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recycling Article

Skateboards as a Sustainable Recyclable Material Dylan T. Willard and Joseph R. Loferski * Department of Sustainable Biomaterials, Virginia Tech, Blacksburg, VA 24061, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-540-231-4405; Fax: +1-540-231-8868 Received: 25 April 2018; Accepted: 11 May 2018; Published: 15 May 2018

Abstract: The exact number of skateboards manufactured every year is unknown, but it is estimated to be in the millions. Most skateboard decks are made from a high grade of maple (Acer spp.) veneer plywood and typically last only a few months before they break or deteriorate beyond use. Millions of used skateboard decks are discarded annually, ending up in landfills when, instead, they could be recycled into new products. But beyond artistic or aesthetic purposes, material properties of the used skateboard decks are unknown. The objective of this paper is to investigate the material properties of wooden composite panels created by reengineering the skateboard deck material. These aesthetically pleasing wooden panels may be a sustainable recycled product. This paper presents a method of analyzing material properties and structural aspects of used skateboard deck material. Tests were developed to measure the stiffness and strength in bending, moisture content, specific gravity, moisture durability, and species identification. The results show that this process of reengineering skateboard decks makes for a strong wood product and may be useful to those interested in developing new products from recycled materials. Keywords: skateboard; sustainability; recycling; plywood; veneer; composite; wood composite; sugar maple; stiffness; strength; moisture content; specific gravity; delamination; moisture durability; species identification

1. Introduction The Skateboard Skateboarding is an action sport that involves riding and preforming tricks on a skateboard. Skateboarding can be a competitive or recreational sport. Skateboards are composed of three main components: the deck, trucks, and wheels. The deck is a wooden plywood material that a rider stands on. The length of a skateboard deck is often between 787–838 mm (31–33 in.) and the width can be between 196–210 mm (7.75–8.25 in.). The shape of a skateboard deck is slightly concave on the longitudinal axis, with a nose and tail that are both curved upward. Examples of the shape of a skateboard deck can be seen in Figures 1 and 2. A sheet of griptape is applied to the top of the skateboard deck to provide friction between the rider and the board. The trucks allow the skateboard to turn, serving as the axles of the skateboard. Trucks connect the wheels to the deck. Trucks vary in size. The width of the truck is typically equivalent to the width of the skateboard deck. The skateboard trucks are attached near the curved ends at the nose and tail. The wheels, which have ball bearings to reduce friction, are attached to the ends of the truck. Examples of skateboard trucks and wheels are shown in Figures 3 and 4.

Recycling 2018, 3, 20; doi:10.3390/recycling3020020

www.mdpi.com/journal/recycling

Recycling 2018, 3, 20

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Recycling 2018, 3, x

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Figure 1. Skateboard Top View.

Figure 1. Skateboard Top View. Figure 1. Skateboard Top View.

Figure 1. Skateboard Top View.

Figure 2. Skateboard Bottom View. Figure 2. Skateboard Bottom View.

Figure 2. Skateboard Bottom View. Figure 2. Skateboard Bottom View.

Figure 3. Skateboard Front Left Wheel. Figure 3. Skateboard Front Left Wheel. Figure 3. Skateboard Front Left Wheel.

Figure 3. Skateboard Front Left Wheel.


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Figure 4. Skateboard Front Truck.

Figure 4. Skateboard Front Truck. The skateboard deck withstands a lot of abuse during riding and even more when the rider is preforming tricks such as ollies, kickflips, or any trick in which the tail of the board hits the ground. The skateboard deck withstands a lot of abuse during riding and even more when the rider is As the skateboard ages and is used aggressively, the skateboard deck loses its elasticity, the nose and preforming suchshape as ollies, kickflips, any trick theeven tail break of theinboard hits the tailtricks lose their or shorten due toor abrasion, and in thewhich deck can half. While a ground. As the skateboard used the itskateboard deck loses the nose and skateboardages deck and may is look wornaggressively, and unusable after has served its purpose to its the elasticity, rider, the inner layers of plywood veneerdue mayto still retain their structural properties. Aside from in abrasion that occurs tail lose their shape or shorten abrasion, and the deck can even break half. While a skateboard on the edges of the board, most of the damage is only on the surface of the plywood. Even if the deck may look worn and unusable after it has served its purpose to the rider, the inner layers of skateboard deck breaks, there may be potential for it to be recycled. This paper explores this plywood veneer may properties. from abrasion on the possibility thatstill the retain plywoodtheir deckstructural can be reengineered into Aside panels which are strong, that stiff, occurs and edges of the board, most of the damage is only on the surface of the plywood. Even if the skateboard aesthetically pleasing. The objectives this paper for are to; deck breaks, there may beofpotential it to be recycled. This paper explores this possibility that the 1. Develop process in whichinto skateboard plywood material stiff, can beand fabricated into panels. plywood deck can beareengineered panelsdeck which are strong, aesthetically pleasing. 2. Determine strength andare stiffness The objectives of this paper to; in bending of strips used to make the panels. 3.

1. 2. 3. 4. 5.

Measure the moisture content and specific gravity of several used skateboard decks.

4. Determine durability of the adhesives usedmaterial in the skateboard plywoodinto and panels. Develop a processthe in moisture which skateboard deck plywood can be deck fabricated evaluate the propensity for splitting of veneers. Determine strength and stiffness in bending of strips used to make the panels. 5. Identify the species of wood used in the plywood. Measure the moisture content and specific gravity of several used skateboard decks. The goal of this project is to determine how skateboards can be used as a sustainable material. Determine moisture the adhesives used in theskateboard skateboard deck plywood and Materialthe properties weredurability measured to of identify potential uses of recycled material. evaluate There the propensity for splitting veneers. are three different types of of veneer in skateboard decks; face veneer (top and bottom), longitudinal veneer, perpendicular veneer. Longitudinal veneer has the orientation of the wood Identify the species ofand wood used in the plywood. grain running from nose to tail on the skateboard. Face veneer is a sheet of veneer that has a

