Foam Injection Molding of Poly(Lactic Acid) with Azodicarbonamide-Based Chemical Blowing Agent ^†

Abstract

In this study, poly(lactic acid) (PLA)-based biopolymer foams were prepared through injection molding using the high-pressure foam injection molding method, also referred to as “breathing mold” technique, with the addition of various amounts (0, 1, 2, and 4 wt.%) of azodicarbonamide-based chemical blowing agent (CBA). The prepared samples were analyzed for their macrostructure using computed tomography (CT) while the mechanical properties were determined by flexural and Charpy impact tests. CT analysis revealed a finer foam cell structure and decreasing shell thickness with increasing CBA content. Regarding the mechanical properties, the specific flexural strength and flexural modulus of PLA were improved as a result of foaming; however, this improvement came at the cost of a slight deterioration in impact strength.

1. Introduction

As a consequence of strict emission and fuel economy regulations, there is a global demand for high-strength lightweight materials, particularly in the automotive industry. Among the many approaches to achieving these goals, reducing the weight of vehicle components is the most significant [1]. A 10% reduction in weight is estimated to decrease CO₂ emissions by 8%, reduce fuel consumption by 5.6–7.6%, and improve the electric range by 13.7% [2]. Therefore, industrial players are motivated to replace metallic components with polymeric counterparts to achieve significant weight savings and thereby comply with stringent governmental and international regulations. To also meet green technology targets in vehicle manufacturing, the industry can shift to using renewable resource-based materials, i.e., biopolymers that exhibit properties equal to or superior to conventional plastics [3,4], while offering other advantageous traits as well. In order to further reduce the weight of fabricated parts, manufacturers of automotive components have adopted foaming as a rational approach during the processing of plastics by forming gas voids in the polymer [1].

Among biopolymers, poly(lactic acid) (PLA) is considered a frontrunner due to its favorable mechanical, physical, and optical properties, as well as its biodegradability [5,6,7]. PLA is a thermoplastic polyester synthesized from lactic acid, which is obtained through the fermentation of carbohydrate-rich renewable resources such as cassava, rice, potatoes, wheat, sugar cane, and sugar beets [8]. However, it also exhibits some major drawbacks, including brittleness, high production costs, and low melt strength. This latter disadvantage is a major challenge when it comes to the foaming of PLA [9].

Foamed plastics can be prepared by various physical, chemical, and bead foaming techniques [10]. Among these, chemical foaming is a widely used method because it does not require costly machinery; polymer foams can be prepared on the same equipment that is used for common polymer processing (extruders, injection molding machines, etc.). Chemical foaming works on the principle that a substance called a chemical blowing agent (CBA) is added into the hopper of the processing machine along with the polymer pellets, and the CBA decomposes and releases gas or gases at a specific processing temperature [11]. When choosing the appropriate CBA, a major consideration is the decomposition temperature of the substance, which should preferably be within the processing temperature range of the polymer. During the chemical foaming of PLA, a commonly used CBA is azodicarbonamide (ADC). ADC has a decomposition temperature of 230 °C; however, it can be tailored by combining it with various activators to be more suitable for specific polymers.

Chemical foaming is a technique that is suitable for use during injection molding. Chemical foam injection molding techniques can be classified into two major groups, namely low- and high-pressure foam injection molding [9]. In the case of low-pressure foam injection molding, the cavity of the mold is only partially filled with the polymer melt, and it becomes fully filled as a consequence of the foaming process. On the other hand, during high-pressure foam injection molding, the mold is entirely filled with the melt, and the foaming is allowed by slightly opening the mold, hence the name “breathing mold” technique. The breathing mold method has been successfully applied in the foam injection molding of various polymers, including acrylonitrile-butadiene-styrene [11] and poly(ethylene terephthalate) [12]; however, the processing of PLA foam using this technique is yet to be investigated.

To provide insights into the applicability of ADC as CBA during the foam injection molding of PLA, foams were prepared using the breathing mold technique by varying the amount of the blowing agent in the range of 1 to 4 wt.%. Additionally, bulk PLA samples were also manufactured to serve as a reference. The fabricated specimens were examined for their macrostructure and mechanical properties. For the analysis of the macrostructure, computed tomography (CT) was employed while the mechanical properties were determined by means of flexural and Charpy impact tests.

