A continuous preparation process for enhanced formaldehyde-free particleboard

Abstract

The present invention provides a continuous preparation process for an enhanced formaldehyde-free particleboard, the steps of which include: preparation of standard particles, separate gluing of particles, precise assembly of slabs, and hot-pressing-cold-pressing linkage molding of slabs. The wood raw materials used in the present invention are prepared and sorted into three types of particles, and an adhesive with isocyanate as the main agent is applied to the three types of particles respectively. Large pieces of thin particles are mixed with 1 to 20% of film-like plastic and placed on the sub-surface layer, and five groups of paving machines are used for full symmetrical paving, and hot-pressing-cold-pressing linkage is used to achieve a significant improvement in the comprehensive performance of the particleboard product. The process provided by the present invention innovates the traditional three-layer particleboard through reasonable structural design and raw material control, improves the efficiency of raw material use and the uniformity of gluing, solves the problem of low moisture content of formaldehyde-free artificial boards, and significantly improves the physical and mechanical properties of the particleboard.

Description

Technical Field

[0001] The present invention relates to the field of wood processing, in particular to a production technology for improving the quality and efficiency of artificial boards, and in particular to a continuous preparation process for enhanced formaldehyde-free particleboards. Background Art

[0002] Particleboard is a log substitute that makes full use of wood resources. The use of raw materials such as branches, processing residues, and recycled wood has important environmental significance. The particleboard industry has developed rapidly in the past 10 years. In 2022, my country's particleboard production exceeded 40 million m 3 , and is widely used in furniture, home furnishing, decoration, packaging and other fields. Lightweight, high strength, environmental protection and safety are the requirements of downstream enterprises and consumers for the use of wood-based materials such as particleboard.

[0003] As a wood composite material, particleboard can achieve different properties with different raw materials, structures and proportions. At present, common particleboards at home and abroad are mainly three-layer structures. The larger particles are used as the core layer to provide strength, and the fine particles are used as the surface layer to facilitate subsequent veneer and coating processing. The density of common three-layer particleboard products is less than 0.64g / cm 3 To meet the performance requirements of furniture-type (P2) or even load-bearing (P3) boards, it is necessary to select small-diameter timber or logs with excellent morphology, and process large particles with a prominent width-to-thickness ratio or length-to-thickness ratio as the core material. This is achieved by processing large particles into fine-surface (also known as veneerable) oriented particle board (OSB) on the market. However, a common problem in production needs to be solved: mixed particles containing excessively long and wide particles lead to poor gluing and paving uniformity. Oriented paving also leads to significant directional differences in particleboard performance, with the difference in longitudinal and transverse mechanical properties exceeding 30%, which easily leads to performance waste.

[0004] If the wood is not processed into large pieces, but the wood raw materials are cut into long strips and then planed, a larger screen is used to screen out a large number of larger-shaped (compared with conventional particleboard) particles. The three-layer structure "super particleboard" prepared with this large particle as the core layer is also a solution to achieve increased strength. However, this method has no obvious effect on the performance improvement and production cost control under the same conditions.

[0005] The environmental performance of particleboard for indoor use mainly refers to the formaldehyde emission. The adhesive is formaldehyde-free and meets consumer demand. Isocyanate adhesive has become the most common adhesive for the preparation of formaldehyde-free particleboard. The formaldehyde emission of isocyanate adhesive particleboard can reach E NFHowever, after tempering, particleboard made with isocyanate adhesive can reach a maximum moisture content of 6%, approximately 2% lower than that of particleboard made with modified urea-formaldehyde adhesive. The industry currently considers a moisture content of around 8% to be optimal for particleboard. However, in industrial production, high moisture content in raw materials can easily cause blistering defects in the board, and isocyanate adhesive reacts with water to generate gas, which is difficult to control.

[0006] Therefore, developing an environmentally friendly particleboard with excellent performance and qualified formaldehyde emission has become an urgent problem to be solved. Summary of the Invention

[0007] In order to solve the above problems, the purpose of the present invention is to provide a continuous preparation process for enhanced formaldehyde-free particleboard, which avoids the problems of uneven gluing and large gluing amount caused by excessive differences in the morphology of the core layer particles of ordinary particleboard and the problem of too low moisture content in the finished product. The particleboard prepared under the same raw materials and the same density conditions has higher physical and mechanical properties.

