Introduction

The polymer foaming transforms solid polymers into cellular structured polymeric composites by incorporation of a large number of very small-sized bubbles to reduce the use of material without a significant effect on the mechanical properties of the product. In the 1930s, the first polymer foam with macrocellular structure was reported [1], and since then the research & development is being consistently pursued towards the smaller cell types of cellular structure. In the 1980s, polymer foam with microcellular structure was reported by Prof. Suh et.al [2] from Massachusetts Institute of Technology and subsequently, in the early 2000s, the nanocellular polymeric structure came into existence. The development in this field of research still continues to endeavour through numerous cutting-edge innovation for several diverse industrial applications [1]. The microcellular plastics are being extensively used in a wide range of applications such as biomedical, automotive, naval, aerospace, safety goods, insulation purpose in construction, packaging, filters, membranes, cushioning owing to its properties like strength to weight ratio, toughness, insulating properties, flexibility, etc. [2,3,4,5,6]. These cellular materials can be classified according to the inter-connectivity of cell structure, cell size, cell density, expansion ratio, elastic modulus [1, 7,8,9,10]. Figure 1 shows a clear depiction of the classification of foamed polymers.

Fig. 1
figure 1

Classification of foamed polymers

Full size image

Mechanism of polymer foaming

The mechanism of polymer foaming typically comprises three distinct stages which are clearly shown in Fig. 2 [1, 3, 11]. The first stage is the dissolution of gas or blowing agent. In this stage, a polymer sample (solid or melt) is loaded with a high-pressure blowing agent such as CO2 or N2. The dissolution of the blowing agent in the polymer matrix occurs over an extended period of time, till full saturation level is achieved. The dissolution process involves the mixing of two different phases (gas & solid) to form a single-phase homogenous solution. The dissolution of gas or blowing agent in polymer also causes plasticization effect which assists in the flow of polymer matrix during the foaming process. Plasticization occurs due to the suppression of glass transition temperature & result in the decrease of stiffness, viscosity and increases the chain mobility of polymer materials which cause the reduction in energy barrier for cell nucleation [12]. The second stage is cell nucleation and cell growth. Thermodynamic instability is the basic principle behind cell nucleation. The thermodynamic instability can be induced either by a sudden rise in temperature or a sudden drop in pressure. Due to this thermodynamic instability, the solubility of CO2 within the polymer matrix drops instantaneously and the dissolved blowing agent emerges out from the polymer matrix, thereby creating a large number of nuclei. Finally, the third stage is the cell stabilization stage. To preserve the so-developed cell structure, the foamed sample is quenched in water or other suitable cooling media. Even though after quenching, cell growth may continue.

Fig. 2
figure 2

Mechanism of polymer foaming

Full size image

The blowing agent plays an important role in the transformation of solid polymer into the cellular structured polymer. Two types of blowing agents are using generally in foam manufacturing namely,- chemical blowing agent (CBA) and physical blowing agent (PBA) [1, 5, 7, 13]. The foaming with PBA has many advantages over foaming with CBA, such as lesser materials usage, no residues, and lower cost. In foaming with PBA, mostly CO2 or N2 are used as blowing agent because of their inertness, easiness to integrate, availability, and notably low cost [14, 15]. The cell density is the function of saturation pressure [16]. The increase in saturation pressure increases the dissolution of the blowing agent within the polymer matrix & leads to higher cell density [17].

Traditional manufacturing technologies of microcellular polymers

The traditional techniques generally used for microcellular polymeric foam manufacturing are; batch solid-state microcellular foaming, extrusion foaming, and injection mould foaming. Out of which mostly extrusion foaming and injection mould foaming are commercially used for foam production and batch solid-state foaming is being used for research purposes.

Batch solid-state microcellular foaming

Batch solid-state foaming is a non-continuous foam formation technique as in this technique a saturation of the polymer sample and the foaming (i.e. cell nucleation and cell growth) occurs individually. The polymer sample to be foamed is in solid-state and is processed in batches/group in a closed vessel at defined saturation parameters, thus named as batch solid-state foaming [1, 3, 9, 12, 18]. The batch solid-state foaming process is further classified as- Pressure-induced (One-step) and Temperature-induced (Two-step). These two techniques are discussed in detail below.

