Critical Factors in Extruding Catheter Tubing from Polyamide

Medical tubing extrusion requires rigorous attention to a number of material and process characteristics. Understanding the importance of these parameters is key to ensuring product quality.

Sedigheh Farzaneh, Erik Andersen, and Abbas Tcharkhtchi

The process of manufacturing catheter tubing and balloons from polyamide can be a complex task. There are a number of parameters that can affect the properties of the finished product. This article describes the importance of these parameters and briefly explains some of the difficulties associated with the extrusion of catheter tubing from polyamide materials. Some of these difficulties are related to the material characteristics of the polymer while others result from the extrusion methods used.

Single- or dual-screw extrusion is used to manufacture catheter tubing. The polymer is transported along the screw channel while passing from a solid to a molten state. During steady extrusion, three distinct zones may exist along the screw length:

The screw diameter gradually increases from the machine's infeed to the die—either over the entire screw length (as in a universal screw), or over a screw section. Figure 1 illustrates a typical tubing extrusion line design.

During the extrusion process, air at a specific pressure is used to obtain the intended tubing shape. A heat-transfer process begins as the polymer exits the die. Air is used as the initial cooling medium, followed by water in a cooling bath. A belt or wheel haul-off (puller) pulls the tube from the die and through the cooling bath at a controlled speed.

Because of the complexity of catheter tube manufacturing, several parameters associated with the extrusion process may affect the properties of the finished product.¹ These parameters include:

Polymer choice can also affect the finished product's properties. One polymer that is widely used for extruded balloon catheters is polyamide (nylon). This material is used in a number of diverse tubing applications because of its unique combination of physical and chemical properties. ²

DIFFICULTIES LINKED TO POLYMER CHARACTERISTICS

Because of its low molecular weight, extrusion of the polyamide studied by the authors (Rilsan, manufactured by Atofina Chemicals Inc., Paris) is very delicate. The polymer's fluidity while in a molten state is relatively high, and a temperature change of a few degrees can affect the process significantly. The extrusion temperature zone is therefore narrow and limited.

If the extrusion temperature is not high enough, the melted polymer will not be sufficiently homogeneous, and some crystalline portions may remain unmelted. This unmelted crystalline phase may then initiate recrystallization during the cooling process, resulting in a heterogeneous morphology. Conversely, if the extrusion temperature is too high, two potential problems arise. The risk of degradation increases, and it may be difficult to extrude the polymer because of its low viscosity.

Gel Formation. Gel in the form of unmelted material may be present in the initial granules, or it may be formed during the extrusion process. The presence of gel is one of the sources of heterogeneity in extruded tubes and balloons. In particular, a balloon's capacity for deformation can change because of this gel. To address the potential risk, it is important to remove gels by screen filtration. The size of the filter and its position in the extruder are quite critical.

Postpolycondensation. Extrusion of polyamides is generally accompanied by postpolycondensation. This is more significant whenever the initial polymerization (synthesis) is not totally completed as a result of inappropriate process conditions. The result may be low molecular weight, broad molecular weight distribution, or the presence of nonreacted monomers or oligomers with very low molecular weights.

Figure 2 provides the viscosity curves of a polymer before and after extrusion, obtained by rheology tests. This figure shows that the Newtonian viscosity of the polymer (and consequently the average molecular weight) increases significantly following extrusion. This increased molecular weight may be the result of postpolycondensation.

Dark Points and Impurities. In some batches of supplied polymer, there may be numerous dark points among the granules that can be a source of heterogeneity (Figure 3). These dark points may have different dimensions, and some may be large enough to be seen directly. These dark points may occur for several reasons:

Aging. If the extrusion temperature is too high, or polymer stays in the extruder for too long, the risk of degradation (thermal or oxidative aging) may become significant. When this degradation occurs, the polymer becomes brittle. Thus it is important to determine the optimal operating conditions.

To ensure that such degradation does not occur, infrared (IR) spectrophotometry may be used. Degradation is indicated by the formation (or increase) of carbonyl bonds (at 1710–1730 cm^-1) or OH bonds (at 3400–3500 cm^-1) on the IR spectrum. Figure 4 shows the IR spectrum of a polymer prior to extrusion. Molecular weight changes, caused by degradation also may be controlled by other methods, such as rheometry or chromatography tests.

DIFFICULTIES ASSOCIATED WITH THE EXTRUSION PROCESS

Drying. The water absorption capacity of polyamide is relatively high. In a saturated state, this type of polymer can absorb between 3 and 8% of its weight in water at ambient temperature. As supplied, the polymer studied had absorbed about 1% of its weight in water. It is thus very important to dry a polymer before extrusion to avoid the formation of bubbles or microholes in the finished balloon catheter.

