Plywood Glue adhesive : Glue Bond classes

Plywood Glue adhesive : Glue Bond classes

Chisel test
The evaluation of bond quality for veneer-based products is usually conducted
using a chisel test. The Australian and New Zealand standard AS/NZS
2098.2.2012 outlines this procedure (‘Methods of test for veneer and plywood,
Method 2: Bond quality of plywood—chisel test’). The test forcibly separates
veneers along the glue line, allowing the percentage of wood fibre on the exposed
surfaces to be estimated and the glue line to be evaluated. A quality glue line will
demonstrate substantial wood fibre failure on the separated veneers, while a poor
bond will have little or no wood fibre remaining indicating the adhesive has failed.
The test procedure uses a pneumatic chisel to force the glue line apart in a
direction perpendicular to the veneer grain (Figure 10.1). Individual veneers
from each glue line are then assessed to estimate the percentage area covered by
wood fibres (Figure 10.2) and a bond quality value from 0 to 10 (where 10 is the
best) is assigned to each glue line (Table 10.2). Under normal circumstances,
a sample pass or fail would be determined in accordance with the specification
outlined in the relevant standard for the product (e.g. AS 6669:2016 ‘Plywood—
Formwork’; AS/NZS 2269.0:2012 ‘Plywood—Structural—Determination of
structural properties—Evaluation methods’; AS/NZS 2271:2004 ‘Plywood and
blockboard for exterior use’; AS/NZS 4357.0:2005 ‘Structural laminated veneer
lumber—Specifications’). Each of these standards requires a sample to have an
average bond quality score of not less than 5, with any individual glue line not less
than a bond quality 

Figure 10.2.
Veneers are separated to assess wood fibre failure.

Veneers are separated to assess wood fibre failure
Veneers are separated to assess wood fibre failure

◆ veneer thickness variation leading to variation in press pressure;
◆ adhesive pre-cure;
◆ adhesive under-cure;
◆ veneer case hardening or surface inactivation.

 

 

Over-penetration of adhesive
The common causes of over-penetration are:
◆ high initial veneer moisture content;
◆ high glue spread;
◆ press open assembly time too short;
◆ high press temperatures causing increased adhesive flow;
◆ low adhesive viscosity.
This problem is identified by:
◆ good adhesive transfer between veneers;
◆ limited glue solids remain in the glue line; however, fillers remain;
◆ blown panel.
Remediation measures include:
◆ check veneer moisture content prior to panel production;
◆ check the adhesive viscosity;
◆ check adhesive spread rates;
◆ check recommended press open assembly time parameters remembering that
this is temperature sensitive;
◆ after pre-pressing, check that the adhesive has transferred between veneers
and that the veneers have ‘tacked’.

Adhesive dry-out
Common causes of adhesive dry-out are:
◆ low initial moisture content in the veneer;
◆ low adhesive spread rate;
◆ veneer temperature elevated due to insufficient cooling and equalisation time
following drying;
◆ press open assembly time too long;
◆ high adhesive viscosity

This problem is identified by:
◆ little or no adhesive transfer between veneers;
◆ little wetting of the un-spread veneer;
◆ no adhesive penetration;
◆ adhesive spreader roller marks are still visible on the veneer;
◆ excessive adhesive remains present on spread veneer.
Remediation measures include:
◆ check veneer moisture content prior to panel production;
◆ check adhesive spread rates;
◆ check the adhesive viscosity;
◆ check press open assembly time procedures, remembering that this is
temperature sensitive;
◆ after pre-pressing, check to see that veneers have ‘tacked’;
◆ check the veneer temperature especially if there is minimal delay between
drying and adhesive application.

Veneer thickness variation
Variation in the thickness of veneers can be due to a number of causes. It can
occur when spliced or jointed veneer has a thickness change between adjacent
veneers. It can also occur during the peeling process due to incorrect settings or
equipment limitations; this can be localised variation (within a veneer sheet) or a
gradual change in thickness during the peel resulting in variation between veneer
sheets. The acceptable variation in veneer thickness is usually up to 5% of the
nominal thickness.

