JPS639974B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for obtaining a molded article with unprecedented properties by further imparting foaming ability to foamed particles that themselves have expansion ability. Conventionally, a mold is filled with cross-linked polyethylene foam particles, which are then heated and foamed to fuse the particles together.
Various methods have been proposed for forming a molded article that conforms to the mold. Due to its high elasticity, the molded products obtained in this way are used as insulation materials in homes, freezers, ships, automobiles, etc., packaging for various goods, cushioning materials for transportation, cushioning materials such as mats and packing materials, etc. There were great expectations for its use in fields such as decorative items and materials for toys. However, in reality, the properties of the foam molded product obtained by this molding method are limited, and the characteristics corresponding to the intended use cannot be achieved, so the demand for it has completely stagnated. For example, in the field of ornaments and toys, the surface of the molded product has many irregularities and is matte and slimy, making it unattractive.In the field of insulation materials, the insulation performance deteriorates significantly over time, and In particular, when used for ceiling materials where the surface of the molded product is exposed to high-temperature atmospheres exceeding approximately 70°C, the dimensions of the molded product will shrink by 4 to 7%, creating gaps between each unit, which will no longer function as a heat insulator. The disadvantage is that it becomes impossible to use. In addition, in the field of cushioning materials and cushioning materials, there is a drawback in that they are particularly poor in heat creep resistance, and the cushioning materials do not lose their performance and may damage the contents in the hold of a ship during export. On the other hand, from a producer's perspective, for example, conventional molded products have a large molding thinness, so the internal dimensions of the mold are usually made larger in consideration of this, but parts with different wall thicknesses are When the purpose is to make a product, the major drawback is that the shrinkage state in each part varies, so the mold dimensions must be determined through repeated trial and error, relying on the experience and intuition of the manufacturer. It has also been attempted to improve productivity by making thick-walled molded bodies exceeding 100 mm and slicing them into thin pieces. However, thick molded bodies obtained by conventional molding methods not only have many sink marks and surface wrinkles, but also have poor particle fusion inside the molded body, so they are not suitable for practical use.
Even if the thickness can be adjusted and sliced, the sliced product from the inside of the molded product has a drawback that it has a large water absorption capacity and cannot maintain its insulation performance. Furthermore, if an attempt is made to reduce the unit cost by forming a molded article with a low density, there is a drawback that the cushioning properties and creep properties deteriorate. It is thought that all of the above-mentioned conventional drawbacks are caused by the insufficient foaming ability of the foamed particles that expand in the mold, which causes excessive stress and resulting distortion in various parts of the resulting molded product. It will be done. The present invention was researched and developed to overcome the current situation, and its first purpose is to provide a molded product that is glossy and has a smooth surface with no depressions between particles. The second purpose is to provide a molded product with a small thermal shrinkage rate and excellent water absorption resistance and heat creep resistance.
The fourth objective is to enable the production of thick-walled molded products without sink marks or wrinkles, and to provide molded products with excellent thermal insulation properties over time, even if the inside of the molded product is sliced. A fifth object of the present invention is to provide a molded product with excellent cushioning properties, compressive strength, and creep resistance, and to provide a method for producing a molded product with excellent mold-to-mold reproducibility. According to the present invention, these objectives are achieved by using crosslinked polyethylene resin as a material and achieving smoothness (S).
Spherical foam particles with a surface of 1.05 or less, an expansion ratio of about 20 to 40 times, and an attenuation coefficient (T) of the air pressure injected into the particles are within the range of (1/1500 to 1/2500) D. This is then compressed to about 95-50% of the original bulk volume of the foamed particles, and then filled into a mold and heated and foamed before the expansion ability decreases, or in this way. This can be achieved by further aging the molded product obtained at a temperature of 60° C. or higher for 6 hours or more. The cross-linked polyethylene resin foam particles used in the method of the present invention have a shell on their surface consisting of a structure that is harder than the internal structure, and when compared with future cross-linked polyethylene resin foam particles, they have a pearl-like appearance that can be identified at a glance. They differ from conventional crosslinked polyethylene resin foam particles in that they have a surface gloss of . In order for the foamed particles to have such characteristics, as mentioned above, the surface should have a smoothness (S) of 1.05 or less, and the expansion ratio (D) should be in the range of about 20 to 40 times. , and the attenuation coefficient (T) of the air pressure injected into the particles must be in the range of D/1500≧T≧D/2500 (). The smoothness (S) here is a factor that represents the degree of unevenness of each cell on the particle surface, and in the case of an ideal smooth surface with no cell unevenness,
It becomes 1.00. Therefore, when looking at the overall surface condition of a particle, it can be said that the closer the smoothness (S) is to 1.00, the smoother the particle is. This smoothness (S) can be determined as follows. That is, the expanded particles are cut into thin pieces, and a microscopic photograph is taken of the cross section, focusing in particular on the surface contour. Next, as shown in the schematic diagram in Figure 1, connect the points of contact of the contour with adjacent cells with a straight line for each cell, and calculate the length of this straight line for each cell.
Find the ratio B/A of the sum A of a ₁ , a ₂ , a ₃ ... and the sum B of the actual length b ₁ , b ₂ , b ₃ ... of the surface layer of each cell, and smooth this. The degree is S. In order to investigate the correlation between the smoothness obtained in this way and the glossiness, five male adults with normal color sense were asked to visually examine the glossiness of foamed crosslinked polyethylene resin particles with various degrees of smoothness. When the evaluation was carried out, the results shown in Table 1 were obtained. The rating in this table is 4 points if you feel that it has a strong gloss, 3 points if you feel that it is shiny, and 2 points if you feel that it is not shiny.
1 point was given to each item, and 1 point was given to those who felt that the product lacked gloss.

