Alternative to Chrome Etching
by
James P. Bell
Department of Chemical Engineering and Polymer Science Program
University of Connecticut
Storrs, CT 06269
EPA Cooperative Agreement No. CR 821875
EPA Project Officer
Paul Randall
Sustainable Technology Division
National Risk Management Research Laboratory
Cincinnati, OH 45268
NATIONAL RISK MANGEMENT RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
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/p. TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing!
. TITLE AND SUBTITLE
Alternative to Chrome Etching Processes for Metals
7. AUTHOR(S)
James P. Bell
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Chemical Engineering and
Polymer Science Program
University of Connecticut
Starrs, CT 06269
12. SPONSORING AGENCY NAME AND ADDRESS
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
15. SUPPLEMENTARY NOTES : " ~
Project Officer: Paul M. Randall (513)569-7673
16. ABSTRACT"
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO."
1. CONTRACT/GRANT NO.
CR-821875
13. TYPE OF REPORT AND PERIOD COVERED
. SPONSORING AGENCY CODE
EPA/600/14
f1 ' 9 *e,Natlonal Center for Manufacturing Science, have initiated programs for chrome abatement
T" reduct"°n by Use of e^n9 **™*&* and do not address the elimination
^60""010^ development It is proposed to evaluate polymeric coupling agents as •
pretreatment processes on aluminum and steel
ffer a Practical alternative and improvements to chrome etching pretreatments presently utilized Ion
? lre moleculestnat h^e the ability to chemically react with both the metal
*"* forming a "bridge" between them- The coupling agents can be tailored to
h tlfs of ** P°*»"* top coat and the reactivity of the metal substrata Bond failure
mH h"midrty. ete-> can be greatiyireduced. In addition, polymeric coupling agents are able to
ure ^ "ear the meta' interfaoe thereby reducing internal stress concentrations that result in
C.ontainin9 both b-diketone and epoxy functional groups: This copolymer is being '
oxidesurface- Etehed coupling agent treated joints exhibited improved joint
ASTM B1l ' C°Uplin9 a9enttreated alumin"m substrates displayed improved corrosion resistance after
ASTM 81 17 salt spray chamber exposure as compared to untreated controls
ItaSSfSSSS^^f^S^ ^ *"f9 6ValUated * P*6"*31 P0^"16^ coupling agents fortne steel-epoxy system.
LSnH P ectrosc°Py and X-ray Photoelectron Spectroscopy indicate chemical interactions of the coupling agent wtth both the
S1±H ^l"^
SSSi X ™ PhS^on S±i0t W3ter "Jf^ and C0rrosion resistance were also observed fofcoupling agent treated
wS the e^ayer Spectroscopy and Scanning Electron Microscopy investigations suggest that failure is occurring
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Pollution prevention
Polymeric coupling agents
Non-chromium pretreatment
18. DISTRIBUTION STATEN
Release to Public
Form 2220.-] (Rat. 4—77) PREVIOUS EDITION is OBSOLETE
b.lDENTIFIERS/OPEN ENDED TERMS
Chrome etching
Surface coating
Corrosion resistance
19. SECURITY CLASS (This Report!
Unclassified
20. SECURITY CLASS (Tills page/
Unclassified
COSATI Field/Group
21. NO. OF PAGES
22. PRICE
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Notice
The U.S. Environmental Protection Agency through its Office of Research and Development
partially funded and collaborated in the research described here under Cooperative Agreement
CR821875 to the University of Connecticut. It has been subjected to the Agency's peer and
administrative review, and has been approved for publication as an EPA document. Mention of
trade names or commercial products does not constitute endorsement or recommendation for
use.
11
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's
land, air, and water resources. Under a mandate of national environmental laws, the Agency
strives to formulate and implement actions leading to a compatible balance between human
activities and the ability of natural systems to support and nurture life. To meet this mandate, EPA's
research program is providing data and technical support for solving environmental problems today
and building a science knowledge base necessary to manage our ecological resources wisely,
understand how pollutants affect our health, and prevent or reduce environmental risks in the
future.
The National Risk Management Research Laboratory is the Agency's center for investigation of
technological and management approaches for reducing risk from threats to human health and the
environment. The focus of the Laboratory's research program is on methods for prevention and
control of pollution to air, land, water and subsurface resources; protection of water quality in public
water systems; remediation of contaminated sites and ground water; and prevention and control of
indoor air pollution. The goal of this research effort is to catalyze development and implementation
of innovative, cost-effective environmental technologies; develop scientific and engineering
information needed by EPA to support regulatory and policy decisions; and provide technical
support and information transfer to ensure effective implementation of environmental regulations
and strategies.
This publication has been produced as part of the Laboratory's strategic long-term research plan.
It is published and made available by EPA's Office of Research and Development to assist the user
community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
ill
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Abstract
Several industries, including the National Center for Manufacturing Science have initiated
programs for chrome abatement. The programs, however, generally focus on chrome
reduction by use of existing technologies and do not address the elimination of chrome in
pretreatment processes by new technology development. It is proposed to evaluate
polymeric coupling agents as replacements for chromium pretreatment processes on
aluminum and steel.
Polymeric coupling agents offer a practical alternative and improvements to chrome
etching pretreatments presently utilized on aluminum and steel. Polymeric coupling
agents are molecules which have the ability to chemically react with both the metal
substrate and wide variety of polymer top coats, thus forming a "bridge" between them.
The coupling agents can be tailored to specific systems, depending on the functionalities
of the polymer top coat and the reactivity of the metal substrate. Bond failure caused by
environmental factors (humidity, etc.) can be greatly reduced. In addition, polymeric
coupling agents are able to distribute mechanical and thermal stresses near the metal
interface thereby reducing internal stress concentrations which result in bond failure.
A copolymer has been polymerized containing both b-diketone and epoxy functional groups.
