4 33-.
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ADHESIVE 13CNIAN4 PROIDERTIES OF VARIOUS
METALS AS AFFECTED 13,' CHEMICAL AND
ANODIZING TREATMENTS OF THE SURFACES
(Part A - Additional tests on Anodized Aluminum and on
Zinc-Chromate-Primed Magnesium)
February 1955
This Veport is Ono of a Series
Issued in Cooperation with
AIRFORCE-NAVE'-CIVIL SUBCOMMITTEE
on
AIRCRAFT DESIGN CRITERIA
Under the Supervision of the
AIRCRAFT COMMITTEE
of the
MUNITIONS !BOARD
No. 1842-A
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
FOREST, PRODUCTS LABORATORY
Madison 5, Wisconsin
•
In Cooperation with the University of Wisconsin
•
ADHESIVE BONDING PROPERTIES OF VARIOUS METALS AS AFFECTED BY
CHEMICAL AND ANODIZING TREATMENTS OF THE SURFACES1
(Part A--Additional tests on Anodized. Aluminum and on
Zinc-Chromate-RPrimed Magnesium)
By
H. W. EICKNER, Chemical Engineer
Forest Products Laboratory, ? Forest Service
U. S. Department of Agriculture
--—
Summary
The concentration of chromic acid (5 or 10 percent) and the length of
the anodizing period (20 or 40 minutes) were not found to influence the
adhesive bonding properties of the chromic-acid-anodized) bare 75S-T6
aluminum alloy. However, seal treatment of the anodized surface by
heating in water greatly interfered with bonding except for one type of
adhesive (high-temperature formulation of neoprene, nylon, and phenol
resins on fabric carrier). This same adhesive also gave unusually good
bond strengths
bonding to zinc-chromate-primed magnesium surfaces with
per
square
inch
on
the
standard 1/2averaging as . high as 2 1 770 . pounds
inch overlap specimens of 0.064-inch-thick FS1-H magnesium.
Introduction
The following tests were made to supplement the previous work on adhesive
bonding of various metal surfaces reported in Forest Products Laboratory
-This progress report is one of a series prepared and distributed by the
Forest Products Laboratory under U. S. Navy Bureau of Aeronautics
Order No. NAer 01628 and U. S. Air Force Order No. AF 18(600)-102,
Amendt. No. A6(55-286). Results here reported are preliminary and may
be revised as additional data become available.
410
2
-Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
Rept. No. 1842-A
-1-
Agriculture-Madison
Report No. 1842. 3 These new tests include work to determine the effect
of chromic acid concentration (5 or 10 percent by weight), length of
treatments (20 or 40 minutes), and hot-water sealing in anodizing bare
75S-T6 aluminum, on the initial bond strength of joints to this metal.
Adhesive bonding tests were also made on magnesium surfaces that had
been treated and zinc-chromate-primed by two aircraft fabricators using
their standard surface treatments. The zinc-chromate-prime coatings
used with these treatments were thinner than those used on magnesium
surfaces in the previous work in Report No. 1842, and were reported to
improve bonding to this metal.
Procedure
Type and Number of Specimens
Small 4- by 5-1/2-inch test panels were prepared by overlapping two 3by 4-inch metal sheets, 0.064-inch in thickness, for 1/2 inch. Metal
sheets of 2 types, bare 75S-T6 aluminum alloy and FS1-H24 magnesium
alloy, were included in these tests. Three test panels were bonded for
each of the variables studied with three 1-inch-wide test specimens cut
from each of the panels.
Preparation of Metal Surfaces
Aluminum.--The bare 75S-T6 aluminum surfaces were prepared for bonding
by first wiping with a cloth saturated with acetone, and then immersing
for 5 minutes at 170° to 190° F. in an alkaline solution (pH approximately 12.0) of the composition:
5.0 ounces sodium metasilicate
0.5 ounce Nacconal NR
1.0 gallon water
The alkaline solution was rinsed from the surfaces in hot water and
then cold water. The sheets were then anodized in a chromic acid
solution (5 or 10 percent by weight) at 90° to 100° F. by first applying a small direct current potential (3 to 5 volts) between the aluminum pieces suspended as anodes and the cathode, which was the lead container for the solution. This voltage was gradually increased (within
5 minutes) to 40 volts. The higher voltage was applied at a current
density of 2.5 to 3.0 amperes per square foot of anode surface for
periods of 20 or 40 minutes.
3Eickner, H. W. Adhesive Bonding Properties of Various Metals as
Affected by Chemical and Anodizing Treatments of the Surfaces.
Forest Products Laboratory Report No. 1842.
Rept. No. 1842-A
-2-
•
•
After removal from the anodizing treatment, the surfaces were either
rinsed in warm water and force dried in front of a circulating fan
(unsealed), or sealed by heating the metal sheets in distilled water
for 1 hour at 180° F. before drying.
