JPS6311975B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a novel molded product formed by fusing together a large number of crosslinked polyethylene foam particles having a fine cell structure, and a method for manufacturing the same. More specifically, the present invention relates to a novel crosslinked polyethylene foam molded product that exhibits excellent physical properties and has a smooth, glossy surface with almost no interparticle gaps, and a method for producing the same. Up to now, various methods have been proposed for obtaining a molded article that conforms to the mold by filling a mold with cross-linked polyethylene foam particles, heating and expanding the particles, and causing fusion between the particles. Molded products are used in a wide range of fields, including insulation materials for homes, ships, and automobiles, packaging for various goods, cushioning materials for transportation, cushioning materials such as mats and packing materials, other decorative items, toys, and floating devices. There is. However, when actually manufacturing foams suitable for these uses, it is not always possible to obtain foams with fully satisfactory properties.
Naturally, there are limits to that demand. For example, when used as a flotation device, there is a disadvantage that the buoyancy decreases significantly during use, and
Toys have disadvantages in that the large number of gaps that occur between particles on the surface of the molded product causes shadows, and the lack of surface gloss reduces the commercial value in terms of appearance. Furthermore, when used as a heat insulating material, the heat insulating performance deteriorates significantly over time, and the dimensions of the molded product, especially for roofing materials used in locations where the surface is exposed to high temperature atmospheres exceeding approximately 70°C, are is 4-7%
The disadvantage is that the cushioning material shrinks and creates gaps between each unit, making it impossible to fulfill its role.
When used as a cushion material, there is a drawback that, due to insufficient heat-resistant creep properties, the cushioning performance is lost during transportation of the product, resulting in damage to the product. On the other hand, when manufacturing packaging, buffer containers, and insulated storage boxes, it is difficult to obtain a shape that is faithful to the mold if the structure is complicated by thick and thin parts due to uneven foaming in the mold or sink marks. In addition, it has the disadvantage of not having buffering properties or heat insulating properties. The present inventors learned that such defects were caused by the lack of expansion ability of the foamed particles that were fused in the mold, and first developed highly expandable foamed particles made of polyethylene resin with specific physical properties as a base material. Or, by using high-expansion polyethylene foam particles with specific physical properties, it has been possible to obtain a foam molded product with improved properties to some extent, but there are various defects due to defects in its appearance and the shape of the target molded product. The improvement of various defects derived from the structure has not yet been fully satisfactory. Therefore, the first object of the present invention is to provide a molded article having a smooth and glossy appearance with substantially no depressions between the particles forming the surface portion, and a high quality appearance. The second purpose is to provide a molded body that has excellent interparticle fusion bonding inside and outside the molded body and does not cause a decrease in buoyancy over time. The purpose is to provide a molded product with excellent properties and excellent heat creep resistance.The fourth objective is to provide a molded product with excellent sustainability of thermal insulation performance over time and excellent dimensional stability over time. The fifth objective is to ensure that even when molded into complex shapes and dimensions, the surface will be smooth, there will be no loss of edges due to shrinkage, and the compressive strength will be high.
The object of the present invention is to provide a wide variety of molded bodies having various properties such as buffering properties and water absorption resistance. According to the present invention, these objects are achieved in a molded article constituted by fusion and aggregation of crosslinked polyethylene foam particles consisting of a large number of fine cell structures: (b) The melting point of the resin forming the cell structure of the foamed particles is at least 107°C; (c) The density of the molded product ( D) is within the range of 0.025 to 0.05 g/cm ³ and the stress (F) when compressing the molded body by 25% is expressed by the formula 1/140 (270-6/D) ≦F≦1/ 140 (315-6/D); (d) the indentation coefficient between the particles forming the surface portion of the molded body has a value of 10 or less; (e) the aging of the molded body A polyethylene foam molded product having substantially no interparticle voids on its surface is characterized by a dimensional change rate of less than 2% at 90°C for 96 hours and a heat-resistant creep of less than 35% at 80°C. can be achieved. The polyethylene resin used as a material for the molded article of the present invention is preferably an ethylene homopolymer, but it may also be a copolymer of ethylene and other monomers as long as its essential properties are not impaired, or a monopolymer of ethylene. It may also be a blend with other polymers. This material is first granulated into granules with a particle size of about 0.5 to 3 mm, and then processed by known crosslinking methods such as electron beam irradiation and organic peroxide impregnation to form crosslinked polyethylene particles.
Foaming is carried out using an organic blowing agent, an inorganic gas, or both by a single-stage method or a multi-stage method to obtain foamed crosslinked polyethylene particles having a multi-microcell structure. The cross-linked polyethylene foam particles thus obtained are then given expansion capabilities for in-mold expansion, including impregnation with an organic blowing agent, increasing the internal pressure of inorganic gas within the particles, and compacting the particles. Any method can be used. The crosslinked polyethylene foam particles imparted with expansion ability in this manner are filled into a mold, heated to expand, and the particles are tightly fused together to form a desired molded article. The molded product of the present invention is a novel crosslinked polyethylene foam molded product characterized by the above-mentioned items (a) to (e). First, requirement (a) is that the crosslinked polyethylene foam particles constituting the molded article of the present invention are composed of a large number of fine cell structures, and have a skin thickness that is larger than the thickness of the cell structure inside. However, this can be easily confirmed from an enlarged microscopic photograph of the cross section of the foamed particles. That is, FIG. 1 of the attached drawings is an enlarged microscopic photograph of the cross-section of the conventional cross-linked polyethylene foam particles, and FIG. 2 is an enlarged micrograph of the cross-section of the cross-linked polyethylene foam particles of the present invention. The film thickness is almost the same both inside the particle and on the skin, but in Figure 2, the film thickness on the skin is clearly thicker than the inside, and the ratio is approximately three times that of the inside. It turns out that the above is the case. Furthermore, the foamed particles of the present invention are clearly different in that they are smooth and shiny, and exhibit considerable elasticity when crushed with a finger, whereas conventional foamed particles have a rough and lackluster surface. It will be done. Only by using such foamed particles can a foamed molded article with no gaps between particles on its surface be obtained. Next, as a requirement (b), the temperature of the resin of the expanded particles constituting the molded article of the present invention is at least 107°C, preferably 107°C.
