Background

Rice is one of the staple crops. Rice is grown over a large part of Asia, South America, and North America. In 2017, one-hundred and seventy-eight million hundredweight (CWT) of rice was produced in the US, and California has 70% of the USA medium grain rice production!. Once the crop is harvested using mechanical combines, it leaves behind significant lengths of straw, that have traditionally been used as fuel (usually for direct burning with deleterious environmental effects), livestock feed, as a substrate for growing mushrooms or for production of biochar for improving soil conditions, and the like.

Although there are many pathways, the use of rice straw as a source of energy is limited. Mechanical devices like choppers quickly wear out, due to the presence of high quantities of abrasive silica in the straw. Ash content and volatile matter content in rice straw is also relatively high, as compared to wood and coal, with lower content of fixed carbon, as compared to coal. Fouling and failure of boiler parts due to melting occurs due to the production of ash having a relatively high alkali and potassium content.

Rice straw fiber is composed of 20% lignin, with 32% cellulose, 28% hemicellulose, 11% ash, and 9% of other materials, where the lignin is the natural binder, the cellulose is the reinforcing fiber and hemi-cellulose is a filler that provides minimal mechanical strength. Methods that have been used to obtain lignin from rice fiber include ultrasound- assisted alkaline extraction^ steam explosion and biological treatments; hydrothermal extractions; solvent extraction using bio-ionic liquid (with cholinium (choline chloride) as the cation and amino acids as the anions). While useful, these methods tend to suffer from inefficiency and the requirement of potentially hazardous chemistry.

Methods that have been proposed using rice fiber in composite boards with binder systems from the chemical industry include rice straw fiber board with modified surface using NaOH and modified soy protein isolate adhesivess; medium and high density rice fiber boards with methylene diphenyl diisocyanate (MDI) adhesives*; fiber board using rice straw and urea formaldehyde resin. Again, while useful, these methods tend to suffer from inefficient use of production energy and the need for unnecessarily complex and potentially hazardous chemistry. For example, the composition of current laminating resins is typically a phenol-formaldehyde mixture. For improved environmental safety both during manufacturing and in the ultimate in-home use as kitchen cabinet and furniture countertops, there is a desire to reduce the amount of both phenol and formaldehyde. In addition, there is a desire to use sustainable green products vs. formaldehyde and phenol which are produced from petrochemicals.

Thus, there remains a need for a way of utilizing rice straw to yield fiber board products that is more efficient and less dependent on complex and hazardous chemistry.

The present novel technology addresses this need. Summary

The objective is to produce a composite board where both the binder and the reinforcing fibers are made from rice straw, which is an agricultural waste product. Two results are detailed herein: (1) the method for catalytically degrading rice straw to obtain a product stream of cellulose that are relatively clean of lignin and a material that includes depolymerized lignin and other products such as waxes, sugars and hemicellulose and (2) formulation of a binder system using the depolymerized along with appropriate curing agents that can be used with a mixture of the clean cellulose fibers and additional rice fibers to produce composite boards with desirable physical properties. One attractive feature of this approach is to produce a formaldehyde free composite board. Initial analysis indicates that the material costs for the formaldehyde-free, rice board is approximately $O.22/kg ($0.10/lb) vs $O.37/kg ($0.17/lb) for the competitive formaldehyde-free product made using a PMDI resin. A $O.15/kg ($0.07/lb) material cost difference in a low margin business like composite boards is significant and there is opportunity to further reduce the material cost of the rice composite board.

The overall manufacturing process is summarized as follows. Steps include:

1. Lignin Resin Mixing Process. Depolymerized lignin produced from rice straw in a high-pressure reactor with a nickel catalyst was combined the appropriate curing agents and other additives.

2. A-Stage Cure. The depolymerized lignin with curing agents was heated to partially advance the cure in to control the viscosity of the resin mixture. Resin Extension. The main component of the laminating resin in a traditional phenol-formaldehyde resin is extended by the addition of depolymerized lignin resin from the A-stage cure. Kraft Paper Saturation. The resin mixture is brushed onto the kraft paper using a small paint brush so that the paper is fully saturated with minimal bleed though of resin through the paper. The kraft paper is not dripping wet, but tacky to the touch, where the resin should not be pooled. B-Stage Cure. The resin impregnated kraft paper is now subjected to a partial cure of the resin. The impregnated paper is heated in an oven at 130 °C for 1 minute on a first side of the laminate, turned over and then heated for an additional 1.5 minutes on the opposite side. The B-stage cure stiffens the kraft paper as well as removes solvent which thereby reduces the flow of the resin. Layup Procedure. The impregnated kraft paper that has been partially cured in the B-Stage cure is laid up as follows: 1) topcoat, 2) decor paper, and 3) impregnated kraft paper that has been B-Staged cured. The three-component laminate is placed between two metal plates, where the plate next to the topcoat layer is often a texture plate that is a stainless-steel plate textured with a wood grain or pattern. This assembly in now ready to be placed in the hot press. Press Process. The laminate assembly is now placed in the hot press where it undergoes a two-step cure procedure. One commonly selected laminate size is 8.5 in. by 11 in. or 21.6 cm. by 28 cm. In the first cycle, 1283 psi pressure is applied as the press is heated from 68°C to 115°C, where the overall process takes 13.5 minutes. The temperature ramp from 68°C to 115°C is typically timed to last about 7 minutes, and the temperature ramp is more typically a linear time/temperature relationship. The laminate assembly is held (soaked) at 115°C for the remainder of the 13.5 minutes. This is immediately followed by the second cycle where the pressure is decreased rapidly to 428 psi and then the press platens are heated from 115°C to 127°C for a total of 8 minutes, where the (typically linear) heating ramp takes approximately 3 minutes after which the sample is soaked at 127°C is for the remainder of the 8 minutes after which the fully cured laminate is removed from the press.

