Advances in 3D-Printed Polymeric Materials for Sustainable Thermal and Acoustic Insulation

The way that three-dimensional printed porous polymers behave thermally depends mainly on the way that those materials were constructed, which is fully customizable during the additive manufacturing process. The primary structural features of these materials (including infill density, pore shape, unit-cell size and porosity, and connectivity) have a significant effect on their effective thermal conductivity (kₑff) and overall thermal transfer processes. The ability to create any possible geometrical configuration through additive manufacturing makes it possible to manipulate the structural parameters systematically and to adjust polymeric structures to provide a combination of effective thermal insulation, lightweight construction, and multi-functionality. Therefore, the interaction between geometrical shape and heat flow is now one of the major areas being researched to develop future architected polymer systems for use in sophisticated thermal management applications [10].

2.1 Effect of infill density

Infill density has a major impact on the heat transfer properties of 3D-printed porous polymers because it specifies the amount of solid material in the printed structure. This factor controls the number of heat-conducting polymer chains relative to the isolated air pockets, thus calculating the effective thermal conductivity (kₑff) and the total insulation capacity of the material. Physical experiments have shown that polymeric lattices made by additive manufacturing can have very low thermal conductivities, generally between 0.03 and 0.09 W m⁻¹ K⁻¹, which is close to the performance of standard insulating materials like glass wool (0.02-0.04 W m⁻¹ K⁻¹) [11].

By means of different AM processes—such as fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering (SLS), and direct ink writing (DIW)—thermally functional structures with controlled porosity and customized heat-transfer properties have been produced, which are also light in weight [12].

Multiple research studies [7, 8, 11] have demonstrated that as the amount of infill density is decreased (thus increasing porosity) when 3D printing polymers, there is a corresponding decrease in effective thermal conductivity. The inclusion of air instead of solids for providing heat-transfer paths creates an increasing discontinuity within the conductive network; therefore, an increase in thermal resistance occurs. Tychanicz‑Kwiecień et al. [7] noted that with a decrease in infill density the keff values are significantly decreased for PLA, Polyethylene Terephthalate Glycol-modified (PET-G) and Acrylonitrile Butadiene Styrene (ABS), while Krapež Tomec et al. [8] showed a similar decline in conductivity, diffusivity, and effusivity when observed with a decrease in infill density for wood–PLA composite materials, and Islam et al. [11] indicated that highly porous PLA lattice structures can achieve extremely low conductivity values in comparison to traditional forms of insulation materials.

2.2 Effect of infill pattern and geometry

Researchers have demonstrated that the infill type used to construct an additively manufactured polymeric structure is critical in determining how that structure responds thermally and mechanically. The internal structure of a polymeric part (the infill pattern) establishes how heat is conducted, how stress is distributed through the material and how energy concurrently dissipates through the material. Lopes et al. [9] comprehensively studied twelve different infill structures made from PET-G materials and found that just by selecting an infill type it can increase/decrease thermal conductivity by up to 70% and increase/decrease the mechanical performance of a part (when compared to an added equivalent structure): over 300%. Additionally, it has been established that the infill type selected has a significant effect on a material's behavior and performance.

Honeycomb type infill structures have consistently produced the best thermomechanical performance when compared to other infill types due to their high stiffness to weight ratio and efficient load transfer mechanisms between strut junctions. Eryıldız [13] found that honeycomb-infused PLA materials displayed the greatest tensile strength (while using a linear testing method) at approximately 29.43 MPa. This is attributed to improved junction integrity and decreased local stress concentrations as compared to other infill types. Conversely, other patterns, specifically patterned infills that present large air gaps (i.e., space-filling infill or loosely packed infill), do not produce as consistent a mechanical stability or thermal conduction capability due to the use of discontinuous heat-transfer paths [14].

At the architectural scale, the shape of the cavity is the major factor that determines the insulation performance. de Rubeis et al. [10], by means of a hot-box thermal apparatus, compared PLA panels with multi-row, square, and honeycomb cavities and demonstrated that the honeycomb configuration resulted in the lowest overall heat-transfer coefficient (U = 1.22 ± 0.04 W·m⁻²·K⁻¹). The reason for this improvement was the increased tortuosity of the internal channels, which effectively elongates the conductive path length and inhibits heat flow without a substantial increase in mass. In the same way, Lopes et al. [9] found that by changing the rib continuity, orientation, and cell topology from cubic to grid and honeycomb layouts there were significant differences in effective thermal conductivity (kₑff) and thus geometric anisotropy was identified as a key factor in the regulation of thermal transport [15].

