Carbon fiber has quickly become the material of choice for high-performance pickleball paddles because of its low weight, high stiffness, and high strength when compared to other materials. Depending on the grade of carbon fiber and its weave pattern, the fibers themselves can provide a rough surface finish that can increase friction between the ball and paddle, enabling a player to put more spin on the ball. These paddle surfaces are marketed under the names of “textured carbon fiber”, “raw carbon fiber”, and “carbon friction surface”, and are becoming a popular alternative to grit-enhanced paddles. It is relatively easy to identify these paddles on the pickleball court, as they are characterized by their flat black appearance with minimal use of paint and logos.
In a previous article, “Pickleball Paddle Materials”, we provided a brief overview of the construction of pickleball paddles, including the materials used in the honeycomb core and face sheets. In a follow-up article, “How is Topspin Generated?” we looked at the interaction of the pickleball with the paddle face at the instant of contact and determined the role of dynamic friction on the amount of spin that can be imparted to a ball. In this article, we will take an in-depth look at carbon fiber face sheet materials to see how their stiffness and friction affect pickleball paddle performance. We will also examine textured carbon face sheet materials that are used in high performance paddles, such as the TMPR Terra TC-16 paddle shown above.
Carbon Fiber Stiffness and Strength
Carbon fibers are made from precursor fibers such as polyacrylonitrile (PAN) and rayon that are chemically treated, heated, stretched, and carbonized to create high-stiffness and high-strength fibers. The elastic modulus is an indication of the stiffness of the fiber, which relates the amount of tension force (or stress) required to achieve a certain amount of elastic elongation (or strain). The tensile strength of a fiber, on the other hand, is an indication of the amount of stress that will cause failure or breakage of the fiber. The elastic modulus and tensile strength are obtained by testing the material on an Instron tensile test machine that precisely records the force (or stress) verses displacement (or strain) as the material is pulled apart (Figure 1). (There are several companies that make tensile test machines, however the brand name ‘Instron’ has become a generic descriptor for these machines.)
The results of the Instron tests are plotted on a stress-strain plot as shown in Figure 2. The elastic modulus (denoted by ‘E’) is the slope of the stress (denoted by the Greek letter sigma or ‘σ’) divided by the strain (denoted by the Greek letter epsilon or ‘ε’). The tensile strength is the value of the stress when breakage of the material occurs. Another important material characteristic is its fracture toughness, which is the ability of a material to absorb energy without fracturing. The fracture toughness is obtained by calculating the area underneath the stress-strain curve. Tougher and more resilient materials will have larger areas than materials that are more brittle or fragile. In our example shown in Figure 2, although Material #1 has a higher stiffness and higher strength than Material #2, it has a lower toughness. Therefore, over continued usage, a paddle made from Material #1 may accumulate more fiber damage and lose stiffness at a faster rate than a paddle made from Material #2.
Carbon Fiber Grades
Most carbon fiber composite materials used for sporting goods come in two different grades: T300 or T700. (The ‘T’ prefix was previously used as an indicator of the amount of tonnage that a 1 cm2 area of the fibers could support; however, it is currently used to differentiate the T-series fibers from the higher modulus M-series.)
The T300 fiber was invented by the Japanese company Toray, in the 1970s, and is widely used in several industrial, consumer, and recreational applications. The T700 fiber has the same stiffness and diameter of the T300 fiber, and both fibers are classified as “standard modulus” fibers. The T700 fiber, however, is more advanced and has a strength that is almost 40% greater the T300 fiber. This means that the T700 fiber can withstand about 40% more stress and strain than the T300 fiber before breakage. There are higher grades of carbon fibers available, such as T800, T1000, and T1100, which are classified as “intermediate modulus” fibers, however, these materials are too brittle to be used in pickleball paddles.
The diameter of a single carbon fiber filament ranges from about 5 to 10 microns (μm). For purposes of comparison, the diameter of a single human hair is considerably larger at about 75 μm! In order to make the carbon fiber filaments more usable, they are arranged in bundles called “tows”, which identify how many thousands of filaments are bundled together. T300 fibers are available in 1K, 3K, 6K and 12K tow sizes, with 3K being the most popular and common. Based on geometric considerations alone, a 3K tow will have a diameter of about 0.4 mm (0.016”). T300 fibers in 3K tows have a higher elongation to failure and better strength than T300 fibers in 6K, 9K or 12K tows.
T700 fibers are available in 6K, 12K, and 24K tow sizes, with 12K being the most popular and common. Based on geometry, a 12K tow will have a diameter of about 0.8 mm (0.032”), or twice that of a 3K tow. Because the tows are larger, a face sheet produced with 12K tow can be easier and less expensive to produce than a similar face sheet made from 3K tow, as one-half the number of layers is required. Furthermore, a 12K tow face sheet will result in a larger weave that can be stiffer and heavier than a similar 3K tow face sheet.
Carbon Fiber Face Sheets
Sheets (or plies) made with carbon fibers that are aligned in a single direction will be very stiff in the direction parallel to the fibers (commonly called the fiber direction), but very compliant in a direction perpendicular to the fibers (commonly called the matrix direction).
