materials
ArticleMechanical, Chemical, and Processing Properties of Specimens
Manufactured from Poly-Ether-Ether-Ketone (PEEK) Using
3D Printing
Maciej Mrówka 1,2 , Tomasz Machoczek 1 , Paweł Jureczko 1 , Kamil Joszko 3, * , Marek Gzik 3 ,Wojciech Wolański 3 and Krzysztof Wilk 4
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2
3

4

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Citation: Mrówka, M.; Machoczek,
T.; Jureczko, P.; Joszko, K.; Gzik, M.;
Wolański, W.; Wilk, K. Mechanical,
Chemical, and Processing Propertiesof Specimens Manufactured from
Poly-Ether-Ether-Ketone (PEEK)
Using 3D Printing. Materials 2021, 14,2717. https://doi.org/10.3390/

Department of Theoretical and Applied Mechanics, Faculty of Mechanical Engineering, Silesian University ofTechnology, 18A Konarskiego Str., 44-100 Gliwice, Poland; maciej.mrowka@polsl.pl (M.M.);
tomasz.machoczek@polsl.pl (T.M.); pawel.jureczko@polsl.pl (P.J.)
Biotechnology Center, Silesian University of Technology, Krzywoustego 8 Str., 44-100 Gliwice, Poland
Department of Biomechatronics, Faculty of Biomedical Engineering, Silesian University of Technology,
Roosevelta 40 Str., 41-800 Zabrze, Poland; marek.gzik@polsl.pl (M.G.); wojciech.wolanski@polsl.pl (W.W.)
3DGence Inc., Graniczna 66 Str., 44-178 Przyszowice, Poland; krzysztof.wilk@3dgence.com
Correspondence: kamil.joszko@polsl.pl

Abstract: As part of the experiments herein, the mechanical properties of specimens made of polyether-ether-ketone (PEEK) material using 3D printing technology were determined. Two populationsof specimens were investigated, the first of which contained an amorphous structure, while theother held a crystal structure. The studies also investigated the influence of the print directionality
on the mechanical properties obtained. Static tensile, three-point bending, and impact tests werecarried out. The results for the effect of the structure type on the tensile properties showed thatthe modulus of elasticity was approximately 20% higher for the crystal than for the amorphous
PEEK form. The Poisson’s ratios were similar, but the ratio was slightly higher for the amorphous
samples than the crystalline ones. Furthermore, the studies included a chemical PEEK modification
to increase the hydrophilicity. For this purpose, nitrite and hydroxyl groups were introduced into thechain by chemical reactions. The results demonstrate that the modified PEEK specimens had worse
thermoplastic properties than the unmodified specimens.

ma14112717
Academic Editors: Juergen Stampfl
and Jun Liu

Keywords: poly-ether-ether-ketone (PEEK); mechanical properties; chemical modification; 3D printing; experimental tests

Received: 24 February 2021
Accepted: 16 May 2021
Published: 21 May 2021

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Licensee MDPI, Basel, Switzerland.
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1. Introduction
Modern industries, especially mechanical and biomedical engineering, are constantly
searching for construction materials that may, due to enhanced useful properties andincreased durability, replace the materials utilized thus far [1,2]. Continuous progress inthe field of material engineering provides an opportunity to produce new or improved
structural materials, which must undergo extensive research before their production can be
optimized and approved for use [2,3]. The selection of material for a specific constructional
application requires knowledge of its physical and chemical properties, ensuring that
it will be susceptible to the required shaping, in order to obtain the finished product.
Furthermore, the material should meet both economic and environmental criteria (i.e.,
for future recycling) [4–6]. Currently, a main focus of the manufacturing industry isdetermining the possibility of composite material applications, which often replaces thosein previous use [7,8]. Polymer materials can become the matrix of the composite, which,depending on the type of filler, can change the physicochemical or biological properties
that are most desirable in a given situation [9,10]. Choosing the right filler can increasemechanical strength and reduce abrasion at the same time [11,12]. An important aspect of4.0/).

