JP5968799B2 - Method for producing cross-linked polyolefin foam - Google Patents

Description

  The present invention relates to a method for producing a polyolefin-based foam having a low odor and an foaming ratio of 6 times or more using an inorganic foaming agent.

  Conventionally, organic foaming agents such as azodicarbonamide (ADCA) have been used for the production of high-magnification polyolefin foams by chemical foaming. As a result, discoloration such as odor, fogging of glass and the like were caused, and the application was limited.

On the other hand, sodium bicarbonate (sodium bicarbonate, NaHCO ₃ ), which is a representative inorganic foaming agent, only generates carbon dioxide and water vapor by thermal decomposition and does not generate corrosive components or odorous components. However, when trying to obtain a high-magnification foam, there is a problem of shrinkage after production, and only a low-magnification foam is obtained.

  As an improvement measure, in order to remove water generated by the thermal decomposition of sodium hydrogen carbonate, an alkali metal or alkaline earth metal oxide is mixed with the compound before foaming to allow it to react with moisture and absorb it. A method (for example, see Patent Document 1), a method for mixing a water-absorbing resin with a compound before foaming to absorb moisture (for example, see Patent Document 2), a method for releasing to the outside by heat treatment after foaming (for example, for example) Patent Document 3) has been proposed, but none of the effects of carbon dioxide was taken into consideration.

  On the other hand, with respect to the processing method and foaming method using a high-pressure inert gas (including a supercritical state), a method of foaming a blend of two types of incompatible resins with different inert gas solubility. (See, for example, Patent Document 4), a method for producing a foam having fine bubbles by mixing a fluorosurfactant with polycarbonate (for example, see Patent Document 5), a cell structure that has a high thickness recovery rate A batch of high-pressure inert gas used in a method for producing a controlled foam (see, for example, Patent Document 6) is brought into contact with a resin, impregnated and dissolved, and then suddenly released to the atmosphere. For example, an inert gas supersaturated in the resin by reducing the pressure is generated as bubbles and foamed.

JP 2007-217505 A JP 2007-217639 A JP 2007-231064 A JP 2005-271504 A JP 2008-127467 A JP 2012-140599 A

  However, there is a problem that shrinkage occurs after the production of a polyolefin foam having a foaming ratio of 6 times or more using sodium hydrogen carbonate as a foaming agent.

  This invention pays attention to such a malfunction, and it aims at providing the manufacturing method of the polyolefin foam of the expansion ratio 6 times or more using sodium hydrogencarbonate as a foaming agent.

  As a result of intensive studies to solve the above problems, the present inventors have come to the idea that the shrinkage is mainly caused by diffusion of carbon dioxide in the bubbles to the outside. The process will be specifically described below. Sodium hydrogen carbonate is thermally decomposed in a pressurizing mold at a high temperature to generate carbon dioxide gas and water vapor. Thereafter, by releasing the mold pressure, the carbon dioxide gas and the water vapor expand and bubbles are grown. The bubbles are stabilized by being cooled. At this time, water vapor present in the bubbles is condensed, and it is considered that the gas inside the bubbles is mainly carbon dioxide, and the atmosphere outside the bubbles is mainly nitrogen and oxygen. And since there is a concentration difference between these gases inside and outside the bubble, diffusion occurs through the bubble wall film, and the gas is replaced. Specifically, carbon dioxide gas diffuses from inside the bubble to outside the bubble, and nitrogen and oxygen diffuse from outside the bubble to inside the bubble. Since the diffusion rate is proportional to the permeability coefficient of the membrane and the gas concentration difference (partial pressure difference) inside and outside the bubble, the initial diffusion rate of carbon dioxide in the polyethylene bubble wall membrane is more than 10 times that of nitrogen. Estimated. For this reason, it is considered that the gas in the bubbles decreases for a while after the gas diffusion starts, the bubble internal pressure decreases accordingly, and the bubbles contract. As time elapses and the concentration of carbon dioxide in the bubbles decreases, the velocity of nitrogen or oxygen flowing into the bubbles exceeds the rate of carbon dioxide flowing out of the bubbles, and the shrinkage of the bubbles is recovered. Eventually, it is considered that the gas concentration is stable when the gas concentration inside and outside the bubble is balanced.

  Based on the above, as a means for solving the shrinkage, it is considered effective to promote the replacement of the gas inside and outside the bubble. Specifically, the treatment is performed by pressurizing the atmosphere with main gas components in the air. It can be achieved. This process is thought to improve shrinkage by the following principle.

