Elsevier

Additive Manufacturing

Volume 34, August 2020, 101373
Additive Manufacturing

Research Paper
Improving the coloring of polypropylene materials for powder bed fusion by plasma surface functionalization

https://doi.org/10.1016/j.addma.2020.101373Get rights and content

Highlights

Abstract

Polypropylene (PP) powder is treated in a fluidized bed reactor with an atmospheric Ar/O2 plasma jet to investigate the influence of the plasma treatment on the coloring of the powders. The oxygen concentration in the plasma gas and duration of the treatment was varied. Contact angle and X-ray photoelectron spectroscopy measurements confirmed and quantified the formation of surface functionalities and their dependence on the process parameters. The plasma treatment contributes to a significant improvement of the dispersibility and coloring of PP. Powder bulk properties such as particle size distribution and flowability were not modified by the process. Effects of the plasma treatment on the material bulk properties are also negligible.

Introduction

Powder bed fusion (PBF) of polymers is a powder-based additive manufacturing (AM) process, which allows the production of functional parts with complex geometries without the use of tools and forms [1]. During the manufacturing process, consecutive layers of powder are applied with a blade or a roller and are sintered with a laser before the next layer of powder is applied [[2], [3], [4]]. The mechanical properties and surface quality of the produced devices depend to a great extent on the intrinsic material (e.g. melting behavior, crystallization) and powder bulk (e.g. particle size and shape, flowability) properties [[4], [5], [6]].

Despite of the progress in the PBF technology in the fields of aerospace, automotive and medical industry, major challenges need to be addressed to push the technology to new frontiers. A main restriction in this regard is the limited selection of commercially available materials, which are mainly based on polyamides (PA) including PA12, PA11 and PA6 with 90–95% of the market share. Other commercially available powders for PBF are polypropylene (PP), polystyrene (PS), polyaryletherketone (PAEK) and thermoplastic polyurethane (TPU) [7]. This restriction is mainly due to the limited understanding of the complex dependencies between the material properties (e.g. powder flowability, degree of crystallinity, polymer chemistry) and the process conditions (e.g. laser intensity, powder bed temperature, deposition mechanism) which define the processability of the powders [8].

In most of the cases, parts produced by PBF require post-processing to tailor specific properties of the final part. Common post-processing techniques involve the removal of unconsolidated powder and the improvement of the surface finishing to eliminate the grainy finish and the stair casing effect resulting from the layer-by-layer manufacturing approach [9]. Coloring of the parts is also widely employed for aesthetics and functional reasons. Coloring of polymers can be achieved by dispersing pigment particles in the bulk polymer, by mixing of dye molecules to the polymer matrix or by direct binding of chromophores to the polymer backbone. Dyeing is one of the most applied techniques for applying color because of the cost-efficiency and short time it takes. For this process the produced part is immersed in a dye bath at elevated temperatures (80 °C). Different additives as surfactants, salts, acids, alkalis are conventional used in the dyeing process as e.g. dispersants, leveling, wetting and soaping agents or buffers to enhance the process. The use and selection of these additives depends on the kind of dyestuff and substrate used [10]. Basically, almost any dyestuff used in the textile industry can be applied for this purpose [7]. However, the dyeing can only be applied for polymers containing polar functionalities (N and O-groups) or aromatic systems in the macromolecular chain, to which the dye molecules can be attached [11,12]. For that reason, dyeing can be successfully applied to PBF parts made of PA systems.

