JP4974924B2 - Recycling method for crosslinked polymer - Google Patents

Description

  The present invention relates to a method for recycling a crosslinked polymer, which is difficult to recycle materials because it does not have thermoplasticity, and makes it possible to thermoplasticize a crosslinked polymer dumped in a large amount and to be recycled.

  Polymers having a continuous C—C bond are widely used, including electric wire / cable coating materials, connection material, hot water pipe materials, and heat storage materials, as represented by polyolefin polymers. The degree of branching of C—C bonds including a crosslinked structure is strongly involved in the molding processability of such polymers.

  In particular, since the crosslinked polymer has a three-dimensional network structure and is not melted by heat, it is difficult to recycle the waste material by molding. Therefore, it is difficult to reuse, and most of the materials after use are landfilled or incinerated.

  On the other hand, recently, cross-linked polymers have been studied for the purpose of recycling due to the growing awareness of global environmental conservation and the depletion of resources. One is to pulverize the crosslinked polymer, collect it as a finely divided fuel with good combustion efficiency, and reuse it as a fuel.

  The other is a method (Patent Document 1) in which a pulverized product is heated to a high temperature, oiled by pyrolysis, and recovered as fuel.

  In addition, a method of obtaining a product by mixing a finely powdered product with an uncrosslinked resin so that it can be melted and extrusion-molded has been studied.

  Furthermore, recently, a method of decomposing a crosslinked polymer using supercritical water or subcritical water has also been proposed (Patent Documents 2 and 3).

  However, when the polymer is thermally decomposed by the above-described method, the molecular weight is greatly reduced, and the decomposition reaction proceeds to the low molecular weight wax or oil, and it is difficult to recycle the material to the original polymer before crosslinking.

  On the other hand, as a method of selectively cutting the cross-links, a method of thermoplasticizing a sulfur cross-linked polymer by cutting only a sulfur bond having a low bond energy using a difference in bond energy of chemical bonds, or supercritical Examples include a method in which only a siloxane bond is cleaved using a chemical reaction in alcohol to thermoplasticize a silane crosslinked polymer.

  However, all of these methods are methods of thermoplasticizing a crosslinked polymer by utilizing the difference in the chemical structure of the sulfur bond or siloxane bond part that forms a crosslink and the part that forms the main chain of the polymer. A crosslinked polymer crosslinked by a technique such as oxide crosslinking or electron beam crosslinking is crosslinked with the same CC bond as the chemical bond contained in the main chain of most polymers.

  For this reason, since the chemical bond between the cross-linked portion and the main chain of the polymer is common, it is difficult to recycle the thermoplastic polymer by preferentially cutting the cross-link by the above method.

JP-A-10-160149 JP-A-6-279762 Japanese Patent Laid-Open No. 10-24274 Japanese Patent No. 3855006

  In addition, as disclosed in Patent Document 4, the present inventor has proposed a method in which a waste polymer or the like is made into supercritical carbon dioxide, and the waste polymer is made into a medium or small molecule by an oxidative decomposition reaction with nitrogen oxides. .

  If it is attempted to thermoplasticize the crosslinked polymer by this method, the product causes a significant decrease in molecular weight, so that the use as a polymer material is limited and recycling is difficult.

  As described above, in the crosslinked polymer, since there is little difference in chemical bond energy between the crosslinked portion and the main chain of the polymer, in the above method, the crosslinking is preferentially cut and recycled as a thermoplastic polymer. difficult.

