Introduction

Fiber optic cables are commonly used in long-distance and high-performance data networks, including telecommunication, military, and medical purposes. Nowadays, optical communications technology becomes dominant medium for data transmission in telecommunication networks, because of high speed of data transmission, lower losses in high distances and high transmission capacity [1]. Typically, optical fibers consist of core and cladding made of silica glass doped with germanium or phosphorus oxides to achieve higher refractive index in core and boron or fluorine oxides to reduce refractive index in cladding. Directly on the cladding surface, a two-layer of acrylate resin is applied to provide mechanical protection of the fiber [1]. The next protection layer is made of thermoplastic materials in the form of loose or tight buffer. The first buffer type is single layer or bilayer, typically filled with hydrophobic gel or water-swellable material to block water penetration into the tube. In tight buffers, the material is directly applied onto a single fiber without a free space between the fiber and the tube [2]. Tensile performance is enhanced by introducing dielectric fiberglass, aramid yarns, dielectric glass-reinforced plastic rod, steel or aluminum armor tape and steel rod [2]. The outer layer of the cable, named jacket, protects fiber optic cable against environmental factors: mechanical damage, UV light, exposure to chemicals and risk of fire spread [2, 3].

Generally, fiber optic cables are classified into two main groups according to their application area: indoor and outdoor [1]. In highly developed countries, indoor cables must meet rigorous fire-safety regulations ensuring high level of human safety during fires in buildings [4, 5]. Flame Retardant, Non-Corrosive (FRNC)/Low Smoke Zero Halogen (LSZH) materials fulfill these criteria. They are highly filled polymer composites with reduced flammability and smoke generation. They are multicomponent, strongly heterogenous materials, consisting of polymer matrix with high amount of flame retardants (FRs) as well as other organic and inorganic additives improving material processability, stability, etc. In accordance with FR additives, the following broad categories could be listed: halogenated compounds, phosphorus-based compounds, intumescent protective systems, mineral fillers, silicon and inorganic oxides, boron compounds, nanocomposite systems and interfacial systems with flame-retardant effects [6,7,8,9]. The extrusion process of such materials is very challenging in optimization, because of many variables influencing both the process itself and the properties of final products [10]. On the other hand, constantly growing telecommunication market requires development of cable manufacturing technology to provide higher production capacity and cost reduction as well. Increase in extrusion speed can cause unpredictable changes in both chemical and physical structure of the processed materials and impact on properties of the cable FRNC sheaths, which could negatively affect the cable quality and limit its applicability.

The most common FRs in fiber optic cable industry consist of the polymer matrix based on linear low-density polyethylene (LLDPE), polypropylene (PP), poly(ethylene-co-vinyl acetate) (EVA), acrylate copolymers, polyolefin elastomers (POE) or in some specific applications ethylene-propylene-diene (EPDM) rubber loaded with high amount (≥ 50 mass%) of inorganic mineral fillers such as aluminum trihydroxide (ATH) and/or magnesium dihydroxide (MDH). Polymer matrix is responsible for processability, flexibility, strength and dimension stability over temperature and time [7, 11,12,13]. FRNC/LSZH materials usually consist of considerable number of other additives added to improve specific properties. One of the most widely used in flame retardant formulations is coupling agent, improving the compatibility between polymer matrix and inorganic filler. Other common additives are heat and UV stabilizers, plasticizers and processing aids [4, 14]. The degradation of polymers EVA/LLDPE is often a complex phenomenon involving many processes such as: depolymerization, main chain-scission, side groups releasing, cross-linking and others. Chemical changes in macromolecule during degradation may affect polymer morphology, cause complex reaction pathways between different additives, and change the form of the fillers and interactions between filler and polymer matrix [6, 15,16,17]. Typical extrusion lines dedicated to fiber optic cable consist of a set of payoffs (unwinding semi-products), a plasticizing system with an extruder head (heating, compressing and conveying material to the extruder head, where molten material is formed into a final shape), cooling trays or baths (length and temperature affect shrinkage of the product), and a take-up device (to collect the final product) [15, 18]. High melt temperatures can reduce viscosity, resulting in possible increase in line output. However, processing at high temperatures can considerably change material structure because of various degradation processes. Referring to cable FRNC/LSZH materials, ATH starts to decompose at approximately 180 °C [7], whereas EVA deacetylation starts at approximately 360 °C [19], resulting in polyenes formation, followed by random chain scission reactions above 400 °C [7]. Material degradation during processing negatively impacts the stability of production processes and the mechanical properties, chemical resistance, or surface quality of the final product [7, 19].

