CA1275620C - High density corrugated wafer board panel product - Google Patents

Abstract

"HIGH DENSITY CORRUGATED WAFER BOARD PANEL PRODUCT"

ABSTRACT OF THE DISCLOSURE
A 'high density' corrugated wafer board panel is provided. The wafer board panel has a substantially uniform density ranging from between about 700 kg/m3 to 900 kg/m3.
As a result of increasing the density of the panel without changing the panel weight per projected unit area, a panel having improved overall flexure performance properties is provided.

Description

1Field of the Invention 2The present invention relates to a 'high density' 3wafer board panel having a corrugated, or wave-like, 4configuration.

6Typically, a wafer board panel comprises layers of 7wood flakes or wafers formed into a composite structure using 8a resinous binder. The preparation of wafer board panels is 9complex, but broadly consists of two principal stages. The 10first stage comprises the preparation of the wafers and 11admixing thereof with the binder to form a loose layer or 12mat; the second stage involves su~sequent compression and 13heating of the mat to cure the resin and form the 4consolidated panel.
5At present, wafer board is usually manufactured in 16the form of planar or flat sheets. ~afer board is a 17recognized structural panel, finding wide application in the 18construction industry, particularly as a plywood substitute 19in residential constructïon.
20Improvement in performance characteristics of flat 21wafer board panels has been attained by optimization of such 22parameters as wafer orientation, wafer geometry, resin 23selection and content, and the like.

2~After exhaustive optimization studies of planar 25wafer board it was postulated that its flexural strength 26characteristics could be improved if a corrugated 27configuration wa~ imparted thereto. q e f=ndamental concep~

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l of corrugating materials to thereby improve the structural 2 properties is not a novel one. Indeed, corrugated wafer 3 board ~er se has previously been manufactured in the 4 industry. However, the wafer board panels prepared by these prior art techniques do not have the capability of economical 6 industrial manufacture or the desired structural strength 7 properties because they do not have a substantially uni~orm 8 density.
9 In a recent advance, as disclosed in my U.S. Patent 4,616,991, an apparatus for manufacturing a corrugated wafer ll board panel ha~ing a substantially uniform density was l2 developed. The ~reakthrough disclosed ~y the patent referred l3 to supra, resided in the apparatus being adapted to avoid 14 haring to 'stretch' a planar mat into a corrugated conformation. Stretching, which had always been present in 16 the prior art methods, would result in a final product 17 exhibiting an uneven distribution of wood flakes and hence 18 non-uni~orm density.
l9 This prior apparatus involved a pair of opposed, spaced-apart, upper and lower platens. Each platen was 21 formed of adjacent lengths of chain-like links. When the 22 lengths were pushed inwardly from the side, they would shift 23 from a planar to a sawtooth-like form. In doing so, the 24 length of the non-undulating space between the platens in the second stage would be generally the same as the length of the 26 planar space ~etween the platens in the first stage. The 27 process involved in using the apparatus was initiated by 28 distributing a mat of loose binder-coated wood wafers between 29 said platens. A pre-compression step was conducted by f~ 7~ 'f~
1 biasing the platens together, the biasing force being applied 2 in a vertical direction, to substantially fix the wafers, 3 thereby limiting their further movement. The platens were 4 then biased from the side to convert them from their planar configuration to the corrugated configuration. Heat and 6 further pressure were applied to cure the binder and produce 7 the panel of uniform density.
8 The density of raw wood varies considerably. HoweYer~
9 by the completion of the compaction and curing processes, the density of the produced wafer board will have been increased, 11 typically by a value of about fifty percent more than that of 12 raw wood.
13 It is recognized that the industry selects the 14 density of wafer board panels so as to provide the optimum structural strength commensurate with the lowest price in 16 terms of raw materials and manufacturing costs. So, for a 7 typical planar (or flat) wafer board panel, its selected 18 density would ~e of the order of about 640 kg/m3.
19 It is generally known that if one were to increase the density of a planar wafer board panel, specific material 21 properties fnamely E - modulus of elasticity, and MOR-22 . modulus of rupture) would improve. ~owever, such 23 improvements would be at the expense of the 'overall flexure 24 strength' properties thereof. By overall flexure strength, bending moment capacity, load capacity, or bending strength, 26 is meant the multiple of 'S' (section modulus) and '~O~' 27 ~modulus of rupture). Additionally, it is accepted that if 28 the density of the planar wafer board panel is increased, its 29 'bending stiffness' will also decrease. Bending stiffness is Sfi2 ~

1 defined as the multiple of E and I (where E is the modulus of 2 elasticity and I is the moment of inertia).
3 By ~high density~, ~normal density , and ~low 4 density~ in the present context is meant wafer board having substantially uniform densities in the ranges of 700-6 900kg/m3; 600-700kg/m3 and 400-600kg/m3 respectively.
7 In summary, therefore, the commonly held belief in 8 the art was that to increase the density of a wafer board 9 panel above the normal would result in a panel having reducsd values in certain important structural properties. Such an 11 increase in density was therefore to be avoided.

