Straw composite silent wooden door and preparation method thereof

Abstract

The invention discloses a straw composite silent wooden door and a preparation method thereof, and belongs to the technical field of wooden door manufacturing. The wood door sequentially comprises a straw core layer, a wood frame layer, a micro-foaming adhesive layer and a covering layer from inside to outside. Wherein the straw core layer is formed by compounding and curing straw or wheat straw and an improved magnesium oxychloride inorganic adhesive, and the curing degree is 65-70%; the wooden frame layer is formed by splicing quadrilateral or equihexagonal unit blocks processed by small-diameter Chinese fir or poplar; and the covering layer is a medium-density fiberboard and a facing material or a facing plywood. The improved magnesium oxychloride adhesive is prepared from light calcined magnesia, anhydrous magnesium chloride, deionized water, phosphoric acid, nano silicon dioxide and highland barley straw ash according to a specific ratio. The preparation method comprises the three steps of straw core layer preparation, composite door core plate preparation and finished product composite door preparation. The formaldehyde-free improved magnesium oxychloride adhesive is adopted, the formaldehyde release amount is smaller than or equal to 0.1 mg / L, and the straw utilization rate reaches 90% or above; through the synergistic effect of the porous structure of the straw core layer and the micro-foaming adhesive layer, the sound insulation amount is larger than or equal to 35 dB and is increased by 40% or above compared with a traditional wooden door; due to the wood frame and straw core layer composite structure, the static bending intensity of the core plate is larger than or equal to 18 MPa, and the internal bonding strength is larger than or equal to 0.8 MPa; the production cost is reduced by 30%-40% compared with that of a solid wood door, and the composite material can be widely applied to the fields of indoor silence doors and environment-friendly home building materials.

Description

Technical Field

[0001] This invention relates to the field of wooden door manufacturing technology, specifically to a composite silent wooden door and its preparation process using agricultural straw as the core raw material and combined with a modified inorganic adhesive, which is suitable for applications such as indoor silent doors and environmentally friendly home building materials. Background Technology

[0002] With tightening environmental policies and exacerbating the shortage of timber resources, the resource utilization of agricultural straw has become a hot research topic in the industry. Traditional wooden doors mostly use solid wood or MDF as the base material, which has drawbacks such as high wood consumption, insufficient sound insulation (usually ≤25dB), and poor moisture resistance. On the other hand, existing straw composite doors generally use organic adhesives such as urea-formaldehyde resin, which pose a risk of formaldehyde release, and the weak interfacial bonding between straw and adhesive leads to unstable mechanical properties.

[0003] Magnesium oxychloride inorganic adhesives have garnered widespread attention due to their formaldehyde-free, environmentally friendly properties and high early strength. However, unmodified products suffer from poor water resistance and high brittleness (softening coefficient typically ≤0.6), limiting their application in wooden door substrates. Meanwhile, existing soundproof doors largely rely on auxiliary structures such as sealing strips, lacking a systematic soundproofing design from core materials to composite structures, making it difficult to meet the high soundproofing requirements (≥30dB) of bedrooms, studies, and other similar settings. Therefore, developing a straw composite wooden door manufacturing technology that combines environmental friendliness, mechanical stability, and high-efficiency soundproofing performance is of significant practical importance. Summary of the Invention

[0004] This invention aims to solve the problems of high wood consumption, poor sound insulation, and insufficient environmental friendliness of existing wooden doors. It provides a silent wooden door with rice straw / wheat straw as the core material and a modified magnesium oxychloride adhesive, as well as its preparation method, to achieve synergistic optimization of straw resource utilization and silent performance.

[0005] To achieve the above objectives, this invention provides a straw composite soundproof wooden door and its preparation method, the specific technical solution of which is as follows: (I) Straw Composite Silent Wooden Door Structure The wooden door comprises, from the inside out: a straw core layer (20mm–40mm thick), a wooden frame layer, a micro-foamed adhesive layer (0.8mm–1.2mm thick), and a surface layer; wherein: Straw core layer: composed of rice / wheat straw and modified magnesium oxychloride adhesive, with a curing degree of 65% to 70%; Wooden frame layer: made of small-diameter fir / poplar wood processed into quadrilateral or equal hexagonal units and spliced ​​together, with a moisture content of 5% to 8%; The surface layer is a composite layer of medium-density fiberboard (MDF) and decorative material or decorative plywood, with a thickness of 3mm to 5mm.

