Cotton in compressed form is taken from bales and is placed in opening hoppers where it
is fed between lattice aprons that contain sharp, large- diameter spikes to begin the
process of breaking up the batt of cotton into tufts to facilitate further processing
and removal of trash. After this initial opening, the cotton typically passes through
one or two additional stages of opening and cleaning in preparation for washing. The
first mechanical cleaner that is almost always used is a "step" cleaner that uses air
to convey the cotton through a series of rotating, spiked cylinders to remove extraneous
material (mostly plant trash). The beating action of the rotating spikes loosens the
heavier trash particles from the lint, and the particles then fall from the air stream
to the waste chambers. Often a second stage of opening and cleaning is used to further
open the cotton stock and remove small trash particles. Machines used in this second
stage usually employ rotating cylinders containing saw teeth. This is a very vigorous
mechanical treatment that can damage fibers and adversely affect cotton processing
quality. Consequently, this second stage of cleaning is usually used only when the
finished washed cotton must pass the USP ash test. Thus, mechanical processing is used
to remove physical impurities from the cotton and to produce small fiber tufts
(1 to 3g) to provide for effective wetting and packing in the cakemaking step. In
the two effective batch kier washing trials described in the body of this report,
mechanical opening and cleaning were accomplished by use of a spiked opener and a step
cleaner in the first trial [Perkins and Berni 1991] and by use of a spiked opener, a
step cleaner, and a fine-cylinder cleaner in the second trial [Perkins and Olenchock
1995].
The small cotton tufts produced in the mechanical process are conveyed by air to a
cakemaking device designed to produce a doughnut-shaped cake of cotton that is
thoroughly wet and packed to controlled, uniform density. This is accomplished by
spraying the incoming tufts with a warm solution of water plus a wetting agent as
they fall into the cylindrical container of the cakemaker. The spray is directed
so that solution that does not contact the cotton tufts falls into the container.
The entire container rotates slowly, and a mechanical tramper packs the wet cotton
to produce the uniform cake. The amount of wetting solution used and the packing
intensity are carefully controlled to produce the desired cake density. The prewetting
during cakemaking is a key step to ensure that the cotton will be thoroughly wet even
in the first cycle in the kier wash step.
Three cakes of prewet cotton are added to a kier for washing. The individual integrity
of the cakes is maintained by separation of the cakes with perforated stainless steel
discs. Kier capacity varies from about 1,500 to 2,300 lb of cotton (dry weight). In
the washing cycle, washing solution (water plus wetting agent) at the selected
temperature is pumped through the cakes from the inside to the outside. The solution
passes through the cakes in the kier to an outside expansion tank and then back through
the cakes. The solution is then dumped into the sewer; a new, clean washing solution
is added to the kier; and the process is repeated. The number of cycles of new washing
solution and the cycle times depend on preselected washing conditions. After the
required number of cycles with washing solution, clean water is used in a number of
rinsing cycles to complete the removal of dissolved impurities and residual wetting
agent. The fact that the wash and rinse solutions in each cycle are not reused results
in maximum removal of dissolved impurities.
The centrifuged cakes are broken up mechanically by a series of large-diameter spiked
cylinders to produce a batt of cotton that then passes through a
tunnel dryer. Heated air, usually in the temperature range of 125° to 150° C, is forced
through the batt to accomplish drying to a preselected moisture content range, usually
about 4% to 8%. After drying, the cotton is fed through a mechanical opener-cleaner to
remove residual trash and to break up the dried batt of cotton for feeding to the bale
press for compression and packaging. The finished, covered bales are weighed (average
weight is about 500 lb) and tagged for identification as appropriate.
Most of the currently envisioned potential end uses for 100% mildly washed cotton would
probably involve novelty or niche markets but could potentially result in significant
expansions into other markets. Advantages of mildly washing raw cotton extend beyond
the potential for achieving compliance with OSHA exposure limits and protecting worker
health. Specifically, while washing generally adversely affects cotton textile
processing, mild washing may actually enhance the textile processing characteristics
of some cotton (e.g., "sticky" cotton). It may also enable value-added special effects
(e.g., differential dyeing characteristics for novelty yarns and improved quality of
rotor-spun yarns). Potential use of mildly washed cotton in cotton and cotton-synthetic
yarns containing only a small proportion of washed cotton would also be possible, but
(as with any
potential application of washed cotton) washing-associated costs and benefits will need
to be considered on a case-by-case basis and may prove limiting.
