Patent Publication Number: US-2023150173-A1

Title: Resin degassing

Description:
The present disclosure relates to degassing of resins, in particular resins for fibre-reinforced composite materials, such as fibre-reinforced composite materials for use in wind turbine blades. 
     BACKGROUND 
     As wind turbines and wind turbine blades increase in size, the risk of lighting striking the wind turbine increases. It is therefore of increasing interest to provide wind turbines and in particular wind turbine blades with the maximum possible strength. 
     Gas and water in resins can negatively impact the strength of fibre-reinforced materials based on such resins. Thus, there is a need for a system that is capable of providing resin with as little gas content as possible. 
     Current degassing systems are based for instance on boiling, heating, membranes configured for degassing, chemical degassing, and vacuum degassing. 
     Voids in the end product negatively affects the mechanical properties, including strength and the quality of the surface of the end product. Such voids may result for several reasons. Composites manufactured using evacuation, such as by vacuum-assisted resin transfer moulding (VARTM), the reduced absolute pressure, routinely below 30-300 mbar, can cause creation of relatively large pockets of gas that was dissolved in the resin before infusion. In that case, a post-infusion process is needed. Although this process ultimately results in a strong product that meets the specifications, the process is very time-consuming. It is therefore desirable to reduce the need for this additional manufacturing step, or at least reduce the need for it. 
     By exposing the resin to a reduced absolute pressure, the solubility of gases decreases. Reducing the absolute pressure also causes bubble formation, which allows removal of much of the gases. However, gases may still be dissolved in the resin, but the amount of gas can be too small for it to escape out of suspension for instance by formation of bubbles. These remaining gases as well as any water negatively affect the mechanical properties of the end product. Some existing methods deliberately add gas to the resin before performing the degassing process. 
     It is therefore an object of the present invention to provide an alternative system process for removing gases and water from resin, such as a process that may be more efficient than existing processes in one or more ways. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention provides a process for degassing a liquid resin in a degassing system, the degassing system comprising:
         a first duct having a resin inlet for receiving liquid resin to be degassed,   a resin pump for pumping liquid resin into the first duct via the resin inlet,   a flow control valve arranged at a downstream end of the first duct, configured to control a flow rate of resin through the first duct and configured or configurable to provide a pressure drop of at least 1.5 bar across the flow control valve,   a second duct in fluid communication with the first duct via the flow control valve,   a resin storage tank having a resin storage tank inlet in fluid communication with an outlet end of the second duct, and   a gas evacuation system operable to reduce an absolute pressure in the resin storage tank to below 100 mbar,       

     the process comprising:
         connecting a liquid resin source to the resin inlet,   pumping liquid resin into the first duct via the resin inlet using the resin pump to achieve a first absolute pressure of at least 1.6 bar in the first duct,   configuring the resin pump and/or the flow control valve to achieve a first pressure drop across the flow control valve of at least 1.5 bar,   maintaining, using the gas evacuation system, a pressure in the storage tank below 100 mbar at least partly concurrently with pumping resin into the first duct.       

     Systems and processes in accordance with the first aspect of the invention can mitigate or even eliminate one or more of the issues of the prior art described above. The high absolute pressure on the input side of the flow control valve assists in creating relatively large bubbles in the liquid resin following the flow control valve. This greatly increases the efficiency with which gas in larger gas pockets are removed. Additionally, and very importantly, the pressure drop acts as a catalyst for diffusion of smaller gas pockets, and even individual gas molecules, out of the resin and into the larger bubbles, whereby they can be extracted. In prior art systems, diffusion is not an efficient mechanism for removing gas pockets in liquid resin. Embodiments of the present invention allows this mechanism to play a much larger role. In a sense, the conditions described in embodiments of the present invention cause creation of a foam-like state that is surprisingly effective in removing air and other gasses from the resin, including by way of diffusion. 
     Another mechanism that, under the process conditions described herein, improves the degassing process compared to known processes is the motion of the resin through the degassing system. The motion further increases the extraction of gas from the liquid resin, including very small gas pockets in the resin. 
