Reaction injection molding of polyurethane foam for improved thermal insulation

Reaction Injection Molding of Polyurethane Foam for Improved Thermal Insulation

MYUNG SOOL KOO, KWANSOO CHUNG, and JAE RYOUN YOUN*

School of Materials Science and Engineering Seoul National University

Seoul, 151-742, Korea Shinlim-dong. Kwanak-gu

A study on the apparent thermal conductivity of polyurethane foam was carried out. A HCFC (hydrochlorofluorocarbon) gas and carbon dioxide were used as the physical blowing agent and ultrasonic excitation was applied to increase the rate of bubble nucleation. The thermal conductivity of the binary gas mixture was predicted theoretically to estimate the apparent thermal conductivity of the polymer foam. Effects of conduction and radiation on the apparent thermal conductivity of the cellular polyurethane were considered with respect to the cell size and the effect of convection was neglected because of the small cell size. A laboratory RIM machine was designed and built for foaming experiments. The foaming experiments were performed at various processing conditions, and density, apparent thermal conductivity, number of cells, and cell sizes were measured. Best results such as low thermal conductivity and small bubbles were obtained when the polyol was mixed with the HCFC gas and saturated with carbon dioxide at 0.3 MPa, and foamed with ultrasonic nucleation.

INTRODUCTION

eaction injection molding (RIM) is carried out R by polymerizing very reactive monomers and oligomers in the mold (1). The monomers and oligomers. which have low viscosity at the processing temperature, are mixed in the mixing head by utilizing turbulent mixing prior to injection into the mold. RIM has some distinct advantages compared with injection molding because it is fast and requires less mold investment especially when a large complex part is to be produced. Polyurethane, polyurea, nylon, epoxy, and unsaturated polyesters may be processed by RIM, but 95% of RIM products are made of polyurethane. For RIM of polyurethane, two components, polyol and isocyanate, are mixed in the mixing head by impingement mixing, injected into the mold, and cured quickly as soon as the mold is filled. A typical RIM machine is shown in m. 1 schematically.

Polyurethane foam, used for thermal insulation of home appliances is produced by reaction injection molding (NMI with blowing agents. Both physical and chemical blowing agents are used to generate bubbles in the polyurethane resin. Qpically water is used as the chemical blowing agent and CFC or HCFC gases

*Corresponding author.

are used as the physical blowing agent. The CFC or HCFC gases, which act as the physical blowing agent by changing phases from liquid to vapor states, are known as harmful to the environment and will be pro- hibited in the future. Water reacts with isocyanate to yield carbon dioxide gas and amine which will further react with isocyanate to form a urea linkage. The carbon dioxide gas and the evaporated vapor will con- tribute to bubble formation. Water, which is used as the chemical blowing agent, is environmentally safe but the volatile liquids used as the physical blowing agent are not. Some other volatile liquids such as cyclo-pentane and hexane are tested and used in the industry for foaming of polyurethane. However, these physical blowing agents show high thermal conductivity and flammability, and require capital investments to modify the existing production line.

A new processing method of polyurethane foam had been proposed by Park and Youn (2-4) to produce a cellular structure by applying ultrasonic excitation to the nitrogen gas supersaturated liquid. Since bubble nucleation and growth are strongly affected by the concentration of dissolved gas in the resin, the saturation pressure should be properly selected. Higher saturation pressure will cause higher rate of nucleation and faster growth of bubbles. For the production of uniform cell structure with small size bubbles, a high

POLYMER ENGINEERING AND SCIENCE, JULY 2001, Vol. 41, No. 7 1177

Myurg So01 Koo, Kwansoo Chung, and Jae Ryoun Youn

solid walls, conduction heat transfer through the solid, conduction heat transfer through the gas mixture in the void, convection heat transfer by the gas in the void, and radiation heat transfer through the void must be considered. Convection plays an important role if there is a large space for gas movement. For polymer foams, the space in each cell is small enough for the contribution of the convection heat transfer to be neglected. The contribution of gas convection to the apparent thermal conductivity of the solid foam can be neglected if the cell diameter is smaller than 1.5 mm (6). In the case of HCFC gases, the convection is negligible for cell sizes less than 4 mm.

