DESCRIPTION
PROCESS FOR MAKING RIGID POLYURETHANE FOAMS
This invention relates to processes for the preparation of rigid polyurethane or urethane- modified polyisocyanurate foams and to foams prepared thereby.
Rigid polyurethane and urethane-modified polyisocyanurate foams are in general prepared by reacting the appropriate polyisocyanate and isocyanate-reactive compound (usually a polyol) in the presence of a blowing agent. One use of such foams is as a thermal insulation medium as, for example, in the construction of refrigerated storage devices.
Polyurethane foam moldings are conventionally manufactured by introducing a polyurethane reactive mixture containing a blowing agent into a mold cavity, with the blowing agent being released in the course of the polyaddition reaction between the polyisocyanate and isocyanate- reactive mixture to foam and fill the mold cavity.
Blowing agents used are either low boiling liquids such as chlorofluorocarbons, methylene chloride, pentane, and the like, which evaporate due to the increase in temperature of the reactive mixture in the course of the exothermic polyaddition reaction, or water, which chemically releases carbon dioxide due to the reaction between the water and the isocyanate. Conventionally the blowing agent is mixed with the polyol component, which is then mixed with the polyisocyanate.
It has also been proposed to use blowing agents, which are gaseous at room temperature. These blowing agents are physically dissolved under pressure in the polyurethane reactive mixture. This creates the problem that, on expansion at ambient pressure, the polyurethane reactive mixture foams almost instantaneously to release the dissolved blowing agent, thereby producing a froth. In these known systems the partially frothed mixture is ejected in a turbulent manner owing to the sudden evaporation of the blowing agent. This leads to a significant loss in blowing agent and also a reduction in foam quality due to the presence of large bubbles, pinholes and irregularly shaped or over-sized cells which make such systems less attractive for use.
It is the object of the present invention to provide an improved method for making rigid polyurethane foam using gaseous blowing agent not showing the disadvantages of the prior art methods.
The present invention involves a method for making rigid polyurethane or urethane- modified polyisocyanurate foams from polyisocyanates and polyfunctional isocyanate- reactive components in the presence of blowing agents having a boiling point at atmospheric pressure of between -70°C and 16°C, comprising the steps of forming a mixture of the reactive components and the blowing agent which is dissolved under pressure in the reactive mixture and feeding the mixture under pressure controlled conditions into the cavity to be foamed.
Distribution of the reactive mixture is done under pressure controlled conditions by maintaining a backpressure on the upstream mixture so as to maintain the blowing agent in a fluid state or in solution, yet permitting frothing of the mixture to be initiated upon discharge; thereafter the mixture discharged in the cavity is allowed to expand into a progressively non-reactive frothing material by progressively releasing the fluid or dissolved blowing agent into the frothing material.
Backpressure on the mixture outside the mixing head to control the expansion rate of the blowing agent can be generated either by putting pressure on the cavity to be foamed or by the use of a restricting tube at the outflow of the mixing head or by injecting the reaction mixture into a smaller cavity which is positioned in the larger cavity to be filled and which releases the reactive mixture gently as pressure builds up.
Thus the present invention provides a process for the manufacture of polyurethane foam moldings by the introduction of a polyurethane reactive mixture containing a gaseous blowing agent into a mold cavity, comprising the steps of a) dissolving the blowing agent under pressure in the polyurethane reactive mixture, b) introducing the reactive mixture into a closed mold cavity, c) maintaining the gas pressure in the mold cavity, or part of the mold cavity, during the introduction of the reactive mixture at a pressure being such that instantaneous evaporation of the gaseous blowing agent present in the reactive mixture is prevented, and d) reducing the gas pressure above the reactive mixture in a way such that uncontrolled evaporation of the gaseous blowing agent is prevented. In step c), the pressure is maintained such that, at most, only a partial foaming occurs.
The mold is pressurized with gas before or during introduction of the reaction components, and depressurization takes place in the mold after introduction of the reaction mixture into the mold and before the foam has set.
Generally the mold is pressurized to a pressure of from 8 bar to 1 bar, in particular from 4 bar to 2 bar, most preferably between 3 and 4 bar.
The mold is depressurized from 8 bar to atmospheric pressure, preferably from 3 bar to atmospheric pressure, with the time for depressurization being the time from 0 seconds after introduction of the reaction mixture into the mold to the string time minus one second or faster. Preferably start of the pressure release is at a time below 10 seconds, more preferably below 8 seconds, after the start of the introduction of the reactive mixture.