Composite face sheet manufacturers will therefore balance the different plies by stacking them in a balanced lay-up. For example, an 8-ply stackup of unidirectional plies might be in orientations of 0°/90°/+45°/-45°//-45°/+45°/90°/0°. Some sheets will already be provided to the manufacturer where the fibers are already in a bidirectional arrangement, providing an equal stiffness in complementary axes.
When the desired number of plies are stacked, one process to make the face sheets is to place them in a plastic bag and subject them to high pressures (through vaccum) and elevated temperatures while infusing resin into the stack-up. When the resin has hardened, the face sheets are removed from the plastic bag and are ready for lamination onto a honeycomb core.
Textured Carbon Fiber Paddles
As we determined in a previous article, “How is Topspin Generated?”, the coefficient of dynamic friction between the paddle face and the ball is a key element in generating spin on the ball. Pickleball paddle manufacturers can increase the friction coefficient of their paddles by using anti-skid compounds, sand, grit, or rubber particles in their paddle face coatings. The USA Pickleball Association (USAPA) certifies paddle surfaces based on two tests, a coefficient of friction test and a surface roughness test. The maximum allowable average coefficient of kinetic friction on a paddle face is 0.1875, whereas the maximum allowable surface roughness is 40 microns based on the highest peak or 30 microns based on the average height of the peaks. Pickleball paddles that exceed these test limits are not approved by the USAPA.
One of the problems in using adhered grit to the paddle face to increase the friction coefficient is that with prolonged use and exposure to the elements, the grit can eventually wear off, giving the paddle less spin capability over time. To address this problem, paddle manufacturers have resorted to using carbon fiber face sheets that they market under the terms of “raw carbon fiber” or “textured carbon fiber”.
When the carbon fiber face sheets are created, a material is placed between the plastic sheet (in a vacuum bagging process) and the impregnated carbon fiber sheets. This material, known as the “peel ply“, is used as a release liner between the plastic sheet and the carbon fiber sheet. Since the peel ply will leave an impression on the surface of the resin, face sheet manufacturers can use different peel plies to obtain a desired surface finish on the face sheet. Rough surface finishes have traditionally been desirable for bonding one composite sheet to another, as the epoxy adhesive can better grab the rougher surfaces, causing a more effective bond.
The rough surface finishes from the peel ply are also useful in pickleball paddles as an alternative to adhered grit to increase the surface roughness and thereby enhance the paddle’s spin capability. Current textured carbon fiber paddles that are currently on the market include the following:
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Introducing the TMPR Terra TC-16
The TMPR company has graciously loaned me a pre-production TMPR Terra TC-16 textured carbon fiber paddle for evaluation. This paddle has been approved by the USAPA and is scheduled to be released for sale in late-December 2022 (Figure 3). This paddle uses the same T700 carbon fibers used in several popular paddles. A cursory (non-scientific) examination of the paddle face finds that the TC-16 surface is among the roughest of similar bare carbon competitor paddles and is about equal to medium-roughness grit-enhanced paddle.
As mentioned above, the impression of the peel-ply provides a rough surface finish to the paddle. Figure 4 shows the TMPR Terra TC-16 paddle surface under a microscope with 2000X magnification. This surface provides a regular pattern that increases the amount of friction between the paddle and ball, thereby enabling a greater amount of spin.
Figure 5 shows a paddle with an adhered grit (Franklin Signature with Max Grit) under a 2000X magnification. While some of the color is attributed to the paint on the paddle face, other colors are a natural artifact of the light reflecting from the rough surface at different angles, resulting in a prismatic color shift. The rough surface appears to be somewhat more gradual than the TMPR Terra TC-16.
Figure 6 shows the face of a relatively smooth paddle (Vulcan V530 Power) under a 2000X magnification at the border of a blue and orange line. Here we see only a slight color variation (pink) that would indicate only a slight roughness of the paddle.
Not All Textured Carbon Fiber Paddles Are Equal
Before you rush out to purchase a textured carbon fiber paddle, it is important to note that a variety of other factors will affect the performance and durability of the paddle face. Some of these factors include the resin system, the weave, and the lay-up used to create the face sheets. The stiffness, deformation, and durability of the face sheets depends on how well the face sheets are matched to the core material, and how the face sheets and core are implemented in the overall paddle design.
The direct exposure of the resin in the face sheet can pose several problems. There are no coatings or paints on the paddle surface that can protect the peel ply texture from dirt and contaminants. Furthermore, without a coating, impact loads from the ball or other objects can be greatly concentrated or localized over a small area, resulting in breakage of the resin. Broken fibers have been attributed by videographers to cause a loss spin capability for raw carbon fiber paddles. However, this observation is not entirely supported by the mathematics behind how spin is generated (see “How is Topspin Generated?”) or how the peel ply texture is added to the paddle face.
Examination of the TMPR Terra TC-16 paddle with the microscope under up to 2000X magnification did not find any damage to the peel ply texture, although the paddle has been vigorously used by several players of skill level 3.0 – 4.5 for an average of at least 2-3 hours every day for the past several weeks. This may be attributed to a highly durable resin system on the face sheets. Other important factors regarding the construction of the face sheets, core, and paddle assembly can also reduce the amount of stress and strain on the paddle surface enabling greater durability. It appears that TMPR has done a very good job in balancing these factors and have developed a winning combination for their Terra TC-16 paddle. A more comprehensive review of this paddle will be provided in a future article.