     In some embodiments, the flow control valve and/or the resin pump are controlled so that an average resin transit time from the output of the flow control valve to the inlet of the storage tank is in the range 10-120 s, such as in the range 15-75 s, such as in the range 20-60 s, such as in the range 30-50 s, such as in the range 30-40 s. This has turned out to be process parameters that allow a very high degree of degassing while maintaining a high flow. Longer transit times would mean that the system would have to be unnecessarily large, and shorter transit times leave the resin insufficiently degassed. 
     In some embodiments, the degassing system further comprises a chamber inline between the flow control valve and the storage tank, the chamber having a smallest flow area A 2  which is at least 20 times a largest flow area A 1  of a duct portion connecting the flow control valve and the chamber. It has been found by the inventor that such an inline chamber further enhances the mechanisms described above. This allows that the system further efficiently degas liquid resin at relatively high flow rates, such as those given in various embodiments of the invention. 
     In some embodiments, the degassing system further comprises a duct inline between the chamber and the storage tank, said duct having a largest flow area A 3  which is at most A 2 /20, such as substantially equal to the largest flow area A 1  of the duct portion connecting the flow control valve and the chamber. As will be described later, this may further enhance the degassing efficiency. 
     In some embodiments, a figure of 10 or even 5 can be used in place of 20 above, but for a high flow rate, the system then must be larger to obtain the same degree of degassing. The diffusion mechanism also turns out to be a little less efficient in such embodiments, 
     In some embodiments, the largest flow area A 1  is in the range 2-6 cm 2 . 
     In some embodiments, the smallest flow area A 2  is in the range 130-200 cm 2 , 
     In some embodiments, the largest flow area A 1  is in the range 2-6 cm 2  and the smallest flow area A 2  is in the range 130-200 cm 2 . 
     In some embodiments, a volume of the chamber is in the range 5-50 L, such as in the range 10-40 L, such as in the range 15-25 L 
     In some embodiments where a chamber as described above is included, the flow control valve and/or the resin pump are controlled so that an average resin transit time through the chamber is in the range 10-120 s, such as in the range 15-75 s, such as in the range 20-60 s, such as in the range 30-50 s, such as in the range 30-40 s. The volume of the chamber may be selected to obtain such transit times under the flow rates required in a given use case. 
     In some embodiments, the first absolute pressure is in the range 3-8 bar, such as in the range 3-5 bar. Such a condition gives a very efficient degassing while being a very manageable pressure. 
     In some embodiments, the first pressure drop is in the range 2.5-5.5 bar, such as in the range 2.5-4.5 bar. 
     Generally, the first absolute pressure is higher than the first pressure drop by an amount in the range 200-700 mbar, such as by an amount in the range 300-600 mbar. By providing a large pressure drop, the gas evacuation system needs not necessarily be powerful. For a larger system, all else being equal, a stronger gas evacuation system is needed. A high flow rate demands a higher capacity of the gas evacuation system, as more gas must be removed per unit time. 
     In some embodiments, the absolute pressure in the storage tank is maintained below 50 mbar. This contributes to an efficient degassing of the foam-like liquid resin prepared upstream of the storage tank in accordance with the process conditions described above. 
     In some embodiments, the first absolute pressure is at least 3 bar, the first pressure drop is at least 2.5 bar, the absolute pressure in the storage tank is maintained below 100 mbar. In some embodiments, a flow rate of resin into the resin inlet is in the range 20-60 L per minute. 
     In some embodiments, the first absolute pressure is at least 4 bar, the first pressure drop is at least 3.5 bar, the absolute pressure in the storage tank is maintained below 50 mbar. In some embodiments, a flow rate of resin at the resin inlet is in the range 20-60 L per minute. 
     In some embodiments, the storage tank has an outlet coupled to an outlet pump for outputting degassed resin during a first time period overlapping with pumping resin into the first duct. Such embodiments can operate inline with a manufacturing area in fluid communication with the outlet, which means that the need for storing degassed resin is strongly reduced or essentially eliminated, except for the degassing system. The high efficiency with which liquid resin is degassed in embodiments of the present invention allows such inline degassing, which in turn makes the logistics associated with providing degassed resin much simpler. The high flowrate of the system, which can be achieved by the process conditions described herein, is suitable for manufacturing large fibre-reinforced composite parts, such as wind turbine blades and aircraft fuselage and wing parts, some of which may have dimensions of more than 100 m and therefore require a large amount of resin over a relatively short period of time. The systems and methods can also be used on a smaller scale. 
