CA1255866A

CA1255866A - Method and apparatus for production of foamed thermoplastic material

Info

Publication number: CA1255866A
Application number: CA000514389A
Authority: CA
Inventors: Hiromu Fujisaki; Motoshige Hayashi; Motokazu Yoshii
Original assignee: Sekisui Kaseihin Kogyo KK
Current assignee: Sekisui Kaseihin Kogyo KK
Priority date: 1985-07-24
Filing date: 1986-07-22
Publication date: 1989-06-20
Also published as: ES2001581A6; FR2585282A1; DE3624909C2; CN1007717B; PT83040A; AT394518B; PT83040B; FR2585282B1; KR870001021A; DE3624909A1; GB8618082D0; KR900003735B1; CN86105207A; GB2179886A; ATA199286A; GB2179886B

Abstract

ABSTRACT OF THE DISCLOSURE
A method and apparatus for the production of a foamed thermoplastic material. A cavity transfer mixer is provided between a screw extruder which melts, or fuses, the thermoplastic material, and a cooler. The cavity transfer mixer enables the formation of uniform and well mixed admixtures of a foaming agent and a thermoplastic material and, optionally, other desired additives.

Description

~2 ~5 ~6 , METHOD AND APPARATUS FOR PROI)UCTION
OF FOAMED THERMOPLASTIC MATERIAL
BACKGROUND OF T~E INVENTION
Field of the Invention This invention relates to a method and apparatus for producing foamed thermoplastic materials by homogeneously mixing thermoplastics with a foaming agent and other desired additives and continuously extruding the mixture.
Description of the Prior Art There are various methods for producing foamed thermoplastic materials and extruders are now in wide use for this purpose. The producing of foamed thermoplastic materials using an extruder is practically advantageous in that, aEter a thermoplastic is homogeneously mixed with a foaming agent or other additive under pressure, the mixture is extruded under low pressure to continuously manufacture foamed thermoplastic materials of desired shapes, e.g., sheets or plates.
In the above method of manufacture, however, importance has been attached to the fact that a foaming agent or other additive should homogeneously be mixed with the molten therm~plastic and the mixture extruded after the molten plastic composition containing the foaming agent is uniformly cooled sufficiently to be suitable for foaming.
As a result, there have been proposed various methods and apparatus for homogeneously mixing a ther~oplastic with a foaming agent or the like and uniformly coollng the plastic composit~on containing the foaming agent. U.S. Patent Mo.
3,751,377 discloses a method and apparatus whe~ein a static ~25S~3~6 1 mixer is installed following an extruder having a screw in order to mix the molten plastic with a foaming agent and uniformly cool the mixture to a temperature fit for foaming.
Although such an arrangement has succeeded in producing thick, lar~e, low-density foamed materials to a certain extent, it has drawba~ks in that the extrusion rate is reduced because the flow resistance of the plastics to the static mixer is strong, the s~:atic mixer is partially deformed or damaged particularly when :it is cooled from the outside and the dispersion properties of the mixed foaming agent are insufficient.
Attempts have been made to remedy such shortcomi.ngs.
As disclosed in U.S. Patent No. 4,454,087, a rotary mixing cooler and a ~îgzag mixer as a kind of static mixer are simult:aneously installed following the extruder. Although the ~ig2ag mixer is prevented from deforming in.this method because it is not cooled, there is still a desire to manufacture further low-density foamed materials and those in which the improvea homogeneous dispersion of an additive is ensured.
On the other hand, U.S. Patent No. 4,419,014 discloses a method for homogeneously mixing molten plastics, rubber and the like with other additives, wherein an extruder-mixer is equipped with a cavity transfer mixer directly coupled to the front end of the screw contained in the extruder.
The present inventors have conceived of the application of the cavity transfer mixer used in the above extruder-.mixer to the manufacture of foamed thermoplastic ~Z ~8 ~

