Hot melt adhesives TKH-Technical Information Sheet
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TKH-Technical Information Sheet
Hot melt adhesives
Status: January 2006
Publisher: Technical Committee on Wood Adhesives (Technische
Kommission Holzklebstoffe - TKH) of Industrieverband Klebstoffe e.V.,
Düsseldorf
This briefing note can be obtained from:
Industrieverband Industrieverband Klebstoffe e.V., P.O. Box 26 01 25, D-40094
Düsseldorf, Phone +49/211/6 79 31-14, Fax +49/211/6 79 31-33,
Klebstoffe e.V. Internet: www.klebstoffe.com
ContentsTKH-Technical Information Sheet Hot melt adhesives • Jan. 2006 Seite 2 von 8
Contents Principally, a hot melt adhesive, like all adhesives,
consists of one or more polymers plus additives
Introduction .................................................... such as pigments, stabilizers etc. More
1. Definitions and terms ..................................... information in chapter 2.2. Chemical
characteristics of adhesives.
2. Characteristics of adhesives ..........................
2.1. Physical characteristics of adhesives ............ Usually, hot melt adhesives are named after the
2.2. Chemical characteristics of adhesives........... base polymers:
3. Properties of substrates .................................
3.1. Physical properties of materials ..................... Many systems can be formulated to be reactive
3.2. Chemical properties of materials .................. and thermoplastic. Thermoplastic bonds with
4. Processing parameters .................................. hot melt adhesives are reversible. Under
sufficiently high thermal stress, they once again
5. Processing methods ...................................... liquefy and consequently loose their cohesion.
6. Test methods .................................................
To counteract this fact (which is sometimes
7. Technical requirements/Specifications ..........
desired – just think of recycling), the polymer
molecules must be cross-linked after setting
(chemical linkage). Loss of cohesion at higher
Introduction temperatures is consequently reduced and
adhesive forces of the hot melt adhesives are
Today's wood and furniture manufacturing is not maintained. These systems submitted to chemical
imaginable without hot melt adhesives. They cross-linkage after the cooling phase are so-called
constitute the only adhesive system which allows reactive hot melt adhesives.
modifications of processing parameters during
processing. For example: viscosity of a hot melt
adhesive can be widely adapted to the desired In melted state, the hot melt adhesive is a liquid,
conditions by controlling temperatures. This the so-called melt. Only in this state, the adhesive
makes hot melt adhesives the perfect adhesives wets the elements to be joined, the so-called
when work processes must be controlled and substrates and develops adhesion with these
automated. elements. However, a liquid cannot transfer
bonding strength, consequently has little
It is the purpose of this data sheet to clarify the cohesion. After cooling, the hot melt adhesive
multitude of terms and definitions processors of becomes a solid (mostly Amorphous, with more or
hot melt adhesives are faced with. less crystalline fraction) with very high cohesion.
Once the bond is prepared, the visco-elastic
1. Definitions and terms polymers ensure that adhesion will be preserved
1)
even after cooling process which results in
According to DIN EN 923 a hot melt adhesive changes of volume and affiliated development of
(German: Schmelzklebstoff; French: Colle mechanical stress. The built-up cohesion provides
thermofusible) is defined as an adhesive system the adhesive strength between the substrates.
which is hot melted and develops cohesion
(internal strength) after cooling.
Table 1: Base polymers for the production of hot melt adhesives
Base polymer Notes
Ethylene-Vinylacetate (EVA) frequently used base polymer
Polyolefine (APAO) For higher thermal resistance
Polyamide (PA) High thermal resistance and easy processibility
Polyurethane (PUR) Frequently reactive system with high thermal and moisture resistanceTKH-Technical Information Sheet Hot melt adhesives • Jan. 2006 Seite 3 von 8
2. Characteristics of adhesives The melt is described based on variables
2.1 Physical characteristics of adhesives specifying a liquid, such as viscosity; the solid
however is specified by mechanical variables
It results from above description of the bonding such as the G-module. In-between are the
process that when describing a hot melt adhesive, parameters describing the phase transition
we must accurately distinguish if referring to the between the two such as for example the
melt, the phase transition or the set solid. Most softening temperature.
application parameters describe the melt, in
contrast, most selection criteria describe the
solid.