[Table] As shown above, it can be seen that the numerical value of smoothness and the visual evaluation completely match. Also, the smoothness
All of the people evaluated particles of 1.05 or less as being shiny, but these are all foamed particles of the present invention, and from this result, the foamed particles of the present invention are clearly distinguishable from conventional ones. I know it will happen. Next, in the present invention, it is further necessary that the following relational expression be satisfied between the expansion ratio (D) of the foamed particles and the damping coefficient (T) of the air pressure inside the particles. D/1500≧T≧D/2500 () (However, 40≧D≧20.) In other words, in order to foam-mold the foamed particles without the step of imparting foaming ability, the particles themselves must have an expansion ability of 1.3 times or more. This is a practical necessity, but for this purpose the above requirements must be met. Figure 2 shows the expansion ratio (D) of various expanded particles with a smoothness (S) of 1.05 or less and the attenuation coefficient (T) of the air pressure inside the particles when air is injected into the particles. The relationship is shown as a graph. Note that each code in the figure indicates the stratification results based on the criteria in Table 2.

【table】

[Table] From FIG. 2, it can be seen that in order for the foamed particles themselves to have an expansion capacity of 1.3 times or more, the requirements shown in the above relational expression () must be satisfied. In other words, in order for the foamed particles of crosslinked polyethylene resin to have an expansion capacity of 1.3 times or more the volume of the original foamed particles, the smoothness (S) must be
Even if the foamed particles have a surface of 1.05 or less and an expansion ratio (D) of about 20 to 40 times, the attenuation coefficient (T) of the air pressure injected into the particles will be at a certain point in relation to (D) above. A[20,
0.0133], B[20,0,0080], C[40,0.0160],
This means that the foamed particles must be located inside the quadrilateral surrounded by the straight line connecting D[40, 0.0267]. Cross-linked polyethylene is a material that forms cell groups, and the smoothness (S) is based on the surface structure of the particles, and the relationship between the expansion ratio (D) and the air pressure damping coefficient (T) is based on the skin. Considering that it is a comprehensive factor that indicates the wall thickness distribution structure and the cell structure inside the particle, including the alteration of the crystal structure, the combination of these factors is one structural index that indicates the structure of the expanded particle. Can be done. The expansion ability of foamed particles as used in the present invention refers to the expansion ability of foamed particles at 110°C under conditions where the gas pressure inside the particles is substantially 0 Kg/cm ² (gauge pressure) or the blowing agent content is substantially 0% by weight.
After heating with water vapor for 5 seconds, heat in a constant temperature room at 70℃ for 5 seconds.
Since it indicates how many times the foaming ratio has become compared to the initial foaming ratio after being left for a certain period of time, it can be said that this is the performance of the particles themselves, which is different from the foaming ability imparted later. Expanded particles with such an expansion capacity of 1.3 times are completely new, whereas conventional particles have an expansion capacity of only 1.11 times or less. The reason for choosing 1.3 times is that if you use foamed particles with at least 1.3 times the expansion ability, you can mold it without the process of imparting expansion ability, and furthermore, the resulting molded product itself is In comprehensive evaluation of properties such as , fusion adhesion, water absorption, surface smoothness, sink marks, dimensional stability over time, heat resistance creep, compression creep, etc., it exhibits the surprising advantage of being an extremely excellent molded product. It is in. As already mentioned, the cross-linked polyethylene resin foam particles of the present invention can be distinguished from the conventionally known cross-linked polyethylene resin foam particles even when observed with the naked eye, but when the cross-section is observed under a microscope, it is possible to distinguish between the two. When will the difference become so obvious? That is, FIG. 3 is a 23 times enlarged micrograph of the cross section of the foamed particles of the present invention (A) and the conventionally known foamed particles (B), and FIG. 4 is a 250 times enlarged micrograph of a partial cross section of the same. However, if you compare the two, the difference is obvious. First, conventional foam particles
In (B), the bubble film inside the particle and the bubble film on the surface are almost uniform, and the size and shape of the bubbles are uniform as a whole, and the bubbles exposed on the surface have their original shape. As a result, the particle surface forms an uneven shape consisting of connected arcs corresponding to the bubble film, whereas the expanded particle (A) of the present invention The particles are smaller and have a flat shape compared to the inner ones, the bubbles that make up the particles are uneven overall, and the bubble film exposed on the surface is thicker than other ones, making the particles The surface is covered with a relatively thick epidermis. In this way, the presence of an epidermis on the particle surface can also be confirmed by touch; when conventional foamed particles are touched with the hand, the surface feels soft and slightly rough, whereas the surface of the present invention In the case of foamed particles, it feels slightly hard, elastic, and smooth. Also,
When a particle surface is pierced using an insect pin with a slightly blunted tip, the conventional pin penetrates without feeling any resistance, but the pin of the present invention shows resistance and produces a faint popping sound. This suggests that in the latter case, an epidermis with a different tissue from the internal tissue is formed. The foamed particles of the present invention having such a special particle structure can be produced by a step of crosslinking polyethylene resin to make crosslinked polyethylene resin, impregnating the crosslinked polyethylene resin particles with a foaming agent, and then foaming them in stages. In the usual method for producing foamed particles, which consists of the steps of: (1) impregnating the resin particles with a foaming agent, placing them under conditions where the foaming agent present on the surface layer of the resin particles is likely to volatilize; 2) It can be produced by selecting conditions that do not cause excessive stress on the particle surface layer at the stage of converting low-magnification cross-linked polyethylene resin foam particles into high-magnification cross-linked polyethylene resin foam particles. As for the specific operation of (1) above, for example, once the resin particles impregnated with the blowing agent are taken out under atmospheric pressure, they are kept for several minutes, usually 1.
