The choice of the right wet strength agent depends mainly on which type of process is currently in use for the paper production. Under neutral und slightly alkaline conditions, the best wet strength agents are the polyamidoamineepichlorohydrine (PAAE) resins, which provide the highest level of wet strength permanence.
Today, PAAE resins are used in practically all types of wet strength papers, including household products such as paper towels, napkins and tissue; packaging materials as liquid and food packaging, corrugated boxes and paper bags; and specialties as industrial filters, decorative laminates and label papers.
Though the chlorine bleaching process mainly contributes to the AOX pollution of paper mills effluents, PAAE resins with a high level of DCP (1,3-dichloropropane-2-ol) and MCPD (3- monochloropropane-1,2-diol) are also AOX contributors. Therefore considerable efforts have been performed, in order to develop environmentally friendly wet strength resins with a reduced level of DCP and MCPD.
Over 20 years at the edge of the wet strength research, BK Giulini offers a wide variety of PAAE resins. GILUTON wet strength agents comply with the high quality requirements of papermakers for tissue, laminates, wall and filter paper production.
Formaldehyde resins
Urea-formaldehyde (UF) and melamine-formaldehyde were the first synthetic polymers which reached commercial success in wet-strength paper application (graph 1). Both resins had been developed in order to fit the traditionally acidic conditions in the papermaking industry. However, in the mid 1980s the usage of these polymers has declined significantly due to the need for applications in neutral medium and environmental concerns about formaldehyde.
Formaldehyde emission from urea-formaldehyde resins, during manufacture of wet strength paper upon curing in the dryer section as well as from the finished paper products, contributes to formaldehyde exposure at the workplace. The U.S. Occupational Safety & Health Administration (OSHA) decided therefore to regulate formaldehyde concentrations at the workplace and in the products sold to consumers.
Due to the concerns on the use of urea- and melamineformaldehyde polymers and the advantages of papermaking under alkaline conditions, another type of wet strength agents with different monomers has been developed.
PAAE resins
The first alkaline-curing wet-strength resins to become commercially available were the polyamidoamineepichlorohydrine (PAAE) resins. These gained immediate acceptance and rapidly started to replace urea-formaldehyde resins in many applications. PAAE resins offer numerous advantages, as the possibility to use less expensive fillers such as calcium carbonate, reduced machine corrosion and the compatibility with alkaline sizes.
PAAE resins are cationic and are thermosetting at near neutral and alkaline pH conditions. The development of wet strength in paper by using PAAE resins can be explained with two mechanisms. The first is referred as homo-crosslinking mechanism where the water-soluble PAAE resin is adsorbed by the cellulose and forms a crosslinked network when the paper is dried. When the paper next comes into contact with water, rehydratation and swelling of the cellulose is restricted by the resin network. The second mechanism imparts on the cocrosslinking of the PAAE resin with the fibres upon covalent bonds, which provides permanent wet strength.
The PAAE resins are produced by the reaction between adipic acid and diethylene triamine, and subsequent derivatization of the resulting pre-polymer (polyamidoamine) with epichlorohydrine. Graph 2 shows the condensation reaction of diethylene triamine with adipic acid during the synthesis of polyamidoamine. The reaction with epichlorohydrine creates highly reactive azetidinium groups along the polymer chain. The supposed intermediate structure of these highly reactive groups is shown on graph 3. These cationic azetidinium groups play the major part in the homo- and co-crosslinking mechanisms, which confer wet strength to paper.
Environmental concerns
Due to the high reactivity of epichlorohydrine, the reaction between polyamidamine and epichlorohydrine does not take place quantitatively. The synthesis of the PAAE resins gives also rise to a small amount of side products such as DCP (1,3- dichloropropane-2-ol) and MCPD (3-monochloropropane-1,2- diol).
DCP an MCPD are generated upon hydrolysis of epichlorhydrine (graph 4). DCP and MCPD are products of low molecular weight, also classified as AOXs (adsorbable organic halides). The AOX level is measurable in accordance with various standards (DIN 38409, SCAN-W9:89, ISO 9562). As DCP is recognized to be genotoxic and carcinogenic, its presence in pulp and paper effluents had to be limited due to the concerns of the regulatory authorities.
The legal framework for water management in Germany is stipulated by the Federal Water Act which regulates waste water generated by various industrial processes. In these Act, the minimum requirements concerning waste water treatment, requirements concerning techniques of analysis and monitoring as well as limits for the content of specific substances are laid down for different industries. Therefore the paper industry has to respect different treshold values in order to conform to the german law. The actual treshold values for the AOX concentration is 100 g/l and the annual authorized freight is limited to 10 kg.
In the mid 1980s the neutral curing wet strength agents were drawn into the general discussion concerning the effects of AOX pollution to the environment. Their concentration in the waste waters of the paper mills had to be limited. All wet strength agents with a DCP content of more than 1000 ppm had to be labelled as “potentially carcinogenic” from then on. The pressure from the lawmakers led to the market introduction of the 2nd generation of neutral wet strength agents, whose content of residual epichlorohydrine and its hydrolysable products DCP and MCPD was greatly reduced. The graph 5 shows the development of DCP and MCPD in wet strength resins from the 1st to the 3rd generation.
The German Federal Institute for Risk Assessment (BfR) published therefore recommendations for the assessment of the health risks of paper and paperboards that come into contact with food products. According to the XXXVI recommendation, papers that come into contact with food are to be produced in compliance with good manufacturing practice in such a way that, under normal or foreseeable conditions of use, they do not transfer their constituents to foods in quantities which could endanger human health.
In order to respect this recommendation, DCP may not be detectable in the paper aqueous extract. The DCP concentration should therefore not exceed 2 g/l. Concerning MCPD; it should be as little present in the aqueous extract as technically possible. On 17th October 2002, the BfR published an upper limit for MCPD of 12 g/l in water extract of paper products.
The GILUTON wet strength agents are high efficient PAAE resins, which comply with the requirements of papermakers in terms of quality and environmental issues. Graph 6 presents the properties of three different GILUTON resins, from the 2nd to the 3rd generation. Wet strength resins of 3rd generation as GILUTON 1100/37 N are particularly valuable for the manufacture of paper for medical, cosmetic, or food contact uses.
There are clear differences between the GILUTON resins and products of 1st generation from local producers. For example, one competitor product which is currently widespread on the Indian market has been compared to GILUTON resins in terms of DCP and MCPD values. Though the competitor product has the same dry content as GILUTON 1100/28 N, it contains about 17 times more DCP than GILUTON 1100/28 N!
Contrary to competitor products, the chemical composition of all GILUTON wet strength resins corresponds to recommendation XXXVI of the BfR (www.bfr.bund.de) and complies with the requirements of the RAL environmental mark for paper(www.blauer-engel.de), the Nordic Swan Label (www.svanen.nu) and with the requirements of the FDA regulations 21 CFR §§ 176.170 and 176.180 (www.fda.gov).
Conclusions:
Today polyamidoamine-epichlorohydrine resins (PAAE) of 2nd and 3rd generations constitute the State of the Art in order to comply with the requirements of papermakers in terms of high quality and environmental issues. The conformity of the products to FDA and BfR represent a real guarantee for the customers, as these products can be used for production of paper or tissue with food contact. Moreover the conformity of wet strength resins to the RAL and Nordic Swan labels permit the production of papers which will obtain ecolabelling.
