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Article

The Development of Sustainable Polyoxymethylene (POM)-Based Composites by the Introduction of Natural Fillers and Melt Blending with Poly(lactic acid)-PLA

by
Anna Soćko
1,2 and
Jacek Andrzejewski
1,*
1
Institute of Materials Technology, Faculty of Mechanical Engineering, Poznan University of Technology, Piotrowo 3 Street, 61-138 Poznan, Poland
2
Faculty of Mechanical Engineering, Poznan University of Technology, Piotrowo 3 Street, 60-965 Poznan, Poland
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2024, 8(8), 315; https://doi.org/10.3390/jcs8080315
Submission received: 14 July 2024 / Revised: 26 July 2024 / Accepted: 8 August 2024 / Published: 10 August 2024
(This article belongs to the Special Issue Progress in Polymer Composites, Volume III)
Browse Figures
Versions Notes

Abstract

:
The conducted study was focused on the development of a new type of technical blend reinforced with natural fillers. The study was divided into two parts, where, in the first stage of the research, unmodified POM was reinforced with different types of natural fillers: cellulose, wood flour, and husk particles. In order to select the type of filler intended for further modification, the mechanical characteristics were assessed. The 20% wood flour (WF) filler system was selected as the reinforcement. The second stage of research involved the use of a combination of polyoxymethylene POM and poly(lactic acid) PLA. The POM/PLA blend (ratio 50/50%) was modified with an elastomeric compound (EBA) and chain extender as the compatibilized reactive (CE). The microscopic analysis revealed that for the POM/PLA system, the filler–matrix interface is characterized by better wettability, which might suggest higher adhesion. The mechanical performance revealed that for POM/PLA-based composites, the properties were very close to the results for POM-WF composites; however, there is still a significant difference in thermal resistance in favor of POM-based materials. The increase in thermomechanical properties for POM/PLA composites occurs after heat treatment. The increasing crystallinity of the PLA phase allows for a significant increase in the heat deflection temperature (HDT), even above 125 °C.