Studies on FRNC materials are relatively rare. Only several papers described degradation and aging processes in commercial FRNC cables. For example, fiber optic cables, where physico-chemical characterization were conducted for samples that were exposed to accelerated and natural aging and the mechanical properties, were affected by overall increase in the crystallinity degree of the EVA copolymer [20]. Another paper described electrical cable coatings containing EVA copolymer, where aging process affects mechanical, flame retardant and thermal properties [21]. Other studies related to the subject of this paper were made on thermal properties of commercial optical cables [22], decomposition and fire retardancy of materials (for example, LDPE/ATH with and without FR additives [17] or EVA/ATH with or without nanoclay [23]), radiation aging of coatings [24]. However, referred above papers provide information about commercially available materials or compounds prepared under laboratory conditions, but do not report the influence of processing conditions in industrial scale on thermal degradation of FRNC materials.

Acceleration of the extrusion process can be done for example by the reduction of the material viscosity with increase of the processing temperature. In a result, lower shear forces can be applied to press the material through the extruder, but the material is liable to thermal degradation. Processing at low temperature protects the material against thermal degradation, but the mechanical degradation may occur. The most often both the extruder parameters (range of accessible temperatures, maximum power consumption, pressure limitations) and material thermomechanical stability determine the range of available production outputs. It is also important to notice that in real industrial environment, the processing parameters can fluctuate in time and differ from the assumed ones due to many reasons. In a result, the local overheating or exceeding the safe shear and strain threshold can occur. Thus, the deep knowledge on changes in materials properties induced by real processing conditions are necessary to optimize the production conditions, as the tests in laboratory scale often do not reflect well the industrial conditions [1, 15].

The aim of this study was to find the processing factors limiting the production speed by verification of thermal stability of selected FRNC/LSZH materials and identification of degradation effects in jacket samples produced under various production conditions. Produced jacket samples were tested in terms of tensile properties (elongation at break and tensile strength), resistance to heat aging and shrinkage according to industry standards. The great added value of this research is to propose a coherent and comprehensive methodology for research on the susceptibility of cable FRNC materials to degradation in processing, including the definition of critical parameters and a way to increase the process output while maintaining satisfactory properties of the final product.

Experimental

Materials

Two commercially available materials (encoded here as: FRNC 1 and FRNC 2) dedicated to fiber optic cables extrusion were examined. Material samples were in the form of granulates with average size 3 mm. Both materials are EVA/LLDPE blends with high filler loading (≥ 50 mass%) ATH and MDH. The full composition of both materials could not be given here because of the trade secret, however, some characteristic technical parameters can be revealed for FRNC 1 and FRNC 2, respectively: density: 1.58 g cm−3 and 1.49 g cm−3, bulk density 1.48 g cm−3 and 1.50 g cm−3, Melt Flow Index 1.95 g/10 min and 4.14 g/10 min (according to ISO 1133, 150 °C, 21.6 kg), LOI (Limited Oxygen Index) 50% and 38%.

