G.L. DiMaio’s research while affiliated with Eastern Idaho Regional Medical Center and other places

For automotive upholstery leather, UV and heat resistance are very important qualities, particularly for non-chrome-tanned (chrome-free) leather. One of our research endeavors has focused on an environmentally friendly finishing process that will improve the UV and heat resistance of automobile upholstery leather. Both tocopherol and butylhydroxytoluene (BHT) are wellknown antioxidants commonly used in the cosmetic and food industries. Tocopherol has been reported as a potent free radical scavenger and highly protective agent for collagen fibers against UV and heat damage. Experiments were conducted by adding 1 to 5% alpha-tocopherol and mixed-tocopherol to the grain layer finishes (topcoat) of chrome-free leather. The treated samples were exposed to artificial sunlight at a high temperature for 72 hours and then evaluated for the efficacy of UV and heat resistance and by mechanical property testing for tensile strength and fracture energy. Tests using 5% alpha-tocopherol or mixed-tocopherol showed a significant improvement in color fading resistance against UV radiation and heat. We also studied the addition of BHT to the fatliquoring drums. Observation, however, showed BHT has no beneficial effect on the UV and heat resistance of finished leather.

Chrome-free leather such as glutaraldehyde-tanned leather behaves very differently from chrome-tanned leather. Information regarding its viscoelasticity has not been previously reported. Hysteresis and stress relaxation are two essential properties associated with viscoelasticity. We have designed a cyclic tensile test to measure these properties to gain insight into the structural difference between chrome-free and chrome-tanned leather. Observations revealed that chrome-free leather tanned with glutaraldehyde has a higher hysteresis than chrome-tanned leather. Stress relaxation experiments, on the other hand showed chrome-free leather has very similar relaxation curves as chrome-tanned leather. Both leathers demonstrate a rapid decrease in stress for the first few seconds followed by a much slower decay thereafter. The chrome-free leather, however, has a greater initial stress than chrome-tanned leather, indicating a higher stiffness than chrome-tanned leather. Moreover, observations showed the viscoelasticity of leather was affected significantly by its fatliquor content. A decrease of loading energy in a cyclic stress–strain experiment resulted from higher fatliquor content in leather.

Milling is a key process to soften leather for adequate compliance. It is, however, still not well understood for its effects on leather structure and mechanical properties. To consistently produce high quality leather, it is essential that the tanner understands the impact and effects of every mechanical operation in the leather-making process. This investigation is devoted to gaining a better understanding of milling effects on pliability and other properties of concern to the leather industry, such as mechanical strength and toughness. Observation showed milling leads to a decrease in Young's modulus, consequently an improved compliance and softness. Data also showed, however, there is little change in tensile strength and toughness. Moreover, leather products in service are constantly being stretched. For understanding the mechanical behavior of leather products under cyclic stretching, we measured the energy loss (hysteresis) during cyclic tensile tests using an advanced computing program. Data showed that hysteresis is the greatest for the first cycle, thereafter, the rest of hysteresis values are relatively unchanged. We also discovered that drum milling significantly decreases the hysteresis. This implies that a structural change occurs during milling, resulting in a removal of the residual stress that was introduced during the leather making process.

At the present time, testing the mechanical properties of leather is a destructive process, wherein cutouts are taken off the tannery floor for mechanical testing. Under a cooperative R&D agreement (CRADA), we recently investigated the feasibility of using the acoustic emission (AE) technique to dynamically and nondestructively measure the mechanical properties of leather. The long-range goal of this collaborative effort is the production of an AE instrument, which will provide the leather industry with a nondestructive way in which to monitor the quality of leather at each of the leather-making stages. In this investigation, a rotational acoustic sensor was rolled across the leather samples to collect their AE quantities such as waveforms, frequency, hits, counts and energy. The software AE-Win™ was utilized to capture and record the data, and the neural network software NOESIS™ was subsequently used for data analysis. Observations showed an excellent correlation between the softness of leather and the corresponding cumulative acoustic counts. We also used this dynamic method to characterize the grain break of leather. Results showed that the difference in grain break could be determined from the amount of acoustic energy collected from moving the AE sensor over a leather sample laid inside a half pipe. The grain break decreased as the AE energy increased. The higher AE energy is an indication of stiffer leather; therefore, the results revealed that stiffer leather is prone to bad grain break. Data also demonstrated that the thicker samples tended to have poorer grain break. We derived a predictive model that could be very useful for nondestructively testing grain break using the AE method described in this report. Results also showed a close relationship between the tensile strength and AE energy or duration obtained from testing. In short, this study demonstrated that the softness, tensile strength and grain break could be nondestructively determined by measuring the acoustic quantities with a rotational sensor rolling over the leather.

A study was conducted on the alkaline calcium peroxide hairburn and hairsave processes, using hide pieces and matched sides and compared to conventional hairburn with sodium sulfide. The studies revealed that shrinkage temperature, physical testing, dye shading, dye levelness and grain appearance were comparable. In addition, several commercially available non-sulfide sharpening agents, added during reliming of hide pieces, were demonstrated to remove residual hair stubble that remained after rapid oxidative hairburn.

