Process for producing cordierite honeycomb structural body and honeycomb structural body molding aid

There is provided a process for obtaining a cordierite honeycomb structural body with thin cell walls and no molding defects such as cell breakage or the like. Cell breakage is prevented by limiting the maximum particle size of the cordierite starting material powder to no greater than 85% of the slit width of the extrusion molding die so that the starting material particles will not clog inside the slits or the introduction port of the slits. As one cordierite starting material, talc with a mean particle size of 5 .mu.m or greater is used to give a honeycomb structural body with a void volume of greater than 30%, for production of a honeycomb structural body, with good moldability, having a small thickness and a low heat capacity. It is preferred to add to the starting material at least a lubricant/humectant, a binder and/or a mixture of a water-soluble polyhydric alcohol derivative and a polyhydric alcohol.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a process for producing a cordierite
 honeycomb structural body used as a catalyst carrier for an exhaust gas
 purification catalyst in an automobile engine such as an internal
 combustion engine, and to a molding aid used for molding of the honeycomb
 structural body.
 2. Description of the Related Art
 With the toughening of exhaust gas standards for automobile engines in
 recent years, there has been a demand for more rapid activation of exhaust
 gas purification catalysts in order to reduce hydrocarbon emissions
 immediately after engines are started. One means of rapid activation of
 catalysts that has been considered is to lower the heat capacity by
 reducing the thickness of the cell walls in cordierite honeycomb
 structural bodies acting as catalyst carriers, but with narrowing of the
 cell walls, an inconvenience has resulted as the cell walls break during
 extrusion molding of the honeycomb structural body. This occurs because
 coarse grains in the starting material of the honeycomb structural body
 clog the lattice-like slits or introduction port of the extrusion mold for
 molding, thus inhibiting provision of the starting mixture, and this
 requires prior removal of the coarse grains in the starting material.
 As concerns the particle size of the starting material powder, Japanese
 Unexamined Patent Publication No. 8-112528 teaches that lattice defects of
 molded bodies can be reduced by limiting the ratio (maximum particle size
 of starting material powder)/(slit width of extrusion mold for molding) to
 1/3. However, because honeycomb structural bodies formed under these
 conditions have low void volume, the effect of reduced heat capacity is
 less significant, while the catalyst carrier property is also weaker. In
 addition, because of a larger thermal expansion coefficient, there is also
 a problem of lower thermal shock resistance.
 Narrowing the cell walls of a honeycomb structural body also tends to
 result in molding defects known as cell wrinkles in the honeycomb
 structural body. These will be explained below. When a honeycomb
 structural body is extrusion molded, the starting mixture is first molded
 into a round bar form, and the round bar is extrusion molded into a
 honeycomb form. A screw-type tug mill such as shown in FIG. 1A is usually
 used for the round bar molding, in order to obtain a homogeneous round bar
 of the starting mixture for molding. A screw-type tug mill has upper and
 lower level screws 1, 2, and is provided with a vacuum chamber 3 and a
 strike-through roller 4 between the upper level screw 1 and the lower
 level screw 2. A resistance plate 5 is fitted in front of the lower level
 screw to create a uniform flow of the starting mixture and, as shown in
 FIG. 1B, the resistance plate 5 has a structure with a plurality of round
 holes 5 opened in a disk. The starting mixture which is kneaded by the
 upper and lower screws 1, 2 and passes through the resistance plate 5 is
 thus converted into a plurality of bar-shaped bodies which are introduced
 into a round bar mold 6 and are bonded together into a round bar as the
 cylinder size of the mold 6 narrows toward the tip.
 The starting mixture used for molding of the round bar has conventionally
 been a cordierite-converted starting material of talc, kaolin, etc. with a
 water-soluble polyhydric alcohol added as a molding aid, but the cohesion
 is insufficient between the starting mixture after it has passed through
 the resistance plate 5 of the screw-type tug mill, and a starting mixture
 interface corresponding to the shape of the resistance plate 5 is formed
 on the round bar. This starting mixture interface presents almost no
 problem when molding a ceramic honeycomb structural body with a cell wall
 thickness of 100 .mu.m or greater, and causes no visible molding defects.
