Coating processes – Coating remains adhesive or is intended to be made adhesive – Pressure sensitive adhesive
Reexamination Certificate
1999-07-30
2004-11-30
Yoon, Tae H. (Department: 1714)
Coating processes
Coating remains adhesive or is intended to be made adhesive
Pressure sensitive adhesive
C427S208600, C427S208800, C524S270000, C524S505000, C524S589000, C428S343000, C428S349000
Reexamination Certificate
active
06824820
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of adhesives, specifically to the field of pressure-sensitive-adhesives and heat-activatable adhesives that are polyurea-based.
BACKGROUND OF THE INVENTION
Adhesives have been used for a variety of marking, holding, protecting, sealing and masking purposes. Adhesive tapes generally comprise a backing, or substrate, and an adhesive. One type of adhesive, a pressure-sensitive-adhesive (PSA) is particularly preferred for many applications.
PSAs are well known to one of ordinary skill in the art to possess certain properties at room temperature including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear strength. The most commonly used polymers for preparation of PSAs are natural rubber, synthetic rubbers (e.g., styrene/butadiene copolymers (SBR) and styrene/isoprene/styrene (SIS) block copolymers), and various (meth)acrylate (e.g., acrylate and methacrylate) copolymers. With the exception of several (meth)acrylates, which are inherently tacky, these polymers are typically blended with appropriate tackifying resins to render them pressure-sensitive.
One disadvantage of (meth)acrylates, however, is that (meth)acrylates often contain residual (i.e., unreacted) monomers and other low molecular weight impurities in the final adhesive compositions. Residual monomers and low molecular weight impurities may be problematic in certain applications, such as medical and electronic applications. In medical and electronic applications, residual monomers and impurities may cause, for example, undesirable odor or potential contamination of substrates/articles (e.g., hard disk drives) in which they are in contact.
Much less frequently described are PSAs based on polyurethane-based polymers, such as the polyether/polyurethane polymers described by Allport and Mohajer in
Block Copolymers
, D. C. Allport and W. H. Janes Ed., (1973) pp. 443-92. Also see U.S. Pat. Nos. 3,718,712 (Tushaus) and 3,767,040 (Tushaus). It has been difficult to obtain balanced viscoelastic properties when using polyurethane-based polymers, however, which may explain their infrequent use when preparing PSAs. For example, U.S. Pat. No. 5,591,820 (Kydonieus et al.) indicates that existing polyurethane-based adhesives function either as weak elastics or high viscosity liquids. The former, weak elastics, tend to fail gradually by peeling away from surfaces to which they have been applied. The latter, high viscosity liquids, typically leave a residue upon removal from a surface and their cohesive strength is too low to withstand stresses applied in many applications.
Polyurethane-based polymers are typically prepared by reacting an isocyanate-functional material with a hydroxy-functional material. Some examples of polyurethane-based polymers used for formulating PSAs include those described in U.S. Pat. No. 3,437,622 (Dahl). Polyurethane-based polymers are not always desirable, however, because they typically require either a catalyst or external heat source to form the urethane linkages. For example, see U.S. Pat. No. 5,591,820 (Kydonieus et al.).
Furthermore, many polyurethane-based polymers must be crosslinked to have adequate cohesive strength as PSAs. There are two general methods used to crosslink polyurethane-based polymers. One method is chemical crosslinking through the formation of covalent bonds. However, the degree of chemical crosslinking must be carefully controlled so that the moduli of the resulting material is not increased to the extent that peel adhesion and tack are adversely affected. Furthermore, premature gellation of the adhesive and limited pot life of the PSA may also be problematic when using chemical crosslinking to bolster the cohesive strength of a PSA.
A common chemical crosslinking method described in the literature is the use of multivalent components to achieve a crosslinked network in the adhesive composition. For example, the crosslinked network may be formed by incorporating urethane or urea linkages between polyurethane polymer chains. Urea linkages are typically incorporated into the material by, for example, using a polyamine. For example, see JP-07-102,233 (Sekisui Chemical) and JP62-297,375 (Kao Corp.). Some of the resulting materials purportedly have PSA properties, either in a partially cured state or in the final composition.
For example, U.S. Pat. No. 4,803,257 (Goel) describes a polyurethane adhesive (i.e., having structural or semi-structural properties) comprising a mixture of a polyisocyanate blocked with a phenolic agent and a polyamine curing agent. The composition may optionally include a polyepoxide. In the partially cured state, this composition is said to exhibit properties similar to those of PSAs. The compositions cure at room temperature to reach the full strength of a structural adhesive.
Also, U.S. Pat. Nos. 3,437,622 and 3,761,307 (Dahl) describe preparation of polyurethane polymers suitable for making PSAs, and which can be crosslinked with certain amines. Suitable amines are taught to be aromatic diamines or polyamines with the amino groups sterically or otherwise hindered by negative groups (Cl, Br, I, OH, etc). These negative groups decrease the reactivity of neighboring amino groups. When crosslinked, it is required that the amino groups be unreactive enough so that polyols and isocyanates can react to form polyurethane polymers before the isocyanates extensively react with the amino groups.
A second method of crosslinking polyurethane-based polymers is physical crosslinking. Physical crosslinking such polymers typically involves incorporation of urea segments in the polyurethane-based polymeric chain using, for example, an amine chain extender. However, polyurea-based polymers are even less likely candidates for formulation into PSAs because polyureas have even higher moduli than the corresponding polyurethanes due to the chemical nature of the urea groups in polyureas. Accordingly, polyureas tend to be more elastic and adhesives prepared therefrom may not have adequate peel adhesion and tack, properties that may be desired for certain applications. For example, see Chen, Z. et al., “The Study of Polyurethanes and Polyureas by Transmission Spectra of Fourier Transform Infrared Spectroscopy,”
Gaofenzi Cailiao Kexue Yu Gongcheng
1993, 9(3), pp. 58-62 and Chen, Z, “Study About Effect of Urea and Urethane Linkages on Phase Separation of Segmented Polyurethanes and Polyureas,”
Gaofenzi Cailiao Kexue Yu Gongcheng
1990, 6(5), pp. 66-71. The higher moduli of polyureas present a problem when trying to formulate the polyureas into PSAs, particularly PSAs having adequate peel adhesion for many applications. Perhaps because of this apparent difficulty, there are very few descriptions of PSAs that are polyurea-based. Accordingly, polyurea-based polymers, particularly silicone polyurea-based polymers are typically used for release materials, such as those described in U.S. Pat. No. 5,866,222 (Seth et al.).
Polyurea-based polymers provide an alternative to polyurethane-based polymers. Polyureas are preparable by reacting an isocyanate-functional material with an amine-functional material. Advantageously, polyurea-based polymers typically do not require a catalyst or an external heat source for their preparation.
Among the few descriptions of polyurea-based PSAs, organosiloxane-polyurea block copolymers useful as PSAs are described by Leir et al. (EP Patent Publication No. 0 250 248 A2). The organosiloxane-polyurea block copolymers described therein are prepared by the condensation polymerization of a difunctional organopolysiloxane amine with a diisocyanate. The reaction may include an optional difunctional amine chain ext
Dastur Meherdil D.
Kinning David J.
Leir Charles M.
Sherman Audrey A.
3M Innovative Properties Company
Fulton Lisa P.
Yoon Tae H.
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