Pressure-induced (one-step) technique-

In this technique, a solid polymer sample is placed inside an autoclave vessel, which is pressurized with blowing agent at high pressure termed as saturation pressure and at a certain temperature termed as saturation temperature for a defined period of time termed as saturation time. Once the polymer sample is fully saturated, the vessel is depressurized rapidly at a high depressurization rate. This gas saturated polymer at a fully saturated state which is defined as, the state above which no more dissolution or absorption of blowing agent in the polymer sample can occur. In this type of batch foaming method, the induction of thermodynamic instability occurs due to the high pressure gradient (\(-\frac{\partial P}{\partial t}\)) which results in cell nucleation and its subsequent growth. The negative sign indicates the drop in pressure with respect to time. Finally, the sample is cooled in water for stabilization of the microstructure [1, 3, 12, 19]. Figure 3 represents a schematic of the typical pressure-induced batch solid-state microcellular foaming process.

Fig. 3
figure 3

Schematic representation of pressure-induced batch solid-state microcellular foaming

Full size image

Temperature-induced (two-step) technique-

In the temperature-induced technique, the sample saturation is typically done at a temperature lower than the glass transition temperature of the polymer matrix. Immediately after depressurization, the sample is taken out from the vessel and is dipped in a hot oil bath or glycerine/silicon bath [20, 21]. The bath temperature is generally kept above the glass transition temperature of the polymer matrix which is termed as foaming temperature. If the foaming temperature is above that glass transition temperature, stiffness or viscosity of polymer matrix decreases thus dissolved gas diffused out fastly and cause nucleation [22]. The time for which the saturated sample is dipped inside the hot bath is termed as the foaming time. In general, with an increase in the foaming temperature, the cell size also increases because higher temperature reduces the polymer viscosity and in turn reduces the resistance to cell growth [1, 4, 18]. In this type of batch foaming, the thermodynamic instability for cell nucleation and cell growth occurs due to the high temperature gradient (\(+\frac{\partial T}{\partial t}\)). The positive sign indicates the rise in temperature with respect to time. Figure 4 shows a schematic of the typical temperature-induced batch solid-state microcellular foaming process. Also, Table 1 depicts the comparison between techniques of batch solid-state microcellular foaming [1, 3, 4, 18, 22].

Fig. 4
figure 4

Schematic representation of temperature-induced batch solid-state microcellular foaming

Full size image
Table 1 Comparison between techniques of batch solid-state microcellular foaming
Full size table

The limitation of batch solid-state microcellular foaming is that it takes a significant amount of processing time for the development of foam and the autoclave vessel capacity also limits its product dimensions. To overcome this drawback, processes like extrusion foaming and injection mould foaming were developed [22], which could manufacture microcellular foamed products at an industrial scale.

Extrusion microcellular foaming

Extrusion microcellular foaming was developed by amalgamating conventional polymer extrusion process and external gas injection system which could disperse the blowing agent within the polymer melt [1]. When compared with the batch foaming process, the extrusion foaming process provides higher productivity, ease of control, versatility in end-product properties and profiles [22]. In extrusion foaming, the polymer pellets are fed into the barrel through hopper. The pellets get melted inside the barrel due to high temperature and the blowing agent at high pressure from the external gas injection system is injected into polymer melt, typically in the compression zone of the extruder and action of shear forms a homogenous mixture.

Generally, two kinds of extrusion processes are used which are single barrel extrusion and tandem (two-barrel) extrusion. In the single barrel extrusion, melting and cooling polymer matrix occurs in the same barrel while in tandem extrusion, separate barrels are there for melting and cooling [1, 4, 7]. Significantly better results are obtained with tandem extrusion than single barrel extrusion but gas leakage possibility, setup cost & power consumption are more in tandem extrusion compared to single barrel extrusion [4, 7]. Figures 5 and 6 depict a typical schematic of single extrusion and tandem extrusion microcellular foaming.

Fig. 5
figure 5

Schematic representation of single barrel extrusion microcellular foaming

Full size image
Fig. 6
figure 6

Schematic representation of tandem extrusion microcellular foaming adapted from Ref [1] with kind permission from Elsevier

Full size image

The screw motion passes the molten mixture into the second barrel, here it gets cooled to a temperature lower than temperature in the first barrel. Further cooling provided to reduce the cell coalescence. The melt pump regulates the amount of molten mixture flowing through the extruder. As the molten mixture exits the extruder die head, a large number of cells begin to nucleate and subsequently grows. The primary driving force for this nucleation is thermodynamic instability due to the high pressure gradient (\(-\frac{\partial P}{\partial t}\)). The cell growth occurs until it stabilizes or ruptures [7, 23]. The dispersion of the blowing agent in polymer melts significantly affects the morphology of the developed foam using extrusion process. The blowing agent injecting location in extrusion barrel affects the residence time of blowing agent which in turn affects the morphology of the developed foam [24].