 When the cell wall thickness is less than 100 .mu.m, however, it has been
 found that cell wrinkles are generated at the sections corresponding to
 the starting mixture interface, wherein the cells of the ceramic honeycomb
 structural body ripple in the direction of extrusion. It is thought that
 this is caused because the thin cell wall results in a higher molding
 pressure, leading to precipitation of moisture, etc. at the starting
 mixture interface and greater flowability of the starting mixture near the
 interface; thus the difference in the flowability at the other sections
 where the flowability of the starting mixture does not change produces a
 change in the cell formation rate of the ceramic honeycomb structural
 body, thus leading to generation of the cell wrinkles.
 Japanese Unexamined Patent Publication No. 7-138076 discloses a method of
 adding emulsified wax and methyl cellulose, as molding aids for reduced
 frictional resistance between the starting mixture and the mold wall
 surface, to improve molding defects such as stripping of the outer
 perimeter surface or cell wrinkles in ceramic honeycomb structural bodies.
 With this method, however, it has not been possible to eliminate the
 starting mixture interfaces on round bars, and thus a difference in
 flowability between the starting mixture interface and the other sections
 is produced. Consequently, while some effect of fewer cell wrinkles is
 seen by lowering the frictional resistance between the starting mixture
 and the mold wall surface, it is not possible to completely eliminate cell
 wrinkles.
 The prior art processes, therefore, have been associated with the problem
 of molding defects such as cell breakage and cell wrinkles when the cell
 wall thicknesses of cordierite honeycomb structural bodies are reduced. It
 has also been necessary to position the catalyst carrier as close as
 possible to the engine in order to take advantage of the engine exhaust
 gas temperature for rapid activation of the catalyst. However, positioning
 the catalyst carrier close to the engine leads to the problem of cracking
 due to thermal shock as the catalyst carrier is exposed to sudden
 temperature variations. Improving the thermal shock resistance of the
 catalyst carrier requires a reduction in its thermal expansion
 coefficient, and specifically, to prevent cracking near the engine it is
 necessary for the cordierite honeycomb structural body to have a thermal
 expansion coefficient of 1.0.times.10.sup.-6 /.degree. C. or smaller.
 It is therefore an object of the present invention to obtain a honeycomb
 structural body with a low cell wall thickness, with good moldability,
 wherein cell breakage is prevented without reducing the void volume and
 cell wrinkles caused by the starting mixture interface formed on the round
 bar are prevented, and to obtain a honeycomb structural body with
 excellent thermal shock resistance having a thermal expansion coefficient
 of 1.0.times.10.sup.-6 /.degree. C. or smaller.
 The first aspect of the invention, designed to solve the problem of cell
 breakage when the cell wall thickness has been reduced, is a process for
 producing a honeycomb structural body composed mainly of cordierite which
 comprises adding a molding aid to a powder of a cordierite starting
 material, kneading the mixture and extrusion molding it with an extrusion
 molding die with honeycomb-shaped slits, and then firing it, the process
 being characterized in that the maximum particle size of the powder of the
 cordierite starting material is limited to no greater than 85% of the slit
 width of the extrusion molding die, at least talc is used as the
 cordierite starting material, and the mean particle size thereof is 5 m or
 greater.
 If the maximum particle size of the cordierite starting material is smaller
 than the slit width of the extrusion molding die the starting material
 particles should not clog between the slits or at the slit introduction
 port; however, clogging in fact occurs if it is only slightly smaller than
 the slit width. The present inventors have found cell breakage due to
 clogging of the starting material particles can be eliminated if the
 particle size of the starting material is restricted so that the maximum
 particle size is no greater than 85% of the slit width. However, if the
 particle size of the starting material is simply reduced, the void volume
 is also smaller and an effect of lower heat capacity by thickness
 reduction cannot be achieved. For a lower heat capacity it is preferred
 for the void volume to be greater than 30%, and according to the first
 aspect talc with a mean particle size of at least 5 .mu.m is used for this
 purpose. Talc forms voids by being melted during firing, thus providing an
 effect of increased void volume. Thus, while talc with a small particle
 size will disappear by contraction during the firing, if the mean particle
 size of the talc is at least 5 .mu.m the disappearance of voids can be
 prevented, to give a honeycomb structural body with a void volume of
 greater than 30%.