8. Cool Down Process. The laminate is then removed from the press and cooled, such as between two aluminum plates that act as heat sinks. Samples are cooled until they can be handled by hand without heat gloves.

After completion of Step 8 the cured laminate is removed from the press and its physical properties are evaluated.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the disclosure, 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 disclosure is thereby intended.

The present novel technology relates to a process for extracting and/or depolymerizing lignin as well as fibrous material from rice straw for use in the production of composite boards.

Production of depolymerized lignin from rice straw

Example

In a first embodiment, a reaction mixture was prepared, consisting of 15 g of rice straw fibers in 300 ml of methanol with 1.5 g of nickel on activated carbon catalyst. The activated nickel catalyst was prepared by incipient wetness impregnation of nickel nitrate hexahydrate solution (2.76 gram salt in 8 mL de-ionized water), on 5 grams of 100 mesh screened activated carbon. This mixture was oven air dried for 24 hours, followed by oven drying at 120oC, and finally treated in 50 standard cubic centimeters flow of nitrogen in a tube furnace, for two hours, with heat ramp of 1 hour. The resulting catalyst sample was cooled in the continuous nitrogen flow to room temperature and stored in glass vials prior to use. The reaction mixture was a fiber slurry that consisted of fibrous chunks settled at the bottom of the slurry. The reaction mixture described above was added at room temperature to a 600 ml stainless steel reactor. The reaction mass was heated over 60 mins to 200°C where the reaction generated a pressure of 40 bar. This reaction was sub-critical, where the critical point for methanol is 239°C at a pressure of 80 bar. The reaction was allowed to proceed for 6 hours. The reaction mass at the end of the reaction had the appearance of dark red slurry, and the viscosity of liquid products was similar to that of methanol, with chunks of fibers settled at the bottom of the reactor, and few lighter chunks floating on liquid surface.

The resulting products from the reaction were filtered using a funnel, separating fibrous residue and catalyst from the liquid products. Fibers were air dried and stored in desiccator, whereas the liquid product was stored in rubber stoppered conical flask. . Excess methanol was removed from liquid product, using a rotary evaporator under vacuum, resulting in viscosity of products similar to honey, along with waxy precipitates sticking to the wall of glass vial.

The reaction scheme described above was repeated 15-20 times to produce several batches of products. On the average 3 g of depolymerized products were obtained per 15 grams of straw fibers charged into the reactor (although this mixture including some quantity of methanol), where 15 g of rice straw contains 2.9 grams of lignin. Qualitative and quantitative analysis of the reaction products in underway, using gas chromatography - mass spectrometry and gas chromatography - flame ionization detection.

One interesting observation is that the fibers of the rice straw clumped to form chunks in the reactor, which may result in less than effective mixing (See Figure 2). We believe that this mixing issue is due to the small tank size, as well as the tank having a significant number of internal baffles. Investigation of the mixing process of rice fiber with methanol was done in a glass tank at room temperature without catalyst without internals. Observation of the resulting mixture revealed some clumping of the fibers, but the mixture could be readily agitated indicating that as one moves to larger processing equipment there is nothing with respect to the mixing that cannot be addressed using slurry mixing technology that is standard in the chemical industry.

The liquid products of the reaction were dark red in color, precipitates were observed to stick to the glassware after removal of excess methanol; however, no precipitates were observed on the inside of the steel reactor. This indicates that the precipitates are likely silica. In the envisaged composite board application of the depolymerized rice fiber mixture the presence of silica particles poses no significant problem; thus, removal of silica is not considered important at this time.

