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Polymer foam materials, with their excellent comprehensive properties such as lightweight and high strength, thermal insulation, sound insulation and shock absorption, buffering and energy absorption, and high specific strength, are widely used in fields such as packaging logistics, building energy conservation, automotive rail transit, aerospace, fire protection, and intelligent equipment. They are an important branch of the lightweight, functional, and green development of polymer materials. As a core functional additive for polymer microcellular molding, foaming agents generate a large number of uniform microporous structures inside the polymer matrix through physical phase transformation or chemical reactions, directly determining the morphology, porosity, pore size distribution, mechanical strength, thermal stability, and long-term service performance of the foamed material. The type, structure, gas production characteristics, interface compatibility, decomposition/phase transition temperature window, and matching degree with the rheological behavior of the matrix of foaming agents are the core keys to regulating the quality of polymer foaming, avoiding defects such as cell collapse, merging, and breaking.

With the upgrading of polymer processing technology and the iteration of environmental policies, traditional high pollution, high energy consumption, and high residue foaming systems are gradually being phased out. New foaming agents with low VOC, low toxicity, green environmental protection, controllable micropores, and wide adaptability have become a research hotspot in the industry. At present, research on polymer foaming systems mainly focuses on single foaming processes or specific material modifications, lacking a systematic review of the classification system of foaming agents, micro foaming mechanisms, performance balance laws, and cutting-edge research progress. Based on this, this article systematically categorizes mainstream polymer foaming agent systems, elaborates on the micro mechanisms of physical foaming, chemical foaming, and composite foaming, summarizes the adaptation scenarios, performance advantages, and inherent shortcomings of various foaming agents, focuses on reviewing new technological advances such as modified foaming, supercritical fluid foaming, microcapsule controllable foaming, and bio based green foaming, analyzes the current technological bottlenecks in the field of polymer foaming, and looks forward to the future development trends of green, precise, and intelligent foaming systems, providing theoretical references for the formulation design and process optimization of high-performance polymer foaming materials.

Classification system and basic characteristics of polymer foaming agents

According to the foaming principle, mechanism of action, and differences in material form, polymer foaming agents can be divided into three categories: chemical foaming agents, physical foaming agents, and composite functional foaming agents. Chemical foaming agents rely on high-temperature thermal decomposition and chemical reactions to release inert gases for foaming; Physical foaming agents rely on phase change gasification, supercritical dissolution and precipitation, and volume expansion to achieve microporous molding; Composite foaming agents combine the advantages of both and achieve precise control of the foaming process through multi-component synergy. There are significant differences among the three types of foaming agents in terms of decomposition/phase transition temperature, gas production efficiency, pore control ability, matrix compatibility, environmental friendliness, and processing cost, which are suitable for different polymer substrates and molding processes.

1.1 Chemical foaming agent

Chemical foaming agent (CFA) is the most mature and industrially adaptable traditional foaming system, mostly consisting of organic or inorganic pyrolyzable compounds. It undergoes irreversible thermal decomposition reactions within the temperature range of polymer melting and processing, releasing inert and flame retardant gases such as N ₂, CO ₂, H ₂ O, which nucleate, grow, and form microporous structures inside the melt. According to the thermal decomposition effect, it can be divided into two types: exothermic and endothermic. The molecular structure is stable, the storage is convenient, the foaming ratio is controllable, and it is suitable for conventional polymer molding processes such as extrusion, injection molding, and compression molding.

Organic chemical foaming agents are mainstream application categories, with typical representatives of azo, sulfonylhydrazine, and urea compounds. Azodicarbonamide (AC/ADC) is a universal and efficient foaming agent, with a decomposition temperature range of 195-215 ℃ and a gas production of up to 220 mL/g. The decomposition products are mainly N ₂ and CO ₂, with high foaming ratio and high nucleation density. It is widely suitable for general polymer materials such as PE, PP, PVC, EVA, etc. However, its decomposition temperature is high, the heat release is concentrated, the instantaneous gas production rate is too fast, which can easily cause local bubble merging and pore size coarsening, and there are trace harmful residues in the decomposition, limiting its environmental protection. Azobisisobutyronitrile (AIBN) has a lower decomposition temperature and smoother gas production, making it suitable for low-temperature processing of polymers. However, it has a lower total gas production and higher cost, and is mostly used for high-end fine foaming products.

