improvement of demold time for rigid polyurethane foam

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Improvement of Demold Time forRigid Polyurethane Foam

H. FUJINO, K. MATSUBARA, N. TOKORO AND T. NOZAWA

Polymer Research LaboratoryCentral Research Institute

2-5, Kasumigaseki 3-Chome

Chiydaku, TokyoJapan

1. INTRODUCTION

igid polyurethanefoam has been used in insulation field such as

refrigeration and building industry because of its excellent ther-mal insulating properties. The need for reduction of demold time has

recently increased to save energy and to enhance productivity, and hasbecome an important research subject in rigid polyurethane foam.Many reports have been found on the demolding of rigid polyure-

thane foam. E. Kuhn et al. [1] investigated the effect of mold geometryon demold time. W. D. Clarke [2] measured foaming pressure, andthrough examining its decay curve, he proposed a method for determin-

ing minimum demold time which enables prevention of unfavorableexpansion and cracks generating immediately after demolding.Rigid polyurethane foams are produced by isocyanates, polyols, blow-

ing agent like CFC-11 and other additives through chemical reactionas well as physical change. The foaming process is characterized by theheat of reaction, foaming pressure and green strength of resultingfoams as illustrated in Figure 1. The minimum demold time estimated

This paper was presented at Polyurethanes 88, Proceedmgs of the SPI-31st AnnualTechnical/Marketing Conference, Philadelphia, PA, October 18-21, 1988. The paper isbeing published herein from the conference proceedings after review by the EditorialBoard, but without the customary peer review process.

JOURNAL OF CELWLAR PLASTICS Volume 25-November 1989

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Figure 1. Foaming profile of rigid polyurethane foam

Figure 2. The concept for shorter demold time.




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531Improvement ofDemold Time for Rigid Polyurethane Foam

from Figure 1 is ideally considered to be the time Tl when the foamingpressure and green strength are in balance. In order to achieve the

minimum demold time, the following two methods are considered to beeffective.

1. Lowering foaming pressure2. Attaining faster green strength

They are illustrated in Figure 2. The authors have focused on the lattermethod and investigated the effect of polyol structure on the greenstrength of the foams. The results obtained will be described in thispaper.

2. EXPERIMENTAL

a) Raw Materials

Polyurethanes are thermosetting resin and the foaming process is anexothermic reaction. The heat evolution is considered to have an impor-tant effect on the rate of polymer formation. In order to keep heat evolu-tion constant in these

experiments, isocyanateand

polyol hydroxylnumber were fixed as polymeric MDI and 400, respectively. Physicalproperties of polyols are shown in Table 1. Polyols A, B and C were pro-duced from mixed initiator of sucrose and amines, mixing ratio ofwhich was different to vary functionality of polyols. An aromatic aminewas used as the initiator for polyols E and F. Another aromatic aminewas used for Polyol D.

b) Formulation

Experiments were carried out by using formulation shown in Table 2.The amount of catalyst was adjusted so as to give the same reactivity,gel time of 70 ±2 seconds.

c) Property Characterization

HEAT OF REACTION

Temperature increase of foam was used as the measurement of heatof reaction. A thermocouple was placed at the center of the alminumclosed mold with a size of 250L x 250W x 100H mm. A predeter-mined amount of polyurethane foaming material was poured into themold, and the temperature of the foam was measured.




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533Improvement of Demold Time for Rigid Polyurethane Foam

Table 2. Typical formulation.

’L 5420 from Union Carbide Chemicals Corp2Tetramethyl hexanediamine3MDI CR from Mitsui Toatsu Chemicals Inc

GREEN STRENGTH

A predetermined amount of polyurethane foaming material waspoured into the mold described above. After a prescribed time, resultingfoam was taken out of mold. Specimens were immediately made by cut-ting foam and stress-strain curve was successively measured. Com-

pressive modulus was calculated by following equation. A higher value

of compressive modulus indicates faster growth of green strength.

where

M = compressive modulus (kg/cmz)L = foam

height(12.5 cm)

OL = displacement in foam height (1.0 cm)S = compressed area (7.07 cm2)F = force required for displacing 1.0 cm in foam height (kg)

CURING RATE

Curing rate of the foam was obtained by dividing compressive modu-lus with time.

DEMOLD TIME

A predetermined amount of polyurthane foaming material was poredinto the mold described above. After a prescribed time, the lids of the




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534 H. FUJINO, K. MATSUBARA, N. TOKORO AND T. NOZAWA

mold were removed. The foam in the mold immediately expandedupward. The foam height in the mold was measured by dial gauge atthe highest part of the foam. The measured value was calculated by

subtracting mold height from the value of the foam height afterdemolding. Smaller foam expansion indicates shorter demold time.

