Sawn-timber and kraft pulp properties of <i>Pinus elliottii × Pinus caribaea</i> var. <i>hondurensis</i> and <i>Pinus patula × Pinus tecunumanii</i> hybrids and their parental species

Abstract

A study was undertaken to evaluate the quality of the timber produced by Pinus elliottii × Pinus caribaea var. hondurensis (PECH) and the Pinus patula × Pinus tecunumanii low elevation (PPTL) and high elevation (PPTH) hybrids and their parent species for both kraft pulp and sawn-timber production. Trees were taken from unthinned tree improvement trials managed for pulpwood, ranging in age between 15 and 19 years. All sawn boards produced by study trees met the minimum wood density requirement for S5-grade structural timber (360 kg m⁻³), but approximately 17% of the boards failed to meet the other requirements for the grade, largely due to knot-related defects. Results of dynamic modulus of elasticity assessments performed on all of the boards suggested that a large percentage of boards would not meet the specified average stiffness (7 800 MPa). This was confirmed by the results of static bending tests performed on a subsample of boards. All boards tested for all species and hybrids met the required fifth percentile bending strength value for grade S7 (15.8 MPa) according to SANS 6122 (2008) specifications. The kraft pulping results indicated that the samples taken from the upper part of the stem yielded slightly better results on average than the samples representing the whole tree with respect to uniformity in the kappa vs charge and temperature, and yield vs kappa traits, with slight improvement of pulp yield (52% vs 50%) and some strength properties compared with whole tree pulping. Samples from the upper part of the stem had a close delignification rate in the 60–80 kappa range. The delignification rate for PPTL in the 60–80 kappa range was slower and the yield was slightly lower than P. patula (53.17% vs 52.72%) despite a higher kappa number. The pulp strengths short-span compressive test, breaking length and tearing strength of PECH were similar to those of P. elliottii, which were in turn generally lower than those of P. patula. The pulp strengths of PPTL and PPTH were similar to those of P. patula, whereas P. caribaea strengths were intermediate between those of P. patula and P. elliottii. With the exception of a slightly lower pulp yield, PPTL emerged as the best all-round hybrid for both pulp and sawn-timber properties.

Keywords: acoustic stress wave, breaking length, kraft pulp yield, modulus of elasticity, modulus of rupture, SCT, short-span compressive test, structural timber, tear index, wood density