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APRILY
By Dr Mark Crisp and Ian Padley
A 30,000 tpy tissue mill in Northern Europe was making
a significant volume of towel on its twin-wire machine
for the German market. With a mixed virgin fiber and
recycled fiber content, it had a very high wet-strength
resin loading on the machine in order to meet exacting
wet tensile targets.
The system had been extensively optimized
over the years and employed a highly anionic material,
CMC (carboxy methyl cellulose), to boost wet strength
resin retention and improve tensile, a common practice.
Switching from Kymene 625 to Kymene 624 wet-strength
resin had given improved efficiency and a higher solids
product, but the move into the German towel market meant
an increased focus on BfR compliance for 1,3 DCP and
3- MCPD in the finished paper.
Laboratory analyses by Hercules
revealed that at best the mill could only achieve borderline
compliance using the existing chemistry. Reduction of
the wet-strength resin loading was not possible due to
quality constraints, and switching to the ultra-pure
Kymene G3-X wet-strength resin could not be considered
because of cost.
Further analysis revealed that the issue
was a high loading of 3-MCPD in the final paper towel.
Investigations by Hercules scientists had previously revealed
that the manufacturing process for Kymene 217LX wet-strength
resin could ‘strip out’ this
unwanted impurity. The product was known to deliver an
excellent cost-of-use profile and, if the science was correct,
it would also yield very low 3-MCPD content in the final
paper. The theory was discussed with the customer and a
controlled trial cautiously authorized.
The results confirmed
the earlier optimism of the scientists. The new wet-strength
resin had such low 3-MCPD content in the active polymer
that almost none was transferred into the sheet (see Illustration
1). Moreover, the high efficiency of the basic product
ensured a competitive cost of use. The application continues
to this day. The point is that a seemingly simple product,
often taken for granted, in fact represents the culmination
of years of accumulated technological experience and that
the application of sound scientific methodology still has
a place even in the manufacture of a product as familiar
as a basic
paper towel.
REGULATION AND EFFICIENCY
The current market for neutral pH curing wet-strength resin
for tissue and towel grades products is surprisingly dynamic
as chemical manufacturers balance the increasing regulatory
requirements with the need for cost effectiveness of their
products and the unprecedented pressure of raw material
costs.The product is a medium molecular weight cationic
polymer, which first attaches to the fibers by electrostatic
attraction in the machine wet-end. During drying, the polymer
can form chemical bonds directly with the fiber, or cross-links
with itself to form a reinforcing network of water-insoluble
polymer around and within the fibers. This typically gives
the finished paper a wet tensile strength of up to 30%
of the dry tensile value.
The basic chemistry of these products
is well known and little changed in the 50 years since
their development by Hercules (see boxed insert). The chemistry
is poly(aminoamide)- epichlorohydrin (PAE) and is made
by the reaction in a two step process. First, a dibasic
acid is reacted with a polyalkylenepolyamine to form a
low molecular weight pre-polymer. This pre-polymer is subsequently
reacted with epichlorohydrin to impart the reactive functionality,
and further develop the molecular weight upon which the
technology rests. Whilst epichlorohydrin is required to
develop the key functional and structural resins characteristics
required to impart wet strength to paper, it also results
in side reactions which give rise to the impurities 1,3-
dichloropropan-2-ol (1,3-DCP) and 3-monochloro-2,3-propandiol
(3-MCPD) that concern regulatory authorities.
| A Brief
History of Kymene® wet-strength
resins |
1956: |
Hercules
begins research programs to find a neutral pH
curing wet strength resin to compliment Aquapel® emulsion
sizing product. Initial investigations are based
on reaction products between polyalkylenepolyamines
and epichlorohydrin, but discontinued after reviewing
earlier patent technology held by American Cyanamid. |
| 1957: |
Investigation now based on the reaction products
between epichlorohydrin and the basic polyamide complexes
produced from polyalkylenepolyamines and dibasic
carboxylic acids. First PAE resins are produced in
lab and scaled up in the summer of 1957.First patent
application filed by Hercules for PAE technology.