It is necessary to have a melting point of ~124°C. This melting point was determined using a differential thermal thermometer (Perkin-Elmer, Differential Scanning Calorimeter, Model 1-B) at a heating rate of 10°C/
This is the value measured under the conditions of min and sample amount of 0.005 g. If the melting point is less than 107°C, satisfactory creep properties cannot be obtained. As a result of the research conducted by the present inventors, we found that if the resin melting point of the expanded particles exceeds 124°C, the properties will deteriorate when molded into a molded product, and if it reaches 127°C, a molded product of high quality that can be put to practical use cannot be obtained. It has been found that a melting point in the range of 107 to 124°C, particularly 110 to 120°C, is advantageous. The melting point of the resin with this cell structure matches the melting point of the resin of the molded body, but does not necessarily match the melting point of the material used to produce the expanded particles. Next, as a requirement (c), the molded article of the present invention has a density (D)
is within the range of 0.025 to 0.05 g/cm ³ and the stress (F) when compressing the molded body by 25%, the formula 1/140 (270-6/D)≦F≦1/ 140 (315-6/D
) It is necessary to satisfy the following relationship. This is a necessary requirement to make the water absorption rate 0.3% by volume or less, and outside this range, it is not possible to obtain a dense molded article with low water absorption. Figure 3 shows that molded bodies with melting points in the range of 107 to 124°C, but with different densities and compressive stresses, are produced by changing the manufacturing conditions, and the water absorption rate, which represents the state of bonding and fusion of the particles, is obtained. This is a graph that evaluates the quality of the molded product. In this figure, the water absorption rate
Based on 0.3 volume%, anything less than that is 〇,
Those exceeding this were stratified by ×. In addition, water absorption rate
The criterion of 0.3% by volume was chosen in relation to the sustainability of the insulation performance. According to this Figure 3, the water absorption rate is at least 0.3.
In order to obtain a molded body with a value of volume % or less, it is necessary that the density of the molded body be in the range of 0.025 to 0.05 g/cm ³ and that the compressive stress (F) satisfy the above formula. It turns out that In other words, even if the melting point of the resin is a molded body of 107 to 124°C, points (D, F) correspond to the graph showing the relationship between the density (D) and compressive stress (F) of the molded body. When expressed in the coordinates of a
(0.025, 0.54), b (0.005, 1.39), c (0.025,
0.21) and d(0.050, 1.07). This compressive stress is 100mm long, 100mm wide, and 25mm thick.
The above sample was compressed at a rate of 12 ± 3 mm/min, and 25%
This was determined by measuring the compressive stress value when strain occurs. The above-mentioned requirement (B) is related to the characteristics of the material constituting the microcells, and requirement (C) is the shape and size of the microcells, the cell structure including the cell film thickness and its distribution, and the state of fusion between particles. This is related to the comprehensive evaluation of can be said to be one indicator of the internal structure of the molded article. Furthermore, as requirement (d), the molded article of the present invention needs to have a depression coefficient, which indicates the surface condition, of 10 or less. FIG. 4 is an enlarged microscope photograph showing the surface condition of a conventional foam molded product that satisfies the relationship (c), and FIG. 5 is an enlarged micrograph showing the surface condition of the foam molded product of the present invention. In addition, while the conventional molded product has gaps or depressions between particles, the molded product of the present invention almost completely fills the spaces between particles and has virtually no interparticle spaces. . In the present invention, the difference between these states is distinguished by a factor called a depression coefficient. This indentation coefficient is determined by the predetermined area (10
x 10cm), draw several ruled lines corresponding to a total length of 1m while avoiding steam holes, and indicate this as the number of depressions (identified by a notch width of 1mm or more) that can be caught on the ridge of the cutting line. It can be done. Figure 6 is a graph showing the relationship between the indentation coefficient determined in this way and the surface gloss and internal water absorption rate of the molded product. Looking at this graph, it can be seen that the larger the indentation coefficient, the lower the surface gloss. However, there is a tendency for the internal water absorption rate to increase. This means that even if a molded object is manufactured using a material that produces surface gloss, if the surface has many depressions, the surface gloss will be damaged due to the shadows and diffused reflection of light, and the gloss level of the surface of the molded object will decrease. The fact that the particle temperature decreases and that many depressions occur on the surface area indicates that the expansion of the particles is insufficient, at least locally, and the bonding between particles and melting point become insufficient, resulting in an increase in the water absorption inside the molded body. ing. This indentation coefficient is a factor that qualitatively indicates the surface condition of the molded product, so even if the value varies within a range of about ±1, there will be no substantial change. The indentation coefficients of all foam molded products are about 20 or more, whereas the indentation coefficients of the molded products of the present invention are all in the range of 4 to 6, which clearly distinguishes them from conventional products. 10. This can also be sufficiently confirmed by observing and comparing the surface conditions of the conventional product and the molded product of the present invention. That is, FIGS. 7A and 7B and FIGS. 8A and 8B are enlarged microscopic photographs of partial cross-sections of the surface of the molded product corresponding to FIGS. 1 and 2, but the conventional product (FIG. 7) The bonding state of the particles in the internal cross-sectional direction is rough and the surface film of the molded product is thin, whereas the molded product of the present invention (No. 8
In Figure), the bonding and adhesion of particles in the same direction is sufficiently homogeneous, and the film of particles forming the surface of the molded product is also slightly thicker. These facts agree well with the results shown in FIG. 6 above. However, the surface gloss of the molded product of the present invention may be due not only to the indentation coefficient of the molded product surface but also to the use of foamed particles that tend to produce surface gloss, that is, special particles with a thick skin film. Conceivable. In any case, such surface gloss can be said to be a unique effect that has not been observed in any of the conventional products, and is a significant cosmetic effect. In addition, as can be seen from Figure 7 A and Figure 8 A, when conventional foamed particles are used, there is almost no difference in the thickness of the cell structure on the particle fusion surface and other areas. On the other hand, in the case of the present invention, the relationship between the thickness of the internal cell structure and the thickness of the surface layer in the expanded particles is maintained even after the foamed particles are formed, and the latter is about three times or more than the former. There is. Furthermore, FIG. 9 is a graph showing the relationship between the water absorbency of a foamed molded article and the sustainability of its heat insulating performance over time (λ'/λ). This is the result of an accelerated test using the device whose cross-sectional view is shown in FIG. 10, and this accelerated test corresponds to a rooftop insulation time of about 2 years. As is clear from FIG. 9, when focusing on the sustainability of thermal insulation performance over time, a water absorption rate of 0.3% by volume has an important technical significance. In addition, Figure 11 shows the dimensional change rate over time, Figure 12 shows the heat resistance creep, Figure 13 shows the surface gloss, Figure 14 shows the sustainability of the insulation performance, and Figure 15 shows the buffer properties. This is a graph comparing the molded product of the present invention (solid line) with two commercially available products (broken line, dashed-dotted line'), and it can be seen from this graph that the molded product of the present invention has much superior properties than conventional products. . Such a molded article of the present invention has a density of 0.925 to 0.940.
