Advancements in Layer-by-Layer (LbL) Flame-Retardant Coating Technique

What is Layer-by-Layer(LbL) technique?

Layer-by-Layer(LbL) involves alternating deposition of oppositely charged materials (e.g., polyelectrolytes, nanoparticles, rods, sheets, biomolecules).

Traditionally relies on electrostatic attraction, but can also use:

• Hydrogen bonding

• Covalent bonding

• Donor–acceptor interactions

How does LbL work?

The substrate (e.g., cotton) is sequentially dipped into baths containing dilute solutions/suspensions of desired materials. Each dip forms a thin nano-layer, repeating in a specific sequence—forming bi-layers (A-B), tri-layers (A-B-C), or quad-layers depending on the design.

Primer layer

Often, a first "primer" layer is applied to improve adhesion for the next layers. This ensures better stability and uniformity of the whole structure.

Spraying vs. Dipping

Dipping: more common in lab-scale applications.

Spraying: better suited for pilot or industrial scale, offering scalability.

How many layers?

Often 10–50+ layers, depending on desired properties.

Layers can be finely tuned to reach specific performances (e.g., flame retardancy, UV protection).

Origins and early advancements of flame-retardant treatments

Establishment of LbL on cellulose-based materials:

Early studies confirmed that the LbL technique could be successfully applied to cotton, a cellulose-rich and inherently flammable material, enabling surface engineering without altering bulk properties.

Exploration of intumescent and thermal shielding mechanisms:

Researchers investigated the use of LbL assemblies composed of polyelectrolytes, clay nanosheets, and phosphorus/nitrogen-based additives. These layers expanded under heat, forming a protective char that insulated the underlying fabric—demonstrated through vertical flame tests and thermogravimetric analysis (TGA).

Identification of key layer constituents and assembly parameters:

Experimental setups compared different bi-layer numbers, pH levels, and ionic strengths to evaluate how these parameters affected flame retardancy. For example, a study showed that a 10-bilayer coating of poly(diallyldimethylammonium chloride) and sodium polyphosphate significantly improved the limiting oxygen index (LOI) of treated cotton.

Verification of nanostructure uniformity and fabric integrity:

Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to visualize the even deposition of nanolayers on cotton fibers, confirming the method's precision while preserving breathability and softness.

Value and Potential of LbL Technique

Currently, the Layer-by-Layer (LbL) nano-structured self-assembly technique has emerged as a highly promising method for functionalizing cotton fabrics, particularly in enhancing flame-retardant properties. This is largely due to:

• Wide material selection: A virtually unlimited choice of components;

• High structural tunability: Excellent flexibility in designing and constructing nanostructures;

• Multifunctional integration: In addition to flame retardancy, properties such as hydrophobicity, self-healing, and antibacterial effects can be incorporated;

• Flame-retardant mechanisms: Primarily based on thermal shielding and the formation of an intumescent carbon layer, with potential synergistic effects between the layered materials.

Major Current Limitations

Scale-up limitations:

The LbL process remains largely laboratory-based. The traditional dip-coating method is complex, time-consuming, and often manual, despite early efforts to automate. In contrast, spray-based methods show greater industrial promise due to faster processing speeds and reduced cross-contamination.

Durability issues:

Most LbL assemblies are held together by electrostatic interactions, which are neither strong nor brittle. Exposure to water, washing, abrasion, or microbial activity can lead to partial delamination of the layered structure and a decline in flame-retardant performance.