1 Introduction

The developing automotive industry puts enormous pressure on both industrial and scientific societies in order to produce optimal components, both in terms of material and design. Compositions of composites based on polymer matrices, which have high strength properties in relation to density, and thus a good price/performance ratio, are mainly optimized. The development of the polymer composites sector in the automotive industry is also related to the increasingly popular electric cars. In the case of electric vehicles, the main focus is on reducing the weight of the vehicle in order to achieve higher fuel efficiency and lower CO2 emissions [1]. It is estimated that currently, composite materials constitute about 10–15% of the weight of a civilian vehicle [2].

Despite the great interest in petrochemical polymer composites, the current environmental situation and legal changes force manufacturers to change their course towards more sustainable materials. There is a lot of interest in the field of biopolymers, mainly in the packaging industry through legal regulations imposed by the European Union. In this context, the European Commission recently published the Packaging and Packaging Waste Directive, setting out a strategy for plastics in a circular economy that includes a ban on single-use plastics by the end of 2022 [3]. However, the SARS-Cov-2 epidemiological situation has postponed the implementation date of this directive. The epidemiological situation has also shown that it is reasonable to look for alternatives to petroleum-based materials, as manufacturers have noticed a reduction in the availability of conventional plastics in industrial markets.

Substitutes for traditional polymer composites are bio-based/biodegradable polymer materials [4]. Biodegradable plastics such as polylactide or polyhydroxyalkanoates are degraded and their decomposition products are harmless to the environment [5]. In contrast, bio-based polymers can be partially or fully synthesized from building blocks of biological origin and offer almost identical chemical structures and properties to their petrochemical counterparts. One such example is polyamide 11 (PA11) which is 100% bio-based. Oliver-Ortega et al. in their work showed that it is possible to produce a composite based on PA11 reinforced with softwood stone ground-wood fibers (SSG) [6]. The research showed that the addition of 50 wt% SSG improved Young's modulus 3.51 times in relation to unmodified PA11, and thus achieved the same values as polypropylene (PP) reinforced with 30 wt% glass fiber (GF). Nanni i Messori [7] produced by injection molding a composite based on PA11 with the addition of solid wine waste called wine lees (WL) (10, 20 and 40 phr). For the highest additive content, the Young's modulus increased by 34%, while for each of the tested composites the tensile strength decreased and the highest decrease was recorded for composites with the addition of 40 phr. In the case of thermodynamic properties, an increase in the parameter values was also achieved up to the Tg limit (40 °C), but also after exceeding the Tg up to a temperature of 70 °C, an improvement in the properties was visible, and for PA11/40WL the value of the storage modulus (E') was higher by 1,3 times than for the unmodified polymer.

Another bio-based plastic is bio-polyethylene (bioPE). The influence of additives on the mechanical properties of bioPE based composites was investigated by Tarres et al. [8]. They produced composites with the addition of 0–30 wt% of thermomechanical pulp (TMP) and 6 wt% of maleic anhydride by injection molding and 3D printing methods. The addition of TMP caused a linear increase in tensile strength and Young's modulus, and for tensile strength, the increase was in the range of 115–127%. Another work containing bioPE modifications was a work on composites with the addition of thyme [9]. The composites were fabricated and characterized by Montana et al. The tested materials contained 0–50 wt% filler and poly(ethylene-co-methacrylate glycidides) compatibilizer (PEGM), and its content was set at 10% in relation to the total filler content. The tests showed that the addition of lignocellulosic filler contributed to the increase in all tested mechanical properties. The highest values were obtained for composites with the highest thyme content and the increase in tensile strength was approximately 32% and flexural strength was over 55% compared to the unmodified polymer. Comparisons between composites based on petrochemical HDPE and bioPE reinforced with abaca fibers were presented by Seculi et al. [10]. Composites with fiber content from 0 to 50 wt% were tested. and 8 wt% compabilizer (MAPE—polyethylene-grafted-maleic anhydride). Mechanical properties increased with increasing filler content, and optimal results were obtained for composites with 20 wt% abaca fibers. Composites based on bioPE showed statistically significantly higher values of Young's modulus than materials based on petrochemical PE. BioPE composites with 50 wt% filler showed Young's modulus 5 times higher than unmodified bioPE and the values of this parameter were 14% higher than for HDPE.

Poly(ethylene terephthalate) (PET) also used on a large scale has also found its green counterpart, which is produced from plant sugars. Although bioPET is becoming more and more popular, it has not yet been widely researched in the world literature and hybrid composites do not exist at all. Work on the bioPET modification was carried out by Montava et al. [11]. They produced composites with the addition of cotton waste (CW) from the textile industry (1–10% by weight). The tests showed that with the increase in fiber content, the Young's modulus increased and for the highest filling the improvement was approximately 50%, and the hardness of the produced composites also increased. However, a negative effect of the additive was observed on the tensile strength where the highest decrease was over 50% compared to unmodified bioPET.

In such advanced applications as automotive components, materials are often needed that have many features at the same time. Achieving high performance and long service life for engineering materials is an urgent need to promote sustainability. Therefore, hybrid polymer materials are appearing on the market more and more often. Hybrid polymer composites are materials that contain at least 3 components (matrix and at least two reinforcement phases). The addition of many systems to composites can offset the disadvantages of another system by valorizing another. Hybrid composites offer a better chance of optimizing the desired properties than non-hybrid composites. In our previous work on hybrid composites, it was possible to see the satisfactory impact of hybridization on the strength, thermal, and fatigue properties [12, 13]. In addition, other scientists discuss the topic of the hybridization of polymeric materials in different ways. In their research, Papageorgiu et al. investigated the effect of the simultaneous addition of graphite nanoplatelets (GNP) and GF [14]. They gave reference points to unmodified polypropylene (PP) and composites with only one type of fiber. Their research showed that the addition of both fibers at the same time resulted in a combination of properties with both GNP and GF. GFs had a decisive effect on increasing the breaking stress and Young's modulus, while GNP improved the thermal resistance of composites and their thermal conductivity. Another good example are wood polymer composites which are very popular, but their properties are too low for highly advanced applications. In the work of Guo et al., efforts were made to create hybrids based on HDPE reinforced at the same time with wood fibers (WF) and carbon fibers (CF) [15]. The addition of only 20 wt% of WF decreased the tensile strength by 17% and improved Young's modulus by 44%. CF composites showed the highest increases and amounted to – 30% for tensile strength, and Young's modulus improved 8 times. In the case of composites with 10 wt% of WF and 10 wt% of CF at the same time, the following increases were noted – 16.3% for tensile strength, and Young's modulus improved over 5 times. As the authors themselves indicate, the decrease in strength properties by the addition of WF can be compensated by the addition of CF without a significant increase in costs.