After cross-linking particles of polyethylene resin of g/cm ³ and then impregnating the particles with a blowing agent,
After preferentially volatilizing the foaming agent on the particle surface, a general foaming treatment is performed to form foamed particles having a surface film thickness that is approximately three times or more larger than the film thickness of the internal cell structure, and then After the expansion ability imparting treatment, the foamed particles are filled into a mold, heated and expanded to cause fusion between the expanded particles, and the resulting molded product is further aged at a temperature of 60°C or higher for at least 6 hours. It can be manufactured using In this method, it is necessary to particularly select the raw material polyethylene, which generally has a density of 0.925 to 0.940.
It is necessary to use polyethylene in the range g/cm ³ . Particularly suitable is the coordinate (density,
Vikatsu softening point), (0.925, 377), (0.925,
369), (0.940, 383), and (0.940, 390). By using such polyethylene, a foamed molded article made of a resin having a melting point of 107 to 124°C can be obtained. This raw material polyethylene is granulated and crosslinked as described above, and then in order to form expanded particles having a skin thickness larger than the thickness of the cell structure inside the particles, After the particles are impregnated with a foaming agent, the foaming agent on the surface of the particles is preferentially volatilized and then subjected to a treatment to cause foaming on the entire surface. Such treatment includes, for example, exposing the expandable particles to the atmosphere in an open state for about 1 to 10 minutes or bringing them into contact with an inert gas, followed by heating and foaming, or at the stage of transferring the expandable particles to a foaming container. After rapidly reducing the pressure to atmospheric pressure or lower, heat and foam the particles, or transfer the expandable particles as they are to a foaming container, and first blow heated gas below the foaming initiation temperature into the container. This is carried out by a method such as raising the temperature of the particles at a temperature of 100° C. and then blowing a heating medium at a foaming temperature to cause foaming. The specific conditions for these methods vary depending on the type of blowing agent used, the target expansion ratio, etc.
Optimal conditions can be determined by simple preliminary experiments. In the case of a high expansion ratio, it is advantageous to first form foamed particles with an expansion ratio of about 10 times, and then foam the particles in several stages until the desired expansion ratio is reached, taking care not to generate excessive stress. FIG. 2 is an enlarged microscopic photograph of the foamed particles obtained in this way. Compared to the conventional foamed particles shown in FIG. 1, the skin part is thicker and smoother, and the surface The effect of preferentially volatilizing the blowing agent contained in the area is clearly shown. In the method of the present invention, the foamed particles obtained in this way are subjected to expansion ability imparting treatment. Alternatively, it can be carried out by adding an inorganic gas such as air into the foamed particles so that the internal pressure of the foamed particles themselves becomes 0.05 to 3 kg/cm ² (gauge pressure). Next, the foamed particles imparted with expansion ability in this manner are filled into a predetermined mold, and heated by passing water vapor or the like to expand and fuse the particles at the same time. When this is further aged at a temperature of 60° C. or higher for at least 6 hours, a desired molded article can be obtained. This aging treatment is necessary for improving the mold size ratio of the molded article, and by performing this aging treatment, the mold size ratio can be brought close to 1. What is particularly important in this method is, as mentioned above, to form special foam particles with different cell structures in the skin and inside, and the other steps are the same as those commonly used in the production of conventional polyethylene foam moldings. You can apply the conditions as they are. By using the special foamed particles mentioned above, the way the particles expand and fuse in the mold changes, resulting in a molded product with a low dent coefficient and a high gloss compared to conventional cross-linked polyethylene foam. The particle in-mold molding method produces unexpected effects. To further explain this point, in-mold molded objects range from relatively simple shapes formed with a thin, almost uniform wall thickness to those with a wall thickness of 80 mm.
A wide variety of materials are available, such as thick plate-like materials such as those with a wall thickness of 10 mm or less and parts with a wall thickness of 80 mm or more. There is something. Molded objects with such complex shapes and large wall thicknesses have a higher
The reality is that the manufacturing conditions for obtaining high quality products are difficult because the distribution state of particles filled in the mold and the transmission behavior of heating temperature are different for each mold. In other words, when performing in-mold foam molding, it goes without saying that it is preferable to increase the expansion ability exerted within the mold, but conventional means for imparting expansion ability, such as adding internal pressure to particles and increasing particle compressibility, alone are insufficient. Because sufficient effective expansion ability cannot be obtained, and instead brings about unfavorable results such as excessive surface fusion and increased density, even if there is no problem in obtaining a molded article with a relatively simple shape, It has been difficult to obtain molded bodies with complex shapes of good quality. In the present invention, in addition to the foamed particles themselves having expansion ability, by additionally performing a treatment to impart expansion ability, the two work synergistically to significantly improve the expansion ability within the mold. This made it possible to obtain complex-shaped molded products with excellent quality that could never be obtained using conventional methods. The foam molded article of the present invention thus obtained has a melting point of the resin constituting the cells of 107°C or higher,
The relationship between compact density (D) and compressive stress (F) is as specified, and the surface depression coefficient is 10 or less.
In addition, this product has excellent heat resistance creep properties, dimensional change rate over heat, surface gloss, and appearance quality (few sink marks, dents, etc.), and has low water absorption, so it has excellent insulation performance connectivity and buoyancy maintenance. It will be excellent. Therefore, the degree of freedom in selecting the physical properties of the polyethylene foam molded product is significantly expanded, and the industrial advantage is that the field of use thereof is widened. Next, the present invention will be explained in more detail with reference to Examples, and the resins used in each example are as follows.