  In view of this, the present inventor disclosed in Japanese Patent Application No. 2006-213554 (Title of Invention: Recycling Method for Crosslinked Polymer) that the crosslinked polymer is cut in supercritical carbon dioxide and the C—C bond is broken by oxidative decomposition reaction with nitrogen oxides. In recycling, a method of preferentially oxidizing the C—C bond branching point of the cross-linked polymer by maintaining it at 100 ° C. or lower for 10 hours or longer, or cutting the C—C bond, or reacting the cross-linked polymer with a reaction vessel After being accommodated in the inside, the air in the reaction vessel is replaced with carbon dioxide, and then nitrogen oxides and carbon dioxide are added to the reaction vessel, and the reaction vessel is kept below the supercritical pressure of carbon dioxide to crosslink the polymer. Then, nitrogen oxides are sorbed (added) to carbon dioxide, and then carbon dioxide is maintained at a supercritical pressure or higher to react nitrogen oxides with the crosslinked polymer to preferentially oxidize the C—C bond branch point. -C bond cut How to proposed.

  However, the recycled polymer obtained in this manner has a problem of yellowing when it is obtained, and when heated to a high temperature, it has a problem of becoming a darker color and foaming and gelling.

  In order to obtain a molded product satisfying more complex and high required properties, it is necessary to selectively cut the C—C bond branch point to improve the processability of a polymer that has been difficult to process.

  The present invention has been made in view of such circumstances, and it is possible to recycle by cross-linking a cross-linked polymer that has been difficult to recycle and has been landfilled or incinerated, and that is C-C. By preferentially decomposing the branch point of the bond, the cross-linked part is preferentially cut and the high molecular weight component of the polymer before cross-linking is maintained, while the cross-linked polymer is thermoplasticized for material recycling. It is possible.

  That is, an object of the present invention is to provide a method for recycling a crosslinked polymer that is capable of being recycled by thermoplasticizing the crosslinked polymer and that is not foamed or discolored even when thermoplasticized.

In order to achieve the above object, according to the invention of claim 1, after the crosslinked polymer is selectively activated with a nitrogen oxide , the crosslinked polymer is crosslinked with hydrogen peroxide in liquid or supercritical carbon dioxide. A method for recycling a crosslinked polymer, characterized in that a thermoplastic polymer is obtained by oxidizing and decomposing a part.

In the invention of claim 2, the nitrogen oxide is nitrogen dioxide and / or dinitrogen tetroxide, and the crosslinking part is selectively controlled by controlling the pressure and temperature to control the reactivity of the nitrogen oxide. a recycling method of the crosslinked polymer of claim 1 wherein the activation.

In the invention of claim 3 , after the cross-linked polymer is accommodated in the reaction vessel, nitrogen oxide and carbon dioxide are added to the reaction vessel, and the reaction vessel is maintained at a supercritical pressure of carbon dioxide or less to form nitrogen in the cross-linked polymer. An oxide is added, and then carbon dioxide is maintained at a supercritical pressure or higher, and hydrogen peroxide and a crosslinked polymer are reacted to preferentially oxidize the C—C bond branch point to break the C—C bond. This is a method for recycling a crosslinked polymer.

A fourth aspect of the present invention, when reacting the hydrogen peroxide with cross-linked polymer, the concentration to the volume of the reaction vessel of the hydrogen peroxide, according to claim 1 to 3 is 0.2g / L~6.0g / L The method for recycling a crosslinked polymer according to any one of the above.

In the invention of claim 5 , the temperature of the step of adding nitrogen oxides and carbon dioxide to the reaction vessel, and maintaining the inside of the reaction vessel below the supercritical pressure of carbon dioxide to add nitrogen oxides to the crosslinked polymer is 100 ° C. It is the following, It is the recycling method of the crosslinked polymer in any one of Claims 3-4 .

The invention of claim 6, crosslinked polymer, peroxide crosslinking, electron beam crosslinking, crosslinked by silane water crosslinking, any of claims 1 to 5 is a polyolefin or an ethylene copolymer containing a tertiary carbon and the quaternary carbon A method for recycling a crosslinked polymer as described above.

  The present invention preferentially decomposes a crosslinked portion of a polymer to enable recycling, and preferentially decomposes a portion where a carbon bond forming a crosslinked bond is branched. Crosslinking can be preferentially decomposed, the high molecular weight component of the polymer before cross-linking is not completely lost, and it is not discolored, foamed or gelled under molding processing conditions, so the resulting recycled resin is material recycled as a polymer. And exhibits an excellent effect that its industrial value is remarkably high.

