Adhesive bond formation High-frequency gluing of...

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

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Presentation Overview

Adhesive bond formation

Flow & Transfer

Penetration

Wetting

Solidification


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.


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”


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


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)


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


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


Wood species

SW vs. HW


Distribution of adhesive

Particleboards, OSB, MDF, …


Technology (conventional / HF), MC


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.


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.


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.


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


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


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


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


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


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.



COST Action FP1006

Bringing new functions to wood through surface

modification

High-frequency (HF) or radio frequency (RF)

USA


Advantages and disadvantages of HF

+ Fast Thicker & different shape Uniform & selective heating

- Expensive Complex regulation


Inductivity Conductors f < 3 MHz

Capacitance Dielectric f = 3 - 30 MHz

Principle of HF heating


+ - 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


tan0

2 rfEV

P

tV

P

crT

1

Dielectric properties of wood


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


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


Selective HF heating


Thank you for your attention!


Adhesive bond formation High-frequency gluing of...

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