Patent Publication Number: US-2007099529-A1

Title: Pressure equalization fabric for hydraulic hot press facilities

Description:
The present invention relates to a pressure equalization fabric, particularly a pressure cushion, for use in hydraulic hot press facilities, comprising metal threads, both as warp threads and also weft threads of the pressure cushion, and high-temperature-resistant elastomer threads, the latter preferably having stabilizing core threads, the elastomer threads being situated alternating with the metal threads either as warp threads or as weft threads.  
      Pressure cushions are used in various types of pressing facilities, for example, single-opening short cycle presses for laminating particle board panels, MDF panels, or HDF panels with duroplastic resins such as melamine, melamine-urea mixed resins, or phenol resins, but also in multi-opening high-pressure and low-pressure presses for producing high-pressure laminate, compact, and plywood panels. In principle, pressure cushions may be used in all pressing facilities. Pressure cushions of this type are typically constructed in the form of a fabric, the materials being highly temperature resistant, since the usage temperatures in the pressing facilities are sometimes above 200° C. Furthermore, if possible, they are to have a large resilience property in the event of intermittent pressure load and possess rapid thermal conductivity.  
      Duroplastic resins are very sensitive in regard to pressure tolerances in the implementation of surfaces on the above-mentioned panel materials. Thus, surface disturbances may result due to unequal pressure distribution. Significant thickness tolerances are known to exist in the coating facilities, which add up from the pressing facility, pressing plates, and the particular pressed product. The pressure cushions therefore have the object of equalizing the thickness tolerances and transmitting the compression pressure uniformly over the entire area onto the pressed product, rapid and uniform heat transmission of the heating plate temperature of the pressing facility to the pressed product being desired simultaneously.  
      For example, pressure cushions according to the species are known from DE 90 17 587 U1. The flexible cushion fabric comprises a yarn made of aromatic polyamide, which is possibly mixed with other yarn materials. The textile fabric contains metal threads, the proportion being between 0 and 70 weight-percent, to set the required thermal conductivity accordingly.  
      A pressure cushion for high-pressure and low-pressure presses made of various materials is disclosed in EP 0 713 762 A2.  
      A pressure cushion also known from the prior art is known from EP 0 735 949 B1, the warp threads and/or weft threads having a silicone elastomer and metal wire in the form of solid threads or having silicone sheaths being contained in the fabric. Furthermore, a pressure cushion comprising a fabric is known from DE 200 08 249 and EP 1 136 248 B1, which has a significant proportion of a blend elastomer, which is produced by cross-linking a mixture made of a silicone rubber and a fluorinated rubber or made of a silicone rubber and a fluorinated silicone rubber. The proportion of fluorinated rubber or fluorinated silicone rubber is to be at least 4 weight-percent, preferably at least 10 weight-percent. The warp and/or weft threads contain metal threads.  
      Furthermore, a pressure cushion which is used for producing circuit boards is known from EP 0 978 528 A1. The previously known cushion may have a fabric, a paper, a film, or a leaf-like structure, which are each combined as a layer with at least one further layer made of a fluoroelastomer. The fluoroelastomer is preferably to have a fluorinated rubber component of the polyol vulcanization system, a vulcanization agent, a vulcanization accelerator, and an acid acceptor.  
      A further pressure cushion is disclosed in EP 1 300 235 B1. This is a metal fabric, comprising core threads having stainless steel sheaths and twisted copper wires having aramid cores, the metal fabric being provided with a silicone rubber coating. To ensure heat transmission, the copper threads project out of the silicone rubber surface.  
      Finally, DE 297 21 495 U1 is also cited, from which a pressure cushion for use in lamination presses having a fabric is known, which has two groups of threads running in parallel, the threads of the first group and the second group intersecting. The threads of the first group comprise metal or at least partially have a metal component, and the threads of the second group comprise rubber-elastic material or at least have a component thereof. To provide a pressure cushion having better thermal conductivity, the thread density of the threads of the second group is so great that the threads of the first group only pass through the second group by compressing the threads of the latter. Because the threads of the first group are forced due to the small distance between the threads of the second group to pass very steeply between these threads, short paths result between the two surfaces of the pressure cushion, so that the heating of the pressed product occurs more rapidly.  
