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Manufacture of temporary wet-strength (TWS) papermaking additives based on renewable resources (ACETAL)

With this project launched on the 1st of April, 2021 and sponsored by the Federal Ministry for Economic Affaires and Energy within the scope of the INNOKOM funding programme „Market-oriented research and development”, PTS pursues the goal to study the production and application of polysaccharide- based products as reversible wetstrength agents for the paper sector. This project is furthermore intended to make a meaningful contribution to achieving sustainable production of papers using such additives and their recyclability.

Project title: » Development of bio-based wet-strength additives as an effective way to substitute conventional additives in papermaking

Project period: » 01/04/2021 – 30/09/2023

Project type: » IK-MF 200135

Research locations: » Papiertechnische Stiftung (PTS),

project leader: Dipl.-Chem. Stefan Möckel

The state of the art mainly comprises permanent wet-strength additives based on formaldehyde resins and amine epichlorohydrin resins that cause problems for the repulping of mill broke or of recovered paper containing such additives. Known concepts for achieving a temporary and reversible wet strength and thus easier disintegration of wetstrength paper products involve the use, for example, of cross-linking aldehydes such as glyoxalated polyacrylamide (g-PAM). The underlying chemical mechanism is based on aldehyde groups capable of forming acetal groups with the hydroxyl groups of cellulosic fi bres, wherein the formation of said acetal groups is reversible.

However, one disadvantage of g-PAM is that it is almost always available in diluted solutions only, because it tends to self-crosslinkage and would otherwise not be storable for a longer period. In most cases, fresh production in paper mills fails because they would need to use glyoxal, which is a mutagenic chemical, and are not normally equipped to the required degree, neither in terms of equipment, nor in terms of personnel.

One specifi c way to overcome said challenges is to use additives forming a temporary wet strength based on renewable resources that show either no or reversible homocrosslinking.

Use of aldehydic polysaccharides for making temporary wet-strength paper products

Aldehyde groups are formed by chemical conversion of polysaccharide base materials under oxidative glycol cleavage mechanisms. As described above and shown in Fig. 1, the aldehyde groups can react with cellulosic paper fi bres to form reversible acetal groups and thus can be applied as temporary wet-strength (TWS) agents.

Fig. 1: Generation of reversible wet strength by “acetal formation”

In comparison with g-PAM, polysaccharide- based products not only are produced from renewable resources according to aspects of green chemistry, but also are safe and harmless to health. As of the project launch, it is also expected that they can be stored in a solid form and can be added to production as freshly prepared solutions or as solutions having a longer shelf life. Furthermore, the wet-strength additives to be developed during the course of the project will be subjected to benchmark tests against conventional temporary and permanent wetstrength agents in terms of possible applications, mode of action and disintegration behaviour on a bench and a pilot scale.

So the potential of the new additives can be assessed and may allow for the substitution of part of the currently permanent wet-strength agents in selected segments. PTS has the vision to improve the recyclability design of wet-strength papers and close another section of the secondary-fi bre circuit, regardless of any impurities, because it would no longer be necessary for selected wet-strength papers to be discarded via the waste stream.

Developments based on accordingly modifieds tarch were already described back in the 1970s, but their application on a broader scale was not possible, because their production costs were higher than those of conventional additives at the time and established production processed did not exist. However, especially since the turn of the millennium, there is a growing trend driven by more efficient automatic controlling systems and improved electrode materials towards the generation and regeneration of inorganic and organic chemicals by electrochemical processes.PTS therefore wants to make use of the progress of these principles during the life of this project.

Innovation potential of the electrochemical generation of aldehyde-terminated polysaccharides

The mechanism for the generation of aldehydic polysaccharides is based on stoichiometric conversion with periodate. It involves an oxidation process on the polymer chains, which causes the ring to be opened and aldehyde groups to be formed. Periodate is reduced to iodate and obtained as a by-product. Considering the price of periodate and the required disposal of iodate-containing waste liquor, the production process would be extremely expensive.

Fig. 2: Production of aldehydic polysaccharides inclusive of coupling the electrochemical chemical regeneration

A possible way to overcome this aspect is to couple the generation of polyaldehydes as shown with an electrochemical iodate/ periodate regeneration. That is illustrated schematically in Fig. 2 and is the core of the current project. Especially, the fact that the electricity available in Germany and required for the electrochemical recovery of the periodate is more and more generated from sustainable sources increases the ecological balance of this approach. Preliminary studies prior to the application for the research project have already confi rmed the feasibility of the idea in principle and do suggest that the manufacturing cost after appropriate process optimisation will be in the range of that of conventional wet-strength agents.

Stefan Möckel,