Table 2: Variables for melt characterization
Variable Abbreviation Unit Measuring method Description
.
Viscosity η Pa s rheometer describes flow
characteristics
Melting Flow Index MFI g/10min MFI measuring device Additional viscosity
feature
Melting Vol. Index MVI ml/10min MFI measuring device Additional viscosity
feature
Table 3: Variables for characterization of phase transition
Variable Abbreviation Unit Measuring method Description
Flow point FP °C Kofler-heating Visual properties of melt
2)
bench during cooling
Softening EP °C Kofler-heating bench Visual properties of
3)
temperature Ring & Ball method solid during heating
Table 4: Variables for characterization of solid
Variable Abbreviation Unit Measuring method Description
Density ρ g/ml volumetric Specific weight
elast. G- module G´ Pa rheometer Memory module describes
elasticity
plast. G-module G´´ Pa rheometer Loss modulus describes
plasticity
Relaxation time τ sec rheometer Describes ratio of plastic and
elastic propertiesTKH-Technical Information Sheet Hot melt adhesives • Jan. 2006 Seite 4 von 8
2.2. Chemical characteristics of adhesives
The somewhat vague term Resin requires
Hot melt adhesives are multi-component systems clarification: following everyday usage we call
which allow to adjust certain characteristics by those polymers resins which during melt are
expert combining of different polymers and mainly responsible for the generation of adhesion.
additives so that, for example, hot melt adhesives Those are mostly low-molecular compounds
can show thermal or mechanical behavior which is (compared to base polymers).
very different from that of the base polymers. Like
with the alloy steel, which is far superior to its raw The term Filler material in this context does not
material iron, properties can be achieved by an constitute a synonym for "cheap blends". Filler
optimal combination of different raw materials materials decisively influence rheology and
which the single substances do not have. structure of the adhesives and substantially
contribute to their profile. Likewise, gravel added
This allows to custom-tailor hot melt adhesives as to asphalt in road construction is not used to
polymer alloys. The table below lists the essential stretch the bitumen, but rather to ensure stability
raw materials used for hot melt adhesives: of the road surface.
Table 5: Essential raw materials used for the fabrication of hot melt adhesives
Name Abbreviation Function, examples
Ethylene Vinyl-Acetate EVA Base polymer
Polyolefin APAO Base polymer
Polyamide PA Base polymer
Polyurethane PUR Base polymer
Filler materials e.g. chalk, barite
Pre-Polymer Cross-linking component, e.g. Isocyanate
Resin Tackifier, e.g. natural or petrochemical
resins
3. Characteristics of the substrates 3.1. Physical properties of materials
The properties of the bond are not only influenced The following table 6 lists as an example the
by the adhesive used, but also by those of the properties of highly demanding furniture edges.
substrates. We will therefore expand our analysis
to the properties of the bond as a system. It is the The last column "Examination" wants to infer that,
system we are interested in, not the individual at least as a comparison test, the behavior of
components, the adhesives and substrates. edges at permanently high temperatures, permit
an estimate regarding stress to be expected:
Table 6: Properties of furniture edges, making high demands on bonds
Problem Source Effect Examination
Internal stress Extrusion, calandering Constant load on Behavior of edge in
adhesive. Discharge at drying cabinet
excess temperature
Frozen-in internal stress Production process Effects only shown Behavior of edge in
much later drying cabinet
Degree of condensation Production Internal stress, Behavior of edge in
resistance to water drying cabinet
when exposed to
moistureTKH-Technical Information Sheet Hot melt adhesives • Jan. 2006 Seite 5 von 8
Please note that many of above materials cannot
Let us look at the bond of a thermoplastic edge on be bonded when left untreated. In some cases,
chipboard. When temperatures increase above manufacturer will already apply a primer which
room temperature, in most cases the hot melt then serves as actual contact surface for the
adhesives as well as the edges soften. The bond adhesive.
will hold as long as the adhesive can transmit the
mechanical stress even under these temperature
conditions. 4. Processing parameters
The stress on the adhesive is determined by the Table 8 on page 7 is a list of "practical"
stress occurring in the substrates – edge and parameters which are often questioned and
board. As a result, edge material which tends to misunderstood. These are variables to be
shrink in high temperatures (shearing stress) or attributed to the phase transition of hot melt
edge material which curls (normal stress) will adhesives from liquid to solid.
mechanically overstress many adhesives. In
everyday usage, this is defined as "low thermal These parameters, which in most cases refer
stability". explicitly or implicitly to time, directly influence
settings of the production machine.
