Paper360 - March/April 2016 - (Page 34)
techlink | CONVERTING
Discovering Hidden Causes of
Standard measurement methods can't always predict the behavior
of paper and board surfaces during converting processes.
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.
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.
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
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
SWM Gets Faster ERP Financial Data
Battling Rejection Burnout
Index of Advertisers
Paper360 - March/April 2016