1 Introduction

In the 1930s, polyamide (PA) was the first fibre to be produced entirely synthetically and became famous under the trade name nylon for its use in the manufacture of stockings [1, 2]. Nowadays, many different PA types exist that are still mainly produced and used as fibres in textiles for apparel or technical applications, as tires, ropes, safety belts, or sailcloth [3]. Among synthetic fibres, PA takes 2nd place in terms of volume with a market size value of USD 33.30 billion in 2020 [4, 5]. In the last decades polyamide, however, continuously extended its field of application to the automotive and building industry, electronics, or the medical sector. The worldwide polyamide market size is therefore expected to increase from 2021 to 2028 with a compound annual growth rate of 6.1%. The most important PA types are still polyamide 66 (PA 66) and polyamide 6 (PA 6) which have with over 90% the highest market share. The much smaller proportion of polyamide fibres consists of high-performance polyamide and bio-based polyamide fibres [5]. The important factors for the success of PA, in particular PA 66 and PA 6, are an economically viable polymer synthesis and also economically viable processing methods as for instance extrusion, injection moulding, and melt fibre spinning. In addition, the polymer properties cover an excellent tensile strength, good elastic recovery, very good electrical resistance and good resistance to chemicals [1, 6]. In contrast, they show an inertness to other chemicals due to their smooth surface and lack of chemical functionalities. Therefore, PA often faces interface stability problems in composites. Surface activation techniques are a suitable means to overcome the interface stability problem by changing the surface chemistry and surface topography of the substrate.

For this work, three surface activation techniques have been chosen to modify the surface of a polyamide 66 fibre. These include a complexation technique with a Lewis acid solvent that contains calcium chloride CaCl2, ethanol EtOH, and water H2O (CEW), a plasma treatment under atmospheric pressure with air (APPA), and a grafting process with 2-hydroxyethyl methacrylate (HEMA) using hydrogen peroxide as initiator. These three techniques differ in terms of activation principle (Fig. 1).

Fig. 1
figure 1

Used surface activation methods for PA 66 fibres

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The CEW treatment represents a wet chemical treatment that induces a roughening due to the solvation of the fibre and does not change the fibres’ surface chemistry by introducing new functional groups. The complexation between Ca2+ and the oxygen atom of the amide group competes with the formation of hydrogen groups between the oxygen atoms in one PA chain with the -NH groups of another PA chain. As Lewis acids have a higher electron-accepting capacity than -NH in the amide group, the hydrogen bonds between the PA chains are disrupted and Lewis acid coordination is established. The PA chains thus contain complexed oxygen atoms, while -NH of the amide group is released. As a result, PA chains get loosen from the surface and go in solution. The process is reversible. By rinsing the complexed PA66 with water, Ca2+ is removed and the decomplexation can take place. The hydrogen bonds are re-established and the polymer chains get re-arranged [7,8,9,10,11]. The plasma treatment is abundantly used in several applications such as surface modification, as it presents an eco-friendly, simple and efficient method. Atmospheric pressure plasma treatment in particular is convenient as it can be operated continuously. Due to its widespread use, the atmospheric pressure plasma with air (APPA) is used as the state-of-the-art reference technique in this work. The plasma treatment represents a physical treatment that induces roughening by etching reactive species in the plasma. At the same time, the plasma introduces new functional groups depending on the used plasma gas. These groups cannot be specifically assigned to one type of functional group. Most likely, hydroxyl, amine, carbonyl, carboxyl and amide groups are introduced during plasma treatment under normal pressure (NP) with air [12,13,14,15,16]. The grafting process represents a non-ablative technique compared to the activation techniques mentioned above. HEMA is polymerised with hydrogen peroxide in solution on the fibre. Defined chemical functionalities are incorporated on the fibre surface. During grafting with HEMA, mainly oxygen groups such as hydroxyl and ether groups are formed on the surface. The topography is changed accordingly [17, 18].

