Method for producing foamable polypropylene

Abstract

To provide a method for producing a foamable polypropylene which can be suitably used even in applications requiring both high expansion ratio and high rigidity (for example, foam to be mounted in vehicles).SOLUTION: A crosslinked polypropylene mixed with an additive is thermally plasticized and then kneaded with a non-crosslinked polypropylene. The additive is a peroxide that imparts foamability to the thermally plasticized crosslinked polypropylene and has a one-minute half-life at 150°C or higher. The foamable polypropylene obtained thereby exhibits high expansion ratio and high rigidity.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for thermoplasticizing crosslinked polypropylene to produce expandable polypropylene. [Background technology]

[0002] Polypropylene (abbreviated as PP) is a thermoplastic resin made by polymerizing propylene monomer. Polypropylene boasts the highest heat resistance of all general-purpose resins, and is characterized by its relatively high strength, excellent chemical resistance (including acid and alkali resistance), and low moisture absorption. As a result, it has a wide range of uses, including stationery, banknotes, automotive parts, packaging materials, textile products, plastic parts, and various containers.

[0003] Cross-linked polypropylene is polypropylene that has been treated by adding a cross-linking agent or by irradiating it with electron beams to form intermolecular bonds between the polymer molecular chains into a three-dimensional mesh structure. Cross-linked polypropylene has improved heat resistance and impact resistance compared to regular polypropylene, and is used in automobile and aircraft parts. However, cross-linked polypropylene does not melt even when heated, making it extremely difficult to recycle.

[0004] Therefore, in order to recycle cross-linked polypropylene, research is being conducted on thermoplasticization technology, which involves applying heat to cross-linked polypropylene to melt it. Thermoplasticization is a technology that applies appropriate heat and shear stress to cross-linked polypropylene to destroy the cross-linked structure, lowering the molecular weight and allowing it to melt when heated. However, it is difficult to properly cut the cross-linking points of cross-linked polypropylene, and simply performing thermoplasticization at high temperature and high shear force results in a decrease in physical properties such as tensile strength and rigidity.

[0005] Furthermore, since polypropylene is not prone to long chain branching, polypropylene produced by conventional methods has an expansion ratio of 200% or less even when heated with the addition of a blowing agent. For industrial use, a high expansion ratio is desired, and a ratio of 300% or more is often preferable, so various methods have been proposed to produce highly expandable polypropylene.

[0006] Known methods for producing expandable polypropylene include those described in Patent Documents 1 to 3, for example. Patent Document 1 discloses a modified polypropylene composition having appropriate fluidity and high melt tension. Patent Document 2 discloses a polyolefin-based resin composition that has properties such as heat resistance and recyclability and can be used to produce high-expansion foams, and a method for producing the same. Patent Document 3 discloses a method for providing a propylene-based resin composition for flexible foams that can produce polypropylene-based flexible foams with excellent flexibility and heat resistance.

[0007] The modified polypropylene composition described in Patent Document 1 comprises a non-crosslinked polypropylene and a weakly crosslinked polypropylene having a melt flow rate in the range of 0.1 to 10 g / 10 min and a gel fraction of 0.01 to 25% by weight or less as determined by boiling paraxylene extraction. The non-crosslinked polypropylene and the weakly crosslinked polypropylene are contained in an amount of 99 to 1% by weight and 1 to 99% by weight, respectively.

[0008] In Patent Document 2, a powdered polyolefin resin (E) is mixed with a foaming polyolefin resin crosslinked composition (D). The polyolefin resin crosslinked composition (D) is obtained by melt-kneading a polyolefin resin (A), a polyolefin resin (B) having a crystalline melting peak (melting point Tmb) of 100°C or higher (polyolefin resins include polypropylene (paragraph 0013)), and a thermally decomposable chemical foaming agent (C). The resulting mixture is then pulverized into powder (Abstract, Claim 7,

[0028] ).

