CA2185221A1

CA2185221A1 - Plastics foam and method of manufacturing same

Info

Publication number: CA2185221A1
Application number: CA002185221A
Authority: CA
Inventors: Graham Ramlu Sanyasi
Original assignee: Individual
Current assignee: Visy Industrial Plastics Pty Ltd
Priority date: 1994-03-11
Filing date: 1995-03-09
Publication date: 1995-09-14
Also published as: MX9603699A; EP0749455A1; PE22495A1; AU1941395A; EP0749455A4; CO4410417A1; IN188233B; WO1995024440A1; NZ282337A; JPH09509975A; BR9507102A

Abstract

A method of producing an extruded plastics foam of enhanced physical strength which comprises: (a) intimately mixing a blowing agent incorporating CO2 and in which the major proportion is a natural gas, in a plastics resin melt to form an homogeneous resin mix;
and (b) extruding the resin mix through an exit die into a region of lower pressure; wherein the temperature of the resin mix is adjusted so that it is below the critical temperature (as hereinbefore defined) at the point of extrusion out of the exit die.

Description

2 l 8 5 2 2 1 PCT/AU95/00~27 Pl ~STICS FOAM Al\lr) MFTHOD OF MANUFACTURI~G ~sA~r This invention relates to the production of a plastics foam which is formed utilising a non-fluu,u~d,~ol- blowing agent. The des~,i,uliull of the invention 5 herei"drLel generally refers to the use of carbon dioxide alone as the blowing agent.
This is only because this is the preferred agent. Other natural gases can be used with CO2 such as nitrogen, air and water.
Until recently the favoured agents for use in the yel)eldtiull of plastics extruded foams, have been fluolu~,dl~ulls such as dichlorofluuru,,,ell,d,,e and 10 trichlorofluo,u,,,etl,d,,e. In recent times, however, there has been substantial movement away from the use of such compounds as some scientific studies have indicated that flUulu~dlbOI15 when released into the: ' "u~,ulle,é may have damaging consequences, in particular to the ozone layer. Altemative leul IllOl~yie~ have been developed utilising natural gases as blowing agents such as carbon dioxide and an 15 example can be found in U.S. 4,436,679.
However, there have been very few, if any cc,ll,,llel.;idl U~Jeld~iUlla reportedusing CO2 as the sole blowing agent in the manufacture of polystyrene foam usingmolten polystyrene. Most cu"""e~-,idl Ope~ , using CO2 as a blowing agent use CO2 in Cullluilldtiuil with a h~dlucdlL~Il blowing agent such as pentane or butane or 20 with a flu~luudluoll product such as Du Pont 152A. It is an object of the present invention to produce a foam using a CO2 or CO2 containing blowing agent.
Preferably the foam produced should have ~ dld~.lelialil and properties at least as good as a fluùlu~,dl uOI1 blown foam. Most preferably the foam produced will have an average cell diameter below 22 microns, a cell wall thickness below 4 microns, a25 density between 1.5 to 3.0 Ibs/ft3 and cells which are suu~ldl ,'i "y uniformly oriented in all three dilllellai~lls.
Hitherto, it has been cOI~aideled that plastic foams such as polystyrene foams when blown using a natural gas should i"c~"uu,_'~, a fairly low molar amount of the blowing agent entrained in the resin. It has been previously l,ùllai.leled that 30 generally as the proportion of blowing agent is increased the density of the foam is de~,,eased and that the physical strength of the foam decreases with the density.
For example polystyrene foams produced using above about 0.1 moles blowing agent per 1û09 of polystyrene have generally been col~side,ed to be too weak to be of cu"""er~,idl value, particularly when the foam is intended for use in making an end 35 product such as a tray or other supporting substrate. It has also been previously col~sid~led that the control of the foaming process is more difficult as the proportion of blowing agent is increased. Most examples previously published suggest the use of a proportion of CO2 (when making a polystyrene foam) in the range 1.5-3.0% byweight (0.034 - 0.068 moles CO2/1 00g polystyrene).

