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Thermally expandable microcapsules for polymer foaming--relationship between expandability and viscoelasticity.

INTRODUCTION

Recently, there have been requirements for greater advances in the weight reduction of automotive parts for mileage improvement. Polymer foaming is one of the promising techniques for realizing further weight reduction. Many automotive parts are made from polypropylene (PP), and they are produced by either injection molding or extrusion in general. Thus, the application of the foaming technique to foam PP injection or extrusion was considered as a straightforward solution for reducing the weight of auto parts. However, several problems must be solved. For auto part production, a superior appearance is one of the quality requirements. Polymer foaming often creates silver streaks on the product surface.

Thermally expandable microcapsules are a class of polymeric particles used for polymer foam production as blowing agents or as lightweight fillers (1), (2). They have a core and shell structure. A low-boiling-point hydrocarbon liquid is encapsulated by a polymeric shell. Mixing the microcapsules with the thermoplastic polymer and letting them thermally expand in a polymer can make polymer foams. When the microcapsules are heated to 80-180[degrees]C, they are expanded to 50-100 times greater than their initial volume, as illustrated in Fig. 1. The different thermo-mechanical behaviors of the various grades provide an opportunity for selecting an optimal grade for each polymer and application. With variety of grades, they have been used not only in foam production but also in wallpaper production, vinyl plastisol formulations, as well as rotational molding of linear low-density polyethylene and ethylene-vinyl acetate, etc (3-7).

[FIGURE 1 OMITTED]

One of the major advantages of using microcapsules for plastic foaming, especially, for foam injection molding, is to prevent the bubble eruption and breakup at the flow front as well as the product surface. Thus, the microcapsules can suppress the occurrence of surface deterioration such as silver streaks on the product surface. That deterioration is often observed conventional physical foaming as well as chemical foaming. However, the current microcapsule cannot completely solve all foaming problems. For example, the capsule could not be applicable to injection molding and extrusion at high operating temperatures over 200[degrees]C, at which foam injection molding and extrusion of PP are often conducted. Thus, it is highly desired in the automotive industry to develop a new microcapsule for PP foam extrusion and injection.

The factors determining the thermal expandability of microcapsules are volatility of encapsulated foaming agent, gas permeability, viscoelasticity of the shell polymer, and thickness of the shell. This study focused on the effect of viscoelasticity on the expandability. Changing the degree of crosslinking of the shell polymer, we controlled the viscoelasticity and investigated both expandability and shrinkage behavior of the microcapsules by means of visual observation of batch foaming, foam injection molding, and extrusion foaming of PP. By using the experimental results as fundamental data, we developed a new microcapsule that could be suitable to PP foam processing.

EXPERIMENTAL

Materials

All monomers, solvents, and reagents except for PEG#200-diacrylate and colloidal silica dispersion were purchased from Wako Pure Chemical Industries Co. Ltd., Tokyo, Japan. PEG#200-diacrylate was purchased from Kyoueisha Chemical Co. Ltd., Osaka, Japan. Colloidal silica dispersion in water was purchased from Asahidenka Industries Co., Ltd., Osaka, Japan.

Preparation of Thermally Expandable Microcapsules

The fundamental manufacturing technique of the expandable microcapsules was first patented by the Dow Chemical Company (8), and several patents followed (9-11). However, only a limited number of studies have been published (12-14). The authors in Sekisui Chemical developed a procedure for creating an acryloni-trile (AN)-based thermally expandable microcapsule (15), (16). In this study, the microcapsules were made by the developed procedure: 5000 g of deionized water, 1685 g of sodium chloride, and 523 g of 20 wt% colloidal silica dispersion in water were loaded into a polymerization reactor (15 L) equipped with an agitator. An aqueous solution of polyvinyl pyrrolidone (17 g of 30 wt%) and a solution containing sodium nitrite (20 g of 10 wt%) were successively added. The pH of the aqueous solution was adjusted within 3 to 4 by hydrochloric acid. AN, metha-crylonitrile, methacrylic acid, and methyl methacrylate were utilized as monomers. An oil phase mixture was prepared utilizing 1968 g of the monomer and containing 24 wt% blowing agents (iso-pentane/iso-octane), 16 g of 2,2'-azobisisobutyronitrile, and 12 g of 2,2'-azobis(2,4-dimethylvaleronitrile) as an initiator. The oil phase mixture was added to the aqueous solution under strong agitation of a blade rotating at about 10,000 rpm. The droplets of about 16-18 [micro]m in diameter were established in the solution. After the initial dispersion, the suspension-type polymerization was maintained at a temperature about 60[degrees]C for a period of 15 hr under 0.5 MPa [N.sub.2] purge. At the end of the polymerization, the temperature was lowered. The resulting beads were filtered and subsequently dried in an air oven at a temperature of about 60[degrees]C. Several thermally expandable microcapsules with different crosslinking degrees were prepared from AN co-polymers by using PEG#200-diacrylate as the crosslinking agent in a suspension-type polymerization process.