Molten polymer absorbs up to 4.5–10% of its weight in water in the cooling bath as it leaves the extruder die. The quantity of absorbed water can be affected by several factors, including the temperature of melted polymer, the distance between the die and the cooling bath, the flow rate of the melted polymer, and the water temperature of the cooling bath. Functioning as a plasticizer, water can modify the mechanical properties of the tube and finished balloon catheter.

Cooling. The morphology of semicrystalline polymers depends on the cooling rate, especially when the crystallization rate is very low (as when using PET, for example). Furthermore, because the thermal conductivity of polymer is low, the cooling rate at the outer surface of the tube may differ from that on the inner surface, particularly when tubing walls are thick. In the case of semicrystalline polymer, it has been shown that this cooling-rate differential can, after extrusion or injection molding, cause the formation of a very thin layer with a different morphology at the surface of the tube. This phenomenon is known as the skin effect.

It must be noted that the crystallization rate of polyamide is relatively high and the extruded tubes have relatively thin walls. As a result, the skin effect appears to be less significant than for other materials. The formation of elliptical defects (fish eyes) on balloon catheters, however, may be caused by this skin effect.³ An example of such a defect is shown in Figure 5.

After extrusion, the tubing is cooled by water. Because water can be absorbed readily by the hot polymer, the tubing should be hot-air dried after passing through the cooling bath to minimize adverse effects upon its properties or those of the finished balloon.

Heat Exchange. The heat-exchange process that takes place between the molten polymer and the cooling water is a potential source of product instability. The cooling water warms up as it pulls heat from the tubing. This heated water then tends to follow the tube, surrounding it like a physical layer. If the water becomes turbulent for any reason, this heated layer is no longer controlled, which introduces another variable into the extrusion process.

Temperatures in the Extruder Zone. The extruder may have three or four different temperature zones, depending on the screw length. Determining the appropriate temperature for each zone can be one of the most important challenges of tube manufacturing. It depends on several different parameters:

Friction between Polymer and Metal. In the feed zone, because of specific rheological behavior, the polymer, either as granules or powder, can be quickly compacted or sintered by pressure and temperature, and can slip into the space between the extruder screw and sleeve. This slipping, however, depends on the friction between the polymer and the metal. This means that, on the one hand, it depends on the polymer viscosity (and shear force) and, on other hand, it relates to the roughness of the metallic surface.

Two extreme examples show the role of the coefficient of friction between polymer and metal. In the first example, the polymer sticks perfectly on the screw but slips on the sleeve. The screw then becomes clogged little by little and, after a few minutes of operation, flow ceases completely.

In the second example, the polymer slips perfectly on the screw, but sticks to the sleeve. As a result, there may be ample flow from the extruder, but the screw can be blocked because of high input torque.

Factors Affecting the Flow of Melted Polymer. The flow of melted polymer is controlled by the screw rate. This rate depends on five factors:

Die. In catheter tube manufacturing, it is important to maintain precise control of the tubing diameter and wall thickness. To accomplish this, the flow rate of material passing from the extruder and through the die must be uniform and constant.⁴

If the flow of melted polymer through the die is not kept beyond certain critical values, it is practically impossible to obtain a uniform flow rate. Flow uniformity depends both on the characteristics of the polymer (that is, its viscoelastic properties) and on the level of shear stress imposed on the polymer during die shaping.

All extrusion systems operating with extremely tight tolerances will exhibit some surging as a result of electrical drive-control fluctuations, screw design, and normal rheological variations in the polymer. Clearly, high reject rates and waste levels will result if the process relies solely on extruder stability. To overcome this, a precision gear pump is used to provide steady pressure and accurate metering so the polymer reaches the die head in a controlled and surge-free manner.

Installed between the extruder and the die, this gear pump becomes the main extruder control device, minimizing the inefficiencies inherent in conventional extrusion operations. Discharge pressure, and thus mass-flow variations, can be held to less than 1%, yielding excellent dimensional accuracy.

If the die is poorly designed, heterogeneity, shrinkage, and residual stress are likely to occur. To optimize performance, the surfaces of the die must be polished. It should generally be manufactured of stainless steel.

Pulling System. To pull the melted polymer through the cooling bath, a haul-off unit called a puller is used. The pulling force exerted by the system must be controlled precisely. To manufacture accurate small-diameter tubes, good motor-speed control is required to maintain an accurate draw-down ratio and haul-off speed.

