JPS5932300B2 - Method for manufacturing molded foam - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for economically producing a molded foamed article by heat-molding expanded ethylene resin particles in a mold.

In recent years, many studies have been conducted on methods for obtaining molded foamed articles using expanded ethylene resin particles.

However, the already established foam molding technology for styrene resin molds cannot be directly applied to ethylene resin foam molding, and furthermore, the existing foam molding technology for ethylene resin molds does not meet market requirements. No technology worthy of commercialization has been developed, either because high-quality molded bodies cannot be obtained or because it is too uneconomical. For example, Japanese Patent Publication No. 48-34591 and Japanese Patent Publication No. 49-8
In the ethylene resin mold foaming technology disclosed in No. 5158, in order to increase the internal pressure of foamed particles used during molding,
It is necessary to use an expensive blowing agent or to carry out gas filling at high pressure for a long period of time using a large amount of pressure-resistant equipment, and in both cases, the increased internal pressure of the foamed particles remains for a very short time under normal conditions. It has the disadvantage that economical molding is not possible because it can only last for a long time.

Furthermore, as another technique, a method is known in which ethylene-based foamed particles are compressed by gas pressure and then foam-molded in a mold.

For example, in U.S. Pat.
%), in JP-A-49-9574, the foamed particles are reduced to 80% or less of the original bulk volume (bulk volume compression ratio of 20% or more),
All of them disclose a method of compressing, heating and foaming to form a molded product.

The present inventors tried the above two methods.

The results are summarized in Table 1. This Table 1 clarifies the difference between the particle compression ratio of the expanded particles used in the present invention and the above-mentioned bulk volume compression ratio, and also shows the difference between the compression ratio (or bulk volume compression ratio) of the expanded particles during molding and the gain. This clarifies the relationship between the quality of the molded product and the quality of the molded product. The expansion ratio in the present invention is the value obtained by dividing the true volume of the foam (foamed particles, foamed molded product) by the weight of the foam (Cc/9) measured by the submersion method.
).

The particle compression ratio used in the present invention is defined by the following formula, where the true volume of the original expanded particles is V1 and the true volume of the expanded particles after compression is V2.

The bulk compression ratio is defined by the following equation, where t is the bulk volume per unit weight of the original foamed particles, and V is the bulk volume per unit weight of the foamed particles after compression.

Here, in general, the bulk volume V per unit weight is as follows, where w is the weight of foamed particles filled into a mold by a pneumatic filling device commonly used for molds having a volume of V.

The bulk volume per unit weight of the compressed particles is determined by filling the mold while maintaining the transport air pressure for the filling device and the internal pressure of the mold at predetermined pressures, and measuring the weight of the filled compressed particles. The first problem in Table 1 is the relationship between the bulk compression ratio and the final molded product.
In the scope of the invention of No. 04068, the bulk compression ratio is 10 to 6.
Although it is stated that a range of 0% can be molded, the first
The table shows that good molded products were obtained only when the bulk compression ratio was 25 or higher.

However, if we objectively analyze this contradiction, we find that the US Patent No. 35
The specification of No. 04068 states that the optimum condition for bulk volume compression ratio is 40 to 50% (60 to 50% of the original bulk volume), and JP-A-49-9574 states that
Describes an example with a bulk compression ratio of 28 (f) and 62%,
Considering the fact that the claim is limited to 20% or more (bulk volume compression ratio), U.S. Patent No. 3504068
It can be seen that the difference between the description in No. 1 and the supplementary test results in Table 1 is due to the fact that the evaluation scale for molded products is stricter in the present invention. Next, when the results in Table 1 are comprehensively examined using the evaluation scale of the present invention, it is found that
The disadvantage of the techniques disclosed in No. 504068 and Japanese Patent Application Laid-open Nos. 49-9574 is that the final molded article obtained has a lower expansion ratio than that of the initial foamed particles.

In order to maintain the compression ratio of 2 bulk volume until immediately before molding, a pressure of at least 2.0k9/d gauge is required, but this pressure exceeds the pressure limit of a normal mold, and the equipment is not suitable. becomes uneconomical.

