Approaches Used for the Additive Manufacturing of Polyolefins: History
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Fotis Christakopoulos
,
Paul M. H. van Heugten
,
Theo A. Tervoort

Additive manufacturing, also called 3D printing, is the process of building up a three-dimensional (3D) object from computer-aided-design (CAD) models through a layer-by-layer fabrication process. Polyolefins are semi-crystalline thermoplastic polymers known for their good mechanical properties, low production cost, and chemical resistance. They are amongst the most commonly used plastics, and many polyolefin grades are regarded as engineering polymers. The two main additive manufacturing techniques that can be used to fabricate 3D-printed parts are fused filament fabrication and selective laser sintering. Polyolefins, like polypropylene and polyethylene, can, in principle, be processed with both these techniques. The semi-crystalline nature of polyolefins adds complexity to the use of additive manufacturing methods compared to amorphous polymers. 

  • polyolefins
  • polyethylene
  • 3D

1. Polypropylene

One of the most commonly used polymers is isotactic polypropylene (iPP) [1]. In additive manufacturing, iPP can be used in both fused filament fabrication (FFF) and fused filament fabrication (SLS). Polypropylene is a polymorphic material in which the mechanical properties of the final product can be tuned through the temperature and cooling rate during processing. This is due to the different crystal morphologies that iPP can form, which, in turn, have a very different mechanical response [2]. The best-known polymorphs are monoclinic α-phase, trigonal β-phase, orthorhombic γ-phase, and mesophase (smectic form) [3][4][5][6][7][8].
There are reports in the literature of iPP being processed via FFF, with several advances in the past few years [9][10][11]. It should be noted that several commercial filaments for FFF are branded as iPP, whereas these are usually copolymers with different additives. In the work of Silva et al. [9], the mechanical properties of FFF-printed samples come within 70–80% of parts processed through injection molding. The two main difficulties when using iPP for FFF are achieving good mechanical properties and minimizing the effect of warpage stemming from the semi-crystalline nature of iPP. The adhesion between two layers and the porosity of FFF-printed parts is dependent, among others, on printing parameters such as the temperature of the nozzle and bed chamber [10]. An increase in the temperature of the nozzle has been reported to result in products with higher values of adhesive fracture toughness [10][12]. The melt viscosity decreases at higher temperatures, thus promoting the contact between the newly deposited and the previous layer. Regarding the effect of the temperature of the bed chamber, a lower value increases the undercooling and so the cooling rate, which results in a larger temperature difference between the newly deposited layer and the previous one. This restricts the formation of entanglements and co-crystallization across the interface and, thus, obstructs the consolidation of the two layers. Further, the porosity of FFF-fabricated samples is affected by the bed temperature, with a higher temperature resulting in decreased porosity. From these considerations, it follows that for a semi-crystalline material like iPP, the bed temperature has to be set close to its melting temperature [10].
According to Lotz et al. [4], the most common morphology found in iPP is the α-configuration. This was also found by van Erp et al. [13], who investigated the crystallization kinetics of iPP homopolymer upon constant cooling as a function of pressure and shear flow conditions. They could distinguish three regimes: quiescent crystallization, flow-enhanced point nucleation, and flow-induced crystallization of oriented structures, resulting in morphologies ranging from spherulitic to oriented shish-kebabs. For commercial grades of iPP at low pressures and moderate shear rates, conditions that are typically relevant for both SLS and FFF, crystallization proceeds toward spherulitic or weakly oriented row structures. In this case, the α-phase was found to be the most common, whereas the β-phase only occurs when flow gradients are