 In addition, the thermal expansion coefficient of the cordierite honeycomb
 structural body can be controlled by utilizing the orientation of the
 plate-crystal talc particles lined up along the cell walls of the
 honeycomb during molding of the honeycomb, and it has been found that a
 larger mean particle size results in easier orientation and a smaller
 thermal expansion coefficient. Specifically, if the mean particle size of
 the talc is at least 5 .mu.m, it is possible to limit the thermal
 expansion coefficient of the cordierite honeycomb structural body to no
 greater than 1.0.times.10.sup.-6 /.degree. C., to thus provide increased
 thermal shock resistance.
 Thus, according to the process of the first aspect, it is possible to avoid
 cell breakage without reducing the void volume, while it is also possible
 to lower the thermal expansion coefficient to 1.0.times.10.sup.-6
 /.degree. C. or smaller. It thus becomes possible to obtain an easily
 moldable honeycomb structural body with thin cell walls, a good catalyst
 carrying property, a low heat capacity and excellent thermal shock
 resistance.
 A lubricant/humectant is preferably added as a molding aid at 2-5 wt % to
 100 wt % of the cordierite starting material. Addition of a
 lubricant/humectant further improves the effect of preventing cell
 breakage. This is because insertion of a substances with low frictional
 resistance between the starting material particles increases the distance
 between the starting material particles, having the effect of lowering the
 frictional resistance and preventing clogging of the starting material
 particles in the slits, and therefore the lubricant/humectant is
 preferably added at a total of 2 wt % or greater. However, if the amount
 of the lubricant/humectant added is too great the hardness of the starting
 mixture will be lowered, thus rendering it difficult to maintain the shape
 of the molded honeycomb structural body. Thus, a range of 2-5 wt % is best
 to achieve both lower frictional resistance and shape retention.
 It is preferred to add a binder as the molding aid at 3-9 wt % to 100 wt %
 of the cordierite starting material. Addition of a binder will also
 provide an effect of reducing the frictional resistance to prevent
 clogging of the starting material particles and thus prevent cell
 breakage, similar to addition of the lubricant/humectant. The binder may
 be added in an amount in the range of 3-9 wt % to achieve this effect with
 shape retention.
 The molding aids used to overcome the problem of cell wrinkles when the
 cell wall thickness is reduced are preferably a mixture of a water-soluble
 polyhydric alcohol derivative and a polyhydric alcohol, and they are
 preferably added so that the mixing ratio is represented by the following
 equation:
 mixing ratio=polyhydric alcohol/(water-soluble polyhydric alcohol
 derivative+polyhydric alcohol) is in the range of 0.895-0.995.
 Addition of the mixture of the water-soluble polyhydric alcohol derivative
 and the polyhydric alcohol with this mixing ratio to the cordierite
 starting material will improve the cohesion of the starting mixture when
 shaping the round bar for molding of the honeycomb structural body. It
 will thus become possible to eliminate the starting material interface in
 the round bar to prevent generation of cell wrinkles caused thereby. The
 mixing ratio may be at least 0.895 in order to achieve this effect, but if
 the mixing ratio exceeds 0.995 the hardness of the starting material will
 be lowered thus reducing the shape retention property which holds the
 shape, and leading to generation of warps and the like; the mixing ratio
 should therefore be in the range of 0.895-0.995.
 Thus, according to the method of using a water-soluble polyhydric alcohol
 derivative and a polyhydric alcohol in this proportion, it is possible to
 eliminate the starting material interface in the round bar and to mold a
 honeycomb structural body with a narrow cell wall thickness without
 producing cell wrinkles.
 The second aspect of the invention is a molding aid added to the starting
 material for a honeycomb structural body during molding of the honeycomb
 structural body, characterized by comprising a mixture of a water-soluble
 polyhydric alcohol derivative and a polyhydric alcohol such that the
 mixing ratio represented by the following equation:
 mixing ratio=polyhydric alcohol/(water-soluble polyhydric alcohol
 derivative+polyhydric alcohol) is in the range of 0.895-0.995.
