There are several features determining the quality of the plywood - the main ones being Stress Grade, Bond Type and Face & Back Grades. Stress Grading is only applied to some plywoods - the most common being F11 & F14.
Bond Types are A - Phenol Formaldehyde. Durable & Permanent For Marine & Structural uses in exposed conditions.
B - Melamine Urea Formaldehyde. Semi-durable in fully exposed area & totally durable in a humid internal area. Maximum exposed life of 2 years.
C - Low Extension Urea Formaldehyde. For use in areas completely protected from weather, wet or damp. Suitable for high humidity areas.
D - High Extension Urea Formaldehyde. For non-structural, fully protected low humidity areas.
Face and Back Grades define the quality of the surfaces with the face grade being the first designated. There are five grades A - Top quality appearance suitable for clear finishing.
B - Good quality appearance suitable for high quality paint finish.
C - Non appearance finish with a solid surface where permissible defects have been filled & sanded.
D - Allow open defects such as splits and holes, useful as a backing for high quality faces.
S - A special grade where natural characteristics such as knots, holes or colour are promoted for effect.
Plywood with a designator CD means a C-grade front and a D-grade back.
Categories : There are five general categories of commonly available plywoods - based on the general use. These are called STRUCTURAL, STRUCTURAL LAMINATED VENEER LUMBER (LVL), MARINE, EXTERIOR and INTERIOR. In addition there are other types of specialised plywoods such as Formply, I-Beams, Tongue & Grooved Flooring & Webbed Beams.
|
Product
|
Standard
|
Bond
|
Adhesive
|
Face
Grades |
Back
Grades |
Application
|
|
Structural
Plywood |
AS/NZS
S2269 |
A
|
Phenol
Formaldehyde |
A,B,C,D,
S |
A,B,C,D
S |
Permanent structural.
|
|
Structural
Laminated Veneer Lumber (LVL) |
AS/NZS
S4357 |
A
|
Phenol
Formaldehyde |
n/a
|
n/a
|
Permanent structural.
|
|
Marine
|
AS/NZS
2272 |
A
|
Phenol
Formaldehyde |
A
|
A
|
Boats, Hulls, Ultralight Aircraft- may need preservative
treatment for durability.
|
|
Exterior
|
AS/NZS S2271 |
A |
Phenol Melamine Urea |
A,B,S
A,B,S |
C,D |
Exterior non-structural Exterior exposure for max 2 years or semi exposed permanently |
|
Interior
|
AS/NZS
S2270 |
C |
Urea
Formaldehyde Extended Urea Formaldehyde |
A,B,S
A,B,S |
C,D
C,D |
Interior non-structural in humid environments.
Interior non-structural in temperate climates |
For a technical summary of Marine Ply see the
Summary Table. Marine Ply is graded AA -A bond and has been designed specifically
for use in the marine environment- particularly in boat hulls. Other plys
will be perfectly suitable for some boating applications - especially where
it is to be painted or glassed or otherwise covered. eg transoms, bulkheads
or flooring.
Like other plywoods, Marine Ply with its cross
laminations gives high strength and stiffness to weight ratio, high dimensional
stability, together with high shear and chemical resistance. It can be sawn,
bored, drilled, screwed, glued and nailed with no need of sophisticated equipment.
Marine Ply resists impact damage caused by floating debris or rough weather.
High quality finishes in either clear or colour can be achieved.
Standard panel sizes of 2400 X 2100 mm and to
a lesser extent 1800 X 1999 mm are available. Lengths of 2100 mm are also
available but usually with a delay. Thicknesses between 1.5 mm and 25 mm are
normally available.
As with any wood, whether
solid or ply, care must be taken to apply the correct protection - especially
to the end grain or ply edges. Where the veneer laminated
product may be subject to fungal or insect attack it should be treated to
beat the hazard. Treatments available include Copper Chrome Arsenate (CCA),
Light Organic Solvent Preservatives (LOSP), Ruply, Ammoniacal Copper Quatemary
compounds (ACQ), Copper Azole or paint-on products such as Copper Napthenate.
When in doubt consult the local plywood expert. (Plywood Express for the Central
Coast - 4323 3771)
2. Do not seal the inside of the hull with fibreglass or paint unless it is necessary to do so for reasons of wear or appearance. If painting inside the hull is necessary, apply copper naphthenate, zinc naphthenate, pentachiorophenol or other brush-on, spray-on preservatives before painting. Check with the paint manufacturer to make sure that the paint selected will be compatible with the preservative.
3. It is good practice to apply water-repellant preservatives based on copper or zinc naphthenate or pentachlorophenol each season, even to unpainted sections that can dry out readily and therefore have a low rot hazard. Some preservatives which are water-repellent last longer than a season. Application of preservative is more important in sections around the stem, transom and under the deck where normal ventilation is poor and access difficult. This treatment can overcome problems of condensation in these areas.
4. Don't, as a regular practice, hose out the boat with fresh water. Fresh water laying about should be carefully sponged out and dried up. Cabin floor and housing covers should be removed and the interior left ventilated so that normal air drying can occur. Washing or hosing down with salt water is a better alternative. Undissolved salt boric acid or copper sulphate in bilge water provide conditions which will inhibit possible fungal attack.
5. Keep boats stored out of the rain, if possible, but avoid covering with a close fitting tarpaulin. If a cover sheet is used it should be raised to allow air to circulate.
