BB,
Here is a very good technical write up on Aluminum Bonding that I thought is appropriate to the subject of this post.
Bonding to Aluminum - Article
I've pasted it below, lest we loose the web link at some future date. It confirms much of what has been already discussed on this post. There are some surprising data as well. No mention is made of methacrylates in this article however.
The formatting got a little goofed up in the copy and paste process...
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Bonding to Aluminum
SpecialChem - Jan 4, 2006
Edward M. Petrie, Member of SpecialChem Technical Expert Team
Introduction
The Basics of Aluminum
Surface Properties of Aluminum
Historical Aluminum Pretreating Processes
Trends in Aluminum Pretreating Processes
Selection of the Adhesive with Regard to Aluminum
Introduction
Aluminum is an almost ideal substrate for adhesives. It has a high surface energy and is very resistant to most environments. It is also a material with good formability and high strength-to-weight ratio that can benefit greatly from properties offered by adhesive joining. As a result, adhesive bonded aluminum joints are commonly used in the aircraft and automotive industries as well as in a multitude of other structural and non-structural applications.
Aluminum's surface oxide forming ability provides excellent corrosion resistance and a surface providing high initial adhesive strength. However, this oxide layer is relatively complex and can be formed in many ways depending on the conditions present during its formation. The oxide layer can also provide significant problems for the adhesive formulator and the end-user - especially when it comes to bond durability in warm and humid climates. As a result, a prebond surface treatment is generally required for aluminum to stabilize and specifically define the oxide structure.
The technical and commercial advantages of using aluminum have significantly encouraged its use as an adhesive substrate, and aluminum is perhaps the most scrutinized of all adhesive substrates. Aluminum joints are commonly used in adhesive research and for comparing different adhesive materials and processes. In their marketing literature, adhesive manufacturers usually describe the properties of their products measured on aluminum substrates.
This article will explore the important considerations necessary when using aluminum as an adhesive substrate. The surface properties of aluminum will be considered as well as the various processing methods used to modify the surface for effective adhesive bonding. The unique and interactive aspects of the adhesive, substrate surface, and service environment will be explained. Because of the plethora of technical data available on aluminum, bonded aluminum joints, and the adhesive-aluminum interface, this article cannot hope to cover all possible factors. It will, however, serve as a useful guide to bonding aluminum and an introduction into the more comprehensive aspects of the subject.
The Basics of Aluminum
The use of aluminum exceeds that of any other metal except iron. It is important in virtually all segments of the world economy. The strength and durability of aluminum varies widely depending on the components of the specific alloy, and also on the particular manufacturing process. Aluminum is generally characterized by its alloy content (a four digit number) and the temper or condition of the alloy (a letter designation). The quality of adhesive joints formed with aluminum greatly depend on these factors.
The corrosion resistance and the durability of joints made with adhesives are primarily dependent on the type of aluminum alloy that is used. Bonds made with relatively corrosion resistant 6061-T6 aluminum alloy will last about four times as long as equivalent joints made with 2024-T3 alloy when exposed to marine environments. The permanence of adhesive bonded aluminum joints is dependent on the alloy type because of their different corrosion degradation rates under extreme environmental conditions. With the proper surface treatment, both initial joint strength and the durability to aggressive environments can be greatly improved as shown in Table 1.
Tensile Shear Strength, psi
Alloy 2036
Alloy 6151
Alloy X5085
A
B
C
A
B
C
A
B
C
Initial, no aging
1930
1850
2200
2550
2690
2530
2250
2270
2110
After 3 mos aging at 23°C, 85% RH
1390
1050
2400
1350
1550
2410
1860
1890
1910
After 3 mos aging at 52°C, 100% RH
420
350
1110
920
1050
1100
1260
1520
1230
After 3 weeks aging in 5% salt spray solution
0
0
1410
80
150
2090
690
530
1210
Table 1: Effect of Surface Treatment on Aluminum Joints Bonded with a Heat Cured Epoxy Adhesive (EC-3443, 3M Company) 1
A - Mill finish
B - Vapor degreased
C - Alodine 401-45
The yield strength of the alloy and other bulk properties will also influence the joint strength to a certain degree. This effect will depend on the specific joint design and type of mechanical stress. Although the bulk material is important in the design and ultimate performance of adhesive joints, the main requirements for high bond strength and lasting durability reside at the adhesive - substrate interface.