13 In accordance with the present invention, I have ~4 determined that for a corrugated wafer board panel~ it is possible to provide an improvement in its overall flexure 16 performance properties by increasing the density thereof.
17 This observation is based on the discovery that the 18 modulus of rupture (MOR), for a corrugated panel, increases 19 proportionately more than the modulus of elasticity (EJ
thereof and that the section properties for corrugated wafer 21 board change less as its density is increased, relative to 22 flat wafer board. Stated otherwise, I have found that if the 23 density of flat and corrugated wafer board are increased 24 without changing the unit panel mass per projected surface area, th- results are approximately aa followa.

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1 Specific Overall 2 Material Section Bending 3 Properties Properties Properties E MOR I S E I S MOR
7 Flat up up down down down equal 8 Waferboard more to 9 than down "E"

13 Waveboardup up down down no up 14 more less less major than than than change 16 "E" flat- flat-17 board board 21 And, as stated earlier:
22 bending stiffness equals: E I
23 bending strength equals: S MOR.

25 By bending stiffness and strength is here meant 26 stiffness and strength performance in one direction namely ths~
27 direction where the wave top parallels the span.
28 I have discovered that the bending strength (S MOR) of 29 corrugated wafer board increases as its density i~ increasea, and the bending stiffness remains essentially unchanged provided the 31 panel weight per unit area is kept constant.
32 Advantageously, by providing a board having a higher~
33 density it is possible to obtain better wood utilization than in 34 lower density corrugated waveboard. This finding is the opposite to the case for flat waferboard.

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l Broadly stated, the invention is a corrugated wafer 2 board formed of binder-coated wafers which have been 3 subjected to heating and compression, which comprises: a 4 board having a wave-like configuration, said board fu.rther having a substantially uniform density resulting ~rom the 6 even distribution of wafers therein, said density ranging 7 from between about 700 kg/m3 to 900 kg/m3 .

8 DESCRIPTION OF l'HE DRAWINGS
g Figure 1 is a plot showing the relative bending strength (S-MOR) and relative bending stiffness fE-I) ll improvement in corrugated wafer board (having the same l2 section properties) versus density change.

l3 DESCRIPTION OF THE P~EFERRED EMBODINENT
The corrugated wafer board panels having a wave-like configuration were prepared using the process and platen l6 system described in U.S. Patent ~,616,991. As stated l7 earlier, the platen system involved a pair of opposed, 18 spaced-apart upper and lower platens. Each platen was formed of adjacent lengths of chain-like links. Upon application of a lateral force thereto, the link assembly would move from a 2l planar to a corrugated form.The final outside dimensions of 22 the prepared panels were 24" x 36", the skin thickness was 23 approximately 11.3 mm ~7/16"J, and the panel depth wave peak 24 to bottom was 63.5 mm (2-l/2"J. Additionally, it can be appreciated that the final panel size can be scaled up to 26 1220 x 4880 mm (4~ x lfi~). Boards having panel densities 27 from 647 kg/m3 up to 768 kg/m3 were prepared.

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1 The process for preparing the ~high density~
2 corrugated wafer board compris~d the following steps:
3 The furnish could be prepared using various wood 4 species. Asp~n logs approximately 8l in length and 6" - 14"
in diameter were used. The logs were cleaned, debarked, 6 waferized and screened. The strand or wafer length averagéd 7 76 mm (3ll) and the thickness was about 0.76 mm t0.03"), 8 however other strand or wafer geometrics can be used.
g The moisture content of the furnish was reduced from the green state to about 5% using commercial dryers. The 1l wafer were screened following drying.
12 At 5% moisture content, the furnish was blended 13 with 3% by weight of powdered phenol formaldehyde resin and 14 1% by weight wax in a laboratory drum blender. Wax was utilized to improve the moisture resistance of the panel.
16 Resin was utilized as a ~inder for the wafers.
17 The wafers and wax/resin in admixture were arranged-18 loosely by hand between two flexible stainless steel screens 19 (cauls) to form the mat. The quantity of wafers and resin used was sufficient to produce a board having the requisite 21 density. The cauls had previously been dusted with talcum 22 powder to prevent bonding of the wafers thereto. Using the 23 cauls, the mat was transferred to the press.
24 In the press, the mat was subjected simultaneously to high temperature, which set the binder, and to high 26 pressure which compressed the mat to specified thickness.
27 More particularly, the corrugated platen temperature was 28 maintained at 205~C. The platen was heated by electrically 29 heated rods extending within the press platens.