Sulfonylhydrazine foaming agents have moderate decomposition temperature, uniform gas production, mild heat release, excellent pore fineness, and can effectively avoid instantaneous boiling foaming defects. They are suitable for high flatness and high-precision foaming materials. Inorganic chemical foaming agents are based on sodium bicarbonate, sodium carbonate, and ammonium bicarbonate, with low cost, low toxicity, and low decomposition temperature. However, they have poor gas production stability, are greatly affected by humidity, have low bubble nucleation density, and large pore size dispersion. They are only suitable for low-end lightweight foaming products and are difficult to meet the requirements of high-performance microporous materials.

1.2 Physical foaming agent

Physical foaming agent (PFA) relies on phase transition, volume expansion, or supercritical dissolution to foam, with no chemical reactions, no solid residue, low toxicity, and environmental protection throughout the process. It is the core development direction of green foaming technology. Traditional physical foaming agents include fluorocarbons, short chain alkanes, low boiling point organic solvents, etc., which have high foaming efficiency and uniform pores. However, they have defects such as ozone layer depletion, greenhouse effect, and flammability and explosiveness, and have gradually been restricted or phased out.

The new environmentally friendly physical foaming agents are represented by supercritical CO ₂, supercritical N ₂, liquid pentane, and inert gas, among which the advantage of supercritical fluid foaming technology is the most prominent. Supercritical CO ₂ combines the dual characteristics of low gas viscosity, high diffusivity, and high liquid solubility density. It can uniformly dissolve in polymer melts under high pressure to form a homogeneous system. By triggering supersaturation of the system through sudden pressure drops or temperature increases, uniform bubble nucleation and growth can be achieved. This system is non-toxic, residue free, green and low-carbon, suitable for the vast majority of thermoplastic and thermosetting polymers, and can be used to prepare microporous and ultra microporous lightweight foam materials. Supercritical N ₂ has a faster diffusion rate and higher nucleation density, making it suitable for ultra-thin microporous, high-strength precision foam products. However, its dissolution capacity is low and the foaming ratio is limited. Overall, physical foaming agents have the advantages of excellent environmental friendliness, controllable foam pores, and no residual pollution. However, they require high precision in equipment pressure and temperature control, and have a higher processing threshold than chemical foaming systems.

1.3 Composite functional foaming agent

Both single chemical or physical foaming agents have inherent performance shortcomings, making it difficult to simultaneously meet the multiple demands of high foaming ratio, uniform micropores, stable structure, green environmental protection, and low cost. The composite functional foaming agent achieves complementary advantages and offset defects through the design concept of “organic-inorganic compounding, chemical physical synergy, and multi-component stepwise gas production”. Common systems include AC sodium bicarbonate complex, supercritical CO ₂ - microsphere foaming synergy, modified nitrogen-containing foaming agent expandable graphite complex, etc. By adjusting the group allocation ratio, the starting temperature, gas production rate, and continuous gas production range of foaming can be precisely regulated, achieving stepwise continuous foaming and effectively solving problems such as unstable gas production, uneven cell size, and structural collapse in a single foaming system. It is currently the mainstream solution for high-performance polymer foaming modification.

2. Microscopic mechanism of polymer foaming core

Polymer foaming molding is a typical dynamic evolution process of gas liquid solid three-phase, which includes five core stages: gas dissolution, bubble nucleation, cell growth, structural stability, and solidification and shaping. The type and characteristics of foaming agent directly dominate the dynamic behavior of each stage, ultimately determining the micro pore structure and macro properties of foaming materials.

2.1 Gas dissolution and supersaturation nucleation mechanism

In a physical foaming system, supercritical fluid or inert gas uniformly permeates and dissolves in the polymer melt under high temperature and high pressure conditions, forming a thermodynamically stable polymer gas homogeneous system; The chemical foaming system continuously releases gas through high-temperature thermal decomposition, leading to the accumulation of gas concentration inside the melt. When the external pressure drops sharply, the temperature rises, or gas is continuously produced, the gas concentration in the system exceeds the saturation threshold, forming a supersaturated thermodynamic instability state, triggering the uniform nucleation of bubbles. Bubble nucleation is divided into homogeneous nucleation and heterogeneous nucleation. The interior of pure melt is homogeneous nucleation, with high nucleation barrier and low nucleation density; Fillers, additives, and grain boundaries in the system can serve as heterogeneous nucleation sites, significantly reducing nucleation barriers and improving the number and uniformity of bubbles and pore sizes. This is also the core mechanism by which additive modification can optimize foaming effects.