With the use of 2 concentrations of anodize solution, at 2 anodizing
periods for both sealed and unsealed surfaces, 8 types of anodized
aluminum surfaces were therefore prepared for investigation in this study.
surfaces of the magnesium sheets were prepared for bonding by two aircraft fabricators by using their established procedures.
Magnesium.--The
A. One method, involving electrolytic treatments, was reported to consist essentially of the following steps:
(1) Vapor degreasing surfaces in stabilized trichloroethylene to
remove oils and contaminants
(2) Cathodic cleaning (4.0 to 6.0 volts direct current at 5 to
25 amperes per square foot of cathode surface) for 3 to 10
minutes at 180° to 200° F. in a solution of the composition:
10.0 ounces sodium hydroxide
1.0 gallon water
(3) Rinse in cold water
(4) Pickling for 3 to 5 seconds at room temperature in a solution
of the composition:
4.3 ounces concentrated sulfuric acid (sp. gr.-1.84)
2.8 ounces concentrated nitric acid (sp. gr.-1.42)
1.0 gallon water
(5) Rinse in cold water
(6) Electrolytic treatment for approximately 15 minutes with a
4.0- to 8.0-volt, 60-cycle alternating current at a current
density of 20 to 30 amperes per square foot of magnesium
surface in a solution of the composition:
40.0 ounces sodium hydroxide
0.5 ounce phenol
5.2 ounces sodium silicate (41° Be.)
Water to make 1 gallon
Operating temperature of the solution was 180° to 200° F.
(7) Rinse in hot water
Rept. No. 1842-A
-3-
(8) Neutralizing for 1 to 5 minutes at 135° to 145° F. in solution
(pH-2.4) of the composition:
0.07 ounce chromic acid
1.0 gallon water
(9) Force drying in hot air
(10) Priming by dipping in a zinc chromate primer (MIL-P-6889A,
Type I) thinned to result in a dry film thickness of
0.00020 to 0.00035 inch
(11) Air drying prime coat for at least 24 hours prior to bonding.
B. The other method, involving only chemical treatments, was reported to
consist essentially of the following steps: (Metal pieces with this
method were slightly larger, 5- by 6-inches, than the standard size.)
(1) Vapor degreasing surfaces in stabilized trichloroethylene to
remove oils and contaminants
(2) Cleaning surfaces by immersion for 10 minutes at 170° to 190°
F. in a commercial phosphoric-silicate cleaner for magnesium
(5 ounces per gallon)
(3) Rinse in cold water
(4) Removing scale by immersion for 1 to 10 minutes at room temperatures it a solution of the composition:
24.0 ounces chromic acid
4.0 ounces sodium nitrate
1.0 gallon water
(5) Rinse in hot water
(6) Pickling for 5 minutes at room temperature in a solution of
the composition:
1 part by volume hydrofluoric acid (50 percent)
2 parts by volume water
(7) Rinse in cold water
(8) Sealing by boiling for 30 minutes in a solution of the
composition:
16.0 ounces sodium dichromate
0.2 ounce calcium fluoride
1.0 gallon water
Rept. No. 1842-A
-4-
I
(9) Rinse in cold water
(10) Forced drying in hot air
(11) Priming by dipping in zinc chromate primer, thinned with
toluene (1 part primer to 3 parts thinner) to a consistency to result in a dry film thickness of 0.00015 to
0.00025 inch
(12) Air drying prime coat 24 hours, and then curing for 30
Minutes at 180° to 200° F.
These two methods were essentially the same as methods M-5 and M-3,
respectively, in the original work of Report No. 1842, except that the
prime coat in the earlier work was much thicker, and different pro
prietary primers were used.
Adhesive Bonding Processes
The following 4 adhesive bonding processes were used it bonding the
lap-joint panels prepared with the 2 metals having several surface
conditions.
Bloomingdale FM-47.--A high-temperature-setting formulation of the
vinyl-phenolic type supplied by Bloomingdale Rubber Company, Delaware
and Flower St., Chester, Pa.
Redux E, Type R.--A high-temperature-setting, two-component formulation
of a phenol-resin solution and vinyl-pdymarpowder supplied by Ciba
Company, 627 Greenwich St.,•New York 14, N. Y.
Metlbond MN3C Nylon Tape.--A high-temperature-setting adhesive formulation of neoprene, nylon, and phenol resins, supported as a film on
nylon-fabric tape, supplied by NARWO Resins and Coatings Company, 600
Victoria St., Costa Mesa, Calif.
Scotchweld Bonding Film AF-6.--A high-temperature-setting formulation
of acryonitrile-butadiene rubber and phenol resin in the form of an
unsupported tape supplied by Minnesota Mining and Manufacturing Company,
411 Piquette Ave., Detroit, Mich.
These adhesives are of the same general types as used in the original
studies of Report 1842 and addendum, but the adhesive products included
are identical in only 1 of the 4 processes.