In this work, the impact of the three techniques on the fibre’s appearance and dimensions, such as fibre fineness, fibre wetted length, and fibre roughness shall be investigated. In addition, the wetting and dewetting behaviour of the pristine and treated fibres will be analysed using a force tensiometer. The wetting behaviour of the fibre for the generation of composites is an important measure to evaluate whether gaps between the fibre and the matrix material will be observed which may lead to a decrease in mechanical properties [19]. The three techniques will be compared and an optimal setting for an optimal wetting will be detected.

2 Experimental

2.1 Materials

For the comparative study, PA66 monofilament kindly provided by Getzner Textil AG, Austria was used to investigate the effect of the surface treatment on the fibre’s properties and to exclude the effect of the macrostructure of e.g. a woven or knitted textile on the fibre's properties. These monofilaments are referred to as pristine or untreated PA66 fibres and serve as the reference substrate in this work. Only for X-ray photoelectron spectroscopy analysis (XPS), a washed woven fabric made from the above filaments (mass per unit area of 80 g m−2 and a yarn count of 110 dtex, provided by Getzner Textil AG, Austria) was used to generate uniformed comparable surfaces.

Calcium chloride dihydrate (CaCl2·2H2O, > 98.0%) was supplied by Merck KGaA, Germany and embittered ethanol (C2H6O, > 98.0%, MEK, 1%) was supplied from Carl ROTH GmbH & Co. KG, Germany. 2-hydroxyethylmethacrylate (C6H10O3, > 97.0%), stabilised with hydroquinone monomethyl ether was supplied by Merck KGaA, Germany and stabilised hydrogenperoxide (H2O2, 30.0%) was supplied from Carl ROTH GmbH & Co. KG, Germany. All chemicals were used without further purification.

2.2 Surface Activation of PA 66 Fibre

2.2.1 CaCl2/EtOH/H2O Treatment

In accordance with the results of Rietzler et al. two different solvent compositions were selected that induce either swelling or dissolution of the fibre (Table 1) [20]. The CaCl2 content remained 12.5 mol%. The swelling and dissolving solvent are characterized by an H2O/EtOH ratio of 3.67 and 0.4, respectively.

Table 1 Selected solvent compositions for the dissolution and swelling process of PA fibres
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Each PA66 monofilament was separately immersed for 1 min, 5 min, and 15 min in a CEW solution at room temperature. After the treatment, the sample was immersed in H2O for 2 h, rinsed with an excess of H2O, and dried at room temperature overnight.

2.2.2 Atmospheric Pressure Plasma Treatment with Air

The plasma treatment was performed continuously with the Piezo Brush® (PZ2), Relyon Plasma GmbH, Germany that is based on a piezoelectric direct discharge technology. The distance between plasma nozzle and substrate was 10 mm. Each fibre was treated with a relative velocity between plasma source and substrate of 300 mm s−1, 150 mm s−1, 50 mm s−1.

The second plasma source is based on pulsed atmospheric arc (PAA®) technology. The fibres were treated with a relative velocity between plasma source and substrate of 150 mm s−1 and a plasma nozzle-substrate distance of 10, 20 and 30 mm.

2.2.3 Grafting Process

Each PA66 monofilament was separately immersed at elevated temperature and atmospheric pressure in the HEMA solution with molar ratios of 5.6 mol% HEMA, 1.1 mol% H2O2, and 93.3 mol% H2O between 1 and 30 min at 63 °C to ensure an operation temperature below the glass transition temperature. Operation temperatures above the glass transition temperature can likely induce tempering and annealing leading to a disorientation of the polymer molecules [21].

2.3 Analysis of PA66 fibres

2.3.1 Scanning Electron Microscopy (SEM)

Scanning electron images were taken using the benchtop Hitachi TM 4000 (Japan) with a voltage of 15 kV. The samples were sputtered with gold prior analysis.

2.3.2 Fibre Properties

Fineness The fibre fineness was measured using the vibroscope 500 from Lenzing Instruments GmbH & Co. KG, Austria with a pre-tension weight of 50 mg. One value represents a mean of 10 single fibre tests.

