CA1209767A - Process for producing foamed plastic articles - Google Patents

Abstract

A B S T R A C T

An improvement is disclosed in a method for producing foamed articles of thermo-plastic or thermo-setting compounds. The improvement reduces the number of steps normally followed to make a foamed product and cuts down on the time. The method comprises intensively mixing and thermokinetically heating a particulate com-position comprising thermo-plastic resin particles and a foaming agent, the foaming agent having a trigger tempera-ture to initiate gas evolution to effect foaming, the mixing and heating occurring in an enclosed mixing cham-ber with a plurality of blades attached to arms rotating about an axis, with a blade tip speed of at least about 18 meters per second, continuing the mixing in the enclosed chamber until the composition is heated to a mixing temperature at or above the trigger temperature of the foaming agent, the mixing temperature being suffi-cient to effect fluxing of the particulate composition to an extent such that the fluxed composition has a viscosity sufficient to contain gas evolved from the foaming agent; discharging the fluxed composition from the mixing chamber when the fluxed composition is at the mixing temperature and when the foaming agent is just beginning to foam; and feeding the discharged fluxed composition to a mold and confining the composition within the mold to mold the composition while foaming of the composition continues, to form a molded foamed plastic article in the mold.

Description

IMPROVED PROCESS FOR PRODUCING FOAMED PLASTIC ARTICLES

The present invention relates to a process for producing foamed articles of thermo-plastic and thermo-setting compounds~
Synthetic polymers generally fall into two classes, thermo-plastic polymers which can be converted from solid to liquid state by heating, and upon cooling resume their previous physical characteristics without undergoing change in chemical composition, and thermo-setting polymers which cannot be converted from thc solid to the liquid form by heat without undergoing some fornt of chemical change.
Virtually every polymer that has ever been made has at some time or another been foamed. Foaming occurs by dispersing gas directly or from chemical foaming agents into the material when it is in viscous form. When the plas-tic material solidifies the gas is either totally incapsulated in discrete cells within the material,-or is unconfined so it can move through interconnected passages in the plastic material. These two types of foaming are known as the so-called closed cell or open cell types of foams.
Foamed materials have many uses including insula-tion,packaging, structural and decorative parts and flotation devices. In some instances foamed materials reduce material costs by produciny more buIk per unit weight. Flexible foams may be used in cushioning, filters, packaging, 2S clothing and many other uses. The densities of foam plastics vary over a wide range. An example of a typical density range-is from 1.6 to 960 kilograms per cubic meter.
Flame retardant versions of foams are readily available.
One method o~ foaming plastic materials is by mixing or injecting the compounds with volatile liquid~ or gases, which expand during processing. The mixing generally has to include a blending step to ensure uniform distribution t~67 throughout the mixture of ~he compounds before processing.
Another method of foaming is by intimately mixin~ a chemical foaming ayent with the polymer. This foaming agent e~olves a gas at a pred~termined trigger temperature and expands in the viscous polymer. However, all the known foaming processes have certain process disad~antages. One such disadvantage is the requirement that the molding of the foamed product usually has to be done under high pressures. Another disadvantage in molding or otherwise shaping foamed plastic articles is the necessity during most of the process to maintain the compounds under the trigger temperature of the foaming ager.t, and then heat the compounds further after they have been placed in the mold or other shaping de~ice to initiate evolution of gas from the foaming agent. ~n flexible foam applications, one method is to disperse PVC compounds in a liquid plasticizer.
Foaming is then accomplished by a chemical foaming agent, - or by mechanical frothing. In both cases, however, high pressures and long cycle times in the final stages of ~ forming tend to make these methods expensive. Flexible foamed PVC compounds may also be made in extruders from special grades of PVC compounds. Flexible foam applications include such items as door seals, gaskets, cable insulation, etc.
Most known processes of making a foamed article include four skeps; the mixing step, wherein the compounds are mixed with a oaming agent, for PVC this includes a milling process to ensure a uniform mixing throughout the mixture, then a mold filling step wherein the mixture is 3Q placed in molds followed by a heaking and cooling cycle in a press under high pressures and generally for a time in excess of 30 minu~es. Foaming occurs in this final step in the mold which can last for as long as 60 minutes.
Thus, for example, the time to make a foamed PVC article may be in excess of 2 hours.

