Low application temperature hot melt adhesive

Abstract

A hot melt adhesive composition, comprising a blend of components including about 10% to about 40% by weight of an elastomeric block copolymer, preferably styrene-isoprene-styrene (SIS) or styrene-butadiene-styrene (SBS), about 15% to about 70% by weight of a first midblock tackifying resin having a softening point of at least about 110° C. and having an aromatic content of at least about 1.5% by weight; about 0 to 55% of second midblock tackifying resin, about 5% to about 35% by weight of a plasticizer; and about 0% to about 20% by weight of an end block resin having a softening point lower than 125° C.; wherein the components total 100% by weight of the composition, the viscosity of the composition is equal to or less than about 20,000 mPa.s at 120° C., and is applied at a temperature lower that 150° C. and initial bond retention of the composition on elastic strands is at least about 60%. Also, the elastic modulus G′ of the composition is higher than about 5000 Pa, the vicous modules G″ is higher than about 50 Pa, and the tan delta value is between about 0.5 and about 60. Laminates, especially those used in disposable soft goods, and methods of making such laminates are also described. The adhesive composition and / or laminate may be used in making a variety of end products such as a disposable diaper, a sanitary napkin, a bed pad, a bandage, a surgical drape, a tape, a label, a plastic sheet, a nonwoven sheet, a paper sheet, a cardboard, a book, a filter, or a package.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to hot melt adhesives, and more particularly to a hot melt adhesive having low viscosity and showing good cohesion level like high initial bond resistance that may be applied at relatively low temperatures for example for making elastic components such as laminates containing elastic strands for use in disposable diapers. [0002] The increasing complexity of manufactured goods, in particular disposable goods, also leads to major improvements and developments in the hot melt adhesive industry. Hot melt adhesives are being used to bond a wider variety of substrates, within a broader adhesive application process window, and for a large end-use portfolio. For example considering the diaper manufacturing industry, materials involved may be non-woven materials, polymeric films, and in general elastomeric components. These elastomeric components can be used in products like diapers, in a form of strands, films, nonwovens or an...

AI Technical Summary

Benefits of technology

[0048] The present invention solves the very important requirement of having a hot melt adhesive applied at relatively low application temperature, i.e. under 150° C., using the same application techniques as currently used, like coating techniques and add-on levels, and providing the end-use application the same level of performances expected with the current technologies, i.e. high bond strength levels in term of creep resistance, peel force and in general bond retention with mechanical resistance and heat resistance. The present invention is based on a unique formulation using styrene block copolymers, particularly for elastic attachment into diaper structures.

[0059] Another aspect of the present invention is the use of low to medium softening point aromatic tackifying resins in conjunction with high softening point mid-block resins. High softening point is defined herein as a softening point equal to or higher than 110° C., preferably equal to or higher than 115° C. Low to medium softening point is defined herein as a softening point below 125° C., preferably between 50 and 120° C., most preferably between 70 and 115° C. Softening point of these aromatic resins is important to control both the molten viscosity at the application temperature and the level of adhesion of the adhesives. It is well known that addition of such aromatic resins affects the Tg of the styrenic phase of a styrene block copolymer (SBc), lowering it or increasing it. However, it has been surprisingly seen in the present invention that this benefit can be expressly useful for both lowering the application temperature of a hot melt adhesive formula and in particular, allowing an application temperature lower than 150° C., preferably equal to or lower than 140° C., most preferably lower than 135° C., and also for getting a high level of bond retention, in conjunction with the use of a mid-block tackifying resin having a softening point high enough for the adhesive bond to withstand mechanical stress like creep effect, potentially at low, ambient or elevated temperature.

Problems solved by technology

Lowering the application temperature presents problems in terms of wet-out, and most of the time 150° C. would be seen as a minimum temperature one can go to attach elastic parts onto the diaper structure.

It is known in the diaper industry that the use of heat sensitive substrates may cause problems if the adhesive temperature is too high because the production line has to be stopped each time the substrate breaks or is damaged by the molten adhesive material (described as a “burn through” phenomenon) and would need to be replaced or fixed before starting the line again.

Heat distortion or deterioration of the film or non-woven substrates can occur easily when the hot adhesive material contacts the substrates' surfaces.

As a result, the functionality of the substrates in the end-use structure is affected in a way that is not acceptable.

Using this amount of oil at the viscosity level claimed, i.e. less than 5,000 mPa.s at 125° C., it is clear that this reference does not teach how to achieve a conventional elastic attachment at low temperature.

Even if the application temperature was low, the viscosity level of the examples described in this patent would be very high and would thus hinder any process where the adhesive needed to be pumped and pushed through conventional components of a hot melt adhesive application device.

Again, this does not help to get bond retention in the way needed for elastic attachment in a diaper structure.

Although this reference mentions that hot melt adhesives for structural or elastic attachment are available on the market, it does not provide any solution to applying them at low temperatures for elastic attachment.

WO 00 / 78886A1 mentions applications at low temperatures of 130° C. to 135° C. Unfortunately, application results are exhibited only for spiral construction or bottle labeling, which are non-demanding applications in terms of cohesion and bond retention, in contrast to the ones needed in an elastic attachment environment.

Besides the fact that such amounts are not practical to maintain bond retention in a hot melt adhesive composition, it describes aromatic modified resins having a softening point approximately equal to or lower than 100° C., which leads to poor cohesion levels, non-aromatic modified resins having a softening point in a broad range of temperature (100 to 140° C.

This allows for a lower application temperature as low as 100° C. to 130° C. This solution may not provide any economic advantage over current technology using conventional synthetic mineral oils, and does not teach how to make a hot melt composition that would be adapted to creep resistance or bond retention in general, and elastic attachment into a diaper structure in particular.

Moreover, resins mentioned in the examples have softening points of 100° C. or below, which does help with decreasing the viscosity of the adhesive, but not building enough cohesion in it.

Numerous references claim the concept of applying an adhesive at low temperature, with a certain lack of precision, i.e. they do not precisely define the temperature domain, or they do not give a clear way of how to practically achieve the low temperature application.

This is not sufficient to teach how to build cohesion in the adhesive material while having a low application temperature.

The description of adhesive composition given in this reference does not allow one to think about a relevant way to get creep resistance from the bond with acceptable performances.

Although it exhibits viscosity measurements at temperatures as low as 100° C., it does not show that this low level of temperature is achieved while applying the adhesive.

WO 00 / 78886A1 mentions the use of Hercolite 290, which is a high softening point aromatic resin that helps to build cohesion into the adhesive material, as pure monomer resins are known for, but definitively does not help with lowering the application temperature.

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