Injection mould microcellular foaming

In the injection mould microcellular foaming process, the polymer pellets get melted in the barrel and the blowing agent is mixed in the molten polymer to form a homogenous mixture. The screw pushes this single phase molten mixture and is injected in the mould, the pressure drops to the atmospheric pressure and this leads to the microcellular nucleation phenomenon. The nucleated cells grow till they are stabilized. Due to the presence of gas, plasticization of polymer chain occurs due to which the viscosity of melt decreases & it leads to decrease in injection pressure. Also, the lesser clamp force is required [11, 23, 25, 26], when compared with the conventional injection moulding process. Figure 7 shows the typical schematic setup for injection mould microcellular foaming.

Fig. 7
figure 7

Schematic representation of injection mould microcellular foaming adapted from Ref [1] with kind permission from Elsevier

Full size image

The cycle time required for microcellular injection mould foaming is significantly lesser than conventional injection moulding. Approximately 20–50% cycle time can be saved with microcellular injection moulding. Figure 8 shows a comparison between conventional and microcellular injection foaming.

Fig. 8
figure 8

Comparison between conventional and microcellular injection foaming adapted from Ref [13] under an open-access license

Full size image

In microcellular injection foaming, the viscosity of polymer matrix reduced due to the presence of blowing agent (i.e. CO2/ N2) which increases the filling speed and reduced filling time. The packing pressure provided by gas in bubbles thus, holding time is almost eliminated. And as cell nucleation & growth required endothermic variation thus, cooling time is also reduced [13, 27]. Table 2 represents the comparison between traditional manufacturing processes of microcellular polymers [1, 3, 4].

Table 2 Comparison between traditional manufacturing processes of microcellular polymers
Full size table

Advanced manufacturing technologies in the microcellular foaming

From the last decade, a number of innovations have occurred in the manufacturing technologies of microcellular foam. These advanced technologies include ultrasound-aided microcellular foaming, bi-modal microcellular foaming, and cyclic microcellular foaming. These advanced technologies are developed either by specific process integration or process modification or variation in the process parameters & their levels to obtain the desired foam structure.

Ultrasound-aided microcellular foaming

Ultrasound-aided microcellular foaming is the advanced foaming technique in which external ultrasound vibration applied within the existing traditional batch foaming process to enhance the cell morphology of the foamed product. This technique is useful to convert the closed cell structure to an open or interconnected cell structure. The main application of this technique could be in the field of tissue engineering scaffold as it is a solvent-free technique and therefore, the new tissue generation ability of biological cells remains unaffected [28,29,30]. Also, it could be utilized for filters and membrane preparation [31]. The various process parameters such as ultrasound frequency, exposure time of ultrasound, the intensity of ultrasound, temperature of water in the ultrasound aided microcellular foaming technique which significantly affects the cell morphology of polymer foam. Figure 9 shows the schematic representation of for ultrasound aided microcellular foaming setup used by Gandhi et al.

Fig. 9
figure 9

Schematic of setup for ultrasound aided microcellular foaming- Adapted from Ref [29] with kind permission from Elsevier

Full size image

Mechanism of ultrasound aided microcellular foaming

The ultrasonication creates the vibrational sinusoidal wave in an elastic medium. During the positive half of the wave, the distance between the molecules of the medium decreases and during negative half that distance increases. When the wave vibration reaches to threshold or peak, it create bubbles that are termed as cavitation bubbles. With time, the size of cavitation bubbles increased, when it reached the critical size it explodes violently and thus creating the packets of energy called microjets. These microjets start continuously striking on the polymer surface that creates the hot spot. The hot spot is confined to a localized area, experience extreme high temperature and pressure about 5000 °C and 1000 atm [31,32,33], which induces high thermodynamic instability.

The significant effect of ultrasonication on cell morphology depends on when ultrasonication assists with traditional foaming technique, i.e. either at the beginning of nucleation or after nucleation.

Ultrasonication applied at beginning of nucleation-

The ultrasonication creates a number of cavitation bubbles which collapse violently once bubbles reach their critical sizes and thus creates hot spots. The extreme conditions of hot spots generate high thermodynamic instability which leads to the nucleation of a large number of small cells i.e. cell density gets enhanced [33].

