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About the Use of Recycled or Biodegradable Filaments for Sustainability of 3D Printing

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Additive Manufacturing (AM) and 3D printing are drivers for material savings in manufacturing. Owing to the continuous diffusion of 3D printing driven by low-cost entry-level material extrusion printers, sustainability of a so popular AM technology is of paramount importance. Therefore, recycling 3D printed wastes and 3D parts again at the end of their life is an important issue to be addressed. Research efforts are directed towards the improvement of the biodegradability of 3D printing filaments and the replacement of oil based feedstock with bio-based compostable plastics. The aim of this work is to describe the state of the art about development and use of recycled or biodegradable filaments in 3D printing. Beyond a critical review of the literature, open issues and research opportunities are presented.
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About the Use of Recycled or Biodegradable Filaments
for Sustainability of 3D Printing
State of the Art and Research Opportunities
Jukka Pakkanen
1,2()
, Diego Manfredi
2
, Paolo Minetola
1
, and Luca Iuliano
1
1Department of Management and Production Engineering, Politecnico di Torino, Turin, Italy
jukka.pakkanen@polito.it
2Center for Sustainable Future Technologies CSFT@PoliTo, Istituto Italiano di Tecnologia,
Turin, Italy
Abstract. Additive Manufacturing (AM) and 3D printing are drivers for material
savings in manufact uring. Owing to the continuou s diffusion of 3D printing dri ven
by low-cost entry-level material extrusion printers, sustainability of a so popular
AM technology is of paramount importance. Therefore, recycling 3D printed
wastes and 3D parts again at the end of their life is an important issue to be
addressed. Research efforts are directed towards the improvement of the biode‐
gradability of 3D printing filaments and the replacement of oil based feedstock
with bio-based compostable plastics. The aim of this work is to describe the state
of the art about development and use of recycled or biodegradable filaments in
3D printing. Beyond a critical review of the literature, open issues and research
opportunities are presented.
Keywords: Additive manufacturing · 3D printing · Fused Deposition Modelling
(FDM) · Biodegradability · Recycling · Bio-based filaments · Sustainability
1 Introduction
Recycling is an important topic brought up by the European Union this year with the
circular economy initiative, wherein “the proposed actions will contribute to close the
loop of product lifecycles through greater recycling and re-use, and bring benefits for
both the environment and the economy” [1]. Unlike in subtractive manufacturing
processes, parts are fabricated layer by layer in Additive Manufacturing (AM) with a
minimum allowance for finishing operations. Since AM allows for greater material
savings than traditional processes, 3D printing can be considered a distributed manu‐
facturing technology for improving sustainability and circular economy worldwide [2].
Most of polymeric materials are produced from exhaustible resources. Around 4% of
worldwide production of oil and gas is used as feedstock for plastics and a further 3–4% is
used to supply energy for the transformation of polymeric materials [3]. On the other hand,
unlike oil-based polymers, bio-plastics are derived from renewable sources i.e. sugars and
natural fibres.
Among AM technologies, Fused Deposition Modelling (FDM) is a low-cost tech‐
nique that uses a thermoplastic filament to build parts layer after layer [4]. The popularity
of FDM started with expiration of the patent of Sir Scott Crump by Stratasys and the
© Springer International Publishing AG 2017
G. Campana et al. (eds.), Sustainable Design and Manufacturing 2017, Smart Innovation,
Systems and Technologies 68, DOI 10.1007/978-3-319-57078-5_73
subsequent open-source Reprap project [5], that ever since has become the preferred
choice of makers. The FDM process is more popularly known as 3D printing and belongs
to the material extrusion category according to the ISO/ASTM terminology [6]. FDM
systems or 3D printers have a simple design consisting of a Cartesian structure with
three controlled axes, up to three hot extrusion heads and a building platform. The price
of consumer-end 3D printers starts from 100 USD and makes these machines affordable
and appealing to many people. It can be forecasted that more and more people will adopt
this technology in the near future. People will 3D print amazing pieces, but also generate
a lot of waste if material is not correctly disposed or recycled at the end of its life.
In terms of AM sustainability, environmental impact of 3D printing should consider
the usage of resources, energy, emitted emissions and waste. Among these energetic
demand is especially critical with industrial FDM machines that use a hot sealed working
volume [7].
Gebler et al. used a two-step model for calculating the possible environmental impact
of all 3D printing technologies globally by 2025 [8]. They estimated that, in the best
case in which improvements are applied to 3D printing and production efficiency
increases, the use of AM for creating new parts can lead to savings of about 5% in energy
and CO
2
emissions in the manufacturing industry worldwide.
