WO2000047384A2

WO2000047384A2 - Method and apparatus for the production of skinless flexible polyurethane foam

Info

Publication number: WO2000047384A2
Application number: PCT/EP2000/000949
Authority: WO
Inventors: Carlo Fiorentini; Paolo Spinelli
Original assignee: Cannon S.P.A.
Priority date: 1999-02-10
Filing date: 2000-02-07
Publication date: 2000-08-17
Also published as: AU3278600A; ITMI990258A1; IT1307947B1

Abstract

A method and apparatus for the continuous production of skinless or low skin flexible polyurethane foam (2). To reduce the thickness of the solidified skin on the surface of the polyurethane foam (2), the foaming occurs in a closed space (11). The environmental conditions during foaming are controlled by maintaining a gas-saturated atmosphere inside the closed space (11) using a gas or a mixture of gases different from ambient air. The closed space (11) is closed by a cover (12) which extend between the side-walls (9), from the mixture feeding point (16) and over a conveying path (4, 5). The reduction of the skin allows a greater percentage of polyurethane mixture to be usefully transformed into an expanded foam (2).

Description

METHOD AND APPARATUS FOR THE PRODUCTION OF SKINLESS FLEXIBLE POLYURETHANE FOAM

BACKGROUND OF THE INVENTION

This invention relates to the production of a skinless or 5. low skin flexible polyurethane foams, particularly but not exclusively for the production of polyurethane foam slabstocks, via continuous process or mould.

Current practice for the production of flexible polyurethane foams using continuous or discontinuous 10. process involves a polyurethane forming mixture being poured or fed into a foaming space where the polyurethane foam is left to freely expand and rise in contact with air, letting it completely polymerise.

Moreover, in the continuous process for the production of 15. polyurethane foam, a mixture of a polyol and isocyanate, complete with reactants and blowing agent, is usually poured or fed into a channel-shaped space, provided by sidewalls, fallplates and conveyors lined with a moving substrate .

20. This leaves a top-open foaming space wherein the reaction mixture can expand and rise freely whilst moving along the conveyor, until it reaches a maximum height, depending on the chemical formulation of the same reaction mixture and on the foaming conditions for the polyurethane foam.

25. Once the polymerisation is completed, the foam is cut into square or rectangular shaped slabs or blocks, which are suitable for subsequent processing.

It is known that during free expansion of a polyurethane forming mixture, the foam is maintained at ambient air conditions and in contact with air; this causes a high density surface skin on the foam, to form thickness ranging about 2 to 5 mm.

5. The skin for many applications has to be cut away and removed, thus resulting in polyurethane waste and loss of useful foam, calculated at around 3-5% in weight, in dependence of block height.

It is therefore advisable, were possible, to avoid the skin 10. phenomenon, or to reduce the thickness of the skin to have a greater exploitation of the polyurethane mixture and a greater height of the expanded foam.

In order to avoid or reduce this disadvantage, it has been suggested to apply one or more covering sheets or plastic 15. films on the polyurethane mixture.

During the expansion, whereto the foam would tend to adhere, thus reducing the tendency of the same foam to collapse and to form a skin. Some systems for the production of polyurethane foam slabstocks can be found for ₂₀. example in US 3,325,823, US 3,870,441, US 4,492,664, GB 1.354.341, GB 1.392.859, EP 0 645 226.

Although many attempts have been made in the past, even achieving acceptable results, the problem of the skin formation still remains and has not yet been satisfactorily 25. resolved. In particular the use of covering sheets, in addition to creating problems during foaming, considerably complicates the process and the production equipment. OBJECTS OF THE INVENTION

The main object of the present invention is to provide a method for the production of flexible polyurethane foams whereby the formation of a surface skin is substantially _5< reduced or prevented, thereby allowing a greater exploitation of the mixture and production of foam blocks having a greater height.

A further object of the present invention is to provide a method for the production of flexible polyurethane foams, ₁₀. suitable both in continuous and discontinuous production plants, whereby the polyurethane foam is let to expand freely in a gas controlled environment, thus minimising the formation of skin on its outer surface.

Yet a further object of the present invention is to provide ₁₅. apparatus for the production of flexible polyurethane foams, according to the method referred to above, which is of simplified construction and use.

BRIEF DESCRIPTION OF THE INVENTION

These and other objectives are achieved by means of a ₂₀. method according to claim 1, as well as by means of apparatus according to claim 7.