WO 95~24440 2 1 8 5 2 2 l PCT/AU95/00127 These previously made foams have been extruded at a stock at die temperature of about 140 to 155C. It has also been found with these foams that if they are extruded at the bottom end of this temperature range they have a tendency to shrink i"""- " ~y after manufacture. This is probably due to the rapid diffusion of CO2 out of the cells causing a partial vacuum in the cells. Shrinkage may be controlled by increasing the stock at die temperature or by de~,ed~i,,g the CO2 COIl~ dlion.
Nut~L;'l:.ld,)ui.,g the accepted wisdom collce",i"y the d~,uru~u,i ' level of blowing agent addition the applicant through a series of trials using higher levels of 10 natural gas blowing agent addition has dsce,ldi"ed that the blowing agent if it i~uu~u~ CO2 can act as a viscosity modiher of the resin (this has various ,d",iriu ~"s as he~i"dll~ described) and also that the temperature at which th~
pld~liC/~ . 19 agent mix is extruded is critical to the strength of the resulting foam.
It has generally been coll:,id~ d that the temperature of foaming is not particularly important. For polystyrene a temperature range from 155C to 135C
has been quoted in the literature, but actual examples of CO2 blown foams have all been limited to above at least 14ûC. In fact the scant regard to the temperature of the material at the exit die in the prior art highlights the fact that it has not previously been dp~,c~ ,d how important this pa,d",~,L. is in the production of strong foams.
20 The temperature of the stock varies col~:.idt:ldl,ly at or near the exit die but there has not previously been any emphasis on the manner of measurement of the temperature or exactly how it is to be measured. The only benefit of lowering the temperature previously reported has been to increase control over the forming process to avoid surface defects in the formed sheet. In general, reduction of 25 lt:",,u~, ~re has not been favoured because the output of the extruder normally reduces with reduction of temperature of the material being extruded. The applicants have now found that the temperature at which the material is extruded is particularly important.
More spe-,iliua:ly, in acculddl~ut: wlth the present invention the applicants 30 have as-,~,l_;.,ed that if the l~"~,u~ ~re of the material, when it is being extruded, is below a particular critical temperature this can enhance the strength of the resulting product.
Quite u"e~ ul~.lly the applicants have found that whilst the physical strength of a blown foam generally decreases, at hrst, with a drop in the temperature of the 35 material at the exit die, the ~IdtiU~sll;!J is not a direct one and in fact there is a temperature (I~ i" ~ called "the critical temperature") below which the strength of the resulting foam sharply increases. With an initial drop in stock at die temperature the viscosity of the mixture in the extruder increases and the mixture is difficult to extrude using normal a,u~Jd~ ~s If the p~,w"ta~e CO2 is increased at the sarrle _ _ _ . . . . _ .... . . _ _ .. . . ...