Apart from the energetic footprint, the material management of FDM waste needs
to be addressed. The waste and excess material from 3D printed parts (e.g. support
structures, filament ends and scraps) is the second important environmental aspect of
FDM. Waste management can be directly dealt by makers or final users by means of
material recycling and production of recycled filaments.
As concerns to material sustainability in 3D printing, the aim of this paper is to review
the state of the art about the use of recycled or biodegradable filaments, discussing open
issues and related research opportunities. The organization of this work is the following:
the different types of materials available as 3D printing filaments are presented in
Sect. 2, the main properties of recycled and biocompatible materials are summarized in
Sect. 3, Sect. 4 deals with the material sustainability and finally research opportunities
from open questions are discussed in the conclusions.
2 Filaments for 3D Printing
Many polymeric materials are available for 3D printing, even though the choice is very
limited when compared to that of injection moulding polymers. There are few similar‐
ities in the properties of all FDM materials: the material needs to have a low melting
point and a reduced viscosity in order to flow out of the nozzle for deposition and adhe‐
sion to the previous layer under the low pressure applied by the extrusion mechanism.
The key point for base plastics is to set the proper extrusion temperature to achieve the
right viscosity for a good flowability, whereas rheological behavior of composite fila‐
ments is more complex. The main issues of composites are related to the type of filler
and the amount of water content [9].
Acrylonitrile butadiene styrene (ABS) is one of the most common materials in 3D
printing. Good mechanical properties and extrudability make it the preferred choice of
About the Use of Recycled or Biodegradable Filaments 777
makers. It is usually not biodegradable, but the company Enviro ABS claims to have
produced a biodegradable ABS [10]. The downside of ABS is the lack of UV resistance.
To overcome this limitation, acrylonitrile styrene acrylate (ASA) has been developed.
The second most popular material is polylactic acid (PLA). PLA is made from poly‐
merization of sugars and starches, so it is biodegradable and recyclable. Other commonly
used polymers are polycarbonate (PC), polyamide (PA) family plastics, high density
polyethylene (HDPE), polyethylene terephthalate (PET) and flexible thermoplastic
polyurethane (TPU).
There are studies about the emission of fumes to close proximity while 3D printing
ABS and PLA [11, 12]. The fumes are ultra-fine aerosol (UFA) or volatile organic
compounds (VOC) particles that might be harmful for humans. In the order to reduce
the unpleasant smells and related health risks, a closed printer design and good venti‐
lation of the room is generally recommended. Bio-plastic filaments like PLA produce
less fumes and smells than oil-based ones, e.g. ABS.
2.1 Bio-degradable FDM Filaments
Apart from PLA, other biodegradable plastics used for FDM filaments are polyhydroxy-
alkanoates (PHA), polyvinyl alcohol (PVA), Polyethylene terephthalate (PET) and High
impact polystyrene (HIPS). The properties of these materials are summarized in Table 1.
Table 1. Bio-degradable materials for 3D printing
Material Produced
from
Properties Extrusion
temperature
Pros Cons
PLA Plants starch Tough, strong 160 ÷ 222 °C Bio-plastic, non-
toxic, odorless,
low-warp
Low heat
resistance,
brittle
PVA Petroleum Water-
soluble, good
barrier
190 ÷ 210 °C Biodegradable,
recyclable, non-
toxic
Expensive,
deteriorates
with moisture,
special storage
PHA Sugars with
biosynthesis
Several
copolymers,
brittle and stiff
~160 °C UV-stable,
stiffness
Elasticity,
brittle
HIPS Petroleum High impact
resistance,
soluble in
limonene
190 ÷ 210 °C Biodegradable,
low cost, similar
to ABS
Warping,
heated
printing bed
PET Petroleum Strong and
Flexible
210 ÷ 230 °C FDA approved,
Recyclable
Absorbs
moistness
PHA can be used as it is or as a mixture with PLA. PVA is a water-soluble and
biodegradable material used for support structures. Duran et al. [13] printed PVA as
support structure for ABS. They found that PVA is printable at dried condition until
45 min before it absorbs moisture from the air and it turns impossible to print.
778 J. Pakkanen et al.
Polyethylene terephthalate (PET) is a common plastic used for food containers and
tools. It is fully recyclable and safe to use with foods. Recycled filament from PET is
sold commercially by B-PET Company since 2015 [14].
High impact polystyrene (HIPS) is similar to ABS with good mechanical properties
and extrusion temperature. It is used as a support material for ABS as it dissolves in
limonene, but ABS does not.
