CA2111326A1 - Improved thermoformable polypropylene-based sheet and process - Google Patents

Abstract

The invention provides a thermoformable sheet comprising a resinous polymer of propylene and an effective amount of a beta-spherulite nucleating agent, a process for making the sheet and articles thermoformed from the sheet.

Description

2 PCI'/US93/03459 h 1 1 1 3 2 6 IlllP~OVED THERUIOFORMABLE POLYPROPYLENE-BASED
SHEET AND PROCESS

Field of the Invention The pres~nt invention relates to an improved thermotormable sheet comprising at leas~ one layer ot a resinous polymer of propylene and an effective amount of beta-spherulites, a proc~ss for preparing such sheet and to articles thermotormed trom such sheets wherein such articles can be thermoformed at faster production rates and such artides can provide 10 improved end use properties such as microwaveability and low-temperature impact resistance.

around ot the Invention Conventional thermotormable thermoplastic resin sheets typically are 15 made from resins such as pdyvinyl chloride, polystyrene-based resins and the like. Among these therrnoplastic resins, however, polyvinyl ch~orides have disa~an~ges in respect to hyaiene, heat ~, moisture proofness and other p operties. Moreover, incineration of these materials causes emission of chlorine-containing gases. Polystyrene-based resins also show deficiencies 20 in respect to heat resistance, impact strength, moisture-proofness and other propenhs. Notw~thstanding these disadvantages and deficiencies, sheets of these thermoplastic resins are widely used as thermotormed packaging material in many fields.
Conventional thermoforming processes generally involve heating a 25 thennoplas~c shset above its sonening point, torming the sotten~d sheet and albwhg the formed dleet to cool and harden. Polypropybne, Wng a highly cry~lline polymer, must be heated up to its meltin~ temperature (Tm~ of about 160C in order to be thermohrmed by conventional themlotormin~ processes.
The flexural modulus ot polypropylene can decrease by more than two orders 30 of ma~nitude at it neus its melting temperature which can thereby cause sheetmade ot polypropylene to undergo excessive sa~ durin~ thermotorming. Also, polypropytene does not exhibit the nubbery plateau that is characteristic o~
~lassy polymers when such polymers are heated above their glass transition temperature (T~). Nevertheless, resinous polymers ot propylene have been 35 used increasin~ly in recent years in place of polyvinyl chloride and polystyrene-based resins by virtue ot their excellent strength, rigidity, h~at resistance, moisture proofness and other desirable properties.

WO 93/21262 PCI`/US93/03459 The market for thermoformed plastic products has undergone rapid growth in recent years, and polypropylene-based resins have the potential to become a premium material tor this market. That such resins are difficult to thermofQnn, as described above, has limited their use in lhis high-growth area.
Various methods have been anempted to lessen these thermoforming ditficulties.
One such method has been developed by Shell Development Co.
wherein a solid phase pressure forrning process, known an SPPF, is utilized.
Although the SPPF process allows a hot, but not molten, sheet to be thermotor ned just below its aystalline melting point, other limitiny conditionsare introduced such as the need for specialized, expensive thermoforming equipment, restricted depth of draw, limited draw ratio, and high levels of built-in stress.
Still other methods ot attempling to overcome the thermoforming dmiculties have taken the approach ot tailoring the molecular weight of the polypropylene resin, thereby making it possible to process extruded sheets on conventional thermotorrning equipment. In order to reduce the degree ot sagging ot the heated sheet as tt nears its ayslallization temperature, the meltnOw rate ot the polypropylene resin has had to be bwered to t actional values, typically less than 0.25 dg/min. The resulting high melt viscosity creates another problem; namely, that ot produdng sheet at economical extrusion production rates. Anempts have been made to overcome this problem by broadening the molecular weight distfibution of the polypropylene to improve extudability.
Thermotormed articles made by the processes described above dependin~ on the p.~oduc~ contained in them and conditions under which the ar~cbs are mic~waved, can undergo chan~e in dimensional integrity.
U.S. Patent No. 4,680,157 discloses a method for preparing a sheet of polypropylene having excellent transparency and surface propelties as well as them~otormability including a slight stretching of the sheet and optionally alpha-spherulite nucleating agents with articles vacuum thermoformed from sheet heated at 153 to 158C.
U.S. Patent No. 4,567,089 discloses a propylene polymer laminated sheet for sunace gloss, appearance and impact resistance with a surface layer comprisin~ crystalline polypropylene and up to 5 wt% of an inorganic or organic alpha-spherulite nucleating agent and a second layer comprising polypropylene, an ethylene polymer and an inorganic filler.

WO 93/21262 PCI`/US93/03459

3 ~ `` ','.~ 11326 Beta-spherulite nucleating agents useful in compositions for forming unstretched 1ilms up to 0.4 mm in thickness which can be made porous by extraction of beta-spherulites, stretching the tilm or a combination of extraction and stretching are disdosed for the production of porous films and processes tor makin~ such films in U.S. Patent Nos. 4,386,129 and 4,975,469, U.S~
Patent Application Serial No. 07/633,087, filed December 21, 1990, in the name ot P. ~lacoby, et al., and U.S. Patent Application Serial No. 07/749,213, filed Au~ust 23, 1991, in the name of P. .~acoby, et al., all commonly assigned to the present assignee.
In spite ot the showing of the use of beta-sphenulite nucleating agents in the tormation of microporous films and the various techniques for thermoforming sheets of polypropylene-based resins including the use ot alpha-spherulite nucleating agents, there remains a need tor resinous polymers of propybne which can be formed into sheet under facile conditions and competitive production rates which can be used for therrnoforrning anides. Such sheet would desirably also be thermotormable on conventional 1hermotorming equipment at increased production rates with the resultant the~otom~ed ar~icles having desirable improved end-use properties such as microwaveability and low-temperature impact resis~ance.
Applicants have unexpectedly found that polymeric compositions comprising a resinous polymer ot propy~ene and an effective amount ot a beta-sphenJlite nucleating agent are usetul tor preparing a thermotormabb sheet, panicularly polypropylene-based compositions having sutticient melt tlow rate tor the facib and efficient production ot sheet, and the therrnofor ning ot a~cles trom such sheet on conventional thermotormin~ equipment.
n is an object of this invention to provide an improved thennotormable she~t.
Another object of this invention is to provide an improved 1hermotormable sheet comprising a resinous polymer ot propylene and an effective amount ot beta-spherulites.
A further object ot this invention is to provide a method for thermofont~ing a sheet comprising a resinous polymer of propylene and an effective amount of beta-spherulites.
A still hrther object of this invention is to provide artides therrnotormed 35 trom such improved thermoformable beta-spherulite nucleated resinous polymers ot propylene.
Advar~geously, the thermotormable sheet ot this invention comprises one or more layers ot a crystalline resinous polymer ot propylene having beta-WO 93/21262 PCI/US93/03"59 .~1;11326 4 .

spherulites present at a K-value o~ about 0.3 to 0.95 which can be thermotormed at significantly higher production rates and the sheet produces therrnoformed articles which exhibit improved sidewall strength, reduced warp, and improved microwaveability compared to articles produced trom convehtional alpha-spherulite nucleated or non-nucleated polypropylene-based resins. Additional advantages are found in embodiments of the invention comprising multiîayer sheets which include interior layers ot beta-spherulite nucleated resinous polymer of propylene and exterior layers ot polypropylene-based resins such as ethylene-propylene impact copolymer for improved low-temperature impact resistance.

~ma~ the Invention This invention provides a thermoformab!e sheet comprising one or more layers ot a crystalline resinous polymer ot propylene having ~-beta-spherulites present at a K-value of about 0.3 to 0.95. In another aspect the invention provides a method tor thermof~rming a resinous polymer ot propylene-containing sheet comprising:
(a) melt torming a polymeric composition comprising a crystalline resinous polymer ot propylene having alpha-sphenulites and an effective amount ot a nucleating agent capable of producing beta-spherulites into a sheet; ;~
(b) quenching the melt-tormed sheet at a.quench temperature sufficient to produce beta-spherulites wherein the beta-sphenJlites are present at a K-value ot about 0.3 to 0.95;
(c) heating the quenched sheet to a thermotorming temperature -~
su11icient to allow thermotorming ot the sheet; and ~ -;
(d) thermoforming an article trom the heated sheet with a ~`
thermotorming means under thermotorming conditions.
In a still turther aspect, this invention provides a thermoformed articîe 30 comprising one or more layers of a polymeric composRion comprising a crystalline resinous polymer of propylene having alpha-spherulites and a residue of an organic beta-spherulite nucleating agent and having improved microwaveability compared to thermoformed articles comprising a rcsinous polymer of propylene without the organic beta-spherulite residue..
Brief Descrietion ot the Invention Crystalline polypropylene (sometimes referred to as isotactic polypropylene) is capabîe of crystallizing in three polymorphic forms. In O 93/21262 PCI`/US93/03459 .. . ~ .
5 ;~

melt-crystallized material the predominant polymorph is the alpha or monoclinic form. The beta or pseudohexagonal torm generally occun at levels ot only a tew percent unless certain heterogeneous nuclei are present or the crystalliz~*ion has occurred in a temperature gradient or in the presenceot shearing torces. The third crysWline modffication is the gamma or triclinic torm, which is typically only obseNed in low-molecular wei~ht or stereoblock tractions that have been crystallked at elevated pressures.
The alpha torm also is identified herein as alpha-spherulites and alpha~ls, while the beta torm also is identitied hersin as beta-spherulites, beta~tals, beta-form spherulites and beta cfystallinity.
In this invention sufficient beta-spherulites are incorporated in a resinous pe~lymer of propylene such that when a sheet is melt-formed trom such polymer it is thermoformable at lower temperature and at higher produc~on rates on conventional thermotorming equipment as compared with shee~t maeb trom alpha torm nucleated or non-nucbated polypropylene. A
typical way to include beta-spherulites within a resinous polymer is to incorporate one or more suitable beta-spherulite nucleating agents into the resinous polymer betore the sheet is tormed.
In the p~ce ot this invention, it is preterred that nucleating agents be used to produce beta-torm spherulites in the polypropylene-based resins.
H. J. Leugering (Makromol. Chem. 109, p. 204 (1967)) and A. Duswalt et al.
(Amer. Chem. Scc. I)iv. Org. Coat., 30, No. 2, 93 (1970)) disclose the use of certain nucleating agents that cause prebrential tormation ot such beta-torm spherulites.
AS discussed by Duswalt et al., only a tew materials are known to prebren~ally nucleate beta-torm sphenJlites. These known beta-nucleators include (a) the gamma-crystalline form ot a quinacridone colorant Permanent ~ed E3B having the structural forrnula H O

