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
2002-07-24
2004-03-09
Wu, David W. (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...
C428S35500R, C428S343000, C526S317100, C526S318000, C526S319000, C526S329200, C526S346000
Reexamination Certificate
active
06703441
ABSTRACT:
BACKGROUND OF THE INVENTION
As a result of ever-increasing environmental regulations and cost pressure, there is at present a trend toward the preparation of PSAs containing small amounts, if any, of solvent. This objective can most simply be realized through the hotmelt technology. A further advantage is the shortening in production time. In hotmelt units, adhesives can be laminated to backings or release paper significantly more quickly, thus saving time and money.
Hotmelt technology, however, is imposing ever-higher requirements on the adhesives. For high-grade industrial applications, polyacrylates in particular are preferred on account of their transparency and weathering stability.
To produce acrylic hotmelts, conventionally, acrylic monomers are polymerized in solution and then the solvent is removed in a concentration process in an extruder. Beside the advantages in transparency and weathering stability, however, acrylic PSAs also have to meet stringent requirements in the area of shear strength. This is achieved by means of polyacrylates of high molecular weight, high polarity, and subsequent efficient crosslinking.
Crosslinking, however, is a limiting factor for these acrylic hotmelts. UV crosslinking is not very suitable, since the depth of penetration of the UV radiation is limited to a maximum of 70 &mgr;m. This value reduces further if resins are included as bond strength promoter additives. An alternative technology available is that of electron beam crosslinking, although for acrylic compositions its crosslinking mechanism is extremely unfavorable, given that these PSAs are generally saturated, with no double bonds present. Consequently, this crosslinking is markedly inferior to the conventional thermal crosslinking for solventborne acrylic compositions, resulting in a lower level of thermal shear strength.
Another property for the coating of acrylic compositions from the melt is the phenomenon of the orientation of the polymer chains. For the properties of PSAs, this orientation of the macromolecules plays a significant part. As a result of the orientation, the corresponding polymers may acquire special properties, but generally at least acquire a planar anisotropy of the properties. Some examples, applying to polymers in general, of properties which can be influenced by the degree of orientation are strength and stiffness of the polymers and of the plastics produced from them, thermal conductivity, thermal stability, and anisotropic behavior with respect to permeability to gases and liquids (I.M. Ward, Structure and Properties of oriented polymers, 2nd ed. 1997, Kluwer, Dortrecht).
Interesting properties have likewise been found for oriented PSAs. For instance, the generation of a partial orientation in partly crystalline, rubber-based PSAs has already been described in U.S. Pat. No. 5,866,249. As a result of the anisotropic adhesive properties, innovative PSA applications are defined.
A disadvantage of the prior art PSAs is firstly that the acrylic hotmelts must be crosslinked in the process in order to obtain the orientation and secondly that the orientation slowly decreases over a prolonged period of time, as a result of structural relaxation.
It is an object of the present invention to provide PSA systems which have a predetermined profile of properties without additional crosslinking, said profile being retained over a prolonged period, without having to accept the disadvantages of the prior art. A further object is to specify a process for preparing such PSAs.
SUMMARY OF THE INVENTION
Surprisingly, and in a manner unforeseeable for the skilled worker, this object is achieved by the oriented, pressure sensitively adhesive systems as specified in the main claim. The subclaims relate to preferred developments of this PSA. Also claimed is a process for preparing PSAs of this kind, and their use.
The invention accordingly provides pressure sensitively adhesive systems at least comprising a pressure sensitive adhesive based on at least one block copolymer, the weight fractions of the block copolymers totaling at least 50% of the adhesive, one block copolymer being composed at least in part on the basis of (meth)acrylic acid derivatives (1), additionally at least one block copolymer comprising at least the unit P(A)-P(B)-P(A) comprising at least one polymer block P(B) and at least two polymer blocks P(A) (2), where
P(A) independently of one another represent homopolymer blocks or copolymer blocks of monomers A, the polymer blocks P(A) each having a softening temperature in the range from +20° C. to +175° C.,
P(B) represents a homopolymer block or copolymer block of monomers B, the polymer block P(B) having a softening temperature in the range from −130° C. to +10° C.,
the polymer blocks P(A) and P(B) are not homogeneously miscible with one another,
a feature of the pressure sensitively adhesive system of the invention being that it is oriented, possessing a preferential direction, with the refractive index n
MD
measured in the preferential direction being greater than the refractive index n
CD
measured in a direction perpendicular to the preferential direction.
DETAILED DESCRIPTION
In the text below the polymer blocks P(A) are sometimes referred to as “hard blocks” and the polymer blocks P(B) as “elastomer blocks”. Moreover, the block copolymers comprising the unit P(A)-P(B)-P(A) are referred to below as triblock copolymers, even when other blocks are present in the block copolymer.
By softening temperature in this context is meant, in the case of amorphous systems, the glass transition temperature and, in the case of semicrystalline polymers, the melting temperature. Glass transition temperatures are stated as results of quasistatic methods such as Differential Scanning Calorimetry (DSC), for example.
Preferably, one, more or all of the properties (1) and/or (2) indicated above for at least one block copolymer apply to two or more, or all, of the block copolymers present.
In one very preferred embodiment of the PSA systems, the difference &Dgr;n in the refractive indices, where &Dgr;n=n
MD
−n
CD
, is ≧1.10
−5
.
Very preferably, the PSA systems have different stress-strain behavior in the lengthwise direction and in the crosswise direction.
Systems which have turned out to be particularly advantageous in the sense of the invention are those having a shrinkback of at least 5%. The shrinkback is measured in accordance with the test method described as method D in the experimental section. (The shrinkback of unbacked PSA strips is defined as the ratio of the length of a test strip, taken along the coating direction, shortly after coating and after one week.)
Particular preference is given to PSA systems in which the structure of at least one block copolymer, preferably of two or more or all the block copolymers, can be described by one or more of the following general formulae:
P(A)-P(B)-P(A) (I)
P(B)-P(A)-P(B)-P(A)-P(B) (II)
[P(B)-P(A)]
n
X (III)
[P(B)-P(A)]
n
X[P(A)]
m
(IV),
where n=3 to 12, m=3 to 12 and X is a polyfunctional branching unit, i.e., a chemical component via which different polymer arms are linked to one another,
the polymer blocks P(A) independently of one another represent homopolymer or copolymer blocks of the monomers A, the polymer blocks P(A) each having a softening temperature in the range from +20° C. to +175° C., and
the polymer blocks P(B) independently of one another represent homopolymer or copolymer blocks of the monomers B, the polymer blocks P(B) each having a softening temperature in the range from −130° C. to +10° C.
The polymer blocks P(A) as described in the main claim or in the advantageous embodiments may comprise polymer chains of one single type of monomer from group A or copolymers of monomers of different structures from group A. In particular, the monomers A used may vary in their chemical structure and/or in their side chain length. The polymer blocks therefore span
Dollase Thilo
Husemann Marc
Cheung William K
Norris & McLaughlin & Marcus
tesa Aktiengesellschaft
Wu David W.
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