Surface Properties of Aluminum
The oxide layer that forms on aluminum is more complex than with other metal substrates. Aluminum is a very reactive surface, and oxide forms almost instantaneously when a freshly machined aluminum surface is exposed to the atmosphere. Fortunately, the oxide is extremely stable, and it adheres to the base metal with strengths higher than could be provided by most adhesives. The oxide is also cohesively strong and electrically nonconductive. Once a very small (on the order of 0.001 mm) layer forms, it protects the base metal from atmospheric corrosion. These surface characteristics make aluminum a desirable metal for adhesive bonding, and they are the reasons why many adhesive comparisons and studies are done with aluminum substrates.
Various interactions between surface oxides and environmental factors (e.g., water, temperature, atmospheric chemicals) can create bonding complications. Under normal conditions the surface oxygen atoms will hydrate to form a layer of hydroxyl groups. These adsorb and retain molecular layers of bound water. The presence of bound water on the surface can be a positive factor since it encourages wetting by polar organic adhesives, such as epoxy. A hydrated metal oxide surface layer is illustrated in Figure 1.
Figure 1: Metal surfaces are actually hydrated metal oxides2
Note:1.The oxide layer is typically 40-80°A thick
2.The hydrated layer is tightly bound
3.A pure metal surface is rarely available for engineering use.
The initial strength of an aluminum joint can be improved by surface pretreatment. This generally involves cleaning the surface to remove contaminants and / or converting the existing surface to a new surface of higher surface energy. In the case of the aluminum surface, chemical conversion can also protect the base metal from corrosion and enhance the durability of the bonded joint in various service environments. The most common surface preparations that have been used for bonding aluminum can be generally segregated into three groups:
- simple cleaning and abrading,
- chemical etching, and
- primers and conversion coatings.
The parameters of the pretreating process and their control can be very important in assuring consistency from joint-to-joint. It has been reported that the temperatures used for rinsing and drying aluminum should not exceed 70°C since different oxides are formed above and below this temperature. The oxide (bayerite, ß-Al2O3.H2O) formed at lower temperatures supposedly provides better adhesion.3 It has also been recognized that the use of distilled or deionized water rather than tap water for cleaning and etching baths is preferred for greater bond strength and consistency.
Alloying elements influence the growth mechanism of the oxide layer and, hence, the corrosion behavior of the surface. In moist air the oxide can undergo corrosion, the products of which are deposited on the top of the oxide layer and form a weak boundary. The fracture of these joints always occurs within the oxide layer. To obtain a strong and stable bond between the metal and the adhesive, the naturally formed surface oxide has to be removed (usually by abrasion or chemically) and replaced with a new oxide layer that is continuous, solid, and corrosion resistant.
In selecting a pretreatment process, for aluminum or any other substrate, both the initial strength and the permanence in a specific operating environment must be considered. Mechanical abrasion is a useful pretreatment in that it removes the oxide and exposes bare aluminum. When this is done, however, many of the benefits of the protective oxide layer are lost. For example, if bare abraded aluminum is bonded, the reactive metal at the joint interface can potentially become hydrolyzed and oxidized, which will displace the adhesive. Hence, this bonded joint may initially be much stronger than one made with unabraided metal, but it will deteriorate rapidly when exposed to a harsh environment such as high heat and humidity.
This is why pretreatments that modify the oxide layer or create a new, stable oxide layer are especially desirable when permanence is a primary consideration. They improve bondability and maintain protection. To protect the aluminum joint from the affects of the environment, especially water and corrosion, an artificially thickened oxide layer is generally formed on the surface. Historically, chemical etching or anodizing surface treating processes have provided the surest way of obtaining durable adhesive bonds with aluminum.
Once the aluminum is treated by one of these processes (discussed below), the work of protecting the interface is still not over. The usual approach to good bonding practice is to prepare the aluminum surface as thoroughly as possible, then wet it with the adhesive as soon afterwards as practical. In any event, aluminum parts should ordinarily be bonded within 48 hours after surface preparation. However, in certain applications this may not be practical, and primers are used to protect the surface between the time of treatment and the time of bonding.
