GB2309700A - Polymer polyol and process for the preparation thereof - Google Patents

Abstract

Process for the preparation of a polymer polyol comprising the steps of: (a) preparing a first polymer polyol from a reaction mixture comprising a monomer component, a polymersation initiator and a polyol; (b) separating the first polymer polyol into a polymer-rich fraction and a fraction which is rich in the polyol; and (c) dispersing the polymer-rich fraction in a polyol to obtain the polymer polyol. When a stabiliser is used in step (a) the product of step (c) is substantially free of stabiliser. The polymer may be an addition or condensation polymer. The polyol used in step (c) may be the polyol used in step (a) or a different polyol.

Description

POLYMER POLYOL AND PROCESS FOR THE PREPARATION THEREOF The present invention relates to a process for preparing polymer polyols.

Polymer polyols are known in the art. In general, a polymer polyol is a stable dispersion of polymer particles in a polyol. Many different polymers and polyols can be used in such systems. Polymer polyols are normally formed by the in-situ polymerization of suitable monomers, if necessary in the presence of a polymerisation initiator. If necessary, a stabilising agent may also be present. Basically, two different polymer types are used in polymer polyols. The first type of suitable polymer is formed by the polymers which are obtained by free radical polymerization.

Typically, the monomers used are ethylenically unsaturated compounds, whereby styrene and acrylonitrile are the most frequently used monomers.

Such polymers are formed in the polyol by in-situ polymerization in the presence of a free radical initiator. Examples of polymer polyols containing this type of polymers are, for instance, described in EP-A0,076,491; EP-A-0,162,588; EP-A-0,343,907 and EP-A-0,495,551.

The second type of polymer is the class of polymers obtained via polyaddition. Polyurea and polyurethanes are the most frequently used polymers of this type in polymer polyols. These polymers are also formed in-situ by polyaddition of diisocyanate or polyisocyanate and diamine in the case of polyurea and by polyaddition of isocyanate and alkanolamine in the case of polyurethanes. Polyurea polyol and polyurethane polyol systems are commonly known as PHD polyol and PIPA polyol, respectively. PHD polyols and methods for their preparation are, for instance, described in US3,325,421; US-4,093,569 and EP-A-0,371,323, whilst PIPA polyols and methods for their preparation are, for instance, described in GB-B-2,072,204.

In the preparation of polymer polyols stabilising agents are often used to ensure that a stable dispersion of polymer in the polyol is obtained. For instance, in US-4,350,780 a method for preparing stable polymer polyols is disclosed, wherein a preformed stabilizer is used. This preformed stabilizer is a graft or addition copolymer of an ethylenically unsaturated polymeric portion chemically bonded to a portion which is soluble in the polyol medium. Since the ethylenically unsaturated polymeric portion of this graft or addition copolymer is compatible with the dispersed polymer, said preformed stabilizer is able to keep the distinct polymer particles stably dispersed throughout the polyol and prevents phase separation.

An alternative method for stabilising polymer polyols is by adding a stabilising agent which, during the polymerization of the monomer(s) present, reacts with one or more of the monomers, thereby forming a block or graft copolymer with said monomer(s). The stabilising agent should be chosen such that it is soluble in the polyol used. In this way the resulting block or graft copolymer contains not only portions which are soluble in the polyol, but also portions which are compatible with the polymer formed. As a result, the same mechanism as described above for the preformed stabiliser applies. Examples of stabilising agents of this type are coupled polyols, also referred to as prepolymers, or macromers. The latter term refers to polyols or coupled polyols having at least one reactive double bond in the carbon chain.

A disadvantage of using a stabilising agent may be that part of it often remains unreacted in the polyol, i.e. in the liquid phase. Due to the high viscosity these stabilising agents normally have, the viscosity of the final polymer polyol is then also relatively high, which is undesired from a processing point of view, particularly in processing of the polymer polyol to form polyurethanes. Accordingly, it would be advantageous if polymer polyols would be available of which the liquid phase is essentially free of any residual stabilising agent so that it has the appropriate -low- viscosity for processing purposes.