  First, by pressurizing the atmosphere, the bubbles are compressed, the bubble internal pressure is increased, the concentration of carbon dioxide gas is increased, and diffusion outside the bubbles is promoted.

  Second, by pressurizing with nitrogen or air, diffusion of nitrogen or air into the bubbles is promoted.

That is, the present invention takes the following means in order to achieve such an object.
(1) A foamable composition obtained by kneading after adding a foaming agent and a cross-linking agent, which decomposes when heated to a polyolefin-based resin to generate carbon dioxide and water vapor, is heated and foamed 6 times. A foam production step for producing a foam having the above initial foaming ratio, a shrinkage step for shrinking the foam produced by this foam production step under normal pressure for a predetermined time, and the foam shrunk by the shrinkage step The foaming ratio of the foam is restored to a value more than two-thirds of the initial foaming ratio by treatment in high-pressure nitrogen gas or high-pressure air under a temperature condition from 35 ° C. to the softening point of the polyolefin-based resin. A method for producing a crosslinked polyolefin-based foam comprising a high-pressure treatment step.
(2) The method for producing a crosslinked polyolefin-based foam according to (1), wherein the predetermined time is 24 hours or more.
(3) The method for producing a crosslinked polyolefin-based foam according to (1) or (2), wherein the pressure in the high-pressure treatment step is 0.2 MPa to 20 MPa.
(4) The method for producing a crosslinked polyolefin-based foam according to (1), (2) or (3), wherein the time for performing the high-pressure treatment step is 1 hour or longer.
(5) The method for producing a crosslinked polyolefin-based foam according to (1), (2), (3) or (4), wherein the foaming agent is sodium hydrogen carbonate.
(6) A crosslinkable foamable composition obtained by adding and kneading a foaming agent, which is a compound that decomposes upon heating to generate carbon dioxide gas and water vapor, and a crosslinking agent is heated and foamed 6 times. After producing a foam having the above expansion ratio, a method of restoring the contracted foam is allowed to elapse for 24 hours or more after production, and then the softening point of the polyolefin resin is 35 ° C. to high pressure nitrogen of 0.2 MPa to 20 MPa. A method for producing a crosslinked polyolefin-based foam, comprising restoring a foam that has shrunk by treatment in gas or high-pressure air for 1 to 48 hours.

  According to the method of the present invention, it is possible to provide a high-magnification polyolefin foam that does not cause odor, metal corrosiveness, and fogging of glass.

The figure which shows the foaming magnification change after the high-pressure process of a 15 times foamed product as a graph. The figure which shows the foaming magnification change after the high-pressure process of a 30 times foaming product as a graph.

  Hereinafter, an embodiment of the present invention will be described.

  The method for producing a crosslinked polyolefin foam according to the present embodiment is obtained by adding a foaming agent, which is a compound that generates a carbon dioxide gas and water vapor by being decomposed upon heating to a polyolefin resin, and kneading after adding the crosslinking agent. A foam production process for producing a foam having an initial foaming ratio of 6 times or more by heating and foaming the obtained crosslinkable foamable composition, and the foam produced by the foam production process is contracted under normal pressure for a predetermined time. The foaming ratio of the foam is reduced by treating the foam that has been shrunk in the shrinking step and in a high-pressure nitrogen gas or high-pressure air under a temperature condition from 35 ° C. to the softening point of the polyolefin resin. And a high-pressure treatment step for restoring the value to at least two-thirds of the initial foaming ratio.

    The polyolefin referred to in the present invention is, for example, polyethylene, ethylene-vinyl acetate copolymer, polypropylene, polytetrafluoroethylene, ethylene-propylene copolymer, poly-4-methyl-1-pentene, polyvinyl chloride, polyvinylidene chloride. , Polyvinylidene fluoride, tetrafluoroethylene, and ethylene copolymer.

  The cross-linking agent as used in the present invention has a decomposition temperature at least equal to or higher than the flow start temperature of the polyethylene resin in the polyethylene-based resin, and is decomposed by heating to generate free radicals between the molecules or within the molecules. Organic peroxides that are radical generators that cause cross-linking, such as dicumyl peroxide, n-butyl 4,4-bis (t-butylperoxy) valerate, 1,1-ditertiary butyl peroxide, 1,1-ditertiary butyl peroxy-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-ditertiary butyl peroxyhexane, 2,5-dimethyl-2,5-ditertiary butyl peroxyhexine , Α, α-ditertiary butyl peroxyisopropylbenzene, Tasha Over butylperoxy ketone, there are such tertiary butyl peroxybenzoate, shall select the optimum organic peroxide with the resin to be used at that time.