[0008] In order to achieve the above object, the present invention provides a continuous preparation process for a reinforced formaldehyde-free particleboard, which comprises the following steps:

[0009] 1) Preparation of standard wood shavings:

[0010] The wood raw materials are chipped, flaked, dried and screened to obtain three types of shavings: thin large shavings, medium shavings and fine shavings;

[0011] Among them, the length of thin large shavings is 20-50mm, the width is 5-40mm, and the thickness is 0.4-1.0mm; the length of medium shavings is 10-30mm, the width is 1-5mm, and the thickness is less than 1.2mm, accounting for more than 85%; the length of fine shavings is 1-10mm, and the thickness and width are less than 0.8mm, accounting for more than 80%; the weight of thin large shavings is 1-20% of the sheet-shaped thermoplastic plastic film with similar format;

[0012] The main function of sheet plastic film is to supplement gluing and improve waterproofing. Since the gluing uniformity of large wood chips is poor, close width sizes help to mix evenly.

[0013] 2) Gluing wood shavings separately:

[0014] Mix glue separately for thin large wood shavings, medium wood shavings and fine wood shavings;

[0015] The thin large wood chips are first adjusted to have a moisture content of 3-6%, and then 1-4% by weight of an isocyanate adhesive is applied; the medium wood chips are first adjusted to have a moisture content of 6-12%, and then 2-10% by weight of a mixed foamable adhesive is applied; the fine wood chips are applied with 2-5% by weight of a thermoplastic adhesive and 2-5% by weight of an isocyanate adhesive, and the moisture content of the fine wood chips is adjusted to 12-20%.

[0016] 3) Precise assembly of slabs:

[0017] The slab is assembled in a symmetrical manner, with the upper and lower surface layers being made of fine wood shavings and the core layer being made of medium wood shavings. Thin sheets of large wood shavings are laid on the upper and lower surface layers between the upper and lower surface layers of fine wood shavings and the core layer of medium wood shavings as reinforcement layers. Each single layer has a gradient structure.

[0018] 4) Slab hot pressing and cold pressing linkage forming:

[0019] After preheating, the slab is sent into a continuous hot press for rapid hot pressing and cold pressing to fix the slab into shape.

[0020] The press is divided into 2 to 5 sections in the length direction for hot pressing parameter control, with a hot pressing time of 3 to 15s / mm, a heating temperature of 160 to 230°C in the first 1 to 4 sections, and a plate surface pressure of 1.0 to 2.0MPa. The last section uses 20 to 60°C cooling heat transfer oil, and a plate surface pressure of 0.5 to 1.2MPa, which is constant until the plate is discharged.

[0021] As mentioned above, the wood raw material in step 1) is branches, small-diameter wood, wood processing residues or recycled wood materials. Before chipping, the raw material is sprayed with water or soaked to control its moisture content to 20-40%.

[0022] As described above, the chipping, flaking, and drying process described in step 1) is specifically as follows: after the wood raw material is chipped by a chipper, large wood chips with a length of 30 to 70 mm and small wood chips with a length of 10 to 30 mm are sorted during transportation by a conveyor. The large wood chips are then conveyed to a flaker for producing large, thin wood chips to be sliced into large flake wood chips, while the small wood chips are conveyed to a flaker for producing narrow, flat wood chips to be sliced into smaller wood chips. The large wood chips are then dried using a belt dryer to a moisture content of 3 to 6%, and the smaller wood chips are dried using a drum dryer to a moisture content of 3 to 6%.

[0023] In step 1), the wood shavings are further screened and selected using a wood shavings screening machine with four layers of screens with different apertures. The aperture of the screens is adjusted according to the size of the wood shavings to select three categories: thin large wood shavings, medium wood shavings, and fine wood shavings. Among them, the first layer of screens is (6-8)×(30-50) mm, the second layer of screens is (4-6)×(4-6) mm, the third layer of screens is (1-2)×(1-2) mm, and the fourth layer of screens is (0.3-0.5)×(0.3-0.5) mm. Excessively large wood shavings are crushed again and used, and excessively fine wood shavings are sent to the fuel area.

[0024] As described above, the sheet-like thermoplastic plastic film in step 1) is a recycled thermoplastic plastic film of low-density polyethylene, polypropylene or polyvinyl chloride with a melting point below 200°C, with a thickness of 0.05 to 0.2 mm, and is processed into thin sheets with a width of less than or equal to 50×50 mm using a crushing device.

[0025] As described above, the hybrid foamable adhesive described in step 2) is composed of a main agent and an auxiliary agent, wherein the main agent is an isocyanate adhesive; the auxiliary agent is a mixture of a polyether polyol, a surfactant, and a delayed catalyst, wherein, by weight, the polyether polyol: surfactant: delayed catalyst = 100: (1-4): (0.05-2.0); before spraying, 10-40% by weight of the auxiliary agent is added to the main agent, mixed evenly, and then sprayed onto the surface of medium wood particles in a glue mixer; wherein the catalyst is a tin-based foaming catalyst or an acidic delayed foaming agent.

