A METHOD FOR PREPARING OPEN-CELLED GROWING MEDIA AND GROWING MEDIA

Abstract: A process for producing hydrophilic growing media, preferably plugs, providing growing media having improved properties and a minimal amount of binding agent. The method forms a resilient, integral, dimensionally stable, cohesive, homogenous mass by contacting an aggregate with a hydrophilic urethane prepolymer and water. Upon curing, the prepolymer binds the aggregate material together. The mixing of the aggregate, prepolymer and water is conducted under specific, carefully controlled conditions, which enables the use of less prepolymer than would otherwise be necessary to obtain a given strength and stable air space.

Description: A METHOD FOR PREPARING OPEN-CELLED GROWING MEDIA AND GROWING MEDIA BACKGROUND OF THE INVENTION The present invention relates to stable growing media, such as plugs, for the germination of seeds and the growing or propagation of plants, and a method of manufacturing such growing media. Plant seedlings or shoots can be planted in and grown in rooting media comprising soil mixture and a synthetic binder therefor, such as an organic resin polymer. Such rooting media or grow plugs eliminates the need to use pots, which add substantial weight and cost to transportation of plants. Such grow plugs are known, as is evident from U.S. Patent No. 3,805,531, which discloses a rooting media formed by mixing a prepolymer with aggregate or soil mixture and adding water to the mixture. Upon curing of the prepolymer, a cohesive mass is formed. Similarly, US Patent No. 4,175,355 discloses soil plugs using a mixture of soil material comprising at least 15% polyurethane by weight on a dry basis of the soil material. A slurry is prepared by intensively mixing the prepolymer, soil material and water. The slurry is then cast into a die and after at least partial hardening of the polyurethane resin forming material, the resulting plug is removed from the die. Because of the relatively high percentage of polyurethane in the plugs of the prior art, growing of the seedling is impeded. Indeed, it is generally believed that the less synthetic material in such plugs, the better the seedlings grow. Presumably the presence of the synthetic material impairs access of water and air to the plant roots. Ideally, therefore, grow plugs would contain no synthetic material. However, the synthetic material is a critical component of such plugs in order to provide a self-supporting, stable plug that does not disintegrate upon handling. Indeed, U.S. Patent No. 4,175,355 discussed above expressly teaches that using less than about 15% polyurethane resin by weight on a dry basis results in the rooting media crumbling and falling apart. Moreover, plugs having a high concentration of polyurethane tend to be rubbery and insufficiently dense. As a result, the water retaining properties of such plugs are poor, and the penetration of air is impaired. It would thus be highly desirable to produce a grow plug that mimics, as closely as possible, growing media that is free of synthetic material. U.S. Patent No. 5,209,014 discloses molds for growing seedlings, the molds being formed of turf, water, nutrients and a synthetic molding materials such as a polyurethane resin. The '014 patent states that the prepolymer is added in the amount of about 6 to 8% by weight to the turf mixture, the turf mixture first having been ground in a turf grinder. It is apparent from the disclosure that in referring to 6-8% prepolymer by weight of the dry mass, the term "dry mass" is being used to indicate turf mixture to which additional water has not yet been added. That is, the turf mixture inherently contains moisture, and the "dry mass" referred to in the '014 patent includes that moisture, but does not include the water introduced to the turf mixture prior to the addition of the prepolymer. Were the amount of prepolymer in the '014 patent calculated on a true "bone dry" basis to be consistent with the other prior art, the percentage of prepolymer would be considerably more than the 6-8% disclosed. It is therefore an object of the present invention to provide cohesive, stable, resilient, homogeneous, integral, self-supporting growing media with minimal amounts of synthetic material. It is a further object of the present invention to provide growing media having improved water retaining properties and increased air space. It is a still further object of the present invention to provide a process for preparing cohesive, stable, resilient, homogeneous, integral, self-supporting growing media with minimal amounts of synthetic binding material. SUMARY OF THE INVENTION The problems of the prior art have been overcome by the present invention, which provides a process for producing hydrophilic growing media, preferably plugs, and provides growing media having improved properties and a minimal amount of binding agent. In general terms, a method is provided for forming a resilient, integral, dimensionally stable, cohesive, homogeneous mass by contacting an aggregate with a hydrophilic urethane prepolymer and water. Upon curing, the prepolymer binds the aggregate material together. The mixing of the aggregate, prepolymer and water is conducted under specific, carefully controlled conditions, which enables the use of less prepolymer to obtain a given strength and stable air space than would otherwise be necessary. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a front view of apparatus suitable for manufacturing grow plugs in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION Suitable aggregate that can be used in the present invention includes peat moss (including reed-sedge peat, sphagnum peat, horticultural sphagnum peat, and wood peat) , soil, coir (coconut husk) , perlite, vermiculite, pumice, baked clay, wood pulp, pine bark, ground tree bark, sawdust, and mixtures thereof. Peat is the preferred aggregate. The term "bone dry aggregate" as used herein means aggregate that has been oven-dried at 105°C for 24 hours in order to remove the water therefrom. In the case of peat moss, since the moisture content of commercially available peat moss varies, the peat should be conditioned prior to use. Specifically, the moisture content of the peat can be standardized by adding water thereto until the peat is saturated. The saturation point can be determined visually; it occurs when the peat ceases to absorb water. Suitable binding agents for the plugs of the present invention include hydrophilic polyurethane prepolymers, such as those that are the reaction product of a polyol, preferably a polyoxyethylene polyol, with a polyisocyanate. Preferred prepolymers are those disclosed in U.S. Patent No. 3,812,619 to Wood (the disclosure of which is hereby incorporated by reference) , and are commercially available under the name HYPOL ® from Hampshire Chemical Corp. Preferably the ratio of prepolymer to as supplied aggregate is 0.03-0.1:1, more preferably 0.05-0.07:1, most preferably 0.07:1. The particular ratio will depend on the nature of the aggregate being used, and can be readily determined by those skilled in the art without undo experimentation. Preferably the amount of water added is about 14-16kg/5kg aggregate. The chart below shows some suitable formulations: MEDIA DENSITY (g/1) Substrate (G) HYPOL® (g) WATER (1) PERLITE (grade 1) 88 1500 120 3.5 AGRAPERLITE 85 1500 120 3.5 (grade 2) VERMICULITE 9. 3500 250 7.5 PUMICE (stones 637 10,000 200 2.0 from Iceland) FLUGZAND (very 10,000 200 2.0 fine sand) Turning now to Figure 1, there is shown suitable apparatus for preparing grow plugs in accordance with the present invention. A plurality of trays are provided with suitable impressions to contain the plugs. A tray destacker 1 can be utilized to consolidate space, and to hold the plurality of trays to be filled with the slurry prepared as discussed below. These trays can assume any desired configuration, each typically including a plurality of apertures or molds shaped depending upon the desired shape of the final plug. Typical shapes include cylindrical and frusto-conical. The destacker 1 unloads individual trays onto a conveyor means, such as a conveyor belt 20, which transports the trays to a sprayer 2. The sprayer 2 sprays a release agent onto each tray in order to facilitate removal of the plug from the tray, once formed. Any conventional release agent that is not phytotoxic can be used, although the preferred release agent comprises lecithin. The trays coated with release agent proceed along conveyor belt 20 to a mixer 3. The mixer 3 mixes the various components of the slurry which ultimately cure in the tray to form the grow plug. The slurry includes the aggregate, the hydrophilic polyurethane prepolymer binding agent, and water, which preferably are each supplied to the mixer 3 separately. In the embodiment illustrated in Figure 1, the binding agent is housed in containers 11 which communicate with the mixer 3 through suitable ducting; the aggregate is housed in container 15, which communicates with the mixer 3 via a conveyors 12 and 13; and water is housed in container 14, which communicates with the mixer 3 through suitable ducting. Once the components are emptied into the mixing chamber of the mixer 3, the mixing is commenced. Although the order of addition of the ingredients is not critical, preferably the aggregate is added to the mixing chamber first, followed by the addition of water and finally binding agent. It is critical to the process of the present invention that the temperature of the slurry to be mixed, the temperature during mixing, and the temperature during curing be low, suitably less than 20°C, preferably less than 10°C, most preferably less than 6°C. The low temperature ensures that the reaction proceeds slowly, which the present inventors have found results in a desirably stable product having a suitable air space. More specifically, by ensuring a slow prepolymer cure rate, a homogeneous mix can be achieved even if the mixing is conducted under low shear so as not to damage the aggregate particles, sufficient air from mixing and carbon dioxide from blowing (resulting from the reaction