1 materials and found that further improved foamed thermoplastic materials can be manufactured by introducing the cavity transfer mixer into the conventional extrusion process.
SUMMARY OF THE I~3VENTION
The present invention is directed to solving the problems heretofore posed in the manufacture of foamed thermoplastic materials based on the above-described knowledge of the prior art. According to the present invention, in a method and apparatus for the production of foamed thermoplastic materials, the apparatus comprising an extruder for melting and e~truding the thermoplastic material and a cooling unit for cooling the molten thermoplastic containing a foaming agent to a temperatu-re suitable for foaming, an improvement is provided wherein a rotor is supported in a stator between the extruder and the cooling unit, the gap between the statcr and the rotor being used as a passageway for the molten thermoplastic and wherein a number o~ isolated cavities are respectively ormed on the inner face of the stator and the outer face of the rotor arranged opposite thereto, the cavities on both the stator and the 3:0tor overlapping one another during their rotations and forming a mixer by causing the molten plastic to be transferred between the cavities. The molten plastic is mixed with the foaming agent or any other additive in the upstream side of the mixer.
An object of the present invention is to provide a method a~d apparatus capable of homogeneously mixing a molten ~2 55~ 6~
1 ~hermoplastic wit~ a large amount of foaming agent so that highly foamed, low-density, thic~ thermoplastic materials are obtained.
Another object of the present invention is to pro~ide a method and apparatus capable oE ensuring the improved homogeneous dispersion of a foaming agent or other additive in a mo]ten plastic, so that uniformly foamed thermoplastic materials having excellent physical properties and quality are obtained. Moreover, ~oamed materials having a homogeneous disp~rsion of a nucleat ~ agent such as f~ powdery talc cells and excellent post processability are also obtainable.
Still another object of the present invention is to provide a method and apparatus capable of homogeneously mixing a plurality of incompatible thermoplastics, so that many kinds o~ ~lerm~ plastics beoome muxable- M~veover, foa~ materials having desired properties can be manufactured.
A further object of the present invention is to provide a method and apparatus capable of homogeneously mixing 2Q thermoplastics with a foaming agent which is considered not easily mixable therewith. This is advantageous to the - manufacture of foamed materials since a foaming agent can be selected from many kinds of materials.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a vertical sectional view of an embodiment of the apparatus of the present invention.
Figs. 2a-h a~e schematic views illustrating the :. - , ' ~

. , .

~2 ~ ~6~

1 principles of mixing and agitation using a cavity transfer mlxer.
Fig. 3 is a vertical sectional view of another embodiment of the apparatus of the present invention.
Fig. 4 is a vertical sectional view of a modified cooling unit of tlle apparatus of Figs. 1 and 3.
Fig. 5 is a view taken on line V-V of Fig. 4.
Fig. 6 is a vertical sectional view of still another embodiment of the apparatus of the present invention~
DETAILED DESCRIPTION OF THE INVF,NTI3N
Thermoplastics which can be subjected to extrusion-~oaming according to the present invention are not parti~ularly _lmited. Representative thermoplastics include polystyrene, styrene-acrylonitrile copolymer, styrene-acrylonitrile-butadiene copolymer, styrene~maleic anhydride copoly~er, styrene-ethylene copolymer, poly-~-methylstyrene, polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, polyvinyl chloride, polymethyl methacrylate, polyamide, etc.
These copolymers may be used independently or in combination.
Since relatively incompatible poly~ers can may be uniformly mixed according to the present invention, a wide range;-o~ ~ermo plastics can be selected. Accordingly, thermoplastics having desired physical properties are readily producible.
Foaming agents which can be used in the present invention are also not limited. Volatile or decomposable foamlng agents are normally used.

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~25S~366 1 As volatile foaming agent tllere may be ~entioned aliphatic hydrocarbons such as propane, butane, isobutane, pentane, neopentane, isopentane~ and the like; alicyclic hydrocarbons such as cyclobutane, cyclopentane, cyclohexene, and the like; methylchloride; methylene chloride;
dichlorofluoromethane; chlorotrifluoromethane;
dichlorodifluoromethane; chlorodifluoromethane;
trichlorofluorome-thane; trichlorotrifluoroethane; and dichlorotetrafluoroethane. As decomposable foaming agents there may be mentioned dinitrosopentamethylenetetramine;
trinilrosotrimethylenetriamine; p,p'-oxybis(benzene sulfonyl hydra~ide); azodicarbonamide, and the like. These foaming agents may be ~lsed independently or in combination.
~ A fnaming agent is added on the upstream side of a mixer according to the present invention. The normal mixing methocl comprises heating and melt~ng thermoplastics i~ an extrucler and supplying a foaming agent to the extruder under press~Lre. Another method is supplying thermoplastics containing a foaming agent to an extruder.
When a large amount of foaming agent is mixed in order to obtain a low-density foamed material, e.g., 100 parts by weight of thermoplastic mixed with 5-50 parts by weight of a volatile foaming agent, it is preferred to pre-knead the molten thermoplastic and a foaming agent in an extruder or by any other mixing means and then supply the mixture to a mixer.
~ccording to the present invention, an additive normally used is added when the foamed material is made.

