Chen Zhizhao, Liu Huaxia, Zhou Kan(Pusaifu Phosphorus Chemical Co., Ltd., Qingyuan)

Abstract

This study evaluates two flame retardant systems - the biphenyl-based system (Nitrophor™ FR1000) and the thermal-resistant system (Nitrophor™ FR2000) - in high-gloss polypropylene (PP) materials. Experimental results demonstrate that both systems meet the UL94 V-2 standard at a 2.5% additive loading. The mechanical properties were comparatively analyzed, with particular focus on the fogging performance of flame-retardant PP. Findings indicate that Nitrophor™ FR2000 exhibits superior overall performance.

Keywords

Polypropylene, Flame retardant, UL94 V-2, High-gloss

Introduction

Polypropylene (PP), one of the most widely used general-purpose plastics in daily life, offers excellent mechanical properties, good insulation performance, and low density, making it extensively applied in electronics and electrical appliances. However, with an oxygen index of only about 18%, PP is highly flammable, releasing substantial heat and smoke during combustion. Therefore, flame-retardant modification is the most critical approach to improving its combustion performance.

In recent years, with rising living standards, smart toilets featuring heating, lighting, cleaning, and self-cleaning functions have gained popularity. The seat components of these smart toilets are typically made of PP materials, which require UL94 V-2 flame retardancy, along with specific thermal conductivity and high-gloss properties.

Commercially available UL94 V-2 flame retardants for PP mainly include bromine-antimony systems [1] and free-radical initiator dripping flame-retardant systems [2,3]. Bromine-antimony systems, often used in high-gloss PP, typically employ less-migrating TBC combined with antimony trioxide, requiring about 8% loading to achieve UL94 V-2. On September 3, 2020, the European Commission issued Regulation (EU) 2020/1245, amending EU10/2011, which took effect 20 days later. A key provision states that "the migration limit for antimony trioxide applies to all sources of antimony trioxide, not just its use as an additive." This restriction on antimony trioxide has impacted the prospects of bromine-antimony systems, which is why they are not the focus of this study.

The most common free-radical initiator dripping flame-retardant system is the bibenzyl system, typically composed of bibenzyl (2,3-dimethyl-2,3-diphenylbutane), aluminum hypophosphite, and melamine hydrobromide. With a total loading of about 2-3%, this system achieves UL94 V-2 flame retardancy, offering advantages such as low additive content and high efficiency.

Figure 1 Chemical structure of bibenzyl (2,3-dimethyl-2,3-diphenylbutane)

Some upgraded versions of the bibenzyl system, referred to herein as thermal-resistant systems, address the low decomposition temperature of bibenzyl (TGA shows ~120°C at 1% weight loss, as in Figure 2). These systems replace bibenzyl with a higher-temperature free-radical initiator, combined with aluminum hypophosphite and melamine hydrobromide, forming a flame-retardant system with improved thermal stability. The required loading (2-3%) is similar to that of the bibenzyl system to achieve UL94 V-2.

Figure 2 TGA curve of bibenzyl (10 mg sample, heating rate 10°C/min, up to 300°C)

This study evaluates two flame-retardant systems in a typical high-gloss flame-retardant PP formulation:

The bibenzyl-based system (Nitrophor™ FR1000, produced by Qingyuan Pusaifu Phosphorus Chemical Co., Ltd.)

2.The thermal-resistant system (Nitrophor™ FR2000, a non-bibenzyl, non-polybibenzyl system from the same company).

Their flame-retardant and other performance characteristics are compared under standardized formulations.

1 Experimental Section

1.1 Materials

Polypropylene low-halogen eco-friendly flame retardant (Nitrophor™ FR1000): Pusaifu Phosphorus Chemical Co., Ltd., Qingyuan

Polypropylene low-halogen eco-friendly flame retardant (Nitrophor™ FR2000): Pusaifu Phosphorus Chemical Co., Ltd., Qingyuan

Homopolymer polypropylene (Z30S): Sinopec Maoming, melt flow rate 28 g/10 min

EBS (Ethylene Bis-Stearamide): Kao Corporation, Japan

Antioxidant 1010: Sanfeng Chemical

Antioxidant 168: Sanfeng Chemical

Carbon black masterbatch: Cabot Corporation

1.2 Instruments and Equipment

Twin-screw extruder: Model SK-36, Nanjing Keya Chemical Equipment Co., Ltd.

Plastic injection molding machine: Haishi HS-1200

Universal testing machine: Model CMT4204, Shenzhen New SANS Materials Testing Co., Ltd.

Melt flow index tester: Model XRL-400A, Chengde Precision Testing Machine Co., Ltd.

