JPH0129684B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for pre-foaming expandable thermoplastic resin particles, and in particular to a method for controlling the temperature distribution inside the can in a batch pre-foaming process in which the resin particles are pressurized and heated inside the can. In addition to stabilizing and achieving a uniform foaming ratio, it also prevents the occurrence of blocking.
The present invention relates to a method for producing homogeneous pre-expanded particles with high commercial value, with improved yield and less adhering moisture. (Prior Art) Conventionally, expandable thermoplastic resin particles, such as beads of expandable polystyrene or polyolefin, used for foam molding are usually pre-expanded to a magnification of several tens of times. In order to perform such pre-foaming, conventionally, a certain amount of expandable thermoplastic resin particles are loaded into a vertical cylindrical can with a built-in stirrer, and while stirring, a heating medium such as heated steam is introduced into the can. heating the resin particles to a temperature equal to or higher than their softening point under pressure equal to or higher than atmospheric pressure;
A batch process is generally used in which the foam is cooled and taken out from the can when a predetermined expansion ratio is reached. In this process, it is necessary to adjust the expansion ratio to a level of a predetermined volume of the can. For this reason, various attempts have been made to control the amount of heat medium supplied by detecting changes in the internal pressure of the can as the total volume increases due to foaming of resin particles within the can. However, adjusting the expansion ratio using such a control method does not sufficiently control the uniformity of the temperature inside the can, resulting in variations in the expansion ratio between the top and bottom of the can, and the so-called blocking phenomenon in which resin particles are fused to each other in a lump. This has also been a factor in deteriorating the quality of foam molded products obtained using the same. In order to homogenize the pre-expanded particles, an auxiliary screw is provided on the stirring shaft inside the vertical cylindrical can,
A device that promotes mixing and agitation of resin particles and eliminates non-uniformity of agitation (publication of Utility Model Publication No. 55-42908), by means of a heating medium inlet pipe arranged parallel to the agitation blades between the agitation blades. Attempt to uniformly heat the inside of a can
(No. 42789 Publication) Alternatively, the stirring shaft can be rotated within the foaming tank, and the body of the foaming tank, which is equipped with blades corresponding to the stirring blades inside, can be rotated around the stirring shaft to improve the stirring efficiency. A device (Japanese Unexamined Patent Publication No. 57-49538) has been proposed. However, since all of these proposed devices have special structures, they not only result in increased equipment and maintenance costs, but are also economically disadvantageous.
A difference in foaming expansion rate occurs between resin particles that come into contact with high-temperature steam in the vicinity of the steam nozzle and are subjected to a rapid heating effect, and resin particles that are subjected to milder heating at a location distant from the steam nozzle. is unavoidable,
In addition, blocking still occurs, and a fundamental solution to the problem has not yet been found. (Problems to be Solved by the Invention) In order to overcome the above-mentioned problems in the pre-foaming technology, the present inventor has conducted intensive research focusing on the performance of the heating medium itself, the supply method, heating efficiency, etc.
This completes the present invention. An object of the present invention is to provide thermoplastic resin particles having a uniform pre-expansion ratio throughout. Another objective is to modify the batch process conventionally applied for pre-expanding expandable thermoplastic resin particles without requiring any particularly large additional capital investment, and to economically advantageously obtain uniform pre-expanded particles. There is something to do. Still another object is to prevent the occurrence of blocking phenomenon in the pre-foaming process and to improve the yield of pre-foamed resin particles. The ultimate objective is to provide a foamed resin molded product that is homogeneous and has high commercial value. (Means for Solving the Problems) The above object is to pressurize the expandable thermoplastic resin particles housed in the can to a pressure higher than atmospheric pressure by supplying a heating medium into the can; Achieved by a method for pre-foaming expandable thermoplastic resin particles, characterized in that compressed air is mixed into the heating medium in the step of pre-foaming by heating to a temperature equal to or higher than the softening point of the resin particles. . Hereinafter, specific embodiments of the method of the present invention will be further described in detail with reference to the accompanying drawings. FIG. 1 is a schematic elevational view showing an example of a conventionally known apparatus used for pre-expanding expandable thermoplastic resin particles. FIG. 2 is a schematic elevational view showing an example of a prefoaming apparatus adapted to carry out a preferred embodiment of the method of the present invention. FIG. 3 is a chart showing the valve opening/closing procedure at each stage of the process of carrying out the present invention using the apparatus shown in FIG. 2. In FIG. 1, a vertical cylindrical can 1 constituting the main body of the pre-foaming machine is concentrically housed with a stirrer 4 consisting of a rotating shaft 3 equipped with a plurality of stirring blades 2. It is rotated by a drive device 6 connected to a power source 5 such as a motor via a machine or the like. Below the raw material hopper 7 provided above the can body 1 is a receiver that measures the expandable thermoplastic resin particles discharged from the raw material hopper 7 and supplies a fixed amount of the particles into the can body 1 through a supply valve 8. 