Preparation, Properties, and Application in Zero-Energy Buildings of Compostable, Fully Biobased Foam Materials Based on Polylactic Acid (PLA) and Microcellulose Fibers (MCF)

Abstract

With the increasing awareness of environmental protection, the construction industry has a growing demand for biobased foam materials. This paper focuses on compostable, fully biobased foam materials prepared from polylactic acid (PLA) and microcellulose fibers (MCF) using supercritical carbon dioxide (sc-CO₂) physical foaming. The influence of MCF on the structure and properties of the foam materials is explored, and their application potential in zero-energy buildings is evaluated. The research indicates that the foam materials have certain advantages in degradability and thermal insulation performance, and although their energy consumption is slightly higher than that of traditional materials, they still demonstrate good application prospects.

Keywords
#Polylactic Acid; #Microcellulose Fibers; #Biobased Foam; #Compostability; #Zero-Energy Buildings

1. Introduction

The development of the construction industry has brought about energy consumption and environmental issues. Traditional petroleum-based polymer foams, such as polyurethane and polystyrene, are widely used in building insulation, but their non-degradability poses a threat to the environment. Biobased polymer foams have attracted considerable attention as alternatives, with PLA becoming a research hotspot due to its advantages of renewable raw materials and low production energy consumption. MCF, characterized by high modulus and large specific surface area, is often used as a reinforcing material to improve polymer performance. This paper aims to study the properties of PLA/MCF foam materials and evaluate their application effects in zero-energy buildings.

2. Experimental Section

2.1 Experimental Materials

Amorphous PLA (Ingeo 4060D) from NatureWorks LLC was selected, with a d-lactide content of 12%, a density of 1.24 g/cm³, and a melting point of 210°C. MCF was provided by TENSTECH Inc. NC, sourced from specific wood pulp and ground to obtain fibers with a size of 10-120 nm and an aspect ratio of 10, with an effective powder density of 0.47 g/cm³. Carbon dioxide with a purity of 99.9% was used as the foaming agent. PLA/MCF blends were prepared in different proportions and labeled as PLA_X (X = 0, A, B, C, corresponding to 0 wt.%, 1.5 wt.%, 2.25 wt.%, and 3 wt.% MCF, respectively).

2.2 Foaming Process

PLA and MCF were melt-blended using a twin-screw extruder to prepare compression-molded samples with a diameter of 12.7 mm and a thickness of 1.5 mm. A two-step carbon dioxide pressure-drop foaming method was employed. The samples were first placed in a pressure vessel preheated to 70°C, where carbon dioxide was introduced and maintained for 5 hours (saturation temperature of 70°C, saturation pressure of 11.72 MPa) to allow the gas to fully dissolve. Subsequently, the temperature was reduced to 58°C, and the pressure was rapidly dropped to 3.45 MPa to promote cell nucleation and growth. A secondary pressure drop was then applied, and the samples were cooled to room temperature to obtain the foam samples.

Figure 1: Schematic diagram of the foaming process

2.3 Performance Testing

(1) Glass Transition Temperature (Tg): The Perkin Elmer DSC 4000 differential scanning calorimeter was used to heat the samples from 30°C to 190°C at a rate of 10°C/min, hold for 2 minutes, and then cool to 30°C at the same rate. This process was repeated for two cycles, and Tg was calculated from the heating scan curve.

(2) Microstructure: The cross-sectional morphology of the samples was observed using an environmental scanning electron microscope (FEI Quanta 200 ESEM) under high vacuum. The frozen-fractured samples were coated with gold/palladium. The foam skin thickness was measured under low vacuum.

(3) Density, Porosity, and Morphological Analysis: According to ASTM standards, an electronic densitometer based on Archimedes' principle was used to measure the density of the unfoamed polymers and foams, and the expansion ratio was calculated. The Ultrapyc 1200e gas displacement porosity meter, based on helium displacement, was used to determine the open porosity. SEM images were analyzed using Image J Pro software to calculate cell density, cell size, and porosity, and then the cell wall thickness was derived.