This copolymer is being evaluated as a potential polymeric coupling agent for bonding
polymers to aluminum. X-ray Photoelectron Spectroscopy indicates interaction of the
coupling agent with the aluminum oxide surface. Etched coupling agent treated joints
exhibited improved joint strengths after immersion in water. Coupling agent treated aluminum
substrates displayed improved corrosion resistance after ASTM B117 salt spray chamber
exposure as compared to untreated controls.
Quinone-amine polyurethanes (QAPs) are being evaluated as potential polymeric coupling
agents for the steel-epoxy system. Infrared Spectroscopy and X-ray Photoelectron
Spectroscopy indicate chemical interactions of the coupling agent with both the steel and
epoxy surfaces. Quinone-amine treated steel substrates exhibited increased joint shear
strengths as compared to untreated controls. Increases in both hot water stability and
corrosion resistance were also observed for coupling agent treated substrates. X-ray
Photoelectron Spectroscopy and Scanning Electron Microscopy investigations suggest
1:hat failure is occurring within the epoxy layer.
This report was submitted in fulfillment of Cooperative Agreement No. CR821875 by the
University of Connecticut. This report covers a period of time from October 1993 to
October of 1995.
IV
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Table of Contents
Foreword iii
Abstract , iv
List of Figures .... vi
List of Tables viii
Chapter 1 Introduction 1
Chapter 2 Conclusions 4
Chapter 3 Recommendations , la
Chapter 4 Experimental 7
Chapter 5 Results and Discusion 10
References 21
v
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List of Figures
II. General structure of an oxygen-bonded b-diketone 2
2. Modified torsional shear joint [19] 8
3. Chemical structures of a) AQM-1, b) TDI, and c) Teirathane 650 8
4. Chemical structure of 7-octen-2,4-dione « 10
5. Reflection-Adsorption Infrared spectra of a) a thick layer of 7-octen-2,4-dione
on an aluminum substrate, and b) the sample from (a) after thorough rinsing with
methanol - • • 10
6. Tautomeric forms of b-diketbnes 10
7. Suggested structure of the b-diketone cheiate with the aluminum oxide
surface [20J 11
8. AI2p region of the XPS spectrum of a) an aluminum substrated following
treatment with 7-octen-2,4-dione and b) a bare aluminum substrate 12
9. Chemical structure of 2-(methacryloyloxy)ether acetoacetate 12
t
10. Shear strength (kPa) as a function of days immersed in 57° C water for degreased
coupling agent treated joints and untreated degreased controls 15
11. Shear strength (kPa) as a function of days immersed in 57° C water for ethched
coupling agent treated joints and untreated etched controls 16
12. Micrograph (20x magnification) of degreased aluminum substrate after 24 hours
in an ASTM B117 salt spray chamber 16
13. Micrograph (20x magnification) of etched aluminum substrate after 24 hours in
an ASTM B117 salt spray chamber 17
14. Water Stability of the quinone-amine poiyurethanes 18
VI
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List of Tables
1. Binding energies (eV) for etched samples after immersion in pure solvent and
coupling agent solution 14
2. Binding energies (eV) for degreased samples after immersion in pure solvent and
coupling agent solution . - 14
3. Dry shear strength of joints treated with CA1 as compared to untreated controls as
a function of coupling agent concentration 17
4. Joint shear stresses of coupling agent treated steel joints 17
5. Dry shear strength of joints treated with organosilanes GS and AS as compared to
untreated controls 19
6. Dry shear strength of joints treated with b-diketone and PETM as compared to
untreated controls 19
7. ASTM B117 Salt spray apparatus corrosion resistance evaluation 20
vn
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Chapter 1
Introduction
The National Center for Manufacturing Sciences and
many industrial companies have initiated major programs
concerning chrome abatement Such programs, however, are
generally limited to reducing the quantity of chromium
compounds utilized annually in existing products and
technologies and do not address the elimination of chrome use
through new, longer term technology development [1].
Acidic chromium oxide, chromic acid or dichromate salts are
widely utilized in etching or sealing pretreatment of
aluminum and steel parts prior to bonding to adhesives or
application of various organic coatings, for the purpose of
promoting durable adhesion. A list of applications for
chrome in surface etching, preparation, and cleansing baths is
shown below [2 and 3].
• Chromic acid deoxidizing/desmutting -
chromic acid is used to remove the natural metal
oxide and other surface contaminants to prepare a
surface for plating, painting, or other surface
treatment.
• Chromic acid etch - chromic acid roughens
surfaces to improve coating adhesion.
• Chromic acid anodize - anodizing is a
process for treating aluminum to give a highly
corrosion-resistant coating that offers an excellent
surface for bonding and painting. Anodizing uses
electrochemical methods to form a thin aluminum
oxide layer that contains chromium ions.
• Sealing - Sodium dichromate as a sealant
enhances fatigue properties by anodizing and by
depositing oxydichromate onto the anodized layers.
• Chemical film - chromate solutions can be
used to chemically deposit a thin film to prepare
metal surfaces for subsequent coating application.
The key factor in these processes for promotion of
durable adhesion is hexavalent chromium ions, which are
water soluble and very toxic to most species of aquatic life as;
well as to humans and animals [4-6]. Present chemical
processes to remove chromium from effluent baths include;
reduction of the hexavalent chromium to the trivalent form by!
sodium sulfite, followed by precipitation, electroplating, ion
exchange or electrowinning, or reverse osmosis [7].
Precipitated chromium compounds are then disposed of in
landfills with possible legal ramifications, or recycled at high
cost Concentrated solutions may be reused or recycled.
Although optimization of standard technologies can >
result in a reduction of the quantity of toxic chromium1
compounds utilized in industrial pretreatment processes, it is
only through the elimination of chromium prelreatment:
processes that contamination will be completely avoided. >
Elimination of chromium pretreatment processes requires the
development of new technologies, through fundamental
research, that yield coating bond strengths and bond
durabilities comparable to those of chrome pretreated. surfaces,
without the use of chromium compounds. The objective of
this research was to investigate polymeric coupling agents as;
potential replacements for chromium compound
pretreatments.
