Method for improving tensile strength and thermal stability of polypropylene filled with oil shale residue

Abstract

The application provides a method for improving tensile strength and thermal stability of oil shale residue filled polypropylene, and belongs to the technical field of polymer materials and solid waste resource utilization. The application optimizes the particle size of oil shale residue (OSA), fills the treated OSA into a polypropylene (PP) matrix, and adopts a melting compounding and hot pressing forming process to prepare a PP / OSA composite material. When the ball milling time is 40 min and the OSA filling amount is 15 wt.%, the comprehensive performance of the composite material is optimal, the tensile strength reaches 19.03 MPa, which is increased by 22.85% compared with that of pure PP, the initial thermal decomposition temperature is delayed by 27 DEG C, the residual mass at 650 DEG C is increased by 12.41%, and the crystallinity is increased by 3.72%. The composite material prepared by the method can be applied to building materials, product packaging and agricultural films, and has wide market prospect and important economic benefits.

Description

Technical Field

[0001] This invention relates to the field of composite materials, specifically to a method for improving the tensile strength and thermal stability of polypropylene filled with oil shale slag. This invention belongs to the interdisciplinary field of polymer material modification and comprehensive utilization technology of solid waste resources. The resulting composite material, due to its excellent comprehensive performance and low cost, is suitable for manufacturing building materials (such as formwork, partitions, and decorative panels), product packaging, agricultural films, and outdoor weather-resistant high-strength products, aligning with national policies on circular economy and green building materials industry. Background Technology

[0002] Polypropylene (PP) is a commonly used thermoplastic material in the construction industry, widely applied due to its low cost and good processability. However, its mechanical strength and thermal stability need further improvement. Filler modification is a common method to enhance PP performance. In existing technologies, glass fiber and nanoclay are commonly used fillers for improving PP performance, but they also have significant drawbacks. For example, glass fiber is prone to breakage during building material processing, resulting in exposed fibers on the surface of the product, and its cost is as high as 8-12 yuan / kg (5-8 times that of OSA). Nanoclay requires complex organic modification (increasing modification costs by 30%) and is prone to agglomeration in PP (agglomeration rate ≥15%), leading to large fluctuations in the mechanical properties of the composite material. Therefore, developing economical and effective PP modification technologies to further improve its comprehensive performance has become a research hotspot.

[0003] Jilin Province has proven oil shale reserves of 108.6 billion tons, accounting for over 80% of the national total. With an average oil content of 5%, the complete utilization of oil shale would generate nearly 100 billion tons of waste residue. The generation of oil shale residue not only damages the ecological environment and endangers human health but also places a heavy burden on enterprises. Therefore, the treatment of oil shale residue is urgent. Oil shale residue is rich in inorganic components such as SiO2 (45-60 wt.%) and Al2O3 (15-28 wt.%), and has high hardness, good chemical stability, and low cost, making it an ideal filler material for construction.

[0004] However, using OSA directly as a filler in PP faces significant challenges. The inertness of the OSA surface and its poor interfacial compatibility with the non-polar PP matrix lead to stress concentration, often resulting in a decline in material properties rather than an improvement. Therefore, improving interfacial bonding is a key technical challenge. Al-Shurafa et al. (Asian J. Chem. 2021, 33:2175-2181) reported the preparation of OSA-filled PP composites, using 63-106 μm OSA to fill PP, but without precise ball milling and interfacial modification. This resulted in decreased mechanical properties (tensile strength decreased by 12.2%, flexural strength decreased by 28%), severely deteriorated crystallinity (decreased by 42.4%), high water absorption (up to 3.98%), agglomeration, and interfacial delamination (OSA agglomeration rate ≥20%, Turcsanyi model B value ≤0.7). Illia Krasnou et al. (J. VinylAddit. Technol. 2022, 28:94-103) investigated the feasibility of using OSA as a filler in low-density polyethylene (LDPE), but also found issues such as complex processing and decreased crystallinity. These studies confirm the potential of OSA as a filler, but most remain at the level of simple mixing and filling, with insufficient research on the morphology control of the filler itself and its fundamental impact on interfacial properties.

