Preparation of highly resilient polyurethane foams

Details Preparation of highly resilient polyurethane foams Preparation of highly resilient polyurethane foams

2002-06-04

2003-12-09

Cooney, Jr., John M. (Department: 1711)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S115000, C521S128000, C521S129000, C521S170000, C521S174000

Reexamination Certificate

active

06660783

ABSTRACT:

The present invention relates to a process for the preparation of highly resilient polyurethane foams.
Highly resilient polyurethane foams, frequently also referred to as HR foams, have long been known. They are generally prepared by reacting filler-containing polyols with isocyanates, in particular TDI, MDI or mixtures thereof, it also being possible to use modified isocyanates, in particular isocyanates containing urethane, urea, biuret and/or allophanate groups. Examples of filler-containing polyols used are:
a) Dispersions of homo- and/or copolymers of styrene and acrylonitrile in polyols, which are grafted with acrylonitrile and/or styrene (SAN polyols),
b) dispersions of polyurea particles in polyols, the polyurea particles being prepared by reacting diamines and diisocyanates in the presence of the polyol (PHD polyols),
c) dispersions of particles which are prepared by reacting alkanolamines and diisocyanates in polyols (PIPA polyols) or
mixtures of at least two of said polyether alcohols.
In addition to these filler-containing polyols, alkanolamines are also generally used as crosslinking agents for the preparation of HR foams. The alkanolamines cause chemical crosslinking of the foam and are required for obtaining foams having sufficient stability. If no alkanolamines are used, the foam generally collapses or settles considerably since the cells are not sufficiently stabilized after the end of the polyisocyanate polyaddition reaction. In contrast to conventional foams, the physical stabilization in the case of HR foams is not sufficient at this time since special silicone stabilizers having comparatively low activity are used for the preparation of HR foams. If, on the other hand, more active stabilizers, as used for the preparation of conventional slabstock foams, are employed, the HR foams, owing to the generally higher crosslinking density, tend to form closed-cell foams, which leads to shrinkage of the foams in the cooling phase.
However, the use of the alkanolamine crosslinking agents is also associated with some changes in the mechanical properties of the foams and thus also disadvantages, so that their content in the foam system must as far as possible be limited.
Thus, it is known from the literature that the addition of alkanolamines, for example diethanolamine, leads to an increase in the crosslinking density of the foam since the diethanolamine molecules are covalently incorporated into the hard phase segments of the foam matrix. This is described, for example, in Dimitros, Wilkes: Journal of Applied Polymer Science 77 (2000), 202-216, and Dounis, Wilkes: Journal of Applied Polymer Science 65 (1997), 525-539.
The incorporation of the diethanolamine molecules into the hard segments leads to a disturbance of the hard phase order by reducing the proportion of intermolecular hydrogen bridge bonds between the urea units in the hard segments. However, these hydrogen bridges are essential for the occurrence of crystalline structures which give the foam its typical rigidity. Since the proportion of hydrogen bridges and hence the proportion of crystalline units in the hard segments are reduced, a reduction in the rigidity of the foams occurs in conventional HR foams as a result of the addition of alkanolamine crosslinking agents. However, the disturbance of the hard segments in the HR foam also has advantages. In the case of the deformation of the foams by the action of force, the hydrogen bridges are broken at a specific force. During the subsequent relief, the hydrogen bridge bonds must form again. This recovery phase lasts longer the larger and more undisturbed the hard phase segments. Consequently, the area between loading curve and relief curve, i.e. the hysteresis, becomes larger, which manifests itself in lower resilience. Diethanolamine disturbs the formation of longer and ordered hard segments, so that fewer hydrogen bridges can form. Consequently, the recovery phase is shorter and the hysteresis smaller.
Furthermore, it is known that the resilience of HR foams increases with increasing rigidity. In principle, the reduction in rigidity is undesirable in many cases. Highly resilient foams tend as a rule to be formulated to be rigid in order to meet the high requirements with respect to the comfort properties, for example high resilience. One criterion which the foams should fulfill is the SAG factor, the quotient of the indentation hardness at 65% deformation and the indentation hardness at 25% deformation (method B). The SAG factor should be >2.5, preferably >3. Furthermore, the disturbance of the hard phase order and hence of the intermolecular hydrogen bridges by alkanolamine crosslinking agents also leads to lower stability of the foams to the action of heat and moisture, since the domains are more labile and hence more mobile. The alkanolamine crosslinking agents are incorporated into the polyurethane hard segments via urethane and urea groups. These additional urethane and urea groups in the hard segments permit the formation of hydrogen bridge bonds to penetrating water molecules, whereas this is not possible in hard segments without alkanolamine since here the urea bonds interact with one another to form crystalline hard segments via hydrogen bridges so that no additional hydrogen bridges to water molecules can be formed. The adverse effect of the penetrated water in the hard segments is evident, for example, in the substantial deterioration in the mechanical properties under or after the action of moisture. In particular, the wet compression set (WCS), a compression set measurement under climatically controlled conditions, the humid aged compression set (HACS), a compression set measurement after exposure to a humid warm atmosphere, are frequently too high in the case of HR foams. The WCS in particular is an important quality criterion for HR foams when used as mattresses and upholstery materials. In addition to the compression set, the rigidity, tensile strength and elongation under the action of heat and moisture also should not deteriorate.
A number of possibilities for improving the WCS of HR foams are known from the prior art. One possibility is the heat treatment of the foams after their preparation is complete. This postcuring at 120° C. is described, for example, by Brasington and Lambach (Handbook of the 35th Annual Polyurethane Technical/Marketing Conference, 1994, pages 261-266) and by Skorpenske et al. (Handbook of the 34th Annual Polyurethane Technical/Marketing Conference, 1992, pages 650-658) and Broos et al. (Journal of Cellular Plastics 36 (2000), 5). The disadvantage of this process is that, after the preparation, the foam has to be subjected to a further processing step in which the action of heat can result in impairment of the mechanical properties, such as the tensile strength and elongation.
According to EP-A-0 903 362, low molecular weight chain extenders or crosslinking agents are used for the preparation of polyurethane foams having improved rigidity and higher stability in a humid and warm atmosphere, for example glycerol, trimethylolpropane, sorbitol or ethylene oxide-rich adducts of these compounds. Owing to their high OH numbers of 400-700 mg KOH/g, these compounds are too reactive for use in HR slabstock foams since the other polyols used for the preparation of these foams already have very high reactivity, so that closed foams tending to shrink result when the claimed low molecular weight crosslinking agents are used.
EP A-731 120 claims a process for the preparation of highly resilient foams using filler-free polyols. A mixture of a polyol having a relatively high functionality and an ethylene oxide content of 10-30% by weight and a highly reactive polyol having predominantly primary OH groups and an ethylene oxide content of 50-95% is used as filler-free polyols. The disadvantage of this type of mixture is the very low elongation at break, owing to the high reactivity and the high functionality of the polyols and hence the high degree of crosslinking of the foam. The high content of ethylene oxide in the high

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