Halogen-free flame retardant PP

This material is made of homopolymer polypropylene (PP, melt flow rate 10g/10min, isotacticity 95%, melting temperature 160℃) as the base material, and mixed with melamine pyrophosphate (MPP, phosphorus content 20%, nitrogen content 35%), expanded graphite (particle size 100 mesh, expansion multiple 250mL/g, carbon content 99%), zinc oxide (active zinc oxide, particle size 0.5μm, purity 99.7%), ethylene-vinyl acetate copolymer (EVA, VA content 18%, melt flow rate 2.5g/10min), silane coupling agent (KH570, purity 98%) and lubricant (ethylene bis stearamide, EBS, melting point 140℃) in an accurate mass ratio of 52:28:10:6:3:1. The preparation process is divided into two steps: first, PP and EVA are put into a 50L internal mixer at a temperature of 170°C and a rotor speed of 60 rpm. They are plasticized for 3 minutes until they are completely melted, so that EVA is evenly dispersed in the PP matrix. The flexibility of EVA is used to improve the impact performance of the material and compensate for the brittleness caused by the flame retardant. Then, MPP, expanded graphite, and zinc oxide are added in turn, the speed is adjusted to 50 rpm, and the mixing is carried out for 5 minutes. Finally, KH570 and EBS are added and mixed for 3 minutes until the material is uniform. KH570 can improve the compatibility of the flame retardant and the matrix, while EBS can reduce the melt viscosity and improve processing fluidity. The mixed materials were pelletized by a single screw extruder (screw length-diameter ratio 25:1), and the barrel temperature was controlled by zones: zone 1 175℃, zone 2 185℃, zone 3 190℃, and die head 185℃. The screw speed was 180 rpm, and the pellets were cut into 3×3mm particles to obtain Halogen-free flame retardant PP. After testing, its density was 1.05g/cm³, melt flow rate was 7g/10min (230℃, 2.16kg), oxygen index was 32% (GB/T 2406), flame retardant performance reached UL94 V-0 at a thickness of 1.6mm, vertical combustion self-extinguishing time was <2 seconds, and there was no dripping phenomenon. Halogen content detection showed that the chlorine and bromine content were both <500ppm (IEC 61249), which fully complies with the requirements of the EU RoHS directive. The core of this halogen-free system is the synergistic effect: MPP decomposes under heat to provide a phosphorus source, reacts with the matrix to generate phosphate esters, and promotes carbonization; expanded graphite expands rapidly at high temperatures (volume expands more than 200 times), forming a loose and porous insulation layer, which physically blocks flame and heat transfer; zinc oxide, as a synergist, can not only catalyze the formation of the carbon layer, but also enhance the oxidation resistance of the carbon layer to prevent further combustion of the carbon layer; EVA improves the toughness of the material through compatibility with PP. The multi-components are like precision gears, and only by cooperating with each other can they achieve efficient flame retardancy, and there is no halogen involved in the whole process, avoiding the problem of traditional bromine flame retardants releasing toxic gases such as hydrogen chloride and hydrogen bromide when burning, which is more environmentally friendly and safe.

This product has excellent performance in the field of electronic and electrical housings. Taking the 1.5mm thick router housing as an example, its injection molding parameters have been repeatedly optimized: mold temperature 50℃, injection pressure 110MPa, holding pressure 70MPa, injection time 3 seconds, cooling time 20 seconds, the surface finish of the product reaches 60GU, there are no weld marks, shrinkage marks and flash, and it meets the appearance requirements. The flame retardant performance has passed a number of rigorous tests: UL 94 vertical burning test reaches V-0 level, and the self-extinguishing time at 1.5mm thickness is less than 3 seconds; in the glow wire test (IEC 60695), the 750℃ glow wire does not ignite after contact for 30 seconds, and there is no continuous combustion after contact with the 960℃ glow wire. The glow wire ignition temperature (GWIT) is ≥850℃, which fully complies with the IEC 60950 "Safety of Information Technology Equipment" standard. The electrical properties are also excellent. According to the GB/T 1410 test, its surface resistance reaches 10¹⁴Ω, and its volume resistance reaches 10¹⁵Ω・cm. Its antistatic performance meets the insulation requirements of electronic equipment. After the 1000V high-voltage breakdown test, there is no breakdown phenomenon. The dielectric constant is 3.2 (1MHz, GB/T 1409), and the dielectric loss tangent is 0.003. It is suitable for high-frequency electronic equipment and will not affect signal transmission. In the long-term use reliability test, after the product was placed in a 100℃ oven for 1000 hours, the tensile strength retained 88% (25MPa), and the impact strength was 4.8kJ/m², which only decreased by 12%, which is much better than the 60% retention rate of pure PP, indicating its excellent thermal stability. Compared with traditional bromine-based flame retardant PP, its smoke density rating (SDR) is only 35 (GB/T 8627), which is 40% lower than 58 of bromine-based ones, and the release of toxic gases (CO) is reduced by 55% (ISO 5659). When a fire occurs in a closed electronic cabinet, it can buy more escape time for personnel and reduce the risk of casualties. In the corrosion resistance test, after 48 hours of 5% NaCl salt spray test (GB/T 10125), there is no trace of rust on the surface. Because it does not contain halogen, it has no corrosive effect on the mold, which can extend the mold life by 50% (from 100,000 molds to 150,000 molds) and reduce production costs. Therefore, this material has become an ideal choice for electronic and electrical housings, taking into account safety, performance and economy, and is widely used in routers, switches, set-top boxes, charging piles and other equipment.