The use of fatliquors is critical to attain the required physical characteristics for leather products. Currently a wide variety of fatliquors are being used in leather manufacturing. There is a lack of information, however, regarding their performance in terms of physical properties of the resultant leather. Such information certainly is very informative for the leather manufacturers to select the right fatliquors to meet quality demands. We have therefore recently conducted a comparison study on the physical properties of leather prepared with various types of fatliquors. Results showed that the type of fatliquor applied in a fatliquoring process does not directly affect the physical properties of leather, instead, the actual concentration of fatliquor will govern the results. Data indicated that a lower Young's modulus induced higher elongation and resulted in increased tear strength. Observation showed an excellent correlation between the softness of leather determined by manual assessment and the Young's modulus. It appears that one of the key functions of fatliquoring is to reduce the initial deformation resistance. Data also showed that the initial strain energy is an improved method to determine the initial deformation resistance, which takes the non-linear viscoelasticity of leather into account and is easier to measure and to define. Therefore the initial strain energy is a better objective method to determine the softness of leather instead of the subjective manual assessment.

A mixture of proteolytic enzymes derived from the bacterium, Streptomyces griseus, was used to unhair bovine hide pieces. The enzymatic unhairing was assisted by a 30-min pretreatment of the pieces with carbonate buffer and the inclusion of a common amphoteric surfactant (N, N-dimethyl-1-dodecanamine oxide) in the enzyme formulation. This unhairing process may serve as a replacement for the hairburn process that relies on hazardous sodium sulfide. The unhairing reaction consisted of drumming pretreated hide pieces with 0.5 mg/mL of the bacterial protease (in 1% N, N-dimethyl-1-dodecanamine oxide) for 4 hours at 37°C in a 200% float. This process causes the majority of hair to either fall out intact or to become extremely loose and hence readily removed through physical/mechanical means. Some fine hairs remain, thus requiring sharpening agents to be used in reliming to achieve total unhairing. Laboratory-scale studies using bovine hide pieces compared this enzymatic treatment to typical hairburn with sodium sulfide. The studies demonstrated that shrinkage temperature, physical properties, dye levelness, dye shading, and grain appearance were comparable if not better for the enzymatic unhairing.

Finishing, the final step in the leather-making process, contributes to the beauty and durability of leather. Choosing the right coating is critical in obtaining a satisfactory finishing result, such as good flexing endurance and no surface cracking. Acoustic emission has been known to be a useful method to characterize the coating properties for the automobile industry. As a continuation of our efforts to investigate the applications of acoustic emission technology to leather manufacture, we exploited the AE technique to measure the flexing endurance of leather coatings. An acoustic sensor was clipped to the grain layer of finished leather in a tensile test to collect various acoustic quantities. Observations showed that differences in the flexibility of the coatings can be analyzed by examining the plot of the AE count rate as a function of time. We have concluded that a quantitative association exists between the flexibility of coatings and the cumulative acoustic counts produced at an initial tensile stretch. The results of this AE research may provide a route to examine the flexing endurance of leather coatings.

The importance of knowing the effect of leather making conditions on the resultant area yield cannot be over-emphasized because the price of a piece of chrome-tanned leather is determined by its area. In our previous vacuum drying studies, we demonstrated that the residual water content is the key factor governing area retention. However this general doctrine can be complicated by additional variables such as the initial water content and the number of staking passes, particularly for the toggled-dried leather because of the mechanical stretch that occurs. We therefore have conducted a systematic study focusing on four factors simultaneously: initial water content, fatliquor concentration, drying time and the number of staking passes. We used the SAS statistical program to formulate their interactive relationship on the area retention of leather. A second order polynomial regression equation was derived to plot 3-D response surfaces that clearly illustrate the relationship between the variables and area retention. Observation showed that the area retention increases steadily with increasing either initial water content or fatliquor concentration. This trend slowly reverses, however, as both of these factors increase further. The effect of staking on area retention is also very intricate. Without the proper amount of fatliquor or initial water content, staking can significantly reduce area retention. This investigation has attained comprehensive information of the effects of these four variables and more importantly their interactions on area retention.

The quality of hides suffers from their treatment by the packing industry as a byproduct, at best, of meat production. A meat packing company, Future Beef Operations, LLC (FBO), is changing this mentality by developing a vertically integrated cattle operation in which the hide is a product in its own right. A key Component of this innovarive operation is the rapid unhairing of stunned cattle carcasses prior to flaying in the slaughterhouse. Since it removes hair-associated manure, dirt, and microorganisms, rapid unhairing greatly reduces the cross-contamination of microorganisms on the hide to the meat, thus serving as a food safety measure while allowing for the rapid grading of hide quality and cost savings in downstream tanning operations. Under a cooperative agreement, Agricultural Research Service and FBO developed an effective rapid unhairing process based upon the earlier work (in 1977) by R. Dorstewitz on a rapid through-feed unhairing system. The rapid unhairing process was refined whereupon optimal conditions included 6.2% sodium sulfide at 35°C applied as a pressurized spray (in two applications) to warmed hide piece samples; an additional spray application of the sodium sulfide was used to remove the partially dissolved hair from the samples and subsequent neutralization of residual sulfide (still on the sample pieces) with 3% hydrogen peroxide was brought about in less than 5 min of total reaction time. The process sufficiently removed hair and hide-associated manure balls, allowing for splitting of green hide. The process was further developed to incorporate the recycling of the unhairing agent (sodium sulfide) and recovery of removed hair. Analysis of the recycled unhairing solution over 16 cycles showed a linear consumption of ca. 20% sulfide with a linear gain of ca. 1% nitrogen (proportional to dissolved hair protein content).