You can leave it for ~10 minutes. Further, as a specific operation for the above (2), for example, during secondary foaming, the rate of temperature increase can be made moderate within a range that does not impair the expansion ability of the foamed particles to be produced. The reason why the foamed particles of the present invention having a special particle structure can be obtained by selecting such manufacturing conditions is considered to be as follows. That is, by impregnating resin particles with a foaming agent and then placing them under atmospheric pressure, the foaming agent on the surface layer of the particles evaporates, reducing the foaming ability in that region.
Next, such resin particles are foamed at a low expansion ratio, and then the temperature is gradually raised when foaming to a higher expansion ratio, so that the hard-to-foam resin in the surface layer is broken there by excessive internal pressure. It stretches without bending, and the formation of internal bubbles is completed. In this way, foamed particles are formed that have a structure that appears to be covered with a skin and have excellent properties. In the present invention, the foamed particles prepared in this way are further given foaming ability, that is, 95 to 50% of the original bulk volume of the particles (compressibility of 5
It is necessary to compress the foam to a temperature of ~50%, then fill it into a mold, heat it, and foam it before the combined expansion capacity of both of them decreases. If the compression rate is less than 5%, it is not possible to obtain a thick, highly foamed molded product without sinkage or wrinkles, or to maintain the sustainability of the insulation performance when the inside of the molded product is sliced and used. On the other hand, if the compression ratio is high, exceeding 50%, the flow of heated steam between the particles will be hindered, resulting in insufficient internal heating and the internal particles will become pomegranate-shaped. Figure 5 shows the molded body () obtained in this manner (density 0.025, expansion ratio approximately 40 times) and the molded body obtained from commercially available expanded particles [company X product (density
0.033, foaming ratio approximately 30 times), Y company product' (density
0.034, foaming ratio of approximately 30 times)], Figure 6 is a graph showing the heat resistant creep properties of the same product, and Figure 7 is a graph showing the buffer properties of the same product. From these graphs, It can be seen that the molded articles obtained from the expanded particles of the present invention are significantly superior in gloss, heat-resistant creep properties, and buffering properties than those obtained from conventional expanded particles. Figure 8 shows the thermal insulation performance of a 30 mm thick molded body obtained by slicing the central part of a 130 mm thick molded body obtained by the method of the present invention and a commercially available molded body with a wall thickness of 50 mm. The durability is compared; the solid line shows the product of the present invention, and the broken line shows the commercially available product. As is clear from FIG. 8, the method of the present invention makes it possible to produce a thick molded body with a well-organized internal structure. As is clear from Figures 5 to 8, the method of the present invention allows the external foaming ability to act additively on the expansion ability of the particles themselves, thereby solving various problems that could not be solved by conventional methods. It can be solved. The compression of particles carried out in the method of the present invention is carried out, for example, in a pressure-resistant container containing expanded particles at room temperature with a pressure of 0.5 to 2.
This can be achieved by pressurizing a gas (for example, air) at a pressure of Kg/cm ² for 3 to 10 seconds, and then filling the mold with an equal pressure. In this case, it is advantageous to install an adjustment device that detects the pressure inside the container and makes the difference from the set pressure substantially zero, so that compression and its management can be performed freely. In the method of the present invention, the molded product obtained as described above is thermally formed at a temperature of 60°C or higher for 6 hours or more, thereby achieving even more excellent properties, such as improved mold reproducibility and thermal shrinkage over time. Benefits such as lower rates can be provided. For example, FIGS. 10 and 11 show improvements in mold reproducibility and thermal shrinkage rate over time, respectively. According to FIG. 10, the molded product that has been cooled and taken out after molding in the mold is slightly smaller than the internal dimensions of the mold, but it is preferably heated at a temperature of 60°C or higher for 6 hours or more. It can be seen that by aging at a temperature of 80°C or more for 8 hours or more, it is possible to make it close to the internal mold dimensions. Note that the aging treatment temperature cannot exceed the melting point of the molded article, and the upper limit of the treatment time should be determined as appropriate from the viewpoint of productivity. The molded body that has undergone such aging will not change its dimensions in an environment near the processing temperature thereafter. The commercially available molded body (dashed line) shown in Figure 11 is 90
