Views: 46 Author: Yinsu Flame Retardant Publish Time: 2025-07-30 Origin: www.flameretardantys.com
Effect of Crosslinking Degree on Tensile and Dielectric Properties of Cross-Linked Polyethylene/Organically Modified Montmorillonite Nanocomposites
I. Introduction
Cross-linked polyethylene (XLPE) is widely used as the insulation layer in power cables because of its excellent electrical, mechanical, and thermal properties. However, variations in the production process can lead to differences in the crosslinking degree of the insulation layer, which in turn affects the microstructure and performance of the material. Scholars have conducted extensive research on the influence of crosslinking degree on the aggregate structure and performance of XLPE. The study by Zhang Qiaofeng et al. showed that as the crosslinking temperature rises, the crosslinking degree of XLPE increases, the three-dimensional network structure formed by crosslinking is strengthened, the spherulitic structure becomes densely arranged, and the polarization current, conduction current, and interfacial polarization current of XLPE decrease, thereby improving the electrical insulation performance of XLPE. Yuan Bao et al. found that adding a co-crosslinking agent can increase the crosslinking degree of XLPE and reduce the amount of dicumyl peroxide (DCP), thereby decreasing crosslinking by-products and improving the dielectric properties of the material. Zhou Quan et al. studied the influence of different crosslinking degrees on space charge and found that crosslinking can increase the charge injection threshold field strength of polyethylene, but as the crosslinking degree increases, the injection threshold field strength of XLPE first increases and then decreases. It is evident that an appropriate crosslinking degree can enhance the dielectric performance of XLPE.
With the increasing voltage levels in power systems, higher demands are placed on cable insulation performance. Polymer-based nanocomposite dielectrics provide new ideas for the development and research of electrical insulation materials. The polymer matrix determines the intrinsic properties of the composite, while good dispersion of nanoparticles is beneficial for improving the performance of the composite. However, the crosslinking process of the polymer may also introduce uncertainty in nanoparticle dispersion, thereby affecting the interfacial bonding between nanoparticles and the polymer matrix, resulting in differences in macroscopic properties.
Montmorillonite (MMT) is a unique type of filler that reaches the nanoscale in only one dimension. This filler has a very large aspect ratio. After compounding with the polymer, the polymer inserts into the interlayer space, fully utilizing the internal and external surface area of MMT to form a stable and robust interfacial region, which restricts molecular chain movement and blocks charge movement, effectively improving the mechanical and electrical properties of the polymer. Therefore, this paper selects organically modified montmorillonite (OMMT) at a mass fraction of 0.5% to modify polyethylene. By adjusting the crosslinking time, XLPE/OMMT nanocomposites with different crosslinking degrees are prepared. Combined with the interfacial characteristics formed by the nanodispersed OMMT, the relationship between the crosslinking process and the microstructural morphology of the composite is analyzed, and the influence of crosslinking degree on the tensile and dielectric properties of the nanocomposite is investigated.
II. Experiment
1. Main raw materials
Organically modified montmorillonite (OMMT), pre-intercalated with octadecyl quaternary ammonium salt intercalant, manufactured by Zhejiang Fenghong New Materials Co., Ltd.; polyethylene (PE), model 2220H, produced by Yangzi Petrochemical-BASF Co., Ltd.; 35 kV XLPE insulation compound, supplied by Nanjing Zhongchao New Materials Co., Ltd.
2. Sample preparation
Graft compound: PE was grafted with maleic anhydride to prepare maleic anhydride-grafted polyethylene (PE-g-MAH).
Masterbatch: PE-g-MAH and OMMT were placed on a two-roll mill at a mass ratio of 8:2 and melt-blended at 100 °C with a screw speed of 40 r/min to produce a masterbatch with an OMMT mass fraction of 20%.
XLPE/OMMT nanocomposite: The masterbatch was melt-blended with XLPE pellets at a mass fraction of 2.5% at 100 °C for 15 min, resulting in an OMMT mass fraction of 0.5%.