Use of polymeric coupling agents offers a practical
alternative and improvement to the chrome pre-treatments
presently utilized on aluminum and steel [8-13]. From the
environmental aspect, the elimination of the chrome etching
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process would greatly reduce a source of toxic hexavalent
chromium. Polymeric coupling agents are typically applied
from solutions of less than 0.1 wt% coupling agent, thus
significantly limiting quantities for disposal. Solution
compositions will be dependent upon the coupling agent
under study, however a major focus during coupling agent
development will be low toxicity. In addition, the polymeric
coupling agent solutions will likely be recyclable.
Polymer coupling agents offer a wide range of
advantages in addition to the environmental issue. Of primary
interest is that coupling agents can be optimized for a specific
metal/polymer system, reacting with the functionalities of the
metal and polymer substrate which are to be bonded. In
addition, the functionalities and their relative concentrations
on the coupling agent backbone can be tailored to improve
environmental resistance to bond failure. For example, the
coupling agent can be made hydrophobic to enhance corrosion
resistance. The porous nature of the oxidized aluminum
surface facilitates both mechanical interlocking and chemical
(ionic or covalent) bonding to active surface moieties.
Another advantage of polymeric coupling agents is that they
are able to absorb and distribute mechanical and thermal
stresses within the bond, thereby reducing internal stress
concentrations which result in bond failure.
In contrast to "primers", coupling agents form true,
verifiable chemical (ionic or covalent) bonds to both the metal
oxide and the polymer substrate, whereas primers typically
enhance wetting or other physical surface interactions. In
addition, a true coupling agent is optimal at a thickness of
150A. Primers are typically applied in layers on the order of
microns in thickness [8].
Scientific evaluation of organic coupling agents on
metals, other than silanes and titanates, has been very limited. \
Silanes and titanates have been discussed in a review by
Comyn [13]; while evidence of chemical attachment of these
two agents has been found, the improvement in mechanical ;
properties of adhesive joints relative to controls is not
substantial in many instances, especiaily when water is
present
The objective of this research was to develop and/or •
identify suitable polymeric coupling agents for bonding
polymers to aluminum and steel, thus eliminating the use of ;
the environmentally unfriendly chromium pretreatments and
ultimately yielding bond strengths and bond durability equal :
or superior to current levels. • •
i
Coupling Agent Selection ;
Aluminum
Investigations of a p-diketone functionalized
copolymer as a potential polymeric coupling agent are
presently in progress, P-diketones have been shown to form !
stable chelates with a variety of metals and metal oxides [14
and 15]. These metal p-diketonates behave as inner
complexes, showing predominantly covalent behavior [15].
Figure 1 shows the general structure of an oxygen-bonded P-
diketone. Low molecular weight p-diketones have also been
shown to exhibit corrosion resistance and improve coating
adhesion on aluminum-substrates [14].
R
CH
M
R1
Figure 1. General structure of an oxygen-bonded
diketone.
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Steel
Quinone-amine polymers have been shown to adhere
strongly to metals and alloys and greatly improve corrosion
resistance of Iron particles [16]. The corrosion resistance has
been attributed to the presence of functional groups that can
function as moisture resistant adhesion promoters [16-18]. A
significant example is that they have been shown to displace
water from wet, rusty steel surfaces [18]. Figure 2 shows the
chemical structures of the components of the copolymer. By
altering the relative ratios of the components in the!
copolymerization we are able to vary the amount of soft
segment, diol, in the copolymer, thus enabling us to produce
an entire range of coupling agents with varying degrees of
molecular flexibility. Reactivity of the QAPs with the
organic top coat is under investigation.
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Chapter 2
Conclusions
Aluminum
A copolymer has been polymerized containing both
P-diketone and epoxy functional groups. This copolymer is
being evaluated as a potential polymeric coupling agent for
bonding polymers to aluminum. X-ray Photoelectron
Spectroscopy indicates interaction of the coupling agent with
the aluminum oxide surface. Etched coupling agent treated
joints exhibited improved joint strengths after 3 and 5 days
immersion in 57° C water. Coupling agent treated aluminum
substrates displayed improved corrosion resistance after 24
hours in an ASTM B117 salt spray chamber as compared to
untreated controls.
Coupling agent treated aluminum substrates will be
sent to Astroseal Products Manufacturing Co., Inc., Old
Saybrook, CT and ALCOA, Alcoa, PA for evaluation during
the first quarter of 1996. Both companies have expressed
interest in sample evaluation and assistance with
commercialization of viable products and technologies.
The results of this research were presented at four
national conferences: Annual Adhesion Society Meeting,
February 17-21, 1995, Hilton Head, SC; SAMPE's 27th
International Technical Conference, October 7-12, 1995,
Albuquerque, NM; AIChE Annual Meeting, November 12-
17, 1995, Miami, FL; and the Annual Adhesion Society
Meeting, February 18-22, 1996, Myrtle Beach, SC.
Steel
Quinone-amine polyurethanes (QAPs) are being
evaluated as potential polymeric coupling agents for the steel-
epoxy system. Infrared Spectroscopy and X-ray Photoelectron'
Spectroscopy indicate chemical interactions of the coupling'
agent with both the steel and epoxy surfaces. Quinone-aminei
treated steel substrates exhibited increased dry joint shear
strengths as compared to untreated controls. The increase in
joint strength is directly related to both the amount of
quinone-amine moieties in the coupling agent and the
concentration of the coupling agent solution. Increases in
i
both hot water stability and corrosion resistance were also
observed for coupling agent treated substrates. X-ray*
Photoelectron Spectroscopy and Scanning Electron
Microscopy investigations suggest that failure is occurring
within the epoxy layer. Preliminary investigation of (J--
diketone, silane and thiol functionalized polymers indicate
good promise for use as coupling agents for the steel-epoxy
system. < j
Samples have been sent to Inland Steel, East
Chicago,IL, United Technologies, East Hartford, CT, and
Hamilton Standard, Windsor Locks, CT, for evaluation.