In the presented work, functional hybrid polymer composites based on bioPET were produced for the first time after a review of the literature. Chemically and thermally resistant basalt fibers and highly modular carbon fibers were used as the reinforcing phase. The effect of the synergistic action of the fibers on the physical, mechanical, and thermal properties of the produced composites was assessed. Our previous research [16] was an introduction to this work, however, it did not directly address interfacial interactions, micromechanical simulations, and thermal properties of the tested materials. Understanding the behavior of composites with respect to the properties of long fiber hybrid materials is desirable not only for the practical application and prediction of composite properties but also for the basic knowledge required in the development of new materials for technical applications.

2 Materials and Methods

2.1 Materials and Preparation of Composites

BioPET under the trade name ECOZEN® BS 400 was purchased from SK Chemicals (Seongnam, South Korea). It is a copolyester with the following properties: density – 1.25 g/cm3, tensile strength – 42 MPa, elongation at the break – 150%, and flexural modulus – 2.13 GPa. The matrix was reinforced by two types of fibers simultaneously: basalt fibers with the length of 6.35 mm, density 2.67 g/cm3, and Young's modulus of 84 GPa were provided by Basaltex Inc. (Wevelgem, Belgium) and carbon fibers PX35 type 45 with a nominal diameter of 7.2 μm, length of 4 mm, density 1.81 g/cm3, and Young's modulus of 242 GPa provided by Zoltek, Toray Group (Bridgeton, MO, USA).

Composites were injection molded into standard dumbbell samples according to ISO 3167 [17] (10 × 4 × 150) using a single screw injection molding machine Engel ES 200/40 (Schwertberg, Austria). The screw speed ranged from 60 to 90 mm/s, and the process temperature was set at 230–260 °C from zone 1 to the die. Prior to this process bioPET and fibers were compounded by means of a twin screw extruder (Maris America Corp., Windsor Mill, MD, USA). The composition ratio of bioPET/basalt fibers/carbon fibers was: 100/0/0%, 90/5/5%, 85/7.5/7.5% and 80/10/10% (w/w).

2.2 Characterization

2.2.1 Density

The density (ρ) of the samples was determined using the RADWAG WAS 220/X (Radom, Poland) electronic analytical balance following the immersion method according to the ISO 1183 standard [18]. Five measurements were taken for each composite to obtain an average value and calculate the relative standard deviation. In addition, to verify the fiber/matrix interaction, the theoretical density was calculated using the formula (1):

$$\uprho ={\uprho }_{f1}V{F}_{f1}+{\uprho }_{f2}V{F}_{f2}+{\uprho }_{m}(1-{VF}_{f1}-V{F}_{f2})$$
(1)

where:

ρf1:

is density of basalt fibers

ρf2:

is density of carbon fibers

ρm:

is density of bioPET

The volume fractions of basalt fibers (VFf1) and carbon fibers (VFf2) were calculated by the following formula (2):

$${VF}_{f1}=\frac{1}{1+\frac{{\uprho }_{f1}}{{\uprho }_{f2}}+\frac{{\uprho }_{f1}}{{\uprho }_{m}}(\frac{1}{{{\text{WF}}}_{f1}}-2)}$$
(2)

WFf1 and WFf2 are the weight fraction of basalt fibers or carbon fibers (5%, 7.5%, or 10%).

2.2.2 Thermal Properties

Vicat softening temperature (VST) was measured by using Ceast HDT and Vicat Tester Type 6520 (Pianezza, Italy). Samples with the dimension of 10 × 4 × 10 were placed in an oil bath in accordance with the ISO 306 standard using the A50 method [19]. The applied force was 10 N and the heating rate was 50 °C/h. The percentage linear shrinkage (LS) of the specimen was calculated by the formula (3):

$$LS=\frac{{L}_{1}-{L}_{2}}{{L}_{1}}\times 100\%$$
(3)

where:

L1:

is the selected dimension in the mold,

L2:

is the same dimension measured on the mold at a certain temperature and pressure.

The thermal expansion of composites was performed on the NETZSCH 402 F1 Hyperion device (Selb, Germany), where the samples were placed vertically. The dilatometric analysis consists in measuring the length of the sample (L) as a function of temperature (T). The length and temperature data were recorded and analyzed with Proteus software. The samples were cooled from 30 °C temperature to -60 °C, heated to 140 °C, and finally cooled to -60 °C. The heating and cooling rates were kept at a constant level – 10 °C/min. The coefficient of linear thermal expansion (αL) was determined:

$$\mathrm{\alpha L}=\frac{1}{2aL}\frac{{\text{dL}}}{dT}, 1/{\text{K}}$$
(4)

The thermal resistance was investigated with a TG 209 F3 Tarsus thermogravimetric analyzer (NETZSCH-Geratebau GmbH, Selb, Germany). Samples weighing 10 mg were heated in Al2O3 crucibles from 40 °C to 700 °C at a scan rate of 10 °C/min under nitrogen atmosphere (a purge of 20 ml/min of N2 protection gas and 30 ml/min of N2 sample gas). The decomposition temperature of the material was measured at 5%, 10%, 50%, and 90% weight loss. The residual mass (R) was defined at about 600 °C.

