Migration and bleeding in footwear and leather goods production

1. Definitions Bleeding This term relates specifically to the running of colours. It is often used to do with the transfer of substances between materials in contact with each other. Migration This refers to displacement of a compound within the material or from one material to another by diffusion at the molecular level (see paragraph 2). In bleeding, the compounds rise to the surface and thus are able to leave their original environment en masse. In the case of migration the compounds are able to make their way through the material to the new site, however far away that may be. They are sometimes obliged to cross several materials or several different layers of differing natures before arriving at their end destination. In practice the distinction between bleeding and migration is not always easy to make. We will be using the term ‘bleeding' for running of colours and ‘migration' for internal transfers. 2. Process A compound that migrates is a compound:

• that is not fixed to the environment where it is found (1)
• is not suited to it (2).

(1) It is understood that ‘non-fixed' is a compound that does not have a strong link with its environment and that the link is either of a physical or chemical nature. (2) The suitability of a compound within its environment is determined by its ability to merge into the environment or to fix to it. This term is adopted due to its general nature; it encompasses most notably its ability to dissolve into its environment. All migrations require:

• a donor environment (3) which contains the migrating compound.
• a receptive environment (4) for which the migrating compound presents an affinity
• it needs a carrier (5) which is a substance that carries the migrating compound.
• a certain number of factors facilitate or accelerate migration (6).

(3) and (4) Many materials can either be donors or receptors depending on what they are in contact with. This depends on the partition coefficient of the compound migrating between the materials (see below). Certain materials including pvc plastics are almost always receptors. This is why they are used in trials. In particular to control the migration of dyes (see 4.2). (5) The carriers that run the most are solvents (residual glues for example) then plastic coatings and fatty matter. Humidity can play the role of a carrier when it concerns watersoluble and absorbent compounds, except when these play the role of accelerating the process. (6) The accelerators of migration are essentially heat and humidity. The combination of heat and humidity is used in prevention trials. (see 4.2) Case no 1. Tacky briefcases A leathergoods manufacturer produces very beautiful briefcases in box calf with an acrylic resin finish and a nitrocellulose fixing agent. The lining consists of a jersey pvc coating imitating the box calf. The glue of the lining is a natural rubber latex mixed with vinyl resins. At the end of production, the product has a beautiful appearance. Certain briefcases, which remain in their box for three months are never separated from the paper they were packed in and become tacky. Others are stacked one on top of the other on retail shelves. This tackiness can be explained as follows: the plastic coatings contained in the pvc lining have migrated through the leather and intrude into the nitrocellulose fixation of the leather. The leather itself is now over-plastified and becomes tacky. Instead of acting as a barrier, the lining glue absorbs the plastics and transforms them into a viscous mass. In this form, the mass assists in the transfer of plastic coatings between the lining and the surface below and further migrates thus exacerbating the tackiness. Conclusions: the pvc must be separated from the cellulose finishes and natural rubber-based glues (latex and rubber solutions). In fact, the pvc contains plastic coatings which are rife in the two products mentioned. These two products absorb the coatings to the point of exhaustion when they become tacky. The manufacturer should be aware of such incompatibility. The accelerated ageing tests will allow them to reproduce the conditions. Migrations do not act in the same way as water flowing along the ground. Instead, they come from permanent exchanges between the materials at the molecular level. It is at this point that the partition coefficient comes into effect. While the two materials are in contact and one of them contains substance X, this is shared between the materials according to its solubility in one or the other. In other words their affinity to one or the other of the materials. ‘Partition coefficient' refers to the relationship between concentrations of substance X in the two materials once their equilibrium is reached. If substance X shows no affinity for the neighbouring material (ie the partition coefficient is close to zero), it does not migrate. If the affinity of substance X for the neighbouring material is very large (partition coefficient is infinite), it migrates until exhaustion. In most cases, the coefficient has an intermediate value which signifies that the substance is shared between the two materials, otherwise known as a partial transfer. Substances qualifying as carriers (plastic coatings, fatty matters, solvents, moisture etc) can also accelerate migration. While donor and receptor materials are kept apart, the presence of a carrier is necessary. Migration is carried out in two steps and two partition coefficients come into force: that between donor and carrier material and subsequently that between the carrier and receptor. It is generally sufficient to eliminate one of these factors to halt migration or prevent it from taking place. It has been verified in a laboratory that migration in leather was completely prevented once the leathers had been degreased (obviously without touching the migrating substance). If at first this seems obscure, the following example will clarify the situation. A true example A footwear manufacturer produces a ladies' summer shoe with a white kidskin upper and beige leather lining. On opening the box in the shop, a pinkish hue was found on the leather upper. In the CTC study, it was found that other than the beige pigments, the finish of the lining contained a pink migrant compound. It was most likely that the tanner mistakenly added an incompatible dye shade to satisfy his customers' requirements. This dye easily crossed the two leather layers (lining and upper) to become concentrated in the finish of the upper leather and, more precisely, in the finishing lacquer (surface layer) made up of acetobutyrate cellulose. Study of the mechanism of this migration:

• The colourant in question does not only have an affinity to the finishing of the lining but is also free to leave its own environment to find another more suitable one.
• The fatliquors undertake the transport of the dye (the upper contains 10% fatty matter compared with the dry leather).
• The latex glue between the upper and the lining has a great affinity for fatty matter which eases the transfer of the dye from one material to the other.
• Heat and humidity also accelerate these exchanges.
• Over the course of their journey, the dye molecules meet the fixation lacquer of the leather upper and find a favourable environment and dissolve into it. Step by step they lead to migration of the dye between the finish of the lining and that of the upper, thus the pinkish hue worsens over time.

The reader will have recognised the different environments presented at the beginning of the paragraph (donor, carrier, receptor) as well as the different stages of migration. What's remarkable about this example is that the dye had crossed two layers of leather as well as the lining glue to find a material where it could disperse. This case is far from being unique. Note: it is important not to be guided by first impressions that, in this case, would have led to the conclusion that the problem lay in the leather upper as it had become discoloured. Conclusions should always be confirmed by appropriate tests that reproduce the phenomenon. 3. The most common migrations: This list is not exhaustive. Dye migration There are several cases to consider:

• Dye migration from one material to another (the most frequent)
• Dye migration within the same material, without distinction
• Dye migration between different layers of the same material, eg in the case of finishing (case number 2).
• Dye rising to surface

This case refers to dyes that crystallize on the surface of the leather giving a metallic sheen of different colours. This phenomenon occurs in both finished and non-finished leathers. In this case, the dye does not present any affinity to the finish and cannot be absorbed. Case number 2 - Car interiors that redden A car manufacturer equips his top of the range vehicle interiors with leather. After a couple of months several clients complain that the leather is reddening in places. Investigations quickly show the presence of a red migrating compound in the leather. However, the leather has not been dyed. The finish is reputed to only contain mineral pigments linked with the polyurethane. The fixing agent is made up of a colourless polyurethane. After a number of trials, at the end of the process the red compound was isolated and was found to be an azo dye. The tanner, experiencing difficulties in maintaining the dye colour through mineral pigments, probably committed the error of introducing a shade of dye in the pigment layer of the finish. There is no doubt that in the following months under the effect of summer heat, this dye migrated towards the pu fixing agent causing a reddening of the leather. Conclusion Automotive leather destined by nature to endure extreme climatic conditions cannot contain any dye which is susceptible to migration. A preliminary trial (Migrakit test) would have allowed the detection of the migration within 24 hours. Migrations of plastic coatings These are found in large quantities in pvc. They can, of course, be harmful through contact but can also migrate towards the leather, attacking the cellulose finishes and destroying them. They can also degrade certain glues. Migration of vegetable tannins These are notably responsible for the appearance of dark stains on the shoe upper, above the leather sole when that, or the first part to be assembled, contains non-fixed tannins. This can be triggered by sweat or by rain water. Badly fixed vegetable tannins can also bleed on contact with certain materials. Migration of glues In the majority of cases, this concerns polychloroprene (neoprene) glues. Migrations appear as yellowy stains which gain intensity in the light. It is not the polychloroprene itself that migrates and causes the yellowing, but the additional resins that accompany it. Migration of fatty matters This is not a question of fatty spue. The leather's fatty matter can sometimes migrate in products and materials that are in contact with each other. Citing the case of natural rubber based glues (latex or rubber solutions) applied on fatty skins: the glue feeds on fatty matter, the macro molecular chains of the natural rubber fragment, to produce a blackish and viscous product capable of crossing the leather articles and resulting in dark stains. Migrations in the pvc PVC is by nature a semi-universal receptor for a large number of compounds including dyes but also many types of substances can be found in the neighbouring materials from where the problems of colouration and yellowing can arise. This ability to absorb is used in migration trials where one often uses pvc as a control. 4 Prevention The majority of the cases presented in this article could surely have been avoided if a prevention system had been put in place. To forsee accidents, it is first necessary to recognise the materials and their possible interaction, evaluate the risks and put into effect the controls that are deemed necessary. That appears to be obvious! However, the number of cases treated by CTC show that significant progress remains to be achieved. Risk assessment By considering just the cases reported in this article, the following question immediately springs to mind: were the manufacturers of the articles in question aware of the risks they were taking? For example by lining a leather with a cellulose finish with a pvc one? Who is still using a latex glue? (Case number one) The same can be said for a tanner incorporating a finishing dye into an automotive leather. Plenty of other examples studied in the past could be mentioned.