[0009] Furthermore, the propylene-based resin composition for flexible foams described in Patent Document 3 contains 50 to 90 parts by weight of a propylene-based random block copolymer (A) that is polymerized in the presence of a metallocene catalyst and is a propylene-based random block copolymer having a melt flow rate of 0.1 to 10 g / 10 min and a melting point in the range of 100 to 155°C, and that is composed of 90 to 30 wt% of a portion insoluble in n-decane at room temperature and 10 to 70 wt% of a portion soluble in n-decane at room temperature, and 50 to 10 parts by weight of a modified polypropylene (B) having a melt tension in the range of 4 to 30 g.

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-60563 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-26937 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-84304 Summary of the Invention [Problem to be solved by the invention]

[0011] However, the expansion ratio of the modified polypropylene composition obtained by the method described in Patent Document 1 is low, at 180 to 200%, as shown in Table 3 in the specification, and is not a polypropylene with a high expansion ratio of 300% or more.

[0012] Furthermore, the polyolefin resin composition obtained by the method described in Patent Document 2 contains polymer compounds with lower rigidity than polypropylene, such as ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, ethylene-diene copolymer, ethylene-propylene-diene terpolymer, ethylene-octene copolymer, and low-density polyethylene (Patent Document 2 (0012), (0043) [Example 1]). Therefore, it is suggested that the rigidity is lower than that of polypropylene alone.

[0013] Furthermore, the propylene-based resin composition for flexible foams described in Patent Document 3 is a propylene-based random block copolymer polymerized in the presence of an expensive metallocene catalyst, and the use of an expensive catalyst results in high production costs. Furthermore, the propylene-based random block copolymer is a copolymer of propylene and ethylene (Patent Document 3 (0019)), and since it contains polyethylene, which has lower rigidity than polypropylene, it is suggested that the rigidity is lower than that of polypropylene alone.

[0014] The present invention has been made to overcome the above-mentioned disadvantages, and aims to provide a method for producing expandable polypropylene having a high expansion ratio and high rigidity. Another object of the present invention is to provide a method for producing expandable polypropylene inexpensively while promoting material recycling by using crosslinked polypropylene, which has conventionally been discarded, as a raw material. The use of this expandable polypropylene makes it suitable for applications requiring both a high expansion ratio and high rigidity (e.g., automotive foams). [Means for solving the problem]

[0015] The method for producing expandable polypropylene according to the present invention comprises a mixing step of mixing a crosslinked polypropylene with an additive to produce a mixture, a thermoplasticizing step of feeding the mixture from a first input section of an extruder and applying heat and shear stress to thermoplasticize the crosslinked polypropylene to produce a thermoplastic product, and a kneading step of feeding non-crosslinked polypropylene from a second input section of the extruder and kneading the thermoplastic product and the non-crosslinked polypropylene by applying heat, wherein the amount of the additive in the mixing step is 0.1 to 5 parts by weight relative to 100 parts by weight of the crosslinked polypropylene, the additive is a peroxide that imparts expandability to the thermoplasticized crosslinked polypropylene and has a 1-minute half-life of 150°C or more, and the shear stress in the thermoplasticizing step is 5,000 to 15,000 s -1 The method is characterized in that the shear rate is generated for 5 to 30 seconds at a shear rate of 100 to 150 parts by weight of the non-crosslinked polypropylene added in the kneading step.

[0016] The additive is also characterized in that it contains at least one peroxide selected from t-butyl peroxylaurate, t-butylperoxyisopropyl monocarbonate, t-hexyl peroxybenzoate, t-butyl peroxybenzoate, dicumyl peroxide, di-t-hexyl peroxide, t-butylcumyl peroxide, di-t-butyl peroxide, dibutyl peroxide, di(2-t-butylperoxyisopropyl)benzene, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane.

