained in the packing material.

[38] The biodegradable fraction comprises hydrophilic small molecular compound, which is favorable for biodegradation of the packing material, likely to be consumed by bacteria, and has small volume not to significantly effect absorption rate of the composite material. Therefore, the plastic product containing the packing material of the present invention does not gather mold unlike starch plastics, has reduced molecular weight and lowered temperature for incinerating the plastic due to specific ingredients added to the composite light sensitizer contained in the packing material.

[39] The food packing material product fabricated by the present inventive process has pure white color without other colors and excellent visual effect. Since CaSO is not dissolved in HAC, acetic acid elution of the packing material product does not exceed the standard level even if CaSO is included in large volumes in the packing material. In addition, the product can be continuously oxidized and decomposed under light- shielded conditions for about 10 to 60 days (which is determined according to thickness and light irradiation strength of the product) until the product is biologically degraded. Due to biological affinity and lubrication of calcium stearate and fatty acid amide they can be used to replace paraffin wax to cover PPG as biodegradable sodium alginate and improve compatibility between PPG and a base plastic material. Since the paraffin wax is not added during combination, amount of cyclohexane elution does not exceed the standard level. Alternatively, large amounts of inorganic powder, that is, CaSO is employed to simultaneously discard the plastic waste as well as municipal solid waste and treat the waste through incineration and/or composting process.

[40] The waste from the present inventive product will never contaminate underground waster and has very superior environmental affinity. More particularly, the present inventive packing material has advantages as follows:

[41] * CaSO which is an inorganic mineral material rich in resource ingredients, has stable chemical properties and contains no heavy metals, thus its waste does not cause contamination of underground water sources thereby preventing dangerous influence to environment. [42] * Petroleum resources can be saved by using the inorganic material in plastic; in the case where an amount of the inorganic material used is 50% by weight, volume of the plastic is reduced to 18 to 20% and consumption of the petroleum is decreased by about 40%. Such results are of significant importance at present in that petroleum resources are nearly exhausted. In view of the development of novel materials accompanied by environmental protection, it will be appreciated that the inorganic material which is obtained from nature and returns to nature, can contribute to environmental protection if the inorganic material is used to fabricate plastic products in place of typical polymeric materials.

[43] * Environmental protection effect of reducing incineration of plastics by using the inorganic material; the plastic waste impossible to be recycled can be ultimately included in a waste treatment system having the same mode of processing to other discarded wastes. Especially, treatment of incinerating the plastic waste is a very important technique and electric power generation through waste incineration is commonly and extensively applied in recovering energy all over the world. Plastic component becomes easily coked and agglomerated into a lump to inhibit sufficient combustion of internal components of the plastic component. Incomplete combustion of the plastic component may cause generation of CO gas in great quantities and result in an extremely intensive greenhouse effect. Also, the incompletely combusted fraction may be adhered to wall or pipeline of a boiler and adversely affect transportation of the waste or air flow stream, thereby accelerating damage of the boiler wall. In contrast, if the plastic contains the inorganic material it can considerably dilute the plastic component and, in addition, CaSO causes an expansion effect during the incineration to very remarkably facilitate complete combustion of the plastic component.

[44] Consequently, the food packing material according to the present invention satisfies hygienic properties required for packing foods as well as known properties of typical photo/biodegradable plastics, so that the material can be treated simultaneously with common municipal solid waste by converting photoinduction degradation into biodegradation of the waste to make the waste into compost or incinerating it. Accordingly, the food packing material of the present invention can be broadly distributed and employed to achieve enormous social and economic benefit.

[45]

Advantageous Effects

[46] As described in detail above, the present inventive food packing material satisfies hygienic properties required for packing foods as well as known properties of typical photo/biodegradable plastics, so that the material can be treated simultaneously with common municipal solid waste by converting photoinduction degradation into biodegradation of the waste to make the waste into compost or incinerating it. Accordingly, the food packing material of the present invention can be broadly distributed and employed to achieve enormous social and economic benefit.