time it has been ~,,ex,ueuleuly found that foam sheet can be produced. However, with the initial drop in stock at die temperature the foam has a lower strength and has a greater shrinkage. The shrinkage can be as high as 3040% of the initial .li",en~iol1s of the foam. This increasing deleriuldliol~ of properties was a barrier to further é~.,uerilllel,ldliol1 in this region. However, surprisingly it has been found that if the stock at die temperature is further reduced (and if the CO2 uullcellLI ' , is simultaneously increased) that below a certain temperature the shrinkage reducesrapidly and the strength increases. This increase in strength and reduction in shrinkage occurs even if one utilises a high proportion of blowing agent. The critical temperature can readily be ascè,ldi"ed by a person skilled in the art for any resin/blowing agent mix by plotting the physical strength and/or shrinkage of the resultant foam against the têmperature of thê material at the exit die through a range of different temperatures. Below a specific temperature the applicant has found that the strength of the resultant foam sharply increases and the shrinkage rapidly reduces.
In addition the applicant has found that for foams extruded at relatively low temperatures and using a relatively high proportion of a natural gas blowing agent the density of the foam produced is not directly plU~il;Ulldl to the molar col1CellLld~iùll of the blowing agent. The applicants have also found contrary to conventional belief, that control of thê process using relatively large amounts of blowing agent is not impaired.
Thus, in acco,dd"ce with a first aspect of this invention there is provided a - method of producing an extruded plastics foam of enhanced physical strength which c~" I,ul i:,es.
(a) intimately mixing a blowing agent i~UU~,UOI ' ~y CO2 and in which the major prupulliuil is a natural gas, in a plastics resin melt to form an l,u",~ge"eous resin mix; and (b) extruding the resin mix through an exit die into a region of lower pressure;wherein the temperature of the resin mix is adjusted so that it is below the critical temperature (as 11e,ei"L,eru,e defined) at the point of extrusion out of the exit die.
The preferred resin is a styrene polymer. Other useful resins in the practice ofthe invention include other polymers which CO2 will plasticize. Most preferably, the resin is a polymer or copolymer having at least 90% styrene ",ol,u",e,~. Other 35 IIIOI1OIIIèliC units present in suitable copolymers and ill~.,uuly,llel~ include acrylic acid, acrylonitrile and other equivalents known in the art. In one elll~odilllel~l, virgin polystyrene polymer (80%) is mixed with regrind polystyrene (20%). In d,u~lu,ulidle circumstances, a nucleating agent may be illCol~uuldLed into the resin mix but this is usually not necessary. At col ICel ~L, .~ ,s of CO2 blowing agent above about 6.û% by WO 95/24440 2 1 8 5 2 2 1 PCTIAU9~/00127 weight, the CO2 will act as its own nucleating agent. Suitable nucleating agentsinclude sodium LiGdl~ull , citric acid or mixtures thereof. If a nucleating agent i5 used it should make up no more than about û.2% of the weight of the resin mix. The polystyrene may be blended with an impact modifier. The melt flow index of the 5 resin is not narrowly critical. It is preferably between 1.5 to 16 and most preferably i~
the range 2.û4.û. (Reference here and throughout this ~ ~ ;r;~ to the melt flow index of the resin is that as tested according to Australian Standard Test Method ASTM D1238-G) Preferably the blowing agent is 1 ûO% CO2 although other natural gases such 1û as nitrogen, air or water or mixtures of these gases can be utilised with CO2. A
natural gas useful in the invention is any naturally occunring d~lllu~ e,iu material which is a vapour at the temperature and pressure at which the foam is produced.Of course in the process of the invention the blowing agent does not need to be introduced in a gaseous state - in fact it is preferred to introduce the substance in a 15 liquid or super critical state.
The applicants have also found that at least some of the benefits of the -rF ll ~ invention are retained if up to 5û% of the amount of CO2 or CO2/naturalgas is replaced by an equivalent molar amount of a IIJ~lucdlbull blowing agent such as butane, pentane or a hydroflu~,uud,l,u,~. It is preferred that the h~dluLdlbul1, if 20 used, be present in a proportion of between 0.01 - 0.06 moles/100g polystyrene.
The applicants have found that a convenient method of addition of a hy.l, U~,dl bUI I blowing agent is via regrind. If the regrind is obtained from polystyrene foam previously blown with hydlucd,L,u,,a, then the regrind will contain n,u,ul~uidùle amounts of residual hydluud,~ol,. Levels of 2% - 3% hydlu-,dl~ùll weight per resin 25 weight are quite usual. A convenient source for such regrind is packaging foam blown from polystyrene pellets using pentane or butane.
If a foam is produced in acc~,dd"ce with the dful~ ed method an increased proportion of blowing agent can be used (so to fonm a foam of lower density) yet still produce a foam of enhanced strength. In fact, the utilisation of a 30 greater amount of blowing agent brings with it important further advantages. First, if a higher proportion of blowing agent is used in the production of the foam it ispossible to produce the product with a smaller cell size and in which the average thickness of the cell walls is reduced. The smaller cell size improves the c".ped,d"~e of the product as the smaller the cell size the smoother the surface of the end 35 product. Furthermore, small cell size foams are of reduced brittleness as compared with foams with larger cells. Secondly, the use of a higher proportion of blowing agent enables one to extrude a resin mix through the exit die of an extrusion apparatus more easily than if a low pe,~"~d~e of blowing agent were used. For example in the case of a CO2 blown polystyrene foam the viscosity reducing effect of WO 95~24440 2 1 8 5 2 2 I PCTIAU9S/OOIZ7 CO2 as previously discussed enables the invention to be worked using known ~u~ .idl apparatus without substantial Illud~fi~.dliull. This is because the viscosity modifying effect of the CO2 counters the contrary effect on the viscosity caused by the reduced extrusion temperature.
In a preferred polystyrene/CO2 blown foam utilizing 10û% styrene polymer having a melt flow index of 3.5, the applicants have found that the critical temperature when measured by an infra-red probe beamed onto the material as it exits the extrusion die to be about 135C. The temperature cited here is the temperature as measured on a Scotchtrak Heat Tracer made by the 3M company.
The instrument was set for the emissivity of opaque white plastic (0.95). The IRprobe was found to be consistent (within + 1C) with a thermocouple placed in a probe extending to the mid point of the feed line i~ ly ~,~edi"~ the die.
The It:" ,,u~ re of the extruded material varies ,;~u"ir,~.d, "y in various regions at or near the exit die. As the temperature of the material as it exits the die has been found to be important, it is critical that a specific location be chosen for themeasurement of the temperature of the material and it is for this reason that the specific procedure outlined above is used. In this ~l.el ;r .~ all reference to the temperature of the resin mix at the point of extrusion out of the exit die is the temperature as measured by an infra-red probe as described above, unless sp~ ic.~"ystated otherwise.
Preferably the temperature of the resin mix at the point of extnusion out of theexit die is between 125 to 140C. The most preferred le:lll,u~ re will depend in part on the nature of the resin used. In general, the lower the glass transition point (or the higher the Melt Flow Index) of the resin, the lower the preferred temperature.
For example, with a polystyrene having a melt flow index of 3.5 (e.g. AUSTREX 112 - a product of Huntsman Chemical Company Australia Limited) the temperature of the resin mix at the exit die is preferably between about 126 to 132C. Below 125C the applicants have found that even with high levels of carbon dioxide addition the material is too cool to form in line. The lower limit for the temperature for polystyrene is about 120C at which only very simple shapes can be formed in line.
For a polystyrene which has a melt flow index of 1.8 (e.g. AUSTREX 103) the temperature of the material at the exit die is preferably between 130 to 137C. For a polystyrene which has a melt flow index of 16 (e.g. AUSTREX 555) the temperature of the material at the exit die is preferabiy between 124 to 130C.The die pressure required to avoid premature foaming for any particular grade of polystyrene increases as the ~,U~ L~ of CO2 increases and stock at die l~:llI,ut:l Ire increases. In practice to avoid excessive die pressures which in turn lead to extremely difficult to control flows of foam exiting the die as the cù,)Cl:llll " 1 of CO2 is increased it is preferred to further reduce the stock at die temperature WO 95/24440 2 1 8 5 2 2 1 PCT/AI~95/00127 below the critical temperature. The applicants have found that using a slit die that a die pressure much in excess of 5,000 p.s.i. Ieads to a foam which is difficult to control.
