The effect of plantation grown pine species selections and processing parameters on veneer recovery for plywood production

Abstract

Plywood producers in South Africa historically used P. patula for veneer production. However, in recent years it has become difficult to plant P. patula at lower altitudes due to large scale mortality induced by the fungal pathogen Fusarium circinatum. Subsequently, alternative species, such as P. elliottii and P. taeda were established for veneer production. The wood properties of P. elliottii and P. taeda are inherently different from P. patula and limited knowledge is available on the optimal processing parameters for these species. This study examined how log size (diameter class), storage duration and steaming time impact the veneer quality of P. elliottii and P. taeda. Wet recovery, peeler waste production, dryer throughput and quality recovery were determined and the results show significant differences in wet recovery between the two pine species. This was mostly caused by differences in taper, as P. taeda logs show more taper than P. elliotti. Extended log storage negatively affected wet recovery for both species as fungal induced blue stain was often mistaken for wane, leading to the unnecessary rounding of logs during the rotary peeling process. Logs stored for longer duration resulted in lower veneer grades after drying compared to those processed within four weeks after felling. Log steaming improved peeling performance, enhancing veneer quality without directly affecting wet recovery. Increasing steaming time from 8 to 12 h did not clearly impact wet recovery, but significantly improved veneer quality after drying. Larger logs (mostly from the pruned section of the tree) generally achieved better veneer grades due to fewer knots, while longer-stored logs resulted in more downgrades due to fungal induced blue stain and splitting, likely from moisture loss. P. taeda showed overall better quality recovery than P. elliotti, however, P. elliottii had better volume recovery. Both P. elliotti and P. taeda had about 4% less face veneer recovery compared to P. patula.