Effects of Elevated Service Temperature
Effects of Elevated Service Temperature
For an adhesive bond to be useful, it must not only withstand
the mechanical forces that act on it, but it must also resist the elements
to which it is exposed during service. One of the most degrading elements
for organic adhesives is heat. To synthesize
high temperature polymer systems for structural adhesives is a never ending challenge.
Let's take a look at the resins capable of withstanding extreme temperatures along with some important class of additives.
All polymeric materials are degraded to some extent by exposure to elevated
temperature. Not only do elevated temperatures lower short-term physical
properties, but properties will also likely degrade with prolonged thermal
aging. Thus, several important questions need to be asked for an adhesive
if high service temperatures are expected.
- What is the maximum temperature that the bond will be exposed
to in service?
- What is the average temperature to which the bond will be exposed?
Ideally, one would like to have a definition of the entire temperature
- time relationship representing the adhesive's expected service
history. This data would include time at various temperatures, number
of temperature cycles, and rates of temperature change.
Creep and Lack of Cohesive Strength
Certain polymers have excellent resistance to high temperatures over
short durations (e.g., several minutes or hours). The short-term effect
of elevated temperature is primarily one of increasing the molecular mobility
of the adhesive. Thus, depending on the adhesive, the bond could actually
show increased toughness but lower shear strength. Certain polymers with
lower glass transition temperatures will show softness and a high degree
of creep at elevated temperatures.
However, prolonged exposure to elevated temperatures may cause several
reactions to occur in the adhesive. These mechanisms can weaken the bond
both cohesively and adhesively. The main reactions that affect the bulk
adhesive material are:
These reactions
generally result in brittleness and loss of cohesive strength. Thermal
aging can also affect adhesion by causing changes at the interface.
These changes include:
- Internal stress on the interface due to shrinkage of the
polymer
- Chemical reactions with the substrate, and
- Reduced peel or cleavage
strength because of brittleness
If heating brings a non-crosslinked adhesive above its glass transition
temperature, the molecules will become so flexible that their cohesive
strength will drastically decrease. In this flexible, mobile condition,
the adhesive is susceptible to creep and greater chemical or moisture
penetration occurs. Generally with a crosslinked adhesive, prolonged heating
at an excessively elevated temperature will have the following effects:
- Split polymer molecules (chain scission) causing lower molecular
weight, degraded cohesive strength, and low molecular weight byproducts.
- Continued crosslinking resulting in bond embrittlement and shrinkage.
- Evaporation of plasticizer resulting in bond embrittlement.
- Oxidation (if oxygen or a metal oxide interface is present) resulting
in lower cohesive strength and weak boundary layers.
Most organic adhesives degrade rapidly at service temperatures greater
than 150°C. However, several polymeric materials have been found to
withstand up to 250-300°C continuously and even higher temperatures
for a short-term basis. To use these materials one must generally pay
a premium in adhesive cost and also be able to provide long, high temperature
cures, often with pressure. Long-term temperature resistance, greater
than 250-300°C, can only be accomplished with inorganic or ceramic-based
adhesives.
Requirements of the Base Polymer
Requirements of the Base Polymer
The base polymer, of course, is a key ingredient in a high temperature
adhesive system. For an adhesive to withstand elevated temperatures it
must have a high melting or softening point, and resistance to oxidation.
1. A High Softening Point or Glass Transition Temperature
Materials with a low melting point, such as many of the
thermoplastic
adhesives, may prove excellent adhesives at room temperature. However,
once the service temperature approaches the glass transition, plastic
flow results in deformation of the bond and degradation of cohesive strength.
Thermosetting adhesives, exhibiting no melting point, consist of highly
crosslinked networks of macromolecules. Because of this dense crosslinked
structure, they show relatively little creep at elevated temperatures
and exhibit relatively little loss of mechanical function when exposed
to either elevated temperatures or other degrading environments. Many
of these materials are suitable for high temperature applications.
