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Views: 40 Author: Yinsu Flame Retardant Publish Time: 2026-06-26 Origin: www.flameretardantys.com
With the continuous improvement of global environmental awareness and the increasingly stringent fire safety regulations, low-smoke halogen-free (LSHF) materials, as environmentally friendly alternatives to traditional halogen-containing flame-retardant materials, have seen growing demand in fields such as wire and cable, building materials, and new energy.
The core advantage of LSHF materials lies in their absence of halogen elements such as chlorine and bromine. During combustion, they do not release toxic corrosive gases such as hydrogen chloride and dioxins, and their smoke emission is extremely low. This effectively reduces secondary injuries at fire scenes, significantly improves the probability of personnel escape and equipment protection, and aligns with the core requirements of modern industrial green and safe development.
Currently, the flame-retardant performance of LSHF materials is mainly achieved by filling inorganic flame-retardant fillers, among which aluminum hydroxide (ATH) and magnesium hydroxide (MH) have become the most widely used inorganic flame-retardant fillers due to their low cost, environmental friendliness, non-toxicity, and stable flame-retardant effects. However, these inorganic fillers have relatively low flame-retardant efficiency, requiring a high filling ratio of 60%–75% to enable the material to meet the UL94 V0 flame-retardant standard.
High filling levels not only alter the material's system structure but also bring a series of difficulties and pain points to the processing and product performance of LSHF materials. These pain points not only reduce production efficiency but also easily lead to quality defects such as rough surfaces, cracking, and powder shedding, severely limiting the promotion and application of LSHF materials in high-end scenarios.In response to the above issues, researchers at home and abroad have conducted extensive studies, mainly focusing on filler modification, formulation compounding, and processing optimization. However, existing research mostly addresses single pain points and lacks systematic analysis and comprehensive solutions for the five major pain points.
Based on this, this article, combining the processing characteristics and practical application requirements of LSHF materials, deeply analyzes the root causes of various pain points brought by high filling, and proposes a comprehensive solution that balances flame-retardant performance, processing performance, and mechanical properties, providing strong technical support for the high-performance and industrialized production of LSHF materials.
The high-filling-related pain points of LSHF materials fundamentally stem from the essential differences between inorganic flame-retardant fillers and polymer matrix resins, as well as the direct impact of high filling levels on system structure and processing.
Inorganic fillers are mostly rigid powders with strong surface polarity and hydrophilicity, while the matrix resins of LSHF materials (such as EVA, PE, TPE, etc.) are mostly non-polar, hydrophobic polyolefin materials. The compatibility between the two is extremely poor. Meanwhile, high filling levels disrupt the continuous phase structure of the resin, hinder molecular chain movement, and consequently lead to significant declines in processing performance and mechanical properties.
The following provides a detailed analysis of the root causes of the five core pain points.
In LSHF materials, the filling level of inorganic flame-retardant fillers such as ATH and MH usually needs to reach 60%–75% to meet the V0 flame-retardant requirement. This high filling ratio is determined by the flame-retardant mechanism of inorganic fillers.
ATH and MH's flame-retardant effect mainly relies on endothermic thermal decomposition and dehydration carbonization, forming a dense char layer to isolate oxygen and heat and suppress the release of combustible gases. However, their flame-retardant efficiency is far lower than that of halogen-containing flame retardants, so a relatively high filling level is necessary to achieve the expected flame-retardant effect.
The primary issue brought by high filling is filler agglomeration. Inorganic filler surfaces contain a large number of hydroxyl groups and are highly polar. During mixing with non-polar resins, due to the large surface energy difference between the two, filler particles tend to attract and aggregate with each other through hydrogen bonds, forming larger agglomerates.
The presence of agglomerates leads to uneven dispersion of fillers in the resin matrix. This not only fails to fully exert their flame-retardant effect but also creates mechanical defect points in the system, exacerbating material brittleness while obstructing melt flow and further increasing processing difficulty.