between the prepolymer and water) can become entrained to provide a low density product, and the resulting homogeneous mix can be suitably emptied into molds. The temperature of the slurry can be best controlled by controlling the temperature of the water added to the aggregate and binding agent. Thus, the temperature of the binding agent can be relatively high, such as 40-50°C, yet upon the addition of suitably cold water to the binding agent and aggregate, the resulting slurry will have the desired temperature as long as the water temperature is properly regulated. Since the amount of water greatly exceeds the amount of binding agent, the initial temperature of the binding agent has virtually no effect on the temperature of the slurry. It is also critical to the present invention that the mixing be conducted under shear low enough so as to minimize and preferably avoid damage to the aggregate fibers. Stated differently, the length of the aggregate fibers prior to mixing should be substantially the same as the length after mixing is completed; virtually no grinding or decimation of the fibers should take place, so that the total surface area of the aggregate prior to mixing is the same or substantially the same as the total surface area of the aggregate after mixing, regardless of the residence time of the mixture in the mixing chamber. This can be measured in terms of particle size distribution; the particle size distribution of the aggregate prior to mixing should be the same or substantially the same as the particle size distribution of the aggregate after mixing. Those skilled in the art will appreciate that the starting aggregate should be virgin aggregate; that is, commercially available "as is" aggregate that has not been ground. Preferably the particle size distribution of the starting aggregate, after particles retained by a size mesh 8 sieve have been removed, has a particle size distribution such that at least more than 40% of the particles are retained by a size mesh 35 sieve, preferably at least 45-65%, more preferably at least 48-60%, even more preferably about 50-58%, most preferably about 54-56%. In order to minimize or prevent particle damage during mixing, especially damage to the larger particles, the mixing should be carried out under low shear. To that end, the mixing chamber is preferably equipped with a four rotating arms mixer, with each mixer arm having a length of about 25cm and is operated at about 150 rpm. Those skilled in the art can easily determine other suitable low shear mixers by trial and error; a mixer meets the requirements of the present invention as long as the mixer does not shred or otherwise substantially damage or grind the aggregate fibers, especially the larger (e.g., greater than 35 mesh) particles, during mixing, yet produces a homogeneous mix during a mixing time that is long enough to allow a sufficient air to become entrained, but is short enough so that the binding agent does not cure in an amount sufficient to prevent filling of molds, or in amount sufficient to cause the media to break into clumps, thereby weakening the product. The present inventors have found that with the HYPOL ® hydrophilic prepolymer binding agent preferred in the instant invention, a residence time in the mixer of from about 30 seconds to about 4.5 minutes, preferably from about 60 seconds to about 2 minutes, most preferably about 1.5 minutes, results in a homogeneous product having ideal, stable air space. Whether a particle mixer is capable of forming a homogeneous mix can be determined visually; in non-homogeneous mixes, large particles of cured polymer are readily visible, and/or the resulting media is relatively weaker than otherwise identical media made from homogeneous mixes. Since the temperature of the mix is sufficiently low to inhibit substantial curing of the prepolymer, the prepolymer has an opportunity during the mixing period to infuse into the interstices amongst the aggregate fibers in an amount sufficient to form a homogeneous composition that upon curing, is strong, cohesive, stable and self-supporting. In view of the lowered temperature of the mix, relatively long mixing times under low shear that may be necessary to achieve a homogeneous mix can be used without allowing the prepolymer to cure enough to deleteriously inhibit mixing or the filling of the mix into suitable molds or trays. The present invention is characterized by a batch mixing operation coupled with a continuous fill operation. Thus, the various components making up the growing media are metered into a mixing chamber on a batch basis, are mixed, and then empty into a tray filling machine 5 (Figure 1) . While in the tray filling machine 5, the mix is slowly tumbled or agitated, and is continuously forced out the bottom of the tray filling machine 5 into suitable trays or molds properly spaced and aligned on conveyor belt 20. The speed of conveyor belt 20 is regulated so that the filling operation is continuous; i.e., it is based upon the predetermined mixing time in the mixer 3, so that there is always some