[Table] In addition, each characteristic was measured and evaluated as follows. (1) Fusion property: Approximately 300mm long, 300mm wide, 80mm thick
Make a cut with a depth of 30 mm in a plate-shaped test piece of a molded product, tear the molded product while bending it, calculate the percentage of the number of cracked particles to the total number of particles present per tear section, and evaluate according to the following criteria. .

[Table] (2) Water absorption rate; approximately 300mm long, 300mm wide, 80mm thick
After making a molded test piece and accurately measuring its volume (V) and weight (W), it was immersed in freshwater at approximately 20°C at a depth of 25 mm below the water surface for 24 hours, and after being taken out, the surface was quickly wiped off. , the weight increase (W) before and after immersion is determined and calculated according to the following formula. Water absorption rate (volume %) = W x 100/V x water density

[Table] (3) Sink mark: Place a horizontal ruler diagonally on the top surface of a molded plate-shaped test piece approximately 300 mm long, 300 mm wide, and 80 mm thick, and measure the maximum distance of the gap between this test piece and the ruler. Find the percentage between the length of the diagonal and the length of the diagonal, and evaluate according to the following criteria.

[Table] (4) Dimensional change rate over heat: A molded product test piece cut into 50 mm cubes was placed in a constant temperature bath controlled at 90°C for 96 hours, taken out, and allowed to cool for 1 hour. The dimensional change rate (%) with respect to the body is determined, and its maximum value is evaluated according to the following criteria.

[Table] (5) Compression creep property: 0.1 kg/kg of test piece cut into 50 mm x 50 mm x 25 mm at 25°C.
Apply a load of cm ² , and the thickness (to) immediately after that, 24
The thickness (t) after the elapse of time is measured and calculated according to the following formula. Compression creep (%) = to-t/to×100

[Table] (6) 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] (7) Glossiness (reflectance) of the molded body; attach the surface of the molded body to Nippon Denshoku Kogyo's Gioss Meter VG-10 model, adjust the irradiation angle to 45°, and measure the reflectance. and will be evaluated according to the following criteria.