In addition, high filling levels cause the continuous resin phase to be segmented by a large number of filler particles, significantly reducing the resin's encapsulation of the fillers and further intensifying agglomeration. This forms a vicious cycle of "filler agglomeration → uneven dispersion → performance degradation," severely affecting the comprehensive performance of the material.
Beyond this, high filling also increases the difficulty of cost control. Excessive inorganic fillers significantly raise material density, reducing lightweight performance, which does not meet the core lightweight requirements of fields such as new energy and rail transit.
At the same time, high filling levels increase energy consumption in processing steps such as mixing and extrusion, reduce production efficiency, further increase industrial production costs, and constrain the scaling of industrial development.
Melt fluidity is a key core indicator in the processing of LSHF materials, directly determining the feasibility of extrusion, injection molding, and other processing techniques, as well as product molding quality.
High filling levels are the core cause of poor melt fluidity in LSHF materials, with roots mainly reflected in two aspects: first, the hindering effect of inorganic fillers on resin molecular chain movement; second, the significant increase in friction resistance between fillers and the resin interface.
The matrix resins of LSHF materials (such as EVA, PE) have a certain degree of fluidity in the molten state. Inorganic fillers are rigid particles. When the filling level is high, a large number of filler particles are uniformly dispersed between resin molecular chains, forming a "barrier layer" that hinders molecular chain sliding and movement. This causes melt viscosity to rise sharply and the melt flow rate (MFR) to decrease significantly.
Typically, unfilled EVA resin has an MFR of 10–20 g/10min (test conditions: 190°C, 2.16 kg). When the ATH filling level reaches 65%, the material's MFR drops to 1–3 g/10min or even lower, failing to meet the basic requirements for melt fluidity in extrusion, injection molding, and other processing techniques, making processing difficult to proceed smoothly.
Furthermore, due to the poor compatibility between inorganic fillers and resin, the interface bonding is not tight. During melt processing, significant friction resistance is generated between filler particles and resin molecular chains, further increasing melt viscosity and causing continuous decline in melt fluidity.
Meanwhile, the presence of filler agglomerates forms "flow obstacles," hindering uniform melt flow. This leads to issues such as uneven flow velocity and insufficient plasticization during processing, directly affecting product molding quality and easily producing surface defects.
The combined effects of poor melt fluidity and filler agglomeration directly lead to significantly increased difficulty in extrusion and injection molding of LSHF materials, specifically manifested as:
During extrusion: high extruder load, excessive current, die carbon buildup, rough product surfaces, and strand breakage; during injection molding: incomplete mold filling, difficult injection, poor demolding, and quality defects such as sink marks and bubbles on product surfaces. These severely affect production efficiency and product yield, increasing the cost of industrial production.
During extrusion, high-filling materials have high melt viscosity and poor fluidity, requiring greater extrusion pressure to push the melt through the die. This increases screw load and current. Long-term operation accelerates equipment wear and shortens equipment lifespan.
At the same time, insufficient melt plasticization and incomplete fusion between filler particles and resin easily cause accumulation and carbonization at the die, forming die carbon buildup. This affects product surface quality and even causes strand breakage, requiring frequent shutdowns for die cleaning, significantly reducing production efficiency.
In addition, high-filling materials have poor melt elasticity. During extrusion, melt fracture phenomena are prone to occur, causing ripples and burrs on product surfaces, affecting product appearance and performance.
During injection molding, poor melt fluidity prevents the melt from rapidly and uniformly filling the mold cavity, easily causing incomplete filling and short shots. Meanwhile, high melt viscosity requires significantly increased injection pressure, which accelerates mold wear and easily causes flash defects.
Furthermore, due to poor compatibility between fillers and resin and loose interface bonding, during injection cooling, the shrinkage rate difference between fillers and resin is large, easily generating internal stress. This leads to cracking and deformation of products, increasing demolding difficulty, further reducing product yield, and constraining scaled production.
High filling levels cause significant degradation in the mechanical properties of LSHF materials, especially toughness deterioration and increased brittleness. This is specifically manifested as substantial reductions in impact strength and elongation at break, with cracking and damage easily occurring during processing, transportation, or use. This fails to meet the basic mechanical performance requirements for practical applications and limits the scope of application.
