1. Introduction

1.2. Polymers in Medical Tubing

When developing medical tubing such as catheters, it is important to select the correct materials to ensure that the catheter performs as required. An appropriate combination of physical and mechanical properties is essential if a product is to be successful in its application [5]. The various types of catheters require different characteristics such as, stiffness/flexibility, kink resistance, softness, smooth surface, elasticity, coefficient of friction etc. Developments into innovative polymers are ongoing, where manufacturers are developing polymer groups with various different properties.

Synthetic polymers have extensive applications as biomaterials in medical implants. They can either be permanent, where their intended duration spans years, or temporary, where they are naturally biodegraded in vivo or removed upon healing [4]. The most common polymers used in catheter applications are Thermoplastic Polyurethane (TPU), Silicone [6], Polyether Block Amide (PEBA) [7] and Polyvinyl Chloride (PVC) [8]. What makes these materials special is that manufacturers have the capabilities to tweak the formulations during synthesis to obtain materials with different properties. When reviewing materials available on the global polymer market, there are a wide variety of TPUs and PEBAs for example with different shore hardness’s and properties such as tensile modulus.

PEBA is one of the most versatile polymeric materials in use in the medical device industries. Poly (ether amide) are a group of Thermoplastic Elastomers (TPEs) that can be processed by injection moulding and profile or film extrusion [9]. Pebax® or poly(ether-b-amide) is a category of thermoplastic elastomeric copolymers prepared through block arrangement of hard polyamide and flexible polyether sections forming by condensation polymerization [10]. What makes Pebax® so suitable for catheter applications is that while it shows excellent processability, it also displays high thermal and chemical stability [3]. The chemical structure of PEBA can be seen in Error! Reference source not found., where the hard segments are typically composed of PA12 and the soft segments were identified by Bardin, et al., (2022) as a PTMO in Pebax® 2533. It is unclear if the same soft segment flexible polyether is used for all Pebax® grades.

Figure 1. PEBA chemical structure.

Figure 1. PEBA chemical structure.

The various PEBA grades from Arkema in the Pebax® range would have similar constituents, with differing compositions to achieve properties to suit particular applications. Pebax® 6333 SA01 MED for example would, in theory, have a greater number of hard block PA 12 segments than Pebax® 2533 SA01 MED to achieve a stiffer, harder material. Error! Reference source not found. shows some of the various grades of Pebax® SA01 MED for comparison purposes. It can be seen that the manufacturer of Pebax®, Arkema S.A., have developed a variety of PEBA thermoplastics suitable for a wide range of applications where either a hard or soft material is required, but also where a manufacturer may wish to focus on other properties such as tensile modulus (Young’s modulus) or impact strength. This has been possible due to the development of these grades by tuning the constituents prior to the condensation polymerisation process.

Table 1. Various Pebax® grades properties [11,12,13,14].

Table 1. Various Pebax® grades properties [11,12,13,14].

Properties	Units	Pebax® 2533 SA01 MED	Pebax® 4033 SA01 MED	Pebax® 6333 SA01 MED	Pebax® 7233 SA01 MED
Shore A Hardness	-	74	89	~100 (estimate)	>100 (estimate)
Shore D Hardness	-	~25 (estimate)	35	58	61
Tensile Modulus	MPa	12	73	307	510
Charpy notched Impact Strength, +23°C	kJ/m2	No Break	No Break	No Break	15

1.4. Inorganic Fillers

When a radiopaque material is used for the single-scan method, it must be homogenous and uniform in appearance and must not create scatter. One such material is barium sulphate, which is commonly used for gastrointestinal (and cardiovascular) imaging [16]. Ingestion of certain forms of barium (e.g. barium carbonate or barium fluoride) in toxic amounts can lead to gastrointestinal (vomiting, diarrhea and abdominal pain), cardiac and skeletomuscular stimulation followed by paralysis [17,18]. The toxicity of barium compounds depends on their solubility. Barium sulphate (BaSO₄), which is often used for medical purposes, remains essentially unabsorbed and is unlikely to cause adverse effects (to the body) [17]. Chemically pure BaSO₄ is nontoxic to humans, due to insolubility, and is frequently used as a contrast agent in the X-ray diagnosis of colorectal and upper gastrointestinal examinations [19]. BaSO₄ is commonly used in medical devices, incorporated homogeneously into a polymer suitable for a particular application. It is typically incorporated at levels of 10-40 wt.% but is known to be incorporated up to 60 wt.% [20].

Bismuth compounds are considered to be poorly to moderately absorbed after inhalation or ingestion, but there are no quantitative data. The highest concentration of absorbed bismuth is typically found in the kidney and liver, and generally excreted through the urine [21]. Similar to BaSO₄, bismuth oxychloride is insoluble in water, reducing the toxicity of the compound. BiOCl is commonly used in medical devices, incorporated homogeneously into a polymer suitable for a particular application. Similar to barium sulphate, bismuth oxychloride is typically incorporated at levels of 10-40 wt.%. High-loading filler fractions up to 60 wt.% are often required to achieve good X-ray visibility, although such amounts of radiopaque filler can strongly influence mechanical properties [22].

Other radiopaque additives such as bismuth subcarbonate (Bi₂OCO₃), bismuth trioxide (Bi₂O₃) and tungsten (W) each yield their own advantages and disadvantages. While Bi₂OCO₃ is the most popular radiopaque filler after BaSO₄ due to its greater radiopacity, its polymer compatibility is limited and is unstable at temperatures in excess of 200°C. Bi₂O₃, similar to Bi₂OCO₃, has excellent radiopacity, however, its yellow appearance may be undesirable, especially when browning occurs at elevated temperatures. Tungsten is different from the other fillers, due to its greater density (19.28g/cm³), it is generally loaded at levels in excess of 80 wt.% [20]. While the occupied volume is similar to those of barium sulphate and bismuth oxychloride, it is an extremely dense compound. It does, however, have excellent radiopaque properties. That being said, the appearance of the compound is dark for those who desire more colourful devices, may not be an option. Some advantages and disadvantages of both BaSO₄ and BiOCl can be seen in Table 2.