It is preferred when using CO2 that the content of the CO2 be above 5.5% and below 10% (0.125 - 0.23 moles/100g polystyrene) by weight to the weight of resirl.
At a level of 10% a stock at die temperature of about 120-125C is required to avoid excessive die pressures for a polystyrene having a melt flow index of 3.5. Most preferably the CO2 uu11C~ dLiull is between 6 to 8% (0.136 - 0.180 moles/100g polystyrene). Similar molar amounts are preferred if using other natural gases.
At the higher end of CO2 CU~C~ levels indicated above as pr~fl.""er,~
it has been found that the density of the foam increases leading to an extremelystrong foam.
At the lower levels of blowing agent addition the resultant foam has a low density. A polystyrene foam formed using about 6% CO2 has a density of between about 2 to 2.5 Ibs/ft3 and a cellular structure where the diameter of each of the cells is below about 0.002 inches. At a CO2 addition rate of 6.5% or more the foam has a cellular structure with a cell size below 0.001 inches. This aspect of the invention brings with it further specific advantages quite apart from the enhanced physical strength of the foam brought about because of the temperature at which the product is extruded. Previous foams formed using only natural gases as blowing agents had relatively high densities resulting in large cell sizes and cell wall Llli-,hll~as~s. In t~le past it has not been possible to make foams by extnusion using a naturai gas blowing agent with a microfine cellular structure. The present invenUon when practiced with the preferred blowing agent addition amounts referred to above enables tlle production of foams of enhanced strength which do not have the brittleness ~ j ' ' with foams having larger cell sizes. The reduced cell wall II,ich"eases of the foams produced in duc~lddllce with the invention result in the foams having reduced density.
An important aspect of the method of this invention is the intimate mixing of the blowing agent in the resin melt. Unless the blowing agent is intimately entrained wlthin the resin a sd~iard~;tuly foam cannot be produced. Commercial equipment for extruding foam may be used in the process of the present invention although some", "~ , may be required to meter the blowing agent into the extruder and to ensure adequate mixing especially at higher p~ "tdges of blowing agent addition.Typically, cull ", l~luidl equipment comprises either a single extruder or two extruders in series (tandem extrusion). In either system, there are access points provided in the apparatus through which materials required to make the foam can be introduced.
In a single extnuder system, resin granules, combined in most cases with a nucleating agent, are introduced into the extruder at or near its upstream end. The resin is melted and mixed in the extruder. A blowing agent is usually introduced into the extruder at some point d~.llall~dlll from the point at which the resin is introduced into the molten resin.
In some systems, a blowing agent is introduced after the IllellllU~Jlda~i~, melt 5 has passed through the extruder at a point illlt:lllledidlt: the extruder and the outlet die in which case a further mixer is i, ,u~, ,uu, ' ' into the line to ensure proper mixing of the blowing agent in the Illellllùpldali~ melt. Tandem extnusion is a variation of this process. In tandem extrusion, the resin is melted and mixed in the first extruder.
Blowing agent is then introduced into the melt prior to being introduced into a second extnuder where mixing and cooling takes place In both systems, the foam is formed by controlled release of the melt with the blowing agent entrained therein, through an exit die into a region of lower pressure. The back pressure of the die is important in this regard. As with all processes of manufacturing foam, the back pressure at the die must be suffficiently high to prevent premature foaming of the mixture as itextrudes through the die. It is also known that if the die back pressure is too low that this will lead to surFace i""ve,rt:.,tivi,s. The die back pressure can be increased by reducing the flow through the die by altering the dimension of the die. It has been found that the die back pressure as measured i"""edidl~:ly behind the die head should preferably be above 3,500 psi. For foams with a blowing agent content of 6%
of more by weight the die back pressure is preferably above 4,000 psi.
The geometry of the die and the treatment of the foam post die should be such that the foam is produced as a smooth uniform sheet at the die exit at the desired die exit It:""ver ' ~re. In general the applicant has found that foam extruded through a slit die and passed over a bar mandrel to flatten out any waves is easier to 2~ control than foam extruded through an annular die and passed over a conicalmandrel. Control of the foam will also be effected by the thickness of the die slit and other r" "' " ns obvious to those skilled in the art. Col1c~,,l,dliulls of the CO2 greater than about 8% by weight are diffficult to control. Control is easier if the foam is extruded into a zone of pressure i"' "~d;..~ that of the mixture before extrusion 30 and dll"os~l,eric.
The take up of foam from the die is preferably such that the foam is not ai~lliri~;dlllly stretched as such stretching will impart undesired ",e~;l,d"iual distortion to the cell structure. In general, the higher the die pressure the faster the foam will emerge and the faster the take up Illt:-,lldlli~lll should nun. A typical take up 35 ",ecl,d"is", is a forming wheel rotating so that the velocity of the circumference of the wheel is suLald, IL;~ IY equal to the speed of the exit of the foam from the die.
In most standard dlldll~ llt~ the apparatus includes a cooling device. This cooling device may be a cooled extruder, or part extruder, a dynamic cooler or other means known to those skilled in the art. By dynamic cooler it is meant a cooler WO 95/24440 2 1 8 5 2 2 l PCT/AU95/00127 having a rotating shaft. If used, the dynamic cooler is located at a point in the extrusion line to receive the resin/blowing agent mix after the blowing agent has been intimately entrained within the resin. The dynamic cooler includes means tocontinue the admixture of the resin and blowing agent while simultaneously reducing its temperature.
For the process of the present invention, the l~",~uer ' Ire of the resin mix ispreferably adjusted to a temperature below the critical I~III,U~ re by means of a dynamic cooler.
An ull~ ,e~ d advantage of the present invention is the suitability of existing apparatus to make the preferred foams. A CO2 blown polystyrene foam is forme~
both conventionally and in the process of the present invention by ill~.OI,UUld~carbon dioxide into the polystyrene melt at a temperature broadly in the range of between 17ûC to 230C. After the blowing agent has been intimately entrained within the resin melt it is cooled to the d,U,UlUj)li ' ~t:lll,lJt:l ' Ire for extrusion out of the exit die. In a polystyrene foam c~,,,,u,i:,;,,y for example 3.5% CO2 the conventional apparatus will reduce the ~ ll,uel ' Ire of the melt through the dynamic cooler to about 155C. vVhen exercising the preferred ~ bodilllt~ of this inventio~
(which involves the use of a higher per~l l~dge of carbon dioxide) the dynamic cooler will operate to bring about a greater drop in the temperature of the mix. This is because of the viscosity modifying effect of the carbon dioxide. The greater pelu~ dge addition of carbon dioxide, the less viscous the resin mix and this means that there is less shear heating when the mix is passed through the dynamic coolel-.
The applicants have found that on conventional apparatus the critical temperature for polystyrene/CO2 foam of about 135C can be achieved through unmodified dynamic cooling apparatus simply by increasing the CO2 addition rate to about 5.5% by weight.
The foams produced by the c~ lled methods have ~,lldld~ ,s of strength/sl,,ou~l,lless and lack of brittleness not previously known in an extruded plastics foam.
Thus, in accu,dd,)ce with a further aspect of the invention there is provided anextruded polystyrene foam sheet illCOl,Ul)ld~ill9 a cellular structure in which the average cell diameter is less than 0.002 inches and which has a density of less thar about 4.0 Ibsm3.
Most preferably the foam sheet has a density of between 2.0 to 3.0 Ibs/ft3.
The average cell diameter is most preferably less than 0.001 inches. The cell structure is preferably closed. The average cell wall thickness is between 1 to 2 microns (0.00004 - 0.0008 inches) and most preferably between 1 to 1.5 microns.
The invention is he,_i,,dl~l described by reference to the preferred o.li~llel~ in which:

WO 95124440 2 t 8 5 2 2 1 PCT~AUgS~OQI~7 Figure 1 is a schematic diagram of a single extnuder extrusion system for practicing the current invention;
Figure 2 is a sul,e",dli~ ,ul~s~:lltdliull of the cross-sectional view of a foamsheet made in acG~,-lal~ce with the invention; and 5 Figure 3 is a cross-sectional view of the mixing tip in the extruder shown in Figure 1.
Polystyrene foam made SULJ:~ldl " 'Iy entirely by virgin and regrind polystyreneand CO2 is prepared on a single extrusion line as shown irl Figure 1.
Resin granules are introduGed into the upstream end of extruder 2 through 1û hopper 3. The resin may be mixed with a nucleating agent such as sodium ~ica,L"~l,dL~, citric acid, hydrocerol, talc or any other nucleating agent as known in the art. The addition rate of nucleator in the praGtice of the present invention is in the range of û% to 1% and preferably between 0% to 0.1%. Regrind material previouslyextruded with CO2 can be added to the resin granules in a ratio preferably between 10 to 40%. A screw extruder rotates within the barrel of extruder 2. The barrel is IllaillLdill~d at a temperature between 170 to 180C to melt the polystyrene and enable it to move easily along the barrel. The extruder 2 has four separate zones dG-aiyl ' ' A, B, C and D in Figure 1. The polystyrene resin is melted in zones A, B
and C and the barrel is I"ai"t~.;.,ed at a pressure of between 3500 to 4500 psi. The 2û pressure in the extruder barrel 2 is checked by a transducer 4 fitted before a screen changer 5.
CO2 is metered and introduced into the polystyrene melt at access point 6 at the end of zone C. The carbon dioxide is added to the polystyrene at a rate of above 5.5% consumption by weight compared with the weight of extruded polystyrene. TheCO2 is preferably injeGted as a liquid at a pressure higher than the pressure within the extruder barrel 2, most preferably at around 5000 psi.
The resin and the CO2 are intimately mixed in zone D of extruder barrel 2. To ensure adequate mixing in this region the applicants have developed a mixing tip 7 which is fitted at the end of the screw in zone D. Mixing tip 7 is shown in more detail 3û in Figure 3. It will be noted that the mixing tip comprises a high density of fixed mixing pegs 8 and an expanded mixing tip head 9 such that the clearance between the outemmost edge of the mixing tip head 9 and the inner wall of the extruder barrel 2 is about 1.4 mil or less. These ",- " " ns to the mixing tip head or similar such 111 "' " 15 are necessary to ensure adequate mixing of the higher proportion of 3~ the blowing agent within the resin. An " , "~c a~a~)yc:l,,e,,L has been previously disclosed in the ~' F' I~ U.S. patent 5,129,728. The applicants have found that it is important that there is no dead space in the line between where the gas is added to where the mixture is extruded through the die. The mixture should be continuously worked.