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The present invention provides a simple and rapid process for the production o foamed thermo-plastic and thermo-setting articles. The process reduces the number of steps necessary in most of,the previously known techniques, avoids the high pressures usually necessary in the molds or other shaping devices and also the long cycle times that are required to produce foamed thermo-setting and thermo-plastic articles.
By using a high in-tensity mixer, it has been found that the plastic compounds and foaming agent can be mixed and fluxed in a very short time cycle and immediately formed in a mold or other shaping means at lower operating temperatures an~ lower pressures than previously thought possible.
The present invention provides a method of making a molded foamed thermo-plastic article from a particulate thermo-plastic resin comprising: intensively mixing and thermokine-ticall~ heating a particuIate composition com-prising thermo-plastic resin partlcles and a foaming agent, the foaming agent having a trigger temperature at which a gas is evolved therefrom to effect foaming, the mixing and heating being effected by mixing the particulate composition in an enclosed mixing chamber with a plurality of blades attached to arms rotating about an axis within said chamber with a blade tip speed of at least about 18 meters per second, continuing the mixing of the particulate composition in the enclosed chamber until the particulate composition is heated to a mixing *emperature at or above the trigger temperature of,the foaming agent, the mixing temperature being suficient to efect fluxing o the particulate com-position to an extent such that the fluxed composition hasa viscosity sufficient to contain gas evolved from the foaming agent; discharging the fluxed composition from the mixing 'cham~er when the fluxed' composition is at the mixing temperature and when the foaming agent is just beginning to 6~

foam; and eeding the discharged fluxed composltion to a mold and confining the composition within the mold to mold the composition while foaming of the composition con-tinues, to form a molded plastic article in the mold.
In further embodiments of the invention, the cqm-pounds and the foaming agent may be fed into the mixing chamber at the same time, or the foaming agent may be added to the mixing chamber after commencement of mixing the com-, pounds. When the plastic compounds are rigid or flexible polyvinyl chloride or a low denslty polyethylene resin, a preferred foaming agent is a~odicarbonamide, or para, para-oxybis (benzenesulfonylhydrazide). The blade tip speed in the mixing chamber is preferably in the range of about '3I - 38 m~ters per second, and the predetermined tempera-ture is preferabl~ in the range of about l90 - 240~C. The batch produced in this preferred range generally reaches this predetermined temperature in about 7 - 20 seconds. In other embodiments the forming means includes a molding means, shaping means, extruder, or a press which operates under a pressure in the range of about 550'- 2200 kilo-pascal , 'and the time the composition is confined in the mold is not more than about 5 minu~es.
High intenslty mixers may be used in the fluxing of thexmo-plastic and thermo-setting materials. One type of high intensity mixer is shown by Goeser et al in U.S.
patent 3,266,738 published August 15, lg66. This patent describes a high intensiky mixer available on the market toda~ ~nder the trade name Drais-Gelimat. The mixer includes a plurality of,blades which rotate about an axls within an enclosed container. In U.S. patent 4,230,615 published October 28, 1980, in the name of Crocker et al, a s~stem is provided for monitoring the batch tempera-ture in a high intensity mixer separately from the mixer tem-perature, and then discharging the ba-tch from the mixer when the batch temperature reaches a final predetermined level.
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In column 5 of this patent at line 34, reference is made to utilizing foaming agents added late in the mixing process to avoid premature -Eoaming in the mixer, and the ~atch tem-perature being set to discharge the batch just below foaming temperature. However, in the present invention it has been found that the foaming agents do not have to be added late in the mixing process, bu~ in some cases may be added at the commencement of the mixiny process. Furthermore, in the present invention the batch of thermo-plastic resin particles and foaming agent is heated to a temperature at or above the trigyer temperature at which a gas is evolved.
Another method of controlling the amount of mixing is disclosed in U.S. patent 4,142,804 issued March 6, 1979 to Crocker. This patent discloses a method of controlling lS the state of flux of a batch issuing from a high intensity mixer by monitoring vibration in the mixer and discharging the batch from the mixer when the vibration reaches a preset level representing a particular state of flux.
In drawings which illustrate embodiments of the invention, the figure is a diagr~mmatic side elevation cross-sectional view of a high intensity mixer and mold suitable for the method of the present invention.
In a series of ~ests, batches of thermo-setting or thermo-plastic compounds were added to a high intensity mixer, together With a foaming agent and processed in a Drais-Gelimat high intensity mixer, which had a blade tip speed above 18 meters per second. In some cases the foaming agent was added at the same time as the compounds, in other cases, the Gelimat had a screw conveyor feeding the compounds into the interior of the mixing chamber, and the foaming agent was added via the screw conveyor after the mixing step had commenced. The total time in the mixer did not exceed 30 seconds, although the weight of the batch fed into the mixer varied from 100 gms to 30~ gms depending on the siz~ of the batch requlred.

