Assessing cartons and cardboard boxes

Packaging used to transport footwear is critical to ensuring that product quality is not compromised during the journey from manufacturer to retailer.

by Steve Rose

Regardless of distances involved, almost all footwear will need to be transported from manufacturer to retailer. Most of these will be transported from major manufacturing regions in Asia to European or American markets, destined to spend many weeks on container ships or in storage and be subject to extremes of temperature and humidity.

The use of appropriate packaging ensures products are delivered in the same condition as they left the factory. Although a number of other alternatives exist, the traditional cardboard box is still used by most shoemakers as it is generally perceived to provide satisfactory physical protection, is easily stackable, represents a relatively efficient use of space when shipping and can be used as an effective and attractive display to the retail customer.

A poor quality carton, however, can mitigate against some or all of these benefits and therefore it is important to ensure that packaging can not only withstand the physical rigours of transportation, but also retain its colour and appearance. In addition, it shouldn’t compromise the footwear through the introduction of damaging or harmful chemicals used in manufacture or in associated packaging.

Shoe manufacturers expect cardboard boxes to be physically durable and some have specifications for assessing compression, burst and edge crush characteristics. In addition, the materials used need to be assessed to ensure they do not cause staining, colour loss or print transfer. SATRA has carried out various shoe box tests over the years and this has led to the development of comprehensive performance specification for these products.

Developed jointly with L&E International, the SATRA specification (see table 1) addresses physical, colour fastness, chemical, colour/print transfer, and moisture absorption characteristics. The specification concentrates on ensuring products get to market reliably so carton end of life and recycling issues have not been included.

Table 1: Specification for shoe boxes and cartons
No.	Property	Shoe box	Outer carton	Test method	Requirement
1	Box compression test (N)	Y	Y	TAPPI T804 SATRA TM439	As specified
2(1)	Diaphragm burst test (Mullen) (kPa)	–	Y	TAPPI T810 SATRA TM170 EN ISO 2759	As specified
3	Cardboard adhesion test	Y	Y	Qualitative	Laminate shall tear, not peel
4(2)	Colour evaluation test (comparison of sample colour against original)	Y	–	LSD chart	Within specified colour range
5	Edge crush test (kN/m)	–	Y	TAPPI T841 SATRA TM440	As specified
6(3)	BHT determination (mg/kg) NOx discolouration test	Y	Y	SATRA TM373 SATRA TM341	None detected No worse than Grey Scale 4
7(4)	Rub fastness test on printed areas (50 dry rubs, 10N load)	Y	–	EN ISO 11640 SATRA TM173	No worse than Grey Scale 4 (colour change); no significant smearing, no loss of legibility
	Rub fastness test on printed areas (150 dry rubs, 0.9kg load)			TAPPI T830
8	Slide angle test (degrees)	Y	–	TAPPI T815	18 ± 1
9(5)	Sun test (colour change after two hours' exposure)	Y	–	Equipment operating instructions	No worse than Grey Scale 4/5
10	Thickness (mm)	Y	Y	Graduated eyepiece, calipers or rule	As specified
11	Water absorption test (per cent)	Y	Y	SATRA TM9	Maximum 200
12	Weight measurement (g/m2)	Y	Y	SATRA TM28 TAPPI T410	As specified
13	Restricted substances	Y	Y	Total digestion and ICP analysis (EN 1122 modified)	Market legislation
1Applicable only to outer cartons for apparel products (not footwear). 2The Light/Standard/Dark (LSD) chart shows the acceptable range of variation for each colour. However, alternative systems, such as L*a*b* ΔE values, may also be used. 3Only one of these tests needs to be carried out. BHT is the preferred test as all packaging should be BHT-free. This includes tissue and spacers used inside boxes. However, NOx  discolouration can be used as an alternative test. Beware: NOx is an extremely toxic gas. 4Only one of these tests needs to be carried out. 5The two-hour ‘sun test’ is a simple and rapid screening procedure for checking the resistance of materials to light-induced colour change. The performance of test lamps deteriorates over time and, therefore, it is important that they are replaced in accordance with the supplier’s specification in order to obtain consistent light exposure levels. (The two-hour ‘sun test’ should not be regarded as a substitute for light fastness tests such as SATRA TM160 and EN ISO 20105:B02, both of which, unlike the two-hour ‘sun test’, incorporate control measures such as the use of reference standards).

The cardboard used to make the individual boxes and the shipping cartons selected to transport goods from manufacturing site to their intended market must be of sufficient strength and durability to protect their contents from damage. Box compression, edge crush and burst tests are already established carton tests widely used in industry, and these test methods, among others, feature in the SATRA specification.

Box crush test

The principle performance requirement of boxes is to be able to protect the contents when stored and transported under normal, foreseeable conditions. The most stressful ‘normal’ condition will be the applied loads during stacking. This will take the form of loads applied to the vertical sheets of cardboard in the sides of a box. The rigidity of the card is, therefore, a key factor.