[0017] Furthermore, the expandable polypropylene produced by the kneading step is characterized by exhibiting an expansion ratio of 300% or more when foamed at 200°C for 3 minutes with the addition of a foaming agent. [Effects of the Invention]

[0018] According to the present invention, crosslinked polypropylene is thermoplasticized using a peroxide with a one-minute half-life of 150°C or more, and then mixed with non-crosslinked polypropylene to obtain expandable polypropylene with a high expansion ratio and high rigidity at low cost. The expandable polypropylene thus obtained can be expanded by adding a blowing agent and used for applications requiring a high expansion ratio and high rigidity, such as automotive interior materials and protective materials for automotive air conditioning ducts. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a cross-sectional schematic view showing the internal structure of a twin-screw extruder according to an embodiment of the present invention. [Figure 2] FIG. 1 is a partial cross-sectional plan view of two screws arranged inside a cylinder of a twin-screw extruder according to an embodiment of the present invention. [Figure 3] 1 is a process diagram of a method for producing expandable polypropylene according to an embodiment of the present invention. [Figure 4] FIG. 1 is a cross-sectional view of a foam obtained by adding a foaming agent to the expandable polypropylene according to Example 1 of the present invention and foaming it at 200° C. for 3 minutes. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0021] <Device Description> The thermoplasticization of cross-linked polypropylene is carried out using an extruder, which can be broadly classified into single-screw extruders with only one screw and twin-screw extruders with two screws. Compared to single-screw extruders, twin-screw extruders have advantages such as superior uniform mixing efficiency of different components such as additives and fillers, reduced heat generation due to less frictional heat, and a more complex internal material flow that allows greater shear stress to be applied to the material. For this reason, tests were conducted using a twin-screw extruder in this invention.

[0022] Fig. 1 is a cross-sectional schematic diagram showing the internal structure of a twin-screw extruder 10 according to an embodiment of the present invention. Fig. 2 is a partial cross-sectional plan view showing two screws 13 arranged inside a cylinder 12 of the twin-screw extruder 10.

[0023] As shown in FIG. 1 , a twin-screw extruder 10 includes a cylinder 12 extending from a drive section 14 and a first input section 16 and a second input section 24 for inputting cross-linked polypropylene or the like. The cylinder 12 is provided with a first conveying section 18, a first kneading section 20, a second conveying section 26, a second kneading section 28, and a third conveying section 32, in that order. Each of the first conveying section 18, the first kneading section 20, the second conveying section 26, the second kneading section 28, and the third conveying section 32 has two paired screws 13. The screws in the first conveying section 18, the second conveying section 26, and the third conveying section 32 have a shape suitable for conveying. The screw in the first kneading section 20 has a shape suitable for thermoplasticization, and the screw in the second kneading section 28 has a shape suitable for kneading.

[0024] The drive unit 14 is provided with a motor (not shown), which is connected to the screw of the first conveying section 18, and each screw is connected to an adjacent screw. Therefore, when the motor in the drive unit 14 is driven, the screws of the first conveying section 18, the first kneading section 20, the second conveying section 26, the second kneading section 28, and the third conveying section 32 rotate simultaneously. Therefore, the screws of the first conveying section 18, the first kneading section 20, the second conveying section 26, the second kneading section 28, and the third conveying section 32 rotate at the same speed. Furthermore, a heating means (not shown), such as a heater, is provided to maintain the interior of the cylinder 12 at a predetermined temperature. Note that in FIG. 1, the first conveying section 18, the second conveying section 26, and the third conveying section 32 are indicated by hatching, and the first kneading section 20 and the second kneading section 28 are indicated by a group of small squares. However, these are merely for the convenience of indicating the positions of the respective sections.

[0025] A first input section 16 is provided above the first transport section 18, and the mixture mixed outside the twin-screw extruder 10 is input from the first input section 16, heated and melted in the first transport section 18 heated by a heater or the like, and transferred to the first kneading section 20.

[0026] The first kneading section 20 has a narrow screw and cylinder width to generate high shear stress. A first degassing section 22 is provided above the first kneading section 20, and the mixture transferred from the first conveying section 18 is subjected to shear stress and heat in the first kneading section 20 to thermoplasticize the cross-linked polypropylene. The gas generated during this process passes through the first degassing section 22 and is discharged to the outside of the twin-screw extruder 10. The thermoplastic mixture is then transferred to the second conveying section 26.