[0026] The above-mentioned mixed foamable adhesive, isocyanate adhesive, polyether polyol, surfactant and delayed catalyst are all formaldehyde-free reagents, which avoid the formaldehyde release problem of particleboard from the root.

[0027] As described above, the thermoplastic adhesive in step 2) is one or more of polyvinyl alcohol (PVA), polyvinyl acetate (PVAC), and ethylene-vinyl acetate copolymer (EVA) emulsion, and the amount of adhesive applied is 2-5% by weight of the wood chips; the thermoplastic adhesive and the isocyanate adhesive are sprayed onto the surface of the fine wood chips through different pipes.

[0028] As described above, in the five-layer structure of the slab described in step 3), the weight ratios of the raw materials of the five-layer symmetrical structure of the slab are: upper surface layer fine wood chips: upper surface layer thin flake large wood chips: core layer medium wood chips: lower surface layer thin flake large wood chips: lower surface layer fine wood chips = (1.5~2): (1~2): (2~5): (1~2): (1.5~2).

[0029] As described above, the method for preparing the gradient structure in step 3) is:

[0030] a) The method for preparing the gradient structure of fine wood shavings is as follows: the fine wood shavings on the upper and lower surfaces are laid using an air flow laying machine, and a unidirectional gradient structure with the finest surface layer and a coarser inner layer is formed by air separation;

[0031] b) The method for preparing the gradient structure of the medium-sized wood chips in the core layer is as follows: using a mechanical paving head with a central axis symmetry to perform paving, and scattering the wood chips symmetrically to form a symmetrical gradient structure with coarse outer surface and fine inner surface;

[0032] c) The large thin shavings of the upper and lower surface reinforcement layers are paved with mechanical paving heads. The paving heads have a three-level structure from top to bottom. The upper level consists of 2 to 5 groups of discharge rollers, the middle level consists of 6 to 10 groups of diamond roller paving heads, and the lower level consists of 4 to 6 groups of squirrel cage paving heads rotating in the same direction. The large shavings are formed into a non-directional unidirectional gradient structure with a fine outer layer and a coarse inner layer by the paving heads.

[0033] As mentioned above, in step 4), the slab is preheated by steam spraying or microwave heating, and the core layer temperature reaches 35-60°C after preheating.

[0034] The characteristics of the present invention are:

[0035] 1) The reinforced formaldehyde-free particleboard prepared by the preparation method of the present invention uses large thin wood particles as reinforcement and is placed on the subsurface of the slab to form a five-layer symmetrical structure. Five special paving equipment are used to achieve non-directional and uniform gradient paving of each layer. The physical and mechanical properties of the particleboard are greatly improved through structural design.

[0036] 2) The enhanced formaldehyde-free particleboard prepared by the preparation method of the present invention is applied with different isocyanate-based adhesives according to the particle shape and position, so as to achieve the purpose of improving the continuity of the particle interaction structure in the core layer, improving the waterproofness and bonding strength of the sub-surface layer, and improving the initial adhesion of the surface layer to prevent loosening.

[0037] 3) The enhanced formaldehyde-free particleboard produced by the preparation method of the present invention is formed using a hot-press-cold-press linkage process. This allows for rapid molding of the slabs at high moisture contents without bubbling, ensuring the product moisture content is within an optimal range. Furthermore, the sizing is applied separately to the particles of varying morphology, minimizing the morphological differences. This avoids the problems of uneven sizing and high sizing requirements associated with the widely varying morphologies of the core particles of conventional particleboard. Consequently, the particleboard produced using the same raw materials and density exhibits superior physical and mechanical properties.

[0038] 4) The reagents used in the present invention are all formaldehyde-free reagents, which fundamentally avoids the problem of formaldehyde release from particleboard caused by external additions.

[0039] The beneficial effects of the present invention are:

[0040] The present invention provides a continuous preparation process for enhanced formaldehyde-free particleboard. The preparation process is achieved by finely separating raw materials and precisely controlling the structure. The process can adopt a higher slab moisture content for production. By precisely controlling the particle shape and paving, the hierarchical structure of the particleboard product is optimized, thereby achieving a significant improvement in the performance of ordinary particleboard. The process is highly practical and has good market competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A schematic diagram of the combined structure of the five-layer board blank of the enhanced formaldehyde-free particleboard provided by the present invention.

[0042] Figure 2 for Figure 1 A partial enlarged view of the five-layer board in the circle.

[0043] Figure 3A This is a cross-sectional view of the enhanced formaldehyde-free particleboard prepared in the laboratory using the preparation method of the enhanced formaldehyde-free particleboard provided by the present invention in Example 1.

[0044] Figure 3B This is a cross-sectional view of the three-layer particleboard used as a control in Example 1.