Table 2. Advantages and disadvantages of Barium Sulphate and Bismuth Oxychloride [23].

Table 2. Advantages and disadvantages of Barium Sulphate and Bismuth Oxychloride [23].

Filler	Advantages	Disadvantages
Barium Sulphate (BaSO4)	Widely used in industryyufenRelatively inexpensiveyufenVery process stableyufenEasy to colour	High loading levelsyufenPoor tinting strength
Bismuth Oxychloride (BiOCl)	Excellent white colouryufenHighly compatible with wide range of polymersyufenSmooth surface finish	Difficult to colouryufenSusceptible to UV degradation

2. Materials and Methods

2.1. Materials

Polyether Block Amide (Pebax® 6333 SA01 MED), Barium Sulphate (Blanc Fixe XR HN) and Bismuth Oxychloride (Bismuth Oxychloride BPC) were obtained from National Chemicals Company (NCC). The properties of the Pebax® 6333 SA01 MED can be seen in Table 3. The BaSO₄ used had a density of 4.5 g/cm³ and an average particle size of approximately 1µm. The BiOCl used had a density of 7.7 g/cm³ and an average particle size of approximately 1.5µm. Both fillers are white in appearance.

Table 3. Pebax® 6333 SA01 MED material properties (Arkema S.A., 2024). Cond (conditioned) for at least 16 h at 23 ±2°C and 50 ±10 %RH (relative humidity).

Table 3. Pebax® 6333 SA01 MED material properties (Arkema S.A., 2024). Cond (conditioned) for at least 16 h at 23 ±2°C and 50 ±10 %RH (relative humidity).

Property	Dry/Cond	Unit	Test Standard
Tensile Modulus	307/240	MPa	ISO 527-1/-2
Yield Stress	19/18	MPa	ISO 527-1/-2
Yield Strain	22/22	%	ISO 527-1/-2
Nominal Strain at Break	?50/>50	%	ISO 527-1/-2
Shore D Hardness, after 15s	58/*	⁻	ISO 868
Charpy Notched Impact Strength, +23°C	⁻/No Break	kJ/m2	ISO 179/1eA
Density	1.01/⁻	g/cm3	ISO 1183

2.4. Injection Moulding

Injection moulding of impact and tensile test specimen was carried out on an Arburg Allrounder 370E Golden Electric injection moulding machine (ARBURG GmbH + Co KG, Germany) located in Ross Polymer Services Ltd, Athlone. The Arburg has a maximum clamping force of 600 kN, a screw diameter of 30mm and a theoretical stroke volume of up to 85 cm³. The machine has four thermocouples along the barrel and one on the nozzle controlling the input temperature in the process. The recommended processing temperatures from Arkema were used in the processing of the composites, 230 °C in zone 1 to 260 °C at the nozzle. A mould temperature of 40 °C was maintained by means of an a Wittmann Tempro basic (Wittmann Technology GmbH, Vienna, Austria) temperature controller. The mould temperature was confirmed using an ATP temperature probe. The mould utilised for in the production of tensile and impact test specimen was a ‘two by two’ mould, with cooling performed for 15 seconds (s). The geometry of the moulded samples can be seen in Figure 10.

Figure 2. Tensile and Impact test specimen, (A) 30 wt.% BaSO₄ & (B) 30 wt.% BiOCl.

Figure 2. Tensile and Impact test specimen, (A) 30 wt.% BaSO₄ & (B) 30 wt.% BiOCl.

3. Results

3.1. Processing Observations

3.1.1. Twin-Screw Extrusion

Observations were noted throughout the extrusion trials. These included the screw speed, machine torque, melt pressure at the die and polymer melt temperature. The data recorded from the compounding trials were graphically represented and can be seen in Figure 3. All graphs show the average value obtained throughout each filler level of both BaSO₄ (orange) and BiOCl (green). While the BaSO₄ remained most consistent throughout compounding, there were no issues in processing. During the compounding of both PEBA/BaSO₄ and PEBA/BiOCl, it can be seen in Figure 3B that there was an increase in torque as the filler level increased. PEBA/BaSO₄ showed a slight increase in torque 69% to 70% torque, while the PEBA/BiOCl display a greater increase, 73% to 77% to 75% torque. It can be seen in Figure 3A that the screw speed was reduced as the filler level increased for each respective filler. It was necessary to reduce the screw speed in an attempt to maintain a consistent torque level. This increase in torque may be due to the addition of polymer and dry filler in the first zone of the extruder. This may put additional pressure on the screws leading to an increase in torque required by the motor to rotate screws. The melted blend may experience; however, it is expected that the viscosity of the composite may reduce due to the shear occurring further in the process. It can be seen in Figure 3C and D that the melt pressure and temperatures were both consistent. Figure 3D show that the melt temperatures in the various PEBA/BaSO₄ composites were lower melt temperature than those of PEBA/BiOCl. This may suggest that the BiOCl has a higher thermal conductivity value than BaSO₄, where the PEBA/BiOCl composites absorb more heat from the extrusion process. At the highest filler loading level (30 wt.%), the melt temperatures of the two formulations were essentially superimposable. It was posited that this was due to the increased volume of BaSO₄ in the composition compared to BiOCl. Though both materials were loaded at 30 wt.%, the density of the BaSO₄ is 4.5 g/cm³, whereas the BiOCl has a density of 7.8 g/cm³, therefore BaSO₄ occupies 42 % more volume in the melt.