FLOWABILITY

A predetermined amount of polyurethane foaming material waspoured into a reversed L shaped mold illustrated in Figure 3 and flowdistance was measured on the cured foam. Longer flow length per unit

weightof foam indicates better

flowability.THERMAL CONDUCTIVITY

The cured foam was cut into pieces with a predetermined size. Ther-mal conductivity was measured with an ANACON Model-88 instru-ment.

3. RESULTS AND DISCUSSION

a) Heat of Reaction

Heat evolution was almost constant as illustrated in Figure 4, even

though foaming was conducted by using different polyols. In any

Figure 3. Reversed L shaped mold.




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Figure 4. Exothermic curve.

polyols, temperature of foam reached 130 ° C after 3 minutes, 150 ° Cafter 5 minutes and maximum temperature 155 ° C after 7 minutes.

b) Green Strength

Table 3 shows compressive modulus after 3 minutes, 5 minutes, and7 minutes respectively from the reaction starts. Compressive modulus

Table 3. Effect of polyol initiator on compressive modulus.




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536 H. FUJINO, K. MATSUBARA, N TOKORO AND T NOZAWA

which indicates green strength was increased with the elapse of time inall polyols. Figure 5 illustrates the growth of compressive modulus pre-pared from polyols A, B and C which have different functionality. The

polyols having higher functionality tend to exhibit higher compressivemodulus. However, the curing rate illustrated in Figure 6 is extremelydecreased as the polyol functionality is lower. Growth of compressivemodulus and curing rate prepared from aromatic amine initiatiedpolyols D, E and F are illustrated in Figure 7 and 8, respectively. Asillustrated in Figure 7, aromatic amine initiated polyols rapidlyincrease compressive modulus with the elapse of time. Figure 8 illus-trates the tendency more clearly. The curing rate is also increased overthe course of time.

Amongthe aromatic amine initiated

polyols, polyolE and F are superior to polyol D on the growth of compressive modulusas well as curing rate.Figure 9 illustrates the effect of average functionality of polyols on

the compressive modulus after 7 minutes from the reaction starts.

Polyols based on higher functional initiators tend to exhibit highercompressive modulus in the process foam formulation. Aromatic amineinitiated polyols, however, are exceptions. Polyols E and F in particularexhibit the highest compressive modulus among the polyols examined.

Table 4 shows the growth ratio of compressive modulus after 7 minutesfrom the reaction starts when the modulus after 24 hours from the

reaction starts is regarded as 100%. Polyols having higher function-ality or aromatic amine intiated polyols indicate higher growth rate.

Figure 5. Sucrose/amine initiated polyol vs. compressive modulus.




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Figure 6. Sucrose/amine initiated polyol vs. curing rate.

Figure 7. Aromatic amine initiated polyol vs. compressive modulus.




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Figure 8. Aromatic amine initiated polyol vs. curing rate.

Figure 9. Average functionality of polyol vs. compressive modulus.




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Table 5. Effect of polyol initiator on foam expansion.

c) Demold Time

The foam expansion after removing the mold lids was immediatelymeasured and the results are shown in Table 5. All polyols give smaller

expansion over the course of time. Figure 10 illustrates the relation-

ships between demold time and foam expansion prepared from polyols A, B and C which have different functionality. The foam prepared frompolyol C, which has higher functionality, tends to exhibit smaller

Figure 10. Sucrose/amine initiated polyol vs. foam expansion.




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Figure 11. Aromatic amine initiated polyol vs. foam expansion.

Figure 12. Average functionality of polyol vs. foam expansion.




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Figure 13. Compressive modulus vs. foam expansion.

expansion. The foam expansion prepared from aromatic amine initi-ated polyols D, E and F is compared in Figure 11. Polyols E and F givesmaller

expansion. Figure12 illustrates the effect of

averagefunction-

ality of poloyols on foam expansion from 7 minutes after the reactionstarts. Polyols based on higher functional initiators tend to givesmaller expansion. Aromatic amine initiated polyols, however, areexceptions. Polyols E and F gave the smallest expansion among thepolyols examined.

d) Green Strength and Demold Time

As described above, the growth of green strength and demold timewere examined by using polyols based on various initiators. The rela-tionship between compressive modulus and foam expansion after 7minutes from the reaction start is plotted as illustrated in Figure 13,identifying that higher compressive modulus leads to smaller expan-sion.

e)Flowability and Thermal Conductivity

Experiments on thermal conductivity and flowability were also car-ried out in order to investigate both the overpacking property of foaminto the panel and the insulation property of the resulting foams. Asshown in Table 6, polyols based on lower functional initiators exhibitbetter flowability. In comparing aromatic amine initiated polyols D, E