  Hereinafter, a preferred embodiment of the present invention will be described in detail.

  In the present invention, a cross-linked polymer is contained in a reaction vessel, carbon dioxide is maintained at a supercritical pressure or higher, hydrogen peroxide and the cross-linked polymer are reacted to preferentially oxidize the C—C bond branch point. It is possible to provide a recycled polymer that does not cause discoloration, foaming, or gelation even at high temperatures by breaking the -C bond, and provides a method for recycling a crosslinked polymer that realizes material recycling by thermoplasticizing the crosslinked polymer. Is.

In the invention of the prior application using NO ₂ as an oxidizing agent, we found that the cause of foaming, crosslinking and discoloration was caused by nitrogen oxides chemically bonded to the polymer.

  Therefore, in the present invention, a product in which the product does not foam at a high temperature can be produced by oxidizing the polymer by a method that does not use nitrogen oxide for the supercritical reaction.

  That is, the present invention is a method for recycling a crosslinked polymer in which a crosslinked polymer is obtained by oxidative decomposition of a crosslinked part using hydrogen peroxide in liquid or supercritical carbon dioxide.

  The present invention includes a step of adding a cross-linked polymer to a reaction vessel, adding nitrogen oxide, and adding the nitrogen oxide to the cross-linked polymer, and reacting the added cross-linked polymer with hydrogen peroxide in supercritical carbon dioxide. The reaction for cleaving the bond may be performed separately.

  That is, the cross-linked polymer is put into a reaction vessel, nitrogen oxide and carbon dioxide are added to the reaction vessel, and the reaction vessel is kept below the supercritical pressure of carbon dioxide to add nitrogen oxide to the cross-linked polymer. Then, after removing excess nitrogen oxides from the reaction vessel, carbon dioxide and hydrogen peroxide are put in the reaction vessel, and the hydrogen peroxide and the cross-linked polymer are reacted while maintaining carbon dioxide supercritically. The -C bond branching point (particularly in the case of having a bridge structure) may be preferentially oxidized to cut the C-C bond.

  Nitrogen oxides include nitrogen dioxide, dinitrogen tetroxide, nitric oxide, dinitrogen monoxide, dinitrogen trioxide, etc., which may be used alone or in combination. Further, it may be used in combination with oxygen, ozone, hydrogen peroxide, sulfur dioxide, etc., among which nitrogen dioxide or dinitrogen tetroxide is preferable.

  In the oxidative decomposition reaction with hydrogen peroxide, metal catalysts such as Ru, Rh, Pd, Pt, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, benzoyl peroxide, azobisisobutyronitrile, N -You may react by adding radical initiators, such as a hydroxyphthalimide, or organic acids, such as formic acid and an acetic acid.

  The reaction temperature at this time is preferably kept below the thermal decomposition temperature or depolymerization temperature. The heat decomposition temperature or depolymerization temperature or lower is 360 ° C. or lower. More preferably, the reaction temperature is 100 ° C. or more and the reaction time is 3 hours or less.

  The crosslinked polymer of the present invention is a polymer having a continuous C—C bond, and the C—C bond branch point is a polyolefin or ethylene copolymer having a chemical structure in which the C—C bond branch point is crosslinked by peroxide crosslinking, electron beam crosslinking, or silane water crosslinking. It is a polymer.

  The polymer having a continuous C—C bond is a polymer typified by polyethylene, and the branch point of the C—C bond refers to, for example, a branch point between a side chain and a main chain of polyethylene or a cross-linked part.

  Generally, it is difficult to preferentially cleave from a bond with a secondary or tertiary carbon only by the difference in the degree of substitution of one carbon of a C—C bond (ie, primary, secondary, or tertiary carbon). .

However, in the present invention, NO ₂ radicals are first dispersed in carbon dioxide gas, whereby the reaction can be preferentially performed from the combination with the secondary or tertiary carbon.