Samples with different crosslinking degrees: XLPE and XLPE/OMMT composite samples were prepared according to the standard JB/T 10437—2004 "Cross-linkable polyethylene insulation compound for wires and cables". The samples were preheated without pressure at 120 °C for 10 min on a flat vulcanizing press and then crosslinked at 175 °C and 15 MPa for 5 min, 10 min, 15 min, 20 min, and 30 min. After cooling, samples with different crosslinking degrees were obtained.
3. Property testing
X-ray diffraction (XRD) analysis.
To characterize the intercalation and dispersion state of montmorillonite, a D8 ADVANCE X-ray diffractometer from Bruker was used to scan the samples in the range of 1.5°–10° at a scan rate of 0.1 s/step. The Bragg equation was used to calculate the interlayer spacing of montmorillonite:
where d is the interplanar spacing, θ is the incident angle, λ is the incident wavelength (0.15406 nm), and n is the diffraction order.
To analyze the aggregate structure changes of XLPE/OMMT nanocomposites, a D8 ADVANCE X-ray diffractometer from Bruker was used to scan the samples in the range of 10°–30° at a scan rate of 10°/min. Equations (2) and (3) were used to calculate the crystallinity and the grain size corresponding to different diffraction peaks.
where χ is the crystallinity, S1 is the area of the amorphous peak, S2 is the area of the (110) diffraction peak, and S3 is the area of the (200) diffraction peak.
where Dhkl is the grain size perpendicular to the (110) or (200) plane, K is a constant (0.89), and β is the half-width.
Gel content testing
According to the method in Appendix A of standard JB/T 10437—2004, the gel content of the samples was used to characterize the crosslinking degree. 0.5 g of the sample was shredded and placed in a 130-mesh (0.14 mm aperture) stainless steel mesh, immersed in xylene at 110 °C for 24 h, then dried in a vacuum oven at 110 °C to constant weight. The gel content was calculated using equation (4):
where W1 is the mass of the mesh, W2 is the mass of the sample and mesh before extraction, and W3 is the mass of the sample and mesh after extraction and drying.
Morphological etching
XLPE/OMMT nanocomposite films were etched in 5% KMnO4 concentrated sulfuric acid solution for 3 h at 50 °C, with stirring every 0.5 h. After etching, the samples were cleaned in an ultrasonic microwave cleaner for 0.5 h, dried, and sputter-coated with gold. A SU8010 scanning electron microscope (SEM) was used to observe the crystal morphology at 1000× magnification.
Tensile property testing
A UTM2103 universal testing machine was used according to the standard GB/T 1040—2006 "Determination of tensile properties of plastics". Type 5 specimens were tested at a tensile speed of (250±50) mm/min with a thickness of (1.0±0.1) mm.
Conductivity-temperature characteristics
A three-electrode system with a ZC36 high-resistance meter was used to measure the volume resistivity (the reciprocal of which is the volume conductivity γ) at 23 °C, 40 °C, 60 °C, 80 °C, and 90 °C. The test voltage was 1 kV, and the sample thickness was 1 mm. The activation energy μ0 of the composite was calculated from the fitted temperature-volume resistivity data.
where k is the Boltzmann constant and B is the slope of the temperature-volume resistivity fitting line.
Dielectric constant and dielectric loss tangent testing
A QS87 dielectric constant measurement system was used to test the dielectric constant and dielectric loss tangent of the samples. The sample diameter was 50 mm and the thickness was 1 mm.
Breakdown field strength testing
A sphere-sphere electrode was used, and the voltage was increased at a rate of 2 kV/s until the sample broke down. The results were processed using a two-parameter Weibull distribution. The samples were circular with a diameter of 150 mm and a thickness of 0.1 mm.