[0005] Therefore, developing a method for preparing composite materials by modifying and filling polypropylene with oil shale slag and improving its tensile strength and thermal stability has significant scientific and practical value. Based on this, this invention proposes a method for improving the tensile strength and thermal stability of polypropylene filled with oil shale slag. By precisely controlling the ball milling parameters, the particle size and surface state of OSA filler are effectively optimized, improving the mechanical and thermal properties of the composite material. The treated material with 15 wt.% OSA exhibits excellent reinforcing effect in the PP matrix, increasing the strength by 32.98% compared to the industrial standard high-quality filler 2500 mesh heavy calcium carbonate powder (2500CC) used under the same conditions. Summary of the Invention

[0006] The purpose of this invention is to provide a method for improving the tensile strength and thermal stability of polypropylene filled with oil shale slag, aiming to solve the problems mentioned in the background art, so as to achieve efficient resource utilization and significant improvement in the performance of composite materials.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for improving the tensile strength and thermal stability of polypropylene filled with oil shale residue (OSA) is characterized by: optimizing the particle size of oil shale residue (OSA) through dry ball milling, filling the treated OSA into a polypropylene (PP) matrix as a functional filler, adding stearic acid to improve interfacial compatibility, and preparing PP / OSA composite materials using melt composite and hot pressing processes; the method specifically includes the following steps:

[0009] S1) The blocky oil shale slag is subjected to primary crushing to obtain OSA small pieces with a particle size of less than 5 mm;

[0010] S2) Place the OSA small pieces obtained in step S1) into a ball mill for dry ball milling, and obtain the particle size of OSA powder by controlling the ball milling parameters;

[0011] S3) Pass the OSA powder obtained in step S2) through a 200-mesh sieve to obtain OSA fine powder with uniform particle size distribution.

[0012] S4) Place the OSA fine powder obtained in step S3) in a drying oven and dry it at 110-130 ℃ for 8-16 hours;

[0013] S5) Weigh polypropylene (PP) granules and lubricant stearic acid according to the predetermined ratio, add them to a small internal mixer, and carry out preliminary melting and mixing at a temperature of 190-210 ℃;

[0014] S6) After the PP has completely melted, add the dried OSA fine powder from step S4) into the internal mixer in batches and continue stirring and mixing for 5-15 minutes.

[0015] S7) The PP / OSA composite material prepared in step S6) is preheated at 200±5 ℃ for 3-5 min on a flat vulcanizing apparatus, and then hot-pressed at 3±0.5 MPa for 3-5 min.

[0016] S8) After hot pressing, immediately switch to cold pressing mode and cool and shape under the same pressure for 3-5 minutes to improve the dimensional stability of the composite material and obtain PP / OSA composite board.

[0017] S9) Cut the PP / OSA composite sheet obtained in step S8) into strips for mechanical and thermal performance testing using a standard cutter.

[0018] Furthermore, in step S2), the rotational speed of the ball mill is 350-450 r / min, the ball-to-material mass ratio is 8:1 to 12:1, and the ball milling time is 35-45 min. Under these conditions, the average particle size of the obtained OSA powder reaches a minimum of 5.5-6.0 μm, the particle size distribution variance is ≤0.8 μm, and the agglomeration rate is ≤5%.

[0019] Furthermore, in step S4), the specific conditions for the drying process are continuous drying at 120°C for 12 hours.

[0020] Furthermore, in steps S5) and S6), the mixing temperature of the small internal mixer is 190-210℃, with a temperature control accuracy of ±2℃; the rotation speed is 40-60 rpm / min, and the total mixing time is 8-12 min.

[0021] Furthermore, in step S6), the filling amount of OSA fine powder in the PP / OSA composite material is 5-30 wt.%, preferably 15 wt.%; the amount of stearic acid added is 0.5-1.5 wt.% of the total mass of the composite material, preferably 1 wt.%.

[0022] Furthermore, in step S7), the preheating temperature of the flat vulcanizing apparatus is 200 ℃, the preheating time is 3 min, the hot pressing pressure is 3 MPa, the hot pressing time is 3 min, and the hot pressing pressure fluctuation is ≤0.1MPa.

[0023] Furthermore, in step S8), the cold pressing time is 3 minutes.

[0024] Furthermore, the PP / OSA composite material specimens prepared in step S9) are subjected to tensile strength testing. The test specimens are Type I, and the standard tensile test specimens for Type I are the standard specimens for building plastics specified in GB / T 1040-2006. The test environment temperature is 25±2℃, the relative humidity is 50±5%, and the tensile speed is the standard speed of 50 mm / min. Five samples are tested in each group, and the average value is taken to avoid errors.