These companies have also expressed interest in sample
evaluation and assistance with commercialization of viable
products and technologies. I
These results have been presented at a poster session
in the 18th Annual Adhesion Society Meeting, February 19-
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22, 1995 at Hilton Head, SC and in a talk at the Society of at the 19th Annual Adhesion Society Meeting, February 1996 ;
Plastics Engineers Meeting (ANTEC 95), May 7-11, 1995 in ' at Myrtle Beach, SC. j
Boston, MA. The final results will also be presented in a talk , i
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Chapter 3
Recommendations
Coupling agent treated steel and aluminum substrates
exhibited both increased joint strengths and improved
corrosion resistance as compared to untreated controls. In
addition, spectroscopic analysis indicates chemical interaction
between the coupling agents with both the metal and polymer
substrates. As a method of direct comparison, similar joint
strength and corrosion resistance evaluations must be
performed on chromium pretreated substrates. Further work is
required in the area of optimizing the coupling agent treatment
process. Development on the quinone-amine coupling agent;
system for steel should continue in direct conjunction with
industrial partners in order to insure optimization proceeds in
a direction toward potential commercializatioa. The
aluminum coupling agent system requires additional research
in the area of coupling agent development as well as process
optimization.
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Chapter 4
Experimental
Aluminum
Synthesis of 7-octen-2,4-dione was performed via
Claisen condensation of ethyl acetate and 5-hexen-2-one. To a
suspension of 0.1 mole of sodium methoxide (Fisher) in 85
ml of tetrahydrofuran was added dropwise 0.1 mole of 5-
hexen-2-one (Aldrich), with stirring. Upon addition, the
mixture was allowed to continue stirring 15 minutes at room
temperature. This was followed by the dropwise addition of
0.1 mole ethyl acetate (Aldrich). The reaction vessel was
fitted with a condenser and heated at reflux for 24 hours. The
tetrahydrofuran was removed via rotary evaporator. The
remaining residue was dissolved in distilled water and the
excess reagents were removed by ether extraction. The (3-
diketone was released from its sodium complex by adjusting
the pH to 5 with a 10% solution of hydrochloric acid. The |J-
diketone was. separated via extraction with ether. Evaporation
of the ether revealed a red-brown liquid. The product was
confirmed by infrared, positive test with iron (111), and the
green copper chelate formation.
The 2-(methacryloyloxy)ethyl acetoacetate copolymer
was formed, via radical polymerization of 0.10 mole 2-
(methacryloyloxy)ethyl acetoacetate (Aldrich), MAEAA, with
0.37 x 10" * mole glycidyl methacrylate (Acros) in 50 ml of
acetone at 68° C for five hours. 0.34 x 10"^ moles of
benzoyl peroxide (Aldrich) were utilized as the initiator. The
copolymer was precipitated in methanol. Solution NMR of
the copolymer suggests copolymerization is in a ratio of 2
epoxy groups to 1 MAEAA group. The presence of the {3-
diketone was verified by formation of the solid green copper
chelate of the copolymer. ;
Two pretreatment processes were employed prior to,
coupling agent application, degreasing and an etch. The.
degreasing process consisted of immersion in methanol for 30
minutes, wiping with a lint-free methanol soaked cloth, and
air drying. Samples subjected to the etch pretreatment were
immersed in a 5% sodium hydroxide solution, at 90° C for 1,
minute, rinsed in distilled water, and desmutted. Desmutting
was accomplished via immersion in a 20% nitric acid solution
at room temperature for 5 minutes followed by a distilled
water rinse and air drying.
X-ray Photoelectron Spectroscopy (XPS)j
experiments were conducted on a 5300 Series Perkin-Elmer
XPS. All data was collected at a pressure less than 2x10" 8
torr, 15.0 kV, and 600 watts. Curve fitting was performed on
high resolution spectra collected with a pass energy of 8.45;
eV. I
Modified torsional shear joints, Figure 2, were'
utilized to evaluate the joint strength and durability. Joint
halves were pretreated and air dried [19]. If the joints were to
receive the coupling agent treatment it was then applied.
44mg ± 1 of epoxy resin (Epon 828 and 4,4'-
methylenedianiline, 28phr) was applied to the raised annulus
of the torsional joint, the joint halves bonded together and the
entire joint cured at 120°C for one hour and 150°C for two
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hours. Joint durability studies were conducted by immersing
cured joints into 57°C water for varying lengths of time (days)
and measuring the joint strength after removal from the water.
CH,
© © © ©
H4- HLK
U- .*iua3 U- -Jua»
Figure 2. Modified torsional shear joint [19].
Steel
Three quinone-amine polyurethanes (QAPs), a low
molecular weight p-diketone (Benzoyl acetone), 2 Silanes (3-
glycidoxypropyltrimethoxysilane and 3-aminopropyl
-triethoxysilane) and PETM (Pentaerythritol
tetramercaptopTopionate) have been used as coupling agents
for the epoxy-steel system. The low molecular weight P-
diketone and the silanes were purchased from the Aldrich
Chemical company. The silanes were purified prior to use.
PETM was purchased from Pfaltz and Bauer Inc.
The QAPs compounds were synthesized by Dr.
Nikles's group at the CMTT, Univ. of Alabama, by a two step
process. The first step gives a prepolymer with a hard block,
to which a soft segment diol is reacted subsequently to give
the block-copolymer. The QAPs contain 40%, 20% and 30%
of the amine-quinone monomer (AQM-1) and stoichiometric
amounts of TDI and Tetrathane 650 (Polytetrahydrofuran of
mol. wt 650) (Figure 3).