2. Materials and Methods

2.1. Materials

Poly(lactic acid) Ingeo 3052D was purchased from NatureWorks LLC in pellet form. It exhibits a density of 1.24 g/cm³, a melt flow rate (MFR) of 14 g/10 min, and a melting temperature of 145–160 °C. The foaming agent used in this study was a Tracel IM 3170 MS type additive, kindly supplied by Interdist Ltd. This grade of CBA has an ADC content of 30%, a decomposition temperature of ~170 °C, and a gas yield of 50 mL/g (at 220 °C). Its effective gases are N₂, CO, CO₂, and NH₃.

2.2. Foam Injection Molding

Prior to processing, both components were dehumidified in a Faithful WGLL-125BE drying chamber (Huanghua, Cangzhou, China) at 80 °C for 4 h. The chemical foaming of PLA was carried out using the breathing mold injection molding technique with an Arburg Allrounder 420C machine (Lossburg, Germany). During the process, the nozzle temperature was set to 200 °C, the mold temperature was maintained at 40 °C, and the injection speed was 40 cm³/min. After filling the cavity, the mold was opened by 0.4 mm to allow the formation of a foamed structure through the decreased pressure. In this way, dumbbell-shaped type 1A specimens were prepared according to ISO 527-2 [13] Overall, four different sets of specimens were fabricated, containing 0, 1, 2, and 4 wt.% of CBA.

2.3. Characterization

The density of the fabricated samples was determined according to the Archimedes principle following the standard ISO 1183-1 [14], using a Kern ABP100-5DM analytical balance (Kern & Sohn, Balingen, Germany) equipped with a YDB03 density measurement kit (Kern & Sohn, Balingen, Germany).

The cellular structure of the foamed samples was examined by computed tomography (CT). For this purpose, CT scans were completed using an YXLON-Modular Y.CT system (Yxlon, Hamburg, Germany) on 5 mm long sections from the middle of the rectangular parts of the dumbbell specimens. The scanner was operated at 190 kV and 0.13 mA tube voltage and current. Projections were reconstructed with a voxel resolution of 7.6 µm. The data were processed using VGStudio MAX 3.4 software to provide a volumetric porosity metric for the samples, consisting of individual cell volumes, cell count, and overall porosity ratio.

Flexural tests were carried out using an Instron 5582 testing machine (Instron, Norwood, MA, USA) with rectangular specimens of 80 × 10 × 4 mm³ according to the ISO 178 standard [15]. The tests were performed under ambient conditions with a span length of 64 mm and a crosshead speed of 5 mm/min. The reported values were obtained by averaging the results of at least five specimens for each composition.

Specimens with a rectangular shape (size: 80 × 10 × 4 mm³) were used to determine the impact strength of the samples. The Charpy impact tests were performed on unnotched samples using a CEAST pendulum impact tester model 6545 (Pianezza, Italy) equipped with an impact hammer of 2 J energy at room temperature according to the ISO 179 standard [16]. The reported values were obtained by averaging the results of at least five specimens for each composition.

In addition to determining the absolute parameters, specific mechanical properties of the fabricated materials were also calculated. The specificity was based on the density of the samples as shown in Equation (1), as follows:

$$SX={\displaystyle \frac{X}{\rho }},$$

(1)

where ρ is the density; SX stands for the specific flexural strength, specific flexural modulus, or specific impacts strength; and X indicates the absolute value of the corresponding parameter.

3. Results and Discussions

3.1. Density and Foam Structure

Table 1. Density of the manufactured PLA foams at different CBA contents and their relative reduction compared to the bulk polymer.

CBA content [wt.%]	0	1	2	4
Density [g/cm3]	1.24	1.18	1.17	1.15
Density reduction [%]	0	5.1	5.8	7.5

shows the effect of CBA loading on the density of PLA and the relative reduction compared to the bulk polymer. The density of the non-foamed polymer was 1.24 g/cm³, aligning with the values provided in the official datasheet. By adding 1 wt.% foaming agent, the density decreased to 1.18 g/cm³, while in the case of 2 and 4 wt.% CBA, it dropped to 1.17 and 1.15 g/cm³, respectively. Accordingly, the relative reduction in density was 5.1%, 5.8%, and 7.5%.

