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Introduction
The sawmill industry is defined as the business of transforming roundwood into sawn wood. In Mexico, this industry processes about 70 % of annual forest production (Ortiz, Martínez, Vázquez, & Juárez, 2016). However, in this transformation process, the sawmill conversion efficiency rates range between 45 and 60 % (Luna et al., 2012), so approximately 40 % becomes waste of little or no economic value.
Mexico has a high number of forest species, ranking it fourth among the 17 so-called megadiverse countries (Villaseñor & Ortiz, 2014). The country has 138 million ha of forest vegetation, equivalent to 70 % of the national territory; of this area, 64.9 million ha correspond to forests and tropical forests (Hernández-Salas et al., 2013). The state of Durango, due to its forest cover, is considered one of the main wood producers. In 2013, the state accounted for 32.8 % of production; other states such as Chihuahua (16.79 %), Michoacán (7.76 %), Oaxaca (7.13 %) and Veracruz (4.93 %) had a smaller share. It should be noted that just Durango and Chihuahua together accounted for 49.6 % of the country's timber production, hence their importance in the forestry sector (Secretaría del Medio Ambiente y Recursos Naturales [SEMARNAT], 2014).
The most used species in Durango are the pines, with approximately 20 species, of which half are the most used, due to their quality and abundance. These include: Pinus durangensis Martínez, P. arizonica Engelm., P. engelmannii Carr. and P. cooperi C. E. Blanco; other representative species are P. leiophylla Schiede ex Schltdl. & Cham. and P. chihuahuana Martínez (González-Elizondo, González-Elizondo, Tena-Flores, Ruacho-González, & López-Enríquez, 2012). The most noteworthy residue is sawdust, the main product of wood sawing, which generally does not have an important application or commercialization; rather, it becomes a disposal and handling problem within the industry. Sawdust ends up in purely traditional and rudimentary uses, mainly as fuel, as a cleaning aid in homes and farms and even abandoned in the field due to the lack of technological proposals for industrial use (Tchehouali et al., 2015). In this context, this paper provides a state-of-the art review of the sustainable alternatives for the development and use of forest industry resources.
Delimitation of the problem
Residues left out in the open, without pretreatment, can be considered hazardous, by being a focus of proliferation of infectious agents (rodents, insects and pathogenic microorganisms) that are harmful to humans and animals, and by deteriorating soil, water and air quality (Saval, 2012).
The final disposal of sawdust and other wastes is a growing problem in the wood industry, since sawmills produce large quantities of sawdust annually. At the local level, SEMARNAT (2012) estimated timber production in the state of Durango at 1 741,212 m3 corresponding to 78 % of average yield, resulting in a total of 491,111 m3 of residual byproduct generated in forest harvesting.
The National Program for the Prevention and Comprehensive Management of Waste operated by SEMARNAT (2008), arising from article 19 of the General Law for the Prevention and Comprehensive Management of Waste (Cámara de Diputados del H. Congreso de la Unión, 2003), includes waste from health and transportation services, among others, but leaves out so-called lignocellulosic byproducts. This results in total disinterest at wood processing centers for the rational management of forestry industry residues; however, there is local legislation such as Mexico City’s Solid Waste Act (Asamblea Legislativa del Distrito Federal, 2003). Article 31 of this Act establishes that "agricultural and forestry by-products are considered as special solid wastes, including the waste from inputs used in this practice."
The production of sawdust and bark on a large scale remains a major problem as only partial solutions have been developed, mainly due to the geographic dispersion of generation sources and potential markets. In addition, transportation problems stemming from the mobilization of large volumes limits profitable returns for such materials (Pérez, Barrera, & Ramírez, 2015).
Uses in the agricultural sector
In recent decades, by-products or waste from various domestic, urban and industrial activities have been used as growth media (Luna, Córdoba, Gil, & Romero, 2013). The incorporation of these materials allows obtaining low-cost products that in the long term have a positive ecological impact (Pineda-Pineda et al., 2012).
