Abstract:
A foamed glass plant growth support structure, including a foamed glass substrate and a plurality of interconnected pores distributed throughout the substrate. The substrate is characterized by a porosity of at least about 65 percent. The pore size is substantially between about 0.2 and about 2 millimeters and the substrate is sufficiently chemically stable such that water filling the plurality of interconnected pores experiences a pH shift of less than 0.5.

Description:
TECHNICAL FIELD 
       [0001]    The novel technology relates generally to the field of ceramic materials and, specifically, to a foamed glass substrate system having including hydrophilic pore walls for water storage and root propagation. 
       BACKGROUND 
       [0002]    Hydroponics is the science of growing plants in a nutrient solution with the mechanical support of an inert medium. Hydroponics is an old art, and a variety of inert media are known as suitable for the germination, rooting and growth of plants. Such substrates include peat, vermiculite, perlite, fly ash, pumice, rock wool, glass wool, organic and inorganic fibers, polymers such as polyurethane, polystyrene, polyethylene, and the like. These substrates have been used for true hydroponics or in quasi-hydroponic environments such as in admixtures with soil. Typically, the inert medium is either in the form of a contained loose particulate, such as sand, or as a rigid and self-supporting structure that can support growth of the plant. The rigid structure has some notable advantages over the loose particulates, in particular the ability to stand alone without a requisite container. However, the loose particulate media tend to offer better pathways for water and gasses to be delivered to and from the root systems. 
         [0003]    One problem common to hydroponic gardening is under/overwatering. Some media offer limited porosity and/or limited means for circulating water into and out of pores. As a result, vegetation growing hydroponically is often underwatered. Conversely, hydroponic techniques lend themselves to the provision of excessive water to the plant root system, often in response to the underwatering that is occurring. Overwatering can result in chlorosis, retarded growth, pallor, and, eventually death. In such situations, the water around the roots becomes stagnant and gasses dissolved therein are only urged to and from the roots through diffusion. Moreover, vital gasses quickly become depleted and waste gasses saturated in the water proximate the roots, exacerbating the situation. Thus, it is desired to reduce the stagnant water around the roots by circulating the water. 
         [0004]    Most of the substrates currently known are solids with limited porosity. Some known substrates have attempted to add or increase the porosity of the substrate in order to better provide for gas exchange to the roots. One such substrate has been produced in the form of a sponge-like or foraminous foamed polymer body with conduits 1-5 millimeter in nominal diameter, spaced about 1-8 mm apart and extending throughout the substrate. The conduits drain water from the substrate and provide reservoirs of oxygen for the plant roots and at the same time allow substrate to hold some water that may then be available to the roots. The porosity of this substrate ranges from between 6 and 53 percent. Soil or the like is deposited on top of the substrate and a seed, cutting or small plant is placed in the soil. With the substrate under the soil layer, over-watering induced problems are prevented, as excess water drains from the substrate, filling the conduits with air and oxygen will be readily available to the roots. 
         [0005]    Another issue with known substrates is pH control. Natural substrates tend to include soluble mineral residue that dissolves at uncontrolled rates, shifting water pH. Man-made substrates likewise may include materials that dissolve over time and at nonlinear rates, shifting pH as they do. Changes in pH can have a drastic and deleterious effects on plant growth. 
         [0006]    While useful in hydroponic and soil amendment applications, the above substrates are still hampered by a lower than optimal porosity, limited wicking capacity, low capacity for water infiltration and retention, and uncontrolled pH arising from mineral dissolution. Thus, there remains a need for a highly porous substrate for supporting plant growth. There also remains a need for improves the aeration of soil and allows for better water filtration and irrigation. The present novel technology addresses these needs. 
       SUMMARY OF THE NOVEL TECHNOLOGY 
       [0007]    The present novel technology relates to a foamed glass material system for supporting plant growth, and the method for making the same. One object of the present novel technology is to provide an improved foamed glass plant support substrate material. Related objects and advantages of the present novel technology will be apparent from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  is a schematic view of a process for mixing a batch of precursors for a foamed glass article according to a first embodiment of the present novel technology. 
           [0009]      FIG. 1B  is a schematic view of a process for firing a foamed glass article mixed according to  FIG. 1A . 
           [0010]      FIG. 1C  is a perspective view of as milled glass powder according to the process of  FIG. 1B . 
           [0011]      FIG. 1D  is a perspective view of rows of milled glass powder mixture ready for firing. 
           [0012]      FIG. 1E  is a perspective view of  FIG. 1D  after firing into a substantially continuous foamed glass sheet. 
           [0013]      FIG. 2  is a partial perspective view of roots infiltrating the porosity of the foamed glass article of  FIG. 1B . 
           [0014]      FIG. 3  is a process diagram of the process illustrated in  FIGS. 1A and 1B . 
           [0015]      FIG. 4  is a cutaway elevation view of the article of  FIG. 2  as partially immersed in water and supporting plant growth. 
           [0016]      FIG. 5  is a plan view of a plurality of crushed foamed glass pebbles according to a second embodiment of the present novel technology. 
           [0017]      FIG. 6  is a cutaway elevation view of the article of a hydroponic system including foamed glass media as shown in  FIG. 5  mixed with soil and supporting root growth. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    For the purposes of promoting an understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates. 