III. Results and Discussion
1. Dispersion state of OMMT in XLPE
The interlayer spacing changes of montmorillonite were analyzed via XRD diffraction patterns. The XRD patterns of MMT, OMMT, masterbatch, and XLPE/OMMT nanocomposite are shown in Figure 1, and the calculated interlayer spacings are listed in Table 1.
Figure 1 and Table 1 show that the initial diffraction peak of MMT is around 6.20°, with an interlayer spacing of 1.42 nm. After treatment with the octadecyl quaternary ammonium salt intercalant, OMMT is obtained, and its diffraction peak shifts leftward to 3.45°, with the interlayer spacing increasing to 2.56 nm, indicating that organic modification enlarges the interlayer spacing of montmorillonite. Compared with OMMT, the initial diffraction peak of the masterbatch further shifts leftward to 1.90°, and the interlayer spacing increases to 4.65 nm. This is due to the strong interaction between the maleic anhydride groups on the polymer molecular chains and the OMMT sheet surfaces, causing PE-g-MAH to intercalate into the OMMT interlayers and further increasing the interlayer spacing. In the XLPE/OMMT composite system, the initial diffraction peak has shifted leftward to 1.54°, and the interlayer spacing of XLPE/OMMT has expanded to 5.73 nm, an increase of 303.5% compared with MMT. Additionally, the initial diffraction peak in Figure 1 has partially disappeared, showing that under the action of PE-g-MAH, OMMT has achieved exfoliated dispersion in XLPE/OMMT.
2. Crystallinity of XLPE/OMMT nanocomposite samples
The XRD patterns of XLPE/OMMT samples under different crosslinking times and the Gaussian fitting diagrams are shown in Figures 2a and 2b. The parameters of the (110) and (200) diffraction peaks and the crystallinity of each sample are listed in Table 2. In XRD analysis, peak shape, half-width β, and crystallinity can be used to analyze changes in the crystalline structure and the degree of structural damage. Under the same crystallization conditions, the half-width can characterize the degree of structural distortion within the crystals. A larger half-width indicates a higher degree of crystal distortion. From Figure 2, it can be seen that the absorption peak positions of the samples do not shift significantly, all around 21.3° and 23.6°, but the intensities differ, indicating that the crystal forms of the samples are essentially the same, but the crystal sizes and crystallinity differ.
Figures 2a and Table 2 show that as the crosslinking time increases, the crystallinity of the samples decreases, and the crystal sizes in both crystal plane directions decrease. After 20 min of crosslinking, the crystallinity decreases significantly, and the half-width β increases, indicating that under excessive crosslinking time, the distortion and damage to the crystalline regions of the samples are significant.
3. Crosslinking degree of XLPE and XLPE/OMMT nanocomposite samples
The crosslinking degree of the samples can be characterized by gel content. The gel contents of XLPE and XLPE/OMMT nanocomposite samples under different crosslinking times are listed in Table 3. It can be seen that the gel content of the samples first increases and then decreases with increasing crosslinking time, reaching a maximum at 15–20 min. This indicates that as the crosslinking time increases, the crosslinking degree gradually increases, and when the crosslinking time reaches 15–20 min, the crosslinking reaction is essentially complete, and the crosslinking degree is the highest. When the crosslinking time is further extended, the crosslinking degree decreases.
In the initial crosslinking stage before 15 min, XLPE and XLPE/OMMT nanocomposite samples have similar crosslinking degrees. This is mainly because under the action of the crosslinking agent, chemical bonds begin to form a three-dimensional network structure between molecular chains, and the crosslinking degree gradually increases. After 15 min, the crosslinking degree of XLPE/OMMT nanocomposite samples is higher than that of pure XLPE. This is because the exfoliated OMMT dispersed in the matrix can act as physical crosslinking points. The combined action of chemical and physical crosslinking promotes an increase in crosslinking degree. However, excessive crosslinking time causes some main chains to break due to thermal oxidation, leading to a decrease in crosslinking degree.
Thus, before 15 min of crosslinking, XLPE and XLPE/OMMT nanocomposite samples are in an "under-crosslinked" state; at 15–20 min, they are in a "properly crosslinked" state; and after 20 min, they are in an "over-crosslinked" state.
