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Adhesive bond formation &
High-frequency gluing of wood
Dr. Milan Šernek
Associate Professor, Department of Wood Science and Technology, University of Ljubljana, Slovenia
COST Action FP1006
Bringing new functions to wood through surface
modification
Presentation Overview
Adhesive bond formation
Flow & Transfer
Penetration
Wetting
Solidification
High-frequency gluing of wood
Principles of HF heating and gluing
Dielectric properties
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012 2/0
Adhesive bond formation
Bond formation between wood and an adhesive involves five distinct actions (Marra 1992):
1. Flowing in the plane of the adhesive bond line in response to pressure and to high and low spots on the surface;
2. Transferring to the opposite surface in the manner of a printing operation;
3. Penetrating the pore and interstitial structure of the wood surface;
4. Wetting the wood substance at available surfaces to produce adhesion;
5. Solidifying to produce strength.
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
1. Flow
Adhesive flow first motion occurs after adhesive application promotes further when the opposing two wood surfaces
are brought together flowing into grosser irregularities and forming a film
This motion is the easiest to perform because it involves only coarse movement of the adhesive as mass flow. There is sufficient amount of adhesive; The adhesive is sufficiently fluid; There is contact pressure with the opposing surface.
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
2. Transfer
Adhesive is applied to:
both bonding surfaces
one bonding surface:
open and closed assembly times
transfer is achieved with slightly more difficulty than flow and as a consequence is more easily inhibited.
The adhesive must have an extra degree of mobility, beyond that needed for flow alone.
This characteristic of the adhesives may be visualised as “tack”
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
3. Penetration
Importance of adhesive penetration
an influence on bond strength
mechanical interlocking
filling gaps and lumens
secondary interactions
reinforcement of damaged wood cells
an impact on wetting area
more effective distribution of stresses
excessive penetration may lead to a starved bond line
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Factors influencing penetration
Adhesive properties (everything that influences the adhesive’s fluidity) viscosity, molecular weight distribution of the resin solids,
solvent content, extender and filler content, pH, etc. Wood properties anatomy, permeability, porosity, roughness, surface
energy, moisture content, direction, thermal conductivity, etc.
Processing variables assembly time, press temperature, pressure, time,
technology (conventional / HF)
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Adhesive mixture I III IV II
Dynamic viscosity (cP) 990.89 1393.78 1415.56 2079.78
EP(mm) 25.70 19.52 19.65 16.09
Viscosity
Resin solids
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
0
5
10
15
20
25
bottom side top side
Eff
ecti
ve P
en
etr
ati
on
( mm
)RADIAL TANGENTIAL
Anatomy and direction
Source: Textbook of Wood Technology
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Wood species
SW vs. HW
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Distribution of adhesive
Particleboards, OSB, MDF, …
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Technology (conventional / HF), MC
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
4. Wetting
A condition of a surface that determines how fast a liquid will wet and spread on the surface or if it will be repelled and not spread on the surface.
An intimate contact on a molecular level is assumed to be
necessary for bond formation to achieve good adhesion between materials.
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Contact Angle ()
Young’s equation:
Young-Dupre’s equation:
WLS … work of adhesion -represents the amount of work which must be expanded to separate a unit of solid surface from liquid
Vapor (V)
Liquid (L)
Solid (S)
LV
SV
SL
cosLVSLSV
)cos1( LVSLW
Wettability
A high surface free energy of substrate and a low surface energy of the adhesive promote wetting and spreading of the adhesive.
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
The amount of wetting depends on:
The molecular nature of the adhesive; The molecular mobility in the adhesive; The quality of the wood surface; The wood characteristics: wood species and composition, extractives wood anatomy, wood surface sections, wood seasoning, moisture content, temperature
The pressure (and temperature) of the adhesive; The time available before hardening of the adhesive
arrests mobility.
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Nature of solid surfaces
Hydrophilic surface; affinity for water, “likes water”
Hydrophobic surface; repels water, “hates water”
< 90°
90° < < 180°
Perfect wetting: = 0°
No wetting: = 180°
Wettability
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
0
50
100
150
200
250
300
350
400
-0.2 0.0 0.2 0.4 0.6
Cos i
Gm
ax (
J/m
2)
Adhesion - wood wettability
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
5. Solidification
Four basic mechanisms for hardening are available: Loss of solvent or liquid carrier; Loss of heat; Chemical reaction; Combinations.
Conversion: from liquid to solid
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Conversion of adhesive vs. T and t
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60
Time (s)
Deg
ree o
f cu
re
120°C
140°C
160°C
180°C
200°C
Press temperature
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Development of the strength
0
1
2
3
4
5
6
7
8
9
0 10 20 30 40 50 60 70 80 90 100 110 120
Time (s)
Sh
ear
stre
ng
th (
N/m
m2)
200°C
180°C
160°C
140°C
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Conclusions – adhesive bond formation
In forming a bond, adhesives execute five separated motions or actions: flow, transfer, penetration, wetting, and solidifying.
Bonding actions begin when adhesive meets wood, bringing together variables from three sources:
wood, adhesive, and process.
This makes the process of adhesive bond formation in wood-based composites very complex.
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
High-frequency gluing of wood
COST Action FP1006
Bringing new functions to wood through surface
modification
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
High-frequency (HF) or radio frequency (RF)
USA
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Advantages and disadvantages of HF
+ Fast Thicker & different shape Uniform & selective heating
- Expensive Complex regulation
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Inductivity Conductors f < 3 MHz
Capacitance Dielectric f = 3 - 30 MHz
Principle of HF heating
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
+ - dipol
Dielectric constant
‘ shows how many times the force of interaction between the electric charges in the given medium is less than that in a vacuum.
0
0'
C
C
V
V
Dielectric loss tangent
Tg is numerically equal to the ratio between the active current and the reactive current in the material.
C
R
I
Itg
IC I
IR
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
tan0
2 rfEV
P
tV
P
crT
1
Dielectric properties of wood
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Material f
(MHz)
T
(°C) ε tgδ
Cellulose/Hemicellulose 1 20 6,0 0,06
Lignin 1 20 4,0 0,01
Water 1 20 80 0,02
Spruce (u=5 %) 9,2 20 2,0 0,10
Dielectric properties of adhesives
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
f v MHz
Liquid adhesive Cured adhesive
D3T EZT D3V D3A EZV EZA
6,310 129,0 141,9 2,85 2,37 2,56 1,78
Higher than wood (10 -100 X)
Faster / intensive heating
Equipment for measurement of dielectric properties
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Selective HF heating
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012
Thank you for your attention!
1st Training School: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Slovenia, 2012