1. Introduction

The development of sustainable types of materials is one of the most common trends in the materials technology of polymer composites. There are a few aspects of the sustainability problems for polymer composites. The first issue is connected with the petroleum-based nature of most of the thermoplastic and thermoset polymers. The second problem concerns the negative environmental impact of the synthesis and preparation methods for polymer-based materials. The last issue regards the problem of polymer waste management [1,2,3,4,5]. The listed problems are related to the polymer processing industry in general; however, for engineering composites, the listed issues should be more exposed to discussion. Unlike the packaging production industry, where plastics processing has been focused on reducing the negative impact on the environment for many years [6,7,8,9,10], the serial production of polymer composites is at the very beginning of the process of conversion to sustainability. An example of the problems discussed is materials produced by injection molding. Materials of this type are used in many areas of the economy, such as machine construction, electrical engineering, transport, or electronics. However, there are still no systemic solutions to many problematic aspects of the use of this type of material [6,11].
Previous research adopts several concepts that can be implemented in industrial practice, and will allow for the reduction of the negative environmental effects of the production of technical composites. The most popular pro-environmental strategy, already widely used in many companies, is the management of post-production waste or post-consumer polymers. Research in this area is carried out in almost every variety of thermoplastic polymers. The advantages of this concept include the ability to quickly implement new material solutions, the simple optimization of material properties, and economic benefits for manufacturers. However, material recycling has certain limitations related to the availability of good-quality polymer materials and the inability to meet selected quality standards in the food or medical industries.
Currently, plastics producers and processors are paying attention not only to the need to recycle polymer composites, but also to the creation of materials with a lower rate of use of petrochemical semi-finished products. Materials of this type include numerous varieties of composites based on biobased polyamides synthesized from vegetable oils [12,13,14], polyolefins [15], or thermoplastic polyesters synthesized from substrates that are the product of the fermentation of natural polysaccharides [16,17,18]. This concept differs significantly from the commonly known strategy involving attempts to use compostable polymers, where products made of thermoplastics are to be disintegrated during storage. This strategy has been strongly criticized for many years for several reasons [15,19,20,21]:
-
A lack of effective methods for separating composted polymers from traditional materials;
-
Requirements for the conditions of an effective composting process;
-
The use of prooxidative additives with traditional thermoplastics (PE/PP) instead of compostable polymers;
-
Difficulties in the recycling procedure due to a lack of compatibility/miscibility with traditional polymers;
-
A lack of sufficient functional properties to replace petroleum-based materials.
The given reasons mean that the concept of using bio-based materials, which are not necessarily suitable for composting, is currently of greater interest.
The development of techniques for producing polymer composites, apart from numerous examples of research on polymer synthesis, currently also focuses on the possibility of developing new types of polymer fillers. In this field, very positive results are obtained for materials of secondary origin, such as carbon and glass fibers recovered in the chemical and thermal decomposition processes of the polymer matrix [22,23,24,25,26]. In addition to the possibility of recycling synthetic fibrous materials, great interest has been observed for many years in the area of natural fiber-based materials processing. Natural fibers that constitute a potentially effective and competitive type of reinforcement include materials based on linen (flax), hemp, bamboo, or cotton; in selected applications, they can be used in the automotive and construction industries [27,28,29]. However, in practical large-volume applications, powdered materials are more commonly used. Wood fibers, sawdust, and cellulose particles are the most popular examples of such additives. They are often a by-product resulting from the processing of wood or other types of lignocellulosic materials, like leaves, shells, or husk. Unlike long fibers, wood-based fillers can be processed without additional processing, similar to other plastic additives such as mineral fillers or concentrates in the form of granules.
The research work that is the subject of this study presents a method of preparing materials based on polyoxymethylene and its blends with PLA and natural fillers. The presented concept for developing composite materials is a combination of two operational strategies. The first is the concept of replacing synthetic varieties of polymer fillers with lignocellulosic materials, while the second is the process of mixing petrochemical POM with partially biobased PLA. A similar approach has already been used for the preparation of PLA-based blends, where the second component was petroleum-based polymers like acrylonitrile-butadiene-styrene (ABS) [30,31], polycarbonate (PC) [32,33,34], or polybutylene terephthalate/polytrimethylene terephthalate (PBT/PTT) [35,36]. The already-investigated examples of mixed bio- and petroleum-based blends reveal that in most cases, the properties of original unmodified blends can be strongly improved by the addition of reactive compatibilizers and elastomeric impact modifiers. However, still, the open question concerns the preparation of a more complex composite composition.
The works discussed are an extension of previously conducted research on the modification of POM/PLA mixture systems [37]. In the current case, the research work was divided into two stages, where composite materials based on unmodified POM were first prepared. Composite additives in the form of wood flour, cellulose, and ground grain husk were introduced in amounts ranging from 10% to 20% and 30%, to allow for a general assessment of the mechanical characteristics of the prepared materials. In the second stage of work, the polymer matrix was modified. Three types of POM modifications were prepared: a POM/PLA blend, POM/PLA with the addition of an elastomer, and a POM/PLA/elastomer blend prepared in the reactive extrusion process. For the developed materials, one of the aspects of the work was to obtain good thermomechanical properties. Therefore, some of the materials were additionally thermally treated (annealed), the intention of which was increasing the level of crystallinity of the matrix and increasing thermal resistance.
A short list discussing the main abbreviations contained in the text is presented below:
POM—polyoxymethylene (polyformaldehyde);
PLA—poly(lactic acid);
CF/WF/BH—cellulose flour/wood flour/buckwheat husk;
EBA—ethylene-butyl acrylate-glycidyl methacrylate copolymer (E/BA/GMA);
DMTA—dynamic mechanical, thermal analysis;
DSC—differential scanning calorimetry;
SEM—scanning electron microscopy.

2. Materials and Methods

2.1. Materials

The polyoxymethylene (POM) resin type used for the study was Tarnoform 300 from Azoty SA (melt flow rate (MFR) = 9 g/10 min (2.16 kg/190 °C)). The poly(lactic acid) (PLA) type was Ingeo 3251D (NatureWorks); MFR = 35 g/10 min (2.16 kg/190 °C). The elastomeric compound, referred to as EBA, was ethylene-butyl acrylate-glycidyl methacrylate copolymer E/BA/GMA, supplied by Du Pont under the trade name of Elvaloy PTW. The chain extender (CE) styrene-acrylic oligomer was used as the reactive extrusion compatibilization agent, grade Joncryl 4368C (BASF, Ludwigshafen, Germany). Four types of natural fillers were used for the composite preparation. The conifer tree-based wood flour (WF), Lignocell C120, had a particle size of 70–150 μm, and a bulk density 100–135 g/dm3. The cellulose flour (CF) Arbocel FD 600/30 had a fiber length of 45 µm, fiber thickness of 25 µm, and bulk density of 220–280 g/dm3. A detailed description of the wood and cellulose flour has already been given in our previous paper [38]. The buckwheat husk particles (BH), supplied by the Sante company, were melt-sieved and divided into two types, where coarse particles had a size above 0.8 mm, and fine particles were below that size. The characterization of the milled BH particles has already been discussed in the literature [39,40,41].

