A method of recycling polypropylene waste into high melt index polypropylene

Abstract

This invention provides a method for recycling polypropylene waste into high melt index polypropylene, relating to the field of polypropylene waste recycling technology. The method includes the following steps: S1, pretreating the polypropylene waste and then extruding and granulating it to obtain pretreated material; S2, mixing the pretreated material with a degradation agent, extruding, granulating, and dehydrating to obtain high melt index polypropylene. The polypropylene obtained by this invention has excellent mechanical properties and toughness, and also has a high melt index with a small melt index fluctuation range and a low yellowness index.

Description

Technical Field

[0001] This invention relates to the field of polypropylene waste recycling technology, and in particular to a method for recycling polypropylene waste into high melt flow index polypropylene. Background Technology

[0002] Recycling polypropylene (PP) waste is an important direction for resource recycling and environmental protection. Currently, PP waste comes from a wide range of sources, mainly including lumpy melt solidified material generated during production, water-containing powder from water ring pelletizing, and loose material. These wastes vary greatly in impurity content, moisture content, and basic mechanical properties. Traditional recycling methods typically involve simple mixing followed by direct melt granulation. However, due to the lack of effective classification and pretreatment of the waste, the recycled granules suffer from problems such as large fluctuations in melt flow index, high yellow index, and severe degradation of mechanical properties, resulting in extremely low added value and often being downgraded for reuse. Summary of the Invention

[0003] Based on the technical problems existing in the background art, the present invention proposes a method for recycling polypropylene waste into high melt index polypropylene.

[0004] This invention proposes a method for recycling polypropylene waste into high melt flow index polypropylene, comprising the following steps:

[0005] S1. Polypropylene waste is pretreated and then extruded and granulated to obtain pretreated material.

[0006] S2. The pretreated material and the degradation agent are mixed, extruded, granulated and dehydrated to obtain polypropylene with a high melt index.

[0007] Preferably, in S1, the polypropylene waste includes impact-resistant polypropylene waste and homopolymer polypropylene waste; the impact-resistant polypropylene waste has an ethylene content of 9%~10% and a melt index of 3g / 10min; the homopolymer polypropylene waste has a melt index of 3g / 10min.

[0008] More preferably, the mass ratio of the impact-resistant polypropylene waste to the homopolymer polypropylene waste is (1~3):(1~3).

[0009] Preferably, in S1, the pretreatment is selected from one or more of crushing, washing, sieving, and drying.

[0010] More preferably, the particle size after pulverization is 8~10mm.

[0011] Preferably, in S1, the extrusion temperature is 180~220℃, the extrusion die pressure is 8~12MPa, and the rotation speed is 400~600rpm.

[0012] Preferably, in S1 and S2, the particle size after granulation is 3~5mm.

[0013] Preferably, in S1 and S2, granulation is carried out using nitrided steel cutters with a die head diameter of 3~5mm.

[0014] Preferably, in step S2, the method for preparing the degrading agent includes the following steps:

[0015] 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane, antioxidant, and calcium stearate were mixed to obtain a mixture; PP-g-MAH was mixed with the mixture, extruded, and granulated to obtain a degradation agent.

[0016] More preferably, the mass ratio of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, antioxidant, calcium stearate, and PP-g-MAH is (10~15):(2~8):(0.01~0.2):100.

[0017] More preferably, the antioxidant is selected from one or more of antioxidant 3114 and antioxidant 168.

[0018] More preferably, the mass ratio of antioxidant 3114 to antioxidant 168 is (1~3):(1~5).

[0019] More preferably, the extrusion temperature is 130~150°C.

[0020] More preferably, the grafting rate of the PP-g-MAH is 0.8%~2.0%, and the melt index at 230℃ / 2.16kg is 10~30g / 10min.

[0021] When the grafting rate is below 0.8%, there are insufficient active sites; when the grafting rate is above 2.0%, the polarity of PP-g-MAH itself is too strong, resulting in poor compatibility with the PP matrix. Within a certain range, the grafting rate of PP-g-MAH enables the prepared degradation agent to exhibit excellent performance.

[0022] Preferably, in step S2, the mass ratio of the pretreatment material to the degradation agent is 100:(0.1~0.3).

[0023] Preferably, in S2, the extrusion temperature is 200~230℃.