longitudinal handpickedhow basedskateboards on aesthetic qualities. theas face veneer is the material. The goal of thisorientation, project isbut to is determine can beSince used a sustainable sheet of wood which is exposed on the bottom and top of the skateboard deck it is important to have Material properties were measured to identify potential uses of recycled skateboard material. a high grade of wood veneer with no knots and smooth grain so it is more appealing to consumers. TherePerpendicular are three different typesthat ofhas veneer skateboard decks; toface veneer (top and bottom), veneer is veneer a grainin orientation perpendicular the longitudinal veneer. longitudinal and perpendicular veneer the orientation of the Thisveneer, provides support from side to sideveneer. along the Longitudinal board and helps to uphold has the decks concavity. The plywood is made from seven different sheets of veneer that are layered with the grain orientation, wood grain running from nose to tail on the skateboard. Face veneer is a sheet of veneer that has from top to bottom, as follows; face, longitudinal, perpendicular, longitudinal, perpendicular, a longitudinal orientation, but is handpicked based on aesthetic qualities. Since the face veneer is the longitudinal, and face. The longitudinal orientation of the skateboard deck is the direction that takes sheet of wood which is exposed the bottom top ofThat the isskateboard it is important the largest impacts and has aon significant stressand in bending. why there aredeck five sheets of veneer to have a high grade wood veneer with no knots and smooth grain sheets. so it isThe more appealing to consumers. withof a longitudinal orientation as opposed to two perpendicular perpendicular veneers are often thinner the longitudinal while maintaining symmetry about the longitudinal neutral axis Perpendicular veneer is than veneer that has a veneers, grain orientation perpendicular to the veneer. providing a balanced construction of the plywood. Skateboard manufactures often use aliphatic resin This provides support from side to side along the board and helps to uphold the decks concavity. The plywood is made from seven different sheets of veneer that are layered with the grain orientation, from top to bottom, as follows; face, longitudinal, perpendicular, longitudinal, perpendicular, longitudinal, and face. The longitudinal orientation of the skateboard deck is the direction that takes the largest impacts and has a significant stress in bending. That is why there are five sheets of veneer with a longitudinal orientation as opposed to two perpendicular sheets. The perpendicular veneers are often thinner than the longitudinal veneers, while maintaining symmetry about the neutral axis providing a balanced construction of the plywood. Skateboard manufactures often use aliphatic resin (wood glue) that is a nontoxic, water resistant, and a strong adhesive. They typically cold press the veneers using molds to create the upswept nose and tail and the concavity of the board.


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There are many reasons to recycle skateboards. The number of skateboards manufactured per year (wood glue) that is a nontoxic, water resistant, and a strong adhesive. They typically cold press the is unknown sincethat industries do not publicize that information, but itThey is estimated in thepress millions (wood glue) is a to nontoxic, water resistant, and atail strong cold the [1]. veneers using molds create the upswept nose and and adhesive. the concavity oftypically the board. For example, theare skateboard deckto manufacturing PSthe Stix™, owned by board. Paul Schmitt CEO veneers using molds create the upswept nose company, and tail concavity of the There many to reasons recycle skateboards. Theand number of skateboards manufactured per and Founder, over 300,000 skateboard a year PS Stix™ofbut manufactures skateboard decks There are many reasons to recycle skateboards. The number skateboards manufactured per for year makes is unknown since industries do notdecks publicize that[1]. information, it is estimated in the millions yearFor is unknown since industries do notToy publicize that information, it is estimated inPaul the millions [1]. example, the skateboard deck manufacturing company, PSbut Stix™, owned by Schmitt Element™, FA™, Welcome™, Quasi™, Machine™, and many more brands [1]. With this large [1]. For example, thedecks skateboard manufacturing company, PS Stix™, owned bymanufactures Paul Schmitt CEO Founder, makes overdeck 300,000 skateboard decks year [1]. PS Stix™ number ofand skateboard being produced every year andaconsidering the “wear and tear” they CEO and Founder, makes over 300,000 skateboard decks a year [1]. PS Stix™ manufactures skateboard decks for Element™, FA™, Welcome™, Quasi™, Toy Machine™, and many more brands experience, many boards end up broken or thrown away. A difficulty of recycling skateboard decks is skateboard decks for Element™, FA™, Welcome™, Quasi™, Toy Machine™, and many more brands [1]. With this large number of skateboard decks being produced every year and considering the “wear the logistics of collecting the broken/used decks in sufficient numbers to warrant a business dedicated [1]. With large number of skateboard being produced every away. year and considering “wear and tear”this they experience, many boardsdecks end up broken or thrown A difficulty of the recycling to manufacturing products from them. Collecting skateboard decks at one large central location and tear” they experience, many boards end up broken or thrown away. A difficulty of recycling skateboard decks is the logistics of collecting the broken/used decks in sufficient numbers to warrant would be desirable but likely impractical, because most skateboarders are dispersed geographically. decks is the logistics of collecting the from broken/used decks in sufficient numbers warrant askateboard business dedicated to manufacturing products them. Collecting skateboard decks attoone large A possible solution could involve local skate shops, who sell skateboards, to provide a a business dedicated tobe manufacturing from them. Collecting decks atdispersed onelocation large to central location would desirable butproducts likely impractical, because mostskateboard skateboarders are returngeographically. broken/used skateboard decks. This would allow artisans and businesses access to a potentially central location would be desirable likely impractical, because areto dispersed A possible solution but could involve local skate shops,most whoskateboarders sell skateboards, provide sustainable resource for littlesolution or no cost. involve Furthermore, landfill waste would be reduced because A possible could shops, who sell skateboards, to provide ageographically. location to return broken/used skateboard decks.local Thisskate would allow artisans and businesses access a location return broken/used skateboard decks. Thiscost. would allow and access to a potentially sustainable resource for or no Furthermore, landfill waste would be a product that to was previously discarded haslittle now found its way in toartisans a market ofbusinesses recyclable materials. to a potentially resource for little or no cost.has Furthermore, waste because sustainable a product was previously discarded nowdiscarded found landfill its way in to a would market ofa way With reduced such a substantial amountthat of potentially usable material being every year, findingbe reduced because a product that was previously discarded has now found its way in to a market of recyclable materials. With such a substantial amount of potentially usable material being discarded to repurpose skateboard decks would decrease waste and provide a structural aesthetic material, recyclable a substantial amount of potentially material discarded every year,materials. finding aWith waysuch to repurpose skateboard decks would usable decrease wastebeing and provide a which is the motivation for this project. every year, findingmaterial, a way to repurpose skateboardfor decks would decrease waste and provide a structural aesthetic which is the motivation this project. structural aesthetic material, which is the motivation for this project. 2. Turning Skateboards into Wood-Based Composite Panels 2. Turning Skateboards into Wood-Based Composite Panels 2. Turning Skateboards intowood Wood-Based Composite Panels Skateboards turned into based composite panels, such as the ones in Figures 5 and 6, Skateboards turned into wood based composite panels, such as the ones in Figures 5 and 6, were were processed processed by cutting, sanding, and gluing. Skateboards turned into wood based composite panels, such as the ones in Figures 5 and 6, were by cutting, sanding, and gluing. processed by cutting, sanding, and gluing.