On the other hand, if the compression is carried out under a pressure of less than 2.0 kg/d, the shrinkage deformation of the resulting molded product will be large, or at least the fusion between the particles will be insufficient, resulting in a molded product with no commercial value.

etc. will be understood.

The present invention has been made in view of the current situation, and is particularly based on U.S. Pat.
The in-mold molding technology was completed using foamed particles with a low compression rate, which was not covered by the technology No. 4, and it became possible to economically and easily produce high-quality foamed molded products.

That is, the present invention provides a method in which foamed ethylene resin particles are filled into a mold and heated and molded, using gas pressure to compress the foamed particles to a particle compression rate of 30% or less (excluding O), The molded product is filled into a mold in a compressed state and heated and molded, cooled until the surface temperature of the molded product becomes at least the softening temperature of the base resin or lower, and then taken out.Then, the molded product is heated to a temperature of 60°C or lower than the softening temperature of the base resin. It is an object of the present invention to provide a method for producing an ethylene resin mold foam, which is characterized in that the foam is placed in an atmosphere at the above temperature, and then slowly cooled to room temperature over a period of 3 hours or more.

The contents of the present invention will be explained in more detail below.

FIG. 1 shows an example of a slow cooling program used in the present invention (particularly its embodiments), and FIG. 2 shows an example of a slow cooling program for comparison.

The most important parts of the present invention are compressing the foamed particles used during molding to a particle compression rate of 30% or less, expanding the compressed foamed particles in a mold, and the molded product obtained. Once the temperature is controlled in a temperature atmosphere from below the base resin softening temperature to 60°C or above, and then slowly cooling to room temperature over a period of 3 hours or more,
The goal is to organically combine the four requirements of

The individual requirements will be explained below to make the reason easier to understand.

First, the limitation of the particle compressibility is, as clarified by the results in Table 1, in the present invention, by using foamed particles with a low compression rate, which have been excluded because conventionally excellent foams cannot be obtained. This is due to the development of a method to make an excellent foam while effectively utilizing the advantages hidden in the foam.

In other words, particles with a particle compressibility of 30(f) or less require as little gas pressure as possible during the compression of the expanded particles and up to the time of molding, and as a result, molds, filling equipment, etc. It has the advantage of being simple and easy to operate. Therefore, the most desirable range is 15% particle compression ratio.
It is as follows. However, the part with a compression ratio of 0 was excluded from the present invention because it was previously developed as a separate invention and has already been filed as Japanese Patent Application No. 16038. It is necessary to compress the particles using gas pressure.

The reason for this is to prevent particle deformation from becoming anisotropic as much as possible. Particles with strong anisotropy may change the filling rate in the mold, or the expansion may be localized, making it difficult to This is because it becomes difficult to obtain a molded body. Further, it is preferable to compress the particles before filling them into the mold, but if the shape of the mold is simple, it is also possible to compress the particles inside the mold while filling the particles inside the mold.

The filling rate of compressed foam particles into the mold is usually about 50 to 50% by volume.
It is desirable to do this within a range of 70%.

Next, the purpose of the compressed foamed particles in the mold can be sufficiently achieved if the compressed state is maintained at least until the closure of the mold is completed, but it is more desirable that the compressed foam particles remain in the compressed state until just before heating and expansion begins. It is desirable that it be maintained.
The reason for this is that the foaming expansion of the present invention is not due to the vaporization expansion force of the substance contained in the particles or the expansion pressure of the gas filled to a certain pressure, but rather due to the normal gas contained in the particles. Since it only expands with a relatively small force such as the heating expansion pressure of a gas close to the pressure, when the expansion force caused by being compressed acts synergistically, the expansion of the particles will occur. It is believed that the effect of filling the gaps between the particles in the mold and increasing the fusion between the particles is further enhanced. The obtained molded body is first cooled and then its temperature is controlled. In this case, the cooling conditions may be either rapid cooling or slow cooling, but generally rapid cooling is often used to shorten the process time. In any case, it is first necessary that at least the surface temperature of the molded body be cooled to below the softening temperature of the base resin. The reason for this is to prevent the molded product taken out of the mold from undergoing irreversible deformation due to external forces or its own contraction force, and if necessary, it is cooled to room temperature or lower. In some cases, the temperature may be cooled to the temperature for the next temperature control step. In this case, if cooling is allowed to proceed excessively, there will be an advantage that the following temperature control and slow cooling steps can be separated into separate processes, but on the other hand, there will be a disadvantage that it will take time to raise the temperature of the molded product in the temperature control step. .