As described above, polymer polyols are normally made by the in-situ polymerization of the monomer component, which may consist of one or more polymerisable monomers. It will be appreciated that by using this preparation method, it could be difficult to determine exactly the final polymer content of the polymer polyol in advance, i.e. prior to the polymerization. Furthermore, the polyol in which the polymerization takes place inevitably is also the polyol present in the polymer polyol. This could be a disadvantage in situations where a polymer is best formed in one polyol, but is better used in another polyol in the production of polyurethanes.

The present invention aims to provide a process for preparing polymer polyols enabling the formation of polymer polyols having excellent viscosity properties.

Furthermore, the present invention aims to provide a process providing a maximum controllability and flexibility having regard to polymer content of the polymer polyol and type of polyol used.

Accordingly, the present invention relates to a process for the preparation of a polymer polyol comprising the steps of: (a) preparing a first polymer polyol from a reaction mixture comprising a monomer component, a polymerisation initiator and a first polyol; (b) separating the first polymer polyol into a polymerrich fraction and a fraction which is rich in first polyol; and (c) dispersing the polymer-rich fraction in a second polyol to obtain a second polymer polyol, which is the product.

The first polymer polyol is suitably prepared from a reaction mixture comprising at least a monomer component, a polymerisation initiator and a first polyol. Suitably, but not necessarily, a dispersion stabilising agent is also present. The first polymer polyol may be prepared by the conventional methods, i.e. by free-radical polymerization or polyaddition, depending on the desired polymer and hence on the type of monomers used.

Polymers, which are useful in polymer polyols and which are prepared by free-radical polymerization, are those based on ethylenically unsaturated monomers.

Polymer polyols containing this type of polymer are normally prepared by polymerising in the polyol one or more ethylenically unsaturated monomers in the presence of a free radical polymerisation initiator and suitably a dispersion stabilising agent. The resulting polymer particles are, accordingly, suitably stabilised by a stabilising agent. As has already been indicated hereinbefore, too high an amount of residual stabilising agent present in the liquid phase of the polymer polyol may result in an increased viscosity of the polymer polyol. On the other hand, if insufficient stabilising agent is used, the polymer particles may not remain stably dispersed in the liquid phase.

Suitable monomers must have at least one olefinic linkage, which is prone to free radical polymerization,' in their molecular structure. Suitable monomers, then include vinyl aromatic compounds, such as styrene and methylstyrene, acrylonitrile, vinyl acetate, acrylic and substituted acrylic monomers, such as acrylic acid, methacrylic acid and various alkyl acrylates, and conjugated dienes, such as butadiene and isoprene.

Combinations of two or more of the aforesaid monomers may also be used, such as a mixture of styrene and acrylonitrile or a mixture of styrene and butadiene and/or isoprene. Of all these ethylenically unsaturated monomers, styrene, acrylonitrile and mixtures thereof (resulting in polystyrene, poly(acrylonitrile) and styrene-acrylonitrile copolymer, respectively, as the dispersed polymers) are preferred. Styreneacrylonitrile copolymer polyols are also commonly known as SAN polyols.

Free radical polymerization catalysts are known in the art and include both peroxide compounds and azo compounds. Examples of suitable peroxide catalysts are dibenzoyl peroxide, lauroyl peroxide, di-t-butyl peroxide, diisopropyl peroxide carbonate, t-butyl peroxy-2-ethylhexanoate, t-butylperpivalate, tbutylperneo-decanoate, t-butylperbenzoate, t-butyl percrotonate, t-butyl perisobutyrate and di-t-butyl perphthalate. For the purpose of the present invention it is preferred to employ peroxyester free radical initiator catalysts. Examples of such catalysts are tamylperoxy-2-ethylhexanoate and t-butylperoxy-2ethylhexanoate amongst others. It is also preferred to employ azo compounds as free radical catalyst. Examples of such catalyst are azobis-isobutyronitrile (AIBN) and azobis-(2-methylbutanenitrile), the first one being most preferably used. The quantity of initiator may range from 0.1% by weight to 5% by weight based on total weight of monomer(s).

The free radical polymerization is usually carried out at a temperature in the range of from 70 to 1300C and at pressure up to 20 bar, while the preferred temperature is in the range of from 80"C to 1200C and the pressure is preferably atmospheric.

Polymers, which are also useful in polymer polyols, are those prepared by polyaddition. Polyurea and polyurethane polymers are examples of such polymers.