Figure 4 shows the appearance of pellets pre- and post-extrusion, where (A) is the Pebax® SA01 MED, (B), (C) and (D) are PEBA/BaSO₄-10, -20 and -30 respectively, while (E), (F) and (G) are PEBA/BiOCl-10, -20 and -30 respectively. The ‘whiteness’ of the composites is obvious from the image. While it is clear that both PEBA/BaSO₄ and PEBA/BiOCl obtain a deeper white colour as the filler level increases, the PEBA/BiOCl has a more pronounced white colour. It is evident that at the lower loading levels, the PEBA/BaSO₄ composites are much more translucent than the lower loadings of PEBA/BiOCl. Both the PEBA/BaSO₄-30 and PEBA/BiOCl-30 displayed a brilliant white colour, however, the PEBA/BiOCl-30 displays a somewhat yellow hue.

3.1.2. Injection Moulding

Similar to the extrusion trials, there were a number of observations made during the injection moulding trials. Two parameters of interest during injection moulding were melt cushion and plasticising time. Figure 16 depicts the average values obtained from the injection moulding trials, with Figure 16A displaying the average melt cushion and Figure 16B the average plasticising time. It can be seen that the trends are similar between the PEBA/BaSO₄ and PEBA/BiOCl, where there is a clear increase in the melt cushion as the filler addition level increases for both PEBA/BaSO₄ and PEBA/BiOCl. In relation to the plasticising time for the various filler levels of PEBA/BaSO₄ and PEBA/BiOCl composites, it was clear that as the loading increased, there was a reduction in plasticising time. This may be as a result of the reduction in viscosity, which is a major variable in the plasticising time in injection moulding [25]. An observation was made, whereby, as the filler content increase, so too did the melt flow index. This may have been due to the shear in the extrusion process, but the viscosity was reduced. In relation to the melt cushion, as materials with higher densities have less compressibility, a smaller volume of melt cushion would be required to maintain consistent pressure in the mould to obtain dimensionally stable components. In contrast to this, with shear in the extrusion processes prior to injection moulding, the molecular weight of the various composites is expected to have reduced, which leads to a lower viscosity, therefore requiring a larger melt cushion to provide more control during the packing phase. It was observed that initially, at the lower loading levels of inorganic filler for both PEBA/BaSO₄ and PEBA/BiOCl composites, the melt cushion had dropped, but increased again as the filler loading within the composites increased. It could be said that due to the increase in the melt density as the filler content increases, the melt cushion increases to compensate for the loss in volume of the composite at higher filler loading. The standard deviation of each of the process parameters explains a lot about the behaviour of the material. Consistency, or low standard deviation, shows that the material is consistent, with little variation throughout. The consistency of the melt cushion and plasticising times for each of the composites was crucial in evaluating the dispersion of the filler throughout the respective polymer matrix. Without consistency in the process, this would signify a lack of consistency in the composites themselves.

Figure 5. Injection moulding observations, (A) average melt cushion, (B) average plasticising time; Pebax® 6333 SA01 MED (blue), BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

Figure 5. Injection moulding observations, (A) average melt cushion, (B) average plasticising time; Pebax® 6333 SA01 MED (blue), BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

3.2. Mechanical Properties

3.2.1. Tensile Properties

There are multiple factors that may influence the tensile properties of a polymer or polymer composite. Bayazian & Schoeppner found that when processing a polymer using extrusion technology, the shear and other stresses that occur in the process causes chain scission to occur, reducing the molecular weight of the polymer [26]. This reduction in molecular weight can cause the mechanical properties of the polymer to be reduced. Another factor that plays a major role in the mechanical properties of polymer composites is a phenomenon known as nucleation. With the incorporation of micro (>1 µm) or nano (1-100 nm) particles into a polymer matrix, it is not uncommon to see an increase in the degree of crystallinity of the polymer due to a nucleating affect brought on by the filler. This can lead to an enhancement of the tensile properties of the polymer due to improved organisation of lamellar crystals. Composite strength and toughness are very much dependent on the adhesion quality, the particle size and the loading levels in the polymer [27]. Figure 6 displays the tensile properties of the various PEBA/BaSO₄ and PEBA/BiOCl composites in comparison to the Pebax® 6333 SA01 MED (PEBA100).

