How Melt Flow Index Affect LDPE Powder Coatings Performance

The melt flow index (MI), also known as the melt flow rate (MFR), is a key indicator to measure the melt fluidity of polyethylene LDPE powder coatings under specific temperature and pressure conditions. Its value directly affects the processing performance, film-forming quality, and final physical properties of the coating. The following are the specific impacts of the melt flow index on the properties of LDPE powder coatings:

1. Affect on Processing Performance

Fluidity and Coating Efficiency

• High MI (e.g., >20g/10min): The molten powder is easy to spread, which is suitable for substrates with complex shapes, deep cavities, or fine gaps (such as pipes and special-shaped parts). It can quickly fill the gaps and reduce coating leakage points.
• Low MI (e.g., <5g/10min): The melt has high viscosity and poor fluidity. During coating, higher temperature or pressure is required; otherwise, problems such as uneven coating and bubbles are prone to occur. It is more suitable for simple planes or thick coating requirements (such as the surface of plates).

Curing Speed and Production Efficiency

LDPE with high MI cools and cures faster after melting, which can shorten the curing time on the production line and increase the output per unit time; low MI is prone to coating cracking due to uneven shrinkage during cooling because of high viscosity, thus requiring a longer curing time.

2. Affect on Coating Appearance

• Flatness and Glossiness: The melt with high MI has good fluidity. After coating, it can flow and level naturally, forming a smooth and high-gloss coating, and reducing defects such as orange peel and pinholes; the melt with low MI is difficult to flow and level, and the coating surface is prone to be rough with low glossiness, and even sagging (during vertical surface coating) may occur.
• Thickness Uniformity: High MI is suitable for thin coatings (e.g., 10-50μm), and uniform thickness can be achieved by controlling the melting time; low MI is more suitable for thick coatings (e.g., 50-200μm), but the coating amount needs to be precisely controlled, otherwise local accumulation or uneven thickness is prone to occur due to poor fluidity.

3. Affect on Physical Properties of Coatings

• Adhesion: LDPE coatings with medium MI (e.g., 5-20g/10min) have better adhesion: the melt can not only penetrate into the tiny pores on the substrate surface to form mechanical bonding, but also will not cause excessive penetration and loss due to too strong fluidity (high MI) or poor bonding due to insufficient fluidity (low MI).
• Hardness and Wear Resistance: The LDPE coating with low MI has tighter molecular chain entanglement, and the coating has higher hardness and better wear resistance after cooling, which is suitable for scenarios with high requirements on mechanical properties (such as floors and tool surfaces); the coating with high MI has loose molecular arrangement, low hardness, and is easy to be scratched, but has better flexibility.
• Flexibility and Impact Resistance: The LDPE coating with high MI has better flexibility and can withstand slight deformation of the substrate (such as bending and vibration) without cracking, which is suitable for easily deformable substrates such as plastics and metal sheets; the coating with low MI has poor flexibility and is prone to brittle fracture when impacted, but has slightly higher tensile strength.

PECOAT® design the melt flow index (MI) according to the product to be coated.

4.Summary

• For complex shapes, thin coatings, and high gloss requirements → high MI (e.g., 20-100g/10min);
• For thick coatings, high hardness, and wear resistance requirements → low MI (e.g., 0.5-5g/10min);
• For balanced adhesion and universal scenarios → medium MI (e.g., 5-20g/10min).

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