O H
hereinaner also referred to as ~Q dye~;

W093/21262 ~11 1326 PCI/US93/03459 (b) the bisodium salt of o-phthalic acid;
(c) the aluminum salt of 6-quinizarin sulfonic acid; and to a lesser de~ree (d) isophthalicacid andterephthalic acid.
Also, German Patent DE 3,610,644, published March 29, 1986, discloses a beta-nucleating a~ent prepared from two components, A and B.
Component A is an organic dibasic acid such as pimelic acid, azelaic acid, o-phth~,o acid, terephthalic acid, isophthalic acid and the like. Component B is an oxide, hydroxide or an acid salt ot a metal ot Group ll such as magnesium, 10 c~cium, strontium and barium. The acid salt of component B may be denved from an or~anic or inor~anic acid and may be a carbonate, stearate and the like. Component B may be one ot t!he additives already added to the resinous polymer of propylene. Components A and B may each be present at up to 5 wt%, based the weight of the polymer, and especially up to 1 wt%.
The nucleating agents are ordinarily used in the torm of powdered solids. To efficiently produce beta-crystallites the powder particles should be less than 5 microns in diameter and prebrably no greater than 1 micron in diameter. Mixtures ot the various beta-spherulite nucleatin~ agents as well as inorganic materials such as calcium carbonate, barium sultate, sodium 20 chbride and the like with Q dye can also be used.
The nucleant can be dispe~ed in the resinous polymer ot propylene by any suitable procedure normally used in the polymer art to effect thorough mixing of a powder with a polymer resin. For example, the nucleant can be powde! blended with resin in powder or pellet torrn or the nucleant can be 25 slurried in an inert medium and used to impregnate or coat the resin in powder or pellet torm. Alternatively, powder and pellets can be mixed at elevated temperatures by usin~, tor example, a roll mill or multiple passes through an extnuder. A prebrred procedure tor mixin~ is the blending ot nucleant powder and base resin pellets or powder and melt compoundin~ this blend in an 30 extruder. Multiple passes through the extruder may be necessary to achieve the desired level ot dispersion ot the nucleant. Ordinarily, this type of procedure~is used lo torm a masterba1ch ot pelleti~ed resin containing suffident nucleant so that when masterbatch is let down in ratios ot 1011 to 200/1 and blended with the base resin the desired level ot nucleant can be 35 obtained in the final product.
For sheet tormed containing beta-spherulites, the beta-spherulite content ot the sheet can be defined qualitatively by optical microscopy, or quantitatively by x-ray diffraction. In the optical microscopy method, a thin WO g3/21262 PCI/US93/03459 ;`` 7 ~111326 section microtomed from the sheet is examined in a polarizing microscope using cfossed polars. The beta-sphenulites show up as being much brighter than the alpha spherulites, due to the much higher biretringence of the beta-spherulites. For the thermoformable sheets ot this invention the 5 beta-spherulites should oocupy at least 50% ot the field of view.
In the x-ray diffraction method the diffraction pattem of the sheet is measured, and the heights of the three stron~est alpha phase diffraction peaks, Hl1o. H130 and Ho40 are determined, and compared to the height of the stron~ beta phase peak, H300. An empirical parameter known as ~K~ is 10 defined by the equation:
K (H300)/l(H3oo)+(H11o)+(Ho~o)+(Hl3o)l The value ot the K parameter can vary trom 0, tor a sample with no beta-aystals, to 1.0 tor a sample with all beta crysWs.
For the thermotormable sheets ot this invention, the preterred 15 beta-spherulite nucleating agent is C~dye present at a level of about 0.1 to about 10 ppm and the value ot the K parameter should be in the range of about 0.3 to 0.95, preterably in the range of 0.4 to 0.85. Above a value ot 0.95for K, there is not sufficient alpha-spherulite content in the sheet to SUppOft the sheet and to prevent the sheet trom sagging during the heating step ot the 20 thermotormin~ process. Below a value of 0.3 tor K, insufficient beta-spherulites are present to allow facile thermoforming of the sheet at the beta phase melting temperature. The optimum range tor the value ot K is about 0.4 to about 0.85. For sheets having K-values of about 0.3 to 0.95, the sheet so1tens at a lower temperature and allows shorter cycle times to be used 25 in the pf~duction ot thermohrmed afticles.
Thermal analysis ot the therrnoformable sheet can be charactefized by Differential Scanning Calorimetry (DSC) to determine the beta-spherulite nucleation effects. Parameters which are measured during the first and second heat scans ot the DSC include the aystallization temperature, Tc~ the 30 meltin~ temperature, Tm, ot the alpha and beta aystal forms, and the heat ot fusion, ~H~, both the total heat ot fusion, ~Hf , and the beta meltin~ peak heatot fusion, ~Hf . The magnitude of the ~H~ parameter provides a measure of how much beta c ystallinity is present in the sample at the start ot the heat scan. Generally, the second heat ~H values are reported, and these values 35 represent the properties of the material after having been melted and WO 93/21262 PCI /US93/0345g .'~111326 8 recrystallized in the DSC at a cool-down rate of 10C/minute. The first heat thermal scans provids information about the state of the material before the heat history of the pr~cessing step used to make the samples had been wiped out.
S In ~reater detail, for the thennofonnabîe sheet of this invention, various types of polyolefin resins can be used as the starting ~ase resin with pafticularly satisfactory results obtained by the use o~ resinous polymers of prowlène. Suitable resinous polymers of propylene include propylene homopolymer, fandom or block copolymers of propylene and ethylene or an a-olefin having 4 to 12 carbon atoms, preferably 4 to 8 carbon atoms, such as butene-1, hexene-1 and mixtures of such a-olefins. Also, blends of propylene homopolymers ~th other polyolefins such as high density polyethylene, low density polyethylene, Iinear low density polyethy!ene and polybutylene can be used. Preferably, the resinous polymer of propylene is selected trom the group consisting of polypropylene, random or block copolymers of propylene and up to 40 mol% of ethylene or an a~ olelin having 4 to 12 carbon atoms and mixtures thereof, blends of polypropylene and low density polyethylene and blends ot polypropylene and linear low density polyethylene.
The re~inous polymer of propylene also reterred to herein as polypropylene-based resin, propylene-based polymer or resin, and in particular, polypf~pylene homopolymer should have a melt now rate (MFR) as measured by ASTM-t238 which is great enough for facile and economical produ~ion ot the thermoforrnable sheet but not so great as to produce a sheet with undesirable physical properties. Typically, the MFR should be in the range of about 0.5 to 20 dg/min and, preferabJy, from about 1.0 to 10 dg/min.
When the MFR ot the resin exceeds 20 dglmin, disadvantages are caused by 1he unduly low rigidity of the resin sheet with increased sagging of the sheet when being the~moformed. When the MFR is less than 0.5 dg/min, difficulties are encountered in shaping o~ the she~t due to the unduly high melt viscosity.
The resinous polyrner of propylene can be admixed according to need with various other kinds of additives including lubricants, antioxidants, ultraviolet'''absorbers, radiation resistance agents, antiblocking agents, antistatic agents, coloring agents such as pigments and dyes, opaciffers such as talc and rlO2, and the like in the usual quantities. Care must be taken to avoid incorporation of other nucleating agents or pigments which might act as nucleatin~ agents since these materials may prevent the proper nudeation ot beta-spherulites. Radical scavengers, such as dihydro~y talcite, should also be avoided since they have some nucleating abilit~r. Mineral materials used WO 93/21262 PCI'/US93/03459 2~ 1t~2~i as whiteners or opacifiers such as r~O2 and CaC03 are not nucleants and do not interfere with the beta-sphenulne nucleation. ~he effective amount ot such additives wiîî depend upon the particular application or end-use intended for the anides thermohrmed from the sheet and can range from 0.005 to abo~ 5 5 wtYo, based on the weight of the polymer. Suitable stabilizers are the usuaî
stabilizing compounds for polypropylene and other a-olefin polymers.
Prebrably, for opaque, white thermoformed anicles TiO2 or CaC03 is added to the beta-nucleated resinous polymer ot propylene at a level of about 0.5 to about 5 wtYo.
Preferred antistatic agents are alkali metal alkane sulfonates, polyether-modified, i.e. ethoxylated and/or propoxylated, polydiorgano-siloxanes andlor substantially linear and saturated, aliphatic tertiary amines containin~ a C1~20 aliphatic radicaJ and substituted ~y two C1 4 hydroxyalkyl ~roups, amon~ which N,N-bis-(2-hydroxyethyl)-alkyl amines containing C
and prebrably C~2 18 alkyl groups are particularly suitable.
SuitaUe antiblocking agents are inorganic additives, such as silicon dioxide, calcium carbonate, magnesium silicate, aluminum silicate, calcium phosphate and the like, nonionic surfactants, anionic surtactants andlor incompatible organic polymers, such as polyamides, polyesters, polycubonates and the like. Examples ot lubricants are hi~her aJiphatic acid amides, hi~her aliphatic acid esters, waxes and metal soaps.
The melting point ot the beta-torrn spherulites ot polypropylene-based resins is generally about 144 to 148C, contrasted with the typical melting point range ot alpha-torm spherulites of about 159 to 163C. When extruded sheet contaifing beta-spherulites is heated above the metting point ot the be~sphenJUte crys~ls, but below the melting point of the alpha-spherulites, the sheet becomes sott enough to thermoforrn, and the unmelted alpha-sphenulite aystals in the she~t act to reinforce the sheet against saggingbefore and durin~ the themnotorrning step. As the thermoformed article cools, the mened put ot the polypropylene-based resin recrystallizes as the higher meltin~ alpha toml so that there are essentially no beta crystals present in thethermoforrnéd a~ticles, thus allowing the thermotormed alticle to have the same high temperature physical prope~ties as articles thermoforrned trom non-beta-spherulite nudeated polypropylene.
Some bda-torrn nucleated resinous polymer ot propylene sheet has b~en shown to have roducsd optical propora~s. Also, ~ Q-dyo lov~ls abov~
about 2 ppm the thermotormed anicles exhibit a ~pinWsh~ color to the human eye. Excellent opaque, white therrnotormed anicles can be prepared from .';