All this collective evidence demonstrates that geometrical complexity can improve the thermal inefficiency associated with thermal insulating/energy-transfer materials, by creating longer and more coiled thermal pathway routes, thereby reducing the thermal conductivity of those materials. Additionally, complementary research completed by Islam et al. [11] shows that the geometry of a material's pores exemplifies an additional function of hollow materials; for example, at the same level of porosity, cubic and irregular pore networks exhibit different heat-transfer/energy-transfer behaviors. Therefore, creating an optimal infill geometry of the printed polymeric system is critical to achieving higher mechanical strength and improved thermal insulation performance.

2.3 Triply periodic minimal surfaces (TPMS) and gyroid architectures

In particular, gyroid structures—have been identified as highly viable architectures for next-generation thermal insulation systems due to their topological and structural characteristics of a kind. These structures are non-self-overlapping, smoothly curved surfaces that by default extend the heat conduction channels and at the same time maintain the mechanical properties of the material, thus allowing an ideal equilibrium to be reached between the mechanical strength and thermal insulation. The periodicity and smooth curvature of TPMS structures naturally eliminate the concentrated stresses at a local level and form tortuous ways that effectively hinder conductive heat transfer.

Anwajler et al. [12] produced photocurable resin (DLP)-printed gyroid structures that show thermal conductivity varying from 0.023 to 0.039 W·m⁻¹·K⁻¹, which are at least as good as, or even better than, those of typical polymeric foams. The same study carried out energy modeling simulations that showed the use of gyroid-based panels in building envelopes could bring the annual heating energy demand down by more than 15 %, thus confirming their real potential in energy-efficient construction. The results speak to the capability of structures based on TPMS to be the source of lightweight, multifunctional insulation materials that are structurally efficient and have good thermal performance. Together, the findings of the studies noted above support the hypothesis that commercial products will benefit from all of the above advantages, due to the ability of these materials to be optimized for multiple applications and thus lead to higher performance (thermal and acoustic) than other alternatives. As indicated by Islam et al. [11], the thermal conductivity of 3D-printed PLA gyroids is very low (as low as 0.037 W·m⁻²·K⁻¹), and they can achieve an STL value as high as 48.27 dB, depending on the density and porosity of the print. The ability of AM to enable the precise control of these microstructural variables through the versatility of design, mass customization, and process-driven multifunctionality elevates the significance of TPMS-structured materials in the production of the next generation of thermally and acoustically efficient materials [16]. Together with results from life cycle cost analysis (LCCA), TPMS-based insulation configurations can yield a substantial energy savings (between 45-67% energy savings) with a return on investment of 8.5 to 14.8 years, demonstrating that both the technical and economic feasibility of TPMS insulation systems [17] is demonstrated. Ultimately, the combination of gyroid and TPMS geometries provides a basis for the development of sustainable, architected polymers for multifunctional energy-efficient uses.

2.4 Influence of unit-cell size and convection suppression

Pore size is a decisive factor which determines the location of the first cells in a porous material where natural convection appears. Alqahtani et al. [18] pointed out that polymer lattice structures with hydraulic diameters less than about 8 mm exhibit pure conduction heat transfer only. Their research also revealed that there was no negative impact on the thermal performance when scaling unit cells to commercial sizes (1 m²), which is an important finding for real-life applications. In a similar manner, Monkova et al. [19] showed that convective heat transport in closed-cell foams is negligible if the Average Pore Size (APS) is Smaller than approximately Six Millimeters (6 mm) versus larger cells (EPS = 6 mm) in size, with the proportion of Heat Transfer through Convection being 20 Percent (20%) for cells >6 mm APS.

The size of cell openings (hydraulic diameter) is the factor that determines when the heat transfer process changes from conduction–dominated to convection-influence [13]. A smaller cell size leads to the elimination of natural convection, so that the conduction-dominated transfer is observed. On the other hand, larger cells may lower the thermal resistance (leading to higher U-values), while smaller cells increase the number of serial interfaces, thus limiting heat transfer [20, 21]. Therefore, the choice of geometry (both shape and size) is equally important as density in the thermal design of 3D-printed insulators.