The resin with the CO2 intimately entrained therein after being mixed thoroughly in the extruder passes through the screen changer 5 to gear pump 10.
Gear pump 10 is operated at an d,U,lJlU,Oli ' rate having regard to the rate of foam output at die ll and the rate of resin production through extruder 2 to balance the 5 pressures on either side of the gear pump. The pressure variation after gear pump 10 is preferably less than 300 psi. Greater variations can cause fluctuation of sheet at the die ll.
The well mixed material is then fed into the dynamic cooler 12 preferably at a pressure above 4000 psi. The dynamic cooler 12 can be of any type as known in the 10 art. Preferably it is a cooler having a rotating shaft with gears and the geared teeth carry small amounts of material allowing the COIresin mix to cool down to the desired temperature. The heat t:x~.l,al~gel 12a is used to cool the dynamic cooler body and shaft. Oil temperature cooling the cooler depends on the cooler output.This oil temperature can be in a range of between 45 to 1 00C. Preferably the resin 15 mix is cooled to a temperature of about 130C. Once the COIresin mix has beencooled down it is extruded out of die ll and is pemmitted to expand into a foam by passage into a region of lower pressure. The pressure at the die is preferably about 4000 psi.
The sheet so fommed can be used in a continuous process to form products 20 i"""edidL~ly after it exits the die or the foam can be used at a later time to II,e""ufu"" products. Foams made in d-c~,d~"ce with the clFvl~l"e:, ~ed method have low expansion on reheating. Therefore conventional foam re-heat Ill~llllufullllillg apparatus needs to be modified for use with such foams. It is preferred to form the foam so produced while it is still hot after exiting the die on a 25 continuous vacuum assist or plug assist II,e""ufu""~l of conventional design.Whilst the preferred method described above is detailed by reference to a specific type of extruder apparatus foam produced in acc~lddll- t: with this invention may be made on any equipment capable of making conventional coarse foam sheet using 100% natural gas blowing agent at an exit die It:lllut:l ~re of between 140 to 30 155C provided that the equipment has adequate mixing capability so to intimately entrain the higher proportion of natural gas blowing agent used in the preferredembodiments of this invention.
When the equipment is running smoothly at for instance 145C making good foam to make foams of the present invention the temperature is de~ ased alld 35 preferably the natural gas COn~t:lllldliul1 simultaneously increased keeping tile viscosity of the cooled molten mixture roughly the same. Viscosity can be monitored from the torque on the cooling screw or dynamic cooler.
If there is excessive fluctuation of pressure occurring as the blowing agent col1ce"l,i ~ is increased (or if bubbles of gas start popping from the die) then there WO 9S124440 2 1 8 5 2 2 1 PCT/AUg5/00127 is not adequate mixing and the equipment must be modified (such as shown by the ill~,Ol,lJl~ld~iUII of a modified mixing tip as shown in Figure 3) to increase mixing capability.
Assuming adequate mixing capability the cell size reduces to a microfine 5 cellular structure as detailed by reference to Figure 2. It will be seen that the foam 13 has a microfine cellular structure wherein each cell 14 has a maximum diameter of about 0.001 inches and the average cell wall thickness is between 1 to 2 microns.
The temperature at the point of extrusion out of the exit die may be reduced to the lowest temperature that the foam sheet can be either Ll,e,,l,ufur,,,ed or othenwise 10 manipulated. Depending on the nature of the post fomming operation, this temperature will be su",~,..'~e~t: in the region between 120 to 127C for polystyrene with a melt flow index of between 2 to 4.
The preferred foam produced by the process described by reference to Figures I and 3 above has a microfine cellular structure as can be seen in Figure 2.
15 The physical ~;lldld~,L~lioli~,S of the foam are as follows: density: 2.0-2.2 Ibs/ft3 average cell size: less than 0.001 inch average cell wall thickness: 1 to 2 microns.
In order to assess the influence of the L~,,,,u~, ' Ire of the resin mix at the point of extrusion out of the exit die the applicant conducted a number of trials using the same apparatus, pc,lyOLy.~"e having the same melt index and using exclusively CO2 20 as the blowing agent. In each case, the polystyrene grade used was AUSTREX 112 having a melt flow index of 3.5 and the CO2 was food grade. The foam produced ineach trial was formed into a meat tray (7 inch x 5 inch x 5/10 inch deep) using a continuous vacuum fommer and the side strength of the tray was tested.
The strength test involved placing the fommed foam tray flat on a jig and 25 measuring the maximum force that ~he side wall of the tray could withstand before it would collapse. The results of this test for foams produced over a range of different exit die temperatures are set out in the table reproduced in Table 1 below.
A second series of tests were conducted using AUSTREX 112 resin in which the average cell size was also measured. The results of these tests are shown in30 Table 2.