W093/21262 2~ ~ ~32~i PCI~/US93/o3459 such sheet by adding TiO2 since such addition does not interfere with the formation of beta-spherulites. Q dye has also been shown to be quite effective at the 0.5 ppm level in inducing high levels of beta crystallinity, and at this concentration in thermoformed articles it is virtually undetectable to the humanS eye. Also, intensive compounding of the resin and dye in a twin screw extruder has been shown to redwe the color imparted to the tinal resin at a given dye concentration. Other potentiai beta nucleating agents that are colorless can be used.
After formation ot a homogeneous compositibn of a resinous polymer of 10 propylene and an effective amount of a beta-spherulite nucleating agent, the composition can be used in the method of this invention tor thermoforming a resinoùs polymer of propylene-containing sheet comprising the steps of:
(a) melt forming a polymeric composition comprising a crystalline resinous polymer of propylene having alpha-spherulites and an effective amount of a nuclea1ing agent capable of producing beta-sphenulites into a sheet;
(b) quenching the melt-tormed sheet at a quench temperature sutticient to produce beta-spherulites at a concentration corresponding to a K-value of about 0.3 to 0.95;
(c) heating the quenched sheet to a thermoforming temperature sufficient to allow thermoforming of the sheet; and (d) thermoforming an article from the heated sheet with a thermoforming means under thermotorming conditions.
The resinous polymer of propylene used in this method is selected from the group consisting of polypropylene, random or block copolymers of propylene and up to 40 mol% of ethylene or an a-olefin having 4 to 12 carbon atoms and mixtures thereof, blends of polypropylene and low density polyethylene and blends ot polypropylene and linear low density pol~ethylene.
The beta-spherulite nucleating agent usetul in the method ot this invention is any inorganic or organic nucleating agent which can produce beta-spher~lites in the melt-fQrrned sheet at a concentration corresponding to a K-value of 0.3 to 0.95. We ha~ e tound that quinac idone colorant Permanent Red E3B is particularly effective as a beta-sphenul;ne nucleating agent when present at a level ot about 0.1 to about 10 ppm, based on the weight of the resinous pdymer of propylene, and which has the stru~ral formula:

211132~i H O

In a broader sense, the therrnoformable sheet need not be limited to one layer but can be two-layered, three-layered or more than three layers.
5 Conventionally, muni-layer and single layer sheets can be melt tormed by coextrusion and extrusion, respectively, by various known shaping methods such as the calender method, the extnusion method and the casting method.
Among these, the melt extrusion sli1-die or T-die process is especially preferred. Extruders used in such a melt-extrusion process can be single-10 screw or twin-screw extnuders. PrebraUy, such machines should be tree of excessively lar~e shearing stress and be capable ot kneading and e~nruding at relalively low resin ~emperatures.
In the preparation ot 1he therrnoforrnable sheet by the slit-die, T-die or other suitable processes, the extruded sheet in the form of molten polymer is 15 quenched or cooled ~o solidify the molten sheet by a suitable quenching means such as a single quench roll or a multi-roll quench stack such as a 2-roll, a 3-roll or a 5-roll quench stack and the like. The quenching me~ns must be capable ot quenching the sheet at a rate equal to or greater than the sheet production rate and the temperature encountered by the sheet in the 20 quenching means must be in a range suitable to promote the development ot beta-spherul~tes. Preterably, a 3-roll vertical quench stack is used with the sheet nipped between the top and middle rolls with the beta-spherulite crys~llinity starlin~ at the middle roll and the sheet wrapping around the middle and boltom rolls. The temperature ot the middlé roll should be at bast 25 80C, prebrably in the range ot 90 to 130C, tor optimum prodwtion ot beta-spherulites. For a single layer sheet having beta-spherulites ~hroughout the sheet the temperature ot the bottom roll should be in the ran~e ot about 80 to 110C. However, it a single layer sheet with a very small amount ot beta-spherulites neu the sheet surtaces and a larger amount ot beta-spherul~es 30 near the center is desired, tha bottom roll temperature should be less than 80C. The temperature ot the top roll of the 3-roll stack is less c~ and can range trom 60 to 120C without adversely affcctin~ t~e beta torm content of the sheet. The quenching means should be positioned relatively close to the WO g3/21262 PCl`/US93/03"59 extruder die, the distance being dependent on factors such as the temperat~re ot the rolls, the sheet extrusion rate, the sheet thickness, and the roll speed.Generally, the distance trom the die to the roll is about 0.25 to S cm. The quenching step can be overdriven relative to the rate ot extruded sheet 5 production to eff0ct a drawdown ot the extruded sheet. Since sheet made by this process is drawn in only one direction, strength properties are not Wanced in the machine and transverse directions.
For producin~ coextruded muni-layer sheet havin~ beta-spherulite nudeated resinous polymer ot propylene as one layer, one extruder may be 10 used to extrude a sheet ot the beta-spherulite nucleat~d resin and a second extruder used to extrude a layer of non-nwleated polymer~ resin as a layer on at least one side ot the nucleated resin layer with the resin layers contac~ed between nip rolls. U a layer ot non-nucleated resin is desired on both sides of the beta-nucleated resin then the non-nwleated polymer melt can be split 15 between two slit dies and a ~econd layer of extnuded sheet contacted with theother side of the beta-nucleated polymer resin layer between a second set of nip rdls. ~tematively, more than one extruder can be u~ed to supply molten pd~ner to a coextrusion die which allows two or more distinct polymer layers to be coextruded from a ~iven slit-die. The temperature at the die exit should 20 be controlled by use ot a die-lip heater to the same or slightly higher .emperature than the resin melt temperature in order to prevent ~freeze-off~ of the polymer at the die lip. The de should be free ot mars and suatches on the w~tace so as to give a sheet having smooth surtaces.
The singb layer sheet or multi-layer sheet prepared by extrusion, 25 laminatbn or other means can have a thickness which is thick enough to be therm~formed without sag~ing too much durin~ themlotorrning and not too thick as not to be able to be the~oformed into an acceptable part. Typically, the thermobtmable sheet ot this invention has a thickness ot 0.25 mm or greater and ranges trom about 10 to about 200 mils. The multi-layer sheets 30 can have a construction in which the beta-nucleated polymer resin occupies from about 10 to about 99.9 percent of the sheet thickness and the non-nucleated polymer resin occupies f`rom about 90 to about 0.1 percent ot the sheet thickness. Prebrably, tor three-layer sheet, the inner layer is the beta-nucleated polyrner and occupies about 50 to about 99.5 percent of the 35 sheet thickness and the outer two layers are non-nucleated polymer and occupy from about 0.5 to about 50 percent ot the sheet thickness. The outer layers can have substantially equal or different thicknesses. Preferably, the outer laye~s each have a thickness of about 0.01 to about 0.1 mm and the ~ 13 ~fll~26 inlermediate layer has a thickness of about 0.23 to about 4.5 mm. Such multi-layer sheet can have a combination of different resins by the use ot two or more extruders. The resinous polymers of propylene for the multi-layer sheet can be, for example, polypropylene homopolymer, random or block 5 polymerized ethylene-propylene copolymer, polypropylenes having different melt ~ow rates, a polypropylene and an adhesive polyoletin modified with an unsaturated carboxylic add or a derivative lhereof, a polypropylene and a polyethylene or an ethylene-vinyl acetate copolymer, a polypropylene and an ethylene-vinyl alcohol copolymer, beta-spherulite nucleated polypr~pylene 10 and polypropylene, and the like. For thermoformable sheet or thermoformed ar~cles o~ three or more layers, an inner layer may be employed as a tie layer to join together exterior polymer layers or the inner layer can be a ~as/chemical barrier layer to provide gas or chemica~ resistance. Altematively, such multi-layer sheets can be tormed by other known means such as the 15 lamination of roll stock sheets together by heat and/or adhesive tie layers, by lamination of loll stock to a sheet as a is bein~ extnuded and the like.
Wth respect to multi-layer thermoformable sheet thermoformed into articles providing gas and chemical barrier protection, the barrier layer typically employs a polymer matrix such as poly(ethylene vinyl alcohol) 20 (EVOH), various hi~h narile polymers such as poly (vinylidene chloride) and the like as the polu polymer and a polymer such as a polyolefin as a moisture resistant, non-pdar polymer. ~-A8 a gas/chemical barner polymer, EVOH polymers can be used with an ethylene oontent varyin~ trom 29 to 44 mol%. Typical copolymers used are 25 EVAL grades supplied by Kuraray Col, Ltd., Soamol ~rades supplied by Nippon Goh~i and Selar OH grades supplied by DuPont Co. Other barrier polyme~ indude high nitrile polymers such as Barex 2t0 and Barex 218 (high acrylonitrile-methy~ acrylate copolymers grafted onto a preformed poll~(butadiene-acrylonitrile) elastomer); high acrylonitrile-styrene co- and 30 terpolymers; high acrylonitrile-indene co- and terpolyme s; and, homo-, co- or terpolymers high in methacrylonitrile content. Another class of barrier polymer~ which can be used is that derived from all common homo-, co-, or terpolymers based on vinylidene chloride.
Representative examples of other barrier type polymers include 35 poly(vinyl chlo~ide); methyl methacrylate-styrene copolymers gratted onto a diene elastomer; amorphous polyamides such as Trogamid T, crystalline polyamides such as nylon-6 and nylon-66; polyesters such as polyethylene terephthalate and poly(ethylene 2,6-naphthalene dicarboxylate);

