Felts for pelts10 January 2001
Felts are used in three important areas of leather production, sammying, setting out and finishing. Thus, there are three basic types of felts, one for each of the processes. They are distinctive with characteristics that are fit for the purpose they are intended for. The 'character' of the felt comes from the mix of synthetic fibres and wool. When Guido Mattei started the company, felts were 100% wool, but over the years he and his colleagues have developed combinations of synthetic and wool. It is this composition that makes the felt suitable for its purpose and where the art is found in felt making. Analogous to the lime-end manager who came into the tannery at the beginning of the shift, sniffed the air and commented that 2% more sulfide would give a perfect liming, so the felt works manager has worked out the best recipe for the felt. This he does by experience, knowledge and an understanding of what the felt will be used for. Thus, a felt is not just a felt. Taking the processes separately, there are wet-end and finishing felts. The process is described followed by the uses the felts are put to. Process Wool from various sources, including South American and Australian grade A and synthetic fibres including polyester and polyamide, are the basis of felt. The wool and synthetic fibres are mixed by weight into the percentages required. This varies depending on the final use of the felt. They are then put into a carding machine and processed. The carded composite is then either woven into a continuous sock or processed to produce a non-woven fabric. Five or six strands of fibre, which give the strength, are woven into a continuous sock, which is used as the basis for the felt. The remaining mix is taken and carded on a machine so that the fibres line up north/south. The direction of carding is then changed and they are carded east/west, before finally being layered to a specific relative density. The density is measured using an americium isotope (241Am), which is an alpha particle emitter. The alpha particles are easily stopped by the felt, so the machine is measuring the number that can penetrate. This will be a measure of the density. The folded fabric is put onto a roller and transferred to a machine for final embedding into the woven sock. The non-woven fabric is meshed into the woven sock using a bed of needles, which push the non-woven into the sock. The needles are like fishhooks and the density of hooks affects the type of felt being produced. The felt is gradually built up until the required product is achieved. The quality of the felt is measured by determining the permeability. This is achieved by blowing air through the felt at a certain rate and pressure and determining how much passes through. The felts are then put through a finish press. This was installed about six months ago. The Bergi built finishing press is used in the textile industry and pre-stabilises the felt before delivery to the tannery. The process puts the felts through a phase of controlled operation at pre-set temperatures and increasing pressures over a period of time, until reaching values well over the tannery's maximum conditions. During the operation the exact thickness is reached and maintained electronically to ensure the desired density is attained. Finally the felts can be finished with resins to give a final guarantee of quality. Wet-end - sammying For sammying, the best composition has been found to be 70% synthetic and 30% wool. All felts are constructed by bringing together woven and non-woven fibres into a mesh that gives the qualities needed. These include the ability to drain water from the wet pelt. But, it is far more than that. The felt needs to accept pelts of different substance, often within one skin; to remove water to a consistent degree, whatever the condition of the leather; to let the leather flow out and reduce the chance of creasing and, hence, reducing the area obtainable. And, to do all this every few seconds, time and time again and still spring back to the same thickness. While the felt will not increase the area gained it can certainly help to limit the loss in area caused by poor processing. Another problem to consider is the build up of extraneous materials such as grease, either natural or from fatliquors used in the tannage, the tannage itself and any other materials that may be extracted during sammying. This build up needs to be cleaned off periodically. Wet-end - setting The problems are similar to those of sammying. There is still a need to drain water and to spring back to a consistent thickness after each pass. The difference is that retanning materials often have a larger particle size and so would be trapped in a sammying felt. To overcome this problem Mattei have developed felts that have a more open weave and so allow the synthetic tan or vegetable tan particles to pass through. The different structure is achieved by varying the machine parameters and by the use of different fibres composition. There are also differences depending on whether the felt is installed on a through feed machines such as those from Escomar, Bauce, Rizzi, 3P, CM, or whether they are installed on round rollers for discontinuous feeding. With the former, the percentage of synthetic fibres is greater at about 85%, compared with 70% for the non through-feed machines. Finish felts Until recently, these felts were 80% wool and 20% synthetic fibres. However, there were a number of problems associated with this. During the embossing and ironing processes there can be a problem with static electricity. This causes unwanted phenomena such as: * a shock is felt by the operator receiving the leather every time s/he takes the leather off the press * dust in the area will be attracted to the surface of the pelt or, worse, to the grain. This can lead to problems of surface dullness or specks of dust seen in the finish * the static electricity causes the leather to stick to the felt, which on softer leathers means that there could be an increase in the formation of pleats along the belly and necks and it may cause grain marks on the hides or skins To reduce this problem, Mattei added an anti static non-metallic superconductor, ASP, to their primary felt mix. Provided the machine is grounded the excess electricity will be will be discharged safely. Mattei also control strictly to ensure the felts are 100% free of any metal parts. To this effect they issue a 'metallic particle absence' (MPA) certificate which states that any felt sent out has undergone the MPA test and is free of needles or metal particles that could seriously damage the leather processed and the equipment used. Redesign However, recent developments obtained through ongoing research have brought about a redesign of the felt. The composition is now 60% synthetic, 27% wool and 13% MTT fibres. It is the latter that makes the 60/27/13 felt unique. The synthetic component provides the anti-static and heat resistance, the wool is a natural anti-static as well as providing the felt with resilience and absorbance. The MTT fibres combine the strength of the synthetic fibres with the positive characteristics of the wool. The other advantages of using the MTT fibre is that the resultant felt is cheaper than the 80/20 felt, it has a longer life because of the synthetic component but maintains the cloned effects of the wool. The success of Mattei Guido e Figli can be seen by their domination of the felt sales in the leather industry, especially with wet-end felts. Many, if not all, the machines in this past edition of Tanning Tech used Mattei felts. The new 60/27/13 should see that domination continue into the finishing felts as well. As if to highlight the growing business, Mattei have recently begun to supply a large group in South America with felts. The continuing success of their business in South America means that from the beginning of 2001 they will have a base in Uruguay to supply that market with felts on a just-in-time basis. The company, Mattei Mercosur, will be set up in a free trade zone and be able to supply the tanners of the Mercosur countries with felts a very short time after they are requested.