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The figure indicates diagrammatically apparatus suitable for the method of the present invention. A high intensity mixer lO, such as that shown by Goeser in U.S. patent 3,266,788, includes an enclosed mixing chamber ll with blades 12 rotating on arms 13:about shaft 14.
The batch is fed into the entry port 15 to a screw conveyor 16 which feeds into the mixing chamber 11. When the de-sired temperature of the batch is reached, a hydraulic cylinder 17 activates discharge flap 18, and the contents drop through a funnel l9 directly into mold 20. The mold 20 is moved from beneath the mixer, and a lid 21 placed over the mold 20 so that the mass is con~ined within the mold.
In a preferred embodiment the temperature of the batch within the mixer was determined by measuring the infra-red radiation from that batch so that the temperature of the batch was measured separately from the temperature of the mixer and mixer blades in accordance with the system and method disclosed in U.S. patent 4,230,615. The temperature of the batch was set to control the instant of discharge.
The temperature was selected so that the plastic compounds as discharged from the high intensity mixer had fluxed and were in a viscous state which could be fed into a mold or other shaping means for forming into a f.inished article.
The temperature of the batch at discharge was higher than the trigger temperature o~ the foaming agent so that foam-ing or evolution of gas from the foaming agent had been initiated in the high intensity mixer before the batch was discharged from the mixer and yet had not progressed extensively due to the short time before discharge from the mixer. As soon as the batch was discharged from the mixer it was immediately and without undue delay fed into a mold and then pressed and contained in the mold until foaming was completea and the article cooled although foaming had already started in the mixèr when the tri.gger temperature $7~

of the foaming agent was reached and the bàtch had a vis-cosity sufficient to contain the gases evolved from the foaming agent. The trigger temperature has to be high enough to initiate evolution of gas from the foaming agent at about the same time or after fluxing of the compound has reached a sufficient state of viscosity to contain the gases of evolution. If the trigger temperature is too low, the evolved gases dissipate into the atmosphere and are not contained in the compound. The batch, as discharged from the mixer, may be placed in a mold, an extruder or other shaping means. If a foamed plate or sheet is to be maae, then the fluxed batch is placed between two flat surfaces, contained and compressed. The pressure between these flat surfaces need not exceed 2200 kilopascals, and the press may be unheated because the temperature of the fluxed material is sufficiently high to ensure that foaming is maintained. Foaming then continues until the material fills the mold and commences to harden. Because the material foams in unheated molds and no exterior heat is necessary, there is no limitation as to the thickness of the material that may be molded. Articles up to almost any thickness may be foamed inasmuch as the heat comes from the interior of the fluxed material and not from exterior surfaces. In some cases the mold surfaces or other shaping devices may be preheated to ensure that there is no excessive cooling of the material adjacent to the mold surfaces. Since foam~
ing occurs by decomposition of the chemical foaming agent to release gases over the period of time that the chemical reaction occurs, loss of available gas for foaming can be limited by precise control in milliseconds of the instant of discharge of the foaming batch and rapid feeding of this foaming batch to the shaping device.
Two types of foaming agents were used in experi-ments illustrating the process of the present invention.

