wood and plastics

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Wood and Plastics

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Major Topics

Classification of Wood Hardwood Softwood

Growth of Wood Wood Defects Wood (Lumber)

Seasoning/Kiln Drying Moisture Content Cutting

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Major Topics con’t

Wood (Lumber) Decay/Insects Grading Sizing (Nominal vs. Actual) Joints Fasteners

Prefabricated Wood (Glue-laminated)

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Major Topics con’t

Plywood Other Panels

Waferboard Composite Particleboard OSB (Oriented Strand Board)

Plastics

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Classification of Wood

Hardwood – trees which are deciduous (shed broad shaped leaves annually) ex: Birch, Ash, Maple, Oak

Softwood – trees which are evergreen (needle-like leaves) ex: Southern Pine, Fir, Spruce, Redwood

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Growth of Wood

Growth is formed from core (pith) in rings. The # of rings and spacing between rings show age and growing conditions of tree.

Wood is made of hollow tubular cells running parallel to the long axis of the tree.

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Wood Defects

Common defects include: knots, stain, pitch pockets, decay, and cracks.

These defects will impact the visual “grading” of wood products

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Examples of Defects

Knot

Wane – irregular rounding caused by cutting too close to outside of log

Resin Pocket

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Lumber

Broad term that applies to all finished or semi-finished wood shaped with parallel longitudinal surfaces

Nominal piece sizing includes: Board -- 11/2” or less thick and 2” or

more wide Dimension – 2” to 5” thick and >2” wide Timbers -- 5” or more thick and wide

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Lumber Measured in “Board Feet”

http://www.woodzone.com/tips/board_feet/board_feet.htm

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Seasoning/Kiln Drying

Seasoning- the process of reducing moisture until a suitable level is achieved (causes shrinking in lumber size)

May be seasoned in the air (2-6 months for softwood and may take 4 years for some hardwoods) or by using a kiln

A chemical (hygroscopic) may be applied to the wood to keep surface moist to reduce shrinkage cracks (checks)

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Moisture Content

The strength of wood increases as the moisture content (m.c.) decreases

M.C. varies depending on the conditions (geographical region & indoor/outdoor) in which the lumber will be used

Typically, m.c. does not exceed 19%

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Cutting

Plain-sawed (flat sawed) – lumber which is cut in parallel slides [less waste & cheaper; warps & splits]

Edge-sawed – lumber which is cut perpendicular to the exterior of the tree

Quarter-sawed – lumber in which the log is 1st cut into quarters and then cut on the diagonal [produces the most attractive wood grains]

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Decay-Insects

Termites – destroy wood by chewing it (chemical or physical barriers should be used to deter them)

Fungi – feed on wood fibers leaving wood weakened with rotting

Avoid placing untreated wood directly in contact with concrete

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Pressure-Treated Lumber

Must meet all EPA (Environmental Protection Agency) requirements

CCA (Chromated Copper Arsenate) is the most common treatment used today

4- EPA approved wood preservatives include Waterborne Oil-borne Creosote Fire-retardant solutions

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Grading

Depends on the appearance ( # of defects) and strength

Lumber pieces are marked with a grade stamp Typical stamp includes

Grading body (WWP – Western Wood Products

Mill identification Grade name (Const – construction) Moisture content Type of wood (D-Fir – Douglas Fir)

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Sizing (Nominal vs. Actual)

Lumber is referred to by nominal size but the actual size is less (see handout) 2”x4” actually measures 1/1/2” x 3

½” 1” x 8” actually measures ¾” x

71/4”

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Joints

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Fasteners

Nails, screws, bolts, staples, anchors, and joist hangers are common fasteners

Sizes, styles, and finishes (coatings) depend on the intended application

Length of nails designated in “pennies” [d]

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Nails

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Prefabricated Wood

Because of the limitations of size of sawn wood – the gluing of smaller pieces together will enable structural members of virtually any length, cross-section, and desired curves to be made

Glu-lam members are widely used in areas of construction using arches (must comply with ANSI 190.1)

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Glu-Lam

Individual laminations are placed so that: Weak spots are separated from each other to

avoid concentration of weakness Appearance flaws in wood are hidden within

the member End joints between lams are separated from

each other to avoid a plane of weakness The strongest wood is placed where stresses

are highest

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Species of Lumber Used for Glu-Lam

Douglas Fir Alaska Cedar Spruce-Pine Fir Southern Pine

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Plywood

Plywood is a type of glued, laminated wood. Thin wood layers of laminations are arranged with the grains of each layer perpendicular to the adjacent one.

Veneers – the actual laminations consisting of face & back, crossbands, and the inner (core) [see fig 6-20, pg. 176]

Usually an odd number of veneers (3-5; may be up to 11)

Thickness may range from ¼” – 1 ¼”

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Advantages of Plywood vs. Sawn Lumber

Has great transverse strength which aids in strengthening/bracing entire structure when used over studs, joists, and rafters for wood frame construction

Less warping and change due to moisture changes Is more easily bent to form curves for concrete forms or

curved wood construction Fabricated in large sheets (4’x8’, typical) which covers

larger areas more quickly Can be worked closer to the edges without splitting Desired appearance can be obtained by using thin

veneers of high quality wood where they are visible

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Grades/Types of Plywood

5 basic grades from best to less desirable finished appearance: A, B, C, C plugged, & D

5 species groups (according to stiffness and strength): Group 1 is the strongest/stiffest

Types of plywood: interior and exterior Interior- made with glue suitable for indoor

use; available in any grade Exterior – made with hot, phenolic resin glue

which is unaffected by water & resists weathering; no veneers below C grade used

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APA – Engineered Wood Products

APA – American Plywood Association – key organization for plywood information

Website link:

http://www.apawood.org/

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Other Panels

Waferboard Composite Particleboard OSB (Oriented Strand Board)

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Plastics

Usually contain synthetic resins. May also contain plasticizers, fillers, and colorants

2 basic classes of plastics; thermoplastics (no chemical change during heating/cooling) & thermosetting plastics (change chemically when heated and solidify while still hot)

Plastic products are strong, light in weight, formable, and resistant to corrosion

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Chemical

NameProperties Forms/Uses

Acrylics

(Plexiglas)

Transparent, hard, weather-resistant, shatter resistant, easily scratched

Cast sheets/Window & skylight glazing

Polyethylene Flexible, tough, translucent, low cost, easily scratched; Film thickness measured in “Mils”

Film & sheet/Vapor barriers, building enclosing ; Open mesh/ window screen

Polystyrene

(Styrofoam)

Hard, clear, brittle, water- and chemical-resistant, low cost; good insulating properties

Tile & sheet/Wall covering & tile

Vinyls

Poly-Vinyl Chloride (PVC)

Tough, wear- and stain-resistant Tile & sheet/Floor & wall tile, sheet covering; Coated glass fiber/ window screen; Plumbing Parts

Polyamides

(Nylon)

Tough, hard, wear-resistant, expensive

Cast/Rollers & Bearings

Thermoplastics

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Thermosetting Chemical

NameProperties Forms/Uses

Alkyds Weather-resistant, tough, good adhesive properties

Liquid and solid/Surface coatings such as paints, enamels, molded products

Melamines Hard, durable, abrasion-resistant, chemical- and heat-resistant

Sheets/Decorative laminates, high-pressure laminates, countertops

Polyesters Weather- and chemical- resistant, stiff, hard

Corrugated and flat translucent laminates, woven glass reinforced/ Window glazing & skylights

Epoxy Resins Applied by using brush, trowel, or spraying equipment

Waterproofing, joint sealers, protective coatings, adhesive