         [0019]      FIGS. 1A-3  illustrate a first embodiment of the present novel technology, a lightweight foamed glass substrate  10  defining a plurality of voluminous, interconnecting pores  15  for supporting plant growth. The pores  15  typically have diameters ranging from about 0.2 mm to about 2.0 mm. The pore walls  17  typically exhibit a crazed or microcracked microstructure  19  to facilitate wicking. As illustrated schematically in  FIGS. 1A-2 , a ground, milled and/or powdered glass precursor  20 , such as recycled waste bottle and/or window glass, is mixed with a foaming agent  22  (typically a finely ground non-sulfur based gas evolving material, such as calcium carbonate) to define an admixture  27 . The foaming agent  22  is typically present in amounts between about 1 weight percent and about 3 weight percent and sized in the average range of about 80 to minus 325 mesh (i.e. any particles smaller than this will pass through—typically, the apertures in 80 mesh are between about 150 and about 200 micrometers across and the apertures in −352 mesh are between about 40 and about 60 micrometers across). More typically, the foaming agent has a particle size between about 5 and about 150 microns. Typically, a pH modifier such as dicalcium phosphate  24  is added to the admixture  27 , wherein the pH modifier  24  becomes effective when the foamed glass product  10  is used in an aqueous environment. The pH modifier  24  is typically present in amounts between about 0.5 and 5 weight percent, more typically between about 1 and about 2 weight percent. Additional plant growth nutrient material may be added to the starting mixture to vary or enhance the plant growth characteristic of the final product  10 . 
         [0020]    Foamed glass, like most ceramics, is naturally hydrophobic. As hydrophobic surfaces are not conducive to wetting and impede capillary action, treatment is typically done to make the pore walls  17  hydrophyllic. In one embodiment, the pore walls  17  are coated to form a plurality of microcracks  19  therein. The microcracks  19  supply increased surface area to support wicking. Alternately, or in addition, an agent may be added to further amend the surface properties to make the foamed glass more hydrophilic. Such an agent may be a large divalent cation contributor, such as ZnO, BaO, SrO or the like. The hydrophilic agent is typically added in small amounts, typically less than 1.5 weight percent and more typically in amounts of about 0.1 weight percent. 
         [0021]    The combination is mixed  26 , and the resulting dry mixture  27  may then be placed into a mold  28 , pressed into a green body and fired without the use of a mold, or, more typically, arrayed into rows  31  of powder mixture  27  for firing and foaming. Typically, whether placed  29  into the mold  28  or not, the mixture  27  is typically arrayed in the form of several rows  31 , such as in mounds or piles of mixture typically having a natural angle of repose of about 15 to 50 degrees, although even greater angles to the horizontal can be achieved by compressing the dry mixture  27 . This arraying of the rows  31  allows increased control, equilibration and optimization of the heating of the powder  27  during firing, reducing hot and cold spots in the furnace as the powder  27  is heated. This combing of the powder  27  into typically rows  31  of triangular cross-sections allows heat to be reflected and redirected to keep heating of the rows generally constant. 
         [0022]    The mold  28 , if used, is typically a refractory material, such as a steel or ceramic, and is more typically made in the shape of a frustum so as to facilitate easy release of the final foamed glass substrate  10 . Typically, the inside surfaces of the mold  28  are coated with a soft refractory release agent to further facilitate separation of the foam glass substrate  10  from the mold  28 . In a continuous process, the powder  27  is typically supported by a fiberglass mesh fleece or the like to prevent fines from spilling as the powder  27  is moved via conveyor through a tunnel kiln; the fleece is burned away as the powder  27  sinters. 
         [0023]    The so-loaded mold  28  is heated  30  in a furnace by either a batch or continuous foaming process. More typically, the mixture  27  is then heated  30  in order to first dry  32 , the sinter  34 , fuse  36 , soften  38 , and foam  40  the mixture  27  and thereby produce a foamed glass substrate  10  having a desired density, pore size and hardness. As the powdered mixture  27  is heated to above the softening point of glass (approximately 1050 degrees Fahrenheit) the mixture  27  begins to soften  38 , sinter  34 , and shrink. The division of the powdered mixture  27  into rows or mounds allows the glass to absorb heat more rapidly and to therefore foam faster by reducing the ability of the foaming glass to insulate itself. At approximately 1025 degrees Fahrenheit, the calcium carbonate, if calcium carbonate has been used as the foaming agent  22 , begins to react with some of the silicon dioxide in the glass  20  to produce calcium silicate and evolved carbon dioxide. Carbon dioxide is also evolved by decomposition of any remaining calcium carbonate once the mixture reaches about 1540 degrees Fahrenheit, above which calcium carbonate breaks down into calcium oxide and carbon dioxide gas. Once the temperature of the mixture  27  reaches about 1450 degrees Fahrenheit, the glass mixture  27  will have softened sufficiently for the released carbon dioxide to expand and escape through the softened, viscous glass; this escape of carbon dioxide through the softened glass mass is primarily responsible for the formation of cells and pores therein. The mixture  27  in the mold  28  is held for a period of time at a peak foaming temperature of, for example, between about 1275 and about 1700 degrees Fahrenheit, more typically between about 1450 and about 1600 degrees Fahrenheit, or even higher, depending on the properties that are desired. By adjusting the firing temperatures and times, the density and hardness as well as other properties of the resultant substrate  10  may be closely controlled. 