NCO
NCO
(a)
HO+ CH2- CH2~ CH2- CH2- O j~ H
(b)
O
OH
(c)
Figure 3. Chemical structures of the components of the
quinone-amine copolymer: a) toluene ;
diisocyanate, b) tetrathane, c) amine-quinone '
monomer 1. '
The hard segment content of these segmented '
polyurethanes were 67%, 34% and 51% respectively. The
AQM was made from 1,4-benzoquinone and 2-(N-
methylamino) ethanol. A low-molecular weight amine
quinone monomer (AQM-2) was made to aid in surface
studies. The polymers have been characterized by IR !
spectroscopy, thermal analyses (TGA and DSC) and GPC. ,
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The conditions for the surface preparation of the
joints were optimized. The following surface treatment was
found to be die best for the steel joints. The joints were
immersed in a 3% citric acid aqueous solution with the pH of
the solution adjusted to the value of 7 using ammonium
hydroxide. This solution was maintained at a temperature of
70° C. After ten minutes, the joints was immersed in distilled
water to remove the excess of the acid followed by an
immersion in methanol to remove the excess of water. Next,
the joints were immersed in a dilute solution of the coupling
agent in the appropriate solvent. The solvents used wereTHF
for the QAP, methanol-water for the silanes (fresh solutions),
and methanol for the low molecular weight (i-diketone and the
PETM. The excess coupling agent was washed off with
solvent and the surface dried. A stoichiometric amount of
Epon 828® epoxy resin and a methylene dianiline (MDA)
curing agent were mixed, and a calculated quantity of this
resin was applied to each joint. The joints were subjected to a
cure cycle of 120°C for 1 hour followed by 150°C cure for 2
hours. Finally, the joints were cooled in air.
The surface of steel coupons were analyzed after the
application of the QAP or the low molecular weight {5-
diketone using IR Reflectance Microscopy, ATR-IR
(Attenuated Total Reflectance Infrared Spectroscopy). IR was
used in the transmittance mode to study the interaction of
QAP with the Epoxy.
X-ray Photoelectron Spectroscopy (XPS)
experiments were conducted on a 5300 Series Perkin-Elmer
XPS. All data was collected at a pressure less than 2x10'°
torr, 15.0 kV, and 600 watts. Curve fitting was performed on
high resolution spectra collected with a pass energy of 8.95
eV. XPS (X-ray Photoelectron Spectroscopy) studies were
done on the bare metal surface and the QAP coupling agent
treated surfaces. The spectra were obtained after treatment of j
the steel coupons with the QAPs and washing off the excess
coupling agent using pure solvent.
The locus of failure analysis was done by analyzing
fracture surfaces (broken in pure shear) of steel-epoxy lap:
shear joints made with the QAPs as the coupling agents. XPS '
and Scanning Electron Microscopy (SEM) were used in
tandem to analyze these fracture surfaces.
The corrosion resistance of steel coupons coated with <
the coupling agent followed by epoxy resin were evaluated in:
accordance to ASTM Bl 17 standard test method of salt spray
(fog) testing using a lab scale salt spray apparatus. The'
samples were held in a mist of salt water for a period of 10,
days and analyzed qualitatively.
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Chapter 5
Results and Discussion
Aluminum
7-octen-2,4-dione Synthesis
We have successfully synthesized a polymerizable P-
diketone, 7-octen-2,4-dione, Figure 4. Preliminary
investigation of the interaction of 7-octen-2,4-dione with an
aluminum oxide substrate via Refection-Absorption Infrared
Spectroscopy suggests the formation of a chelate between the
p-diketone and aluminum in the aluminum oxide layer.
Figure 5a is a spectrum of 7-octen-2,4-dione on an aluminum
substrate. The peak at 1703 cm"1 corresponds to that of the
free ketone and the peak at 1612 cm"1 to the enolic form of
the diketone tautomer, Figure 6. The absence of the peak at
1703 cm"1 in Figure 5b, after through washing of the
substrate, suggests that the only p-diketone that remains on
the aluminum substrate after washing is in a 6-membered ring
formation or is chelated with aluminum in the oxide layer.
Figure 7. Similar results have been reported by Allara for
2,4-pentanedione on aluminum substrates [20].
S CH- CH2-
o o
II II
C- CH2- O CH3
Figure 4 Chemical structure of 7-octen-2,4-dione
Figure 5.
1703
1400
•'I
Reflection-Adsorption Infrared spectra of a)
a thick layer of 7-octen-2,4-dione on an
aluminum substrate, and b) the sample from \
(a) after thorough rinsing with methanol.
R'
H,C
-^»
R
Figure 6. Tautomeric forms of |}-diketones.
0..,-\0 0/\
I °\ ^—^ I II
-------
Figure 7. Suggested structure of the p-diketone chelate
with the aluminum oxide surface [20].
The A12p region of the XPS spectrum of an
aluminum substrates following treatment with 7-octen-2,4-
dione is shown in Figure 8a. The spectrum of the bare
aluminum oxide is shown in Figure 8b. The appearance of a
second oxide peak at lower binding energies in the A12p
region of the XPS spectrum of an aluminum substrates
following treatment with 7-octen-2,4-dione suggest that either
the P-diketone is promoting dissolution of the weak outer
oxide layer while exposing an underlying layer with a different,
chemical structure or that the second peak may be due to
chelate formation of the P-diketone with aluminum in the,
aluminum oxide layer. Further study is necessary to
determine the origin of the second oxide peak.
As stated, our goal was to copolymerize 7-octen--
2,4-dione with an epoxy reactive functionalized monomer and.
to then evaluate the polymer in terms of joint strength and
corrosion resistance imparted to coated aluminum substrates..
Unfortunately, after extensive experimentation,
polymerization proved futile. Polymerization was;
complicated due to the hydrogen atom in the alpha position to \
the allyl functionality. This highly reactive hydrogen atom
promoted chain transfer, thereby halting polymer growth.