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1 The open or fully extended surface area of the 2 platens was 920 x 920 mm.
3 To obtain pre-compression and corrugation the press 4 was operated in a manual control mode. Once the mat was in place on the platens, a vertical pre-compression force of 6 less than 3.4 x 106 Newton's was applied. Application of 7 this force brought the top and bottom platens towards one 8 another. At this displacement, the platens were, following g pre-compression, actuated into the corrugated configuration by application of a horizontal side force of less than 0.52 x 11 1~6 Newtons thereto.
12 A final compression was applied by bringing the 13 press platens closer together, until the latter reached their 14 stops. The panel was retained between the press platens for four minutes to allow the resin to set.
16 Prior to removal of the finished wafer board panel ~7 from the press, the pressure was released slowly to avoid 18 steam release damage.
19 The panels were then cooled.
It is to be noted that if a section of the panel 21 prepared in accordance witA the procedure outlined hsreabove 22 was taken at any point along its length and its density was 23 measured, the density value was su~stantially uniform.

24 Experimental Example I
26 Table I and Fi~ure 1 exemplify the improvement in 27 bending strength (5 NOR) and bending stiffness (E-I) as the 28 density of corrugated wafer board panels are increased. The `7~i62~

1 panels were prepared using 3" (76 mm) long aspen flakes and 2 3% powdered phenol formaldehyde resin.
3 The wavelengths of a71 the panels were 189 mm, the 4 panel depths were 64 mm and the skin thicknesses were 11.3 mm. The section properties for all four panel types 6 mentioned in this example are therefore the same. The wafer 7 lengths were 104 mm.

9 Unit Panel Unit Bending Bending 11 ~ Density StrengthStiffness 12 S-NOR E-I Relatire Relative 13 UNITS kq/m3 N.mm/mm N mm2/mm MOR E
14 581 3350 l7,200,000 84% 84%
W ~90%) (8496) (84%) 16 V ~47 4000 20,400,000 100~6 100%
17 E (100%) (100%) (100%) B

1 8 O 700 4720 22,500,000 118% 110%
19 A(108%)(118%) (110%) D 768 5560 24,400,000 139% 120%
21 (119%)(13~%) (120%) 22 Example II
23 Table II given herebelow, demonstrates that for 24 twoflat wafer board panels, one having a 'high densityi and one having a 'normal density',both the overall flexure 26 strength ralue, (S MOR) and the bending stiffness (B I) 27 decreased in the 'high density' sample. The table further 28 illustrates the increase in overall flexure strength (S NOR) 29 when the density is increased for corrugated wafer board without increasing the amount of wood and binder used.

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2 Waveboard* Flat Waferboard 3 Normal ~ighNormal ~igh 4 Density DensityDensity Density Value ValueValue Value 6 Unit Panel Weight 7 (kg/m2) 8.3 8.26.8 6.8 8 Panel Density g (kg/m3) 667 846651 846 Thickness (mmJ 10.2 8.0 10.5 8.0 ll Unit Bending l2 Strength 3 (N.mm/mmJ S-MOR 3,247 3,609 398 349 l4 Unit Bending Stiffness 16 (N.mm~/mm)16,470,000 16,300,000462,000 279,600 17 * The wave peak to wave bottom depth was approximately 64 mm l8 and wavelength was 188 mm. A11 the panels were manufactured l9 using 3" ~76 mmJ long aspen flakes and ?.5% powdered phenol formaldehyde resin.

2l Example III
22 Table III below provides a~comparison of the .
23 properties of waveboard having a control density value and a 2~ high density value wherein the panels were manufactured using 4" aspen flakes and 3% isocyanate ~MDI) resin. The peak to 26 peak depth was approximately 6~ mm and the wavelength was 188 27 mm. The wafer lengths were l04 mm.

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2 Waveboard 3 Control High 4 Density Density Value Value 6 Unit Panel Weight 7 (kg/m2) 9.4 9.4 Panel Density (kg/m3) 691 835 Thickness (mm) 11.2 9.2 ll Unit Bending Strength l2 (Nmm/mm) S-MOR 4762 5220 13 Unit Bending Stiffness l4 (Nmm2/mm) E-I 22,503,000 22,154,000 '~

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Claims (2)

1. A corrugated wafer board formed of binder-coated wafers which have been subjected to heating and compression which comprises:
a board having a wave-like configuration, said board further having a substantially uniform density resulting from the generally even distribution of wafers therein, said density ranging from between about 700 kg/m3 to 900 kg/m3.

2. A board as set forth in claim 1 which is manufactured using wood wafers or strands which are longer than 3" (75 mm).