Upcycling Spent Coffee Grounds-Based Composite for 3D Printing: A Review of Current Research

Abstract

Driven by the growing demand for sustainable materials, spent coffee grounds have emerged as a promising bio-based reinforcement in polymer composites, particularly for additive manufacturing applications. As a readily available byproduct of the coffee industry, spent coffee grounds contain cellulose, hemicellulose, lignin, proteins, and oils, making them attractive fillers for both thermoplastic and thermoset matrices. Incorporating spent coffee grounds into composites supports waste valorization, cost reduction, and environmental sustainability by transforming organic waste into functional materials. This review first examines the issue of spent coffee ground waste, addressing its environmental footprint and disposal challenges. It then explores the composition and properties of spent coffee grounds. The paper provides a comprehensive overview of composites based on spent coffee grounds for 3D printing, covering processing methods, potential applications, and current challenges in additive manufacturing. Special attention is given to the preparation and processing of these composites, including key steps such as drying, grinding, sieving, and surface modification to enhance compatibility with polymer matrices. Various additive manufacturing techniques influence the printability, processability, and mechanical performance of such composites. While spent coffee grounds offer notable sustainability advantages, challenges such as weak interfacial adhesion, moisture sensitivity, and reduced mechanical properties necessitate optimized processing conditions, surface treatments, and tailored material formulations. This review highlights recent advancements and outlines future research directions, emphasizing the need for stronger interactions between spent coffee grounds and polymer matrices, improved recyclability, and scalable additive manufacturing solutions to establish spent coffee grounds as a viable and eco-friendly alternative for 3D printing applications.

1. Introduction

The rapid development and widespread adoption of Additive Manufacturing (AM) have revolutionized prototyping and small-scale production across numerous industries [1]. However, this technological progress has also led to significant material waste. Temporary-use prototypes, failed prints, support structures, and discarded components due to design iterations all contribute to a growing volume of post-processing waste [2]. As AM technologies continue to scale, the accumulation of polymer waste, particularly from non-biodegradable, fossil-based feedstocks, poses a serious environmental challenge that demands sustainable and circular alternatives [3].

In response, the search for bio-based and renewable feedstock has intensified. One particularly promising approach lies in the valorization of agro-industrial waste streams, such as spent coffee grounds (SCGs), which are generated in massive quantities estimated at over 7–8 million tons annually worldwide [4]. Most of this biomass ends up in landfills or is incinerated, practices that release greenhouse gases and squander valuable organic resources [5]. The circular approach involves a progressive process from the initial waste generation through reutilization and recycling, ultimately leading to an eco-friendlier lifecycle for spent coffee grounds (SCGs) as shown in Figure 1. SCGs have been explored for various applications across multiple sectors, including the food industry, agriculture, renewable energy, wastewater treatment (as adsorbents), electronic devices, construction materials, the production of biopolymers, and as fillers in polymer composites. One promising strategy is the integration of SCGs into additive manufacturing (AM) feedstocks, which provides a dual benefit: it mitigates the environmental burden of organic waste while contributing to the development of sustainable materials. Indeed, SCGs are rich in cellulose, hemicellulose, lignin, proteins, and residual oils, making it an attractive candidate for reinforcing polymer matrices [6,7]. Their incorporation into thermoplastic or thermoset composites can enhance mechanical properties, thermal stability, and, potentially, biodegradability [8]. By converting this abundant waste into functional fillers, the environmental impact of both the coffee and additive manufacturing industries can be mitigated. Moreover, the integration of SCGs into AM supports circular economy principles by promoting waste valorization, reducing dependency on virgin fossil-based materials, and enabling the fabrication of eco-friendly, cost-effective products [9,10]. This valorization of organic waste not only reduces landfill burden and greenhouse gas emissions but also contributes to the diversification of raw material sources in additive manufacturing [11]. Moreover, the use of SCGs as bio-based filler enhances material sustainability by decreasing dependence on virgin polymers, while potentially improving specific mechanical or thermal properties when properly treated and incorporated. Such integration fosters a closed-loop system where waste is transformed into value-added products, reinforcing the transition toward greener and more resilient manufacturing practices [10].

AM techniques offer new avenues for the tailored fabrication of SCG-based composites. Among them, Fused Deposition Modeling (FDM) is the most widely employed method due to its compatibility with thermoplastics such as polylactic acid (PLA) and polypropylene (PP) [13,14,15]. Alternatives like Fused Granulate Fabrication (FGF) [16,17,18] and Direct Writing (DW) [19] also present valuable options, offering advantages in terms of material throughput or spatial resolution. However, each technique introduces specific challenges related to printability, interlayer adhesion, and final mechanical performance.

Despite these advances, the successful implementation of SCG-based composites in AM requires a deep understanding of their physicochemical properties and tailored processing strategies. Due to their hydrophilic nature, SCGs typically exhibit poor interfacial compatibility with hydrophobic polymers, which can result in diminished mechanical strength and moisture sensitivity. To address this, surface treatments such as alkali treatment [14], silane functionalization [13], or solvent extraction are employed to improve interfacial adhesion and dispersion within the matrix. Furthermore, particle size, aspect ratio, filler concentration, and extrusion parameters play critical roles in determining the final properties of printed parts.

This review presents a comprehensive evaluation of SCG-derived composites for additive manufacturing, encompassing waste management strategies, chemical composition, upcycling potential, and the influence of different 3D printing techniques. By synthesizing recent advancements and identifying technical barriers, this study highlights the potential of SCGs as viable, sustainable resources for the fabrication of bio-based AM materials. Continued research into formulation optimization, surface chemistry, and scalable processing methods will be essential for unlocking their full potential and establishing spent coffee grounds as a key contributor to greener manufacturing practices.

2. SCG Waste Threats and Opportunities

Coffee continues to thrive as one of the most consumed beverages globally, with Brazil, Vietnam, Colombia, and Indonesia leading the world in coffee production. Among these, Brazil remains the largest producer, followed by Vietnam and Colombia [20]. In 2023 alone, global coffee production reached approximately 178 million 60 kg bags, while global consumption stood at around 177 million 60 kg bags, highlighting the massive scale and steady growth of the coffee industry (Figure 2).

However, alongside this booming consumption, significant volumes of organic waste are generated at every stage of coffee processing, including harvesting, pulping, roasting, and brewing.

These byproducts include coffee silverskin (CS), coffee husks (CH), coffee pulp (CP), and most notably, spent coffee grounds (SCGs), as shown in Figure 3 [5]. SCGs are the primary residue obtained after brewing coffee or during the production of instant (soluble) coffee. It is estimated that nearly 50% of global coffee production is directed toward soluble coffee manufacturing [21], and the preparation of just 1 kg of soluble coffee can generate approximately 2 kg of wet SCG [22].

In recent years, SCGs have attracted growing attention due to their chemical richness and functional potential, which enable their reuse in various high-value applications. Their composition, discussed in detail in the next section, includes carbohydrates, proteins, lipids, polyphenols, and caffeine, making them suitable for bio-based product development across diverse sectors. However, without proper management, SCGs also pose serious environmental and health risks. Typically discarded in landfills or incinerated, this waste stream is often mismanaged, resulting in environmental contamination and the release of harmful pollutants [23].

Indeed, the direct disposal of SCGs into the environment can lead to soil and water contamination. The caffeine content, tannins, and other compounds in SCGs have been shown to inhibit enzymatic activities, suppress microbial growth, and negatively impact soil fertility by altering parameters such as microbial biomass carbon and nitrogen [24]. Moreover, elevated concentrations of SCGs in soil can reduce nitrogenase activity and impair plant growth and seed germination [25]. Leachates containing caffeine and polyphenols also threaten aquatic ecosystems by disrupting biodiversity and water quality [26].

Another major concern arises from greenhouse gas emissions. When SCGs are landfilled under anaerobic conditions, they release methane, a potent greenhouse gas with a global warming potential many times greater than carbon dioxide. While methane is odorless and difficult to detect, its cumulative environmental impact is profound [5]. Incineration of SCGs, a commonly used disposal method, also contributes to air pollution by emitting toxic gases such as carbon monoxide (CO) and nitrogen oxides (NOx), which are linked to respiratory and cardiovascular health issues [27].