 By using a molding aid containing the mixture of the water-soluble
 polyhydric alcohol derivative and polyhydric alcohol for molding of a
 honeycomb structural body, such as a cordierite honeycomb structural body,
 it is possible to improve the cohesion of the starting material for
 molding of the honeycomb structural body and prevent generation of cell
 wrinkles caused by the interface of the starting mixture on the round bar.
 If the mixing ratio of the water-soluble polyhydric alcohol derivative and
 polyhydric alcohol is within the range specified above, it is possible to
 eliminate the interface of the starting mixture on the round bar and thus
 prevent generation of cell wrinkles in the honeycomb structural body in
 cases where the honeycomb structural body has narrow cell walls.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 The present invention will now be explained in detail. A honeycomb
 structural body fabricated according to the invention has a theoretical
 composition represented by 2MgO.multidot.2Al.sub.2
 O.sub.3.multidot.5SiO.sub.2, and it usually contains, as the main
 component, cordierite with a composition comprising a ratio of 49.0-53.0
 wt % SiO.sub.2, 33.0-37.0 wt % Al.sub.2 O.sub.3 and 11.5-15.5 wt % MgO.
 The cordierite honeycomb structural body is obtained by adding and
 kneading the aforementioned molding aids with the cordierite starting
 mixture prepared with the desired cordierite composition, and then molding
 and firing it in a honeycomb shape.
 Here, at least talc (Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2) is used as the
 cordierite starting material and, in particular, if its mean particle size
 is at least 5 .mu.m, the void volume of the honeycomb structural body can
 be increased to over 30%. If the mean particle size of the talc is smaller
 than 5 .mu.m the voids formed by melting of the talc during firing will
 disappear by contraction during the firing, making it impossible to obtain
 an effect of increased void volume. Also, if the mean particle size of the
 talc is 5 .mu.m or greater it is possible to limit the thermal expansion
 coefficient of the cordierite honeycomb structural body to no greater than
 1.0.times.10.sup.-6 to thus provide improved thermal shock resistance.
 Cordierite starting materials other than talc are not particularly
 restricted, and kaolin (Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4), alumina
 (Al.sub.2 O.sub.3), aluminum hydroxide (Al(OH).sub.3) and the like are
 suitable for use. In addition to these there may also be used Mg-based,
 Al-based and Si-based oxides, hydroxides and chlorides. Such compounds
 include serpentine (Mg.sub.3 Si.sub.2 O.sub.5 (OH).sub.4), pyroferrite
 (Al.sub.2 Si.sub.4 O.sub.10 (OH).sub.2) and brucite (Mg(OH).sub.2).
 When these cordierite starting materials are used to fabricate a cordierite
 honeycomb structural body, the talc is first combined with the other
 cordierite starting materials to the desired cordierite composition.
 According to the invention, the maximum particle size of the cordierite
 starting material powder containing talc is controlled to be no greater
 than 85% of the slit width of the mold for extrusion molding. This can
 prevent clogging of the starting material particles at the slit
 introduction port or inside the slits, as well as occurrence of cell
 breakage during extrusion molding.
 Next, the molding aid is added to and kneaded with the cordierite starting
 material to prepare a starting mixture for extrusion molding. It is
 generally preferred to use a screw-type tug mill, as shown in FIG. 1, for
 the kneading and for molding into a round bar shape. The starting mixture
 shaped into a round bar is further extrusion molded into a honeycomb shape
 using a publicly known mold for extrusion molding. The molding aid used
 may be a common lubricant/humectant, or a binder, etc. Specifically, as
 lubricant/humectants there may be mentioned waxes, water-soluble
 polyhydric alcohol derivatives, surfactants, etc. and as binders there may
 be mentioned methyl cellulose, polyvinyl alcohol and the like.
 According to the invention it is possible to minimize cell breakage during
 extrusion molding by adding the molding aids within specified ranges.
