WO2002079292A1

WO2002079292A1 - Low monomer prepolymer

Info

Publication number: WO2002079292A1
Application number: PCT/GB2002/001485
Authority: WO
Inventors: Rodney Bicknell; John Grimshaw
Original assignee: Baxenden Chemicals Limited
Priority date: 2001-03-30
Filing date: 2002-03-28
Publication date: 2002-10-10
Also published as: GB0108060D0

Abstract

A prepolymer obtainable by polymerising a mixture comprising: (A) an isocyanate component (B) a polyol component comprising one or more rigid foam polyols; and (C) a low viscosity component which is non-reactive towards isocyanate and hydroxyl groups; and subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight.

Description

LOW MONOMER PREPOLYMER

The present invention relates to processes for producing low monomer prepolymer for use in the production of polyurethane foam and prepolymers so produced.

Polyurethane foam products are well known and are used in a variety of applications, for example as sound and/or vibration absorbers or dampers, as thermal insulation including in automotive assembly, white consumer goods, insulation panels, refrigeration units and tank insulation, and in gap filling and void filling including in construction, automotive, DIY and other markets. Prepolymers which are formed from an isocyanate component and a polyol component may be used in the production of the foams.

A high monomeric diisocyanate content has previously been considered to be a necessary component of the prepolymer in order to produce a polyurethane foam with useful properties and sufficient mechanical strength. The free monomer level in a conventional foamable composition, which is typically in the range of 2 to 25%, contributes significantly to the rigidity of the finished product. This is due to the fact it is a low molecular weight chain extender or crosslinker that forms a hard block segment in the finished product. Diisocyanate monomers are also of low viscosity and therefore help to reduce the final viscosity of the isocyanate component. Further, in conventional foams the monomeric diisocyanate also significantly contributes to the reactivity of the foam. Therefore prepolymers for use in the production of polyurethane foams have, until now, contained significant levels of free monomeric diisocyanate.

This currently causes practical problems in use, as the volatile monomeric diisocyanate can escape into the atmosphere. This is clearly undesirable in view of the adverse health effect of exposure to isocyanates, which is well known and a recognised concern within the industry. In particular there is difficulty in situations where such products are required to be used in areas of poor ventilation.

The present invention overcomes the above problems by providing a prepolymer which has a low monomeric diisocyanate content yet still affords a foam which has a sufficiently low initial viscosity and cures to a sufficiently rigid product that may be used in construction and related applications. Thus a prepolymer product is provided that offers a significantly reduced hazard due to monomeric diisocyanate but can be used to produce a polyurethane foam that retains rigidity, has reactivity equal to conventional products and is produced at a manageable viscosity. The prepolymer product can also suitably be used in other polyurethane applications such as coatings and adhesives.

Specifically, the present invention provides, in a first aspect, a prepolymer obtainable by polymerising a mixture comprising: (A) an isocyanate component

(B) a polyol component comprising one or more rigid foam polyols; and

(C) a low viscosity component which is non-reactive towards isocyanate and hydroxyl groups; and subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight, preferably 0.5% or less by weight.

The invention also provides, in a second aspect, a prepolymer obtainable by polymerising a mixture comprising:

(A) an isocyanate component and (B) a polyol component comprising one or more rigid foam polyols; then adding

(C) a low viscosity component which is non-reactive towards isocyanate and hydroxyl groups; and subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight, preferably 0.5% or less by weight. The invention also provides, in a third aspect, a prepolymer obtainable by polymerising a mixture comprising:

(A) an isocyanate component and

(B) a polyol component comprising one or more rigid foam polyols; subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight, preferably 0.5% or less by weight; then adding

(C) a low viscosity component which is non-reactive towards isocyanate and hydroxyl groups.

The use of such prepolymers in one component foam dispensers is described and claimed in British Patent Application No. 0108061.3.

The isocyanate utilised in the present invention is preferably a diisocyanate, for example toluene diisocyanate (TDI), 4,4'-diphenyl methane diisocyanate (MDI), isophorone diisocyanate, hexamethylene diisocyanate, para-phenylene diisocyanate, 1,3 bis(l-isocyanato-l-methylethyl) benzene (m-TMXDI) or 4,4'-methylene-bis cyclohexyl diisocyanate (Desmodur W) or a mixture thereof. Most preferably the isocyanate is toluene diisocyanate (TDI) or 4,4'-diphenyl methane diisocyanate (MDI), or a mixture thereof. The 2,4- or the 2,6- isomer of toluene diisocyanate, or a combination of the two isomers, may be used. Preferably toluene diisocyanate of grade TDI 80/20 (80% by weight 2,4- isomer and 20% 2,6- isomer) is used. The 4,4'-diphenyl methane diisocyanate may be pure MDI with a functionality of 2, modified pure MDI with a functionality of from 2.0 to 2.2 or crude MDI with a functionality of less than 3.0.