Figure 5. Composite Panels made by cutting skateboard decks into strips and gluing together. Figure 5. Composite Panels made by cutting skateboard decks into strips and gluing together.

Figure 5. Composite Panels made by cutting skateboard decks into strips and gluing together.

Figure 6. Another view of the composite panels in Figure 5. Note the colorful veneers dyed by the Figure 6. Another view of for theaesthetics. composite panels in Figure 5. Note the colorful veneers dyed by the skateboard manufacturers Figure 6. Another view of the composite panels in Figure 5. Note the colorful veneers dyed by the skateboard manufacturers for aesthetics.

skateboard manufacturers for aesthetics.


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Details of the process follow: 1.

2.

3.

4.

5.

6.

Preparation: After initial disassembly of the trucks from the skateboard, the griptape is removed, and leftover adhesive is scraped off the top of the deck. Once completed, the deck is ready for processing. Sanding: The skateboard decks were smoothed using sand paper to remove dirt, paint, hydrophobic material, or stain that may be left on the wood. Sanding also prepares the wood material for gluing. Rough Sizing: Due to the awkward shape of a skateboard deck, cutting using wood working machines, such as a table saw or radial arm saw, can be difficult or dangerous. Because the board is concave, it cannot be placed flat on the bottom side of the skateboard deck. Likewise, due to the nose and tail projecting upward the board cannot lie flat on its top side. A band-saw can be used to safely remove the nose and tail from the ends, allowing the center portion to lay flat for further processing with a radial arm saw or similar tool. At this point the board is cut into three pieces: a nose, tail, and middle section. The middle section of the skateboard is used to create the panels in this project. The nose and tail can be saved for other purposes. Rough cutting: After making the rough cut to remove the nose and tail, the middle portions of the deck can be cut on a radial arm saw. The nose and tail were removed in step 3, the skateboard deck can be placed top side down (i.e., concave down), which allows the board to remain stable when cutting, because the board can sit on the two parallel edges of the concave surface. The fence of the radial arm saw is perpendicular to the direction in which the saw cuts, the edge of the skateboard deck can be held against the fence and ends can be precisely cut to 90◦ . The cut made by the radial arm saw improves the rough cut previously made by the band saw and produces a rectangular middle section. The skateboard deck is now ready to be cut into strips. If the skateboard deck has suffered severe damage or has cracks, these areas can be cut off by the radial arm saw and smaller panels can be made from these zones. Final Cutting: A band saw is used to cut strips out of the middle section of the board, which was made using the radial arm saw in step 4. The band saw has a thin kerf compared to a table saw, which is an advantage when making multiple cuts, because less waste material is produced. Due to the board’s concavity, the strips must be cut with square cross sections and parallel edges. If the middle section is not cut properly, the strips will have an uneven parallelogram like shape, making them difficult to process into panels. Cutting parallel to the long axis of the board (i.e., rip cutting) through the center, multiple times, helps to reduce the skew in the shape of the strip, because the cut will always be made along the high point on the arch of the board’s concavity. Skateboards are manufactured in widths between 196–210 mm (7.75–8.25 in.), with the most common size of 203 mm (8 in.). If the boards are rip cut four times, each strip will be approximately 12.7 mm ( 12 in.) thick. Since the thickness of a skateboard deck is also 12.7 mm ( 12 in.) thick, the strips will be approximately square in cross section. This process will make 16 strips from a middle section of a skateboard deck, of which only 14 strips will be usable, because the edge strips, which were exposed during the skateboards lifetime, are rounded and damaged from usage. Once all of the strips have been cut, they are ready to be glued and pressed together. Gluing: The strips are edge glued into panels, by rotating them so that the face veneers of the original skateboard deck can be glued together. This is a change in orientation from face wise to edge wise, which can be seen in Figures 7 and 8. This change in the orientation of the plywood strips provides increased load capacity and adds an aesthetic quality to the panels. Many skateboard companies dye some veneers various colors, which are exposed when the strips are reoriented in the final product of the panels. Adhesive is the applied to the face veneers of the strips, which are then clamped and left to cure for 24 h. Aliphatic resin was used as the adhesive, because it is water resistant, non-toxic, and easy to use.


7.

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Finishing: After the adhesive cured, a scraper is used to remove excess glue from the panels. A progression of abrasive sanding grits from 80 to 220 was used to smooth rough edges and imperfections. Examples of the finished product can be seen in Figures 5 and 6. Recycling 2018, 3, x

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Figure 7. Face Wise Strip Orientation.

Figure 7. Face Wise Strip Orientation. Figure 7. Face Wise Strip Orientation.

Figure 8. Edge Wise Strip Orientation. Figure 8. Edge Wise Strip Orientation.

3. Measuring Material Properties Figure 8. Edge Wise Strip Orientation. 3. Measuring Material Properties 3.1. Stiffness and Strength in Bending