℃, shows approximately 5% shrinkage over 96 hours, whereas the shrinkage rate of the molded product aged at 80℃ for 10 hours to approximate the mold dimensions changes only by about 0.7% over time. I see that it has stopped. The reason why a molded article with excellent properties can be obtained by using specific expanded particles in the method of the present invention is considered to be due to the following reasons. First, the foaming of the surface portion of the foamed particles is suppressed compared to the inside, and the surface tension of the particles suppresses the air bubbles on the particle surface, forming a relatively thick skin with a smooth surface. This skin has crystals oriented in three axes, giving it luster and rigidity, as well as low gas permeability. Therefore, when these particles are filled into a mold, fortunately due to their smoothness, they are uniformly filled into every corner of the mold in a state close to close-packed, and when heated, the thermal expansion of the internal gas will act almost directly as the expansion ability of the particles. In general, it is said that approximately spherical foamed particles have 20% or more voids between the particles, assuming they are packed close-packed, but the particles of the present invention have an expansion capacity of more than 1.3 times ( i.e. 30
% or more of voids).
These voids can be filled and particles can be closely fused together. It is presumed that even if the pressure inside the bubbles is then reduced in the cooling stage, the rigidity of the skin supports this and brings in outside air, resulting in a molded product without sinkage or distortion. The polyethylene resin used as a material in the present invention is preferably an ethylene homopolymer, but copolymers containing other monomers or mixed resins containing other resins may be used as long as the characteristics of the present invention are not impaired. You can also do it. In addition, the smoothness, air pressure damping rate, and expansion ability referred to in the present invention are not necessarily related to the type of resin, but from the performance of the obtained molded product, the density is 0.925 to 0.935, and the Vikat softening point is 371. ~385〓
It is advantageous to use polyethylene resins in the (absolute temperature) range. According to the method of the present invention, it is possible to obtain a cushioning material with a compact cushioning design that does not lose its cushioning performance in a ship's hold during loading. It brings beneficial advantages to various industrial fields, such as being able to obtain beautiful containers. Next, the present invention will be explained in more detail with reference to Examples. In addition, the measurement and evaluation of the characteristics in each example are as follows:
I did it as follows. (1) Smoothness of particles: Slice the expanded particles to a thickness of approximately 20 μm and enlarge them 250 times to obtain a cross-sectional photograph (focus in particular on the contours). Connect the boundary points with the cells in sequence with straight lines, and let the length formed by these straight lines be A (see Figure 1). Let the length of the surface layer of the cell corresponding to the section A above be B. The smoothness (S) of the particles is determined by the formula: S=B/A (2) Attenuation coefficient of pressurized air: The foamed particles were stored in a pressure-resistant container and left for 24 hours at 23°C and 10Kg/ ^cm2 , then taken out and quickly divided into 5 containers in an amount of about 10g. After accurately weighing the weight (Wi), each was connected to five water column pipes with one end open to atmospheric pressure, and the amount of gas escaping from the foamed particles (V _G ) was measured over time. , calculate each value according to the following calculation formula and use this as the internal pressure of the foamed particles measured for each container. Internal pressure of foamed particles = V _G /V _S W/d where d is the density of the polyethylene used, and V _s is the weight of the foamed particles that was actually measured by calculating the conversion factor between weight and volume using a large number of samples obtained from the same population. This is the volume of expanded particles calculated from The end point of the measurement in this case is the time when the difference in internal pressure between before and after one hour becomes less than 0.01 Kg/cm ² . P ₁ is the end point pressure in the relationship between the obtained internal pressure and elapsed time, P ₂ is the pressure after 1 hour has passed from the start of measurement, and V _G1 is the amount of gas dissipated during 1 hour from the start of measurement. Assuming that the number of particles distributed in each container is n, the value for each container is calculated using the following formula, and the average value is taken as the attenuation coefficient (T) of the pressurized air. (3) Cell internal pressure: Approximately 10 g of expanded particles taken out from the pressurized atmosphere were quickly divided into 5 containers, their weight (W) was accurately weighed, and then the 5 containers were placed into 5 containers with one end open to atmospheric pressure. Each is connected to an underwater pipe,
The amount of gas (V _G ) escaping from the expanded particles is measured over time, each value is determined according to the following calculation formula, and the average value is taken as the cell internal pressure. Cell internal pressure = V _G /V _S -W/d where d is the density of the polyethylene used, and V _S is the weight and volume conversion factor obtained from a large number of samples obtained from the same population. This is the volume of the expanded particles calculated from the weight. In this case, the end point of the measurement is the time when the difference in internal pressure between before and after one hour becomes less than 0.01 Kg/cm ² . (4) Glossiness (reflectance) of the molded body: The surface of the molded body was mounted on a Nippon Denshoku Gioss Meter VG-10 model, and the irradiation angle was adjusted to 20°, 45°, and 75°, and the reflectance was measured. Measure. (5) Compressive strength; molded product test piece (100×100×25mm
t) at a speed of 12 ± 3 mm/min, 25
Measure the compressive stress value when % strain occurs. (6) Cushioning properties: A molded specimen (50 mm
A weight (of several types with different weights) with a flat bottom is dropped vertically from a height of 60 cm.
Then, attach an acceleration pick-up to this weight and measure the magnitude of the acceleration at the moment of collision, and graph it with maximum acceleration (G) on the vertical axis and static stress on the horizontal axis. (7) Fusion properties: Make a cut with a depth of 20 mm in a plate-shaped test piece of a molded product measuring 300 mm long, 300 mm wide, and 50 mm thick, tear the molded product while bending it, and calculate the total number of particles present per cross section of the torn part. The percentage of the number of cracked particles is determined and evaluated based on the following criteria.

