Microsphere application in the leather industry

Dr Tegtmeyer, head of product development & application in LANXESS's Leather business unit, told delegates that the technology had been developed over the previous 15 months in Leverkusen. The product has been named Levotan X-Cel and the Leather business unit were in the process of installing pilot plants for processing 'X-Cel leathers' in their laboratories in each geographical region in order to carry out initial trials on customers' material in situ. The next step was to develop full production-scale processing plants that correspond to tannery requirements regarding size and speed in collaboration with the Italian machine manufacturers Fratelli Carlessi SpA. I. Introduction The use of microspheres in the leather industry is currently most commonly applied as an auxiliary in special finishing binders. Several applications have been studied and have already been introduced into the market. However, a number of   interesting new application possibilities are still open for further investigation. There are two general forms of microsphere containing polymer dispersions for upgrading in the market:

• dispersions with small compact microspheres with an average diameter of between 5-20m. These spheres are filled with a liquid gas which can expand up to 40 times their original size to a much bigger microcell by a thermo activation treatment (the expanded form is referred to as m microcells to differentiate them).
• Secondly, dispersions containing already pre-expanded microcells of an average diameter of between 40-60m. These cells are already filled with a gas and cannot be further expanded.

The use of the unexpanded form - the microsphere - always has to be followed by an expansion step. Due to this expansion process the liquid gas in the unexpanded sphere converts into its gas phase, the pressure inside the sphere is reduced and the density of the particle changes significantly from around 1kg/cm3 in the unexpanded form to approximately 30g/cm3 in the expanded form. It is important to know that in both the unexpanded and expanded form, the shell of the microcell is absolutely impermeable and the gas cannot escape without the destruction of the cell itself. Microspheres and expanded microcells in both forms have a high resistance to chemicals such as retanning polymers. The expanded form of the cell especially is not hard but highly flexible and resilient. For a better understanding of dimensions, it is possible to compare this process of expansion with the idea of a tennis ball which is filled with a liquid gas and expands to the size of a big, elastic gymnastic ball. The expanded spheres are perfectly suited to fill loose areas and defects because they fill without increasing the density of a leather and have an elastic structure. II. Use of microcells containing polymers Currently there are two main reasons for the use of microcells containing polymers in the leather making process: 1. Upgrading leather by the elimination of defects 2. Low density finishing which results in better aesthetics of the final leather. In terms of upgrading leather, we again differentiate between two completely different applications: 1a. Upgrading the surface with a simple spray process to cover holes and scratches in the grain in order to increase cutting yield 1b. Upgrading the internal structure of the leather Tegtmeyer focused on the second of these which he referred to as structural upgrading. He said that this completely new application for the leather industry offered the tanner a lot of new opportunities to upgrade lower quality leather; something which tanners have long wished for.  This new application consists of the addition of a special auxiliary containing the unexpanded  microspheres in the wet-end process, and an expansion step of the leather at a later point in the process with a customized, new and patented expansion technology. The microcells, through expansion, can fill open and loose areas inside the leather. It could be described as a type of ‘internal stucco' to upgrade the structure of leather. II.1. Incorporation of the microspheres in the leather    A number of different trials have been carried out to determine the right product composition for this new application. The microsphere containing auxiliary is added in the retanning step at a relatively early stage of the wet-end process. It is a combination of several parameters, eg the right size of the microsphere, the right product formulation, the required auxiliaries etc, and obtaining a high exhaustion rate for the microspheres of more than 95%. Temperature and pH condition remain comparable to a normal wet-end process together with a conventional retanning process, whatever the respective application might be for, eg, furniture, automotive or shoe upper etc. Due to the relatively long but normal running time in the drum during the retanning process the micro-spheres have enough time to penetrate the leather and to be correctly distributed inside the leather. The distribution is based on a normal physical diffusion process and it has been shown that, after the wet-end process, the unexpanded micro-spheres are ending up especially in loose areas and open channels of the corium layer, eg in prominent veins. The retanning process is followed by a normal drying step through vacuum, toggling or hanging. Investigations of the leather have clearly shown that no significant expansion happens to the microspheres inside the leather during these drying steps under normal conditions. It looks as if the temperature inside the leather is not high enough to activate the micro-spheres at this stage.    II.2. Activation of the micro-spheres in the leather    Since the standard drying process through vacuum or toggling is not sufficient to activate the unexpanded microspheres, an additional process step is required for activation. Based on a new expansion technology by steam, including special machinery, it becomes possible to expand the microspheres inside the leather. The principle is based on a big steam powered iron similar to the smaller versions used in the home. The dry crust passes through a channel, where it is treated with hot, saturated steam for several seconds only. The steam penetrates the leather without making it wet and the humidity transports the  temperature within seconds inside the leather to the unexpanded spheres. These special conditions (humidity and temperature) lead to a softening of the shell of the microspheres and they immediately expand inside the leather to form microcells. The size of expansion depends on the structural environment of the particle; the looser the area, the greater the increase in size - up to 40 times - if it is in a tighter area, the expansion will be much more moderate. Once the micro-spheres are expanded they cannot be removed and you achieve an almost permanent elastic filling of the leather. Through this operation open holes/channels and loose parts in the corium get filled with microcells without making the leather become heavy and  hard. Depending on the amount of microspheres offered during retanning, open areas are not only filled after expansion, there is also a significant increase in the overall thickness of the final leather. The leather could be ‘pumped up' and you can achieve an increase in thickness of 0.2 mm up to 0.8 mm, eg from 1.2 mm up to 2 mm in thickness.    The absolute values for tear strength don't decrease with the increase in thickness and the leather retains its strength. However, this also means that the tear strength in relation to the thickness becomes smaller. On the other hand, other important values such as water permeability and water absorption (eg. water proofing) are going to be improved through filling voids etc with microcells. Since open areas are filled evenly and the soft and elastic microcells are not fixed inside with a binder, the leather becomes soft after milling while maintaining the increase of the thickness and filling. It is also possible to carry out this expansion step at a later point of the process, eg after finishing. In particular a milling prior to an expansion has led in some cases to a significantly better structural filling. However, the right timing and parameters for this expansion step have to be evaluated case by case, and depend on  the article and the specific upgrading requirement, which will be achieved.       III. Possible Applications on Leather    The feasibility of this new technology was tested in the pilot plant at LANXESS.