Postextrusion Heating. Polymers generally exhibit viscoelastic behavior; their return to equilibrium after being disturbed is not immediate and depends on relaxation time. Additionally, semicrystalline polymers have a morphology that can be influenced by cooling rate. These characteristics enable manufacturers to address shrinkage or postshrinkage issues in tube extrusion in several ways. To deal effectively with shrinkage, it is necessary to heat the extruded tubing beyond its glass transition temperature, or Tg, for a few hours. This treatment can eliminate the residual stress and accelerate relaxation of the polymer.

CONTROLS

In manufacturing balloon catheter tubing, all process parameters must be closely controlled. This is necessary to keep the product within specified quality limits. The manufacturing process must be stable, to ensure precise extrusion performance and to reduce variability within the key parameters.

No process can be controlled, however, until there is a clear determination of what to measure and how to measure it. The first step is to determine how to measure quality, and to determine which process variables can be manipulated to influence that quality. Of the various parameters involved, dimensional control and air pressure inside the extruded tube are the most critical.

Dimensional Control. During the tube extrusion process, air at atmospheric pressure enters the tube through the core tube on a crosshead die, or the spider leg of an in-line die. The tube is sized by the combination of extruder and puller speeds, and the distance between the extruder die and the cooling water.

For example, if the puller speed increases while the extruder speed remains constant, the tube diameter will become smaller. On the other hand, if the distance between the hot die and the cooling tank is decreased, the tubing diameter will be increased because of the draw-down from the die outlet. In addition, adjustments made to this distance can affect the cooling rate and, thus, tubing properties.

In fact, the key quality parameters in medical tube production are dimensional stability and tolerances in all dimensions. Accurate and rapid measurement of the outside diameter of extruded tubing can be performed by laser gauges that measure a shadow created when the tube obscures a fine beam of rapidly scanning light. Dual-plane laser gauges measure outside diameters in two planes. This provides measurements of both average outside diameter and ovality with a resolution of 1 µm. Tubing wall thickness can be determined by gamma backscatter probes. These systems can measure wall thicknesses greater than 0.05 mm with a resolution of 1 µm for tubes with diameters 1 mm or larger. Another method, based on ultrasonic reflection, can measure the dimensions of tubes with outside diameters greater than 1.0 mm and a wall thickness of 0.13 mm or less, with an accuracy of ±5 µm.

Air Pressure inside the Tube. There are several different methods for using air pressure inside tubing during extrusion. These include

For each method, it is important to precisely control the outside and inside pressures.

With single-lumen tube designs, the wall is typically concentric. Roundness is relatively easy to maintain because any internal pressure will tend to exert an equal force against all sides. But if the air-blow axis is not in alignment with the theoretical central axis of the cylindrical tube, that section of the tube wall will not be homogeneous, and the internal pressure can cause unequal stress on tube's internal wall. This results in an oval shape. In addition, if the air pressure (or vacuum) is not constant, but the extruded tube wall section will not be homogeneous.

CONCLUSION

Tubing used for medical applications has well-defined requirements and needed characteristics that must be monitored and controlled by appropriate physicochemical or mechanical means. Various methods may be used to assess tubing properties following extrusion. These include optical microscopy, tensile testing, rheometry testing, IR spectrophotometry, and enthalpy differential analysis. These methods can be used to measure the effects of extrusion parameters on the final product and for comparison with the original polymer characteristics.

REFERENCES

1. K Sauerteig and M Giese, "The Effect of Extrusion and Blow Molding Parameters on Angioplasty Balloon Production," Medical Plastics and Biomaterials 5, no. 3 (1998): 46–49.

2. M I Kohan, Nylon Plastics Handbook (Munich: Hanser, 1995), 209–236.

3. A Tcharkhtchi and E Andersen, "Examining Elliptical Surface Defects on Angioplasty Balloons," Medical Device & Diagnostic Industry 24, no. 5 (2002): 103–110.

4. J Colbert, "Achieving Precision Tube Extrusion for Medical Applications," Medical Plastics and Biomaterials 3, no. 2 (1996): 22–29.

Sedigheh Farzaneh, PhD, is an associate professor at Leonardo da Vinci University (Paris) and specializes in biomaterials and quality control. Erik Andersen is general manager of CathNet-Science (Paris). Abbas Tcharkhtchi, PhD, is a professor at ENSAM (Paris) and specializes in polymer processing and properties.

Copyright ©2002 Medical Device & Diagnostic Industry