2.2. Sample Preparation

The materials’ compounding was conducted using the twin-screw extrusion method; we used a Zamak EH16.2D-type machine equipped with 16 mm-diameter screws (Zamak Mercator, Skawina, Poland). The process was conducted at a temperature of 200 °C and a screw speed of 80 rpm. Before processing, the natural fillers were dried for 12 h at 90 °C. Pellets were dried separately for 6 h—the POM resin at 90 °C, and the PLA resin at 60 °C. All materials were initially dry-blended and placed inside the machine hopper. The melt-blended materials were cooled down using a water bath and pelletized. Before the next processing step, the pellets were dried again at 80 °C for 6 h. The list of samples prepared during the compounding procedure is presented in the Table 1, the collected data includes the percentage composition of individual materials.
Samples were prepared using the injection molding method, where an Engel ES 80/20 HLS hydraulic press was used. Samples were molded using the standard procedure, where the injection temperature was set to 210 °C, the mold temperature was 40 °C, the injection/holding pressure was 1050/550 bar, and the holding/cooling time was 10/30 s. Parts of the samples were treated by the annealing procedure; we used a cabined oven, wherein the temperature was set to 120 °C, and the time of annealing was 3 h.

2.3. Characterization

The mechanical properties of the prepared materials were characterized using the universal testing machine, model Zwick/Roell Z010. (Zwick/Roell GmbH, Ulm, Germany) The tensile tests were conducted according to ISO 527 standards. The impact resistance measurements were conducted using the notched Izod method, according to ISO 180 standard [42,43].
Thermomechanical properties were evaluated using the dynamic mechanical thermal analysis method (DMTA); we used Anton Paar MCR301 apparatus (Anton Paar GmbH, Graz, Austria) equipped with a torsion clamp system. The tests were conducted from 25 °C to 150 °C, with a heating rate of 2 °C/min. The strain amplitude was set to 0.01%, while the frequency was 1 Hz. The additional HDT (heat deflection temperature) test was conducted using the HDT/Vicat RV300C machine (Testlab, Warsaw, Poland), wherein the measurements were performed according to ISO 75 standard [44].
The DSC analysis was performed using the Netzsch Phoenix 204 F1 apparatus (Netzsch Geratebau GmbH, Selb, Germany). The measurements were carried out under a protective nitrogen atmosphere, with a flow rate of 10 mL/min. Tests were conducted using a standard heating/cooling/heating procedure, with the test temperature ranging from 20 to 230 °C and the heating/cooling rate set to 10 °C/min. Due to the complex nature of the prepared blend, the crystallinity level was not calculated, and the results have been presented in the form of first heating thermograms.
The fractured surface of the impact test sample was subjected to SEM analysis. The specimen surface was sputter-coated with a layer of gold, and the analysis was performed using a Carl Zeiss EVO 40 apparatus (Carl Zeiss GmbH, Jena, Germany).