In light of these issues, the valorization of SCGs is both an environmental necessity and an opportunity for innovation. There is growing political and societal pressure to reduce industrial pollution and adopt sustainable practices. Consequently, researchers worldwide are investigating a wide array of applications for SCGs and their derivatives, ranging from biofuels, bioactive compounds, adsorbents, and nanocomposites to polymers, cosmetics, compost, animal feed, food ingredients, and composite materials, many of which are discussed in detail in Section 4.

Recycling SCGs into value-added products and energy sources offers a viable path to alleviate the environmental burden posed by this abundant waste. Such efforts align with the principles of the circular economy by turning waste into resources, generating social and economic value while minimizing ecological harm [24,28,29,30]. Ultimately, sustainable SCG management requires the deployment of innovative, interdisciplinary strategies that optimize the recovery of useful components while mitigating negative environmental impacts.

3. SCG Composition and Properties

The composition of spent coffee grounds (SCGs), as shown in Table 1, varies depending on several factors, including coffee variety, cultivation conditions, and brewing method. Nevertheless, most SCGs share a broadly similar chemical profile [6]. Polysaccharides, primarily cellulose and hemicellulose, are among the most abundant constituents, together accounting for approximately 50% of the dry weight, chemical, functional, and structural properties of Spent Coffee Grounds and Coffee Silverskin [7]. Cellulose is mainly composed of glucose, while hemicellulose includes sugars such as arabinose, mannose, galactose, and xylose.

SCGs also contain a significant amount of lignin, typically ranging from 20 to 30 wt% of the dry mass (ash-free basis), which contributes to their structural rigidity [31,32]. Proteins make up about 13–17 wt%, while lipids are present in concentrations exceeding 15 wt%, making SCG a rich source of residual oils [33,34].

In addition to macronutrients, SCGs contain minor yet valuable bioactive compounds, such as phenolics (approximately 12.0 mg/g), caffeine (14.5 µg/g), and chlorogenic acid (31.8 µg/g) [35]. Their mineral content is also notable, with ashes containing elements such as potassium, calcium, magnesium, phosphorus, iron, manganese, boron, and copper [27].

Table 1. Chemical and mineral compounds found in SCGs, according to the literature. (¹ Dry weight; ² Only the highest values are reported).

Chemical Compounds	Content (wt. %) 1	References	Minerals Compounds	Content 2 (mg/g) 1	References
Hemicellulose	30–40		Potassium	3.7
Lignin	20–30		Phosphorus	1.47
Proteins	13–17		Calcium	1.38
Cellulose	8–15	[7,31,34]	Magnesium	1.29	[36,37,38]
Lipids	7–21		Aluminum	0.28
Ashes	1–2		Iron	0.12
			Sodium	0.07
			Manganese	0.05
			Copper	0.03
			Zinc	0.01

Table 1. Chemical and mineral compounds found in SCGs, according to the literature. (¹ Dry weight; ² Only the highest values are reported).

Chemical Compounds	Content (wt. %) 1	References	Minerals Compounds	Content 2 (mg/g) 1	References
Hemicellulose	30–40		Potassium	3.7
Lignin	20–30		Phosphorus	1.47
Proteins	13–17		Calcium	1.38
Cellulose	8–15	[7,31,34]	Magnesium	1.29	[36,37,38]
Lipids	7–21		Aluminum	0.28
Ashes	1–2		Iron	0.12
			Sodium	0.07
			Manganese	0.05
			Copper	0.03
			Zinc	0.01

The major properties of spent coffee grounds relevant for AM can be summarized into the following key aspects: water and oil holding capacity, emulsifying activity and emulsion stability, antioxidant potential, crystallinity, thermal behavior, morphology, and porosity.

3.1. Water and Oil Holding Capacity

Water holding capacity (WHC) and oil holding capacity (OHC) are crucial functional properties representing the material’s ability to retain water or oil after external forces such as gravity or compression. According to Murthy et al. (2012) [39], WHC and OHC are influenced by the particle size of the material, with smaller particle sizes leading to increased holding capacities due to higher packing density. Several studies suggest that particle sizes with diameters ≤ 500 μm are optimal for evaluating these functional properties [40,41]. Ballesteros et al. (2014) [7] reported that SCGs with particle sizes ≤ 500 μm exhibited a WHC of 5.73 ± 0.10 g water/g dry sample and an OHC of 5.20 ± 0.30 g oil/g dry sample.

3.2. Emulsifying Activity and Emulsion Stability

Emulsifying activity (EA) refers to the ability of a compound to form a homogeneous dispersion of two immiscible liquids, while emulsifying stability (ES) describes the capacity to maintain a thermodynamically stable emulsion [42]. Comparative studies show that SCGs and CS have similar EA values of 54.72% and 57.50%, respectively, with SCGs displaying slightly higher ES (92.38%) than CS (88.18%) [7]. Both residues demonstrate higher ES than other plant materials such as lima bean [41], papaya kernel flour (58%), corncob (80%), and wheat straw (86.94%) [43]. This behavior is attributed to the type and proportion of soluble and insoluble fibers and the protein fraction, which plays a key role in anchoring fibers at the oil-water interface [44].

3.3. Antioxidant Potential

Studies comparing SCGs and CS reported comparable antioxidant potential, with SCGs and CS exhibiting 20.04 and 21.35 μmol TE/g dry material, respectively, based on the DPPH assay. However, the FRAP assay revealed that SCGs have a 2.3-fold higher antioxidant potential than CS [7]. Current literature indicates that various analytical methods are available to assess antioxidant activity in food and biological systems. These findings highlight the potential for valorizing coffee residues, particularly spent coffee grounds, as a sustainable source of antioxidant compounds.

3.4. Crystallinity

Crystallinity, indicating the degree of structural order in a solid, significantly influences properties such as hardness, density, transparency, and diffusion. X-ray diffraction (XRD) studies reveal that SCGs and CS share similar crystalline regions compared to cellulose references [7]. Cellulose molecules have both crystalline regions, responsible for tensile strength and chemical resistance due to strong hydrogen bonding, and amorphous regions, primarily hemicellulose and other components, which are more chemically accessible [45]. Despite CS containing higher cellulose content (23.77 ± 0.09 g/100 g dry material) than SCGs (12.40 ± 0.79 g/100 g dry material), SCGs exhibit higher crystallinity, likely due to thermal treatments during coffee roasting that promote transformation of cellulose polymorphs and dehydration of crystal fractions [46].

3.5. Thermal Behavior

Thermogravimetric analysis (TGA) is extensively used to study thermal decomposition and mass changes in lignocellulosic materials under controlled atmospheres (air or nitrogen) [47]. Fermoso et al. (2018) [48] demonstrated that increasing the heating rate (from 5 to 100 °C/min) during pyrolysis of SCGs up to 500 °C accelerates thermal decomposition. Kwon et al. (2013) [49] used TGA to determine fatty acid contents in lipids extracted from SCGs. Polysaccharides from SCGs show thermal behavior similar to known polysaccharides like cellulose and hemicellulose when analyzed by TGA combined with differential scanning calorimetry (DSC) [48,49]. Comparisons between SCGs and CS indicate similar melting points at approximately 76.89 °C and 74.60 °C, respectively [7]. Thermal stability studies following oil extraction from SCGs have demonstrated potential for biodiesel production and subsequent composting of residual solids [30].

3.6. Morphology and Porosity

Scanning electron microscopy (SEM) analyses by Ballesteros et al. (2014) [7] showed that CS particles possess a denser, sheet-like morphology resembling sawdust. In contrast, SCG particles exhibit a more porous and granular structure due to component removal during brewing. Coffee chaff exhibits a fibrous morphology with a dense, smooth surface, contrasting with SCG’s porosity [50]. Hydrochar derived from SCGs presents increased porosity and a smoother surface after hydrolysis [51,52]. Particle sizes of SCGs typically range from 297 to 430 µm, with pore sizes between 20 and 30 µm [53]. Biochar from SCGs shows a sponge-like morphology with smaller pore sizes (10–20 µm) relative to untreated SCGs [54].