Another example is a water resistant hot melt Strictly speaking, these variables are not
adhesive – it will not be able to keep an edge on a parameters relating to the adhesive, but rather
chipboard which will swell in water, once the settings, i.e. production parameters, defined by
chipboard cover layer detaches when exposed to the production environment. Consequently, for
water. different applications, these parameters may
These two examples clearly show that "thermal widely vary for one and the same hot melt
stability" and "water resistance" are typical adhesive.
properties of a bond, not of the adhesive alone. This explains why any attempt to standardize
This fact must be considered with regard to these values must fail.
requirement parameters.
On the other hand, these variables have emerged
from practical experience and are strictly speaking
3.2. Chemical characteristics of materials the only information which really interest a user.
Based on a simple model, we will try to submit a
For coating materials and edges as well, the base tool to assess the problematic variables (which
material is also used for naming. Table 7 lists cannot be measured) with the help of measurable
some current materials with no claim to be adhesive parameters and measurable
complete. Information regarding recycling environmental parameters.
behavior and mechanical flexural strength are
contained in the technical data sheets issued by
the respective manufacturer. Application areas We consider the following model:
pertinent to our context are listed in the second
column. A hot melt adhesive wets the substrates while still
liquid. Wetting depends on viscosity and surface
tension in liquid state. Both are measurable
Table 7: Base materials for coating materials functions of the temperature.
and furniture edges
Name Application By releasing heat to substrate and environment,
ABS (Acrylonitrile–butadiene– stryene) edge
the adhesive cools off. At the same time, its
PVC (Polyvinyl chloride) edge, surface
viscosity and surface tension increase, wetting
PP –PE (Polypropylene – polyethylene) edge, surface
power is reduced and cohesion (strength)
Laminates edge, surface
increases. Speed of these processes is a
PET / polyester edge
measurable characteristic defined by the heat
Veneers + solid wood edge, surface
Aluminum edge
flow. (Slow, if the environment poorly abducts
Chipboard support
heat and the adhesive has a high thermal
MDF / HDF support capacity, fast when the environment is very cold,
Material mix bond edge, because in this instance the heat flow is great).
supportTKH-Technical Information Sheet Hot melt adhesives • Jan. 2006 Seite 6 von 8
Table 8: “Practical” processing parameters for hot melt adhesives
Term Definition Controllable variables Implications
Open time Time after application of Heat flow Maximum time after
adhesives during which proper Ambient temperature application of
wetting of substrates is ensured. Substrate temperature adhesive until joining
of the substrates
Can be controlled by
user
Setting time Complete cohesion development Substrate properties Minimum time after
Viscosity Ambient conditions application of
unmeasurably high. adhesive until
Cross-linking – physical and Can be controlled by mechanical
chemical – has noticeably set in. user load rating of glue
joint
Surface tension Temperature-dependant variable Adhesive temperature Important for
for wetting behavior of liquids adhesive
Can be controlled by discharge on rolls
user
Heat Cohesion and adhesion Viscosity profile Effect on mechanical
adhesiveness with elevated temperatures load rating during
Initial tack Can be controlled by joining process
user
Time for % of final strength Viscosity profile Effect on mechanical
development of load rating right after
cohesion Can be controlled by joining process
user
Ability for Remelting behavior Reactivation Joining behavior
reactivation temperature
Can be controlled by
user
Milling behavior Viscosity profile Machining of finished
"Spreading" parts, tool
Can be controlled by
adhesive manufacturer
Stringing Adhesive temperature Contamination of
"Angel hair" finished parts and
Can be controlled by tools
user
5. Processing methods ambient moisture in order to prevent premature
reactions. The same applies for melting devices.