4. Influence of crosslinking degree on the micro-morphology of XLPE/OMMT nanocomposites
To investigate the influence of crosslinking degree on the micro-morphology of the composite, XLPE/OMMT samples with crosslinking times of 5 min, 15 min, and 30 min were selected for observation under scanning electron microscopy. The results are shown in Figure 3. The crystal sizes were statistically analyzed, and the data are listed in Table 4. From Figure 3 and Table 4, it can be seen that compared with the 5 min crosslinked sample, the average crystal size of the composite decreases from 32.06 μm to 20.58 μm at 15 min, the number of crystals increases significantly, and the crystal size distribution becomes more uniform. At 30 min, the average crystal size decreases to 18.98 μm, but the dispersion of crystal sizes increases.
This trend is consistent with the XRD results. In the under-crosslinked state, the uniformly dispersed OMMT sheets in the matrix can act as heterogeneous nucleating agents, accelerating the crystallization rate, increasing the number of crystals, and forming strong interfacial interactions with the matrix, restricting segmental motion and reducing the crystal growth rate, resulting in smaller crystal sizes. As the crosslinking degree increases, more long-chain molecules are formed, the regularity of molecular chains deteriorates, and flexibility decreases, making it difficult for the chains to fold regularly into the crystal lattice, thereby inhibiting crystal growth and leading to non-uniform crystal sizes. With further improvement of the crosslinking network, the distribution of crystals becomes uniform again, reducing the difference in crystal sizes. In the over-crosslinked state, the breaking of some molecular chains leads to segmental viscous flow, and this thermodynamic instability affects the intercalation and dispersion of molecular chains in OMMT, worsening the dispersion of OMMT in the matrix, causing overlapping nuclei, concentrated crystal growth, and increased differences in crystal sizes. Additionally, prolonged high temperatures cause some molecular chains to break due to thermal oxidation, reducing the symmetry of the crosslinking network and hindering the ordered arrangement of segments during crystallization, leading to poor uniformity in crystal size distribution.
5. Influence of crosslinking degree on the tensile properties of XLPE/OMMT nanocomposites
Tensile tests can reflect the elastic deformation, plastic deformation, and fracture stages of a material from loading to fracture. The stress-strain curve of XLPE/OMMT nanocomposites can be divided into two parts: ① the elastic deformation region where stress and strain have a linear relationship, and ② the plastic deformation region where stress and strain have a non-linear relationship. The elastic deformation parameters of XLPE/OMMT nanocomposites are listed in Table 5.
From Table 5, it can be seen that as the crosslinking time increases, the elastic modulus E of the samples first increases and then decreases, while the yield strength σY and yield strain εY decrease to varying degrees. In the under-crosslinked state, first, the uniformly dispersed OMMT in the matrix acts as physical crosslinking points, increasing the number of load-bearing points and improving the composite's resistance to deformation; second, the crosslinking degree is low, and the network structure formed inside the composite is close to ideal, with each network chain bearing the load evenly, and as the crosslinking degree increases, the load-bearing capacity increases, so E increases; additionally, the elastic deformation capacity of the composite is related to the molecular weight. As the crosslinking degree increases, the molecular weight of the polymer increases, and under the entanglement of molecular chains, the composite's resistance to deformation is enhanced, further increasing E. In the over-crosslinked state, the asymmetry of the crosslinking network caused by thermal oxidative scission exacerbates the non-uniform load distribution, reducing the composite's resistance to deformation. Moreover, the large number of crosslinking bonds reduces the distance between OMMT sheets, weakening the load-dispersing ability of OMMT, so E decreases. The yield strength is mainly affected by crystallinity. Crosslinking reactions inhibit crystallization and reduce crystallinity, increasing the amorphous region and making molecular chain orientation easier, so σY and εY decrease.