present. In this study, crystallization of the γ-phase only took place at elevated pressures. Varga [3] stated that the formation of the β-phase of iPP is also promoted by high undercooling, crystallization in a temperature gradient, and, most efficiently, by the use of special β-nucleating agents. Wang et al. [11] showed that, in FFF, the formation of β-crystals occurs, with several explanations being given for its formation. Namely, the favorable temperature of 130°C, at which the bed temperature is set, is beneficial for the formation of β-phase and the re-crystallization of α- to β-phase [14][15]. β-phase can only form on aligned α-phase or by means of nucleating agents. The formation of an aligned α-phase is caused by shear flow at the extrusion nozzle [16][17]. Due to the shear flow, the polymer chains are stretched and oriented, which gives rise to flow-induced nucleation [18][19]. The temperatures of the nozzle and the chamber, together with the extruding velocity of the thermoplastic material, thus have a great influence on the crystallization kinetics and the resulting morphology. A benefit of using FFF over SLS with iPP is the possibility of using a reinforced filament as feedstock, for example, by adding glass fibers in order to improve the mechanical properties of the printed object [1].
A second additive manufacturing technique used with iPP is SLS. An advantage of using iPP for SLS is the large sintering window of around 35 °C, as can be seen through the differential scanning calorimetry thermograph of Figure 1. In an SLS set-up, it is known that the temperatures can fluctuate, and due to the wide sintering window, the early crystallization can be delayed [20]. Nonetheless, temperature gradients can influence the type of crystals that are formed, as the degree of undercooling influences the crystallization kinetics of the different crystal morphologies [4]. Thus, a temperature gradient can give rise to different crystallization kinetics along the 3D-printed part. As a result, the mechanical properties can vary along the produced part.
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Figure 1. Differential scanning calorimetry thermograph of neat iPP (Advance3d materials) at a heating and cooling rate of 10 °C min−1. The sintering window is 35.1 °C, and the overall degree of crystallinity is 39.58%. Reproduced with permission [20], Copyright 2018, MDPI.
Processing of iPP homopolymers and copolymers (CoPP) through SLS has been reported by Zhu et al. [21] and Tan et al. [22]. As expected, iPP homopolymer predominantly crystallized in the α-configuration. However, both works claim that using SLS for CoPP resulted in a substantial formation of γ-phase crystals. This is a special result, as the formation of γ-phase normally occurs under the condition of slow cooling and high pressure, with the application of high pressure being unlikely to occur during SLS [23][24][25]. Although the modulus and yield stress of the γ-phase are higher than for α-phase, the tensile strength of the γ-phase is lower [25]. Besides the evident appearance of the γ-phase, it was also found that the total crystallinity for iPP parts fabricated through SLS printing is higher compared to injection-molded counterparts. The increase in crystallinity can be explained through reorganization and crystal perfection as the 3D-printed part is kept at elevated temperatures during the SLS process. In addition, the cooling of the fabricated part to room temperature is performed in a more gradual manner compared to injection molding. Even though a higher tensile strength would be expected for specimens with a higher crystallinity, that is not the case, possibly due to the high porosity of the fabricated specimen [21].
In general, iPP can be processed through both SLS and FFF, but the main complexity is controlling the crystallization kinetics, temperature gradients, and porosity. An increase in adhesive fracture energy is challenging due to the early formation of crystals, either induced by flow or temperature, that prevent chain diffusion across the weld-line interfaces. The total crystallinity can be enhanced through post-printing thermal treatments, where the printed component is held at an elevated temperature between the crystallization and melting temperature [26].

