The thermoforming of pickleball paddles provides several advantages. For paddle manufacturers extra materials, adhesives, and labor are not required to bond the edge guard to the paddle. For pickleball players, the stiff carbon fiber edge acts like a frame that is similar to a tennis racquet, which allows the center of the paddle to behave like a trampoline that can increase “pop” and the velocity of serves and returns. In recent weeks, however, the delamination of thermoformed pickleball paddles has caused a lot of controversy and has received a lot of attention.
Conventional wisdom suggests that the thermoformed composite paddle face sheets are de-bonding at the interface with the inner honeycomb core materials. In response, paddle manufacturers have asked their vendors to apply more or better adhesives to bond the face sheets to the cores. But is this the answer? If both conventional and thermoformed paddles use similar glues to bond the face sheets to the cores, one would expect conventional paddles to also delaminate; however, this does not appear to be the case. Delamination appears to affect thermoformed paddles exclusively. Why?
Types of Delamination
We discussed the construction of pickleball paddles in our article, “Pickleball Paddle Materials“. In short, pickleball paddles have a carbon fiber front and back face sheets, that are adhesively bonded to a polypropylene honeycomb core (Figure 1).
Typical carbon fiber face sheets are comprised of a lay-up of unidirectional plies, where the fiber runs in one direction. The mechanical properties (such as the stiffness) of a ply are different in the direction parallel to the fiber (called the “fiber direction”) verses in the direction perpendicular to the fiber (called the “matrix direction”). In order to obtain the same mechanical properties in the plane of the face sheet, the plies in a face sheet are arranged in a balanced or symmetric pattern. For example, an eight-ply face sheet might have the plies laid up in a configuration of 0°/+45°/-45°/90°||90°/-45°/+45°/0° plies (Figure 2).
As discussed above, delamination of the paddle can occur through debonding of the face sheets from the honeycomb core. This failure, commonly called interfacial delamination, can occur if the face sheet separates from the honeycomb core. It can be caused by inadequate adhesion between the face sheet and core due to use of inferior or insufficient adhesives, degradation of the adhesives during processing, or accelerated aging of the adhesives due to environmental factors, such as temperature extremes or moisture.
Another type of delamination failure can occur between multiple plies of a face sheet or within a single ply, termed interlayer and intralayer delamination, respectively. For our purposes, we will lump these together as “interply delamination”. For example, an interlayer crack can develop between the plies within a face sheet that that later branches off and propagates as an intralayer crack within another ply. The mechanism by which interply cracks develop is somewhat complicated and has to do with stress singularities that develop at free edges of the plies.
Thermoformed pickleball paddles seem to be more susceptible to delamination than conventional paddles, but why? Thermoformed sandwiched composite materials have been successfully implemented in several demanding applications in the aerospace, defense, and aeronautics industries. These applications use similar manufacturing processes, materials, and adhesives that must be subjected to extreme environmental conditions without failure. So why would thermoformed pickleball paddles be any different? To answer this question, we must first look at the thermoforming process.
Thermoforming Delamination
Traditionally, there have been two types of plastics – thermoplastics and thermosets. The major difference between the two is that thermoplastics can be re-melted and re-formed, whereas thermosets are heavily cross-linked and will permanently retain their shape once formed. Virtually all conventional pickleball paddles with unidirectional continuous carbon fibers use thermosetting plastics and epoxies. This is the reason that your pickleball paddle will not “melt” or soften when it is left out in the hot sun, or become brittle if it is left in the trunk of your car during the winter.
The fact that you cannot re-form a thermosetting plastic is the fundamental reason that conventional pickleball paddles must be cut from flat composite panels, which require plastic edge guards to be installed. How then, is it possible to have pickleball paddles made from carbon fiber face sheets that are thermoformed? Although it might be possible to hand-layup balanced plies with rolled edges and laminate the plies together, such a manufacturing process would require an inordinate amount of labor which would make their production costs prohibitive. So how is it possible for paddle manufacturers to mass produce thermoformed carbon fiber paddles?
Vitrimers are a new class of polymeric materials that were recently discovered that have properties of both thermoplastics and thermosets. Like thermosets, vitrimers can be embedded with continuous unidirectional carbon fibers and made to crosslink, creating stiff face sheets. However, like thermoplastics, vitrimers can be softened and re-formed at elevated temperatures. This enables composite sheet manufacturers to economically produce flat carbon fiber sheets and ship them to thermoforming manufacturers who can cut the sheets to the desired size and mold them into pickleball paddles.
Thermoformed paddles are created by first cutting the honeycomb core and face sheets to their desired shapes. Although some sources claim that the foam around the edge of the honeycomb core is “injected” around the edge after thermoforming, it is likely placed around the honeycomb core prior to thermoforming to provide mechanical support to the face sheets and allow them to conform around the curvature of the foam during the thermoforming process.
The honeycomb core and foam edge are coated with epoxy adhesives and sandwiched between the top and bottom face sheets. The entire assembly is then placed into a two-piece mold that provides heat and pressure to the paddle assembly, cross-linking the adhesives and bonding the top and bottom face sheets together with the honeycomb core. After a prescribed cooling time, the adhesives become cured and the paddle is removed from the mold.
The thermoforming of carbon fiber reinforced composite sheets requires that the fibers be capable of moving during the forming process. If the viscosity of the polymer matrix is too high, the fibers cannot move, leading to stretching, buckling, breakage, voids, and wrinkles in the fibers, which can cause high residual stresses to remain in the face sheets after thermoforming. If the viscosity of the polymer matrix is too low, the fibers can move too much, causing resin-rich areas that are brittle and may be prone to cracking. A delicate balance must therefore be achieved by the thermoforming manufacturers to precisely control the temperature, pressure, and molding time to create defect-free paddles.
Although Pickleball Science has not completed an in-depth scientific study or analysis of delaminated paddles, it is likely that high residual stresses are created in the edges of the pickleball paddle where the face sheets have been folded during thermoforming. These high stresses may cause cracking of the matrix material between the fibers or between adjacent plies. These cracks then propagate along ply or fiber boundaries as the paddle is subjected to repeated impact loads from striking the ball. The cracks can coalesce and propagate towards the center of the paddle, thereby reducing face stiffness, resulting in greater “pop” from the paddle.
While delamination is a current problem with thermoformed paddles, it is likely that thermoforming manufacturers will eventually develop the ideal combination of face sheet materials, fold geometry, and adhesives, together with an optimum combination of thermoforming temperature, pressure, and processing time to avoid delamination of future paddles.
Future Articles
In a conventional paddle, face sheet delamination causes “dead spots”, that lack reactivity when striking the ball, resulting in a lower ball return velocity. Why then, does delamination of a thermoformed paddle increase the “pop” and ball velocity? In the next article of this series (“Jansen vs Devidze Revisited“) we will examine why this occurs, and discuss why the current USAPA face stiffness test might not be adequate to identify delaminated paddles. We also test delaminated paddles in our post, “Thermoformed Paddle Delamination Revisited“, where we demonstrate an easy-to-use tap test to identify potential delamination problems and why a three-point bending test is the most effective means of determining the extent of face sheet delamination.