Differential scanning calorimetry (DSC) thermal analysis of bioPET composites was carried out using a DSC1 Mettler Toledo calorimeter (Columbus, OH, USA). Measurements were made in an inert gas (nitrogen) environment under dynamic conditions. The samples which weighed about 10 mg were crimped in aluminum crucibles and an empty aluminum crucible was used as a reference. At the first step, the samples were heated from 25 °C to 340 °C (at a scanning rate of 20 °C/min) and held at this temperature for 5 min to eliminate their thermal history. Then samples were cooled to -80 °C at a cooling rate of 20 °C/min. Next, the samples were heated from 25 °C to 340 °C (heating rate 20 °C/min), held at this temperature for 5 min, and cooled to 25 °C at a cooling rate of 20 °C/min. The data recorded in the last heating were used for calculations. The obtained results allowed to determine the characteristic temperatures of the material: glass transition temperature (Tg), cold crystallization during heating (Tcc), and the peak temperature of melting (Tm). Glass temperature (Tg) were taken as the inflection point of the step change of the calorimetric signal, while Tm and Tcc were defined as the highest rate of melting and recrystallization, respectively.

2.2.3 Wide Angle X-ray Scattering (WAXS)

Structural investigations of the obtained samples were carried out by means of a wide-angle X-ray scattering method (WAXS, Carl Zeiss AG, Jena, Germany), using CuKα radiation. The accelerating voltage and the applied current were 30 kV and 25 mA, respectively. The samples were scanned with step 0.04° during time 3 s at an angular range 2 theta: 10—60°. For X-ray measurements, dumbbells samples and samples obtained during non-isothermal crystallization were used. This second samples used for structure tests were obtained during heating between a cover slip and a glass slide on a microscopic hot stage up to 335 °C, after which the samples were cooled and the temperature dropped to room temperature.

2.2.4 Hot-Stage Polarized Light Microscopy

Hot-stage polarized light microscopy was used to observe isothermal crystallization of bioPET in the presence of BF and CF. For this purpose, a Labophot-2 polarizing optical microscope (Nikon, Tokyo, Japan) coupled with a Panasonic CCS camera (Kadoma, Japan) and the Linkam TP93 thermal attachment (Tadworth, UK) was used. The analyzed samples were placed on a microscope slide and heated to the temperature of 250 °C (rate of 20 °C/min), in which the melting took place. Then the bioPET and composite samples were cooled to 140 °C.

2.2.5 Fourier-transform infrared (FTIR)

Fourier-transform infrared (FTIR) spectra of the sample were obtained using the attenuated total reflectance (ATR) mode (a Nicolet 5700 equipped with a ZnSe crystal ATR unit, Thermo Fisher Scientific Inc., (Bartlesville, OK, USA), and a Bruker Tensor 27 equipped with a SPECAC Golden Gate diamond ATR accessory, Bruker Optik GmbH, (Ettlingen, Germany). The spectra were recorded with a resolution of 4 cm−1 and an accumulation of 64 spectra.

2.2.6 Scanning Electron Microscope (SEM)

The morphology of composites was investigated by the scanning electron microscope SEM (JEOLJSM5510LV, Tokyo, Japan) 20 kV. Prior to SEM, the samples were coated with a thin layer of gold, under vacuum, using an auto vacuum coater (Cressington, Watford, UK) to avoid electrical charge accumulation.

2.2.7 Contact Angel

In order to test the surface characteristics of the hybrid composites, the contact angle was measured using a contact angle goniometer (Contact Angle System OCA, Filderstadt, Germany) at room temperature (about 25 °C) and with accuracy ± 0.01 mN/m. 2-μm drops of water were placed by means of an automatic pipette. The values of the contact angle were read 5 s after the drops were placed on the sample (Dataphysis, OCA15EC).

2.2.8 Mechanical Properties

The tensile properties of the samples were determined using an MTS Criterion Model 43 universal testing machine (Eden Prairie, MN USA) at a constant crossed speed of 10 mm/min according to the ISO 527 standard [20] with load cell 30 kN. For the accurate measurement of displacement allowing the determination of the elastic modulus, an MTS 634.31F axial extensometer was used. The flexural tests were conducted in the three-point loading mode using a universal testing machine MTS Criterion Model 43 with a displacement rate of 5 mm/min according to the ISO 178 standard [21] with the load cell 30 kN and span length of 80 mm. Unnotched samples were used to measure impact strength by Charpy method using Zwick/Roell MTS-SP testing machine (Ulm, Germany) (ISO 179 standard [22]). The applied energy was 1.5 J. The mechanical tests were performed at various temperatures of potential application. Tests were carried out at—24, 23, and at 80 °C using the temperature chamber (Instron, Norwood, USA). Five standard dumbbell samples (10 × 4 × 150) of each composite were tested, and the average value was reported.

2.2.9 Hydrodegradation

Water absorption was carried out according to ISO 62 standard [23]. The standard dumbbell samples (10 × 4 × 150) of bioPET and its composites were immersed in distilled water at room temperature and weighed after 1 and 30 days using an electronic weighing balance (RADWAG WAS 22W, Radom, Poland). Samples were removed from the water bath and their weight was measured after surface drying using tissue paper. Water absorption was calculated using formula (5):

$${M}_{t}=\left[\frac{{W}_{t}-{W}_{0 }}{{W}_{0 }}\right]\times 100$$
(5)

where:

Mt:

stands for the percentage of water content,

Wt:

for the instantaneous weight of the sample,

W0:

for the initial weight of the sample.

Additionally, to assess the influence of absorbed water all compositions were tested after 30 days of soaking in water. The samples were tested after the water was removed from the surface of the samples with tissue paper.