[0045] Figure 4A These are milled solid images at different depths in the thickness direction of the particleboard produced in a factory pilot test using the method for producing the enhanced formaldehyde-free particleboard provided by the present invention in Example 4.

[0046] Figure 4B These are the actual milling images of conventional three-layer particleboard at different depths in the thickness direction. DETAILED DESCRIPTION

[0047] The embodiments of the present invention will be described in detail and comprehensively below so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more precise definition of the protection scope of the present invention.

[0048] In the present invention, percentages (%) not explicitly stated are all percentages by weight.

[0049] The isocyanate adhesive used in the present invention is polymethylene polyphenyl isocyanate, with an isocyanate content of 30-32%, and is referred to as isocyanate glue.

[0050] The polyether polyol is a polyether polyol with high hydroxyl value and high functionality, and is one or more of polyether 310, polyether 8350, polyether MN500, and polyether R8243.

[0051] The surfactant is one or both of amino silicone oil OFX8803 and dimethyl silicone oil.

[0052] The tin catalyst is stannous octoate or dibutyltin dilaurate.

[0053] The acidic catalyst is oxalic acid, phosphoric acid or citric acid.

[0054] Example 1 (Laboratory)

[0055] 1) Purchase poplar wood chips with a length of 60mm;

[0056] 2) The wood chips are prepared into large thin flakes using a low-speed flaker in the factory and dried to a moisture content of 2-5%;

[0057] 3) The dried wood shavings are sieved with a laboratory screening machine. Four layers of screens are set from top to bottom, namely 6×40 mm, 4×4 mm, 1×1 mm and 0.3×0.3 mm. The wood shavings are sieved into three forms: 4-6 mm (sub-surface shavings), 1-4 mm (core shavings) and 0.3-1.0 mm (surface shavings). A glue mixer is used to spray water to adjust the moisture content of the surface shavings to 15%, the moisture content of the sub-surface shavings to 3%, and the moisture content of the core shavings to 12%, and the shavings are sealed and maintained.

[0058] 4) The five-layer structure had a ratio of 2:1.5:3:1.5:2. Under laboratory conditions, an isocyanate adhesive was applied to the surface layer of fine wood particles (3% isocyanate adhesive and 2% PVAC). The sub-surface layer of large wood particles was mixed with 10% polyethylene film and then coated with 3% adhesive. The core layer was coated with a mixed foamable adhesive (4%). The mixed foamable adhesive consisted of a 15% by weight mixture of isocyanate adhesive and polyether R8243, amino silicone oil OFX8803, stannous octoate, and citric acid. The weight ratios of polyether R8243, amino silicone oil OFX8803, stannous octoate, and citric acid were 100:3:0.3:0.5. The control three-layer particleboard consisted of a mixture of 1-7 mm wood particles from the inner three layers of the five layers, then coated with 4% isocyanate adhesive. The surface material morphology and adhesive application were consistent.

[0059] 5) Use a 40×45 cm mold frame to weigh the raw materials, then manually lay the raw materials layer by layer in the frame, compact them, and then send them into the laboratory press. The hot pressing temperature is 190°C, the hot pressing time is 10 s / mm, and then cool them with cold water for 30 s before taking them out.

[0060] 6) After the boards were cut and the surface was sanded, a reinforced five-layer particleboard was prepared. At the same time, a three-layer particleboard of the same density (with a core layer of large particles and a surface layer of fine particles) was prepared as a control group. After standing for one week in an indoor environment, the test was carried out in accordance with the standard GB / T 17657-2013 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels". The results are shown in Table 1.

[0061] Table 1 Performance comparison of enhanced five-layer particleboard and ordinary three-layer particleboard

[0062] performance Reinforced five-layer particle board Three-layer particle board <![CDATA[Density (g / cm 3 )]]> 0.58 0.58 Moisture content (%) 6.3 6.5 24h water absorption thickness expansion rate (%) 9.6 10.4 Static bending strength (MPa) 13.1 8.1 Elastic modulus (MPa) 2100 1450 Internal bonding strength (MPa) 0.50 0.41 Surface bonding strength (MPa) 1.2 0.9

[0063] like Figure 3A This is a cross-sectional view of the five-layer board of the prepared reinforced formaldehyde-free particleboard. Figure 3B The three-layer particle board is the control group.

[0064] From Table 1, Figure 3A and Figure 3B It can be seen that the prepared reinforced formaldehyde-free particleboard has a thickness of 18 mm and an average density of only 0.58 g / cm 3 The flexural elastic modulus is 40% higher than that of the control group poplar three-layer particleboard, and the static bending strength is more than 60% higher.