The pressure is generated by means of substances which are gaseous at room temperature and which are inert towards the reaction components, for example, air, nitrogen, carbon dioxide, dinitrogen oxide, argon, helium or neon.
To carry out the process of the present invention, it is advantageous to likewise keep the reservoir for the polyurethane components as well as the product lines to the mixing head under pressure. The pressure here should be from 0 bar to 26 bar, in particular from 0.5 bar to 15 bar.
By using the process of the present invention for foam systems blown with gaseous blowing agents an improvement in foam quality, expressed in significant reduced number of blowholes, is obtained with the blowholes present being significantly smaller in size. Finer foam cells are obtained since less nuclei are destroyed during the initial injection process. All of this leads to better flow and lower thermal conductivity of the obtained foam.
Applying a pressure in the mould during injection of foam systems with low froth levels or foam systems blown with liquid blowing agents has no effect at all or a negative effect on the foam quality.
The rigid polyurethane or urethane-modified polyisocyanurate foam produced according to the process of the present invention generally is closed-celled, i.e. the open cell content is less than 20 %.
Suitable isocyanate-reactive compounds to be used in the process of the present invention include any of those known in the art for the preparation of rigid polyurethane or urethane- modified polyisocyanurate foams. Of particular importance for the preparation of rigid foams are polyols and polyol mixtures having average hydroxyl numbers of from 300 to 1000, especially from 300 to 700 mg KOH/g, and hydroxyl functionalities of from 2 to 8, especially from 3 to 8. Suitable polyols have been fully described in the prior art and include reaction products of alkylene oxides, for example ethylene oxide and/or propylene oxide, with initiators containing from 2 to 8 active hydrogen atoms per molecule. Suitable initiators
include: polyols, for example glycerol, trimethylolpropane, triethanolamine, pentaerythritol, sorbitol and sucrose; polyamines, for example ethylene diamine, tolylene diamine (TDA), diaminodiphenylmethane (DADPM) and polymethylene polyphenylene polyamines; and aminoalcohols, for example ethanolamine and diethanolamine; and mixtures of such initiators. Other suitable polymeric polyols include polyesters obtained by the condensation of appropriate proportions of glycols and higher functionality polyols with dicarboxylic or polycarboxylic acids. Still further suitable polymeric polyols include hydroxyl-terminated polythioethers, polyamides, polyesteramides, polycarbonates, polyacetals, polyolefins and polysiloxanes.
Suitable organic polyisocyanates for use in the process of the present invention include any of those known in the art for the preparation of rigid polyurethane or urethane-modified polyisocyanurate foams, and in particular the aromatic polyisocyanates such as diphenylmethane diisocyanate in the form of its 2,4'-, 2,2'- and 4,4'-isomers and mixtures thereof, the mixtures of diphenylmethane diisocyanates (MDI) and oligomers thereof known in the art as "crude" or polymeric MDI (polymethylene polyphenylene polyisocyanates) having an isocyanate functionality of greater than 2, toluene diisocyanate (TDI) in the form of its 2,4- and 2,6-isomers and mixtures thereof, mixtures of MDI and TDI, 1,5-naphthalene diisocyanate and 1,4-diisocyanatobenzene. Other organic polyisocyanates, which may be mentioned, include the aliphatic diisocyanates such as isophorone diisocyanate, 1,6-diisocyanatohexane and 4,4'-diisocyanatodicyclohexylmethane.
The quantities of the polyisocyanate compositions and the polyfunctional isocyanate-reactive compositions to be reacted will depend upon the nature of the rigid polyurethane or urethane- modified polyisocyanurate foam to be produced and will be readily determined by those skilled in the art.
The blowing agent that is used in the present invention has a boiling point at atmospheric pressure of between -70 and 16°C, preferably between -50 and -5°C, most preferably between -30 and -20°C .
Preferably the blowing agent is first dissolved under pressure in the polyol component before the latter is mixed with the polyisocyanate component to produce the polyurethane reactive mixture.
The amount of gaseous blowing agent dissolved in the polyurethane reactive mixture is generally between 4 and 50 %, preferably between 4 and 30 % by weight based on the isocyanate-reactive component.
