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
2000-06-22
2002-10-15
Mullis, Jeffrey (Department: 1711)
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...
C524S487000, C525S505000
Reexamination Certificate
active
06465557
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to hot melt pressure sensitive positioning adhesives for use with absorbent articles which are based on hydrogenated block copolymers of styrene and butadiene or isoprene and exhibit improved viscosity and viscosity/temperature profile.
BACKGROUND OF THE INVENTION
Positioning adhesives are used on disposable articles (absorbent articles) such as sanitary napkins, incontinent pads, bed pads, feminine pads, panty shields, and diaper inserts, where an adhesive layer is used to attach the article to a woven fabric substrate such as a supporting undergarment or bed sheet. The positioning adhesive is commonly applied to a release liner and transfer coated to the garment facing surface of the disposable article. The positioning adhesive must be capable of attaching to the undergarment to hold the article in place without transferring to or otherwise being deposited on the undergarment. Furthermore, the adhesive must not discolor, damage, or disturb the fibers of the garment.
The positioning adhesive must be a pressure sensitive adhesive that has an application viscosity that permits it to readily flow onto and partially penetrate the particular surface to which it is applied. It must have good bond strength and high tack for initial placement of the article on the undergarment but also must have the ability to avoid loss of adhesion over time due to temperature conditions. Finally, these articles are sometimes used for long periods of times at body temperature and they can have the drawback that the hot melt adhesive gradually softens and penetrates into the undergarment to which the article is adhering. In this case, the adhesive force greatly increases and the cohesive force is reduced. This causes the adhesive layer to suffer cohesion breakdown when the article is removed and some adhesive remains on the undergarment. Prevention of this deposit of adhesive on the undergarment is accordingly a necessary prerequisite for a successful positionable hot melt adhesive composition.
Block copolymers of styrene and dienes such as butadiene or isoprene have been used for a number of years in positionable hot melt adhesive formulations. More recently, the material of choice for such adhesives in feminine care applications has been hydrogenated block copolymers of styrene and butadiene such as KRATON® G1650 SEBS hydrogenated styrene-butadiene-styrene) block copolymer. These SEBS block copolymers that have been used in this application (see U.S. Pat. No. 5,777,031) generally have a vinyl content of 18 to 50% by weight. KRATON G1650 has a vinyl content of about 35% by weight. Formulations based on these SEBS block copolymers have been found to have excellent adhesion to fabrics like cotton and nylon and have the advantage that they leave no residue after peeling. The application viscosity i.e. melt viscosity, of formulations using these polymers is acceptable but it would be advantageous to have a positionable adhesive formulation which has a lower melt viscosity in order to be able to lower the application temperature. This reduces the risk of degradation, char forming, and filter plugging. This also results in energy and cost savings, decreases maintenance costs, and reduces the amount of odor due to any volatiles coming from the adhesive formulation. The present invention provides such an improved positionable hot melt adhesive formulation.
SUMMARY OF THE INVENTION
The present invention is a hot melt pressure sensitive positioning adhesive for use with an absorbent article. The adhesive comprises:
(a) from greater than 6 to less than 15 percent by weight of the total of (a), (b) and (c) of a hydrogenated styrene-(butadiene and/or isoprene)-styrene block copolymer having a vinyl content of from greater than 50 to 90%, preferably 70 to 80% by weight of the total butadiene and/or isoprene block; and
(b) from 50 to 80 percent by weight of the total of (a), (b) and (c) of a tackifying resin which has an aromaticity such that the MMAP is at least 45° C.; and
(c) from 5 to 35 percent by weight of the total of (a), (b), and (c) of a plasticizer.
The term “vinyl content” refers to the fact that a conjugated diene may be polymerized via 1,2-addition (in the case of butadiene—it would be 3,4-addition or 1,2-addition in the case of isoprene). Although a pure “vinyl” group is formed only in the case of 1,2-addition polymerization of 1,3-butadiene, the effects of 3,4-addition polymerization of isoprene (and similar addition for other conjugated dienes) on the final properties of the block copolymer will be similar. The term “vinyl” refers to the presence of a pendant vinyl group on the polymer chain. The purpose here is to introduce chain branching and to reduce the size of the main polymer backbone (since some of the carbons in the diene are in the pendant group) which reduces the end to end length of the molecule and, in turn, its viscosity in the cement.
DETAILED DESCRIPTION OF THE INVENTION
One of the primary components of the positioning adhesive composition of the present invention is the above-described hydrogenated block copolymer which has two polystyrene endblocks and a saturated or hydrogenated polybutadiene and/or polyisoprene midblock. This hydrogenated base block copolymer provides the primary load bearing capability of the adhesive composition. It is important that the polymer be hydrogenated so that the structural integrity of the polymer is preserved even if outside forces that cause degradation are encountered. The block copolymer may be hydrogenated as generally described in the prior art, preferably so as to reduce at least 90 percent of any olefinic double bonds in the polymer chains. Suitably at least 50 percent, preferably at least 70 percent, and more preferably at least 90 percent, most preferably at least 95 percent of the original olefinic unsaturation is hydrogenated.
Polymerisation of butadiene (and/or isoprene) must produce a polymer block with a high vinyl content. This typically involves anionic polymerisation in an apolar solvent in the presence of a structure modifier, as is well known in the art. The vinyl content should be greater than 50% by weight, preferably 50-90%, more preferably 70 to 80%wt. At 50% or below, the polymer viscosity is similar to conventional polymers and there is no advantage. Above 90%wt, the viscosity decrease has reached a plateau and no longer drops with higher vinyl content. Therefore, there is no further advantage.
Anionic polymerization of conjugated diene hydrocarbons with lithium initiators is well known as described in U.S. Pat. Nos. 4,039,593 and Re. 27,145 which descriptions are incorporated herein by reference. Polymerization commences with a monolithium, dilithium, or polylithium initiator which builds a living polymer backbone at each lithium site. Typical living polymer structures containing polymerized conjugated diene hydrocarbons are:
X—B—Li
X—A—B—Li
X—A—B—A—Li
Li—B—Y—B—Li
Li—A—B—Y—B—A—Li
wherein B represents polymerized units of one or more conjugated diene hydrocarbons such as butadiene or isoprene, A represents polymerized units of one or more vinyl aromatic compounds such as styrene, X is the residue of a monolithium initiator such as sec-butyllithium, and Y is the residue of a dilithium initiator such as the diadduct of sec-butyllithium and m-diisopropenylbenzene. Some structures, including those pertaining to polylithium initiators or random units of styrene and a conjugated diene, generally have limited practical utility although known in the art.
The anionic polymerization of the conjugated diene hydrocarbons is typically controlled with structure modifiers such as diethylether or ethyl glyme (1,2-diethoxyethane) to obtain the desired amount of 1,2-addition. (i.e. vinyl content). A 1,2-addition of about 40% may be achieved during polymerization at 50° C. with about 6% by volume of diethylether or about 200 ppm of ethyl glyme in the final solution. A 1,2 addition of about 47% may be achieved during polymerization by the presence of about 250 ppm of ortho-dimethoxybenzene (ODMB)
De Keyzer Noël Raymond Maurice
Stoner Carolyn Ann
Kraton Polymers U.S. LLC
Mullis Jeffrey
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