Radiation between the surfaces of the cell is less significant than thermal conduction through the solid walls or through the gas mixture emerged from the blowing agent. However, more than 2Ooh of the overall thermal conductivity is due to radiation in the case of commercially available plastic foams which consist of bubbles larger than 200 pm. As the size of the void is decreased below 10 pm, contribution of the radiation can be neglected. The apparent thermal conductivity of a polyurethane foam is determined by conduction and radiation heat transfer, and is expressed by the following equation (7).

Flg. 1. SchematiCdiagramoftheRIMmachine.

rate of cell nucleation and slower growth of bubbles are desired. Relatively low saturation pressure was used to lower bubble growth and ultrasonic excitation was employed to promote bubble nucleation. The ultrasonic excitation creates a sufficient negative pressure to yield copious bubble nucleation by reducing the critical activation energy for nucleation. Carbon dioxide gas was also used by Kim and Youn (5) for saturation of the polyol resin, and cellular polyurethane was produced by RIM with the application of ultrasonic bubble nucleation. They reported that the environmentally friendly processing method generated polyurethane foam with good cell structure and com- petitive thermal insulation capability. Bubble nucleation and bubble growth were modeled theoretically to predict the rate of bubble nucleation and the final cell Size.

In this study. polyurethane foams are produced by using a binary mixture of carbon dioxide and HCFC gas as the blowing agent. An experimental RIM machine was built to prepare the specimens under various foaming conditions. Thermal conductivity was predicted theoretically and compared with the measured values. The goal of this study was to improve the thermal insulation properties of the currently used polyurethane foam and to reduce the amount of HCFC gas that is harmful to the atmosphere.

Thermal Conductiwity of Polyurethane Foam

Thermal conductivity of a polymer foam is determined by the three modes of heat transfer mecha- nisms-conduction, convection, and radiation. Be- cause a polymer foam consists of many voids and

where Am is the apparent thermal conductivity, [ASas + A S 9 - is the conductivity caused by the conduction through the gas and solid, and A- is the conductivity caused by the radiation in the cells.

Conduction Heat Transfer

Thermal conduction through the solid and the gas mixture in the cell is the most important contribution to the apparent thermal conductivity of the foam. Since a mixture of HCFC-141b and carbon dioxide is used as the blowing agent in this study, thermal conductivity of the binary gas mixture is of interest. Ther- mal conductivity of a gas mixture can be calculated by the Wassiljewa equation (8,9).

where A, is the thermal conductivity of the gas mixture, yi is the mole fraction of the gas i, A is the Was- siljewa constant, and A, is the thermal conductivity of the gas i. For a binary gas mixture, J3q 2 becomes

(3) Y l X 1 + Y2 A2 A,,, =

Y1 + YzA12 Yz + YlA21 by assuming A, and 42 are unity ( 10, 1 1). To predict the thermal conductivity of a gas mixture, the Wassil- jewa constant should be determined. The Wassiljewa constant is determined by several theoretical rela- tions. For example, Wassiljewa (12) derived the following equation

1178 POLYMER ENGINEERING AND SCIENCE, JULY 2oO1, Voi. 41, No. 7

Reaction Injection Molding of Polyurethane Foam

A12 = 1 ( y)2 J"' + m,

(4) m,

A21 = & ( T ) 2 d F where s1 and s, are the diameter of each gas mole- cule, and rn, and m, are the molecular mass of each component. Mason and Sexana (1 1) also suggested the following equation.

where M is the molecular weight, q is the viscosity, and K is a numerical constant which is close to 1. An empirical relation is also proposed by Brokaw (1 1).

Am = XrnL + (1 - q)h, (6)

where

hrnL = ~ 1 h 1 + ~ 2 x 2 (7)

y1 and y2 are constants and determined by the experiment.