mixed growing media composition in the tray filling machine 5 during the operation. The amount of product emptying into each tray is controlled by a weighing device 4. The product cures in the trays in about 5 minutes at room temperature. The batch mixing/continuous filling operation allows for different molds or trays without retooling. The trays containing cured product can then proceed to a dibbler station 9, where a suitable cutting or hole is made in each plug to accommodate a seedling or the like. Tray stackers 6 and 10 and tray destacker 8 can be placed along the path of travel to conserve space. Those skilled in the art will recognize that other additives can be included in the growing media, such as nutrients, wetting agents, fertilizers, fungicides, pesticides. pH adjusters such as lime, etc. Preferably such additives are added during the batch mixing stage, and if water soluble, can be rapidly water soluble or can be released over an extended period of time. The invention will be further illustrated by the following non-limiting examples. EXAMPLE 1 A slurry is prepared from the following components: 5 kg peat soil (containing 47% by weight moisture) 200 g HYPOL® 2002 15 kg water The temperature of the water added was 5°C. The resulting slurry was mixed in a mixer having four arms 25 cm long at 100 rpm for 1.5 minutes. The resulting dispersion was casted in packaging trays and hardened within 3-4 minutes at a temperature of about 10°C. The resulting plug had excellent density and did not crumble or fall apart. EXAMPLE 2 The procedure of Example 1 was repeated, except that 6 kg of ground coconut fiber was used instead of peat soil, and the slurry was prepared using 210 g resin forming solution and 4 kg of water. The resulting plug obtained was a coherent body having excellent density. EXAMPLE 3 Berger blonde golden spaghnum peat moss was used. It was fluffed for three minutes in a Hobart mixer at 100 RPM. The peat was then sieved with standard mesh sizes 8, 20, 35, 50 and 80 for five minutes. All material left on the mesh 8 screen was discarded and the other fractions were collected and placed in separate bags for each fraction. The peat was then subjected to dry mixing simulating low shear (Hobart mixer for 90 seconds at 500 rpm) and high shear (modified hand drill with a homogenizing blade inside a plastic container having an inside diameter (3.5 inches; height of 7 inches) slightly larger than the blade diameter (2.75 inches) to ensure a high degree of contact between the blade and the media) . The resulting particle size distributions for each are set forth in Table 1: TABLE 1 CONTROL LOW SHEAR HIGH SHEAR MESH 20 31.4% 28.2% 14.77% <20 >80 56.2% 54.5% 65.1% <80 12.4% 17.3% 20.2% The data demonstrates that under low shear, little damage is done to the particles. In contrast, under high shear, substantial damage is done, especially to the larger fibers, i.e., only 14.77% of the fibers were retained by a mesh 20 sieve, compared to 31.4% in the control. The fluffed and screened peat of the various particles sizes was weighed and re-combined in its exact original particle size distribution. The peat having this recreated particle size distribution was conditioned by adding sufficient water to bring the density from its original 100 grams/liter to 180 grams/liter. (This conditioning step prepares the media for the addition of a standard amount of water in order to produce mixtures that are consistent from batch to batch, regardless of initial (preconditioned) density/moisture content.) The resulting compositions were mixed in a Hobart mixer for 2 minutes at 100 RPM. Table A sets forth various data recorded for different particle sizes of peat, and shows that for smaller particle sizes, significantly more water is required: TABLE A CONCEPT CONTROL PEAT PEAT MESH SIZE 20 PEAT MESH SIZE 35 PEAT MESH SIZE 50 SAMPLE WEIGHT (g) 100.0 100.0 80 41.5 INITIAL DENSITY (g/1) 100.0 79.8 78.6 83.0 WATER ADDED (g) 110.0 130.0 118.0 110.0 FINAL DENSITY (g/1) 180 178 180 185 WATER TO ADD/100 g 110/100 130/100 147.5/100 266/100 PEAT The conditioned peat was sieved using standard mesh sizes 8 and 35. The larger meshes were used because the conditioned peat had swelled as a result of absorbing the water used in the conditioning step. The conditioned peat was sieved for five minutes, and the results of low and high shear mixing (as carried out above) are shown in Table 2: TABLE 2 CONTROL LOW SHEAR HIGH SHEAR MESH 20 6.0% 2.8% 0.2% <80 >35 93.2% 95.1% 97.6% <35 0.8% 2.1% 2.2% Again, the data confirms that substantial damage is done to the fibers of the aggregate at high shear, especially the larger fibers. As a control, 200 grams of conditioned peat was placed in a mixing bowl. 500 grams of water at a temperature of 4-5°C was added. 