[Table] (8) Water absorption rate: After making a 50 mm cubic molded test piece and accurately measuring its volume (V) and weight (W), it was placed at a position 25 mm below the water surface in fresh water at approximately 20°C.
After soaking for a period of time and taking out, the surface is quickly wiped, and the weight increase (W) before and after soaking is determined and calculated according to the following formula. Water absorption rate (volume %) = W x 100/V x water density

[Table] (9) Surface smoothness: Select a 50 mm square area anywhere on the surface of the molded product placed horizontally, surround it with a frame, then pour in the agar solution, cool, and solidify. Next, tear it off, place it on a flat plate with the adhesive side facing up, cut out the protruding part on the horizontal plane, calculate the number of cut agar blocks, and compare it with the number of foam particles present on the surface in the above area. Evaluate the percentage according to the following criteria: The height when cutting on a horizontal plane is 1/5 of the average particle diameter.

[Table] (10) Sink mark: Place a horizontal ruler diagonally on the top surface of a molded plate-shaped test piece measuring 300 mm long, 300 mm wide, and 50 mm thick, and calculate the maximum distance of the gap between this test piece and the ruler. The percentage between the length of the diagonal is determined and evaluated based on the following criteria.

[Table] (11) Dimensional stability over time: A 50 mm cubic molded specimen was placed in a constant temperature bath controlled at 90°C for 96 hours.
After taking it out, it is left to cool for 1 hour, the dimensional change rate (%) with respect to the initial molded body is determined, and its maximum value is evaluated according to the following criteria.

[Table] (12) Compression creep property; molded product test piece (50×50×25
Apply a load of 0.1Kg/cm ² to the 25°C (mm) at a temperature of 25°C, measure the thickness immediately after (to) and the thickness after 24 hours (t), and calculate according to the following formula. calculate. Compression creep (%) = to-t/to×100

[Table] (13) Heat resistance creep: Perform the same operation as in the previous section on compression creep property at a temperature of 80°C to determine the compression creep and evaluate it according to the following criteria.

[Table] (14) Comprehensive evaluation: The product value is determined by integrating the evaluation of each characteristic.

[Table] The types of polyethylene resins used in each example are shown in the table below.