     In some embodiments, the resin pump and/or the flow control valve and/or the outlet pump are controlled to maintain a substantially constant amount of degassed resin in the storage tank during the first time period. The latter can be achieved either by manually adjusting the system, or it can be achieved automatically based on data from the storage tank. In some embodiments, the system comprises measuring means for measuring a parameter representing the amount of degassed resin in the storage tank, and the system further comprises control means configured to receive measurements from the measuring means and control the resin pump and/or the flow control valve and/or the outlet pump to maintain the substantially constant amount of degassed resin in the storage tank. As an example, the control means may control the resin pump and the flow control valve and the outlet pump. In another example, a user controls the outlet pump in order to withdraw the amount of degassed resin necessary. In response, the control means controls the resin pump and/or the flow control valve to ensure the constant amount of degassed resin in the storage tank. The control means may also control the outlet pump, at least partly, so that the amount of degassed resin provided at the outlet is controlled at least in part by the control means. This allows the system to reconfigure to respond to a higher or lower demand for degassed resin, as opposed to the system having to respond only once resin is withdrawn, or not, from the storage tank. Instead, the system can increase or decrease pump pressure and adjust the flow control valve setting before activating the outlet pump, whereby production of degassed resin can be is initiated to replace any resin that is withdrawn from the storage tank when the control means opens the outlet pump. Similarly, when less resin is withdrawn, the resin pump and/or the flow control valve are controlled to reduce production of degassed resin. 
     It is noted that in relation to controlling the flow of resin into the system and/or out of the storage tank, the resin pump may be a system that comprises a valve in addition to a pump, or even several pumps, optionally coupled to the control means and controllable by the control means. Similarly, the outlet pump may be a system that comprises a valve in addition to a pump, or even several pumps, optionally coupled to the control means and controllable by the control means. Thus, the control means is coupled to and controls a number of pumps and valves in the system, which above are grouped into an “inlet pump”, a “flow control valve”, and an “outlet pump”. 
     The measuring means may for instance comprise a storage tank load cell that can provide a signal representative of a weight of the storage tank including the degassed resin. As another example, liquid level measurement means, such as a pressure transducer, can be installed in the storage tank. The signal from such a transducer is directly relatable to the amount of liquid in the storage tank. 
     In some embodiments, the outlet is in fluid communication with a manufacturing area where a fibre-reinforced composite part, such as a wind turbine blade part, is being manufactured at least during the first time period. As described above, the present systems and method enables an inline degassing of liquid resin, whereby the challenges of producing and handling degassed resin are significantly mitigated. 
     In some embodiments, no gas, such as air, is provided into the resin during the degassing process, apart from the gas contained in the resin when pumped into the first duct. Some prior art relies on the addition of gas to the resin in order to improve the degassing process. Embodiments of the present invention completely eliminates the need for adding gas. 
     In a second aspect, the invention provides another process for degassing resin, closely related to the first aspect. The process comprises:
         providing liquid resin into a first duct to achieve a first absolute pressure of at least 1.6 bar in the first duct,   passing the liquid resin from the first duct to a second duct through a flow control valve, wherein the first absolute pressure and the flow control valve are configured to cause cavitation on a downstream side of the flow control valve at least when the first absolute pressure is at least 1.6 bar, and   transferring the liquid resin to a storage tank in fluid communication with the second duct and maintaining an absolute pressure in the storage tank below 100 mbar.       

     In some embodiments of the processes described above, the liquid resin comprises at least one of: epoxy resin, polyurethane resin, polyester resin, unsaturated polyester resin, vinyl ester resin, thermosetting resin, and/or thermoplastic resin, such as thermoplastic infusion resin. 
     A third aspect of the invention provides a degassing system for degassing liquid resin. The degassing system comprises:
         a first duct having a resin inlet for receiving liquid resin to be degassed,   a resin pump for pumping liquid resin into the first duct via the resin inlet,   a flow control valve arranged at a downstream end of the first duct, configured to control a flow rate of resin through the first duct and configured or configurable to provide a pressure drop of at least 1.5 bar across the flow control valve,   a second duct in fluid communication with the first duct via the flow control valve,   a resin storage tank having a resin storage tank inlet in fluid communication with an outlet end of the second duct, and   a gas evacuation system operable to reduce an absolute pressure in the resin storage tank to below 100 mbar.       