4. SCG Upcycling

As mentioned previously, due to the chemical composition of spent coffee grounds (SCGs), numerous efforts have been made to produce value-added products from this waste material.

Table 2. SCG Potential Applications.

Fields	Process	Products	References
Food industry	Extraction of Bioactive Compound	Feed ingredient	[55,56]
		Food products and ingredients
Agriculture	Composting	Fertilizers, soil improvers	[57,58]
	Substrate	Mushroom growth
Renewable energy	Extraction	Biodiesel	[7,59,60]
	Pressing	Solid fuel pellets
Adsorbent	Synthesis	Biochar	[57,61,62,63]
Electronic devices	Extraction	Biobattery	[64,65]
Construction materials	Mixing	Bricks	[66,67,68,69]
Biopolymers production	Hydrolysis, Synthesis	PLA	[69,70,71,72,73,74,75,76]
Composite manufacturing	Extrusion, injection molding	Biocomposite	[13,16,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96]

summarizes the broad range of potential applications for SCG recycling.

In the food industry, earlier studies explored the use of SCGs as a feed ingredient [55]. Additionally, SCGs have been investigated as a functional ingredient in sponge cake formulations. Replacing flour with SCGs at levels of 2%, 4%, and 6% resulted in a reduction in browning, which was attributed to a lower glycemic sugar-protein content compared to the control. Cakes supplemented with SCGs exhibited significantly higher volume, weight, and specific volume than the control. Texture profile analysis revealed that adding SCGs decreased hardness, resilience, cohesiveness, gumminess, and chewiness in the sponge cakes [56].

Beyond food applications, SCGs can be used as fertilizer, in composting, and as a soil amendment. However, their raw form contains phytotoxic compounds such as caffeine, tannins, and polyphenols, which may negatively affect soil fertility and plant growth. To mitigate these effects, studies recommend combining SCGs with other organic materials when used as organic amendments, thereby reducing phytotoxicity and improving soil biology and function [57,58,97].

SCGs are also rich in nutrients like protein, potassium, magnesium, and phosphorus, making them ideal substrates for composting and fermentation processes [7,57]. Their low ash content further positions SCGs as a promising alternative energy source [7]. For example, oil extracted from SCGs can be used for biodiesel production, and SCGs can be processed into solid fuel pellets, contributing to sustainable fuel options [59].

Due to their excellent absorbent properties, SCGs serve as effective filters for removing heavy metals such as cadmium, copper (II), and zinc. Additionally, their high water and oil retention capacities make SCGs suitable precursors for biochar production via pyrolysis [57]. SCG-derived biochar has demonstrated effectiveness in adsorbing heavy metals and metal ions, making it a promising material for environmental remediation [57,98,99].

Byproducts from SCGs also show potential as functional materials in various electronic applications [64]. For instance, Pawarangan and Jefriyanto [65] reported fluctuating current, voltage, and electrical power generated by arabica and robusta coffee grounds over seven days. Their bio-battery device characterization revealed that robusta coffee grounds produced a maximum voltage of 1.11 ± 0.09 V and power of 0.38 ± 0.08 mW, while arabica coffee grounds generated a maximum current of 0.39 ± 0.07 mA.

SCGs have also been explored in construction materials. They were evaluated as additives for brick-and-mortar production. Giada La Scalia et al. [66] proposed a method to assess the potential of SCGs as an additive in mortar, aiming to develop an alternative circular process for recycling coffee biowaste. Mortars were produced using two types of binders: a green geopolymer and ordinary hydraulic lime, with SCGs partially replacing sand in different proportions. The designed aggregate mixtures yielded mortars with properties suitable for industrial processing. Some bio-composites demonstrate enhanced mechanical characteristics. The inclusion of SCGs increased water absorption but significantly improved insulation performance, thereby reducing environmental impact. Notably, adding just 5% SCGs resulted in a significant reduction in thermal conductivity, thus enhancing thermal insulation.

The chemical composition of spent coffee grounds (SCGs) makes them promising feedstocks for bioplastics production. Among the most common biopolymers derived from SCG is polyhydroxyalkanoates (PHA), produced by culturing SCG oil with Cupriavidus necator [70,71]. Since pure poly (3-hydroxybutyrate) (PHB) homopolymer tends to be brittle, there is interest in engineered microbial strains capable of producing PHB copolymers with enhanced strength and flexibility [72,73]. However, only one study to date has reported the mechanical properties of PHB derived from SCG oil, describing it as a brittle, crystalline polymer with an elastic modulus of approximately 1.3% [76]. This highlights the need for further investigation into PHA production from SCG oil, especially to elucidate the mechanical performance of these biopolymers, which remains insufficiently explored.

In addition to PHAs, SCGs have been used in the production of lactic acid, the monomer for polylactic acid (PLA), a widely employed bioplastic in food packaging [79,80,81]. This process involves acid hydrolysis and cellulase treatment of SCGs, followed by fermentation with Lactobacillus rhamnosus, achieving high lactic acid yields of up to 98%.

An emerging trend in composite materials involves using SCGs as reinforcements or fillers. However, the characteristics and applications of SCG-based composites vary depending on the polymer matrix. Polyethylene (PE) and polypropylene (PP) are the most common synthetic matrices combined with SCGs, owing to their widespread use in packaging, low cost, and favorable properties [90,91].

Cestari and Mendes [100] investigated the effect of particle size and soluble extraction on high-density polyethylene (HDPE) composites incorporating four types of SCGs (integral, extracted, large, and small) at 10 wt% filler loading. Their results showed that integral SCGs exhibited similar properties to small-size SCGs and outperformed extracted SCGs. Thermal analysis indicated no degradation of HDPE during processing between 160 and 190 °C. Composites with integral and small SCGs degrade at higher temperatures than those with extracted or large SCGs. The melting temperature of all composites remained close to that of neat HDPE (134 °C), while crystallinity decreased slightly except in large SCG composites.

In a more recent study, Mendes et al. [93] produced ecological composites of HDPE with SCG contents ranging from 10 to 30 wt% via extrusion and injection molding. Incorporating SCG fibers increased the modulus of elasticity by 49% and the flexural modulus by 108%, significantly enhancing composite stiffness. Additionally, composites with 10 wt% SCG showed an approximate 13% increase in elasticity. SCG particles up to 20 wt% were homogeneously dispersed in the HDPE matrix, demonstrating good compatibility without needing a compatibilizer.

PP composites reinforced with SCGs have also been explored. Incorporating 10, 20, and 30 wt% SCGs into PP homopolymer and copolymer matrices led to some mechanical property reductions compared to neat PP. However, a notable 77% increase in impact strength was observed with 10 wt% SCGs in PP homopolymer. The study emphasized the importance of SCG surface treatment to improve interfacial adhesion and suggested further work to enhance tensile strength. These composites show promise as sustainable alternatives to virgin PP in applications requiring improved impact strength [94].

Several studies have focused on treated SCG fillers to improve interfacial adhesion [77,78,94,95,96]. For instance, recent research developed biocomposites based on a PP matrix containing untreated and alkaline-treated SCGs at 0, 10, 15, and 20 wt%. Biocomposites with treated SCGs exhibited superior mechanical properties compared to untreated ones. The use of PP-grafted maleic anhydride (PP-g-MA) as a compatibilizer was crucial in achieving uniform SCG dispersion and preventing agglomeration, thereby significantly enhancing interfacial bonding and mechanical performance. These findings indicate that PP-based composites containing SCGs are promising sustainable alternatives to conventional plastics, offering reduced weight and improved biodegradability [78].

Beyond PE and PP, SCG composites have been prepared with other synthetic polymers such as polyurethane (PU) [79], epoxy [80], and rubber [81]. Researchers have also explored biodegradable polymer matrices, including poly (butylene adipate-co-terephthalate) (PBAT) [82], polyvinyl alcohol (PVA) [83], and PLA [13,84,85]. PLA remains the most widely used biodegradable polymer in packaging due to its favorable mechanical and barrier properties [86].