In biocompatible and medical applications, 3D printing filaments are made of poly‐
mers with low melting temperatures. These materials can be used in FDM to create parts
that integrate within the human tissues, such as scaffolds for example. Chia et al. [15]
and Serra et al. [16] list some of these materials.
Biodegradable 3D printed hollow capsules made of hydroxypropyl cellulose (HPC)
for drug delivery systems have been presented by Melocchi et al. [17] and Pietrzak et al.
[18]. These capsules are orally consumed and the degrading of the capsule in stomach
releases the drugs concealed inside.
2.2 Bio-composite FDM Filaments
Bio-composites filaments available on the market consist of a biodegradable polymeric
matrix and bio-based fillers. Fillers can be fibres or particles. Filler contents starts from
few % up to 40% in volume. The most used thermoplastic is PLA and the filler can be
sawdust, cellulose fibres or other natural fibres. Filament manufacturers have developed
many wood like filaments for FDM [9] using different types of fibres: Bamboo, Birch,
Cedar, Cherry, Coconut, Cork, Ebony, Olive, Pine, Willow. These are used to provide
a wood-like tactile feeling to aesthetic parts. Seppänen et al. [19] have printed thermo‐
plastic cellulose derivatives on an FDM machine.
An example of filament modification is given by Kuo et al. [20] who prepared a
biomass mixture of thermoplastic starches and ABS (TPS/ABS). Plastic pellets of ABS
were first created and infiltrated with a compatibilizer for joining the two polymers
together. These pellets were then molten and filtrated with TPS and TiO
2
particles into
new pellets that were extruded into a new 3D printing filament.
At MIT self-assembly lab, David et al. [21] used a multi-material approach to apply
wood-inspired design to 3D printing. By exploiting the hydrophilic nature of wood-
based filaments as base layer and non-hydrophilic ABS or PA as reinforcement for top
layers, they produced hydro-induced actuation. A similar work in fibre actuation was
done by Duigou et al. [22], but with one deposited material only.
Xhang et al. [23] have created high conductive PLA filaments by mixing graphite
flakes into filaments by melt extrusion. Graphene mixed well with PLA and conductive
PLA filament was successfully printed in 2D and 3D.
A binder made of potato-starch was used by Marina Ceccolini, who graduated at the
University of Design of San Marino, to fabricate AgriDust. This is a material composed
of food waste for 64.5% and of the potato binder for the rest. AgriDust requires a cold
technology and can be 3D printed by replacing the classic extruder with a syringe.
Because of its constituents, the material is biodegradable and non-toxic [24].
A soy-based filament named FilaSoy was developed by students of the Purdue
University in Indiana, USA [25]. The 20–25% soy additive improves PLA performances
About the Use of Recycled or Biodegradable Filaments 779
by providing anti-microbial properties and reducing the brittleness without inhibiting
compostability and recycling.
3 Tensile Properties of 3D Printed Materials
A limited selection of materials is available for 3D printing and 3D printed parts usually
have a worse mechanical behavior than the same materials processed by extrusion,
compression or injection moulding. However, 3D printing empowers engineers and
designer with more design freedom than other traditional processes [9, 22]. Tensile
properties of FDM tensile test specimens manufactured from plastics, composites and
recycled filaments are reported in Table 2 when available from the literature (NA = Not
Available).
Table 2. Properties of tensile test specimens made from different filament materials
Material Ultimate tensile
strength (MPa)
Elongation (%) Young modulus (MPa)
ABS [26] 19.9–29.1 1.5–8.9 1910–2050
PC [26] 29.5–36.9 3–6.7 1620–2000
PLA [27] 49.1–65.5 1.7–5.0 2800–3600
PLA recycled once [28] 51 1.88 3093 ± 194
PLA recycled 5 times [28] 48.8 1.68 3491 ± 98
PLA/PHA+10–20% fibre [22] 20–30 0.9–1.1 3500–4000
PLA/PHA+10–20% fibre water
saturated [22]
15–20 0.5–0.7 3100–3600
PLA+5% pine lignin [29] 40.2–43.6 2.31–2.83 2160–2200
TPS/ABS biomass [20] 34.8–46.8 NA NA
PLA+graphite 2% [23]50 8.1 NA
PLA+graphite 8% [23]62 6.1 NA
HDPE virgin [30] 25.5 16.1 463.4
HDPE recycled once [30] 25.6 16.1 428.4
Mechanical properties of 3D printed biodegradable plastics or composites are not as
good as pure matrix materials. Fillers usually increase the melt viscosity, and therefore
flowability problems arise. Lack of fusion between layers, porosity and swelling induced
by natural fibres are also problematic [22]. Filler content in biodegradable filaments is
usually below 40% in volume; layer adhesion and extrudability are reduced increasing
this content, as well as mechanical properties [22].