     In some embodiments, the system further comprises a chamber inline between the flow control valve and the storage tank, the chamber having a smallest flow area A 2  which is at least 20 times a largest flow area A 1  of a duct portion connecting the flow control valve and the chamber. 
     In some embodiments, a duct inline between the chamber and the storage tank has a largest flow area A 3  which is at most A 2 /20, such as substantially equal to the largest flow area A 1  of the duct portion connecting the flow control valve and the chamber. 
     A fourth aspect of the invention provides use of a process in accordance with the first or second aspect of the invention in the manufacturing of a fibre-reinforced composite part, such as a wind turbine blade part. 
     A fifth aspect of the invention provides use of a system in accordance with the third aspect of the invention in the manufacturing of a fibre-reinforced composite part, such as a wind turbine blade part. 
     The features described in relation to the first aspect of the invention may also be applied in the other aspects of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described in more detail in the following with reference to the accompanying figures. The figures show selected ways of implementing the present invention and shall not to be construed as being limiting the scope of the invention. 
         FIG.  1    is a schematic diagram illustrating an exemplary wind turbine. 
         FIG.  2    is a schematic view of a system for degassing liquid resin in accordance with an embodiment of the invention. 
         FIG.  3    is a schematic view of a system for degassing liquid resin in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF SELECTED EMBODIMENTS 
     Unless otherwise indicated, the drawings are not necessarily drawn to scale. 
       FIG.  1    illustrates a conventional modern upwind wind turbine  2  according to the so-called “Danish concept” with a tower  4 , a nacelle  6  and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub  8  and three blades  10  extending radially from the hub  8 , each blade having a blade root  16  nearest the hub and a blade tip  14  furthest from the hub  8 . The invention is not limited to use in wind turbines of this type. 
     The blades  10  are usually made at least partly of fibre-reinforced composites, such as glass fibres and/or carbon fibres in a resin matrix. The strength of such blades is very dependent on the amount of gases and also water remaining in the resin when the fibres are impregnated with the resin, whatever the method of impregnation. It is therefore important that the resin be degassed as much as possible before use. 
       FIG.  2    illustrates a system  200  for degassing liquid resin in accordance with an embodiment of the invention. The system comprises a first duct  205  having a resin inlet  202  for receiving liquid resin to be degassed. In the present example, the resin inlet  202  of the system  200  is connected to a resin source  201  comprising liquid resin to be degassed. A resin pump  203  pumps liquid resin from the resin source  201  farther into the system. The system also comprises a flow control valve  208  arranged at a downstream end of the first duct  205 . The first duct  205  is connected to an input port  207  of the flow control valve  208 . The flow control valve  208  is configured to control a flow rate of resin through the degassing system  200 , particularly to provide a pressure drop of at least 1.5 bar. The resin pump  203  and/or the flow control valve  208  are controlled to allow the absolute pressure in the first duct  205  to reach a desired level and to obtain a desired pressure drop across the flow control valve  208 , as will be described below. A second duct  211  is in fluid communication with the first duct  205  via the flow control valve  208 . The second duct is connected to an output port  209  of the flow control valve  208 . Accordingly, the absolute pressure in the second duct  211  is significantly lower than the absolute pressure in the first duct  205 . 
     The second duct  211  connects to an inside of a resin storage tank  213  via a storage tank resin inlet  212 . The inside of the storage tank  213  is also connected to a gas evacuation system  214  comprising a gas duct  215  connected to the inside of the storage tank  213 , to a vacuum pump  217 , and to an exhaust  219 . The vacuum pump  217  is operable to maintain a low absolute pressure in the resin storage tank  213  by removing gases from the inside of the storage tank  213 . The system in  FIG.  2    also includes a pump  221  for pumping degassed resin to a part manufacturing area via a storage tank outlet  220 . In this way, degassed resin can be provided directly to the manufacturing area. As described previously, the system  200  may even run inline, producing degassed resin while the part manufacturing is taking place. 
     The process of degassing resin from the resin source  201  includes pumping resin into the first duct  205  via the resin inlet  202  using the resin pump  203 . The resin pump may itself comprise the inlet  202 . This is a matter of design. 