More specifically, according to the invention, a method has been provided for the production of flexible polyurethane foam, whereby it is possible to prevent or substantially ₂₅. reduce the formation of the skin on the surface of the foam. According to this the polyurethane forming mixture is let to foam into a closed space, and in which said closed space is maintained at a gas-saturated condition by supplying a gas or gases different from the air. For the purpose of the present invention, the statement * gas-saturated condition" for the closed foaming space, means a state that occurs when the closed foaming space is completely filled with gas or gases. These gasses are 5. deliberately introduced from the out side with those released from the polyurethane reaction, substantially preventing the ambient air entry from the outside.

Although any gas or mixture of gases different from air are suitable, and since now carbon dioxide is also used as an

10. added blowing agent in manufacturing polyurethane foams, unexpected results have been achieved by appropriately using carbon dioxide or a gas mixture comprising carbon dioxide for controlling the environmental conditions of the closed space in which the polyurethane forming mixture can

₁₅, foam and rise.

Even if not strictly necessary, it is however preferable for the molecular weight of the gas, or of the gas mixture, to be higher than the average molecular weight of the nitrogen and oxygen mixture in the air.

₂₀. According to another aspect of the invention, an apparatus has been provided for the continuous production of flexible polyurethane foam slabstocks, whereby the foaming space is defined by an enclosure extending from the feeding zone of the polyurethane mixture, for the entire length of

₂₅. expansion and growth path of the polyurethane foam. This creates a closed chamber connected to a source of gas or mixture of gases different from the air, in order to control the environmental conditions in a suitable manner for preventing or reducing the formation of the skin.

30. It is important to prevent drops of liquid falling into the foam, as this will cause damage or imperfections in the top surface. As previously stated, the gases in the enclosure will consist of both those introduced deliberately from outside the enclosure and those released from the polyurethane reaction.

5. The drops of liquid may be caused by temperature changes causing the gases to condense. Condensation may already exist within the atmosphere of the enclosure around the foaming space. This will collect on any surface of the enclosure and when the drops are large enough they will

₁₀. fall into the foam.

In addition when the gases are in contact with a cooler surface, some types of gases will condense directly on the surface of the enclosure,.

To prevent these drops falling into the foam, it will be ₁₅. necessary to control the temperature of the surface of the enclosure by providing heating and/or cooling to prevent condensation, or by designing the enclosure such that the condensed gas runs down towards collection points.

It may also be advisable to control the temperature of the 20. gases, the flow and the velocity of the same within the enclosure .

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features of the invention, will be made clearer by the following description with reference to the ₂₅. accompanying drawings, in which:

Fig. 1 is a perspective view of a first apparatus for the continuous production of polyurethane foam slabstocks, according to the present invention; Fig. 2 is a top view of the apparatus of Figure 1;

Fig. 3 is a cross-sectional view along line 3-3 of Figure 2;

Fig. 4 is a view similar to that of Figure 1, for a ₅. second embodiment of apparatus according to the invention;

Fig. 5 is a perspective view of a mould for the production of polyurethane foam slabstocks according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

_10> We will describe here the general features of the method according to the invention, as well as some embodiments of apparatus for continuous production of flexible polyurethane slabstocks, where foaming occurs m an environment having a controlled atmosphere of gas,

15. according to the invention.

Referring initially to Figures 1 to 3, an apparatus for the continuous production of polyurethane foam is shown, comprising a trough 1 wherein a liquid polyurethane mixture is continuously fed, let to foam and chemically react m 20. order to produce a flexible foam 2 which is left to polymerise while it is moving along a conveying path.

A polyurethane forming mixture is continuously fed at the bottom of the trough 1 from a mixing device 3 appropriately connected to tanks for storing the polyurethane components ₂₅. A and B to be mixed with reactants and a blowing agent. The trough 1 has an outlet at its upper edge, to allow the foaming mixture to overflow onto and down an angled fall- plate 4. The foam expands on the fall-plate and is carried by the moving substrate onto a conveyor 5. The conveyor is controlled to convey the foam 2 at a constant speed.

A moving substrate in the form of a paper or polyethylene band 6, is continuously fed from a roll 7, and is appropriately guided in order to run towards the upper edge of the trough 1, along the fall-plate 4 and on the conveyor 5, as shown.

The polyurethane foam 2 is retained on both sides of the fall-plate 4 and of the conveyor 5, by additional side

10. paper or polyethylene sheets 8, which run inside sidewalls 9, as shown in Figures 2 and 3.