2. Resistance to Oxidation Degradation
When considering thermosets, the critical factor is the rate of strength
reduction due to thermal oxidation or pyrolysis. Thermal oxidation can
result in chain scission or crosslinking. Crosslinking causes the polymer
to increase in molecular weight, leading to brittleness and decreased
elongation.
Progressive chain scission of molecules results the following losses within the bulk adhesive:
- Weight
- Strength
- Elongation, and
- Toughness
Figure below, illustrates the effect of oxidation by comparing adhesive joints
that are aged in both high temperature air and inert gas (nitrogen) environments.
The rate of bond strength degradation in air depends on the temperature,
the adhesive, the rate of airflow, and even the type of adherend.
The Effect of 260°C Aging in Air and Nitrogen on an
Epoxy-phenolic Adhesive
Some metal adhesive interfaces are chemically capable of accelerating
the rate of oxidation. For example, it has been found that nearly all
types of structural adhesives exhibit better thermal stability when bonded
to aluminum than when bonded to stainless steel or titanium
3. Resistance to Thermally Induced Chain Scission
Pyrolysis is simple thermal destruction of the molecular chain of the
base polymer in the adhesive or sealant formulation. Pyrolysis causes
chain scission and decreased molecular weight of the bulk polymer. This
results in both reduced cohesive strength and brittleness. Resistance
to pyrolysis is predominantly a function of the intrinsic heat resistance
of the polymers used in the adhesive formulation. As a result, many of
the polymers that are used as base resins in high temperature adhesives
are rigidly crosslinked or are made of a molecular backbone referred to
as a "ladder" structure as shown in the figure below.
Degradation of a Ladder Polymer and
Straight Chain Polymer due to Thermal Aging
The ladder structure is made from aromatic or heterocyclic rings in the
main polymer structure. The rigidity of the molecular chain decreases
the possibility of chain scission by preventing thermally agitated vibration
of the chemical bonds. The ladder structure provides high bond dissociating
energy and acts as an energy sink to its environment.
Notice in the figure above that to have a complete chain separation (resulting in a decrease in
the molecular weight) two bonds must be broken in the ladder polymer.
Whereas, only one needs to be broken on a more conventional linear or
branched chain structure.
In order to be considered as a promising candidate for high temperature
applications, an adhesive must provide all of the usual functions necessary
for good adhesion (wettability, low shrinkage on cure, thermal expansion
coefficient similar to the substrate, toughness, etc.)
Conventional High Temperature Polymers for Structural Adhesives
Conventional High Temperature Polymers for Structural Adhesives
High temperature adhesives are usually characterized by a rigid polymeric
structure, high glass transition temperature, and stable chemical groups.
The same factors also make these adhesive relatively difficult to process.
Only certain epoxy phenolic, bismaleimide, polyimide, and polybenzimidazole
adhesive can withstand long-term service greater than 177°C. However,
modified epoxy and even certain cyanoacrylate adhesives have moderately
high short-term temperature resistance. Silicone adhesive also have excellent
high temperature permanence, but they exhibit low shear strength and may
not be applicable for "structural" applications.
Properties
of these adhesive systems are compared in the table below.
Property
|
Modified Epoxy
|
Epoxy-Phenolic
|
Cyano-acrylate
|
Polyimide
|
Silicone Rubber
|
Pressure Sensitive
Silicone
|
Temperature Range, °C |
-55 to 177
|
-251 to 260
|
-40 to 246
|
-251 to 315
|
-73 to 232
|
-40 to 260
|
Optimum Cure Condition
- Time,
min
- Temp, °C
- Pressure, psi
|
|
|
- Seconds
- Room Temp
- Contact
|
|
- 24 hours
- Room Temp
- Contact
|
|
Tensile shear, psi at
|
|
|
|
|
|
|
Short Term Strength and Cure
Properties of High Temperature Structural Adhesives
|
There are even fewer sealants suitable for long term, high temperature
service. The thermal endurance requirement of a highly crosslinked polymer
generally is counter to the requirement that a sealant must be flexible.