Ultrasonication applied after cell nucleation & growth -

The generated microjets continuously strike on the foam surface and break the surface cell walls. Then the water along with cavitation bubbles enters into inner foam. Further, in the same manner, cell walls get rupture and the interconnected or open cell structure achieved [33]. Even if there is heat generation at hot spots, it is consized to the localized area so that the overall temperature is less, and hence the foam shape or structure is not distorted [34]. The schematic representation of the mechanism of ultrasound aided microcellular foaming is shown in Fig.10.

Fig. 10
figure 10

Schematic representation of the mechanism of ultrasound aided microcellular foaming -Adapted from Ref [29] with kind permission from Elsevier

Full size image

Wang et al. demonstrated that ultrasound irradiation (UI) enhanced the non-uniform cell structure to uniform cell structure of the semi-crystalline or crystalline polymer if UI introduces at the beginning of cell nucleation. The delay in the introduction of UI leads to non-uniformity of structure and if the exposure time of UI increases, it leads to cell uniformity and enhances the cell density [28]. The literature review on ultrasound aided microcellular foaming is given in Table 3.

Table 3 Literature review on ultrasound aided microcellular foaming
Full size table

Bi-modal microcellular foaming

The bi-modal structure also termed as bi-cellular or complex cellular, as it consists of both large sized cells and small ones [40, 41]. The small sized cells provide mechanical strength, thermal insulation, whereas large sized cells provide low bulk density [42]. The bi-modal structured foam has better thermal insulation compared to uni-modal structured foam. The two different nucleating mechanisms are required to develop the bi-modal cell structure [42]. Generally, the bi-modal foam structure is developed by; two-step depressurization technique [43,44,45,46,47,48,49,50,51], co-blowing technique [40, 42, 52,53,54], polymer blend technique [55,56,57,58]. Along with this, some researchers developed bi-modal structure using ultrasound excitation [33], multiple soaking technique (MST) [59], temperature rising and depressurization [60], etc.

The schematic of two-step depressurization is shown in Fig. 11. In this technique initially, sample saturated at pressure (P1) and temperature (T1) for a defined time (t1). Then depressurized the vessel to an intermediate pressure (P2) and hold it for some time (t2). Finally depressurized vessel to atmospheric pressure. This stepped depressurization causes large and small bubbles. The intermediate pressure also termed as holding pressure.

Fig. 11
figure 11

Schematic representation of two-stage depressurization

Full size image

The co-blowing agent with a primary blowing agent also develops bi-modal cell structure. In this case, the different nucleating mechanism is induced by two different blowing agents. Generally, the large cell size is obtained by the co-blowing agent and small cell size is obtained by a primary blowing agent which may be due to the diffusion difference of blowing agent in the polymer matrix. The polymer blending technique leads to the formation of the bi-modal structure due to the non-homogeneity of polymer blend and also due to the difference in stiffness. This leads to time inclusion in first cell nucleation and second cell nucleation.

Gandhi et al. [33] shown that bi-modal structure could be created with ultrasound excitation in which the ultrasound frequency is a significant parameter that affects the bi-modal microcellular structure of ABS foam as shown in Fig. 12. The author found that the low ultrasound frequency (25 kHz) led to uniform cell morphology and high ultrasound frequency (45 kHz) led to bi-modal cell morphology. The high ultrasound frequency generated the number of microjets. These microjets continuously hit the polymer surface and formed a number of small cells. But microjets continued to strike the surface and forming new cells around the growing cells eventually which led to bi-modal microcellular morphology.

Fig. 12
figure 12

Schematic representation of the formation of bi-modal ABS foam structure using ultrasound excitation

Full size image

Huang et al. [59] developed another new technique called multiple soaking temperature (MST) to generate a bi-modal structure. In this method, first sample was sealed in the autoclave chamber at room temperature and pressurized with saturation pressure of P1. Then raised temperature to T1 which is the first soaking temperature, kept it for time t1. After that, decreased the temperature to second soaking temperature T2 kept for time t2. Again temperature increased to third soaking temperature T3 kept for time t3, followed by a reduction in temperature to fourth soaking temperature or foaming temperature T4 kept for time t4, later depressurized chamber to ambient pressure. Finally cooled autoclave chamber to room temperature and foamed sample taken out from chamber. Here, soaking temperature array as T1 ˃T3 ˃T4 ˃T2. With this method, the author successfully fabricated porous bi-modal PLA foam with open-cell structure had porosity, open-cell content, average large and small cell size are 87.9%, 82.4%, 150 μm, and 8 μm respectively.

