A preparation method of transparent polypropylene

Abstract

This application relates to the field of polypropylene materials, and specifically relates to a preparation method of transparent polypropylene. A preparation method of transparent polypropylene includes (1) the preparation of dimethylbenzaldehyde; (2) the preparation of a transparent nucleating agent; (3) the preparation of polypropylene. By not using 3,4-dimethylbenzaldehyde with high purity as the raw material of the transparent nucleating agent and adding a nucleating agent stabilizer to the polypropylene processing raw materials, this application reduces the production cost of the transparent nucleating agent, and the finished polypropylene has both excellent transparency and no obvious peculiar smell.

Description

Technical Field

[0001] This application relates to the field of polypropylene materials, and particularly relates to a method for preparing transparent polypropylene. Background Art

[0002] Polypropylene (PP) is a commonly used thermoplastic with excellent chemical stability and electrical insulation. At the same time, it is light in weight, good in strength, and has good heat resistance. Therefore, it is widely used in many fields such as packaging, the automotive industry, household appliances, and medical devices. However, polypropylene is a crystalline polymer. When cooled under natural conditions, it will form relatively large crystals, resulting in the material presenting a milky white or opaque state. This opacity limits the use of polypropylene in some applications, especially for products that require high transparency, such as transparent packaging materials, transparent containers, and medical devices. To overcome this limitation, adding a transparent nucleating agent during the processing of polypropylene helps to control the crystallization process, enabling polypropylene to form finer crystals during crystallization, reducing light scattering inside the material, and thus improving the transparency of the material. Currently, the transparent nucleating agents on the market are mainly the third-generation sorbitol product 1,3,2,4-di(3,4-dimethyl)benzylidene sorbitol (DMDBS), which is prepared by the aldol condensation reaction of 3,4-dimethylbenzaldehyde and sorbitol.

[0003] However, during the preparation process of 3,4-dimethylbenzaldehyde, isomers will be produced. These by-products are mainly substances such as 3,5-dimethylbenzaldehyde, 2,3-dimethylbenzaldehyde, and 2,5-dimethylbenzaldehyde. Since the sorbitol acetals obtained by the reaction of these isomers with sorbitol are prone to decomposition, the resulting polypropylene will produce an odor. Therefore, 3,4-dimethylbenzaldehyde with a purity of more than 99% is usually used in the preparation of DMDBS. However, due to the extremely similar properties of these isomers and 3,4-dimethylbenzaldehyde, it is extremely difficult to purify them by distillation separation, which greatly increases the production cost of DMDBS, thus limiting the further application of DMDBS in the market. And due to the bottleneck of distillation technology, it is very difficult to completely separate 3,4-dimethylbenzaldehyde and its isomers. This leads to the decomposition of the sorbitol acetal formed by the reaction of trace amounts of isomers with sorbitol when DMDBS is used as a nucleating agent to prepare polypropylene, thereby releasing aldehyde substances and generating an odor, which further limits the market of DMDBS. Summary of the Invention

[0004] To solve the problems of high production cost of transparent nucleating agents and the presence of peculiar smell in polypropylene, the present application provides a method for preparing transparent polypropylene. The preparation method includes: (1) preparation of dimethylbenzaldehyde; (2) preparation of transparent nucleating agent; (3) preparation of polypropylene. By not using 3,4-dimethylbenzaldehyde with a purity of more than 99% as the raw material of the transparent nucleating agent and adding a nucleating agent stabilizer to the polypropylene processing raw materials, the production cost of the transparent nucleating agent is reduced, and the prepared polypropylene has both less peculiar smell and excellent transparency.

[0005] In a first aspect of the present application, there is provided a method for preparing transparent polypropylene, comprising the following steps:

[0006] (1) Preparation of dimethylbenzaldehyde: Mix o-xylene with 1,2-dichloroethane, add aluminum trichloride and hydrochloric acid, and react under a carbon monoxide atmosphere; wash the reacted solution, let it stand and separate layers, then remove the lower layer solution to obtain the upper oily substance, and remove the solvent in the oily substance to prepare dimethylbenzaldehyde, which includes 3,4-dimethylbenzaldehyde and 3,5-dimethylbenzaldehyde; (2) Preparation of transparent nucleating agent: Mix the dimethylbenzaldehyde with sorbitol, cyclohexane, methanol, and p-toluenesulfonic acid, and react to prepare a transparent nucleating agent; (3) Preparation of polypropylene: Mix the transparent nucleating agent, the compound shown in Formula I, an auxiliary agent with a polypropylene matrix resin to obtain a polypropylene processing raw material, add it to an extruder for extrusion to prepare polypropylene.

[0007] Formula I

[0008] Wherein, m, n, x, and y in Formula I are each independently an integer greater than or equal to 0.

