WO2001030895A1

WO2001030895A1 - Process for the preparation of a flexible polyurethane foam

Info

Publication number: WO2001030895A1
Application number: PCT/EP2000/010723
Authority: WO
Inventors: Jian Chen; Jeremy Hallam Shears
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 1999-10-29
Filing date: 2000-10-27
Publication date: 2001-05-03
Also published as: AU2154001A

Abstract

Process for the preparation of a flexible polyurethane foam, which process comprises pouring a polyurethane reaction mixture into a closed reaction space where the foaming reaction occurs in an atmosphere having a relative humidity of at least 90 %. Additional gases may be present as well and include carbon dioxide and volatile acids. The process reduces the thickness of the high density surface skin normally formed on a flexible slabstock foam, thereby increasing the overall yield of useful foam.

Description

PROCESS FOR THE PREPARATION OF A FLEXIBLE POLYURETHANE FOAM

The present invention relates to a process for the preparation of a flexible polyurethane foam, in particular a flexible slabstock foam.

Flexible polyurethane slabstock foams can be made in a continuous process or in a discontinuous process. The continuous and discontinuous slabstock production processes are well known.

In continuous slabstock production the components of a foam formulation are normally brought together in a mixing head where they are mixed and from which the formulation is subsequently poured onto a moving conveyor belt, normally covered with paper. Often a traversing mixing head is used in order to pour the foam formulation onto the entire breadth of the conveyor belt . From the mixing head the foam formulation may also be charged into a trough from which the foam formulation subsequently flows via a downwardly sloping fall plate onto a lower situated, horizontally moving conveyor belt. The foam formulation expands on the conveyor belt and may be covered with a top layer of paper or plastic film. After a certain length the foam thus produced is cut.

In discontinuous slabstock production either the components of a flexible polyurethane formulation are poured into a shaped box (e.g. a square or cylindrically shaped box) and mixed or the components are mixed and then poured into the shaped box, after which the foam is formed. After expansion the foam is removed from the box for subsequent handling.

Shaped articles made of flexible polyurethane foam are widely used in numerous applications. Main sectors of application are the automotive and aircraft industry, upholstered furniture, mattresses and technical articles. For instance, full foam seats, top pads for the seats and restraints for back and head, all made from flexible polyurethane foam, are widely used in cars and aeroplanes . Other applications include the use of flexible polyurethane foam as carpet backings, foamed seat saddles for motorbikes, gaskets between a car body and its lights, lip seals of air filters for engines and insulating layer on car parts and engine parts to reduce sound and vibration.

An undesired, but up to now inevitable, phenomenon occurring in both type of slabstock processes described above is the formation of a high density skin on the surface of the slabstock foam prepared. Due to its high density this surface skin is not suitable for most slabstock foam applications and consequently has to be removed before the foam can be processed for its envisaged application. Generally, this may result in a loss of foam of up to approximately 5% by weight based on the total amount of slabstock foam prepared.

The present invention aims to provide a method for preparing a flexible slabstock polyurethane foam wherein the thickness of the surface skin layer is reduced, thereby reducing the amount of polyurethane waste and increasing the yield of useful product.

It was found that when the foam expansion takes place in an atmosphere of high humidity the yield of useful foam considerably increases. Accordingly, the present invention relates to a process for the preparation of a flexible polyurethane foam, which process comprises pouring a polyurethane reaction mixture into a closed reaction space where the foaming reaction occurs in an atmosphere having a relative humidity of at least 90%. The "polyurethane reaction mixture" referred to above is a mixture containing all components necessary for making a polyurethane foam. It is a chemically reactive mixture which has been prepared by mixing all necessary components together just before the resulting reaction mixture is poured into the reaction space. Typically, such reaction mixture will contain a polyol component, a polyisocyanate component and the usual ancillary chemicals, like urethane catalysts, blowing agents, surfactants, fillers and the like.