Therefore, when using a continuous process to shorten the process time, the molded body is cooled to the lower limit of the temperature control temperature, which is below the softening temperature of the base resin to 60°C, and then transferred to the next temperature control process. This is desirable. The temperature control step is completed by placing the molded body in an atmosphere at a temperature below the softening temperature of the base resin and above 60°C.

The necessity of this temperature control process is to minimize the variation in the temperature of the cooled molded product and to stabilize the subsequent slow cooling conditions, or to restore the once contracted molded product to its original shape. It is important in its role of expanding. Therefore, for example, if the temperature control conditions are such that the temperature of the molded object exceeds the softening temperature of the base resin, the molded object will shrink again in the next slow cooling process, and on the other hand, the molding If the temperature of the molded body is only below 60°C, there is a disadvantage that it is not possible to measure the re-expansion of the contracted molded body. The above-mentioned temperature control process is completed by placing the molded body in an atmosphere adjusted to a certain temperature within the above-mentioned range for an appropriate period of time. It is possible to use a method in which the molded product is moved continuously, a method in which the molded product placed on a trolley is stagnated in a conditioning chamber for a necessary period of time, and the like.

Next, in the slow cooling step, it is necessary to slowly cool the molded body over a period of at least 3 hours to bring the temperature of the compact from the above-mentioned temperature control temperature to room temperature (approximately 25°C or less). be.

In this case, the slow cooling program requires cooling in stages with relatively small temperature differences, as illustrated in Figure 1. For example, even if the cooling time is 3 hours or more, the cooling Rapid cooling or cooling that is close to rapid cooling is inappropriate, and even if the cooling time is 3 hours or more, cooling that repeatedly subcools or raises the temperature to reach room temperature is inappropriate. . The slow cooling of the present invention requires 3 hours or more to obtain a molded product without sink marks, but in order to keep the required time to the minimum necessary, the temperature of the molded product transferred to the cooling process is kept low (however, 60°C (above), and when the molded product is on the high temperature side during slow cooling, the temperature difference is made relatively small and the temperature is gradually cooled in many stages, so that the temperature of the molded product approaches the room temperature side. It is better to choose a gradual cooling program that increases the cooling temperature difference as the temperature increases.

The above-mentioned specific temperature and time conditions are appropriately selected within the above-mentioned range of conditions depending on the type of base resin used, the thickness of the molded article, and the like. Further, specific examples of the above-mentioned cooling method include, for example, a method in which the molded body is moved in a plurality of temperature-controlled rooms having a cooling gradient in stages for a necessary residence time;
An effective method is to place the molded body in one room and gradually change the room temperature according to a cooling program.

Although the effects of the series of steps in the present invention have not yet been fully elucidated, the functions are expected to be as follows.

In other words, the in-mold foaming of expanded particles observed in the present invention is caused by a very small amount of foaming gas remaining in the final particles after foaming and/or by infiltration from the atmosphere into the expanded particles after foaming, as described above. This is thought to be due to the foaming force based on the thermal expansion of gas at near normal pressure.

Therefore, if the molded product is rapidly cooled, the volume of gas within the bubbles will rapidly and significantly decrease, causing the molded product to shrink.

However, during this cooling, at least until the temperature of the molded body is cooled to below the softening point of the base resin,
If the molded product is cooled while minimizing the application of external force to the molded product, the molded product will be given a restoring force depending on the post-process conditions. Furthermore, as the molded product ages after being cooled, the penetration of atmospheric gas into the cells of the molded product progresses, and the presence of the blowing agent gas within the cells has the effect of increasing the gas pressure within the cells to 1 atmosphere or more. Probably.

