Wood Flour Research Articles

Biocomposites Based on Polyethylene/Ethylene–Vinyl Acetate Copolymer/Cellulosic Fillers

This work studied biocomposites based on a blend of low-density polyethylene (LDPE) and the ethylene–vinyl acetate copolymer (EVA), filled with 30 wt.% of cellulosic components (microcrystalline cellulose or wood flour). The LDPE/EVA ratio varied from 0 to 100%. It was shown that the addition of EVA to LDPE increased the elasticity of biocomposites. The elongation at break for filled biocomposites increased from 9% to 317% for microcrystalline cellulose and from 9% to 120% for wood flour (with an increase in the EVA content in the matrix from 0 to 50%). The biodegradability of biocomposites was assessed both in laboratory conditions and in open landfill conditions. The EVA content in the matrix also affects the rate of the biodegradation of biocomposites, with an increase in the proportion of the copolymer in the polymer matrix corresponding to increased rates of biodegradation. Biodegradation was confirmed gravimetrically by weight loss, an X-ray diffraction analysis, and the change in color of the samples after exposition in soil media. The prepared biocomposites have a high potential for implementation due to the optimal combination of consumer properties.

Enhanced Production of Furfural via Methanolysis of Wood Biomass with HCl Gas.

This study explores the production of furfural, xylose and methylxylosides through the methanolysis of wood flour using anhydrous HCl gas. The process involves methanolysis of wood flour with HCl gas under pressure to generate methylxylosides, which are subsequently converted to xylose and furfural via autohydrolysis in a Parr batch reactor system. The methanolysis was conducted in temperature-controlled HCl gas reactor employing 24 h reaction time and 50 % methanol content in wood flour. During the methanolysis step with HCl gas, 65 % of the available xylan in wood flour was converted to water-soluble methylxylosides, xylose, xylooligosaccharides (XO) and water-soluble methyl xylooligosaccharides (MXO). Methanolysis filtrates were then autohydrolyzed with Parr 50 mL batch reactor system to xylose and furfural in two different pH values at 180 °C. The highest furfural yield of 91 % from methanolysis filtrate was achieved with pH 1.2 and 25 min reaction time.

Influence of halloysite nanotubes on processing, structural and thermal properties of poly(vinyl chloride)/high-density polyethylene composites with wood flour

ABSTRACT The aim of this work was to investigate the effect of halloysite nanotubes (HNT) on the processing, structural and thermal properties of poly(vinyl chloride) (PVC), high-density polyethylene (HDPE), and PVC–HDPE composites with various proportions of both polymer components containing 30 wt% wood flour (WF) and 5 wt% halloysite processed in the Brabender measuring mixer. The HNT content in the composites leads to a slight increase in the maximum torque and a reduction in the gelation time for PVC and PVC–HDPE matrix composites with the 90:10 component ratio. In addition, microscopic images of these materials showed a homogeneous distribution of HNT in the polymer matrix. The addition of mineral filler to the matrix, regardless of the polymer type, resulted in a slight increase in the torque values at the kneading endpoint. The PVC–HDPE–wood flour composites are characterized by higher thermal stability at processing temperature when HNT is loaded. The introduction of 5 wt% HNT into the PVC–HDPE 90:10 30 wt% WF blend resulted in an increase of the 2% weight loss temperature by 16°C. The simultaneous use of wood flour and halloysite as fillers for PVC–HDPE blends could be an eco-efficient solution for the new composite material sector.

FDM 3D Printing and Properties of WF/PBAT/PLA Composites

Fused deposition molding (FDM) is a commonly used 3D printing method, and polylactic acid (PLA) has become one of the most important raw materials for this technology due to its excellent warping resistance. However, its mechanical properties are insufficient. Polybutylene adipate terephthalate (PBAT) is characterized by high toughness and low rigidity, which can complement the performance of PLA. The biodegradable polymers produced by blending the two have thus been used to replace petroleum-based plastics in recent years, but the high cost of the blends has limited their wide applications. Introducing plant fibers into the blends can not only maintain biodegradability and improve the overall performance of the plastics but also reduce their costs greatly. In this study, the PBAT/PLA blends with a mass ratio of 70/30 were selected and mixed with wood flour (WF) to prepare ternary composites using a FDM 3D printing technique. The effects of WF dosage on the mechanical properties, thermal properties, surface wettability, and melt flowability of the composites were investigated. The results showed that the proper amount of WF could improve the tensile and flexural moduli of the composites, as well as the crystallinity and hydrophobicity of the printed specimens increased with the content of WF, while the melt flow rate decreased gradually. Compared to PBAT/PLA blends, WF/PBAT/PLA composites are less costly, and the composite containing 20 wt.% WF has the best comprehensive performance, showing great potential as raw material for FDM 3D printing.