[47]

Best Mode for Carrying Out the Invention

[48] Features of the present invention described above and other advantages will be more clearly understood by the following non-limited examples and comparative examples. However, it will be obvious to those skilled in the art that the present invention is not restricted to the specific matters stated in the examples below.

[49] EXAMPLE 1

[50] Example 1 relates to the production of PE film product.

[51] Calcium sulfate was pulverized into particulate powder having particle size of 800 mesh. 45kg of the particulate powder was taken and fed in a high speed mixer, heated to 8O⁰C under stirring, added with 135g of DL-411-A as the aluminate coupling agent and 27Og of stearate rare-earth La, and homogeneously mixed together during stirring.

[52] 5 lkg of PE (7000F), 500g of stearate rare-earth La, 500g of iron dibutyldithio- carbamate, 75Og of calcium stearate, 75Og of oleamide (Ningbo Union Biotech Authority), and 1.5kg of sodium alginate were placed in the high speed mixer and mixed together during stirring for 10 minutes. The mixture was charged in the parallel twin-screw type extrusion granulator to form granules. Temperature for the production of granules was controlled in steps such that the temperatures were set to 16O⁰C, 18O⁰C and 200⁰C in the steps of: adding individual materials; mixing the materials; and transporting the mixture, respectively and, in addition, to 17O⁰C at the extrusion head of the extrusion granulator to prepare the final granules from the mixture. The resulting granules were formed into a sheet type of packing product with thickness ranging from 30 to 4OD through film blowing.

[53] EXAMPLE 2

[54] Example 2 relates to the production of PP based food box.

[55] Calcium sulfate was pulverized into particulate powder having particle size of 1000 mesh. The particulate powder was fed in the high speed mixer, heated to 9O⁰C under stirring, added with 672g of DL-411-AF as the aluminate coupling agent and 224g of salicylic rare-earth Ce, and homogeneously mixed together during stirring.

[56] 20kg of each of PP(1300) and PP(8303), 333g of salicylic rare-earth Ce, 666g of iron dimethyldithiocarbamate, 75Og of calcium stearate, 75Og of linoleamide, and 1.5kg of sodium alginate were placed in the high speed mixer and mixed together during stirring for 10 minutes. The mixture was charged in the parallel twin-screw type extrusion granulator to form granules. Temperature for the production of granules was controlled in steps such that the temperatures were set to 12O⁰C, 22O⁰C and 18O⁰C in the steps of: adding individual materials; mixing the materials; and transporting the mixture, respectively and, in addition, to 19O⁰C at the extrusion head of the extrusion granulator to prepare the final granules from the mixture. The resulting granules were formed into a box type of disposable food packing product with thickness ranging from 0.8 to 1.0mm through plastic suction molding.

[57] EXAMPLE 3

[58] Example 3 relates to the production of PE film product.

[59] Calcium sulfate was pulverized into particulate powder having particle size of 1200 mesh. 30kg of the particulate powder was taken and fed in the high speed mixer, heated to 8O⁰C under stirring, added with 180g of DL-411-DF as the aluminate coupling agent and 150g of linoleic acid rare-earth Pr, and homogeneously mixed together during stirring.

[60] 66kg of PE(1F7B), lOOOg of linoleic acid rare-earth Pr, lOOOg of iron diethyldithio- carbamate, 500g of iron stearate, 75Og of stearate amine, and lOOOg of sodium alginate were placed in the high speed mixer and mixed together during stirring for 5 minutes. The mixture was charged in the parallel twin-screw type extrusion granulator to form granules. Temperature for the production of granules was controlled in steps such that the temperatures were set to 135⁰C, 200⁰C and 195⁰C in the steps of: adding individual materials; mixing the materials; and transporting the mixture, respectively and, in addition, to 18O⁰C at the extrusion head of the extrusion granulator to prepare the final granules from the mixture. The resulting granules were formed into a sheet type of food packing product with thickness ranging from 40 to 50D through film blowing.

[61] EXAMPLE 4

[62] Example 4 relates to the production of PP based food box.

