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

2004-10-05

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...

C521S050500, C521S057000, C604S358000, C604S369000

Reexamination Certificate

active

06800666

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 the action of microwave energy (ultrahigh frequency irradiation) on an aqueous solution or dispersion each containing 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resins as absorbent interlayer are extensively described in prior DE application no. 100 34 505.0, unpublished at the priority date of the present invention.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Prior DE application no. 100 27 770.5, unpublished at the priority date of the present invention, describes the preparation of foams from low-formaldehyde open-celled melamine-formaldehyde resins having a molar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, 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
)alkylstyrenes as monomers, preferably styrene and ethylstyrene as comonomers, also aromatic polyvinyl compounds as crosslinkers; optionally polyfunctional acrylates, methacrylates, acrylamides and methacrylamides and mixtures thereof as additional cross-linker 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. Nos. 5,318,554 and 5,550,167 concern the production of open-celled foams based on HIPE emulsions and their use in hygiene articles to absorb aqueous body fluids. However, the open-celled foams are always used together with other components responsible for ultimate absorption (immobilization) of the body fluids. The materials have good application advantages, but also clear 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 wash 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 is therefore certainly not 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, characterized by a droplet absorption rate of less than 5seconds and an EU standard EN ISO 14184-1 formaldehyde emission of less than 100 mg of formaldehyde/kg of foam.
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, which comprises
(a) heating 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 and crosslink the precondensate,
(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 hydrophilicizer and/or with ozone, a corona discharge or a plasma.
Process step (a) is 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 molar ratio of melamine to formaldehyde is, for example, in the range from 1:1.0 to 1:5 and is preferably in the range from 1:1.0 to 1:1.9. The preferred range is known from prior DE application no. 100 27 770.5, unpublished at the priority date of the present invention. To produce low-formaldehyde and melamine-formaldehyde resins it is particularly advantageous for the molar ratio of melamine to formaldehyde to be within the range from 1:1.3 to 1:1.8. 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 viscosity 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 or 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 “by the action of microwave energy” according to the process known from EP-A-0 037 470.
Structure and mechanical properties of the foams are known from 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 than 0.04 [W.m
−1
.K
−1
];
the DIN 53 577 compression hardness on 60% compression, divided by the density, is less than 0.3, preferably less than 0.2 [N.cm
−2
/g.l
−1
], the determination of the compression hardness at 60% compression having to be followed by a recovery of the foam to at least 70%, preferably at least 90%, especially 95%, of its original dimensions;
the modulus of elasticity on the lines of DIN 53 423, divided by the density, is less than 0.25, preferably less than 0.15 [N.mm
−2
/g.l
−1
].
the DIN 53 423 bending travel on fracture is more than 10, preferably more than 15 [mm];
the DIN 53 527 compression set on 50% compression is less than 45%, preferably less than 30%, especially less than 10%;
the DIN 18 165 dynamic sti

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