[0027] Here, shear stress τ is the stress that acts when one surface inside an object is deformed in a parallel direction. If the force applied to deform is F and the area of ​​the deformed surface is A, shear stress is expressed as F / A, and its unit is τ (Pa Pascal) = F (N Newton) / A (area m 2 )

[0028] Also, the shear stress τ (Pa) is proportional to the shear rate γ (s -1 ) and the viscosity μ (Pa·s) of polypropylene. In other words, τ = μγ. The unit of shear rate γ is expressed in reciprocal seconds (1 / s). Here, the viscosity μ of polypropylene is determined by the type of polypropylene and the temperature, so the shear stress τ is generally controlled by the shear rate.

[0029] Here, the shear rate γ(s -1 ) is expressed as γ=v / t, where t (mm) is the clearance between the extruder cylinder and the screw and v (mm / s) is the speed at the tip of the screw. The speed at the tip of the screw, v, is v=πND / 60, where N (rpm) is the screw rotation and D (mm) is the diameter. In other words, the shear rate is expressed as γ=πND / 60t, where π represents the constant of the circumference of the circle. According to the shear rate formula, the larger the clearance, the smaller the shear rate; conversely, the smaller the clearance, the larger the shear rate. Since the clearance of the first kneading section 20 is smaller than that of the second kneading section 28, the shear rate of the first kneading section 20 is set to be higher than that of the second kneading section 28.

[0030] The second transport section 26 is connected to the first kneading section 20 and the second kneading section 28, and a second input section 24 is provided above the second transport section 26. The solid material input from the second input section 24 is melted in the second transport section 26 heated by a heater or the like, and is transferred to the second kneading section 28 together with the mixture transferred from the first kneading section 20.

[0031] The second kneading section 28 is connected to the second conveying section 26 and the third conveying section 32, and has two screws that perform mixing and kneading. The second kneading section 28 can uniformly knead the thermoplastic mixture transferred from the second conveying section 26 with the input material. The kneaded expandable polypropylene is then transferred to the third conveying section 32. Note that "mixing" here generally refers to mixing two or more materials. Also, "kneading" refers to mixing at least one material with a relatively high viscosity (here, thermoplastic cross-linked polypropylene in a molten state) with another material (here, non-cross-linked polypropylene in a molten state) and then kneading them.

[0032] The third conveying section 32 is connected to the second kneading section 28 and the discharge section 34. A second degassing section 30 is provided above the third conveying section 32, and is configured to discharge gas generated inside the cylinder 12 to the outside. The expandable polypropylene transferred from the second kneading section 28 passes through the third conveying section 32 and is discharged to the outside from the discharge section 34.

[0033] <Process Description> Next, a method for producing expandable polypropylene according to an embodiment of the present invention will be described with reference to Fig. 3. Fig. 3 is a process diagram of the method for producing expandable polypropylene according to an embodiment of the present invention.

[0034] (Preparation process: S1) At the start, the twin-screw extruder 10 is powered on. This causes the motor in the drive unit 14 of the twin-screw extruder 10 to rotate, which in turn causes the screws in the first conveying section 18, the first kneading section 20, the second conveying section 26, the second kneading section 28, and the third conveying section 32, all of which are connected to the drive unit 14, to rotate. At the same time, the inside of the cylinder 12 is heated to a predetermined temperature by the heating mechanism, and this temperature is maintained for a predetermined time until the device is sufficiently stabilized.

[0035] As mentioned above, the shear rate is expressed by the formula γ = πND / 60t, and the shear rate in the first kneading section is 5,000 to 15,000 s -1 , preferably 6,000 to 8,500 s -1The screw rotation speed is set to 5,000 s in the first kneading section. -1 At shear rates below 15,000 s, thermoplasticity does not progress sufficiently. -1 If a shear rate exceeding this value is applied, the physical properties of the resulting expandable polypropylene will be significantly reduced.

[0036] Furthermore, the shear stress in the first kneading zone is preferably applied for 5 to 30 seconds, more preferably 10 to 15 seconds. If the shear stress is applied for less than 5 seconds, the thermoplasticization does not proceed sufficiently, and if the shear stress is applied for more than 30 seconds, the physical properties of the resulting expandable polypropylene are significantly reduced.