3 Result and Discussion

3.1 Density and Thermal Properties

The values of density, Vicat softening temperature, and shrinkage are summarized in Table 1. As the fiber content increased, the density of the produced composites increased (Table 1). It is mainly related to the addition of fibers with a higher density than the matrix: basalt fibers – 2.67 g/cm3 and carbon fibers – 1.81 g/cm3. The experimentally calculated density values correspond to the calculated theoretical values, which proves good fiber/matrix adhesion. Good adhesion between the fiber and matrix means that no pores are formed in the material, which results in a stable density of composites. Sufficient adhesion and good processing are also confirmed by the results of TG (Table 3), where the residual mass coincided with the amount of introduced fibers which, due to their high thermal properties, did not burn (Fig. 1).

Table 1 Comparison of compositions, theoretical density, experimental density, Vicat softening temperature, and shrinkage of bioPET and its composites
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Table 2 Linear coefficient of thermal expansion, softening temperature, maximum of extension, and shrinkage temperature of bioPET and its composites
Full size table
Table 3 The results of thermal decomposition of polymer matrices and composite materials
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Fig. 1
figure 1

Thermogravimetric analysis TGA a) and its derivative mass (DTG) b), curves of bioPET and its composites

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In the case of engineering composites made of thermoplastic plastics, operational thermal properties are very important parameters. Therefore, the study determined the Vicat softening temperature (VST) and shrinkage. The addition of fibers increased the VST and its values increased as the fiber content increased – an improvement of 10% for bioPET/10B10C was observed. In general, there are three options for increasing the heat resistance of a polymer: increasing the glass transition temperature (Tg), increasing the crystallinity, and strengthening [24]. In the case of the produced composites in this study, the increase in thermal stability was influenced by Young's modulus growth and good fiber/matrix interaction (Fig. 6). The combination of basalt and carbon fibers gives a synergistic effect on improving the thermal properties of the composite. The addition of fibers will reduce the possibility of movement of the polymer chains.

From the properties of individual fibers, it can be concluded that modular CFs contribute to increasing the stiffness of composites, while rigid and heat-resistant BF for an increase in thermal stability [25, 26]. A similar situation was observed in the case of shrinkage. The reduction in the shrinkage value ranged from 46 to 62%. The reduction in shrinkage can be attributed to the length of the fibers that can join distant areas in the composite and to their much lower shrinkage than the polymer matrix. The improvement of processing properties with a simultaneous slight increase in weight significantly extends the range of applications of the produced composites.

The values of the linear thermal expansion coefficient (LTEC) of the tested materials are presented in Table 2. The highest value was obtained by neat bioPET. This is because thermoplastic polymers have high linear thermal expansion coefficients due to weak secondary bonds between the chains [27, 28]. As the content of filler added to the polymer matrix increased, the LTEC decreased – the value decreased almost threefold. This phenomenon is associated with a much lower fiber LTEC: BFs – 6.5–8.0 × 10–6 °C and CFs – lower than 1.0 × 10–6 °C [29]. The addition of fibers reduced the possibility of deformation of the bioPET molecular chain and hindered the flow of the polymer matrix. This is a positive phenomenon due to the dimensional stability of elements made of such composites, better compatibility, and accuracy of fixing.

3.1.1 TGA

Reinforcing bioPET with fibers can create high-performance materials. The sample of neat polymer and its composites are subjected to thermogravimetric analysis (TGA), which determining thermal stability, i.e. the ability to maintain constant structure under changing environmental conditions, in this case, under exposure to high temperatures. TGA is studied to understand the decomposition behavior (mass loss as a function of temperature or time) of the bioPET and its composites.

The difference in thermal decomposition of the bioPET is clearly visible in Fig. 1 (TGA (a) and DTG (b) thermograms of the research samples) and in Table 3 where are summarized the selected parameters obtained from the TGA/DTG thermograms. DTG analysis can give us an information about the decomposition rate of the samples during the heating process. Tmax is defined as the temperature of the maximum rate of decomposition sample.

In general, the thermal stability of polymeric materials strongly depends on the chemical structure, molecular weight, and degree of its crystallinity [30]. It can be seen, that bioPET and its composites containing BF and CF fibers presented a good thermostability. The results of the study showed that there was no significant mass loss until 360 °C, so it is the limit for practical application of these composites. At a temperature of 378–380 °C, the weight loss was approximately 5%. As can be seen, bioPET and its composites undergo single stage decomposition. BFs, according to literature data, do not decompose under 8 000 °C [31], while CFs undergo decomposition (by oxidation) at temperature 600 °C [32]. The incorporation of fibers slightly improves the thermal stability of the bioPET matrix by a small increase in the temperature of thermal degradation as compared to neat polyester. However, there is no significant change in filled composite with different basalt/carbon concentrations, indicating that the content of BF and CF fibers within the scope of the test had a slight influence on the bioPET characteristics of the pyrolysis process temperature. The highest rate of decomposition of neat polymer took place at about 408 °C and polymer composites containing fibers at 413 °C. The presence of fiber in the polymer matrix of bioPET affects the temperature of the decomposition above T90, e.g. for the neat polymer and composite with 10% loading (bioPET/5B5C), a weight loss of 90% is observed at 437 and 627 °C, respectively. As can be seen, the residual mass, especially at higher fiber content in the polyester matrix, corresponds to the content of the fibers added to the polymer matrix, but the mass percentage of the residue in the composite containing 5 wt% BF and CF fibers is slightly lower as that of the added basalt/carbon fiber. Therefore, it can be considered that basalt/carbon fibers have a slight effect on increasing the carbonization effect of bioPET.

3.1.2 Differential Scanning Calorimetry (DSC)

Crystallization is a very important property affecting all mechanical and physical properties of bioPET and highly affects processing conditions like processing temperature, cooling rate, stretching process, nucleating agents, etc. Moreover, crystallization abilities depend on the polymer tactility, bulkiness, stiffness of the polymer chains, the polarity of side groups, and microstructure of polymers. In this case of study, bioPET has a melting point (Tm) at about ~ 327 °C and a glass transition temperature (Tg) at about ~ 82 °C. Composite materials did not crystallize from the melt but developed a cold crystallization process during heating.