Second patent filed in 1959. Both are granted in
1960. |
| 1958: |
Kymene® 557 wet-strength resin makes commercial
debut. First example of Generation 1 PAE resins.
Two years later Kymene® 557H wet-strength resin
(12.5% TS) is launched. |
| 1966: |
American
Cyanamid attempts to sue Hercules for patent
infringement. District Court finds in favour
for Hercules. The final judgement describes the
Kymene® wet-strength
resin as a ‘wholly unique invention’. |
| 1980’s: |
Suppliers
begin research efforts to develop products containing < 1,000
ppm of 1,3-DCP. In Europe Hercules engages the
services of the University of Kent to develop
a biologically based post reaction cleaning process. |
| 1990: |
Kymene® SLX wet-strength resin launched in
Europe – first example from Hercules of Generation
2 PAE resin technology (< 1,000 ppm of DCP). The
wet-strength performance characteristics are slightly
compromised with this first product offering (typically
90% of the performance, relative to Kymene® 557H
wet-strength resin). In the following 2–3 years,
Hercules launches Kymene® 617 wet-strength resin
in Europe and Kymene® 557 LX wet-strength resin
in North America which are examples of more efficient
Generation 2 PAE resins. |
| 1993: |
Kymene® ULX wet-strength resin launched in
Europe, an example of a ‘biodehalogenated’ PAE
resin developed by Hercules and the University of
Kent, UK. This process utilizes naturally occurring
micro-organisms to convert 1,3-DCP and 3- MCPD into
glycerol which is then consumed as food. This process
is able to reduce the levels of 1,3-DCP and 3- MCPD
in Generation 2 PAE resins to < 10 ppm. |
| 1999: |
Kymene® G3 wet-strength resin launched in Europe
- first example, from Hercules, of Generation 3 PAE
technology capable of meeting the limits for epi
residuals permitted in paper products being considered
by the BgVV. The process is based upon combination
of advanced chemical processing and the ‘biodehalogenation’ process
developed in Kymene® ULX wetstrength resin. This
initial product offering has lower wet-strength performance
relative to Generation 2 PAE resins, but research
efforts continue to address this, resulting in the
launch of Kymene® G3-X wet-strength resin in
2001, and Kymene® G3 X-Cel wet-strength resin
in 2004, which are products with wet strengthening
performance better than Kymene® SLX wet-strength
resin. |
| 2000: |
Kymene® 625 wet-strength resin launched in
Europe. This is a high solid (25%) Generation 2
PAE resin with performance characteristics equivalent
to Kymene® SLX wet-strength resin. In the 2 – 3
years that follow Hercules develops a range of
high solids (20-25%) Generation 2 PAE resins with
improved performance characteristics, culminating
in the launch of Kymene® 25 X-Cel wet-strength
resin in 2003. |
| 2005: |
Hercules
launches Kymene® 217LX wet-strength
resin, a high performance PAE resin with substantially
lower levels of AOX, in particular PBOX. |
In 1957, when the Generation 1 PAE wet-strength
resins were developed, the impurities, specifically the
residual by-products 1,3-DCP and 3-MCPD, were greater than
1%. In Europe, these residual compounds became the focus
of much scrutiny in the latter part of the 1980s for two
reasons: first, products containing more than 1,000 ppm
in the delivered product need to be labelled as ‘potentially
carcinogenic’; and ,second, 1,3- DCP was identified
a major component of the AOX that wet strength resins contributed
to the effluent streams from paper mills.
Scientists in
both North America and Europe were engaged in a research
effort to reduce the levels of the by-products to less
than 0.1%, with particular focus placed on 1,3-DCP. The
products that were developed were the first of a range
of low-AOX-containing products now referred to as Generation
2. This development relied on a revision of the basic Kymene® wet-strength
resin formulation and significant changes to the manufacturing
process in order to utilize the epichlorohydrin more efficiently.
Within a year, almost 90% of the European PAE wet-strength
resin market had switched to Generation 2 technology.