The use of primers for structural aluminum joints in the aerospace and automotive industries is especially common. Primers are applied as a low viscosity solution which enables them to chemically wet a metal surface more effectively than more viscous, higher solid content adhesives. Corrosion resistant primers are often used to protect the etched surface during assembly operations and exposure to environmental conditions when in service.4
Historical Aluminum Pretreating Processes
When bonding aluminum to itself or to other materials, the optimal surface preparation should be determined for the application based on the initial strength and durability required, and then the process should be rigidly followed. Over-specifying the strength requirements should be avoided since it could result in the selection of a surface preparation process that is time consuming, difficult to control, and expensive. Table 2 serves as a guideline for selecting which pretreatment to try first. Conventional pretreating processes for aluminum are described in ASTM D2651.
Surface Treatment
Type of Bond
Solvent wipe (MEK, MIBK, trichloroethylene)
Low to medium strength
Abrasion of surface (sandblasting, coarse sandpaper, etc.) plus solvent wipe
Medium to high strength
Hot vapor degrease (trichloroethylene)
Medium strength
Abrasion of surface plus vapor degrease
Medium to high strength
Alodine treatment
Low strength
Anodize
Medium strength
Caustic etch
High strength
Chromic acid etch (sodium dichromate - sulfuric acid)
Maximum strength
Table 2: Effect of Substrate Treatment on Strength of Aluminum Joints Bonded with Epoxy Adhesives 5
The simplest form of aluminum pretreatment consists of (1) solvent wiping, (2) vapor degreasing, or (3) either of these methods combined with mechanical abrading. In each instance, care must be taken to assure that the cleaning materials themselves do not become unknowingly contaminated, thus providing ineffectual cleaning or cross-contamination resulting in poor bond performance. For low to moderate strength aluminum joints, vapor degreasing and alkaline cleaning are often used. These processes will generally remove mill oil and debris from the surface of the substrate. The processes also provide a uniform surface for subsequent bonding.
Sandblasting is commonly used for treating aluminum surfaces prior to adhesive bonding because of its simplicity and economics. This process removes the existing aluminum oxide surface layer and allows a new one to develop. The new oxide layer will depend on the environmental conditions during which it forms.
Chemical pretreating processes, such as etching, produce higher reliability and longer service life in a bonded assembly. If aluminum adherends are first cleaned, then sandblasted, and finally chemically treated, the surface area is increased, the contaminants are removed, and a new oxide layer forms which is conducive to excellent initial and long-term strengths. However, this three step process is often not necessary when only low-to-moderate strengths (500-2000 psi) are required or if the finished adhesive joint will not be exposed to aggressive service environments (especially high moisture or humidity conditions). Useful bonds in these low-to-medium strength applications can be achieved simply with cleaning and / or abrasion.
While various acidic or caustic procedures can be employed with or without vapor degreasing, the most recognized etching pretreatment for bonding aluminum has been the sulfuric acid - dichromate solution used by the aircraft industry and described in ASTM D 2651. This process is sometimes known as the FPL etch, named after its developers, Forest Products Laboratories.6 The first step in this process is vapor degreasing followed by alkaline cleaning and then chemical immersion. The substrates are finally forced air-dried. There are several modifications of this treatment including a paste-like etching solution to allow for parts that cannot be immersed in the acid solution.
Other important methods of pretreating aluminum for adhesive bonding include anodizing and chromate conversion coating.7 In anodizing, the aluminum is immersed in various concentrations of acids (usually phosphoric or chromic) while an electrostatic charge is applied. The oxide reacts with the etchant to form a compound that protects the surface and is compatible with the adhesive. In this way the aluminum oxide is retained, but it is rendered more receptive to bonding. It has been shown that an anodized surface on aluminum alloy can constitute a very durable surface for epoxy adhesive bonding with excellent resistance to seacoast or other saltwater types of exposure. Examples of widely used anodizing processes are the Boeing phosphoric acid anodize (PAA) process8 and the chromic acid anodize (CAA) process . 9
The landmark U.S. Air Force Primary Adhesively Bonded Structure Technology (PABST) Program in the late 1970s demonstrated that properly designed and manufactured bonded fuselage panels made from the correct aluminum materials can actually operate safely at higher stress levels than comparable rivet joined aluminum structures.10,11 The results of this program show phosphoric acid anodizing as an optimal way to achieve durable aluminum bonds.