[0006] (II) Improved magnesium oxychloride inorganic binder formulation (by mass parts) The mixture consists of 100 parts of lightly calcined magnesium oxide, 45-50 parts of anhydrous magnesium chloride, 80-90 parts of deionized water, 1 part of phosphoric acid, 3 parts of nano-silica, and 5-10 parts of highland barley straw ash. The composite modification of phosphoric acid and nano-silica can increase the softening coefficient of the adhesive to over 0.95, while the highland barley straw ash fills the pores through a secondary hydration reaction, enhancing water resistance.

[0007] (III) Preparation method Straw core preparation Raw material pretreatment: Cut rice straw or wheat straw into short fibers of 10mm to 15mm, dry them at 105℃ until the moisture content is ≤10%, and remove impurities; Mixed paving: Straw and modified magnesium oxychloride binder are mixed evenly at a mass ratio of 1:0.3 to 0.4, and a continuous core layer blank is formed by paving in the same direction; Curing and cutting: Place the blank in a constant temperature and humidity chamber with a temperature of 25℃~30℃ and a relative humidity of 60%~70% for curing. When the curing degree reaches 65%~70%, take it out and saw it horizontally to the designed thickness (i.e., the thickness of the inner core layer of the silent door).

[0008] Composite door core board preparation Wood unit processing: small-diameter fir or poplar wood is sawn into blanks with a length 2mm to 3mm longer than the thickness of the inner core layer, dried to a moisture content of 5% to 8%, and then processed into unit blocks with a cross-section of quadrilateral (side length 20mm to 30mm) or equal hexagonal (side distance 20mm to 30mm) by die cutting or milling. Frame splicing: Water-based polyurethane adhesive (solid content ≥45%) is applied to the wooden units (adhesive layer thickness 0.2mm~0.3mm) to form the door stop and door frame. The splicing pressure is 0.8MPa~1.2MPa, the clamp spacing is ≤300mm, and the joint gap is ≤0.2mm. Core layer assembly: The straw core layer prepared in step 1 is embedded in the middle of the frame and placed in a wide-band sander for fixed-thickness sanding. After sanding, the thickness error of the core board is ≤ ±0.5mm and the surface flatness is ≤ 0.5mm / 2m.

[0009] Preparation of finished composite doors Glue application and lamination: Apply a micro-foamed adhesive layer evenly to the upper and lower surfaces of the core board, with an adhesive application amount of 200g / m²~250g / m², and then lay medium-density fiberboard + decorative material or decorative plywood respectively. Pressure curing: Curing at room temperature under 0.5MPa pressure for 24 hours to ensure complete curing of the adhesive layer without bubbling or peeling; Post-processing: The edges of the door leaf are milled and trimmed, and magnetic silent locks and sealing strips are installed. The sound insulation of the finished product is tested to be ≥35dB.

[0010] (iv) Beneficial effects Environmental friendliness: It uses formaldehyde-free modified magnesium oxychloride adhesive, with formaldehyde release ≤0.1mg / L, and straw utilization rate of over 90%, realizing the resource utilization of agricultural waste; Sound insulation performance: The porous structure of the straw core layer and the micro-foamed adhesive layer work together to improve sound insulation by more than 40% compared with traditional wooden doors, meeting the requirements for high quietness; Mechanical properties: The composite structure of the wooden frame and straw core layer makes the static bending strength of the core board ≥18MPa and the internal bonding strength ≥0.8MPa, which meets the requirements of the superior grade of GB / T11718-2009 standard. Economic efficiency: Small-diameter wood and straw raw materials have low costs, the process is simple and can be mass-produced, and the production cost is 30% to 40% lower than that of solid wood doors. Detailed Implementation Example

[0011] Preparation of modified magnesium oxychloride adhesive: Raw material selection: Lightly calcined magnesium oxide (particle size 200 mesh) with an activity content ≥85% is selected to ensure sufficient hydration reaction; anhydrous magnesium chloride with a purity ≥98% is used to avoid impurities affecting the bonding performance; hydrophobic nano-silica with a particle size of 50nm is selected to enhance compatibility with inorganic substrates; barley straw ash is calcined at 600℃ for 2 hours and then pulverized to 300 mesh to increase the content of active ingredients. Accurately weigh 100 parts by weight of lightly calcined magnesium oxide, 48 parts by weight of anhydrous magnesium chloride, 85 parts by weight of deionized water, 1 part by weight of phosphoric acid, 3 parts by weight of nano-silica, and 8 parts by weight of barley straw ash. First, anhydrous magnesium chloride is added to deionized water and stirred for 30 minutes until completely dissolved to prepare a magnesium chloride aqueous solution. Then, lightly calcined magnesium oxide is added and stirred at high speed for 15 minutes to form a uniform slurry. Next, phosphoric acid, nano-silica, and highland barley straw ash are added in sequence and stirred for another 20 minutes. Finally, the mixture is placed in an ultrasonic disperser (power 500W, frequency 40kHz) and ultrasonically dispersed for 15 minutes to eliminate air bubbles inside the slurry, resulting in a uniformly dispersed and clumpy modified magnesium oxychloride binder. When ready for use, it should be placed in a constant temperature environment of 25°C to avoid moisture evaporation.

