According to the latest data released by Wohlers in 2022, the additive manufacturing industry increased by 19.5% in 2021, while polymer powder materials (mainly nylon, TPU, etc.) accounted for 16% of the 3D industry material market, and the shipment volume exceeded that of photosensitive resin materials for the first time. This means that 3D printing technology has evolved from

prototype verification application to production level application, and is getting better and better.

Meanwhile, according to the 2021 in-depth research report on the global and China 3D printing elastomer Industry issued by xinsijie Industrial Research Center, the global 3D printing elastomer market will grow from US $180 million in 2021 to US $620 million in 2026, with a CAGR of 28.1%.

In 2020, TPU material has dominated the 3D printing elastomer market, and it is expected to maintain its dominant position in the future. From 2021 to 2026, TPE material will become the

leading part of the 3D printing elastomer market.

With the wide application of 3D printing TPU, higher requirements are put forward for the performance of TPU. At present, there are many kinds of TPU developed by people, and the span of mechanical properties is also very large. It has some excellent properties of rubber and plastic. At the same time, TPU also has good wear resistance, aging resistance and high resilience. It is widely used in industry, life, medical treatment, military industry and so on.

However, TPU also has an obvious disadvantage. Ordinary TPU is flammable, and its oxygen index (LOI) is only 16% ~ 18%. In case of fire, it will burn rapidly and decompose to produce a large amount of toxic smoke.

The flame-retardant TPU is through the introduction of inorganic or organic flame-retardant elements into the TPU substrate, such as some units containing phosphorus, nitrogen, boron, aluminum, magnesium and halogen. In the early stage, halogen flame retardants were widely used in the flame retardant modification of polymer materials, but halogen will produce a large number of toxic gases during combustion, so it will be eliminated slowly.

At present, people have turned their research to more environmentally friendly halogen-free flame retardant technology. The flame retardant modification of TPU can be divided into reactive intrinsic flame retardant modification and additive flame retardant modification according to the combination between flame retardant and polymer substrate.

This paper will introduce the research progress of flame retardant TPU from the following aspects.

Reactive flame retardant modification

Reactive flame retardant modification refers to the introduction of flame retardant functional elements or chemical functional groups into the structure of polyurethane polymer chain through chemical bonds, so that TPU polymer chain itself has flame retardant characteristics. At present, the commonly used reactive flame retardants are polyols or isocyanate units containing phosphorus, nitrogen and other elements.

For example, the intrinsic flame retardant TPU prepared with phosphorus containing polyols as polymerization monomers. Phosphorus based flame retardant polyols are used to modify TPU. Phosphorus is introduced into the polymer chain through polymerization. As a part of the polyol structure, phosphorus will release and capture the free radicals generated by the combustion of polymer matrix in the form of Po · free radicals during the combustion process, so as to quench the combustion reaction, promote the carbon formation of matrix and achieve the effect of flame retardant.

Nitrogen containing flame retardants are mainly produced by decomposition at high temperature, while NH3, N2 and other non combustible materials play a flame retardant effect.
For example, the flame retardancy and thermal stability of reactive TPU prepared with diisocyanate, dihydroxymethylpropionic acid, polyether diol and diol (frc-5) containing phosphorus / nitrogen as monomers are significantly improved. Dihydroxy liquid phosphate (bbhp) was synthesized from n-butanol, phosphorus oxychloride and 1,4-butanediol, and then reacted with 4,4 '- diphenylmethane diisocyanate (MDI) to form flame retardant TPU. When the mass fraction of bbhp is 10% ~ 12%, the oxygen index of flame retardant TPU reaches 27%.

Reactive flame retardant modification generally has the advantages of long-lasting and stable flame retardant effect and little impact on other properties of materials, but the modification process is relatively complex and involves polymerization.

At the same time, the requirements for modifiers are also high. Only some flame retardant elements or functional groups can be introduced into the TPU molecular chain, so there are few research and practical applications. In addition, the flame retardant efficiency of reactive flame retardant technology needs to be further improved.

Additive flame retardant modification

The advantage of additive flame retardant modification is that the preparation of flame retardant TPU is physical mixing, does not involve chemical reaction, the process is relatively simple, and the flame retardant has a wide range of sources and low cost. Therefore, the research and application of additive flame retardant modification are very extensive.

It should be noted that the added flame retardant needs to consider the compatibility with the matrix, otherwise it is easy to precipitate, which will affect the flame retardant effect and the mechanical properties of TPU. Additive flame retardant modification can be divided into inorganic flame retardant addition, organic flame retardant addition and organic-inorganic composite addition according to the category of flame retardant.

Addition of inorganic flame retardant

Inorganic flame retardants mainly include inorganic compounds containing aluminum, boron, silicon, magnesium, titanium and other elements. The flame retardant mechanism of inorganic flame retardants is mainly to reduce the heat generated by TPU combustion or improve the carbon layer strength and thermal insulation effect.

Inorganic flame retardants can be ground into powder or nano size. After surface modification, they can be mixed with TPU resin. Some complex chemical reactions will occur when TPU matrix materials are burned. For example, the commonly used inorganic flame retardant aluminum hydroxide, when TPU is burned, the crystalline water in the aluminum hydroxide molecule will be released to form water vapor, reduce the oxygen concentration and absorb heat at the same time. Alumina particles generated after dehydration of aluminum hydroxide will also combine with the carbon generated by the combustion of polymer materials to form a solid composite carbon layer to isolate oxygen, making it difficult for the internal polymer to continue combustion.

