Industrial tannin/hexamine low-emission exterior particleboards

Subject Low formaldehyde emission and high moisture tolerance of tannin bonded particleboard can be achieved by using hexamine as hardener. Materials and Methods 17 Industrial three layer particleboard panels of dimensions 3750 x 1500 x 21 mm were prepared on a Pagnoni particleboard industrial line using a tannin adhesive based on a mix of 30/70 pecan/pine industrial tannin extracts used as a 36% water solution at pH 6.2 and using as hardener 6.5% on tannin extract solids of hexamethylenetetramine (hexamine) as a 40% water solution added through a separate pump line to a Drais glueing blender. Adhesives solids content on dry wood was of 11% for the core particles and of 13% for the surface particles. The pressing conditions used were 14 s/mm at 167 degrees C in a non-simultaneous closure multidaylights press with a cycle max/medium/degassing pressure of respectively 1/1/3 minutes at 20.5/11/5 kg/cm(2). The moisture content of the particles mat just before hot pressing was of 29.6% for the core particles and of 31.5% for the surface particles. The proportion of surface to core material was of 50:50 by weight. 17 control particleboard panels were also produced under the same conditions but at 185 degrees C with a commercial quebracho tannin adhesive using traditional tannin adhesives hardener technology based on 5% paraformaldehyde and 5% urea. The boards were tested according to DIN standards for internal bond (I.B.) strength dry and after boiling and for formaldehyde emission by perforator test. The results are shown in the Table. Results and Discussion The results in the Table confirm at the industrial level that exterior grade particleboard (and other types of panel products) meeting the V100 requirements can be produced when using hexamine as hardener for fast tannins such as pine and pecan. The use of this hardener is instead only capable of yielding interior grade particleboard with slower reacting tannin adhesives such as those based on mimosa and quebracho tannins. It must be noted (i) the exceptional tolerance to high moisture content of both types of adhesives (experimental and control), that in the case of hexamine-hardened adhesives has been shown to also be a necessity to yield good exterior results, and (ii) the coupling of low maximum pressure, relatively longer degassing time during hot pressing, and particularly the lower pressing temperature necessary to yield good results with hexamine hardeners. While the need for the latter has been already shown at laboratory level, the need for the two former could only be shown during the industrial plant trial reported here. Formaldehyde emission is much lower than for the traditional tannin adhesives technology of the control and is near to what obtainable by just heating the wood alone. At these low perforator values HCHO air concentration in chamber tests is likely to be 0 ppm or very near to it.