Thermogravimetric analysis

Thermogravimetric analysis (TG) was performed with TA Instrument Q500 apparatus in platinum pans. Dynamic analysis was carried out in the range of 30–650 °C under nitrogen atmosphere with the purge flow 70 mL min−1. In dynamic mode, all samples of 6.0–8.0 mg underwent an isotherm at 25 °C for 10 min and next were linearly heated with rate 10 °C min−1. Isothermal analysis was carried out under nitrogen atmosphere with purge flow 70 mL min−1. In case of isothermal measurements, the maximum efficiency of the equipment was used to heat up the sample from room up to target temperature, and time of heating was in each case 7 ± 0.5 min. Measurements were made in following target set points 180 °C, 200 °C, 220 °C or 250 °C.

Rheological measurements

Measurements were performed with TA Instrument Discovery HR20 modular compact rheometer, in oscillatory shear mode with parallel plates (25 mm in diameter with a gap of 1000 µm). Initial measurements were carried out at 200 °C at the following frequencies 1 rad s−1, 10 rad s−1, 100 rad s−1 and 628 rad s−1 corresponding to shear rates 12.5, 125, 1250 and 7850 1 s−1. Then, isothermal measurements were performed at: 200 °C, 220 °C and 250 °C with a constant angular frequency 10 rad s−1 (corresponding to shear rate of 125 1 s−1) and constant strain 1%, during 30 min in time sweep mode under air atmosphere in convectional heating chamber. Time of the measurement was designated based on maximum time of the material in the extruder in typical manufacturing environment. Samples in a form of 100 × 100 × 3 mm plates were prepared initially by compression molding at 180 °C for 10 min—next cut to round disk shape (25 mm diameter) at ambient temperature. Samples were loaded to the rheometer and compressed to a gap size of nearly 1000 µm. For all samples, the conditioning time was approximately 5 min at 160 °C to ensure desired sample arrangement between plates.

Cable samples production

The cable production line consists of set of a payoffs, an extruder and thermoplastic pellet dosing system, a water cooling system and a take-up system [18]. The plasticizing system was made of a barrel (without grooving), a screw (dedicated to FRNC materials with a barrier zone that improves melting efficiency [15]) and a crosshead (dedicated to FRNC materials) with extrusion tools. The screw diameter was 60 mm, the L/D ratio (screw length divided by diameter) was 25 and the compression ratio (ratio of the feed channel depth to the meter channel depth in the extruder screw [15]) was 1.37. A set of extrusion tools (tip and die) was dedicated to the semi-compression extrusion process. The final outer diameter of testing cable was 2.9 mm. Selection of process parameters were based on maximum available speed in defined a temperature profile. Absolute value of maximum line speed has not been disclosed as sensitive technical data. Thus, the maximum observed line speed was assumed as 100%, while other line speeds under investigation were shown as the relative values in relation to the maximum one. However, the main limitation was melt pressure, which could not exceed 600 bar. Table 1 presents all production trials, including successful and unsuccessful ones because of the exceeding the pressure limit before reaching desired line speed.

Table 1 Selected parameters for cables production
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Mechanical measurement

Mechanical properties of cable products are essential for working conditions because of temporary or permanent stresses (e.g., tension, shear, compression) in a wide range of temperatures. Mechanical tests of cable samples were made according to internal procedures based on fiber optic cable industry standards IEC 60811-501 [25] and PN EN ISO 527-2 [26] using a universal testing machine Zwick Roell Z005, equipped with a 2.5 kN load cell and non-contact extensometer.

Cable jacket samples were prepared by gently removing of reinforced elements and optical fibers. Test cable design is “loose”, ensuring free movement of cable components. Tensile tests were performed on the cable jacket samples in their initial state (a fresh sample after production) and after thermal aging. Thermal aging conditions are specified by standard IEC 60811-401 [27]. The following standard aging conditions were used: 100 °C in a circulating air oven for seven days. General requirements for FRNC cable jacket materials are determined by standard EN 50290-2-27 [28]. Elongation at break and tensile strength were reported as the averages of ten independent tests for each sample. Measurements were conducted according to the following settings: pre-load 0.3 MPa, test speed 250 mm min−1, extensometer distance 65 mm. Jacket samples prepared on an industrial extrusion line were characterized and evaluated according to Table 2.