In order to improve the thermal insulation capability of the polyurethane foam, a binary mixture of the HCFC gas and carbon dioxide is used as the blowing agent in this study. For commercially available polyurethane foam, CFC or HCFC gases are used with water as the foaming agent. Because water acts as a chemical blowing agent by chemical reaction with isocyanate and yields carbon dioxide for a relatively long period of time, cells grow to large sizes. If carbon

dioxide is used as the blowing agent and the ultrasonic excitation is applied, the rate of nucleation will be increased and the cell growth will be reduced to end up with smaller cells. A mixture of the HCFC gas and carbon dioxide is selected for the experimental in- vestigation. Thermal conductivity of the HCFC gas and carbon dioxide mixture is predicted by using Eq 6 and is plotted in Fig. 2. If the cell size in the polymer foam is fixed, the thermal conductivity will increase as the volume fraction of carbon dioxide in the binary gas mixture increases. However, if the ultrasonic bubble nucleation reduces the cell size (2-5). the overall thermal conductivity will be reduced.

Contribution of the thermal conduction through the solid walls to the apparent thermal conductivity depends on the volume fraction of the solid. The volume fraction of the solid in the polyurethane foam is determined by the density of the foam. As the density is reduced to a small value, thermal conduction through the solid wall becomes negligible. When the density of the urethane foam is smaller than 40 kg/m3, the contribution of thermal conduction through the solid wall is less than 0.001 W/mK. Therefore, thermal conduction through the solid wall is neglected in this study. Thermal conduction of the binary gas mixture and radiation in the cell will be considered to predict the overall thermal conductivity of the polyurethane foam.

Radiation Heat Traaafer

There have been many theoretical modelings of the contribution of radiation heat transfer to the apparent conductivity of the plastic foam (7-24). Helte (13) predicted the radiation heat transfer by assuming that the solid polymer completely blocks the radiation. Jones (14) calculated the radiation heat transfer by considering emissivity of the polymer and the thick- ness of the gas layer. Williams and Alado (15) proposed

Fig. 2. Theoretically cakulated thermal conductivity of HCFC- 141 b/C02 mixtures according to Brokaw empirical method

00 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 0

volume fraction of HCFC-141 b


Myurg Sool Koo, Kwansoo Chung, and Jae Ryoun Youn

0.014

0.012

P z 0.010 5 c 0.008 > 0 3 u 0.006 c 0 0 C 0.004 0 m

0.002

.- c

.- c.

IY I

0 100 200 300 400 500 600

Cell size (pm)

Rg. 3. Radiation con&.ctiuity us. cell sue when density is m

a model by assuming that the solid layers and the gas layers are parallel. Other theoretical studies (16-24) have been reported on radiation effects in the polymer foam. The following equation is selected in this study to predict the contribution of the radiation to the apparent thermal conductivity because it is relatively ac- curate and simple to use.

where k is a correction factor to be determined by experiments. u is the Stefan-Boltzmann constant, E is the emissivity, and L is the cell size. In this study, k was experimentally determined to be 3.7, and the radiation heat transfer was calculated as a function of cell size and shown in Flg. 3 with some experimental

results (25) reported in the literature. As shown in the Figure, the radiation conductivity can be reduced as the cell size is decreased. If the cell is smaller than 10 pm, the t h e d conductivity caused by the thermal radiation in each cell may be neglected. Therefore, the apparent thermal conductivity can be reduced if the cell size is decreased by increasing the rate of nucleation and suppressing the bubble growth.