9 are set in succession. A steam chamber 10 is provided at the bottom of the can body 1, partitioned by a perforated disc 11. The perforated disc 11 has small holes or slits, etc., of a size that allows the passage of gas but prevents the passage of resin particles. It is equipped with At the top of the can body 1, there is an exhaust port 15 having an exhaust valve 12, a trip valve 13 for pressure regulation, and an emergency valve 14, and at the upper side of the can, there is a level of resin particles whose volume has increased due to preliminary foaming. A level meter 16 for detecting the temperature, a pre-expanded grain outlet 18 equipped with a reciprocating opening/closing mechanism 17 such as a plunger at the lower side, and a steam supply port 18 in the bottom steam chamber 10 of the can body 1. Piping 19, compressed air supply piping 20, and drain discharge piping 21 are provided, respectively. These pipes each include a steam solenoid valve 22, an air solenoid valve 23, and a drain solenoid valve 24, and are opened and closed manually or automatically according to the detected pressure inside the can. In the conventionally known method of performing pre-foaming using such a device, first, a fixed amount of foamable resin particles is measured using a raw material hopper 7, and the foamable resin particles are dropped into a funnel-shaped receiver 9, and then a supply valve 8 is used.
The liquid is then supplied to the can body 1. At this time, the exhaust valve 12 and the drain solenoid valve 24 are opened, and the can body 1 is kept open to the outside air. The supply amount of resin particles is
The can body volume is calculated from the height at which the upper surface of the pre-expanded resin particle layer reaches the level meter 16, and a predetermined pre-expansion ratio is set in advance based on actual measurement and correction. After finishing the supply and charging of the raw material resin particles, temporary heating is performed while the stirrer 4 is operated. Primary heating is performed to preheat to a temperature just before reaching the resin softening temperature, thereby relaxing the temperature gradient in the next secondary heating stage and avoiding as much as possible uneven expansion ratio due to localized heating. The steam electromagnetic valve 22 is opened with the exhaust valve 12 open, and a heating medium such as heated steam is passed into the can. For expandable polystyrene beads,
It is generally considered that treatment for 30 seconds to 1 minute at a temperature of about 75 to 80°C under atmospheric pressure is sufficient. The resin particles then proceed to a secondary heating stage. The heating at this stage is performed by closing the exhaust valve 12 to create a closed system, and pressurizing the heating medium through the steam supply pipe 19 to heat the resin particles to their plasticization temperature, that is, a temperature higher than their softening point. In order to control rapid expansion effects, the process is carried out under a suitable pressure above atmospheric pressure. For example, in the case of polystyrene beads, the temperature is 90-100℃, preferably 95-98℃, and the pressure inside the can is
0.2~0.8Kg/ ^cm2G (gauge pressure), preferably 0.2~
The treatment is normally carried out for 40 seconds to 80 seconds under the condition of 0.5 kg/cm ² G. At this time, the pressure inside the can is regulated by the operation of the trip valve 13, and the heating medium is The supply amount is automatically controlled, and the temperature is usually adjusted while maintaining the pressure. The pre-expanded resin particles that have undergone the above secondary heating are
Next, it is subjected to a cooling process. At this stage, first, the steam solenoid valve 22 is closed to stop the supply of heating medium, the exhaust valve 12 is opened to reduce the pressure inside the can to atmospheric pressure, and the air solenoid valve 23 is opened to supply compressed air. Pass. At this step, the preliminary foaming is completely stopped, and when heated steam was used as a heat medium in the preceding step, the condensed water adhering to the resin particles is vaporized and removed by residual heat and dried. The cooled and dried resin particles are discharged from the pre-expanded grain discharge port 18 which is opened by the action of the opening/closing mechanism 17. The above is an overview of the conventionally known pre-foaming process, in which careful attention is paid to uniform temperature distribution, such as by continuing stirring throughout the period from primary heating to discharge, and heating in two stages. In addition, as mentioned above, despite numerous improvement efforts such as improving stirring efficiency and distributing heat medium supply pipes, asymmetry in pre-expansion ratio is still observed. ing. However, according to the method of the present invention, compressed air is mixed with the heating medium and supplied into the can in the heating stage of the conventional method, especially in the secondary heating stage, thereby making the temperature distribution inside the can uniform and foaming. It is truly surprising that we were able to achieve uniform magnification all at once. That is, in the heating step of the conventional method, the method of the present invention simultaneously carries out the feeding of the heating medium from the steam supply pipe 19 and the pressurization of compressed air from the compressed air supply pipe 20. The heating medium and compressed air are mixed in the steam chamber 10, passed through the slits in the perforated disk 11, and supplied into the can body 1 to heat the resin particles. At this time, the original pressure of the compressed air is 4 to 6 kg/cm ² G, and the blowing pressure is adjusted to 0.1 to 0.5 kg/cm ² G by a needle valve. Table 1 compares the effects of the conventional method and the method of the present invention in pre-foaming polystyrene. However, in this table, the blowing pressure of steam and air are both
It is 0.5Kg/cm ² G. In addition, the temperature inside the can is not the initial state, but the results obtained when the initial 5 batches were discarded and the results were repeated after 6 batches are shown.