The bottom paper 6, whose edges are folded upward, and the side papers 8 thus form a moving foaming channel along the fall-plate and the conveyor 5. The polyurethane mixture is ₁₅. allowed to freely expand vertically and fall downwards along the fall-plate 4, until achieving a preset height of the foam.

As previously discussed, wherein the foam 2 freely expands in an open top environment and in direct contact with air, 20. the upper surface of the foam would form a skin, consisting of a dense layer of foamed polyurethane material. This has to be removed, with the consequent being the loss of useful foam.

By substantially reducing the thickness of the skin, or ₂₅ eliminating its formation, it would therefore be possible to convert a higher percentage of polyurethane mixture into useful foam for commercial purposes.

This is made possible, according to the present invention, by maintaining the polyurethane foam, during expansion, in a closed space 11 filled with a gas-saturated atmosphere. The foam is kept in direct contact with a gas or a mixture of gases different from air, in order to prevent any contact with ambient air during the expansion and until ₅. consolidation of the foam structure has been reached.

The gas used can be any gas or mixture of gases suitable for this purpose. For example, good and unexpected results have been achieved by the use of carbon dioxide, or a mixture of gases comprising carbon dioxide. The gas is 1₀. continuously fed into the closed space 11, in a suitable manner to control the environmental conditions above the foam during rising or its expansion phase and prevent direct contact with ambient air. In this way the skin formation on the foam surface is reduced or eliminated.

₁₅_ In the case of the apparatus shown in Figure 1, the space above the expanding foam 2 is enclosed 11: by the two guide walls 9 and by an upper cover 12, eventually extendible side wise and which extends longitudinally from the trough 1, along the fall-plate 4 and for an initial portion of the

20. conveyor 5, up to a point wherein the structure of foam 2 can be considered consolidated.

The space 11 is connected to a gas source 13, via a piping 14 comprising a gas distributor 15, which transversely extends for part or for the entire width of the cover 12.

_25> The gas for controlling the environmental conditions above the foam 2, can be fed at one or more points at the top of the space 11, in such a way as not to interfere with or cause damages to the expanding foam.

In the case of Figure 1, the polyurethane forming mixture ₃₀. already in the expansion phase is fed from the trough 1 onto the fall-plate 4, so that the foam can vertically expand and fall downwards, m order to obtain foam blocks having a substantially flat upper surface.

However, other types of apparatus or other systems for

₅. feeding the polyurethane mixture may be used m the production of polyurethane foam slabstocks, where by the improvements of the present invention can be applied.

Another example of apparatus for the continuous production of flexible polyurethane foams is shown m Figure 4. Figure

10. 4 differs from Figure 1 substantially due to the fact that the expansion and free rising of the foam 2 take place vertically from the bottom, upwards, whilst on the conveyor 2. Moreover the trough 1 of figure 1 has been replaced by a mixing device 16, which can either be of the high or low

₁₅. pressure type, and which feeds the polyurethane mixture 2 m the liquid or pre-expanded phase, directly from above onto the bottom paper or polyethylene substrate 6, as shown .

Otherwise the apparatus of Figure 4 corresponds

_20> substantially to that of Figure 1. Therefore m Figure 3 the same reference numerals have been used to denote parts which are similar or equivalent to those of the previous example .

In this case, an enclosed space 11 with a cover 12 has been ₂₅. provided over the expanding foam 2. This extends between the side-walls 9, along a portion of the conveyor 5, from the feeding point of the polyurethane mixture at the mixing head 16, along the whole length of the rising portion for the foam and up to a position where the expansion or growth 30. of the foam can be considered substantially stabilised. The cover 12 for closing the space 11 can be fixedly supported, m relation to the fall-plate 4, and/or to the conveyor 5, or it can be supported m a height-adjustable manner by a suitable support system. For example, as ₅. schematically shown m Figure 3, the cover 12 can be supported by side arms 17, which can slide and be locked by clamps 18 directly attached to the side walls 9, or m another suitable manner.

The present invention, m addition to the continuous ₁0. production of polyurethane foam, can also be applied to discontinuous processes for production m moulds, as schematically shown m Figure 5.

Figure 5 shows the production of a block 19 of polyurethane foam m an appropriate mould 20. The mould 20 is closed by

_15, a removable cover 21 provided with a conduit 22 for feeding a gas different from air, to control the internal environment, and a port 23 for venting the air. In this case the height of the mould 20 must be extended m relation to a conventional mould, to be greater than the

₂₀. height of the foam block 19 and in order to form a closed space 24 which is filled with a gas or mixture of gases different from air, for example carbon dioxide or another suitable gas.