The preparation and construction of biomimetic mineralization compatible interface of wood fiber/foamed magnesium oxychloride lightweight composites

The purpose of this study is to improve the application performance of foamed magnesium oxychloride cement (FMOC) and solve the problem of poor interfacial compatibility between the organic-inorganic interface of traditional inorganic composite materials. Inspired by the adhesion mechanism of barnacles to inorganic substrates through amyloid nanofibers and phosphoprotein, a simple green method was proposed to improve the strength and water resistance of the system. The FMOC composites (30×30×30 mm) were prepared by adding wood flour (WF) and phytic acid (PA) to the mixture of magnesium oxide and magnesium chloride. The influences of WF and PA on the compressive strength, composition, microstructure, pore structure, water resistance and flame retardant properties of FMOC were investigated. WF was used as a skeleton structure to induce cement particle aggregation, the PA molecule was used as a connecting bridge to activate the fiber skeleton to increase the active sites, and promote crystal nucleation and aggregation through hydrogen and ionic bond interactions to construct the biomimetic mineralized structure. The results show that the introduction of PA improves the compressive strength and water resistance of the FMOC/WF/PA composite system. When the PA addition amount is 0.8 %, the compressive strength of FMOC/WF/PA-0.8 % composites reached 6.64 MPa, which was 3 times compared to the FMOC composites. The softening coefficient and water absorption of the obtained FMOC/WF/PA-0.8 % composites were 0.81 and 13.84 %, respectively, which were superior to FMOC composites and presented better behavior in water resistance. Meanwhile, the modified composites have more stable pore structure and flame retardant properties, thus these green FMOC/WF/PA composites displayed potential application value in building energy conservation, aerospace and other fields.

Effect of the filler content on some physical and mechanical properties of virgin- and recycled thermoplastic polyurethane composites

Effects of thermoplastic polyurethane (TPU) types (Recycled (R) and Virgin (V)) composites with 15 wt% and 30 wt% oakwood flour addition were studied. Selected physical, mechanical, morphological, and thermal properties of resulting polymer composites were analyzed. Test samples were manufactured using injection molding, except that abrasion resistance specimens were manufactured using a compression molding process. The findings indicated that the types of TPU and filler contents played a significant role in the density and mechanical properties of the TPU test samples. The increased oak wood flour contents in both TPU types showed improvement in density, tensile modulus, hardness, flexural strength flexural modulus, dynamic impact strength, and yield strength of the composite while decreasing the elongation at break values. In addition, both TPU types and filler contents significantly affected the densities of V-TPU and R-TPU. The TPUs type, filler content, and cycle-rpm affected Taber’s abrasion resistance values. Weight loss, which increased with the number of cycles for the control samples, decreased with increasing wood flour content. This study aimed to provide an overview of the effect of the wood flour content in the manufacturing of thermoplastic-reinforced composites and to provide a basis for further research and development efforts.

Physical, Mechanical, and Flammability Properties of Wood-Plastic Composites (WPC) Containing Beech-Wood Flour and Flame-Retardant Additives.

This study aims to develop a recyclable, economical, and flame-retardant composite material using polypropylene, beech flour, tetrabromobisphenol A bis (TBBPA), and antimony trioxide (ATO). Flame-retardant additives (TBBPA and ATO) were initially added into polypropylene at different rates, and masterbatch (MB) samples were produced by the extrusion method. Subsequently, different percentages of wood flour (10%, 15%, 20%, 25%, and 30%) along with 60% MB were added to the polypropylene to create wood-polymer composites (WPC) using the injection method. The TBBPA, ATO, and wood flour were introduced through side-feeding hoppers during injection to ensure a homogeneous distribution within the WPC. Physical, thermal, and mechanical tests were conducted on the WPC samples. Additionally, TGA, FTIR, and SEM analyses were performed. The results indicated that the optimal ratios for TBBPA and ATO additives were 20% and 10%, respectively. It was observed that increasing the wood flour content in the WPC samples led to enhanced density, water absorption, hardness, impact, and abrasion resistance. Conversely, MFI, bending strength, and tensile strength decreased with higher wood flour content. It was observed that WPC samples exhibited flame resistance up to 725 °C. The produced WPC materials can be used in flooring applications, interior furniture, decorative wall panels, and aesthetic structural elements due to their fire behavior, good mechanical properties, low water-absorption rates, and aesthetic appearance.