[Table] As shown in the table above, the method of the present invention not only eliminates most of the variations in can temperature and expansion ratio difference caused by the conventional method, but also has the remarkable effect of completely preventing the occurrence of blocking phenomena. was recognized. There is no doubt that such remarkable effects are due to the excellent action of the method of the present invention, but the physicochemical function when compressed air is mixed with the heating medium is not known in detail, and it is likely that This is due to the increase in fluidity of the heating medium and the change in thermodynamic properties, which increases the propagation speed of thermal energy, homogenizes the temperature distribution inside the can, and provides stable and slow heating throughout the can. it is conceivable that. FIG. 2 shows a conventionally known pre-foaming device modified to provide a more preferred embodiment for applying the method of the present invention. In the figure, a compressed air pipe 25 is branched and connected to an appropriate location on the outlet side of the steam solenoid valve 22 on the steam supply pipe 19, and the amount of compression controlled by the compressed air solenoid valve 26 is compressed from the compressed air pipe 25. Air is arranged to be mixed into the steam supply pipe 19 under pressure, for example through a needle valve. 2
7, 28, and 29 are pressure gauges. Using such a device, the compressed air is mixed with the heating medium prior to supplying the heating medium into the can, and is more evenly mixed than when mixing in the steam chamber 10. Complete mixing is achieved, making it easier to stabilize and homogenize the temperature distribution. Therefore, in such a preferred embodiment of the method of the present invention, if compressed air is mixed with the heating medium only in the secondary heating stage, the purpose can be sufficiently achieved and the process can be simplified. The valve opening/closing procedure at each stage of the process of carrying out the method of the present invention using the apparatus shown in FIG. 2 will be explained with reference to FIG. The material feeding stage and the primary heating stage are similar to the above explanation regarding the known method. Regarding primary heating, compressed air can be mixed into the heating medium by operating the compressed air solenoid valve 26, but it may be omitted as in the conventional method. In the secondary heating stage, the compressed air solenoid valve 26
When the compressed air is opened and mixed with the heating medium in the steam supply pipe 18, the uniformly mixed heating medium is supplied under pressure to the resin particle layer through the steam chamber 10. In the cooling stage, the conventional method of opening the exhaust valve 12 and the air solenoid valve 23 was shown, but the compressed air solenoid valve 2
6 can be opened to promote cooling and drying action,
Further, when only the compressed air pipe 25 is provided without adding the compressed air supply pipe 20, this process step is performed by operating the compressed air solenoid valve 26. The discharge stage of the pre-expanded particles is no different from the conventional method. (Example) Next, the method of the present invention will be described with reference to Examples. Example 1 Using expandable polystyrene beads (manufactured by Kanebuchi Kagaku Co., Ltd., trade name: Kanepal GM), pre-foaming was carried out according to the procedure shown in Fig. 3 using a pre-foaming device with a can capacity of 1600 as shown in Fig. 2. Ta. The process conditions were as follows, and compressed air was mixed with heated steam for both primary heating and secondary heating. Raw material bead input amount: 1700Kg Steam blowing pressure: 0.5Kg/cm ² G Air source pressure: 5.0Kg/cm ² G Air blowing pressure: 0.2Kg/cm ² Primary heating time: 48 seconds Primary heating setting temperature: 78℃ Expansion ratio Setting value: 64 times Comparison example 1 For comparison, the steam pressure is set to 075Kg/cm ² G,
Pre-foaming was carried out under the same conditions as in Example 1, except that compressed air was not mixed. Example 2 Pre-foaming was carried out using expandable polystyrene beads (manufactured by Kanebuchi Kagaku Co., Ltd., trade name: Kanepaul GM) using the pre-foaming device with a can capacity of 1600 as shown in Fig. 2 according to the procedure shown in Fig. 3. . The process conditions were as follows, and compressed air was mixed with heated steam only in the secondary heating stage. Raw material bead input amount: 17.80Kg Blowing steam pressure: 0.5Kg/cm ² G Blowing air pressure (secondary heating only): 0.2Kg/cm ² G Primary heating time: 40 seconds Primary heating setting temperature: 80℃ Expansion ratio setting value 60x Comparative Example 2 Pre-foaming was carried out under the same conditions as in Example 2, except that compressed air was not mixed in the secondary heating stage. Each of the above Examples and Comparative Examples was carried out consecutively, the first 5 batches were discarded, and the conditions after the 6th batch were observed in 4 batches. The results obtained are shown in Table 2.