Any of the physical blowing agents known for the production of rigid polyurethane foam having a boiling point in the above described range can be used in the process of the present invention. Examples of these include dialkyl ethers, cycloalkylene ethers and ketones, fluorinated ethers, chlorofluorocarbons, perfluorinated hydrocarbons, hydrochlorofluorocarbons, hydrofiuorocarbons, and hydrocarbons.
Examples of suitable chlorofluorocarbons include dichlorodifluorornethane (CFC 12).
Examples of suitable hydrochlorofluorocarbons include 1-chloro- 1,1-difluoroethane (HCFC 142b), monochlorodifluoromethane (HCFC 22) and 1-chloro- 1,2,2,2-tetrafluoroethane (HCFC 124).
Examples of suitable hydrofiuorocarbons include 1,1,1,2-tetrafluoroethane (HFC 134a), 1,1,2,2-tetrafluoroethane (HFC 134), 1,1,1-trifluoroethane (HFC 143a), 1,1-difluoroethane (HFC 152a), 1,1,1,2,2-pentafluoroethane (HFC 125), difluoromethane (HFC 32), 1,1,1,2,2- pentafluoropropane (HFC 245cb), 1,1,1,3,3-pentafluoropropane (HFC 245fa), 1,1,2,2- tetrafluoropropane (HFC 254cb), 1,1,1,2,3,3,3-heptafluoropropane (HFC 227ea), 1,1, 1,2,2,3, 3 -heptafluoropropane (HFC 227ca), 1,1,1,3,3,3-hexafluoropropane (HFC 236fa), 1,1,1,2,3,3-hexafluoropropane (HFC 236ea), 1,1,1,2,2,3-hexafluoropropane (HFC 236cb) and 1,1,2,2,3,3-hexafluoropropane (HFC 236ca).
Examples of suitable hydrocarbon blowing agents include n-propane, iso-propane, n-butane, iso-butane, cyclobutane, neopentane, 1-butene and any mixture of the above.
Also liquid carbon dioxide can be used as blowing agent.
Preferred blowing agents are isobutane, HFC 134a, HCFC 22, HCFC 142b and any mixture thereof.
Generally water or other carbon dioxide-evolving compounds are used together with the physical blowing agents. Where water is used as chemical co-blowing agent typical amounts are in the range from 0.2 to 5 %, preferably from 0.5 to 3 % by weight based on the isocyanate-reactive compound.
The total quantity of blowing agent to be used in a reaction system for producing cellular polymeric materials will be readily determined by those skilled in the art, but will typically be from 2 to 25 % by weight based on the total reaction system.
In addition to the polyisocyanate and polyfunctional isocyanate-reactive compositions and the blowing agents, the foam-forming reaction mixture will commonly contain one or more other auxiliaries or additives conventional to formulations for the production of rigid polyurethane and urethane-modified polyisocyanurate foams. Such optional additives include crosslinking agents, for examples low molecular weight polyols such as triethanolamine, urethane catalysts, for example tin compounds such as stannous octoate or dibutyltin dilaurate or tertiary amines such as dimethylcyclohexylamine or triethylene diamine, isocyanurate catalysts, surfactants, fire retardants, for example halogenated alkyl phosphates such as tris chloropropyl phosphate, and fillers such as carbon black.
In operating the process for making rigid foams according to the invention, the known one- shot, prepolymer or semi-prepolymer techniques may be used together with conventional mixing methods.
The various aspects of this invention are illustrated, but not limited by the following examples.
EXAMPLES
Closed celled rigid polyurethane foams are produced from the following ingredients:
Polyol: a mixture of polyether polyols having an average functionality of 5.1 and average OH value of 400 mg KOH/g Surfactant: a silicone surfactant
Catalyst: an amine catalyst package Isocyanate: a polymeric MDI
The amounts of the different ingredients in the polyol blend (in pbw) are indicated in Table 1 below.
Table 1
Foams were made using a Cannon HP40 machine in a mould suitable to apply to it an internal pressure of up to 9 bar. The following machine parameters were kept constant: components temperature 20°C, components pressure 130 bar, machine output 180 g/s, mould temperature 40°C. To pressurize the mould nitrogen (in case flammable blowing agents are used) or compressed air (in case non-flammable blowing agents are used) were used. The mould is put under pressure, then the mixture is injected and after a few seconds the pressure is released by a release valve positioned at the last point to fill.