Suitable polyol components are those comprising at least one polyol normally applied for preparing flexible polyurethane foams, i.e. a flexible polyol. The flexible polyol may be a polyester polyol or a polyether polyol. Very good results, however, have been achieved when polyether polyols (or polyoxyalkylene polyols) are used. Suitably the polyol component comprises one flexible polyol or a blend of two or more of such polyols. Alternatively, the polyol component can also comprise, or even completely consist of, a so called polymer polyol. A polymer polyol consists of a base polyol with a polymer stably dispersed therein. The base polyol is a flexible polyol, while the polymer normally is a styrene- acrylonitrile polymer, polystyrene, polyurea and polyurethane polymers and the condensation product of triethanol amine and toluene diisocyanate (TDI). The dispersed polymer is suitably present in an amount of from 5 to 40% by weight based on total weight of polymer polyol . Flexible polyether polyols, then, typically are based on polymerization products of propylene oxide, optionally together with ethylene oxide. Such polyols typically have a molecular weight of at least 2,500 and suitably at most 6,500, and an average nominal functionality (Fn) of at least 2.0, more suitably from 2.5 to 6.0. Primary hydroxyl content and ethylene oxide content are not particularly critical and may also depend on the application foreseen for the polyurethane foam to be prepared. Thus, both primary hydroxyl content and ethylene oxide content may have any value from 0% to 100%.

In addition to the flexible polyol described above, a hydrophilic flexible polyol may be present. Suitable hydrophilic polyols typically have a molecular weight in the range of from 2000 to 5500, an ethylene oxide content of at least 40% by weight and a primary hydroxyl content of at least 50%. Preferred hydrophilic polyols to be used as component (b) are those having a molecular weight in the range of from 3000 to 5000, an ethylene oxide content of from 50 to 80% by weight and a primary hydroxyl content of from 70 to 95%.

Ancillary chemicals that are normally present include the conventional additional components and auxiliaries useful in the production of flexible polyurethane foams. Additional components are foaming (or polyurethane) catalysts and/or crosslinking agents. Auxiliaries like fillers, flame retardants, foam stabilisers (surfactants), blowing agents and colorants may be present as well. The nature of these components are well known in the art. An extensive list of polyurethane catalysts is, for instance, given in U.S. patent specification No. 5,011,908 and includes tin catalysts and tertiary amine catalysts, normally used in an amount of 0.01 to 2.0 php (parts by weight per 100 parts by weight of polyol) . Crosslinking agents often applied in the production of polyurethane foams are polyfunctional glycol amines, such as diethanol amine. If used at all, the crosslinking agent is applied in amounts up to 3.0 php. Suitable blowing agents include water, acetone, (liquid) carbon dioxide, halogenated hydrocarbons, aliphatic alkanes and alicyclic alkanes. The amounts wherein the blowing agents other than water are to be used are those conventionally applied, i.e. between about 0.1 and 20 php. Water can be used in amounts up to 6 php or even higher, while in flexible foam formulations for casing applications 4 php is normally considered the upper limit. Frequently applied foam stabilisers are the organosilicone surfactants and they are used in an amount of up to 5% by weight based on the reaction mixture of polyol reactant and polyisocyanate reactant.

Polyisocyanates that may be used are those conventionally applied in the production of flexible polyurethane foams . Useful polyisocyanates should contain at least two isocyanate groups and include both aliphatic -usually alkylene- and aromatic di-, tri-, tetra- and higher isocyanates known in the art to be suitably applied in the production of flexible polyurethane foams . Mixtures of two or more of such aliphatic and/or aromatic polyisocyanates may also be applied. Examples of suitable polyisocyanates, then, include 2,4-toluene diisocyanate (2,4-TDI), 2,6-TDI, mixtures of 2,4-TDI and 2,6-TDI, 4 , ' -diphenylmethane diisocyanate (MDI) and polymeric MDI, a mixture of polyisocyanates with MDI as the main component. The reaction between polyol and polyisocyanate typically takes place at an isocyanate index (i.e. the equivalence ratio of isocyanate groups to hydroxyl groups) of from 80 to 130, suitably 90 to 120. Reaction conditions like temperature and pressure may be those normally applied in slabstock processes, i.e. ambient temperature (typically 15-30 °C) and atmospheric pressure. However, the pressure in the reaction space is not critical and will typically range from 0.1 to 15 bara, more suitably 1 to 10 bara . Higher than ambient temperatures may suitably be applied as well. For instance, it will be appreciated that when the high humidity atmosphere in the reaction space is created by steam, the temperature in the reaction space will normally be above 40 °C . Suitably, however, it will not exceed 100 °C and more suitably it will not exceed 90 °C . Preferably, the reaction temperature is in the range of from 45 to 80 °C, more preferably from 45 to 65 °C . In any event, due to the strongly exothermic character of the polyurethane formation reaction the actual temperature inside the foam will quickly rise to values that may exceed 100 °C . Depending on the rate at which the atmosphere wherein the foaming occurs is replaced, this temperature rise of the foam will also have an effect on the temperature of this surrounding atmosphere. In general, the closed reaction space wherein the present process takes place should continuously contain an atmosphere having a relative humidity of 90% or more. This can for instance be achieved by using a reaction space which is substantially gas tight. This means that the reaction space need not necessarily be essentially 100% gas tight, but should be tight to such extent that continuously 90 volume% or more of the gases which make up the reaction atmosphere also remain in the reaction space. Thus, traces of air surrounding the reaction space may still be allowed to flow into the reaction space and may eventually constitute up to 10 volume% of the reaction space. Alternatively, the closed reaction space contains means for continuously supplying the appropriate reaction atmosphere to the reaction space. In this case the supply of reaction atmosphere should be approximately in balance with the reaction atmosphere leaving the reaction space. If the closed reaction space is not gas tight no specific means for withdrawing reaction atmosphere from the reaction space are needed: the supply rate should be sufficiently high to compensate for the rate at which reaction atmosphere gases escape from the reaction space. If the closed reaction space is substantially gas tight, means for removing reaction atmosphere (like suction devices) may be applied.