         [0024]    As the mixture  27  reaches foaming temperatures, each mass of foaming  40  glass, originating from one of the discrete rows or mounds, expands until it comes into contact and fuses with its neighbors. The fused mass of foaming glass then expands to conform to the shape of the walls of the mold  28 , filling all of the corners. The shapes and sizes of the initial mounds of mixture are determined with the anticipation that the foaming  40  mixture  27  exactly fills the mold  28 . After the glass is foamed  40  to the desired density and pore structure, the temperature of the furnace is rapidly reduced to halt foaming  40  of the glass. When the exterior of the foamed glass in the mold has rigidified sufficiently, the resultant body  10  of foamed glass is removed from the mold  28  and is typically then air quenched to thermally shock the glass to produce a crazed microstructure  19 . Once cooled, any skin or crust is typically cut off of the foamed glass substrate  10 , which may then be cut or otherwise formed into a variety of desired shapes. Pore size can be carefully controlled within the range of about 5 mm to about 0.5 mm, more typically within the range of between about 2.0 mm and 0.2 mm, and is typically controlled such that the interconnected or open porosity may readily accommodate a typical plant root  135  (see  FIG. 4 ). Substrate density can be controlled from about 0.4 g/cc to about 0.26 g/cc. Typically, the bulk density of the crushed foam may be as low as 50% of the polyhedral density. 
         [0025]    The substrate  10  may be either provided as a machined polyhedral shape  10  or, more typically, as a continuous sheet that may be impacted and/or crushed to yield aggregate or pebbles  50  (typically sized to be less than 1 inch in diameter). The crushed substrate material  50  may be used to retain water and increase air volume in given soil combinations. The polyhedrally shaped substrate bodies  10  are typically sized and shaped as growing media for seeds and immature plants, such as for use in soil or hydroponic systems  140  (see  FIGS. 4-6 ). The foamed glass substrate material  10  is thus used to improve aeration and water retention in agricultural systems, and the porous polyhedral material  10  also provides a sufficiently spacious path for root growth and attenuation. The foamed glass material  10  itself is typically resistant to aqueous corrosion and has minimal impact on solution pH. In order to provide better pH control, the foamed glass material  10  is typically doped (in batch stage, prior to foaming) with specific dicalcium phosphate or a like pH stabilizing material  24  which dissolves in water to help stabilize the pH. The foamed glass substrate  10  can typically hold between about 1.5 and about 5 times its own weight in water in the plurality of interconnected pores  17 . 
         [0026]    Crushed foam bodies  50  may be rapidly made by an alternate method. Using soda-lime glass frit or powder as the glass component  22 , the processing is similar to that described above but without the annealing step. The alternate method employs the same foaming temperature ranges as related above. The batch material  27  consists of up to 8 percent by mass limestone, magnesite, or other applicable foaming agent  22 , usually less than 2 percent by mass dicalcium phosphate  24 , with the balance being a borosilicate, silicate, borate or phosphate glass frit  22 . The batch  27  is then placed in a typically shallow mold  28 , more typically having a configuration of less than 2″ batch for every square yard of mold surface. The mold  28  is typically then heated to approximately 250° C. above the dilatometric softening point for soda-lime glass (or the equivalent viscosity for other glass compositions) and allowed to foam. The mold  28  is held at the foaming temperature for less than 30 minutes and then pan quenched, i.e. substantially no annealing is allowed to occur 
         [0027]    This method typically yields a material  10  of density less than 0.25 g/cc, and more typically as low as about 0.03 g/cc. This material  10  is then crushed into pebbles  50 , with a corresponding lower bulk density as per the above-described method. Material made by this alternate method has similar chemical properties as described above, can accommodate an even larger nutrient content, but has substantially lower strength. 
         [0028]    Still another alternate method of preparing foamed glass substrate material  10  is as follows. A batch  27  is prepared as discussed above and pressed into small (typically less than 5 mm diameter) pellets. The pellets are rapidly heated, such as by passage through a flame source, passage through a rotary furnace, or the like. Typically, the pellets are heated to about 1500 degrees Fahrenheit, such as to cause the pellet to expand as a foam particulate without the need for a mold. This material yields the weakest, but least dense foam particles. The typical density may be as low as 0.02 g/cc or as high as 0.2 g/cc, or higher. 
         [0029]    The foamed glass substrate  10 , either in polyhedral body form or crushed product form, may serve as a root system support and an aeration and/or water retention aid. The material  10  typically enjoys a void fraction of at least about 80 percent. The substrate  10  is typically mechanically mixed with as little as 15% by volume soil or soil mixture and this new mixture will still generate solution chemistry dominated by soil. 
         [0030]    The polyhedral product  10  typically functions as a supporting substrate for both soil based and hydroponic applications. The polyhedral substrate  10  may be tailored to be compatible with mildly acidic, neutral, or mildly alkaline solution pH. The pore network  15  is compatible with root propagation through the material  10 . The pore network typically has surface adhesion or adsorption properties such that chemical additives  51 , such as plant growth nutrients, herbicides, pesticides or combinations thereof, may be physically adsorbed thereonto for later release. Typically, these additives are added in small amounts, such as a few weight percent (0.5 1, 2, 5, or the like) of the substrate product  10 . 
         [0031]    The foamed glass substrate  10  typically has a porosity in the range of between about sixty-five and about eighty-five percent. Air holding capacity is typically between about forty and about fifty-five percent. Water holding capacity typically ranges from between about twenty and about thirty-five percent. 
         [0032]    Porosity is measured during the production process using a parometer testing protocol. Bulk density and pH are also measured during production, with the results providing active feedback for a control loop. 
         [0033]    The pore size is typically between about 0.2 mm and about 2.0 mm in diameter, with a relatively tight pore size distribution. The finished substrate  10  is typically processed through a series of conveyors and crushing equipment to yield a desired size spread of pellets  50 . 
         [0034]    The precursor glass material is typically recycled or post-consumer waste glass, such as plate, window and/or bottle glass. The glass is ground or milled to a fine mesh profile of minus 107 microns. A typical sieve analysis of the precursor glass is given as Table 1, and a compositional analysis of the glass is given as Table 2. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sieve Analysis 
               