  In the present invention, for the purpose of improving the physical properties of the composition to be used or reducing the price, a compounding agent (filler) that does not significantly adversely affect the cross-linking, such as zinc oxide, titanium oxide, calcium oxide, magnesium oxide, oxidation Add metal oxides such as silicon, carbonates such as magnesium carbonate and calcium carbonate, fiber materials such as pulp, or various dyes, pigments and fluorescent materials, other conventional rubber and plastic compounding agents as necessary. Can do.

  And, it is desirable that the predetermined time in the shrinking step is 24 hours or more. Here, if the predetermined time is less than 24 hours, the shrinkage is further continued after the high-pressure treatment step, which is not desirable.

  The pressure in the high-pressure treatment step is desirably 0.2 MPa to 20 MPa. Here, if the pressure is lower than 0.2 MPa, the gas cannot be sufficiently diffused into the bubbles, which is not desirable. Further, even if the pressure is higher than 20 MPa, the gas is diffused excessively into the bubbles, and no further improvement is seen in the recovery of shrinkage. Therefore, it is desirable because the labor and labor involved in the processing increase, and consequently the cost increases. Absent.

  The time for performing the high-pressure treatment step is 1 hour or longer, preferably 1 to 48 hours. If the time for performing the high-pressure treatment step is shorter than 1 hour, the gas cannot be sufficiently diffused into the bubbles, which is not desirable. Further, even if the time for performing the high-pressure treatment process is longer than 48 hours, no further improvement is seen in the amount of gas diffusing into the bubbles, which increases the labor and time involved in the treatment, which in turn increases the cost. Absent.

  The foaming agent referred to in the present invention refers to a compound that decomposes upon heating to generate carbon dioxide gas and water vapor, and sodium bicarbonate is particularly preferable.

  Although the embodiment of the present invention has been described above, the specific configuration of each unit is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further in detail, this invention is not limited to these.

  First of all, Examples 1 to 5 of the present invention, which are 15-fold foamed products, and Comparative Examples 1, 2 and 3, were produced as follows.

<Foam production process>
Low-density polyethylene (trade name: Novatec LF441B, density 0.924 g / cm ³ , melt flow rate 2.0 g / 10 min, manufactured by Nippon Polyethylene Co., Ltd.) 100 parts by weight, sodium hydrogen carbonate (trade name: Cellbon FE507, Eiwa Chemical Industries) 13.5 parts by weight, 0.6 parts by weight of dicumyl peroxide, n-butyl 4,4-bis (t-butylperoxy) valerate (trade name: Perhexa V-40, NOF Corporation) A composition comprising 1.4 parts by weight and 0.5 parts by weight of stearic acid was kneaded with a 115 ° C. roll and kneaded into a mold (23.8 × 168 × 168 mm) in a press heated to 160 ° C. The product was filled and heated under pressure for 60 minutes to form an intermediate foam.

  Next, the intermediate foam is placed in a mold (46.5 × 350 × 350 mm) in a press heated to 140 ° C. at approximately the center and heated under pressure for 20 minutes to leave the remaining foaming agent and crosslinking agent. Decomposition yielded a 15-fold foam.

<Shrinking process>
By placing the foam produced by the above-mentioned foam production process for 24 hours or more at normal pressure and normal temperature, shrinkage was promoted together with cooling of the foam.

<High pressure treatment process>
A test piece of 3 cm × 1 cm was cut out from the foam obtained through the shrinking step, placed in a high-pressure vessel, heated to 60 ° C., pressurized to 5 MPa with high-pressure air, and held for 21 hours after stabilization of temperature and pressure. did. Thereafter, the atmosphere was released, and the foam was taken out of the pressure vessel, and Example 1 was obtained. Thereafter, the expansion ratio with respect to the elapsed time was measured.

  In the high-pressure treatment step in Example 1, the heating temperature was set to 35 ° C. as Example 2.

  In the high-pressure treatment step in Example 1, the heating temperature was set to 90 ° C. as Example 3.

  In the high pressure treatment step in Example 1 described above, Example 4 was used in which the high pressure air pressure was 10 MPa.

  In the high pressure treatment step of Example 1 above, Example 5 was designated as a holding time of 48 hours.

  Comparative Example 1 was obtained by cutting out a 3 cm × 1 cm test piece from the foam sheet without performing the high-pressure treatment step in Example 1 above.