(4) Mechanical Properties: At room temperature, compression tests were performed on unfoamed composites and foams using the Shimadzu AG-X plus series material testing machine, following ASTM standards. Due to the brittleness of the samples, a crosshead speed of 0.5 mm/min was selected to measure the compressive modulus and compressive strength.

(5) Thermal Conductivity: The Hot-Disk thermal constants analyzer was used to conduct transient thermal conduction tests on foam samples cut to specific sizes, using a Kapton sensor as both the heat source and temperature sensor. The thermal conductivity was automatically calculated over 160 seconds, and multiple measurements were averaged to calculate the thermal resistance R-value.

(6) Biodegradability: According to ASTM D 5388 standards, an automatic multi-unit composting system (AMUCS) was used for composting experiments. The foam samples were mixed with compost and cultured under specific temperature and humidity conditions for 50 days. The moisture content, total solids, and volatile solids content of the compost were regularly measured. Elemental composition before and after composting was determined by CHN elemental analysis, and the carbon dioxide release was calculated to assess the biodegradation rate of the foams.

(7) Energy Simulation: Using the zero-energy research laboratory at the University of North Texas as a model, EnergyPlus software was used to simulate and analyze the heating and cooling energy consumption when PLA/MCF foam materials replaced traditional polyurethane foam as building insulation materials.

3. Results and Discussion

3.1 Effect of MCF on the Glass Transition Temperature of the Foam

As the MCF content increased, the second Tg value of the unfoamed PLA/MCF blends gradually decreased, indicating that MCF had a plasticizing effect on PLA. After foaming, the Tg values of the foams further decreased due to the plasticizing effect of sc-CO₂. The Tg values of all foams were approximately 45±2°C, indicating that the combined effect of sc-CO₂ and MCF influenced the mobility of the polymer chains.

Table 1: First Tg values of solid PLA/MCF composites and second Tg values of foam blends

3.2 Microstructure Analysis

MCF acted as a nucleating agent in the PLA matrix, reducing the free energy required for bubble formation, increasing cell density, and decreasing cell size. Pure PLA foam had a bimodal cell structure, while the addition of MCF resulted in a more uniform cell size distribution. At 1.5 wt.% MCF, the microcellulose fibers were loosely distributed at the PLA interface, reducing the foam density. However, as the MCF content increased, "hornification" aggregation occurred, leading to an increase in the density of the unfoamed composites, a decrease in foam porosity and open porosity, and hardening of the foam, which limited carbon dioxide adsorption and expansion.

Table 2: Foam shape parameters
Figure 2: SEM micrographs and cell size and frequency distribution: (a) PLA_0f, (b) PLA_Af, (c) PLA_Bf, (d) PLA_Cf

3.3 Mechanical Properties

Figure 3 and Table 3 show the compression test results for unfoamed and foamed samples. In unfoamed materials, the addition of an appropriate amount of MCF (such as PLA-A) increased the modulus and strength by 48% and 35%, respectively, but further increases in MCF content resulted in decreased performance. The mechanical properties of the foam materials also showed a trend of increasing and then decreasing. The modulus and strength of the PLA-A foam were higher than those of the unfoamed material, but overall, foaming significantly reduced the modulus and strength of the material (by about 99%), which was related to changes in the foam pore structure and differed from the performance changes observed in polypropylene foams.

Figure 3: Mechanical (compression) properties: (a) unfoamed composites, (b) foam composites
Table 3: Mechanical and thermal properties of PLA/MCF composites with different MCF weight fractions

3.4 Thermal Performance

The addition of MCF affected the density of the unfoamed composites, and as the MCF content increased, the effective thermal conductivity increased linearly. For the foam materials, the foam with 1.5 wt.% MCF had the lowest thermal conductivity (0.04926 W/m∙K), which was lower than that of pure PLA foam. This was because at this content, MCF was located at the interface, increasing the open porosity and porosity, reducing the density, and improving the thermal insulation performance. However, as the MCF content further increased, its aggregation led to an increase in cell wall thickness and skin thickness, resulting in an increase in effective thermal conductivity. This conclusion was