WO g3/21262 PCI/US93/03459 S~111326 t4 polyurethanes; polycarbonates; polyphenylene oxide; polyphenylene oxide/polystyrene blends; polystyrene; polyetherimide and polyalkyl methacry ates.
Polymers for the inner layer can be selected for other functions such as.
5 for instance, to provide systems with hi~h-temperature resistance charactenstics. In that case, polymers that can be employed are selected trom - the group consisting of polycarbonate, polyethylene terephthalate, poly(ethylene 2,6-naphthalate dicarboxylate), polyphenylene oxide, polysulfone, polyetherimides, thermoplastic polyimides and 10 polybenzimidazoles. The additional polymer layer for the inner layer should not adversely affect the improved therrnoforrnin~ characteristics of the beta-sphenulite containing resinous polymers of propylene. Preferably, the intermediate layer additionally comprises a crystalline resinous polymer of propylene and a residue of an organic beta-sphenulite nucleating agent or an 15 ethylenevinyl alcohol copolymer.
Pa~ticular polymer composition combinations can be used for one or both ot the outer two layers for sheets of three or more layers. For a thermoformable sheet comprising an intermediate layer ot the beta-spherulite-containing resinous polymer ot propylene and two outer layers of a 20 thermoplastic resin, the resinous polymer of propylene is selected trom the group consisting ot polypropylene, random or bk~ck copolymers ot propylene and up to 40 mol% of ethylene or an a-olefin having 4 to 12 carbon atoms and mixtures thereof, blends of polypropylene and low density polyethylene and blends of polypropylene and linear low density polyethylene and the 25 thermoplastic resin is selected trom the group consisting of polypropylene, random or block copolymers of propylene and up to 40 mol% of ethylene or an a-olefin having 4 to 12 carbon atoms, blends of polypropylene and low density polyethylene, blends ot polypropylene and linear low density polyethylene, a block ethylene-propylene copolymer having an ethylene 30 conlent of about 1 to 20 wth, blends of ethylene-propylene nJbber polymer and high density polyethylene and blends of ethylene-propy~ene rubber polyrner aiid low density polyethylene.
For example, impact modified polypropylene copolymers can be used for the outer layers and beta-nucleated material can be used for the middle 35 layer lO produce a thermoformable sheet with increased thermoforming ra~e and thermoformed articles with improved low tempe~ature impact resistanc0.
For optimum formation of beta-sphenJlites in the melt-formed sheet the quench temperature ot step (b) is about 90 to about 130C. The ~ 15 ~111326 thermoforming temperature of step (c) should be sufficient to melt the beta-sphenulites but not the alpha-spherulites. Typically, the beta-spherulite form of polypropylene has a melting point of about 144 to 148C and the alpha-spherulite torm of propylene hæ a melting point of about 159 to 163C.
5 ey heating the quenched sheet to a temperature in the ran~e of about 144 to 148C, the beta-spherulites sotten and allow thermotorming of the sheet. The alpha-spherulites remain in the solid phase, provide integrity to the sheet and prevent excessive sagging of the sheet during thermoforming.
In a particular embodiment ot the method ot this invention when the 10 thermohrming temperature ot step (c) is less than the melting temperature of the beta-spherulites of 144 to 148C, the thermotorm0d article of step (d) can exhibit a tendency to undergo ~mhrovoiding~. By microvoiding is meant the formation of ve y small voids within the sidewalls of the therrnotormed articles.
This microvoiding pf~duces an owue, white thermofo~ed article without the 15 presence of a filler. The sidewalls ot these containers have a density from about 2 to about 20% less than the sheet trom which they were formed.
~though the microvoiding provides a less dense sidewall the articJe still has integrity and vapor barrier pf~perties.
The thermoformable sheet of this invention can be thermoformed by 20 conven1ional thermoformin~ equipment and processes including thermotorming in-line with a sheet casting extruder or off-linè using a roll-fedthermoformer. Conventional thermotorming processes include vacuum forfning, pressure forming, plug assist pressure forming and matched-mold the~moforming which are described in The Encyclopedia of Polymer Science 25 & Engineering, John ~lley & Sons, Vol. 16, p. 807-832, 1989. Such .thermofofming is the pr~cess of manufacturing pr~ducts from thermoplastic sheet generalîy invdving the sequential steps of (a) heating a thermoplastic sheet until it softens, (b) tormin~ the softened sheet under the influence of gfavi~y, pressure and/or vacuum in a mold, and (c) allowing the formed sheet 30 to cool, harden and be die cut from the sheet, stacked and packaged.
Variations of basio therfnohrming include processing cut-to-size, r~ll-feed, or in-line extfuded sheet; matefials of sheet heating such as metal-sheath radiant heaters, quar~e radiant heatin~ panels, ceramic heaters, convection ovens, con~ heating and the like; type of mold; vacuum or air pressure forming; trim 35 in place or separately; and pa~a~ing. For the method ot thermo~orming sheet ot this invention generally lower pressures can be us~d as compared to the thermotormin~ of non-beta-nucleated polypropybne sheet. Also, the method ot thermoforming ot this invention can be done in-line during the preparation of WO 93/21262 PCl`/US93/03459 21~1326 16 the thermoformable sheet or it can be done off-line trom rolls ot sheet material.
Preferred thermoforming processes include vacuum forming and plug assis~
pressure torming.
Various combinations of polymers and layers can be used in 5 combination with the beta-nucleated crystalline resinous polymers of propylene to torm the thermotormable sheet and thermotorrned anicles ot this invention. For example, in multi-layer sheet with beta-nucleated material as the intermediate layer, the intermediate layer can aWitionally comprise an ethylene vinyl alcohol copolymer or regAnd material compAsing a crystalline 10 resinous polymer ot propylene and a residue ot an organic beta-spherulite nucleating agent such as ~dye.
Such sheet can be thermotormed at lower temperatures and at taster cycle times relative to that required tor resinous polymers of propylene containing no beta-spherulite nucleating agent. Under these therrnoforming 15 conditions ot lower temperature and taster cycle time, sheet sag is less ot a problem and less heat is needed to be removed from the sheet thereby producing a more rapid set-up ot the thermoformed anicle and allowing wWer unsuppo ted sheet to be used in the thermotdrmin~ operation. -Thermoformed articles ot this invention typically are used in 20 applications including: automotive applications swh as bumpers, truck-bed liners, tender wells, door panel insens, glove box doors, and the like; -consumer items such as luggage, trays, storage trailers, ice cooler liners, ice cube trays, toys, signs, and the like; appliance applications such as retrigerator door liners, treezer panels and the like; housewares such as cups, shower 25 stalls, sinks, tubs, and the like; recreational materials such as boat hulls, golf cart canopies, bicycle wheel covers, hoods and shrouds tor skimobiles an be containers and lids tor containers tor toods and beverages in general; and the like and packaging applications such as food containers including such items as yO~un cups, margarine tubs, cottage cheese containers, deli containers, 30 trozen food trays, lids, and the like, meat trays, fast tood disposables, and the like. ~Iso, articles having a deeper draw and tormed 1rom thicker sheets can be made. Preterred thermotormed anicles are food containers and anicles having low-temperature impact resistance.
X-ray diffraction data was determined on specimens taken trom various 35 locations on thermoformed anicles made trom different resins. No evidence ot the beta diffraction peak was obsenred tor any ot the thermotormed sampl~s. It is known that when beta phase is melted without melting the remaining alpha phase, the molten polymer recr~stallizes as the alpha phase only. This occurs WO 93/21262 PCI`/I~S93/0345g ~11.132b because the un-melted alpha crystals direct the re-crystallization process.
Therefore, the thermotorrned articles contain virtually no beta crystallinity and the article has the same temperature properties as the non-beta-nucleated material. Althou~h the thermotormed articles contain virtually no 5 beta-spherulRes, the beta-spherulite nucleating agent and residue ot the nucleating agent remain in the article so that the anicle can be analyzed fsr the nucleating a~ent. Alternatively, therrnotormed articles made trom beta-nucleated material can be reground and tormed into sheet having beta-spherulRes if quenched under appropriate condRions described above.
10 Thus regfind matefial can be used by itselt or in continuation with virgin beta-nudeated matenal with the proviso that such materials are sufficiently compatible and that the re~find material does not contain material which might interfere with the nucleation ot beta-spherulites.
The following examples further elaborate the present invention although it will be understood that these examples are for purposes of ~
illustration and are not intended to limit the scope of the invention. ~ -1~ and Control ExamDle A
Cast sheets were made from nucleated polypropylene resins containing 20 dtferent levels ot beta-spherulite nudeating a~ent. The beta nucleant was a red quinacridone dye, E3B, commercially a~ailable trom Hoechst-Celanese. A
masterb~ch of ~dye at a level o~ 200 ppm was prepared as a powder blend o1 Q dye vnth polypropylene powder havin~ a MFR as determined by ASTM
D1238 o13.1 dg/min. The masterbatch was let down to final concentraffons of 25 1.0, 1.5 and 2.0 ppm o1 ~dye in a polypropylene resin having a MFR of 3.1 d~min. The resin Uends were stabilized with 0.18 wt96, based on the weight ot the resin, ot a stabilizer package of a hindered phenol, a phosphonite, and calcium stearate and pelletked with a 63.5 mm Prodex extruder. The above reSins as well as a control polyprowlene resin having a MFR ot 2.5 dg/min 30 and no beta nucleant were processed into cast sheet on a 38 mm Davis Standard extruder cast sheet line with the following processing conditions:
Polymer men temperature, C 227 ExtnJder screw rotation speed, rpm 25 ExtnJder die gap, mm 0.508 Sheet production rate, mls 0.017 Sheet thickness, mm 0.406 Chill roll temperature, C 108 Air knife pressure, psi 40 WO 93/21262 PCl`/US93/03459 326 18 ~

