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High-tech hides make better leather.

High-Tech Hides Make Better Leather

When the first loincloth was donned in prehistoric days, it marked the beginning, albeit simple, of hides and leather technology.

From those rudimentary raiments, an $8.9 billion per year industry has evolved, with products ranging from automobile upholstery to clothing and, of course, shoes.

Agricultural Research Service scientists are looking for more efficient ways to make leather goods while extending the qualities of strength and elasticity for several new leather products.

The agency's researchers are also working to replace outdated leather processing techniques with methods that are environmentally friendly.

At the ARS Eastern Regional Research Center in Philadelphia, scientists are studying ways to preserve hides with electron beam irradiation. David G. Bailey, a chemist at the Philadelphia center's Hides, Leather, and Wool Research Unit, says the method could replace salt or brine curing.

Currently, hides are immersed in a saturated salt solution within a few hours after slaughter of the animal. This prevents bacterial growth that breaks down the hide's structure.

Deterioration of the hide seriously hinders the tanner's ability to convert it into leather. The primary function of leather tanning is to stabilize protein fibers so the hide will not readily degrade.

Under proper conditions of cool temperatures and low humidity, brine curing will stabilize the hide from microbial growth for up to 2 years.

Electron beam irradiation uses the same sort of electronic beam used in a television to convert broadcast signals from the atmosphere to a picture on the screen. Accelerating the electrons to very high speeds and subjecting the hide to these electrons destroys the DNA of bacteria lurking there.

"Our method is bringing high technology to what has always been low-technology preservation," he says. "We have some rawhide samples that have not been tanned, but have been irradiated, and they've lasted for 5 years."

"The hide is laid on a moving belt and passes under the beam," Bailey says. "The electron beam can penetrate deeply into the hide."

This type of irradiation is now used to sterilize prepackaged bandages and surgical dressings, Bailey notes.

In addition to its environmental advantages, electron beam preservation is fast as well, allowing the processing of 3,000 to 4,000 hides per day for each machine.

Unfortunately, some physical strength is lost when subjected to this process, but Bailey describes it as "minimal."

Salt curing has several negative side effects related to water pollution and the corrosive nature of concentrated salt solutions. Bailey says about 1 gallon of brine is released for every hide that is brine-cured.

Typically, a fresh hide contains about 64 percent water. Salt curing reduces the moisture content to about 45 percent but adds as much as 14 percent salt to the hide. The tanner removes this by soaking, which creates further salt-solution waste.

Bailey has also discovered that temperature has a direct relationship to the hide's salt absorption rate.

But as the temperature of the salt bath drops, so does the rate of salt absorbed by the hide.

"There's no doubt that low temperatures reduce the rate of salt penetration," Bailey says. "What a salt bath can accomplish in June, it can't do as quickly in December."

"In one study, more than 75 percent of the cattle hides cured during the winter months in colder regions of the country had unsatisfactory levels of salt saturation," Bailey says. "Samples held at the lower temperatures did not achieve a satisfactory cure in 24 hours."

It is important that the proper salt saturation and curing time be coordinated with temperature. If curing time isn't long enough during cold weather, the partially cured hides may look preserved but will spoil more quickly at higher temperatures.

Bailey observed hides cured at 36 [degrees] F, 50 [degrees] F, 60 [degrees] F, and 80 [degrees] F to reach his conclusions. He found that mechanical tumbling in the bath also increases the rate of salt penetration in the hide.

Collaring Collagen

When it comes to using tanning agents to manipulate the natural structure of fiber in hides for various leather uses, tanners and leather processors currently have no simple formula for success.

But ARS scientists are working to improve those odds with a computer model that offers a three-dimensional view of collagen--animal skin's fibrous proteins. Collagen is responsible for the strength and toughness of rawhide and the leather made from it.

While the basic shape of collagen is understood, it's unclear how these molecules pack into fine fibers known as fibrils. These molecular reactions determine the strength and flexibility of collagen fibers.

"Each collagen fibril is like a very thin, long string," says James M. Chen. "An enormous number of these are packed together to form skin.