After the material undergoes elastic deformation and the applied stress exceeds its elastic limit, the material enters the plastic deformation stage. The plastic deformation parameters of XLPE/OMMT nanocomposites are listed in Table 6. From Table 6, it can be seen that the tensile strength σB of the composite first increases and then decreases with increasing crosslinking time, reaching a maximum of 19.5 MPa at 15 min. It is believed that in the under-crosslinked state, the gradually formed crosslinking bonds enhance the intermolecular forces, and the crosslinking reaction increases the number of entangled molecular chains between crystal lamellae, thus improving σB. However, dense crosslinking points shorten the chain length between crosslinking points, restricting molecular chain movement and making orientation along the tensile direction difficult, reducing relative motion and decreasing elongation at break.
The tensile strength of the polymer is also related to microvoids formed during crystallization and deformation of the amorphous region (resulting in internal stress). During crystallization, molecular chain ends and defects tend to concentrate in the amorphous region. In the over-crosslinked state, the crosslinking degree decreases, crystallinity further decreases, the amorphous region increases, and thermal oxidative scission of molecular chains affects the bonding between OMMT and the matrix, causing phase separation at the interface and inducing microcracks. These factors lead to increased microvoids and defects in the amorphous region, increased internal stress, brittleness, and reduced strength.
Fracture energy S is the work done by tensile stress on a unit volume of material during tensile fracture. A larger value indicates better toughness and a lower likelihood of brittle fracture. As the crosslinking time increases, S first increases and then decreases. In the under-crosslinked state, as the crosslinking degree increases, S increases, indicating that the crosslinking reaction improves the toughness of the composite. This is attributed to the crosslinking bonds formed by the crosslinking reaction, which enhance intermolecular forces. Additionally, the large specific surface area of OMMT increases the interaction with the polymer, and the exfoliated OMMT acts as physical crosslinking points, effectively alleviating external impacts and improving the toughness of the composite. However, with excessive crosslinking, on one hand, the non-uniformity of the crosslinking network caused by thermal oxidative scission leads to a few molecular chains bearing the load first and breaking under external forces, accelerating the degradation of the composite; on the other hand, microcracks or cracks between OMMT and the matrix tend to concentrate stress, causing the composite to fracture and reducing S.
Combined with the XRD results, before and at the properly crosslinked state, as the XLPE/OMMT crosslinking degree increases, the crosslinked network structure is gradually established, enhancing intermolecular forces and increasing load-bearing capacity, thereby improving tensile properties. At this time, although crystallinity decreases and the amorphous region increases, the uniformly dispersed OMMT sheets in the matrix act as physical crosslinking points, increasing physical entanglement under the action of PE-g-MAH, further enhancing the ability to disperse load and improving tensile strength, fracture energy, and elastic modulus. In the properly crosslinked state, the high elastic deformation imparted by the three-dimensional network maintains the bonding force at the phase interface and the stability of the OMMT intercalation and dispersion state, which is also an important reason for the increase in elastic modulus.
In the over-crosslinked state, increased molecular chain scission reduces the crosslinking degree, deteriorates the OMMT intercalation and dispersion state, reduces interfacial bonding force, and induces microcracks. Additionally, the degree of crystal damage increases, crystallinity decreases, the amorphous region increases, and the difference in crystal size distribution increases, exacerbating stress concentration. Nanocomposite samples exhibit a trend of decreasing tensile strength, fracture energy, and elastic modulus.
6. Influence of crosslinking degree on the dielectric properties of XLPE/OMMT nanocomposites
Influence of crosslinking degree on the conductivity-temperature characteristics of XLPE/OMMT nanocomposites
The fitted conductivity-temperature relationship of XLPE/OMMT under different crosslinking degrees is shown in Figure 4. The activation energies of each sample were calculated based on Equation (5) and Figure 4, and the data are listed in Table 7.
