Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-10-05
2001-10-16
Mulcahy, Peter D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S503000
Reexamination Certificate
active
06303680
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a flame-retardant adhesive composition. More particularly it relates to an adhesive composition for dielectric films and is useful in the fabrication of solderable flexible circuits.
2. Description of the Related Art
The flexible circuit industry requires adhesives for dielectric film and metal foils (usually copper). The laminate, and hence the adhesive, must be able to withstand elevated temperatures and a variety of harsh solvents and chemicals. During the many preparation and processing steps for circuit manufacture, these solvents and chemicals can cause an adhesive to swell, leading to blister formation and/or delamination, which results in reduced circuit yields. The swelling can also open the adhesive matrix to ionics which can result in metal dendrite formation, reduced electrical resistance, and localized darkening or staining. The application of heat, such as in soldering, can similarly cause circuit failures and/or staining. In addition, it has become increasingly important for the adhesive compositions to be flame retardant. External fire sources or short circuits can cause a flexible circuit to burn if it is not flame retardant.
It is known that flame retardancy can be achieved by the addition of materials such as halogen-containing compounds, antimony oxide, or phosphorous compounds. In particular, brominated compounds have been used to achieve flame retardancy in hot melt adhesives, thermoplastic materials, and reinforcing resins, as described in, e.g., U.S. Pat. Nos. 4,727,107, 5,041,484, 5,376,718 and 5,443,911. However, to be useful as a flame retardant in an adhesive for electronic applications, a flame retardant material must satisfy several property requirements which are not necessary for hot melt adhesives or thermoplastic materials. For example, the flame retardant additive must not adversely affect the electrical properties of the composite structure. The additive must also not degrade the adhesive performance of the composite structure, both before and after soldering steps, i.e., the peel strength of the composite structure must not be substantially lower. And, the additive must be stable to processing solvents and chemicals.
Acrylic adhesive compositions with flame retardant additives or fillers for use in flexible circuits have been described in U.S. Pat. No. 5,331,040 and European Patent 429027. However, the addition of the flame retardant filler tends to make the adhesive more brittle, which adversely affects the peel strength of composite structures made with the filled adhesive. The brittleness generally can be overcome by the addition of plasticizers. However, the plasticizers must be chosen such that they are not attacked by processing chemicals, are stable at soldering temperature, etc. In addition, the fillers are generally composed of particles of a micron or more in size and tend to make the adhesive films opaque. Opaque films make inspection processes difficult.
Thus, there is a need for a relatively clear, flame-retardant adhesive composition for use with dielectric films in composite structures for electronic applications, which provides good adhesion throughout the life of the composite structure, and retains all the necessary properties, including electrical properties, of non-flame-retardant adhesives.
SUMMARY OF THE INVENTION
This invention relates to an adhesive composition comprising:
A. 80-98% by weight, based upon the total amount of adhesive dry solids, of at least one latex copolymer made from monomers comprising:
(1) 20-55% by weight, based upon the weight of the copolymer, of at least one brominated aromatic monomer,
(2) 5-15% by weight, based upon the weight of the copolymer, of a monomer selected from the group consisting of glycidyl acrylate, glycidyl methacrylate, and mixtures thereof, and
(3) 40-60% by weight, based upon the weight of the copolymer, of at least one non-brominated monofunctional(meth)acrylate monomer;
(4) 0-10% by weight, based upon the weight of the copolymer, of a monomer selected from the group consisting of acrylonitrile, methacrylonitrile, and mixtures thereof;
B. 0-10% by weight, based upon the total amount of adhesive dry solids, of at least one crosslinking agent;
C. 1-10% by weight, based upon the total amount of adhesive dry solids, of at least one antimony oxide; and
D. 0-15% by weight, based upon the total amount of adhesive dry solids, of at least one viscosity modifier.
In a second aspect, this invention relates to a dielectric film or a release support coated with one or more coatings of the above adhesive composition.
In a third aspect, this invention relates to a composite structure comprising at least one substrate adhered to at least one above-described adhesive coated dielectric film.
DETAILED DESCRIPTION OF THE INVENTION
Component A comprises at least one latex copolymer. A single latex copolymer or a combination of two or more latex copolymers can be used. The term “copolymer” is intended to mean a polymer made from two or more different monomers, and includes terpolymers, tetrapolymers, etc. The flame retardancy for the adhesive is largely incorporated directly into the copolymer, by the use of brominated aromatic monomers. This eliminates the need to add separate brominated flame retardant fillers, thus maintaining the transparency of the adhesive film.
The first monomer in the copolymer is a brominated aromatic monomer. A single brominated aromatic monomer or a combination of two or more brominated aromatic monomers can be used. This monomer provides flame retardancy. In addition it provides high temperature stability and hardness. A preferred type of brominated monomer is a styrene or styrene derivative with bromine substitution in the phenyl nucleus. Mono-, di-, tri- and tetra- substitution can be used. Representative brominated aromatic monomers include styrene, methylstyrene, alpha-methylstyrene, alpha-methyl methylstyrene, ethylstyrene, alpha-methyl ethylstyrene, phenylacrylate, phenylmethacrylate, and mixtures thereof, each having bromine substitution in the aromatic ring.
The brominated aromatic monomer is present in an amount of from about 20 to 55% by weight, preferably from about 30 to 40% by weight, based on the weight of the copolymer. Preferably there is from about 10 to 40% bromine by weight, most preferably from 15 to 25% bromine by weight, based on the weight of the copolymer.
The second monomer in the copolymer is glycidyl acrylate, glycidyl methacrylate, or mixtures thereof, collectively referred to as glycidyl (meth)acrylate. This monomer also provides toughness and high temperature resistance. The glycidyl (meth)acrylate is present in an amount of from about 5 to 15% by weight, preferably from about 8 to 12% by weight, based on the weight of the copolymer.
The third monomer is a non-brominated monofunctional alkyl acrylate, alkyl methacrylate, or mixture thereof, collectively referred to as alkyl (meth)acrylates. As used herein, (meth)acrylate refers to esters and does not include the free acids. A single (meth)acrylate or a combination of two or more (meth)acrylates can be used. This component provides the flexibility and necessary softness to the adhesive composition. In general, the alkyl (meth)acrylates are esters of (meth)acrylic acid with alcohols having 1 to 20 carbon atoms, preferably from 2 to 10. Examples of suitable alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, n-amyl (meth)acrylate, n-hexyl (meth)acrylate, isohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-heptyl (meth)acrylate, isoheptyl (meth)acrylate, 1-methylheptyl (meth)acrylate, n-octyl (meth)acrylate, 6-methylheptyl (meth)acrylate, n-nonyl (meth)acrylate, n-decyl (meth)acrylate, and lauryl (meth)acrylate. Preferred alkyl (meth)acrylates include ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, lauryl methacrylate, octyl acrylate, and heptyl acrylate. The third
E.I. Du Pont de Nemours and Company
Mulcahy Peter D.
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