Example 1 was prepared tr~m a polypropylene composition having a O-dye concentration of 1.0 and Examples 2 and 3 had Q-dye concentrations of 1.0 and 2.0 ppm, respectwely. Control Example A had no O-dye. Example 4 had the same polyprowlene and ~dye concentration as Example 3 except that 5 the extruder screw rpm was increased trom 25 to 40 rpm and the sheet production rate increased from 0.017 to 0.028 m/s. Both the polypropylene-based compositions and the sheet were characterized by DSC.
The beta cr~stal content ot the examples ot sheet was characterized from the determination of the K-value trom x-ray diffraction measurements. Because 10 the sheets were thick enough to poæibly be anisotropic, x-ray diffraciion measurements were taken on both the air-knife and chill-roll sides ot each sheet. The polymer composition properties are summarked in Table I
including ~dye concentration, composiffon MFR, melting temperature of the alpha-spherulite phase, Tam, melting temperature of the beta-spherulite phas~, ~ . ~
1~ ~m~ and crystallization temperature, Tc. The sheet propenies are summarized in Table ll, including K-value and c~linity on both the air-knNe and chill-roll sides ot each sheet. -Table I
Polymer Compos~ion Proeerties p~perty 1 ~ ;~ Contr~l A
~dye, wm 1.0 1.5 2.0 O
MFR, d~hnin 3.10 3.20 2.85 2.55 T~, C 157.7 157.8 158.3 NM