Results of DSC investigations of all bioPET and those of composite materials containing a mixture of BFs and CFs are presented in Table 4 and Fig. 2. The investigation was performed for a series of compositions containing 0, 5, 7.5, and 10 wt% of the BF and CF fibers. The melting of bioPET (semi crystalline polymers) was a very broad process. The peak temperature of melting indicates the temperature at which the melting proceeds with the maximum rate and not the highest temperature at which crystals melt. As can be seen, bioPET and its composites were melting in the range of the temperatures 275 – 340 °C with the maximum rate of melting at 326 °C. The analysis of the obtained data has shown that modification of bioPET with fibers mixture causes significant changes in the course of thermal transformations of the polymer matrix. The addition of fiber resulted in a broadening of the rate of melting temperature towards lower temperature. This behavior is most likely related to the formation of smaller, less perfectly formed crystalline structures, as well as to changes in the uniformity of their size due to the introduction of the filler. Smaller and/or less perfectly formed structures melt at lower temperatures [33].

Table 4 Thermal characterization of neat polymer and its composites
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Fig. 2
figure 2

Compilation of calorimetric curves for produced composites with different basalt/carbon fibers content; a glass transition temperature Tg; b the melting temperature and recrystallization peak temperature

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The modification of bioPET with fibers had a significant effect on the crystallization properties of the materials (Fig. 2b). The introduction of basalt and carbon fibers into the bioPET matrix caused a slightly shift of the Tc peak maxima (the highest rate of recrystallization) towards higher temperatures and extending the crystallization range to lower temperatures. This phenomenon is often explained by the strong nucleation occurring on the fiber surfaces, which shortens the time required for crystallization [34]. Some consider that, in case of cold crystallization volumetric crystallization dominates and transcrystalline growth may be suppressed or completely absent [35]. Therefore, in our case, presence of additives may affect bulk crystallization by increasing the nucleation of the polymer network and thus influence on the size and degree of homogenization of crystalline structures. As the density usually changes during crystallization, stresses are created which are perhaps better relaxed in the presence of fibers in the matrix.

Tg is one of the most important parameters characterizing polymeric and composite materials because it is the temperature, at which the ductile or melt state changes into the glassy state and that's why determines the highest and lowest application temperature of the materials. In semi crystalline polymers (composed of crystalline and amorphous regions), like bioPET, the Tg area was very broad and partially smeared out (Fig. 2a). In this case, Tg could be determined through various ways, the most popular of them being the inflection (midpoint) temperature. With an increasing mixture of fibers content, the Tg of the bioPET phase slightly shifted to lower temperatures. The deviation in Tg of polymer composites from the neat bioPET is associated with the influence of the fibers on the motion of polymer chains: fibers may decreases the free volume of the polymer chains and enhances the polymer crosslinkage [36].

3.2 WAXS

The results of X-ray examinations (Fig. 3) are in agreement with the examinations performed with the DSC technique. As it results from the thermograms, the crystallization of bioPET takes place only in the process of recrystallization during heating at temperature of about 220 °C. The crystallization process does not take place during the cooling of the samples. Therefore, the tests of unmodified bioPET moldings shows that no crystal structure has formed. On the other hand, the number of recorded impulses at the angle 2 theta 20° indicates the phenomenon of diffraction on trace amounts of ordered areas. However, differences in the shape of X-ray images can be observed. It can be noticed that the X-ray image of the composite (bioPET/10B10C) shows the crystallization beginnings at 2 theta angles of 18.5 and 24.5 [37]. This points to an analogy with crystalline of standard PET X-ray pattern having two diffraction maxima at 16.5–8.5 2 theta and another two at angles from 23.0 to 26.0.

Fig. 3
figure 3

X-ray diffraction pattern of bioPET and bioPET/10B10C

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Since analogous results were recorded for the samples and freely crystallized samples, it can be concluded that it was not the shear forces occurring during the injection process, but the mere presence of the used fillers, which has a slight influence on the nucleation of the crystallization process. Moreover, the X-ray patterns for the inside of the molded part and the outer layer (skin) are the same, which confirms the lack of influence of shear forces on the bioPET crystallization process.

3.3 FTIR Spectroscopy

ATR-FTIR spectroscopy was performed to determine crystallinity and the molecular interactions between bioPET and BF/CF fibers. As shown in Fig. 4 the characteristic bands of the neat bioPET were observed at 2924 cm−1, 2862 cm−1 and can be attributed to C–H stretching of the CH2 groups. The region of 1578 cm−1, 1505 cm−1, and 1407 cm−1 corresponds to aromatic skeleton stretching. The peak at ~ 1243 cm−1 and 1096 cm−1 corresponds to C–C–O stretching of the ester group and the peak at ~ 1257 cm−1 corresponds to C–O–C vibrations of the matrix. Bio-based PET show bands at 1017 cm−1 (C-O stretching), 1451 cm−1, and 872 cm−1 (CH2 bending and CH2 rocking), which are attributed to the gauche/amorphous conformation [38]. Other spectral features confirm the bioPET traces of crystallinity, such as the strong band corresponding to the carbonyl stretching band at 1715 cm − 1 (C = O stretching of carboxylic ester group) – this spectral pattern is determined by the ordered structure of bioPET where the carbonyl groups are coplanar with the benzene rings [38]. The bioPET/fibers samples exhibited absorption bands relatively similar to that of the neat matrix. However, a new peak or peak shift was not observed, which implies that PET and fiber may have not strong interaction, it was observed only that the peaks intensity decreases with increasing fibers content.

Fig. 4
figure 4

Experimental infrared absorption spectra of neat bioPET and its composites

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3.4 Morphological Studies

3.4.1 Hot-Stage Polarized Light Microscopy

The hot-stage polarized light microscopy analysis was carried out in order to determine the nucleation activity of the fillers: carbon and basalt fibers in the bioPET polymer matrix. In the microscopic images taken, no growth of spherulitic structures in the matrix was observed. However, the performed analysis allowed for the characterization of the obtained composite systems. The dimensions of the filler have been dimensioned in the obtained images (Fig. 5). The conducted analysis allowed to evaluate the dispersion of the filler in the polymer matrix. The filler was found to be uniformly dispersed regardless of the concentration added. This is a satisfactory effect that positively influences the properties discussed below.