      For modern coating technology, as currently occurs in single-opening short cycle presses, for example, the cited pressure cushions have disturbances in their physical structure.  
      Especially in highly sensitive HDF (high-density fiberboard) panel coating having melamine resin overlay and Al 2 O 3  filler and a melamine resin decorative film for producing floor panels, extremely high requirements are placed on the pressure cushion, so that in the event of heat transmission from heating plate to pressing plate which is not adapted, surface disturbances of the condensed melamine resins occur. Since currently the shortest pressing times are desired, of course, the resin flow must nonetheless be controlled in such way that the formaldehyde vapor and water vapor arising during the resin condensation may also diffuse sufficiently rapidly into the paper web and the panel surface.  
      During the coating in the press, the melamine resins are first meltable at low viscosity and free-flowing and then cross-link to form insoluble, unmeltably hard, abrasion resistant, and thermally resistant plastics. This is a polycondensation, water and formaldehyde arising. The water and formaldehyde arising during the condensation reaction may not be released in the form of vapor, since the compression pressure which exists in the press is above the vapor pressure of the water and also presses the two gas products into the film layer or into the support plate. The flow time and the melt viscosity of the resin are essentially determined by the degree of condensation, the reactivity, and the temperature and must be tailored in such way that the vapor bubbles may travel off before the resin melt solidifies and the surface is implemented. If these vapor bubbles are not able to diffuse sufficiently into the support plate, cloudy or milky surfaces result. This cloudiness is caused by the different indices of refraction of the predominant materials. The existing temperatures are relatively constant in the pressing facilities, so that the resin flow may not be influenced through temperature change, one not wishing to change the predefined pressing times, of course. Since the heat flow is at difference speeds, but is uniform in all pressure cushions, no positive influence of the resin viscosity or the resin flow may be achieved in this way either.  
     OBJECT  
      The present invention is based on the object of suggesting a pressure cushion, with the aid of which better diffusion of the vapor bubbles is ensured and thus a transparent and closed melamine resin surface is achieved. In particular, the liquid resin phase is to be lengthened.  
      Achievement  
      The object is achieved in that the thickness in the uncompressed state of the pressure cushion is determined by the elastomer threads and thus the contact points of the pressure cushion with parts of the hot pressing facility at moderate pressure buildup are first formed by the elastomer threads, the elastomer threads being compressible in such way that upon further pressure increase up to the entire compression pressure, further contact points are formed by the metal threads.  
      The advantage of the pressure cushion according to the present invention is controlled heat flow like a pressure-dependent heat flow.  
      The parts of the hot pressing facility which come into contact with the pressed product, in particular the pressing plates, already cause melting of the melamine resin because of the temperature existing at the beginning of the pressing procedure, which simultaneously represents a viscosity reduction of the resin. The heat flow occurs during this time essentially only through the elastomer threads, since the metal threads do not yet come into contact with the heating plate and the pressing plate. Since the viscosity or the degree of cross-linking of the resin is temperature-dependent, the liquid resin phase is lengthened until the metal threads of the pressing cushion come into contact with the heat-delivering heating plate and the pressing plate due to the increasing compression pressure. Only when the total pressure of the hot pressing facility acts does an increased heat flow arise (only now do the metal threads come into contact with both the heating plate and also the pressing plate, through which the thermal conductivity of the cushion significantly increases). Therefore, there is an increase of the resin viscosity and/or the degree of cross-linking of the resin. This results in a transparent and closed surface being formed—in contrast to known pressure cushions. Even with deep embossed textures and relatively short total pressing times, the optimum pressure distribution causes the implementation of perfect surfaces. The pressing cushion according to the present invention thus allows a time-delayed increase of the temperature existing at the surface of the pressing cushion, to provide more time for flowing of the resin via lengthening of the “low temperature phase”.  
      The pressing cushion according to the present invention has metal threads both as warp threads and also as weft threads, because of which the pressing cushion is very dimensionally stable.  
