Method of manufacturing molded pulp product

Abstract: A moldable pulp material, which contains a main constituent of pulp comprising strips of paper having a size of millimeters and a fibrous component defibrated from waste paper, a starch binder in which a portion of the hydroxyl group in molecules of starch is modified, and thermally expandable hollow particles, is mixed with water which is effective to gelatinize the starch binder. The moldable pulp material is filled in a mold assembly and compressed in the mold assembly. The compressed moldable pulp material is then heated to at least a gelatinization temperature at which the starch binder is gelatinized, for thereby gelatinizing the starch binder to produce a molded pulp product for use as a shock absorbing material.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a method of manufacturing a molded pulp product which is composed mainly of pulp. Description of the Related Art: Heretofore, packages for electric appliances including television sets, video tape recorders, cassette tape recorders, etc., and other packages such as food trays, fish containers, etc. have primarily been made of foamed styrol. Since foamed styrol is a good shock absorbing material, it is suitable especially for use as a package material for electric appliances. However, packages molded of foamed styrol present various problems after use, particularly when they are thrown away after use. For example, when foamed styrol packages are burned in a furnace, they tend to damage the furnace due to the intensive heat they generate. When the foamed styrol is buried in soil, it remains unchanged chemically and physically because it is not degradable in soil. Therefore, the widespread use of foamed styrol packages is causing serious concern about their disposal in view of the present growing interests in environmental protection and the conservation of natural resources. There have been demands for moldable materials, as substitutes for foamed styrol, for use as package materials which are less conducive to environmental pollution and more effective to utilize natural resources. One of the substitute package materials is a moldable pulp material which is chiefly composed of waste paper such as waste newspaper. One molded product formed of such a moldable pulp material is manufactured by applying a moldable pulp slurry produced from waste paper such as newspaper to a porous mold or a mold with a mesh screen, and then dehydrating, pressing, and drying the applied layer of moldable pulp slurry in several stages. However, the molded product thus manufactured finds limited applications because it has a relatively small thickness ranging from 1 mm to 5 mm and is not strong enough to hold heavy objects. Inasmuch as the moldable pulp material has a relatively small shock absorbing capability of its own, it fails to meet shock absorbing requirements for packages for holding television sets, video tape recorders, cassette tape recorders, etc., which have weights of several Kg or greater. At present, the moldable pulp material is mainly used as a package material for small packages including egg cases, small machine tool packages, seedling raising pots, packages for cassette tape recorders which have weights of 1 kg or smaller, etc.. It has been proposed to increase the shock absorbing capability of molded products formed of a moldable pulp material by adding a small amount of binder and hollow particles to a main constituent of pulp (see Japanese laid-open patent publication No. 6-10300 (and its counterpart US-A-5,360,825), for example) . Prior art document EP-A-0 598 372 describes a cylindrical composite paper board cushion core which comprises a paper board substrate and a cushion layer formed on the cylindrical paper board substrate from an expanded paper sheet. The above already mentioned document US-A-5,360,825 discloses a pulp molding which is formed by molding a feedstock consisting of a pulp and some binder. The feedstock contains plastic microspheres which form uniformly distributed voids in the pulp molding. The microspheres contain an organic solvent having a low boiling point where expanded microspheres have diameters four to five times those of the original microspheres. The microspheres are formed of a thermoplastic resin. Prior art document WO-A-94 18 384 describes a method to separate cellulose based fibers from each other in water, wherein a raw material is first wetted with water. By a combination of addition of a hydrocolloide which binds water, and strong kneading at high solid content, that mixture is transformed into a modulable paste. Shock absorbing products which are molded of a moldable pulp material have the same level of shock absorbing capability and moldability as products molded of foamed styrol. However, since the shock absorbing products are molded of a pulp slurry formed of waste paper pulp and containing only 10 % or more of a solid material, it takes several hours to mold them through the process of filling the pulp slurry in a mold, dehydrating and drying the pulp slurry, and removing the molded product from the mold. Accordingly, the rate of production of shock absorbing products molded of a moldable pulp material has been low. For the above reasons, there has been a demand for a moldable pulp material which has a high shock absorbing capability and can be molded into a molded product at a rate of production with is equivalent to the rate of production of molded products formed of foamed styrol. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of manufacturing a molded pulp product which resolves the above-mentioned problems. To solve this object, the present invention provides a method of manufacturing a molded pulp product as specified in claim 1. Preferred embodiments of the invention are described in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of an SEM photographic representation of a moldable pulp material; FIG. 2 is an exploded perspective view of a mold assembly for molding a product of the moldable pulp material; FIG. 3 is a perspective view of a pressing apparatus for compressing the moldable pulp material filled in the mold assembly shown in FIG. 2; FIG. 4 is a cross-sectional view of a central portion of the pressing apparatus shown in FIG. 3; FIG. 5 is a diagram of a heating apparatus according to an embodiment; FIG. 6 is a perspective view of a main housing of the heating apparatus shown in FIG. 5; and FIG. 7 is a graph showing the relationship between the compressive distortion ratio and the compressive stress plotted based on the results of a dynamic pressure test conducted on molded masses of moldable pulp materials. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A moldable pulp material and a method of forming a molded pulp product will be described in detail below with reference to the drawings. The method of forming a molded pulp product will be described below as a method of forming a shock absorbing material as a molded pulp product. First, the moldable pulp material will be described below. Waste newspaper is defibrated for 10 minutes by a defibrator (HFC-23 manufactured by Orient Co. Ltd.) and then passed through a filter having a mesh size of 2.5 mm, thereby producing a mass of defibrated pulp. The mass of defibrated pulp mainly comprises a mixture of small pieces of paper each having a size of about 2 by 2 mm and a fibrous component. The defibration of the waste newspaper is stopped before all the waste newspaper is defibrated into the fibrous component, more specifically, when the proportion of the fibrous component becomes greater than the proportion of the small pieces of paper. Since the mass of defibrated pulp comprises a mixture of small pieces of paper and a fibrous component, the mass of defibrated pulp can contain more water than if it were composed of only the fibrous component. To the mass of defibrated pulp, there were added 50 weight % of water, 3 weight % of foamed thermally expandable hollow particles (f-80D manufactured by Matsumoto Yushi Seiyaku Co., Ltd.), and 10 weight % of modified starch (EAT-4, the degree of acetylization: 3 % ∼ 4 %, the gelatinization temperature: 57°C ∼ 63°C, manufactured by Honen Co., Ltd.). The mixture is kneaded for 5 minutes by a planetary mixer, producing a moldable pulp material. Because the waste newspaper contains about 9 % of water in an air-dried state, the overall water content of the moldable pulp material is a little bit over about 36 % by weight. FIG. 1 shows an SEM photographic representation of a moldable pulp material 1 thus produced. As shown in FIG. 1, the moldable pulp material 1 comprises defibrated pulp 2, modified starch 3, and thermally expandable hollow particles 4. The hollow particles serve as a cushioning agent in a shock absorbing material which is formed of a moldable pulp material by a molding apparatus (described later on). Specifically, when the shock absorbing material is pressed under external pressure, the hollow particles are deformed to perform a shock absorbing function. The hollow particles are produced as follows: Particles known as thermally expandable microcapsules each have an outer shell of polymer and an inner shell of hydrocarbon which is covered with the outer shell. When the particles are heated, the outer shell of polymer is softened, and the inner shell of hydrocarbon is gasified. Therefore, the particles are expanded to increase their volumes several tens times. The particles thus expanded are used as the hollow particles. The hollow particles should preferably have their outer shells highly elastic to absorb stresses applied thereto and be highly resistant to heat with their heat distortion temperature being of 130°C or higher. Specifically, thermally expandable microcapsules having a distribution of particle diameters ranging from 10 µm to 30 µm, which comprise an organic solvent of isobutane, pentane, petroleum ether, hexane, or the like, which