  In Comparative Example 2, a heating temperature of 105 ° C. in the high pressure treatment step in Example 1 was used.

  After the foam preparation step in Example 2 above, the shrinkage that had not undergone the shrinkage after less than 0.5 hours, in other words, the foam that had not undergone the shrinkage step was subjected to the high pressure treatment step was set as Comparative Example 3. .

  In each of Examples 1 to 5, Comparative Examples 1 and 2, and Comparative Example 3, the density of the foam was measured by an underwater substitution method (based on JIS K-7112 5.1), and the expansion ratio was calculated by the following formula. .

そして第二に、３０倍発泡品である実施例６、比較例４を以下のように作製した。

Secondly, Example 6 and Comparative Example 4 which are 30-fold foamed products were produced as follows.

<Foam production process>
Low-density polyethylene (trade name: Novatec LF441B, density 0.924 g / cm ³ , melt flow rate 2.0 g / 10 min, manufactured by Nippon Polyethylene Co., Ltd.) 100 parts by weight, sodium hydrogen carbonate (trade name: Cellbon FE507, Eiwa Chemical Industries) 30 parts by weight, 0.6 parts by weight of dicumyl peroxide, n-butyl 4,4-bis (t-butylperoxy) valerate (trade name: Perhexa V-40, NOF Corporation) A composition consisting of 4 parts by weight and 0.5 parts by weight of stearic acid was kneaded with a 115 ° C. roll, and the kneaded product was filled into a mold (12 × 105 × 105 mm) in a press heated to 160 ° C. The mixture was heated under pressure for 60 minutes to form an intermediate foam. The foaming ratio of the molded product was 9 times.

  Next, the intermediate foam was placed in the mold (23 × 290 × 290 mm) in the press heated to 145 ° C., and heated under pressure for 40 minutes to decompose the remaining foaming agent and crosslinking agent. 30 times the foam was produced.

<Shrinking process>
By placing the foam produced by the above-mentioned foam production process for 24 hours or more at normal pressure and normal temperature, shrinkage was promoted together with cooling of the foam.

<High pressure treatment process>
A 6 cm × 11 cm test piece was cut out from the obtained foam, put into a high-pressure vessel, heated to 60 ° C., pressurized to 5 MPa with high-pressure nitrogen, and held for 21 hours after stabilization of temperature and pressure. After that, the atmosphere was released and the foam was taken out of the pressure vessel, and Example 5 was obtained. Thereafter, the expansion ratio with respect to the elapsed time was measured.

  Comparative Example 4 was obtained by cutting out a 6 cm × 11 cm test piece from the foam sheet without performing the high pressure treatment step in Example 6.

  In both Example 6 and Comparative Example 4, the density of the foam was calculated from its weight and the volume obtained by dimensional measurement, and the foaming ratio was calculated by the following formula.

実施例１〜６、比較例１〜４の組成、収縮工程条件、高圧処理工程条件を表１にまとめる。

Table 1 summarizes the compositions, shrinkage process conditions, and high-pressure treatment process conditions of Examples 1 to 6 and Comparative Examples 1 to 4.

図１に実施例１〜５及び比較例１、比較例２比較例３の発泡体の経過時間に対する発泡倍率の変化を示す。高圧処理工程品は処理直後を時間０、未処理品は試験片の切り出し時を時間０としている。比較例１は時間経過に対して自然に気泡内外のガスの置換が起こることにより、若干寸法が回復し、１０日程度で９．５倍程度の発泡倍率で安定した。一方、高圧処理工程を行った実施例１は処理直後１４倍程度に膨張したが、その後若干収縮し１０日程度で約１３倍の発泡倍率で安定した。

FIG. 1 shows changes in the expansion ratio with respect to the elapsed time of the foams of Examples 1 to 5 and Comparative Examples 1 and 2 and Comparative Example 3. The high-pressure processed product is time 0 immediately after processing, and the untreated product is time 0 when the test piece is cut out. In Comparative Example 1, due to the natural substitution of the gas inside and outside the bubbles over time, the dimensions were slightly recovered and stabilized at a foaming ratio of about 9.5 times in about 10 days. On the other hand, Example 1 which performed the high-pressure treatment process expanded to about 14 times immediately after the treatment, but then contracted slightly and stabilized at an expansion ratio of about 13 times in about 10 days.

  In Example 5, the treatment time was increased to 48 hours, but the shrinkage recovery effect was not changed. The gas replacement is considered to be almost completed in 24 hours.