[0037] The temperature in the first kneading section 20 is set to 150 to 280°C, preferably 220 to 250°C. If the temperature in the first kneading section 20 is below 150°C, the reaction does not proceed sufficiently, and the cross-linked polypropylene is not thermoplasticized sufficiently. If the temperature exceeds 280°C, the expandable polypropylene will be thermally deteriorated, resulting in a significant decrease in its physical properties.

[0038] (Mixing process: S2) Next, the cross-linked polypropylene and additives are mixed outside the twin-screw extruder 10 to produce a mixture. At this time, it is preferable to mix 0.1 to 5 parts by weight of additives with 100 parts by weight of cross-linked polypropylene. If the amount of additive is less than 0.1 part by weight, the thermoplastic reaction does not proceed sufficiently, and if it is more than 5 parts by weight, the physical properties of the finally obtained expandable polypropylene will decrease.

[0039] The additive is preferably a peroxide with a one-minute half-life of 150°C or higher. If the one-minute half-life is less than 150°C, the peroxide will decompose before thermoplasticization begins, and thermoplasticization of the cross-linked polypropylene will not proceed sufficiently. The one-minute half-life refers to the temperature at which half (50%) of the peroxide decomposes in one minute.

[0040] When cross-linked polypropylene is thermoplasticized using a peroxide with a one-minute half-life of 150°C or more, the cross-linking points are properly cut, producing polypropylene with many long-chain branches. Polymers have a main chain and side chains extending from the main chain, and molecular chains with side chains containing six or more carbon atoms are called long-chain branches. Polypropylene with many long-chain branches is known to have a high expansion ratio.

[0041] Therefore, the additive preferably contains at least one peroxide selected from t-butyl peroxylaurate, t-butylperoxyisopropyl monocarbonate, t-hexyl peroxybenzoate, t-butyl peroxybenzoate, dicumyl peroxide, di-t-hexyl peroxide, t-butylcumyl peroxide, di-t-butyl peroxide, dibutyl peroxide, di(2-t-butylperoxyisopropyl)benzene, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. Only one type of additive may be used, or two or more types may be used.

[0042] (First input process: S3) The mixture of crosslinked polypropylene and additives is fed into the first feeding section 16 , heated and melted while rotating around the twin screws in the first conveying section 18 , and introduced into the first kneading section 20 .

[0043] (Thermoplasticization process: S4) The mixture is subjected to shear stress and heat in the first kneading section 20, which breaks the crosslinking points of the crosslinked polypropylene and thermoplasticizes it. Gas generated as the thermoplasticization progresses is discharged to the outside of the cylinder 12 from the first degassing section 22 located above the first kneading section 20. As the generated gas is discharged, the pressure in the first kneading section 20 becomes close to atmospheric pressure. The temperature in the first kneading section 20 is preferably in the range of 150°C to 280°C, and more preferably 220 to 250°C. This is because the thermoplasticization of the crosslinked propylene does not proceed sufficiently at temperatures below 150°C, and the polypropylene itself decomposes at temperatures above 280°C.

[0044] (Second input process: S5) Non-crosslinked polypropylene is fed from the second feeding section 24 and introduced into the second kneading section 28 from the second transport section 26 together with the mixture thermoplasticized in the first kneading section 20. The amount of non-crosslinked polypropylene is preferably 50 to 150 parts by weight per 100 parts by weight of crosslinked polypropylene fed from the first feeding section.

[0045] If the amount of non-crosslinked polypropylene fed from the second feeding section 24 is less than 50 parts by weight, the physical properties of the resulting expandable polypropylene will be low and the molded product will be prone to defects. If the amount is more than 150 parts by weight, the expansion ratio of the resulting expandable polypropylene will be low and an appropriate foam will not be obtained.