has a boiling point ranging from 50°C to 100°C, encapsulated by a thermoplastic resin of vinylidene chloride, acrylonitrile, acrylic ester, methacrylic ester, or the like, are heated to 100 °C to 150°C for a short period of time to increase their diameter 4 to 5 times and their volume 50 to 100 times, thus producing elastically deformable hollow particles having a true specific gravity ranging from 0.015 to 0.025, a distribution of particle diameters ranging from 40 µm to 200 µm, an average diameter of up to 100 µm, and a pressure resistance of 300 Kg/cm 2 or higher. The thermally expandable microcapsules may be expanded into the hollow particles before or after they are mixed with the defibrated pulp. In an embodiment and experimental examples described below, hollow particles that have already been expanded are mixed with the defibrated pulp. From the standpoint of the cost, however, it is preferable to expand the hollow particles after they, together with the modified starch and water, have been mixed with the defibrated pulp. For expanding the hollow particles after having been mixed with the defibrated pulp, such conditions should be maintained as to allow the hollow particles to be expanded sufficiently. The amount of hollow particles to be mixed with the defibrated pulp may be determined appropriately depending on the type of pulp that is used. For example, since the cost increases with the proportion of mixed hollow particles, the proportion of hollow particles to be mixed is determined in relation to the modified starch which serves as a binder. Specifically, if the proportion of hollow particles to be mixed is reduced and the amount of modified starch to be added is increased as much, then the bonding strength between the defibrated pulp and the hollow particles is increased, and the cost is lowered. The modified starch which serves as a binder is produced by modifying a portion of the hydroxyl group in molecules of starch. Specifically, a portion of the hydroxyl group in molecules of starch may be replaced with an acetyl group or an ester group as indicated by the chemical formulas given below. Starch (a portion of the hydroxyl group is replaced): Acetylized modified starch (produced by replacing a portion of the hydroxyl group with an acetyl group X: -COCH 3 ): Esterified modified starch (produced by replacing a portion of the hydroxyl group with an ester group X': -COR'): The modified starch should preferably have a low gelatinization temperature ranging from about 55°C to 65°C. If the added amount of modified starch were too large, then a shock absorbing material molded of the moldable pulp material would be unduly hard, and its shock absorbing capability would be unduly low. If the added amount of modified starch were too small, then the mechanical strength of a shock absorbing material molded of the moldable pulp material would be unduly low. Therefore, the upper limit of the range for the added amount of modified starch is determined depending on the shock absorbing capability and the cost which are desirable, and the lower limit of the range for the added amount of modified starch is determined depending on the desirable mechanical strength of the shock absorbing material. The moldable pulp material may be produced from other waste paper than waste newspaper. For example, waste corrugated cardboard may be shredded into narrow strips or milled into fibers, and a mass of defibrated pulp may be produced from those narrow strips or fibers. A mold assembly for molding a product of the moldable pulp material, a pressing apparatus for compressing the moldable pulp material filled in the mold assembly, and a heating apparatus will be described below with reference to FIGS. 2 through 4. As shown in FIGS. 2 through 4, a mold assembly 11 comprises a lower mold 12 made of an aluminum alloy and having an upper surface as a main molding surface which has a predetermined three-dimensional shape, an upper mold 13 made of an aluminum alloy and comprising a plate having a flat pressing surface, and a boxed-shaped frame 14 having upper and lower openings. The lower and upper molds 12, 13 are fitted in the frame 14. The lower and upper molds 12, 13 and the frame 14 have a number of vent holes 12a, 13a, 14a defined respectively therein at a spacing or pitch of about 15 mm, each of the vent holes 12a, 13a, 14a having a diameter of 10 mm. The upper mold 13 has a pair of vertical attachment guide bars 15 mounted on an upper surface thereof at respective opposite ends thereof. The attachment guide bars 15 have respective vertical guide slots 15a defined therein. The upper mold 13 is fastened to the frame 14 by screws 16 with handles, which are inserted through the respective vertical guide slots 15a and threaded into respective end walls of the frame 14. The frame 14 has a flange 14b extending along an upper edge thereof, and a pair of grips 17 mounted on the flange 14b above respective side walls of the frame 14. As