In the context of increasing ecological, sustainable, and resource-efficiency demands placed on industrial sectors such as the paper industry, the pursuit of automation and papermaking 4.0 has become paramount. For the tissue industry, research and development efforts have been crucial, not only in pulp and paper testing equipment, but also in optimizing paper production processes and ensuring final paper quality while avoiding broke and inefficient process interruptions. When embracing intelligent solutions in our quest to protect the environment and our world, even small steps can make a significant difference.
Fig. 1. A comparison of the effects of WSA on the softness of tissue paper.
With ongoing global industrialization and the heightened focus on hygienic standards, particularly in metropolitan areas, the importance of clean environments and responsible social conduct has become evident. The rapid worldwide spread of COVID-19 served as a stark reminder of the necessity to maintain a certain level of hygiene. Consequently, the tissue industry has gained even more significance than ever before.
To address these developments, tissue paper manufacturers must prioritize optimizing and maximizing production output while ensuring product quality. Whether they are producing kitchen rolls, paper towels, toilet paper, napkins, facials, or handkerchiefs, each product has specific desired and expected properties that may vary significantly.
THE MAIN DESIRE: STRENGTH AND SOFTNESS
Strength and softness are two of the main desires of tissue paper makers worldwide. Strength is the result of several factors, including the type of used fibers (virgin or recycled, long or short) and the chemical bonds holding the fibers together. On the other hand, softness is a special and difficult parameter to measure, because it depends largely on the perception of the individual user. Softness is not necessarily perceived as the same by different people or markets. People also have different expectations for different tissue products. For instance, toilet paper or facials require a soft feel, while kitchen paper needs to be tear-proof and resistant, even when exposed to moisture. There are various characteristics affecting softness, often in conflict with the requirements tissue makers need to improve dry or wet strength.
The wet strength property of a tissue product depends on the degree of molecular bonding between the fibers that compose it. To maintain the wet strength of these products as long as possible, additives are used to add or replace the hydrogen bonding of the fibers. In this way it can be ensured that the tissue paper retains up to half of its dry strength when wet. Additives are chemicals and usually mixed with the pulp in the paper mill at the wet end of the process.
The addition of Wet Strength Agent (WSA) can help tailor specific characteristics, but it affects the softness properties of tissue. However, in some cases it is important to strike a balance, as increasing the WSA content reduces the material’s softness.
Measuring the softness of tissue paper can be achieved using the new TSA Tactile Sensation Analyzer. This device simulates the human hand and measures various individual parameters that combine to provide an objective assessment of the subjective hand feel. The WSA primarily impacts the softness parameter (TS7 value) and the deformation value (D), while the surface structure (TS750 value), also known as smoothness or roughness, remains less affected. Softness is determined by the flexibility of individual fibers on the tissue paper’s surface, which is measured using a sound analysis. Flexible fibers produce less noise, indicating increased softness, while stiffer fibers create more noise, meaning reduced softness. The WSA binds with the fibers, making them harder.
The flexibility or stiffness (in-plane) of the base or finished tissue paper product can be measured by subjecting it to a deformation measurement with a certain pressure force. The easier it is to press down the sample, the more flexible the material.
When producing kitchen rolls, wet strength properties are prioritized over softness. In contrast, for toilet paper, softness is crucial, and wet strength is less desirable due to water solubility and flushability concerns. Tissue paper handkerchiefs need to be soft on noses but strong enough not to tear during use. Achieving this delicate balance requires precise chemical dosing. Insufficient WSA content leads to inadequate wet strength in the final handkerchief, while excessive WSA can cause process interruptions due to anionic pulp turning cationic. Continuing production with excessive WSA adversely affects felt softness, necessitating additional lotion in the converting process to meet the required softness. However, this solution increases production costs through superfluous chemical consumption, without improving material flexibility.
To avoid such issues, constant monitoring of the production process is essential from the beginning. The surface charge of fibers in the pulp, as measured by the zeta potential, reliably indicates whether the desired quality can be achieved. Armed with this knowledge, tissue makers can optimize chemical dosage and preemptively address any potential issues. Real-time information provided by the emtec FPO Fiber Potential Analyzer Online enables process managers to take immediate countermeasures, which ensures process stability and a high-quality end product.
The emtec FPO Fiber Potential Analyzer Online measures the zeta potential online and automatically transfers the data to the process control system or a web server in real time. This data includes the surface charge of fibers (fiber zeta potential), streaming potential, conductivity, pH value, and temperature.
Constantly monitoring the production process becomes particularly important when adjusting process chemicals and additives while changing the tissue paper grade, such as transitioning from kitchen roll to toilet paper. Furthermore, the device has already proven its efficacy during napkin production, where various properties, including wet strength, softness, as well as certain optical designs require the use of additional additives (e.g. dyes) to adhere to fibers and color the final napkin.
The emtec FPO is fully automatic, performing sampling, measuring, and cleaning with a very low maintenance effort. It constantly provides relevant process data to the mill’s control system or a data cloud, accessible from anywhere globally by authorized personnel. The device can be installed at various points in the tissue paper machine, alternately taking samples before and after dosing points for additives, for example. It can perform a maximum of six measurements per hour, though four measurements per hour are standard, depending on process adjustments.
Online tools such as the emtec FPO, which provide real-time data, enable the industry to further digitize and automate the papermaking process. The device does exactly what a test device in the Industry 4.0 should do. Given the importance of constant quality control and the ability to access data from anywhere, several countries support their economies’ path toward automation through various funding programs.