Process for preparing a flexible polyurethane foam

Details Process for preparing a flexible polyurethane foam Process for preparing a flexible polyurethane foam

1999-08-06

2002-07-09

Sergent, Rabon (Department: 1711)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S130000, C521S133000, C521S160000, C521S174000, C521S904000

Reexamination Certificate

active

06417241

ABSTRACT:

The present invention is concerned with a process for preparing flexible polyurethane foams.
It is widely known to prepare flexible polyurethane foams by reacting an organic polyisocyanate and a high molecular weight isocyanate-reactive compound in the presence of a blowing agent. More in particular it has been disclosed in EP 111121 to prepare flexible polyurethane foams from a polyisocyanate composition comprising a semi-prepolymer. The polyisocyanate composition is prepared by reacting a diphenylmethane diisocyanate and a polyol; a polymethylene polyphenylene polyisocyanate (polymeric MDI) is used as well. In EP 392788 flexible foams are prepared by reacting semi-prepolymers or prepolymers with an isocyanate-reactive composition containing a high amount of water. In EP 296449 flexible foams are prepared by reacting polyisocyanates, polyols and water at a relatively low NCO-index.
Copending application PCT/EP95/02068 is concerned with a process for making flexible foams using a semi-prepolymer which has been made by reacting a portion of polymeric MDI with a polyol and adding the other part to the reaction product so obtained.
Although useful flexible foams based on MDI and polymeric MDI and polymeric MDI are obtainable, room for improvement exists. In particular foams made in a closed mould which foams are to be used as cushioning material in automotive seating could be improved as to foam strength, resilience (ball rebound) particularly at low density of the foam.
In the past some of these improvements have been obtained by using tolylene diisocyanate (TDI) instead of MDI. In particular such foams show a high resilience, good foam strength at low density. However, due to its vapour pressure and toxicity special measures need to be taken to handle TDI. Further TDI based foams show relatively poor hardness especially at low density and a slow cure and narrow processing range (isocyanate index).
More recently, proposals have been made to avoid the disadvantages of both MDI-based and TDI-based foams by using combinations of MDI and TDI.
In EP 439792 the use of a polyisocyanate has been proposed which comprises 21-95% by weight of TDI in order to improve the tensile strength; the amount of TDI used still is relatively high.
In EP 679671 the use of a mixture of polymeric MDI and TDI comprising 3-20% by weight of TDI has been proposed to prepare a low density foam having an enhanced impact resilience, improved compression set and an excellent ability to reduce 6 Hz vibration transmissibility. The polymeric MDI used has a high three benzene ring compound content compared to the four or more benzene ring compound content + less active ingredient. The use of polymer polyols has been proposed in very general terms.
In EP 694570 the use of a polyisocyanate prepolymer has been proposed comprising MDI, polymeric MDI and 5-15% by weight of TDI. The polyisocyanate prepolymer has improved flowability; the foams made thereof show improved ILD, compression set and flammability characteristics. The use of a graft polymer dispersion in a polyol has been proposed as well.
In WO 97/19971 it has been proposed to use an MDI prepolymer and TDI to improve resilience and foam stability, comfort properties and mechanical strength. The amount of TDI may be 2-25% by weight of the polyisocyanate composition, which has an MDI+TDI functionality of 2.05-2.35. The use of polymer polyol prepared by the in situ polymerisation of styrene and/or acrylonitrile in polymeric polyols or by the in situ reaction between a polyisocyanate and triethanolamine in a polymeric polyol (PIPA polyol) has been proposed in general terms. The polymeric polyol may contain 5-50% by weight of dispersed polymer. In co-pending application PCT/EP98/0867 it has been proposed to lower the level of TDI and PIPA polyol.
EP 555721 discloses a process for preparing cold-cure, moulded flexible foam by reacting a polyisocyanate of the MDI-type in a one-shot process with a polyol composition comprising 5-30% of a polyol having at least 50% by weight of oxyethylene groups and an OH value of less than 150. The foams show a high resilience (about 65%) only at elevated density (at least about 55%).
However, there remains room for improvement. In particular there is a need for further reduction of the density, while maintaining or even improving other physical properties, like elongation, tear strength, dry compression set and hysteresis loss, of moulded foams which have been made according to the one-shot process using water as the sole blowing agent and MDI-type polyisocyanate as the polyisocyanate.
Also, foams made in a closed mould, which foams are to be used as cushioning material in automotive seating, could be improved as to resilience at low density, especially in the case of TDI-free MDI based foams.
Surprisingly, it has been found that this may be achieved by the process according to the present invention.
The present invention is concerned with a process for preparing a flexible polyurethane foam at an NCO index of 70-120 by reacting in a closed mould
a) a polyisocyanate composition consisting of diphenylmethane diisocyanate and homologues thereof having an isocyanate functionality of 3 or more, the amount of diphenylmethane diisocyanate being 81 to 100% by weight calculated on the amount of diphenylmethane diisocyanates and homologues, and the diphenylmethane diisocyanate comprising 40-60% by weight, calculated on the weight of this diphenylmethane diisocyanate, of diphenylmethane diisocyanate containing at least one NCO group in the ortho position, and the amount of homologues having an isocyanate functionality of 3 or more being 19-0% by weight, and
b) a polyol composition comprising
1) a polyoxyethylene-polyoxypropylene-polyol, having an average nominal hydroxyl functionality of 2-6 , an average equivalent weight of 1000-4000 and containing 10-25% by weight of oxyethylene groups;
2) 4 to 8 parts by weight of water as the sole blowing agent or optionally together with C
O
2
as the sole blowing agents; and
3) 2 to 20, preferably 2 to 10 parts by weight of a polyether polyol having an average nominal hydroxyl functionality of 2-6, an average equivalent weight of 200-600 and containing at least 60% by weight of oxyethylene groups; and optionally
4) up to 30 and preferably 4-25 parts by weight of particulate material which is dispersed in said polyol composition; the amounts of b2) to b4) being calculated per 100 parts by weight of b1); and
5) optionally auxiliaries and additives known per se.
In the context of the present application the following terms have the following meaning:
1) isocyanate index or NCO index or index:
the ratio of NCO-groups over isocyanate-reactive hydrogen atoms present in a formulation, given as a percentage:
[NCO]×
100/(%)
[active hydrogen]
In other words the NCO-index expresses the percentage of isocyanate actually used in a formulation with respect to the amount of isocyanate theoretically required for reacting with the amount of isocyanate-reactive hydrogen used in a formulation.
It should be observed that the isocyanate index as used herein is considered from the point of view of the actual foaming process involving the isocyanate ingredient and the isocyanate-reactive ingredients. Any isocyanate groups consumed in a preliminary step to produce the semi-prepolymer or other modified polyisocyanates or any active hydrogens reacted with isocyanate to produce modified polyols or polyamines, are not taken into account in the calculation of the isocyanate index. Only the free isocyanate groups and the free isocyanate-reactive hydrogens (including those of the water) present at the actual foaming stage are taken into account.
2) The expression “isocyanate-reactive hydrogen atoms” as used herein for the purpose of calculating the isocyanate index refers to the total of hydroxyl and amine hydrogen atoms present in the reactive compositions in the form of polyols, polyamines and/or water; this means that for the purpose of calculating the isocyanate index at the actual foaming proc

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