Annealing is a term normally used to describe a heat treatment that is widely used in science polymers to improve mechanical and thermal properties by modifying the morphology. This consists of a thermal process where polymers are heated in a desiccated environment to a temperature between the glass transition and melting temperature for a specific period of time (normally between 1 and 10 hours). From a microstructural point of view, when a semi-crystalline polymer is annealed, there is an increase in laminar crystal thickness due to the reduction in free energy gained by lowering the surface area of lamellar crystals when it becomes thicker and less wide, and this change in the microstructure leads to new mechanical and thermal properties. Balloon forming is one of many science polymers fields where annealing processes can help to improve the development and properties of balloon angioplasty. To help better understand what the implications of annealing during balloon forming are, we proceed to answer the following key questions:
How balloon angioplasty operates and what are the required mechanical and thermal properties?
Many different types of catheters are currently used in the medical field, but all of them have similar working methods. When catheters are inserted into the human body, they are commonly slid in and guided through by using openings or ducts such as arteries or the urethra until they reach the deploying position. Previous and during the catheter insertion, the balloon is folded and located in a circumscribe passing guide. Once the catheter is set in place, the balloon is deployed by inflating it to nominal pressure (specified by manufacture). These operating conditions require the balloon to have thin walls (~ 0.001 to 0.002 in) to freely move through the guide but also to be high pressure resistant with burst pressure between 10 and 20 atm, nominal pressures between 5 and 10 atm, high dimensional stability (high modulus and low viscoelasticity), and high resistance to fatigue (for multiple deployments applications). Finally, balloons angioplasty also requires high temperature stability since most balloons are conducted to thermal manufacturing process after being blown such as laser bounding.
What are the annealing effects on the mechanical and thermal properties?
Knowing the balloon’s mechanical and thermal properties that are needed to create successfully operational catheters, we can analyze the annealing effects on such properties and how this thermal treatment can contribute to the final product during balloon forming. The annealing effects on the mechanical properties of polymers have been already investigated by many researchers because of their impact on the science polymer field and related industries [1-6]. Polymer structures can present an increase in Young modulus and ultimate tensile strength of almost x2 when they are exposed to an annealing process for 6 hours due to an increase on the crystallinity of the material [1-3]. An increase in the modulus and tensile strength allow balloons to be fabricated with small wall thickness and still be used under the required internal pressure conditions without undesired deformations or failure. Viscoelastic effects are decreased by annealing leading this to a better dimensional stability since the polymer’s properties are less time dependent [4]. From an operational point of view, this decrease on the viscoelastic behavior of polymers allows the balloon to be hold at a constant internal pressure, showing minimal growth as a function of time. One more aspect to take into consideration is the desiccation process that is inherently conducted during annealing. Since most polymers are hydrophilic and their mechanical and thermal properties change as a function of moisture content, it is always convenient to make sure that their moisture content is always controlled [5]. Annealing balloons can help to eliminate H2O molecules that could have been absorbed during the balloon blowing forming process [6]. Finally, it is worth mentioning that the melting temperature of the polymer structure and the heat capacity also increase [7,8] this increase in the melting temperature helps the balloon to have a better dimensional thermal stability. All these changes on the mechanical and thermal properties are strongly related to the annealing time, thus a greater change in magnitude is shown for longer anneals.
How does the annealing process affect the internal stresses?
The annealing process also helps to release internal stresses that could have been induced during the balloon blowing by thermally relaxing the microstructure that is found under tensions. Most of the time, residual or internal stresses could compromise the final use performance of balloons, causing irregularities on the stress distribution of the balloon that could lead to dimensional distortion. Therefore, adding an annealing treatment to the blowing forming will help to reset the internal stresses to zero in the final product.
Annealing process
Different techniques are found for annealing polymers (Batch annealing, Conveyorized Forced Hot Air Annealing (CFHA), and Infrared annealing (IR)). Here, we explain the batch annealing process for being the most common process. The stages of this process are:
· Placing the balloon in an annealing oven under a desiccated environment.
· Heating the balloon to the annealing temperature at a controlled rate. The balloon can be also set into the oven after this has been preheated.
· Hold the balloon at the annealing temperature for a specific period of time
· Cooling the balloon to the ambient temperature at a specific rate or letting it cool down by free convention
References
[1] Cho, Ah‐Ra & Shin, Dong Myeong & Jung, Hyun & Hyun, Jae & Lee, Joo & Cho, Daehwan & Joo, Yong. Effect of Annealing on the Crystallization and Properties of Electrospun Polylatic Acid and Nylon 6 Fibers, Journal of Applied Polymer Science, 2011.
[2] Nicholson TM, Ward IM. A comparison of the effect of annealing on two liquid crystalline polymers, Polymer, Volume 39, Issue 2,1998, 315-317.
[3] Takayama T, Todo M, Tsuji H. Effect of annealing on the mechanical properties of PLA/PCL and PLA/PCL/LTI polymer blends, Journal of the Mechanical Behavior of Biomedical Materials, Volume 4, Issue 3, 2011, 255-260.
[4] Drozdov AD, Christiansen JD, The effect of annealing on the elastoplastic and viscoelastic responses of isotactic polypropylene, Computational Materials Science, Volume 27, Issue 4, 2003, 403-422.
[5] Alvarez, VA, Fraga, AN, Vázquez, A. Effects of the moisture and fiber content on the mechanical properties of biodegradable polymer–sisal fiber biocomposites, Journal of Applied Polymer Science, Volume 91, 2004.
[6] Higueras-Ruiz D, Feigenbaum H, Shafer M, Moisture’s significant impact on Twisted PolymerActuation. Smart Materials and Structures, Manuscript submitted for publication (2020).
[7] Yeh SY, Hosemann R, Loboda-Čačković J, Čačković H, Annealing effects of polymers and their underlying molecular mechanisms, Polymer, Volume 17, Issue 4, 1976, 309-318.
[8] Young RJ, Lovell PA. Introduction to Polymers: Third Edition. 3 ed. Boca Raton, FL, USA: CRC Press, 2011.