[0024] More preferably, the extrusion temperature is divided into two temperature zones: the first temperature zone has a temperature of 210~230℃, and the second temperature zone has a temperature of 200~210℃.

[0025] Preferably, in S2, the extrusion equipment is a twin-screw extruder; the twin-screw extruder has a length-to-diameter ratio of 42 to 46, a nominal diameter of 71 mm, and a rotational speed of 500 to 600 rpm.

[0026] More preferably, the die pressure of the twin-screw extruder is 8~12MPa.

[0027] Preferably, in step S2, the dehydration parameters include: a vacuum degree of -0.1 to -0.01 MPa, and a moisture content of no more than 0.3% after dehydration.

[0028] A high melt index polypropylene is prepared by the above method.

[0029] The beneficial effects of this invention are as follows:

[0030] This invention proposes a method for recycling polypropylene waste into high melt flow index (MFI) polypropylene. The resulting polypropylene exhibits excellent mechanical properties and toughness, along with a high MFI, a small MFI fluctuation range, and a low yellowness index. The degradation agent provided by this invention involves loading peroxide onto the surface of a PP-g-MAH carrier. Utilizing the carrier's dual "slow-release" and "targeted" effects, degradation is controllable, yellowing is minimal, and mechanical properties are strong. This invention avoids side reactions caused by excessively high local free radical concentrations through slow release via the carrier. Simultaneously, the PP-g-MAH in the carrier can capture some residual free radicals, inhibiting the oxidative color development pathway. Furthermore, the stable slow-release degradation process results in minimal MFI fluctuation in the polypropylene. PP-g-MAH preferentially distributes in the amorphous region, and the peroxide targets the molecular chains in the amorphous region, reducing damage to the crystalline region, thus preserving higher rigidity. Simultaneously, the degraded carrier acts as an in-situ compatibilizer, improving matrix uniformity and simultaneously increasing impact strength, breaking the traditional dilemma of "increasing rigidity inevitably reduces toughness" in degradation modification. Detailed Implementation

[0031] The technical solution of the present invention will be described in detail through specific embodiments.

[0032] Impact-resistant polypropylene waste: ethylene content 9%~10%, melt index 3g / 10min.

[0033] Homopolymer polypropylene waste: melt flow index is 3 g / 10 min.

[0034] Unless otherwise specified, all materials and reagents used in the following examples and comparative examples are commercially available.

[0035] Example 1

[0036] The preparation method of the degradation agent includes the following steps:

[0037] 12 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 2 parts of antioxidant 3114, 3 parts of antioxidant 168, and 0.1 parts of calcium stearate were mixed to obtain a mixture. 100 parts of PP-g-MAH (grafting rate 1.5%, melt index 20 g / 10min (230℃ / 2.16kg)) were mixed with the mixture and extruded at 140℃. The granulated particles had a particle size of 3 mm, thus obtaining the degradation agent.

[0038] A method for recycling polypropylene waste into high melt flow index polypropylene includes the following steps:

[0039] S1. After crushing, washing, screening, and drying, impact-resistant polypropylene waste and homopolymer polypropylene waste are extruded (temperature 200℃, pressure 10MPa, speed 500rpm) and granulated (granulation uses nitrided steel cutters, die orifice diameter 4mm, granulated particle size is 4mm) to obtain pretreated materials; the mass ratio of impact-resistant polypropylene waste to homopolymer polypropylene waste is 1:1.

[0040] S2. Mix 100 parts of pretreated material with 0.2 parts of degradation agent evenly, and extrude the mixture through a twin-screw extruder (the barrel adopts segmented temperature control, the front section is 220℃, the rear section is 200℃, the die pressure is 12MPa, the twin-screw extruder has an L / D ratio of 44, a nominal diameter of 71mm, and a rotation speed of 550rpm). The mixture is then granulated (granulation is carried out using nitrided steel cutters, the die orifice diameter is 4mm, and the particle size after granulation is 4mm). The granulated material then enters the dehydration system, which has a vacuum degree of -0.09MPa. The moisture content after dehydration is 0.2%, resulting in polypropylene with an ethylene content of 3% and a melt index of 41.1±1.2g / 10min.

[0041] Example 2

[0042] The only difference between Example 2 and Example 1 is that the amount of degradation agent added is 0.1 parts, and the rest is the same as in Example 1.