◆ check for mechanical issues with the lathe such as worn bearings on the
rollers;
◆ segregate and sort veneers based on thickness to reduce thickness variation
during product manufacture.
Adhesive pre-cure
Adhesive pre-cure occurs when the adhesive cures before sufficient press pressure
is applied. This results in the adhesive not flowing correctly and too little wetting
occurs to form an adequate bond. This can occur if a panel sits too long before
being pre-pressed, or the panel remains on a hot platen of the hot press for too
long before pressure is applied.
In un-prepressed plywood this problem is identified by:
◆ limited adhesive transfer;
◆ limited adhesive wetting;
◆ no adhesive flow (e.g. spreader marks still visible);
◆ no adhesive penetration.
In pre-pressed plywood this problem is identified by:
◆ adequate adhesive transfer;
◆ poor adhesive penetration;
◆ adequate quantity of adhesive on both surfaces;
◆ adhesive in the glue line has cured and is difficult to scrape off

This problem is identified by:
◆ no adhesive transfer in failed zone;
◆ no wetting in failed zone;
◆ no penetration of adhesive into the veneer in failed zone;
◆ a distinct demarcation between a good/bad bond zone;
◆ spreader roller marks are still visible on spread veneer in the failed zone.
Remediation measures include:
◆ check for thickness variations along the width of veneers to be pressed;
◆ check for thickness variation at spliced zones;
◆ check the lathe settings;

check for mechanical issues with the lathe such as worn bearings on the
rollers;
◆ segregate and sort veneers based on thickness to reduce thickness variation
during product manufacture.
Adhesive pre-cure
Adhesive pre-cure occurs when the adhesive cures before sufficient press pressure
is applied. This results in the adhesive not flowing correctly and too little wetting
occurs to form an adequate bond. This can occur if a panel sits too long before
being pre-pressed, or the panel remains on a hot platen of the hot press for too
long before pressure is applied.
In un-prepressed plywood this problem is identified by:
◆ limited adhesive transfer;
◆ limited adhesive wetting;
◆ no adhesive flow (e.g. spreader marks still visible);
◆ no adhesive penetration.
In pre-pressed plywood this problem is identified by:
◆ adequate adhesive transfer;
◆ poor adhesive penetration;
◆ adequate quantity of adhesive on both surfaces;
◆ adhesive in the glue line has cured and is difficult to scrape off.
Remediation measures include:
◆ check press protocols to ensure sufficient and even pressure;
◆ check the hot press loading protocols to ensure panels are not left too long in
the hot press before pressure is applied or loading time is excessive.
Adhesive under-cure
Adhesive under-cure is caused by insufficient pressing time or by insufficient
temperature.
This problem is identified by:
◆ adequate adhesive transfer;
◆ adequate adhesive quantity on both veneer faces;
◆ outer glue lines are satisfactory but the inner glue lines show poor adhesion

Remediation measures include:
◆ check the hot press platens with a thermocouple to ensure adequate
temperature has been achieved;
◆ check that the press time is adequate—measuring the temperature of the
middle glue line in the panel during hot pressing can assist in calibrating the
press protocols.
Veneer surface inactivation or case hardening
Inactivated veneer surfaces are indicated when the wood resists wetting. This
problem is usually caused by drying veneer at excessive temperatures or by overdrying.
The components that make up wood are held together by strong molecular
forces. When wood is machined, the molecular bonds are broken. The newly
created open bonding sites are unstable and have strong attractive forces, and
the higher the number of available bonding sites the greater the total attractive
force. It is this attraction that gives freshly machined wood its wettability.
Water, gases, microscopic dust and dirt particles, extractives in the wood and,
of course, adhesives are all likely candidates to bind with open bonding sites.
The longer freshly machined wood is exposed to the atmosphere, the more of
these bonding sites will be taken by gases and pollutants and the fewer will be
available for the adhesive. This is why wood loses its wettability over time and the
wood surface becomes inactivated. When the wood is heated, the chance of an
inactivated surface increases because heat increases the movement of extractive
(non-soluble) compounds, potentially moving to the surface and attaching to
open bonding sites. Extreme heat can alter the molecular chemistry, destroying
available bonding sites.
This problem is identified by:
◆ veneer has discoloured after drying;
◆ poor adhesive transfer or wetting;
◆ no adhesive penetration into the spread veneer;
◆ the failed area resembles a dried-out bond;
◆ the glue line has an imprint of the opposite surface (often appearing as a wood
grain pattern) and an occasional loose fibre imbedded in the glue

Remediation measures include:
◆ check that the dryer temperature is not excessive;
◆ check the veneer moisture content at the completion of drying to determine
suitability of drying time;
◆ reduce the veneer storage period

Plywood Glue adhesive : Glue Bond classes
Plywood Glue adhesive : Glue Bond classes

 

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