Substrate in agriculture
Pine (Pinus spp.) sawdust and bark from the timber industry have potential as a substrate, and have high availability due to the large amounts produced in Mexico (Ortega-Martínez, Sánchez-Olarte, Díaz-Ruiz, & Ocampo-Mendoza, 2010).
Sawdust used properly can compete with other substrates that have comparatively limited availability (Raviv, 2011). All sawdust types improve the physical characteristics of the growth medium, since the particle size is easily manageable with the components of the medium. The favorable effects can be compared with those of peat, as much for the mass density as for the porosity and aeration of sandy soils and water retention of clayey soils (Martínez-López, Fernández-Concepción, Álvarez-Lazo, García-González, & Rodríguez-Álvarez, 2012). In Mexico, sawdust has been evaluated as a substrate in Solanaceae; its effect is reflected in an efficient root system, contributing to the production of a good quality plant (Ortega-Martínez et al., 2010).
On the other hand, pine bark has adequate physical and chemical characteristics for the production of plants of various species (Jackson & Wright, 2009). The use of this material helps reduce damage caused by pathogenic organisms, due to the low moisture retention (Reis, 1995). The bark mixed with other mineral or industrial substrates increases water availability and nutrient efficiency without affecting the root or plant growth (Vargas-Canales, Castillo-González, Pineda-Pineda, Ramírez-Arias, & Avitia-García, 2014).
Substrate in forest production
Raw sawdust has proved to be a good substrate for the production of Cedrela odorata L. plants in a nursery, without toxic effects during the greenhouse period (Mateo-Sánchez, Bonifacio-Vázquez, Pérez-Ríos, Mohedano-Caballero, & Capulín-Grande, 2011). The mixture of conventional substrates and sawdust provides a satisfactory growth medium for the production of forest species with modern production systems (Maldonado-Benitez, Aldrete, López-Upton, Vaquera-Huerta, & Cetina-Alcalá, 2011).
Hernández-Zarate, Aldrete, Ordaz-Chaparro, López-Upton, and López-López (2014) evaluated the growth of P. montezumae Lamb. seedlings in a nursery in Puebla, Mexico. The physical and chemical characteristics of the substrates composed of pine bark and sawdust provided conditions appropriate for the growth of the seedlings that at the age of 10 months showed morphological characteristics suitable for field transplant. In this regard, Mateo, Capulín, Araujo, Suárez, and Mitjans (2015) evaluated the effect of three substrates based on pine sawdust on the growth of Acacia retinodes Schltdl. in a nursery. Sawdust (67 %) mixed with vermicompost (33 %) and slow release fertilizer (4 kg·m-3), as a complement, produced root dry weight, shoot weight, total biomass, plant height and stem diameter values greater than those obtained with conventional substrates.
In addition to the products obtained from the conversion of lignocellulosic waste, products such as biochar could be a variant to consider, since it provides beneficial effects as a soil conditioner, by improving its quality and productivity. The use of biochar has improved soil chemical and physical properties (moisture retention, nutrient content and retention, soil permeability and biological properties), contributing to increased crop yields and soil establishment for reforestation (Woolf, Amonette, Street-Perrott, Lehmann, & Joseph, 2010).
Livestock feed supplement
Ruminants have adapted to various feeding systems because microbial species living in their rumen, such as bacteria, protozoa and fungi, transform low-quality foods, such as cereal straws or even urea, into others with high protein content (Rodríguez & Rodríguez, 2011). Agroindustrial and forest residues, due to their volume and composition, can constitute an alternative to deal with the lack of food. Due to these characteristics, it is possible to use unconventional fiber sources without affecting productive variables (Perea, Guardia, Medina, & Hinestroza, 2013).