Polyurea polyols (PHD polyols) are prepared by the insitu polyaddition reaction of diisocyanate and diamine, thus forming a polyurea as the dispersed polymer.

Polyurethane polyols (PIPA polyols) are obtained by reacting diisocyanate and polyhydric alcohol, thus forming a dispersion of polyurethane particles as the polyaddition product.

Any dispersion stabilising agent used in preparing the first polymer polyol in step (a) of the present process may be selected from those stabilising agents known in the art, for instance, from US-4,350,780. A very suitable class of stabilising agents, particularly in polystyrene polyols and SAN polyols, is formed by coupled polyols comprising high molecular weight polyols. Examples of these coupled polyol stabilisers are the polyisocyanate coupled polyols described in US4,357,430; EP-A-0,495,551and EP-A-0,076,491 and the epoxy resin coupled polyols described in US-4,855,330 and EP-A-0,437,348. Another class of suitable dispersion stabilising agents are those of which the molecules comprise at least one reactive olefinic bond and at least one group, which is compatible (or soluble) with the polyol serving as the liquid phase of the polymer dispersion. Such stabilising agents are often described as macromers and can react with ethylenically unsaturated monomer molecules, thus becoming part of a polymeric chain. In this way molecules are formed containing on the one hand groups which are compatible with the polyol medium and on the other hand polymerised monomer molecules which are compatible with the polymer formed. Examples of such macromer stabilisers are acrylate functionalised polyols, such as, for example, described in US4,390,645, and the maleate/fumarate functionalised polyols described in, amongst others, US-5,364,906and EP-A-0,461,800.

The polyol to be used as the first polyol may be any polyol having a functionality of 2 or higher known to be suitably applied in polymer polyols. In general, such polyol is obtained by the alkoxylation of at least one polyhydric alcohol. Ethylene oxide and/or propylene oxide are most frequently used as the alkoxylating agents. Suitable polyhydric alcohols include diols like diethylene glycol, monoethylene glycol, monopropylene glycol, dipropylene glycol, diphenylol propane and diphenylolmethane, and polyols like glycerol, trimethylol propane, sucrose, sorbitol, pentaerythritol and diglycerine. Mixtures of two or more of these polyhydric alohols may also be used. Likewise, mixtures of two or more polyols, i.e. alkoxylated polyhydric alcohols, may also be used as the first polyol.

Suitable, commercially available polyols are sold under the trade names CARADOL (ex Shell), VORALUX (ex Dow), DESMODUR (ex ICI), ARCOL (ex Arco) and POLYORAX (ex B.P.).

In step (b) of the process according to the present invention the first polymer polyol is separated into a polymer-rich fraction and a fraction which is rich in first polyol. Accordingly, this step involves separation treatment of the first polymer polyol resulting from step (a), wherein this first polymer polyol is separated into a fraction having a polymer concentration which is higher than the polymer concentration in the first polymer polyol and a fraction having a polymer concentration which is lower than the polymer concentration in the first polymer polyol. Consequently, this latter fraction will have a concentration of first polyol which is higher than in the first polymer polyol. It is desirable to remove as much of the polymer particles as possible. It is therefore preferred to remove at least part of the liquid phase comprising the first polyol from the first polymer polyol to obtain the polymer-rich fraction. The concentration of polymer in the polymer-rich fraction selfevidently depends on the amount of liquid phase removed. It is preferred that so much liquid phase is removed that the polymer-rich fraction is a polymer slurry or is even essentially free of any liquid, i.e.

it consists of distinct polymer particles.

The separation of the polymer particles can be effected by any known technique of physical separation, such as centrifugation, membrane filtration, screen filtration or flocculation. For example, in US4,283,500 techniques for removal of polymer particles from a polymer polyol are disclosed, wherein the polymer particle are as small as 30 microns.

Filtration of polymer polyols to remove polymer particles is described in US-4,500,675, while separation by centrifuge is disclosed in US-4,350,780.

However, none of these patent specifications refers to redispersion of the removed polymer particles in the same or another polyol.

The preferred method of separation to be applied in step (b) is centrifugation, although other techniques of separation are also applicable. The polymer particles can be removed from the liquid by passage through a membrane which is selectively permeable for the liquid phase and which restrains the passage of the polymer particles. The separation can also be attained by screen filtration whereby the pores of the screen have such diameter that the polymer particles are trapped while allowing the liquid to pass through the screen. It is also possible to treat the polymer polyol to flocculate the polymer, which can then be more easily removed by filtration.