~552'3~i6 1 As aclditives, there are nucleatin~ agents retardants, stabilizers, lubricants, plasticizers, coloring agents, fillers, etc.
Extruders for use in melting and extruding thermoplastics accordi.ng to the present invention are single or twin screw extruders, whose screws are preferably equipped with pins or other mixing means after the foaming agent is supplied under pressure.
Cooling units usable according to the present invention are those developed and conventionally used for the manufacture of foamed thermoplastic materials J such cooling units being provided with heat exchanging means and being capable of regulatin~ the temperature of various plastics. It is preferred to employ a cooling unit having a rotary shaft lS equipped with vanes inside the outer cooling cylinder as disclosed in U.S. Patent Nos. 4,~54,087 and 2,669,~51 and Japanese Patent Nos. 544/73 and 42026/79. It is also preferred to use a cooling extruder which is larger in dia~eter than the eY~I~er for melting the thermo plastics so as to uniformly cool the thermo plastics by slowly turning it with the screw with a smaller pumping action.
Mixers usable according to the present invention are cavity transfer mixers for mel~ing thermo plastics having rotor supported in stator the gap between the stator and the rotor being used as a passageway for the ~olten plastics. A number of isolated cavities are formed on the inner face of the stator and the outer face of ~ ~5~ ~6 1 the rotor , respectively. The cavities formed on the stator and the rotor are positioned so that they overlap one another during rotation. The cavities may be semi-spherical, cylindrical and rhombical but should preferably be a semi-spherical shape which causes little congestion of molten plastics. The cavities should be cross-stitched (refer to the drawings) on the inner face of the stator and the outer face of the rotor in the respective axial and circumferential directions. The total area of the opening of the ~avities should be increased, in terms of the transfer face thereof, to over 60% of the area of the inner face of the ~-sta-tor or the outer face of the rotor The mixer shou:!d be positioned in a location where the temperature and pressure are highest during the extrusion-foaming process.
Accordingly, the mixer may be coupled to the front end of the screw of the extruder so that the mixer may rotate synchronously with the screw or installed so that the m~xer can rotat:e independently. In case of the latter, heating and mixing are conveniently regulated because the number of rotations is freely controllable according to the kind of thermoplastics and kind of foaming agents and other additives.
In other words, the mixer may be rotated at a low speed when a fire retardant which is subject to heat decomposition and a fibrous material readily cut upon the application of high shear force are used and may be rotated at a high speed when a volatile foaming agent having a viscosity different from that of the plastic is employed.
As dies usable according to t~e present invention, ~s~

l there are ordinarily T-shaped dies, coat hanger dies, Eish -tal~dies, circular dies, etc. with sizing dies as occasion demands.
The construction and efEects of the present invention will now be described in concrete terms according to selected em~odiments of the invention.
Referring to Fig. 1, an embodiment o~ the apparatus of the present invention will be described. In Fig. 1, there is shown an arrangement of a main extruder 1, a cooling extruder 2 installed parallel to the main extruder 1 and having its a~is shifted from that of the latter and a mixer 3 installed between the extruders 1, 2.
The main extruder 1 comprises first barrel 4; a first cantilever screw 5 rotatably inserted in the first barrel 4; a first kneader means 6 installed at the front end o the first screw 5; a column 7 installed at the front end of the screw -shaft; a number of pins 8 which protrude from the periphery of the column 7; an inlet 9 for introducing a foaming agent under pressllre, the inlet 9 being positioned opposite to the boundary between the first screw 5 of the first barrel 4 and the first kneader means 6; an outlet 11 formed at the end where the screw of the first barrel 4`floats; an inlet 12 for materials, the inlet 12 being formed at the end where the screw of the first barrel 4 is supported; a material hopper 13 installed at the inlet 12 and heaters 14 installed on Lhe periphery of the first barrel 4.
~le coolinO e~truder 2 co~prisas a se^ond bar-ei 16 . .