The
development of the Generation 2 Kymene® wet-strength resins allowed for a significant reduction in the AOX content in the effluent of paper mills. In the latter part of the 1990s, further attention was placed on 1,3-DCP and 3-MCPD but this time on the levels that were found to be present in paper products. Scientists devised techniques to reduce the 1,3- DCP and 3-MCPD to ‘non detect’ levels. This class of ultra clean PAE resin technology, capable of meeting the strictest limits being considered for 1,3-DCP and 3-MCPD in paper, is now known as Generation 3, and Hercules’ first product offering in this area was Kymene® G3
wet-strength resin.
In 2001, the German Federal Institute
for Consumer Health Protection and Veterinary Medicine
(BgVV) finalized its recommendations for the levels of
1,3-DCP and 3-MCPD to be found in the water extracts of
finished paper products. The newer higher solids Generation
2 wet-strength resins, such as Kymene® 625 wet-strength
resin being developed in a parallel program to Kymene® G3
wet-strength resin, were able to meet these recommendations.
This gave tissue makers an alternative to the Generation
3 wet-strength resin technologies coming to the market.
In
recent years, there has been more focus again on AOX in
mill effluent, and also on the total OX content in the
final paper products, especially in the German market.
In order to be classed as Total Chlorine Free (TCF) the
OX content of the paper product has to be <30mg/kg of
paper. Hercules scientists have worked to develop a product
that reduces the total AOX level of PAE resins to meet
the requirements of TCF qualification. This latest product
offering is called Kymene® 217LX
wetstrength resin.
EFFECIENCY NEEDS
In the latter part of the 20th century and the first years
of the new millennium, much of the attention of the chemical
industry’s scientific community was focused on making
PAE wet-strength resin compliant with regulatory demands.
During recent years we have seen sustained cost increases
in the basic raw materials for PAE resins (see Illustration
2) and at the same time increased demand from tissue manufacturers
to contain or even reduce costs.
The Hercules solution to this predicament
was to undertake further development programs resulting
in higher delivered solids and improved efficiency of the
resins. The increased solids have reduced the freight impact,
and meanwhile the improved reactive functionality of the
wetstrength resin has also allowed customers to reduce
dosage levels whilst maintaining the same wet-tensile effect.
The end result has been the potential for reduced cost
of use and less epi-residual loading in the papermaking
systems. Both of these are of great benefit to the tissue
maker.
However, achieving this was not straightforward,
as it is accepted that increasing solids and functionality
can lead to reduced product stability, especially at the
elevated temperatures found in southern Europe. Nevertheless,
recent years have seen a stream of new products with increased
efficiency from Hercules and other manufacturers. Hercules
has continually worked to minimize any less desirable product
characteristics and, through a succession of developments,
now has a Kymene® X-Cel wet-strength resin that is
both stable and highly efficient and is available in a
range of active solids up to 25%. The high solids, high
efficiency strategy was also combined with techniques to
reduce Organo-halogen levels to yield the ultra low AOX
wet-strength resin, Kymene® 217LX
wet-strength resin. TW
Dr Mark Crisp is senior research scientist
and Ian Padley is global segment manager Tissue & Towel
with Hercules Paper Technology and Ventures in Manchester,
UK
Illustration 2
PAE resin material
costs
• Ethylene amines started to increase at the start of the 21st Century
• Some easing but are expected to hold at the current high level
• Hurricane season resulted in additional tightness of the market in 2005.
Force majeure announcement released by major producers as a result of hurricanes
Epichlorohydrin
• Propylene and chlorine are the main feedstocks for ‘Epi’.
Record prices for propylene continued in 2005. Chlorine prices continue at very
high levels and are the drivers pushing ‘Epi’ costs
• Strong demand from China has also increased prices. Since 2002, Epi prices
have almost doubled Adipic Acid
• Very stable 2002-03 but began to increase in late 2003. The driver for
the increase is the dramatic rise in benzene costs
• Benzene costs of $1.20 to $1.35 per gallon in early 2003 have increased
to more than $4.00/gallon in Q3 2004
• Recent Hurricanes have caused Force Majeure to all major NA Adipic Acid
producers
• European suppliers have also declared Force Majeure several times in recent
years
• With no new capacity being announced and strong demand from China, the
situation is not expected to ease |
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