Method for producing foamable polypropylene

Abstract

[Problem] 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] The present invention involves thermally plasticizing a crosslinked polypropylene having an additive mixed thereto and then kneading the same together with a non-crosslinked polypropylene. The additive is a peroxide that imparts foamability to the thermally plasticized crosslinked polypropylene and exhibits a one-minute half-life at 150°C or higher. The foamable polypropylene obtained thereby exhibits high expansion ratio and high rigidity.

Description

Method for producing expandable polypropylene

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

[0002] Polypropylene (PP for short) is a thermoplastic resin made by polymerizing propylene monomer. Among general-purpose resins, polypropylene boasts the highest heat resistance, relatively high strength, excellent chemical resistance (including acid and alkali resistance), and low moisture absorption. Therefore, it has a wide range of uses, including stationery, banknotes, automobile 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 and lower the molecular weight, thereby promoting thermal dissolution. 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 unlikely to undergo long chain branching, polypropylene produced by a conventional method 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, and therefore, various methods for producing highly expandable polypropylene have been proposed.

[0006] Methods for producing expandable polypropylene are known, for example, as described in Patent Documents 1 to 3. Patent Document 1 discloses a modified polypropylene composition having appropriate fluidity and high melt tension. Patent Document 2 discloses a polyolefin-based resin composition having properties such as heat resistance and recyclability and capable of being made into a high-magnification foam, and a method for producing the same. Patent Document 3 discloses a method for providing a propylene-based resin composition for flexible foams, which can produce polypropylene-based flexible foams having 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 amounts of 99 to 1% by weight and 1 to 99% by weight, respectively (Abstract, Claim 1).

[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 (the polyolefin resin (B) includes polypropylene (paragraph 0013)), and a thermally decomposable chemical foaming agent (C). The resulting mixture is then pulverized into a powder (Abstract, claim 7,

[0028] ).

[0009] Furthermore, the propylene-based resin composition for flexible foams described in Patent Document 3 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. The propylene-based random block copolymer (A) 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 the propylene-based resin composition contains 50 to 90 parts by weight of a propylene-based random block copolymer (A), and 50 to 10 parts by weight of a modified polypropylene (B) having a melt tension in the range of 4 to 30 g (Abstract, Claim 1).

[0010] JP 2002-60563 A JP 2004-26937 A JP 2009-84304 A

[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 of Patent Document 1, and is not a high expansion ratio polypropylene 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 (Example 1 of

[0012] and

[0043] in Patent Document 2). This suggests 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 (Abstract, Claim 1), 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 expandable polypropylene of the present invention can be used suitably for applications requiring both a high expansion ratio and high rigidity (e.g., automotive foams), for example.

[0015] The method for producing expandable polypropylene according to the present invention is a method for producing expandable polypropylene by thermoplasticizing crosslinked polypropylene using an extruder (for example, a single-screw extruder or a twin-screw extruder), the method comprising: a mixing step of mixing the crosslinked polypropylene with an additive to produce a mixture; a thermoplasticizing step of feeding the mixture from a first input section of the 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 (More preferably 5,000 to 8,500 s -1 The method is characterized in that the shear rate is generated at a shear rate of 100 to 500 rpm for 5 to 30 seconds, and 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.

[0016] The additive may also contain 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 may exhibit an expansion ratio of 300% or more when foamed at 200° C. for 3 minutes with the addition of a foaming agent.

[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 the resulting mixture is kneaded 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 foamed 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.

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

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

[0021] <Description of the Equipment> 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, less influence of frictional heat, which reduces heat generation, and a more complex internal material flow, which allows greater shear stress to be applied to the material. For this reason, tests were conducted using a twin-screw extruder in the present invention.

[0022] Fig. 1 is a schematic cross-sectional view 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 view showing, in plan view, 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 pairs of screws 13. The screws of the first conveying section 18, the second conveying section 26, and the third conveying section 32 have a shape suitable for conveying. The screw of the first kneading section 20 has a shape suitable for thermoplasticization, and the screw of 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 conveyance 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 conveyance section 18, the first kneading section 20, the second conveyance section 26, the second kneading section 28, and the third conveyance section 32 rotate simultaneously. Therefore, the screws of the first conveyance section 18, the first kneading section 20, the second conveyance section 26, the second kneading section 28, and the third conveyance 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 conveyance section 18, the second conveyance section 26, and the third conveyance 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 narrow screw and cylinder widths 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, thereby thermoplasticizing 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 and another surface inside the object are deformed in alternating parallel directions. If the force applied to deform is F and the deformed area is A, shear stress is expressed as F / A, and its unit is τ (Pa Pascal) = F (N Newton) / A (area m ２ )

[0028] In addition, the shear stress τ (Pa) is proportional to the shear rate γ (s -1 It is expressed as the product of the shear stress τ 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 v at the tip of the screw is expressed as v = πND / 60, where N (rpm) is the rotation of the screw and D (mm) is the diameter. That is, 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, a larger clearance results in a smaller shear rate, and conversely, a smaller clearance results in a larger 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 greater 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] <Explanation of Processes> 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 step: 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, causing the screws of the first conveying section 18, first kneading section 20, second conveying section 26, second kneading section 28, and third conveying section 32 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 -1 The rotation speed of the screw is set so that -1 If the shear rate is less than 15,000 s, the thermoplasticity does not proceed 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 is preferably generated for 5 to 30 seconds, more preferably 10 to 15 seconds, in the first kneading section 20. If the shear stress is generated 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 less than 150°C, the reaction does not proceed sufficiently, and the thermoplasticization of the cross-linked polypropylene does not proceed sufficiently. If the temperature exceeds 280°C, the expandable polypropylene will thermally deteriorate, resulting in a significant decrease in its physical properties.