Materials 2021, 14, 2717. https://doi.org/10.3390/ma14112717

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Materials 2021, 14, 2717

2 of 13the new materials is their non-toxicity to the environment, understood as both the impactof the entire material and that of products of possible degradation [13,14].
Poly-ether-ether-ketone (PEEK), owing to its special properties, has become a polymer that has been utilized within many industries, including the medical industry [15].
This semi-crystalline thermoplastic, belonging to the poly-ether-ether-ketone family, ischaracterized by a unique combination of high mechanical properties and resistance tochemicals [16–18]. An additional advantage of PEEK is the lack of cytotoxicity towards
normal cells of the human body. Thus, it can be successfully used as a biomaterial [19,20].
Thanks to its high mechanical strength, Young’s modulus similar to that of bone, improved
load distribution compared to titanium biomaterials, and lower rigidity in comparison tosteel implants, PEEK has become an alternative to classical solutions [21–23]. Poly-etherether-ketone is also characterized by its high thermal stability, and due to its melting point
oscillating at approximately 340 ◦ C, PEEK is suitable for use at temperatures reaching
260 ◦ C. With the development of additive technologies for high-temperature printing,
PEEK began to be used in 3D printing of thermoplastics by liquid thermoplastic molding(fused deposition modeling, FDM) [24–26]. However, PEEK itself is highly hydrophobic. It
promotes adhesion of bacterial cells from the environment to its surface [27]. One way to
bypass the tedious and costly process of frequent sterilization is to modify the chemical
structure of PEEK to give it more hydrophilic characteristics. This effect can be achieved byintroducing nitro groups to the structure of the poly-ether-ether-ketone chain in different
degrees of saturation, reducing the ketone group to an alcohol one, and introducing thehydroxyl group to the main chain [28].
Considering the above advantages of PEEK, especially its mechanical properties and
its Young’s modulus indicating similarity to bone properties, gives high hopes for the
possibility of printing dedicated implants with a better load distribution compared tosteel implants. However, in order to clearly determine whether this material is suitablefor 3D printing and whether it is possible to improve its biological properties, a series ofexperiments involving standard test methods [29,30] must be carried out. There are known
studies focused on the analysis of printed PEEK samples [31–33]. Most of them concerned
the parameters of the printing process, such as printing speed and temperature, layer
thickness, and the degree of filling and laying of layers. According to these sources, it ispossible to obtain optimal processing parameters ensuring the best mechanical properties.
However, there is a lack of research analysing the influence of the structure and chemicalparameters on the properties of the printed PEEK.
Therefore, the aim of the work was to evaluate the mechanical properties of specimens
made of poly-ether-ether-ketone material using 3D printing technology. We assessed
the impact of 3D printing direction and crystal or amorphous structure of the materialon the mechanical properties. Furthermore, the hydrophilic nature of this material was
improved by adding nitro groups with various degrees of saturation into the structure of
the PEEK chain.
2. Materials and Methods
The material called PEEK 151G was purchased from Victrex, (Victrex, Lancashire,UK). The samples were manufactured according to the ASTM D790-02 Standard Test
Method [29] with the use of incremental technology through FDM, which is a method of
forming liquid thermoplastic.
The printer used to produce the samples was an INDUSTRY F340 from 3DGence(3DGence, Przyszowice, Poland). Optimal parameters of the printing process were taken
according to [24]. The using printing parameters were speed of 30 mm/s, layer thicknessof 0.15 mm, temperature of 425 ◦ C, and filling ratio of 100%.
The sample orientation during the manufacturing process is shown in Figure 1. The
specimens were created in three mutually perpendicular directions:Materials 2021, 14, 2717

3 of 13

Figure 1. (a) Geometric orientation of the printed samples with the marked printing directions:(A) through thickness,(B) transverse, (C) longitudinal; (b) printed specimens used for: I—tensile testing, II—three-point bending testing, III—
impact testing.

