Putting the shine on1 December 2009
BLC Leather Technology Centre present an overview of patent and metallic fashion finishing effects that can be seen on more and more leathers these days. Every few years patent and other highly fashionable finishes applied by foils such as metallic become heavily demanded but often present a range of technical difficulties to the tanner or finisher
Leather is a fashion item subject to the seasonal and annual variations that occur in the fashion industry. As a result, every few years there is a demand on the leather industry to produce patent and metallic leathers. These trends certainly add sparkle to a wardrobe either in the form of shoes, handbags, belts or even full garments.
The potential downside of fashion trends being imposed on a traditional industry such as leather is that new products and effects will invariably have their own unique problems and technical issues. The recent metallic trend has imposed the development of metallic finishes on leather suppliers resulting in an increasing number of technical problems and complaints within the industry.
Patent leathers are also currently highly fashionable, but tanners, retailers and end users need to take care to avoid the many potential problems and issues with the manufacture of this type of leather. It is important to ensure good practice is employed in the coating system used. Patent leathers were traditionally produced in black or white for evening wear and children’s footwear, but increasing fashion trends have led to the production of patent in a myriad of colours. New patent leathers are also being developed to provide two tone, metallic and pull-up patent finishes.
During the year 1818, Seth Boyden of Newark, New Jersey, began to investigate the possibility of creating a version of leather that would retain its desirable qualities of protection and durability whilst at the same time having a more dressy appearance.
Originally, this was accomplished by applying layers of a linseed oil finish to fine grain leather, gradually creating the sleek appearance which resulted in the production of glossy leather that was quickly adopted as a complement to formal dress.
Coating with linseed oil required considerable time and effort. Many individual applications were needed with a long drying period for each coat, which meant that it could take up to four weeks of production time. As time went on, the invention of plastics impacted the methods for producing patent leather.
Plastic finishes were able to produce effects similar to the application of several treatments with linseed oil, with the advantage of considerably less monetary investment on the part of the producer. The original methods of production as well as being time consuming also had poor fastness properties. Technology has moved on considerably and genuine patent leathers are produced by a solvent based polyester/isocyanate cross-linking system. There are also water based finishes to produce the same patent ‘wet look’ which are easier to produce with greatly reduced problems and hazards.
The standard method of finishing, which applies to the majority of patent leathers, is outlined in the next section. However, it is also possible to use transfer papers to produce the patent effect. Great care needs to be taken when using these papers, which can either be the ‘whole’ finish including base coats or simply the patent top coat. The papers are applied using a conventional tannery press at 125°C for 3 to 5 seconds at 100 bar pressure.
Standard method of production for patent leathers
It is the final coat applied that gives the required gloss, but the base coats and leather preparation also have a profound impact on the final properties.
Most patent leathers are produced on corrected grain. Careful preparation is required and the leather needs to have good buffing properties and a tight flat grain. One of the main reasons for the grain correction is that the production of such a high gloss finish needs a surface free from blemishes. The surface is then heavily impregnated, covered with pigment coats prior to the application of the final gloss. Vegetable retannages are used to enhance the deep buffing and often the final surface deposition of an acrylic resin to enhance the tightness. It is also important to ensure that the fatliquors used penetrate to ensure the surface is free from grease.
Dust is a major enemy of patent leathers so it is good practice to spray the flesh side of the leather with a light seal to avoid subsequent cross-contamination.
The finishing sequence comprises of various coats and it is essential that the formulae promote good inter-coat adhesion. The normal sequence is impregnation, base coat, intermediate coat and patent top coat.
The formula needs to be carefully controlled and includes:
- Impregnating resin of the correct particle size
- Wetting agent to reduce the surface tension without creating poor wet adhesion
- Strict control of the balance of the resin to penetrator (typically 10% penetrator by volume).
- Controlled and consistent deposition of the resin at the grain corium junction (typically between 3-5g/sq ft).