It is clear that the PEBA/BiOCl composites outperform the PEBA/BaSO₄ composites in all tensile properties at the respective filler loading. It is, however, noted that the PEBA100 performs exceptionally well, almost outperforming the composites in all tensile properties. Figure 6A displays the tensile strengths of the PEBA100 and the various PEBA/BaSO₄ and PEBA/BiOCl composites. A 7% reduction was observed in the tensile strength of PEBA/BaSO₄-10 (36.95 MPa) when compared to PEBA100 (39.78 MPa), however, there was only a 0.3 % reduction in tensile strength of the PEBA/BiOCl-10 (39.67 MPa) when compared to the PEBA100. These reductions in tensile strength were also observed by Mahdi & Dean when analysing the tensile behaviours of PP/cotton fibre (CF) composites [28]. It was found that as the filler loading increased from 0 wt.% to 50 wt.% in increments of 10 wt.%, the tensile strength of the composite displayed a reduction in each step. This reduction in tensile strength from PEBA100 to filled PEBA composites may be as a result of the reduction in molecular weight post-processing, but also due to the decreased crystallinity in the composites and poor interfacial adhesion between filler and polymer. As for the lesser reduction of strength in the PEBA/BiOCl composite in contrast to the PEBA/BaSO₄ filled, this may be as a result of the volume % of filler in the polymer matrix. As the BiOCl is almost twice as dense as the BaSO₄, the volume of filler in the polymer is less in the PEBA/BiOCl composites, potentially leading to lesser reduction of strength. When comparing the lowest filler loading for each of the inorganic fillers, it is apparent that the PEBA/BaSO₄ suffers more so with greater loading than PEBA/BiOCl. An 11% reduction of tensile strength was observed in PEBA/BaSO₄-30 (32.88 MPa) when compared to PEBA/BaSO₄-10. In stark contrast, PEBA/BiOCl-30 only experienced a 2% reduction in tensile strength when compared to PEBA/BiOCl-10. This could again be associated with the volume of filler in the composite, the potentially excessive loading of BaSO₄ yields a greater reduction in strength of the composite. Upon reviewing the stress at break of the PEBA100 and various fillers in Figure 6B, it can be seen that the trend is similar to that of the tensile strength. While there was an observed reduction in stress at break of 5.5% for PEBA/BaSO₄-10 (35.74 MPa) when compared to PEBA100 (37.85 MPa), there was a greater reduction in the stress at break value PEBA/BaSO₄10 to PEBA/BaSO₄-30 (31.41 MPa). The PEBA/BaSO₄-30 seen a reduction of 12% when compared to PEBA/BaSO₄-10, and even an 11.7% reduction when compared PEBA/BaSO₄- 20 (35.61 MPa). Similar to the results obtained in the tensile strength of the PEBA/BiOCl composites, an overall improvement in stress at break values was exhibited by the PEBA/BiOCl composites in comparison to the PEBA100, with a 2% improvement of PEBA/BiOCl-10 (38.69 MPa) vs. PEBA100 (37.85 MPa). From there, the PEBA/BiOCl-30 (39.62 MPa) showed an increase over the PEBA/BiOCl-10 of 2.4%. It could be said that with the PEBA/BiOCl, due to the lower volumetric loading of inorganic filler, the polymer elongation or deformation is not impeded as it may be with the higher volume loading of BaSO₄.

Figure 6C depicts the results obtained for the strain at break in each of the samples. As the % strain at break is a representation of the maximum change in length from the original length prior to a sharp drop in stress, it is essentially a metric for the ability of the material to deform until it breaks. The PEBA100 (330.35 MPa) outperformed both the PEBA/BaSO₄ and PEBA/BiOCl at all addition rates, with a 4 % reduction in strain at break from PEBA100 to PEBA/BaSO₄-10 (316.82 MPa), and a 3.7 % reduction when compared to PEBA/BiOCl-10 (318.03 MPa). With the PEBA/BaSO₄ composites, there was a 10 % reduction from PEBA/BaSO₄-10 to PEBA/BaSO₄-30 (279.68 MPa), while in the case of PEBA/BiOCl, there was only a 1 % reduction from PEBA/BiOCl-10 to PEBA/BiOCl-30 (314.79 MPa). When comparing PEBA/BaSO₄-30 with the PEBA/BiOCl-30, the PEBA/BiOCl outperforms the PEBA/BaSO₄ by 10 %. When reviewing the reduction in strain at break for PEBA100 to the higher filler loaded composites, it is apparent that the PEBA100 behaves much better in relation to deformation that the PEBA/BaSO₄ than that of the PEBA/BiOCl. The PEBA/BaSO₄-30 sample experienced a significant reduction in strain at break, with a decrease of 15 % when compared to PEBA100. In contrast, the PEBA/BiOCl-30 composite only exhibited a 4.7 % reduction when compared to PEBA100. This may be as a result of the excessive volume of BaSO₄ in the polymer leading to an increase in the brittleness of the composite. The Young’s modulus of the samples is a representation of the ease of deformation or ability of a material to bend. It is a measurement of a materials stiffness, where a high Young’s modulus indicates a stiff material.

Figure 6D displays the results from the Young’s modulus of the samples. It can be seen that PEBA100 and the composites with the lowest loading level of inorganic filler offer the lowest Young’s modulus. This shows that with no filler, or low levels of filler, the polymer has the ability to deform, showing low stiffness when compared to the higher loading of fillers. While the difference between the 10 wt.% filled composites is miniscule in comparison to PEBA100, the PEBA/BaSO₄-30 composite exhibits a 25 % increase in Young’s modulus, while the PEBA/BiOCl-30 exhibits a 30 % increase. A similar trend was observed by Mahdi & Dean upon analysis of the Young’s modulus of PP/CF composites. It was observed that as the filler content (wt.%) increased, so too did the Young’s modulus [28]. The difference between PEBA/BaSO₄-30 and PEBA/BiOCl-30 respective composites is 6 %, where the PEBA/BaSO₄ exhibits a greater Young’s modulus. This depicts the PEBA/BaSO₄ as a stiffer composite due to its reduced ability to deform as exerted stress increases on the sample. The cause of this increase in stiffness of the PEBA/BaSO₄ composites with respect to the PEBA/BiOCl may be due to the excessive volume of filler. Salmah, et al., had similar findings when analysing the Young’s modulus of palm kernel shell filled LDPE. It was found that as the filler level increased, so too did the Young’s modulus [29]. It could be seen that the addition of both inorganic fillers at the various loading levels leads to changes in the polymer’s tensile behaviour. While there was no significant change in the tensile properties between PEBA100 and the PEBA/BaSO₄-10 and PEBA/BiOCl-10 composites, there were changes in the composites as the filler level increased. It was clear that as the loading levels of PEBA/BaSO₄ increased, there were greater reductions in tensile strength, stress at break and strain at break than those observed within the PEBA/BiOCl composites. The PEBA/BiOCl composites appear to have performed better than the PEBA/BaSO₄ at the various loading levels, with less variation in properties between filler contents. While the PEBA/BiOCl experience minimal increase in tensile strength from 10 wt.% to 30 wt.% filler content, 2.4 %, the PEBA/BaSO₄ composites experienced a large shift of 11 %. The PEBA/BaSO₄ composites seen a large shift in Young’s modulus with the increase from 10 wt.% to 30 wt.%, an increase of 32 %, while the PEBA/BiOCl seen a 25 % increase in Young’s modulus with a filler content increased from 10 wt.% to 30 wt.%. This shows that with the increase in filler content of both PEBA/BaSO₄ and PEBA/BiOCl, the respective composites experience a significant increase in stiffness. As mentioned by Shah, as the barium content moves beyond about 20 % by volume, composites begin to show losses of the base polymer's tensile strength and other mechanical properties [30]. This is the equivalent to > 40 wt.% BaSO₄ in the composite, which experiences a significant reduction in properties compared to the other composites.