      According to the present invention, the high-temperature-resistant elastomer threads are situated alternating with the metal threads either as warp threads or as weft threads. Either one elastomer thread and one metal thread may be situated alternately, or one elastomer thread and two or three metal threads in each case, or vice versa, the present invention not being restricted to the above-mentioned variations.  
      It is advantageous if the diameter of the elastomer threads is at least twice, preferably three times the diameter of the metal threads. With increasing ratio of the diameter of the elastomer threads to that of the metal threads, the length of the liquid resin phase increases and thus the time in which the vapors may diffuse into the paper web during the pressing procedure. However, too large diameters of the elastomer threads are opposed by the need for short pressing times.  
      Because of the high temperatures which act on a pressing cushion, it is advantageous if the elastomer threads comprise high-temperature-resistant elastomers such as silicone rubber, fluorinated silicone rubber, fluorinated rubber, or a copolymer made of the above-mentioned types of rubber.  
      The stabilizing core threads may comprise single filaments, it again being advantageous if these are high-temperature-resistant core threads, for example, made of polyaramids such as Kevlar, or Nomex filaments.  
      In addition, it is especially favorable for stability reasons if the elastomer threads are equipped with metallic core threads either made of single threads or stranded metal threads such as copper strands, brass strands, (stainless) steel strands, etc.  
      To further improve the thermal conductivity properties of the pressing cushion, the elastomer threads may contain heat conductive materials, such as metal powder, in the elastomer sheath.  
      According to DIN 7724, elastomers are polymer materials which have entropy elastic (rubber elastic) behavior in the usage temperature range. Elastomers which are also referred to in common usage as rubber (with the exception of hard rubber) or vulcanizates, arise through primary-valency, wide-mesh cross-linking of rubbers or through cross-linking copolymerization of low-molecular-weight starting products. Elastomers may not be significantly deformed by the effect of heat or through moderate pressure. In general, their tension set is below 2%.  
      Exemplary Embodiment 
    
    
      The present invention is explained in greater detail in the following on the basis of an exemplary embodiment of the pressing cushion which is illustrated in the drawings.  
       FIG. 1 : shows a strand made of brass threads,  
       FIG. 2 : shows an elastomer thread having a core thread,  
       FIG. 3 : shows a pressing cushion fabric according to the present invention in the top view, and  
       FIG. 4 : shows a section through the pressing cushion fabric from  FIG. 3 . 
    
    
       FIG. 1  shows a metallic strand made of individual stranded brass threads  2 , this metal thread  1  being processed both as warp thread and also as weft thread of a pressure equalization fabric  6 .  
       FIG. 2  illustrates an elastomer thread  3 , which comprises a core thread  4  and an elastomer sheath  5 , made of a copolymer fluorinated silicone/silicone rubber, for example. The core thread  4  comprises stranded copper threads. In the present example, the elastomer thread  3  is processed as a weft thread of the pressure equalization fabric  6 .  
      The elastomer threads  3  in the warp, which are bent upward, and the bent metal threads  3  in the warp and the weft of the pressing cushion fabric may be recognized in the top view of the pressure equalization fabric  6  according to the present invention illustrated in  FIG. 3 . The elastomer threads  3  have a significantly larger diameter than the metal threads  1 , the diameter of the elastomer threads  3  advantageously being at least twice as large as that of the metal threads  1 .  
       FIG. 4  shows a section through the pressing cushion according to the present invention from  FIG. 3 , which is situated in a pressing facility between a heating plate  7  and a pressing plate  8 . The contact points of the pressing cushion are first formed only by the elastomer threads  3  at moderate pressure buildup. The metal threads  1  all have a distance, even at their extreme points (“wave peaks”), from the plane formed by the extreme points (“wave peaks”) of the elastomer threads  3 , whereby the thermal conductivity is initially lower. Only upon further pressure increase up to the total compression pressure are further contact points  9  formed by the stranded brass threads  2 , so that a more rapid heat flow is then produced.  
      List of Reference Numerals  
       1  metal thread  
       2  brass thread  
       3  elastomer thread  
       4  core thread  
       5  elastomer sheath  
       6  pressure equalization fabric  
       7  heating plate  
       8  pressing plate  
       9  contact point