[0046] (Kneading process: S6) The thermoplastic mixture and the non-crosslinked polypropylene fed from the second feeding section 24 are mixed and kneaded in the second kneading section 28 to obtain a uniform foamable polypropylene. The reason for kneading the non-crosslinked polypropylene after the thermoplastic step is to improve moldability. (Discharge process: S7) Thereafter, the expandable polypropylene passes through the third transport section and is discharged from the discharge section 34 to the outside of the twin-screw extruder 10, whereby the expandable polypropylene is obtained.

[0047] The expandable polypropylene obtained in this manner has excellent moldability and mechanical strength, with a tensile strength of 20 MPa or more, a tensile elongation of over 600%, and a modulus of elasticity of 400 MPa or more. The rigidity of polypropylene is indicated by its high modulus of elasticity. Furthermore, since it has an expansion ratio of 3 times or more, it can be used in applications requiring physical strength and high expandability. The modulus of elasticity here is a constant calculated from the relationship between elongation and force when an object is pulled.

[0048] <Examples and Comparative Examples> The results of the foamable polypropylene will be shown below by way of examples of the present invention and comparative examples, but these examples do not limit the present invention.

[0049] [Tensile test and elongation test] Tensile and elongation tests were performed on the foamed polypropylene. The tensile and elongation test equipment used was the Shimadzu Autograph AGS System. The tensile tests were performed in accordance with Japanese Industrial Standards (JIS) K6922-2, where foamed polypropylene was press-molded into a 1 mm thick sheet and punched into a No. 3 dumbbell shape, using a tensile tester at a speed of 50 mm / min.

[0050] [Gel fraction] The gel fraction was determined by extracting the sample in boiling hot xylene for 8 hours in accordance with JIS-K6796. The sample was then vacuum dried at 140°C for 3 hours, after which it was weighed and the gel fraction was calculated from the ratio of the weight before extraction to the weight before extraction using the following formula. Gel fraction (%) = (weight after extraction (g) / weight before extraction (g)) × 100 The gel fraction of the crosslinked polypropylene used as the raw material was 50 to 80%.

[0051] [Expansion ratio] The expansion ratio (%) was calculated by mixing the foamable polypropylene with a foaming agent, passing the mixture between two rolls heated to 150°C with a gap of 2 mm between them, and then heating it in an oven at 200°C for 3 minutes to cause foaming. The expansion ratio (%) was calculated using the following formula: Expansion ratio (%) = (cross-sectional area length after foaming (mm) / cross-sectional area length before foaming (mm)) x 100 The foaming agent may be one or more selected from ADCA (azodicarbonamide), DPT (N,N'-dinitropentamethylenetetramine), OBSH (4,4'-oxybisbenzenesulfonylhydrazide), bicarbonate, carbonate, sodium bicarbonate-based foaming agents, etc., but is not limited to these.

[0052] Next, Examples 1 to 8 and Comparative Examples 1 to 4 will be described in detail.

[0053] [Table 1] XPP: Cross-linked polypropylene, MI: Melt flow index, ○: Very smooth appearance, △: Rough appearance, Maximum stress: Maximum tensile strength in tensile test conducted in accordance with JISK6922-2

[0054] Examples 1 to 8 are explained below. In Examples 1 to 8, a mixture was produced by mixing 0.5 to 5.0 parts by weight of peroxide as an additive with 100 parts by weight of cross-linked polypropylene (XPP) as shown in Table 1. After that, the mixture was subjected to thermal plasticization using a twin-screw extruder according to the process described above, and then non-cross-linked polypropylene was kneaded to obtain an expandable polypropylene, and the physical properties and expansion ratio were measured.