3. Results

3.1. Mechanical Performance Evaluation—Static Tensile Tests, Charpy Impact Resistance

The properties of the manufactured composites are presented separately for POM-based composites (see Figure 1) and modified POM-PLA blend materials (Figure 2). The results of the initial investigation for POM composites present the mechanical characteristics of materials with the addition of 10%, 20%, and 30% of different types of natural fillers. The results for POM/PLA blends (Figure 2) reflect the properties of the unreinforced blends and 20% wood flour (WF) composites. Many factors justify the selection of 20% wood fiber composition, and the first one is the price and availability of wood fiber-based materials. The reason for BH filler exclusion is the aesthetic properties, as the color of BH-modified samples was visibly changed compared to wood and cellulose fillers. The color appearance of the injection-molded materials needs to be considered as significant, since limitations in color possibilities strongly limit the application potential of most producers. Besides other reasons for WF filler selection, the number of samples required for a full set of filler types will increase drastically when introducing the PLA/EBA/CE modification system. We assume that the properties of the WF-based composite system will reveal the main differences between the pure POM and matrix-modified composites. The results should relatively effectively reflect the main changes in properties for CF- and BH-based composites.
The combination of highly crystalline POM resin and natural fillers results in visible stiffness improvement. The observed changes were significantly lower than those in the reinforcing efficiency of glass or carbon fibers [45,46,47], while the highest value of the tensile modulus was close to 3400 MPa, compared to 2100 MPa for unmodified POM. Interestingly, the differences between the individual varieties of fillers used are small despite large differences in the particle morphology. The fibrous nature of the cellulose and wood flour particles should result in a more effective reinforcement effect, but the examples presented will not confirm this theory. The analysis of the tensile strength values confirms the lack of favorable changes for composites. The strength for all of the prepared composites is visibly lower than for the reference POM samples (58 MPa). The results recorded for samples with 30% filler loading are close to 35 MPa, which means that the reduction reached around 40%. It is clear that this kind of behavior must be considered as an important drawback. Similarly to the changes observed for the tensile modulus, the differences for individual types of fillers are very slight; more evident are the changes caused by the filler content. An increase in the tensile modulus value is an obvious consequence of the rule of mixture principle, and occurs for most of the composite materials even when the other mechanical factors are strongly reduced. In the case of natural fillers, this issue is more complex, because apart from the fundamental influence of the filler content and its own stiffness (modulus), other factors may be decisive, such as fiber orientation, particle morphology and porosity [48,49,50,51].
The results of the elongation at break and Charpy impact strength confirm that the presence of natural filler leads to ductility deterioration. The strain at break for pure POM was close to 23%, while even the 10% content of the filler particles reduced this value to around 4%. When the filler content was increased, the elongation was even lower, while for POM/30% filler specimens, the recorded elongation was below 2%. Interestingly, in the impact strength results, the downward trend was not revealed, apart from the obvious difference between the results for unmodified POM (6.5 kJ/m2) and the composite samples. The Charpy impact strength ranged from 1.5 kJ/m2 to 3.0 kJ/m2, which results were already reported for composites prepared from the selected POM resin type [52,53]. Summarizing the results of mechanical properties for the initial part of the research, it is worth noting that none of the used types of filler revealed a visible advantage. The expected favorable properties of fibrous fillers, cellulose, and wood flour were not confirmed by the results. The second stage of research was conducted using 20% wood flour. Considering the price and availability of this type of material, it seems to be the optimal solution.
For the main part of the study, the prepared blends were compared to the pure POM materials. The results collected in Figure 2 show the changes in mechanical characteristics after the introduction of PLA and modifications in the reference POM. The plots show the results for unfilled blends and WF-reinforced materials. For the tensile modulus, it is clear that the addition of the PLA component leads to visible stiffness improvement, which was observed for both the POM/PLA blend and the POM/PLA-WF20 composite. As expected, for EBA-modified samples, the tensile modulus was reduced, which is typical behavior for elastomer-toughened materials, especially for dedicated copolymers like E/BA/GMA compound [35,54,55,56]. Similar to stiffness, the tensile strength results for toughened samples are visibly reduced. Interestingly, for EBA-modified specimens, the tensile strength values for unloaded and WF-reinforced samples are very similar. In contrast, for pure POM and POM/PLA materials, there is a visible difference in tensile strength values, where the results for WF-reinforced samples are slightly lower for unloaded blends. This effect usually occurs for materials with a high degree of compatibilization, in particular for biocomposites subjected to surface modification of the filler or reactive processes during processing [57,58,59,60]. However, for the prepared materials, the phenomena occurring at the boundary of the matrix–filler phases do not significantly determine the properties due to the relatively low content of wood flour, much lower than for extruded WPC materials [60,61]. Since the main reason for the addition of the E/BA/GMA compound and CE reactive compound was the compatibilization and toughening of the POM/PLA blend system, the most important results of the modification are reflected by elongation at break and impact strength results [62,63,64]. The maximum strain of 5% for POM/PLA blends was significantly reduced compared to the reference POM (22%). The main reason for this is the lack of ductility for PLA components, since the elongation for pure PLA reached only 4%. As confirmed by our previous study and the other research reports [37,65,66,67], the miscibility of the POM/PLA blends is relatively high, which suggests the partial self-compatibilization of this polymer system.
Unfortunately, for the described blend system, good miscibility does not translate into the possibility of obtaining optimized mechanical properties. Therefore, the improvement in elongation value occurs only after the introduction of the elastomeric phase. The addition of the EBA component led to a large improvement in toughness since the elongation at break for the POM/PLA/EBA and POM/PLA/EBA/CE samples strongly increased, reaching at least 15%. The trends observed for the stain values are simultaneously reflected for impact strength. Taking into account the results for the POM/PLA blends themselves, the visible effectiveness of the used modifications was revealed. However, in the case of the conducted research, the key aspect is the addition of natural fillers. In this context, the elongation at break and impact strength indicate that the type of used matrix is not decisive, since both results indicate a very high brittleness of composites with the addition of wood flour. The recorded strain at break results for WF-modified materials is below 4%, lower than the result for the unmodified POM/PLA blend. The same conclusion applies to the impact test results, where the results for all composite materials ranged from 2.5 kJ/m2 to 3 kJ/m2. Such results clearly indicate that in the case of the tested composites based on the POM/PLA system, it is not advisable to use them in applications where impact strength will be a key operational parameter. Taking into account that filled materials have increased stiffness and strength comparable to unreinforced mixtures, the use of this type of system still seems reasonable. Considering that the most important reason for using a PLA-based matrix in the discussed research was the bioderived nature of this polymer, the obtained results confirm that the POM/PLA system can be used as an effective matrix for natural fibers.