- PAA is the most durable pretreatment for aluminum that is processable within reasonable production tolerances.
- PAA with a corrosion resistant primer provides the best corrosion resistance.
- The adhesive should be selected on the basis of best durability as defined by slow cyclic testing in a hot / humid environment.
The chemical conversion coating method is also commonly used to treat aluminum substrates prior to bonding. Chemical conversion coating is an amorphous phosphatization process where the aluminum is treated with a solution containing phosphoric acid, chromic acid and fluorides. Chromate conversion coatings on aluminum constitute an effective way to enhance the surface bondability and also improve the corrosion resistance of the bond line.12 The resulting durability observed with mechanical abrasion and chemical conversion coatings are variable and depend on the particular processing conditions. Whereas, anodizing and etching processes produce consistent and generally durable aluminum joints.
Trends in Aluminum Pretreating Processes
The recognition that chrome is carcinogenic has forced alterations in surface treatment processes. Chromic acid anodizing and FPL etching are being phased out in many locations.
There have been several processes that are suggested as more environmentally friendly alternatives. One such process is a chromate-free etching process (designated PT2) for improved environmental and occupational health and safety perspectives.13
Work on sulfuric acid anodizing and sulfuric boric acid anodizing is now in progress.14 In the automotive industry, a pretreatment has been developed for aluminum coil that is nontoxic and compatible with weld bonding. This proprietary treatment is claimed to be as effective as chromium based pretreatment processes on exposure to salt spray.15
An environmentally benign sol-gel process has also been developed to provide aluminum with good bondability and corrosion resistance.16 In addition this process is claimed to be low cost and energy saving. The sol-gel consists of a ceramic inorganic coating to establish and aluminum oxide based coating. The ingredients are alumina and organosilane.
Selection of the Adhesive with Regard to Aluminum
Once the surface considerations are taken care of, there are many types of adhesives that can bond well to aluminum. The selection will depend on the strength needed, the type of stress involved (e.g., peel or shear; static or dynamic), and the operating environment. Reynolds Metals Company 17 offers some general rules of thumb in selecting an adhesive for aluminum bonding.
- Bonds to aluminum are generally stronger than bonds to steel
- A chromic - sulfuric acid etch gives the best resistance to weathering and salt water environments
- Room temperature curing epoxies offer the best salt water resistance
- Higher strengths are usually obtained with heat curing epoxies than with room temperature curing epoxies
- Modified phenolic films give the highest peel and shear strength combinations
- The most severe adhesive environment is a hot, humid climate (temperature 30-50?; humidity +90%)
- Structural adhesives are strong in shear; weakest in peel and cleavage
- Heat curing adhesives are less sensitive to surface preparation than room temperature curing adhesives
The commercial epoxy, acrylic, and polyurethane adhesives will all bond well to aluminum and to a wide variety of other materials. Sell 18 has also ranked aluminum adhesives in order of decreasing durability as follows: nitrile-phenolics, high temperature epoxies, elevated temperature curing epoxies, elevated temperature curing rubber modified epoxies, vinyl epoxies, two-part room temperature curing epoxy paste with amine cure, and two-part urethanes.
The starting formulations presented in Table 3 are designed for general purpose bonding of aluminum where other substrates may also be involved. Note that aluminum powder is a key ingredient in these formulations to provide for a closer match in coefficient of thermal expansion between the adhesive and the substrate.