[Table] Reference example 1 2 parts by weight of magnesium carbonate and dicumyl peroxide (crosslinking agent) in 180 parts by weight of water in a pressure-resistant container.
Finely disperse 0.5 parts by weight, add 100 parts by weight of (G) resin (average particle size 1.2 mm), replace the inside of the container with nitrogen, and heat treat at 100°C for 2 hours, then at 135°C for 7 hours. This was done to create crosslinked polyethylene particles with a gel fraction of approximately 60%. The cross-linked polyethylene particles are placed in a pressure-resistant container, dichlorodifluoromethane (foaming agent) solution is added,
After raising the temperature to 90℃, impregnation treatment is performed for 2 hours, and the
Expandable crosslinked polyethylene particles containing % blowing agent by weight. The expandable particles were exposed to atmospheric pressure for 3 minutes and then placed in a foaming device to yield 0.7Kg/cm ² (gauge pressure).
was heated and foamed with water vapor. The foaming conditions in this case were that the temperature was raised for 35 seconds and then heated for 5 seconds, and the foaming ratio of the particles obtained was approximately 6 times (primary foamed particles). The primary foamed particles were treated at 80° C. for 24 hours in pressurized air at 5 kg/cm ² to form expandable particles containing air (blowing agent gas), and heated and foamed with water vapor at 0.7 kg/cm ² . The foaming conditions in this case were such that the heating rate was 35 seconds, and the foaming ratio of the particles obtained was approximately 17 times (secondary foamed particles) by heating for 5 seconds after heating. The same air impregnation foaming treatment as above was repeated on these secondary foamed particles to obtain crosslinked polyethylene foamed particles (No. 1) with an expansion ratio of about 30 times. On the other hand, as a comparative example, Sumikasen G202 with a particle size of 2 to 6 mm was prepared using a pressure container according to the method described in Japanese Patent Publication No. 45-32622.
60 parts by weight of pellets (low-density polyethylene manufactured by Sumitomo Chemical) are dispersed and suspended in 300 parts by weight of water in which 0.6 parts by weight of GH-23 polyvinyl alcohol (polyvinyl alcohol manufactured by Nippon Gosei Chemical) is dissolved while stirring. Separately dicumyl peroxide as a crosslinking agent
After adding 0.6 parts by weight dissolved in 6 parts by weight of xylene to the above suspension system, 18 parts by weight of butane was further added, pressurized to 5 kg/cm ² with nitrogen gas, and heated at 125 to 130°C.
React for 10 hours. The crosslinking degree of the obtained expandable particles was 43% in terms of xylene insoluble content (gel fraction). As a result of heating the expandable particles with a particle size of 3 to 6 mm separated from the suspension system after cooling to 100℃ with steam, the foaming degree was 25.
cc/g (expansion ratio of about 25 times) crosslinked polyethylene resin foam particles (No. 2) were obtained. Comparing the foamed particles No. 1 and 2 above, No. 1
The product (of the present invention) had a beautiful pearl-like luster, whereas the product of No. 2 (conventional) had no luster at all. When the internal structures of particles No. 1 and 2 were observed under a microscope, it was found that No. 1 had a relatively thick skin-like substance on its surface, whereas No. 2 had a relatively thick skin on its surface. Nakatsuta. An example of an enlarged photo (approx. 23x)
250x) are shown in Figures 3 and 4, respectively. Note that the particle luster referred to here refers to a translucent raw material resin that has become cloudy due to foaming, but has newly developed luster due to the action of something like an epidermis on its surface. The feeling you get is completely different. Reference Example 2 Among the conditions used in Reference Example 1, the time of exposing the expandable particles to atmospheric pressure before forming them into primary expanded particles was 1 minute,
The experiment of Reference Example 1 was repeated, except that the temperature was changed to 2/4 level, and the heating temperature increase rate at each foaming stage was changed to two levels, 20 seconds and 50 seconds. The smoothness (S) of the four types of particles obtained and the particles of Examples were evaluated. The results are shown in Table 1 above along with visual evaluation. According to the results in Table 1, the gloss ranking based on the total score matches the tendency for the smoothness (S) to be small. In addition, there is no evaluation of particles with a smoothness (S) of 1.05 or less of 2 points or less. From the above, it can be said that smoothness (S), although only a structural observation under a microscope, clearly represents the requirements that foamed particles should have to be judged to be glossy. Reference example 3 The number of resin types used was increased to five (A), (B), (C), (D), and (E), and the crosslinking conditions were selected so that the gel fraction was uniform at approximately 60% for each type. While adjusting each of the relationships between the aeration time of the foamed particles before making them into primary foamed particles and the heating temperature increase rate at each foaming stage so that the smoothness (S) of the foamed particles obtained is a value of 1.05 or less. , the magnification of the final foamed particles, respectively.
The same experiment as in Reference Example 1 was repeated except for multi-stage foaming aiming at 15, 20, 25, 30, 35, 40 and 45 times, and the obtained particles were No. 7 to 41 in the order of the resin used. A serial number is attached. (No.24 is No.1
) For each sample particle, the melting temperature, expansion ratio (D),
The smoothness (S), the air pressure damping coefficient (T), and the expansion ability were evaluated, and the results are shown in Table 3.

【table】

[Table] According to the results in Table 3, it can be seen that foamed particles having an expansion capacity of 1.3 times or more were not obtained from those using (A) resin (E) resin. In addition, the smoothness (S) obtained from resins (B), (C), and (D)
Even if the particle size is 1.05 or more, it can be seen that in the evaluation of the expansion ability, there are particles less than 1.3 times and particles larger than 1.3 times. The results of Table 3 are plotted in FIG. 2 as a relationship diagram between expansion ratio (D) and air pressure damping coefficient (T). Also, the symbols in the diagram (◎〇●×□
etc.) are stratified based on the combination of resin type and expansion capacity. According to the results in Figure 2, even if the smoothness (S) is 1.05 or less, the overall surface structure, that is, each expansion ratio (D) and air pressure damping coefficient (T), is determined by the formula D/1500≧T. It can be seen that unless the relationship of ≧D/2500 (where D is 20 to 40) is satisfied, it cannot have an expansion capacity of 1.3 times or more. In other words, this range is the coordinates indicated by the point [expansion ratio, air pressure damping coefficient], A[20, 0.0133]B
It must be within the quadrilateral connecting [20, 0.0080] C [40, 0.0160] D [40, 0.0267] with a straight line. Reference Example 4 Using expanded particles shown in Nos. 10, 17, 18, 24, 32, and 39 listed in Table 3 (substantially 0 internal pressure, 0 foaming agent content, confirmed), they were used as a closure metal with small holes. Fill the mold (inner dimensions 300 x 300 x 50 mm) as it is,
Heating for 20-30 seconds under water vapor pressure of 1.2-2.0Kg/ ^cm2 ,
The particles were foamed and fused, and after cooling with water at 20°C, the molded body was taken out. The fusion properties, water absorption rate,
Surface smoothness, sink marks, thermal aging dimensional stability, compression creep, and heat resistance creep were measured and evaluated, and the results are shown in Table 4.