Fig. 5
figure 5

Microscopic image for a sample of bioPET/10B10C composite with dimensioned fibers (taken at 250 °C)

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3.4.2 Scanning Electron Microscopy (SEM)

Figure 6 shows SEM images of tensile fractured specimens of the tested composites: bioPET/5B5C and bioPET/10B10C. The resulting fractures are fragile as evidenced by the layered arrangement of the planes. BFs have a significantly larger diameter from 11 μm to 14 μm, while CFs, according to the manufacturer's description, approx. 7 μm. As can be seen, the fibers are distributed evenly without significant agglomeration and their arrangement is random to each other. There is satisfying interaction between the fibers and the matrix as evidenced by the matrix residues on the fibers for both BF and CF. The sufficient interaction between the fiber and the matrix is confirmed by the high tensile strength properties described later in the text. Additionally, for the shorter CFs, the fracture plane runs close to the fracture plane, which proves the good possibility of transferring the load from the matrix to the fiber. In the case of long BF fibers, the pull-out phenomenon occurs more often than in the case of CF, as evidenced by the holes left in the matrix. In addition, this phenomenon is exacerbated by a much smoother BF surface than CF, which favors the removal of the fibers from the matrix. The higher roughness of the CF is related to the surface treatment, which contributed to the larger specific surface area of the CF. According to the manufacturer, silane sizing with a content of 2.75 wt% was used for CF, while BF was not modified. Moreover, the length of the fibers had a significant effect on the increase of the strength properties of the produced composites and thus the load-carrying capacity.

Fig. 6
figure 6

The SEM image of bioPET/5B5C (top) and bioPET/10B10C (bottom)

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3.5 Contact Angle

The hydrophilic or hydrophobic character of the composite material surface can be evaluated by measuring the water contact angle (ƟW). In our study, this parameter enabled the evaluation of changes in the hydrophobicity of polymer and basalt/carbon – modified composite.

Contact angle, determining surface wettability, is the main parameter that characterizes the drop shape on the solid surface. Contact angle and the wetting behavior of bioPET composite containing BF and CF fiber are influenced by many physical and chemical factors such as surface roughness and heterogeneity as well as the size/length of fiber. The value of the contact angle of bioPET is about 78.3°, which means that matrix has hydrophilic properties and was well wetted with demineralized water. As previously mentioned, interactions between the polymer matrix and the fibers are good, so some of the fibers migrate to the surface, which results in an increase in surface roughness. Surface inhomogeneities cause the drop to wets the surface of the polymer composite to a lesser extent, which consequently increases the hydrophobic properties of the materials. The highest increase in the contact angle (about 4%) was observed on the surface of composites containing 10 wt% of BF and CF fibers (Fig. 7).

Fig. 7
figure 7

The image of contact angle ƟW for the composite containing: bioPET, bioPET/5B5C, bioPET/7B7C and bioPET/10B10C

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3.6 Mechanical Properties

3.6.1 Tensile Properties

Figure 8 shows the results obtained during the tensile and flexural tests at three different temperatures: -24, 23, and 80 °C. The strength properties increased with the increase in the fiber content – the higher the fiber content, the higher the properties. At room temperature, the highest increase was recorded for composites with a total fiber content of 20%, and it was an increase of 85% and 366% in relation to the unmodified composite for tensile strength and Young's modulus, respectively (Fig. 8 (a) and (b)). The addition of a mixture of stiff fibers, on the other hand, resulted in a sharp decrease in strains at the break to the level of 2–2.5% from the initial 60% for neat bioPET, which was reflected in the high results of the stiffness of composites impressed by Young's modulus. Such high strength values were obtained by adding two types of fibers with high input properties – high modulus CFs and high strength BFs and in addition as shown in the SEM images (Fig. 6) through sufficient fiber/matrix interaction. A significant increase in strength was achieved by the addition of long BFs, and stiffness by the addition of modular CFs. The addition of CFs was compensated by the addition of chemically very resistant BFs and contrariwise. It is worth noting that the most effective fiber addition was 15 wt%, where the increase in tensile strength was improved by 81%, while Young's modulus by 337%.

Fig. 8
figure 8

Mechanical properties of bioPET and its composites at different temperatures (-24, 23 and 80 °C): a tensile strength, b Young's modulus, c flexural strength and d) flexural modulus

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Looking at the work of the predecessors, the hybridization showed satisfactory results in this work because the obtained results were comparable or much higher than in the case of adding the same amount of only one type of fiber: basalt or carbon. In the work of Ronkay and Czigany, composites based on petrochemical PET with the addition of 15–45 wt% BFs were produced [39]. The input parameters of unmodified PET were comparable to our study, however, the addition of 45 wt% of BFs increased tensile strength by only 8% and Young's modulus by 159%. Considering only adding CFs to PET, in the work of Shi et al., the results of the influence of CF treatment on PET properties are presented [40]. Composites with various contents from 2 to 15 wt% of short CFs were prepared. The addition of 15 wt% CFs improved the tensile strength by 48%, flexural strength by 63%, and flexural modulus by 156%, what is still lower than in the case of the presented study.

3.6.2 Flexural Properties

A similar tendency as in the case of tensile strength was obtained for the three-point flexural test (Fig. 7 (c) and (d)). The strength values are higher than for tensile strength due to the complex state of stresses in the sample and the good adhesion of the fibers to the matrix. With the increase in the amount of filler, the properties increased. The highest increases were recorded for bioPET/10B10C– 84% and 292% for flexural strength and flexural modulus, respectively. Lowering the test temperature contributed to the improvement of the properties, however, it was not as noticeable as in the tensile test. Increasing the test temperature resulted in a decrease in properties and the lowest was for unmodified bioPET – 5% (flexural strength) and 13% (flexural modulus), while the highest decrease was noted for bioPET/7B7B – 27% (flexural strength) and 31% (flexural modulus).