Tenacity The single fibre tensile tests were performed using vibrodyn 500, Lenzing Instruments GmbH & Co. KG using a pre-tension weight of 50 mg, gauge length of 20 mm, and test speed of 15 mm min−1. One value represents a mean of 10 single fibre tests.

Surface roughness The fibre surface topography before and after activation was investigated with the confocal 3D laser-scanning microscope (Keyence VK-X150, Keyence Corporation, Japan) under a magnification of 100x. The arithmetic average roughness value Ra in µm was determined via the MultiFileAnalyzer with the line roughness feature. For every measuring point, three different PA66 fibres were analysed, and an average roughness value was determined.

Wetted length The wetted length of the fibre was measured using the force tensiometer K100 from KRÜSS GmbH, Germany. The measurement is based on the Wilhelmy plate method with n-hexane as a wetting agent. For one wetted length determination, four fibres were measured simultaneously at room temperature with an immersion speed of 5 mm min−1 and an immersion depth of 2 mm. Each value represents a mean of nine wetted length measurements.

2.3.3 FTIR

The attenuated total reflectance (ATR) of untreated and treated PA66 samples was recorded on the PA66 fibre surface in the spectral range of 4000 to 600 cm−1 using an FTIR microscope Bruker Lumos FTIR Microscope, Bruker Optik GmbH, Germany with a resolution of 2 cm and 64 scans per measurement. The ATR stage was equipped with a Ge crystal. The results are presented as the mean value of three measurements.

2.3.4 X-ray photoelectron spectroscopy (XPS)

XPS analysis was performed with the Thermo MultiLab 2000 spectrometer Thermo Fischer Scientific, USA using the alpha 110 hemispheric analyser Thermo Electron Scientific Instruments LLC, USA. Kinetic energies of the electrons were detected in Fixed Analyzer Transmission (FAT) mode, pass energy of 25 eV or 100 eV and resolution of 2.2 eV. A 4 mm area of the fabrics was analysed.

2.3.5 Wetting Behaviour

The dynamic contact angle was measured using the force tensiometer K100 from KRÜSS GmbH, Germany. The measurement is based on the Wilhelmy plate method with deionized (DI) water as the wetting agent. For DI water, a surface tension of 72 mN m−1 was determined. For one dynamic contact angle measurement, four fibres were measured simultaneously at room temperature under an immersion and receding speed of 5 mm min−1 and an immersion depth of 2 mm. Each value represents a mean of three dynamic contact angle measurements.

3 Results and Discussion

3.1 Change in Fibre Topography and Dimensions Due to Surface Treatment

3.1.1 Optical Change

Figure 2 shows the SEM images before and after surface activation. It can be observed that the pristine PA66 fibre shows a smooth and clean surface while the CEW treatment using a solvent with an H2O/EtOH ratio of 3.67 leads to the formation of pores. Using a solvent with an H2O/EtOH ratio of 3.67, swelling can occur due to a higher diffusion rate of the CEW components into PA66 compared to the disentanglement rate of polyamide chains. The reason for this might be an increased amount of small and mobile water molecules. As a result of the swelling process, an increased amount of pores are present on the fibre surface [22, 23]. The CEW treatment using a solvent with an H2O/EtOH ratio of 0.4 leads to deposition of undefined re-complexed PA on the substrate surface and has a dissolving effect that leads to a decrease in fibre diameter. Since the rate of disentanglement of the polymer chains at the polymer surface is higher than the rate of diffusion of the CEW components, the CEW components cannot diffuse into the polymer fast enough to cause significant swelling [20, 22, 23]. As a result, mainly dissolution of the fibre occurs that resulting in a decreased fibre diameter. Both plasma sources lead to the formation of groovings and voids due to etching processes during the plasma treatment. The grafting process only changes the surface slightly. It appears that a thin new layer has formed that might be attributed to the newly formed pHEMA layer.