[Table] As is clear from the results, if the expansion capacity of the expanded particles was less than 1.3, a good molded product could not be obtained. In other words, a molded product with good performance according to this result has a smoothness (S) of 1.05 or less and a foaming ratio (D) of 1.05 or less.
It can be seen that the relationship between and the damping coefficient (T) of air pressure is obtained when using expanded particles of D/1500≧T≧D/2500 (where D is 20 to 40). It can also be seen that these foamed particles are unprecedented particles that can yield molded products with better performance even without being imparted with foaming ability unlike conventional foamed particles. For comparison, foamed particles No. 2 of Reference Example 1 (expansion capacity: 1.06) were placed in a closed mold with small holes (inner dimension: 300 mm).
×300×50mm) and steam at 105 to 115℃ for 10 minutes.
The density, fusion properties, and
Although surface smoothness, sink marks, etc. were evaluated, it was not possible to obtain a good molded product. The evaluation results of the molded bodies are shown in Table 5 (additional test to Japanese Patent Publication No. 45-32622).

【table】

[Table] Under the above conditions, no matter what you do, the fusion properties are good. It was impossible to obtain molded bodies without flattened surfaces or without sink marks. Example 1 Using expanded particles of sample No. 25 shown in Table 3 of Reference Examples, this was packed in a sealed container and kept at room temperature 0.5 to 2.
Treat with Kg/ ^cm2 air for about 3 seconds to reduce the original bulk volume.
Compressed particle samples No. 42, No. 43, No. 44, No. 45 and No. 46 were produced by compressing to 97, 95, 80, 50 and 55%. Next, add this particle to a depth of 300mm vertically and 300mm horizontally.
The particles are filled into a closed mold with an internal dimension of 130 mm and equipped with small holes, and heated with steam at 1.2 to 2.0 Kg/cm ² (gauge pressure) for 20 to 30 seconds to foam and expand the particles. After being fused together and molded, it was heated to 20°C.
After cooling with water, the molded body was taken out. Each of the obtained molded bodies was examined for density, fusion adhesion, sink marks, surface smoothness, gloss, and sustainability of thermal insulation performance over time. The results are shown in Table 6. For comparison, we used the foamed particles of sample No. 2 according to the method of Japanese Patent Publication No. 45-32622, and left them in a closed container at 10 kg/cm ² at 80°C for 30 minutes to 5 hours. Internal pressure 0.05 or 1.5Kg/cm ² (gauge pressure)
Prepare expandable additive particles No. 47 and 48 and use the same method as above.
The same evaluation was performed on a molded product that was hot-formed in a mold of 300 mm x 300 mm x 130 mm and taken out after cooling, and the results are added to Table 6.

[Table] Shows the sustainability of performance over time.
As is clear from the results in Table 6, as the internal pressure of the foamed particles is increased to 0.05 or more and the foaming capacity is added, the visibility of the molded body becomes smaller and the degree of sinkage is reduced, but when the internal pressure of the cell is 3.5 When the pressure reaches Kg/cm ² (gauge pressure), expansion and fusion of the surface rapidly progresses during molding and heating, resulting in insufficient internal heating and a molded product with poor fusion properties, making it unusable.
In the case of a cell internal pressure of 3.0 Kg/cm ² (gauge pressure), the above tendency is somewhat observed, but there is no practical problem and this is considered to be the upper limit of the present invention. Moreover, in the case of the comparative examples, good results were not obtained in any case. From the above results, the method of the present invention could not be obtained conventionally. It can be seen that a molded article with a high wall thickness and high foaming (thickness and low density) can be obtained. Example 2 Compressive strength was measured for the molded product of sample No. 44 obtained in Example 1, the cushioning material board manufactured by Company X, and the cushioning material board manufactured by Company Y, and the results are shown in Table 7. .