EXPERIMENT3

Reaction Injection Moiding

For reaction injection molding of polyurethane foam, an experimental setup was designed and built in the laboratory. Two hydraulic pumps are used to pressurize the liquid components and two accumula- tors are also employed for steady supply of the pressurized liquid component. The pressurized components are transferred to the mixing chamber for impingement mixing by opening two ball values attached between the accumulator and the mixing chamber. The two values are opened and closed at the same time by operating the pneumatic cylinders. The polyol and the isocyanate were stored in the storage tank and pressurized by carbon dioxide for 24 hours. The resins saturated with carbon dioxide were transported to the accumulator by the hydraulic pump. Once the pressure inside the accumulator reached the desired level, the ball values are opened simultaneously for a fixed period of time. The mixed Polyurethane was injected to the mold and expanded in the mold until it fdled the mold cavity.

Foamhg of P-e A mixture of the HCFC gas and carbon dioxide was

used as the blowing agent to produce a polyurethane foam, and ultrasonic excitation was applied to increase the rate of nucleation. Carbon dioxide gas was selected because it has high solubility in the resin and will give a higher rate of nucleation under ultrasonic

(4 (b) Flg. 4. SEM pictures of the samples produced by using the p l y 0 1 containing 3 1 g of HCFC gas at CO, saturation pressure of 0.1 Mpa; [a) with ultrasonic excitation, (b] without ultrasonic excitation.

1180 POLYMER ENGINEERING AND SCIENCE, JULY 2001, Vol. 41, No. 7


Amount (9) of HCFC-141b per polyol 100 g

Industry formula (IP)

excitation compared with water used as the chemical blowing agent. The amount of the HCFC gas, the saturation pressure of carbon dioxide, and application of the ultrasonic excitation were varied for production of specimens. Experimental conditions are listed in Table 1. As shown in the table, water was removed from the formulation and different amount of the HCFC gas was added to the polyol. The polyol resin was saturated with carbon dioxide under a Merent pressure. Although water is used widely as a chemical blowing agent, it is omitted for the foaming of polyurethane because water contributes to growth of bubbles as it reacts with isocyanates for a certain period of time. If carbon dioxide is used with ultrasonic excitation, the rate of nucleation will be increased and the growth will be restrained.

The stoichiometric ratio of the two components, polyol and isocyanate, was satisfied by controlling the pressure of each fluid. The volume flow rate was calculated by assuming a Newtonian fluid. Since the diameter and length of the nozzle, the viscosity of each resin at the operating temperature, and the pressure inside each accumulator are known, the volume flow rate and the Reynolds number can be predicted. Be- cause the ball values are opened and closed at the same time, the amount of each component supplied

CO, pressure (saturated for 12 hours)

0 MPa

Ultrasonic excitation

Yes

no

into the mixhead was controlled by setting the pressure difference. A complete mixing must be achieved in the mixhead for good quality polyurethane (26). If the viscosity of the resin is increased by temperature drop along the tube between the accumulator and the nozzle, the Reynolds number will be decreased and poor mixing wiU occur. The temperature of the experimental setup was maintained by insulating the accu- mulators and tubes.

The mixed resin in the mixhead was transferred into a small reservoir attached to a rectangular mold with a vent hole at the end. The ultrasonic wave was applied into the reservoir whenever needed. Foamed polyurethane samples were detached from the mold and density was measured. The samples were cut into specimens of 300 mm X 300 mm X 25 mm and the apparent conductivity was measured by using a heat flow meter instrument (HC-074, EKO Instruments). The specimens were cut into small pieces and observed with a scanning electron microscope to measure the number and size of bubbles. The SEM pictures were scanned by an image scanner and an image processing program (Image-Pro Plus V. 2.0) was used to determine the average diameter of cells. The number of cells per unit volume was also determined by using the image processing results.

Table 1. Experimental Conditions Employed for the Study.