2. Low- and High-Density Polyethylene

LDPE is branched polyethylene with a substantial number of long-chain branches. These branches do not fit in the polyethylene crystal lattice, resulting in a lower crystallinity compared to high-density polyethylene (HDPE) [27]. The lower crystallinity results in a lower density, hence the name “LDPE”. Because of its adjustable properties and low density, LDPE can be found in a wide range of applications, such as food packing and rigid containers [28]. Additive manufacturing of low-density polyethylene (LDPE) is considered to be possible but uninteresting due to its inferior mechanical properties. For this reason, only minimal research has been conducted on this matter.
Its low mechanical properties and poor adhesion pose difficulties toward a straightforward utilization of LDPE in additive manufacturing. The processing of LDPE through FFF has mainly been researched toward the formation of composites, where the polymer is reinforced with, for example, ceramic nano-particles or metal particles to improve the mechanical properties [29]. FFF processing of pure LDPE is explored in the work of Bedi et al. [30]. The main downsides of 3D printing with pure LDPE are reported to be a large amount of shrinkage and low mechanical properties. However, it was found that the reinforcement of LDPE, for instance, with alumina particles, was beneficial. Besides the mechanical properties, the crystallinity of the fabricated specimen was also increased, probably because the alumina particles act as nucleating agents during crystallization. SLS printing of LDPE has not been explored yet, probably due to a combination of reasons, such as bad adhesion, low crystallinity, low mechanical properties, and low melt viscosity, leading to dimensional instabilities during printing).
One of the most well-known polyolefins, and one of the leaders in commodity plastics, is high-density polyethylene (HDPE) due to its great potential for sustainability and recycling. HDPE is a linear polymer that can be processed by many common melt-processing means, for instance, through injection molding or extrusion [31]. HDPE is the second-most recycled plastic, and the ability to recycle HDPE in additive manufacturing, through FFF, has been demonstrated [32][33].
Schirmeister et al. [34] investigated the FFF of HDPE parts using a commercially available FFF printer. They found that they were able to match the mechanical properties of HDPE injection-molded specimens by carefully tuning the FFF process parameters. Extensive warping and porosity of the 3D-printed parts were avoided by using an appropriate build plate material and by increasing the extrusion rate during the printing process so that the formation of cavities is minimized and shrinkage is compensated for. A general problem with 3D printing HDPE that is encountered is the adhesion of the print object to the print surface. Schirmeister et al. [34] evaluated several print-surface materials and found that HDPE bonds best to a build plate made from HDPE but that detaching an HDPE FFF-printed sample from an HDPE build plate without damaging the build plate, or the printed object was virtually impossible, especially at higher nozzle temperatures. However, a good compromise was found by using a build plate made of poly(styrene-block-etheneco-butene-block-styrene) thermoplastic elastomer (SEBS, KRATON© FG1901 G), which exhibited good adhesion but still allowed for easy detachment of the printed object.
In the work of Mejia et al. [32], it is shown that HDPE can be recycled using FFF. The disadvantage of recycling HDPE is that the melt viscosity becomes higher, most probably due to the cross-linking that takes place during the processing cycles. This increasing viscosity makes the deposition of a filament with a constant diameter more difficult.
The processing of HDPE using SLS has been researched to a large extent in terms of blends, for example, with polyamide 12 (PA12), which is a standard material used in SLS. Through the use of PA12/HDPE blends, it has been shown that the sintering of HDPE particles is feasible [35][36]. Inter-diffusion of the macromolecular chains is impeded by the short sintering time [37][38]. The reptation time of HDPE, though depending strongly on molecular weight, is typically in the order of seconds, while the residence time of the material in the molten state during processing by SLS is mostly less than one second, thus limiting the amount of re-entanglement [39][40][41]. Moreover, porosity is a returning issue in SLS. It originates from the porous nature of polyethylene powder and the lack of consolidation of the powder upon printing, making SLS processing of HDPE even more challenging [42]. The porous naturem together with the relatively high crystallinity of HDPE, give rise to a large amount of shrinkage followed by warpage upon melting and subsequent cooling. In the work of Hoelzel et al. [43], the causes of the difference in the amount of warpage over each layer are investigated. In general, it was found that a higher scanning speed of the laser results in less warpage, which is due to a lower amount of powder melting, leading to less consolidation. When the scanning speed is lowered, more warpage occurs since the higher specific energy density applied to the scanned area penetrates the powder layer more. However, while more shrinkage and warpage are encountered, a larger specific energy density would improve the coalescence of the particles. The absorbance coefficient of HDPE can be improved by adding, for instance, a small amount of carbon black.
Another parameter that has to be considered is the flowability of the powder, which is typically low. In the work of Wencke et al. [44], the flowability was shown to be improved by coating the HDPE particles with nano-silica particles. This results in a printed object with a higher bulk density than expected; however, caking, i.e., the adhesion of particles to the sintered part around the printed object, is clearly present. Caking results from the melting of particles outside the melting region arising from irradiation scattering [22][44]. It is suggested that the addition of infrared-radiation-absorbing particles, such as carbon black or dyes, could decrease this scattering [44]. The mechanical properties of HDPEproduced samples through SLS have not been presently reported yet, possibly partly due to the inability to extract single, well-defined specimens due to caking.