Currently, the use of sawdust in ruminant feeding has an impact on the important weight gain variable in meat production, reducing the cost of common fiber sources. In this regard, Mateo-Sánchez, Cobos-Peralta, Trinidad-Santos, Cetina-Alcalá, and Vargas-Hernández (2002) isolated a culture of ruminal bacteria capable of degrading pine sawdust. These authors suggest that, in addition to providing the necessary fiber to the diet, sawdust can be used as an energy source if there are adequate ruminal conditions for the activity of cellulolytic and hemicellulolytic bacteria. Also, Guerra-Medina, Pérez-Sato, Cobos-Peralta, and Montañez-Valdez (2010) reported that the inclusion of 30 % sawdust of Pinus patula Schiede ex Schltdl. & Cham., in lieu of corn stover as a fiber source, improved daily weight gain in finishing lambs, which had a similar response in voluntary feed intake and, consequently, better food efficiency. Recently, Guerra-Medina et al. (2015) evaluated the effect of adding agave bagasse and sawdust to the diet for sheep on average daily gain, dry matter intake, feed conversion and ruminal pH. Experimental diets were: 15 % corn stover (control), 15 % agave bagasse and 15 % pine sawdust. The results suggest that pine sawdust and agave bagasse can be used as alternative sources of fiber without affecting productive variables.
Use as biofuel
Combustion of biomass from forest waste (branches, needles, sawdust and woodchips), as well as of conventional fossil fuels, can be considered as raw material in fuel production and electric, thermal and potential energy generation, for use in the industrial, commercial, family and transportation sectors (García, Pizarro, Lavín, & Bueno, 2014). Bioenergy can be obtained from solid biofuels such as firewood, charcoal, agricultural and forest residues, pellets and briquettes; from liquid biofuels such as bioethanol and biodiesel; and from gaseous sources such as biogas (Vega-Nieva, Fernández-Lorenzo, Ortiz- Torres, & Corral-Rivas, 2015).
Raw material in bioethanol production
The increase in ethanol production in the world has been linked to the development of new technologies that enable obtaining it from wood waste, solid waste and materials containing cellulose and hemicellulose, which allows revaluing the waste from several industries (Mora et al., 2015).
Interest in the use of lignocellulosic materials as raw material in transformation processes by microorganisms has increased over the last several decades (Viñals-Verde, Bell-García, Michelena-Álvarez, & Ramil-Mesa, 2012). Sawdust is used for the production of ethanol, fuel and other chemicals (Palonen, Tjerneld, Zacchi, & Tenkanen, 2004) due to being the dominant lignocellulosic material in the northern hemisphere (Galbe & Zacchi, 2002). Wood waste is efficient for the production of ethanol from lignocellulosic materials (bioethanol) (Koppram & Olsson, 2014); however, a limiting factor for production is the recovery of the sugars, which is determined by the hydrolysis procedure used. The enzymatic hydrolysis is limited by the presence of lignin and the crystallinity of the cellulose (Åkerholm, Hinterstoisser, & Salmén, 2004); therefore, several pretreatments, highlighted by alkaline, acid and steam explosion, have been used (Farías-Sánchez et al., 2015). It has been determined that alkaline pretreatment is the one that most favors enzymatic hydrolysis, since it produces the highest yields of reducing sugars (López-Miranda, Soto-Cruz, Rutiaga-Quiñones, Medrano-Roldán, & Arévalo-Niño, 2009; Schell, Farmer, Newman, & McMillan, 2003). With this pretreatment, López-Miranda et al. (2009) obtained 134 % higher saccharification yields than pretreating with sulfuric acid, and 246 % higher than using steam explosion. The authors indicate that their results allow visualizing the development of a technology for the use of pine sawdust in the generation of biofuels for the automotive industry and of clean technologies that could eliminate a pollutant from the soil that poses a health risk.
In the state of Durango, Pérez-Verdin, Navar-Chaidez, Grebner, and Soto-Álvarez (2012) estimated the production, gathering, extraction and transport of biomass with Monte Carlo simulations. According to the authors, about 322,000 t are used for the annual production of 38 million liters of ethanol, at an average cost of 23.8 USD·t-1 of forest waste (0.20 USD·L-1 ethanol), while for industrial waste it is 22.6 USD·t-1 (0.19 USD·L-1 ethanol). Therefore, the differential cost adjustment still limits the feasibility of this technology.