Polypropylene is one of the most important commodity polymers used in the industry because of its tensile strength, abrasion and chemical resistance. Coloring of polypropylene represents a challenge. On the one hand, pigments act as nucleating agents affecting considerably the degree of the crystallinity and crystallization kinetics of PP [13], thus leading to problems during PBF processing (warpage, shrinkage, and curling) [8]. On the other hand, due to chemical structure of PP of solely hydrogen and carbon atoms with absence of hetero atoms or reactive radicals, polypropylene cannot be easily dyed [12,14]. This reduces the market sector and application fields of this material for the additive manufacturing industry. Some approaches to improve the coloring of PP include the incorporation of reactive sites for the easy attachment of dye molecules as for example chlorination, bromination, nitration and chlorosulphonation [14]. Other approaches included the addition of dyestuff acceptors in the polymer melt during extrusion or copolymerization [[15], [16], [17]]. These methods are, however, undesirable as they imply considerable ecological problems and are very expensive [18]. A cheaper and environmentally friendly alternative is the plasma treatment and dyeing of polymer powders prior to the PFB process [19].

The plasma surface treatment of planar polymeric substrates (foils and films) is well-known [20,21]. Dependent on the composition of the plasma gas and the resulting plasma chemistry, various effects of the treatment on the polymer surface can occur: When using noble gases, such as helium (He) and argon (Ar), simple cleaning of the surface and breakage of polymer chains followed by crosslinking may take place. Using reactive gases, such as oxygen (O2) or nitrogen (N2), results in the formation of surface groups (e.g. COOH, OH, Cdouble bondO and NH2), which increase the surface energy and, thus, improve the hydrophilicity, wettability and adhesion of the polymers without affecting their bulk properties [20,[22], [23], [24], [25]].

The plasma treatment of polymer powders is a more involved process due to the large surface areas to be treated [26], although the principles are the same: the challenge is to achieve a homogenous functionalization and to avoid particle aggregation [27,28]. Due to the temperature sensitivity of polymers, typically only low temperature (non-thermal/non-equilibrium) plasmas can be applied for the functionalization of thermoplastic powders [20]. Arpagaus et al. reviewed the different processes used for the plasma treatment of polymer powders, providing an extended summary of the different types of reactor concepts, polymer materials, and applications to which the plasma treatment has been applied over the last 40 years [22]. Atmospheric pressure plasmas are more relevant for industrial application, as they facilitate the continuous integration of the plasma treatment into other process steps and reduce the costs associated with vacuum equipment [26,29]. There are fewer reports on the plasma treatment of polymer powders using atmospheric pressure plasmas are reduced compared with studies at low pressure. According to Arpagaus et al. [22], approximately 75 % of the reported studies on the plasma treatment of polymer powders used low pressure reactors. The main difficulty lies in the fact that atmospheric pressure plasmas are highly unstable and limited to a very small discharge gap [30]. Atmospheric pressure plasma reactors have been successfully applied for the plasma functionalization of powders of polypropylene, polyethylene (PE), polystyrene (PS), poly (methyl methacrylate) (PMMA), polyethylene terephthalate (PET), polyamide 12 (PA12) and polytetrafluoroethylene (PTFE) as well as acrylic and epoxy powders [22]. Up to now, only dielectric barrier discharge (DBD) plasmas have been applied successfully for the treatment of polymer powders [22]. Commonly used reactor designs are downer [29,30], fluidized bed [31,32] and circulating fluidized bed [26,33] reactors. Fluidized bed reactors are used due to the intensive solid mixing and good heat transfer, which allows for an homogenous powder treatment and fast removal of heat generated by the plasma [31,34].

Considering the plasma treatment of PP powders, different examples of reactors can be found in the literature. Fang et al. [35] used an inductively coupled argon low-pressure plasma reactor for the treatment of 160 μm PP particles. The powder was mixed using a magnetic stirrer. Even though the powders were treated with Ar-plasma, a considerable decrease of the water contact angle and an increase of the oxygen concentration in the surface was observed with increasing treatment time. Gilliam et al. [36] reported on an atmospheric plasma reactor with a treatment time of less than 1 s for the treatment of PP particles of 250 μm. The plasma jet was generated using a DBD with air or N2 as plasma gases. The powders were fed directly into the plasma by gravity. The plasma contributed to an increase in the dispersability in water of the PP particles. The atomic oxygen concentration on the surface increased from 5 % to 18 % after plasma treatment upon the additional injection of water. Matoušek et al. [37] used a fluidized bed atmospheric plasma reactor using air as plasma gas for the treatment of 355 μm PP particles. The treatment led to an increase of the oxygen concentration on the surface from 5% for the untreated material to about 22 % after 20 s treatment at 50 W. Abourayana et al. [38] used a rotatory barrel atmospheric plasma reactor to treat PP particles of 5 mm. Mixtures of He/O2 were used as plasma gas. The treatment contributed to a considerable decrease of the water contact angle after 5 min of treatment.

