Paper and Cardboard Testing with a Texture Analyser – Part 3

Reliable Testing for Packaging and Print

Tensile Test
Example standard: ASTM D828

Tensile testing is applied to paper products for various reasons to assess strength in use; standards such as ASTM D828 cover the methods required to measure the tensile properties of paper and paperboard. Tensile tests are used for combined corrugated board but not as frequently as bending, compression and puncture.

In a tensile test, a sample (usually cuboidal or dogbone-shaped) is placed between two grips. The Texture Analyser pulls the grips apart at a constant speed until failure occurs. Properties that can be determined from a tensile test include force and displacement to failure, stiffness and work to failure, and force and distance results may be converted to stress and strain if dimensions can be reliably measured.

There are many variations of paper tensile testing for use as measurement solutions, including perforation testing and wet paper testing.

Perforation Tensile Test
Example standard: ASTM D4987

Until recently, one-part continuous form paper was in common use with personal computers until laser and inkjet printers were introduced. It is still used in some industrial and commercial situations due to its low cost and high speed printing, but at the sacrifice of quality. 

Tests are still used today to measure the tensile strength of the perforation. This is an important measurement, ensuring that the form will not break apart during printing and handling. In this test, a 1-inch width sample containing a perpendicular perforation is pulled apart using tensile grips and the breaking load recorded.

This type of measurement can be applied to any perforated paper or card. The perforations act as a stress concentrator and so tensile measurements are simple to perform (there is little risk of breaking at the grips, as with testing many samples in tension).

Wet Paper Test
Example standard: ISO 3781

Ordinary paper tends to lose its mechanical properties if it becomes wet. However, some applications require enhanced wet strength. The wet strength of paper and paperboard is of interest to manufacturers of paper towels and liquid packaging and is a measure of how well the paper structure holds together in tension.

‘Wet strength’ often refers to the percentage ratio of wet tensile strength to dry tensile strength, and a paper is generally classified as strong when wet if this ratio is more than 15%.

Some wet strengthening techniques can be so successful that the resulting paper is not recyclable due to the large amount of energy needed to break down its structure.

In a wet test, the paper or board is immersed in water under set conditions, then tested in tension.

 

Tear Test
Example standard: TAPPI T470

Tearing resistance is measured by manufacturers as a means to test paper’s durability. As with puncture testing, it is commonly measured using a pendulum tester, but there are many advantages associated with the use of a Texture Analyser.

In a tear test, a sample is held in tensile grips and a sample containing a pre-existing notch is pulled apart. ‘Trouser’ tests are also a common testing configuration. Tearing resistance is the force needed to progress a tear perpendicular to the paper sheet. A specific displacement is generally used so the work done can be calculated from the area under the force-distance graph.

Fluctuating Humidity Test

Humidity has been mentioned above as a problem to overcome for some adhesives used in box making. There are also issues associated with the integrity of the board as a whole when the humidity fluctuates. In these conditions, paper fibres contract and swell with each variation in humidity, causing the board to creep over time due to the fluctuating stresses caused, and this can cause the structure to droop and fail.
In these circumstances, it is useful to perform tests under fluctuating humidity conditions on both the paper components and the finished corrugated board to find the combination of materials that is most resistant to humidity-induced creep.

Other options from the Standard Method approach

A standard test method is a definitive procedure that produces a test result.  In order to ensure accurate and relevant test results, a standard test method is explicit, un-ambiguous, and experimentally feasible, as well as effective and reproducible.  

A standard test method can be considered an experiment that determines one or more characteristics of a given sample and the detail of the test method is usually as a result of creation from a number of experts in that measurement field.

Within the materials testing industry, for example, there are an enormous number of standard methods (e.g. ASTM, ISO) that have been developed to measure specific sample characteristics including those mentioned already for paper and board testing.

When reporting results in an academic paper or industrial report these standard methods will have been adhered to for the collection of the results and are more globally understood by others using the same standard methods.  It is for this reason that standard methods have their place in the laboratory.

The limitations of applying standards, however, is that their biggest advantage (of being specific) can be a major disadvantage (or restriction) to the measurement.  All too often, operators are forced to use a standard method to measure a property of their product when:

• The property that the standard method measures is not the most important, or interesting, measurement that could be performed on the sample, or
  
  
• The sample preparation procedure may be difficult (or impossible) to conform with given the sample to be tested is different to the sample for which the test method was developed.

A well-written test method is important. However, even more important is choosing a method of measuring the correct property or characteristic. Not all tests and measurements are equally useful; usually a test result is used to predict or imply suitability for a certain purpose. However, it would seem more important to measure the most important characteristics of a product than to follow a seemingly pointless standard method that does not ideally characterise the product.

If you are not bound by any existing constraints and are looking for the most simple testing solution, sometimes the easiest approach is to set up a test that closely imitates the way the product is evaluated in real life. This is called an ‘Imitative Test’.  This type of test usually also makes data interpretation easier for you to understand.

For certain industries consensus regarding the method of measuring the physical characteristics of products is virtually impossible and whilst procedures can be agreed upon internationally, they are not always universally accepted.  Standardisation of methods is essential if investigations carried out by different groups are to be directly comparable.

The methods used for physical testing should measure the desired characteristics necessary to monitor the process, but need not always be comparable with other laboratories, where different criteria may be important.
 
Where international comparison is important it is essential that methodologies be standardised.  This would include all aspects of the testing procedure and this is an area of which the reference methods are primarily directed.

In contrast, where direct measurements are being made of the physical properties of a product, the experimental methodologies should not be constrained by reference methods. Instead researchers are encouraged to develop and use methodologies that enhance the precision and accuracy of testing methods and lead to an understanding of the basic mechanisms. It is likely that new understanding will lead eventually to methods that more closely predict consumer assessment of product characteristics.  

The Stable Micro Systems range of Texture Analysers provide the flexibility of testing principles (e.g. compression, penetration, tension, extrusion, etc.) and the endless option of probes, fixtures and data analysis techniques within the software for texture analysis and fully customisable test settings.

Whilst standard methods are particularly useful for traditional material forms that have not changed over the decades and are fully accepted there lies another world where creativity has taken off into new materials and material forms for which these standard methods cannot be applied.  

Not only are new non-standard methods viable where sample dimensions cannot be prepared according to the standard but also where the operator may wish to modify the test parameters (such as the test speed) to more closely mimic the situation under which the sample is to be typically used.

 

For help putting your packaging to the test, contact Stable Micro Systems today.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

No-one understands texture analysis like we do!

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