Production of solid fuels
Authors such as Styles et al. (2015) believe that the importance of biomass has increased worldwide because it is viewed as a renewable energy source, which reduces the harmful effects of fossil fuels considerably.
Residues from forest and industrial uses are an important source of energy, because they have high caloric content, high density and low moisture content. These attributes are desirable in the transformation of waste into bioenergy (Thiffault, Béchard, Paré, & Allen, 2015). Charcoal is a solid fuel with a calorific value that fluctuates between 121,336 and 146,440 kJ·kg-1, a value superior to that presented by wood that oscillates between 50,208 and 87,864 kJ·kg-1. The conversion of waste by pyrolysis, to obtain products with fuel characteristics, generates a product called biochar, which has been widely used as a management alternative, reducing the volume of solid waste (Yamato, Okimori, Wibowo, Anshori, & Ogawa, 2006). In terms of yield, the biochar weight obtained depends on the operating conditions and parameters of the pyrolysis process, the final temperature and the composition of the biomass. The yield decreases rapidly with the increase in temperature and the biomass heating rate, by favoring the generation of gases and the formation of a very reactive carbon of high porosity, which evolves towards the formation of volatile compounds. Under these rapid heating conditions, it has also been shown that tar formation increases (Angin, 2012). On the other hand, the higher the lignin content in the initial biomass the greater the yield in biochar weight due to its high thermal stability. Therefore, the three key factors favoring the production of biochar in a biomass pyrolysis process are low process temperatures, a slow heating rate and higher lignin content (Kim-Kwang, Jae-Young, Tae-Su, & Weon-Choi, 2012). In this regard, Arteaga-Crespo, Carballo-Abreu, García-Quintana, Alonso-López, and Geada-López (2012) obtained biochar from timber species endemic to Cuba with yields higher than 40 %, at 300 °C and at low heating rates.
Pellets and briquettes made from forest waste
One way to use wood residues is by converting them into pellets or briquettes, also known as densified solid biofuels. For this, the initial moisture, chemical composition of the ashes and their granulometric distribution are important (Correa-Méndez et al., 2014a, 2014b). These biofuels have a cylindrical shape with nominal diameters between 7 and 22 mm and lengths of 3.5 to 6.5 cm, are manufactured at high pressure without the need for adhesive and have a calorific value above 17,572 kJ·kg-1 (López et al., 2008).
Pellets and briquettes are made from sawdust compacted at pressures greater than 147,100 kPa; they are widely used in Europe and North America, in countries with high forest development (Dávila, Amador, Morazan, & Rugama, 2013). Combustion of pellets is environmentally friendly because it can reduce CO2 emissions by 50 % compared to the burning of firewood or woodchips; it also has low concentrations of sulfur (0.004 to 0.007 %) and nitrogen (0.05 to 0.16 %), with respect to the final dry weight of each pellet. The raw material contains between 8 and 12 % moisture, obtaining an energy efficiency of 18,828 kJ·kg-1 as the highest calorific value (Ortíz, Tejada, Vázquez, & Piñeiro, 2004).
In calorific efficiency studies, it has been found that the combination of 47.5 % charcoal and 52.5 % sawdust exceeds 21,307 kJ·kg-1, resulting in an energy gain of 24.25 % with respect to the highest calorific value of the sawdust pellets (17,148 kJ·kg-1) (Soto & Núñez, 2008).
Mixed compounds based on forest and plastic residues
The manufacture of products based on wood and polymers (composites) has taken on increased importance in recent years (Moya-Villablanca, Oses-Pedraza, Poblete-Wilson, & Valenzuela-Hurtado, 2014). Kuang, Kuang, Zheng, and Wang (2010) evaluated a reinforced polypropylene composite, prepared by the polypropylene extrusion process with wood, at proportions of 15, 25 and 40 % by weight at two different granulometries. The products were made by injection processes with dimensions according to the ASTM standard. The authors observed that with the increase in fiber content, properties such as the melt flow index decreased as the modulus of elasticity, hardness and density increased. The results show that the properties of the composites are highly efficient when compared with other commercial systems reinforced with inorganic fillers.