  This is considered to utilize the fact that the secondary or tertiary carbon radical is more stable than the primary carbon radical.

  It is considered that such a reaction can be used for thermoplasticizing a crosslinked polymer that is crosslinked by peroxide crosslinking or electron beam crosslinking and has a C—C bond in the crosslinked structure.

  That is, as shown in Chemical Formula 1, the structure of the part forming the cross-link has quaternary carbon in the polymer molecular chain (main chain) of the secondary carbon. Since the tertiary carbon radical is more stable than the primary carbon radical, it is considered that the C—C bond at the bridge portion is cleaved to generate a radical.

  Therefore, it is expected that the oxide is added by preferentially reacting the C—C bond branching point of the cross-linked structure. As a result, the added carbon is activated and easily oxidized, and the cross-linked structure Is preferentially cleaved, so that it is possible to recycle the crosslinked polymer as a regenerated polymer while minimizing degradation of the polymer main chain, that is, deterioration.

  Further, in the present invention, for example, when a alkoxysilane is grafted to a polymer using vinylsilane and then crosslinked by a condensation reaction of silanol groups in the presence of moisture, a C-C bond branch point is generated. The present invention can be used effectively.

  Furthermore, it is considered that the C—Si bond can be selectively cleaved under the reaction conditions of the present invention because the C—Si bond has a lower binding energy than the C—C bond in the silane water-crosslinked polymer.

  For this reason, it is possible to preferentially cut the crosslink even when, for example, those crosslinked with vinylsilane and peroxide crosslinks are mixed with each other.

Nitrogen dioxide (NO ₂ ) is a substance in a chemical equilibrium state with dinitrogen tetroxide (N ₂ O ₄ ), and since the equilibrium can be controlled by pressure and temperature, the reaction can be easily controlled by using these substances.

  Since carbon dioxide has a critical pressure of 7.38 MPa and a critical temperature of 31.1 ° C., it has a low critical point, is easy to handle as a supercritical fluid, and is more permeable and diffusible than liquid. It is effective in carrying out a selective decomposition reaction because it efficiently transports hydrogen peroxide dissolved in the solution.

  Therefore, it is more desirable to react under the reaction conditions in which hydrogen peroxide solution dissolves in supercritical carbon dioxide, and the hydrogen peroxide solution becomes excessive and cannot be completely dissolved in supercritical carbon dioxide. When the polyethylene and the cross-linked polyethylene react with each other, the permeability is inferior and the reaction efficiency is lowered. That is, considering the solubility in supercritical carbon dioxide, the concentration of hydrogen peroxide with respect to the volume in the reaction vessel is preferably 0.2 g / L to 6.0 g / L.

To material recycling, the additional temperature 100 ° C. of nitrogen oxides, more preferably preferably to 60 ° C. or less, thereby preferentially added is NO ₂ radicals to tertiary or quaternary carbon that It is thought that the carbon bond is likely to be broken.

In addition, once the cross-linked polymer to which the NO ₂ radical has been added is taken out to remove excess nitrogen oxides, the branch point of the C—C bond is cut by heating in supercritical carbon dioxide to which hydrogen peroxide has been added. .

  Further, when the reaction temperature with hydrogen peroxide is equal to or higher than the thermal decomposition temperature of the C—C bond, the polymer is randomly decomposed, so that the molecular weight is remarkably lowered, and the parts other than the cross-linked bonds are decomposed. Therefore, since the mechanical strength, elongation, and the like of the polymer are remarkably reduced, it is difficult to reuse the recycled material as a polymer. Therefore, the reaction temperature is preferably 100 ° C. or higher and 360 ° C. or lower, and preferably 180 ° C. or lower.

  In order to efficiently cut the cross-linked bond, the cross-linked polymer may be supplied in the form of pulverized pellets or powder.

  In order to promote decomposition by hydrogen peroxide, two or more peroxides or nitrogen oxides may be mixed with hydrogen peroxide, or an inert gas other than carbon dioxide may be mixed.

