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novel processing of lbba appliance systems

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novel processing of lbba appliance systems

    2003中国聚氨酯行业整体淘汰ODS国际论坛论文集

    Novel Processing of LBBA Appliance Systems

J. FEIGHAN E. SHENG J. DESCHAGHT, F. MAGNANI

     Huntsman Polyurethanes Huntsman Polyurethanes Huntsman Polyurethanes

     286 Mantua Grove Road 452 Wen Jing Road, Shanghai Via Mazzini, 58

     West Deptford, New Jersey PR China 200245 21020 Ternate (VA)

     USA 08066-1732 Italy

ABSTRACT

     With the phase out of HCFC-141b, appliance manufacturers have had to decide on a zero Ozone Depletion Potential (ODP) blowing agent option. In North America, HFC-245fa and HFC-134a are the leading candidates, however there is a limited use of hydrocarbons. None of these options are an ideal drop in for HCFC-141b. Both HFC-245fa and HFC-134a are considered low boiling blowing agents (LBBA) and require additional handling measures. One of the hydrocarbons being used, iso-butane, is also an LBBA. The much lower boiling points of these options lead to major challenges for the end user. Of these options, HFC-134a is the most difficult to work with.

     Polyol blends containing HFC-134a have an elevated vapor pressure that is significantly higher than current HCFC-141b systems. This is primarily due to the low boiling point in combination with the poor solubility of HFC-134a in polyol blends. Because of their high vapor pressure, blends containing HFC-134a need to be kept under pressure during every phase of their use.

     Blends containing HFC-245fa develop much lower vapor pressures because of the better polyol solubility of the blowing agent, and because its boiling point is much closer to ambient temperatures. Isobutane is typically used as a co-blowing agent with other hydrocarbons, so the effect of its low boiling point is somewhat diluted. However, in spite of their differences, any LBBA will present processing issues that can have a negative impact on foam quality.

     Appliance foams are typically mixed under high pressure between 1500~2000 psi. At the mix-head, there is a sudden pressure drop as the foam goes through a transition from the high pressures that are used for mixing, down to atmospheric pressure as the foam exits the mixhead. The effect of this sudden pressure drop results in foam with blowholes as the blowing agent escapes from the foam mixture. These blowholes have a negative effect on the insulation and flow properties of the foam.

     This paper describes a processing technique that provided improved foam properties when compared to standard foaming techniques. The process innovation involved the use of a pressurized foam mold. This technique eliminated the blowholes from foam blown with HFC-134a. When using polyol blends that have been optimized for HFC-134a, the improvement in foam quality resulted in k-factor improvements of about 7%, or 0.01 BTU-in/hr-;F-ft?. Minimum fill weights showed reductions of 10%. These improvements are due to the increased retention of the HFC-134a. Measurements confirmed a 20% increase in the amount of HFC-134a that remained in the final foam. Similar improvements were seen with HFC-245fa, as well as isobutane systems.

     This paper does not address the feasibility of a commercial implementation of these techniques. However, it is believed that the kind of improvements that were seen in this project provide a glimpse of what is achievable with LBBAs, once the processes are optimized.

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    INTRODUCTION

     Appliance manufacturers are making preparations towards the imminent phase-out of HCFC blowing agents. Most appliance manufacturers in North America are currently using HCFC-141b or HCFC-141b/22 blends as the physical blowing agent. Those that have not yet made the change are expected to go to a zero ODP (Ozone Depletion Potential) blowing agent during the time period from Q4 2002 through Q3 2003. The ability to stockpile quantities of HCFC-141b is a factor in the changeover timing. If significant quantities of HCFC-141b can be secured, manufacturers would have the option of changing to the zero ODP systems more gradually.

    Table 1. Blowing Agent Properties

    Property HCFC-141b HFC-134a HFC-245fa Isobutane

    Chemical Formula CClFCH CFCHF CFCHCHFCH(CH) 2332322 33

    Molecular Weight 116.95 102.03 134.05 58.12

    Boiling Point (? 32.2 15.3 26.5 12.0

    Ozone Depletion Potential 0.13 0 0 0

     In North America, the next generation foam systems that are being developed are based on either HFC-245fa or HFC-134a. Both of these blowing agents are considered to be Low Boiling Blowing Agents (LBBA) because they are gases under typical ambient conditions. At 1 atmosphere, HFC-245fa has a boiling point of 59.5 ;F

    (15.3?), HFC-134a boils at 15.1;F (-26.2?), and isobutane boils at 10.4;F (-12.0?). Blowing agent

    properties are listed in Table 1.