[0009] For the preparation of the transparent nucleating agent of the present application, the product after the reaction of o-xylene is directly reacted with sorbitol. This dimethylbenzaldehyde can be used as a raw material to prepare the transparent nucleating agent without going through the process of rectification and purification that takes a long time and requires precise equipment, which greatly reduces the raw material cost of the transparent nucleating agent and thus the production cost of the transparent nucleating agent. Since there is an isomer of 3,4-dimethylbenzaldehyde in the above-prepared dimethylbenzaldehyde, the sorbitol acetal obtained by its reaction with sorbitol is prone to the attack of the oxygen atom on its acetal bond due to the acidity, moisture, etc. in the reaction environment for preparing polypropylene, thereby forming a hemiacetal intermediate and ultimately leading to the cleavage of the acetal bond and the release of the parent aldehyde. Based on this, the preparation method of the present application further adds a nucleating agent stabilizer shown in Formula I to the raw materials for preparing polypropylene. The ester group in this nucleating agent stabilizer can consume water molecules and acidic substances in the reaction environment, thereby preventing their attack on the acetal bond of sorbitol acetal and playing a role in inhibiting the decomposition of sorbitol acetal, thus avoiding the generation of peculiar smells in the finished polypropylene. Moreover, the alcohol substances generated after the nucleating agent stabilizer shown in Formula I decomposes by consuming water molecules and acidic substances can further combine with trace aldehyde substances that may exist in the reaction environment, thereby further avoiding the peculiar smells brought by aldehyde substances. And the applicant unexpectedly found in the experiment that compared with the sorbitol acetal transparent nucleating agent formed by reacting 3,4-dimethylbenzaldehyde with a purity of more than 99% with sorbitol, the sorbitol acetal transparent nucleating agent formed by reacting dimethylbenzaldehyde without rectification and purification with sorbitol can further improve the transparency of polypropylene. Based on this phenomenon, the applicant found through experiments that the transparency-enhancing effect of the transparent nucleating agent obtained by reacting 3,5-dimethylbenzaldehyde with sorbitol is better than that of 3,4-dimethylbenzaldehyde, and the transparency-enhancing effects of the transparent nucleating agents obtained by reacting 2,3-dimethylbenzaldehyde and 2,5-dimethylbenzaldehyde with sorbitol are close to those of the transparent nucleating agent obtained by reacting 3,4-dimethylbenzaldehyde. Therefore, by adopting the preparation method of polypropylene of the present application, while reducing the production cost of the transparent nucleating agent, the finished polypropylene has both excellent transparency and no obvious peculiar smell generated.

[0010] In any embodiment, m, n, x, and y in Formula I are each independently greater than or equal to 10 and less than or equal to 15.

[0011] When m, n, x, and y in Formula I are each independently greater than or equal to 10 and less than or equal to 15, the length of the alkyl chain of the nucleating agent stabilizer is within a suitable range. This can avoid too short an alkyl chain, which would reduce its compatibility and dispersibility with non-polar molten polypropylene, making the addition of the nucleating agent stabilizer unfavorable for the transparency of polypropylene. It can also avoid too long an alkyl chain, which would lead to a significant steric effect and be unfavorable for the uniform crystallization of crystal nuclei, also having a negative impact on the transparency performance of polypropylene due to the addition of the nucleating agent stabilizer. When m, n, x, and y in Formula I are each independently greater than or equal to 10 and less than or equal to 15, the nucleating agent stabilizer shown in Formula I can have good dispersibility, easily form a homogeneous phase with molten PP, inhibit the decomposition of sorbitol acetal, reduce the odor of polypropylene, and have no negative impact on the transparency of polypropylene when added in trace amounts as a nucleating agent stabilizer.

[0012] In any embodiment, in step (3), the amount of the compound shown in Formula I is 0.1% - 0.5%, based on the mass of the polypropylene matrix resin.

[0013] When the amount of the compound shown in Formula I is within the above range, it can consume substances such as water and acid present in the preparation process of polypropylene, avoid the decomposition of unstable sorbitol acetal in polypropylene to produce aldehyde substances, and can also consume trace aldehyde substances that may be present in polypropylene, thereby avoiding obvious odor in the polypropylene finished product. It can also avoid too many amorphous regions in polypropylene due to excessive use, which is beneficial to reducing the scattering and refraction of light inside polypropylene, and the addition of the nucleating agent stabilizer has no obvious adverse effect on the transparency of polypropylene.

[0014] In any embodiment, step (1) specifically includes: mixing o-xylene and 1,2-dichloroethane, adding aluminum trichloride and hydrochloric acid, reacting at a temperature of -10°C to -5°C under a carbon monoxide atmosphere for 2 h - 3 h; washing the reaction solution with water at -10°C to -5°C, standing for separation, removing the lower-layer solution, obtaining the upper-layer oily substance, adding an imidazole-based ionic liquid to the oily substance, carrying out a catalytic reaction, then washing the reaction solution again with water at -10°C to -5°C, standing for separation, removing the lower-layer solution, obtaining the upper-layer liquid, and removing the solvent in the liquid to obtain the prepared dimethylbenzaldehyde, and the dimethylbenzaldehyde includes 3,4-dimethylbenzaldehyde and 3,5-dimethylbenzaldehyde.