Perhaps the most obvious method of stacking behaviour assessment is to directly simulate the loads applied to a representative sample of the boxes in question. This means applying a compressive force on a test machine to a complete box. This test, commonly known as the ‘box crush test’ (BCT), shown in figure 1, will determine the force (loading) under which the box or carton will buckle and collapse. The maximum stacking height (number of cartons) can then be calculated from this value. However, particularly for a carton, this calculation cannot be performed if it is tested empty – it must be tested fully loaded with identical contents. This is because the weight of the carton, the type of contents (for example, boxes or loose bags) and also how tightly they are packed will all affect the crush load.

It is clear that if a carton is packed tightly with well-fitting contents, these items will ‘reinforce’ the carton walls and significantly increase the crushing load. If, however, products are loosely packed in the carton (for instance, in a plastic bag), little support will be given to the external carton walls. Loosely packed cartons will also allow movement and settling of the contents. This could permit (or even cause) distortion and bulging of the cartons, increasing the risk of collapse under load. Indeed, some cartons will have internal edge and corner supports to reinforce the structure where the contents are loose.

While individual boxes can be tested against the BCT on a standard laboratory tension/compression machine, large shipping cartons will require more specialised test equipment.

Cardboard edge crush test (ECT)

A key factor in determining stacking stability is the load which the card can support when vertically aligned. This can be achieved by simply applying a load to a small sample of card.

This test, commonly known as the ‘edge crush test’ (ECT), shown in figure 2, has been used in the paper and packaging industries for many years and is widely accepted as a definitive standard for cardboard quality and hence box strength. A small square of the cardboard box material (usually corrugated in several layers for a higher strength-to-weight ratio) is cut and a load applied to its edge when it is stood on end.

It is critical here that the sample is cut accurately with clean, undistorted edges. If the corrugations are damaged in any way, the sample will collapse prematurely and the load will not be representative of the material in service.

Burst test

Adverse pressure on the boxes can be damaging to footwear during transport and storage. If packed in individual shoe boxes, the individual shoes must not be crushed against each other, which could cause permanent marks.

The ‘Mullen burst’, is a simple diaphragm distension method where the sample of cardboard is domed until it bursts. The hydraulic (or pneumatic) pressure at failure is recorded as the burst strength. The test determines the pressure required to rupture or burst the side of a cardboard box. This then gives an indication of the box’s ability to withstand external or internal forces.

Butylated hydroxy toluene (BHT)

Cardboard packaging materials can contain BHT or similar anti-oxidants (as can plastic bags and tissue paper). This relatively mobile chemical can migrate from the packaging into the footwear and cause discolouration. Therefore, BHT-free packaging is essential – especially for storing white and light coloured materials.

For many years, SATRA has been able to test for the presence of BHT in packaging materials. The test method (SATRA TM373:2016 – ‘The quantitative determination of butylated hydroxytoluene in packaging materials’) has recently been revised and published to ensure that the latest techniques are used to accurately determine the levels of this chemical.

Colour fastness of printed areas

The shoebox is often an important element in the overall presentation of the footwear product and therefore it is important that it maintains its appearance. For this reason the specification includes a number of tests to assess colour change after exposure to a xenon lamp for two hours to simulate strong sunlight and rub fastness tests on printed areas.

Nitrogen oxides

Shoe boxes can be adversely affected by ‘NO_x’ – a group of oxides of nitrogen – compounds such as NO₂ (nitrogen dioxide) and NO (nitric oxide) which are highly reactive atmospheric pollutants generated by the combustion of fossil fuels. In factories and warehouses, typical sources include heating systems and vehicle emissions. Although the most common form of NO_x discolouration is observed as yellowing of white or light coloured materials, ‘gas fading’ of coloured materials can also occur.

Slide angle

The slide angle test determines the coefficient of static friction of most packaging materials by measuring the angle at which one test surface begins to slide against another inclined surface as the incline is increased at a constant and prescribed rate.

The coefficient of friction of packaging materials indicates how packages made from that material will perform in many critical situations. A high coefficient of the surface of the box to a surface of similar size and material on another box means that they will tend to resist sliding over one another when stacked. A low coefficient may indicate potential problems with the packages slipping from a stack.

Moisture

Perhaps the most damaging agent of shoes in storage is moisture. Many shoemaking countries are in tropical or sub-tropical climates, and during many seasons the humidity in the air will be very high. Leather in particular is prone to absorb moisture and can hold large quantities without feeling damp. However, this absorbed moisture, as the temperature within the packaging changes, can easily evaporate from the leather and condense elsewhere within the shoe box during transportation and storage. The specification includes a simple test – SATRA TM9:1992 – ‘Surface water absorption and repellency’ – to determine water absorption. A piece of box material is exposed to water for a given time after which its increase in mass is determined.

Environmental legislative considerations

Footwear products bound for many major markets will be subject to restricted substances legislation in some form. Likewise the boxes they are supplied in also need to comply. For instance, in Europe the sum of the concentration levels of cadmium, mercury, lead and hexavalent chromium should not exceed 100 parts per million.

Publishing Data

This article was originally published on page 46 of the February 2018 issue of SATRA Bulletin.

Other articles from this issue »

---