[63] Calcium sulfate was pulverized into particulate powder having particle size of 800 mesh. 70kg of the particulate powder was taken and fed in the high speed mixer, heated to 85⁰C under stirring, added with 630g of DL-411-A as the aluminate coupling agent and 350g of stearate rare-earth La, and homogeneously mixed together during stirring.

[64] 4.5kg of PP(1300), 20kg of PP(8303), 20Og of stearate rare-earth La, 300g of iron dibutyldithiocarbamate, 1.25kg of calcium stearate, 1.25kg of oleamide, and 2.5kg of sodium alginate were placed in the high speed mixer and mixed together during stirring for 10 minutes. The mixture was charged in the parallel twin-screw type extrusion granulator to form granules. Temperature for the production of granules was controlled in steps such that the temperatures were setup to 145⁰C, 195⁰C and 185⁰C in the steps of: adding individual materials; mixing the materials; and transporting the mixture, respectively and, in addition, to 185⁰C at the extrusion head of the extrusion granulator to prepare the final granules from the mixture. The resulting granules were formed into a box type of disposable food packing product with thickness ranging from 0.5 to 0.7mm through plastic suction molding.

[65] EXPERIMENTAL EXAMPLE 1 [66] This example is to conduct light emission test for monitoring the product from Example 3 compared with the photodegradable film without addition of the inorganic materials.

[67] The photodegradable film with no inorganic material added is prepared by employing the same method and desired volume in Example 3 except that it does not contain calcium sulfate powder.

[68] Procedure: A thin film sample was fixed to a frame and placed on a porch of an outer roof to receive sunlight. The same was positioned in the south direction at the slope of 45 angle. This experiment was conducted in Beijing in the season from July to September.

[69] The following Table 1 shows the result of experiments for the present invention indicated as time passed under a condition that the photodegradable PE film sample has specific CI (carbonyl group index).

[70] Table 1

[71] * CI: carbonyl group index [72] Conclusion: The inorganic mineral has activity of accelerating plastic decomposition.

[73] Mechanism: As O penetration rate of PE photodegradable film itself was increased after filling the film with a great amount of inorganic mineral, it means that the film is more prone to oxidation and decomposition.

[74] COMPARATIVE EXAMPLE 1 [75] This example is to compare the waste of the product from Example 2 according to the present invention with the waste of the known plastic product (containing starch) according to conventional process, more particularly, with respect to influence of the waste to chemical oxygen demand COD of water. From the result, the environmental protection effect of the product according to Example 2 was demonstrated as shown in the following Table 2.

[76] Under the same condition, the present inventive product according to Example 2 deteriorated into small pieces less than 5mm within the time period ranging from 15 to 20 days. The influence of the plastic waste containing starch on generation of COD in water is illustrated as shown in the following Table 2.

[77] Table 2

[78] COMPARATIVE EXAMPLE 2 [79] This example gives data of decomposition experiments for comparing the present inventive product from Example 1 with the conventional product.

[80] CI of a typical photodegradable PE film was 20 after light irradiation for 10 days and 28 even at 6 months after landfill of the film. Thus, the PE film still had viscosity average molecular weight M greater than 10,000, which was little altered.

[81] Conversely, the PE film prepared by using the present inventive material had CI of 15 after light irradiation for 10 days and 50 at 6 months after landfill of the film resulting in decrease of M η to 4,000.

[82] The above experiments show that the present inventive material can have oxidation degradation properties as well as noticeable light- shielding continuation and biodegradability after inducing light irradiation to the material. In particular, M η became lowered to 4,000 at 6 months after landfill of the plastic containing the present inventive material, which indicates the period required to deteriorate the plastic. The deteriorated product may be permeable to water and possibly subjected to proliferation of microorganisms on relatively small molecular chains thereof. As the result, the plastic entered the biodegradation stage.