T~m~ C 144.7 144.6 145.2 NM

Tc,C 116.5 115.3 116.6 NM
NM - not measured WO 93J21262 PCl IUS93/03459 ~-. 19 2:1 11326 Ia~ ;' ..
Example Pro~ertv 1 2 ~ 4Control A
K-value Air-knife side0.374 0.489 0.639 0.743 0.203 Chill-roll side0.4U 0.562 0.748 0.876 0.283 Crystall~nity, %
Air-knife side 53 55 59 63 54 Chill-roll side 57 61 64 66 56 ExamDIes ~a Polypropylene-based compositions containing four different levels of ~dye, NA-10 nucleant (sodium bis(4-t-butylphenyl)phosphate) and a control with no nudeant were prepared f~om nwleated polypropylene resin stabilized 10 with O.t8 wt%, based on the weight ot the redn, of commercially available antioxidan~ and processin~ stabilize~s as desc~ibed in Examples 1~. The composilions were melt compounded and p~elletized on a 63.5 mm Prodex extruder. After compounding and pelletizing, 5 wt% ot a TiO2 pellet concentrate, polypropylene resin with 50 wt% TiO2, was dry blended with each 15 ot the resin compositions. The resin blends were then compounded and extnJded into a 40 mil thick sheet on a 63.5 mm D NRM PMIV sheet extrusion ~\ne. The te~er~ures tor the ~roll ver~ical quench stack in the extrusion line were 110C tor the top roll and 104C tor the middle and bo~tom rolls.
The compo~ition ot Example S had a Q-dye concentration of 0.5 ppm 20 The oomposilions ot Examples 6, 7 and 8 had Q-dye concentrations ot 1.0, 2.0 and 4.0 ppm, r spsclively. Control Example C had a composition with no Q-dye added. Control Example D had NA-10 nucleating agent present at a bvel ot 850 ppm. The polyprowlene-based resins and extruded sheets were characterized by DSC and x-ray diffraction. The polymer composition - 25 proper~ies-are summarized in Table lll inc~uding Q dye composition, meltingt mperature ot the alpha-spherulite phase, Tam, melting bmperatur~ of the beta-sphenulite phase, Tm~ crystallization temperature, Tc, total heat ot fusion~Htt and heat ot tusion of the beta phase melting peak, ~Ht . The sxtn~ded WO 93/21262 PCI /US93/0345g '.)..111'~26 20 sheet properties including thermal properties for first and s~cond heat scans otDSC are summar~zed in Table IV.
Table lll ~_ =~ .
Example Pr~ee ~Y ~i ~i 7 ~ C ontrol B C ontrol C ::-Nucleanttype Qdye Q~ye Qdye Qdye None NA-10 Level, ppm 0.5 1.0 2.0 4.0None B50 Tc, C 114.4 113.5 115.8 119.2 108.0 127.6 Tm~ C 157.3 157.7 157.9 158.4 158.3 160.6 -Tm~ C 143.8 144.2 144.2 - - -~Htfat,caU~ 18.9 19.4 19.6 20.4 20.4 20.6 ;
~HB, caUg 0.2 2.1 0.1 Table IV
E~nn.ded Sheet Proeerlies - Example Proeerty ~ Con~rol B Control C
Fi~ Heat Tc, C 115.6 t l 7.6 118.6 120.9 110.3 128.4 -, C 161.7 164.1 161.2 164.5 161.8 161.8 Tm,C 146.6 147.5 146.8 t46.5 145.0 ~Hf , caUg 19-4 19.1 19.2 19-4 19.4 20.3 ~HB ~9 2.5 2.1 1.6 0.4 0.2 -TC~ C ~ 160.2 162.0 161.3 162.6 159.9 162.3 TBm~ C 146.4 145.8 148.4 - 145.3 tft caU~ 20.6 20.6 21.3 21.4 19.4 21.7 ~Hf~, caU~ 0 9 0.2 - - - -WO 93/21262 P~/US93/03459 21 ~1 ~t326 From the data in Tables lll and IV, the parameters ot greatest interest from the thermal analysis relative to nucleation effects are the crystallizationtemperature Tc, and the heat ot fusion of the beta melting peak, ~Hf . As the density of nucleation centers increased, Tc increased. The magnitude of the 5 ~Hf parameter provides a measure of how much beta crystallinity is present in the sample at the start of that heat scan. Generally, the second heat ~H values are reported, and lhese are representative of the propenies of the material aner having been melted and re-crystallized in the DSC at a cool-down rate of 10C/minute. The first-heat thermal scans provide information about the state 10 of the material before the heat history of the proceæing step used to make the samples had been wiped out.
From the second heat scan data in Tables~ lll and IV, it can be seen that the polymer compositions showed a peak in beta crystallinity at the 1.0 ppm nwleant level, while the extn)ded sheet showed this peak at a beta nucleant 15 content at the 0.5 ppm level. The trend of the existence of a maximum level ot beta c ystallinity with increasing nucieant concentration is believed to be due to C~dye nudeating both alpha and beta crystalline forms of polypropylene.
The alpha torm begins to c ystallize before the beta bml, and can dominate the morpholo~y U a w!ficiently high concentration ot nudeant particles is not 20 present. The level ot beta crystallinity that develops depends not only on nucleant concentration, but also on the degree ot dispersion ot the nucleant particles and the thermal conditions used to crystallize the material. The samph made trom the extruded sheet differed in two ways trom the base resin sample. Fl~y, the sheet sampb had undergone an additional compounding 25 step, and this may have served to alter the dispersion of the nuclei particles~
Secondl~r, the sheet sample contained 2.5% TtO2, and this may have also affected the crystallization behavior.
The important effect of thermal history on the cry~talline morphology ot the sample can be seen by comparing the ffrst and second heat scans ot the 30 extn~ded s~ieet samples. n can be seen that the first heat scans of Examples 5, 6 and 7, with beta-nucleant levels of 0.5, 1.0 and 2.0 ppm, respectively, all have prominent beta melting peaks relative to the other sheet samples. This result suggests that the beta eontent maximum is broader for the sheets as compared to the resins, and extends from Q dye levels ot 0.5 - 2.0 ppm. It is 35 also noteworthy that low levels of beta crystallinity are seen in the WO 93/21262 PCI`/US93/03'159 2~32 , ' ~ , un-nucleated Example Control B, and no evidence ot a beta phase is seen for the conventionally alpha-tonn nucleated material Example Control C.
Articles were thermotormed trom the extnuded sheets ot Examples 5, 6 and 7 and Control Examples B and C using a Plastitorm Labtorm Model 1620 5 PVICP thermoformer with a 12 ounce ~cottage cheese~ cup mold. The thermotormer heater was set at three different settings: 315C (600F), 371C
(700F) and 42PC (800F). The thermotorming evaluation was pertormed at three different heater settings with the 427C setting bein~ the standard setting used to evaluate the thermohrrnability of polypropylene. The lower the heater 10 temperature, the longer the cycle times that were required to produce acceptable looking cups. At each temperature, heating times were varied to determine an upper limR, best operating range, and lower limit heating times.
The lower limit value represents the minimum time needed to produce an acceptable part wherein uniformity of wall thickness and sharp duplication ot 15 mold contours define the criteria of an acceptable part. Above the limit ot the uwer time the sheet becomes too soft, and excessive draw-down and sticking ot the sheet to the plug was observed.
The x-ray data obtained on the extruded sheet samples are summanzed in Table V. X-ray measurements were pertonned on both sides of 20 the sheet because the thermal history ot the two sides were somewhat different. From this data it can be seen that the highest levels ot be~a crystallinity as measured by the K-value were for the sheets that contained 0.5,1.0, and 2.0 wm ot ~dye, which is consistent with the thennal data discussed above. ExarnpJe 8 containing 4.0 ppm ot ~dye showed a large discrepancy 25 in the K-value trom one side ot the sheet to the other, suggesting that this material was p~icularly sensitive to differences in thermal history.
Resins which showed the greatest degree ot mold fflling were Examples 5, 6 and 7. These ue the Examples which exhibited the highest K-values and the highest amount ot beta crystallinity on the tirst heat DSC scans. The 30 sample with the poorest de~ree of mold tilling was the conventionally alpha-torm nucleated resin, Control C, which had no beta crpallinity evidenced by x-ray or therrnal analysis. Control B, which had a low level of beta crl~stallinity, had interrnediate mold tilling behavior. Clearly the thermotormability ot these sheets at low heating times accurately mirrors the level ot beta crystallinity that 35 is present in the sheet.
The optimum thermotorrning window data tor these resins at the 427C
heater setting are given in Table Vl and demonstrates that a broader processing window exists for Examples 5, 6 and 7 for those resins whose extruded sheets contained the highest level of beta crystallinity. Similar data `:;
obtained at heater settings of 316 and 371C are also summarized in Table Vl Table V
X-Rav Data on ExtmdQd Sheets 2~m~1e Pro~erty ~ g ~ ~ Control B Control C
T~pe Q dye Q dye ~dyeQ dye None NA-10 Level, ppm 0.5 1.0 2.04.0 None 850 K-Value Bottom side of sheet 0.71 0.60 0.500.57 0.29 0 Top side of sheet 0.77 0.67 0.670.31 0.22 0 Table Vl ~--~--Thermoforming ~m~l~
Tlme. sec. ~ ~ 7 ~ Control B Control C ~-~
~16C
Upper Limit 46 46 46 46 46 47 Optimum Range 40-44 40-44 40-44 40-44 40-44 40-46 Lower Umit 38 38 38 38 38 38 Window 8 8 8 8 8 9 ~71C
Upper Umit 31 30 31 31 30 30 Optimum Range 25-30 25-29 25-30 26-30 26-29 26-29 Lower Umit 24 24 24 25 t5 25 ~ :~
Wmdow 7 6 7 6 5 5 UpperLimit 21.5 21.5 21.5 20.5 21 22 -Optimum Ran~e 19-21 18-21 18-21 18.~20 20-20.5 20-21 Lower Umit 18 18 18 18 19 19 ndow 3.5 3.5 3.5 2.5 2 3 :-~

WO 93/21262 PCI`/US93/03459 2111'~6 24 Examples 9-11 Polypropylene-based compositions were prepared from nucleated polypropylene resin stabilized with 0.18 wt/O, based on the weight ot the resin, ot a stabilizer package of a hindered phenol, a phosphonite, and calcium 5 stearate. These compositions contained two different concentrations of Q-dye or a concentration of an alpha-spherulite nucleant, NA-10 (sodium bis(4-t-butylphenyl)phosphate) and were prepared as described in Example 1.
These compositions were melt compounded and pelletized on a Japan Steel Works CIM extruder. After compounding and pelletizing, r~O2, in the form of a 10 50 wPh Tl02 concentrate (P-8555 available from A. Schulman Co.) was added to the composition to obtain a 1 wtYo level rlo2. The compositions were extnuded into various thickness sheets. The Example 9 composition had a ~dye concentration ot 0.75 ppm and no r~O2. Example 10 had a Q-dye conoentration of 0.75 ppm and 1 wtYo of TiO2. The Example 11 composition 15 had a Q-dye concentration ot 1.5 ppm and Control Example D had a non-beta-spherulite nucleating agent concentraffon ot 850 Wm. The 70 mil sheet was extnJded on a 89 mm Welex extruder using a 3-roll quench stack with a top roll temperature range ot 71.7 to 73.3C, a middle roll temperature range ot 101.1 to 102.8C and a bottom roll temperature range of 79.4 to 20 80.6C. Sheets with thicknesses ot 17 mils, 25 mils and 48 mils were extnJdedusin~ a ~roll stack on a 114 mm Welex extruder with a top roll temperature of 60C, à middle roll temperature range of 97.8 to 102.8QC and a bottom roll temperature range ot 81.1 to 85.6C.
Otf-line thermoforming of the 25 mil sheet was done on an Armac 25 thermoformer with a rectangular tray mold. Off-line themloforming of the 48 mil sheet was done on a Gabler 743 thennotormer with a 16 ounce deli cup mold and IWs forthe 16 ounce cup were made on the Gabler lid thermoformer using the 17 mil sheet.
01f-line extrusion was performed on a Welex 114 mm extruder to 30 produce sheets having thicknesses of 17, 25 and 48 mils. These sheets were therrnoformed, respectively, into lids for 16 ounce containers, rectangular trays and 16 ounce deli cups. All ot these sheets were made trom the polymer compositions with 2 wt~6 ot the P-~555 r~O2 for a rO2 tinal content of t%.
The major differences in the sheet appearance involved the slight pink 35 coloration o1 the Example 9 and 10 sheets, and the som0what lower level ot gloss on the bonom side of these two beta-nucleated sheets. The bottom side ot the sheet was the side in contact with the middle chrome roll, and a microscopic examination ot sections cut trom the sheet showed a higher WO 93/21262 PCI`/US93/03459 .~ 25 ~I II 326 concentration of beta spherulites on this side ot the sheet. The beta spherulites probably created minor imperfections on this side ot the sheet, which w~re responsible tor the lower gloss.
A subsequent x-ray analysis of samples taken trom these sheets was 5 performed to characterize the distribution of crystal types. The K-values obtained on both the top and bottom surfaces ot each sheet are given in Table Vll. The Control D sheets contained no beta crystals. For sheets made from the beta nucleated resins, Examples 9, 10 and 11, the beta crystal contenl was generally higher on the bottom side of the sheet. This effect was probably due 10 to the higher temperature that the bottom side of the sheet experienced in ~ontact with the middle roll. For the 25 mil thick sheets one sheet with no nO2 was also made and its K-values were almost identical with that of the Example 9 sheet made from the same resin after r~O2 was added, indicating that there was li~e or no intluence ot the r~O2 on crystal nucleation but the r~O2 did 15 contribute to a more uni1Ormly white, opaque sheet. For the 25 mil thick sheet, increasing the C~dye l~vel from 0.75 to 1.5 ppm had a very marginal effect on the K-value. The 17 mil thick sheet had the lowes~ K-values and this may haYe been due to the hbher line speed and shorter contact ffme ot this sheet with lhe middb ch~ome roll. The 48 mil thick sheet had a high K-value of 0.79 that 20 was the same on both sides of the sheet.
Trays using 25 mil thick sheet trom Example 11 were made using standard conditbns ot 14.5 cycles per minute (cpm). Under these conditions the lrays boked quite acceptable. When the production rate was speeded up above 15 cpm, some bss ot sidewall definilbn was observed. Trays made 25 from Example 9 sheet appeared to have better material distribution than Example 11 t-ays, and speeds above 16 cpm with no loss in sidewall detinition were obtained. The beta nucleated sheets afforded a 10-15% improvement in the produc~on rate ot ar~cles on this thermofo~mer, and gave a tray with bener overall material distribution and sidewall strength, compared to neat 30 polypropylene resin. Lids tor 16 ounce cups were made using 17 mil sheet f~om Exam~pb 11 resin in the Gabler lid machine. This machine was operated at a maximum speed ot 15 cpm using nonbeta-nucleated polypropylene resin.
At higher speeds warping of the lids was obsen~ed. With Example 11 sheet speeds were obtained up to 20 cpm with no warping, tor a productivity 35 increase ot 33~ The 16 ounce containers were thermoformed from the 48 mîl thick sheet using the Gabler 743 thermotormer. Thermotorming was started with neat polypropybne sheet at speeds ot 14.9 cpm. When the proc~ss was switched over fo Example 9 sheet, the appearance ot the containers improved WO 93/21262 PCI'/USg3/03'~59 ? ~ ~326 26 ~ ~