The thermodynamic instability could be generated by sudden pressure drop or sudden temperature rise which results in the cell nucleation and in general foaming process, either one of them is enough for nucleation. But Lin-Qiong Xu et al. [60] used both thermodynamic instability aspects synergistically and successfully created the bi-modal structure in polystyrene foam.

Radhakrishna et al. used a step-wise depressurization technique (Four-step), and each depressurization step induced different nucleation phenomenon led in the development of bi-modal and multi-modal ABS foam microstructure [61].

The literature review on bi-modal microcellular foaming by two-step depressurization, co-blowing agent, polymer blend technique is given in Tables 4, 5, and 6 respectively.

Table 4 Literature review on bi-modal microcellular foaming (Two-step depressurization)
Full size table
Table 5 Literature review on bi-modal microcellular foaming (Co-blowing agent)
Full size table
Table 6 Literature review on bi-modal microcellular foaming (Polymer blend technique)
Full size table

Cyclic microcellular foaming

The cyclic foaming is repetitive foaming. In the cyclic foaming first, a neat polymer sample converts into a foamed sample then again foamed sample processed under the required levels of foaming parameters. From literature, it has seen that for cyclic microcellular foaming generally batch foaming process was used as; easy to control processing parameters and repeatability [65]. In the cyclic foaming process, saturation pressure is a crucial parameter; consider primary saturation pressure as P1 and secondary saturation pressure as P2. Also, the sequence of P1 and P2 plays a significant role in controlling cell morphology [65, 66]. Gandhi et al. developed a novel technique by applying the ultrasound assistance to cyclic foaming to produce ultralow density foam with interconnected cell structure as shown in Fig. 13. Table 7 shows the literature review on cyclic microcellular foaming.

Fig. 13
figure 13

Schematic representation of ultrasound-induced cyclic foaming process- adapted from Ref [38] with kind permission from Elsevier

Full size image
Table 7 Literature review on cyclic microcellular foaming
Full size table

Manufacturing technologies of advanced microcellular polymers

The microcellular polymers have enhanced properties owing to its micron-sized cell structure and therefore the application range gets widened compared to macron-sized cell structure. The advanced applications of the microcellular polymer such as EMI shielding, microcellular auxetic foam for sports & safety equipment, scaffolds for controlled drug released, wooden foam composites for high strength applications. The manufacturing technologies of advanced microcellular polymers, its properties and applications are discussed in detail.

Electromagnetic interference shielding microcellular polymers

The radiation generated by electronic devices interferes with each other causing the disruption in the functionality of devices is known as Electromagnetic Interference (EMI) [69, 70]. The ringing cellphone near the television causes the fluctuations in video or audio quality of television is one of the examples of EMI. EMI may lead to the adverse effect on the functioning of the important electronic systems/devices in the field of aerospace, defence, intelligence department, significant scientific research and also on human health due to prolonged radiation exposure [69,70,71]. The term EMI Shielding is referred to as the protection against the interruption of electromagnetic waves. The conductive material possesses good EMI shielding capability thus generally, the metal-based materials are used as EMI shield material but their weight to shielding effect ratio is more. Therefore nowadays, polymer-based conductive materials replaced them owing to their less weight, corrosion resistance, ease of processing, flexibility, and cost [69, 70, 72].

There are two kinds of conductive polymers; intrinsic conductive polymers (ICP) which have good electrical conductivity and extrinsic conductive polymers (ECP) in which electrical conductivity has to generate i.e. by conductive coating or conductive filler addition. There is a large range of conductive fillers are available categorized as; carbon-based and metal-based. Carbon-based fillers comprise as; CNT, CNF, CB, graphene, GR, GO, GNRs, GNPs, etc. And metals based metal nanowires, metal granules metal nanoflakes, etc. [69, 70].

The various structures were reported to enhance the absorption of electromagnetic waves as, segregated structure, multilayer structure, sandwich structure and foam structure [71]. The density of polymer composite is less than metal composite but due to the addition of conductive filler, the density and the viscosity of polymer composite may increase. The researchers strive to further reduce the density of polymer composites by developing a porous structure in the polymer matrix [69, 71]. The reflection, absorption, multiple-reflection, and transmission are the mechanisms that play a significant role in locking the electromagnetic waves in the shielding material [69]. EMI shielding effectiveness (EMI SE) is a measure of the shielding material’s performance, which is the sum of losses associated with all the above mechanisms and given in terms of decibel (dB) [69, 70, 72, 73]. In this study, this section is focussed on EMI shielding of the foam structure. The multiple-reflection mechanism occurs in foamed EMI shielding material because the foamed EMI shielding material has a large interface area and surface area so multiple-reflection possible [69].