 Specifically, the lubricant/humectant may be added in a range of 2-5 wt %,
 and the binder in a range of 3-9 wt %, to 100 wt % of the cordierite
 starting material. When added to the cordierite starting material, the
 lubricant/humectant or binder is dispersed between the starting material
 particles, thus reducing the frictional resistance to provide an effect of
 preventing clogging of the starting material particles in the slits. This
 effect is not achieved if the lubricant/humectant is added at less than 2
 wt % or the binder is added at less than 3 wt %. However, if the
 lubricant/humectant or binder is added in too great an amount the hardness
 of the starting mixture will be lower, making it difficult to retain the
 shape of the molded honeycomb structural body, and therefore they are
 preferably not added in excess of the ranges specified above. A
 lubricant/humectant and binder may be used in combination, and an
 excellent effect against cell breakage can be achieved when each is within
 the ranges specified above.
 According to the invention, a mixture of a water-soluble polyhydric alcohol
 derivative and a polyhydric alcohol may be used as a molding aid. For
 example, as a water-soluble polyhydric alcohol derivative there may be
 mentioned polyalkylene glycol, etc. and as polyhydric alcohols there may
 be mentioned glycerin, diethylene glycol, etc. In particular, the starting
 mixture can be given an improved cohesive property if the water-soluble
 polyhydric alcohol derivative and polyhydric alcohol are used in such a
 combination that the mixing ratio as represented by the following
 equation:
 mixing ratio=polyhydric alcohol/(water-soluble polyhydric alcohol
 derivative+polyhydric alcohol) is in the range of 0.895-0.995. This
 provides an effect which prevents formation of a starting mixture
 interface during molding of the round bar for molding of a honeycomb
 structural body, or generation of cell wrinkles during the honeycomb
 molding.
 If the aforementioned mixing ratio is smaller than 0.895, the cohesive
 property of the starting mixture as it passes and is pressed through the
 resistance plate of the tug mill will be insufficient, thus leaving a
 starting mixture interface in the round bar. A mixing ratio of greater
 than 0.895 can eliminate the starting mixture interface, but if the mixing
 ratio is greater than 0.995 the hardness of the starting mixture is
 reduced, resulting in lower shape retention leading to warping, etc. In
 other words, if the mixing ratio is in the range of 0.895-0.995 it is
 possible to eliminate the starting mixture interface while maintaining
 shape retention, so that it is possible to prevent cell wrinkles caused by
 the interface.
 A mixture of a water-soluble polyhydric alcohol derivative and a polyhydric
 alcohol mixed in the aforementioned mixing ratio also has an effect as a
 lubricant/humectant, and can therefore be used as a lubricant/humectant.
 In this case, the mixture of a water-soluble polyhydric alcohol derivative
 and a polyhydric alcohol as a lubricant/humectant may be added in the
 range of preferably 2-5 wt % to 100 wt % of the cordierite starting
 material to effectively prevent both the cell breakage and cell wrinkles
 described above.
 The honeycomb-shaped mold obtained in this manner is then fired at above
 the firing temperature of cordierite to obtain a cordierite honeycomb
 structural body. According to the process of the invention it is possible
 to fabricate a cordierite honeycomb structural body with thin cell walls,
 without resulting in cell breakage or cell wrinkles.
 The case described above was a molding aid for the first aspect comprising
 a mixture of a water-soluble polyhydric alcohol derivative and a
 polyhydric alcohol, used for molding of a cordierite honeycomb structural
 body; however, the molding aid is not limited to a cordierite honeycomb
 structural body and can of course be used for molding of other ceramic
 honeycomb structural bodies (second aspect).
 Examples 1 and 2, Comparative Examples 1 to 3
 A cordierite starting material was prepared by mixing 38 wt % of talc, 42
 wt % of kaolin, 5 wt % of alumina and 15 wt % of aluminum hydroxide, and
 then 2.8 wt % of a lubricant/humectant, 5.5 wt % of a binder and a
 suitable amount of water were added to 100 wt % of the cordierite starting
 material and kneaded to obtain a starting mixture. The mean particle size
 and maximum particle size of the talc and the maximum particle size of the
 other starting materials were as shown in Table 1. Here, a 5% aqueous
 solution of polyalkylene glycol was used as the lubricant/humectant, and
 methyl cellulose was used as the binder.