[0015] When the aldehyde group and methyl group on dimethylbenzaldehyde have uneven charge distribution on the benzene ring (such as 3,4-dimethylbenzaldehyde, 2,3-dimethylbenzaldehyde, 2,5-dimethylbenzaldehyde, etc.), they tend to rearrange in the most stable manner with the lowest energy structure (such as 3,5-dimethylbenzaldehyde). Based on the discovery that the transparent nucleating agent obtained from the reaction of 3,5-dimethylbenzaldehyde and sorbitol has better transparency-enhancing performance for polypropylene than other isomers, the applicant further uses imidazole-based ionic liquids as catalysts to catalyze the electrophilic rearrangement reaction of dimethylbenzaldehyde, thereby increasing the content of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde, and then enhancing the transparency-enhancing performance of the transparent nucleating agent for polypropylene, which is beneficial to further reducing the haze of polypropylene. The cation of the imidazole-based ionic liquid can exhibit certain acidity and can act as a proton donor to promote the conversion of the methyl group on the benzene ring into an electrophilic CH 3 + , and then through rearrangement with the benzene ring to form 3,5-dimethylbenzaldehyde with lower energy and more stable structure. The polar environment provided by the imidazole-based ionic liquid also helps to stabilize the electrophilic intermediate. Compared with proton acids such as hydrochloric acid, imidazole-based ionic liquids have the advantages of being green and environmentally friendly, recyclable, structurally designable, and mild, which can reduce the occurrence of side reactions and improve the selectivity of the target product.

[0016] In any embodiment, the imidazole-based ionic liquid includes one or more of 1-methyl-3-butylimidazolium hydrogen sulfate, 1-butyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methyltetrafluoroborate.

[0017] The anions of the above imidazole-based ionic liquids such as hydrogen sulfate HSO 4 - , hexafluorophosphate PF 6 ⁻ and tetrafluoroborate BF 4 ⁻ are further beneficial to the conversion of the methyl group on the benzene ring into an electrophilic CH 3 + , thus further facilitating the electrophilic rearrangement reaction of dimethylbenzaldehyde catalyzed by the imidazole-based ionic liquid and improving the selectivity of the target product. Among them, hydrogen sulfate HSO 4 - can further provide protons to promote the conversion of the methyl group on the benzene ring into an electrophilic CH 3 + because it can dissociate hydrogen ions, which makes the catalytic effect of 1-methyl-3-butylimidazolium hydrogen sulfate on the electrophilic rearrangement reaction of dimethylbenzaldehyde more excellent and is beneficial to further improving the transparency of polypropylene.

[0018] In any embodiment, the dosage of the imidazole-based ionic liquid is 0.01% - 0.1%, based on the total mass of the oily substance.

[0019] When the dosage of the imidazole-based ionic liquid is within the above range, it can avoid the catalytic electrophilic rearrangement reaction rate being too slow due to too little catalyst dosage, resulting in too low mass proportion of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde, which is not conducive to improving the transparency of polypropylene; it can also avoid the reaction rate being too fast or affecting the normal progress of the reaction due to too much catalyst dosage, leading to too high mass proportion of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde or too many side reactions. Since the addition amount of the nucleating agent stabilizer needs to be controlled within a certain range during the preparation of polypropylene, the presence of too much unstable sorbitol acetal will cause its decomposition to be unable to be inhibited, resulting in an odor in the polypropylene finished product. When the dosage of the imidazole-based ionic liquid is within the above range, polypropylene has excellent transparency and no odor.

[0020] In any embodiment, the temperature of the catalytic reaction is -10°C to -5°C, and the reaction time is 1h - 6h.

[0021] When the temperature of the catalytic reaction is within the above range, it is conducive to the occurrence of the electrophilic rearrangement reaction, improving the content of 3,5-dimethylbenzaldehyde in the prepared dimethylbenzaldehyde; when the reaction time of the catalytic reaction is within the above range, the content of 3,5-dimethylbenzaldehyde in the prepared dimethylbenzaldehyde can be within a suitable range, avoiding too little content of 3,5-dimethylbenzaldehyde due to too short reaction time, which is not conducive to improving the transparency of polypropylene; it can also avoid too much content of 3,5-dimethylbenzaldehyde or too many side reactions due to too long reaction time, reducing the production efficiency and also making polypropylene produce an obvious odor. When the reaction time of the catalytic reaction is within the above range, the transparent nucleating agent makes polypropylene have excellent transparency and no obvious odor.

[0022] In any embodiment, based on the total mass of the dimethylbenzaldehyde, the mass proportion of 3,5-dimethylbenzaldehyde in the dimethylbenzaldehyde is 8% - 40%.

