Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
1999-09-27
2002-09-10
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C526S081000, C526S319000, C524S804000, C524S833000
Reexamination Certificate
active
06447900
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a pressure-sensitive adhesive which is used for various labels, tapes, sheets and the like, and is excellent in quality stability, processability and storage stability, and pressure-sensitive adhesive properties such as adhesion, holding power and bonding power to a curved surface; a pressure-sensitive adhesive product; and a process for producing the same.
BACKGROUND ART
Hitherto, as a pressure-sensitive adhesive, acrylic or rubber type polymer has been used. The pressure-sensitive adhesive has, however, the following problems. Firstly, if thickness of the pressure-sensitive adhesive is not strictly controlled, at the time of coating the pressure-sensitive adhesive to a backing in the production step, the resultant products come to have very different adhesions. In the case of any acrylic pressure-sensitive adhesive, the thickness of the coated pressure-sensitive adhesive is usually controlled at about 20-25 g/m
2
. The pressure-sensitive adhesive has a property that the larger the coated thickness becomes, the larger the adhesion becomes; and the smaller the coated thickness becomes, the smaller the adhesion becomes. For this reason, common knowledge in the prior art is that if the coated thickness varies, the adhesion of the products also varies. In order to prevent troubles occurring particularly by the fact that the coated thickness inclines to become thin, coating-makers adapt so that the pressure sensitive adhesive is coated so as to become thick by 20-30% (25-30 g/m
2
) in the present circumstances.
However, if the layer of the pressure-sensitive adhesive becomes thick in such a way, costs rise up and productivity falls. Moreover, in various steps subsequent to the coating, the following problems occur.
Namely, a pressure-sensitive adhesive sheet is usually slit by using a rotating sharp-edged tool to cut the raw roll thereof coated with a pressure-sensitive adhesive into an appropriate width, and the sheet is die cut into an appropriate size at the time of printing. In these slit-or die-cutting procedures, the pressure-sensitive adhesive remains or adheres on the edged tool, or the pressure-sensitive adhesive spreads out and is not cut. Thus, the pressure-sensitive adhesive transfers to the product or other parts, so that the product gets dirty or the sheet is not satisfactorily cut. Such various troubles arise.
And then, concerning a tape or sheet raw-roll, in the step of storing the products, a pressure sensitive adhesive is oozed from its edge-face by its weight and/or rolling-tightening pressure. Such phenomena arise, too. Besides, concerning raw-roll products die-cut into a label form after printing, their pressure-sensitive adhesive is oozed from their backing in the same manner so that the raw-rolls adhere to each other or the labels themselves shift on release paper and get out of their proper-position. In any case, the pressure-sensitive adhesive transfers onto an actual item for sale during the step of sticking the label, so that the item itself gets dirty. During the process of affixing the labels with a machine, the adhesive tends to shift the label out of proper-position. Such troubles tend to arise more frequently in the case that the backing is a plastic film than in the case that the backing is paper.
When a pressure-sensitive adhesive is coated to have a large thickness in this way, various troubles are caused. Therefore, coating-makers take, as an adoption, that a hardening agent (crosslinking agent) is added in a larger amount to, e.g., a solvent-type pressure-sensitive adhesive so that the pressure-sensitive adhesive is made hard and its cohesion is made high. According to the method, however, its original and high adhesion is suppressed so that the real performances of pressure-sensitive adhesive are not exhibited. As a result, there arises a dilemma that the pressure-sensitive adhesive should be coated thicker and thicker.
As for an emulsion-type pressure-sensitive adhesive, holding power can be obtained if a cohesive component is added thereto. There is, however, caused a phenomenon that its adhesion is extremely lowered.
Many emulsion pressure-sensitive adhesives are produced by emulsifying-polymerizing (meth)acrylate such as butyl acrylate and 2-ethylhexyl acrylate. However, a pressure-sensitive adhesive using, as a monomer, only (meth)acrylate has insufficient holding power necessary after that a tape or the like coated with the pressure-sensitive adhesive is stuck on an adherent.
Thus, the method, which aims to raise the cohesive power of the emulsion and which improves the holding power of the emulsion, is conducted by adding a polar monomer, such as acrylic acid or adding a crosslinking monomer having two or more reactive groups, such as divinylbenzene, to (meth)acrylate and copolymerizing them. The addition of the polar monomer, however, causes a fall in the water-resistance of the pressure-sensitive adhesive, thereby resulting in such a drawback that the adhesion onto any non-polar adherent such as polyolefin is largely lowered. Moreover, when a cohesive component such as these polar monomers and these crosslinking agents is added to the pressure-sensitive adhesive, the pressure-sensitive adhesive becomes elastic so that adhesion and tackiness are largely lowered.
As the above-mentioned, conventionally, it is difficult to produce an emulsion-type pressure-sensitive adhesive having good balance of pressure-sensitive adhesive performances so as to satisfy sufficiently both properties of holding power (cohesion) and adhesion.
Besides, it is important to have sufficient bonding power to a curved surface as one of the properties of a pressure-sensitive adhesive. In the case that an adherent has a curved surface, the larger the curvature thereof is, the more easily a pressure-sensitive adhesive sheet is stripped from its edge face. In other words, there arises a so-called edge lifting phenomenon that the force of repulsion of the backing against the pressure-sensitive adhesive causes stress for stripping the pressure-sensitive adhesive sheet from the curved surface of the adherent so that the edges of the pressure-sensitive adhesive sheet are gradually lifted up with the passage of time. In the present situation, there is not known a clear theory on such bonding power to a curved surface. A trial-and-error method has been repeated in the industry. The bonding power to a curved surface, which may be referred to as edge lifting resistance, is one of the important performances required for pressure-sensitive adhesives. It should be not lacking for the quality of a final product being stuck with such a pressure-sensitive adhesive label or sheet to satisfy this bonding power sufficiently. In the present situation, however, there is insufficient bonding power to a curved surfaces by various kinds of adherents, in particular an adherent made of polyolefin type compound.
Then, JP-A 7-330813 and WO097/07174 disclose an emulsion for a pressure-sensitive adhesive, comprising 50% or more by weight of a long-chain alkyl (meth)acrylate. However, the emulsion cannot sufficiently meet the above-mentioned demands.
DISCLOSURE OF THE INVENTION
The inventors have found that the above-mentioned object can be attained by a pressure-sensitive adhesive exhibiting a specific storage modulus in dynamic viscoelasticity measurement and having a specific gel fraction. The inventors have found that the above-mentioned pressure-sensitive adhesive can be produced by a process for producing an emulsion-type pressure-sensitive adhesive in which the manner of adding a polymerization-initiator is devised.
The present invention is composed of the pressure-sensitive adhesive whose adhesion does not fall even when it is thin coated, and the present invention provides the excellent pressure-sensitive adhesive which can solve fluctuation in quality based on fluctuation in the coated thickness of the pressure-sensitive adhesive, which is excellent in slitting property and property for die-cutting into a label, a tape, a sticker or the
Doi Yasuhiro
Ishikawa Yoshinobu
Ohtani Shoji
Sakuda Toshitaka
Shiba Kenichi
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Keehan Christopher M
Toyo Ink Manufacturing Co. Ltd.
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