Yang et al. [74] integrated hot pressure compression moulding and salt leaching process to develop PS/CNT foamed composite having a density of 0.45 and 0.27 g/cc.

Ling et al. [75] used the water vapour induced phase separation (WVIPS) process shown in Fig. 14 to produce microcellular nanocomposite foams for enhancement of EMI Shielding. Figure 15 depicts microwave transfer across nanocomposite foam that enables composite foam as an excellent microwave absorber. Table 8 represents the literature review on EMI shielding microcellular polymers.

Fig. 14
figure 14

Schematic representation of WVIPS process to prepare nanocomposite foam- adapted from Ref [75] with kind permission from American Chemical Society

Full size image
Fig. 15
figure 15

Schematic representation of the microwave transfer across nanocomposite foam- adapted from Ref [75] with kind permission from American Chemical Society

Full size image
Table 8 Literature review on EMI Shielding microcellular polymers
Full size table

Auxetic foamed polymers

The auxetic polymers are processed materials having negative Poisson’s ratio (NPR) by the virtue of which it explores the exceptional properties [89,90,91,92]. The material with NPR indicates, the material expands laterally when stretched linearly and vice versa [93,94,95]. The first polymer auxetic foam was synthesized by Lakes R. in 1987, had Poisson’s ratio of −0.7 and Young’s Modulus of 72 kPa from open-celled PS foam [89]. The auxetic foam has superior properties over conventional as; resilience, synclastic, indentation resistance, shear resistance, etc. which leads to the wide range applications in the field of aerospace, military, sports, biomedical, etc. [89, 90, 92, 96,97,98,99].

Generally, the thermo-mechanical technique is used for the transformation of conventional polymer foam to auxetic polymer foam. The thermo-mechanical technique is comprised of three steps; first is volumetric compression of conventional foam using a mould of which shape is to be achieved. In this step, the buckling of cell ribs or struts occurs which leads to a re-entrant cell structure. Then second is a heating step, in which compressed foam along with mould heated in a furnace at a temperature above the softening temperature to maintain the buckled cell structure for a defined time. Finally in third step, mould takes out from the furnace, cooled it to room temperature and remove the foam from it. Sometimes cell ribs stick to each other which may affect the functionality of final foam (auxetic) thus stretch the foamed sample in the opposite direction of compression [89, 100,101,102,103]. It is recommended to lubricate inside the mould or use of wire or tweezers or redesign the mould to avoid the surface wrinkling or surface creasing [97].

To eliminate the high temperature processing, Garima et al. [104] developed a novel chemo-mechanical technique. In this technique, the first compression step was similar to as in thermo-mechanical technique. Then compressed foam sample wrapped in filter paper and placed in the organic solvent (acetone) for a defined time. Finally taken out the sample and dried it in the air. The formed foam was auxetic in nature. Also, the researchers successfully re-convert auxetic foamed structure to the original foamed structure using the same organic solvent through the re-expansion of the auxetic foamed sample in the solvent.

The simple geometry auxetic foams are easy to manufacture than complex and curved geometry auxetic foams using above mentioned conventional techniques. As the solution for this, Binachi et al. [105] developed a new method called half mould manufacturing process. In this method, the author used an open curved mould instead of a closed mould. The conventional foamed sample placed in mould on which layer of non-porous fluorinated ethylene propylene (FEP) film and over that the medium weighs polyester nonwoven breather blanket placed such a that it covers foamed sample with mould. The assembly closed in the flexible membrane and sealed it properly then using a vacuum pump reduced internal pressure to 0.7 Bar. Due to vacuum, cell ribs get bucked and re-entrant structure was formed. To preserve the cell structure, all assembly was kept in a furnace of temperature 200 °C by maintaining inside vacuum. After a definite heating time of half-hour, assembly was taken out and immersed in water. Once the temperature reached to room temperature foamed sample outstretched orthogonally to relive cell sticking. The resultant foam was in auxetic nature.