[0023] When the mass proportion of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde is 8% - 40%, the transparent nucleating agent prepared from it and sorbitol makes polypropylene have excellent transparency, and the nucleating agent stabilizer added to the polypropylene raw material effectively inhibits the decomposition of sorbitol acetal. Polypropylene has excellent transparency and no obvious odor.

[0024] In any embodiment, in step (3), the dosage of the transparent nucleating agent is 0.1% - 0.5%, based on the mass of the polypropylene matrix resin.

[0025] Based on the mass of the polypropylene matrix resin, when the dosage of the transparent nucleating agent is within the above range, the polypropylene has excellent transparency and no obvious peculiar smell is generated.

[0026] In any embodiment, in step (3), the auxiliary agent includes one or more of antioxidant 1010, antioxidant 168, and calcium stearate.

[0027] The above-mentioned auxiliary agent is beneficial to improving the mechanical properties of polypropylene and prolonging the service life of polypropylene.

[0028] In summary, the present application has the following beneficial effects:

[0029] (1) In the preparation of the transparent nucleating agent of the present application, the product after the reaction of o-xylene is directly reacted with sorbitol. The dimethylbenzaldehyde does not need to go through the process of rectification and purification that takes a long time and requires precise equipment, and is used as a raw material to prepare the transparent nucleating agent, which greatly reduces the raw material cost of the transparent nucleating agent, thereby reducing the production cost of the transparent nucleating agent. Further, by adding the nucleating agent stabilizer shown in formula I to the raw material for preparing polypropylene, the ester group in the nucleating agent stabilizer can consume water molecules and acidic substances in the reaction environment, thereby preventing their attack on the acetal bond of sorbitol acetal, playing a role in inhibiting the decomposition of sorbitol acetal, and thus avoiding obvious peculiar smell in the finished polypropylene. And the applicant unexpectedly found in the experiment that compared with the sorbitol acetal transparent nucleating agent formed by reacting 3,4-dimethylbenzaldehyde with a purity of more than 99% with sorbitol, the sorbitol acetal transparent nucleating agent formed by reacting dimethylbenzaldehyde without rectification and purification with sorbitol can further improve the transparency of polypropylene. Therefore, by using the preparation method of polypropylene of the present application, while reducing the production cost of the transparent nucleating agent, the prepared polypropylene has both less odor and excellent transparency.

[0030] (2) The applicant uses imidazole-based ionic liquid as a catalyst to catalyze the electrophilic rearrangement reaction of dimethylbenzaldehyde, thereby increasing the content of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde, and then improving the transparency-enhancing performance of the transparent nucleating agent for polypropylene, which is beneficial to further reducing the haze of polypropylene. Specific Embodiments

[0031] The present application will be further described in detail below with reference to examples and comparative examples.

[0032] In the following examples, those not specified in detail were carried out under conventional conditions or conditions recommended by the manufacturer. Except as otherwise specified, the raw materials used in the following examples were all available from ordinary commercial sources. Examples Example 1

[0033] A method for preparing polypropylene, comprising the following steps:

[0034] (1) Preparation of dimethylbenzaldehyde

[0035] Add 100 parts by weight of o - xylene and 200 parts by weight of 1,2 - dichloroethane. Start the stirrer and pre - cool the materials in the reaction flask to - 20°C to - 10°C. Then, add 3 - 4 drops of concentrated hydrochloric acid and 130 parts by weight of the catalyst anhydrous AlCl 3 , and introduce nitrogen as the displacement gas. Control the operating temperature at - 10°C to - 5°C, introduce carbon monoxide, keep the partial pressure of carbon monoxide at 1.0 Mpa, and start timing the reaction. After reacting for 2.5 h, stop the reaction, close the carbon monoxide ventilation valve, continue stirring, and then pour the above reactants into an ice - brine flask with a temperature of - 10°C to - 5°C that has been prepared. After stirring, place it in a separatory funnel, extract and shake well, let it stand for stratification, separate the lower acid water layer, and obtain the upper oily substance. Put it into a distillation flask and distill it under normal pressure to remove the solvent to obtain the prepared dimethylbenzaldehyde. Through testing, it is calculated that the mass proportion of 3,5 - dimethylbenzaldehyde in dimethylbenzaldehyde is 8%.

[0036] (2) Preparation of transparent nucleating agent

[0037] Add 400 parts by mass of cyclohexane and 60 parts by mass of methanol. Then, add 30 parts by mass of sorbitol, 50 parts by mass of the above - prepared dimethylbenzaldehyde, and 3 parts by mass of p - toluenesulfonic acid catalyst in sequence. Heat up to 70°C and carry out an aldol condensation reaction under a nitrogen atmosphere. Separate the mixture of methanol and water by a liquid - liquid separator. After reacting for 6 hours, cool down, and when the temperature drops to 50°C, add an alkaline solution to neutralize and remove the acidic catalyst. Distill off the solvent, filter, and vacuum - dry to obtain the transparent nucleating agent product.