shown in FIG. 3, a pressing apparatus 21 has an air cylinder 23 mounted on an upper support base 22a of a machine frame 22, and a mold assembly holder 24 mounted on a lower support base 22b of the machine frame 22. The air cylinder 23 has a cylinder rod 23a extending downwardly and supporting on its lower end a presser disk 25 which can be inserted downwardly into the frame 14 when the cylinder rod 23a is lowered. The mold assembly holder 24 includes a pair of confronting mold assembly clamps 26 for clamping the mold assembly 11 therebetween. A heating apparatus according to an embodiment , for forming a molded pulp product, i.e., a shock absorbing material, of the moldable pulp material described above, will be described below with reference to FIGS. 5 through 7. According to this embodiment, the heating apparatus heats the molded mass of the moldable pulp material with hot air. The molded mass of the moldable pulp material which is to be heated with hot air by the heating apparatus according to the embodiment is produced by the mold assembly 11 shown in FIG. 2 and the pressing apparatus 21 shown in FIGS. 3 and 4. The pulp mold material to be heated with hot air by the heating apparatus according to the embodiment contains more water than a pulp mold material to be heated with steam. Specifically, the pulp mold material to be heated with hot air by the heating apparatus according to the embodiment contains 80 weight %, for example, of water with respect to the mass of defibrated pulp. In the embodiment, the pulp mold material may contain 30 to 80 weight % of water. The lower limit of the range for the water content, i.e., 20 weight %, represents an amount required to knead the mixture of the small pieces of paper and the fibrous component easily with the hollow particles and the modified starch, and to allow the modified starch to be gelatinized with heat from hot air. The upper limit of the range for the water content, i.e., 80 weight %, is determined by the period of time for which the molded mass is dried and the ease with which the molded mass is separated from the mold assembly. In the embodiment, the upper and lower limits of the range for the water content have taken into account an amount of water which will be lost when heated with hot air. As shown in FIGS. 5 and 6, a heating apparatus 131 according to the embodiment comprises a main housing 132 having a mold assembly storage chamber 133 defined therein, an upper discharge chamber 134 defined above the main housing 132, and a lower pressure regulating chamber 135 (see FIG. 5) defined below the main housing 132. The main housing 132 is supported on a machine frame 136. As shown in FIG. 6, the main housing 132 has a hinged front wall 133a which can be opened and closed to open and close the mold assembly storage chamber 133. The discharge chamber 133 has a frustopyramidal shape, and is hinged so as to be opened about a rear edge thereof with respect to the mold assembly storage chamber 133. The mold assembly storage chamber 133 includes a lower space 137 defined below a mold assembly attachment plate 137a for receiving the mold assembly 11 therein, the lower space 137 communicating with the lower pressure regulating chamber 135. The mold assembly attachment plate 137a has a central opening 138 defined therein for receiving therein the frame 14 of the mold assembly 11 when the mold assembly 11 is placed in the lower space 137. When the mold assembly 11 is placed in the lower space 137 with the frame 14 received in the opening 138, the flange 14b is fastened to the mold assembly attachment plate 137a by a plurality of toggle clamps 139 which are mounted on the mold assembly attachment plate 137a around the opening 138. As shown in FIG. 5, the lower pressure regulating chamber 135 has an inverted frustopyramidal shape, and houses a water droplet stop net 140 kept taut horizontally. The main housing 132 of the heating apparatus 131 is associated with a piping system as described below. The discharge chamber 134 has an upper wall 134a fitted with an outlet port 141. The pressure regulating chamber 135 has a lower wall 135a fitted with first and second inlet ports 142, 143 combined with respective valves and a water outlet port 144 combined with a valve. The piping system includes a discharging pipe 145 connected to the outlet port 141, a first pipe 146 connected to the first inlet port 142 for supplying hot air, a second pipe 147 connected to the second inlet port 143 for supplying air under pressure at normal temperature, and a drain pipe 144a connected to the water outlet port 144. The discharging pipe 145 is connected to a discharge blower 148. The first pipe 146 is connected through a shutoff valve 149 to a hot air blower 150. The second pipe 147 is coupled to a compressor 151. The drain pipe 144a is connected to a drain. A process of molding a shock absorbing material of the moldable pulp material using the mold assembly 11, the pressing apparatus 21, and