Type A: Specimen at the incremental manufacturing stage adjacent to the printer tablewith the largest surface area—through the thickness direction;
Type B: Specimen at the incremental manufacturing stage adjacent to the printer tablewith a smaller surface area—transverse direction;
Type C: Specimen at the incremental manufacturing stage adjacent to the printer tablewith the smallest surface area—longitudinal direction.
The surface of the samples after manufacturing was not modified with any chemical
substances (solvents) or by any mechanical treatment. This allowed us to obtain an
amorphous structure that was characteristic of the tested samples. In addition, a populationof samples in crystalline form was prepared by the same manufacturing method. Due tothe heat treatment consisting of heating the material in an induction furnace (Nabertherm,
Lilenthal, Germany) for 120 min to 200 ◦ C, then heating at this temperature for 180 min
and free cooling under ambient conditions, the samples were brought to a crystalline
form. Nitric acid (V) 65% [HNO3 ], sulfuric acid (VI) 96% [H2 SO4 ], hydrochloric acid
35–38% [HCl], N,N-dimethylformamide [DMF], dimethyl sulfoxide [DMSO], and methanol[CH3 OH] were purchased from ChemPur (Piekary Slaskie, Poland). Sodium hydrochloride[NaBH4 ] was purchased from Acros Organics (Saint Louis, MO, USA), technical acetone
70% was purchased from POCH™ (brand from Avantor™ Performance Materials, Gliwice,
Poland), and demineralized water was obtained in the laboratory.
2.1. Strength Analysis Using a Static Uniaxial Tensile Method
The test population consisted of 10 specimens for each printing direction. Due tothe complexity of the specimen structure resulting from the manufacturing method, an
experimental test was carried out using an MTS Insight 2 machine (MTS Systems, Eden
Prairie, MN, USA) and a DANTEC Q400 vision system (Dantec Dynamics A/S, Skovlunde,Denmark) for non-contact deformation analysis (Figure 2). The using speed of the test was
50 mm/min. The stress at yield, elongation at yield, tensile strength at break, elongation
at break, and modulus of elasticity were determined. In order to perform static tensile
measurements with the use of a vision system, it was necessary to prepare a suitablebackground for stochastically placed markers on the measuring portion of individual
samples. This action was carried out on the basis of a special preparation in aerosol,
which, by definition, did not react with the material, but provided only a primer for
dynamically sprayed black markers. These markers were utilized to determine the relative
elongations (deformations) of the specimen in both the longitudinal and the transverse
directions. Moreover, thanks to the applied vision system, Poisson’s coefficients were
determined, which in the case of the analyzed, heterogeneous structures were a serious
concern. The analysis was limited to only five specimens in each group, one for eachdirection of manufacture, as the range of forces in some cases exceeded the measuring
range of the MTS Insight 2 tensile machine. The experiment was carried out at room

Materials 2021, 14, 2717

4 of 13temperature (i.e., 22 ◦ C, humidity: 50%). As a result of the trials, a set of mechanicalparameters describing the structure of the material from which the samples were madewas obtained and is presented in Table 1.

Figure 2. Strength test stand including (a) MTS Insight 2 tensile machine and DANTEC Q400 visionsystem; (b) grips used to fix a sample.
Table 1. Results of uniaxial tensile testing of the crystalline-form samples.
Name ofSamples

Stress at Yield[MPa]

Strain at YIELD[mm/mm]Stress at Break[MP]

Strain at Break[mm/mm]Modulus of Elasticity[GPa]

Poisson’s
Ratio

Sample A
Sample B
Sample C

26.86 ± 0.56
27.19 ± 1.11
5.80 ± 1.49

0.0096 ± 00.0096 ± 0
0.0022 ± 043.67 ± 0.07
38.76 ± 1.9
12.21 ± 2.4

0.016 ± 00.013 ± 00.0043 ± 0

2.79 ± 0.03
2.83 ± 0.15
2.69 ± 0.51

0.38 ± 00.38 ± 00.35 ± 0.02

2.2. Three-Point Bending Experiments
Three-point bending tests were carried out on specimens produced according to the
ASTM D790-02 Standard Test Method [29]. The experiment was carried out on an MTS
Insight 2 tensile machine. The speed utilized during the analysis was 1.65 mm/min, andthe distance between the supports was 52 mm.
Each procedure was carried out at room temperature (i.e., 22 ◦ C, humidity: 50%). The
method of fixing a sample is shown in Figure 3.

Figure 3. Stand for the three-point bending tests.