[0038] (3) Preparation of polypropylene

[0039] Mix the prepared transparent nucleating agent, the compound shown in formula I, antioxidant 1010, antioxidant 168, calcium stearate, and polypropylene matrix resin evenly in a high - speed mixer according to the mass ratio of 2:3:0.5:1:0.5:1000. Extrude the mixture through a twin - screw extruder to prepare polypropylene. For the compound shown in formula I, m, n, x, and y are 12, and the compound shown in formula I is pentaerythritol tetramyristate. Among them, the temperatures of each zone of the extruder from the feeding port to the head are 190°C, 200°C, 210°C, 218°C, 210°C, 200, 190°C respectively.

[0040] Prepare the standard specimens required for haze test and odor test by injection molding the prepared polypropylene in an injection molding machine. The temperature from the first stage to the head of the injection molding machine is 200°C - 230°C.

[0041] Comparative Example 1

[0042] The preparation method of the polypropylene in Comparative Example 1 was basically similar to that in Example 1, except that in the preparation of the transparent nucleating agent in step (2), commercially available 3,4-dimethylbenzaldehyde with a purity of over 99% was used.

[0043] Comparative Example 2

[0044] The preparation method of the polypropylene in Comparative Example 2 was basically similar to that in Example 1, except that in the preparation of the polypropylene in step (3), the compound shown in Formula I was not added.

[0045] Comparative Example 3

[0046] The preparation method of the polypropylene in Comparative Example 3 was basically similar to that in Example 1, except that in the preparation of the polypropylene in step (3), the compound shown in Formula I was not added, and instead, pentaerythritol was added.

[0047] Performance testing

[0048] 1. Haze

[0049] Splinters with a specification of 50 mm × 50 mm × 1 mm were prepared and tested according to the standard of ASTM D1003-07.

[0050] 2. Odor

[0051] Using the odor test standard VDA270-2018, splinters with a specification of 50 mm × 50 mm × 20 mm were prepared and placed in a 500-ml glass bottle, and the bottle cap was tightened. The glass bottle and the bottle cap were cleaned before use to keep them clean and the odor neutral. At ambient temperature, it was placed for one day and then the odor evaluation was carried out. Five people participated in the odor evaluation, and the odor evaluation used a 6-level evaluation standard:

[0052] Level 1 indicates no odor, Level 2 indicates there is an odor but it is not disturbing, Level 3 indicates there is an obvious odor but it is still not disturbing, Level 4 indicates there is a disturbing odor, Level 5 indicates there is a strong disturbing odor, and Level 6 indicates unbearable; the evaluation result can be described in half-level, and the average value is taken as the final odor level result. The lower the level, the smaller the odor.

[0053] 3. Content of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde

[0054] The prepared dimethylbenzaldehyde was detected by gas chromatography (GC) to calculate the mass proportion of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde.

[0055] The polypropylene samples of Example 1 and Comparative Examples 1-3 were prepared for performance testing, and the test results are shown in Table 1 respectively.

[0056] Table 1 Performance Detection Table of Polypropylene in Example 1 and Comparative Examples 1-2

[0057] Serial number Mass ratio of 3,5-dimethylbenzaldehyde Haze / % Odor Example 1 8% 8.7 1 Comparative Example 1 / 10.6 2 Comparative Example 2 8% 9.9 5 Comparative Example 3 8% 10.3 4

[0058] Referring to Table 1, it can be seen from the comparison of Comparative Example 1 and Comparative Examples 1, 2, and 3 that the preparation method of the present application directly uses the dimethylbenzaldehyde after the reaction and adds the compound shown in Formula I, which greatly reduces the production cost of the transparent nucleating agent, avoids the generation of odor, and further reduces the haze of polypropylene. Comparative Example 1 uses 3,4-dimethylbenzaldehyde with a purity of more than 99% as a raw material, which is costly, to prepare a transparent nucleating agent for the preparation of polypropylene. Since the raw material may still contain trace isomers that cannot be completely separated, when the compound shown in Formula I is not added, the polypropylene in Comparative Example 1 has an odor. Comparative Example 2 directly uses the dimethylbenzaldehyde after the reaction as a raw material to prepare a transparent nucleating agent, but in the case of not adding the compound shown in Formula I to inhibit the decomposition of sorbitol acetal and remove aldehyde substances, the polypropylene has a strong peculiar smell, which is not conducive to the application of polypropylene products in the fields of food, cosmetics, pharmaceuticals, etc. Comparative Example 3 directly uses the dimethylbenzaldehyde after the reaction as a raw material to prepare a transparent nucleating agent, and pentaerythritol is added to the reaction raw material for preparing polypropylene. Although it plays a certain role in inhibiting odor, the effect is weak, and it has a negative impact on the transparency of polypropylene.