The isocyanate component may alternatively be a polyisocyanate, for example a biuret, an allophanate or a trimer (isocyanurate). Such polyisocyanates are typically produced from their corresponding diisocyanate and therefore the isocyanate component may contain some diisocyanate in addition to the polyisocyanate. The polyol component of the present invention comprises a rigid foam polyol. As used herein the term "rigid foam polyol" refers to all those polyols known in the industry as being suitable for the production of rigid polyurethane foams; examples include polyols used in the art as the polyol component of two component foam systems which are not primarily designed for use in prepolymer manufacture. Preferably the rigid foam polyol is blended with other polyols.

Typical polyols, or combinations of polyols, for use in the current invention comprise hydroxyl terminated compounds having an average functionality of greater than 2.0, and up to 5, for instance 2.1, 2.2 or 3, and up to 5.0, and a molecular weight of from 100 to 6000, preferably from 200 to 3000. Diols may suitably be utilised provided that they are blended with one or more higher functionality polyols. The polyol may suitably be selected from hydroxyl terminated polyethers, such as polyoxypropylene, ethoxylated polyoxypropylene and propoxylated sucrose derivatives, and other polyether polyols, for example polyether polyols with a functionality above 2, for instance from 4 to 5 or more, preferably about 4.6. Hydroxy terminated polyester polyols, natural oils such as castor oil and mannich bases may also be used.

The isocyanate component and polyol component are used in relative amounts such that the isocyanate index (NCO/OH) is greater than 2.0, preferably from 2.1 to 3.0, for example 2.6.

Significantly, a third essential component, a low viscosity component, is included in the production of the prepolymer in order to reduce the viscosity of the product. This material is included in an amount such that the resultant prepolymer has a viscosity of 300 000 cps or lower at 40°C (Bohlin), preferably of from 1000 to 280 000 cps. The material must be non-reactive towards isocyanate and hydroxyl groups and should not adversely affect the desired properties of the end product. If the low viscosity component is to be added prior to the process of removing residual diisocyanate monomer from the prepolymer, it is also important that the low viscosity component is not removed during this process and remains in the product stream. In this case, it is necessary that the component has a boiling point higher than the distillation temperature at the distillation pressure used during the process of removing residual diisocyanate monomer, for example a boiling point of 180°C or greater at 5mm Hg (666.6 Pa), preferably a boiling point of 160°C or greater at 2mm Hg (266.6 Pa). Alternatively or additionally, the low viscosity component can be added after the process of removing residual diisocyanate monomer, in which case the materials used are not restricted to those having a boiling point above the distillation temperature at the distillation pressure of the removal process.

The low viscosity component preferably comprises one or more inert plasticisers, for example non-halogenated phosphates, adipates or phthalates and/or one or more flame retardants. Suitable plasticisers include trixylenyl phosphate (TXP); trisoctyl phosphite (TiOP); diisooctylphthalate (DiOP); dibutylphthalate (DBP); dimethylphthalate (DMP); and dioctyladipate (DOA). Suitable flame retardants include tri(2-ethylhexyl)phosphate; tris (2-chloropropyl) phosphate (TCPP); triethyl phosphate (TEP); and organo phosphonate esters. Preferably the low viscosity component comprises one or more compounds selected from tri (2-ethylhexyl) phosphate, di-2-ethylhexyl adipate and di-2-ethylhexyl phthalate.

The polyurethane prepolymer is prepared by reacting the isocyanate component with the polyol component, optionally in the presence of the low viscosity component, under carefully controlled conditions to produce a material with the correct and consistent molecular weight distribution. If the low viscosity component is not incorporated during prepolymerisation, it may be added to the prepolymer thus formed, or it may be added after the subsequent step of removing free diisocyanate.