~ 5~866 l having a spiral reErigerant passage~ay 17; an inlet 18 and an outlet 19 of the refrigerant passageway 17; a second cantilever screw 20 rotatably inserted into the second barrel 16; die 21 fixed to the end where the screw 20 of the second barrel 16 floats, the die 21 being equipped with an outlet forthe~ plas-tics;
a bearing 23 for rotatably supporting the base of the second screw 20; packing 24; a packing pusher 25; a passageway 26 of a refrigerant for cooli.ng the packing; an inlet 27 for the injection of khermo-plastics,th~inlet 27 being formed at the end where the screw 20 o:E the second barrel 16 is supported; and a pipe 28 for supplying a refrigerant to the inside of the second screw 20.
. The mixer 3 comprises a fixed cylinder 30; a cant~lever rotor 31 rotatably inserted in the fixed cyli:~der 30j' the axls thereof being perpendicular to those of the ~irst and second screws 5, 20; an inlet 32 formed on one side where the rotor 31 in the fixed cylinder 30 is supported and which communicates with the outlet 11 of the main extnlder l; an outlet 33 formed on the other side where the 2Q roto:- 31 in the fixed cylinder 30 floats and which communicates with the inlet 27 for the injection of thermo-plastics to th2 coolingextruder 2; a stator 34 installed on the inner face of the fixed cylinder 30; a number of isolated semi-spherical cavities 35, 36 for~ed on the inner face of the stator 34 and on the outer face of the rotor 31, the cavities 3S on the inner face of the stator 34 partially overlapping the cavities 3~ on the periphery of the rotor 31 during l rotation; a bearing 37 for ro~atably supporting the rotor 31; packing 38; a packing pusher a passageway 40 of a reErigerant for cooling the packing; a heater 41 arranged on the periphery of the fixed cylinder 30, and a pipe 42 for supplying a refrigerant to the inside of the rotor 3l.
lhe operation of the apparatus shown in Fig. l will now be described. By rotating the first and second screws 5, 20 in the directions of arrows A, B, and by rotating the rotor 31 separately from the first and second screws 5, 20 in the direction of arrow C, a thermoplastic.material is supplied from the material hopper l3 to the first barrel 4. The material, i.e., plastic is transported by the first screw 5 in the direction of arrow D and heated and caused to melt by the heater l4. A oaming agent is added through the inlet 9 to the molten plastic ~lnder pressure and the oaming agent added is pre-mixed with the thermo plastic by pins ~ ofthe first kne~der m.eans 6. The thermoplastic containing the foaming agent is subsequently passed through the outlet l1 and inlet 32 before being supplied into the fixed cylinder 30 of the mixer 3. In

2~. the mix~r 3, the ther o plasti~ is agitated and muxed by the cavities 36 o the rotor 31 and the cavities 35 of the stator 34 ar.d the foaming agent is homogeneously dispersed in the thermo plastic.
Referring to Figs. 2a-h, the typical mixing and agitating principle will be described using linear material.
The striation extruded from the bottom of the cavi~y 35 on the left of Fig. 2a is extended along the inner periphery .

1 thereof and, as shown in F'ig. 2b, the front end of the ~- ~-striation is pulled by the edge i of the cavity 36 oE the rotor 31 rotating in the direction of arrow C and caused to change its direction so as to conform to the state shown in Fig. 2c. As shown in Fig. 2d, the front end of the striation is folded by the edge ii and, as shown in Fig. 2e, the fron~ end thereof is cut by the edge ii and the stator 34 and, as sho~l in Fig. 2f, the front end thereof is bent by the edge iii and, as shown in Fig. 2g, the front end thereof is cut by the edge iii and the stator 34 and, as shown in Fig. 2h, the front end thereof is bent by the edge iv. The same operation is repeated thereafter and the front end of the linear material is successively cut and the cut portion is accumulated within the cavities.
According to this principle, the plastic material is extended to form a;thin plate and cut to pieces wherein a foa~ing agent or any other addicive is ho~ogeneously dispersed in the thermo plastics. ~hel~ plastic wi~ the foa~ng agent homog~neously dispersed therein is supplied to the second barrel 16 of ~he cooling extruder 2 through the outlet 33 and the inlet 270 The thermo plastic containing ~e foc~ming age~t is transported by the rotation of the second screw 20 in the direction of arrow E and is cooled by refrigerant passing through the refrigerant passage 17 to a temperature suitable for foaming. It is conveyed and extruded out of the outlet 22 for foaming purposes.
Re.erring to Fig. 3, another e~bod~ment o' the present invention will be described. In Fig. 3, like reference *2 5 ~ 6~
numbers designate like parts as shown in Fig. 1 and 2 description of these components will be omitted. In the apparatus of Fig. 3 a cooling unit 44 comprising a couplerl45 is coupled to the outlet of the stator 34 of the mi.xer 3; an intermediate body 46 is coupled to the coupler 45; an inner core 47 is provided in the center of the side of the intermediate body 46; an external cylinder 48 is concentrically fitted to the inner core 47 with one end coupled to the intermediate body 46; and a die 49 is fixed to the other end of the external cylinder 48. A passageway 50 for the therm~ plastic material formed between the inner core 47 and the external cylinder 48 communicates with the mixer 3 through a communicating passageway 51 formed in the coupler 45 and the intermediate body 46. A cooling space 52 is formed in the ~5 inner core 47 and an inlet 53 and an outlet 54 for refrigerant.
are passed from the periphery of the intermediate body 46 up to the cooling space 52 therethrough. A spiral refrigerant passageway 55 is formed in the external cylinder 48 and an orificè. 56 is formed in the ~ie 49. An L-shaped pipe 58 is used to communicate the outlet 11 of the main extruder 1 with the inlet 32 of the mixer 3. A static mixer 59 is arranged on the mixer end side of the communicating pipe 58 and a throttle nozzle 60 is arranged on the extruder side of the communicating pipe 58. Numeral 61 designates a heater arranged on the periphery of the com~unicating pipe 58~
The operation of the apparatus shown in Fig. 3 will now be described. The velocity of the molten plastic e~.truded :.

