How to prevent the impacts of wet strength additives on the papermaking process: Clogging felts and wet strengthened paper recycling

The high quality of some paper products is directly correlated to the wet strength property of such materials. Based on the purpose of the final product – whether tissue paper, paper towel, paper for liquid packaging, and so on – wet strength agents must be added in different amounts to the papermaking process to achieve appropriate characteristics.

Carla Veríssimo - R&D Manager - Contech

A number of chemical products have been used to impart wet strength to paper, including mainly polymers based on polyamide epichlorohydrin, polyamine epichlorohydrin, and polyamide-amine epichlorohydrin (PAE). The application of such additives to paper manufacturing causes the occurrence of contaminants with hydrophobic surfaces and the need of specific chemicals for the cleaning treatment. It goes without saying that accurate contamination control programs are required in order to avoid deleterious effects on the process caused by such contaminants that often result in non-programmed stops and production delays with an increase in the cost of the paper.

IN THE EVENT THAT WET STRENGTH AGENTS DO NOT BECOME FIXED ON THE PAPER SHEET, the residual resins in the aqueous fiber suspension will be pulled into the felt pores, causing problems to the process. To avoid such deposition, which will clog the felt, different strategies can be employed to establish a specific program to keep the felt clean. Chemical cleaning done by the direct application of a cleaning product as well as the use of a conditioner on the felt (resulting in a felt conditioning treatment that prevents PAE deposition and occlusion of the pores) constitute important approaches to a cleaning program. The maintenance of a clean felt can be achieved by batch or continuous treatment. In the case of a continuous treatment, a conditioning product is applied on the felt with the aim of preventing wet strength resin contaminants from depositing.

CONTECH´S PORTFOLIO PRESENTS VARIOUS PRODUCTS suitable for batch and continuous treatment that are effective in the removal of wet strength resin contaminants. As part of the cleaning solution, the company also has equipment for their chemical application under high pressure and temperature that significantly improves the efficiency of the treatment. There are several major improvements that can be observed in the process: increase in clothing life time; shut-down time reduction, increase in machine productivity and paper quality (improved cross direction profile), cleaner felts without contamination, product application with the machine running (saving time), specific steam consumption reduction; moreover, the products are formulated to preserve the environment.

FIGURES 1 AND 2 SHOW OPTICAL MICROGRAPHS OF CROSS CUTS OF USED FELTS COLLECTED FROM DIFFERENT PAPER MACHINES. Quantification analysis of the felt sample presented in Figure 1A shows 13% (w/w) of contaminants. A batch test performed in the laboratory using a specific product leads to a cleaning efficiency of 45% (Figure 1B). In the case presented in Figure 2 it is also possible to observe an image of a felt free of contaminants after a cleaning test in the laboratory using an association of different products (Figure 2B). The cleaning efficiency obtained in this case was 89%. In addition, Contech offers conditioning products as a preventive treatment for felts that promote a passivation which, in turn, forms a protective layer on the surface, preventing the deposition and subsequent clogging by free species of wet strength resins.

IT IS A KNOWN FACT THAT RECYCLED FIBERS HAVE BEEN INCREASINGLY USED IN THE PAPERMAKING PROCESS AS AN IMPORTANT RAW MATERIAL, conferring sustainability to the process as far as environmental and economic aspects are concerned. The use of waste materials containing wet strength agents in their composition as raw material for papermaking can be a problem. The available wet strengthened paper products for use in the production of recycled fibers are usually difficult to repulp since they contain relatively high amounts of wet strength agents that cause a delay in the repulping. Thus, increasing the repulpability of such waste materials is a challenge and overcoming it is currently being studied.

DIFFERENT MECHANISMS HAVE BEEN DEVELOPED TO EXPLAIN HOW CHEMICAL ADDITIVES IMPART WET STRENGTH PROPERTIES TO PAPER. The protection mechanism explains wet strength characteristics as a result of the distribution of a wet strength agent like PAE on cellulosic fiber, followed by crosslink reactions with itself, forming a water insoluble material deposited over and through the fiber. According to the reinforcement mechanism, such property is achieved by the reaction of the wet strength resins with cellulose to form linkages among cellulose molecules, resin molecules, and fibers. In both cases, chemical reactions of the additives are decisive to impart wet strength to the final product, with the formation of insoluble material difficult to degrade in a repulping process. To increase the recycling of wet strengthened papers, chemicals and pulping conditions must lead to more effective depolymerization of the wet strength resin. Different chemical approaches can be employed and the application of oxidizing agents under appropriate conditions is needed for chemical reactions to be able to degrade the crosslinked polymers responsible for wet strength.

IN A GENERAL WAY, THERE IS NO STANDARD PROCEDURE TO INCREASE THE REPULPABILITY OF WET STRENGTHENED PAPERS, and each paper product may have an individual necessity to make its reuse easier. The type and amount of wet strength resins will require different chemical treatments and conditions for repulping, such as different pH values, temperatures, and chemical additives. Repulping aids must facilitate this process with savings in time and energy costs. Choosing the right product, application technology, and process condition is crucial for the implementation of a successful repulping program and can be achieved when the chemical supplier works along with the customer, looking for solutions to obtain synergistic results. Besides offering cleaning felt solutions for the wet strength additive contaminants present on the papermaking machines and repulping aids based on different chemical technologies, Contech works hard to assist its customers with tailor-made products and exclusive services.

Login or Register to publish a comment