3. Measuring Material Properties

3.1. Stiffness and Strength in Bending To determine the stiffness and strength of used skateboard deck plywood material, samples were made from the skateboard decks which were approximately 303 mm (12 in.) long (longitudinal To Strength determinein theBending stiffness and strength of used skateboard deck plywood material, samples 3.1. Stiffness and direction of the face veneer) and 12.7 mm (0.5 in.) by 12.7 mm (0.5 in.) in cross section (d). The samples were made from the skateboard decks which were approximately 303 mm (12 in.) long (longitudinal were tested in third point bending to failure using a MTS Systems Corporation™ 22,000 kg (10,000 To determine and of in.) used deck plywood material, samples were direction ofthe thestiffness face veneer) andstrength 12.7 mm (0.5 by skateboard 12.7 mm (0.5 in.) in cross section (d). The samples lbs.) capacity computer controlled testing machine over a span (L) of 254 mm (10 in.) (L/d = 20), where tested in thirddecks point bending to failure using a MTS Systems Corporation™ 22,000 kg (10,000 direction made fromwere the skateboard which were approximately 303 mm (12 in.) long (longitudinal the load points were 95.3 mm (3.75 in.) apart. The advantage of third point bending is that it provides lbs.) capacity computer controlled testing machine over a span (L) of 254 mm (10 in.) (L/d = 20), where of the faceaveneer) 12.7 mm of (0.5stress in.) byover 12.7 amm (0.5 in.) cross center sectionpoint (d). The samples were greater and distribution larger areain than bending. the load points were 95.3 mm (3.75 in.) apart. The advantage of third point bending is that it provides Figures 9 and bending 10 show thetotesting arrangement. tested in third point failure using a MTS Systems Corporation™ 22,000 kg (10,000 lbs.) a greater distribution of stress over a larger area than center point bending. Figures 9 and 10 show the testingmachine arrangement. capacity computer controlled testing over a span (L) of 254 mm (10 in.) (L/d = 20), where the

load points were 95.3 mm (3.75 in.) apart. The advantage of third point bending is that it provides a greater distribution of stress over a larger area than center point bending. Figures 9 and 10 show the testing arrangement.


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Two material properties were measured, the stiffness (EI) and moment at failure (FS), which is a measure of strength. A computer data acquisition system continuously recorded load and deformation during the test. Load was measured by an electronic load cell and deflection was recorded by a 25.4 mm (1 in.) travel LVTD (Linear Variable Differential Transducer). Stiffness of the board is used to calculate deflection for a given load in a linear range of the stress/strain behavior. Stiffness Recycling(EI) 2018, is 3, xcomputed by the following formula: 7 of 18 Two material properties were EImeasured, = Pa(3L2the − stiffness 4a2 )/(24(EI) × and ∆) moment at failure (FS), which is a measure of strength. A computer data acquisition system continuously recorded load and 2 ) measured by an electronic load cell and deflection was deformation during the 2test. was where: EI = Stiffness (kg-mm ) or Load (lbs-in recorded by a 25.4 mm (1 in.) travel LVTD (Linear Variable Differential Transducer). Stiffness of the is used deflection for a or given load in a linear range of the stress/strain behavior. P = Q/2board = The load to atcalculate each load point (kg) (lbs.) Stiffness (EI) is computed by the following formula:

Q = Total load in the linear range below proportional limit (kg) or (lbs.) EI = Pa(3L2 − 4a2)/(24 × Δ) L = Span (mm) or (in.) a = Distance load(kg-mm points2) (mm) or2)(in) where:between EI = Stiffness or (lbs-in ∆ = Deflection at load Q (mm) or (in) P = Q/2 = The load at each load point (kg) or (lbs.)

(1)

(1)

Q = Total load in the linear range below proportional limit (kg) or (lbs.)

Moment (FS) is computed by the following formula: L = Span (mm) or (in.) a = Distance between load points (mm) or (in) FS = MMax Δ = Deflection at load Q (mm) or (in)

(2)

Moment (FS) is computed by the following formula:

where: FS = Maximum Moment (kg-mm) or (lbs.-in.)

FS = MMax

(2)

F = Stress at failure (kpa) or (psi.) where: FS = Maximum Moment (kg-mm) or (lbs.-in.) S = Section modulus (mm3 ) or (in.3 ) F = Stress at failure (kpa) or (psi.) MMax = PMax × L/3 (mm-kg) 3or (in.-lbs.) S = Section modulus (mm ) or (in.3) PMax = Q = ×Maximum atoreach load point MMax Max =/2 PMax L/3 (mm-kg) (in.-lbs.) Max = Q Max/2 = Maximum at each loadby point QMax = PTotal maximum load measured load cell (kg) or (lbs.) Q Max = Total maximum load measured by load cell (kg) or (lbs.) L = Span (mm) or (in.) L = Span (mm) or (in.)

Figure 9. Third point bending setup in the face wise direction showing load points and linear variable

Figure 9. Third point bending setupmeasuring in the face wise direction load points and linear variable differential transformer (LVDT) deflection at center showing line. differential transformer (LVDT) measuring deflection at center line.


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Figure 10. Third point bending setup in the edge wise direction showing load points and LVDT

Figure 10. Thirddeflection point bending setup in the edge wise direction showing load points and LVDT measuring at centerline. measuring deflection at centerline. Two orientations of the plywood strips were tested, where force was applied parallel to the width (edge wise) and parallel to the thickness where the face grain is parallel to the span (face wise) Two orientations of the plywood strips were tested, where force was applied parallel to the width as shown in Figures 7–10. This provided data on the board strength and stiffness parallel and (edge wise) and parallel to axis the thickness where the face to the (facefrom wise) as shown perpendicular to the of the plywood. Because thegrain stripsisinparallel the panels are span orientated face in Figures Thiswise provided on the board and stiffness and perpendicular to wise7–10. to edge during data the assembly of the strength panels (Figure 7 and 8), parallel tests were done in both to compare material the properties. Thethe results of face orientation are shown in Table 1. wise the axisdirections of the plywood. Because strips in panels arewise orientated from face wise to edge 2 (3740.9 lbs-in.2) and the strength was 1840 kg-m stiffness facepanels wise was 1,095,0007 kg-mm during The the average assembly of the (Figures and 8), tests were done in both directions to compare (159.7 lbs.-in.). The results of edge wise orientation are shown in Table 2. The average stiffness face material properties. The results of face wise orientation are shown in Table 1. The average stiffness face 2 (4771.2 lbs-in.2) and the strength was 2112 kg-mm (183.3 lbs.-in.). wise was 1,396,000 kg-mm 2 2