C
~ N~
r ~ Y ~ U
~O ~ C ~ ;Q ~D
~ ~ r ~ .~ ae ~ ~ u~ ~ O
3 .. ~ ' ~ ~ `I O C~ ~ ~ ~ ~ ~
t~ ~ ~ o~a~ 1~0~I0t~l~OC~ O o o _ ~, +I
o - . - o CD ~ r~ ~ ~ co ~ o ~ o OD a~ 5 E ~5 ON~7~, ~NNNN~ o ,~ ~ . O ~
N ~ ~ 0 o 3 ~ ~ O N ~ U~ ~ N U~ 3 j~ , 2 o~ i E~ 3~
.N _ N O _ ~ ~ ~ W
t~ ' 3 ~~ ' .C
tO cn o o 0 CD ~ O ~ cn ~D 10 ~ cn ~ ~ o tO ~ cn ~ 3 ~ ~ .
~ o tD 1~ CC) cn ~ ~~ N ~ C0 CJ~ O ~ ~ N

WO 95/24440 2 1 8 5 2 2 l PCTIAU95/00127 -.~ --C
a~ c) o lo ~ ~ ~ ~ o co U~ ~ o ~; 5 _ , -- N N N N N
a) ~,0 ~ ~ O, O ~ ~ ~ ID 0, N 0, 0 0 0 0 0 0 _ 0 5 ~ N N ~ _ ~ C~ O N ~ ~ ~ N ON N N _ c~l ~5 ~ N
o ,X t~
5 ~
Q
--X ~ tD N 1` 0 C~ N ~ ~ ;~5 ~ C) o _ N

WO 95/24440 21 8 5 ~1 PCT/Al~95/00127 As can be seen from the above test results for polystyrene/CO2 mixes the resultant foam is of reduced strength with a decrease in the die temperature to about 135C vJ,le,l:dr~l further reduction in the temperature of the material as it exits the die results in a quite significant increase in the physical strength of the foam5 produced. In particular, it is to be noted from the trials shown in Table 1 that the side strength of the foams produced at temperatures between 131 to 134C were all 20 newtons or greater which is s~b~ld~ ," 'Iy the same or better than the side strength of the trays produced from a foam having a much higher density such as those in trials 1,2and3.
At lower exit die temperatures it is possible to i~U~,UUI ' higher levels of CO2and thus produce foams of lower density yet having enhanced side strength.
The sharp increase in the strength to weight ratio for temperatures below 135C indicates that for this resin/gas mixture, the critical l~",,ut:, ' Ire is about 1 35C.
1~ It will be noted that the side strength of the material extruded at 128C is greaterthan the side strength of the material extruded at 150C n ' 'haldlldi,lg that the density of the material is some 30% less than the material which was extnuded at 1 50C.
A third series of tests were conducted by the applicant using a polystyrene 20 resin having a higher melt flow index. For this series of tests, the applicant used Austrex 555 resin which has a melt flow index of 16. Line conditions for the production of a foam tray were set so to be su~dll~ the same as in previous trials using Austrex 112 resin. CO2 addition was at 6.8%. Good trays were produced at a stock at die temperature below 130C. The optimum temperature 2., appeared to be about 128 C at which the foam trays produced were found to have a density of 2.11 Ibs/ft3 and the foam was found to have a microfine structure having a cell size of less than O.û01 inch.
Whilst the sheet quality was found to very slightly d~t~liuldl~ using a resin having such a high melt flow index, the trays formed were found to have similar 30 properties to those made with a styrene having a much lower melt flow index such as Austrex 112.
The present invention provides many flow on benefits. Any grade of polystyrene can be used because the viscoslty of the material can be lowered by use of a higher pe,~ tdge carbon dioxide. This reduces the stress on the equipment. A
35 foam sheet can be made having lower density without sacrificing tensile properties.
It is possible to achieve microfine cellular structure and this has ad~/dll~d~u,~s of sl"~u~l",ess, reduced brittleness and enhanced insulation properties. In addition microfine cell foams allow increased extruder output due to reduced material viscosity as a result of the higher CO2 content. This enables equipment to run faster _ _ _ _ _ _ .

with low stress.
Finally microfine cell foams are flexible.
To produce the foams of the present invention on conventional apparatus it was necessary for the applicants to recognize the viscosity modifying effect of CO2 5 and to increase the proportion of blowing agent and make a lower density foam so to permit extrusion below the critical L~,nu~ re. The reduction of the materials density to thereby enhance strength was counter-intuitive but lead to a superiorproduct having several advantages as detailed above.
It will be u, ,de,~luod that the abov~ iol ,ed preferred des~ u~;Jn of the 10 process of the invention may be modified (for exampie by i"-,~, fJUI ,~ additional mixing by a way of a tandem extrusion system) or by the i, ,cul ~u,~iu,, of additional materials (such as nucleating agents) without departing from the spirit and scope of the invention.