3.2.2. Flexural Properties

The flexural behaviour is similar to the tensile behaviour of a material, but rather than the resistance to stretching as is the case with tensile, the flexural behaviour of a material is a measure of its resistance to bending deformation. This measurement of stiffness provides in depth knowledge about a material and how it behaves when a simple beam load is exerted upon the material and can be comparable to the stiffness observed in the Young’s modulus. Figure 7 displays the various flexural properties of PEBA100 and the PEBA/BaSO₄ and PEBA/BiOCl composites.

It can be seen from the graphs that the composites perform very well in comparison to the polymer, and in the case of flexural strength and modulus, there is a noticeable trend whereby the higher the loading of inorganic filler, the greater the measured values. There were, however, virtually no changes in the flexural strain when the composites are compared to PEBA100. One noteworthy result from the flexural testing was that there was no measurable data obtained for the PEBA/BiOCl-30 in relation to flexural strength and flexural strain. In Figure 7A, the values for flexural strength of PEBA100 and composites can be seen. It is clear in the graph that the composites outperform PEBA100 at all loadings of filler. This is a contrast to the tensile strength of the composites, which explains that the addition of filler provides stiffness to the composites, reducing the flexing that occurs in the samples. There was an observed 7.3 % increase in the flexural strength PEBA/BaSO₄ -10 (11.93 MPa) when compared to PEBA100 (11.06 MPa), and a 7 % increase in flexural strength of the PEBA/BiOCl (11.89 MPa) when compared to the PEBA100 (11.06 MPa). This clearly demonstrates the enhancement of the stiffness of the polymer composites with the incorporation of inorganic filler, even at the lowest addition level. There was an 11.4 % increase in the flexural strength from PEBA/BaSO₄-10 (11.93 MPa) to PEBA/BaSO₄-30 (13.47 MPa), demonstrating greater stiffness with the added volume of filler. A similar trend was observed from PEBA/BiOCl-10 (11.89 MPa) to PEBA/BiOCl-30 (12.85 MPa), where no measured data was obtained for the PEBA/BiOCl-30 for comparison. There was minimal difference between the PEBA/BaSO₄ and PEBA/BiOCl at the respective filler loading, with a difference of 0.3 % in favour of the PEBA/BaSO₄.

Figure 7B shows the flexural strain values obtained from the flexural test for PEBA100 and composites. This metric evaluates how much a material can bend, changing its length from the original before failure. The graph clearly shows that there has been virtually no change in the flexural strain across all tested samples. Across all samples tested with measured data, a minimum of 0.05 % to a maximum of 1.9 % difference was observed between all samples, PEBA100 and composites (13.028 – 13.281 % strain). It is clear from this that the polymer, irrespective of filler content, behaves the same in relation to its change in the length due to the applied load. This shows that, although as previously indicated, the material becomes stiffer with the incorporation of filler into the polymer matrix, the samples still deform in a similar manner as without filler, showing that the polymer retains its inherent flexibility. The results for the highest loading of PEBA/BiOCL-30 were inconclusive as there was no measured data for this composite.

The flexural moduli of the samples can be seen in Figure 7C. The flexural modulus of a material explains how much stress is required to bend or deform the material. More stress required, means that the material is stiffer. The results obtained from the flexural testing on the flexural strength confirm the Young’s moduli of the various samples obtained during tensile testing. It can be seen that as the filler content increases in the composites, the materials become much stiffer. When comparing PEBA100 (126.05 MPa) with the PEBA/BaSO₄-10 (141.5 MPa), there was a 10.9 % increase in the flexural modulus with the incorporation of BaSO₄. There was less of a difference when compared to the PEBA/BiOCl-10 (140.01 MPa), where the observed difference in flexural modulus was 9.97 %. However, in contrast, at the higher loadings of filler in the composites, the PEBA/BiOCl-20 (158.36 MPa) displayed a 0.98 % increase in flexural modulus vs. PEBA/BaSO₄-20 (156.82 MPa). Similarly, the PEBA/BiOCl30 (181.35 MPa) displayed a 2.87 % increase in flexural modulus vs. PEBA/BaSO₄-30 (176.13 MPa). While the differences in flexural moduli are miniscule when comparing the composites to the polymer, it is seen that the PEBA/BiOCl composite stiffer at the higher loading than the PEBA/BaSO₄. It is clear that with the addition of inorganic filler to the polymer matrix, the composites become much stiffer. This is exemplified by the tensile strength and Young’s modulus, but also by the values of the flexural strength and modulus. As seen with the flexural strength, as the filler level increased, so too did the flexural strength. Bociong, et al. made similar observations, whereby at 40 wt.% silanized silica filled dental resin, the flexural strength increased. However, as the filler loading increased further, there was a noticeable reduction in flexural strength [31]. This can be associated with the higher volume of filler in the matrix creating a lack of interfacial adhesion between filler and polymer, causing weak areas around the filler. Zabihzadeh et al. evaluated the flexural behaviour of LLDPE/rapeseed composites at loadings levels of 30 wt.%, 45 wt.% and 60 wt.%. At these higher loadings, it was clear that there was a reduction of flexural strength as the filler content increased, showing that as the volume of filler increases in the composite, the weak interfacial adhesion between filler and polymer matrix leads to stress concentration around the filler particles [32]. With the flexural modulus, it was clear that as the filler level increased, so too did the modulus, showing enhanced stiffness in the composite. A similar trend was seen by Omidvar, et al. whilst evaluating the flexural behaviour of LLDPE/rapeseed composites at various filler loadings. It was found that as the filler loading increased by weight, so too did the flexural modulus [32].