2.5 Theoretical models to explain the effect of shape

Theoretical models provide workable ways to explain experimental results and predict a system's behavior. A simple model, such as the Maxwell-Eucken model anticipates an almost linear drop of the effective thermal conductivity (k_eff) with growing porosity. In reality, architected structures like honeycombs and gyroids can be considered as heterogeneous composites with orientation-dependent properties, which account for the need of advanced modeling methods. To illustrate this, Hrițuc et al. [22] developed an empirical mathematical power function model for 3D-printed PLA panels, showing that sound volume is the factor that influences the acoustic pressure level the most. The same authors, in a parallel study, applied Taguchi L18 factorial experiments to demonstrate that sound frequency has the greatest influence and that PLA panels can reduce sound pressure levels by about 45% [22].

Islam et al. [11] backed up these theoretical methods by thorough experiments in which they obtained thermal conductivity values as low as 0.037 (W/m·K). Monkova et al. [23] went on to say that theoretical models and molecular dynamics simulations offer micro- and macro-level bases, which, when combined with AM, can both confirm theoretical correctness and open up new practical applications.

More sophisticated effective medium theories (E-M-T) include shape factors to better depict the non-spherical or rib-like pores. Nonetheless, contradictions are often encountered among experimental measurements as well as theoretical calculations because of pore interconnectivity, anisotropy arising from the process, and interfacial resistances that are 3D printing process intrinsic. Sophisticated models are necessary to unravel these complexities. For example, a percolation model is able to factor in the tortuosity and connectivity of the polymer network [24], thus producing results that are in a much closer agreement with the experimental data, particularly when the polymer network is near discontinuity.

The theoretical Maxwell-Eucken and Johnson-Champoux-Allard (JCA) models are useful ways to predict how porous materials behave thermally and acoustically. However, currently we have limited ability to apply the theory of each of these models analytically. Therefore, we can perform a simple numerical comparison of the predictions of the theory with actual experimental data. For instance, the theoretical Maxwell-Eucken prediction for PLA (polylactic acid) at 50% porosity would be k = 0.042 (W/m·K) whereas experimentally determined k ≈ 0.048 (W/m·K) for FDM (fused deposition modeling) printed PLA samples of the same density. The 12-15% difference between the two numbers relates to some of the interfacial defects that occur in 3D-printed parts and to print anisotropy and so indicates that model calibration is necessary in order to use these models effectively in practice.

Researchers have closely looked into the dual thermal and acoustic performance of polymeric materials in various systems, from typical PLA panels to high-tech aerogels and eco-friendly composites. The aim is to pinpoint materials that present a synergistic combination of properties for multifunctional applications.

Engineered Polymers: Islam et al. [11] carried out the investigation of 3D-printed PLA and documented thermal conductivity as low as 0.037 (W/m·K) along with sound transmission loss (STL) values of ~48 dB at 1600 (Hz). Their data indicate that infill densities of the medium range (40-60%) yield the most advantageous compromise between heat resistance and broadband sound absorption.

Advanced Aerogel Systems: As a result of these breakthroughs, advanced aerogel systems have evidenced excellent multifunctionality beyond the board. To illustrate, polyimide aerogels, also SiO₂-reinforced hybrids [49, 50], have been able to achieve ultra-low thermal conductivities (< 0.025 W/m·K), very high absorption coefficients (up to 0.9), and STL values over 50 dB.

Bio-Composites: One of the most promising variants of composite materials is those made from natural fibers and agro-industrial residues. The use of bio-fillers in polymer matrices is the fundamental approach in the creation of environmentally friendly composites with given characteristics. Pop et al. [30] experimented on the biocomposites which are made from natural fibers and measured k ≈ 0.045 (W/m·K) alongside absorption coefficients higher than 0.7 at the middle-frequency range. In the same manner, Segura et al. [31] and Ali et al. [32] utilized the waste of fruits, tea bags, as well as date palm fibers to produce the boards with k ≈ 0.036–0.04 (W/m·K) and α > 0.8 at 2000–4000 (Hz) intervals. These outcomes attest that plant residues can be converted into efficient thermal along with acoustic insulators, thus, achieving the goal of combining eco-friendliness and technical performance.

Nonwoven Polymer Fabrics: Nonwoven materials have been the subject of numerous studies to find cheap insulation alternatives. Usta et al. [33] observed that the mixture of poplar/PET can deliver an R-value ≈ 0.12 (m²·K /W) alongside α up to 0.78 at 6300 (Hz). Katsura et al. [34] improved the nonwoven fabric by the addition of aerogel granules and thus decreased the k value to ~0.03 (W/m·K) and increased α to 0.85. Karimi et al. [35] investigated polypropylene nonwoven mats and disclosed that k was close to 0.046 (W/m·K) with the acoustic absorption level around 42 (dB). The mentioned experiments indicate that the fiber morphology and aerogel reinforcement can bring nonwoven structures at the same level as multifunctional systems.