     The low boiling points of these blowing agents lead to higher vapor pressures of the polyol blends. The lack of stability of the polyol / blowing agent solution complicates material transfer at every stage of handling. Extra care is required when LBBA polyol blends are drained from foam machines since the material froths as soon as it experiences the reduced pressure of 1 atmosphere. It is this inherent instability of the LBBA polyol blend that also leads to foam defects during the mixing operation.

     When high loadings of gaseous blowing agents are used, the foam mixture exits the mixing head with much higher turbulence than HCFC-141b systems. The increased turbulence is caused by the rapid expansion of the LBBA. Typical mixing pressures for high-pressure machines range from 1500-2000 psi. As long as the polyol/LBBA remains under pressure, the polyol/blowing agent solution is in a liquid state. Once the pressure is reduced however, the blend becomes a froth.

    Blowholes During foam mixing, the pressure drop has yet another

    repercussion. As it exits the mixing chamber, the expanding

    LBBA coalesces to form large pockets of gas know as

    blowholes. The blowholes have a negative impact on the

    thermal insulation of the foam by acting as conductive

    regions. The cell coalescence also reduces the amount of

    LBBA that is available for foam insulation as a large

    proportion of it escapes into the atmosphere. These large

    cells are shown in Figure 1. Figure 1. LBBA Blowholes The objective of this paper is to demonstrate improved

    processing of LBBA via a controlled expansion rate of the blowing agent. The controlled expansion of the LBBA gas is achieved by the use of a pressurized mold. By introducing the foam mixture into a pressurized environment, it is possible to ease the transition of the LBBA as it equilibrates from 1500-2000 psi to atmospheric pressure. Significant foam improvements are seen when the mold pressure is reduced at a time and a rate that is optimized relative to the foam reactivity. Because of its low boiling point, this technique has shown the greatest potential for use with HFC-134a, although similar improvements were seen with HFC-245fa. The

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    benefits that were seen include improvements to k-factor, foam yield, foam appearance, as well as better

    retention of the blowing agent in the foam cell.

     The factors that were investigated were the amount of pressurization, the time to initiate depressurization,

    and the rate of depressurization.

    Table 2. Machine Parameters.

     Machine Cannon HP 40

    Chemical temperature (Iso / Polyol) 68/68 ;F(20/20?)

    Mixing pressure(Iso / Polyol) 1885/1885 psi.(130/ 30 bar)

    Machine throughput 24 lb./min.(180 g/sec.)

    Mold temperature 104 ;F(40?)

EXPERIMENTAL

     This project was carried out at the Huntsman Polyurethanes global appliance center, which is located in Ternate, Italy. The foaming was done with a Cannon HP 40 high-pressure machine. The processing parameters are listed in Table 2. The mold that was used for the pressurized foaming is shown in Figure 2. The mold can be operated at internal pressures as high as 130 psi (9 bar). This mold was foamed in the horizontal position, and was heated to 104 ;F (40?).

     Because of the sensitivity of the foam quality to pressure variations, the mold needed to be airtight so that the pressure did not dissipate prematurely. This allowed for reproducibility of the technique. Compressed air was used to pressurize the mold when nonflammable blowing agents were used while nitrogen was utilized for hydrocarbons. The maximum pressure was controlled via a pressure gauge that was built into the mold. The experimental mold

    Figure 2. Pressurized mold was provided by Cannon.

     The formulations are listed in Table 3. The study looked at three HFC-134a loadings that worked out to be 13%, 16%, and 22% of the polyol side. The water level was adjusted

    in order to maintain a constant level of foam blowing.

    Table 3. Formulation Data.

     1 2 3

    Polyol 85.0 84.2 77.3

    Water 2.0 1.8 0.7

    Isocyanate 110 101 88

    HFC-134a 13 16 22

    Cream (sec) Froth Froth Froth

    Gel (sec) 66 71 80

    Free rise density (pcf / g/l) 1.61/25.8 1.54/24.7 2.12/34.7

RESULTS

    Process Optimization

     The first phase of the study looked at the optimization of the mold pressurization technique. The foam

    system that was evaluated was developed for use with HFC-134a. In order to challenge the effect of the LBBA

    pressure drop, the system had a relatively high loading of the blowing agent. The HFC-134a made up 22% of the

    polyol blend. The goal of this part of the study was to determine the pressure level and depressurization timing

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    that produced the best results. During this phase, the pressure was varied from 0 up to 56 psi. At each pressure level, the depressurization time was varied between 2 to 10 seconds. The results are listed in Table 4.

