In this work, uniaxial tensile tests were carried out on standardized dog bone specimens and extruded PA 12 tubes used in the manufacturing of PTCA balloon catheters. The tensile tests were conducted at five different temperatures \({T}_{\mathrm{i}}\), ranging between 23 and 100 °C. At each temperature level, 10 specimens were tested, whereby half the 10 specimens were conditioned in water for 40 days at 23 °C, and the other half was stored under ambient laboratory conditions (approx. 23 °C and 50% relative humidity). In a previous study it was found that the influence of a solution with physiological buffers, such as a 0.9% saline solution, over the duration of an intervention (approximately 1–2 min) is neglectable (see Appendix) and therefore not considered in the further conditioning.
In addition, 20 PA 12 tubes were processed by a necking process in which the tube is pulled through a nozzle that reduces the tube dimensions. The necked tubes were also subjected to a tensile load at 23 °C and 37 °C, respectively. The crystallinity, the glass transition temperature \({T}_{\mathrm{G}}\), and the melting temperature \({T}_{\mathrm{m}}\) were assessed for each specimen geometry (dog bone, tube, and necked tube) by DSC of type DSC 3 + (Mettler-Toledo AG, Switzerland) on one sample. The DSC analyses were performed in a nitrogen environment during one cycle with a heating/cooling ramp of 10 °C/min between 0 and 300 °C.
Experimental setup for the dog bone specimens
The specimens (ISO 527-2, Type 1A, EMS-CHEMIE AG, Switzerland) were fabricated by injection molding of Grilamid® L25 granules. Their initial cross section \({A}_{0}\) was roughly \(40 {\mathrm{mm}}^{2}\). For each specimen, the width and thickness of the middle section were measured with a caliper.
In order to evaluate the water absorption (cf. Eq. 1), the mass of the specimen was measured with a scale of type MC21S (Sartorius AG, Goettingen, Germany) before and after the immersion, \({m}_{\mathrm{amb}}\) and \({m}_{\mathrm{wet}}\), respectively.
$$\mathrm{Increase in weight}=\frac{{m}_{\mathrm{wet}}-{m}_{\mathrm{amb}}}{{m}_{\mathrm{amb}}}\bullet 100 [\mathrm{\%}]$$
(1)
The uniaxial tensile tests were carried out on a material testing machine Zwick Z250 (ZwickRoell GmbH & Co. KG, Ulm, Germany) with a temperature chamber (± 1 °C) (cf. Fig. 5). The forces were measured in [N] with a 5-kN load cell. Local deformations in transverse and longitudinal direction were measured using the Zwick VideoXtens video-extensometer with glued-on measuring marks at an initial gauge length of \({L}_{0}=75 \mathrm{mm}\).
The tests were performed based on ISO 527–2. The ISO defines a test speed of \(1 \mathrm{mm}/\mathrm{min}\) for measuring the Young's Modulus \(E\) and the Poisson's ratio ν and a test speed of \(50 \mathrm{mm}/\mathrm{min}\) for the mechanical properties until fracture. Therefore, at 0.6% strain, the specimen was relieved and re-loaded with a test speed of 50 mm/min until breakage. The change in test speed is required on one hand, to allow for a good accuracy of the Young’s Modulus and on the other hand limits the effect of a time depended behavior onto the mechanical properties.
Necking of tubes
Necking is a process commonly used for balloon forming. It is done prior to the stretch–blow molding to reduce the dimensions of the catheter and to control the location of the balloon [15]. During the necking process, the polymeric tube is heated locally and stretched in axial direction. The axial stretch causes an alignment of the molecule chains within the tube. As explained by Roesler [6], the strength of semi-crystalline polymers can be increased either by a rise in crystallinity or by orientation of chain molecules (stress-induced crystallization). Therefore, the necking process is expected to influence the mechanical strength of the PA 12.
Standard single-lumen monolayer Grilamid® L25 PA 12 tubes, with an outer diameter \({D}_{\mathrm{a}}\) of 0.92 mm, for catheter manufacturing, were delivered by Putnam Plastics, USA. In the necking process used during this study, the PA 12 tubes were manually pulled through an unheated tapered nozzle (cf. Fig. 6). For achieving better control of the inner diameter, a calibration wire was inserted, and in order to relieve inner tensions the necked samples were tempered for 60 min at 50 °C.