Izod impact tester: Model ZBC-25B, Shenzhen New SANS Materials Testing Co., Ltd.

Vertical burning tester: Custom-built by the company

Fogging tester: Horizon FTS, Guining (Shanghai) Laboratory Equipment Co., Ltd.

1.3 Experimental Procedure

1.3.1 Sample Preparation

The polypropylene (PP) resin was dried at 90°C for 3 hours prior to processing. The dried PP was then uniformly mixed with flame retardants and additives according to the formulation, followed by extrusion and pelletization using a twin-screw extruder. The resulting pellets were dried again before being injection-molded into standard test specimens for various performance evaluations.
 

1.3.2 Experimental Formulation

1.3.3 Performance Testing

Density testing was conducted according to ASTM D792 (Standard Test Methods for Density and Specific Gravity of Plastics by Displacement)

Tensile properties were measured following ASTM D638 (Standard Test Method for Tensile Properties of Plastics)

Flexural properties were evaluated per ASTM D790 (Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials)

Izod impact strength was determined using ASTM D256 (Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics)

Melt flow index was tested in compliance with ASTM D1238 (Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer)

Vertical burning tests were performed according to the UL94 standard

Fogging tests were carried out following DIN 75201:2011-B

 

2 Results and Discussion

2.1 Performance Testing of Two Flame-Retardant High-Gloss PP Materials

2.2 Fogging Performance Comparison

Figure 3 Aluminum foil surface after fogging test with Nitrophor™ FR1000

Figure 4 Aluminum foil surface after fogging test with Nitrophor™ FR2000

Experimental data in Table 3 demonstrate that the bibenzyl-based system (Nitrophor™ FR1000) exhibits significantly greater fogging deposition compared to the thermal-resistant system (Nitrophor™ FR2000), indicating FR2000's superior stability and migration resistance in PP products.

The fogging test evaluates condensable volatiles from materials under controlled thermo-optical conditions (per automotive standards DIN 75201). This study employed the method to assess potential exudation issues in UL94 V-2 compliant flame-retardant high-gloss PP. Mechanistically, the key difference lies in the decomposition temperatures of the two flame retardants. As shown in Figure 5, Nitrophor™ FR1000's lower decomposition temperature versus FR2000 proves critical. During processing (170-200°C) and fogging testing (prolonged 100°C exposure), bibenzyl in FR1000 decomposes into low-MW species trapped within the polymer matrix, which subsequently migrate and condense on testing substrates.

Figure 5 TGA curves of both flame retardants (10mg sample, 10°C/min heating rate, 700°C)

 

3 Conclusions

(1) Both low-halogen eco-friendly flame retardants, Nitrophor™ FR1000 and Nitrophor™ FR2000, can meet the UL94 V-2 flammability requirements when applied in high-gloss polypropylene (PP) materials.

(2) Owing to its higher decomposition temperature and superior thermal stability, Nitrophor™ FR2000 demonstrates better fogging performance than Nitrophor™ FR1000 in flame-retardant high-gloss PP applications, making it the more effective solution.
 

REFERENCES

[1] J. Kaspersmaa,C. Doumena, S. Munrob, A. Prinsa. Fire retardant mechanism of aliphatic brominecompounds in polystyrene and polypropylene[J]. Polym. Degrad. Stab. 2002,77,325–331;

[2]G. Bertelli, G.Camino, L. Costa & R. Locatelli. Fire Retardant Systems Based on MelamineHydrobromide: Part I--Fire Retardant Behaviour[J]. Polym. Degrad. Stab. 1987,18, 225-236;

[3] G. Bertelli,G. Camino, L. Costa & R. Locatelli. Fire Retardant Systems Based onMelamine

Hydrobromide: Part2 Overall Thermal Degradation [J]. Polym.Degrad. Stab.1987, 18, 307-319;

Comparison of two types of flame retardants used in highgloss flame retardanted polypropylene

Zhizhao Chen, Huaxia Liu, Kan Zhou

Presafer (Qingyuan) Phosphor Chemical Company Limited

Abstract 
In this paper, We use two types of flame retardants in high gloss polypropylene, Dicumyl typing(NitrophorTMFR1000 from Presafer) and anti-aging typing(NitrophorTM FR2000 fromPresafer). The experimental results show that they can meet the standard ofUL94 V-2 when the additions are 2.5%, and the mechanical properties are alsotested. We focus on the atomization performance of the two high gloss flameretardanted polypropylenes, and found that anti-aging typing(NitrophorTMFR2000 from Presafer) has the best performance

Key words 
polypropylene, flameretardant, UL94 V-2, high gloss