Chemical use
Studies on wood and bark extracts from conifers report chemical structures called secondary metabolites, such as: monoterpenes, sesquiterpenes, sesquiterpenlactones, diterpenes, triterpenes, flavonoids and lignans (Otto & Wilde, 2001). In some cases, these compounds are responsible for the resistance of wood to attack by insects, fungi and bacteria; therefore, they could have applications as pesticides, wood preservatives and antibiotics (Kubo, Muroi, & Himejima, 1992; Kubo, Muroi, & Kubo, 1995). Plant extracts, as biological control agents, and in combination with chemical and natural processes are emerging as partial solutions for the control of degrading organisms (fungi, bacteria and termites).
Fungicidal effect of chemical extracts
Liquids obtained in pyrolysis, as a consequence of the condensation of the gases, are known by a variety of terms: pyrolysis oil, bio-oil, biofuel oils, pyroligneous acid, wood liquids and wood distillates (Bridgwater, 2003). Navas (2002) obtained a pyroligneous liquid from sawmill waste and found fungicidal efficiency in in vitro tests, demonstrating the toxicity of the liquid to certain wood-rotting fungi. The toxicity limit values were 3 % volume of preservative:pyroligneous liquid, per medium volume, for the fungi Coriolopsis polizona (Pers.) Ryvarden and Fomitella supina (Sw.) Murrill, and 4 % volume of preservative: pyroligneous liquid, per medium volume, for the fungi Pycnoporus sanguineus (L.) Murrill and Trametes villosa (C. P. Robin) Berkhout. These values, once converted into their organic fraction content equivalent, coincided with toxicity values in similar studies. Vargas-Muñoz (2008) found antifungal activity of the crude methanolic extract of the bark of Pinus caribaea Morelet var. hondurensis, showing a significant increase from the fractionation of this extract, for the fungi Gloeophyllum trabeum (Pers.) Murrill and Trametes versicolor (L.) Lloyd.González-Laredo, Ochoa, Guzmán, and Castañeda (1989) reported that when pine samples and woodchips are treated with condensed tannins from their own bark, resistance increases against attack by the fungus G. trabeum, which causes brown rot. The authors explained that this phenomenon is perhaps due to a formation of covalent bonds between the flavonoids of the extract and the cellulosic material of the wood. Moreover, the cost of carrying out procedures for preserving tannins, extracted from the bark of forest material, represents a relatively low price, around $4.00·kg-1. The material is obtained from recyclable forest activity products, which confers an economically viable marginal advantage (González-Laredo et al., 1989).
Antibiotic effect of sawmill material
The species Pinus douglasiana Martínez and P. pseudostrobus Lindl. var. pseudostrobus, originating from the Sierra Madre Occidental, among others from different parts of the country, contain extracts with antimicrobial activity, but there are no scientific reports showing effective doses against some strains of pathogenic microorganisms (Stefanova-Nalimova, Coronado-Izquierdo, & Rizo-Peña, 2005).
Becerra et al. (2002) isolated 10 diterpenes from the bark and wood of the Chilean Podocarpaceae species: Podocarpus nubigena Lindl., P. saligna Zeller, Prumnopitys yina (Poepp. et Endl.) de Laub. and Saxegothaea conspicua Lindl. Six diterpenes had strong activity against Staphylococcus aureus Rosenbach and Pseudomonas sp., with the most efficient being totarol, ferruginol, dehydroferruginol and acetylferruginol. A similar investigation by Kubo and Himejima (1992) reported antimicrobial activity of totarol against Gram positive and negative bacteria.
