Influence of modified phosphogypsum content on the properties of HDPE/phosphogypsum composites

Phosphogypsum has received increasing attention for solid waste with high production and environmental pollution. This study treated phosphogypsum with the silane coupling agent 3-(methacryloyloxy) propyltrimethoxysilane (KH570) to create modified HDPE/phosphogypsum composites. The modified phosphogypsum was then melted and blended with high-density polyethylene (HDPE). Thermal weight loss (TG), differential scanning calorimetry (DSC), and rheological and mechanical property testing were used to study the impact of the changed phosphogypsum content on the properties of the composites. The outcomes demonstrate that even at low filling levels, the modified phosphogypsum may be evenly distributed throughout the matrix. Furthermore, the addition of increased phosphogypsum filling enhances the mechanical characteristics of the HDPE matrix.


Introduction
High-density polyethylene (HDPE) is widely used as a plastic material due to its cost-effectiveness, ease of processing, and minimal requirements [1] .However, HDPE has drawbacks, including easy deformation, poor mechanical qualities, and low surface hardness [2] .To address these drawbacks, modifying the filling of HDPE can lower costs while enhancing HDPE performance [3] .Calcium sulfate dihydrate (CaSO 4 ꞏ2H 2 O), a by-product of the manufacturing of phosphoric acid [4] , is the major component of phosphogypsum.Phosphogypsum is often dumped in piles due to factors such as high production costs, waste of resources and pollutants (such as F, S, and heavy metals) polluting the environment, and low recovery rate.(e.g.F, S, and heavy metals).Researchers have done a lot of research on the use of phosphogypsum as a filler.The use of phosphogypsum as HDPE filler not only enhances the comprehensive properties of the material but also realizes the solid waste utilization of phosphogypsum, which is of great significance for broadening the application field of phosphogypsum.However, phosphogypsum powder is easy to agglomerate and has poor dispersion [5] , and the surface modification of phosphogypsum is required to enhance the interface compatibility with polymers.
Recently, Luo et al. [6] have tried to use phosphogypsum as a filler powder to prepare composite materials.Li et al. [7] prepared Nano-CaSO 4 by using phosphogypsum and found that Nano-CaSO 4 had a significant reinforcing effect on linear HDPE.Sun et al. [8] found that the modified phosphogypsum whiskers had a good reinforcing effect on HDPE.Zhang et al. [9] modified phosphogypsum by using KH550, KH560, and KH570 and prepared HDPE/phosphogypsum composites, and found that the modified phosphogypsum could significantly improve the mechanical properties of the composites and KH560 and KH570 had the best modification and strengthening effect.Despite the positive outcomes of the research above, the process of converting phosphogypsum into whiskers for utilization proved to be labor-intensive.Based on the aforementioned findings, the effect of phosphogypsum filling quantity on composite properties is examined in this study to offer a theoretical foundation for the use of phosphogypsum in polymers.

Sample preparation
Firstly, a certain amount of HDPE was weighed and dried at a constant temperature of 100°C for 12 h, and 17 mL of absolute ethanol, 2 mL of distilled water, and 1 mL of silane coupling agent KH570 were taken and placed in a magnetic mixer for a constant reaction for 4 h.Secondly, HDPE composites were prepared with modified phosphogypsum as filler.We weigh the dry phosphogypsum, add a modifier with a volume of 20% of the solid volume, slowly add it drop by drop, stir magnetically at 60°C for 3 h to modify, set the centrifuge speed of 3, 000, centrifuge for 3 minutes, dry the precipitate, and obtain the modified phosphogypsum.Then, the dried HDPE and phosphogypsum were put into the high-speed mixer for mixing evenly according to Table 1 and then put into the twin-screw extruder to control the temperature of 170~200°C.After being chopped into pellets by a cutting machine, the samples were then dried for 12 h at 100°C before being inserted into an injection molding machine to create sample strips.