[0059] Examples 2-5

[0060] The preparation methods of the polypropylene in Examples 2-5 are basically similar to that of Example 1, except that the structure of the compound shown in Formula I added in step (3) has changed, specifically as follows:

[0061] In Example 2, m, n, x, and y of the compound shown in Formula I are 10, and the compound shown in Formula I is pentaerythritol tetralaurate;

[0062] In Example 3, m, n, x, and y of the compound shown in Formula I are 14, and the compound shown in Formula I is pentaerythritol tetrapalmitate;

[0063] In Example 4, m, n, x, and y of the compound shown in Formula I are 5, and the compound shown in Formula I is pentaerythritol tetra-n-heptanoate;

[0064] In Example 5, m, n, x, and y of the compound shown in Formula I are 16, and the compound shown in Formula I is pentaerythritol tetrastearate;

[0065] The polypropylene samples in Examples 2-5 were prepared for performance detection, and the test results are shown in Table 2 respectively.

[0066] Table 2 Performance Detection Table of Polypropylene in Example 1 and Examples 2-5

[0067] Serial number Mass ratio of 3,5-dimethylbenzaldehyde Haze / % Odor Example 1 8% 8.7 1 Example 2 8% 9.2 1 Example 3 8% 9.7 1 Example 4 8% 9.9 1 Example 5 8% 10.4 1

[0068] Referring to Table 2, by comparing Examples 1 - 3 and Examples 4 - 5, it can be seen that when m, n, x, and y of the compound shown in Formula I are greater than or equal to 10 and less than or equal to 15, it is beneficial to further reduce the haze of polypropylene. This may be because when the length of the carbon chain of the compound shown in Formula I connected to the ester group is within a suitable range, it is easy to form a homogeneous phase with molten PP and will not affect the crystallization of polypropylene. Thus, while avoiding the generation of odor in polypropylene, polypropylene also has excellent transparency. When m, n, x, and y of the compound shown in Formula I are less than 10 and greater than 15, the transparency of polypropylene may decrease because the carbon chain is too short to be conducive to its dispersion and too long to be conducive to the crystallization of polypropylene.

[0069] Examples 6 - 9

[0070] The preparation methods of the polypropylenes in Examples 6 - 9 are basically similar to that in Example 1, except that the dosage of the compound shown in Formula I in step (3) is changed, specifically as follows:

[0071] The dosage of the compound shown in Formula I in Example 6 is 0.1% based on the mass of the polypropylene matrix resin;

[0072] The dosage of the compound shown in Formula I in Example 7 is 0.5% based on the mass of the polypropylene matrix resin;

[0073] The dosage of the compound shown in Formula I in Example 8 is 0.05% based on the mass of the polypropylene matrix resin;

[0074] The dosage of the compound shown in Formula I in Example 9 is 0.8% based on the mass of the polypropylene matrix resin;

[0075] The polypropylenes of Examples 6 - 9 were made into samples for performance testing, and the test results are shown in Table 3 respectively.

[0076] Table 3 Performance test table of the polypropylenes of Example 1 and Examples 6 - 9

[0077] Serial number Mass ratio of 3,5-dimethylbenzaldehyde Haze / % Odor Example 1 8% 8.7 1 Example 6 8% 8.3 2 Example 7 8% 9.8 1 Example 8 8% 8.1 3 Example 9 8% 10.3 1

[0078] Referring to Table 3, by comparing Example 1, Examples 6, 7 and Examples 8, 9, it can be seen that when the dosage of the compound shown in Formula I is 0.1% - 0.5%, it is beneficial to both inhibit the generation of odor in polypropylene and make polypropylene have excellent transparency. When the addition amount of the compound shown in Formula I is less than 0.1%, the inhibition of the decomposition of sorbitol acetal is limited, making the prepared polypropylene prone to obvious odor; when the addition amount of the compound shown in Formula I is greater than 0.5%, there will be too many amorphous regions in polypropylene, thus reducing the transparency of polypropylene. Example 10

[0079] The preparation method of the polypropylene in Example 10 is basically similar to that in Example 1, except that in step (1), an imidazole-based ionic liquid is further added. The specific preparation method of step (1) is as follows:

[0080] Add 100 parts by weight of o-xylene and 200 parts by weight of 1,2-dichloroethane, start the stirrer, pre-cool the materials in the reaction flask to -20°C to -10°C, and then add 3 - 4 drops of concentrated hydrochloric acid and 130 parts by weight of the catalyst anhydrous AlCl 3 , and introduce nitrogen as the displacement gas; control the operating temperature at -10°C to -5°C, introduce carbon monoxide, keep the partial pressure of carbon monoxide at 1.0 Mpa, and start timing the reaction. After reacting for 2.5 h, stop the reaction, close the carbon monoxide gas inlet valve, continue stirring, and then pour the above reactants into an ice-salt water flask with a prepared temperature of -10°C to -5°C. After stirring, place it in a separating funnel, extract and shake well, let it stand for stratification, separate the lower acid water layer, and obtain the upper oily substance; add 0.05% of 1-methyl-3-butylimidazolium hydrogen sulfate based on the total mass of the oily substance to the oily substance, carry out a catalytic reaction at -5°C for 4 h, and then pour the above reactants into an ice-salt water flask with a prepared temperature of -10°C to -5°C. After stirring, place it in a separating funnel, extract and shake well, let it stand for stratification, separate the lower layer solution, and obtain the upper layer solution; put the upper layer solution into a distillation flask and distill it at atmospheric pressure to remove the solvent to obtain the prepared dimethylbenzaldehyde. It is tested that in the obtained dimethylbenzaldehyde, the mass proportion of 3,5-dimethylbenzaldehyde is 24%.