Over the past decade, numerous studies have explored SCG valorization as a filler for PLA composites to mitigate SCG waste accumulation. Baek et al. [85] examined the influence of SCG content (10, 20, 30, and 40 wt%) on PLA composites with 4,4-methylene diphenyl diisocyanate (MDI) as a coupling agent. While mechanical strength generally decreased with increasing filler content, MDI addition improved strength at 30 wt% SCGs, attributed to urethane bond formation between PLA and SCGs. Wu et al. [87] studied the biodegradation of PLA/SCG composites with and without coupling agents, finding that 20 wt% SCG-filled PLA exhibited higher mass loss after 60 days compared to neat PLA and composites containing coupling agents. Microscopy revealed increased voids and surface disruptions in PLA/SCGs without coupling agents. A photodegradation test preceding biodegradation accelerated the degradation process, reduced crystallinity and impact strength, and increased water absorption. SEM images confirmed surface roughening, cracks, and erosions resulting from this sequential treatment.

SCG oil has been proposed as a plasticizer for PLA composites, notably in formulations containing 40 wt% recycled coffee cup paper. The addition of 30 wt% SCG oil plasticizer enhanced hydrophobicity and reduced brittleness, increasing elongation at break by 86%. This composite showed balanced mechanical properties and non-toxic behavior suitable for food applications, particularly in the coffee beverage industry [88].

Additionally, SCG-derived luminescent quantum dots (QDs) have been used as fillers in PLA composites. A 1 wt% QD addition provided excellent UV shielding and visible light transmission, along with substantial improvements in tensile strength (+69%) and elastic modulus (+67%) compared to neat PLA. This material is promising for high-performance nanocomposites requiring transparency and UV protection [89].

A notable study employed PP and lignin as compatibilizers for PLA/SCG composites, resulting in enhanced mechanical, thermal, and morphological properties relative to neat PP or lignin alone. The synergistic effect of this combination demonstrated efficient compatibilization for PLA/SCG composites [101].

Composites have also gained widespread attention in additive manufacturing (AM) due to their versatility and processability. Recent research has focused on developing cost-effective polymer-based AM materials by incorporating various fillers, including SCG waste. The following section examines the feasibility of using SCGs in different AM techniques, assessing their effects on printability, mechanical properties, thermal properties, and morphology.

5. SCG-Based Composite for 3D Printing

As a sustainable materials development approach, integrating SCGs into polymer composites for 3D printing has gained significant attention. The preparation process varies depending on the printing technique to ensure optimal performance. For FDM, SCGs are blended with a polymer matrix and extruded into continuous filaments. In contrast, FGF uses SCG/polymer granulates directly with large-format 3D printers. Meanwhile, DW employs SCG-based resin formulations for the precise deposition of highly viscous materials. Each method requires specific processing conditions to enhance printability, mechanical integrity, and adhesion properties. This section reviews current literature on SCG composite 3D printing, covering key processing steps, material characteristics, and the challenges in developing these sustainable composites.

Figure 4 illustrates the flow diagram of the 3D printing process using SCGs. The process begins with the mixing of SCGs and a polymer matrix. Depending on the chosen printing technique, the material can follow different pathways: it may undergo granulation and extrusion to produce filaments for FDM printing, be used directly as granules in FGF, or be mixed into a resin for Direct Writing (DW) applications. The figure highlights the versatility of SCG-based composites and how each printing approach involves specific processing steps to achieve a final 3D-printed object.

5.1. Composite Preparations

According to the literature, the process begins with the preparation of SCGs, which are first dried in an oven to remove moisture and then sieved to achieve the desired particle size. Subsequently, SCGs are mixed with polymer pellets at varying concentrations via extrusion to ensure a homogeneous distribution within the matrix [102]. In FDM, the production of suitable filaments is a prerequisite before the printing process. For instance, Chang et al. (2019) [13] investigated the fabrication of SCG/PLA composite filaments, focusing on material blending, extrusion parameters, and filament quality. Pulverized PLA powder was mixed with oil-extracted SCGs (Ox-SCGs) in concentrations ranging from 0 to 20 wt% using a V-type powder mixer. The blend was dried in a vacuum oven at 40 °C and then extruded at 162 °C using a single-screw extruder, producing filaments with a controlled diameter of 1.75 ± 0.05 mm. In a study by Boughanmi et al. (2024) [103], PLA was blended with various amounts of SCGs (up to 15 wt%) and extruded using a twin-screw extruder (Figure 5a) to produce pellets, which were subsequently processed using a 3Devo Filament Maker (Figure 5b) to fabricate filaments.

Similarly, Li et al. (2021) [14] prepared MN-DSCGs/PLA filaments using a single-screw extruder. The SCGs were treated with 5% NaOH at 90 °C for 3 h, bleached with 3% NaClO₂ at 80 °C, and dried at 40 °C for 10 h. Both the modified SCGs and PLA were dried at 50 °C, blended at various ratios (0–20 wt%), and extruded at 180 °C. The filaments were cooled in a water bath and subsequently dried at 70 °C.

In another study, W. Yu et al. (2023) [15] used a precision drawing machine to fabricate SCG/PLA filaments. Dried SCGs were mixed with PLA at concentrations of 1%, 3%, 5%, and 7%, with each 100 g mixture homogenized in a V-shaped mixer for 4 h. After pre-drying the PLA at 50 °C for 4 h, the filament was extruded with fine-tuned settings (temperature, extrusion speed, and traction speed) to maintain a diameter of 1.75 mm. The drawing temperature was reduced to 140 °C. Likewise, I. K. M. Yu et al. (2023) [104] utilized a single-screw extruder set to 155 or 185 °C for filament production. SCGs were ground and sieved to a particle size of 0.1–0.2 mm and premixed with PLA (≤0.9 mm) at room temperature. The composite was then extruded and passed through a filament puller (3DPANY, Zhengzhou, China) equipped with cooling fans and a diameter sensor to ensure a consistent 1.75 mm filament.

Wang, Yong, and Loo (2024) [105] also reported on SCG/PLA filament fabrication. SCGs were sieved through a 45-mesh screen, dried at 55 °C for at least 24 h, and blended with PLA. The filaments, including PLA-TPS and PLA–food waste blends, were extruded using a Noztek Pro single-screw extruder (Noztek, West Sussex, England) at 178 °C and cooled with a fan.

More recently, the focus has expanded toward FGF as an alternative to filament-based printing. Studies have explored the use of SCG/polymer pellets (Figure 5c) instead of filaments [16,17,18]. Lage-Rivera et al. (2024) [16], for example, developed pellets containing 3, 5, 10, 15, and 20 wt% SCG biofiller. These composites were melt-mixed using a co-rotating twin-screw extruder with two mixing zones, operated manually at 15 rpm and with barrel temperatures between 150 and 175 °C. After extrusion, the filaments (Figure 5d) were cooled down by an impinging air current and winded with an EX6 spooler (Filabot, Barre, VT, USA), ensuring a diameter of 1.75 ± 0.01 mm.

In addition, DW 3D printing has emerged as a viable method for resin-based formulations that do not require filament preparation. Alhelal et al. (2021) [19] utilized SCG-derived biochar to reinforce epoxy composites for DW 3D printing. After washing and drying, the SCGs were subjected to pyrolysis in an autogenic high-pressure/high-temperature reactor. The resulting biochar was dispersed into part A of an epoxy resin using ultrasonication and mixed with the hardener before being printed using a DW system to fabricate composite structures.

5.2. Three-Dimensional Printing Techniques for SCG-Based Composites

After the preparation of SCG-based composites, their suitability for 3D printing must be assessed to ensure successful fabrication. This section explores the AM techniques applicable to SCG composites, along with the key processing parameters reported in the literature that influence printability and part performance. AM enables the fabrication of three-dimensional parts by depositing material layer by layer [106]. Among the available AM technologies, extrusion-based methods are widely applied to composite materials [107]. According to the literature, three primary AM techniques have been employed for SCG-based composites: FDM, FGF, and DW.