0 MPa

31 9

40 9

no

t

i

Yes

no

Yes

no

0.1 MPa

0.3 MPa

0 MPa

Yes

no

Yes

no

0.1 MPa I

0.3 MPa I 1

I Yes

no 0 MPa

50 9 Yes

no

Yes

no

0.1 MPa

0.3 MPa

POLYMER EffG\N€ER/NG AND SCIENCE, JULY 2007, Vof. 41, No. 7 1181

Myung Soot Koo, Kwansoo Chung, and Jae Ryoun Youn

Q. 5. SEM pictures of the samples produced by using the p l y 0 1 Containing 3 1 g of HCFC gas at CO, saturation pressure of 0.3 MPcq (4 with ultrasonic excitation, (b) without ultrasonic excitation

RESULTS AND DISCUSSION

Different amounts of the HCFC gas were added to the polyol. i.e., 31 g, 40 g, and 50 g per 100 g of polyol. For comparison, commercially formulated polyol for industrial processing (IP) was also used to produce the polyurethane foam. The IP formula con- tains both water and the HCFC gas as the blowing agent and the ultrasonic excitation was not applied for foaming of the IP formula. When polyols were saturated with carbon dioxide at 0.1 and 0.3 MPa, ultrasonic excitation was applied to some specimens but not to others. Polyurethane foams produced with the various experimental conditions shown in Table 1 were evaluated to determine the density, cell size, number of cells in the unit volume, and the apparent thermal conductivity.

Cross sections of the specimens were observed by scanning electron microscope: see Figs. 4 and 5. The samples were produced by using the polyol containing 31 g of the HCFC gas and saturated with carbon dioxide at 0.1 and 0.3 MPa. Smaller cells are produced when ultrasonic excitation is applied because the rate of nucleation is increased by the negative environ- mental pressure generation [2, 3) due to the ultrasonic wave. As the saturation pressure was raised from 0.1 to 0.3 m a , cell sizes were reduced. It was also observed that the carbon dioxide saturated polyol generated smaller and more uniform cells than the IP formula. The average size of the cells was measured by an image processing technique and plotted in Figs. 6 and 7. The size of cells that are generated by applying the ultrasonic excitation is smaller than that of cells produced without ultrasonic excitation, as

Q. 6. Average cell diameter as a function of CO, saturation pressure with ultrasonic excitation

650

550

450

c

6 300 A,

250

150

+ 409-polyol

loo ' 0.0 0.1 0.2 0.3

COP pressure (MPa)



Flg. 7. Average cell diameter asa function of CO, saturation pressure without ulfrasonic ewcitation.

700

650 1 600

550 - 500 450

5 400

350

300

250

200

5 k c m

Fig. 8. Average cell diameter of the samples produced by using the polyol contau2ing 50 g of HCM: gas as afunction of CO, saturation pressure.

31g-pOlyOl + 409-polyol + 5og-polyol

shown in Figs. 8 and 9. As the saturation pressure of carbon dioxide is increased, the average cell size is reduced because the critical free energy for bubble nucleation will be lower. In the case of the polyol containing 50 g of HCFC gas, the cell size was increased slightly when it was saturated at 0.1 MPa compared with the cells produced without carbon dioxide saturation. It will influence the apparent thermal conductivity of the polyurethane foam.

The apparent thermal conductivity of the polyurethane specimens produced by the experimental conditions listed in Table 1 was measured. In Fig. 10, the apparent thermal conductivity is plotted as a function

0.0 0.1 0.2 0.3

C02 pressure (MPa)

of the amount of the HCFC gas, saturation pressure of carbon dioxide, and the ultrasonic application. As mentioned earlier, the thermal conductivity of the specimens produced with the polyol containing 50 g of the HCFC gas with ultrasonic excitation was higher than that of the specimens produced without carbon dioxide saturation. As shown in the Figure, the thermal conductivity of the specimens produced with ultrasonic excitation is lower than that of those produced without ultrasonic excitation. As the cell size becomes smaller, the apparent thermal conductivity of the polyurethane foam becomes lower. If the density of a plastic foam is in the range of 30 to 80 kg/m3

-+- with ultrasonic excitation -3 without ultrasonic excitation

650 - 600 - 550 -

3 s E 500 - 5 450 -

400 -

350

300 -

-

250 ' 0.0 0.1 0.2 0.3

C02 pressure (MPa)