The gas pressure within these bubbles is related to the foaming power during thermal expansion during temperature control (reheating). Next, in the slow cooling process, if cooling can be done to such an extent that the shrinkage deformation force generated in the same molded product does not at least exceed the rigidity of the molded product while cooling has not progressed. , shrinkage will not occur due to the rigidity of the molded body, and if the infiltration of gas from the atmosphere is promoted in summer,
Since the shrinkage deformation force will be further reduced, both of these together enable the next cooling in stages, so that a series of slow cooling programs can be formed.

The parts of the equipment used to carry out the present invention, which are usually considered to be difficult, include filling the compressed foamed particles into a mold in a compressed state, or keeping the foamed particles in a compressed state until they begin to heat and expand. It is part of the device that maintains the state.

However, in the case of the present invention, since the pressure of the gas used for compression is low and the compressibility of the particles is low, there is an advantage that it can be carried out using a relatively simple device and method. That is, to give one example, foamed particles are compressed using gas pressure in a pressure-resistant hopper, and the particles are placed in a mold equipped with a particle filling port placed under the same gas pressure atmosphere as described above. The gas is transported using the following gas.

When the inside of the mold reaches a predetermined degree of filling with compressed particles, the mold is closed and the particle filling port is closed, allowing the inside of the mold and the pressurized atmosphere area where the mold is placed to become independent from other parts. The particles will be under a gas pressure that does not cause them to expand. Therefore, if heating steam is introduced into the pressurized atmosphere in which the mold is placed while releasing the pressure, the compressed particles will maintain their compressed state at least until the completion of filling the mold. If the pressure of the pressurized atmosphere in which the mold is placed is not released, but instead is heated by replacing it with heating steam under the same pressure, the compressed particles will maintain their compressed state at least until they begin to heat and expand. Become. The ethylene resin used in the present invention is a general term for homopolymers, copolymers, and blend polymers containing 60% or more of ethylene, such as high-pressure polyethylene, low-pressure polyethylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, and ethylene-ethyl Examples include methacrylic acid copolymers, ethylene ionomers, mixtures thereof, and blend polymers of these and other resins.

Among them, choosing a resin with a softening temperature of 84°C or higher that exhibits particularly remarkable effects can shorten the annealing time,
It has the advantage of widening the optimum range of temperature control and slow cooling conditions. In the case of the present invention, the above-mentioned ethylene resin is used after being crosslinked and modified. The reason is that the expansion ratio of the foamed particles is 1.
This is to increase the number by 0 times or more. The crosslinking method is conventionally known, such as a method of incorporating an organic peroxide into resin particles and heating it, a method of irradiating the resin and foam resin with ionizing radiation, and a method of appropriately combining these methods. used. The foamed particles of the present invention are obtained by foaming the crosslinked ethylene resin. This foaming method can be carried out, for example, by directly contacting the resin particles with the vapor or liquid of a volatile blowing agent, or by dispersing the resin particles in a suspension and indirectly contacting the blowing agent. A method is used in which the particles are impregnated with the foaming agent and then heated and foamed. Furthermore, if specific conditions are selected, foamed particles can also be produced using an extrusion foaming method. Volatile blowing agents include, for example, propane, butane, methyl chloride, methylenetalolide, dichlorodifluoromethane,
Trichloromonofluoromethane, monochlorotrifluoromethane, dichlorotetrafluoroethane, etc. are used.

The amount of these blowing agents to be used is 5 to 30 parts by weight per 100 parts by weight of the ethylene resin, and is appropriately determined depending on the target expansion ratio and the foaming ability of the blowing agent used.
The foamed particles usually have an expansion ratio of 5 to 40 times. The shape of the foamed particles is spherical, pseudo-spherical, cylindrical, pseudo-cylindrical, etc., and the diameter obtained by converting to the spherical volume is 2 to 10 m.
The type I is generally easy to use, and the spherical type is the easiest to use because it allows smooth transfer by gas and filling in the mold. It is preferable that the mold used in the present invention be a closable mold having a large number of small holes in its wall to allow gas to pass through. When steam is used as the heating heat source, the small holes can linearly heat the resin in the mold, which is effective in terms of thermal efficiency. The present invention enables in-mold molding of so-called foamed particles whose internal pressure is close to atmospheric pressure without actively containing a blowing agent or foaming gas in the foamed particles. It goes without saying that this invention can also be applied to in-mold molding of expanded particles having an internal pressure greater than atmospheric pressure.