[0025] Furthermore, the PP / OSA composite material prepared in step S9) was tested according to ASTM E1131 standard, and its thermal stability was tested using a microcomputer differential thermal balance at a heating rate of 10 °C / min. The test conditions were room temperature to 650 °C at a heating rate of 10 °C / min.

[0026] Furthermore, the PP / OSA composite material prepared in step S9) was subjected to brittle fracture under liquid nitrogen freezing, and the dispersion of OSA and its adhesion to the matrix in the low-temperature frozen fracture surface were observed using field emission scanning electron microscopy.

[0027] Furthermore, the PP / OSA composite material prepared in step S9) is fitted with the interface interaction parameters using the Turcsanyi model to determine the interfacial interaction between the filler and the matrix.

[0028] The beneficial effects of this invention are as follows:

[0029] This invention, through ball milling and sieving of OSA, precisely controls the particle size of the OSA filler and its dispersion behavior in the matrix, preparing a PP / OSA composite material with excellent mechanical properties and thermal stability. This solves the problems of decreased strength and crystallinity deterioration in OSA-filled PP in existing technologies, achieving a triple improvement in tensile strength (+22.85%), crystallinity (+3.72%), and thermal stability (+27℃). The finest powder particle size (5.791 μm) was obtained after ball milling for 40 minutes (OSA-40), and at this particle size, the composite material achieved optimal tensile strength and thermal stability at a filler content of 15 wt.%. Appropriate ball milling not only sufficiently refines the particles and increases the contact interface with PP, but also avoids secondary agglomeration caused by over-grinding. Attached Figure Description

[0030] Figure 1 These are the stress-strain curves of the PP / OSA composite material of the present invention, wherein (a) is PP / OSA-20, (b) is PP / OSA-40, (c) is PP / OSA-60, and (d) is PP / 2500CC.

[0031] Figure 2 This is a tensile strength diagram of the PP / OSA composite material of the present invention.

[0032] Figure 3 These are SEM images of the cryogenic fracture surfaces of the PP / OSA composite material filled with 15 wt.% OSA according to the present invention, where (a) is PP, (b) is PP / OSA-20-15, (c) is PP / OSA-40-15, and (d) is PP / OSA-60-15.

[0033] Figure 4 This is the Turcsanyi model fitted to the PP / OSA composite material of this invention, where B in the figure represents the fitting parameter.

[0034] Figure 5 These are the DSC curves of the PP composite material, where (a) is the cooling crystallization curve and (b) is the secondary heating melting curve.

[0035] Figure 6 This is the XRD pattern of PP composite material.

[0036] Figure 7 This is the thermogravimetric curve of the PP / OSA composite material of the present invention. Detailed Implementation

[0037] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0038] Example 1: Preparation of OSA packing material with different ball milling times

[0039] Oil shale residue (OSA) was subjected to primary crushing. Equal amounts of OSA pieces were placed in a ball mill for dry ball milling at a speed of 400 r / min and a ball-to-material mass ratio of 10:1. Different ball milling times were set: 10 min, 20 min, 40 min, and 60 min. After ball milling, all samples were passed through a 200-mesh sieve to obtain four powders: OSA-10, OSA-20, OSA-40, and OSA-60. Particle size analysis showed that OSA-40 had the smallest average particle size, at 5.791 μm.

[0040] Example 2: Preparation and properties of OSA-40 filled polypropylene (PP) composite material (compared with pure PP)

[0041] Pure PP, PP / OSA-10-15, PP / OSA-20-15, PP / OSA-40-15, and PP / OSA-60-15 composite materials were prepared according to the method of claim 1 (10-60 represents ball milling time, and 15 represents wt.% filling amount).

[0042] Mechanical property test results show that the tensile strength of PP / OSA-40-15 composite material reaches 19.03 MPa, which is 22.85% higher than that of pure PP (15.49 MPa) and significantly better than samples with other ball milling times.

[0043] Thermogravimetric analysis (TGA) showed that the initial thermal decomposition temperature of the PP / OSA-40-15 composite was delayed by 27°C compared to pure PP, and the residual mass fraction at 650°C increased by 12.41%, which was also better than that of the PP / 2500CC-15 composite.

