1. Anti-hydrolysis agents primarily aim to block the hydrolysis process of polyester polymers.

In applications using polymers containing ester bonds, such as PBT, PET, PLA, and polyurethanes (TPU, CPU), water molecules readily attack the ester or urethane bonds in the molecular chain under high temperature and humidity conditions. This leads to chain breakage and hydrolysis, a decrease in polymer molecular weight, and consequently, brittleness, cracking, and loss of performance. Anti-hydrolysis agents are used to counteract this hydrolysis process. Anti-hydrolysis agents are mainly divided into two categories: reactive and physical. Reactive anti-hydrolysis agents eliminate the initiation sites or products of hydrolysis through chemical reactions, representing the mainstream and highly efficient method. Physical anti-hydrolysis agents, on the other hand, block or absorb moisture through physical action.

Physical hydrolysis inhibitors do not participate in chemical reactions but prevent moisture penetration through physical means. Representative types include zeolites, calcium oxide (CaO), diatomaceous earth, silanes, and waxes. Zeolites and calcium oxide, through their porous structure or chemical reactions, absorb and lock in moisture absorbed by the polymer during processing and use, primarily protecting materials from degradation due to trace amounts of moisture before processing (such as injection molding and extrusion), essentially acting as "desiccant" properties. Silanes and waxes, on the other hand, migrate to the surface of the product, forming a hydrophobic barrier, or extend the moisture penetration path through layered fillers (such as clay), primarily protecting the material surface.

Reactive hydrolysis inhibitors can react with the carboxyl groups (-COOH) at the ends of polymer chains or with carboxyl groups generated during hydrolysis, interrupting the autocatalytic process of hydrolysis and thus achieving a fundamental stabilizing effect. These mainly include carbodiimide, oxazoline, epoxy, and aziridine hydrolysis inhibitors.

2. Carbodiimide is the most advantageous and widely used reactive hydrolysis inhibitor.

Carbodiimides are currently the most widely used and effective class of anti-hydrolysis agents. They react with the carboxyl groups produced by polymer hydrolysis to form stable N-acylurea, thereby eliminating the catalyst for the hydrolysis reaction and interrupting the autocatalytic cycle. Oxazoline derivatives, another important class of reactive anti-hydrolysis agents, have an oxazoline ring as their reactive functional group. The oxazoline ring can react with both carboxyl and hydroxyl groups to form ester amides or diesters, thus stabilizing the polymer ends. Epoxy-functionalized polymers utilize the high reactivity of the epoxy groups to provide stabilization. The epoxy groups can react with carboxyl, hydroxyl, and even amino groups, thereby capping these reactive groups.

Table: Comparison of Common Reactive Hydrolysis Resistants

Types of anti-hydrolysis agents	carbodiimide	Epoxy functional group polymers	Oxazolinides
Core Mechanism	It reacts with the carboxyl groups produced by hydrolysis to generate stable N-acylurea, thus interrupting the autocatalytic cycle.	Its epoxy group can react with various groups such as carboxyl, hydroxyl, and amino groups.	Its oxazoline ring can react with carboxyl and hydroxyl groups.
Main advantages	●Extremely high resistance to hydrolysis, with the most significant effect.	●Multifunctionality: It combines the functions of chain extension and repairing degraded molecules.	● Bifunctional reaction, with a wide range of applications
	The addition amount is small (0.5%-2.0%), with minimal impact on the material's intrinsic properties.	●Can improve melt strength and viscosity	● Can be used as a compatibilizer in certain systems.
	● Relatively good safety	● Good compatibility with polymers
Main disadvantages	● Relatively high cost	●As a single anti-hydrolysis agent, its efficiency is not as specific as that of carbodiimide.	● Costs are usually the most expensive
	● Primarily targets carboxyl groups; does not directly react with hydroxyl groups.	● Excessive addition may lead to cross-linking or gelation.	● Lacks efficiency advantage in general-purpose applications
Typical applications	● Polyester: PBT, PET, PLA, PBAT	● Plastic recycling: Repairing rPET , etc.	● Polyester (PET, PBT)
	● Polyurethane: TPU, CPU (shoe soles, hoses, etc.)	● Polyamide (Nylon)	●Polyamide
		● Polyester systems requiring simultaneous thickening	● Polymer alloy (as a compatibilizer)

 

3. Carbodiimide blocks the hydrolysis process by reacting with carboxylic acids to form acylurea structures.

Polyester polymers exhibit poor moisture stability. Under high temperature and humidity conditions, the ester bonds in the polymer react with water, causing the long-chain structure of the macromolecule to break and generate terminal carboxyl groups. These terminal carboxyl groups can ionize H+ ions, further catalyzing the hydrolysis reaction with acid, ultimately leading to a significant reduction in various material properties and a greatly shortened service life. Carbodiimide compounds, containing carbodiimide (N=C=N) functional groups, can react with the carboxyl groups generated during polymer hydrolysis to form stable acylurea structures, simultaneously reducing the carboxyl group concentration and preventing further hydrolysis. They are among the most commonly used anti-hydrolysis agents currently available.