The transformation of closed cell conventional foam to auxetic foam is difficult than open cell conventional foam to auxetic foam which may be due to isolated cell or thick cell walls. Fan et al. [106] proposed a new technique using water steam called steam penetration and condensation (SPC). In this technique, the foamed sample of defined shaped placed in water steam environment which maintains at a specific temperature for a defined time. Due to steam, all cells contracted inside leads to cell bucking, re-entrant structure. Then the sample was taken out and cooled to room temperature. The foam structure resulting from this technique has NPR i.e. showing uniform auxetic nature due to the pressure difference.

In order to enhance the mechanical performance of auxetic foam compared to the performance of auxetic foam produced from the thermo-mechanical technique, the new technique was developed by Quadrini et al. [107] called solid-state foaming for auxetic foam. In this method, first the epoxy resin powder tablets of re-entrant hexagonal shape were prepared by cold compaction. Here the number of small tablets were arranged in an ordered manner with defined space between them because the large size tablet leads to poor foaming. After that foaming of tablets was done using the oven at 320 °C for 8 min in the presence of air. Then the foamed samples were taken out and cooled in the air which showed auxetic behaviour.

For the manufacturing of large auxetic foam blocks, Chan et al. [108] proposed a multi-stage thermo-mechanical technique in which volumetric compression and heating occurred in stages. The advantage of this multi-stage method is that the surface ceasing is eliminated. Table 9 shows the literature review on auxetic foamed polymer.

Table 9 Literature review on Auxetic foamed polymers
Full size table

Porous microcellular polymers

The large surface area, interconnected cell structure, small and uniform cell size increased demand of the porous microcellular polymer preferably for tissue engineering scaffolds, separation membranes, cushioning, etc. The scaffolds used for the transformation of nutrients, the excretion of cell wastes, cell interaction, tissue formation which required porous structure [109, 110]. The various techniques used for the fabrication of porous structure for scaffold as; solvent casting/ particulate leaching, thermally induced phase separation, freeze-drying, etc. [110, 111]. These are conventional solvent-based techniques which may affect the ability of the biological cell to generate new cells [31], less pore-interconnectivity, uneven pore distribution, less porosity, etc. [109]. These problems are resolved by the gas foaming technique as it is a solvent-free technique [109, 110, 112].

Presently, advanced techniques such as electrospinning and 3D printing or rapid prototyping are using to achieve better specific structure properties with ease of processing at economical cost. Electrospinning is used to fabricate a porous scaffold using fibers, it’s a simple and cost-effective fiber production technique. In this technique, the high voltage supply applied to polymer droplet held at tip of needle, which charges the droplet & formed repulsive force. The charged jet of polymer solution erupts and reaches to a collector, solvent evaporates and jet solidified into thin fibers [111, 113,114,115]. Another technique called, 3D printing or rapid prototyping technique is used to fabricate complex scaffold structures precisely from biodegradable materials [110, 116,117,118,119,120,121]. Also, some researchers modified or integrated two or more techniques to achieve the advance structure [122,123,124,125,126]. This section focused on porous microcellular plastics using a gas foaming technique.

To fabricate the porous or open interconnected structure, the ultrasound assistance was also used by various researchers such as Gandhi et al. [38], Wang et al. [31], H Wang et al. [34], X Wang et al. [36] for different applications. Table 10 contains the literature review on porous microcellular plastic.

Table 10 Literature review on Porous microcellular polymers
Full size table

Wood fiber reinforced microcellular polymer composites

Wood fillers or flour as reinforcement in polymer matrix led to the new branch of composites termed as wood fiber reinforced polymer composite (WFRP). At present, WFRP composites are the most demandable composites in different fields like construction for interior decoration & furniture, automotive for decking, window, and door lineals, railing, auto parts, etc. due to its mechanical properties [135,136,137,138,139]. The WFRP composites are generally manufactured by injection moulding, extrusion and compression moulding [135, 137]. The reason behind the huge demand of WFRP composite is, readily available wood filler at low cost, has specific strength and biodegradability [135,136,137]. The various wood fillers, with different aspect ratio are available in the form of fiber or flour [137]. There are some constraints for the utilization of wood fibers such as; thermal degradation, fiber breakage, and moisture content which limits the selection of processing parameters [135, 137, 140]. The WFRP composites possess good strength and stiffness so it could be utilized for static applications but also has low impact strength, brittleness, and high bulk density which limits its utilization in dynamic applications [140,141,142,143]. Therefore to advance its utilization in dynamic applications, the mechanical properties have to be enhanced (impact strength & ductility has to enhance with a reduction in bulk density), which could be achieved by the creation of microcellular structure in composites [136, 141, 144, 145].