Enhancement and Compatibilization of Waste-Sourced Biocomposites Through Elastomer Blending and Matrix Grafting Modification

A novel elastomer-modified multicomponent, multiphase waste-sourced biocomposites, was prepared for converting waste biomass and plastic into value-added products. The effects of blending elastomer–olefin block copolymer (OBC) and maleic anhydride (MAH), and divinylbenzene (DVB) co-grafting of recycled polypropylene (rPP) matrix on the adhesion interface, structure, and properties of high wood flour-filled (60 wt.%) composites were thoroughly investigated. The results indicated that DVB introduced branched structures into the polymer matrix molecular chain and increased the MAH grafting rate. Co-grafting rPP/OBC blends enhanced the interfacial adhesion among rPP, OBC, and wood flour. Additionally, MAH-grafted OBC was prone to encapsulating rigid wood flour, thereby forming an embedded structure. Notably, the tensile modulus and impact strength of the final three-component composites increased by 60% and 125%, respectively, compared with the unmodified composites. Additionally, dynamic mechanical analysis revealed that DVB-induced branching promoted the formation of microvoids in the OBC shell layer surrounding the wood, which in turn induced significant plastic deformation in the polymer matrix. This work offers a facile and efficient method for preparing high-toughness, high-stiffness, and low-cost waste PP-based composites for automotive interiors, and indoor and outdoor decoration.

Physicochemical, mechanical properties, and biodegradation studies of poly(3-hydroxybutyrate) composites reinforced with bacterial nanocellulose or wood flour

Physicochemical, mechanical properties, and biodegradation studies of poly(3-hydroxybutyrate) composites reinforced with bacterial nanocellulose or wood flour

Multiple Recycling of Wood–Plastic Recycled Composite (WPRC): Developing a Method to Evaluate the Degree of Degradation of Used WPRC

Wood–plastic recycled composite (WPRC) are composites obtained by heating and mixing the main raw material, wood flour, with thermoplastic resin, containing at least 40% by mass of recycled material in the raw material. In order to promote the multiple-recycling of WPRC to reduce greenhouse gas emissions and ensure the sustainability of resources, three types of WPRC decking materials with different exposure conditions (outdoor-exposed product, unexposed product and product stored in the factory for a long time) and samples after accelerated weathering tests (WPRC and recycled plastics from raw materials) were evaluated and compared by a TG-DTA in order to develop a method for evaluating the degree of degradation of used WPRC. Exothermic behavior with weight loss was observed in the temperature range of 30–500 °C for the WPRC product in two temperature ranges. In order to focus on the change in the first exotherm by oxidative degradation, where the rapid weight loss begins, this paper will focus on the exothermic behavior that develops in the temperature range of 150–300 °C on the lower temperature side. The results obtained are as follows. (1) Initial oxidation temperature (IOT) measurement from DTA behavior suggested that it is possible to evaluate the degree of degradation of WPRC. (2) On the exposed surface of WPRC exposed outdoors for more than 9 years and 8 months, significant decreases in the IOT were observed up to 1 mm from the surface, and a slight decrease in the IOT was observed between 1 and 2 mm from the surface. On the other hand, for the indoor long-term storage of 11 years and 6 months, there were almost no changes in the IOT with respect to the depth from the surface. Regarding the outdoor long-term-exposed WPRC, significant decreases in the IOT were observed not only on the exposed surface but also on the hollow and ribbed surfaces up to a depth of 1 mm from the surfaces. (3) A similar decrease in the IOT with increasing accelerated degradation time was observed for the WPRC and raw recycled plastic samples after accelerated weathering tests as for outdoor exposure. Furthermore, FTIR-ATR spectra also revealed that accelerated degradation caused oxidative degradation of the plastic. Therefore, it is thought that the decrease in the IOT can be used as an indicator to evaluate the degree of degradation of the plastic raw material in WPRC.