This is the main fully pigmented covering coat. Selection of resins is important and must reflect the final properties required in the final leather, such as resistance to cracking and solvents, good ironing properties plus wet and dry adhesion.
Adhesion problems can be minimised by avoiding waxes, oils and silicones. The base coat needs to be applied at levels of between 14-16g/sq ft and is usually applied by several rollercoater applications with an intermediate ironing.
On conventional leathers this would be called the top coat. This is a heavy coat of nitrocellulose lacquer which provides a bond between the base coat and the patent top coat, improving adhesion. It is usually applied by spraying. Within this coat it is possible to produce ‘effects’, eg antique, two-tone, tips, by the addition of dyes and pigments.
Patent top coat
The application must be carried out in dust free conditions with up to 20g/sq ft (lower amounts for soft leathers). It is important to ensure that there are no air bubbles in the finish, resulting in the application methods being airless spray or curtain coater. Solvent selection is crucial and any solvents that contain water will give a tendency to bloom. The drying can take longer than 12 hours and it is essential to use an isolated room. The area used must be sealed to prevent airborne dust contamination in the surface finish. The drying is usually 2 hours at 25°C followed by around 10 hours at 50°C. Great care is required in handling the final product to prevent the flesh coming into contact with the grain surface.
What can go wrong?
- Tacky feel and fingermarking
This is probably caused by insufficient crosslinker in the top coat, resulting in a soft surface. To clean, wipe the whole surface with a cloth impregnated with a silicone-based cleaner.
This can be caused by the combination of adding a ‘thick’ finish to a thin loose leather. The finish accentuates the looseness problem resulting in creasing.
- Cloudy dull appearance can be caused if the patent coat is contaminated with water. Consider changing solvents.
- Air bubbles in the finish. This can be caused either by dirt on the surface prior to the patent top coat or incorrect viscosity of the patent top coat, affecting the flow out.
- Poor flex resistance. Check the failure to determine which coat has failed. It may be that there is too much crosslinker in the patent coat.
- Problems with transfer papers. Peeling or lifting due to poor bond with leather. Defects in transfer papers allowing humidity to penetrate and lift the transfer papers.
Production of foiled leathers
There are many types of foils and transfers that can be applied to leathers including:
- printed effects
- metallic foils
- crackle foils
- multilayer foils
- lacquer/coated films
- holographic films etc
The production of metallic foiled leathers can be split into a series of stages which are summarised below.
Before the foil is applied the following considerations must be made:
- The leather should be clean and dry crust (can be dyed) without any type of finish.
- The leather should be free from dust and with a dry, non-greasy surface. Regular checks at the tannery are recommended.
- The leather should be flat before application with pleats or folds removed by trimming.
- The plate of the press should be pre-heated to the required temperature before transfer.
- The felt in the press should be covered with a sheet of Teflon to improve handing and efficiency.
Application of the adhesive
- The amount of adhesive applied is important and should only be sufficient to ensure good adhesion without peeling. This may initially require development and the amount applied should be monitored for consistency during production. Too much adhesive can result in the whole film peeling off with the foil, this is particularly likely under wet conditions. Too little adhesive can mean low film strength, resulting in low adhesion and ‘powdering off’ of the foil particles.
- The adhesive is usually applied by spray although rollercoaters can also be used. The absorption to the surface is important and must be consistent from pack to pack. Unfortunately, a process of trial and error is required to find the optimum amount for each particular type of crust.
- The choice of adhesive is important and is determined by the type and absorption of the crust. The adhesive is usually an aqueous polyurethane, which may need diluting slightly for spray application or thickening if applied by rollercoater.
- Usually the adhesive is dried prior to application so it is tacky. Sometimes the foil is applied without the adhesive being dried (wet application). The best route for individual tanners depends on their leathers, circumstances and preferences. Trials should always be carried out to determine the best method and control checks to be used within the tannery. This is to ensure that processing
conditions are maintained. The conditions of manufacture of the end product should also be taken into account, including processes such as moulding, lasting in shoe manufacture and even shipping. Changes in heat and humidity can affect the adhesion of the leather/ adhesive/ foil bonds.