In 2008, Amaya-Gutiérrez, Toledo-González, Ruiz-García, Flores-Machuca, and Casas-Solís evaluated the sensitivity of the bacteria S. aureus, Salmonella enteritidis (SE), Escherichia coli (Escherich) and the yeast Candida albicans (C. P. Robin) Berkhout to different concentrations of extracts of P. pseudostrobus, P. douglasiana and Pinus spp. The results showed that P. pseudostrobus, at concentrations of 25 %, inhibited 50 % of S. enteritidis, whereas the extract of Pinus spp. was effective with all four microorganisms in inhibition ranges from 83 to 97 %. Finally, P. douglasiana, at a concentration of 25 %, achieved 73 % growth control of S. aureus and about 60 % of E. coli and C. albicans.
Insecticidal effect of wood chemical compounds
Wood products need chemical agents for their preservation, but they create environmental problems and adversely affect many beneficial organisms and insects (Dutta, 2015). Extracts isolated from the heartwood of some pine species may provide alternatives in pest control, due to the high content of bioactive chemicals. In addition, the extracts are biodegradable and could thus help to resolve the environmental problems caused by synthetic pesticides (Montico & Di Leo, 2015). Several extracts present toxicity and repellency against some termite species (Manzoor et al., 2011); the mechanism is not entirely clear, so it is suggested to adapt methodologies that can contribute knowledge. The activity of most natural extractives against termites is generally low relative to commercial insecticides.
Antioxidants present in industrial wood waste
Polyphenols and especially flavonoids are secondary metabolites that form naturally in all plants (Riveros & Inga, 2015). Some of these compounds have been identified in pine species originating from Durango and in seeds of fruits from the same state (González-Elizondo et al., 2012; González-Laredo et al., 2007). Flavonoids are natural antioxidants that have a significant effect on wood durability (Dai & Mumper 2010). According to Schultz and Nicholas (2000), flavonoids protect the heartwood against colonization of fungi by a dual function: fungicidal activity and elimination of free radicals (antioxidant activity). Flavonoids have received particular attention because of their role in the neutralization and scavenging of free radicals (Gupta & Prakash, 2009). Pietarinen, Willför, Vikström, and Holmbom (2006) showed that the antioxidant capacity of flavonoids is particularly important because it is responsible for neutralizing reactive species that cause cell wall decay during white and brown rot. On the other hand, several researchers have reported that phenols extracted from Pinus radiata D. Don are effective preservatives in the food industry due to their antioxidant properties (Jerez, Touriño, Sineiro, Torres, & Núñez, 2007; Raghavendra, Kumar, & Prakash, 2007). These antioxidant effects have also been described for bark extracts of Pinus marítima Ait. (Packer, Rimbach, & Virgili, 1999) and Pinus pinaster Ait. (Jerez, Selga, Sineiro, Torres, & Núñez, 2007), under different extraction conditions.
Rosales-Castro and González-Laredo (2003) evaluated the content of condensed tannins and total phenols, expressed as tannic acid, in ethanolic and aqueous extracts of the bark of eight pine species abundant in the state of Durango: P. arizonica, P. ayacahuite Ehrenb. ex Schltdl., P. cooperi, P. chihuahuana, P. durangensis, P. engelmannii, P. leiophylla and P. teocote Schiede ex Schltdl. & Cham. The yields in total extract (total solids extracted) varied with respect to the extraction solvent and species. In all species, the extract yield obtained with 50 % ethanol was higher than that obtained with water, with P. ayacahuite and P. leiophylla being the species with the highest concentration of phenolic compounds, and P. engelmannii and P. cooperi those with the lowest concentration. Even so, the concentration of flavonoids determined in this study, which ranged from 13 to 16 % (P. durangensis, P. ayacahuite and P. leiophylla), can be considered viable for use in biomedical areas (Cortés, Pulgar, Sanhueza, Aspé, & Fernández, 2010).
