Red Phosphorus Application

Introduction

Red phosphorus, also known as phosphorus, is an excellent flame retardant. However, it has some drawbacks such as low ignition point, flammability, explosive dust, easy absorption of moisture and oxidation into acids, and the release of phosphine (PH3). The surface modification of red phosphorus particles has become an important topic in flame retardant research. Organic phosphorus-coated red phosphorus products from Japan Phosphorus Chemical Company, products from Albright & Wilson Company in the UK, as well as products from the United States, Germany, and other countries, have overcome some of the drawbacks of red phosphorus as a flame retardant to varying degrees and have been commercialized and put into practical use. In China, red phosphorus as a flame retardant has not been widely used, mainly due to the lack of mass production of high-quality coated red phosphorus and insufficient research on the technology of adding red phosphorus to polymer products. This article discusses the research and application of coated red phosphorus flame retardants. Author/Yong Wu.

Trends in Halogen-Free Flame Retardants and Requirements for High Efficiency, Low Toxicity, and Low Smoke Flame Retardants have led to increased attention to red phosphorus as a standalone high-efficiency halogen-free flame retardant. Coated red phosphorus (or microencapsulated red phosphorus) is a new type of high-efficiency, non-toxic, additive flame retardant that has developed in the past decade.

2. Flame Retardant Mechanism of Red Phosphorus Flame Retardants

When coated red phosphorus is burned in a polymer, the capsule material ruptures, releasing the flame retardant effect of red phosphorus. Different polymers have different structures and therefore exhibit different flame retardant effects with red phosphorus, and red phosphorus acts on them in different ways.

The flame retardant mechanism is as follows: Red phosphorus is first thermally decomposed into Pn→P4→P2. It can capture oxygen-containing cross-linked carbonized layers and can also react with ambient oxygen to generate oxygen-containing phosphoric acid (mainly hypophosphoric acid). This oxygen-containing acid has strong hygroscopicity, which carbonizes the surface of the burning polymer. The existence of the carbonized layer can isolate the polymer, reduce the release of flammable volatile components, and also has heat absorption properties, reducing the surface oxidation heat of the polymer, achieving the purpose of condensed phase flame retardancy. At the same time, the thermal decomposition product of red phosphorus, PO· free radicals, can capture a large number of H· and HO· free radicals in the burning flame, cutting off the flame oxidation chain reaction, achieving gas-phase flame retardancy.

3. Red Phosphorus Flame Retardants and their Coating Treatment

3.1 Why does red phosphorus need surface treatment?

In practical applications, red phosphorus cannot be used directly because:

3.1.1 Red phosphorus easily absorbs moisture in the air, generating substances such as H3PO4, H3P03, and H3PO2, making red phosphorus sticky, losing fluidity, and producing phosphoric acid that is even more hygroscopic. If red phosphorus is present in high polymer materials, the red phosphorus absorbs moisture and oxidizes, corroding the surface of the product, losing its luster and original properties, and gradually corroding the inner layers. The generated oxygen-containing acids can also corrode processing equipment.

3.1.2 Red phosphorus has poor compatibility with resins and can cause separation and sedimentation, increasing the viscosity of the resin. This brings difficulties to the casting, impregnation, and operation of the resin and also leads to a decrease in the performance of synthetic materials.

3.1.3 Long-term exposure of red phosphorus to air can release highly toxic PH3 gas, polluting the environment.

3.1.4 Red phosphorus's hygroscopicity has adverse effects on the leakage of weak electrical components and the insulation of high-voltage components.

3.1.5 Red phosphorus's purple-red color can easily cause discoloration of products.

3.1.6 Explosiveness of dry red phosphorus dust.

3.2 Stabilization of Red Phosphorus Treatment

Adding stabilizing agents was an early treatment method, but it could not solve the problems of hygroscopicity and corrosiveness. It is now rarely used.

3.3 Surface Coating Treatment of Red Phosphorus

This is a new method that developed in the late 1970s. The principle is to coat a single or multiple layers of continuous and dense inorganic or organic protective film on the surface of red phosphorus through physical or chemical means, encapsulating red phosphorus particles, and forming microencapsulated red phosphorus flame retardant. According to the material (capsule material) used for red phosphorus coating, the treatment methods for microencapsulated red phosphorus are divided into three types: inorganic coating method, organic coating method, and inorganic-organic composite coating method.

3.3.1 Inorganic Coating Method

The method of depositing inorganic materials on the surface of red phosphorus particles through appropriate means is called the inorganic coating method. Inorganic materials usually include Al(OH)3, Mg(OH)2, Zn(OH)2, etc. The typical inorganic coating method is to suspend red phosphorus in an aqueous solution containing Al2(SO)3, add NaOH to adjust the pH value to a range of 6 to 8, deposit the formed Al(OH)3 uniformly and densely on the surface of red phosphorus, and finally wash with water, dry, and complete the treatment process. Compared with ordinary red phosphorus, inorganic-coated red phosphorus has improved ignition point, hygroscopicity, and PH3 generation to varying degrees. However, since the capsule material is an inorganic material, when it is used as a polymer flame retardant, it still has the disadvantages of poor compatibility with resins, a small increase in ignition point, and the release of a certain amount of PH3.

