Shelling of Growth Rings at Softwood Surfaces Exposed to Natural Weathering
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coatings Article Shelling of Growth Rings at Softwood Surfaces Exposed to Natural Weathering Lukie H. Leung 1 and Philip D. Evans 1,2,* 1 Department of Wood Science, Faculty of Forestry, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; [email protected] 2 Department of Applied Mathematics, The Australian National University, Canberra, ACT 0200, Australia * Correspondence: [email protected]; Tel.: +1-604-822-0517; Fax: +1-604-822-9159 Received: 13 August 2020; Accepted: 2 September 2020; Published: 5 September 2020 Abstract: Shelling is the delamination of growth rings and the projection of woody tissue from wood surfaces. Shelling disrupts coatings and makes refinishing difficult, and a better understanding of the phenomenon is needed to help alleviate its unwanted effects. We tested whether confocal profilometry could quantify shelling in flat-faced and profiled-faced western larch deckboards exposed to natural weathering and examined the effects of growth-ring orientation and angle on shelling. Confocal profilometry was able to quantify shelling in both deckboard types. Shelling developed at the surface of flat-faced deckboards oriented pith-side-up, whereas it was absent from boards oriented bark-side-up. We found an inverse correlation between the height of shelled growth rings and the angle of growth rings to the surface of flat-faced boards. Shelling occurred in profiled-faced boards oriented pith-side-up due to the delamination of growth ring tips and projection of latewood from wood surfaces. A superficially similar although less pronounced phenomenon was seen in profiled-faced boards oriented bark-side-up. The shelling of profiled-faced boards oriented pith-side-up created lanceolate-shaped slivers of latewood that projected from the peaks of profiles. Some of these latewood tips were sharp and, for this reason, we suggest that profiled-faced western larch deckboards should always be oriented bark-side-up rather than pith-side-up. Keywords: shelling; western larch; confocal profilometry; profiling; growth rings; latewood; earlywood; pith-side-up; bark-side-up; natural weathering 1. Introduction Wood that is exposed outdoors is subject to surface degradation by solar radiation (U.V. and visible light), water, heat, environmental pollutants, and mould [1]. The most obvious manifestation of such degradation is the greying of wood exposed outdoors [2]. Another highly visible effect of “weathering” is the surface cracking (checking) of wood [3]. Checks develop when moisture-induced surface stresses exceed the tensile strength of wood perpendicular to the grain [4]. The severity of checking depends, in part, on the orientation of growth rings to wood surfaces exposed to the weather. For example, checking is less severe in coniferous wood (softwood) whose growth ring are perpendicular to the exposed surface (radial or quarter-sawn boards) when compared to wood whose growth rings are mainly parallel to the surface (tangential or flat-sawn boards) [5]. Checks in quarter-sawn softwoods often develop at the interface between denser bands of summer wood (latewood) and lower density springwood (earlywood) [6]. In flat-sawn softwoods, checks develop in and propagate radially into wood via rays [4]. Flat-sawn wooden boards can be classified into ones where growth rings either curve away from, or curve towards, their upper wide surface [7] (Figure1). These boards are commonly known as bark-side-up and pith-side-up boards, respectively [7] (Figure1). The checking Coatings 2020, 10, 862; doi:10.3390/coatings10090862 www.mdpi.com/journal/coatings Coatings 2020, 10, 862 2 of 15 of wood during exposure to the weather is less pronounced in wooden boards that are oriented pith-side-up [5,8]. Coatings 2020, 10, x FOR PEER REVIEW 3 of 15 FigureFigure 1.1. OrientationOrientation of of growth growth rings rings to to the the upper upper exposed exposed surface surface of flat-sawn of flat-sawn (tangential) (tangential) western western larch deckboards.larch deckboards. The darker The darker bands withinbands growthwithin ringsgrowth are rings latewood are latewood and the lighter and the bands lighter areearlywood: bands are (earlywood:a) flat board (a with) flat growthboard with rings growth that curve rings towards that curve the towards exposed the upper exposed surface. uppe Theser surface. are knownThese are as pith-side-upknown as pith-side-up boards. Growth boards. rings Growth can rings make can shallow make (shallowα) or high (α) (orβ) high angles (β) toangles the exposed to the exposed upper surface;upper surface; (b) flat board(b) flat with board growth with ringsgrowth that rings curve that away curv frome away the exposedfrom the upper exposed surface. upper These surface. are knownThese are as bark-side-upknown as bark-side-up boards; (c) boards; profiled (c pith-side-up) profiled pith-side-up board; and, board; (d) profiled and, (d bark-side-up) profiled bark-side- board. up board. In addition to checking, another less commonly known defect, shelling or