Performance testing and structural characterization
We weigh 5-10 mg of the sample strip in an aluminum crucible and heat the sample from 50°C to 600°C at a temperature increase rate of 10 K/min under the atmosphere of N 2 to obtain the thermal degradation curve.Then, we weighed 5-10 mg of the sample strip in the aluminum crucible, and under the atmosphere of N 2 , the sample was heated from 50℃ to 250℃ with a temperature rise and fall rate of 10 K/min and then decreased to 50℃ after a certain time of constant temperature, and increased to the end of 250℃ to observe the crystallization and melting curve of the sample.Rheological performance test: We cut 1 cm by 1 cm samples on the testing platform, separated by 0.5 mm, and frequency testing is conducted at the temperature of 230℃.Impact performance: Under GB/T 1843-2008 "Determination of Impact Strength of Plastic Cantilever Beams", the impact performance test of the specimen is carried out.Tensile properties: Referring to the tensile standard GB/T1040-79, the tensile standard specimen is dumbbell-shaped, and the corresponding dimensions are 180×10×4 mm.Bending performance: Referring to the bending standard GB/T1042-79, the bending standard specimen is a rectangular strip with a size of 80×10×4 mm.The impact of phosphogypsum on the mechanical characteristics of composites made of HDPE and phosphogypsum is depicted in Figure 1.The tensile strength of pure HDPE is 32.69 MPa, as shown in Figure 1 (a).When the mass fraction of phosphogypsum is 40%, the tensile strength of the composite increases to 37.89 MPa, and the modified phosphogypsum has high adhesion to the surface, improving the stiffness of the composite.The composites will have a reduction in fracture expansion under stress.The HDPE will gradually become stronger as the modified phosphogypsum is added, and the tensile strength of the composite will gradually increase.It can be seen from Figures 1 (b) and (c) that the bending strength and impact strength of HDPE composites are significantly higher than those of pure HDPE.As the mass fraction of phosphogypsum content increases, the bending strength of HDPE/phosphogypsum composites increases by 6.96%, 13.50%, 31.34%, and 49.79%, respectively, while the impact strength increases by 12.33%, 15.66%, 15.99%, and 17.52%.The enhancement of the bending properties is because the modified phosphogypsum has better interfacial compatibility with the matrix, while the phosphogypsum powder dispersed in the matrix increases the stress point [10] .The enhancement of impact strength is because the presence of inorganic particles produces a stress concentration effect on the composites, which can easily cause microcracks in the surrounding resin and absorb certain deformation work, thus improving the notched impact strength of the composites.  2 shows the TG and DTG curves of HDPE/phosphogypsum with different phosphogypsum mass fractions, and the corresponding thermal decomposition temperatures are listed in Table 2.As shown in Figure 2, the weight loss curves of the composite HDPE/phosphogypsum are essentially the same as those of HDPE, with both exhibiting a single weight loss step, and the residual rate of HDPE/phosphogypsum-10% is close to 10%, with residues filled with modified phosphogypsum powder.When comparing the T 2% , T 5% , T 10% , and T DTG peaks of the materials, phosphogypsum has no significant effect on the thermal stability of HDPE/phosphogypsum, with HDPE/phosphogypsum-30% composite having the best thermal performance [11] .Table 3. DSC melting and crystallization data of pure phosphogypsum and HDPE/phosphogypsum (10%-40%) composites.

Crystallization and melting behavior of HDPE/phosphogypsum composites
(Note: T c : peak temperature of crystallization; ∆H m : enthalpy of crystallization; χ c : degree of crystallization.) As shown in Figure 3 and Table 3, the peak crystallization temperature of the modified HDPE/phosphogypsum composite ranges from 117.97°C to 116°C.At 94°C, the melting point decreases and the crystallization ability deteriorates.As can be seen from Table 3, the crystallinity of the modified HDPE composites decreases to 62.46%, the flexibility of the material chain increases, the intermolecular force decreases, the movement of the molecular chain segments is easier, and the impact strength of the material becomes significantly stronger.The toughness increases after the impact load is applied to the material, which is consistent with the law of influence in Figure 1.  4, the addition of 10% phosphogypsum resulted in a more significant decrease in the energy storage modulus, loss modulus, and viscosity of the composites, which was due to the small amount of phosphogypsum acting as a lubricant in the system.However, the viscosity of HDPE/phosphogypsum composites was increased by the phosphogypsum content, which led to a decrease in the rheological rate of the composites and a decrease in flowability.The addition of phosphogypsum filler hinders the movement of HDPE molecular chains, and more energy is required to make the molecular chains move.When phosphogypsum increases, the loss modulus of the composite material increases.The loss factor of pure HDPE is low, and the addition of phosphogypsum increases the loss factor and the composite viscosity.The storage modulus also increased with the increase of the added amount, indicating that the stiffness of the material increased with the increase of the filling amount.

Conclusion
The incorporation of modified phosphogypsum yields measurable improvements in the properties of HDPE.As the filling quantity increased, the mechanical properties of HDPE/phosphogypsum composites improved and reached the optimum at 40% mass fraction of phosphogypsum, which was also confirmed by scanning electron microscopy.Impacts on thermal properties remain relatively subtle, and a discernible upward trend in energy storage modulus indicates increased material stiffness.This research remains nascent with areas requiring refinement including addressing deficiencies and enhancing precision through subsequent experiments focused on optimizing the phosphogypsum mass fraction gradient.

Figure 3 .
Figure 3. DSC curves of HDPE/phosphogypsum with different mass fractions of phosphogypsum content.

Figure 4 .
Figure 4. Rheology curves of HDPE/phosphogypsum with different mass fractions of phosphogypsum content.As shown in Figure4, the addition of 10% phosphogypsum resulted in a more significant decrease in the energy storage modulus, loss modulus, and viscosity of the composites, which was due to the small amount of phosphogypsum acting as a lubricant in the system.However, the viscosity of

Figure 5 (
a) shows the SEM image of pure HDPE, which has strong continuity and flat sections.Phosphogypsum modified by KH570 is well dispersed in the matrix, and there is no gap at the interface between phosphogypsum and matrix, indicating that the modification has improved the interfacial compatibility between phosphogypsum and HDPE.Phosphogypsum particles are larger after KH570 modification, and the fracture surface is flat.As shown in Figures5 (b) and (c), phosphogypsum was uniformly distributed in the HDPE matrix, and as the filling amount increased, more powder could be observed in the fracture out.

Table 2 .
TG and DTG data for HDPE/phosphogypsum with different mass fractions of phosphogypsum.