[0065] Example 2 (laboratory):

[0066] 1) Using eucalyptus branches and wood cores, make wood chips about 60mm in length and store them;

[0067] 2) feeding the wood chips into a low-speed flaker to prepare large thin flakes, and drying the mixed flakes to a moisture content of 2-5%;

[0068] 3) The dried wood shavings are screened in a screening machine to select three types of wood shavings: 4-6 mm (sub-surface shavings), 2-4 mm (core shavings), and 0.3-1.0 mm (surface shavings). A glue mixer is used to spray water to adjust the moisture content of the surface shavings to 15%, the moisture content of the sub-surface shavings to 3%, and the moisture content of the core shavings to 12%, and the shavings are sealed and maintained.

[0069] 4) The designed sheet thickness is 18mm, and the weight ratio of the five-layer structure is 1.8:1.2:4:1.2:1.8. The surface layer glue is 1% EVA and 4% isocyanate glue. The sub-surface layer is mixed with 10% polyethylene film scraps and glued at 3%. The core layer glue is 3%. The isocyanate glue is added with 20% by weight of a mixture of polyether MN500, dimethyl silicone oil, stannous octoate, and phosphoric acid. The weight ratio of polyether MN500, dimethyl silicone oil, stannous octoate, and phosphoric acid is 100:3:0.5:0.5.

[0070] 5) Use a 40×45cm mold frame to lay the slabs in layers. After completion, send them to the hot press. The hot pressing temperature is 200℃, the hot pressing time is 6s / mm, and the thickness of the board is controlled by a thickness gauge. After the hot pressing is completed, cool it with cold water for 30s and then release the pressure and take it out.

[0071] 6) Cool, cut and sand the board after it is produced, and test its performance after one week.

[0072] The eucalyptus particleboard prepared in this example has a thickness of 18 mm and an average density of 0.63 g / cm 3The reinforced board has a moisture content of 7.0%, a static bending strength of 16.8 MPa, an elastic modulus of 2700 MPa, an internal bond strength of 0.50 MPa, a 24-hour water absorption thickness expansion rate of 7.6%, and a surface bonding strength of 1.3 MPa, meeting the requirements for load-bearing particleboard (Type P3, GB / T4897-2015 "Particleboard") in dry environments. Conventional particleboard at this density only meets the requirements for furniture-grade particleboard (Type P2). The reinforced board exhibits over 40% higher bending resistance, over 20% higher dimensional stability, and over 30% higher surface bonding strength than conventional particleboard.

[0073] Example 3 (factory pilot):

[0074] 1) The poplar branches are made into wood chips and then sent to the silo;

[0075] 2) The wood chips are screened by a disc and separated into two types: chips less than 30 mm in length and chips greater than 30 to 60 mm in length. The chips are fed into a high-speed flaker for producing narrow flat chips and a low-speed flaker for producing large thin chips. The small chips are fed into a drum dryer and dried to a moisture content of 3 to 5%, while the large chips are dried in a belt dryer to a moisture content of 3 to 5%.

[0076] 3) The dried wood chips are screened. Large flakes are 30-50 mm long, 10-40 mm wide, and 0.4-1.0 mm thick. Medium flakes pass through a 2-4 mm sieve. Fine flakes pass through a 0.3-1.0 mm sieve. The large flakes are mixed with 10% recycled polyethylene film shreds before entering the silo.

[0077] Recycled polyethylene greenhouse film is waste. The crushed material after using the recycled polyethylene greenhouse film can be used as sheet-like thermoplastic plastic film and added to thin large wood chips. It can not only supplement the bonding and improve the waterproofness, but also realize the recycling of recycled materials.

[0078] 4) The board thickness is 15 mm, and the weight ratio of the five-layer structure is 1.6:1.4:4:1.4:1.6. The medium wood chips in the core layer are first watered to a moisture content of 11%, and then a mixed foamable adhesive is sprayed in at a total amount of 4%. The mixed foamable adhesive is composed of isocyanate glue and 10% by weight of polyether 310, dimethyl silicone oil, dibutyltin dilaurate, and phosphoric acid. The weight ratio of polyether 310, dimethyl silicone oil, dibutyltin dilaurate, and phosphoric acid is 100:3:0.5:0.6; 3% isocyanate glue and 2% EVA emulsion are added to the upper and lower surface fine chips, and the moisture content of the fine chips is adjusted to 15%; the glue amount of the upper and lower surface thin large chips is 2.5%.

[0079] 5) Precise assembly of slabs:

[0080] The slab is assembled in a symmetrical manner, with the upper and lower surface layers being made of fine wood shavings and the core layer being made of medium wood shavings. Thin sheets of large wood shavings are laid on the upper and lower surface layers between the upper and lower surface layers of fine wood shavings and the core layer of medium wood shavings as reinforcement layers. Each single layer has a gradient structure.