  Here, the polymer having a continuous C—C bond is, for example, a polyolefin such as polyethylene or polypropylene, a chlorinated polyethylene, an ethylene-vinyl acetate copolymer, an ethylene-ethyl acrylate copolymer, an ethylene- Examples thereof include ethylene copolymers such as propylene rubber and ethylene-octene rubber.

  Examples of the present invention and comparative examples will be described below.

Example 1:
Peroxide-crosslinked polyethylene having a gel fraction of 85% (base; low-density polyethylene) was molded into a 1 mm sheet shape, and pulverized into pellets of 2 to 3 mm. After filling 0.5 g of this pellet into a 50 ml autoclave, the air in the autoclave was replaced with carbon dioxide, and then 30% hydrogen peroxide (1.1 g) and carbon dioxide gas were added, and Table 1 was added. Reaction with peroxide-crosslinked polyethylene at a reaction temperature (160 ° C.), a pressure (10 MPa), and a time (3 hours).

Examples 2-6:
Peroxide-crosslinked polyethylene having a gel fraction of 85% (Example 6 is silane-crosslinked polyethylene having a gel fraction of 65%) was molded into a 1 mm sheet shape and pulverized into pellets of 2 to 3 mm. After filling 0.5 g of this pellet into a 50 ml autoclave, the air in the autoclave was replaced with carbon dioxide, and then NO ₂ (1.1 g) and carbon dioxide gas were added, and 60 ° C. for 1 hour at 3 MPa. NO ₂ was added to the peroxide crosslinked polyethylene. Then, after removing excess NO ₂ under reduced pressure, 30% hydrogen peroxide (0.1 to 0.5 g) was added and the temperature shown in Table 1 in carbon dioxide (120 ° C., 110 ° C.), It was heated and pressurized again under pressure (10 MPa) and time (3 hours, 1 hour in Example 6), and reacted with hydrogen peroxide.

Example 7:
Instead of the peroxide cross-linked polyethylene in Example 2, electron beam cross-linked polyethylene was used.

Example 8:
The reaction was carried out by increasing the amount of 30% hydrogen peroxide water in Example 2 to 1.1 g.

Example 9:
The amount of 30% hydrogen peroxide solution in Example 2 was reduced to 0.03 g for reaction.

Comparative Examples 1 and 2:
Instead of using hydrogen peroxide water in the reaction step in Example 2, NO ₂ (0.5 g) was used as an oxidant, Comparative Example 1 was conducted at a reaction temperature of 120 ° C., and Comparative Example 2 was conducted at 140 ° C. It was.

  After the reaction, the reaction vessel was cooled, the polymer was recovered, and the gel fraction serving as an index of molecular weight distribution and degree of crosslinking was measured.

  These measurement conditions are as follows.

  The molecular weight distribution was measured using high temperature GPC (gel permeation chromatography) using o-dichlorobenzene as a solvent. As a result, even when the number average molecular weight of the recovered product was lowered, a case where a high molecular weight component of 300,000 or more remained was marked with ◯, and a case where a high molecular weight component did not remain was marked with x.

  In accordance with JIS C3005, the gel fraction was obtained by immersing the sample after the reaction in 110 ° C. xylene for 24 hours, vacuum-drying the remaining sample, and obtaining the ratio from the initial weight.

  As for moldability, the obtained polymer was heated to 200 ° C., and the change of the polymer at that time was observed.

  Regarding discoloration, the white polymer was discolored yellow and the undiscolored material was marked with ◯.

  As for foaming, the case where bubbles could be confirmed with the naked eye was marked with x, and the case where bubbles could not be confirmed was marked with ◯.

  As for gelation, x indicates that the material does not undergo plastic deformation by applying force with tweezers, and ○ indicates that it does not.

  Table 1 shows the experimental conditions and results of Examples 1 to 9 and Comparative Examples 1 to 3.

  In Examples 1 to 9, the gel fraction was 40% or less, and foaming, discoloration, and gelation did not occur.

