When considering material strength, high strength values are often discussed in absolute terms, like “as strong as steel.”
In practical use, a material only needs to be as strong as required, with factors like lightness, ease of manufacture, and sustainability guiding selection.
Although paper, as a raw material, is on the lower end of the strength scale, resourceful processing, creative design, and appropriate quality assurance (QA) testing make it an ideal choice.
Tensile Strength Test
Paper is a finished product and a raw material for other products. Its physical properties are measured like any other test specimen, and test criteria is published to ensure performance and functionality standards are met.
The Technical Association of the Pulp & Paper Industry outlines various test standards and recommended methods used globally by paper and packaging producers.
The standard tensile strength test for metals and plastics utilizing a dumbbell-shaped specimen has its equivalent for paper. A strip of paper is held and stretched to measure the force and elongation it withstands before rupture.
The test requires a tensile testing machine capable of constant elongation and secure grip clamping surfaces that prevent local damage to the paper.
An important measure from this test is tensile energy absorption (TEA), calculated from the area under the stress-strain curve. TEA represents the work done per unit area as the paper stretches to rupture. This test is conducted in the manufacturing machine direction and the cross direction.
Influence of Moisture
Paper sometimes needs to withstand moisture. Examples include applying printing or writing inks, adverse weather conditions, or a product’s intended purpose, such as absorbent tissues.
Due to the likelihood of rupture when wet, tests are often conducted on wet samples. A sample is soaked in the “Finch Cup” and subjected to tensile loading to gauge wet strength. The paper is looped around a lower pin fixture, and the upper end is clamped and pulled away at a constant elongation rate.
Key results from this test include tensile strength and “wet-tensile-strength” retention (the ratio of strength that the wetness has influenced).
Samples from a single sheet of paper undergoing strength tests (R to L:) tensile strength stress-strain and wet strength of tissue paper measurements on UTMs and a dedicated Elmendorf test standard tear tester. Image Credit: Mecmesin
Beyond Tensile Strength Testing
QA testing also investigates other stresses to assess resilience thoroughly.
- Burst assesses penetration resistance by the human thumb related to stresses in multiple directions.
- Friction measures surface coefficient of friction characteristics for smooth processing, sorting, and stacking by dragging a weighted sled across the surface and recording resistance.
- Peel checks test the adhesive strength of the paper and the labeling product’s adhesive. Good results ensure resistance to peeling in transit and ease of dispensing from the roll.
Another key failure mode for paper (and other sheet materials, such as fabrics and textiles) is tear. The “Elmendorf” tear test measures the tear resistance perpendicular to the plane of the sheet, simulating accidental tearing by clamping a small sample, initiating a cut, and recording the force as a pendulum propagates the tear. A dedicated tester performs this method.
Boards
The advantages of paper as a raw material have led to the development of forms with increased absolute strength and, as a result, wider applications.
Adding thickness, plies, and fluted corrugations transforms paper into boards, such as paperboard, cardboard, corrugated paperboard, and fiberboard, which are widely used as packaging materials.
Quantifying the strength of packaging materials is critical for selecting their specific implementations. Test standards define “crush” tests, many of which take cut samples of the board subjected to compression forces in significant planes, typically “flat” (perpendicular to the board surface) and edgewise.
Board rigidity is evaluated using edge crush testing, flat crush testing, ring crush testing, and corrugating medium testing methods.
Layered paper in the forms of paperboard and corrugated cardboard is itself a raw material for packaging products and is crush strength tested flat and edgewise with specialist fixtures. Image Credit: Mecmesin
The tensile strength of board in the perpendicular plane is also tested, evaluating the resistance to being pulled apart as a measure of the effectiveness of the internal bonding adhesion. This is called adhesive bond for paperboard and pin adhesion for corrugated forms.
Boxes
After selecting the appropriate configuration of board grade (number of plies), flute type, and number of walls (single, double, or triple), an ultimate quality check determines the suitability of the final product.
To test assembled boxes, crush, and impact resistance are assessed. Due to this packaging solution’s effectiveness, surprisingly large products are often housed within cardboard boxes.
A compression tester simulates expected loads due to stacking and handling over the product’s lifetime. The box is crushed between platens located on swivel joints to maintain an even contact face, and the peak load at the point of collapse is recorded, enabling the recommended stacking guidance to be published.
The ubiquitous cardboard box is a common final destination for corrugated board and is compression-crush tested in all sizes to protect its contents from stacked, tracked to unpacked. Image Credit: Mecmesin
The most strenuous event that could occur to a box in the distribution environment is being dropped. This dynamic loading is replicated by dedicated drop testers, with industry standards specifying drop height, specimen orientation, and number of drops. Like the paper industry, a specialized body is in place to recommend test standards in this domain, the International Safe Transit Association.
Conclusions
Regardless of the paper industry sector, a robust software environment gives manufacturers the advantages of protected data, optimized processes, and operational efficiency.
Industry standards can be consistently applied worldwide, which is especially critical in sectors like medical packaging, where a secure audit trail is essential. QA results can be shared electronically.
The Future
Paper is increasingly being used for packaging products traditionally made of plastics.
Paper’s potential to improve the manufacturing sustainability of other items, such as bottles, is optimized by focused testing methods to ensure suitability.
This information has been sourced, reviewed, and adapted from materials provided by Mecmesin.
For more information on this source, please visit Mecmesin.