The present invention will be explained below with reference to Examples, but the present invention is not limited thereto.

Example 1 High pressure polyethylene (density 0.9219/Cc.MI3
．． O. dicumyl peroxide (melting point: 113°C, softening temperature: 92°C) is heated to achieve a crosslinking degree of 60%.
crosslinked polyethylene particles were obtained.

Add dichlorodifluoromethane solution to these particles in a pressure vessel for 8 minutes.
Direct contact impregnation for 1 hour, cooled, taken out, transferred to a pre-foaming machine, heated and foamed with steam at 110°C, foaming ratio 2
0cc/9 spherical foam particles with a diameter of 7 mI were obtained.

The foamed particles were transferred to a hopper tank, compressed using air pressure, and then transported by air while maintaining the compressed state to fill the mold at a filling rate of about 69%.

After closing the mold, the atmosphere surrounding the mold is reduced to an original pressure of 1.01<g/
d (gauge pressure), heated for about 20 seconds to expand and mold the expanded particles, cooled once to the controlled temperature, and then placed inside the cage adjusted to each temperature. was gradually cooled down to room temperature while being transferred on a conveyor. The conditions at this time are as shown in the table below, and the mold used was a box type with outer dimensions of 20CrrLX20CTnX7 (1-JMoV! inner dimensions of 15 (7L x 15CTIL x 5cm), and the molding machine used was , an ECHO 120 automatic molding machine manufactured by Toyo Kikai Kinzoku Co., Ltd.

The evaluation of the obtained molded bodies is shown in Table 2.

Example 2 High pressure polyethylene (density 0.922 poly TC
．． MllO soft (temperature: 94°C) was supplied to the extruder, and dichlorotetrafluoroethane was pressurized into the middle region of the extruder and kneaded with the molten polyethylene.Then, the molten resin was cooled to 110°C while passing through a stirring cooling device. Adjust to ℃, diameter 1
The foam was discharged into the atmosphere from a nozzle with a diameter of 5 mm and a foamed filament with a foaming ratio of 25 cc/J was extruded.

This filament continuum was continuously irradiated with 4 Mrad radiation using an electron beam accelerated by an electron beam accelerator with an output of 500 KV, and the length of the posterior striae was 5 m7! It was cut into L to obtain crosslinked polyethylene foam particles. The degree of crosslinking of the expanded particles was 59%. Using these expanded particles, foam molding, temperature control, and slow cooling were performed in the same manner as in Example 1 except for the conditions shown in the table below to obtain an excellent molded product.

Note that the molded product used in this example before temperature controlled slow cooling was a foamed product in a contracted state. The evaluation of the obtained molded bodies is shown in Table 2.

Comparative Example 1 Only the conditions shown in the table below were changed as stated in the table,
The method of Example 1 was carried out.

The results are summarized in Table 3. The evaluation of the obtained molded bodies is shown in Table 3.

The present invention has the above-described structure, so that even if the blowing agent is press-fitted under high pressure for a long time, the molded product with a higher expansion ratio than the foamed particles will not have defects such as distortion and sink marks. It is an invention that is effective in industry and has the advantage that it can be manufactured by using a method of manufacturing, and since the gas handled during the process is on the low pressure side, manufacturing and operating methods can be extremely simplified.

[Brief explanation of drawings]

FIG. 1 shows an example of a slow cooling program used in the present invention;
The figure shows an example slow cooling program for comparison.

Claims (1)

[Claims] 1 In a method of filling foamed ethylene resin particles into a mold and heat forming them, the foamed particles are compressed to a particle compression ratio of 30% or less (excluding 0) using gas pressure, and then placed in the mold in the compressed state. The molded body is filled with heat and molded, cooled until the surface temperature of the molded body becomes at least below the softening temperature of the base resin, and then taken out.
1. A method for producing an ethylene resin mold foam, which comprises placing it in an atmosphere at a temperature of 0° C. or higher, and then gradually cooling it to room temperature over a period of 3 hours or more.