Comparing Example 1 and Examples 2 to 6, when NO ₂ was added to peroxide-crosslinked polyethylene (Examples 2 to 6), the amount of hydrogen peroxide added was small and the reaction temperature was low. (110, 120 ° C.), the gel fraction can be lowered.

  However, Example 8 was obtained by adding hydrogen peroxide water in excess of Examples 2 and 3. However, the decomposition of cross-linking was less likely to occur than in Examples 2 and 3, and the amount of hydrogen peroxide was set to Example 2. In the case of Example 9, which was significantly reduced from 3, the reaction did not proceed sufficiently and the gel fraction of the product was high. Therefore, the amount of hydrogen peroxide water is 0.03 g (the concentration of hydrogen peroxide with respect to the volume in the reaction vessel is 0.18 g / L) and 1.1 g (the concentration of hydrogen peroxide with respect to the volume in the reaction vessel is 6.6 g). / L), it is understood that the reaction is difficult to proceed. Therefore, the concentration of hydrogen peroxide with respect to the volume in the reaction vessel is preferably in the range of 0.2 g / L to 6.0 g / L.

  In Comparative Examples 1 and 2, nitrogen dioxide was used as the oxidizing agent. As a result, the crosslinking is not cut at a reaction temperature of 120 ° C., so the gel fraction is high. On the other hand, when reacted at 140 ° C., the gel fraction can be reduced to 0% and the crosslinking can be cut, but the product is heated to the molding conditions. As a result, problems such as foaming, discoloration, and gelation occurred and processing conditions could not be set freely.

  As described above, the present invention enables recycling by preferentially decomposing a cross-linked portion of a cross-linked polymer which has been difficult to recycle and has been landfilled or incinerated in large quantities, and the carbon bond forming the cross-linked bond is branched. Disassemble the part that has been preferentially disassembled. Since the cross-linking can be preferentially decomposed, the high molecular weight component of the polymer before cross-linking is kept completely, and even when heated to a high temperature, no discoloration, foaming or gelation occurs. Material recycling is possible as a polymer, and its industrial value is remarkably high.

Claims (6)

After selectively activating the cross-linked portion of the cross-linked polymer with nitrogen oxides, the cross- linked portion is oxidatively decomposed with hydrogen peroxide in liquid or supercritical carbon dioxide to obtain a thermoplastic polymer. A method for recycling a cross-linked polymer characterized. The nitrogen oxides are nitrogen and / or nitrogen tetroxide dioxide, that by controlling the pressure and temperature to control the reactivity of the nitrogen oxides, according to claim 1, wherein selectively activating the bridge Recycling method for crosslinked polymer.   After storing the crosslinked polymer in the reaction vessel, nitrogen oxide and carbon dioxide are added to the reaction vessel, and the reaction vessel is kept below the supercritical pressure of carbon dioxide to add nitrogen oxide to the crosslinked polymer. A crosslinked polymer characterized in that carbon dioxide is maintained at a supercritical pressure or higher and hydrogen peroxide and the crosslinked polymer are reacted to preferentially oxidize the CC bond branch point to break the CC bond. Recycling method. The cross-linking according to any one of claims 1 to 3, wherein when hydrogen peroxide and the cross-linked polymer are reacted, the concentration of hydrogen peroxide with respect to the volume in the reaction vessel is 0.2 g / L to 6.0 g / L. Polymer recycling method. Nitrogen oxides and carbon dioxide was added to the reaction vessel, according to claim 3 temperature steps of the reaction vessel is added the nitrogen oxides in the crosslinked polymer is held under the supercritical pressure of carbon dioxide is 100 ° C. or less 5. The method for recycling a crosslinked polymer according to any one of 4 above. The crosslinked polymer according to any one of claims 1 to 5 , which is a polyolefin or ethylene copolymer containing tertiary carbon and quaternary carbon crosslinked by peroxide crosslinking, electron beam crosslinking, or silane water crosslinking. Recycling method.