Carbodiimide antihydrolysis agents are diverse and can be broadly classified into monomeric and polymeric types. Monomeric carbodiimide compounds contain only one carbodiimide functional group and are small molecule compounds. Polymeric carbodiimide compounds typically contain two or more carbodiimide functional groups, have a relatively high molecular weight, and belong to the long-chain polymer structure type.

Monomeric carbodiimide antihydrolysis agents are bright yellow to brown liquids or crystals at room temperature. They are soluble in organic solvents but insoluble in water, and have advantages such as high purity, simple preparation, and high reactivity. 2,6-Diisopropylphenyl)carbodiimide is the most commonly used commercially available monomeric carbodiimide antihydrolysis agent.

 

Polymeric carbodiimides are yellow to brown powders or viscous liquids at room temperature, with a relative molecular mass generally greater than 1000, while the relative molecular mass of oligomers is controlled at around 2000. Polymeric carbodiimides are typically obtained by reacting diisocyanate monomers, catalysts, solvents, and end-capping agents at suitable temperatures. First, the diisocyanate monomers undergo a condensation reaction under a catalyst to obtain a prepolymer containing multiple carbodiimide groups and isocyanate end groups. Then, the isocyanate groups react with active hydrogen from the end-capping agent to obtain polycarbodiimides. Typical polycarbodiimides are obtained by condensing 2,4,6-triisopropylphenyl-1,5-diisocyanate and end-capping with 2,6-diisopropylphenyl monoisocyanate.

 

4. Typical application areas of carbodiimide

PET, as the most common polyester material, possesses excellent mechanical properties, dimensional stability, chemical resistance, and optical properties, and is widely used in agriculture, industry, construction, medical, and automotive fields. PET is produced through the polycondensation of PTA and ethylene glycol; the ester bonds are highly susceptible to hydrolytic degradation, leading to a decrease in polymer viscosity and severe performance deterioration. PET hydrolysis limits the application of its downstream products in high-temperature, humid, or outdoor environments. Related research has found that incorporating monomeric anti-hydrolysis agents into PET masterbatch to prepare film samples improves the heat resistance, damp heat aging, and elongation at break of the film products. Aromatic carbodiimide shows particularly good hydrolysis performance .

Polyurethane synthesis utilizes a wide variety of monomers, allows for controlled reactions, and offers advantages such as high strength, abrasion resistance, good temperature resistance, and ease of processing. It is widely used in adhesives, coatings, elastomers, foamed plastics, and synthetic fibers. Polyester-type polyurethane is prepared from oligomeric polyester polyols, which contain many ester bonds in their molecular chains, resulting in poor hydrolysis resistance. Carbodiimide anti-hydrolysis agents have minimal adverse effects on polyurethane synthesis and can be added to the polyester polyol during the synthesis process. Furthermore, polymeric carbodiimides prepared by isocyanate condensation contain -N=C=O end groups, enabling them to participate in the reaction to prepare hydrolysis-resistant polyurethane. Additionally, carbodiimides can be added during polyurethane blending. Related studies have demonstrated that the addition of carbodiimides can lower the initial acid value of the polyester polyol, inhibit polyester hydrolysis, and effectively improve the hydrolysis resistance of TPU.

Polyester-based biodegradable polymers such as PBAT, PLA, and polyglycolic acid (PGA) possess good biocompatibility, biodegradability, safety, non-toxicity, and good physical and mechanical properties, showing great promise in medical devices, packaging materials, and agriculture. However, these biodegradable materials all suffer from poor hydrolytic and thermal stability , readily degrading during processing, storage, and use, leading to performance degradation and failing to reach their expected lifespan. Carbodiimide can undergo a capping reaction with the terminal carboxyl groups in the molecular chains of PBAT, PLA, and PGA to generate a relatively stable acylurea structure, simultaneously inhibiting hydrolysis and improving thermal stability.

Carbodiimide-modified MDI (also known as liquefied MDI) is one of the main modified products of diphenylmethane diisocyanate (MDI). It is produced by the condensation reaction of MDI under the action of a catalyst to generate carbodiimide groups. Carbodiimide-modified MDI is characterized by being liquid at room temperature, easy to store, and having a long shelf life. At the same time, it can significantly improve the hydrolysis resistance of polyurethane materials.

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