It is not uncommon when observing film containers that have been in archival vaults or when handling film containers in a laboratory after they have been taken from storage vaults, to find the labels only partially attached or missing. This should not have to be so.
Label material
Some early labels with water-based adhesives curled and fell off the film cans as the adhesive dried out or if the labels became humid or wet. Paper labels will absorb humidity and will do so at a rate different from the film container on which they are attached. This introduces differential shrinkage and expansion in relation to the surface to which they have been attached. This means that labels that are unable to absorb humidity do not expand and shrink as radically and as differently as the surface to which they have been attached. The shelf life of a label may be short-term instead of permanent. Some labels maintain their integrity in spite of immersion in water, such as some labels used on bottles, while others soak up water and slide off. In film archives, challenges include variations in humidity, drying and when not properly cycled, may even include moisture that condenses on the exterior of the container that transfers water to the label. Labels made of white polyester with durable laser imprintable matte top coating are one type that are recommended for long-term use.
How adhesive is the label?
Some label adhesives have more ‘peel strength’ required to break the bond than others. This is the measure of the force required to peel back a label, once it has been attached to a flat surface. It is not uncommon to indicate at what temperature an adhesion test was done. Another measure of label adhesion is the ‘shear strength’, that is measured by attaching a weight to the bottom edge of a label and waiting until the label slides off an application rack. The time it takes before that happens is the measure of shear strength. For example, tests done according to PSTC # 1 – 15 minute dwell time. It should take 45 oz/inch for a polyester film container to lose the recommended label.
The adhesive
There are four types of pressure sensitive adhesives, solvent, hot melt, rubber and water-based. Some label adhesives are designed to perform well at low temperatures, but they may not perform as well under hot conditions. An excellent solvent-free permanent adhesive is acrylic emulsion. Labels used in film archives will need to perform under cold and dry conditions as well as human comfort zone conditions. In addition, the adhesive on the label must have been designed for the surface on which it is to be applied. Standard adhesives may work well with wood, metal and painted surfaces, but may fail with low-energy surfaces like polypropylene. Some adhesives stick well to metal, but fail to stick to low surface energy plastics. Plasticizers contained in film containers to be labeled, may be absorbed by the adhesive, soften it and cause it to lose its internal bond strength. This is not always obvious to the user, so be sure to know the make-up of the material your film container is made of. Many “quick stick” adhesives are made specifically for these low surface energy plastics. But they may sacrifice peel and shear strength at higher temperatures as they soften much quicker. A thicker adhesive layer can also make a significant difference, by providing good peel strength to a bond with a rough surface. When the layer is thin, half or more of the surface to which it is to attach itself is not in contact with it because of the surface roughness of the container. Being attached to a rounded surface, the label adhesive will be subjected to stress induced by the memory of the state in which it was applied to the label when it was flat. Such labels may require a stronger adhesive. They may not have been designed for the temperature and relative humidity differentials encountered when cycling the film container between the storage vault and work areas. Label manufacturers produce labels with different adhesives that are suitable for a wide range of temperatures, that can be used to bond with various surfaces.
How do you print on the label?
On paper, the life of laser printing ink is limited. If white polyester label material is used, the carbon fibers of the laser printer melt into the polyester coat and will last longer than thermal transfer printed information. To assure sufficient carbon deposit, do not use recycled toner cartridges and replace an exhausted cartridge. The printed label should resist smudging (immediate finger smear), heat moisture, and should have excellent abrasion resistance using the Taber Abraser Test.
The degree of permanence
The definitions for permanence used by various manufacturers may vary between ten and fifteen years. Few if any adhesives or label manufacturers have done accelerated testing to verify the expected or calculated life expectancy of their adhesive, or may be unwilling to share documented evidence of such test results. Therefore, labels must be tested for the worst possible case conditions under accelerated life conditions, at elevated temperatures and high relative humidity and should also be cycled between those extremes and between cold and normal human comfort zone conditions. Short-term tests will probably fail to identify potential long-term problems. One test to verify the ability of the white polyester labels (or any other selected label) to resist water is determined by immersion for a period of 7 days at 21ºC. Labels and adhesive used should remain in excellent condition. They can also be tested for their ability to resist humidity, by exposing them in an oven to a 100% relative humidity environment, at 38ºC during 14 days. The labels and their adhesives should remain excellent. Dimensional stability should be <1% for a (6″ X6″) sample exposed to 71ºC of heat for 42 hours. The above recommendations should be used as a guide only. This information does not constitute a warranty of any kind but is intended to assist in the verification using appropriate criteria when making choices. The user should independently verify whether the materials selected are suitable for the intended purpose.
Testing labels and their adhesives
To make sure that a film container label meets extended-term life expectancy requirements, the label vendor should be asked to supply a white paper that indicates that longevity has been tested, the manner in which it has been tested, the temperature, relative humidity and duration of the test. In the event vendors are unable or unwilling to supply such test information, it is advisable not to trust their marketing claims, but have the labels tested. To make the test cost-effective, test multiple sample products from different vendors that are recommended for long life expectancy. Split the cost estimate for the test, and ask each vendor to pay its share for the costs. In return, provide each vendor with a copy of the test report with all samples listed under anonymous indicators, i.e. samples A-Z. Each vendor should know by which letter their sample was known, but not receive the information about their competitors, nor which competitors and which products were tested. Another approach could be for multiple users to collectively have their labels tested at the same time, thus sharing the cost and the results. This requires unbiased coordination of course, and an exchange of vendor names and label types, finish, thickness, adhesive name (if known) and which type of label ink application is used.
Accelerated aging is typically approximately 50 degrees C and 85% relative humidity, for a specified number of weeks. During this test, the adhesives should neither soften nor run, remain water-resistant and shall continue to hold firmly on the film container used.
© Innovaid-e 2001 (INSIDE TRACK 3)
Don’t forget that STIL Casing Solution sells the best quality labels available…call us for a sample or to order!