The progression from an initial acute and reversible pulmonary effect to an eventual
chronic and irreversible effect has been implicit in clinical descriptions of byssinosis
and is a key underlying assumption of the OSHA cotton dust standard [43 Fed. Reg. 27351
(1978), 50 Fed. Reg. 51120 (1985)]. Yet as recently as 1986, it was emphasized that "a
clear relationship between acute and chronic respiratory disease in cotton workers has
not been established, and a prospective study is necessary to investigate this relationship"
[ASPH 1986]. Such a study has recently been completed, and the results of this large
prospective study of 1,664 U.S. cotton textile workers indicate a significant association
between acute (across-shift) decline and longitudinal decline in lung function
[Glindmeyer et al. 1994]. This is an extremely important observation with respect
to results of the cotton washing studies because it supports the contention that the
acute respiratory response measured in washed cotton studies is a valid predictor
for the chronic hazard potential of cotton dust.
The search for a measure of the acute and chronic respiratory toxicity of cotton
dust more specific than airborne gravimetric dust concentration has increasingly centered
on bacterial endotoxin. Several studies have documented an association between endotoxin
concentration and respiratory symptoms of exposed individuals; even more clearly, they
have demonstrated a relationship between endotoxin and across-shift FEV1 decrement among
humans exposed to cotton dust [Rylander and Haglind 1983,
1986;
Castellan et al. 1984,
1987;
Rylander et al. 1985]. The most definitive findings were reported by NIOSH
investigators, who observed a clear exposure-
response relationship between mean FEV1 response and endotoxin concentration
(P<0.00001), though dust concentrations from the same set of exposures were not
correlated with FEV1 change (P=0.43) [Castellan et al. 1987]. All 51 exposures
above 50 ng/m3 endotoxin resulted in statistically significant mean FEV1 responses,
whereas none of the eight exposures below 10 ng/m3 endotoxin did so, and a linear
regression model based on the observed data predicted a "threshold" at approximately
9 ng/m3 for the FEV1 response [Castellan et al. 1987]. (As discussed above, because
of differences in endotoxin extraction and assay methods between laboratories,
results of endotoxin measurements from other laboratories may not be directly comparable
to this "threshold.")
Although these experimental results do not by themselves prove that endotoxin is
causal, the very clear exposure-response relationship between airborne endotoxin
concentration and acute decline in FEV1 is very unlikely to have been observed
unless a predominant causal role is played by endotoxin or some other cotton dust
component (or components) in a concentration that closely parallels that of
endotoxin. On the basis of this exposure-response relationship, NIOSH concluded
in a letter to OSHA that although "it is not now possible to offer a definitive
opinion regarding chronic health effects, ... airborne endotoxin is a valid
surrogate for the level of acute respiratory hazards of cotton dust" [Niemeier
1990].
The demonstrated relationship between acute and chronic respiratory responses to cotton
dust [Glindmeyer et al. 1994] and the demonstrated relationship of acute respiratory
response and endotoxin [Rylander and Haglind 1983,
1986;
Castellan et al. 1984,
1987;
Rylander et al. 1985] together offer a basis for accepting the endotoxin measurements
made during the washing studies as a surrogate for the chronic respiratory hazard of
cotton dust, as well as for the acute respiratory hazard. Additional evidence for
considering endotoxin inhalation a risk factor for chronic lung effects is provided
by other studies that have demonstrated quantitative relationships between chronic
respiratory effects and exposure to airborne, endotoxin-contaminated organic dust.
These studies have involved textile mill workers [Kennedy et al. 1987;
Sigsgaard et al. 1992],
Dutch animal feed mill workers [Smid et al. 1992], and workers in the swine
confinement industry [Zejda et al. 1994], an occupational setting in which an
exposure-effect relationship of airborne endotoxin exposure with across-shift FEV1
decrement has been reported [Donham et al. 1988;
Heederick et al. 1991].