the heating apparatus 131 will be described below. First, the lower mold 12 is fitted into the lower open end of the frame 14, and an air- and water-permeable sheet 18 such as of nonwoven cloth is laid on the upper surface of the lower mold 12 and inner wall surfaces of the frame 14. Then, a predetermined amount of the moldable pulp material 1 is charged into the frame 14 and forcibly filled so as to fully cover the three-dimensional upper surface of the lower mold 12. After the moldable pulp material 1 is charged, the sheet 18 is also placed over the upper surface of the moldable pulp material 1, and then the upper mold 13 is inserted into the frame 14 through its upper open end and placed on the moldable pulp material 1 with the sheet 18 interposed therebetween, as shown in FIG. 4. Thereafter, the screws 16 are inserted through the guide slots 15a in the attachment guide bars 15 and threaded into the end walls of the frame 14. At this time, the screws 16 are not fully tightened so that the upper mold 13 can slide vertically with respect to the frame 14. The air- and water-permeable sheet 18 such as of nonwoven cloth is laid on the upper surface of the lower mold 12, the inner wall surfaces of the frame 14, and the lower surface of the upper mold 13. However, the air- and water-permeable sheet 18 may be dispensed with if the vent holes 12a, 13a, 14a defined in the lower and upper molds 12, 13 and the frame 14 are sufficiently small in diameter or a lubricating layer is disposed on the upper surface of the lower mold 12, the inner wall surfaces of the frame 14, and the lower surface of the upper mold 13. The mold assembly 11 which has thus been filled with the moldable pulp material 1 is then placed in the mold assembly holder 24 of the pressing apparatus 21, and set in position in the pressing apparatus 21 by the mold assembly clamps 26 which sandwich the mold assembly 11 in engagement with lower opposite sides of the mold assembly 11. Thereafter, the ,air cylinder 23 is actuated to lower the cylinder rod 23a, pressing the presser disk 25 downwardly into the frame 14 against the upper surface of the upper mold 13. The air cylinder 23 is continuously operated to cause the presser disk 25 to press the upper mold 13 for thereby compressing the moldable pulp material 1 in the mold assembly 11 under a predetermined pressure. The moldable pulp material 1 is now adjusted in density such that its density will be about 0.125 g/cm 3 (= 125 Kg/m 3 ) after it is dried, and has its lower portion shaped complementarily to the three-dimensional shape of the upper surface of the lower mold 12. When the moldable pulp material 1 is thus compressed, air trapped in the moldable pulp material 1 is removed, and the bonding force of the moldable pulp material 1 is increased. Then, the screws 16 are fully tightened to fasten the upper mold 13 to the frame 14 through the attachment guide bars 15. Thereafter, the cylinder rod 23a is lifted back, and the mold assembly 11 is removed from the mold assembly holder 24 and transferred into the heating apparatus 131. The molded mass of the moldable pulp material 1 in the mold assembly 11 is dried with heat into a shock absorbing material by the heating apparatus 131. Operation of the heating apparatus 131 will be described below. The mold assembly 11 taken out of the pressing apparatus 21 is placed in the mold assembly storage chamber 133 of the heating apparatus 131. Specifically, the front wall 133a is opened, and the upper discharge chamber 134 is lifted open, exposing the mold assembly attachment plate 137a. The mold assembly 11 is fitted in the central opening 138 defined in the mold assembly attachment plate 137a, with the flange 14b placed on the mold assembly attachment plate 137a. The flange 14b is then fastened to the mold assembly attachment plate 137a by the toggle clamps 139. After the mold assembly 11 is fixedly positioned in the heating apparatus 131, the front wall 133a and the upper discharge chamber 134 are closed, sealing the mold assembly storage chamber 133. Now, the shutoff valve 149 is opened, and the hot air blower 150 is operated to generate hot air. At this time, the valve of the water outlet port 144 is closed. The generated hot air, which is set to a temperature ranging from 100°C to 110°C, as converted to heat, under a 1 ∼ 1.5 atmospheric pressure, is delivered through the first pipe 146, and introduced from the first inlet port 142, opened by its valve, into the pressure regulating chamber 135. In the pressure regulating chamber 135, the hot air is uniformized in pressure. Then, the hot air is supplied to the lower space 137 in the mold assembly storage chamber 133. The hot air is supplied from the lower space 137 to the molded mass of the moldable pulp material 1 in the mold assembly 11. The supplied hot air flows into the mold assembly 11 through the vent holes 12a, 14a of the lower mold 12 and the frame 14, passes through and heats the molded mass of the moldable