Materials 2021, 14, 2717

5 of 132.3. Impact Experiments
Impact testing was performed on the specimens adjacent to the largest surface of the
3D printer table during the manufacturing phase. Due to the innovative nature of theissue, and due to the recommendations of the ASTM D256-02 Standard Test Method [30],the experimentation on the specimens was carried out with three different notch types(Figure 4). Additionally, specimens were tested without the recommended stress concentration component. These specimens, similar to those in the static tensile and flexural analysis,
were in the form of amorphous and crystalline structures. The trials were performed
using a universal impact testing machine 400/27 (VEB Werkstoffprufmaschinen, Liepzig,
Germany), with a hammer weight of 0.937 kg and a hammer impact speed of 1.3048 m/s.
The trials were carried out at room temperature (i.e., 22 ◦ C, humidity: 50%).

Figure 4. Specimens for impact testing: (a) Notch shape type A, (b) notch shape type B, (c) notch shape type C, (d) samples
without notches N, (e) 3D printer.

2.4. Chemical Modifications of PEEKThe PEEK samples remaining after the mechanical tests were used for chemical
modification. Poly-ether-ether-ketone was obtained by grinding unsuccessful prints andwaste from the 3D printing process in a FRITSCH’s PULVERISETTE 19 mill machine(FRITSCH GmbH, Idar-Oberstein, Germany). As a result of grinding, a powder with agrain diameter of 1 mm was obtained. Afterwards, PEEK functionalized with nitro groups
was prepared in 93% as described in the paper [28]. The reaction was carried out according
to Conceiacao et al., 2008 [28]. A quantity of 30 g of ground PEEK was suspended in a
mixture of 300 mL of HNO3 and 75 mL of H2 SO4 (4:1 v/v) placed in a round-bottomed
750 mL magnetic stirrer flask (Labo24.pl, Gliwice, Poland). The flask was placed under areflux condenser (Labo24.pl, Gliwice, Poland), on a heated magnetic stirrer (IKA, Staufen
im Breisgau, Germany), and was heated at 65 ◦ C for 60 min. Upon completion, the flask
was removed and cooled, the liquid decanted, and the product, obtained in the form ofprecipitate, dissolved in DMF. Dissolved, modified PEEK_NO2 (93%) was precipitated in
HCl, filtered, and then washed several times with distilled water, methanol, and acetone
until the precipitate became neutral. The final product obtained was in the form of a yellow
powder and was transferred to a porcelain evaporator (Labo24.pl, Gliwice, Poland) anddried for several hours in a laboratory dryer (Binder GmbH, Tuttlingen, Germany), heated
to 100 ◦ C.
The next step was the preparation of PEEK functionalized with nitro groups in 247%
as described in the paper [28]. The procedure as follows was dictated by Conceiacao et al.,2008 [28]. Similar to the above procedure, 30 g of ground PEEK was suspended in a mixtureof 300 mL of HNO3 and 75 mL of H2 SO4 (4:1 v/v). The mixture was placed in a 750 mL

