Compatibilizer: an analysis of key additives in the field of plastic alloys

I. Basic definition of compatibilizer

In the plastic material system, different types of polymers often exhibit thermodynamic incompatibility due to differences in molecular structure and polarity, making it difficult to directly form a stable blend system. Compatibilizers, also known as compatibilizers, are a class of functional additives that improve the interfacial compatibility of different polymers through intermolecular interactions (including chemical bonding, physical entanglement, etc.). Their core function is to promote the uniform dispersion of incompatible polymers and form a blend material with a stable micro-phase structure.

Taking maleic anhydride grafted polyolefin compatibilizers (such as PE-g-ST, PP-g-ST) as an example, the strong polarity of the maleic anhydride groups can form hydrogen bonds or undergo chemical reactions with polar polymers, significantly enhancing the comprehensive performance of blended materials in applications such as halogen-free flame retardancy and filler reinforcement.

II. Mechanism of action of compatibilizers

The fundamental role of a compatibilizer is to reduce the free energy at the interface of incompatible polymers and enhance the interfacial adhesion strength. Its molecular structure typically exhibits amphiphilic characteristics: one end forms physical entanglements or chemical covalent bonds with the molecular chains of polymer A, while the other end interacts similarly with the molecular chains of polymer B, thereby creating a transition layer at the interface between the two phases.

This process can effectively reduce the particle size of the dispersed phase, increase the thickness of the interfacial layer, and enable the blending system to form a stable structure characterized by "macroscopic homogeneity and microscopic phase separation", ultimately achieving synergistic improvements in mechanical properties and processing performance.

III. Performance characteristics of compatibilizers

From a thermodynamic perspective, compatibilizers share similar interfacial enrichment characteristics with surfactants, but their molecular weights are typically higher (generally ranging from thousands to tens of thousands), enabling the formation of a more stable interfacial layer during high-temperature mixing. By adjusting the molecular structure of the compatibilizer (such as molecular weight, type and content of functional groups), the interfacial tension (which can be reduced by 30%-70%) and dispersed phase particle size (usually controlled between 1-5 μm) of the blend system can be precisely controlled, thereby optimizing the comprehensive performance of the material.

IV. Classification system of compatibilizers

Based on their mechanisms of action, compatibilizers can be primarily categorized into two major groups:

(I) Non-reactive compatibilizer

This type of compatibilizer achieves compatibilization through physical effects, such as intermolecular forces and segment entanglement. Common types include block copolymers, graft copolymers, and random copolymers. Its advantage lies in its simple process, which requires no additional chemical reaction conditions, and it is widely used in non-polar polymer blending systems such as PE/PP and PS/ABS. For example, in polyolefin alloys, adding 3%-5% ethylene-propylene block copolymer can significantly improve the interfacial compatibility between the two phases.

(II) Reactive compatibilizer

In addition to physical effects, such compatibilizers can chemically react with the polar groups of polymers through the active functional groups (such as hydroxyl, epoxy, anhydride groups, etc.) on their molecular chains, forming chemical bonds, thereby achieving a stronger compatibilizing effect. The typical structure is a non-polar polymer backbone (such as polyolefin segments) grafted with polar functional groups. For example, in the PA/PP blend system, maleic anhydride grafted polypropylene (PP-g-MAH) can increase the tensile strength of the system by 40%-60% through the reaction of anhydride groups with the amino groups of PA.

V. Typical Compatibilizer Varieties and Characteristics

(I) Cyclic anhydride type (taking maleic anhydride as an example)

Maleic anhydride grafted polyolefin compatibilizers are representative varieties of reactive compatibilizers, with a grafting rate typically controlled at 0.8%-1.0%. In engineering plastic alloys such as PP/PA6 and PP/PA66, adding 5%-8% can increase the interfacial bonding strength by 3-5 times. It should be noted that the anhydride groups may undergo transesterification reactions with ester groups in the system, resulting in a decrease in the heat distortion temperature of the material by 5-10°C. Therefore, it is necessary to optimize the processing temperature and residence time.

(II) Carboxylic acid type

Acrylic acid grafted polyolefin compatibilizers share similar polar groups with maleic anhydride-based compounds and can be used as substitutes in systems such as PE/PS and PP/ABS. Their acid value range is typically 15-30 mgKOH/g, making them suitable for applications sensitive to anhydrides.