pulp material 1 in the mold assembly 11, and flows from the vent holes 13a of the upper mold 13 into the upper discharge chamber 134 above the mold assembly storage chamber 133. Thereafter, the hot air which now contains steam is forcibly drawn and discharged from the upper discharge chamber 134 through the outlet port 141 and the discharging pipe 145 by the discharge blower 148. The hot air supplied to and passing through the molded mass of the moldable pulp material 1 is forcibly drawn and discharged under a negative pressure, e.g., a 0.5 atmospheric pressure, by the discharge blower 148, while heating the molded mass. The hot air is supplied and forcibly discharged continuously for a given period of time, e.g., of several tens seconds. Then, while the discharge blower 148 is in operation, the first inlet port 142 is closed by its valve, and the second inlet port 143 is opened by its valve. A gas having a temperature lower than the temperature at which the modified starch is gelatinized, e.g., dry air at normal temperature under a predetermined pressure, e.g., a 1.4 atmospheric pressure, is supplied from the compressor 151 through the second pipe 147 to the pressure regulating chamber 135 continuously for several tens seconds. After the dry air is supplied and forcibly discharged, the mold assembly storage chamber 133 is opened as described above and the mold assembly 11 is removed from the heating apparatus 131. Immediately thereafter, the molded mass of the moldable pulp material 1 is removed from the mold assembly 11. As described above, the molded mass of the moldable pulp material 1 is heated with the hot air while in the main housing 132 of the heating apparatus 131, during which time the modified starch added as a binder to the moldable pulp material 1 is gelatinized. When water is evaporated from the gelatinized modified starch, the bonding strength of the gelatinized modified starch becomes strong enough to allow the molded mass to be removed in one piece from the mold assembly 11. Specific experimental examples in which a moldable pulp material was molded into a shock absorbing material by the above steam-heating process according to the second embodiment will be described below. Experimental example 1: A moldable pulp material which was prepared from the above constituents at the above proportions was filled in the mold assembly, and adjusted to a specific gravity of 0.125. The mold assembly was installed in the heating apparatus, and heated with hot air at 100°C to 110°C under a 1 ∼ 1.4 atmospheric pressure, supplied from below the mold assembly. The hot air passed through the molded mass of the moldable pulp material in about 1 minute after the hot air started to be supplied to the heating apparatus, and then the hot air which contained steam was discharged from the outlet port. After the hot air was further supplied for 30 seconds, the valve of the first inlet port was closed, and the mold assembly was removed from the heating apparatus. While the molded mass was being heated with the hot air for 1 minute and 30 seconds, the modified starch added as a binder was gelatinized. After the molded mass was heated with the hot air, the gelatinization of the modified starch progressed due to the heat which had been imparted by the hot air, and water was evaporated from the gelatinized modified starch, for about 1 minute while the molded mass was being retained in the mold assembly prior to removal therefrom. It was therefore confirmed that the molded mass developed enough bonding forces to remove itself from the mold assembly. The forced discharge of the hot air is effective to shorten the period of time required to dry the molded mass. In this experimental example, when the molded mass was heated with the hot air for less than 20 seconds, the entire molded mass was not sufficiently gelatinized, and the mechanical strength of the dried product was insufficient. The period of time for which the molded mass is to be heated with the hot air depends on the thickness of the molded mass and the structure of the mold assembly, and the modified starch starts being gelatinized when supplied with water and heated to a temperature higher than the gelatinization temperature thereof. It is necessary in the experimental example 1 to heat the molded mass with the hot air for at least about 20 seconds after the entire molded mass is filled with the steam and hold the molded mass in the mold assembly for about 30 seconds, if no forced discharge of hot air is employed. The molded mass removed from the mold assembly was then heated at 60°C for 4 hours by an air blower drier, so that it was dried enough to be practically usable as a shock absorbing material. Experimental example 2: The mold assembly filled with the above moldable pulp material which was adjusted in density was installed in the heating apparatus, and heated with hot air at 100°C to 110°C supplied from below the mold assembly and forcibly discharged under a negative