CHARACTERIZATION OF GLUED LAMINATED PANELS PRODUCED WITH STRIPS OF BAMBOO (Guadua magna) NATIVE FROM THE BRAZILIAN CERRADO ABSTRACT: Panels were produced with strips of bamboo (Guadua magna) in layers crossed at angles of 90° and bonded with phenol-formaldehyde or PVA based resin,

Panels were produced with strips of bamboo (Guadua magna) in layers crossed at angles of 90° and bonded with phenol-formaldehyde or PVA based resin, glued in three and five plies. The panels were tested and the physical and mechanical properties determined. The tests were primarily related to the commercial adhesives used as well as the number of layers of bamboo used. The density ranged from 0.690 to 0.768 g.cm-3. Panels bonded with PVA resin showed low mechanical strength, with MOR between 6.7 and 7.8 MPa. Those bonded with phenol-formaldehyde adhesive showed high strength, with modulus of rupture (MOR) ranging from 55.5 to 87.0 MPa, which is excellent for boards similar to wood-based panels. Panels with phenolic resin also had reduced thickness swelling, up to 8.4%. The panels made with three layers showed higher resistance to bending than those made with five layers. CARACTERIZAÇÃO DE PAINÉIS LAMINADOS COLADOS PRODUZIDOS COM TALISCAS DE BAMBU (Guadua magna) NATIVO DO CERRADO RESUMO: Painéis foram produzidos com taliscas de bambu (Guadua magna) montados em três ou cinco camadas cruzadas entre si em ângulo de 90° e coladas com resina à base de fenol-formaldeído ou PVA. Os painéis foram testados e as propriedades físicas e mecânicas determinadas. Os testes foram quanto aos adesivos comerciais usados, bem como a quantidade de camadas dos painéis. A densidade variou entre 0,690 e 0,768 g.cm-3. Painéis colados com resina PVA mostraram baixa resistência mecânica, apresentando MOR entre 6,7 e 7,8 MPa. Os colados com resina fenol-formaldeído foram mais resistentes, com módulo de ruptura (MOR) entre 55,5 e 87,0 MPa, excelente para painéis similares à base de madeira. Painéis com resina fenólica apresentaram também reduzido inchamento em espessura, até 8,4%. Os painéis produzidos em três camadas apresentaram resistência maior à flexão que os com cinco camadas. DOI: 10.1590/01047760201521041893


INTRODUCTION
Bamboo is a monocotyledon plant of the grass family Poaceae subfamily Bambuseae, producer of flowers and seeds, and having axial fibrous and vascular systems (ALMEIDA, 2004).
The aerial part of bamboo corresponds to the tree trunk and is called stem, and is basically a straight vertical piece.The anatomical structure of the bamboo stem is composed of nodes and walls (or internodes), with cells and fibers oriented axially.This structure influences directly on the physical and mechanical properties.The wall of the stem possesses two distinct transversal parts: an outer, tough and waterproof layer formed by an epidermal cell; and an inner part, consisting of parenchyma tissue, where capillary vessels or sap conductors, deposits of starch and fibers are the main components.The outer layer of the bamboo stem contains high proportions of silicate (about 4%), being resistant to insect attacks.The bamboo wall is composed of 50% of parenchyma, 40% of fiber and 10% of vessels.The nodes have from 40% to 50% of fibers (DUNKELBERG, 1985).
The bamboos of the genus Guadua have a crucial importance in the economy of Ecuador and Colombia, and as construction material it has species with high strength, such as Guadua angustifolia.In Brazil there are many natural Guadua inhabited forests, in the Amazon and in vast areas of the Brazilian territory, from south to north.The species G. magna was recently described by Filgueiras and Londoño (2006) as Guadua magna Londoño & Filg.
Due to the good physical and mechanical properties of bamboo, as well as the peculiar geometrical shape, low cost and ease to be obtained, the plant is widely used as construction material in various countries in which it grows abundantly, especially in tropical and subtropical areas of Asia and in some Latin American countries.Bamboo has high impact resistance and flexural strength, in part due to the high strength of its fibers in tension (BERALDO; RIVERO, 2003;SILVA, 2004).Recent studies demonstrate the technical feasibility of using bamboo for production of composites panels (ARRUDA, 2011;ARAÚJO, 2011).
Brazil has presented a very strong demand in the construction and residential renovations area, furniture industry, packaging and others.This fact combined with the decreasing supply estimated for timber from native forests in the near future, either from tropical and planted areas, demands for the search of lignocellulosic materials that can furnish the forest based industry in the coming decades (ABIMCI, 2009;IWAKIRI et al., 2005 ;IWAKIRI et al., 2009).
Bamboo, when compared to timber and in particular to planted forests, has many advantages such as rapid growth, high productivity, fast maturation, excellent properties and versatility of uses.The characteristics of silviculture, growth and mechanical resistance of the bamboo, opens the possibility to introduce this material for the manufacture of laminates (MOIZÉS, 2007;STAMM, 2002).As an alternative, the cultivation of bamboo can help avoid the cutting of woods under pressure used for these purposes.
Recently the Legislative Law 12484 was approved in Congress in 2011, which provides for a national policy to encourage the sustainable management and cultivation of bamboo.The law provides for research and technological development and applications of the products and byproducts of bamboo (REPÚBLICA FEDERATIVA DO BRASIL, 2011).
This study aimed to determine the physical and mechanical properties of laminated panels produced with strips of bamboo Guadua magna bonded with two types of glues.

