When we talk about material strength we tend to think of high strength values in absolute terms—as strong as steel. In real-world use, though, a material only needs to be as strong as necessary, with other properties such as lightness, manufacturability and sustainability influencing its selection. As a raw material, paper is seemingly at the other end of the strength spectrum, but with resourceful processing, innovative final product design and appropriate quality assurance testing, paper ticks all the boxes as the ideal choice.

From pulp to paper

Paper is both a raw material for other products and a product in itself. Its physical properties are measured just as any other test specimen and test standards are published to ensure that it meets the performance and functionality needed to do its job. TAPPI (Technical Association of the Pulp & Paper Industry) defines a range of test standards and recommended methods applicable to paper and packaging makers worldwide.

The classic material tensile strength test using a dumbbell-shaped specimen which is performed on metals and plastics has its equivalent in the world of paper. A strip of paper is gripped and stretched, measuring the force it can withstand and the elongation undergone until rupture. A tensile testing machine capable of a constant rate-of-elongation is required, and the grips’ clamping surfaces must ensure a secure hold but eliminate local damage to the paper at the gripping location. One resulting property of interest is referred to as TEA (tensile energy absorption), calculated by the area under the stress-strain curve, representing the total work done per unit area of paper when stretching to rupture. This test is usually performed in both the manufacturing machine direction (MD) and the cross direction (CD).

Paper may need to endure some level of wetness—printing or writing inks on the surface coating, adverse weather conditions when exposed on packaging or labels, or as a result of its intended purpose in the case of absorbent tissue products. The likelihood of rupture when wet has led to tests being conducted on wet samples. The wet strength test again subjects the specimen to tensile loading after soaking in the ‘Finch Cup.’ The paper is looped around a lower pin fixture and the upper end clamped and pulled away at a constant elongation rate. Wet tensile strength and ‘wet-tensile-strength’ retention (the ratio of strength that the wetness has influenced) are the salient results from this type of test.

Further to stretching a single-ply paper specimen, quality assurance testing also focuses on other types of loading or in-service simulation to thoroughly check the resilience of this light and versatile material:

• Burst—dealing with stresses in multiple directions, this test quantifies penetration resistance by the human thumb.
• Friction—for smooth processing, sorting and stacking, the surface COF characteristics of paper are measured, by dragging a weighted sled along the surface and measuring the resistance.
• Peel—Labelling products are peeled from their liner to check the strength of the paper itself and the adhesive that is an integral part of the product. Good results ensure resistance to peeling in transit and ease of peeling in dispensing from the roll.

One other fundamental 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. This test simulates tearing the paper into strips or accidentally, when turning the page of a book, by clamping a small sample, initiating a cut and then recording the force as a pendulum propagates the tear. This specific method is performed by a dedicated tester.

From sheet to board

The benefits of paper as a raw material have promoted its development into forms where its absolute strength is increased—to increase its applications. By adding plies and fluted corrugations, paper becomes board—paperboard, cardboard, corrugated paperboard and fiberboard. By increasing thickness and creating an internal ‘fluted’ structure a capable and widely used packaging material is formed.

Naturally, quantifying the strength of these packaging materials is paramount in selecting their specific implementations and test standards define these ‘crush’ tests.  Many of these procedures take cut samples of the board and subject them to compression forces in significant planes, typically ‘flat’ (perpendicular to the board surface) and edgewise.

The rigidity of the complete board—outer/inner liners with interior flutes—and individually, the all-important corrugated element are evaluated by Edge Crush Test (ECT), Flat Crush (FCT), Ring Crush (RCT) and Corrugating Medium (CMT) methods.

Paper to board

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— Adhesive Bond (ZDT) for paperboard and Pin Adhesion (PAT) for corrugated forms.

From board to box

Once the configuration of board grade (number of plies), flute type and number of walls (single, double or triple) is selected, the proof of suitability for the final product is the ultimate quality check. As with the board, the testing of assembled boxes focuses on crush and impact resistance. Such is the appropriateness of this packaging solution, that surprisingly large products are housed in the ubiquitous cardboard box.

To simulate the loads that an individual box may be exposed to in its lifetime, from stacking in storage, through handling (and mishandling) in transit, a suitably accommodating test space in a compression tester is required. 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 reported. This information enables recommended stacking guidance to be published—often on a peel-tested paper label.

Possibly the most perilous event that a package could encounter in the distribution environment is that of being dropped. This dynamic loading is replicated by dedicated drop testers, with industry standards specifying drop height, specimen orientation and number of drops. As with the paper industry, a specialized body exists for recommending test standards in this domain, the ISTA (International Safe Transit Association).

No paper trail

No matter in what sector of the paper industry a manufacturer operates, the benefit of a robust software environment is the assurance of protected data, optimized processes and operational efficiency. Industry standards can be applied consistently, globally, and in the medical packaging sector—provide a secure audit trail. Quality assurance results can be shared electronically, or if required … sent to a printer.

The final page

Paper is finding its way into packaging products long held to be the realm of plastics. Its potential to bring improved sustainability to the manufacture of other items, such as bottles, is being optimized by focused testing methods to ensure its suitability. In this way, high-quality results “on paper” translate into real-world performance.