Flavonoids present in the bark of forest species and their effect as nutraceuticals have been investigated by Nakayama et al. (2015). Previously, Devaraj et al. (2002) patented an extract from P. pinaster bark with the name Pycnogenol, showing that a 150-mg dose of this extract per day for six weeks significantly reduces low-density lipoprotein (LDL-cholesterol) levels and increases high-density lipoproteins (HDL) in humans. Methods and techniques for obtaining these extracts are described in some U. S. patents (55,720,956, 8,697,749 and 5,720,556). In this regard, Rosales et al. (2009) evaluated the concentration of total phenols, flavonoids and proanthocyanidins in 70 % aqueous acetone extracts (crude extract) and semi-purified extracts by liquid-liquid partition with ethyl acetate (organic extract) in bark of P. cooperi, P. engelmannii, P. leiophylla and P. teocote. In this way, the authors determined the antioxidant activity of the extracts by the ABTS•+ and deoxy-d-ribose (hydroxyl radical sequestration) radical techniques, as well as by the inhibition of LDL oxidation. This allowed the identification of the flavanol catechin in all four species, but at low concentrations; even so, all the extracts had high antioxidant activity, as they inhibited free radicals, proving that the extracts are natural products with high value as biologically active phytochemicals.
Tannins and their uses as natural adhesives
Tannins are chemical compounds with very complex structures formed by phenolic groups of plant origin, and they have the ability to react and precipitate with alkaloids, gelatins and other proteins. The highest concentrations of tannins are found in woody or xylem tissues, which contain a high proportion of parenchyma cells, especially early wood parenchyma, and in the wood radii (Pedraza-Bucio & Rutiaga-Quiñones, 2011).
Tannins have been used in the formulation of adhesives with mixtures of other chemical compounds such as formaldehyde (Pedraza-Bucio & Rutiaga-Quiñones, 2011; Vázquez, González-Álvarez, López-Suevos, & Antorrena, 2003). Vázquez et al. (2003) developed phenol-formaldehyde-tannin adhesives using P. pinaster bark tannins with promising results in their application to eucalyptus plywood boards. Adhesives based on forest residues have been formulated with marginal success, as part of the search for ways to decrease the use of chemical components. Encinas, Paredes, and Tiburzi (2007) prepared adhesives with tannic extracts of Caribbean pine bark that withstood more than 196.133 kPa, which is the minimum accepted as resistance to shear of wood boards. Adding sulphite in the preparation of the glue improves its resistance to shear, since the failure of the samples occurs in the wood and not in the bonds (Encinas et al., 2007).
Esteves et al. (2015) evaluated liquefied P. pinaster sawdust using a polyvalent method with acid catalysis. They found that bond strength decreases with increased liquefied wood content; however, at a concentration of 20 % liquefied wood, the reduction of internal bond strength is relatively small and still within the minimum standards required When 70 % liquefied wood is used there is a significant decrease in bond strength. The authors concluded that it is possible to use a small amount of maritime pine liquefied wood (sawdust) as a partial substitute in the formulation of urea-formaldehyde and melamine-urea-formaldehyde resins for the production of adhesives, thus decreasing the formaldehyde content.
Vázquez, López-Suevos, González-Álvarez, and Antorrena (2005) proved that adding tannins to prepolymers modifies the rheological characteristics from a Newtonian behavior to a pseudoplastic behavior in the adhesives. This allowed researchers to develop products that exceeded European quality standards for outdoor use boards.
Conclusions
Interest in the use of lignocellulosic materials from sawmill industry waste, as raw material in transformation processes for a more comprehensive use, has increased over the last several decades. Knowledge of the chemical and physical properties of forest by-products and the concentrations of the elements that comprise them is of vital importance for the sustainable development of the forest industry, which could make it more profitable by creating new sources of income. At the national level, and particularly in Durango, there are initial efforts to harness the potential of sawdust and bark, which prioritize the following applications: a) as a substrate in forest and agricultural production, under greenhouse conditions; b) as a less polluting energy material and mixtures of forest residues that produce more specific heat, achieving a potential ecological use capable of counteracting the pollution generated by the traditional disposal of by-products; the initial viability points towards the manufacture of pellets or briquettes. We suggest that the necessary methodologies to determine the quality of wastes generated in the state be adopted, and that an in-depth study on the viability of the extractables for use as fungicide and wood preservative and for the formulation of adhesives be carried out.