[Table] As is clear from Table 2 above, according to the method of the present invention, the temperature inside the can is significantly more uniform than in the known comparative example, and the expansion ratio of the resin particles is also lower than that of the bulk upper layer. It can be seen that the uniformity is achieved to the extent that there is almost no difference between the layer and the lower layer, and that an extremely excellent effect is produced in that no blocking is observed at all. Furthermore, at the end of the primary heating, although the temperature level is approximately the same as that of the secondary heating,
The time required to reach the foaming ratio set value by secondary heating, that is, the time required for secondary heating, is longer in the method of the present invention, which means that the heating action progresses slowly, causing a rapid local temperature rise. This leads us to infer that there is a reason why this is not the case. Comparative Example 3 Next, in the secondary heating stage, the upper exhaust valve 12 was opened using the device shown in FIG. 2, the inside of the can was kept at atmospheric pressure, and the compressed air was converted to heated steam under the same conditions as in Example 2 above. The mixture was mixed and prefoamed. As a result, the pressure in the tank of Example 2 was 0.2Kg/
cm ² G, there was almost no difference in the temperature inside the can, there was no heat loss, the thermal energy was propagated into the can on average, and the foaming ratio difference between the upper and lower layers was about 0.8 times on average. In the above comparative example, heat was dissipated with the exhaust gas, increasing heat consumption, and there was a maximum temperature difference of 1° between the upper and lower layers, and there was an average difference of about 1.8 times in the foaming ratio between the upper and lower layers. Further, in the case of the example, most of the water adhering to the resin particles was removed, but in the above example, a small amount of adhering water still remained. (Effects of the Invention) As described above, according to the method of the present invention, in the batch pre-foaming of expandable thermoplastic resin particles, it is possible to stabilize and uniformize the temperature distribution inside the can, which was previously considered to be extremely difficult to solve, and to achieve this. The equalization of the pre-expansion ratio,
By using existing equipment or making minimal modifications to it, it can be realized industrially easily and economically, improving the quality of pre-expanded resin particles and, by extension, foaming molding using them. It greatly contributes to improving the accuracy and quality of products. In addition, the present invention successfully prevents the blocking phenomenon that has often been a hindrance in the pre-foaming process and foam molding process, resulting in an unexpected side effect of improving the yield. His contributions to this field are significant.

[Brief explanation of drawings]

FIG. 1 is a schematic elevational view showing an example of a conventionally known apparatus used for pre-foaming expandable thermoplastic resin particles, and FIG. FIG. 2 is an elevational view schematically showing an example of the device. FIG. 3 is a chart showing the valve opening/closing means at each stage of the process of carrying out the present invention using the apparatus shown in FIG. 1... Can body, 2... Stirring blade, 3... Rotating shaft, 4
... Stirrer, 5 ... Power source, 6 ... Drive device, 7
... Raw material hopper, 8 ... Supply valve, 9 ... Receiver, 1
0... Steam chamber, 11... Perforated plate, 12... Exhaust valve, 13... Trip valve, 14... Emergency valve, 15
...Exhaust port, 16...Level meter, 17...Opening/closing mechanism, 18...Pre-expanded grain discharge port, 19...Steam supply piping, 20...Compressed air supply piping, 21...Drain discharge piping, 22 ...Steam solenoid valve, 23...
Air solenoid valve, 24...Drain solenoid valve, 25...Compressed air pipe, 26...Compressed air solenoid valve, 27...Steam pressure gauge, 28...Air pressure gauge, 29...Inner pressure gauge.

Claims (1)

[Claims] 1. The expandable thermoplastic resin particles housed in the can are pressurized to a pressure higher than atmospheric pressure by supplying a heating medium into the can, and at the same time A method for pre-foaming expandable thermoplastic resin particles, characterized in that compressed air is mixed into the heating medium in the step of pre-foaming by heating to a temperature. 2. The method for pre-foaming expandable thermoplastic resin particles according to claim 1, wherein the heating medium is mixed with compressed air prior to supplying the heating medium into the can body. 3. A method for pre-foaming expandable thermoplastic resin particles according to claim 1 or 2, which comprises adjusting the flow rate of compressed air.