Currently, there are proposals to coat red phosphorus with quinones and cobalt salts. For example, a patent application by a Japanese chemical industry company suggests coating red phosphorus with hydrated titanium-cobalt hydroxide. It is claimed that the PH3 generation of red phosphorus can be reduced to below 0.05 mg/g.

3.3.2 Organic Coating Method for Red Phosphorus

In the early days, organic coating methods used paraffin to inhibit the contact of red phosphorus with moisture. However, the mechanical strength and thermal strength of the coating were weak, and it was easily peeled off with slight impact or heating, with poor stability.

Currently, the organic coating method commonly uses thermosetting resin interface polymerization or in-situ polymerization to coat red phosphorus.

The use of melamine-formaldehyde resin coating is widely adopted due to its fast curing speed, high tensile and compressive strength of the coating, strong water and acid-alkali resistance.

The in-situ polymerization coating of melamine-formaldehyde resin on red phosphorus generally consists of three steps: Firstly, uniformly disperse red phosphorus powder in an aqueous system to form a red phosphorus suspension. Secondly, prepare melamine-formaldehyde prepolymer. This prepolymer is alkylated and soluble in water. Finally, mix the red phosphorus suspension with the melamine-formaldehyde prepolymer and heat it to further crosslink and cure the melamine-formaldehyde resin on the surface of red phosphorus. The speed of red phosphorus encapsulation depends on the temperature and pH value of the mixed solution. The encapsulation temperature is generally controlled at 60°C to 80°C, and the pH value is in the range of 4.0 to 6.5. Electron microscopy can be used to observe whether capsules have formed (usually satisfactory encapsulation can be achieved in about 1 hour). After the encapsulation process is completed, the temperature is lowered to room temperature, the pH value is adjusted to near neutral, and finally, vacuum drying or spray drying is used to obtain powdered encapsulated red phosphorus.

The outstanding advantage of organic coating red phosphorus is the low PH3 generation, high ignition point of the product, and good compatibility with resins. The disadvantages are strong hygroscopicity and significant impact on the electrical insulation properties of polymer products.

3.3.3 Inorganic-Organic Composite Coating of Red Phosphorus

Given the advantages and disadvantages of inorganic or organic-coated red phosphorus, some companies in Japan and Western Europe have proposed inorganic-organic composite coating methods.

The composite coating method selects appropriate polymer materials to re-coat the capsule material on the basis of inorganic coating red phosphorus. It is currently the most ideal method among the various red phosphorus modification methods.

The composite coating method selects appropriate polymer materials to re-coat the capsule material on the basis of inorganic coating red phosphorus. It is currently the most ideal method among the various red phosphorus modification methods.

Similar to single-layer coating, the composite coating material usually uses A1(OH)3 or Zn(OH)2 as the inorganic layer, and phenolic resin, melamine-formaldehyde resin, or epoxy resin as the organic layer. For example, the Japan Phosphorus Chemical Industry Company proposes using A1(OH)3 or Zn(OH)2 as the base layer of the composite coating, and then using thermosetting resin as the second layer of coating. The Japan Chemical Industry Company proposes using A1(OH)3, other metal hydroxides, and organic coating agents as the three layers of coating for red phosphorus.

4. Application of Red Phosph

According to reports, the annual usage of red phosphorus flame retardants in Japan reached 200 t to 250 t in the mid-1980s. As shown in Table 1, the encapsulated red phosphorus has strong adaptability and a wide range of applications. Red phosphorus flame retardants are used in thermosetting resins, especially epoxy resins. Germany, the United States, Japan, and other countries have used encapsulated red phosphorus as an additive flame retardant to synthesize flame retardant fibers such as chlorinated fibers, viscose fibers, acrylic fibers, polyester fibers, nylon, and polypropylene. Various flame retardant textiles have been produced, fundamentally solving the flame retardancy issue of synthetic fiber textiles.

5. Conclusion

Encapsulated red phosphorus flame retardants can be regarded as both high-concentration phosphorus-based flame retardants and stable inorganic flame retardants. It is emerging with its high efficiency, low cost, and non-toxic characteristics. In order to better utilize the flame retardant properties of this flame retardant, it is recommended to consider adding other synergistic flame retardants to enhance flame retardant performance and reduce costs. After more than ten years of research and development, encapsulated red phosphorus flame retardants have been widely used in synthetic resins and rubber. The application of red phosphorus flame retardants in China has just begun, and it is hoped that this new type of flame retardant can better serve the three major synthetic material industries. Research and application of encapsulated red phosphorus flame retardants by author/Wu Yong.