After separation of the polymer particles as a polymer-rich fraction from the first polymer polyol, the remaining fraction may be recycled to a reaction mixture for forming the first polymer polyol.

After step (b) and prior to step (c) it may be desired to wash the recovered polymer-rich fraction to further remove any residual dispersion stabiliser, if present, and first polyol. Accordingly, such additional washing step is particularly useful if the polymer-rich fraction is in the form of a highly concentrated polymer slurry containing the polymer particles, such as for instance obtained after centrifugation in the preceding step (b). Washing can suitably be attained by redispersion of the polymer particles in a suitable low boiling liquid, such as a low molecular weight monohydric alcohol and subsequent separation of the polymer particles, suitably by centrifugation. Examples of suitable low molecular weight monohydric alcohols are methanol, ethanol and propanol. This washing step may be repeated several times until the required amount of first polyol and stabilising agent, if any, has been removed. In practice, it will be sufficient to perform the washing step one to ten times. It will be understood that the exact number of times will also be determined by the volume of the polymer-rich fraction to be washed. If the polymer-rich fraction has been separated by a technique such as screen filtration, it is also convenient to wash the said fraction whilst it is still present on the filtration screen. This may be simply accomplished by the passage of a suitable low boiling point liquid through the separated polymer-rich fraction and screen immediately after the initial filtration of the first polymer polyol. In all of the washing procedures the use of a suitable low boiling liquid as the washing medium aids its subsequent removal from the polymer-rich fraction after completion of the washing process.

In step (c) of the present process the polymer-rich fraction recovered in step (b), and optionally washed subsequently, is dispersed in a second polyol to obtain the polymer polyol endproduct. The second polyol may be the same polyol as the first polyol, but may also be a different polyol. As the second polyol any polyol mentioned above to be suitable as the first polyol may be used. When first and second polyol are the same, the polymer polyol product will normally have a lower viscosity than the first polymer polyol at the same polymer content. This is particularly true if a stabilising agent is used in the preparation of the first polyol. The dispersion or redispersion of the polymer-rich fraction can be effected by any known technique of dispersion. For example, the polymer-rich fraction can be diluted with portions of the polyol.

Dispersing the polymer-rich fraction in the second polyol may also be accomplished by high shear mixing using a mixer fitted with a suitable dispersion head.

If a washing step is applied after step (b), the polymer-rich fraction to be dispersed in step (c) suitably is essentially free of residual stabiliser and first polyol and, accordingly, may be in the form of a (wet) powder of polymer particles. After such washing step the polymer particles may stick together and be recovered in the form of lumps. Such lumps are suitably converted into a fine powder of the distinct polymer particles prior to (re)dispersion step (c). Such conversion can be conveniently achieved by crushing or milling the lumps via any suitable means known in the art.

Dispersing the polymer-rich fraction in the second polyol is suitably performed at a temperature in the range of from 0 C to 1000C.

In general, average particle size and particle size distribution are important characteristics of polymer polyols, particularly with regard to their use for preparing polyurethane foams. The average particle size of the polymer particles in the polymer polyol should be relatively small to be suitable for most polyurethane end use applications. Typically, the average particle size should be in the range of from 0.5to 5.0 micron, suitably from 0.8to 3.0 micron. With respect to particle size distribution it is important that the number of large polymer particles (i.e. having a size greater than 10 micron) is minimised. The presence of substantial quantities of such large polymer particles in polymer polyols, namely, can adversely affect the properties of the polyurethane foams prepared. The present process allows the preparation of polymer polyols having a particle size distribution span (PS span) of between 1.3 and 4.0, which is relatively narrow. The PS span is defined as PS span = d(90%) - d(10) d(50%) wherein d(x%) is the particle diameter in micron at which x% by volume of the particles has a smaller particle diameter. Particle size measurements are on a volume basis and can be carried out by methods known in the art, such as the laser light scattering technique.

The final polymer polyol may comprise from 2 to 60% by weight of polymer and preferably from 15% to 50% by weight of polymer. The weight percentage is based on the total weight of polymer polyol.