3.2.3. Impact Strength

It was important to analyse the effects of incorporating inorganic filler at the various loadings on the impact strength or rigidity of the composites. In order to evaluate this, each sample was subjected to Charpy notched impact testing. The impact values obtained for the PEBA100, and the various composites can be seen in Figure 19. It is clear from the graph that while there is a small increase in the impact strength of the filled PEBA grades (4.7 – 5.33 % increase) in comparison to the PEBA100, there is an observed reduction in impact strength between PEBA100 and PEBA/BaSO₄-30. While the incorporation of inorganic filler into the PEBA increases the impact strength, it must be noted that there is a point whereby the addition of filler causes the polymer to become too brittle, leading to catastrophic failure. This was the case for PEBA/BaSO₄-30 (33.88 kJ/m²), where the samples broke, requiring much less energy to achieve the break than the energy absorbed by the PEBA/BaSO₄-10, PEBA/BaSO₄-20 and all PEBA/BiOCl composites. The breakage of PEBA/BaSO₄-30 demonstrates the volumetric overloading of inorganic filler, leading to a brittle composite. There was minimal difference between the PEBA/BiOCl composites of various filler loadings, showing that there was no clear reduction in impact strength as the loading increase, contrary to the results obtained for the PEBA/BaSO₄-30. A case could be made for the PEBA/BiOCl composites, where due to the higher density of the BiOCl compared. BaSO₄, the volume of BiOCl in the PEBA/BiOCl-30 composite does not cross into the brittle point as the PEBA/BaSO₄-30 does.

Figure 8. Impact properties of all tested samples (n=6); Pebax® 6333 SA01 MED (blue), BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

Figure 8. Impact properties of all tested samples (n=6); Pebax® 6333 SA01 MED (blue), BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

An important factor in the toughness of a polymer composite is the not only the addition level of filler, but also the particle size and particle-matrix interaction. Sreekanth, et al. conducted research on the effects of particle size and filler level on the mechanical properties of TPE composites. It was established that larger particle sizes led to a reduction in the toughness of the composites. Similarly, as the filler content increase, there was a clear reduction in toughness of the composites. It was summarised that the greater the addition of filler, the greater the reduction in elasticity and ability of the composite to absorb energy, reducing the deformability of the polymer matrix [33]. In a similar study, Lakkundi, et al. confirmed the findings of Sreekanth, et al., whereby an increased loading of filler in the polymer caused a weaker interfacial interaction between polymer and filler due to the high loading of filler, leading to a less ductile composite [34].

3.3. Thermal Analysis

3.3.1. Melt Flow Analysis

The rheological properties of the polymer and polymer composites are crucial information for processing methods such as injection moulding, and tube extrusion. Figure 9 displays the melt flow index of PEBA100 and the various PEBA/BaSO₄ and PEBA/BiOCl composites. It is clear from the graph that irrespective of filler content, the MFI has increased for all composites. This shows that the shear as a result of the twin screw compounding has a more pronounced effect on the flowability of the composites in comparison to the filler content. The increase in melt flow of the PEBA/BaSO₄ composites with respect to PEBA100 were 11.9 %, 17.5 % and 21.8 % with the respective increases in filler loading. Similarly, the PEBA/BiOCl composites experienced increases of 13.5 %, 16 % and 25 % with the respective increases in filler loading. There was minimal difference in melt flow between the PEBA/BaSO₄ and PEBA/BiOCl composites, showing that the % volume of filler in the composites had minimal impact on the melt flow.

While it may be expected that the composites viscosity would increase with the incorporation of large volumes of filler, this is clearly not always the case. There are other factors that influence the rheological characteristics of the composite. While the addition of fillers into the polymer matrix may enhance certain properties of the polymer, the rheological properties are not as straight forward. The additional processing step, and therefore thermal history, adds additional shear into the polymer, reducing the molecular weight which leads to a reduction in viscosity, or increase in melt flow. This shear, along with adequate mixing, can lead to a very well dispersed filler within the composite, which leads to a reduction in viscosity. If the mixing was inadequate, agglomerates within the polymer matrix would lead to an increase in viscosity. A case could be made for excessive shear in the polymer composites, which may have caused an excessive breakdown of molecular weight of the composites. Hnatkova, et al. analysed the influence of molecular weight on the viscosity of polyethylene glycol (PEG). It was established that as the molecular weight of PEG decreased, there was a reduction in viscosity, which mirrors the results obtained in this study [35].