Visual representations of the cells in the cross-sections of the different samples, based on the CT scans, are shown in Figure 1. The reconstructed 3D models (Figure 1a–c) are presented in a cross-section perpendicular to the melt flow direction, while specific scan images are shown for all samples parallel to the melt flow direction (Figure 1d–f). In the 3D reconstructed model, a color code was applied to differentiate the cells based on the size of their volumes. It can be concluded that the lowest amount (1 wt.%) of CBA (Figure 1a,d) resulted in the largest cells, while the number of cells was the lowest in this case; a total of 3033 individual voids were detected in the analyzed section of 5 mm length. By increasing the concentration of CBA in the polymer, the size of cells gradually decreased and their count increased, resulting in a finer cell structure. Specimens prepared with 2 and 4 wt.% foaming agent contained 7596 and 48,202 individual voids, respectively. It can be concluded that with the growing CBA concentration, the number of nucleated cells also increases. The porosity of the samples was also determined by calculating the total volume of the voids relative to the volume of the analyzed section. The porosity was found to be 4.15%, 4.75%, and 9.06% for samples containing 1, 2, and 4 wt.% CBA. The porosity values are consistent with the density reduction values presented in Table 1. In addition to the finer cell structure, the shell of the foamed parts, consisting of bulk polymer, also decreased, as indicated in Figure 1a–c.

Distribution diagrams of the cells based on their sizes are presented in Figure 2 for the three foamed samples. It can be observed that for all CBA concentrations, the number of cells decreased with increasing size; however, in the case of 1 wt.% (Figure 2a) and 2 wt.% (Figure 2b) foaming agent, a significant proportion of larger (>0.006 mm³) voids can be found, which contribute most to the overall porosity. On the other hand, the sample prepared with 4 wt.% CBA appears to contain a large number of small-volume cells.

3.2. Flexural and Impact Mechanical Properties

The quasi-static flexural and impact mechanical properties of the prepared samples as a function of CBA content are presented in Figure 3. Figure 3a shows the flexural strength and the specific flexural strength values. Bulk PLA exhibited the highest flexural strength of 102 MPa, which decreased when foaming was performed. Among the foamed samples, the lowest strength values were found for those with 1 and 2 wt.% CBA. It is important to note, however, that the foam structure did not compromise the strength of PLA, as the relative reduction in flexural strength was approximately proportional to the reduction in density showcased in Table 1. This fact can also be validated by comparing the specific strength values, which were very similar for bulk PLA and its 1–2 wt.% CBA-containing foams. Interestingly, the foam prepared with 4 wt.% foaming agent exhibited higher flexural strength than the other two foamed samples, which can be ascribed to its much finer cell structure as presented in Figure 1. Consequently, its specific strength became even higher than that of bulk PLA. Litauszki et al. [10] prepared PLA foams of identical composition using extrusion as a processing technique. The authors reported similar findings, namely a finer cell structure at 4 wt.% CBA; however, in their case, some cell collapse was also detected at this CBA loading. Apparently, the lower residence time when performing injection molding prevents this phenomenon from occurring, ultimately leading to more advantageous flexural mechanical properties. The flexural modulus (Figure 3b) of all samples, regardless of the CBA loading, was rather similar at ~3.4 GPa. However, with increased porosity, the specific modulus values gradually increased from 2.7 GPa to 3.0 GPa. It is assumed that the increasing cell wall orientation due to the increasing total specific surface area of smaller cells was responsible for the improved flexural modulus.

Figure 3c shows the impact strength values of bulk PLA and the PLA foams prepared with 1–4 wt.% foaming agents. Based on the results, the presence of voids inherently decreased the toughness of PLA. It is assumed that during an impact load, the voids act as failure sites, leading to the early failure of the material. Apparently, rather than the size of the cells, the overall porosity was more prominent considering the impact resistance of PLA, as evidenced by a gradually decreasing impact strength with increasing CBA content, regardless of cell sizes.

4. Conclusions

In this study, PLA-based injection molded biopolymer foams were prepared through chemical foaming using the “breathing mold” injection molding technique by varying the CBA concentration in the range of 0–4 wt.%. Subsequently, the samples were characterized for their physical, macrostructural, and mechanical properties. It was found that with increasing CBA loading, the average size of the cells gradually decreases while their number exponentially grows, resulting in a much finer cell structure. Consequently, the sample containing 4 wt.% CBA exhibited the most beneficial specific flexural strength and modulus values, even surpassing those of bulk PLA, suggesting that these biopolymer foams could serve as an excellent sustainable alternative to conventional petrochemical plastics in the automotive industry. However, the impact strength of the samples gradually decreased with increasing porosity without showing any clear correlation with cell size.