Fig. 2
figure 2

SEM pictures of untreated (a), CEW 5 min/3.67 (b), CEW 5 min/0.4 (c), APPA PZ2/10 mm/50 mm s−1 (d), APPA PAA/10 mm/150 mm s−1 (e), grafted 5 min (f) fibres

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These observations already show a change in the dimension of the fibre. While the plasma and the grafted samples show similar diameters as the pristine fibre, the CEW-treated fibre using a solvent with an H2O/EtOH ratio of 0.4 shows a decrease in value. In addition, it can be observed that the topography of the fibre changed due to the treatment. These pictured changes shall now be verified and quantified by analysing the fineness, roughness, and wetted length of the fibre.

3.1.2 Surface Roughness Measurements

Surface roughness profiles using LSM were taken to analyse the effect of the activation on the substrates’ surface topography. In this study, the arithmetic average roughness Ra is determined that describes "the arithmetic mean of the deviations, yi, from the mean height of the surface profile over n measurement positions" (Eq. 1) [24]. It is therefore a suitable measure of the overall height deviation from a mean height across a surface profile. However, it cannot distinguish between peaks and valleys, wavelengths (high slopes, low slopes), or identify the location of the height change. It is therefore possible that surfaces with the same Ra value show completely different topographies [25].

$$ R_{a} = \frac{1}{n} \cdot \mathop \sum \limits_{i = 1}^{n} \left| {y_{i} } \right|. $$
(1)

In a previous work [23], we have already analysed the roughness of the untreated and CEW-treated fibres (H2O/EtOH = 3.67). The CEW treatment induced an increase in value with increasing treatment time. The values are displayed in Table 2. In this comparative study, we want to further explore the roughness change due to other surface activation techniques and conditions.

Table 2 Wetted length of untreated and modified PA66 fibres
Full size table

A significant increase in Ra value for the CEW treatment with the H2O/EtOH ratio of 0.4 is observed for increased treatment times of 5 and 15 min with values of 0.131 and 0.233 µm, respectively. The increased roughness can be assigned to an unevenly reprecipitation of PA after rinsing the substrate with water and inducing a decomplexation. The extent of the surface roughness correlates with treatment time. Moreover, a higher H2O/EtOH ratio leads to an increased roughness. The difference can be assigned to the different behaviour of the fibre in solution. According to Rietzler et al., an H2O/EtOH ratio of 3.67 leads to swelling, while an H2O/EtOH ratio of 0.4 leads to the dissolution of the fibre [20]. It is, therefore, possible that swelling induces an increased roughness while solvation of the fibre increases the roughness to a smaller extent.

Higher plasma intensities show an increase in Ra value of 0.098 ± 0.013 µm for the PZ2 (d = 10 mm, v = 50 mm s−1) and an increase in Ra value of 0.156 ± 0.063 mm for the PAA® source (d = 10 mm, v = 100 mm s−1). The roughness is increased by decreasing the nozzle-substrate distance or the velocity between plasma source and substrate. The increased surface roughness by plasma can be assigned to etching processes since highly energetic species in the plasma can induce fragmentation of the polymer backbone and chain scission [26,27,28]. It is also shown that the PAA® source leads to a higher roughness increase than the PZ2 source for the same treatment distance and velocity that can be assigned to different plasma temperatures. Etching processes of the substrates may be enhanced using the PAA® source with a plasma temperature of approximately 300 °C compared to the PZ2 source with a plasma temperature below 50 °C. In addition, PA can start melting locally during the PAA® treatment, since the polymer shows a melting temperature of approximately 260 °C [29]. After cooling it can solidify more unevenly.

The deposition of the pHEMA layer changes the topography of the PA fibre surface, resulting in increased roughness by a treatment time of longer than 5 min. This can be attributed to an unevenly polymerization of HEMA on the fibre. Especially at higher treatment times, this effect is highly visible, e.g. high standard deviations. For instance, a 10 min-treated fibre shows a Ra value of 0.215 µm with a standard deviation of 0.111 µm.