Table 2 Tensile requirements for FRNC materials for optic fiber cable jacket from the standards
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Shrinkage

Shrinkage tests on jacket samples were performed according to the international standard IEC 60794-2-50 [29]. The samples were conditioned in circulating air oven, and reduction of material length was measured with an accuracy of 0.05 mm. The following conditions were used to prepare jacket samples: sample length of 150 mm, four cycles of aging at 70 °C (each cycle consists of 24 h of aging followed by 1 h at ambient temperature). The results were averaged over ten independent tests.

Results and discussion

The studies on the thermal and mechanical stability of various polymers are very common. However, their objects are mainly neat polymers or composites with simple composition [8]. Moreover, the material stability is usually tested in the laboratory scale. Scaling problems are well known in industrial practice, and they relate also to the prediction of the impact of processing conditions on the final properties of products [30]. In this paper, two commercially available FRNC/LSZH polymer materials were used to test an influence of conditions of extrusion process (temperature profile type, temperature range and speed) performed at the industrial scale on their mechanical properties (tensile strength, elongation at break and shrinkage).

Raw material characterization

Thermogravimetric analysis

In the first step, the thermogravimetric analysis of raw materials was done to establish critical temperatures for material processing that cannot be exceeded. TG curves for raw FRNC 1 and FRNC 2 materials are shown in the Fig. 1 and results of thermogravimetric analysis are compared in Table 3. The 3 steps degradation is observed for both tested materials. The first stage occurs in the range of 296–344 °C with maximum rate at 323 °C for FRNC 1 and 293–343°C with maximum rate at 316 °C for FRNC 2, respectively. It may correspond to dehydration of ATH fillers with formation of aluminum oxide [31, 32], starting of MDH fillers dehydration with formation of magnesium oxide (with reported onset of decomposition 300–320 °C) [7], and the EVA deacetylation with formation of unsaturated bonds and probably polyene domains [9]. Referring to literature for EVA/ATH blends, maximum ATH dehydration rate is 342 °C [33]. Water as a decomposition product creates a thermal shield and protects bulk material from a source of heat and oxygen diffusion, which is positive effect in terms of flame retardancy properties [17]. The second stage of decomposition is visible in range of 344–406 °C with maximum rate at 364 °C for FRNC 1 and in range of 343–396 °C with maximum rate at 364 °C for FRNC 2. It can be assigned to further decomposition of MDH and EVA—chain scission and conversion of ethylene entities to low molecular weight aliphatic compounds, and the conjugation of double bonds to aromatic volatile structures [34]. The third stage takes place between 406 and 550 °C for FRNC 1 and 396–550 °C for FRNC 2. The maximum rate of this stage was found at 470 °C for FRNC 1 and at 467 °C for FRNC 2, respectively. This step can be ascribed to the degradation of EVA unsaturated backbone and LLDPE main chains [35]. Subsequently, above 500 °C char residues are formed that contain aluminum and magnesium oxides as well. Above 550 °C degradation of the char layer is continued at much lower rate [33].

Fig. 1
figure 1

TG and DTG curves acquired for FRNC 1 and FRNC 2 materials under inert atmosphere in the range of 80–650 °C: solid lines–TG curves, dashed lines–DTG curves

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Table 3 Decomposition temperatures and mass loss for FRNC 1 and FRNC 2 determined by TG/DTG analysis (dynamic mode)
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As a measure of thermal stability of tested materials, the temperature corresponding to 5% mass loss, Td, was chosen, as it is commonly used procedure in thermogravimetric investigations [33]. For FRNC 1, Td was found at 296 °C, while for FRNC 2 at 293 °C.

Dynamic thermogravimetric measurements have shown that decomposition of both FRNC 1 and FRNC 2 materials started at temperatures below 300 °C, however the mass loss changes were 1.48% and 1.26% up to 250 °C. It is worthy to underline herein, that dynamic measurements could not be considered as relevant indication of material degradation in real extrusion conditions as the results are heating rate dependent. Thus, it was decided to perform isothermal measurements to characterize influence of both temperature and time on decomposition progress. In isothermal mode, mass loss of a sample can be used to quantify the degree of degradation in defined temperature over time. Tests were performed at several temperatures significantly lower than Td (compare to Table 3), and equal or higher than typical processing temperature (recommended by material suppliers).