[0012] Straw core preparation: Rice straw harvested that year (free from mold and pests) was selected and cut into 12mm short fibers using a straw cutter. The fibers were then fed into a belt dryer and continuously dried at 105℃ for 4 hours until the moisture content dropped to 8% (real-time monitoring was performed using a halogen moisture analyzer). Subsequently, impurities such as soil and stones were removed by a vibrating screen (2mm mesh diameter). The straw and modified magnesium oxychloride binder were added to a twin-shaft paddle mixer at a mass ratio of 1:0.35, with a mixing speed of 300 rpm and a mixing time of 10 minutes, ensuring the binder evenly coats the straw fiber surface without any exposed fibers. A tracked, co-directional laying unit was used, with a laying speed of 0.5m / min and a laying thickness of 40mm (allowing for subsequent sawing allowance), forming a continuous core layer blank. During the laying process, the blank temperature was monitored using an infrared thermometer and maintained between 20℃ and 25℃. The blanks were cut into 1220mm × 2440mm slabs and placed in a constant temperature and humidity curing chamber. The temperature was set at 28℃ and the relative humidity at 65% for 48 hours. During this period, the degree of curing was checked every 12 hours (using a Shore hardness tester; a hardness of D60 corresponds to a degree of curing of 68%). After curing, a precision sliding table saw (saw blade speed 4500r / min, feed speed 5m / min) was used to saw transversely to a thickness of 30mm to obtain the straw core layer. After sawing, the surface was lightly sanded with a belt sander (P120 mesh belt) to remove burrs.

[0013] Core board preparation: Small-diameter fir trees with a diameter at breast height (DBH) of 8-12cm were selected and cut into 33mm long blanks (3mm thicker than the straw core layer) using a band saw. These blanks were then fed into a hot air circulating drying kiln using a stepped drying process: first, drying at 60℃ for 12 hours; then increasing the temperature to 80℃ for 24 hours; and finally, cooling to 40℃ for 8 hours to equilibrate. The moisture content after drying was controlled at 6% (measured using a wood moisture meter at 5 points on different parts of the blank, and the average value was taken). The blanks were then processed into 25mm side-length regular quadrilateral unit blocks using a CNC die-cutting machine. The dimensional error of each unit block was ≤±0.2mm, and the perpendicularity deviation of the edges was ≤0.1mm. Water-based polyurethane adhesive with a solid content of 48% was selected and applied to the splicing surfaces of the quadrilateral unit blocks using a roller coater. The adhesive layer thickness was controlled at 0.25mm (adjusted by the gap between the coating rollers). The frame was assembled within 30 minutes after application using a splicing machine with a splicing pressure of 1.0MPa and a holding time of 30 minutes. The clamp spacing was 250mm. During the splicing process, a feeler gauge was used to check the joint gaps to ensure they were ≤0.2mm. After splicing, the frame was allowed to cure naturally for 24 hours to form the gate frame and door frame. Then, the straw core layer was embedded in the middle of the frame and placed in a wide-band sander (sand belt specification P150 mesh). The sanding speed was set to 3m / min, with a single sanding amount of 0.5mm. Sanding was done in two passes to the designed thickness of 40mm. After sanding, the core board thickness error was ≤±0.5mm, and the surface flatness was ≤0.5mm / 2m (checked using a 2m straightedge and feeler gauge).

