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
1996-05-30
2001-11-20
Henderson, Christopher (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...
C525S902000
Reexamination Certificate
active
06319992
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for preparing aqueous dispersions comprising alkyl (meth)acrylates which are suitable as pressure sensitive adhesives.
2. Description of the Background Art:
Pressure sensitive adhesives (also known as “self-adhesive compounds”) form a permanently adhesive film which, under slight pressure, adheres immediately to various surfaces at room temperature. (See 1985, “Ullmann's Encyclopedia of Industrial Chemistry”, 5th Ed., Vol. A1, pub. VCH, pp. 235-236.)
Pressure sensitive adhesives are used principally for coating of supports, such as those based on paper or polymer films or sheets. These coated supports provide products such as self-stick labels, adhesive tapes and self-stick film which have a characteristic broad spectrum of properties, including:
short-contact-time adhesion of the adhesive film to the substrate surface (“tack”) and
water-resistance of the adhesive layer.
Understandably, a single pressure sensitive adhesive cannot display all desirable properties, particularly those properties which are mutually exclusive. Accordingly, a compromise must be made and an “optimized” set of properties must be found for each application. Often, product optimization is accomplished by the use of various auxiliary substances, such as synthetic resins, tackifiers, crosslinking agents and viscosity regulators. The particular adhesives, depending on their compositions, are applied to the substrates in the form of solutions, dispersions, or even melts. After the adhesive sets, the layer of pressure sensitive adhesive is present as a film on the support. Prior to use, the film is often covered, with a suitable anti-adhesive material, such as a silicone-based film. However, adhesive tapes may also be employed with no covering layer (see “Ullmann's Encyclopedia”, loc.cit., 258). Natural and synthetic rubbers, in modified form, may be used as adhesive materials for pressure sensitive adhesives. Also frequently used are adhesives prepared from poly(meth)acrylic acid esters, polyvinyl ethers, and polyisobutylene, often in combination with phenolformaldehyde-or hydrocarbon resins.
Polyacrylate dispersions or vinyl acetate copolymers are also often used as so-called “dispersion pressure sensitive adhesives”, to which resins are often added.
The problem with most pressure sensitive adhesives can be better understood by way of an illustrative example based on adhesive tapes made of (corona treated) polypropylene strip material. Such adhesive tapes are often used as packaging tape for cardboard boxes. The most important requirements in this area of application are for good adhesion of the tape to the surface of the box, and good internal strength (or cohesion) of the adhesive layer. In most cases, it has not been possible to satisfy both of these requirements simultaneously, because many of the means of producing good cohesion are detrimental to good adhesion. For example, while it is known that cohesion increases with increasing molecular weight of the polymer, adhesion decreases correspondingly.
In Ullmann, loc.cit., it is stated:
“There are indications that a pressure sensitive adhesive must always be comprised of
a high polymer principal resin, which contributes cohesion and the specific adhesion, and
so-called tackifiers, wherewith in many systems the latter may be replaced by low molecular weight components of the principal polymer. To increase cohesion, in many systems the principal resin is crosslinked (or vulcanized, in the case of rubbers) after application.”
Certain techniques for preparing the adhesive by emulsion polymerization have found favor, despite the fact that a number of variants are known. Thus, the use of ammonium peroxydisulfate (APS) as an initiator has become standard procedure. Conventionally peroxydisulfate compounds are used as initiators in amounts of 0.001-0.05 wt. % (based on the weight of the monomers) (see Houben and Weyl, 1961, “Methoden der Organischen Chemie”, 4th Ed., Vol XIV/1, pub. G. Thieme, p. 1049; and 1968, Rauch-Puntigam, H. and Voelker, Th., “Acryl- und Methacrylvergindungen”, pub. Springer-Verlag, 221).
Thus, it is desired in the art to improve the overall set of properties of the known acrylate pressure sensitive adhesives, particularly their adhesion and cohesion, without requiring the use of auxiliary additives as noted above.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a method for preparing a pressure sensitive adhesive composition which has both high adhesion and high cohesion.
A further object of the present invention is to provide a pressure sensitive adhesive prepared by the method of the present invention, in which the adhesive composition has both good adhesion and good cohesion.
A further object of the present invention is to provide a method for preparing a pressure sensitive adhesive having both high adhesion and high cohesion, without the use of conventional auxiliary additives to provide such adhesion and cohesion.
These and other objects of the present invention have been satisfied by the discovery that dispersions for pressure sensitive adhesives can be prepared by a method for preparing a dispersion, wherein the dispersions comprise a copolymer (CP) comprised of units of n-butyl acrylate and (meth)acrylic acid; the method comprising the steps of polymerizing an aqueous emulsion (EM) containing butyl acrylate, an anionic emulsifier and a non-ionic emulsifier, in the aqueous phase, under heating and in the presence of at least one water-soluble initiator (IN) of formula (I)
M R
1
(I)
where M represents a metal cation, and
R1 represents an anion of a peroxyacid or azo-group-containing acid,
wherein said (IN) is present in an amount of 0.5-1.5 wt. % (based on weight of the monomers);
emulsifying-in (meth)acrylic acid while maintaining an elevated reaction temperature; and
adding a second initiator (RI) to complete the polymerization.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The starting point for the present invention is a principal resin comprised of butyl acrylate in combination with relatively small amounts (<15 wt. %) of (meth)acrylic acid.
The present invention relates to a method of producing dispersions for pressure sensitive adhesives, which dispersions are based on a copolymer CP comprised of units of n-butyl acrylate and small amounts of (meth)acrylic acid, wherewith, in a first step, butyl acrylate is polymerized, preferably as a core, in aqueous phase preferably under the conditions of a core and shell polymerization, in the presence of water-soluble initiators IN of formula I
M R
1
(I)
where M represents a metal cation such as sodium or potassium, and
R1 represents the anion of a peroxyacid or azo-group-containing acid,
which initiators are present in the amount of 1±0.5 wt. % (based on the weight of the aqueous phase); and in a second step butyl acrylate and (meth)acrylic acid are polymerized, in an overall ratio in the range 99.5:0.5 to 90:10 in terms of parts by weight (pbw), preferably 98:2 to 96:4 in terms of pbw, wherewith the (weight) ratio of core to shell is preferably in the range 30:70 to 70:30, particularly preferably 50:50, and the polymerization is carried out to completion, wherewith preferably at the end of the polymerization the (meth)acrylic acid groups are neutralized with ammonia (neutralization pH 9.5). The procedure is performed according to customary methods of emulsion polymerization, such as in a semicontinuous “feed” process or in a batch process (see Houben and Weyl, loc.cit., Vol. XIV/I).
According to the present invention, the internal strength of the adhesive layer can be increased by copolymerization with various crosslinking agent systems in various concentrations (one might mention, e.g., allyl (meth)acrylate, ethylene glycol dimethacrylate, methacrylamide, N-(hydroxymethyl) methacrylamide, and triallyl cyanurate) (see 1968, Rauch-Puntigam, H. and Voelker, Th., loc.cit.). However, systems crosslinked in
Fink Herbert
Heyne Joachim
Mueller Gunther
Henderson Christopher
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Roehm GmbH Chemische Fabrik
LandOfFree
Pressure sensitive adhesive dispersion of a copolymer of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Pressure sensitive adhesive dispersion of a copolymer of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Pressure sensitive adhesive dispersion of a copolymer of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2602849