[Table] (8) Sustainability of thermal insulation performance over time: A test piece of the molded product cut out from the center of the molded product to a length of 200 mm, width of 200 mm, and thickness of 25 mm is measured using the apparatus shown in Figure 10. That is, a temperature regulator 3 surrounded by a heat insulating material 2
50° C. hot water 4 is poured into a container 1 equipped with a container 1, and the opening side of the container is closed with the above-mentioned test piece via a packing 6. At this time, place the test piece so that there is a distance of approximately 30 mm between the bottom surface of the test piece and the hot water surface in the container. In addition, the upper surface of the test piece is cooled by cooling water circulated from circulation water ports 7 and 8.
It is in close contact with the cooling plate 9 which is cooled to ℃.
After maintaining this condition for 30 days, wipe the surface of the sample piece lightly with gauze.
The thermal conductivity λ' of the material is measured in accordance with ASTM C518, and the rate of change λ'/λ from the thermal conductivity λ measured under the same conditions before the test is determined and evaluated according to the following table.

[Table] (9) Comprehensive evaluation; evaluate the product value by integrating the evaluation of each characteristic.

[Table] Example 1 Polyethylene resins A to G were impregnated with dicumyl peroxide as a crosslinking agent and crosslinked by heating to prepare crosslinked polyethylene particles having an average particle size of 1.2 mm. Next, the crosslinked polyethylene particles were placed in a pressure container, dichlorodifluoromethane solution was added thereto, and an impregnation treatment was performed while heating to obtain expandable crosslinked polyethylene particles. After the expandable particles were exposed to atmospheric pressure, they were placed in a foaming device and heated and foamed using water vapor. These foamed particles (primary foamed particles) were held in pressurized air to form air-containing foamable particles, which were further heated and foamed using water vapor. The range of conditions used in the above operations was as shown below, and the optimum conditions were selected to obtain approximately 30 times as many expanded particles for each resin. Average particle size of unfoamed particles 1.2mm Gel fraction 58-62% Blowing agent impregnation temperature 80℃ Blowing agent impregnation time 1-1.5 hours Standing time in atmosphere 1-3 minutes Primary foaming water vapor pressure 0.6-0.9Kg/cm ^2Primary foaming heating time 20-50 seconds Air addition pressure 10Kg/cm ^2Air addition temperature 80-90℃ Air addition time 4-6 hours Secondary foaming water vapor pressure 0.4-1.0Kg/cm ^2Secondary foaming time 20 ~50 seconds Leave these foamed particles (secondary foamed particles) under atmospheric pressure for one week, and after confirming that there is no foaming agent or internal pressure higher than atmospheric pressure inside the particles, place the particles in a pressure-resistant container. Treated with air at room temperature for about 5 seconds to compress it to 88% of the original bulk volume (compression ratio 12%), fill it into the mold as it is, heat it, cool it, and then remove it from the mold.
A molded body was obtained by aging in a hot air drying oven at 70°C for about 6 hours. The mold used was a closed mold with small holes (inner dimensions
300mm x 300mm x 80mm) and the molding temperature is 1.2~2.0
Kg/cm Heat fused with water vapor at ² pressure for about 20 to 30 seconds, approx.
Cooling was performed for 30-60 seconds in water at ℃. Resin melting point, fusion,
The results of measuring and evaluating sink marks and heat resistance creep are the first
Shown in the table.

[Table] As is clear from Table 1, when the properties of the molded product obtained by the method described were evaluated in terms of fusion properties, sink marks, and heat resistance creep, the melting point of the resin forming the molded product was 107 to 124. It can be seen that good results are obtained when the temperature is in the range of 110°C to 120°C, and that the molded product is even more desirable when the temperature is in the range of 110°C to 120°C. Example 2 Expandable crosslinked polyethylene particles were prepared by the method shown in Example 1 using resin symbols B, C, D, E, and F, and then aerated under atmospheric pressure for 0 to 4 minutes, and then placed in a foaming device. The primary expanded particles were obtained by adjusting the heating rate to 20 to 50 seconds and the heating time after raising the temperature to 5 seconds using water vapor of 0.5 to 1.0 Kg/cm ² . The primary expanded particles were treated in pressurized air at 10 kg/ ^cm2 at 80°C for 6 hours to form expandable particles containing air.
Using 0.5 to 1.0 Kg/ ^cm2 of water vapor, the heating rate was set to 20.
~50 seconds, and the heating time after temperature rise was adjusted to 5 seconds to obtain secondary expanded particles. The same air impregnation foaming treatment as above was repeated on these secondary foamed particles to obtain crosslinked polyethylene foamed particles with an expansion ratio of 15 to 45 times. After leaving the expanded particles under atmospheric pressure for one week and confirming that the particles do not contain a foaming agent or internal pressure higher than atmospheric pressure, the particles are placed in a pressure-resistant container and air pressure is applied at room temperature. It is compressed to 80% (compression ratio 20%) or 88% (compression ratio 12%) of the original bulk volume, and then molded into a closed mold with small holes (inner dimensions
300 x 300 x 80 mm) and fill it as it is with 1.2 to 2.0
Kg/cm2 After heating and fusing with steam at ² pressure for 20 to 30 seconds and cooling with water at about 20°C for 30 to 60 seconds, it was taken out of the mold and aged in a hot air drying oven at 70°C for 6 hours to obtain a molded body. . The obtained molded bodies were numbered (1) to (37), and the melting point, density, stress at 25% compression and water absorption were measured and shown in Table 2. In addition, the contents of Table 2 were stratified based on the criteria described above and indicated by symbols. The results are plotted in Figure 3. According to the results shown in Figure 3, a molded body with a water absorption rate of 0.3% by volume or less has a relationship between its density (D) and compressive stress (F), where (D) is in the range of 0.025 to 0.05 g/ ^cm2 . , and the coordinates expressed by the point (D, F) are a(0.025,
0.54), b (0.050, 1.39), c (0.025, 0.21), d
(0.050, 1.07).