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and F, polyols E and F were found to have somewhat lower flowabilitythan polyol D.On the other

hand, many reportshave been

publishedto date on the

thermal conductivity of foams. The authors [3] have also investigatedthe effect of initiators on the thermal conductivity of rigid polyure-thane foams. Aromatic amine initiated polyols which have excellent

compatibility with isocyanate and a faster increase rate of viscosityhave been reported to result in lower thermal conductivity. Similarresults are also obtained in this paper, and aromatic amine initiated

polyols D, E and F lead to lower thermal conductivity of resultingfoams.

f) Preparation of Large Foam Panel

The above research was carried out by preparing foams with a handmixing method. Machine pouring into a large sized panel with dimen-sions of 900W x 1800L x 100H mm was conducted by using sucrose/amine initiated polyol B and aromatic amine initiated polyol F. Thecorrelation between hand mixing and machine pouring was examinedand the results are illustrated in

Figure14. In machine

pouring, polyolF is found to give smaller expansion as in the case of hand mixing.

Figure 14. Polyol B, F vs. foam expansion by using foaming machine.




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4. CONCLUSION

Improvementof

demold time for rigid polyurethane foam, particu-larly the effect of polyol structure on the green strength was examined.The results are summarized as follows:

1. Demold time has a good relationship with growth of green strength.Higher compressive modulus, which indicates faster growth ofgreen strength, results in shorter demold time.

2. The growth of green strength is remarkably influenced by the func-tionality and the initiator of polyol. The following polyols are recom-

mended for improvement of demold time.-polyols based on high-functional initiators-polyols initiated with aromatic amines

3. Results of hand mixing are good coincidence with those of machinefoaming in the production line.

4. In addition to shorter demold time, aromatic amine initiated polyolprovides the foam with excellent thermal conductivity.

As described above, polyols initiated with some kind of aromatic

amines were found to be excellent in the growth of green strength. Forfurther improvement of demold time, investigations on the hydroxylnumber of polyols are required as well as better flowability.

REFERENCES

1. Kuhn, E. and P. Schindler. Proceedings of the 31st Annual SPITechnical/Marketing Conference. p. 756 (1987).

2. Clarke, W. D. Proceedings of the 29th Annual SPI Technical/Marketing Con-ference. p. 194 (1985).3. Nozawa, T., K. Matsubara, N. Tokoro and H. Fujino. Proceedings of the 30th Annual SPI Technical/Marketing Conference. p. 393 (1986).

BIOGRAPHIES

Hiroshi Fujino

Hiroshi Fujino received a B.S. degree in Synthetic Organic Chemistryfrom Kumamoto University in 1981. He joined Mitsui Toatsu Chemi-cals Inc. as a research chemist for the urethane foam group at PolymerResearch Laboratory in 1981. He has been engaged in the developmentof rigid polyurethane foams. He is currently the chief chemist.




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546 H. FUJINO, K. MATSUBARA, N TOKORO AND T NOZAWA

Kiyoshi Matsubara

KiyoshiMatsubara received a

B.S. degreein Technical

Chemistryfrom Kyoto University in 1964. He joined Mitsui Chemical Co., Ltd. asa research chemist for the urethane group-polyether polyol-atNagoya Research Division in 1964. In 1970 he transferred to PlasticsResearch and Technical Service Laboratories of Mitsui Toatsu Chemi-

cals Inc. He has been engaged in the development of flexible foam andRIM elastomers. He is currently general manager and also groupleader of polyurethane foam and RIM in the Polymer Research Labora-tory.

Nobuo Tbkoro

Nobuo Tokoro received a B.S. and M.S. degree in Synthetic OrganicChemistry from Nagoya University in 1967 and 1969 respectively. Hejoined Mitsui Tbatsu Chemicals Inc. as a research chemist of polyetherpolyol at Nagoya Research Division in 1969-1977. In 1977 hetransfered to Polymer Research Laboratory of Mitsui Toatsu ChemicalsInc. He is currently the senior research scientist and responsible for

rigid polyurethane foams.

Tbshio Nozawa

Toshio Nazawa received a B.S. and M.S. degree in Industrial Chemis-

try from Seikei University in 1974 and 1976 respectively. He joinedMitsui Toatsu Chemicals Inc. in 1976. He worked five years as staff

officer of Polyisocyanate Production Division at Ohmuta factory of Mit-

sui Toatsu Chemicals Inc. in 1976-1981. In 1981 he transferred toPolymer Research Laboratory and has been engaged in the develop-ment of rigid polyurethane foam. He is currently the senior researchscientist.

improvement of demold time for rigid polyurethane foam

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