    Table 4. Effect of Mold Pressure / Depressurization Time.

    HFC-134a at 22% of Polyol Blend

    Mold Pressure Depressurization Time Appearance Rating

    (psi. / bar) (sec.) (0 worst / 4 best)

    0 / 0 - Reference 1

    14 / 1 2 through 8 Improved 2

    14 / 1 10 Coarse / collapse 0

    29 / 2 2 through 8 Improved 2

    29 / 2 10 Coarse / collapse 0

    43 / 3 2 through 8 Fine cells 4

    43 / 3 10 Coarse / collapse 0

    58 / 4 2 or 4 Coarse / collapse 0

     The reference foam was produced without any mold pressure and is seen in Figure 3. While the majority of the foam is fine celled, there were many large blowholes throughout the sample. The blowholes were formed by the coalescence of the HFC-134a as it exited the mixing chamber.

Figure 3. Reference Foam No Pressure Figure 4. 43 psi., Depressurization at 2-8 seconds

     For the next seven experiments, the mold pressure was incrementally increased from 14 to 58 psi. At 14 psi, the depressurization time was varied from 2 to 10 seconds. With a mold pressure of 14 psi, the foam showed a slight reduction in blowholes when the pressure release was initiated between 2 to 8 seconds after shot time. When the depressurization was delayed until 10 seconds, the foam quality became coarse and showed signs of collapse. Similar effects were seen at the next pressure increment. At 29 psi, a slight improvement was seen at the shorter depressurization times, while coarse foam was produced when the pressure release did not occur until 10 seconds. This trend was seen throughout the study, and is an indication of the balancing act between the amount of pressure that is added to the mold, and the depressurization time. The most significant effect was seen when the mold pressure was increased to 43 psi. At this pressure, when

    the depressurization took place between 2 to 8 seconds, the foam quality

    increased dramatically. The blowholes were almost completely

    eliminated. This improvement is seen in Figure 4.

     At 58 psi it was seen how there is a point where the initial mold

    pressure can be too large. At this pressure, the foam quality became

    coarse and glassy and showed signs of collapse. It would seem that the

    higher pressure inhibits the expansion of the foam, and causes the cells to

    collapse upon each other. This is seen in Figure 5.

    Figure 5. 58 psi., Depressurization

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    Effect on Thermal Conductivity

     When the pressurized mold conditions were optimized, the foam quality improvements also translated into thermal conductivity and flow improvements. Table 5 shows the k-factor data for the HFC-134a system. This data shows a comparison for an HFC-134a formulation that was run at three different blowing agent loadings. In each case, the formulation was run both with and without pressurization.

    Table 5. Thermal Conductivity Results -- HFC-134a System.

    k-factor at 50;F (10?) Mean Temperature

    HFC-134a in Polyol Mold Pressure Depressurization k-factor

    (%) (psi / bar) (sec.) (mW/mK) (BTU-in / hr-ft2-;F)

    13 0 - 0.155 21.1

    13 43 / 3 5 0.145 19.6

    16 0 - 0.153 20.7

    16 43 / 3 5 0.145 19.5

    22 0 - 0.151 20.5

    22 43 / 3 6 0.141 19.0

     Building on the work from the last section, the pressure levels that were used were those that were determined to be optimum for the 134a system, 43 psi of pressure and a depressurization time of 5 seconds. In all three cases, there was a significant improvement from the use of the pressurization technique. The k-factors

    2improved by 0.008 to 0.010 BTU-in/hr-ft-;F. This equated to improvements of 5-7%. It is clear that the

    retention of additional blowing agent is contributing to the thermal conductivity improvement.

Effect on Flow

     The flow study was based on determining the minimum fill weights that were required to fill the mold. Using the same HFC-134a system and blowing agent loadings that were used in the k-factor study, the flow project was designed to determine if the injection into a mold that was under a positive pressure had a negative effect of foam flow. Surprisingly, when the conditions were optimized, the pressurized mold resulted in a flow improvement. The flow results are listed in Table 6.

     When process conditions were optimized, less foam was required to fill the mold under the pressurized conditions. The density reductions ranged from 7-9%. It is believed that the increased retention of the blowing agent led to the increase in the foam yield.

     The Just Fill density for the 22% HFC-134a formulation was higher than the densities for the other two HFC-134a loadings. This difference is due to the lower water level of the 22% system. This is also seen in the formulation data of Table 3 as the free rise density was higher for the 22% loading. Although the moles of total blowing agent are kept constant, a decrease of blowing efficiency is often seen at low water levels.