[0043] Example 3

[0044] The only difference between Example 3 and Example 1 is that the amount of degradation agent added is 0.3 parts, and the rest is the same as in Example 1.

[0045] Comparative Example 1

[0046] The only difference between Comparative Example 1 and Example 1 is that the degradation agent is 0.03 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and the rest is the same as in Example 1.

[0047] Comparative Example 2

[0048] The only difference between Comparative Example 2 and Example 1 is that the degradation agent is 0.03 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and 0.17 parts of PP-g-MAH, and the rest is the same as in Example 1.

[0049] The polypropylene was tested according to ASTM D638 for tensile strength, ASTM D790 for flexural modulus, ASTM D256 (23℃) for notched impact strength, ASTM D1238 (230℃ / 2.16kg) for melt flow index, and the melt flow index stability of 10 batches was tested (CV = standard deviation / mean × 100%). The yellowness index (YI) was tested according to ASTM E313. The test results are shown in Table 1.

[0050] Table 1

[0051]

[0052] The polypropylene provided by this invention possesses excellent mechanical properties and toughness, as well as a high melt index with a small melt index fluctuation range and a low yellowness index. In Comparative Example 1, the degrading agent was 0.03 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, slightly higher than the content of the corresponding peroxide in Example 1. As can be seen from Comparative Example 1 and Example 1, the polypropylene prepared using 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane alone as the degrading agent exhibited a large melt index fluctuation range, a high batch-to-batch CV value, and a significantly increased yellowness index. In Comparative Example 2, the degrading agent consisted of 0.03 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and 0.17 parts of PP-g-MAH. When applied to recycled polypropylene, it was simply a blend. As can be seen from Comparative Example 2 and Comparative Example 1, the melt index fluctuation and yellowness index of Comparative Example 2 were better than those of Comparative Example 1, but still significantly worse than those of Example 1. It can be seen that by loading peroxides onto the PP-g-MAH carrier, the present invention significantly improves the controllability of degradation, yellowing, mechanical properties, and toughness.

[0053] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for recycling polypropylene waste into high melt flow index polypropylene, characterized in that, Includes the following steps: S1. Polypropylene waste is pretreated and then extruded and granulated to obtain pretreated material. S2. The pretreated material and the degradation agent are mixed, extruded, granulated and dehydrated to obtain polypropylene with a high melt index.

2. The method according to claim 1, characterized in that, In S1, the polypropylene waste includes impact-resistant polypropylene waste and homopolymer polypropylene waste; the impact-resistant polypropylene waste has an ethylene content of 9%~10% and a melt index of 3g / 10min; the homopolymer polypropylene waste has a melt index of 3g / 10min.

3. The method according to claim 2, characterized in that, The mass ratio of the impact-resistant polypropylene waste to the homopolymer polypropylene waste is (1~3):(1~3).

4. The method according to claim 1, characterized in that, In S1, the pretreatment is selected from one or more of the following: crushing, washing, sieving, and drying.

5. The method according to claim 1, characterized in that, In S1, the extrusion temperature is 180~220℃, the extrusion die pressure is 8~12MPa, and the rotation speed is 400~600rpm.

6. The method according to claim 1, characterized in that, In S2, the method for preparing the degradation agent includes the following steps: mixing 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, an antioxidant, and calcium stearate to obtain a mixture; mixing PP-g-MAH with the mixture, extruding, and granulating to obtain the degradation agent.

7. The method according to claim 6, characterized in that, The mass ratio of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, antioxidant, calcium stearate, and PP-g-MAH is (10~15):(2~8):(0.01~0.2):

100.

8. The method according to claim 6, characterized in that, The antioxidant is selected from one or more of antioxidant 3114 and antioxidant 168; the extrusion temperature is 130~150℃; the grafting rate of PP-g-MAH is 0.8%~2.0%; and the melt index of PP-g-MAH at 230℃ / 2.16kg is 10~30g / 10min.

9. The method according to claim 1, characterized in that, In S2, the mass ratio of pretreated material to degradation agent is 100:(0.1~0.3); the extrusion temperature is 200~230℃.

10. The method according to claim 1, characterized in that, In S2, the dehydration parameters include: a vacuum degree of -0.1 to -0.01 MPa, and a moisture content of no more than 0.3% after dehydration.

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