3.1.3 Change in Fibre Fineness and Wetted Length

Analysis of fibre fineness and wetted length is carried out to determine the change in dimensions and topography of the pristine and modified fibres. Before investigating the change due to surface treatment, a pristine fibre is analysed and diameters are calculated by assuming that the fibre has a homogeneous and cylindrical shape in order to show the good correspondence of the techniques. Fibre fineness measurements are performed via a vibroscope. The diameter d in cm can be determined as follows with m as a mass in g, L as the length of the fibre in cm, ρPA as the density of the fibre in g cm–3. The density of PA 66 is assumed to be 1.14 g cm−3 [29].

$$ {\text{Fineness}} = \frac{m}{L} = \frac{{\rho_{{{\text{PA}}}} \cdot V}}{L} = \frac{{\rho_{{{\text{PA}}}} \cdot \left( \frac{d}{2} \right)^{2} \cdot \pi \cdot L}}{L} $$
(2)
$$ d = \sqrt {\frac{{4 \cdot {\text{Fineness}}}}{{\rho_{{{\text{PA}}}} \cdot \pi }}} $$
(3)

The wetted length l in cm is determined using the Wilhelmy plate method with n-hexane. When using n-hexane as a liquid for contact measurements, a complete wetting due to its low surface tension is assumed which results in a contact angle θ of zero. The wetted length can be then determined with the Wilhelmy equation as followed by FWilhelmy as the Wilhelmy force in N and σ as the surface tension of n-hexane in mN m–1 [30,31,32].

$$ l = \frac{{F_{{{\text{Wilhemy}}}} }}{{\cos \Theta \cdot \sigma_{{\text{n - hexane}}} }} = \frac{{F_{{{\text{Wilhemy}}}} }}{{\sigma_{{\text{n - hexane}}} }}. $$
(4)

The diameter of the fibre from wetted length determinations is calculated as followed:

$$ d = \frac{l}{\pi }. $$
(5)

Diameters of 12.2 µm, 12.3 µm, and 14 µm were calculated for the pristine fibre using a vibroscope, force tensiometer, and SEM imaging, respectively. The diameter determined by SEM imaging is in the same range, but slightly higher than the diameter determined by vibroscope and force tensiometer. In this case, it is assumed that the imaging technique is less accurate because the vibroscope and the tensiometer measure the diameter along a certain fibre length and a larger number of fibres were analysed.

The fibre fineness of a pristine fibre shows a value of 1.33 ± 0.06 dtex (Table 2). The CEW activation with 1 min treatment time does not change the fineness significantly. It is therefore assumed that neither the diameter nor the density have changed significantly. A slight decrease in fineness to a value of 1.16 ± 0.06 dtex and 1.21 ± 0.07 dtex is observed after 5 min treatment using the solvent with an H2O/EtOH ratio of 3.67 and 0.4, respectively. The decrease in fibre fineness is more pronounced at higher treatment times for the solvent with an H2O/EtOH ratio of 0.4, while the solvent with an H2O/EtOH ratio of 3.67 causes a slight increase in fineness at a treatment time of 15 min. The decrease in fibre fineness may be attributed to a decrease in fibre diameter or a decrease in fibre density. Using optical image analysis, Rietzler et al. showed for a PA fibre with similar dimensions, a diameter decrease of approximately 8% and 27% after a 5 and 15 min treatment, respectively (H2O/EtOH ratio of 0.4) [20]. For our samples, a 4.6% and a 12.4% decrease in fibre diameter after a 5 min and 15 min treatment can be calculated by assuming no change in fibre density. The values of our study are smaller but follow the same trends. Differences can be for instance attributed to the different substrate and the different measuring technique (optical image analysis, vibroscope). Rietzler et al. observed for the solvent with an H2O/EtOH ratio of 3.67 an increase in outer and a decrease in inner diameter of the fibre with increasing treatment time. The decrease for the 5 min treated fibre may be therefore attributed to a decrease in fibre density as swelling of the fibre occurs and a sponge structure is formed [20]. However, it cannot be explained why the fineness does not decrease for a 15-min treatment. Considering the high standard deviation, an inhomogeneous surface treatment could be the reason for the observed value.

