Polymer Char Formation And Anti-Drip Flame Retardant Technology

Polymer Char Formation and Anti-Drip Flame Retardant Technology

Char formation is one of the key methods for solid-phase flame retardancy in polymers. By creating a carbonaceous layer on the polymer surface, it prevents contact between the polymer interior and flammable gases or oxygen, thereby achieving flame retardancy.

In flame retardant mechanisms, phosphorus-containing flame retardants can promote char formation in oxygen-containing polymers such as epoxy resins, polyurethanes, and unsaturated polyesters by dehydrating and forming char. Boron-containing flame retardants, such as zinc borate, also contribute to char formation. Intumescent flame retardants achieve flame retardancy through char formation, making them particularly effective in this regard.

I. Polymer Combustion Process and General Principles of Char Formation

For every 1/3 increase in char yield, smoke generation decreases by 1/2. To achieve a maximum specific optical density (Dm) of less than 100 in polymers, the char yield should be at least 30%. To meet the UL94V-0 flame retardancy rating, the char yield must exceed 30%.

Most polymers have low inherent char-forming properties and char yields, as detailed in the table below.

Generally, compounds with high double bond content, such as aromatic groups, hydroxyl groups, ether groups, and ester groups, exhibit higher char-forming properties. Examples include polyurethane (PU), polyacetylene, pentaerythritol, cellulose and its derivatives, melamine derivatives, and isocyanurate.

II. Methods to Improve Polymer Char Yield

Adding char-forming flame retardants: Examples include phosphate esters, boron-based agents, and intumescent flame retardants.

Crosslinking to promote char formation: Adding crosslinking agents can enhance char formation in polymers such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS). For acrylonitrile-butadiene-styrene (ABS), graft copolymerization can be used to achieve char formation.

Adding char-forming agents: Typical char-forming agents include pentaerythritol, isocyanates, melamine derivatives, cellulose and its derivatives, polyphenylene oxide (PPO), phenol formaldehyde (PF), polyetherimide (PEI), polycarbonate (PC), polyimide (PI), polyamide (PA), and thermoplastic polyurethane (TPU). These agents can only char themselves and cannot catalyze char formation in other materials, so their addition should be at least 5%.

Adding catalytic char-forming synergists: Examples include metal oxides and aluminosilicates such as zeolite, talc, kaolin, and hydrotalcite. These can catalyze char formation in other components and are typically added in amounts below 5%. For instance, adding 1% zinc oxide can increase the oxygen index of an ABS flame retardant formulation by 5%.

III. Polymer Anti-Drip Flame Retardant Technology

Polymer dripping during combustion refers to the phenomenon where molten polymer falls during burning. This is strictly prohibited in flame retardant polymers, as dripping can ignite objects below the burning material, contributing to fire spread. The vertical combustion UL test specifies that dripping must not ignite objects below.

Many polymers exhibit melt-dripping characteristics, such as PE, PP, high-impact polystyrene (HIPS), polyamide (PA), and polycarbonate (PC). Therefore, anti-drip modifications are necessary. Polymer char formation technology is one of the primary methods to prevent dripping, and anti-drip technology often aligns with char formation. The following methods are introduced:

Adding crosslinking agents: Crosslinking is an effective method to promote char formation and a primary means of preventing dripping. For example, adding crosslinking agents such as triallyl isocyanurate (TAIC) or dicumyl peroxide (DCP) to PP can induce crosslinking reactions. This increases char formation on the burning surface, effectively preventing molten droplets at high temperatures.

1. Adding cross-linking agent

Crosslinking is an effective way to promote the formation of charcoal, but also the main technical means of anti-drip, such as adding triallyl triisocyanate (TAIC) or diisopropyl peroxide benzene (DCP) crosslinking agent in PP, can make the PP crosslinking reaction, the surface of the combustion object in the combustion of charcoal increases, can effectively prevent the melt from dripping at high temperatures.

2. Add anti-drip agent

More effective anti-drip agent for polytetrafluoroethylene (F4), the principle of the use of F4 in polymer processing in the fibrillating effect, a significant increase in melt strength and modulus of elasticity, the melt, even if the elongation of the melt because of the strength of the high also will not drip, you can play a role in the anti-drip role to reduce the spread of the flame. In addition, it can also increase the melt strength and anti-collapse properties of the polymer.

There are two types of anti-drip agents on the market today: pure PTFE and modified PTFE. Pure polytetrafluoroethylene is directly utilized with poor dispersibility, it is easy to agglomerate, so it is difficult to use the process, and poor dispersion will lead to poor surface of the product and poor anti-drip effect.

At present, there is a kind of anti-drip agent on the market is a special modified PTFE powder, typical products such as the anti-drip agent PA-5935 developed by the U.S. 3M company, which is based on PTFE fluorine additives, specific for the use of SAN for the treatment of the package, and the general-purpose resins have a better compatibility and transparency, the mass ratio of PTFE and SAN is 7:3~5:5.

In addition to PTFE anti-drip agent, some inorganic fillers can play the role of anti-drip, specifically such as talc, organic montmorillonite and metal oxides, etc., of which the organic montmorillonite also has both charcoal-forming properties and anti-migration properties.

3. Add compatibilizer

Add compatibilizer anti-drip principle: add compatibilizer after the compatibility of the components of the formula to improve the amount of charcoal, can reach 15.88%, thereby improving the densification of the combustion charcoal layer, play a role in anti-drip.

Adding appropriate amount of PP-g-MAH in PP/Inflated Flame Retardant (IFR) 70:30 formula not only improves the flame retardant performance, but also gives non-dripping performance, while the pure PP/Inflated Flame Retardant (IFR) 70:30 formula has dripping phenomenon.