[0038] (Mixing step: S2) Next, the crosslinked polypropylene and additives are mixed outside the twin-screw extruder 10 to produce a mixture. At this time, it is preferable to mix the additives in a ratio of 0.1 to 5 parts by weight per 100 parts by weight of the crosslinked 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 (1 minute half-life or 1 min half life) refers to the temperature at which half (50%) of the peroxide decomposes in one minute.

[0040] When crosslinked polypropylene is thermoplasticized using a peroxide with a one-minute half-life of 150°C or more, the crosslinks are appropriately cleaved, resulting in 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 charging step: S3) The mixture of crosslinked polypropylene and additives is charged into the first charging 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 step: 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. Furthermore, 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 progress sufficiently at temperatures below 150°C, and the polypropylene itself decomposes at temperatures above 280°C.

[0044] (Second charging step: S5) Non-crosslinked polypropylene is charged from the second charging 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 charged from the first charging section 16.

[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 step: S6) The thermoplastic mixture and the non-crosslinked polypropylene fed from the second feeding section 24 are then mixed and kneaded in the second kneading section 28 to obtain a uniform expandable polypropylene. The non-crosslinked polypropylene is kneaded after the thermoplasticization step to improve moldability. (Discharge step: S7) The expandable polypropylene then passes through the third conveying section 32 and is discharged to the outside of the twin-screw extruder 10 from the discharge section 34, obtaining the expandable polypropylene.

[0047] The foamable polypropylene obtained as described above has excellent moldability and mechanical strength, with a tensile strength (maximum stress) of 20 MPa or more, a tensile elongation of over 600%, and an elastic modulus of 400 MPa or more. The rigidity of polypropylene is indicated by its high elastic modulus. Furthermore, since it has an expansion ratio of 300% or more, it can be used in applications requiring physical strength and high expandability. The elastic modulus here is a constant determined from the relationship between elongation and force when an object is pulled.

[0048] <Examples and Comparative Examples> Hereinafter, the results of the expandable polypropylene will be shown 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 test and elongation test were performed on the foamed polypropylene. The tensile test and elongation test device used was an Autograph AGS System manufactured by Shimadzu Corporation. The tensile test was performed in accordance with Japanese Industrial Standard (JIS) K6922-2, in which the foamed polypropylene was press-molded into a 1 mm thick sheet and punched into the shape of a No. 3 dumbbell, using a tensile tester at a speed of 50 mm / min.

[0050] [Gel Fraction] The gel fraction was measured by extraction 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 the weight was measured and the gel fraction was calculated from the ratio 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 measured by mixing a foamable polypropylene with a foaming agent, and then passing the mixture between two rolls heated to 150°C with a gap of 2 mm between them. The mixture was then heated 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 the group consisting of ADCA (azodicarbonamide), DPT (N,N'-dinitropentamethylenetetramine), OBSH (4,4'-oxybisbenzenesulfonylhydrazide), bicarbonate, carbonate, and sodium bicarbonate-based foaming agents, but is not limited to these.

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

[0053]

[0054] In Table 1, XPP: crosslinked polypropylene, MI: melt flow index, substance 1: dicumyl peroxide, substance 2: dibutyl peroxide, substance 3: 2,5 dimethyl 2,5 di(t-butylperoxy)hexane, substance 4: dilauroyl peroxide, A: very smooth appearance, B: uneven appearance, maximum stress: maximum tensile strength in a tensile test conducted in accordance with JIS K6922-2.

[0055] Examples 1 to 8 are described below. In Examples 1 to 8, 100 parts by weight (pbw) of cross-linked polypropylene (XPP) was mixed with 0.5 to 5.0 parts by weight of peroxide as an additive, as shown in Table 1, to produce a mixture. The mixture was then 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.

[0056] (Example 1) Using the twin-screw extruder 10 of FIG. 1, the temperature of the first kneading section 20 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 ( Figure 3, S1: preparation step). 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 ( Figure 3, S2: mixing step). The mixture was then fed into the twin-screw extruder 10 through the first feeding section 16 ( Figure 3, S3: first feeding step). Kneading was then performed for 15 seconds in the first kneading section 20 to produce a mixture in which the cross-linked polypropylene was thermoplasticized ( Figure 3, S4: thermoplasticization step). Next, 100 parts by weight of non-cross-linked polypropylene was fed through the second feeding section 24 ( Figure 3, S5: second feeding step). The thermoplasticized mixture and the non-cross-linked polypropylene fed through the second feeding section 24 were transferred from the second conveying section 26 to the second kneading section 28, where they were mixed and formed into a uniform expandable polypropylene ( Figure 3, S6: kneading step), passed through the third conveying section 32, and discharged from the discharge section 34, yielding the expandable polypropylene ( Figure 3, S7: discharge step).