3.3.2. Differential Scanning Calorimetry

To evaluate the thermal properties, DSC analysis was carried out on the polymer, two inorganic fillers and various composites. Due to the semi-crystalline nature of PEBA, it is of interest to understand how the addition of inorganic fillers affects the crystallinity and other thermal properties. Figure 10 and Figure 11 display the overlaid thermograms for the BaSO₄ and BiOCl composites respectively. To provide a comprehensive overview of the thermal characteristics, PEBA100, the various composites of BaSO₄ and BiOCl, and the inorganic fillers were all analysed via DSC. Upon initial review of the graphs, it could be seen that while there is minimal movement in the temperature at which endo and exothermic peaks occur in PEBA/BaSO₄ composites, there were greater shifts in the temperature at which the exothermic peaks occurred for the PEBA/BiOCl composites. These shifts in temperature for exothermic peaks are caused by crystallisation of the polymer, and as these occur during cooling with crystallisation occurring at a greater temperature. This signifies a reduction in the degree of crystallinity of the polymer., suggesting that the PEBA/BiOCl composites have a lower degree of crystallinity than both the polymer and PEBA/BaSO₄ composites. Twin crystallisation peaks were observed during cooling of the PEBA/BiOCl composites, however, not in the thermograms of PEBA/BaSO₄. One aspect to consider when understanding the effects of filler on the degree of crystallinity is the nucleating that occurs in the polymer due to the incorporation of fillers. Pingping & Dezhu conducted similar trials on the effects of various addition rates of calcium carbonate (CaCO₃) on the cold crystallisation peaks of poly (ethylene terephthalate) (PET). It was found that with an increase in the addition of CaCO₃, there was a clear reduction in the degree of crystallinity of the composite. It was summarised that the amorphous regions between spherulites decrease considerably with the addition of CaCO₃. These results conform well with the mechanism of the effect of nucleation of CaCO₃ on the PET crystallisation process [36]. It could be said that the BaSO₄ filler induces a greater degree of crystallisation in comparison to the BiOCl due to an enhanced level of nucleation.

Figure 10. Overlay thermogram of Pebax® 6333 SA01 MED, BaSO₄ and PEBA/BaSO₄ composites; PEBA-100 (green), PEBA/BaSO₄-10 (blue), PEBA/BaSO₄-20 (mauve), PEBA/BaSO₄-30 (magenta) & BaSO₄-100 (teal).

Figure 10. Overlay thermogram of Pebax® 6333 SA01 MED, BaSO₄ and PEBA/BaSO₄ composites; PEBA-100 (green), PEBA/BaSO₄-10 (blue), PEBA/BaSO₄-20 (mauve), PEBA/BaSO₄-30 (magenta) & BaSO₄-100 (teal).

Figure 11. Overlay thermogram of Pebax® 6333 SA01 MED, BiOCl and PEBA/BiOCl composites; PEBA-100 (green), PEBA/BiOCl-10 (blue), PEBA/BiOCl-20 (mauve), PEBA/BiOCl-30 (magenta) & BiOCl-100 (teal).

Figure 11. Overlay thermogram of Pebax® 6333 SA01 MED, BiOCl and PEBA/BiOCl composites; PEBA-100 (green), PEBA/BiOCl-10 (blue), PEBA/BiOCl-20 (mauve), PEBA/BiOCl-30 (magenta) & BiOCl-100 (teal).

Table 6 provides the values obtained from the DSC analysis. It can be seen in the table that while there was minimal change in melt temperature of the composites in comparison to the polymer (PEBA100), there was in fact a change in the melting enthalpies and the crystallisation temperature, where subsequently, the degree of crystallinity has shifted in the composites. The reduction in degree of crystallinity for the PEBA/BaSO₄-10, -20 and -30 composites were 10.1 %, 30 % and 28.9 % respectively. On the other hand, the PEBA/BiOCl-10, -20 and -30 composites experienced a 10.6 %, 38.8 % and 37.6 % reduction in crystallinity respectively when compared to the PEBA100, showing that the PEBA/BiOCl composites experienced a greater reduction in crystallinity in comparison to the comparatively loaded PEBA/BaSO₄ composites. These results may signify a greater degree of crystallinity of the BaSO₄ inorganic filler itself when compared to the BiOCl. This was also investigated by performing DSC on the fillers, however, there were no observed endothermic or exothermic peaks, possibly due to the relatively low temperatures at which the testing was conducted. It may be a case whereby the BaSO₄ has overlapping crystallinity when incorporated into the polymer, therefore obtaining a more crystalline composite.

Liu, et al. assessed the effects of melt compounding and the addition of various loading of inorganic fillers on the thermal properties of PLA. It was established that first and foremost, the shear introduced into the polymer during melt compounding had caused the degree of crystallinity to decrease when compared to the PLA. The observed reduction in the degree of crystallinity was significant, at 64.3 % [37]. This reduction in crystallinity showed the effects of shear imparted into the polymer, which, when compared to the results obtained in this study, were mirrored by the reduction in the degree of crystallinity of the melt compounding composites, however, not to such an extent. Similarly, in this study, it was found that as the density of the inorganic fillers increased, the degree of crystallinity decreased significantly. While the trend was similar, it was not to such an extent as observed by Liu, et al. In this study, the observed difference between the two inorganic fillers at the respective loadings was 0.61 %, 12.63 % and 12.33 % respectively. It was also noted that, while the degree of crystallinity decreased with the increase in filler content, they increased slightly at the highest level of loading when compared to the respective middle loading (20 wt.%).