wise was 1,095,000 kg-mm (3740.9 lbs-in. ) and the strength was 1840 kg-m (159.7 lbs.-in.). The results of edge wise orientation are shown in Table 2. The 1,396,000 kg-mm2 Table 1. Maximum Load at failure, Strength, andaverage stiffness ofstiffness Face Wise face Stripswise tested was in bending. 2 (4771.2 lbs-in. ) and the strength was 2112 kg-mm (183.3 lbs.-in.). FS EI EI (kg-mm) (lbs-in.²) (kg-mm2) Table 1. Maximum Load 105.6 at failure, Strength, Wise Strips tested in bending. 1F 47.9 and stiffness 178.2 of Face2053 4336 1,269,000 2F 98.1 44.5 165.6 1908 4018 1,176,000 Sample3F QMax 99.5 QMax 45.1 FS 167.9 FS 1934 EI EI 4201 1,229,000 2 2 Number4F (lbs.) 66.5 (kg) 30.2 (lbs.-in.)112.2 (kg-mm) (kg-mm 1293 (lbs-in. 3354) 982,000 ) 3332 975,000 1F 5F 105.6 78.6 47.9 35.7 178.2 132.7 2053 1529 4336 1,269,000 793 232,000 2F 6F 98.1 89.6 44.5 40.6 165.6 151.2 1908 1742 4018 1,176,000 4948 1,448,000 3F 7F 99.5 119.4 45.1 54.2 167.9 201.5 1934 2322 4201 1,229,000 4946 1,447,000 4F 8F 66.5 99.7 30.2 45.2 112.2 168.2 1293 1938 3354 982,000 5F 78.6 35.7 132.7 1529 3332 COV (%) 975,000 Face Wise Average STDV 6F 89.6 40.6 151.2 1742 793 EI (lbs-in.2) 3741 1253 33.5 232,000 7F(kg-mm2) 119.4 54.2 201.5 2322 4948 1,448,000 EI 1,095,000 367,000 8F 99.7 45.2 168.2 1938 4946 FS (lbs.-in.) 159.7 25.79 16.11,447,000 (kg-mm) FaceFSWise Average 1840 STDV 297 COV (%) Sample Number

EI (lbs-in.2 ) EI (kg-mm2 ) FS (lbs.-in.) FS (kg-mm)

QMax (lbs.)

3741 1,095,000 159.7 1840

QMax (kg)

FS (lbs.-in.)

1253 367,000 25.79 297

33.5 16.1


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Table 2. Maximum Load at failure, Strength, and stiffness of Edge Wise Strips tested in bending. Sample Number

QMax (lbs.)

QMax (kg)

FS (lbs.-in.)

FS (kg-mm)

EI (lbs-in.2 )

EI (kg-mm2 )

9E 10E 11E 12E 13E 14E 15E 16E 17E 18E 19E 20E 21E 22E 23E

116.1 113 118.8 119.7 105 116.7 113.6 102.8 75.3 73.9 110.7 121.4 117.2 105.2 119.8

52.7 51.3 53.9 54.3 47.6 52.9 51.5 46.6 34.2 33.5 50.2 55.1 53.2 47.7 54.3

196 190.7 200.4 201.9 177.2 196.9 191.6 173.5 127.1 124.7 186.7 204.9 197.8 177.6 202.2

2258 2197 2309 2326 2042 2269 2207 1999 1464 1437 2151 2361 2279 2046 2330

4950 4707 5122 5254 4407 5486 4728 5084 3181 2996 5032 5137 5419 4689 5376

1,449,000 1,377,000 1,499,000 1,538,000 1,290,000 1,605,000 1,384,000 1,488,000 931,000 877,000 1,473,000 1,503,000 1,586,000 1,372,000 1,573,000

Edge Wise EI (lbs-in.2 ) EI (kg-mm2 ) FS (lbs.-in.) FS (kg-mm)

Average 4771 1,396,000 183.3 2112

STDV 721.6 21,100 24.39 281

COV (%) 15.1 13.3

3.2. Moisture Content and Specific Gravity Moisture content (MC) and specific gravity (SG) were measured according to ASTM D 4442 [2] and ASTM D 2395 [3] on 12 samples of skateboard decks. The samples were approximately 76.2 mm (3 in.) long by 102 mm (4 in.) wide and were weighed to the nearest 0.01 g before placing them in the oven. After 24 h in the oven, the oven dry mass of the samples was recorded and the volume was measured by the water immersion method using paraffin wax coating to minimize water absorption into the sample. Specify gravity was measured using oven dry mass and oven dry volume. Moisture content was calculated according to Equation (3): MC% = (AD − OD)/OD × 100

(3)

where: MC% = Moisture Content in Percent AD = Air Dry initial Mass (g) OD = Oven Dry Mass (g) The MC results are in Table 3. The mean MC was 9.76%, standard deviation (STDV) was 1.07%, and the coefficient of variation (COV) was 11.0%. The MC ranged from 6.4% to 10.5%. Specific gravity was then calculated according to Equation (4): SG = (OD/V)/D

(4)

where: SG = Specific Gravity V = Volume (cm3 ) D = Density of Water (1 g/cm3 ) The SG results are in Table 3. The mean SG was 0.72, standard deviation was 0.018, and the coefficient of variation was 2.5%. The SG ranged from 0.698 to 0.755.


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Table 3. Moisture Content and Specific Gravity Data. Sample Number

Moisture Content %

Specific Gravity

1 2 3 4 5 6 7 8 9 10 11 12

10.12 10.09 10.1 10.14 9.72 9.72 6.36 9.35 10.44 10.22 10.46 10.33

0.714 0.699 0.711 0.712 0.745 0.749 0.755 0.728 0.698 0.709 0.716 0.712

Moisture Content Average Moisture Content STVD Moisture Content COV % Specific Gravity Average Specific Gravity STVD Specific Gravity COV %

9.76 1.07 10.97 0.721 0.0183 2.541

3.3. Moisture Durability The moisture durability of the skateboard deck was tested using ANSI/HVPA EF 2009 [4] soak/dry test except the soak times were modified. The purpose of this test is to determine the moisture resistance of the adhesive and skateboard deck plywood material. Defects such as delamination and splitting of the veneers were evaluated. Four different skateboard decks were tested. The test uses a 4 h soak and 24 h dry cycle at 53 ◦ C. The second cycle uses a 24 h soak and a 24 h dry cycle at 53 ◦ C. At the end of each cycle the samples are evaluated for delamination, splits, and surface checks. “The sample is considered as failing when any single delamination between two plies is greater than 50.8 mm (2 in.) long, more than 6.35 mm (0.25 in.) deep and 0.076 mm (0.003 in.) in width as determined with a feeler gauge.” (ANSI/HVPA EF 2009 [4]). The MC after each cycle was computed. The data was entered into a spreadsheet and is shown in Table 4. After completing the test, all of the samples warped and split, as seen in Figures 11 and 12, which show the top and bottom surfaces after the test. Two of the samples (samples 2 and 4) delaminated and failed the test (Figures 13–16). Moisture contents of the samples and failure evaluation can be seen in Table 4. Table 4. Moisture Durability Test. Sample Number

Initial Moisture Content (%)

MC After 4 h Soak/24 h Dry (%)

MC After 24 h Soak/24 h Dry (%)

1 2 3 4

6.96 6.34 6.18 5.66

5 4.23 4 3.93

4.78 4.23 3.89 3.58

Delamination (Yes/No) No Yes No Yes

Top Split (Yes/No)

Bottom Split (Yes/No)

Yes Yes Yes Yes

Yes Yes Yes Yes


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Figure 11. Top view of samples after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]).