The conductivity of the composite depends on the concentration and mobility of charge carriers within the material, and the mobility is influenced by the binding barrier (i.e., activation energy). From Figure 4, it can be seen that the conductivity of each sample increases with temperature, indicating that as the temperature rises, more charge carriers are thermally excited to participate in conduction, consistent with ionic conduction characteristics. In XLPE/OMMT, at low temperatures, the exfoliated OMMT, with its high surface energy, forms strong interfacial bonding with the XLPE matrix under the action of PE-g-MAH, restricting the directional movement of charge carriers and adsorbing charge carriers and impurity ions in XLPE, making charge carrier migration difficult and resulting in low conductivity. As the temperature rises, the bound charge carriers are thermally excited to overcome the barrier and participate in migration, increasing conductivity. Additionally, the interfacial bonding between OMMT and XLPE weakens, and the intermolecular forces decrease, reducing the restriction on charge carrier movement. Therefore, conductivity increases with temperature.
From Table 7, it can be seen that as the crosslinking time increases, the activation energy first increases and then decreases, reaching a maximum of 0.35 eV at 15 min. The activation energy mainly characterizes the energy required for charge carriers to overcome the barrier height when participating in conduction. In the under-crosslinked state, as the crosslinking reaction proceeds, the sample transforms from a linear molecular structure to a three-dimensional network structure. Internal traps shift from shallow traps to deep traps, and the robust interfacial region between exfoliated OMMT sheets and the matrix also introduces more deep traps, increasing the probability of charge carrier capture, making it difficult for trapped charges to de-trap, and raising the barrier and activation energy. From the SEM results, it can be seen that in the properly crosslinked state, the sample has a uniform crystal distribution and a well-developed phase structure. The well-dispersed OMMT sheets and the XLPE matrix form strong interfacial bonding, enhancing the restriction on charge carrier directional movement, reducing charge carrier mobility, and further increasing activation energy. In the over-crosslinked state, the dispersion of OMMT deteriorates, affecting the interfacial region between OMMT and the matrix, reducing the number of deep traps, and the thermal oxidative scission of molecular chains weakens intermolecular forces, reducing the restriction on charge carrier movement, thus lowering the barrier and activation energy.
As the crosslinking degree of XLPE/OMMT increases, the crystallinity of the samples decreases, the amorphous region increases, facilitating charge carrier transport, and the internal structure tends to be more complete, reducing traps caused by defects. Additionally, differences in the base resin type of the composite determine differences in trap energy levels, so the barrier energy level of XLPE/OMMT samples is lower, and the activation energy is smaller.
Influence of crosslinking degree on the dielectric constant and dielectric loss of XLPE/OMMT nanocomposites
The variation of relative dielectric constant and dielectric loss tangent of XLPE/OMMT with crosslinking time is shown in Figure 5. It can be seen that as the crosslinking time increases, the relative dielectric constant of XLPE/OMMT first decreases and then increases, reaching a minimum at 15–20 min, while the dielectric loss tangent shows an "N"-shaped change with increasing crosslinking time. The relative dielectric constant of the nanocomposite depends on the polar groups introduced by the nanoparticles and the bonding degree of the filler-matrix interface. As crosslinking proceeds, the crosslinking degree increases, the three-dimensional crosslinked network structure of the composite becomes more complete, restricting viscous flow caused by segmental cooperative jumps, preventing OMMT sheet aggregation, increasing the stability of OMMT sheet dispersion, further restricting the orientation of dipoles, and hindering the orientation of side groups and molecular chain segments. Meanwhile, the polymer matrix is connected to OMMT sheets via PE-g-MAH, and the polar groups on the surface intercalant interact with the anhydride groups on PE-g-MAH, enhancing the interfacial interaction between the polymer and OMMT sheets, reducing molecular polarizability, decreasing the relative dielectric constant, and lowering the dielectric loss tangent. The initial increase in dielectric loss tangent may be due to increased structural losses caused by the relatively loose crosslinking structure at this stage.