[0055] Example 1 Using the twin-screw extruder 10 shown in FIG. 1, the temperature of the first kneading section was set to 230°C, the screw rotation speed was set to 250 rpm, and the maximum shear rate was set to 8,500 s -1 The resin temperature in the second kneading section 28 was set to 220°C, the screw rotation speed was set to 250 rpm, and the maximum shear rate was set to 700 s -1 The kneading time was set to 20 seconds (Fig. 3, S1: preparation step). Then, a mixture was obtained by mixing 100 parts by weight of cross-linked polypropylene with 0.5 parts by weight of dicumyl peroxide as an additive (Fig. 3, S2: mixing step). The mixture was then fed into the twin-screw extruder 10 through the first feeding section 16 (Fig. 3, S3: feeding step). Then, kneading was performed for 15 seconds in the first kneading section 20 to produce a mixture in which the cross-linked polypropylene was thermoplasticized (Fig. 3, S4: thermoplasticization step). Next, 100 parts by weight of non-cross-linked polypropylene was fed through the second feeding section 24 (Fig. 3, S5: second feeding step). The thermoplasticized mixture and the second non-cross-linked polypropylene fed through the second feeding section were transferred from the second conveying section 26 to the second kneading section 28, where they were mixed and formed into a uniform foamable polypropylene (Fig. 3, S6: kneading step). The mixture passed through the third conveying section 32 and was discharged from the discharge section 34, yielding foamable polypropylene (Fig. 3, S7: discharge step).

[0056] The elastic modulus, which indicates the rigidity of the obtained expandable polypropylene, was 483 MPa, and other physical properties (gel fraction, MI, elastic modulus, maximum stress, and elongation) were as shown in Table 1. The expansion ratio was 450%. The cross-sectional view of the obtained foam is shown in Figure 4, and it can be seen that independent cells were formed.

[0057] Example 2 Additive: 0.5 parts by weight of dicumyl peroxide, screw speed: 350 rpm, shear rate: 12,000 s -1 The test was carried out in the same manner as in Example 1, except that the above-mentioned conditions were changed, to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 485 MPa, and other physical property values ​​were as shown in Table 1. The expansion ratio was 600%.

[0058] Example 3 Except for using 0.1 parts by weight of dicumyl peroxide as an additive, the same test as in Example 1 was carried out to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 439 MPa and the expansion ratio was 350%.

[0059] Example 4 Except for using 1.0 part by weight of dicumyl peroxide as an additive, the same test as in Example 1 was carried out to obtain an expandable polypropylene. The elastic modulus of the expandable polypropylene obtained was 453 MPa, and the expansion ratio was 310%.

[0060] Example 5 Except for using 2.0 parts by weight of dicumyl peroxide as an additive, the same test as in Example 1 was carried out to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 538 MPa and the expansion ratio was 470%.

[0061] Example 6 Except for using 5.0 parts by weight of dicumyl peroxide as an additive, the same test as in Example 1 was carried out to obtain an expandable polypropylene. The elastic modulus of the expandable polypropylene obtained was 464 MPa, and the expansion ratio was 320%.

[0062] Example 7 Except for using 1.0 part by weight of dibutyl peroxide as an additive, the same test as in Example 1 was carried out to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 549 MPa and the expansion ratio was 450%.

[0063] Example 8 The same test as in Example 1 was carried out except that 1.0 part by weight of 2,5 dimethyl-2,5 di(t-butylperoxy)hexane was used as the additive, to obtain expandable polypropylene. The elastic modulus of the expandable polypropylene obtained was 472 MPa, and the expansion ratio was 380%.

[0064] The results of Examples 1 to 6 indicate that by adding 0.1 to 5 parts by weight of dicumyl peroxide as an additive and then thermoplasticizing, polypropylene was obtained with a modulus of rigidity of 400 MPa or more and an expansion ratio of 310 to 600%, demonstrating high expandability. Furthermore, the results of Examples 7 and 8 indicate that by adding 1.0 part by weight of dibutyl peroxide or 1.0 part by weight of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane as an additive and then thermoplasticizing, polypropylene was obtained with a modulus of rigidity of 472 to 549 MPa and an expansion ratio of 380 to 450%, demonstrating high expandability. Furthermore, as shown in Figure 4, a cross-sectional view of a foam obtained by adding a blowing agent to the expandable polypropylene obtained in Example 1 and foaming at 200°C for 3 minutes reveals that the cells are closed cells, not interconnected. Closed cells are preferable for achieving high rigidity.