Figure 11. Top view of samples after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]). Figure 11. Top view of samples after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]).

Figure 12. Bottom view of samples after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]). Figure 12. Bottom view of samples after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]).

Figure 12. Bottom view of samples after soak/dry moisture durability test (ANSI/HVPA EFFailure 2009 [4]). Samples 2 and 4 delaminated, as seen in Figures 13–16 and therefore failed the test. in sample 4 occurred adhesive. Failure sample 2 occurred in both the adhesive and in the wood. Samples 2 andin4the delaminated, as seeninin Figures 13–16 and therefore failed the test. Failure in Normally, an adhesive bond has 100% woodand failure and no failed delamination. sample 4 occurred in the adhesive. Failure ininsample 2 occurred in both the adhesive and theFailure wood. Samples 2when and 4stressed, delaminated, as seen Figures 13–16 therefore theintest. Failure in should occur in in the wood because the adhesive should stronger than the itself. Therefore, Normally, when stressed, an adhesive bondinhas 100% be wood failure and nowood delamination. Failure sample 4 occurred the adhesive. Failure sample 2 occurred in both the adhesive and in the the adhesives used inwood samples 1 andthe 3 had good water resistance andthan samples 2 and 4itself. had poor water should occur in the because adhesive should be stronger the wood Therefore, wood. Normally, when stressed, an adhesive bond has 100% wood failure and no delamination. resistance. the adhesives used in samples 1 and 3 had good water resistance and samples 2 and 4 had poor water Failure should occur in the wood because the adhesive should be stronger than the wood itself. Delamination can be detrimental to the recycled skateboard panels. If the panels are exposed to resistance. Therefore, the adhesives used in samples 1 and 3 had good waterofresistance and samples 2 andof4 had a high moisture environment, delamination could occur because the poor Delamination can be detrimental to the recycled skateboard panels. If themoisture panels areresistance exposed to poor athe water resistance. adhesive which was useddelamination by the original skateboard deck manufacturer. The panels can be high moisture environment, could occur because of the poor moisture resistance of Delamination be detrimental to original the recycled skateboard panels. Ifdamage theThe panels are exposed finished with which a can water resistant such as skateboard polyurethane, to manufacturer. minimize from moisture. the adhesive was used material by the deck panels can be to Testing the moisture with the such simple test can identify plywood material suitableof the a highfinished moisture environment, delamination could occur because the poor moisture resistance with a waterdurability resistant material assoak/dry polyurethane, toof minimize damage from moisture. for dry or moisture environments. Testing thehigh moisture durability with the simple soak/dry test can identify plywood material adhesive which was used by the original skateboard deck manufacturer. The panels cansuitable be finished

dry orresistant high moisture environments. with for a water material such as polyurethane, to minimize damage from moisture. Testing the moisture durability with the simple soak/dry test can identify plywood material suitable for dry or high moisture environments.


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Figure 13. Cross section view of sample 2 showing delamination after soak/dry moisture durability

Figure 13. Cross section view of sample 2 showing delamination after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]).of sample 2 showing delamination after soak/dry moisture durability Figure 13. Cross section view test (ANSI/HVPA EF 2009 [4]). test (ANSI/HVPA EF 2009 [4]).of sample 2 showing delamination after soak/dry moisture durability Figure 13. Cross section view test (ANSI/HVPA EF 2009 [4]).

Figure 14. Face wise close up view of sample 2 showing delamination after soak/dry moisture durability (ANSI/HVPA EF 2009 Figure 14. test Face wise close up view[4]). of sample 2 showing delamination after soak/dry moisture Figure 14. Face wise close up view of sample 2 showing delamination after soak/dry moisture durability (ANSI/HVPA EF 2009 Figure 14.test Face wise close up view[4]). of sample 2 showing delamination after soak/dry moisture

durability test (ANSI/HVPA EF 2009 [4]).

durability test (ANSI/HVPA EF 2009 [4]).

Figure 15. Face wise wide view of sample 2 showing delamination after soak/dry moisture durability test (ANSI/HVPA EFwide 2009view [4]). of sample 2 showing delamination after soak/dry moisture durability Figure 15. Face wise test (ANSI/HVPA EF 2009 [4]). Figure 15. Face wise wide view of sample 2 showing delamination after soak/dry moisture durability

Figure 15. Face wise wide view of sample 2 showing delamination after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]). test (ANSI/HVPA EF 2009 [4]).

Recycling Recycling 2018, 2018, 3, 3, 20 x

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Figure 16. Face wise view of sample 4 showing delamination after soak/dry moisture durability test

Figure 16. Face wise view of sample 4 showing delamination after soak/dry moisture durability test (ANSI/HVPA EF 2009 Figure 16. Face wise view[4]). of sample 4 showing delamination after soak/dry moisture durability test (ANSI/HVPA EF 2009 [4]). (ANSI/HVPA EF 2009 [4]). Splitting parallel to the grain occurred in each sample penetrating through the first two layers of longitudinal veneer, as seen in Figures 17 and 18. Because the grain of the topthe twofirst veneers and of Splitting through two layers Splitting parallel parallel to tothe thegrain grainoccurred occurredinineach eachsample samplepenetrating penetrating through the first two layers bottom two veneers is orientated parallel to the length of the skateboard, formations of splits due to longitudinal veneer, as seen in Figures 17 and 18. Because the grain of the top twotop veneers and bottom of longitudinal veneer, as seen in Figures 17 and 18. Because the grain of the two veneers shrinkage are not restrained by a cross veneer. Therefore, splits that occur in the top two layers of and two veneers is orientated parallel to the length of length the skateboard, formations of splits due to shrinkage bottom twostop veneers orientated parallel to the the skateboard, splits due to veneer whenisthey reach the perpendicular veneer of (Figures 17 and 18).formations In sample 4,ofthere was are not restrained by a cross veneer. Therefore, splits that occur in the top two layers of veneer stop shrinkage are not restrained by a cross veneer. Therefore, splits that occur in the top two layers of one case where a split penetrated through the perpendicular cross veneer and into the middle of when they reach the perpendicular veneer (Figures 17 and 18). In sample 4, there was one case where longitudinal veneer, as seenthe in Figures 19 and 20. The split occurred where theInskateboard veneer stop when they reach perpendicular veneer (Figures 17 and 18). sample 4,trucks there was aone split penetrated the cross veneer and into middle of longitudinal veneer, were installed. This area is perpendicular highlythrough stressed the while the skateboard is the in use and and the holes are case where athrough split penetrated perpendicular cross veneer into which the middle of as seen in Figures 19 and 20. The split occurred where the skateboard trucks were installed. This area drilled through theasplywood weaken area.occurred where the skateboard trucks longitudinal veneer, seen inalso Figures 19 the andsurrounding 20. The split