;7 r - ~ 8 An azodicarbonamide sold under the trade mark Ficel AC/4 which has a trigger temperature in the range of 190 -220C. This trigger temperature ~an be lowered to 170C
by me-tal sralt stabili~ers often present in flexible PVC
compounds.
The second foaminy agent is a para, para-oxybis (benzenesulfonylhydrazide) sold under the trade mark Celogen orr. This foaming agent has a trigyer temperature in the range of 13~ - 160C and most efficiently this trig~er temperature was above 150C. This particular foam-ing agent is recommended for injectlon and compression molding in PVC and low density polyethylene. It is parti-cularly useful for obtaining celluIar structures with thin or thick non-eellular skins.
The compounds used in the tests were, a rigia PVC
powder idqntified by the trade name CTR-12D whlch contains 0.5% Ficel-ACJ4 ~oaming agent, a rigid PVC compound used in film manufacture referred to as CGP-llO0, a rigid vinyl siding compound referred to as BPV5 and a riyid PVC piping ~ compound containing 300 parts o 366 resin, 1~8 parts of TM692, 1.2 parts of CAST ~, 3.0 parts of Klo25 and 6.0 parts of Atomite~* Another plastic compound was a plasti-cized PVC product known as R7203A. A low density poly-ethylene coded EXPY ~34-175 had a melt ~ndex of 4.4 and was also foamed.

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7~7 Example 1 As may be seen from the ~able, 300 grams of rigid PVC compound identified as C~R-12D was processed in a high intensity mixer, the foaming agent was present at 0.5~ by weight of the compounds, the high intensity mixer had a blade tip speed of 38 meters per second, and no cooling of the mixer was included. ~n this first preliminary~test the foaming agent was already premixed with the compounds when they were added to the mixer although not necessarily in the optimum amount, and the discharge temperature was 215C. After discharging from the mixer, approximately half the ~atch was formed in a press with a temperature of about 180C and at a pressure of 2200 kilopascals. The resuI-ting foamed product had a specific gravity of 1.29 as compared to an unfoamed value of 1.35, indicating that some foaming had occurred. In the second test an additional 1.5 grams o~ foaming agent was added to the 300 grams of com-pounds after the batch in the mixer~reached a temperature of 220C. The discharge temperature was 240C and the 20: proauct after being formed had a specific gravity of less than one because it floated in water.
Example 2 ~ Specific gravities of approximately 0.7 were ; obtained with foams made from CGP-llO0 compounds with Ficel AC/4 foaming agent. The foaming agent was added in amounts of 0.3 - 1.2% by weight of the compounds. In other examples carbon black and titanium dioxide were added to the compounds without affecting the foaming properties. Pressures in the press as low as 550 kilo-3Q pascals were satisfactory to produce the foamed product.In one particular simplified test, 170 grams of the com-pounds, plus 1 gram of the foaming agent were formed in a slightly preheated shoe sole mold to a density less than one by simply having a person stand on the retaining top plate of the mold. A slight preheat of the mold was 6~7 needed to remove the heat sink effect of this rather heavy metal mold.
The foamea rigid PVC material responded favourably to most normal wood working and joining techni~ues. It can be drilled, sawed, planed, sanded~ routed and otherwise shaped. It also showed excellent retention of integrity as evidenced by the ability to reset numerous times large and small screws without loss of holding strength.
Examplé 3 A rigid PVC formuIation used for vinyl siding foamed well with 0.'6%'foaming agent added 12 seconds after' commen'cement of the high lntenslty mixing cycle. The measured specific gravity of the foamed sheet was 1.17 indicating a marked reduction in the value of 1.49 for the fluxed material without oaming agent.
Example 4 A'rigid PVC piping compound foamed well under the tabuIated conditions to give a specific gravity of just under l.O'as contrasted with the unfoamed value of 1.38.
Exa~ple 5 This flexible PVC compound has a specific gravi~y of 1.21 in the unfoamed state and readily foamed under a pressure of approximately 550'kilopascals to a measured specific gravity of o.713.
Example 6 .

