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
2000-06-20
2003-12-02
Mulcahy, Peter D. (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...
C524S270000, C524S274000, C524S515000, C524S528000
Reexamination Certificate
active
06657000
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 and/or isoprene. More particularly, the invention relates to such adhesives which incorporate interpolymers of ethylene and at least one C
3
-C
20
alpha olefin for 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, diaper inserts, etc. 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 a 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 and low pressure 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. 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 of formulations using these polymers is acceptable but it would be advantageous to have a positionable adhesive formulation which has a lower viscosity in order 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 the volatiles coming from the adhesive. 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 5 to less than 15 percent by weight, basis the total of (a), (b), and (c), of a blend of
(i) a hydrogenated styrene-(butadiene or isoprene)-styrene block copolymer having a polystyrene block molecular weight of 20,000 or less, and
(ii) a homogeneous linear or substantially linear interpolymer of ethylene and at least one C
3
-C
20
alpha olefin having a density from 0.85 to 0.91 grams per cubic centimeter; and
(b) from 50 to 80 percent by weight, basis the total of (a), (b), and (c), of a tackifying resin which has an aromaticity such that the MMAP cloud point is at least 45° C.; and
(c) from 5 to 35 percent by weight, basis the total of (a), (b), and (c), of a plasticizing oil.
The weight ratio of the block copolymer and the interpolymer in (a) may range from 90:10 to 10:90, preferably from 80:20 to 20:80, more preferably from 60:40 to 40:60. In the case where the polystyrene block number average molecular weight is from 5500 to 9500 and the total number average molecular weight is from 65,000 to 95,000, the weight ratio may range from 60:40 to 90:10, preferably 80:20 to 90:10. The block copolymer of (i) may also be one in which the polystyrene block molecular weight is greater than 20,000. In such case, the tackifying resin must have an aromaticity wherein the MMAP cloud point is at least 70° C. In each embodiment, a second block copolymer can be added to the block copolymer of (i) to lower the viscosity. One option is to add a second polymer which has a diblock content of 20 to 70 percent by weight. A second option, irrespective of diblock content is if the base hydrogenated styrene-butadiene or isoprene-styrene block copolymer has a polystyrene content of 25 percent by weight or more, preferably 30 percent by weight or more, the polymer which is added to it to decrease the viscosity has a polystyrene content of less than 25 percent by weight, preferably 22 percent by weight or less.
Another option is to add more than two block copolymers. For example, a blend of a styrenic triblock copolymer, plus a styrenic block copolymer containing diblock, and a styrenic block copolymer containing less than 25% polystyrene could be used. Additionally, styrenic diblock copolymers could be added or copolymers with an ethylene/propylene multiblock could be added. This list is not limitative and therefore, it can be appreciated that a wide variety of block copolymers are useful in this invention including ABA triblock structures, ABA/AB triblock/diblock structures, AB diblock structures, (B)
n
structures, ABAB tetrablock structures where the A endblock is a non-elastomeric polymer block, typically polystyrene, and B is ethylene/butylene (hydrogenated butadiene) or ethylene/propylene (hydrogenated isoprene). Also included in the list are polymers which are grafted with maleic anhydride.
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 conventional 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.
The butadiene (isoprene) used in this invention should be one which forms a polybutadiene (polyisoprene) block which is largely amorphous at use temperatures (usually body temperature) and does not contain excess crystallinity which would interfere with flexibility. For hydrogenated polybutadiene blocks, it is generally best that the vinyl content before hydrogenation not be less than 18 percent by weight, preferably not less than 30 20 percent by weight because below that number, the crystallinity of the polymer is too high, giving a stiff polymer which is unsuitable for use in pressure sensitive adhesives.
The term “vinyl content before hydrogenation” refers to the fact that a conjugated diene is polymerized via 1,2-addition (in the case of butadiene—it would be 1,2-addition or 3,4-addition in the case of isoprene). Although a pur
De Keyzer Noël Raymond Maurice
Stoner Carolyn Ann
Kraton Polymers U.S. LLC
Mulcahy Peter D.
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