1993 Volume 1

During production, converting and usage, paper and board products may be exposed to environmental conditions of both constant and variable nature. As these paper and board products are ultimately composites of the natural polymers cellulose, hemicelluloses and lignin, they are, due to their hydrophilic nature and also to the existence of thermal transitions, highly influenced by the surrounding environment. The paper properties are accordingly affected by moisture and temperature and it is these changes of a physical nature that are discussed in this paper.

In this paper, an attempt is made to present a philosophy of how the total performance of paper products can be reduced to the question of how moisture and temperature interact with wood polymers on a molecular level. The multicomponent nature of the wood fibre and the consequences of the build up of dried-in stresses are particularly emphasised. Thus the sorptive properties of the wood polymers are described from the standpoint of molecular interaction and a plasticizing effect which reduces the glass transition temperatures of the wood polymers. The consequences that this softening have on mechanical properties and on hygroexpansivity are presented together with descriptions of the effect of drying stresses and the creep behaviour during moisture cycling.

Practical examples are given, discussing high temperature processes such as hot calendering, corrugating and press drying. Effects related to printing and converting operations, such as dimensional stability, surface roughening and linting are also commented on. Finally some ideas regarding future research are presented.

Four commercial grades of food packaging board were analysed by chromatographic techniques and sensory evaluation. Attention was paid to providing as similar conditions as possible for the sample preparation and sample pre-treatment in the two types of analysis. A mathematical function for the dependence of sensory data on instrumental data with a prediction error of 12.3 % was found. Hexanal and pentanal were to a significant degree responsible for the odour impact perceived from the board. The developed method has been used to estimate the odour level in boards with fibres from various sources.

Wax based hot melts used for food packaging are blends of paraffin waxes, microcrystalline waxes and low density polyethylene whose characteristics are adapted to each specific application. The results of differential scanning calorimetry analysis at different cooling rates show that the crystallisation temperature of the polyethylene is a linear function of the logarithm of the rate of cooling. By extrapolating these results to very much greater cooling rates, we have assumed the existence of a critical rate of cooling for which the polyethylene and the paraffin wax crystallise simultaneously. By using finite element methods we have produced a heat transfer model that permits the rate of cooling at all points in the paper hot melt structure to be predicted throughout a manufacturing process

Physical aging is a general process appearing in all amorphous or partially amorphous materials below the glass transition and is a fundamental characteristic of the glassy state. It is shown here that paper is no exception and the same behaviour is observable as in other materials. The aging process represents the slow movement of a glassy molecular network towards equilibrium and displays the ‘universal’ characteristics of strong age-dependence of some properties, scaling behaviour, reversibility, and relaxations which follow a stretched exponential or Kohlrausch law.

Physical aging in paper is readily reversed by moisture sorption, which lends the aging effect particular practical importance. Small-strain mechanical viscoelastic properties such as creep and stress relaxation rates are strongly affected by age; this has very significant implications for the performance of paper webs or structures subject to short or long term endurance loading. A measurable but less important effect is also demonstrated for a very large-strain property, CD Ring Crush. The rate at which moisture is sorbed is strongly age dependent. Attempts to measure the glass-transition temperature in paper, and its dependence on moisture content, by dynamic mechanical thermal analysis are described; the results were inconclusive.

A four-point bending stiffness tester was used to study the creep behaviour of corrugated board panels under constant and cyclic relative humidity. By substituting one linerboard in a single-wall corrugated board with steel ribbons, creep tests on linerboard under compression and under tension were also carried out. The results were evaluated using isochronous stress-strain curves relating to both constant and cyclic relative humidities. The isochronous curves showed a straight line relationship at low stress levels, for both constant and cyclic relative humidities. This means that the linerboard could be described as a linear viscoelastic material in both tension and compression at constant relative humidity.

The initial slope of the isochronous curves, the creep stiffness index, was lower under the cyclic relative humidity than at the constant relative humidity equal to the highest used in the cyclic creep experiments, a phenomenon called “mechano-sorptive creep”. It is worth noting that, as opposed to many other investigations, mechano-sorptive creep was demonstrated for paper subjected to comparatively low strain levels. No difference was observed between creep stiffness indices in tension and compression under a constant climate, but under a cyclic relative humidity the compression creep stiffness index was found to be lower i.e. the creep rate was higher in compression. A freely dried paper was found to have a lower compression creep stiffness index both at constant and under cyclic relative humidity than a paper dried under restraint. In machine-made sheets, the compression creep stiffness index was higher in the MD than in the CD both at constant and under cyclic relative humidities. For panels of corrugated board, isochronous bending moment-curvature curves were used to describe creep behaviour under pure bending. The results are discussed in relation to a model previously proposed (1)to describe the cyclic creep behaviour of paper.

Accurate measurement of the shear stiffness of corrugated fibreboard and other high performance sandwich panels is difficult because most loading strategies required to introduce shear stresses also introduce secondary effects, which complicate the measurement.

This paper analysis the strain field introduced by twisting a strip of material and demonstrates a simple relationship for accurately determining the shear stiffness of corrugated fibreboard and other high performance sandwich panels.

A suggested method to describe the creep behaviour of paperboard in edgewise compression works for paperboard in the same way as for other polymers. The relation between stress, strain and time maybe determined by a simple equation involving two factors, a power function of time and a factor describing the non-linear behaviour of the stress-strain curve.

For engineering purposes, it is an advantage to be able to design a given product in terms of stress and strain in isochronous curves. In some applications the critical design criteria maybe governed by a critical strain. For such applications isometric curves, i.e. stress versus time at different strains, are used.

The strain at break in compression of the investigated paperboards was found to be independent of time. The strain to break in creep was equal to the strain at break for stress-strain testing under strain control. By inserting the strain at break in creep in the creep equation, the isometric curve at break could be derived which by definition gives the relation between stress and lifetime. The data in this investigation indicate that a rather small number of specimens and a reasonably short creep time are sufficient to provide a good prediction of the long-term creep behaviour. In other words, the creep of paper follows the fundamental behaviour of the material already established at short times, and described by the creep equation.