Hydrophilic open-cell, elastic foams with a...

Details Hydrophilic open-cell, elastic foams with a... Hydrophilic open-cell, elastic foams with a...

2003-03-10

2004-12-07

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S188000, C521S064000, C604S359000, C604S368000

Reexamination Certificate

active

06828354

ABSTRACT:

The present invention relates to hydrophilic open-celled resilient foams based on melamine-formaldehyde resins, their preparation and their use in hygiene articles.
EP-A-0 017 621 and EP-A-0 017 672 disclose open-celled resilient foams based on melamine-formaldehyde condensation products and to processes for their preparation. The process known from EP-A-0 037 470 produces open-celled resilient foams from melamine-formaldehyde condensation products in a particularly advantageous manner by reaction of microwave energy (ultra high frequency irradiation) on an aqueous solution or dispersion containing in each case a melamine-formaldehyde precondensate, an emulsifier, a blowing agent and a curing agent. The solution or dispersion is heated in such a way that it foams up and cures the precondensate. The foams thus obtainable emit small amounts of formaldehyde at a rate which increases with increasing foam temperature and moisture content.
The construction of hygiene articles and use of open-celled foams of melamine-formaldehyde resins as adsorbent interlayer is extensively described in prior DE application No. 100 34 505.0, unpublished at the priority date of the present invention. The melamine-formaldehyde resin foams recited therein are highly hydrophilic, but they give off a comparatively large amount of formaldehyde on contact with body fluids. This substantially limits the possibility of using such foams in hygiene articles.
Prior DE application No. 100 27 770.5, unpublished at the priority date of the present invention, describes the preparation of foams from low-formaldehyde open-celled melamine-formaldehyde resins having a molar ratio of melamine to formaldehyde in the range from 1:1.0 to 1:1.9. These foams emit less than 30 mg of formaldehyde per kg of foam even under the warm-moist conditions customary in the hygiene sector (EU Standard EN ISO 14 184-1, water immersion at 40° C. for 1 h). They consequently meet the baby clothing requirements of Oeko-Tex Standard 100 (quality mark of textiles tested for harmful substances). However, the appreciable reduction in formaldehyde emission comes at the expense of a partial loss of hydrophilic properties of the foam, as a result of which the liquid acquisition rate of such foam layers decreases.
WO-A-96/21682 discloses foams which, owing to their open-celled structure with relatively large openings and channels, are very useful for absorbing aqueous body fluids, especially blood. The foams are obtained by polymerization of (C
4
-C
14
)alkyl acrylates, (C
6
-C
16
)alkyl methacrylates, (C
4
-C
12
)alkyl-styrenes as monomers, preferably styrene and ethyl-styrene as comonomers, also aromatic polyvinyl compounds as crosslinkers; optionally polyfunctional acrylates, methacrylates, acrylamides and methacryl-amides and mixtures thereof as additional crosslinker substances. The polymerization takes place within a High Internal Phase Emulsion (HIPE) of the W/O type in which the weight ratio of water phase to oil phase is in the range from 20:1 to 125:1. After the polymerization has ended, the polymer foams are washed and dried.
WO-A-97/07832, U.S. Pat. No. 5,318,554 and U.S. Pat. No. 5,550,167 concern the production of open-celled foams based on HIPE emulsions and their use for absorbing aqueous body fluids. However, the open-celled foams are always used together with other components responsible for the ultimate absorption (immobilization) of the body fluids. The materials have good application advantages, but also clear in the hygiene article disadvantages.
For instance, the production of these materials is an extremely complicated process which is difficult to control. The enormous amount of aqueous phase (aqueous salt solution) required is neither economically nor ecologically sensible. Moreover, the materials are hydrophilicized at the surface with a salt layer. This layer can become detached during use and pass into the storage medium of the absorbent core. The storage medium is generally made of superabsorbents. It is known that superabsorbents are susceptible to “salt poisoning,” i.e., their absorbency decreases dramatically with the increasing salt content of the solution to be absorbed. It can, therefore, certainly not be desirable to additionally increase the salt load in the body fluids to be absorbed.
It is an object of the present invention to provide open-celled resilient foams based on melamine-formaldehyde resins that are hydrophilic and whose formaldehyde emissions are substantially reduced compared to existing foams of melamine-formaldehyde resins.
We have found that this object is achieved by hydrophilic open-celled resilient foams comprising melamine-formaldehyde resins, obtainable by
(a) heating and crosslinking an aqueous solution or dispersion each containing at least a melamine-formaldehyde precondensate, an emulsifier, a blowing agent and a curing agent to form a foam,
(b) then conditioning the foam at from 120° C. to 300° C. for from 1 to 180 minutes to remove volatiles, and
(c) treating the foam during the conditioning or thereafter with at least one polymer containing primary and/or secondary amino groups and having a molar mass of not less than 300.
The foams are prepared using, for example, melamine-formaldehyde precondensates where the molar ratio of melamine to formaldehyde is in the range from 1:1.0 to 1:5.0. Preference was given to melamine-formaldehyde precondensates where the molar ratio of melamine to formaldehyde is in the range from 1:2.0 to 1:5.0, especially in the range from 1:2.5 to 1:3.5. The molar mass of the polymers which contain primary and/or secondary amino groups and are present as modifiers on the hydrophilic open-celled resilient foams is, for example, in the range from 500 to 5 million, preferably in the range from 1,000 to 100,000. These foams preferably include vinylamine polymers, polyethyleneimines, polyallylamines, lysine condensates or mixtures thereof to reduce formaldehyde emissions with or without hydrophilic properties being imparted.
Such foams have, for example, a density of from 5 to 200 g/l, a specific surface area (determined according to BET) of more than 0.5 m
2
/g and a Free Swell Capacity of more than 20 g/g. They have, for example, in the wet state a tensile strength of >60 J/m
2
.
The invention also provides a process for preparing hydrophilic open-celled resilient foams comprising melamine-formaldehyde resins, which comprises
(a) heating and crosslinking an aqueous solution or dispersion each containing at least a melamine-formaldehyde precondensate, an emulsifier, a blowing agent and a curing agent to form a foam,
(b) then conditioning the foam at from 120° C. to 300° C. for from 1 to 180 minutes to remove volatiles, and
(c) treating the foam during the conditioning or thereafter with at least one polymer containing primary and/or secondary amino groups and having a molar mass of not less than 300.
Process steps (a) and (b) are known from the prior art, cf. the above-discussed references EP-A-0 017 621, EP-A-0 017 672 and EP-A-0 037 470. The foaming as per step (a) is effected by heating the mixture to a temperature above the boiling point of the blowing agent and is carried out, for example, in such a way that initially there is little increase in the viscosing and a steep rise in the viscosity and crosslinking substantially does not occur until the foaming process has ended. However, foaming of the mixture and crosslinking of the precondensate may also be effected concurrently. Heating of the mixture is effected, for example, using hot air, steam and/or by utilizing heat of reaction. The foaming of the aqueous mixture of melamine-formaldehyde precondensate, emulsifier, blowing agent and curing agent is preferably effected by means of microwaves according to the process known from EP-A-0 0 37 470.
Structure and mechanical properties of the foams are known from and EP-A-0 017 672:
the DIN 53 420 density is in the range from 1.6 to 30, preferably from 2 to 20 [g/l];
the DIN 52 612 coefficient of thermal conductivity is less than 0.06, preferably less t

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