The core root of this problem lies in the poor compatibility between inorganic fillers and resin, as well as the destruction of the continuous resin phase structure by high filling levels.
Inorganic fillers are rigid particles with strong surface polarity. The interface bonding force with non-polar resin is weak. When the material is subjected to external forces, stress tends to concentrate at the filler-resin interface, leading to interface peeling and cracking, which then triggers overall material fracture. Meanwhile, high filling levels destroy the continuous phase structure of the resin. The connections between resin molecular chains are cut by a large number of filler particles, preventing effective transmission of external forces. This leads to decreased toughness and increased brittleness, making it difficult to withstand external impact and bending.
Experimental data shows that when the ATH filling level increases from 30% to 65%, the elongation at break of LSHF EVA materials drops from over 500% to below 50%, and the Charpy impact strength drops from 20 kJ/m² to below 5 kJ/m². The material transitions from a tough state to a brittle state, unable to withstand bending, impact, and other external forces. This severely limits its application in scenarios such as wire and cable and automotive parts that require a certain degree of toughness.
In addition, the presence of filler agglomerates becomes stress concentration points, further exacerbating material brittleness. This causes the material to crack under slight external forces, affecting product service life.
High-filling LSHF materials exhibit poor surface performance, mainly manifested as insufficient scratch resistance and abrasion resistance. Slight friction can cause scratches and whitening, and even filler detachment and powder shedding. This severely affects product appearance quality and service life, making it particularly unsuitable for high-end application scenarios with high surface performance requirements, limiting the improvement of product added value.
The root cause of this problem mainly lies in the destruction of the continuous resin phase by high filling levels and the loose bonding between fillers and resin. When the inorganic filler filling level is too high, the resin cannot completely encapsulate filler particles. Some filler particles are exposed on the material surface, resulting in insufficient surface density and reduced hardness. When the material surface is subjected to friction, exposed filler particles are easily scraped off, forming scratches. Meanwhile, separation occurs at the filler-resin interface, causing surface whitening. In addition, the presence of filler agglomerates causes uneven凹凸 on the material surface, further reducing scratch and abrasion resistance and exacerbating surface defect generation.
Moreover, insufficient plasticization and poor melt fluidity during processing also lead to rough and loose material surfaces, further exacerbating surface whitening and powder shedding. For example, in the production of LSHF wire and cable sheaths, surface powder shedding reduces the adhesion between the sheath and conductor, affecting cable electrical performance and service life. In automotive interior applications, poor scratch resistance affects product appearance aesthetics, reducing user experience and limiting application in the high-end automotive field.
In response to the above five core pain points, combining the flame-retardant requirements, processing characteristics, and practical application needs of LSHF materials, this article proposes targeted solutions from three core dimensions: filler modification, formulation compounding, and processing optimization. These solutions aim to effectively improve material processing performance, mechanical properties, and surface performance while ensuring V0 flame-retardant performance, achieving a balance between high filling and high performance, and promoting the industrial application of LSHF materials.
The core of solving high filling and agglomeration problems is to improve filler-resin compatibility, reduce the filling level of single fillers, and optimize filler dispersion. The following three methods can be used synergistically to fundamentally alleviate the negative impacts brought by high filling and agglomeration.
Filler surface modification is a key means to improve filler-resin compatibility and reduce agglomeration. Its core principle is to coat a layer of modifier on the filler surface through physical or chemical methods, reducing filler surface polarity, enhancing interface bonding with non-polar resin, reducing hydrogen bonding between filler particles, thereby improving filler dispersion and ensuring uniform distribution in the resin matrix.
Currently, commonly used surface modifiers mainly include coupling agents, surfactants, and compatibilizers, among which coupling agents have the most significant modification effect and are the most widely used.