Materials 2021, 14, 2717

6 of 13round-bottomed flask with a magnetic stirrer. The flask was positioned under a reflux
condenser, on a magnetic stirrer, and was heated at 75 ◦ C for 90 min. After heating, theflask was removed and cooled, the liquid decanted, and the product, obtained in the formof sludge, dissolved in DMF. Dissolved, modified PEEK_NO2 (247%) was precipitated in
HCl, filtered, and then washed several times with distilled water, methanol, and acetone
until the sludge was neutralized. The final product obtained in the form of yellow powder
was transferred to a porcelain evaporator and dried for several hours in a laboratory dryer
heated to 100 ◦ C.
In order to prepare PEEK_OH, the reaction was carried out according to Conceiacao et al., 2008 [28]. A quantity of 30 g of ground PEEK was added in portions, forapproximately 5 min, to a 9 g NaBH4 solution in 750 mL DMSO placed in a 1000 mL
round-bottomed flask. The flask was placed under a reflux condenser, on a magnetic stirrer,
with heating and was heated at 120 ◦ C for 180 min. After this time, the flask was removed
and cooled, the liquid decanted, and the product, obtained in the form of a grey-beige
powder, filtered and washed several times with distilled water, methanol, and acetone.
After cleaning, the powder was transferred to a porcelain evaporator and dried for several
hours in a laboratory dryer heated to 100 ◦ C.
2.5. Determining Processing Properties
Approximately 10 g weights were prepared from each of the specimens obtained
and placed in capsules (CORMAK, Siedlce, Poland). The capsules and their contents
were placed into a KS 520/14 electro-inductive hardening furnace (Nabertherm, Lilenthal,
Germany). The furnace was heated from room temperature (i.e., 20 ◦ C) to a temperature of450 ◦ C at intervals of 20 ◦ C. The weights of the modified products were checked at each levelof temperature. As a result of the thermal treatment carried out in the KS 520/14 quenching
furnace at a temperature of ~420◦ C, the waste from PEEK was plasticized in a container.
FT-IR spectra were recorded using a Perkin-Elmer Spectrum Two spectrometer (Waltham,
MA, USA). The examinations were carried out in the radiation range of 400–4000 cm−1 .
3. Results
3.1. Results of the Uniaxial Tensile Testing
Figure 5 presents the average values of mechanical properties for which visible changes
were recorded in the uniaxial tensile testing. In the case of stress at yield, the highest
mean value was recorded for samples with B-type crystal structures, with the result being
27.19 MPa. On the other hand, the lowest average value for this parameter was recorded
for samples with an amorphous structure of type A, this value was 4.82 MPa. For theparameter of tensile strength at break, the highest mean value was obtained for samplesof types A and B and was found to be 43.67 MPa. For the A-type sample, this number
occurred in the crystal structure, and for the B-type sample, it occurred in the amorphous
structure. In the case of the modulus of elasticity, it was observed that for all types ofsamples, i.e., A, B, and C, a higher mean value of the modulus was noted for the crystal
structure, approximately 20% higher than that for the amorphous structure. The Poisson’s
ratio for all samples was at a similar level and, on average, was 0.38.
Other properties, for which no significant differences were found, are summarizedin Table 1 for samples with a crystalline structure. An example of the stress-strain curve
which was obtained using the DANTEC Q400 vision system for specimen A1 is shown in
Figure 6. In Table 2, summarized results are presented for samples with an amorphous
structure, and Figure 7 shows an example of the stress-strain curve of amorphous PEEK
for specimen A1.

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Figure 5. Comparison of the mechanical properties obtained during the uniaxial tensile experiments of samples type A, B
and C: (a) Strength at Yield, (b) Strength at break, (c) Modulus of elasticity, (d) Poisson’s ratio.

Figure 6. Example of the stress-strain curve of crystal poly-ether-ether-ketone specimen A1.
Table 2. Results of the uniaxial tensile experiment on the amorphous-form samples.
Name ofSamples

Stress at Yield[MPa]

Strain at Yield[mm/mm]Stress at Break[MPa]

Strain at Break[mm/mm]Modulus of Elasticity[GPa]

Poisson’s
Ratio

Sample A
Sample B
Sample C

26.73 ± 0.52
22.51 ± 0.99
4.82 ± 0.26

0.011 ± 00.0094 ± 0
0.0022 ± 0

43.08 ± 0.12
43.67 ± 0.13
10.03 ± 0.87

0.022 ± 00.021 ± 00.0045 ± 0

2.25 ± 0.05
2.38 ± 0.03
2.18 ± 0.17

0.40 ± 0.01
0.39 ± 0.01
0.36 ± 0.02

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Figure 7. Example of the stress-strain curve of amorphous PEEK specimen A1.

3.2. Results of Three-Point Bending Testing
Figure 8 displays graphs containing the average values of mechanical properties
recorded during the static, three-point bending tests. For the flexural stress parameter,
it was observed that for all types of samples, A, B, and C, higher average values ofapproximately 57% were obtained for the crystal structure. The highest mean value wasrecorded for the B-type sample and was 58.63 MPa. The lowest mean value for the flexuralstress parameter was found to be with the A sample, and it was 29.79 MPa. Furthermore,in the case of strain, the highest average values were determined to be with the crystal
structure. This parameter was higher by an average of 23%. The highest mean strain,
0.73%, was recorded for sample B with a crystal structure. The lowest mean strain value
was observed to be 0.48% for sample A in the amorphous phase. Additionally, for theparameter of flexural stress at break, the highest mean value was found to be 144.52 MPafor the B-type sample in the crystalline phase. The lowest mean value for flexural stress at
break was recorded for the C-type sample in the amorphous phase and was 67.69 MPa. It
was also observed that the modulus for all types of samples was higher for their crystal
structures by an average of 27%. The highest mean value of modulus was recorded forsample A, in the crystalline phase, and was found to be 3.37 GPa. Finally, the lowest mean
modulus value, 2.3 GPa, was recorded for C-type samples with an amorphous structure.
All of the mechanical properties determined during the three-point bending experiments are summarized in Tables 3 and 4.
Table 3. Results of three-point bending analysis of the crystalline-form samples.
Name ofSamples