[0044] SEM observations showed that OSA-40 particles were uniformly dispersed in the PP matrix, with tight interfacial bonding. Turcsanyi model calculations showed that the PP / OSA-40-15 composite material had the largest interfacial interaction parameter B value (3.98), which was better than the B value of PP / 2500CC (2.07), further confirming the strongest interfacial bonding force of PP / OSA.

[0045] Example 3: Performance of OSA-40 filled polypropylene (PP) composite material (compared with 2500 mesh heavy calcium carbonate powder)

[0046] The PP / 2500CC-15 composite material was prepared using the same formulation and process as in Example 2.

[0047] Test results show that the tensile strength (19.03 MPa) of the PP / OSA-40-15 composite material is 32.98% higher than that of the PP / 2500CC-15 composite material, achieving a significant breakthrough over the industry benchmark and demonstrating the non-obviousness and major progress of the present invention.

[0048] Example 4: Effect of OSA-40 on PP crystallization behavior

[0049] Differential scanning calorimetry (DSC) analysis was performed on the composite materials prepared in Examples 2 and 3, and the crystallinity of the composite materials was determined using formula (1). X c,DSC Perform the calculation: (1)

[0050] In the formula, ΔH m The enthalpy of fusion in DSC testing. ΔH m 0 This is the theoretical enthalpy of fusion of the sample. ΔH m 0 =209 J / g, φ w It is the percentage of matrix mass in a composite material.

[0051] The crystallinity of PP, PP / OSSC composite material, and PP / 2500CC-15, calculated based on the above formula, is shown in the table below:

[0052] Table 1 Crystallinity of PP composite materials

[0053] Composite materials <![CDATA[ T c (℃)]]> <![CDATA[φ w (%)]]> <![CDATA[ T m (℃)]]> <![CDATA[ ΔH m (J / g)]]> <![CDATA[ X c,DSC (%)]]> PP 115.92 99.0099 165.82 83.46 40.33 PP / OSSC-20-15 120.52 84.1584 163.35 66.97 38.07 PP / OSSC-40-15 120.38 84.1584 164.12 73.57 41.83 PP / OSSC-60-15 120.15 84.1584 163.26 71.61 40.71 PP / 2500CC-15 119.37 84.1584 163.44 68.64 39.02

[0054] The results showed that the crystallinity of PP / OSSC-40-15 was increased by 3.72% compared to pure PP, and by 7.2% compared to PP / 2500CC-15. OSA-40, as a heterogeneous nucleating agent, effectively promoted PP crystallization and lowered its crystallization temperature (…). T c ) and crystallinity ( X c Both OSA-40 and PP / 2500CC-15 composites showed higher nucleation rates than pure PP and PP / 2500CC-15 composites. XRD analysis further confirmed that OSA-40 had a better nucleation effect than 2500CC, resulting in finer grains.

[0055] Example 5: Effect of different ball milling times on the properties of composite materials

[0056] The samples from Example 2 with different ball milling times were compared. The results showed that the tensile strength continuously increased as the ball milling time increased from 10 min to 40 min; however, the tensile strength decreased when the ball milling time was extended to 60 min. This proves that the optimal ball milling time is 40 min, and excessive grinding will cause particle agglomeration, weakening the reinforcing effect.

[0057] Example 6: Effect of different filler contents on the properties of PP / OSA-40 composite materials

[0058] The OSA was ball-milled for 40 minutes, with varying filling amounts (5, 10, 15, 20, 25, 30 wt.%).

[0059] Test results show that the tensile strength first increases and then decreases with the increase of filler content, reaching a peak at 15 wt.%.

[0060] Example 7: Analysis of Interface Interaction Parameters (B-value)

[0061] The Turcsanyi model was used to fit all PP composite material samples.

[0062] The results showed that the PP / OSA-40-15 composite material had the highest B value (3.78), which was higher than that of pure PP and PP / 2500CC, proving that there was the strongest interfacial bonding force between its filler and matrix.

[0063] Figure 1 The stress-strain curves for the PP / OSA composite material are shown. Figure 1 As can be seen, the maximum tensile stress of PP / OSA composite material gradually increases with the decrease of OSA particle size, and the composite material filled with 15 wt.% OSA-40 exhibits the highest tensile stress.

[0064] Figure 2 This is a tensile strength diagram of the PP / OSA composite material. From... Figure 2 It can be seen that the PP / OSA composite material filled with 15 wt.% OSA-40 has the best tensile strength, reaching 19.03 MPa, which is 22.85% higher than that of pure PP; compared with the 2500CC / PP composite material, the tensile strength is increased by 32.98%.