 The resulting starting mixture was extrusion molded using an extrusion mold
 with honeycomb-shaped slits. The slit width of the extrusion mold used was
 75 .mu.m. The resulting molded product was then fired in an electric
 furnace at 1390.degree. C. in an air atmosphere to fabricate a cordierite
 honeycomb structural body. The void volume and presence or absence of cell
 breakage in each of the resulting cordierite honeycomb structural bodies
 are listed in Table 1.
 TABLE 1
 Mean
 Thermal
 particle Maximum particle size
 expansion Void
 size (.mu.m) Aluminum
 Cell coefficient volume
 Talc Talc Kaolin Alumina hydroxide
 breakage (.times. 10.sup.-6 /.degree. C.) (%)
 Comp. Ex. 1 17.6 60.2 67.5 26.1 6.7 yes
 0.39 35.9
 Comp. Ex. 2 17.6 60.2 51.5 65.8 6.7 yes
 0.36 35.6
 Example 1 17.6 60.2 51.5 26.1 6.7 no
 0.38 34.3
 Example 2 6.1 45.3 51.5 26.1 6.7 no
 0.89 31.6
 Comp. Ex. 3 4.2 30.9 51.5 26.1 6.7 no
 1.11 27.4
 In Examples 1 and 2 wherein the maximum particle size of the starting
 material was no greater than 63 .mu.m and the mean particle size of the
 talc was at least 5 .mu.m as shown in Table 1, no cell breakage was
 observed and the void volume exceeded 30%. In contrast, cell breakage was
 observed in Comparative Examples 1 and 2 wherein the maximum particle size
 of the kaolin and alumina in the cordierite starting material was larger
 than 85% of the slit width of the extrusion mold (63 .mu.m). In
 Comparative Example 3 wherein the maximum particle size was less than 63
 .mu.m, no cell breakage was observed but the mean particle size of the
 talc was smaller than 5 .mu.m, and the void volume was less than 30%. In
 Comparative Examples 1 and 2 wherein the mean particle size of the talc
 was greater than 5 .mu.m, the void volume was greater than 30%. It was
 thus demonstrated that cell breakage can be prevented without reducing the
 void volume, if the maximum particle size of the starting material is no
 greater than 85% of the slit width and the mean particle size of the talc
 is at least 5 .mu.m.
 The thermal expansion coefficients of the obtained cordierite honeycomb
 structural bodies were also measured, and are listed in Table 1. Table 1
 shows that in Example 1 and Comparative Examples 1 and 2 wherein the mean
 particle size of the talc was as large as 17.6 .mu.m, the thermal
 expansion coefficients were small values under 0.4.times.10.sup.-6
 /.degree. C. Example 2 wherein the mean particle size of the talc was as
 small as 6.1 .mu.m had a thermal expansion coefficient of
 0.89.times.10.sup.-6 /.degree. C. which was larger than Example 1, but
 this was smaller than the usable limit of 1.0.times.10.sup.-6 /.degree. C.
 However, in Comparative Example 3 wherein the mean particle size was 4.2
 .mu.m, or smaller than 5 .mu.m, the thermal expansion coefficient was
 1.11.times.10.sup.-6 /.degree. C. which exceeded the usable limit of
 1.0.times.10.sup.-6 /.degree. C., and resulted in poor thermal shock
 resistance of the cordierite honeycomb structural body. FIG. 2 is a graph
 showing the relationship between the mean particle size of the talc and
 the thermal expansion coefficient, based on the results given above, and
 it shows that with a talc mean particle size of 5 .mu.m it is possible to
 limit the thermal expansion coefficient to no greater than
 1.0.times.10.sup.-6 /.degree. C. for improved thermal shock resistance.
 Next, talc, kaolin, alumina and aluminum hydroxide with the same mean
 particle size, maximum particle size and weight ratios as in Example 1
 were used as cordierite starting materials (see Table 1), changing the
 amount of lubricant/humectant added and the amount of binder added to 100
 wt % of cordierite starting material according to (condition 1) to
 (condition 5) listed in Table 2, and the effects thereof were examined.