2.2 Bubble Growth and Dynamic Evolution Mechanism

After bubble nucleation, the gas inside the melt continues to diffuse towards the bubble interface, driving the continuous growth and expansion of the bubbles. The growth rate of the bubbles is determined by the gas diffusion rate, the viscoelasticity of the melt, and the interfacial tension. Excessive viscosity of the melt can inhibit the expansion and expansion of the foam cells, resulting in insufficient foaming ratio and stiff foam cells; If the viscosity of the melt is too low, it cannot support the bubble structure, causing the bubble wall to stretch and break, and the bubbles to merge and collapse. A high-quality foaming system needs to achieve precise matching between gas diffusion rate and melt rheological properties, ensuring uniform and stable growth of foam cells. Instantaneous gas production of chemical foaming agent is easy to cause rapid expansion of bubbles and rupture of interface facial mask; Physical supercritical foaming produces gas smoothly and evenly, with better stability in cell growth.

2.3 Bubble Stability and Curing Forming Mechanism

In the later stage of cell growth, as the system temperature decreases, the melt crosslinks and solidifies, or the pressure stabilizes, the viscosity of the polymer melt sharply increases, the molecular chains freeze and solidify, and the cell structure is permanently locked, forming a stable microporous foaming system. This stage is the key to suppressing secondary collapse of bubbles and coarsening of pore size. Flexible polymer matrix can reduce structural defects by buffering pore growth stress through molecular chain slip and elastic deformation; Rapid crosslinking of rigid matrix can lead to incomplete cell shaping and structural cracking. The timing and rate of gas production of foaming agents directly determine the integrity and uniformity of the final pore structure by matching the curing window of the matrix.

Performance advantages and inherent shortcomings of three types of foaming agents

The structural characteristics of different foaming systems determine their adaptation scenarios and performance shortcomings. Clarifying the performance balance rules of various foaming agents is the basis for formula optimization and process control.

Chemical foaming agents have strong adaptability to the process, low equipment requirements, high foaming ratio, and low cost, making them suitable for industrial large-scale production of general foaming materials. But it has obvious shortcomings: the exothermic foaming agent produces concentrated gas and releases uneven heat, which can easily cause local overheating and bubble collapse; Residual decomposition products can easily affect the thermal stability, weather resistance, and mechanical properties of polymers; Some organic foaming agents have toxicity, odor, and pollution issues, which do not meet the requirements for the development of green materials; Poor foaming controllability and large pore size dispersion make it difficult to prepare high-precision ultra microporous materials.

Physical supercritical foaming agent is green, non-toxic, residue free, with fine and uniform foam pores, and controllable foaming process. It can achieve micro – and nano scale ultra micro pore molding, suitable for high-end precision, lightweight, and high insulation foam products. However, its processing equipment is expensive, the precision requirements for process control are high, and the foaming ratio is relatively limited, making it difficult to adapt to the demand for ultra-high ratio foaming; Some alkane based physical foaming agents are flammable and explosive, posing high construction safety risks.

Composite foaming agents can effectively compensate for single system defects, balance high foaming ratio and microporous uniformity, and balance process cost and product performance. However, the complex distribution of multiple groups and low formula error tolerance can cause foaming timing mismatch and performance fluctuations with small fluctuations in group distribution ratio, requiring precise process control system support.

Research progress on modification of polymer foaming agents and new technologies

In response to the problems of poor controllability, numerous cell defects, insufficient environmental friendliness, and outstanding performance balance of traditional foaming agents, the industry has conducted extensive research in recent years on foam agent modification, foam process upgrading, and system collaborative optimization. New controllable foaming, green foaming, and intelligent foaming technologies have rapidly iterated, promoting the development of polymer foaming materials towards refinement, high performance, and low carbonization.