3. Ultra-High Molecular Weight Polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) is an engineering polymer used in a wide range of high-end applications ranging from medical implants to ballistic protection equipment due to its exceptional properties, such as high impact strength, high abrasive resistance, high chemical resistance, and biocompatibility. However, the main reason for the exceptional mechanical properties, namely the ultra-long macromolecular chains, result in ultra-high melt viscosity, rendering melt processing of UHMWPE a tedious process. For this reason, UHMWPE is regarded as an intractable polymer that cannot be processed by common melt-processing techniques used for polymers, such as injection molding and melt extrusion. UHMWPE is typically sold as as-polymerized reactor powder and processed through compression molding or ram extrusion into plates and rods, followed by machining to obtain the desired parts. Both processes severely hamper the design flexibility of acquired parts and have low throughput. Hence, the fabrication of UHMWPE parts through additive manufacturing would significantly widen the processing pathways for UHMWPE and the design flexibility necessary for specific applications, such as, for example, personalized medical implants.
Due to the limitation of melt extrusion, processing through FFF of pure UHMWPE is not feasible. However, there have been several attempts to process UHMWPE through SLS. The sintering window for UHMWPE can be seen in Figure 2, where a differentialscanning calorimetry thermograph of a standard UHMWPE grade (GUR4120 supplied from Celanese) is depicted. The experiments were conducted on a Discovery 2500 differential scanning calorimeter (TA Instruments), with a heating/cooling rate of 10 °C min−1, under nitrogen flow. UHMWPE is known to depict superheating, i.e., the melting peak temperature of the as-polymerized powder is higher (141 °C) than that of subsequent melting runs (∼132 °C) [45][46][47]. Since the powder used for SLS is reactor powder, and the melt viscosity of UHMWPE is so high that it is practically intractable, the high melting peak temperature of about 141 °C is used for estimating the temperature window for sintering.
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Figure 2. Differential scanning calorimetry thermograph of reactor powder UHMWPE (GUR4120, molecular weight of 4.5 ×106g mol−1) at a heating and cooling rate of 10 °C min−1. The red solid and red dashed lines correspond to the melting of the as-polymerized and melt-crystallized powder, respectively. The sintering window is approximately 7 °C.
The works of Goodridge et al. [48], Khalil et al. [49], Song et al. [50], and Ullsperger et al. [51] suggest that there is considerable room for improvement in the SLS printing of UHMWPE. A common denominative in the aforementioned works is a significant decrease in mechanical properties, such as tensile strength and elongation at break, of the printed specimen compared to the specimen fabricated through compression molding. A typical value for the tensile strength of UHMWPE processed through compression molding is about 50 MPa, while the values found in the literature for specimens fabricated through SLS range between 5 and 14 MPa [49][50][51]. This could be due to the low level of re-entanglement between the sintered polymer chains. Re-entanglement, through chain diffusion, is essential for the development of good mechanical properties [52][53][54][55][56][57]. As mentioned earlier, re-entanglement of the macromolecular chains in polymer melts occurs through reptation of the polymer chains with the characteristic reptation time scaling with the Mw. Selfdiffusion, i.e., reptation, of the polymer chains is a slow process for UHMWPE due to its ultra-high molecular weight, which is typically higher than 3,000,000 g mol−1, resulting in a high amount of steric hindrance between the polymer chains. This steric hindrance is often expressed as the number of entanglements per chain. Given a molecular weight between entanglements for polyethylene of 1250 g/mol, this means that UHMWPE has more than 2400 entanglements per chain, which is high compared to melt-processable grades of polyethylene that typically have about 100–200 entanglements per chain.
The ultra-long polymer chains of UHMWPE result in the longest relaxation time in the melt being in the order of hours, which is orders of magnitude larger than the residence time in the melt when processed through SLS [58]. However, it should be noted that full re-entanglement across the interface upon welding of two UHMWPE surfaces might not be necessary to obtain a good adhesion at room temperature. Xue et al. [53] reported that good adhesion between UHMWPE surfaces, expressed as the strength of a weld as determined by T-peel testing at room temperature, can be achieved by co-crystallization of the macromolecular chains across the interface. However, the effect of this mechanism on 3D-printed specimens has not been observed yet.
The first-reported example of UHMWPE sintering into specimens with sufficient mechanical strength that could be removed from the powder bed is from Goodridge et al. [48], where the heating of the powder bed to a temperature close, but below, the melting temperature was implemented in order to promote melting. Due to the brittle nature of the specimen, the flexural strength, instead of the more commonly evaluated tensile strength, was measured for these samples, with the average value being only about 0.52 MPa. Differential scanning calorimetry experiments of these sintered parts showed that the powder was only partially molten during sintering, as still a large fraction of the original, hightemperature, melting peak of nascent UHMWPE powder could be observed. In general, incomplete fusion of particles appears to be an important factor that limits the ultimate tensile strength and appears to be directly related to the laser power used to sinter the powder during SLS printing. This graph suggests an optimum laser power of 10 W. However, the influence of wavelength, hatch spacing, and laser speed must also be taken into account [59].
As mentioned before, the specific energy density is related to the amount of energy provided per unit area. The specific energy density Es is defined as follows [48]:
E s = P av v l · h ,
where Pav is the average laser power, vl is the speed of the laser, and h is the hatch spacing. Degradation of the polymer is the result of the specific energy density being too high, such that thermal degradation occurs, leading to chain scission and decomposition of the macromolecular chains due to irradiation and buildup of superfluous heat [51][59]. Nonetheless, the incident energy is not to be mistaken with the absorbed energy. The processing of UHMWPE through SLS is hindered by the fact that the absorption coefficient is very low, about 0.01 cm−1 at a wavelength of 1064 nm [60]. As a result, the excessive heat gives rise to a decrease in resolution, increases warpage, and leads to oxidative degradation processes [61].
In the work of Ullsperger et al. [51], a characterization of the consolidation regime related to the specific energy density has been performed. Four different consolidation regimes were identified, ranging from weakly sintered particles to complete melting. The optimal specific energy density was determined to be between 6 and 8 J mm−2.
Their work highlights the inherent difficulty of processing UHMWPE through SLS [51]. It was found that the narrow sintering window is not only dependent on temperature but also on the laser speed, hatch spacing, and layer height. Furthermore, tensile bars were fabricated at the optimal sintering conditions, where a well-sintered layer is achieved. However, the mechanical properties still remain low compared to the compression molded specimen, possibly due to embedded pores and fusion defects at the layer boundaries. In addition, the porous nature of the UHMWPE reactor powder causes additional shrinkage when achieving a well-sintered layer.

This entry is adapted from the peer-reviewed paper 10.3390/polym14235147

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