Exemplary results for isothermal TG measurements are shown in Fig. 2a. The region marked in blue was selected to analyze the kinetics of degradation processes resulting in sample mass reduction. In this region the temperature is stable as temperature profile showed. Moreover, the first derivative of normalized mass lost (m/m0 = f(t), where m0 is an initial mass of tested sample) showed that the process of mass lost was not strongly disturbed by temperature variation in chosen region. Analogously, fitting regions were selected for all tested conditions.

Fig. 2
figure 2

a Exemplary results of isothermal degradation for FRNC 1 at 250 °C. Charts form bottom to top: temperature profile, sample mass loss normalized to initial mass (m0), derivative of the sample mass loss curve. The blue area corresponds to the data used for kinetic data calculations. Temperature marked in red (180 °C) corresponds to the degradation onset point. b Results of the fitting exponential decay function for FRNC 1 at 250 °C. Inset shows the Arrhenius plot for both tested materials

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To estimate the kinetics parameters of observed degradation processes the model of 1st order reaction was chosen as it was the simplest model predicting well the acquired mass loss profiles. However, many authors assume pseudo-zero order kinetics of polymer materials degradation [36, 37], it is necessary to remind that this model assumes that the area of degraded objects is constant [38]. Considering that the first step of degradation of FRNC materials relates to inorganic fillers dehydration such assumption is too far-reaching simplification.

The mass changes in time for first order reaction model are described by Eq. (1):

$$m = m_{{\text{o}}} \text{e}^{{\left( { - \frac{t}{\tau }} \right)}}$$
(1)

where τ is characteristic time (half-time reaction). This equation was used to fit TG results as Fig. 2b shows. The reaction constants k was calculate based on the results of fitting procedures according to Eq. (2):

$$k = \ln 2/\tau$$
(2)

As expected, the highest degradation rates (in a sense of mass lost) were observed at 250 °C for both tested materials. These rates were estimated to be equal approximately 5% h−1 and 3.5% h−1 for FRNC 1 and FRNC 2, respectively, at the beginning of degradation process. Both materials exhibited the highest stability at 180 °C, and at this temperature the degradation rates did not exceed 0.4% h−1. As 180 °C is typical melt processing temperature for both materials, the mass lost at the level of 0.4% was here accepted as the upper limit for material degradation. In time range defined by the production conditions, acceptable degradation value can be achieved in higher temperatures by shortening of time exposition (faster line speed). In terms of 200 °C mass loss of 0.4% was achieved after 12.6 min and 29 min for FRNC 1 and FRNC 2, respectively. Mass loss of 0.4% was achieved in temperature 220 °C after 6.5 min and 12 min and in temperature 250 °C after 3.4 min and 4.6 min for FRNC 1 and FRNC 2, respectively.

Based on the isothermal TG results the Arrhenius plot was constructed (see Fig. 2b). It shows that the FRNC 1 decomposes faster in comparison to FRNC 2. It is clear taking into account smaller activation energy of degradation processes for this material (Ea = 5.35 kJ mol−1 for FRNC 1 and Ea = 18.23 kJ mol−1 for FRNC 2). These values of activation energy are significantly lower in comparison to activation energies of dehydration processes of Al(OH)3 and Mg(OH)2 (37 kJ mol−1 [17] for Al(OH)3 and 85.8 kJ mol−1 [39] for Mg(OH)2, respectively). Observed differences between measured and reported in literature values (measured for pure substances) can result from different form of tested samples, grain size, eventual impurities and interactions with polymer matrix and other components present in tested herein industrial materials. In case of tested commercial materials surface-active additives can significantly change surface energy of FR fillers improving their miscibility with polymer matrix.