     In the present example, the pump maintains an absolute pressure in the first duct  205  around 3 bar, at least during a substantial part of the degassing process when resin is fed through the system. The vacuum pump  217  of the gas evacuation system  214  is at the same time operated to maintain a reduced absolute pressure below 50 mbar. 
     This process results in a very efficient degassing of the resin, and as described previously, this is not just by traditional bubble formation. The process conditions described herein cause creation of bubbles that have a size that allows gas and water to very efficiently diffuse out of the resin. In known systems, the primary mechanism behind removal of gases and water is the formation of bubbles due to a reduced absolute pressure. The gas from such bubbles is eventually removed by a gas evacuation system. However, dissolved resin is not efficiently removed in known systems, including systems that add gas as part of the degassing process. 
     Embodiments of the present invention treats the resin just long enough and under pressure conditions that result in a more thorough removal of gasses and water from the resin before it reaches the storage tank  213 , while maintaining a high flow rate. 
     The fact that the processes described herein are different from known processes is also, surprisingly, observable by evaluating the degassing system equipment after degassing liquid resin. For reasons that are not well-understood, known systems and corresponding processes cause significant build-up of residues in various parts of those systems. The build-up rate itself is rather unpredictable, and monitoring the state of the degassing system is therefore a rather time-consuming task. Removing the residue and replacing damaged parts is even more time-consuming and contributes to downtime. 
     The process conditions described in the present specification result in much less build-up of residues, possibly due to the foamy state of the resin and the motion of the resin through the system under the process conditions described herein. The present invention therefore makes the degassing more efficient not only with respect to the amount of residual gasses, but also with respect to the maintenance load and associated downtime. 
     In another exemplary process, performed in the system shown in  FIG.  3   , a chamber  330  modifies the flow of the resin by providing a change, in some cases a relatively large change, in the flow area between the control valve  208  and the storage tank  213 . As shown in  FIG.  3   , the chamber  330  between the flow control valve  208  and the storage tank  213  provides a larger surface area A 2  for the resin compared to a largest flow area A 1  in the duct  310  between the control valve  208  and the chamber  330 . This further contributes to removal of gasses, not just by allowing bubbles to more easily form, but also by increasing removal of gas from the resin by diffusion. 
     A ratio between the flow area A 2  and the flow area A 1  in the range 20-60 results in a very efficient diffusion of gasses out of the resin, including single gas molecules otherwise trapped and unable to form gas bubbles by which they can escape. The large flow area allows the resin to be in the particularly advantageous foamy state that characterizes embodiments of the present invention for a longer time. 
     This is further enhanced by providing that the resin after the chamber  330  flows in a relatively narrow duct  311  as illustrated in  FIG.  3   . It turned out that such a narrowing gives an even foamier resin in the chamber and also leads to smaller and more uniform bubbles in the chamber, which increases the rate at which small gas pockets and individual molecules diffuse out of the resin. Gasses and water trapped in very small amounts, down single molecules, as described above, can diffuse out of the resin, which allows them to be removed together with larger pockets of gas. Without the narrower duct after the chamber, bubbles tend to be somewhat larger and vary more in size, and the diffusion component of the degassing process is lower. As a consequence, more gas remains suspended in the resin especially on a molecular level. 
     The invention has been described with reference to selected embodiments. However, the scope of the invention is not limited to the illustrated embodiments, and alterations and modifications can be carried out without deviating from the scope of the claims. 
     LIST OF REFERENCES 
       2  wind turbine 
       4  tower 
       6  nacelle 
       8  hub 
       10  blade 
       14  blade tip 
       16  blade root 
       200 ,  300  degassing system 
       201  resin source 
       202  resin inlet 
       205  first duct 
       203  resin pump 
       207  flow control valve input port 
       208  flow control valve 
       209  flow control valve output port 
       211  second duct 
       212  storage tank resin inlet 
       213  resin storage tank after degassing 
       214  gas evacuation system 
       215  gas duct 
       217  vacuum pump 
       219  gas exhaust 
       220  resin outlet 
       221  pump to part manufacturing area 
       230  part manufacturing area 
       310  duct between control valve and chamber 
       311  duct between chamber and storage tank 
       330  chamber 
     A 1  largest flow area of first duct 
     A 2  smallest flow area of chamber 
     A 3  largest flow area of duct between chamber and storage tank