M y w So01 Koo, Kwansoo Chung, and Jae Ryoun Youn

32 -

2 E 30- 3

Fig. 9. Average cell diameter of the samples produced by using different amounts of blowing agent at CO, saturation pressure of 0.3 m a

-+- 31 g-poyol(with ul.excitation) u 31 g-polyol(without ul.excitation) --t 40g-polyol(with ul.excitation) -0- 40g-polyol(without ul.excitation) -+- 50g-polyol(with ul.excitation)

i I f I

-o- with ultrasonic excitation -0- without ultrasonic excitation

300

280 -

- 260 -

a, E 240 - m

220 -

200 -

h

5 k)

ii

c

160 ' I

0 10 20 30 40 50

HCFC-14lb (9)

[27), density is not an important parameter influenc- ing the thermal conductivity. Cell size is a critical factor determining the apparent thermal conductivity, because the thermal radiation heat transfer strongly depends upon the cell size.

Some of the thermal conductivity measurement results are plotted in Fig. 11 as a function of the HCFC gas amount for the saturation pressure of 0.3 ma. Compared with the IP specimens, lower thermal con-

ductivity is obtained when the polyol containing 31 g of the HCFC gas and saturated with carbon dioxide was used for foaming of polyurethane with ullrasonic application. The apparent thermal conductivity of each spec- imen was predicted by using Eqs I, 6, and 9 as shown in Rg. 12. There is some Merence between the measured conductivity and the predicted ones because it is difficult to determine the volume fi-action of HCFC gas in each bubble. It will be possible that lower thermal

Q. 10. Thermal conductivity as a function of CO, saturation pressure.



25

h

- 1 t

E 5 22-1

18 ' I I 1

0 10 20 30 40 50

HCFC-14lb (9) Rg. 1 I. Thermnl conductivity of di@rent samples produced by using various amount of the physical blowing agent at CO, saturation pressure of 0.3 MPa.

conductivity can be achieved if the right amount of the HCFC gas and carbon dioxide is used for ultrasonic foaming. It is expected that the thermal insulation capability of commercdly available polyurethane will be improved by applying the experimental results. If the

HCFC gas is removed &om the foaming process and the same level of thermal conductivity is obtained by utilizing other physical blowing agents for ultrasonic bubble nucleation, environmentally safe processing of polyurethane foam will be realized.

-0- without ultrasonic excitation

\

I I

0 10 20 30 40 50

HCFC-141 b (9)

Rg. 12. ?hermal condudiuity predicted for different samples ptoduced with uarious amounts of the physical blowing agent at CO, saturation pressure of 0.3 Wa.


Myung Sool Koo, Kwansoo Chung, and Jae Ryoun Youn

CONCLUSIONS

Various polyurethane foam specimens were produced by using binary gas mixtures as the physical blowing agent. Instead of using water as the chemical blowing agent, Carbon dioxide was used for foaming of polyurethane. The apparent thermal conductivity of the binary mixture was calculated based on the Was- siljewa equation. If the cell size remains constant, the apparent conductivity of a polymer foam will be increased as the volume fraction of carbon dioxide in the binary mixture increases. However, it was shown that the conductivity would be reduced if the cell size was also reduced. When the polyol was saturated with carbon dioxide at higher pressures, the cell size be- came smaller. For the same composition of the binary gas mixture, the apparent thermal conductivity of the polyurethane foam was proportional to the cell size if the density of the foam is fured. When 31 g of the HCFC gas was added to the polyol and the polyol was saturated with CO, at 0.3 ma, the minimum thermal conductivity was obtained. It will be possible to produce polyurethane foam with better thermal properties if a proper mixture of physical blowing agents and the proper processing conditions are employed.

ACKNOWLEDGMENT

This study was supported by the Korea Science and Engineering Foundation (KOSEF) through the Applied Rheology Center and some experimental materials were supplied by the Daewoo electronics company. The au- thors are grateful for the support.

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Reaction injection molding of polyurethane foam for improved thermal insulation