Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2001-11-13
2004-06-15
Metzmaier, Daniel S. (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S418000, C222S129000
Reexamination Certificate
active
06749938
ABSTRACT:
BACKGROUND OF THE INVENTION
The automobile industry utilizes outer skin/body panels made from sheet molding compounds (SMC) as an alternative to steel panels in an effort to reduce weight and corrosion susceptibility of the vehicle, i.e., an automobile, van, truck or the like. Sheet molding compound is typically comprised of various resin compositions such as a polyester resin reinforced with, for example, glass fibers. The sheet molding compound is molded under heat and pressure in order to prepare a rigid, fiber-reinforced structure. Structural assemblies of body panels based on sheet molding compounds requires the use of curable adhesive compositions to affix or bond the panel to structural metal or composite fixtures. The bond formed by the adhesive between the SMC panel and structural member must meet certain adhesion and aesthetic requirements. After being bonded for use in automobile applications, the bonded SMC is often exposed during paint bake cycles to temperatures of 205° C. for periods of time up to one hour. Surface imperfections occur in the use of bonding adhesives.
Certain structural adhesive compositions based on epoxy curing compounds used for bonding SMC for automotive shaped bodies, e.g. body panels, among other vehicle sub-assemblies are disclosed in U.S. Pat. Nos. 4,578,424; 4,695,605; 4,740,539; 4,803,232; and 4,921,912. Many traditional epoxy-based structural adhesive compositions such as those disclosed in the above patents suffer from the disadvantage of undergoing foaming or disintegration during the high temperature paint baking process. This foaming or disintegration of the adhesive composition can cause cohesive failure of the adhesive or interfacial failure between the parts being bonded, rendering them undesirable for industrial applications.
Epoxy adhesives of the state of the art exhibit bond line shrinkage which is referred to herein as “read-through” in the exposed SMC surface from underlying bond line on the backside of the panel. This imperfection can occur in original equipment manufacture or in the repair of damaged panels and appears as a slight crevasse along the bond line. Noticeable read-through is commercially unacceptable and requires sanding, and/or refilling prior to painting of the body panel.
U.S. Pat. No. 5,929,141 discloses a one-component, flexible epoxy adhesive useful as an encapsulant comprising the following components: (a) at least one polyepoxide resin having a hardness not exceeding a Shore D durometer reading of about 45 when cured with a stoichiometric amount of diethylene triamine; (b) a substantially stoichiometric amount of curing agent including at least one amine-terminated butadiene-acrylonitrile polymer; and (c) an electrically-conductive filler.
U.S. Pat. No. 5,385,990 discloses an epoxy structural adhesive useful for bonding of paint-baked SMC, and comprising an epoxy component; an amine hardener, selected from the group consisting of polyamidoamines, aliphatic polyamines, alicyclic polyamines, tertiary amines and mixtures thereof; and a hydroxy-substituted aromatic compound having a specified pKa and boiling point range. In a worked example the first Part contains 60 parts of diglycidyl Ether of bisphenol A and 39 parts of talc, and the second part contains:
4.29 parts of diethylenetriamine
19.36 parts of a polyamidoamine (1)
9.5 parts of p-Chlorophenol
34.15 parts of ATBN (liquid nitrile polymer) (2)
and 32.68 parts of talc.
The first and second parts are metered and mixed together in a volume ratio of 1:1.25. This adhesive formulation contains 29.7 wt. % of (1) and (2) and results in unacceptable read-through on SMC.
U.S. Pat. No. 5,019,608 discloses rubber toughened epoxy adhesives. In a worked example Component A and Component B are as follows:
Grams
Component A:
Bisphenol A epoxy resin
32.5
(Epon ® 828 of Shell Chemical Co.)