is highly stressed while theisskateboard is in use andthe theskateboard holes whichisare drilled thewhich plywood were installed. This area highly stressed while in use andthrough the holes are also weaken the surrounding area. drilled through the plywood also weaken the surrounding area.

Figure 17. Cross Section View Showing Splitting in Sample 2.

Figure 17. Cross Section View Showing Splitting in Sample 2. Figure 17. Cross Section View Showing Splitting in Sample 2.


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Figure 18. Cross Section View Showing Splitting in Sample 3. Figure 18. Cross Section View Showing Splitting in Sample 3.


Figure 19. Top View Showing Splitting in Sample 1. Figure 19. Top View Showing Splitting in Sample 1.

Figure 19. Top View Showing Splitting in Sample 1. Figure 19. Top View Showing Splitting in Sample 1.

Figure 20. Cross Section View Showing Four Layer Split in Sample 1. Figure 20. Cross Section View Showing Four Layer Split in Sample 1. Figure 20. Cross Section View Showing Four Layer Split in Sample 1.

Figure 20. Cross Section View Showing Four Layer Split in Sample 1.


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3.4. Species RecyclingIdentification 2018, 3, x

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The most common type of wood used to make skateboards is sugar maple (Acer saccharum). 3.4. Species Identification Sugar3.4. maple is used because it is a very stiff and strong wood with an average specific gravity of Species Identification The most common type of wood to make is sugar maple (Acer saccharum). 0.63 [5], it works well with aliphatic resin used adhesives [6],skateboards and is abundant in North America. Microscopic SugarThe maple used because a veryused stiff and strong wood withisansugar average specific gravity of 0.63 mostis common typeitofiswood to make skateboards maple (Acer saccharum). wood identification techniques were used to confirm that the wood in the skateboards used in this [5], it works with aliphatic adhesives andwood is abundant North specific America. Microscopic Sugar maple well is used because it isresin a very stiff and [6], strong with anin average gravity of 0.63 project was sugar maple. Samples from five different boards were examined. wood identification techniques to confirm that the wood in North the skateboards used in this [5], it works well with aliphatic were resin used adhesives [6], and is abundant America. Microscopic First, awas stereo microscope was used at lower magnification (10 × to 40×) to view the general projectidentification sugar maple. Samples from fivetodifferent examined. wood techniques were used confirmboards that thewere wood in the skateboards used in this characteristics the maple. wood. A razorblade usedmagnification to make a clean cut through section First, stereo microscope was used atdifferent lower (10× to 40×) to viewthe thecross general project wasaof sugar Samples from fivewas boards were examined. of thecharacteristics plywood to reveal the wood anatomy. Figures 21–24 show the cross sections of two parallel of themicroscope wood. A razorblade used to make a clean(10× cut to through cross section of First, a stereo was used was at lower magnification 40×) tothe view the general longitudinal veneers, which top and bottom veneers. Several anatomical were the plywood to theare wood anatomy. Figures 21–24 thecut cross sections of two parallel characteristics ofreveal the wood. A the razorblade was used to makeshow a clean through the features cross section of used longitudinal veneers, are theanatomy. top andof bottom Several features were used the plywood toofreveal the wood Figures 21–24 the anatomical cross of atwo parallel to confirm that all the which boards were made sugarveneers. mapleshow including; thesections wood is diffuse porous to confirm that all of the boards were made of sugar maple including; the wood is a diffuse porous longitudinal veneers, which are the top and bottom veneers. Several anatomical features were used hardwood with uniseriate and multiseriate rays of two sizes, larger rays that are approximately the hardwood with uniseriate and multiseriate rays of rays two sizes, larger rays that are at approximately the confirm that vessel all of the boards were made of sugar maple thevisible wood is alow diffuse porous sameto width as the diameter and the smaller thatincluding; are barely magnifications, same widthwith as the vessel diameter and the smaller barely visible at low magnifications, hardwood uniseriate and multiseriate rays of rays two that sizes,are larger rays that are approximately the and most of the vessels are solitary as compared to red maple, which often has radial multiples of and most of as thethe vessels solitaryand as the compared redthat maple, which visible often has radial multiples of same width vesselare diameter smallerto rays are barely at low magnifications, vessels up to four cells wide [5]. Color was not used as an identification feature because veneers in the vessels upof to the fourvessels cells wide [5]. Coloraswas not usedtoasred an maple, identification because in the and most are solitary compared which feature often has radialveneers multiples of skateboard decks are cells often dyed totovivid colors for marketing purposes. skateboard decks often vivid colors for marketing purposes. feature because veneers in the vessels up to four are widedyed [5]. Color was not used as an identification skateboard decks are often dyed to vivid colors for marketing purposes.

Figure 21. Species Identification Sample 1 20×.

Figure 21. Species Identification Sample 1 20×. Figure 21. Species Identification Sample 1 20×.