The polymerization difficulty prompted a change in
research plans in mid-May 1995. We purchased a
commercially available p-diketone, 2-(methacryloyloxy)ethyl
acetoacetate (MAEAA), Figure 9. , We have successfully!
homopolymerized and copolymerized MAEAA
11
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*s
**k
ac
77
76
75 7-1 73
B1WISC ENERGY. eV
71
77
7E
75 7-1 73
BM1IHG EW3GY. eV
71
Figure 8. A12p region of the XPS spectrum of a) an aluminum substrated following treatment with 7-octen-2,4-dione;
andb) a bare aluminum substrate.
CH
HoC=(
3 o
II
C-
-o—
c-
H2
•c-
H2
-O—
O
II
c-c-
H2
O
II
-c
CH,
Figure 9. Chemical structure of 2-(methacryloyloxy)ether acetoacetate.
12
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Copolymer Synthesis
The copolymer was formed via radical
polymerization of 2-(methacryloyloxy)ethyl acetoacetate,
MAEAA, with glycidyl methacrylate in of acetone, as
described in the Experimental Section of this report. The
copolymer has a Tg of 47° C and a decomposition temperature
of 232° C. The chemical interaction of the copolymer with
an aluminum oxide substrate was evaluated with X-ray
Photoelectron Spectroscopy.
X-ray Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy was utilized to
investigate the nature of the interaction between the
copolymer coupling agent and an aluminum substrate. Etched
and degreased samples were prepared as described in the
Experimental section of this paper. Verification of the
presence of the coupling agent on the substrate surface
following coupling agent treatment was accomplished by
comparing the Cls regions of the spectra of a solvent cast
copolymer film to the spectra of an aluminum substrate
following coupling agent treatment. Due to the complexity
of the Cls spectrum for the copolymer, limited information
regarding the nature of the coupling agent/aluminum substrate
interaction was obtained. If chelation of the {J-diketone was
occurring with aluminum ions in the aluminum oxide layer of
the substrate then this would result in an alteration in the
electronic environment of the affected aluminum atoms.
Thus, the A12p region of the spectrum was investigated at
high resolution. In addition, to eliminate any changes in
binding energies that may be a result of solvent interactions,;
coupling agent treated substrates were compared to aluminum
substrates that were subjected to the same preteeatment
process and soaked hi pure solvent for 1 hour. Tables 1 and 2'
list the binding energies for two sets on samples, etched and!
degreased, respectively. For the etched samples, mere is no'
effect of the coupling agent on the binding energies of either!
the oxide or the metallic aluminum species with their binding;
energies at 75.31 and 72.78 respectively. The degreased!
samples, however, showed a distinct shift to lower binding;
energies for the oxide peak as a result of coupling agent
treatment The sample soaked in pure solvent displayed an
oxide peak at 75.55 and a metallic aluminum peak at 72.65;
eV whereas the coupling agent treated sample had peaks at
74.41 and 72.65 eV respectively. Two explanations are,
offered for the shift hi the binding energy for the oxide peak:'
1) the peak is due to chelate formation of the p-diketone with;
the aluminum atoms in the aluminum oxide layer, or 2)
dissolution of the outermost layer of the aluminum oxide;
occurred during immersion in the coupling agent, exposing an
underlying aluminum oxide layer, of different chemical'
composition. Further investigation is necessary to;
distinguish between these two possible causes for the peak'
shift. This shift in binding energy has been noted previously
for studies of the low molecular weight P-diketone 7-octen-;
2,4-dione [9]. Reflection-Adsorption Infrared Spectroscopy
experiments are currently in progress.
13
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Table 1.
Binding energies (eV) for etched samples after immersion in pure solvent and coupling agent solution.
NMP
eV
72.78
75.31
NMP / CA
eV
72.76
75.32
Assignment
metallic Al
Al oxide
Table 2. Binding energies (eV) for degreased samples after immersion in pure solvent and coupling agent solution.
NMP
eV
72.65
75.55
NMP / CA
eV
72.65
74.41
Assignment
metallic Al
Al oxide •
Joint Strength and Durability
Joint strengths and durabilities for the copolymer
treated joints were evaluated via torsional shear experiments
[21]. Figure 10 illustrates the joint strengths of degreased
copolymer treated torsional shear joints in comparison to
untreated controls. Each experimental data point is the
average of at least four joints and the error bars are the
standard deviation of the data set. Dry joint strengths for the
coupling agent treated and untreated joints are comparable.
Strengths after one day of immersion in 57° C water revealed
a significantly lower joint strength for the coupling agent
treated joints as compared to the untreated controls. Joint'
strengths after 3 and 5 days immersion, however, again
revealed comparable strengths for both the coupling agent
treated and the untreated controls. The initial decrease in
joint strength following water immersion is generally
attributed to the plasticization of the epoxy due to moisture,
uptake. The increased rate of joint strength loss over the first,
one to two days for the coupling agent treated joints suggests
that the presence of the coupling agent influences the initial
rate of moisture uptake of the resin. ;
14
-------
Sa
to
I
00
00
MAEAA
Degreased
Oegrsase/CA
Days Immersed in 57 °C Water
Figure 10. Shear strength (kPa) as a function of days immersed in 57° C water for degreased coupling agent treated joints
and untreated degreased controls.