FDM has been the most extensively explored method for processing SCG/polymer composites. This technique involves the layer-by-layer deposition of melted thermoplastic filament through a nozzle. It is commonly used for small-format systems and is compatible with a variety of thermoplastic-based composites. Several studies have investigated the incorporation of SCGs into thermoplastic matrices for FDM printing [13,14,15,102,103,104,105]. Yu et al. (2023) [104] used a CR200B FDM printer (Creality, Shenzhen, China) to print dumbbell-shaped specimens (ASTM D368, Type IV) [108] at 100% infill and 200 °C. More complex structures, such as joints, display items, and cup holders, were also successfully printed using SCG/PLA and STL/PLA filaments (Figure 6). W. Yu et al. (2023) [15] employed an FDM printer (Figure 7b) for SCG-based composite fabrication.

FGF (also known as Fused Particle Fabrication, FPF) offers several advantages over FDM, such as cost-effective use of recycled materials, flexibility in composite formulation, and elimination of filament extrusion during feedstock preparation [109,110]. Although research in this area is still emerging, three recent studies have evaluated SCG-based composites for FGF printing [16,17,18]. Lage-Rivera et al. (2024) [16] printed 3Dbenchy samples (Figure 8) using a CR-10 v2 printer (Creality, China) fitted with a pellet extruder. Samples were printed at a layer height of 0.3 mm, nozzle diameter of 0.8 mm, bed temperature of 60 °C, and a maximum speed of 10 mm/s, with 100% infill and three perimeters. Romani et al. (2024) [18] used a large-format FGF printer (1 m³ build volume) with a single-screw extruder and a 3 mm nozzle (Figure 7a). They optimized parameters using both extrusion trials and vase-mode prints. Samples included tensile specimens (ASTM D638-22 Type I) [108] and complex 3D parts with nonplanar slicing (Figure 9). PLA was processed at 180–190 °C, and LDPE/HDPE at 160–170 °C. Paramatti et al. (2024) [17] printed a full-sized coffee table (300 × 300 × 450 mm) (Figure 10) using nonplanar slicing, highlighting the potential of FGF for large-scale applications. Despite limited research, FGF presents a promising alternative to FDM due to its use of pelletized feedstock, reduced material costs, and broader formulation flexibility.

DW differs significantly from FDM and FGF, as it does not require melting or extrusion of the feed material. In the case of thermoset-based composites, the curing process starts shortly after deposition, resulting in improved interlayer bonding and enhanced strength in the build direction [111,112,113]. Alhelal et al. (2021) [19] utilized a DW printer equipped with a syringe-based system to print a biochar-reinforced epoxy composite (Figure 7c). Printing was conducted with a 0.19-inch nozzle, 50 mm/s speed, and 100% rectilinear infill at a 90° angle. The bed temperature was initially set to 60 °C and increased by 3 °C per layer to improve interfacial adhesion. Post-curing was carried out at 100 °C for 2 h, resulting in samples approximately 5.2 mm thick.

Table 3. Comparative overview of 3D printing techniques used for SCG-based composites.

3D Printing Technique	Full Name	Principle	Advantages	Limitations	Ref
FDM	Fused Deposition Modeling	Deposition of a melted filament layer by layer through a heated nozzle	Widely used, Low cost, Easy to adapt for SCG-based filaments	Requires good filament quality, Risk of nozzle clogging	[13,14,15,102,103,104]
FGF	Fused Granular Fabrication (Pellet-based)	Direct extrusion of melted polymer pellets without filament	Uses raw pellets, Lower material preparation, and Cost-effective	Less common, more complex process control	[17,18]
DW	Direct Writing	Extrusion of viscoelastic inks or pastes through a fine nozzle	Allows for printing of bio-based or water-based pastes, a low-energy process	Low resolution, Slow speed, Requires optimized paste formulation	[19]

provides a comparative overview of 3D printing techniques used for SCG-based composites.

5.3. Composite Characterizations

5.3.1. Mechanical and Physical Properties

The mechanical and physical properties of 3D-printed SCG/polymer composites are essential for evaluating their performance and applicability in AM. These properties are influenced by both the material composition and processing conditions, including strength, flowability, and printability. From composite preparation to printing, critical factors such as SCG dispersion, SCG/polymer interfacial adhesion, extrusion parameters, and print settings must be optimized to ensure high-quality printed parts.

Table 4. SCG/Polymer composite in AM and its impact on mechanical and physical performance (PLA = polylactic acid; EP = epoxy; HDPE = high-density polyethylene; LDPE = low density polyethylene).

Polymer Matrix	Filler	Filler Content (wt%)	Filler Size	Treatment	Methods	Results	Ref
PLA	SCG	0, 5, 10, 15, 20	_	Oil extraction	FDM	At 20 wt%, Ox-SCG loading resulted in a remarkable 418.7% increase in toughness, reaching 25.24 MJ/m3, while only incurring a 26% reduction in storage modulus compared to pure PLA.	[13]
PLA	SCG	2, 4, 6, 8, 10, 15, 20	700 μm	Alkaline treatment	FDM	PLA/MN-DSCG composites show good mechanical performance up to 10% MN-DSCGs. Viscosity decreases with MN-DSCG content up to 15%, improving melt flow and processability, but increases beyond 15% due to the rigid structure of MN-DSCGs.	[14]
EP	SCG	1, 3	<100 μm	Pyrolysis	DW	Storage modulus at 1 wt% biochar filler improved up to 27.5%, flexural modulus improved by 55.55%, and flexural strength improved by 43.30%. At 3 wt% biochar, the storage and flexural modulus decrease significantly due to agglomeration and poor polymer crosslinking.	[19]
PLA, LDPE and HDPE	SCG	10	>900 µm	_	FGF	PLA/SCG composites exhibit stable tensile strength (13.9 MPa) and higher elongation at break (3%) compared to conventional filaments. The elastic modulus varies, with rLDPE/SCGs reaching 107.4 MPa and HDPE/SCGs 587 MPa. The composites show non-Newtonian behavior, with decreasing viscosity improving printability.	[18]
PLA	SCG	5, 10	_	_	FGF	PLA/SCG composites maintain good tensile strength (49 MPa for 95PLA5CM, ~47 MPa for 90PLA10CM) with a slight decrease in elastic modulus (10%). They exhibit non- Newtonian behavior, improving printability, and interlayer adhesion.	[17]

summarizes the findings from recent studies on SCG/polymer composites in AM and their impact on mechanical and physical behavior. Key steps during feedstock preparation, such as drying, particle size control, filler content, mixing, dispersion, and filament diameter calibration, are typically well-controlled. Drying reduces moisture content, while effective mixing ensures homogeneous dispersion of SCGs. Extrusion parameters (temperature, speed, and cooling) are adjusted to maintain a filament diameter of 1.75 ± 0.05 mm, ensuring print consistency. Additionally, chemical modifications to SCGs improve dispersion and interfacial adhesion with the matrix.

Print parameters also significantly influence the resulting properties. For example, the extrusion temperature is typically set at 220 °C within the processing range of many thermoplastics. Still, it must be carefully managed, as excessive heat can degrade lignocellulosic SCGs, thereby compromising mechanical performance.

Most studies have examined the mechanical behavior of PLA-based composites reinforced with SCGs using FDM. Boughanmi et al. (2024) [103] reported that low SCG content (3–5 wt%) increases density and stiffness, with Young’s modulus rising by 12.37% at 3 wt%. However, higher SCG contents (10–15 wt%) result in decreased density (due to porosity) and tensile strength (from 23 MPa to 6.28 MPa at 15 wt%) due to poor SCG/PLA adhesion. Elongation at break also drops, reducing ductility. Yu et al. (2023) [104] found that SCGs improve stiffness but reduce flexibility and tensile strength. At 20 wt% SCG, tensile strength reached 18.2 MPa at 185 °C. Lower extrusion temperatures (155 °C) improved strength to 21.6 MPa. Young’s modulus increased, while elongation at break decreased. Additionally, the melt flow rate and viscosity dropped, improving flowability. Another study evaluated PLA composites reinforced with micro/nano-structured decolorized SCGs (MN-DSCGs). Up to 10 wt% MN-DSCGs preserved comparable tensile and flexural strength to neat PLA. At 20 wt%, flexural modulus increased by 42%, but higher contents led to mechanical deterioration due to microvoids and poor adhesion. Rheological tests confirmed reduced viscosity up to 15 wt% MN-DSCGs, improving melt flow and printability. Excessive filler increased viscosity due to the rigid SCG structure [114]. Chang et al. (2019) [13] studied oil-extracted SCGs (Ox-SCGs) and found that up to 20 wt% Ox-SCGs enhanced impact toughness by 418.7% (25.24 J/m³) while reducing storage modulus by 26% at room temperature. The composites displayed ductile fracture, and viscosity measurements confirmed improved flowability and printability.