Modified lignin by deep eutectic solvent as a novel coupling agent for wood-plastic composites

In this work, the influence of modified lignin by Deep Eutectic Solvent (DES) as a novel bio coupling agent on the various properties of wood flour- polypropylene composites (WPCs) was investigated. For this purpose, extracted lignin from Bagasse Soda black liquor was modified with Choline chloride–ZnCl2 deep-eutectic solvent (ChCl–ZnCl2 DES) and then different ratios of unmodified and modified lignin (0, 1, 3 and 5 wt%) were added to the wood flour/polypropylene mixture. Different properties of lignin after modification by DES along with physical and mechanical properties of the manufactured composites were measured according to the relevant standard methods. The FTIR analysis indicated that DES- modified lignin has a higher proportion of phenolic hydroxyl groups (1213 cm−1) and a lower one of methoxy groups (2854 cm−1) when compared to virgin lignin. DSC analysis also showed that the glass transition temperature (Tg) of lignin decreased from 90 °C to 80 °C by DES treatment. Based on the results obtained, the physical and mechanical properties of WPCs were continuously improved by increasing the modified lignin content from 1 to 5 wt%. The composites with DES-treated lignin presented lower water absorption and thickness swelling as well as higher flexural, tensile and impact properties compared to those prepared with unmodified lignin at the same fixed level of lignin. Moreover, based on the finding of this research, the composites with 5 wt% DES-treated lignin have relatively better physical and mechanical properties than those made from 2 wt% maleic anhydride polypropylene (MAPP) as a control sample. Modification of lignin with DES improves its uniform dispersion in the polymer matrix according to Scanning Electron Microscopy (SEM) images.

COMPARATIVE EFFECT OF WOODFLOUR AND GRANULATED RICE HUSK ON THE THERMAL INSULATION OF TERMITE HILL CLAY

This research work covered the comparative effect of wood flour (saw dust) and granulated rice husk on the thermal insulation of termite hill clay. The raw clay samples were mined from the Federal University of Technology Akure Campus and processed. Likewise, the saw dust and rice husk were also processed and grinded. The clay samples and the additives were sieved to obtain 300 microns and were mixed in predetermined proportions of 0, 2, 4, 6, 8 and 10 wt. %. Cylindrical samples were developed from the mixtures which were compacted and then air dried before oven dried and finally fired. The fired samples were subjected to selected tests to determine their insulation and refractory properties. These tests include porosity, linear shrinkage, volumetric shrinkage, bulk density, thermalconductivity, thermal shock resistance and cold crushing tests. It was discovered that the saw dust samples meet the ASTM standard for insulating bricks while the rice husk samples did not. Saw dust samples with 2 and 8 wt. % were ones with optimum properties.

Structure–property correlation assessment of LLDPE‐based biocomposites with Azadirachta Indica wood flour

AbstractThis research explores the development of new composite material by integrating Azadirachta Indica (AI) with LLDPE to create wood‐plastic composites using the rotational molding process. By examining various proportions of AI wood flour blended with LLDPE, we investigated their impact on mechanical and physical properties. Our tests elucidate a clear correlation between mechanical properties and composite morphologies. Despite identical molding conditions, higher wood particle concentrations reduced mechanical properties compared to lower concentrations. Remarkably, a 12% wood content emerges as optimal, yielding a tensile modulus of 3.69 MPa and a flexural modulus of 468.5 MPa, with an acceptable reduction of 11% density and 13% porosity versus pure LLDPE. Additionally, we observed declines in tensile strength, impact strength, and hardness by up to 23%, 62%, and 11%, respectively, compared to neat LLDPE. Natural fillers enhance aesthetics, making these materials ideal for consumer products like garden equipment and furniture accessories.

