Views: 0 Author: Site Editor Publish Time: 2025-08-13 Origin: Site
Polyurethane soles have become a vital component in modern footwear, offering the perfect balance of lightweight comfort, cushioning, and design versatility. From athletic sneakers to heavy-duty safety boots, these soles must withstand constant stress and exposure to various environmental conditions. However, one of the biggest challenges facing polyurethane soles, especially those made from polyester-based materials, is hydrolysis — a chemical breakdown caused by moisture that can lead to premature cracking, brittleness, and failure. Addressing this issue is critical to ensuring the durability and performance of footwear products. This article explores how high molecular polycarbodiimide anti-hydrolysis agents play a crucial role in enhancing the lifespan and reliability of polyurethane sole systems.
Polyurethane (PU) soles have become a cornerstone in the footwear industry, powering everything from high-performance sports shoes to rugged safety boots and everyday casual footwear. Their popularity stems from a combination of lightweight construction, excellent cushioning properties, and remarkable design flexibility. Unlike traditional rubber soles, PU soles can be molded into complex shapes with varying hardness levels, allowing manufacturers to tailor comfort, durability, and aesthetics to exact specifications.
However, despite these advantages, PU soles face a significant durability challenge: hydrolysis degradation. This chemical process gradually weakens the sole material when exposed to moisture, especially under high temperatures and humidity, which can lead to cracking, brittleness, and eventual sole failure. For manufacturers seeking to produce footwear that performs reliably across diverse environments, overcoming hydrolysis is essential.
Hydrolysis is a chemical reaction in which water molecules attack the ester bonds present in polyester-based polyurethane soles. These ester linkages are vulnerable because moisture can penetrate the polymer matrix and cleave these bonds, breaking down the polymer chains. This chain scission leads to a progressive loss of mechanical integrity.
Hydrolysis in PU soles is particularly problematic in tropical climates, wet environments, or applications where the shoes are exposed to sweat, rain, or immersion. The degradation process is accelerated by elevated temperatures, which increase the reaction rate and worsen the material’s breakdown.
As a result, hydrolyzed soles exhibit:
Drastic reductions in mechanical strength, making the sole less able to absorb impact or support the foot.
Loss of flexibility, leading to stiffness and discomfort.
Visible cracking or crumbling, which compromises the sole’s appearance and function.
Premature failure, requiring costly replacements or leading to dissatisfied customers.
Addressing hydrolysis is thus critical for footwear manufacturers aiming to deliver durable, long-lasting products.
Anti-hydrolysis agents are specialized chemical additives designed to protect polyurethane chains from moisture-induced degradation. They work by neutralizing or reacting with the reactive sites generated during hydrolysis, thereby stabilizing the polymer structure.
Common types of anti-hydrolysis agents include:
Polycarbodiimides: Highly effective compounds that react with carboxylic acid groups formed during hydrolysis.
Carbodiimide oligomers: Similar to polycarbodiimides but with shorter chain lengths.
Hindered amines and other stabilizers: Often used for UV protection but can offer some hydrolysis resistance.
Among these, polycarbodiimide-based agents have become the gold standard for improving hydrolysis resistance in polyester-based PU soles, due to their efficiency and long-lasting effects.
High molecular polycarbodiimide is a polymeric compound containing multiple carbodiimide functional groups along its backbone. This structure contrasts with low molecular weight carbodiimides, which are smaller molecules or oligomers.
The key characteristics of high molecular polycarbodiimide include:
Greater stability: Its larger molecular size reduces volatility and migration, allowing it to remain active in the PU matrix over longer periods.
Extended protection: Because it is less likely to evaporate or leach out, it provides more durable hydrolysis resistance throughout the sole’s lifetime.
Typical forms: Usually supplied as a fine powder or as masterbatch granules that can be easily incorporated into polyurethane formulations.
This form is especially suited for demanding applications such as footwear soles, where long-term durability is a priority.
Using high molecular polycarbodiimide offers several significant benefits over lower molecular weight alternatives and other stabilizers:
High molecular polycarbodiimide provides enhanced and prolonged hydrolysis protection, helping PU soles maintain their mechanical strength and flexibility even after extended exposure to humidity and moisture. This results in soles that resist cracking and crumbling, extending product life and user satisfaction.
Because of its large molecular structure, high molecular polycarbodiimide exhibits very low volatility, meaning it does not easily evaporate or migrate out of the polymer matrix. This quality ensures that the stabilizer remains active inside the sole throughout its service life, unlike low molecular weight carbodiimides which can gradually lose effectiveness.
High molecular polycarbodiimide is designed to integrate seamlessly into PU formulations without negatively affecting the sole’s important physical properties. It maintains:
Hardness: The sole’s resistance to deformation under pressure.
Rebound: The sole’s energy return during walking or running.
Flexibility: Essential for comfort and performance.
Prepolymers or systems containing high molecular polycarbodiimide often show longer shelf life, as the stabilizer helps prevent premature hydrolysis during storage, reducing waste and manufacturing risks.
High molecular polycarbodiimide is generally incorporated by blending it with the polyol component of the polyurethane formulation before it reacts with the isocyanate. By adding it during this early stage, manufacturers ensure the stabilizer is thoroughly and evenly distributed throughout the polymer matrix. Once the isocyanate is introduced and the polyurethane cures, the high molecular polycarbodiimide becomes an integral part of the polymer network. This method is straightforward, does not require special equipment, and seamlessly integrates into existing footwear sole production lines. The result is a finished PU sole system that has consistent hydrolysis protection throughout its entire structure, reducing the chance of weak spots that could fail under moisture exposure.
The effective dosage of high molecular polycarbodiimide typically falls between 0.5% and 1.5% by weight, calculated relative to the polyol component in the formulation. The right amount depends on several factors:
Severity of hydrolysis risk: Shoes intended for tropical climates, frequent outdoor use, or prolonged exposure to moisture benefit from the higher end of the dosage range.
Specific polyurethane formulation: Some formulations may be inherently more or less prone to hydrolysis, requiring dosage adjustments.
Manufacturing and performance priorities: While increasing dosage improves hydrolysis resistance, manufacturers also consider cost implications and must balance enhanced durability with overall production budgets.
High molecular polycarbodiimide is highly compatible with standard polyester-based MDI polyurethane systems, which are widely used for manufacturing durable footwear soles. It does not disrupt common catalysts, fillers, or pigments when properly blended, helping preserve the desired mechanical properties and color of the final product. However, for formulations that use unique additives, specialty fillers, or proprietary color systems, manufacturers are advised to run small-scale compatibility trials to ensure the stabilizer integrates smoothly without unintended interactions, safeguarding consistent product quality.
Polyurethane soles offer a lightweight, flexible, and comfortable solution for modern footwear, but hydrolysis remains a persistent threat to their durability, especially in humid or wet conditions. The use of high molecular polycarbodiimide anti-hydrolysis agents represents a critical advancement in protecting polyester-based PU soles from moisture-induced degradation.
By incorporating high molecular polycarbodiimide, manufacturers benefit from superior, long-lasting hydrolysis resistance, minimal impact on essential physical properties, and improved shelf stability of their formulations. This translates into footwear products that maintain strength, flexibility, and appearance throughout their expected lifespan, reducing failures and enhancing customer satisfaction.
Footwear producers aiming for durability and performance in challenging environments should strongly consider adopting high molecular polycarbodiimide additives to future-proof their polyurethane sole systems.