As has already been mentioned before, one of the main advantages of the present invention is that the content of polymer in the polymer polyol can be set at any desired value within the practical limits, whilst another major advantage is that there is a very high flexibility in the choice of polyol for the polymer polyol. The latter is very important for manufacturers of polymer polyols, who always seek to introduce as much flexibility as possible in commercial processes to make the polymer polyols. Maximum flexibility results in a reduction of capital expenditures and in polymer polyol preparation time. The process according to the present invention greatly increases a manufacturer's flexibility with regard to type of polyol and type and quantity of polymer in the polymer polyol endproduct.

In addition, there is an increased flexibility in the manufacture of polyurethanes, where the properties of the polyol used are very important for the properties of the final polyurethane product. The process of the present invention allows the manufacture of tailor-made polymer polyols for polyurethane production, particularly in terms of polyol type and polymer content.

Yet another major advantage is that at similar polymer content, the polymer polyols prepared according to the present process have a significantly lower dynamic viscosity at 25"C than the polymer polyols prepared according to the prior art methods, if it is necessary to use a dispersion stabiliser when preparing the polymer in the polymerization step. This is an important difference with the polymer polyols prepared according to prior art methods. As has been explained above, this is the result of the fact that the polymer polyols prepared according to the present process are substantially free of any residual dispersion stabiliser in the liquid polyol phase. Accordingly, the present invention also relates to polymer polyols comprising a polymer dispersed in a liquid polyol phase obtainable by the process described hereinbefore, wherein the liquid polyol phase is essentially free of residual dispersion stabiliser. In a preferred embodiment the dispersed polymer is polystyrene.

The invention is further illustrated by the following examples without restricting the scope of the present invention to these particular embodiments.

Example 1 A first polymer polyol was prepared by adding to a stirred mixture of 87.5 by weight of CARADOL SC46-02 polyol and 12.5% by weight of a diisocyanate coupled polyol (as dispersion stabiliser) at 900C, via linear dosing over a period of 3 hours, 30% by weight -based on total weight of final reaction mixture- of styrene monomer and 150 mmol/kg styrene of LUPEROX 575 (tamylperoxy-2-ethylhexanoate; LUPEROX is a trade name).

After all styrene monomer and peroxide initiator were added, the polymerization reaction was allowed to proceed to completion for 2 hours. Subsequently, volatiles were stripped from the product (at 120 "C for 6hours under a light nitrogen purge). The resulting polystyrene polyol was a white dispersion having a solids content of 29.6% by weight.

The diisocyanate coupled polyol was prepared by mixing CARADOL SC46-02 polyol and MDI (4,4'diphenylmethane diisocyanate) in an amount of 49.5g MDI per kg CARADOL SC46-02 at 40 "C until the mixture was homogeneous. A triethylamine catalyst was then added and the stirred mixture was heated to 90 OC over 30 minutes. Total reaction time was 5 hours. The product was a clear, viscous liquid having a viscosity between 50,000 and 70,000 mPa.s.

The resulting polystyrene polyol was subsequently diluted with ethanol in a proportion of 67% ethanol:33% polystyrene. Separation of the polymer particles was effected by centrifugation at 15,000 rotations per minute (rpm; 36,000 G) until a clear separation was observed between the polystyrene particles and the liquid phase.

The supernatant liquid was then decanted and the polystyrene-rich slurry recovered was dispersed in ethanol. The mixture was again separated by the same centrifugation treatment. This washing/separation step was repeated four times to remove all traces of the coupled polyol and first polyol.

After the final washing/separation step the polystyrene particles were recovered in lumps of these particles sticking together. These lumps were crushed in order to obtain a fine powder of the distinct polystyrene particles. This fine powder was subsequently dispersed in CARADOL SC46-02 polyol in quantities of 10%, 20%, 30%, 40% and 46% by weight based on total weight of polymer polyol. Dispersion was effected with a Silverson high shear mixer fitted with an emulsifying dispersion head.

Dynamic (Brookfield) viscosities of each of the polystyrene polyols prepared were determined at 25"C (Vd,25) using a Brookfield LVDV-3 viscometer. The results are listed in Table I.