     The last column of Table 6 lists the Flow Index of each formulation. The Flow Index is the ratio of the Just Fill density to the Free Rise density. This allows for system comparisons in cases where there are differences in the free rise density of the formulations. This data also confirms the improvement from the pressurized mold.

    Table 6. Flow Results -- HFC-134a System

    HFC-134a Mold Pressure Depressurization Just Fill Density Flow Index 3(% in polyol) (psi / bar) (sec.) (lb/ft) (g/l) JF/FRD

    13 0 - 2.32 37.1 1.43

    13 43 / 3 5 2.12 33.9 1.31

    16 0 - 2.13 34.1 1.38

    16 43 / 3 5 1.94 31.0 1.25

    22 0 - 2.75 44.0 1.26

    22 43 / 3 6 2.57 41.1 1.18

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    Effect on Cell Gas Content

     The mold pressurization had a positive effect on the amount of LBBA that was retained in the foam. Cell gas measurements were made to quantify this effect. The cell gas results are summarized in Table 7. Cell gas measurements were made on a foam system that had HFC-134a added at 16% of the polyol blend. The control foam was molded without any pressure. The amount of HFC-134a that was detected in the control was determined to be 4.40%. The theoretical amount of HFC-134a that is calculated to be in the foam is 7.90%.

     The difference between the theoretical value and the actual gives an idea of the amount of HFC-134a that is lost to the atmosphere. The numbers reported are for the HFC-134a in the cell, they do not include the blowing agent dissolved in the polymer matrix.

     When the same system was processed with a mold pressurization of 43 psi, the cell gas content was determined to be 5.46%, an increase of 24%. It is evident that the mold pressurization had a significant effect on the LBBA retention.

     It was believed that the amount of LBBA that is dissolved in the polymer matrix would not be affected by the molding technique, so therefore it was not measured. Earlier studies have shown that the amount of HFC-134a that is contained in the matrix average between 1-3%. Assuming that the amount of HFC-134a trapped in the polymer averages about 2%, than the total amount of HFC-134a that is contained in the foam ranges from about 80% for the standard molding technique to about 95% for the pressurized molding.

    Table 7. Cell Gas Content

    HFC-134a Contained in Cell + Polyol

    HFC-134a Pressure Release Time Cell Gas Content *Total HFC-134a

    (% in polyol) (psi. / bar) (sec.) (wt % of total foam) (% of theoretical)

    16 0 - 4.40 81

    16 43 / 3 5 5.46 94

    Theoretical amount of HFC-134a contained in foam equals 7.9% (cell gas + matrix)

    * Assumes additional 2% of HFC-134a contained in polymer matrix

Other LBBA

     The improvements that were seen with the pressurized mold technique are not limited to HFC-134a. Work has been done with HFC-245fa and isobutane and similar improvements to thermal conductivity and foam yield were seen. This study focused on HFC-134a because prior studies have shown that this LBBA had the greatest potential for improvement due to the combination of its poor polyol solubility and its low boiling point.

CONCLUSIONS

     The data from this study has clearly demonstrated the magnitude of improvements that are possible when the process conditions of an LBBA system are optimized. The use of a pressurized mold, or any other technique that would control the pressure of the foam as it exits the mixhead, provides great insight into the cell gas retention mechanism of LBBA systems.

     This study determined some of the optimal conditions for the pressurized mold technique. For each foam system, there is a combination of mold pressurization level and depressurization timing that will produce the best results. The work has shown that too low of a pressure will not eliminate the LBBA blowholes, while too high of a mold pressure will lead to cell collapse and coarse foam.

     For the HFC-134a system the following improvements were seen with the pressurized mold technique:

    - reduction / elimination of blowholes.

    - improvement of k-factors by 5-7%.

    - increase in foam yield by 7-9%.

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    - increase of LBBA in cell gas by more than 20%.

     For the HFC-134a system, the optimal mold pressure was about 40 psi. (3 bar). The foam quality was optimized when the depressurization of the mold began within 2-8 seconds of the foam injection. The foam quality was seen to deteriorate when the pressure release was delayed beyond 8 seconds.

     At this time, additional studies are needed to determine if the advantages of the mold pressurization technique out weigh the cost for pressurized jigs. The cost of retrofitting the many fixtures of a typical foam line can be high. However, it is believed that this study has shown some key insights into the LBBA foaming process, as well as the magnitude of improvements that can be attained via process optimization.

     The authors believe that the understanding that has been gained from this work will lead to more practical processing techniques that can deliver foam improvements.