[83] On the other hand, the conventional photodegradable PE film did not have the same properties described above. [84]

Industrial Applicability

[85] As described above, the food packing material according to the present invention achieves enormous social and economic benefit by satisfying hygienic properties required for packing foods and, at the same time, exhibiting photo/biodegradable properties to convert photoinduction degradation into biodegradation of the waste containing the food packing material. [86]

Claims

[1] Food packing material with eco-friendly decomposable properties which is decomposable by light, calcium and/or organisms, and prepared from the following raw materials in specific composition by weight: CaSO 4 (gypsum powder anhydrite) 10 to 70%

Composite light sensitizer 0.1 to 5% Biodegradable fraction 0.1 to 5% the balance of residual ingredients such as polypropylene or polyethylene.

[2] The food packing material according to claim 1, wherein CaSO 4 is preferably selected from 30 to 60% by weight; the composite light sensitizer is preferably selected from 0.5 to 2% by weight; and the biodegradable fraction is preferably selected from 2 to 3% by weight.

[3] The food packing material according to claim 1 or 2, wherein CaSO is prepared

4 by steps in series including ore-dressing, washing the treated ore material, calcining the washed material and grinding the calcined material; has whiteness not less than 90% and particle diameter not less than 800 mesh; and is necessarily subjected to surface treatment by using any coupling agent before the melting process.

[4] The food packing material according to claim 3, wherein the coupling agent comprises 0.3 to 1.2% by weight of aluminate and 0.4 to 0.6% by weight of rare- earth based soap, which are calculated relative to total weight of CaSO .

[5] The food packing material according to claim 1, wherein the composite light sensitizer comprises rare-earth based soap and organic light sensitizer, which have the relative ratio by weight of the rare-earth based soap and the organic sensitizer to the composite sensitizer ranging from 1:1 to 1:2.

[6] The food packing material according to claim 5, wherein the rare-earth based soap is preferably selected from a group consisting of: stearate rare-earth chlorides; salicylic rare-earth chlorides; lauric rare-earth chlorides; olein rare- earth chlorides; linolein rare-earth chlorides, etc.; more preferably, selected from stearate rare-earth La, salicylic rare-earth Ce and linolein rare-earth Pr among them; and the organic light sensitizer is selected from a group consisting of iron dimethyldithiocarbamate, iron diethyldithiocarbamate and iron dibutyldithio- carbamate and, more preferably, comprises iron dibutyldithiocarbamate.

[7] The food packing material according to claim 1, wherein the biodegradable fraction is a mixture of sodium stearate, fatty acid amide and sodium alginate, with the relative ratio by weight of individual ingredients of 1 : 1 : 2; sodium stearate is preferably selected from calcium stearate and iron stearate; and fatty acid amide is preferably selected from a group consisting of oleamide, linoleamide and stearate amide.

[8] A process of manufacturing food packing material with eco-friendly decomposable properties that is decomposable by light, calcium and/or organisms, which comprises steps of: treating surface of granular CaSO by using the coupling agent after grinding CaSO into the granular state with particle size larger than 800 mesh; mixing the surface treated CaSO granules with other

4 materials in a high speed mixer for 5 to 10 minutes; and forming the mixture into final granules by an extrusion granulator, in which the temperature for obtaining the final granules is controlled in steps such that the temperatures are set to 120 to 16O⁰C in a step of adding individual materials, 180 to 22O⁰C in a step of mixing the materials, 180 to 200⁰C in a step of transporting the mixture, and 170 to 19O⁰C at the extrusion head of the extrusion granulator, respectively, in order to prepare the final granules from the mixture, and wherein the resulting granules are subjected to further processes to manufacture various packing products such as food bag, disposable food box, etc.

[9] The method according to claim 8, wherein the coupling agent comprises 0.3 to

1.2% by weight of aluminate and 0.4 to 0.6% by weight of rare-earth based soap; the surface treatment is carried out by heating CaSO powder up to 80 to 90? in the high speed mixer during agitating to remove moisture from surface of the powder, adding the aluminate and the rare-earth based soap in amounts of 0.3 to 1.2% and 0.4 to 0.6%, respectively, to the heated CaSO powder and homogeneously mixing the mixture; and the content ratio of the aluminate and the rare-earth based soap is calculated relative to total weight of CaSO .

[10] The method according to claim 8 or 9, wherein the extrusion granulator is preferably a parallel type of twin-screw extruder.