Silane coupling agents (products from Shanghai Juju Company) are the most widely used filler modifiers in LSHF materials. Their molecular structure contains inorganic-philic and organic-philic groups. The inorganic-philic group can react chemically with hydroxyl groups on the filler surface to form strong chemical bonds, while the organic-philic group can react with or physically entangle with resin molecular chains, significantly improving filler-resin compatibility.
During modification, dry or wet processes can be used to uniformly coat the coupling agent on the filler surface. For example, using the dry modification method, ATH powder and 0.5%–1.5% silane coupling agent are added to a high-speed mixer and stirred at 80–100°C for 10–15 minutes to uniformly coat the coupling agent on the ATH surface. The modified ATH shows significantly reduced surface polarity, greatly improved compatibility with EVA resin, significantly reduced agglomeration, and markedly improved dispersion.
In addition to silane coupling agents, titanate and aluminate coupling agents can also be used for filler modification. Their modification mechanisms are similar to silane coupling agents. The appropriate coupling agent type and dosage can be selected according to resin type and processing requirements to ensure modification effectiveness.
Furthermore, surfactants (such as stearic acid and zinc stearate) can be used for auxiliary modification to further reduce filler surface energy, reduce agglomeration between filler particles, and improve dispersion.
By compounding flame-retardant systems and introducing synergistic flame retardants, the filling level of single inorganic fillers such as ATH and MH can be significantly reduced while ensuring that material flame-retardant performance meets standards. This fundamentally alleviates various problems caused by high filling, achieving a balance between flame-retardant performance and processing performance.
Commonly used synergistic flame retardants mainly include phosphorus-nitrogen intumescent flame retardants, silicon-based synergists, and zinc borate, which produce significant synergistic flame-retardant effects when used in combination with ATH and MH.
Phosphorus-nitrogen intumescent flame retardants (such as ammonium polyphosphate APP and melamine cyanurate MCA) have high flame-retardant efficiency and are environmentally friendly and non-toxic. When used in combination with ATH and MH, they produce synergistic flame-retardant effects: ATH and MH undergo endothermic thermal decomposition and dehydration, with the formed oxides serving as char layer skeletons; the ammonia gas and phosphoric acid substances produced by phosphorus-nitrogen flame retardant thermal decomposition promote char layer formation, enhance char layer density, and simultaneously suppress combustible gas release. Thus, while reducing inorganic filler filling levels, the material can still meet the UL94 V0 flame-retardant standard.
For example, when the ATH filling level is reduced from 65% to 50%, combined with 10%–15% APP and 5% MCA compounding, the material's flame-retardant performance can still reach UL94 V0 level. Meanwhile, the total filler filling level drops below 65%, effectively alleviating agglomeration and processing difficulties while balancing flame-retardant performance and processing performance.
Silicon-based synergists (such as silane coupling agents and silicone resins) can not only improve filler-resin compatibility but also form siloxane protective layers during combustion, enhancing char layer stability and further improving flame-retardant effects. When used in combination with ATH and MH, they can further reduce inorganic filler filling levels.
Zinc borate, as a synergistic flame retardant, can synergize with ATH and MH to effectively suppress smoke generation while improving flame-retardant efficiency, reducing inorganic filler usage, and achieving dual improvements in environmental protection and performance.
Using graded matching of inorganic fillers with different particle sizes can effectively improve filler packing density, reduce internal voids in the system, thereby reducing filler agglomeration and improving dispersion at the same filling level, while reducing melt viscosity and improving processing performance.
Specifically, large particle size (10–20 μm), medium particle size (5–10 μm), and small particle size (1–5 μm) ATH or MH powders can be matched in certain proportions. Large particle size powders can reduce material costs, medium particle size powders can fill the voids between large particle size powders, and small particle size powders can fill the voids between medium and large particle size powders, forming a dense packing structure. This reduces internal voids in the system, thereby reducing filler agglomeration, increasing filler-resin contact area, and improving dispersion and processing performance.
Experiments show that using large, medium, and small particle size ATH in a 3:4:3 ratio, with a filling level of 60%, filler packing density increases by more than 20%, agglomerate size significantly decreases, and material melt fluidity and mechanical properties are both significantly improved.