Expansion Behavior of Thermally Expandable Microcapsules

To evaluate the expandability of the prepared microcapsules, the change in volume of a specified amount of microcapsules loaded in a cup was measured by varying the temperature with a Thermomechanical Analyser (TMA2940 TA Instruments Inc., New Castle, DE). The amount of the microcapsules charged in the measurement cup was 24 [micro]g. A force of 0.1 N (3.4 kPa) was loaded on the capsules by a probe and heated up at the rate of 5[degrees]C/ min. When the microcapsules started expanding, a probe was moved up by the expansion. The temperature at the onset of the expansion is denoted by [T.sub.s], while the temperature at which maximum volume of expansion observed is denoted by [T.sub.max]. The maximum dimension change is represented by [D.sub.max].

In addition to the TMA expandability measurements, visual observation experiments were conducted (17-19). A small amount of microcapsules was loaded in a visual observation cell and heated up from room temperature to 200[degrees]C either in the atmospheric pressure of air or in contact with 6 MPa [CO.sub.2]. A high-speed CCD camera was equipped to the view cell. The foaming behavior was recorded through the CCD camera by a computer. When experiments with pressurized [CO.sub.2] were carried out, the pressure was reduced from a given level to atmospheric pressure 1 min after the temperature reached 200[degrees]C.

Viscoelastic Properties

To measure the storage modulus, G', and loss modulus, G", of the polymer, which composes the microcapsule shell, the hydrocarbon blowing agent was first removed by dissolving the microcapsule into N,N-dimethylformamide (DMF) for 24 hr and drying in vacuum at 60[degrees]C for 24 hr. As a result, polymer powder was obtained. Then, a sheet was made from the powder by a compression molding at 190[degrees]C under a 15 MPa mechanical force for 5 min. These rheological properties were measured at a temperature of 200[degrees]C using a rotational rheometer (UBM Inc.) with parallel plates in the dynamic oscillatory mode in the range of 0.02-62.8 rad/sec.

Measurement of the Degree of Crosslinking

The degree of crosslinking was measured by the DMF extraction method. DMF extraction was conducted by following the procedures A and C described in ASTM D 2765 (20): At first, 1 g of the microcapsule was completely dissolved in 29 g of DMF for 24 h at room temperature. The polymer gel was separated from DMF extraction by centrifugation at 3000 rpm for 10 min, and it was dried at 130[degrees]C in vacuo until the weight was not changed. On the other hand, the polymer was also retrieved from the DMF extraction by simply drying the solution without centrifugal treatment. The degree of crosslinking was calculated by 100 [1 - ([[[W.sub.d1] - [W.sub.d2]])/[W.sub.d2]]], where [W.sub.d1] is weight of dried polymer and [W.sub.d2] is the weight of gel.

Molding Experiments

A mixture of the microcapsules and PP was prepared by dry blending 2 wt% of microcapsules into PP. The density of PP alone was 0.9 g/[cm.sup.3]. The MFR of PP was 30 g/10 min used for injection grade and 3 g/10 min for extrusion grade. By using the mixture of PP and microcapsules, foam injection molding and foam extrusion were conducted. The injection molding machine (TOSHIBA, IS 350E) with 350 ton of clamping force was used for the foam injection molding. The barrel temperatures were set in the range of 200-220[degrees]C and the mold temperature at 50[degrees]C. Foam was made by performing core back expansion operation with the processing condition (Back pressure = 10 MPa, Injection speed = 160 mm/sec, Injection pressure = 110 MPa, Injection time = 0.46 sec). In the foam extrusion, the foams were made by a single screw extruder (UNION PLASTICS) with screw LID = 2.0 and the full fright screw geometry. The barrel temperatures of the extruder were set in the range of 200-220[degrees]C. The density of the resulting foam was measured by an electronic densimeter (Alfa Mirage Co., Ltd).