3.6.3 Influence of Temperature on Mechanical Properties

As shown in Fig. 8, the results of the tensile and flexural strength are temperature dependent. The highest properties were obtained for composites tested at reduced temperatures (-24 °C), for the unmodified polymer, the increase over room temperature was 18% and 4%, for tensile strength and Young's modulus, respectively. The addition of fibers will not significantly change the value compared to neat bioPET. A similar situation was observed by Chu et al. [41]. They attribute the phenomenon of improving the mechanical properties at lowered temperatures to the thermal shrinkage of the matrix, which contributed to a stronger interaction of the fiber/matrix, and thus a better load transfer from the matrix to the fibers.

Increasing the test temperature resulted in a decrease in the strength properties of the tested materials. This is a typical phenomenon of thermoplastics, where, after exceeding the Tg, the material is easily deformable [42]. For PET, Tg is around 80 °C, which is also confirmed by these tests [11]. It is worth noting that the drops in strength properties were not significant due to the testing performed at a temperature close to the Tg of the composites. The lowest decrease in properties was obtained for unmodified bioPET– 7% and 9%, for tensile strength and Young's modulus, respectively. The addition of fibers contributed to the deterioration of the results at 80 °C and the highest decrease was observed for bioPET/7B7C – 25% (tensile strength) and 29% (Young's modulus). These results perfectly correspond with the DSC results, where the lowest Tg value was recorded for bioPET/7B7C – 73.4 °C.

3.6.4 Theoretical Calculation

The effectiveness of the reinforcement and the estimation of the adhesion of individual fibers to the matrix were verified with the use of three models. The first model to estimate the values of Young's modulus was the modified Mixture Rule (mRoM), which one of them is Parallel (Voigt) model [43]. The Parallel model assumes perfect adhesion of the filler to the matrix, which results in an overestimation of the properties (6):

$${E}_{c}={E}_{m}{V}_{m}+{E}_{B}{V}_{B}+{E}_{C}{V}_{C}$$
(6)

where:

Ec, Em, Eb, and Ec – Young's modulus of composites, matrix, basalt fibers, and carbon fibers, respectively,

Vm, Vb, and Vc- volume friction of matrix, basalt fibers, and carbon fibers, respectively.

Another model is the Series model which inverts the RoM and assumes no contact between the particles (7) [44]:

$$\frac{1}{E}_{c}=\frac{{V}_{m}{E}_{B}{E}_{C}+{V}_{B}{E}_{m}{E}_{C}+{V}_{C}{E}_{B}{E}_{m}}{{E}_{m}{E}_{B}{E}_{C}}$$
(7)

The third model, the modified Haplin-Tsai (H-T) equation, is used to predict unidirectional composite stiffness as a function of filler load and shape factor (8) [45]:

$${E}_{c}=\frac{3}{8}{E}_{11}+\frac{5}{8}{E}_{22}$$
(8)
$${E}_{11}=\left\{\frac{1+{\left(L/D\right)}_{{\text{B}}}{H}_{L(B)}{V}_{B}}{1-{H}_{L(B)}{V}_{B}}{E}_{m}\right\}+\left\{\frac{1+{\left(L/D\right)}_{{\text{C}}}{H}_{L(C)}{V}_{C}}{1-{H}_{L(C)}{V}_{C}}{E}_{m}\right\}$$
(9)
$${E}_{22}=\left\{\frac{1+{H}_{T(B)}{V}_{B}}{1-{H}_{T(B)}{V}_{B}}{E}_{m}\right\}+\left\{\frac{1+{H}_{T(C)}{V}_{C}}{1-{H}_{T(C)}{V}_{C}}{E}_{m}\right\}$$
(10)
$${H}_{L(B,C)}=\frac{{E}_{(B,C)}/{E}_{m}-1}{{E}_{(B,C)}/{E}_{m}+2{L}_{(B,C)}/{D}_{(B,C)}}$$
(11)
$${H}_{T(B,C)}=\frac{{E}_{(B,C)}/{E}_{m}-1}{{E}_{(B,C)}/{E}_{m}+2}$$
(12)

where:

L and D are length and diameter of the basalt (B) and carbon (C) fibers, respectively.

Figure 9 presents the experimental data of Young's modulus for the produced composites and the data obtained from theoretical calculations: a) Parallel, b) Series and c) Haplin-Tsai. Although the work did not produce composites containing only one filler, the mathematical calculations included composites containing only one filler: 10 wt% of BF or CF (bioPET/10B and bioPET/10C), 15 wt% of BF or CF (bioPET/15B and bioPET/15C) and 20 wt% of BF or CF (bioPET/20B and bioPET/20C).

Fig. 9
figure 9

Comparison of experimental Young's modulus results with a) Parallel, b) Series and c) Halpin–Tsai equations

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None of the used models is 100% consistent with the experimental results, however, based on these data, it can be concluded that there was sufficient fiber/matrix adhesion in the produced composites. The experimental results are between the results calculated with the Voigt model, where ideal adhesion of the matrix fiber is assumed, and the results from the Ressus model where there is no adhesion.

In the case of RoM, a linear increase in Young's modulus is observed. However, both in the case of the experimental results and the results calculated on the basis of the H-T model, we observe that after exceeding 10 in wt% of filling, where up to this value is a sharp increase in Young's modulus, only a slight increase in the value of Young's modulus is visible. On this basis, it can be concluded that the optimal amount of filler is 7.5 wt% of BF and 7.5 wt% CF. It is worth noting that the simultaneous addition of basalt and carbon fibers showed a more pronounced stabilization of the results above 10 wt% of the total fiber content than in the case of adding the same content of only one type of fiber – linear increase (H-T model).