3.2. Structure Evaluation—Scanning Electron Microscopy Observations

The structure of the obtained composites was characterized using the scanning electron microscopy method, where the investigated surface was derived from the impact-fractured samples. The structural appearance of the POM-based composite is revealed in Figure 3A–D. The obtained pictures present the general view for different types of fillers and the loading contents of 10% and 30%. The POM/PLA-based composites are presented in Figure 3E, and the modified samples are presented with higher magnification to reveal the WF–filler matrix interphase.
The differences in the structural appearance of fibrous cellulose and wood fibers are negligible. For both types of fillers, the 10% loaded samples have similar irregular surfaces in the fracture, and the appearance of the fibers is manifested by the presence of relatively randomly distributed short fibers. The pull-out mechanism typical for synthetic fibers, like carbon or glass fibers, cannot be observed. Due to the relatively low length-to-width ratio of cellulose and wood filler, the used natural fibers behave in a manner similar to mineral particles with spherical morphology. This is even more visible for materials with 30% filler content, where the structure takes on a more chaotic appearance. Interestingly, the fibers do not agglomerate and are usually evenly distributed in the matrix. The structures of materials with the addition of BH particles have a different character. The size of the filler particles is visibly larger since the size of the coarse particles can reach around 1.4 mm, which was the size of the sieve used during the preparation procedure. The surface of the matrix component is more smooth, which partly confirms the presence of more brittleness; the distance between the BH particles is much bigger than for wood or cellulose fibers. Interestingly, there is no visible difference between the materials with 10% and 30% of the BH filler, which is mainly due to the sizes of the filler particles themselves, since they occupy a significant volume of the observed section of the sample. In summarizing the microstructure analysis, it is worth emphasizing that fibrous reinforcement seems to be more favorable in the context of structure homogeneity. It is worth adding that the large particle size of the BH filler has a significant negative impact here because, in previous works, it was possible to obtain a more homogeneous composite structure [39,68].
The modification of the matrix blend using the PLA component was performed in order to increase the sustainability of the composite system. However, the previously reported results suggest some additional features like self-compatibility of the blend after EBA addition [38]. That phenomenon was already confirmed for many other types of thermoplastic blends, since the addition of a polyester component leads to the formation of copolymers, especially when the reactive component of the modifier forms the chemical bond with the PLA hydroxyl end groups [33,35,69]. Interestingly, for the prepared blends, the self-compatibilization of the unmodified POM-PLA blend was confirmed since the matrix structure was characterized by a smooth surface and a lack of visible phase separation. That kind of behavior was already described in our previous research, while for the presented study, we focused on the evaluation of the WF filler/matrix interactions. It seems that the direct comparison of the POM-WF and POM/PL-WF composites revealed a difference in filler surface wettability. For the POM-based samples, the boundary region was cracked, forming a visible gap at the interface. For POM/PLA blends, the interface region was bonded to the surface of the fiber particle, which suggests stronger interface adhesion for the POM/PLA blend system. Similar results, revealing favorable interface adhesion, were observed for POM/PLA/EBA-WF and POM/PLA/EBA/CE-WF samples; however, for toughened samples, the additional structural changes were connected with the presence of the elastomeric phase. For both types of composites, the E/BA/GMA phase’s inclusion was visible. Additionally, the fractured surface was covered with visible fibrous structures. The observed fibers were formed during the plastic deformation of the toughened structure, which is a typical behavior when a surface fracture is subjected to standard temperature conditions.