Component
Parts by Weight
A
B
C
Part A
DGEBA epoxy resin (EPON 828, Resolution Performance Products)
100
100
50
Ground tubular alumina (Alumina T-60/T-64, Alcoa)
230
Aluminum powder
100
50
Part B
Aromatic amine eutectic (60/40 blend of MDA and MPDA)
12
Aliphatic amine (EPI-CURE 3234, Resolution Performance Products)
5
Aliphatic amine (EPI-CURE 3245, Resolution Performance Products)
5
Polyamide (EPI-CURE 3125)
54.5
Properties:
Mix ratios, Part A : Part B
19.4:1.0
9.6:1.0
100:27
2:2.1
100:5
---
Viscosity, cps at 25°C
Paste
Paste
Paste
Working life at 25°C
1-2 hrs
60 mins
60 mins
Cure schedule
Elevated Temperature
Room Temperature
Room Temperature
Tensile shear strength, psi, measured at 25°C on aluminum when cured:
- 7 days at 25°C
- 10 mins at 204°C
- 100 mins at 121°C
----
2250
2300
2050
1050
Table 3: Starting Formulations for Epoxy Adhesive for Bonding Aluminum19
The durability of epoxy bonded aluminum joints that were immersed in water are shown in Figure 2. The anodized and grit blasted surface treatments, although giving different initial joint strengths, showed no deterioration after two years' exposure. Both the vapor degreased and conversion coating treatments were significantly degraded by the moist environment. Exposure of similarly prepared specimens to a more aggressive soak-freeze-thaw cycle gave rise to even greater differences in performance with only the anodized treated aluminum joint showing a high percentage of joint strength after a two year period.20
Figure 2: Effect of surface treatment on the durability of epoxy / aluminum joints exposed to room temperature water immersion. (1) anodized, (2) girt blasted plus vapor degrease, (3) vapor degrease, (4) chromate conversion coating.21
References
1. Adhesive Bonding of Aluminum Automotive Body Sheet Alloys, The Aluminum Association, Inc. 1975.
2. Schneberger, G.L., "Adhesives for Specific Substrates", Chapter 21 in Adhesives in Manufacturing, G.L. Schneberger, ed., Marcel Dekker, New York, 1983.
3. Murphy, J.F. and Page, H.A., Am. Chem. Soc. Div. Paint Plastics Printing Ink Chem. Papers, vol. 15, no. 1, 1955 , p. 27.
4. Petrie, E.M., "The Use of Primers with Adhesives and Sealants", SpecialChem4Adhesives.com, August 27, 2003.
5. Adhesive Bonding of Aluminum, Reynolds Metals Company, 1966.
6. Eichner, H. W., and Schowalter, W. E., Forest Products Laboratory Report 1813, 1950.
7. Minford, J. D., "Surface Preparations and Their Effect on Adhesive Bonding", Adhesives Age, July 1974.
8. Kabayashi, G. S., and Donnelly, Boeing Company Report DG-41517, February 1974.
9. Bijlmer, P. F. A., Journal of Adhesion, 6 (1973), p. 319.
10. Thrall, E., "Bonded Joints and Preparation for Bonding, AGARD Lecture Series 102 5.1, 1979.
11. Shannon, et. al, "Primary Adhesively Bonded Structure Technology (PABST) General Materials Property Data", Douglas Aircraft Co., McDonnell Douglas Corp., Air Force Flight Dynamics Laboratory, Technical Report AFFDL-TR-77-107, September 1977.
12. Kim, G., and Ajersch, F., "Surface Energy and Chemical Characteristics of Interfaces of Adhesively Bonded Aluminum Joints", Journal of Materials Science, Vol. 24, 1994, pp. 676-681.
13. Wilson, I., et. al., "Pretreatment for Bonded Aluminum Structures", Advanced Materials and Processes, August, 1997.
14. Browne, J., "Aerospace Adhesives in the 90s", 38th International SAMPE Symposium, May 10-13, 1993.
15. Wilson, I., et. al., "Pretreatment for Bonded Aluminum Structures", Advanced Materials and Processes, August, 1997.
16. Zheng, H., et. al., "An Environmentally Benign Aluminum Alloy Surface Pretreatment Process for Adhesive Bonding", 28th International SAMPE Technical Conf., November, 1996.
17. Adhesive Bonding Aluminum, Reynolds Metals Company, Richmond, VA, 1966.
18. Sell, W. D., "Some Analytical Techniques for Durability Testing of Structural Adhesives", Proceedings 19th National SAMPE Symposium and Exhibition, Vol. 19, New Industries and Applications for Advanced Materials Technology, April 1974.
19. Resolution Performance Products, Starting Formulations 4011 and 4012.
20. Hartshorn, S. J., "Durability of Adhesive Joints", in Structural Adhesives, S. R. Hartshorn, ed., Plenum, Press, New York, 1986. Also in Minford, J. D., Treatise on Adhesion and Adhesives, R. L. Patrick, ed., vol. 5., p. 45, Marcel Dekker, New York, 1981.
21. Hartshorn, S.J., "Durability of Adhesive Joints", in Structural Adhesives, S.R. Hartshorn, ed., Plenum Press, New York, 1986.