(III) Epoxy type

This type of compatibilizer is prepared by graft copolymerization of epoxy resin or monomers containing epoxy groups with polymers. The epoxy groups can undergo ring-opening reactions with amine and carboxyl groups to form covalent bonds, making it suitable for polar-nonpolar blending systems such as PA/PE and PC/ABS. Its compatibilization efficiency is 20%-30% higher than that of non-reactive compatibilizers.

(IV) Oxazoline type

Taking oxazoline grafted polystyrene (RPS) as an example, its grafting rate is about 1%. The oxazoline group in the molecule can react efficiently with polymers containing amino, carboxyl, and carbonyl groups, exhibiting wide applicability. In PC/polyester blends, it can achieve "in-situ compatibilization" and simplify the processing flow.

(V) Other types

Imide type: suitable for high-performance engineering plastic alloys such as PA/PO and PC/PO, capable of withstanding temperatures above 250℃, with an interfacial bonding strength of over 20MPa.

Isocyanate type: It reacts with hydroxyl and carboxyl groups through the -NCO group, and is primarily used in polyurethane blending systems.

Low molecular weight type: Such as acrylate small molecule compatibilizers, which are characterized by low cost and easy dispersion, but require high shear dispersion ability of the extruder, and are usually used in high-filling systems with a filling content greater than 30%.

VI. Application Fields of Compatibilizers

(I) Plastic alloy field

Compatibility agents are the core elements of polymer alloy technology. By regulating the micro-phase structure of the blend system (such as the particle size of the dispersed phase and the thickness of the interfacial layer), high performance materials can be achieved. Typical applications include:

PP/PE universal plastic alloy: Enhances impact resistance by 30%-50%.

ABS/PC engineering plastic alloy: Improves heat resistance and processing fluidity.

PBT/PA composite material: Enhance interfacial bonding strength.

(II) Polymer modification field

By introducing compatibilizers, non-polar polymers (such as PE and PP) can be modified into polar polymers, which can then be blended with polar fillers or polymers. For example, after introducing maleic anhydride grafting groups into PE, it can form chemical bonds with calcium carbonate fillers, resulting in a 25%-35% increase in the tensile strength of the filled system.

(III) Waste plastics recycling field

Compatibilizers play a crucial role in the recycling and reuse of waste plastics, enabling the blending and compatibility of different types of plastics. For instance, by adding 5%-10% of compatibilizers to a mixture of PE/PP/PS waste plastics, a stable sea-island structure can be formed, enabling the preparation of recycled plastic alloys. The tensile properties of these alloys can reach over 80% of the original resin, effectively addressing the issue of "white pollution".

(IV) Coupling of plastics and fillers

As a "macromolecular coupling agent", the polymer segment of the compatibilizer is compatible with the polymer matrix, while the polar group reacts with the surface of the filler, significantly improving the filler's dispersibility. In the PE/CaCO₃ system, the use of a compatibilizer can reduce the dispersed particle size of calcium carbonate from 10μm to below 2μm, and increase the bending modulus of the material by 40%-60%.

(V) Polar resin toughening field

Thermoplastic elastomers (such as EPDM and SEBS), after being modified with compatibilizers, can serve as toughening agents for polar resins. For instance, EPDM grafted with MAH, as a toughening agent, can still maintain the impact strength of the PP system above 20 kJ/m² even at a low temperature of -45°C, with a typical dosage of 5%-10%.

(VI) Improving plastic surface properties

The compatibilizer can improve the surface energy of plastics through surface enrichment, thereby enhancing surface properties such as adhesion (surface tension increased by 10-15 mN/m), antistatic property (surface resistance reduced by 2-3 orders of magnitude), and printability, broadening the application scenarios of the material.

Ⅶ. Conclusion

As a key functional additive in the field of plastic processing, compatibilizers achieve synergistic enhancement of different polymer systems through molecular design and interfacial regulation. With the development of polymer materials technology, compatibilizers are moving towards high efficiency (high compatibilization efficiency with low addition), multifunctionality (simultaneously achieving compatibilization, flame retardancy, reinforcement, and other functions), and greenification (development of biodegradable compatibilizers). In the future, compatibilizers will exhibit broader application prospects in frontier fields such as new energy materials, biobased plastics, and electronic information materials, continuously driving technological innovation in the polymer materials industry.