STT® for high-end
tissue
To counter rising costs of wood fiber and energy,suppliers
have developed new technologies thatmaintain or even improve
functionality and qualitywhile allowing for reductions in
basis weight that cansignificantly save costs. The reduction
in basis weight for agiven end product not only saves fiber
but also results in areduction of energy usage.
One such development is the new STT® technology, theresult
of the partnership between Metso Paper, Hercules andAlbany
International.This technology, based on Metso Paperpatents,
has now been in commercial operation since the endof 2005 and
is producing results that could previously beachieved only
with TAD. STT® offers lower capacity thanTAD. It is considered
appropriate for a developing market,where the needed capacity
is below 35-40,000 tpy. The relativeease of rebuilding an older
machine and thus upgrading the product performance offered
to the market by the machine is a major advantage of STT® technology.
Sheet conditions exiting the Yankee press nip determinewhether
a conventional or structured paper is produced. Ina conventional
machine, with a felted nip designed to subjectthe web to uniform,
high pressure, the result is a thin, strongand compact web
uniformly glued to the Yankee cylinder.The press nip generally
gives dryness after of around 40%. The high heat transfer from
the Yankee cylinder, incombination with the high temperature
Yankee Hood makesthe rest of the drying.
Developments have been made in an attempt to improveconventional
pressing by replacing a double press systemwith a long nip
press, for example a shoe press or the morerecent development,
a ViscoNipTM press. The idea behindthese technologies is to
reduce sheet compaction withoutsignificantly decreasing sheet
dryness.
The most widely used technology for producing structured tissue
today is through air drying (TAD). In machines making structured
paper, the sheet is transferred to the drying sectionon a particular
type of fabric. The sheet is dewatered beforereaching the Yankee
cylinder using vacuum and steam as well as TAD or other pre-drying
processes.
In the STT®process, the sheet is molded by being embedded
in theSTT® fabric. The final product mirrors the pattern
of the fabric. The STT®fabric, developed in cooperation
with Albany, has an open structureoptimized to enhance sheet
properties as well as improve the dryness characteristics of
the base sheet. The sheet is exposed to vacuum anddewatered
in this molding process in which the three-dimensional structureof
the sheet is created.
The sheet is then transferred to the Yankee cylinder. This
is done gently without compacting the sheet as is done in a
conventional feltednip, so as to retain high bulk and flexible
sheet properties.
Advantage™ STT® is a patented concept capable of
producing the fullrange of conventional and structured products.
The production of eitherconventional or structured tissue can
be altered back and forth through arelatively small machine
modification. This machine is targeted at producersseeking
to gradually expand emerging structured product markets.
In conventional mode the machine operates as an Advantage "C" wrap.
To convert to structured mode requires about 12 hours of downtime.
Thefelt is replaced by an STT® fabric. The suction pressure
roll is exchangedwith a solid pressure roll. The vacuum boxes
are raised to engage thefabrics. The Yankee spray system is
exchanged for the STT® system,tailored for the new chemical
package. This STT® coating package hasbeen developed in
cooperation with our partner Hercules. Yankee doctorsare specially
designed to meet the requirements of the STT® process.
The AdvantageTM STT® concept is suitable for making all
tissueproducts, such as bath, facial and towel, of different
basis weights. It canalso be used for rebuilds. The new equipment
is installed in the formerand press section. The framing in
the press is modified tomake installation of the STT® fabric
easy and safe.
To increase drying capacity, a new Yankee cylinderand wet end
AdvantageTM AirCapTM can be installed. Are build case is an
opportunity to use old assets to make high quality structured
products. The rebuild can also be done in such a way that the
machine is capable of swinging between Structured and Conventional
Tissue modes.
Crescent former configurations can also be rebuilt into the
AdvantageTM Advantage STT® concept for makinghigh quality
structured tissue products.
The AdvantageTM STT® concept makes a high quality structured
sheet. Both TAD and STT® products are superior to conventional
products. Absorption and bulk are significantly higher, which
allows tissue makersto reduce basis weight, usually with about
a 15-20% reduction. This givesa significantly lower fiber cost
per customer tissue roll. Basis weightreductions also positively
affect energy and chemical costs.New concepts should not only
make better quality products, but should also reduce manufacturing
costs. The STT® concept consumes more energythan a conventional
product if compared per ton. However, with STT®bulk is
significantly higher, giving finished product production cost
equalto or less than that of conventional product. TW