9 grams of HYPOL® 2002 prepolymer was then added, and the materials were mixed using a Hobart mixer at a rate of 500 RPM for 90 seconds. The resulting mixture was poured into two cylinders having inside diameters of 3 inches and a height of 3.5 inches. Any excess mixture was poured into a graduated beaker. The wet density (and drained) and strength were determined, and are recorded below. The wet/drained density was determined by allowing the cylinders to drain for between three and four hours at 30°C. Each cylinder with media was weighed. (The media was removed from the cylinder for use in the strength and dry density tests (below) ) . The cylinder tare weight was subtracted from this total weight and the resulting weight was divided by the volume of the cylinder (403.29 cc) . The strength of the wet (and drained) media was determined by sieving using screens with mesh 3/8 inch, 8 and 20. The amount of media remaining on each screen after sieving was recorded to measure the relative strength of the samples. A mixing time experiment was carried out comparing the control (90 seconds of mixing) with samples that were mixed for only 30 seconds (Sample 2) and 300 seconds (Sample 3) . The results are shown in Table 3 below. The results demonstrate that mixing for too short a period of time (Sample 2) results in a more dense media (21% in this case) , and that mixing for too long a period of time (300 seconds) results in an even denser (54% in this case) media. Higher density translates to less air space, media that is too wet (which is deleterious to the roots of the plants or seedlings, and can result in root rot) , and higher media manufacturing costs (i.e., lower yields). A particle size experiment was carried out by preparing samples having only relatively large particle size aggregate (Sample 4), smaller particle size (Sample 5), and very small particle size (Sample 6) to the control. The data in Table 3 demonstrate that using large particle size media results in good media having properties similar to the control. However, as the particle size decreases, air space begins to decrease, and the media becomes markedly weaker. As particle size becomes very small (simulating ground aggregate), the media becomes even denser, air space is reduced, and the media remains visually very wet. A water amount experiment was conducted wherein a sample containing too little water (Sample 7) and a sample containing too much water (Sample 8) were compared to the Control. Table 3 shows that although using too little water results in a low density, high air space media, the air space is not stable (i.e., it decreases over time), and the media was not cohesive but rather was very weak. Using too much water results in a high density, low air space media that retains too much water is would be deleterious to plantings. TABLE 3 ** Density after drying for 4 hours at 30°C. N.D. = not determinable N.A. = not applicable

Claims: What is claimed is: 1. A method for preparing cohesive, stable, self- supporting, open celled growing media, said method comprising forming a slurry consisting essentially of aggregate, hydrophilic polyurethane prepolymer, and water; mixing said slurry at a temperature below about 20°C, said mixing being conducted with a shear sufficiently low so as to avoid substantial damage to said aggregate during said mixing, and for a time sufficient to create a homogeneous mix; and allowing the resulting mix to harden. 2. The method of claim 1, wherein said slurry has a temperature below about 10°C. 3. The method of claim 1, wherein said aggregate is selected from the group consisting of peat, soil, coir (coconut husk) , perlite, vermiculite, pumice, baked clay, wood pulp, ground tree bark, sawdust, and mixtures thereof. 4. The method of claim 1, wherein said aggregate is peat. 5. The method of claim 1, wherein said prepolymer and aggregate are present in a ratio of 0.03-0.1:1. 6. A method for preparing cohesive, stable, self- supporting, open celled growing media, comprising forming a slurry consisting essentially of aggregate having a particle size distribution, hydrophilic polyurethane prepolymer, and water, said slurry having a temperature below about 20°C; mixing said slurry at a temperature below about 20°C, said mixing being conducted with a shear sufficiently low so that said particle size distribution of said aggregate is not materially altered, and for a time sufficient to create a homogeneous mix; and allowing the resulting mix to harden. 7. The method of claim 6, wherein said slurry has a temperature below about 10°C. 8. The method of claim 6, wherein said aggregate is selected from the group consisting of peat, soil, coir (coconut husk) , perlite, vermiculite, pumice, baked clay, wood pulp, ground tree bark, sawdust, and mixtures thereof. 9. The method of claim 6, wherein said aggregate is peat. 10. The method of claim 6, wherein said prepolymer and aggregate are present in a ratio of 0.03-0.1:1. 