ONE COMPONENT FOAM
The present invention relates, to processes for producing one component flexible polyurethane foam and flexible foams so produced. Flexible polyurethane foams are well known and are, in particular, extensively used in a variety of applications in the automotive, furniture and packaging markets. Such foams are traditionally produced by a two component method which comprises mixing together an isocyanate component and a polyol component. Additives, such as cell stabilisers, foaming agents and catalysts are also required and are usually pre-mixed with the polyol component. However, such foams are generally applied via heavy, two component dispensing equipment which is often fixed and located on production lines or factory floors. Further, such equipment requires regular maintenance and calibration to ensure correct .flmctioning and that the correct ratio of components is achieved at the mixing head. In contrast, one component foams are very simple to apply and are dispensed from readily portable equipment, requiring no metering or mixing at the point of use. Generally, one component foams may be dispensed from a pressurised container, typically an aerosol can, via a plastic rube or gun attached to the valve. As such, one component foam dispensers can be used on production lines or carried and used by individuals as required. Such products are readily available and are used in many DIY and factory applications.
One component polyurethane foams are generally of very low density, which leads to problems of low strength and higher susceptibility to shrinkage. These problems are traditionally overcome by incorporating sufficient cross-linked density to give a dimensionally stable polymer network. A proportion of open cells within a foam also facilitates dimensional stability, because less force is exerted during curing.
In such foams, sufficient cross-linked density is provided by forming branched prepolymers with low molecular weight polyols. Isocyanates having high functionality, such as polymeric '4,4' diphenylmethane di-isocyanate (MDI) with a functionality of 2J, are used to ensure that even if the polyol is only di-functional,
the resultant prepolymer will be lightly branched or cross-linked. However, it is more usual to use tri- or higher functional polyols to ensure sufficient cross-linked density. Di-functional polyols are used in order to give a low viscosity prepolymer, but then it is necessary to include low molecular weight cross linkers to reinforce the foam and ensure the resultant foam has sufficient dimensional stabihty. In this case, the highly cross-linked nature of the polymer network produced inherently leads to the production of rigid foam without elastic properties.
Therefore one component foam dispensers have, until now, been limited to the dispensing of rigid foam. Such foams are dispensed from a pressurised container to give a thixotropic froth, which expands as it reacts with atmospheric moisture. ' When the reaction mixture cures, the material obtained is low-density rigid polyurethane foam, which exhibits high strength and is non-elastic. Rigid foams are typically used as construction sealants, gap fillers and fixing products. Such foams are totally unsuitable for flexible applications where a degree of resilience and recovery is required. Therefore one component polyurethane foams have mainly been used in the construction industry, in applications such as cavity filling, insulation and structure reinforcement.
The present invention provides a one-component polyurethane foam which overcomes the problems of the prior art in that it is flexible whilst not susceptible to shrinkage. The foam exhibits a high level of elasticity and resilience, together with good physical properties in terms of, for example, tensile strength, elongation and tear strength, and is tactually pleasing. The elongation at break is a satisfactory ' indicator of the flexibility of the foam. Preferably the foam will have an elongation at break (ASTM D624) of 20% or more, more preferably 40% or more. The one component polyurethane foams of the present invention are formed according to the normal methods known in the art, that is to say a prepolymer is formed from an isocyanate component and a polyol component, and this prepolymer is exposed to atmospheric moisture at the point of use to produce polyurethane foam. Specifically, the present invention provides, in a first aspect, a prepolymer obtainable by polymerising a mixture comprising:
(A) a polyol component having a functionality of 3 or less;
(B) an isocyanate component having-a flnctionality of from 2.0 to 2J; and
(C) a low molecular weight monohydric alcohol.
In a second aspect of the invention there is provided a one component polyurethane foam dispenser, which comprises a sealed vessel containing the above mentioned prepolymer, said vessel being pressurised and having a gas-tight dispenser valve.
In a third aspect the invention provides a flexible one component polyurethane foam obtainable by a process comprising expelling prepolymer from the above mentioned foam dispenser; said polyurethane foam being suitable for use in automotive, furnishing or packaging applications.
In order to form a flexible foam, a linear or lightly branched prepolymer is required. In particular it is important that the number of potential cross linked sites is controlled, as excessive cross linking reduces the flexibility and elasticity of the foam. Therefore in the present invention, lower functionality isocyanates are utilised ■ as the isocyanate component. Specifically, isocyanates having a functionality of * from 2.0 to 2 J are utilised, for example commercially available isocyanates having a functionality of from 2.4 to 2J, preferably isocyanates having a functionality of from 2.0 to 2.3. More preferably, isocyanates having a flinctionality of from 2.0 to 2.2 are used. For example, the isocyanate may suitably be selected from toluene di-isocyanate(TDI), 4,4'-diphenylmethane di-isocyanate (MDI) or modified derivatives of these products such as Isonate M340 (modified MDI with a functionality of 2.1), Isonate M342 (modified MDI with a functionality of 2.0) and Isonate M143L (liquified pure MDI with a functionality of 2.2). The polyols utilised in the present invention may be pure single compounds or mixtures. The total polyol content of the mixture should have a functionality (average number of hydroxyl groups per molecule) of three or less in order to promote a flexible foam product. High molecular weight polyols are preferably used in the polyol component, to provide the resultant foam with the desired resilience and elasticity. Typical polyols, or combinations of polyols, for use in the present invention are hydroxyl terminated compounds having a functionality of three or less,
preferably with a molecular weight of from 400 to 10 000, for example from 400 to 3000, 3000 to 7000 or 7000 to 10 000. For example, the polyol may suitably be selected from hydroxyl terminated polyoxypropylenes, polyesters, polyoxyethylenes, polytetramethylene glycols, polycaprolactones and copolymers of polyoxypropylene- polyoxyethylene.