Composite boards production using rice straw with lignin from rice straw

Materials used in the rice straw fiberboard including a combination of depolymerized rice lignin reactor product, urea, ethylene urea, glyoxal, glutaraldehyde, Polycup, maleic anhydride, succinic anhydride, potassium hydroxide, and a paraffin wax. POLYCUP is a registered trademark of Solenis Technologies, LP, a Delaware Limited Partnership, 3 Beaver Valley Road, Suite 500, Wilmington, DELAWARE, 19803, reg. no. 0863338. Solvents in the process consist of water and methanol. The various components of the resin and associated costs are given in Table 1 for No. 106 resin formulation. The solids content of the liquid phase (i.e. non fiber) from the reactor described above was approximately 1/3 or the total mass. Thus, if rice fiber costs $O.15/kg, then the cost per pound of rice resin is $0.46/kg for the material. Making a very generous assumption that there is $0.02/kg for the capital equipment for a 45M kg/year plant and $0.02/lb for operating costs, the total rice resin cost is $O.53/kg, where using the $0.55 number in Table 1 is overly conservative.

The composite boards were made with 11 wt% of the resin package given in Table 1. The 89% of fibers was composed of 1/3 from the fibers used to produce the rice resin and 2/3 from new rice fibers. Thus, the total cost of materials is:

0.11*$0.36 + 0.66*$0.07 = $0,086 Ib/composite board

This should be compared to the alternative PMDI with rice fiber board

7% PMDI at $3.3O/kg + 93% rice fiber at $0.15/kg = $O.37/kg

Thus, the total material cost to the board manufacturer using the instant rice resin system is approximately 50% of that of the PMDI formaldehyde free alternative. Moreover, there is considerable room for optimization, such as, for example, reduction of succinic anhydride, better cost on production of rice resin, efficiencies of scale, and the like.

Estimated cost is $0.79 per kilogram of rice resin cure package

The process of first making rice straw fiberboard may be envisioned to begins with making a Resin Cure Package (RCP) and then a Wax + Surface Modification Package (WSMP).

Resin Cure Package. First, solid urea pellets and/or ethylene urea powders are added to a capable vessel. The depolymerized rice lignin product is mixed with methanol solvent to reach 60% “solids” content and this mixture is then added to the vessel and subsequently both glyoxal and glutaraldehyde crosslinkers are added. Then, additional methanol is used to adjust the viscosity of this mixture so as to yield a sprayable mixture, where the amount of methanol added is typically 2-3 ml per ml of the lignin, glyoxal and glutaraldehyde solution. The resulting mixture is subsequently heated in an oven at 85°C for about five minutes. Then an aqueous solution of maleic anhydride is added to the mixture, followed by the Polycup crosslinking agent. Finally potassium hydroxide is added to bring pH to the neutral condition that is desired for optimal reaction of the Polycup.

Wax + Surface Modification Package. The WPSMP is made in a separate vial. First 0.1 g of succinic anhydride is dissolved in 0.3 ml of a 1:5 by mass water-methanol mixture with slight heating to facilitate the dissolution of the succinic. Paraffin based wax is then added to this mixture. This mixture already is at or near the proper viscosity for spraying.

When manufacturing the board, the first step is to determine the required amount of fiber for the desired density and size of board. The WPSMP is sprayed on the fibers; the wetted fibers are mixed; and the mixture is dried in an oven, for example at 75oC for ten minutes. Once this mixture is at the optimal moisture content (approximately 10 to 12 wt %) as may be determined by touch, the RCP is sprayed on these fibers; and, the fiber with resin mixture is further agitated to ensure uniform coating of the fibers with the RCP. This RCP-fiber pre-composite is then dried in the oven, for example at 85oC for two minutes, to remove some additional moisture content so that the pre-composite have appropriate tack (i.e. the ability for fibers and resin to form the appropriate shape for the mold). Ideal conditions for pressing are 10-12% moisture content.

Current procedure for 5x7.6 cm (2x3 inch) boards employs a heated press with the bottom platen at 170°C and the top platen at 175°C. This temperature differential helps prevent board warping during manufacturing. Aluminum foil with a mold release agent is used to prevent the board from sticking to the press platens. Pressure profile for the first press at 400+25 PSI for 3 minutes and 700+25 PSI for 1 minute. The removal of the board between pressings allows for solvent to leave the system. Once the board is cooled after coming out of the press, the edges are trimmed. EXAMPLE

A formaldehyde-free fiber-based composite board can be produced as described above, wherein the board includes a fiber portion with a weight percentage of 80-95% and a resin portion with a weight percentage of 5-20%. The resin portion further includes a resin cure package and a wax-based surface modification package. The resin cure package defines a mixture of a catalytically depolymerized product of a fiber-based lignin, wherein the catalytically depolymerized product includes at least one compound selected from the group consisting of:

Other variations of the biomass of rice straw material are available and may be relevant. The wax-based surface modification package includes at least one wax material with a formula of CnH2n+2, wherein n is between 15 and 40. The fiber is typically a rice-straw- based fiber. The mixture of the resin cure package typically also includes at least one dialdehyde with a formula of OHCfCHsJnCHO, wherein n is between 0 and 6. The mixture of the resin cure package typically incudes an anhydride, and that anhydride is typically maleic anhydride. The mixture of the resin cure package may also include a polyamideepichlorohydrin as a crosslinking agent. The mixture of the resin cure package typically has a pH value of 6.5-7.5.

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.