4.1 Surface modification of foaming agents and microcapsule encapsulation technology

Microcapsule encapsulation modification is a core technology for regulating the thermal decomposition timing of chemical foaming agents, smoothing the gas production rate, and improving system stability. By using shell materials such as polyurethane, polyurea, silica, and polymer resin to coat the surface of AC, melamine, and sulfonylhydrazine foaming agents, external thermal interference can be isolated, low-temperature premature decomposition can be suppressed, the foaming starting temperature can be accurately delayed, the thermal decomposition temperature range can be narrowed, and a stepped and stable gas production can be achieved. The coating layer can effectively buffer the instantaneous gas production impact force, avoid the collapse and merger of bubbles, and significantly improve the refinement and uniformity of bubbles. Simultaneously, surface modification can enhance the interfacial compatibility between foaming agents and polymer matrices, reduce agglomeration defects, minimize the negative impact of residual small molecules on the mechanical and weather resistance properties of the matrix, and significantly broaden the window of foaming agent addition and formula tolerance.

4.2 Supercritical fluid precise foaming technology

Supercritical CO ₂/N ₂ foaming technology is currently a cutting-edge hotspot in the field of polymer foaming, relying on the special dissolution and diffusion characteristics of fluids to achieve residue free and high-precision micro pore forming. By regulating the system pressure, temperature, holding time, and release rate, gas solubility, nucleation density, and pore growth size can be accurately controlled to prepare ultra microporous foam materials with uniform pore size, controllable porosity, and dense structure. In recent years, research has found that the use of CO ₂/N ₂ composite foaming strategy can achieve performance synergy. CO ₂ provides high expansion driving force to ensure foaming ratio, while N ₂ improves nucleation density and refines microporous structure, effectively solving the problem of insufficient foaming ratio or large pore size in a single supercritical system. At the same time, the high elasticity controllable foaming technology can construct a sparse cross-linked network, match the ductility of the melt molecular chain, suppress the brittle fracture of the matrix during the foaming process, and simultaneously improve the lightweight and mechanical strength of the foaming material.

4.3 New technology for physical foaming of thermal expansion microspheres

Thermal expansion microsphere foaming agent is a new type of functional physical foaming agent with a core-shell structure. The outer shell is a thermoplastic polymer, and the inner core encapsulates low boiling point liquid alkanes. At room temperature, the microsphere structure is stable and has excellent storage properties. After heating, the shell softens and the core liquid vaporizes and expands, achieving a volume increase of tens of times. This foaming agent has a wide foaming temperature range, uniform expansion, no decomposition residue, and controllable foam pores. It can be adapted to low-temperature molding systems such as water-based coatings, elastomers, and soft polymers. Compared with traditional foaming systems, microsphere foaming has no violent chemical reactions, mild molding, high product flatness, and no coarse defects. It is suitable for high-end lightweight, soundproof, and heat-insulating functional coatings and flexible foaming products, and is a key breakthrough direction in the field of flexible polymer foaming in recent years.

4.4 Research and development progress of bio based green foaming agents

Driven by the dual carbon policy, non-toxic, biodegradable, and environmentally friendly bio based foaming agents have become an emerging research direction. The green foaming agent, prepared based on natural plant protein, biomass polysaccharides, and microbial metabolites, has the advantages of low VOC, no pollution, and natural degradation. The foaming process is mild, the pores are delicate and stable, and it is suitable for foaming biodegradable polymer materials. Biobased foaming agents can perfectly match biodegradable polymer systems such as PLA, PHA, PBAT, etc., solving the problem of traditional foaming agent residues affecting material degradation performance. They are green and low-carbon throughout their entire lifecycle and can be widely used in fields such as green packaging, medical protection, and environmentally friendly building materials. The future industrialization potential is enormous.

4.5 Collaborative controllable foaming technology with additives

Through the synergistic combination of nucleating agents, foam stabilizers, rheological agents, and foaming agents, multi-dimensional regulation of the foaming process can be achieved. Inorganic nano fillers such as nano silica, talc powder, and montmorillonite can serve as efficient heterogeneous nucleation sites, significantly improving bubble nucleation density, refining pore size, and optimizing pore structure uniformity; Polymer foam stabilizers can enhance the viscoelasticity of the melt interface, suppress pore drainage, coalescence, and collapse; Rheological additives can accurately regulate the high-temperature viscosity of the melt, achieve perfect matching between the rheological behavior of the melt and the timing of gas production, solve the industry pain points of “too thin collapse and too thick foam insufficient” in foaming, and achieve synchronous optimization of foaming ratio, pore structure stability, and material mechanical properties.

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