Significantly, a third essential component is included in the present invention. A low molecular weight monohydric alcohol is reacted with the isocyanate component and the polyol component when producing the prepolymer. This alcohol acts as a chain terminator, terminating a. polymerising chain and hence effectively ending a chain reaction within the prepolymer. As such, a proportion of non-reactive terminations are introduced, having the effects that the shelf life of the prepolymer product is increased and its viscosity is lowered. Further, the alcohol increases the proportion of open cells present, and hence reduces shrinkage in the resultant polyurethane foam. Without the alcohol present in the reaction mixture, the resultant prepolymer
' has a short shelf life, tending to cross-link, even at storage temperature, sufficiently rapidly so as to become too viscous to be dispensed from the container only about a week after manufacture. In contrast, when an appropriate amount of alcohol is used, the prepolymer product is stable in its container for six months. However, the alcohol effectively blocks a portion of the isocyanate end groups, such that these groups are not available to take part in the curing or cross-linking reaction following application of the product and it would be expected that the resultant foam would not be suitable for use. Surprisingly, it has been found that the presence of alcohol does not prevent a useable foam from being produced from the prepolymer and thus such an alcohol may be used in the present invention to improve the properties of the prepolymer without detriment to the resultant foam.
The alcohols utilised in the present invention are monohydric and contain no other functional groups capable of cross-linking. Generally the alcohols have a molecular weight of less than 200 and may be straight chain or branched, and typically have from 2 to 10 carbon atoms, preferably 4 or 5 carbon atoms. Specific examples of suitable alcohols are l-methoxy-2-propanol, l-ethoxy-2-propanol and 2- '
butanol.
Conventional additives may be incorporated into the foam as is customary" in the art. For example, cell stabilisers, foaming agents and catalysts may be added as required to the polyol component of the foam. . The prepolymer may be produced by reacting the components inside the container which will be used for dispensing the foam or by reacting the components outside the container and then placing the product into the container. The stoichiometry of the components is designed to be such that the prepolymer product has terminal isocyanate groups. These reactive sites are relatively stable whilst in anhydrous conditions such as in the container, but when the prepolymer is expelled from the container they react with atmospheric moisture to produce a cured polymer. In the first method, the components (isocyanates, polyols, alcohols and other additives) are placed into a container suitable for use in one component foam applications. The container is then sealed, for example with an airtight dispenser valve such as a straw valve or a gun valve. Blowing agents and propellants are then. ' injected in order to pressurise the container. The components are allowed to react to produce a polyurethane prepolymer within the container.
In the second method, the components (isocyanates, polyols, alcohols and other additives) are placed in a vessel and the isocyanates and polyols are reacted. For example, the components may be placed in a batch reactor to carry out the prepolymerisation reaction. The product is then transferred to a container suitable for use in one component foam applications. The container is then sealed, for example with an airtight dispenser valve such as a straw valve or a gun valve. The container is then pressurised by injecting blowing agents and propellants into the container, and then prepolymer is allowed to equilibrate to room temperature. Water is excluded during these operations.
The present invention is illustrated further by means of the following Examples:
Example 1
The following components were premixed, being added to the mixing vessel in the given order to facilitate mixing, to produce the polyol component.
The foam components were then combined in the amounts indicated in the table below as follows:
The polyol component was added to a vessel suitable for use in one-component foam applications. The isocyanate component (a modified pure 4,4' diphenylmethane- diisocyanate) was weighed into the vessel, which was then crimped and sealed with an airtight dispenser valve. Dimethyl ether was added to the vessel, followed by the propane/ butane mixture. The mixture of components was agitated vigorously for three minutes, and then left to react at room temperature for three days. The vessel containing the prepolymer was then suitable for use in a one component polyurethane foam dispenser.
A free rise foam was subsequently produced by expelling the prepolymer from- the vessel onto flat card. The foam had the following properties:
Example 2
The following components were premixed, being added to the mixing vessel in the given order to facilitate mixing, to produce the polyol component.
The foam components were then combined in the amounts indicated in the table below as follows: The polyol component was added to a dry round bottom flask. The isocyanate component was then weighed into the vessel, which was then sealed airtight. The reaction mixture was stirred and then allowed to exotherm to no more than 50°C over two hours. The resulting prepolymer was then transferred into a vessel suitable for use in one-component foam applications, which was then crimped and sealed with an' airtight dispenser valve. Dimethyl ether was then added to the vessel, followed by the propane/ butane mixture. The mixture of liquid gases and prepolymer was agitated vigorously for three minutes, and then left at room temperature for two hours to equilibrate to room temperature. The vessel containing the prepolymer was then suitable for use in a one component polyurethane foam dispenser.