Stress at Yield[MPa]

Strain at Yield[%]

Stress at Break[MPa]

Strain at Break[%]Modulus of Elasticity[GPa]

Sample A
Sample B
Sample C

47.75 ± 9.06
58.63 ± 13.87
49.08 ± 16.79

0.59 ± 0.12
0.73 ± 0.17
0.66 ± 0.22

129.02 ± 13.45
144.52 ± 3.61
88.24 ± 53.29

2.11 ± 0.46
2.62 ± 0.07
1.48 ± 0.99

3.37 ± 0.11
3.36 ± 0.15
2.94 ± 1.01

Table 4. Results of three-point bending tests of the amorphous-form samples.
Name ofSamples

Stress at Yield[MPa]

Strain at Yield[%]

Stress at Break[MPa]

Strain at Break[%]Modulus of Elasticity[GPa]

Sample A
Sample B
Sample C

29.79 ± 4.81
35.01 ± 4.01
33.74 ± 14.94

0.48 ± 0.08
0.54 ± 0.06
0.60 ±0.29

91.58 ± 2.77
103.37 ± 3.74
67.69 ± 39.52

2.52 ± 0.04
2.30 ± 0.06
1.53 ± 0.95

2.54 ± 0.06
2.79 ± 0.12
2.30 ± 0.81

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Figure 8. Comparison of the mechanical properties obtained during the three-point bending tests of samples types A, B andC: (a) Flexural stress, (b) Strain, (c) Flexural stress at break, (d) Strain at break, (e) Modulus of elasticity.

3.3. Results of Impact Testing
During the impact testing, the impact resistance of each sample was recorded, which
is presented in Table 5. The highest mean value of 112.09 kJ/m2 was observed for the
N-type sample with an amorphous structure. The smallest result was determined for asample of type C, in the crystalline phase, and was 6.57 kJ/m2 .
Table 5. Results of impact testing for each sample.
Name of Samples
Sample A
Sample B
Sample C
Sample N

Impact Resistance [kJ/m2 ]Amorphous Form

Crystalline Form

12.80
21.16
12.60
112.09

7.66
14.59
6.57
23.96

3.4. Infrared Spectroscopy
Samples were taken in the form of a fragment of an unground filament made of PEEK,
ground PEEK grains, and the three modification products. During the measurements, thespectra of unmodified PEEK were recorded, in which the carbonyl group was reduced toalcoholic (PEEK_OH) and nitrated (PEEK_NO2 —93% and PEEK_NO2 —247%) samplesof poly-ether-ether-ketones. The results are presented in Figures 9 and 10. The first figure
shows the spectra of PEEK and PEEK_OH, while the second shows the spectra of PEEK,PEEK_NO2 —93%, and PEEK_NO2 —247%. In all spectra, the dependence of transmittanceMaterials 2021, 14, 2717

10 of 13[%] on the position of the band [cm−1 ] was determined. In Figure 9, band loss in the rangeof 1680–1700 cm−1 was observed in the PEEK_OH spectrum. This band, present on the pure
PEEK spectrum in the same range, comes from the carbonyl group, and its disappearance
proves the reduction of the carbonyl group to a hydroxide group. Figure 10 shows the
combined spectra of PEEK, PEEK_NO2 —93%, and PEEK_NO2 —247%. The occurrence ofintense bands characteristic for nitro groups at 1540 and 1720 cm−1 was observed thereon.
These bands do not occur in the unmodified PEEK spectrum, which proves the success ofthe nitriding process. Both bands are much more intense for PEEK_NO2 —247% than for
PEEK_NO2 —93%. In all spectra, bands characteristic of aromatic compounds, differing in
intensity, were also observed. Bands of aromatic compounds can be observed in the range
from 1600 to 1500 cm−1 and at 3000 cm−1 . In the case of the PEEK_NO2 —247% spectrum,
the band at 3000 cm−1 shows additionally strong intensity, coming from a high content of
nitro groups in the poly-ether-ether-ketone chain.