Experimental setup for the tubes
Since conventional measurements with a caliper impose a risk to harm the tube, the cross section was instead determined by measuring the mass \(m\) and length \(L\) of the samples (cf. Eq. 2). The density of the material is specified by the manufacturer (\(\rho =1010\frac{kg}{{m}^{3}}\)):
$$A= \frac{m}{\rho \bullet L} \left[{\mathrm{mm}}^{3}\right].$$
(2)
The uniaxial tensile tests of the PA 12 tubes were conducted on a Zwick Z010 (ZwickRoell GmbH & Co. KG, Ulm, Germany) using a temperature chamber. Various clamping devices, such as pneumatic grips, with pyramid or rubber jaws, as well as capstan grips which are usually used for yarn testing, were evaluated to find the most suitable device for preventing any sliding of the tubes during testing. The rubber jaws proved to be the best solution against slippage, and simultaneously, allowed for testing of short samples with \({L}_{0}=45 \mathrm{mm}\) and measuring failure in the limited travel range of the machine. In order to stabilize the lumen from collapsing between the clamps, wire inserts were implemented. Four markers were applied at 10 mm distances onto the specimen to measure local deformations with a video extensometer. Whenever fracture occurred close or inside the clamps, test results were discarded, and the experiment was repeated.
As in the tests of the dog bone specimens, a test speed of \(1 \mathrm{mm}/\mathrm{min}\) was defined for the start of the measurement. After a strain of \(0.3\%\) was reached, the test speed was increased to \(50 \mathrm{mm}/\mathrm{min}.\)
The tensile tests were carried out at 23 °C, 37 °C, 50 °C, 80 °C and 100 °C for the standard tubes and at 23 °C and 37 °C for necked as well as conditioned tubes.
Material properties
For each material parameter, the median as well as the IQR were determined. The characteristic nominal stress \((\sigma )\) (cf. Eq. 3) vs. longitudinal strain \(({\varepsilon }_{\mathrm{l}})\) (cf. Eq. 4) curves are obtained by converting the measured force \(F\) and video-extensometer displacement \((\Delta L)\) by means of the initial cross section (\({A}_{0})\) and the initial gauge length \(({L}_{0})\), respectively:
$$\sigma = \frac{F}{{A}_{0}} \left[MPa\right],$$
(3)
$${\varepsilon }_{l}= \frac{\Delta L}{{L}_{0}} \left[-\right].$$
(4)
The Poisson's ratio \((\nu )\) describes the ratio of transverse contraction \(({\varepsilon }_{\mathrm{t}})\) and longitudinal extension \(({\upvarepsilon }_{\mathrm{l}})\) (cf. Eq. 5) (ASTM D638). For most materials under tension, \(\nu\) lies in a range of 0 to 0.5, whereas 0.5 is for incompressible materials (e.g., rubber). The Poisson’s ratio \(\nu\) was obtained from the slope of a linear fit (Matlab R2019a, MathWorks, Massachusetts, United States) between \({\varepsilon }_{l}\) = 0.25–0.6% for 1 mm/min and \({\varepsilon }_{\mathrm{l}}=0.6-0.95\) for 50 mm/min for the measurements of the dog bone sample:
$$\nu = \frac{\Delta {\varepsilon }_{t}}{\Delta {\varepsilon }_{l}} \left[-\right].$$
(5)
For each specimen, the Young's Modulus \(E\) (cf. Eq. 6) was obtained from the slope of a linear fit of \(\sigma\) and \({\varepsilon }_{\mathrm{l}}\) between \({\varepsilon }_{\mathrm{l}}=0.05-0.3 \%\) for 1 mm/min and \({\varepsilon }_{\mathrm{l}}=0.6-0.8 \%\) for 50 mm/min:
$$E= -\frac{\Delta \sigma }{\Delta \varepsilon } \left[MPa\right].$$
(6)
The proportional limit is specified as a deviation from the proportionality of the stress–strain curve and marked by \({\sigma }_{\mathrm{p}}\) and \({\varepsilon }_{\mathrm{p}}\), respectively. To evaluate the proportional limit an offset-criterion is used. A line with a slope equal to the samples Young's Modulus is shifted by a strain-offset of 0.05%. The intercept between the nominal stress–strain curve and the offset-line defines the proportional limit A (cf. Fig. 7). The point B defines the yield point, which describes the stress \({\sigma }_{\mathrm{y}}\) and elongation \({\varepsilon }_{\mathrm{y}}\) after which the specimen starts to neck, i.e., the cross-sectional area decreases. The drop ratio (DR) is defined as the ratio between the stress at the point of neck stabilization (C) and the yield point (B) (\(\mathrm{DR}=C/ B\)). Point D is defined as the ultimate tensile stress \({\sigma }_{\mathrm{u}}\) and elongation at fracture \({\varepsilon }_{\mathrm{u}}\), respectively.
The complex chain alignment during the tensile test has been explained by a simple model. Since the bond strength in the crystalline region is higher than in the amorphous region, the amorphous molecules begin to lengthen (elastic region) and to untangle first. At larger strains (yield point), micro- and nano-voids start to form within the amorphous region, while the crystalline domains begin to orientate into the stress direction. At large strains the crystalline regions start to separate from each other, form blocks and finally form microfibrils [6, 13, 14].