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~ S~ 6 l from the o~tlet ll of the extruder l is accelerated by the throttle nozzle 60 and the foaming agent supplied from the inlet 9 is added to the accelerated the~ plastic under pressure.
The thermo plastic containin~ ~e fo~ng agent is ~ansported to the static mixer 59 wherein the the~mo plastic and ~e ~oaming agen-t are preliminarlly mixed. Subsequently, -~e therm~ plastic containing the foam:ing agent is discharged from the static mixer 59 and is coveyed to mix~r 3 ~-here the therno plastic and the foaming agent are thoroughly kneaded under the same action as in the case o~ the firsl: embodiment. The plastic with the foaming agent homoc,eneously dispersed in the mixer 3 is then supplied to the thenm~ plastic passage 50 of the cooling unit 44. The therm~ plastic conta:~ing the:Fcaming agent is cooled in the ~erm~ plastic passage 50 b~ the refrigerant passing through the passageway 55 while movi~g in the direction of the arrow E and is then discharged from the outlet 56 for foaming.
In carrying out the method of the present invention, a cocler 63 as shown in Fig. 4 or the cooling unit 44 as shown in Fig. 3 may be used in place of the cooling extruder 2 shown in Fio. 1. The cooler 63 shown in Figs. 4 and 5 and installed on the downstream side of the mixer 3 according to the first and second embodiments of the invention is described in detail below. The cooler 63 comprises an outer cylinder 64 having a spiral refrigerant passageway 65; a cantilever main shaft 66 rotatably inserted in the outer cylinder 6~; a ~erm~ plastic inlet 67 formed through the neighborhood o~ the end where the main shaft of the outer cylinder 54 is supported; a die 68 wl.h an l4 ~>~ 6~

orifice 69; and a pipe 70 for supplying a refrigerant to a cooling space provided in the main shaft 66. The main shaft 66 comprises a large diameter end portion 66A rotatably supported on the outer cylinder 64 through a bearing 7.1, a central small diameter portion 66B and a large diameter ront end portion 66C, an annular protrusion 66D being installed in a pOSi!iOn slightly closer to the downstream side of the central s~al:l diameter portion 66~ facing the plastic inlet. A nymber of hllrdle type kneading bars 72 are protruded from a portion farther downstream than the annular protrusion 66D of the central small diameter portion 66B.
In the cooler 63 shown in Figo 4 an.d thus const:ructed, t~e plastic containing the foaming agent and introduced in the outer cylinder 64 from the mixer 3 through the F~lastic inlet 67 is forced over the annular protrusion 66D
in t~.e direction of arrow E and kneaded by the knea~.ing bars 72 of t~.e main shaft 66 rotating in the direction of arrow F and then extrudec from the orifice 69 through the gap between the larve diameter front end portion 66C and the outer cylinder 64.
Exa~ples 1, 2 and 3 and Comparative Example 1 . _ _ _ .
The apparatus of Fig. 1 was used in Examples 1, 2 and

3 and had the following specifications: an internal diameter of 50 mm of the first bar.rel 4 of extruder 1; an internal diameter o~ 65 mm of the second barrel 16 of cooling extruder 2; an internal diameter of 50 mm of the stator 34 of mixer 3; a gap of 0.4 mm be.tween the stator 34 and the ro.or 31; six cavities respectively in the circumferential directions of the s~ator 34 and the rotor ~ , .

~, .