dramatically. Production rates were achieved up to 18 cpm with excellent mate~ial distribution and no visible warp. The cups made from this sheet had a shiny outside appearance, with more of a matte type finish on the inside. Here the outside of the cup corresponded to the top side of the extruded sheet.
5 When the sheet roll was flipped over, so that the top side ot the sheet becamethe inside of the cup, the matte finish was seen on the outside of the container.
Ta~e Vll Sheet Bottom Thick- ~oll Sheet Level, ness, Temp.
Fxample Side IiQ2 ~ ~m m~ ~Vah~e C
9 Top No Q-dye 0.75 ~ 25 0.69 79 9 Bottom No Q~e 0.75 25 0.84 79 Top Yes O dye 0.75 25 0.69 79 1 O ~ottom Yes Q dye 0.75 25 0.86 79 11 Top Yes Q dye 1.5 25 0.76 79 11 Bottom Yes Q dye 1.5 25 0.88 79 11 Top Yes Q-dye 1.5 17 0.49 83 11 Bonom Yes ~dye 1.5 17 0.62 83 Control D Top Yes NA-10 850 25 0 83 Control D Bottom Yes NA-10 850 25 0 83 Top Yes Q dye 0.75 48 0.79 86 Bonom Yes Q dye 0.75 48 0.79 86 Control D Top Yes NA-10 850 48 0 86 Control D Bottom Yes NA-10 850 48 0 86 -10~ Various analyses were performed on these 16 ounce cups, and a summary ot the results is ~iven in Table Vlll. In terms of sidewall proïïle cupsmade from the beta nucleated resin at 16, 17, and 18 cpm showed improved wall thickness relative to neat polypropylene. The increase in cycle rate trom 14.9 to 18 cprn for the beta nudeated resin, did not produce a significant 15 drop-off in any ot the cri~ical properties ot the container.

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Tests were performed on these containers using both water and spaghetti sauce in a microwave oven at a setting ot High tor 5 minutes. The containers made from the compositions containing 0.75 ppm Q-dye, Example 9, exhibited virtually no warpage or distortion following the microwaveability 5 test, whereas the containers made from the sheet of Control Example D were significantly distort~d during the test. This difference in behavior is believed to be due to lower molded-in stress in the containers thermoformed from the beta-nucleated sheet due to the beta phase being melted prior to thermoforming.
ExamplQ 12 A multi-layer thermoformable sheet was prepared having polypropylene homopolymer composition with no beta-sphemlite nucleation for the two outer or skin layers and a beta-spherulite nucleated polypropylene homopolymer 15 composition as the middle or core layer. Both compositions were stabilized with 0.18 wP/O, based on the weight of the polymer, of a stabilker package of a hindered phenol, a phosphonite, and calcium stearate. The composition of the skin layer was 12-5013 grade of polypropylene, available from Amoco Chemical Company, and had a MFR of 3.8 dg/min. The polypropylene-based 20 resin of the composition ot the core layer was a polypropylene homopolymer havin~ a MFR of 3.0 dg/min and a bet~-spherulite nucleating agent of 0.75 ppm of Q-dye. The compositions were melt compounded and p~lletized at conventional polypropylene operating conditions using a Japan Steel Works CIM extruder. After compounding and pelletizing, 5 wt/O, based on the weight 25 of the polymer, of P-8555, a concentrate of 50 wt% r~O2, was added to the compositions. The core layer of the multi-layer sheet was extnJded using a 89 mm Welex extruder and the skin layers were extruded using a 63.5 mm extruder. The total thickness of the sheet was 48 mils including a 2 mil thick skin layer on each side of the core layer. The throe layer sheet was extruded 30 onto a three-roll quench stack wRh temperatures measured on the surface ot the rolls of: top roll, 74C; middle roll, 98C, and bottom roll, 82C. Dynatup impact strength was measured on the extruded sheet at a temperature of -20C with a measured peak load of 41.3 Ibs and a measured peak energy of 0.21 ft-lbs.
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A three-laysr thermoformable sheet was prepared by the same process conditions and equipment as described in Example 12. The composition of WO 93/21262 PCI'/US93/03459 2111326 - . `
the core layer was 50 w~% ot a polypropylene homopolymer having a MFR of 3.0 d~/min and 0.75 ppm Q-dye nucleating agent and 50 wt% ot a normal 50/50 regrind blend of 3.0 dg/min MFR polyprowlene resin with residual levels of 0.75 ppm and 1.5 ppm ~dye nucleating agent. The composition of 5 the skin layers was an impact ethylene-propylene copolymer, grade PD7292N
from Exxon, having a MFR ot 3.5 dg/min. l)ynatup impact strength was measured on the extruded sheet at a temperature of -20C with a measured peak load ot 206.8 Ibs and a measured peak energy of 2.08 ft-lbs.
The sheets of Examples 12 and 13 were thermoformed using a 16 10 ounce deli cup mold with both in-line and off-line thermoforming processes.
Durin~ off-line thermoforrning of both Example 12 and Example 13 sheets, production rates ot 18 cpm and greater were achieved. The containers produced trom lhe sheet of Examples 12 and 13 had excellent appearance, good contour definition and good sidewall distribution. The 18 cpm production 15 rate represents a 20.8% increase in production compared to the typical production rate ot 14.9 cpm for non-nucleated polypropylene compositions with the 16 ounce deli cup mold. Various analyses were pertormed on these 16 ounce cups, and a summary ot the results is given in Table IX.
Microwave oven tests were pertormed on the containers of Example 12.
20 Fifteen ot the 16 oz deli containers of Example 12 produced at a rate ot 18.1 to 18.3 cpm containing water were placed in a microwave oven at a setting of - High for 5 minutes. All 15 containers held up well without any visible indicalion of warpage or shrinkage. fifteen of the 16 oz deli containers ot Example 12 prodwed at the 18.7 cpm production rate containing water were 25 placed in a microwave oven hr 5 minutes on a setting of High. Bottoms on some ot these containers ~bubbled out~ aner the microwave test. Containers produced at the higher thermoforming rate of 18.7 cpm appear acceptable tor one-ffme use. The higher thermotorming rate achieved using the coextruded sheet with the core layer ot beta-spherulite nucleated polypropylene 30 demonstrated the improved thermoformability of such coextruded sheet. The 16 ounc~ bowls prodwed from both Example 12 and 13 sheets had no detectable pink coloration to the human eye and only a slight hint of pink coloration when a number of the bowls were stacked together.