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Figure 24.using Species Identification Sample 4 20 . The species was also identified higher magnification (100× to× 400×) with a compound light microscope. Samples wereidentified preparedusing by producing a thin layer(100× (10 μm) through tangential light face The species was also higher magnification to 400×) withthe a compound using a microtome. The samples were placed onmagnification a glass microscope with of water and a light microscope. Samples were prepared by producing a thin layer (100 (10 slide, μm) the tangential face The species was also identified using higher × tothrough 400×a) drop with a compound glass cover slip. Microscopic features such as alternate intervessel pitting, simple perforation plates, using a microtome. The samples were placed on a glass microscope slide, with a drop of water and a face microscope. Samples were prepared by producing a thin layer (10 µm) through the tangential and spiral thickenings were observed at 100× and 400×intervessel magnification andsimple are shown in Figures 25 glass cover slip. Microscopic features such as alternate pitting, perforation plates, using a microtome. The samples were placed on a glass microscope slide, with a drop of water and and 26. spiral thickenings were observed at 100× and 400× magnification and are shown in Figures 25 a glass cover slip. Microscopic features such as alternate intervessel pitting, simple perforation plates, and 26.

and spiral thickenings were observed at 100× and 400× magnification and are shown in Figures 25 and 26.

Recycling 2018, 3, 20 Recycling Recycling 2018, 2018, 3, 3, xx

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Figure Figure 25. 25. Tangential Tangential view view of of face face veneer veneer showing showing alternate alternate intervessel intervessel pitting, pitting, spiral spiral thickenings, thickenings, Figure 25. Tangential view of face veneer showing alternate intervessel pitting, spiral thickenings, and and and cross cross section section of of sample sample perforation perforation plates plates (400×) (400×) that that are are characteristic characteristic of of sugar sugar maple maple (Acer (Acer cross section of sample perforation plates (400×) that are characteristic of sugar maple (Acer saccharum). saccharum). saccharum).

Figure 26. Tangential view face veneer showing alternate intervessel pitting, spiral thickenings, Figure 26. 26. Tangential Tangentialview viewofof offace faceveneer veneer showing alternate intervessel pitting, spiral thickenings, Figure showing alternate intervessel pitting, spiral thickenings, and and cross section of sample perforation plates (100×) that are characteristic of sugar maple (Acer and cross section of sample perforation plates (100×) that are characteristic of sugar (Acer cross section of sample perforation plates (100 ×) that are characteristic of sugar maple (Acermaple saccharum). saccharum). saccharum).

4. Summary and Conclusions 4. 4. Summary Summary and and Conclusions Conclusions 4.1. Summary 4.1. 4.1. Summary Summary It is estimated that millions of skateboards are produced annually and often have a lifetime of It is estimated that of skateboards are annually and have aa lifetime of is months estimated that millions millions skateboards are produced produced annually and often often lifetime only It few because of wear of and tear on the deck. The decks of skateboards arehave typically made of of only few months because of wear and tear on the deck. The decks of skateboards are typically made only few laminated months because of wear and(Acer tear on the deck. veneers. The decks of skateboards areseveral typically made plywood from sugar maple saccharum) This paper presents methods of of plywood plywood laminated laminated from from sugar sugar maple maple (Acer (Acer saccharum) saccharum) veneers. veneers. This This paper paper presents presents several several methods of testing material properties for use in reengineering skateboard plywood decks into methods of testing material properties for use in reengineering skateboard plywood decks into small small panels, panels, which which can can be be used used as as aa raw raw material material for for aa variety variety of of purposes. purposes.


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of testing material properties for use in reengineering skateboard plywood decks into small panels, which can be used as a raw material for a variety of purposes. Panels were fabricated from used skateboard decks by a process of initial preparation, sanding, rough sizing, cutting, gluing, and finishing. The curvature of the original skateboard decks was eliminated by selectively sawing strips for laminating the panels. Because the skateboard panels have a myriad of beautiful colors, many possible applications exist for using the panels. For example, artisans can produce wood composite items such as parquet flooring, furniture, and other artistic creations where color is of interest. This paper demonstrates that panels made from recycled skateboard decks have excellent material properties (Strength (FS) and Stiffness (EI)), which allows them to be utilized in load bearing applications. A limitation of the panels made and tested in this project is their relatively small physical size. However, the strips can be offset or staggered during panel manufacturing to produce larger panels for a wide variety of applications limited only by the imagination of the user. 4.2. Conclusions (1)

(2) (3)

(4)

Strength and Stiffness of the strips in two orientations were measured in third point bending. The average stiffness (EI) of the strips face wise and edge wise were 1,095,000 kg-mm2 (3740.9 lbs-in.2 ) and 1,396,000 kg-mm2 (4771.2 lbs-in.2 ) respectively. The average strength (FS) of the strips face wise and edge wise were 1840 kg-m (159.7 lbs.-in.) and 2112 kg-mm (183.3 lbs.-in.) respectively. Average moisture content of the skateboard decks at the time of testing was 9.76%. Average specific gravity of the skateboard decks was 0.721. Moisture durability tested according to ANSI/HVPA EF 2009 showed that half of the samples failed by delamination. Therefore, adhesives used in skateboard deck manufacturing have poor to moderate moisture resistance. All the test samples had splits which penetrated through the top and bottom veneers. The species used in all the samples tested was sugar maple (Acer saccharum).

Author Contributions: D.W.: Conception of idea, development and implementation of research plan, fabrication of samples, testing, co-writer of manuscript. J.L.: Project supervisor and advisor, testing, photo micrographs, co-writer of manuscript. Conflicts of Interest: The authors declare there is no conflict of interest.

References 1. 2. 3. 4. 5. 6.

Schmitt, P. (CEO and Founder, Los Angles, CA, USA); Stix, P.S. (Los Angles, CA, USA). Personal Communication, 2017. ASTM D4442. Standard Test Methods for Direct Moisture Content Measurement of Wood and Wood-Based Materials; Annual Book of ASTM Standards 2016; ASTM International: West Conshohocken, PA, USA, 2016. ASTM D2395. Standard Test Methods for Density and Specific Gravity of Wood and Wood-Based Materials; Annual Book of ASTM Standards 2016; ASTM International: West Conshohocken, PA, USA, 2016. American National Standard. American National Standard for Engineered Wood Flooring (ANSI/HVPA EF 2009); Bond Line Test; American National Standard Institute (ANSI): New York, NY, USA, 2009; 12p. Bruce Hoadley, R. Understanding Wood: A Craftsman’s Guide to Wood Technology; The Taunton Press: Newtown, CT, USA, 2000; p. 280. The Nitty Gritty Materials. Skateboard Builder Directory, Ministry of Wood. Available online: ministryofwood. com/the-nitty-gritty-materials/ (accessed on 21 February 2018). © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).