Claims

1. A method of producing a polystyrene foam article of enhanced physical strength in which the article is thermoformed from an extruded polystyrene foam said article being produced by:
(a) intimately mixing a blowing agent consisting essentially of a natural gas orgases in a polystyrene melt to form an homogeneous mix said blowing agent containing between 5.5% to 10% by weight CO2 to the weight of the resin;
(b) extruding the resin mix through an exit die into a region of lower pressure whilst maintaining the temperature of the resin mix below the critical temperature (as hereinbefore defined) at the point of extrusion out of the exit die to form a polystyrene foam sheet; and (c) thermoforming said polystyrene sheet without reheating the sheet so to form an article immediately after extrusion of the resin mix through the exit die.

2. A method as claimed in claim 1 wherein said blowing agent includes CO2 mixed with another natural gas (as hereinbefore defined).

3. A method as claimed in claim 2 wherein said blowing agent includes CO2 mixed with nitrogen.

4. A method as claimed in claim 3 wherein said blowing agent consists exclusively of CO2 and nitrogen.

5. A method as claimed in claim 1 wherein said blowing agent includes between 0.01 - 0.06 moles/100g resin of any one or more of pentane butane or a hydro- flurocarbon and wherein the remainder of the blowing agent is CO2.

6. A method as claimed in claim 1 wherein said blowing agent is exclusively carbon dioxide.

7. A method as claimed in any one of claims 1 to 6 wherein said polystyrene melt includes virgin polystyrene polymer and regrind polystyrene polymer.

8. A method as claimed in any one of claims 1 to 7 wherein the regrind polystyrene is reground foam blown with a hydrocarbon blowing agent.

9. A method as claimed in claim 1 in which the plastics resin is a styrene polymer having a melt flow index of between 2 to 4 and the blowing agent is exclusively CO2 wherein the critical temperature of the resin mix at the point of extrusion out of the exit die is about 135°C as measured by an infra-red probe beamed onto the material as it exits the extrusion die.

10. A method as claimed in claim 9 wherein the temperature of the resin mix at the point of extrusion out of the exit die is between 125 to 135°C.

11. A method as claimed in claim 10 wherein the temperature of the resin mix at the point of extrusion out of the exit die is about 128°C.

12. A method as claimed in claim 1 in which the plastics resin is a styrene polymer having a melt flow index of between 1.5 to 2 and the blowing agent is exclusively CO2 wherein the critical temperature of the resin mix at the point of extrusion out of the exit die is about 140°C as measured by an infra-red probe beamed onto the material as it exits the extrusion die.

13. A method as claimed in any one of claims 1 to 12 wherein the content of carbon dioxide blowing agent is between 6 to 8% by weight to the weight of the resin.

14. A method as claimed in claim 1 wherein the resin mix is maintained at a pressure of between 3,500 to 5,000 psi immediately prior to extrusion through the exit die and is permitted to expand into a foam by passage into a region maintained at atmospheric pressure.

15. A method as claimed in claim 1 wherein a nucleating agent is added to the resin prior to incorporation with a blowing agent; said nucleating agent being present in the amount of no more than 0.2% by weight of the resin mix.

16. A method as claimed in claim 15 wherein said nucleating agent is sodium bicarbonate, citric acid talc or mixtures thereof.

17. A method as claimed in claim 1 wherein the blowing agent is intimately mixed with the plastics resin melt in a screw extruder and wherein the temperature of the resin mix is reduced to below the critical temperature by a dynamic cooler which is adapted to reduce the temperature of the mix whilst simultaneously maintaining an homogenous mix of the resin and blowing agent.

18. A method as claimed in either one of claims 1 or 17 wherein the resin mix is extruded through a slit die and is passed over a bar mandrel.

19. A method as claimed in claim 1 wherein the extruded plastics foam is thermoformed into an article atmospheric pressure immediately after the extrusion of the foam out of the exit die.

20. An extruded polystyrene foam sheet made in accordance with the method of any one of claims 1 to 19 which incorporates a cellular structure in which the average cell diameter is less than 0.002 inches and which has a density less than or equal to 4.0 lbs/ft3.

21. A foam as claimed in claim 20 wherein the density of the foam is between 2.0 to 3.0 lbs/ft3.

22. A polystyrene foam sheet as claimed in claim 21 wherein said density is between 2.2 to 2.4 lbs/ft3.

23. A polystyrene foam sheet as claimed in any one of claims 20 to 22 wherein the average cell diameter is less than or equal to 0.001 inches.

24. A polystyrene foam as claimed in claim 20 wherein the average thickness of the cell walls of the foam is between 1 to 2 microns.

25. A polystyrene foam as claimed in claim 24 wherein the average thickness of the cell walls of the foam is between 1 to 1.5 microns.

26. A polystyrene foam as claimed in claim 20 wherein the foam is predominantly of a closed cellular structure.

27. A foam tray formed from an extruded polystyrene foam as claimed in claim 20.