Experimental hybrid – bioPET, bioPET/5B5C, bioPET/7B7V and bioPET/10B10C; Parallel hybrid – bioPET, bioPET/5B5C, bioPET/7B7V and bioPET/10B10; Parallel only basalt – bioPET/10B, bioPET/15B and bioPET/20B; Parallel only carbon – bioPET/10C, bioPET/15C and bioPET/20C; Series hybrid – bioPET, bioPET/5B5C, bioPET/7B7V and bioPET/10B10; Series only basalt – bioPET/10B, bioPET/15B and bioPET/20B; Series only carbon – bioPET/10C, bioPET/15C and bioPET/20C; Halpin–Tsai hybrid – bioPET, bioPET/5B5C, bioPET/7B7V and bioPET/10B10; Halpin–Tsai only basalt – bioPET/10B, bioPET/15B and bioPET/20B; Halpin–Tsai only carbon – bioPET/10C, bioPET/15C and bioPET/20C.

3.6.5 Charpy Impact Strength

Figure 10 shows the results obtained during the impact test according to the Charpy method at three different temperatures: -24, 23, and 80 °C. The addition of fibers increased the impact strength at room temperature, which confirms a very good adhesion of the fibers to the matrix. The addition of up to 15 wt% of the fibers improved the properties by 25%, while the addition of 20 wt% did not cause any changes – improvement at the same level as for bioPET/7B7C. The improvement in impact strength is an unexpected result, as one would expect that stiff fibers tend to reduce the impact strength in the ductile matrix, which is the highly flexible PET– strain at the break of about 60%. However, in the case of bioPET, the glass transition temperature of which is approximately 80 °C, it should be noted that the material works beyond the glass transition temperature and is more brittle than ductile. Therefore, the introduction of stiff fibers improves the impact strength. Moreover, on the one hand, this is due to the good adhesion of the fibers to the matrix, and, on the other hand, the lower fiber content contributed to the production of smaller spherulites and thus greater flexibility [46]. Lowering the test temperature increased the brittleness of the material by about 30%, the addition of fibers had no significant effect on the differences in values. Increased temperature contributed to higher mobility of polymer chains which translates into higher deformability and the highest increased was for unmodified material – 28%. The addition of fibers partially inhibited the movements of polymer chains and as the fiber content increased, the differences between the properties obtained at room temperature and + 80 °C decreased – ~ 1% for bioPET/7B7C and bioPET/10B10C.

Fig. 10
figure 10

Charpy impact strength of bioPET and its composites at different temperatures: -24, 23, and 80 °C

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3.7 Hydrodegradation

In order to determine the life cycle of the produced composites, the samples were subjected to hydrolytic degradation for 30 days. Determining the effect of water on polymer composites is a very important aspect in determining the viability of potential products. The water environment may contribute to the degradation of the polymer matrix, while in the case of fiber-reinforced composites, interfacial detachment may occur, leading to osmotic cracking [47]. The results of the water absorption and the properties obtained in the tensile test after 30 days of incubation in water are summarized in Table 5.

Table 5 Results of water immersion of bioPET and its composites: water absorption after 1 day and 30 days, tensile strength, and Young's modulus after 30 days of immersion in water
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As the number of days increased, the amount of absorbed water increased. The addition of fibers that do not absorb water caused a slight decrease in water absorption. As the fiber content increases, the water absorption capacity of the composite decreases. These results confirm the increase in the hydrophobicity of the composites, which was also manifested in the wettability angle results, where the hydrophobicity increased with increasing fiber content (Fig. 7). Additionally, the SEM images (Fig. 6) confirm the sufficient fiber-matrix interaction, which contributed to a lower micro tunneling capacity between the phases.

The results obtained during the tensile test after 30 days of incubation in water show that the water environment had only a slight influence on the deterioration of the strength properties. The decrease in tensile strength ranged from 4–6%, while in the case of Young's modulus, the decrease was even smaller, from 1 to 3%. The lowest values of the decrease in properties were obtained for bioPET/10B10C– 4% (tensile strength) and 1% (Young's modulus).

4 Conclusion

The presented work shows that the simultaneous addition of two types of fibers will positively affect the mechanical, thermal, and performance properties. Composites produced by injection molding on the basis of bioPET were reinforced with basalt and carbon fibers (5/5, 7.5/7.5, and 10/10 wt%). As shown by mechanical tests, the property values increased with increasing fiber content. The optimal setting was noted for composites with 7.5 wt% of basalt and carbon fibers + 81% and 337% for tensile strength and Young's modulus, respectively. It should be emphasized that the addition of the same fiber content of only one type of fiber did not result in the properties that were obtained for hybrid composites. A significant increase in strength was achieved by the addition of long basalt fibers, and stiffness by the addition of modular carbon fibers. The addition of fibers will have a positive effect on the impact strength values and the improvement for bioPET/7B7C composites was 25%, while further increasing the fiber content did not change this parameter.

For all the produced composites, an improvement in thermal resistance was observed, reflected in shrinkage which was improved by 62% for bioPET/10B10C, and the linear coefficient improved by over 65% for all composites. In addition, the incorporation of fibers slightly improves the thermal stability of the matrix by a small increase in the temperature of thermal degradation. SEM images and hot-stage polarized light microscopy showed good adhesion of the fibers to the matrix, which translated into high strength properties. In addition, the processing process used was appropriately selected because the fibers were evenly distributed in the matrix and the residual mass coincided with the initial assumptions of the addition of fibers. The presence of the used fillers has a slight influence on the nucleation of the crystallization process, the crystallization of bioPET and its composite takes place only in the process of recrystallization during heating at a temperature of about 220 °C.

Incubation of the materials in water did not contribute to the rapid increase in water absorption. This is mainly due to the addition of fibers that do not absorb water and good fiber/matrix adhesion. Also, mechanical tests carried out after 30 days of incubation in water have shown that bioPET composites reinforced with BF and CF can be used for materials intended for the production of articles with a long-life cycle in extreme conditions.

The application of hybridization to bio-based polymer composites appears to be very promising for the development of sustainable engineering materials. Obtaining optimal parameters will not only affect the spread of bio-based materials but also contribute to the creation of reliable compositions that work well in various operating environments. Such comprehensive research in the future could be helpful in the development of sustainable materials in the near future.