Typically, the foil is cut to the approximate size of the leather ensuring a slight overlap
without too much wastage. The first step is to place a Teflon sheet on the felt and then add the leather followed by the cut piece of foil. The latter two are often assembled in advance and placed into the opening on the hydraulic press. Application temperatures and pressure vary but table 1 shows an example of normal settings.
After the foil has been applied it is advisable to leave the leather to mature for around 4 hours to achieve the best results in terms of adhesion, scuff and gloss. The backing carrier paper from the foil should be removed in one constant movement. Further operations such as staking, milling and ironing can be carried out on the foiled leathers to achieve the required handle.
As with other leather types, foiled leathers are subject to their own unique series of problems. Some of these are described here with the potential causes.
There are several reasons for foil peeling:
- If the surface of the leather is not correctly prepared, (for example too oily or dusty) the foil may not adhere correctly to the surface
- Incorrect application of foil, for example too high temperatures or pressure
- Poorly made foil
- Foil damage during product manufacture: high temperatures/pressures and solvents from glues
- High temperature and humidity during transport
- Leather with too much stretch, which causes the foil to crack when pulled out
- Cleaning of leather with a product which is not suitable
Damage, during one part of leather or product manufacture can make the foil become more susceptible to damage during subsequent operations. These problems can be observed under magnification as small cracks. This allows moisture (or other contaminants) to penetrate the foil leading to further breakdown and loss of adhesion.
Foil can fail in several ways. Foils are usually an aluminium layer with a coloured coating to produce a gold or silver ‘colour’. Failure can occur between the leather and the foil (caused by one of the reasons above) and the indication for this will be the base leather colour showing. The other area can be the foil itself, where there is failure between the aluminium and coloured coating on the foil, this shows as silver colour with the coloured component being removed.
Often foils can be ‘rubbed away’ from the leather and this can be caused by failure of the leather/adhesive/foil bond as outlined before. Another potential problem may, however, be related to over finishing (re-finishing) the foil to create fashion effects. It is important that the adhesion properties of the foil are tested to ensure suitability in wear.
In one example observed by BLC a foil had been sprayed with a nitrocellulose top coat followed by a coloured wax coat. The leather was then buffed to allow the silver foil to show through on the raised areas of the leather producing a worn antique look. A final protective nitrocellulose coat was then applied. Finish loss was observed and was caused by a failure in adhesion between the layers of the foil used. The foil was sensitive to the heat and humidity conditions that occurred during the over finishing and this resulted in a deterioration of the finish.
Application of any extra finish to a foil needs caution. It is important to ensure sufficient adhesion between coats to prevent layers from separating. To get good adhesion between layers it is necessary to get penetration of one layer into another. This in itself can cause a problem as migration of water/ humidity and solvent can affect the adhesion bond already in place. If the finish layer being applied to the foil is penetrating, then it may also be causing some damage. This then makes the finish more sensitive to further changes in conditions and more likely to show finish problems.
Fashion demands made on the tanner will always create situations where normal rules and standards are not applied. It is important that the tanner installs rigorous checks in their production processing to ensure that for metallic leathers the bonds between leather/adhesive/ foil are consistent. Once leather is foiled, care needs to be taken in subsequent processing as over-sprays and after treatments can affect the original bonds that have been formed. These problems may not surface until the leathers are subjected to variable heat and humidity situations, such as shipping or shoe manufacture. The tanner needs to implement sufficient safeguards and tests to account for this and BLC can provide advice on the areas of testing that should be considered.
BLC can help to identify the cause of problems experienced in the production of specialised leathers and offer technical advice and support to improve production procedures and ensure that appropriate product performance characteristics are achieved. For further information contact Stuart Booth on [email protected] or +44 (0) 1604 679956.n