Recycled and Reused Materials: Consistent with the principles of the circular economy, tests have been conducted on reused and recycled materials to see if they can be used as dual insulation. Neri [36] fabricated the panels from waste polyester and felt, and obtained k ≈ 0.05 (W/m.K) and STL ~45 (dB). Although their performance is lower than that of the engineered PLA or aerogels, the results show that recycling strategies can provide good insulation while being sustainable.

The polymeric materials spectrum of the literature is the basis for the materials that can be used as insulation. These materials can be engineered (PLA, aerogels), eco-sourced (bio-composites, agro-waste), or recycled (nonwovens, polyester), and they can be gradually adapted to serve the multifunctional insulation purpose [49]. Biodegradable polymers such as PLA, polyhydroxyalkanoates (PHA), and polybutylene succinate (PBS), which are mostly processed through FDM, are at the core of this green production turn, however, there still exist some problems of mechanical performance and biodegradability that need to be solved. The factors that determine the interaction between thermal conductivity and sound absorption are porosity, density, and microstructural design, and these apply to all the systems [50].

Future advancements in 3D-printed insulation will likely stem from innovations in materials, design, and manufacturing processes. Key areas of opportunity include multi-material printing, smart systems, and the establishment of clear performance benchmarks.

6.1 Multi-material and hierarchical architectures

Using multiple materials to create three-dimensional prints allows for the creation of structures that have multiple layers, which can help improve both the insulation qualities of heat and sound, and are typically designed after observing nature's designs. By following the basic principles of natural design and taking into account how nature has combined many different materials together in order to create stronger and more functional composite materials, we can produce some of the best-performing and most versatile materials on the market today [63]. To date, hierarchical porous structures have been utilized effectively for performing thermal management tasks. Examples include creating ceramics using clay as a base material combined with a foam-like ink that has been stabilized with particles, resulting in creating hierarchical structures with micropores that are controlled by the temperature at which the ceramics are sintered and that create thermal insulating and evaporative cooling properties [64]. In addition, the use of cellular structures created by 3D printing has enabled lightweight products to be made from materials that combine a combination of high mechanical strengths and specific thermal and acoustic response characteristics based on porosity [65]. One extremely unique and innovative application developed is the development of hybrid silica voxels, which are porous silica particles combined with elastomeric materials. These hybrid silica voxels have been demonstrated to have a very low thermal conductivity (19.1 (W/m·K)) and possess mechanical flexibility with tunable strength (71.6 kPa to 1.5 Mpa) [66-76]. These innovative approaches hold great potential for building thermal management applications as well as battery thermal aging.

6.2 Smart and 4D printing for adaptive insulation

Smart polymers with shape memory, self-healing, or stimuli-responsive porosity features can be the reason for adaptive insulation, e.g., tunable vents or panels that stiffen under dynamic loads [67-70]. Low electrical, magnetic, or photothermal power can be used for indirect heating to actuate 4D printed elements [71, 72]. But it will take a hefty amount of progress in material robustness, safety, and lifecycle validation to make this technology feasible for applications at the scale of buildings.

Four-dimensional printing is an emerging additive manufacturing technology that combines 3D printing along with the usage of smart materials that undergo time-dependent transformations in response to a variety of external stimuli such as heat, moisture, pH, magnetic fields, or light [73, 74]. The range of applications includes deployable structures for the polar regions, tissue engineering, and drug delivery systems [75, 76], thus, the potential of building envelopes that adjust to environmental conditions.

6.3 Benchmarks and comparative studies

The advent of functional 3D printing is an innovative technology for creating multidimensional functional materials that can be tailored to numerous applications, such as sensors, actuators, and construction materials [38]. Thermal conductivity values have been reported for a variety of AM processes and materials ranging from 0.03 to 0.09 (W/m·K), e.g., silivoxel composites, cellulose nanocrystals, PU-cork, etc., and PLA lattices can exhibit k ≈ 0.037 (W/m·K) and STL ≈ 48 (dB) [11]. Based on LCA data, there are potential energy savings ranging from 45% to over 67% over an 8.5-14.8-year payback time with optimized panel thicknesses of approximately 4-10 mm for typical building conditions [11]. A new category of functional composite materials that incorporate either conductive or sensing components has great potential for developing integrated condition monitoring solutions [74].