 TABLE 2
 Amount added (wt %)
 Lubricant/humectant Binder
 .gradient.: Condition 1 1.8 2.7
 .diamond.: Condition 2 1.8 5.5
 .DELTA.: Condition 3 4.2 2.7
 .smallcircle.: Condition 4 2.8 5.5
 : Condition 5 5.2 9.5
 FIG. 3 shows the relationship between the number of moldings and the cell
 breakage when the resulting starting mixture was extrusion molded into a
 155-mm long honeycomb mold using an extrusion mold with a slit width of 75
 .mu.m.
 The results shown in FIG. 3 show that when the amount of
 lubricant/humectant added is less than 2 wt % and the amount of binder
 added is less than 3 wt % (condition 1), no cell breakage is observed with
 up to 2 moldings, but some cell breakage is observed with 3 or more. When
 the amount of lubricant/humectant added is less than 2 wt % and the amount
 of binder added is 3-9 wt % (condition 2), or the amount of
 lubricant/humectant added is 2-5 wt % and the amount of binder added is
 less than 3 wt % (condition 3), no cell breakage is observed with up to 4
 moldings, but some cell breakage is observed with 5 or more. When the
 amount of lubricant/humectant added is 2-5 wt % and the amount of binder
 added is 3-9 wt % (condition 4), absolutely no cell breakage is observed
 with up to 6 moldings. When the amount of lubricant/humectant added is
 greater than 5 wt % and the amount of binder added is greater than 9 wt %
 (condition 5), no cell breakage was observed with up to 6 molded rods, but
 the starting mixture becomes soft, and this led to easier deformation of
 the molded honeycomb structural body.
 Thus, it is preferred for the amount of lubricant/humectant added to be in
 the range of 2-5 wt %, and the amount of the binder added to be in the
 range of 3-9 wt %, for an excellent effect of preventing cell breakage. It
 is also clear that cell breakage is even further minimized if the
 lubricant/humectant and the binder are used in combination in the ranges
 specified above.
 Examples 4 to 6, Comparative Examples 4 to 8.
 An experiment was conducted to determine the effect of using mixtures of
 water-soluble polyhydric alcohol derivatives and polyhydric alcohols as
 molding aids with cordierite starting materials. The cordierite starting
 materials used were talc, kaolin, alumina and aluminum hydroxide with the
 same mean particle sizes, maximum particle sizes and weight ratios as in
 Example 1 (see Table 1), and then 2.8 wt % of a lubricant/humectant, 5.5
 wt % of methyl cellulose as a binder and a suitable amount of water were
 added to 100 wt % of the cordierite starting material, and kneaded to
 prepare a round bar-shaped starting mixture. The lubricant/humectant used
 was a water-soluble polyhydric alcohol derivative and polyhydric alcohol,
 or either alone, with the different mixing ratios listed in Table 3. A 5%
 aqueous solution of polyalkylene glycol was used as the water-soluble
 polyhydric alcohol derivative, and glycerin or diethylene glycol was used
 as the polyhydric alcohol.
 Each of the resulting round bars was extrusion molded using a mold for
 extrusion molding with a 75 .mu.m slit width, to obtain a honeycomb
 structural body. The presence or absence of starting mixture interfaces on
 the round bars and the shape retentions were examined, giving the results
 listed in Table 3. Here, the presence or absence of starting mixture
 interfaces on the round bars were evaluated based on whether or not
 non-continuous sections of the starting mixture were present when a 10-mm
 thick disk sliced from the round bar was bent. Samples with interfaces are
 indicated by "B", and those without by "G". The shape retention was
 evaluated by the state of peripheral deformation when the honeycomb
 structural body was molded. Samples with no deformation are indicated by
 "G" and those without by "B".