• A flap of a bag lined with pvc is pressing on a leather surface containing a cellulose finish.
• The reverse is also true. A flap of a bag lined with a leather with a cellulose finish pressing on a pvc surface
• Leathers of different colours find themselves in contact in the same article.
• A polychloroprene (neoprene) based positioning glue causes a yellowing of white or clear leathers or fabrics.
• Prevention begins at the design stage.

Case number 3 - Purplish reflections on black leather Observed under certain circumstances, the leather presents an unpleasant purple reflection which has appeared over time. Studies show the presence of a light deposit of dye colour on the surface of the finish. The study also showed that the colour itself came from the finish and not from the dye. Introduced for whatever reason this colour does not have a real affinity to the finishing resins used and looks for a way out. Once free, it crystallises and the reflections can be observed. This case calls to mind bronzing, reddening and other often observed phenomena on the surface of the leather. It is a matter of colour rising to the surface from either the dye or the finish. In all cases it is caused by non-fixed colours which present little affinity to the finish from where they have a tendency to migrate to the surface. Conclusions Take care with badly fixed dyes. These are easily detectable, notably by the Migrakit test. This problem is of particular interest for the clothing sector.  Certain choices should tell the tanner that he needs to take certain precautions. For example:

• presence of pvc (even if just a thin coating is used)

-    risk of absorbing dyes (or other products) -    risk of cellulose finishes degrading -    risk of certain glues degrading (latex in particular) -    risk of darkening of certain materials (by the absorption of plastic coatings)

• dyes for leather, fabrics and other materials -    risk of bleeding
• different materials in contact -   risk of bleeding or interaction, sometimes leading to degradation
• use of glues -   polychloroprene: risk of yellowing -    natural latex or rubber solutions: risk of tackiness or dark shadows from absorption of fatty matters or plastic coatings.

It is necessary to have a good knowledge of the materials that you are planning to use, their composition and properties, in order to prevent any potential interactions. 4.2 Trials Trials are often necessary either as a preventative measure or after the event to understand any unforeseen phenomena. Tests can be carried out internally, if the company has the equipment, or outsourced to a laboratory such as CTC. Preventative tests The tests to be carried out depend on the risks described in 4.1. The aim of these tests is to:

• ensure the quality of a material or its manufacture;
• allay doubts about the compatibility of the material with the other components of the article and detect any potential migration;
• test a prototype article to ensure its behaviour over time.

When a potential risk is present, you cannot cut any corners. With regards to migration, there is a normalised test method: EN ISO 15701. Beyond some material difficulties in carrying out the tests, the principal fault in this method is that tests are only carried out in a dry atmosphere which does not bring to light all migrations nor all reactions between materials. This is why CTC created the Migrakit test more than twenty years ago. The kit comprises: a set of tongs for holding the materials together to be tested, an oven to bring the test tubes to the trial temperature and a humid container to place into the oven. This has been adopted by numerous companies, particularly in leathergoods production. The tests are generally carried out at 60°C and testing time can range from a few hours to seven days (mostly in exceptional cases). A visual examination of the samples is carried out at the end of the tests as standard. The results are fairly similar to those attained according to the norm (of equal duration) but by carrying out the tests in humid conditions, in addition to the dry test, the possibility of detecting potential migration and interaction is considerably increased. It is fitting to conclude by reiterating the importance of the ageing test carried out on a prototype or on an article withdrawn from the manufacturing chain, particularly when a change has been made. This enables us to detect and thus prevent the majority of problems that can crop up over time, particularly migration. Evaluation Proper assessment, after the fact, determines the cause of a problem or a fault in the articles (most often returns from customers). Many of the cases could have been avoided by establishing a prevention policy. Case number 4 - Even fabrics can bleed We tend to think that migrations only occur between component parts within a manufactured article. The following example shows that we should also be wary of internal migration, including textiles and in particular in the presence of a coating. A leathergoods manufacturer makes three pretty evening bags in red and beige brocaded fabric. To be more precise, in the brocade the patterns produced are woven by playing with the warp and weft yarn of different colours. The fabric is then re-covered with a thin layer of transparent pvc meant to protect it from dirty marks and give it a shine. Very quickly the bags are returned to the retailer because, through carrying, the bags become dark red in places, and the original pattern is wearing off. It can be explained as follows: the red thread used in the weaving bleeds in warm, humid conditions into the pvc coating, causing a complete reddening of the coating. As the bag has no handle, when carried it is indirect contact with the hand, which transmits the warmth and humidity referred to. Conclusions: verify the robustness/reliability of dyes. Be vigilant when pvc is present, even in the form of a thin layer, particularly if the pvc is in contact with a dyed material (in this instance the fabric). The test would have detected the defect in two hours (the duration necessary for it to be detected in the warmth humidity test carried out by CTC). A simple artificial ageing test on a prototype article, could have also immediately revealed the problem.