Although most studies focused on FDM, recent work has explored the potential of FGF for large-scale applications. Paramatti et al. (2024) [17] evaluated the mechanical behavior of PLA/SCG composites for FGF printing. The elastic modulus decreased by ~10% compared to neat PLA, while ultimate tensile strength remained stable (49 MPa for 95PLA5CM; 47 MPa for 90PLA10CM). Elongation at break remained ~2%. SCG addition reduced viscosity, improved melt flow, and enhanced interlayer bonding, reducing defects and improving printability. Romani et al. (2024) [18] studied PLA/SCG (10 wt%) and reported a tensile strength of 13.9 MPa, elastic modulus of 1546.8 MPa, and elongation at break of 3%. rLDPE/SCG (10 wt%) showed lower tensile strength (6.5 MPa) but higher elongation (48.9%), indicating greater flexibility. HDPE/SCG (10 wt%) exhibited intermediate strength (10.9 MPa) and modulus (587 MPa) but suffered from poor adhesion and porosity. Melting temperatures of all blends remained stable (PLA/SCG: 153.5 °C; rLDPE/SCG: 106.6 °C; HDPE/SCG: 131.5 °C), and all displayed shear-thinning behavior favorable for extrusion.

In addition, DW printing based on resin systems has also been explored. Alhelal et al. (2021) [19] demonstrated that adding 1 wt% SCG-derived biochar to epoxy enhanced storage modulus (by 27.5%), flexural modulus (by 55.55%), and flexural strength (by 43.3%). However, performance declined at 3 wt% due to agglomeration and poor crosslinking. While increased viscosity improved shape stability, it also risked nozzle clogging. Optimizing biochar content is crucial to balancing stiffness, printability, and thermal stability.

5.3.2. Morphology and Surface Quality

The morphology of SCG-reinforced composites has been widely investigated using scanning electron microscopy (SEM), which remains the primary technique for analyzing fracture surfaces and microstructural features [13,14,15,17,19,104,105]. SEM observations provide valuable insights into filler dispersion, interfacial adhesion, and the failure mechanisms of SCG/polymer composites.

W. Yu et al. (2023) [15] examined the SEM morphology of PLA composites reinforced with spent coffee grounds (SCGs). Their results revealed poor dispersion of coffee grounds within the PLA matrix, as illustrated in Figure 11(a1–a3). This microstructural inhomogeneity explains the observed variations in mechanical properties, such as the modulus of elasticity, modulus of rupture, tensile strength, and density.

Chang et al. (2019) [13] investigated the morphology of oil-extracted spent coffee grounds (Ox-SCGs) incorporated into PLA. SEM images (Figure 11(b1)) revealed that Ox-SCG particles exhibit predominantly spherical and polygonal morphologies, which contribute to improved load transfer and uniform stress distribution. In Figure 11(b2), the fracture surface of a pure PLA specimen shows a smooth, brittle fracture. In contrast, the 20 wt% Ox-SCG composite (Figure 11(b3)) displays an elongated and rough fracture surface, indicating enhanced ductility due to the embedded Ox-SCG particles.

In the study conducted by Alhelal et al. (2021) [19], the morphology of SCG-derived biochar in 3D-printed epoxy composites was analyzed. The neat epoxy sample (Figure 11(c1)) showed excellent interlayer adhesion. At 1 wt% biochar loading (Figure 11(c2)), the biochar particles were uniformly dispersed, contributing to improved mechanical properties. However, at 3 wt% loading (Figure 11(c3)), particle agglomeration became apparent. These agglomerates act as stress concentrators and compromise both the mechanical performance and surface quality of printed parts, potentially leading to premature failure under load.

Li et al. (2021) [14] further explored the effects of chemical treatment on the morphology of PLA composites reinforced with micro/nano-structured decolorized SCGs (MN-DSCGs). SEM observations showed that the fracture surfaces became rough and textured, with a slight yellowish hue in filaments containing MN-DSCGs (Figure 12). Importantly, homogeneous dispersion was maintained across the matrix, even at high filler loadings up to 20 wt%. No signs of particle agglomeration were detected, demonstrating the effectiveness of chemical treatment in promoting uniform filler distribution and enhancing interfacial bonding.

However, Paramatti et al. (2024) [17] used optical microscopy to examine the morphology of PLA/SCG 3D printed samples. From the micrographs of the sample surfaces (Figure 13a,c,e), the SGC content has increased, as well as their inhomogeneous dispersion into the PLA matrix as seen in the fracture cross section (Figure 13b,d,f).

For 3D printable materials incorporating biomass residues, the optimal filler size usually ranges from 50 to 500 μm; larger particles may lead to clogging and affect the composite’s properties. However, the use of large-format 3D printers, as demonstrated in this study, helps minimize these issues by employing larger nozzle diameters, which improve material flow and ensure more consistent extrusion. This is shown by the fracture cross sections, which reveal a variable granulometry of SCGs, up to a maximum dimension of 900 μm. Also, Chang et al. (2019) [13] used transmitted optical microscopy to evaluate the Ox-SCG distribution in the PLA matrix from 0 to 20 wt%. They observed that the Ox-SCG particles were distributed uniformly with a minor agglomeration throughout the matrix. As the Ox-SCG loading approaches 20 wt%, the light becomes much less permeable through the specimen.

Nevertheless, optimization parameters have never been explored in the literature, and there is no clear understanding of how researchers ensure the extrusion of filaments with uniform dimensions. Moreover, studies on the use of SCGs in AM technologies remain limited, with little focus on processing conditions, material compatibility, and printability. This gap underscores the need for further research to improve the consistency, mechanical properties, and overall performance of SCG-based composites in additive manufacturing.

5.3.3. Thermal Properties

Understanding the thermal properties of SCG-based composites is essential to evaluate their performance in temperature-dependent applications, particularly in 3D printing. Thermal analysis refers to any measurement technique in which a change in the property of a sample is related to a controlled variation in temperature. Among the various thermal analysis techniques, the most commonly used are Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), Differential Scanning Calorimetry (DSC), Thermomechanical Analysis (TMA), and Dynamic Mechanical Analysis (DMA) [115]. Thermal behavior usually relies on the composition of the fiber, including components like cellulose, pectin, lignin, and ash. Moreover, moisture content influences the thermal response [116]. Several studies have employed TGA to investigate the thermal stability of SCG-based composites [13,14,19,104]. Chang et al. (2019) [13] evaluated the thermal stability of Ox-SCG (oil-extracted spent coffee ground) PLA composites. Thermal degradation of polymeric composites involves three main stages: evaporation of water, decomposition of organic constituents, and final carbonaceous degradation. This degradation profile has been reported for Ox-SCG materials: the evaporation of water at sub-100 °C, the breaking down of polysaccharides at 250 °C, and the mass drop at 450 °C (Figure 14a). The authors found that an increase in Ox-SCG concentration resulted in a gradual reduction in the onset temperature of thermal degradation of the composite compared to pure PLA. The effect follows a linear trend and is most noticeable in a 20% Ox-SCG specimen. Based on TGA results, Ox-SCG/PLA composites have little to no degradation at 195 °C during 3D printing. As a result, the composite is thermally stable at its processing temperature. Li et al. (2021) [14] reported that the thermal behavior of PLA-based filaments reinforced with MN-DSCGs (micro/nano-structured decolorized SCGs) assessed their suitability for 3D printing. They found a similar pattern of thermal decomposition in all specimens since PLA is the main component (Figure 14b). However, the composites showed lower thermal degradation temperatures than pure PLA. Regarding the effect of MN-DSCGs content, the onset degradation temperature of the composites gradually decreases as the filler content increases. Even with 20 wt% MN-DSCGs loading, the specimen shows an onset degradation temperature of 255 °C, which remains higher than the proper 3D printing temperatures for PLA (~200 °C), suggesting reliable thermal stability of the composite filaments throughout the printing process. Recently, the thermal behaviors of SCG/PLA composites at 20 wt% were reported by I. K. M. Yu et al. (2023) [104]. The results of the composite reveal good thermal stability up to 185 °C, with a limited mass loss of 0.67%, confirming the absence of significant amounts of volatile compounds or moisture within this temperature range (Figure 14c). The maximum degradation temperature is recorded at 361 °C, representing a slight decrease compared to pure PLA (384 °C), suggesting limited interaction between the polymer matrix and the coffee grounds filler. This reduction in thermal stability can be attributed to an incompatibility between the hydrophilic nature of the SCG components (fibers, carbohydrates) and the hydrophobic character of PLA. Furthermore, the mass loss at the degradation peak is higher (68.2%) than that of pure PLA (63.7%), indicating that the introduction of SCGs alters the thermal degradation kinetics of the composite. In summary, although the addition of SCGs slightly affects the overall thermal stability, the composite maintains thermal properties compatible with the extrusion-based 3D printing process.