Accelerating development in wood‐plastic composites: Presenting key material properties and demonstrating statistical methods

AbstractIn a time when global initiatives to lower CO2 emissions are accelerating the shift towards bio‐based products, various efforts are being made to develop wood plastic composites (WPCs) and similar bio‐composites using a variety of raw materials. This paper first showcases benchmark material properties serving as standards and models the composition, for polypropylene‐based WPCs integrating wood flour and maleic anhydride‐modified polypropylene. Subsequently, by applying statistical modeling techniques, we demonstrate an adaptive experimental design that efficiently and rapidly adjusts formulations. Beginning with an empirical dataset (24 samples), we established high‐performing models that articulate the individual contributions of the constituents on ten vital properties of WPCs. Our adaptive experimental design, leveraging nonlinear partial least squares regression and the Bayesian optimization, successfully refined formulations to enhance bending and impact strengths. Notably, we proposed optimal conditions starting from just eight samples with minimal iterations. This study shows that statistical techniques can quickly optimize WPC formulations, making the overall development process faster. By addressing the multiple conflicting properties, these techniques can greatly reduce the time and effort needed for development.Highlights Applied several regression methods to analyze ten properties of PP‐based WPCs. Showcased distinct impacts of the formulations of WF and MAPP on WPCs. Implemented the adaptive experimental design for formulation optimization. Enhanced bending and impact strengths in the formulations of WPCs. Advanced sustainable material development with data‐driven approach.

Biocomposites reinforced with wood flour and eggshell powder: production and properties

Abstract Wood plastic composite (WPC) is one example of a biocomposite that has been applied commercially as structural materials either in the building industry and in the automobile interior parts. In this work, WPC was developed based on polypropylene (PP) from used medical mask, polyethylene (PE) from plastic pocket reinforced with wood flour mixed with eggshell powder (ESP). The study aims to target similar properties as commercial WPC with its tensile strength of 14.27 MPa, flexural strength of 37.57 MPa, water absorption 2.61 %, and flammability (burning rate) < 100 mm/min. The composition of the composites investigated was PP (45 wt. %), PE (5 wt. %), wood flour (30 wt. %), ESP (20 wt. %). The effect of calcined and uncalcined ESP on the properties was first studied and the results show no significant differences in their properties. That finding leads to prepare biocomposite samples using hot press with various applied pressures, i.e. 0.55; 0.65; and 0.75 MPa. The tests show that the samples prepared with hot press pressure of 0.55 MPa can meet all the limit target of the above properties.

Cellulose Nanofibrils Dewatered with Poly(Lactic Acid) for Improved Bio-Polymer Nanocomposite Processing.

Cellulose nanofibrils (CNFs) have theoretically ideal properties for bio-based composite applications; however, the incorporation of these materials into polymers is made challenging by the strong binding of water to CNFs and the irreversible agglomeration of CNFs during drying. Previous methods used "contact dewatering", wherein the addition of wood flour (WF) to CNFs facilitated the mechanical removal of water from the system via cold pressing, which showed potential in producing dried CNF fibrils attached to wood fibers for biocomposite applications. In this work, the viability of contact dewatering with poly(lactic) acid (PLA) powder for PLA/CNF composites was evaluated. The energy efficiency of dewatering, preservation of nanoscale CNF morphology, and mechanical properties were examined by mixing wet CNFs with pulverized PLA at various loading levels, pressing water out of the system, and compression molding and shear mixing composites for testing. The most impressive results from this dewatering method were the preservation of micron-to-nanoscale fibers with high aspect ratios in PLA-CNF composites; increased strength and modulus of 1.7% and 4.2%, respectively, compared to neat PLA; equivalent or better properties than spray-dried nanocellulose at similar loading levels; and an 11-194x reduction in drying energy compared to spray-drying CNFs.

Application of Abrasive Water Jet Cutting on Rubber Wood Plastic Composites

Wood-plastic composites (WPCs) possess a diverse arrange of advantageous characteristics resulting from the combination of wood and plastic. These include a prolonged lifespan, cost-effectiveness, and environmental friendliness. However, the net shape can only be roughly achieved by the extrusion method used to make WPCs. WPCs require additional machining to comply with the dimensional accuracy and assembly conditions. As a result, more research into the cutting process of this composite is needed. In this work, abrasive water jet cutting was used to cut WPCs manufactured from rubber wood flour and polypropylene (PP). The emphasis was on two distinctive architectures of 1-layered and 3-layered WPCs with a thickness of 10 mm. The effects of cutting factors including water pressure, traverse speed, and abrasive mass flow rate were taken into account. The study employed a full factorial analysis to investigate the correlation between cutting parameters and cutting performance, including kerf characteristics and machined surface roughness. The abrasive and water pressure in the experiment had a significant effect in cutting both materials. In some cutting conditions, without an abrasive, non-through cutting happens, particular for the 3-layered WPC structures, that are particularly strong. An increase in water pressure decreased machining surface roughness and kerf width. The cutting parameters of 350 MPa water pressure, 30 mm/s traverse speed, and 6.67 g/s abrasive flow rate were suggested for both the 1-layered and 3-layered WPCs structures. This procedure was considered suitable for post-cutting without requiring for surface finishing.

