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
Contents
TKH-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 resistance
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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 properties
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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
                                                               moisture
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                                                               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).
                                       support
TKH-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 from
TKH-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 test
TKH-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.
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