[0081] The preparation method of the gradient structure is as follows:

[0082] a) The method for preparing the gradient structure of fine wood shavings is as follows: the fine wood shavings on the upper and lower surfaces are laid using an air flow laying machine, and a unidirectional gradient structure with the finest surface layer and a coarser inner layer is formed by air separation;

[0083] b) The method for preparing the gradient structure of the medium-sized wood chips in the core layer is as follows: using a mechanical paving head with a central axis symmetry to perform paving, and scattering the wood chips symmetrically to form a symmetrical gradient structure with coarse outer surface and fine inner surface;

[0084] c) The large thin shavings of the upper and lower surface reinforcement layers are paved with mechanical paving heads. The paving heads have a three-level structure from top to bottom. The upper level is 4 groups of discharge rollers, the middle level is 7 groups of diamond roller paving heads, and the lower level is 5 groups of squirrel cage paving heads rotating in the same direction. The large shavings are formed into a non-directional unidirectional gradient structure with a fine outer layer and a coarse inner layer by the paving heads.

[0085] 6) After the slab is laid, it is preheated to 50℃ in the core layer and then sent to the continuous hot press. The hot pressing temperature is 200℃, the hot pressing time is 3s / mm, and the temperature of the cold pressing section pipe is controlled at 40℃.

[0086] 7) After the board is produced, it is subjected to post-processing such as cutting, cooling and sanding to prepare the enhanced formaldehyde-free particleboard.

[0087] The particleboard prepared in this example has a thickness of 15 mm and a density of 0.62 g / cm 3 , with a moisture content of 6%. It boasts a static flexural strength of 16.2 MPa, an elastic modulus of 2550 MPa, an internal bond strength of 0.56 MPa, a 24-hour water absorption thickness expansion of 7.5%, and a surface bonding strength of 1.2 MPa, meeting the requirements for load-bearing particleboard (Type P3, GB / T4897-2015 "Particleboard") in dry environments. Its bending resistance is over 40% higher than that of conventional particleboard of the same density, its dimensional stability is over 20% higher, and its surface bonding strength is over 20% higher.

[0088] Example 4 (factory trial):

[0089] 1) The poplar branches are made into wood chips and then sent to the silo;

[0090] 2) The wood chips are screened by a disc and separated into two types: chips less than 30 mm in length and chips between 30 and 50 mm in length. The chips are then fed into a high-speed flaker for producing smaller chips and a low-speed flaker for producing larger, thinner chips. The smaller chips are then fed into a drum dryer and dried to a moisture content of 3-5%. The larger chips are then dried using a belt dryer to a moisture content of 3-5%.

[0091] 3) The dried wood chips are screened. Large flakes are 30-40 mm in length, 10-30 mm in width, and 0.4-0.8 mm in thickness. Medium flakes pass through a 2-4 mm sieve, and fine flakes pass through a 0.3-1.0 mm sieve. The large flakes are mixed with 5% recycled polyethylene film shreds before entering the silo.

[0092] 4) The board is 18mm thick, with a five-layer structure in a weight ratio of 2:1:4:1:2. The medium-sized wood chips in the core layer are first watered to an 11% moisture content, then sprayed with a mixed foamable adhesive at a total application rate of 5%. The mixed foamable adhesive consists of a cyanate ester adhesive mixed with a 10% by weight mixture of polyether 8350, polyether 310, dimethyl silicone oil, dibutyltin dilaurate, and phosphoric acid. The weight ratios of polyether 8350, polyether 310, dimethyl silicone oil, dibutyltin dilaurate, and phosphoric acid are 80:20:3:0.6:0.9. A 3% EVA-diluted water mixture and 3% isocyanate are added to the fine wood chips in the upper and lower surface layers to increase their moisture content to 15%. The large wood chips in the upper and lower surface layers are glued at a 3% rate.

[0093] 5) Precise assembly of slabs:

[0094] The slab is assembled in a symmetrical manner, with the upper and lower surface layers being made of fine wood shavings and the core layer being made of medium wood shavings. Thin sheets of large wood shavings are laid on the upper and lower surface layers between the upper and lower surface layers of fine wood shavings and the core layer of medium wood shavings as reinforcement layers. Each single layer has a gradient structure.