Recently Sachs et al. [31] reported on a fluidized bed reactor with a secondary remote DBD plasma jet to functionalize polymer powders. The reactor concept was applied successfully to functionalize high-density polyethylene (HDPE), polypropylene (PP) and PA12 powders using Ar/O2 mixtures and air as plasma gas [31]. Contact angle measurements via the sessile drop method demonstrated an improvement of the wettability of the plasma-treated powders.

In the present study the plasma functionalization of commercial PP for improving the coloring using the fluidized bed reactor developed by Sachs et al. [31] is investigated. The influence of process parameters, such as the duration of the plasma treatment and the Ar/O2-concentration of the plasma gas on (i) the material and surface properties, such as contact angle, chemical composition of the surface, coloring and (ii) on the powder bulk properties, such as flowability and particle size of the treated powders is addressed. Secondly, the effect of the dyeing process on material and powder bulk properties is investigated as well. 3D parts produced by using untreated and treated powders before and after dyeing are produced to assess to total performance of this approach.

Section snippets

Plasma treatment

Batches of 300 g PP (Coathylene® PD0580, Axalta Polymer Powders, Switzerland) were processed in the fluidized bed reactor depicted in Fig. 1. The fluidized bed has an inner diameter of 0.1 m and a height of 0.8 m. In the bottom of the fluidized bed a sintered metal plate (SIKA-R20, GKN Sinter Metals Engineering GmbH, Germany) is used as distributer plate for the fluidization gas (Nitrogen, N2, 5.0, Linde AG, Germany) over the whole cross-section. The volumetric flow rate of the fluidization gas

Fluidization behavior under the influence of the plasma jet

The measured density and Sauter mean diameter of the tested powder is reported in Table 3. According to the determined properties, all the tested powders are classified as group A (aeratable) powders according to the Geldart classification of powders. Powders of group A are characterized by the formation of a homogenous fluidized bed expansion after minimum fluidization and before the formation of bubbles starts [49].

Deviations from the theoretical fluidization curves of group A powders in a

Conclusions

In this contribution, PP powders were treated in a fluidized bed equipped with an atmospheric Ar/O2 plasma jet. The fluidization behavior of the powders was characterized and the temperatures reached during treatment were monitored during the process. During the plasma treatment the powders are only exposed to low global temperatures (T<100 °C). The effect of the concentration of oxygen in the plasma gas and duration of the treatment duration on the surface, material and powder properties were

CRediT authorship contribution statement

Juan S. Gómez Bonilla: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Tim Szymczak: Formal analysis, Investigation. Xuemei Zhou: Formal analysis, Investigation. Stefan Schrüfer: Formal analysis, Investigation. Maximilian A. Dechet: Formal analysis, Writing - original draft. Patrik Schmuki: Validation, Resources. Dirk W. Schubert: Validation, Resources. Jochen Schmidt: Conceptualization, Methodology, Formal analysis, Writing

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 61375930 – CRC 814 (Additive Manufacturing), sub-project A2 and Bundesministerium für Wirtschaft und Energie (BMWE, Federal Ministry for Economy and Energy)in the frame of the Central Innovation Programme for small and medium-sized enterprises (SMEs) – Project-ID 16KN073002 – HiEPP for SLS is gratefully acknowledged.

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