11. The method of claim 6, wherein said aggregate particle size distribution is such that at least about 40% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 12. The method of claim 6, wherein said aggregate particle size distribution is such that at least about 50% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 13. A method for preparing cohesive, stable, self- supporting, open celled growing media, comprising: a. combining aggregate having a particle size distribution, hydrophilic polyurethane prepolymer, and water, said combination having a temperature sufficiently low so as to allow time for the mixing set forth in step b without said prepolymer curing in an amount sufficient to prevent such mixing; b. mixing said combination at said temperature, said mixing being conducted with a shear sufficiently low so that said particle size distribution of said aggregate is not materially altered, and for a time sufficient to create a homogeneous mix; c. filling a mold with said homogeneous mix; and d. allowing the resulting mix to harden. 14. The method of claim 13, wherein said combination has a temperature below about 10°C. 15. The method of claim 13, wherein said aggregate is selected from the group consisting of peat, soil, coir (coconut husk) , perlite, vermiculite, pumice, baked clay, wood pulp, ground tree bark, sawdust, and mixtures thereof. 16. The method of claim 13, wherein said aggregate is peat. 17. The method of claim 13, wherein said prepolymer and aggregate are present in a ratio of 0.03-0.1:1. 18. The method of claim 13, wherein said aggregate particle size distribution is such that at least about 40% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 19. The method of claim 13, wherein said aggregate particle size distribution is such that at least about 50% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 20. A method for preparing cohesive, stable, self- supporting, open celled growing media, comprising: a. providing aggregate having a pre-mix particle size distribution; b. mixing said aggregate with hydrophilic polyurethane prepolymer and water, said water having a sufficiently low temperature so that when said water is mixed with said aggregate and prepolymer, the temperature of the mix is adjusted to below about 20°C, said mixing being conducted with a shear sufficiently low so that said pre-mix particle size distribution of said aggregate is not materially altered during said mixing, said mixing being conducted for a time sufficient to create a homogeneous mix; and c. allowing the resulting mix to harden. 21. The method of claim 20, wherein said mix has a temperature below about 10°C. 22. The method of claim 20, wherein said aggregate is selected from the group consisting of peat, soil, coir (coconut husk) , perlite, vermiculite, pumice, baked clay, wood pulp, ground tree bark, sawdust, and mixtures thereof. 23. The method of claim 20, wherein said aggregate is peat. 24. The method of claim 20, wherein said prepolymer and aggregate are present in a ratio of 0.03-0.1:1. 25. The method of claim 20, wherein said water temperature is 4-5°C. 26. The method of claim 20, wherein said pre-mix particle size distribution is such that at least about 40% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 27. The method of claim 20, wherein said aggregate particle size distribution is such that at least about 50% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 28. Stable, self-supporting, open celled growing media formed from aggregate having a first particle size distribution, said growing media comprising said aggregate bound by hydrophilic polyurethane polymer uniformly distributed therein, said aggregate of said growing media having a second particle size distribution substantially the same as said first particle size distribution. 29. The growing media of claim 28, wherein said aggregate is selected from the group consisting of peat, soil, coir (coconut husk) , perlite, vermiculite, pumice, baked clay, wood pulp, ground tree bark, sawdust, and mixtures thereof. 30. The growing media of claim 28, wherein said first particle size distribution is such that at least about 40% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 31. The growing media of claim 28, wherein said first particle size distribution is such that at least about 50% of the particles are retained by a size mesh 35 sieve after particles larger than size mesh 8 sieve have been removed. 32. A method for preparing cohesive, stable, self- supporting, open celled growing media, comprising: a. mixing, in a batch, aggregate, hydrophilic polyurethane prepolymer and water, said water having a sufficiently low temperature so that when said water is mixed with said aggregate and prepolymer, the temperature of the mix is adjusted to below about 20°C, said mixing being conducted with a shear sufficiently low so that said pre-mix particle size distribution of said aggregate is not materially altered during said mixing, said mixing being conducted for a time sufficient to create a homogeneous mix; b. transferring the resulting homogeneous mix into a vessel; c. continuously filling molds with said homogeneous mix from said vessel; and d. allowing the resulting mix to harden in said molds.

Inventors: LEERDAM FRANS, VAN ZANTEN EVERT, LEERDAM, FRANS, VAN ZANTEN, EVERT
Applicants: LEERDAM FRANS, PREFORMA WESTLAND BV, ZANTEN EVERT VAN, LEERDAM, FRANS, PREFORMA WESTLAND B.V., VAN ZANTEN, EVERT
Application year: 1996
Main classification: A01G9
IPC codes: A01G1, A01G9