In the over-crosslinked state, under prolonged high temperatures, increased molecular chain scission reduces the crosslinking degree, weakens intermolecular forces, facilitates orientation of dipoles, and increases polar groups due to oxidation and removal of side groups and aging by-products such as carbonyl groups, increasing the relative dielectric constant and dielectric loss. Additionally, OMMT sheets undergo micro-rearrangement due to strong intermolecular forces in XLPE, overlapping interface regions, reducing interfacial forces, weakening restrictions, and further increasing the relative dielectric constant and dielectric loss.
Influence of crosslinking degree on the breakdown characteristics of XLPE/OMMT nanocomposites
The two-parameter Weibull distribution of power frequency breakdown field strength of the samples is shown in Figure 6, and the relevant shape and scale parameters of each sample are listed in Table 8. From Figure 6 and Table 8, it can be seen that as the crosslinking time increases, the breakdown field strength first increases and then decreases, reaching a maximum at 15 min with the smallest data dispersion. As the crosslinking reaction proceeds and the crosslinking degree increases, the number of deep traps inside the samples increases, the probability of free electron capture increases, and the number of free electrons decreases. The addition of OMMT and the increase in crosslinking degree jointly improve the microstructure of XLPE, reduce free volume, shorten the mean free path of free electrons, and make it difficult to form conductive channels, thereby increasing the breakdown field strength of the composite and reducing data dispersion. Additionally, combined with the elastic modulus test results, it can be found that in the properly crosslinked state, the toughness of the composite molecular chains is enhanced, enabling them to resist the compressive force under an electric field, reducing the probability of electro-mechanical breakdown. With further extension of crosslinking time, the crosslinking degree decreases, long-chain molecules break, some molecular chain lengths and bond angles change, and the chains can no longer fold regularly around nuclei, causing micro-defects in the crystals and facilitating the development of local conductive channels. The weakened interaction between OMMT and the matrix allows more free electrons to escape, and under the action of the electric field, these electrons continuously collide with atoms on lattice nodes, destroying the crystal structure of the composite and reducing the breakdown field strength and increasing data dispersion. Overlapping interfacial regions between OMMT and the matrix also provide pathways for free electron movement, and the decreased crystallinity and increased amorphous region accelerate charge transport, further reducing the breakdown field strength of the composite.
IV. Conclusions
1. XLPE/OMMT nanocomposites reach a "properly crosslinked" state at 15–20 min. The crosslinking degree affects the uniformity of crystal sizes and crystallinity. However, crosslinking bonds reduce the distance between OMMT sheets, affecting OMMT dispersion, leading to increased crystal size differences in the over-crosslinked state.
2. The increase in crosslinking bonds, the improvement of the three-dimensional network structure, and the role of OMMT as physical crosslinking points jointly enhance the tensile strength and toughness of the material. In the over-crosslinked state, decreased crystallinity, thermal oxidative scission of molecular chains, and micro-rearrangement of OMMT sheets reduce the tensile properties of the composite.
3. The conductivity of the nanocomposite is closely related to its crosslinking degree and the interfacial interaction between OMMT and the matrix. The rate of conductivity change with temperature reflects the strength of the interfacial interaction's restriction on molecular movement. In the properly crosslinked state, the combined action of crosslinking bonds and interfacial forces reduces the dielectric constant and dielectric loss tangent of the composite. Appropriate crosslinking promotes the integrity of the composite's microstructure, and together with the barrier effect of OMMT, enhances the electrical strength of the composite.
The 15 min "properly crosslinked" window has been verified: the synergistic effect of exfoliated OMMT sheets and the crosslinked network can simultaneously enhance the mechanical and dielectric limits of XLPE. If the specially supplied carbon-forming agent from YINSU Flame Retardant is further added, dense carbon layers can be rapidly formed at the initial stage of combustion, creating a dual barrier with OMMT and achieving a low-smoke halogen-free flame-retardant safety upgrade for cables—making high-voltage cables both strong and safe, ready for industrial deployment with one click.