Epi-Rez ®
19.8
(Celanese Corp., a 60/40 blend
of Epon ® 828 and polyacrylate
ester)*
Diethylene glycol
1.3
Fumed silica
1.7
TiO
2
0.8
White Talc
43.9
Component B:
Genamid ® 2000
26.1
(1)
ATBN 1300X42
45.6
(2)
White Talc
11.5
Aluminum Powder
14.0
Fumed Silica
2.8
The first and second parts are metered and mixed together in a weight ratio of 1:1.09. This adhesive formulation contains 33.9 wt. % of (1) and (2) and results in unacceptable read-through on the surface of SMC bonded to a substrate.
A need exists for a structural adhesive composition which can withstand high temperature paint bake conditions and which improves bond shrinkage over state of the art approaches as to avoid costly and time-consuming repair or rebonding. SMC panels of gauge thickness of 125 mils, the current auto industry standard. A panel gauge thickness of 90-110 mils, especially 90-100 mils is desirable. Such gauge thickness of 110 mils or less, especially with “toughened” SMC compounds places an added burden on the adhesive composition to achieve the critical performance parameters of good (i.e, low or no) read-through, and a bond strength of at least 200 p.s.i. at 180° F., and at least 44 p.s.i. bond strength at 400° F., along with fiber tearing bonds after long term water soaking of the bonded composite. Present epoxy adhesives used to bond thinner gauge SMC show undesirable read-through. A Balance of adhesive bond performance and read-through is needed.
SUMMARY OF THE INVENTION
The present invention is an epoxy adhesive composition that meets the critical requirements for bonding conventional SMC panels, and especially SMC panels, outer-body skins, and the like, to structural supporting substrates, the panels preferably having a thickness of 90-110 mils (0.009-0.010 in.). The preferred panels are further subjected to E-coat paint-baking conditions after bonding to the substrates takes place. According to the invention a two-part epoxy adhesive that has acceptable SMC read-through and bonding performance exhibits a Young's modulus in a critical range of from 25,000 to 200,000, psi, when the adhesive contains a wt. % total of liquid rubber and polyamide and/or polyamidoamine of from 20% to 60 wt. % on total adhesive weight and the adhesive is proportioned in the epoxy side (A) to curative side (B) in a volume ratio range of from 1:1.4 to 1:2.2.
In another aspect, the invention is directed to a bonded shaped body of sheet mold compound, having one surface which is a class A surface, and a backside surface which is bonded to a substrate with an epoxy adhesive in mix-proportioned parts, wherein said adhesive contains a liquid elastomer having a terminal epoxy-reactive group, and the adhesive part A comprises an epoxy compound, and part B comprises a polyamide or polyamidoamine. The adhesive contains from 30 to 60 wt. % combined of a liquid epoxy-reactive elastomer and polyamide or polyamidoamine and is applied to bond the shaped body in a volume mix-proportion ratio of part A to part B of from 1:1.4 to 1:3.0.
DETAILED DESCRIPTION OF THE INVENTION
The present 2-part epoxy adhesive composition comprises an epoxy compound, an optional amine hardener, optional hydroxy-substituted aromatic compound, and from 20% to 60% of the total weight of combined adhesive of polyamide and/or polyamidoamine and a liquid epoxy-reactive terminal group containing elastomer. The most preferred liquid elastomers contain epoxy-reactive volume:volume ratio of the first epoxy (A) part to second curative (B) part of from 1:1.4 to 1:3.0, and preferably from 1:1.8 to 1:2.5.
The epoxy compound of the present invention can be any compound that contains an epoxy group having the formula:
has a viscosity of about 200 centipoise or higher at 25° C. Such epoxy materials include monomeric epoxy compounds and epoxide resins of the polymeric type and can be aliphatic, cycloaliphatic, aromatic or heterocyclic. These materials generally have, on the average, at least 1.5 polymerizable epoxy groups per molecule (preferably two or more epoxy groups per molecule). The polymeric epoxides include linear polymers having terminal epoxy groups (e.g., a diglycidyl ether of a polyoxyalkylene glycol), polymers having skeleta
Damico Dennis J.
Pugne David R.
Dearth Miles B.
Keehan Chistopher
Lord Corporation
Metzmaier Daniel S.
LandOfFree
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