             
          
           
               
                   
                 Class up to 
                 Pass 
                 Remaindser 
                 Incidence 
               
               
                   
                 (μm) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 0.7 
                 1.3 
                 98.7 
                 1.3 
               
               
                   
                 0.9 
                 1.6 
                 98.4 
                 0.3 
               
               
                   
                 1 
                 1.8 
                 98.2 
                 0.2 
               
               
                   
                 1.4 
                 2.8 
                 97.2 
                 1.0 
               
               
                   
                 1.7 
                 3.7 
                 96.3 
                 0.9 
               
               
                   
                 2 
                 4.6 
                 95.4 
                 0.9 
               
               
                   
                 2.6 
                 6.4 
                 93.6 
                 1.8 
               
               
                   
                 3.2 
                 7.9 
                 92.1 
                 1.5 
               
               
                   
                 4 
                 9.9 
                 90.1 
                 2.0 
               
               
                   
                 5 
                 12.0 
                 88 
                 2.1 
               
               
                   
                 6 
                 14.0 
                 86 
                 2.0 
               
               
                   
                 8 
                 17.5 
                 82.5 
                 3.5 
               
               
                   
                 10 
                 20.5 
                 79.5 
                 3.0 
               
               
                   
                 12 
                 23.3 
                 76.7 
                 2.8 
               
               
                   
                 15 
                 27.3 
                 72.7 
                 4.0 
               
               
                   
                 18 
                 31.1 
                 68.9 
                 3.8 
               
               
                   
                 23 
                 37.2 
                 62.8 
                 6.1 
               
               
                   
                 30 
                 45.1 
                 54.9 
                 7.9 
               
               
                   