[Table] According to this table, it can be seen that molded bodies having almost the same compressive strength can be obtained despite the large density difference. In addition, the glossiness (reflectance), heat-resistant creep characteristics, buffering characteristics, and sustainability of the heat insulation performance of the central portion over time of these molded bodies were investigated, and the results are shown in graphs in FIGS. 5 to 8. From these graphs, it can be seen that the molded product obtained by the present invention has a lower density and gloss than any commercially available product, and is also a molded product with excellent heat resistance creep and excellent buffering performance. Also, according to FIG. 8, according to the method of the present invention,
It can be seen that a thick-walled molded body with a well-organized internal structure can be obtained. Here, the sustainability of the thermal insulation performance of the central portion over time was measured as follows. In other words, slice parts corresponding to 1.5 mm above and below the center of the molded body in the thickness direction, and
A plate-shaped body with a thickness of 30 mm was obtained, and three sample pieces with vertical, horizontal, and thickness sizes of 200 mm, 200 mm, and 30 mm were cut out, and a heat insulating material 2 was applied in the apparatus shown in Fig. 9, and the temperature was adjusted. Hot water 4 at 50° C. is poured into a container 1 having a machine 3, and the opening side of the container is closed with a prepared test piece 5. In this case, sealing is sufficiently performed with a packing 6. Also, there should be a space of approximately 30 mm between the bottom surface of the test piece and the surface of the hot water in the container. Furthermore, the upper surface of the test piece was kept in close contact with the surface of the cooling plate 9, whose surface was cooled to 3°C by the cooling water circulated from the circulation ports 7 and 8, and this condition was maintained for 10 days and 20 days, respectively. The sample was maintained for 30 days and used as a sample for measuring insulation performance. The surface of the above test piece was gently wiped with gauze and the thermal conductivity λ' of each sample was measured using the method specified in ASTM C 518. The state of change was calculated as λ'/λ and evaluated as "sustainability of insulation performance over time." Example 3 Using the expandable additive particles of sample No. 44 obtained in Example 2, a molded article 21 with a length, width, and height of 50 mm was made.
The pieces were produced, and when they were taken out of the mold, they were subjected to the following different treatments. (1) No treatment (2) Remain in a constant temperature room controlled at 50℃ for 4, 6, 8, 10 or 12 hours. (3) Remain in a constant temperature room controlled at 60℃ for 4, 6, 8, 10 or 12 hours. (4) Remain in a constant temperature room controlled at 70℃ for 4, 6, 8, 10, or 12 hours. (5) Remain in a constant temperature room controlled at 80℃ for 4, 6, 8, 10 or 12 hours. The results are shown in FIG. 10 as a graph showing the relationship between residence time and mold size ratio. In addition, this mold size ratio is determined by taking the molded product out of the thermostatic chamber at the time of taking it out from the mold for (1), and after the predetermined residence time has elapsed for all other cases, and letting it cool for 4 hours at room temperature and normal pressure. Afterwards, its dimensions were determined. As is clear from this graph, heat treatment is required after molding in order to bring the mold-to-mold size ratio close to 1.0, and the heating conditions at that time are 60
℃ or higher for 6 hours or more, preferably 80℃ or higher for 8 hours or more. Example 4 Four 50 mm cubic test pieces were made from the molded product obtained in Example 7 that had been heat-treated at 80°C for 10 hours, and they were kept in a thermostatic chamber controlled at 90°C to give 3, 5, and 10 test pieces, respectively.
Or take out the sample after 96 hours and store it at room temperature and pressure for 1 hour.
The sample was left to cool for a while and its dimensional change was measured. The results are shown in FIG. 11 as a solid line as a graph showing the relationship between residence time and dimensional change rate based on the molded product before residence. As is clear from this graph, the molded product obtained according to the present invention has a very small dimensional change rate, with 96
The change was only 0.7% even after the residence time. For comparison, the results of an experiment conducted on a crosslinked polyethylene resin molded body for cushioning material manufactured by Company X (density 0.033) are shown by a broken line. This material had a large dimensional change rate, and shrank by 5% after being retained for 96 hours.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the cross-sectional surface layer of foamed beads, Figure 2 is a graph showing the relationship between expansion ratio and damping coefficient of various foamed beads, and Figure 3 is a 23-fold enlargement of the foamed beads of the present invention and conventional foamed beads. A cross-sectional micrograph, Figure 4 is a 250x magnification micrograph of the same thing, Figure 5 is a graph showing the glossiness of a molded product obtained from the expanded particles of the present invention and a commercially available product, and Figure 6 also shows the heat-resistant creep properties. Graph, Figure 7 is a graph showing the same buffering properties, Figure 8 is a graph showing the relationship between water absorption time and the sustainability of insulation performance over time, and Figure 9 is a cross-sectional view of the device that measures the sustainability of insulation performance over time. , FIG. 10 is a graph showing the relationship between the heat treatment time after molding and the size ratio to the mold, and FIG. 11 is a graph showing the dimensional change over time. The symbols in the figure are as follows. 1... Container, 2
...Insulation material, 3...Temperature controller, 5...Test piece,
6...Putskin, 9...Cooling plate.

Claims (1)

[Claims] 1. Using crosslinked polyethylene resin as a material,
It has a surface with a smoothness (S) of 1.05 or less, a foaming ratio of about 20 to 40 times, and an attenuation coefficient (T) of the air pressure injected into the surface in the range of (1/1500 to 1/2500)/D. The spherical foamed particles inside are adjusted, and then compressed to about 95-50% of the original bulk volume of the foamed particles, filled into a mold, heated before the expansion ability decreases, and foamed. A method for producing a polyethylene resin foam molded article, the method comprising: 2 Using cross-linked polyethylene resin as a material,
It has a surface with a smoothness (S) of 1.05 or less, a foaming ratio of approximately 20 to 40 times, and an attenuation coefficient (T) of the air pressure injected into the surface within the range of (1/1500 to 1/2500)D. The spherical foamed particles are adjusted, and then compressed to about 95-50% of the original bulk volume of the foamed particles, filled into a mold, heated and foamed before the expansion ability decreases, and then A method for producing a polyethylene resin foam molded product, which comprises aging the molded product thus obtained at a temperature of 60° C. or higher for 6 hours or more.