【table】

[Table] Example 3 Among the molded bodies (1) to (37) shown in Example 2, F
The relationship between and D is within the fan-shaped range surrounded by a, b, c, and d in Figure 3.
Table 3 and FIG. 6 show the results of selecting molded bodies and measuring the relationship between the gloss of the molded body surface, the internal water absorption rate, and the indentation coefficient.

[Table] According to the results shown in Figure 6, it can be seen that as the indentation coefficient increases, the surface gloss tends to decrease and the internal water absorption rate tends to increase. Furthermore, when determining the internal water absorption rate and gloss using the indentation coefficient as used in the present invention, a commercially available molded product is located around approximately 20, and a molded product that is evaluated as particularly excellent is located around approximately 5. do. On the other hand, the dent coefficient has a probabilistic element in the evaluation method itself, so there is no critical significance such as competing for ±1. However, it will be understood that setting 10, which is on the safer side between 20 and 5, as one limit has the significance of indicating the level of technology. Note that the internal water absorption rate shown here is calculated from the center of the molded body wall thickness by 15 mm in the vertical direction of the thickness (total 30 mm).
Figure 2 shows the water absorption rate measured by cutting out a molded body of thickness). In addition, the conventional product (classification code
34) and the product of the present invention (distinction code 17) were cut and enlarged cross-sectional photographs were taken. The results are shown in FIGS. 7 and 8. According to FIGS. 7 and 8, the film thickness of the individual outer shell portions of the expanded particles constituting the compact with classification code (17) having a small indentation coefficient is greater than that of the compact with classification code (34). It can be seen that it is slightly thicker and the degree of adhesion between the particles is also tighter. Furthermore, enlarged cross-sectional photographs were taken of the expanded particles used in the molding of (34) and (17) above. The results are shown in FIGS. 1 and 2. According to FIGS. 1 and 2, the difference in film thickness on the particle surface described in the enlarged cross-sectional photograph of the molded body can be more clearly identified. In other words, in the cross section shown in Figure 2, the thickness of the cell membrane that constitutes the surface part and the inside is clearly different.According to the reference scale shown in the photograph, the thickness of the cell membrane at the surface part reaches a maximum of 15μ. On the other hand, the cross section shown in FIG. 1 has a thickness of about 8μ at most. In addition, according to Figure 2, some of the cells that make up the surface area are extremely small, and this occurred because sufficient foaming gas could not be held during foaming (because it was dissipated in advance). There are phenomena that remind us of the process of their creation. Comparative Example 1 180 parts by weight of water containing 0.25 parts by weight of potassium phosphate prepared from an aqueous sodium phosphate solution and an aqueous potassium chloride solution was placed in a pressure-resistant container, and while stirring, dicumyl par was added to 20 parts by weight of water containing 0.1 part by weight of Neoverex. After heating 0.38 parts by weight of oxide and adding the finely dispersed product while stirring,
Add 100 parts by weight of polyethylene (manufactured by Mitsui Polychemicals, trade name Mirason-9) with a particle size of 2.5 mm, replace the inside of the container with nitrogen, and treat at 100°C for 2 hours, then at 135°C for 7 hours to remove the contents. When taken out, crosslinked polyethylene particles with a gel fraction of about 50% were obtained. [] The crosslinked particles were placed in a pressure container and impregnated with 12% dichlorodifluoromethane to form heat-foamed particles.The foamed particles were held under dichlorodifluoromethane gas at a pressure of 18 kg/ ^cm2 , and then the pressure was removed to calculate the apparent volume. Approximately 45 c.c./g of expanded particles were obtained. These foamed particles were placed in pressure containers of 5000 c.c., 5625 c.c., and 6430 c.c.
After compressing the expanded particle volume to 4000, 4050, and 4050 c.c. respectively by pressurizing with air at Kg/cm ² pressure, they are placed in a mold with internal dimensions of 300 mm x 300 mm x 50 mm (inner volume 4500 cm ³ ). It was heated with water vapor at Kg/cm ² pressure and then cooled to obtain a molded body. These molded bodies are designated as (38), (39), and (40). [] The above crosslinked particles were placed in a pressure container and impregnated with dichlorodifluoromethane at 60°C ^for 2 hours to obtain impregnated beads with a weight of 13.5%. Heat for 20 seconds,
Expanded particles with an apparent volume of about 18 c.c./g were obtained. The particles were placed in a pressure-resistant container and held in air at 18 kg/cm ² pressure and 75°C for 15 minutes, 20, and 25 minutes, respectively, and then the pressure was removed to atmospheric pressure to form a foam with an apparent volume of approximately 27 c.c./g. Made into particles. The average internal pressure within this particle is
They were 0.43, 0.84, and 1.22Kg/cm ² (gauge pressure).
The foamed particles are directly shaped into inner dimensions of 300mm x 300mm x 50mm.
(Inner volume: 4500 cm ² )
A molded body was obtained by heating with water vapor at Kg/cm ² pressure for 20 seconds. These molded bodies are designated as (41), (42), and (43). Regarding (38) to (43) above, the melting point, molded body density, stress at 25% compression, and indentation coefficient of the cell membranes were measured and are shown in Table 4. Note that the internal pressure of the particles here was measured as follows. That is, about 10 g of expanded particles taken out from a pressurized atmosphere were quickly divided into 5 containers, their weight (W) was accurately weighed, and then each was placed into 5 water column pipes with one end open to atmospheric pressure. 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 internal pressure. Internal pressure of foamed particles = V _G /V _S -W/D [unit: Kg/cm ² ], where D is the density of the polyethylene used, and V _S is the weight and volume conversion using a large number of samples obtained from the same population. This is the volume of the foamed particles calculated from the weight of the foamed particles that was actually measured after determining the coefficient. 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 .