In addition, the addition of ultrafine powders (particle size less than 1 μm) can further improve flame-retardant effects and reduce filler usage. However, it is necessary to control the addition amount of ultrafine powders to avoid exacerbated agglomeration caused by excessively high surface energy of ultrafine powders, which would affect modification effectiveness.
Test conditions: EVA matrix, silane coupling agent addition 1.0%, mixing temperature 160°C
The core of improving LSHF material melt fluidity is reducing system melt viscosity and enhancing molecular chain fluidity. This can be approached from two core aspects: formulation optimization and resin selection. Through scientific and reasonable adjustments, melt fluidity can be significantly improved to meet processing requirements. Specific measures are as follows.
Flow modifiers can effectively improve resin molecular chain sliding performance, reduce melt viscosity, and increase melt flow index. They are an effective means to solve poor melt fluidity.
Commonly used flow modifiers mainly include lubricants and plasticizers, among which lubricants are the most widely used. The appropriate lubricant type and dosage can be selected according to processing requirements.
Polyethylene wax and zinc stearate are commonly used lubricants in LSHF materials, which can be divided into internal lubricants and external lubricants. Internal lubricants (such as zinc stearate) can interact with resin molecular chains, reducing intermolecular friction, improving molecular chain sliding performance, and enhancing internal melt fluidity. External lubricants (such as polyethylene wax) can form a uniform lubricating film on the melt surface, reducing friction between the melt and processing equipment (screw, die), while reducing friction resistance between fillers and resin, improving overall melt fluidity.
Lubricant dosage needs to be reasonably controlled according to filler filling level and resin type, typically 0.5%–2.0% of total material mass. For example, when the ATH filling level is 65%, adding 1.0%–1.5% EBS combined with 0.5% zinc stearate can increase the material's melt flow index from 1–3 g/10min to 5–8 g/10min, with significantly improved melt fluidity, capable of meeting extrusion, injection molding, and other processing requirements.
It should be noted that excessive addition of single lubricants should be avoided, as this can cause lubricant bleeding and blooming, affecting material surface quality and mechanical properties, which is counterproductive to product molding.
Test conditions: EVA 18-3 matrix, 190°C, 2.16 kg
Matrix resin viscosity directly affects system melt fluidity. Selecting low-viscosity, high-melt-flow-index resins can reduce overall system viscosity from the matrix level, improve melt fluidity, and provide a good foundation for the processing process. Commonly used matrix resins for LSHF materials include EVA, PE, TPE, TPU, etc., among which EVA resin has become the most widely used matrix resin due to its good compatibility and excellent processing performance.
In actual production, EVA resins with higher melt flow indexes (such as EVA 18-3, MFR 18 g/10min) can be selected as the matrix to replace low-MFR EVA resins (such as EVA 14-2, MFR 14 g/10min), significantly improving system melt fluidity.
In addition, low-viscosity resins such as POE (polyolefin elastomer) and LDPE (low-density polyethylene) can also be added to the system as matrices or modifiers to further reduce melt viscosity.
For example, adding 5%–10% POE to the EVA matrix can reduce system melt viscosity by 15%–25%, significantly increasing melt flow index, while also improving material toughness, achieving dual improvements in processing performance and mechanical properties.
Single internal or external lubricants are difficult to simultaneously meet the requirements for melt fluidity and surface quality. A synergistic combination of internal and external lubrication is needed to optimize the lubricant system ratio, balancing fluidity and surface quality, ensuring smooth processing while improving product appearance quality.
Typically, the ratio of internal to external lubricants is controlled between 1:1 and 2:1, which can be flexibly adjusted according to processing technique and filler filling level to adapt to different processing requirements.