Comparative Example 1 A comparative polystyrene polyol, having a polystyrene content of 40% by weight, was manufactured according to the in-situ polymerisation described in Example 1. Three other comparative polystyrene polyols having polystyrene contents of 10%, 20% and 30% by weight were made from this 40% by weight polystyrene polyol via dilution with CARADOL SC46-02 polyol.

Dynamic viscosities of each of these polystyrene polyols were determined in the same way as described in Example 1. The results are listed in Table I.

Table I Dynamic viscosities of polystyrene polyols

Amount of Vd,25(mPa.s) Vd,25(mPa.s) polystyrene (% wt) Example 1 Comp. Example 1 10 903 1002 20 1425 1848 30 2610 3625 40 5684 11900 46 2017 The data in Table I show that at equal polystyrene contents the measured dynamic viscosities of the redispersed polystyrene polyols (Example 1) are significantly lower than the dynamic viscosities of the comparative polystyrene polyols, where redispersion has not taken place (Comparative Example 1).

Example 2 Preparation of a first polymer polyol, obtaining a polymer-rich fraction therefrom and washing of this polymer-rich fraction to obtain polystyrene particles was carried out in the same way as described in Example 1.

After the final washing/separation step the polystyrene particles were recovered in lumps of these particles sticking together. These lumps were crushed in order to obtain a fine powder of the distinct polystyrene particles. Samples of this powder were then dispersed in three different polyol grades: CARADOL SC46-02, CARADOL MC36-03 and CARADOL MH56-03. Molecular weights (MW) and hydroxyl value (OHV) of each of these polyols are listed in table II. In each case the polymer content of the dispersion was 40% by weight based on total weight of polymer polyol. Dispersion was again effected with a Silverson high shear mixer fitted with an emulsifying dispersion head.

Dynamic (Brookfield) viscosities of each of the polystyrene polyols prepared were determined at 25C (Vd,25) using a Brookfield LVDV-3 viscometer. Particle size characteristics (average particle size, APS and particle size span, PS Span) of each of the polystyrene polyols were determined with a Malvern Mastersizer MS20 (trade name) laser light scattering apparatus. The results are listed in Table II.

TABLE II Polystyrene polyols in different polyols

Polyol MW OHv (mmol Vd,25 APS PS Span polyol KOH/g) (mPa.s) (micron) SC46-02 3500 46 5684 0.89 2.76 MC36-03 4700 36 6590 1.33 5.62 MH56-03 3000 56 6061 0.91 3.55 From Table II it can be seen that the present process results in 100% polystyrene polyols having excellent particle size characteristics as well as viscosities for different polyols.

Claims (12)

1. Process for the preparation of a polymer polyol comprising the steps of: (a) preparing a first polymer polyol from a reaction mixture comprising a monomer component, a polymerisation initiator and a first polyol; (b) separating the first polymer polyol into a polymerrich fraction and a fraction which is rich in first polyol; and (c) dispersing the polymer-rich fraction in a second polyol to obtain a second polymer polyol, which is the product.

2. Process according to claim 1, wherein the reaction mixture in step (a) also comprises a dispersion stabiliser.

3. Process according to claim 2, wherein the dispersion stabiliser is a coupled polyol.

4. Process according to any one of the preceding claims, wherein the first and second polyol are the same or different polyether polyols.

5. Process according to any one of the preceding claims, wherein the monomer component used in step (a) consists of at least one ethylenically unsaturated monomer.

6. Process according to claim 5, wherein the monomer component is styrene and/or acrylonitrile and the dispersed polymer is polystyrene, poly(acrylonitrile) or styrene-acrylonitrile copolymer.

7. Process according to any one of claims 1 to 4, wherein the dispersed polymer is polyurea.

8. Process according to any one of claims 1 to 4, wherein the dispersed polymer is polyurethane.

9. Process according to any one of the preceding claims, wherein the separation of the polymer-rich fraction from the first polymer polyol is effected by centrifugation, membrane filtration, screen filtration or flocculation.

10. Process according to any one of the preceding claims, wherein after step (b) and prior to step (c) a washing step is performed.

11. Polymer polyol comprising a polymer dispersed in a liquid polyol phase obtainable by the process according to any one of claims 2 to 10, wherein the liquid polyol phase is essentially free of residual dispersion stabiliser.

12. Polymer polyol according to claim 11, wherein the dispersed polymer is polystyrene.