Gkartzou et al. [29] investigated the processability and the mechanical properties of
a PLA filament containing 5 wt.% of craft pine lignin. The addition of lignin makes PLA
more brittle and reduces the elongation at break. Moreover, the filler causes a remarkable
increase of the superficial roughness of the PLA filament.
Some strengths and weaknesses in bio-composites with different cellulose fibres are
reported by Li et al. [9]. These are, for example, strength, hardness, flexibility and mois‐
ture sensitivity. Some of these issues might be altered through mixing differently refined
780 J. Pakkanen et al.
cellulose fibres. Markstedt et al. have printed pure cellulose with a modified 3D printer,
but the process was liquid-based instead of filament based [31].
4 Material Sustainability
Sustainability of 3D printing materials is of utmost importance for the future, since the
3D printing market is projected to grow at an annual rate of about 26% till 2020 [32].
Recycling the excess and unwanted material primarily into new feedstock or finding
new methods for the material to degenerate or compost into harmless building blocks
in nature is imperative. The same issues faced in general plastic recycling should be
dealt with while recycling FDM wastes.
Recycling by transforming the waste material into new filaments is a good primary
recycling method, especially for thermoplastics with similar resin properties and homo‐
genous source. Thermoplastic materials have been recycled since 1970’s and today there
is more knowledge and expertise about the recycling process [3335].
Nevertheless, material transformation implies also degradation. Degradation is an
irreversible process leading to a significant change in the structure of the filament mate‐
rial resulting in loss of properties. Cruz et al. [28] investigated the degradation of a 3D
printing PLA filament along 5 complete recycling cycles. They observed a trend of a
slight reduction of mechanical performances through the cycles as in conventional recy‐
cling [36, 37]. Their results show a decrease of the polymer molecular weight of about
47% after 5 recycling cycles increasing crystallinity of the plastic. The loss in molecular
weight caused a lower tensile strength, but did not affect the yield stress. In particular,
the strain at break reduced from 1.88% to 1.68%. On the contrary, the reduction of
molecular weight improved the material flowability and processability with a viscosity
reduction of 80% after 5 recycling cycles.
Within the communities of makers, Kreiger [38] and Baecher [39] have created their
own filament extruder and using a commercial manufacturing extruder Hamod [30]
demonstrated that whoever owns a shredder and an extruder can directly turn waste
plastics into new feedstock.
Kreiger et al. [38] have studied HDPE plastic collection in the USA. The study
compared the environmental effect of centralized and decentralized filament making
with recycled material available from the makers’ community. Baechler et al. [39]
calculated that the cost of producing recycled HDPE filament is between 2 and 3 USD
per kilogram with homemade extruder, while the commercial filament is sold at an
average price of 38 USD per kilogram. After calculating the environmental impact from
both recycling cases of HDPE, Kreiger et al. [38] concluded that it would be better to
centralize the plastic recycle into industrially made recycled filament as the scale would
be larger and more viable. In their research, Baechler et al. [39] found out that more
automation and product control for the extruder is needed to produce homogeneous
filament. The properties of parts printed with recycled HDPE were lower than those of
the same pieces made of virgin material. However, different results were reported by
Hamod [30], whose study achieved similar mechanical properties for both virgin HDPE
and recycled HDPE. The HDPE was only produced and recycled once and both virgin
About the Use of Recycled or Biodegradable Filaments 781
and recycled filaments were made using the same equipment. Further recycling and
production cycles as well as impurities introduced into the material by recycling make
the filament properties decay as described for PLA by Cruz et al. [28] or reported in the
case of injection moulding [40].
To help identifying the thermoplastic resins after 3D printing, recycling material
dependent coding is proposed and investigated by Hunt et al. [41]. Codes like those of
conventional plastic industry could be added on the surface of 3D printed parts to make
recycling straight forward by easy identification of the plastic blends. In industrial
sorting, since plastics are sorted out by means of density and other properties, these
markings are not relevant as in visual sorting.
5Conclusions
Unlike injection moulding materials, not all thermoplastics are available or suitable for 3D
printing. Moreover, 3D printed materials usually have worse properties than injection
moulded ones. So far, the efforts of many researchers were directed to improve the 3D
printing feasibility for new bio-based or compostable filaments, but there is a lack of
performance testing for these filaments in the literature. The degradation of filaments
during repeated recycling needs to be studied more in detail like Cruz et al. [28] did for PLA.