To investigate degradation effects under production conditions, FRNC 1 was selected for production of jacket samples as “more challenging” cable material with lower thermal stability. Based on thermogravimetric analysis, it was concluded that maximum processing temperature could be extended from 180 °C (suggested by suppliers in technical data sheets) to higher temperatures in short period of time (e.g., 200 °C for maximum 12 min for FRNC 1 or 220 °C for maximum 12 min for FRNC 2).

Rheological measurements

Rheology is an accurate method for distinguishing structural changes caused by degradation processes in polymers, such us: molecular mass lost, changes in molecular weight distribution, branching and/or crosslinking. Time-sweep rheometry was successfully and widely used to explore the degradation of various polymers [40,41,42], and could be used to evaluate polymer degradation in terms of processing optimization [16, 43].

Initially, rheological measurements in oscillatory mode were carried out at the following frequencies 1 rad s−1, 10 rad s−1, 100 rad s−1 and 628 rad s−1, corresponding to shear rates 12.5, 125, 1250 and 7850 s−1 at 200 °C. Shear rate was calculated from the angular velocity Ω and geometry strain constant Kγ (Kγ = r/z, where r is radius of the plate and z gap between plates) according to Eq. (3):

$$\dot{\gamma } = \Omega K_{{\upgamma }}$$
(3)

It was found that at 200 °C complex viscosity was not significantly changed over time in broad shear rate range for 125, 1250 and 7850 s−1, see Fig. 3. However, at shear rate 12.5 s−1 monotonic growth of complex viscosity was noticed. For further testing, only one shear rate was selected − 125 s−1, because of the estimated maximum shear rate in the plasticizing system of the testing industrial line estimated by Eq. (4) [15]:

$$\gamma_{{{\text{screw}}}} = \frac{\pi DN}{h}$$
(4)

where D is screw diameter, N is screw rotational speed and h describes distance between the barrel wall and shearing surface of the screw flight.

Fig. 3
figure 3

The complex viscosity results as a function of time at 200 °C at different share rate. Complex viscosity in time of a FRNC 1 and b FRNC 2

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Isothermal measurements were performed at following temperatures 200 °C, 220 °C, and 250 °C. Complex viscosity measured at 200 °C varies around initial value for FRNC 1 and increased insignificantly for FRNC 2, see Fig. 4. For FRNC 1 at temperature 220 °C increase of complex viscosity is visible in the first 7 min of the measurement, then its growth slows down. In case of FRNC 2 minor variations at the beginning of measurement are visible, nonetheless complex viscosity remains on initial level. Significant changes of complex viscosity over time are observed at 250 °C for both materials. More intense increase of complex viscosity was observed for FRNC 1 indicating worse thermal resistance of this material in comparison with FRNC 2.

Fig. 4
figure 4

The complex viscosity as a function of time at 200 °C, 220 °C and 250 °C at constant shear rate 125 1 s−1 a FRNC 1 and b FRNC 2

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Generally, negligible changes of complex viscosity over time are visible during isothermal measurements at 200 °C and 220 °C. At 250 °C complex viscosity for both materials significantly increases over time, starting from the beginning of isothermal test for FRNC 1 and accelerating after ~ 14 min or starting after ~ 10 min for FRNC 2. According to the literature [44], degradation of LLDPE is accompanied by an increase of number of vinyl groups, carbon structures with conjugated double carbon–carbon bonds, and carbonyl groups over time. Moreover, chain scission can cause amorphous chains to become more mobile and promote crystallization when the degradation of many chains occurs simultaneously [44]. As already mentioned, thermal decomposition of EVA copolymer proceeds in two steps, initially by deacetylation resulting in acetic acid release, and next by formation of unsaturated bonds in polymer backbone. These EVA and LLDPE fragments with multiple bonds can participate in crosslinking reactions, however the intensity of this process is limited by the vinyl acetate content (typically in commercial materials 18–40% wt) [7].