RESULTS AND DISCUSSION

Properties of Thermally Expandable Microcapsules

Figure 1 shows SEM images of microcapsules before and after expansion. It can be easily seen that the microcapsule has a hollow structure. The average capsule diameter is about 20 [micro]m, and the density is 1 - 1.1 g/[cm.sup.3] before expanding. After expansion, the diameter was increased to about 100 [micro]m, and the density was reduced to 0.002-0.02 g/[cm.sup.3]. Overheating sometimes induces shrinkage of the capsules due to a breakup or leakage of the foaming agent from the thinned polymer shell.

Preventing shrinkage while keeping good expandability could be a key factor of making the microcapsule usable for high-temperature polymer foaming processes. In this study, modification of the polymer properties such as viscoelasticity and permeability was attempted to control the capsule expandability and the shrinkage. Microcapsules with different viscoelasticities were prepared by changing the degree of crosslinking, and the relationships among the degree of crosslinking, viscoclasticity, expandability, and shrinkage were investigated through the foaming experiments. Five kinds of microcapsules with different degrees of crosslinking in the range from 0 to 78.6% were prepared. The measured rheological properties of the five kinds of shell polymer with different degrees of crosslinking are summarized in Table 1. The measurements were conducted at 200[degrees]C and at two different frequencies, 63 and 0.045 rad/s. A mixture of iso-pentane and iso-octane was used as the foaming agent sealed in the capsules. The vapor pressure of the foaming agent at 200[degrees]C was calculated by Antoine's equation to be 2.23 MPa.
TABLE 1. Shell properties of thermally expandable microcapsules.

                                 Viscoelasticity

                            200 [degrees]C 63 rad/sec

Sample  Degree of cross-linking (wt%)     G'        G"     tan [delta]

  1                     0              1.49E+05  5.33E+04     0.36
  2                  65.5              1.40E+05  6.28E-04     0.45
  3                  76.6              1.97E+05  6.08E+04     0.31
  4                  77.5              1.10E+05  3.04E+04     0.28
  5                  78.6              1.40E+05  3.84E+04     0.27

                                 Viscoelasticity

                          200 [degrees]C 0.045 rad/sec

Sample  Degree of cross-linking (wt%)     G'        G''    tan [delta]

  1                     0              3.64E+04  1.91E+04     0.53
  2                  65.5              3.52E+04  1.49E+04     0.42
  3                  76.6              7.90E+04  2.38E+04     0.30
  4                  77.5              5.58E+04  1.32E+04     0.24
  5                  78.6              8.54E+04  1.57E+04     0.18


Figure 2 shows the frequency dependency of G', G", and tan [delta] at 200[degrees]C for the five kinds of shell polymer. The storage modulus, G', increased at low frequency as the degree of crosslinking increased. Tan [delta] at low frequency decreased with an increase in the degree of cross-linking, which corresponds to the fact that as the degree of crosslinking increased, the capsule shell became tougher. In this study, tan [delta] was used as an index of toughness of the polymeric shell.

[FIGURE 2 OMITTED]

Figure 3 shows G' at two frequencies (0.045 and 62.8 rad/s) for five shell polymers. G' increased significantly at the low frequency with an increase in the degree of cross-linking, especially for the polymer samples #3, #4, and #5. On the other hand, G' at the high frequency was in the range from [10.sup.5] to 2 X [10.sup.5] Pa and did not show prominent changes with varying degrees of crosslinking.