Figure 11 illustrates the joint strengths of etched
copolymer treated joints as compared to etched untreated
controls. Dry joint strengths for the coupling agent treated
and untreated joints are comparable. Strengths after one day
of immersion in 57° C water revealed a significantly lower
joint strength for the coupling agent treated joints as
compared to the untreated controls, 'similar to that observed
for the degreased samples in Figure 10. Joint strengths after 3
and 5 days immersion, however, revealed improved strengths
for the coupling agent treated as compared to the untreated
controls. Thus at longer immersion times the coupling agent
improves joint durability. ;
15
-------
a
•b
w>
ss
s
+->
CO
JS
00
Days Immersed in 57 °C Water
Figure 11. Shear strength (kPa) as a function of days immersed in 57° C water for ethched coupling agent treated joints
and untreated etched controls. <
Corrosion Resistance
Corrosion resistance of a coupling agent treated
substrate was evaluated by visual inspection of two aluminum
substrates following exposure to an ASTM B117 salt spray
chamber for 24 hours. One substrate was degreased and served
as a control sample while the second sample was degreased
and treated with the coupling agent. Micrographs (20x
magnification) of the two sample surfaces are shown in
Figures 12 and 13. The untreated aluminum substrate showed
extensive discoloration and pitting. The coupling agent
treated sample exhibited significantly less discoloration and
pitting as compared to the untreated control. The
discoloration and pitting on the treated sample appeared to be
concentrated in distinct region suggesting that the coupling
agent coating was not uniform over the entire surface. This
incomplete coverage may be due to incomplete cleaning
during the degreasing process.
Figure 12. Micrograph (20x magnification) of'
degreased aluminum substrate after 24 hours
in an ASTM B117 salt spray chamber. :
16
-------
Figure 13. Micrograph (20x magnification) of etched
aluminum substrate after 24 hours in an
ASTM Bl 17 salt spray chamber.
Steel
Joint Strength
The AQM-1 (model compound) and the quinone-
amine polyurethanes (QAPs) were found effective in very low
concentrations e.g., 0.05%, 0.2% etc. by weight in THF, and
caused an increase in the joint strength with increased
concentration. The average (sets of seven) measured joint
shear strengths of joints made using AQM-model as the
coupling agent (CA) are found in Table 3 below.
The QAPs were found to be effective, even in low
solution concentrations, in increasing the joint shear stress of
the steel-epoxy joints. Joints showed an increase in strength
with increased concentration of the coupling agent solution.
The joint shear stresses of the coupling agents (CAs) QAP-;
( :
IB, -2B and -3B (with tetrahydrofuran as the solvent) treated
joints are found in Table 4.
Table 4. Joint shear stresses of coupling agent treated
steel joints I
% CA in
solution
(by wt)
0
0.1
0.2
0.5
1.0
QAP1B
as CA
(psi)
8760
9830
9855
9950
10475
QAP2B
as CA
(psi)
8760
9855
9995
10240
10890
QAP3B
as CA
(psi)
8760
9835
9955
10145
10745
Table 3. Dry shear strength of joints treated with
CA1 as compared to untreated controls as a
function of coupling agent concentration.
Sample
Shear Strength (psi)
Controls
0.2% CA1
0.4% CA1
8760
9365
9665
The increase in the dry joint shear strength from the
low molecular weight compound (AQM) to the polymeric
coupling agents (QAP-1B and QAP-2B) is due to the ability
of a polymer to distribute stresses better. In the polymers, the
increase in joint shear strength with increase in concentration'
of the coupling agent used probably indicates the presence of a{
good amount of entanglement of the coupling agent with the
epoxy topcoat
Among the polymers used, the QAP-2B has a lower.
melting point and a higher soft segment content and so is;
17
-------
much more flexible than the QAP-3B and -IB copolymers.
This, probably leads to much more entanglement of chains at
the interface and crack tip energy dissipation when this
polymer is used, with consequent improvement in dry joint
strength.
Water durability tests also indicated that the QAP
coupling agents could be used to make joints that were
extremely stable in hot water (60°C). The comparative water
stability data for the three QAPs is shown in Figure 14.
1.2 10
g 8000
s_X
| 6000
I 4000
C/3
20001
0
Controls
QAP-1B
QAP-2B
QAP-3B
0246
Number of days in hot water (at 60Q
8
Figure 14. Water Stability of the quinone-amine polyurethanes.
The QAP-1B outperformed the QAP-3B and QAP-2B
(in the same order) in strength retention over longer periods of
immersion in hot water. Quinone-amines are known for their
hydrophobicity and these polymers are indeed very stable in
water. The reason for the initial drop in the shear stress in
both the cases may be more to do with the plasticization
effects of water on the epoxy and coupling agent than any
actual bond rupture across the interface. The higher strength
retention of the QAP-IB may be due to its inherent rigidity
which results from its stiffer backbone and higher hard
segment content when compared to QAP-3B and QAP-2B'.
This allows a much lower quantity of water to penetrate, thus
reducing the plasticization effect.
Two silanes, 3-glycidoxypropyltrimethoxy-silane
(GS) and 3-aminopropyl -triethoxysilane (AS) were evaluated
for their effectiveness as coupling agents for steel. A
methanol-water solvent system was utilized. The average (sets
18
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of seven) dry joint shear strengths of the silane treated joints
are found in Table 5.
Table 6. Dry shear strength of joints treated with p"-
diketone and PETM as compared to;
untreated controls. '
Table 5. Dry shear strength of joints treated with
organosilanes GS and AS as compared to
untreated controls.
Sample
Controls
0.2% GS treated
0.2% AS treated
Shear Strength (psi)
8760
9255
9205
Sample
Controls
0.2% BK
0.2% PETM
Shear Strength (psi)
8760
9400
9325
Of the other compounds used, only the 3-
aminopropyltriethoxysilane and benzoyl acetone treated joints
were moderately stable in water.
The low molecular weight p-diketone (BK) and the
PETM treated joints showed improvements in the joint
strength compared to the controls. Methanol was used as a
solvent in both cases. The average (sets of seven) dry joint
shear strengths of the joints are found in Table 6.