The fineness of the APPA-treated fibres range between 1.29 ± 0.08 and 1.38 ± 0.04 dtex and thus shows similar values as the pristine fibre. A correlation between the treatment conditions and the fineness cannot be established for this activation technique. It is therefore assumed that the surface treatment has not significantly changed either the diameter or the density of PA 66 by the applied activation conditions.

An increase in fineness of the grafted fibres is observed above a treatment time of 5 min compared to pristine fibres that can be attributed to the additional grafted pHEMA layer on the fibre surface. In contrast to the expected higher yield of polymerisation by increasing treatment time, a decrease in fineness is observed. It is possible that treatment times of 10–30 min cause swelling and dissolution of the fibres in the monomer solution at elevated temperatures, which might affect the measurement results and lead to a mass loss of the fibre. This assumption will be verified in future works. It is, therefore, possible that a 5 min-treated fibre shows a higher value of 1.52 ± 0.11 dtex than a 30 min-treated fibre with a value of 1.42 ± 0.08 dtex. Inhomogeneities of the grafting treatment at especially high treatment times might be also a reason for these observations.

The determined wetted length of the reference sample shows a value of 38.60 ± 5.34 µm. The CEW-treated samples show values between 46.06 ± 5.18 µm and 50.07 ± 3.59 µm slightly higher wetted lengths than the reference sample with no correlation between treatment conditions and wetted length. Although the fibre fineness decreases, especially at higher treatment times, the wetted length of the fibres is slightly higher than the wetted length of the untreated fibres. An example shall demonstrate these dimensions: Assuming no changes in fibre density after 5 min CEW treatment with an H2O/EtOH ratio of 0.4, a wetted length of 36.52 µm can be calculated from vibroscope analysis. The wetted length determined by the force tensiometer shows an approximately 10 µm higher value which is almost the diameter of a single fibre. Therefore, the CEW leads to a strong increase in waviness and roughness confirming Ra value measurements. The differences between wetted length determinations by vibroscope and force tensiometer vary between 8.7 and 13.7 µm for the CEW treatment with no correlation between solvent or treatment time. This may be due to uncertainties in the calculation, such as the change in fibre density, fibre shape or inhomogeneity of the treatment.

The wetted length of the APPA-treated samples is with values between 42.58 ± 3.03 µm and 48.92 ± 4.24 µm slightly higher than the wetted length of the untreated sample, but still lower as the wetted length of CEW-treated samples. As the fibre fineness does not change significantly, the slight increase may be attributed to an increased waviness and roughness of the fibre due to treatment.

The activated fibres after 1–10 min HEMA grafting also show similar values as the plasma-treated samples, most likely due to the addition of pHEMA on the fibre surface. After 30 min treatment, the fibre shows a wetted length of 29.94 ± 10.09 µm that is a strong decrease compared to the reference sample. The decrease could be attributed to dissolution of the fibre in the reaction solution during treatment at higher treatment times.

3.2 Wetting and Dewetting Behaviour

Small contact angles are an indication of good wetting behaviour. The contact angle of unmodified and modified single fibres was determined with a force tensiometer using the measurement principle based on the Wilhelmy method based on the known wetted length of the fibre. For the calculation, the wetted length previously determined with n-hexane is used [32]. The contact angle determined through measurements of the wetted length, and through estimations of fiber diameter from SEM images, differed only slightly.

The untreated PA66 fibres show a contact angle of 73.77 ± 2.33°. After 1 min CEW treatment with an H2O/EtOH ratio of 3.67, the fibre shows a contact angle value of 62.30 ± 1.86° that decreases with increasing treatment time to a value of 56.11 ± 0.72° for 15 min (Fig. 3). The contact angle values for fibres treated with an H2O/EtOH ratio of 0.4 decreases to values between 50.62 ± 2.77° and 58.25 ± 1.97°. For both solvents, the highest decrease in contact angle value can be observed within the first minute of the treatment. Between a treatment time of 1 to 15 min, it appears that an equilibrium is reached. Furthermore, the solvent with an H2O/EtOH ratio of 0.4 induces a higher contact angle decrease than the solvent with an H2O/EtOH ratio of 3.67. As shown in Fig. 3, the contact angle decreases with increasing arithmetic average roughness value for both solvents. Surface features such as pores, voids, and grooves as observed in SEM pictures can increase the capillary effect and therefore the adsorption capability of water molecules.