Khalaf conducted DSC analysis on composites with various loading levels of filler. It was established that as the percentage of filler loading increased, the degree of crystallinity decreased. This was attributed to the lower free volume within the polymer due to the greater volume of filler, thereby reducing the crystallisation ability of the polymer [38]. Changes in a polymer’s morphology such as degree of crystallinity can lead to phenomena such as shrinkage in parts. When designing a medical device product, it is important to understand the crystallinity of the polymer or polymer composite, so that accommodations may be made for shrinkage that may occur during process, either post extrusion, or while moulding.

Table 6. Thermal transitions identified from resultant thermograms. Χ_c = (ΔH/ΔH*); ΔH is the melting enthalpy calculated through integration of the melting peak determined via DSC; ΔH* = 65 J g⁻¹ is the melting enthalpy of a 100% crystalline Pebax®.

Table 6. Thermal transitions identified from resultant thermograms. Χ_c = (ΔH/ΔH*); ΔH is the melting enthalpy calculated through integration of the melting peak determined via DSC; ΔH* = 65 J g⁻¹ is the melting enthalpy of a 100% crystalline Pebax®.

Sample	Tm (°C)	ΔHm (J g−1)	Tc (°C)	ΔHcc (J g−1)	Χc (%)
PEBA100	172.30	58.12	133.67	62.29	89.41
PEBA/BaSO410	172.59	52.25	136.17	54.95	80.38
PEBA/BaSO420	172.65	40.70	136.90	44.69	62.62
PEBA/BaSO430	173.71	41.35	136.89	45.89	63.61
PEBA/BIOCl10	173.53	51.93	144.201, 137.022	0.871, 5.292	79.891
PEBA/BIOCl20	173.36	35.56	147.68	37.72	54.71
PEBA/BIOCl30	171.44	36.25	143.50	36.39	55.77

3.4. Physical Properties

3.4.1. Density

Density is a crucial characteristic to consider when evaluating composites of various filler content for medical devices. When comparing various filler types of dissimilar densities, it is important to look at the densities of the final composites, showing clearly the effects of loading rates a particular filler into a polymer system. In this case, where BaSO₄ and BiOCl were incorporated into the polymer at various rates, with such as difference in density of the inorganic fillers, the densities of each formulation of composites were analysed. Knowing the density of a material or composite is important as it provides critical understanding about how the material would behave during processing, but also for design of a medical device. It has also been shown that incorporation of fillers of varying densities at similar weight percentages may have differing effects on the reinforcement of the base polymer. As such modification of the weight percentage of filler added may require alteration [39]. The reinforcement effect herein was, however, not affected by the differing densities of the filler.

Figure 12 displays the densities of the various composites produced during compounding while Table 7 shows a comparison of theoretic and actual densities. All experimentally obtained values were within 0.01% of the theoretical values, showing the effectiveness of the extrusion and moulding processes in creating uniform blends. It is clear from the graph that with the incorporation of both inorganic fillers into the respective composites, the densities increased from that of the polymer. When comparing the composites against PEBA100 (1.01 g/cm³), there was an increase of 8.73 %, 14.65 % and 23.23 % for the PEBA/BaSO₄-10 (1.107 g/cm³), -20 (1.183 g/cm³) and -30 (1.316 g/cm³) composites respectively. Similarly, a 9.39 %, 16.67 % and 26.85 % increase was observed for PEBA/BiOCl-10 (1.115 g/cm³), -20 (1.212 g/cm³) and -30 (1.381 g/cm³) respectively. It was established that the PEBA/BiOCl composites were denser than the PEBA/BaSO₄, with an increase of 0.72 %, 2.37 % and 4.71 % at the respective increases in filler loading. It was expected that the BiOCl composites would have a greater density at the respective loadings than the PEBA/BaSO₄ composites.

Table 7. Comparison of theoretical densities of the composites prepared and the values obtained experimentally. ρ_theor: theoretical density & ρ_act: actual density.

Table 7. Comparison of theoretical densities of the composites prepared and the values obtained experimentally. ρ_theor: theoretical density & ρ_act: actual density.

	ρtheor (g/cm³)	ρact (g/cm³)	Deviation (%)
PEBAX100	1.010	1.010	0.0100
PEBAX90BaSO₄10	1.095	1.107	0.0101
PEBAX80BaSO₄20	1.195	1.183	0.0099
PEBAX70BaSO₄30	1.316	1.316	0.0100
PEBAX90BiOCl10	1.106	1.115	0.0101
PEBAX80BiOCl20	1.222	1.212	0.0099
PEBAX70BiOCl30	1.366	1.381	0.0101

Figure 12. Density analysis; Pebax® 6333 SA01 MED (blue), BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

Figure 12. Density analysis; Pebax® 6333 SA01 MED (blue), BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

3.4.2. Ash Content

Ash content is an excellent method for evaluation of the content of inorganic filler in a polymer composite after compounding. While this method does not offer as much in-depth data, it does provide an indication of the level of filler within the composite. Figure 24 displays the results obtained from the ash content test. It is clear from the graph that the desired filler loadings for the composites were achieved by compounding. There was some variation in the addition of filler for PEBA/BaSO₄-10, -20 and -30 of 0.5 %, 0.84 % and 0.07 % respectively. Similarly, the discrepancy in filler loading of PEBA/BiOCl-10, -20 and -30 was 1.66 %, 0.15 % and 0.76 % respectively, displaying the accuracy of feeding during the compounding process.

Figure 13. Ash content analysis; BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

Figure 13. Ash content analysis; BaSO₄ filled Pebax® 6333 SA01 MED (orange), BiOCl filled Pebax® 6333 SA01 MED (green).

4. Conclusion

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