[FIGURE 3 OMITTED]

Relationship Between Expandability of Microcapsule and PP Foaming

The results of the TMA measurements of the expandability are summarized in Table 2 and shown in Fig. 4. Figure 4a shows the expansion curves of the five microcapsules with different degrees of crosslinking. The dimension change, i.e., the volume change by expansion, was used as an indicator of expandability. [T.sub.s], [T.sub.max], and [D.sub.max] were determined from the TMA expansion curves. [T.sub.max] can be used as a heat resistance index, as [T.sub.max] reflects the glass transition temperature of the shell polymer. The polymeric shells of the capsule were made from the same chemicals but with varying crosslinking degree.
TABLE 2. Expandability of microcapsules measured by TMA.

Sample  [T.sub.s]   [T.sub.max]  [D.sub.max] ([micro]m)
        [degrees]C  [degrees]C

  1         181         225                735
  2         178         229                913
  3         177         224               1235
  4         176         218                398
  5         179         214                106


[FIGURE 4 OMITTED]

The dimension change, [D.sub.max], reached a maximum for the microcapsules with shells made from polymer sample #3. This indicates that the microcapsule made from polymer sample #3 showed the highest expandability. Figure 4b shows the relationship between tan [delta] at frequency 0.045 rad/s and the dimension change. The result showed that there was an optimal tan [delta] value for the capsule expandability.

From the TMA measurements, it could be said that the capsule made from the polymer sample #3 provided the highest expandability. The behavior of the capsule might be different in practice from the TMA data. The TMA measurements were conducted under atmospheric pressure, but in practice, capsule expansion occurs in a polymer under some stress or in contact with other physical foaming agents such as [CO.sub.2]. Thus, we further investigated the expandability under a stress by using a view cell. The stress was created by pressurizing the cell with high-pressure [CO.sub.2] after placing the microcapsule in the view cell. Figure 5 shows the change in the average capsule diameter against time in the foaming experiments. The experiments were conducted by heating the microcapsules from room temperature to 200[degrees]C under 6 MPa [CO.sub.2], and then [CO.sub.2] was released to atmospheric pressure 1 min after the temperature reached 200[degrees]C. Some capsules were not expanded, and the expandability of some capsules was reduced under the pressurized [CO.sub.2]. The samples #1, #2, and #3 showed the higher expandability even under some stress. However, the microcapsules made from polymer sample #1 started shrinking before [CO.sub.2] pressure was reduced, and their expansion was not observed even when pressure was reduced. The microcapsules made from polymer sample #2 showed the biggest expansion among the five kinds of microcapsules in the experiments. However, it was broken and shrunk after the secondary expansion induced by releasing [CO.sub.2]. As a result, the capsule size became comparatively small 30 min after the pressure reduction. As for microcapsules made from polymer samples #4 and #5, they showed little shrinkage even after depressurization. However, the expandability was reduced, producing the smallest final capsule size.

[FIGURE 5 OMITTED]

The diameter of unexpanded capsules and maximum and final diameters of the final expanded capsules were also shown in Table 3.
TABLE 3. Diameter of expanded microcapsules measured by visual
observation (6 MPa [CO.sub.2] at 200[degrees]C and depressurization).

Sample    Unexpanded particle    Max expanded        Final expanded
                  Size           particle size    particle size after
                                  ([micro]m)      reducing pressure

  1               35.0               152                   91
  2               34.5               212                  113
  3               34.2               178                  151
  4               35.1               130                  113
  5               37.9                97                   78


Figure 6 shows the relationship between tan [delta] and the final and maximum capsule diameters obtained in the visual observation experiments. Tan [delta] has an optimal value in terms of expandability even in the presence of stress. The microcapsules made from polymer sample #2 provided better expandability from the viewpoint of maximum expansion. The microcapsules made from polymer sample #3 provided better expandability from the viewpoint of final expansion.

[FIGURE 6 OMITTED]

It could be concluded that the modification of microcapsule shell properties by crosslinking has a trade-off relationship between shrinkage protection and expandability enhancement. The microcapsule made from the polymer sample #3 could be a compromised solution in the trade-off relation.

