Consolidation of Dry Formed Web Through Hot Pressing : Effects of Pressing Parameters and Pulp Freeness on Tensile Strength

Abstract

Airlaying is a dry web forming technology that is used for manufacturing hygiene products. Originally, the technology was developed to substitute traditional papermaking due to its many advantages, including low or no water consumption, meaning that the airlaying technology does not require an energy-intensive drying process compared to conventional papermaking. However, the technology has disadvantages, such as weak bonding ability that induces low strength in the fiber web structure. Generally, the fiber web bonding is enhanced by mechanical interlocking, chemical additives, or thermal methods which are based on the mixing of thermoplastic material that bind the structure upon melting.

This Thesis aims to study improving the strength of the airlaid webs through hot-pressing method without additives. The objective is to determine how pressing parameters including moisture content and press temperature, and pulp freeness affect the strength of airlaid webs. The subject was studied both by means of a literature review and experimental research.

The literature review discusses the main properties of paperboard, the characteristics of airlaying technology, and its current state-of-the-art. Additionally, it considers the effects of pressing, pressing parameters, and fiber properties on fiber bonding and the strength of the fiber web structure. The experimental part includes raw material preparation, formation of the sample sheets, and the measurements. The raw material used was chemi-thermomechanical pulp (CTMP) with 3 different freeness numbers. The pulps were dry disintegrated for airlaying, and wet disintegrated for the determination of pulp fiber geometrical properties and formation of wetlaid reference sheets. The airlaid sheets were produced through airlaying and hot pressing using different moisture contents and press temperatures. The reference sheets were formed by wetlaying following the standard, without hot pressing. The measurements included the determination of fiber geometrical properties and pulp freeness, imaging the sheets for optical formation analysis, and measuring the tensile test and brightness of the sheets.

From the results, it was observed that the fiber’s geometrical properties were very close to each other, and the pulp freeness numbers were a bit higher than the ones provided by the manufacturer. It was observed that the higher press temperature and the moisture content resulted in lower brightness, but the lowest hot press temperature used did not affect the brightness. The tensile strength results showed that hot pressing can be used to improve airlaid webs strength if the moisture content is increased in the airlaid structure. The moisture content has a significant effect on the airlaid web strength, but the used hot press temperatures have only a minor effect. The strength increased as the sheet's initial moisture content increased. The freeness affected strength so that the pulp with a lower freeness produced denser structures thus resulting in higher strength. However, when comparing the hot-pressed airlaid sheets to the conventional wetlaid sheets, the strength of the airlaid sheet was lower, especially when the moisture content and the pulp freeness were low. With fibers of the highest freeness number, the moisture content of about 37% in the airlaid sheet produced the same strength as in the wetlaid sheet.