Silicone based elastomers and some very special elevated temperature elastomers
are the only products that will provide both thermal endurance and a significant
degree of flexibility.
There are many
polymers that are not mentioned here because they are used in relatively
small amounts or are still considered in the developmental stage. The
reader may want to consider a large number of textbooks and research papers
on the development of high temperature polymers.
These high temperature resins will provide the main elements in the adhesive
formulator's recipe. However, as will be shown in following sections
of this paper, there are also additives, fillers, etc. that can further
enhance the thermal properties of these adhesives. These additional components
will improve thermal resistance by providing oxidation resistance, toughening,
and control of bondline stress.
Oxidation Resistance
Oxidation Resistance
Oxidation in high temperature adhesive joints involves reaction of the
adhesive polymer with oxygen in the air as well as reaction with certain
metal surfaces (e.g., ferrous metals). Oxidative degradation is initiated
by the action of highly reactive free radicals caused by heat or metallic
impurities. The function of the antioxidant is to prevent propagation
or the reaction of these free radicals with oxygen to form unstable species.
Using Antioxidants
Antioxidants should be included in high temperature adhesive formulations in order to achieve optimum thermal aging properties. Antioxidants use
in structural adhesives differ from those used to improve thermal stability
of thermoplastic materials. Here, they must be less volatile, resistant
to higher temperatures, longer acting, and of course compatible with the
base polymer. Antioxidants used in structural adhesives are generally
of inorganic origin; whereas, antioxidants used to prevent oxidation during
polymerization, processing, or fabrication of plastics are of organic
origin.
Arsenic based antioxidants, such as arsenic pentoxide and arsenic thioarsenate,
had been used extensively in the past to retard oxidation. In a polyimide
adhesive formulation, for example, arsenic compounds were found to improve
thermal resistance. At 315°C no loss in strength was exhibited after
1000 hrs and substantial strength (1300 psi) was retained after 2000 hrs
exposure. Without the arsenic additive there was marked reduction after
only 200 hrs at 315°C.
The use of arsenic compounds have been greatly curtailed
because of health and safety concerns. Antimony trioxide and similar compounds
are now commonly found in high temperature adhesives to forestall as best
as possible the effects of oxidation Compounds found to improve thermal
aging include Bi2O3 and
Sb2O3 and others belonging
to Group V and having secondary valances of 3 and 5. Usually, concentrations
of less than 1% are effective.
Oxidative stability depends on the adherend surface as
well as on the adhesive itself. Some metal adhesive interfaces are chemically
capable of accelerating the rate of oxidation. For example, it has been
found that nearly all types of structural adhesives exhibit better thermal
stability when bonded to glass or aluminum than when bonded to stainless
steel or titanium.1 For any given metal, the
method of surface preparation can also determine oxide characteristics,
and hence bond durability. Thus, the use of primers is common practice
with high temperature structural adhesives.
Using Chelating Agents
Chelating agents are sometimes used as scavengers to capture undesirable
metal ions. These compounds react directly with the metallic substrate,
thereby inhibiting its catalytic effects on oxidation. The effect of several
different chelating agents on the resistance of epoxy-phenolic bonded
aluminum joints to thermal aging is shown in the table below.
Chelating Agent at 1% By Weight |
Shear Strength, psi*
|
None |
670
|
n-propyl gallate |
1074
|
Gallic acid |
820
|
Acetyl Acetone |
985
|
Catechol |
980
|
Aluminum Triacetonylacetonate |
960
|
Ethylenediamine |
835
|
Effect of Several Chelating
Agents on the Resistance of an Epoxy-Phenolic Adhesive to Thermal Aging2
* Measured on aluminum adherends;
tested at 23°C after aging 200 hrs at 286°C
Improving Toughness in High Heat Structural Adhesives
Improving Toughness in High Heat Structural Adhesives
For many years, the typical method of improving the toughness of high
temperature structural adhesives was to add elastomeric resins to rigid
high temperature base polymer to create a hybrid product such as epoxy-nitrile
or phenolic-nitrile systems. However, the toughening of high temperature
adhesives can provide a difficult challenge, since the service temperatures
usually exceed the degradation point of most rubber additives. Also, the
addition of an elastomer generally resulted in lowering of the glass transition
temperature of the base polymer.