The Development of Sustainable Polyoxymethylene (POM)-Based Composites by the Introduction of Natural Fillers and Melt Blending with Poly(lactic acid)-PLA

The conducted study was focused on the development of a new type of technical blend reinforced with natural fillers. The study was divided into two parts, where, in the first stage of the research, unmodified POM was reinforced with different types of natural fillers: cellulose, wood flour, and husk particles. In order to select the type of filler intended for further modification, the mechanical characteristics were assessed. The 20% wood flour (WF) filler system was selected as the reinforcement. The second stage of research involved the use of a combination of polyoxymethylene POM and poly(lactic acid) PLA. The POM/PLA blend (ratio 50/50%) was modified with an elastomeric compound (EBA) and chain extender as the compatibilized reactive (CE). The microscopic analysis revealed that for the POM/PLA system, the filler–matrix interface is characterized by better wettability, which might suggest higher adhesion. The mechanical performance revealed that for POM/PLA-based composites, the properties were very close to the results for POM-WF composites; however, there is still a significant difference in thermal resistance in favor of POM-based materials. The increase in thermomechanical properties for POM/PLA composites occurs after heat treatment. The increasing crystallinity of the PLA phase allows for a significant increase in the heat deflection temperature (HDT), even above 125 °C.

Methacrylated Wood Flour-Reinforced Gelatin-Based Gel Polymer as Green Electrolytes for Li-O2 Batteries.

With its very high theoretical energy density, the Li-O2 battery could be considered a valid candidate for future advanced energy storage solutions. However, the challenges hindering the practical application of this technology are many, as for example electrolyte degradation under the action of superoxide radicals produced upon cycling. In that frame, a gel polymer electrolyte was developed starting from waste-derived components: gelatin from cold water fish skin, waste from the fishing industry, and wood flour waste from the wood industry. Both were methacrylated and then easily cross-linked through a one-pot ultraviolet (UV)-initiated free radical polymerization, directly in the presence of the liquid electrolyte (0.5 M LiTFSI in DMSO). The wood flour works as cross-linking points, reinforcing the mechanical properties of the obtained gel polymer electrolyte, but it also increases Li-ion transport properties with an ionic conductivity of 3.3 mS cm-1 and a transference number of 0.65 at room temperature. The Li-O2 cells assembled with this green gel polymer electrolyte were able to perform 180 cycles at 0.1 mA cm-2, at a fixed capacity of 0.2 mAh cm-2, under a constant O2 flow. Cathodes post-mortem analysis confirmed that this electrolyte was able to slow down solvent degradation, but it also revealed that the higher reversibility of the cells could be explained by the formation of Li2O2 in the amorphous phase for a higher number of cycles compared to a purely gelatin-based electrolyte.

Effects of Heat Treatment on the Chemical Composition and Microstructure of Cupressus funebris Endl. Wood

The effects of heat treatment on Cupressus funebris Endl. wood were examined under different combinations of temperature, time, and pressure. The chemical composition, crystallinity, and microstructure of heat-treated wood flour and specimens were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Vacuum heat treatment led to changes in the functional groups and microstructure of C. funebris wood, and the relative lignin content decreased with increasing treatment temperature, which was significant at lower negative pressures. Cellulose crystallinity showed a change rule of first increasing and then decreasing throughout the heat treatment range, and the relative crystallinity ranged from 102.46% to 116.39%. The cellulose treated at 120 °C for 5 h at 0.02 MPa had the highest crystallinity of 44.65%. These results indicate that although heat treatment can improve cellulose crystallinity, very high temperatures can lead to decreased crystallinity. The morphology and structure of the cell wall remained stable throughout the heat treatment range; however, at elevated temperatures, slight deformation occurred, along with rupture of the intercellular layer.