~s~
l 31 and seven rows of cavities 35, 36 in the axial directions thereof; respective diameters of 23 and 24.5 mm of the semi-spherical cavities 35, 36; depths of 8 and 9.5 mm of the cavities 35, 36; distances oE 22 mm between the centers of the cavities in the axial direction; l06 rpm of the rotor of mixer 3; a temperature of 123C o~ the molten material at the exit of the cooling extruder 2; a width of lO0 mm and a ~ height of 1 mm of the orlfice 22 o~ the die 21.
i~ lO0 parts by weight of polystyrene [Styron 679 of Asah:~ Kasei ] as a base resin was uniformly mixed with 0.3 part by weight of fine powdery talc as a nucleating agent and 2.0 part~ by wei~ht of hexabromocyclododecane as a fire retardant and supplied to the main extruder l, which was operated so as to e~:trude the material at a rate of 55 kg per hour. In addition, l2.5 parts by weight of dichlorodifluoromethane were added as 2 Eoaming agent from the inlet 9 for the foaming agent to lO0 parts by weight of the base therm~ plastic under pressure. As a result, foamed pla~es about 250 mm wide, 25 mm thiclc and 40 kg/m3 in density were obtained through a sizer attached to the 2a. d~e 21 (see Table 1)O
The results shown for Examples 2 and 3 were obtained through similar tests carried out by changing the quantity of the foaming agent and the rpm of the rotor 310 The comparative example of Table 1 shows a. case where the mixer 3 is removed from the apparatus of Fi~. 1.

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-rl h al C ~ al ~ h QJ C C ~

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~a2~iS86~i Example 4 and Comparative Exam~ 2 The apparatus of Fig. 1 was used in this exaTnple and had the following specifications: an internal diameter of 50 mM of the firs t barrel 4 of extruder 1; an internal diameter of 5 65 mm of the second barrel of cooling extruder 2; an internal dia~eter o 90 mm of the stator 34 of mixer 3; a gap of 0 . 2 rnm between the stator 34 and the rotor 31; ten cavities respectively in the circumferential directions of the stator 34 and t:he rotor 31 and seven rows of cavities 35, 36 in 10 the axial directions thereof; respective diameters o~ 27 and 28 mm of the semi-spherical cavities 35, 36; depths of 8 and 9 . 5 mm of the cavities 35, 36; distances of 25 mm between the cent~rs of the cavities in the axial direction; 100 rpm of the rotor of mixer 3, .a temperature of 159 C of the molten 15 material passing through the cooling extruder 2, a diameter of 60 mm and a gap of 0. 6 mm of orifice 22 of the die 21 in the form of a circular slit.
100 parts by weight of polystyrene ~Styron 691 of Asahi. Kasei3 as a base thermo plastic was unifor~y m~ed with 2.0 20 parts by weight of fine powdery talc as a nucleat.ing agen-t ar.d supplied to the main extruder 1, which was operated so as to extrude the material at a rate of 28 kg per hour. In addition, 3 . 5 parts by weight of butane were added as a foaming agent from the inlet 9 for the foaming agen~ to 100 parts by weight ~5 of the base thermo plastic under pressure. As a result, uniformly minu~ely foam.ed sheets about 633 ~n wide, 25 mn thick and 179 Kg/m2 in the unit weight were cooled and sized through a cooling I[~drel attached to the die at the dis-tance and slit at one place in the extrusion direction.

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~z~ s 1 Mbreover, the foamed materials thus obta~ed were aged for seven days in ~e rcom te~era-ture and then the cell diameter was measured according to ASTM D 2842-69, wherein the materials were heated at 120C for 12 seconds for examining secondary foaming thickness as assessing theformability The results obtained are shown in Table 2.
A similar test with the mixer 3 removed from ~he apparatus used in Example 4, was conducted t:o obtain foamed sheets ab~lt 2 mm thick, 633 mm wide and 176 y/m2 in the unit weight adjusting the temperature of thermoplastic to 158C. However, a number of concentrated ~owdery talc were seen in the sheets with rough cells. Secondary foaming was also inferior.
Table 2 Diameter o cell (mm): Secondary foam Machine Traverse ~ertical direction direction direction thlc~ness (mm) Exa~ple 4 0.22 0.21 0.21 3.28 Comparative Exa~ple 2 0.28 0.31 0~27 3.04 Example 5 The apparatus used in Exa~ple 4 was used in this example with the exception that the die 21 and the cooling mandrell according to the Example 1 were use~.
In the apparatus thus arranged, polyethylene (Yukaron HE~30 made by Mitsubishi Yuka K.K.) was ~J used as a base th ~ o plastic. lO0 parts by weight of the base thermo plastic was mixed with lO parts by weight of a mutual impreg-nation polymer (Piocelan (trade mark) of Sekisui Kaseihin Kogyo K.K.) obtained by impregnating styrene monomer in a polyethylene plastic while polymerizing the monomer, which ~ ~`rc~ 4rk -, 19 .