Alhelal et al. (2021) [19] performed a TGA on reinforced Epoxy resin by semi-crystalline carbon biochar derived from spent coffee grounds (SCGs). They compared the thermal stability of neat epoxy resin with that of 1 wt% and 3 wt% biochar-reinforced epoxy, as shown in Figure 14d. The onset of degradation, defined as 5% weight loss, was recorded at 325 °C for both neat epoxy and epoxy loaded with 1 wt% biochar. However, in samples containing 3 wt% biochar, it occurred at around 328 °C. Based on derivative weight change graphs, the maximum rate of decomposition values were 365.47 °C for neat epoxy with a 46% weight loss, 365.86 °C with a weight loss of 46%, and 366.96 °C with a weight loss of 49% for 1 wt% and 3 wt% loaded samples, respectively. The overall thermal properties did not show significant improvement, indicating negligible effects on thermal stability. The residue of the epoxy systems was higher for biochar-loaded epoxy composites due to the presence of carbon, which is inherently more stable.

Differential scanning calorimetry (DSC) is also a widely used analytical technique for studying the thermal properties of biocomposites. It enables the measurement of thermal parameters such as the glass transition temperature (Tg), melting temperature (Tm), crystallization temperature (Tc), heat of fusion (ΔHf), and degree of crystallinity (Xc) [117]. Several studies have been conducted using DSC to investigate the thermal properties of SCG-based composites for 3D printing applications [15,17,18,116].

Table 5. DSC data for SCG-based composites.

Matrix	Process Methods	Composition	Tg (°C)	Tcc (°C)	ΔHcc (J/g)	Tm (°C)	ΔHm (J/g)	Xc (%)	Ref
PLA	-	PLA	60.4	108.2	−29.6	169.5	35.0	5.8	[15]
		PLA/1% SCG	60.6	110.4	−25.0	170.2	29.8	5.2
		PLA/3% SCG	60.2	109.7	−28.8	169.4	31.8	3.3
		PLA/5% SCG	60.5	108.6	−25.2	169.5	33.3	9.2
		PLA/7% SCG	60.2	110.3	−21.2	169.8	26.5	6.1
PLA	Alkaline treatment	PLA	61	106.13	-	151.93	-	1.60	[14]
		PLA/2% MN-DSCGs	60.76	105.72	-	151.69	-	1.81
		PLA/4% MN-DSCGs	60.53	104.87	-	151.05	-	1.83
		PLA/6% MN-DSCGs	60.77	104.73	-	151.78	-	1.86
		PLA/8% MN-DSCGs	60.86	104.65	-	152.07	-	2.54
		PLA/10% MN-DSCGs	60.58	104.80	-	151.26	-	2.66
		PLA/15% MN-DSCGs	60.42	104.95	-	150.71	-	3.09
		PLA/20% MN-DSCGs	60.28	105.27	-	151.23	-	2.66
PLA		PLA	61.2	133.6	0.6	154.2	1.9	2	[18]
		PLA/10% SCG	59	131.4	5.8	153.5	6.7	7.2
rLDPE	-	rLDPE	109.0	-	-	112.0	107.0	39.0
		rLDPE/10% SCG	72.3	-	-	106.6	82.9	28.3
HDPE	-	HDPE	-	-	-	134.1	187.4	63.8
		HDPE/10% SCG	-	-	-	131.5	171.5	58.5
PLA	-	PLA	361.2	133.6	0.6	154.2	1.9	-	[17]
		CM	61.8	-	-	121.2	6.9	-
		95PLA5CM	61.8	131.1	8	154.7	10.6	-
		90PLA10CM	59.5	130	6.8	154	8.3	-

summarizes thermal properties of SCG-based composites obtained by DSC. According to Yu et al. (2023) [15], the DSC analysis showed that incorporating SCGs into PLA had little impact on the crystallinity of PLA (less than 10%), with the glass transition temperature (Tg) of pure PLA recorded at 60.4 °C, due to the large amount of coffee grounds. Although SCGs were dispersed in the PLA matrix, the molecular chain spacing and intermolecular forces remained largely unchanged, resulting in no significant change in Tg. When a small amount of SCGs is added, it can slightly increase chain spacing, reduce intermolecular forces, and enhance chain mobility, potentially lowering the glass transition temperature. The melting temperature also decreased due to the increased mobility of chain segments, which reduced the energy required for the melting process. SCGs have little effect on the crystallinity of PLA (below 10%), while mechanical tests indicate that their content significantly influences the mechanical properties of 3D printed materials. This suggests that the mechanical behavior of PLA/SCG composites is not directly related to their crystallinity. Additionally, the glass transition, cold crystallization, and melting temperatures of PLA/SCG materials show minimal variation compared to pure PLA, indicating similar thermal performance. Therefore, standard PLA filament processing and FDM printing temperatures are suitable for PLA/SCG composites [118].

In study by Li et al. (2021) [14], Adding MN-DSCGs to PLA had minimal influence on the thermal transitions of the composites, with only slight variations observed in the glass transition temperature (Tg) and melting temperature (Tm) compared to neat PLA. In contrast, the cold crystallization temperature (Tcc) tends to decrease with increasing MN-DSCGs content, which could be due to the nucleating effect of MN-DSCGs, which promotes PLA crystallization [119]. Moreover, the degree of crystallinity (Xc) increased progressively with MN-DSCGs content up to 15%, indicating enhanced crystal formation. However, at higher filler loadings, a reduction in Xc was noted, likely due to the excessive presence of MN-DSCGs within the matrix, which limits the available free volume for crystallite development [120,121]. Paramatti et al. (2024) [17] used spent SCGs in the form of a coffee masterbatch (CM). This masterbatch, consisting of a PLA carrier, a dispersant, and dehydrated SCGs, facilitates the uniform dispersion of the powder into the polymer matrix. The PLA pellet showed no crystallization or melting during the second heating scan, while the composite masterbatch (CM) exhibited only melting. This difference is likely due to their distinct thermal histories and processing steps, such as pelletizing. The absence of a melting peak suggests the cooling rate was too fast for PLA polymer chains to reorganize, preventing recrystallization. For the 3D printed samples, the recorded temperatures (Tg, Tcc, and Tm) showed no clear trend, and differences between the formulations were minimal. Overall, the addition of spent coffee grounds (SCGs) has a limited effect on the thermal behavior of PLA/SCG composites, indicating that the thermal properties of the composites are comparable to those of pure PLA. In addition to PLA, numerous other polymers are widely employed in additive manufacturing, including rLDPE and HDPE [18]. DSC analysis of the three formulations (PLA/SCGs, rLDPE/SCGs, HDPE/SCGs) revealed two distinct thermal behaviors depending on the polymer matrix. PLA/SCGs composites exhibited all thermal transitions (Tg, Tcc, Tm), whereas PE-based composites showed only melting. The melting temperatures aligned well with literature values and those of the pure polymer matrices. The degree of crystallinity varied among formulations, with a slight increase observed for PLA/SCGs and a decrease for PE-based composites, likely due to the irregular particle size of SCGs causing crystallization defects. Overall, SCG addition had a limited effect on thermal behavior but influenced crystallinity, which can impact mechanical properties and failure modes.