For example, in extrusion processing, the proportion of external lubricants can be appropriately increased to reduce friction between the melt and the screw and die, avoiding die carbon buildup and improving product surface smoothness. In injection molding, the proportion of internal lubricants (such as zinc stearate) can be appropriately increased to improve internal melt fluidity, enhance mold filling efficiency, and avoid defects such as incomplete filling. Meanwhile, a small amount of compatibilizing lubricant can be added to enhance compatibility between the lubricant, resin, and filler, avoiding lubricant bleeding and ensuring stable comprehensive material performance.
Solving the difficulties in extrusion and injection molding requires combining formulation optimization and processing adjustments, starting from three core aspects: reducing processing resistance, improving plasticization effects, and optimizing the molding process. Through comprehensive measures, processing difficulty can be reduced, and production efficiency and product yield can be improved. Specific measures are as follows.
Introducing elastomers and compatibilizers into the formulation can effectively improve system processing shear characteristics, reduce melt shear resistance, and improve processing performance, providing a good foundation for extrusion and injection molding.
Elastomers such as POE, SEBS (styrene-ethylene-butylene-styrene block copolymer), and EPR (ethylene-propylene rubber) have good compatibility and flexibility. After being added to the system, they can improve the interface bonding between resin and filler, reduce melt viscosity, enhance melt elasticity and fluidity, reduce processing resistance during extrusion and injection molding, and avoid processing defects.
Adding grafted compatibilizers (such as Juju Company's PE-g-MAH and Juju Company's EVA-g-MAH) can further enhance the interface bonding force between inorganic fillers and resin, reduce processing internal friction, improve plasticization effects, and ensure smooth processing.
For extrusion processing, equipment structure and process parameters need to be optimized to reduce processing resistance, improve plasticization effects, and ensure product molding quality.
In terms of equipment, low-shear screw structures can be used to reduce screw shear effects on the melt, avoiding melt degradation and filler agglomeration. Meanwhile, die design can be optimized by increasing die exit area to reduce melt exit resistance and improve extrusion efficiency.
In terms of process parameters, segmented temperature increase is adopted. Barrel temperature gradually increases from the feeding section to the die section: feeding section temperature controlled at 120–140°C, melting section at 150–170°C, and die section at 160–180°C. This avoids melt degradation caused by excessively high temperatures and insufficient plasticization caused by excessively low temperatures. Meanwhile, screw speed is reduced and plasticization time is extended to ensure complete fusion between fillers and resin, reduce agglomerates, and improve melt uniformity.
For injection molding processing, barrel temperature, injection parameters, and mold design need to be optimized to improve mold filling efficiency and product quality. Barrel temperature is similar to extrusion processing, using segmented temperature increase to ensure sufficient melt plasticization. For injection parameters, injection pressure is appropriately increased (controlled at 80–120 MPa) and injection speed is increased to enhance melt mold filling capability, avoiding incomplete filling and short shots. Meanwhile, holding pressure time is extended (controlled at 5–10 s) to reduce product sink marks and bubbles and improve product molding quality.
In terms of mold design, gate and runner design is optimized by increasing gate size and runner cross-sectional area to reduce melt flow resistance. Meanwhile, reasonable draft angles are set to reduce demolding resistance, avoid product cracking, and improve demolding efficiency and product yield.
The quality of mixing processes directly affects filler dispersion and melt plasticization effects. Optimizing mixing processes can effectively improve processing performance and reduce processing defects.
During mixing, a "dry mixing first, then melting" approach is adopted. First, raw materials such as resin, modified filler, lubricant, and compatibilizer are added to a high-speed mixer and dry mixed at 80–100°C for 10–15 minutes to achieve uniform mixing of all components and reduce filler agglomeration. Then, the dry mixed material is added to an internal mixer or open mill and melted and mixed at 150–170°C for 15–20 minutes. Mixing pressure and speed are strictly controlled to ensure complete fusion between fillers and resin, forming a uniform dispersion system and improving melt plasticization effects.
In addition, a secondary mixing process can be used. The material after initial mixing undergoes secondary melt mixing to further improve filler dispersion and plasticization effects, reduce agglomerates in the system, lower processing difficulty, ensure smooth processing, and improve product quality stability.