In addition, the visual observation experiments in contact with [CO.sub.2] elucidated the possibility of improving expandability with the coexistence of [CO.sub.2] and hydrocarbon in the microcapsule. As 6 MPa [CO.sub.2] pressure was higher than the vapor pressure of the hydrocarbon foaming agent, no expansion could be expected in the visual observation experiments. However, as can be seen in Fig. 5, the microcapsules were expanded by heating even at 6 MPa [CO.sub.2] pressure. It can be considered that [CO.sub.2] diffused into the microcapsules, mixed with the hydrocarbon, and then promoted the expansion. The coexistence of [CO.sub.2] and hydrocarbon foaming agent can be realized by performing foam injection moldings or extrusion using microcapsules together with [CO.sub.2] gas or chemical blowing agents that liberate [CO.sub.2] gas.

The relationship between tan [delta] and expandability was further examined by conducting the foam PP injection molding and foam PP extrusion experiments. The foaming experiments were conducted without using extra blowing agent such as [CO.sub.2]. The density and the surface appearance of foam injection and those of foam extrusion are summarized in Tables 4 and 5. Figures 7 and 8 show the relationship between tan [delta] at the low frequency of 0.045 rad/s and the density of foams. The foams were made by injection molding and extrusion at 200 and 220[degrees]C, respectively. In foam injection molding, the highest density reduction was achieved at 200[degrees]C by the microcapsules made from polymer sample #2. The density reduction was 34-36% at 200[degrees]C for injection molding. The microcapsules made from the polymer sample #3 could provide the second highest expansion. In the foam extrusion process, the highest density reduction was achieved at 220[degrees]C by the microcapsules made from polymer sample #2. The highest density reductions were 39% at 200[degrees]C and 44-46% at 220[degrees]C. In the foam extrusion process, the microcapsules with the degree of crosslinking of 65% expanded well, which corresponds to the microcapsules with tan [delta] of 0.42. By comparing Figs. 7 and 8 with Table 1 and Fig. 6, it can be seen that the optimal tan [delta] was the same, i.e., 0.42, in all foaming processes except for the case of injection molding at 220[degrees]C. The optimal tan [delta] for injection molding at 220[degrees]C was 0.3.
TABLE 4. Density of PP foams made by injection molding with thermally
expandable microcapsules (Back pressure: 10 MPa, Injection speed: 160
mm/see, Injection pressure: 110 MPa, Injection time: 0.46 sec).

                                200 [degrees]C

Sample  Density (g/[cm.sup.3])  Density reduction (%)  Appearance

  1             0.620                   31.1           Orange peel
  2             0.577                   35.9           Orange peel
  3             0.594                   34.0           Smooth
  4             0.656                   27.1           Smooth
  5             0.679                   24.6           Smooth

                               220 [degrees]C

Sample  Density (g/[cm.sup.3])  Density reduction (%)  Appearance

  1             0.794                   11.8           Orange peel
  2             0.709                   21.2           Smooth
  3             0.677                   24.8           Smooth
  4             0.741                   17.7           Sink mark
  5             0.781                   13.2           Sink mark

TABLE 5. Density of PP foams made by extrusion with thermally
expandable microcapsules.

                                200 [degrees]C

Sample  Density (g/[cm.sup.3])  Density reduction (%)  Appearance

  1             0.625                   30.6             Rough
  2             0.552                   38.7             Rough
  3             0.614                   31.8             Smooth
  4             0.627                   30.3             Smooth
  5             0.798                   11.3             Glossy

                                220 [degrees]C

Sample  Density (g/[cm.sup.3])  Density reduction (%.)  Appearance

  1             0.504                    44.0             Rough
  2             0.486                    46.0             Rough
  3             0.692                    23.1             Smooth
  4             0.641                    28.8             Glossy
  5             0.802                    10.9             Glossy


[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The sample #2 could provide the better performance than the sample #3. Because of the breakup and shrinkage of the capsules were prevented by the matrix polymer surrounding the capsule. Temperature dependency of microcapsule's expandability in injection foaming was different from extrusion foaming. The injection molding at temperature 200[degrees]C could provide better expandability than that at 220[degrees]C, but the extrusion molding at 220[degrees]C could give better results. We assumed that some microcapsules could be broken by higher shear rate at high-temperature condition. Thus, the higher shear rate in the injection molding might cause lower expandability of foams, i.e., lower density reduction at 220[degrees]C.