Summarizing, changes in complex viscosity should be considered as several simultaneous processes. Firstly, crosslinking in LLDPE domains and deacetylation of EVA and secondly continuation of crosslinking of LLDPE and EVA because of occurrence of unsaturated bonds after deacetylation. From the other hand, oxides, generated by endothermic decomposition of ATH or MDH, change the size of filler particles and their morphology (e.g., MDH decomposes from spherical like to plate-like particles, what may explain observed increase in complex viscosity of the melt. Furthermore, thermal decomposition of fillers may lead to an increase of their surface to volume ratio and as a result can impact on kinetics of further degradation processes [45]).

Production of cable samples

Optimization of polymer viscosity in an extruder is one of the crucial challenges. Considering turbulent flow of fluid (melt) through the extruder channels and non-Newtonian behaviors of heterogeneous FRNC compounds (which viscosity is temperature, time, and flow rate dependent), it is difficult to predict the impact of local conditions on the properties of processed material. Moreover, the viscous properties of FRNC materials are affected by many factors such as: type of base polymer, molecular weight distribution, type of flame-retardant fillers, its size, shape and loading, type of plasticizers, stabilizers and other additives [13, 46]. The variety of commercially available thermoplastic compositions causes that the processing conditions should be adjusted individually for each material, extrusion line and product, as the melt viscosity determines the values of extruder melt pressure, screw motor load and rotational speed.

Initial production tests allowed to find maximum process speed based on pressure limitation (max 600 bars) and product dimensions stability (maximum permissible deviation of outer diameter was 0.05 mm). First production trails allowed also to determine speed–pressure dependency. Higher production speed at higher processing temperatures resulted in decrease of outer diameter stability and, in some cases, material inclusions or roughness on the jacket surface appeared, disqualifying a set of parameters from further tests. Raw material testing gave valuable information about material stability in time and was a guideline to extend processing temperature, achieving higher production speed. Based on combined TG and complex viscosity results, it was concluded that 250 °C setpoint generates high risk of intensive degradation in the manufacturing conditions. Such a high temperatures should be generally avoided but if necessary to use to increase production output, time of exposure should be limited to maximum 3–4 min and strictly controlled. On the other hand, laboratory test results showed that processing temperatures may be likely extended even up to 220 °C for limited and controlled time of exposure (up to 6.5 min for FRNC 1 and 12 min for FRNC 2). Jacket samples were produced with different configurations of parameters, see Table 1, that were used for further investigations to verify mechanical properties of the jacket defined in industry standards. Material during production trial was processed in extruder within the time range 4–14 min, depending on the line speed.

Mechanical measurements of cable samples

Elongation at break of a cable jacket is an essential property and gives information about structural integrity, such us flexibility and ability to withstand a wide range of environmental conditions. Reduction of elongation at break and tensile strength of cables is an indicator of degradation of polymers as a result of processing [47]. During processing of FRNC materials macromolecules are oriented by flowing and stretching inside extruder channels and subsequently molten polymer is rapidly frozen in a water-cooling system. Molecular orientation of polymer chains can have influence on mechanical performance of the cables, especially with increasing extrusion line speed and processing temperature [48].

Results of mechanical tests have shown that elongation at break decreases if the line speed increases, Fig. 5. Increasing temperature profile gave satisfactory results only for the lowest line speed (28%). Increasing profile type with higher line speed (60%) passed only one criterion-elongation at break after aging. However, flat temperature profile gave results meeting the standard requirements [28] only for speed 28% and for processing temperature not exceeding 190 °C. A flat temperature profile for the same speed, but for 200 °C, gave significant decrease in elongation at break. Promising result was achieved for hooked profile, where temperature was locally increased up to 210 °C on the barrier zone introduced to reduce material viscosity. Maximum production speed with hooked profile was found at 48% speed. All samples produced with line speed higher than 60% did not meet cable standard requirement [28] for tensile performance independently of the temperature profile.