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~'2 S''~
1 is co~,~sed of 30/O by ~eicJht of ethylene and 70% by~eight o~ styrene and has 18.6~ by weight of cross-l~ing ratio, ~hecross~ ng r~t:io;c~
for examble,be obtained by mP,asuring an amount of the insoluble co~ponent of the unit sample in boilin~ xylene, and Wl~ 0.5 part by ~-eight of talc as a nucleating agent. A m~ure ~us obtained was supplied to the ~x~der at a ra~e of 30 kg ~er hour.
On the other hand, l4 parts by weight of a mixture of 70% by weight of dichlorodi~luoromethane and 30% by weight of butane were supplied às a ~oaming agent under pressure. The extrusion was then carried out at 'l00 rpm for the mixer 3 and coo:ed a temr~ra-ture of llOaC for the molten thermo plastics passing through the cooling extruder for foaming purposes. The foamed materials obtained were externally fine, uniform foamed plates about 20 mm thlck, 230 mm wide and 33 kg/m3 in dénsity.
Comparative Example 3 This example was carried out according to the method of Example 5 except that the mixer 3 was removed from the apparatus. High density foamed spots due to inferlor mixing and dispersion of PIOC~N Resin and a number of foa~ing agent gas ac~mulations were occured. Only foamed plates having uneven surfaces were obtained.
Referring to Fig. 6, another emdobiment of the present invention will now be describ,ed. In Fig. 6, like reference characters designate like and corresponding parts of Figs. l and 4, wherein an extruder 1 and a cooler 63 are arranged in parallel but their axes are shifted from each other. A pipe 32a for supplying the thermoplastic material connects the outlet 11 of the extruder l and the injection port ' ' ' ' ' , ' . . .

~Z55~3fi6 67 of a cooler 63. Numeral 31 designat.es a rotor concentrically connected to a column 7 installed at the front end of a screw 5, and with a number of recessed isolated semi-spherical cavities 35, 36 respectively formed on the outer face of the rotor 31 and the inner face o:E a barrel 4 arranged opposite thereto. ~e cavities 35 of ~le ban eï 4 and ~he cavities of 36 of the rotor 31 respectively overlap one another therebetween to form a mixer where the plastic is transferred.
The length of the rotor 31 should be two to eight times (preferably four to eight times) as large as the diameter of the screw S . If the length is less than twice as large as the diameter, kneading will be insufficient, whereas heating will become too great if it is more than eight times the l~tter.
The length of the column 7 is normally one to seven times (preferably two to five times) as large as the diameter of the screw S. If the length is less than the diameter of the screw, pre-kneading will be lnsufficient, whereas the 2Q kneading effect will not be further improved if it is greater than :;even times.
The cross-sectional area where therm.o plastic is al10~7ed to pass ~hrough the column 7 is made greater than (preferably 1.5 to 3 times as large as) that at the front end of the screw 5.
Otherwise, the quantity of the ther~ plastic supplied to the col~r~ 7 will be excessive and make sufficient kneading impossible. As the protrusions 8 provided on the column 7, there are columnar -. 21 l pins, cutout screw ~ hts and Dulmage screws.
The screw 5 and the main shaft 66 are resyectively rotated in the directions of arrows A and F and the ~aterial, i.e., therm~ plastic is supplied to the barrel 4 throuyh the ;nlet 12.The th~ plastic is sent in ~he direction of arrow D by the screw 5 and heated and neltingby the heater l4 during that time. The foaming agent is added from the inlet 9 t.o the n~lten thermo plastic under pressure, whereby the foaming agen-t and the therm~ plastic are preliminarily mixed together. Subsequently, the thermo plas-tic containing the foaming agent is forced to enter the gap between the rotor 3l and the barrel 4 and is kneaded by the cavities 35, 36 so that the foaming agent may be uniformly dispersed in ~he thermo plastic.The kneadin~ principle is the sa~e as that described in conjunction with Fig. 2. The ~herm~ plastic wher~in the foaming agent has been disp'ersed uniformly is conveyed t~ the outer cylinder 64 of the cooler 63 through the supply pipe 32a and then in the direction of arrow E over the annu'lar prot.usions 66D. The thermo plastic is kneaded by hurd'le-shaped kneading bars rotating in the direction of arrow F and, after being appropriately cooled, is passed through the uap l)etween the large diameter front end portion 66C and the oute~- cylinder 64 and extruded from the outlet 69 for foaming.
Example 6 and Comparative Example _ The apparatus shown in Fig. 6 was used and had the following specifications: an internal diameter of 50 mm of the first barrel 4 of extruder 1; a diameter of S0 mm of the screw 5; a length of 250 mm of the colu;nn 7; a length or 250 mm oL

l the ro-tor 31; a gap of 0.4 mm between the ro-tor 31 and the ba~rel 4; six cavities respectively in the circumferential direction of the ro-to.r 31 and the barrel.

