KEEPING WARM 
Thermographs recorded changes in 
heat distribution as a pair of chilled 
human hands, above, regained nearly 
normal warmth, right. Color changes 
occurred at intervals of about four 
degrees Fahrenheit, and darker colors 
indicate lower temperatures. 
perature in a room where the ambient 
air temperature is also precisely con- 
trolled, scientists can measure the insu- 
lation capability of the clothing put on 
the “copper man.” 
Improvements in infrared scanning 
cameras also make it possible to take 
photographs of the “heat leaks” in var- 
ious items of clothing. These cameras 
create ten-color images, with each color 
representing a specific temperature 
range. Such photographs may reveal 
significant differences in the protection 
offered by two seemingly similar items 
of clothing. With a computer coupled to 
such a camera, a quantitative assess- 
ment can be made of the total heat leak 
shown in the various areas (at various 
temperatures) in the photographs. 
Yet another tool that is becoming 
more and more useful in the evaluation 
of clothing materials is the scanning 
electron microscope, which examines 
the textures of fiber surfaces in precise 
detail. That, in turn, provides clues to 
the ways in which two apparently simi- 
lar materials may vary considerably in 
their ability to trap air and thus in their 
insulation properties. 
Given two insulation materials having 
fibers with the same surface features, 
the one with the finer fibers provides 
more surface area and hence a greater 
proportion of still air. With two materi- 
als of the same fiber diameter, the one 
possessing small surface discontinuities 
on the fiber will present the greater 
surface area and thus a higher propor- 
tion of still air. 
One way to examine the effect of 
surface area is to look at some compara- 
tive dimensions. Fiber length (or, in the 
case of feathers, vane length) per gram 
is the pertinent measure. Synthetic fi- 
bers in modern insulation materials may 
contain as much as a mile of fiber length 
per gram. By contrast, feathers might 
yield eight miles of fiber length per 
gram and high-quality down thirty-two 
miles per gram. 
A comparison of feathers with down 
demonstrates the insulation limitations 
imposed by the feather’s quill. A quill 
can represent 25 percent of the feather’s 
total weight and provide very little sur- 
face area. In eiderdown, on the other 
hand, the comparable portion of the 
structure that contributes little to its 
insulation is only 8 percent of the total 
weight. Feathers and down also differ 
markedly in structure. Feathers are two- 
dimensional, with vanes emerging on 
opposite sides of each shaft. Down, on 
the other hand, is three dimensional. 
Small wonder, then, that feathers are 
more apt to pack and compress, while 
down can be fluffed and tends to remain 
fluffed up. 
Among the natural fibers used in 
clothing, particularly for warmth, wool 
offers many desirable characteristics. 
Fine wools, with a fiber width of 17 
microns (about 0.0007 inch), present a 
large surface area, and wool fibers pos- 
sess a wavy structure, or crimp, that 
serves several useful purposes. Crimped 
fibers interlock more readily, so that 
they hold together when twisted into 
yams. More important, the crimp yields 
a strong, not easily matted yarn without 
much twist — just what is needed to pro- 
duce a porous, low-density structure for 
insulation. 
In designing clothes for warmth, the 
synthetic-fiber industry cannot be over- 
looked. Considerable progress has been 
made in developing materials that begin 
to duplicate some of the desirable char- 
acteristics of natural fibers. Polyester 
filaments — fibers made of a long-chain 
synthetic polymer — can be produced 
with fine diameters, and can also be 
crimped to provide lofty yarns. Yet an- 
other refinement is hollow polyester fi- 
bers, which offer significantly greater 
surface area per unit of length or weight 
than solid fibers because they are hol- 
low. For example, DuPont’s Hollofil 
(also marketed under the name Hollo- 
bond) is an insulation material consist- 
ing of hollow polyester fibers that are 
staple, or short cut (as opposed to the 
continuous-filament synthetic materi- 
als), and have an average length of 2.5 
inches and a diameter on the order of 23 
microns (about 0.0009 inch). With such 
short cut fibers, the batting must be 
quilted to keep the fibers from migrat- 
ing. 
Although quilting has the disadvan- 
tage of producing areas of reduced insu- 
lation because of compression, it has the 
advantage of assuring that the fibers 
will be kept in proper distribution dur- 
ing laundering. Maintaining a uniform 
distribution of insulation material within 
a garment is important. Even the best 
and most expensive insulation materials, 
such as high-quality down, become rela- 
tively useless if they become matted (as 
94 
