Additives & Polymers for High Temperature Structural Adhesives

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:

Oxidation
Pyrolysis

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

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.

Effect of Aging in an Epoxy-phenolic Adhesive

Effect of Aging in an Epoxy-phenolic Adhesive

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

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	60 177 10-50	60 177 10-100	Seconds Room Temp Contact	90 288 to 371 50	24 hours Room Temp Contact	5-10 100 Minimal
Tensile shear, psi at 20°C 175°C 1260°C	4330 2300 --	3800 2500 2000	3120 970 430	3300 -- 2300	275 -- 275	3-10 piw -- 3-10 piw

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.

Structural Epoxy Adhesives Formulation Optimization Part 1

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

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

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.