However, newer adhesives systems having moderate temperature resistance
have been developed with improved toughness without sacrificing other
properties. When cured, these structural adhesives have discrete elastomeric
particles embedded in the matrix. The most common toughened hybrids using
this concept are acrylic and epoxy systems. The elastomer is generally
a vinyl or carboxyl terminated acrylonitrile butadiene copolymer
Within the past several years, improvements in the toughening of high
temperature epoxies and other reactive thermosets, such as cyanate esters
and bismaleimides, have been accomplished through the incorporation of
engineering thermoplastics. Additions of poly(arylene ether ketone), PEK,
and poly(aryl ether sulfone), PES, have been found to improve fracture
toughness. Direct addition of these thermoplastics generally improves
fracture toughness but result in decreased tensile properties and reduced
chemical resistance.
Chemical functionalization of the thermoplastics was found to improve
toughness without such detractions. High molecular weight resins based
on amine terminated PES oligomers or chain extension of bismaleimide resin
with the same amine terminated PES were found to have improved fracture
resistance and reduced thermal shrinkage.3 Also a mechanism was found
to toughen cyanate esters by incorporating epoxy resins, which can react
with a cyanate ester.4
Get inspired: Improve the toughness of your structural epoxy adhesives while keeping environmental & temperature resistance in target
Reducing Internal Stress
Reducing Internal Stress
Internal stresses are common in joints made with high temperature adhesives.
These stresses can be due to:
- The high temperature bonding processes generally used
- The temperature excursions and cycling between ambient and service temperature,
and
- Thermal shrinkage that occurs after the adhesive is aged for a period
of time at elevated temperatures
Stresses caused by (1) and (2) factors above, are exasperated by the mismatch
in thermal expansion coefficients between the adhesive and the substrate.
Incorporating fillers into the adhesive formulation can often reduce these
stresses. Fillers reduce the thermal shrinkage during aging by bulk displacement
of the polymeric resin.
Flexibilizers generally cannot be used to counteract internal stress
because of their relatively low glass transition temperature and thermal
endurance properties. However, most high temperature adhesive systems
incorporate metallic fillers (generally aluminum powder) to reduce the
coefficient of thermal expansion and degree of shrinkage.
It is usually not possible to match the adhesive's coefficient
of thermal expansion to the substrate, because of the higher filler loadings
that would be required. High loading volumes increase viscosity to the
point where the adhesive could not be easily applied or wet a substrate.
For some base polymers, filler loading values up to 200 parts per hundred
may be employed, but optimum cohesive strength values are usually obtained
with lesser amounts.
Metal fillers for high temperature adhesives must be carefully selected
because of their possible affect on oxidation as indicated in the previous
section. Carrier films, such as gloss cloth, are generally used to facilitate
the application of the adhesive, but they also provide a degree of reinforcement
and lowering of the coefficient of thermal expansion. Thus, they also
reduce the degree of internal stress experienced at the joint's
interface.
References
- Krieger, R.
B., and Politi, R. E., "High Temperature Structural Adhesives",
in Aspects of Adhesion, vol. 3, D. J. Alner, ed., London University
Press, 1967.
- Black, J. M., and Bloomquist, R. F., "Metal Bonding Adhesives
for High Temperature Service", Modern Plastics, June 1956.
- Wilkinson, et. al., Polymer Preprints, 33 (1), 1992, p. 425.
- Shimp, D. A., et. al., "Co-Reaction of Epoxide and Cyanate Resins,"
33rd SAMPE Symposium and Exhibition, Anaheim, CA, Mar. 7-10, 1988.