[0065] Figure 3 SEM images of the cryogenic fracture surface of a PP / OSA composite material filled with 15 wt.% OSA. From Figure 3As can be seen, the fracture surface becomes rougher after adding OSA, indicating that more energy absorption and tensile deformation occurred during fracture. OSA-40 exhibits the best particle dispersion in the composite material, with no obvious agglomeration or porosity. The interface between OSA-40 and PP is relatively continuous, maximizing its reinforcing effect on PP and significantly improving tensile strength.

[0066] Figure 4 To predict the interfacial properties of filler-filled composite materials using the Turcsanyi model. From Figure 4 As can be seen, the larger the B value, the stronger the interfacial bonding, enabling the filler to effectively transfer stress. This helps prevent stress concentration and crack formation, significantly improving the strength of the composite material. The PP / OSA-40 composite material has the highest B value.

[0067] Figure 5 The DSC curves for the PP composite material are shown. Figure 5 It can be seen from this that the PP / OSA composite material has T c The concentrations were all higher than those of pure PP, indicating that the filler played a nucleation role in the PP matrix, which helped promote the crystallization of PP molecular chains. The PP / OSA composite material... T c The higher crystallinity of OSA compared to PP / 2500CC composites indicates that OSA is more effective as a heterogeneous nucleating agent than 2500CC, allowing PP molecular chains to begin crystallization earlier and increasing the crystallization rate. The PP / OSA-40-15 composite exhibits the highest crystallinity (41.83%). The crystallinity of PP / OSA and PP / 2500CC composites... T m Both are slightly lower than pure PP.

[0068] Figure 6 The image shows the XRD pattern of the PP composite material. Figure 6 As can be seen, the grain size of the PP / OSA composite material (15.222nm) is smaller than that of the PP / 2500CC composite material (20.681nm), indicating that OSA has a superior nucleation ability.

[0069] Figure 7 Thermogravimetric analysis (TGA) curves for PP / OSA composite materials. From... Figure 7 It can be seen that the decomposition temperature of the composite material filled with 15 wt.% OSA-40 is delayed by 27 ℃ compared with PP, and the residual mass at 650 ℃ increases by 12.41%; compared with 2500CC / PP composite material, the thermal decomposition temperature is delayed by 18 ℃, the residual mass increases by 4.07%, and the thermal stability is significantly improved.

[0070] The above embodiments are preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any equivalent changes or modifications made to the present invention by those skilled in the art without departing from the spirit and essence of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for improving tensile strength and thermal stability of polypropylene filled with oil shale slag, characterized in that: S1: The blocky oil shale residue is subjected to primary crushing to obtain small pieces of oil shale residue with a particle size of less than 5 mm; S2: Place small pieces of oil shale slag into a ball mill for dry ball milling. The ball mill speed is 350-450 r / min, the ball-to-material mass ratio is 8:1 to 12:1, and the ball milling time is 35-45 min to obtain oil shale slag powder with an average particle size of 5.5-6.0 μm, a particle size distribution variance of no more than 0.8 μm, and an agglomeration rate of no more than 5%. S3: The treated oil shale slag powder is passed through a 200-mesh sieve to obtain fine oil shale slag powder with uniform particle size distribution. S4: Dry the fine powder of oil shale residue at 120 ℃ for 12 h, controlling the moisture content to be no more than 0.5%; S5: Mix the dried oil shale slag fine powder with polypropylene particles and stearic acid in a certain proportion. The amount of oil shale slag filling is 5-30 wt.% and the amount of stearic acid added is 0.5-1.5 wt.% of the total mass of the composite material. Melt and mix for 8-12 min at a temperature of 190-210 ℃ and a rotation speed of 40-60 r / min. S6: Preheat the mixture at 195-205 ℃ for 3-5 min, then hot press it at 2.5 to 3.5 MPa for 3-5 min, and then cold press it at the same pressure for 3-5 min to obtain polypropylene / oil shale slag composite material. When the ball milling time is 40 min and the oil shale slag content is 15 wt.%, the tensile strength of the obtained polypropylene / oil shale slag composite material is 19.03 MPa, the interfacial interaction parameter B is 3.78, and the crystallinity is 41.83%.

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