                 36 
                 51.2 
                 48.8 
                 6.1 
               
               
                   
                 45 
                 59.2 
                 40.8 
                 8.0 
               
               
                   
                 56 
                 67.6 
                 32.4 
                 8.4 
               
               
                   
                 63 
                 72.3 
                 27.7 
                 4.7 
               
               
                   
                 70 
                 76.6 
                 23.4 
                 4.3 
               
               
                   
                 90 
                 86.5 
                 13.5 
                 9.9 
               
               
                   
                 110 
                 92.7 
                 7.3 
                 6.2 
               
               
                   
                 135 
                 97.1 
                 2.9 
                 4.4 
               
               
                   
                 165 
                 99.3 
                 0.7 
                 2.2 
               
               
                   
                 210 
                 100.0 
                 0 
                 0.7 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Glass oxide 
                 Wt. % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SiO 2   
                 71.5 
               
               
                   
                 Na 2 O 
                 12.6 
               
               
                   
                 K 2 O 
                 0.81 
               
               
                   
                 Al 2 O 3   
                 2.13 
               
               
                   
                 CaO 
                 10.1 
               
               
                   
                 MgO 
                 2.3 
               
               
                   
                 TiO 2   
                 0.07 
               
               
                   
                 Fe 2 O 3   
                 0.34 
               
               
                   
                 BaO 
                 0.01 
               
               
                   
                 SO 3   
                 0.05 
               
               
                   
                 ZnO 
                 0.01 
               
               
                   
                   
               
             
          
         
       
     
       Example 1 
       [0035]    A plant growth support substrate  10  having a pore network  15  and pH control agent  24  embedded therein is produced from a precursor batch of about 1.25 weight percent calcium carbonate, about 1.25 weight percent dicalcium phosphate anhydrous, about 0.2 weight percent iron oxide colorant, and about 97 weight percent recycled post-consumer glass ground or milled to minus 107 microns, are mixed together. The resulting mixture  27  is placed into a stainless steel mold  28  having inside dimensions of 4.25 inches by 4 inches by 8.25 inches. The mold  28  is covered with a one-half inch stainless steel plate. The mold  28  with the mixture  27  therein is fired  30  in a tunnel kiln having a plurality of zones with a travel rate of about 10 inches per minute and a residence time in each zone of about 9 minutes. The first zone typically has a temperature of about between about 200 and about 500 degrees Fahrenheit, more typically between about 400 and about 450 degrees Fahrenheit, and still more typically about 225 degrees Fahrenheit, wherein drying  32  of the mixture  27  is accomplished. Zones 2-4 have temperatures increasing from about 1300 to about 1500 degrees Fahrenheit, and sintering  34  is accomplished therein. Foaming  40  occurs primarily in zones 4 and 5, with zone 5 having a temperature of about 1580 degrees Fahrenheit. In zone 6, temperature about 1570, foaming  40  is finalized and the resultant soft foamed body  42  is cured  44 . In the final zone, cooling begins as the temperature is decreased to about 1400 degrees Fahrenheit. Upon exiting the tunnel kiln, the foamed body  42  is air quenched  46  to produce the desired crazed microstructure  19 . The cooled block of foamed glass  10  is removed from the mold  28 , and the outer layer of crust is removed (such as with a band saw) to expose the open porosity. The substrate  10  is further cooled and quenched until it is approximately at room temperature. The resulting block typically has a density of about fourteen pounds per cubic foot and a pore size distribution ranging from about 0.5 to 2 mm. The resulting block has open, interconnected pore cells. 
         [0036]      FIGS. 4 and 5  illustrate other embodiments of the novel technology, hydroponic systems  40 ,  55  for growing plants. A hydroponic system  40  is disclosed wherein a substrate block  10  is partially immersed in a fluid medium  42 , such as water or an aqueous nutrient solution. Roots  35  extend downwardly into the top of the substrate block  10  through the interconnected open pores  15 . Nutrient solution  42  is carried upwardly into the block  10  via capillary action through the open pores. 
         [0037]      FIG. 6  illustrates another embodiment of the present novel technology, a system  155  including foamed glass pebbles or pellets  50  prepared as detailed above and used in a soil-pellet mixture  157  into which plant roots  135  may extend. The mixture  157  may, in addition to the pellets  50 , include soil, clay, peat moss, nitrates, fertilizer, manure or the like and mixtures thereof. Likewise, the pellets  50  may be used in place of perlite in soil admixtures. The pellets  50  contribute by picking up, holding, and slowly releasing water, as well as by providing a network of ‘anchors’ for the roots  135  to extend into and through. 
         [0038]    While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.