6. Stripping down and repainting according to paint manufacturers' instructions is recommended. Time for
refinishing varies up to five years, depending on conditions of use. Painted hull bottoms require more constant care and maintenance. The boat should be slipped regularly.
7. Despite puncture resistance higher than in most other boat materials, any craft can become accidentally holed. Because of the cross laminated construction of plywood any such hole is confined to a limited area. Here is how easy repairs can be made. First remove all damaged wood by cutting an exact square (with a key hole saw) slightly larger than the hole. Then cut from plywood of the same thickness as the hull planking a square patch the same size as the hole in the hull. Then cut another plywood square 75 mm larger all round. Fix the small square patch to the centre of the larger piece with resorcinol glue and clench nail for pressure. Now fit the patch exactly into the hole with liberal resorcinol glue on all edges on the inside surfaces of the 75 mm wide surround. Clench nail all sides of the surround to the hull with nails in two rows approximately 40 mm apart. When glue has set sand and fibreglass the outside if desired.
8. The golden rule for boat construction is "Glue all joints well with resorcinal glue"; No nailed or screwed joint is as sound as as a glued joint or good bond. In theory nails and screws could be removed when the glue is set as they serve no further purpose - however, in many cases this would be impractical and unnecessary.
Preservation of the high rot hazard areas of the boat is relatively cheap insurance to ensure a satisfactory service life. This is particularly significant as the conditions under which the boat will operate are not foreseeable.
There are two ways of preservatively treating marine plywood - in the factory or at the boat building site.
The two most common types of factory applied preservative treatments are pressure treatment with waterborne preservatives of the copper chrome arsenic (CCA) type and light oil solvent preservatives of the copper naphthenate, zinc naphthenate, tributyl tin oxide and pentachlorophenol types. All these treatments are effective against fungal attack provided the correct preservative retention and penetration is obtained. Full details on preservation are given in Australian Standard A.S.1604 Preservative Treated Sawn Timber, Veneer and Plywood.
On site preservative treatments are usually carried out by brushing, spraying or immersing of the plywood in a solvent based preservative such as copper naphthenate, zinc naphthenate, pentachlorophenol or tributyl tin oxide. It is important that the chemical manufacturers instructions are followed in detail to get maximum preservative benefits and minimise the health hazard. All cuffing and boring should be carried out prior to the application of the preservative.
Generally the factory applied preservative
treatments are more effective as they are carried out under controlled conditions
and therefore give better preservative retention and penetration than those
carried out at the building site. The on-site treatments are ideal for low
hazard areas of the boat or for applying extra protection for plywood which
has been cut or bored in the fabrication process. Brush-on preservatives also
should be applied as part of the regular maintenance of the boat.
With either treatment method it is a good
policy to lightly sand the surface prior to gluing or finishing to remove
surface salt However, in the case of brush-on preservatives it is preferable
to carry out all gluing prior to the application of the preservative.
The Engineered Wood Products Association
can assist in providing additional information on preservation. Alternatively
assistance also may be obtained from State Forestry Departments, Division
of Building Research C.S.l.R.O. or the Timber Preservers Association.
|
RESORCINOL
FORMALDEHYDE & RESORCINOL PHENOL-FORMALDEHYDE |
EPOXY RESIN
|
|
|
USES
|
Laminating marine work, plywood structural components | Gluing of non-cellulosic materials to plywood or where extreme chemical resistance is required |
|
ADVANTAGES
|
Complete water resistant bond. Used for all joints subjected to extreme exposure. Not subject to bacterial attack | Excellent gap filling, wetting & flow properties. Good water resistance but inferior to resorcinol glues |
|
DISADVANTAGES
|
Relatively expensive, dark colour. Poor gap filling properties | Relatively expensive. Mixed resins not water soluble & therefore more difficult to use than other glues. Resins toxic & may cause dermatitis or internal poisoning. |
|
PREPARATION
|
Mix resin with hardener | Mix resin & hardener very carefully |
|
WORKING REQUIREMENTS
|
Dry wood (8 to 15% moisture content) Wet glue spread 35 Kg per 100 Sq Mtrs of glueline. Assembly time to 60 mins. Pressure from 500 to 1400 KPa. Clamping time of 24 hrs at not less than 20°C and 10 mins at 90°C. Pressure must be applied while glue is still wet. | Dry wood (8 to 15% moisture content) Wet glue spread 20 to 30 Kg per 100 Sq Mtrs of glue line. Pressure from 350 to 900 KPa. Clamping time up to 48 hrs at 20°C and about 1 hour at 90°C |
|
COMPOSITION
|
Combination of resorcinol and formaldehyde or phenol-formaldehyde. Extra formaldehyde sets glue by polymerisation. | Combination of resins.- one of which serves as a catalyst. Glue sets by polymerisation. |
|
TYPES
|
Cold or medium temperature formulations. Formulations also available for high frequency heating | Cold and medium temperature formulations available. Also available in wide viscosity range |
|
COMMERCIAL
|
Viscous reddish brown liquid with liquid or powder hardener. | Honey like liquid of coloured paste with liquid hardener. |
All information about plywood was supplied
by the Engineered Wood Products Association of Australasia (ABN : 009 704 901). Contact details
are : "PLYWOOD HOUSE"
Head Office - 3 Dunlop St., Newstead, Brisbane
Postal
- PO Box 2108, Fortitude Valley, B.C. Qld 4006
Phone
- (07) 3854 1228 Fax - (07) 3252
4769
E-Mail
- folk@plywoodassn.com.au