In recent years, in addition to traditional inorganic flame retardants, a large number of new inorganic flame retardants have been developed by scientific researchers for TPU flame retardant.

In addition to strengthening the carbon layer and catalyzing the formation of carbon, some inorganic compounds containing special metal ions also have good smoke suppression effect and have their advantages in environmental protection. Therefore, inorganic flame retardants have attracted more and more attention. However, the compatibility between inorganic particles and organic polymer TPU is not good, and the addition amount is generally low. A large amount of addition will damage the mechanical properties of TPU.

Add organic flame retardant

Organic flame retardants mainly include early halides and phosphorus and nitrogen organic compounds of general concern. The flame retardant mechanism of organic flame retardants varies with different components. The high flame retardant efficiency of halides is due to the fact that during combustion, halides can produce free radical polymer combustion, generate a large amount of non combustible flue gas and dilute combustible gas to achieve the purpose of flame retardant, but the disadvantage is that the generated flue gas is highly toxic, so it is gradually eliminated.

The flame retardant mechanism of phosphide is similar to that of halogen. It can also generate free radicals to prevent the basic reaction of combustion (oxidation reaction). Its advantage is that it will not produce toxic gas, but also promote carbon formation and improve the strength of carbon layer. Therefore, it has attracted much attention.

Nitrogen containing flame retardants are mainly gas-phase flame retardants. During combustion, a large number of non combustible gases are generated, oxygen is diluted, and the oxidation reaction is carried out. Some nitrogen-containing compounds, such as hindered amines, can also produce free radicals to prevent the oxidation reaction.

In recent years, organic flame retardants containing phosphorus and nitrogen have obvious flame retardant effects, so they have been deeply studied. For example, BDP flame retardant modified TPU was prepared by mixing bisphenol a-bis (diphenyl phosphate) (BDP) with monomer by one-step embedding method. The results show that within the research scope, the oxygen index and UL 94 flame retardant grade of flame retardant TPU increase with the increase of the content of flame retardant BDP, but its mechanical properties such as tensile strength and 100% modulus of elongation show a trend of increasing and then decreasing with the increase of the amount of flame retardant. When the mass fraction of flame retardant BDP is 9%, the comprehensive performance of flame retardant TPU is good, its oxygen index reaches 26%, and the flame retardant grade of UL 94 reaches V-1.

Previous studies have shown that organic flame retardants have obvious flame retardant effect and good compatibility with TPU substrate. Their addition amount can be more than inorganic flame retardants, and their influence on mechanical properties is also less than inorganic flame retardants. However, their effect on smoke suppression is not prominent, and only a small amount of organic flame retardants have certain smoke suppression effect.

Organic inorganic compound addition

Both inorganic flame retardants and organic flame retardants have their own advantages and disadvantages. Therefore, people pay more and more attention to the combination of organic flame retardants and inorganic flame retardants to give play to synergistic effects, develop strengths and avoid weaknesses, and achieve better flame retardant effects.

Aluminum hypophosphite (AHP) and melamine cyanurate (MCA) were added to TPU to prepare flame retardant TPU materials. When 11% flame retardant was added (the mass ratio of AHP to MCA was 1 ∶ 2), the vertical combustion of flame retardant TPU reached UL 94 V-0 and LOI was 25.2%. The addition of flame retardant AHP / MCA can improve the thermal stability of the composites and promote the carbonization of the composites.

A series of TPU composites were prepared by melt blending method with ammonium polyphosphate (APP), aluminum hypophosphite (AHP), aluminum diethylphosphinate (ADP) as flame retardants and 1-ethyl-3-methylimidazole hexafluorophosphate ionic liquid as synergistic flame retardant and smoke suppressant. The results show that [EMIM] PF6 alone as a flame retardant has better flame retardant and smoke suppression effects on TPU materials, and as a synergistic flame retardant, it has better flame retardant and smoke suppression effects on TPU composites in synergy with app, AHP and ADP flame retardants.

After organic and inorganic flame retardants are combined to form hybrid materials in a certain way, their flame retardant effect is significantly improved compared with a single flame retardant, but the flame retardant modification mechanism involved is also more complex, especially the inorganic organic synergistic effect, which needs to be further studied.

Generally speaking, starting from the monomer structure, the reactive flame retardant modification by introducing flame retardant groups through chemical bonds during polymerization can effectively improve the structural stability and flame retardant durability of materials, but this method has complex process and great limitations. The additive modification has relatively simple process, wide sources of flame retardants, large room for improving the properties of composites, and relatively more research and application.

The added flame retardants have their own advantages and disadvantages. Inorganic flame retardants generally have the advantages of catalytic flame retardant effect, strengthening carbon layer structure and smoke suppression, but the disadvantages are poor compatibility with TPU matrix, poor dispersion, low durability, and the addition amount should not be too large. Organic flame retardants are generally easy to mix with the matrix and also have catalytic flame retardant effect, but the efficiency is generally not high and the stability is also lacking. Through organic-inorganic composite use, such as organic flame retardant coating inorganic flame retardant, or loading organic flame retardant on two-dimensional inorganic flame retardant, it can not only improve the compatibility, but also increase the stability of organic matter and strengthen the flame retardant efficiency. Therefore, the hybrid of organic and inorganic flame retardants and their synergistic effect will be an important development direction of TPU flame retardant modification research in the future.