Bamboo species
The used bamboo species Guadua magna is popularly known as "Taquaruçu", and was collected in the municipality of Santa Teresa, in the state of Goiás (Center region of Brazil) in a clump with 4 years of age.

Preparation of the bamboo strips
The stems were cut with a chainsaw, and then transformed into pieces covering the entire wall thickness, using a bandsaw.The bamboo diaphragms were also removed using the bandsaw.The pieces were then planned in the inner side without bark and the width adjusted between 20 and 30 mm (Figure 1).
Next, strips were removed from each piece on a router, equipped with a circular saw.From each piece an average of two strips were cut.The final strips exhibited between 20 and 30 mm in width and between 500 and 600 mm in length.The thickness of the strips obtained varied from 1.0 and 2.4 mm.

Manufacture of the laminated panels
The laminated panels of Guadua magna were produced from bamboo strips, glued laterally together on an uneven number of layers arranged adjacent and perpendicular to each other, and having two outer layers (face and backface) composed of one veneer of wood of ipê champagne (Dipteryx odorata) as decorative purposes as shown in Figure 2. A total of nine laminated panels measuring 300 x 300 mm were produced as follows: three of them with three inner layers of Taquaruçu and having a fi nal thickness of 6.0 mm and three with fi ve internal and two external layers presenting a fi nal thickness between 9.0 mm and 10.0 mm, all bonded with the commercial phenolic resin type Cascophen HL-2080.The three other panels were made the same way as mentioned before, having three inner layers of bamboo and bonded with the PVA type adhesive Rhodopas (Table 1).
The pressing was performed in a hydraulic press at a pressure of 0.76 MPa and a temperature of 140°C for 8 minutes for panels glued with phenolic resin Cascophen HL-2080.The panels bonded with the PVA glue were pressed at the same pressure at room temperature.After pressing, the panels were stored for 24 hours, then squared and taken to a conditioning room with controlled temperature of 20°C and 60% of relative humidity to reach equilibrium moisture content.

Adhesives
Two commercial adhesives were used and applied with a foam paint roller.One of these adhesives is suitable for hot pressing and moisture resistant and the other for cold pressing and not moisture resistant.The preparation of each adhesive followed the recommendation of the manufacturer.
-Phenol-Formaldehyde (Cascophen HL-2080) -PF: phenolic resin normally applied to plywood for external use.The formulation of the glue used was 100 parts of resin, 10 parts of water to adjust the viscosity and 10 parts of wheat fl our, as an extender, both mixed to the resin to obtain a homogeneous mixture.The amount of glue used was 360 g .m -².The viscosity of the adhesive was adjusted to 590 cPs.

Panels testing
The following properties were determined: -Bending strength: Modulus of Rupture (MOR) and modulus of elasticity (MOE) in accordance with EN 310 (1993).Each panel was cut in four samples of 140x50 mm in the parallel direction to the fi bers and three in the perpendicular direction, a total of 21 specimens per treatment, or type of panel, and grand total of 63. -Specific mass (M.E.A.): determined in the static bending specimens.
-Water absorption and thickness swelling: according to ASTM D1037 (1999).Seven specimens of 50 x 50 mm were cut from each panel, totaling 21 specimens per treatment, or type of panel, and 63 in the grand total.
-Janka hardness: according to ASTM D1037 (1999).Seven specimens were cut from each panel bonded with PF resin and six from each PVA bonded panel.The overall total tested was 60 specimens.
The data analysis was performed with the aid of SPSS 13.0 software.The experiment was set up as a randomized experimental design with three treatments: two layers (3 and 5 ply) assigned to the PF resin and one layer (3 ply) assigned to the PVA.An analysis of variance with 5% significant level was used to compare results and the Tukey test for means comparison.

RESULTS AND DISCUSSION
The G. magna bamboo culms used in this study have the dimensions shown in Table 2.

Physical Characterization
The apparent specific mass (M.E.A.) The density at 12% moisture content (M.E.A.) of the panels ranged from 0.69 to 0.77 g .cm -3 .The Tukey test showed a significant difference between the panels produced with PVA glue (0.77 g .cm -3 ) and with the phenolic adhesive.To eliminate the effect of M.E.A., a correlation test was performed correlating this property with others studied.Only MOR significantly correlated with M.E.A.
The moisture content was 6.8% with PVA glue for the laminated panels and 9.8% and 10.6% for PF with three and five layers, respectively.

Water absorption (WA) and thickness swelling (TS)
The Tukey test for the panels' water absorption showed that this property was linear along all treatments with no significant difference.The absorption of the panels varied from 46% to 46.9%.There was no influence of adhesive type or number of layers in the water absorption.
Regarding TS, the adhesives influenced this property significantly.Laminated panels bonded with PVA were less stable dimensionally, with an average of 14.4%.Panels bonded with phenolic resin were not significantly different and more stable with average TS ranging from 8.0% to 8.4%.Marton (2008) obtained TS and WA values of 33.7% and 6%, respectively.The overall results of the G. magna laminated panels are shown in Table 3.

Figure 2
Figure 2 Laminated panel with three layers of bamboo in the core and two wood veneer faces.Figura 2 Painel laminado com três camadas de bambu no miolo e duas lâminas de madeira nas faces.

Table 1
Experimental design of the study.

Table 3
Physical and mechanical properties of G. magna bamboo laminated panels.Tabela 3 Propriedades físicas e mecânicas dos painéis laminados de bambu G. magna.