[0057] The elastic modulus, which indicates the rigidity of the obtained foamable 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 closed cells were formed.

[0058] (Example 2) 0.5 parts by weight of dicumyl peroxide as an additive, screw rotation 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 expandable polypropylene obtained was 485 MPa, and other physical property values ​​were as shown in Table 1. The expansion ratio was 600%.

[0059] (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 expandable polypropylene obtained was 439 MPa, and the expansion ratio was 350%.

[0060] (Example 4) Except for using 1.0 part by weight of dicumyl peroxide as an additive, the test was carried out in the same manner as in Example 1 to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 453 MPa and the expansion ratio was 310%.

[0061] (Example 5) Except for using 2.0 parts by weight of dicumyl peroxide as an additive, the test was carried out in the same manner as in Example 1 to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 538 MPa and the expansion ratio was 470%.

[0062] (Example 6) Except for using 5.0 parts by weight of dicumyl peroxide as an additive, the test was carried out in the same manner as in Example 1 to obtain an expandable polypropylene. The elastic modulus of the expandable polypropylene obtained was 464 MPa, and the expansion ratio was 320%.

[0063] (Example 7) Except for using 1.0 part by weight of dibutyl peroxide as an additive, the test was carried out in the same manner as in Example 1 to obtain an expandable polypropylene. The elastic modulus of the expandable polypropylene obtained was 549 MPa, and the expansion ratio was 450%.

[0064] (Example 8) A test was carried out in the same manner as in Example 1, except that 1.0 part by weight of 2,5 dimethyl-2,5 di(t-butylperoxy)hexane was used as the additive, to obtain an expandable polypropylene. The elastic modulus of the obtained expandable polypropylene was 472 MPa, and the expansion ratio was 380%.

[0065] 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 that exhibited high expandability, with a modulus of rigidity of 400 MPa or more and an expansion ratio of 310 to 600%. 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 that exhibited high expandability, with a modulus of rigidity of 472 to 549 MPa and an expansion ratio of 380 to 450%. Furthermore, as shown in Figure 4, a cross-sectional view of the foam obtained by adding a blowing agent to the expandable polypropylene obtained in Example 1 and foaming it at 200°C for 3 minutes reveals that the individual cells are closed cells, not interconnected. Closed cells are preferable for achieving high rigidity.

[0066] 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%.

[0067] 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 expandable polypropylene obtained was 364 MPa, and the expansion ratio was 110%.

[0068] 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 an expandable polypropylene. The elastic modulus of the expandable polypropylene obtained was 437 MPa, and the expansion ratio was 110%.

[0069] 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%.

[0070] 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 considerably lower than in the Examples, and almost no foaming occurred.

[0071] 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, it is possible to obtain foamable polypropylene that exhibits high foaming performance with an expansion ratio of 300% or more.

[0072] According to the present invention, expandable polypropylene with high foaming performance can be produced by using an extruder and 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. Furthermore, as shown in Figure 4, the expandable polypropylene foam has closed cells, making it suitable for applications requiring high rigidity and thermal insulation.

[0073] REFERENCE SIGNS LIST 10 Twin-screw extruder 12 Cylinder 13 Screw 14 Drive section 16 First input section 18 First conveying section 20 First kneading section 22 First degassing section 24 Second input section 26 Second conveying section 28 Second kneading section 30 Second degassing section 32 Third conveying section 34 Discharge section

Claims

1. A method for producing expandable polypropylene by thermoplasticizing crosslinked polypropylene using a twin-screw extruder, comprising: a mixing step of mixing crosslinked polypropylene with an additive to form a mixture; a thermoplasticizing step of feeding the mixture into a first input section of the twin-screw extruder and applying heat and shear stress to thermoplasticize the crosslinked polypropylene to form a thermoplastic product; and a kneading step of feeding non-crosslinked polypropylene into a second input section of the twin-screw 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 per 100 parts by weight of the crosslinked polypropylene; the additive is a peroxide that imparts foamability 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 8,500 s. -1 and generating a shear rate of 5 to 30 seconds at a shear rate of 50 to 150 parts by weight of the non-crosslinked polypropylene added in the kneading step relative to 100 parts by weight of the crosslinked polypropylene.

2. A method for producing expandable polypropylene according to claim 1, characterized in that the additive contains at least one peroxide selected from the group consisting of t-butylperoxylaurate, t-butylperoxyisopropyl monocarbonate, t-hexylperoxybenzoate, t-butylperoxybenzoate, 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. A method for producing expandable polypropylene according to claim 1, 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.

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