From what has been said and shown m the accompanying ₂₅. drawings, it is clear that the innovative aspect of the invention consists of causing the expansion of the polyurethane foam m an atmosphere saturated with a gas or mixture of gases different from ambient air and maintaining a high degree of saturation. It is not excludeα that m ₃0. certain cases a small quantity of ambient air can be tolerated m proportion to the quantity of gas or gas mixture used to saturate of the foaming space.

Claims

1. A method for the production of a skinless or low skin flexible polyurethane foam comprising the steps of feeding a chemically reactive polyurethane foaming mixture

₅. into a foaming space (11, 24) in which the foaming of the polyurethane mixture occurs, and in which the resulting foam (2) is let to expand freely, characterised by the steps of foaming the polyurethane foaming mixture in a closed space (11, 24), and maintaining the closed space

₁₀. (11, 24) at gas-saturated conditions by a gas atmosphere comprising at least one gas different from ambient air.

2. A method according to claim 1, wherein said gas has a molecular weight greater than ambient air.

3. A method according to claim 1 characterised in that 15. the gas atmosphere comprises carbon dioxide.

4. A method according to claim 1 characterised in that said gas-atmosphere comprises a mixture of gases including carbon dioxide.

5. A method according to claim 1, for the continuous ₂₀. production of flexible polyurethane slabstock foam, in which an expandable polyurethane foaming mixture is fed onto a moving substrate (6) and let to expand along a conveying path (4, 5) , characterised by comprising in general a closed foaming space (11) along said conveying ₂5. path (4, 5) and gas feeding means (14) to feed a gas being to maintain the closed space (11) at gas-saturated condition, the gas or mixture of gases being different from the ambient air.

6. A method according to claim 1, characterised in that said foaming closed space (24) comprises the cavity of a mould (20) .

7. Apparatus for the production of skinless or low skin flexible polyurethane foam by the method of the 5. previous claim 1, in particular for the continuous production of polyurethane slabstock foams, comprising:

a movable bottom substrate (6), for the foam (2) and side walls (8, 9) longitudinally extending along a conveying path (4, 5) to define a foaming area comprising a 10. foaming space;

feeding means (1, 16) for feeding a foamable polyurethane mixture into said foaming space defined by the moving substrate (6) and by side walls (8, 9);

a cover member (12) extending longitudinally along the 15. conveying path (4, 5) and between the side-walls (8, 9); and

feeding equipment (14, 15) for feeding a gas or a mixture of gases different from air into said closed space (11) .

20.

8. Apparatus according to claim 7, characterised in that said cover member (12) is adjustable in height.

9. Apparatus according to the previous claim 7, characterised in that said cover member (12) is longitudinally extending from a position close to the

₂₅. feeding point (1, 16) of the polyurethane forming mixture, along a portion of the conveying path (4, 5) .

10. Apparatus according to the previous claim 7, in which said conveying path (4, 5) comprises a downwardly slanted fall-plate (4), characterised in that the cover member (12) extends along said fall-plate (4) and for at least a portion of the remaining conveying path (5) .

₅.

11. Apparatus according to the previous claim 7, in which the conveying path (4, 5) comprises a foaming portion longitudinally extending from the feeding point (16) of the polyurethane foaming mixture, characterised in that said cover member (12) is upwardly slanting in respect to the

₁₀. conveying path (4, 5) .

12. Apparatus according to one or more of the previous claims, characterised in that said cover member (12) is provided with gas feeding means (15) opening into said closed space (11).

15.

13. Apparatus according to claim 7, characterised by means to prevent drop formation of condensed gas inside the closed space (11) .

14. Apparatus according to claim 13, characterised in that said means to prevent drop formation comprises of

₂₀. means of temperature control of the inner surface of said closed space (11 ) .

15. Apparatus according to claim 13, characterised by means to run the drops of condensed gas towards collecting points .

25.

16. Apparatus for the production of skinless flexible polyurethane foam blocks (10) into a mould (20) according to the method of claim 1, the mould (20) comprising bottom and side walls and a cover member (21), characterised in that the side walls of the mould (20) extend beyond the maximum allowed rise of the polyurethane foam, and that the cover member (21) is provided with gas feeding means (22) .

17. Apparatus according to claim 16, characterised in that said cover member (12) is provided with air venting ₅. means (23 ) .