The free diisocyanate monomer is removed from the prepolymer using elevated temperatures and/or reduced pressures by suitable methods known in the art, such as wiped film distillation. The resulting "stripped" product has levels of free diisocyanate monomer of less than 2% by weight, preferably 0.5% or less by weight. Preferably the level of free diisocyanate monomer is reduced to below the level considered to be harmful for the specific diisocyanate in question. It is preferred that the stripped prepolymer contains no more than 0.5% by weight, preferably 0.1% or less, of TDI and that it contains less than 2% by weight, preferably 0.5% or less, of MDI.

The stripped prepolymer may then be used to produce polyurethane foams, coatings or adhesives having a low level of monomeric diisocyanate using methods known in the art.

The present invention is illustrated further by means of the following Examples:

Example 1 Toluene diisocyanate and tri(2-ethylhexyl) phosphate were mixed in a sealed reactor under a nitrogen blanket. The mixture was then heated to 60°C and a preblended mixture of RH 360 and P400 was added slowly to the mixture such that the temperature was maintained below 70°C. The resulting mixture was stirred constantly and kept at this temperature for a minimum of one hour. The reaction was allowed to continue until the theoretical isocyanate value (13.2%) was reached.

Prepolymer Index(NCO/OH ratio) 2.60 Polyol Blend Functionality 3.3 (approx.)

The resulting prepolymer was charged to a Canzler wiped film evaporator of 316 stainless steel construction in order to remove the excess diisocyanate. The distillation chamber was set to 160°C and a vacuum of approximately 2 mm Hg (266.6 Pa) applied. The material was run through at a rate of approximately 1 to 2 kg/hr and the finished prepolymer product was collected and stored in a watertight container capped with dry nitrogen.

The finished prepolymer product was a pale straw coloured liquid with an isocyanate (NCO) content of 8.50% and a free TDI content of less than 0.1 %. Further properties of the prepolymer are shown in Table 2 below.

Examples 2 and 3

The method of Example 1 was repeated using different ratios of RH360 to P400 in the polyol blend used in the prepolymer. The compositions of Examples 2 and 3 are shown in Table I and the properties of the prepolymers obtained are shown in Table 2 below.

Table 1

Table 2

Claims

1. A prepolymer obtainable by polymerising a mixture comprising:

(A) an isocyanate component (B) a polyol component comprising one or more rigid foam polyols; and

(C) a low viscosity component which is non-reactive towards isocyanate and hydroxyl groups; and subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight.

2. A prepolymer obtainable by polymerising a mixture comprising:

(A) an isocyanate component and

(B) a polyol component comprising one or more rigid foam polyols; then adding

3. A prepolymer obtainable by polymerising a mixture comprising:

(A) an isocyanate component and

(B) a polyol component comprising one or more rigid foam polyols; subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight; then adding

4. A prepolymer according to any one of claims 1 to 3, wherein the isocyanate component comprises a diisocyanate.

5. A prepolymer according to any one of the preceding claims, wherein the free diisocyanate monomer is removed from the prepolymer to a level of 0.5% or less by weight.

6. A prepolymer according to any one of the preceding claims, wherein toluene diisocyanate is removed from the prepolymer to a level of 0.1% or less by weight.

7. A prepolymer according to any one of the preceding claims, wherein the isocyanate of component (A) comprises one or more diisocyanates selected from 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and 4,4'-diphenyl methane diisocyanate.

A prepolymer according to any one of the preceding claims wherein the polyol of component (B) comprises one or more hydroxyl terminated compounds having a functionality of from greater than 2.0 to 5 and a molecular weight of from 100 to 6000.

A prepolymer according to any one of the preceding claims wherein component (C) comprises one or more non-halogenated phosphates, adipates or phthalates.

10. A prepolymer according to claim 9, wherein component (C) comprises one or more compounds selected from tri (2-ethylhexyl) phosphate, di-2-ethylhexyl adipate and di-2-ethylhexyl phthalate.

11. A prepolymer according to any one of the preceding claims, wherein free diisocyanate monomer is removed from the prepolymer by wiped film distillation.

12. A process for the production of a prepolymer according to any one of the preceding claims, which process comprises mixing isocyanate component (A), polyol component (B) and low viscosity component (C) under controlled conditions and subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight.

13. A process for the production of a prepolymer according to any one of the preceding claims, which process comprises mixing isocyanate component (A) and polyol component (B) under controlled conditions and subsequently removing free diisocyanate monomer from the prepolymer to a level of less than 2% by weight, then adding low viscosity component (C).

14. A process according to claim 12 or 13, substantially as hereinbefore described with reference to the Examples.