[0095] The preparation method of the gradient structure is as follows:

[0096] a) The method for preparing the gradient structure of fine wood shavings is as follows: the fine wood shavings on the upper and lower surfaces are laid using an air flow laying machine, and a unidirectional gradient structure with the finest surface layer and a coarser inner layer is formed by air separation;

[0097] b) The method for preparing the gradient structure of the medium-sized wood chips in the core layer is as follows: using a mechanical paving head with a central axis symmetry to perform paving, and scattering the wood chips symmetrically to form a symmetrical gradient structure with coarse outer surface and fine inner surface;

[0098] c) The large thin shavings of the upper and lower surface reinforcement layers are paved with mechanical paving heads. The paving heads have a three-level structure from top to bottom. The upper level is 4 groups of discharge rollers, the middle level is 7 groups of diamond roller paving heads, and the lower level is 5 groups of squirrel cage paving heads rotating in the same direction. The large shavings are formed into a non-directional unidirectional gradient structure with a fine outer layer and a coarse inner layer by the paving heads.

[0099] 6) After the slab is laid, the surface is preheated by microwave, and the core layer temperature is raised to 50℃ before being sent to the continuous hot press. The hot pressing temperature is 190℃, the hot pressing time is 4s / mm, and the temperature of the cold pressing section pipe is controlled at 40℃.

[0100] 7) After the board is produced, it is subjected to post-processing such as cutting, cooling and sanding to prepare the enhanced formaldehyde-free particleboard.

[0101] The prepared reinforced formaldehyde-free particleboard is milled in the thickness direction as follows Figure 4A As shown in the figure, we can see the difference in the morphology of the particles at different layers. The fine particles at the lower surface are too thin and fragile, so they are not reflected. At the same time, the conventional three-layer particle board is milled in the thickness direction, as shown in the figure. Figure 4B As shown, it can be seen that the wood chips in the inner core layer have no hierarchical differences.

[0102] The particleboard prepared in this example has a thickness of 18 mm and a density of 0.63 g / cm 3 , with a moisture content of 7%. It boasts a static flexural strength of 14.5 MPa, an elastic modulus of 2400 MPa, an internal bond strength of 0.52 MPa, a 24-hour water absorption thickness expansion of 8.5%, and a surface bonding strength of 1.3 MPa, meeting the requirements for load-bearing particleboard (P3 type, GB / T4897-2015 "Particleboard") in dry environments. Its bending resistance is over 30% higher than that of conventional three-layer particleboard of the same density, its dimensional stability is over 20% higher, and its surface bonding strength is over 30% higher.

[0103] It can be seen from the above embodiments that the present invention provides a continuous preparation process for enhanced formaldehyde-free particleboard. The process uses ordinary particleboard to produce the same low-grade wood raw materials, avoiding the problems of uneven gluing and large gluing amount caused by excessive differences in the morphology of the core layer particles of ordinary particleboard. The particleboard prepared under the same raw materials and the same density conditions has higher physical and mechanical properties, achieving the purpose of improving the comprehensive performance of ordinary particleboard, and has important social value and economic benefits.

[0104] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A continuous preparation process for enhanced formaldehyde-free particleboard, characterized in that: The process includes the following steps: 1) Preparation of standard wood shavings: The wood raw materials are chipped, flaked, dried and screened to obtain three types of shavings: thin large shavings, medium shavings and fine shavings; Among them, the length of thin large shavings is 20-50mm, the width is 5-40mm, and the thickness is 0.4-1.0mm; the length of medium shavings is 10-30mm, the width is 1-5mm, and the thickness is less than 1.2mm, accounting for more than 85%; the length of fine shavings is 1-10mm, and the thickness and width are both less than 0.8mm, accounting for more than 80%; the weight of thin large shavings mixed accounts for 1-20% of the sheet-shaped thermoplastic plastic film of similar format; 2) Gluing wood shavings separately: Mix glue separately for thin large wood shavings, medium wood shavings and fine wood shavings; The thin large wood chips are first adjusted to have a moisture content of 3-6%, and then 1-4% by weight of an isocyanate adhesive is applied; the medium wood chips are first adjusted to have a moisture content of 6-12%, and then 2-10% by weight of a mixed foamable adhesive is applied; the fine wood chips are applied with 2-5% by weight of a thermoplastic adhesive and 2-5% by weight of an isocyanate adhesive, and the moisture content of the fine wood chips is adjusted to 12-20%. 3) Precise assembly of slabs: The slab is assembled in a symmetrical manner, with the upper and lower surface layers being made of fine wood shavings and the core layer being made of medium wood shavings. Thin sheets of large wood shavings are laid on the upper and lower surface layers between the upper and lower surface layers of fine wood shavings and the core layer of medium wood shavings as reinforcement layers. Each single layer has a gradient structure. 4) Slab hot pressing and cold pressing linkage forming: After preheating, the slab is sent into a continuous hot press for rapid hot pressing and cold pressing to fix the slab into shape. The press is divided into 2 to 5 sections in the longitudinal direction for hot pressing parameter control, with a hot pressing time of 3 to 15s / mm. The heating temperature of the first 1 to 4 sections is 160 to 230°C, and the plate surface pressure is 1.0 to 2.0 MPa. The last section uses 20 to 60°C cooling heat transfer oil, and the plate surface pressure is 0.5 to 1.2 MPa, which is constant until the plate is discharged. 5) cutting, cooling and sanding the board after it is produced to prepare a reinforced formaldehyde-free particleboard; Wherein, the preparation method of the gradient structure described in step 3) is: a) The method for preparing the gradient structure of fine wood shavings is as follows: the fine wood shavings on the upper and lower surfaces are laid using an air flow laying machine, and a unidirectional gradient structure with the finest surface layer and a coarser inner layer is formed by air separation; b) The method for preparing the gradient structure of the medium-sized wood chips in the core layer is as follows: using a mechanical paving head with a central axis symmetry to perform paving, and scattering the wood chips symmetrically to form a symmetrical gradient structure with coarse outer surface and fine inner surface; c) The large thin shavings of the upper and lower surface reinforcement layers are paved with mechanical paving heads. The paving heads have a three-level structure from top to bottom. The upper level consists of 2 to 5 groups of discharge rollers, the middle level consists of 6 to 10 groups of diamond roller paving heads, and the lower level consists of 4 to 6 groups of squirrel cage paving heads rotating in the same direction. The large shavings are formed into a non-directional unidirectional gradient structure with a fine outer layer and a coarse inner layer by the paving heads.

2. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: The wood raw material in step 1) is branch wood, small-diameter wood, wood processing residue or recycled wood material. Before chipping, the raw material is sprayed with water or soaked to control its moisture content to 20-40%.

3. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: The chipping, flaking and drying process described in step 1) is specifically as follows: after the wood raw material is chipped by a chipper, large wood chips with a length of 30 to 70 mm and small wood chips with a length of 10 to 30 mm are sorted during transportation by a conveyor device; the large wood chips are then conveyed to a flaker for producing large thin wood chips to be sliced into large flake wood chips; the small wood chips are conveyed to a flaker for producing narrow flat wood chips to be sliced into smaller wood chips; the large flake wood chips are then dried using a belt dryer to a moisture content of 3 to 6%, and the smaller wood chips are dried using a drum dryer to a moisture content of 3 to 6%.

4. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: In step 1), the wood shavings are further screened and selected using a wood shavings screening machine with four layers of screens with different apertures. The aperture of the screens is adjusted according to the size of the wood shavings to select three categories: thin large wood shavings, medium wood shavings, and fine wood shavings. Among them, the first layer of screens is (6-8)×(30-50) mm, the second layer of screens is (4-6)×(4-6) mm, the third layer of screens is (1-2)×(1-2) mm, and the fourth layer of screens is (0.3-0.5)×(0.3-0.5) mm. Excessively large wood shavings are crushed again and used, and excessively fine wood shavings are sent to the fuel area.

5. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: The sheet-like thermoplastic plastic film described in step 1) is a recycled thermoplastic plastic film of low-density polyethylene, polypropylene or polyvinyl chloride with a melting point below 200°C, with a film thickness of 0.05 to 0.2 mm, and is processed into a sheet with a width of less than or equal to 50×50 mm using a crushing device.

6. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: The hybrid foamable adhesive described in step 2) is composed of a main agent and an auxiliary agent, wherein the main agent is an isocyanate adhesive; the auxiliary agent is a mixture of a polyether polyol, a surfactant, and a catalyst, wherein, by weight, the polyether polyol: surfactant: catalyst = 100: (1-4): (0.05-2.0); before spraying, 10-40% by weight of the auxiliary agent is added to the main agent and mixed evenly, and then sprayed onto the surface of the medium wood particles in a glue mixer; wherein the catalyst is a tin foaming catalyst and an acidic delayed foaming agent.

7. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: Step 2) The thermoplastic adhesive is polyvinyl alcohol, polyvinyl acetate or polyethylene-vinyl acetate emulsion, and the amount of adhesive applied is 2-5% of the weight of the wood chips; the thermoplastic adhesive and the isocyanate adhesive are sprayed onto the surface of the fine wood chips through different pipes.

8. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: The five-layer structure of the slab described in step 3) has a weight ratio of raw materials of the five-layer symmetrical structure of the slab: upper surface fine wood chips: upper surface thin flake large wood chips: core layer medium wood chips: lower surface thin flake large wood chips: lower surface fine wood chips = (1.5-2): (1-2): (2-5): (1-2): (1.5-2).

9. The continuous preparation process of the reinforced formaldehyde-free particleboard according to claim 1, characterized in that: Step 4) The slab is preheated by spray steam or microwave equipment, and the core layer temperature reaches 35-60°C after preheating.

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