3.3. Thermomechanical Properties—Dynamic Mechanical Thermal Analysis (DMTA), Heat Deflection Temperature (HDT)

The thermomechanical properties of the developed materials were investigated using the dynamic mechanical thermal analysis method (DMTA) and heat deflection temperature tests (HDTs). The DMTA method allows the investigation of the changes in material stiffness (modulus), while the HDT results are widely accepted by industrial users as a valuable measurement of thermal resistance changes. The results of both types of measurements are shown in Figure 4. Since the most important differences in HDT results were observed for the PLA-modified materials, the presented results are focused on the evaluation of the thermomechanical properties of POM/PLA-based blends and composites. The initial study conducted for the POM-based composites revealed only some minor differences between the composites modified with the use of wood flour, cellulose, and BH particles. The HDT results ranged from 121 °C to 132 °C, with no clear trends caused by the amount or type of filler.
The results of the DMTA analysis for unfilled POM/PLA blends (see Figure 4) reveal a visible difference in material stiffness. The unmodified POM resin is characterized by a constant decrease in stiffness over temperature; however, considering the differences between the temperature at the beginning of the measurement (around 25 °C) and the end (150 °C), the dynamics of these changes are very small. In contrast, for pure PLA, a rapid drop in the storage modulus values could be observed at 60 °C. The observed behavior reveals the typical difference between the thermal resistance of highly crystalline and amorphous polymers. Interestingly, for the untoughened POM/PLA blend, the initial modulus is very close to the values represented by pure PLA. In contrast, for EBA-modified blends, the room temperature values are closer to those of the stiffness of pure POM.
For all of the POM/PLA type blends, the initial stiffness values started to decrease as the temperature increased. The plot comparison reveals that the modulus drop was less rapid than for pure PLA; however, visible stiffness reduction was observed at 40 °C. The faster softening of the POM/PLA blends can be confirmed when analyzing the tan δ plots, where the peak of the curve was recorded around 63 °C, compared to 70 °C for pure PLA. In contrast to the sharp peak for PLA, the shapes of the plots for POM/PLA-based blends are broader, which suggests that the glass transition for this type of material is more gradual. Nevertheless, the position of the peak on the curve confirms at least the partial miscibility of the POM/PLA system. The DMTA results obtained for the WF-reinforced samples (see Figure 4B) were collected for molded materials and annealed specimens. It is quite clear that the appearances of the plots for molded samples are very similar to those of the unreinforced samples. The only noticeable difference is a visible increase in the storage modulus values, especially at the initial temperature range of the test. More visible differences are recorded for annealed materials since the difference in crystallinity strongly influences the thermomechanical performance. The storage modulus values observed close to the room temperature are almost similar for molded and annealed samples; however, the main differences are revealed in the glass transition region. In contrast to the reference POM/PLA materials, the change in stiffness in the glass transition area does not differ from the intensity change seen in other areas of the plots. For the untoughened POM/PLA blend, the storage modulus values are only slightly lower than those for pure POM. The tan δ plots reveal a large reduction in the Tg peak area, since the increasing crystallinity of the PLA phase leads to a reduction in amorphous phase mobility. Nevertheless, the position of these peaks has not changed in relation to the reference samples, which confirms that the heating (annealing) procedure mainly affects the content of the crystalline phase and not the phase transition occurrence.
The DMTA analysis allows for a better understanding of HDT measurement results (Figure 4C). The comparison was prepared for unfilled blends and WF-reinforced composites, wherein the results reflect the properties before and after the annealing procedure for both types of materials. The HDT values for molded blends range from 50 °C to 55 °C, which are very close to the results reported for pure PLA (HDT ≈ 54 °C). Interestingly, the HDT for pure POM resin was 91 °C, while the annealing procedure resulted in an increase to 105 °C. This confirms that even for highly crystalline materials such as POM or polyamides, it is possible to further improve the properties by thermal processes. For POM/PLA blends, the annealing also increased the HDT results, where the best results for unmodified POM/PLA samples reached 77 °C. However, the observed increase was relatively small, and still, the HDT results are far below the thermal resistance for pure POM. After the introduction of the WF filler, all of the results obtained for unreinforced samples significantly increased. For example, the HDT results for POM/WF reached 126 °C, which is 35 °C higher than the results for unmodified POM. For POM/PLA-based composites, the HDT values also increased by a minimum of 25 °C. The additional annealing treatment was even more effective than WF addition, especially for POM/PLA-based composites. Again, the highest HDT of 138 °C was recorded for POM/WF samples; however, even for toughened POM/PLA composites, the results are close to 125 °C, which is an increase of over 45 °C compared to untreated WF composites.