Table 9 lists typical applications for hot melt
adhesives in the furniture industry and the related
processing methods. The processing parameters 6. Test methods
described carry different weight depending on
application. In each case, the most important In table 10, we are listing the primary methods for
parameters are listed in the last column. testing a bond with hot melt adhesives. Most of
them are used at the customer site for quick
Depending on application method and type of quality control. All methods have in common that
adhesive, the form of delivery of the adhesive they are object tests. They make sense to test
varies. It is important that application method and bonds, however do not claim to provide
adhesive type are well coordinated. Some typical information on an adhesive without substrate. In
forms of delivery are granulates, cartridges or terms of above statements this would in any case
blocks. With reactive hot melt adhesives, be futile.
packaging must protect the adhesive fromTKH-Technical Information Sheet Hot melt adhesives • Jan. 2006 Seite 7 von 8
The two last columns constitute the TKH use it to characterize the potential of the relevant
assessment. Discrimination/resolution is a term process to detect subtle differences in quality.
from measurement and test engineering and we
Table: 9: Typical uses of hot melt adhesives in the furniture industry
Application Characteristics Mode of application Important parameters
Straight edge Application of adhesive Applicator roll, nozzle,
to HolzHer
base plate or edge adhesive application
material systems
Softforming Application of adhesive Applicator roll, nozzle Open time, initial
to edge material strength
Stationary treatment Workpiece is clamped Applicator roll Initial strength
and machined and an
edge is added
Pre-coated edges Application of adhesive Applicator roll Reactivation
to edge material;
adhesive is reactivated
Postforming Shaping of surface Applicator roll Initial strength
coating around edge Application with nozzle
Coating Application of adhesive Applicator roll, nozzle Open time
to coat
Assembly bonding Assembly tool "hot nail" Nozzle, hand operated Open time
gun
Table 10: Primary methods for testing of a hot melt adhesive bond
Term Method / feature Discrimination Validity
4)
WPS 68 Creep behavior poor very limited
Peel strength Peeling 90 degrees Contingent on edge/foil good
Flexural strength of material
edge/foil
Manual adhesion test In-house methods varying good
Ascending heat test Proof stress defined by moderate moderate
edgefoil material
Cold testing In-house methods very limited limited
Alternating climate test Proof stress defined by contingent on edge/foil moderate
edge/foil material material
Long-term testing Proof stress defined by adequate good
edge/foil material
Water resistance In-house methods poor very limited
7. Technical methods (chapter 6) help in comparing test
requirements/Specifications results.
Based on the processing methods (chapter 5) and Suitability of an adhesive for a specific area of
the relevant processing parameters (chapter 4), application not only depends on the raw material
the appropriate adhesive can now be selected. In base or other individual parameters. In each
addition, technical requirements for the different
areas of application can be defined. Different testTKH-Technical Information Sheet Hot melt adhesives • Jan. 2006 Seite 8 von 8
individual case, the interaction of all features must
be considered. Frequently, certain adhesives The effect of the adhesive on test results is
might be designated as the standard for certain considerably lower than generally presumed. In
areas of application. This means that they have fact, it is rather processing during bonding, as well
been proven and tested over the years and as resistance of coating and substrate materials
comply with the main stress classifications listed which play the crucial role.
in table below.
Consequently, the result of these tests only show
the behavior of the component under certain
climatic conditions. These tests help to define the
quality achieved. The results may later be used as
reference for quality control testing.
Table 11: Primary methods for testing of hot melt adhesive bonds
Technical
Area of application Main stress class
requirements/parameters
Standard furniture sector Variety of substrates, adhesion Processing parameters
requirements
Bath and kitchen furniture Heat resistance Different test methods (chapter 6)
Vapor resistance
Extreme climatic conditions Heat resistance Different test methods (chapter 6)
vapor resistance
Low temperature resistance
Literature:
1) DIN EN 923 "Adhesives – Terms and
definitions", German version EN 923:2005,
Publisher: Beuth Verlag, Berlin
2) Kofler Heat Bank: Metal plate with defined
temperature gradients
3) ASTM E 28: Test method for softening point;
Ring-and-Ball method
4) WPS 68: Testing of heat resistance with
ascending temperature under defined load
The content of this briefing note is based on the best knowledge and the current state of the art. It is for information only and without
responsibility. Warranty claims cannot be derived from this industry guideline.You can also read