The conditions of bonding with each of the adhesives were as follows:
411
Bloomingdale FM-47.--Three spray coats of the adhesive, thinned with
1-1/2 parts by volume of adhesive solvent to 1 part of adhesive, were
Rept. No. 1842-A
-5-
applied to the metal with 1-hour air drying between coats, and overnight
air drying after the final coat. The adhesive film was then precured
for 1 hour at 150° F. in an oven. Following the precure, the joint was
assembled and placed in a hot press, where it was preheated without
pressure for 5 to 9 minutes at 300° F. before it was given the final
cure for 15 to 25 minutes at 300° F. and 200 pounds of pressure per
square inch.
R.--One medium coat of the liquid component was brushed
on the metal, and the powdered component was sprinkled immediately into
the wet spread of adhesive. Any excess powder was brushed from the
surface. The adhesive film was air dried overnight, and the joint was
assembled and pressed at 200 pounds per square inch of pressure for 25
minutes in a hot press at a temperature of 300° F. Approximately 10
minutes of the pressing period were required to bring the temperature
of the glue line to that of the press platen.
Redux E, Type
Metlbond MN3C Nylon Tape.--Four spray coats of the priming component
(M3C) were applied to the metal pieces to result in a 0.001- to 0.002inch film of adhesive. The adhesive film was dried for 30 minutes between coats and 2-1/2 to 4 hours after the final coat. The joint was
assembled with a single layer of tape adhesive and pressed at 50 pounds
per square inch pressure for 42 minutes in a hot press at a temperature
of 335° F. Approximately 12 minutes of the pressing period were required
to bring the temperature of the glue line to that of the press platen.
Scotchweld Bonding Film AF-6.--The single film of adhesive was assembled
in the joint. The assembly was pressed for 45 minutes at 150 pounds per
square inch of pressure in a hot press at a temperature of 325° F.
Approximately 10 minutes of the pressing period were required to bring
the temperature of the glue line to that of the press platen.
To insure uniform pressure distribution, all joints were pressed by
using cauls of 0.027-inch-thick chipboard between the press platen
and the metal pieces.
Testing
The 3 lap-joint panels prepared with each metal, surface treatment, and
bonding process were sawn into individual 1-inch-wide specimens. Cutting
was done with a metal-cutting bandsaw using a slow rate of feed and a
holding jig to minimize any mechanical damage or overheating of the
joint. The lap-joint specimens were tested to failure by loading them
in tension at a rate of 300 pounds per minute. The ends of the specimens were held in 1-inch-wide Templin-type grips that extended down
from the ends of the specimens to within 1-inch of the edge of the
overlap. Testing was done at a temperature of 72° to 76° F. The
failing load (calculated as unit stress on the measured test area)
Rept. No. 1842-A
-6-
and estimated areas (expressed as percentage of the total area) of
adhesion, cohesion, and primer and coating failures were recorded.
Test Results
The results of the bonding tests made to the chromic-acid-anodized
aluminum, unsealed and sealed, are given in table 1, and to the zincchromate-primed magnesium in table 2.
The results in table 1 definitely show that the hot-water sealing of
the aluminum oxide film formed during chromic-acid anodizing interferes
with bonding. This interference is much greater for types of adhesives
represented by Bloomingdale FM-47, Redux E, and Scotchweld Bonding Film
AF-6 than it is for the type of adhesive represented by Metlbond MN3C
Tape. The concentration of the chromic-acid anodizing solution (5 or
10 percent) and the length of the anodizing period (20 or 4o minutes)
were not found to influence the bonding properties of the anodized
surfaces significantly.
The results in table 2 again confirm the results indicated in the
original work and given in Forest Products Laboratory Report No. 1842;
namely, that adhesives of the type of Metlbond MN3C Tape produce bonds
to zinc-chromate-primed magnesium sheets that usually have higher bond
strengths than those obtained with the Redux E and Bloomingdale FM-47type adhesives. The latter two types are normally judged by lap-joint
tests with clad aluminum to have higher bond strengths than the first
type. The type of solvent in an adhesive, and its reaction with the
primer, might be expected to influence the bond strengths to the zincchromate-primed magnesium but this possible effect was not investigated.
The average shear strength of 2,770 pounds per square inch for the bonds
made with Metlbond Tape to the zinc-chromate-primed magnesium, treated
by method B, is considered by present standards to be unusually good
bonding to magnesium. The Metlbond Tape adhesive, and also Scotchweld
AF-6 film, gave bond strengths of about 1,950 pounds per square inch to
the magnesium treated by method A.
Rept. No. 1842-A
-7-
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