Fig. 5
figure 5

Elongation at break for production samples made of FRNC 1. Type of temperature profile: I—increasing, F—flat, H—hooked; for details see Table 1

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Temperature profiles with 200 °C set point in the extruder (barrel zone) influence strongly the tensile strength, see Fig. 6. For line speed 28% all temperature profiles met standard requirement except flat profile with maximal used temperature set point 200 °C. Similarly, to the elongation at break results, hooked profile (H165-190 °C) passed the requirement at 48% for both fresh and aged samples. Within samples produced at 60%, increased (I160–200 °C) and flat 190 °C (F190 °C) profiles gave acceptable results, while the flat profile at 200 °C (F200 °C) did not meet standard criteria [28]. The highest tested line speeds—92% and 100% gave unsatisfactory tensile test results and were excluded from further testing.

Fig. 6
figure 6

Tensile strength comparison for production samples made of FRNC 1. Type of temperature profile: I—increasing, F—flat, H—hooked; for details see Table 1

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Shrinkage

Conditions of cable extrusion process, such us: temperature, pressure and shear rate have high impact on final product properties including its dimensions. Post-extrusion shrinkage, commonly known in literature as shrinkback, is one of the most important properties for FRNC materials. Physically, shrinkage describe the shortening of a cable sample during temperature cycling. The phenomenon is non-reversible contraction that occurs after annealing of a sample. Shrinkback can lead to an increase of excess fiber length (EFL) in the cable and in consequence increased signal attenuation [49].

There are known different mechanisms of shrinkback. One of them assumes that polymer chains or their parts are specifically oriented inside an extruder due to the high shear forces. Such orientation can remain after cooling of the cable to room temperature. Another shrinkback mechanism considers different cooling rates between the material surface and bulk. In consequence, different contraction of stresses occurs across the cable [50].

Results of shrinkage tests have shown that the shrinkage increases with increasing melt pressure (Table 4), what is associated primarily but not only with increased line speed. The highest values of shrinkage were achieved for highest line speed (92 and 100%), that is aligned with findings from other studies [49, 51, 52]. Maximum acceptable shrinkage according to the standard [29] for indoor applications is 5%. According to data shown in Fig. 7 all samples passed standard requirements independently of the production conditions.

Table 4 Pressure and shrink comparison for different processing conditions
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Fig. 7
figure 7

Shrinkage for production samples made of FRNC 1. Type of temperature profile: I—increasing, F—flat, H—hooked; for details see Table 1

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The results showed that the primary limiting factors for line speed increase were the melt pressure and jacket tensile performance, but neither the shrinkage nor extruder motor load.

Conclusions

The main goal of the presented herein study was to find the processing factors limited the production speed in case of extrusion of FRNC materials. Two commercially available FRNC materials were characterized in laboratory scale and the more challenging one was selected for industrial scale tests. The cable samples were prepared with different combinations of processing parameters taken under consideration various process limitations.

Comparison of the results of isothermal TG and viscosity measurements showed that the processing temperature of tested materials can be extended from 180 to 200 °C without significant evidence of degradation and keeping a broad margin for local sample overheating.

It has been also shown that, higher line speed deteriorates mechanical properties of the cable samples. Higher temperatures reduced melt pressure, however, the combination of higher output and higher temperature in production conditions caused stronger degradation as evidenced by decrease in tensile strength and elongation at break. Flat temperature profiles are non-useful for production with speed higher than 28%, as the mechanical properties of obtained products do not meet the standard requirements [28].

Maximum line speed (60%) with acceptable mechanical performance of jackets was found for hooked profiles. These temperature profiles are especially interesting as their usage results in lower pressure in comparison to flat profiles.

Finally, it should be pointed out that the results reported herein are specifically collected for FRNC materials dedicated for cable applications. However, they can be easily extended to other industrial sectors processing FRNC materials by extrusion. Also, proposed methodology can be reproduced for any class of materials in order to investigate the effects of degradation in defined processing conditions and search for optimal processing conditions to maximize production output.