In case of a discontinuous slabstock process the reaction space may be a box comprising means for making it substantially gas tight, such as a close fitting, optionally sealed, lid.

In order to create a closed reaction space, which optionally is substantially gas tight, in a continuous slabstock process the initial part of the moving conveyor belt, on which most of the foam expansion occurs, may be passed through a closed space, so that substantially all of the expansion of the polyurethane foam takes place inside this space. This closed and optionally substantially gas tight space then forms the closed reaction space within the meaning of the present invention.

In both discontinuous and continuous slabstock processes the closed reaction spaces applied suitably contain means for continuously or discontinuously supplying reaction atmosphere gases thereto and optionally means for actively withdrawing or letting escape reaction atmosphere gases therefrom.

It is essential that the atmosphere of the reaction space has a relative humidity of at least 90%. The water accounting for this high humidity may be present in the form of vapour and/or droplets. Water vapour suitably is introduced into the reaction space in the form of steam. If water droplets are present in the reaction space they will form a suspension of (microscopic) water droplets or a mist in the reaction atmosphere. Such a suspension of water droplets in air may be produced by means known in the art, e.g. by ultrasonic means or by passing air through a sinter in water. In any case the water vapour and droplets should cause the reaction space to have a relative humidity of at least 90%. In a preferred embodiment the atmosphere in the reaction space has a relative humidity of 95% or higher, more preferably 98% or higher, while complete saturation of the reaction space with water, resulting in a relative humidity of typically 100%, is most preferred. In addition to water other gases may be present in the reaction space. A particularly preferred gas in this connection is carbon dioxide. Thus, in a preferred embodiment of the present invention the atmosphere in which the foam expansion takes place additionally comprises carbon dioxide. It was found that a reaction atmosphere having a relative humidity of at least 90% and additionally comprising carbon dioxide significantly reduces the skin thickness of the slabstock foam prepared. The amount of carbon dioxide used is not particularly critical. If used, it is suitably used in such amount that its partial pressure in the reaction atmosphere is from 0.1 to 10 bara.

Another class of preferred additional gases are volatile acids. Such volatile acids include low molecular weight organic acids containing up to 5 carbon atoms, such as acetic acid, propionic acid, butanoic acid and pentanoic acid. Carbonic acid and mineral acids like hydrochloric acid may also be applied. These acids may be used as such, but suitably they are used in an aqueous solution which is subsequently used to make steam.

Typical concentrations in such solutions may vary within broad limits, i.e. between 0.01N to 5N. Suitably, the concentration of an acid solution in water used to prepare steam will be in the range of from 0.01N to 0.5N, more suitably from 0.05N to 0.25N. It was found particularly effective to use steam prepared from an aqueous acetic acid solution. It was found particularly advantageous to carry out the present process with a reaction atmosphere comprising carbon dioxide and steam derived from an aqueous acetic acid solution as described herein before.

Other additional gases may also be present. Such gases typically include nitrogen, oxygen, helium, hydrogen and noble gases like argon. In the case only steam or only a suspension of microscopic water droplets in air (i.e. a mist) is used, the remainder of the atmosphere may be air introduced together with the water. Normally such air will then be saturated with water.

The invention will be further illustrated with the following examples without limiting the scope of the invention to these particular embodiments.

