A mechanistic analysis of surface shrinkage, excessive closed holes, and pinhole issues during solvent-free leather production

 With increasingly stringent global environmental regulations and growing consumer awareness of health and safety, solvent-free polyurethane finished leather Due to its advantages such as zero VOCs (volatile organic compounds) emissions during production and no toxic solvent residues, it has become a compound finished leather The main direction of green transformation in the industry.

However, compared to traditional solvent-based or water-based polyurethane processes, the production of solvent-free leather demands more stringent raw material ratios, process windows (temperature, humidity), and equipment precision. During actual mass production, Surface shrinkage, excessive closure rate, and pinholes (dark bubbles) These are the three main quality defects that cause product degradation and affect surface appearance and tactile feel. This article will conduct an in-depth analysis of the formation mechanisms of these three typical problems from the perspectives of chemical reaction kinetics and fluid rheology, and explore their solutions.

Excessive shrinkage and closed pores on the coating surface

“Surface shrinkage” usually manifests as uneven leather, exhibiting phenomena similar to orange peel or localized collapse, and is often accompanied by “excessive closed pores.”

Foaming and gel balance are out of control

Solvent-free polyurethane foam coatings are a typical competitive reaction system. isocyanates (-NCO) and polyols (-OH). Gel reaction It determines the growth and crosslinking density of molecular chains; And isocyanates are combined with water Foaming reaction (or the vaporization of physical foaming agents) determines the generation of gases.

      Phenomenal mechanism : In the early stages of the reaction, if the gel reaction rate is too fast (usually due to excessive organotin or triethylene diamine catalysts), the viscosity of the system will rise rapidly, causing the membrane on the cell walls to become too elastic and tough. The bubbles generated at this time cannot burst under the continuous pressure generated by the gas, resulting in numerous independently closed cells (closed pores).

      Contraction is the cause : When the coating leaves the high-temperature oven and enters the cooling stage, the gas inside the closed pores (CO₂ or foaming agent vapor) drops sharply due to the temperature drop. Because the hole walls are sealed and tough, outside air cannot enter to replenish them, creating negative pressure inside the holes and causing the cell walls to cave inward, which on a macroscopic scale manifests as surface shrinkage or even deformation of the coating.

The effect of foam stabilizers

The main function of silicone oil-based foam stabilizers is to reduce surface tension, emulsify various components, and control the size of the bubbles. If the dosage is too high, the cell walls become too stable, suppressing the natural bursting (opening) process of bubbles at the end of growth, which also exacerbates the closure rate and triggers cooling shrinkage.

Puncture and dark blisters

Pin holes refer to small depressions or micropores on the leather surface, which, when severe, can reveal bright spots or shadows when viewed through light, seriously affecting the product’s barrier performance and aesthetics.

Excessive system viscosity and poor leveling properties

Solvent-free resins typically have a high initial viscosity. If the raw material temperature is not properly controlled or the formula itself has too high viscosity (recommended in the range of 1500~1800 mPa·s), the material cannot be quickly leveled and wetted quickly when coated on the substrate (release paper or base fabric).

      Bubble entrainment ： During high-speed mixing or coating, mechanical action can draw air into the material. If the system viscosity is too high, bubbles will float to the surface and rupture for a long time. When the coating surface begins to cross-link and shape, bubbles will not yet be expelled, causing puncture holes or trapping inside to form “dark holes.”

Initial foaming speed is too fast

The “latency period” after the material enters the oven is extremely short. If the catalyst ratio is imbalanced or the oven temperature is too high, the material begins to react violently and foam before leveling.

      Surface curing ： The surface material is first heated, causing its viscosity to rise sharply, forming a dense “film.” At this point, the gas produced from the lower layer continues to react and rises upward, but since the upper layer has solidified, the gas can only break through the epidermis and be expelled, leaving an irreparable pinhole.

Moisture interference (NCO/water reaction out of control)

Solvent-free systems are extremely sensitive to moisture. In production environments with high humidity, or when the moisture content in polyol components exceeds the standard, isocyanates will preferentially react with water to produce CO₂ gas and produce amines.

      Heterogeneous foaming ： This uncontrolled side reaction disrupts the normal foaming rhythm, causing a large number of uneven bubbles to instantly form locally. If these bubbles are not broken or expelled in time, they will cure and form surface defects or internal pinholes.