By Rogério Berardi and Thomas T Scherb
The main tissue products
in terms of worldwide
consumption are bathroom
tissue and kitchen
towel. The most important physical properties are bulk (volume)
and softness for bathroom tissue while water absorption capacity
is the most important property for kitchen towel paper.
Bathroom tissue and
kitchen towel can be divided into three quality categories according
to the global requirements of different markets: standard, intermediate
and premium as shown on Graphs 1 (bathroom tissue) and 2 (kitchen
towel). The quality categories are produced on different technologies,
such as dry crepe, single recrepe, TAD and ATMOS as well as wet
crepe technologies.
Standard Quality
The standard tissue quality is produced on dry crepe tissue machines
as shown on Picture 1. This technology has been available in the
market for many decades, normally as Crescent formers or Duoformer
machines with 1 or 2 presses, as well as shoe presses against
the yankee cylinder and a standard reeling system.
The main factors that influence quality generation in this concept
are the flat sheet
formation and pressing
of the sheet followed by the dry creping at the yankee. The major
limitation is the fact that 100% of the sheet is pressed during
yankee transfer in order to maximize solid contents before thermal
drying. The result is pressed fibre and a compacted sheet while
on the other hand the high solids allow less thermal energy requirements
for drying. An important advantage of the standard dry crepe process
is the flexibility of running different raw materials, from bleached
virgin to recycled fibers.
Premium Quality
The premium quality is the best tissue quality available in the
market and is produced on through air drying (TAD) and ATMOS machines
(Pictures 2 and 3).
TAD technology for structured tissue products: For more than 30
years, premium tissue
quality was reserved
exclusively for TAD
technology machines. Here, the main principles of TAD technology
in respect to quality generation is to transfer an originally
flat formed sheet into a structured fabric at low solids (typically
around 23%), generating a tri-dimensional structure by vacuum
application and a wet crepe effect resulting from a speed difference
between outer forming wire and structured fabric. The structured
tissue is dried mainly with hot air on the TAD drum and transferred
to the yankee with only a minor part of the sheet area being pressed.
Some 75% of the tissue is protected in the structure of the fabric,
avoiding fibre and sheet compaction during transfer to the yankee.
TAD technology was developed over the years, such as the P&G
TAD technology, the UCTAD from Kimberly Clark, the TAD webs with
one or two TAD drums.
The low mechanical dewatering participation in the overall drying
process results in high thermal energy consumption on the air
system used in the TAD drum(s). The process is considered to be
of high operational complexity. Furthermore, TAD is not known
to operate efficiently with a fibre mix using more than 25% standard
quality recycled fibre.
ATMOS - Forming on a structured fabric: Since 2006 premium tissue
has also been produced with ATMOS technology and since the end
of 2007 is freely available in the market. With this concept,
quality is generated using a crescent former configuration of
an outer wire and inner structured fabric. The tissue is mechanically
dewatered to 35-40% solids by combined vacuum and low pressure
field application in the ATMOS module. The sheet is carried by
the structured fabric from the headbox to the yankee, avoiding
tensile losses of fabric-to-fabric transfer. As with TAD technology,
only a minor part of the sheet surface area is pressed during
the yankee transfer, so 75% of the sheet remains protected in
the structure of the fabric.
Commercially, the ATMOS technology proved to be operationally
simple and robust, allowing a high level of flexibility for grade
changes over the complete range from premium to standard tissue.
The high solids allow considerably lower energy consumption values
than TAD machines.
Besides, ATMOS machines can easily swing between premium, intermediate
and standard qualities by changing a coarser or finer mesh structured
fabric to a classical crescent former, allowing the tissue producer
to adapt to regional market quality requirements. As a final benefit,
the ATMOS technology operates efficiently on virgin fiber as well
as on 100% recycled fibre.
Intermediate Quality
Historically, 'intermediate' tissue qualities have rarely been
found, as only few technologies can narrow the quality gap between
standard dry crepe and TAD. The technology known to produce the
intermediate quality category is the single recrepe (SRC), basically
a dry crepe standard technology using two yankees in line. Few
industries have resorted to SRC and this technology is well protected
by patents.
Wet crepe technologies: In recent years, technologies have been
developed to produce intermediate tissue quality based on the
principle of wet creping. The tissue sheet is formed between flat
fabrics and pressed 100% using shoe press technology to achieve
solids above 40%. During pressing, the sheet is transferred to
a transfer element (heated or nonheated roll, transfer belt, etc).
The sheet is wet creped into a structured fabric by running a
negative draw between the transfer element and a structured fabric
before being transferred to the yankee cylinder for thermal drying.
The wet creping effect is to be considered the main tool for quality
generation. A tri-dimensional structure is created, but the effect
is limited as it is generated at relatively high solids and as
fibres and base sheet have been highly compacted during pressing
before and have lost much of their absorptive capacity. Besides,
wet creping at high solids breaks up fibre bonds and reduces the
tensile strength of the finished product. The operational complexity
of the process, the necessity to transfer the sheet from a transfer
element with a smooth surface into a structured fabric and running
speed differentials are the major challenges for runnability,
fabric lifetime and overall process efficiency. The higher solids
achievable through high mechanical dewatering can help lower energy
consumption for a structured product near to a level achievable
with standard dry crepe. It is still open to question whether
reduced process efficiency counterbalances that effect.
ATMOS for intermediate qualities: An alternative to produce intermediate
tissue quality is to operate an ATMOS machine with fine mesh structured
fabrics. The result is a structured paper offering quality between
premium and standard dry crepe. The finer mesh of the fine structured
fabrics increases the intimacy between the fabrics in the ATMOS
module, allowing better dewatering to dryness levels typical for
standard dry crepe machines, and this results in structured tissue
production with lower overall energy consumption.
Apparently, the world of tissue quality which, in the past was
polarized into TAD
premium and standard dry crepe qualities, is changing rapidly.
New developments are allowing alternatives for premium and intermediate
tissue quality production, giving tissue producers the chance
to operate an optimum quality/cost ratio in their market region.
These developments are driven by the tissue producers' need for
regionally differentiated product innovation as well as by fibre
cost increases and sustainability issues, not the least of which
is the reduction of energy consumption. TW
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Rogério Berardi (left)
is responsible for Tissue Technology Marketing and Thomas
T Scherb is General Manager R&D Tissue, both with
Voith Paper.