In order to fully verify its weather resistance and aging resistance, a number of rigorous tests were conducted, and the results showed that it can adapt to a variety of complex environments. In the QUV aging test, the test piece was placed under a UVB-313 lamp with an irradiance of 0.71W/m², and a cycle mode of 8 hours of illumination (60°C) and 4 hours of condensation (50°C) was used. After 1680 hours of continuous testing, the tensile strength retention rate was 75%, which was 50% higher than the 50% of pure PP; the elongation at break was 60%, which was twice that of pure PP; the impact strength retention was 65%, which was much higher than the 40% of pure PP, indicating that its resistance to UV aging was significantly enhanced. Thermogravimetric analysis (TGA) shows that in a nitrogen atmosphere (heating rate 10℃/min), its initial decomposition temperature is 330℃, 50℃ higher than the 280℃ of pure PP, and the residual carbon rate is 15% at 500℃, which is 3 times that of pure PP, indicating that it has excellent thermal stability and can withstand higher temperatures without decomposition. The outdoor exposure test was carried out in the hot and humid southern region. The 1.5mm thick test piece was fixed on an exposure rack at a 45° angle facing south. After 12 months of wind, sun and rain, it was observed that there was no powdering on the surface, the gloss retention rate was 70%, the color difference ΔE=2.5 (pure PP ΔE=5.8), the impact strength was retained by 70%, and the bending strength was retained by 75%, which still met the use requirements. These excellent properties are due to the composite anti-aging system added to the formula: 0.2% UV absorber (UV-531) can absorb 290-330nm ultraviolet rays and reduce damage to the molecular chain; 0.3% antioxidant 1010 and 0.6% antioxidant 168 are compounded to inhibit thermal oxidative aging, capture free radicals, and delay degradation; 2% carbon black (particle size 20nm) further enhances weather resistance by scattering ultraviolet rays. According to Fourier infrared spectroscopy analysis, the degree of molecular chain breakage of the aged material is lighter than that of pure PP, and the carbonyl index grows slowly, indicating that its anti-aging ability is significantly improved. Therefore, this material is not only suitable for indoor electronic equipment, but also for outdoor scenes such as charging pile shells and outdoor distribution boxes, with long-lasting and stable performance.

From the perspective of the full life cycle cost analysis, although the initial raw material cost of this product is higher, the overall benefit is better. In terms of raw material cost, since the price of halogen-free flame retardants (such as MPP and expanded graphite) is higher than that of brominated flame retardants, their raw material cost is 15% higher than that of brominated flame retardant PP. However, the processing link saves significantly: the halogen-free system is non-corrosive to the equipment, the wear rate of the screw and barrel is reduced by 30%, and the equipment maintenance cost is reduced by 25%; the mold life is extended by 50%, because the hydrogen halide produced by the decomposition of the brominated flame retardant will corrode the mold surface, while the halogen-free system does not have this problem, and the mold replacement cycle is extended from 100,000 molds to 150,000 molds. In terms of processing loss, the melt flow stability of halogen-free PP is better, and the scrap rate is reduced by 8%, which further saves raw materials compared with 5% of the brominated system. In terms of product design, its mechanical properties are better, and the product thickness can be reduced by 10% (such as from 2mm to 1.8mm). Although the density is 3% higher than the brominated system, the unit product material consumption is reduced by 7%, reducing material consumption. In terms of environmental certification, since there is no need to test halogen-related indicators, the REACH and RoHS certification fees for entering the EU market are reduced by 30%, and trade barriers in halogen-restricted areas can be avoided, broadening sales channels. In long-term use, its aging resistance reduces the frequency of product replacement. Taking outdoor distribution boxes as an example, the service life is extended from 5 years to 8 years, and the life cycle cost is reduced by 12%. Comprehensive calculation, in the case of mass production (annual consumption of 1,000 tons), the full life cycle cost is only 5% higher than that of brominated flame retardant PP, but in areas with strict environmental protection requirements, its social benefits and market competitiveness far exceed traditional products, so it has a better cost-effectiveness.

Due to the addition of a large amount of flame retardants, its processing technology needs to be specially adjusted for characteristics such as fluidity and thermal stability. During injection molding, the barrel temperature needs to be 10-15℃ higher than that of pure PP, specifically: 190℃ in zone 1, 200℃ in zone 2, 210℃ in zone 3, and 205℃ in nozzle. Because flame retardants increase melt viscosity, higher temperatures can improve fluidity. The screw speed should be controlled at 200-250 rpm. Too high (>300 rpm) will cause excessive shear heat, causing MPP to decompose above 230℃, releasing ammonia, affecting flame retardant properties and product appearance; too low will cause uneven mixing and flame retardant agglomeration. The holding time needs to be extended by 20% (from 15 seconds to 18 seconds) compared to pure PP. Because the melt cools faster, sufficient holding pressure is required to fill the shrinkage space and prevent shrinkage marks. For the extrusion blow molding process, the die temperature is strictly controlled at 185℃, and the traction ratio is 1:3.5 to ensure uniform film thickness (deviation <5%), because expanded graphite is prone to premature expansion at too high a temperature, resulting in bubbles in the film. Special attention should be paid to moisture-proofing in the storage process. Because expanded graphite has a porous structure, it is easy to absorb moisture and make the moisture content exceed 0.2%, affecting the processing fluidity. Therefore, the product needs to be packaged in moisture-proof aluminum foil bags, 25kg per bag, with built-in desiccant, and stored in a warehouse with a relative humidity of <50%. The shelf life is 12 months. If moisture is accidentally absorbed, it needs to be dried in an 80℃ oven for 4 hours, and the moisture content can be reduced to below 0.1% before it can be used. Ventilation equipment needs to be installed in the processing workshop. Because MPP may release a small amount of ammonia when heated, operators need to wear protective masks to ensure a safe working environment. Through these process adjustments, the performance of the product can be ensured to be stable, and the qualified rate can reach more than 98%.