[0065] (Comparative Example 1) Comparative Examples 1 to 4 are explained below. In Comparative Example 1, the test was carried out in the same manner as in Example 1, except that 1.0 part by weight of an additive, dilauroyl peroxide (1 minute half-life 113°C), was used in the crosslinked polypropylene, to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 517 MPa, and the expansion ratio was 120%.

[0066] (Comparative Example 2) In Comparative Example 2, the test was carried out in the same manner as in Example 1 except that no additive was used, to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 364 MPa, and the expansion ratio was 110%.

[0067] (Comparative Example 3) In Comparative Example 3, the test was carried out in the same manner as in Example 1, except that no additive was used and the rotation speed was set to 350 rpm, to obtain expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 437 MPa, and the expansion ratio was 110%.

[0068] Comparative Example 4 In Comparative Example 4, the test was carried out in the same manner as in Example 1, except that no additive was used and the rotation speed was set to 450 rpm, to obtain expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 505 MPa, and the expansion ratio was 110%.

[0069] As shown in Comparative Examples 1 and 2 to 4, when crosslinked polypropylene was thermoplasticized using an additive with a 1-minute half-life of 150°C or less or without any additive, the expansion ratio was 110 to 120%, which was significantly lower than in the Examples, and almost no foaming occurred.

[0070] From the above results, it was found that by using as an additive a peroxide with a 1-minute half-life of 150°C or more in an amount of 0.1 to 5 parts by weight per 100 parts by weight of crosslinked polypropylene to produce foamable polypropylene, foaming ratios of 300% or more and high foaming performance can be obtained. [Industrial Applicability]

[0071] According to the present invention, a twin-screw extruder is used to produce expandable polypropylene using 0.1 to 5 parts by weight of a peroxide with a one-minute half-life of 150°C or more per 100 parts by weight of crosslinked polypropylene, thereby producing expandable polypropylene with high expansion performance. Furthermore, as shown in Figure 4, the expandable polypropylene foam has closed cells, making it suitable for applications requiring high rigidity and thermal insulation. [Explanation of symbols]

[0072] 10 Twin-screw extruder 12 cylinders 13 Screw 14 Drive unit 16 First input section 18 First Transportation Department 20 First kneading section 22 First degassing section 24 Second input section 26 Second Transport Department 28 Second mixing section 30 Second degassing section 32 Third Transport Department 34 Discharge section

Claims

1. a mixing step of mixing the crosslinked polypropylene with the additives to form a mixture; a thermoplasticizing step of feeding the mixture through a first input section of an extruder and applying heat and shear stress to thermoplasticize the crosslinked polypropylene to form a thermoplastic product; a kneading step of feeding non-crosslinked polypropylene from a second feeding portion of the extruder and kneading the thermoplastic material and the non-crosslinked polypropylene by applying heat, the amount of the additive in the mixing step is 0.1 to 5 parts by weight relative to 100 parts by weight of the crosslinked polypropylene; the additive imparts foaming properties to the thermoplasticized crosslinked polypropylene and is a peroxide having a one-minute half-life of 150°C or more; The shear stress in the thermoplastic process is set to 5,000 to 15,000 s -1 for 5 to 30 seconds at a shear rate of A method for producing expandable polypropylene, characterized in that the amount of the non-crosslinked polypropylene added in the kneading step is 50 to 150 parts by weight per 100 parts by weight of the crosslinked polypropylene.

2. 2. The method for producing expandable polypropylene according to claim 1, The method for producing expandable polypropylene, wherein the additive contains at least one peroxide selected from the group consisting of t-butyl peroxylaurate, t-butylperoxyisopropyl monocarbonate, t-hexyl peroxybenzoate, t-butyl peroxybenzoate, dicumyl peroxide, di-t-hexyl peroxide, t-butylcumyl peroxide, di-t-butyl peroxide, dibutyl peroxide, di(2-t-butylperoxyisopropyl)benzene, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane.

3. 3. The method for producing expandable polypropylene according to claim 1 or 2, The method for producing expandable polypropylene, characterized in that the expandable polypropylene produced by the kneading step exhibits an expansion ratio of 300% or more when foamed at 200°C for 3 minutes with the addition of a foaming agent.

Images