  In Example 4, the air pressure was increased to 10 MPa, but the shrinkage recovery effect was improved. This is probably because the amount of air diffused into the bubbles increased.

  In Example 2, the treatment temperature was lowered to 35 ° C., but the degree of shrinkage improvement after treatment was lower than in other examples, and the expansion ratio after 10 days was also reduced to about 11 times. The recovery effect itself was confirmed. On the other hand, in Example 3, the treatment temperature was increased to 90 ° C., which is near the softening point of the resin, but the re-shrinkage after the treatment was suppressed, and the expansion ratio after 10 days was increased to about 15 times. Even if the temperature is raised, the amount of gas in the bubbles does not increase, but this is thought to be because when the bubbles are re-expanded when the atmosphere is released to the atmosphere, the resin softens and the bubbles expand when the temperature is high. In Comparative Example 2, the temperature was further increased to 105 ° C. in the vicinity of the melting point, but the foaming ratio after the treatment was about 7 times lower than that of the untreated. This can be attributed to reasons such as bubble breakage and bubble coalescence due to the high degree of softening. Thus, it can be seen that the higher the treatment temperature is near the softening point, the higher the shrinkage improving effect, but the lower the treatment temperature is, the lower the temperature is.

  In Comparative Example 3, the high pressure treatment step was performed without the shrinkage step after the foam preparation step, but the foaming ratio after the treatment was as low as about 7 times. This is because the foam is put into the high-pressure vessel with sufficient carbon dioxide in the bubbles, so the carbon dioxide concentration in the high-pressure vessel is increased by the carbon dioxide diffused from the bubbles to the outside, and the gas inside and outside the bubbles is replaced. This is thought to be due to insufficient performance. As described above, when the shrinkage step is sufficiently shrunk at room temperature and normal pressure for 24 hours or more, the shrinkage improvement effect by the high pressure treatment step is high. It turns out that it will not improve.

  FIG. 2 shows changes in the expansion ratio with respect to the elapsed time of the foams of Example 6 and Comparative Example 4. As compared with Comparative Example 4 which is about 10 times after a sufficient time has elapsed, Example 6 has an expansion ratio of about 20 times, and it can be seen that the treatment effect can be obtained with a tendency similar to the above even for a relatively large test piece. .

  INDUSTRIAL APPLICABILITY The present invention can be used as a method for producing a polyolefin foam having a low odor and an expansion ratio of 6 times or more using an inorganic foaming agent.

Claims (6)

The foamable composition obtained by kneading after adding a foaming agent and a crosslinking agent, which decomposes when heated to generate a carbon dioxide gas and water vapor to a polyolefin resin, is heated and foamed to produce an initial of 6 times or more. A foam production process for producing a foam having an expansion ratio;
A shrinking step of shrinking the foam produced by this foam production step under normal pressure for a predetermined time;
By treating the foam that has been shrunk in the shrinking step in a high-pressure nitrogen gas or high-pressure air under a temperature condition from 35 ° C. to the softening point of the polyolefin-based resin, the foaming ratio of the foam is the initial foaming ratio. A method for producing a crosslinked polyolefin-based foam, comprising: a high-pressure treatment step for restoring to a value of 2/3 or more.   The method for producing a crosslinked polyolefin foam according to claim 1, wherein the predetermined time is 24 hours or more.   The method for producing a crosslinked polyolefin-based foam according to claim 1 or 2, wherein the pressure in the high-pressure treatment step is 0.2 MPa to 20 MPa.   The method for producing a crosslinked polyolefin-based foam according to claim 1, 2 or 3, wherein the time for performing the high-pressure treatment step is 1 hour or more.   The method for producing a crosslinked polyolefin-based foam according to claim 1, 2, 3, or 4, wherein the foaming agent is sodium hydrogen carbonate.   6 times or more foaming by heating and foaming a crosslinkable foamable composition obtained by kneading after adding a foaming agent and a crosslinking agent, which is a compound that decomposes upon heating to generate carbon dioxide gas and water vapor to a polyolefin resin. After producing a foam having a magnification, after 24 hours or more have elapsed after production as a method for restoring the contracted foam, 35 ° C. to the softening point of the polyolefin resin, high pressure nitrogen gas of 0.2 MPa to 20 MPa or high pressure A method for producing a crosslinked polyolefin-based foam, comprising restoring a foam that has shrunk by treatment in air for 1 to 48 hours.

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