loosened grain, develops at2. Materials wood surfaces, and Methods especially those that are exposed to the weather [7,9–11]. Shelling occurs when mechanical or moisture-induced stresses at the surface of flat-sawn boards are sufficiently large to2.1. cause Preparation delamination and Weathering of growth of Larch rings Deckboards at the boundary between latewood and earlywood [9,10]. SuchSix delamination flat-sawn, J-grade, of growth kiln-dried, rings resultsand dressed in the western projection larch of boards latewood cut from above trees the growing surface ofin adjacentthe interior earlywood. of British Two Columbia, forms of BC, shelling Canada, have and been measuring observed 38 at ×flat-sawn 140 × 3658 (tangential) mm3, were surfaces donated that by resultTolko’s from Lavington either thePlaner separation Mill Ltd. of (6200 tips or Jeffers edges Driv of latewoode, Lavington, from British earlywood, Columbia, respectively BC, Canada). [9,10]. ShellingEach parent mainly board develops was cross-cut at the surfaceusing a pendulum of boards orientedsaw (Stromab pith-side-up. ps 50/f, Campagnola It can also occur Emilia, beneath Italy) opaqueto produce coatings six samples, and disrupt each 600 paint mm films in length. on siding Samples (cladding), were then and randomly this is one assigned of the reasonsto one of why five woodendeckboard siding profiles is fixed or the to flat building control, with Table growth 1. Sample rings boards oriented were bark-side then profiled facing on outwards both sides [10 using,11]. Shelling,a moulding particularly machine shelled(Weinig tips Powermat of latewood, 700, canMichae createl Weinig sharp splintersInc., Mooresville, that protrude NC, fromUSA) the equipped surface ofwith wood. a 125 These mm diameter, splinters are two-wing, objectionable cylindrical and dangerous rotary cutter according head (Great-Loc to Koehler SG [9], Positive particularly Clamping when theyUniversal occur Tool at the System, surface Great of deckboardsLakes Custom that Tool people Mfg. walkInc., Pestig on. Shellingo, WI, USA). also After makes double-sided refinishing andprofiling, cleaning each of sample wood surfaceswas cut diintoffi culttwo [using9,10]. a Shelling pendulum is most saw, pronouncedas above. The in sub-samples softwoods with were a pronouncedoriented either diff pith-side-uperence in cell or wall bark-side-up thickness betweenduring the latewood subsequent and earlywood,natural weathering for example, trial. species,A total suchof seventy-two as southern deckboard pine (Pinus samplesspp.), larch each (Larix withspp.), a final and dimension Douglas firof ( Pseudotsuga32 × 130 × menziesii292 mm(Mirb.)3 were Franco)manufactured. [9,10,12 ],The but ends it has of also deckboard been observed samples in hardwoodwere coated with with relatively an epoxy uniform sealer structure, (Intergard such® 740, as yellowInternational poplar Paint (Liriodendron Singapore tulipfera Pty Ltd.,L.) Jurong, [10]. These Singapore) wood speciesto prevent are end very checking important of boards commercially. during Furthermore,conditioning and the woodweathering. products The that decking are negatively samples were affected stored by shellingin a conditioning such as decking room at and 20 siding± 1 °C areand especially 65 ± 5% r.h. important. for at least For three example, months the before market the for outdoor wooden weathering decking and trial. siding in the USA had a combined value in 2018 of ~US 2.8 billion, and each year over 300 million square feet (~28 million m2) of wood siding is installedTable on 1. houses Geometry in theof profiles USA [ 13in ,western14]. However, larch deckboards. wooden siding and decking are losing market share, because they require more maintenance particularly refinishing and cleaning than Profile Type 1 Groove Depth (mm) Groove Radius (mm) Peak Radius (mm) products made from alternative materials, such as plastic and composites (wood-plastic and wood-fibre Flat - - - cement composites) [13,14]. Hence, there is a need to better understand and develop solutions to Rib 2.0 0.16 2.4 problems suchShort as shelling rib that reduce1.5 coating durability and0.16 increase the frequency2.4 of refinishing of wood products usedTall rib for siding and decking.2.5 0.15 2.2 Our understandingRibble of shelling2.0 is based on a small0.65 number of studies1.3 [7 ,9,10], but, unlike grain raising, whichRipple was until recently2.0 a similarly neglected1.0 surface phenomenon1.2 that affects the performance of coatings1 Number on of wood, peaks per no method150 mm was has 20 been for all developed profiles except to quantify flat samples. shelling at weathered wood surfaces. Grain raising was first quantified using stylus profilometry [15], and recent studies haveProfiled used non-contact and flat deckboard profilometry sub-samples to quantify from and image the same grain parent raising western [16,17], larch and examineboard (board the shape 1, 2, recovery3, 4, 5, or of6) earlywoodwere screwed and onto latewood separate beneath sub-frames machined made and from painted preservative-treated radiata pine (Pinus 2 × radiata4-dimensionalD. Don) lumber.