Advantages of prefreezing for reducing shrinkage-related degrade in eucalypts: General considerations and review of the literature

Abstract

Summary Difficulties associated with the drying of ash eucalypts including collapse and internal checking, are discussed briefly. Prefreezing is one method that has been used successfully as a pretreatment for the drying of both hardwoods and softwoods from temperate and tropical regions.Prefreezing has produced marked reductions in shrinkage, collapse and drying degrade of the heartwood in the following species: California redwood, black walnut, black cherry, tanoak, toon, bamboo, and eucalypts. Little or no collapse reduction has been observed in New Zealand red beech, Pacific madrone, white birch, sitka spruce, and white ash. Limited response has been observed for numerous other species notably red oak and white oak.Reduced drying time in response to prefreezing has been observed in jarrah, karri, black walnut, Asian oak, toon, and California redwood; in Pacific madrone and tanoak the drying time increased. Not all species which respond with a reduction in shrinkage show reduced drying rates.Prefreezing wood at -20 ~ appears to be the most practicable temperature, although some species respond better at lower temperatures. However, in all cases, it is critical to ensure that the wood freezes and remains frozen for a number of hours. Indications are that the effect is retained for days to weeks and that the length of time of freezing need not exceed 12-24 hours.A number of explanations have been put forward to explain the behaviour of prefrozen wood. It is suggested that the main mechanism responsible for reduced shrinkage is due to the migration of moisture from the cell wall onto frozen lumen water. The moisture loss from the cell wall produces a 'cold shrinkage'; water to ice transformation leads to an expansion of liquid water in the lumen, thus imparting a compressive stress to the cell wall, which together with the moisture loss, make the cell more rigid, and therefore likely to shrink less. There is some evidence that certain types of wood extractives migrate into the cell wall during freezing and may play a role in the reinforcement of the wall. Reduced shrinkage after prefreezing has also been attributed to a reduction of the plasticising effect of wood extractives in wood dried at higher temperatures and low humidities; this effect does not occur at low temperatures.

Physical and Mechanical Properties of Fast-Growing Wood Subjected to Freeze-Heat Treatments

“…On the other hand, the freezing stage significantly increased the EMC of the rose gum (both the control and sample heated at 180 °C) and the Gympie messmate. The improved permeability of wood after freezing (Glossop 1994;Ilic 1995) could lead to an increase of the fractional volume of wood vessels and, consequently, to an increase of longitudinal permeability and EMC (Siau 1984). This phenomenon was also highlighted in other studies (Bal 2014;Cademartori et al 2014;Esteves et al 2014;Missio et al 2016).…”

“…The absence of strength loss for the Gympie messmate wood can be related to its lower moisture content (~70% dry basis) during freezing, in comparison with the Rose gum wood (~140% dry basis). The mechanical changes of wood promoted by the freezing treatment were associated with compression forces against the cell wall of wood attributed to the water expansion (Ilic 1995). This phenomenon is aggravated by the faster freezing rate (Szmutku et al 2013).…”

“…On the other hand, wood freezing has been used to reduce the shrinkage and the wood drying defects (Ilic 1995;Awoyemi 2006), to increase the diffusion or the permeability of wood (Glossop 1994;Ilic 1995), to reduce the wood collapse (Chen and Cooper 1974;Ilic 1995) and reduce wood cracks (Chen and Cooper 1974;Ilic 1999). However, this technique has not been fully explored.…”

Effects of Pre-Freezing Prior to Drying upon Some Physical and Mechanical Properties of Eucalyptus urophylla × Eucalyptus grandis Wood

“…Pre-freezing produces marked reductions in shrinkage, collapse, and drying degradation of the heartwood of eucalypts; however, no significant improvement regarding internal checking was obtained (Ilic 1999b). Pre-freezing wood for 12 to 24 h at -20 °C appears to be the most practicable conditions for eucalyptus wood (Ilic 1995). There was less change in wood strength when spruce wood was subjected to freezing at -10 °C/h.…”

“…Of the three groups, one was chosen as control and the other samples underwent prefreezing for 72 h at -20 or -40 °C by household refrigerator and 596 litres of vertical cryogenic refrigerator, respectively. Some research data indicated (Ilic 1995) that being kept frozen in a freezer for 72 h could change the structure of wood. The rate of temperature decrease for freezing was -10 °C/h.…”