In the Examples the following materials were used: Polyol A a polyether polyol having a molecular weight of 3000, a hydroxyl value of

56 mg KOH/g, a functionality of 3 and a random ethylene oxide content of

7 wt%

NIAX L580 silicone surfactant (NIAX is a trademark)

DABCO 3LV 33 wt% solution of triethylenediamine in dipropylene glycol (DABCO is a trademark)

DABCO T9 stannous octoate TDI-80 a blend of 80% weight 2,4-isomer and

20% weight 2,6-isomer of toluene di-isocyanate Examples

Five foams were prepared by a standard hand-mixing technique as follows. Polyol, water, silicone surfactant and amine catalyst were pre-blended in a 800 ml beaker for 30 seconds. The tin catalyst was subsequently added and mixed for an additional 10 seconds. Finally, the toluene di-isocyanate was added and stirring was continued for a further 8 seconds. The reaction mixture was then poured into a cardboard box of dimensions

30 x 20 x 15 cm, which was placed inside a 55 x 55 x 50 cm box.

In all experiments the following foaming mixture was applied

Polyol A 100 pbw

Water 4.8 pbw

NIAX L580 1.2 pbw

DABCO 33LV 0.15 pbw

DABCO T9 0.15 pbw

TDI-80 59.1 pbw

Isocyanate index 107 Different atmospheres for the foam production were created by saturating the large box with different gases each time. Foaming experiments were carried out in the following atmospheres : Experiment 1 (for comparison) : air Experiment 2 steam

Experiment 3 steam and carbon dioxide

Experiment 4 steam from 0.16N acetic acid solution

Experiment 5 steam from 0.16N acetic acid solution and carbon dioxide In all experiments the large box was first saturated with steam, which was generated by boiling water (Experiments 2 and 3) or an aqueous acetic acid solution (Experiments 4 and 5) in a kettle and subsequently passing it into the box. The steam was prevented to escape from the box with a cardboard lid placed on the large box. In Experiments 3 and 5 carbon dioxide was added to the box saturated with steam by passing carbon dioxide gas (obtained through purified grade liquefied carbon dioxide in a pressurised cylinder) into the box at a pressure setting of 7.6 bara. Upon pouring the foaming mixture into the smaller box, the lid was quickly lifted and closed immediately after the complete foaming mixture had been poured into the smaller box. Meanwhile the supply of steam and carbon dioxide, if used, continued.

After the foam had attained full rise, it was left to stand in the fume hood for 5 minutes before it was placed in a forced air oven at 80 °C to further accelerate the curing process. The foams produced were then left to fully cure for another 24 hours prior to evaluation.

The weight of each individual foam block was measured. The foam block was then sliced into two pieces (in the longitudinal direction) and the maximum foam block height was measured. The core density of the foam (taken from the top and middle section) produced under the different set conditions were measured. The starting point in time for measuring the foam reactivity (full rise time) was the moment at which the di-isocyanate solution was added. Table 1 Foams prepared in different atmospheres

* of the middle section of the foam

From the physical measurements of the foam blocks obtained, it can be seen that a 7 to 15% higher foam block height (over the reference Experiment 1) was obtained when the surrounding humidity and temperature were high (relative humidity of 100%, temperature around 55 °C) .

As can be seen from table 1, when the foam expansion took place in an atmosphere saturated with water already a significant increase in foam block height (and hence reduction of thickness of surface skin) was achieved relative to normal foam expansion in air. Further improvements were achieved when additionally using carbon dioxide and a volatile acid (acetic acid) . The combination of steam from a dilute acetic acid solution together with carbon dioxide gas, however, generated the most significant increase in foam block height obtained, thus indicating that a thinner skin was formed, while the density of the middle section of the foam remained essentially the same.

Claims

C L A I S

1. Process for the preparation of a flexible polyurethane foam, which process comprises pouring a polyurethane reaction mixture into a closed reaction space where the foaming reaction occurs in an atmosphere having a relative humidity of at least 90%.

2. Process as claimed in claim 1, wherein the reaction atmosphere has a relative humidity of at least 95%.

3. Process as claimed in claim 2, wherein the reaction atmosphere is completely saturated with water.

4. Process as claimed in claim 3, wherein the water present in the reaction space consists of steam obtained from an aqueous acetic acid solution.

5. Process as claimed in any one of the preceding claims, wherein the atmosphere in the reaction space additionally comprises carbon dioxide.

6. Process as claimed in any one of the preceding claims, wherein the temperature in the reaction space is higher than 40 °C.