4 and seven rows of cavities 35, 36 in the axial directions thereof; a respective diar~ter of 23.0 and 24.5 mm of the se.~i-spherical caviti.es 35, 36; a ~epth of 3 and 9.S mm of-~e cavities 35, 35; a distance of 22 mm bet~,een the cavities in the axial directions; 106 rpm oE ~e scr~w 5, a te~-erature of 123C oE the ~olten material passing through the cool.er 63, a width of 100 mm and a height of 1 mm of the ori.fice 69 of the die 6~.
100 parts by weight of polystyrene [Styron 679 of Asahi.~asei] as a base ~lerm~ plastic was uniformly mixed with C.3 part by weight of talc as a nucleating age~ and , 2.0 parts by weight of hexabromocyclododecane as a fire retardant and supplied to the main extruder 1, which was operated so as to extru~e the material at a rate of 55 kg per hour. In addition, 12.5 parts by weight of dichlorodifluoromethane were added as a foami.~g agent from theinlet 9 for the foaming agent to 100 parts by we:ight of the base thermo plastic under pressure. As a result, foamed plates about 250 mm wide, 25 mm thick and 40 kg/m3 in densi y were obtained through a sizer attached to the die 21 (see Table 33.
In Comparative Example 4, the rotor 31 was removed to dispense with the cavities 35 of the barrel 4.

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' ~55~366 1~s is obvious from Table 3, uniformly foamed materials can be obtained according to the method and apparatus of the present inven~ion.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing a foamed thermoplastic material comprising melting and extruding said thermoplastic material by means of a screw extruder, adding a foaming agent to the molten thermoplastic; mixing said foaming agent and molten thermoplastic in a mixing means and cooling the resultant mixture of foaming agent and thermoplastic to a temperature suitable for foaming in a cooling means; said mixing means comprising a rotor supported in a stator so as to form a gap for the passage of the mixture of foaming agent and thermoplastic during rotation of the rotor; said rotor having semi-spherical cavities formed on the outer face thereof and said stator having semi-spherical cavities formed on the inner face thereof, the cavities on said rotor and the cavities on the stator being arranged to overlap with each other during rotation of the rotor and to cause transfer of the mixture of foaming agent and thermoplastic between the cavities during passage of the mixture through said mixing means.

2. A method for the production of a foamed thermoplastic material as claimed in claim 1, wherein the thermoplastic and the foaming agent are premixed before being further mixed in said mixing means.

3. A method for the production of a foamed thermoplastic material as claimed in claim 1, wherein the rotor of said mixing means is rotated independently the screw of said extruder.

4. A method for the production of a foamed thermoplastic material as claimed in claim 1, wherein the rotor of said mixing means is connected to the front end of the screw of said extruder.

5. A method for the production of a foamed thermoplastic material as claimed in claim 1, wherein said foaming agent is a liquid or solid at room temperature.

6. A method for the production of a foamed thermoplastic material as claimed in claim 1, wherein said mixture of foaming agent and thermoplastic is heated during mixing in said mixing means.

7. An apparatus for producing a foamed thermoplastic material comprising screw extruder for melting and extruding said thermoplastic material; mixing means communicating with said screw extruder for receiving a mixture of a foaming agent and molten thermoplastic from said screw extruder and for mixing said foaming agent and said molten thermoplastic material and cooling means communicating with said mixing

Claim 7 continued....

means for cooling the mixture of foaming agent and thermoplastic formed in said mixing means to a temperature suitable for foaming, said mixing means comprising a rotor supported in a stator so as to form a gap for the passage of the mixture of foaming agent and thermoplastic during rotation of the rotor; said rotor having semi-spherical cavities formed on the outer face thereof and said stator having semi-spherical cavities formed on the inner face thereof, the cavities on said rotor and the cavities on the stator being arranged to overlap with each other during rotation of the rotor and to cause transfer of the mixture of foaming agent and thermoplastic between the cavities during passage of the mixture through said mixing means, an injection port of a foaming agent being provided upstream of said mixing means.

8. An apparatus for the production of a foamed thermoplastic material as claimed in claim 7, wherein said mixing means further comprises a passageway for refrigerant provided at a portion wherein said rotor is supported in said stator.

9. An apparatus for the production of a foamed thermoplastic material as claimed in claim 7, wherein the

Claim 9 continued....

rotor of said mixer is capable of rotation independently of rotation of the screw of said extruder.

10. An apparatus for the production of a foamed thermoplastic material as claimed in claim 7, wherein the rotor of said mixer is connected to the front end of the screw of said extruder, and said injection port of a foaming agent is provided for the extruder.

11. An apparatus for the production of a foamed thermoplastic material as claimed in claim 10, further comprising preliminary kneading means between said screw extruder and said mixing means.