Solving the TPU Flame Retardancy Challenge: Inorganic Flame Retardants + Nano-Compositing — Achieving Both Physical Properties and Flame Retardancy

Solving the TPU Flame Retardancy Challenge: Inorganic Flame Retardants + Nano-Compositing — Achieving Both Physical Properties and Flame Retardancy

Due to limitations in raw material costs, technological development, and usage conditions, the mineral flame retardants currently applied on a large scale are primarily aluminum hydroxide flame retardants and magnesium hydroxide (brucite) flame retardants. Aluminum hydroxide is the most widely used mineral flame retardant globally in terms of volume, accounting for approximately 45% of total flame retardant consumption. In both domestic and international markets, the aluminum hydroxide used for flame retardancy is mainly alpha-alumina trihydrate (ATH，-Al2Os·3H2O). Magnesium hydroxide is another major environmentally friendly flame retardant following aluminum hydroxide. The United States leads in both the production volume and variety of magnesium hydroxide flame retardants. Although China possesses abundant magnesium hydroxide resources, research into magnesium hydroxide flame retardants started relatively late, and its output, scale, variety, and quality have consistently lagged behind international standards.

For inorganic flame retardants, achieving a high level of flame retardancy necessitates a large addition of the flame retardant. However, inorganic substances typically have poor compatibility with polymer matrices. Their substantial incorporation can negatively impact the mechanical and thermal properties of materials like TPU. To improve compatibility, appropriate surface treatment of inorganic flame retardants is necessary.

When mica and ATH are added together as additives to polyurethane materials, ATH demonstrates a clear flame-retardant effect but reduces the material's tensile strength and hardness. Mica, possessing excellent thermal insulation properties, can partially compensate for the negative impact on the mechanical properties of TPU caused by the introduction of ATH when used in combination, making it a beneficial supplement to ATH.

Clay minerals can be uniformly dispersed in polymers at the nanoscale. The nanoscale lamellae of clay minerals act as barriers in two dimensions, hindering the small molecules, combustible vapors, and released heat generated during polymer combustion. This significantly influences the degradation and combustion of the polymer's condensed phase. Furthermore, the clay lamellae in two dimensions can also obstruct the feedback of heat produced during gas-phase combustion to the condensed phase, thereby enhancing the polymer's flame retardancy. Nanoscale dispersed clay lamellae significantly restrict the mobility of polymer macromolecular chains, resulting in higher decomposition temperatures for these chains when heated compared to completely free molecular chains.

Flame retardancy was imparted to polyester-based TPU using a domestic nitrogen-phosphorus flame retardant (FRs) combined with organoclay. When the FRs addition was 18%, the material achieved a UL94 V-0 rating and an LOI of 32.8%. However, tensile strength, elongation at break, and tear strength decreased by 52.0%, 44.0%, and 12.3% respectively, while Shore A hardness increased. Compared to pure TPU, when the FRs addition was 18% and the clay addition was 1%, the tensile strength, elongation at break, and tear strength decreased by 48.7%, 39.4%, and 11.1% respectively. Meanwhile, the flame retardancy still reached UL94 V-0 with an LOI of 32.8%.

Expandable graphite (EG) is a special graphite intercalation compound formed by chemically treating natural flake graphite. Graphite has a layered structure, allowing alkali metals, strong oxidizing oxyacids, and others to be inserted between the layers, forming intercalation compounds. These begin to expand around 200°C due to the decomposition, gasification, and expansion of the intercalated compounds, reaching maximum expansion around 900°C, with an expansion ratio potentially reaching 280 times. The expanded graphite transforms from flakes into low-density "worm-like" structures. It enhances the stability of the char layer through a cross-linked network, preventing the char layer from detaching. This forms an efficient heat-insulating and oxygen-barrier layer on the material's surface, capable of blocking heat transfer to the surface and hindering the diffusion of small molecule flammable gases produced by internal decomposition to the combustion zone. This prevents further polymer degradation, thereby interrupting the combustion chain and providing efficient fire protection and flame retardancy.

Graphite does not chemically react with polyurethane, but between 200°C and 300°C, it forms a wormhole-like char layer that encapsulates the decomposing polyurethane. This layer remains relatively stable at high temperatures, preventing heat transfer from the heat source to the inner material and hindering the migration of decomposition products from the inner material to the heat source, thereby improving the thermal stability of the material.

During its R&D process, Yinsu Flame Retardants discovered that expandable graphite is also a highly efficient synergistic flame retardant. The "worm-like" char layer formed by its instantaneous expansion upon exposure to fire provides effective thermal insulation and oxygen barrier. When combined with systems such as inorganic flame retardants and nano-clays mentioned in the article, it offers a more diverse approach to solving the challenge of balancing flame retardancy and physical properties in materials like TPU. Welcome to inquire for more details~