Contamination of extruded filament should be prevented. On one side, contamination
can negatively affect material properties and mechanical performances. On the other
side, contaminants can be the source for the development of UFA and VOC emissions
and thus cause health hazards and risks.
Further development and testing of 3D printing filaments is needed to better char‐
acterize their properties in terms of mechanical behaviour and compostability. Because
biodegradable plastics have lower mechanical properties and restricted life cycles, user
should find short life-cycle applications. These applications for biodegradable filaments
need to consider the environmental conditions (e.g. moistness, exposure to UV radia‐
tions, etc.) that can negatively affect the filament extrusion or the part swelling when
part is in use. However, in some cases, swelling might also be a desired effect for part
functionality as shown by David et al. [21]. On the contrary, the advantage of these
biodegradable plastics is that printed components can be disposed at the end of their life
in a more environmental-friendly manner by recycling them to make new filament or
by composting. The characteristic of recyclability is fundamental for the sustainability
of 3D printing, since the adoption and popularity of this AM process has been constantly
increasing in recent years.
Filament recycling should be aimed at preserving interesting material properties
from the engineering point of view allowing for a noble use of the recycled polymer. It
might not be possible to use fully recycled filament as the mechanical properties
decrease, but with a blend of virgin and recycled material may allow finding an accept‐
able trade-off. If additives are used for this purpose, they should not jeopardize the
opportunity for further recycling cycles. Great losses in mechanical properties do not
restrain the use of the recycled filament for models, visual prints, packaging and other
short life-cycle products without structural demands or durability requirements. In
782 J. Pakkanen et al.
addition to this, recycled filaments will need a retuning of 3D printing parameters to
cope with the increase of flowability and reduced viscosity resulting from material recy‐
cling.
Up to date, research efforts about the development of bio-degradable or compostable
3D printing filaments have been primarily focused toward achieving material printa‐
bility, whereas material recycling and the whole product life cycle has not been fully
considered. In the circular economy framework, at least one or two recycling cycles for
material re-use should be ensured as an alternative to direct eco-friendly disposal.
Since correct and successful filament recycling is a much more complex operation
than 3D printing, the use of low-cost or crowdfunded extruders for makers appears
restrictive. As a matter of fact, such kind of recycling devices are not optimized because
of constrained development costs and production costs savings. Thus, their suitability
is limited to the recycling of a few materials through control of a limited set of processing
parameters.
As for the success of any other recycling process, the correct consumer behaviour
and attitude towards waste management is not questioned, but specific policies or incen
tives might be proposed for material disposal in the 3D printing sector. Material trace‐
ability and the possibility of including specific marking into the design of 3D printed
parts is still an open question. The best way to impose it could be by including an auto‐
matic marking function in the 3D printer software. The software should automatically
add the mark to the part during the computation of the printing path, i.e. during the slicing
operation. The selection of the area for marking should be left up to the user in order
not to affect the aesthetics of the part significantly. Nonetheless, in open 3D printers the
identification of the right material is not error free, because the software cannot auto‐
matically identify the filament material as with chipped filament spools.
In order to solve some of these issues, Chong et al. conceived a distributed recycling
platform for 3D printed products to achieve the goal of zero waste production [42].
Beyond improvement of filament recyclability, recycled FDM material certification and
other regulations about 3D printing waste management should be implemented to reach
such a noble objective.
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About the Use of Recycled or Biodegradable Filaments 785
... The use of biodegradable composite materials in the 3D printing process offers multiple advantages. Besides being environmentally friendly and sustainable, these materials can be used in various applications, from the production of unique or prototype biodegradable packaging and medical devices to components used in the automotive and aerospace industries [51]. ...
... Furthermore, these biodegradable composite materials offer reasonable mechanical and thermal properties, making them suitable for a wide range of applications. For example, in the field of regenerative medicine, these materials can be used to produce gradually degrading artificial tissues and organs within the human body [50,51]. Another benefit of using biodegradable composite materials in 3D printing technologies is that they contribute to reducing the negative impact on the environment. ...
... Another benefit of using biodegradable composite materials in 3D printing technologies is that they contribute to reducing the negative impact on the environment. Since the materials are biodegradable, the printed objects can be recycled or naturally decompose, thus reducing waste and pollution [51]. ...
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... Although the formwork is reported to be recyclable and creates minimal waste, its removal requires a heat gun and could still include concrete particles after removal, making recycling difficult [5,38]. Also, Polylactic Acid (PLA) or other biodegradable plastics commonly used as filaments in FDM might not be fully recyclable as they could lose their mechanical properties after a few cycles [39]. ...
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