A Tale of Two Footballs
©Cambridge Polymer Group, Inc.(2017). All rights reserved
Synthetic or natural leather football? Why does it matter and what is the difference? Surely, synthetic materials can work as well as natural ones? The football we have all come to know, with its signature white stitches, can be traced back to early origins in Greece and Rome. The early “footballs” of antiquity, were made from inflated animal bladders (often times obtained from pigs) but later generations included a leather cover over the bladder, giving rise to the football’s “pigskin” nickname. The development of vulcanized rubber in the mid-1800s led to the replacement of the natural bladder insert with an inflatable rubber bladder. It is only in more recent years that the external covering has transitioned from natural animal hide to synthetic leather materials. The synthetic material is lower cost, making footballs cheaper to manufacture, but at what cost? Although modern elite soccer and rugby balls are now synthetic, largely because of weight gain issues in the wet state, professional and college football league regulations in the United States require the use of natural leather footballs. We sought to determine how different natural leather and synthetic leather footballs really are. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and friction testing were utilized to examine the differences in chemical composition, surface topography, and effective “feel” of synthetic and natural leather footballs.
Fourier Transform Infrared Spectroscopy
Not surprisingly, the FTIR spectra for the leather and synthetic material are markedly different (see Figure 1). The leather shows the presence of amides from the leather protein (at 1655, 1550 and 1270 cm-1) and ester groups from the tanning process (at 1734 cm-1). The spectrum for the leather ball examined in this study does not show waxes (peak at approximately 1815 cm-1) and residual tanning agents (peaks at approximately 3010 and 1710 cm-1) that might otherwise be expected in natural leather. This indicates that care was taken to clean the material prior to assembly, an example of the exacting requirements for the leather.
The modern synthetic football is usually composed of a fabric coated with a polymer. Early choices for the coating were polyvinyl chloride as it can be quite pliable and stitches easily when blended with plasticizers, although it does scuff easily and is more affected by the climate (temperature differences). As a result of these drawbacks, polyurethanes are often considered a better choice. The method of the polyurethane manufacture creates a material with different properties, and the football can be constructed with many layers to achieve the desired properties of grip, elasticity, durability and weather resistance.
The FTIR spectrum for the synthetic material, when compared to a spectral library, indicates that the material of the football used in this study was polyurethane based (see Figure 2).
Surface Analysis by Microscopy
Small bumps covering the surface of a football are visible by eye, but what do those bumps look like at higher magnification and what is their purpose? Optical microscopy images (see Figure 3) revealed finer details in the surface topography of the football. The dimensions of the raised features (outlined) of the leather and synthetic ball are comparable, with diameters of about 2 mm. However, the natural leather ball shows a non-uniform surface with recesses and flattened regions within the leather. The synthetic surface appears more uniform yet still has a bumpy texture. When imaged in the SEM (see Figure 4), distinctions between the materials become apparent. The leather surface appears fibrous, while the synthetic material clearly shows the small (~10 µm) bumps within the overarching macroscopic texture. These differences in surface morphology point back to the natural and man-made origins of the two materials.
What is the purpose of the macroscopic bumps on the surface of the football? Natural animal hide is embossed to add the raised features into the leather for improved traction. The added topography increases the surface area of the football, making it easier to grip, throw, and catch. Of course, synthetic materials were designed to mimic the same embossed texture, yet the details of the microscopic fibrous nature of the leather are not captured in the man-made football.
Coefficient of Friction
The argument of synthetic over natural leather in footballs and other sports balls, such as rugby or soccer, ultimately comes down to feel and grip. Rugby has transitioned to synthetic surfaces (and anyone who ever caught a high ball made from leather in wet conditions is grateful for that), but in football the preferred elite ball composition is leather. It is reasonable to ask then if that choice is advantageous for gripping the ball, or is it purely tradition? In the data below, we have determined the friction on a synthetic and natural leather ball using a conventional method to yield a coefficient of friction (CoF) that allows ranking of relative frictional properties, and also how those frictional properties vary with speed and under varying levels of wetness. The CoF is defined as the force required to slide a surface against a test surface (“frictional force”), divided by the amount of compression (“normal force”). Often this CoF is plotted versus the Sommerfeld number (a dimensionless number that incorporates the contact area and viscosity of the lubricant) in a graph called the Stribeck curve. Here, we present the CoF against shear velocity. In the experiments performed here, synthetic and leather footballs were tested intact on a TA Instruments AR-G2 shear rheometer (see Figure 5). A custom annular stainless steel fixture was used as the counterface. The choice of this geometry allows the contact region to experience a constant contact pressure and an approximately constant velocity everywhere.
In the first set of data (see Figure 6), the two balls are compared dry under two compressive loads, namely 1 N and 10 N of compression, across a range of speeds. For both balls the friction appears to increase with shear speed, with very similar friction at the lowest speed, and a peak in frictional force near 100 mm/s. After that speed the friction drops fast, suggesting that both balls appear at their “grippiest” when the ball is moving moderately fast (100 mm/s would translate to 4 inches/s, which with a regulation ball of 22” circumference, this speed would imply a rotation rate of ~0.2 rev/s, or 12 rpm). This rate is well below the reported rotation speed in an NFL game of 600 rpm (220 inches/s or over 5,000 mm/s). Perhaps more interestingly, the synthetic material appears to have higher friction than the leather at all speeds. In both cases, the friction force increases with compression load, suggesting the grip gets better the harder the ball is held. This could simply be a result of compression of the texture on the surface, or a better contact between the test geometry and the ball. Note that the synthetic high load data reaches the maximum instrument threshold for shear force before the experiment completes.
The picture changes somewhat when the balls are wetted with distilled water (see Figure 7). In these data, there is still a pronounced maximum in friction at approximately the same speed, but now the reported friction appears independent of compressive force and neither ball appears to have an advantage. For reference, the average speed that an NFL player can throw a ball is 20 m/s (20,000 mm/s), which is the relative speed that a receiver would experience when catching the ball. However, under these conditions, the synthetic ball actually exhibits a marked drop in frictional force at higher loads (the leather ball also sees a slight decrease). The leather ball would therefore have fairly consistent levels of grip, irrespective of how hard the ball were held, but the synthetic ball would have less grip at higher loads. Although minor, this observation already indicates that the leather ball may have an advantage in less-optimal playing conditions.
The picture becomes even more distinct when the ball is soaked in water. Visual comparison of the two balls after 30 minutes of soaking in water already indicates a difference between the two materials (see Figure 8). Clearly the synthetic ball is essentially unchanged (visually) after the water soak. In contrast, the leather ball is substantially darkened. This qualitative observation is supported by the friction data shown in Figure 9. It is clear from these data that in fact the synthetic ball is impacted by the soak, but to the detriment of grip, with a gradual decrease in frictional force. In contrast, the frictional force on the leather ball goes up as the ball gets wet, suggesting in fact that this ball should have improved grip in wet conditions. The weakness in the discussion here is the choice of counterface. Most players either use bare hands (in the case of the quarterback) or coated gloves (in the case of receivers). The steel counterface used here was a pragmatic choice, and may not represent the frictional force, which can be very sensitive to both the surface chemistry, and the conformability of the material. Nonetheless, this simple non-destructive ranking experiment yields insights into why the leather ball is still the choice of the elite sport divisions.
 Robert G. Watts and Gary Moore, “ The Drag Force on an American football”, Am. J. Phys., 71, (8), 791–793 (2003).