3.4. Thermal Behavior and Phase Transition Changes for POM/PLA Blends—Differential Scanning Calorimetry Analysis (DSC)

The prepared WF-modified composites were characterized by visibly higher thermomechanical resistance (HDT). The increase in heat deflection factor for the POM/PLA blend-based materials was visible for molded samples; however, the most significant changes appeared after the introduction of the annealing procedure. The possible reason for this is the change in phase morphology of the matrix structure, since both of the used polymeric components are semi-crystalline. Since the SEM analysis did not reveal any visible correlations between the molded and annealed samples, the additional DSC thermal analysis was applied to investigate possible changes in crystallinity and phase. The most important results are presented in Figure 5, where first heating scans for molded and annealed samples are revealed. The results of the prepared DSC analysis are strongly in line with those of our previous study, where unreinforced POM/PLA blends were subjected to testing. This is why the presented discussion is limited to the most important results. Since the melting peak signals for the used types of resin are overlapping, it is not possible to calculate the crystallinity level. This fact makes the whole analysis less quantitative, and most of the conclusion needs to be formulated based on the appearance of the DSC plot.
The plots comparison presented in Figure 5A collects the DSC signals for molded WF-modified composites. The observed results confirm that for POM/PLA (50%/50% ratio) blends, the dominant melting peak signal did not reveal bimodal morphology, which can be observed when the POM content is reduced. The direct comparison of the Tm for POM/20WF and POM/PLA-based blends shows a small decrease in the melting peak position. This kind of difference can be considered minor. Additionally, the same shift was already observed in our previous research for unreinforced blends, which means that the addition of WF particles does not lead to large changes in the phase morphology of the crystalline phase. Besides the melting peak area, the heating plots reveal the presence of a cold crystallinity phenomenon, which can be observed at around 105 °C. The size of the recorded peaks is relatively small, which is an obvious consequence of using a maximum of 50% of the PLA resin in the matrix. However, when compared to the pure POM/PLA blends in our previous study, the addition of WF particles enhanced the cold crystallization effect.
The annealing treatment that was conducted at 120 °C slightly changed the appearance of the DSC plots (Figure 5B). The first visible change can be seen in the PLA phase cold crystallization region, where previously reported exothermic peaks are not present after annealing. The observed changes are in line with many other research studies where the annealing procedure is involved during the processing of PLA-based materials. Reheating above the Tcc limit enables chain mobility and results in structure reorganization. This phenomenon is temperature- and time-dependent, which means that many types of polyesters require long heat treatment [70,71,72]. Unfortunately, for many types of PLA-based materials, the significant softening that occurs before structure stiffening leads to deformation, which means that annealing cannot be used as an effective method of heat treatment. The presence of WF-based reinforcement can be considered a favorable factor for annealing treatment since the softening of the molded samples was strongly reduced for POM/PLA-20WF composites. The cooling stage plots analysis did not reveal visible changes for tested materials, while all of the recorded crystallization peaks were recorded at 145 °C, which is the typical value for the pure POM phase.
Summarizing the DSC test results, it is clear that the annealing procedure lead to the increase in PLA crystalline structure. Unfortunately, a detailed analysis of the crystallinity level was not possible for both POM and PLA resin. However, the obtained results clearly reveal that, for the prepared materials, changes in the crystallinity were mainly observed for the PLA phase, which does not exclude possible changes in the morphology of the POM phase after the annealing process.

4. Discussion

Taking into account the previous publications covering the use of natural fillers as an additive for POM [47,73,74,75], it is worth noting that the main emphasis in the previous research was on obtaining the highest possible reinforcement factor, while the main advantages of using POM as a technical polymer, i.e., its high thermal resistance, were often omitted. Preliminary tests conducted for unmodified POM showed that, in the case of fillers in milled and powdered form, the effectiveness of reinforcement is very insignificant for unmodified POM, which is made visible by the relatively small increase in stiffness and a decrease in strength. This is a relatively typical behavior due to the small shape factor of the particles of this type. The length-to-diameter ratio is very low, which negatively affects the mechanics of the material deformation process. It is clear that for POM/PLA blends, some further research regarding the utilization of traditional glass/carbon reinforcement might reveal the higher effectiveness of synthetic materials [76,77,78]. However, the sustainability factors of the resulting composite will be less favorable, especially for carbon fiber usage [79].
The second stage of the conducted research involved the modification of POM with the addition of PLA. The concept of using PLA as a component of a blend with engineering polymers is always associated with the risk of reducing the thermal resistance of the resulting material [31,33,80,81,82]; in particular, the second component of the system is a high-crystalline polymer such as PP, PE, PA, or PBT [83,84,85,86,87]. The results confirm this for the obtained materials because the thermomechanical properties of unmodified POM/PLA blends are very similar to those of pure PLA. Therefore, in the discussed research, we decided to use an important feature of PLA and polyester, i.e., the ability to cold-crystallize. The annealing procedure turned out to be very effective, even for materials without the addition of wood flour.

5. Conclusions

The results of the study reveal that the utilization of natural fibers during the processing of POM/PLA blends could improve the thermal resistance of the obtained composites; however, other characteristics are less favorable. The values of the HDT coefficient are still the highest for materials based on unmodified POM; for the discussed research, the key concept included minimizing the content of this polymer. Hence, the results for POM/WF materials can be treated as a reference point.
Unlike previous research reports, where POM resin was used as the matrix material for different types of natural fillers, the study revealed that the reinforcing efficiency of cellulose or wood fibers is relatively small; however, when considering the thermomechanical properties, the 20% WF addition strongly improved the heat deflection (HDT) results. For the most complex systems based on a POM/PLA blend with the addition of an EBA impact modifier, it was possible to obtain elongation and impact resistance values close to those of pure POM.
When summarizing the results of the work carried out, it should be emphasized that the research conducted was of a multifaceted nature. The primary goal of trying to produce a polymer material with functional properties similar to commercially available WPC materials has been achieved. An additional aspect related to obtaining increased thermomechanical properties was achieved by the heat treatment of materials. However, an equally important value in the discussed research is related to the indication of the potential for materials of this type; the results indicate the possibility of using the potential of both main components of the POM and PLA mixture to obtain a material with a very wide possible range of modifications and potential properties.
The results of the work suggest some potential applications for the discussed materials, but in the case of research planned in the near future, we would like to check whether it is possible to use waste and post-consumer polymers in material systems based on POM/PLA mixtures. A separate topic concerns the issue of using more effective fibrous fillers instead of lignocellulosic particles, which we also plan to investigate.