Figure 9. FT-IR spectrum of PEEK_OH compared to that of pure PEEK.

Figure 10. FT-IR spectra of PEEK_NO2 —93% and PEEK_NO2 —247% compared to that of pure PEEK.

3.5. Results of Chemical Modifications of PEEKIn order to investigate the specific processing properties of PEEK modification products, capsules with specific weights of substances were placed in a quenching furnace andheated from 20 to 450 ◦ C. The temperature was constantly controlled with a thermometer
placed next to the furnace and was increased by 20 ◦ C per interval. When the furnace
reached the set temperature, the condition of the modification products placed in it wasMaterials 2021, 14, 2717

11 of 13determined. No changes were observed in all three modified products at 80 ◦ C, whileat 100 ◦ C, dense, eye-catching smoke escaped from the chamber when the furnace was
opened, and all the samples placed in the capsules were charred. The experiment was
repeated three times, but each time, the PEEK modification products charred in the temperature range of 80–100◦ C. Images of samples before and after the heat treatment are shownin Figure 11.

Figure 11. Results of thermal treatment of the obtained PEEK modification products.

4. Conclusions
The resulting properties of the specimens manufactured using the 3D printing technology based on the fused deposition modeling (FDM) method showed repeatability,
dependent upon the method, of their processing parameters. The effects of printing direction of layers and structural form on the mechanical properties were noted. Static uniaxial
tensile testing showed that the specimens produced in direction A (i.e., with the largest
surface adjacent to the printer table) had similar properties to the specimens marked as B
(i.e., with the minor surface adjacent to the printer table). The lowest tensile strength wasobserved for specimens made in direction C (i.e., with the smallest surface adjacent to theprinter table).
An analysis of literature sources [31–33] also showed the possibility of correcting the
exemplary printing modes with the aim to improve their mechanical properties. However,the conducted research provided new information on the influence of the printing positions,structure, and chemical parameters of specimens on the properties of the printed PEEK.
Thanks to the applied vision system, the Poisson’s coefficients were also determined.
The highest value of the modulus of elasticity was observed for specimens A in
crystalline form during both experimental tests. The samples with a crystalline structure
also had higher stiffness and, thus, increased brittleness in comparison to the amorphous
samples. These tendencies were observed in the static three-point bending and uniaxial
tensile tests. According to [31–33], our results obtained for amorphous-form samples are
similar. The tensile strength had the same level of value, at about 40 MPa, but the modulus
of elasticity was different due to the filling ratio. Our specimens of type A had a filling
ratio of 100%, which caused a higher modulus of elasticity. The results showed that theprinted PEEK exhibits exceptional tensile properties that can be achieved while maintaining
a crystalline structure. This form of PEEK provides an excellent balance of mechanicalproperties combined with flexural and tensile characteristics.
Along with the change in the chemical structure of the modification products, thephysicochemical properties of the materials changed. The resulting modification productceased to be a thermoplastic and became a thermosetting polymer, as such, it cannot
be thermally processed. While PEEK is a thermoplastic polymer, the obtained modified
specimens became duromers as a result of the heat treatment.

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Author Contributions: Conceptualization, M.M., T.M., K.J., M.G., and W.W.; methodology, K.J., P.J.
and T.M.; software, K.W.; validation, P.J., M.G. and W.W.; formal analysis, T.M., M.G. and W.W.;
investigation, M.M., T.M., K.J. and P.J.; resources, K.W.; data curation, K.J. and T.M.; writing—originaldraft preparation, M.M., T.M., P.J. and K.W.; writing—review and editing, K.J., M.G., and W.W.;
visualization, M.M., and K.J.; supervision, M.G and W.W.; project administration, M.M.; All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are contained within the article.
Conflicts of Interest: The authors declare no conflict of interest.

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