pressure of about a 0.5 atmospheric pressure from above the mold assembly. The hot air passed through the molded mass of the moldable pulp material in about 20 seconds after the steam started to be supplied to the heating apparatus, and the hot air which contained steam was discharged from the discharge blower connected to the outlet port of the heating apparatus. Thereafter, the hot air was supplied and forcibly discharged for 20 seconds. Then, while the discharge blower was in operation, dry cold air was supplied under about a 1.4 atmospheric pressure to the molded mass for 30 seconds. Immediately after the mold assembly was taken from the heating apparatus, the molded mass could be removed from the mold assembly. The bonding strength of the modified starch contained in the molded mass removed from the mold assembly proved large enough to allow the mass removed in one piece from the mold assembly as with the experimental example 1., Since the amount of water contained in the heated molded mass was smaller than with the experimental example 1, it was found that the period of time required for the hot air to enter the molded mass in its entirety was reduced to about half (from about 1 minute to about 20 seconds) and the period of time for which the molded mass was heated with the hot air was reduced to about half (from 30 seconds to 20 seconds) by forcibly drawing and discharging the hot air. After the molded mass was heated with the hot air, the dry cold air which was supplied and forcibly discharged promoted evaporation of water from the molded mass, resulting in a reduction of the time (from 1 minute to 30 seconds) required before removing the molded mass from the mold assembly. Inasmuch as the dry cold air which was supplied to the molded mass was effective to cool the molded mass, the molded mass removed from the mold assembly was easily handled. It was confirmed that, in both the experimental examples 1, 2, it was possible to form a shock absorbing pulp molding in a molding cycle time ranging from about 1 minute and 10 seconds to 2 minutes and 30 seconds, which was less than the molding cycle time of about 3 minutes for molding foamed styrol. Evaluation of shock absorbing capability: The molded masses or blocks produced in the experimental examples 1 and 2 were tested in a dynamic compression test, and a stress vs. distortion diagram was plotted based on the results of the dynamic compression test for the evaluation of a shock absorbing capability. There was no difference between shock absorbing capabilities of the molded masses produced in the experimental examples 1 and 2, and the compressive stress at a compressive distortion of 50 % of the molded masses produced in the experimental examples 1 and 2 had a value equivalent to the value of foamed styrol expanded 40 times in size. It was confirmed that it was possible to form a molded mass having the same shock absorbing capability, i.e., the shock absorbing coefficient: Cf = compressive stress/distorting energy, as that of foamed styrol. FIG. 7 shows the relationship between the compressive distortion ratio and the compressive stress based on the results of the dynamic compression test conducted on the experimental examples 1 and 2 (dry type including 3 % of thermally expandable hollow particles) and foamed styrol expanded 40 times in size. In the experimental examples 1 and 2 described above, the defibrated pulp, the thermally expandable hollow particles, and the modified starch as a binder are uniformly mixed with an amount of water small enough to keep the materials unseparated, e.g., 30 to 80 weight %, preferably, 30 to 50 weight % of water. The mixture is then filled in the mold assembly, and hot air is supplied to one side of the mold assembly and passed through the mold assembly at a temperature higher than the gelatinization of the modified starch. When the molded mass in the mold assembly is heated with the hot air, the modified starch is gelatinized, and water is evaporated from the gelatinized modified starch with the heat of the applied hot air. As a result, the molded mass develops sufficient bonding forces. At the same time that the hot air is applied to heat the molded mass, the hot air is forcibly drawn and discharged from the opposite side of the molded mass. Thus, the period of time in which the entire molded mass in the mold assembly is heated up to a temperature higher than the gelatinization temperature of the modified starch is shortened, making it possible to reduce the period of time spent after the heating of the molded mass in the molded assembly to the removal of the molded mass from the mold assembly. After the gelatinization of the modified starch heated by the hot air is completed, the supply of the hot air is cut off, and dry cold air or nitrogen at a temperature lower than the gelatinization temperature of the modified starch is applied to one side of the molded mass to cool the molded mass. This is also effective to shorten