 TABLE 3
 Ceramic
 starting Methyl Water-soluble polyhydric Polyhydric
 Mixing Shape
 material cellulose alcohol derivative alcohol
 ratio Interface retention
 Comp. Ex. 4 100 wt % 5.5 wt % 5 wt % aqueous solution of --
 0.000 B G
 polyalkylene glycol
 derivative
 2.80 wt %
 Comp. Ex. 5 " " 5 wt % aqueous solution of glycerin
 0.870 B G
 polyalkylene glycol 0.70 wt %
 derivative
 2.10 wt %
 Comp. Ex. 6 " " 5 wt % aqueous solution of diethylene
 0.870 B G
 polyalkylene glycol glycol
 derivative 0.70 wt %
 2.10 wt %
 Example 3 " " 5 wt % aqueous solution of glycerin
 0.896 G G
 polyalkylene glycol 0.84 wt %
 derivative
 1.96 wt %
 Example 4 " " 5 wt % aqueous solution of glycerin
 0.979 G G
 polyalkylene glycol 1.96 wt %
 derivative
 0.84 wt %
 Example 5 " " 5 wt % aqueous solution of diethylene
 0.979 G G
 polyalkylene glycol glycol
 derivative 1.96 wt %
 0.84 wt %
 Example 6 " " 5 wt % aqueous solution of glycerin
 0.994 G G
 polyalkylene glycol 2.52 wt %
 derivative
 2.80 wt %
 Comp. Ex. 7 " " -- glycerin
 1.000 G B
 2.80 wt %
 Comp. Ex. 8 " " -- diethylene
 1.000 G B
 glycol
 2.80 wt %
 In Comparative Example 4 wherein the 5% aqueous solution of polyalkylene
 glycol was added at 2.8 wt % and no polyhydric alcohol was added, with a
 mixing ratio of 0 as shown in Table 3, the shape retention was
 satisfactory but the presence of an interface resulted in cell wrinkles at
 the sections corresponding to the interface. In Comparative Example 5
 wherein the 5% aqueous solution of polyalkylene glycol was added at 2.1 wt
 % and glycerin at 0.7 wt % as the polyhydric alcohol, with a mixing ratio
 of 0.870, the round bar interface was reduced but was not sufficiently
 minimized, and cell wrinkles corresponding to the interface were observed
 in the honeycomb structural body molded using the round bar.
 In contrast, in Example 3 wherein the 5% aqueous solution of polyalkylene
 glycol was added at 1.96 wt % and glycerin at 0.84 wt %, with a mixing
 ratio of 0.896, no non-continuous sections were found in starting mixture
 upon bending of a disk sliced from the round bar, and the interface in the
 round bar disappeared. Also, no cell wrinkles corresponding to the
 interface were observed when the round bar was used for a honeycomb
 structural body. Examples 4 and 6 wherein the mixing ratio was from 0.895
 to 0.995 also exhibited no cell wrinkles corresponding to the round bar
 interface, and shape retention was maintained.
 However, when the mixing ratio exceeded 0.995, for example in Comparative
 Example 7 with a mixing ratio of 1 wherein no 5% aqueous solution of
 polyalkylene glycol was used and glycerin was added 2.8 wt %, no cell
 wrinkling was observed corresponding to the round bar interface, but the
 starting mixture was soft resulting in low shape retention, and therefore
 deformation occurred in the honeycomb structural body. FIG. 4 shows the
 relationship between the mixing ratio and the starting mixture hardness,
 where it is seen that a larger mixing ratio with glycerin results in a
 gradually softening starting mixture hardness, but with a mixing ratio of
 greater than 0.995 it becomes drastically softer, contributing to lower
 shape retention of the honeycomb structural body. The hardness of the
 starting mixture was determined, as shown in FIG. 5, by filling a holder
 11 having an inner diameter of 24 mm and a depth of 25 mm with about 20 g
 of the starting mixture, completely burying a steel ball having a diameter
 of 4 mm attached to the tip of a push-pull gauge 13, moving the holder in
 the direction of the steel ball at a rate of 2 mm/min with a micro feeder,
 not shown in the figure, to thereby press the starting material to the
 steel ball, and then reading the starting mixture hardness as the load
 indicated by the gauge after one minute.
 The same results for the mixing ratio with polyalkylene glycol, the
 presence or absence of the round bar interface and changes in the shape
 retention of the honeycomb structural body were obtained even when
 diethylene glycol was used instead of glycerin as the polyhydric alcohol
 (Example 5, Comparative Examples 6 and 8).