[0081] And in the preparation of polypropylene in step (3), the mass ratio of the compound shown in formula I to the polypropylene matrix resin is 5:1000.

[0082] Examples 11 - 14

[0083] The preparation methods of the polypropylenes in Examples 11 - 14 are basically similar to that in Example 10, except that the dosage of 1-methyl-3-butylimidazolium hydrogen sulfate in step (1) is changed, specifically as follows:

[0084] In Example 11, the dosage of 1-methyl-3-butylimidazolium hydrogen sulfate is 0.01% of the total mass of the oily substance;

[0085] In Example 12, the dosage of 1-methyl-3-butylimidazolium hydrogen sulfate is 0.1% of the total mass of the oily substance;

[0086] In Example 13, the dosage of 1-methyl-3-butylimidazolium hydrogen sulfate is 0.005% of the total mass of the oily substance;

[0087] In Example 14, the amount of 1-methyl-3-butylimidazolium hydrogensulfate is 0.2% of the total mass of the oily substance;

[0088] The polypropylene samples of Examples 10-14 were prepared for performance testing, and the test results are shown in Table 4 respectively.

[0089] Table 4 Performance test table of polypropylene in Example 1 and Examples 10-14

[0090] Serial number Mass ratio of 3,5-dimethylbenzaldehyde Haze / % Odor Example 1 8% 8.7 1 Example 10 24% 7.1 1 Example 11 16% 7.7 1 Example 12 38% 6.8 2 Example 13 12% 8.2 1 Example 14 43% 6.6 4

[0091] Referring to Table 4, it can be seen from the comparison between Example 1 and Example 10 that further adding imidazole-based ionic liquid to catalyze the electrophilic rearrangement reaction is beneficial to increasing the content of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde. When used for preparing a transparent nucleating agent for the preparation of polypropylene, in combination with the compound shown in Formula I to inhibit the decomposition of sorbitol acetal, while further reducing the haze of polypropylene, no peculiar smell is generated. It can be seen from the comparison between Examples 10-12 and Examples 13, 14 that when the amount of imidazole-based ionic liquid is 0.01% to 0.1% of the total mass of the oily substance, the polypropylene has excellent transparency and no obvious peculiar smell. When the amount of imidazole-based ionic liquid is less than 0.01%, the content of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde is too small, which is not conducive to improving the transparency of polypropylene; when the amount of imidazole-based ionic liquid is greater than 0.1%, since the content of 3,5-dimethylbenzaldehyde in dimethylbenzaldehyde is too large, the content of unstable sorbitol acetal in the transparent nucleating agent is too large, resulting in peculiar smell in polypropylene.

[0092] Examples 15-18

[0093] The preparation methods of the polypropylene in Examples 15-18 are basically similar to that of Example 10, except that the catalytic reaction time in step (1) is changed, specifically as follows:

[0094] The catalytic reaction time of Example 15 is 1 h;

[0095] The catalytic reaction time of Example 16 is 6 h;

[0096] The catalytic reaction time of Example 17 is 0.5 h;

[0097] The catalytic reaction time of Example 18 is 8 h;

[0098] The polypropylene samples of Examples 15-18 were prepared for performance testing, and the test results are shown in Table 5 respectively.

[0099] Table 5 Performance test table of polypropylene in Example 10 and Examples 15-18

[0100] Serial number Mass ratio of 3,5-dimethylbenzaldehyde Haze / % Odor Example 10 24% 7.1 1 Example 15 17% 7.6 1 Example 16 35% 6.9 2 Example 17 10% 8.6 1 Example 18 33% 7.0 3

[0101] Referring to Table 5, it can be seen from the comparison of Examples 10 and 15 - 18 that the time for the catalytic electrophilic rearrangement reaction is 1 h - 6 h, which is beneficial to controlling the content of 3,5 - dimethylbenzaldehyde in dimethylbenzaldehyde within a suitable range. While the polypropylene has excellent transparency, there is no obvious odor generated. When the time for the catalytic electrophilic rearrangement reaction is less than 1 h, the content of 3,5 - dimethylbenzaldehyde in dimethylbenzaldehyde is too small, which is not conducive to improving the transparency of polypropylene; when the time for the catalytic electrophilic rearrangement reaction is greater than 6 h, it is not conducive to further increasing the content of 3,5 - dimethylbenzaldehyde in dimethylbenzaldehyde, and may lead to the generation of by - products, resulting in the generation of odor in polypropylene. Example 19

[0102] The preparation method of the polypropylene in Example 19 is basically similar to that in Example 10, except that the type of imidazole - based ionic liquid in step (1) is changed. The imidazole - based ionic liquid used in Example 19 is 1 - butyl - 3 - methylimidazolium hexafluorophosphate.