[Table] According to the results in Table 4, all molded products made by the current molding method that is generally practiced are outside the range of the products of the present invention in terms of melting point, relationship between density and compressive stress, and indentation coefficient. I understand something. Example 4 Molded bodies with classification code (17) obtained in Example 2, molded bodies with classification codes (39) and (43), and a board for cushioning material manufactured by Company X (900 mm x 1200 mm x 50 mm, expansion ratio 30 times)
and Y company cushioning board (900mm x 1200mm x 50
(mm foaming magnification 29 times), the melting point of the foam membrane of the molded product, the density of the molded product, the compressive stress, the indentation coefficient, the fusion property, the water absorption rate, the sink mark, the dimensional change rate over heat, the compression creep, the heat-resistant creep, the surface gloss, The sustainability of the insulation performance over time was investigated and the results are shown in Table 5.

[Table] Evaluated.
From the results shown in Table 5, it is clear that the molded product according to the present invention exhibits far superior performance in all aspects to the conventional product. For the following molded bodies, raw data measured during evaluation of dimensional change rate over heat, heat resistance creep, surface gloss, sustainability of heat insulation performance over time, and buffer properties are shown in Figures 11, 12, 13, 14, and 15, respectively. This is shown in a graph. The display in the figure is as follows.

[Table] Example 5 Resin D (about 1.2 mm in diameter) was impregnated with 0.4% dicumyl peroxide and heated to 150°C to form crosslinked polyethylene particles with a gel fraction of 60%. This product was impregnated with about 12% dichlorodifluoromethane to form expandable crosslinked polyethylene particles, which were divided into two parts, designated as J and K, respectively. First, Particles J are directly transferred to a foaming device and heated to 115°C with water vapor at a temperature increase rate of 4°C/sec to form foamed (primary foamed) particles with an expansion ratio of approximately 8 times. ℃, kept under air pressure of 5Kg/cm ² (gauge pressure) for 6 hours, and then heated at a rate of 4 with water vapor.
The particles are heated to 115°C at a rate of ℃/sec to form foamed particles (secondary foaming) with an expansion ratio of approximately 17 times, and then air impregnation and heat foaming (tertiary foaming) are performed under the same conditions to obtain a molded product with an expansion ratio of approximately 30 times. It was made into foamed particles. On the other hand, the expandable particles K are the same as the expanded particles except that they are exposed to room temperature and atmospheric pressure for about 4 minutes before being transferred to the foaming device, and the temperature increase rate during primary foaming is 2°C/sec. This operation was performed to obtain foamed particles with an expansion ratio of approximately 30 times. A detailed observation of the above two types of expanded particles reveals that the expanded particles have different structures, properties, etc., in the expanded particles from which molded object No. 17 of Example 2 was created and the expanded particles from which molded object No. 34 was created. The results were very close. That is, the foamed particles had a thick skin and were glossy, whereas the foamed particles had a thin skin and were dull. Using the above expanded particles, molded bodies having the following shapes were created. Molded object: A plate-shaped object with a wall thickness of 100 mm. Molded object: A rectangular container with a high rise (length 470 x width 265 x height 300 mm) with a wall approximately 14 mm thick. Molded object: Wall thickness 5 m /m, with an internal structure of 9 partition plates 32m/m high arranged in a grid at intervals of 16.5m/m (vertical 235 x horizontal)
180 x height 48) container, molded body; most of the wall thickness is approximately 10 m/m
Streamlined shape with no corners (vertical: 270 x horizontal: 210 x
Container with a height of 140 mm (machine cover) The molding conditions are: (i) Particle compression degree (imparting expansion ability in the mold)
0.5-35% [95-65% of original bulk volume] (ii) In-mold heating temperature (maximum steam pressure)
1.2 to 1.5 Kg/cm ² (iii) In-mold heating time 10 to 15 sec (iv) In-mold cooling time 30 to 40 sec (v) Molded article aging conditions Optimum conditions were selected from the range of 70° C. 6 hr. As a result, no matter how you choose the conditions for particles,
A molded body obtained from particles could not be obtained. Table 6 shows the results of comparing each molded article with what seems to be the best one in terms of appearance.