the period of time spent after the heating of the molded mass in the molded assembly to the removal of the molded mass from the mold assembly. In this case, the dry cold air is forcibly discharged from the opposite side of the molded mass to promote the evaporation of water from the molded mass for thereby accelerating the solidification of the modified starch and increasing the bonding force of the molded mass. Consequently, the period of time spent after the heating of the molded mass in the molded assembly to the removal of the molded mass from the mold assembly can further be reduced. The molded mass thus heated and cooled is removed from the mold assembly, and then dried by an air blower drier until the water content of the molded mass reaches about 15 weight %. In the embodiment, as described above, the moldable pulp material is mainly composed of pulp made from waste paper such as waste newspaper and contains a binder of modified starch and thermally expandable hollow particles. The moldable pulp material is pressed into a molded mass and heated with hot air, producing a shock absorbing pulp molding having an excellent shock absorbing capability in a short period of time. The molded mass can be produced in a cycle time equivalent to the cycle time needed to produce foamed styrol, and exhibits a shock absorbing capability equivalent to the shock absorbing capability of foamed styrol. The moldable pulp material can be used to produce environmentally oriented shock absorbing packages as a substitute for foamed styrol packages, and hence can find useful industrial applications in view of the effective utilization of waste paper and the conservation of natural resources. In the above embodiment, the moldable pulp material is primarily composed of a mass of defibrated pulp produced from waste paper. As described above, the mass of defibrated pulp comprises a mixture of small pieces of paper and a fibrous component. The ratio between the small pieces of paper and the fibrous component depends op the period of time during which the waste paper is to be defibrated by the defibrator. If the waster paper is defibrated for too a long period of time, then the moldable pulp material will be too costly to manufacture, and the waste paper will be defibrated almost entirely or fully into a fibrous component, which cannot absorb a sufficient amount of water. The ratio between the small pieces of paper and the fibrous component should, therefore, be determined in view of the cost and the each with which the defibrated pulp can absorb water.

Claims: A method of manufacturing a molded pulp product, comprising the steps of: mixing a moldable pulp material comprising a main constituent of paper pulp, a starch binder, and thermally expandable hollow particles with water which is effective to gelatinize the starch binder, wherein said thermally expandable hollow particles comprise particles each having an outer shell of polymer and an inner shell of hydrocarbon which is covered with said outer shell; filling the moldable pulp material mixed with the water in a mold assembly and compressing the moldable pulp material in the mold assembly; and heating the compressed moldable pulp material with hot air for at least 20 seconds under a 1 to 1.5 atmospheric pressure to a temperature of 100 °C to 110 °C at which the starch binder is gelatinized, for thereby gelatinizing the starch binder to produce a molded pulp product of the moldable pulp material. A method according to claim 1, further comprising the step of drawing and forcibly discharging the hot air through said compressed moldable pulp material. A method according to claim 1 or 2, further comprising the step of supplying a gas at a temperature lower than said gelatinization temperature to said compressed moldable pulp material after the compressed moldable pulp material is heated with said hot air. A method according to anyone of claims 1 to 3, wherein said moldable pulp material contains water in an amount ranging from 30 to 80 weight %. A method according to anyone of claims 1 to 4, wherein said pulp comprises-pieces of paper having a size of millimeters and a fibrous component defibrated from waste paper, said fibrous component having a proportion greater than said pieces of paper. A method according to anyone of claims 1 to 5, wherein when said particles are heated, the outer shell of polymer is softened, and the inner shell of hydrocarbon is gasified, expanding the particles.

Inventors: IKEDA HIROMU, KAWANO MITOSHI, MAHAMA KAZUTOSHI, NOGUCHI TSUTOMU, SUZUKI SUNAO, TAN MASARU, IKEDA, HIROMU, KAWANO, MITOSHI, MAHAMA, KAZUTOSHI, NOGUCHI, TSUTOMU, SUZUKI, SUNAO, TAN, MASARU, Ikeda, Hiromu, c/o Rengo Co., Ltd., Kawano, Mitoshi, c/o Sony Corp., Mahama, Kazutoshi, c/o Sony Corp., Noguchi, Tsutomu, c/o Sony Corp., Suzuki, Sunao, c/o Rengo Co., Ltd., Tan, Masaru, c/o Sony Corp.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1996
Main classification: C08J9
IPC codes: C08J9, C08L1