The microcellular structure could be generated in wood composites by foaming processes such as; batch foaming, injection mould foaming, extrusion foaming, compound Moulding [136, 141, 145]. Some advancement is still going on to develop a new process or modify the existing ones as; co-extrusion, foaming during extrusion, and inline coating technologies, etc. [138]. The advantages of foamed wood composite over the unfoamed wood composite are low processing temperature, high production speed which in turn reduced manufacturing cost also better surface smoothness, sharp counters, and corners, etc. [136, 144, 146].

The process of WFRP composite foaming is similar to the general foaming process comprised of three stages; gas dissolution & saturation, cell nucleation, and cell growth & stabilization. In wood-composite foaming, blowing agent is not dissolved in wood fillers thus only polymer matrix gets plasticized. Subsequently, due to the generation of thermodynamic instability, cell nucleation occurs. In this case, both types of nucleation occur; homogenous as well as heterogeneous. The homogenous nucleation due to the gas diffusion from the composite whereas heterogeneous nucleation due to the entrapped gas at micro-voids near polymer-filler interfaces. The percentage of heterogeneous nucleation is more than the homogenous nucleation because of the more filler content, the number of interfaces available and restricted the gas dissolution to the polymer matrix. The continue cell growth may lead to cell coalescence and cell collapse, [136, 141]. To avoid this, the cells are stabilized using a cooling medium.

A wide variety of additives are used to form wood composites such as coupling agents to improve the dispersion of fillers, chemical surfactants to enhance the bonding or adhesion between filler and polymer matrix, flame retardants to enhance resistance to the outbreak of fire, etc. [139,140,141, 144, 145, 147,148,149,150].

Some researchers fabricated wood composites from the wastes; PS foam as polymer matrix and agricultural waste as a wood filler. Koay et al. [151] prepared novel wood composite from recycled PS foam and durian husk fiber. In this, first PS foam sample was dissolved in acetone to remove air from it. Later it was dried at 70 °C using oven followed by filtration; subsequently recycled PS was cut into small samples for further process. For wood fiber, durian husks were collected, cleaned and cut into small pieces then dried it in an oven at the same 70 °C and prepared short fiber with the grinder. The fibers dried again to avoid high moisture content. Subsequently, WPC compound formed using torque rheomix and moulded into sheets using hot press. Similarly, Tawfik et al. [152] formed a wood composite with recycled PS and rice straw. The tensile strength, water absorption, and acoustic resistance test were performed. The tensile strength decreased with an increase in rice straw content above 30%. Chun et al. [153] used agricultural waste to formed wood composite with PS foam. The tensile, thermal and morphological evaluation was done to analyze the performance of composites. Table 11 represents the literature review on wood fiber reinforced microcellular polymer composite.

Table 11 Literature review on wood fibre reinforced microcellular polymer composite
Full size table

Conclusions and future direction

  • The manufacturing of customized polymer foam with controlled cell morphology using general microcellular foaming is quite difficult. Therefore, the integration of advancement to traditional methods has been made more convenient and effective.

  • The advancement in the microcellular foaming process such as ultrasonication has been found to be much effective in enhancing both nucleation rate as well as interconnected open-cell structure. It depends on the time of application of the ultrasonication (i.e. at the beginning of nucleation or after nucleation)

  • The microcellular polymer composites exhibit a better EMI Shielding effect than neat polymer composites with a reduction in weight density too.

  • Carbon-based fillers are the most suitable fillers to enhance the electrical conductivity of polymer foam in EMI shielding applications due to their wide range of properties.

  • In wood fiber reinforced polymer foaming, the cell nucleates by homogenous nucleation as well as heterogeneous nucleation. Homogenous nucleation occurs due to gas diffusion from polymer matrix whereas, heterogeneous nucleation takes place at polymer-filler interface due to entrapped gas at micro-voids.

  • The buckling of cells governs the auxetic behaviour and the performance of auxetic foam polymer is independent of the cell size of the foam structure.

  • The bi-modal cell structure provides superior properties compared to the uni-modal cell structure. Moreover, the tri-modal cell structure may add more value to properties governed by the bi-modal structure. Hence, there is scope to further develop the polymeric foam with tri-modal or multi-modal cell structure.

  • Few literatures are available on the cyclic foam manufacturing process which builds up research focus to explore more and more on the cyclic foaming technique to manufacture ultralow density foam with desired properties.