Dynamic Mechanical Analysis (DMA) is currently used to evaluate the SCG-based composites. DMA measures viscoelastic properties of a material in terms of stiffness and damping. However, Dynamic mechanical properties belong to the behavior of a material when it is subjected to cyclic load. These properties are reported as storage modulus, loss modulus, damping, and glass transition temperature [122]. Only two studies have reported the use of DMA to characterize SCG-based composites. Alhelal et al. (2021) [19] investigated the effect of biochar loading on the dynamic mechanical properties of epoxy resin. The DMA results (Figure 15a) show that the storage modulus of epoxy resin improved by about 27.5% with 1 wt% biochar, indicating increased stiffness at room temperature. However, with 3 wt%, the modulus decreased by ~21%, likely due to poor print quality, nozzle clogging, and weak interlayer bonding. The tan delta peak, which corresponds to the glass transition temperature (Tg), showed little change with biochar loading. As temperature increased, the storage modulus decreased for all samples, with a notable sharp drop between 45 and 65 °C in the 1% biochar sample, possibly caused by poor interfacial bonding between biochar and polymer chains. Although higher filler content typically increases stiffness, this effect was less evident in 3D printed parts made by the Direct Writing method. Chang et al. (2019) [13] examined the effect of Ox-SCG loading on the dynamic mechanical properties of PLA composites (Figure 15b). They reported that incorporating 20 wt% Ox-SCG/PLA composite exhibits a 26% decrease in storage modulus at room temperature compared to pure PLA, likely due to the plasticizing effect of the fiber additive. Increasing Ox-SCG content shifts the onset of storage modulus reduction to lower temperatures. At low loadings (5–10 wt%), Ox-SCG acts as a nucleating agent, enhancing storage modulus during cold crystallization, but at higher loadings (10–20 wt%), it hinders polymer chain mobility and inhibits crystallization. The glass transition temperature (Tg) slightly decreases with higher Ox-SCG content, and the damping factor reduces, indicating increased material stiffness and reduced molecular mobility at higher filler concentrations.

6. Overall Comparison Between SCG Composites and Fibers

Composites based on spent coffee grounds (SCGs) offer a promising balance between mechanical performance, environmental impact, and cost. Compared to traditional fillers such as wood flour and glass fibers, SCGs provide moderate mechanical properties, which are suitable for many 3D printing applications, while significantly reducing the environmental footprint. Glass fibers deliver excellent mechanical reinforcement but are associated with high energy consumption and carbon emissions during production. Wood flour, although derived from a renewable resource, still requires energy-intensive processing and chemical additives. In contrast, SCGs are a readily available, biodegradable post-consumer organic waste that requires minimal treatment (drying and grinding), making it an economical and sustainable option. Therefore, incorporating SCGs into PLA composites represents a promising strategy for developing environmentally friendly materials while keeping production costs low.

Table 6. Comparative summary of PLA composites in 3D printing [104,123,124,125] *.

Filler Types	Tensile Strength (MPa)	Tensile Modulus (GPa)	Thermal Stability	Environmental Impact	Cost
SCG	41.6	1.3	Low	Low CO2 footprint, circular waste valorization	Very low (waste material)
Wood sawdust	7.08	3.5	Medium	Low to moderate	Very low (waste material)
Flax	50	5	Good	Low	Moderate—more expensive than waste
Jute	40	3	Medium	Low-biodegradable	Low-widely used in packaging/textiles
Sisal	30	2	Low	Low	Moderate
Kenaf	45	4	Medium	Low	Low to moderate
Ramie	35	3	Low	Low to moderate	Moderate to high
Cotton (lyocell)	25	2	Low	Moderate	High processing
Glass fibers	47	-	Good	Higher pollutant emissions	Expensive production process

* For comparison’s sake, the values corresponding to the mechanical properties were chosen as the median value for the respective composite type at the closest to 5 wt. % fiber loading.

presents a comparative summary of PLA composites in 3D printing.

7. Challenges and Outlook for SCG-Based Composites in AM

SCG waste management has emerged as a significant environmental concern due to the increasing global consumption of coffee. In response, researchers have explored various upcycling pathways for SCGs, with one of the most promising being its incorporation into polymer composites for AM. This approach not only contributes to sustainable material development but also aligns with the goals of a circular economy by reducing reliance on fossil-based polymers and minimizing organic waste. SCG-reinforced composites have the potential to serve as sustainable alternatives for producing customized, lightweight, and eco-friendly 3D-printed products in both industrial and consumer markets.

With advancements in AM technologies such as FDM and FGF, the integration of SCGs into polymer matrices offers a novel route for sustainable fabrication. However, this integration is technically challenging and demands careful consideration of material formulation, processing conditions, and printability.

One of the foremost challenges is achieving homogeneous dispersion of SCG fillers within the polymer matrix. Poor dispersion can lead to defects in the final printed parts, such as weak interfacial bonding and non-uniform mechanical properties. The control of particle size is equally critical. Oversized particles tend to clog the extrusion nozzle, compromise flow stability, and produce inconsistent filament diameters. As noted by Li et al. (2021) [14], SCG particles larger than 0.5 mm are generally unsuitable for filament fabrication due to extrusion inconsistency. To mitigate this, mechanical milling techniques have been used to obtain micro/nano-decolorized SCGs (MN-DSCGs) with particle sizes ranging from a few hundred nanometers to several micrometers. Likewise, Yu et al. (2023) [15] employed SCG particles sized between 0.1 and 0.2 mm, while other studies used particles between 180 µm and 400 µm to avoid jamming and enhance extrusion smoothness [13,14,15,16,106,108,113].

Moisture sensitivity is another major issue. SCGs are inherently hygroscopic and can retain significant amounts of moisture, which negatively impacts polymer processing and part performance. Thorough drying of SCGs before compounding is therefore essential to maintain material integrity and ensure reliable 3D printing outcomes.

Interfacial adhesion between SCG particles and the polymer matrix also plays a decisive role in determining the composite’s mechanical performance. Inadequate adhesion results in reduced strength and poor structural cohesion. Various surface treatments have been employed to improve compatibility, with alkaline treatment showing particularly promising results. Li et al. (2021) [14] demonstrated that MN-DSCGs treated with alkaline solutions exhibited excellent dispersion and interfacial bonding in a PLA matrix, even at high filler loadings of up to 20 wt%, without signs of agglomeration.

In the context of FGF, additional challenges arise, such as irregular pellet feeding, extrusion instability, and maintaining consistent print quality over large-scale parts. The rheological behavior of the material is especially important in this case; poor melt flow characteristics can result in uneven layer deposition and delamination, particularly in overhanging or unsupported structures.

To overcome these technical hurdles, a multifaceted research approach is required:

• Improved filler dispersion using a combination of mechanical grinding and chemical modification techniques.
• Enhanced interfacial adhesion through advanced compatibilizers and surface treatments.
• Optimization of filament and pellet production to ensure uniform diameter and improved flow behavior during extrusion.
• Refinement of extrusion and printing parameters to prevent common defects such as nozzle clogging, poor layer bonding, and inconsistent mechanical performance.

Moreover, machine learning (ML) techniques offer promising tools for intelligent process control. By leveraging experimental data, ML algorithms can predict optimal extrusion and printing settings for both FDM and FGF systems, thereby enhancing repeatability and part quality.

Finally, comparative studies between FDM and FGF techniques are essential to fully understand their respective advantages and limitations when processing SCG-based composites. Such investigations can inform the selection of the most suitable manufacturing strategy depending on application requirements, material properties, and production scale.

In summary, while the integration of SCGs into AM offers a sustainable and innovative path forward for bio-composite development, realizing its full potential requires continued research into material formulation, process optimization, and intelligent manufacturing technologies.