Fig. 3
figure 3

Advancing contact angle of CEW-modified fibres

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The PZ2 plasma treated samples show smaller contact angles as the reference sample that decreases with increasing treatment intensity (Fig. 4). The PAA® treated samples only show a distinct decrease in contact angle value at a treatment distance of 10 mm and a treatment velocity of 150 mm s−1. They, however, show for this treatment condition, a smaller contact angle than the PZ2 treated samples. The decrease in the contact angle of plasma-treated fibres can be assigned to a change in topography and surface chemistry. As shown in Fig. 4 b, d, the contact angle decreases with increasing surface roughness that induces an enhanced capillary effect and adsorption of water molecules [33]. The PAA® source induces a higher surface roughness value than the PZ2 source and therefore might lead to a lower contact angle value. According to the literature, highly reactive species in the plasma interact with the substrate surface and induce several interface reactions. Air as plasma gas leads to the formation of oxygen- and nitrogen-containing groups [12,13,14]. For verification, XPS measurements were performed for this study. The (O + N)/C ratio increased from 0.265 to 0.422 for the PZ2 and to 0.407 for PAA® plasma source (distance: 10 mm, velocity: 150 mm s−1) (Supporting information Fig. 1 and Table 1). In addition, especially an increase in -C=O/-CONH and -COOH groups was observed (Supporting information Fig. 2 and Table 2). The newly formed polar groups can enhance the intermolecular interactions between the PA surface and the water molecules surface and improve wettability [34, 35].

Fig. 4
figure 4

Advancing contact angle of APPA-modified fibres

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The HEMA grafted PA fibres show similar contact angle values as the untreated fibre for activation times below 5 min (Fig. 5). A slight decrease can be observed for the 10 min grafted PA fibre, while for the 30 min fibre a sharp decrease to a value of 57.13 ± 3.77° is monitored. The decrease in contact angle may be attributed to newly formed functional groups due to the grafting process. FTIR analysis confirmed that new -C=O groups are formed on the surface after the grafting process. Additionally, there is also evidence of -C–OH, and -C–O–C groups at the fibre surface after 30 min treatment (Supporting information Fig. 2). XPS analysis confirmed these observations with an increase in the (O + N)/C ratio to 0.401 for a 30 min grafted fabric that can be assigned to a strong increase in -OH and -COOH groups (Supporting information Fig. 3 and Table 2). However, a correlation between surface roughness and contact angle cannot be observed for the grafted samples.

Fig. 5
figure 5

Advancing contact angle of grafted fibres

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The receding contact angle is smaller than the advancing contact angle due to a contact angle hysteresis. The hysteresis can be assigned to the anchoring of the liquid on defects of the sample and can be in the range of several tens of degrees [36]. Butt et al. assign the hysteresis to four factors of real surfaces: Heterogeneity, roughness, deformation of the substrate by the contact line and adaption [37]. The samples of this work show a significant hysteresis with values between 10.24 to 31.78° with a big scatter. Moreover, a correlation between treatment or treatment intensity cannot be observed [32, 38].

3.3 Fibre Tenacity

Generally, influencing factors of the tenacity of polymer fibres are the composition of the polymer, its molecular weight and processing conditions that influence the degree of crystallisation, the orientation, and the intermolecular bonds. High molecular weights and higher degrees of crystallinity usually increase the fibres tenacity [39, 40]. The crack formation will usually be initiated by defects that lead to a void in the polymer. The void grows and turns into a crack that spreads across the fibre cross-section until break. Defects and inhomogeneities in the structure will therefore decrease and the weakest cross-section will determine the tenacity of the sample [41,42,43]. Unevenness in load-sharing and stress concentration due to inclusions will also enhance crack formation [42].