    REFERENCES

     This work utilized an experimental mold that was designed by Cannon, and the authors would like to acknowledge their contribution.

BIOGRAPHIES

    Jack Feighan

    Jack Feighan has been with Huntsman Polyurethanes since 1979. He obtained a BS in Chemistry from Drexel University, and an MBA from LaSalle University. During his time with Huntsman he has been involved in a number of technical roles ranging from flexible slabstock through elastomers. He is currently a Technical Associate in the Appliance Group, based in West Deptford, NJ.

    Joris Deschaght

    Joris Deschaght joined Huntsman Polyurethanes in 1982 after obtaining a degree in

    Industrial Engineering from the KIH Ostend, Belgium. He initially worked on polyol

    development at the R&T Center at Everberg. He became involved in rigid foam

    development during the CFC phase-out, and most recently has specialized in appliance

    foam. He has had a secondment in Singapore as a Technical Manager, and is currently the Global Appliance Technical Manager, based in Ternate, Italy.

Franco Magnani

    Franco Magnani joined Huntsman Polyurethanes in 1984 after receiving his First Level degree in Plastics Engineering at the Technical Institute in Varese (Italy). During this time he has been active in several areas within the organization. He is currently working in the Appliance Group, based in the Global Appliance Center in Ternate, Italy. His recent work has specialized in HFC and hydrocarbon technology.

Enshan Sheng

    Enshan Sheng received a PhD degree in Surface and Interface Chemistry in 1992 from Loughborough University of Technology (UK). After completing his post-doctoral

    research on carbon black reinforced rubber in the same university, he joined Huntsman Polyurethanes (Asia Pacific) in Singapore in 1995 as a Technical Service Specialist. He is now the Technical Development Manager based in Shanghai.

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    低沸点发泡剂用于家电聚氨酯发泡中的新加工工艺

    J. Feighan, J. Deschaght, F. Magnani, 盛恩善

    (亨斯迈聚氨酯公司)

摘要,

    随着HCFC-141b的淘汰?发达国家的家电行业在发泡剂的采用上必须面临抉择。在北美?HFC-245faHFC-134a成为首选?而碳氢化合物的使用极为有限。这些发泡剂中没有一种发泡剂可以直接取代HCFC-141bHFC-245faHFC-134a是低沸点发泡剂(简称LBBA)?因而需要采取额外的措施才能使用。碳氢化合物之一异丁烷也是一种LBBA。使用LBBA?对聚氨酯生产厂家来讲?确实是一种挑战。在常用LBBA中?HFC-134a是使用最困难的一个。

    含有HFC-134a的组合聚醚的蒸汽压比含有HCFC-141b的组合聚醚的蒸汽压要大得多。这主要是因为?一方面HFC-134a的沸点较低?另一方面?HFC-134a在组合聚醚里的溶解性较差。因为含有HFC-134a的组合聚醚的蒸汽压较大?所以该组合聚醚在使用过程中必须储存在压力容器中。HFC-245fa的沸点较高?且它在组合聚醚里的溶解性较好?所以含有HFC-245fa的组合聚醚的蒸汽压较小。异丁烷一般是用于与其它碳氢化合物共混发泡?因此?其低沸点的特性对发泡的负面影响并不大。然而?任何LBBA对发泡的工艺条件都有特别的要求?并对泡沫的质量产生负面的影响。

    在家电行业中?发泡一般是采用高压发泡技术?其混合压力介于1015 MPa (100150大气压)间。在混合枪头?泡沫从这样很高的混合压力骤降至大气压。这种压力的突然降低会导致原是闭孔的泡孔内的发泡剂冲破泡孔?从而产生泡沫内较大的空洞。这些空洞对泡沫的导热及流动性能会产生负面的影响。

    本篇文章论述了一种新型的发泡工艺。该工艺采用一个可加压的模具。它可以消除HFC-134a发泡过程中产生的泡沫空洞。这种技术可降低导热系数7%左右。泡沫的流动性也得到大幅度的改善?恰填量可降低10%左右。这主要是因为留在泡孔内的HFC-134a增加了。测试表明?留在最终泡孔内的HFC-134a增加了20%。实验结果同时表明?采用HFC-245fa或异丁烷发泡时?也可得出类似的结果。

    本篇文章并没有涉及该新型技术在实际生产应用中的可行性。但是?该技术从一个侧面说明?若工艺条件得到优化?采用LBBA发泡也可以生产出性能优异的泡沫。

    关键词,发泡剂!低沸点!聚氨酯泡沫塑料!家电

    (英文题目,Novel Processing of LBBA Appliance Systems)

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