"One thing we need to understand is how these are packed. This will tell us what kind of tanning agent could penetrate and bind."

He says variations in skin packing are similar to building a structure with building blocks. If all of the blocks--collagen molecules--were the same, then normally all structures would be the same. The same is true of animal hides.

But in reality, the collagen molecule is more complex than a building block. It is a large protein molecule with many chemical characteristics; scientists need to know more about its molecular structure. Once Chen, a research chemist at the Philadelphia center develops a computer model of collagen that mirrors collagen in skin, scientists will then be better able to evaluate how different tanning agents affect that structure.

Chen's three-dimensional model has already shown that interactions between collagen molecules are strongly influenced by the number of molecules present at that particular site on the collagen protein. This explains why tanning agents used to modify collagen perform differently at various sites on the protein.

"What you may be able to do is design a tanning agent that will modify the skin for strength or softness without damaging the hide," Chen says. "The three-dimensional model allows you to isolate different sites where tanning agents may be reacting."

Chen is also using experimental techniques such as NMR (nuclear magnetic resonance) and circular dichroism to compare information in the computer models with experimental data. His work should be instrumental in designing new tanning agents that give leather characteristics like softness and strength without damaging the fiber.

"It's a well-defined starting point," Chen says of the experiments. "All these studies are matched with the modeling system to see what kind of answers we're getting."

Not only should this research give the leather industry more opportunities to expand the type and quality of leather products. It could also have medical implications. For example, an inheritable disease such as osteogenesis imperfecta, characterized by bone fragility, is due to mutations of fiber-forming collagen. If researchers knew the three-dimensional structure of collagen, they would have a clearer understanding of how these mutations affect it.

Hides That Don't Measure Up

Paul L. Kronick, a research chemist at the Philadelphia lab, has found a way to reduce the number of poor-quality hides that currently make their way into leather processing. Fifteen percent of these hides are rejected after tanning because they are not strong enough to be made into leather products.

He sorts the hides with laser light-scattering photometry, which evaluates the orientation of fibers in the hide.

Kronick says leather tends to be weaker when the hide's fibers run perpendicular to the hide's surface. Currently, the only way to detect the direction of hide fibers is by looking at them through a microscope.

Even with the aid of a microscope, Kronick says, it requires "quite a skill" to detect vertical hide fibers. Typically, fibers that crisscross at an angle of about 45 degrees are preferred by leather manufacturers.

"What the laser does it allow the hide buyer to identify the orientation of fibers before a tanner wastes money and time tanning the hide," Kronick says. "It clearly picks them out, reducing post-tanning rejections because of this problem."

Processors sometimes try to control the direction of fibers by stretching the tanned hide, he says. This pulls fibers in a horizontal direction and makes leather stronger, stiffer, and more suitable for products like leather straps.

The laser technique would be useful in computer-controlled manufacturing for indicating how much a particular piece of hide should be stretched, Kronick says.

To measure the average fiber direction in a piece of hide or leather, Kronick sends a laser beam through a thin slice taken from the sample.

The pattern of light coming through the slice is read electronically and sent to a desktop computer, which calculates the fiber angle. The results can be used to determine which hides to reject, Kronick says.

"The whole thing can be done with desktop computers," he adds. "It ensures that the hide buyer gets what is paid for."

PHOTO : Chemist Frank Scholnick (left) and research associate James Chen examine the results of experimental treatments for leather. (K-4388-1)

PHOTO : Research associate James Chen studies the stages in which collagen models interact to form larger microfibril units. (K-4387-17)

PHOTO : Chemist Eleanor Brown uses a nuclear magnetic resonance spectrometer to detect the molecular structure of collagen peptides. (K-4387-2)

David G. Bailey, James M. Chen, and Paul L. Kronick are at the USDA-ARS Eastern Regional Research Center, 600 Mermaid Lane, Philadelphia, PA 19118. Phone (215) 233-6585.
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Author:Kinzel, Bruce
Publication:Agricultural Research
Date:Jan 1, 1992
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