[Table] According to the results in Table 6, even if the same expanded particles are used, the density (D), compressive stress (F), and indentation coefficient of the obtained molded product depend on the structure of the molded product to be obtained. changes. However, the same type of foamed particles that completed molded article No. 17 of the present invention satisfy the structural characteristics and properties of a molded article as defined in the present invention, even though the molded article has a shape and structure that has been considered difficult to produce in the past. It has been demonstrated that it can be realized in various situations. A molded body of particles obtained in this way,
, , are not only novel;
It is extremely useful because it is a molded article with a shape and structure that can take advantage of the advantages of in-mold molding, which was previously desired but could not be completed. Example 6 Using the same foamed particles as in Example 5, the internal pressure inside the foamed particles was increased to a range of 0.05 to 2.5 Kg/cm ² (gauge pressure) by holding them under pressurized air for the required time. Various samples were prepared and foam molded under the same molding conditions as in Example 5. The various properties of the molded products thus obtained were measured and compared, and as in Example 5, results using foamed particles were much better than those using foamed particles. Indicated. Example 7 In order to confirm the effect of the aging treatment of the method of the present invention, the same foamed particles as used in Example 5 were used, and under the same molding conditions as in Example 5, the length, width, and height were changed. Twenty-one 50 mm blocks were manufactured, and after being removed from the mold, they were subjected to aging treatments under the following different conditions. (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. 16 as a graph showing the relationship between processing 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. As clarified from the above experimental results, the present invention has excellent heat resistance as typified by dimensional change over heat and heat-resistant creep, etc., as well as excellent durability in buffering properties and heat insulation performance, and has a smooth and glossy surface. It is an unprecedented molded product that can be made into a variety of shapes and structures, so it has high practical value and is an invention that can make a significant contribution to industry. be.

[Brief explanation of the drawing]

Figures 1 and 2 are micrographs of partial cross-sectional views of the skin of conventionally used foamed particles and foamed particles used in the present invention, respectively, and Figure 3 shows the relationship between the density and compressive stress of the foamed molded product. The graphs shown in Figures 4 and 5 are micrographs of the surface portions of the conventional foam molded product and the foam molded product of the present invention, respectively, and Figure 6 shows the relationship between the depression coefficient, surface gloss, and internal water absorption rate. Graphs, Figure 7 A and B and Figure 8 A and B are micrographs with increased magnification of the surface of the molded body corresponding to Figures 4 and 5, respectively, and Figure 9 is the water absorption rate of the molded body. Graph showing the relationship between and sustainability of insulation performance,
Fig. 10 is a cross-sectional view of a device for measuring the sustainability of thermal insulation performance over time, Fig. 11 is a graph showing dimensional changes over time in a heated state of a compact, and Fig. 12 is a compression diagram of a compact in a heated state. Graph showing the changes in creep; Figure 13 is a graph showing the glossiness (reflectance) of the surface of the molded body; Figure 14 is a graph showing the relationship between the water absorption time of the molded body and the sustainability of heat insulation performance over time; Figure 15 The figure is a graph showing the cushioning properties of the molded article, and FIG. 16 is a graph showing the effect of aging treatment in the method of the present invention. In FIG. 10, reference numeral 1 indicates a container, 2 indicates a heat insulating material, 3 indicates a temperature controller, 5 indicates a test piece, 6 indicates packing, and 9 indicates a cooling plate.

Claims (1)

[Scope of Claims] 1. In a molded article constructed by fusion and aggregation of crosslinked polyethylene foam particles consisting of a large number of fine cell structures, (a) the foam particles have a thickness relative to the thickness of the cell structure therein; (b) The melting point of the resin forming the cell structure of the foamed particles is at least 107°C; (c) The density of the molded product (D ) is within the range of 0.025 to 0.05 g/cm ³ and the stress (F) when compressing the molded body by 25% is expressed by the formula 1/140 (270-6/D)≦F≦1/140 (315-6/D) (d) The indentation coefficient between the particles forming the surface of the molded body has a value of 10 or less; (e) The molded body changes over time. A polyethylene foam molded article having substantially no interparticle gaps on its surface, characterized by having a dimensional change rate of less than 2% at 90°C for 96 hours and a heat-resistant creep of less than 35% at 80°C. 2 Particles made of polyethylene resin with a density of 0.925 to 0.940 g/cm ³ are cross-linked, then the particles are impregnated with a foaming agent, and the foaming agent on the surface of the particles is preferentially volatilized, followed by a general foaming treatment. Then, foamed particles are formed that have a skin thickness that is about three times or more larger than the thickness of the cell structure inside, and then these foamed particles are filled into a mold after being treated to give expansion ability, and heated and expanded. A polyethylene foam molded product having substantially no interparticle gaps on its surface, characterized in that the foamed particles are fused together by fusion, and the resulting molded product is further subjected to an aging treatment at a temperature of 60°C or higher for at least 6 hours. manufacturing method.