Improving LSHF material toughness and solving the problem of high brittleness requires enhancing filler-resin interface bonding force, improving material impact resistance, absorbing impact energy, and reducing stress concentration. The following three methods can be used synergistically to significantly improve material toughness and elongation at break, avoiding cracking problems.
Adding elastomers to the system can construct a "sea-island structure" (elastomer as islands, resin as sea). When the material is subjected to external impact, elastomer particles can effectively absorb impact energy and inhibit crack propagation, thereby significantly improving material toughness and elongation at break, solving the problems of high brittleness and easy cracking. Commonly used elastomers include POE, SEBS, TPU, and EPR, among which POE has become the most widely used toughening agent due to its good compatibility and significant toughening effects.
Elastomer dosage needs to be reasonably controlled according to filler filling level and toughness requirements, typically 5%–15% of total material mass. For example, when the ATH filling level is 65%, adding 10%–12% POE can increase material elongation at break from below 50% to above 150%, and Charpy impact strength from below 5 kJ/m² to above 15 kJ/m². Material toughness is significantly improved, capable of effectively withstanding bending, impact, and other external forces, avoiding cracking during processing, transportation, or use.
Meanwhile, SEBS and TPU also have significant toughening effects. The appropriate elastomer type can be selected according to material application scenarios. For example, TPU toughening can simultaneously improve material abrasion resistance and toughness, suitable for scenarios with abrasion resistance requirements.
Test conditions: EVA 18-3 matrix, silane coupling agent 1.0%
Weak interface bonding force is an important cause of high material brittleness. Through dual modification with coupling agents and grafted compatibilizers, filler-resin interface bonding force can be significantly enhanced, interface peeling and cracking can be reduced, and material toughness can be improved, fundamentally solving the easy cracking problem.
As mentioned earlier, silane coupling agents can form chemical bonds between fillers and resin, enhancing interface bonding force. The grafted groups of grafted compatibilizers can react with hydroxyl and amino groups on the filler surface, while entangling with resin molecular chains, further reinforcing interface bonding and improving interface stability.
Improving the surface performance of LSHF materials requires ensuring the integrity of the continuous resin phase, improving material surface density and hardness, and reducing surface defects. The following three measures can be used synergistically to significantly improve material scratch and abrasion resistance, avoid whitening and powder shedding, and improve product appearance quality and service life.
Excessive filler filling levels destroy the continuous resin phase, causing filler exposure. Therefore, the upper limit of filler filling needs to be strictly controlled. While ensuring flame-retardant performance, filler filling levels should be reduced as much as possible to ensure that resin can completely encapsulate filler particles, forming a complete continuous phase, laying the foundation for improving surface performance. Through the flame-retardant system compounding mentioned earlier, total filler filling can be controlled below 65%. At this point, resin can form a continuous encapsulation layer, avoiding filler exposure, thereby improving material surface density and hardness, improving scratch and abrasion resistance, and reducing whitening and powder shedding.
In addition, fillers with smaller particle sizes and better dispersion can be selected to reduce damage to the continuous resin phase while improving material surface smoothness. For example, using ATH powder with 5–10 μm particle size to replace 10–20 μm ATH powder can make the material surface smoother, significantly improving scratch and abrasion resistance, and effectively reducing surface defect generation.
Adding scratch and abrasion resistance additives to the formulation can form a wear-resistant protective layer on the material surface, significantly improving surface hardness and scratch resistance, reducing whitening and powder shedding, and improving product surface quality.
Commonly used scratch and abrasion resistance additives include special wear-resistant agents, silicone scratch-resistant agents, and micro powder waxes. The appropriate additive type and dosage can be selected according to surface performance requirements.
Silicone scratch-resistant agents (such as silicone resins and silicone masterbatches) have good lubricity and wear resistance. After being added to the system, they can form a smooth siloxane protective layer on the material surface, reducing surface friction coefficient, improving scratch and abrasion resistance, while improving material surface gloss and reducing whitening. The typical addition amount is 1%–3% of total material mass. For example, adding 2% silicone scratch-resistant agent can increase material surface hardness by more than 20%, with significantly improved scratch resistance. Slight friction will not cause scratches or whitening, effectively improving product appearance quality.
