Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-09-13
2003-12-02
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C524S522000
Reexamination Certificate
active
06657011
ABSTRACT:
The present invention relates generally to improved adhesive compositions. More particularly, the present invention relates to improved adhesive compositions containing low molecular weight, polymeric additives and methods for forming the same.
The properties of tack, peel strength and shear resistance, which are frequently mutually exclusive properties, may be highly important in tailoring an adhesive composition that is suitable for a particular application. Tack is generally a measure of viscous flow under conditions of fast strain rates and low stress magnitudes. Peel strength is a measure of resistance to flow at intermediate strain rates and moderate to high stress magnitudes, as well as the cohesive strength of the adhesive. Shear resistance is a measure of resistance to flow at intermediate stress magnitudes.
Polymeric additives, such as tackifiers or plasticizers, are often added to adhesive compositions to modify the properties of the adhesive including, for example, the viscoelastic behavior of the finished adhesive. The particular polymeric additive selected may vary depending, for example, on the adhesive involved and the modification sought.
Traditionally, natural and synthetic rubbers have been used as pressure-sensitive adhesives, but these rubbers alone may not have the necessary balance of properties of tack, shear, and peel adhesion. For example, rubber-based adhesives typically require the addition of tackifiers, such as rosin acid derivatives or aliphatic/aromatic resins, to impart useful properties as pressure-sensitive adhesives.
Acrylic polymers have also been useful as pressure-sensitive adhesives. Acrylic polymers have typically not required compounding with tackifier resins to achieve desirable pressure sensitive characteristics. Recently, however, it has been shown that tackifiers may provide a convenient and useful tool for modifying pressure-sensitive adhesives as discussed by T. G. Wood, “The Effects of Tackification on Waterborne Acrylic Pressure-Sensitive Adhesive”, Adhesive Age, Vol. 30, pp. 19-23 (1987). The Handbook of Pressure-Sensitive Adhesive Technology, edited by Don Satas, Van Nostrand Reinhold Co., pp 353-369, (1982) provides a discussion on pressure-sensitive adhesives. This reference teaches, inter alia, that the addition of tackifiers to acrylic, pressure-sensitive adhesive polymers may improve the performance characteristics for these materials.
Generally speaking, to enhance performance characteristics of adhesive compositions, the tackifier should possess a certain range of properties with respect to the adhesive polymer or copolymer to which it is added. For example, the tackifier should generally have a lower molecular weight as compared to that of the adhesive polymer or copolymer. In addition, the tackifier should generally have a glass transition temperature (T
g
) that is higher than that of the adhesive polymer or copolymer. The softening temperature of the tackifier should generally be higher than that of the adhesive polymer or copolymer, and the tackifier should generally have a minimum degree of compatibility with the adhesive polymer or copolymer.
Plasticizers may be added to an adhesive composition, for example, to increase its workability, flexibility, and/or distensibility. Depending upon the involved adhesive system, the addition of a plasticizer may lower the melt viscosity, elastic modulus, and glass transition temperature (T
g
) of the overall system. Plasticizers may also be added to polymer formulations to increase flexibility by promoting bond formation between polymer molecules and the plasticizer rather than between the polymer molecules themselves. In doing so, the plasticizer may also increase the specific volume and change the index of refraction of the resulting composition. The addition of the plasticizer to the adhesive composition may also raise the viscosity of the adhesive system.
Japanese Patent Disclosure No. 54-3136 discloses a pressure-sensitive adhesive containing an acrylic polymer and a tackifier. The tackifier is prepared by solution polymerizing 25 to 75 weight % of vinyl aromatic compound and 75 to 25 weight % acrylic or methacrylic acid ester. The disclosed tackifier has a number average molecular weight of 500 to 3,000 and a softening point of less than 40° C.
U.S. Pat. No. 3,867,481 discloses a low molecular weight acrylic polymer containing (1) an alkyl methacrylate and (2) one or more additional acrylic methacrylic acid ester and/or a styrene for use as a processing modifier for rigid polyvinyl halide resins.
U.S. Pat. No. 4,056,559 discloses low molecular weight polymers of alkyl methacrylates having an average chain length of about 6 to about 50 mers and having a variety of uses in films, coatings, adhesives and inks.
U.S. Pat. No. 4,912,169 discloses low molecular weight tackifiers that have a number average molecular weight less than about 35,000 and a softening point greater than about 40° C. The tackifiers are prepared with high levels, or 1 to 20 mole percent, of chain transfer agents such as C
1
-C
15
alkyl mercaptans, benzyl mercaptan, 3-mercaptoproprionic acid and esters thereof, mercaptoethanol, benzyl alcohol, alpha-methyl benzyl alcohol, and ethyl mercaptoacetate.
Mercaptans are often effective for use as chain transfer agents and in reducing the molecular weight of polymers. Such uses of mercaptans is problematic, however, due to, inter alia, the expense of these materials and their frequently offensive odors. Moreover, the use of mercaptans during processing can impart thiol functionalities into the resulting polymer, thereby affecting the properties of the polymer. The use of other common chain transfer agents such as, for example, hypophosphites, sulfates, and alcohols, may also be undesirable in that they may add to the cost of the process, impart undesired functionality to the polymer, introduce undesired salts into the process, and introduce additional process steps, including product separation.
The present invention seeks to provide improved adhesive compositions containing polymeric additives for use, for example, as adhesives. The polymeric additives of the present invention can be prepared without the use of high levels of chain transfer agents. Moreover, the polymeric additives of the present invention can be prepared without mercaptans or alcohols as chain transfer agents.
The present invention is directed, in part, to improved adhesive compositions. Specifically, in one embodiment, there are provided adhesive compositions comprising (a) adhesive polymers or copolymers prepared from monomers selected from the group consisting of styrene, butadiene, acrylonitrile, ethylene, vinyl acetate, acrylic acid, esters of acrylic acid, methacrylic acid, esters of methacrylic acid and combinations thereof; and (b) from 1 to 55% on a dry weight basis of a polymeric additive comprising a low molecular weight polymer having a number average molecular weight of 20,000 or less and which comprises: (i) polymerized units of at least one ethylenically unsaturated monomer; and (ii) 5 weight percent or less of a polymerized acid-containing monomer wherein the polymeric additive is polymerized in the presence of a metal chelate chain transfer agent.
Another aspect of the invention relates to a method of modifying one or more properties of an adhesive comprising combining with the adhesive polymer or copolymer from 1 to 55% on a dry weight basis of a polymeric additive comprising a low molecular weight polymer having a number average molecular weight of 20,000 or less and which comprises: (i) polymerized units of at least one ethylenically unsaturated monomer; and (ii) 5 weight percent or less of a polymerized acid-containing monomer wherein the polymeric additive is polymerized in the presence of a metal chelate chain transfer agent.
Still another aspect of the invention relates to methods for the in-situ generation of a polymeric additive within an adhesive composition, the method comprising: (a) polymerizing at least one first ethylenically unsaturated monomer to form an ad
Ho Kim Sang
Lau Willie
Van Rheenen Paul Ralph
Clikeman Richard R.
Lipman Bernard
Rohm and Haas Company
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