Author Contributions

Conceptualization, J.A.; methodology, J.A. and A.S.; software, J.A. and A.S.; validation, J.A. and A.S.; formal analysis, A.S.; investigation, A.S.; resources, J.A. and A.S.; data curation, J.A. and A.S.; writing—original draft preparation, A.S. and J.A.; writing—review and editing, A.S. and J.A.; visualization, A.S. and J.A.; supervision, J.A.; project administration, J.A.; funding acquisition, J.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Education and Science in Poland as part of the statutory subsidy for the Poznan University of Technology, project No. 0613/SBAD/4888.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The results of static tensile tests and Charpy impact resistance measurements. The results presented on the charts reflect the properties of POM-based composites reinforced with different types of fillers: (A) tensile modulus, (B) tensile strength, (C) elongation at break, and (D) Charpy impact strength.
Figure 1. The results of static tensile tests and Charpy impact resistance measurements. The results presented on the charts reflect the properties of POM-based composites reinforced with different types of fillers: (A) tensile modulus, (B) tensile strength, (C) elongation at break, and (D) Charpy impact strength.
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Figure 2. The results of static tensile tests and Charpy impact resistance measurements for POM/PLA blends: (A) tensile modulus; (B) tensile strength; (C) elongation at break; (D) Charpy impact strength. Plots reflect the results for unmodified and wood flour (WF)-reinforced samples.
Figure 2. The results of static tensile tests and Charpy impact resistance measurements for POM/PLA blends: (A) tensile modulus; (B) tensile strength; (C) elongation at break; (D) Charpy impact strength. Plots reflect the results for unmodified and wood flour (WF)-reinforced samples.
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Figure 3. (A–D) The structure of POM-based composites with different types of fillers; (E) the structure of modified POM/WF composites.
Figure 3. (A–D) The structure of POM-based composites with different types of fillers; (E) the structure of modified POM/WF composites.
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Figure 4. The results of the DMTA analysis (storage modulus and tan δ): (A) for unmodified samples; (B) for WF-modified materials. (C) The results of the head deflection temperature (HDT) measurements.
Figure 4. The results of the DMTA analysis (storage modulus and tan δ): (A) for unmodified samples; (B) for WF-modified materials. (C) The results of the head deflection temperature (HDT) measurements.
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Figure 5. The results of the DSC analysis for 20% WF reinforced composites. Both charts present the first heating scans: (A) for untreated samples; (B) for annealed composite materials.
Figure 5. The results of the DSC analysis for 20% WF reinforced composites. Both charts present the first heating scans: (A) for untreated samples; (B) for annealed composite materials.
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Table 1. The list of prepared samples and blend composition (presented as wt. %).
Table 1. The list of prepared samples and blend composition (presented as wt. %).
SamplePOMPLAE/BA/GMACellulose Four (CF)Wood Flour (WF)Buckwheat Husk
(BH Fine)		Buckwheat Husk
(BH Coarse)
POM100------
POM/CF1090--10---
POM/CF2080--20---
POM/CF3070--30---
POM/WF1090---10--
POM/WF2080---20--
POM/WF3070---30--
POM/10BH(fine)90----10-
POM/20BH(fine)80----20-
POM/30BH(fine)70----30-
POM/10BH(coarse)90-----10
POM/20BH(coarse)80-----20
POM/30BH(coarse)70-----30
POM/PLA5050-----
POM/PLA/EBA404020----
POM/PLA/EBA/CE *404020----
POM/PLA-20WF4040--20--
POM/PLA/EBA-20WF323216-20--
POM/PLA/EBA/CE-20WF *323216-20--
* The chain extender (CE) content was limited to 0.5 phr for each composition.
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MDPI and ACS Style

Soćko, A.; Andrzejewski, J. The Development of Sustainable Polyoxymethylene (POM)-Based Composites by the Introduction of Natural Fillers and Melt Blending with Poly(lactic acid)-PLA. J. Compos. Sci. 2024, 8, 315. https://doi.org/10.3390/jcs8080315

AMA Style

Soćko A, Andrzejewski J. The Development of Sustainable Polyoxymethylene (POM)-Based Composites by the Introduction of Natural Fillers and Melt Blending with Poly(lactic acid)-PLA. Journal of Composites Science. 2024; 8(8):315. https://doi.org/10.3390/jcs8080315

Chicago/Turabian Style

Soćko, Anna, and Jacek Andrzejewski. 2024. "The Development of Sustainable Polyoxymethylene (POM)-Based Composites by the Introduction of Natural Fillers and Melt Blending with Poly(lactic acid)-PLA" Journal of Composites Science 8, no. 8: 315. https://doi.org/10.3390/jcs8080315

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