- Polyethylene wa5 foamed with the Celogen OT
foaming agent, having a lower triggering temperature then previously used. This produd-t was foamed to a specific gravity of 0.65 as compared to the value of 0.'90 for the same compound ~ithout a foaming agent put through the high intensity mixer at the same conditions.
Precise temperature control of this process by means of being able to discharge the ~atch from the high intensity mixer' at the instant that the batch reaches a prec'ise temperature,'allows the condition where the foaming agent is just beginning to start foaming, even though the compound is at or near the melt state. If the foaming agent is added just before the batch reaches its dlscharge temperature, then minimum foaming occurs in the high inten-sity mixer, and most of the foaming occurs as soon as theheated batch is discharged and is being formed into the desired shape.
The foaming step of the present invention may be applied to other known processes used to feed various shaping means whereby the foaming is presently initiated by a further subsequent heating step~ Various amendments may be made to the process deflned herein without departing from the scope of the present invention, which is limited only by the scope of the claims.

Claims (11)

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

1. A method of making a molded foamed thermo-plastic article from a particulate thermo-plastic resin comprising:
intensively mixing and thermokinetically heating a particulate composition comprising thermo-plastic resin particles and a foaming agent, said foaming agent having a trigger temperature at which a gas is evolved therefrom to effect foaming, said mixing and heating being effected by mixing said particulate composition in an enclosed mixing chamber with a plurality of blades attached to arms rotating about an axis within said chamber with a blade tip speed of at least about 18 meters per second;
continuing said mixing of said particulate com-position in said enclosed chamber until said particulate composition is heated to a mixing temperature at or above said trigger temperature of said foaming agent, said mixing temperature being sufficient to effect fluxing of said particulate composition to an extent such that the fluxed composition has a viscosity sufficient to contain gas evolved from said foaming agent;
discharging the fluxed composition from said mixing chamber when the fluxed composition is at said mixing temperature and when said foaming agent is just beginning to foam; and feeding the discharged fluxed composition to a mold and confining the composition within said mold to mold the composition while foaming of said composition continues, to form a molded foamed plastic article in said mold.

2. A method according to claim 1 wherein said foaming agent and said resin particles are introduced into said mixing chamber at the same time.

3. A method according to claim 1 wherein said foaming agent is present in an amount of about 0.5 to 1.2% by weight of the composition.

4. A method according to claim 1 wherein said resin particles comprise a resin selected from the group consisting of polyvinyl chloride and low density poly-ethylene.

5. A method according to claim 4 wherein said blade tip speed is about 31 - 38 meters per second.

6. A method according to claim 4 wherein said foaming agent is selected from the group consisting of azodicarbonamide and para-oxybis (benzenesulfonylhydrazide).

7. A method according to claim 5 wherein said mixing temperature is about 190 to 240 °C.

8. A method according to claim 7 wherein said mixing and heating is effected for about 7 to 20 seconds.

9. A method according to claim 1 wherein said composition is confined within said mold under a pressure of 550 - 2200 kilopascals.

10. A method according to claim 9 wherein said composition is confined within said mold under said pressure for not more than 5 minutes.

11. A method according to claim 1 wherein said foaming agent is introduced into said mixing chamber after said resin particles are introduced therein.