Infrared Spectroscopy
I
It was seen spectroscopically that the quinone-amine;
polyurethane coupling agent had chemical interaction with the'
steel and epoxy surfaces. IR spectroscopy in the reflectance
mode was used to investigate the interaction of the coupling'
agent with steel (shift in the C=C or the carbonyl peak : from;
1622 to 1604 wavenumbers in QAP-1B; from 1730 to 1743|
wavenumbers in QAP-2B and ,from 1722 to 1703,
wavenumbers in QAP-3B), while the transmission IR mode.
was used to investigate the interaction of the coupling agent'
with epoxy (disappearance of the amine peaks at 1633 and
: i
1554 wavenumbers in the spectra of the QAP-1B, after
reaction with epoxy). These seem to indicate interactions of
the coupling agent with the two surfaces.
19
-------
if.
it
X-ray Photoelectron Spectroscopy
XPS studies were done on the bare metal surface and
the QAP coupling agent treated surface after washing with
copious quantities of solvent. The QAP-IB treated surface
showed Iron(II) oxide and benzoquinone like oxygen peaks in
addition to a third oxygen peak. The presence of oxygen from
the coupling agent indicates that it was not washed off
completely from the surface by the solvent. These 3 kinds of
oxygen peals probably indicate the interaction of the oxygen
from the quinone moiety with the steel surface. There was
only a single nitrogen peak, so the amine moiety is free to
react with the epoxide ring of the top coat in the system. A
similar behavior was seen with the QAP-2B and -3B as well.
The: model compound (AQM-1) showed the presence
of 3 types of oxygen peaks as well, confirming the interaction
of the quinone moiety with the steel surface. The presence of
more than one nitrogen peak in the model compound probably
indicates that the nitrogen .interacts with the surface as well in
this case, the compound probably forming a chelate with the
metal. This type of behavior was not observed for the QAPs.
Fracture Surface Analysis
The XPS results of fractured lap shear joints show
that the number and type of oxygen and nitrogen peaks of the
fractured joints were different from those seen with the
coupling agent alone on steel. These additional peaks are
expected only if there was the presence of amine cured epoxy
on the outer surface of the fractured joints. Both sides of the
fractured joint showed the above mentioned behavior. This
leads us to believe that the bond could be rupturing in the
epoxy layer. This belief is further corroborated by SEM
results which show large bundles of epoxy visible in what
otherwise looks like exposed metal surface on the fractured
joint. Vast areas of the fracture surface also show coverage by
epoxy. Preliminary XPS results obtained after immersing the
lap shear joints in hot water also support this belief.
Corrosion Resistance
The qualitative corrosion resistance results of steel
coupons coated with the coupling agent followed by epoxy
resin, after evaluation in accordance to ASTM Bl 17 standard
test method of salt spray (fog) testing using a lab scale salt
spray apparatus, are shown in Table 7. The samples were held
in a mist of salt water for a period of 10 days
Table 7. ASTM B117 Salt spray apparatus corrosion
resistance evaluation.
Sample Streaks/Spots
Result
Steel-epoxy
Steel-QAPlB-
epoxy
Steel-QAP2B-
epoxy
Steel-QAP3B-
epoxy
Completely
corroded
None
Streaks
Spots
Completely
corroded
Inhibition
Retardation
Better retardation
We are investigating the effects of flexibility and of
primary amine groups in the polymer chain on the bonding
process by using various other types and percentage
compositions of the AQM in the quinone amine
polyurethane.
20
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References
1. M. Jaworowski, United Technologies Research
Center, East Hartford, CT., Personal
Communication.
2. E. W. Thrall, R. W. Shannon, Ed., Adhesive
Bonding of Aluminum (Dekker, N.Y., 1985).
3. C.G. Marvin, American Ceramic Society Bulletin , 72,
66-68(1993).
4. J. O. Nriagu, E. Nieboer, Ed., Chromium in the
Natural and Human Environments, vol. 20 (Wiley,
N.Y., 1988).
5. National Research Council Committee on Biological
Effects of Atmospheric Pollutants, Chromium.
(National Academy of Sciences, Washington, 1974).
6. J. P. Bell, R. Schmidt, U.S. Patent No. 4,812,363 ,
(1989).
7. P.B. Schultz, Aluminum Company of America, Alcoa
Center, PA., Personal Communication.
8. J. P. Bell, R. G. Schmidt, A. Malofsky, D. Mancini,
Journal of Adhesive Science and Technology, 5,
927-944(1991).
9. A. J. DeNicola, Ph.D., University of Connecticut
(1981).
10. D. P. Mancini, M.S., University of Connecticut
(1990).
11. W. T. McCarvill, J. P. Bell, J. Adhesion 6,185-193
(1974).
12. J. D. Minford, Handbook of Aluminum Bonding
Technology and Data. (Marcel Dekker, Inc., New
York, 1993).
13. J. Comyn, in Structural Adhesives A. J. Kinloch, Ed.,
(Elsevier, N.Y., 1986).
14. V. P. Wystrach and F. C. Rauch, U.S. Patent*
3,615,888, (1971).
15. R. C. Mehrotra, R. Bohra, D. P. Gaur, Metal
b-Diketonates and Allied Derivatives.(Academic
Press Inc., New York, 1978). ,
16. K. Kaleem, F. Chertok, and S. Erhan, J. Polymer
Science: Part A: Polymer Chemistry, 27, 865-871
(1989) ;
17. K. Kaleem, F. Chertok, and S. Erhan, Progress in '.
Organic Coatings , 15,63 (1987). :
18. S. Erhan, US Patent # 4,882,413 (1989). !
19 J.P. Bell and C.J. Lin, J. Appl. Polym. Sci., 16,
1721-1733(1972). :
20. D.L Allara, "Organic Monolayer Studies Using
Fourier Transform Infrared Reflection Spectroscopy":
in Vibrational Spectroscopies for Adsorbed Species,
A.T. Bell and M.L. Hair, Eds., (American Chemical
Society, Washington, D.C., 1980),
21. R.G. Schmidt and J.P. Bell, J. Adhesion, 27,
135-142(1989).
21
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