Micro powder waxes (such as polyethylene micro powder wax and PTFE micro powder wax) can fill tiny voids on the material surface, improving surface density while reducing surface friction and improving abrasion resistance. When used in combination with silicone scratch-resistant agents, surface performance can be further improved, avoiding whitening and powder shedding. Special wear-resistant agents (such as polyamide wear-resistant agents and polyurethane wear-resistant agents) can react with resin molecular chains, enhancing material surface hardness and abrasion resistance, suitable for high-end application scenarios with high surface performance requirements.
Test conditions: EVA 18-3 matrix, ATH particle size 5–10 μm
Insufficient plasticization and unreasonable cooling and shaping during processing lead to rough and loose material surfaces, exacerbating whitening and powder shedding. Therefore, molding and shaping processes need to be optimized to improve surface quality and ensure that product appearance meets requirements.
In extrusion processing, die temperature and cooling water temperature are strictly controlled. Excessively high die temperature causes bubbles and burrs on the material surface, while excessively low temperature causes rough surfaces. Cooling water temperature is controlled at 20–30°C, using segmented cooling to ensure uniform material cooling and shaping, avoiding roughness and looseness caused by uneven surface shrinkage, and improving surface smoothness.
In injection molding processing, holding pressure time and cooling time are optimized. Extended holding pressure time reduces product surface sink marks and looseness, while extended cooling time ensures complete product shaping, avoiding surface softening and whitening. Meanwhile, molds are polished to improve mold surface smoothness, thereby improving product surface quality and reducing surface defects.
In addition, post-treatment (such as hot air drying and surface coating) can be applied to formed products to further improve surface density and scratch and abrasion resistance, avoiding powder shedding and ensuring stable product quality.
Conclusion and Outlook
The high-filling processing pain points of LSHF materials fundamentally stem from the poor compatibility between inorganic flame-retardant fillers and resin, as well as the destruction of system structure and processing by high filling levels. These are mainly manifested as five problems: filler agglomeration, poor melt fluidity, difficulties in extrusion and injection molding, high material brittleness, and poor surface scratch resistance. These problems are interrelated and mutually influential, severely constraining the industrial application and high-performance development of LSHF materials.
In response to these pain points, through comprehensive solutions including filler surface modification, flame-retardant system compounding, elastomer toughening, and processing optimization, material processing performance, mechanical properties, and surface performance can be effectively improved while ensuring V0 flame-retardant performance, achieving a balance between high filling and high performance, and providing feasible technical paths for its industrial production.
Specifically, using silane coupling agents for inorganic filler surface modification, combined with phosphorus-nitrogen and silicon-based synergistic flame retardants, can effectively reduce single-filler filling levels, reduce agglomeration, and improve flame-retardant performance and compatibility.
Adding efficient flow modifiers and selecting low-viscosity matrix resins can significantly improve melt fluidity to meet processing requirements.
Introducing elastomers and compatibilizers, and optimizing processing techniques, can reduce processing difficulty, improve material toughness, and avoid cracking problems.
Controlling filler upper limits, adding scratch and abrasion resistance additives, and optimizing molding and shaping processes can improve material surface performance, reduce whitening and powder shedding, and improve product quality.
With the rapid development of new energy, rail transit, photovoltaic energy storage, and other fields, higher requirements have been put forward for the high-performance, lightweight, and low-cost development of LSHF materials.
Future research directions mainly focus on three aspects:
First, developing efficient synergistic flame-retardant systems to further reduce filler filling levels, achieving material lightweight and high-performance development.
Second, researching new modification technologies to improve filler-resin compatibility, achieving a better balance between high filling and high performance, and expanding application scenarios.
Third, optimizing industrial processing techniques, reducing production costs, improving production efficiency, promoting the widespread application of LSHF materials in more high-end scenarios, and supporting green and safe industrial development.