[0103] Comparative Example 4

[0104] The preparation method of the polypropylene in Comparative Example 4 is basically similar to that in Example 10, except that concentrated hydrochloric acid is used as the catalyst in Comparative Example 4.

[0105] The polypropylene samples of Example 19 and Comparative Example 4 were prepared for performance testing, and the test results are shown in Table 6 respectively.

[0106] Table 6 Performance test table of the polypropylene of Example 19 and Comparative Example 4

[0107] Serial number Mass ratio of 3,5-dimethylbenzaldehyde Haze / % Odor Example 10 24% 7.1 1 Example 19 17% 7.7 1 Comparative Example 4 37% 6.8 5

[0108] Referring to Table 6, it can be seen from the comparison of Example 10 and Example 19 that when the imidazole - based ionic liquid is 1 - methyl - 3 - butylimidazolium hydrogensulfate, it is more conducive to catalyzing the electrophilic rearrangement reaction, which is beneficial to increasing the content of 3,5 - dimethylbenzaldehyde in dimethylbenzaldehyde, thereby reducing the haze of polypropylene. It can be seen from the comparison of Example 10, 19 and Comparative Example 4 that when hydrochloric acid is used as the catalyst, the rate of the catalytic electrophilic rearrangement reaction is too intense and by - products are easily generated, resulting in obvious peculiar smell in polypropylene. Compared with hydrochloric acid, using imidazole - based ionic liquid as the catalyst has mildness, can reduce the occurrence of side reactions, and while improving the transparency of polypropylene, polypropylene does not generate peculiar smell.

[0109] This specific embodiment is only an interpretation of the present application, and it is not a limitation of the present application. Those skilled in the art can make modifications without creative contributions to this embodiment according to needs after reading this specification, but as long as it is within the scope of the claims of the present application, it is protected by the patent law.

Claims

1. A method for preparing transparent polypropylene, characterized in that: The following steps are involved: (1) Preparation of dimethylbenzaldehyde: o-xylene and 1,2-dichloroethane are mixed, aluminum chloride and hydrochloric acid are added, and the mixture is reacted at a temperature of -10°C to -5°C for 2h-3h under a carbon monoxide atmosphere; the reaction solution is washed with water at -10°C to -5°C, the lower layer of solution is removed after static stratification to obtain an upper layer of oily substance, an imidazole ionic liquid is added to the oily substance, and a catalytic reaction is carried out, and the reaction solution is washed again with water at -10°C to -5°C, the lower layer of solution is removed after static stratification to obtain an upper layer of liquid, and the solvent in the liquid is removed to obtain the prepared dimethylbenzaldehyde, wherein the dimethylbenzaldehyde includes 3,4-dimethylbenzaldehyde and 3,5-dimethylbenzaldehyde; (2) Preparation of a transparent nucleating agent: mixing the dimethylbenzaldehyde with sorbitol, cyclohexane, methanol, and p-toluenesulfonic acid, and reacting the mixture to prepare a transparent nucleating agent; (3) Preparation of polypropylene: The transparent nucleating agent, the compound represented by formula I, the auxiliary agent and the polypropylene matrix resin are mixed to obtain a polypropylene processing raw material, and the raw material is added into an extruder for extrusion to prepare polypropylene. Formula I Wherein, m, n, x, and y in Formula I are each independently greater than or equal to 10 and less than or equal to 15.

2. The preparation method according to claim 1, characterized in that: In step (3), the amount of the compound represented by formula I is 0.1%-0.5%, based on the mass of the polypropylene matrix resin.

3. The preparation method according to claim 1, characterized in that: The imidazolium ionic liquid includes one or more of 1-methyl-3-butylimidazolium hydrogen sulfate, 1-butyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methyltetrafluoroborate.

4. The preparation method according to claim 1, characterized in that: The amount of the imidazole ionic liquid used is 0.01%-0.1%, based on the total mass of the oil.

5. The preparation method according to claim 1, characterized in that: The temperature of the catalytic reaction is -10°C to -5°C, and the reaction time is 1h-6h.

6. The preparation method according to any one of claims 1 to 5, characterized in that Based on the total mass of the dimethylbenzaldehyde, the mass proportion of 3,5-dimethylbenzaldehyde in the dimethylbenzaldehyde is 8%-40%.

7. The preparation method according to claim 1, characterized in that: In step (3), the amount of the transparent nucleating agent is 0.1%-0.5%, based on the mass of the polypropylene matrix resin.

8. The preparation method according to claim 1, characterized in that: In step (3), the auxiliary agent includes one or more of antioxidant 1010, antioxidant 168, and calcium stearate.

Images