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Microwave oven and freeze drop tests were performed on ~he containers of Example 13. Twenty containers thermoformed at a production rate of 15-18 cpm were packed with 340 9 of product, sealed, frozen in a freezer at about -18C, removed from the freezer and dropped from a three foot height onto a concrete floor. The containers held up well during the test with some trays landing on their comers and some bouncing and flipping over. None of the tested 20 containers cracked. These containers were then subjected to a microwave oven test with eight minutes on a setting of high. All of the microwaved containers held up well and retained an excellent appearance.
ExamDle 14 and Control Exameles E-K
The composition ot Examples 14 and control Examples E-K compare the beta-spherulite nudeating efficiency of Q-dye to other beta nucleating ~-agents and to controls with no nucleating a~ent. All of the nucleants were in the form of fine powders. The nucleants and 0.18 w~/O based on the weight of the polymer of a stabilizer package, were added to various resinous polymers of prowlene and compounded using a 19 mm Brabender extruder~ The --Brabender extruder was also used to ~ast sheets from a 15.2 cm wide slit die and a 3-roll quench stack with a center cast roll temperature of 90C.
Control Example E was prepared from a polypropylene homopolymer having a nominal MFR of 2.0 dç,i/min and a nucleating agent composed of 50/50 blend by weight of terephthalic acid and calcium oxide at levels of 1, 10,and 100 ppm.
Control Example F was prepared trom a polypropylene homopolymer having a nominal MFR of 2.0 dg/min and a nucleating a~ent composed of a 50/50 blend by weight ot azelaic acid and barium oxide at levels of 1, 10, 100 and 1000 ppm.
Control Example G was prepared from a polypropylene having a nominal MFR ot 2.0 dg/min and no nucleating agent.
Control Exampb I was prepared from a blend of resinous polymers of propylene 27.4 wrh of polypropylene homopolymer having a nominal MFR ot 2.0 dg/min, 50 wt% ot an ethylene-propylene block copolymer having an ethylene content ot 40 wt%, and a nominal MFR of 1.0 dg/min, 5 wt% of a low molecular weight polypropylene having a melt viscosity ot 112 poise ~;
measured at 190~C and at a shear rate ot t36 sec1 and 17.6 wt% CaC03. ~ ;
Control Example H was prepared trom a polypropybne homopolymer having a nominal MFR ot 2.0 dg/min and a nucleating agent composed of a 50/50 bbnd by weight ot 1000 ppm azelaic acid and 1000 ppm CaCO3. -WO 93/21262 PCl`/US93/0345g ~111326 32 Control Example J was prepared from the blend of resinous polymers of propylene of Control Example I and 1000 ppm of azelaic acid.
Control Example K was prepared from the blend of resinous polymers of propylene ot Control Example I and a 50/50 blend by weight of 1000 ppm barium oxide and 1000 ppm of azelaic acid.
Example 14 was prepared trom the blend of resinous polymers of propylene ot Control Example I and 2 ppm of ~dye.
The results of Differential Thermal Colorimetry and x-ray diffraction measurements for Example 14 and Control examples E-K are summarized in Table X.
In Control Examples E and F, polypropylene homopolymer was blended with 1, 10, and 100 ppm of either terepmhalic acid with CaO, or azebic acid with BaO. No elevation of the Tc value was observed with either nucleant system, and only trace amounts of beta c~ystallinity were detected on the second heat scan. The two samples containing 100 ppm ot either nucleant were compression molded into thin fflms and examined under crossed polars.
Large alpha-type spherulites were seen, with only a scattering of beta spherulites.
In another study, levels ot 1000 ppm of different nucleants were used and blends were made up using both polypropylene homopolymer, as well as a blend containing polypropylene homopolymer, ethylene-propylene block copolymer, low molecular weight polypropylene. As controls, blends were also prepared with no nucleant present, and one sample contained 2 ppm of the Q-dye. One of the tilled samples was made with only the azelaic acid present, since a hi~h level ot CaCO3 was already there by virtue ot the tiller -par~cles. The DSC and x-ray data on these Control Exarnples are given in Table X.
The DSC data shows that the un-nucleated resins had Tc values in the ran~e ot 115-116C, and none ot the mixed oxide/acid nucleants show~d any significant eleva~on ot this value. Only the sample wilh the O-dye showed a significant~rise in the Tc value, and a signitkant beta melting peak. The x-ray - data on the cast films showed that only the presence ot the Q~dye produced a significant rise in the K-value.
:

WO 93/21262 PCl-/US93/03459 Ta~e X
Nucleatin~ A~ent Com~anson P~ee~y Co~l E Co~l F
5 Type TAJCaO A2~VBaO
Level, ppm 1 10 100 1 10 100 1000 Tc,C 112.8 112.7 112.6 112.3112.1 11.25 116.7 Tm~ C 158.2 158.5 158.6 158.3158.7 158.1 160.0 Tm~ C 144.3 144.9 144.8 144.8144.7 144.1 146.2 tft, caU~ 21.0 21.3 21.5 21.4 21.2 21.2 21.7 ~H~, caUg 0.2 Tr Tr Tr 0.1 Tr 0.5 K-value NM NM NM NM NM NM 0.16 Table X ICo~inued) Proee~ty Cont. G Control H ~Q~ Cont. J Control K
~an Type None AzA/CaC03 None AzA AzAlBaO Q dye Levei, ppm - 1000/1000 - 1000 1000/1000 2 Tc,C 116.0 116.2 115.3 115.4 115.2 120.9 Tm~C 159.8 160.6 160.4 159.4 158.2 160.2 Tm~ C 146.2 146.8 145.8 145.2 144.7 147.7 tf-t caU~ 22.4 æ.2 10.1 10.0 10.1 10.5 ~HB, caU~ 0.4 0.3 Tr 0.1 Q1 2.6 ' K-v~ue 0.11 0.15 0.16 0.06 0.06 0.72 10 TA - Terephthalic acid AzA - Azelaic acid Tr - Trace amount detected NM- Not Measur~d

Claims (20)

We Claim:

1. A thermoformable sheet comprising one or more layers of a crystalline resinous polymer of propylene having beta-spherulites present at a K-value of about 0.3 to 0.95.

2. The thermoformable sheet of claim 1 wherein said beta-spherulites are included within said resinous polymer by incorporating one or more suitable beta-spherulite nucleating agents into said resinous polymer before said sheet is formed.

3. The thermoformable sheet of claim 2 wherein said beta-spherulite nucleating agent is present at a level of about 0.1 to about 10 ppm and has the structural formula:

4. The thermoformable sheet of claim 3 wherein said resinous polymer of propylene is selected from the group consisting of polypropylene, random or block copolymers of propylene and up to 40 mol% of ethylene or an .alpha.-olefin having 4 to 12 carbon atoms and mixtures thereof, blends of polypropylene and low density polyethylene and blends of polypropylene and linear low density polyethylene.

5. The thermoformable sheet of claim 4 comprising an intermediate layer of said beta-spherulite-containing resinous polymer of propylene and two outer layers of a thermoplastic resin.

6. The thermoformable sheet of claim 5 having a thickness of about 0.25 mm or greater and said two outer layers each have a thickness of about 0.01 to about 0.1 mm and said intermediate layer has a thickness of about 0.23 to about 4.5 mm.

7. The thermoformable sheet of claim 6 wherein said thermoplastic resin is selected from the group consisting of polypropylene, random or block copolymers of propylene and up to 40 mol% of ethylene or an .alpha.-olefin having 4 to 12 carbon atoms, blends of polypropylene and low density polyethylene, blends of polypropylene and linear low density polyethylene, a block ethylene-propylene copolymer having an ethylene content of about 1 to 20 wt%, blends of ethylene-propylene rubber polymer and high density polyethylene and blends of ethylene-propylene rubber polymer and low density polyethylene.

8. The thermoformable sheet of claim 7 wherein said intermediate layer additionally comprises ethylene vinyl alcohol copolymer.

9. The thermoformable sheet of claim 7 wherein said intermediate layer additionally comprises a regrind material comprising a crystalline resinous polymer of propylene and a residue of an organic beta-spherulite nucleating agent.

10. A polymer composition suitable for forming the thermoformable sheet of claim 1 comprising a resinous polymer of propylene, one or more suitable beta-spherulite nucleating agents and about 0.05 to about 5 wt% TiO2 or CaCO3.

11. A method for thermoforming a resinous polymer of propylene-containing sheet comprising:
(a) melt forming a polymeric composition comprising a crystalline resinous polymer of propylene having alpha-spherulites and an effective amount of a nucleating agent capable of producing beta-spherulites into a sheet;
(b) quenching said melt formed sheet at a quench temperature sufficient to produce beta-spherulites wherein said beta-spherulites are present at a K-value of about 0.3 to 0.95;
(c) heating said quenched sheet to a thermoforming temperature sufficient to allow thermoforming of said sheet; and (d) thermoforming an article from said heated sheet with a thermoforming means under thermoforming conditions.

12. The method of claim 11 wherein said resinous polymer of propylene is selected from the group consisting of polypropylene, random or block copolymers of propylene and up to 40 mol% of ethylene or an .alpha.-olefinhaving 4 to 12 carbon atoms, blends of propylene and low density polyethylene and blends of polypropylene and linear low density polyethylene and said beta-spherulite nucleating agent is present at a level of about 0.1 to about 10 ppm based on the weight of the resinous polymer of propylene and has the structural formula:

13. The method of claim 12 wherein said quench temperature of step (b) is about 100° to about 130°C.

14. The method of claim 12 wherein said thermoforming temperature of step (c) is sufficient to melt said beta-spherulites but not sufficient to melt said alpha-spherulites.

15. The method of claim 11 wherein said thermoforming temperature of step (c) is less than the melting temperature of said beta-spherulites and said thermoformed article of step (d) has a side-wall density of about 2 to 20%
less than said quenched sheet of step (c).

16. A thermoformed article comprising one or more layers of a polymeric composition comprising a crystalline resinous polymer of propylene having alpha-spherulites and a residue of an organic beta-spherulite nucleating agent and having improved microwaveability compared to thermoformed articles comprising a resinous polymer of propylene without said organic beta-spherulite residue.

17. The article of claim 16 wherein said residue comprises and is present in said article in an amount of about 0.1 to about 10 ppm.

18. The thermoformable sheet of claim 1 in the form of a thermoformed food container.

19. The thermoformable sheet of claim 7 in the form of a thermoformed article having improved low-temperature impact resistance.

20. The thermoformed article of claim 16 comprising an intermediate layer of said beta-spherulite-containing resinous polymer of propylene and two outer layers of a thermoplastic resin selected from the group consisting of polypropylene, random or block copolymers of propylene and up to 40 mol of ethylene or an .alpha.-olefin having 4 to 12 carbon atoms, blends of polypropylene and low density polyethylene and blends of polypropylene and linear low density polyethylene.