Process for preparing polyurethane foam

Details Process for preparing polyurethane foam Process for preparing polyurethane foam

1999-01-05

2002-05-28

Cooney, Jr., John M. (Department: 1711)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S130000, C521S131000, C521S170000, C521S174000

Reexamination Certificate

active

06395796

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing polyurethane foam. The invention is especially adapted for making polyurethane foam using the one-shot foaming process, the quasi-prepolymer process and the pre-polymer process. The invention specifically relates to using reaction product of a tertiary amine and a halogenated carboxylic acid with optional hydroxyl functionality as a catalyst for promoting reactions involved in the production of polyurethanes, preferably one-shot polyurethanes, and particularly flexible polyurethane foams.
2. Background
Polyurethane foams are produced by reacting a di- or polyisocyanate with compounds containing two or more active hydrogens, generally in the presence of catalysts, silicone-based surfactants and other auxiliary agents. The active hydrogen-containing compounds are typically polyols, primary and secondary polyamines, and water. Two major reactions are promoted by the catalysts among the reactants during the preparation of a polyurethane foam. These reactions must proceed simultaneously and at a competitively balanced rate during the process in order to yield a polyurethane foam with desired physical characteristics.
Reaction between the isocyanate and the polyol or polyamine, usually referred to as the gel reaction, leads to the formation of a polymer of high molecular weight. This reaction is predominant in foams blown exclusively with low boiling point organic compounds. The progress of this reaction increases the viscosity of the mixture and generally contributes to crosslink formation with polyfunctional polyols. The second major reaction occurs between isocyanate and water. This reaction adds to urethane polymer growth, and is important for producing carbon dioxide gas which promotes foaming. As a result, this reaction often is referred to as the blow reaction. The blow reaction is essential for avoiding or reducing the use of auxiliary blowing agents.
Both the gel and blow reactions occur in foams blown partially or totally with carbon dioxide gas. In fact, the in-situ generation of carbon dioxide by the blow reaction plays an essential part in the preparation of “one-shot”, water blown polyurethane foams. Water-blown polyurethane foams, particularly flexible foams, are produced by both molded and slab foam processes.
As noted above, in order to obtain a good urethane foam structure, the gel and blow reactions must proceed simultaneously and at optimum balanced rates. For example, if the carbon dioxide evolution is too rapid in comparison with the gel reaction, the foam tends to collapse. Alternatively, if the gel extension reaction is too rapid in comparison with the blow reaction generating carbon dioxide, foam rise will be restricted, resulting in a high-density foam. Also, poorly balanced crosslinking reactions will adversely impact foam stability. In practice, the balancing of these two reactions is controlled by the nature of the promoters and catalysts, generally amine and/or organometallic compounds, used in the process.
Flexible and rigid foam formulations usually include a polyol, a polyisocyanate, water, optional blowing agent (low boiling organic or inert gas (CO
2
)), a silicone type surfactant, and catalysts. Flexible foams are generally open-celled materials, while rigid foams usually have a high proportion of closed cells.
Historically, catalysts for producing polyurethanes have been of two general types: tertiary amines (mono and poly) and organo-tin compounds. Organometallic tin catalysts predominantly favor the gelling reaction; while amine catalysts exhibit a more varied range of blow/gel balance. Using tin catalysts in flexible foam formulations also increases the quantity of closed cells contributing to foam tightness. Tertiary amines also are effective as catalysts for the chain extension reaction and can be used in combination with the organic tin catalysts. For example, in the preparation of flexible slabstock foams, the “one-shot” process has been used wherein triethylenediamine is employed for promoting the water-isocyanate reaction and the cross-linking reaction; while an organic tin compound is used in synergistic combination to promote the chain extension reaction.
Flexible polyurethane foams are commercially prepared as slabstock foam or in molds. Some slabstock foam is produced by pouring the mixed reactants in large boxes (discontinuous process), while other foam is prepared in a continuous manner by deposition of the reacting mixture on a paper lined conveyor. The foam rises and cures as the conveyor advances and the foam is cut into large blocks as it exits the foam machine. Some of the uses of flexible slabstock polyurethane foams include: furniture cushions, bedding, and carpet underlay.
In the discontinuous processes, the initiation of the reaction must be delayed to allow uniform laydown of the reacting mixture and allow excess air entrapped during reactant mixing to escape. Otherwise, foam splitting caused by the tardy release of such entrapped air may occur. In such situations, delayed action catalysts can be used to achieve the required reactivity profile. The problem also can be acute with slabstock foam produced by the continuous process on a machine with a short conveyor. In this case, the formulation has to be highly catalyzed in order to be sufficiently cured when the foam reaches the cutting saw. Thus, not only is delayed action necessary for a uniform laydown, but once activated, rapid catalytic action is critical.
The process for making molded foams typically involves the mixing of the starting materials with polyurethane foam production machinery and pouring the reacting mixture, as it exits the mix-head, into a mold. The principal uses of flexible molded polyurethane foams are: automotive seats; automotive headrests and armrests; and also in furniture cushions. Some of the uses of semi-flexible molded foams include automotive instrument panels, energy managing foam, and sound absorbing foam.
Modern molded flexible and semi-flexible polyurethane foam production processes have enjoyed significant progress. Processes such as those used in Just-in-Time (JIT) supply plants have increased the demand for rapid demold systems. Gains in productivity and/or reduced part cost result from reduced cycle times. Rapid cure High Resilience (HR) molded flexible foam formulations typically achieve demold times of three minutes. This is accomplished by using one, or a combination of the following: a higher mold temperature, more reactive intermediates (polyols and/or isocyanate), or increasing the quantity and/or the activity of the catalysts.
High reactivity molded polyurethane systems give rise to a number of problems, to however. The fast initiation times require that the reacting chemicals be poured into a mold quickly. In some circumstances a rapid build-up of the viscosity of the rising foam causes a deterioration of its flow properties and can result in defects in the molded parts. Additionally, rapidly rising foam can reach the parting line of the mold cavity before the cover has had the time to close resulting in collapsed areas in the foam. In such situations, delayed action catalysts potentially can be used to improve the initial system flow and allow sufficient time to close the mold. Delayed action catalysts which exhibit high catalytic activity following activation are especially useful.
Another difficulty experienced in the production of molded foams, which is usually worse in the case of rapid cure foam formulations, is foam tightness. Foam tightness is caused by a high proportion of closed cells at the time the molded foam part is removed from the mold. If left to cool in that state, the foam part will generally shrink irreversibly. A high proportion of open cells also are required if the foam is to have a desired high resiliency. Consequently, foam cells have to be opened either by physically crushing the molded part or inserting it in a vacuum chamber. Many strategies have been proposed, both chemical and mechanical, to

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