The fibre tensile strength of an untreated fibre shows a value of 44.03 ± 3.05 cN tex−1 (Fig. 6). For the CEW treatment the fibre tensile strength decreases for both solvents within the first 5 min to a value of 38 cN tex−1. While the value does not change significantly for the solvent with an H2O/EtOH ratio of 0.4 with a longer treatment time, the solvent with an H2O/EtOH ratio of 3.67 induces a further decrease to a value of 22.71 ± 1.59 cN tex−1. The decrease in tensile strength can be attributed to several factors. In a previous study, we observed a slight decrease in melting enthalpy due to the CEW treatment by differential scanning calorimetry that may be assigned to a decrease in crystallinity by the decomplexation process with water [23]. Furthermore, an unevenly dissolution of PA during complexation and an unevenly reprecipitation of polymer during decomplexation leads to inhomogenities such as voids, grooves and pores that can act as defects and lead to an enhanced crack formation. The solvents show similar behaviour except for a treatment time of 15 min. The solvent with an H2O/EtOH ratio of 3.67 leads to a strong decrease in tenacity that may be attributed to an advanced formation of pores and domination of pores along the fibre cross-section.

Fig. 6
figure 6

Fibre tensile strength of reference, CEW modified (a), APPA modified (b, c), and grafted (d) fibres

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The plasma treatment only induces a small decrease in fibre tensile strength. However, the PZ2 source induces with a value of 43.37 ± 1.68 cN tex−1 a smaller decrease in value than the PAA® treatment with a value of 36.8 ± 2.3 cN tex–1 for the same treatment distances and velocities. It is assumed that the decrease can be mainly attributed to inhomogeneities of the fibre cross-section due to etching processes from the plasma source.

The same can be applied to the grafting process. It appears that an increasing treatment time enhances the decrease in value. The cross-section of the grafted PA fibre is more susceptible to defects than the cross-section of a pristine fibre as an interface between PA 66 and pHEMA is formed.

4 Conclusion

The comparative study of different surface activation techniques was carried out on polyamide single fibre using different analytical techniques. It was shown that fibre fineness measurements using the vibroscope are in good accordance with wetted length determinations using the force tensiometer and are therefore comparable for a pristine fibre. CEW treatments tend to decrease the fibre fineness due to swelling and dissolution processes more than plasma treatments due to etching processes. The grafting process increases the fineness, as an additional polymer layer is formed. Despite differences in fibre fineness and roughness values, the wetted length values are similar or higher than the wetted length of the pristine fibre. We assume that an increased waviness due to the treatments leads to this observation.

All treatments lead to an improved wetting behaviour of the fibre either due to an increased surface roughness or the incorporation of new functionalities. The surface roughness increase was mainly responsible for a decreased contact angle for the CEW-treated fibres due to an enhanced capillary effect. However, also for the plasma-treated fibres a correlation between increasing surface roughness and decreasing contact angle was observed. Additionally, XPS measurements confirmed an increase in (O + N)/C ratio due to plasma treatment. For the grafted samples, the improved wettability can be mainly attributed to increased oxygen-containing functional groups on the fibre surface. In contrast to the wettability improvement, the surface treatment induces a decrease in mechanical properties of the fibre that is predominant for the CEW treatment using a solvent with an H2O/EtOH ratio of 3.67, the PAA® source or the grafting process at high treatment intensities. Possible reasons could be the formation of material defects or decreased crystallinity of the polymer, respectively.

In the emerging field of textile-based composites, the effectiveness of these treatments in improving the interface stability of PA66 with other materials will be investigated in future works. These may include polymer hybrids with rubber or epoxy matrices and coatings with polyurethane, silicon, acrylates or conductive polymers. The surface modifications can also help to enhance the interaction of PA66 with dyes and pigments or the interaction in metal-polymer composites to enhance the metallisation ability of the fibre and the interphase stability in the composite. However, it needs to be considered that for some activated single fibres, bulk properties such as the tensile strength were negatively affected. In future works, the processes need to be optimised to minimise the impact on fibre bulk properties. The excellent tensile strength and good elastic recovery of polyamide fibres need to be maintained to provide these properties in composites and to avoid problems in their processing and manufacturing.