Page 333 - Handbook of Properties of Textile and Technical Fibres

306 Handbook of Properties of Textile and Technical Fibres

temperature ranges, and therefore cannot be used globally. After harvesting, the plants
remain in the field (spread out in uniform and thin nonoverlapping swaths) for the micro-
organisms to separate fibers from cortex and xylem. The plants are turned over on a reg-
ular basis to ensure homogeneous retting. The retting process should be monitored and
stopped at the right time to prevent degradation of cellulose by microorganisms; if this
occurs, it is called as overretting. Overretting reduces the mechanical performance of the
fibers while underretting makes further fiber processing difficult. Dew retting depends on
weather conditions and normally takes 3e6 weeks. There are several disadvantages of
this process such as poor quality of fibers compared with other processes, dirty dark fi-
bers due to contact with soil, and it is limited to geographical locations, uncontrollable
weather conditions, and occupation of land until the retting is complete (Natural fibers,
biopolymers, and biocomposites, 2005). Dew retting remains the choice of the farmers of
various plant fiber crops, due to low labor cost and being sustainable.

9.3.2 Stand retting
A modified field retting process was explored so as to overcome the limitations of dew
retting in Northern Ireland. A preharvest desiccant, glyphosate (N-phosphonomethyl
glycine), was used to facilitate retting. It was found that fungal spread and retting
were slower than dew retting, and in comparison the fibers had better strength when
the stand retting method was used (Mussig, 2010). Another stand retting method is
the thermally induced stand retting. The plant growth is terminated by open gas flames,

and the plant bases are heated approximately to 100 C. The plants dry in a couple of
days, which allows the replacement of weather-vulnerable dew retting. The risk of
weather and crop damage is reduced by using this method, but the cost of retting in-
creases (Natural fibers, biopolymers, and biocomposites, 2005).

9.3.3 Cold water retting

Conventional water retting involved steeping the fiber section of the plant in running wa-
ter such as rivers or streams. Later, this was replaced by water tanks that are sealed or
opened depending on the weather and the water change takes place every 2 days. This
process utilizes anaerobic bacteria that break down the pectin, and it is mainly employed
in Eastern Europe. Cold water retting normally takes 7e14 days but the duration largely
depends on the temperature of the water, the water type, and the bacterial function. This
retting process produces higher quality fibers than field retting. There are some draw-
backs of this process such as the high amount of water consumption, high cost of artifi-
cial drying after retting, environmental pollution from organic wastewater, and malodor
of fermentation gases. This process has been terminated in most parts of Europe due to
environmental concerns (Natural fibers, biopolymers, and biocomposites, 2005).

9.3.4 Warm water retting

This is an accelerated water retting process, which can produce clean and high-quality

fibers in 3e4 days. The water tanks are heated between 28 and 40 C. The process and