Author:
Dipl.-Ing. (FH) Stefan Muller - Director Nonwovens of BRUCKNER Plant Technologies GmbH & Co. KG / Leonberg (Germany)
BRUCKNER Power-Frame - innovative thermal treatment with stenter technology
The company BRUCKNER as market and technological leader supplied more than 5.000 stenter all over the world. With its innovative stenter technology BRUCKNER is also a competent partner for convective thermal treatment in the nonwovens industry. Combined with an efficient and customer-oriented air circulation concept from BRUCKNER's range of products the required product features can be achieved with a minimum consumption of resources. In the field of nonwovens web bonding the use of stenter technology is concentrated on the drying or the additional thermal and chemical consolidation of nonwovens which are already pre-bonded. In the field of nonwovens finishing the stenter can be used almost without limits and universally.
The stenter technology opens additional possibilities of thermal nonwovens treatment
Until the beginning of the nineties it was sufficient to describe the production of nonwovens as two-stage process. Since then the production of nonwovens changed and became a three-stage process. web forming ¡V web bonding ¡V web finishing
1. process stage: web forming
By means of different well-known processes a planiform fibre matrix is made out of individual fibres. With its insufficient stability this fibre matrix does, however, not meet the requirements of a use in practise.
2. process stage: web bonding
There are mechanical, thermal or chemical processes for the bonding of the nonwovens webs. Formerly the nonwovens satisfied often the requirements already after the consolidation process. But the pressure of competition increases also in the nonwovens branch for many years. The expectations of the final consumer increased as well, which forces the nonwovens producers to improve permanently their products and to make innovations.
3. process stage: web finishing
In many cases the classic processes of textile finishing are used to achieve such aims, of course after being very well modified for the special needs of nonwovens.
The thermal treatment of nonwovens plays an important part not only in the field of nonwovens bonding but also in the finishing of nonwovens and can be used in the form of contact heat, radiation heat, convection heat or in combined systems. Continuously working heat treatment chambers comprise mainly the machinery frame with insulation housing, the material transport system, the heating system and the air circulation system in case of convection machines. The term "stenter" or "stentering machine" is generally connected with a thermal treatment tunnel with convective system (circulation of hot air) through which the nonwovens material is continuously transported by means of two stenter chains.
Possibilities of process-oriented use of stenters
A basic pre-condition for the use of stenter chains for material transport in the field of nonwovens bonding and nonwovens finishing is a certain tensile strength in cross direction of the material to ensure a reliable hold of the fabric web during the thermal treatment.
Use in nonwovens bonding
In the web bonding the use of stenter technology is limited to the further treatment of nonwovens which have already been pre-consolidated (by needle punching, knitting or hydroentangling). The aims of this additional process stage are:
¡E thermal fusion (melting of thermoplastic fibres or powders to improve the mechanical characteristics)
¡E heat-setting (heating of the material to heat-setting temperature in order to increase the dimensional stability and to eliminate the residual shrinkage / for hi-tech fibres BRUCKNER even supplies lines with a treatment temperature of up to 400 ¢XC)
¡E drying (evaporation of process-related water and possibly curing of chemical agents).
This will improve the strength and elongation values of the final product or the product will be provided with additional specific features. As an example we can name nonwovens for geotechnical use, wipes, filtration media and automotive applications.
Use in nonwovens finishing
In the field of the nonwovens finishing the stenter technology is the optimum solution since the nonwovens have almost always a sufficient strength in cross direction. The material guidance via stentering chains is in this case superior to all other transport systems (drum, belt, air cushion nozzles):
¡E Beside the reliable fixing of the nonwoven web when passing the thermal treatment chamber (prevention of edge turnover, no fluttering of the web, no uncontrolled shrinkages) only the use of stentering chains allows a reproducible and controlled influence on the web dimensions (stretching in cross direction, shrinking in cross direction, shrinking in material flow direction).
¡E Another essential advantage is the extremely low soiling of the chain transport system since an impregnated nonwovens web touches the stenter chain or pin bar only at the outermost edges. In drum type or belt dryers the impregnated nonwovens web is supported over the entire surface, which often leads to marks on the substrate surface or to strong soiling of the drum surface or the transport belt. This leads to lower product qualities and correspondingly higher cleaning efforts and thus to higher production costs.
Hence, the stenter technology is perfectly suitable for a great number of nonwovens applications such as finishing, coating, printing, dyeing, impregnating, condensing etc.
The patented SPLIT-FLOW system and the DUO-THERM technology
For an air impingement onto the web from above and below and for special requirements BRUCKNER has the patented SPLIT-FLOW System and the DUO-THERM technology in its range of products.
¡E The SPLIT-FLOW system is characterized by the fact that the circulating air quantity led into the upper and lower air plenums can be regulated continuously and independently of each other via two fans in each partial section with 1500 mm length. This unique and innovative technology prevents unnecessary additional costs for electrical power which would otherwise be caused by the normally used dampers and the connected pressure drops.
¡E With the DUO-THERM technology the customer has in addition the possibility to set the circulating air temperature above and below the fabric web differently of each other. This is particularly interesting in case of nonwovens which have two different material sides (for example composites materials made of different fibres or one-side coated webs. The setting of the optimum treatment parameters for each nonwovens side (temperature and circulating air volume) leads to a clear increase of the throughput and to the best possible final product characteristics.
Through-air stenter with SUPRA-FLOW technology
BRUCKNER offers with its SUPRA-FLOW technology a special feature regarding energy efficiency and process control for through-air stenters. This is a combined material transport system comprising a horizontally circulating roller chain and an air-permeable transport belt as well as an air circulation system in which the circulating air flows always from the top down through the nonwovens and the transport belt.
The SUPRA-FLOW system has clear advantages. The modular structure with several heating zones allows to set optimally the circulating air quantity, the circulating air temperature and the exhaust air flow in each phase of the thermal treatment process. Different parameter adjustments in the different process phases can have considerable advantages. Additionally, the complete process air streams in the SUPRA-FLOW technology in counter-flow to the material flow direction. Therefore a great part of the cooling air is re-used for thermal treatment and the exhaust air can be charged with maximum humidity. This leads to a clear reduction of the energy costs, particularly in drying processes.
Resume
The increasing competition forces also the nonwovens sector to permanent product improvements and diversifications. For this purpose textile finishing processes are particularly suited. The stenter technology is however, by many nonwovens producers still considered as finishing machine for classic woven and knitted fabric textile industry. Innovative producers, however, recognized the advantages for the treatment of nonwovens already a long time ago. Thanks to the stenter technology they create new products and get considerable competitive advantages. With 60 years of experience in the finishing of textile products and more than 5000 delivered stenters BRUCKNER is the undoubted market and technology leader in this sector.
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