Paper360 - March/April 2016 - (Page 34)

techlink | CONVERTING Discovering Hidden Causes of Converting Problems Standard measurement methods can't always predict the behavior of paper and board surfaces during converting processes. SANDRA VOGT Paper and board is often further converted: glued, printed or coated. For optimal runnability of the converting process and to assure the finished product's quality, the producer and customer agree on certain specifications. Usually, these specifications are based on standard measurement methods like the Cobb test, Hercules Sizing Test HST, Bendtsen/ Gurley porosimeter and Smoothness Beck. Yet often, even though the producer meets the agreed-upon specifications, converting problems occur and the reason is unclear. CONVERTING PROBLEMS AND CAUSES The main problems in the converting process occur during gluing, printing and coating. Highly simplified, the process of gluing can be separated into three phases (Fig. 1). Phase I: Application - In Phase I, a liquid such as adhesive is applied onto the surface of a paper/board. During that process, parts of the applied liquid are pushed more or less deeply into the surface, due to the extremely short pressure pulse, within milliseconds or even microseconds in the applicator nip. The penetration depth depends on the surface porosity of the paper/board. Other influences are the pressure pulse value (configuration of the application unit, machine speed) and the rheological characteristics of the applied liquid (which we won't consider here). Phase II: Transport - In Phase II, water or oil will be transported from the applied layer into the deeper surface areas of the paper/ board; this occurs within milliseconds or seconds. This process is mainly dependent on surface sizing/hydrophoby, if water-based liquids are used. Phase III: Oxidation - In Phase III, the process liquid dries or will be hardened by oxidation. During the gluing process, this signifies the final fixation of the bonded parts; it takes place within seconds. This shows that the surface parameters of paper/board are primarily important for the Figure 1: Simplified schematic illustration of the gluing process. 34 Paper360º MARCH/APRIL 2016 respective converting processes. In particular, surface porosity and surface hydrophoby/ sizing are critical. WHEN STANDARD TESTS FALL SHORT As described earlier, converters base their specifications on several standard testing methods. Yet, because these tests do not provide sufficient information - and sometimes even provide misinformation - there are often unexplained problems during converting. This article presents testing results from two other testing systems: PDA.C 02 and EST12. Emtec Electronics GmbH developed both the modular measuring system PDA.C 02 Penetration Dynamics Analyzer and the EST12 Surface & Sizing Tester to determine converting-relevant paper parameters. The innovative measuring principle is based on the assessment of the dynamics of ultrasound transparency of paper/board during liquid absorption. Just as in the real process, the measurement of the interaction between test liquid and sample surface occurs during the first milliseconds, which is important for the characterization of the surface porosity; and within seconds after contact, which is significant for surface hydrophoby/sizing. DETERMINING SURFACE POROSITY Problems in the converting process can occur because of non-optimal surface porosity; causes include too much or too little starch application in the size press, or fiber quality/refining. Converting machine settings are usually defined for one paper - that is, a certain liquid amount with certain rheological characteristics. Issues arise when the pore structure on the surface changes, in either direction - for example, too little application of glue could inhibit bonding.

Table of Contents for the Digital Edition of Paper360 - March/April 2016

Over the Wire
Engineering: A Continuing Evolution
RISI’s European CEO of the Year
Sappi Europe: Serious About the Future of Paper
Reliability and Maintenance Beliefs – Part IV
Discovering Hidden Causes of Converting Problems
Barrier Technologies: The New Revolution in Food Packaging
Standard Bleaching Sequences Including an Ozone Stage – Part I
TAPPI Journal Summaries
Consolidation Watch
SWM Gets Faster ERP Financial Data
Battling Rejection Burnout
Association News
Index of Advertisers

Paper360 - March/April 2016