Strength and toughness
Strength and toughness are composite terms used to describe one or several of the paperboard’s physical (mechanical) properties. They play an important role, directly or indirectly, in a large number of paperboard applications and users’ situations. Strength and toughness are important characteristics for the paperboard’s ability when it comes to:
Sophisticated surface design, such as embossing, or complicated box construction, creative shape and functional shape.
Physical protection and resistance to stacking, handling and stresses from the environment.
Conversion and treatment of paperboard such as embossing, cutting, creasing, and folding. Important for packaging and conversion efficiency.
The physical properties of paperboard can be split into two groups:
• well-defined physical properties
• complex (less well-defined) physical properties.
Examples of well-defined properties are those where generally accepted definitions and methods (from physics) are applied to paperboard. An example is tensile strength. Complex, or less well-defined, properties are those where one or several properties are combined into a functional characteristic or where methods are used that try to simulate a user’s situation. Box compression strength and tearing resistance are examples of this category.
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The paperboard choice
In packaging applications, protection is the primary paperboard function. Therefore the paperboard’s ability to withstand a range of applied stresses is an important consideration in the choice of paperboard. This need is achieved by the strength and toughness properties of the paperboard.
A first step in choosing a paperboard is to identify the applied stresses to which the paperboard will be exposed in printing, conversion, packaging, distribution, and consumer use. Any exposure to extremes of temperature or humidity in the environment results in changes to the moisture content of the paperboard. If high moisture contents occur there will be a significant loss in strength. The weight of the product, its shape, whether it has sharp edges, whether it is a solid, granular, a powder or a liquid, are all features which influence the paperboard requirement.
Strength and toughness are complex properties, which are defined by characteristics such as elasticity, elongation, stiffness, tensile strength, box compression strength, etc. Some properties are well defined and easy to measure, such as tensile strength. Others are more difficult, such as box compression strength. The contribution of the structural design to complex properties such as box compression strength is best examined by box compression tests, travel tests or other realistic handling testing.
Converters often have requirements on the strength of paperboard, in order to perform certain operations, such as printing and cutting, without stoppages and production delays. Sometimes the demands from the conversion operations on the strength properties are contradictory. A low tearing resistance might be needed when a pull tab is to be used for opening but a higher value would be better if the package is to be opened and closed a number of times. Because paperboard is a visco-elastic material, permanent deformation occurs during creasing and embossing but that must be avoided during printing. Delamination strength must be balanced to allow the paperboard to delaminate when creased but to stay intact during other operations and use.
The different types of fibres have an effect on the strength properties, the longer the fibres the higher the strength. Solid Bleached Board (pure bleached chemical pulp) has good strength properties and Folding Box Board (mechanical pulp with bleached chemical pulp in the surface plies) has high stiffness. The printing, conversion and end-use application determines the relative importance of the various strength properties. For any given type of paperboard strength properties vary with grammage and thickness.