The increase in G' and decrease in tan [delta] could not increase the expandability monotonically at temperatures over 200[degrees]C. In other words, the microcapsules made from polymer sample #2, which had a degree of crosslinking of 65%, could be chosen as the best expandable capsule from the injection molding and extrusion data. However, from the viewpoint of surface appearance, the microcapsules made from polymer sample #2 were not suitable. The samples #1 and #2 showed orange-peel or rough surface as described in Tables 4 and 5.

The poor surface appearance was due to leakage of the gas from the surface, shrinkage, collapse, and rupture of microcapsules. The microcapsules made from polymer samples #4 and #5 showed sink marks or glossy surface. The microcapsules made from polymer sample #3 showed a smooth surface. As a result, the microcapsules made from polymer sample #3 could provide a compromise solution between density reduction and good surface appearance. This corresponds to the results of the final microcapsule diameter obtained by visual observation under 6 MPa [CO.sub.2] at 200[degrees]C followed by a pressure quench, as shown in Fig. 6. This also corresponds to the results obtained from TMA measurements, as shown in Fig. 4b.

Figure 9 shows some pictures of orange peels that appeared on the surface of foam injection molding products. The orange peels were often observed on the surface of sample #2 foams, whereas they were seldom observed on the sample #3. Figure 10 shows pictures and SEM images of the foam extrusion products made from both sample #2 and #3. As seen in SEM images and pictures, the sample #2 gives rougher surface than the sample #3. The surface was roughened due to aggregation of ruptured microcapsules. From the viewpoint of appearance, the sample #3 is more appropriate microcapsule for PP foaming.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

CONCLUSIONS

We developed a new microcapsule suitable for use at high-temperature foaming process. Controlling the rheological properties, G', and tan [delta] of the shell polymer through the degree of crosslinking could significantly affect the expandability and shrinkage of microcapsules. The relationship between tan [delta] of the shell polymer and the expansion behavior was thoroughly investigated via TMA expansion curves, foaming behavior under pressurized [CO.sub.2] obtained by visual observation, and density measurements of the foams produced by both injection and extrusion processes. The relationships obtained by these different types of experiments were very similar. They showed the existence of an optimal tan [delta] or optimal degree of crosslinking as a compromise solution between shrinkage protection and expandability enhancement as well as between expandability enhancement and surface appearance improvement. By using the developed microcapsules, greater than 30% density reduction could be achieved while keeping a smooth surface in PP foam injection molding and extrusion processes.

It was also found in this study that the use of the microcapsules and the physical foaming agent [CO.sub.2] together were able to improve expandability.

To design the optimal microcapsule suitable to the process and operating conditions, a better understanding of the relationship between expandability and shrinkage with the viscoelasticity of the polymer was very useful.

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Yasuhiro Kawaguchi, (1) Daichi Ito, (1) Yoshiyuki Kosaka, (2) Masazumi Okudo, (2) Takeshi Nakachi, (3) Hiroshi Kake, (3) Jae Kyung Kim, (4) Haruo Shikuma, (4) Masahiro Ohshima (4)

(1) Speciality Chemicals Division (Tokuyama) High-Performance Functional Plastics Company, Sekisui Chemical Co., Ltd, 4560, Kaisei-Cho, Shunan, Yamaguchi 746-0006, Japan

(2) Speciality Chemicals Division (Minase) High-Performance Functional Plastics Company, Sekisui Chemical Co., Ltd, 2-1, Shimamoto-Cho, Mishima-Gun, Osaka 618-0021, Japan

(3) Department of Polymer Production, Tokuyama Sekisui Co., Ltd, 4560, Kaisei-Cho, Shunan, Yamaguchi 746-0006, Japan

(4) Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan

Part of the paper was presented at the ANTEC meeting (Chicago), 2009.

Correspondence to: Masahiro Ohshima; e-mail: oshima@cheme.kyotou.ac.jp

DOI 10.1002/pen.21595

Published online in Wiley InterScience (www.interscience.wiley.com).

[C]2009 Society of Plastics Engineers
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Author:Kawaguchi, Yasuhiro; Ito, Daichi; Kosaka, Yoshiyuki; Okudo, Masazumi; Nakachi, Takeshi; Kake, Hirosh
Publication:Polymer Engineering and Science
Article Type:Report
Date:Apr 1, 2010
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