[0014] Finished product preparation: A 60% solids content polyether-based microfoam adhesive was selected and evenly applied to the upper and lower surfaces of the core board using a sprayer. The application rate was 220 g / m² (the application rate was calibrated by weighing). Within 10 minutes of application, a 3 mm thick medium-density fiberboard (MDF, density 0.75 g / cm³) was applied. The MDF surface was pre-covered with a natural wood veneer (0.15 mm thick). The veneered door panel was then placed in a cold press at a pressure of 0.5 MPa and cured at room temperature for 24 hours. During the curing period, the pressure was checked every 6 hours to ensure there was no pressure loss. After curing, the edges of the door leaf were milled and trimmed using a CNC milling machine to create a door frame groove (5 mm wide, 3 mm deep). Subsequently, a silicone magnetic silent lock and EPDM sealing strip were installed, with the sealing strip flush with the edge of the door leaf after being embedded in the groove. Finished product testing: The sound insulation was tested at 36dB according to GB / T50121-2005 "Standard for Evaluation of Sound Insulation of Buildings"; the formaldehyde emission was tested at 0.08mg / L according to GB18587-2017 "Formaldehyde Emission Limits in Wood-based Panels and Their Products for Interior Decoration and Renovation"; and the static bending strength was tested at 18.5MPa and the internal bond strength was tested at 0.85MPa according to GB / T11718-2009 "Medium Density Fiberboard", all of which meet the requirements for superior products. Example

[0015] Raw material pretreatment and adhesive preparation: Wheat straw harvested that year (with longer fiber length and better toughness than rice straw) was selected, cut into 15mm short fibers, dried at 105℃ to a moisture content of 9%, and impurities were removed before use. The modified magnesium oxychloride adhesive formula was adjusted to include 5 parts of highland barley straw ash, while the remaining components and preparation process were the same as in Example 1. The softening coefficient of the prepared adhesive was tested to be 0.96.

[0016] Preparation of straw core layer and frame: The mass ratio of straw to adhesive was 1:0.3. The mixing, laying, and curing processes were the same as in Example 1. After curing for 48 hours, the degree of curing reached 65%, and the straw was sawn to a thickness of 35 mm. Small-diameter poplar wood (initial moisture content 18%) was selected as the raw material and sawn into blanks with a length of 38 mm (3 mm thicker than the straw core layer). The blanks were dried in a hot air circulating drying kiln to a moisture content of 7%, and then processed into hexagonal unit blocks (side distance 22 mm, side length 12.7 mm) by a CNC milling machine. The processing accuracy of the unit blocks was controlled within ±0.2 mm. Using the same water-based polyurethane adhesive and coating process as in Example 1, the straw core layer was embedded after splicing the frame and sanded to a thickness of 45 mm.

[0017] Finished product lamination and testing: The surface layer uses 4mm thick veneer plywood (poplar core, surface covered with engineered wood veneer), with a micro-foaming adhesive application rate of 230g / m². After cold pressing and curing for 24 hours, the edges are trimmed. Finished product testing results: sound insulation 35.2dB, static bending strength 19.5MPa, internal bond strength 0.9MPa, softening coefficient 0.96, formaldehyde release 0.09mg / L. All indicators meet the requirements for use as an indoor soundproof door. Furthermore, the combination of the poplar frame and straw core layer reduces the door weight by 5% compared to Example 1, making it easier to install and use.

Claims

1. A straw composite silent wooden door, characterized in that, From the inside out, it includes a straw core layer, a wooden frame layer, a micro-foamed adhesive layer, and a covering layer; the straw core layer is composed of rice straw / wheat straw and modified magnesium oxychloride adhesive, with a curing degree of 65%-70%; the wooden frame layer is made of small-diameter fir / poplar wood processed into quadrilateral or equal hexagonal units and spliced ​​together, with a moisture content of 5%-8%.

2. The straw composite silent wooden door according to claim 1, characterized in that, The modified magnesium oxychloride adhesive comprises the following components by weight: 100 parts of lightly calcined magnesium oxide, 45-50 parts of anhydrous magnesium chloride, 80-90 parts of deionized water, 1 part of phosphoric acid, 3 parts of nano-silica, and 5-10 parts of highland barley straw ash.

3. A method for preparing a straw composite soundproof wooden door as described in claim 1, characterized in that, Includes the following steps: (1) Cut rice straw / wheat straw into short fibers of 10-15mm, mix with modified magnesium oxychloride adhesive at a mass ratio of 1:0.3-0.4, lay in the same direction, and cure at 25-30℃ and 60%-70% humidity until the curing degree reaches 65%-70%, and then saw to the designed thickness; (2) Small-diameter fir / poplar wood is processed into quadrilateral or equal hexagonal units, dried to a moisture content of 5%-8%, glued and spliced ​​into a frame, embedded with the straw core layer of step (1), and sanded to a certain thickness to make a core board; (3) Apply micro-foamed adhesive layer to the top and bottom of the core board, cover it with medium density fiberboard + decorative material or decorative plywood, and then pressurize and cure to obtain the finished product.