The reaction profile is followed by injecting one shot of about 400 g of foam into a plastic bag and checking the cream and string time. The same bag is used to measure the free rise density by cutting a piece of foam, weighing it and measuring it by caliber.
The just fill weight on the mould is determined as the minimum amount of foam needed to fill the mould leaving empty the corners near the end of rise.
Physical testing is carried out on the foam panels: density (in g/1) according to ISO 845, compression strength (in kPa) according to ISO 844, thermal conductivity at 10°C (in mW/mK) according to ASTM C 518 (measured after 24 hours).
EXAMPLE 1 : HFC 134a as blowing agents
The following foam systems as indicated in Table 2 having varying amounts of HFC 134a blowing agent were used.
Table 2
All of these foam systems were run with different pressures being applied to the mould. The pressure is applied from the beginning of the injection and kept constant till a defined release time which is 2, 4, 6 or 8 seconds. The release time is the time interval between the start of the injection and opening of the mould's pressure release valve.
In the following tables the parameters that were used and also the results achieved in terms of foam structure (blowholes and cell structure) are given. A comparison is made between the cell structure of the reference foam (made using an unpressurised mould) and the foams made by applying pressure to the mould): 0 means "worst in terms of blowholes and cell size", 4 means "best fine and uniform cell size".
Table 3: Foam system 1
Table 4: Foam system 2
Table 5: Foam system 3
Table 6: Foam system 4
These results show that if low amounts of HFC 134a (foam system 1) are used the best result is achieved without pressure or with a little bit of pressure; the maximum time to keep the pressure is between 2 and 8 seconds; after that the cell structure becomes coarse and the cell windows are broken so that an open cell system is achieved.
With systems showing a strong froth effect (foam systems 2, 3, 4 and 5) the cell structure improves when the pressure is applied. Best performance is generally obtained when a pressure of 3 to 4 bar is applied.
Thermal conductivity was measured for these different foam systems and parameters. The results are given in Table 8.
Table 8
Thermal conductivity is improved with 5 to 8 % when pressure is applied to the mould.
Flow is determined by measuring the minimum fill weight. Results are given in Table 9.
Table 9
Flow is improved in all of these systems when a pressure is applied. The improvement is about 7 to 10 % for systems co-blown with water and about 20 % for systems without water being added.
The backpressure in the mould avoids the loss of blowing agent from the polyol blend during the reaction and the flow is improved. Compression strength is not influenced by applying backpressure.
The timing to start pressure release is related to the reaction profile of the system or the real start of the reaction between polyol/water and iso (cream time and not froth effect). Keeping the pressure too long always gives bad foam quality and foam collapse.
There is no influence by the speed of pressure release if the release speed is an average between 5 and 10 seconds per bar.
EXAMPLE 2: isobuta e as blowing agents
The following foam system as indicated in Table 10 was used.
Table 10
This foam system was run with different mould pressures. The pressure is applied from the beginning of the injection and kept constant till a defined release time which is 2, 4, 6 or 8 seconds.
In table 11 the parameters that were used and also the results achieved in terms of foam structure (blowholes and cell structure) are given.
Table 11 : Foam system 6
This isobutane blown system gives a strong froth effect; the cell structure is improved by applying the backpressure. The best performance is obtained with 3 and 4 bar. Also with this system the time to keep the pressure on is between 2 and 8 seconds.
Thermal conductivity was also measured for this foam system. The results are given in Table 12.
Table 12
Thermal conductivity is improved with about 5 %. Also flow is improved with about 7 % as shown in Table 13.
Table 13
COMPARATIVE EXAMPLE 3: Liquid blowing agents
The following foam systems as indicated in Table 14 blown with liquid blowing agents were used.
Table 14
These foam systems were run with different pressures being applied to the mould. The pressure is applied from the beginning of the injection and kept constant till a defined release time which is 2, 4, 6 or 8 seconds
In the following tables the parameters that were used and also the results achieved in terms of foam structure (blowholes and cell structure) are given.
Table 15: Foam system 7
When liquid blowing agents are used the best result is achieved without pressure being applied to the mould. When a little bit of pressure is applied the cell structure becomes worse and cell arduous appears to be as glass. Further increasing the pressure leads to breaking of the cells and open celled foam.
Both foam systems 7 and 8 also showed a worse flow when a backpressure is applied.